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A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. The word clathrate is derived from the Latin clathratus (clatratus), meaning 'with bars, latticed'.[1] Most clathrate compounds are polymeric and completely envelop the guest molecule, but in modern usage clathrates also include host–guest complexes and inclusion compounds.[2] According to IUPAC, clathrates are inclusion compounds "in which the guest molecule is in a cage formed by the host molecule or by a lattice of host molecules."[3] The term refers to many molecular hosts, including calixarenes and cyclodextrins and even some inorganic polymers such as zeolites.

Clathrate cavities. For example, 512 (dodecahedral) and 51262 (tetrakaidecahedral) make up a Type I (sI) structure.[4]

Clathrates can be divided into two categories: clathrate hydrates and inorganic clathrates. Each clathrate is made up of a framework and guests that reside the framework. Most common clathrate crystal structures can be composed of cavities such as dodecahedral, tetrakaidecahedral, and hexakaidecahedral cavities. Unlike hydrates, inorganic clathrates have a covalently bonded framework of inorganic atoms with guests typically consisting of alkali or alkaline earth metals. Due to the stronger covalent bonding, the cages are often smaller than hydrates. Guest atoms interact with the host by ionic or covalent bonds. Therefore, partial substitution of guest atoms follow Zintl rules so that the charge of the overall compound is conserved. Most inorganic clathrates have full occupancy of its framework cages by a guest atom to be in stable phase. Inorganic clathrates can be synthesized by direct reaction using ball milling at high temperatures or high pressures. Crystallization from melt is another common synthesis route. Due to the wide variety of composition of host and guest species, inorganic clathrates are much more chemically diverse and possess a wide range of properties. Most notably, inorganic clathrates can be found to be both an insulator and a superconductor (Ba8Si46). A common property of inorganic clathrates that has attracted researchers is low thermal conductivity. Low thermal conductivity is attributed to the ability of the guest atom to "rattle" within the host framework. The freedom of movement of the guest atoms scatters phonons that transport heat.[4]

Crystal structure of Na8Si46. Example of a Type I clathrate consisting of dodecahedral (orange) and tetrakaidecahedral (yellow) silicon cavities containing sodium atoms.[4]

Examples

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Portion of the lattice of the clathrate xenon-paraquinol.[5]

Clathrates have been explored for many applications including: gas storage, gas production, gas separation, desalination, thermoelectrics, photovoltaics, and batteries.

  • Clathrate compounds with formula A8B16X30, where A is an alkaline earth metal, B is a group III element, and X is an element from group IV have been explored for thermoelectric devices. Thermoelectric materials follow a design strategy called the phonon glass electron crystal concept.[6][7] Low thermal conductivity and high electrical conductivity is desired to produce the Seebeck Effect. When the guest and host framework are appropriately tuned, clathrates can exhibit low thermal conductivity, i.e., phonon glass behavior, while electrical conductivity through the host framework is undisturbed allowing clathrates to exhibit electron crystal.
  • Methane clathrates feature the hydrogen-bonded framework contributed by water and the guest molecules of methane. Large amounts of methane naturally frozen in this form exist both in permafrost formations and under the ocean sea-bed.[8] Other hydrogen-bonded networks are derived from hydroquinone, urea, and thiourea. A much studied host molecule is Dianin's compound.
Cd(CN)2·CCl4: Cadmium cyanide clathrate framework (in blue) containing carbon tetrachloride (C atoms in gray and disordered Cl positions in green) as guest.

See also

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References

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Clathrate compound

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A clathrate compound, also known as a clathrate or compound, is a in which molecules or atoms of one component (the guest) are physically trapped within the three-dimensional lattice or cage-like framework formed by another component (the host), without the formation of covalent chemical bonds between them. The term "clathrate" derives from the Latin word clathratus, meaning "lattice-enclosed," reflecting the enclosing structure that holds the guest in place through der Waals forces or other weak interactions. These compounds require a size match between the guest and the host's cavities, and the guest is typically mobile within the cage at elevated temperatures. Clathrate compounds can be classified into several types based on their host materials and structures. The most prominent are clathrate hydrates, where water molecules form the host lattice, creating polyhedral cages that enclose guest gases such as (CH₄), (CO₂), or like (Xe); these occur naturally in and ocean sediments, forming vast reserves of natural gas hydrates. Other types include organic clathrates, such as those formed by or , which encapsulate or hydrocarbons in hexagonal or channel-like frameworks, and inorganic clathrates, like type-I and type-II silicon- or germanium-based structures (e.g., Ba₈Ga₁₆Ge₃₀), where or alkaline-earth metals serve as guests in semiconducting cages. Clathrates exhibit diverse structures, including cubic (sI, ) and hexagonal (sH) forms for hydrates, with the host lattice maintaining integrity even when guests are removed under certain conditions. The study of clathrate compounds dates back to the 19th century with early observations of gas hydrates, but systematic understanding emerged in the mid-20th century through crystallographic analyses by researchers like H.M. Powell, who coined the term in 1948. These materials are notable for their applications in , such as hydrates as potential clean sources (estimated to exceed conventional gas reserves), in hydrate forms achieving up to 5-6 wt% capacity, and thermoelectric devices where inorganic clathrates like Sr₈Ga₁₆Ge₃₀ exhibit low thermal conductivity due to "rattling" guest atoms. Additionally, clathrates play roles in processes, gas separation (e.g., CO₂ capture), and even , as they are hypothesized to exist in icy bodies like Europa and Titan. Recent advancements as of 2025 include the synthesis of boron-stabilized carbon clathrates and observation of guest-free frameworks, expanding potential applications in advanced materials. Their unique stability under specific pressure and temperature conditions underscores their importance in both fundamental chemistry and practical technologies.

