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Characterization Of Acid Catalysts By Use Of Model Reactions
B. Imelik et al. (Editors), Catalysis by Acids and Bases
© 1985 Elsevier Science Publishers B. V., Amsterdam - Printed-in The Netherlands
283
CHARACTERIZATION OF ACID CATALYSTS BY USE OF MODEL REACTIONS GUISNET Laboratoire de Catalyse en Chimie Organique (U.A. C.N.R.S. 350), Universite de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers (France) ~1.
RE L'utilisation de reactions modeles permet de caracteriser 1 'acidite d'un solide et de verifier son interet comme catalyseur. Pour cela les proprietes des sites actifs - nature, disposition spatiale, force de leurs composants ... doivent etre connues. Les reactions doivent etre simples, leur vitesse initiale facile a mesurer avec precision (desactivation lente ... ) et les transformations secondaires limitees. Par ail leurs ces reactions modeles ne doivent pas etre catalysees par plusieurs types de centres actifs. On montre que de nombreuses reactions d'hydrocarbures olefiniques, aromatiques ou satures satisfont ces exigences. On dispose donc d'une large gamme de reactions modeles parmi lesquelles on peut choisir la mieux adaptee aux catalyseurs a caracteriser (en particulier a la force de leurs sites acides). Si la vitesse initiale des reactions est 1'information essentielle il est aussi important d'examiner en detail leur selectivite. En effet certains produits secondaires meme en faible quantiimportante les vitesses de reaction. C'est en te peuvent modifier de fa~on particulier ce qui rend si compliquee la caracterisation des catalyseurs bifonctionnels metal-acide. Sot~MAI
ABSTRACT Model reactions allow one not only to verify rapidly the suitability of a solid acid as a catalyst but also to characterize its acidity. For this purpose all the properties of the active sites must be known. Moreover these reactions must be simple and not catalyzed by several different types of active centers. Their initial rate has to be easy to measure accurately (slow deactivation ... ) and any side reaction must be insignificant. It is shown that numerous reactions of olefinic, aromatic and saturated hydrocarbons satisfy these requirements. Within this range of model reactions it will be possible to choose the one(s) best adapted to the catalyst to be characterized in particular taking into account its acid strength. If pure acid catalysts can be accurately characterized by this simple method, the presence of hydrogenating sites complicate considerably the process. INTRODUCTION Catalyst manufacturers and users must dispose of simple accurate methods for characterizing their catalysts. There exists for acid catalysts various physicochemical methods generally based on chemisorption of basic compounds (1-3). If the interest of these methods is quite obvious, it must be noted that they lead to information which generally does not permit to judge completely of the quality of the catalyst. Indeed the surface characterized is often far different
284
from that of the catalyst in the industrial reaction particularly because the operating conditions are not identical. Moreover a distinction must be made between adsorption sites and active sites (4). Indeed the surface of acid catalysts comprises a large variety of species which differ by their chemical nature and which can exist in many different configurations (surface sites). For these species, alone or in combination, to be active in a given process (adsorption or reaction) they must satisfy numerous geometric, configurational and energetic requirements. This can only be the case for a very low percentage of these surface sites. The requirements depending obviously on the process, it follows that adsorption sites are not necessarily active sites. Thus whereas for oxides the density of the chemisorption sites varies form 1012 to 1013 sites/cm 2 (4) that of the active centers can be as low as 3.10 3 sites/cm 2 (value determined by the transition state method for t-butylbenzene cracking on a silica-alumina catalyst (5)). The best way to characterize acid catalysts is therefore through model reactions. The densities of active sites can be determined, at least in simple cases by means of a kinetics method which however requires a great number of experiments (5). That is why it is often preferred to proceed in a simpler way and characterize the acid catalysts by their activity in various simple reactions. GENERALITIES
Model reactions allow one not only to verify the suitability of a solid acid as a catalyst but also to characterize its surface acidity. Verification of the suitability of catalysts Model reactions constitute for catalyst manufacturers and users an efficient means for verifying the suitability of their catalysts before use in industrial plants. A comparison of all the main characteristics of the catalyst - activity, selectivity and stability - with those of a reference can be made in operational conditions similar to those of the industrial process. It is consequently easy to eliminate those catalysts which do not present the appropriate characteristi cs. If the transformation of a real feed is generally preferred, a model compound reaction can also be used. This reaction must be chosen so as to reproduce as closely as possible the transformations (primary and secondary) involved in the industrial process. For example a great deal of information can be obtained concerning cracking catalysts from alkane transformation: activity of the catalyst for C-C bond scission but also for the secondary reactions : hydrogen transfer, coke formation ... and stability.
285
Characterization of the active sites The use of model reactions allows one to go beyond the simple screening of catalysts. Their superficial sites can be defined and thus one can discover why such and such a solid is a better catalyst than another. To obtain this result, the properties of the sites - nature; spatial disposition and strength of the components - must be known. However if the model reactions are to be of use, it i~ obvious that it must not be catalyzed by several different types of sites. Moreover these reactions must be as simple as possible. Their initial rate has to be easy to measure accurately (slow deactivation, no thermodynamic limitations ... ) and any side reaction must be insignificant. We shall study various reactions for the purpose of determining the interest thereof for the characterization of acid catalysts. We have limited ourselves to a certain number of hydrocarbon reactions although other reactions of hydrocarbons or of functional compounds (such as alcohols) can be also used. HYDROCARBON REACTIONS FOR ACID CATALYST CHARACTERIZATION Olefin isomerization Double-bond shift and cis trans isomerization. Because of its great simplicity, n-butene isomerization was one of the model reactions most used. This reaction can occur on metals and bases as well as on acids and this by a large variety of mechanisms. Thus on alumina, by the use of deuterated reactants and poisoning techniques we have been able to show the existence of at least three types of reactions (6,7) : - double-bond shift and cis trans isomerization occurring with exchange of hydrogen between the olefin and the catalyst on purely acid sites (Lewis + Bronsted). - cis trans isomerization without exchange independent of double-bond shift catalyzed by weak Lewis acid-base sites. - double-bond shift and cis trans isomerization without exchange catalyzed by strong Lewis acid-base sites. The selectivity of nondeuterated butene transformation does not allow to distinguish between these mechanisms and a fortiori, when several of them are involved to determine their role in the isomerization. For this reason the transformation of nondeuterated butenes will not enable us to characterize definitely the acidity of the catalysts. However this will be possible if selectively deuterated butenes such as [1,4 2H 6] cis-but-2-ene are used. Unfortunately the study of the transformation of these butenes is lengthy and complicated. Skeletal isomerization. Contrarily to double-bond shift isomerization, skeletal isomerization occurs neither on metals nor on bases. Moreover it is clearly shown that it occurs on Bronsted acid sites via carbenium ion inter-
286
mediates (8,9) by a three-step mechanism : a) protonation of the reactant b) rearrangement of the carbenium ion formed c) desorption of the product. Step b is the rate-limiting step. If this step does not imply a change in the degree of branching, the carbenium ion rearrangement occurs by alkyl transfer (type A rearrangement (10)).
If it does imply a change (type B rearrangement) an alkyl transfer would need the formation of a highly unstable primary carbenium ion.
