The fabrication of gas-sieving amorphous silica membranes exhibiting superior thermal/hydrothermal/chemical stability by the facile, scalable and controllable synthesis strategies is highly desired to bring the silica membranes a step closer to large scale deployment. Sol-gel modification of bis(triethoxylsilyl)ethane (BTESE) membrane pore network with Ti substitution resulted with a shift in the molecular level cut-off to smaller molecules establishing ideal selectivity improvement from 13 ± 2 to 20 for H2/N2 and 17 ± 4 to 28 for H2/CH4 at 200 °C. BTESE and analogous Ti-substituted BTESE membranes (BTESE-Ti) exhibited H2 permeances of (0.9–1.2)x10−6 mol/(m2sPa) and 5.8 × 10−7 mol/(m2sPa), respectively, reflecting a difference in their thickness. Decrease in the sol concentration by 2-fold led to an increase in the H2 permeance value to 8.9 × 10−7mol/(m2sPa) at 200 °C for BTESE-Ti membranes. We further theoretically designed amorphous BTESE and BTESE-Ti structures, and simulated their interactions with the gas molecules along with the pore diffusion of H2 as a model system to elucidate the membrane performance through the density functional theory (DFT) calculations. Theoretical findings suggest that the only transport mechanism is the direct transport from the gas phase to the pore entrance by excluding the surface diffusion possibility.