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Mechanisms in the reaction between olefins and alcohols catalyzed by ion exchange resins
37
Journal of Molecular Catalysis, 4 (1978) 37 - 48 @I Elsevier Sequoia S-A_, Lausanne - Printed in the Netherlands
MECHANISMS N THE REACTION BETWEEN ALCOHOLS CATALYZED BY ION EXCHANGE
FRANCESCO ANCILLOTTI, and LUIGI ROMAGNONI** Snamprogetti (Received
May
S_p_A_. Direzione
MARCELLO Ricerca
MASS1
e Suiluppo.
OLEFNS AND RESINS*
MAURI, 2009
ERMANNO
7 S_ Donate.
BESCAROLLO Milan
(Italy)
27,1977)
Summary An experimental study was carried out on the addition of alcohols to tertiary olefins catalyzed by a macroporous sulfonic acid resin. Initial rates of the reaction between methanol or n-butanol with isobutene show different mechanisms depending on the ratio of the reactants. At stoichiometric or higher aIcohol/isobutene ratios, the initial rates show a zero order on the alcohol and a first order on the olefin; in this case the experimental data agree with an ionic mechanism wherein the protonation of the olefin by the solvated proton is the rate determining step, In such conditions the influence of the alcoho1 is displayed only by control of the protonation power of the catalyst, and the higher reactivity of n-butanol us. methanol should reflect the acidity order of the corresponding solvated proton_ On reducing the alcohol/olefin ratio, a negative order on the methanol and an increase in the rates are observed, until a maximum in the rate is reached; a further reduction leads to a zero order on the olefin and a first order on the alcohol. The mechanism agrees in this case with a concerted reaction on the associated network of sulfonic groups. The lower reactivity found for the n-butanol can be attributed to the difficulty of the polymeric structure adapting to the higher steric hindrance of n-butanol.
Introduction R is known from recent literature that reactions catalyzed by acid ion exchange resins can show different mechanisms according to the polarity of the reactants or reaction medium_ It is also known that these reactions can *Lecture presented at the International Donato, Milan, May 25 - 26,1977_ **On secondment to Snamprogetti from
Conference
on Polymers
Pavia University
as Reagents,
for the graduation
San
thesis-
38
undergo a transition mechanism when the partial pressure of reactants or their ratio is varied over a wide range_ This behaviour was first shown by Gates et al. [II in the dehydration of butyi alcohols. They pointed out that the dehydration of the t-b&y1 aIcoho1, performed in liquid phase in the absence of water, was catalyzed by the undissociated matrix-bound SO&I groups. Water added to the reactant competed for SOsH groups, strongly inhibiting the reaction; the reaction was catalyzed by hydrated protons, Moreover, Gates ef aZ. 121 showed that in the gas phase dehydration of n-hutyl aIcoho1, there is an initial increase of the reaction rate with the substrate partial pressure (first orderon the reactant) to a maximum, after which a further pressure increase of the substrate affected the rate negatively (negative order). A subsequent further pressure increase had very little influence on the reaction. Prom IR spectra obtained on membranes of the working catalyst, the authors attributed such transitions to the breaking of the associated network of sulfonic groups and to their dissociation, operated by the protonic subStrate-
A rather similar behaviour could be hypothesized from kinetic data of the reaction between a protic reactant, such as alcohol, and an aprotic tertiary olefin as reported in our previous paper [3 3 _ Particuhu~y, in the reaction between methanol and isobutene we observed that, starting from a stoichiometric ratio between reactants, or with an excess of alcohol, initial rates showed a zero order on the alcohol and a first order on the olefin, whereas operating with a large excess of isobutene the kinetics of the system evolved towards a negative order on the methanolWe considered it worthwhile to explore a wider range of concentrations of protic and aprotic reactants in order to define the role of their ratio in the reaction mechanism_ The study is based on the determination of the initial rates of the reaction between methanol and/or n-butanol with isobutene diluted by lbutene, The reactions were performed in liquid phase and catalyzed by the macroporous acidic resin Amberlyst 15 (A X5)_ In this system other side reactions are possible in addition to the main formation of tertiary alkyl ethers, i-e_, - isobut&e dimerization; - doubIe bond isomerization in I-butene; - methanol and n-b%tanol condensation’ to symmetric alkyd ethers; -reaction between alcohols and I-butene. Only the first two reactions were observed practically and, since their rates may strongly depend on the catalyst. array, their values, too, have been recorded_ Experimental Catalyst preparation, analytical methods, apparatus, and procedure have been described in detail previously [3]. Briefly, the experiments were per-
