Transport mechanisms of water and organic solvents through microporous silica in the pervaporation of binary liquids

Pervaporation experiments were performed on microporous silica membranes in the temperature range 30–80 °C using binary liquids containing 5–22 wt.% water. The organic solvents used were methanol, N,N-dimethylformamide (DMF) and 1,4-dioxane. The dependency of flux and selectivity on temperature and feed composition were investigated. The results were interpreted in terms of the Maxwell–Stefan theory, and Maxwell–Stefan diffusion coefficients at 60 °C were estimated. Water, methanol and DMF were found to diffuse through silica by a surface diffusion mechanism, while 1,4-dioxane was transported mainly by viscous flow through mesopores or small defects. DMF inhibited the transport of water, which may be explained by micropore blocking by strongly adsorbed DMF molecules. The flux of methanol appears to be dominated by a dragging effect by the larger water flux.