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 CH 3 CH 2 OOH (hydroperoxide) and Mn III ···OOCH 2 CH 3 complexes resulting from the propagation reactions. CH 3 CH 2 OOH intermediates can only be decomposed into the oxidation products by Mn II sites; thus, a reaction pathway in which Mn sites in Mn III ···OOCH 2 CH 3 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 ···OOCH 2 CH 3 complexes are transformed into a molecule of aldehyde and Mn III ···OH complexes by an intramolecular H transfer from the methylene C to the terminal O in the peroxo radical in a slow process that requires a high activation energy of 141 kJ/mol. Mn sites in Mn III ···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 ···OOCH 2 CH 3 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 CH 3 CH 2 OH is transferred to the CH 3 CH 2 OO ligand to give CH 3 CH 2 OOH and CH 3 CH·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.