Quantitative Equilibrium Constants between CO2and Lewis Bases from FTIR Spectroscopy

Abstract

Equilibrium constants measured from the ν 2 bending mode of CO 2 by FTIR spectroscopy are reported for the electron donor−acceptor interactions of CO 2 with three Lewis bases: triethylamine (TEA), pyridine (PYR), and tributyl phosphate (TBP). The average K c values are 0.046 (CO 2 −TEA), 0.133 (CO 2 −PYR), and 1.29 (CO 2 −TBP) L/mol at 25 °C in the solvent pentane. For the CO 2 −TBP system, the average enthalpy of association, Δ H °, is −4.7 kcal/mol. Ab initio calculations indicate that steric repulsion of the ethyl groups in TEA cause the binding energy of the CO 2 −TEA complex to be weaker than that of the CO 2 −PYR complex by 1.34 kcal/mol, a trend that is in agreement with our spectroscopic data. The lattice fluid hydrogen bonding model was used in conjunction with the spectroscopically determined K c values to predict bubble points for the CO 2 −TEA and CO 2 −TBP systems and CO 2 sorption in a hypothetical polymer. These calculations indicate that these relatively weak specific interactions have a measurable effect on phase behavior and can influence sorption of CO 2 in polymers.