CH4 Hydrate Formation between Silica and Graphite Surfaces: Insights from Microsecond Molecular Dynamics Simulations

Multifunctional Amphiphobic Coating toward Ultralow Interfacial Adhesion of Hydrates

Hydrate adhesion is a challenging issue in some practical applications. However, most current anti-hydrate coatings fail to maintain their properties when subject to crude oil and corrosive contaminants. In addition, the effect of surface properties on the nucleation of hydrates is still unexplored from a microscopic perspective. In this study, a multifunctional amphiphobic PF/ZSM-5 coating consisting of 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane modified ZSM-5 zeolite (F/ZSM-5) and adhesive polyethersulfone was fabricated by the spraying method. The interfacial nucleation and adhesion of hydrates on substrates were studied from a microscopic perspective. The coating exhibited excellent repellencies to various liquids, including water, edible oil, liquid paraffin, vacuum pump oil, n-hexadecane, and crude oil. The tetrabutylammonium bromide (TBAB) hydrate is readily nucleated on the bare Cu surface. In contrast, the coated substrate effectively inhibited the hydrate nucleation on the surface and even reduced the adhesion force to 0 mN/m. Furthermore, this coating was fouling- and corrosion-resistant and can maintain an ultralow hydrate adhesion force even after immersion in crude oil or TBAB solution for 20 and 300 d, respectively. The durable anti-hydrate performance of the coating was attributed mainly to the unique architecture and excellent amphiphobic properties enabling stable air cushions between the solid-liquid interface.

Effects of marine environments on methane hydrate formation in clay nanopores: A molecular dynamics study

In nature, CH₄ hydrates are mainly buried in marine sediments. The complex marine environments on the seafloor continuously affect hydrate formation. Herein, systematic molecular simulations have been performed to investigate CH₄ hydrate formation in clay nanopore, mainly affected by several marine environmental factors, including seawater salinity, pressure and temperature. Simulation results show that these factors exert different effects on hydrate formation in the nanopore and the outside bulk solutions by affecting the mass transfer and phase separation inside and outside of the nanopore. Specifically, high salinity hinders the diffusion of CH₄ molecules from nanopores into the outside bulk solutions, promoting hydrate formation in nanopore and inhibiting hydrate formation in bulk solution; salinity has a dual effect on hydrate formation in the whole system by changing the local CH₄ concentration via the formation of the hydration of salt ions. High pressure favors the diffusion of CH₄ molecules from nanopore into outside bulk solutions, promoting hydrate formation in bulk solution and inhibiting hydrate formation in nanopore; high pressure promotes hydrate formation at the nanopore throats by increasing CH₄ concentration and reducing ion concentration therein. In contrast, temperature significantly affects hydrate formation in the system by causing phase separation, i.e. high temperature promotes the aggregation of CH₄ molecules to form nanobubbles and inhibits hydrate formation. Under high temperature conditions, the nanobubble in the nanopore gradually decomposes, while the nanobubble in the outside bulk solution grows an extra-large cylindrical nanobubble. These molecular insights into the formation behavior of CH₄ hydrates in clay nanopores are helpful for understanding the formation process of natural gas hydrates in marine sediments and the development and utilization of CH₄ hydrates.

A Review of the Effect of Porous Media on Gas Hydrate Formation

Most gas hydrates on the earth are in sediments and permafrost areas, and porous media are often used industrially as additives to improve gas hydrate formation. For further understanding its exploration and exploitation under natural conditions and its application in industry, it is necessary to study the effect of porous media on hydrate formation. The results show that the stacked porous media affects the phase equilibrium of hydrate formation depending on the competition water activity and large specific surface areas, while integrated porous media, such as metal foam, can transfer the hydration heat rapidly and moderate the hydrate phase equilibrium. A supersaturated metal-organic framework is able to significantly improve gas storage performance and can be used as a new material to promote hydrate formation. However, the critical particle size should be studied further for approaching the best promotion effect. In addition, together with the kinetic accelerators, porous media has a synergistic effect on gas hydrate formation. The carboxyl and hydroxyl groups on the surface of porous media can stabilize hydrate crystals through hydrogen bonding. However, the hydroxyl radicals on the silica surface inhibit the combination of CH₄ and free water, making the phase equilibrium conditions more demanding.

