Methane hydrate formation in confined nanospace can surpass nature

Phase Stability of CH4 and CO2 Hydrates under Confinement Predicted by Machine Learning

Understanding the phase stability of gas hydrates under confinement is fundamental to the geological stability evolutions of gas hydrate systems on Earth. Herein, the phase stability of CH4 and CO2 hydrates under confinement is predicted by machine learning. Three machine learning models, including support vector machine, random forest, and gradient boosting decision tree, are constructed to predict the phase stability of CH4 and CO2 hydrates under confinement. Our machine learning results show that the prediction accuracy of the support vector machine model is highest, yet the prediction accuracy of the random forest model is lowest among those machine learning models in determining the phase stability of confined gas hydrates. Based on their performance in predicting the phase stability of confined gas hydrates, the support vector machine model with a training set fraction of 0.7 is finally chosen to deal with the unknown phase stability of confined gas hydrates. Importantly, the average accuracy of the support vector machine model can reach more than 90% in predicting the unknown phase stability of both CH4 and CO2 hydrates. The trained machine learning models can help us to quickly and accurately determine the phase stability of CH4 and CO2 hydrates under confinement in future applications.

Gas Hydrate Dynamics with Parameter-Free Clathrate Phase Description: Validation for Hydrate Formation and Dissociation

One of the major challenges involved in clathrate hydrate science that has remained for more than six decades lies in highly parametric clathrate phase estimation. In this contribution, a recently developed parameter-free hydrate phase statistical equilibrium model is employed for the first time to formulate the formation and dissociation dynamics of clathrates and predict their experimental observation at diverse geological conditions. This rigorous thermokinetic model takes into account various practical issues, notably hydrate formation in nanometer-sized pores (confirmed through seismic survey studies), irregularity in porous particle shape and pore size, renewal of the particle surface over which hydrate majorly forms and decays, and nth-order phase transformation. The model parameters are identified by formulating the genetic algorithm-based optimization strategy. Finally, this multicomponent hydrate model is tested by predicting the formation and decomposition data having pure water as well as saltwater with and without porous media. The proposed formulation secures a promising performance with a lower absolute average relative deviation for a wide variety of data sets over the latest hydrate models.

Surface modification of mesostructured cellular foam to enhance hydrogen storage in binary THF/H2 clathrate hydrate

This study introduces solid-state tuning of a mesostructured cellular foam (MCF) to enhance hydrogen (H2) storage in clathrate hydrates. Grafting of promoter-like molecules (e.g., tetrahydrofuran) at the internal surface of the MCF resulted in a substantial improvement in the kinetics of formation of binary H2-THF clathrate hydrate. Identification of the confined hydrate as sII clathrate hydrate and enclathration of H2 in its small cages was performed using XRD and high-pressure 1H NMR spectroscopy respectively. Experimental findings show that modified MCF materials exhibit a ∼1.3 times higher H2 storage capacity as compared to non-modified MCF under the same conditions (7 MPa, 265 K, 100% pore volume saturation with a 5.56 mol% THF solution). The enhancement in H2 storage is attributed to the hydrophobicity originating from grafting organic molecules onto pristine MCF, thereby influencing water interactions and fostering an environment conducive to H2 enclathration. Gas uptake curves indicate an optimal tuning point for higher H2 storage, favoring a lower density of carbon per nm2. Furthermore, a direct correlation emerges between higher driving forces and increased H2 storage capacity, culminating at 0.52 wt% (46.77 mmoles of H2 per mole of H2O and 39.78% water-to-hydrate conversions) at 262 K for the modified MCF material with fewer carbons per nm2. Notably, the substantial H2 storage capacity achieved without energy-intensive processes underscores solid-state tuning's potential for H2 storage in the synthesized hydrates. This study evaluated two distinct kinetic models to describe hydrate growth in MCF. The multistage kinetic model showed better predictive capabilities for experimental data and maintained a low average absolute deviation. This research provides valuable insights into augmenting H2 storage capabilities and holds promising implications for future advancements.

Insights into the mechanical stability of tetrahydrofuran hydrates from experimental, machine learning, and molecular dynamics perspectives

Natural gas hydrates (NGHs) hold immense potential as a future energy resource and for sustainable applications such as gas capture and storage. Due to the challenging formation conditions, however, their mechanical properties remain poorly understood. Herein, the mechanical characteristics of tetrahydrofuran (THF) hydrates, a proxy for methane hydrates, were investigated at different ice contents, strain rates, and temperatures using uniaxial compressive experiments. The results unveil a distinct behavior in the peak strength of THF hydrates with a varying ice content, strain rate and temperature, exhibiting an increase as the strain rate and temperature decrease, in contrast to the peak strength-strain rate relationship observed in polycrystalline ice. Based on the experimental data, four machine learning (ML) models including extreme gradient boosting (XGboost), multilayer perceptron (MLP), gradient boosting decision tree (GBDT) and decision tree (DT) were developed to predict the peak strength. The XGboost model demonstrates superior predictive performance, emphasizing the significant influence of ice content and temperature on the peak strength of hydrates. Furthermore, molecular dynamics (MD) simulations were employed to gain insights into the dissociation and formation processes of clathrate cages, as well as phase transitions and amorphization occurring at grain boundaries (GBs) involving diverse unconventional clathrate cages, including 51265, 4151062, 4151064, 425861 and 425862, with 425861 and 425862 cages being predominant. This study enhances our understanding of the mechanical properties and deformation mechanisms of hydrates and provides a ML-based predictive framework for estimating the compressive strength of hydrates under diverse coupling conditions. The findings have significant implications for stability assessments of NGHs and the exploitation of NGH resources.

