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Zeolite beta: selective molecular sieve for synthesis of xylenes from trimethylbenzenes
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
2627
Zeolite Beta: Selective molecular sieve for synthesis of xylenes from trimethylbenzenes Jifi (~ejkaI and Andrea Krej~,i1'2 1j. Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejgkova 3, CZ-182 23 Prague 8, Czech Republic 2Department of Organic Technology, Institute of Chemical Technology, Technick/l 5, CZ- 166 28 Prague 6, Czech Republic The impact of channel geometry of zeolite Beta on the activity and selectivity in trimethylbenzenes disproportionation and their transalkylation with toluene was investigated. Both reactions are strongly controlled by the transport of bulky trimethylbenzene reactants and tetramethylbenzene products which are accumulated in the inner channel system of zeolite Beta. These adsorbed molecules block the access of toluene reactant molecules which decreases the xylene yield for longer time-on-stream values. The addition of toluene to the reaction feed dramatically decreases the rate of trimethylbenzene dealkylation. The equimolar ratio of trimethylbenzenes to toluene leads to the highest xylenes yield in transalkylation reaction. 1. INTRODUCTION Zeolite molecular sieves have been recognized as unique catalysts for both acid-base and redox catalyzed reactions and number of their industrial applications is continuously increasing. Various transformations of organic compounds are intensively investigated over zeolites and other crystalline microporous molecular sieves due to the increasing number of their well-defined pore structures, their shape selective properties, the possibility of tailoring their chemical and structural properties and environmental tolerance [ 1-4]. The understanding of the mechanisms of zeolite synthesis [5,6] together with a knowledge of reaction mechanisms in transformations of various hydrocarbons [2,3,7,8] significantly increases our possibilities to choose or even rationally design a proper catalyst for a given organic reaction. Although the aromatic chemistry on zeolites is well-established [9] and many reactions have been industrially used, the transformation of C9 aromatic hydrocarbons represents another challenge in zeolite catalysis in the synthesis of more demanded products, namely xylenes. It is evident that particularly transalkylation processes are attractive for those refineries possessing BTX extraction units, where excessive C9+ aromatic hydrocarbons are produced. In our study we have used 1,2,4-trimethylbenzene (1,2,4TMB) as a model reactant as all three TMB isomers are always in thermodynamic equilibrium with about 65 % of this isomer [ 10]. This contribution is focused on the utilization of zeolite Beta for the selective synthesis of xylenes from trimethylbenzenes via their disproportionation and transalkylation with toluene. The effect of molecular sieve channel structure and acid number is presented.
2628 2. E X P E R I M E N T A L
Zeolite Beta with Si/AI ratio ranging from 12.5 to 30, synthesized in our laboratory from Cab-O-Sil M5, sodium aluminate and sodium hydroxide using tetraethylammonium hydroxide or bromide as organic template, was used in this study. XRD diffraction patterns (Siemens D5005), FTIR spectroscopy of skeletal vibrations (Nieolet Magna 550) and SEM (Jeol) confirmed a high crystallinity and phase purity of these zeolites possessing the crystals size lower than 0.5 ~tm. For comparison zeolite Y supplied by the Research Institute for Oil and Hydrocarbon Gases, Slovak Republic, and mordenite purchased from PQ Corporation, USA, were used. The acid strength and number of OH groups were determined by TPD of ammonia and FTIR spectroscopy of adsorbed d3-acetonitrile providing quantitative information on the number of Bvoensted and Lewis sites. Disproportionation of trimethylbenzenes (TMB) and their reaction with toluene were performed in a glass down-flow mieroreaetor under atmospheric pressure at the temperature range of 473 - 723 K with WHSV ranging from 0.35 to 15 h -~. TMBs and/or toluene were fed into the reactor using saturators and hydrogen as carrier gas with concentration of TMB = 5.2 vol. % (disproportionation) and 4.5 vol. % (reaction -with toluene). TMB/toluene molar ratios ranging from 0.4 to 2.5 were investigated. The reaction products were analyzed using an "on-line" gas chromatograph (Hewlett Packard 5890 Series II) equipped with FID and MSD detectors and a high-resolution capillary column (Supelcowax 10, 30 m long). The first analysis was performed after 15 minutes of time-on-stream (T-O-S) with others followed with 40 minutes interval. 