Clathrate hydrates are crystalline solid compounds formed by the physically stable interaction between host molecules and relatively small guest molecules under specific conditions, generally low temperature and high pressure. The gas hydrates are divided into three distinct structures Ⅰ, Ⅱ and Η by formational condition of hydrate, cavity size and shape. In particular, carbon dioxide forms structure Ⅰ hydrate.
The four-phase hydrate equilibria and the phase behavior of CO2 + 3-methyl-1-butanol + water, CO2 + 1,4-dioxane + water and CO2 + THF + water were investigated using both a high-pressure equilibrium cell and a mechanical stirring system in the temperature range of 270-292K and the pressure range of 0.1-4.0MPa. The dissociation pressures of CO2 + 3-methyl-1-butanol + water were identical to those of pure CO2 hydrate, indicating that CO2 is not acting as a help gas for structure H hydrate formation, thus the formed hydrate is pure CO2 structure I hydrate. Interestingly, at lower concentrations than 3mol% of 1,4-dioxane, unstable dissociation behavior was observed for binary CO2 and 1,4-dioxane hydrates. The dissociation pressures of CO2 + THF + water were lower than those of CO2 + water at all experimental conditions. This indicates that the CO2 molecules could be encaged in small cavities of the sⅡ hydrate framework formed with THF.
The Raman spectroscopy of binary hydrate samples was measured to confirm the crystallographic structure and guest occupation in cage of hydrate. The results show that CO2 was existed in small cavity the sⅡ hydrate framework.