Structure, Composition, and Thermal Expansion of CO2 Hydrate from Single Crystal X-ray Diffraction Measurements

Hydrate Technologies for CO2 Capture and Sequestration: Status and Perspectives

CO2 capture and sequestration based on hydrate technology are considered supplementary approaches for reducing carbon emissions and mitigating the greenhouse effect. Direct CO2 hydrate formation and CH4 gas substitution in natural gas hydrates are two of the main methods used for the sequestration of CO2 in hydrates. In this Review, we introduce the crystal structures of CO2 hydrates and CO2-mixed gas hydrates and summarize the interactions between the CO2 molecules and clathrate hydrate/H2O frames. In particular, we focus on the role of diffraction techniques in analyzing hydrate structures. The kinetic and thermodynamic properties then are introduced from micro/macro perspectives. Furthermore, the replacement of natural gas with CO2/CO2-mixed gas is discussed comprehensively in terms of intermolecular interactions, influencing factors, and displacement efficiency. Based on the analysis of related costs, risks, and policies, the economics of CO2 capture and sequestration based on hydrate technology are explained. Moreover, the difficulties and challenges at this stage and the directions for future research are described. Finally, we investigate the status of recent research related to CO2 capture and sequestration based on hydrate technology, revealing its importance in carbon emission reduction.

Discovery of the final primitive Frank-Kasper phase of clathrate hydrates

In weakly bound materials such as water, one of the three primitive Frank-Kasper (FK) phases, the Z phase, is long absent due to the relatively unstable framework. The Z phase in clathrate hydrate, which is known as the HS-I structure, has now been found by precise tuning of the molecular guest structure. In the crystal structure, the never stabilized combination water cage of two 15-hedra and two 14-hedra formed with its original symmetries, providing sufficient gas capacity to the 12-hedral cages. With the discovery of the final FK clathrate hydrate, guest design now enables engineering of weak interactions in any mix of the three, illuminating how to leverage properties of clathrates in the broadest sense.

Three-phase equilibria of hydrates from computer simulation. III. Effect of dispersive interactions in the methane and carbon dioxide hydrates

In this work, the effect of the range of dispersive interactions in determining the three-phase coexistence line of the CO2 and CH4 hydrates has been studied. In particular, the temperature (T3) at which solid hydrate, water, and liquid CO2/gas CH4 coexist has been determined through molecular dynamics simulations using different cutoff values (from 0.9 to 1.6 nm) for dispersive interactions. The T3 of both hydrates has been determined using the direct coexistence simulation technique. Following this method, the three phases in equilibrium are put together in the same simulation box, the pressure is fixed, and simulations are performed at different temperatures T. If the hydrate melts, then T > T3. Conversely, if the hydrate grows, then T < T3. The effect of the cutoff distance on the dissociation temperature has been analyzed at three different pressures for CO2 hydrate: 100, 400, and 1000 bar. Then, we have changed the guest and studied the effect of the cutoff distance on the dissociation temperature of the CH4 hydrate at 400 bar. Moreover, the effect of long-range corrections for dispersive interactions has been analyzed by running simulations with homo- and inhomogeneous corrections and a cutoff value of 0.9 nm. The results obtained in this work highlight that the cutoff distance for the dispersive interactions affects the stability conditions of these hydrates. This effect is enhanced when the pressure is decreased, displacing the T3 about 2-4 K depending on the system and the pressure.

CO2 competes with radioactive chemicals for freshwater recovery: Hydrate-based desalination

Here, we introduce CO2 hydrate-based desalination (CHBD) technology for freshwater recovery from radioactive wastewater, for water particularly containing Cs and Sr. The hydrate equilibrium curves of CO2 hydrates shift towards lower temperature and higher pressure regions as the concentrations of CsCl and SrCl2 increase. X-ray diffraction and Raman spectroscopy measurements found that neither CsCl nor SrCl2 can affect the structure of CO2 hydrates. The high-pressure micro-differential scanning calorimetric results demonstrate that CO2 hydrates in the presence of CsCl and SrCl2 started to dissociate at lower temperatures due to the enrichment of CsCl and SrCl2 in the remaining solutions. The formation kinetics results indicate that increases in the concentrations of the radioactive chemicals lead to a decrease in the initial reaction rate and sub-cooling temperature. Solid-state nuclear magnetic resonance spectroscopy was utilized to confirm the exclusion of radioactive isotopes from solid gas hydrates. Importantly, the CHBD technology proposed in this study is applicable to radioactive wastewater containing Cs+ and Sr2+ across broad concentration ranges, spanning from a percent to hundreds of parts per million (ppm), and even sub-ppm levels, with comparable recovery efficiency. This study presents new insights into the potential of environmentally sustainable technologies to overcome the challenges posed by radioactive wastewater.

