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Cumene disproportionation over zeolite β
Applied Catalysis, 77 (1991) 199-207 Elsevier Science Publishers B.V., Amsterdam
199
Cumene disproportionation over zeolite p I. Comparison of catalytic performances and reaction mechanisms of zeolites Tseng-Chang Tsai’, Chin-Lan Ay” and Ikai Wang Department of Chemical Engineering, (Taiwan, ROC), tel. (+886-35)715131,
National Tsing Huu University, Hsinchu 30043
fax. (+886-35)715408
(Received 3 September 1990, revised manuscript received 17June 1991)
Abstract Cumene disproportionation has been demonstrated as an accurate probe reaction for zeolite structures. Over twelve-membered ring zeolites, cumene undergoes a bimolecular mechanism; but over tenmembered ring xeolites, it undergoes a monomolecular mechanism. Zeolite j?,a highly siliceous twelvemembered ring xeolite, shows excellent activity, disproportionation selectivity and stability in cumene disproportionation and therefore has practical potential in the production of diisopropylbenzenes. Keywordx catalytic performance, cumene disproportionation, diisopropylbenxene, reaction mechanism, xeolite 8.
INTRODUCTION
Cumene cracking has been well demonstrated as a useful probe reaction for zeolite acidity [ 11. However, there are only a few studies that have been carried out on cumene disproportionation. Absil et al. proposed that cumene disproportionation can be applied to probe zeolite acidity [ 21. Nevertheless, Tsai and Wang demonstrated that zeolite pore structures can affect the reaction mechanism and catalytic activity [ 31. Lishchiner et al. concluded from the transalkylation reaction of diisopropylbenzene (iiPB ) and benzene that transalkylation and alkylation are catalyzed by different active sites [ 41. These studies revealed that interactions between disproportionation reaction characteristics and zeolite properties are complicated There should be some zeolite characteristics other than acidity which affect zeolite catalytic performance. Further studies on cumene disproportionation would be worthwhile in the sense of exploring the fundamental understanding. ‘On leave from the Chinese Petroleum Corporation, 239 Ming-Shen S. Road, Chiayi, Taiwan. ‘Present address: China Technical Consultants, Inc.
0 1991 Elsevier Science Publiahem B.V. All rights reserved. 0166-9634/91/$03.50
200
In addition, iiPB, especially m- andp-isomers, are important intermediates for polymer industries [ 51. They can be produced with two methods. The first method is benzene propylation over catalysts such as Friedel-Crafts [ 61, ZSM5 [7] and ZSM-12 [El]. The second method is cumene disproportionation by applying catalysts including Al& [9 1, ZSM- 12 [ 8 ] and mordenite [ 10 1. Generally speaking, disproportionation has better reaction selectivity than alkylation. Meanwhile, zeolites are better than AlC& in terms of environmental pollution problems and selectivity of p- and m-isomers. Therefore, zeolites have practical potential in manufacturing iiPB. The objective of this study is to demonstrate that cumene disproportionation can be applied as a probe reaction for zeolite structures. Our earlier study showed that zeolite /I, being a highly siliceous zeolite with a twelve-membered ring pore opening [ 111, is a promising catalyst for transalkylation of toluene and trimethylbenzenes [ 121. It is revealed in the present study that zeolite p has an application potential in the production of iiPB. EXPERIMENTAL
Zeolite /3 was prepared by the hydrothermal method from silica-aluminasodium hydroxide-tetraethylammonium hydroxide-water mixtures, by following the procedures reported by Pererz-Pariente et al. [ 131 and by Wadlinger et al. [ 141. Solid products from each preparation were filtered, water washed and dried overnight at 120°C. The products were then characterized by a Simens D-500 X-ray diffractometer. The synthesized products were calcined at 450°C and then repeatedly ion exchanged with 1 mol/l ammonium nitrate solution six times. The sample was then calcined at 450°C and finally zeolite H-/3 was obtained. ZSM-5 was prepared according to the procedures documented by Chao et al. [ 151. H-mordenite (HM) and Na-Y were supplied by Stream Chemicals. ZSM-5 and Y were NH.,+ ion-exchanged with the same ion-exchange procedures as zeolite fi. All the H-form zeolites were then pelletized and crushed into 12-20 mesh particles. Reactions were conducted in a continuous fixed-bed reactor system. Mixtures of zeolite of 2.54 g in 12-20 mesh and ceramics of 4.65 g were packed into the reactor and activated in air 6 h at 450” C. The reactor was then cooled under a nitrogen stream to the desired reaction temperatures. In disproportionation experiments, cumene (Jassen Chemica) was charged into the reactor by a metering pump. Reaction products were collected in intervals of 15 min and analyzed with a Hewlett-Packard 5890 gas Chromatograph, equipped with a dimethylsiloxane capillary column, HP Ultra 1. The conversion of cumene, Xc, is defined as X,(wt.-%;) =lOO-
