Thermodynamic Stability of Structure II Methyl Vinyl Ketone Binary Clathrate Hydrates and Effects of Secondary Guest Molecules on Large Guest Conformation.

Clathrate hydrates have received massive attention because of their potential application as energy storage materials. Host water frameworks of clathrate hydrates provide empty cavities that can capture not only small molecular guests but also radical species induced by γ-irradiation. In this work, we investigated structure II methyl vinyl ketone (MVK) binary clathrate hydrates with CH, O, and N and the effects of secondary guest species on MVK conformation in the cavity of hydrate and on the thermodynamic stability of unirradiated and γ-irradiated hydrate phases.

Spectroscopic Observation of the Hydroxy Position in Butanol Hydrates and Its Effect on Hydrate Stability.

In this study, we investigate the crystal structures and phase equilibria of butanols+CH +H O systems to reveal the hydroxy group positioning and its effects on hydrate stability. Four clathrate hydrates formed by structural butanol isomers are identified with powder X-ray diffraction (PXRD). In addition, Raman spectroscopy is used to analyze the guest distributions and inclusion behaviors of large alcohol molecules in these hydrate systems.

Incorporation of ammonium fluoride into clathrate hydrate lattices and its significance in inhibiting hydrate formation.

The stability of hydrate frameworks is influenced by guest molecules capable of hydrogen bonding with surrounding water molecules. Four remarkable features from the ammonium fluoride incorporation into a crystalline hydrate matrix provide important information on the thermodynamic stability, formation kinetics, structural characteristics, and molecular behavior in clathrate hydrate systems.

Kinetics of methane hydrate replacement with carbon dioxide and nitrogen gas mixture using in situ NMR spectroscopy.

In this study, the kinetics of methane replacement with carbon dioxide and nitrogen gas in methane gas hydrate prepared in porous silica gel matrices has been studied by in situ (1)H and (13)C NMR spectroscopy. The replacement process was monitored by in situ (1)H NMR spectra, where about 42 mol % of the methane in the hydrate cages was replaced in 65 h. Large amounts of free water were not observed during the replacement process, indicating a spontaneous replacement reaction upon exposing methane hydrate to carbon dioxide and nitrogen gas mixture.

Structural transformation and tuning behavior induced by the propylamine concentration in hydrogen clathrate hydrates.

The structures and the guest-host distributions of iso-propylamine (i-PA) and n-propylamine (n-PA) hydrates with hydrogen as a secondary guest were identified by powder X-ray diffraction and Raman spectroscopic analysis. The structure of 11.1 mol% i-PA + H2 hydrate was identified to be hexagonal (space group P63/mmc) with a few unindexed diffraction peaks, while 5.

Unexpected carbon dioxide inclusion in water-saturated pores of metal-organic frameworks with potential for highly selective capture of CO2.

Unusual CO2 storage in water-saturated MOFs was investigated by combining experiment and simulation. It was found that the micropores of HKUST-1 saturated with water provide an environment that is thermodynamically and kinetically favorable for CO2 capture, but not for N2 and H2 capture. We expect that this phenomenon have potential to be used for successful separation of CO2 from versatile flue streams and pre-combustion gas.

Nondestructive natural gas hydrate recovery driven by air and carbon dioxide.

Current technologies for production of natural gas hydrates (NGH), which include thermal stimulation, depressurization and inhibitor injection, have raised concerns over unintended consequences. The possibility of catastrophic slope failure and marine ecosystem damage remain serious challenges to safe NGH production. As a potential approach, this paper presents air-driven NGH recovery from permeable marine sediments induced by simultaneous mechanisms for methane liberation (NGH decomposition) and CH₄-air or CH₄-CO₂/air replacement.

Inhibited phase behavior of gas hydrates in graphene oxide: influences of surface and geometric constraints.

Porous materials have provided us unprecedented opportunities to develop emerging technologies such as molecular storage systems and separation mechanisms. Pores have also been used as supports to contain gas hydrates for the application in gas treatments. Necessarily, an exact understanding of the properties of gas hydrates in confining pores is important.

Experimental verification of methane-carbon dioxide replacement in natural gas hydrates using a differential scanning calorimeter.