3. RESULTS AND DISCUSSION In our previous paper [11] we have shown that TMB molecules are too bulky to penetrate into the 10-member ring channels of medium-pore zeolites and their transformations can proceed only on their "external" surface. In contrast, for large pore zeolites higher temperatures and longer contact times significantly increased the rate of undesirable dealkylation at the expense of disproportionation. Dealkylation and deactivation were promoted by the more open structure exhibiting large cavities in the case of zeolite Y and by a high acid strength and only one-dimensional channels of mordenite. 3.1.1,2,4-Trimethylbenzene disproportionation The broad spectrum of reaction products is obtained in 1,2,4-TMB transformation due to the complexity of the reaction pathways starting from 1,2,4-TMB. These reactions include disproportionation, isomerization and, particularly at temperatures above 673 K, also dealkylation. The temperature dependence of selectivities to disproportionation, isomerization and dealkylation products is depicted in Figure 1. It is clearly seen, that in the reaction temperature range studied both disproportionation and isomerization proceeds easily already from 473 K. On the other hand, dealkylation is minimized up to 623 K. As we have found almost the same results for both 1,2,4-TMB and 1,3,5-TMB [11] under all reaction conditions used it indicates that fast isomerization of TMB isomers represents the first step of the reaction mechanism. This is followed by the consecutive reactions such as disproportionation and/or dealkylation. The maximum of the rate of TMB disproportionation is located between 573 and 673 K with the significant competition of dealkylation at the higher temperature of this interval, especially for short T-O-S values.
2629 80 I
=~
m
60
=.==
o
E
._~
40
.>_
~
!
_.e t~ 20
! I
0
, 450
500
-
, 550
-
, 600
, 650
, 700
i 750
Temperature [K] Figure 1 Temperature dependence of selectivities to disproportionation (11), isomerization ( , ) and dealkylation ( e ) in 1,2,4-TMB transformation over zeolite Beta (Si/Al- 12.5, WHSV = 5 hl, T-O-S = 55 min, TMB conversion- 30.2 % at 473 K, 46.6 % at 523 K, 51.7-56.4 % in the range 573-723 K). It is well known that synthesis and transformations of xylenes are strongly influenced by transport of reactants and products over ZSM-5 zeolite and it seems that similar situation is also in the case of transformations of trimethylbenzenes over large pore zeolites. Although xylene to tetramethylbenzene molar ratio (X/TeMB) should be one in the case of bimolecular TMB transalkylation, this ratio changes significantly for different reaction temperatures (cf. Figure 2). While at temperatures around 523 and 573 K higher concentrations of tetramethylbenzenes have been found, with increasing reaction temperature xylenes started to dominate. At lower temperatures it is strongly connected with a slow diffusivity of larger tri- and particularly tetramethylbenzenes which are accumulated in the channel system of zeolite Beta. This significantly changes the concentration of aromates in the channels and it is assumed that the methyl transfer between adsorbed trimethylbenzenes and tetramethylbenzenes proceeds in a larger extent. Thus, this leads to the preferential formation of tetramethylbenzenes at the account of xylenes. On the other hand, an increase in X/TeMB ratio at reaction temperatures of 673 and particularly 723 K is strongly influenced by the significant rate of TMB dealkylation, which favours the formation of xylenes. Only small differences between overall selectivities to xylenes were found aider both 15 and 215 minutes of T-O-S in the whole temperature range, while significantly different selectivities to tetramethylbenzenes for the same T-O-S values were observed at 673 K and particularly 723 K. This is a result of the effect of reaction temperature on the overall reaction mechanism as xylenes are products of both disproportionation and dealkylation of TMBs in contrast to tetramethylbenzenes being formed only via TMB disproportionation. This is clearly demonstrated in Figure 1 presenting a high onset of dealkylation at temperatures above 623 K. In addition, differences in tetramethylbenzene concentrations for 15 and 215 minutes of T-O-S are due to the blocking of the most active sites and a formation of some coke deposits.
2630
55~ 50"6 4 5 -
E 40ram
35t~ 30 25 500
I
I
I
I
550
600
650
700
750
Temperature [K] Figure 2 Temperature dependence of selectivities to xylenes (T-O-S = 15 min - VI, 215 min - O) and tetramethylbenzenes (T-O-S = 15 min - m , 215 min - O) in 1,2,4-TMB disproportionation over zeolite Beta (Si/AI = 12.5, WHSV = 5 hl).