(cumene, wt.-o/o)r
(I)
201
RESULTS AND DISCUSSION
Two disproportionation mechanisms,the bimolecular mechanismand the monomolecularmechanism,have been proposed When cumeneproceeds disproportionation with the bimolecular mechanism, cumene first forms a carbenium ion, then an i-propylphenylcumeneion and finallycracksinto benzene and dipropylbenxene(DPB ) (Pii. 1) . Since crackingtakes place after the formation of the biphenyl complex, the disproportionationproducts,benzeneand dipropylbenzene, should be stoichiometrically equal in mols. The reaction products should show a near unity benzene-to-dipropylbenxenemolar ratio (B/DPB) and disproportionation selectivity (Sn), which is defined in eqn. (2). SD(mol/mol) =
2(DPB, wt.-%)r/162 xc/120
(2)
where (cumene, wt.-%)r and (DPB, wt.-%)r is the composition of cumene and DPB in the reactor effluents, respectively.In addition, there is no rearrangementof the propyl group and hence only the iiPB among the DPB isomers would be observed. However,with the monomolecularreactionmechanism,cumenefirst cracks into benzene and an i-propyl ion. The i-propyl ion then alkylatesto another cumenemolecule,which is in a free molecularstate, and forms iiPB. When the extent of the cracking reaction is greaterthan that of the alkylationreaction, there will be an excessivebenzene yield with respectto DPB yields and the Sn
0
SNl
0
+
cp
-
1
c&,c_c&A ji!s
\
\
C
C C+
Fig. 1. Cumene disproportionation mechanisms.
202
will be low. The cracked i-propyl ion prior to alkylation can isomerize to an npropyl ion. Therefore, in addition to iiPB, n-propyl-i-propyl benzene (niPB) can be obtained among the DPB isomers. The disproportionation mechanism can then be identified by examining the DPB isomer distribution and Sn. Cumene disproportionation was demonstrated as a probe reaction for zeolite structures, as shown in Table 1. At low conversion levels, over zeolite /3,Y and mordenite, which are twelve-membered ring zeolites, cumene undergoes disproportionation at a Sn of nearly 100 mol-%. However, over ZSM-5, which is a ten-membered ring zeolite, the Sn is much lower. Meanwhile, its benzene molar yield is greater than the iiPB molar yield and the B/DPB ratio greatly exceeds unity. We have concluded from n-propylbenzene disproportionation that disproportionation over wide pore twelve-membered ring zeolites such as zeolite j3 and Y undergoes a bimolecular mechanism whereas over restricted twelvemembered ring zeolites such as mordenite, disproportionation proceeds a combined mechanism of the monomolecular and bimolecular mechanisms. Over ten-membered ring zeolites such as ZSM-5, disproportionation undergoes a monomolecular mechanism [ 31. The different reaction mechanisms over varTABLE 1 Comparison of activity, selectivity and stability of zeolites in cumene disproportionation conversion levels WHSV, 3.4 g h-’ g-,-l;
nitrogen/cumene,
Reaction temperature ( “C) Conversion (wt.-% )
0.19 mol/mol; time-on-stream, HY
HM
HZSM-5”
118 14.0
205 13.0
200 5.9
120 6.3
0.99 1.0
(Y
“Time-on-stream, 30 min.
60 min
H-B
Selectivity (mol/mol) Disproportionation Benzene/DPB m/p-iiPB
Product yields (wt.-%) Benzene Cumene n-Propylbenzene o-iiPB m-iiPB p-iiPB p-niPB Other aromatics
at low
0.95
0.93
0.69
1.75
1.05 2.10
1.09 1.71
2.07 0.23
0.07
0.15
0.35
4.50 86.05 0.11
4.21 87.04 0.13 0.19 5.49 2.62
1.95 94.10 0.15 0.10 2.28 1.33
5.94 3.4 -
-
0.32
0.09
2.92
93.69 0.05 0.52 2.28 0.13 0.41
203
ious zeolites can account for their difference in Sn in cumene disproportionation (Table 1). We have suggested that the biphenyl complex for the bimolecular mechanism is too large to be formed inside the ZSM-5 pores [ 31. In addition, the ipropyl group of cumene would have a size close to the i-butane molecular size of 5.0 A [161 which is critical to the pore opening of ZSM-$5.1 A [171. Therefore cumene disproportionation over ZSM-5 is a monomolecular reaction. The monomolecular mechanism results from the shape selectivity of the transition complex and the reactant. The same reaction mechanism was also proposed by Fukase and Wojciechowski [ 18 1. In the monomolecular mechanism, the cracked i-propyl ion tends to polymerize to form coke which plugs pore mouths and deactivates catalytic activity [ 191. Therefore, when comparing with twelve-membered ring zeolites, ZSM-5 exhibits an unusually poor stability in cumene disproportionation. In addition, because of the