(375b) Computational Design of Zr and Hf Organometallic Catalysts for Direct Propene Epoxidation Using Molecular Oxygen Oxidant

Propylene oxide (PO) is a key intermediate in the chemical industry and one of the most produced chemicals worldwide by mass. All of the existing commercial methods for PO production consume H₂ as a co-reactant (e.g., H₂O₂ and cumene processes) or produce co/by-products. Direct propene epoxidation utilizing O₂ as oxidant without co-reactants or co-products could potentially give economic and environmental benefits. We performed computational screening using density functional theory (DFT) calculations to identify suitable catalysts for this process.

Seven Zr/Hf-based catalysts have been studied. Complete reaction cycles for each catalyst were computed, including ground state and transition state energies. Chemical potential diagrams were calculated that describe the relative energies of different catalyst forms as a function of the oxygen atom chemical potential. Energetic spans have been computed and possible side reactions have been tested for all seven catalysts. The enthalpy energetic span is an estimate of the effective activation barrier for the entire catalytic cycle. Our best direct propylene epoxidation catalysts have a computed enthalpy energetic span of <30 kcal/mol.To the best of our knowledge, this is lower than any of the previously reported direct propene epoxidation catalysts.

These catalysts operate via a completely new selective oxidation mechanism that passes through η³-ozone intermediates. This catalytic cycle involves: (a) the η³-ozone group releases an O atom to form substrate oxide and generate a peroxo or weakly adsorbed O₂ group, (b) the peroxo or weakly adsorbed O₂ group releases an O atom to form substrate oxide and an oxo group, (c) an oxygen molecule adds to the oxo group to form an η²-ozone group, (d) the η²-ozone group rearranges to regenerate the η³-ozone group.

We also used DFT calculations to investigate potential catalyst deactivation processes and other side reactions. While these are important for some of the catalysts, others show very little tendency for side reactions and catalyst deactivation processes.