Enhancement of CO2 capture capacity of mesoporous sorbents via functionalization with an amino acid ionic liquid

The development of high-performance depuration processes for CO2 removal from flue-gases deriving from fossil-fueled power plants represents one of the most stimulating scientific and technological challenges of the third millennium, to face the global climate change. Post-combustion CO2 capture by means of ionic liquids (IL) supported onto porous substrates is a promising strategy to cope with the main issues associated with IL, and related to their generally high viscosity and market costs. Despite the great scientific interest for IL properties, such as high thermo-chemical stability and selectivity for CO2 when containing basic functional groups, few studies investigated the effect of the support textural properties on the CO2 capture efficiency of IL confined in nanospaces, under typical flue-gas conditions. This work analyses the CO2 capture performances of two mesoporous substrates, namely γ-Al2O3 (termed as Al-raw) and Amberlite XAD1180N (termed as Amb-raw), both raw and functionalized with 1-ethyl-3-methylimidazolium glycinate [Emim][Gly] IL. The functionalized sorbents were prepared by wet impregnation of each substrate with [Emim][Gly] in ethanol solutions, at different active phase concentrations (i.e. C0=7.8, 14.3 and 25.0 g L-1) followed by solvent evaporation. Functionalized sorbents were termed as Al-[Emim][Gly]5%, Amb-[Emim][Gly]5%, Al-[Emim][Gly]9%, Amb-[Emim][Gly]9%, Al-[Emim][Gly]16% and Amb-[Emim][Gly]14%, where 5, 9, 14 and 16% represent the IL mass fractions effectively dispersed onto the support, as determined by thermogravimetric analysis. CO2 adsorption tests on the selected sorbents were performed in a fixed-bed column at 303, 323, 353 and 373 K and for mimicking flue-gas effluents (CO2 15% vol., balance N2). Experimental results highlighted a monotonic enhancement of the CO2 capture performances with the IL loading for both the parent substrates, mainly at lower process temperatures. For example, at 303 K Amb-[Emim][Gly]14% displayed a value of the adsorption capacity nearly 5-times greater than the one obtained for Amb-raw. In the case of Al-[Emim][Gly]16% tested at 303 K, the capture capacity increased by a factor 3 with respect to Al-raw. Dynamic adsorption tests testified slower saturation kinetics of each functionalized sorbent when compared to its unsupported counterpart, likely due to a partial pore clogging effect determined by the active phase. Faster adsorption dynamics and a greater IL utilization factor were observed for sorbents belonging to the Amb-[Emim][Gly] series at each impregnation condition. These experimental evidences are possibly ascribable to a greater mean pore diameter and surface area values retrieved for the Amberlite support ensuring a greater dispersion of the IL film and faster CO2 diffusion rates within the pores network. In general, the obtained results are very encouraging to quicken the applicability of supported ionic liquid sorbents in post-combustion CO2 capture systems.