isomerization of the i-propyl ion, ZSM-5 produces a great proportion of niPB among the DPB products. In contrast, zeolite 8, Y and mordenite have Sn of near unity (Table l), indicating that those three twelve-membered ring zeolites catalyze a bimolecular reaction. In addition, only iiPB among the DPB isomers was obtained. Although mordenite catalyzes n-propylbenzene disproportionation with a reaction mechanism combining a monomolecular mechanism and a bimolecular mechanism [ 3 1, it catalyzes cumene disproportionation with a bimolecular mechanism. The shift of the reaction mechanism could be due to the great stability of the biphenyl intermediates stabilized by the x-electrons of the benzene rings (Fig. 1) . The bimolecular mechanism in cumene disproportionation over mordenite leads to a high Sn. The So increases from 0.75 in n-propylbenzene at a conversion of 5.47 wt.-% [ 31 to 0.91 in cumene at a conversion of 5.9 wt.-% (Table 2). The deactivation of the zeolite activity with time-on-stream (TOS) was observed. The deactivation behavior can be expressed as a power law, eqn. (3 ) . xt=x()t
--(y
(3)
where X, is the conversion measured at TOS oft; X,, is the initial conversion extrapolated to zero time-on-stream and aris a decay exponent. The lower the value of (Y,the greater is the stability. Among the twelve-membered ring zeolites studied, zeolite /3has the best stability whereas mordenite has the worst one (Table 1) . However, twelve-membered ring zeolites always have a better stability than ten-membered ring zeolites (Table 1) . As discussed earlier, the difference in stability results from the difference in the reaction mechanism. For twelve-membered zeolites whose reaction mechanisms are the same, zeolite pore structures could affect their stability. Guisnet and Magnoux have studied zeolite stability in hexane cracking [ 201. They proposed a concept of
204 TABLE 2 Comparison between cumene disproportionation mordenite DPB =Dipropylbenzene; di-n-propylbenzene
and n-propylbenzene
disproportionation
iiPB, Diieopropylbenzene; niPB, n-propyl-i-propylbenzene; nnPB/DPB,
Feedstock
Cumene
n-Propylbenzene
SiOz/A1203 Reaction temperature ( ‘C) WHSV (g/g,& 1 Conversion (wt.-% )
7.0 200 3.4 5.9
19.0 199 2.0 5.47
Selectivity (mol/mol) Dispropoxtionation Benzene/DPB iiPB/DPB niPB/DPB nnPB/DPB
over
0.93 1.09 1.00 0 0
0.75 1.23 0 0.26 0.74
7.0 356 3.4 11.6 0.40 4.16 0.14 0.50 0.36
“coke toxicity”. For ZSMd which has interconnecting channels and is without cavities, coke forms in the intersections. However, for mordenite whose pore system is a monodimensional channel, coke forms and completely plugs its pore mouth. Therefore, ZSM-5 can tolerate much higher coke deposition than mordenite. According to the proposal of G&net and Magnoux, the coke toxicity for ZSM-5 is lower than that for other zeolites. Similarly, the good stability of zeolite /9 can be attributed to its pore channel systems which are also interconnecting with no cavities. A comparison of zeolites in cumene disproportionation at high conversion levels is shown in Table 3. Among the zeolites studied, zeolite jl has the greatest activity and Sn. The best Sn can be attributed to its low reaction temperature requirements. It can also be attributed to the loose structure of zeolite /3 such as we proposed in n-propylbenzene disproportionation [ 31. For zeolites with the same reaction mechanism, their catalytic activity mainly depends on acidity. Hedge et al. concluded from infrared measurements that zeolite B has a higher acidity than Y [ 211. The results that zeolite j? has a higher activity than Y is consistent with the acidity measurement. Kaeding recently reported that zeolite ZSM-12, being another high siliceous twelve-membered ring zeolite, can also selectively catalyze cumene disproportionation [ 81. This result further supports our proposal that cumene disproportionation can be selectively catalyzed by twelve-membered ring zeolites. For zeolite p and ZSM-12, being high siliceous zeolites, their maximum attainable conversion with a minimum Sn of 0.90 mol/mol are much higher than zeolite Y and mordenite (Table 3 ) . It is shown in Fig. 2 that in cumene disproportionation over zeolite j3,zeolite activity decreases and the maximum attainable conversion increases with in-
205 TABLE 3 Comparison of activity, selectivity and stability of zeolitea at high disproportionation conversion of cumene WHSV, 3.4 g h-r bt-‘;
nitrogen/cumene,
Reaction temperature ( oC ) Conversion (wt.-%) Coke (g/g,,% )*
0.19 mol/mol; time-on-stream, 60 min
H-8
HY
HM
HZSM-12”
143 40.1 4.72
220 22.5 4.54
240 14.5 6.62
250 45.1 -
Selectivity (mol/mal) Disproportionation Benzene/DPB m/p-iiPB
0.98 1.06 2.05
0.91 1.16 2.18
0.95 1.12 2.01
o!
