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Acid zeolites as catalysts in organic reactions: condensation of acetophenone with benzene derivatives
i
~
ELSEVIER
APPLIED CATALYSIS A:GENERAL
Applied Catalysis A: General 130 (1995) 5-12
Acid zeolites as catalysts in organic reactions: condensation of acetophenone with benzene derivatives Marfa J. C l i m e n t a, A. Corma b, H. Garcia b,:~, S. Iborra a, J. Primo a lnstituto de Tecnologia Quimica UPV-CSIC, Universidad Polit~cnica de Valencia, Apartado 22012, 46071Valencia, Spain b Departamento de Qufmica, Universidad Polit~cnica de Valencia, Apartado 22012, 46071-Valencia, Spain
Received 22 November 1994; revised 13 May 1995; accepted 13 May 1995
Abstract Acetophenone in aromatic solvents may undergo two different processes in the presence of large pore acid zeolites as catalysts such as Y,/3 and ~. Thus, acetophenone and toluene react with high conversion to give a complex mixture containing predominantly methyl derivatives and positional isomers of 1,1-diarylethanes, together with dypnone, 1,3-diphenylbutadienes and minor amounts of 1,1,1-triarylethanes. The formation of diarylethanes closely resembles the previously reported product distribution pattern for the analogous reaction using aromatic aldehydes. In contrast, in benzene or tetrachloromethane the aldolic condensation of acetophenone to yield dypnone was the major pathway. Although dypnone in toluene also reacts to form diarylethanes, we have established that the reaction with toluene and the aldolic condensation are independent processes. Influence of zeolite crystalline structure and catalyst particle size on the course of the reaction reveal that a severe diffusion restriction is controlling the conversion and the selectivity of this process. Accordingly, ,O was found a very inefficient catalyst, while at similar conversions/3 was much more selective to the less-space demanding dypnone than the Y zeolite. Keywords." Hydroxyalkylation; Aldolic condensation; particle size influence; Zeolite/3; Zeolite f2
I. Introduction We have reported [ 1] that acid faujasites catalyze the hydroxyalkylation of benzaldehyde with substituted benzene derivatives to give rise to complex mixtures of positional isomers of diarylmethanes. Comparison of these results with those * Corresponding author. 0926-860X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved S S D I O 9 2 6 - 8 6 0 X ( 9 5 ) 001 22-0
6
MaNa J. Climent et al./Applied Catalysis A: General 130 (1995) 5-12
attained using AICI3 as catalysts showed that zeolite Y displays a higher selectivity, including shape-selectivity to para substitution. In the present work, we have studied the analogous reaction of acetophenone with benzene and toluene catalyzed by large pore acid zeolites, including Y,/3 and Y2. It can be anticipated that the presence of a methyl group susceptible to undergo an aldolic reaction attached to the carbonyl functionality, must have some influence on the course of the hydroxyalkylation. In fact, there are some precedents in the literature reporting that aldolic condensations of ketones can be catalyzed by modified zeolites [2 ]. Actually, it is known that Y zeolite may promote the aldolic condensation of acetophenone to dypnone with a high selectivity [ 3]. We have already shown that acid faujasite is an efficient catalysts for the Claisen-Schmidt condensation of acetophenone with benzaldehyde in the liquid phase and at moderate temperatures [4]. Herein we report that both types of processes, hydroxyalkylation like benzaldehyde, and aldolic condensation can be observed in the reaction of acetophenone with arenes. The relative ratio depends on the aromatic solvent and on the crystalline structure of the catalyst.
2. Experimental 2.1. Catalysts The HY-100 zeolite was prepared starting from a commercial NaY sample (SK40, Union Carbide) with a 0.25 M aqueous solution of ammonium acetate using a solid-liquid weight ratio of 1:4 at 298 K for 30 min. The resulting zeolite was dried at 383 K for 6 h and deep-bed calcined at 823 K for 3 h. This partially exchanged NaHY was submitted to two additional exchange-calcination treatments first with a 0.40 solution and then with a 0.60 M solution of ammonium acetate following the above experimental procedure. The average crystal size of SK-40, as determined by scanning electron microscopy (SEM) was 0.8/xm. The framework Si/A1 ratio of HY zeolite was 9.2. A NaY sample with the same framework Si-to-Al ratio as SK-40 but with an average crystallite size of 0.30/xm as measured by SEM, was NH2- exchanged at 353 K until all the Na ÷ was removed and then deep-bed calcinated at 823 K. The resulting sample (HYC) ( S i / A I = 6 ) showed a final Na ÷ content of less than 2 wt.-% of the amount of Na in the starting material. H/3 zeolite was prepared starting from TEA-/3 [5] (Si/A1 = 13, crystal size 0.2 /zm) by heating at 773 K in N2 stream, followed by calcination in air at 823 K and twice NH~- exchange and calcination (823 K). Zeolites Hg2 [crystal size: Hg2-1 (0.1 × 0.3 /xm)] polycrystalline aggregates, HO-2 (0.1 × 4.0/xm) prismatic single crystals were kindly provided by Prof. F. Figueras.
