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Table 3. Summary of energies of optimized structures. Table 4. Optimized energies of HBA, HBD, and natural deep eutectic solvents (NADES) interactins with CO, at various spatial positions.

Table 3 Summary of energies of optimized structures. Table 4. Optimized energies of HBA, HBD, and natural deep eutectic solvents (NADES) interactins with CO, at various spatial positions.

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Table 1. Structures of hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD) compounds. 2.1. Experimental CO) sorption performance of NADES samples were reported in a previous study, in which the contactless magnetic suspension gravimetric method was used for high-pressure gas absorption-desorption measurements via Rubotherm apparatus. A magnetic suspension apparatus that is equipped with an automated syringe pump was used to conduct both low- and high-pressure gas experiments. In a typical procedure, a few ml of sample is placed in a measurement chamber and it is evacuated overnight at temperatures around 70 °C. Once the material preparation is complete, then solubility measurements take place in the apparatus, starting from the lower pressures toward higher pressures, with stepwise gradual pressure increments. It takes about 60 min to establish equilibrium thermal in between each pressure increment. Once the highest pressure is measured, then few pressure points are re-experimented during the desorption period, in order to observe whether hysteresis occurs during the reversed conditions. The described method was used to obtain HBA + CO, and HBD + CO, sorption performances in this work, and further details of the measurement technique, calibration, and overall uncertainty can be obtained elsewhere [53].
Table 1. Structures of hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD) compounds. 2.1. Experimental CO) sorption performance of NADES samples were reported in a previous study, in which the contactless magnetic suspension gravimetric method was used for high-pressure gas absorption-desorption measurements via Rubotherm apparatus. A magnetic suspension apparatus that is equipped with an automated syringe pump was used to conduct both low- and high-pressure gas experiments. In a typical procedure, a few ml of sample is placed in a measurement chamber and it is evacuated overnight at temperatures around 70 °C. Once the material preparation is complete, then solubility measurements take place in the apparatus, starting from the lower pressures toward higher pressures, with stepwise gradual pressure increments. It takes about 60 min to establish equilibrium thermal in between each pressure increment. Once the highest pressure is measured, then few pressure points are re-experimented during the desorption period, in order to observe whether hysteresis occurs during the reversed conditions. The described method was used to obtain HBA + CO, and HBD + CO, sorption performances in this work, and further details of the measurement technique, calibration, and overall uncertainty can be obtained elsewhere [53].
Figure 1. CO) capture performance for hydrogen bond acceptors and hydrogen bond donors prior to mixing to form deep eutectic solvent at 298.15 K isotherm. form the deep eutectic solvent at 298.15 K isotherm. At a low-pressure side (p < 5bars), there is no distinct effect observed on the sorption performance and the data are clustered around ~0.3 mmol CO>,/g sorbent. As the pressure is increased to moderate to high pressures, there is a clear separation between ChCl + CO2, Be + CO? (~4.95 mmol CO2/g) cases in comparison to Al + CO2, La + COo, Ma + COz, Ca + CO? (~3.77 mmol CO>/g) cases. However, Fr + CO? falls in between of the trend of these two groups at high pressures yielding 4.45 mmol CO /g performance. Furt hermore, absorption capacity of DES is provided in electronic supporting information for CO) loading amounts in mol/mol units. Figure 51 reveals significant trends on the effect of hydroxyl groups. As This is re and the CO; molecule. For La, Ma, Fr, Ca there are 4, 3, 2, 1 hydroxyl groups in the HBD structure, and the al the amoun ated bsor ption ca of hydroxyl groups in the HBD increases, the absorption of capacity of the CO) increase. to the intramolecular and intermolecular hydrogen-bonding forces between the HBA pacity in Figure S1 overlaps with this argument. On the other hand, for the case of HBA, as the volume of the of the group that is attached on the N atom in the HBA structure increases, ita ffects the d istance between center of mass and the CO2 molecule and, thus, develops a hydrogen bond easier. This leads to the hydrogen-bonding force of DES and improved solvent properties. This phenomenon is also observed in this work when the molecular structures of ChCl, Be, and Al are considered, and t heir CO? sorption capacities are examined in Figure $1. The development of hydrogen bonding is discussed in further detail, from the molecular point of view, in the following section of this manuscript.
