942
I n d . Eng. Chem. Res. 1988, 27, 942-947 Villadsen, J.; Livbjerg, H. Catal. Reu.-Sci. Eng. 1978, 17, 203.
Shaw, 1.4. Ph.D. Dissertation, Northwestern University, Evanston, IL,1985. Smidt, J.; Witlafner, R.; Jira, J.; Sedlmier, J.; Seiber, R.; Kajer, H. Angew. Chem. 1959, 71,176. Szonyi, G . Adv. Chem. Ser. 1968, 70, 53.
Received for review May 4, 1987 Revised manuscript received December 18, 1987 Accepted February 10, 1988
Isomerization of Ethylbenzene and m -Xylene on Zeolites Y. S. Hsu, T. Y. Lee,*and H. C. Hu Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, 30043, R.O.C.
Simultaneous isomerization of ethylbenzene and m-xylene on zeolite catalysts, including Pt/mordenite, Pt/USY, Pt/ZSM-5, and Pd/ZSM-5, was studied. Experimental results indicated that Pt/ZSM-5 was the superior catalyst for these reactions. Pd/ZSM-5 is better than Pt/USY, although, both are good enough for the reactions, in comparison with Pt/mordenite. A kinetic model with plausible reaction paths was proposed for the isomerization. The estimated reaction rate constants and activation energies indicated that the transformation of m-xylene to o- or p-xylene might be limited by the mass-transfer rate of the diphenylmethane-type intermediate, and the formation of o-xylene from ethylbenzene could be restricted by the smaller protonated cyclopropane intermediate. reactor: the first section was the preheating zone filled with 1-mm glass beads; the middle section was the reaction zone where the catalyst bed was located and a thermocouple was inserted into the middle of the catalyst bed in order to measure and control the temperature of the reaction; the last section was filled with glass beads of the same size as in the first section to maintain and stabilize the temperature and flow inside the reactor. The pipings were wrapped with heating tape to avoid condensation. B. Materials. Ultrastable Y (USY) was obtained from the dealuminization of NaY purchased from Strem Chemicals. The detailed procedure is described elsewhere (Hsu, 1986). ZSM-5 was synthesized in our laboratory and has a Si/Al ratio of 70-90 with approximately 30% Alz03 binder. Pt/Mordenite was obtained from Chinese Petroleum Corp., which contained 0.4% Pt. Pt/USY was obtained by ion exchange in an aqueous solution of Pt(NHJ4C12 at 25 "C for 48 h and reduced in hydrogen atmosphere at 400 "C for 3 h. The extent of exchange was measured by atomic absorption spectroscopy of the concentrations of the solution before and after the exchange. Pt/ZSM-5 was prepared by the same procedure except that the reduction temperature was at 500 "C. The resulting Pt/USY and Pt/ZSM-5 contained 0.31% and 0.3% Pt, respectively. Pd(NH3)4C12solution was used for Pd/ZSM-5 ion exchange instead of Pt(NH3)&12,and all other conditions remained the same. The resulting Pd/ ZSM-5 contained 0.34% Pd. All the particle sizes of the zeolite are smaller than 0.85 mm (20 mesh). H2PtClg6H20 and H2PdCk6H20were purchased from Higuchi Chemical Laboratory, Tokyo, Japan. NH40H was purchased from Union Chemical Laboratories, ITRI, Taiwan, Republic of China. NzH4.2HC1,m-xylene, and ethylbenzene of research grade were purchased from Merck. C. Procedure. Normally the feed ratio of ethylbenzene to m-xylene was fixed at 113. The total feed rate and amount of catalyst placed in the reactor were varied to study the level of conversion. Before the run, the reactor was purged with N2 for h and then heated to the desire temperature. The H2/hydrocarbonsratio was adjusted to 10. The outlet samples were not taken until the reaction was stabilized for 3 h. In case the catalyst needed regeneration, air was pumped in for 5 h at 400 "C and then purged with N2 for 1 h and reduced in H2 for 3 h. D. Analysis. Samples were analyzed by a chromatograph from Shimadzu Co., Kyoto, Japan, Model GC-GA,
The C8 aromatics consist of four isomers: o-, m-, and p-xylene and ethylbenzene. Their boiling points are very close together, but their melting points are relative far apart. Consequently, in a petrorefinery, m-xylene and p-xylene are traditionally separated by crystallization. Recently, an adsorptive column for the separation of xylenes and ethylbenzene has become widely used. Mixed xylenes have limited usage, while p - and o-xylene are extensively used for the plasticizer, synthetic fiber, resin, dye, and paint industries. After the removal of p - and o-xylene from the reformate of a refinery, the remaining m-xylene and ethylbenzene are taken back into the catalytic isomerizer and isomerized again (Hancock, 1982). The standard catalyst for the isomerization is precious metals on solid acid supports, and normally hydrogen is added to promote the isomerization and prevent coking on the catalyst. There are many postulations concerning the roles of precious metal and solid support on the catalytic isomerization. But the general consensus is that the metal site is used for hydrogenation and dehydrogenation and the solid acid support is served for skeletal rearrangement. In fact, the reactions are very complicated. They may involve naphthenic intermediates, and many side reactions such as disproportionation, transalkylation, and hydrodealkylation, etc., may also occur. There were extensive studies in the literature concerning the isomerization of xylenes and ethylbenzene. Most of the studies dealt with xylenes or ethylbenzene individually (Robschlager and Christoffel, 1979; Nitta and Jacobs, 1980; Sosa et al., 1984; Chutoransky and Dwyer, 1973; Collins et al., 1982,1983; Cortes and Corma, 1978, 1979; Corma and Cortes, 1980). In this report, two precious metals, Pd and Pt, and three solid supports, Y, ultrastable Y, and ZSM-5, with an increasing Si/A1 ratio were selected for study with the intention of elucidating the best possible performing catalyst and of illustrating a few possible reaction paths on the roles of metals and acid supports for the complex reactions of isomerization of m-xylene and ethylbenzene.
