CO<sub>2</sub>在气体水合物结构特性的固体核磁共振波谱与拉曼光谱实验研究

doi:10.3964/j.issn.1000-0593(2022)02-0464-06

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Abstract 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, ¹³C solid-state nuclear magnetic technology (NMR) and Raman spectroscopy (Raman) technology were used to characterize CO₂ molecules from CH₄ hydrates replaced by CO₂ gas and the synthesized ¹³CO₂-H₂-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 CH₄ hydrates replaced by CO₂ gas to obtain CO₂ molecules occupied in the 5¹²6² cages and CH₄ molecules occupied in the 5¹² and 5¹²6² 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 high-frequency resonance is also used to obtain CO₂ molecules occupied in the 5¹²6² cages and CH₄ molecules occupied in the 5¹²and 5¹²6² 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 (5¹²6² 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 (5¹² 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 ¹³CO₂-H₂-CP hydrates and the occupancy rates of H₂, CO₂ in 5¹² cages, and CP in 5¹²6² 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 H₂, CO₂ in 5¹² cages, and CP in 5¹²6² 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 ¹³C-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 ¹³C-labeled NMR result. (3) This paper assigns the ¹³C NMR chemical shift of CO₂. The results in this paper provide a reference for CO₂ hydrate research used by ¹³C NMR technology. In addition, combined with the comparative analysis of Raman and ¹³C NMR, it provides another reference for the quantitative study of CO₂ hydrate used by Raman technology.

Key words：Raman; Solid-state ¹³C NMR; CO₂; Gas hydrate; Structure feature; Quantitative analysis

Received: 2020-12-24 Accepted: 2021-03-10

ZTFLH:

O657.37

Corresponding Authors: SU Qiu-cheng E-mail: suqc@ms.giec.ac.cn

Cite this article:

FU Juan,MO Jia-mei,YU Yi-song, et al. Experimental Study on the Structure Characteristics of CO₂ in Gas Hydrate by Solid-State Nuclear Magnetic Resonance and Raman Spectroscopy[J]. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2022, 42(02): 464-469.

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2022)02-0464-06 OR https://www.gpxygpfx.com/EN/Y2022/V42/I02/464