Highly selective encaging of carbon dioxide molecules in the mixed carbon dioxide and nitrogen hydrate at low temperatures

The structural identification and guest compositions of the mixed CO ₂ and N₂ hydrates at low temperature conditions were investigated by both theoretical predictions and experimental measurements. From the model calculations, at very low temperatures, the highly CO ₂-concentrated hydrates over 95 mol % CO₂ on the basis of water-free concentration could coexist with the gas mixtures of low CO ₂ concentrations in equilibrium. X-ray diffraction measurements of the hydrates formed with the gas mixture of 3.16 mol % CO₂ and balanced N₂ indicate that the formed hydrates at all conditions considered in this study were identified as structure I, whereas the model predicts a structural transition to structure II around 220 K. However, it was also found that the formed hydrate samples contain a considerable amount of hexagonal ice resulting from incomplete conversion of ice to the hydrates. The compositional analysis suggests that a favorable encaging of CO₂ in the mixed hydrate can be obtained by the hydrate formation at low temperatures and relative amount of CO₂ molecules in the mixed hydrates increases with a decrease of temperature.