Computational Screening of Physical Solvents for CO<sub>2</sub> Pre-combustion Capture (2021)

A computational scheme was used to screen physical solvents for CO<sub>2</sub> pre-combustion capture by integrating the commercial NIST database, an in-house computational database, chem-informatics, and molecular modeling. A commercially available screened hydrophobic solvent, diethyl sebacate, was identified from the screening with favorable physical properties and promising absorption performance. The promising performance to use diethyl sebacate in CO<sub>2</sub> pre-combustion capture has also been confirmed from experiments. Water loading in diethyl sebacate is very low, and therefore, water is kept with H<sub>2</sub> in the gas stream. The favorable CO<sub>2</sub> interaction with diethyl sebacate and the intermediate solvent free volume fraction leads to both high CO<sub>2</sub> solubility and high CO<sub>2</sub>/H<sub>2</sub> solubility selectivity in diethyl sebacate. An in-house NETL computational database was built to characterize CO<sub>2</sub>, H<sub>2</sub>, N<sub>2</sub>, and H<sub>2</sub>O interactions with 202 different chemical functional groups. It was found that 13% of the functional groups belong to the strong interaction category with the CO<sub>2</sub> interaction energy between -15 and -21 kJ/mol; 62% of the functional groups interact intermediately with CO<sub>2</sub> (-8 to -15 kJ/mol). The remaining 25% of functional groups interact weakly with CO<sub>2</sub> (below -8 kJ/mol). In addition, calculations show that CO<sub>2</sub> interactions with the functional groups are stronger than N<sub>2</sub> and H<sub>2</sub> interactions but are weaker than H<sub>2</sub>O interactions. The CO<sub>2</sub> and H<sub>2</sub>O interactions with the same functional groups exhibit a very strong linear positive correlation coefficient of 0.92. The relationship between CO<sub>2</sub> and H<sub>2</sub> gas solubilities and solvent fractional free volume (FFV) has been systematically studied for seven solvents at 298.2 K. A skewed bell-shaped relation was obtained between CO<sub>2</sub> solubility and solvent FFV. When an organic compound has a density approximately 10% lower than its density at 298.2 K and 1 bar, it exhibits the highest CO<sub>2</sub> loading at that specific solvent density and FFV. Note that the solvent densities were varied using simulations, which are difficult to be obtained from the experiment. In contrast, H<sub>2</sub> solubility results exhibit an almost perfect positive linear correlation with the solvent FFV. The theoretical maximum and minimum physical CO<sub>2</sub> solubilities in any organic compound at 298.2 K were estimated to be 11 and 0.4 mol/MPa L, respectively. An examination of 182 experimental CO<sub>2</sub> physical solubility data and 29 simulated CO<sub>2</sub> physical solubilities shows that all the CO<sub>2</sub> physical solubility data are within the maximum and minimum with only a few exceptions. Finally, simulations suggest that in order to develop physical solvents with both high CO<sub>2</sub> solubility and high CO<sub>2</sub>/H<sub>2</sub> solubility selectivity, the solvents should contain functional groups which are available to interact strongly with CO<sub>2</sub> while minimizing FFV.