(735d) Porphyrinic Metal-Organic Framework As Catalyst for Styrene Epoxidation Reaction: A First Principle Study

Recent experiments have shown that the mesoporous metal organic framework containing accessible porphyrinic sites, known as ZnMn-RPM, is catalytically active for oxidation of alkenes and alkanes. We have investigated the reaction mechanism and calculated the energy barriers with spin-polarized periodic planewave density functional theory along with the nudged elastic band and dimer method for finding locate the transition states. We have identified a three-step mechanism for oxidation of styrene to form an epoxide in ZnMn-RPM. The first step involves the transfer of an oxygen atom from the oxygen carrier to the manganese site of ZnMn-RPM. The oxygen atom is transferred from Mn to the styrene molecule in the second step, with the oxygen bound primarily to a single carbon atom forming a radical intermediate. The third step involves rearrangement of the oxygen on the styrene group to form an epoxide. The energy barrier for the first step is calculated to be 0.15 eV using the Perdew-Burke-Ernzerhof (PBE) functional, and the inclusion of van der Waals (vdW) interactions with the PBE-D2 approach increases the barrier to 0.25 eV. The reaction barrier for the second step was computed to be 0.13 eV from PBE, and 0.08 eV from PBE-D2. The barrier for the third step was found to be 0.21 (0.15) eV from the PBE (PBE-D2). We found that the oxygen transfer reactions accompanied by a significant rearrangement of the spin population. Our calculations indicate that the vdW energy is important to accurately describe these systems. For comparison, we have computed the reaction barriers for direct epoxidation without the catalytic ZnMn-RPM to be 1.95 (1.55) eV from PBE (PBE-D2).