Enzyme-inspired catalysts for ethylene epoxidation

Ethylene oxide (EO) is a primary industrial precursor for synthesizing several chemicals and fuels. In recent years, efficient design techniques for catalysts to reduce the cost of epoxidation and improve product selectivity have been of prime interest. This study uses the Density Functional Theory method and electronic structure analysis to investigate the efficiency of six enzyme-inspired catalysts for epoxidation of ethylene under ambient temperature and mild oxidizing agent (N₂O). We consider a metal-embedded porphyrin framework mimicking the micro-environment of the cytochrome P450 enzyme to build the model catalysts. The three transition metal atoms, Mn, Fe, and Co, are chosen as the active sites. The axial cysteine linkage is modified with a thiol ligand to reduce the computational cost. Mn and Fe complexes show intermediate to higher spin states as the most stable, whereas Co complexes show low to intermediate spin states as the preferred ground state. Comparing the reaction thermodynamics and kinetic energy barriers, we determine the feasible reaction mechanism and the active intermediates involved during the reaction. The activation barriers vary based on the occupancy of the frontier d-states of the metal centers. The Mn and Fe-embedded porphyrin framework without the axial thiol ligand at the quartet (IS) and quintet spin (HS) states shows the most efficient activity and selectivity for epoxide formation. The spin density analysis of the intermediates shows that the reaction prefers a radical pathway.