Aerobic Oxidation of Hydrocarbons Catalyzed by Mn-Doped Nanoporous Aluminophosphates (III): Propagation Mechanism

We apply electronic structure methods based on hybrid-exchange DFT functionals under periodic boundary conditions to study the catalytic aerobic oxidation of hydrocarbons in Mn-doped aluminophosphates. In particular, we focus on the mechanism of the propagation reactions. Hydrocarbon oxidation is achieved via a succession of H abstraction, O 2 addition, and desorption reactions occurring on Mn III ···OX complexes (X = H, CH 2 CH 3, or OCH 2 CH 3 ). The complexes Mn III ···OH and Mn III ···OCH 2 CH 3 result from the decomposition of CH 3 CH 2 OOH by preactivated Mn II sites, whereas Mn III ···OOCH 2 CH 3 is formed in preactivation or propagation routes. The radical nature of the oxo-type ligands (OX) allows for the homolytic H abstraction from new hydrocarbon molecules, leading to XO–H (HO–H, CH 3 CH 2 O–H and CH 3 CH 2 OO–H) and to CH 3 CH 2 · radicals that are stabilized by interaction with the H atoms transferred. Subsequent stereospecific O 2 additions yield free peroxo radicals CH 3 CH 2 OO· that undergo a propagation subcycle to produce further CH 3 CH 2 OOH; these hydroperoxide molecules re-enter the oxidation cycle by reacting with Mn II . The different H abstraction ability of the Mn III ···OX complexes is related to the stability of the oxo radicals that act as ligands. Our results demonstrate that the role of the Mn sites in the propagation reactions is to stabilize the oxo radicals by forming complexes, but no redox process involving Mn takes place in this stage of the reaction; Mn III is the only active species throughout the propagation steps.