Preparation of hydrophobic nanoporous methylated SiO2 membranes and application to nanofiltration of hexane solutions

Methylated SiO2 colloidal sol solutions were prepared by hydrolysis and condensation reactions of mixed tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) using EtOH as a solvent with NH3 as a catalyst. Organic/inorganic hybrid membranes were prepared by coating porous substrates with the methylated SiO2 solutions and calcinating at 400–600 °C in an N2 atmosphere. Nanopermporometry characterization using water and hexane as a condensable vapor confirmed the successful preparation of hydrophobic nano-sized pores of 2–4 nm (hexane-based). The hexane permeability of methylated SiO2 membranes was almost constant following the addition of water in concentrations of 0–80 ppm (saturated), but the permeability of hydrophilic TiO2 membranes was decreased by the addition of water. Methylated SiO2 membranes were applied for the nanofiltration of polyolefin oligomers in hexane solutions. The hexane permeability of polyolefin oligomers/hexane solutions was approximately the same as that of pure hexane, which suggested there was no fouling of the hydrophobic SiO2 membranes and hexane solutions. Methylated SiO2 membranes showed a hexane permeability of 2.0–7.58 × 10−11 m3/(m2 s Pa), which corresponds to 7.2 and 27 L/(m2 h bar), with molecular weight cut-offs of 1000–2000.