Definition and Fundamentals

Definition

A clathrate compound is an inclusion compound in which guest molecules are physically enclosed within cage-like voids formed by a lattice of host molecules. The term "clathrate" originates from the Latin clathratus, meaning "latticed" or "with bars," derived from the Greek klêithra (bars or lattice), evoking the cage-like enclosure of the guests. Key characteristics of clathrate compounds include the formation of a three-dimensional framework by host molecules, such as or other substances, which create polyhedral cages that trap guest molecules through physical entrapment rather than covalent bonding. The guest molecules remain mobile within these cages, and there is no between the host and guest, with stability primarily maintained by weak van der Waals forces. Clathrate compounds differ from solvates, which involve stoichiometric incorporation of molecules into a lattice often with stronger interactions, and from interstitial compounds, where small atoms occupy gaps in a metal lattice without forming distinct . Instead, clathrates emphasize the structure and reliance on van der Waals stabilization for guest retention. The of clathrate compounds is generally expressed as nM⋅mGn \mathrm{M} \cdot m \mathrm{G}, where M represents the host molecule and G the guest, though the ratios n:m vary depending on the specific cage sizes and occupancy, often non-stoichiometric due to partial filling of voids.

Historical Development

The discovery of clathrate compounds traces back to the early , when British chemist identified chlorine in 1810 while investigating the properties of what was then called "oxymuriatic acid gas" (). Davy demonstrated that this solid phase consisted of molecules combined with chlorine, marking the first recognition of a gas as a distinct chemical entity. In 1823, provided a detailed chemical of chlorine , determining its composition and , which laid foundational empirical data for subsequent studies. By the early , practical implications emerged in industrial contexts. In 1934, engineer E.G. Hammerschmidt published observations linking gas hydrate formation to blockages in transmission pipelines, attributing these issues to the solidification of water-gas mixtures under high-pressure, low-temperature conditions; this work shifted attention from laboratory curiosities to engineering challenges. Concurrently, structural investigations advanced the field. German chemist Max von Stackelberg conducted pioneering diffraction studies in the late , elucidating the cage-like frameworks of gas hydrates and proposing their inclusion-compound nature. In 1948, British crystallographer H.M. Powell coined the term "clathrate" (from the Latin clathratus, meaning "provided with a lattice") based on his crystallographic analysis of quinol () clathrates, which trapped guest molecules like within a host lattice without covalent bonding; Powell's work formalized the concept of clathrate compounds beyond hydrates. The saw a pivotal in and theoretical understanding, transitioning from the term "gas hydrates" to the broader "clathrates" to encompass non-hydrate inclusion compounds, driven by revelations of their shared caged structures. This period also featured early hypotheses, such as S.P. Nikitin's 1936 suggestion that gas hydrates were clathrate-like, later confirmed by von Stackelberg's structural models. A major theoretical milestone came in 1959 with the statistical thermodynamic model developed by J.H. van der Waals and J.C. Platteeuw, which described clathrate stability using Langmuir adsorption isotherms to account for guest occupancy in host cavities; published in Advances in , this framework remains foundational for predicting hydrate phase behavior. The marked the recognition of clathrates' natural abundance and global significance. Expeditions by the in the late 1970s and early confirmed vast deposits of hydrates in marine sediments and beneath in regions, highlighting their role as resources and influencers; seismic and core sampling data from sites like the Blake Ridge and Siberian permafrost provided direct evidence of these occurrences. Powell's structural insights and Hammerschmidt's engineering observations continued to influence research, bridging early empirical findings with modern applications.

Structural Characteristics

Lattice Formation

The host lattice in clathrate compounds forms an architectural framework composed of polyhedral cages constructed from host molecules, which create voids capable of enclosing guest . These lattices are typically stabilized by intermolecular forces, such as hydrogen bonding in clathrate hydrates and organic clathrates or covalent interactions in inorganic frameworks. In clathrate hydrates, for instance, the host lattice adopts cubic or hexagonal symmetries, with polyhedral cages exemplified by pentagonal dodecahedra in structure I arrangements. Framework stability relies on the cooperative arrangement of host units, where empty lattices often collapse without guests to maintain the open structure. Cage geometries in clathrate lattices are predominantly , defined by the number and type of faces formed by host atoms or molecules. A common small is the 5125^{12} , featuring 12 pentagonal faces and accommodating 20 host atoms, while larger like 512625^{12}6^{2} incorporate 12 pentagons and 2 hexagons, resulting in 24 host atoms per . These geometries arise from the geometric constraints of host , with the and of the —ranging from approximately 0.5 nm for small polyhedra to over 0.7 nm for larger ones—directly influencing the dimensions of guests that can be stably enclosed without distorting the lattice. Clathrate frameworks exhibit varying dimensionality, from three-dimensional networks of interconnected polyhedra to one-dimensional channel systems. In three-dimensional topologies, cages link via shared faces or edges to form extended porous structures, such as face-centered cubic arrangements in some lattices. One-dimensional variants, conversely, consist of parallel tunnels running through the , as seen in certain organic hosts. The role of host molecules is central to lattice formation, as they self-assemble into the defining network. Water, in hydrates, links via tetrahedral bonds to build rigid, ice-like cages that persist under specific pressure-temperature conditions. Urea molecules, through bonding between carbonyl oxygen and amino s, form a hexagonal host lattice with linear channels of about 0.52 nm diameter. Similarly, hydroquinone assembles into a body-centered tetragonal framework via bonds between hydroxyl groups, generating nearly spherical cages of roughly 0.4 nm radius within a three-dimensional . In inorganic clathrates, the host framework consists of covalently bonded polyhedra formed by elements like or , as in type I structures with cubic symmetry and cages such as 5125^{12} and 512625^{12}6^{2}.

