Functional Silica Aerogels with High Specific Surface Area: Influence of Preparation Conditions on Structure Properties

Abstract

A series monolithic silica aerogels modified by amino-substituted organic groups were synthesized by sol-gel processing of tetraethoxysilane (TEOS) and 3-aminopropyltriethoxysilane (APTES) mixtures, followed by drying of the wet gels with supercritical carbon dioxide (SCCO2). The molar ratios of EtOH/TEOS and DMF/ (TEOS+APTES) were fixed at 5 and 0.4, respectively, and APTES/TEOS ratio was varied between 1:9 and 3:7. HCl was added to obtain a narrow pore distribution, and the ratio of HCl/ (TEOS+APTES) was varied between 0 and 0.004. When APTES/TEOS ratio was kept at constant, gelling of the precursor mixtures was accelerated with an increasing ratio of HCl/ (TEOS+APTES). The physical properties of the aerogels have been investigated in detail by the bulk density, porosity, nitrogen sorption measurements and scanning electron microscopic (SEM). The results showed that adding HCl did increase the specific surface areas of the aerogels and make the aerogels have narrow pore distributions, but also resulted in some increase in bulk densities and decrease in porosities. The bulk density of the aerogels was 0.13-0.32 g/cm3, the corresponding porosity between 80% and 92%. Microscopically, the framework of the functional aerogels consists of spherical secondary nano-particles, which stack into grape-like aggregate and interconnect in different directions into a network. The specific surface area of the aerogels were in the range of 362-1044 m2/g, and uniform pore distribution aerogels were obtained when the molar ratio of APTES/ (TEOS+APTES)/HCl at 1/10/0.002 and 1/10/0.004.