In Situ Nuclear Magnetic Resonance Mechanistic Studies of Carbon Dioxide Reactions with Liquid Amines in Non-aqueous Systems: Evidence for the Formation of Carbamic Acids and Zwitterionic Species

In a previous study, we reported the use of in situ 1H and 13C nuclear magnetic resonance (NMR) to elucidate mechanistic pathways for the reaction of carbon dioxide with a broad range of amines (pKa ∼ 4.5–15.5), including alkanolamines of commercial interest, in water. In the aqueous systems of that study, water most importantly functions as a Brønsted acid/Lewis base and, as the amine is consumed and pH decreases, hydrolyzes the initially formed carbamate species (1:2 CO2/amine stoichiometry), into the alkyl ammonium bicarbonate with a more beneficial 1:1 CO2/amine stoichiometry. This study has been extended herein to amines, amidines, and guanidines dissolved in non-aqueous solvent systems, such as dimethyl sulfoxide, sulfolane, toluene, 1-methyl-2-pyrrolidinone, and the ionic liquid 1-ethyl-3-methyl-imidazolium acetate. The use of non-aqueous organic solvents shuts off some CO2 reaction pathways available in aqueous solution. However, more importantly, it opens up new possibilities and reaction pathways for amine-based carbon capture. Two important aqueous system pathways are eliminated: the direct hydration of CO2 with tertiary amines or guanidines to form bicarbonates and the hydrolysis of carbamates at lower pH to form bicarbonates. In non-aqueous solution, the initial step for the reaction of primary and secondary amines with CO2 is the same as in aqueous solution: nucleophilic attack by the amine nitrogen on CO2. However, additional mechanistic pathways are enabled in non-aqueous solvents, particularly the stabilization of carbamic acid(s) (rather than carbamates) products in certain organic solvents. The formation of carbamates requires no water and is favored by higher amine concentrations and basicities (higher amine pKa). In contrast, carbamic acid/zwitterion formation is favored by lower amine concentrations, higher CO2 partial pressures, lower amine pKa, and selection of more polar organic solvents that promote hydrogen bonding. The new amine–CO2 reaction pathways enabled here by the use of non-aqueous solvents introduce stabilizing interactions between the non-aqueous solvent and the amine–CO2 reaction products, facilitating higher capacity and selectivity for carbon capture than in water solutions. The effects of the temperature, amine basicity, solvent electronic structures, and concentration on amine–CO2 reaction products (carbamic acid/zwitterion/carbamate and equilibria between neutral and ion-paired forms) are discussed in detail herein.