Published papers

Understandings of structure-based properties of porous materials, such as gas storage and gas separation performance, are important. Here, the crystal structures of the canonical structure II (sII) clathrate hydrates encapsulating cyclic molecules (tetrahydrofuran, cyclopentane, furan, and tetrahydropyran) are studied. To understand the effect of guest molecules on the host water framework, we performed powder X-ray diffraction measurements where the hydrate structures and guest distribution within 51264 cages were obtained by the direct-space technique followed by the Rietveld refinement. It was shown that the sizes of the 512 and 51264 cages of sII hydrates expand, as its unit-cell size is enlarged by the guest. In this process, it is revealed that the shape of 51264 cages with larger guest molecules became more spherical and volume ratio of empty small 512 cages in the unit cell decreases. Our findings from crystallographic point of view may give insights into better understanding of the thermodynamic stability and higher gas storage capacity of binary clathrate hydrates.

This paper demonstrates the diversity in crystal growth dynamics and crystal morphology of structure-H hydrate formed with methane and methylcyclohexane (MCH) correlated to the driving force. ΔTsub, which is the difference between the experimental temperature of the crystal growth and the phase equilibrium temperature, was used as an index of the driving force. At ΔTsub = 1.6 and 3.2 K, hydrate crystals were initially formed at the water/MCH interface and grew into the hydrate film covering the interface. After the entire interface was covered by the hydrate film, the hydrate growth stopped. The shape of individual hydrate crystals was polygonal. The size of hydrate crystals was smaller with increasing ΔTsub. At ΔTsub = 5.3 K, the first hydrate crystals were observed at the water/MCH interface and crystals grew not only along the interface but also toward the interior of MCH phase, even after the water/MCH interface was completely covered by the hydrate film. The dendritic hydrate crystals were observed at the interface at ΔTsub = 5.3 K. The implications of these observations on crystal morphology on determining the conditions for methane capture for the purpose of storage and transportation are discussed.

Since its first discovery, technologies utilizing the structure H (sH) clathrate hydrates have been proposed. To develop the technology, understanding the mechanism of thermophysical property manifestation would be of fundamental importance. It is reported in the literature that the molecular properties of guest molecules had a significant influence on the crystal structure and the resulting thermophysical properties of the sH hydrate. In the present study, the effect of linear N2 help-gas molecule on the crystal structure was experimentally and computationally investigated to focus on the shape of the guest molecule in the sH binary clathrate with neohexane. Powder X-ray diffraction (PXRD) measurements were performed in temperatures from 93 to 183 K. The lattice constant values on a and c axes, the cage occupancies of small 512 and 435663 cages, and guest distributions in the host cages were determined. Parrinello-Rahman molecular dynamics (MD) simulations were performed on the sH clathrate hydrate phase. The simulated values of the lattice constants indicated the same tendency as the experimental results. The MD simulations provided detailed descriptions of the molecular motions of guest molecules. On the basis of these results, the influence of a small guest molecule on the crystal structure of sH hydrate is discussed.

The hydrate-based thermal energy storage system is one of the most effective supplemental technologies to stabilize the energy supply for renewable energies. The system is expected to solve the global energy-management issues related to energy-demand increase and energy supply chain. In fact, this eco-friendly technology requires prospective hydrate candidates for the thermal energy storage media, followed by the extensive investigation of the thermophysical properties of the selected hydrates. In this study, to satisfy these requirements, the equilibrium temperature and the dissociation heat of tetrabutylphosphonium acrylate (TBPAc) ionic semi-clathrate hydrate were studied. It was found that the highest equilibrium temperature was 14.4 °C at the mass fractions from 0.37 to 0.40. It was also found that the largest dissociation heat was (210.4 ± 5.3) kJ kg−1 at the mass fraction 0.39. These values indicate that TBPAc hydrate could be a new promising thermal-storage medium for the general or data-centre air-conditioning systems. The experimental results of this study also revealed an underlying tendency between the thermophysical properties of the tetrabutylphosphonium (TBP) + carboxylate salt hydrates and the molar mass of the guest anion in salt.

In the present study, Raman spectroscopic measurements were performed using clathrate hydrate samples composed of ethanol, propan-1-ol, or propan-2-ol + deuterated methane + deuterium oxide. The temperature of the samples was stepwise increased from 93 K to 263 K in steps of 10 K or 20 K to decompose the clathrate hydrate samples during the measurements. The Raman spectra of the alcohols + deuterated methane hydrate sample in the C-H symmetric vibration region were similar to that of pure vapor alcohols, which indicates the weak guest alcohol-host water interaction although alcohols are hydrophilic compounds. In the propan-2-ol + deuterated methane + deuterium oxide system, the formation of two compounds having different composition from structure II propan-2-ol + deuterated methane clathrate hydrate was observed.

