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.

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.

Recovery of N2O: Energy-Efficient and Structure-Driven Clathrate-Based Greenhouse Gas Separation.

N2O has 300 times more global warming potential than CO2 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 N2O from gas mixtures. Clathrate formation between α-form/ß-form hydroquinone (α-HQ/ß-HQ) and gas mixtures reveals guest-specific and structure-driven selectivity, revealing the preferential capture of N2O in ß-HQ and the molecular sieving characteristics of α-HQ. With a maximum gas storage capacity and cage occupancy of 54.1 cm3 g-1 and 0.86, respectively, HQ clathrate compounds including N2O are stable at room temperature and atmospheric pressure and thus can be easily synthesized, treated, and recycled via commercial CBGS processes. High selectivity for N2O recovery was observed during ß-HQ clathrate formation from N2O/N2 gas mixtures with N2O concentrations exceeding 20%, whereas α-HQ traps only N2 molecules from gas mixtures. Full characterization using X-ray diffraction, scanning electron microscopy, Raman spectroscopy, solid-state nuclear magnetic resonance, and compositional analysis and the formation kinetics of HQ clathrates was conducted to verify the peculiar selectivity behavior and to design the conceptual CBGS process. These results provide a new playground on which to tailor host-guest materials and develop commercial processes for the recovery and/or sequestration of greenhouse gases.

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.

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.

CO2 Capture from Flue Gas Based on Tetra-n-butylammonium Fluoride Hydrates at Near Ambient Temperature.

Semiclathrate hydrates of tetra-n-butylammonium fluoride (TBAF) are potential CO2 capture media because they can capture CO2 at near ambient temperature under moderate pressure such as below 1 MPa. In addition to other semiclathrate hydrates, CO2 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 CO2 capture properties of TBAF hydrates for simulated flue gas, that is, CO2 + N2 gas, by the gas separation test with three different parameters for each pressure and aqueous composition of TBAF in mass fraction (w TBAF). The CO2 capture amount in TBAF hydrates with w TBAF = 0.10 was smaller than that obtained with w TBAF = 0.20 and 0.30. The results found that gas pressure greatly changed the CO2 capture amount in TBAF hydrates, and the aqueous composition highly affected CO2 selectivity. The crystal morphology and single-crystal structure analyses suggested that polymorphism of TBAF hydrates with congruent aqueous solution may lower both the CO2 capture amount and selectivity. Our present results proposed that an aqueous solution with w TBAF = 0.20 is advantageous for the CO2 capture from flue gas compared to near congruent solutions of TBAF hydrates (w TBAF = 0.30) and dilute solution (w TBAF = 0.10).

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.

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

Semiclathrate hydrates are a crystalline host-guest material, which forms with water and ionic substances such as tetra-n-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. Measurements for melting temperatures and a heat of fusion and a crystal structure analysis were performed. In comparison with the other similar materials, such as acetates, propionates, lactates, and hydroxybutyrates, the glycolate greatly changed the melting temperature and the heat of fusion. The preliminarily determined crystal structure showed that the glycolate anion builds a relatively porous structure compared to the previously reported hydrates formed with hydroxycarboxylates. The present study showed that substitution of a hydrophobic group by a hydrophilic group is an effective method to control the thermodynamic properties as well as to improve environmental, biological, and chemical properties.

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., Xe, Kr and Ar, were characterized by crystal structure analyses. Stabilization mechanisms of the semiclathrate hydrates by the D cages are discussed.

Structure-driven CO2 selectivity and gas capacity of ionic clathrate hydrates.

Ionic clathrate hydrates can selectively capture small gas molecules such as CO2, N2, CH4 and H2. We investigated CO2 + N2 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 CO2 selectivity of TBAC hydrates was remarkably higher than those of the other hydrates despite less gas capacity of TBAC hydrates. The TBAB hydrates also showed irregularly high CO2 selectivity at a low pressure. X-ray diffraction and Raman spectroscopic analyses clarified that TBAC stably formed the tetragonal hydrate structure, and TBPB and TBPC formed the orthorhombic hydrate structure. The TBAB hydrates showed polymorphic phases which may consist of the both orthorhombic and tetragonal hydrate structures. These results showed that the tetragonal hydrate captured CO2 more efficiently than the orthorhombic hydrate, while the orthorhombic hydrate has the largest gas capacity among the basic four structures of ionic clathrate hydrates. The present study suggests new potential for improving gas capacity and selectivity of ionic clathrate hydrates by choosing suitable ionic guest substances for guest gas components.

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. Phase equilibrium measurements and optical observations of the crystal morphology and crystal structure analysis were performed. The TBAL hydrate has a melting temperature of 284.8 K suitable for cool energy storage applications. The actual hydration patterns around a lactate anion are shown in the form of ionic clathrate hydrate structure.

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. Each hydrate sample was stored in an air-filled container at atmospheric pressure and a constant temperature in the range between -20 and 2 degrees C and was continually subjected to iodometric measurements of its fractional ozone content. Such chronological measurements and structure analysis using powder X-ray diffraction have revealed that ozone can be preserved in a hydrate-lattice structure for more than 20 days at a concentration on the order of 0.1% (hydrate-mass basis).