Absorption performance and reaction mechanism study on a novel anhydrous phase change absorbent for CO2 capture

Carbon capture and utilization technologies are urged more than ever due to the excessive emissions of CO2 that could lead to serious environmental problems. Phase change absorbents are quite competitive in reducing the energy consumption of carbon capture. In this work, tetramethylammonium glycinate ([N1111][Gly]), an efficient absorbent, was dissolved in ethanol and 1-propanol, respectively, to absorb CO2. After CO2 absorption, the saturated solution separated into two phases. CO2 was enriched in the lower phase, so that only the CO2-rich phase needed to be regenerated and the energy consumption of regeneration could be thus reduced. Under the optimal conditions, the CO2 loading of fresh absorbents could reach 0.85 mol CO2/mol IL. On the basis of the 13C NMR analysis and quantum chemical calculation, a three-step reaction mechanism of CO2 capture with [N1111][Gly]/ethanol was proposed. CO2 reacted with [N1111][Gly] to form carbamate, which was similar to the two-step Zwitterionic mechanism of MEA with CO2. Then the formed carbamate could react with ethanol to form ethyl carbonate and [N1111][Gly], which resulted in higher CO2 loading. Moreover, hydrogen bond characteristics were analysed to clarify the phase change mechanism. The number and bonding energy of intramolecular hydrogen bonds were increased after capturing CO2, leading to the formation of precipitation.