3.2. Transalkylation of 1,2,4-trimethylbenzene with toluene The results of transalkylation of 1,2,4-TMB with toluene confirmed the previous data reporting a much higher reactivity of TMB compared to toluene [10]. While overall TMB conversion after 15 minutes of T-O-S is almost 50 - 54 % in the whole temperature range of 573 - 723 K for TMB/toluene ratio 1 : 1, toluene conversion depends very significantly on the reaction temperature (Figure 3). In addition, the decrease in TMB conversion with T-O-S is at all temperatures significantly slower compared to the decrease in toluene conversion. E.g. the decrease in TMB conversion at 723 K was only from 54 % to 45 % during the whole kinetic run, while toluene conversion decreased from 44 % to less than 27 %. This strongly resembles disproportionation of pure 1,2,4-TMB the conversion of which is controlled by the accumulation of slowly diffused large reactants and products. This again supports our conclusion concerning the very important role of adsorption and transport of TMB molecules as a rate determining step of this transalkylation. While toluene is at lower reaction temperatures almost exclusively transalkylated with TMB into xylenes (Figure 4), at temperatures above 673 K toluene also diproportionates into xylenes and benzene (benzene yield about 3 %). On the other hand, TMB disproportionation proceeds with almost constant selectivity at all reaction temperatures studied. In contrast to pure 1,2,4-TMB disproportionation, the addition of toluene to the reaction mixture significantly decreases the rate of dealkylation which was at 723 K less than 2 % (cf. Fig. 1). About 70 % of xylene molecules at 723 K are formed via transalkylation of TMB with toluene, while the remaining 30 % are produced via toluene or TMB disproportionation and these concentrations are practically constant during the whole experiment. At lower temperatures for low T-O-S
2631 values xylenes are much more formed via TMB transalkylation with toluene, however, with increasing T-O-S more than 50 % of xylenes are produced by the disproportionation of toluene (Figure 3). This is a result of a significantly lower toluene reactivity and also of an accumulation of TMB and tetramethylbenzene molecules in the zeolite channels. Under all reaction conditions studied the concentrations of individual xylene isomers were near to the thermodynamic data.
-551 *~ 50 c45
5O
t
~
t ~ t
t'l}
30 tn o
25 =
w
c
20 ~
t
~
~
E .-.
0
I i
~15 >'10 e-
15 = 10~ 5
i I
0
I
0
e..
~-35 ~
c 3 5 -t o 30
~ 5 N 220 c
40 ~"
I
~ 40
60
120
180
t0
,
240
0
T-O-S
[min]
60
120
180
240
Figure 3 Temperature dependence of TMB and toluene conversion in TMB transalkylation with toluene (zeolite Beta Si/AI = 12.5, TMB/toluene = 1 1, , - 573 K, II - 623 K, 9 - 673 K, A _ 723 K). The TMB/toluene molar ratio was varied from 0.4 to 2.5. It is seen from Figure 4 that the increase in this ratio above one does not lead to the further growing of xylene yield. On the other hand, decreasing this ratio below one causes a significant decrease in xylene yield. This is probably a result of the adsorption competition between TMB molecules and toluene and/or the "diluting" of the feed with an excess of toluene lowering the probability of TMB disproportionation. In contrast to TMB disproportionation which conversion on zeolite Beta does not depend significantly on the concentration of active sites controlled by Si/AI ratio [11,12], in the case of TMB transalkylation with toluene the conversions and xylene yield are strongly influenced by Si/AI ratio. While with Si/AI = 12.5 higher conversions and xylene yields were obtained (the highest xylene yield = 4 1 % ) compared to Si/A1 = 30 (xylene y i e l d - 33 %), zeolite Beta with a higher Si/AI ratio exhibited significantly lower decrease in conversion and xylene yield with T-O-S. Zeolite Beta exhibits stable activity in time-on-stream experiments in TMBs disproportionation with the highest yields of xylenes and tetramethylbenzenes compared to other large pore molecular sieves. In transalkylation reaction of TMB with toluene, the presence of toluene significantly decreases the rate of dealkylation. The resulting
2632 concentrations of low hydrocarbons and benzene among the reaction products are significantly lower compared to TMB disproportionation. The highest xylene yields are achieved at equimolar ratio of trimethylbenzene and toluene. 45
45
40-
40
35-
35
*~ 30 -
~20"
25 ~ E 20 E
~15
15~
"~ 25 -
~X t
10
5
5 i
. 0
60
120
180
T-O-S [min]
240
0
.
