CO2 absorption using two morpholine protic ionic liquids: measurement, model, and quantum chemical calculation

Two kinds of protic ionic liquids, N-methylmorpholinium formate ([NMMH][For]) and N-ethylmorpholinium formate ([NEMH][For]), were synthesized by the acid-base neutralization method. The solubility of carbon dioxide (CO₂) in the two ionic liquids was measured at temperatures of 298.15 to 338.15 K and pressures of up to 900 kPa. The solubility increases linearly with pressure, indicating that CO₂ is physically absorbed in ionic liquids. The solubility of CO₂ in [NEMH][For] is higher than that of [NMMH][For]. Absorption behavior was investigated by thermodynamic properties, such as the Henry's law constant, partial molar Gibbs free energy, partial molar enthalpy, and partial molar entropy. The Pitzer's model and the Soave-Redlich-Kwong (SRK) cubic equation of state were used to fit the solubility data, respectively. The Pitzer's model is found to have better prediction accuracy. The interaction of two protic ionic liquids with CO₂ was analyzed using quantum chemistry. The molecular mechanism of CO₂ absorption by two protic ionic liquids was explained from the microscopic point of view. CO₂ + [NEMH][For] has more hydrogen bonds and a higher interaction energy. The cation–anion interaction of [NEMH][For] diminishes in the presence of CO₂, resulting in an increase in the cation–anion distance. These factors result in higher solubility of CO₂ in [NEMH][For].