Nowadays, many researchers are focused on finding a solution to the problem of global warming. Carbon dioxide is considered to be responsible for the “greenhouse” effect. The largest global emission of industrial CO₂ comes from fossil fuel combustion, which makes power plants the perfect point source targets for immediate CO₂ emission reductions. A state-of-the-art method for capturing carbon dioxide is chemical absorption using an aqueous solution of alkanolamines, most frequently a 30% wt. solution of monoethanolamine (MEA). Unfortunately, the usage of alkanolamines has a number of drawbacks, such as the corrosive nature of the reaction environment, the loss of the solvent due to its volatility, and a high energy demand at the regeneration step. These problems have driven the search for alternatives to that method, and deep eutectic solvents (DESs) might be a very good substitute. Many types of DESs have thus far been investigated for efficient CO₂ capture, and various hydrogen bond donors and acceptors have been used. Deep eutectic solvents that are capable of absorbing carbon dioxide physically and chemically have been reported. Strategies for further CO₂ absorption improvement, such as the addition of water, other co-solvents, or metal salts, have been proposed. Within this review, the physical properties of DESs are presented, and their effects on CO₂ absorption capacity are discussed in conjunction with the types of HBAs and HBDs and their molar ratios. The practical issues of using DESs for CO₂ separation are also described.

This paper demonstrates the assessment of physicochemical and thermodynamic properties of aqueous solutions of novel deep eutectic solvent (DES) built of tetrabutylammonium chloride and 3-amino-1-propanol or tetrabutylammonium bromide and 3-amino-1-propanol or 2-(methylamino)ethanol or 2-(butylamino)ethanol. Densities, speeds of sound, refractive indices, and viscosities for both pure and aqueous mixtures of DES were investigated over the entire range of compositions at atmospheric pressure and T = (293.15 ‒ 313.15) K. It was concluded that the experimental data were successfully fitted using the Jouyban–Acree model with respect to the concentration. Obtained results showed that this mathematical equation is an accurate correlation for the prediction of aqueous DES properties. Key physicochemical properties of the mixtures—such as excess molar volumes, excess isentropic compressibilities, deviations in viscosity, and deviations in refractive indices—were calculated and correlated by the Redlich–Kister equation with temperature-dependent parameters. The non-ideal behavior of the studied systems were also evaluated by using the Prigogine−Flory−Patterson theory and the results were interpreted in terms of interactions between the mixture components.

Deep eutectic solvents (DESs) became an object of a great interest as an alternative to ionic liquids (ILs) and commonly used in CO₂ capture amine solutions. In the present study, five different DESs based on 3-amino-1-propanol as physical-chemical CO₂ absorbents were used. The composition was chosen in order to estimate the effects of hydrogen bond acceptor:hydrogen bond donor (HBA:HBD) molar ratio, anion type and length of alkyl chain of composing salt. The Fourier Transform Infrared (FTIR) spectroscopy was used to confirm chemical reaction. The solubility of CO₂ was measured at low pressures up to 170 kPa at the temperature range of 293–318 K. Viscosity, polarity and Kamlet–Taft parameters were determined in order to estimate the dependences of the parameters and the CO₂ capacity. CO₂ uptake was observed to improve with decreasing molar ratio of hydrogen bond donor. Comparing the CO₂ capacity of [TBAC]-based DESs, at the approximate pressure of 50 kPa, it was observed that the capacity increased in the following order of molar ratios—1:8 < 1:6 < 1:4 and a decrease in molar ratio from 1:8 to 1:4 resulted in about a 100% increase of capacity. Compared to [TBAC][AP] DESs, the [TEAC][AP] 1:4 and [TBAB][AP] 1:4 exhibited higher CO₂ uptake, though the best results were obtained for [TBAB][AP].

The aim of this research was to assess the impact of the components of alkanolamine deep eutectic solvents (DESs) on the physical properties of those DESs and their carbon dioxide capacity. To achieve this goal, novel deep eutectic solvents were synthesized by using 2-(butylamino)ethanol (BAE) as the hydrogen bond donor (HBD), along with tetrabutylammonium bromide TBAB), tetrabutylammonium chloride (TBAC), or tetraethylammonium chloride (TEAC) as the hydrogen bond acceptors (HBA) at various molar ratios (1:6, 1:8, and 1:10). To confirm the presence of hydrogen bond interactions between the components Fourier Transform Infrared Spectroscopy measurements were conducted. Furthermore, thermal properties, including melting points and thermal stability, of these deep eutectic solvents as well as key physical properties, such as density, viscosity, refractive index, and sound velocity, within the temperature range of 293.15–333.15 K and at a pressure of 0.1 MPa were examined. The effect of the molar ratio of HBA to HBD, the type of anion, and the length of the alkyl chain were studied and analysed in regard to physicochemical properties. In this work, the solubility of carbon dioxide in DESs derived from 2-(butylamino)ethanol, 3-aminopropan-1-ol (AP), and 2-(methylamino)ethanol (MAE) was measured. The highest CO₂ capacity was found for TEAC:MAE 1:10 DES characterized by the shortest alkyl chain length in both HBA and HBD molecules, the highest amine content, and the lowest viscosity. Additionally, the effect of water addition on carbon dioxide solubility was explored. The results showed that the influence of water on CO₂ solubility varies with the type of DES. In general, this work highlighted that DESs can serve as effective media for carbon dioxide capture, and their performance can be tailored by changing the type of hydrogen bond acceptor or donor, their molar ratio and by the addition of water.