Guest-Host Interactions

In clathrate compounds, the interactions between guest molecules and the host lattice are primarily governed by weak, non-covalent forces, with van der Waals interactions serving as the dominant mechanism for entrapment. These forces arise from the dispersion between the guest and the inner walls of the host cages, without involving charge transfer, covalent bonding, or significant hydrogen bonding between guest and host in molecular clathrates and . In inorganic clathrates, however, electropositive guests often transfer charge to the framework, contributing to stability via electrostatic interactions. In clathrate specifically, the further contributes to stabilization by reducing the structured around non-polar guests, promoting cage formation through entropic gains in the surrounding solvent. This combination ensures that guests are physically confined within the polyhedral voids of the host framework, such as the dodecahedral or tetrakaidecahedral cages typical of hydrate structures. Guest occupancy in clathrate cages varies from partial to full filling, depending on the size and shape compatibility between the guest and the cavity dimensions. Effective enclathration requires guests with diameters roughly matching the cage radii, typically in the range of 0.1 to 0.9 nm for small and large cages in hydrate systems, allowing for stable van der Waals contact without excessive strain. For instance, (approximately 0.38 nm) fits well in small 5¹² cages, while larger molecules like (about 0.47 nm effective size) occupy larger 5¹²6⁴ cages. Multiple guests can occupy a single cage in cases of smaller species, such as diatomic pairs in larger voids, enhancing overall lattice occupancy up to stoichiometric ratios like 1/5.75 for structure I hydrates. The dynamics of guest molecules within the host lattice involve translational diffusion and rotational motion, influenced by the cage geometry and guest size. Guests exhibit significant rotational freedom, often nearly isotropic at low temperatures, due to the spherical symmetry of van der Waals potentials inside the cages, as seen in hydrogen molecules undergoing free quantum rotations. Diffusion occurs via rattling within cages or hopping between adjacent voids, with rates increasing for smaller guests; however, larger guests induce lattice expansion to accommodate their size, slightly distorting the host framework and reducing mobility. This expansion, typically on the order of 0.1-0.5% in unit cell volume for varying guest radii, helps maintain stability but can limit long-range diffusion at low temperatures. Stabilization of clathrate compounds relies on these guest-host interactions under specific conditions, where formation is -driven at low temperatures and high pressures, as the incorporation of guests maximizes configurational by filling otherwise unstable empty . Empty host lattices, such as pure , are thermodynamically unstable and collapse without guests to provide dispersive stabilization, leading to phase transitions to or . The high-pressure environment compresses the system to favor cage occupancy, while low temperatures minimize thermal disruption of the weak interactions, ensuring the overall persists as a metastable inclusion compound.

Classification

Clathrate Hydrates

Clathrate hydrates are non-stoichiometric crystalline inclusion compounds in which molecules form a hydrogen-bonded host lattice that encloses guest molecules, typically gases or volatile liquids, within polyhedral cages. These structures are stabilized by van der Waals interactions between the guests and the framework, without covalent bonding. Unlike general clathrates, the host in hydrates relies on bonding to create cage-like voids, enabling the entrapment of hydrophobic guests under specific thermodynamic conditions. The primary structural subtypes of clathrate hydrates are distinguished by their cage architectures and suitable guest sizes. Structure I (sI) adopts a cubic lattice ( Pm3n) composed of 46 molecules per , featuring two small pentagonal dodecahedral (5¹²) cages and six larger tetrakaidecahedral (5¹²6²) cages, which accommodate small guests with molecular diameters of 4.2–6 , such as (CH₄) or (CO₂). Structure II (sII), also cubic ( Fd3m), contains 136 molecules per with 16 small 5¹² cages and eight larger hexakaidecahedral (5¹²6⁴) cages, suited for larger guests (6–7 ) like (THF) or (C₃H₈). Structure H (sH) forms a ( P6/mmm) with 34 molecules per , including three small 5¹² cages, two medium irregular (4³5⁶6³) cages, and one large (5¹²6⁸) cage, requiring a of small help guests (e.g., CH₄) and larger molecules (7–9 ) such as or halogenated cyclohexanes to stabilize it. Formation of clathrate hydrates generally occurs under low-temperature and elevated-pressure conditions that favor the hydrogen-bonded lattice over liquid . For gas guests like , stability is achieved at temperatures below approximately 10°C and pressures exceeding 3 MPa, with the exact boundaries defined by phase diagrams showing equilibrium lines where hydrate, , and guest phases coexist. These conditions are influenced by guest type and , but the process involves of cage structures followed by growth, often requiring of the guest in . In nature, clathrate hydrates predominantly occur as hydrates in oceanic sediments along continental margins and in regions of the and sub-Arctic. These deposits form where cold temperatures and high pressures from overlying and columns preserve the structures, with oceanic hydrates concentrated in depths greater than 300–500 meters. Global estimates indicate that natural clathrate hydrates store 500 to 2,500 gigatons of carbon, representing a significant portion of Earth's reserves and a potential feedback if destabilized. Distinctive traits of clathrate hydrates include their high water-to-guest ratios, which reflect the cage occupancy; for example, fully occupied sI has a stoichiometric ratio of approximately 5.75 molecules per guest (46 waters for 8 cages). They exhibit an ice-like crystalline appearance but possess lower densities than pure ice Ih (0.917 g/cm³), typically around 0.90 g/cm³ for filled sI structures due to the expanded lattice from guest inclusion, making them less dense and more buoyant in sediments.

