Investigations of primary and secondary amine carbamate stability by ¹H NMR spectroscopy for post combustion capture of carbon dioxide

Carbamate formation is one of the major chemical reactions that can occur in solution in the capture of CO₂ by amine-based solvents, and carbamate formation makes a significant enthalpy contribution to the absorption-desorption of CO₂ that occurs in the absorber/stripper columns of the PCC process. Consequently, the formation of carbamates of selected series of primary and secondary amines over the temperature range (288 to 318) K has been investigated by equilibrium ¹H NMR studies, and the stability constants (K₉) for the equilibrium: [formula could not be replicated] are reported. van’t Hoff analyses have resulted in standard molar enthalpies, ΔH^o_m, and entropies, ΔS^o_m of carbamate formation. A ΔH^o_m - ΔS^o_m plot generates a linear correlation for carbamate formation (providing a mean standard molar free energy, ΔG^o_m for carbamate formation of about -7kJ · mol^-1), and this relationship helps provide a guide to the selection of an amine(s) solvent for CO₂ capture, in terms of enthalpy considerations. A linear ΔH^o_m - ΔS^o_m plot also occurs for carbamate protonation. The formation of the carbamates has been correlated with systematic changes in composition and structure, and steric effects have been identified by comparing molecular geometries obtained using density functional B3LYP/6-311++G(d,p) calculations. Trends in steric effects have been identified in the series of compounds monoethanolamine (MEA), 1-amino-2-propanol, 2-amino-1-propanol (AP) and 2-amino-2-methyl-1-propanol (AMP). In the case of 2-piperidinemethanol, 2-piperidineethanol and 3-piperidinemethanol, strong intramolecular hydrogen bonding is shown to be the likely cause for lack of carbamate formation, and in the ring systems of pyrrolidine, morpholine, piperidine and thiomorpho- line trends in carbamate formation (as given by K₉) have been correlated with the internal ring angle at the amine nitrogen, as well as the planarity of the environment around the nitrogen atom