The methane (CH4) - carbon dioxide (CO2) swapping phenomenon in naturally occurring gas hydrates is regarded as an attractive method of CO2 sequestration and CH4 recovery. In this study, a high pressure microdifferential scanning calorimeter (HP μ-DSC) was used to monitor and quantify the CH4 - CO2 replacement in the gas hydrate structure. The HP μ-DSC provided reliable measurements of the hydrate dissociation equilibrium and hydrate heat of dissociation for the pure and mixed gas hydrates.

Multiple guest occupancy in clathrate hydrates and its significance in hydrogen storage.

We report a new concept of structural transformation combined with tuning phenomena which together result in a significant increase in the hydrogen storage capacity in an icy material. It is necessary to investigate the use of a fully water-soluble structure H (sH) former so as to observe how hydrogen molecules are stably loaded into hydrate cages.

Electric double-layer capacitor based on an ionic clathrate hydrate.

Herein, we suggest a new approach to an electric double-layer capacitor (EDLC) that is based on a proton-conducting ionic clathrate hydrate (ICH). The ice-like structures of clathrate hydrates, which are comprised of host water molecules and guest ions, make them suitable for applications in EDLC electrolytes, owing to their high proton conductivities and thermal stabilities. The carbon materials in the ICH Me4NOH⋅5 H2O show a high specific capacitance, reversible charge-discharge behavior, and a long cycle life.

Recovery of methane from gas hydrates intercalated within natural sediments using CO(2) and a CO(2)/N(2) gas mixture.

The direct recovery of methane from massive methane hydrates (MHs), artificial MH-bearing clays, and natural MH-bearing sediments is demonstrated, using either CO(2) or a CO(2)/N(2) gas mixture (20 mol % of CO(2) and 80 mol % of N(2), reproducing flue gas from a power plant) for methane replacement in complex marine systems. Natural gas hydrates (NGHs) can be converted into CO(2) hydrate by a swapping mechanism. The overall process serves a dual purpose: it is a means of sustainable energy-source exploitation and greenhouse-gas sequestration.

Atomic hydrogen production from semi-clathrate hydrates.

Atomic hydrogen has received recent attention because of its potential role in energy devices, silicon devices, artificial photosynthesis, hydrogen storage, and so forth. Here, we propose a highly efficient route for producing atomic hydrogen using semi-clathrate hydrates. Two major hydrogen radical sources, derived from guest/host materials, are closely examined.

Superexchange-like interaction of encaged molecular oxygen in nitrogen-doped water cages of clathrate hydrates.

Clathrate hydrates are a highly prospective material in energy and environmental fields, but the inherent nature of inclusion phenomena occurring in the stacked water cages has not been completely resolved yet. Investigating the magnetism of guest molecules is a new experimental approach in clathrate hydrate research to open the possibility of icy magnetic applications as a novel material as well as to understand the unrevealed host-guest interactions in icy inclusion compounds. In this study, we observed an indirect spin coupling between encaged dioxygen molecules via a nonmagnetic water framework through the measurement of guest magnetization.

Abnormal thermal expansion of clathrate hydrates induced by asymmetric guest molecules.

We investigated for the first time the abnormal thermal expansion induced by an asymmetric guest structure using high-resolution neutron powder diffraction. Three dihydrogen molecules (H(2), D(2), and HD) were tested to explore the guest dynamics and thermal behavior of hydrogen-doped clathrate hydrates. We confirmed the restricted spatial distribution and doughnut-like motion of the HD guest in the center of anisotropic sII-S (sII-S=small cages of structure II hydrates).

Guest gas enclathration in semiclathrates of tetra-n-butylammonium bromide: stability condition and spectroscopic analysis.

In this study, guest gas enclathration behavior in semiclathrates of tetra-n-butylammonium bromide (TBAB) was closely investigated through phase equilibrium measurement and spectroscopic analysis. The three-phase equilibria of semiclathrate (H), liquid water (L(W)), and vapor (V) for the ternary CH(4) + TBAB + water and CO(2) + TBAB + water mixtures with various TBAB concentrations were experimentally measured to determine the stability conditions of the double TBAB semiclathrates. Equilibrium dissociation temperatures for pure TBAB semiclathrate were also measured at the same concentrations under atmospheric conditions.

