Aerobic Oxidation of Hydrocarbons Catalyzed by Mn-Doped Nanoporous Aluminophosphates (II): Hydroperoxide Decomposition

Electronic structure methods based on hybrid-exchange functionals in Density Functional Theory (DFT) and periodic boundary conditions have been applied to study the reaction mechanism of the aerobic oxidation of hydrocarbons catalyzed by Mn-doped nanoporous aluminophosphates. In this paper we examine the decomposition of hydroperoxide intermediates (ROOH). The reaction takes place on Mn II acid sites, charge-balanced by a proton on a nearest neighbor framework oxygen, resulting from the preactivation step. In this stage, the Mn II sites catalyze the homolytic decomposition of the hydroperoxide molecules to produce Mn III and oxo-containing radical species, stabilized by complexation to Mn III, yielding in addition alcohol and water molecules. Two parallel reaction pathways have been identified for this process, through alkoxy (RO·) or hydroxy (HO·) radical-like intermediates. The occurrence of the two mechanisms depends on the stereochemistry of the initial adsorption of ROOH onto the active site, which takes place through H-bonding with the framework acid proton: adsorption via the hydroxylic O atom of ROOH leads to RO· intermediates, while adsorption via the nonterminal O atom in ROOH, which is less energetically stable, drives the decomposition toward HO· intermediates. In both cases, the hydroperoxide decomposition is assisted by the Mn II sites, in a concerted mechanism that consists of a H-transfer from the framework to the O atom of the adsorbed ROOH involved in the H-bond, prompting the oxidation of Mn II, and the O–O homolytic cleavage in ROOH, leading to the formation of RO· or HO· radicals that are stabilized by binding the oxidized Mn III site. The relative energetics of the two reaction pathways is explained in terms of the relative stability of the oxo-radicals produced: the higher stability of RO· radicals causes a more favorable decomposition of the hydroperoxide intermediate through this pathway. Our results demonstrate the crucial role of Mn in this stage of the aerobic oxidation of hydrocarbons, due not only to its redox activity but also, and fundamentally, to its coordinative unsaturation, that allows for the stabilization of the radicals produced by complexation.