()+
C-C-C-C-C-C
+ C
c-t-c-C-C
~
By the use of 13C labelled molecules it can be shown that this rearrangement occurs then through protonated cyclopropane intermediates without primary carbenium ion formation (11,12).
+
c-c-c-c-c-c
r,
~
0;;--
b
c7'Hir c-C-t-t-c-c a
~ ~c
c-l-c-c-c (~)
c-~-~-c-c
Type A rearrangements are definitely faster than type B (12,13) : thus reaction l is about 20 times faster than the isomerization of n-hexenes into methylpentenes (reaction ~). Type B rearrangements require stronger acid sites than type A : thus an alumina, active in reaction l can be inactive in cyclohexene into methylcyclopentenes isomerization (reaction 1 type B)
0 -+H+
..---
-H+
0+
-"'"
...;--
U~
O +Qt ~O
Q T
c1
-- cJ
-H+
'""+ +H
Q 0
(1)
287
Double-bond shift and cis trans isomerization being generally much faster than skeletal isomerization (14), the reactional mixtures are often very complex: thus n-hexenes into methylpentenes isomerization leads to a mixture difficult to analyze for it contains the 14 olefinic isomers with n-hexane or methylpentane skeletons. The analysis can be simplified if the mixture obtained is previously hydrogenated. However model reactions leading to a more limited number of products will be preferred. Such would be the case of 3,3 dimethylI-butene (type A) or cyclohexene (type B) isomerization (15-18). The apparently is used very rarely because it very simple isomerization n-butenes ~isobutene is a very slow reaction and accompanied by formation of secondary products (19). This slow rate is due to the fact that a primary carbenium ion intermediate must be formed (12) : +
c-c=c-c
C-C-C-C
-a
~
Olefins can undergo various secondary reactions: cracking, oligomerization, polymerization, cyclization, hydrogen transfer, coke formation ... Moreover, the catalysts can be rapidly deactivated by coke. The significance of these reactions bei ng greater than the catalyst is more acid, it would be difficult to use skeletal isomerization of olefins to characterize highly acid catalysts. It must be noted however that secondary reactions (and deactivation) can be minimized by working at low olefin partial pressure since these reactions imply bimolecular steps whereas on the contrary isomerization occurs intramolecul arly. Aromatic transformations On acid catalysts, alkylaromatics can undergo several reactions namely dealkylation, isomerization and disproportionation. The relative significance of these reactions depends on the aromatic, on the catalyst and the operating conditions. Dealkylation. Cumene dealkylation into benzene and propene is the most used reaction for characterizing acid catalysts. This very simple reaction can be considered as a reaction of electrophile substitution of an isopropyl ion by a proton
+
~
-H
C-C-C
©
+ C-C=C
(~)
288
A very complete kinetic study of this reaction was carried out on silicaalumina by Prater and Lago (20) who show that all their results are compatible with a kinetic reaction scheme in which the cumene adsorption follows a Langmuir isotherm and the rate of cracking is proportional to the number of cumene molecules chemisorbed on active sites. As can be expected from this scheme, the reaction exhibits zero order kinetics at high cumene pressure and it is thus possible by means of absolute rate theory (21) to calculate the number of active sites. Moreover, a highly significant inhibiting effect of h~droperoxide is found (reaction rate divided by 2 for 0.05 % of hydroperoxide in cumene). This shows that it is essential that cumene be completely purified before use. Various secondary reactions can accompany the transformations of cumene in propene and benzene. Thus on alumina between 350 and 550°C it has been shown that cumene cracks roughly 50 % by an ionic mechanism and 50 % by a radical type mechanism with formation of styrene and methyl styrene (22). This radical pathforcracking is also found at temperatures above 500°C on alkali and alkaline earth ion exchanged Y zeolites (23). At low temperature (180°C) on the support of a hydrocracking catalyst, disproportionation becomes the prevailing reaction. Finally on a highly active lanthanum exchanged Y type zeolite catalyst (24) over 60 reaction products have been observed ; under initial conditions at 360°C cumene dealkylation (based on propene yield) accounts for only 64 % of the total cumene converted (24). On all the catalysts, coke is a significant product, which is easily understood since the reactional mixture contains aromatics and olefins (propene but also styrene and methyl styrene) well known as coke precursors (25). The Bronsted acidity of catalysts can also be characterized by using other alkylbenzenes. Indeed several studies of the influence of the alkylaromatic structure on their reactivity have shown that the order of reactivity is what is expected when cracking occurs through mechanism (i) (26-29). In all the series of hydrocarbons used: monoalkylbenzenes, substituted cumenes ... linear free energy relationships were applied successfully to the interpretation of the results. For example, good linear relationships were observed between the rate constants logarithms of monosubstituted alkylbenzenes (@- R) dealkylation and the enthalpy of formation of the corresponding carbenium ion R+ (26, 27) .
Isomerization and disproportionation. Disproportionation and isomerization mechanisms of aromatics depend on the catalyst employed, on the operating conditions and on the alkyl substituents of the aromatic ring. Thus the isomerization of methyl benzenes occurs by an intramolecular mechanism involving as limiting step a 1,2 alkylshift in a benzenium ion:
289
whereas t-butylbenzene isomerization occurs by transalkylation or via dealkylation-alkylation (10). Xylene isomerization is very much employed to characterize acid catalysts. This very simple reaction occurs on Bronsted acid centers as demonstrated by Ward and Hansford's work (30). For two series of silica-alumina catalysts the authors observed a linear relationship between the o-xylene isomerization rate and the Bronsted acidity. This acidity was measured by the intensity of the absorption band at 1545 cm- 1 (arising from pyridinium ions) in the spectrum of chemisorbed pyridine recorded after evacuation at 125°C. Oisproportionation into toluene and trimethylbenzenes and coke formation are the only reactions which accompany xylene isoemrization. Oisproportionation being a bimolecular reaction, it is possible to reduce its significance by decreasing the xylene partial pressure. But this reaction can also give information concerning the acidity of the catalyst. Two types of mechanisms have been proposed to explain this reaction (10). The first, like isomerization, occurs through benzenium ion intermediates with two possibilities: i) dealkylation-alkylation ii) direct alkyl transfer from one aromatic molecule to the other
the second occurs through benzylic carbenium ions which mechanism (Fig. 1) is the most likely in the case of xylenes (31). The active sites are most probably different for disproportionation and isomerization since their rate ratio (0/1) depends to a great extent of the catalyst employed. For example all the treatments of a protonic mordenite (dealumination, wet air treatment, exchange by various cations) modify the 0/1 value (32). It does not seem as if the active sites are of a different nature: it has been proved that Bronsted acid sites are responsible for disproportionation (33-35) as for isomerization. Moreover we were able to show by pyridine poisoning that the difference between the active sites was not the result of a difference between their acid strengths (31) : a HY zeolite presenting sites with very different acid strengths was
290
