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

Lanyun Wang; Yu Chen; Yongliang Xu; Yajuan Zhang; Yao Li; Yan Wang; Jianping Wei; Tingxiang Chu

doi:10.1021/acsomega.2c06621

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

ACS Omega. 2022 Dec 24;8(1):1191-1205. doi: 10.1021/acsomega.2c06621. eCollection 2023 Jan 10.

Authors

Lanyun Wang^{1

2

3}, Yu Chen¹, Yongliang Xu^{1

2

3}, Yajuan Zhang⁴, Yao Li^{1

2

3}, Yan Wang^{1

2

3}, Jianping Wei^{1

2

3}, Tingxiang Chu⁵

Affiliations

¹ School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo454003, China.
² Collaborative Innovation Center for Coal Safety Production & High-Efficient-Clean Utilization for Coal by Provincial and Ministerial Co-Construction, Jiaozuo454003, China.
³ State Key Laboratory Cultivation Base for Gas Geology and Gas Control in Henan Polytechnic University, Jiaozuo454003, China.
⁴ College of Oceanography, Hohai University, Nanjing210098, China.
⁵ School of Safety Engineering, North China Institute of Science and Technology, Beijing101601, China.

Abstract

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.