Reaction intermediates and mechanism of the zeolite-catalyzed transalkylation of 1,2,4-trimethylbenzene with toluene

Abstract

We report the catalytic properties of a series of large-pore (H-Y, H-beta, H-mordenite, and H-UZM-35) and medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites with different framework structures for the transalkylation of 1,2,4-trimethylbenzene (1,2,4-TMB) with toluene. H-NU-87 with intersecting 10- and 12-ring channels, but in which access to the inner part of the crystal can only occur through the 10-ring pores, was found to show a significantly higher xylene yield (40% vs. 23% at 673 K and 10 h on stream) and catalyst stability (31% vs. 17% 1,2,4-TMB conversion after 30 h on stream at 673 K) than the cage-based large-pore zeolite H-Y, the current commercial transalkylation catalyst. GC–MS analyses of organic species occluded in used zeolite catalysts reveal that the type of diphenylmethane derivatives serving as key reaction intermediates of 1,2,4-TMB-toluene transalkylation is strongly influenced by the pore architecture of the zeolite catalyst. A bimolecular diphenylmethane-mediated reaction mechanism for this transalkylation is proposed and discussed based on both experimental and theoretical results.