Thermodynamic inhibition of 4-methylmorpholine while forming sH hydrate with methane

• 4-methylmorpholine as a new large-molecule guest substance of structure H hydrate.
• Thermodynamic inhibition effect of 4-methylmorpholine at various concentrations.
• The ratio of the molecular diameter to cage diameter is 0.87.
• Explanation for enclathration behaviors of 4-methylmorpholine.

4-methylmorpholine (4-mMPL), a nitrogen-containing heterocyclic compound, is an organic base which acts as a proton acceptor. In this study, we characterized structure H (sH) clathrate hydrates with 4-mMPL by measuring the hydrate-phase equilibria and a series of microscopic analyzes (powder X-ray diffraction, solid-state 13C NMR and Raman spectroscopy). 4-mMPL was found to have lower dissociation temperature at a given pressure compared to that of pure CH4 hydrate, simultaneously with a co-guest in sH hydrates (hexagonal P6/mmm). Such a change of thermodynamic stability is attributed to an oxygen atom in the molecular structure of 4-mMPL, which prevents hydrogen-bonding among water molecules. The ratio of the molecular diameter to cage diameter is 0.87, implying that the guest molecules can fit into cavities without distortion. The thermodynamic and chemical characteristics of 4-mMPL can be extended to other guest species in order to explain molecular behaviors related to inherent mechanisms of hydrate stability and host-guest interactions.

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