39
formed in an autoclave provided with a mechanical stiier. Reaction heat was removed by maintaining the reaction -mixture boiling at the temperature of the experiment by a device controlling the system overpressure. In this way, the temperature was kept within t- 0.5 “C. During these experiments, samples of products were taken for GLC injections.. The kinetic analyses were performed on the initial rates expressed as moles of formed product per acid equivalent per second. The calculation of initial rates was made from the slopes of initial straight lines of experimental product concentrations plotted versus time_ Reagents: isobutene and I-butene, poIymerization grade, purity 3 99% Methanol and n-butanol, reagent grade, dried and stored over 4 A molecular sieves. Catalyst: standard A 15 and samples partially neutralized by sodium ions. Results (a) Initiai rates of methyl-t-butyl ether (MTBE) synthesk (i) Reaction with standard A 15 Data on the initial rates of the reaction between methanol and isobutene are shown in Fig, 1. At constant isobutene concentration we can see the
Fig_ 1. Methyl-t-butyl ether (MTBE) synthesis_ Initial rate dependence on initial methanol concentration at various constant isobutene contents. Temperature: 60 “C; catalyst: Amberlyst 15; isobutene (mol/l): o,6; AP 4; n, 2; X. I_
40
effect of methanol concentration. At low values, the rate increases, increasing the methanol concentration up to a maximum depending on isobutene concentration, after which a further increase in alcohol causes a drop in the rate to a minimum vahxe which remains constant even in the event of a further increase of methanol_ Regarding the isobutene concentration, the rate is in&pendent along the initial straight linesection of Fig. 1, but after the maximum, when the rate decreases, the kinetic order on the olefin becomes positive and reaches a stable first order when the kinetics are independent from the methanol_ The points on the initial straight line section, where the maximum is reached, are characterized by a constant ratio between the olefin and alcohol concentrations, this vaIue being about 10. Likewise, the points where the influence of methanol changes from a negative value to zero are characterized by a constant ratio betieen isobutene and methanol concentrations, the value being about 1.7. (ii)
Reactian
OR
partially
neutralized A 15
Experiments were performed on A 15 partially neutralized by sodium ions, starting from two solutions having both 6 molfitre of isobutene con-
6
2 1 Acid Group conc_b
0 ra~in_5m6e&8~
0,23-i5678 rz-f3utcmot
initial
concentration.mdllitre
Fig. 2. Methyl-t-b&y1 ether (MTJ3E) synthesis. -Initial rate dependence on catalystacid groups concentration. CataIyst: Amberlyst 15 and partially neutralized Amberlyst 15; temperature: 60 “C; initial cone- (moI/I): A, 0.5 methanol, 6 isobutene; 0,4 methanol, 6 isobutene. Fi& 3_ n-Butyl-t-butyi ethe- (II-BTBE) synthesis. Initial rate dependence on initial IIbutanol concentr$ion at various constant %obut.ene contents_ Temperature: 60 “C; catalyst: Amberlyst 15; isobutene (moI/l)t o, 8-7; o, 6; A, 1.
41
centration and, respectively, 4 and 0.5 mol/litre of methanol. Data are shown in Fig. 2. At high methanol concentration, the dependence of the rate on the concentration of SOaH groups agrees with a third order as previously found for a solution having 4 mol/litre both in isobutene and methanol 131. At low methanol concentrations the dependence of the rate on SOaH groups is initially stronger, but a transition to a lower order can be evidenced along the reduction profile of the concentration of SOsH groups. In fact, the same rate can be observed, independently from the alcohol concentration, when the SOsH groups have been extensiveIy neutrabzed. (b) Initial rateof n-ButyZ-t-ZmtyZ ether (n-BTBE)
synthesis Data on the initial rates between n-butanol and isobutene are indicated in Fig. 3. It will be seen that at const&t isobutene the dependence of the rate on n-butanol concentration, though qualitatively similar to that evidenced for methanol, is different quantitatively: - At low alcohol concentration the rates of the formation of n-BTBE are lower than in the case of MTBE. The maximum in the rate is lower too, as shown in Fig. 4. - The ratio of isobutene and n-butanol at maximum points is lower. - The inhibiting effect of n-butanol, after the maximum, is alto found, but it is Iess marked; with an excess of alcohol the rate of formation of n-BTBE is higher than that of MTBE.
d
0
.
1
.
.
3 4ccmCe~tratio6. 2 Alcohol initid
m&Iitre
Fig. 4. MTBE and n-BTBE syntbe~is. tration at constant kobutene content 15; 0, MTBE; 0, n-BTEE.
