Improved methane dehydrocondensation reaction on HMCM-22 and HZSM-5 supported rhenium and molybdenum catalysts

Abstract

As one of a few possible catalysts for the methane dehydrocondensation reaction towards benzene and naphthalene, novel Re/HMCM-22 was investigated. Results were compared with those from other HMCM-22 and HZSM-5 supported rhenium and molybdenum catalysts. Re/HMCM-22 was similarly active and selective catalyst for the reaction as Mo/HZSM-5, Re/HZSM-5 and Mo/HMCM-22 catalysts. The catalytic performances of HMCM-22 supported rhenium and molybdenum catalysts were characteristic of higher benzene selectivities and lower naphthalene selectivities as well as of higher catalytic stability than those of HZSM-5 supported catalysts. The differences of catalytic behavior between HMCM-22 and HZSM-5 supported catalysts are mainly related to their different channel structures, where HMCM-22 has large cavities and slit-like pore openings. The catalytic stabilities of HMCM-22 and HZSM-5 supported catalysts were improved greatly by the addition of a few percent of carbon dioxide in methane feed and by the acid-reflux pre-dealumination of HMCM-22 and HZSM-5 supports, owing to the effective suppression in the formation of carbonaceous deposit, which was revealed by the temperature-programmed oxidation (TPO) study of used catalysts. The analysis of solid-state aluminum and proton NMR studies shows that Brønsted acid sites on zeolite support promotes the formation of harmful carbonaceous deposit on the catalyst surface and, hence, that the partial reduction of the Brønsted acid sites by acid-reflux treatment enhances their catalytic stabilities. Extended X-ray absorption fine structure (EXAFS) analysis reveals that both molybdenum carbide and metallic rhenium on HMCM-22 are responsible for the methane activation, similar to the case of supporting on HZSM-5.