0.23
0.14
0.31
13.5 59.88 0.10
7.74 77.53 0.17 0.42 9.17 4.21 0.76
4.95 35.55 0.14 0.11 6.08 3.03 0.14
0.90 1.17 1.81
Product yields (wt.-%) Benzene Cumene n-Propylbenzene o-iiPB m-iiPB p-iiPB Other aromatics
-
17.76 8.68 0.08
“Data from Kaeding [8]. *Coke deposition after 3 h of time-on-stream
“-o1l
15.42 54.9 0.32 0.23 17.54 9.70 1.89
reaction conditions.
0
Fig. 2. Plot of the maximum attainable conversion and their reaction temperatures plotted against the silica-to-alumina ratio in cumene disproportionation over zeolite 8. (* ) Maximum conversion; (n ) reaction temperature.
206
0
10
20 Gmersicqwt
30
40
50
*I.
Fig. 3. Dependence of the m/p-diisopropylhenzene ratio of zeolites on catalytic conversion [weight hourly space velocity (WHSV), 3.4 g h-’ gu,-‘; nitrogen/cumene, 0.19 mol/mol; time-on-stream, 60min]. ([7)8; (A) mordenite; (0) Y; (A) ZSM-12; (0) ZSM-5.
creasing SiOz-to-AlgO ratio. This could be the result of the characteristics of the spatial distribution of the acid sites. The distances between neighboring sites inside the pores of zeolites increase with their SiO,-to&O3 ratio. Thus, the pore systems of higher siliceous zeolites can accommodate a greater number of diphenylcarbenium ions without cracking. Accordingly, it is possible to use the maximum disproportionation conversion as a measurement of acid site distribution. It was found that the formation of iiPB isomers is highly shape-selective (Fig. 3 and Table 1). The m-iiPB to p-iiPB ratio, m/p-iiPB, is greater for zeolites with larger pore openings. This ratio increases with increasing conversion levels and eventually reaches parity, indicating a thermodynamic equilibrium value which was reported nearly 1.8 [ 221. The shape selectivity results from the relatively bulky iiPB molecules in terms of the zeolite pore openings. CONCLUSION
The type of reaction mechanism in cumene disproportionation was derived from the fact that the bimolecular mechanism catalyzes a disproportionation selectivity and diisopropylbenzene fraction among the dipropylbenzene isomer of near unity. It can be concluded that twelve-membered zeolites take a bimolecular reaction mechanism whereas ten-membered ring zeolites take a monomolecular reaction mechanism. A zeolite family which catalyzes a bimolecular mechanism has a better catalytic stability than those which catalyze a monomolecular mechanism. Cumene disproportionation can also be used as a measurement of acid site distribution. It is shown that high siliceous zeolites can obtain high disproportionation conversion and maintain good disproportionation selectivity. It is noted that the disproportionation reaction mechanism could be deter-
207
mined not only by zeolite structures, but also by the types of alkylbenzenes. Over mordenite, the disproportionation mechanism shifts from a reaction mechanism combining a monomolecular mechanism and a bimolecular mechanism in n-propylbenzene disproportionation to a bimolecular mechanism in cumene disproportionation. In terms of activity, disproportionation selectivity and stability, zeolitej3 has been shown as a promising catalyst for diisopropylbenzene production by cumene disproportionation. ACKNOWLEDGMENT
We would like to thank F.J. Yang for the preparation of the manuscript. This study was financially supported by the Taiwan Styrene Monomer Company.
