First principles investigation of manganese catalyst structure and coordination in the p-xylene oxidation process

The oxidation of p-xylene to terephtalic acid has global importance, with the product used as a precursor for polyethylene terephthalate (PET). The oxidation of p-xylene proceeds via a redox cascade that involves cobalt, manganese, and bromide, with a synergy allowing for high selectivity and reactivity; however, the equilibrium coordination environment of the catalyst species remains uncertain due to the hostile industrial operating conditions. To build knowledge of the catalyst speciation and develop understanding of the reaction process, a density functional theory approach is applied herein to determine the static and dynamic properties of the divalent (reduced) and trivalent (oxidized) manganese catalysts in the redox cascade. The Gibbs free energy has been calculated for manganese as a function of ligands in the inner coordination sphere, with the octahedrally-coordinated Mn(OAc)₂(HOAc)₂ and Mn(OAc)₃(H₂O)₁ identified as the most thermodynamically stable coordination environments for Mn(II) and Mn(III), respectively. Dynamic properties of these catalysts in the presence of an explicit solvent environment have been determined using first principles molecular dynamics simulations. The simulations indicate 0–2 coordinating water ligands are present in the inner coordination sphere under standard industrial temperatures and pressures. The dynamical simulations have been extended to include HBr, which couples with Mn in the redox cascade, and the bromide species does not enter in the inner-coordination sphere of the oxidized Mn(III) catalyst, providing evidence that the electron transfer between bromide and Mn(III) proceeds via an outer sphere mechanism. Our results suggest that oxidation of Mn(II) has the potential for facilitating L-type ligand exchange in the inner-sphere coordination environment. The results are a platform for developing a more complete knowledge of the reaction mechanism at the atomistic scale.