Abstract
Large molecules such as 2-methylbutane (C5H12) or 2,2-dimethylbutane (C6H14) form structure H (sH) hydrates with methane (CH4) as a help gas. In this study, the Raman spectra of the C–H symmetric stretch region of CH4 enclathrated within various sH hydrates and structure I CH4 hydrates were analyzed in the temperature range 137.7–205.4 K. Thermal expansions of these sH hydrate samples were also measured using powder X-ray diffraction. Symmetric stretch vibrational frequencies of CH4 in host–water cages increased because of varying temperature, and the sizes of the host–water cages also increased; variation of CH4 in small cages was less than in larger cages. Comparing the variations of the C–H symmetric stretching frequencies of CH4 in gas hydrates with varying pressure and temperature, we suggest that the observed trend is caused by thermal vibrations of the CH4 molecule in water cages.
Highlights
Clathrate hydrates are crystalline inclusion compounds that consist of guest molecules of suitable sizes and shapes trapped in well-de ned cages formed by water molecules
Small guest molecules such as methane (CH4) or ethane form sI-type hydrates, whereas larger molecules like propane or 2-methylpropane (C4H10) form structure II (sII) hydrates. structure H (sH) hydrates are obtained from even larger molecules, e.g., 2-methylbutane (C5H12) or 2,2-dimethylbutane (C6H14), in the presence of a help gas as CH4.1 sH hydrate that is capable of encapsulating these larger molecules in natural gas than either structure I or II hydrates was found at the Barkley Canyon.[5]
powder X-ray diffraction (PXRD) measurements showed that the crystal structure of the six different gas hydrates was sH (Fig. S1†)
Summary
Clathrate hydrates are crystalline inclusion compounds that consist of guest molecules of suitable sizes and shapes trapped in well-de ned cages formed by water molecules. Various sH CH4 and large-molecule mixed hydrates and an sI CH4 hydrate were prepared, and the Raman spectra in the C–H symmetric stretch region of enclathrated CH4 were studied in a temperature range of 137.7–205.4 K.
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