Anisotropic Lattice Expansion of Structure H Clathrate Hydrates Induced by Help Guest: Experiments and Molecular Dynamics Simulations

The structure H (sH) clathrate hydrates of neo-hexane with argon, krypton, and methane help gases are synthesized to study the effect of the help gas on the crystal lattice structure. Powder X-ray diffraction (PXRD) measurements on these hydrates were performed for temperatures in the range of 93–183 K, and the <i>a</i>-axis and <i>c</i>-axis lattice constants and the small and medium sH cage occupancies were determined. The PXRD results show that the <i>a</i>-axis lattice constants of the three clathrate hydrates are close in magnitude, but at each temperature, for the <i>c</i>-axis lattice constants <i>c</i>(CH₄) > <i>c</i>(Kr) > <i>c</i>(Ar). Parrinello–Rahman molecular dynamics (MD) simulations were performed on the three sH clathrate hydrate phases. The PXRD observed trends in the <i>a</i>-axis and <i>c</i>-axis lattice constants at different temperatures were reproduced by the simulations. The dynamics of the small cage guests are characterized by the velocity autocorrelation functions. The experiments and computations show the complex interplay of the molecular size and interaction energies in determining the lattice structure and stability of even relatively simple clathrate hydrates of nonpolar, hydrophobic molecules

Molecular Storage of Ozone in a Clathrate Hydrate: An Attempt at Preserving Ozone at High Concentrations

<div><p>This paper reports an experimental study of the formation of a mixed O₃+ O₂+ CO₂ hydrate and its frozen storage under atmospheric pressure, which aimed to establish a hydrate-based technology for preserving ozone (O₃), a chemically unstable substance, for various industrial, medical and consumer uses. By improving the experimental technique that we recently devised for forming an O₃+ O₂+ CO₂ hydrate, we succeeded in significantly increasing the fraction of ozone contained in the hydrate. For a hydrate formed at a system pressure of 3.0 MPa, the mass fraction of ozone was initially about 0.9%; and even after a 20-day storage at −25°C and atmospheric pressure, it was still about 0.6%. These results support the prospect of establishing an economical, safe, and easy-to-handle ozone-preservation technology of practical use.</p> </div

The initial ozone fraction in the formed hydrate versus the gas-phase composition.

<p>The mole fraction of ozone, <i>X</i>_O3, shown here is for the gas phase inside the reactor when the hydrate formation ceased. The legend inserted in the graph indicates the system pressure <i>p</i> during each hydrate-forming operation. The error bar for each data point represents the uncertainty of the ozone-fraction measurement by iodometry.</p

Results of the ozone preservation tests.

<p>This graph shows the time evolution of ozone fraction (mass basis) in each O₃+ O₂+ CO₂ hydrate stored under an aerated atmospheric-pressure (0.101 MPa) condition temperature-controlled at −25°C. The comparison of the ozone preservation test data obtained in this study (marked by closed symbols) and those from a previous study <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0048563#pone.0048563-Nakajima1" target="_blank">[7]</a> (marked by open symbols) are compared. The legend inserted in the graph indicates the O₃+ O₂ versus CO₂ molar ratio in the feed gas and the system pressure <i>p</i> for each hydrate-forming operation. The error bar for each data point represents the uncertainty of the ozone-fraction measurement by iodometry.</p

PXRD profile of an O₃+ O2+ CO₂ hydrate at 98 K.

<p>The solid curve shows the intensities observed using Cu−Kα radiation. The top row of tick marks represent the calculated peak positions for the structure I hydrate, and the lower two rows represent those for the hexagonal ice Ih and cubic ice Ic, respectively. The hydrate sample (accompanied by ice crystals) used in this PXRD measurement was formed from a mixture of O₃+ O2 and CO₂ in a nearly 2∶ 8 molar ratio at the condition of <i>p</i> = 2.0 MPa and <i>T</i> = 0.1°C.</p

Schematic illustration of the experimental setup for forming the O₃+ O₂+ CO₂ hydrates.

<p>This setup consists of (a) an oxygen cylinder, (b) a carbon-dioxide cylinder, (c) an ozone generator, (d) a gas-mixing chamber, (e) a pressure gauge, (f) and (g) pressure gauges, (h) and (i) gas-pressurizing chambers, (j) a gas-sampling chamber, (k) a pressure gauge, (l) a hydrate-forming reactor, (m) a Pt-wire resistance thermometer, (n) a stirrer, (o) a data logger, (p) an immersion cooler, (q) a PID-controlled heater, (r) a vacuum pump, (s) an ozone monitor, (t) a vacuum pump, and (u) an ozone decomposer.</p

Engineering Investigation of Hydrogen Storage in the Form of Clathrate Hydrates: Conceptual Design of Hydrate Production Plants

This paper describes a part of our feasibility study on the storage of hydrogen in the form of clathrate hydrates. The specific objective of this paper is to present conceptual designs of hydrogen-hydrate production plants applicable to large-scale in situ storage of hydrogen produced in an industrial complex area or to smaller-scale urban-area storage of hydrogen which is to be transported from the industrial complex area by container trucks. The plants were so designed as to produce either a simple hydrogen hydrate under a pressure of 35 MPa and a temperature of 140 K or a mixed hydrogen + tetrahydrofuran hydrate under a pressure of 30 MPa and a temperature of 223 K. In either case, the rate of hydrogen uptake into the hydrates during their production in each plant was targeted for 3000 Nm³/h (for use in an industrial complex area) or 500 Nm³/h (for use in an urban area). For each type of plant, we have prepared a process flow diagram accompanied by material-balance, heat-balance, and machinery specifications. The energy consumption in plant operation has also been evaluated, assuming that the cool energy generated by adjacent LNG facilities may or may not be available for cooling the hydrate-forming assemblies in each plant

Distortion of the Large Cages Encapsulating Cyclic Molecules and Empty Small Cages of Structure II Clathrate Hydrates

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

Phase Behavior and Structural Characterization of Ionic Clathrate Hydrate Formed with Tetra‑<i>n</i>‑butylphosphonium Hydroxide: Discovery of Primitive Crystal Structure

This paper reports phase equilibrium measurements and crystal structure analysis on the ionic clathrate hydrate formed from tetra-<i>n</i>-butylphosphonium hydroxide (TBPOH). Phase equilibrium temperatures were measured in the mole fraction range of TBPOH in aqueous solution from 0.0072 to 0.0416. The highest ionic clathrate hydrate–solution equilibrium temperature was determined to be 290.2 K at a TBPOH mole fraction of 0.0340, which corresponds to the congruent composition. Single-crystal X-ray diffraction measurements were performed on the crystal formed at 288.7 K, and the chemical composition of the TBPOH hydrate crystal was determined to be TBPOH·29.6H₂O, which is consistent with the congruent composition obtained by the phase equilibrium measurement. The crystal structure of the TBPOH hydrate has a superstructure identical with Jeffrey’s type I cubic structure, with an <i>I</i>4̅3<i>d</i> space group with a lattice constant of 24.5191(13) Å. The TBPOH hydrate structure is compared with the same hydrate structure formed by the tetra-<i>n</i>-butylammonium fluoride. We provide a comprehensive overview of the dissociation temperature, the counteranion, and the hydrate structure regarding TBP and TBA salt hydrates. The dissociation temperatures decrease linearly with the increase in the partial molal volume of anions for TBA and TBP salt hydrates, changing the hydrate structures from the primitive cubic one that has the minimum hydration number