Nanofiltration in non-aqueous solutions by porous silica–zirconia membranes

Abstract

Porous membranes having various average pore sizes, ranging from 1 to 4 nm, were prepared from silica–zirconia composite colloidal sols by sol–gel processes, and were used for nanofiltration (NF) experiments in non-aqueous solutions of ethanol and methanol. Silica–zirconia membranes, which were tested in pure alcohol solutions for the first time after the preparation of the membrane, showed a gradual decrease in flux for approximately 100 h and then reached a steady flux. When the feed, after reaching the steady flux with ethanol, was changed to another alcohol, steady flux was attained after only several hours. Ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol (PEG) of various molecular weights (PEG400, 600, 1000, and 2000) were nanofiltrated in methanol and ethanol solutions at 50°C. Rejections in non-aqueous solutions increased with applied pressure, which is similar to aqueous solutions. Control of pore size of silica–zirconia membranes showing molecular weight cut-offs in methanol solutions at 300, 600, 1000, and >1000, respectively, was possible by the appropriate choice of colloidal particle sizes. Rejection in methanol solution showed a tendency similar to that in ethanol solution, while rejection in methanol was slightly larger than in ethanol solutions. In addition, rejection in water was much smaller than in methanol solution. For example, the rejection of PEG600 in water and methanol was 0.03 and 0.74, respectively. These results suggest that solvent type plays an important role in determining rejection, as a result of the interaction with solvents and/or membrane surface.