Marfa J. Climent et al. /Applied Catalysis A: General 130 (1995) 5-12
~3
~ ~H3 ,h-c-c.-c-P,
PhCOCH 3 H+
Zeol-
Ph r
+
Ph-~>cH-e-cH2
%-/~c._~,? +
2 3a-c
~ \
c.3 +
cs:
Ph
CH3
5 R2
a b
CH3
CH 3
H H
CH 3 H
Scheme 1.
HZSM-5 zeolite was synthesized as reported [6]. After calcination to remove the organic materials, it was exchanged twice with ammonium acetate, followed by calcination at 823 K. The average crystal size was 2/~m. 2.2. R e a c t i o n p r o c e d u r e
The substrate-to-catalyst ratio and the experimental procedure was the most usual for liquid-phase, batch reactions. Activation of the catalyst was performed in situ by heating the solid ( 1.00 g) under vacuum ( 1 Torr approximately) for 3 h. After this time, the system was left at room temperature and then a solution of nitrobenzene ( 100 mg) as internal standard and the acetophenone (200 mg) in toluene (50 ml) as solvent was poured onto the activated catalyst. The resulting suspension was magnetically stirred at reflux temperature for 20 h. At the end of the reaction the catalyst was filtered, washed with dichloromethane, the organic solution concentrated in vacuum, and the residue weighed and analyzed by gas chromatography-mass spectrometry ( G C - M S ) using a Hewlett-Packard 5988A spectrometer provided with a 25 m capillary column of cross linked 5% phenylmethylsilicone. The 1H-NMR analysis was carried out with a 60 MHz Varian 360 EM-spectrometer in tetrachloromethane as solvent and TMS as internal standard or alternatively using a 400 MHz Varian VXR-400 S spectrometer in deuterated trichloromethane as solvent and TMS as intemal standard. The zeolite was submitted to continuous solid-liquid extraction with dichloromethane using micro-Soxhlet equipment. After removal of the solvent the residue was also weighed and analyzed by G C - M S and ' H-NMR spectroscopy. In all cases the recovered material accounted for more than 90% of the starting acetophenone. Isolation of pure compounds was accomplished by high performance liquid chromatography using an isocratic Waters apparatus provided with a semipreparative Microporasil g column and mixtures of hexane-ethyl acetate as eluent. The chemical structures of all the products obtained in this study are presented in Scheme 1 and Scheme 3 and were in agreement with the spectroscopic data.
3. Results and discussion When acetophenone was reacted with toluene on the HY-100 zeolite at 115°C a complicated reaction mixture of more than ten products including positional iso-
8
Maria J, Climent et al./Applied Catalysis A: General 130 (1995) 5-12
Table 1 Results of the reaction of acetophenone (200 mg) in toluene (50 ml) at 110°C in the presence of acid zeolites ( 1.00 g) as catalysts Catalyst
HY-100 Hfl HYC Hg2-1 Hg2-2 HZSM-5
Total product yield ( % )
Product distribution (%)
76 35 85 2 9 <2
2
3a (op:ppY
3b
3c
4
2 28 16 2 8
30(7:23) 7(0:7) 33(6:27)
20 l 20
5
13
3
13
5
1(0:1)
The values in brackets indicate the contribution of the ortho-para (op) and para-para (pp) positional isomers of 3a.
mers was obtained (Scheme 1 and Table 1). The main reaction products that could be characterized can be classified into four groups of compounds: dypnone (2), 1,1-diarylethanes (3), 1,3-diphenylbutadienes (4) and 1,1,1-triarylethanes (5). The turnover numbers (calculated as the initial rate divided by the amount of acid sites per gram of catalyst, mmol × h - 1 x g - ' ) measured using HY-100 were 2.50, 1.64 and 0.23 for compounds 2, 3, and 4, respectively. When the yields of the different products were plotted versus the total conversion (Fig. 1) diarylethanes (3) appeared as primary products, trans-dypnoneshowed a primary but unstable nature, while diphenylbutadienes (4) are secondary unstable, and the triarylethanes are secondary products. According to the general reaction mechanism accepted for the hydroxyalkylation of activated arenes with carbonyl compounds catalyzed by conventional acids, [ 7 ] acetophenone and toluene should give rise 1,1-diarylethanols (6) as reaction intermediates (Scheme 2). These tertiary alcohols (6) would be quite unstable in the presence of zeolite HY-100 and would react further to give the more stabilized 1,1-diarylethylium 50 Yield(%) 40-
30 -
20 -
10-
0T 0
20
40
Total Conversion
60
80
(%)
Fig. 1. Yield-total conve~ion plot ~ r ~ e reaction of acetophenone (1) with toluene in the presence of HY-100: 2 ( O ) , 3( x ), 4 ( O ) , 5 ( A ) .
