NEW FINDINGS ON GUEST ENCLATHRATION IN STRUCTURE-H HYDRATES BY MEANS OF THERMODYNAMIC AND SPECTROSCOPIC ANALYSIS

Abstract

Among the three common gas hydrate structures, structure-H (sH) hydrate has been regarded as forming only in the laboratory since it was first reported in 1987. However, natural gas hydrate samples obtained from the Cascadia margin showed that sH hydrate can form naturally. Not only was the sH hydrate found in natural samples, but it was also discovered that n-alkanes such as npentane and n-hexane, considered to have too large molecular size to be sH hydrate formers, can act as co-guests of sH hydrates in mixtures with other sH hydrate formers. In this study, thermodynamic measurements and spectroscopic analysis of powder X-ray diffraction and 13 C solid-state NMR methods, were performed for synthetic hydrate samples in order to identify the accommodation of n-alkanes with five or more carbon atoms. In addition, some new hydrate guests were found to form sH hydrates. From the present results, it is clear that, so far, our understanding of gas hydrates and guest enclathration needs to be revised and expanded in order to explain new findings.