Reaction kinetics of CO2 absorption in to phosphonium based anion-functionalized ionic liquids

Abstract

The reaction kinetics between CO₂ and trihexyl(tetradecyl)phosphonium ([P₆₆₆₁₄]) – based ionic liquids (ILs) with prolinate ([Pro]), 2-cyanopyrrolide ([2-CNpyr]), and 3-(trifluoromethyl)pyrazolide ([3-CF₃pyra]) anions are studied at temperatures from 22–60 °C. The absorption of CO₂ is carried out in a stirred reactor under pseudo first order conditions. ILs are diluted to concentrations of 0.05, 0.1 and 0.15 M with tetraglyme – a nonreactive, low volatility solvent with much lower viscosity than the ILs. Physical solubility of CO₂ in the mixtures is calculated using correlations developed from CO₂ solubility measurements in tetraglyme and the N₂O-analogy for ILs and dilute IL solutions. The diffusivity of CO₂ is estimated from viscosity-dependent correlations chosen after a thorough literature review. The results indicate partial first order reaction kinetics with respect to IL with values ranging from 19 500 L mol⁻¹ s⁻¹ ([P₆₆₆₁₄][Pro]) to 3200 L mol⁻¹ s⁻¹ ([P₆₆₆₁₄][3-CF₃pyra]) at 22 °C. The second order reaction rate constants follow Arrhenius behavior with the highest activation energy of 43 kJ mol⁻¹ measured for [P₆₆₆₁₄][Pro]. ILs with aprotic heterocylic anions (AHA), on the other hand, show small activation energies of 18 and 11 kJ mol⁻¹ for [P₆₆₆₁₄][3-CF₃pyra] and [P₆₆₆₁₄][2-CNpyr], respectively. The ILs studied in this work exhibit reactivity comparable to or higher than common aqueous amines. High reaction rates and tunable capacity make ILs, and AHA ILs in particular, attractive solvents for CO₂ separations.