Thermodynamic Properties of Double Semiclathrate Hydrates Formed with Tetrabutylphosphonium Chloride + CH4

Phase equilibrium conditions for tetrabutylphosphonium chloride (TBPC) + CH4 double semiclathrate hydrates (SCHs) were determined to evaluate the stabilization effect of TBPC for potential applications of hydrate-based gas separation and storage. The equilibrium data revealed that TBPC strongly enhances the stability of CH4 hydrates by increasing the equilibrium temperature at a given pressure, and the system showed the strongest stabilization effect at a TBPC mass fraction of 0.30. The dissociation enthalpies of (TBPC + CH4) SCHs were calculated from the experimental data obtained in this work using the Clausius-Clapeyron equation and the Peng-Robinson equation of state. It was found that the mean dissociation enthalpies for (TBPC + CH4) SCHs with TBPC mass fractions of 0.05 to 0.50 were (135.49 to 158.32) kJ.(mol of CH4)(-1), which were much higher than that of the pure CH4 hydrate. In addition, to identify the molecular behavior and hydrate structure, Raman spectroscopy measurements were conducted at 203 K. The spectra demonstrated that (TBPC+CH4) SCHs were formed in tetragonal structure I with small 5(12) cages trapping CH4 molecules.