Fig. 1. Oisproportionation mechanism of xylenes via benzylic carbocation i ntermedi ates. chosen. The ratio of the a-xylene transformation rates on the pyridine poisoned catalyst and on the fresh catalyst was determined as a function of the desorption temperature of this poison. Very slightly different curves were obtained for isomerization and disproportionation. It is therefore most likely the "demanding" character of the different reactions which explains the change of the 011 value with the acidity : the bimolecular disproportionation reaction would demand a pair of adjacent Bronsted acid sites whereas isomerization would require only one. The D/I value brings therefore significant information regarding the density of the acid sites. Trimethylbenzenes can also be used for characterizing acid catalysts. They have a faster - roughly twice - transformation rate than xylenes (36). By pyridine poisoning it can be shown that the acid strength required for 1,2,4trimethylbenzene transformations is weaker than that required for o-xylene (37). Oisproportionation of toluene (38-40) of ethyl benzene (34) and of cumene (41) have also been proposed for characte r iz inq acid catalysts. In the two latter cases, the choice of operating conditions and especially of the temperature is very important for limiting secondary reactions and deactivation by coke. Alkane transformations Over acid catalysts, alkanes undergo three main reactions: isomerization, cracking and disproportionation. All three of them involve carbenium ions as intermediates and their relative significance depends both on the characteris-
291
tics of the alkane and of the catalyst. For heavy alkanes (~ C7) cracking is practically the only reaction, for C5-C6 isomerization accompanies cracking and for C3-C4 disproportionation is often the main reaction. In all the cases, the active centers are probably Bronsted acid sites. Alkane cracking occurs through steps 1-4 of Fig. 2. Px-
-
Px
Fig. 2. Isomerization and cracking of paraffins on acid catalysts. P : paraffin; 0 : olefin; C+ : carbocation ; x,y : number of carbon atoms. In stationary state, thecarbenium ion formation (step 1) results from hydride transfer between a reactant molecule and a preadsorbed carbenium ion. The cracking rate is determined by the stability of the C; and C; carbenium ion intermediates. Thus on HY zeolite, at 400°C (42) the isooctane cracking which involves only tertiary carbocations
+
is 10 to 15 times faster than the cracking of 2 and 3-methylpentane, 30 to 40 than n-hexane and 150 to 200 than n-pentane cracking this latter involving an ethyl primary carbocation +
C=C-C
By pyridine poisoning we have shown (37) that the more difficult the reaction the greater must be the acid strength of the sites necessary for its catalysis: thus n-hexane cracking at 350°C demands sites capable of keeping the pyridine adsorbed at least at 520°C whereas isooctane cracking requires sites capable of keeping the pyridine adsorbed at 350°C. It can be noted that a cracking mechanism involving a pentacoordinated carbonium ion intermediate was recently proposed (43) to explain the formation of C1 and C2 hydrocarbons by cracking of C6 alkanes at high temperatures and low pressures on zeolites and on silica-alumina. n-Hexane cracking is one of the most employed reaction for characterizing the acidity of catalysts (a value (44,45)). It is very interesting to note that
292
the apparent activation energy of this reaction is the same on catalysts differing strongly by their acidity. This allows the reaction temperature to be chosen as a function of the acidity of the catalysts to be characterized from 300 DC for the more acid to over 500 DC for the less acid thus covering a range of activities of more than four orders of mangitude (45). The constancy of the apparent activation energy in spite of large variations in selectivity suggests a uniformity in the character of the rate limiting step (in this case the formation of the carbenium ion, step 1 Fig. 2) ; product variations result from subsequent secondary reaction processes which depend on the reaction temperature as well as on the acidity. The selectivity can therefore give interesting information on the acidity of the catalysts provided it is determined at the same temperature. Butane transformation which leads to much less products than that of n-hexane could seem simpler (46-48). However the reactional scheme of this transformation is generally complex involving as a first step butane disproportionation into propane and pentanes, then rapid cracking of pentanes (49). Butane isomerization occurs also by disproportionation (50,51). Fig. 3. shows as an example how propane and isopentane can be formed by n-butane disproportionation. +
2(C-C-C-C
C-C-C-C +
+
C-C=C-C C-C-C-C + C-C-C-C + C-C=C-C ~ 'VCH3,'VH
+
C-C-~-~-C-C
C=~-C-C + C-C-C
+ H+
~
~
RH)
+ H+ +
C-C-~-~-C-C
()t:
> C-~-C-c-C-C
~
+
C-C-C +
C=~-C-C
C
C-~-C-C
C-C-C +
R+
C-c-C-C
~
Fig. 3.
n-Butane disproportionation. Simplified mechanism.
t
Butane disproportionation at 350°C requires extremely strong acid sites (capable of keeping pyridine adsorbed at 550 DC (52)) and therefore could only be employed for characterizing very acid catalysts. Moreover it is more or less certain that this reaction requires, like the disproportionation of aromatics two adjacent acid sites (49).
293
CATALYST CHARACTERIZATION BY USE OF MODEL REACTIONS Choice of the reaction As has been seen, there exists a large variety of reactions which allow the characterization of active sites of acid catalysts. In the reactions that have been exa~ined here, the active sites can be classified in two categories : acidobasic sites and Bronsted acid sites. The Lewis acid sites alone do not seem to be responsible for these reactions but probably could catalyze other reactions. Acidobasic centers catalyze facile reactions such as double-bond shift and cis trans isomerization of olefins whereas Bronsted acid sites are responsible for more difficult transformations. The acid strength that Bronsted acid sites must have to be active depends a great deal on the reaction considered ; thus for catalyzing 3,3-dimethyl-l butene isomerization at 250°C, it is sufficient for the sites to be able to retain pyridine adsorbed at 290°C whereas for catalyzing isobutane disproportionation at 350°C, they must be able to retain it at 550°C (37,52). Moreover, bimolecular reactions of alkane and aromatic disproportionation seem to demand pairs of adjacent Bronsted sites for their catalysis whereas one single Bronsted site is enough for catalyzing monomolecular reactions. The choice of the reaction will depend firstly on the acid strength of the catalyst to be characterized. For acidobasic oxides such as alumina the olefin isomerizations are the reactions the best adapted; the other reactions can be used but radical path transformations can make the interpretation of the results difficult. For highly acid catalysts alkane and aromatic transformations are to be preferred. Isomerization of olefins must be avoided for these compounds can undergo many side reactions - cracking, oligomerization, polymerization, coke formation ... - which can disturb the characterization of the catalysts. Moreover as the reactions will be very fast, they could be limited by transport phenomena. In each of these two series of catalysts the samples to be characterized can differ highly by their acidity. It is hence very interesting to employ the reactant which transforms itself to several parallel or successive reactions demanding sites of different strengths. This is particularly the case for 3,3-dimethyl-l butene whose the rearrangement will be limited to 2,3-dimethylbutenes on slightly acid catalysts but could lead to n-hexenes on highly acid catalysts (15). One can also use two or more reactions belonging to a same class in which the reactant changes systematically in its acid strength requirement. Good examples are cracking of substituted cumenes (29) - or of t-butyl, s-butyl and n-butyl - benzenes (28), isomerization of xylenes and trimethylbenzenes (36), cracking of n-alkanes (49) ... Lastly in the particular case of shape-selective zeolites, it is obvious that the size of the reactants and of the products must be such that the reaction rate will be
