Microstructural investigation of morphology and kinetics of methane hydrate in the presence of tetrabutylammonium bromide: Insights for preservation and inhibition.

Developing highly efficient methane (CH4) hydrate storage methods and understanding the hydrate dissociation kinetics can contribute to advancing CH4 gas storage and transport. The effects of tetrabutylammonium bromide (TBAB) (a thermodynamic promoter) addition on the kinetics of CH4 hydrate were evaluated on the microscopic scale using synchrotron x-ray computed tomography (CT) and powder x-ray diffraction. Microscopic observations showed that a 5 wt. % TBAB solution facilitated the nucleation of CH4 hydrate owing to the initial growth of TBAB semi-clathrate hydrate particles. The CH4 hydrate crystals in the CH4 + TBAB hydrate sample were sponge-like with many internal pores and exhibited slightly enhanced self-preservation compared to the pure CH4 hydrate, both in the bulk and after pulverization to a fine powder. This study demonstrates the feasibility of controlling the rate of CH4 hydrate formation and preservation by using aqueous TBAB solutions in CH4 hydrate formation.

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-n-butylammonium cation is known for best-fit in hydrogen-bonded water structures. Few investigations have been conducted for other cations, while there are numerous candidates. Relationships between the molecular structures of ionic guest substances and their hydrate structure and relevant thermodynamic properties are yet to be understood. In this study, the semiclathrate hydrates formed with two variations of tetra-n-butylammonium chloride (N4444Cl) that are n-propyl, tri-n-butylammonium chloride (N3444Cl) and tri-n-butyl, n-pentylammonium chloride (N4445Cl) were investigated. Structure analyses found that both salts formed Jeffrey's type III tetragonal hydrate structure which is the same as that of tetra-n-butylammonium chloride hydrate, although their lattice parameters are significantly different. The present data found that this hydrate structure can cover a wide range of melting temperature compared to the other two main semiclathrate structures. The present N4445Cl hydrate is an example in which its melting temperature was adjusted to be suitable for air conditioning, i.e., ∼282 K, compared to that of the N4444Cl hydrate, the melting temperature of which is slightly too high for this purpose. The results provide insight that the thermal properties of the tetragonal P42/m hydrate structure can be widely tuned by ionic guests for various practical requirements.

Multi-phase retrieval of methane hydrate in natural sediments by cryogenic x-ray computed tomography.

In this study, we observed natural methane (CH4) hydrate sediments, which are a type of unconventional natural gas resources, using x-ray computed tomography (CT). Because CH4 hydrates are formed by hydrogen bonding of water molecules with CH4, material decomposition becomes challenging when CH4 hydrates coexist with liquid or solid water in natural sediments. Tri-contrast (absorption, refraction, and scattering) imaging was performed via diffraction enhanced x-ray CT optics using monochromatic synchrotron x rays. The quantitative characterization of the contrast changes successfully enabled the decomposition of CH4 hydrates coexisting with frozen seawater (ice) in natural sediments obtained from the Okhotsk Sea. This study reveals complementary structural information about the microtexture and spatial relation among CH4 hydrates, ice, and pores by utilizing the distinct physical properties of x rays when passing through the materials. These results highlight the exceptional capabilities of high-resolution multicontrast x-ray tomography in materials science and geoscience applications.

Reversible Transition between Discrete and 1D Infinite Architectures: A Temperature-Responsive Cu(I) Complex with a Flexible Disilane-Bridged Bis(pyridine) Ligand.

A thermoresponsive structural change based on a disilane-bridged bis(pyridine) ligand and CuI is reported. Single-crystal X-ray analysis revealed that there are two polymorphs in the Cu(I) complex: octanuclear copper(I) complex at 20 °C and 1D staircase copper(I) polymer complex at -173 °C. The formation of these polymorphs is due to the flexibility of the ligand. Cu-I bond formation is observed upon cooling the sample from -10 °C to -170 °C. The temperature-induced phase transition progression was clarified by DSC, VT-PXRD, and VT-photoluminescence measurements and indicated a reversible temperature-controlled crystal-to-crystal phase transition. Observation on a VT-stage using a high-speed camera showed crystal cracking during single-crystal to single-crystal transitions between these polymorphic forms.

Topological dual and extended relations between networks of clathrate hydrates and Frank-Kasper phases.