Methane hydrate phase equilibrium considering dissolved methane concentrations and interfacial geometries from molecular simulations

Natural gas hydrates, mainly existing in permafrost and on the seabed, are expected to be a new energy source with great potential. The exploitation technology of natural gas hydrates is one of the main focuses of hydrate-related studies. In this study, a large-size liquid aqueous solution wrapping a methane hydrate system was established and molecular dynamics simulations were used to investigate the phase equilibrium conditions of methane hydrate at different methane concentrations and interfacial geometries. It is found that the methane concentration of a solution significantly affects the phase equilibrium of methane hydrates. Different methane concentrations at the same temperature and pressure can lead to hydrate formation or decomposition. At the same temperature and pressure, in a system reaching equilibrium, the size of spherical hydrate clusters is coupled to the solution concentration, which is proportional to the Laplace pressure at the solid-liquid interface. Lower solution concentrations reduce the phase equilibrium temperature of methane hydrates at the same pressure; as the concentration increases, the phase equilibrium temperature gradually approaches the actual phase equilibrium temperature. In addition, the interfacial geometry of hydrates affects the thermodynamic stability of hydrates. The spherical hydrate particles have the highest stability for the same volume. Through this study, we provide a stronger foundation to understand the principles driving hydrate formation/dissociation relevant to the exploitation of methane hydrates.

Ab Initio Studies of Structural and Mechanical Properties of NH3, NO, and N2O Hydrates

The investigation on the mechanical properties of clathrate hydrate is closely related to the exploitation of hydrates and gas transportation. In this article, the structural and mechanical properties of some nitride gas hydrates were studied using DFT calculations. First, the equilibrium lattice structure is obtained by geometric structure optimization; then, the complete second-order elastic constant is determined by energy-strain analysis, and the polycrystalline elasticity is predicted. It is found that the NH₃, N₂O, and NO hydrates all have high elastic isotropy but are different in shear characteristics. This work may lay a theoretical foundation for studying the structural evolution of clathrate hydrates under the mechanical field.

On the phase behaviors of CH4-CO2 binary clathrate hydrates: Two-phase and three-phase coexistences

We develop a statistical mechanical theory on clathrate hydrates in order to explore the phase behaviors of clathrate hydrates containing two kinds of guest species and apply it to CH4-CO2 binary hydrates. The two boundaries separating water and hydrate and hydrate and guest fluid mixtures are estimated, which are extended to the lower temperature and the higher pressure region far distant from the three-phase coexisting conditions. The chemical potentials of individual guest components can be calculated from free energies of cage occupations, which are available from intermolecular interactions between host water and guest molecules. This allows us to derive all thermodynamic properties pertinent to the phase behaviors in the whole space of thermodynamic variables of temperature, pressure, and guest compositions. It is found that the phase boundaries of CH4-CO2 binary hydrates with water and with fluid mixtures locate between simple CH4 and CO2 hydrates, but the composition ratios of CH4 guests in hydrates are disproportional to those in fluid mixtures. Such differences arise from the affinities of each guest species to the large and small cages of CS-I hydrates and significantly affect occupation of each cage type, which results in a deviation of the guest composition in hydrates from that in fluid on the two-phase equilibrium conditions. The present method provides a basis for the evaluation of the efficiency of the guest CH4 replacement to CO2 at the thermodynamic limit.