Molecular Insights into Factors Affecting the Generation Behaviors, Dynamic Properties, and Interfacial Structures of Methane Gas Bubbles

Molecular dynamics simulations were performed to study the effects of temperatures, pressures, and methane mole fractions on the generation behaviors, dynamic properties, and interfacial structures of methane gas bubbles. Methane gas bubbling can be promoted by high temperatures and high mole fractions of methane, which come from the generation of larger methane clusters in solution. Bubbles were found to be highly dynamic, with more methane molecules exchanging between bubbles and the surrounding solution at high pressures and in systems with high mole fractions of methane. The interfacial structures between bubbles and the surrounding solution were rough at a molecular level, and the roughness of the outermost methane and water molecules was high at high temperatures, low pressures, and in systems with high methane mole fractions. The dissolution of methane molecules depended on the interactions between the outermost methane and water molecules, which would become stronger with decreasing temperatures, increasing pressures, and decreasing methane mole fractions. The results obtained can help in understanding both the generation behaviors of bubbles when gas hydrates decompose and the re-nucleation behaviors of gas hydrates in the presence of bubbles.

Effects of surface property of mixed clays on methane hydrate formation in nanopores: A molecular dynamics study

HYPOTHESIS

Mixed clays (e.g. montmorillonite, illite and kaolinite) are ubiquitous in hydrate-bearing sediments under seafloor, and their surfaces inevitably affect the formation of natural gas hydrates therein. Nevertheless, the actual effects of clay surfaces on hydrate formation remain elusive.

EXPERIMENTS

Systematic molecular dynamics simulations have been performed to investigate CH₄ hydrate formation in mixed clay nanopores of montmorillonite, illite and kaolinite, to examine the effects of surface property and layer charges of mixed clays.

FINDINGS

Simulation results indicate that the surfaces of mixed clays affect CH₄ hydrate formation in the nanopores by changing the CH₄ concentration (x_CH4) and ion concentration (x_ions) in the middle region of the nanopores via surface adsorption for CH₄, H₂O and ions. Specifically, the surfaces of montmorillonite and illite, the siloxane and gibbsite surfaces of kaolinite show different affinities for adsorbing CH₄, H₂O and ions, which can significantly affect the x_CH4 and x_ions in the interfacial and middle regions of the nanopores. Moreover, hydrate growth shows certain surface preference. These molecular insights into the effect of mixed clay surfaces on CH₄ hydrate formation can help to understand the formation mechanism of natural gas hydrate in marine sediments.

Unraveling Adhesion Strength between Gas Hydrate and Solid Surfaces

Natural gas hydrate is a promising future energy source, but it also poses a huge threat to oil and gas production due to its ability to deposit within and block pipelines. Understanding the atomistic mechanisms of adhesion between the hydrate and solid surfaces and elucidating its underlying key determining factors can shed light on the fundamentals of novel antihydrate materials design. In this study, large-scale molecular simulations are employed to investigate the hydrate adhesion on solid surfaces, especially with focuses on the atomistic structures of intermediate layer and their influences on the adhesion. The results show that the structure of the intermediate layer formed between hydrate and solid surface is a competitive equilibrium of induced growth from both sides, and is regulated by the content of guest molecules. By comparing the fracture behaviors of the hydrate-solid surface system with different intermediate structures, it is found that both the lattice areal density of water structure and the adsorption of guest molecules on the interface together determine the adhesion strength. Based on the analysis of the adhesion strength distribution, we have also revealed the origins of the drastic difference in adhesion among different water structures such as ice and hydrate. Our simulation indicates that ice-adhesion strength is approximately five times that of lowest hydrate adhesion strength. This finding is surprisingly consistent with the available experimental results.

