Carbonation and decarbonation of non-aqueous solutions with different compositions of ethylene glycol and various amidines

A mixture of equivalent amounts of amidine, alcohol, and carbon dioxide forms alklycarbonate amidinium salts. Here we studied the effect of the composition of solutions consisting of ethylene glycol (EG) as the alcohol and one of various amidines on their ability to absorb and desorb carbon dioxide. Compounds containing single or multiple formamidine groups were synthesized and dissolved in EG. The ability of these solutions to absorb carbon dioxide was studied for various ratios of EG to amidine and at various temperatures. An excess of at least ten equivalents of EG relative to the amidine was required for rapid and complete reaction of the base with carbon dioxide. Excess EG also reduced the viscosity of the solution, and in this way increased the absorption rate. However, the excess EG appeared to have stabilized the formed alkyl carbonate salts via hydrogen bonding, resulting in a relatively low degree of decarbonation of the solution at elevated temperatures. This effect would cause the working capacity to be decreased were this type of material be used for carbon dioxide capture applications. Our results suggest that, in general, the compositions of the alcoholic solutions of superbases need to be optimized to achieve high-efficiency absorption and desorption of carbon dioxide.