Methane conversion to aromatics on Mo/H-ZSM5: Structure of molybdenum species in working catalysts

The structure and density of Mo species in Mo/H-ZSM5 during catalytic CH4 reactions was investigated using in-situ X-ray absorption spectroscopy (XAS), temperature-programmed oxidation after reaction, and the isotopic exchange of D-2 with OH groups in H-ZSM5 before and after CH4 reactions. These methods reveal that CHI reactions cause exchanged Mo2O52+ dimers, formed from physical mixtures of MoO3 and H-ZSM5, to reduce and carburize to form small (0.6-1 nm) MoCx clusters with the concurrent regeneration of the bridging OH groups that were initially replaced by Mo oxo dimers during exchange. In this manner, catalytically inactive Mo oxo species activate in contact with CH4 to form the two sites required for the conversion of CH4 to aromatics: MoCx for C-H bond activation and initial C-C bond formation and acid sites for oligomerization and cyclization of C2+ hydrocarbons to form stable aromatics. These MoCx clusters resist agglomeration during methane reactions at 950 K for > 10 h. The Bronsted acid sites formed during carburization and oligomerization of MoCx species ultimately become covered with hydrogen-deficient reaction intermediates (H/C similar to 0.2) or unreactive deposits. The highly dispersed nature of the MoCx clusters was confirmed by detailed simulations of the XAS radial structure function and by the low temperatures required for the complete oxidation of these MoCx species compared with bulk Mo2C. Initial CH4 reactions with MoOx precursors are stoichiometric and lead first to the removal of oxygen as CO, CO2, and H2O and to the introduction of carbidic carbons into the reduced structures. As carbidic carbon passivates the surface, C-H bond activation reactions become catalytic by the coupling of this activation step with the removal of the resulting CH, species to form C-2 hydrocarbons, which desorb to re-form the MoC, sites required for C-H bond activation steps.