. 1
.
.
i
2
0
3
TMBItoluene ratio
Figure 4 The T-O-S dependence of xylene yield at different temperatures over zeolite Beta (Si/AI = 12.5) and the effect of TMB/toluene ratio on xylene yield at different reaction temperatures ( , - 573 K, II - 623 K, 9 - 673 K, & - 723 K, WHSV (TMB) = 5
h-l). ACKNOWLEDGEMENT This study was supported by a grant from the Grant Agency of the Czech Republic (No. 104//99/0840). REFERENCES .
2. 3. 4. 5. 6.
.
10. 11. 12.
P.B. Venuto, Micro porous Mater. 2 (1994) 297. B. Wichterlov/t, J. Cejka, J. Catal. 146 (1994) 523. A. Corma, Chem. Rev. 95 (1995) 559. A. Corma, H. Garcia, Catal. Today 38 (1997) 257. C.S. Cundy, Collect. Czech. Chem. Commun. 63 (1998) 1699. Molecular Sieves, Science and Technology: Synthesis (H.G.Karge, J.Weitkamp - Eds.), Vol. 1, Springer Berlin, 1998. R.A. Sheldon, I.W.C.E. Arends, H.E.B. Lempers, Collect. Czech. Chem. Commun. 63 (1998) 1724. J. Cejka, N. Zilkov/l, J.E. Sponer, B. Wichterlov/~, Collect. Czech. Chem. Commun. 63 (1998) 1769. N.Y. Chen, W.E. Garwood: Catal. Rev.- Sci. Eng. 28 (1986) 185. I. Wang, T.C. Tsai, S.T. Huang, Ind. Eng. Chem. Res. 29 (1990) 2005. J. Cejka, A. Krej~i, J. Hanika, Stud. Surf. Sci. Catal. 125 (1999) 351. S.J. Chu, Y.W. Chen, J. Chem. Technol. Biotechnol. 72 (1998) 1.
2627
Zeolite Beta: Selective molecular sieve for synthesis of xylenes from trimethylbenzenes Jifi (~ejkaI and Andrea Krej~,i1'2 1j. Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejgkova 3, CZ-182 23 Prague 8, Czech Republic 2Department of Organic Technology, Institute of Chemical Technology, Technick/l 5, CZ- 166 28 Prague 6, Czech Republic The impact of channel geometry of zeolite Beta on the activity and selectivity in trimethylbenzenes disproportionation and their transalkylation with toluene was investigated. Both reactions are strongly controlled by the transport of bulky trimethylbenzene reactants and tetramethylbenzene products which are accumulated in the inner channel system of zeolite Beta. These adsorbed molecules block the access of toluene reactant molecules which decreases the xylene yield for longer time-on-stream values. The addition of toluene to the reaction feed dramatically decreases the rate of trimethylbenzene dealkylation. The equimolar ratio of trimethylbenzenes to toluene leads to the highest xylenes yield in transalkylation reaction. 1. INTRODUCTION Zeolite molecular sieves have been recognized as unique catalysts for both acid-base and redox catalyzed reactions and number of their industrial applications is continuously increasing. Various transformations of organic compounds are intensively investigated over zeolites and other crystalline microporous molecular sieves due to the increasing number of their well-defined pore structures, their shape selective properties, the possibility of tailoring their chemical and structural properties and environmental tolerance [ 1-4]. The understanding of the mechanisms of zeolite synthesis [5,6] together with a knowledge of reaction mechanisms in transformations of various hydrocarbons [2,3,7,8] significantly increases our possibilities to choose or even rationally design a proper catalyst for a given organic reaction. Although the aromatic chemistry on zeolites is well-established [9] and many reactions have been industrially used, the transformation of C9 aromatic hydrocarbons represents another challenge in zeolite catalysis in the synthesis of more demanded products, namely xylenes. It is evident that particularly transalkylation processes are attractive for those refineries possessing BTX extraction units, where excessive C9+ aromatic hydrocarbons are produced. In our study we have used 1,2,4-trimethylbenzene (1,2,4TMB) as a model reactant as all three TMB isomers are always in thermodynamic equilibrium with about 65 % of this isomer [ 10]. This contribution is focused on the utilization of zeolite Beta for the selective synthesis of xylenes from trimethylbenzenes via their disproportionation and transalkylation with toluene. The effect of molecular sieve channel structure and acid number is presented.
