Effect of Temperature and Water Concentration on CO₂ Absorption by Tetrabutylphosphonium Formate Ionic Liquid

The CO₂ absorption capacities for binary solutions of tetrabutylphosphonium formate ionic liquid (IL) and water are measured at 0.1 MPa of CO₂ for various compositions (up to 10 in mole ratio of water to the IL) as a function of temperature (−24 to 60 °C). The capacities measured as the mole of CO₂ with respect to 1 mol of the IL varied in a wide range from 0.01 to 1.0. Capacities decreased monotonously with temperature at a fixed absorbent composition. When the water concentration is varied at a fixed temperature, capacity takes a maximum when the water mole ratio is approximately 1. In contrast to CO₂ absorbents made from acetate ILs, the present system loses affinity to CO₂ in the absence of water. The equilibrium constant for the chemisorption is defined in two ways by assuming that CO₂ is captured as a free bicarbonate ion (HCO₃^–) or a complex ion with formic acid ([(HCOO) (HCO₃)­H] ^–) . The van’t Hoff plot of these two equilibrium constants are both linear. However, the estimated enthalpy change of absorption is ca. 20 kJ/mol larger for the former scenario than the latter