Spectroscopic analysis and structure identification of ion entrapped clathrate hydrate

Abstract

Unlike widely known non-ionic clathrate hydrates stabilized by van der Waals interaction between a host framework and guest molecule, ionic clathrate hydrates are a type of inclusion compound in which an ionic interaction exists between a cation or anion guest and the surrounding host cage. In this study, spectroscopic analysis and structure identification of ionic double hydrate and binary hydrate were investigated using NMR, Raman and X-ray diffraction spectrometer. The research scope of this study can be divided into two major fields. The first one is complex inclusion phenomena occurring in ionic clathrate hydrates, the second one is molecular dynamics of guest molecules with highly flexible lattice. First, extraordinary structural transition in cage-like as well as channel-like host frameworks of the double $(CH_4 + tetramethylammonium hydroxide (Me_4NOH))$ and $(H_2 + Me_4NOH)$ ionic clathrate hydrates and mixed Me4NOH clathrate hydrates are reported. In particular, intracrystalline channel patterns created in complex ionic host-guest networks are considered to play an important role in providing pathways and windows for small guest diffusion and sorption. Although the vacant channels formed by the linkage of specific cages have not received any attention in the inclusion phenomena of clathrate hydrates, it is essential to realize that these channels can play an important role in guest diffusion pathways and occupancy occurring in a complex clathrate hydrate matrix. Second, in other to investigate the molecular dynamics of guest molecules in clathrate hydrate, the binary $(H_2 + CH_4)$ and $(H_2 + Xe)$ structure-I (sI) clathrate hydrates were used. A notable observation appears in the spectra measured at temperatures above 223K, the proton signal implies that the H_2 diffusion is likely to occur in the hydrate phase to the direction of either sI-S to sI-L or sI-L to sI-S. This observation implies that the migration pathway of H_2 molecules is quite t...