2628 2. E X P E R I M E N T A L
Zeolite Beta with Si/AI ratio ranging from 12.5 to 30, synthesized in our laboratory from Cab-O-Sil M5, sodium aluminate and sodium hydroxide using tetraethylammonium hydroxide or bromide as organic template, was used in this study. XRD diffraction patterns (Siemens D5005), FTIR spectroscopy of skeletal vibrations (Nieolet Magna 550) and SEM (Jeol) confirmed a high crystallinity and phase purity of these zeolites possessing the crystals size lower than 0.5 ~tm. For comparison zeolite Y supplied by the Research Institute for Oil and Hydrocarbon Gases, Slovak Republic, and mordenite purchased from PQ Corporation, USA, were used. The acid strength and number of OH groups were determined by TPD of ammonia and FTIR spectroscopy of adsorbed d3-acetonitrile providing quantitative information on the number of Bvoensted and Lewis sites. Disproportionation of trimethylbenzenes (TMB) and their reaction with toluene were performed in a glass down-flow mieroreaetor under atmospheric pressure at the temperature range of 473 - 723 K with WHSV ranging from 0.35 to 15 h -~. TMBs and/or toluene were fed into the reactor using saturators and hydrogen as carrier gas with concentration of TMB = 5.2 vol. % (disproportionation) and 4.5 vol. % (reaction -with toluene). TMB/toluene molar ratios ranging from 0.4 to 2.5 were investigated. The reaction products were analyzed using an "on-line" gas chromatograph (Hewlett Packard 5890 Series II) equipped with FID and MSD detectors and a high-resolution capillary column (Supelcowax 10, 30 m long). The first analysis was performed after 15 minutes of time-on-stream (T-O-S) with others followed with 40 minutes interval. 3. RESULTS AND DISCUSSION In our previous paper [11] we have shown that TMB molecules are too bulky to penetrate into the 10-member ring channels of medium-pore zeolites and their transformations can proceed only on their "external" surface. In contrast, for large pore zeolites higher temperatures and longer contact times significantly increased the rate of undesirable dealkylation at the expense of disproportionation. Dealkylation and deactivation were promoted by the more open structure exhibiting large cavities in the case of zeolite Y and by a high acid strength and only one-dimensional channels of mordenite. 3.1.1,2,4-Trimethylbenzene disproportionation The broad spectrum of reaction products is obtained in 1,2,4-TMB transformation due to the complexity of the reaction pathways starting from 1,2,4-TMB. These reactions include disproportionation, isomerization and, particularly at temperatures above 673 K, also dealkylation. The temperature dependence of selectivities to disproportionation, isomerization and dealkylation products is depicted in Figure 1. It is clearly seen, that in the reaction temperature range studied both disproportionation and isomerization proceeds easily already from 473 K. On the other hand, dealkylation is minimized up to 623 K. As we have found almost the same results for both 1,2,4-TMB and 1,3,5-TMB [11] under all reaction conditions used it indicates that fast isomerization of TMB isomers represents the first step of the reaction mechanism. This is followed by the consecutive reactions such as disproportionation and/or dealkylation. The maximum of the rate of TMB disproportionation is located between 573 and 673 K with the significant competition of dealkylation at the higher temperature of this interval, especially for short T-O-S values.
2629 80 I
=~
m
60
=.==
o
E
._~
40
.>_
~
!
