Structure Transition from Semi- to True Clathrate Hydrates Induced by CH4 Enclathration

Abstract

Diethylamine and n-propylamine, known as semiclathrate hydrate formers, are found to show structural transition when a help gas, CH,, was introduced, The diethylamine.8.67H(2)O semiclathrate hydrates (orthorhombic Pbcn) were changed to sH type (hexagonal P6/mmm) true clathrate hydrates, while the n-propylamine.6.5H(2)O semiclathrate hydrates (monoclinic P2(1)/n) turned into sII clathrate hydrates (cubic Fd3m). Irregularly distorted voids in the semiclathrate hydrate phases were transformed to conventional ones after changing their structures to gas hydrate phases. The different shape of large voids in the semiclathrate hydrates changed to the typical shape of sH or sII large voids, and pentagonal dodecahedra were formed so as to capture CH4 molecules. Transition pattern and molecular behavior from semiclathrate hydrate to true clathrate hydrates were analyzed with PXRD, NMR, and Raman methods. In addition, the liquid mixture-CH4 hydrates-CH4 vapor (L-H-V) thermodynamic equilibrium conditions were measured.