Two alternative mechanisms of aromatization of light hydrocarbons on Ga/ZSM-5 and Zn/ZSM-5 zeolites

By means of IR diffuse reflectance spectroscopy, it was shown that aromatization of light paraffins on the GaZSM-5 and ZnZSM-5 catalysts occurs through different routes. The first step of transformation on GaZSM-5 is the oxidative addition of paraffin molecules to Ga⁺ ions resulting in homolytic dissociation and the appearance of particles [H-Ga-R]⁺, whereas dehydrogenation on zinc-containing high-silica zeolites results from heterolytic dissociative adsorption on acid-base pairs yielding Brönsted acid centers and alkyl groups bonded to zinc. The subsequent degradation of the alkyl fragments leads to the formation of zinc hydrides or gallium dihydrides and olefins. The oligimerization of olefins in the case of ZnZSM-5 can occur on both acidic hydroxyl groups produced via paraffin dissociation and on the Zn²⁺ cations per se already at ambient temperature. In the case of GaZSM-5, the appearance of Brönsted sites in the catalyst is achieved via incomplete replacement of OH groups with gallium cations.