Efficient and Reversible Absorption of CO₂ by Functional Deep Eutectic Solvents

Extremely low-volatility functional deep eutectic solvents (DESs), based on ethylene glycol (EG) and diethylene glycol (DG) as hydrogen-bond donor and the ammonium salts obtained from triethylenetetramine (TETA) and HCl at different mole ratios as hydrogen-bond acceptor, were designed and used to capture CO2. All of the designed DESs can efficiently capture CO2 even at low partial pressures. CO2 absorption capacity of [TETA]Cl-EG DES with n[TETA]Cl/nEG 1:3 is high up to 17.5 wt % (1.456 mol CO2/mol [TETA]Cl) at 40 °C and 1 atm. CO2 absorption capacity decreases with increasing temperature and decreasing CO2 partial pressure. Regeneration experiments show that CO2 absorption capacities in [TETA]Cl-EG DES and [TETA]Cl-DG DES do not vary after five absorption/desorption cycles. It is found that EG or DG can increase the absorption capacity via activating −NH– or −NH2 on [TETA]Cl and enhance the basicity of DESs. In addition, CO2 absorption mechanism in [TETA]Cl-EG DES based on the change of its viscosity during absorption and FTIR analysis indicates that there is a chemical interaction between CO2 and [TETA]Cl, and the stoichiometry for the reversible absorption is 1.5 molecules of CO2 per [TETA]Cl-EG DES molecule.