Silica aerogels formed from soluble silicates and methyl trimethoxysilane (MTMS) using CO₂ gas as a gelation agent

Silica aerogel has been formed using CO₂ gas as the gelation agent through the low-cost ambient pressure drying technique. With water wash and the following solvent exchange process for Na⁺ removal, this synthesis route has no ion exchange resin used and becomes more straightforward. In order to reduce the silica particle and pore sizes, different hydrolysis-condensation rate control agents including dimethylformamide (DMF), methyltriethoxysilane (TMES), Glycerol and methyltrimethoxylsilane (MTMS) have been applied to modify the structural groups of silica network. Results show that MTMS is the most effective control agent and the optimized MTMS/silica sol volume ratio has been found at 1% by testing the specific surface area of the aerogel. In addition, the effects of chlorotrimethylsilane (TMCS) amount and the calcining temperature on the physicochemical properties of the aerogel have been also investigated. When the TMCS/silica sol volume ratio is 0.4, Thermogravimetric and Differential Scanning Calorimetry analysis (TG/DSC) measurement shows that there is an apprent exothermic peak at calcining temperture of 420 °C, caused by the transformation of Si-CH₃ group to Si-OH group. X-ray photoelectric spectroscopy (XPS) results show that the percentages of Si-C(-H) bonds with binding energy of 102.4 eV have decreased from 25.5% to 7.2% after 420 °C calcining. The contact angle of the as-dried aerogel is as high as 154° and dramatically decreases to ca. 109° after calcining at 450 °C. The pore volume and BET specific surface area both increase first and then decrease with the increase of calcining temperature, and the pore volume reaches the maxmium of 1.45 cm³/g at 400 °C and it confirms Si-CH₃ group release. This study offers a new facile route to the synthesis of silica aerogel and presents a promising way of the cost-effective large-scale manufacturing.