Surface modification of γ-Al2O3/TiO2 multilayer membranes for applications in non-polar organic solvents

This paper reports the surface modification of hydrophilic γ-Al2O3/anatase-TiO2 multilayer membranes by a silane coupling treatment, in order to obtain NF membranes applicable in non-polar solvents. Special care was given to optimization of the pore structure of the anatase toplayer (microporous–mesoporous) and selection of the reaction medium (ethyl acetate) and silane coupling reagent ((CH3)2SiCl2, C1 silane; C8H17CH3SiCl2, C8 silane). Hexane and water permeability were studied before and after silane treatment as an indication of the effect of the modification. No hexane permeability was obtained before modification. For microporous membranes fired at 300 °C, the internal pore structure remained hydrophilic upon silane treatment, as evidenced by the fact that the hexane permeability remained zero. On the contrary, the more open membrane structure with mesopores obtained by firing at temperatures higher than 400 °C, could be modified by a silane coupling reaction at the internal pore structure. The ratio of permeability, hexane/water, was altered from 0/1 for a firing at 300 °C and a C1 silane treatment to 1/0 for a firing at 500 °C and a C8 silane treatment. Standard PEG-retention measurements in water showed that the membranes fired at 400 and 450 °C and modified with a C8 and C1 silane, respectively, have pore sizes corresponding to those of nanofiltration (NF) membranes. These membranes were characterized by a molecular weight cut-off (MWCO) of 410 and a hexane permeability of 3 l h−1 m−2 bar−1 (400-C8) and a MWCO of 650 and a hexane permeability of 5 l h−1 m−2 bar−1 (450-C1).