_.e t~ 20
! I
0
, 450
500
-
, 550
-
, 600
, 650
, 700
i 750
Temperature [K] Figure 1 Temperature dependence of selectivities to disproportionation (11), isomerization ( , ) and dealkylation ( e ) in 1,2,4-TMB transformation over zeolite Beta (Si/Al- 12.5, WHSV = 5 hl, T-O-S = 55 min, TMB conversion- 30.2 % at 473 K, 46.6 % at 523 K, 51.7-56.4 % in the range 573-723 K). It is well known that synthesis and transformations of xylenes are strongly influenced by transport of reactants and products over ZSM-5 zeolite and it seems that similar situation is also in the case of transformations of trimethylbenzenes over large pore zeolites. Although xylene to tetramethylbenzene molar ratio (X/TeMB) should be one in the case of bimolecular TMB transalkylation, this ratio changes significantly for different reaction temperatures (cf. Figure 2). While at temperatures around 523 and 573 K higher concentrations of tetramethylbenzenes have been found, with increasing reaction temperature xylenes started to dominate. At lower temperatures it is strongly connected with a slow diffusivity of larger tri- and particularly tetramethylbenzenes which are accumulated in the channel system of zeolite Beta. This significantly changes the concentration of aromates in the channels and it is assumed that the methyl transfer between adsorbed trimethylbenzenes and tetramethylbenzenes proceeds in a larger extent. Thus, this leads to the preferential formation of tetramethylbenzenes at the account of xylenes. On the other hand, an increase in X/TeMB ratio at reaction temperatures of 673 and particularly 723 K is strongly influenced by the significant rate of TMB dealkylation, which favours the formation of xylenes. Only small differences between overall selectivities to xylenes were found aider both 15 and 215 minutes of T-O-S in the whole temperature range, while significantly different selectivities to tetramethylbenzenes for the same T-O-S values were observed at 673 K and particularly 723 K. This is a result of the effect of reaction temperature on the overall reaction mechanism as xylenes are products of both disproportionation and dealkylation of TMBs in contrast to tetramethylbenzenes being formed only via TMB disproportionation. This is clearly demonstrated in Figure 1 presenting a high onset of dealkylation at temperatures above 623 K. In addition, differences in tetramethylbenzene concentrations for 15 and 215 minutes of T-O-S are due to the blocking of the most active sites and a formation of some coke deposits.
2630
55~ 50"6 4 5 -
E 40ram
35t~ 30 25 500
I
I
I
I
550
600
650
700
750
Temperature [K] Figure 2 Temperature dependence of selectivities to xylenes (T-O-S = 15 min - VI, 215 min - O) and tetramethylbenzenes (T-O-S = 15 min - m , 215 min - O) in 1,2,4-TMB disproportionation over zeolite Beta (Si/AI = 12.5, WHSV = 5 hl).
3.2. Transalkylation of 1,2,4-trimethylbenzene with toluene The results of transalkylation of 1,2,4-TMB with toluene confirmed the previous data reporting a much higher reactivity of TMB compared to toluene [10]. While overall TMB conversion after 15 minutes of T-O-S is almost 50 - 54 % in the whole temperature range of 573 - 723 K for TMB/toluene ratio 1 : 1, toluene conversion depends very significantly on the reaction temperature (Figure 3). In addition, the decrease in TMB conversion with T-O-S is at all temperatures significantly slower compared to the decrease in toluene conversion. E.g. the decrease in TMB conversion at 723 K was only from 54 % to 45 % during the whole kinetic run, while toluene conversion decreased from 44 % to less than 27 %. This strongly resembles disproportionation of pure 1,2,4-TMB the conversion of which is controlled by the accumulation of slowly diffused large reactants and products. This again supports our conclusion concerning the very important role of adsorption and transport of TMB molecules as a rate determining step of this transalkylation. While toluene is at lower reaction temperatures almost exclusively transalkylated with TMB into xylenes (Figure 4), at temperatures above 673 K toluene also diproportionates into xylenes and benzene (benzene yield about 3 %). On the other hand, TMB disproportionation proceeds with almost constant selectivity at all reaction temperatures studied. In contrast to pure 1,2,4-TMB disproportionation, the addition of toluene to the reaction mixture significantly decreases the rate of dealkylation which was at 723 K less than 2 % (cf. Fig. 1). About 70 % of xylene molecules at 723 K are formed via transalkylation of TMB with toluene, while the remaining 30 % are produced via toluene or TMB disproportionation and these concentrations are practically constant during the whole experiment. At lower temperatures for low T-O-S
2631 values xylenes are much more formed via TMB transalkylation with toluene, however, with increasing T-O-S more than 50 % of xylenes are produced by the disproportionation of toluene (Figure 3). This is a result of a significantly lower toluene reactivity and also of an accumulation of TMB and tetramethylbenzene molecules in the zeolite channels. Under all reaction conditions studied the concentrations of individual xylene isomers were near to the thermodynamic data.
