Temperature effect on C–H symmetric stretching frequencies of CH4 in water cages of sI and sH clathrate hydrates were clarified.

Knowledge of carbon isotope fractionation is needed in order to discuss the formation and dissociation of naturally occurring CO2 hydrates. We investigated carbon isotope fractionation during CO2 hydrate formation and measured the three-phase equilibria of 12CO2–H2O and 13CO2–H2O systems. From a crystal structure viewpoint, the difference in the Raman spectra of hydrate-bound 12CO2 and 13CO2 was revealed, although their unit cell size was similar. The δ13C of hydrate-bound CO2 was lower than that of the residual CO2 (1.0–1.5‰) in a formation temperature ranging between 226 K and 278 K. The results show that the small difference between equilibrium pressures of ~0.01 MPa in 12CO2 and 13CO2 hydrates causes carbon isotope fractionation of ~1‰. However, the difference between equilibrium pressures in the 12CO2–H2O and 13CO2–H2O systems was smaller than the standard uncertainties of measurement; more accurate pressure measurement is required for quantitative discussion.

Large molecules such as 2-methylbutane (C 5 H 12 ) or 2,2-dimethylbutane (C 6 H 14 ) form structure H (sH) hydrates with methane (CH 4 ) as a help gas. In this study, the Raman spectra of the C-H symmetric stretch region of CH 4 enclathrated within various sH hydrates and structure I CH 4 hydrates were analyzed in the temperature range 83-183 K. Thermal expansions of these sH hydrate samples were also measured using powder X-ray diffraction. Symmetric stretch vibrational frequencies of CH 4 in host water cages increased because of varying temperature, and the sizes of the host water cages also increased; variation of CH 4 in small cages was less than in larger cages. Comparing the variations of the C-H symmetric stretching frequencies of CH 4 in gas hydrates with varying pressure and temperature, we suggest that the observed trend is caused by thermal vibrations of the CH 4 molecule in water cages.

Hydrate equilibrium conditions for 13CH4 were measured for temperatures ranging from 270 to 278 K and for pressures ranging from 2.3 to 4.2 MPa. The equilibrium pressure values in a 13CH4–H2O system were nearly the same as those in a 12CH4–H2O system. The similarity between the equilibrium pressures for these two systems can be used to explain why an existing study found nearly no fractionation of CH4 carbon isotopes during the formation of synthetic CH4 hydrates. We also measured the hydrate equilibrium conditions for singly deuterated CH4 (CH3D) in a temperature range of 270–273 K and pressure range of 2.3–2.6 MPa. The quadruple points (the coexistence of ice, liquid water, solid hydrate, and gaseous methane) for the 13CH4–H2O, CH3D–H2O, and CH4–H2O systems were determined using phase equilibrium (p–T) data and the Clausius–Clapeyron equation. The difference between the equilibrium pressures for CH3D and CH4 hydrates below the quadruple point (∼0.02 MPa) was slightly smaller than above the quadruple point (∼0.04 MPa) as reported by an existing study. The results of the phase equilibrium p–T conditions for hydrates encapsulating CH4 isotopologues can be used to explain the difference in fractionation behaviors for carbon and hydrogen isotopes of CH4 during the formation of the CH4 hydrate. Moreover, we characterized the crystal structure and the cage occupancies of the 13CH4 hydrate using powder X-ray diffraction and Raman spectroscopy. Our results suggest that the effect of encapsulating 13CH4 molecules in hydrate crystals was nearly the same as for CH4.

Temperature effects on the C–H symmetric stretch of hydrocarbons in various cages of sI and sII clathrate hydrates were observed.