The nature of the selective species in Fe-HZSM-5 for non-oxidative methane dehydroaromatization

Abstract

Conversion of natural gas to aromatics via non-oxidative dehydroaromatization (DHA) is a promising route to replace oil with a cheap and abundant domestic resource as feed-stock for the petrochemical industry. However, the reaction is still lacking a suitable catalyst system to-date. We present results from a study into the nature of the active species in Fe-containing HZSM-5 catalysts through the preparation of Fe-HZSM-5 via three different synthetic approaches: isomorphous substitution, wet ion exchange, and core–shell synthesis. By carefully investigating the impact of the preparation methods on the distribution of Fe species, and studying the catalytic performance of the resulting materials in methane DHA, we show that coking is reduced with increasing Fe dispersion and that highly (atomically) dispersed Fe²⁺ inside the zeolite micropores constitutes the selective species for DHA. However, the presence of Brønsted acid sites in the zeolite micropores results in continued coke formation due to secondary reactions of the aromatic product. Further optimization of the catalyst hence requires careful adjustment of this acidity.