Chapter 3 Heterogeneous Eliminations, Additions and Substitutions

263 Chapter 3

Heterogeneous Eliminations, Additions and Substitutions L. BERANEK and M. KRAUS

1. General features 1.1CORRESPONDENCE BETWEEN HOMOGENEOUS AND HETEROGENEOUS

REACTIONS

Elimination, addition and substitution reactions over solid catalysts are treated together in this chapter on the basis of some common features of their mechanisms and the acid-base nature of the catalysts. They behave in such an analogous way t o liquid phase reactions, both catalysed and uncatalysed, that electron shifts solely in pairs (heterolytic) have never been seriously doubted and free radical-like (homolytic) mechanisms have been considered only by few authors. The discovery of parallelism between acid-base reactions in solution and over solids helped t o advance the understanding of reaction mechanisms in this branch of heterogeneous catalysis much more than, for example, in catalysis over metals. The theory of organic reactions has been developed mostly with the help of experimental material concerning substitution and elimination in the liquid phase and the accumulated knowledge and proven research methods were utilised in interpretation of transformations over catalysts with acidic and basic properties. The first step in this approach was the recognition [l--31 that the cracking reactions of hydrocarbons over strongly acidic silica-alumina catalysts have patterns similar t o the reactions in the liquid phase catalysed by strong mineral Brplnsted or Lewis acids for which the carbonium ion mechanism has been suggested [4]. It took some time t o adopt a similar view of other heterogeneous elimination and substitution reactions. Most efficient experimental tools have been found in stereochemical studies, correlation of structure effects on rates and measurement of deuterium kinetic isotope effects. The usual kinetic studies were not of much help due t o the complex nature of catalytic reactions and relatively large experimental error. The progress has been made possible also by the studies of surface acid-base properties of the solids and their meaning for catalysis (for a detailed treatment see ref.

5).

The analogy between homogeneous and heterogeneous eliminations and substitutions has been pursued further. Joint action of an acidic and a basic site, suggested quite early for the heterogeneous dehydration of alcohols [ 6 ] ,has been gradually accepted as a general mode of operation

264 in acid-base catalysis over solids (e.g. refs. 7-9). No basic difference is now seen between the action of a surface acid-base double-centre and heterolysis of the bonds in an organic molecule caused by an attack of a base (or an acid) assisted by the solvent acting as a conjugated acid (or base) [ 91. Also, the nomenclatures for homogeneous elimination and substitution mechanisms have been adopted for heterogeneous reactions with only a slightly modified meaning. Of course, steric requirements are more restrictive in surface processes than in solution because the surface sites are immobile. On solid acid-base catalysts, beside elimination, addition and substitution, some other reactions also proceed. Of these, especially skeletal isomerisation of hydrocarbons and double bond shift should be mentioned. The latter can influence the product composition in olefin-forming eliminations and thus distort the information on orientation being sought. 1 . 2 NATURE OF THE CATALYSTS

From the point of view of chemical composition, the solid acid-base catalysts are oxides (like alumina, silica, thoria, magnesia), mixed oxides (like silicaalumina, silica-magnesia), crystalline aluminosilicates (zeolites), metal salts and ion exchange resins. The last type differs from the others in the character of reactant transport into the catalyst grain. With organic ion exchangers, which may or may not possess pores in the dry state, the important component of the reactant penetration into the grain is the diffusion through the more or less swollen macromolecular mass; then, in a favourable case, almost all acidic and basic functional groups may serve as active centres. With inorganic solid catalysts, the reactants reach the internal surface of a porous catalyst grain by means of diffusion through the pores; the bulk of the solid is not utilised for catalysis. Therefore, for understanding the ways in which a catalyst influences a reactant, the surface chemistry of the inorganic solids is important. In spite of much effort, the nature of the active sites on acid-base inorganic catalysts is still not completely understood. However, the work on this problem has shown how complicated the surface structure may be and that several types of active centres may be simultaneously present on the surface; the question is then which type plays the major role in a particular reaction. Also, the catalytic activity may be influenced t o a large extent by impurities present in the feed (catalytic poisons) or by-products of the reaction. The last point is often not taken into account and it will be discussed specially in Sect. 1.2.6. First, the models of surface sites on the most important and best-studied catalysts will be described.

1.2.1 Silica The surface of silica (for detailed description of results see refs. 5, 11 and 1 2 ) contains a variable amount of hydroxyl groups and adsorbed wa-

265 ter molecules. Even after heating t o 900°C in vacuum, it retains some OH groups (e.g. ref. 10). The absolute number of hydroxyl groups may differ from sample t o sample according t o the methods of preparation resulting in different participation of various crystal planes on the surface. Extensive research by means of IR spectroscopy and chemical reactions has shown (e.g. refs. 10-18) that two types of surface hydroxyl groups are present: single or free (A) and paired (B). Their relative proportions on partially dehydroxylated surfaces were estimated t o be 1 : 1 [14],1 : 2 [ 181, 1 : 9 [ 171. The nature of the paired hydroxyl groups is still a matter of discussion: vicinal ( B l ) and geminal (B2) structures are both possible. With vicincl OH groups, interhydroxyl hydrogen bonding is assumed H H I I 0 0 \ I

/St

/o\

0 0 0 I\ I \ I Si Si Si Si Si

0 0 0 I \ I\ Si Si Si B2

A

H I 0 I Si \ 0 \

H

H I 0 \

/si

0

I Si

B1

The surface of silica is highly reactive and hydroxyl groups exchange hydrogen for deuterium with DzO [ 14-16] but not with Dz. They can be replaced by C1 from Clz or CC14 [16] and they react with silanes and aluminium chloride [ 15,191. Surface alcoholates are formed when silica is contacted with primary or secondary alcohols [20] either by the reaction with hydroxyl groups \ \ -Si--OH + CH30H = S i - O - C H 3 + H,O l I

or by the rupture of a surface siloxane bond [21]

R f f r rc nccs u p . 38 5-3 98

266

A number of other substances react with surface hydroxyl groups forming surface compounds I221. However, for catalysis, the hydrogen bonding seems to be more important. With alcohols, the hydrogen bonds are formed in such a way that surface hydroxyl groups act as donors of hydrogen [ 231, viz. Si-O-H...O-R

I

H 1.2.2 Alumina

Aluminium oxide exists in many crystalline modifications, usually designated by Greek letters, some with hexagonal and some with cubic lattices (cf. refs. 11 and 24). The best known and mostly used forms are a- and y-alumina but practical catalysts are seldom pure crystailographic specimens. This makes the surface chemistry of aluminas rather complicated. Moreover, the catalytic activity of alumina depends very much on impurities. Small amounts of sodium (0.08-0.65%) poison the active centres for isomerisation but do not affect dehydration of alcohols [ 101. On the other hand, traces of sulphates and silica may increase the number of strong acidic sites and change the activity pattern. The surface hydroxyl groups and adsorbed water are important factors determining the surface properties of alumina (e.g. refs. 5, 11and 24). At present, we have at our disposal a model of the (100) plane which is probably exposed on the surface of spinel-type y-alumina (cf. ref. 24). The model is due to Peri [25] and is based on his detailed investigations by IR spectroscopy [ 261, by gravimetry [ 261, by ammonia adsorption [ 27 1 and by Monte Car10 modelling of surface dehydration [ 251. According t o Peri [ 2 5 ] , the (100) plane of alumina, fully hydrated at low temperatures, exposes a square lattice of OH groups [Fig. l ( a ) ] . If the dehydration were ideal, a regular surface of equally spaced 0 2 -ions would be formed [Fig. l(b)]. However, the splitting off of water molecules is a random process and, consequently, only two-thirds of the original OH groups can be removed without disturbing the original order. Further dehydration is possible only at the expense of some disorder. Ultimately, only isolated hydroxyl groups, which have no partner in the neighbourhood for the formation of water, remain on the surface. Five different types of these isolated hydroxyl groups can be distinguished according t o the number of neighbouring oxygen atoms in the surface layer (Fig. 2); their frequency depends on the degree of dehydration. The hydroxyl groups act as Br@nsted acidic sites and the exposed aluminium atoms in the second layer [Fig. l ( b ) ] as Lewis acidic sites. Rehydration of the surface changes the Lewis into Brqhsted sites. The Peri model of alumina also demonstrates that basic sites of various strength, consisting of oxygen atoms in various arrangements (isolated

267

0000 0000 000

(A)

Fig. 1. Ideal surface (100) plane of alumina after Peri [ 25 1. (A) T o p layer viewed perpendicualrly to the plane; (B) section through the three t o p layers. (a) Fully hydrated surface. ( b ) dehydroxylated surface. Open circles denote oxygen, filled circles hydroxyl, small black points aluminium,

0

8 00 (d)

0

00

o@o (e)

Fig. 2. Schematic representation of five different arrangements of oxygen atoms around the surface hydroxyl groups (filled circles) o n t h e (100) plane of alumina after Peri [25]. References p p . 385-398

268 atoms in the upper layer or two or three oxygen atoms on adjacent sites), are available on the surface. Experimental evidence for the presence of basic sites comes from adsorption of BF3 [ 281, titration with benzoic acid [29] and poisoning of the dehydration of alcohols over alumina by tetracyanoethylene [ 81 and by acetic acid [ 301. Different types of hydroxyl groups and oxygen atoms have different properties and the surface is therefore non-homogeneous. This heterogeneity manifests itself not only in the varying acid and base strengths of the sites but, and this might be more important for catalysis, in the frequency of suitably spaced pairs of acidic and basic sites. Strong evidence from mechanistic studies shows that such pairs are a prerequisite for the concerted elimination mechanism which predominates over alumina. The surface of alumina is highly reactive, not only t o water, ammonia or acetic acid, but also t o a number of other substances. Surface alcoholates are products of the interaction with alcohols [31] and carboxylate surface structures are formed from a fraction of adsorbed alcohol molecules [ 321. The action of hydrofluoric acid [ 33-35], as well as impregnation by BF3 [ 31,341, increases the acidity of alumina. 1.2.3 A lu m inosilica tes Aluminosilicates are the active components of amorphous silicaalumina catalysts and of crystalline, well-defined compounds, called zeolites. Amorphous silica-alumina catalysts and similar mixed oxide preparations have been developed for cracking (see Sect. 2.5) and quite early [36,37] their high acid strength, comparable with that of sulphuric acid, was connected with their catalytic activity. Methods for the determination of the distribution of the acid sites according t o their strength have been found, e.g. by titration with t-butylamine in a non-aqueous medium using adsorbed Hammett indicators for the Ho scale [ 381. The chemistry of silica-alumina catalysts has been reviewed several times (e.g. refs. 39-41) and the nature of acidic active sites has been discussed in numerous papers, very often from the point of view of whether Lewis or BrQlnsted sites are responsible for catalytic activity. The experimental methods for their separate determination are not very conclusive and in the actual catalytic process one type of centre may be converted t o another by the action of reagents, products or impurities. The experiments with various substances added t o the feed indicate (see the following sections dealing with individual reactions) that different types of reaction require sites of different strength. The great variety of Lewis and Brgnsted sites which may exist on the surface of silicaalumina has been demonstrated by Peri [42] on the basis of a simplified model of the reaction of AlCl, with a silica surface and subsequent hydrolysis. Peri has constructed eight different surface sites by combining possible groups on the surface of silica with possible aluminium ion structures; more arrange-

269 ments can probably be thought of. Some of the Peri sites are Al’ I \ 0 0

‘si’

0

‘ A1 I \

0 0 \ I Si

do‘ lAf

I \ 0 0 I di si \ I 0

0 I

0 I si s i \ I 0

The original view, that in the reaction of silica with aluminium hydroxides a strong aluminosilicic acid, which possesses a dissociable proton (e.g. ref. 2), is formed has not been proved. H-aluminosilicates are unstable and spontaneously convert t o aluminium aluminosilicates [ 191. Crystalline aluminosilicates (zeolites, molecular sieves) catalyse a number of organic reactions [43] and the striking difference between them and amorphous silicaalumina is that they are active for cracking even in the form of Na’ of CaZ+salts [ 44,451 ; these cations are poisons for silicaalumina. However, metal salts of zeolites exhibit strong acidity [ 51. This acidity is of both the Lewis and Brq5nsted type and strong Lewis sites are converted t o Brgnsted by water [46]. The catalytic activity of zeolites depends on the nature of the cation but it seems (cf. refs. 47 and 48) that the active centres are not metal cations or hydroxyl groups attached t o such ions. As with amorphous silica-alumina, the active centres in zeolites are probably situated on the aluminosilicate surface. The function of metal cations is not clear; they might stabilise the structure and influence the degree of hydroxylation and hydration of the surface which are important factors for catalysis. In the section dealing with alumina and silica, the necessity of basic sites on the surface, which cooperate with acidic sites, has been stressed. Also, for both amorphous silica-alumina and zeolites, the simultaneous presence of acidic and basic sites has been proved and it has been suggested that OH groups act as amphiprotic centres according t o the nature of the adsorbed species [ 491. 1.2.4 Metal salts

Solid metal sulphates and phosphates also exhibit acid-base properties; their acid strength is lower than that of silicaalumina but they are stronger acids than some oxide catalysts [ 51. Correlation of activity with electronegativity of cations has been obtained for several reactions [ 9, 50,511.

270

1.2.5 Ion exchange resins Organic ion exchangers are macromolecular substances containing chemically welldefined acidic or basic functional groups, The macromolecular skeleton may be formed by polycondensation or, more frequently, by copolymerisation. The use of basic (anion) exchangers as catalysts (e.g. in aldol condensation) is rather rare; the main representatives of acidic sites in cation exchangers are sulphonic (-SO,H), phosphonic (-PO(OH),) and carboxylic (-COOH) groups. In the kinetic studies reported in this chapter, sulphonated styrenedivinylbenzene copolymers were used almost exclusively. They may be of two types: (i) non-porous (standard) ion exchangers whose grains do not possess internal porosity in the sense usual in catalysis, and (ii) porous (macroreticular) ion exchangers with artificially developed porous structure (pores of about 10-20 nm prevailing) and a large inner surface area. Ion exchangers can be used as catalysts both for liquid (standard ion exchangers are preferred) and vapour phase (macroreticular ion exchangers are more convenient) reactions. The main factors determining the catalytic activity of ion exchangers are: (i) the acid strength of the functional groups (sulphonated resins are much more active than the others), (ii) the concentration of functional groups in the protonated form (ion exchangers fully neutralised with cations are catalytically inactive) and (iii) the degree of crosslinking of the copolymer, i.e. the content of divinylbenznee (DVB). There is no doubt that the functional groups in ion exchangers are responsible for the catalytic activity. Although they are chemically defined, it is not clear in which form they participate in the catalytic reaction, since a certain amount of water is always present in the resin which cannot be removed easily. It has been proven by IR spectroscopy [52541 that in polystyrenesulphonic acid several hydrated forms of the --S03H groups may occur (mono-, di-, tri- and tetra-hydrates) and, in consequence, the mobility of the proton of a -S03H group and also the catalytic activity may change. Lower hydration states are also possible through hydrogen bonding of one water molecule to two or more sulphonic acid groups. Even fully dehydrated sulphonic groups may be hydrogen bridged, for example in the form

P-H***o o=\\

-S=O

ii 0.e.H-O

\\

-

The lack of information about relative activities of different forms and the unknown dependence of their relative concentrations on catalyst pretreatment and reaction conditions, and the influence of reactants, products (water) and solvents, introduce uncertainty into the interpretation of kinetic measurements.

271

It seems probable, in view of the idea presented in Sect. 1.1,that, in elimination, addition and substitution reactions over ion exchangers, also, two types of catalytic sites are involved, viz. acidic (protons of the functional groups) and basic, which are likely t o be represented by oxygen atoms of the functional groups. A typical property of ion exchange resins which distinguish them from inorganic catalysts is swelling; this is the more important factor the lower is the degree of crosslinking of the copolymer. Due t o swelling, a considerable amount of reactants, products and solvents can be retained (absorbed) by the resin and the functional groups inside the polymer mass may also be utilised for catalysis. Thus, the accessibility of the catalytically active groups can be facilitated, not only by a artificial porous structure (which increases only the number of the groups on the surface of the polymer mass), but also by swelling. In this situation, the rate of reactant transport (diffusion), not only through the pores (if their are present), but also through the more or less swollen polymer mass, may become important. If the rate of diffusion through the polymer is much larger than that of the chemical reaction, then, in the extreme case, all functional groups may be utilised for catalysing the reaction. In the opposite case, when the diffusion through the polymer mass is much slower than the chemical reaction, only the surface groups will act as catalytic sites. This latter was observed with highly crosslinked ion exchangers and large reactant molecules and the term “sieve effect” was used t o describe it. 1.2.6 The working surface

The surface structures outlined in the preceding sections have been determined under conditions very far from those of an actual catalytic reaction. At partial pressures of reactants used in flow reactors and in the steady state, the catalyst surface is very probably almost covered by starting substances and products. This is indicated by the type of kinetics found for various reactions (see following section); very often zero-order expressions or Langmuir-Hinshelwood type rate equations with high values of adsorption coefficients have been found. Some products of the catalytic reactions are of special interest in this connection. Water formed in elimination, esterification or condensation reactions is present in sufficient quantities to change almost all Lewis sites into Br@nsted sites. Much more fundamental changes can be caused by hydrogen halides produced in the decomposition of alkyl halides on oxides; it is well known that the catalytic activity of alumina can be enhanced by the action of hydrochloric or hydrofluoric acids. It is evident that the study of free surfaces and of surfaces covered only partially by various substances at temperatures much lower than those needed for a catalytic reaction to proceed can give only indirect inforReferences P P . 385-398

272 mation about possible states on working surfaces. Better evidence is obtained by observing the influence of substances added t o the feed which can interact with some surface sites a t reaction conditions. For example, in this way the importance of basic sites has been confirmed. Linear correlations of effects of reactant structure on rate and adsorptivity are also helpful and especially the interpretation of their slopes may yield valuable information (e.g. refs. 55 and 56). The transient-response technique, in which the changes in product composition after an abrupt stop or start of the feed flow are observed, is also promising. 1.3 TYPE OF KINETICS

The complex nature of heterogeneous catalytic reactions, which consist of a sequence of at least three steps (adsorption, surface reaction and desorption), the possible intervention of transport processes and the uncertainty about the actual state of the surface makes every attempt t o obtain a complete formal kinetic description without simplifying assumptions futile. In this situation, some authors prefer fully empirical equations of the type

r

=

kpi&

...

(1)

which bear no connection to the mechanism. With the exception of zero-, first- and second-order expressions, the interpretation of the constants h, a, b, ..., cannot be used as a basis for the elucidation of the laws governing catalytic reactions. However, simple kinetic models, especially of the Langmuir-Hinshelwood type, can serve with advantage for correlation of experimental data in spite of simplifying assumptions which are necessary for their derivation. Experience shows that heterogeneous acid-base catalysis is the very field where they fit best. Their most frequent general form

where Ki denotes the adsorption coefficient of the substance i, a, b, ..., = 1 o r $ and n = 1, 2, 3, .,., is well suited to the estimation of the competition of all substances present in the system for active centres. However, because the same equation may be obtained on the basis of various different assumptions (cf. ref. 57), its form cannot be used as a proof of a certain mechanism. Of the assumptions accepted for the generation of LangmuirHinshelwood type and related equations, the most controversial seems t o be that the surface is homogeneous. It has been shown in the preceding section that inorganic oxide catalysts and even ion exchangers contain a number of differing acidic and basic sites, i.e. they possess an inherent heterogeneity. The question is how this “static” non-homogeneity manifests itself

273 under the dynamic conditions of a catalytic reaction. Some sites may be ineffectual for steric reasons when they d o not find a basic (or acidic) partner site within a suitable distance. Out of the residual spectrum of sites differing in strength, some are probably too weak t o be able t o initiate bond reorganisation in adsorbed molecules. Other sites can bind the reactants or products too strongly and thus be blocked out. Working sites come, therefore, from a band which is narrower then the original one estimated on the basis of adsorption measurements (including determination of the number of acidic sites by titration with a base etc.). The position and width of this working band must depend on the chemical nature of the reagent (e.g. cracking of alkanes requires other sites than dehydration of alcohols) and on the form of the distribution curve of sites according t o their strength. Some experimental results are available which show the influence of surface heterogeneity on the kinetics and the contribution of sites of different strength t o the over-all rate. The surface of acidic catalysts has been divided into several fractions by acidimetric [ 581 or thermochemical [ 591 titrations and on the basis of group analysis [59]and partial poisoning [58,60] the contribution of these fractions has been calculated. It has been found that the over-all rate of dehydration is determined by the performance of a single narrow fraction, the contribution of the others being almost negligible. Another approach t o this problem involved modelling of acidic catalysts with different sites by mixing ion exchangers containing functional groups of different acidity [ 611. For dehydration; the over-all activity was again given by the activity of the strongest (-S03H) group. For re-esterification, the contribution of weaker centres (-PO(OH),) could not be neglected but the over-all kinetics could still be correlated by a single Langmuir-Hinshelwood rate equation. Summarising, it seems that the surface heterogeneity is not such a serious problem for the formal kinetic description of acid-base catalysis on solids as would be expected from the studies of the surface by non-kinetic methods. Moreover, the rate equations for non-homogeneous surfaces, developed by the Russian school (Temkin, Roginskii and Kiperman, see ref. 62) are similar t o eqn. (2); the term 1 is not present and n can have any value greater than 0 (cf. also ref. 63). Only their further drastic simplification leads to equations of type (1). The next problem of the LangmuirHinshelwood kinetics, the validity of the ratedetermining step approximation, has not been rigourously examined. However, as has been shown (e.g. refs. 57 and 63), the mathematical forms of the rate equations for the LangmuirHinshelwood model and for the steady-state models are very similar and sometimes indistinguishable. This makes the meaning of the constants in the denominators of the rate equations somewhat doubtful; in the Langmuir-Hinshelwood model, they stand for adsorption equilibrium constants and in the steady-state models, for rate coefficients or products and quotients of several rate coefficients. References P P . 385-398

274 The problem discussed in Sect. 1.2.6, i.e. what composition the working surface has, also has its kinetic counterpart. If the number of active sites of a certain type depends on the partial pressures of some reaction components, then the question arises whether rate equations of type (2) are sufficient for the description of such changes. All these facts and unsolved problems require that the rate equations of type (2) be taken as semi-empirical expressions. They may be directly utilised for engineering purposes with higher certainty than eqn. (l), but they reflect the actual react.ion mechanism only in general features. However, the constants are a good source of values for comparison of reactivities and adsorptivities of related reactants on the same catalyst. Such interpretations of experimental data are usually quite meaningful as is confirmed by successful correlations of the constants with other independent quantities. 2. Elimination reactions In organic chemistry, elimination processes are those decompositions of molecules whereby two fragments are split off and the multiplicity of the bonds between two carbon atoms or a carbon atom and a hetero atom is increased. Such a broad definition also embraces the dehydrogenation of hydrocarbons and alcohols which is dealt with in Chap. 2. Here we shall restrict our review t o the olefin-forming eliminations of the t Y Pe I I I I --(+?-C,=Cc =

X

+ HX

H

Although some observations (e.g. ref. 7) indicate that the process need not (Y, 0-(or 1 , 2-) elimination, practically all experimental results have been interpreted on the assumption that 1,3- and 1,4-eliminations d o not participate significantly. The substituents X may have very different structures but heterogeneous catalytic eliminations with X = halogen, OH, alkoxyl, NR2 (R = H or alkyl), SH, OCOCH3 and alkyl or aryl only have been described. The individual reactions are usually named according to the compound HX which is the product, i.e. dehydrohalogenation, dehydration etc. but some exceptions exist (e.g. cracking). The reverse reactions are additions t o the C--C multiple bonds which will be dealt with in Sect. 3 of this chapter. Homogeneous olefin-forming eliminations have been studied extensively, especially in the liquid phase and comprehensive treatments of the subject are available [ 64,651. The rules governing the course of homogeneous eliminations and their mechanisms are well established and the interpretation of the results obtained with heterogeneous catalytic sys-

to be always an

275 tems can obtain useful assistance from these. In this connection, a recent review on catalytic eliminations is especially valuable [9]. 2.1 COMMON FEATURES O F HETEROGENEOUS CATALYTIC ELIMINATIONS

2.1.1 Mechanism

In discussing the mechanism of eliminations over solids, the nomenclature which has been developed for homogeneous reactions will be used. Therefore the basic mechanisms of olefin formation have first to be outlined and their meaning in heterogeneous catalysis defined. The E2 mechanism is so called because the process is bimolecular and in solution consists of an attack by a base on the P-hydrogen atom with synchronous splitting of the substituent X in the form of an anion. In heterogeneous catalysis, the most important feature is the timing of the fission of the two bonds C,-X and CB-H: in the E2 or E2-like mechanism, these bonds are broken simultaneously. Because this can be achieved only by the action of two different centres, a basic one and an acidic one with both present on the sudace, the kinetic distinction of the mechanism loses its original sense under these circumstances. The E l mechanism has, as the ratedetermining step in solution, the ionisation of the reactant forming a carbonium ion which then decomposes rapidly. For heterogeneous catalytic reactions, the important features are the occurrence of the reaction in two steps and the presence on the solid surface of carbonium ions or species resembling them closely. Again, the kinetic characterisation by way of an unimolecular process is of little value. Even the relative rates of the two steps may be reversed on solid catalysts. A cooperation of an acidic and a basic site is also assumed, the reaction being initiated by the action of the acidic site on the group

X.

The ElcB mechanism is a two-step process beginning with the abstraction of a proton from the P-position by a base to give a carbanion. The second step is the loss of the group X as an anion. In heterogeneous catalysis, the corresponding mechanism consist of the primary action of a basic site assisted later by an acidic site which temporarily accomodates the group X-. It is evident that the simple model of heterogeneous catalytic eliminations assumes the same adsorption complex for all mechanisms, written schematically as

-c-c-

I I X. H.

. . 00 The only distinction between various mechanisms is the timing of the References p p . 385-398

276 fission of the bonds C,-X and C,-H. Usually, a continuous spectrum of mechanisms is assumed in which E l , E2 and ElcB are processes with a clearly defined character. This idea, which has been slowly developed during the last decade, has been discussed in detail in a recent review [ 91. It is certainly compatible with views on the nature of elimination catalysts. These solids are typically oxides o r metal salts which have positively and negatively charged atoms on their surface. In the array of electron-donating and electron-accepting centres, pairs of required acidic and basic sites with suitable spacings can be found. Because the strength of the sites is different on individual catalysts according t o their structure, the catalysts can be put into a sequence, from those where the basic character predominates through those where basic and acidic properties are in balance to those with prevailing acidic nature. It is d e a r that a catalyst wifl transform a reactant by means of the mechanism which corresponds t o the predominating acid o r base strength of the sites. It is well known from homogeneous reactions that the mechanism depends also on the strength of the C,-X and C,-H bonds and this applies also t o heterogeneous catalysis. The double influence on the “choice” of mechanism, i.e. of the nature of the catalyst and of the reagent, has been graphically represented by Mochida et al. [66] (Fig. 3). They have

-

-

Cd-H Bond strength Fig. 3 . Schematic representation of the influence of reactant structure, of catalyst nature and of temperature on the elimination mechanism. Numbers in parentheses denote the rate-determining steps on Scheme 1 .

Cp- H

277

*

xI

I

H .

A h <=CX-H' I I A B

. . X. H. . .

A B

*=C-

X- H' I 1 A B

-

1

x.

