Mechanistic Insights into CO₂ Sequestration via Clathrate Hydrate Formation from Molecular Simulations: Influence of Temperature and Pressure

Complex temperature/pressure conditions are present in the process of CO2 sequestration in hydrate form in subsea sediments; how temperature and pressure affect CO2 hydrate formation remains unclear. To address this issue, molecular dynamics simulations were conducted to explore CO2 hydrate formation in a CO2/water two-phase system under a series of temperature/pressure conditions, i.e., 250, 255, 260, 263, 265, 268, 270, and 273 K (all at 50 MPa) and 5, 10, 20, 30, 40, and 50 MPa (all at 250 K). Simulation results show that CO2 hydrate formation is more sensitive to the change in temperature than to pressure: Hydrate forms only at 250 K and 50, 40, 30, and 20 MPa, but not at higher temperatures. CO2 hydrate formation is closely related with the evolution of CO2 hydration shells, where hydrogen bond formation is the predominant reaction. Adsorption of sufficient CO2 molecules to the CO2 hydration shells significantly prolongs the lifetimes of hydrogen bonds and is crucial to cage formation and hydrate nucleation. Such a formation mechanism of CO2 hydrate is identified in the nucleated systems. Pressure and temperature affect the formation of CO2 hydrate via directly impacting the stability of hydrogen bonds in the CO2 hydration shells. Furthermore, pressure and temperature indirectly influence the stability of hydrogen bonds via changing the concentration of CO2 in water, which could modulate the number of CO2 molecules adsorbing to the CO2 hydration shells and thus affect the lifetime of hydrogen bonds therein. Specifically, increasing the temperature significantly destabilizes the stability of hydrogen bonds, while variation in pressure has little influence. Additionally, some square-face-containing metastable cages (4151062, 4151063, and 4151064) in the incipient CO2 hydrate solids can transform into sI hydrate cages (512 and 51262) via removal, insertion, or rotation of a pair of water molecules.