Phase Equilibria and Stability of an HS-I Hydrate and a Related Phase Formed with Tri- <i>n</i> -butyl, <i>n</i> -Hexylammonium Salts under Methane, Carbon Dioxide, and Nitrogen Gas Pressures

Phase equilibrium data of clathrate hydrates are essential for evaluating their potential in gas storage and separation. In this study, we determined three-phase (hydrate–gas–liquid) equilibrium points for the systems (N4446Cl or N4446Br) + H2O + (CH4, CO2, or N2) at pressures of up to 10 MPa. N4446Cl, previously reported to stabilize the hexagonal structure-I (HS-I) phase, formed hydrates with all three gases even at aqueous compositions lower than those reported earlier. The P–T slope analyses suggest gas uptake potentials for CH4 and CO2 comparable with N4446Cl hydrates and N4446Br hydrates. The slope analysis suggests that N4446Br likely forms a phase similar to the HS-I structure under CH4 and CO2 but shows reduced capacity for N2. These results expand the known guest species capable of stabilizing HS-I hydrates and highlight the distinct gas-specific behavior of bromide and chloride salts. The phase equilibrium data reported here provide a basis for designing HS-I hydrate-based gas storage and separation applications, including the capture of CO2 from lean mixtures.