CO2 capture using ZIF-8/water-glycol-2-methylimidazole slurry with high capacity and low desorption heat

By dispersing zeolitic imidazolate framework-8 (ZIF-8) in different liquids composed of water, glycol, and 2-methylimidazole (mIm), a series of ZIF-8/water-glycol-mIm slurries were formed. The sorption isotherms of CO2 in these slurries, as well as in aqueous solution of mIm, were measured in the temperature range from 293.15 to 313.15 K. Next, the enthalpies of CO2 sorption in these slurries were correspondingly determined using the Clausius-Clapeyron equation. At the same time, the sorption kinetics and regeneration performances of the slurries were investigated. The results show that CO2 absorption in aqueous mIm solutions is dominated by chemical absorption, while in the ZIF-8/mIm-water-glycol slurry, CO2 uptake is determined by both chemical absorption and physical adsorption. Increasing ZIF-8 content in slurry can significantly increase the sorption capacity and decrease the sorption heat by increasing the physical adsorption contribution. Increasing ZIF-8 content can even increase the sorption rate within a certain content range. Although the presence of glycol decreases the sorption capacity and increases viscosity to a certain extent, it also significantly decreases the desorption heat. By considering sorption capacity, desorption heat, and viscosity, a particular ZIF-8 slurry recipe, i.e., solid fraction of 15-25.0 wt% and liquid composition of approximately mIm 25 wt% + water 40 wt% + glycol 35 wt%, is recommended, for which the CO2 sorption capacity can reach 1.6 mol/L (higher than that of some alkanol amine solutions) under ambient pressure and 313.15 K, while the desorption heat could be as low as 32 kJ/mol. The structure of ZIF-8 can remain intact in the CO2 capture process in the presence of mIm, i.e., the slurry could be regenerated perfectly.