294
limited neither by transport phenomena nor by steric constraints in the formation of intermediates or transition states. Method for characterization The essential information for the characterization of active sites is obviously the reaction rate which must be accurately determined {generally in a flow reactor) before the deactivation of the catalyst. As has already been indicated. the right choice of the reaction can often avoid a significant deactivation-by coke. Operating conditions: temperature. partial pressure of reactant ... are of great importance. Lastly by use of pulse technique it is sometimes possible to measure the initial activity of catalysts highly sensitive to deactivation. However it is well known that this technique gives exact values only in the case of linear reactions (53). Moreover the reaction rate must be measured in absence of diffusional limitations in the porous catalyst particle. Diffusion effects can be considered as negligible (54) for isothermal reactions if
where r is the reaction rate (moles/cm 3.sec). c the concentration of the reactant (moles/cm3), R the radius of the catalyst particles (cm), De the effective diffusivity of the reactant in the porous catalyst at the reaction temperature (cm 2/sec). Unfortunately. De is very often difficult to determine. In order to obtain an accurate rate value. high conversions too close to equilibrium must be avoided. In the case of a reversible transformation, it is of interest to study the reaction in a thermodynamically favorable direction; for example the transformation
c-~-c=c
(55)
will be studied in the forward rather than in the reverse direction. Problems created by the acidity characterization of bifunctional catalysts These catalysts which associate a hydrogenating component (metal or sulfur) with an acid support are employed in numerous industrial processes. Their acidity plays a very significant role in their activity and selectivity (see for example ref. 56 for alkane hydrocracking). It is therefore important to be able to characterize the acidity not only before the introduction of hydrogenating
295
component but also on the bifunctional catalyst and if possible in the conditions of its utilisation. Karge et al (57) propose to characterize bifunctional catalyst acidity by ethylbenzene disproportionation. However they found for this reaction an inhibiting effect of hydrogen on the activity of bifunctional zeolitic catalysts which effect was attributed to an acidity decrease. Another interpretation of this inhibiting effect however was proposed (58) : the decrease in activity would be related to a decrease in the concentration of benzylic carbocations (intermediates of the disproportionation) due either to hydrogen (40)
©
+ H2
...-~
©
+ ft
or to a low quantity of branched alkanes produced by ethyl benzene hydrogenation (59) •
©
+
c5
~
...,....--
© c5 +
At the present time it is difficult to attribute definitely the inhibiting effect of hydrogen to the decrease in acidity or in benzylic carbocation concentration. In n-hexane cracking, the activity of PtHY catalysts, measured in the absence of hydrogen in order to avoid bifunctional catalysis, is definitely greater than that of a platinum-free HY zeolite (60). This can be connected to the formation of a small amount of hexenes by dehydrogenation of n-hexane on platinum sites (61). Indeed it is a well known fact that olefins can increase the alkane cracking rate by increasing the number of chain initiators. One of the significant criteria in the choice of an acidity characterization reaction of bifunctional catalysts will be that reactants and products undergo the least possible side reactions on hydrogenating sites. Poisoning of these sites can be envisaged but then it must be made sure than the acidity will not be affected (60). The problems created by bifunctional catalyst characterization are evidently met again for acid catalysts presenting hydrogenating impurities. This is particularly the case for cracking catalysts which after use contain appreciable amounts of nickel and vanadium. It is therefore always essential to control in detail the reaction selectivity and in particular to take into consideration the formation of any new secondary product and its possible effect on the reaction rate.
296
ACKNOWLEDGEMENT The author thanks ELF FRANCE (Centre de Recherche de Solaize) for supporting thi s research. REFERENCE 1 K. Tanabe, "Solid Acids and Bases", Academic Press, New York (1970). 2 H.A. Benesi and B.H.C.Winquist, in D.D. Eley, H. Pines, P.B. Heisz (Eds}, Advances in Catalysis, Academic Press, New York, San Francisco, London, Vol. 27, 1978, p. 97. 3 L. Forni, in H. Heineman (Ed.),Catalysis Reviews, M. Dekker, New York, Vol. 8, 1974, p. 65. 4 H. Knozinger, in D.D. Eley, H. Pines, P.B. Weisz (Eds), Advances in Catalysis, Academic Press, New York, San Francisco, London, Vol. 25, 1976, p. 184. 5 R.W. Maatman, in H. Heineman (Ed.), Catalysis Reviews, M. Dekker, New York, Vol. 8, 1974, p. l. 6 M. Guisnet, J.L. Lemberton, G. Perot and R. Maurel, J. Catal .,48 (1977) 166. 7 J.L. Lemberton, G. Perot and M. Guisnet, J. Chern. Research,S (1979) 94, M(1979) 1290. 8 C.S. John, C. Kemball and R.A. Rajadhyaksha, J. Catal .,57 (1979) 264. 9 E.A. Irvine, C.S. John, C. Kemball, A.J. Pearman, M.A. Day and R.J. Sampson, J. Catal.,61 (1980) 326. 10 M.L. Poutsma, Zeolite Chemistry and Catalysis, J.A. Rabo Ed. ACS Monograph 171, American Chemical Society, Washington D.C., 1976, Ch 8, p. 437. 11 D.M. Brouwer, Rec. Trav. Chim.,87 (1968) 1435. 12 F. Chevalier, M. Guisnet and R. Maurel, Proceedings.6 t h Int. Congr. Catal., G.C. Bond P.B. Wells F.C. Tompkins Eds , The Chemical Society, London, 1977, Vol. 1, p. 478. 13 J. Weitkamp and H. Farag, Proceedings of the Symposium on Zeolites, Szeged 1978, p. 327. 14 G. Perot, J.L. Lemberton et M. Guisnet, Bull. Soc. Chim., (1977) 355. 15 H. Pines, J. Catal .,78 (1982) 1. 16 W.O. Haag and H. Pines, J. Amer. Chern. Soc.,82 (1960) 2488. 17 L. Bassery, Bull. Soc. Chim. ,(1969) 1077. 18 L. Ponsolle, L. Bassery, J.P. Joly et D. Vaillant, Bull. Soc. Chim., (1969) 1083. 19 J. Dubien, L. de Mourgues et Y. Trambouze, Bull. Soc. Chim., (1967) 108. 20 C.D. Prater and R.M. Lago, in D.D. Eley, H. Pines, P.B. Weisz (Eds), Advances in Catalysis, Academic Press, New York, San Francisco, London, Vol. 8, 1956, 293. . 21 S. Glasstone, K.J. Laidler and H. Eyring, "The Theory of rate processes", Mac Graw Hill, New York, Ch 7, 1941. 22 S.E. Tung and E. Mc Ininch, J. Catal .,4 (1965) 586. 23 J.T. Richardson, J. Catal.,9 (1967) 182. 24 D. Best and B.W. Wojcieckowski, J. Catal .,47 (1977) 11. 25 D.E. Walsh and L.D. Rollmann, J. Catal.,49 (1977) 369. 26 I. Mochida and Y. Yoneda, J. Catal.,7 (1967) 386; 7 (1967) 393; 8 (1967) 223. 27 H. Matsumoto, J.I. Take and Y. Yoneda, J. Catal .,19 (1970) 113. 28 P. Andreu and M.M. Ramirez, Proceedings 6th Int. Congr. Catal., G.C. Bond P.B. Wells, F.C. Tompkins Eds, The Chemical Society, London, 1977, Vol. 1, p, 593. 29 N.S. Gnep, These Ingenieur, Poitiers 1974. 30 J.W. Ward and R.C. Hansford, J. Catal., 13 (1969) 154. 31 M. Guisnet and N.S. Gnep, "Zeolites: Science and Technology" NATO ASI Series, F.R. Ribeiro et al Eds, Martinus Nijhoff Publishers, The Hague, Boston, Lancaster (1984) p. 571.
