Complex Structure Transition Induced by Swapping Process Occurring in Natural Gas Hydrate Layer

In this study, we examine the swapping pattern occurring between two external guest molecules, N2 and CO2, and crystalline (sI and sII) CH4 hydrate. For sI methane hydrate, the spectroscopic result shows that N2 molecules mainly attack CH4 molecules occupying small cages while CO2 molecules play an active role in replacing most of CH4 molecules in large cages. In the other case for sII and sH methane hydrates, we observe the spontaneous structure transition of sII to sI during replacement and cage-specific distribution of guest molecules. We present here that under strong attacks of external CH4 guest molecules the sII and sH methane hydrates are structurally transformed to the crystalline framework of sI, leading to favorable change of the lattice dimension of the host-guest networks.