REFERENCES
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
A. Corma and B.W. Wojciechowski, CataI. Rev. Sci. Eng., 24 (1982) 1. R.P.L. Absil, J.B. Butt and J.S. Dranoff, J. Catal., 85 (1984) 415. T.C. Tsai and I. Wang, J. CataI., submitted for publication. 1.1.Lischiner, V.A. Plakhotnik, D.Z. Levin, E.S. Mortikov and V.F. Ilyin, React. Kinet. Catal. Lett., 23 (1983), 261. H.A. Covin and J. Muse, Chemtech, (1986) 500. Mitaui Petrochemical Industry, Jpn., Pat. 57 040 423 (1982). C.D. Chang and J.N. Miale, U.S. Pat. 4 577 048 (1986). W.W. Kaeding, J. Catal., 120 (1989) 409. M.S. BelenkiI, G.P. Pavlov and N.V. Ulitakaya, SU Pat. 192 190 (1964). R.M. Suggit and W. Falls, US Pat. 3 780 123 (1973). M.M. Treaty and J.M. Newsam, Nature (London), 332 (1988) 249. I. Wang, T.C. Tsai and S.T. Huang, Ind. Eng. Chem. Res., 29 (1990) 2005. J. Peren-Pariente, J.A. Martens, and P.A. Jacobs, Appl. Cat& 31 (1987) 35. R.L. Wadlinger, G.T. Kerr and E.J. Rosinski, US Pat. 4 333 859 (1982). K.J. Chao, T.C. Tsai, M.S. Chen and I. Wang, J. Chem. Sot., Faraday Trans. 1, 77 (1981) 547. S.M. Cscisery, Zeolites, 4 (1984) 202. W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Apes, International Zeolite Association, PolycryetaI Book Service, Pittsburgh, PA, 1978. S. Fukase and B.W. Wojciechoweki, J. CataI., 102 (1986) 452. T.C. Tsai and I. Wang, Appl. CataI., 77 (1991) 209. . M. Guisnet and P. Magnoux, Appl. CataI., 54 (1989) 1. S.M. Hedge, R. Kumar, R.N. Bhat and P. Ratnasamy, Zeolites, 9 (1989) 231. W.M. Kaeding and R.E. Holland, J. Cat&, 109 (1988) 212.
199
Cumene disproportionation over zeolite p I. Comparison of catalytic performances and reaction mechanisms of zeolites Tseng-Chang Tsai’, Chin-Lan Ay” and Ikai Wang Department of Chemical Engineering, (Taiwan, ROC), tel. (+886-35)715131,
National Tsing Huu University, Hsinchu 30043
fax. (+886-35)715408
(Received 3 September 1990, revised manuscript received 17June 1991)
Abstract Cumene disproportionation has been demonstrated as an accurate probe reaction for zeolite structures. Over twelve-membered ring zeolites, cumene undergoes a bimolecular mechanism; but over tenmembered ring xeolites, it undergoes a monomolecular mechanism. Zeolite j?,a highly siliceous twelvemembered ring xeolite, shows excellent activity, disproportionation selectivity and stability in cumene disproportionation and therefore has practical potential in the production of diisopropylbenzenes. Keywordx catalytic performance, cumene disproportionation, diisopropylbenxene, reaction mechanism, xeolite 8.
INTRODUCTION
Cumene cracking has been well demonstrated as a useful probe reaction for zeolite acidity [ 11. However, there are only a few studies that have been carried out on cumene disproportionation. Absil et al. proposed that cumene disproportionation can be applied to probe zeolite acidity [ 21. Nevertheless, Tsai and Wang demonstrated that zeolite pore structures can affect the reaction mechanism and catalytic activity [ 31. Lishchiner et al. concluded from the transalkylation reaction of diisopropylbenzene (iiPB ) and benzene that transalkylation and alkylation are catalyzed by different active sites [ 41. These studies revealed that interactions between disproportionation reaction characteristics and zeolite properties are complicated There should be some zeolite characteristics other than acidity which affect zeolite catalytic performance. Further studies on cumene disproportionation would be worthwhile in the sense of exploring the fundamental understanding. ‘On leave from the Chinese Petroleum Corporation, 239 Ming-Shen S. Road, Chiayi, Taiwan. ‘Present address: China Technical Consultants, Inc.
0 1991 Elsevier Science Publiahem B.V. All rights reserved. 0166-9634/91/$03.50
200
In addition, iiPB, especially m- andp-isomers, are important intermediates for polymer industries [ 51. They can be produced with two methods. The first method is benzene propylation over catalysts such as Friedel-Crafts [ 61, ZSM5 [7] and ZSM-12 [El]. The second method is cumene disproportionation by applying catalysts including Al& [9 1, ZSM- 12 [ 8 ] and mordenite [ 10 1. Generally speaking, disproportionation has better reaction selectivity than alkylation. Meanwhile, zeolites are better than AlC& in terms of environmental pollution problems and selectivity of p- and m-isomers. Therefore, zeolites have practical potential in manufacturing iiPB. The objective of this study is to demonstrate that cumene disproportionation can be applied as a probe reaction for zeolite structures. Our earlier study showed that zeolite /I, being a highly siliceous zeolite with a twelve-membered ring pore opening [ 111, is a promising catalyst for transalkylation of toluene and trimethylbenzenes [ 121. It is revealed in the present study that zeolite p has an application potential in the production of iiPB. EXPERIMENTAL
