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 alpha-H5PV2Mo10O40 polyoxometalate as catalyst. Emphasis was placed on the initiation of the r adical chain reaction by investigation of the aldehyde-polyoxometalate inte raction. Using P-31 NMR and ESR spectroscopy one could differentiate betwee n the reactivity of the five inseparable isomers of alpha-H5PV2Mo10O40. Con trary to previous belief, the 1,11 isomer with vanadium in distal positions is the most abundant. The P-31 NMR and ESR spectra supported by UV-vis abs orption-time profiles of the reduction of alpha-H5PV2Mo10O40 indicated that isomers with vanadium in vicinal positions were most kinetically viable in the alkane oxidation. Addition of isobutyraldehyde to alpha-H5PV2Mo10O40 g ave in the V-51 NMR spectrum a new downfield peak attributed to the formati on of an aldehyde-polyoxometalate intermediate. The alkane/aldehyde/O-2 oxi dizing system was found to be quite effective and selective for ketone form ation. Reaction probes indicated that acyl peroxo radicals were the active oxidizing intermediates. Five pathways for its reaction were identified: ch ain propagation, alkane oxidation, decomposition to form oxygen, decomposit ion to acyl oxo radicals leading to CO2 and ketone, and capture and inhibit ion by the polyoxometalate. (C) 1999 Academic Press.