Distortion of the Large Cages Encapsulating Cyclic Molecules and Empty Small Cages of Structure II Clathrate Hydrates

Understandings of structure-based properties of porous materials, such as gas storage and gas separation performance, are important. Here, the crystal structures of the canonical structure II (sII) clathrate hydrates encapsulating cyclic molecules (tetrahydrofuran, cyclopentane, furan, and tetrahydropyran) are studied. To understand the effect of guest molecules on the host water framework, we performed powder X-ray diffraction measurements where the hydrate structures and guest distribution within 5¹²6⁴ cages were obtained by the direct-space technique followed by the Rietveld refinement. It was shown that the sizes of the 5¹² and 5¹²6⁴ cages of sII hydrates expand, as its unit-cell size is enlarged by the guest. In this process, it is revealed that the shape of 5¹²6⁴ cages with larger guest molecules became more spherical and volume ratio of empty small 5¹² cages in the unit cell decreases. Our findings from crystallographic point of view may give insights into better understanding of the thermodynamic stability and higher gas storage capacity of binary clathrate hydrates.