Preparation and characterization of sol-gel-derived unsupported Al2O3-SiO2 composite membranes

The synthesis of unsupported Al2O3-SiO2 composite membranes has been achieved by controlling hydrolysis and condensation of aluminium iso-propoxide and tetraethyl silicate under acidic conditions. The phase transformation and pore structural evolution of composite membranes are investigated by X-ray diffraction and nitrogen adsorption measurement, respectively. After sintering at 600 degreesC for 10 h, the composite membranes consist of only amorphous phase as SiO2 concentration ranges from 25 to 100 mol% and crystalline gamma -Al2O3 appears at lower SiO2 content. Both micropores and mesopores contribute to the total pore volume and BET surface area, and mesopores become dominant. At high SiO2 content (50 mol%), the composite materials exhibit a I-nm pore diameter decrease and about 100 m(2) g(-1) surface area increase, as well as only a negligible pore volume drop of 0.0164 cm(3) g(-1), compared with the single Al2O3 membrane. For the composite membrane with 40 mol% SiO2 concentration, sintering at higher temperature leads to phase transformation and grain growth, and both densification and phase transformation result in a decrease in both total pore volume and surface area, as well as an increase in mean pore diameter and finally the collapse of pores. Further study is required to improve the thermal stability of the composite membrane. (C) 2001 Elsevier Science B.V. All rights reserved.