Aerobic Oxidation of Hydrocarbons Catalyzed by Mn-Doped Nanoporous Aluminophosphates (IV): Regeneration Mechanism

Electronic structure methods based on periodic DFT with hybrid-exchange functionals are applied to study the reaction mechanism of the aerobic oxidation of hydrocarbons catalyzed by Mn-doped nanoporous aluminophosphates. Here, we focus on the regeneration of the active sites that closes the oxidation cycle. At this stage, the catalyst pores are accumulated with CH3CH2OOH (hydroperoxide) and Mn-III center dot center dot center dot OOCH2CH3 complexes resulting from the propagation reactions. CH3CH2OOH intermediates can only be decomposed into the oxidation products by Mn-II sites; thus, a reaction pathway in which Mn sites in Mn-III center dot center dot center dot OOCH2CH3 are reduced is essential for the oxidation cycle to proceed. We demonstrate that two different regeneration mechanisms take place at different times of the oxidation reaction: at the beginning, Mn-III center dot center dot center dot OOCH2CH3 complexes are transformed into a molecule of aldehyde and Mn-III center dot center dot center dot OH complexes by an intramolecular H transfer from the methylene C to the terminal 0 in the peroxo radical in a slow process that requires a high activation energy of 141 kJ/mol. Mn sites in Mn-III center dot center dot center dot OH can then be regenerated by a H-transfer from a new hydrocarbon molecule to a framework O nearest neighbor to Mn, followed by coupling between the resulting alkyl radical and the OH ligand to give a molecule of ethanol and Mn-II sites. At later stages of the oxidation, when alcohol molecules are accumulated within the pores of the catalyst, Mn sites in Mn-III center dot center dot center dot OOCH2CH3 can be regenerated by a more favorable mechanism with a double H-abstraction from the alcohol. First, a H atom in the methylene C of CH3CH2OH is transferred to the CH3CH2OO ligand to give CH3CH2OOH and CH3CH center dot OH radicals, followed by H transfer from this radical to a framework O to yield finally a molecule of aldehyde and Mn-II sites. The occurrence of alternative regeneration mechanisms along the oxidation reaction explains the variation of the alcohol-to-aldehyde ratio observed experimentally as a function of the reaction time.