Formation of 1, 2, 4-trimethylbenzene by shape-selective alkylation of xylenes with methanol on H-ZSM-5 zeolite

Formation of 1, 2, 4-trimethylbenzene by shape-selective alkylation of xylenes with methanol on H-ZSM-5 zeolite Seitaro Nmnba, Akira Inaka and Tatsuaki

Yashima

Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan (Received 18 October 1982) The three xylene isomers have been alkylated with methanol on H-ZSM-5. The main aromatic products were xylene isomers and 1,2,4-trimethylbenzene, the latter constituting more than 99% of the trimethylbenzene fraction. The yield of 1,2,4-trimethylbenzene depended on the xylene isomer used initially. Keywords: Zeolite ZSM-5; 1,2,4-trimethylbenzene; alkylation, shape-selectivity; xylene; methanol

The molecular dimension (minimum cross-section) of 1,2,4-trimethylbenzene (1,2,4-TMB) is smaller than the other TMB isomers, 1,2,3- and 1,3,5TMBsL Therefore, 1,2,4-TMB should be predominantly produced by some shape-selective reactions on suitable caudysts. In our previous paper 2, 1,2,4-TMB is predominantly formed by the shapeselective disproportionation of xylene on Cu-Hmordenite and the fraction of 1,2,4-isomer in TMB formed is 78% at 20% of the TMB yield. In the conversion of methanol into hydrocarbons on H-ZSM-5 zeolite at 644 K 3, the fraction of 1,2,4-isomer in TMB formed is about 79%, ~dthough the xylenes formed are essentially in thermodynamic equilibrium with each other. Therefore, we expect that 1,2,4-TMB is selectively formed by the alkylation of xylene with methanol on H-ZSM-5 zeolite at reaction temperatures lower than 644 K. The catalyst used was H-ZSM-5, whose Si/AI and Na/AI atomic ratios were 44 and less than 0.005, respectivcly. The H-ZSM-5 synthesized as previously described < s with the particle size of about 58 nm determined from the broadening of X-ray diffraction. The alkylation of o-, m-, p-xylene with methanol was carried out in a fixed-bed type apparatus with a continuous llow system at atmospheric pressure using helium as a carrier gas. The reaction conditions were as follows; reaction temperature = 548 K, amount of catalyst = 1.44 g, W/F = 16.2 (g-cat.) h tool -1 (where W was catalyst weight and F was molar feed rate of reactants and helium), molar ratio of methanol to xylene = 2, partial pressure of the reactants = 20.3 kPa. As the alkylation products, TMBs, tetramethylbenzenes and dimethylethylbenzenes were obtained together with the isomerization products, xylene 0144-2449/83/030106-03503.00 © Butterworth & Co. (Publishers) l t d .

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isomers. In the conversion of methanol under similar reaction conditions, the yield of aromatic hydrocarbons was extremely low. Therefore, the aromatic hydrocarbons formed in the alkylation were essentially derived from tile reactant xylene. In the alkylation of m-xylene with methanol on H-ZSM-5 zeolite at 548 K, the changes of yields of the alkylation and isomerization products with process time are shown in Figure 1. The yields of o- and p-xylenes and of dimethylethylbenzcnes decreased with process time. On the other hand, the yields of TMBs and tetranaethylbenzenes did not change with process time for 4 h. Dimethylethyl-

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Process time /h Figure 1 Yields of (o) trimethylbenzenes, (.) tetramethylbenzenes, (o) dimethylethylbenzenes, (za) o-xylene and (A) p-xylene as a function of process time. The reaction conditions are described in the text

Short papers

Table 1 Yield of trimethylbenzenes and fraction of 1,2,4-isomer in trimethylbenzenes formed*

Reactant

Yields of TMB (%)

Fraction of 1,2,4-isomer in TMB formed (%)

o-xylene m-xylene p-xylene

10.1 13.3 13.4

99.7 99.6 99.7

* Reaction conditions are described in the text

benzenes might be formed by the alkylation of xylene with ethylene, which was derived from methanol. The fraction of 1,2,4-isomer, the smallest isomer, in TMB formed was always more than 99%. Similar phenomena were observed in the alkylation of o- or p-xylene.

1,2,4-TMB there are only two positions. Moreover, it has been reported 6 that the diffusivity o f p xylene is much higher than that of o- or m-xylene. In the case of rn-xylene, as mentioned above, there are two positions to form 1,2,4-TMB. At these positions, m-xylene is more easily alkylated than por o-xylene, because in the alkylation of alkylbenzenes para/ortho orientation generally predominates. From above discussions, the order of the yields for three kinds of xylene can be explained. The fraction of 1,2,4-isomer in TMB formed for o-, rn- or p-xylene is more than 99.5%, so the shape-selectivity of H-ZSM-5 is very high in this aikylation.

Table 1 shows the yield of TMB and fraction of

REFERENCES

1,2,4-isomer in TMB formed in the alkylations of three kinds of xylene. The yield of 1,2,4-TMB changed with kinds of xylene. The order of the yields for the three xylene isomers was: p- = m - > o-.

1 Csicsery, S. M. d. Catal, 1970, 19,394 2 Namba, S., Iwase, O., Takahashi, N., Yashima, T. and Hara, N. J. Catal. 1979, 56,445 3 Chang, C. D. andSilvestri, A.J.J. Catal. 1977,47,249 4 Yashima, T., Sakaguchi, Y. and Namba, S. 'Studies in Surface Science and Catalysis', Elsevier, Amsterdam, 1981, Vol. 7, p. 739 5 Yoshimura, A., Namba, S. and Yashima, T. Shokubai 1981,23, 232 6 Weisz, P. B. "Studies in Surface Science and Catalysis', Elsevier, Amsterdam, 1981, Vol. 7, p. 3

Forp-xylene, to form 1,2,4-TMB there are four positions in the benzene ring, where alkylation can occur. However, for o- and m-xylenes, to form

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