Maria J. Climent et al. / Applied Catalysis A: General 130 (1995) 5-12
o
%.
lJ
Ph-C-CH3
,t
~÷
~
Ph_CH3 [
OH
CH3 ]
II
Ph-C-CH3
-~÷
Ic
1%__¢
CH 3
I
.o
J -n 2
6 H +
CH3
CH3
Ctt 3 3b
CH3 7
Iprotolyticcleavage
_ H + ~h-oH3
~cH3
,CH3
protolytic P CH3
CH3
cleavage CH3
CH~ 5
3a
S c h e m e 2.
cations (7). Precedents concerning the formation of intermediates 7 on acid solids by protonation of 1, 1-diarylethylene is well documented [ 8 ]. These carbocations can now desorb through a hydride transfer process giving diarylethanes (3), which will be accounted as primary products. Alternatively, carbenium ions 7 can alkylate a second toluene molecule to give the triarylethanes
<5). A striking feature of the product distribution is the formation as apparent primary products of ditolylethanes (3a). In order to explain the formation of these ditolyl derivatives one must assume that the triarylethanes (5) can suffer a protolytic cleavage of the C-Ph bond (see Scheme 2). A similar process has been proposed earlier to account for the formation of triphenylmethylium cation after adsorption of triphenylmethane on silica-alumina [9]. The most intriguing fact is that 3 appear as primary products while chemically it is clear that a series of consecutive reactions should be involved in their formation from secondary triarylethanes. This apparent contradiction can be explained assuming in this case a disguised kinetics, probably caused by the much slower diffusion through the zeolite micropores of the larger triarylethanes relative to the diarylethanes. This restricted diffusion would also justify why the amount of triarylethanes detected is too small to account for the chemical yields observed for products 3. It is, however, possibly an alternative mechanism which would involve dypnone as the key intermediate. Indeed, when aromatic hydrocarbons are absent from the reaction mixture acetophenone might be able under these experimental conditions to undergo an aldolic condensation producing dypnone as a primary product. In order to address this possibility acetophenone was reacted in CC14and in benzene at reflux temperature. The results obtained are given in Scheme 3. As a
10
Mar(a J. Climent et al. /Applied Catalysis A: General 130 (1995) 5-12 O
H + Zeol-
II Ph-C-CB3
~I
O CH3 II Ph-C-CH=C-Ph-
~
H ÷ Zeol-
¢~/~Ph
1= -H20
1
2
solvent
Dypnone(2)
8
~-Phenylnaphthalene(8)
CC14
37
4
C6H6
22
5
Scheme 3.
matter of fact, trans and cis dypnone together with/3-phenylnaphthalene (8) were formed. Results from Fig. 2 show that, as expected, dypnone 2 are primary and unstable products, while naphthalene 8 is a secondary stable product. This could indicate that naphthalene 8 is formed through an intramolecular acid-catalyzed cyclization of dypnone 2 (Scheme 3). We were interested in disclosing if dypnone when dissolved in toluene can give rise to a similar product distribution as that observed from acetophenone under the same conditions. In order to see this, an experiment was carried out using dypnone (0.20 g) and toluene (50 ml) as reactants on HY-100 zeolite ( 1.00 g) as catalyst. Practically the same complex reaction mixture was obtained starting from dypnone or from acetophenone. This observation raises the question if dypnone can be the key intermediate in the formation of products 3-5. However, the fact that dypnone is rapidly transformed back into acetophenone when the former reacts in the presence of HY zeolite with a deactivated aromatic compound such as chlorobenzene instead of toluene rules out this possibility. Finally a small degree of methyl migration between toluene and the reaction products was established by treating diphenylmethane with toluene in the presence of HY-100, whereby appreciable amounts of phenyltolymethane were formed. This transmethylation process would lead to some level of interconversion between 30 Yield(%) 20
10
0
v
0
.
10
20
Time (h)
Fig. 2. Yield of dypnone (2) ( • ) and/3-phenylnapthalene (8) (O) versus the reaction time when acetophenone (200 mg) is heated in C6H6 at 78°C in the presence of HY-100.
Marfa J. Climent et al. /Applied Catalysis A: General 130 (1995) 5-12
11
diaryl and triarylethanes 3 and 5 as well as would explain the formation of small amounts of 1,1-diphenylethane (3c) [ 10].
