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PublicationACS Catalysis
Year2011
Volume1 (11)
Pages1487-1497
International

Aerobic oxidation of hydrocarbons catalyzed by Mn-doped nanoporous aluminophosphates (III): propagation mechanism

Authors:Luis Gómez-Hortiguela Sáinz, Furio Corà , C. Richard A. Catlow
Groups of research:Molecular Sieves

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, O2 addition, and desorption reactions occurring on MnIII···OX complexes (X = H, CH2CH3, or OCH2CH3). The complexes MnIII···OH and MnIII···OCH2CH3 result from the decomposition of CH3CH2OOH by preactivated MnII sites, whereas MnIII···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· radicals that are stabilized by interaction with the H atoms transferred. Subsequent stereospecific O2 additions yield free peroxo radicals CH3CH2OO· that undergo a propagation subcycle to produce further CH3CH2OOH; these hydroperoxide molecules re-enter the oxidation cycle by reacting with MnII. The different H abstraction ability of the MnIII···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; MnIII is the only active species throughout the propagation steps.

Keywords:oxidation; heterogeneous catalysis; nanoporous aluminophosphates; zeolites; molecular modeling; aerobic; reaction mechanism
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