Experimental Section A. Experimental Setup. The equipment employed was a laboratory-scale integral reactor of stainless steel tube 2 cm in diameter. There were three sections in the
* To whom
correspondence should be addressed.
0888-588518812627-0942501.50I O
Q
1988 American Chemical Societv
Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988 943 25
20
5
0 W I F (9-cot hrlg-HCI
Figure 3. Product distribution on Pt/USY. W I F (9-col h r l g - H C ,
75
25 EB
Figure 1. Comparison of activity. 0
m-xylene
A
ethylbenzene
0
o-xy1ene toluene
W/F W I F i g - c a t hrtg-HCl
I
19-cat h r / g - H C
Figure 4. Product distribution on Pt/ZSM-B at 300 "C, compared with model prediction (-).
Figure 2. Product distribution on Pt/mordenite at 400 "C.
with FID and a Hewlett-Packard Model 3390 integrator. The GC column used was 5% SP-1200/1.75% Bentone 34 on 100/120 Supelcoport, l/s in. X 6 ft. Operation conditions were injector temperature, 220 OC; detector temperature, 220 "C; and oven temperature, from 65 to 125 "C programmed at 2 OC/min. Components of benzene, toluene, ethylbenzene, p-xylene, m-xylene, o-xylene, 1,3,5-trimethylbenzene, 1,2,4-trimethylbenzene, and 1,2,3-trimethylbenzenewere well separated by the column.
Results and Discussion The total conversion of the four catalysts, Pt/mordenite (MN), Pt/USY, Pd/ZSM-5 and Pt/ZSM-5, is shown in Figure 1, where the total conversion is defined as [ (ethylbenzene (EB) and m-xylene converted)/ (initial EB and m-xylene)]lOO. In comparison with the other three catalysts, the activity of Pt/MN was extremely small. The activity of the four catalysts was in the following order: Pt/MN meta > ortho and was more pro-
Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988 945 Table.11. Fitted Rate Constants on Pt/ZSM-5 rate constant, k" (x103) temp, "C
kl
k-1
kz
k-2
k3
k4
k-4
k6
k-5
activation energy, E,b preexponent factor, A"
145 302 513 9.4 566
64.6 135 231 9.4 268
33.6 61.1 88.5 7.1 17.5
79.9 138 192 6.4 23.2
1.56 7.17 24.4 20.5 115000
4.16 13.5 31.1 14.5 1590
1.04 4.06 10.9 16.9 3430
7.80 32.8 104 19.3 198 000
8.25 33.3 101 18.7 121000
300 350 400
In mol/(g of catalyst-h). *In kcal/mol. 100
350'C
p-xylene A 300'C
A
extremely fast and reached pseudoequilibrium state. (e) Transalkylation and disproportionation reactions were extremely small and were neglected. (f) The four isomers have the same rate of dealkylation to become benzene and methane. The resultant mass balance for species i in the integral reactor is dXi/d(W/Ft,) = (1 - XM)(Rj - XiRM)
LWt
The system equations for the reactions are dX,
ATE of m-xylene
Figure 10. ATE on Pt/USY.
found at higher temperatures. The results were consistent with the study of Collins et al. (1983) on HZSM-5. The same characteristics of relative reaction rate, para > meta > ortho, were observed on Pt/ZSM-5 and Pd/ZSM-5 also.
