Structure-driven CO₂ selectivity and gas capacity of ionic clathrate hydrates

Ionic clathrate hydrates can selectively capture small gas molecules such as CO₂, N₂, CH₄ and H₂. We investigated CO₂ + N₂ mixed gas separation properties of ionic clathrate hydrates formed with tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), tetra-n-butylphosphonium bromide (TBPB) and tetra-n-butylphosphonium chloride (TBPC). The results showed that CO₂ selectivity of TBAC hydrates was remarkably higher than those of the other hydrates despite less gas capacity of TBAC hydrates. The TBAB hydrates also showed irregularly high CO₂ selectivity at a low pressure. X-ray diffraction and Raman spectroscopic analyses clarified that TBAC stably formed the tetragonal hydrate structure, and TBPB and TBPC formed the orthorhombic hydrate structure. The TBAB hydrates showed polymorphic phases which may consist of the both orthorhombic and tetragonal hydrate structures. These results showed that the tetragonal hydrate captured CO₂ more efficiently than the orthorhombic hydrate, while the orthorhombic hydrate has the largest gas capacity among the basic four structures of ionic clathrate hydrates. The present study suggests new potential for improving gas capacity and selectivity of ionic clathrate hydrates by choosing suitable ionic guest substances for guest gas components.