I型和II型结构气体水合物的记忆效应

利用水合物二次生成实验装置, 采用“定容法”对I型(甲烷、二氧化碳)和II型(丙烷)结构气体水合物的二次生成进行了实验, 研究了不同结构水合物(I型、II型)彼此间的记忆效应, 发现水合物生成过程存在明显的诱导期, I型结构水合物间在二次生成过程中存在着记忆效应. I型与II型结构水合物之间在相互二次生成过程中存在着显著的记忆效应.     

Unusual crystalline and polycrystalline structures in methane hydrates

We present previously unreported crystalline and polycrystalline structures for methane hydrates obtained at relatively low pressures. These structures, which contain unusual cages with 12 pentagonal faces and 3 hexagonal faces, were observed during the atomistic simulations of the crystal growth of sI and sII methane hydrates. These 51263 cages have a significant impact on the structure of the resulting crystal and could explain several experimental observations regarding in-situ transformations between sI and sII hydrates. We document a previously unidentified structure of methane hydrates which we designate structure sK. Additionally, we predict a polycrystalline structure consisting of this new hydrate and sI and suggest a mechanism for the formation of a polycrystalline structure consisting of sequences of sI and sII hydrates.     

Thermal expansivity for sI and sII clathrate hydrates

Knowledge of thermal expansivity can aid in the understanding of both microscopic and macroscopic behavior of clathrate hydrates. Diffraction studies have shown that hydrate volume changes significantly (as much as 1.5% over 50 K) as a function of temperature. It has been demonstrated previously via statistical mechanics that a minor change in hydrate volume (e.g., a 1.5% change in volume or 0.5% change in lattice parameter) can lead to a major change in the predicted hydrate formation pressure (e.g., >15% at >100 MPa for methane). Because of this sensitivity, hydrate thermal expansivity measurements, for both Structures I and II with various guests, are needed help quantify volume distortions in hydrate lattices to ensure accurate hydrate phase equilibria predictions. In addition to macroscopic phase equilibria, the thermal expansion of different hydrates can give information about the interactions between the guest molecules and the host lattice. In this work, the hydrate lattice parameters for four Structure I (C2H6, CO2, 47% C2H6 + 53% CO2, and 85% CH4 + 15% CO2) and seven Structure II (C3H8, 60% CH4 + 40% C3H8, 30% C2H6 + 70% C3H8, 18% CO2 + 82% C3H8, 87.6% CH4 + 12.4% i-C4H10, 95% CH4 + 5% C5H10O, and a natural gas mixture) systems were measured as a function of temperature. The lattice parameter measurements were combined with existing literature values. Both sI and sII hydrates, with a few exceptions, had a common thermal expansivity, independent of hydrate guest. Many guest-dependent correlations for linear thermal expansivity have been proposed. However, we present two guest-independent, structure-dependent correlations for sI and sII lattices, which have been developed to express the normalized hydrate lattice parameters (and therefore volume) as a function of temperature.     

Structure transition and tuning pattern in the double (tetramethylammonium hydroxide + gaseous guests) clathrate hydrates

In this study, we present an extraordinary structural transition accompanying the occurrence of more than two coexisting clathrate hydrate phases in the double (CH4 + tetramethylammonium hydroxide (Me(4)NOH)) and (H2 + Me(4)NOH) ionic clathrate hydrates using solid-state NMR spectroscopy (high-powered decoupling and CP/MAS) and powder X-ray diffraction. It was confirmed that structure-I (sI) and structure-II (sII) hydrates coexist as the water concentration increases. In the Me(4)NOH-depleted region, the unique tuning phenomenon was first observed at a chemical shift of -8.4 ppm where relatively small gaseous CH4 molecules partly occupy the sII large cages (sII-L), pulling out large cationic Me(4)N+ that is considered to be strongly bound with the surrounding host lattices. Moreover, we note that, while pure Me(4)NOH.16H(2)O clathrate hydrates melted at 249 K under atmospheric pressure conditions, the double (CH4 + Me(4)NOH) clathrate hydrate maintained a solid state up to approximately 283 K under 120 bar of CH4 with a conductivity of 0.065 S cm(-1), suggesting its potential use as a solid electrolyte. The present results indicate that ionic contributions must be taken into account for ionic clathrate hydrate systems because of their distinctive guest dynamic behavior and structural patterns. In particular, microscopic analyses of ionic clathrate hydrates for identifying physicochemical characteristics are expected to provide new insights into inclusion chemistry.     