,
Reaction Paths and Model The simultaneous reactions of xylenes and ethylbenzene on Pt/zeolite were investigated by Robschlager and Christoffel (1980) and the following reaction paths were proposed: m-xylene e===== o-xylene
/ ~'
other products
1 pxylene dl
other products
ethylbenzene
Their model fit the experimental data quite well. But in this study, the reactions of ethylbenzene and m-xylene only, not all the xylenes, on zeolites were investigated. m-xylene I \\
.%
k.2
- o-xylene -. /A \
k4 k-L
- ethylbenzene /
I I
141
k3
.benzene
Y + methane
The crushed pellets were packed into the reactor; hence strictly speaking, the crushed pellets consist of bidispersed pore structure. Consequently, in the treatment of the kinetic model, all the diffusional effects are lumped into the reaction constants. The film resistance between the gas phase and the solid phase is probably eliminated by the fast flow of the feed. After elaborate model selection and comparison, the following model was selected for Pt/ZSM-5 catalyst based on the smallest total error of the sum of squares between the models and the experimental data. Other assumptions in the model were as follows: (a) The rate equation was expressed by mass action law. (b) H2/HC was large, and the consumption of Hz was less than 3%; hence, the effect of hydrogen partial pressure was combined in the rate constants. (c) Plug flow was assumed. (d) Toluene observed was very small; hence, the cracking rate of toluene to benzene and methane was considered
dXb d(W/J'tJ
1 = -ks(l 11
- X,)(Xe
+ x, + x, + Xm)
where Ft,= inlet HC flow rate (mol/h), X, = mole fraction of species i (a), W = catalyst weight (g), Ri = rate of appearance of species i (mol/(g of catalyst-h)), k = rate constant (mol/(g of catalyst-h)),and subscripts e, p, m, 0, b, and M denote ethylbenzene, p-xylene, m-xylene, xylene, benzene, and methane. This system of equations was solved by the Himmelblau et al. (1967) method, and the rate constants obtained by using experimental data on Pt/ZSI 1-5 are listed in Table 11. If the Arrhenius form is employed, the activation energies of reactions can be obtained by the least-squares method and are listed in Table I1 also. Comparisons of the model and experimental data on Pt/ZSM-5 are shown in Figures 4-6. The deviations were small at 300 "C and increased slightly at 350 and 400 "C. In general, the agreement was reasonably good. Particularly, the isomerization of ethylbenzene is believed to go through hydrogenation, skeletal rearrangement, and dehydrogenation steps (Pearce and Patterson, 1981; Robschlager and Christoffel, 1979; Nitta and Jacobs, 1980). Robschlager and Christoffel (1979) studied the isomerization of ethylbenzene on Pt/r-Al,O, and Pt/ zeolites in hydrogen atmosphere. On a low acidity catalyst such as Pt/r-Al,O,, the isomerization mechanism might
Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988
946
- -
--
consist of the following steps: ethylbenzene ethylcyclohexene 1-ethyl-2-methylcyclopentene 1,2-dimethylcyclohexene o-xylene. On a high acidity catalyst such as Pt/zeolites, in addition to the above steps, it might also consist of the secondary carbocation step and produce other xylenes. Nitta and Jacobs (1980) investigated the ethylbenzene isomerization on Pt/NaHY, They agreed with the postulation of Robschlager and Christoffel(l979) that the mechanism involved the protonated cyclopropane as intermediate and showed the reaction steps as follows:
6;Pj=&=b+ (a)
11
/
Increasing the acidity of the zeolite would increase the side reactions of dealkylation and disproportionation and hence reduce the yields of xylenes. Sosa et al. (1984) found that ethylbenzene isomerization on ZSM-5 and ZSM-11 was not taking reaction path a which was energetically favored; instead, paths b and c were taken. It was postulated that the pore size on ZSM-5 or ZSM-11 was smaller than Y zeolite; consequently, the isomerization was forced to take paths b and c due to the internal mass-transfer restriction. The isomerization of xylenes might be explained by the intermolecular or intramolecular methyl transfer, or through bimolecular transalkylation via the diphenylmethane as the intermediate state. One of the methyl groups in m-xylene might shift to the adjacent positions through a series of reactions and become o-xylene or pxylene (Lanewala and Bolton, 1969; Collins et al., 1982; Cortes and Corma, 1978, 1979; Corma and Cortes, 1980). Some of the studies (Collins et al., 1983; Chutoransky and Dwyer, 1973) indicated a triangle reaction path where p-xylene could be converted into o-xylene also. This could be explained by the relatively fast movement of the para isomer inside the porous catalyst and might cause an apparent 1,3 shift of the methyl group in the benzene ring as cited by Young et al. (1982). Judging by the magnitude of the activation energies, it appeared that k,, k+ k,, and k-2 were controlled by intrinsic diffusion and k3, k4, k-*, k5, and k-5 were in the transition regime of diffusion and reaction, both of them seemingly important. For those reactions with an activation energy of less than 10 kcal/mol such as k,, k+ k2, and k-,, diphenylmethane-type transition intermediates might occur before the diphenylmethane fmally is broken up into two rings. And for those reactions with an activation energy greater than 14 kcal/mol such as k3, k4, k-4, k5, and k-5, carbonium ions might be formed as the intermediates and chemical reaction played a bigger role.
Conclusion From the experimental results of this study, we conclude the following: (a) The activity of the catalyst is proportional to the temperature and is in the following order: Pt/MN
