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Methane Conversion to Aromatics on Mo/H-ZSM5: Structure of Molybdenum Species in Working Catalysts

2000; American Chemical Society; Volume: 105; Issue: 2 Linguagem: Inglês

10.1021/jp0030692

1520-6106

Weiping Ding, Senzi Li, G. Meitzner, Enrique Iglesia,

Zeolite Catalysis and Synthesis

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 D2 with OH groups in H−ZSM5 before and after CH4 reactions. These methods reveal that CH4 reactions cause exchanged Mo2O5 2+ dimers, formed from physical mixtures of MoO3 and H−ZSM5, to reduce and carburize to form small (0.6−1 nm) MoC x 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: MoC x 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 MoC x clusters resist agglomeration during methane reactions at 950 K for > 10 h. The Brönsted acid sites formed during carburization and oligomerization of MoC x species ultimately become covered with hydrogen-deficient reaction intermediates (H/C ∼ 0.2) or unreactive deposits. The highly dispersed nature of the MoC x clusters was confirmed by detailed simulations of the XAS radial structure function and by the low temperatures required for the complete oxidation of these MoC x species compared with bulk Mo2C. Initial CH4 reactions with MoO x 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 x species to form C2 hydrocarbons, which desorb to re-form the MoC x sites required for C−H bond activation steps.