Zeolite /3 was prepared by the hydrothermal method from silica-aluminasodium hydroxide-tetraethylammonium hydroxide-water mixtures, by following the procedures reported by Pererz-Pariente et al. [ 131 and by Wadlinger et al. [ 141. Solid products from each preparation were filtered, water washed and dried overnight at 120°C. The products were then characterized by a Simens D-500 X-ray diffractometer. The synthesized products were calcined at 450°C and then repeatedly ion exchanged with 1 mol/l ammonium nitrate solution six times. The sample was then calcined at 450°C and finally zeolite H-/3 was obtained. ZSM-5 was prepared according to the procedures documented by Chao et al. [ 151. H-mordenite (HM) and Na-Y were supplied by Stream Chemicals. ZSM-5 and Y were NH.,+ ion-exchanged with the same ion-exchange procedures as zeolite fi. All the H-form zeolites were then pelletized and crushed into 12-20 mesh particles. Reactions were conducted in a continuous fixed-bed reactor system. Mixtures of zeolite of 2.54 g in 12-20 mesh and ceramics of 4.65 g were packed into the reactor and activated in air 6 h at 450” C. The reactor was then cooled under a nitrogen stream to the desired reaction temperatures. In disproportionation experiments, cumene (Jassen Chemica) was charged into the reactor by a metering pump. Reaction products were collected in intervals of 15 min and analyzed with a Hewlett-Packard 5890 gas Chromatograph, equipped with a dimethylsiloxane capillary column, HP Ultra 1. The conversion of cumene, Xc, is defined as X,(wt.-%;) =lOO-
(cumene, wt.-o/o)r
(I)
201
RESULTS AND DISCUSSION
Two disproportionation mechanisms,the bimolecular mechanismand the monomolecularmechanism,have been proposed When cumeneproceeds disproportionation with the bimolecular mechanism, cumene first forms a carbenium ion, then an i-propylphenylcumeneion and finallycracksinto benzene and dipropylbenxene(DPB ) (Pii. 1) . Since crackingtakes place after the formation of the biphenyl complex, the disproportionationproducts,benzeneand dipropylbenzene, should be stoichiometrically equal in mols. The reaction products should show a near unity benzene-to-dipropylbenxenemolar ratio (B/DPB) and disproportionation selectivity (Sn), which is defined in eqn. (2). SD(mol/mol) =
2(DPB, wt.-%)r/162 xc/120
(2)
where (cumene, wt.-%)r and (DPB, wt.-%)r is the composition of cumene and DPB in the reactor effluents, respectively.In addition, there is no rearrangementof the propyl group and hence only the iiPB among the DPB isomers would be observed. However,with the monomolecularreactionmechanism,cumenefirst cracks into benzene and an i-propyl ion. The i-propyl ion then alkylatesto another cumenemolecule,which is in a free molecularstate, and forms iiPB. When the extent of the cracking reaction is greaterthan that of the alkylationreaction, there will be an excessivebenzene yield with respectto DPB yields and the Sn
0
SNl
0
+
cp
-
1
c&,c_c&A ji!s
\
\
C
C C+
Fig. 1. Cumene disproportionation mechanisms.
202
will be low. The cracked i-propyl ion prior to alkylation can isomerize to an npropyl ion. Therefore, in addition to iiPB, n-propyl-i-propyl benzene (niPB) can be obtained among the DPB isomers. The disproportionation mechanism can then be identified by examining the DPB isomer distribution and Sn. Cumene disproportionation was demonstrated as a probe reaction for zeolite structures, as shown in Table 1. At low conversion levels, over zeolite /3,Y and mordenite, which are twelve-membered ring zeolites, cumene undergoes disproportionation at a Sn of nearly 100 mol-%. However, over ZSM-5, which is a ten-membered ring zeolite, the Sn is much lower. Meanwhile, its benzene molar yield is greater than the iiPB molar yield and the B/DPB ratio greatly exceeds unity. We have concluded from n-propylbenzene disproportionation that disproportionation over wide pore twelve-membered ring zeolites such as zeolite j3 and Y undergoes a bimolecular mechanism whereas over restricted twelvemembered ring zeolites such as mordenite, disproportionation proceeds a combined mechanism of the monomolecular and bimolecular mechanisms. Over ten-membered ring zeolites such as ZSM-5, disproportionation undergoes a monomolecular mechanism [ 31. The different reaction mechanisms over varTABLE 1 Comparison of activity, selectivity and stability of zeolites in cumene disproportionation conversion levels WHSV, 3.4 g h-’ g-,-l;
nitrogen/cumene,
Reaction temperature ( “C) Conversion (wt.-% )
0.19 mol/mol; time-on-stream, HY
HM
HZSM-5”
118 14.0
205 13.0
200 5.9
120 6.3
0.99 1.0
(Y
“Time-on-stream, 30 min.