Temperature- and Pressure-induced Structural Transition of Binary Clathrate Hydrates

We discover new structure II (sII) hydrate forming agents of two C₄H₈O molecules (2-methyl-2-propen-1-ol and 2-butanone) and report the abnormal structural transition of binary C₄H₈O+CH₄ hydrates between structure I (sI) and sII with varying temperature and pressure conditions. In both (2-methyl-2-propen-1-ol+CH₄) and (2-butanone+CH₄) systems, the phase boundary of the two different hydrate phases (sI and sII) exists at the slope change of the phase-equilibrium curve in the semi-logarithmic plots. We confirm the crystal structures of two hydrates synthesized at low (278 K and 6 MPa) and high (286 K and 15 MPa) temperature and pressure conditions by using high-resolution powder diffraction and Raman spectroscopy. 2-Methyl-2-propen-1-ol and 2-butanone can occupy the large cages of sII hydrate at low temperature and pressure conditions; however, they are excluded from the hydrate phase at high temperature and pressure conditions, resulting in the formation of pure sI CH₄ hydrate.     

A new method for screening potential sII and sH hydrogen clathrate hydrate promoters with model potentials

A new predictive computational method for classifying clathrate hydrate promoter molecules is presented, based on the interaction energies between potential promoters and the water networks of sII and sH clathrates. The motivation for this work is identifying promoters for storing hydrogen compactly in clathrate hydrates. As a first step towards achieving this goal, we have developed a general method aimed at distinguishing between molecules that form sII clathrate hydrates and molecules that can-together with a weakly interacting help gas-form sH clathrate hydrates. The new computational method calculates differences in estimated formation energies of the sII and the sH clathrate hydrate. Model interaction potentials have been used, including the electrostatic interactions with newly calculated partial charges for all the considered potential promoter molecules. The methodology can discriminate between the clathrate structure types (sII or sH) formed by each potential promoter with good selectivity, i.e., better than achieved with a simple van der Waals diameter criterion.     

Micro-Raman investigations of mixed gas hydrates

We report laser Raman spectroscopic measurements on mixed hydrates (clathrates), with guest molecules tetrahydrofuran (THF) and methane (CH₄), at ambient pressure and at temperatures from 175 to 280 K. Gas hydrates were synthesized with different concentrations of THF ranging from 5.88 to 1.46 mol%. In all cases THF molecules occupied the large cages of sII hydrate. The present studies demonstrate formation of sII clathrates with CH₄ molecules occupying unfilled cages for concentrations of THF ranging from 5.88 to 2.95 mol%. The Raman spectral signature of hydrates with 1.46 mol% THF are distinctly different; hydrate growth was non-uniform and structural transformation occurred from sII to sI prior to clathrate melting.     

Heterogeneous crystal growth of methane hydrate on its sII [001] crystallographic face

This paper presents a systematic molecular simulation study of the heterogeneous crystal growth of methane hydrate sII from supersaturated aqueous methane solutions. The growth of sII hydrate on the [001] crystallographic face is achieved through utilization of a recently proposed methodology, and rates of crystal growth of 1 A/ns were sustained for the molecular models and specific conditions employed in this work. Characteristics of the crystals grown as well as properties and structure of the interface are examined. Water cages with a 5(12)6(3) arrangement, which are improper to both sI and sII structures, are identified during the heterogeneous growth of sII methane hydrate. We show that the growth of a [001] face of sII hydrate can produce an sI crystalline structure, confirming that cross-nucleation of methane hydrate structures is possible. Defects consisting of two methane molecules trapped in large 5(12)6(4) cages and water molecules trapped in small and large cages are observed, where in one instance we have found a large 5(12)6(4) cage containing three water molecules.     