Non-Hydrate Clathrates

Non-hydrate clathrates encompass a diverse class of inclusion compounds where host frameworks composed of organic or inorganic encapsulate guest molecules or atoms within cavities, without forming chemical bonds between host and guest. Unlike water-based hydrates, these structures exhibit greater flexibility in host composition and synthesis conditions, enabling tailored applications in . Organic hosts, such as and , form stable lattices through hydrogen bonding, while inorganic examples, like silicon-germanium frameworks, rely on covalent bonding to create cage-like enclosures. Analogs in zeolites and metal-organic frameworks (MOFs) extend this concept with microporous architectures that mimic clathrate behavior, offering adjustable pore sizes for selective guest inclusion. Urea-based clathrates represent a prominent organic variant, featuring one-dimensional hexagonal channels that trap linear guest molecules, such as hydrocarbons like (C₂H₆) or n-octane (n-C₈H₁₈). The host lattice, with P6₁22, consists of hydrogen-bonded molecules forming tunnels approximately 0.52 nm in diameter, which require guest occupancy for . Formation typically occurs in solvent-based processes at ambient temperatures and moderate pressures (e.g., 49 MPa for at 253 K), often facilitated by auxiliary solvents like to lower activation barriers. This channel morphology contrasts with the discrete polyhedral cages in structures, allowing for easier guest exchange in linear configurations. Hydroquinone clathrates, another key organic example, adopt cage-like architectures in their β-form, where hydrogen-bonded rings of six hydroquinone molecules create interconnected voids accommodating small spherical guests like noble gases (Ar, Kr, Xe) or methane (CH₄) at a 3:1 host-to-guest ratio. The lattice belongs to space group R3̅ and stabilizes through van der Waals interactions, with the α-form serving as an empty precursor stable without guests. These are synthesized via crystallization from solutions at room temperature, showing less sensitivity to pressure than hydrates and enabling formation under milder conditions. Structural variations include interpenetrated networks that enhance cage isolation, distinguishing them from the more open channel systems in urea. Inorganic non-hydrate clathrates, such as those based on group 14 elements (Si, Ge, Sn), feature covalently bonded frameworks forming polyhedral cages analogous to hydrate polyhedra but with eclipsed tetrahedral arrangements. Type I structures, exemplified by Na₈Si₄₆, consist of 46 atoms creating two types of cages (dodecahedral and tetrakaidecahedral) filled by guests like sodium, which donate electrons to the host . Type II variants, like NaₓSi₁₃₆ (3 ≤ x ≤ 11), incorporate larger cages for varied guest sizes. Synthesis involves high-temperature reactions with metals, yielding materials with tunable electronic properties due to guest/framework . Zeolites, as analogs, provide three-dimensional channel networks for and adsorption, while MOFs offer hybrid organic-inorganic cages with designer ligands for precise guest selectivity, often formed solvothermally at moderate temperatures. Semi-clathrates in these systems exhibit partial host-guest bonding, blending inclusion with weak coordination for enhanced stability in synthetic hosts.

Physical and Chemical Properties

Thermodynamic Properties

Clathrate hydrates exhibit distinct phase behavior characterized by equilibrium curves that delineate the conditions under which formation and decomposition occur, typically plotted as pressure versus . These curves describe the coexistence of the clathrate phase with liquid and guest phases, such as vapor or liquid, and are governed by the Clapeyron equation, which relates the slope of the equilibrium line to thermodynamic changes: dPdT=ΔHTΔV\frac{dP}{dT} = \frac{\Delta H}{T \Delta V} where ΔH\Delta H is the change, TT is the , and ΔV\Delta V is the volume change for the dissociation process. For clathrate hydrates, the positive slope indicates that higher pressures stabilize the clathrate at elevated temperatures, with ΔV>0\Delta V > 0 due to the expansion upon guest release. The formation of clathrates is an , with negative ΔH\Delta H, releasing as guest molecules are incorporated into the host lattice; typical dissociation enthalpies for hydrates range from 50 to 55 kJ/mol, implying formation enthalpies of comparable magnitude but opposite sign. Accompanying this, the overall change ΔS\Delta S for formation is negative, as the confinement of guest molecules reduces translational freedom, though the host framework experiences an increase transitioning from the ordered structure of to the more disordered clathrate lattice. This compensation is central to the statistical thermodynamic description of stability. For non-hydrate clathrates, such as inorganic types, thermodynamic properties differ, often focusing on lattice stability and guest rattling rather than P-T equilibria. Theoretical modeling of these properties for hydrates relies on the van der Waals-Platteeuw theory, a statistical mechanical framework that predicts guest occupancy in cages using Langmuir-type isotherms and partition functions to compute chemical potentials. This theory integrates with equations of state for fluid phases to yield phase equilibria and occupancy distributions, emphasizing the role of guest-host interactions in determining thermodynamic stability. Extensions incorporate statistical thermodynamics to account for multiple occupancy and rotational contributions from guests, enhancing predictions of and effects. Thermodynamic properties of hydrates are influenced by and , with increasing favoring formation by countering the volume contraction, while temperature rises promote due to the endothermic nature of dissociation. Inhibitors such as salts (e.g., NaCl) destabilize clathrates thermodynamically by shifting equilibrium curves to higher pressures and lower temperatures, primarily through salting-out effects that reduce and alter guest ; for instance, NaCl increases the required for methane hydrate stability. Promoters, conversely, may lower the threshold for formation by enhancing occupancy.

