Large amounts of CH4 in the form of solid hydrates are stored on continental margins and in permafrost regions. If these CH4 hydrates could be converted into CO 2 hydrates, they would serve double duty as CH4 sources and CO2 storage sites. We explore here the swapping phenomenon occurring in structure I (sI) and structure II (sII) CH 4 hydrate deposits through spectroscopic analyses and its potential application to CO2 sequestration at the preliminary phase. The present 85% CH4 recovery rate in sI CH4 hydrate achieved by the direct use of binary N 2 ؉ CO2 guests is surprising when compared with the rate of 64% for a pure CO 2 guest attained in the previous approach. The direct use of a mixture of N2 ؉ CO2 eliminates the requirement of a CO2 separation͞purification process. In addition, the simultaneously occurring dual mechanism of CO 2 sequestration and CH4 recovery is expected to provide the physicochemical background required for developing a promising large-scale approach with economic feasibility. In the case of sII CH 4 hydrates, we observe a spontaneous structure transition of sII to sI during the replacement and a cage-specific distribution of guest molecules. A significant change of the lattice dimension caused by structure transformation induces a relative number of small cage sites to reduce, resulting in the considerable increase of CH 4 recovery rate. The mutually interactive pattern of targeted guestcage conjugates possesses important implications for the diverse hydrate-based inclusion phenomena as illustrated in the swapping process between CO2 stream and complex CH4 hydrate structure.clathrate ͉ CO2 sequestration ͉ methane ͉ swapping phenomenon ͉ NMR B ecause the total amount of natural gas hydrate was estimated to be about twice as much as the energy contained in fossil fuel reserves (1, ʈ), many researchers have tried to find a way to exploit CH 4 hydrates deposited worldwide as a new energy source. For recovering them at various conditions in an efficient way, several strategies such as thermal treatment, depressurization, and inhibitor addition into the hydrate layer have been proposed (2). However, all of these methods are based on the decomposition of CH 4 hydrate by external stimulation, which can trigger catastrophic slope failures (3). Furthermore, if CH 4 hydrate decomposes rapidly, it is also possible that the CH 4 released from the hydrate could transfer to the air and significantly accelerate the greenhouse effect (4).Recently, the replacement of CH 4 hydrate with CO 2 has been suggested as an alternative option for recovering CH 4 gas. When CO 2 itself is put under certain pressure, a solid CO 2 hydrate can be formed according to the stability regime (5). In addition, the formation condition of CO 2 hydrate is known to be more stable than that of CH 4 hydrate. Therefore, the swapping process between two gaseous guests is considered to be a favorable approach toward long-term storage of CO 2 . It not only enables the ocean floor to remain stabilized even after recovering the CH 4 ga...

On the basis of crystallographic analysis results, a recent study reported that structure H (sH) hydrate exists in the natural environment, providing direct evidence from hydrate samples recovered from Barkley canyon, on the northern Cascadia margin. It was further indicated that sH is more stable than sI and may thus potentially be found in a wider pressure−temperature regime than are methane hydrate deposits. Accordingly, it is worthwhile to examine whether a swapping process can spontaneously occur between gaseous CO2/(N2 + CO2) and sH (isopentane + CH4) gas hydrate. From high-power decoupling 13C NMR and Raman spectra, we observed the structural transition of sH to sI hydrate. It was found that N2 molecules considerably promoted this structural transition during swapping, because N2 molecules prefer to attack CH4 molecules imprisoned in small cages. Due to this favorable structural transition and N2-induced guest exchange, more than 92% CH4 can be recovered from methane hydrate deposits. The microscopic and macroscopic phenomena together imply that the swapping process between carbon dioxide and methane can be effectively used in the recovery of energy resources that are widely deposited in deep ocean sediments as well as for the sequestration of carbon dioxide to the methane hydrate layer.

Natural methane hydrates occurring in marine clay sediments exhibit heterogeneous phase behavior with high complexity, particularly in the negatively charged interlayer region. To date, the real clay interlayer effect on natural methane hydrate formation and stability remains still much unanswered, even though a few computer simulation and model studies are reported. We first examined the chemical shift difference of 27Al, 29Si, and 23Na between dry clay and clay containing intercalated methane hydrates (MH) in the interlayer. We also measured the solid-state 13C MAS NMR spectra of MH in Na-montmorillonite (MMT) and Ca-montmorillonite (MMT) to reveal abnormal methane popularity established in the course of intercalation and further performed cryo-TEM and XRD analyses to identify the morphology and layered structure of the intercalated methane hydrate. The present findings strongly suggest that the real methane amount contained in natural MH deposits should be reevaluated under consideration of the compositional, structural, and physical characteristics of clay-rich sediments. Furthermore, the intercalated methane hydrate structure should be seriously considered for developing the in situ production technologies of the deep-ocean methane hydrate.