:

H' I

*=CX- H' I 1 A B

Scheme 1. A = acidic site, B = basic site.

inserted into the mechanism scale two additional fixed points, E2cA and E2cB, and attributed to every mechanism a single ratedetermining step. Scheme 1 and Fig. 3 are modified versions of their original representations; as will be shown later in sections dealing with individual elimination reactions, their hypothesis can accomodate various experimental facts very well. This somewhat simplified picture of possible transitions from one mechanism to another can be expanded and supplemented by a finer differentiation of the factors influencing bond strength and catalyst acidbase properties. Such structural parameters are the number and nature of substituents on C, and C, and the nature of the group X. The action of a catalyst depends on its cation charge and radius, on anion basicity and on lattice and surface arrangement (for some details see ref. 67). A temperature increase usually shifts the mechanism in the direction of E l . 2.1.2 Orientation

Most heterogeneous catalytic eliminations proceed according to the Saytzeff rule, i.e. the most stable olefin is formed. This is in agreement with the prevailing situation in E2 and E l homogeneous eliminations and the reasons might be the same (cf. refs. and 64 and 65). The transition state is probably considerably in the direction of the double bond formation, and hyperconjugation with the groups on C, and C, helps t o stabilise the species with more alkyl substituents on the double bond. The notable exception is the dehydration of alcohols on thorium oxide which is governed by the Hofmann rule. However, in this case the ElcB mechanism has been shown to occur [ S S ] . References P P . 385--398

278 More complex is the problem of syn and anti eliminations (cis and trans in the older nomenclature), i.e. whether the substituents on C, and C, leave the parent compound from synperiplanar or from antiperiplanar positions, viz. H

XH

X

SYn anti The mode of the elimination can be recognised by the composition of the products if the olefin formed has such substituents on the double bond that cis and trans stereoisomers may be distinguished. The question is of interest with respect to the concerted E2 mechanism, because in the pure E l and ElcB processes, the intermediate carbonium ion or carbanion, respectively, usually have enough time to rotate around the C,-C, bond, equilibrate and give the same cidtrans ratio from different conformers. The factors influencing the syn/anti elimination ratio were extensively studied with various catalysts, using pairs of compounds with different steric arrangements like meso and df forms of 2,3-dihalobutanes, threo and ery thro forms of 2deutero-3-X-butanes or cyclic stereoisomers. The syn/anti ratio depended on the nature of the catalyst and was low where a more or less concerted mechanism could be assumed (i.e. on not too strongly acidic catalysts) but, in general, a preference for anti-elimination was observed (cf. refs. 9 and 69): Whereas the syn-elimination proceeding on the surface of a solid catalyst can be easily visualised, there were considerable difficulties in explaining the anti-elimination where the leaving components of HX are on the opposite sides of the C,--C, bond. This problem is not encountered in liquid phase eliminations since the leaving groups can be temporarily bound by other reactant molecules or by the solvent. The first explanation of the anti mode of heterogeneous elimination was that it takes place in narrow pores or crevices, the fragments X and H being bound to opposite walls of the openings [ 71. However, this seems improbable since pores or crevices with a suitable distance between the walls must be rare and therefore the reaction rates would be rather low. Noller and Kladnig [ 91 considered the possibility of hydrogen tunelling through the electron cloud of the other atoms of the reactant; however, this hypothesis lacks support for which it would be necessary to do quantum chemical calculations. The proton can be taken and transported t o the surface by means of another molecule of the reactant [70] or by the product HX [9]. The feasibility of such assistance has been confirmed by quantum chemical cal-

279 culations for the dehydration of 2-propanol [ 701. Knozinger e t al. [71] have suggested a model of anti-elimination on the surface of solids which has been considered further by Noller and Kladnig [9] and by SedlaEek [72]. It explains, without further assumptions, the anti-elimination over heterogeneous catalysts as a natural reaction course. On most oxides, water or its components (H' and OH-) are firmly bound to the surface under the conditions used for elimination. It can be shown by suitable atomic models or drawings that a molecule with X and H in antiperiplanar positions can easily find a pair of sites, an acidic site (formed, for example, by a surface hydroxyl group) and a basic site (formed by a oxygen atom), to which it fits well without notable deformation of bond angles and interatomic distances, and thus gives rise t o the anti-eliminaaxis must not lie parallel or tion. The only condition is that the C,-C, perpendicular to the surface plane. Figure 4 shows the model of such adsorption of isopropanol on an alumina surface. Also, for syn-elimination, suitably spaced acidic and basic sites may be found on the surface of alumina [ 721. The question then arises, what is the reason for the preference for anti-elimination when conditions exist for both modes? This has been explained on the basis of quantum chemical calculations [73]. An

Fig. 4. Adsorption complex of 2-propanol o n a partially dehydroxylated (100) plane of alumina after SedlaEek [72]. Small open circles denote H, medium circles with thin hatching 0, medium circles with dense hatching C, large open circles Al. F o r consistency, covalent atomic radii have been used f o r 0 in alumina although its structure is partly ionic ; therefore aluminium atoms appear unusually large. The diagram shows the possibility of a two-point interaction of a n alcohol with a surface hydroxyl group and a surface oxygen without any distortion of t h e molecule. References p p . 385-398

280

attack by a positively charged species on X o r by a negatively charged one on H changes the distribution of electrons in the parent molecule and its bond lengths. The C,--CB bond is strengthened and the C,-X and CD-H bonds are weakened. The rotation of substituents around the C a - C o axis brings about two minima of total energy, a smaller one for the synperiplanar conformation of X and €3 (rotation angle 0") and a larger one for the antiperiplana; conformation (rotation angle 180").The unti-elimination is therefore energetically more favoured than the syn mode and the syn/ anti ratio must depend mostly on the difference in the energy of the two conformations of the particular substance. Of course, the relative frequency of suitable pairs of sites for anti and syn eliminations must also play a certain role, as the variation of syn/unti ratios from catalyst t o catalyst show.

2.1.3 Kinetics For elimination reactions of the general form

A=R+S the simplest Langmuir-Hinshelwood type rate equation, assuming a surface reaction on a single centre as the ratedetermining step

has been found suitable in a number of cases (see sections on individual elimination reactions). However, more often it has been applied in simplified forms. The first one is valid for the case where the adsorption of reactant A is very strong and, in consequence, the surface is almost entirely covered by it all the time, i.e. KAPA >> 1 + K R p R + &pS. This gives the zero-order rate equation

r=k (4) The second simplified form corresponds t o the case where A, R and S are all weakly adsorbed, i.e. 1>> KApA + KRpR + K s p s . Then eqn. ( 3 ) is reduced t o a first-order rate expression

r

= kKAp, = k'p,

(5)

The complications begin when one of the products (the HX molecules particularly) can influence the reaction rate, not only by adsorption on active sites blocking a fraction of them but by forming new active centres of a different nature. Then the parameter k is no longer a constant, but changes with the composition of the reaction mixture. This possibility has received only limited attention until now, but could explain some unusual empirical rate equations which have been found for the dehydration of alcohols on oxide catalysts [8,69].As has been outlined in

281 Sect. 1.2, the surface of an oxide can change its structure quite easily. For example, cracking catalysts exchange oxygen with water almost instantaneously at higher temperatures [74,75]; the interaction of water with such catalysts must therefore be a complex process, involving not only an adsorption in molecular form. In the literature on elimination reactions, it is found that mechanistic conclusions are quite frequently made on the basis of the values of the activation energy. This is a dubious practice, especially when the data have been obtained by the gas chromatographic (pulse-flow) technique, i.e. when there is a non-stationary state on the catalyst surface, and on the basis of supposed first-order kinetics. True activation energies are obtained when the reaction order is zero and probably also when the rate coefficient, k, and adsorption coefficient, K A , have been separated by treatment of rate data by means of eqn. (3). In the case of the first-order rate equation, the apparent activation energy, calculated from k ’ values [eqn. (5)] by means of the Arrhenius equation, is the difference between the true activation energy and the adsorption enthalpy of the reactant A

(6) Therefore there is little emphasis in this chapter on the values of the activation energy. = Etrut- - AHA

Eapp.

2.2 DEHYDRATION

2.2.1 Types of dehydration reactions In general, dehydration means loss of water molecules from chemical substances, irrespective of their structure. Even if all cases where water is bonded in hydrate form are excluded, a number of reactions remain which also include formation of nitriles from amides, lactones from hydroxy acids etc. However, the present treatment will concentrate on the heterogeneous catalytic decomposition of alcohols in the vapour phase, which can be either olefin-forming or ether-forming reactions, and on the related dehydration of ethers t o olefins. The dehydration of alcohols on solid catalysts is one of the first catalytic reactions discovered and has been studied intensively for many decades. During years of experimental work, a general parallel-consecutive reaction scheme (Scheme 2 below) has been developed by gradual addition of /ether

2 alcohols

[H*O\ olefin + alcohol + H,O

\2 olefins + 2 HPY2@0

Scheme 2. Refereiices p p . 385-398

further steps t o the original parallel scheme [ 76-84]. The relative participation of the various steps depends on a number of factors. Prominent is the structure of the starting alcohol. Alcohols which have no @hydrogens, like methanol or benzyl alcohol, yield only ethers. The tendency t o give an olefin increases with the substitution on C, ; with secondary alcohols, the olefin/ether ratio is much higher than with primary and from tertiary alcohols only olefins are formed. Also substituents on C, may influence the ratio. The next important factor is the temperature, on increasing which more olefin is formed than ether. Although all steps are reversible, the thermodynamic calculations for ethanol have shown that, over 2OO0C, the equilibria are shifted t o the production of ethylene [ 831. Other factors influencing the olefin/ether ratio are the partial pressure of the starting alcohol because olefin-forming and ether-forming reactions obey different kinetics (see Sect. 2.2.3) and the nature of the catalyst. The dehydration of alcohols over solids has been the subject of several excellent reviews which summarise most of the vast literature [ 7,69,85871. Therefore in this chapter, reference will be made only to the papers which are most significant, those that are newer or which have not obtained adequate attention in preceding reviews.

2.2.2 Catalysts Hundreds of substances of many types have been tested as dehydration catalysts and found active. Lists can be found in the literature [69,76,85] and we need t o name here only such catalysts which show high activity and selectivity. The latter parameter is more important because a number of solids, especially oxides, can catalyse both the dehydration and the dehydrogenation of alcohols. The formation of aldehydes or ketones is then a parallel reaction t o the dehydration, and the ratio of the rates depends on the nature of the catalyst. Only few oxides are clean dehydration or dehydrogeneration catalysts, but the selectivity may be shifted t o some extent in either direction by the method of catalyst preparation. The important groups of dehydration catalysts are oxides, aluminosilicates (both amorphous and zeolitic), metal salts and cation exchange resins. Most work on mechanisms has been done with alumina.

2.2.3 Experimental kinetic results

( a ) Formal rate equations The complicated reaction scheme for the dehydration of alcohols (Scheme 2) makes kinetic analysis rather difficult. However, initial reaction rates have been measured, without special problems, for secondary

and tertiary alcohols, and even for primary alcohols. Low conversions are also desirable because water is adsorbed more strongly on alumina than are the alcohols [ 881 and modifies its surface. Initial reaction rates can be obtained separately for olefin and ether formation. Complete kinetic descriptions of a somewhat simplified Scheme 2 have been attempted several times [ 82,831 for the reaction of ethanol on the basis of data from integral o r differential flow reactors. Tables 1 and 2 summarise published results on the kinetics of alcohol and ether dehydration. The data are organised according t o the rate equation found or assumed t o be the best one. Table l shows that, for olefin TABLE 1 Rate equations for t h e catalytic dehydration of alcohols t o olefins Alcohol Catalyst Temperature (“C)

Ref.

r=k 1-Propanol, 2-propanol, 1-butanol, 2-butanol, tert-butanol Ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-l-propanol, tert-butanol 4-Heptanol, 2-methyl-3-hexano1, 2,4-dimethyl-3-pentanol,cyclopentanol, cyclohexanol, cycloheptanol, cyclooctanol, cisand trans-2-methylcyclohexanol, cis- and trans-4-tert-butylcyclohexanol 2-Methy1-3-butano1, 2-methyl3-hexanol, 2,4-dimethyl-3-pentanol, 2-methyl-4-ethyl-3-hexano1 1-Phenylethanols (p-CH3, p-F, H, rn-OCH3, rn-F) Tert-butanol 3-Deutero-2-butanol,cisand trans-2-methylcyclohexanol Ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-l-propanol, tert-butanol, 1-pentanol, 2-pentanol, 2-methyl-l-butanol, 3-methyl2-butanol, 2,2-dimethyl-l-propanol, 2-methyl-2-butanol Refererices p p . 385-398

Bauxite

100-250

89,90

A1203

300-4 00

88, 91-93

A1203

180-210

94

300

55

220

95

50 2 10-350

8 96

28 2-39 5

56

A1203 + NaOH Ti02 Si02 Zr02 A1203 + NaOH

Ti02

SiO? Zr02 A12 03-Si02 A12 0 3

hydroxyapatite O3

hydroxyapatite

TABLE 1 (continued) Alcohol

Temperature ("C)

Ref.

350,400 380,400 2 5 0-3 00 346-465 274-3 1 4 425 121-177 232-343 204-260 204-371

79 a 98,99 100 101 82 98 102 102 103 104

200 300-360 170-195

105 106 107

A12 O3

Tert-butanol

Ion exchanger Ion exchanger

346-4 6 5 2 50-350 210-240 90-110 95

108 109 110 111 112

Tert-butanol

A1203

130-180

113

r

=

Catalyst

kKpgA/(l+ CKipi) i

Ethanol

A12 O3

Tert-butanol 1-Hexanol, 1-heptanol 2-Butanol Ethanol, 1-propanol, l - b u tan01 1-Phenylethanols 2-Phenylethanol 1-Propanol

AI203-Si02 A1203-Si02 Alz03-Si02 Alz03-Si02 Al203-Si02 A12 0 3

ZnO BP04

r = k K o A / (1 + C K i p i ) 2 i

Ethanol CyclohexanoI 2-Propanol

a

A12 0 A120

3 3

Without 1 in the denominator of the rate equation.

pw denotes partial pressure of water. In its absence, the equation is reduced t o the zero-order expression.

formation, the zero-order rate equation and its parent (see Sect. 2.1) single-site Langmuir-Hinshelwood type expression predominate. The few cases where the Langmuir-Hinshelwood rate equation with the second power in the denominator has been found are rather puzzling; only for the reaction on an ion exchanger has some explanation been suggested [ 1121. The smaller number of results in Table 2 concerning ether formation are less consistent. The rate equations with the square-root of the partial pressure of the alcohol are either empirical expressions without an underlying kinetic model or are based on a very complicated assumption about the mechanism.

285 TABLE 2

Rate equations for the catalytic dehydration of alcohols to ethers

Alcohol

Methanol Ethanol 2-Propanol

Catalyst

Temperature (“C)

Ref.

Al2 O3

2 50-3 00

100

Ion exchanger A12 O3 Ion exchanger

Ion exchanger

119 274-314 80-120 210-240 90-110

131 82 113 110 111

Ion exchanger

111-150

139

A1203

152-279 150-195 130-240

115 116 117

AIZ O3

r = kKApA/[l + ( K f l ~ ) ” ~ ] ~ Methanol

r = k p i 5 / @ i 5+ bpw) a Methanol Ethanol 1-Propanol, 2-methyl-1-propanol, cyclohexanol, phenylmethanol

r = k p y / ( l + ap:’

+

Methanol a

0 3 0 3

bpw) a Alz03--Si02

160

8

p w denotes partial pressure of water.

For the reaction of diethylether giving either ethylene and ethanol or ethylene and water, the validity of the Langmuir-Hinshelwood type rate equations has again been confirmed [ 821. Different approaches to the kinetics of alcohol dehydration were attempted by two groups of authors [ 118,1191. In one case, it has been assumed that the active surface of alumina is formed either by free hydroxyl groups or by surface alkoxyl groups. The rate equation was then derived on the basis of the steady-state assumption; a good fit t o the experimental data was obtained [1118]. The second model was based on the fact that water influences the adsorption of an alcohol and diminishes the available surface. The surface concentrations of tert-butanol and water were taken from independent adsorption measurements and put into the first-order rate equation; a good description of integral conversion data was achieved [ 1191. References p p . 385-398

( b ) Structure and reactivity

Kinetic deuterium isotope effects have been measured with alcohols deuterated in various positions and have yielded information about ratedetermining steps. As expected, deuterium placed on C, does not influence the rate; this has been shown by comparing the dehydration of 2-butanol and 2deutero-2-butanol on some phosphate [ 1201 and sulphate catalysts [121]. In contrast, deuterium atoms on C, change the rate in comparison with non-deuterated compounds [ 96,122,1231. The value of the kinetic isotope effect depends on the catalyst [123] and on the temperature [122], as both factors affect the mechanism (cf. Sect. 2.1.1). Also, deuterium in the hydroxyl groups can influence the rate to various extents according t o the nature of the catalyst. Table 3 shows how the kinetic isotope effects in the dehydration of differently deuterated 2-propanols change with the catalyst, indicating a change of mechanism. The secondary isotope effect was not observed in catalytic dehydration [96, 120,1211. The dehydration rate depends very strongly on substitution on C , . Large differences in reactivity of primary, secondary and tertiary alcohols over solid catalysts were reported as early as in 1931 by Dohse [go]. Also, substituents on C, affect the rate. Both influences can be quantitatively described by the Hammett and Taft relationships; the published correlations are summarised in Table 4. Of special interest is the extensive set of alcohols of the type R'R2R3COH [56], which includes primary, secondary and tertiary alcohols and gives a single Taft correlation with an excellent fit. The values of p and p * which can give information about the mechanism and catalyst nature will be discussed in the following sections. The structure of the reactant also influences the orientation, i.e. the ratio of 1-t o 2-alkenes in the dehydration of 2-alkanols and the ratio of cis t o trans alkenes. Table 4 shows that these ratios can also be correlated by the Taft equation. For the cis/trans ratio, a better fit was obtained with steric E , constants of substituents than with polar constants [ 1271. TABLE 3 Kinetic isotope effects of deuterium ( h H / h D ) in the dehydration of 2-propanol o n various catalysts at 300°C [ 1 2 3 ] Catalyst A1203 + NaOH Zr02

Ti02

SiOz

~ ( C H .JCHOH . CH3) h(CH3 . CHOD . CH3)

h(CH3 . CHOH . CH3) k ( C D 3 . CHOH . CD3)

1 .o 1.1 1.2 1.5

1.5 1.4 1.2 1.0

287 The steric requirements of the surface during the formation of the adsorption complex or transition state also manifest themselves in the dehydration of rigid alcohols with fixed conformations, e.g. of cyclic alcohols. Cis- and trans-2- and 4-alkylcyclohexanoIs differ markedly in their rate of dehydration on alumina (see Table 5). Most significant are the data on 4-tert-butylcyclohexanols where the bulky tert-butyl group is in an equat6rial position, and thus the differences in the reactivity of the cis and trans isomers indicate the differences in the reactivity of axial and equatorial hydroxyls. The high reactivity of cis-2-tert-butylcyclohexanol is caused most probably by steric acceleration of the elimination, which is, however, absent in the case of 2,2-dimethylcyclohexanol. Table 5 also gives data on the effect of ring size on the rate which are consistent with observations on other reactions of cyclic compounds. This influence can be explained by the change of the strain in the ring (Istrain) in consequence of the change from sp3 t o sp2 hybridisation [ 1291. The above results give a clear picture of structure-reactivity relationships at a chosen temperature. The published information about the influence of temperature on these relationships is less consistent. In cases where the kinetics were clearly established (usually being zero order), the activation energy for individual members of a homologous series of compounds (e.g. 1-alkanols) has been found t o be structure-independent within the experimental error [92,94,104,124,128], On the other hand, the slightly different activation energies for a series of primary alcohols have been correlated successfully, together with the considerably different activation energies for secondary and tertiary alcohols, by means of the Taft equation [ 561. A similar decrease of the activation energy with substitution on C, was reported very early [90,92]. The activation energy of dehydration is further influenced by steric effects as indicated by the data in Table 5.

(c) Direct ion of elimination Over most catalysts, with the notable exception of thoria [130], the thermodynamically more stable olefins are formed (Saytzeff rule) as primary products when two elimination directions are possible. This is in agreement with the results concerning other elimination reactions, both in the liquid phase (cf. refs. 6 4 and 65) and over solid catalysts. The striking difference in the action of thoria has been explained on the basis of a different mechanism [ 681. The frequently observed preference for anti-elimination over syn-elimination on alumina (for a summary of earlier results see ref. 7 , later especially ref. 96) has been a cause of much controversy. However, as has been explained in Sect. 2.1.2, it is a natural reaction course for concerted elimination, provided that suitably spaced acidic and basic sites are available on the surface. Catalysts which operate by means of the El-like mechanism References p p . 385-398

f\3

00 00

TABLE 4 Linear free energy relationships for the dehydration of alcohols over solid catalysts Substi tuen t s (X o r R )

Alcohols

X . C,H4

. CHOH . CH3b

H, p-CH3, p t - C 4 H 9 , m-F, p-F, m-OCH3 H, p-CH3, rn-F, p-F, m-OCH3

Catalyst

A1203

Zr02

+ NaOH

Ti02 SiOz A12

O3

A1203 + NaOH ZrOz TiOz Si02 ROH

A12 0 3

A1203-Si02 + NaOH

Temp. ("C)

Correlation of a

porp*

Ref.

200

log k-0'

-2.6

105

220 220 220 220 380

log k-u' log k-' log k-0' log k-<+

-2.2 -2.1 -2.2 -2.4 -15.8

95 96 95 95 124,125

300 300 300 300 220 250

log k-0' log k-0' log k-' log k-' log k-' log r-u'

1.2 0.3 -0.8 -2.8 -2.0 -13.3

55 55 66 55 56 126

log F

0'

Non-stoichiometric hydroxyapatite

R . CH2 . CHOH . CH3b

= RZ = H, R3 = CH,; = R 2 = H , R 3 = C2 H5 ; = RZ = H, R3 = l - C 3 H 7 ; = RZ = H, R3 = 1C4H9 ; = R2 = H, R3 = 2-C3H7; = R2 = H, R3 = [-C4 H 9 ; = H, RZ R3 = CH 3 ; = H, R2 = CH3, R 3 = C 2 H S ; = H, R2 = CH3, R3 = 1-CjH7; R' = H, R2 = CH3, R3 = 2 - c ~ H 7 ; R'=HR2=R3=C 2 H5 ; R' = Rz = R3 = CH 3 ,. R' = R2 = CH,, R3 = C2H5 CH,, C2HS, l-C3H7, 2-C3H7,1-C4Hg

ROH

CH,, C2HS, 1 G H 7 , C6HSCH2

A1203

R'R~R~COH

tJ

P

a

R' R' R' R' R' R' R' R' R'

1

230

log h-u*

Hydroxyapatite

395

log h-u*

A1203

210 210 160

log S21-u* log Sct--E, log t-u*

282 350

-5.1

-4.5 -3.9

56

-2.3

2.2 0.3 2.9

127 128

k , rate coefficient; r , reaction rate; S 2 1 , ratio of rates of 2-alkene and 1-alkene formation; S,,, ratio of rates of cis- and trans-2-alkene formation; u*, Taft substituent constant; u+, Hammett substituent constant for mesomeric interaction between reaction centre and aromatic ring; E,, Taft steric constant. Olefin formation. Ether formation. Value of t h e 6 parameter.

290 TABLE 5 Effect of ring size and substitution on dehydration of cyclic alcohols at 200°C [94] ____

Alcohol

kRl

____ Cyclopentanol Cyclohexanol Cy cloheptanol Cyclooctanol 2,2-Dimethylcyclohexanol trans-2-Meth ylcyclohexanol cis-2-Meth ylcyclohexanol trans-4-Methylcyclohexanol cis-2-Tert-bu tylcyclohexanol trans-4-Tert-butylcyclohexanol cis-4-Tert-butylcyclohexanol

-

Activation energy (kJ mol-')

150 145 145

1.9 1.0 2.3 8.6 0.9 1.8 12.1 1.9 40 2.1 13.6

140 180 85 160 80 160 115

give cis and trans products in comparable quantities as experiments with threo- and erythro-3deutero-2-butanols have revealed [68,96,132].

2.2.4 Mechanism

( a ) Surface complex A number of contradictory views have been published concerning the structure of adsorbed alcohols and the nature of adsorption sites (for review see ref. 69). Experimental evidence from IR investigations has shown that, on alumina, alcohols form several surface complexes of very different chemical natures (e.g. refs. 31, 32, 117, 133-137): (i) alcohol molecules weakly bonded t o the surface, very probably by hydrogen bonds (I) (such complexes are sometimes denoted as physically sorbed alcohols); (ii) surface alkoxides (alcoholates) (11); (iii) surface carboxylates (111). Less certain is the existence of species with partial double bonds or of ketone-like species. The formation of the various surface complexes is dependent on the structure of the alcohol. For examples, weakly bonded species (I) have been found with all alcohols, alkoxides (11) mostly with primary alcohols, sometimes also with secondary alcohols, but have never been reported for tertiary alcohols.

sp

IH

H I 0 Al

A1

T0 I

R I 0

y

0 0

0 A1 A1

A1

R I C

(-%a

0 0 0 A1 A1

291 The question now is which of these complexes are intermediates in the dehydration of alcohols. There is general agreement that the carboxylate structure (111) is a product of a side reaction and has therefore no connection with dehydration. Species (Ia) and (Ib) could be natural precursors of the surface alkoxides (11) but such a transformation has not been observed t o be the main reaction. Hydrogen-bonded species either desorb without any change in the alcohol structure (or configuration [135]) or dehydrate directly t o an olefin [ 1371. However, the alkoxides (11) are very probably intermediates in the formation of ethers as will be shown later. Also, on an ion exchanger with sulphonic groups, the adsorption complex of methanol is hydrogen bonded [ 1381. ( b ) Mechanism of the surface reaction

For the olefin-forming reaction, two alternative paths have been considered with minor variations concerning the number and nature of surface metal and oxygen atoms which take part in the elementary steps. (i) The alkoxide mechanism assumes complex (11) as the intermediate; it was suggested by Sabatier [76] and advocated mostly by Topchieva et al. [81]. \I

A1

A1

A1 A1

A1

A1

(ii) The cyclic mechanism assumes cooperation of an acidic and a basic site; it was suggested by Eucken and Wicke [6] and supported by Pines and Manassen [ 71 and numerous other authors.

A1 A1 References p p . 385-398

A1 A1

Al A1

292 All recent results suggest the second mechanism. The arguments for its validity may be summarised as: (1)high stereospecificity of elimination on a number of catalysts; (2) existence of both basic and acidic sites on dehydration catalysts; (3) the possibility of treating all elimination reactions in a common way from the point of view of mechanism (cf. Sect. 2.1). Recently, a new argument has been added on the basis of quantum chemical calculations [70]which has shown that the attack of an acid on the hydroxyl group activates the hydrogenes on C, for elimination whereas the loss of hydrogen from the hydroxyl or its substitution by a metal ion (corresponding t o the formation of the surface alkoxide 11) activates the C,-H bond for dehydrogenation. The cyclic mechanism is probably seldom a fully concerted (E2) process, and the different timing of individual electron shifts results in a transition towards the E l or ElcB mechanisms (cf. Sect. 2.1.1). The “choice” of the mechanism depends on the reactant structure as well as on the catalyst nature. As an indicator of the mechanism, either the degree of stereoselectivity (see refs. 68, 121, 132 and 141) or the value of the reaction parameter of a linear free energy relationship, e.g. p or p * constants of the Hammett and Taft equations (cf. ref. 551, may be used. Pines and Manassen [ 71 suggested that tertiary alcohols are dehydrated by the E l mechanism involving the formation of more or less free carbonium ions, secondary alcohols by a mechanism lying somewhere between E l and E2 (i.e. synchronous with a ionic contribution) and primary alcohols by a concerted E2 mechanism. However, the large kinetic isotope effect for the dehydration of fully deuterated tert-butanol on alumina [122] indicates that, even in this case, some synchrony must exist. Alumina strongly prefers the concerted process and with other catalysts the situation may differ. In some cases, the effect of reactant structure may outweigh the influence of catalyst nature. This is seen by comparison with the dehydration of aliphatic secondary alcohols and substituted 2-phenylethanols on four different oxide catalysts (Table 4). With aliphatic alcohols, the slope of the Taft correlation depended on the nature of the catalyst (A1203 + NaOH 1.2, ZrOz 0.3, TiO2-O.8, Si02-2.8 [55]) whereas for 2-phenylethanols, the slope of the corresponding Hammett correlation had practically the same value (from -2.1 t o -2.4) for all catalysts of this series [95]. The resonance stabilisation of an intermediate with a positive charge on C, clearly predominates over other influences. In contrast to olefin-forming dehydrations, the transformation of alcohols t o ethers very probably includes surface alkoxides as intermediates. It is assumed that one molecule of the alcohol forms the alkoxide which is then attacked by the second alcohol molecule either from the gas phase or from a weakly adsorbed state. Again, cooperation of acidic and basic sites seems to be necessary [ 116,142,1431. The important step of ether forma-

293 tion is a nucleophilic substitution, viz. \I/ C \I/ I c ... 0 I 3 I

O

H

The arguments for the suggested mechanism are: (1) Similar products are obtained by the decomposition of metal alkoxides containing no 0-hydrogens and by the reaction of corresponding alcohols on alumina at lower temperatures [ 1421. (2) Acetic acid and pyridine are poisons for the formation of ethers [ 1431. (3) The different degrees of water inhibition on the ether and olefin formation from ethanol on alumina, and the agreement of etherlethylene selectivity ratios found experimentally with those calculated by the Monte Car10 simulation of the hydrated surface of alumina [ 1441. (4) Correlation between the rate of ether formation from ethanol and the surface concentration of ethoxide species determined by IR spectroscopy [ 1361. (5)The positive value of the Taft reaction parameter for the formation of ether in contrast t o negative values for the olefin formation on the same catalyst (Table 4).

(c) Influence of catalyst nature The necessity of cooperation between surface acidic and basic sites for splitting off the elements of water from alcohol molecules was intuitively suggested quite early [ 6 ] and used as a working hypothesis by an increasing number of authors. However, it took some time t o recognise which type of acidity and basicity is suitable for dehydration. The experiments with reversible poisoning of alumina by small amounts of bases like ammonia, pyridine or piperidine revealed [ 8,137,142,145, 1461 relatively small decreases of dehydration activity, in contrast t o isomerisation activity which was fully supressed. It was concluded that the dehydration requires only moderately strong acidic sites on which weak bases are not adsorbed, and that, therefore, Lewis-type sites d o not play an important role with alumina. However, pyridine stops the dehydration of tert-butanol on silica-alumina [ 81. Later, poisoning experiments with acetic acid [ 1431 and tetracyanoethylene [ 81 have shown the importance of basic sites for ether formation, but, surprisingly, the formation of olefins was unaffected. The picture has been clarified by surface acidity and basicity distribution measurements for several catalysts using thermometric titrations with References p p . 385-398

294

1-butylamine and trichloracetic acid [ 59,601. A fine balance between acidic and basic strengths of the working sites is necessary: ethylene formation requires the cooperation of moderately strong acidic sites with weak basic sites, whereas for diethylether formation, moderately strong basic sites play an important role. Moreover, the poisoning by bases increases the basicity; the disappearance of moderately basic sites manifests itself in a decrease of ether formation. These relations seem to be valid for the dehydration of primary alcohols, but secondary and tertiary alcohols may need other combinations of acidic and basic sites. It has been observed that the dehydration of tertbutanol was more sensitive t o the presence of strongly acidic sites than the reaction of methanol, but both processes required basic sites [ 81. All this is in accordance with the dynamic mode1 of elimination mechanisms presented in Sect. 2.1, which allows transition from E l t o E2 or further t o ElcB according t o the structure of the reactant and the nature of the catalyst. The relation between the acid strength of the catalysts and the mechanism has also been demonstrated by correlations [55,123] of the reaction parameter, p * , of the Taft equation for the dehydration of secondary alcohols on A1,03 + NaOH, ZrO,, TiOz and SiO, (see Table 4) with the sensitivity to pyridine poisoning, the heat of adsorption of water and diethylether and the kinetic isotope deuterium effects (Table 3) on the same catalysts (Fig. 5). The parameter p* reflects the mechanism being

-AHads Log 7 log a Fig. 5. Correlation of the Taft reaction parameter for the dehydration of secondary

alcohols (see Table 4) on four different oxide catalysts with the heat of adsorption,

AH,,,, of water and diethylether, with the sensitivity of the rate to pyridine poisoning 7,[55] and with the value of the deuterium kinetic isotope effect [ 1 2 3 ] for the same catalysts.

