Complementary Mechanistic Properties of Fe- and Mn-doped Aluminophosphates in the Catalytic Aerobic Oxidation of Hydrocarbons

Abstract

State-of-the-art electronic structure methods based on hybrid-exchange functionals in Density Functional Theory and periodic boundary conditions are extensively applied in order to unravel the reaction mechanism of the oxidation of hydrocarbons catalysed by microporous aluminophosphates (MAPO) doped with transition metals (Mn and Fe).1 2 The reactionmechanism involved is rather complicated, and involves a series of H-abstractions, 02 insertions, 0-0 bond dissociations and complex formation/ decompositions assisted by the redox activity of the active sites and their coordinative insaturation in MAPO frameworks. These reactions occur through two consecutive stages, a preactivation step, where the transition metals are transformed into active species by reduction or complex formation, and a self-sustained pro pagation cycle, where hydroperoxide intermediates formed during the course of the reaction are transformed into the primary oxidation products (alcohols). 3,4 Due to their higher reactivity, these are then further transformed into secondary oxidation products (acids) by similar reactions.5 Though showing a similar overall reaction mechanism, Mn- and Fe-doped aluminophosphates show distinct features at particular elementary steps depending on their intrinsic electronic configuration. 6 Preactivation of Mn catalysts is easier due to the higher stability of Mn in the reduced oxidation state with 5 d electrons. In contrast, preactivation of Fe is more difficult due to the lower stability of Fe11 ( d6 ), and occurs through formation of Fe111 complexes, preventing a direct Fe reduction. On the contrary, the pro pa gation cycle occurs more favourably with Fe than with Mn sites because of lower activation barriers, higher adsorption energies of the reactants and lower desorption energies of the products on the active sites. Ali the mechanistic information gained by this series of studies can be applied in a n attempt to rationally improve the activity of these oxidation catalysts.