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)(.)6H(2)O 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 50-300degreesC. A fresh Ni-doped silica membrane (Si/Ni = 2/1) fired at 500degreesC showed a large He permeance of about 2.6 x 10(-5) [m(3) (STP) /(m(2) (.) s (.) kPa)] with a selectivity of 600 (He/CH4) at 300degreesC. After the Ni-doped silica membrane was left in humid air (40degreesC, 60% RH) for 4 days, the He permeance decreased slightly (by 5%) with a larger selectivity of 800 (He/CH4) at 300degreesC. 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 H-2 and He can permeate. Humid He and CH4 showed smaller permeabilities, especially at temperatures below 150degreesC, than those of dry gases because of condensed and/or adsorbed H2O 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 300degreesC gave relatively good results and coincided well with the predicted values with the ideal permeance ratio, 600.