The storage of large quantities of hydrogen at safe pressures is a key factor in establishing a hydrogen-based economy. Previous strategies--where hydrogen has been bound chemically, adsorbed in materials with permanent void space or stored in hybrid materials that combine these elements--have problems arising from either technical considerations or materials cost. A recently reported clathrate hydrate of hydrogen exhibiting two different-sized cages does seem to meet the necessary storage requirements; however, the extreme pressures (approximately 2 kbar) required to produce the material make it impractical. The synthesis pressure can be decreased by filling the larger cavity with tetrahydrofuran (THF) to stabilize the material, but the potential storage capacity of the material is compromised with this approach. Here we report that hydrogen storage capacities in THF-containing binary-clathrate hydrates can be increased to approximately 4 wt% at modest pressures by tuning their composition to allow the hydrogen guests to enter both the larger and the smaller cages, while retaining low-pressure stability. The tuning mechanism is quite general and convenient, using water-soluble hydrate promoters and various small gaseous guests.

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...

Natural gas hydrates are a potential source of energy and may play a role in climate change and geological hazards. Most natural gas hydrate appears to be in the form of 'structure I', with methane as the trapped guest molecule, although 'structure II' hydrate has also been identified, with guest molecules such as isobutane and propane, as well as lighter hydrocarbons. A third hydrate structure, 'structure H', which is capable of trapping larger guest molecules, has been produced in the laboratory, but it has not been confirmed that it occurs in the natural environment. Here we characterize the structure, gas content and composition, and distribution of guest molecules in a complex natural hydrate sample recovered from Barkley canyon, on the northern Cascadia margin. We show that the sample contains structure H hydrate, and thus provides direct evidence for the natural occurrence of this hydrate structure. The structure H hydrate is intimately associated with structure II hydrate, and the two structures contain more than 13 different hydrocarbon guest molecules. We also demonstrate that the stability field of the complex gas hydrate lies between those of structure II and structure H hydrates, indicating that this form of hydrate is more stable than structure I and may thus potentially be found in a wider pressure-temperature regime than can methane hydrate deposits.

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