Hydrophobic silica aerogel derived from wheat husk ash by ambient pressure drying

Abstract

Silica aerogels from wheat husk ash (WHA) were prepared via a sol–gel process by ambient pressure drying. Silica was extracted from WHA by NaOH solution to form sodium silicate, which was used as precursor for aerogels. Silica wet gels were synthesized by resin-exchange-alkali-catalysis of the sodium silicate solution, followed by solvent exchange with ethanol (EtOH) and hexane in turn. Consequently, a mixture of trimethylchlorosilane, EtOH and hexane was used for surface modification of the wet gels in order to obtain hydrophobic silica aerogels. The density, pore structure, hydrophobic property and thermal insulation property of the obtained silica aerogels were investigated in detail. The results show that the formation of silica aerogels can be successfully realized at a SiO2/H2O weight ratio varying from 0.065 to 0.167. Silica aerogels possess a desirable pore structure with a surface area ranging from 513 ± 5 to 587 ± 6 m2/g, a pore volume from 2.3 ± 0.3 to 4.0 ± 0.1 cm3/g and a pore size from 9 ± 2 to 15 ± 1 nm, an outstanding hydrophobic property with a water contact angle of 147 ± 0.1° and a distinguished thermal insulation property with a low thermal conductivity ranging from 0.009 ± 0.0001 to 0.012 ± 0.0002 W/(m·K).