Interfacial Heat and Mass Transfer Effects on Secondary Hydrate Formation under Different Dissociation Conditions

Secondary hydrate formation or hydrate reformation poses a serious threat to the oil and gas transportation safety and natural gas hydrate exploitation efficiency. The hydrate reformation behaviors in porous media have been widely studied in large simulators due to their importance in traditional industries and new energy resources. However, it is difficult to understand the interfacial effects of hydrate reformation on the surface and in micropores of the porous media via a basic experimental apparatus. In this work, in situ X-ray computed tomography (X-CT) technology is used to detect the period, distribution, volume, and morphology characteristics of secondary hydrate formation during hydrate dissociation under depressurization, thermal stimulation, and the combined conditions. It is found that the secondary hydrate formation is inevitable under any conditions of hydrate dissociation. The secondary hydrate morphology varies among porous, grain-enveloping, grain-cementing, granular, and patchy structures, which are closely correlated to the hydrate reformation region and gas/water saturated conditions during hydrate dissociation. Accordingly, we revealed that the interfacial superheating phenomenon before hydrate dissociation could provide a supercooling condition for hydrate reformation. The gas flow along the interface of pores and inside the liquid water, as well as gas accumulation in noninterconnected pores, would exaggerate the hydrate reformation by increasing the local pore pressure. Meanwhile, the hydrate reformation aggravates the nonuniform distribution of gas hydrates in pores. In order to avoid hydrate reformation during dissociation, we further compared hydrate reformation and dissociation behaviors under three hydrate dissociation conditions. It is revealed that the combination of thermal stimulation and depressurization is an effective method for hydrate dissociation by retarding secondary hydrate formation. This study provides visual evidence and an interaction mechanism between interfacial heat and mass transfer, as well as secondary hydrate formation behaviors, which can be favorable for future quantitative research on secondary hydrate formation in different scales under various dissociation conditions.

Evidence of Spontaneous Formation of Two-Dimensional Amorphous Clathrates on Superhydrophilic Surfaces

Clathrate hydrates reserved in the seabed are often dispersed in the pores of coarse-grained sediments; hence, their formation typically occurs under nanoconfinement. Herein, we show the first molecular dynamics (MD) simulation evidence of the spontaneous formation of two-dimensional (2D) clathrate hydrates on crystal surfaces without conventional nanoconfinement. The kinetic process of 2D clathrate formation is illustrated via simulated single-molecule deposition. 2D amorphous patterns are observed on various superhydrophilic face-centered cubic surfaces. Notably, the formation of 2D amorphous clathrate can occur over a wide range of temperatures, even at room temperature. The strong water-surface interaction, the characteristic properties of guest-gas molecules, and the underlying surface structure dictate the formation of 2D amorphous clathrates. Semiquantitative phase diagrams of 2D clathrates are constructed where representative patterns of 2D clathrates for characteristic gas molecules on prototypical Pd(111) and Pt(111) surfaces are confirmed by independent MD simulations. A tunable pattern of 2D amorphous clathrates is demonstrated by changing the lattice strain of the underlying substrate. Moreover, ab initio MD simulations confirm the stability of 2D amorphous clathrate. The underlining physical mechanism for 2D clathrate formation on superhydrophilic surfaces is elucidated, which offers deeper insight into the crucial role of water-surface interaction.

Methane hydrate efficient formation in a 3D-rGO/SDBS composite

The optimization of storage space and material composition can significantly improve the generation rate and storage capacity of methane hydrate, which is important for the industrial application of solidified natural gas (SNG) technology. In our report, the effects of the presence of SDBS (sodium dodecylbenzene sulfonate), GO (graphene oxide), 3D-rGO (3D-reduced graphene oxide) and 3D-rGO/SDBS (3D-reduced graphene oxide/sodium dodecylbenzene sulfonate) on the methane hydrate generation process are investigated. The results show that the heterogeneous effect on the solid-phase surface of 3D-rGO/SDBS and its interconnected three-dimensional (3D) structure can achieve rapid nucleation. In addition, the presence of 3D-rGO/SDBS can increase the dissolution and dispersion of gas in solution and further enhance the gas-liquid mass transfer, thus realizing efficient methane storage. The maximum methane storage capacity of 188 v/vw is obtained with 600 ppm of 3D-rGO/SDBS in water, reaching 87% of the theoretical maximum storage capacity. The addition of 3D-rGO/SDBS also significantly reduces the induction time and accelerates the formation rate of methane hydrate. This study reveals that 3D graphene materials have excellent kinetic promotion effects on methane hydrate formation, explores and enriches the hydrate-promoting mechanism, and provides essential data and theoretical basis for the research of new promoters in the field of SNG technology.

Recent progress of MOF-functionalized nanocomposites: From structure to properties

Metal-organic frameworks (MOFs) are novel crystalline porous materials assembled from metal ions and organic ligands. The adaptability of their design and the fine-tuning of the pore structures make them stand out in porous materials. Furthermore, by integrating MOF guest functional materials with other hosts, the novel composites have synergistic benefits in numerous fields such as batteries, supercapacitors, catalysis, gas storage and separation, sensors, and drug delivery. This article starts by examining the structural relationship between the host and guest materials, providing a comprehensive overview of the research advancements in various types of MOF-functionalized composites reported to date. The review focuses specifically on four types of spatial structures, including MOFs being (1) embedded in nanopores, (2) immobilized on surface, (3) coated as shells and (4) assembled into hybrids. In addition, specific design ideas for these four MOF-based composites are presented. Some of them involve in situ synthesis method, solvothermal method, etc. The specific properties and applications of these materials are also mentioned. Finally, a brief summary of the advantages of these four types of MOF composites is given. Hopefully, this article will help researchers in the design of MOF composite structures.

