Effect of zeolite pore structure on the diffusion and catalytic behaviors in the transalkylation of toluene with 1,2,4-trimethylbenzene

Abstract

Effects of pore structures of ZSM-5, MOR, Beta and USY on their catalytic properties in the transalkylation of toluene with 1,2,4-trimethylbenzene (1,2,4-TMB) were investigated by studying the catalytic mechanism and the diffusion behavior of reactants and products. The medium-pore ZSM-5 shows low catalytic activity as a result of slow decomposition of diphenylmethane (DPM) intermediates and occurrence of severe cracking of 1,2,4-TMB via the monomolecular reaction pathway. In contrast, the large-pore MOR, Beta and USY zeolites allow rapid formation and transformation of bulky DPM species, consequently exhibiting higher catalytic activity. The diffusion experiments show that 1,2,4-TMB and 1,2,4,5-tetramethylbezene (1,2,4,5-TeMB) more easily diffuse into/out of the pores of MOR zeolite than of Beta and USY. Thus, the disproportionation of 1,2,4-TMB and coking were inhibited in MOR, which makes it exhibit higher catalytic stability and xylene selectivity. A detailed analysis of the deposited coke species with GC-MS indicates that the transalkylation of toluene with 1,2,4-TMB occurs via the bimolecular intermediate mechanism.