Dynamic Surface Structures and Reactivity of Vanadium-Containing Molybdophosphoric Acid (H3+xPMo12 xVxO40) Keggin Catalysts during Methanol Oxidation and Dehydration

Molybdenum-based Keggin polyoxometalates (POMs) containing vanadium, with the formula H3+xPMo12 xVxO40 (MPAV) and x varying from0 to 3, exhibit unique catalytic properties because of their bifunctional nature and have been investigated as catalysts both for acidic and for oxidative reactions. 3 The addition of vanadium centers are beneficial for redox catalysis, shifting the molybdophosphoric acid (MPA) reactivity from acidic to redox character as evidenced by the shift in selectivity of the methanol oxidation reaction from formation of CH3OCH3 (DME) to HCHO, respectively. The addition of vanadium to the molybdenum-based POMs also affects the reactivity toward methanol oxidation/dehydration with the maximum activity attained for MPAV2 and substitution of more vanadium causing a decrease in reactivity. This catalytic observation was rationalized by concluding that the MPAV Keggin structure can only tolerate one or two vanadium cation(s) per Keggin unit since higher vanadium contents destabilize the crystalline MPA molecular structure. Recent in situ characterization studies by EPR, UV vis, Raman, FTIR, and NMR during heat treatment up to 350 C have revealed that the vanadium cation can be expelled from the MPAV Keggin. 10 Even when the vanadium cation is in the primary structure after synthesis, it can migrate to the secondary structure as a surface vanadyl group under reaction conditions. The migration of vanadium from the primary Keggin structure to the secondary cationic position depends on the content of vanadium, treatment temperature, and reactive environments. Furthermore, the MPAV Keggin structure can also decompose at elevated temperatures and in certain reaction environments. In situ UV vis spectroscopic characterization during methanol oxidation, O2/CH3OH = 10/1, 3/1, and 0.5/1, revealed that the MPAV Keggin structure decomposes at