Advances in Nanomaterials for Sustainable Gas Separation and Storage: Focus on Clathrate Hydrates

ConspectusClathrate hydrates, also known as gas hydrates, are a type of inclusion compound formed in highly developed nanoporous lattice spaces created by water molecules, where gas molecules such as CO2, H2, CH4, and other low-molecular-weight liquid molecules are trapped. The nanoporous cage formed by water molecules serves as the "host", while the trapped gas or low-molecular-weight liquid molecules such as tetrahydrofuran act as "guests". Early on, clathrate hydrates drew attention as a potential replacement for conventional natural gas due to their natural gas hydrate form, which contains natural gases as guests and exists in permafrost or sea floors. Recently, based on the unique physicochemical properties of clathrate hydrates, efforts are being made to utilize synthetic clathrate hydrates in various separation processes such as post- and pre-combustion CO2 capture, H2 storage, natural gas storage and transportation, wastewater desalination, and more. While it is undeniable that clathrate hydrates are based on principles that are beneficial for the separation and storage of gas molecules, there are several challenges that must be addressed for their practical application. These challenges include (i) the limitation of gas storage capacity due to the confined size of nanoporous cages, (ii) the relatively high-pressure and low-temperature thermodynamic storage conditions typically required for clathrate hydrate formation, and (iii) slow formation kinetics and low gas hydrate conversion, which are also essential issues that need to be resolved for the meaningful implementation of clathrate hydrates. In this Account, we aim to introduce recent noteworthy research findings, including those from our research team, focusing on addressing these challenges. We explored the untapped potential of clathrate hydrates by bridging the gap between macroscopic and microscopic properties. This has led to breakthroughs in sustainable gas separation and storage applications. By revealing the hidden nature of these hydrates, we have effectively mitigated their inherent limitations, setting the stage for more feasible and efficient H2 storage solutions through the introduction of hydrogen-natural gas blends to clathrate hydrates. Additionally, we have demonstrated the tuning effect on all naturally formed hydrate structures, offering new insights into their underlying properties and macroscopic behavior. Furthermore, our research has proposed a highly efficient hydrate-based pre-combustion CO2 capture approach that leverages porous media with appropriate wettability and considers the implications of microstructure properties. This emphasizes the crucial connection between nano-structure and macroscopic properties, underscoring the significance of understanding their interplay for economic feasibility. We believe that our efforts to unveil the hidden nature of gas hydrates provide strategies to address challenges and lay the groundwork for practical applications.

Enabling hydrate-based methane storage under mild operating conditions by periodic mesoporous organosilica nanotubes

Biomethane is a renewable natural gas substitute produced from biogas. Storage of this sustainable energy vector in confined clathrate hydrates, encapsulated in the pores of a host material, is a highly promising avenue to improve storage capacity and energy efficiency. Herein, a new type of periodic mesoporous organosilica (PMO) nanotubes, referred to as hollow ring PMO (HR-PMO), capable of promoting methane clathrate hydrate formation under mild working conditions (273 K, 3.5 MPa) and at high water loading (5.1 g water/g HR-PMO) is reported. Gravimetric uptake measurements reveal a steep single-stepped isotherm and a noticeably high methane storage capacity (0.55 g methane/g HR-PMO; 0.11 g methane/g water at 3.5 MPa). The large working capacity throughout consecutive pressure-induced clathrate hydrate formation-dissociation cycles demonstrates the material's excellent recyclability (97% preservation of capacity). Supported by ex situ cryo-electron tomography and x-ray diffraction, HR-PMO nanotubes are hypothesized to promote clathrate hydrate nucleation and growth by distribution and confinement of water in the mesopores of their outer wall, along the central channels of the nanotubes and on the external nanotube surface. These findings showcase the potential for application of organosilica materials with hierarchical and interconnected pore systems for pressure-based storage of biomethane in confined clathrate hydrates.

Molecular Dynamics Study of Clathrate-like Ordering of Water in Supersaturated Methane Solution at Low Pressure

Using molecular dynamics, the evolution of a metastable solution for "methane + water" was studied for concentrations of 3.36, 6.5, 9.45, 12.2, and 14.8 mol% methane at 270 K and 1 bar during 100 ns. We have found the intriguing behavior of the system containing over 10,000 water molecules: the formation of hydrate-like structures is observed at 6.5 and 9.45 mol% concentrations throughout the entire solution volume. This formation of "blobs" and the following amorphous hydrate were studied. The creation of a metastable methane solution through supersaturation is the key to triggering the collective process of hydrate formation under low pressure. Even the first stage (0-1 ns), before the first fluctuating cavities appear, is a collective process of H-bond network reorganization. The formation of fluctuation cavities appears before steady hydrate growth begins and is associated with a preceding uniform increase in the water molecule's tetrahedrality. Later, the constantly presented hydrate cavities become the foundation for a few independent hydrate nucleation centers, this evolution is consistent with the labile cluster and local structure hypotheses. This new mechanism of hydrogen-bond network reorganization depends on the entropy of the cavity arrangement of the guest molecules in the hydrate lattice and leads to hydrate growth.

