Catalytic Reaction Mechanism of Mn-Doped Nanoporous Aluminophosphates for the Aerobic Oxidation of Hydrocarbons

In this work we apply state-of-the-art electronic-structure-based computational methods based on hybrid-exchange density functional theory to study the mechanism of the aerobic oxidation of hydrocarbons catalysed by Mn-doped nanoporous aluminophosphates (Mn-AlPOs). We compare our results with available experimental data. We show that the catalytic efficiency of Mn-AlPOs in oxidation reactions is intrinsically linked to 1) the Mn redox activity, in particular between 2+ and 3+ oxidation states, and 2) the coordinative insaturation of tetrahedral Mn embedded in AlPO frameworks, which facilitates the reaction by stabilising oxo-type radicals through the formation of Mn complexes. Our mechanism demonstrates the crucial role of both Mn-III and Mn-II in the reaction mechanism: Mn-III sites undergo an initial reaction cycle that leads to the production of the alkyl hydroperoxide intermediate, which can only be transformed into the oxidative products (alcohol, aldehyde and acid) by Mn-II. A preactivation step is required to yield the reduced Mn-II sites able to decompose the hydroperoxide intermediates; this step takes place through a transformation of the hydrocarbon into the corresponding peroxoderivative, stabilised by forming a complex with Mn-III and yielding at the same time reduced Mn-II sites. Both species enter a subsequent propagation cycle in which Mn-II catalyses the dissociation of the hydroperoxide that proceeds until the formation of the oxidative products by two parallel pathways, through alkoxy- or hydroxy-radical-like intermediates, whilst the Mn-III-peroxo complex enables further production of the hydroperoxide intermediate.