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 (H 2 O 2) as oxidant and methanol (MeOH) as solvent under mild operating conditions (35 °C and 50 bars). No CO 2 as byproduct 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 H 2 O 2 decomposition and Nb leaching were observed in all cases. Computational studies employing minimal models of the catalytically active sites, suggest how the Brønsted acidity may lead to these detrimental pathways. Indeed, lowering the metal loading to significantly reduce the Brønsted acidity results in a dramatic increase in H 2 O 2 utilization towards EO formation (4304 g EO h-1 kg-1 Nb). The increased EO productivity either matches or surpasses that observed on the conventional Ag-based heterogeneous catalyst (with O 2 as oxidant) as well as a Re-based homogeneous catalyst (with H 2 O 2 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 H 2 O 2 decomposition and metal leaching to practically viable levels.