Unusual temperature-promoted carbon dioxide capture in deep-eutectic solvents: the synergistic interactions

Reversible fixation–release of carbon dioxide (CO2) has attracted much attention during the last few decades due to an excessive increase in the consumption of fossil fuels as the main source of energy, thus causing an upsurge in CO2 emissions, leading to global warming. The current state-of-the-art industrial technology uses amine-based solvents in reversible CO2 capture. 1 This methodology has critical issues such as high regeneration energy, amine instability, high volatility, corrosion and high maintenance costs. In order to optimize the CO2 uptake, the flue gas is cooled to room temperature before absorption, whereas high temperatures are required in the desorption–regeneration step. Both these steps increase the cost of the setup and hence developing a solvent system that can absorb CO2 directly from hot flue gas and undergo rapid desorption is the most attractive solution but have not been introduced until now. The last century witnessed a surge in the development of a number of ‘‘green and sustainable’’ alternatives to volatile solvents. Among them, ionic liquids (ILs) and deep-eutectic solvents (DESs) have been the most explored ones for various applications because of their advantageous properties such as their negligible vapour pressure, wide liquidus range, low flammability, high thermal stability and recyclability. Blanchard et al. reported for the first time the potential of ILs in CO2 capture. 6 Later, various ILs differing in terms of cations, anions and alkyl chain length were applied to optimize CO2 uptake and to gain understanding about the functioning of these neoteric media. The basicity of the anion was identified as the key factor in achieving high CO2 uptake and has been most explored to date. For example, in carbonyl-containing anionfunctionalized ILs, high CO2 intake was favoured by H-bonding interactions. Pan et al. reported an equimolar CO2 uptake in anion-functionalized hydroxypyridine-based ILs owing to intramolecular proton transfer. However, the high molar uptake in ILs is attributed to their large molar mass and thus is less impressive when converted to the gravimetric values. Similarly, task-specific ILs for CO2 capture are expensive and require a long time for synthesis and are therefore rendered unsuitable for industrial applications. In contrast, DESs, a new generation of solvent analogous to ILs, provide a more promising alternative in CO2 capture and involve cost-effective, easy preparation, exhibiting properties similar to ILs, and resulting in higher gravimetric CO2 uptake. 19,20

Reversible fixation-release of carbon dioxide (CO 2 ) has attracted much attention during the last few decades due to an excessive increase in the consumption of fossil fuels as the main source of energy, thus causing an upsurge in CO 2 emissions, leading to global warming. The current state-of-the-art industrial technology uses amine-based solvents in reversible CO 2 capture. 1 This methodology has critical issues such as high regeneration energy, amine instability, high volatility, corrosion and high maintenance costs. 2,3 In order to optimize the CO 2 uptake, the flue gas is cooled to room temperature before absorption, whereas high temperatures are required in the desorption-regeneration step. Both these steps increase the cost of the setup and hence developing a solvent system that can absorb CO 2 directly from hot flue gas and undergo rapid desorption is the most attractive solution but have not been introduced until now.
The last century witnessed a surge in the development of a number of ''green and sustainable'' alternatives to volatile solvents. Among them, ionic liquids (ILs) and deep-eutectic solvents (DESs) have been the most explored ones for various applications because of their advantageous properties such as their negligible vapour pressure, wide liquidus range, low flammability, high thermal stability and recyclability. 4,5 Blanchard et al. reported for the first time the potential of ILs in CO 2 capture. 6 Later, various ILs differing in terms of cations, anions and alkyl chain length were applied to optimize CO 2 uptake and to gain understanding about the functioning of these neoteric media. [7][8][9][10] The basicity of the anion was identified as the key factor in achieving high CO 2 uptake and has been most explored to date. [11][12][13][14] For example, in carbonyl-containing anionfunctionalized ILs, high CO 2 intake was favoured by H-bonding interactions. 15 Pan et al. reported an equimolar CO 2 uptake in anion-functionalized hydroxypyridine-based ILs owing to intramolecular proton transfer. 16 However, the high molar uptake in ILs is attributed to their large molar mass and thus is less impressive when converted to the gravimetric values. 10,17 Similarly, task-specific ILs for CO 2 capture are expensive and require a long time for synthesis and are therefore rendered unsuitable for industrial applications. 18 In contrast, DESs, a new generation of solvent analogous to ILs, provide a more promising alternative in CO 2 capture and involve cost-effective, easy preparation, exhibiting properties similar to ILs, and resulting in higher gravimetric CO 2 uptake. 19,20 In a short time-span, DESs have found application in organic synthesis, polymerization, stabilization of biomolecules, nanotechnology, separation, extraction of target compounds and so on. [21][22][23][24][25][26] The performance of DESs as sorbents in CO 2 capture relies on pairing an efficient hydrogen bond donor (HBD) with hydrogen bond acceptor (HBA) with an optimal molar ratio. [27][28][29] Later developments have shown moderate to high CO 2 uptakes for different classes of DESs. [30][31][32][33] We recently presented an in-depth discussion about the impact of intermolecular interactions on the CO 2 uptake in ethylenediamine ([EDA])-, 3-amino-1-propanol ([AP])-, and polyamine-based DESs. 33 Unlike ILs and amine-based solvent systems where basicity is a prerequisite for high CO 2 capture, the interplay between the hydrogen bond donor acidity (a) and hydrogen bond acceptor basicity (b) governs the CO 2 absorption in DESs. 33 Additionally, the reaction conditions also influence the CO 2 solubility and maximum uptake takes place at low temperatures and high pressures. 34 In line with the current technology, flue gas passes through a cooler before traveling to the absorption tank. 35 Therefore, employing a medium that has a low enthalpy of absorption (DH abs ) and can reversibly absorb CO 2 in the post-combustion process, at typical flue gas temperatures, will eliminate the necessity of a cooler unit and extra steam (for desorption), thus bringing down the cost of the overall setup and making the whole process more lucrative.
In general, an increasing temperature results in a lower viscosity but increases the activation energy of the gas molecule at the same time. The resultant of these opposing effects directs the course of CO 2 capture, at higher temperatures. Therefore, the temperature-favoured CO 2 uptake in the [EDA]-based DESs cannot be attributed to the reduction in viscosity only.

