Thermodynamic and Kinetic Effects of Quaternary Ammonium and Phosphonium Ionic Liquids on CO2 Hydrate Formation

LY Wang and Y Chen and YL Xu and YJ Zhang and Y Li and Y Wang and JP Wei and TX Chu, ACS OMEGA, 8, 1191-1205 (2023).

DOI: 10.1021/acsomega.2c06621

The paper elaborates the effects of ionic liquids (ILs) on the phase equilibrium temperature, induction time, gas consumption, gas consumption rate, and water to hydrate conversion in the presence of 0.25, 0.63, 0.95, 1.25, 3.75, 6.25, and 10.00 wt % ethyltributylphosphonium hexafluorophosphate (P2 4 4 4PF6), tributylhexylphosphonium hexafluorophosphate (P6 4 4 4PF6), tetraethylammonium bromide (N2 2 2 2Br), tetraethylammonium bistrifluoromethanesulfonimide (N2 2 2 2-NTf2 ), and tetraethylammonium hexafluorophosphate (N2 2 2 2PF6) under a pressure of 2 MPa. The results indicate that all five ILs could increase CO2 consumption and enhance the water to hydrate conversion. Compared with the pure water system, P2 44 4PF6 and P6 44 4PF6 shifted the phase equilibrium temperature of CO2 hydrates to a slightly higher temperature with reduced induction times by boosting CO2 hydrate nucleation, showing the dual function promotion effects. In contrast, N2 2 2 2Br, N2 2 2 2NTf2, and N2 2 2 2PF6 shifted the phase equilibrium temperature of CO2 hydrates to a lower temperature and prolonged the induction time by slowing down CO2 hydrate nucleation. The inhibition effects of anions on CO2 hydrates follow an order of Br- > NTf2- > PF6-. Besides, the density functional theory and molecular dynamic calculations were conducted to explain the inconsistent influences of N2 2 2 2Br and N4 4 4 4Br on CO2 hydrate formation. It was found that the anion-cation interaction of N2 2 2 2Br was stronger than that of N4 44 4Br, and Br- in N2 2 2 2Br is less likely to participate in the formation of hydrate cages in the N2 22 2Br + H2O + CO2 system according to the intermolecular anion-water, anion-CO2, and water-water radial distribution function in N2 2 2 2Br + H2O + CO2 and N4 4 4 4Br + H2O + CO2 systems.

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