Stability and Decomposition

The stability of clathrate compounds is governed not only by thermodynamic equilibria but also by kinetic barriers that control their persistence and eventual . For hydrates, decomposition typically proceeds as a involving the collapse of metastable partial cavities in the host lattice, followed by the of guest molecules out of the . This rate-determining step of guest creates significant kinetic hindrance, with activation barriers for through hexagonal faces in clathrate hydrates estimated at approximately 1.0-1.5 eV under neutral conditions. For clathrates, dissociation rates accelerate markedly with increasing temperature, often leading to rapid release above 0°C under , while higher pressures suppress these rates by stabilizing the lattice. Inorganic clathrates, in contrast, show stability up to high temperatures due to covalent frameworks, with involving guest migration rather than lattice collapse. Environmental and operational factors profoundly influence clathrate kinetics. ramping above equilibrium conditions promotes faster cavity collapse and guest escape, whereas sudden drops—common in pipelines—can trigger rapid dissociation by reducing lattice stability. Mechanical stresses, such as those from flow or shear in pipelines, further exacerbate by disrupting the host framework, potentially leading to blockages or hazardous gas releases. The exothermic nature of rapid gas expansion during poses risks, including overpressurization in confined systems. Decomposition mechanisms for hydrates primarily involve sequential guest diffusion from cages and subsequent host lattice collapse, often forming nanobubbles with a radius of ~11 (diameter ~22 ) for that facilitate further breakdown. In clathrate hydrates, empty cages decompose more readily than filled ones, with the presence of adjacent empty large cages accelerating the process through reduced van der Waals stabilization. Quantum simulations reveal that initial lattice disruption occurs via breakage and water fragment attacks on guests, leading to full structural failure. These mechanisms highlight the role of interfacial areas and guest occupancy in dictating pathways. For non-hydrate clathrates, stability is influenced by guest-framework interactions, with "rattler" atoms providing low thermal conductivity without at operational temperatures. Clathrate stability windows vary widely, from hours in settings under forced conditions to in natural deep-sea or deposits due to effects, where surface layers slow gas . Additives such as kinetic hydrate inhibitors (KHIs), including water-miscible polymers like , extend these windows by adsorbing at interfaces, hindering guest and delaying dissociation for periods sufficient to mitigate industrial risks. can maintain clathrates up to 75 K above their temperature, underscoring the dominance of kinetics in long-term persistence.

Synthesis and Preparation

Natural Occurrence

Clathrate compounds occur naturally primarily as gas hydrates in geological environments where specific temperature and pressure conditions allow water molecules to form cage-like structures enclosing guest gas molecules, such as . These formations are concentrated in two main settings: marine sediments along continental margins and beneath in polar regions. Approximately 99% of global gas hydrate reserves are located in marine sediments, particularly in fine-grained deposits on ocean floors at depths ranging from 200 to 1,000 meters, where cold seawater and high hydrostatic pressure stabilize the clathrates. The remaining reserves, about 1%, exist in permafrost-associated sediments in regions like and , often at shallower depths of 100 to 500 meters below the surface. Formation of these natural clathrates occurs through the interaction of dissolved gases with water under cold, high-pressure conditions in organic-rich sediments. , the dominant guest molecule, originates from either biogenic processes—where microbial decomposition of produces gas near the surface—or thermogenic processes, in which deeper hydrocarbons are generated by heat and pressure from breakdown and migrate upward into the hydrate stability zone. Biogenic hydrates form predominantly in shallower marine and settings, while thermogenic ones are more common in deeper marine environments, leading to disseminated or layered deposits up to several meters thick. These processes have been ongoing for millions of years, trapping gases that would otherwise escape to the atmosphere. Global estimates suggest that hydrates contain between 500 and 10,000 gigatons (Gt) of carbon, vastly exceeding the carbon in conventional reserves and representing a significant portion of Earth's organic carbon pool. Release of this , a potent with a 28-36 times that of over a century, could amplify if destabilized by warming oceans or thaw, potentially creating feedback loops that accelerate global temperatures. Detection of natural clathrate hydrates relies on geophysical and direct sampling methods, including seismic imaging that identifies bottom-simulating reflectors (BSRs)—acoustic boundaries marking the base of the hydrate stability zone—and drilling cores that recover hydrate samples. Significant historical discoveries occurred in the 1980s in the , where seismic surveys and coring expeditions first confirmed substantial marine hydrate deposits at depths exceeding 500 meters, prompting global research into their extent.