8
Initial rate dependence (6 mol/l). Temper&G-e:
on initial alcohol 60 “C; catalyst:
concenAmberlyst
42
(c) Initial rates of MTBE and n-ETBE syntheses performed by reacting methanol and n-bufanol blends with isobutene Data on the rates of simultaneous formation of MTBE and n-BTBE are given in Table 1. Two sets of vaIues at low and high alcohols concentrations are indicated. _ At high alcohols concentrations it can be seen that the presence of the two akohols causes a pronounced drop in n-BTBE formation, and a slight increase in MTBE formation with regard to the separate reactions at the same level of individual akohol concentration. Therefore, operating with a blend of alcohols, the formation of MTBE is preferred, in spite of the lower rate in the reaction with the singIe alcohol, At Iow alcohol concentrations a quite different behaviour is noted. In fact, with the blend of alcohols the MTBE formation rate decreases with regard to the rate obtained with methanol aZone; on the contrary, the n-BTBE rate is not influenced by the presence of the methanoI. TABLE
1
InitiaI reaction (Temperature:
Reactant
rates of the simultaneous syntheses 60 “C; catalyst- Amberlyst 15)
concentrations
of MTBE
and n-BTBE
x 103
(mol/l)
1
Isobutene
Methanol
n-Butanol
MTBE
n-BTBE
Overall
6 6 6 6 6 6 6 6 6 6
6 4 4 O-4 0.2 0.2 -
2 2 6 O-2 0.2 0.4
20 22 29 -
3.5 50 50 18
20 22 32.5 50 50 71 34 37
18
18
29
29
3’: 19 -
(d)
Initial rate of isobufene dimerization Data on the initial rates of this side reaction in the synthesis of MTBE are shown in Fig. 5. It can be seen that at low isobutene/methanoI ratios, isobutene dimerization is not detectable, even at high isobutene concentrations, but this rate quickIy increase; as the isobutene/methanol ratio reaches the value of about 3.5, and becomes more evident as the isobutene concentration rises, (e)
Inih-aZ ratesof I-butene double-bond ikcmerikation When I-butene is used as solvent in the reaction between isobutene and methanol, it can undergo a double-bond isomer&ration. The isomerization
lnillal Aale,
moles of 2.bulono x ,g acid oq. x eooond
lnitlal Role,
mole8 of dlrmr ,@ aald eq. I 8eaond
44
ratio is not independent of 1-butene butene concentration decreases-
concentration
and increases as the l-
Discussion The observed transitions of rates in the MTBE synthesis enable us to locate on the plot “Initial rate us_ methanol concentration”, severaI areas delimited by defined ratios between the reactants and characterized by the same mechanism which is believed to be operating inside them, Fig. 7, A similar picture couId be drawn for the synthesis of n-BTBE, but we prefer to limit the general discussion to the MTBE synthesis for which we have more experimentaI data_ We will point out analogies or differences between the syntheses of the two ethers where the experimental data ahow such a comparison. In the area characterized by isobutene/methanol ratios lower than l-7, we found a zero order on the alcohol and a first order on the olefin. An area with the same orders on the reactants is present aIso in the n-BTBE synthesis, although delimited by a higher and less constant ratio between the reactants_ In our previous work [3 3 we hypothesized for such conditions an ionic mechanism operating through the protonation of the olefin by the sohated proton, followed by the interaction of the carbon ion with the nucleophile; the first reaction was expected to be the rate determining step.
Fig_ 7. MTBE synthesis. Typical areas delimited-by ratio of reactants at several hypothesized mechanisms.
45
Many experimental observations agree with this assumption: - The alcohol is strongly in excess in the liquid pores owing to a selective swelling, and in these conditions it can break the network of hydrogenbonded SOsH groups, dissociating and solvating the proton. This possibility is confirmed by the high immersion heat of resins in alcohols [4] and by NMR and IR studies [5 - ?] . - The protonation of the olefin as the rate determining step agrees with the first order found for the isobutene and, as previously described [3], with the.behaviour of the isoamylenes in the etherification with methanol on the same catalyst_ In fact, the observed bond isomerization occurring in the isoamylenes during the reaction implies the existence of a carbonium ion as a common intermediate; moreover, the higher overall reactivity of 2methyl-1-butene us_ 2-methyl-2-butene shows that the carbon ion formation is kinetically more important than its interaction with the nucleophile. In such an ionic mechanism, where the olefm protonation is believed to be the most important kinetic step, the rate is expected to depend on the olefin basicity and on the acid strength of the catalytic species_ The less basic I-butene is unreactive toward the alcohols, and under these conditions l-butene undergoes double-bond isomerization to avery Iow extent, independently of its concentration_ Ethers formation rates are independent of alcohol concentration but depend on its nature, as shown by the different rates of MTBE and IL-BTBE at the same level of reactants_ In these conditions, the alcohol is expected to be the true solvent at reaction sites, strongly conditioning the acidity of the solvated proton. The less basic n-butanol produces more acid and more active ROH; catalytic species. Such an order in the alcohols basicity agrees with a number of experimental observations, and mainly with the NMR chemical shift for the hydroxylic proton of resin-adsorbed alcohols that follows the order [7,3] :
n-butanol > n-propanol > ethanol > methanol. It is interesting to examine the reaction of isobutene with a blend of methanol and n-butanol. We observed a pronounced drop in the rate of nBTBE formation and a slight increase in that of MTBE_ The overall reactivity lies between that of the individual alcohols, and it can be explained by an intermediate resin acidity; the higher reactivity of methanoI has to be at&ibuted to its preferential absorption in the resin rather than to its greater nucleophilicity. Such a hypothesis agrees with the distribution coefficient found to be more favourabIe for methanol [3], and it is confirmed by a comparison of the rates of MTBE and n-BTBE syntheses heterogeneously c&alyzed by the resin and homogeneously catalyzed by p-toluenesulphonic acid, Table 2. In a homogeneous system, no selective etherification for the methanol was found, the ratio of the rates being very similar to the ratio of the concentrations of the alcohols. Ah our experimental data seem to confirm the acid-base nature of the driving force in such an area of the reactants’ ratio. Nevertheless, as previously