Clathrate hydrates are a class of ordered structures that are stabilized via the delicate balance of hydrophobic interactions between water and guest molecules, of which the space-filling network of hydrogen-bonded (H-bonded) water molecules are closely related to tetrahedrally close-packed structures, known as Frank-Kasper (FK) phases. Here we report an alternative way to understand the intricate structures of clathrate hydrates, which unveils the diverse crystalline H-bonded networks that can be generated via assembly of one common building block. In addition to the intrinsic relations and pathways linking these crystals, we further illustrate the rich structural possibilities of clathrate hydrates. Given that the topological dual relations between networks of clathrate hydrates and tetrahedral close-packed structures, the descriptors presented for clathrate hydrates can be directly extended to other ordered materials for a more thorough understanding of their nucleation, phases transition, and co-existence mechanisms.

Thermally Induced Phase Transition of Cubic Structure II Hydrate: Crystal Structures of Tetrahydropyran-CO2 Binary Hydrate.

We report a thermally induced phase transition of cubic structure II hydrates of tetrahydropyran (THP) and CO2 below about 140 K. The phase transition was characterized by powder X-ray diffraction measurements at variable temperatures. A dynamical ordering of the CO2 guests in small pentagonal dodecahedral 512 host water cages, not previously observed in the simple CO2 hydrate, occurs simultaneously with the symmetry lowering transition from a cubic structure II (space group Fd-3m with cell dimensions a = 17.3202(7) Å at 153 K) to a tetragonal (space group I41/amd with cell dimensions a = 17.484(4) Å and c = 12.145(1) Å at 138 K) unit cell. The effect of guest molecules on the phase transition at low temperatures is discussed, which demonstrates that the clathrate hydrate structures and thermodynamic properties can be modified by adjusting the size and chemical structure of larger and smaller guest molecules.

Transformation process of ice crystallized from a glassy dilute trehalose aqueous solution.

Metastable forms of ice and their crystal growth play an important role in meteorology, cryobiology, and planetary science. However, it is difficult to investigate the effects of solute on the crystal growth of ice in a dilute aqueous solution due to the segregation. Herein, we made a non-segregated glass of dilute trehalose aqueous solution (0.023 mole fraction) and examined the transformation of crystalline ice in the aqueous solution with increasing temperature using powder X-ray diffraction measurements. The ice formed immediately after the crystallization is nano-sized stacking disordered ice (ice Isd) with few stacking faults and has high cubicity. The crystal growth of ice Isd in the trehalose aqueous solution was remarkably slower than those of ice Isd in a glycerol aqueous solution and pure ice Isd. The ice Isd survived up to ∼230 K which is higher than the transformation temperature from ice Isd to hexagonal ice (ice Ih) of pure water (∼200 K). The existence of trehalose inhibits the crystal growth of ice Isd and, as a result, the ice sublimates easily under vacuum conditions. Moreover, the occurrence of macroscopic segregation at ∼245 K is related to the Isd-to-Ih transformation. These results are important for the improvement of thawing techniques for cryopreserved biological tissues and for the understanding of the mechanism of ice cloud formation in the Earth's atmosphere.

A Clathrate Hydrate Structure Hidden in Plain Sight.

Clathrate hydrates have diverse crystal structures, and among them, the three (sI, sII, and sH) most prevalent ones cover nearly all known structures, while the norm is to consider other structures only when specific guest molecules are present. Here we report the observation of a hidden clathrate structure: the tetragonal structure (TS-I) in commonly formed gas hydrates, as evidenced from molecular dynamics simulations. We show that when two (or more) sI crystal grains with different growth directions come into contact or when the growth of a sI crystal encounters geometrical frustration, the TS-I results as a cocrystal. We give evidence that TS-I may also play an important role in the combination and/or transition between sI and sII. These results imply that this previously neglected structure may be commonly present whenever sI or sII is formed. This hidden structure must be identified, experimentally and in simulations; confining the possible structures may hinder an in-depth understanding of clathrate hydrates.

A Series of D-A-D Structured Disilane-Bridged Triads: Structure and Stimuli-Responsive Luminescence Studies.

A series of σ-π extended octamethyltetrasilanes, which have phenothiazine, 9,9-dimethyl-9,10-dihydroacridine, or phenoxazine (1, 2, and 3) groups as donor moieties and thienopyrazine or benzothiadiazole (a and b) groups as acceptor fragments, has been prepared, and their optical properties have been studied as an extension of our work. All six compounds exhibited fluorescence in the solid state with maximum wavelengths centered in the range of 400 and 650 nm upon excitation by a UV lamp. Compound 2b showed apparent dual emission behavior in solution, which depends on solvent polarity, and a reversible photoluminescent change under mechanical and thermal stimuli in the solid state. Quantum chemical calculations suggest the contribution of a quasi-axial conformer of the 9,9-dimethyl-9,10-dihydroacridine moiety in 2b to the dual emission in solution and the mechanofluoroluminescence in the solid state, similarly to 1a. These studies provide new insight into the preparation of disilane-bridged triads capable of responding to multiple stimuli.

