Nanostructured titania membranes with improved thermal stability

Abstract

Nanostructured titania membranes have received significant attention in recent years because of their unique characteristics such as high water flux, semiconductance, photocatalysis, and chemical resistance over other membrane materials including γ -alumina, silica and zirconia [1, 2]. The potential applications of titania membranes include ultrafiltration processes and catalytic/photocatalytic membrane reactor systems for liquid and gas separations [3, 4]. However, their practical applications are limited due to the reduction of porosity and surface area at the elevated temperatures. Hydrothermal process is a low temperature crystallization method, which can produce thermally stable crystalline titania particles [5]. Although hydrothermal process has been used as an independent process to produce ultrafine crystalline oxide particles, this process can be also combined with sol-gel process to promote crystallization of sol-gel products under mild conditions. In this work, hydrothermal process was combined with sol-gel process to improve the thermal stability of titania membranes. Titania sol was prepared by hydrolysis and condensation of titanium isopropoxide (TTIP, Ti(OC3H7)4, 97%), followed by peptization [6]. The TTIP was first dissolved in anhydrous ethyl alcohol at a C2H5OH/TTIP molar ratio of 5. Hydrolysis and condensation were conducted at room temperature, by adding the mixed solution slowly into distilled water up to an H2O/TTIP molar ratio of 200. The white precipitates were centrifuged and washed with distilled water. After redispersing the precipitates in the certain amount of water, a 1 M HNO3 solution was added to the white precipitates to attain a [H+]/[Ti] molar ratio of 0.3–0.5. Then, the solution was refluxed at 80 ◦C for 12 hr. For hydrothermal treatment, the peptized sol was transferred into Teflon beaker in the metal jacket (autoclave) and the autoclave was aged at 135 ◦C for 12 hr. Unsupported titania membranes were prepared by pouring peptized sol and hydrothermally treated sol into petri dishes, followed by drying under the controlled atmosphere. The dried membranes were fur-