Molecular dynamics study of structure H clathrate hydrates of methane and large guest molecules

Methane storage in structure H (sH) clathrate hydrates is attractive due to the relatively higher stability of sH as compared to structure I methane hydrate. The additional stability is gained without losing a significant amount of gas storage density as happens in the case of structure II (sII) methane clathrate. Our previous work has showed that the selection of a specific large molecule guest substance (LMGS) as the sH hydrate former is critical in obtaining the optimum conditions for crystallization kinetics, hydrate stability, and methane content. In this work, molecular dynamics simulations are employed to provide further insight regarding the dependence of methane occupancy on the type of the LMGS and pressure. Moreover, the preference of methane molecules to occupy the small (5(12)) or medium (4(3)5(6)6(3)) cages and the minimum cage occupancy required to maintain sH clathrate mechanical stability are examined. We found that thermodynamically, methane occupancy depends on pressure but not on the nature of the LMGS. The experimentally observed differences in methane occupancy for different LMGS may be attributed to the differences in crystallization kinetics and/or the nonequilibrium conditions during the formation. It is also predicted that full methane occupancies in both small and medium clathrate cages are preferred at higher pressures but these cages are not fully occupied at lower pressures. It was found that both small and medium cages are equally favored for occupancy by methane guests and at the same methane content, the system suffers a free energy penalty if only one type of cage is occupied. The simulations confirm the instability of the hydrate when the small and medium cages are empty. Hydrate decomposition was observed when less than 40% of the small and medium cages are occupied.     

Can amorphous nuclei grow crystalline clathrates? The size and crystallinity of critical clathrate nuclei

Recent studies reveal that amorphous intermediates are involved in the formation of clathrate hydrates under conditions of high driving force, raising two questions: first, how could amorphous nuclei grow into crystalline clathrates and, second, whether amorphous nuclei are intermediates in the formation of clathrate crystals for temperatures close to equilibrium. In this work, we address these two questions through large-scale molecular simulations. We investigate the stability and growth of amorphous and crystalline clathrate nuclei and assess the thermodynamics and kinetic factors that affect the crystallization pathway of clathrates. Our calculations show that the dissociation temperature of amorphous clathrates is just 10% lower than for the crystals, facilitating the formation of metastable amorphous intermediates. We find that, at any temperatures, the critical crystalline nuclei are smaller than critical amorphous nuclei. The temperature dependence of the critical nucleus size is well described by the Gibbs-Thomson relation, from which we extract a liquid-crystal surface tension in excellent agreement with experiments. Our analysis suggests that at high driving force the amorphous nuclei may be kinetically favored over crystalline nuclei because of lower free energy barriers of formation. We investigated the role of the initial structure and size of the nucleus on the subsequent growth of the clathrates, and found that both amorphous and sI crystalline nuclei yield crystalline clathrates. Interestingly, growth of the metastable sII crystal polymorph is always favored over the most stable sI crystal, revealing kinetic control of the growth and indicating that a further step of ripening from sII to sI is needed to reach the stable crystal phase. The latter results are in agreement with the observed metastable formation of sII CO(2) and CH(4) clathrate hydrates and their slow conversion to sI under experimental conditions.     