46 TABLE 2 MTBE/n-BTBE rates ratio in homogeneous and heterogeneous catalytic systems (Temperature: 60 “C) CafaIyst Amberlyst 15
p-Toluenesulpbonic acid
alcohols ratio Methanol/n-Butanol
2
2
rates ratio MTBE/n-BTBE
8
2.2
stated, an interaction between ” ion and an induced dipoIe could explain the higher reactivity of n-butanol, owing to its lower dielectric constant 133 _ However, an ion-dipole interaction cannot explain the selective reactivity of isobutene and the absence of reactivity for Iinear butenes. In fact, au iondipole interaction should affect the retention times in gas-solid chromatography, as evidenced by Hirsch ef aZ_ [8] ; C4 olefin retention times on lithium exchanged A 15, however, did not differ enough to account for their great difference in reactivity [9 J _ If an ion-dipole interaction is operative, it is not the main determining factor for the greater reactivity of isobutene, When the isobutene/methanoI mtio becomes higher than 1.7 the rate greatly increases as the methanol decreases, and it can be inferred that a more active mechanism, supported by the reduction of methanol, can be operative_ We can hypothesize that the equilibrium of the reaction, SO,H
+ ROHZ
ROHS i_ SO,,
which is compIetcIy shifted to the right at ratios Iower’than l-7, begins to move back to the left, so that the isobutene can take the proton directly from the sulphonic group_ According to Gates [1] the SOs H group is a more acidic species than the solvated proton, and this can account for the increased rate- Experimental data show that at ratios higher than 1.7 and at a low isobutene level, the I-butene isomedzation increases too, confirming that a more active cataIyst is becoming operative_ The commencement of 1-butene isomerization requires higher isobutene/methanoI ratios, as the isobutene level increases because an increase of isobutene concentration is obtained by a reduction in I-butene concentration_ At ratios higher than 3.5 the dimerization of isobutene also occurs. We were unable to distinguish if a new mechanism was becoming operative or if the internal concentration of isobutene was becoming so high as to allow the reaction between the t-butyl cation and the isobutene. The new mechanism couId be the concerted one evidenced by Gates ef aZ_in the gas phase butyl alcohol dehydration at a low IeveI of protic substance, in such conditions that the SOsH groups are bridged by hydrogen bonds [Zj .
47
We have no experimental support to confirm that such a mechanism is operative already at a ratio of 3-5 It can be observed that if at a 3.5 ratio, SOsH groups were completely associated, as required by an exclusive concerted mechanism, it would be difficult to explain the maintenance of the positive effect of the methanol reduction on the rate, This positive effect suddenly stops when the ratio rises to 10. For higher values methanol behaves as a reactant, determining the kinetics of first order, while the isobutene becomes kinetically non-influencing- We can infer that at this point a real concerted mechanism has been reached, wherein isobutene is coordinated to the associated SO3H groups and the interaction with alcohol is the rate determining step_ It is interesting to compare the rates of MTBE formation as a function of the SOsH group concentration_ At ratios lower than l-7 it is observed that the rates depend on the acid groups, constantly with a third order, indicating that their partial neutralization does not influence the mechanism. The same plot at a ratio of 12 shows a much higher dependence at a high level of acid groups, but shows a transition of the order along the reduction of the concentration of SOaH groups, so that at a very low Ievei of acid groups the same dependence of the third order is reached_ We can infer that the neutralization destroys the concerted mechanism, shifting it toward the ionic one, as is expected to exist at low reactant ratios_ For n-BTBE synthesis, we found that the extreme areas with zero order on a reactant and a first order on the other also exist. However, we can observe now that the transitions are not so clear as in the previous case, and the maximum in the rate is at a lower level than that reached by MTBE_ In order to find an explanation of such a difference we can hypothesize that the higher steric hindrance of n-butanol makes it difficult to reach, in the n-BTBE synthesis, the more active concerted mechanism_ The strong influence of the matrix flexibility on the rate in acid-catalyzed reactions, like the isopropyl alcohol dehydration, has already been recorded [I] _
Conclusions In concIusion, we evidenced in the reaction between Iinear alcohols and tertiary olefins, catalyzed by an ion exchange resin, a remarkable transition in kinetic orders which we related to the ratio between the reactants, and which we tried to explain by mechanistic transitions, operating an analogical extention of mechanisms evidenced by Gates in dehydration of butyl alcohols_ It would be interesting to support our hypotheses, chiefly based on kinetic studies, by an independent chemical-physical examination of the catalyst array in the various evidenced kinetic situations_ Our attempts to obtain an IR support failed, due to the difficulty of operating in liquid phase and of reproducing thin membranes of a macroporous resin with the same properties as A 15.
48
References 1 2 3 4 5 6 7 8
B. C. Gates and W_ Rodriguez, J_ Cat& 31(1973) 27_ R_ Thornton and B. C. Gates, LCatal., 34 (1974) 275, F. AncilIotti, M. Massi Mauri and E. Pescarol:o, J_ Catal., 46 (1977) 49, D, J_ Pietrzyk, Anal. Chem., 44 (1972) 676_ L_ S_ Franked, J_ Phys_ Chem., 75 (1971) 1211_ J. E. Gordon, J. Phys. Chem., 66 (1962) 1150. W_ J_ Casey and D. I Pietrzyk, Anal. Chem., 45 (1973) 1404. R_ F_ Hirsch, H_ C_ Stober, M_ Kowblausky, F_ N. Hubner and A. W. O’ConneI, AnaI. Chem_, 45 (1973) 2 100, 9 L_ Romagnoni and F_ Ancillotti, Unpublished data, Internal Rep. No. 5 495,1976.
