In Situ Nuclear Magnetic Resonance Mechanistic Studies of Carbon Dioxide Reactions with Liquid Amines in Mixed Base Systems: The Interplay of Lewis and Brønsted Basicities

A new approach to non-aqueous CO2–amine carbon capture has been elucidated on the basis of the utilization of a combination of a nucleophilic amine CO2 sorbent (Lewis base) with a second, non-nucleophilic Brønsted base, a “mixed base” system. The nucleophilic amines, typically alkanolamines, e.g., ethanolamine, react directly with CO2 in the gas stream, while the typically stronger nitrogenous Brønsted non-nucleophilic proton-acceptor base, e.g., a guanidine, then forms a more stabilized mixed carbamate reaction product. The proper choice of these bases allows for tailoring absorbent structure and properties and reaction conditions (T and P) to specific applications. Significant increases in absorption capacity are achieved because CO2 capture ratios greater than 1:1 on a molar basis (CO2 per amine group) can be obtained, resulting also in enhanced cyclic regeneration efficiency. In non-aqueous solutions, primary amines are carboxylated by reaction with one or two CO2 molecules, forming either mono- or di-N-carboxylated products. These carbamic acids, unstable in aqueous media, are then stabilized as guanidinium carboxylates. A total of 2 mol of CO2 per mol of a primary alkanolamine is thereby captured. In addition, under non-aqueous conditions, the hydroxyl group of alkanolamine reacts with CO2 (O-carbonation) to form an alkylcarbonic acid that is subsequently stabilized by forming the corresponding alkylbicarbonate salt on reaction with a guanidinine. Each hydroxyl group thereby also absorbs up to 1 mol of CO2. Thereby, enhanced capacity is achieved at both basic N and OH sites of monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), etc. These products may be decomposed by thermal treatment or CO2 partial pressure decrease to liberate CO2 and regenerate the liquid sorbent suitable for reuse in carbon capture operations.