On the dynamic nature of Mo sites for methane dehydroaromatization

For methane dehydroaromation (MDA), a great deal of research has focused on the most promising systems Mo/HZSM-5 and Mo/HMCM-22. The active phase for this catalyst, molybdenum carbide (MoC) was previously found to form in an initial period where no desired products are formed yet. This contribution shows that by pre-carburizing Mo/HZSM-5 with carbon monoxide (CO), a catalyst immediately active to form MDA products can be produced. This CO treatment does not only allow to independently study the interaction of methane (CH4) with the catalytic site, but also allows to better characterize the active site. Observing the active site formation in the initial phase of MDA is difficult because of the simultaneous fast coking. In contrast, using CO as carburizing agent, many characterization techniques finally become useful and can lead to full insight into the nature of the active site on Mo/HZSM-5. Fourier-transform infrared spectroscopy using CO as a probe molecule revealed that the reduced Mo site produced through this route is equivalent to the one produced under CH4 atmosphere. By means of isotopic labelling experiments on a pre-carburized catalyst, we investigated how CH4 is activated on the catalytic site and demonstrate that carbon from MoC is incorporated into the final products, ethylene and benzene. This confirms the dynamic behavior of the active site. Rather than acting as an adsorption site to lower the activation barrier of CH4, MoC actively takes part in the catalytic reaction. This is similar to the Mars-van-Krevelen mechanism for oxidation and what was recently observed for the Fischer-Tropsch reaction.