9 research outputs found
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>(CH<sub>4</sub>) > <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 O<sub>3</sub>+ O<sub>2</sub>+ CO<sub>2</sub> hydrate and its frozen storage under atmospheric pressure, which aimed to establish a hydrate-based technology for preserving ozone (O<sub>3</sub>), a chemically unstable substance, for various industrial, medical and consumer uses. By improving the experimental technique that we recently devised for forming an O<sub>3</sub>+ O<sub>2</sub>+ CO<sub>2</sub> 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><sub>O3</sub>, 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 O<sub>3</sub>+ O<sub>2</sub>+ CO<sub>2</sub> 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 O<sub>3</sub>+ O<sub>2</sub> versus CO<sub>2</sub> 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 O<sub>3</sub>+ O2+ CO<sub>2</sub> 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 O<sub>3</sub>+ O2 and CO<sub>2</sub> 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 O<sub>3</sub>+ O<sub>2</sub>+ CO<sub>2</sub> 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
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 5<sup>12</sup>6<sup>4</sup> cages were obtained
by the direct-space technique followed by the Rietveld refinement.
It was shown that the sizes of the 5<sup>12</sup> and 5<sup>12</sup>6<sup>4</sup> 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 5<sup>12</sup>6<sup>4</sup> cages with larger guest molecules
became more spherical and volume ratio of empty small 5<sup>12</sup> 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·29.6H<sub>2</sub>O, 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>4Ì…3<i>d</i> space group with a lattice constant of 24.5191(13) Ã….
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
Enhanced Hydrogen-Storage Capacity and Structural Stability of an Organic Clathrate Structure with Fullerene (C<sub>60</sub>) Guests and Lithium Doping
An
effective combination of host and guest molecules in a framework
type of architecture can enhance the structural stability and physical
properties of clathrate compounds. We report here that an organic
clathrate compound consisting of a fullerene (C<sub>60</sub>) guest
and a hydroquinone (HQ) host framework shows enhanced hydrogen-storage
capacity and good structural stability under pressures and temperatures
up to 10 GPa and 438 K, respectively. This combined structure is formed
in the extended β-type HQ clathrate and admits 16 hydrogen molecules
per cage, leading to a volumetric hydrogen uptake of 49.5 g L<sup>–1</sup> at 77 K and 8 MPa, a value enhanced by 130% compared
to that associated with the β-type HQ clathrate. A close examination
according to density functional theory calculations and grand canonical
Monte Carlo simulations confirms the synergistic combination effect
of the guest–host molecules tailored for enhanced hydrogen
storage. Moreover, the model simulations demonstrate that the lithium-doped
HQ clathrates with C<sub>60</sub> guests reveal exceptionally high
hydrogen-storage capacities. These results provide a new playground
for additional fundamental studies of the structure–property
relationships and migration characteristics of small molecules in
nanostructured materials