Figure 1. CO) capture performance for hydrogen bond acceptors and hydrogen bond donors prior to mixing to form deep eutectic solvent at 298.15 K isotherm. form the deep eutectic solvent at 298.15 K isotherm. At a low-pressure side (p < 5bars), there is no distinct effect observed on the sorption performance and the data are clustered around ~0.3 mmol CO>,/g sorbent. As the pressure is increased to moderate to high pressures, there is a clear separation between ChCl + CO2, Be + CO? (~4.95 mmol CO2/g) cases in comparison to Al + CO2, La + COo, Ma + COz, Ca + CO? (~3.77 mmol CO>/g) cases. However, Fr + CO? falls in between of the trend of these two groups at high pressures yielding 4.45 mmol CO /g performance. Furt hermore, absorption capacity of DES is provided in electronic supporting information for CO) loading amounts in mol/mol units. Figure 51 reveals significant trends on the effect of hydroxyl groups. As This is re and the CO; molecule. For La, Ma, Fr, Ca there are 4, 3, 2, 1 hydroxyl groups in the HBD structure, and the al the amoun ated bsor ption ca of hydroxyl groups in the HBD increases, the absorption of capacity of the CO) increase. to the intramolecular and intermolecular hydrogen-bonding forces between the HBA pacity in Figure S1 overlaps with this argument. On the other hand, for the case of HBA, as the volume of the of the group that is attached on the N atom in the HBA structure increases, ita ffects the d istance between center of mass and the CO2 molecule and, thus, develops a hydrogen bond easier. This leads to the hydrogen-bonding force of DES and improved solvent properties. This phenomenon is also observed in this work when the molecular structures of ChCl, Be, and Al are considered, and t heir CO? sorption capacities are examined in Figure $1. The development of hydrogen bonding is discussed in further detail, from the molecular point of view, in the following section of this manuscript.
Figure 2. CO2 capture performance of studied deep eutectic solvents (DES) systems and comparison to their constituents. (a) ChCl:La + COz, (b) Be:La + COo, (c) Al:La + CO, (d) Al:Ma + CO>. in comparison to La + CO» case (Figure 2c). For A:MA + COy case, it has a similar trend with ChCl:La + COz and the NADES + CO, trend overlaps again with the HBA trend (Figure 2d), with the exception of a slight departure of NADES towards higher performance at pressures higher than 35 bars However, for the case of Al:La + CO3, there is a distinct segregation of HBA+CO 7, HBD+COy) and NADES+CO) trends (Figure 2c). In this specific case, pressures above 10 bars, CO capture performance trend was observed as La + CO? > NADES + CQ > Al + COz, or in other words HBD + CO2 > NADES + CO > HBA + CO,. Maximum solubility performances were obtained via the highest achieved experimental pressure at 50 bars, reported in Table 2. For all cases, except for Be:La + CO> (Figure 2b), HBA experimental sorption performances was superior than that of HBD, which was also mentioned by D.O. Abranchesn et al. (that Be possess weak interaction with itself, but act as excellent HBA) [66] ChCl+CO, showed the best performance with 4.96 mmol CO /g, whereas Al + CO, showed the worst capture performance with 3.86 mmol CO>/g. In the case of Be:La + CO2, the NADES + CO, profile was observed to be lower than its constituents (Be and La), which can be explained due to the negative excess volume that was created via mixing the HBA and HBD [67]. Likewise, in the case of Al:Ma + CO», the NADES profile was observed to be higher than its HBA and HBD, which is a sign of positive excess volume when Al and Ma was mixed to form NADES.
Figure 2. CO2 capture performance of studied deep eutectic solvents (DES) systems and comparison to their constituents. (a) ChCl:La + COz, (b) Be:La + COo, (c) Al:La + CO, (d) Al:Ma + CO>. in comparison to La + CO» case (Figure 2c). For A:MA + COy case, it has a similar trend with ChCl:La + COz and the NADES + CO, trend overlaps again with the HBA trend (Figure 2d), with the exception of a slight departure of NADES towards higher performance at pressures higher than 35 bars However, for the case of Al:La + CO3, there is a distinct segregation of HBA+CO 7, HBD+COy) and NADES+CO) trends (Figure 2c). In this specific case, pressures above 10 bars, CO capture performance trend was observed as La + CO? > NADES + CQ > Al + COz, or in other words HBD + CO2 > NADES + CO > HBA + CO,. Maximum solubility performances were obtained via the highest achieved experimental pressure at 50 bars, reported in Table 2. For all cases, except for Be:La + CO> (Figure 2b), HBA experimental sorption performances was superior than that of HBD, which was also mentioned by D.O. Abranchesn et al. (that Be possess weak interaction with itself, but act as excellent HBA) [66] ChCl+CO, showed the best performance with 4.96 mmol CO /g, whereas Al + CO, showed the worst capture performance with 3.86 mmol CO>/g. In the case of Be:La + CO2, the NADES + CO, profile was observed to be lower than its constituents (Be and La), which can be explained due to the negative excess volume that was created via mixing the HBA and HBD [67]. Likewise, in the case of Al:Ma + CO», the NADES profile was observed to be higher than its HBA and HBD, which is a sign of positive excess volume when Al and Ma was mixed to form NADES.