Dissociation enthalpies of synthesized multicomponent gas hydrates with respect to the guest composition and cage occupancy

This study presents the influences of additional guest molecules such as C2H6, C3H8, and CO2 on methane hydrates regarding their thermal behavior. For this purpose, the onset temperatures of decomposition as well as the enthalpies of dissociation were determined for synthesized multicomponent gas hydrates in the range of 173-290 K at atmospheric pressure using a Calvet heat-flow calorimeter. Furthermore, the structures and the compositions of the hydrates were obtained using X-ray diffraction and Raman spectroscopy as well as hydrate prediction program calculations. It is shown that the onset temperature of decomposition of both sI and sII hydrates tends to increase with an increasing number of larger guest molecules than methane occupying the large cavities. The results of the calorimetric measurements also indicate that the molar dissociation enthalpy depends on the guest-to-cavity size ratio and the actual concentration of the guest occupying the large cavities of the hydrate. To our knowledge, this is the first study that observes this behavior using calorimetrical measurements on mixed gas hydrates at these temperature and pressure conditions.     

Thermal expansivity of tetrahydrofuran clathrate hydrate with diatomic guest molecules

The guest dynamics and thermal behavior occurring in the cages of clathrate hydrates appear to be too complex to be clearly understood through various structural and spectroscopic approaches, even for the well-known structures of sI, sII, and sH. Neutron diffraction studies have recently been carried out to clarify the special role of guests in expanding the host water lattices and have contributed to revealing the influence factors on thermal expansivity. Through this letter we attempt to address three noteworthy features occurring in guest inclusion: (1) the effect of guest dimension on host water lattice expansion; (2) the effect of thermal history on host water lattice expansion; and (3) the effect of coherent/incoherent scattering cross sections on guest thermal patterns. The diatomic guests of H 2, D 2, N 2, and O 2 have been selected for study, and their size and mass dependence on the degree of lattice expansion have been examined, and four sII clathrate hydrates with tetrahydrofuran (THF) have been synthesized in order to determine their neutron powder diffraction patterns. After thermal cycling, the THF + H 2 clathrate hydrate is observed to exhibit an irreversible plastic deformation-like pattern, implying that the expanded lattices fail to recover the original state by contraction. The host-water cage dimension after degassing the guest molecules remains as it was expanded, and thus host-guest as well as guest-guest interactions will be altered if guest uptake reoccurs.     

Structures of hydrocarbon hydrates during formation with and without inhibitors

The formation of hydrates from a methane-ethane-propane mixture is more complex than with single gases. Using nuclear magnetic resonance (NMR) and high-pressure powder X-ray diffraction (PXRD), we have investigated the structural properties of natural gas hydrates crystallized in the presence of kinetic hydrate inhibitors (KHIs), two commercial inhibitors and two biological ice inhibitors, or antifreeze proteins (AFPs). NMR analyses indicated that hydrate cage occupancy was at near saturation for controls and most inhibitor types. Some exceptions were found in systems containing a new commercial KHI (HIW85281) and a recombinant plant AFP, suggesting that these two inhibitors could impact the kinetics of cavity formation. NMR analysis confirmed that the hydrate composition varies during crystal growth by kinetic effects. Strikingly, the coexistence of both structures I (sI) and II (sII) were observed in NMR spectra and PXRD profiles. It is suggested that sI phases may form more readily from liquid water. Real time PXRD monitoring showed that sI hydrates were less stable than sII crystals, and there was a conversion to the stable phase over time. Both commercial KHIs and AFPs had an impact on hydrate metastability, but transient sI PXRD intensity profiles indicated significantly different modes of interaction with the various inhibitors and the natural gas hydrate system.     

Microscopic observation and in-situ Raman scattering studies on high-pressure phase transformations of Kr hydrate

Direct observations through a microscope and in-situ Raman scattering measurements of synthesized single-crystalline Kr hydrate have been performed at pressures up to 5.2 GPa and 296 K. We have observed that the initial cubic structure II (sII) of Kr hydrate successively transforms to a cubic structure I (sI), a hexagonal structure, and an orthorhombic structure (sO) called "filled ice" at 0.45, 0.75, and 1.8 GPa, respectively. The sO phase exists at least up to 5.2 GPa. In addition to these transformations, we have also found the new phase behavior at 1.0 GPa, which is most likely caused by the change of cage occupancy of host water cages by guest Kr atoms without structural change. Raman scattering measurements for observed phases have shown that the lattice vibrational peak at around 130 cm(-1) disappears in the pressure region of sI, which enables us to distinguish the sI phase from sII and sH phases.     

