Mechanism and Energetics of Secondary Oxidation Reactions in the Aerobic Oxidation of Hydrocarbons Catalyzed by Mn-Doped Nanoporous Aluminophosphates

Abstract

Electronic structure calculations employing hybrid exchange DFT methods under periodic boundary conditions are applied to unravel the mechanism and energetics of the secondary oxidation reactions of hydrocarbons catalyzed by Mn-doped nanoporous aluminophosphates. Secondary oxidations are favored by the activated nature of the primary oxidative products, alcohols, which have weaker C–H bonds than the corresponding hydrocarbon molecules. Our model accounts for the secondary oxidations of terminal alcohols; a double H-abstraction from these compounds, first by MnIII complexes with radical-type oxo-based ligands, and then by framework O atoms nearest neighbor to Mn, yields aldehyde molecules or ketones if nonterminal alcohols are oxidized. These aldehydes are then transformed first into carbonyl-containing α-hydroperoxide derivatives through H-abstraction, addition of O2, and a subsequent H-transfer. These hydroperoxides can then only be decomposed by the action of MnII sites which, depending on the stereochemistry of the hydroperoxide adsorption onto the Mn sites, yield the carboxylic acid final products directly or after a new H-abstraction. The knowledge gained in our study allows us to propose a mechanism for the secondary oxidation of ketones to give carboxylic acids, a route that involves cleavage of C–C bonds adjacent to the carbonyl group and gives place to carboxylic acids of shorter chain-length than the initial hydrocarbon, products commonly found during the experimental reaction.