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. In this paper, we studied semiclathrate hydrates formed from n-propyl, tri-n-butylammonium bromide (N3444Br) and tri-n-butyl, n-pentylammonium bromide (N4445Br). Their cation side chains are decremented or incremented from tetra-n-butylammonium (N4444 or TBA), which is one of the best fits for semiclathrate hydrate structures. The use of the widely used tetra-n-butylammonium bromide (N4444Br or TBAB) as an ionic guest, an increment of the carbon chain, i.e., N4445Br, caused disorders in its hydrate structure due to the oversizing of the cation. This suitably oversized cation selectively stabilized the orthorhombic structure, whose hydration number is relatively high. As a result, the fusion heat at the congruent composition of the hydrate phase was higher than that of the widely used N4444Br (TBAB) hydrates. The N3444Br hydrate showed both significantly decreased melting temperature and fusion heat compared to the N4444Br (TBAB) hydrates. The phase behaviour of the N3444Br hydrate was found to be analogous to that of the N4444Br (TBAB) hydrates. It was demonstrated that the semiclathrate hydrate structures and relevant properties can be modified by adjusting the alkyl side chain length of quaternary ammonium salts.

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

The superheated state of methane (CH4) 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 CH4 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. It was observed that placing the CH4 hydrate in CP converted the outer layer of CH4 hydrate to a thin layer of CP hydrate at around 270 K under atmospheric pressure, which is ∼80 K higher than the usual dissociation temperature. It was also observed that sI CO2 hydrate and C2H6 hydrate could be preserved by CP hydrate.

Advanced X-ray imaging at beamline 07 of the SAGA Light Source.

The SAGA Light Source provides X-ray imaging resources based on high-intensity synchrotron radiation (SR) emitted from the superconducting wiggler at beamline 07 (BL07). By combining quasi-monochromatic SR obtained by the newly installed water-cooled metal filter and monochromatic SR selected by a Ge double-crystal monochromator (DCM) with high-resolution lens-coupled X-ray imagers, fast and low-dose micro-computed tomography (CT), fast phase-contrast CT using grating-based X-ray interferometry, and 2D micro-X-ray absorption fine structure analysis can be performed. In addition, by combining monochromatic SR obtained by a Si DCM with large-area fiber-coupled X-ray imagers, high-sensitivity phase-contrast CT using crystal-based X-ray interferometry can be performed. Low-temperature CT can be performed using the newly installed cryogenic system, and time-resolved analysis of the crystallinity of semiconductor devices in operation can be performed using a time-resolved topography system. The details of each instrument and imaging method, together with exemplary measurements, are presented.

Carbon Isotope Fractionation during the Formation of CO2 Hydrate and Equilibrium Pressures of 12CO2 and 13CO2 Hydrates.

Knowledge of carbon isotope fractionation is needed in order to discuss the formation and dissociation of naturally occurring CO2 hydrates. We investigated carbon isotope fractionation during CO2 hydrate formation and measured the three-phase equilibria of 12CO2-H2O and 13CO2-H2O systems. From a crystal structure viewpoint, the difference in the Raman spectra of hydrate-bound 12CO2 and 13CO2 was revealed, although their unit cell size was similar. The δ13C of hydrate-bound CO2 was lower than that of the residual CO2 (1.0-1.5‰) in a formation temperature ranging between 226 K and 278 K. The results show that the small difference between equilibrium pressures of ~0.01 MPa in 12CO2 and 13CO2 hydrates causes carbon isotope fractionation of ~1‰. However, the difference between equilibrium pressures in the 12CO2-H2O and 13CO2-H2O systems was smaller than the standard uncertainties of measurement; more accurate pressure measurement is required for quantitative discussion.

Dissociation kinetics of propane-methane and butane-methane hydrates below the melting point of ice.

Understanding the dissociation mechanism of gas hydrates below the melting point of ice is crucial for expanding the practical applications of solid hydrates in gas storage. The kinetic processes for gas hydrates have not been clarified, except for those of pure CH4 hydrate and CO2 hydrates. In this study, using in situ X-ray diffraction analysis, the low-temperature onset of the dissociation of C3H8 and C4H10 hydrate fine particles encapsulating CH4 as a secondary guest was investigated during temperature ramping. At ∼200 K, the C3H8 + CH4 hydrate, n-C4H10 + CH4 hydrate, and iso-C4H10 + CH4 hydrate all dissociated in a single step, similar to pure C3H8 and pure iso-C4H10 hydrate. The dissociation of C3H8 hydrate was also found to accelerate the dissociation of CH4 hydrate. Based on the experimental results, it was confirmed that the C3H8 and C4H10 molecules released from the dissociating hydrates accelerated hydrate dissociation.