Solubility of carbon dioxide in water: Some useful results for hydrate nucleation

In this paper, the solubility of carbon dioxide (CO2) in water along the isobar of 400 bar is determined by computer simulations using the well-known TIP4P/Ice force field for water and the TraPPE model for CO2. In particular, the solubility of CO2 in water when in contact with the CO2 liquid phase and the solubility of CO2 in water when in contact with the hydrate have been determined. The solubility of CO2 in a liquid-liquid system decreases as the temperature increases. The solubility of CO2 in a hydrate-liquid system increases with temperature. The two curves intersect at a certain temperature that determines the dissociation temperature of the hydrate at 400 bar (T3). We compare the predictions with T3 obtained using the direct coexistence technique in a previous work. The results of both methods agree, and we suggest 290(2) K as the value of T3 for this system using the same cutoff distance for dispersive interactions. We also propose a novel and alternative route to evaluate the change in chemical potential for the formation of hydrates along the isobar. The new approach is based on the use of the solubility curve of CO2 when the aqueous solution is in contact with the hydrate phase. It considers rigorously the non-ideality of the aqueous solution of CO2, providing reliable values for the driving force for nucleation of hydrates in good agreement with other thermodynamic routes used. It is shown that the driving force for hydrate nucleation at 400 bar is larger for the methane hydrate than for the carbon dioxide hydrate when compared at the same supercooling. We have also analyzed and discussed the effect of the cutoff distance of dispersive interactions and the occupancy of CO2 on the driving force for nucleation of the hydrate.

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.

Modeling of Structure H Carbon Dioxide Clathrate Hydrates: Guest-Lattice Energies, Crystal Structure, and Pressure Dependencies

We performed first-principles computations to investigate the complex interplay of molecular interaction energies in determining the lattice structure and stability of CO₂@sH clathrate hydrates. Density functional theory computations using periodic boundary conditions were employed to characterize energetics and the key structural properties of the sH clathrate crystal under pressure, such as equilibrium lattice volume and bulk modulus. The performance of exchange-correlation functionals together with recently developed dispersion-corrected schemes was evaluated in describing interactions in both short-range and long-range regions of the potential. Structural relaxations of the fully CO₂-filled and empty sH unit cells yield crystal structure and lattice energies, while their compressibility parameters were derived by including the pressure dependencies. The present quantum chemistry computations suggest anisotropy in the compressibility of the sH clathrate hydrates, with the crystal being less compressible along the a-axis direction than along the c-axis one, in distinction from nearly isotropic sI and sII structures. The detailed results presented here give insight into the complex nature of the underlying guest-host interactions, checking earlier assumptions, providing critical tests, and improving estimates. Such entries may eventually lead to better predictions of thermodynamic properties and formation conditions, with a direct impact on emerging hydrate-based technologies.

Investigation of crystal growth of CO2 hydrate in aqueous fructose solution for the potential application in carbonated solid foods

CO₂ hydrate is applicable to solid carbonated foods. The hydrate crystal morphology, which represents the crystal size and shape, is an important characteristic that changes the texture of foods. We report an observational study of the crystal growth of CO₂ hydrate in aqueous fructose solution. The difference between the phase equilibrium temperature and the experimental temperature, ΔT_sub, is applied as an index of the driving force. Experiments were performed at ΔT_sub range of 0.9 K to 5.4 K. At all ΔT_sub, initial crystal formed at the gas-solution interface and grew along the interface. After covering the interface, the crystals grew in the liquid phase The individual crystals were identified as polyhedral with facets (ΔT_sub = 0.9 K), skeletal crystals (ΔT_sub = 2.0 K) and dendrites (ΔT_sub = 3.0 K and 5.4 K). Based on these results, the potential effect of gas hydrate morphology on texture of foods has been discussed.

Instrumental Methods for Cage Occupancy Estimation of Gas Hydrate

Studies revealed that gas hydrate cages, especially small cages, are incompletely filled with guest gas molecules, primarily associated with pressure and gas composition. The ratio of hydrate cages occupied by guest molecules, defined as cage occupancy, is a critical parameter to estimate the resource amount of a natural gas hydrate reservoir and evaluate the storage capacity of methane or hydrogen hydrate as an energy storage medium and carbon dioxide hydrate as a carbon sequestration matrix. As the result, methods have been developed to investigate the cage occupancy of gas hydrate. In this review, several instrument methods widely applied for gas hydrate analysis are introduced, including Raman, NMR, XRD, neutron diffraction, and the approaches to estimate cage occupancy are summarized.

