Systematic Characterization of Catalytic Active Sites and Complete Catalytic Cycles for Methane to Methanol Transformation

Abstract

Methane to methanol (MTM) transformation is a very significant chemical transformation for its scientific, industrial, and commercial importance. Currently, MTM transformation is done by following a high energy procedure which is economically viable only in a large industrial scale. Despite of a search of half a century, an effective low energy MTM transformation is still formidable and active research is ongoing in search of that holy grail. In this project, we have employed high-level electronic structure theory calculations to elucidate some of the profound complexities of MTM transformation and theoretically investigated a new complete catalytic cycle for MTM transformation. Transition metal oxides have attracted considerable attention for their catalytic potency for MTM transformation. Specially FeO and their charge variants have been considered the best potential candidates for MTM transformation because of their presence in the biocatalytic active sites for MTM transformation. We have studied the ground and excited states FeO2+ using high-level electronic structure theory. The ground electronic state of FeO2+ is 3Δ which is of oxyl character followed by the low-lying excited states 5Δ and 5Σ+ which have oxo electronic configuration. Then we have introduced ligands in the system and found that the strong field ligands such as NH3 stabilize both the oxo and oxyl states whereas the weak field ligands such as H2O has stabilized only the oxo states. For the methane activation mechanism, the oxo states undergo oxidative addition route and oxyl states follow the radical mechanism. So, it is possible to select the reaction mechanism by the choice of ligands coordinating the catalytic active site. We have also investigated the ground and excited states of a second-row transition metal oxide dication, RhO2+ using electronic structure theory and found that the ground state is 2Π which is of oxo character. RhO2+ in coordination with four NH3 ligands has exceptionally low activation barrier (13.6 kcal/mol) for MTM transformation because of the rearrangement of electronic configuration. One of the prominent limitations of the MTM transformation using transition metal oxide catalysts is the selectivity issue that means the product methanol is more susceptible to further oxidation that causes over oxidation of methanol and catalyst poisoning. To address this limitation, we have studied a new complete catalytic cycle for MTM transformation using metal methoxide as a catalyst. At first, we have employed FeOCH3+ as a catalyst and used N2O as an oxidant. This combination of catalyst and oxidant has produced a promising energy landscape for MTM transformation. Then we have further optimized the catalytic cycle by incorporating ligands with the active site ((NH3)4FeOCH32+) and using a more potent oxidant (O¬3). Also, a comparison of the C-H bond activation energy between methane and methanol is studied for the complete catalytic cycle. The presence of four NH3 ligands has destabilized the methanol activation channel and the activation barrier of the C-H bond activation of methanol is found higher than the C-H activation of methane.