Heteropolyacid-based catalysts for selective alkane oxidation: mechanism of formation of maleic anhydride from propane

Highly active and selective alkane oxidation catalysts, obtained by heating pyridinium- and niobium-exchanged phosphomolybdate and phosphovanadomolybdate salts under inert atmosphere, can under some conditions lead to maleic anhydride as a major product of propane oxidation. Mechanistic studies based on oxidation of postulated intermediates as well as selectively isotopically labeled propane implicate a route involving oxidative dehydrogenation to propylene followed by acid-catalyzed dimerization to branched C6 hydrocarbons, the main precursors to maleic anhydride.