Surface-enhanced CO2 capture in ionic liquid-silica nanocomposites via sol-gel synthesis in the low partial pressure range

Abstract

Ionic liquid-containing silica nanocomposites enable the capture of carbon dioxide from gas mixtures containing nitrogen, oxygen, and methane. Synthesis methods explored for such nanocomposites include impregnating porous silica and one-pot synthesis via a sol-gel process. This research investigates a non-hydrolytic sol-gel route for nanocomposite materials enabling CO2 capture at low partial pressures (0.1-0.4 bar). The tetraethyl orthosilicate (TEOS) precursor condensation resulted in a silica matrix formed around ionic liquid domains, for bis (trifluorosulfonylimide) (TFSI-)-based ionic liquids with 1-butyl-1-methylpyrrolidinium (BMP+), 1-butyl-3methylimidazolium (BMI+), 1-ethyl-3-methylimidazolium (EMI+) and 1-hexyl-3-methylimidazolium (HMI+) cations. Using a one-pot synthesis method enables exploring CO2 sorption in such nanocomposites for ionic liquid-to-silica contents up to fourteen times higher than in previously reported studies. Moreover, the selected synthesis method provides greater tunability in deposition methods and their control. The silica host matrix was characterized by N2 adsorption isotherms at 77 K after solvent extraction and supercritical drying of the material for ionic liquid removal. The pore size distribution of the freestanding silica network was observed via Scanning Electron Microscope (SEM) imaging and assessed with the Barrett-Joyner-Halenda (BJH) method for nanocomposites of different [BMP][TFSI]-to-silica ratio. The CO2 uptake at pressures down to 0.1 bar was evaluated from CO2 adsorption isotherms at 303 K. The confinement of [BMP][TFSI] resulted in a beneficial effect for the CO2 uptake at lower partial pressures, with an uptake five times higher than the sum of the individual uptake expected from the contained ionic liquid and silica. The reported results show the advantage of a one-pot synthesis method for broader tunability of the nanocomposite, both regarding its content and application, as well as increased performance at lower partial pressures compared to the nanocomposite's individual constituents.