CO2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

天然气水合物是蕴含着巨大能源潜力的非常规能源，2017年和2020年两次我国南海探索性试采的成功，加快了天然气水合物项目的进展。二氧化碳置换开采法，既能开发CH₄，又能封存CO₂。同时水合物法分离烟气中CO₂具有很好的应用前景，而CO₂在气体水合物的微观结构和特性尚不明确，实际应用存在一定的未知影响。为了考察其特性，利用13C固体核磁技术（NMR）和拉曼光谱（Raman）进行CO₂置换CH₄水合物、合成13CO₂-H₂-CP混合水合物实验表征，讨论CO₂在水合物中的定量问题，研究CO₂分子在笼型结构中的分布，探讨CO₂分子在气体水合物中的结构类型和特性。结果表明：（1）利用Raman费米低频共振1 277.5 cm-1峰积分得到CO₂在I型大笼（51262笼）的占有率为0.978 2，CH₄在Ⅰ型小笼（512笼）和大笼（51262笼）的占有率为0.059 3和0.009 5，水合数7.61，Raman费米高频共振1 381.3 m-1峰积分得到CO₂在51262笼的占有率为0.984 3，CH₄在512笼和51262笼的占有率为0.023 7和0.003 3，水合数7.70，CO₂几乎占满了大笼，CO₂气体的加入会导致水合物中，CH₄的大、小笼占有率均大幅度降低，置换后水合数略低于纯甲烷水合物，未标记的CO₂水合物在核磁中较难测出信号，CO₂气体置换后CH₄在小笼的占有率仅0.097 5，大笼占有率为0.317 2，两种方法差异主要原因为核磁的CO₂未出峰。（2）利用拉曼费米低频共振1 273.4 cm-1峰积分得到H₂、CO₂在512笼、CP在51262的占有率分别为0.124 8，0.304 2和0.997 8，水合数9.16；Raman费米高频共振1 380.6 cm-1峰积分得到H₂、CO₂在512笼、CP在51262的占有率分别为0.123 6，0.577 1和0.985 1，水合数7.12。13C标记CO₂分子在水合物中达到较好的固体核磁分辨率，首次确认CO₂在Ⅱ型小笼中的化学位移为124.8 ppm，计算得到CO₂的小笼占有率为0.783 1，CP的大笼占有率为0.971 8，水合数6.70，Raman高频频费米共振峰（1 380.6 cm-1）定量计算与13C NMR结果更接近。（3）对CO₂的13C NMR化学位移进行了归属，并结合Raman与13C NMR的对比分析，为CO₂水合物的13C NMR研究和拉曼定量提供参考。

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, 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 512 and 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.