Spectroscopic measurements on binary, ternary, and quaternary mixed-gas molecules in clathrate structures

To reveal the preferential enclathration of various guest molecules of natural gas into the hydrate structure, systematically prepared hydrate samples from mixed gases such as ethane-propane (C ₂H ₆-C ₃H ₈), methaneethane-propane (CH ₄-C ₂H ₆-C ₃H ₈), and methane-ethane- propane-iso-butane (CH ₄-C ₂H ₆-C ₃H ₈-i-C ₄H ₁₀) with powdered ice were analyzed by spectroscopic techniques. Powder X-ray diffraction analysis at approximately 150 K was used to determine the crystal structure of the sample. Microscopic Raman spectroscopy measured at approximately 120 K revealed the enclathration of guest molecules into various cages. Gas chromatographic analysis on feed gases and the gases retrieved from hydrate samples supported the mass balance estimations derived from the spectroscopic measurements. These results, together with previous studies, revealed the preferential cage-occupation rule of hydrocarbon molecules. The collection of the Raman spectra of the guest molecules in these hydrate samples and in various phases of pure systems are useful for natural sample analysis by this technique.