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.