Ditetraalkylammonium Amino Acid Ionic Liquids as CO2 Absorbents of High Capacity

By grafting butyl or ethyl onto tetramethylethylenediamine, quaternary ammonium salts with two positive charge centers were formed at the first step. Metathesis with Ag2O followed. Through neutralization with glycine, L-alanine, or valine, a series of new ditetraalkylammonium amino acid ionic liquids (DILs) for CO2 capture were generated. The structures of DILs, as shown in Figure 1, were verified by using H-1 NMR and EA. These DILs were found to be of quite high viscosity which militated against their industrial application in CO2 removal. Drawing on the experience of mixed amines' aqueous solutions, these DILs were blended with water or N-methyldiethanolamine (MDEA) aqueous solutions to act as special absorbents of CO2. Using a Double-Tank Absorption System, the absorption performance of these DIL solutions was investigated in detail. The experimental results indicated that among the three aqueous solutions of DILs (20%, 40%, and 80 wt %), the solution of 40% DIL had a higher absorption rate of CO2 than the other two, demonstrating the different effects of concentration and viscosity on the absorption. The solution of 40% DIL or the 15% DIL + 15% MDEA had much higher capacity for CO2 than the corresponding monocation tetraalkylammonium AAILs, due to the special structure of the dication which could influence the solubility of CO2 in the aqueous solution.