Conversion of Recovered Ammonia and Carbon Dioxide into Urea in the Presence of Catalytically Active Copper Species in Nanospaces of Porous Silica Hollow Spheres

The present study firstly reported porous silica hollow spheres as a host material for recovery of ammonia and carbon dioxide and conversion of the compounds into urea. These compounds were effectively introduced into the hollow spheres from an aqueous solution including ammonium and carbonate ions accompanied with catalytically active copper ions from the analyses of diffuse reflectance infrared Fourier transform (DRIFT) spectra and diffusion reflectance ultraviolet-visible and near-infrared (DR UV-vis-NIR) spectra. The ammonium and carbonate ions were converted into urea in the hollow spheres at 323 K under 0.5 MPa of argon atmosphere from the results of the DRIFT spectra. From the results of nitrogen sorption isotherms and X-ray photoelectron spectra (XPS) spectra, the amount of the generated urea depended on the amount of the introduced ammonium ions and the size distribution of the nanospaces in the hollow spheres. Urea was highly generated in the hollow spheres with a high amount of ammonium ions and well-ordered nanospaces from the reactants at high density.

CH/CH2 Group Clusters Doping Methane Hydrate Cages

Methane hydrate is a crystalline compound with methane molecules as guest species trapped in host water cages. In this study, we detected methane hydrate with water cages doped by (C_aromatic-H)₅ clusters, (C_aromatic-H)₆ clusters, and (3C_aliphatic-H₂ + 2H₂O) clusters using current spectroscopic techniques and differential scanning calorimetry (DSC). Methane molecules are trapped in the doped cages with type sI forming in nanoscale silica gel pores. The relative quantity ratio of host carbon to guest carbon in the doped hydrate sample reaches approximately 3.58. Methane hydrate doped by CH/CH₂ group clusters greatly improves the ability of the hydrate unit cell to store methane and increases the stability of methane hydrate. Fast proton diffusion in the doped methane hydrate was confirmed. The results of this study will provide efficient and energy saving technical support for disruptive changes in hydrate storage and transportation of methane gas technology with a doped and dense solid phase.

Rapid and efficient hydrogen clathrate hydrate formation in confined nanospace

Clathrate hydrates are crystalline solids characterized by their ability to accommodate large quantities of guest molecules. Although CH₄ and CO₂ are the traditional guests found in natural systems, incorporating smaller molecules (e.g., H₂) is challenging due to the need to apply higher pressures to stabilize the hydrogen-bonded network. Another critical limitation of hydrates is the slow nucleation and growth kinetics. Here, we show that specially designed activated carbon materials can surpass these obstacles by acting as nanoreactors promoting the nucleation and growth of H₂ hydrates. The confinement effects in the inner cavities promote the massive growth of hydrogen hydrates at moderate temperatures, using pure water, with extremely fast kinetics and much lower pressures than the bulk system.

"Nanoreactors" for Boosting Gas Hydrate Formation toward Energy Storage Applications

Hydrogen and methane can be molecularly incorporated in ice-like water structures up to mass fractions of 4.3% and 13.3%, respectively. The resulting solid structures, called gas hydrates, offer great potential for the efficient storage of hydrogen and natural gas. However, slow gas encapsulation by bulk water hinders this application. Porous structures have been shown to effectively promote gas hydrate formation and are a potential enabler for the development of hydrate-based gas storage technologies. Here, we offer an insightful perspective on using porous structures as nanoreactors for achieving fast gas hydrate formation for gas storage applications. We critically discuss and elucidate the working mechanisms of nanoreactors and identify the criteria for efficient nanoreactors. Based on the concepts founded, we propose a theoretical framework for designing next-generation porous materials for delivering better promoting effects on gas hydrate formation.

Methane Hydrate Formation in Hollow ZIF-8 Nanoparticles for Improved Methane Storage Capacity

Methane hydrate has been extensively studied as a potential medium for natural gas storage and transportation. Due to their high specific surface area, tunable porous structure, and surface chemistry, metal–organic frameworks are ideal materials to exhibit the catalytic effect for the formation process of gas hydrate. In this paper, hollow ZIF-8 nanoparticles are synthesized using the hard template method. The synthesized hollow ZIF-8 nanoparticles are used in the adsorption and methane hydrate formation process. The effect of pre-adsorbed water mass in hollow ZIF-8 nanoparticles on methane storage capacity and the hydrate formation rate is investigated. The storage capacity of methane on wet, hollow ZIF-8 is augmented with an increase in the mass ratio of pre-adsorbed water and dry, hollow ZIF-8 (RW), and the maximum adsorption capacity of methane on hollow ZIF-8 with a RW of 1.2 can reach 20.72 mmol/g at 275 K and 8.57 MPa. With the decrease in RW, the wet, hollow ZIF-8 exhibits a shortened induction time and an accelerated growth rate. The formation of methane hydrate on hollow ZIF-8 is further demonstrated with the enthalpy of the generation reaction. This work provides a promising alternative material for methane storage.

