Quest for an Optimal Methane Hydrate Formation in the Pores of Hydrolytically Stable Metal–Organic Frameworks

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.

Metal-Organic Frameworks for Greenhouse Gas Applications

Using petrol to supply energy for a car or burning coal to heat a building generates plenty of greenhouse gas (GHG) emissions, including carbon dioxide (CO₂ ), water vapor (H₂ O), methane (CH₄ ), nitrous oxide (N₂ O), ozone (O₃ ), fluorinated gases. These up-and-coming metal-organic frameworks (MOFs) are structurally endowed with rigid inorganic nodes and versatile organic linkers, which have been extensively used in the GHG-related applications to improve the lives and protect the environment. Porous MOF materials and their derivatives have been demonstrated to be competitive and promising candidates for GHG separation, storage and conversions as they shows facile preparation, large porosity, adjustable nanostructure, abundant topology, and tunable physicochemical property. Enormous progress has been made in GHG storage and separation intrinsically stemmed from the different interaction between guest molecule and host framework from MOF itself in the recent five years. Meanwhile, the use of porous MOF materials to transform GHG and the influence of external conditions on the adsorption performance of MOFs for GHG are also enclosed. In this review, it is also highlighted that the existing challenges and future directions are discussed and envisioned in the rational design, facile synthesis and comprehensive utilization of MOFs and their derivatives for practical applications.

Durable Hydrate-phobic Coating with In Situ Self-Replenishing Hydrocarbon Barrier Films for Low Clathrate Hydrate Adhesion

Clathrate hydrate growth, deposition, and plug formation during oil and gas transportation causes blockage of pipelines. An effective strategy to solve this problem is to mitigate the hydrate formation and reduce its adhesion on pipe walls through a coating process. However, durability failure, corrosion, inability to self-heal, high cost, and strong hydrate adhesion remain unsolved issues. To address these challenges, in this work, we present an in situ self-replenishing nonfluorinated durable hydrate-phobic coating of candle soot particles. The candle soot coating reduces hydrate adhesion by promoting a thick barrier film of hydrocarbons between the hydrate and the soot coated substrate. The hydrocarbons permeating the soot coating display a high contact angle for water and inhibit the formation of water bridges between the hydrate and soot coated substrate. The spherical cyclopentane hydrate slides off easily on the candle soot coating inside the cyclopentane environment. The hydrate former, cyclopentane-water emulsion, and THF-water mixture have high contact angles as well as low hydrate adhesion on soot coating simultaneously. In addition, the coating is flow-induced long-term slippery, durable, low cost, anticorrosion, self-cleaning, and suitable for practical applications.

Progress in Hybridization of Covalent Organic Frameworks and Metal-Organic Frameworks

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) hybrid materials are a class of porous crystalline materials that integrate MOFs and COFs with hierarchical pore structures. As an emerging porous frame material platform, MOF/COF hybrid materials have attracted tremendous attention, and the field is advancing rapidly and extending into more diverse fields. Extensive studies have shown that a broad variety of MOF/COF hybrid materials with different structures and specific properties can be synthesized from diverse building blocks via different chemical reactions, driving the rapid growth of the field. The allowed complementary utilization of π-conjugated skeletons and nanopores for functional exploration has endowed these hybrid materials with great potential in challenging energy and environmental issues. It is necessary to prepare a "family tree" to accurately trace the developments in the study of MOF/COF hybrid materials. This review comprehensively summarizes the latest achievements and advancements in the design and synthesis of MOF/COF hybrid materials, including COFs covalently bonded to the surface functional groups of MOFs (MOF@COF), MOFs grown on the surface of COFs (COF@MOF), bridge reaction between COF and MOF (MOF+COF), and their various applications in catalysis, energy storage, pollutant adsorption, gas separation, chemical sensing, and biomedicine. It concludes with remarks concerning the trend from the structural design to functional exploration and potential applications of MOF/COF hybrid materials.

"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.

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.

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.

MOF-enabled confinement and related effects for chemical catalyst presentation and utilization

A defining characteristic of nearly all catalytically functional MOFs is uniform, molecular-scale porosity. MOF pores, linkers and nodes that define them, help regulate reactant and product transport, catalyst siting, catalyst accessibility, catalyst stability, catalyst activity, co-catalyst proximity, composition of the chemical environment at and beyond the catalytic active site, chemical intermediate and transition-state conformations, thermodynamic affinity of molecular guests for MOF interior sites, framework charge and density of charge-compensating ions, pore hydrophobicity/hydrophilicity, pore and channel rigidity vs. flexibility, and other features and properties. Collectively and individually, these properties help define overall catalyst functional behaviour. This review focuses on how porous, catalyst-containing MOFs capitalize on molecular-scale confinement, containment, isolation, environment modulation, energy delivery, and mobility to accomplish desired chemical transformations with potentially superior selectivity or other efficacy, especially in comparison to catalysts in homogeneous solution environments.

