CO2 and N2 adsorption performance of [Rmim][NO3] ionic liquids impregnated onto mesoporous silica at ambient pressure

Abstract

The CO₂ and N₂ sorption performances in neat and mesoporous silica supported ionic liquids were studied at ambient pressure. The prepared ionic liquids were four derivatives of 1-alkyl-3-methylimidazolium nitrates, where the alkyl groups were n-C₆H₁₃, n-C₈H₁₇, n-C₁₀H₂₁ or n-C₁₂H₂₅, respectively. These ionic liquids were immobilized onto porous amorphous silica and high-ordered MCM-41 via wet impregnation–vaporization method. The sorbents were characterized using ¹H NMR, N₂ ad/desorption, thermogravimetric analysis (TGA) and X-ray powder diffraction (XRD) methods. By passing dry CO₂ at 25 °C with 12 mL flow rate through either the neat ionic liquids or ionic liquid-loaded solid supports, [C₆mim][NO₃] and (MCM-41)-[C₁₀mim][NO₃](20) showed the highest sorption capacities, with 2.39 and 2.44 (wt%), respectively. The effects of ionic liquid loading, temperature, inlet gas flow rate, gas humidity and alkyl chain length on the CO₂/N₂ sorption capacities were also evaluated. In contrast to blank solid supports, impregnated solid supported ionic liquids lost their mesoporosity, causing a decrease in CO₂ and N₂ adsorption capacity, but an increase in CO₂/N₂ selectivity. For example, the CO₂/N₂ selectivity in (MCM-41)-[C₆mim][NO₃](20) found to be 5.6, but by increase of the ionic liquid portion in (MCM-41)-[C₆mim][NO₃](50), the CO₂/N₂ selectivity increased to 17.2, proving that the ionic liquid plays a decisive role in selective adsorption of CO₂.