-551 *~ 50 c45
5O
t
~
t ~ t
t'l}
30 tn o
25 =
w
c
20 ~
t
~
~
E .-.
0
I i
~15 >'10 e-
15 = 10~ 5
i I
0
I
0
e..
~-35 ~
c 3 5 -t o 30
~ 5 N 220 c
40 ~"
I
~ 40
60
120
180
t0
,
240
0
T-O-S
[min]
60
120
180
240
Figure 3 Temperature dependence of TMB and toluene conversion in TMB transalkylation with toluene (zeolite Beta Si/AI = 12.5, TMB/toluene = 1 1, , - 573 K, II - 623 K, 9 - 673 K, A _ 723 K). The TMB/toluene molar ratio was varied from 0.4 to 2.5. It is seen from Figure 4 that the increase in this ratio above one does not lead to the further growing of xylene yield. On the other hand, decreasing this ratio below one causes a significant decrease in xylene yield. This is probably a result of the adsorption competition between TMB molecules and toluene and/or the "diluting" of the feed with an excess of toluene lowering the probability of TMB disproportionation. In contrast to TMB disproportionation which conversion on zeolite Beta does not depend significantly on the concentration of active sites controlled by Si/AI ratio [11,12], in the case of TMB transalkylation with toluene the conversions and xylene yield are strongly influenced by Si/AI ratio. While with Si/AI = 12.5 higher conversions and xylene yields were obtained (the highest xylene yield = 4 1 % ) compared to Si/A1 = 30 (xylene y i e l d - 33 %), zeolite Beta with a higher Si/AI ratio exhibited significantly lower decrease in conversion and xylene yield with T-O-S. Zeolite Beta exhibits stable activity in time-on-stream experiments in TMBs disproportionation with the highest yields of xylenes and tetramethylbenzenes compared to other large pore molecular sieves. In transalkylation reaction of TMB with toluene, the presence of toluene significantly decreases the rate of dealkylation. The resulting
2632 concentrations of low hydrocarbons and benzene among the reaction products are significantly lower compared to TMB disproportionation. The highest xylene yields are achieved at equimolar ratio of trimethylbenzene and toluene. 45
45
40-
40
35-
35
*~ 30 -
~20"
25 ~ E 20 E
~15
15~
"~ 25 -
~X t
10
5
5 i
. 0
60
120
180
T-O-S [min]
240
0
.
. 1
.
.
i
2
0
3
TMBItoluene ratio
Figure 4 The T-O-S dependence of xylene yield at different temperatures over zeolite Beta (Si/AI = 12.5) and the effect of TMB/toluene ratio on xylene yield at different reaction temperatures ( , - 573 K, II - 623 K, 9 - 673 K, & - 723 K, WHSV (TMB) = 5
h-l). ACKNOWLEDGEMENT This study was supported by a grant from the Grant Agency of the Czech Republic (No. 104//99/0840). REFERENCES .
2. 3. 4. 5. 6.
.
10. 11. 12.
P.B. Venuto, Micro porous Mater. 2 (1994) 297. B. Wichterlov/t, J. Cejka, J. Catal. 146 (1994) 523. A. Corma, Chem. Rev. 95 (1995) 559. A. Corma, H. Garcia, Catal. Today 38 (1997) 257. C.S. Cundy, Collect. Czech. Chem. Commun. 63 (1998) 1699. Molecular Sieves, Science and Technology: Synthesis (H.G.Karge, J.Weitkamp - Eds.), Vol. 1, Springer Berlin, 1998. R.A. Sheldon, I.W.C.E. Arends, H.E.B. Lempers, Collect. Czech. Chem. Commun. 63 (1998) 1724. J. Cejka, N. Zilkov/l, J.E. Sponer, B. Wichterlov/~, Collect. Czech. Chem. Commun. 63 (1998) 1769. N.Y. Chen, W.E. Garwood: Catal. Rev.- Sci. Eng. 28 (1986) 185. I. Wang, T.C. Tsai, S.T. Huang, Ind. Eng. Chem. Res. 29 (1990) 2005. J. Cejka, A. Krej~i, J. Hanika, Stud. Surf. Sci. Catal. 125 (1999) 351. S.J. Chu, Y.W. Chen, J. Chem. Technol. Biotechnol. 72 (1998) 1.