Molecular dynamics study of the stability of methane structure H clathrate hydrates

Molecular dynamics simulations are used to study the stability of structure H (sH) methane clathrate hydrates in a 3 x 3 x 3 sH unit cell replica. Simulations are performed at experimental conditions of 300 K and 2 GPa for three methane intermolecular potentials. The five small cages of the sH unit cell are assigned methane guest occupancies of one and large cage guest occupancies of one to five are considered. Radial distribution functions, unit cell volumes, and configurational energies are studied as a function of large cage CH(4) occupancy. Free energy calculations are carried out to determine the stability of clathrates for large cage occupancies. Large cage occupancy of five is the most stable configuration for a Lennard-Jones united-atom potential and the Tse-Klein-McDonald potential parametrized for condensed methane phases and two for the most stable configuation for the Murad and Gubbins potential.     

常见客体分子对笼型水合物晶格常数的影响

Natural gas hydrates are considered as ideal alternative energy resources for the future, and the relevant basic and applied research has become more attractive in recent years. The influence of guest molecules on the hydrate crystal lattice parameters is of great significances to the understanding of hydrate structural characteristics, hydrate formation/decomposition mechanisms, and phase stability behaviors. In this study, we test a series of artificial hydrate samples containing different guest molecules (e.g. methane, ethane, propane, iso-butane, carbon dioxide, tetrahydrofuran, methane + 2, 2-dimethylbutane, and methane + methyl cyclohexane) by a low-temperature powder X-ray diffraction (PXRD). Results show that PXRD effectively elucidates structural characteristics of the natural gas hydrate samples, including crystal lattice parameters and structure types. The relationships between guest molecule sizes and crystal lattice parameters reveal that different guest molecules have different controlling behaviors on the hydrate types and crystal lattice constants. First, a positive correlation between the lattice constants and the van der Waals diameters of homologous hydrocarbon gases was observed in the single-guest-component hydrates. Small hydrocarbon homologous gases, such as methane and ethane, tended to form sI hydrates, whereas relatively larger molecules, such as propane and iso-butane, generated sⅡ hydrates. The hydrate crystal lattice constants increased with increasing guest molecule size. The types of hydrates composed of oxygen-containing guest molecules (such as CO₂ and THF) were also controlled by the van der Waals diameters. However, no positive correlation between the lattice constants and the van der Waals diameters of guest molecules in hydrocarbon hydrates was observed for CO₂ hydrate and THF hydrate, probably due to the special interactions between the guest oxygen atoms and hydrate "cages". Furthermore, the influences of the macromolecules and auxiliary small molecules on the lengths of the different crystal axes of the sH hydrates showed inverse trends. Compared to the methane + 2, 2-dimethylbutane hydrate sample, the length of the a-axis direction of the methane + methyl cyclohexane hydrate sample was slightly smaller, whereas the length of the c-axis direction was slightly longer. The crystal a-axis length of the sH hydrate sample formed with nitrogen molecules was slightly longer, whereas the c-axis was shorter than that of the methane + 2, 2-dimethylbutane hydrate sample at the same temperature.     

Molecular simulation of nanoclusters of gas hydrates in a water shell. The mechanical state of the system

The molecular dynamics method is employed to study hydrates of methane (sI), and krypton hydrate (sII), as well as an ice nanocluster in a supercooled water shell. The main attention is focused on the local structure and the mechanical state of two-phase nanosized systems, which is described using the local pressure tensor. Analysis of the temperature dependence of the local pressure allows one to compare two possible mechanisms responsible for the anomalous stability of gas hydrates at ambient pressure. According to the first mechanism, the water shell plays the role of a barrier that prevents the gas from escaping from the hydrate core. The second mechanism implies that the water shell generates additional pressure, which transfers the hydrate to a thermodynamically stable state. Results of molecular dynamics simulation indicate that both mechanisms are simultaneously involved in the stabilization of the hydrate nanocluster.     

