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

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 ([P_2 4 4 4][PF₆]), tributylhexylphosphonium hexafluorophosphate ([P_6 4 4 4][PF₆]), tetraethylammonium bromide ([N_2 2 2 2]Br), tetraethylammonium bistrifluoromethanesulfonimide ([N_2 2 2 2][NTf₂]), and tetraethylammonium hexafluorophosphate ([N_2 2 2 2][PF₆]) 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, [P_2 4 4 4][PF₆] and [P_6 4 4 4][PF₆] 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, [N_2 2 2 2]Br, [N_2 2 2 2][NTf₂], and [N_2 2 2 2][PF₆] 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 [N_2 2 2 2]Br and [N_4 4 4 4]Br on CO₂ hydrate formation. It was found that the anion-cation interaction of [N_2 2 2 2]Br was stronger than that of [N_4 4 4 4]Br, and Br^- in [N_2 2 2 2]Br is less likely to participate in the formation of hydrate cages in the [N_2 2 2 2]Br + H₂O + CO₂ system according to the intermolecular anion-water, anion-CO₂, and water-water radial distribution function in [N_2 2 2 2]Br + H₂O + CO₂ and [N_4 4 4 4]Br + H₂O + CO₂ systems.