295 related t o the free energy change on the interaction between the reactant's reaction centre and the catalyst's active centre. The kinetic isotope effect also depends directly on the mechanism and the other quantities show the acid strength of the active centres. It is believed that, in this series of catalysts, the transition toward the E l mechanism is observed in the direction from alkalised alumina t o silica. The question remains open whether the addition of alkali t o alumina shifts the mechanism fully t o ElcB. Another diagnostic method for determining the effect of the nature of the catalyst on the mechanism is the observation of the stereoselectivity of elimination [68,121,132]. It has been found, using the reaction of threo- and ery thro-3deuter0-2-butano1, that, in a series of salt catalysts, only the phosphates Ca3(P04)*,CaHP04, Ba3(P04)*and A1P04 preferred the E2 mechanism while on other metal phosphates and all carbonates, the E l mechanism predominated [ 1321. Another fine distinction among salt catalysts was obtained by following the activity and olefinlether selectivity of metal sulphates in the dehydration of ethanol and 1-propanol. A linear correlation between the electronegativity of the metal ion and the activity has been found, but the selectivity gave a curve with a minimum [ 511. 2.3 DEAMINATION

2.3.1 Types of deamination reactions The transformation of amines over acidic catalysts is more complicated than that of other compounds which can undergo elimination. Like alcohols, amines form either an olefin or a higher substituted amine (the formal counterpart of an ether). With trivalent nitrogen, the reaction scheme includes more compounds than divalent oxygen allows, viz. RNH,

J

.

olefin + NH3

-NH3 +NH3

. R2NH

J

olefin + RNHz

1

etc.

Scheme 3 .

-NH3 +NH3

R3N

J.

olefin + R2NH

1

etc.

At temperatures above 250" C, the olefin-forming reactions are irreversible but the transformations in the first line of Scheme 3 are reversible. Thus, starting with an arbitrary amine, all other derivatives are obtained by these reactions, called disproportionations, transalkylations or dismutations (the nomenclature is also inconsistent in that the analoguous formation of ethers from alcohols is named dehydration). Similarly, like secondary and tertiary alcohols, amines with the alkyl groups branched on a-carbon atoms (i.e. containing the grouping R'R'CHReferences P P . 385-398

296 N- and R’RZR3C-N-) tend more to olefin formation than t o disproportionate. N-Alkylidenalkylamines were found in the reaction products of the transformation of l-butylamine [ 147,1481, cyclohexylamine and isopropylamine [149] on alumina, and were probably formed by the dehydrogenation of the primary amine to an imine, followed by its condensation with a second molecule of the amine [ 1481, rather than by the dehydrogenation of the dialkylamine [ 1471. The N-alkylidenalkylamines R=N-R decompose to an olefin and an imine; a cyclic process has been postulated [ 1481 which explains the increases in reactivity of amines with secondary alkyls. Also butyronitrile has been detected in appreciable amounts in the reaction products of l-butylamine on alumina at 500°C [ 1471. Much less information is available on the deamination and related reactions over solid catalysts than on some other elimination reactions but it suffices for comprehension of the general features.

2.3.2 Catalysts Only fews solids have been used as catalysts for deamination and disproportionation reactions. Among them, alumina has been studied most frequently, and some attention has also been paid to silicaalumina and t o molecular sieves [ 1491. The activity of alumina for the disproportionation of weakly basic aniline t o diphenylamine can be enhanced by impregnation with HC1 [ 1491 or H3B03 [ 1501.

2.3.3 Experimental kinetic results ( a ) Formal rate equations In most cases, the rate equation, eqn. (3) (p. 280), was suitable t o describe the deamination of alkylamines t o alkenes on alumina. It has been applied t o the decomposition of mono-, di- and trialkylamines [ 1511 and of cyclohexylamine [ 1481. In some cases, the simpler zero-order rate expression (cf. Sect. 2.1.3) has been observed [ 149,1531. Equation (3) can also describe the disproportionation of amines such as aniline [ 151,1541 and diethylamine [ 1531. However, the related expression

~KAPA (6) (1+ K A P A + K B P B + KCPC)’ where A denotes the starting amine, B the amine formed by the reaction and C ammonia, also suits the data on disproportionation quite well [ 152,1531. In two cases, more elaborate kinetic models were necessary in order to

r=

297 obtain a good fit of the experimental data. For the deamination of triethylamine and diisopropylamine on alumina, the superposition of two parallel processes [ 1553, one of zero order and the second described by eqn. (3), has been suggested; the final expression had the form

To explain the retardation by the very weakly basic aniline of the deamination and disproportionation of diethylamine and diisopropylamine on alumina, the acidic and basic sites were separately balanced in the derivation of the rate equation and the following expression was obtained [156], viz.

where the subscript P denotes aniline and a, b the acidic and basic sites, respectively. The complete system of deamination and disproportionation reactions has been treated with success by means of eqns. (3) and (6) on the basis of integral data with the exclusion of the time variable, i.e. with relative concentrations of reactants and products [ 1521. ( b ) Structure and reactivity

Two extensive sets of data on the reactivity of various amines in deamination and disproprotionation on alumina give an insight into the influence of structure on rate [ 149,1521. However, the picture is complicated by different effects of the number and nature of the alkyl substituents on the reaction rate coefficient and on the adsorption coefficient. Adsorption studies have revealed [157-1601 that the amount of adsorbed amine depends mostly on the size of the molecule. A linear correlation was obtained between the reciprocal cross-section of the molecule and the adsorptivity [ 1571. However, practically no differences have been observed for the series of primary amines RNHz with the straight chain alkyl groups; this was explained by a perpendicular orientation of the adsorbed amine molecule with respect t o the surface [160]. On the contrary, the adsorptivity decreased with branching of the alkyl group [160] and with the number of alkyl groups on the nitrogen [157--1591; steric hinderance seems t o be the obvious cause. Similarly, large differences in adsorptivity of pyridine and 2,6-dimethylpyridine on alumina have been reported [ 1581. Also, the adsorption coefficients, Ki, determined by means of eqn. (3) from kinetic data [ 1521 show the same trend. The rate coefficient of deamination increases with the inductive effect of the alkyl group as has been demonstrated by the published kinetic data [152] and their correlation by the Taft equation [125]. This was later References p p . 385-398

298 confirmed by experiments which were not complicated by disproportionation and in which the size of the amine molecule was the same or almost the same in the whole series. The deamination of N-ethyl-N-propyl-Nbutylamine showed an increase in the reactivity of the alkyl groups in the order ethyl < propyl < butyl [ 1491. The same trend was observed within the series of alkyldimethylamines [ 1491. It has also been found that the reactivity of the isopropyl group is influenced by other (less reactive) alkyl groups on nitrogen in the order methyl < ethyl < propyl [ 1491. No data are available on the direction of elimination in the deamination on acidic catalysts.

2.3.4 Mechanism The adsorption of ammonia and amines has been studied many times as a method of estimation of the acidity of solid surfaces. Some of the results are pertinent t o the mechanism of amine transformation on these catalysts. Depending on the structure of the catalyst surface, several types of adsorbed species have been observed. Nitrogen bases can be adsorbed on both Lewis and Br@nstedacid sites by means of their free electron pair. Other modes are realised by hydrogen bonding, the hydrogen originating either from a surface OH group or from NH groups of primary and secondary amines. Under suitable conditions, dissociative adsorption of NH3 has been observed by IR spectroscopy; surface NH; ions are formed [7]. Ammonium ions NH'4 have also been found [ 1611. Methylamine [ 1621, aniline [ 1631 and pyridines [ 1361 are adsorbed by means of their nitrogen free electron pairs on Lewis acid sites. Several types of adsorbed species can exist simultaneously on the surface. Measurements of the heat of adsorption of ammonia on silicaalumina catalysts indicated two types of adsorbed forms, one with a low energy of adsorption of about 30 kJ mol-', the second form having double that value [ 1641. Thermal desorption and gas chromatographic studies of adsorption of amines on alumina led t o the suggestion that three types of adsorption have t o be distinguished: below the coverage of 0.1 mmol g-l, strong irreversible adsorption on A13+ sites, in the range 0.1-0.3 mmol g-', reversible adsorption on OH groups and over 0.5 mmol g-' adsorption o n 02-ions also [159]. The reaction mechanism of amine deamination and disproportionation has been put forward by analogy with other eliminations, namely dehydration and dehydrochlorination [ 149,1551, its characteristic feature being the cooperation of acidic and basic sites. In the deamination, 0-hydrogen and the NR, group ( R is hydrogen or alkyl) are eliminated by an E2-like mechanism on alumina, but by an El-like mechanism on silicaalumina. The nature of the acidic sites is not clear, protons from surface hydroxyls o r aluminium ions are possible candidates. However, it seems

299 quite probable that strong A13+sites are poisoned at a very early stage by bases present in the system and consequently do not contribute much t o the reaction (cf. ref. 159). Saturation of the dehydrated alumina surface with water, creating new hydroxyl groups, increased the rate of deamination [ 1491. All this is in favour of the surface hydroxyl groups as acidic sites. The basic sites are most probably the surface 0'- ions. Then the surface complex can be written as

Quantum chemical calculations for some alkylamines, RNH', and their protonised forms supported this model [ 1651. As in the case of alcohols [139], the action of an acid on the nitrogen atom activates the @hydrogens and the weakening of the C,-H bond is most pronounced when one hydrogen atom is in the antiperiplanar position with respect t o nitrogen. Thus, the anti (or trans) elimination is again suggested as the most favoured mode of elimination. The disproportionation of amines is visualised as a nucleophilic substitution at the 0-carbon atom of an amine adsorbed on an acidic site [149,153]. The attacking species is an amine adsorbed by a hydrogen bond onto a basic site

This is again a direct analogy with ether formation from alcohols (see Sect. 2.2.4). The acidic sites might be the A13+ions because rehydration of the alumina surface does not enhance the rate, in contrast t o deamination [ 1491. Very little is known about the behaviour of different catalysts; only a few comparisons of alumina and silica-alumina have been made. On Alz03-Si02, the disproportionation of diethylamine is more rapid by one order of magnitude than its deamination; on A1203, the rates are comparable [ 1491. The activity of alumina for aniline disproportionation is higher than of silica-alumina [150]. The steric demands of the alumina surface are higher than those of silica-alumina as the comparison of the chemisorption of pyridine and 2,6dimethylpyridine has shown [ 1581. Rc9ferences p p . 385-398

The complex interplay of basic and acidic sites in the deamination and disproportionationn of amines is the probable cause of the “stop-effect” which has been observed in the reaction of triethylamine on alumina [155] and, more recently, of other amines [149]. When the steady state on the catalyst surface in a flow reactor is rapidly changed by substituting the amine feed for a nitrogen stream, a rapid temporary increase in olefin production is observed. This phenomenon has been explained as the result of the increased availability of basic centres which were previously blocked by adsorbed molecules [ 149,1551. 2.4 DEHYDROHALOGENATION

2.4.1 Types of d e hy d rohalogena t ion reactions The 0-elimination of hydrogen halides HX from organic halogen compounds yields olefins o r acetylenes, depending on the structure of the starting substance and the number of HX molecules which have split off, viz. X-A-C-H I

I I =C=C + H X I I

(A)

The reaction is reversible and, in general, higher temperatures and lower partial pressures favour the decomposition by shifting the equilibrium t o the right-hand side. The position of the equilibrium is influenced by the groups attached to C, and C,; for example, the heating of 2,2,2-trichloro1,l-bis(pchloropheny1)ethane with FeC13 t o 115--120°C results in a rapid and quantitative conversion t o 2,2dichloro-l,l-bis(p-chlorophenyl)ethylene [ 1661, whereas for high conversions of 1,2dichloroethane t o vinyl chloride, temperatures over 400°C are zeccssary. The equilibrium of reaction (B) is less favourable for the formation of an alkyne and, in order t o achieve equal conversion, much higher temperatures would be required than are necessary for the olefin-forming elimination (A). Therefore little attention has been paid to this type of reaction and this section will be devoted solely to type (A) dehydrohalogenation.

2.4.2 Catalysts A large variety of catalysts, both homogeneous and heterogeneous, has been found active for dehydrohalogenation. The catalysts include a number of Br4nsted and Lewis acids (liquid or soluble, as well as solid), metal oxides, active carbon, carbides, nitrides and some metals. However, in the latter case, the actual catalysts are most probably surface metal halides

301 TABLE 6 Comparison of the activity of different catalysts for the dehydrochlorination of 1chlorobutane [ 1 7 4 ] Catalyst

Temperature ("C) of incipient decomposition

of appreciable decomposition

~.

Thoria Zirconia Calcium chloride Calcium phosphate Alumina Barium chloride Bone charcoal, HCI washed Active carbon

205 205 224 240 24 5 24 5 255 280

220 220 245 260 260 210 275 305

formed from the metal and the reactant [ 1671. Even glass shows some catalytic effect [ 1681 making purely thermal rate measurements impossibly in glass vessels; again, surface metal halides might be the cause of the activity. From the point of view of the present review, the most important catalysts are metal salts and metal oxides which have been used for dehydrohalogenation of simpler organo halides in the gas phase and studied with respect t o kinetics and mechanism. The metal salt group is represented mainly by A1F3, FeF, and MgF, [ 169,1701 and chlorides, sulphates, carbonates and phosphates of the Group Ia, I1 and 111 metals which were employed by many authors but systematically studied by the Noller school (for review see ref. 67) and by the group of Mochida and Yoneda. Occasionally, alkali and alkali earth metal bromides were used. Oxides of the Group Ia, 11, I11 and IV elements are active catalysts and comparative descriptions of their properties has been published by several authors (e.g. refs. 67, 1 7 1 and 172). However, there is some difficulty in finding out the relative activities since most papers have dealt with selectivity problems or with comparisons of the activity in closely related catalyst groups (e.g. in a series of barium salts with different anions [173]). Some information about the activity of quite different types of catalyst is brought together in Table 6.

2.4.3 Experimental kinetic results ( a ) Formal rate equations Only a few kinetic studies on dehydrochlorination and dehydrobromination have been published. They are summarised in Table 7 and the general impression is that the more complicated rate equations have resulted References p p . 385-398

TABLE 7 Kinetics of catalytic dehydrohalogenation Catalyst

Chloroethane A12 0 3

SiOz CaClz Na2S04 CaS04 SrC12 Alz 0 3 MgS04

Technique

Temperature Rate equation range (“C)

Activation energy (kJ mol-’)

Ref.

Flow Flow Flow Flow Flow Flow Flow Flow

520-600. 368-420 355-404 38 7-42 1 354-4 13 404-438,

100 54 46 96 63 120 88 117

175 175 175 175 175 175 176 176

r=

kKAPA (1 + C K i p i ) i

1 -Chloropropane

MgS04

177

Flow

1,2-Dichloroethane Active Flow carbon

r

=

kp2’

or

120-140

178

1,1,2-Trichloroethane Pulse-flow NiS04 1 -Chlorobutane Zeolite Flow 2-Chlorobutane Zeolite Flow

48

180

38

180

1 -Brornobutane A1203-KBr

a4

181

63

181

Stop-flow Stop-flow Stop-flow Stop-flow Stop-flow

79 125 120 84 79

181 182 182 182 182

2-Brorno-2-rnethylpropane Al203-LiCl Stop-flow

130

182

Stop-flow

2-Brornobutane A1203-KBr Stop-flow 1-Brorno-2-methylpropane

Al203-KBr A1203-LiCl A1203-KCl A1203-NaCl A1203--C~Cl

179

r = kpA

from more careful work with continuous flow reactors in which the conditions were varied in a broader range. The chromatographic (pulseflow) method usually requires the postulation of first-order kinetics as a basis for the treatment of data. However, Langmuir-Hinshelwood types

303 TABLE 8 Deuterium kinetic isotope effects in the dehydrobromination of the 1,2-dibromoethanes CH2Br. CH2Br and CDzBr. CDzBr [ 1 7 2 J

SrO NaOH-SO2 CaO K2S04-Si02

2.15 1.83 1.65 1.65

KOH-Si02 NaOH-SiO? NiS04-Si02 Al203-Bz03

--_ 1.59

1.31 1.23 1.05

M a O3 Si02-AI2O3 NiSO4

1.02 1.00 1.00 1.00

rate equations suggested by some authors can easily transform t o the firstorder expression if the adsorption of reactants is small (see Sect. 2.1.3). f b ) Structure and reactivity

The rate of catalytic dehydrohalogenation is influenced by the structure of the reactants, but the extent of this effect varies from one catalyst to another with change of mechanism, i.e. with the timing of the fission of the C,-X and C,-H bonds. This is best seen from the published data on the deuterium kinetic isotope effect in Table 8. Their significance for the elucidation of the mechanism will be dealt with in Sect. 2.4.4 and here we can simply state that the value of the isotope effect depends on the nature of the catalyst. However, with a different reactant and within a series of related catalysts, k H / k D values independent of the catalyst were obtained (Table 9) [ 1831. A clearer picture emerges from studies of substituent effects on the rate with a single catalyst. A series of alkyl chlorides (C, to C,) was decomposed on a barium sulphate catalyst [184] and the rate data were correlated by the Taft equation. Large negative values of p* were obtained, viz. -34.3 at 220°C and -40.3 at 280°C. Similarly, for a series of three alkyl bromides (ethyl, propyl and isopropyl) on silicaalumina, aluminaTABLE 9 Deuterium kinetic isotope effects in the dehydrochlorination o f 2-chloropropane ~ 8 3 1

_ _ _ _ _ ~

Catalyst

kH/kD

for the pair

CH3CHCICH3 CH3CDCICH3

CH3CHCICH3 CD3CHClCD3 1.8 1.75 1.8 1.6

References P P . 385-398

304 TABLE 10 Order of reactivities in the dehydrochlorination of chloroethanes o n different catalysts

I

Reactant

Product

CHCl=CH:! CHCI=CH, CCl2=CH2 CC12=CH1 CICH=CHCI CCl2=CHC1

Cl2 CH-CH3 ClCH24HZCI C13C-CH 3 Cl;, CH-CHZCI

ClZCH-CHC12

Order of reactivity on

SrO and

A1203

A1203-Si02

6 5 3 2 4 1

3 5 1 6 4 2

2 5 1 6 3

4

boron oxide and alumina at 300”C, a p* value of about -20 was found [172]. This is in accordance with similar correlations of substituent effects on the rate of pyrolysis of alkyl chlorides and bromides at 400°C and 350°C ( p * = -23.5 and -22.3 respectively) [185]. The unusually high p* values can be attributed to the fact that the substituents were defined in both cases as R in RX instead of the more correct R ’ in R‘CH2CH2X and t o the high polarity of the transition states. The complex influence of both the reactant structure and catalyst structure is evident from attempts t o correlate the reactivities of a series of chloroethanes (see Table 10) on basic catalysts (SrO and BaO), on alumina and on strongly acidic catalysts (A1203-Si02, A1203-B203) with the delocalisability of hydrogen atoms [l86]. The delocalisability of an atom is a quantum chemical reactivity index calculated by the simple LCAO-MO method according t o Fukui et al. [187]. For basic catalysts, a good linear relationship was observed between the rate corrected by the number of equivalent hydrogen atoms and the delocalisability of the hydrogen atoms in the @position t o chlorine atoms for the nucleophilic abstraction. However, no correlation was obtained for acidic catalysts and alumina. The reactivity of a halogen compound in dehydrohalogenation over solid catalysts also depends on its steric arrangement. This was shown by studying the dehydrohalogenation of the rigid molecules l-bromo-1,2diphenylethylene [ 1881 and l-chloro-1,2-diphenylethylene [ 1711 on catalysts of the type of metal salts and metal oxides: the cis-compound was always more reactive than the tram-derivative.

H

,c6H5

C6H5,

/c=c\ >

Br

.

cis

H

/c=c Br

\

trans

C6H,

The work with 1-bromo-2-chloroethane allowed the influence of the nature of the halogen on its reactivity to be observed as either vinyl bromide or vinyl chloride are formed. The ratio of the chloride t o the bromide in the products changed with the nature of the catalyst, being around 0.1 for sulphates of Ni, Co, Mn, Cu, Zn and for silica-alumina, 0.6 for alumina and 5 for KOH-Si02 [ 1791. (c) Direction o f elimination

A large amount of work has been devoted t o the problem of the rules governing the dehydrohalogenation of halogen derivatives which can form several olefins. Although some regularities may be observed, the general picture is clouded by the following facts. (a) The direction of elimination depends very strongly on the nature of the catalyst because on different catalyst types, different mechanisms operate. (b) The nature of a catalyst (and the mechanism) may be changed by the hydrogen halide which is produced by the reaction [ 1893. (c) The composition of the product (and the participation of different mechanisms) is temperaturedependent and, as various catalysts sometimes differ appreciably in activity, a comparison of selectivities under the same conditions is impossible in a broader series of catalysts. (d) The composition of the product may be changed by a secondary isomerisation of the olefins formed. The point (a) is demonstrated by the data in Table 11for 2-halobutanes which give three products: 1-butene, cis-2-butene and trans-2-butene. The differences in selectivities can be even larger than indicated by these data. The dehydrochlorination of l,l,Z-trichloroethane yields 1,2dichloroethylene (I) and trans- and cis-l,2-dichloroethylene(11). On silica-alumon alumina, 0.30 and on KOHina, the value of the ratio 1/11 was SiOz, 1 0 [66]. The data in Table 11show some facts, the mechanistic consequences of which will be discussed in Sect 2.4.4. The 2-alkene/l-alkene ratio for the catalytic reaction differs significantly from that for the homogeneous decomposition. On all catalysts, this ratio is higher for the 2-bromo- than for the 2-chloroderivative; therefore the orientation also depends on the nature of the halogen. On some catalysts, both ratios (the 241- and cis/ trans) are equal or approximately the same as the equilibrium values, but on other catalysts, significant differences appear. The influence of temperature on the ratio of the products of the dehydrochlorination of 2-chlorobutane is seen from Fig. 6 [ 1901. Other examples may be found in the literature [190,194,195]. Some ratios are almost temperature-independent while some show large changes. Moreover, the data from various sources differ sometimes appreciably (cf. refs. 190 and 914 for 2-chlorobutane and refs. 66 and 195 for 1,1,2-trichloroethane). This might be caused by secondary isomerisation on strongly acidic catalysts of the olefins first formed such a reaction was proved at References p p . 385-398

306 TABLE 11 Examples of product ratio obtained from 2-halobutanes on different catalysts Catalyst

Temp. (“C)

2-Chlorobutane MgS04 Bas04

2-Butene 1-Butene

cis-2-Butene trans-2-Butene

Ref.

200 240 24 0 270 345 200 150 350

6.7 8.1 6.1 5.7 8.1 2.8 9.0 1.5

0.58 0.43 0.69 2.15 1.07 1.8 2.0 0.53

190 190 190 190 190 19 1 191 192

200 180 210 230 300 150 160

11.5 10.1 8.1 6.7 9.0 5.3 3.2

0.53 0.58 0.62 2.0 1.04 1.00 0.94

190 190 190 190 190 193 193

Equilibrium composition [I 911 100 13.3 200 6.7 300 4.3 400 3.3

0.43 0.58 0.62 0.67

A12 (s04

Li2SO4 K2S04 CUO CaO None

)3

2-Bromobutane MgS04 Bas04 A12 (so413 &SO4 KzS04 Si02 KO H-S i 02

least for MgS04 [190]. The water content of metal salts as dehydrohalogenation catalysts also influences the selectivity [ 1961 and consequently the thermal history of the solids must play a role. Therefore, all selectivity data should be judged with caution, especially in cases where the over-all conversion of the haloalkane was high and the catalyst illdefined. Further information about the direction of dehydrohalogenation was obtained with threo- and erythro-2deutero-3-bromobutanes on Si02 and Si02-KOH catalysts [ 193,1951. The determination of deuterium content in the butenes formed allowed the estimation of the extent of the synand anti-eliminations. The values of the deuterium kinetic isotope effect showed that the C,-H (or C,-D) bond is split in the rate-determining step. Over KOH-Si02, the anti-elimination was preferred, but at 3OO0C, syn-elimination was the peferential reaction mode. With SiOz, synelimination was favoured under all conditions. Further stereochemical studies with di- or tri-haloalkanes corroborated the general picture of a strong dependence of the direction of elimination on the nature of the catalyst. The data in Table 1 2 may serve as an exam-

307 I

I

200

100

300 Temperature

400

("C)

Fig. 6. Dependence of the selectivity ratios of the dehydrochlorination of 2-chlorobutane on temperature for different catalysts [ 1901.

ple and additional results were reported for 2,3-dibromobutanes on alkalised silica [ 1981. Large changes of stereoselectivity with the catalyst nature were also demonstrated in the dehydrohalogenation of 1,1,2-trichloroethane t o cis- and trans-dichloroethylene [ 66,172,179,199,2001 and 1,2dihalopropanes [ 1901.

TABLE 12 Ratio of cis- to trans-2-chloro-2-butenes from the dehydrochlorination of meso- and dZ-2,3-dichlorobutanes on different catalysts [ 197 ] Catalyst

K2C03

BaC03 NiC03 LiCO3 CaC12 Ca3(P04)2

Temp.

cisltrans ratio from

("C)

meso

dl

250 330 250 330 350 220 220

11.5 49 6.7 3.0 0.90 0.28 0.20

0.075 0.53 0.23 0.67 0.90 0.30 0.16

References p p . 385-398

308 2.4.4 Mechanism

The hypothesis of a continuous transition of the elimination mechanism from the extreme E l through concerted E2 t o the other extreme E l c B with the change of reactant structure and catalyst nature, described in Sect. 2.1, can be easily adopted for dehalogenation also. The data summarised in Sect. 2.4.3 show some inconsistencies but the over-all picture is clear. This can be demonstrated for some selected examples. Typical basic catalysts are strontium oxide and alkalised silica (studied by the group of Mochida and Yoneda) o r potassium carbonate (studied by the Noller group). With SrO and NaOH-SO2, a large deuterium kinetic isotope effect was observed for 1,2-dibromoethane [ 1721, which shows that the C,-H bond is split in the ratedetermining step. The trans/cis olefin selectivities in the reaction of 1,2-dichloropropane and 1,1,2-trichloropropane [179,189] on SrO and alkalised silica give the greatest values indicating the high stereoselectivity of the elimination. The same results are obtained in experiments with meso- and dl-dihalobutanes on NaOHSiOl [ 1981 and K2C03 [ 1971. All this indicates an E2-type mechanism, very probably an E2cB mechanism in which the fission of the C,-X bond is slightly preceded by the fission of the C,-H bond. However, the structure of the reactant may shift the timing of the two steps of the elimination in the direction of the extreme E l c B mechanism. Evidence for this can be seen in the lower value of the deuterium kinetic isotope effect for the dehydrohalogenation of 1,1,2,2-tetrachloroethane(1.2) than of 1,2dibromoethane (1.6) on KOH-Si02 [66]. The cause of this transition is the increased acidity of hydrogen atoms in the compound with more halogen atoms. Typical acidic catalysts are silica-alumina, transition metal sulphates o r chlorides, calcium phosphate etc. They are characterised by low deuterium kinetic isotope effects and low stereoselectivity (see Tables 8, 11 and 12). These results correspond t o the E2cA or E l mechanisms, between which a transition may be observed due t o the influence of the structure of the reactants, i.e. according t o the polarity of the C,-X and CD-H bonds. Again, the reactions of 1,2dibromoethane and 1,1,2,2-tetrachloroethane yielded the evidence. The deuterium kinetic isotope effect on silica-alumina was 1.0 for the dibromo-derivative, which indicates a pure E l mechanism, whereas for the tetrachloroderivative, the value of 1.5 was found. Alumina is a catalyst which shows intermediate behaviour and over which the concerted E2 mechanism is accepted [66] with slight transition either t o the E2cA or E2cB mechanisms according t o the structure of the reactant. Salts of strong acids and bases also show similar intermediate properties. The concerted or partly concerted mechanisms require two-site adsorption and because the mechanisms are ionic, the active centre must consist of a pair of an acidic and a basic site. Metal salts fulfil this

309 condition; their relative activities should depend on factors like acid and base strength of the surface atoms, lattice spacing, concentration of suitable site pairs etc. Another important factor is the contamination of the surface by adsorbed species like catalytic poisons [ 1791, water [ 1961 or product hydrogen halide [ 1891. Therefore all studies which attempted to find correlation between some property of the catalysts and their activity ended with inconclusive results (cf. refs. 67, 172 and 194). The most reliable data concern dehydrochlorination on zeolites containing different monovalent and divalent cations. A good linear relationship was obtained between the activation energy of the first-order reaction and the electrostatic field of the cation [180] and between the cis/trans selectivity and acid strength [ 2001. Both correlations are in agreement with the proposed shifts in mechanism. The catalysts which operate by means of an E2 mechanism give a high proportion of reaction products which are formed by the anti-elimination. This fact has been discussed in Sect. 2.1 and only few remarks need t o be added here. Quantum chemical calculations [73] on the transition state of the dehydrochlorination of chloroethane, initiated by an attack of a basic species, confirmed the preference of the anti-elimination over the syn-mode. On the contrary, calculations on the transition state for non-catalytic (homogeneous) thermal elimination [ 201,2021 confirmed the syn-elimination path. 2.5 DEALKYLATION BY CRACKING

2.5.1 Types of cracking reactions

In the chemical and petroleum industries, the term cracking is used t o describe a chemically complex process in which the decomposition of larger hydrocarbon molecules into smaller fragments plays a dominant role but is accompanied by a number of other reactions (isomerisation, cyclisation, polymerisation, disproportionation etc.). In this section, under catalytic cracking, only the primary fission of a C--C bond, which yields an alkene and a fragment with a C-H bond in the place of the former C-C! bond

I

R-C-C-H

I

=

R-H

I I + C=C I I

(A)

will be considered. The group R must therefore be alkyl or aryl. Reaction (A) resembles other olefin-forming elimination, not only formally but also in all general features. Of course, there are some special characteristics of the cracking reaction which are due t o the nature of the eliminated groups, R. The general chemistry of the cracking over solid catalysts was studied Rcfcrenccs p p . 385-398

310 intensively in the short period during the forties which followed the introduction of the cracking process and of aluminosilicate catalysts into the production of petrol (for a review see ref. 203). Two types of cracking reactions are of interest for the present treatment: (i) dealkylation of alkylaromatic compounds and (ii) fission of a saturated hydrocarbon chain. The first reaction is characterised by the fission of the bond between an aromatic ring and an alkyl group, e.g. O ( f 4 - H

= O

I I H + C=C I I

(B)

The second type of cracking can take place at any C-C bond of a saturated hydrocarbon with the exception of the bonds t o the terminal CH3 groups, e.g. CH3*H,*H2-CH,-CH,
/ CH3*HZ*H2*H3

+ CH2=CH2

LCH3--CH2--CH1 + CH2=CH-CH3 (C)

Alkenes are very reactive under the conditions of catalytic cracking and easily undergo secondary transformations even at low conversions of the initial reactant. Therefore the products have always been found t o contain less alkenes than arenes o r alkanes. The reader may notice a terminological inconsistency with respect to other olefin-forming eliminations. The reaction is called dealkylation in spite of the fact that the second product is an alkane or arene. The reasons are historical because the aromatic hydrocarbons (and similarly the alkanes) were held t o be the more important components of the products. Dealkanation and dearenation are the proper names for the reactions. The cracking of hydrocarbons requires high temperatures for thermodynamic reasons and 350-550°C is the suitable range when solid catalysts are used. 2.5.2 Catalysts

The cracking reactions are catalysed by strong acids, both liquid or solid, like sulphuric acid, aluminium trichloride, aluminosilicates (including zeolites) and similar two- or three-component oxide mixtures. Solid oxide catalysts are more suitable than other acids because they withstand the necessary high temperatures better. A close parallel between the action of inorganic Brqjnsted and Lewis acids, like sulphuric acid, hydrofluoric acid and aluminium trichloride on the one side, and aluminosilicates or natural clays on the other, was recognised very early [l]. Compositions like Al,03-Si0,, A1203-BZ03, MgO-SiO, and A1,03-Zr02

311 were the first synthetic catalysts employed, besides acid-washed natural clays, for cracking studies (for an early review see ref. 2). Later, preparations containing BF3 on an alumina carrier were used in laboratory work [ 204,2051.

