Microstructural properties of non-supported microporous ceramic membrane top-layers obtained by the sol-gel process

Dried and calcined non-supported membrane top-layers of SiO2, SiO2/TiO2, SiO2/ZrO2 (10, 20 and 30 mol% TiO2 and ZrO2, respectively) and SiO2/Al2O3 (10 mol% AlO1.5) were prepared using acid catalyzed hydrolysis and condensation of alkoxides in ethanol. The microstructure was determined using nitrogen physisorption. The modified Horváth-Kawazoe model for nitrogen adsorption in cylindrical pores was used for pore size assessment. SiO2 non-supported membrane top layers were 100% microporous with an average porosity of 30-37%, depending on drying conditions. The bimodal pore size distribution shows a maximum at an effective pore diameter of 0.5 nm, and a broader tail with a weaker maximum around 0.75 nm. Microporous non-supported binary systems can be prepared with porosities between 15 and 40%. The high reactivity of the Ti, Zr, Al-alkoxides requires carefully chosen conditions. Too much water results in dense materials. The pore size distributions (PSDs) of the binary systems resemble the PSDs for silica.