Spectroscopic observations and thermodynamic calculations on clathrate hydrates of mixed gas containing methane and ethane: Determination of structure, composition and cage occupancy

Clathrate hydrates of methane-ethane mixed gases have two crystal structures depending on their composition. To study their compositions and cage occupancies and how their structure is determined, we synthesized hydrate samples from methane-ethane mixtures. Analysis of the samples using X-ray diffraction, Raman spectroscopy, and gas chromatography revealed their structures, compositions, and cage occupancies. Experimentally, hydrate structure II existed in samples formed when the gas equilibrated with hydrates was approximately. 2% C2H6 (molar fraction) whereas both structures I (sI) and II (sII) coexisted for 12 to 22% C2H6. The structures below 2% and above 22% of C2H6 existed only as sI hydrates. Volume ratios of both structures were obtained from the ratio of the peak intensities of the C-C stretching peaks in the Raman spectra. In the transition zone containing both structures, the volume ratio of the sII structure gradually decreased with increasing C2H6 concentration. Cage occupancies of guest molecules in the hydrate cages were determined by the relative intensity ratio of Raman spectra. C2H6 molecules occupied only large cages in both structures, whereas CH4 molecules occupied the remaining cages. The experiments agree with the structure and molecular distributions that were predicted by minimizing the Gibbs free energy of the sample. This model calculation provides insight into the structural transition mechanism.