Structural CO2 capture preference of semiclathrate hydrate formed with tetra-n-butylammonium chloride

CO2 capture preference of semiclathrate hydrate analyzed by single crystal XRD. Asymmetrically distorted cages preferentially capture CO2.

Molecular Simulations of CO2/CH4, CO2/N2 and N2/CH4 Binary Mixed Hydrates

Abstract

Grand canonical Monte Carlo simulations were performed to study the occupancy of structure I multicomponent gas hydrates by CO2/CH4, CO2/N2, and N2/CH4 binary gas mixtures with various compositions at a temperature of 270 K and pressures up to 70 atm. The presence of nitrogen in the gas mixture allows for an increase of both the hydrate framework selectivity to CO2 and the amount of carbon dioxide encapsulated in hydrate cages, as compared to the CO2/CH4 hydrate. Despite the selectivity to CH4 molecules demonstrated by N2/CH4 hydrate, nitrogen can compete with methane if the gas mixture contains at least 70% of N2.

Exploring CO2 @sI Clathrate Hydrates as CO2 Storage Agents by Computational Density Functional Approaches

The formation of specific clathrate hydrates and their transformation at given thermodynamic conditions depends on the interactions between the guest molecule/s and the host water lattice. Understanding their structural stability is essential to control structure-property relations involved in different technological applications. Thus, the energetic aspects relative to CO₂ @sI clathrate hydrate are investigated through the computation of the underlying interactions, dominated by hydrogen bonds and van der Waals forces, from first-principles electronic structure approaches. The stability of the CO₂ @sI clathrate is evaluated by combining bottom-up and top-down approaches. Guest-free and CO₂ guest-filled aperiodic cages, up to the gradually CO₂ occupation of the entire sI periodic unit cells were considered. Saturation, cohesive and binding energies for the systems are determined by employing a variety of density functionals and their performance is assessed. The dispersion corrections on the non-covalent interactions are found to be important in the stabilization of the CO₂ @sI energies, with the encapsulation of the CO₂ into guest-free/empty cage/lattice being always an energetically favorable process for most of the functionals studied. The PW86PBE functional with XDM or D3(BJ) dispersion corrections predicts a lattice constant in accord to the experimental values available, and simultaneously provides a reliable description for the guest-host interactions in the periodic CO₂ @sI crystal, as well as the energetics of its progressive single cage occupancy process. It has been found that the preferential orientation of the single CO₂ in the large sI crystal cages has a stabilizing effect on the hydrate, concluding that the CO₂ @sI structure is favored either by considering the individual building block cages or the complete sI unit cell crystal. Such benchmark and methodology cross-check studies benefit new data-driven model research by providing high-quality training information, with new insights that indicate the underlying factors governing their structure-driven stability, and triggering further investigations for controlling the stabilization of these promising long-term CO₂ storage materials.

Computational density-functional approaches on finite-size and guest-lattice effects in CO2@sII clathrate hydrate

We performed first-principles computations to investigate guest-host/host-host effects on the encapsulation of the CO₂ molecule in sII clathrate hydrates from finite-size clusters up to periodic 3D crystal lattice systems. Structural and energetic properties were first computed for the individual and first-neighbors clathrate-like sII cages, where highly accurate ab initio quantum chemical methods are available nowadays, allowing in this way the assessment of the density functional (DFT) theoretical approaches employed. The performance of exchange-correlation functionals together with recently developed dispersion-corrected schemes was evaluated in describing interactions in both short-range and long-range regions of the potential. On this basis, structural relaxations of the CO₂-filled and empty sII unit cells yield lattice and compressibility parameters comparable to experimental and previous theoretical values available for sII hydrates. According to these data, the CO₂ enclathration in the sII clathrate cages is a stabilizing process, either by considering both guest-host and host-host interactions in the complete unit cell or only the guest-water energies for the individual clathrate-like sII cages. CO₂@sII clathrates are predicted to be stable whatever the dispersion correction applied and in the case of single cage occupancy are found to be more stable than the CO₂@sI structures. Our results reveal that DFT approaches could provide a good reasonable description of the underlying interactions, enabling the investigation of formation and transformation processes as a function of temperature and pressure.

