Investigating the Support Effect for Catalytic Elimination of Methyl Mercaptan: Role of Hydroxyl Groups over Cr-based Catalysts

Abstract

The support effect for HZSM-5 and Al2O3-supported chromium (Cr) catalysts on the catalytic decomposition of methyl mercaptan (CH3SH) is investigated. Characterization results reveal that the distribution, reducibility, oxidation states and coordination environment of chromium species mightily depend on the nature of support. Al2O3 support is covered by surface hydroxyl groups, thus conducing to the formation of monochromatic Cr(VI) species with tetrahedral coordination, which remarkably increases the reducibility and dispersion of chromium species. In contrast, plenty of inactive α-Cr2O3 particles are formed on the surface of Cr/HZSM-5 catalyst due to the lack of adequate hydroxyl sites. Furthermore, a positive correlation is established between the content of active Cr(VI) species and the number of surface hydroxyl groups over Cr/Al2O3 catalysts. Reactivity data suggest that the addition of chromium species can observably enhance the conversion of CH3SH for both two supports. More importantly, 5% Cr/Al2O3 catalyst features the superior catalytic performance at 400 °C (100% conversion). The promoting effect can be attributed to the high-content hydroxyl groups on Al2O3, which are proven to stabilize monochromatic Cr(VI) species. This result also provides evidence for the active sites of CH3SH decomposition reaction.