Additionally, [MEAÁCl][EDA] and [HMIMÁCl]
[EDA] possess low viscosity, which further decreases upon increasing the HBA : HBD ratio from 1 : 1 to 1 : 4. In the latter case, the viscosity is already low so the influence of temperature is hard to detect. To discern the impact of viscosity on CO 2 uptake more clearly, we calculated the activation energy of viscous flow (E a,Z ) from the temperaturedependent viscosity data using the Arrhenius-type equation (ESI, † Fig. S7) as shown in Table 1. E a,Z accounts for the hindrance in the diffusion of CO 2 by the DES. At HBA : HBD = 1 : 1, the [EDA]-based DESs possess lower E a,Z than the [AP]-class of DESs but, at higher HBA : HBD ratios, the trend in E a,Z reverses ( Table 1)    between 43.2 and 60.6 kJ mol À1 despite the positive effect of temperature on CO 2 capture (ESI, † To account for the impact of intermolecular interactions on the course of CO 2 capture, at elevated temperatures, the thermodynamics (standard enthalpy change (DH1) and standard entropy change (DS1)) of capture along with the solvent polarity parameters (E T (30), a and b) are discussed further to unravel the underlying mechanisms 33,37 (see the ESI, † Fig. S8 and S9 and Table S2).
As shown in Table 1 Table S1). More importantly, a very small value of DH1 for the [EDA]-based DESs makes regeneration of the absorbent energysaving, which is a key factor in practical applications. 38 The nature of DES-CO 2 interactions was then investigated in terms of the Kamlet-Taft parameters a and b, which measure donor and acceptor strengths, respectively (ESI, † Table S2 and Fig. S10). The positive effect of CO 2 uptake indicates ''synergistic interaction'' of the donor and acceptor sites in the [EDA]-based DESs. The synergistic action is widely acknowledged in catalysis and polarity measurements of binary systems. 39,40 ''Hyperpolarity'' was observed in a binary mixture of ILs when the donor (a) and acceptor (b) strengths become similar and favour synergistic interaction. 41 In other words, for synergistic interaction, the relative difference between the donor and acceptor (a -b) should be zero. If ab a 0, the system will have acidic/basic characteristics depending on the relative differences in the values of a and b. This will cause an energy difference in the HBA and HBD in a DES and lower the probability for synergistic interactions (Scheme 1). The synergy in the DES results in weak interactions, which is reflected in the low polarity (E T (30)) and DH1 and DS1 values.
As shown in Table 1 Table 1). The large ab gives rise to differences in the ground state energy of the donor and acceptor and forbids synergistic interactions (Scheme 1). Thus, the greater CO 2 uptake in the [AP]-class of DESs, at higher HBA : HBD ratios, is due to their reduced viscosity.
The In summary, we, for the first time, have reported temperaturepromoted CO 2 capture in novel [EDA]-based DESs. The positive effect of temperature on CO 2 capture arises from the synergistic interactions between the donor and acceptor moieties. The synergistic interactions in DESs are caused by small ab and E T (30) and positive DH1 and DS1. Lower CO 2 uptake in the [AP]-class of DESs, at elevated temperatures, is due to the large ab and negative DH1 and DS1. Very low DH1 indicates quick regeneration of the [EDA]-based DESs and hence these DESs evolve as excellent substitutes for the amine-based technology in a CO 2 capture plant.
We are thankful to the Wallenberg Wood Science Center (WWSC), Kempe Foundations, and the Bio4Energy programme. This work is also part of the activities of the Johan Gadolin Process Chemistry Centre at Åbo Akademi University.

Conflicts of interest
There are no conflicts to declare.