60 min
H-B
Selectivity (mol/mol) Disproportionation Benzene/DPB m/p-iiPB
Product yields (wt.-%) Benzene Cumene n-Propylbenzene o-iiPB m-iiPB p-iiPB p-niPB Other aromatics
at low
0.95
0.93
0.69
1.75
1.05 2.10
1.09 1.71
2.07 0.23
0.07
0.15
0.35
4.50 86.05 0.11
4.21 87.04 0.13 0.19 5.49 2.62
1.95 94.10 0.15 0.10 2.28 1.33
5.94 3.4 -
-
0.32
0.09
2.92
93.69 0.05 0.52 2.28 0.13 0.41
203
ious zeolites can account for their difference in Sn in cumene disproportionation (Table 1). We have suggested that the biphenyl complex for the bimolecular mechanism is too large to be formed inside the ZSM-5 pores [ 31. In addition, the ipropyl group of cumene would have a size close to the i-butane molecular size of 5.0 A [161 which is critical to the pore opening of ZSM-$5.1 A [171. Therefore cumene disproportionation over ZSM-5 is a monomolecular reaction. The monomolecular mechanism results from the shape selectivity of the transition complex and the reactant. The same reaction mechanism was also proposed by Fukase and Wojciechowski [ 18 1. In the monomolecular mechanism, the cracked i-propyl ion tends to polymerize to form coke which plugs pore mouths and deactivates catalytic activity [ 191. Therefore, when comparing with twelve-membered ring zeolites, ZSM-5 exhibits an unusually poor stability in cumene disproportionation. In addition, because of the isomerization of the i-propyl ion, ZSM-5 produces a great proportion of niPB among the DPB products. In contrast, zeolite 8, Y and mordenite have Sn of near unity (Table l), indicating that those three twelve-membered ring zeolites catalyze a bimolecular reaction. In addition, only iiPB among the DPB isomers was obtained. Although mordenite catalyzes n-propylbenzene disproportionation with a reaction mechanism combining a monomolecular mechanism and a bimolecular mechanism [ 3 1, it catalyzes cumene disproportionation with a bimolecular mechanism. The shift of the reaction mechanism could be due to the great stability of the biphenyl intermediates stabilized by the x-electrons of the benzene rings (Fig. 1) . The bimolecular mechanism in cumene disproportionation over mordenite leads to a high Sn. The So increases from 0.75 in n-propylbenzene at a conversion of 5.47 wt.-% [ 31 to 0.91 in cumene at a conversion of 5.9 wt.-% (Table 2). The deactivation of the zeolite activity with time-on-stream (TOS) was observed. The deactivation behavior can be expressed as a power law, eqn. (3 ) . xt=x()t
--(y
(3)
where X, is the conversion measured at TOS oft; X,, is the initial conversion extrapolated to zero time-on-stream and aris a decay exponent. The lower the value of (Y,the greater is the stability. Among the twelve-membered ring zeolites studied, zeolite /3has the best stability whereas mordenite has the worst one (Table 1) . However, twelve-membered ring zeolites always have a better stability than ten-membered ring zeolites (Table 1) . As discussed earlier, the difference in stability results from the difference in the reaction mechanism. For twelve-membered zeolites whose reaction mechanisms are the same, zeolite pore structures could affect their stability. Guisnet and Magnoux have studied zeolite stability in hexane cracking [ 201. They proposed a concept of
204 TABLE 2 Comparison between cumene disproportionation mordenite DPB =Dipropylbenzene; di-n-propylbenzene
and n-propylbenzene
disproportionation
iiPB, Diieopropylbenzene; niPB, n-propyl-i-propylbenzene; nnPB/DPB,
Feedstock
Cumene
n-Propylbenzene
SiOz/A1203 Reaction temperature ( ‘C) WHSV (g/g,& 1 Conversion (wt.-% )
7.0 200 3.4 5.9
19.0 199 2.0 5.47
Selectivity (mol/mol) Dispropoxtionation Benzene/DPB iiPB/DPB niPB/DPB nnPB/DPB
over
0.93 1.09 1.00 0 0
0.75 1.23 0 0.26 0.74
7.0 356 3.4 11.6 0.40 4.16 0.14 0.50 0.36
“coke toxicity”. For ZSMd which has interconnecting channels and is without cavities, coke forms in the intersections. However, for mordenite whose pore system is a monodimensional channel, coke forms and completely plugs its pore mouth. Therefore, ZSM-5 can tolerate much higher coke deposition than mordenite. According to the proposal of G&net and Magnoux, the coke toxicity for ZSM-5 is lower than that for other zeolites. Similarly, the good stability of zeolite /9 can be attributed to its pore channel systems which are also interconnecting with no cavities. A comparison of zeolites in cumene disproportionation at high conversion levels is shown in Table 3. Among the zeolites studied, zeolite jl has the greatest activity and Sn. The best Sn can be attributed to its low reaction temperature requirements. It can also be attributed to the loose structure of zeolite /3 such as we proposed in n-propylbenzene disproportionation [ 31. For zeolites with the same reaction mechanism, their catalytic activity mainly depends on acidity. Hedge et al. concluded from infrared measurements that zeolite B has a higher acidity than Y [ 211. The results that zeolite j? has a higher activity than Y is consistent with the acidity measurement. Kaeding recently reported that zeolite ZSM-12, being another high siliceous twelve-membered ring zeolite, can also selectively catalyze cumene disproportionation [ 81. This result further supports our proposal that cumene disproportionation can be selectively catalyzed by twelve-membered ring zeolites. For zeolite p and ZSM-12, being high siliceous zeolites, their maximum attainable conversion with a minimum Sn of 0.90 mol/mol are much higher than zeolite Y and mordenite (Table 3 ) . It is shown in Fig. 2 that in cumene disproportionation over zeolite j3,zeolite activity decreases and the maximum attainable conversion increases with in-
205 TABLE 3 Comparison of activity, selectivity and stability of zeolitea at high disproportionation conversion of cumene WHSV, 3.4 g h-r bt-‘;
nitrogen/cumene,
Reaction temperature ( oC ) Conversion (wt.-%) Coke (g/g,,% )*
0.19 mol/mol; time-on-stream, 60 min
H-8
HY
HM
HZSM-12”
143 40.1 4.72
220 22.5 4.54
240 14.5 6.62
250 45.1 -
Selectivity (mol/mal) Disproportionation Benzene/DPB m/p-iiPB
0.98 1.06 2.05
0.91 1.16 2.18
0.95 1.12 2.01
o!
