Sulfated hafnia as a support for Mo oxide: A novel catalyst for methane dehydroaromatization

One of the most challenging aspects of modern day catalysis is the conversion of methane. Natural gas is an abundant source of supply of methane and is currently being used mostly for electricity generation, and much of it is simply flared. The potential to convert methane into higher-value hydrocarbons is significant. Among other reactions, direct conversion of methane via dehydroaromatization (MDHA) can be used to produce hydrogen and valuable hydrocarbons like benzene. Mo oxide supported on ZSM-5/MCM-22 has been studied extensively in recent years for MDHA. It has been reported that Mo carbides are responsible for activating methane by forming CHx species. These are dimerized into C2Hy and then oligomerized on the strong ZSM-5/MCM-22 Bronsted acid sites to form benzene. Related work has shown that sulfated zirconia (SZ) provides the acid sites for Mo needed to produce benzene in MDHA [1]. The similarity of sulfated hafnia (SH) with sulfated zirconia is a logical novel support for MDHA. Although SH has been used for other acid-catalyzed reactions, we are aware of no systematic study of Mo-SH for the MDHA reaction, despite the appeal of such a catalyst. Here, Mo/SH catalysts have been tested and characterized using SEM-EDS, XPS, XANES, DRIFTS, HR-TEM, BET and temperature programmed techniques. DRIFTS confirms that SH acidity is unaffected by the addition of Mo to the catalyst. SEM-EDS and XPS have been used to characterize the loading of Mo and sulfur on SH support. XANES as well as HR-TEM analysis are used to study the formation of molybdenum carbide/ oxycarbide species, which are generally regarded to be the active sites for MDHA. MDHA runs were carried out to study the effect of Mo loading, temperature and space velocity. 5% Mo-SH is catalyst more active than 1% Mo-SH, as measured by methane conversion. Conversion increased with higher temperature and lower space velocity and gradually deactivated with time. This can be attributed to catalytic surface coking, confirmed with subsequent TPO analysis. Products observed were primarily ethylene and benzene. Ethylene selectivity increased with time for higher Mo loading and lower space velocity. Benzene product selectivity increased with higher Mo loading, lower temperature, and lower space velocity, while gradually decreasing with time. A direct experimental comparison of conventional Mo-HZSM5 synthesized here showed that the Mo-SH catalyst is more active than Mo-HZSM5.