Free energies of carbon dioxide sequestration and methane recovery in clathrate hydrates

Classical molecular dynamics simulations are used to compare the stability of methane, carbon dioxide, nitrogen, and mixed CO(2)N(2) structure I (sI) clathrates under deep ocean seafloor temperature and pressure conditions (275 K and 30 MPa) which were considered suitable for CO(2) sequestration. Substitution of methane guests in both the small and large sI cages by CO(2) and N(2) fluids are considered separately to determine the separate contributions to the overall free energy of substitution. The structure I clathrate with methane in small cages and carbon dioxide in large cages is determined to be the most stable. Substitutions of methane in the small cages with CO(2) and N(2) have positive free energies. Substitution of methane with CO(2) in the large cages has a large negative free energy and substitution of the methane in the large cages with N(2) has a small positive free energy. The calculations show that under conditions where storage is being considered, carbon dioxide spontaneously replaces methane from sI clathrates, causing the release of methane. This process must be considered if there are methane clathrates present where CO(2) sequestration is to be attempted. The calculations also indicate that N(2) does not directly compete with CO(2) during methane substitution or clathrate formation and therefore can be used as a carrier gas or may be present as an impurity. Simulations further reveal that the replacement of methane with CO(2) in structure II (sII) cages also has a negative free energy. In cases where sII CO(2) clathrates are formed, only single occupancy of the large cages will be observed.     

Water cavities of sH clathrate hydrate stabilized by molecular hydrogen

X-ray diffraction and Raman spectroscopic measurements confirm that molecular hydrogen can be contained within the small water cavities of a binary sH clathrate hydrate using large guest molecules that stabilize the large cavity. The potential increase in hydrogen storage could be more than 40% when compared with binary sII hydrates. This work demonstrates the stabilization of hydrogen in a hydrate structure previously unknown for encapsulating molecular hydrogen, indicating the potential for other inclusion compound materials with even greater hydrogen storage capabilities.     

Effect of Guest–Host Hydrogen Bonding on the Structures and Properties of Clathrate Hydrates

To provide improved understanding of guest–host interactions in clathrate hydrates, we present some correlations between guest chemical structures and observations on the corresponding hydrate properties. From these correlations it is clear that directional interactions such as hydrogen bonding between guest and host are likely, although these have been ignored to greater or lesser degrees because there has been no direct structural evidence for such interactions. For the first time, single‐crystal X‐ray crystallography has been used to detect guest–host hydrogen bonding in structure II (sII) and structure H (sH) clathrate hydrates. The clathrates studied are the tert‐butylamine (tBA) sII clathrate with H₂S/Xe help gases and the pinacolone + H₂S binary sH clathrate. X‐ray structural analysis shows that the tBA nitrogen atom lies at a distance of 2.64 Å from the closest clathrate hydrate water oxygen atom, whereas the pinacolone oxygen atom is determined to lie at a distance of 2.96 Å from the closest water oxygen atom. These distances are compatible with guest–water hydrogen bonding. Results of molecular dynamics simulations on these systems are consistent with the X‐ray crystallographic observations. The tBA guest shows long‐lived guest–host hydrogen bonding with the nitrogen atom tethered to a water HO group that rotates towards the cage center to face the guest nitrogen atom. Pinacolone forms thermally activated guest–host hydrogen bonds with the lattice water molecules; these have been studied for temperatures in the range of 100–250 K. Guest–host hydrogen bonding leads to the formation of Bjerrum L‐defects in the clathrate water lattice between two adjacent water molecules, and these are implicated in the stabilities of the hydrate lattices, the water dynamics, and the dielectric properties. The reported stable hydrogen‐bonded guest–host structures also tend to blur the longstanding distinction between true clathrates and semiclathrates.