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32 M.L. Martin de Armando, N.S. Gnep and M. Guisnet, J. Chem. Research,S (1981) 8 ; ~1 (1981) 243. 33 P.A. Jacobs, H.E. Leeman and J.B. Uytterhoeven, J. Catal., 33 (1974) 3l. 34 H.G. Karge, J. Ladebeck, Z. Sarbak and K. Hatada, Zeolites, 2 (1982) 94. 35 H.G. Karge, K. Hatada, Y. Zhang and R. Fiedorow, Zeolites, 3 (1983) 13. 36 N.S. Gnep, J. Tejada and M. Guisnet, Bull. Soc. Chim.,(1984) 5. 37 G. Bourdillon, C. Gueguen and M. Guisnet, in preparation. 38 H.A. Benesi, J. Catal. ,8 (1967) 368. 39 N.S. Gnep, M.L. Martin de Armando, C. Marcilly, B.H. Ha and M. Guisnet, "Catalyst Deactivation" Studies in Surface Science and Catalysis 6, Elsevier, Amsterdam (1980) p. 79. 40 N.S. Gnep and M. Guisnet, Applied Catalysis, 1 (1981) 329. 41 R.P.L. Absil, J.B. Butt and J.S. Dranoff, J. Catal .,85 (1984) 415. 42 G. Lopez, These Poitiers, 1977. 43 W.O. Haag, R.M. Dessau, Proceedings 8t h Int. Congr. Catal., Dechema, 1984, Vo1. 2, p. 305. 44 P.B. Weisz, J.N. Miale, J. Catal., 4 (1965) 527. 45 J.N. Miale, N.Y. Chen and P.B. Weisz, J. Catal., 6 (1966) 278. 46 H. Rastelli, B.M. Lok, J.A. Duisman, D.E. Earls and J.T. Mullkaupt, Canad. J. Chem. Eng., 60 (1982) 44. 47 J. Blanco, A. Ramos and J. Soria, J. Catal., 54 (1978) 365. 48 G.B. Mc Vicker, G.M. Kramer and J.J. Ziemak, J. Catal., 83 (1983) 286. 49 F. Avendano, These Poitiers, 1984. 50 G.A. Fuentes and B.C. Gates, J. Catal., 76 (1982) 440. 51 C. Bearez, F. Chevalier and M. Guisnet, React. Kinet. Catal. Lett., 22 (1983) 405. 52 C. Bearez, F. Chevalier and M. Guisnet, in preparation. 53 T. Hattori and Y. Murakami, J. Catal., 10 (1968) 114. 54 J.M. Smith, "Chemical Engineering Kinetics", Mac Graw Hill, Second Edition (1970) . 55 Americao Petroleum Institute, Research Project 44, Selected Values of Properties of hydrocarbons and related compounds. 56 M. Guisnet and G. Perot, "Zeolites: Science and Technology" NATO ASI Series; F.R. Ribeiro et al Eds, Martinus Nijhoff Publishers, The Hague, Boston, Lancaster (1984), p. 397. 57 H.G. Karge, Z. Sarbak, K. Hatada, J. Weitkamp, P.A. Jacobs, J. Catal., 82 (1983) 236. 58 M. Guisnet, J. Catal., 88 (1984) 249. 59 N.S. Gnep and M. Guisnet, React. Kinet. Catal. Lett., 22 (1983) 237. 60 G. Bourdillon, G. Giannetto, C. Gueguen.and M. Guisnet, in preparation. 61 M. Guisnet, G. Giannetto, P. Hilaireau and G. Perot, J. Chern. Soc. Chern. Comm.,(1983) 1411.
© 1985 Elsevier Science Publishers B. V., Amsterdam - Printed-in The Netherlands
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CHARACTERIZATION OF ACID CATALYSTS BY USE OF MODEL REACTIONS GUISNET Laboratoire de Catalyse en Chimie Organique (U.A. C.N.R.S. 350), Universite de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers (France) ~1.
RE L'utilisation de reactions modeles permet de caracteriser 1 'acidite d'un solide et de verifier son interet comme catalyseur. Pour cela les proprietes des sites actifs - nature, disposition spatiale, force de leurs composants ... doivent etre connues. Les reactions doivent etre simples, leur vitesse initiale facile a mesurer avec precision (desactivation lente ... ) et les transformations secondaires limitees. Par ail leurs ces reactions modeles ne doivent pas etre catalysees par plusieurs types de centres actifs. On montre que de nombreuses reactions d'hydrocarbures olefiniques, aromatiques ou satures satisfont ces exigences. On dispose donc d'une large gamme de reactions modeles parmi lesquelles on peut choisir la mieux adaptee aux catalyseurs a caracteriser (en particulier a la force de leurs sites acides). Si la vitesse initiale des reactions est 1'information essentielle il est aussi important d'examiner en detail leur selectivite. En effet certains produits secondaires meme en faible quantiimportante les vitesses de reaction. C'est en te peuvent modifier de fa~on particulier ce qui rend si compliquee la caracterisation des catalyseurs bifonctionnels metal-acide. Sot~MAI
ABSTRACT Model reactions allow one not only to verify rapidly the suitability of a solid acid as a catalyst but also to characterize its acidity. For this purpose all the properties of the active sites must be known. Moreover these reactions must be simple and not catalyzed by several different types of active centers. Their initial rate has to be easy to measure accurately (slow deactivation ... ) and any side reaction must be insignificant. It is shown that numerous reactions of olefinic, aromatic and saturated hydrocarbons satisfy these requirements. Within this range of model reactions it will be possible to choose the one(s) best adapted to the catalyst to be characterized in particular taking into account its acid strength. If pure acid catalysts can be accurately characterized by this simple method, the presence of hydrogenating sites complicate considerably the process. INTRODUCTION Catalyst manufacturers and users must dispose of simple accurate methods for characterizing their catalysts. There exists for acid catalysts various physicochemical methods generally based on chemisorption of basic compounds (1-3). If the interest of these methods is quite obvious, it must be noted that they lead to information which generally does not permit to judge completely of the quality of the catalyst. Indeed the surface characterized is often far different
284
from that of the catalyst in the industrial reaction particularly because the operating conditions are not identical. Moreover a distinction must be made between adsorption sites and active sites (4). Indeed the surface of acid catalysts comprises a large variety of species which differ by their chemical nature and which can exist in many different configurations (surface sites). For these species, alone or in combination, to be active in a given process (adsorption or reaction) they must satisfy numerous geometric, configurational and energetic requirements. This can only be the case for a very low percentage of these surface sites. The requirements depending obviously on the process, it follows that adsorption sites are not necessarily active sites. Thus whereas for oxides the density of the chemisorption sites varies form 1012 to 1013 sites/cm 2 (4) that of the active centers can be as low as 3.10 3 sites/cm 2 (value determined by the transition state method for t-butylbenzene cracking on a silica-alumina catalyst (5)). The best way to characterize acid catalysts is therefore through model reactions. The densities of active sites can be determined, at least in simple cases by means of a kinetics method which however requires a great number of experiments (5). That is why it is often preferred to proceed in a simpler way and characterize the acid catalysts by their activity in various simple reactions. GENERALITIES
Model reactions allow one not only to verify the suitability of a solid acid as a catalyst but also to characterize its surface acidity. Verification of the suitability of catalysts Model reactions constitute for catalyst manufacturers and users an efficient means for verifying the suitability of their catalysts before use in industrial plants. A comparison of all the main characteristics of the catalyst - activity, selectivity and stability - with those of a reference can be made in operational conditions similar to those of the industrial process. It is consequently easy to eliminate those catalysts which do not present the appropriate characteristi cs. If the transformation of a real feed is generally preferred, a model compound reaction can also be used. This reaction must be chosen so as to reproduce as closely as possible the transformations (primary and secondary) involved in the industrial process. For example a great deal of information can be obtained concerning cracking catalysts from alkane transformation: activity of the catalyst for C-C bond scission but also for the secondary reactions : hydrogen transfer, coke formation ... and stability.