3.1. Influence of zeolite structure The reaction mechanism given in Scheme 2 involves, in several cases, relatively bulky intermediates. Thus, one may expect that the geometrical constraints imposed by different zeolite geometries can have a strong impact on the reaction selectivity. In order to see this, the reaction between acetophenone and toluene was carried out using a series of 12 membered ring (MR) zeolites with unidirectional as well as tridirectional systems of channels, and a 10 MR zeolite with a MFI structure. The results from Table 1 show that HY-100 is the most active zeolite, followed by the other tridirectional 12 MR zeolite Hfl, and then by unidirectional H/2. Finally, it was found that the geometrical constraints presented by HZSM-5 did not allow any of the bimolecular reactions occurring in this process to take place, and no products were detected. The fact that the conversion on H/2 zeolite formed by polycrystalline aggregates (H/2-1) was comparatively much smaller than either on H/3 or HY-100 is an indication that there is a strong diffusion resistance in the pores of the zeolite. In order to check this, the reaction was also carried out using H/2-2 sample formed by prismatic single crystals. It has been shown [ 11 ] that the rate of diffusion of a given molecule is two orders of magnitude larger on this type of samples (H/2-2) than on the polycrystalline aggregates (H/2-1) and therefore, a higher conversion should be expected for the former catalyst. As a matter of fact, the results obtained here (Table 1 ) agree with the hypothesis of a severe diffusion control for unidirectional/2 zeolites. In the case of tridirectional Y zeolites, we have seen that some kind of diffusion control does also exist. Indeed the conversion obtained on the HY- 100 with average crystal size of 0.80/xm, was lower than that obtained on a HYC zeolite with similar framework Si/A1 ratio but with an average crystallite size of 0.30/zm. The diffusion restrictions should be maximum for products triarylmethanes or ethanes which, in addition, are known to be prone to generate very stable triarylmethylium cations under strong acidic conditions. In fact, we have been able to synthesise and characterize these types of triaryl cations encapsulated within zeolites Y and/3 by reaction of benzaldehydes with aromatic compounds [ 12]. These data together with molecular modeling simulation strongly suggest that triarylethanes should be extremely immobilized within the cages of tridirectional Y and/3 zeolites. The amounts observed here on the liquid phase must correspond to the very minor fraction formed in the external surface of the crystallites. From the point of view of the influence of zeolite structure on the selectivities to the different products, it becomes difficult to compare the results from Table 1 since the total conversion obtained on the different zeolite structures are quite different. Of course, in order to get reliable conclusions about the product selectivity
12
Mar(a J. Climent et al. /Applied Catalysis A: General 130 (1995) 5-12
of different catalysts, product distributions at the same level of conversion have to be compared. This is of particular importance in a reaction system like the one studied here where a series of consecutive reactions occurs. According to Fig. 1, where the yields at different levels of conversion obtained on the HY-100 zeolite were given, it can be seen that for a conversion of 35%, H/3 zeolite is much more selective for the formation of dypnone. Formation of products involving bulkier transition states and leading to compounds 3 and 5 appears to be disfavored in H/3. Notice that the ortho isomers the 3a were not detected. These results must be a direct consequence of the smaller space available in the micropores of the H/3. This effect is also seen using HI2 as catalyst, since in this case only very minor amounts of 3a already found on HY-100 at 10% conversion.
4. Conclusions The predominant formation of diarylethanes in the reaction of acetophenone with toluene catalyzed by HY zeolites closely resembles the product distribution pattern previously reported for the analogous reaction using benzaldehyde as carbonylic compound [ 1 ]. The aldolic condensation of acetophenone was the major reaction pathway only in tetrachloromethane, benzene. It was found that these two different processes are independent. Finally, the influence of the crystalline structure of the zeolite and the particle size on the conversion and selectivity evidence the strong diffusion limitations imposed by the crystalline framework to these bulky products.
Acknowledgements Financial support by the European Commission within the program Human Capital and Mobility (Grant ERB4050PL93-2451 ) is gratefully acknowledged.