2.5.3 Experimental kinetic results

( a ) Formal rate equations Several rate equations, some of them in the integrated form, have been used for a kinetic description of cracking. A critical comparison of them yielded the consistent kinetic model [ 2061 discussed below. Cracking is a reaction of the type

A=R+S where A denotes the starting compound, R the alkane or arene, and S the alkene formed. Most authors agree [204-2181 that the reaction is best represented by the simple Langmuir-Hinshelwood type rate equation, eqn. (3) (p. 280). Some authors have carefully tested it in the differential form [ 204,209,210,216-2181 , others followed the procedure given by Frost [ 2071 who integrated eqn. (3) and after some algebraic manipulation obtained the expression

where F/W denotes the space velocity and x the conversion. ( Y ~ . and OF are complex parameters containing rate coefficients and/or adsorption coefficients

where P is the total pressure in a flow reactor into which the pure compound A is injected. The Frost equation is evaluated [ 205,207,208,213, 2141 by plotting ln(1 - x ) - ' against x and taking the value of the intercept o F as a measure of reactivity or catalyst activity. The rate equation, eqn. (3), has been simplified for catalytic cracking on the basis of various assumptions. When all components are strongly adsorbed, the term C,K,p, in the denominator is much greater than 1and can be omitted. Then the expression [ 212,2191

r

=

R c ferciirrs

kKAPA- -~ KAPA + KRPR + K S P S 911.

385-398

(10)

312 is obtained. When only one component is strongly adsorbed, for example the alkene in the cracking of an alkane (cf. ref. 206), a simple expression can be used, i.e.

The drastic simplification based on the assumption that all reaction components are weakly adsorbed, which leads t o the first-order rate equation, has been found acceptable by some authors [ 206,220-2231. Only a few authors have arrived at different conclusions [224,225]. For example, the empirical rate equation, which fitted the data on cumene cracking, was

where C and C' are constants. In the presence of catalytic poisons, that is substances which are strongly adsorbed on the catalyst surface, the rate equation, eqn. (3), has t o be expanded by adding the term K p P p t o the denominator

Table 13 collects values of adsorption coefficients of some compounds determined by means of this equation from experiments on the cracking of cumene on an aluminosilicate catalyst. ( b ) Structure and reactiuity

The published data give a clear picture of effect of structure on reactivity in the cracking of alkylaromatic compounds. The reactivity increases with the size of the alkyl group, and with its branching, as the studies of TABLE 13 Adsorption coefficients (in bar-') of various substances on a aluminosilicate catalyst at 420°C [ 2 2 6 ]

_~____

Compound

KP

Cumene Benzene Naphthalene Methanol Acetone Phenol Py rid i ne Quinolint?

0.685 0.274 1.03 X lo2 1.28 X lo2 5.22 X lo2 7.32 X lo2 1.67 x 105 1.24 X lo6

313

Fig. 7 . Hammett correlation of the cracking of 1,l-diarylethanes on a kaolin catalyst at 500OC. (Data by May et al. [220].)

- 0.3

-0.2

by

-O*’

Fig. 8 . Taft correlation of the cracking of alkylphenols o n an aluminium fluoroborate catalyst at 425OC (Data by Schneider et al. [204].) References P P . 385-398

314 the cracking of alkylbenzenes on aluminosilicate catalysts [ 210,213,218, 2271 and of alkylphenols on an aluminium fluoroborate catalyst [204] have shown. Substituents on the aromatic nucleus influence the cracking of an alkyl group in alkylbenzenes in a similar way t o aromatic electrophilic substitution; this follows from the experiments with substituted ethylbenzenes [ 2161, isopropylbenzenes [ 2281, diphenylmethanes (a special case of elimination yielding substituted benzenes and toluenes) [220] and diphenylethanes [ 2291. When the series of compounds contain a sufficient number of derivatives, a linear free energy relationship can be applied for correlation of the rate data [ 125,227,229,2301. Two examples of such correlations are presented in Figs. 7 and 8. A summary of all correlations together with the values of the reaction parameters p and p' from the Hammett and Taft equations is given in Table 14. Other linear correlations were also sought, e.g. of adsorption coefficients of alkylbenzenes against the bond strength alkyl-CH, [ 2101 of of the apparent activation energy against enthalpy change of carbonium ion formation [ 2301. The most successful was the correlation of the dealkylation rate against the enthalpy of hydride ion abstraction from the corresponding alkane [ 2271. Less information is available about the cracking of alkanes. Three sources [ 212,232,2331 confirm that, in the series of straight-chain alkanes, the rate increases with the molecular weight. The data could be correlated by the Taft equation when the molecule was divided into the reaction centre and such substituents as R - C H 2 - C H 3 [ 1251. The reactivity of hexane isomers was studied at 550°C on an A1203-Si02-Zr0, TABLE 14 Linear free energy relationships for catalytic cracking React ants

Catalyst

P or P"

Ref.

Hammett equation Ethylbenzenes 2-Propyl benzenes

Aluminium fluoroborate Aluminosilicate

-9.5 -2.0 -1.0 -4.2 -3.6 -5.0 -2.8 -2.4

216,125 213,125 221,228 230 230 231,125 220,125 229

-9.7 -22.6 -5.0 -18.8 -22.4

232,125 210,125 227 204,125 204,125

Aluminosilicate + alkali Zeolite Kaolin Aluminosilicate

1,l-Diphenylethanes

T a f t equation Alkanes A1kylbenzenes

Aluminosilicate Aluminosilicate

o-Alkylphenols p-A1kylphenols

Aluminium fluoroborate Aluminium fluoroborate

~

_

_

~

_

.

_

_

_

_

~~

_

.~

~

315 catalyst [ 2341 ; the conversion increased in the order 2,2dimethylbutane

< hexane < 2-methylpentane < 3-methylpentane < 2,3-dimethylbutane.

There is disagreement in the literature on the influence of reactant structure on activation energy. Some authors (e.g. refs. 212, 213 and 227) have found different activation energies for different reactants but their treatment of the rate data was rather simplified and the rate coefficients obtained were not separated from adsorption coefficients. However, in the studies where the kinetic analysis was more detailed and a true rate coefficient was calculated, the same activation energy was determined for all members of a series of alkylbenzenes [210] and a series of alkylphenols [ 2041. 2.5.4 Mechanism

The mechanism of catalytic cracking was very soon recognised t o differ basically from that of thermal cracking, where free radicals are the active intermediates [ 31. The basis for the interpretation of the processes taking place on the surface of cracking catalysts were (i) the observation that these catalysts possess centres with high acid strength [235,236], (ii) the discovery of a strong similarity between the catalytic action of strong acids and aluminosilicates and (iii) Whitmore's theory of carbonium ion reactions [ 41. The distribution of products suggested the formation of carbonium ions on the surface, which then undergo further transformations according to the well known rules for the homogeneous acid-catalysed reactions [237]. However, while it was quite easy t o explain the way in which various products are formed from the starting hydrocarbon and even qualitatively the order of experimentally observed reactivities of isomers or homologues, for a long time it has been controversial as t o how the carbonium ions are formed. The hydride ion abstraction

suggested by some authors (e.g. ref. 2) seemed improbable t o others, who preferred the formation of the carbonium ions by addition of a proton from the surface t o an alkene present in the feed as an impurity [ 31 I I I I C=C + H' = -C+I t

A'

The experimental evidence for the second hypothesis was the observed increase of the cracking rate of alkanes after addition of small amounts of alkenes t o the feed. Both early theories assumed the continuation of the cracking reaction by intermolecular transfer of the charge from the products t o fresh starting molecules, that is like an ionic chain mechanism with the catalyst acting only as an initiator. The problem was further clouded by the fact that two types of acid centres exist on the surface, the Br@References p p . 385-398

316 sted sites (protonic) and Lewis sites (uncoordinated surface metal ions). In spite of great efforts, it was not possible t o find out the part which they play in the catalysts of the cracking and their relative proportions on the working surface. Only quite recently, an attempt was made t o explain the mechanism of catalytic cracking within the framework of the theory of olefin-forming eliminations outlined in Sect. 2.1 [238]. Before going into details, the acidity of the cracking catalysts will be considered. The close connection between the number of acidic sites and the activity was soon discovered [2,208]. Further studies revealed that the sites can have various acid strengths, which was attributed t o the difference in the nature of Brqhsted and Lewis sites. The experiments with catalysts partially poisoned by alkali showed that the sensitivity to the amount of the poison depends on the structure of the cracked hydrocarbon. The conversion of tert-butylbenzene decreased more slowly with increasing alkali content than did the conversion of isopropylbenzene [ 2391. The pretreatment of a zeolite catalyst with ammonia had no effect on the cracking of tert-butylbenzene, whereas the cracking of isooctane was strongly diminished [240]. Also, the change in total acidity caused by variation in the ratio of A1203t o SiOz had a smaller influence on the cracking of tert-butylbenzene than of 2-butylbenzene [238]. This interplay of catalyst acidity and reactant basicity is in excellent agreement with the dynamic model of elimination mechanisms presented in Sect. 2.1. The strongly acid centres are preferentially poisoned and the remaining sites are weak for the activation of the less polar molecules, e.g. isooctane. The alkylbenzenes, especially tert-butylbenzene, need a smaller activation for the reaction. The linear correlation, with a negative slope, of the reactivity of alkylbenzenes against the enthalpy of hydride ion abstraction from an alkane [227,240] supports the view that the ratedetermining step is the splitting off of the alkyl group. The predominance of trans-2-butene in the products of the cracking of 2-butylbenzene at lower temperatures indicates an at least partially concerted mechanism (E2) [238]. The cracking of alkylbenzenes can be treated as a case of aromatic electrophilic substitution (for recent views on this type of reaction see ref. 241) where the attacking agent is either a proton from a surface Brqinsted site or a coordinatively unsaturated surface cation acting as a Lewis site (cf. ref. 238)

I/

PH -C \

317 or

I/ C-H

\/

-C I M' I

+

I

/

-C-C-H

J L + i M

.__c

The next step is the fission of the bond between the aromatic nucleus and the alkyl group; assistance of a basic site (surface oxygen atoms -0-, -0- or - O H ) which takes the leaving proton from the C, atom is very probable. The last step is the desorption of the arene. Alkanes are less basic than arenes and therefore, according t o the general rules for elimination reactions (see Sect. 2.1),we may expect a strong adherence to the E l mechanism. The suggested carbonium ion formation from alkanes by the action of a strong acidic site [2,237] has been doubted [ 3 ] but has obtained support from studies of the interaction of alkanes with very strong liquid acids such as HF-SbF, or FSO3-SbF5 [ 2421. The abstraction of a hydride ion from an alkane by these systems, which can act either as Brqhsted or Lewis acids according to the conditions, takes place even at very low temperatures. A complete separation of a carbonium ion from the hydride ion is very probably not necessary. It has been shown [73] by MO calculations that any attack by a charged species on an atom bonded t o a carbon atom causes activation of the bonds from a 0-carbon atom t o the substituents. In this way, the splitting of the C,-C, bond can be induced by adsorption of the alkane on a strongly acidic site. The preferential cracking of a saturated hydrocarbon chain in 0-positions t o the position where a carbonium ion might be formed was observed early and named the 0-rule by Thomas [ 2 ] . The question remains open as t o which type of acidic centre is able t o activate an alkane molecule. The fact that an aluminosilicate catalyst is poisoned for the cracking of alkanes by irreversibly adsorbed ammonia suggests a Lewis site [ 2401, viz.

&?

The charge which develops on the y-carbon atom after the rupture of the Cp-C, bond can either be neutralised by a hydride ion forming an alkane or the carbonium ion-like residue may react further, i.e. isomerise, crack etc. References p p . 385-398

318 The electrophilic displacement mechanism for catalystic cracking is further confirmed by the negative slopes of the Hammett and Taft correlations (Table 14) and has been supported by MO calculations in which electrophilic, nucleophilic and free radical mechanisms were compared [ 2281. The formal kinetic description of the cracking of alkanes and alkylbenzenes is in a good agreement with the above mechanistic considerations and structure effects on the rate. For the reaction of alkylbenzenes, the Langmuir-Hinshelwood type rate equation (3), which is based on the assumption of an adsorption equilibrium preceding the fission of the Carom.-Caliph.bond as the ratedetermining step (see ref. 21O), was adequate. On the other hand, the order of reactivities of isomeric hexanes (see above) [234] indicates that, with alkanes, the first step, the attack on the hydrocarbon molecule by an acidic site, might be the rate-determining process; for this reaction, the simple first-order rate equation was sufficient either in the original form or in the form expanded by a term expressing the retardation by alkenes [eqn. ( l l ) ] (cf. ref. 206). 2.6 DEHYDROSULPHIDATION

2.6.1 T y p e s of dehydrosulphidation reactions and catalysts Dehydrosulphidation of thiols and sulphides is directly analogous to dehydration of alcohols and ethers. Thiols (mercaptans) yield both alkenes and sulphides RSH = alkene + H2S

2 RSH

= RSR

+ H2S

and the ratio of the reactions depends mainly on the temperature [243]. Sulphides give alkenes and thiols which can further split off hydrogen sulphide [ 2441 RSR = alkene + RSH The reactions are reversible and the equilibrium is less favourable for the decomposition of a thiol t o an alkene and H2S than of the corresponding alcohol t o an alkene and H 2 0 under the same conditions; data on equilibria in the reactions of propene with water [245] and with hydrogen sulphide [ 2461 indicate that the equilibrium constant of propene hydration is smaller than that for propene sulphidation by approximately two orders of magnitude. The same catalysts as for dehydration are suitable for the dehydrosulphidation, i.e. alumina, silica-alumina, zeolites, metal oxides and metal sulphides. (For a comparison of their activities, see ref. 247.)

319 2.6.2 Experimental kinetic results

No detailed kinetic study of thiol or sulphide dehydrosulphidation has been reported, but first-order kinetics have been assumed for measurements with a pulse flow reactor and found acceptable [248,249]. Effects of structure on reactivity have been studied several times. The sulphides are more stable than the thiols [248,250]. In both series of thiols and of sulphides, the reactivity increases with the inductive effect of the alkyl group [248,251,252], in accordance with other elimination reactions. A linear relation between the logarithm of the rate coefficient and the enthalpy change on carbonium ion formation from the corresponding alkanes has been observed [ 2481. As Fig. 9 shows, linear correlation of the same rate data by means of the Taft equation is also possible. 2.6.3 Mechanism By analogy with other elimination reactions and on the basis of observed structure effects on the rate, the E l and E2-like mechanisms may also be accepted for the dehydrosulphidation. Sugioka and Aomura [ 248, 249,2531 have proposed mechanisms which correspond t o the above designations. Their results on ethanethiol decomposition over a series of

- 0.2

-0.1

6*

0

Fig. 9 . Taft correlation of the dehydrosulphidation of alkanethiols (line 1, p' = -33) and of dialkylsulphides (line 2 , p * = -38). (Data by Sugioka and Aomura [248].) References p p . 385-398

320

2

6

lo

x

14

Fig. 10. Dependence of the dehydrosulphidation rate of ethanethiol over zeolite catalysts with different cations on the electronegativity of the cation x (after ref. 249).

metal-exchanged Y zeolites [249] indicate a change in timing of the fission of the C-S and C,-H bonds with the nature of the metal. The reaction rate was influenced to different degrees by the addition of pyridine; n o influence was observed with NaY and the largest influence with A1Y. A linear relationship was found between the relative decrease and the electronegativity of the metal. The dependence of the rate coefficient on the electronegativity of the metal was volcano-shaped with a maximum for Zn2+and Cd2+(Fig. 10).

3. Addition reactions In this section, additions t o the multiple bonds C=C, C-C and C=O as well as t o the epoxide bond C-C are described. Sections 3.1-3.4 deal ‘0’ predominantly with the reverse reactions t o olefin-forming eliminations which are the subject of Sect. 2, viz.

I

I

C=C + HX I I

I

= -C-C-

I

I I H X X can be halogen, OH, O-alkyl, alkyl and aryl. The published data on

321 reverse reactions t o deamination and dehydrosulfidation (X = NR2, SH and SR) is so small, especially with respect to kinetics, that a review of these reactions is not included. All additions of HX t o the carbon-arbon double bond, treated here as a part of heterogeneous catalysis, can also be catalysed homogeneously by BrBnsted or Lewis acids like sulphuric acid o r aluminium chloride. Because of the thermodynamics, the additions require relatively low reaction temperatures and, sometimes, elevated pressures for good preparative results and the homogeneous alternative has had much more attention. Since the introduction of some heterogeneous catalytic processes into industrial practice (hydration of alkenes, alkylation of aromatic hydrocarbons by alkenes), the kinetics and mechanism of these types of additions have been studied. Several concepts developed originally for the elucidation of homogeneous additions have been accepted for the heterogeneous case but sometimes with doubtful results. Relatively little use has been made of the knowledge of elimination mechanisms over solids, which must have common features with addition mechanisms, because, in most cases, the same catalysts are active for processes in both directions. In Sect. 3.5, the heterogeneous aldol reaction I -C-H

\

I

I

+ C=O =-C--C+-H

I I I I is reviewed. This acid- o r base-catalysed addition t o the carbon-oxygen double bond is a well known example of homogeneous catalysis and a vast literature on its kinetics and mechanism exists. The heterogeneous catalysis of the reaction has less practical significance, but its study helps in the understanding of the general features of acid-base catalysis over solids. 3.1 HYDRATION

Hydration means, in general, addition of the elements of water to a substance. Most of these reactions are nm-catalytic or homogeneously catalysed processes. In this section, only hydration of olefins t o alcohols, of acetylene t o acetaldehyde, and of alkene oxides t o glycols will be treated, since they are typical reactions where the application of solid catalysts has become important.

3.1.1 Hydration of olefins to alcohols Hydration of olefins I I C=C + H 2 0 = <-CI \ I H is the reverse reaction t o \

References p p . 385-398

I

(A) I OH olefin-forming dehydration of alcohols dealt with

322 in Sect. 2.2. The reaction is exothermic with AHo ranging from about -38 t o -46 kJ mol-' in the vapour phase [ 2541 and may be accompanied by the formation of the corresponding ether, aldehyde and olefin oligomers. In contrast t o the formation of olefins from alcohols, which has never become a large-scale industrial operation but has attracted much theoretical interest, the preparation of alcohols by hydration of olefins is one of the most important petrochemical processes (see, for example, refs. 255-260); the kinetics and the mechanism of olefin hydration, however, have been less thoroughly investigated. Originally, the hydration of olefins t o alcohols was carried out with dilute aqueous sulphuric acid as the catalyst. Recently, the direct vapour phase hydration of olefins with solid catalysts has become the predominant method of operation. From the thermodynamic point of view, the formation of alcohols by the exothermic reaction (A) is favoured by low temperatures though even at room temperature the equilibrium is still in favour of dehydration. To induce a rapid reaction, the solid catalysts require an elevated temperatue, which shifts the equilibrium so far in favour of the olefin that the maximum attainable conversion may be very low. High pressures are therefore necessary t o bring the conversion to an economic level (Fig. 11).To select an optimum combination of reaction conditions with respect t o both rate limitation and equilibrium limitation,

I

I

I

I

225

250

275

300

50 xeq

40

30

20

10

200

325

Tem p ero tu r e ("C)

Fig. 1 1 . Calculated equilibrium conversion, x e q (%), in the vapour phase hydration with equimolar mixture of ethylene and water (after ref. 2 6 7 ) .

the thermodynamics of reaction (A) were thoroughly investigated together with the catalyst activity studies, The equilibrium data can be found in the literature [ 261-2781. The catalytic hydration of olefins can also be performed in a threephase system: solid catalyst, liquid water (with the alcohol formed dissolved in it) and gaseous olefin [258,279,280]. The olefin conversion is raised, in comparison with the vapour phase processes, by the increase in solubility of the product alcohol in the excess of water [ 2581. For these systems with liquid and vapour phases simultaneously present, the equilibrium composition of both phases can be estimated together with vapourliquid equilibrium data [ 2811. For the three-phase systems, ion exchangers, especially, have proved to be very efficient catalysts [ 260,2801. With higher olefins (2-methylpropene), the reaction was also performed in a two-phase liquid system with an ion exchanger as catalyst [282]. It is evident that the kinetic characteristics differ according to the arrangement (phase conditions), i.e. whether the vapour system, liquid-vapour system or two-phase liquid system is used. However, most kinetic and mechanistic studies of olefin hydration were carried out in vapour phase systems.

( a ) Catalysts The best catalysts for olefin hydration are not necessarily those which have proved most satisfactory for the reverse reaction. Some of the successful hydration catalysts are not typical dehydration catalysts. The more obvious reasons are: (i) different adsorption characteristics of the catalyst is desirable, e.g. stronger adsorption of olefin relative to alcohol. (ii) under the conditions used for the hydration, ether formation cannot be suppressed as readily as in the dehydration, (iii) at high pressures, the olefins tend to polymerise much more than at the low pressures used for the dehydration. The first solid catalysts used for the direct olefin hydration were inorganic acids (H3P04,H2SO4,H3B03)supported on a porous material (“solid acids”). Heteropolyacids (e.g. silicotungstic), acid clays, zeolites and several acidic oxides were also used. Tungstic oxide ( W 0 3 ) , alone or mixed with other oxides, unreduced or in a reduced state (so-called “blue-oxide”), was often reported to be an efficient catalyst. Metal phosphates and metal sulphates were also investigated in several studies. Comprehensive reviews [ 254,2571 have been published, summarising the literature concerning the different types of solid catalysts used for olefin hydration. The use of synthetic ion exchanger resins as hydration catalysts has also been reviewed [283]. These catalysts are more effective than the inorganic solids [280, 284,2851 and allow the use of lower temperatures where the position of the equilibrium of hydration is more favourable. It is apparent that all the catalysts cited are acidic in nature. The relation between the acidity and activity of catalysts was investigated and Rcfercnces P P . 385-398

324 demonstrated, e.g. for boron phosphate [ 2861 and cation exchange resins [ 2841. With solid H3P04 catalysts, the ethylene hydration rate was found to be linearly dependent on the acidity of the supported phosphoric acid [287,288], and the rate decreased in the course of the reaction due t o the dilution of the acid by the reactant water [287]. The relation between acidic properties and catalytic activity was thoroughly investigated with metal sulphates and other acidic catalysts [ 289,2901. For ethylene hydration [290], a good linear correlation was found for the series of metal sulphates between the initial reaction rate and the number of centres of acid strength -8.2 < Ho < -3.0, whereas there was no correlation with the number of centres of other acid strength. For the hydration of propene [ 2891, which is considered to be more basic than ethylene, the best correlation was found with centres of -3.0 < H o < +1.5. Both Brqjnsted and Lewis acid sites are assumed t o be active, but a conversion of Lewis t o BrQnsted sites is probable under the reaction conditions (presence of water). With Si0,-supported metal sulphates (MX), the formation of Br#nsted centres according t o the reaction

I -Si-OH I

I

+ MX = -%OM + H' + XI

is possible. With cation exchanged zeolites, proton formation by interaction of water vapour with the cation seems likely, viz.

M"+

+ H 2 0 = [M(OH)]("pl)' + H'

For ethylene hydration [ 2901, a correlation between the catalytic activity of cation-exchanged zeolites and the electronegativities of the cations was established. It may be concluded from all these results that the presence of acid centres is unavoidable for a catalyst t o be active in olefin hydration. The possible role of basic centres is less clear; they might participate in a fast step which follows the ratedetermining step. ( b ) Experimental kinetic results

The several attempts, published in the literature, t o describe the kinetics of vapour phase olefin (mostly ethylene) hydration can be classified into two groups according t o the basic model used. One model, for reactions catalysed by phosphoric acid supported on solids, treats the kinetics as if the process were homogeneous acid catalysis and takes into account the acid strength of the supported acid. Thus, a semiempirical equation for the initial reaction rate [ 2881 f J

=

kh OfA

B

was used for ethylene hydration at 250-330°C

and 10-80 bar; ho is the

325 acid strength of phosphoric acid, f A the fugacity of ethylene and aB the activity of water. For the same reaction under similar conditions and on supported H3P04, another equation was applied [ 2873. I t is based on Taft’s mechanism for homogeneously catalysed olefin hydration [ 291,2921 [see scheme (B), p. 3271 according t o which water enters into the reaction scheme in a fast step which follows the ratedetermining step and therefore appears in the rate expression in a negative term, viz. =

khO(pA

-PR/PBKp)

(14)

where p A ,pB and pR are the partial pressures of olefin, water and alcohol, respectively. Since. the reactant water can dilute the phosphoric acid and change its acid strength ho (defined by log ho = -Ho where Ho is the Hammett acidity function), a relationship between the acid strength and the partial pressure of water was derived, which after substitution into eqn. (14)gives the expression (k’/Pg5)(PA- P R / P B K p ) (15) which was also used for reactor optimisation purposes [ 293-2951. The other approach is based on the Langmuir-Hinshelwood kinetics. In all the work using this approach, the surface reaction of adsorbed olefin and water was found, or postulated, to be the ratedetermining step. The corresponding rate equation has the form =

and fitted well the hydration data on W03-Si02 (265-305°C) [277], metal sulphates (160-220°C) [ 289,2901, boron and chromium phosphates (260-320°C) [ 296,2971 and solid phosphoric acid [ 2981. Speculatively, a very similar model was also proposed for 2-butene hydration over boric and phosphoric acids on alumina [299]. Whereas in the reaction over W03-SiOz the reactants were assumed t o be adsorbed t o an equal extent on the catalyst surface [ 2771, a preferential adsorption of water was found on boron and chromium phosphates 1296,2971. On metal sulphates [289], the adsorption of all three reaction components was so weak that eqn. (16) could be reduced to a second-order rate term for hydration and a first-order term for dehydration; viz. +

t

r = k PAPB- k PR Only in one case (the hydration of 2-methylpropene over a H,S04-SiOz catalyst [ 2781) was the so-called “Rideal mechanism” proposed as a preferable model and expressed by the rate equation for single-site adsorption with retardation by the product alcohol, viz.

R r fcrc,n CL’S u p . 3 8 5-39 8

326 With ion exchangers as catalysts for olefin hydration, special attention was paid t o transport problems within the resin particles and t o their effects on the reaction kinetics. In all cases, the rate was found to be of the first order with respect to the olefin. The role of water is more complicated but it is supposed that it is absorbed by the resin maintaining it in a swollen state; the olefin must diffuse through the water or gel phase t o a catalytic site where it may react. The quantitative interpretation depends on whether the reaction is carried out in a vapour system, liquidvapour system o r two-phase liquid system. In the vapour system [284, 2851, the amount of water sorbed by the resin depends on the H 2 0 partial pressure; it was found a t 125--170°C and 1.1-5.1 bar that 2-methylpropene hydration rate is directly proportional to the amount of sorbed water

(17) where pA is the partial pressure of olefin and u B the volume of water sorbed per equivalent of resin. It is believed that this effect was caused by changes in the ionisation of the resin acid groups, by the dependence of the olefin diffusivity on the degree of the resin swelling and by the influence of water upon the solubility of the olefin in the resin. The results are consistent with the Wheeler model for simultaneous diffusion through a spherical particle and a first-order reaction. In the liquid-gas [ 2801 or liquid-liquid [ 2821 systems where the pressure was high enough t o maintain water in the liquid state at the temperatures used, water was present in such a large excess within the resin phase, relative to the olefin concentration, that reaction rates independent of water concentration could be found; an order of one with respect to olefin (propene and 2-methylpropene) was again observed. High reaction rates of 2-methylpropene hydration [282] were found t o result from a high diffusivity of the olefin within the resin catalyst. The analysis of the diffusion model used revealed that this finding was consistent with the transport mechanism involving the surface diffusion of 2-methylpropene in an adsorbed state, There have been no kinetic studies performed with the aim of comparing the reactivity of olefins of different structures. Only for the threephase hydration over reduced tungsten oxide [ 2791 are conversion data reported; from these, the reactivity order at 270°C, C3H, > C4HB (mixture of isomers) 3 C2H4, could be estimated. However, at temperatures above 300"C, a higher yield of alcohol can be obtained from ethylene than from the other two olefins. 2-Methylpropene is reported to be about one hundred times more reactive than n-butenes in vapour phase hydration at 115°C over a H2S04-Si02 catalyst [278]; for the butenes, the reactivity order cis-2-> 1->truns-2- has been established. The addition of water t o alkenes proceeds according t o the Markownikoff rule, i.e. 2-propanol from propene [280,279,289], 2-butanol from 1-and 2-butenes =kpA%

327 [ 278,279,2991 and 2-methyl-2-propanol from 2-methylpropene [ 278,282, 2851 are formed.