285
Characterization of the active sites The use of model reactions allows one to go beyond the simple screening of catalysts. Their superficial sites can be defined and thus one can discover why such and such a solid is a better catalyst than another. To obtain this result, the properties of the sites - nature; spatial disposition and strength of the components - must be known. However if the model reactions are to be of use, it i~ obvious that it must not be catalyzed by several different types of sites. Moreover these reactions must be as simple as possible. Their initial rate has to be easy to measure accurately (slow deactivation, no thermodynamic limitations ... ) and any side reaction must be insignificant. We shall study various reactions for the purpose of determining the interest thereof for the characterization of acid catalysts. We have limited ourselves to a certain number of hydrocarbon reactions although other reactions of hydrocarbons or of functional compounds (such as alcohols) can be also used. HYDROCARBON REACTIONS FOR ACID CATALYST CHARACTERIZATION Olefin isomerization Double-bond shift and cis trans isomerization. Because of its great simplicity, n-butene isomerization was one of the model reactions most used. This reaction can occur on metals and bases as well as on acids and this by a large variety of mechanisms. Thus on alumina, by the use of deuterated reactants and poisoning techniques we have been able to show the existence of at least three types of reactions (6,7) : - double-bond shift and cis trans isomerization occurring with exchange of hydrogen between the olefin and the catalyst on purely acid sites (Lewis + Bronsted). - cis trans isomerization without exchange independent of double-bond shift catalyzed by weak Lewis acid-base sites. - double-bond shift and cis trans isomerization without exchange catalyzed by strong Lewis acid-base sites. The selectivity of nondeuterated butene transformation does not allow to distinguish between these mechanisms and a fortiori, when several of them are involved to determine their role in the isomerization. For this reason the transformation of nondeuterated butenes will not enable us to characterize definitely the acidity of the catalysts. However this will be possible if selectively deuterated butenes such as [1,4 2H 6] cis-but-2-ene are used. Unfortunately the study of the transformation of these butenes is lengthy and complicated. Skeletal isomerization. Contrarily to double-bond shift isomerization, skeletal isomerization occurs neither on metals nor on bases. Moreover it is clearly shown that it occurs on Bronsted acid sites via carbenium ion inter-
286
mediates (8,9) by a three-step mechanism : a) protonation of the reactant b) rearrangement of the carbenium ion formed c) desorption of the product. Step b is the rate-limiting step. If this step does not imply a change in the degree of branching, the carbenium ion rearrangement occurs by alkyl transfer (type A rearrangement (10)).
If it does imply a change (type B rearrangement) an alkyl transfer would need the formation of a highly unstable primary carbenium ion.
()+
C-C-C-C-C-C
+ C
c-t-c-C-C
~
By the use of 13C labelled molecules it can be shown that this rearrangement occurs then through protonated cyclopropane intermediates without primary carbenium ion formation (11,12).
+
c-c-c-c-c-c
r,
~
0;;--
b
c7'Hir c-C-t-t-c-c a
~ ~c
c-l-c-c-c (~)
c-~-~-c-c
Type A rearrangements are definitely faster than type B (12,13) : thus reaction l is about 20 times faster than the isomerization of n-hexenes into methylpentenes (reaction ~). Type B rearrangements require stronger acid sites than type A : thus an alumina, active in reaction l can be inactive in cyclohexene into methylcyclopentenes isomerization (reaction 1 type B)
0 -+H+
..---
-H+
0+
-"'"
...;--
U~
O +Qt ~O
Q T
c1
-- cJ
-H+
'""+ +H
Q 0
(1)
287
Double-bond shift and cis trans isomerization being generally much faster than skeletal isomerization (14), the reactional mixtures are often very complex: thus n-hexenes into methylpentenes isomerization leads to a mixture difficult to analyze for it contains the 14 olefinic isomers with n-hexane or methylpentane skeletons. The analysis can be simplified if the mixture obtained is previously hydrogenated. However model reactions leading to a more limited number of products will be preferred. Such would be the case of 3,3 dimethylI-butene (type A) or cyclohexene (type B) isomerization (15-18). The apparently is used very rarely because it very simple isomerization n-butenes ~isobutene is a very slow reaction and accompanied by formation of secondary products (19). This slow rate is due to the fact that a primary carbenium ion intermediate must be formed (12) : +
c-c=c-c
C-C-C-C
-a
~
Olefins can undergo various secondary reactions: cracking, oligomerization, polymerization, cyclization, hydrogen transfer, coke formation ... Moreover, the catalysts can be rapidly deactivated by coke. The significance of these reactions bei ng greater than the catalyst is more acid, it would be difficult to use skeletal isomerization of olefins to characterize highly acid catalysts. It must be noted however that secondary reactions (and deactivation) can be minimized by working at low olefin partial pressure since these reactions imply bimolecular steps whereas on the contrary isomerization occurs intramolecul arly. Aromatic transformations On acid catalysts, alkylaromatics can undergo several reactions namely dealkylation, isomerization and disproportionation. The relative significance of these reactions depends on the aromatic, on the catalyst and the operating conditions. Dealkylation. Cumene dealkylation into benzene and propene is the most used reaction for characterizing acid catalysts. This very simple reaction can be considered as a reaction of electrophile substitution of an isopropyl ion by a proton
+
~
-H
C-C-C
©
+ C-C=C
(~)
288
A very complete kinetic study of this reaction was carried out on silicaalumina by Prater and Lago (20) who show that all their results are compatible with a kinetic reaction scheme in which the cumene adsorption follows a Langmuir isotherm and the rate of cracking is proportional to the number of cumene molecules chemisorbed on active sites. As can be expected from this scheme, the reaction exhibits zero order kinetics at high cumene pressure and it is thus possible by means of absolute rate theory (21) to calculate the number of active sites. Moreover, a highly significant inhibiting effect of h~droperoxide is found (reaction rate divided by 2 for 0.05 % of hydroperoxide in cumene). This shows that it is essential that cumene be completely purified before use. Various secondary reactions can accompany the transformations of cumene in propene and benzene. Thus on alumina between 350 and 550°C it has been shown that cumene cracks roughly 50 % by an ionic mechanism and 50 % by a radical type mechanism with formation of styrene and methyl styrene (22). This radical pathforcracking is also found at temperatures above 500°C on alkali and alkaline earth ion exchanged Y zeolites (23). At low temperature (180°C) on the support of a hydrocracking catalyst, disproportionation becomes the prevailing reaction. Finally on a highly active lanthanum exchanged Y type zeolite catalyst (24) over 60 reaction products have been observed ; under initial conditions at 360°C cumene dealkylation (based on propene yield) accounts for only 64 % of the total cumene converted (24). On all the catalysts, coke is a significant product, which is easily understood since the reactional mixture contains aromatics and olefins (propene but also styrene and methyl styrene) well known as coke precursors (25). The Bronsted acidity of catalysts can also be characterized by using other alkylbenzenes. Indeed several studies of the influence of the alkylaromatic structure on their reactivity have shown that the order of reactivity is what is expected when cracking occurs through mechanism (i) (26-29). In all the series of hydrocarbons used: monoalkylbenzenes, substituted cumenes ... linear free energy relationships were applied successfully to the interpretation of the results. For example, good linear relationships were observed between the rate constants logarithms of monosubstituted alkylbenzenes (@- R) dealkylation and the enthalpy of formation of the corresponding carbenium ion R+ (26, 27) .