Structural Stability of the CO2 @sI Hydrate: a Bottom-Up Quantum Chemistry Approach on the Guest-Cage and Inter-Cage Interactions

Through reliable first-principles computations, we have demonstrated the impact of CO₂ molecules enclathration on the stability of sI clathrate hydrates. Given the delicate balance between the interaction energy components (van der Waals, hydrogen bonds) present on such systems, we follow a systematic bottom-up approach starting from the individual 5¹² and 5¹² 6² sI cages, up to all existing combinations of two-adjacent sI crystal cages to evaluate how such clathrate-like models perform on the evaluation of the guest-host and first-neighbors inter-cage effects, respectively. Interaction and binding energies of the CO₂ occupation of the sI cages were computed using DF-MP2 and different DFT/DFT-D electronic structure methodologies. The performance of selected DFT functionals, together with various semi-classical dispersion corrections schemes, were validated by comparison with reference ab initio DF-MP2 data, as well as experimental data from x-ray and neutron diffraction studies available. Our investigation confirms that the inclusion of the CO₂ in the cage/s is an energetically favorable process, with the CO₂ molecule preferring to occupy the large 5¹² 6² sI cages compared to the 5¹² ones. Further, the present results conclude on the rigidity of the water cages arrangements, showing the importance of the inter-cage couplings in the cluster models under study. In particular, the guest-cage interaction is the key factor for the preferential orientation of the captured CO₂ molecules in the sI cages, while the inter-cage interactions seems to cause minor distortions with the CO₂ guest neighbors interactions do not extending beyond the large 5¹² 6² sI cages. Such findings on these clathrate-like model systems are in accord with experimental observations, drawing a direct relevance to the structural stability of CO₂ @sI clathrates.

Effects of cage occupancy in hydrate by first-principles calculation and modification of the van der Waals-Platteeuw hypothesis

The changes of methane hydrate lattice with the decrease of cage occupancy were calculated by first-principles methods. The calculation results show that the decrease of the cages occupancy in sII and sH hydrate does not lead to large deformation in the lattice. Even if all the methane molecules are removed so that the hydrates have become new types of ice, the sII and sH lattices remain stable. The same conclusion is also true when the occupancy of the small cages in sI hydrate is reduced. However, the sI hydrate lattice will deform and almost collapse as the large cage occupancy decreases. These calculation results suggest that sI hydrate cannot exist with empty cages. Since the van der Waals-Platteeuw theory is based on the assumption that the stability of host lattice is independent of the occupancy of guest molecule, it would be applicable to sII and sH lattices, but not to sI hydrates. We propose a modification to the van der Waals-Platteeuw hypothesis so that the theory seems more reasonable.

Co-deposition of gas hydrates by pressurized thermal evaporation

Gas hydrates are usually synthesized by bringing together a pressurized gas and liquid or solid water. In both cases, the transport of gas or water to the hydrate growth site is hindered once an initial film of hydrate has grown at the water-gas interface. A seemingly forgotten gas-phase technique overcomes this problem by slowly depositing water vapor on a cold surface in the presence of the pressurized guest gas. Despite being used for the synthesis of low-formation-pressure hydrates, it has not yet been tested for hydrates of CO₂ and CH₄. Moreover, the potential of the technique for the study of hydrate decomposition has not been recognized yet. We employ two advanced implementations of the condensation technique to form hydrates of CO₂ and CH₄ and demonstrate the applicability of the process for the study of hydrate decomposition and the phenomenon of self-preservation. Our results show that CO₂ and CH₄ hydrate samples deposited on graphite at 261-265 K are almost pure hydrates with an ice fraction of less than 8%. Rapid depressurization experiments with thin deposits (approx. 330 μm thickness) of CO₂ hydrate on an aluminum surface at 265 K yield identical dissociation curves when the deposition is done at identical pressure. However, hydrates deposited at 1 MPa almost completely withstand decomposition after rapid depressurization to 0.1 MPa, while samples deposited at 2 MPa decompose 7 times faster. Therefore, this synthesis technique is not only applicable for the study of hydrate decomposition but can also be used for the controlled deposition of a super-preserved hydrate.

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.

