[MIS21-P01] Raman spectroscopic analysis of isotopologue methane hydrates
Keywords:methane hydrate, Raman spectroscopy, isotopologues
Raman spectroscopy is one of the useful tool to get information of gas hydrate crystals. Natural gas hydrates in submarine/sublacustrine sediment mainly encage methane. Methane molecule is composed of carbon and hydrogen, and three kinds of isotopologues, 12CH4, 13CH4, and CH3D exist in nature. Ozeki et al. (2018) first reported Raman spectra of CH3D and CD4 hydrates and compared with CH4 (mainly 12CH4) hydrate. We report Raman spectra of 13CH4 hydrate and summarize Raman spectra of isotopologue methane hydrates.
13CH4 hydrate sample was synthesized in a small pressure cell (volume: 5 mL). Fine ice powder (1g) was put in the pressure cell, and introduced appropriate amount of 13CH4 (purity: 99.5%, Taiyo-Nissan). 13CH4 hydrate was formed by melting the fine ice powder at the temperature of 273.2 K under high pressure of 13CH4. We retrieved the hydrate sample at 77 K and its Raman spectra was obtained at 123 K in the range 2,500-3,300 cm-1 using a Raman spectrometer (RMP-210, JASCO Corporation). The Raman peaks were fitted in the range 2,800-3,000 cm-1 for the C-H stretching peaks of methane using a Voigt function to obtain the integrated intensities of the two peaks corresponding to methane encaged in the large and small cages of the cubic structure I.
Raman shifts for the C-H stretching and bending modes of 13CH4 was 0.8 cm-1 and 14 cm-1 smaller than those of 12CH4, respectively, suggesting that 13C-H bonds affect its vibrational frequency. Hydration number of 13CH4 was estimated as 6.00±0.02, almost the same as that of 12CH4 (6.02±0.02). Therefore, Cage occupancies of 13CH4 and 12CH4 hydrates showed no difference between them.
Reference
Ozeki T, Kikuchi Y, Takeya S, Hachikubo A (2018) Phase equilibrium of isotopologue methane hydrates enclathrated CH3D and CD4. J Chem Eng Data 63(6): 2266-2270, doi: 0.1021/acs.jced.8b00203
13CH4 hydrate sample was synthesized in a small pressure cell (volume: 5 mL). Fine ice powder (1g) was put in the pressure cell, and introduced appropriate amount of 13CH4 (purity: 99.5%, Taiyo-Nissan). 13CH4 hydrate was formed by melting the fine ice powder at the temperature of 273.2 K under high pressure of 13CH4. We retrieved the hydrate sample at 77 K and its Raman spectra was obtained at 123 K in the range 2,500-3,300 cm-1 using a Raman spectrometer (RMP-210, JASCO Corporation). The Raman peaks were fitted in the range 2,800-3,000 cm-1 for the C-H stretching peaks of methane using a Voigt function to obtain the integrated intensities of the two peaks corresponding to methane encaged in the large and small cages of the cubic structure I.
Raman shifts for the C-H stretching and bending modes of 13CH4 was 0.8 cm-1 and 14 cm-1 smaller than those of 12CH4, respectively, suggesting that 13C-H bonds affect its vibrational frequency. Hydration number of 13CH4 was estimated as 6.00±0.02, almost the same as that of 12CH4 (6.02±0.02). Therefore, Cage occupancies of 13CH4 and 12CH4 hydrates showed no difference between them.
Reference
Ozeki T, Kikuchi Y, Takeya S, Hachikubo A (2018) Phase equilibrium of isotopologue methane hydrates enclathrated CH3D and CD4. J Chem Eng Data 63(6): 2266-2270, doi: 0.1021/acs.jced.8b00203