Interfacial study of clathrates confined in reversed silica pores

Storing methane in clathrates is one of the most promising alternatives for transporting natural gas (NG) as it offers similar gas densities to liquefied and compressed NG while offering lower safety risks. However, the practical use of clathrates is limited given the extremely low temperatures and high pressures necessary to form these structures. Therefore, it has been suggested to confine clathrates in nanoporous materials, as this can facilitate clathrate's formation conditions while preserving its CH4 volumetric storage. Yet, the choice of nanoporous materials to be employed as the clathrate growing platform is still rather arbitrary. Herein, we tackle this challenge in a systematic way by computationally exploring the stability of clathrates confined in alkyl-grafted silica materials with different pore sizes, ligand densities and ligand types. Based on our findings, we are able to propose key design criteria for nanoporous materials favoring the stability of a neighbouring clathrate phase, namely large pore sizes, high ligand densities, and smooth pore walls. We hope that the atomistic insight provided in this work will guide and facilitate the development of new nanomaterials designed to promote the formation of clathrates.

Gas hydrates in confined space of nanoporous materials: new frontier in gas storage technology

Gas hydrates (clathrate hydrates, clathrates, or hydrates) are crystalline inclusion compounds composed of water and gas molecules. Methane hydrates, the most well-known gas hydrates, are considered a menace in flow assurance. However, they have also been hailed as an alternative energy resource because of their high methane storage capacity. Since the formation of gas hydrates generally requires extreme conditions, developing porous material hosts to synthesize gas hydrates with less-demanding constraints is a topic of great interest to the materials and energy science communities. Though reports of modeling and experimental analysis of bulk gas hydrates are plentiful in the literature, reliable phase data for gas hydrates within confined spaces of nanoporous media have been sporadic. This review examines recent studies of both experiments and theoretical modeling of gas hydrates within four categories of nanoporous material hosts that include porous carbons, metal-organic frameworks, graphene nanoslits, and carbon nanotubes. We identify challenges associated with these porous systems and discuss the prospects of gas hydrates in confined space for potential applications.

Does Confinement Enable Methane Hydrate Growth at Low Pressures? Insights from Molecular Dynamics Simulations

Natural methane hydrates are estimated to be the largest source of unexploited hydrocarbon fuel. The ideal conditions for methane hydrate formation are low temperatures and high pressures. On the other hand, recent experimental studies suggest that porous materials, thanks to their confinement effects, can enable methane hydrate formation at milder conditions, although there has not been a consensus on this. A number of studies have investigated methane hydrate growth in confinement by employing molecular simulations; however, these were carried out at either very high pressures or very low temperatures. Therefore, the effects of confinement on methane hydrate growth at milder conditions have not yet been elaborated by molecular simulations. In order to address this, we carried out a systematic study by performing molecular dynamics (MD) simulations of methane water systems. Using a direct phase coexistence approach, microsecond-scale MD simulations in the isobaric-isothermal (NPT) ensemble were performed in order to study the behavior of methane hydrates in the bulk and in confined nanospaces of hydroxylated silica pores at external pressures ranging from 1 to 100 bar and a simulation temperature corresponding to a 2 °C experimental temperature. We validated the combination of the TIP4P/ice water and TraPPE-UA methane models in order to correctly predict the behavior of methane hydrates in accordance to their phase equilibria. We also demonstrated that the dispersion corrections applied to short-range interactions lead to artificially induced hydrate growth. We observed that in the confinement of a hydroxylated silica pore, a convex-shaped methane nanobuble forms, and methane hydrate growth primarily takes place in the center of the pore rather than the surfaces where a thin water layer exists. Most importantly, our study showed that in the nanopores methane hydrate growth can indeed take place at pressures which would be too low for the growth of methane hydrates in the bulk.

Effects of Layer-Charge Distribution of 2:1 Clay Minerals on Methane Hydrate Formation: A Molecular Dynamics Simulation Study

Molecular dynamics simulations were used to investigate the effects of the external surface of a 2:1 clay mineral with different charge amounts and charge locations on CH₄ hydrate formation. The results showed that 5¹², 5¹²6², 5¹²6³, and 5¹²6⁴ were formed away from the clay mineral surface. The surface of the clay mineral with high- and low-charge layers was occupied by Na⁺ to form various distributions of outer- and inner-sphere hydration structures, respectively. The adsorbed Na⁺ on the high-charge layer surface reduced the H₂O activity by disturbing the hydrogen bond network, resulting in low tetrahedral arrangement of H₂O molecules near the layer surface, which inhibited CH₄ hydrate formation. However, more CH₄ molecules were adsorbed onto the vacancy in the Si-O rings of a neutral-charge layer to form semicage structures. Thus, the order parameter of H₂O molecules near this surface indicated that the arrangement of H₂O molecules resulted in a more optimal tetrahedral structure for CH₄ hydrate formation than that near the negatively charged layer surface. Different nucleation mechanisms of the CH₄ hydrate on external surfaces of clay mineral models were observed. For clay minerals with negatively charged layers (i.e., high and low charge), the homogeneous nucleation of the CH₄ hydrate occurred away from the surface. For a clay mineral with a neutral-charge layer, the CH₄ hydrate could nucleate either in the bulk-like solution homogeneously or at the clay mineral-H₂O interface heterogeneously.

