Novel acid-catalyzed sol-gel synthesis route to control the crystallinity and phase formation of highly porous TiO2 aerogels.

Abstract

TiO2 aerogels are three-dimensional, open-porous materials, which are synthesized in a sol-gel process based on alkoxide precursor hydrolysis and condensation with subsequent drying using supercritical CO2, to prevent pore collapsing. The advantages of this procedure are the preservation of the high surface area and pore volume and results in highly porous aerogels with a defined pore size distribution. This leads to good accessibility of reactants to the reactive sites and makes aerogels attractive for heterogeneous catalysis. Crystallinity is a crucial factor for efficient catalysis, e.g., in photocatalysis. However, aerogels are naturally amorphous after drying with supercritical CO2, which means that crystalline phases have to be obtained by subsequent heat-treatment at temperatures above 300 °C or even high temperature supercritical drying1, accompanied by a strong decrease of porosity and surface area. Meanwhile a novel acid-catalyzed sol-gel procedure of mesoporous crystalline TiO2 aerogel is developed. Here, high crystalline aerogels can be obtained directly depending on the amount of acid based on a low-temperature synthesis procedure. High surface areas above 450 m2/g and pore volumes above 4 m3/g are obtained. The corresponding crystallinity of the aerogels is approx. 40-60 wt.% confirmed by Rietveld analysis. It is shown that the surface area and pore volume decrease with increasing crystallinity. Even at very high crystallinity levels, the surface area and pore volume are found to be still very high, between 200-350 m2/g and 1.5-2-5 m3/g respectively. Mainly anatase and brookiteare verified by X-ray diffraction. The simple and low-temperature synthesis procedure for crystalline and mesoporous TiO2 aerogels provides a possibility to control the microstructure and crystallinity by changing the acid concentration, which has not been reported yet. 1 Moussaoui et al., Journal of Saudi Chemical Society, 21, 751-760, 2017.