Effect of water activity on carbon dioxide transport in cholinium-based ionic liquids with carbonic anhydrase

• Cholinium-based ionic liquids are extremely selective towards carbon dioxide (CO2).
• The highest CO2 solubility was obtained with cholinium hexanoate.
• A CO2 diffusion coefficient correlation for the cholinium-based ionic liquids.
• CO2 transport enhancement is promoted by carbonic anhydrase enzyme.
• The highest CO2 permeability enhancement was observed in cholinium propionate.

The present work reports a study of the affinity of carbon dioxide towards biocompatible cholinium-based ionic liquids, aiming the development of a sustainable process for carbon dioxide removal. Moreover, the enhancement on carbon dioxide transport by the addition of carbonic anhydrase, able to catalyze the reversible reaction of carbon dioxide with water, is also evaluated.Cholinium acetate, cholinium propionate and cholinium hexanoate were the selected ionic liquids for this study. Carbon dioxide solubility and diffusion coefficients were determined in the three cholinium ionic liquids, at different water activities, in order to identify optimal conditions for the enzyme activity and stability. A carbon dioxide diffusion coefficient correlation for the cholinium-based ionic liquids with different water activities is proposed, allowing for estimation of carbon dioxide diffusion coefficients.The results obtained show that the selected cholinium-based ionic liquids possess high affinity towards carbon dioxide, when compared with most used room temperature ionic liquids. Also, it was demonstrated that a small amount of the carbonic anhydrase (0.1 mgCA/gIL) promotes an enhancement of 63% on the carbon dioxide transport rate, for cholinium propionate with a water activity of 0.753.