Mechanistic insights for enhancing activity and stability of Nb-incorporated silicates for selective ethylene epoxidation

Significant ethylene epoxidation activity was observed over niobium (Nb) incorporated mesoporous silicate materials Nb-KIT-5, Nb-MCM-48, and Nb-TUD-1, with hydrogen peroxide (H2O2) as oxidant and methanol (MeOH) as solvent under mild operating conditions (35 degrees C and 50 bars). No CO2 as by-product was detected at these conditions. The measured ethylene oxide (EO) productivity over Nb-TUD-1 materials (342-2539 g EO h(-1) kg(-1) Nb) spans a greater range than those observed with Nb-KIT-6 (234-794 g EO h(-1) kg(-1) Nb), Nb-KIT-5 (273-867 g EO h(-1) kg(-1) Nb) and Nb-MCM-48 (71-219 g EO h(-1) kg(-1) Nb) materials at similar operating conditions. However, significant H2O2 decomposition and Nb leaching were observed in all cases. Computational studies employing minimal models of the catalytically active sites, suggest how the Bronsted acidity may lead to these detrimental pathways. Indeed, lowering the metal loading to significantly reduce the Bronsted acidity results in a dramatic increase in H2O2 utilization toward EO formation (4304 g EO h(-1) kg(-1) Nb). The increased EO productivity either matches or surpasses what was observed on the conventional Ag-based heterogeneous catalyst (with O-2 as oxidant) as well as a Re-based homogeneous catalyst (with H2O2 as oxidant). These results are paving the way for further computational and experimental investigations aimed at the rational design of improved epoxidation catalysts that reduce H2O2 decomposition and metal leaching to practically viable levels. (C) 2016 Elsevier Inc. All rights reserved.