Amine Solvents
Aqueous amine solvents such as monoethanolamine absorb CO2 chemically, mainly by forming carbamates (favored for primary and unhindered secondary amines, consuming up to two amines per CO2) or bicarbonate (favored for hindered and tertiary amines, with the amine acting as a base). The strength of these reactions sets the heat needed to regenerate the solvent, so tuning amine structure is a central route to lower energy capture. See also CO2 Capture, Solid Amine Sorbents and Conceptual Density Functional Theory.
Substituent effects and descriptors for amine-CO2 reaction energies (2012)
Lee and Kitchin computed, with B3LYP/6-31+G(d,p) and the IEFPCM water model, the energies of bicarbonate and carbamate products for alkylamines, alkanolamines and trifluoroalkylamines of one to three carbon chains (Lee 2012). Further alkylation favored bicarbonate but disfavored carbamate, CF3 groups destabilized both with an effect that weakened with distance from nitrogen, and zwitterions were always above carbamic acid in energy. The amine electronegativity relative to carbonic acid described bicarbonate complex energies across all three families, whereas nitrogen local softness described carbamic acid energies only within separate family groupings. Summary: lee2012.
Solvent rankings from optimized process models (2013)
At a fixed 11 mol% amine, optimized process models found that DEA reaches a higher working capacity than MEA and so a lower reboiler duty, which the authors link to a less stable DEA carbamate that favors the bicarbonate route (Lee et al. 2013). The hindered amine AMP, restricted to bicarbonate, had reaction enthalpy near DEA but working capacity near MEA while needing less stripping steam, suggesting hindered or tertiary amines with higher working capacity as a design target. Summary: lee2013.
Amino acid salt solvents screened against MEA (2015)
Hallenbeck and coworkers compared 0.5 M aqueous potassium salts of glycine, taurine, proline and lysine with MEA (Hallenbeck et al. 2015). Lysinate, with two primary amines, reached 0.70 mol CO2/mol in microfluidic tests (MEA 0.46, glycinate and taurinate 0.43, prolinate 0.34) and 1.32 versus 0.70 mol/mol for MEA in a stirred reactor at 40 C, though protonation of its high-pKa side chain kept the gain below a doubling. On a constant water fraction basis 30 wt% MEA absorbed more per gram than 1.48 M lysinate because of lysine’s higher molecular weight, so the basis of comparison matters. Raman spectra showed carbamate, bicarbonate and carbonate products, with a shifted lysinate carbamate band hinting at a weaker carbamate. Summary: hallenbeck2015.