Discovery of the final primitive Frank-Kasper phase of clathrate hydrates.

In weakly bound materials such as water, one of the three primitive Frank-Kasper (FK) phases, the Z phase, is long absent due to the relatively unstable framework. The Z phase in clathrate hydrate, which is known as the HS-I structure, has now been found by precise tuning of the molecular guest structure. In the crystal structure, the never stabilized combination water cage of two 15-hedra and two 14-hedra formed with its original symmetries, providing sufficient gas capacity to the 12-hedral cages.

Guest size effects on a robust structure of semiclathrate hydrates and their thermophysical properties.

The ability to tune the pore size, shape, and functionality of semiclathrate hydrates, host-guest materials formed from aqueous solutions of ionic guest materials and water, makes them attractive materials for thermal storage and gas storage applications. The flexibility of semi-clathrate hydrates and their guest-molecule-dependent reactions produce these unexpected and desirable properties. As an ionic guest, tetra--butylammonium cation is known for best-fit in hydrogen-bonded water structures.

Designing the structure and relevant properties of semiclathrate hydrates by partly asymmetric alkylammonium salts.

Semiclathrate hydrates are host-guest materials that form from ionic guests and water. There are numerous options for ionic guests, such as quaternary ammonium salts, to tune the functional properties of these materials such as melting temperature, fusion heat, and gas capacity and selectivity. To design these materials, the stabilization mechanism of the side chains of quaternary ammonium salts must be understood based on both thermodynamic and crystallographic properties and relevant host-guest dynamics.

Superheating of Structure I Gas Hydrates within the Structure II Cyclopentane Hydrate Shell.

The superheated state of methane (CH) hydrate that exists under the surface ice layer can persist for considerable lengths of time, which showed promise as a method for storing and transporting natural gas. This study extends this further by coating sI CH hydrate with one of several sII hydrates, thus eliminating the need for a defect-free continuous interface between the sI and sII hydrates. Gas hydrate crystals were kept intact above their dissociation temperature by immersing them in liquid cyclopentane (CP), as observed with powder X-ray diffraction and X-ray CT methods.

Recovery of NO: Energy-Efficient and Structure-Driven Clathrate-Based Greenhouse Gas Separation.

NO has 300 times more global warming potential than CO and is also one of the main stratospheric ozone-depleting substances emitted by human activities such as agriculture, industry, and the combustion of fossil fuels and solid waste. We present here an energy-efficient clathrate-based greenhouse gas-separation (CBGS) technology that can operate at room temperature for selectively recovering NO from gas mixtures. Clathrate formation between α-form/β-form hydroquinone (α-HQ/β-HQ) and gas mixtures reveals guest-specific and structure-driven selectivity, revealing the preferential capture of NO in β-HQ and the molecular sieving characteristics of α-HQ.

X-Ray attenuation and image contrast in the X-ray computed tomography of clathrate hydrates depending on guest species.

In this study, X-ray imaging of inclusion compounds encapsulating various guest species was investigated based on the calculation of X-ray attenuation coefficients. The optimal photon energies of clathrate hydrates were simulated for high-contrast X-ray imaging based on the type of guest species. The proof of concept was provided by observations of Kr hydrate and tetra-n-butylammonium bromide (TBAB) semi-clathrate hydrate using absorption-contrast X-ray computed tomography (CT) and radiography with monochromated synchrotron X-rays.

Correction: X-ray CT observation and characterization of water transformation in heavy objects.

Correction for 'X-ray CT observation and characterization of water transformation in heavy objects' by Satoshi Takeya et al., Phys. Chem.

CO Capture from Flue Gas Based on Tetra--butylammonium Fluoride Hydrates at Near Ambient Temperature.

Semiclathrate hydrates of tetra--butylammonium fluoride (TBAF) are potential CO capture media because they can capture CO at near ambient temperature under moderate pressure such as below 1 MPa. In addition to other semiclathrate hydrates, CO capture properties of TBAF hydrates may vary with formation conditions such as aqueous composition and pressure because of their complex hydrate structures. In this study, we investigated CO capture properties of TBAF hydrates for simulated flue gas, that is, CO + N gas, by the gas separation test with three different parameters for each pressure and aqueous composition of TBAF in mass fraction ( ).

X-ray CT observation and characterization of water transformation in heavy objects.

Nondestructive observations and characterization of low-density materials composed of low-Z elements, such as water or its related substances, are essential for materials and life sciences. However, visualizing these compounds and their phase changes is still challenging. In this study, an approach to X-ray imaging of water-related substances in heavy objects without the use of contrast agents is proposed.

Thermodynamic Properties and Crystallographic Characterization of Semiclathrate Hydrates Formed with Tetra--butylammonium Glycolate.

Semiclathrate hydrates are a crystalline host-guest material, which forms with water and ionic substances such as tetra--butylammonium (TBA) salts. Various anions can be used as a counter anion to the TBA cation, and they can modify thermodynamic properties of the semiclathrate hydrates, which are critical for applications, for example, cold energy storage and gas separation. In this study, the semiclathrate hydrates of the TBA glycolate were newly synthesized.

Anisotropy of dodecahedral water cages for guest gas occupancy in semiclathrate hydrates.

Anisotropic dodecahedral (D) water cages found in semiclathrate hydrates have unique gas selectivity due to their varied shapes. Herein, the D cages incorporating ideally isotropic rare gases, i.e.

Structure-driven CO selectivity and gas capacity of ionic clathrate hydrates.

Ionic clathrate hydrates can selectively capture small gas molecules such as CO, N, CH and H. We investigated CO + N mixed gas separation properties of ionic clathrate hydrates formed with tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), tetra-n-butylphosphonium bromide (TBPB) and tetra-n-butylphosphonium chloride (TBPC). The results showed that CO selectivity of TBAC hydrates was remarkably higher than those of the other hydrates despite less gas capacity of TBAC hydrates.

Selective occupancy of methane by cage symmetry in TBAB ionic clathrate hydrate.

Methane trapped in the two distinct dodecahedral cages of the ionic clathrate hydrate of TBAB was studied by single crystal XRD and MD simulation. The relative CH4 occupancies over the cage types were opposite to those of CO2, which illustrates the interplay between the cage symmetry and guest shape and dynamics, and thus the gas selectivity.

Hydration structures of lactic acid: characterization of the ionic clathrate hydrate formed with a biological organic acid anion.

Ionic clathrate hydrates are water-based materials that have unique properties, such as a wide range of melting temperatures and high gas capacities. In their structure, water molecules coordinate around ionic substances, which is regarded as the actual hydration structure and also linking of the hydrate clusters, giving insight into the dynamics of the water molecules and ions. This paper reports the synthesis and characterization of the ionic clathrate hydrate of tetra-n-butylammonium lactate (TBAL), the anion of which is a biological organic material.

Clathrate hydrates for ozone preservation.

We report the experimental evidence for the preservation of ozone (O(3)) encaged in a clathrate hydrate. Although ozone is an unstable substance and is apt to decay to oxygen (O(2)), it may be preserved for a prolonged time if it is encaged in hydrate cavities in the form of isolated molecules. This possibility was assessed using a hydrate formed from an ozone + oxygen gas mixture coexisting with carbon tetrachloride or xenon.