X射线粉晶衍射-拉曼光谱法研究含甲烷双组分水合物结构及谱学特征

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^-10 m和17.2241×10^-10 m,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。

Characterization of Binary Hydrates Containing Methane by X-ray Diffraction and Microscopic Laser Raman Spectroscopy

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.