Raman Signatures and Thermal Expansivity of Acetylene Clathrate Hydrate

The vibrational signatures for the υ₂ C≡C and υ₁ symmetric C-H stretches of acetylene in cubic structure I clathrate, synthesized under ambient pressure, are reported for the first time. The most diagnostic features are at 1966 for υ₂ and 3353 cm^-1 for υ₁, respectively, and are assigned to acetylene trapped in the large 5¹²6² cages. In addition, the υ₂ mode for acetylene occupying the small 5¹² cages is observed at 1972.5 cm^-1, a red shift of 1.5 cm^-1 from its gas phase frequency. Unit cell parameters and thermal expansion coefficients are determined via powder X-ray diffraction between 195 and 225 K and are found to be in good correlation with previous single crystal data at 143 K. The calculated density for acetylene clathrate is also reported, with values ranging from 0.985 g/cm³ at 195 K to 0.976 g/cm³ at 225 K. These results are relevant for spectral detection of acetylene-containing compounds on planetary bodies, as well as providing additional insights on the thermal behavior and physical properties of acetylene clathrate.

Unraveling the metastability of the SI and SII carbon monoxide hydrate with a combined DFT-neutron diffraction investigation

Clathrate hydrates are crystalline compounds consisting of water molecules forming cages (so-called "host") inside of which "guest" molecules are encapsulated depending on the thermodynamic conditions of formation (systems stable at low temperature and high pressure). These icelike systems are naturally abundant on Earth and are generally expected to exist on icy celestial bodies. Carbon monoxide hydrate might be considered an important component of the carbon cycle in the solar system since CO gas is one of the predominant forms of carbon. Intriguing fundamental properties have also been reported: the CO hydrate initially forms in the sI structure (kinetically favored) and transforms into the sII structure (thermodynamically stable). Understanding and predicting the gas hydrate structural stability then become essential. The aim of this work is, thereby, to study the structural and energetic properties of the CO hydrate using density functional theory (DFT) calculations together with neutron diffraction measurements. In addition to the comparison of DFT-derived structural properties with those from experimental neutron diffraction, the originality of this work lies in the DFT-derived energy calculations performed on a complete unit cell (sI and sII) and not only by considering guest molecules confined in an isolated water cage (as usually performed for extracting the binding energies). Interestingly, an excellent agreement (within less than 1% error) is found between the measured and DFT-derived unit cell parameters by considering the Perdew-Burke-Ernzerhof (denoted PBE) functional. Moreover, a strategy is proposed for evaluating the hydrate structural stability on the basis of potential energy analysis of the total nonbonding energies (i.e., binding energy and water substructure nonbonding energy). It is found that the sII structure is the thermodynamically stable hydrate phase. In addition, increasing the CO content in the large cages has a stabilizing effect on the sII structure, while it destabilizes the sI structure. Such findings are in agreement with the recent experimental results evidencing the structural metastability of the CO hydrate.

A Systematic Protocol for Benchmarking Guest-Host Interactions by First-Principles Computations: Capturing CO2 in Clathrate Hydrates

Clathrate hydrates of CO₂ have been proposed as potential molecular materials in tackling important environmental problems related to greenhouse gases capture and storage. Despite the increasing interest in such hydrates and their technological applications, a molecular-level understanding of their formation and properties is still far from complete. Modeling interactions is a challenging and computationally demanding task, essential to reliably determine molecular properties. First-principles calculations for the CO₂ guest in all sI, sII, and sH clathrate cages were performed, and the nature of the guest-host interactions, dominated by both hydrogen-bond and van der Waals forces, was systematically investigated. Different families of density functionals, as well as pairwise CO₂ @H₂ O model potentials versus wavefunction-based quantum approaches were studied for CO₂ clathrate-like systems. Benchmark energies for new distance-dependent datasets, consisting of potential energy curves sampling representative configurations of the systems at the repulsive, near-equilibrium, and asymptotic/long-range regions of the full-dimensional surface, were generated, and a general protocol was proposed to assess the accuracy of such conventional and modern approaches at minimum and non-minimum orientations. Our results show that dispersion interactions are important in the guest-host stabilization energies of such clathrate cages, and the encapsulation of the CO₂ into guest-free clathrate cages is always energetically favorable. In addition, the orientation of CO₂ inside each cage was explored, and the ability of current promising approaches to accurately describe non-covalent CO₂ @H₂ O guest-host interactions in sI, sII, and sH clathrates was discussed, providing information for their applicability to future multiscale computer simulations.