Thermal expansivity of ionic clathrate hydrates including gaseous guest molecules.

Although thermal expansion is a key factor in relation to the host-guest interaction of clathrate hydrates, few studies have investigated the thermal behavior of ionic clathrate hydrates. The existence of ionic species in these hydrates creates a unique host-guest interaction compared to that of nonionic clathrate hydrates. It was revealed that X-ray diffraction cannot be used for research of tetramethylammonium hydroxide clathrate hydrates due to damage of the cations by the X-ray, which results in abnormal thermal expansion of the ionic clathrate hydrates.

Direct observation of atomic hydrogen generated from the water framework of clathrate hydrates.

We demonstrate that the N(2)-induced ionic hydrate system can be a solution to produce the hydrogen radical from water without direct energy sources such as H(2) and CH(4).

Superoxide ions entrapped in water cages of ionic clathrate hydrates.

In the present work, we first described the stable entrapment of the superoxide ions in gamma-irradiated (Me(4)NOH + O(2)) clathrate hydrate. Owing to peculiar direct guest-guest ionic interaction, the lattice structure of gamma-irradiated (Me(4)NOH + O(2)) clathrate hydrate shows significant change of lattice contraction behavior even at relatively high temperature (120 K). Such findings are expected to provide useful information for a better understanding of unrevealed nature (such as icy nanoreactor concept, ice-based functional material synthesis and lattice tuning by specific ionic guests) of clathrate hydrate fields.

Molecular cage occupancy of clathrate hydrates at infinite dilution: experimental determination and thermodynamic significance.

This study focuses on the cage occupancy of guest molecules in the infinitely dilute state. At the extreme conditions of highly diluted guest concentrations the direct measurements of the cage occupancy ratio representing the competitive inclusion of multiguest species appear to be so difficult because of spectroscopic intensity limitation, but its thermodynamic significance might be considerable due to the fact that the infinite-dilution value of the cage occupancy ratio can provide the valuable thermodynamic information as a very unique and guest-specific parameter. To experimentally identify gaseous guest populations in structure I (sI) and structure II (sII) cages, we used the solid-state nuclear magnetic resonance (NMR), gas chromatography, and direct gas measurements.

Physicochemical properties of ionic clathrate hydrates.

Ionic clathrate hydrates are known to be formed by the enclathration of hydrophobic cations or anions into confined cages and the incorporation of counterions into the water framework. As the ionic clathrate hydrates are considered for their potential applicability in various fields, including those that involve solid electrolytes, gas separation, and gas storage, numerous studies of the ionic clathrate hydrates have been reported. This review concentrates on the physicochemical properties of the ionic clathrate hydrates and the notable characteristics of these materials regarding their potential application are addressed.

Spectroscopic identification of amyl alcohol hydrates through free OH observation.

In this study, we identify the crystal structures of amyl alcohol + CH(4) hydrates and demonstrate that the free OH observation of alcohol hydrates provides evidence of OH incorporation into the host framework occurring in some amyl alcohols. While two amyl alcohols, 3-methyl-2-butanol and 2-methyl-2-butanol, were identified as encaged in the 5(12)6(8) large cage of structure-H hydrate, as expected from their molecular sizes above 7.5 A, two other amyl alcohols, 3-methyl-1-butanol and 2,2-dimethyl-1-propanol, were identified to be abnormally included in the 5(12)6(4) large cage of structure-II hydrate in spite of their too large sizes of 9.

Tuning the composition of guest molecules in clathrate hydrates: NMR identification and its significance to gas storage.

Gas hydrates represent an attractive way of storing large quantities of gas such as methane and carbon dioxide, although to date there has been little effort to optimize the storage capacity and to understand the trade-offs between storage conditions and storage capacity. In this work, we present estimates for gas storage based on the ideal structures, and show how these must be modified given the little data available on hydrate composition. We then examine the hypothesis based on solid-solution theory for clathrate hydrates as to how storage capacity may be improved for structure II hydrates, and test the hypothesis for a structure II hydrate of THF and methane, paying special attention to the synthetic approach used.