( c ) Mechanism

It has been assumed that, with phosphoric acid-based catalysts where the active component is liquid aqueous phosphoric acid adsorbed in the pores of the support, the reaction probably follows the scheme proposed by Taft [ 291,2921 for the hydration of olefins in aqueous solution, viz.

(fast)

n-complex

L

I I - H-C-C-O'-H I I I +H20

+H2O &

-H20

4 5 2 0

(fast)

(fast)

H

(rate-de termining step)

I I H-C-C-OH

I

I

+ H30'

Adopting this mechanism and making some simplifying assumptions, Gelbsthein et al. [287] derived the kinetic equation, eqn. (14), or its alternative form, eqn. (15), mentioned in Sect. 3.1.1.(b) which fitted the experimental data on H3P04-Si02 catalyst well. The Langmuir-Hinshelwood rate equation (16) used by several authors for other types of catalyst was interpreted by Tanabe and Nitta [ 2901 on the basis of their results with NiSO,. The authors assume that the surface reaction of adsorbed ethylene and water molecules, which was found t o be the slowest process, may be written in greater detail as

It has been concluded from deuterium exchange experiments, using ethylene and heavy water, that the addition of an adsorbed proton t o adsorbed ethylene is the actual rate-determining step. It can be seen that the two schemes differ, mainly in that the latter includes dissociative adsorption of water on the surface of the catalyst and does not specify the adsorption of ethylene, but they are consistent in that they assume the formation of a carbonium ion as the ratedetermining step. 3.1.2 Hydration of acetylene t o acetaldehyde

The reaction CH=CH + H 2 0 = [CH2=CHOH] = CH3CH0 References p p . 385-398

328 is exothermic (M"= -150 to -167 kJ mol-' [300,301]). The isomerisation of the intermediate vinyl alcohol to acetaldehyde makes the over-all process practically irreversible since the equilibrium between the enol and carbonyl forms is strongly shifted in favour of the latter. The formation of acetaldehyde from acetylene is accompanied by several side reactions, the most important of them being the aldolisation of acetaldehyde with subsequent dehydration t o crotonaldehyde, and the polymerisation of acetylene. For a long time, only a liquid phase process was employed industrially for the hydration of acetylene t o acetaldehyde; mercury salts in acidic solution were used as catalysts. Only recent reports can be found in the literature (e.g. ref. 300) on the industrial utilisation of the direct vapour phase hydration of acetylene over solid catalysts. It has been reported that solid acids and oxides or salts of different metals can catalyse the vapour phase hydration of acetylene. Most typical are phosphoric acid and phosphates of bivalent metals, such as Zn or Cd. Organic ion exchangers and synthetic zeolites exchanged for Zn2+,Cd2+, Hg2+ and Cu2+ions were also employed. A survey of inorganic catalysts [254] or of organic ion exchangers [283] catalysing the hydration of acetylene or its derivatives can be found in literature. The temperatures reported in the kinetic studies range from 260 to 350°C. In most of the investigations, the hydration rates were found t o be of the first order with respect to acetylene [300,302-3041. With zinc phosphate [ 3031, cadmium-calcium phosphate [ 3001 and cation-exchanged zeolites [ 3041, the rates were independent of the concentration of water. Thus the simple kinetic equation

r = kpA (18) where p A is the partial pressure of acetylene, is valid in these cases. However, with phosphoric acid supported on carbon [ 3021, an increase of water partial pressure caused a decrease of the reaction rate. This was explained in a similar way as in ethylene hydration over a H3P04-Si02 catalyst [ 2871, i.e. by dilution of the supported acid with the water present in the reactant mixture (p. 325). The acidities, h o ,corresponding t o different partial pressures of water could be evaluated and it was established that the reaction rate could be then expressed by the equation (19) r = kh#A = k'p, Thus, the partial pressure of water is not really involved in the rate expression for the reaction catalysed by supported phosphoric acid, as for the other mentioned catalysts [ 300,303,3041 [ eqn. (IS)]. It was proposed [302] to explain this form of reaction kinetics on the basis of a homogeneous mechanism. The authors assume that the reaction proceeds in the film of phosphoric acid containing dissolved acetylene and they adopt the reaction scheme of Taft [291,292] for the hydration of

329 olefins in such a way that a carbonium ion CH2=C+His formed from a .rr-complex of acetylene; the other steps are formally analogous t o those in scheme (B). In the catalysis by zinc phosphate [303] instead of vinyl carbonium ion, the formation of a corresponding carbonium ion H--C'=C-H

I

Zn' is assumed which then reacts with water in the same way as the ion CHI = C'H. Another explanation was offered for the C d - C a phosphate catalyst [ 3051 ; the participation of acidic groups of the phosphate in the reaction mechanism was assumed, since a dependence of the activity of the catalyst on its acidity had been found. With cation-exchanged zeolites [ 3041, the first-order kinetics [eqn. ( I S ) ] is explained by the degeneration of the Langmuir-Hinshelwood equation for monomolecular transformation of adsorbed acetylene in the rate-determining step

r=

~KAPA

1+ C K i p i

when the adsorption of all the reaction components is weak, i.e. CKipi << 1. Quite another type of kinetics was found for a zinc phosphate-phosphoric acid-activated carbon catalyst at 350°C [ 3061. The rate equation

where p B is the partial pressure of water, found experimentally, can be interpreted by assuming that the reaction occurs when acetylene in the gas phase attacks adsorbed water and a steady state is established in which the rate of the removal of water by chemical reaction is equal t o the rate of the adsorption of water. However, the same form of rate equation would result if it were supposed that water vapour attacks adsorbed acetylene. The latter interpretation would be more consistent with the mechanism based on the Taft scheme, according to which non-adsorbed water reacts with a carbonium ion formed from the n-bonded unsaturated compound. 3.1.3 Hydration of alkene oxides t o glycols

The reaction R-CH--CHz + H20=R--CH(OH)-CH20H \ ' 0 is exothermic and irreversible. Most investigations were carried out with ethylene oxide ( R = H) for which AHo = -96 kJ mol-' and log Kp 11 at 25°C and = 3 a t 300°C [ 3071. Usually, the reaction does not stop at the Refererices P P . 385-398

330 stage of monoglycol but this can react further with other molecules of alkene oxide and di, tri- and polyglycols may be formed. Other side reactions which may accompany the main reaction are isomerisation of the alkene oxide t o the corresponding aldehyde or the polymerisation t o ether polymers. In homogeneous liquid media, the hydration is catalysed by acids or bases. With solid catalysts, the reaction may be performed with both reactants either in the vapour [285,307-3111 or in the liquid [312-3141 phase. In the former case, temperatures of about 12O-25O0C, and in the latter case of about 25-90°C, were used. The reaction can also be conducted under conditions which establish a heterogenous system of vapour and liquid phases [ 3081. The solid catalysts for the alkene oxide hydration can also be of an acidic or basic nature. Inorganic solids such as SiO,, A1203,T h o 2 , supported H3P04, silica-alumina and molecular sieves did not prove t o be efficient catalysts [ 307,3111. Good results were obtained with silver oxide on an alumina carrier [307]. More intensively than the inorganic solid catalysts, however, organic polymer ion exchangers were investigated and used as catalysts for the alkene oxide hydration [285,308-3141. The catalytic activity of cation exchangers depended on their acid strength : sulphonated resins were much more active than those of the phosphonic o r carboxylic acid type [ 308,309,313,3141. Anion exchangers were found t o be less active than cation exchangers [313]; a strong base (with an amine group), however, proved to be a good catalyst [ 3081. Formal kinetic investigations (performed only with acidic ion exchange catalysts) revealed, in most cases, the first-order rate law with respect t o the alkene oxide [285,310,312] or that reaction order was assumed [ 309,3111. Strong influence of mass transport (mainly internal diffusion in the polymer mass) was indicated in several cases [285,309, 310,312,3141. The first-order kinetics with respect t o alkene oxide is in agreement with the mechanism proposed for the same reaction in homogeneous acidic medium [ 309,315-3171, viz. CH,\ rH2;O+HfCH2

1

CH,

P'H+

C'H2

CH20H

CHzOH

CH20H

I"'"/

+ H'

(C)

if the rate-controlling step is the decomposition of a protonates substrate t o a carbonium ion. This mechanism represents a classical S N 1 substitution; however, an alternative mechanism was proposed [ 317,3181 which is today most widely accepted for homogeneous acid catalysis, viz.

331 This mechanism does not involve the formation of a free carbonium ion but rather a nucleophilic displacement on a carbon atom in the oxonium complex. The reaction thus becomes a limiting case of a SN2substitution. There are, however, very few reliable kinetic measurements available which would allow the effect of water on the reaction kinetics to be determined and thus distinguish between the two alternative mechanisms in the catalysis by ion exchangers. The role of water is rather complex here: it is not only a reaction partner but it also greatly modifies the catalyst as has already been mentioned in connection with ion exchanger-catalysed hydration of olefins [Sect. 3.1.1.(b)). Water solvates the resin acid groups, causes swelling of the polymer and thus strongly affects the transport situation in the catalyst particle [285,310]. Therefore, the true reaction order with respect to water could not be established. It has been found rather empirically that the reaction rate is proportional to the volume of water retained by the resin catalyst [285,310], as in the hydration of olefins [eqn. (17)]. With ethylene oxide, the volume of sorbed water could be expressed by the BET isotherm which was substituted into eqn. (17) thus relating the reaction rate also to the water partial pressure, pB, ViZ.

where a is a constant and pB,sis saturation vapour pressure of water. The strong effect of transport processes in the polymer particle was further evidenced by the low values of the activation energy [310,312] and by the dependence of reaction rate on particle size [310], on the outer surface area of the resin particles [312), on the crosslinking degree of the copolymer [314] and on the cations introduced [314]; moreover, the lower rates observed with ion exchangers than with an equivalent of sulphuric acid [312,314] indicated that only a part of the acid groups in the polymer was easily accessible. The macrokinetic situation may therefore be characterised as follows: the resin particle is in a more or less swollen state (in the liquid system as well as in contact with water vapour) retaining a considerable amount of water. The alkene oxide is adsorbed on contacting the resin surface, probably by protonation; direct adsorption measurements [ 309 J revealed the occurrence of both physical adsorption and chemisorption. The alkene oxide then reacts on the surface with water according to mechanism (C) or (D) or diffuses within the swollen resin mass [ 3121 to the internal acid groups where it reacts in the same way. With an inorganic catalyst (Ag,0/A1203 [ 3071) a simple surface process is assumed for the vapour phase hydration: water adsorbed on the surface of silver oxide reacts with gaseous ethylene oxide to form adsorbed glycol which is then desorbed; this is obviously an oversimplification of the actual mechanism. References p p . 385-398

3.2 HYDROHALOGENATION

3.2.1 Types of hydrohalogenation reactions and catalysts The addition of hydrogen halides t o unsaturated organic compounds is called here hydrohalogenation in order t o stress that it is the reverse reaction t o dehydrohalogenation. Two types of the hydrohalogenation reaction have to be considered, the addition to a carbon-carbon triple bond --C=C-

+ HX = -CH=CX-

(A)

and the addition t o a double bond I I <=C-

I

I

+ HX = -CH-CX-

(B)

For both reactions, low temperatures and elevated pressures are favourable from the point of view of equilibria but high conversons of acetylene can be obtained even at atmospheric pressure and around 150°C. The same catalysts are active for the hydrohalogenation as for the dehydrohalogenation (see Sect. 2.4). Metal halides are preferred [ 319,3201 but other metal salts [ 3211 and alumina [ 3221 may be also used. When metal halides are the active components of the hydrohalogenation catalysts, they should have the same halogen as the reactant HX because exchange is easy [ 322,3231. For reaction (A), other catalysts are more active than for reaction (B). This difference and the less favourable equilibrium of the second step allow the reaction of acetylene with hydrogen chloride to be stopped at the stage of vinyl chloride. HgC12 is the proven catalyst for this industrially important process, with active carbon serving as the carrier. When a mixture of HgCl, and ZnC12 is used, both vinyl chloride and 1,l-dichloroethane are obtained [ 3201. 3.2.2 Experimental kinetic results The reported rate equations for the hydrohalogenation of acetylene, ethylene, propene and vinyl chloride are summarised in Table 15. Of special interest is the last entry; it is based on a model which assumes two types of active centres, the first one for the adsorption of acetylene, the second for the adsorption of hydrogen chloride and vinyl chloride. The addition of HX to alkenes proceeds according t o the Markownikoff rule, i.e. the halogen is attached to the more substituted carbon atom [321-3231. The reactivity order of butenes was found t o be dependent on the nature of the catalyst. Over MgS04, the order was isobutene > trans-2-butene > l-butene > cis-2-butene but with CaCI, , the reactivity decreased in the order isobutene > cis-2-butene > l-butene > truns-2butene [321]. Propene is more reactive than ethylene [318]. Earlier reports that tert-butylchloride is formed from l-butene and hydrogen

333 TABLE 15 Rate equations for hydrohalogenation reactions @ A , p B and pc denote partial pressures of the unsaturated compound, hydrogen halide and halogenated hydrocarbon, respectively.) ~

Reactants

Catalyst

r = k P d B -k ' p c 1-Butene, 2-butene (Y isobutene + HCI

MgSO, or CaC12

25-150

324

HgCl2

75-180

325* 326**

r = kpQgf5 Acetylene + HCI

Temp. ("C)

2 56-302 80-140

Acetylene + HCI

A1203

HgC12 ZrOCl2-Si02 A1203 ZnCl2

Ethylene + HCI Propene + HCl Vinyl chloride + HCI r = ~KAKBPAPBI(+ ~ KBPB Acetylene + HCl

+

KCpC)(1+ KAPAI

HgCl2

256-302

Ref.

327 328 329,330

121-293 44-80 102-148

327 33 1 332 333 334

75-125

335

* a = 0.65-0.89, b = 0.25-0.38. ** a = 0.80, b = 0.37. chloride [336,337] were corrected later by the same group of authors [ 3211 ; 2-chlorobutane is the sole product as expected.

3.2.3 Mechanism Only a few speculations concerning the mechanism of the hydrohalogenation reactions can be found in the literature. They are mostly based on the better knowledge of the reverse reaction, the decomposition of haloalkanes. It seems evident that catalytic hydrohalogenation must also involve transfer of paired valence electrons and that a free radical-like process is highly improbable. The existence of .rr-complex intermediates has been suggested [ 321,336,3371 but the hypothesis lacks experimental evidence. Adsorption studies revealed that both reactants can be adsorbed on active catalysts. For the reaction of acetylene with hydrogen chloride, the References PP. 385-398

334 activity order of a series of metal halides was the same as the adsorptivity order for both components, HgC12 being the best catalyst and adsorbent [321,338]. 3.3 ALKYLATION BY OLEFINS

3.3.1 Types of alkylation reactions and catalysts Alkylation is a very broad reaction type and it can, depending on the nature of the alkylating agent, proceed either as a substitution or as an addition reaction. The alkylation by substitution of, for example, aromatic hydrocarbons, phenols or amines is based on the reaction with alkyl halides or alcohols. Some evidence indicates that, at least partly, the alkylation proceeds through the intermediate formation of alkenes from the alkylating agent when the reaction is conducted at atmospheric pressure and at high temperature. In this section we shall deal only with the reverse reaction t o dealkylation by cracking (Sect. 2.5), that is with additions of alkanes and aromatic compounds to the carbon-carbon double bond. The former reaction is described only in a single paper [ 3391; the formation of 2,2,4-trimethylpentane from isobutane and isobutene CH3 CH3 I I CH3+H + CH2=C-CH3 = CH,--C--CHZ--CH-CH3 I I I 1 CH, CH3 CH3 CH3 was catalysed by the strongly acidic rare-earth-exchanged crystalline aluminosilicates (zeolites) in the temperature range 25-lOO"C, and in all its features resembled the same reaction catalysed by H2S04 or HF. The alkylation was accompanied by polymerisation of isobutene, hydride transfer reactions etc. The reaction of aromatic compounds with alkenes giving alkylaromatic compounds has obtained more attention. A typical transformation is the alkylation of benzene by lower alkenes, e.g.

Not only benzene and its alkyl derivatives can be used as the aromatic component but also naphthalene [ 3411, phenol [ 340,3411 and thiophene 1341,3421. Low-molecular weight alkenes, C1 to C,, cyclohexene and dodecene have served as alkylating agents. Only strongly acidic solids can catalyse the heterogeneous alkylation of aromatic compounds. Amorphous aluminosilicates were the first catalysts

335 used for this purpose [342-3441 but they are distinctly less active than the zeolites [ 341,345-3501. Solid phosphoric acid [ 3431, iron phosphate [343], aluminium oxide activated by BF3 [351] or B203 [352] and sulphonated polymers [ 340,353-3551 are also suitable catalysts. The activity of the Al2O3-SiO2 catalysts can be enhanced by adding CC14 or other organic halogen compounds into the feed [ 3561. Both liquid phase and gas phase alkylations over solid catalysts under atmospheric and elevated pressures have been described. In order to achieve high conversions, it is necessary to operate at low temperature and pressures above atmospheric.

3.3.2 Experimental kinetic results Only a few authors have attempted t o describe the rate of the alkylation by kinetic equations [ 349,352,355-3571. Table 1 6 shows the conditions of experiments and the rate equations applied by various groups of authors. N o conclusions can be made on the basis of this small set obtained with different approaches. More information is available about orientation, when a second alkyl group is introduced into the aromatic ring, and about relative rates. As might be expected, propene reacts more easily than ethylene [ 342,3461 and isobutene more easily than propene [ 3421. Normal butenes are sometimes isomerised in the process; practically the same product composition, consisting mainly of 2,2,4-trimethylpentane, is obtained in the alkylation of isobutane whether the olefin component is isobutene or 2-butene [339]. In the alkylation of aromatic hydrocarbons, this side reaction is negligible. Toluene is more reactive than benzene [ 3581 as is phenol [ 3481. Alkyl groups and,hydroxyl direct the new entering alkyl group mostly into the TABLE 16 Rate equations for heterogeneous catalytic alkylation of aromatic hydrocarbons Reactants

Conditions and catalyst

Benzene + ethylene Benzene + propene

Gas phase, 300"C, 20 bar, A1203-Si02 Liquid phase, 55"C, 1 bar, ion exchanger Liquid phase, 1O-5O0C, 1 bar, A12 03-B2 0 3 Gas phase, 423-483"C, 1 bar, Al203-Si02

2-Propylbenzene + propene a

Rate equation

a

Indices denote: A aromatic hydrocarbon, B alkene, R product(s), S solvent. The second term in the denominator describes t h e swelling of the polymer.

References p p . 385-398

Ref. 357 356 352

349

336 ortho and para positions [ 342,351,3581. The ortho/para ratio decreases with the size of the alkyl group: CH3 > C2H5> 2-C3H, [347]; it is also influenced by the nature of the catalyst [ 343,3591. The metdpara ratio determined for the ethyiation of toluene at low conversions (in order to supress the consecutive isomerisation of the ethyltoluenes formed) fitted the Brown selectivity relationship for electrophilic aromatic substitution well [ 3581. This relationship correlates relative reactivity of toluene with respect t o benzene and the ratio of reactivities of the meta and para positions of toluene [ 3601 ; many homogeneous substitution reactions conform t o it. However, there are some contradictory reports on the composition of the products of toluene alkylation or benzene dialkylation at high conversions. In some cases, compositions corresponding to the thermodynamic equilibrium between ortho, meta and para isomers were found, and in other cases, kinetic control of orientation, giving mostly the ortho + para substitution, prevailed. Consecutive isomerisation of the ortho and para isomers t o the more stable meta isomer seems t o be the cause of the disagreement. More active catalysts gave more meta derivatives than the less active ones [343] and increasing the temperature has the same effect [ 3511. 3.3.3 Mechanism The observed effects of structure on rate and on orientation, confirmed by the Brown selectivity relationship, show that there is no basic difference between heterogeneous catalytic alkylation of aromatic compounds and homogeneous electrophilic aromatic substitution, cf. nitration, sulphonation etc. This agreement allows the formulation of the alkylation mechanism as an electrophilic attack by carbonium ion-like species formed on the surface from the alkene on Brqhsted acidic sites. The state of the aromatic compound attacked is not clear; it may react directly from the gas phase (Rideal mechanism ) [348] or be adsorbed weakly on the surface [ 3591. It seems that other acidic sites are the most efficient for the alkylation of aromatic compounds than for the reverse reaction, the cracking of alkylaromatic compounds [ 3611. For the forward process, a linear correlation was observed between the activity of decationized Y zeolites and the number of acidic sites corresponding t o H o < + 3.3, whereas for the cracking, the sites corresponding t o Ho < -3.0 correlated with the activity. 3.4 ADDITION OF ALCOHOLS TO ALKENES

The reaction

R’ R3 R’ R3 I I I I C=C + R’OH = H-C-C-0-R’ 1 1 I I R2 R4 RZ R4

R’ R3 I I + R’-O-C-C-H I I RZ R4

337 which is an old method for the preparation of alkyl-tert-butylethers by the action of isobutene on an alcohol or glycol catalysed by H2S04 [362, 3631 has only recently been conducted with heterogeneous catalysts. A great number of patents deal with suitable solid catalysts and conditions for this reaction. Ion exchange resins seem t o be the best catalysts, but aluminosilicates may be also used though with lower selectivity [ 3641. The principal side reactions are oligomerisation of the alkene and dehydration of the alcohol t o the ether. The addition itself is the reverse reaction to the first step of the dehydration of ethers t o olefins (see Sect. 2.2). Two recent papers report the main features of the heterogeneously catalysed addition of alcohols t o alkenes [ 364,3651. The reaction proceeds both in the liquid and gas phase [ 3641, and the temperature must be kept well under 150°C with respect t o the position of the equilibrium [ 3641. The reactivity of isobutene and 2-methyl-1-butene is much higher than that of propene, 2-butene and 3-methyl-1-butene [364,365]. 2-Methyl-1-butene reacts faster than 2-methyl-2-butene [ 3651. The reactivity of alcohols with isobutene decreases in the order methanol > ethanol > 1-propanol > 1-butanol [ 3651. The initial rate in the liquid phase reaction is zero-order in methanol, first-order in isobutene and about third-order in the sulphonic groups of the ion exchanger [ 3651. A comparison of catalysis by an ion exchanger and anhydrous p-toluenesulphonic acid indicated an efficiency of between 5 and 8, depending on temperature, according t o the Hammett definition [366]. The observed structure effects are similar, as in the reaction catalysed by sulphuric acid. On this basis and with the notion of the strong acidity of the heterogeneous catalysts used, it is possible to assume a mechanism similar to olefin hydration (Sect. 3.1) or alkylation (Sect. 3.3). Olefin protonation by the catalyst seems to be the first step, which is followed by the interaction with the nucleophile, in this case the alcohol. 3.5 ALDOL CONDENSATION AND RELATED REACTIONS

3.5.1 Types of reaction

In this proper sense, aldol condensation includes reactions producing P-hydroxyaldehydes or P-hydroxyketones by self condensation or mixed condensation of aldehydes and ketones; these reactions are, in fact, additions of a C-H bond activated by the carbonyl t o the C=O bond of the other molecule, viz.

R4 \

R3

I

CH-C=O +

I

R5' References p p . 385-398

R'

\

R1OH R4 R3 \I

I I

I

c=o= c - c - c = o

I R2

/ R2

R5

338 where R’ to R5 are H, alkyl or aryl. Depending on the conditions, the reaction stops either at the stage of the P-hydroxy carbonyl compound or, if R4 or R5 are hydrogen atoms, this is further transformed (dehydrated) t o a ,P-unsaturated aldehydes or ketones

R L O HR4 R3

R1 R4 R3 \ I I C--C--C=O = C=C--C=O + H 2 0 I / I R2 R2 H

\I

I

I

In several cases, the intermediate hydroxy compound may be formed in an undetectable amount so that the reaction appears as a direct formation of a ,P-unsaturated carbonyl compound by condensation of two carbonyl compounds R1 R1 R4 R3 \ \ I I R4CH2-C=0+ C=O = C=C-C=O + H 2 0 I I R2 R2

R3

I

In a broader sense, the term aldol condensation has sometimes been applied t o many so-called “aldol-type” condensations involving reactions of an aldehyde or ketone with a substance containing a mobile hydrogen, namely R4(or X) CH2Y +

R1 OH R4(or X) \I I C=O = /C--CHY

R1\

I

RZ R4(or X)

R1 \ I = ,C=CY

RZ

+ HzO

RZ where X or Y is an activating group such as COOR, CONHR, CN, NO*, S02C H3;in a similar way t o the true aldol condensations, these reactions may produce a hydroxy compound or its dehydration product. The condensation represented by reaction (D) is very probably involved as the first step in the synthesis of bis( ary1)alkanes from carbonyl compounds and substituted benzenes (e.g. phenol)

This reaction has become industrially important as a large scale process for the production of bisphenol A [ 2,2-bis(p-hydroxyphenyl)propane] from acetone and phenol.

cl,

TABLE17 Equilibrium data for some aldolic reactions Reactants

Product

Reaction scheme

Reaction conditions

Equilibrium constant

Acetaldehyde

fi-Hydroxybutyraldehy de

(A)

Aqueous medium, 25°C

5.7 x 10'4 x 102

Acetaldehyde

Crotonaldehyde

(C)

Vapour phase, 25°C

6.2-32.5

Acetone

Diacetone alcohol

(A)

Liquid phase, 20-30" C

-10-2 -104

Vapour phase

0.19 (300 K ) 2.2 (600 K )

Acetaldehyde + benzaldeh yde a

Cinnamaldehyde

(C)

A€r

(kJ mot-')

-41.2

Ref.

367

-9.7

a

368 C

369 ~

_

_

Calculated from thermochemical data in ref. 367. Evaluated from experimental data. Calculated from free energy data in ref. 368, obtained by t h e group contribution method.

w

w

co

Aldol condensations are reversible and slightly exothermic reactions. The values of equilibrium constants and reaction enthalpies of some aldol reactions reported in the literature are listed in Table 17. Aldol condensations were originally carried out in the liquid phase and catalysed homogeneously by acids or bases; this way of operation is still predominant. Solid-catalysed aldol reactions can also be performed in the liquid phase (in trickle or submerged beds of catalyst), but in many cases vapour phase systems are preferred; the factors determining the choice are the boiling points and the stability of the reactants at elevated temperatures. At higher temperatures, the formation of (Y ,P-unsaturated aldehydes or ketones [reactions (B) and (C)] is preferred t o aldol (ketol) formation [reaction (A)]. A side reaction, which may become important in some cases, is the self-condensation of the more reactive carbonyl compound if a mixed condensation of two different aldehydes or ketones is occurring. The Cannizzaro reaction of some aldehydes or polymerisation t o polyols or other resin-like products can also accompany the main reaction.

3.5.2Catalysts The solid substances catalysing aldol condensations are similar to the homogeneous catalysts in that they may be of an acidic or basic nature, but the latter are preferred. Alkali and alkaline earth metal hydroxides o r phosphates (supported or unsupported), Ca- or Sr-exchanged zeolites and anion exchange resins are typical examples of efficient base catalysts. As acidic catalysts, cation exchanger resins and zeolites in the hydrogen form were used; calcium hydrogen phosphate was also assumed t o act as an acidic catalyst. Condensations of carbonyl compounds with simple aromatic compounds giving bis(ary1)alkanes [reaction (E)] represent a particular case where acidic ion exchange resins are the most successful catalysts. The use of ion exchange resins (mainly of basic types) as catalysts for various types of liquid phase aldol condensation and related reactions has been reviewed [ 370,3711. The effect of the basicity of aldol condensation catalysts on their activity was thoroughly investigated by Malinowski et al. [ 372-3791. The observed linear dependence of the rate coefficients of several condensation reactions on the amount of sodium hydroxide contained in silica gel (Figs. 1 2 and 13) supported the view that the basic properties of this type of catalyst were actually the cause of its catalytic activity, though the alkali-free catalyst was not completely inactive. The amphoteric nature of the catalysis by silica gel, which can act also as an acid catalyst, was demonstrated [380]. By a stepwise addition of sodium acetate t o a HN0,-pretreated silica gel catalyst the original activity for acetaldehyde self-condensation was decreased to a minimum (when an equivalent amount of the base was added); by further addition of sodium acetate, the activity increased again because of the transition t o a base

341 I

I

I

1

I

i

0.8

0

OD 2

OD4

0.00

0.06 mN13

Fig. 1 2 . Dependence of apparent rate coefficient, k (sec-I), on sodium content, m N a (mol Na per 100 g cat), in silica gel catalysts for the vapour phase,condensation of formaldehyde with (1)acetaldehyde, (2) acetone, ( 3 ) acetonitrile, a t 275OC [ 3721.

mNCi Fig. 1 3 . Dependence of apparent rate coefficient, k (sec-I), on sodium content, mNa (mol Na per 100 g cat), in silica gel catalysts for the vapour phase condensation of acetaldehyde with (1) formaldehyde, ( 2 ) acetaldehyde, ( 3 ) benzaldehyde, at 3OO0C [376]. References PP. 385-398

342 type of catalysis. The groups -Si-ONa in the catalyst were assumed t o be the active sites [372,378]. The effect of different alkali metal ions was compared and the activity order Na < K < Cs established [377]. When ion exchange resins were employed as catalysts in liquid phase condensations, a dependence of the activity on the base strength of their functional groups was observed. In self-condensations of some aldehydes and ketones, only the strongly basic ion exchangers, such as Amberlite IRA-400 were active [ 381,3821. In a more detailed study [ 3831, an activity decrease of the functional groups in the order CN > OH > C1 was found; the acetate group was found t o be inefficient. A non-linear relatinship between the exchange capacity and the reaction rate was observed [384], from which it could be concluded that only part of the hydroxyl groups of the resin was accessible and took part in the catalytic action. In benzaldehyde-acetophenone condensation [ 3701, a decrease of the rate coefficient with increasing degree of crosslinking of the resin was found. With acidic ion exchangers, the same effect of the resin degree of crosslinking was observed [ 385,3861 for bisphenol A synthesis [reaction type (E)]. These results indicate that the accessibility of the resin functional groups and the transport phenomena may become important factors in the kinetics of condensation reactions catalysed by ion exchangers (see also ref. 387), as will be discussed in greater detail in Sects. 4.1.3, 4.2.1 and 4.2.2 in connection with esterification and hydrolysis.

