Reaction mechanisms using non-aqueous solutions – brief discussion

https://doi.org/10.1016/j.seppur.2020.117789

“Several studies have been conducted on non-aqueous absorption systems using the sterically hindered amine 2-amino-2-methyl-1-propanol (AMP) [2], [4], [5], [6], [7], [8], [9]. In the absence of water, the proposed reaction mechanism for AMP and CO₂ results in the formation of a carbamate (Reactions 1–2). Physical absorption of CO₂ takes place (Reaction 1) as the gaseous (g) CO₂ is dissolved in the liquid solution (sol). This is followed by the chemical part of the absorption process as the reaction between AMP and CO₂ takes place (Reaction 2) to form the AMP carbamate. The steric hindrance of this carbamate makes it possible to regenerate the solution at temperatures between 70 and 90 °C [2], [10].(Reaction 1)CO₂ (g) ⇄ CO₂ (sol)(Reaction 2)CO₂ (sol) + 2 RNH₂ (sol) ⇄ RNH₃⁺ (sol) + RNHCOO⁻ (sol)

In some organic solvents, the AMP carbamate can also induce phase-changing behavior as it is prone to precipitate as solid (s) crystals (Reaction 3).(Reaction 3)RNH₃⁺ (sol) + RNHCOO^– (sol) ⇄ RNH₃⁺RNHCOO⁻ (s)

If alcohols are used as the solvent, additional formation of carbonate species (Reaction 4) will be possible in the presence of a base, such as AMP.(Reaction 4)CO₂ (sol) + RNH₂ (sol) + RʹOH ⇄ RNH₃⁺ (sol) + RʹOCOO⁻ (sol)

The reaction mechanism for several primary amines, not including AMP, in non-aqueous solutions with both polar and non-polar solvents has been studied by Kortunov et al. [11], using in situ nuclear magnetic resonance (NMR) to determine the chemical species in solution during the reaction with CO₂. They found that the choice of solvent played an important role in determining which protonated species were formed in solution during the reaction with CO₂. Polar solvents such as N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO), which were expected to favor the formation of the carbamate species due to their polarity, instead seemed to favor the protonated carbamic acid in solution. The carbonated species formed by AMP in several non-aqueous solutions have also been investigated previously. Barzagli et al. reported the formation of AMP carbamate in non-aqueous ethylene glycol monoethyl ether [9] and diethylene glycol monomethyl ether (DEGMME) [5], [9]. In mixtures of AMP with ethylene glycol (EG) + ethanol [12], EG + propanol and EG + DEGMME [8] only small amounts of AMP carbamate were detected, suggesting that the formation of carbonates by the reaction between CO₂ and the organic solvents was promoted. Barzagli et al. [5] also reported on the formation of the AMP carbonate in aqueous solutions of AMP. “