Nickel-Doped Silica Membranes for Separation of Helium from Organic Gas Mixtures

:One of the problems in the use of inorganic silica membranes is their instability against water or water vapor, a problem that results from the dissolution and rearrangements of silica networks. In this work Ni(NO3)-6H2O was added to silica sol for fabrication of Ni-doped silica membranes by sol-gel techniques in order to prevent the densification of amorphous silica networks in a humid atmosphere at 50-3()0°C. A fresh Ni-doped silica membrane (Si/Ni = 2/1) fired at 500 C showed a large He permeance of about 2.6 × 10−5 [m3 (STP)/(m2 ·s · kPa)] with a selectivity of 600 (He/CH4) at 300’C. After the Ni-doped silica membrane was left in humid air (40°C, 60% RH) for 4 days, the He permeance decreased slightly (by 5%) with a larger selectivity of 800 (He/CH4) at 300’C. However, little change was observed in the activation energy of He permeation, suggesting that nickel oxides added to silica can preferably prevent the densification of silica networks through which only H2 and He can permeate. Humid He and CH4 showed smaller permeabilities, especially at temperatures below 150°C, than those of dry gases because of condensed and/or adsorbed H20 molecules in silica networks and on grain boundaries.Separation of He/CH4 mixtures with the fresh Ni-doped silica membrane (Si/Ni = 2/1) at 300’C gave relatively good results and coincided well with the predicted values with the ideal permeance ratio, 600.