3.5.3 Experimental kinetic results

(a) Formal rate equations As with homogeneous aldol reactions, simple power-type rate equations have been frequently used t o describe the kinetics of solid-catalysed condensations. For several liquid phase reactions, second-order kinetics was established, viz. r

=

kcAcB

Examples are the formation of diacetone alcohol from acetone. [reaction type (A)] catalysed by barium or strontium hydroxide at 20-30°C [368] o r by anion exchange resin at 12.5-37.5"C [387], condensation of benzaldehyde with acetophenone [type (C)] catalysed by anion exchangers at 25-45°C [ 3701 and condensation of furfural with nitromethane [type (D)] over the same type of catalyst [ 3841. The vapour phase self-condensation of acetaldehyde over sodium carbonate or acetate at 50°C [388], however, was found to be first order with respect t o the reactant. Langmuir-Hinshelwood-type equations were applied in some cases. The kinetics of t h e vapour phase condensation of acetaldehyde with formaldehyde t o acrolein at 275-300°C over sodium-containing silica gel

343 [ 373-3751 was interpreted by means of the equation

where the subscripts A and B denote the reactants and R and S the products (acrolein and water). The equation was derived on the assumption that acetaldehyde is adsorbed on a basic site and reacts with formaldehyde from the gaseous phase (by analogy with the mechanism of homogeneously catalysed aldol condensation). The equation was slightly modified for the self-condensation of acetaldehyde on the same catalyst [ 3791. On the basis of a series of kinetic studies of aldolisation reactions in which either the hydrogen donor (acetaldehyde, acetone, acetonitrile) [ 3721 or the hydrogen acceptor [ 3761 (formaldehyde, acetaldehyde, benzaldehyde) and the catalyst basicity [ 372-374,378,3791 were systematically varied (Figs. 1 2 and 13), the authors have concluded [376] that the rate coefficient is proportional t o the catalyst basicity, K B , the acidity of the hydrogen donor, K H , and t o a factor Y which is related t o the oxygen basicity of the acceptor, viz.

k = /3KBKH Y (22) where p is the proportionality constant. A similar relation has been proposed and discussed on the basis of a kinetic study of benzaldehyde condensation reactions [ 3701. For the liquid phase ketolisation of acetone t o diacetone alcohol over barium hydroxide-sodium silicate or barium hydroxidesodium hydroxide-borax at 12--40°C [ 3891, the equation

r=

k(cA - CR/K) 1 + K A c ~+ K ~ c ,

from a set of seven Langmuir-Hinshelwood equations, was found t o give the best fit t o the experimental data. It is consistent with a model, according to which the desorption of the product is the ratedetermining step. The kinetic data for phenolacetone condensation t o bisphenol A [reaction type (E)] in the liquid phase at 9 1 ° C over sulphonated ion exchanger [ 3851 were best represented by the equation

kcAck r=(1+ KAcA + KBcB + K s p s + K ~ C , ) ’ where A is acetone, B phenol, S water and I methylcyclohexane (added as a non-polar substance). The equation corresponds t o a model, in which the surface reaction is the ratedetermining step and the reactants, water and methylcyclohexane are competitively adsorbed. This semi-empirical model had to be slightly refined in order t o be in closer agreement with the reaction mechanism proposed (Sect. 3.5.4). References p p . 385-398

344 ( b ) Structure and reactivity

With regard t o the effect of the structure of the hydrogen donor, Malinowski et al. [372] obtained, for vapour phase reactions with formaldehyde over Na-modified silicagel, the reactivity order (see Fig. 1 2 ) CH3CH0 > CH3COCH3 > CH3CN. The authors pointed out that the acidities of these substances decreased in the same order and suggested that the rate coefficient be expressed as a function of the hydrogen donor acidity [eqn. (22)]. Kraus [ 1251, using Malinowski’s data, has demonstrated that there is a very good linear relationship between log k for these reactions and the acidities expressed as the pK of the hydrogen donor. In the liquid phase reactions of substituted acetophenones with benzaldehyde over an anion exchanger [ 370 3 , the order of the effect of substituents on the reactivity was found to be p-OCH3 < rn-OCH3 < no substituent < p-F < p-Br < rn-Br. With another group of hydrogen donors, the reactivity order malonitrile > benzyl cyanide > acetophenone > ethyl cyanoacetate was established. Contrary t o the above view [ 3721, these authors [ 3701 came t o the conclusion that the reaction rate was not an unequivocal function of the hydrogen donor activity. The reactivity of hydrogen acceptors in vapour phase reactions with acetaldehyde over alkalised Si02 [376] decreased in the order (see Fig. 13) CH20 > CHJCHO > C,H,CHO. It has been assumed that it is the basicity of the acceptor oxygen which affects the reaction rate, since oxygen basicity is a measure of the ease with which a proton adds to oxygen and, subsequently, of the ease with which the carbonyl carbon attaches an anion [see scheme (F)]. The positive charge on the acceptor carbonyl carbon is dependent on the nature of the closest neighbour of the carbonyl group. Since formaldehyde has no substituent, its carbonyl group has the largest positive charge. In acetaldehyde, hyperconjugation reduces the densities of negative and positive charges on the oxygen and the carbonyl carbon, respectively, and in benzaldehyde, the mesomeric and steric effects suppress the basicity of the carbonyl oxygen and the positive charge on the carbonyl carbon still more. The lower reactivity of benzaldehyde with respect t o acetaldehyde was found also in the vapour phase aldolisation over lithium phosphate [ 3901. Over the same catalyst, the reactivity order in the self-condensations of aldehydes could be estimated as CH3CH0 > CH3CH2CH0>> (CH,),CHCHO. The reactivity of isobutyraldehyde in the self-condensation was almost undetectable, probably due to steric hindrance on the &-carbon, but this substance was able t o react as a hydrogen acceptor with cyclohexanone. With propionaldehyde over a calcium hydroxide catalyst, a Cannizzaro-type reaction occurred t o some extent simultaneously with the aldolisation [ 3901. This unexpected result was also recorded by other authors [391], who established that the tendency t o aldolisation decreased, and the tendency t o the Cannizzaro reaction increased, with

345 increasing chain length of the aldehyde; both tendencies appear to be in agreement with the results above.

3.5.4 Mechanism In the work concerning the mechanism of solid-cataiysed aldol reactions, the analogy between the homogeneous and heterogeneous mechanisms is usually assumed [370,372-3751. The mechanism of base-catalysed condensations, which has received much attention (cf. ref. 371), may be pictured in general as

R4--CH2Y+ B * R4-€-)HY

H R' I I R4-C-C-0'-) I I Y R2

+ B(+)H

(1)

H R' I I + B'+'H + R4-C--C-0H + B I I Y R2

(3)

using the notation of scheme (D). The hydroxy compound formed can be dehydrated to an unsaturated product by a carbanion elimination ElcB mechanism [see Sect. 2.11

H R' I I R4-C-C-0H I 1 Y R2 R' I R4--C(-)+--OH

I

Y

I

R2

R' + B * R4--C'-'&H

I Y

I

+ B'"H

R2

R' I + B'+'H =+R4--C=C + H 2 0 + B I I Y R2

It is not necessary that the acid B(+'H involved in the aldolisation (F) and dehydration (G) mechanisms must be just the conjugated form of the basic catalytic site B acting in steps (1)and (4), but it is possible that a cooperation of basic and acidic sites originally present on the surface of the catalyst as pairs of suitable mutual distance, occurs, though the basicity of the catalyst is the main property determining its catalytic activity. The activating group Y can stabilise the carbanion RC'-)HY or R4Y C(-)C(R'R2)-OH; in the case of true aldolisation when the activating group is a carbonyl, R3C=0,the stabilisation can occur via a keto-enol tautomerReferences p p . 385-398

346 ism

R3 I R4C(-)H-C=0 R4

R3 I + R4CH=C-O-)

R'

\

C(-)-&oH I I R3-C R2 \\0

R4

+

\

R'

I

C-C-OH 0 1 R3--C RZ &(-)

The mechanism presented by schemes (F) and (G) is consistent with the particular mechanisms suggested by different authors for the condensations catalysed by solid basic catalysts [ 370,372,376,3921, and seems to be supported by the effects of the structure of either hydrogen donors or hydrogen acceptors [Sect. 3.5.3.(b) and eqn. (22)]. For acid-catalysed aldol condensations (which are less frequent), the homogeneous mechanism [371] can again be accepted. The enol form of the hydrogen donor interacts with the protonated form of the hydrogen acceptor, viz. R3 I R4RSCH--C =+R4R5C=C

P3

\

\\

OH

0

R1\

C=O I R2

R1\

+ H' *

C")--OH

I

R2

R1\ C'"-OH I R2

(Hf

P3 * R1oH \I R4 I I C--C!-C=O

+ R4R5C=C \

OH

R3

/ R2

I R5

+ H'

The dehydration of the aldol (ketol) proceeds more rapidly with acidic than with basic catalysts, and this is the reason why, with the former, the CY $-unsaturated carbonyl compounds are the products most frequently encountered. The dehydration follows one of the elimination mechanisms discussed in Sect. 2.1, depending on the particular nature of the used catalyst and on the temperature. With solid acid catalysts, it is possible that the protonation of the carbony1 compound does not lead to a fully ionic form, but that by interaction with surface protons, a hydrogen-bonded intermediate can be formed. Such a mechanism, not differing in the other features from scheme (H), was proposed for acetaldehyde self-condensation over a CH 3 / CaHPO, catalyst [ 3931; a surface complex P--O...H..-O--C was

'H

347 assumed. The formation of hydrogen-bonded intermediates on the surface active sites is not unlikely t o occur even in the base-catalysed reactions [372]; the first step of scheme (F) would then lead t o the formation of an intermediate surface complex of the type R4YCH...H.-B . The mechanism proposed for the acid-catalysed synthesis of bis(ary1)alkanes [ 3941 [scheme (E)] follows the main features of the aldolisation scheme (H). The protonised form of the carbonyl compound reacts by an electrophilic attack with the quinonoid structure of the aromatic molecule (e.g. phenol), viz.

The intermediate tertiary carbinol could not be detected (with the exception of bis(trifluoromethyl)(hydroxyphenyl)carbinol from hexafluoroacetone and phenol [ 3951) and reacts readily with another molecule of phenol; this second stage of the reaction is, in fact, an alkylation of phenol by the tertiary carbinol, or by the carbonium ion formed from it, by a common carbonium ion alkylation mechanism (Sect. 3.3). The mechanism represented by scheme (J) was accepted for heterogeneous catalytic reactions and only slightly modified for the reactions catalysed by hydrogen forms of zeolites [395] or for bisphenol A synthesis catalysed by sulphonated ion exchangers [ 385,3961. In the former case, a Rideal-like mechanism was assumed, in which the chemisorbed (by a H-bond) conjugated acid of the carbonyl compound reacts with the nonadsorbed aromatic molecule [395]. In the latter case, acetone is considered to be chemisorbed by hydrogen bonding of its carbonyl group to a resin -SO,H group; chemisorbed acetone then reacts with phenol from the surrounding non-polar matrix of the resin [385]. The tertiary carbonium ion intermediate is assumed in both cases [385,395] t o react in References P P . 385-398

348 the chemisorbed form by an electrophilic attack on the other aromatic molecule. 4. Substitution reactions Substitution reactions represent a very large group of transformation of organic molecules. Substitutions at a saturated carbon atom as well as on the aromatic nucleus are classified according t o mechanism into nucleophilic and electrophilic and are well known under the notation SN and SE. Ester formation and hydrolysis, which are, in fact, also substitutions, namely in the carboxyl group, are usually treated separately in monographs and textbooks (see, for example, refs. 397, 398). In homogeneous media, the kinetics and mechanism of all these substitution reactions have been very thoroughly studied. In heterogeneous catalysis, however, the situation is different: intensive kinetic and mechanistic investigations of esterification and hydrolysis have been performed, while papers concerning the classical aliphatic and aromatic substitutions catalysed by solid substances are scarce and the results diffuse. The substitutions for the OH group in alcohols by amino groups, aryl and halogens can serve as examples. These reactions, catalysed by acidic or basic solid catalysts, frequently proceed by an eliminationaddition pathway with intermediate formation of carbonium ions or highly polarised species and follow the mechanisms which were treated in Sects. 2 and 3 concerning elimination and addition reactions. This is another reason, besides the lack of systematic studies on kinetics and mechanism, why we have not included the solidcatalysed aliphatic and aromatic substitutions in this chapter. Section 4 is therefore limited t o heterogeneous esterification, transesterification and hydrolysis (including hydrolysis of compounds other than only esters) where the extensive published literature permits a review of their kinetics and mechanisms. 4.1 ESTERIFICATION AND TRANSESTERIFICATION

4.1.1 T y p e s of reactions and catalysts In a broader sense, the term esterification may include all reactions in which esters, both of organic and inorganic acids, are formed. We sha!l limit the discussion in this section, however, t o ester formation from organic carboxylic acids and alcohols RCOOH + R’OH = RCOOR’ + H,O

(A)

In the literature concerning solid-catalysed esterifications, kinetic studies of other ester-forming reactions are scarcely reported. Reactions of

organic esters with alcohols (alcoholysis)

RCOOR" + R'OH

RCOOR' + R"OH

(B) are similar to reaction (A) with respect t o the mechanism and catalysts used. They are called transesterifications and will be included in this chapter. A special case of this substitution reaction of esters, hydrolysis (R' = H), will be dealt with separately (Sect. 4.2.1) because of its specific importance. Reactions (A) and (B) are reversible and this must be taken into account in deriving rate aquations; the effect of the reverse reaction is frequently suppressed by working at low conversion or in excess of one of the reactants. For illustration, esterification equilibrium data for some pairs of acids and alcohols in liquid phase are given in Table 18. For vapour phase esterifications, different values can be found, as is evident, for example, by comparing the equilibrium constant for acetic acidethanol esterification in the liquid phase at 155°C ( K , = 3.96) with that determined in the vapour phase at 150°C (KP = 30.9-33.6 [400]). This discrepancy may be due to different activities (fugacities) of reactants in liquid and vapour phases or to errors in determining the conversions caused by adsorption of some gaseous reactant on the solid catalyst [400]. Since the equilibrium constants for esterification of a given acid with homologous alcohols do not usually differ significantly, the equilibrium constants for transesterification, which are given by the ratio of esterification equilibrium constants of the acid with the two alcohols in question =

- Kst.1

Ktrans --

Kst.2

will not be far from unity in most cases (see some experimental values in Table 19). TABLE 18 Equilibrium constant, K,, and limit conversions, xeq, of esterification [ 399 ] (155"C, initial molar ratio of reactants 1 : 1 . ) ~

Acetic acid with various alcohols

2-Methyl-1-propanol with various acids

Alcohol

KC

xeq

Acid

K C

Methanol Ethanol 1-Propanol 2-Propanol I-Butanol 2-Butanol Ally1 alcohol

5.24 3.96 4.07 2.35 4.24 2.12 2.18

69.6 66.6 66.85 60.5 67.3 59.3 59.4

Formic Acetic Propionic Butyric Isobutyric Methylethylacetic Trimethylacetic Benzoic

3.22 4.27 4.82 5.20 5.20 7.88 1.06 7.00

References p p . 385-398

(%)

xeq

(%)

64.2 67.4 68.7 69.5 69.5 73.7 72.65 72.6

TABLE 1 9 Equilibrium constants ( K , o r Kp) of transesterification. Ester

Alcohol

Phase

Temperature ("C)

K , or K ,

Ref.

Ethyl acetate

Methanol

Vapour

Methanol 1-Butanol

Liquid Liquid

0.365 0.550 0.790 0.924 1.35 1.3

401

Ethyl acetate Ethyl acrylate

170 200 220 240 60 100

402 403,404

The physical properties of most acids (esters) and alcohols allow the reaction t o be carried out either in the liquid or in the vapour phase. In the liquid phase, t h e effects of solvents and of transport phenomena may play a more important role than in the vapour phase. On the other hand, the side reactions (mainly the ether and/or olefin formation from the alcoTABLE 20 Reactants and inorganic catalysts used in kinetic studies of esterification (transesterification) Acids (Ester)

Alcohols

Catalysts

Temperature ("C)

Ref.

CH3COOH CH3COOH

C2 H5 OH C2 H5 OH

425 350

405 406

C2-€, normal and branched

250

126

CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH CH3COOH and C3H7COOH CH3COOH CH3COOH

CI-C4 primary, secondary, tertiary CZHsOH C2 Hs OH C2 Hs OH n-C3H7 0 H n-C3H,OH C2 H5 OH Cz H5 OH and n-C4HgOH n-CsH, 7 0 H n-C4 H, OH

Si02-A1203 (SA) Si02-A1203 (SA) Al203-B203 (AB) Na-poisoned SA Na-poisoned AB O3 Na-poisoned SA Si02 SiO, Si02 SiO, SiOz wo 3 /A12 0 Bauxite

200-260 246-286 150-270 170-230 200-260 255

407 408 409 410 41 1 412 413

218-241 155-197

414 415

CH3COOH HCOOH CH3COOCzHs

Cz HSOH C2 HSOH CHjOH

120-140

416 417 401

a

3

Bauxite Acid activated Korvi earth H3P04/C Ca-metaphosphate SiOz -~.._.I_.~__

a

Transesterification.

351 hol) are usually unimportant in the liquid phase, whereas they may become significant in the vapour phase because of the higher temperatures used. In homogeneous media, esterifications are catalysed by acids. Similarly, for kinetic studies of heterogeneously catalysed esteifications, the use of solids of acidic character is reported. These catalysts may be divided into two groups: (i) inorganic acid catalysts and (ii) organic polymer-based ion exchangers in the acid form. With inorganic catalysts, the majority of kinetic studies were performed in the vapour phase, whereas the main use of ion exchanger resins is for liquid phase processes.

4.1.2 Reactions catalysed b y inorganic catalysts The reactants and inorganic catalysts used in kinetic studies of heterogeneous catalytic esterifications (transesterifications) are summarised in Table 20. As can be seen, no systematic comparative study with more than one catalyst (with the exception of paper [ 4061 ) has been performed by any one worker. The greatest attention was paid t o silica gel [4074111. The reactants were usually low molecular weight acids and alcohols; a typical pair of reactants is acetic acid-ethanol. Only in one study [ 1261 was the structure of the reactants systematically varied in order to establish the effect on the reactivity.

( a ) Formal kinetics For a formal kinetic description of vapour phase esterifications on inorganic catalysts (Table 21), Langmuir-Hinshelwood-type rate equations In were applied in the majority of cases [ 405-408,410-412,414,4151. some work, purely empirical equations [413] or second-order power lawtype equations [ 401,4091 were used. In the latter cases, the authors found that transport phenomena were important: either pore diffusion [ 4011 or diffusion of reactants through the gaseous film, as well as through the condensed liquid on the surface [ 4091, were rate-controlling. The Langmuir-Hinshelwood-type rate equation correspond, in most cases, t o the assumption of surface reaction being the rate-determining step [ 405,406,410-412,414,4151. However, the details of the model differ in individual cases: either one (acid or alcohol) [410,411,415] or both reactants [ 405,406,4141 are assumed to be adsorbed. Of rate-determining steps other than surface reaction, only adsorption of the acid is reported [407,408,415,416]. In several cases with silica gel, an activating effect of water was observed; this was described either by including a Langmuir isotherm for adsorption of water into the rate equation on the assumption that each adsorbed water molecule creates an additioiial active site [ 410, 4111, or by including an empirical function L = rn f (pR)for the number, L, of active sites (pRis partial pressure of water) [ 407,4081. References p p . 385-398

TABLE 21 Equations reported as best fitting esterification (transesterification) data o n inorganic catalysts Catalyst Si02-Al203 Bauxite Si02-Al20 A1203-B203 A1203

Equation

1

I

SiO2

Rate-determining step

Ref.

S R between molecularly adsorbed reactants

405,4 1 4

S R between molecularly adsorbed reactants

406

SR between molecularly adsorbed alcohol and acid reacting from t h e vapour phase

4 10,411

AdsA (alcoho: reacts from t h e vapour phase, water increases the number of sites)

407,408

AdsA (alcohol reacts from the vapour phase; temperature, 176 and 197°C) S R between molecularly adsorbed acid and alcohol reacting from t h e vapour phase (temperature, 155°C)

415

(water increases t h e number of sites)

SiOz Acid activated Korvi earth

a

i

415

412

Bauxite a

Power-law type equation (second order)

401 409

Empirical rate equation

413

Symbols: k,, k A = rate coefficients for surface reaction and adsorption of acetic acid, respectively; E = effectiveness factor; L = total number of active sites; p i , Ki = partial pressure and adsorption coefficients of reactant ( A = acid, B = alcohol, R = water, S = ester); KK = adsorption coefficient of water in the second layer; L' = concentration of active sites a t zero water partial pressure; m, a, b = constants. SR = surface reaction, AdsA = adsorption of the acid. Ref. 401 concerns transesterification ( A denotes the ester).

As appears from the examination of the equations (giving the best fit to the rate data) in Table 21, no relation between the form of the kinetic equation and the type of catalyst can be found. It seems likely that the equations are really semi-empirical expressions and it is risky t o draw any conclusion about the actual reaction mechanism from the kinetic model. In spite of the formalism of the reported studies, two observations should be mentioned. Maatman et al. [ 4101 calculated from the rate coefficients for the esterification of acetic acid with 1-propanol on silica gel, the site density of the catalyst using a method reported previously [ 4181. They found a relatively high site density, which justifies the identification of active sites of silica gel with the surface silanol groups made by Fricke and Alpeter [411]. The same authors [411] also estimated the values of the standard enthalpy and entropy changes on adsorption of propanol from kinetic data; from the relatively low values they presume that propanol is weakly adsorbed on the surface, retaining much of the character of the liquid alcohol.

( b ) Effect o f reactant structure on reactivity Mochida et al. [126] have compared the reactivity of eight alcohols with acetic acid and of seven carboxylic acids with ethanol over sodiumpoisoned silicaalumina at 250°C using a gas chromatographic technique; their results correlated by the Taft equation are shown in Figs. 14 and 15. The effect of the structure of both alcohols and carboxylic acids on their reactivity in heterogeneous catalysed esterification is very small. In contrast, the polar effect of substituents in the alcohol molecule in its intramolecular dehydration to olefin, accompanying the esterification, is much larger (Fig. 14) suggesting different ratedetermining steps in these two reactions. Figure 15 demonstrates the difference in the steric effect of substituents in carboxylic acid molecules in heterogeneous and homogeneous esterifications; however, extrapolation of the published data [ 4191 for homogeneous esterification t o higher temperatures indicates that the difference would become less significant if closer temperature for both types of catalysis were used [ 1261. No other systematic studies of the structure-reactivity relations for esterifications on solid inorganic catalysts have been reported. Only Fricke and Alpeter [411] compared their own results obtained for 1-propanol with those of other authors concerning methanol and ethanol in esterification of acetic acid over silica gel. The rate drops sharply with the increasing chain length (in contrast to the findings of Mochida et al. [126] and in agreement with Heath [420]), and the kinetic model changes from a dual site one (with both reactants adsorbed) for methanol to a single site one for 1-propanol (only alcohol is adsorbed). An interpretation which could explain this change is that steric hindrance by the larger molecules of 1-propanol prevents adsorption of the acid; this changes the kinetics and lowers the reaction rate. Reference6 p p . 385-398

354 4

I

I

I

1

3

2 L

F

4

1

0

I

- 03

I

- 0.2

I

- 0.1

I

bsc

0

Fig. 14. Taft correlation with polar substituent constants ( u * ) of the vapour phase esterification of acetic acid with alcohols ( 0 )and of the olefin formation from alcohols (0) over Na-poisoned silica-alumina at 25OoC [126]. 1,Methanol; 2, ethanol; 3, l-propanol; 4, 1-butanol; 5 , 2-methyl-1-propanol; 6, 2-propanol; 7 , 2-butanol; 8, 2-methyl2-propanol.

Fig. 15. Taft correlation with steric substituent constants (E,) in the vapour phase esterification of carboxylic acids with ethanol over Na-poisoned silicaalumina at 25OoC ( 0 ) [126] and in homogeneous acid-catalysed esterification at 4OoC (0)[419]. Acids: 1, acetic, 2, propionic, 3, butyric, 4, isobutyric, 5, isovaleric, 6, pivalic, 7, 2ethylbutyric.

355 (c) Mechanism

The type of catalyst must be taken into account when considering the reaction mechanism. Mochida e t al. [ 4061 investigated several oxide or mixed-oxide catalysts for esterification of acetic acid with ethanol. For silica-alumina, alumina-boria and sodium-poisoned silicaalumina, the authors assumed that active sites for esterification were the protonic sites. Kinetic experiments revealed that the more basic ethanol was more strongly adsorbed than acetic acid, and poisoning experiments with organic bases led to the conclusion that esterification proceeded on even weaker sites. The authors suggested a mechanism (similar t o that used for homogeneous esterification [421,422]) for which the attack of the oxonium ion, formed from acetic acid, on the adsorbed ethanol was ratedetermining. They have excluded the possibility of carbonium ion participation on the basis of a number of arguments, including the formation of ethyl thioacetate and no ethyl acetate from acetic acid and ethanethiol over the silicaalumina catalyst. However, the investigations of the mechanism of olefin and ether formation from alcohol (Sect. 2.2) revealed the importance of basic sites. It is feasible that, for esterification also, pairs of acidic and basic sites might be necessary. A mechanism similar t o that proposed by Mochida for the above-mentioned group of catalysts, though not so explicitely formulated, might also be valid for acetic acid-ethanol esterification over a H3P04/C catalyst [ 4161. According t o the author, the adsorbed acid in a polymolecular film on the surface of the catalyst reacts with protonated molecules of the adsorbed ethanol. These mechanisms are in formal agreement with kinetic equations assuming surface reaction between molecularly adsorbed reactants; besides the group of catalysts used by Mochida e t al. [ 4061, such equations were also found t o fit the kinetic data for silicaalumina [405] and bauxite [414] (see Table 21). An activating effect of water was observed for the catalyst H3P04/C [ 4161 and for silica gel; for the latter, the effect seems to be more general since it has been established by several authors [ 407,408,4111. A plausible explanation of the promoting effect of water on silica gel was suggested by Fricke and Alpeter [411]. The authors assume that water is adsorbed on silica gel in two layers. In the first layer, it is adsorbed strongly, hydrating the surface according t o the recation

The silanol groups formed may act as active sites, since it has been obReferences PP. 385-398

served [423] that surface hydroxyl groups can act as adsorption sites for molecules having electron donor atoms. This idea is in harmony with the mechanism proposed by Mochida et al. [126,406] for esterification on other oxide or mixed-oxide catalysts. The concentration of surface silanol groups can be expressed as a function of water partial pressure and introduced into the rate equation [see Sect. 4.1.2.(a)]. As to the second layer of water, it is assumed to be less strongly adsorbed and to cause the free catalyst surface to decrease by competition with adsorption of reactants. This second, inhibiting effect is expressed by the corresponding Langmuir term K k p , in the denominator of the rate equation (see Table 21). The proposed mechanism of the effect of water can be supported by two other findings: (i) the calculations of Maatman et al. [410] revealed that the active sites could be identified with surface silanol groups [Sect. 4.1.2.(a)] and (ii) independent studies of other authors [ 424-4261 showed that silica gel could actually adsorb two layers of water; the first layer is strongly chemisorbed whereas the second is less strongly adsorbed and retains much of the character of free water. The standard enthalpy and entropy changes on adsorption determined from kinetic adsorption coefficients, KR and Kk,for the first and second layer, respectively [411], are consistent with this observation. It follows from all the above considerations that the acidic character of the surface is necessary for the esterification reaction. This view is supported by the parallel found by some workers [ 405,4061 between the rate of esterification and that of other typical acid-catalysed reactions. A linear correlation was established between the rate of acetic acid-ethanol esterification and that of deisopropylation of isopropylbenzene on a series of silica-alumina, alumina-boria and alumina catalysts [ 4061 ; a similar relation was found between the rate coefficient of the same esterification reaction and the cracking activity of a series of silicaalumina catalysts prepared in a different way [ 4051.