Journal of Molecular Catalysis, 4 (1978) 37 - 48 @I Elsevier Sequoia S-A_, Lausanne - Printed in the Netherlands
MECHANISMS N THE REACTION BETWEEN ALCOHOLS CATALYZED BY ION EXCHANGE
FRANCESCO ANCILLOTTI, and LUIGI ROMAGNONI** Snamprogetti (Received
May
S_p_A_. Direzione
MARCELLO Ricerca
MASS1
e Suiluppo.
OLEFNS AND RESINS*
MAURI, 2009
ERMANNO
7 S_ Donate.
BESCAROLLO Milan
(Italy)
27,1977)
Summary An experimental study was carried out on the addition of alcohols to tertiary olefins catalyzed by a macroporous sulfonic acid resin. Initial rates of the reaction between methanol or n-butanol with isobutene show different mechanisms depending on the ratio of the reactants. At stoichiometric or higher aIcohol/isobutene ratios, the initial rates show a zero order on the alcohol and a first order on the olefin; in this case the experimental data agree with an ionic mechanism wherein the protonation of the olefin by the solvated proton is the rate determining step, In such conditions the influence of the alcoho1 is displayed only by control of the protonation power of the catalyst, and the higher reactivity of n-butanol us. methanol should reflect the acidity order of the corresponding solvated proton_ On reducing the alcohol/olefin ratio, a negative order on the methanol and an increase in the rates are observed, until a maximum in the rate is reached; a further reduction leads to a zero order on the olefin and a first order on the alcohol. The mechanism agrees in this case with a concerted reaction on the associated network of sulfonic groups. The lower reactivity found for the n-butanol can be attributed to the difficulty of the polymeric structure adapting to the higher steric hindrance of n-butanol.
Introduction R is known from recent literature that reactions catalyzed by acid ion exchange resins can show different mechanisms according to the polarity of the reactants or reaction medium_ It is also known that these reactions can *Lecture presented at the International Donato, Milan, May 25 - 26,1977_ **On secondment to Snamprogetti from
Conference
on Polymers
Pavia University
as Reagents,
for the graduation
San
thesis-
38
undergo a transition mechanism when the partial pressure of reactants or their ratio is varied over a wide range_ This behaviour was first shown by Gates et al. [II in the dehydration of butyi alcohols. They pointed out that the dehydration of the t-b&y1 aIcoho1, performed in liquid phase in the absence of water, was catalyzed by the undissociated matrix-bound SO&I groups. Water added to the reactant competed for SOsH groups, strongly inhibiting the reaction; the reaction was catalyzed by hydrated protons, Moreover, Gates ef aZ. 121 showed that in the gas phase dehydration of n-hutyl aIcoho1, there is an initial increase of the reaction rate with the substrate partial pressure (first orderon the reactant) to a maximum, after which a further pressure increase of the substrate affected the rate negatively (negative order). A subsequent further pressure increase had very little influence on the reaction. Prom IR spectra obtained on membranes of the working catalyst, the authors attributed such transitions to the breaking of the associated network of sulfonic groups and to their dissociation, operated by the protonic subStrate-
A rather similar behaviour could be hypothesized from kinetic data of the reaction between a protic reactant, such as alcohol, and an aprotic tertiary olefin as reported in our previous paper [3 3 _ Particuhu~y, in the reaction between methanol and isobutene we observed that, starting from a stoichiometric ratio between reactants, or with an excess of alcohol, initial rates showed a zero order on the alcohol and a first order on the olefin, whereas operating with a large excess of isobutene the kinetics of the system evolved towards a negative order on the methanolWe considered it worthwhile to explore a wider range of concentrations of protic and aprotic reactants in order to define the role of their ratio in the reaction mechanism_ The study is based on the determination of the initial rates of the reaction between methanol and/or n-butanol with isobutene diluted by lbutene, The reactions were performed in liquid phase and catalyzed by the macroporous acidic resin Amberlyst 15 (A X5)_ In this system other side reactions are possible in addition to the main formation of tertiary alkyl ethers, i-e_, - isobut&e dimerization; - doubIe bond isomerization in I-butene; - methanol and n-b%tanol condensation’ to symmetric alkyd ethers; -reaction between alcohols and I-butene. Only the first two reactions were observed practically and, since their rates may strongly depend on the catalyst. array, their values, too, have been recorded_ Experimental Catalyst preparation, analytical methods, apparatus, and procedure have been described in detail previously [3]. Briefly, the experiments were per-
