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 center dot center dot center dot OX complexes (X = H, CH2CH3, or OCH2CH3). The complexes Mn-III center dot center dot center dot OH and Mn-III center dot center dot center dot OCH2CH3 result from the decomposition of CH3CH2OOH by preactivated Mn-II sites, whereas Mn-III center dot center dot center dot OOCH2CH3 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, CH3CH2O-H and CH3CH2OO-H) and to CH3CH2 center dot radicals that are stabilized by interaction with the H atoms transferred. Subsequent stereospecific O-2 additions yield free peroxo radicals CH3CH2OO center dot that undergo a propagation subcycle to produce further CH3CH2OOH; these hydroperoxide molecules re-enter the oxidation cycle by reacting with Mn-II. The different H abstraction ability of the Mn-III center dot center dot center dot 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.