Unsupported membranes of 3Al2O3-2SiO2 composition prepared by sol-gel processs

-Colloidal gels of mullite composition (74Al2O3-26SiO2 wt%) have been prepared using aqueous mixture of a colloidal dispersion of boehmite (AlOOH) and TEOS (Si(OC2H5)4) with pH varying between 0,5 to 3,0. After drying at 60°C the gels have been heat treated up to 1100°C. The temperature evolution of porosity has been studied by gas and mercury porosimetry. Dried gels made with pH ≤ 1,5 show type I isotherm with H4 hysteresis, evidencing microporous structure (diameter (25 Å). This gels heat treated at 500 and 980°C exhibit a narrow monomodul pores distribution with diameter around 40 Å. At higher pH the isotherms are of type II with H2 hysteresis. The pore distribution is larger but the peak maximum is always below 150 Å. The surface area is almost independent of the pH (~250 m2/g) below 980°C and drops to 60-160 m2/g at higher temperature. The sol-gel preparation allows to tailor the porosity of mullite material with unique properties which can be exploited in applications such as filtration, separation, etc.