Preparation of Carbon Aerogels from Polymer-Cross-Linked Xerogel Powders without Supercritical Fluid Drying and Their Application in Highly Selective CO₂ Adsorption

Carbon aerogels are well-known materials for their high porosity and high surface areas. They are typically made from pyrolysis of carbonizable polymeric aerogels. Here, we report an alternative route to monolithic carbon aerogels starting from xerogel powders. Use of powders speeds up solvent exchanges along sol–gel processing, and xerogelling bypasses the supercritical fluid drying step that is needed for making polymeric aerogels. Overall, this alternative route results in time, energy, and materials efficiency in the fabrication of carbon aerogels. Specifically, polymer-cross-linked silica xerogel powders were prepared via free-radical surface-initiated polymerization of acrylonitrile (AN) on a suspension of silica particles derived from tetramethylorthosilicate (TMOS) surface modified with 3-aminopropyltriethoxysilane (APTES)-derived initiator. Alternatively, cross-linked silica xerogel powders were prepared with a carbonizable polyurea (PUA) derived from the reaction of an aromatic triisocyanate (tris(4-isocyanatophenyl)methane) with −OH, −NH2, and adsorbed water on the surface of a TMOS/APTES-derived silica suspension. Wet-gel powders by either method were dried under vacuum at 50 °C to xerogel powders, which were compressed into discs. In turn, these discs were carbonized and then they were treated with HF to remove silica and with CO2 to create microporosity. The resulting monolithic carbon aerogels had porosities up to 83% v/v, Brunauer–Emmett–Teller (BET) surface areas up to 1934 m2 g–1, and could uptake up to 9.15 mmol g–1 of CO2 at 273 K, with high selectivity over H2, N2, and CH4.