Structural characterization based on spectroscopic methods and phase equilibria of 1,4-Dioxane + $H_2$ double hydrate

Abstract

Clathrate hydrates are crystalline nonstoichiometric compounds having various structures such as sI, sII and sH. They are mainly composed of guest and host molecules in which guest is a light-weight one and host is water. They are stable only at high pressures and low temperatures, so that they are found under the ground or the ocean. One of the characteristics of hydrates is they have cavities which can be applied to storage and separation processes. Even though many researches have been done, there are lots of areas that had to be studied. In this study, structural characterization based on spectroscopic methods and phase equilibria of 1,4-dioxane + H2 double hydrate were examined. Spectroscopic methods were solid NMR, Raman and X-ray diffraction (XRD) which are recognized as powerful tools in hydrate research. And also gas chromatography was employed to verify NMR data. To get phase equilibria data temperature-search method was utilized at different 1,4-dioxane’s concentrations. XRD experiments were done first of all to know the structure of 1,4-dioxane and 1,4-dioxane + H2 hydrate to show the same structure and lattice parameter. In order to recognize the proton positions of 1,4-dioxane and hydrogen, solid NMR were performed by using deutrated 1,4-dioxane from 0.5 to 5.6 mol %. It was found that 1,4-dioxane exclusively occupies the large cages while hydrogen does the small ones. Besides, to calculate absolute cage occupancy NMR experiments were done again using normal 1,4-dioxane instead of deutrated 1,4-dioxane from 1.0 to 10.0 mol %. By help of chemical potential equation, absolute cage occupancies were obtained separately. 1,4-dioxane shows almost 1 occupancy, however hydrogen doesn’t go into the small cages effectively except for 5.6 mol %. At 5.6 mol % the absolute cage occupancy is the maximum. This trend was again confirmed by gas chromatography (G.C.) experiments and the data from G.C. mostly coincides with the NMR data. It can be said that 1,4-di...