Absorption of CO2 in the aqueous solutions of functionalized ionic liquids and MDEA

Four ionic liquids (ILs)—tetramethylammonium glycinate (=[Gly]), tetraethylammonium glycinate ([N2222][Gly]), tetramethylammonium lysinate ([N1111][Lys]) and tetraethylammonium lysinate ([N2222][Lys]) were synthesized and mixed with water or N-methyldiethanolamine (MDEA) aqueous solutions to form a new type of solvents for the uptake of CO2. The solubility or absorption of CO2 in these IL + MDEA aqueous solutions was investigated over a wide range of IL concentrations (5–100%), temperature (298–318 K) and partial pressure of CO2 (4–400 kPa). The results indicated that ionic liquid could greatly enhance the absorption and increased the absorption rate of CO2 in MDEA aqueous solutions. It had been found that the aqueous solutions of 15% IL and 15% MDEA had higher absorption rate and larger uptake than other IL + MDEA solutions of 30% total amines. Temperature (298–318 K) seemed to have some influence on the absorption of CO2 in IL + MDEA aqueous solutions. Noticeably, due to the two amino groups in a molecular, the mole absorption of the 30% lysine based ionic liquids aqueous solutions was 0.98 ([N1111][Lys]) and 1.21 ([N2222][Lys]) mole CO2, being about 2–3 times the absorption capacity of MDEA under the same condition. Regeneration under the condition of temperature 353 K, 4 kPa for 240 min showed that aqueous solution of 15% [N1111][Gly] + 15% MDEA had significant regeneration efficiency (over 98%).