Pervaporation dehydration of an isopropanol aqueous solution using microporous TiO2-SiO2-OCL (Organic chelating Ligand) composite membranes prepared under different firing conditions

Abstract

Microporous TiO2-SiO2 composite membranes intended for use in the pervaporation dehydration of a water/isopropanol mixture were prepared via the sol–gel method using acetylacetone (ACA) as an organic chelating ligand (OCL). TiO2-SiO2-ACA sols were prepared by using titanium (IV) propoxide (TiTP), an alkoxide of Ti, tetraorthoethylsilicate (TEOS), an alkoxide of Si, and ACA, as a composite of TiO2 and SiO2 with coordinating OCL to Ti. The dried gels were fired at 300 °C under N2 or at 500 °C under air, and were analyzed via FT-IR. Both powders had Ti-O-Si bonds, however, OCL in the powder fired at 500 °C in air has decomposed and removed. The TiO2-SiO2-ACA sols were applied to alumina plates and fired at either 300 °C under N2 or at 500 °C under air to check the water-contact angle, which indicated that the OCL was retained on the plates fired at 300 °C under N2 and were more hydrophobic than when fired at 500 °C under air. TiO2-SiO2-ACA composite membranes were fabricated by coating either the TiO2-SiO2-ACA sol layer fired at 300 °C under N2 or that fired at 500 °C under air onto a TiO2-SiO2 intermediate layer with an α-alumina support. The pore sizes of both membranes were estimated via the NKP method for single-gas permeance at 200 °C. The mean pore sizes of the membranes fired under N2 at 300 °C and under air at 500 °C were 0.47 nm and 0.68 nm, respectively, which suggested the possibility of pore size control via firing conditions. Pervaporation tests of the TiO2-SiO2-ACA composite membranes in a water/isopropanol mixture (IPA: 90 wt%) showed that the membranes fired under air at 500 °C and under N2 at 300 °C had values for permeation flux of 0.90 and 0.13 kg/(m2 h) and separation factors of 667 and 98 at 50 °C, respectively. The higher permeation flux of the membrane fired under air at 500 °C was due to the influence of a larger pore size as well as to a more hydrophilic surface. On the other hand, the separation factor was also higher for the membrane fired under air at 500 °C, because the hydrophilic surface facilitated water permeation. These results indicate the possibility of using firing conditions to control both the pore size and the surface properties of microporous TiO2-SiO2-ACA composite membranes for use in PV separation.