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Process improvements in silica membrane fabrication, especially the use of clean-room techniques, resulted in silica membranes without detectable mesoscopic defects, resulting in significantly improved transport properties. Supported membranes calcined at 400°C were 30 nanometers in thickness, showed a H2 permeance at 200°C of 2 × 10−6 moles per square meter per second per Pascal (mol m−2 s−1 Pa−1), and had a CH4 permeance more than 500 times smaller. Molecules larger than CH4 were completely blocked. Silica membranes calcined at 600°C showed no detectable CH4 flux, with a H2 permeance of 5 × 10−7 (mol m−2 s−1 Pa−1) at 200°C. These results signify an important step toward the industrial application of these membranes such as purification of H2 and natural gas as well as the selective removal of CO2.