Aqueous Synthesis of Compressible and Thermally Stable Cellulose Nanofibril–Silica Aerogel for CO₂ Adsorption

Cellulose nanofibrils (CNF)–silica aerogels have been facilely synthesized via a one-step in situ aqueous sol–gel process of polymerizing and aging the silica precursor in the presence of CNFs to encompass the superior dry compressive strength and flexibility of CNF aerogels and the thermal stability of silica aerogels. Sodium silicate (Na2SiO3) was hydrolyzed and polymerized in the presence of CNFs at varied ratios to synthesize hydrogels whose storage and loss modulus confirmed CNFs to function as the structural skeleton. At the optimal 8:2 CNFs/Na2SiO3 composition, the hydrogels with homogeneously dispersed silica and CNF can be freeze-dried into hierarchically mesoporous aerogels with ultralow density of 7.7 mg/cm3, high specific surface of 342 m2/g, and pore volume of 0.86 cm3/g. This robust sol–gel approach employs naturally abundant silica and cellulose in aqueous system to generate improved CNF–silica aerogels that had much higher compressive strength and modulus of up to 28.5 and 177 kPa and structural flexibility than silica aerogel and enhanced thermal stability and specific surface over CNF aerogel. Further functionalization of CNF–silica aerogels via organosilane reaction introduced primary amine groups capable of capturing CO2 with an adsorption capacity of 1.49 mmol/g.