Laboratory Synthesis

Laboratory synthesis of clathrate compounds involves controlled conditions to form host-guest structures, typically replicating or enhancing natural processes through , temperature, and compositional adjustments. For clathrate hydrates, common methods include high-pressure reactors where and guest gases or liquids are combined under elevated (up to several hundred bar) and low temperatures (often below 10°C) to promote and growth. These techniques are classified by guest solubility: soluble guests like dissolve in before pressurization and cooling, while insoluble gases like require direct contact via stirring or bubbling in sealed vessels. An alternative ice-to-hydrate approach grinds into and exposes it to guest gas under , enabling uniform formation throughout the volume without liquid phases. Equipment for hydrate synthesis ranges from small-scale stirred autoclaves (milligram to gram quantities) to larger systems (up to scale), often incorporating magnetic stirrers for enhanced and pressure monitoring. cycling, such as rapid cooling from 273 to 77 followed by warming, accelerates clathrate formation from and gas, achieving rates over 100 times faster than conventional methods. For non-hydrate clathrates, synthesis emphasizes from solutions or melts. Organic clathrates, such as hydroquinone-guest compounds, form via gas-solid reactions where the host is crystallized under guest gas exposure, or by melting the host (e.g., at 170–175°C) followed by cooling in the presence of the guest. Inorganic examples like clathrates use arc melting of elemental mixtures (e.g., Sr, , Si) in atmospheres, followed by annealing, or floating-zone melting for single crystals, involving heating and controlled rotation at rates of 5 mm/h. methods, employing excess as a , heat stoichiometric mixtures (e.g., Ba, Cu, Ni, , Si) to 1000°C for 12 hours with slow cooling to yield single crystals of type-I clathrates like Ba₈Cu₁Ni₂.₅Ga₁₀Si₃₃.₅. Challenges in laboratory synthesis include slow nucleation kinetics for hydrates, often requiring extended times (hours to days), and difficulties in controlling polymorphism (e.g., structure I vs. II hydrates) which affects guest occupancy and stability. Purity issues arise from phase impurities or air entrainment, altering thermodynamic properties, while non-hydrates demand high energy inputs for melting and precise stoichiometry to avoid parasitic phases. Scale-up from micrograms to kilograms is limited by heat and mass transfer inefficiencies in reactors. Recent advances post-2010 include surfactant addition, such as (SDS) at 0.1–0.2 mass%, which promotes formation by enhancing gas solubility and forming porous structures, reducing required pressure by up to 20% and increasing conversion rates. For CO₂ hydrates, SDS at 1500 ppm accelerates growth in porous media, though effects vary by guest. Microwave-assisted methods using metal-organic framework (MOF) nanoreactors, like Zr-porphyrin PCN-222, enable rapid clathrate formation at milder conditions (1.35 kbar, 280 ) in 30 minutes, achieving near-complete conversion via confined in mesopores. More recent developments as of 2025 include the laboratory synthesis of a previously predicted phase structure, potentially advancing material science applications, and progress in binary clathrate hydrates for efficient under mild conditions.

Applications and Significance

Energy and Storage

Clathrate hydrates, particularly hydrates, offer a promising method for gas storage due to their ability to encapsulate large volumes of within a solid lattice . Under conditions, one volume of hydrate can store approximately 160 to 180 volumes of gas. This high storage capacity arises from the hydrate's cage-like framework, which traps molecules efficiently at moderate pressures and temperatures, making it suitable for applications such as vehicles (NGVs). In NGVs, hydrates enable safer onboard storage compared to traditional systems, as they operate at lower pressures while achieving comparable or higher volumetric densities. For transportation, solid-state methane hydrate carriers represent an alternative to (LNG) methods, where gas is stored as a solid at and temperatures around -20°C to 0°C. Unlike LNG, which requires cryogenic cooling to -162°C and specialized vessels, hydrate carriers can use standard refrigerated ships, potentially reducing infrastructure costs for shorter routes or stranded gas fields. A notable pilot project occurred in 2013 off 's , where the Japan Oil, Gas and Metals National Corporation (JOGMEC) successfully extracted gas from seabed hydrates using depressurization, demonstrating feasibility for offshore production and . This trial confirmed gas flow rates and recovery, highlighting hydrates' potential for maritime solid-state shipping. As of 2025, Japan's MH21-S R&D consortium continues efforts with planned offshore production tests to advance toward commercialization in the late 2020s. Key advantages of clathrate-based storage include higher energy density than (CNG), which requires pressures up to 250 bar for similar capacities, and enhanced safety due to the non-explosive solid form that eliminates high-pressure risks. However, challenges persist, including the energy-intensive processes for hydrate formation (requiring cooling and compression) and dissociation (needing controlled heating). Economic viability remains a barrier, with production and transport costs estimated above $10 per million British thermal units (MMBtu) in the , exceeding LNG benchmarks in many scenarios. Ongoing research focuses on additives and process optimizations to address these hurdles.

Environmental and Biological Roles

Clathrate hydrates, particularly methane varieties, play a critical role in dynamics by storing vast amounts of —a about 28 times more potent than over a 100-year period—in marine sediments and regions. Warming temperatures and thawing can destabilize these deposits, leading to methane dissociation and potential release into the atmosphere, which may amplify global warming through loops. For instance, models indicate that dissociation could release 5–21 Tg of methane per year from warming bottom waters, though microbial oxidation in sediments and the consumes 80–90% of this methane before it reaches the surface, mitigating atmospheric impacts. In the , subsea -associated hydrates, estimated to hold about 20 Gt of carbon, face intermediate vulnerability from ongoing thaw, with observations of seepage on continental shelves potentially contributing up to 17 Tg of methane annually, heightening risks of abrupt emissions. Environmentally, clathrate hydrates contribute to carbon sequestration by trapping methane and other gases in stable oceanic deposits, effectively locking away carbon that would otherwise cycle into the atmosphere and exacerbate warming. Global reserves are estimated at around 1,800 Gt of carbon, primarily in marine sediments where hydrates form under high pressure and low temperature, preventing methane oxidation and release over geological timescales. Additionally, air clathrate hydrates in polar ice cores serve as vital archives for paleoclimate reconstruction, preserving ancient atmospheric compositions including CO₂ levels dating back over 800,000 years; these hydrates form as air bubbles in ice transform under depth-induced pressure, enabling precise analysis of past greenhouse gas concentrations and climate variability. Studies of the EDML ice core in Antarctica reveal that hydrate number concentrations vary with climatic periods, higher in cold intervals (up to 290 cm⁻³) and lower in warm ones (down to 190 cm⁻³), providing high-resolution proxies for atmospheric changes. Biologically, clathrate hydrates support unique ecosystems at deep-sea cold seeps and hydrothermal vents, where seepage from dissociating hydrates fuels chemosynthetic that form the base of food webs, sustaining dense communities of tubeworms, clams, and other in otherwise nutrient-poor environments. At sites like Barkley Canyon, exposed hydrate outcrops release up to 300 meters into the water column, enabling methanotrophic microbes to thrive and support higher trophic levels, including commercially important species such as crabs and fish. In synthetic contexts, clathrate-binding proteins derived from deep-subsurface mimic enzymatic interactions by stably adhering to surfaces, altering morphology from single octahedrons to polycrystalline or plate-like structures, which could stabilize hydrates in biological or industrial settings and inform designs for enzyme-like catalysts in harsh conditions. Recent studies from the 2020s underscore growing concerns over clathrate hydrate destabilization, driven primarily by ocean warming but with compounding effects from acidification. Observations link contemporary warming to hydrate dissociation on continental margins, such as in the Svalbard region, where bottom-water temperature rises of 1–3°C have triggered methane leakage at rates of 9–118 × 10⁶ mol per year. Furthermore, modeling shows that hydrate dissociation releases methane that oxidizes to CO₂, prolonging ocean acidification by delaying pH recovery; for example, an estimated 1,600 Gt of carbon release over 13,000 years could keep surface pH ~0.12 units lower than preindustrial levels for millennia, deepening corrosive conditions in the deep ocean. These findings highlight hydrate systems as potential tipping elements in the global carbon cycle, with acidification exacerbating sediment destabilization and ecological disruptions.

