Carbon Dioxide Capture Into [C₂NH₂MIm][Lys] Ionic Liquids Aqueous Solution

Fossil fuel combustion and many other industrial processes generate large amounts of CO₂, which contributes significantly to greenhouse effect and global warming. It is necessary to explore a cost-effective, low energy-consumption technique for CO₂ capture. Recently, ionic liquids（ILs） have received growing attention for CO₂ capture due to their extremely low vapor pressure, wide liquid temperature range, nonflammability, chemical stability, and tunable structure and properties. Probably the most relevant feature of ILs is its adjustable and designable structure and properties. ILs can be designed as specific structures for specific properties and particular applications by taking advantage of structure- property relationships.In the present work, a novel multi-amine functionalized IL（[C₂NH₂MIm][Lys]）had been developed for CO₂ capture, in which the amine and lysine functionalized groups were both introduced to the imidazole ionic liquid（IL）. 13 C nuclear magnetic resonance spectrometer(¹³C NMR), Fourier-transform infrared（FT-IR） and Thermal gravimetric analysis（TG） were used to investigated the characterizations of[C₂NH₂MIm][Lys], identifying the functioning groups and its stabilities. The absorption-desorption performances of [C₂NH₂MIm][Lys] were studied. The reaction mechanism and the chemical reaction kinetics between [C₂NH₂MIm][Lys] and CO₂ were identified, respectively.The results indicated that, the initial CO₂ absorption rate of [C₂NH₂MIm][Lys]（mol·L-1, 0.5 M） was 0.95 mmol·min-1 at 313 K, and the maximum CO₂ loading was1.62 mol CO₂/mol ILs. The maximum CO₂ loading of [C₂NH₂MIm][Lys] was 3-4times as much as that of the MEA solution. The optimum regeneration temperature of CO₂-saturated [C₂NH₂MIm][Lys] was 393 K. After 6th regeneration cycle,[C₂NH₂MIm][Lys] solution still remained 89.07% of its original absorption capacity.The absorption-desorption performance indicated that this triple amine functionalized ionic liquid [C₂NH₂MIm][Lys] had a higher absorption capacity and a better regeneration performance compared with MEA absorbent.The CO₂ absorption mechanism of [C₂NH₂MIm][Lys] was determined depending on the change of CO₂ loading, temperature and pH during the absorption process. 13 C NMR analysis was used to determine the intermediate product during absorption process. The results indicated that there were two procedures in the CO₂ absorption process. In the first period, [C₂NH₂MIm][Lys] formed anion and cation in the aqueous solution, and CO₂ could react with all-NH2 on both anion and cation,producing carbonate as an intermediate production. This procedure was an exothermic process. In the second period, the pH of the solution decreased, and the chemical reaction slowed down. The intermediate production carbamate hydrolyzed into carbaminate. Meanwhile, the hydrolysis of CO₂ increased, producing HCO₃^-or CO₃^2-. This procedure was an endothermic process. In the CO₂-saturated[C₂NH₂MIm][Lys] solution, the major forms of C was HCO₃^-or CO₃^2-with low concentration of carbamate.Based on the mechanism study, the CO₂ mass-transfer kinetic was determined by a double-stirring CO₂ absorber. The results showed that CO₂ absorption into[C₂NH₂MIm][Lys] solution was a fast pseudo first order reaction. Combining the experimental data to the classic double-film theory and the mass-transfer model, the CO₂ mass-transfer parameters were calculated. For example, at 303.15 K, the reaction rate constant k2 was11046.11 m3·kmol-1·s-1, the enhancement factor E was115.25. The enhancement factor E had a positive linear relation with [C₂NH₂MIm][Lys]concentration and reaction temperature. The reaction energy was calculated by Arrhenius fomular, which was 24.6 KJ·mol-1.