High-efficiency nano [Zn,Al]ZSM-5 bifunctional catalysts for dimethyl ether conversion to isoparaffin-rich gasoline

Abstract

The development of nonpetroleum routes for producing high-octane clean gasoline rich in isoparaffins is still an important challenge today. Here, nanosized ZSM-5 (NZ5) and zinc isomorphously substituted ZSM-5 ([Zn,Al]NZ5) zeolites with different Si/Me ratios in initial gels (Me = Al or Al and Zn) were prepared in situ by a seed-induced method and were used as high-efficiency catalysts in the reaction of dimethyl ether (DME) to gasoline (DTG). It was found that the formation of isoparaffins and aromatics are competitive processes. The long-chain olefin intermediates can be converted to aromatics by cyclization and hydrogen transfer on the Brønsted acid site (BAS), whereas they undergo the hydroisomerization to form isoparaffins on the hydrogenation species. Excellent catalytic activity and high isoparaffin selectivity were obtained for the [Zn,Al]NZ5 samples due to the reduced BAS and the unique ZnOH+ species with high ability for the heterolysis of hydrogen and hydrogenation activity under a syngas atmosphere. When the reaction atmosphere changes from syngas to H2, the isoparaffin selectivity in gasoline component over the [Zn,Al]NZ5(20) sample increased further, from 67.3 wt% to 76.5 wt%, which is superior to the NZ5 series of samples and to ever reported catalyst.