Theoretical Investigation of the Fusion Process of Mono-Cages to Tri-Cages with CH4/C2H6 Guest Molecules in sI Hydrates

Owing to a stable and porous cage structure, natural gas hydrates can store abundant methane and serve as a potentially natural gas resource. However, the microscopic mechanism of how hydrate crystalline grows has not been fully explored, especially for the structure containing different guest molecules. Hence, we adopt density functional theory (DFT) to investigate the fusion process of structure I hydrates with CH4/C2H6 guest molecules from mono-cages to triple-cages. We find that the volume of guest molecules affects the stabilities of large (51262, L) and small (512, s) cages, which are prone to capture C2H6 and CH4, respectively. Mixed double cages (small cage and large cage) with the mixed guest molecules have the highest stability and fusion energy. The triangular triple cages exhibit superior stability because of the three shared faces, and the triangular mixed triple cages (large-small-large) structure with the mixed guest molecules shows the highest stability and fusion energy in the triple-cage fusion process. These results can provide theoretical insights into the growth mechanism of hydrates with other mono/mixed guest molecules for further development and application of these substances.

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.

Spatial state distribution and phase transition of non-uniform water in soils: Implications for engineering and environmental sciences

The physical behaviors of water in the interface are the fundamental to discovering the engineering properties and environmental effects of aqueous porous media (e.g., soils). The pore water pressure (PWP) is a key parameter to characterize the pore water state (PWS) and its phase transition in the micro interface. Traditionally, the water in the interface is frequently believed to be uniform, negative in pressure and tensile based on macroscopic tests and Gibbs interface model. However, the water in the interface is a non-uniform and compressible fluid (part of tensile and part of compressed), forming a spatial profile of density and PWP depending on its distance from the substrate interface. Herein, we introduced the static and dynamic theory methods of non-uniform water based on diffuse interface model to analyze non-uniform water state dynamics and water density and PWP. Based on the theory of non-uniform water, we gave a clear stress analysis on soil water and developed the concepts of PWS, PWP and matric potential in classical soil mechanics. In addition, the phase transition theory of non-uniform water is also examined. In nature, the generalized Clausius-Clapeyron equation (GCCE) is consistent with Clapeyron equation. Therefore, a unified interpretation is proposed to justify the use of GCCE to represent frozen soil water dynamics. Furthermore, the PWP description of non-uniform water can be well verified by some experiments focusing on property variations in the interface area, including the spatial water density profile and unfrozen water content variations with decreasing temperature and other factors. In turn, PWP spatial distribution of non-uniform water and its states can well explain some key phenomena on phase transition during ice or hydrate formation, including the discrepancies of phase transition under a wide range of conditions.

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.

Reduced phase stability and faster formation/dissociation kinetics in confined methane hydrate

The mechanisms involved in the formation/dissociation of methane hydrate confined at the nanometer scale are unraveled using advanced molecular modeling techniques combined with a mesoscale thermodynamic approach. Using atom-scale simulations probing coexistence upon confinement and free energy calculations, phase stability of confined methane hydrate is shown to be restricted to a narrower temperature and pressure domain than its bulk counterpart. The melting point depression at a given pressure, which is consistent with available experimental data, is shown to be quantitatively described using the Gibbs-Thomson formalism if used with accurate estimates for the pore/liquid and pore/hydrate interfacial tensions. The metastability barrier upon hydrate formation and dissociation is found to decrease upon confinement, therefore providing a molecular-scale picture for the faster kinetics observed in experiments on confined gas hydrates. By considering different formation mechanisms-bulk homogeneous nucleation, external surface nucleation, and confined nucleation within the porosity-we identify a cross-over in the nucleation process; the critical nucleus formed in the pore corresponds either to a hemispherical cap or to a bridge nucleus depending on temperature, contact angle, and pore size. Using the classical nucleation theory, for both mechanisms, the typical induction time is shown to scale with the pore volume to surface ratio and hence the pore size. These findings for the critical nucleus and nucleation rate associated with such complex transitions provide a means to rationalize and predict methane hydrate formation in any porous media from simple thermodynamic data.

Formation of a Low-Density Liquid Phase during the Dissociation of Gas Hydrates in Confined Environments

The large amounts of natural gas in a dense solid phase stored in the confined environment of porous materials have become a new, potential method for storing and transporting natural gas. However, there is no experimental evidence to accurately determine the phase state of water during nanoscale gas hydrate dissociation. The results on the dissociation behavior of methane hydrates confined in a nanosilica gel and the contained water phase state during hydrate dissociation at temperatures below the ice point and under atmospheric pressure are presented. Fourier transform infrared spectroscopy (FTIR) and powder X-ray diffraction (PXRD) were used to trace the dissociation of confined methane hydrate synthesized from pore water confined inside the nanosilica gel. The characterization of the confined methane hydrate was also analyzed by PXRD. It was found that the confined methane hydrates dissociated into ultra viscous low-density liquid water (LDL) and methane gas. The results showed that the mechanism of confined methane hydrate dissociation at temperatures below the ice point depended on the phase state of water during hydrate dissociation.

Local structure and distortions of mixed methane-carbon dioxide hydrates

A vast source of methane is found in gas hydrate deposits, which form naturally dispersed throughout ocean sediments and arctic permafrost. Methane may be obtained from hydrates by exchange with hydrocarbon byproduct carbon dioxide. It is imperative for the development of safe methane extraction and carbon dioxide sequestration to understand how methane and carbon dioxide co-occupy the same hydrate structure. Pair distribution functions (PDFs) provide atomic-scale structural insight into intermolecular interactions in methane and carbon dioxide hydrates. We present experimental neutron PDFs of methane, carbon dioxide and mixed methane-carbon dioxide hydrates at 10 K analyzed with complementing classical molecular dynamics simulations and Reverse Monte Carlo fitting. Mixed hydrate, which forms during the exchange process, is more locally disordered than methane or carbon dioxide hydrates. The behavior of mixed gas species cannot be interpolated from properties of pure compounds, and PDF measurements provide important understanding of how the guest composition impacts overall order in the hydrate structure.

