The passivation of Bronsted acid sites in Nb-TUD-1 to improve H2O2 utilization and resistance to Nb leaching during the liquid-phase epoxidation of ethylene under mild operating conditions (35 degrees C and 50 bar) was studied. Our first strategy was to create hydrophobic ion pairs through the base treatment of Nb-TUD-1 using two organic bases, namely, tetraethylammonium hydroxide and diisopropylethylamine. Although these systems significantly enhanced H2O2 utilization toward ethylene oxide (EO) formation (70% and 90%, respectively), resistance to metal leaching was only moderately improved. We therefore investigated an alternative strategy using four different covalently bound capping groups, namely, methyl (Me), trimethylsilyl (TMS), tert-butyl (t-Bu), and benzyl (Bn); ranging from moderate to high hydrolytic stability. The catalyst capped using the benzyl group, Bn-Nb(40), showed the best performance in terms of significantly improved H2O2 utilization toward EO formation (similar to 60-71%) and reduced metal leaching (similar to 3%). The EO selectivity was >98%. Further, the structure of the Bn-Nb(40) catalyst was found to be stable under reaction conditions even after several recycle runs, thus confirming its potential as a viable epoxidation catalyst.