Structural Transition of the Methane-Ethane Mixture Hydrate in a Hydrate/Water/Hydrocarbon Three-Phase Coexistence System: Effect of Gas Concentration

Hydrate formation of a natural gas mixture is fascinating. Whereas both pure methane and ethane form a structure I (sI) hydrate, their mixture may form a structure II (sII) hydrate at certain compositions. Here, we investigated the underlying mechanisms of the methane-ethane mixture hydrate structural transition using an sII-hydrate-water-hydrocarbon three-phase interface system. The results indicate that sII hydrate formation is a function of methane concentration with a maximum at a mole concentration of 83-90%. In contrast, a significant amount of the sI hydrate forms at concentrations below 75% or equal to 100%. In addition, we unveil the mechanism of the rarely discussed structure conversion from sII to sI. The 15-hedron (mainly 5(12)6(3)) cages, which may originate from both aqua and the 16-hedron (5(12)6(4)) cage transition, play a vital role as a transitional bridge in the conversion at the transition interface. Owing to the specific structure of 15-hedron cages, the 14-hedron (5(12)6(2)) cages can grow, and thus, the sI hydrate grows. Furthermore, the methane occupancy in the 14-hedron cages and the concentration for the energetically favorable formation of the sII structure were quantified based on the formation energies for hydrates with methane and ethane molecules trapped inside. We show that the adsorption preference of guest molecules at the interfaces or cages can be attributed to the difference between the formation energy prediction and the reaction process results. Our findings have both academic and engineering significance for sustainable chemistry related to flow assurance, gas separation, gas transportation, and natural hydrate development in reservoirs with mixed gas hydrate structures.