Characterization of Binary Hydrates Containing Methane by X-ray Diffraction and Microscopic Laser Raman Spectroscopy
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摘要:
天然气水合物的晶体结构主要取决于客体分子种类与组成，目前单组分水合物的结构和谱学特征较为明确，但多组分水合物相关研究较少。为解决多组分水合物的结构识别问题，探讨其谱学特征，本文实验合成了甲烷-丙烷（CH4-C3H8）和甲烷-四氢呋喃（CH4-THF）两种含CH4双组分水合物以及CH4、C3H8和THF等三种单组分水合物，并采用低温X射线粉晶衍射（PXRD）和显微激光拉曼光谱进行了表征。结果表明：CH4-C3H8和CH4-THF双组分水合物的晶格常数a分别为17.2312×10-10m和17.2241×10-10m，为典型的Ⅱ型结构水合物，与相应C3H8和THF单组分水合物结构相同。在CH4-C3H8水合物中，CH4在大、小笼中均有分布，呈现两个特征拉曼峰（2900cm-1和2911cm-1）；C3H8仅分布在大笼，与单组分水合物相比，其C—H伸缩振动峰峰位几无变化，而C—C伸缩振动峰（873cm-1）向低频迁移约3cm-1。在CH4-THF水合物中，大笼被THF占据，CH4仅填充在小笼中（2910cm-1）；双组分水合物中，THF分子C—C和C—H伸缩振动峰峰位均与单组分水合物基本一致。分析认为，含CH4双组分水合物的结构类型与其相应的大分子水合物一致，大分子对双组分水合物的晶体结构特征具有决定作用。同时，大分子影响了CH4分子在笼型结构中的分布，致使双组分水合物的拉曼光谱特征存在显著差异。研究结论对基于谱学特征识别多组分水合物微观结构具有重要的指导意义。
Abstract:BACKGROUND The crystal structure of natural gas hydrate mainly depends on the species and composition of guest molecules. At present, the structure and spectral characteristics of single-component hydrate are relatively clear, but the studies of multi-component hydrates are relatively scarce.
OBJECTIVES To solve the problem of structure identification of multi-component hydrate, and understand its spectral characteristics.
METHODS Methane-propane (CH4-C3H8) and methane-tetrahydrofuran (CH4-THF) binary hydrates, as well as CH4, C3H8 and THF (molar ratio 1:17) single-component hydrate samples were synthesized and characterized by low-temperature X-ray powder diffraction (PXRD) and Raman spectroscopy.
RESULTS Both of the binary hydrates were typical structure Ⅱ hydrates, which were the same as those of C3H8 and THF hydrate. The crystal cell parameters a for CH4-C3H8 and CH4-THF binary hydrates were 17.2312×10-10 m and 17.2241×10-10 m, respectively. For CH4-C3H8 hydrate, CH4 was distributed in both large and small cages, showing two characteristic Raman peaks (2900cm-1 and 2911cm-1); C3H8 was only distributed in the large cage. Compared with C3H8 hydrate, the C-H stretching vibration peak was almost unchanged while the C-C stretching vibration peak (873cm-1) shifted by 3cm-1 to low frequency. For CH4-THF hydrate, the large cage was occupied by THF and CH4 was only filled in the small cage (2910cm-1). The peak positions of C-C and C-H stretching vibrations of THF in CH4-THF hydrate were consistent with those of THF hydrate.
CONCLUSIONS The structure types of binary hydrates are consistent with those of single component hydrates. Molecules with larger sizes play a key role in the crystal structure of binary hydrates containing CH4. Moreover, the distribution of CH4 molecules in the cages are also influenced by larger molecules, and the Raman spectral characteristics of binary hydrates are significantly different. The conclusion of this study has important guiding significance for identifying the microstructure of multi-component hydrate based on spectral characteristics.
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表 1 水合物样品生成前后压力变化
Table 1. Pressure change before and after hydrate formation
水合物样品 初始压力(MPa) 终止压力(MPa) CH4 8.0 6.6 C3H8 0.5 0.3 THF(5.56mol·%) 常压 常压 CH4-C3H8 7.0 4.9 CH4-THF 8.3 5.5 
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表 2 CH4-C3H8水合物样品客体分子拉曼峰及其归属
Table 2. Raman peaks and assignment of guest molecules in CH4-C3H8 hydrate samples
峰位(cm-1) 客体分子 振动模式 笼型分布 2980 C3H8 C—H伸缩 Ⅱ，51264 2932 C3H8 C—H伸缩 Ⅱ，51264 2911 CH4 C—H伸缩 Ⅱ，512 2900 CH4 C—H伸缩 Ⅱ，51264 2875 C3H8 C—H伸缩 Ⅱ，51264 2866 C3H8 C—H伸缩 Ⅱ，51264 1449 CH4，C3H8 CH2剪式 Ⅱ，51264 1445 CH4，C3H8 CH2剪式 Ⅱ，51264 1155 C3H8 (CH3)2CH伸缩 Ⅱ，51264 1050 C3H8 C—C骨架 Ⅱ，51264 873 C3H8 C—C伸缩 Ⅱ，51264
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表 3 CH4-THF水合物主要拉曼峰及其归属
Table 3. Raman peaks and assignments of guest molecules in CH4-THF hydrate samples
峰位(cm-1) 客体分子 振动模式 笼型分布 2993 THF C—H伸缩振动 Ⅱ，51264 2964 THF C—H伸缩振动 Ⅱ，51264 2940 THF C—H伸缩振动 Ⅱ，51264 2910 CH4 C—H伸缩振动 Ⅱ，512 2875 THF C—H伸缩振动 Ⅱ，51264 2861 THF C—H伸缩振动 Ⅱ，51264 1026 THF C—O—C伸缩振动 Ⅱ，51264 915 THF C—C—C—C伸缩振动 Ⅱ，51264
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表 4 水合物样品晶体结构参数
Table 4. Crystal structure parameters of hydrate samples
水合物样品 晶系 空间群 晶格常数
a=b=c(×10-10m)结构类型 CH4 体心立方 Pm3n 11.8927 Ⅰ C3H8 面心立方 Fd3m 17.2029 Ⅱ THF(摩尔比1∶17) 面心立方 Fd3m 17.2180 Ⅱ CH4-C3H8 面心立方 Fd3m 17.2312 Ⅱ CH4-THF 面心立方 Fd3m 17.2241 Ⅱ
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