Aerobic Oxidation of Hydrocarbons Catalyzed by Mn-Doped Nanoporous Aluminophosphates(I): Preactivation of the Mn Sites

We apply state of the art electronic structure techniques, based on hybrid exchange-functionals in DFT and periodic boundary conditions, to unravel the reaction mechanism responsible for the initial stages of the aerobic oxidation of hydrocarbons catalyzed by Mn-doped aluminophosphates. In this preactivation step of the catalyst, which precedes the catalytic propagation cycle in which the final oxidation products (alcohol, aldehyde, and carboxylic acid) are formed, the Mn III ions initially present in the activated (calcined) catalyst are transformed by interaction with one alkane and one O 2 molecule into new Mn-bearing species: a reduced Mn II site and a Mn III ···peroxo complex, which are active for the subsequent propagation cycle. The preactivation step has a high activation energy, calculated as 135 kJ/mol, explaining the long induction time observed experimentally. Our results further show that Mn III sites are able to produce the hydroperoxide intermediate from the reactants; however, this intermediate can be transformed into the oxidative products only through reduced Mn II sites. The latter are formed from Mn III in the preactivation step, via a H-abstraction from the hydrocarbon, also yielding an alkyl radical (R•) that subsequently adds O 2 in a stereospecific way to form a free peroxo radical, ROO•. Migration of ROO• in the AlPO nanopores frees Mn II for the propagation cycle and forms Mn III ···ROO• complexes also needed for propagation. We demonstrate the essential role of Mn III active sites at the initial stages of the reaction for activating the hydrocarbon molecules; such hydrocarbon activation catalyzed by Mn requires much lower activation energies than through noncatalytic pathways, where the hydrocarbon is activated by O 2 alone.