Ultrafast Realization of Ionic Liquids with Excellent CO2 Absorption: A trinity study of machine learning, synthesis, and precision measurement

Abstract

Efficient CO2 capture is indispensable for achieving a carbon-neutral society while maintaining a high quality of life. Since the discovery that ionic liquids (ILs) can absorb CO2, various solvents composed of molecular ions have been developed and their CO2 solubility has been studied. However, it is challenging to optimize these materials to realize targeted properties as the number of candidate ion combinations for designing novel ILs is of the order of 1018. In this study, electronic- structure informatics was applied as an interdisciplinary approach to quantum chemistry calculations, and combined with machine learning to search 402,114 IL candidates to identify those with better CO2 solubility than known materials. Guided by the machine-learning results, trihexyl(tetradecyl)phosphonium perfluorooctanesulfonate was synthesized and it was experimentally confirmed that this IL has higher CO2 solubility than trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide, which is the previous best IL for CO2 absorption. The method developed in this study could be transferable to gas-absorbing liquids in general, such as deep eutectic solvents (hydrogen-bonded mixed organic solvents in a broad sense), which also have numerous practical applications. Therefore, we believe that our method for developing functional liquids will significantly contribute to the development of a carbon-neutral society.

CO2/Hydrocarbon Selectivity of Trihexyl(tetradecyl)phosphonium-Based Ionic Liquids

“…The enthalpy of the PFOS anion is higher than that of the other two anions in the CO 2 , CH 4 , and C 2 H 4 + IL systems. Despite having a lower fluorine number than the PFOS anion, the PF 6 anion has a greater negative enthalpy than that of the PFOS anion only in the case of the C 2 H 6 + IL system . Therefore, the lower enthalpy can explain the fact that fluorination of anions decreases the solubility of C 2 H 6 .…”

“…The solubility data for CO 2 and CH 4 were not measured at the same mole fractions or pressures; all selectivity calculations were made using eq . The Henry’s law constants have already been reported for CO 2 . The ideal IL CO 2 /hydrocarbon selectivities, such as S CO 2 /CH 4 , S CO 2 /C 2 H 6 , and S CO 2 /C 2 H 4 , were calculated using the solubility data for CO 2 and hydrocarbons and eq …”

“…More importantly, the volumetric expansion of the IL must be taken into account while calculating gas absorption; if not, the highest estimate would be approximately 8% lower. The gas absorption-induced IL volumetric expansion swelling ( S w ) was calculated using the Sanchez–Lacombe equation of state. , The Tait equation and Sanchez–Lacombe equation of state coefficients were obtained by fitting the phosphonium-based IL experimental densities obtained at 313.15 and 333.15 K in the range of 0.1–10 MPa …”

“…22 Specifically, they found that the Kamlet−Taft β parameter can be used as the basis for the choice of ILs that maximizes the selectivities of sour gases/CH 4 . 20 2.2. Apparatus and Procedure.…”

“…Despite having a lower fluorine number than the PFOS anion, the PF 6 anion has a greater negative enthalpy than that of the PFOS anion only in the case of the C 2 H 6 + IL system. 20 Therefore, the lower enthalpy can explain the fact that fluorination of anions decreases the solubility of C 2 H 6 . 35 As explained in the previous section, the solubility of CH 4 , C 2 H 6 , and C 2 H 4 increases in the order CH 4 < C 2 H 4 < C 2 H 6 , which is also consistent with the order of their entropies.…”