Molecular dynamics study of gas permeation through amorphous silica network and inter-particle pores on microporous silica membranes

Abstract

A boundary driven non-equilibrium molecular dynamics simulation method was used to study gas permeation mechanisms through microporous amorphous silica membranes. Two types of silica membranes were prepared, one by random atom-removing and the other by regular pore-digging procedures. The former was a dense membrane that served as a model for network pores formed by silica polymers and the latter had a penetrating cylindrical pore which simulated an inter-particle pore. The permeances of He, H₂ and Ne through network models with densities of 1.7 and 1.8 g cm⁻³ increased with decreasing temperature, while activated permeation was observed for the denser models. Deviations in the permeation properties from those predicted by the Knudsen model became greater with increasing membrane density as the result of molecular sieving effects. The permeance of H₂ through a cylindrical pore 0.6 nm in diameter was greater than that for He at all temperatures examined as predicted by the Knudsen model, and the greater interaction of CO₂ with the pore surface yielded a larger temperature-dependency curve for permeance, compared to He and H₂. The simulated permeation properties of several gases were in agreement with experimental data on actual microporous silica membranes, indicating the qualitative validity of the microporous structure model composed of small openings in a silica network phase and larger inter-particle pores.