4.1.3 Reactions catalysed by organic polymer-based cation exchangers Organic cation exchangers are copolymers (mostly of the styrenedivinylbenzene type) with acid functional groups, such as -S03H. The close chemical analogy between these groups and inorganic acids used as homogenous catalysts (e.g. H,SO,) led to the idea of using organic ion exchangers as solid catalysts in proton-catalysed reactions, such as esterification. The fact that homogenously catalysed esterifications are carried out in the liquid phase determined the method of operation in the majority of esterification studies using ion exchanger catalysts: only a few kinetic studies of esterification and transesterification in the vapour phase with these catalysts were performed. The same fact also influenced the approach t o the kinetic analysis of esterifications catalysed by ion exchangers. As for homogeneous reactions, power law-type rate equations

357 (second- or pseudo-first-order) have been used in almost all published work, in spite of the fact that the presence of the solid catalyst may introduce changes into the kinetic relationships and complicate the analysis by the effects of slow diffusion of reactants to the solid surface and/or through the polymer mass. In order t o account for the well known sorption and swelling properties of polymer ion exchangers, Helfferich’s model [ 4271 is frequently used for liquid phase reactions. According to this, the pore liquid of the resin, where the reaction occurs, is treated as a homogeneous system and the reactant is assumed t o be distributed according t o a distribution coefficient

between the pore liquid and the supernatant solution; creSand csol are the respective reactant concentrations. The concentrations c,,, in the pore liquid are introduced instead of cSo1into the second-order kinetic law. In Helfferich’s relation it is implicitly assumed that the diffusion of the reactants through the pore liquid is fast enough so that equilibrium between the pore liquid and the supernatant solutions exists. Several papers, however, report a particle size effect on the esterification rate [4284331 indicating diffusional limitations; attempts t o describe,quantitatively the transport situation in the polymer mass are less frequent (e.g. ref. 434). For the liquid phase kinetic studies of esterification, with a few exceptions [ 402,435-4371 only the standard (non-porous, see Sect. 1.2.5) ion exchangers were used. The macroreticular (porous) ion exchangers with a large inner surface area are prefered for vapour phase reactions, especially in more recent studies [ 436-4431. The authors claimed that diffusion was not the limiting process under their conditions. This observation cannot be generalised, however, and even with vapour phase reactions and macroreticular polymers, the possibility of transport limitations through the pores o r the polymer mass cannot be excluded a priori. As with inorganic solid catalysts, the most extensively studied system was acetic acid-ethanol [ 428,432,434,444-4481.Other alcohols used in kinetic studies were methanol [ 430,449,4501,2-propanol [ 4381, l-butanol [ 429,431,433,451-4581,ally1 alcohol [ 4591,l-pentanol [ 4341 and ethyleneglycol [ 4601 ; besides acetic acid, the reactions of formic [ 4501, propionic [443,461],salicylic [ 430,4491,benzoic [ 453-4571 and oleic acids [430,451-4531 and of phthalic anhydride [462]have been reported. Investigation of a greater variety of reactants is reported in only one paper [463]:six alcohols (C4, C, and C,) and five acids (mainly dicarboxylic were studied. Transesterification kinetic studies were performed with ethyl formate [437,439,441],isobutyrate [ 437,439-4411 acetate [ 402, 435-437,439-4421, methoxyacetate [ 4411 and acrylate [ 403,404,464, 4651 ; the alcohols used were methanol [ 402,435,437,439-442,4501, References PP. 385-398

358 2-methoxyethanol [441], l-propanol [435-437,439-4411, l-butanol [ 403,4041, 3-methyl-l-butanol [ 464],2,2-dimethyl-l-propanol [ 437,439, 4411 and ally1 alcohol [ 4641. (a) Formal

kinetics

As has been already said, the majority of esterification (transesterification) kinetic data measured in the liquid phase were treated by using second- or pseudo-first-order rate equations. The concentrations of reactants were corrected in some cases [ 403,404,434,449,454-457,4641by using the Helfferich distribution coefficient, h. Some authors have recognised the oversimplification involved in the Helfferich model, which takes into account the concentration differences inside and outside the polymer particle but neglects the heterogeneous character of the chemical interaction between the liquid reactants and the functional groups of the solid catalysts. Interaction of such a type can be described by an adsorption isotherm; the Langmuir isotherm can be used, because all the functional groups of a given ion exchanger are chemically identical. Bochner et al. [ 4491 have developed a rate equation for salicyclic acid-methanol esterification (Dowex 50W catalyst) using this approach and assuming that all the reaction components can be associated with the protons of the -S03H groups in the polymer (competitive chemisorption). By using some simplifications, they obtained eqn. (24) in which it was assumed that the reaction between chemisorbed salicyclic acid molecules and methanol molecules in the pore liquid was the rate-determining step, r=

kbl[SA] [CH,OH] 1 + bz[HzOl

where bl and b2 are the association affinities for salicyclic acid (SA) and water, respectively. Equation (24) is, in fact, a Langmuir-Hinshelwood-type equation. Similar models with a single site surface reaction as the ratedetermining step were used for other liquid phase esterifications [ 448,451 1. Experimental data for the l-butanol-oleic acid system were best fitted by eqn. (24) [452] or eqn. (25) [451]

where Ki are adsorption coefficients and N i mole fractions of the reaction components (A = acid, B = alcohol, R = water, S = ester) and K is the thermodynamic equilibrium constant. Langmuir-Hinshelwood equations, with a surface reaction as the ratedetermining step, were also found suitable for liquid phase transesterifications [435-4371. With ethyl acetate and l-propanol in dioxan as a sol-

359 vent, the data obeyed eqn. (26)

where A is ester, B alcohol and I solvent. It is assumed that the ester reacts from the liquid phase with the adsorbed alcohol; the solvent competes with the alcohol for the active sites. With cyclohexane, there is no interaction of this inert solvent with the active sites and both reactants, alcohol and ester, are adsorbed, as it has been found for the same reaction in the vapour phase [439] [eqn. (27)].

If methanol, as a less basic alcohol than 1-propanol, is used in dioxan as solvent, then, contrary t o the model giving rise t o eqn. (26), the ester is adsorbed (protonated) and alcohol reacts directly from the liquid phase, and thus

Considerably fewer kinetic studies were performed with reactants in the vapour phase than in the liquid phase. The second-order rate equation was only used for acetic acid-ethanol esterification at 130°C and 175°C on a KU-2 standard ion exchanger [ 444,4451. A semiempirical second-order rate equation with slight inhibiting effect of reaction products, viz.

kPAPB 1 + aps ibPR was proposed for the vapour phase esterification of acetic acid with butanol over an oxidised phenol-formaldehyde carbon [ 4581. Other equations. For the vapour authors used Langmuir-Hinshelwood-type phase esterification of acetic acid with 2-propanol [ 4381 or ethanol [ 4461 and of propionic acid with ethanol [443], a dual site model with both reactants adsorbed was found to fit the experimental data r=

Herrman [ 4461 established an order-of-magnitude agreement between the values of the adsorption coefficients obtained by direct measurements of adsorption of alcohol and water vapour and those evaluated from kinetic data as KB and KR , which, in the author’s opinion, supported the physical meaning of these constants. The physical meaning of the Langmuir-Hinshelwood model was also examined by means of several transesterification reactions in the vapour phase at 120°C on a macroreticular ion exchanger [439,440,442]. The References p p . 385-398

TABLE 22 Values of the parameters of rate equation ( 2 7 ) for vapour phase transesterification catalysed with macroreticular ion exchanger at 120"C [ 4 3 9 ] Reaction components A

B

Ethyl acetate Ethyl acetate Ethyl acetate

Methanol l-Propanol 2,2-Dimethylpropanol l-Propanol l-Propanol l-Propanol

Ethyl acetate Ethyl isobutyrate Ethyl formate

Rate coefficient, h (mol kg-' h - l )

Adsorption coefficients KA

KB

2625 284 80.4

1.3 1.0 0.9

0.3 2.8 7.6

284 55.8 37050

1.0 0.55 0.4

2.8 3.1 1.8

(bar-')

(bar-')

kinetics were expressed by the rate equation (27) and the resulting values of the parameters are summarised in Table 22. As can be seen, the adsorption coefficients of ethyl acetate obtained by kinetic analysis of its reaction with three differently reactive alcohols have very closely similar values; the same is true for the adsorption coefficients of l-propanol in its reaction with ethyl acetate and isobutyrate. Thus, the adsorption coefficients do not depend on the nature of the second reaction partner and are not empirical constants valid only for one particular reaction but they characterise the compound in question more generally. Kinetic analyses of systems where two esters compete for one alcohol or two alcohols for one ester were also performed [440]. In spite of the fact that more reactants are present in the reaction system and are adsorbed competitively on the surface of the catalyst, good agreement between the values of the parameters of eqn. (27), determined by two independent methods (kinetic analysis of isolated and competitive reactions), was again obtained. The results of both studies [439,440] demonstrate the applicability of the Langmuir-Hinshelwood model to reactions catalysed by ion exchangers.

( b ) Effect o f reactant structure

A quantitative correlation of structural effects of four esters and four alcohols in the vapour phase transesterification on a macroreticular ion exchanger at 120°C was made using the Taft equation [ 4411. The authors found that rate coefficients [from eqn. (27)] yielded better correlation with steric (E,) than with polar (u*) parameters, while there was no significant difference between the correlations of the adsorption coefficients of alcohols, K,, with both parameters. The correlations with E , yielded the slopes 1.4 and 0.6 for the reactivity of the esters and the alcohols, respectively, and - 0 . 4 for the adsorptivity of the alcohols. The observed

TABLE 23 Ratios of initial transesterification rates on the least and the most cross-linked ion exchanger [ 4 3 7 ] (M = macroreticular polymer, S = standard (non-porous) polymer.) Reactants Ester

A1c oh 01

Liquid phase (in dioxan, 52°C)

Vapour phase (120°C)

Ma

Sb

Ma

Sb

Ethyl formate Ethyl acetate Ethyl isobutyrate

1-Propanol 1-Propanol 1-Propanol

2.18 1.01 1.31

78.9 62.2 24.1

0.44 0.36 0.21

9.28 8.33 9.20

Ethyl acetate Ethyl acetate Ethyl acetate

Methanol 1-Propanol 2,2-Dimethylprop ano1

2.22 1.01 1.02

16.5 62.2 82.1

0.21 0.36 0.11

1.58 8.33 16.0

.-

a

Resins with 10 and 60% DVB were compared. Resins with 2 and 50% DVB were compared.

decrease in the reactivity of alcohols with the increasing chain length is analogous to that discussed by Fricke and Alpeter [411] for esterification over a silica gel catalyst. The importance of steric hindrance was also stressed in a comparative esterification study in the liquid phase [ 4631. As will be discussed later [Sect. 4.1.3.(c)], the reactivity in reactions catalysed by ion exchangers depends on the degree of crosslinking of the polymer. If these effects were different for reactants of different structures, one could expect changes in the relative reactivities (selectivities) of reactants on varying the degree of crosslinking. Table 23 summarises the ratios of initial transesterification rates [437] on the least and the most cross-linked ion exchanger (from 2 to 60% divinylbenzene content) for the same reactants as in Table 22. It is apparent that the selectivity in transesterification, e.g. of ethyl acetate with methanol and 2,2dimethylpropanol on standard ion exchangers, would increase five times (82.1/ 16.5 = 4.98) in the liquid phase, or ten times (16/1.58 = 10.1) in the vapour phase, in favour of methanol in going from the lowest t o the highest degree of crosslinking. With macroreticular ion exchangers, the effect would be reversed. When reactants of large molecular size, such as oleic acid, were reacted over an ion exchanger catalyst, a direct proportionality between the reaction rate and the surface area of the catalyst was found [433]. The authors explain the result by assuming that, for the bulky reactant molecules, only acid groups at or near the surface of the catalyst particle can be effective catalysts. The efficiency of the catalyst (the rate coefficient with the resin compared to the rate coefficient with the same stoichiometric amount of dissolved inorganic acid) was found to be considerably References p p . 385-398

362 lower for oleic than for benzoic acid [ 4531. The structure of the reactants can affect the relative adsorptivities of ester and alcohol and perhaps, according t o the view of Setinek and Rodriquez [ 4351, also the kinetic mechanism, as already discussed in Sect. 4.1.3.(a) [see eqns. (26) and ( 2 8 ) ] . It was found in transesterification of ethyl acrylate in the liquid phase over a non-porous KU-2 catalyst [ 4641, that the structure of the alcohol influenced the value of the limiting sorption of alcohol by the ion exchanger, the logarithm of this value being a linear function of the dielectric constant of the alcohol. As the second-order rate coefficients yielded the same sequence as the limiting sorption values, viz. ally1 alcohol > l-butanol > 3-methyl-1-butanol, Filippov et al. [ 4641 assumed a relation between the dielectric constant and the reactivity of the alcohols.

( c ) Effect o f ion exchanger properties With one exception [447], only sulphonated resins were used as catalysts in kinetic studies of esterification and transesterification, the resins being almost exclusively styrene-divinylbenzene copolymers; in one case, a sulphonated phenol-formaldehyde resin was also used [ 4331. The main factors determining the catalytic activity are (i) the concentration of functional groups in protonated form (-S03H groups) and (ii) the degree of crosslinking of the copolymer (characterised by the divinylbenzene content).

(i) Concentration of ucid groups. It is generally accepted that the protons of the acid functional groups in the polymer are responsible for the catalytic activity in esterification. According t o expectation, the activity was always found t o drop when the number of acid groups decreased [428, 455,456,4601. However, this dependence is not linear and the activity of the remaining protons in a partially neutralised catalyst is lower than in a catalyst fully in the H'-form [ 428,455,4561. This observation indicates that the dependence of the reaction rate on the proton concentration is of an order higher than unity and that more than one proton or protonated species might participate in the formation of the activated complex in the ratedetermining step. The activity of the remaining protons is dependent, not only on their concentration, but also on the nature of the cation used for neutralisation. For the esterification of acetic acid with ethylene glycol on a KU-2 catalyst, the inhibiting effect of cations increased in the order Na' < K' < Ca2' < Li+< A13+[460]; from data on the esterification of benzoic acid with 1-butanol over Dowex 5 0 W catalyst [ 4551, the order Mg2' < Na' < Ba2' < Cs' can be estimated. However, the activation energy (-75 kJ mol-') was found t o be independent of the type of cation and the degree of neutralisation [ 4551. The changes in activity are attributed by the authors t o the changes in activation entropy, which decreases with

363 the increasing concentration of inorganic ions in the resin. Since, with higher ion concentrations, the sorption of benzoic acid also increases, the authors assume that the activation entropy is the more reduced the stronger is the adsorption of this reactant and the more restraints upon the transition state. A similar view was expressed in a similar study performed with the cations Li', Na', K', A13', C2HSNH: and (C2H,),NH', [456].In the vapour phase esterification of propionic acid with ethanol over a macroreticular catalyst [443],the effect of cations was not quite consistent with that found in the liquid phase reactions: an increasing inhibiting effect with decreasing cation valency was found (Me3' < Me2+ < Me').

(ii) Degree of polymer crosslinking. The degree of crosslinking of the copolymer is controlled by the amount of divinylbenzene (DVB) added t o the copolymerisation mixture, The effect of crosslinking upon the catalytic activity of the ion exchanger is dependent on the type of resin (standard or macroreticular) and on the conditions of its use (liquid or vapour phase). With the exception of macroreticular catalyst in vapour phase reactions [ 436,4371, the catalysts with a higher degree of crosslinking always exhibited lower activity in esterification or transesterification than (see the less crosslinked ones [ 404,428,429,431,436,437,445,451,4571 also Table 24). The difference in the effects of crosslinking in liquid and vapour phase processes, and especially the different behaviour of standard and macroTABLE 24 Effect of degree of crosslinking of the resin and comparison of heterogeneous and homogeneous rates (a) Esterification of acetic acid with ethanol at 75°C [ 4 2 8 ] Catalyst

Reaction rate (mol equiv-' h-')

Ion exchanger with DVB content (%) 20

1

22.8

82.8

H2S04

108

( b ) Transesterification of ethyl acetate with 1-propanol at 52°C [ 4 3 6 ]

Catalyst

Reaction rate (mol equiv-' h - l ) a

50

25

15

8

2

0

p-toluenesulphonic acid

0.19

0.25

0.46

1.88

4.07

8.50a

9.40

Ion exchanger with DVB content (%)

Extrapolated value.

References p p . 385-398

364

0

10

20

30

40

50 "la

60

DVB

Fig. 1 6 . Effect of degree o f crosslinking ( % DVB) of standard (non-porous) ion exchanger on initial transesterification rate, ro (mol kg-1 h-1 ), of ethyl acetate with 1-propanol [436]. ( 1 ) Liquid phase at 52OC; initial composition (mole%),0.4 ethyl acetate, 0.4 1-propanol, 0 . 2 dioxan (solvent). ( 2 ) Vapour phase at 12OoC; partial pressure of reactants, 0 . 5 bar (ester-alcohol ratio 1:l).

reticular catalysts, was demonstrated (Figs. 1 6 and 17) for transesterification of ethyl acetate with 1-propanol [436]. From Fig. 16, it follows that, with standard ion exchangers, the effect of crosslinking is greater for liquid than for vapour phases processes. This can be explained by the swelling of the polymer, which increases with decreasing crosslinking and is more extensive in the liquid phase. The swelling of standard polymers is very probably the reason why in the liquid phase the reaction rates are higher than in the vapour phase in spite of the lower temperatures used (see also ref. 428). The behaviour of macroreticular ion exchangers (Fig. 17) is different from that of the standard ones. The porosity (pores are mainly about 1 5 nm in diameter) of the macroreticular type, and correspondingly the internal surface area, are higher with higher crosslinked polymers. This would lead to higher reaction rates, but the effect of increasing surface area is compensated in the liquid phase by decreased swelling. Consequently, the catalytic activity of the ion exchangers is only slightly dependent on the degree of crosslinking (Fig. 17, curve 1).On the other hand, for vapour phase reactions, swelling is much less important and, since the surface area is larger with higher crosslinked polymers, the rate increases with the

365

Fig. 1 7 . Effect of degree o f crosslinking (W DVB) of macroreticular ion exchanger o n initial transesterification rate, ro (mol kg-1 h - l ) , o f ethyl acetate with 1-propanol [436].( 1 ) Liquid phase. (2)Vapour phase (reaction conditions the same as in Fig. 16). Dotted curves (1’) and ( 2 ’ ) represent reaction rates o n a surface o f area equal t o the surface area of the ion exchanger with 10%DVB.

degree of crosslinking (Fig. 17, curve 2). If, however, the rates are related to unit surface area (dotted curves in Fig. 17), they decrease with the degree of crosslinking. The crosslinking effect also depends on the structure of the reactants [437] as already discussed [Sect. 4.1.3.(b)]. In liquid phase reactions, the importance of the swelling properties and the related sorption capacities for the catalytic activity of ion exchangers was demonstrated. The rate coefficient of 1-butanolacetic acid esterification [431] decreased with the degree of crosslinking in the same manner as did the water sorption capacity and the solvation coefficient of l-butanol. A similar effect was found for the transesterification of ethyl acrylate with 1-butanol [ 4041. Davini and Tartarelli [ 4571 found an increase of activation energy and activation entropy with degree of crosslinking of the resin in the liquid phase esterification of benzoic acid with 1-butanol. This finding is in contradiction to the view of Bernhard and Hammett [366] according to which the resin structure imposes the more severe restraints upon the transition state the higher is its degree of crosslinking. Nevertheless, Davini and Tartarelli [ 4571 try to explain their observation by reference t o their sorption data. References pp. 385-398

It appears from the relations between the catalytic activity, the structure of the reactants, the degree of crosslinking and the swelling and sorption properties of the resin that all these factors might influence the accessibility of the reactants to the acid groups of the ion exchanger. Thus, the rate of transport of reactants (diffusion) t o the active groups may also become an important factor in the kinetics, especially with larger reactant molecules and more highly crosslinked polymers.

( d ) Effect o f water and solvents (i) Effect o f water. An inhibiting effect of water on esterification (greater than that which corresponds t o the reversibility of the reaction) was observed for both liquid [ 428,431,433,449,4521and vapour phase reactions [438,445,446].In the liquid phase where the resin is in a swollen state, the effect can be ascribed to the interaction of water molecules (in competition with the reactants) with the active protons of the catalyst rendering them less active or inactive. The effect of water has generally been considered qualitatively. However, the competition was expressed quantitatively in salicylic acid-methanol esterification [ 4491 by a Langmuir-Hinshelwood model and eqn. (24)is a simplified expression based on this model. In a similar manner, the inhibition by water was expressed by the Langmuir-Hinshelwood equation (29)for vapour phase esterification of acetic acid with 2-propanol [ 4381 or ethanol [ 4461. With vapour phases processes, however, some other effects were observed. Setinek and Ber6nek [439]found an activating effect of water in acetic acid-ethanol esterification when an ion exchanger, predried in vacuo at a higher temperature, was used. Cunningham et al. [438] observed a loss in catalyst weight (initial water content was 15-2096) and a decrease of the catalytic activity when the esterification was carried out in a flow of nitrogen at a partial pressure higher than 0.6 bar; the authors ascribed the effect t o a partial dehydration or “deswelling” of the resin, which decreased the site accessibility. Herrman [ 4461 reported an increase in the effective rate coefficient when the pressure was increased or the temperature decreased; he suggested that the number of active sites might change with the equilibrium amount of water in the ion exchanger which increases with pressure and decreases with temperature. All the cited experimental observations [ 438,439,4461could have a common explanation: the first effect of water vapour contacting a dry catalyst may consist in slight swelling of the resin thus increasing the amount of accessible sulphonic groups; this effect is reversible and dependent on the partial pressure of the water and on the temperature. With a stabilised resin with a water content above a certain limit, the accessibility of the active groups does not change significantly, however, and the effect of water then consists of competition for the active groups with other reactants, as mentioned above.

367 (ii) Effect of solvents. Because liquid phase esterifications or transesterifications are usually carried out in excess of one of the reactants (most frequently of alcohol) which serves also as a solvent, the influence of other solvents has not been thoroughly investigated. Nevertheless, the available results indicate that there may exist a relationship between the dielectric constant and the polar properties of the solvent, and the reaction rate. The solvents investigated can be classified into three groups: (a) non-polar (heptane, benzene, toluene), (b) polar solvents negatively charged at the oxygen atom and capable of solvating cations (tetrahydrofuran, dipropyl ether, dioxan) and (c) polar solvents not capable of solvating cations (nitropropane, nitroethane, nitromethane). The effect of group (a) (non-polar solvents) was examined in the esterification of benzoic acid with 1-butanol over a Dowex-W X2 catalyst [ 4541. The solvent affected both the Helfferich distribution coefficients and the esterification rates. Dielectric constants, corresponding t o the composition of the pore liquid, were estimated and the kinetic data related t o the polar properties of the medium within the catalyst. In Fig. 18 are plotted specific rate coefficients versus the reciprocal value of dielectric constant of the pore liquid. The slope of the correlation is positive as for

-5

-C -6

3

4

104/ DT

5

6

Fig .18.Dependence of specific rate coefficient, h of the esterification benzoic acid + 1-butanol at 3OoC o n the reciprocal of the dielectric constant, D, of the liquid reaction medium [ 4541. ( 1 ) Homogeneously catalysed reaction. ( 2 ) Reaction catalysed by Dowex-W X2 ion exchanger. T,Absolute temperature; 00,without s o l v e n t ; o l , in heptane; a m , in benzene; AA, in toluene. R e f e r e n c e s p p . 385-398

368 the data on homogeneous catalysis, which is consistent with a reaction mechanism involving a positive ion and a dipolar molecule. The effect of solvents of groups (b) and ( c ) was investigated in transesterification of ethyl acrylate with l-butanol over a KU-2-8 catalyst [465]. It was established for group (b) and for n-heptane that the higher the sorption capacity of the resin for a given solvent, the lower was the rate of reaction in that solvent. This can be explained by the formation of a hydrogen bond between the polar aprotic solvent and the -S03H group, e.g. (C3H,),=O,\

--.H-SO3-

(C)

which lowers the activity of the catalyst, The sorption of non-polar heptane is negligible and the rate of reaction in thia solvent is the highest of the series. A linear dependence of the rate coefficient on the reciprocal of the dielectric constant was found, which indicates that the polarity of the medium determines its ability to solvate the active species which are assumed to be the protonated alcohol molecules, C4H90Hf, thus affecting the reaction rate. An interaction of the type above [scheme (C)], i.e. the formation of hydrogen bond between the oxygen of the solvent and the -S03H group of the catalyst, was proposed by Rodriguez [435,466] to explain the lower reaction rates observed in transesterification of ethyl acetate with l-propanol in dioxan compared with those in cyclohexane [see also eqn. (26)]. As t o solvents in group (c) (njtroalkanes), which hardly solvate cations, higher reaction rates were observed than with other solvents [465]; the rates increased with increasing dielectric constant and the sorption capacity of the resin for the solvent. This result indicates that a solvation of the anions -SO; by the nitroalkane molecules might occur by the formation of a hydrogen bond -S06,-....H6+-CH2N02 ; consequently, the activity of the protons of the sulphonic groups increases and the transesterification rate is enhanced.

( e ) Mechanism It follows from the dependence of the catalytic activity of ion exchangers on the amount of functional groups in the H+-form [Sect. 4.1.3.(c)] and on their acid strength [447], that acidity is the essential property of esterification catalysts and plays a decisive role in the reaction mechanism (see also ref. 438). However, lower specific rates were observed in catalysis by ion exchangers than if the reaction was catalysed by an equivalent amount of p-toluenesulphonic, sulphuric or hydrochloric acids [ 431,453, 454,460,462,4631. The difference between the homogeneous and heterogeneous catalytic reaction rates decreases, however, with decreasing degree of crosslinking of the polymer [Sect. 4.1.3.(c)] and by extrapolation to

369

zero divinylbenzene content, both rates were found t o be approximately equal (Table 24). Therefore, the mechanisms of both homogeneous and heterogeneous reactions might be expected t o be the same and the reduced reaction rate with ion exchanger catalysts could be simply ascribed to a lower number of sulphonic acid groups of the resin accessible to reactants. An equal effect of the dielectric constant of solvents, found for both types of catalytic esterification (Fig. 18), further supports the similarity of both mechanisms. The homogeneous acid-catalysed esterification has been thoroughly studied and a lot of information on its mechanism has been accumulated. The assumed close relation to the heterogeneous mechanism can, therefore, be made use of in mechanistic considerations about the esterification catalysed by ion exchangers. In the homogeneous mechanism, the reaction is assumed to start by protonation of one of the reactants, either ester (mechanisms denoted as AAcl and AAc2 [397,398]) or, less frequently, alcohol (mechanism AALl). It seems likely that protonation of reactants is an important step in esterification catalysed by ion exchangers, too. This follows from all that has been said above about the effect of the acidic properties of ion exchangers on their catalytic activity and is further supported by the effect of the dielectric constant of solvents (Fig. 18), which indicates that the reaction mechanism involves a positive ion and a dipolar molecule [ 4541. In most homogeneously catalysed esterifications, the protonated species is considerdd to be formed from the acid (or from the ester in transesterification) [ 397,398,4671. Starting with this view, Bochner et al. { 4491 assumed a mechanism for heterogeneous esterification [see eqn. (24)] in which the reaction between protonated salicylic acid and methanol in solution was the ratedetermining step. In oleic acid esterification with 1-butanol [452], an analogous mechanism was indicated [see eqn. (25)] by the kinetic analysis. On the other hand, the sorption of the acid was considered to be nonionic and found to decrease with increasing concentration of functional groups of the ion exchanger in the H+-form [455]. The amount of the alcohol sorbed was higher than that of the acid [455] or that of the ester in transesterification [ 403,404,464,4651. The rate coefficients of transesterification varied in the same way as did the amount of the sorbed alcohol when different alcohols [ 4641 and solvents [ 4651 were used or when the degree of crosslinking of the ion exchanger was altered [404]; such a relation did not exist between the reaction rate and the sorbed amount of the ester. These results could be interpreted on the basis that it is the alcohol that is preferentially adsorbed and protonated; the formation of the ion ROH; and its role as a reactive intermediate were explicitely formulated [ 404,4651. Two arguments, however, that weaken this interpretation may be raised: (i) the amount of a reactant sorbed in the bulk of the ion exchanger References p p . 385-398

370 is not necessarily a measure of the chemical interaction of the reactant with Lhe sulphonic acid groups; (ii) chemical interaction of the --S03H groups with the alcohol can consist not only in the proton transfer to the alcohol [scheme (a)] but can also take place through the formation of hydrogen bond between the hydroxyl group of the alcohol and the oxygen atom of the sulphonic group acting as a basic site [scheme (b)].

OH

RS-H /\o=s=o/ ' \ It has also been suggested that an alcohol molecule can be bound t o two o r more sulphonic groups by combined interaction of both types; simultaneous interaction with two -S03H groups, for example, is illustrated by the scheme (c).

OH

0 ''

Interactions of this types are assumed t o be involved in the mechanism of alcohol dehydration over sulphonated ion exchangers [ 112,131,138,4684701 from spectroscopic evidence [ 138,4711. Therefore, it cannot be ruled out that in esterification also more than one active site (-S03H group) may, in general, be involved in the reaction mechanism. Alcohol can be adsorbed on one or more active sites according t o schemes (a)-(c), and acid be protonated as represented by scheme (d). This assumption seems t o be supported by the dependence of the esterification rate on the concentration of sulphonic groups which is of an order higher than unity [see Sect. 4.1.3.(c)]. In a particular case, the adsorption of either the acid (ester in transesterification) or of the alcohol may prevail, and, in an extreme case, only one reactant would be adsorbed, the other reacting from a non-adsorbed state. These considerations can be formulated by schemes (e)-(g) below (R' = H o r alkyl). Scheme (e), in which the acid (ester) is protonated and alcohol reacts in non-adsorbed state, corresponds t o the mechanisms AAcl or AAC2 proposed for homogeneous esterification and hydrolysis; with ion exchanger catalysts, the mechanism (e) was assumed to be operating in the liquid phase esterification of salicyclic acid with methanol [ 4491 and in t h e transesterification of ethyl acetate with the same alcohol in dioxan as

371 solvent [435]. Scheme (f), according t o which alcohol is adsorbed and the acid (ester) reacts from the non-adsorbed state, was proposed for the reaction of ethyl acetate with 1-propanol in dioxan [435] and may perhaps also be operating in cases where protonation of the acid (ester) was not considered to be a likely step [404,454,457,465]. Mechanism AAll the first step of which is protonation of the alcohol, is rare even in homogeneous catalysis. There is no reason t o suppose that it would be more probable in heterogeneous catalysis. A more general scheme with both reactants adsorbed is that represented by scheme (g). It was considered probable in the liquid phase transesterification in a non-polar solvent (cyclohexane) [435] and it may correspond to all vapour phase esterifications and transesterifications where the rate equations from the kinetic analysis (see eqns. (27) and (29) suggested the involvement of dual- or triple-sites.