Coulomb interactions between dipolar quantum fluctuations in van der Waals bound molecules and materials

Mutual Coulomb interactions between electrons lead to a plethora of interesting physical and chemical effects, especially if those interactions involve many fluctuating electrons over large spatial scales. Here, we identify and study in detail the Coulomb interaction between dipolar quantum fluctuations in the context of van der Waals complexes and materials. Up to now, the interaction arising from the modification of the electron density due to quantum van der Waals interactions was considered to be vanishingly small. We demonstrate that in supramolecular systems and for molecules embedded in nanostructures, such contributions can amount to up to 6 kJ/mol and can even lead to qualitative changes in the long-range van der Waals interaction. Taking into account these broad implications, we advocate for the systematic assessment of so-called Dipole-Correlated Coulomb Singles in large molecular systems and discuss their relevance for explaining several recent puzzling experimental observations of collective behavior in nanostructured materials.

Metal-Organic Framework HKUST-1 Promotes Methane Hydrate Formation for Improved Gas Storage Capacity

The large demand of natural gas consumption requires an effective technology to purify and store methane, the main component of natural gas. Metal-organic frameworks and gas hydrates are highly appealing materials for the efficient storage of industrially relevant gases, including methane. In this study, the methane storage capacity of the combination of methane hydrates and HKUST-1, a copper-based metal-organic framework, was studied using high pressure differential scanning calorimetry. The results show a synergistic effect, as the addition of HKUST-1 promoted hydrate growth, thus increasing the amount of water converted to hydrate from 5.9 to 87.2% and the amount of methane stored, relative to the amount of water present, from 0.55 to 8.1 mmol/g. The success of HKUST-1 as a promoter stems mainly from its large surface area, high thermal conductivity, and hydrophilicity. These distinctive properties led to a kinetically favorable decrease in hydrate growth induction period by 4.4 h upon the addition of HKUST-1. Powder X-ray diffraction and nitrogen isotherm suggests that the hydrate formation occurs primarily on the surface of HKUST-1 rather than within the pores. Remarkably, the HKUST-1 crystals show no significant changes in terms of structural integrity after many cycles of hydrate formation and dissociation, which results in the material having a long life cycle. These results confirm the beneficial role of HKUST-1 as a promoter for gas hydrate formation to increase methane gas storage capacity.

Promotion of Activated Carbon on the Nucleation and Growth Kinetics of Methane Hydrates

Due to the hybrid effect of physical adsorption and hydration, methane storage capacity in pre-adsorbed water-activated carbon (PW-AC) under hydrate favorable conditions is impressive, and fast nucleation and growth kinetics are also anticipated. Those fantastic natures suggest the PW-AC-based hydrates to be a promising alternative for methane storage and transportation. However, hydrate formation refers to multiscale processes, the nucleation kinetics at molecule scale give rise to macrohydrate formation, and the presence of activated carbon (AC) causes this to be more complicated. Although adequate nucleation sites induced by abundant specific surface area and pore texture were reported to correspond to fast formation kinetics at macroperspective, the micronature behind that is still ambiguous. Here, we evaluated how methane would be adsorbed on PW-AC under hydrate favorable conditions to improve the understanding of hydrate fast nucleation and growth kinetics. Microbulges on AC surface were confirmed to provide numerous nucleation sites, suggesting the contribution of abundant specific surface area of AC to fast hydrate nucleation and growth kinetics. In addition, two-way convection of water and methane molecules in micropores induced by methane physical adsorption further increases gas-liquid contact at molecular scale, which may constitute the nature of confinement effect of nanopore space.

Unraveling nucleation pathway in methane clathrate formation

Methane clathrates are widespread on the ocean floor of the Earth. A better understanding of methane clathrate formation has important implications for natural-gas exploitation, storage, and transportation. A key step toward understanding clathrate formation is hydrate nucleation, which has been suggested to involve multiple evolution pathways. Herein, a unique nucleation/growth pathway for methane clathrate formation has been identified by analyzing the trajectories of large-scale molecular dynamics (MD) simulations. In particular, ternary water-ring aggregations (TWRAs) have been identified as fundamental structures for characterizing the nucleation pathway. Based on this nucleation pathway, the critical nucleus size and nucleation timescale can be quantitatively determined. Specifically, a methane hydration layer compression/shedding process is observed to be the critical step in (and driving) the nucleation/growth pathway, which is manifested through overlapping/compression of the surrounding hydration layers of the methane molecules, followed by detachment (shedding) of the hydration layer. As such, an effective way to control methane hydrate nucleation is to alter the hydration layer compression/shedding process during the course of nucleation.

Non-porous organic crystals and their interaction with guest molecules from the gas phase

Some organic molecules encapsulate solvents upon crystallization. One class of compounds that shows a high propensity to form such crystalline solvates are tetraaryladamantanes (TAAs). Recently, tetrakis(dialkoxyphenyl)-adamantanes have been shown to encapsulate a wide range of guest molecules in their crystals, and to stabilize the guest molecules against undesired reactions. The term ‘encapsulating organic crystals’ (EnOCs) has been coined for these species. In this work, we studied the behavior of three TAAs upon exposition to different guest molecules by means of sorption technique. We firstly measured the vapor adsorption/desorption isotherms with water, tetrahydrofuran and toluene, and secondly, we studied the uptake of methane on dry and wet TAAs. Uptake of methane beyond one molar equivalent was detected for wet crystals, even though the materials showed a lack of porosity. Thus far, such behavior, which we ascribe to methane hydrate formation, had been described for porous non-crystalline materials or crystals with detectable porosity, not for non-porous organic crystals. Our results show that TAA crystals have interesting properties beyond the formation of conventional solvates. Gas-containing organic crystals may find application as reservoirs for gases that are difficult to encapsulate or are slow to form crystalline hydrates in the absence of a host compound.

