Zeolite-Catalyzed Disproportionation of iso-Propylbenzene: Identification of Reaction Intermediates and Mechanism

Abstract

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 are compared in iso-propylbenzene (iPB) disproportionation. Among the zeolite catalysts studied here, H-UZM-35 with a three-dimensional framework consisting of one type of straight 12-ring channels and two types of tortuous 10-ring channels was found to show a di-iso-propylbenzenes (DiPBs) yield comparable to that of H-beta with two intersecting 12-ring channels, the best catalyst tested for this reaction to date. Gas chromatography mass spectrometry analysis of used zeolite catalysts demonstrates that while mono-iso-propylated 2,2-diphenylpropane derivatives serve as real reaction intermediates of iPB disproportionation over large-pore zeolites, mono-iso-propenylated 2,2-diphenylpropane species, which contain a double bond in the alkyl chain, are intermediates of its side reaction. Unlike that of other aromatic hydrocarbons such as m-xylene, ethylbenzene, and n-propylbenzene, the formation of di-iso-propylated derivatives was not observed as reaction intermediates. A new bimolecular diphenylpropane-mediated reaction pathway, which includes both intermediates of main and side reactions of iPB disproportionation, is proposed based on the experimental and theoretical results.