39
formed in an autoclave provided with a mechanical stiier. Reaction heat was removed by maintaining the reaction -mixture boiling at the temperature of the experiment by a device controlling the system overpressure. In this way, the temperature was kept within t- 0.5 “C. During these experiments, samples of products were taken for GLC injections.. The kinetic analyses were performed on the initial rates expressed as moles of formed product per acid equivalent per second. The calculation of initial rates was made from the slopes of initial straight lines of experimental product concentrations plotted versus time_ Reagents: isobutene and I-butene, poIymerization grade, purity 3 99% Methanol and n-butanol, reagent grade, dried and stored over 4 A molecular sieves. Catalyst: standard A 15 and samples partially neutralized by sodium ions. Results (a) Initiai rates of methyl-t-butyl ether (MTBE) synthesk (i) Reaction with standard A 15 Data on the initial rates of the reaction between methanol and isobutene are shown in Fig, 1. At constant isobutene concentration we can see the
Fig_ 1. Methyl-t-butyl ether (MTBE) synthesis_ Initial rate dependence on initial methanol concentration at various constant isobutene contents. Temperature: 60 “C; catalyst: Amberlyst 15; isobutene (mol/l): o,6; AP 4; n, 2; X. I_
40
effect of methanol concentration. At low values, the rate increases, increasing the methanol concentration up to a maximum depending on isobutene concentration, after which a further increase in alcohol causes a drop in the rate to a minimum vahxe which remains constant even in the event of a further increase of methanol_ Regarding the isobutene concentration, the rate is in&pendent along the initial straight linesection of Fig. 1, but after the maximum, when the rate decreases, the kinetic order on the olefin becomes positive and reaches a stable first order when the kinetics are independent from the methanol_ The points on the initial straight line section, where the maximum is reached, are characterized by a constant ratio between the olefin and alcohol concentrations, this vaIue being about 10. Likewise, the points where the influence of methanol changes from a negative value to zero are characterized by a constant ratio betieen isobutene and methanol concentrations, the value being about 1.7. (ii)
Reactian
OR
partially
neutralized A 15
Experiments were performed on A 15 partially neutralized by sodium ions, starting from two solutions having both 6 molfitre of isobutene con-
6
2 1 Acid Group conc_b
0 ra~in_5m6e&8~
0,23-i5678 rz-f3utcmot
initial
concentration.mdllitre
Fig. 2. Methyl-t-b&y1 ether (MTJ3E) synthesis. -Initial rate dependence on catalystacid groups concentration. CataIyst: Amberlyst 15 and partially neutralized Amberlyst 15; temperature: 60 “C; initial cone- (moI/I): A, 0.5 methanol, 6 isobutene; 0,4 methanol, 6 isobutene. Fi& 3_ n-Butyl-t-butyi ethe- (II-BTBE) synthesis. Initial rate dependence on initial IIbutanol concentr$ion at various constant %obut.ene contents_ Temperature: 60 “C; catalyst: Amberlyst 15; isobutene (moI/l)t o, 8-7; o, 6; A, 1.
41
centration and, respectively, 4 and 0.5 mol/litre of methanol. Data are shown in Fig. 2. At high methanol concentration, the dependence of the rate on the concentration of SOaH groups agrees with a third order as previously found for a solution having 4 mol/litre both in isobutene and methanol 131. At low methanol concentrations the dependence of the rate on SOaH groups is initially stronger, but a transition to a lower order can be evidenced along the reduction profile of the concentration of SOsH groups. In fact, the same rate can be observed, independently from the alcohol concentration, when the SOsH groups have been extensiveIy neutrabzed. (b) Initial rateof n-ButyZ-t-ZmtyZ ether (n-BTBE)
synthesis Data on the initial rates between n-butanol and isobutene are indicated in Fig. 3. It will be seen that at const&t isobutene the dependence of the rate on n-butanol concentration, though qualitatively similar to that evidenced for methanol, is different quantitatively: - At low alcohol concentration the rates of the formation of n-BTBE are lower than in the case of MTBE. The maximum in the rate is lower too, as shown in Fig. 4. - The ratio of isobutene and n-butanol at maximum points is lower. - The inhibiting effect of n-butanol, after the maximum, is alto found, but it is Iess marked; with an excess of alcohol the rate of formation of n-BTBE is higher than that of MTBE.
d
0
.
1
.
.
3 4ccmCe~tratio6. 2 Alcohol initid
m&Iitre
Fig. 4. MTBE and n-BTBE syntbe~is. tration at constant kobutene content 15; 0, MTBE; 0, n-BTEE.
