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

Abstract

The influence of introducing VOx sites into the primary, bulk structure of molybdenum-containing polyoxometalate H3+xPMo12–xVxO40 (x = 0, 1, 2, 3) and secondary, surface structure of supported VOx/H3PMo12O40 Keggins was investigated to establish their structure–reactivity/selectivity relationships. The resulting Keggins were physically characterized (solid-state 51V NMR and in situ FT-IR, Raman and UV–vis spectroscopy) and chemically probed with CH3OH (CH3OH-TPSR spectroscopy and steady-state methanol oxidation/dehydration). The introduction of the VOx sites into the primary Keggin structure resulted in structural disorder that facilitated decomposition of the Keggins at elevated temperatures and under the corrosive methanol reaction environment. The decomposition was reflected in the expelling of the VOx units from the primary Keggin structure into the secondary surface VOx sites where they reacted with surface acidic hydroxyls. For the first time, it was demonstrated that methanol is present as both sorbed species within the Keggin structure as well as surface reactive species during methanol reaction conditions at elevated temperatures (T < 400 °C). Introduction of the VOx sites increased the formaldehyde selectivity and decreased the dimethyl ether selectivity. The VOx units in the primary Keggin structure were slightly more active than the surface VOx species, and both VOx sites were significantly more active than the MoOx sites in the primary Keggin structure. The relatively constant TOFredox values with number of VOx sites in the Keggin reflect that only one VOx site is involved in methanol oxidation to formaldehyde. Although introducing VOx into the Keggin decreased the UV–vis edge energy (Eg) and increased the TOFredox relative to the V-free H3PMo12O40 Keggin, a direct relationship between TOFredox and Eg was not found to be present for the H3PMo12–xVxO40 Keggins. These new fundamental insights demonstrate the importance of performing in situ molecular spectroscopy characterization studies under reaction conditions and reveal that the Keggin structures are dynamic and should not be assumed to be ideal, static model catalytic structures.