ABSTRACT:

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.

INTRODUCTION

Although numerous hydrate studies, covering both macroscopic and microscopic approaches, have recently been conducted for a variety of purposes, and to a certain extent have yielded notable success, little attention has been paid to cage dynamics exploring guest distributions within the sensitive host-guest networks. Moreover, the complex hydrate behavior occurring under strong attacks of external guest molecules to the existing cages has not yet been fully considered, and no detailed study exists even at a very fundamental level. In our previous study, we explored the replacement mechanism of CH4 hydrate with CO2 using spectroscopic methods and found that when a CH4 hydrate is exposed to gas mixtures containing CO2, CH4 is replaced by CO2 in mainly of the large cages (Lee, H., 2003). If the CH4 hydrates could be converted into CO2 hydrates, they would serve double duty as CH4 sources and CO2 storage sites. Here, we further extend our investigations to consider the occurrence of CO2 replacement phenomena on sII hydrate, which is thought to exist in the seabed.

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