8
Initial rate dependence (6 mol/l). Temper&G-e:
on initial alcohol 60 “C; catalyst:
concenAmberlyst
42
(c) Initial rates of MTBE and n-ETBE syntheses performed by reacting methanol and n-bufanol blends with isobutene Data on the rates of simultaneous formation of MTBE and n-BTBE are given in Table 1. Two sets of vaIues at low and high alcohols concentrations are indicated. _ At high alcohols concentrations it can be seen that the presence of the two akohols causes a pronounced drop in n-BTBE formation, and a slight increase in MTBE formation with regard to the separate reactions at the same level of individual akohol concentration. Therefore, operating with a blend of alcohols, the formation of MTBE is preferred, in spite of the lower rate in the reaction with the singIe alcohol, At Iow alcohol concentrations a quite different behaviour is noted. In fact, with the blend of alcohols the MTBE formation rate decreases with regard to the rate obtained with methanol aZone; on the contrary, the n-BTBE rate is not influenced by the presence of the methanoI. TABLE
1
InitiaI reaction (Temperature:
Reactant
rates of the simultaneous syntheses 60 “C; catalyst- Amberlyst 15)
concentrations
of MTBE
and n-BTBE
x 103
(mol/l)
1
Isobutene
Methanol
n-Butanol
MTBE
n-BTBE
Overall
6 6 6 6 6 6 6 6 6 6
6 4 4 O-4 0.2 0.2 -
2 2 6 O-2 0.2 0.4
20 22 29 -
3.5 50 50 18
20 22 32.5 50 50 71 34 37
18
18
29
29
3’: 19 -
(d)
Initial rate of isobufene dimerization Data on the initial rates of this side reaction in the synthesis of MTBE are shown in Fig. 5. It can be seen that at low isobutene/methanoI ratios, isobutene dimerization is not detectable, even at high isobutene concentrations, but this rate quickIy increase; as the isobutene/methanol ratio reaches the value of about 3.5, and becomes more evident as the isobutene concentration rises, (e)
Inih-aZ ratesof I-butene double-bond ikcmerikation When I-butene is used as solvent in the reaction between isobutene and methanol, it can undergo a double-bond isomer&ration. The isomerization
lnillal Aale,
moles of 2.bulono x ,g acid oq. x eooond
lnitlal Role,
mole8 of dlrmr ,@ aald eq. I 8eaond
44
ratio is not independent of 1-butene butene concentration decreases-
concentration
and increases as the l-
Discussion The observed transitions of rates in the MTBE synthesis enable us to locate on the plot “Initial rate us_ methanol concentration”, severaI areas delimited by defined ratios between the reactants and characterized by the same mechanism which is believed to be operating inside them, Fig. 7, A similar picture couId be drawn for the synthesis of n-BTBE, but we prefer to limit the general discussion to the MTBE synthesis for which we have more experimentaI data_ We will point out analogies or differences between the syntheses of the two ethers where the experimental data ahow such a comparison. In the area characterized by isobutene/methanol ratios lower than l-7, we found a zero order on the alcohol and a first order on the olefin. An area with the same orders on the reactants is present aIso in the n-BTBE synthesis, although delimited by a higher and less constant ratio between the reactants_ In our previous work [3 3 we hypothesized for such conditions an ionic mechanism operating through the protonation of the olefin by the sohated proton, followed by the interaction of the carbon ion with the nucleophile; the first reaction was expected to be the rate determining step.
Fig_ 7. MTBE synthesis. Typical areas delimited-by ratio of reactants at several hypothesized mechanisms.
45
Many experimental observations agree with this assumption: - The alcohol is strongly in excess in the liquid pores owing to a selective swelling, and in these conditions it can break the network of hydrogenbonded SOsH groups, dissociating and solvating the proton. This possibility is confirmed by the high immersion heat of resins in alcohols [4] and by NMR and IR studies [5 - ?] . - The protonation of the olefin as the rate determining step agrees with the first order found for the isobutene and, as previously described [3], with the.behaviour of the isoamylenes in the etherification with methanol on the same catalyst_ In fact, the observed bond isomerization occurring in the isoamylenes during the reaction implies the existence of a carbonium ion as a common intermediate; moreover, the higher overall reactivity of 2methyl-1-butene us_ 2-methyl-2-butene shows that the carbon ion formation is kinetically more important than its interaction with the nucleophile. In such an ionic mechanism, where the olefm protonation is believed to be the most important kinetic step, the rate is expected to depend on the olefin basicity and on the acid strength of the catalytic species_ The less basic I-butene is unreactive toward the alcohols, and under these conditions l-butene undergoes double-bond isomerization to avery Iow extent, independently of its concentration_ Ethers formation rates are independent of alcohol concentration but depend on its nature, as shown by the different rates of MTBE and IL-BTBE at the same level of reactants_ In these conditions, the alcohol is expected to be the true solvent at reaction sites, strongly conditioning the acidity of the solvated proton. The less basic n-butanol produces more acid and more active ROH; catalytic species. Such an order in the alcohols basicity agrees with a number of experimental observations, and mainly with the NMR chemical shift for the hydroxylic proton of resin-adsorbed alcohols that follows the order [7,3] :
n-butanol > n-propanol > ethanol > methanol. It is interesting to examine the reaction of isobutene with a blend of methanol and n-butanol. We observed a pronounced drop in the rate of nBTBE formation and a slight increase in that of MTBE_ The overall reactivity lies between that of the individual alcohols, and it can be explained by an intermediate resin acidity; the higher reactivity of methanoI has to be at&ibuted to its preferential absorption in the resin rather than to its greater nucleophilicity. Such a hypothesis agrees with the distribution coefficient found to be more favourabIe for methanol [3], and it is confirmed by a comparison of the rates of MTBE and n-BTBE syntheses heterogeneously c&alyzed by the resin and homogeneously catalyzed by p-toluenesulphonic acid, Table 2. In a homogeneous system, no selective etherification for the methanol was found, the ratio of the rates being very similar to the ratio of the concentrations of the alcohols. Ah our experimental data seem to confirm the acid-base nature of the driving force in such an area of the reactants’ ratio. Nevertheless, as previously
