Thermodynamic and structural features of chlorodifluoromethane (a sI–sII dual hydrate former) + external guest (N2 or CH4) hydrates and their significance for greenhouse gas separation

Abstract

In this study, a new sI–sII dual hydrate former [chlorodifluoromethane (CHClF₂); an important greenhouse gas with a global warming potential of 1810], which forms sI hydrate by itself and forms sII hydrate in the presence of external help guests such as CH₄ and N₂, was introduced and closely investigated for its potential significance in gas hydrate-based gas separation. The phase equilibria of CHClF₂ hydrate, binary CHClF₂ (5%) + N₂ (95%) hydrate, and binary CHClF₂ (5%) + CH₄ (95%) hydrate were measured to examine the formation conditions and thermodynamic stability regions of CHClF₂ + external guest hydrates. Nuclear magnetic resonance and in situ Raman spectroscopic results confirmed the formation of sII hydrates for CHClF₂ + external guest (N₂ or CH₄) mixtures. Powder X-ray diffraction patterns clearly demonstrated a structural transition of sI to sII hydrates and a preferential incorporation of CHClF₂ molecules in the hydrate phase when external guests (N₂ or CH₄) were involved in CHClF₂ hydrate formation. The measured dissociation enthalpy values of CHClF₂ hydrate, binary CHClF₂ (5%) + N₂ (95%) hydrate, and binary CHClF₂ (5%) + CH₄ (95%) hydrate using a high-pressure micro-differential scanning calorimeter also indicated preferential CHClF₂ enclathration. The experimental results provide new insights into the thermodynamic and structural features of the CHClF₂ (sI–sII dual hydrate former) + external guest hydrates for understanding and designing the hydrate-based CHClF₂ separation process.