Transparent Cellulose–Silica Composite Aerogels with Excellent Flame Retardancy via an in Situ Sol–Gel Process

Cellulose–silica composite aerogels were prepared by in situ formation of silica nanoparticles via a two-step sol–gel process in cellulose gel, which was prepared by dissolving cotton pulp in 1-allyl-methylimidazole chloride ionic liquid (AmimCl) and then regenerating from AmimCl/water bath. Related properties were investigated with ultraviolet–visible spectrometer, scanning electron microscopy, compression tests, thermogravimetric analysis, and ignition tests as well as a microscale combustion calorimeter. The results showed that the composite aerogels displayed increasing transparency, compressive properties, and thermal and thermal-oxidative stability with increasing of silica content. The incorporation of silica nanoparticles also improved the mesoporous characteristics of aerogels including specific surface area and mesopore volume and significantly delayed the decomposition of cellulose, suppressing the heat release during combustion. The composite aerogels with high silica content (33.6% or more) exhibited good transparency with light transmittance as high as 78.4% at 800 nm, even higher than the neat cellulose aerogels, and presented excellent flame retardant performance, achieving self-extinguishment after ignition. The transparent cellulose–silica composite aerogels with enhanced mechanical performance and improved flame retardancy might show great potential in a wide variety of applications.