Investigations into the Mechanisms of Zeolite-Catalyzed Transalkylation of iso-Propylbenzene with Toluene

Abstract

The catalytic properties of four large-pore H-beta, Hmordenite, and H-UZM-35) and three medium-pore (H-NU-87, H-TNU-9, and H-ZSM-5) zeolites for the transalkylation of iso-propylbenzene (iPB) with toluene are investigated. Among the zeolite catalysts employed here, I-I-UZM-35 with a 12 x 10 x 10-ring channel system was found to exhibit a comparable cymenes yield to that of H-beta with a 12 x 12 x 12-ring channel system, the most widely studied catalyst for this reaction. GC-MS analysis reveals thatmonomethylated2,2,-diphenylpropane species, whose existence has not been experimentally verified yet, are serving as the main reaction intermediates of the bimolecular iPB-toluene transalkylation. Also, the intrazeolitic buildup of dimethylated 2,2-diphenylpropane and 2-methylphenyl-2-iso-propylphenylpropane species, which must be involved in the formation of 2-tolylpropanylium cations and thus in the simultaneous consumption and production of the reactant molecules (i.e., toluene and iPB), was observed. The formation of these three different groups of diphenylpropane species, which has been further supported by the DFT calculation results, allowed us to propose a new bimolecular reaction mechanism for this transalkylation. To our knowledge, our study is the first example where the repetitive mechanism is ascertained in zeolite-catalyzed reactions.