On the mechanism for ethyl transfer and removal from ethylbenzene during commercial xylene isomerization

Abstract

Abstract Ethylbenzene (EB), a structural isomer of xylenes, is present in feedstocks for commercial xylene isomerization. EB cannot be economically separated from the close boiling xylenes by distillation. It must be converted to lower and/or higher boiling byproducts that can, to prevent its buildup in the xylene isomerization unit recycle loop. Several routes are available for converting EB, including ethyl transfer to another aromatic via transalkylation, and EB dealkylation. A mechanism initiated by hydride abstraction has often been cited for ethyl transfer over large pore molecular sieves, based on old data developed for liquid phase conversion over HF/BF3 catalysts which are strong Lewis acids. However, ethyl transfer occurs readily over large pore molecular sieves that contain only Bronsted acid sites, and little or no Lewis acidity via a mechanism in which the ethyl group never leaves the first ring before being transferred to a second. I propose reconsideration of an SN2 mechanism instead. Also, a published mechanism proposed for EB dealkylation/realkylation that occurs over medium pore sieves is corrected to show ethylene as a stable intermediate and expanded to provide the mechanism for EB dealkylation over catalysts comprising medium pore sieves and a mild hydrogenation catalyst. The role of dual bed catalysts that employ a combination of transition state and product shape selectivity to further reduce xylene loss is explained.