Wet tetraaryladamantane crystals take up methane in form of methane hydrate structure I, even though they appear non-porous to argon.

Direct Synthesis of Mesoporous Organosilica and Proof-of-Concept Applications in Lysozyme Adsorption and Supported Catalysis

Mesoporous materials represent a useful alternative for exploiting the effects of confinement on molecular trapping and catalysis. Their efficiency often depends on the interactions between the surface and the targeted molecules. One way to enhance these interactions is to adjust the hydrophobic/hydrophilic balance of the surface. In the case of mesoporous silica, the incorporation of organic groups is an efficient solution to adapt the material for specific applications. In this work, we have used the co-condensation method to control the hydrophobicity of mesoporous organosilica. The obtained materials are methyl- or phenyl-containing silica with a pore size between 3 and 5 nm. The surface chemistry control has shown the enhanced performance of the materials in two proof-of-concept (PoC) applications: lysozyme adsorption and supported catalysis. The lysozyme adsorption is observed to be over 3 times more efficient with the phenyl-functionalized material than MCM-41, due to π-π interactions. For the catalysis, copper(II) was immobilized on the organosilica surface. In this case, the presence of methyl groups significantly enhanced the product yield for the catalyzed synthesis of a triazole derivative; this was attributed to the enhanced hydrophobic surface-reactant interactions. It was also found that the materials have a higher water adsorption capacity and an improved resistance to hydrolysis. The modulation of water properties in confinement with hydrophobic surfaces, consistently with the water as tuneable solvent (WaTuSo) concept, is a crucial aspect in the efficiency of mesoporous materials for dedicated applications.

Novel Hydroquinone-Alumina Composites Stabilizing a Guest-Free Clathrate Structure: Applications in Gas Processing

Organic clathrates formed by hydroquinone (HQ) and gases such as CO₂ and CH₄ are solid supramolecular host-guest compounds in which the gaseous guest molecules are encaged in a host framework of HQ molecules. Not only are these inclusion compounds fascinating scientific curiosities but they can also be used in practical applications such as gas separation. However, the development and future use of clathrate-based processes will largely depend on the effectiveness of the reactive materials used. These materials should enable fast and selective enclathration and have a large gas storage capacity. This article discusses the properties and performance of a new composite material able to form gas clathrates with hydroquinone (HQ) deposited on alumina particles. Apart from the general characterization of the HQ-alumina composite, one of the most remarkable observations is the unexpected formation of a guest-free clathrate structure with long-term stability (>2 years) inside the composite. Interestingly enough, in addition to a slight improvement in the enclathration kinetics of pure CO₂ compared to powdered HQ, preferential capture of CO₂ molecules is observed when the HQ-alumina composite is exposed to an equimolar CO₂/CH₄ gas mixture. In terms of gas capture selectivity toward CO₂, the performance of this new composite exceeds that of pure HQ and HQ-silica composites developed in a previous study, opening up new opportunities for the design and use of these novel materials for gas separation.

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.

Enhanced methane gas storage in the form of hydrates: role of the confined water molecules in silica powders

Methane hydrates are promising materials for storage and transportation of natural gas; however, the slow kinetics and inefficient water to hydrate conversions impede its broad scale utilisation. The purpose of the present study is to demonstrate rapid (2-3 h) and efficient methane hydrate conversions by utilising the water molecules confined in the intra- and inter-granular space of silica powders. All the experiments were conducted with amorphous silica (10 g) powders of 2-30 μm; 10-20 nm grain size, to mimic the hydrate formations in fine sand and clay dominated environments under moderate methane pressure (7-8 MPa). Encasing of methane molecules in hydrate cages was confirmed by Raman spectroscopic (ex situ) and thermodynamic phase boundary measurements. The present studies reveal that the water to hydrate conversion is relatively slower in 10-20 nm grain size silica, although the nucleation event is rapid in both silicas. The process of hydrate conversion is vastly diffusion-controlled, and this was distinctly observed during the hydrate growth in nanosize silica.

Two-dimensional hydrogen hydrates: structure and stability

The structure and stability of two-dimensional hydrogen hydrate were investigated in this work using density functional theory. The results are in line with expectations that the occupied cages are more stable after their confinement between two parallel hydrophobic sheets. The four two-dimensional hydrogen hydrate crystals - BLHH-I, BLHH-II, BLHH-III and BLHH-IV - that we predicted were much more stable in a restricted environment than in a free environment, even close to or exceeding conventional hydrogen hydrates. Besides, we found that the stability of two-dimensional hydrates is inversely related to the increase in temperature. Our work highlights that two-dimensional hydrates provide a new research idea in the field of hydrogen storage.

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.

