Transformation of lower alkanes into aromatic hydrocarbons over ZSM-5 zeolites.

Abstract

The yields of aromatic hydrocarbons in the transformation of lower alkanes over ZSM-5 zeolites are greatly enhanced by incorporation of gallium or zinc cations into the zeolites. Effects of contact time on the product distribution in the conversion of hexane over H-ZSM-5 and Ga-ZSM-5 were examined in detail in order to obtain information on the reaction pathway. When contact time is short, only cracking of hexane prevails and when the time is extended, the olefins formed by cracking are converted into aromatics. It is concluded that gallium cations do not affect the capacity for hexane cracking, but they enhance the activity and efficacy for aromatization of olefins. Aromatization of propylene proceeds much more effectively over Zn-ZSM-5 than over H-ZSM-5. The lower yield of propane over Zn-ZSM-5 indicates that zinc cations serve as a catalyst for dehydrogenation of olefinic species. While Zn-borosilicate is not effective for promoting propane activation, it is effective for promoting propylene aromatization. It gives high benzene selectivity at low conversion level, indicating the intermediary of allylic species in the aromatization reaction.