Towards highly selective ethylene epoxidation catalysts using hydrogen peroxide and tungsten- or niobium-incorporated mesoporous silicate (KIT-6)

Abstract

Significant ethylene epoxidation activity was observed over Nb- and W-incorporated KIT-6 materials with aqueous hydrogen peroxide (H₂O₂) as the oxidant and methanol as solvent under mild operating conditions (35 °C and 50 bar) where CO₂ formation is avoided. The Nb-KIT-6 materials generally show greater epoxidation activity compared to the W-KIT-6 materials. Further, the ethylene oxide (EO) productivity observed with these materials [30–800 mg EO h⁻¹ (g metal)⁻¹] is of the same order of magnitude as that of the conventional silver (Ag)-based gas phase ethylene epoxidation process. Our results reveal that the framework-incorporated metal species, rather than the extra-framework metal oxide species, are mainly responsible for the observed epoxidation activity. However, the tetrahedrally coordinated framework metal species also introduce Lewis acidity that promotes their solvolysis (which in turn results in their gradual leaching) as well as H₂O₂ decomposition. These results and mechanistic insights provide rational guidance for developing catalysts with improved leaching resistance and minimal H₂O₂ decomposition.