Controlling the Evolution of Active Molybdenum Carbide by Moderating the Acidity of Mo/HMCM-22 Catalyst in Methane Dehydroaromatization

Abstract

Effect of framework Bronsted Acidity by varying SiO2/Al2O3 ratio (SAR) of HMCM-22 zeolite is studied with respect to the formation of active molybdenum carbide and its anchoring over zeolite (HMCM-22) channels of Mo/HMCM-22 catalyst tested for methane dehydroaromatization (MDA) reaction. For this purpose, HMCM-22 is synthesized by conventional methods with varying SAR (30, 40 & 55) and is studied for MDA reaction with 5 wt% Mo loading. XRD, BET, NH3-TPD, H2-TPR, 27Al MAS NMR, Raman spectroscopy and XPS techniques are used to characterize 5Mo/HMCM-22 catalyst having different SAR. XPS analysis of carburized 5Mo/HMCM-22 (SAR-30, 40 & 55) catalyst confirms that higher content of molybdenum carbide (Mo2C) forms over HMCM-22 channels at SAR-30 as compared to SAR-40 and SAR-55 due to effective binding of initial MoOx species. Interaction of initial MoOx species with HMCM-22 zeolite framework is analyzed using 27Al MAS NMR and Raman spectroscopic studies which confirm that MoOx species bind differently at SAR-30, 40 and 55 which affects the catalytic performance. Lower reducibility of MoOx species at SAR-30 confirms that MoOx species strongly interact with HMCM-22 channels at SAR-30 as confirmed by H2-TPR study. Maximum transformation of MoOx species into active molybdenum carbide over HMCM-22 at lower SAR (30) during carburization results in higher activity of 5Mo/HMCM-22 (SAR-30) catalyst with lower coke content.