Revealing Activity of Symmetrical and Asymmetrical Ag2O-MOx (M = Cu, Fe, Zn) Catalysts via DFT Calculations for Direct Propylene Epoxidation

Abstract

Direct propylene epoxidation (DPE) with molecular oxygen is an effective process of conversion of propylene into propylene oxide (PO), which is an essential intermediate compound for many petrochemical products such as propylene glycol, propylene carbonate, polyurethane foams, etc. The modification of Ag-based catalysts which prevents overoxidation and improves catalytic activity for PO production is studied by simulating Ag₂O and mixed metal oxides of Ag₂O-CuO (Ag₂CuO₂), Ag₂O-ZnO (Ag₂ZnO₂) and Ag₂O-Fe₂O₃ (Ag₂Fe₂O₄) catalysts using density functional theory (DFT). Instantaneous adsorption of propylene was observed over Ag₂ZnO₂ and Ag₂CuO_2. DPE with oxygen via the Mars-Van Krevelen mechanism (MVKM) was investigated considering propylene and oxygen as alternatively adsorbed species and molecular species. DPE with molecular oxygen and adsorbed propylene required 1.7 times more activation energy than DPE with molecular propylene and adsorbed oxygen. DPE with oxygen via the Lagmuir-Hinshelwood mechanism (LHM) required 3–4 times more activation energy than MVKM for Ag₂O and Ag₂CuO₂, whereas it is comparable for Ag₂ZnO₂. This showed that both mechanisms are followed for DPE over Ag₂ZnO₂. Ag₂ZnO₂ was found to be the optimum catalyst for DPE with instantaneous adsorption of propylene and instantaneous desorption of PO.