Table 2. Gas sorption data and binding energies ranking. As clearly reported in Table 1, the CO capture performance of NADES is lower in comparison to ole HBA and HBD cases, for most of the studied cases. When the melting point of the studied HBA and IBD are considered, they are solid in room temperatures, and even at elevated process temperatures, it which typical pre- and post-combustion CO capture operations take place. However, the NADES compounds exist in liquid phase at mentioned temperatures, thus, making their processability much asier than their constituents. One of the main objectives of considering solvents for CO? capture is to be ble to utilize existing infrastructures that were built with the consideration of current-state-of-the-art apture agents (e.g., monoethanolamine-based solvents), with minimum retrofitting requirements on he equipment infrastructure.
Table 2. Gas sorption data and binding energies ranking. As clearly reported in Table 1, the CO capture performance of NADES is lower in comparison to ole HBA and HBD cases, for most of the studied cases. When the melting point of the studied HBA and IBD are considered, they are solid in room temperatures, and even at elevated process temperatures, it which typical pre- and post-combustion CO capture operations take place. However, the NADES compounds exist in liquid phase at mentioned temperatures, thus, making their processability much asier than their constituents. One of the main objectives of considering solvents for CO? capture is to be ble to utilize existing infrastructures that were built with the consideration of current-state-of-the-art apture agents (e.g., monoethanolamine-based solvents), with minimum retrofitting requirements on he equipment infrastructure.
Table 4. Cont. Once the binding energy results are deeply analyzed, HBA > HBD trend is observed for CO? interactions, which is similar to the experimental behavior CO, sorption performance in HBA/HBD. The only exception was observed in Be + CO and La + CO) comparisons, for which the binding energy is higher for HBD + CO, than HBA + CO) case. There is no particular correlation between the maximum sorption amount and the binding energy for the NADES + CO, cases. On the other hand, the maximum sorption amounts and the binding energies for both HBD and HBA follows the same rank. In other words, it was observed that as the binding energy for CO) increases, the CO. sorption performance also increases for both HBD and HBA cases.
Table 4. Cont. Once the binding energy results are deeply analyzed, HBA > HBD trend is observed for CO? interactions, which is similar to the experimental behavior CO, sorption performance in HBA/HBD. The only exception was observed in Be + CO and La + CO) comparisons, for which the binding energy is higher for HBD + CO, than HBA + CO) case. There is no particular correlation between the maximum sorption amount and the binding energy for the NADES + CO, cases. On the other hand, the maximum sorption amounts and the binding energies for both HBD and HBA follows the same rank. In other words, it was observed that as the binding energy for CO) increases, the CO. sorption performance also increases for both HBD and HBA cases.
Quantum theory atom-in-a-molecule (QTAIM analysis and RDG isosurfaces for NADES was presented for NADES + COz cases in Figures 4 and 5, respectively. For most of the cases, CO) established critical binding with HBA of the NADES structure, except for the ChCl:Fr + CO, case. In Figure 4, it is evident that -O site of CO, is the most recurring interaction between the HBA -H site. The experimental values for the CO? solubility in NADES can be ranked as ChCl:La > Al:La ~ ChCl:Ma > Be:La > Al:La. Whereas the binding energies that are calculated via DFT simulations for CO) are ranked as ChCl:Fr > ChCl:La ~ ChCl:Ma > Be:La > Al:La. These two comparisons do not overlap with each other. A more logical comparison on overall solubility performance, with respect to bind prior to the mixing, since they form excess volume capture performance. ing energies, would be between the NADES constituent pockets once they are mixed, which leads to excess CO
Quantum theory atom-in-a-molecule (QTAIM analysis and RDG isosurfaces for NADES was presented for NADES + COz cases in Figures 4 and 5, respectively. For most of the cases, CO) established critical binding with HBA of the NADES structure, except for the ChCl:Fr + CO, case. In Figure 4, it is evident that -O site of CO, is the most recurring interaction between the HBA -H site. The experimental values for the CO? solubility in NADES can be ranked as ChCl:La > Al:La ~ ChCl:Ma > Be:La > Al:La. Whereas the binding energies that are calculated via DFT simulations for CO) are ranked as ChCl:Fr > ChCl:La ~ ChCl:Ma > Be:La > Al:La. These two comparisons do not overlap with each other. A more logical comparison on overall solubility performance, with respect to bind prior to the mixing, since they form excess volume capture performance. ing energies, would be between the NADES constituent pockets once they are mixed, which leads to excess CO
Figure 4. Density functional theory (DFT) figures for each structure final optimized geometry prior to the mixing, since they form excess volume pockets once they are mixed, which leads to excess CO2
Figure 4. Density functional theory (DFT) figures for each structure final optimized geometry prior to the mixing, since they form excess volume pockets once they are mixed, which leads to excess CO2
Figure 5. Reduced density gradient (RDG) isosurfaces for HBA + CO and HBD + COz cases
Figure 5. Reduced density gradient (RDG) isosurfaces for HBA + CO and HBD + COz cases
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