Study of the pore structure and size effects on the electrochemical capacitor behaviors of porous carbon/quinone derivative hybrids

We demonstrate the hybridization of a redox-active quinone derivative, 2,5-dichloro-1,4-benzoquinone (DCBQ), and porous carbons with different pore structures for aqueous electrochemical capacitor electrodes. The hybridization is performed in the gas phase, which enables accurate porous carbon/DCBQ weight ratios. This method is advantageous over conventional liquid phase adsorption, in terms of facile optimization of the porous carbon/DCBQ weight ratio to obtain high-performance aqueous electrochemical capacitor electrodes, dependent on the kind of porous carbons; moreover, complete adsorption in the liquid phase cannot be achieved by the conventional liquid phase adsorption method. Their electrochemical capacitor performances are evaluated using an aqueous 1 M H2SO4 electrolyte, and the adsorbed DCBQ undergoes redox reactions inducing pseudocapacitance within the pores of porous carbons. To study the effect of the pore size on the electrochemical capacitor behavior, two kinds of activated carbon (AC) with different pore sizes are examined: the microporous AC and the AC with both micro- and mesopores. Additionally, we examine ordered microporous carbon with a uniform pore size of 1.2 nm and a three-dimensionally (3D) ordered and mutually connected pore structure. The results reveal that mesopores facilitate proton conduction inside the DCBQ-constrained carbon pores, whereas the 3D-ordered and mutually connected micropores balance high volumetric capacitance enhancement with excellent rate capability. Such high proton conduction inside such constrained spaces can be explained only by the Grotthuss mechanism.

Constraint spaces in carbon materials

Nano-sized pores in carbon materials are recently known to give certain constraints to the encapsulated materials by keeping them inside, accompanied with some changes in their structure, morphology, stability, etc. Consequently, nano-sized pores endow the constrained materials with improved performances in comparison with those prepared by conventional processes. These pores may be called "constraint spaces" in carbon materials. Here, we review the experimental results related to these constraint spaces by classifying as nanochannels in carbon nanotubes, nanopores and nanochannels in various porous carbons, and the spaces created by carbon coating.

Pressure Effects with Incorporated Particle Size Dependency in Graphene Oxide Layers through Observing Spin Crossover Temperature

This research highlights the pressure effects with the particle size dependency incorporated in two-dimensional graphene oxide (GO)/reduced graphene oxide (rGO). GO and rGO composites employing nanorods (NRs) of type [Fe(Htrz)2(trz)](BF4) have been prepared, and their pressure effects in the interlayer spaces through observing the changes of the spin crossover (SCO) temperature (T1/2) have been discussed. The composites show the decrease of interlayer spaces from 8.7 Å to 3.5 Å that is associated with GO to rGO transformation. The shorter interlayer spaces were induced by pressure effects, resulting in the increment of T1/2 from 357 K to 364 K. The pressure effects in the interlayers spaces estimated from the T1/2 value correspond to 24 MPa in pristine [Fe(Htrz)2(trz)](BF4) NRs under hydrostatic pressure. The pressure observed in the composites incorporating NRs (30 × 200 nm) is smaller than that observed in the composite incorporating nanoparticles (NPs) (30 nm). These results clearly demonstrated that the incorporated particle size and shape influenced the pressure effects between the GO/rGO layer.

Understanding the Pathway of Gas Hydrate Formation with Porous Materials for Enhanced Gas Separation

The reason that the stoichiometry of gas to water in artificial gas hydrates formed on porous materials is much higher than that in nature is still ambiguous. Fortunately, based on our experimental thermodynamic and kinetic study on the gas hydrate formation behavior with classic ordered mesoporous carbon CMK-3 and irregular porous activated carbon combined with density functional theory calculations, we discover a microscopic pathway of the gas hydrate formation on porous materials. Two interesting processes including (I) the replacement of water adsorbed on the carbon surface by gas and (II) further replacement of water in the pore by gas accompanied with the gas condensation in the pore and growth of gas hydrate crystals out of the pore were deduced. As a result, a great enhancement of the selectivity and regeneration for gas separation was achieved by controlling the gas hydrate formation behavior accurately.

Low-cost disposable high-pressure setup for in situ X-ray experiments

A low-cost, flexible and fast method to create disposable sample cells suitable for in situ catalytic or material synthesis studies based on standard quartz capillaries, heat-shrinkable tubing and standard Swagelok components is described.

Heavy oil oxidation in the nano-porous medium of synthetic opal

Increasing interest to study hydrocarbon behavior in fine porous media, awakened by the shale revolution, requires the application of suitable model porous media. In the current study we prepared nano-porous synthetic opal, profoundly investigated its morphological and textural properties, and studied the kinetics of combustion of heavy oil impregnated into nanopores. Comparison of kinetic parameters of the oil oxidation process for nano-porous and coarse-porous media revealed that nanoconfinement affects the reactivity of oil.

Formation of Methane Hydrate in the Presence of Natural and Synthetic Nanoparticles

Natural gas hydrates occur widely on the ocean-bed and in permafrost regions, and have potential as an untapped energy resource. Their formation and growth, however, poses major problems for the energy sector due to their tendency to block oil and gas pipelines, whereas their melting is viewed as a potential contributor to climate change. Although recent advances have been made in understanding bulk methane hydrate formation, the effect of impurity particles, which are always present under conditions relevant to industry and the environment, remains an open question. Here we present results from neutron scattering experiments and molecular dynamics simulations that show that the formation of methane hydrate is insensitive to the addition of a wide range of impurity particles. Our analysis shows that this is due to the different chemical natures of methane and water, with methane generally excluded from the volume surrounding the nanoparticles. This has important consequences for our understanding of the mechanism of hydrate nucleation and the design of new inhibitor molecules.