Generation of reductive Zn species over Zn/HZSM–5 catalysts for n–pentane aromatization

Abstract

The effects of hydrogen treatment on Zn/HZSM–5 catalysts to the evolution of zinc species and their activity in n-pentane aromatization were studied. By heating Zn/HZSM–5 catalysts under inert atmosphere, most of zinc species appeared as (ZnOH)+. The XPS and IPA–TPD results showed that using H2 as a treatment gas, the hydrodehydroxylation of (ZnOH)+ generated (ZnH)+ species with the recovery of some Brønsted acid sites. It was found that under the hydrogen atmosphere, the H2 molecule was dissociated on this Zn species and generated the H2-chemisorbed Zn species. Compared with HZSM-5 catalyst, the presence of (ZnOH)+ species improved the aromatics selectivity from 11% to 22%. Nevertheless, after pre-treating Zn/HZSM–5 with H2, the formation of H2-chemisorbed Zn species remarkably increased the BTX aromatics selectivity to 31%. It was found that the H2-chemisorbed Zn species was not stable in the absence of H2 however it could be regenerated upon H2 treatment.