Notable Examples

Gas Hydrates

Gas hydrates represent a prominent class of clathrate compounds where molecules form cage-like structures that encapsulate guest gas molecules, primarily under conditions of and low temperature. Among these, hydrates are the most extensively studied due to their prevalence in natural environments such as regions and continental margins. These hydrates predominantly adopt the cubic structure I (sI) configuration, consisting of two types of polyhedral cages—pentagonal dodecahedra and tetrakaidecahedra—that accommodate molecules in a stoichiometric ratio of approximately 1:5.75 (CH₄·5.75H₂O). Global reserves of hydrates are estimated to exceed 1.2 × 10¹⁵ cubic meters of gas equivalent at standard conditions (as of 2014), primarily concentrated in marine and onshore , representing a potentially vast energy resource that dwarfs conventional reserves. However, extraction poses significant geomechanical risks, including seafloor due to the collapse of the hydrate-bearing framework following depressurization or thermal stimulation, which can lead to structural instability and potential landslides. Carbon dioxide hydrates also form in the sI structure under similar thermodynamic conditions to methane hydrates, with CO₂ molecules occupying the cages more efficiently due to their larger size and higher polarity, resulting in a of approximately 1.1 g/cm³ compared to about 0.9 g/cm³ for methane hydrates. This advantage facilitates their application in , where CO₂ can be injected into subseafloor reservoirs to form stable hydrates, potentially locking away greenhouse gases for millennia. Additionally, semi-clathrate variants involving alkylamines, such as , enhance CO₂ enclathration by partially integrating the into the water lattice, lowering formation pressures and improving selectivity for CO₂ over other gases in mixed streams. Other hydrocarbon gases, such as and , typically form structure II () hydrates, featuring larger cages that allow for double occupancy in some polyhedra, with favoring sII due to its molecular size. In natural settings, mixed gas hydrates often occur, incorporating with or in sII structures, as observed in seafloor deposits like those at , where thermogenic gases lead to variable occupancy ratios that stabilize the lattice under ambient pressures. Notable field and investigations have advanced understanding of these systems. The 2002 Mallik Gas Production Research Well in Canada's Mackenzie Delta targeted a high-concentration deposit, recovering over 200 meters of hydrate-bearing core with pore saturations exceeding 90%, and demonstrated short-term gas production via depressurization without immediate collapse. demonstrations of CO₂ replacement for have shown feasibility for simultaneous recovery and sequestration, with experiments confirming that CO₂ spontaneously exchanges into sI cages, releasing up to 50% of encased while forming a mixed phase stable under reservoir conditions.

Zeolite-Based Clathrates

Zeolite-like clathrates, known as clathrasils, consist of rigid all-silica (SiO₂) frameworks characterized by interconnected micropores and cage-like structures that physically trap small guest molecules without chemical bonding. These frameworks are built from corner-sharing SiO₄ tetrahedra, resulting in uniform cage sizes typically between 0.3 and 0.6 nm that function as molecular sieves for non-polar guests. Prominent examples include melanophlogite, with its MEP framework featuring dodecahedral and tetrakaidecahedral cages accessible via small windows, and chibaite, possessing an MTN framework with larger polyhedral voids, both ideal for guest enclathration of gases like or . Common guests encompass , hydrocarbons such as CH₄, or N₂, particularly in natural occurrences where they stabilize the otherwise metastable silica lattice. Shape-selective inclusion is a defining feature, wherein the cage geometry discriminates based on molecular size and shape, permitting entry only to fitting guests while excluding larger ones. These clathrasils demonstrate high thermal stability, retaining framework integrity up to around 1000°C in some cases before structural collapse. Guest inclusion is reversible, driven by physisorptive van der Waals forces that allow desorption upon heating or evacuation without framework degradation, supporting their study in gas storage and separation. In scientific contexts, clathrasils serve as models for understanding hydrate structures and have been synthesized hydrothermally to explore potential applications in and membranes. The study of clathrasils emerged in the late , with natural examples like melanophlogite identified in the and synthetic variants developed in the , building on zeolite synthesis techniques but focusing on pure silica compositions for clathrate behavior. Contemporary research extends to hypothetical silica clathrates isostructural with gas hydrates, aiding insights into planetary ices and energy materials.