Isomerization and disproportionation. Disproportionation and isomerization mechanisms of aromatics depend on the catalyst employed, on the operating conditions and on the alkyl substituents of the aromatic ring. Thus the isomerization of methyl benzenes occurs by an intramolecular mechanism involving as limiting step a 1,2 alkylshift in a benzenium ion:
289
whereas t-butylbenzene isomerization occurs by transalkylation or via dealkylation-alkylation (10). Xylene isomerization is very much employed to characterize acid catalysts. This very simple reaction occurs on Bronsted acid centers as demonstrated by Ward and Hansford's work (30). For two series of silica-alumina catalysts the authors observed a linear relationship between the o-xylene isomerization rate and the Bronsted acidity. This acidity was measured by the intensity of the absorption band at 1545 cm- 1 (arising from pyridinium ions) in the spectrum of chemisorbed pyridine recorded after evacuation at 125°C. Oisproportionation into toluene and trimethylbenzenes and coke formation are the only reactions which accompany xylene isoemrization. Oisproportionation being a bimolecular reaction, it is possible to reduce its significance by decreasing the xylene partial pressure. But this reaction can also give information concerning the acidity of the catalyst. Two types of mechanisms have been proposed to explain this reaction (10). The first, like isomerization, occurs through benzenium ion intermediates with two possibilities: i) dealkylation-alkylation ii) direct alkyl transfer from one aromatic molecule to the other
the second occurs through benzylic carbenium ions which mechanism (Fig. 1) is the most likely in the case of xylenes (31). The active sites are most probably different for disproportionation and isomerization since their rate ratio (0/1) depends to a great extent of the catalyst employed. For example all the treatments of a protonic mordenite (dealumination, wet air treatment, exchange by various cations) modify the 0/1 value (32). It does not seem as if the active sites are of a different nature: it has been proved that Bronsted acid sites are responsible for disproportionation (33-35) as for isomerization. Moreover we were able to show by pyridine poisoning that the difference between the active sites was not the result of a difference between their acid strengths (31) : a HY zeolite presenting sites with very different acid strengths was
290
Fig. 1. Oisproportionation mechanism of xylenes via benzylic carbocation i ntermedi ates. chosen. The ratio of the a-xylene transformation rates on the pyridine poisoned catalyst and on the fresh catalyst was determined as a function of the desorption temperature of this poison. Very slightly different curves were obtained for isomerization and disproportionation. It is therefore most likely the "demanding" character of the different reactions which explains the change of the 011 value with the acidity : the bimolecular disproportionation reaction would demand a pair of adjacent Bronsted acid sites whereas isomerization would require only one. The D/I value brings therefore significant information regarding the density of the acid sites. Trimethylbenzenes can also be used for characterizing acid catalysts. They have a faster - roughly twice - transformation rate than xylenes (36). By pyridine poisoning it can be shown that the acid strength required for 1,2,4trimethylbenzene transformations is weaker than that required for o-xylene (37). Oisproportionation of toluene (38-40) of ethyl benzene (34) and of cumene (41) have also been proposed for characte r iz inq acid catalysts. In the two latter cases, the choice of operating conditions and especially of the temperature is very important for limiting secondary reactions and deactivation by coke. Alkane transformations Over acid catalysts, alkanes undergo three main reactions: isomerization, cracking and disproportionation. All three of them involve carbenium ions as intermediates and their relative significance depends both on the characteris-
291
tics of the alkane and of the catalyst. For heavy alkanes (~ C7) cracking is practically the only reaction, for C5-C6 isomerization accompanies cracking and for C3-C4 disproportionation is often the main reaction. In all the cases, the active centers are probably Bronsted acid sites. Alkane cracking occurs through steps 1-4 of Fig. 2. Px-
-
Px
Fig. 2. Isomerization and cracking of paraffins on acid catalysts. P : paraffin; 0 : olefin; C+ : carbocation ; x,y : number of carbon atoms. In stationary state, thecarbenium ion formation (step 1) results from hydride transfer between a reactant molecule and a preadsorbed carbenium ion. The cracking rate is determined by the stability of the C; and C; carbenium ion intermediates. Thus on HY zeolite, at 400°C (42) the isooctane cracking which involves only tertiary carbocations
+
is 10 to 15 times faster than the cracking of 2 and 3-methylpentane, 30 to 40 than n-hexane and 150 to 200 than n-pentane cracking this latter involving an ethyl primary carbocation +
C=C-C
By pyridine poisoning we have shown (37) that the more difficult the reaction the greater must be the acid strength of the sites necessary for its catalysis: thus n-hexane cracking at 350°C demands sites capable of keeping the pyridine adsorbed at least at 520°C whereas isooctane cracking requires sites capable of keeping the pyridine adsorbed at 350°C. It can be noted that a cracking mechanism involving a pentacoordinated carbonium ion intermediate was recently proposed (43) to explain the formation of C1 and C2 hydrocarbons by cracking of C6 alkanes at high temperatures and low pressures on zeolites and on silica-alumina. n-Hexane cracking is one of the most employed reaction for characterizing the acidity of catalysts (a value (44,45)). It is very interesting to note that
292
the apparent activation energy of this reaction is the same on catalysts differing strongly by their acidity. This allows the reaction temperature to be chosen as a function of the acidity of the catalysts to be characterized from 300 DC for the more acid to over 500 DC for the less acid thus covering a range of activities of more than four orders of mangitude (45). The constancy of the apparent activation energy in spite of large variations in selectivity suggests a uniformity in the character of the rate limiting step (in this case the formation of the carbenium ion, step 1 Fig. 2) ; product variations result from subsequent secondary reaction processes which depend on the reaction temperature as well as on the acidity. The selectivity can therefore give interesting information on the acidity of the catalysts provided it is determined at the same temperature. Butane transformation which leads to much less products than that of n-hexane could seem simpler (46-48). However the reactional scheme of this transformation is generally complex involving as a first step butane disproportionation into propane and pentanes, then rapid cracking of pentanes (49). Butane isomerization occurs also by disproportionation (50,51). Fig. 3. shows as an example how propane and isopentane can be formed by n-butane disproportionation. +
2(C-C-C-C
C-C-C-C +
+
C-C=C-C C-C-C-C + C-C-C-C + C-C=C-C ~ 'VCH3,'VH
+
C-C-~-~-C-C
C=~-C-C + C-C-C
+ H+
~
~
RH)
+ H+ +
C-C-~-~-C-C
()t:
> C-~-C-c-C-C
~
+
C-C-C +
C=~-C-C
C
C-~-C-C
C-C-C +
R+
C-c-C-C
~
Fig. 3.
