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 + H-2 hydrate was identified to be hexagonal (space group P6(3)/mmc) with a few unindexed diffraction peaks, while 5.6 mol% i-PA + H-2 hydrate had a cubic structure (space group Fd (3) over barm). Similarly, the structure of 13.3 mol% n-PA + H-2 hydrate was found to be monoclinic (space group P2(1)/n), while 5.6 mol% n-PA + H-2 hydrate had a cubic structure (space group Fd (3) over barm). 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 + H-2 hydrate only when the amine concentration was lower than the stoichiometric value of structure II hydrate. The three-phase (H-Lw-V) equilibria for alkylamine + H-2 + water mixtures were also measured to investigate their thermodynamic stability.