Temperature- and Pressure-induced Structural Transition of Binary Clathrate Hydrates

Area of Science:

• Materials Science
• Chemical Engineering
• Crystallography

Background:

• Hydrates are crystalline solids that trap guest molecules within their cages.
• Structure II (sII) hydrates are a specific type of clathrate hydrate with unique cage structures.
• Methane (CH4) hydrates are of significant interest for energy storage and transport.

Purpose of the Study:

• To identify new structure II (sII) hydrate forming agents among C4H8O molecules.
• To investigate the abnormal structural transition of binary C4H8O+CH4 hydrates between structure I (sI) and sII.
• To understand the influence of temperature and pressure on hydrate phase behavior.

Main Methods:

• Synthesis and characterization of binary hydrates using C4H8O molecules (2-methyl-2-propen-1-ol and 2-butanone) with methane.
• High-resolution powder diffraction to determine crystal structures.
• Raman spectroscopy to confirm hydrate composition and structure.

Main Results:

• Discovery of 2-methyl-2-propen-1-ol and 2-butanone as novel sII hydrate formers.
• Observation of an abnormal phase transition between sI and sII methane hydrates in binary systems.
• Identification of specific temperature and pressure conditions governing the sI-sII phase boundary.
• Demonstration that C4H8O molecules occupy sII large cages at low T/P but are excluded at high T/P, leading to pure sI CH4 hydrate.

Conclusions:

• 2-Methyl-2-propen-1-ol and 2-butanone are effective structure II (sII) hydrate formers.
• Binary C4H8O+CH4 hydrate systems exhibit complex phase behavior with a distinct sI-sII transition.
• Understanding these transitions is crucial for predicting and controlling hydrate formation in various applications.