n-Butane disproportionation. Simplified mechanism.
t
Butane disproportionation at 350°C requires extremely strong acid sites (capable of keeping pyridine adsorbed at 550 DC (52)) and therefore could only be employed for characterizing very acid catalysts. Moreover it is more or less certain that this reaction requires, like the disproportionation of aromatics two adjacent acid sites (49).
293
CATALYST CHARACTERIZATION BY USE OF MODEL REACTIONS Choice of the reaction As has been seen, there exists a large variety of reactions which allow the characterization of active sites of acid catalysts. In the reactions that have been exa~ined here, the active sites can be classified in two categories : acidobasic sites and Bronsted acid sites. The Lewis acid sites alone do not seem to be responsible for these reactions but probably could catalyze other reactions. Acidobasic centers catalyze facile reactions such as double-bond shift and cis trans isomerization of olefins whereas Bronsted acid sites are responsible for more difficult transformations. The acid strength that Bronsted acid sites must have to be active depends a great deal on the reaction considered ; thus for catalyzing 3,3-dimethyl-l butene isomerization at 250°C, it is sufficient for the sites to be able to retain pyridine adsorbed at 290°C whereas for catalyzing isobutane disproportionation at 350°C, they must be able to retain it at 550°C (37,52). Moreover, bimolecular reactions of alkane and aromatic disproportionation seem to demand pairs of adjacent Bronsted sites for their catalysis whereas one single Bronsted site is enough for catalyzing monomolecular reactions. The choice of the reaction will depend firstly on the acid strength of the catalyst to be characterized. For acidobasic oxides such as alumina the olefin isomerizations are the reactions the best adapted; the other reactions can be used but radical path transformations can make the interpretation of the results difficult. For highly acid catalysts alkane and aromatic transformations are to be preferred. Isomerization of olefins must be avoided for these compounds can undergo many side reactions - cracking, oligomerization, polymerization, coke formation ... - which can disturb the characterization of the catalysts. Moreover as the reactions will be very fast, they could be limited by transport phenomena. In each of these two series of catalysts the samples to be characterized can differ highly by their acidity. It is hence very interesting to employ the reactant which transforms itself to several parallel or successive reactions demanding sites of different strengths. This is particularly the case for 3,3-dimethyl-l butene whose the rearrangement will be limited to 2,3-dimethylbutenes on slightly acid catalysts but could lead to n-hexenes on highly acid catalysts (15). One can also use two or more reactions belonging to a same class in which the reactant changes systematically in its acid strength requirement. Good examples are cracking of substituted cumenes (29) - or of t-butyl, s-butyl and n-butyl - benzenes (28), isomerization of xylenes and trimethylbenzenes (36), cracking of n-alkanes (49) ... Lastly in the particular case of shape-selective zeolites, it is obvious that the size of the reactants and of the products must be such that the reaction rate will be
294
limited neither by transport phenomena nor by steric constraints in the formation of intermediates or transition states. Method for characterization The essential information for the characterization of active sites is obviously the reaction rate which must be accurately determined {generally in a flow reactor) before the deactivation of the catalyst. As has already been indicated. the right choice of the reaction can often avoid a significant deactivation-by coke. Operating conditions: temperature. partial pressure of reactant ... are of great importance. Lastly by use of pulse technique it is sometimes possible to measure the initial activity of catalysts highly sensitive to deactivation. However it is well known that this technique gives exact values only in the case of linear reactions (53). Moreover the reaction rate must be measured in absence of diffusional limitations in the porous catalyst particle. Diffusion effects can be considered as negligible (54) for isothermal reactions if
where r is the reaction rate (moles/cm 3.sec). c the concentration of the reactant (moles/cm3), R the radius of the catalyst particles (cm), De the effective diffusivity of the reactant in the porous catalyst at the reaction temperature (cm 2/sec). Unfortunately. De is very often difficult to determine. In order to obtain an accurate rate value. high conversions too close to equilibrium must be avoided. In the case of a reversible transformation, it is of interest to study the reaction in a thermodynamically favorable direction; for example the transformation
c-~-c=c
(55)
will be studied in the forward rather than in the reverse direction. Problems created by the acidity characterization of bifunctional catalysts These catalysts which associate a hydrogenating component (metal or sulfur) with an acid support are employed in numerous industrial processes. Their acidity plays a very significant role in their activity and selectivity (see for example ref. 56 for alkane hydrocracking). It is therefore important to be able to characterize the acidity not only before the introduction of hydrogenating
295
component but also on the bifunctional catalyst and if possible in the conditions of its utilisation. Karge et al (57) propose to characterize bifunctional catalyst acidity by ethylbenzene disproportionation. However they found for this reaction an inhibiting effect of hydrogen on the activity of bifunctional zeolitic catalysts which effect was attributed to an acidity decrease. Another interpretation of this inhibiting effect however was proposed (58) : the decrease in activity would be related to a decrease in the concentration of benzylic carbocations (intermediates of the disproportionation) due either to hydrogen (40)
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+ H2
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+ ft
or to a low quantity of branched alkanes produced by ethyl benzene hydrogenation (59) •
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+
c5
~
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At the present time it is difficult to attribute definitely the inhibiting effect of hydrogen to the decrease in acidity or in benzylic carbocation concentration. In n-hexane cracking, the activity of PtHY catalysts, measured in the absence of hydrogen in order to avoid bifunctional catalysis, is definitely greater than that of a platinum-free HY zeolite (60). This can be connected to the formation of a small amount of hexenes by dehydrogenation of n-hexane on platinum sites (61). Indeed it is a well known fact that olefins can increase the alkane cracking rate by increasing the number of chain initiators. One of the significant criteria in the choice of an acidity characterization reaction of bifunctional catalysts will be that reactants and products undergo the least possible side reactions on hydrogenating sites. Poisoning of these sites can be envisaged but then it must be made sure than the acidity will not be affected (60). The problems created by bifunctional catalyst characterization are evidently met again for acid catalysts presenting hydrogenating impurities. This is particularly the case for cracking catalysts which after use contain appreciable amounts of nickel and vanadium. It is therefore always essential to control in detail the reaction selectivity and in particular to take into consideration the formation of any new secondary product and its possible effect on the reaction rate.
296
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