New perspectives and insights into direct epoxidation of propylene using O₂ and silver-based catalysts

A series of Ag-based catalysts were studied for direct epoxidation of propylene using molecular O₂ as the oxidant. Ag supported on CaCO₃ and α-Al₂O₃ were prepared as base materials to study. Different amounts of alkali earth metal promoter (K) and feed additives, ethyl chloride (EtCl) and nitric oxide (NO), were added to the catalyst and feed stream, respectively, to evaluate their effects on catalyst performance in terms of PO selectivity and propylene conversion. A conventional catalyst supported on α-Al₂O₃ performed similarly to the ARCO Ag (50 wt%) catalyst, supported on CaCO₃, at a much lower Ag loading (12 wt%). EtCl inhibited consecutive oxidation presumably by Cl deposition on Ag sites and NO increased activity possibly by Ag-catalyzed oxidation of NO to NO_x which was the selective oxidant. K⁺ promoter loadings as high as 1.9 wt% were essential to stabilize both Cl and NO_x on the catalyst surface. PO Selectivities as high as 50 % were obtained.