Supramolecular Organization in Confined Nanospaces

Empty spaces are abhorred by nature, which immediately rushes in to fill the void. Humans have learnt pretty well how to make ordered empty nanocontainers, and to get useful products out of them. When such an order is imparted to molecules, new properties may appear, often yielding advanced applications. This review illustrates how the organized void space inherently present in various materials: zeolites, clathrates, mesoporous silica/organosilica, and metal organic frameworks (MOF), for example, can be exploited to create confined, organized, and self-assembled supramolecular structures of low dimensionality. Features of the confining matrices relevant to organization are presented with special focus on molecular-level aspects. Selected examples of confined supramolecular assemblies - from small molecules to quantum dots or luminescent species - are aimed to show the complexity and potential of this approach. Natural confinement (minerals) and hyperconfinement (high pressure) provide further opportunities to understand and master the atomistic-level interactions governing supramolecular organization under nanospace restrictions.

Elasticity and Stability of Clathrate Hydrate: Role of Guest Molecule Motions

Molecular dynamic simulations were performed to determine the elastic constants of carbon dioxide (CO₂) and methane (CH₄) hydrates at one hundred pressure–temperature data points, respectively. The conditions represent marine sediments and permafrost zones where gas hydrates occur. The shear modulus and Young’s modulus of the CO₂ hydrate increase anomalously with increasing temperature, whereas those of the CH₄ hydrate decrease regularly with increase in temperature. We ascribe this anomaly to the kinetic behavior of the linear CO₂ molecule, especially those in the small cages. The cavity space of the cage limits free rotational motion of the CO₂ molecule at low temperature. With increase in temperature, the CO₂ molecule can rotate easily, and enhance the stability and rigidity of the CO₂ hydrate. Our work provides a key database for the elastic properties of gas hydrates, and molecular insights into stability changes of CO₂ hydrate from high temperature of ~5 °C to low decomposition temperature of ~−150 °C.

Computational study of the interplay between intermolecular interactions and CO2 orientations in type I hydrates

Carbon dioxide (CO₂) molecules show a rich orientation landscape when they are enclathrated in type I hydrates. Previous studies have described experimentally their preferential orientations, and some theoretical works have explained, but only partially, these experimental results. In the present paper, we use classical molecular dynamics and electronic density functional theory to advance in the theoretical description of CO₂ orientations within type I hydrates. Our results are fully compatible with those previously reported, both theoretical and experimental, the geometric shape of the cavities in hydrate being, and therefore, the steric constraints, responsible for some (but not all) preferential angles. In addition, our calculations also show that guest-guest interactions in neighbouring cages are a key factor to explain the remaining experimental angles. Besides the implication concerning equation of state hydrate modeling approximations, the conclusion is that these guest-guest interactions should not be neglected, contrary to the usual practice.

Effects of the CO₂ Guest Molecule on the sI Clathrate Hydrate Structure

This paper analyzes the structural, energetic and mechanical properties of carbon dioxide hydrate clathrates calculated using finite cluster and periodic ab initio density-functional theory methodologies. Intermolecular interactions are described by the exchange-hole dipole moment method. The stability, gas saturation energetics, guest–host interactions, cage deformations, vibrational frequencies, and equation of state parameters for the low-pressure sI cubic phase of the CO₂@H₂O clathrate hydrate are presented. Our results reveal that: (i) the gas saturation process energetically favors complete filling; (ii) carbon dioxide molecules prefer to occupy the larger of the two cages in the sI structure; (iii) blue shifts occur in both the symmetric and antisymmetric stretching frequencies of CO₂ upon encapsulation; and (iv) free rotation of guest molecules is restricted to a plane parallel to the hexagonal faces of the large cages. In addition, we calculate the librational frequency of the hindered rotation of the guest molecule in the plane perpendicular to the hexagonal faces. Our calculated spectroscopic data can be used as signatures for the detection of clathrate hydrates in planetary environments.