Characterization of the Clathrate Hydrate Formed with Fluoromethane and Pinacolone: The Thermodynamic Stability and Volumetric Behavior of the Structure H Binary Hydrate.

To reveal the relation of guest dynamics within the structure H clathrate hydrate and its macroscopic physical properties, experimental and computational works have been conducted on the system of fluoromethane (HFC-41) and pinacolone coexisting with water. The phase boundaries of the hydrate formed from HFC-41 and pinacolone within the pressure range of (0.25-2.48) MPa and the temperature range of (277-293) K were measured. The equilibrium hydrate formation pressure incorporating HFC-41 was lowered by adding the pinacolone as a large guest molecule compound to form a sH phase compared to the HFC-41 single hydrate. Powder X-ray diffraction measurements confirmed the formation of the structure H hydrate with the HFC-41 and pinacolone binary hydrate. The lattice constants of the sH hydrate were also measured to see the effect of the help guest molecular size, which showed a different trend from that of the previous studies of sH pinacolone hydrates. Molecular dynamics simulations of the binary sH phase indicate weak hydrogen bonding of the pinacolone molecules with the water in the cages in the phase with HFC-41. The oblate HFC-41 molecules showed strong orientational preference to the equatorial planes of the D' cages, which may explain some of the trends in the behavior of this phase.

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. The radiographic image of the Kr hydrate also revealed a sudden change in its attenuation coefficient owing to the K-absorption edge of Kr as the guest element. With a photon energy of 35 keV, X-ray CT provided sufficient segmentation for the TBAB semi-clathrate hydrate coexisting with ice. In contrast, the simulation did not achieve the sufficient segmentation of the CH4 and CO2 hydrates coexisting with water or ice, but it revealed the capability of absorption-contrast X-ray CT to model the physical properties of clathrate hydrates, such as Ar and Cl2 hydrates. These results demonstrate that the proposed method can be used to investigate the spatial distribution of specific elements within inclusion compounds or porous materials.

Slow Crystal Growth of Cubic Ice with Stacking Faults in a Glassy Dilute Glycerol Aqueous Solution.

Control of ice formation is an important issue as catastrophic ice growth influences our life activities and many industrial systems. We prepared a homogeneous glass of a dilute glycerol aqueous solution by a pressure liquid cooling vitrification method and examined the effect of solute on the ice formation of solvent water using a powder X-ray diffraction method. The solvent water immediately after the crystallization is composed of nanosized pure cubic ice (ice Ic). The crystal growth of ice Ic with stacking faults is much slower than that of pure water. The presence of glycerol molecules dispersing homogeneously may hinder crystal growth. The macroscopic segregation occurs rapidly during the transformation from stacking disordered ice to hexagonal ice. The results suggest that ice formation can be controlled by changing the solute type and concentration. This study has implications for thawing technology in cryobiology and frozen food engineering.

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. Chem. Phys., 2020, 22, 3446-3454, DOI: 10.1039/c9cp05983k.

Gas hydrates in sustainable chemistry.

Gas hydrates have received considerable attention due to their important role in flow assurance for the oil and gas industry, their extensive natural occurrence on Earth and extraterrestrial planets, and their significant applications in sustainable technologies including but not limited to gas and energy storage, gas separation, and water desalination. Given not only their inherent structural flexibility depending on the type of guest gas molecules and formation conditions, but also the synthetic effects of a wide range of chemical additives on their properties, these variabilities could be exploited to optimise the role of gas hydrates. This includes increasing their industrial applications, understanding and utilising their role in Nature, identifying potential methods for safely extracting natural gases stored in naturally occurring hydrates within the Earth, and for developing green technologies. This review summarizes the different properties of gas hydrates as well as their formation and dissociation kinetics and then reviews the fast-growing literature reporting their role and applications in the aforementioned fields, mainly concentrating on advances during the last decade. Challenges, limitations, and future perspectives of each field are briefly discussed. The overall objective of this review is to provide readers with an extensive overview of gas hydrates that we hope will stimulate further work on this riveting field.

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. The implementation of the approach is based upon X-ray phase shift, in which the optimal photon energy is simulated for high-contrast X-ray imaging. Proof of concept is provided by observations of resins, water, and clathrate hydrates such as CO2 hydrate and tetrahydrofuran (THF) hydrate in an aluminum container by diffraction-enhanced X-ray imaging with synchrotron X-rays of 35 keV. These results suggest that the proposed approach is a unique method for visualizing the transformation of these clathrate hydrates and is also applicable to in situ observations of other objects composed of multiphase materials with small density differences.