4.2 HYDROLYSIS

4.2.1 Hydrolysis of esters Hydrolysis of esters, producing acids and alcohols RCOOR' + H 2 0 = RCOOH + R'OH

(D)

is the reverse reaction t o esterification [scheme (A)] treated in Sect. 4.1. The conversion in such a reversible reaction may be limited by thermodynamic equilibrium (see equilibrium constants of esterification in Table 18). Kinetic studies of ester hydrolysis, however, have been almost exclusively performed in an excess of water which causes the reaction t o be practically irreversible. Although, in homogeneous media, ester hydrolyses are catalysed both by acids and bases, for kinetic investigations of heterogeneously catalysed hydrolysis, the use of solids with acidic properties only is reported, most of them being organic polymer ion exchangers. No side reactions, accompanying the main reaction (D), were observed under References p p , 385-398

37 2 the conditions used. The analogy with homogeneously catalysed ester hydrolysis determined the method of operation (liquid phase, mainly batch reactors) and the kinetic approach. With solid catalysts, the kinetics of reaction (D) were also assumed to be of the second order (see, for example, refs. 472, 473); in excess of water, the order degenerates and pseudo-first-order kinetic equations were used in most cases. In two papers concerning the hydrolysis of ethyl acetate over sulphonated styrenedivinylbenzene copolymers as catalysts [ 448,4511, LangmuirHinshelwood single site models with surface reaction as the ratedetermining step were applied [see eqn. (25)]. (a) Catalysis by inorganic catalysts The activities of six mixed oxides, Si02-A1203, SiO, -Zr02, SiO, TiOz, A1203-Zr02, TiOz-ZrO, and A1203-Ti02, in the hydrolysis of methyl and ethyl acetates at 25 and 35°C were compared and related to the acidic properties of the catalysts [474]. It was found that the pseudofirst-order rate coefficients for both esters and both temperatures were proportional t o the number of acid centres of pK, < -3.0 as determined by the l-butylamine titration of the used catalysts. Since the points for HC1 lay on the same straight line as did the points €or inorganic catalysts, it was believed that the acid centres of pK, < -3.0 of these catalysts behaved even in the presence of water, very much like hydrochloric acid or any other mineral acid. This view was further supported by the fact that, by poisoning the catalysts with dicinnamalacetone (pK, = -3.0), the hydrolysis was fully suppressed. The values of the observed activation energies for all solid catalysts and HC1 (24.7-25.5 and 33.9-35.5 kJ mol-' for methyl and ethyl acetate, respectively) were closely similar and indicate a similarity of the mechanism of hydrolysis with all catalysts and again support the view that there is no substantial difference in the action of dissolved HCl and solid acid catalysts in ester hydrolysis. Specific rate coefficients (related to unit amount of acid centres) were approximately the same for solid catalysts as well as for HC1 [474]. However, when a montmorillonite clay activated by adsorption of protons on its surface was used as the catalyst in ethyl acetate hydrolysis [475], a higher specific rate coefficient (about 1.8 times at 25°C) was found for the reaction catalysed by adsorbed protons than by dissolved acid, this result being explained by the authors by an increase of activation entropy in the former case. (b) Catalysis by organic ion exchangers

The majority of kinetic studies of ester hydrolyses using this type of catalyst were performed with sulphonated styrenedivinylbenzene copolymers. Only in a few cases [476-4781 was the use of phenol-formalde-

373 hyde polycondensates with either -S03H or -€OOH groups reported. The main interest in kinetic investigations with organic ion exchanger catalysts was concentrated on the effects of reactant structure, solvents, the degree of cross-linking of the polymer and partial neutralisation of acid groups with different cations. These effects are usually interrelated and t o discuss each of them separately is of little value. Since, with ion exchangers, the specific properties of the solid acid are of major interest, the efficiency of the catalyst for a given reaction was defined [366] by the relation 4

khet

=-

khom

where khet is the rate coefficient for the reaction catalysed by a solid resin and khom that for the same reaction catalysed by a dissolved inorganic acid, such as HC1, both coefficients being related t o the same amount of -S03H groups or protons in a unit reaction volume.

(i) Effect of reactant structure and solvent. It has been shown that the

effect of the ester structure on both the absolute values of the rate coefficients and the efficiencies is strongly dependent on the properties of the reaction medium used (addition t o water, of solvents which are capable of solvating cations). If the reactions are carried out in acetone-water mixtures [366,479-4811, a decrease of the reactivity as well as of the. efficiency (this being always less than unity) with increasing length of the aliphatic chain in the acyl or in the alkoxy groups was observed. Similar trends were established for the structure effects using water-dioxan mixtures [ 482-4841 o r in excess of the reacting ester [ 485-4871. When, on the other hand the reaction was carried out in excess of water without any other solvent added, other effects of the ester structure on the reactivity and the efficiency of the catalyst were observed [476,488, 4891. The efficiencies, q, were higher than unity (see also ref. 490), which means that the resin-catalysed reaction was faster than that catalysed by HCl; the q values increased with increasing chain length of the alkyl group [476], contrary t o what was found with mixtures of water and other solvents. Several attempts have been made t o explain the variations in efficiency of ion exchanger catalysts for different esters and reaction media. Hammett et al. [ 366,479,4881 suggested that the difference in efficiency for different esters arises from a difference in the magnitude of the loss in internal entropy of the ester molecule which accompanies its fixation on the resin catalyst in the formation of the transition state. It can be shown that the ratio of efficiencies for two esters, 1and 2, is given by

*

-RT ln(q*/q1) = (G:,het - G2,horn) - (G:,het - G:,hom) where the G*’s are the standard free energies of the transition states and References p p . 385-398

37 4

I

01

40

I 60

I

I 100

ao

- 2.c

- 1.5 0-

0" -1.0

- 0.5

0 40

I 60

I 80

I

100

Fig. 1 9 . The efficiency, q , of t h e Amberlite IR-120 ion exchanger catalyst in ester hydrolysis as a function of t h e entropy, S , of t h e parent hydrocarbon RH or R'H of the substituents. ( a ) Hydrolysis (at 25-45OC) of methyl esters RCOOCH3 [ 3661 : 1, acet a t e ; 2, chloroacetate; 3, benzoate; 4, cyclopentanecarboxylate; 5 , phenylacetate; 6, a-naphthylacetate; 7 , l-octanoate. ( b ) Hydrolysis (at 35OC) of acetates CH3COOR' [480]:1 , methyl; 2, ethyl; 3 , cyclopentyl; 4 , cyclohexyl; 5 , 1-butyl; 6 , 2-pentyl; 7 , 1-pentyl; 8, 1-hexyl; 9 , 1-octyl.

37 5 the subscripts refer either t o heterogeneous or t o homogeneous systems. Since the differences in activation energy between the two systems were found [479] (for reactions in 70% aqueous acetone) to be small (of about -5.4 k J mol-I) and were little, if at all, affected by the chain length of the ester, it follows that it is the entropy difference that is controlling the efficiency of the resin for different esters (values of -23.0 and -37.6 J mol-' K-' were observed [479] for methyl acetate and ethyl butyrate respectively). It seems reasonable that the loss in entropy should be the smaller, the smaller is the entropy of the ester. This hypothesis has a surprising degree of quantitative validity, as is shown in Fig. 19, where, in (a), the efficiencies, 4, of the hydrolysis of seven methyl esters, RCOOCH,, are compared with the entropy, S, at 298 K of their simplest structural analogues, the gaseous compounds RH and, in (b), the efficiencies in the alkyl acetate series CH,COOR' are plotted against the S values of the parent hydrocarbon, R'H. A drop in efficiency with increasing degree of crosslinking of the resin was also observed [366], this effect being much greater with the larger molecules of ethyl hexanoate than with methyl acetate. The results are considered by Hammett et al. to be consistent with their hypothesis: the structure of the more highly crosslinked resin imposes more severe restraints in the formation of the transition state than does the structure of the less crosslinked resin. Bernhard and Hammet [ 4881 tried t o explain the apparent discrepancy between the results [366,479] for 70% aqueous acetone and those with water (Table 25). They found that, with water, the activation entropies were, in contrast t o those using aqueous acetone, higher for the resin than for HCl. The observation is consistent with the following hypothesis. In the mixed solvents, the electrically charged transition state is subjected t o restraints arising from solvation, which are stronger than those acting on the ester. Still greater restraints imposed by the resin network lead t o a still greater decrease in the entropy of activation with increasing chain length. In water solution, however, the solvation of the polar group in the ester itself is so powerful that no further important restraints are imposed on the internal motion of the ester by the solvation of the charged transiTABLE 25 Effect of ester structure o n the efficiency of ion exchanger in water [ 4 7 6 ] (Catalyst: Amberlite IR-100; temperature: 25°C.) Ester

Efficiency

Methyl acetate Ethyl acetate 1-Butyl acetate Benzyl acetate

1.8 2.3 10 20

___.

References p p . 385-398

~

~

_

_

-~ ._ _

_

~

_

-

_

__-

376 tion state or by the resin network. Consequently, the entropy change involved in the conversion of the ester t o the transition state is no longer more negative for esters of a greater chain length. Another approach attempts t o explain the different effect of the ester structure in different reaction media simply by the changing ability of the esters t o be absorbed by the resin. Qualitatively, this approach was used [476] t o interpret the results for water and aqueous acetone and a similar idea was suggested for the hydrolysis of dicarboxylic acid esters in water-dioxan mixtures [ 482,4831. Quantitative interpretation was based [481,489] on Helfferich’s model [427]. It follows from eqn. (30) and from the relation -

(31) resulting from the Helfferich model [eqn. (23)] ( h h e t is the rate coefficient corresponding to the concentration of ester in the pore liquid) that hhet = h h e t h

-

4 =-

hhet

hhom

(32)

The experimental evidence of the validity of eqn. (32) was illustrated by a plot of the experimental q versus data; the slope obtained, z h e t / h h o m = 1, means that the reactivity of the ester in the pore liquid (i;h,t) of the resin is the same as in the H2S04-catalysedreaction ( h h o m ) for all the esters investigated [ 4891. Thus, the effect of ester structure is believed to consist only of influencing the distribution coefficients, A , of the ester between the pore liquid and the supernatant solution, which is in accordance with Helfferich’s model (see also ref. 491). Investigating the effect of acetone concentration in acetone-water mixtures on ethyl, 1-propyl and 2-propyl acetate hydrolysis with Dowex X50 containing 10% DVB, Tartarelli e t al. [481] found that the rate coefficients and the efficiencies decreased with increasing acetone concentration (cf. ref. 477). The inversion of the efficiencies, q , from positive to negative values took place at about 45-5576 acetone concentration (see also refs. 478 and 482). The authors [481] plotted the rate coefficients found at different acetone concentrations for the three esters in the homogeneous and heterogeneous reactions and the corresponding distribution coefficients as hhomh versus k h e t and obtained a straight-line dependence with the slope equal t o unity. This means that k h o m x = h h e t ; from this and the relation (31), it again follows that jr;het = h h o m , in agreement with the results of Tartarelli e t al. [489] and with Helfferich’s model.

(ii) Partial neutralisation of ion exchanger. The two theories, that of Hammett and that based on the Helfferich concept, were also used t o interpret the effect on the hydrolysis rate of the partial neutralisation of acid

377 groups of the ion exchanger by different cations. When metal ions (Na+, MgZ+,Ba2+)and NH4' were used for neutralisation, the dependence of the rate coefficient on the proton concentration in the resin was found t o be first order [ 489,4921. In another study [ 4931, a slight decrease of the specific rate coefficient with increasing degree of neutralisation with Na' ions was reported. When cations of larger dimensions such as ethylenediammonium ions or other quarternary ammonium ions with larger organic substituents were used for the neutralisation, the specific rate coefficients were found to change with the degree of neutralisation [ 492,494,4951, this effect being more pronounced in the case of an ester with a longer acyl chain [ 4921. The effect of partial neutralisation by cations of the last mentioned type was, in general, negative. An unexpected enhancement of the specific rate coefficient ( h h e t )was, however, established when the neutralising quarternary ion and the ester had some prominent structural features in common [ 4941 (e.g. hexadecyltrimethylammonium ion in the hydrolysis of ethyl hexanoate, an ester with a rather long aliphatic chain). The interpretation presented by Riesz and Hammett [494] follows from the old principle that like dissolves like. In the case above, this is t o be interpreted in the sense that increasing incorporation into the resin of longchain aliphatic structures lowers the standard free energy of the transition state for the hydrolysis of an ester containing similar structures relative t o standard free energies of the transition states of esters of a different structure. The alternative approach to the problem is based on Helfferich's model. Tartarelli et al. [495] measured the distribution coefficients, A, and specific rate coefficients, k h e t , of the hydrolysis of 1-propyl acetate and ethyl 1-hexanoate; the ion exchanger catalyst was neutralised t o a different degree with benzyldimethylhexadecyl- and trimethylbenzyl-ammonium ions. The specific rate coefficients for the pore liquid of the resin, h h e t , were calculated according t o eqn. (31).The distributioncoefficients, A, of the aliphatic ester were increased and the coefficients hhet lowered when the ion exchanger contained ions with long chain aliphatic groups (benzyldimethylhexadecylammonium ions) and the reverse was true when it contained trimethylbenzylammonium ions (without any long aliphatic chain). The authors explain the results by assuming that the matrix of the resin interacts with the ester being sorbed, involving its reactive group. Hence, the stronger these intereactions are, the higher is the distribution coefficient but the lower is the reactivity.

(iii) The degree of crosslinking and diffusion. The efficiency of the ion exchanger catalyst in ester hydrolysis, as in esterification (Sects. 4.1.3.(b) and (c)], decreases as the degree of crosslinking of the resin increases [ 366,483,484,488,490,4931, this effect being more pronounced with larger ester molecules. Bernhard and Hammett [366] tried t o explain the References p p . 385-398

378 phenomenon by restraints imposed by the resin on the activated state (see p. 375). However, Goldenshtein and Freidlin [ 483,4841 advanced another idea, relating the influence of crosslinking simply to geometric effects of the resin network. Smaller molecules are able t o penetrate into the polymer mass whereas the reaction of the larger ones can take place only on the outer surface of the resin particles, the permeability of the resin being dependent on its degree of crosslinking. A similar idea was put forward [433] in the discussion of the effects of the catalyst surface area on the esterification of oleic acid, and will also appear in the discussion of other hydrolytic reactions [Sect. 4.2.2(b)]. This “sieve effect’’ cannot be considered statically as a factor that only determines the amount of accessible acid groups in the resin in such a way that the boundary between the accessible and non-accessible groups would be sharp. It must be treated dynamically, i.e. the rates of the diffusion of reactants into the polymer mass must be taken into account. With the use of the Thiele’s concept about the diffusion into catalyst pores, the effectiveness factors, Thiele moduli and effective diffusion coefficients can be determined from the effect of the catalyst particle size. The apparent rates of the methyl and ethyl acetate hydrolysis [490] were corrected for the effect of diffusion in the resin by the use of the effectiveness factors, the difference in ester concentration between swollen resin phase and bulk solution being taken into account. The intrinsic rate coefficients, h i n t r ,

TABLE 26 Effect of diffusion and adsorption in methyl and ethyl acetate hydrolysis in water at 40°C (Adsorption coefficients, K A , (dimensionless), effective diffusion coefficients, D , f , (cm2 sec-‘) and intrinsic rate coefficients, k i n t r , (cm3 equiv-’ min-’) for Amberlite ion exchanger catalysts of different degrees of crosslinking [490].) Ester

DVB in resin

KA

D,f

X

lo6

kintr

(%I Ethyl acetate

Methyl acetate

8 10 12

1.23 1.07 1.02 1.02

4 8 10 12

1.oo 0.88 0.83 0.85

4

0.513 0.250

32.6 32.5 32.0 32.3

0.852 0.452

33.1 35.5 38.0 35.3

From hintr and data for inorganic acid, the mean values of intrinsic efficiency q i n t r were calculated: for ethyl acetate qintr = 1.28 and for methyl acetate q i n t r = 1.45.

379 were calculated according t o the formula

where khet is the measured specific rate coefficient, 7 the effectiveness factor (ratio of diffusion limited reaction rate t o the non-limited one) and K A the ratio of ester concentration in the volume of the swollen resin to that in the bulk solution. As can be seen from Table 26, the intrinsic rates of hydrolysis are independent of the degree of crosslinking, whereas the adsorption of ester by the resin and its diffusivitity in the resin decrease with crosslinking. It might, therefore, be assumed that the lower hydrolysis rates observed with higher crosslinked polymers in this [ 4901 as well as in other studies [ 366,483,484,4881 were very probably due t o increased diffusion resistance and decreased adsorption capacity of the more tightly crosslinked resins. I t is not impossible that some effects of solvents, ester structure, or introduced cations, interpreted either by changes of activation free energy [366,479,488,492] o r reactant concentration in the pore liquid [476, 481,489,4951, could also be satisfactorily explained by the changes in the accessibility of catalytically active groups and in the diffusivity of reactants. Higher hydrolysis rates and resin efficiencies were always observed, for example, in reaction media in which the resins swell more, e.g. in water [ 476,481,4951 and alcohols [478]. In solvents in which the resins are less swollen (acetone, dioxan, esters), the rates were lower [ 366,477, 479-481,485-487,4951. The lower rates observed in solvents with negatively charged oxygen atoms, such as acetone and dioxan, might also be due, at least partially, t o the competition of the solvent with the reacting ester for the --S03H groups via an interaction illustrated by scheme (C) (p. 368), as was presumed [ 435,465,4661 for transesterification.

(iu) Mechanism. N o special investigations of the mechanism of the ion

exchanger-catalysed hydrolysis of esters were reported. In most papers, the analogy with the mechanism of acid-catalysed hydrolysis in homogeneous medium [ 397,3981 is impIicitly assumed. Haskell and Hammett [479] pointed out that, in resin-catalysed hydrolysis, as in that catalysed by strong aqueous acids, the mechanism involves a positively charged transition state in which a proton has been transferred t o the ester molecule from the oxonium ion of the acid. The view seems t o be in harmony with the acidic properties of ion exchanger catalysts and with the dependence of their activity on the strength of the acid groups in the polymer [476] as well as on their concentration [489,492,494,495]. Some analogies established between homogeneous and heterogeneous hydrolyses in the structure effects of the ester [366,478-480,4881 and in the effects of solvents [478,481] can be considered to support the concept of the identity of both mechanisms. R e f e r e n c e s p p . 385-398

380 4.2.2 Other hydrolyses

In comparison with ester hydrolysis, little attention has been paid to kinetic investigation of other solid-catalysed hydrolytic reactions, though they could be numerous with respect to the types of functional groups to be hydrolysed. With inorganic solid catalysts, some studies were devoted to the hydrolysis of carbonic acid derivatives, aryl chlorides and compounds with metalloid-hydrogen bonds. With organic ion exchangers, the reactions studied served rather as a model for the investigation of the catalytic action of, and transport phenomena in, the polymer catalysts. A typical hydrolytic reaction used for this purpose is sucrose inversion; limited attention was paid t o acetal hydrolysis. (a) Hydrolysis over inorganic catalysts

The kinetics of the hydrolysis of diethyl carbonate

CO(OC2HS)Z + H2O

=

C02 + 2 CzH50H

was investigated at 160-270°C in the vapour phase (excess of water) in a flow system [496]. About fifteen oxide or metal salt catalysts were compared. The first step in the formation of the activated complex over the majority of catalysts (those with ionic surfaces) is assumed t o involve reaction with the hydroxyl groups of water dissociated on the surface. The activated complex may be pictured as a semi-ionic system of water and diethyl carbonate polarised by the dissociative adsorption of water on an ionic surface. The hydrolysis of carbonyl sulphide

COS + H2O = C02 + H2S in the vapour phase in flow systems was investigated [497-4991. Firstorder kinetics with respect t o carbobyl sulphide and zero-order with respect to water (used in excess) were found. Catalysts with basic properties exhibited high activity whereas those with acidic properties were almost inactive [ 4971 ; alumina, being a good catalyst, is assumed t o act as a basic catalyst. Addition of sodium hydroxide t o the catalyst enhanced the activity considerably [ 498,4991. Alumina and cobalt molybdate supported on alumina were found to be equally active catalysts [499]. The surface of the catalyst is believed t o be partially covered with hydroxyl ions which are formed by dissociative adsorption of water on basic sites [4981 HOH + B(s)+ BH'(s)+ OHwhere B(s) is the basic site on the surface of the catalyts. The kinetics of vapour phase hydrolysis of aryl chlorides

X,C,H,-Cl+

H20 = XnC6H,,0H

+ HCI

381 was studied on catalysts containing lanthanum, cerium and other rareearth phosphates [500]; X was a methyl group and the number, n, of the substituents on the aromatic ring was 0, 1 or 2. The relative reactivity increased from chlorobenzene t o chloroxylenes. The reaction was inhibited at high conversions by the product HCl. The hydrolysis occurs via a reaction of adsorbed aryl chloride and a surface hydroxyl group to give a phenolic product and a surface chloride. The reaction of steam with the surface chloride produces HCl and regenerates the surface hydroxyls. Hydrolytic reactions of compounds with Si-H or B-H bonds in the liquid phase with great excess of water were investigated; zero reaction order with respect t o HzO was found. In the hydrolysis of para-substituted phenyldimethylsilanes at 20°C + H,O p-XC6H4Si(CH3)*H

= p-XC,H,Si(CH,),OH

+ H,

a Pd/C catalyst was used and the effect of substituents (X = H, C1, CH3, CH,O and CzH,O) on the reaction rate (first order with respect to the silane) was correlated by the Hammett equation [ 5011. The value of the p constant was +0.73, which indicated a nucleophilic substitution mechanism with a slightly polarised transition complex; very probably a SN2 mechanism was operating. The hydrolysis of the borohydride anion BH, + 4 H,O

=

B(OH), + 4 H2

was catalysed by rhodium, ruthenium and iron at 20-60"C in 1 N NaOH solution [ 502-5041. The reaction was formally of a fractional order (0 < n < 1) with respect t o the borohydride anion. The variation of the rate with initial concentration of borohydride led t o the conclusion that the monomolecular surface reaction of adsorbed BH, is the ratedetermining step according to a Langmuir-Hinshelwood mechanism. ( b ) Hydrolysis catalysed b y ion exchangers

Sucrose inversion, a typical proton-catalysed irreversible reaction C,zH,ZOl, + HZO sucrose

H+

=

C ~ H U O+ , C6H1206 glucose

fructose

has been investigated by several authors with the use of ion exchanger catalysts, mainly in order t o get specific information on the catalytic action of this type of solid catalyst when a large reactant molecule is used. The reaction in excess water was studied a t temperatures in the range 25-100°C (the order with respect t o sucrose concentration was found to be one). The effects of the particle size, degree of crosslinking and partial neutralisation of sulphonic groups indicated the strong influence of the diffusion of the reaction components into or out of the resin particle on References p p . 385-398

382 TABLE 27 Effect of degree of crosslinking of the polymer catalyst o n the rate coefficient ( k ) of sucrose inversion a t 50°C [505]

DVB

k

X

lo4

(%)

(min-')

1

199.2 110.3 26.3 3 .O 0.7

4

10 15 20

the reaction rate. With increasing particle size, the reaction rate or the first-order rate coefficients decreased [ 505-5071 . Table 27 illustrates the effect of the degree of crosslinking of the resin; the effect on sucrose inversion rates is much greater than was observed for esterification and transesterification (p. 363) or for ester hydrolysis (p. 377). The difference can be attributed to the larger size of sucrose molecules compared with that of acids and esters used in the other reactions mentioned. Figure 20,

Fig. 20. Effect of degree of crosslinking (% DVB) of a standard ion exchanger on the diffusivities, D,f (cm2 min-I), and t h e selectivity ratio, S = k e f S / k e f A c ( k , f = effective rate coefficient, S = sucrose, Ac = ethyl acetate). Data were obtained by rate measurements and Wheeler-Thiele analysis of simultaneous sucrose and ethyl acetate hydrolysis a t 70°C [508].

383 presenting the results obtained by rate measurements of sucrose inversion and ethyl acetate hydrolysis proceeding simultaneously in the same reaction system [ 5081, clearly demonstrates the situation. The decrease of the diffusivity values with increasing degree of crosslinking, obtained by Wheeler-Thiele analysis, is much steeper for sucrose, the reactant with the larger molecule, than for ethyl acetate. This is the reason for the changes of the selectivity ratio k e f S / k e f A cwith degree of crosslinking in favour of the reactant with smaller molecules. The diffusion of the large sucrose molecule may be so slow that a large proportion of the sulphonic acid groups inside the polymer become inaccessible and cannot participate in the reaction [506,508], as was also assumed for esterification (p. 361) and ester hydrolysis (p. 377). Other results also confirm the important role of internal diffusion. Experimental activation energies (67-75 kJ mol-') of the sucrose inversion catalysed by ion exchangers [ 506-5091 were considerably lower than those of a homogeneously catalysed reaction (105-121 kJ mol-') [ 505, 506,5081 and were close t o the arithmetic average of the activation energy for the chemical reaction and for the diffusion in pores. The dependence of the rate coefficient on the concentration in the resin of functional groups in the H'-form was found t o be of an order lower than unity. A theoretical analysis based on the Wheeler-Thiele model for a reaction coupled with intraparticle diffusion in a spherical bead revealed [ 510,5111 that the dependence of the experimental rate coefficient on acid group concentration should be close to those found experimentally (orders, 0.65 and 0.53 for neutralisation with Na' and K' ions respectively [511] or -0.5 with Na' ions [510]). Intrinsic rate coefficients, k i n t r , of sucrose inversion catalysed by Dowex 50W-X8 were evaluated [ 5061 by correcting the experimentally observed coefficients, k O b s using , the relation

hkintr 7 where X is the experimentally determined absorption coefficient of sucrose in the resin and 77 is the catalyst effectiveness factor calculated by Wheeler-Thiele analysis from rate data on catalyst beads of different size. Good agreement between the calculated effective diffusivities of sucrose and those obtained by non-equilibrium sorption measurements demonstrated that the diffusion model used was reasonable. The intrinsic rate coefficients of the reaction in the resin phase were only 60% as large as those determined in 3 N benzenesulphonic, p-toluenesulphonic or hydrochloric acid solution. According t o Gilliland et al. [506] one possible explanation of this discrepancy is that steric hindrance prevents close approach or favorable alignment of a sucrose molecule at an active site for inversion. A similar idea was suggested by Murakami and Mori [ 5121, who assume that only a number of protons smaller than one tenth of those present in the resin grains may effectively catalyse the sucrose kobs =

R e f e r e n c e s p p . 385-398

384 inversion because protons are much too close to each other on the resin surface compared with the dimensions of a sucrose molecule. The hydrolysis of acetals acetal + H,O

=

2 alcohol + aldehyde (ketone)

is another acid-catalysed reaction that was used to investigate transport phenomena in ion exchange resins [ 508,5131. First-order rate coefficients of Dowex 5OW-catalysed reaction ( k r e s ) of a series of acetals at 20°C were plotted against the rate coefficients of reactions catalysed by dissolved HCl ( k h o m ) . As Fig. 21 shows, the hydrolysis rate of less reactive acetals is controlled by a chemical reaction: the slope is near unity and the coefficients k,,, and khom are directly proportional t o one another kres = C k h o m

The hydrolysis of more reactive acetals is influenced by the rate of intraparticle diffusion, since the slope was found to be close to 0.5, which is in agreement with eqn. (33) derived from the Wheeler-Thiele model for a

-1

I

I

I

I

I

-3

-2

I

I -1

0

I

I

:-2 a! m 0

-3

log

khorn

Fig. 21. Hydrolysis of acetals at 2OoC o n a Dowex 5 0 W X10 resin catalyst [513 1. Rate coefficients of the resin-catalysed reaction (kres) versus rate coefficients of the reaction catalysed by dissolved inorganic acid (hhom). 1, Formaldehyde dimethylacetal ; 2 , formaldehyde diethylacetal; 3, formaldehyde di-2-propylacetal; 4 , acetaldehyde ethyleneacetal; 5 , acetone ethyleneacetal; 6 , acetaldehyde dimethylacetal; 7 , acetalde0.5. hyde diethylacetal. The slope for acetals 1-3 is 1 , for the acetals 3-7

-

-

385 TABLE 28 Effect of acetal structure o n the diffusivity (D,f) in a bead of ion exchanger [ 5 0 8 ] (Catalyst: Dowex 5OW-X12; temperature: 20°C.) Acetal

DefX lo6 (cm2 min-’)

Acetone ethyleneacetal Acetaldehyde dimethylacetal Acetaldehyde diethylacetal Benzaldehyde di( 2-buty1)acetal

10.0 6.6 2.6 0.008 -.

first-order reaction coupled with intraparticle diffusion in spherical beads

(33) The transition from the kinetic t o the internal diffusion region was also shown by the change in the activation energy [ 508,5131. The rate coefficient for an acetal with a much larger molecule, benzaldehyde di( 2-butyl) acetal, was, however, so low that it could not be correlated with the corresponding khom by the above relationship. This can be explained by a steric effect of this bulky reactant whose diffusion is much more hindered than that of smaller molecules (“sieve effect”, see also pp. 361 and 378). Effective diffusion coefficients of some acetals were estimated [ 5081 from the kinetic data for Dowex 5OW-X12 catalyst using the Wheeler-Thiele model (Table 28). The great difference (by about three orders of magnitude) between the diffusivity of benzaldehyde di-(2-buty1)acetal and that of the other acetals is evident. kres

=C’(hh~m~ef)~’~

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