References [ 1] M.J. Climent, A. Corma, H. Garcia and J. Primo, J. Catal., 130 ( 1991 ) 138. [2] G.P. Hagen, U.S. Pat., 4 433 174 (1984). [3] P.B. Venuto, Microporous Mater., 2 (1994) 297. [4] M.J. Climent, A. Corma, H. Garcia and J. Primo, Catal. Lett., 4 (1990) 85. [5] J. P6rez-Pariente, J. Martens and P.A. Jacobs, Appl. Catal., 31 (1987) 35. [6] R.J. Argauer and G.R. Landolt, U.S. Pat., 3 702 886 (1982). [7] J. March, Advanced Organic Chemistry. Reactions, Mechanisms and Structures, McGraw-Hill, New York, 3rd edn., 1983. [8] W.K. Hall, J. Catal., 1 (1962) 53. [9] C.Y. Wu, W.K. Hall, J. Catal., (1967) 394, [ 10] P. Magnoux, C. Canaff, F. Machado and M. Guisnet, J. Catal., 134 (1992) 286. [ 11 ] B. Chauvin, F. Fajula, F. Figueras, C. Gueguen and J. Bousquet, J. Catal., 111 (1988) 94. [ 12] M.L. Cano, A. Corma, H. Garcia and M.A. Miranda, submitted for publication.
~
ELSEVIER
APPLIED CATALYSIS A:GENERAL
Applied Catalysis A: General 130 (1995) 5-12
Acid zeolites as catalysts in organic reactions: condensation of acetophenone with benzene derivatives Marfa J. C l i m e n t a, A. Corma b, H. Garcia b,:~, S. Iborra a, J. Primo a lnstituto de Tecnologia Quimica UPV-CSIC, Universidad Polit~cnica de Valencia, Apartado 22012, 46071Valencia, Spain b Departamento de Qufmica, Universidad Polit~cnica de Valencia, Apartado 22012, 46071-Valencia, Spain
Received 22 November 1994; revised 13 May 1995; accepted 13 May 1995
Abstract Acetophenone in aromatic solvents may undergo two different processes in the presence of large pore acid zeolites as catalysts such as Y,/3 and ~. Thus, acetophenone and toluene react with high conversion to give a complex mixture containing predominantly methyl derivatives and positional isomers of 1,1-diarylethanes, together with dypnone, 1,3-diphenylbutadienes and minor amounts of 1,1,1-triarylethanes. The formation of diarylethanes closely resembles the previously reported product distribution pattern for the analogous reaction using aromatic aldehydes. In contrast, in benzene or tetrachloromethane the aldolic condensation of acetophenone to yield dypnone was the major pathway. Although dypnone in toluene also reacts to form diarylethanes, we have established that the reaction with toluene and the aldolic condensation are independent processes. Influence of zeolite crystalline structure and catalyst particle size on the course of the reaction reveal that a severe diffusion restriction is controlling the conversion and the selectivity of this process. Accordingly, ,O was found a very inefficient catalyst, while at similar conversions/3 was much more selective to the less-space demanding dypnone than the Y zeolite. Keywords." Hydroxyalkylation; Aldolic condensation; particle size influence; Zeolite/3; Zeolite f2
I. Introduction We have reported [ 1] that acid faujasites catalyze the hydroxyalkylation of benzaldehyde with substituted benzene derivatives to give rise to complex mixtures of positional isomers of diarylmethanes. Comparison of these results with those * Corresponding author. 0926-860X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved S S D I O 9 2 6 - 8 6 0 X ( 9 5 ) 001 22-0
6
MaNa J. Climent et al./Applied Catalysis A: General 130 (1995) 5-12
attained using AICI3 as catalysts showed that zeolite Y displays a higher selectivity, including shape-selectivity to para substitution. In the present work, we have studied the analogous reaction of acetophenone with benzene and toluene catalyzed by large pore acid zeolites, including Y,/3 and Y2. It can be anticipated that the presence of a methyl group susceptible to undergo an aldolic reaction attached to the carbonyl functionality, must have some influence on the course of the hydroxyalkylation. In fact, there are some precedents in the literature reporting that aldolic condensations of ketones can be catalyzed by modified zeolites [2 ]. Actually, it is known that Y zeolite may promote the aldolic condensation of acetophenone to dypnone with a high selectivity [ 3]. We have already shown that acid faujasite is an efficient catalysts for the Claisen-Schmidt condensation of acetophenone with benzaldehyde in the liquid phase and at moderate temperatures [4]. Herein we report that both types of processes, hydroxyalkylation like benzaldehyde, and aldolic condensation can be observed in the reaction of acetophenone with arenes. The relative ratio depends on the aromatic solvent and on the crystalline structure of the catalyst.
