Formation of the structure-II gas hydrate from low-concentration propane mixed with methane

It has been recognized that a small amount of propane mixed with methane can change greatly in not only the thermodynamics but also the structural properties of gas hydrate. However, its mechanism is still not well understood yet. In this research, structure-II (sII) hydrate is synthesized using a methane-propane gas mixture with an initial mole ratio of 99:1, and it is found that large (51264) cages are co-occupied by multiple gases based on the rigid structure analysis of neutron diffraction data. The first principles calculation and molecular dynamics simulation are conducted to uncover the molecular mechanism for sII methane-propane hydrate formation, revealing that the presence of propane inhibits the formation of structure-I (sI) hydrate but promotes sII hydrate formation. The results help to understand the accumulation mechanism of natural gas hydrate and benefit to optimize the condition for gas storage and transportation in hydrate form.