Highly hydrothermally stable microporous silica membranes for hydrogen separation

Fluorocarbon-modified silica membranes were deposited on gamma-Al2O3/alpha-Al2O3 supports by the sol-gel technique for hydrogen separation. The hydrophobic property, pore structure, gas transport and separation performance, and hydrothermal stability of the modified membranes were investigated. It is observed that the water contact angle increases from 27.2 +/- 1.5 degrees for the pure silica membranes to 115.0 +/- 1.2 degrees for the modified ones with a (trifluoropropyl)triethoxysilane (TFPTES)/tetraethyl orthosilicate (TEOS) molar ratio of 0.6. The modified membranes preserve a microporous structure with a micropore volume of 0.14 cm(3)/g and a pore size of similar to 0.5 nm. A single gas permeation of H-2 and CO2 through the modified membranes presents small positive apparent thermal activation energies, indicating a dominant microporous membrane transport. At 200 degrees C, a single H-2 permeance of 3.1 X 10(-6) mot m(-2) s(-1) Pat and a H-2/CO2 permselectivity of 15.2 were obtained after proper correction for the support resistance and the contribution from the defects. In the gas mixture measurement, the H-2 permeance and the H-2/CO2 separation factor almost remain constant at 200 degrees C with a water vapor pressure of 1.2 x 10(4) Pa for at least 220 h, indicating that the modified membranes are hydrothermally stable, benefiting from the integrity of the microporous structure due to the fluorocarbon modification.