| Global warming has caused increasing concern on the emissions of carbon dioxide (CO2). It is well recognized that carbon capture and sequestration (CCS) technology is one of the most challenging issues of this century. However, there are still a few inherent drawbacks with currently used aqueous solutions of amines, including volatilization and degradation of amine absorbents, corrosion of equipments and high energy consumption. Therefore, the search for new superior absorbents is still vigorously ongoing, especially for those low-cost, low-volatile and energy-saving ones.In this study, in order to search superior CO2 catchers, various supported functionalized ionic liquids and solution systems of ionic compounds were prepared, through introducing basic functional groups into ionic liquids and simple dissolution of ionic compounds in nonvolatile solvents respectively. For these new sorbents, their structures and properties were well characterized and their CO2 capture performances were studied with emphasis. With these efforts, great progresses have been made in improving the sorption capacities and rates, decreasing the regeneration energy consumption and reducing the preparation costs. The paper covers the followings.Firstly, for the purpose of enhancing CO2 sorption capacities and rates, six multi-amino (3-4) functionalized ionic liquids ([apaeP444][AA]) were synthesized and immobilized into porous silica support as highly efficient CO2 sorbents. Besides good thermal stabilities (Td>200℃), these sorbents (Sorb-AA) retain reasonably high specific surface area and porosity and therefore exhibit rapid sorption and desorption rates as well as excellent sorption capacity and selectivity, and can be used repeatedly. Especially, the sorbent Sorb-Gly at 1:1 IL/SiO2 weight ratio can capture 1.37 mmol or 60.4 mg CO2 per gram sorbent from simulated flue gas without clear loss of sorption performance over 1380 sorption/desorption cycles.Then, in order to reduce the preparation costs of task specific ionic liquids (TSILs), two basic ionic liquids ([Ch][Triz] and [Nin8][Triz]) were prepared through a simple one-step ion-exchange reaction. The simplification of synthesis process and inexpensive raw materials used dramatically decreased the preparation cost of the ILs. Due to the nucleophilicity of [Triz]- anions, both ILs trap CO2 effectively, with rapid absorption rates and high absorption capacities. [Ch][Triz] could absorb 3.62 mmol (159.2 mg) CO2/g IL, while [N1118][Triz] could capture 3.33 mmol (146.4 mg) CO2/g IL. However, the application of these two ILs may suffer from their imperfect thermal stability.Next, to hunt for CO2 absorbents with low regeneration energy consumption, simple solution systems of alkali metal salts of 1,2,4-triazole (TrizM) in polyethyelene glycol (PEG) were prepared and assessed for CO2 capture. These aprotic heterocyclic anions (AHAs) containing solutions are easy to prepare, low-volatile and highly thermal stable. Such TrizM-PEG solutions exhibit excellent CO2 capture performances. The TrizM salts chemically absorb equimolar CO2 chemically, and PEG itself has certain physical absorption of CO2. Attractively, the CO2 chemical absorption enthalpies are as low as -32.5～-20.5 kJ/mol, close to that of physical absorption by traditional ILs. This will make the regeneration of the absorbents highly energy-saving.Finally, to seek better solvents for commercially available TrizNa to improve its solubility and decrease the solution viscosity, a great variety of high-boiling or low-volatile solvents were tested. Among them, dimethylsulfoxide (DMSO) dissolves TrizNa best, with a solubility as high as 36.8wt%. Although the volatility of DMSO is not low enough, the simple, inexpensive and low viscous TrizNa-DMSO solution exhibits unique and superior CO2 capture properties. It has high absorption capacities (124.3 mg CO2/g solution for 30wt%TrizNa-DMSO) contributed by the 1:1 stoichiometric chemical absorption of TrizNa and the physical absorption of DMSO, high absorption rates due to the low viscosity and low desorption enthalpy (30 kJ/mol) as same as observed in the TrizM-PEG systems. Interestingly, the reaction product of TrizNa and CO2 precipitates from the solutions after CO2 capture. This makes it possible to avoid heating large amount of solvent during CO2 desorption because the solvent can be easily separated by centrifugation or filter pressing before regeneration. Accordingly, a four-step process (absorption-centrifugation-regeneration-dissolution) similar to the desulfurization by lime water was designed, and the energy consumption was estimated. The two important factors, namely, the low chemical desorption enthalpy and much less solvent presence (less than 10wt%) during regeneration make the regeneration of absorbents highly energy-saving. Its total regeneration energy consumption is as low as 37-48 kJ/mol CO2, which is only 10-13% of the value for 30wt% aqueous solution of ethanolamine. Therefore, such TrizNa-DMSO solution appears to be very promising candidates for highly efficient, energy-saving and economical CO2 catchers. |
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