2. Experimental 2.1. Catalysts The HY-100 zeolite was prepared starting from a commercial NaY sample (SK40, Union Carbide) with a 0.25 M aqueous solution of ammonium acetate using a solid-liquid weight ratio of 1:4 at 298 K for 30 min. The resulting zeolite was dried at 383 K for 6 h and deep-bed calcined at 823 K for 3 h. This partially exchanged NaHY was submitted to two additional exchange-calcination treatments first with a 0.40 solution and then with a 0.60 M solution of ammonium acetate following the above experimental procedure. The average crystal size of SK-40, as determined by scanning electron microscopy (SEM) was 0.8/xm. The framework Si/A1 ratio of HY zeolite was 9.2. A NaY sample with the same framework Si-to-Al ratio as SK-40 but with an average crystallite size of 0.30/xm as measured by SEM, was NH2- exchanged at 353 K until all the Na ÷ was removed and then deep-bed calcinated at 823 K. The resulting sample (HYC) ( S i / A I = 6 ) showed a final Na ÷ content of less than 2 wt.-% of the amount of Na in the starting material. H/3 zeolite was prepared starting from TEA-/3 [5] (Si/A1 = 13, crystal size 0.2 /zm) by heating at 773 K in N2 stream, followed by calcination in air at 823 K and twice NH~- exchange and calcination (823 K). Zeolites Hg2 [crystal size: Hg2-1 (0.1 × 0.3 /xm)] polycrystalline aggregates, HO-2 (0.1 × 4.0/xm) prismatic single crystals were kindly provided by Prof. F. Figueras.
Marfa J. Climent et al. /Applied Catalysis A: General 130 (1995) 5-12
~3
~ ~H3 ,h-c-c.-c-P,
PhCOCH 3 H+
Zeol-
Ph r
+
Ph-~>cH-e-cH2
%-/~c._~,? +
2 3a-c
~ \
c.3 +
cs:
Ph
CH3
5 R2
a b
CH3
CH 3
H H
CH 3 H
Scheme 1.
HZSM-5 zeolite was synthesized as reported [6]. After calcination to remove the organic materials, it was exchanged twice with ammonium acetate, followed by calcination at 823 K. The average crystal size was 2/~m. 2.2. R e a c t i o n p r o c e d u r e
The substrate-to-catalyst ratio and the experimental procedure was the most usual for liquid-phase, batch reactions. Activation of the catalyst was performed in situ by heating the solid ( 1.00 g) under vacuum ( 1 Torr approximately) for 3 h. After this time, the system was left at room temperature and then a solution of nitrobenzene ( 100 mg) as internal standard and the acetophenone (200 mg) in toluene (50 ml) as solvent was poured onto the activated catalyst. The resulting suspension was magnetically stirred at reflux temperature for 20 h. At the end of the reaction the catalyst was filtered, washed with dichloromethane, the organic solution concentrated in vacuum, and the residue weighed and analyzed by gas chromatography-mass spectrometry ( G C - M S ) using a Hewlett-Packard 5988A spectrometer provided with a 25 m capillary column of cross linked 5% phenylmethylsilicone. The 1H-NMR analysis was carried out with a 60 MHz Varian 360 EM-spectrometer in tetrachloromethane as solvent and TMS as internal standard or alternatively using a 400 MHz Varian VXR-400 S spectrometer in deuterated trichloromethane as solvent and TMS as intemal standard. The zeolite was submitted to continuous solid-liquid extraction with dichloromethane using micro-Soxhlet equipment. After removal of the solvent the residue was also weighed and analyzed by G C - M S and ' H-NMR spectroscopy. In all cases the recovered material accounted for more than 90% of the starting acetophenone. Isolation of pure compounds was accomplished by high performance liquid chromatography using an isocratic Waters apparatus provided with a semipreparative Microporasil g column and mixtures of hexane-ethyl acetate as eluent. The chemical structures of all the products obtained in this study are presented in Scheme 1 and Scheme 3 and were in agreement with the spectroscopic data.
3. Results and discussion When acetophenone was reacted with toluene on the HY-100 zeolite at 115°C a complicated reaction mixture of more than ten products including positional iso-
8
Maria J, Climent et al./Applied Catalysis A: General 130 (1995) 5-12
Table 1 Results of the reaction of acetophenone (200 mg) in toluene (50 ml) at 110°C in the presence of acid zeolites ( 1.00 g) as catalysts Catalyst
HY-100 Hfl HYC Hg2-1 Hg2-2 HZSM-5
Total product yield ( % )
Product distribution (%)
76 35 85 2 9 <2
2
3a (op:ppY
3b
3c
4
2 28 16 2 8
30(7:23) 7(0:7) 33(6:27)
20 l 20
5
13
3
13
5
1(0:1)
The values in brackets indicate the contribution of the ortho-para (op) and para-para (pp) positional isomers of 3a.