This paper reports the vapor-liquid-hydrate three phase equilibrium conditions of ionic semiclathrate hydrates formed in the system of water + tetra-n-butylammonium bromide pressurized with carbon dioxide. The three phase equilibrium temperatures were measured in the ranges from 278.8 to 287.4 K of temperature and from 0.05 to 0.45 of mass fraction of tetra-n-butylammonium bromide (wTBAB). Measurements were performed at the system pressures of 0.3 MPa, 0.6 MPa, and 1 MPa. For the three system pressures, the equilibrium temperatures increased with the increase in wTBAB for wTBAB < 0.35 and decreased for wTBAB > 0.40. The highest equilibrium temperatures were obtained at wTBAB = 0.35 and 0.40. The equilibrium temperatures of the tetra-n-butylammonium bromide + carbon dioxide hydrates were higher than those of the simple tetra-n-butylammonium bromide hydrates formed under atmospheric pressure. The improvement of the thermodynamic stability of the tetra-n-butylammonium bromide + carbon dioxide hydrates is ascribed to the incorporation of carbon dioxide molecules into the hydrate cages. The data obtained in the present study may be utilized to specify the thermodynamic conditions to form the tetra-n-butylammonium bromide + carbon dioxide hydrates without forming the simple tetra-n-butylammonium hydrates.

This paper reports the experimental study on measuring the phase equilibrium temperatures and the dissociation heats of ionic semiclathrate hydrate formed with tetrabutylphosphonium acetate (TBPAce) under atmospheric pressure. The phase equilibrium temperature and the dissociation heat measurements were performed in the range of the mass fraction of TBPAce aqueous solution from 0.10 to 0.42. The highest phase equilibrium temperature was 11.0 °C at the mass fraction range from 0.35 to 0.39. The largest dissociation heat of pure TBPAce hydrate was 192.0 kJ kg–1 at the mass fraction 0.36. The obtained results indicate that TBPAce hydrate would be suitable for a new thermal energy storage medium for general air conditioning systems.

This paper reports the experimental values of the interfacial tension between a gas mixture (CO2 + H2) and water measured with the pendant drop method. The measurements were conducted for temperatures from 271.2 K to 280.2 K and pressures up to 7.0 MPa, at increments of 1 MPa. These temperature-pressure ranges were nearly close to the (CO2 + H2) hydrate-stability-zone but each result is obtained without existence of any hydrate. The temperature dependency of the interfacial tension in this system could not be observed, but the interfacial tension decreased almost linearly with increasing pressure, indicating that the observed decrease was caused by an increase in adsorption of the gas phase onto the liquid water with pressure. The gradient of the interfacial tension, with respect to the pressure, was confirmed to be determined by the gas phase composition. The interaction between gases has a marginal influence on the interfacial tension between the (CO2 + H2) gas mixture and water.

The phase equilibrium conditions in the (propan-2-ol + carbon dioxide + water) system were measured. The measurements were performed using the batch, isochoric method at the temperatures between 250.8 K and 265.7 K. The equilibrium pressures were measured between 0.275 MPa and 0.995 MPa, and increased with increasing temperatures. Propan-2-ol is known as thermodynamic inhibitor for hydrate formation. However at the temperatures below 262 K, the equilibrium pressures in the (propan-2-ol + carbon dioxide + water) system are lower than those without propan-2-ol (carbon dioxide + water system). The measurements indicate the formation of the hydrates containing propan-2-ol and carbon dioxide in this system.

This paper reports the thermophysical properties of hydrates in the (krypton + 1-methylpiperidine, 2,2-dimethylbutane or methylcyclohexane + water) system. Phase equilibrium conditions were measured in the temperature ranges from 274.6. K to 297.3. K at the corresponding equilibrium pressures between 0.532. MPa and 6.246. MPa in the (krypton + 1-methylpiperidine + water) system. In the temperature ranges from 291.2. K to 297.0. K and 290.6. K to 296.8. K, the equilibrium pressure was between 4.035. MPa to 8.936. MPa and 3.993. MPa to 9.128. MPa, in the (krypton + 2,2-dimethylbutane + water) system and (krypton + methylcyclohexane + water) system, respectively. These results indicate that hydrates can be formed under the condition of lower temperature and pressure in the (krypton + 1-methylpiperidine + water) system than in the other two systems. The crystallographic structure of (krypton + 1-methylpiperidine) hydrate was thereby identified to be structure H at 183. K based on the Powder X-ray diffraction (PXRD) measurements. Dissociation heat of (krypton + 1-methylpiperidine) hydrate was estimated to be 73.5. kJ/mol, that is smaller than that of structure II hydrates containing krypton. These results support the potential for (krypton + 1-methylpiperidine) structure H hydrate to be utilized in hydrate-based technologies.