Indium-Organic Framework with soc Topology as a Versatile Catalyst for Highly Efficient One-Pot Strecker Synthesis of α-aminonitriles

An In(III) based metal-organic framework (MOF), In-pbpta, with soc topology was constructed from the trigonal prismatic [In3(μ3-O)(H2O)3(O2C-)6] secondary building unit (SBU) and a custom-designed tetratopic linker H4pbpta (pbpta = 4,4',4″,4‴-(1,4-phenylenbis(pyridine-4,2,6-triyl))-tetrabenzoic acid)). The obtained MOF shows a Brunauer-Emmett-Teller surface area of 1341 m2/g with a pore volume of 0.64 cm3/g, which is the highest among the scarcely reported In-soc-MOFs. The constructed MOF demonstrates excellent performance as a heterogeneous Lewis acid catalyst for highly efficient conversion in a one-pot multicomponent Strecker reaction for the preparation of α-aminonitriles under solvent-free conditions, which can be easy to separate and recycle without significant loss of activity for up to seven cycles. The computational modeling studies suggest the presence of the three substrates in close vicinity to the In-oxo cluster. The strong interactions of the aldehyde/ketone and the amine with the In-oxo cluster together with the readily available cyanide ion around the In-oxo cluster lead to high catalytic conversion within a short period of time for the MOF catalyst. Our work therefore lays a foundation to develop MOF as a new class of efficient heterogeneous catalyst for one-pot Strecker reaction.

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.

Clathrate Adhesion Induced by Quasi-Liquid Layer

The adhesive force of clathrates to surfaces is a century-old problem of pipeline blockage for the energy industry. Here, we provide new physical insight into the origin of this force by accounting for the existence of a quasi-liquid layer (QLL) on clathrate surfaces. To gain this insight, we measure the adhesive force between a tetrahydrofuran clathrate and a solid sphere. We detect a strong adhesion, which originates from a capillary bridge that is formed from a nanometer-thick QLL on the clathrate surface. The curvature of this capillary bridge is nanoscaled, causes a large negative Laplace pressure, and leads to a strong capillary attraction. The microscopic capillary bridge expands and consolidates over time. This dynamic behavior explains the time-dependent increase of measured capillary forces. The adhesive force decreases greatly upon increasing the roughness and the hydrophobicity of the sphere, which founds the fundamental basics for reducing clathrate adhesion by using surface coating.

Introducing High Density of Very Active Sites and Stepwise Postmodification for Tailoring the Porosity of Highly Demanding Cr3+-Based Metal-Organic Frameworks

Chromium(III)-based metal-organic frameworks (Cr-MOFs) are highly robust and porous and have been very attractive in a wide range of investigations. However, the harsh direct synthetic conditions not only impede the synthesis of new Cr-MOFs but also restrict the introduction of functional groups into them. Postsynthetic modification has somewhat alleviated such difficulties; nevertheless, it still suffered from procedures that are tedious and conditions that are not mild, which often result in low concentration of the functional groups introduced. To overcome these shortcomings, here, in this paper, we supplied a new route and prepared a benzyl alcohol functionalized Cr-SXU-2 from the judiciously designed benzyl alcohol functionalized Fe-SXU-2 through solvent-assisted metal metathesis strategy. The functionalized Cr-SXU-2 shows well-preserved crystallinity, porosity, and high chemical stability. The benzyl alcohol group can be converted into a very active benzyl bromide group in an almost quantitative yield and thus for the first time produce the benzyl bromide functionalized MOF, Cr-SXU-2-Br, in which the -Br group can be exchanged by a nucleophilic group. As a proof of concept, -N₃ was introduced and transformed into other active sites via "click reaction" to further tailor the interior of Cr-SXU-2. All these functionalized Cr-MOFs showed improved adsorption performance in contrast to the nonfunctionalized one. This step-by-step postmodification process not only diversifies the functionalization of robust MOFs but also opens a new route to employ many different functional groups in the demanding highly stable Cr-MOF platforms.

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.

Dynamics & Spectroscopy with Neutrons-Recent Developments & Emerging Opportunities

This work provides an up-to-date overview of recent developments in neutron spectroscopic techniques and associated computational tools to interrogate the structural properties and dynamical behavior of complex and disordered materials, with a focus on those of a soft and polymeric nature. These have and continue to pave the way for new scientific opportunities simply thought unthinkable not so long ago, and have particularly benefited from advances in high-resolution, broadband techniques spanning energy transfers from the meV to the eV. Topical areas include the identification and robust assignment of low-energy modes underpinning functionality in soft solids and supramolecular frameworks, or the quantification in the laboratory of hitherto unexplored nuclear quantum effects dictating thermodynamic properties. In addition to novel classes of materials, we also discuss recent discoveries around water and its phase diagram, which continue to surprise us. All throughout, emphasis is placed on linking these ongoing and exciting experimental and computational developments to specific scientific questions in the context of the discovery of new materials for sustainable technologies.

Highly selective detection of TNP over other nitro compounds in water: the role of selective host–guest interactions in Zr-NDI MOF

A fluorescent Zr-NDI based MOF shows the selective sensing of TNP in water, over other classes of nitro compounds. DFT studies reveal favourable host–guest interactions behind this phenomenon.

The state of the field: from inception to commercialization of metal–organic frameworks

We provide a brief overview of the state of the MOF field from their inception to their synthesis, potential applications, and finally, to their commercialization.