Anisotropic Thermal Expansion of Zirconium Diboride: An Energy-Dispersive X-Ray Diffraction Study

Zirconium diboride (ZrB2) is an attractive material due to its thermal and electrical properties. In recent years, ZrB2 has been investigated as a superior replacement for sapphire when used as a substrate for gallium nitride devices. Like sapphire, ZrB2 has an anisotropic hexagonal structure which defines its directionally dependent properties. However, the anisotropic behavior of ZrB2 is not well understood. In this paper, we use energy-dispersive synchrotron X-ray diffraction to measure the thermal expansion of polycrystalline ZrB2 powder from 300 to 1150 K. Nine Bragg reflections are fit using Pseudo-Voigt peak profiles and used to compute the a and c lattice parameters using a nonlinear least-squares approximation. The temperature-dependent instantaneous thermal expansion coefficients are determined for each a-axis and c-axis direction and are described by the following equations: αa = (4.1507×10-6 + 5.1086 × 10-9(T-293.15))/(1+4.1507 × 10-6(T-293.15) + 2.5543×10-9(T-293.15)2) and αc = (4.5374×10-6 + 4.3004×10-9(T-293.15))/(1+4.5374×10-6(T-293.15) + 2.1502×10-9(T-293.15)2). Our results are within range of previously reported values but describe the temperature anisotropy in more detail. We show that anisotropic expansion coefficients converge to the same value at about 780 K and diverge at higher temperatures. Results are compared with other reported values.

CO₂ processing and hydration of fruit and vegetable tissues by clathrate hydrate formation

CO2 hydrate can be used to preserve fresh fruits and vegetables, and its application could contribute to the processing of carbonated frozen food. We investigated water transformation in the frozen tissue of fresh grape samples upon CO2 treatment at 2-3 MPa and 3°C for up to 46 h. Frozen fresh bean, radish, eggplant and cucumber samples were also investigated for comparison. X-ray diffraction indicated that after undergoing CO2 treatment for several hours, structure I CO2 hydrate formed within the grape tissue. Phase-contrast X-ray imaging using the diffraction-enhanced imaging technique revealed the presence of CO2 hydrate within the intercellular spaces of these tissues. The carbonated produce became effervescent because of the dissociation of CO2 hydrate through the intercellular space, especially above the melting point of ice. In addition, suppressed metabolic activity resulting from CO2 hydrate formation, which inhibits water and nutrient transport through intercellular space, can be expected.

Disorder of Hydrofluorocarbon Molecules Entrapped in the Water Cages of Structure I Clathrate Hydrate

Water versus fluorine: Clathrate hydrates encaging hydrofluorocarbons as guests show both isotropic and anisotropic distributions within host water cages, depending on the number of fluorine atoms in the guest molecule; this is caused by changes in intermolecular interactions to host water molecules in the hydrates.

Preservation of carbon dioxide clathrate hydrate in the presence of trehalose under freezer conditions

To investigate the preservation of CO₂ clathrate hydrate in the presence of sugar for the novel frozen dessert, mass fractions of CO₂ clathrate hydrate in CO₂ clathrate hydrate samples coexisting with trehalose were intermittently measured. The samples were prepared from trehalose aqueous solution with trehalose mass fractions of 0.05 and 0.10 at 3.0 MPa and 276.2 K. The samples having particle sizes of 1.0 mm and 5.6–8.0 mm were stored at 243.2 K and 253.2 K for three weeks under atmospheric pressure. The mass fractions of CO₂ clathrate hydrate in the samples were 0.87–0.97 before the preservation, and CO₂ clathrate hydrate still remained 0.56–0.76 in the mass fractions for 5.6–8.0 mm samples and 0.37–0.55 for 1.0 mm samples after the preservation. The preservation in the trehalose system was better than in the sucrose system and comparable to that in the pure CO₂ clathrate hydrate system. This comparison indicates that trehalose is a more suitable sugar for the novel frozen carbonated dessert using CO₂ clathrate hydrate than sucrose in terms of CO₂ concentration in the dessert. It is inferred that existence of aqueous solution in the samples is a significant factor of the preservation of CO₂ clathrate hydrate in the presence of sugar.