Small Guest-Free Structure II Hydrate Enclathrating Isoxazole

Clathrate hydrates hold promising potential as porous materials for energy storage and conversion, featuring safer and environmentally friendly alternatives to existing hydrate systems. Here, we report the impact of isoxazole, a five-membered molecule with two heteroatoms, on various clathrate hydrate systems, including methane (CH₄), carbon dioxide (CO₂), and ethane (C₂H₆) small guest molecules. We demonstrate that the isoxazole molecule forms structure II (sII) hydrates with CH₄ and CO₂ and acts as a thermodynamic inhibitor for structure I C₂H₆ hydrate. Interestingly, our results show that isoxazole forms an sII hydrate with C₂H₆ under low-pressure and low-temperature conditions. Moreover, it can form a small guest-free sII hydrate with a vacant 5¹² cage at much lower temperatures than those required for forming tetrahydrofuran and cyclopentane hydrates. Our findings demonstrate that sII (isoxazole) hydrate exhibits no or very weak interaction between isoxazole and the water framework, unlike tetrahydrofuran hydrate implying guest–host interaction.