SALT HYDRATES - NEW REVERSIBLE ABSORBENTS FOR CARBON-DIOXIDE

Melts of salt hydrates such as tetramethylammonium fluoride tetrahydrate, [(CH3)(4)N]F.4H(2)O, and tetraethylammonium acetate tetrahydrate, [(C2H5)(4)N]CH3CO2.4H(2)O, have been found to exhibit unexpectedly large carbon dioxide absorption capacities. For example, [(CH3)(4)N]F.4H(2)O at 50 degrees C and 100 kPa CO2 absorbs 0.28 mol of gas/mol of salt corresponding to a CO2 concentration of about 1.9 M. Absorption is fully reversible and gas is desorbed by reducing the CO2 pressure above the melt. A survey of salt hydrates revealed that those which contain relatively basic anions, such as malonate or citrate, likewise exhibit relatively large CO2 absorption capacities while those which contain relatively neutral anions, such as chloride, do not. Further, the CO2 absorption capacity of salt hydrates is dependent on the water content of the salt and decreases with increasing water content. Characterization by NMR, FTIR, and Raman spectroscopy of [(CH3)(4)N]F.4H(2)O Containing absorbed CO2 is consistent with the presence of bicarbonate and bifluoride ions in the melt. Exposure of [(CH3)(4)N]F containing (H2O)-O-18 to CO2 resulted in incorporation of the label into the gas phase as (COO)-O-18-O-16 and (CO2)-O-18, implying that CO2 reacts reversibly with the melt via a hydration reaction. Spectroscopic evidence suggests that carboxylate-containing salt hydrates also bind CO2 as bicarbonate but the nature of the protonated anion species remains undetermined. However, modeling of isotherm data suggests a one-to-one salt hydrate-to-CO2 reaction stoichiometry. In contrast, modeling of fluoride-containing salt hydrate isotherms supports a two-to-one salt hydrate-to-CO2 reaction stoichiometry, which is consistent with spectroscopic and reactivity studies of the underlying chemistry. The heats of absorption of CO2 by [(CH3)(4)N]F.4H(2)O and [(C2H5)(4)N]CH3CO2.4H(2)O are relatively low, -4.5 and -8.4 kcal/mol, respectively, suggesting that these salts may be attractive, more energy efficient, alternatives to commercial amine based absorbents for the removal of CO2 from process gas streams.