Formation of CH4 Hydrate in a Mesoporous Metal-Organic Framework MIL-101: Mechanistic Insights from Microsecond Molecular Dynamics Simulations

The formation of CH₄ hydrate in a mesoporous metal-organic framework MIL-101 is investigated by microsecond molecular dynamics simulations. CH₄ hydrate is observed to form preferentially in the outer space of MIL-101 cavities rather than inside the cavities; only when the hydrate formation is nearly complete in the outer space can stable hydrate form in MIL-101 cavities. The underlying reason is revealed to be the easy dissociation of small hydrate clusters formed in the nanospace of the cavities, because of the diffusion of CH₄ molecules out of the cavities into the outer space. Compared with dry MIL-101, the CH₄ storage capacity of H₂O-saturated MIL-101 is drastically reduced as the cavities are occupied by H₂O. When oversaturated with H₂O, however, extra H₂O molecules in the outer space of the cavities can form considerable CH₄ hydrate, significantly promoting CH₄ storage capacity. This study provides important mechanistic insights into the formation mechanism and process of CH₄ hydrate in MIL-101 and will facilitate the design of emerging materials for energy storage.

Methane Hydrate Nucleation within Elastic Confined Spaces: Suitable Spacing and Elasticity Can Accelerate the Nucleation

Elastic materials are candidates for process intensification of gas storage by forming gas hydrate. In this work, molecular dynamics simulations of hydrate nucleation in elastic silica double layers were performed to study the effect of elastic confined spaces on hydrate formation. It is found that in narrow confined spaces, hexagonal rings dominated the hydrogen bond network of water molecules established rapidly by a multisite nucleation mechanism. With molecules added, a bilayer water structure was formed finally because elastic space can adapt the volume expansion. In medium and wide confined spaces, hydrates were formed from a series of "pseudo cages" which are considered as precursors of complete hydrate cages. Moreover, the induction time for nucleation was a minimum when the elasticity of the silica layer changes: nucleation is fastest in the weak-elastic system. When the elasticity increases, it becomes hard to adapt the volume expansion during nucleation and also difficult to nucleate in very weak-elastic systems because of the fluctuation of the layers.

CH4 Nanobubbles on the Hydrophobic Solid-Water Interface Serving as the Nucleation Sites of Methane Hydrate

The surface hydrophobicity of solid particles plays a critical role in the nucleation of gas hydrate formation, and it was found that the hydrophobic surface will promote this nucleation process, but the underlying mechanism is still unveiled. Herein, we proposed for the first time our new theory that the formation of methane nanoscale gas bubbles on the hydrophobic surface provides the nuclei sites for further formation of methane hydrate. First, we studied the effect of hydrophobicity of particles on the nucleation of hydrate. It was found that the hydrophobic graphite and silica particles would promote the nucleation of hydrate, but the hydrophilic silica particles did not promote the methane hydrate nucleation. Then, we designed the atomic force microscopy experiment to explain this mechanism from a nanometer scale. The results showed that the methane nanobubbles were formed on the hydrophobic highly ordered pyrolytic graphite surface, but they were hard to form on the hydrophilic mica surface. These results indicated that the methane nanobubbles on the hydrophobic surface could provide the gas hydrate nucleation sites and may induce a rapid nucleation of methane hydrate.

The molecular mechanism of the inhibition effects of PVCaps on the growth of sI hydrate: an unstable adsorption mechanism

Adsorption and non-binding-hydrate sites produce synergistic effects that lead to unstable adsorption and inhibition effects (recurring destruction of hydrate).