Structural transformation and tuning behavior induced by the propylamine concentration in hydrogen clathrate hydratesElectronic supplementary information (ESI) available: The Le Bail fittings of the patterns and additional PXRD patterns. See DOI: 10.1039/c4cp03972f

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 P 6 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&cmb.macr; m ). Similarly, the structure of 13.3 mol% n -PA + H 2 hydrate was found to be monoclinic (space group P 2 1 / n ), while 5.6 mol% n -PA + H 2 hydrate had a cubic structure (space group Fd 3&cmb.macr; 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 + H 2 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 + H 2 + water mixtures were also measured to investigate their thermodynamic stability. The structures and the guest-host distributions of propylamine hydrates provide useful information on the hydrophilic guest-host interactions in clathrate hydrates.