Thermal Behavior of Alkanolamine-Based Deep Eutectic Solvents: Towards Carbon Dioxide Capture Applications

In this study, the specific heat capacities, heat of absorption, extent of chemical absorption of CO2 and thermal stability of amine based deep eutectic solvents (DESs) namely, choline chloride-monoethanolamine (ChCl-MEA), choline chloride-diethanolamine (ChCl-DEA), and choline chloride methyl-diethanolamine (ChCl-MDEA) were determined experimentally and compared with aqueous amine solution and solid adsorbents. The estimation of these quantities would aid in the determination of the regeneration energy and thermal stability requirements of the amine based DESs. This study is significant because the conventional aqueous amines are known to exhibit a prohibitive regeneration energy, and the instability of the solvents under CO2 absorption conditions has been widely reported. It is notable to point out that the specific heat capacity of the amine based deep eutectic solvents are all lower than that of the aqueous amine solution of MEA 30% weight. Moreover, when compared to solid adsorbents, e.g. polyethylenimine (PEI) impregnated mesoporous precipitated silica, amine based DESs exhibited a good level of closeness in the specific heat capacity and sometimes having less specific heat as in the case of ChCl-MDEA. Furthermore, HPLC analysis of the DESs before and after CO2 absorption showed that only part of the amine in the DES reacted with CO2. This is an indication that both chemical and physical absorption of CO2 existed. This result is of great importance since physical absorption of the carbon dioxide will significantly reduce the regeneration energy requirements for the capture process. The Thermo-Gravimetric Analysis (TGA) results showed that amine based DESs are more thermally stable compared to the corresponding aqueous solution of amines. The degradation temperature of amine based DESs is higher than that of stand-alone amines. For instance, the degradation temperature of ChCl-MEA 1:6 is around 140 °C while that aqueous solution of MEA is 65 °C. This result is of great importance in the CO2 capture using aqueous amine solution, especially in the regeneration step where the temperature is about 120-140 °C.