Investigation on the Structure, Formation Mechanism, and Surface Morphology of CP–CH4 Binary Hydrate

Abstract

Cyclopentane (CP) is often used as a hydrate former, which shows an excellent promotion effect in hydrate-based desalination. To elucidate the guest molecule distribution and microcosmic mechanism during hydrate formation, the in situ Raman spectroscopy of CP–methane binary hydrates is performed. The Raman spectra for the intramolecular vibration mode of host and guest molecules reveal that sII CP–methane hydrate formed with CP and CH4 encaged in large and small cages, respectively. But the small cages were not completely filling in. Because of the structure of CP with a five-membered ring, hydrogen-bonded water molecules arranged around CP forming a metastable structure, which accelerates the nucleation rate of hydrate. The peak O–H stretching vibration (ROH–H = I3170/I3400) in hydrate phase is an indication of hydrogen-bond ordering. At 3.5 MPa, the CH4 occupancy in the small cages was 0.9978 and the CP occupancy in the small cages was 0.1751. According to cage occupancies and the thermodynamic model based on van der Waals–Platteeuw, the calculated hydration number was 12.6. Cryo-scanning electron microscopy was applied to observe the surface morphology of hydrate. Energy-dispersive spectroscopy was used to distinguish hydrate from ice by identifying element C.