Effect of immobilization methods and the pore structure on CO2 separation performance in silica-supported ionic liquids

Abstract

The CO2 separation performance in ionic liquids modified porous silica gels prepared by the impregnation and grafting methods were investigated. The effects of impregnation amount and pore size distribution of adsorbents with different types of ionic liquids on the CO2/N2 selectivity and water vapour adsorption and diffusion coefficient were analyzed in detail. The porous structures were characterized by CO2 adsorption isotherms at 0 °C. Adsorption behavior of CO2 (at 25, 40 and 60 °C) and N2 (at 40 °C) was measured using a gravimetric method. Compared with the impregnated samples, the grafting of ILs on the support surface only causes a weak loss of microporosity, leading to a slight decrease in CO2 adsorption capacity, and has much better recycling. CO2/N2 selectivity and water resistance are however enhanced, especially samples with the longer chain on imidazolium cation. A conventional homogeneous surface diffusion model for ILs grafted silica was used to estimate the diffusivities and good agreement between experimental values and fitting curves were obtained. For ILs modified silica prepared with impregnation method, the CO2 diffusivity constants include diffusion coefficients in ionic liquid phase and silica phase simultaneously. The diffusion coefficients are at a level of 10−7-10−9 m2 s−1 and about two or three orders of magnitude higher than that of neat ILs. The grafted silica exhibits diffusivities faster than those of the impregnated silica, indicating that more ionic liquids accessed into inner pore is unfavorable for CO2 diffusion.