https://doi.org/10.1021/acs.energyfuels.0c00880
“We have carried out NMR analyses in unloaded and loaded solutions of ethylene glycol and N-methyl-2-pyrrolidone with N,N-diethylethanolamine to verify if one is able to qualitatively identify peaks relating to alkylcarbonate formation. The results of these studies are given in the Supporting Information. In short, both alkylcarbonates of ethylene glycol and N,N-diethylethanolamine could be observed in the loaded MEG + DEEA solvent, but no alkylcarbonate whatsoever has been observed in the NMP + DEEA solvent. Meanwhile, molecular carbon dioxide can be observed experimentally in loaded NMP + DEEA, accounting for physical absorption of CO2 in that formulation.
Figure 9 shows the two possible alkylcarbonate formation pathways for nonaqueous solutions of MEG and DEEA, where either the deprotonated solvent or the deprotonated amine itself can react directly with CO2 to form carbonate species. For the first pathway to be viable, the solvent must be capable of being deprotonated, which ties in directly with its self-ionization constant. At the same time, both alternatives benefit from having a strong base in solution so that the deprotonation step can be taken to its maximum extent. Therefore, having a mixture of a diluent with low pKS and an amine with high pKa is the best possible combination for enabling CO2 absorption in tertiary amines.”
“Figure 9. Two possible alkylcarbonate formation mechanisms for nonaqueous ethylene glycol + 10 wt % N,N-diethylethanolamine.”