mers was obtained (Scheme 1 and Table 1). The main reaction products that could be characterized can be classified into four groups of compounds: dypnone (2), 1,1-diarylethanes (3), 1,3-diphenylbutadienes (4) and 1,1,1-triarylethanes (5). The turnover numbers (calculated as the initial rate divided by the amount of acid sites per gram of catalyst, mmol × h - 1 x g - ' ) measured using HY-100 were 2.50, 1.64 and 0.23 for compounds 2, 3, and 4, respectively. When the yields of the different products were plotted versus the total conversion (Fig. 1) diarylethanes (3) appeared as primary products, trans-dypnoneshowed a primary but unstable nature, while diphenylbutadienes (4) are secondary unstable, and the triarylethanes are secondary products. According to the general reaction mechanism accepted for the hydroxyalkylation of activated arenes with carbonyl compounds catalyzed by conventional acids, [ 7 ] acetophenone and toluene should give rise 1,1-diarylethanols (6) as reaction intermediates (Scheme 2). These tertiary alcohols (6) would be quite unstable in the presence of zeolite HY-100 and would react further to give the more stabilized 1,1-diarylethylium 50 Yield(%) 40-
30 -
20 -
10-
0T 0
20
40
Total Conversion
60
80
(%)
Fig. 1. Yield-total conve~ion plot ~ r ~ e reaction of acetophenone (1) with toluene in the presence of HY-100: 2 ( O ) , 3( x ), 4 ( O ) , 5 ( A ) .
Maria J. Climent et al. / Applied Catalysis A: General 130 (1995) 5-12
o
%.
lJ
Ph-C-CH3
,t
~÷
~
Ph_CH3 [
OH
CH3 ]
II
Ph-C-CH3
-~÷
Ic
1%__¢
CH 3
I
.o
J -n 2
6 H +
CH3
CH3
Ctt 3 3b
CH3 7
Iprotolyticcleavage
_ H + ~h-oH3
~cH3
,CH3
protolytic P CH3
CH3
cleavage CH3
CH~ 5
3a
S c h e m e 2.
cations (7). Precedents concerning the formation of intermediates 7 on acid solids by protonation of 1, 1-diarylethylene is well documented [ 8 ]. These carbocations can now desorb through a hydride transfer process giving diarylethanes (3), which will be accounted as primary products. Alternatively, carbenium ions 7 can alkylate a second toluene molecule to give the triarylethanes
<5). A striking feature of the product distribution is the formation as apparent primary products of ditolylethanes (3a). In order to explain the formation of these ditolyl derivatives one must assume that the triarylethanes (5) can suffer a protolytic cleavage of the C-Ph bond (see Scheme 2). A similar process has been proposed earlier to account for the formation of triphenylmethylium cation after adsorption of triphenylmethane on silica-alumina [9]. The most intriguing fact is that 3 appear as primary products while chemically it is clear that a series of consecutive reactions should be involved in their formation from secondary triarylethanes. This apparent contradiction can be explained assuming in this case a disguised kinetics, probably caused by the much slower diffusion through the zeolite micropores of the larger triarylethanes relative to the diarylethanes. This restricted diffusion would also justify why the amount of triarylethanes detected is too small to account for the chemical yields observed for products 3. It is, however, possibly an alternative mechanism which would involve dypnone as the key intermediate. Indeed, when aromatic hydrocarbons are absent from the reaction mixture acetophenone might be able under these experimental conditions to undergo an aldolic condensation producing dypnone as a primary product. In order to address this possibility acetophenone was reacted in CC14and in benzene at reflux temperature. The results obtained are given in Scheme 3. As a
10
Mar(a J. Climent et al. /Applied Catalysis A: General 130 (1995) 5-12 O
H + Zeol-
II Ph-C-CB3
~I
O CH3 II Ph-C-CH=C-Ph-
~
H ÷ Zeol-
¢~/~Ph
1= -H20
1
2
solvent
Dypnone(2)
8
~-Phenylnaphthalene(8)
CC14
37
4
C6H6
22
5
Scheme 3.
matter of fact, trans and cis dypnone together with/3-phenylnaphthalene (8) were formed. Results from Fig. 2 show that, as expected, dypnone 2 are primary and unstable products, while naphthalene 8 is a secondary stable product. This could indicate that naphthalene 8 is formed through an intramolecular acid-catalyzed cyclization of dypnone 2 (Scheme 3). We were interested in disclosing if dypnone when dissolved in toluene can give rise to a similar product distribution as that observed from acetophenone under the same conditions. In order to see this, an experiment was carried out using dypnone (0.20 g) and toluene (50 ml) as reactants on HY-100 zeolite ( 1.00 g) as catalyst. Practically the same complex reaction mixture was obtained starting from dypnone or from acetophenone. This observation raises the question if dypnone can be the key intermediate in the formation of products 3-5. However, the fact that dypnone is rapidly transformed back into acetophenone when the former reacts in the presence of HY zeolite with a deactivated aromatic compound such as chlorobenzene instead of toluene rules out this possibility. Finally a small degree of methyl migration between toluene and the reaction products was established by treating diphenylmethane with toluene in the presence of HY-100, whereby appreciable amounts of phenyltolymethane were formed. This transmethylation process would lead to some level of interconversion between 30 Yield(%) 20
10
0
v
0
.