46 TABLE 2 MTBE/n-BTBE rates ratio in homogeneous and heterogeneous catalytic systems (Temperature: 60 “C) CafaIyst Amberlyst 15
p-Toluenesulpbonic acid
alcohols ratio Methanol/n-Butanol
2
2
rates ratio MTBE/n-BTBE
8
2.2
stated, an interaction between ” ion and an induced dipoIe could explain the higher reactivity of n-butanol, owing to its lower dielectric constant 133 _ However, an ion-dipole interaction cannot explain the selective reactivity of isobutene and the absence of reactivity for Iinear butenes. In fact, au iondipole interaction should affect the retention times in gas-solid chromatography, as evidenced by Hirsch ef aZ_ [8] ; C4 olefin retention times on lithium exchanged A 15, however, did not differ enough to account for their great difference in reactivity [9 J _ If an ion-dipole interaction is operative, it is not the main determining factor for the greater reactivity of isobutene, When the isobutene/methanoI mtio becomes higher than 1.7 the rate greatly increases as the methanol decreases, and it can be inferred that a more active mechanism, supported by the reduction of methanol, can be operative_ We can hypothesize that the equilibrium of the reaction, SO,H
+ ROHZ
ROHS i_ SO,,
which is compIetcIy shifted to the right at ratios Iower’than l-7, begins to move back to the left, so that the isobutene can take the proton directly from the sulphonic group_ According to Gates [1] the SOs H group is a more acidic species than the solvated proton, and this can account for the increased rate- Experimental data show that at ratios higher than 1.7 and at a low isobutene level, the I-butene isomedzation increases too, confirming that a more active cataIyst is becoming operative_ The commencement of 1-butene isomerization requires higher isobutene/methanoI ratios, as the isobutene level increases because an increase of isobutene concentration is obtained by a reduction in I-butene concentration_ At ratios higher than 3.5 the dimerization of isobutene also occurs. We were unable to distinguish if a new mechanism was becoming operative or if the internal concentration of isobutene was becoming so high as to allow the reaction between the t-butyl cation and the isobutene. The new mechanism couId be the concerted one evidenced by Gates ef aZ_in the gas phase butyl alcohol dehydration at a low IeveI of protic substance, in such conditions that the SOsH groups are bridged by hydrogen bonds [Zj .
47
We have no experimental support to confirm that such a mechanism is operative already at a ratio of 3-5 It can be observed that if at a 3.5 ratio, SOsH groups were completely associated, as required by an exclusive concerted mechanism, it would be difficult to explain the maintenance of the positive effect of the methanol reduction on the rate, This positive effect suddenly stops when the ratio rises to 10. For higher values methanol behaves as a reactant, determining the kinetics of first order, while the isobutene becomes kinetically non-influencing- We can infer that at this point a real concerted mechanism has been reached, wherein isobutene is coordinated to the associated SO3H groups and the interaction with alcohol is the rate determining step_ It is interesting to compare the rates of MTBE formation as a function of the SOsH group concentration_ At ratios lower than l-7 it is observed that the rates depend on the acid groups, constantly with a third order, indicating that their partial neutralization does not influence the mechanism. The same plot at a ratio of 12 shows a much higher dependence at a high level of acid groups, but shows a transition of the order along the reduction of the concentration of SOaH groups, so that at a very low Ievei of acid groups the same dependence of the third order is reached_ We can infer that the neutralization destroys the concerted mechanism, shifting it toward the ionic one, as is expected to exist at low reactant ratios_ For n-BTBE synthesis, we found that the extreme areas with zero order on a reactant and a first order on the other also exist. However, we can observe now that the transitions are not so clear as in the previous case, and the maximum in the rate is at a lower level than that reached by MTBE_ In order to find an explanation of such a difference we can hypothesize that the higher steric hindrance of n-butanol makes it difficult to reach, in the n-BTBE synthesis, the more active concerted mechanism_ The strong influence of the matrix flexibility on the rate in acid-catalyzed reactions, like the isopropyl alcohol dehydration, has already been recorded [I] _
Conclusions In concIusion, we evidenced in the reaction between Iinear alcohols and tertiary olefins, catalyzed by an ion exchange resin, a remarkable transition in kinetic orders which we related to the ratio between the reactants, and which we tried to explain by mechanistic transitions, operating an analogical extention of mechanisms evidenced by Gates in dehydration of butyl alcohols_ It would be interesting to support our hypotheses, chiefly based on kinetic studies, by an independent chemical-physical examination of the catalyst array in the various evidenced kinetic situations_ Our attempts to obtain an IR support failed, due to the difficulty of operating in liquid phase and of reproducing thin membranes of a macroporous resin with the same properties as A 15.
48
References 1 2 3 4 5 6 7 8
B. C. Gates and W_ Rodriguez, J_ Cat& 31(1973) 27_ R_ Thornton and B. C. Gates, LCatal., 34 (1974) 275, F. AncilIotti, M. Massi Mauri and E. Pescarol:o, J_ Catal., 46 (1977) 49, D, J_ Pietrzyk, Anal. Chem., 44 (1972) 676_ L_ S_ Franked, J_ Phys_ Chem., 75 (1971) 1211_ J. E. Gordon, J. Phys. Chem., 66 (1962) 1150. W_ J_ Casey and D. I Pietrzyk, Anal. Chem., 45 (1973) 1404. R_ F_ Hirsch, H_ C_ Stober, M_ Kowblausky, F_ N. Hubner and A. W. O’ConneI, AnaI. Chem_, 45 (1973) 2 100, 9 L_ Romagnoni and F_ Ancillotti, Unpublished data, Internal Rep. No. 5 495,1976.