Experimental Study on the Structure Characteristics of CO2 in Gas Hydrate by Solid-State Nuclear Magnetic Resonance and Raman Spectroscopy

Natural gas hydrate is unconventional energy with huge energy and source potential. In 2017 and 2020, two exploratory trials of marine hydrate in the South China Sea were successful. The incident accelerated the development of China's natural gas hydrate project. Carbon dioxide replacement and recovery technology can develop natural gas energy sources in a dense solid phase stored in natural gas hydrates and store CO2 greenhouse gases in the ocean. The separation of CO2 from flue gas by forming hydrates is becoming a promising new separation technology. The microstructure and properties of CO2 molecules in gas hydrates are still unclear, and the practical application of CO2 technology has certain unknown effects. In this paper, C-13 solid-state nuclear magnetic technology (NMR) and Raman spectroscopy (Raman) technology were used to characterize COz molecules from CH4 hydrates replaced by CO2 gas and the synthesized (CO2)-C-13-H-2-CP hydrates. The content of CO2 molecules stored in hydrate crystals was tested, the distribution of CO2 molecules stored in the hydrate cage was analyzed, and the structureal characteristics of CO2 molecules in gas hydrates were obtained. The results show that: (1) The 1 277. 5 cm (1) peak integration of the Raman Fermi low-frequency resonance is used in CH 4 hydrates replaced by CO2 gas to obtain CO2 molecules occupied in the 5(12)6(2) cages and CH4 molecules occupied in the 5(12) and 5(12)6(2 )cages. They are 0. 978 2, 0. 059 3, and 0. 009 5, respectively. The hydration number of the hydrate formed is 7. 61. The 1 381. 3 cm (1) peak integration of the Raman Fermi highfrequency resonance is also used to obtain CO2 molecules occupied in the 5(12)6(2) cages and CH4 molecules occupied in the 5(12) and 5(12)6(2) cages. They are 0. 984 3, 0. 023 7, and 0. 003 3, respectively. The hydration number of the hydrate formed is 7. 70. The large cages (5(12)6(2) cages) of the CO2 hydrate formed are almost filled with CO2 molecules. After the replacement, the addition of CO2 molecules in hydrate crystals will cause occupancies of CH4 in the large cages and small cages (5(12) cages) of the CH4 hydrate crystals formed by replacement to be greatly reduced. The hydration number of the CH4 hydrate formed by replacement is slightly lower than that of methane hydrate before the replacement. NMR is difficult to detect that the CO2 molecular signal was coming from the CO2 hydrate formed by unlabeled CO2 molecules. After CO2 gas replacement, the occupancy rate of CH4 in the small cage and the large cage is only 0. 097 5 and 0. 317 2, respectively. The occupancy rates obtained by the above two peak integration methods are not the same. The main reason for this difference is that NMR detected no unlabeled CO2 molecular signal. (2) The Raman Fermi low-frequency resonance 1 273. 4 cm (1) peak integration method was used the synthesized (CO2)-C-13- H-2-CP hydrates and the occupancy rates of H-2, CO2 in 5(12) cages, and CP in 5(12)6(2) cages were obtained with results of 0. 124 8, 0. 304 2, and 0. 997 8, respectively. The hydration number from the hydrate formed is 9. 16. The Raman Fermi high-frequency resonance peak integration method of 1 380. 6 cm (1) was also used, and the occupancy rates of H-2 CO2 in 5(12) cages, and CP in 5(12)6(2) were obtained, respectively, which were 0. 123 6, 0. 577 1, and 0. 985 1, respectively. The hydration number from the hydrate formed was 7. 12. The results show that C-13-labeled CO2 molecules can obtain better solid-state NMR resolution in the synthesized hydrates. This paper confirms for the first time that the chemical shift of CO2 molecules from type II small cages is 124. 8 ppm, and it is calculated that the small cage occupancy rate of CO2 is 0. 783 1, the large cage occupancy rate of CP is 0. 971 8, and the hydration number is 6. 70. The results show that the Raman high-frequency Fermi resonance peak (1 380. 6 cm( 1)) is closer to the C-13-labeled NMR result. (3) This paper assigns the C-13 NMR chemical shift of CO2. The results in this paper provide a reference for CO2 hydrate research used by 13 C NMR technology. In addition, combined with the comparative analysis of Raman and C-13 NMR, it provides another reference for the quantitative study of CO2 hydrate used by Raman technology