Gas hydrate formation by allyl alcohol and CH4: Spectroscopic and thermodynamic analysis

We discovered a new structure II (sII) hydrate forming agent, allyl alcohol (AA), in the presence of methane (CH4) for the first time, and characterized the crystal structure, guest distribution, and phase equilibria of the (AA+CH4) hydrate. Using solid-state C-13 NMR and Raman spectroscopy, the crystal structure of the (AA+CH4) hydrate was confirmed to be a sII hydrate, and the CH4 molecule was found to be encapsulated in both the large and small cages of the sII hydrate. In addition, AA was found to be included in the large cages of the sII hydrate in the Gauche-Gauche form based on the measured- and calculated-NMR spectra. Notably, we investigated the free OH signal of AA in the Raman spectra to determine whether hydrogen bonding occurred between host and guest molecules; however, we could not determine whether the existence of the free OH signal was consistent with this host-guest interaction. To clearly identify the crystal structure and possible host-guest interactions, a high-resolution powder X-ray diffraction (HRPD) pattern of our (AA+CH4) hydrate sample was analyzed using Rietveld analysis with the direct space method. The crystal structure of the (AA+CH4) hydrate was assigned as the cubic Fd3m structure with a lattice constant of 17.1455 angstrom. In particular, the shortest distance between the AA molecule in the hydrate cages and an oxygen atom in the host water was estimated to be 2.55 angstrom; thus, we concluded that the hydroxyl group of the AA molecule was hydrogen-bonded to the host water framework. Finally, we measured the phase equilibrium conditions of the binary (AA+CH4) hydrate and found that the equilibrium pressure conditions of the binary (AA+CH4) hydrate were slightly higher than those of the pure CH4 hydrate.