Structural transformation and tuning behavior induced by the propylamine concentration in hydrogen clathrate hydrates

The structures and the guest-host distributions of iso-propylamine (i-PA) and n-propylamine (n-PA) hydrates with hydrogen as a secondary guest were identified by powder X-ray diffraction and Raman spectroscopic analysis. The structure of 11.1 mol% i-PA + H₂ hydrate was identified to be hexagonal (space group P6₃/mmc) with a few unindexed diffraction peaks, while 5.6 mol% i-PA + H₂ hydrate had a cubic structure (space group Fd3¯m). Similarly, the structure of 13.3 mol% n-PA + H₂ hydrate was found to be monoclinic (space group P2₁/n), while 5.6 mol% n-PA + H₂ hydrate had a cubic structure (space group Fd3¯m). The 'tuning' phenomenon, multiple occupancy of hydrogen in the large cage at the pressure and temperature regions outside of pure hydrogen hydrate stability, was observed in the i-PA + H₂ hydrate only when the amine concentration was lower than the stoichiometric value of structure II hydrate. The three-phase (H-L_w-V) equilibria for alkylamine + H₂ + water mixtures were also measured to investigate their thermodynamic stability.