The interplay of electrostatic and covalent effects in 1-butene oxidation over vanadyl pyrophosphate

Quantum mechanical reactivity indices were computed from B3LYP calculations on 1-butene and active site clusters representing stoichiometric and phosphorous-enriched (VO)₂P₂O₇ (100) surfaces. Results indicate that 1-butene may chemisorb via formation of a τ-cation complex at the stoichiometric (Open) surface, with comparison of local softnesses in 1-butene and the Open active site indicating a strong interaction. Coulombic repulsion at the phosphorous-enriched (Cleft) surface blocks vanadium sites from the C=C region of 1-butene, precluding τ-cation complex formation in the Cleft active site. The phosphorous -enriched surface naturally features a greater concentration of nucleophilic terminal P-O oxygen species, which may promote C-H, rather than C-C, bond rupture in subsequent oxidation of adsorbed 1-butene. Hence, a rationale may be offered for both the ease of adsorption, and the low selectivity to maleic anhydride, observed experimentally when 1-butene undergoes oxyfunctionalisation over vanadyl pyrophosphate. Results also indicate that oxidative transformation of n-butane over (VO)₂P₂O₇ is unlikely to proceed via an olefin intermediate.