Role of Hydrogen-Bond Interactions in CO2 Capture by Wet Phosphonium Formate Ionic Liquid: A Raman Spectroscopic Study.

The mechanism of CO2 absorption by a formate ionic liquid, [P4444 ]HCOO, was studied by Raman spectroscopy. The band area for the symmetric CO2 stretching of the formate anion linearly decreases with the CO2 loading. From the slope of the decrease, 1 : 1 stoichiometry is proven between CO2 and the formate anion. The result favors the mechanism we proposed in a preceding work [J. Chem. Eng. Data 61, 837 (2016)]: HCOO- +CO2 +H2 O→HCOOH+HCO3- →[HCOOH…HCO3- ]. Further support for the mechanism is obtained by the observation of antisymmetric vibration of CO for the proposed hydrogen-bonded complex between HCOOH and HCO3- . The bands appeared as a doublet (1677 and 1730 cm-1 ) as this complex has two carbonyl groups. Based on DFT calculations, the [HCOOH…HCO3- ] complex is supposed to be the most abundant form of chemisorbed CO2 .