0.23
0.14
0.31
13.5 59.88 0.10
7.74 77.53 0.17 0.42 9.17 4.21 0.76
4.95 35.55 0.14 0.11 6.08 3.03 0.14
0.90 1.17 1.81
Product yields (wt.-%) Benzene Cumene n-Propylbenzene o-iiPB m-iiPB p-iiPB Other aromatics
-
17.76 8.68 0.08
“Data from Kaeding [8]. *Coke deposition after 3 h of time-on-stream
“-o1l
15.42 54.9 0.32 0.23 17.54 9.70 1.89
reaction conditions.
0
Fig. 2. Plot of the maximum attainable conversion and their reaction temperatures plotted against the silica-to-alumina ratio in cumene disproportionation over zeolite 8. (* ) Maximum conversion; (n ) reaction temperature.
206
0
10
20 Gmersicqwt
30
40
50
*I.
Fig. 3. Dependence of the m/p-diisopropylhenzene ratio of zeolites on catalytic conversion [weight hourly space velocity (WHSV), 3.4 g h-’ gu,-‘; nitrogen/cumene, 0.19 mol/mol; time-on-stream, 60min]. ([7)8; (A) mordenite; (0) Y; (A) ZSM-12; (0) ZSM-5.
creasing SiOz-to-AlgO ratio. This could be the result of the characteristics of the spatial distribution of the acid sites. The distances between neighboring sites inside the pores of zeolites increase with their SiO,-to&O3 ratio. Thus, the pore systems of higher siliceous zeolites can accommodate a greater number of diphenylcarbenium ions without cracking. Accordingly, it is possible to use the maximum disproportionation conversion as a measurement of acid site distribution. It was found that the formation of iiPB isomers is highly shape-selective (Fig. 3 and Table 1). The m-iiPB to p-iiPB ratio, m/p-iiPB, is greater for zeolites with larger pore openings. This ratio increases with increasing conversion levels and eventually reaches parity, indicating a thermodynamic equilibrium value which was reported nearly 1.8 [ 221. The shape selectivity results from the relatively bulky iiPB molecules in terms of the zeolite pore openings. CONCLUSION
The type of reaction mechanism in cumene disproportionation was derived from the fact that the bimolecular mechanism catalyzes a disproportionation selectivity and diisopropylbenzene fraction among the dipropylbenzene isomer of near unity. It can be concluded that twelve-membered zeolites take a bimolecular reaction mechanism whereas ten-membered ring zeolites take a monomolecular reaction mechanism. A zeolite family which catalyzes a bimolecular mechanism has a better catalytic stability than those which catalyze a monomolecular mechanism. Cumene disproportionation can also be used as a measurement of acid site distribution. It is shown that high siliceous zeolites can obtain high disproportionation conversion and maintain good disproportionation selectivity. It is noted that the disproportionation reaction mechanism could be deter-
207
mined not only by zeolite structures, but also by the types of alkylbenzenes. Over mordenite, the disproportionation mechanism shifts from a reaction mechanism combining a monomolecular mechanism and a bimolecular mechanism in n-propylbenzene disproportionation to a bimolecular mechanism in cumene disproportionation. In terms of activity, disproportionation selectivity and stability, zeolitej3 has been shown as a promising catalyst for diisopropylbenzene production by cumene disproportionation. ACKNOWLEDGMENT
We would like to thank F.J. Yang for the preparation of the manuscript. This study was financially supported by the Taiwan Styrene Monomer Company.
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