10
20
Time (h)
Fig. 2. Yield of dypnone (2) ( • ) and/3-phenylnapthalene (8) (O) versus the reaction time when acetophenone (200 mg) is heated in C6H6 at 78°C in the presence of HY-100.
Marfa J. Climent et al. /Applied Catalysis A: General 130 (1995) 5-12
11
diaryl and triarylethanes 3 and 5 as well as would explain the formation of small amounts of 1,1-diphenylethane (3c) [ 10].
3.1. Influence of zeolite structure The reaction mechanism given in Scheme 2 involves, in several cases, relatively bulky intermediates. Thus, one may expect that the geometrical constraints imposed by different zeolite geometries can have a strong impact on the reaction selectivity. In order to see this, the reaction between acetophenone and toluene was carried out using a series of 12 membered ring (MR) zeolites with unidirectional as well as tridirectional systems of channels, and a 10 MR zeolite with a MFI structure. The results from Table 1 show that HY-100 is the most active zeolite, followed by the other tridirectional 12 MR zeolite Hfl, and then by unidirectional H/2. Finally, it was found that the geometrical constraints presented by HZSM-5 did not allow any of the bimolecular reactions occurring in this process to take place, and no products were detected. The fact that the conversion on H/2 zeolite formed by polycrystalline aggregates (H/2-1) was comparatively much smaller than either on H/3 or HY-100 is an indication that there is a strong diffusion resistance in the pores of the zeolite. In order to check this, the reaction was also carried out using H/2-2 sample formed by prismatic single crystals. It has been shown [ 11 ] that the rate of diffusion of a given molecule is two orders of magnitude larger on this type of samples (H/2-2) than on the polycrystalline aggregates (H/2-1) and therefore, a higher conversion should be expected for the former catalyst. As a matter of fact, the results obtained here (Table 1 ) agree with the hypothesis of a severe diffusion control for unidirectional/2 zeolites. In the case of tridirectional Y zeolites, we have seen that some kind of diffusion control does also exist. Indeed the conversion obtained on the HY- 100 with average crystal size of 0.80/xm, was lower than that obtained on a HYC zeolite with similar framework Si/A1 ratio but with an average crystallite size of 0.30/zm. The diffusion restrictions should be maximum for products triarylmethanes or ethanes which, in addition, are known to be prone to generate very stable triarylmethylium cations under strong acidic conditions. In fact, we have been able to synthesise and characterize these types of triaryl cations encapsulated within zeolites Y and/3 by reaction of benzaldehydes with aromatic compounds [ 12]. These data together with molecular modeling simulation strongly suggest that triarylethanes should be extremely immobilized within the cages of tridirectional Y and/3 zeolites. The amounts observed here on the liquid phase must correspond to the very minor fraction formed in the external surface of the crystallites. From the point of view of the influence of zeolite structure on the selectivities to the different products, it becomes difficult to compare the results from Table 1 since the total conversion obtained on the different zeolite structures are quite different. Of course, in order to get reliable conclusions about the product selectivity
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of different catalysts, product distributions at the same level of conversion have to be compared. This is of particular importance in a reaction system like the one studied here where a series of consecutive reactions occurs. According to Fig. 1, where the yields at different levels of conversion obtained on the HY-100 zeolite were given, it can be seen that for a conversion of 35%, H/3 zeolite is much more selective for the formation of dypnone. Formation of products involving bulkier transition states and leading to compounds 3 and 5 appears to be disfavored in H/3. Notice that the ortho isomers the 3a were not detected. These results must be a direct consequence of the smaller space available in the micropores of the H/3. This effect is also seen using HI2 as catalyst, since in this case only very minor amounts of 3a already found on HY-100 at 10% conversion.
4. Conclusions The predominant formation of diarylethanes in the reaction of acetophenone with toluene catalyzed by HY zeolites closely resembles the product distribution pattern previously reported for the analogous reaction using benzaldehyde as carbonylic compound [ 1 ]. The aldolic condensation of acetophenone was the major reaction pathway only in tetrachloromethane, benzene. It was found that these two different processes are independent. Finally, the influence of the crystalline structure of the zeolite and the particle size on the conversion and selectivity evidence the strong diffusion limitations imposed by the crystalline framework to these bulky products.
Acknowledgements Financial support by the European Commission within the program Human Capital and Mobility (Grant ERB4050PL93-2451 ) is gratefully acknowledged.
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