Reaction of aldehydes with the H₅PV₂Mo₁₀O₄₀ polyoxometalate and cooxidation of alkanes with molecular oxygen

The oxidation of alkanes with molecular oxygen using aldehydes as reducing agents (aldehydes are cooxidized) was studied using the α-H₅PV₂Mo₁₀O₄₀ polyoxometalate as catalyst. Emphasis was placed on the initiation of the radical chain reaction by investigation of the aldehyde-polyoxometalate interaction. Using ³¹P NMR and ESR spectroscopy one could differentiate between the reactivity of the five inseparable isomers of α-H₅PV₂Mo₁₀O₄₀. Contrary to previous belief, the 1,11 isomer with vanadium in distal positions is the most abundant. The ³¹P NMR and ESR spectra supported by UV-vis absorption-time profiles of the reduction of α-H₅PV₂Mo₁₀O₄₀ indicated that isomers with vanadium in vicinal positions were most kinetically viable in the alkane oxidation. Addition of isobutyraldehyde to α-H₅PV₂Mo₁₀O₄₀ gave in the ⁵¹V NMR spectrum a new downfield peak attributed to the formation of an aldehyde-polyoxometalate intermediate. The alkane/aldehyde/02 oxidizing system was found to be quite effective and selective for ketone formation. Reaction probes indicated that acyl peroxo radicals were the active oxidizing intermediates. Five pathways for its reaction were identified: chain propagation, alkane oxidation, decomposition to form oxygen, decomposition to acyl oxo radicals leading to CO₂ and ketone, and capture and inhibition by the polyoxometalate.