References

  1. https://goldbook.iupac.org/terms/view/C01097
  2. https://pubs.acs.org/doi/10.1021/cr50029a002
  3. https://www.sciencedirect.com/topics/engineering/clathrate
  4. https://onlinelibrary.wiley.com/doi/abs/10.1002/recl.19560750806
  5. https://www.science.org/doi/10.1126/sciadv.adv6867
  6. https://www.nature.com/articles/s41467-025-64277-2
  7. https://www.dictionary.com/browse/clathrate
  8. https://www.cambridge.org/core/journals/mrs-energy-and-sustainability/article/inorganic-and-methane-clathrates-versatility-of-guesthost-compounds-for-energy-harvesting/791134A819DAAFC025D5456EAE4F3D7D
  9. https://www.sciencedirect.com/topics/chemistry/clathrate
  10. https://www.ias.ac.in/article/fulltext/reso/009/07/0018-0031
  11. https://pubs.acs.org/doi/10.1021/ie50296a010
  12. https://pubs.rsc.org/en/content/articlelanding/1948/jr/jr9480000061
  13. https://pubs.usgs.gov/circ/1980/0825/report.pdf
  14. https://www.sciencedirect.com/topics/engineering/clathrate-hydrate
  15. https://www.nature.com/articles/1781410a0
  16. https://pubs.acs.org/doi/abs/10.1021/acs.jpcb.5b08369
  17. https://chemistry-europe.onlinelibrary.wiley.com/doi/abs/10.1002/chem.200304837
  18. https://pubs.rsc.org/en/content/articlehtml/2015/cp/c4cp05732e
  19. https://pubs.acs.org/doi/abs/10.1021/jp7114274
  20. https://www.nature.com/articles/s41598-017-01369-0
  21. https://www.pnas.org/doi/10.1073/pnas.2430913100
  22. https://www.sciencedirect.com/science/article/abs/pii/S0009261414000797
  23. https://tohoku.elsevierpure.com/en/publications/thermal-expansion-and-lattice-distortion-of-clathrate-hydrates-of
  24. https://pubs.acs.org/doi/10.1021/jp903439a
  25. https://physicstoday.aip.org/features/clathrate-hydrates-under-pressure
  26. https://pubs.acs.org/doi/10.1021/acs.jpcc.9b04691
  27. https://www.usgs.gov/centers/whcmsc/science/gas-hydrates-primer
  28. https://pmc.ncbi.nlm.nih.gov/articles/PMC6358667/
  29. https://pubs.acs.org/doi/10.1021/jp511749q
  30. https://www.researchgate.net/publication/286131523_The_Early_Development_of_Inorganic_Clathrates
  31. https://pmc.ncbi.nlm.nih.gov/articles/PMC6644575/
  32. https://pubs.rsc.org/en/content/getauthorversionpdf/c5cp07202f
  33. https://www.nature.com/articles/s41557-022-01128-3
  34. https://www.sciencedirect.com/science/article/pii/S0016236125029746
  35. https://pmc.ncbi.nlm.nih.gov/articles/PMC299752/
  36. https://adsabs.harvard.edu/full/1985ApJS...58..493L
  37. https://pubs.acs.org/doi/10.1021/acs.chemrev.0c00494
  38. https://pmc.ncbi.nlm.nih.gov/articles/PMC10687932/
  39. https://www.mdpi.com/2305-7084/8/6/124
  40. https://www.sciencedirect.com/science/article/pii/S0167732221027501
  41. https://pmc.ncbi.nlm.nih.gov/articles/PMC9052115/
  42. https://arxiv.org/pdf/1105.5780
  43. https://www.sciencedirect.com/science/article/pii/0009250995003703
  44. https://pubs.acs.org/doi/10.1021/acs.energyfuels.5c00772
  45. https://pubs.aip.org/aip/jcp/article/142/21/214701/967435/Formation-of-methane-nano-bubbles-during-hydrate
  46. https://pubs.acs.org/doi/10.1021/acs.energyfuels.4c01375
  47. https://www.nature.com/articles/s41598-019-42336-1
  48. https://pubs.usgs.gov/fs/fs021-01/fs021-01.pdf
  49. https://pubs.usgs.gov/of/1996/of96-272/ch03s07.html
  50. https://www.usgs.gov/publications/methane-hydrate-a-major-reservoir-carbon-shallow-geosphere
  51. https://19january2021snapshot.epa.gov/ghgemissions/understanding-global-warming-potentials_.html
  52. https://www.netl.doe.gov/sites/default/files/netl-file/fe0009949-final-report.pdf
  53. http://deepseadrilling.org/66/volume/dsdp66_20.pdf
  54. https://onlinelibrary.wiley.com/doi/10.1002/9783527695058.ch4
  55. https://netl.doe.gov/sites/default/files/netl-file/ESD05-048_HydrateFormationMethods.pdf
  56. https://pubs.aip.org/aip/apl/article/88/13/131909/330893/Fast-synthesis-method-and-phase-diagram-of
  57. https://pubs.acs.org/doi/10.1021/acs.cgd.6b00834
  58. https://www.sciencedirect.com/science/article/pii/S0925838801018400
  59. https://pmc.ncbi.nlm.nih.gov/articles/PMC10912696/
  60. https://www.sciencedirect.com/science/article/abs/pii/S1359028616300183
  61. https://www.mdpi.com/2571-6255/7/7/240
  62. https://onlinelibrary.wiley.com/doi/full/10.1002/anie.202315280
  63. https://phys.org/news/2024-07-elusive-laboratory.html
  64. https://www.sciencedirect.com/science/article/pii/S1385894725075345
  65. https://pmc.ncbi.nlm.nih.gov/articles/PMC9053743/
  66. https://asmedigitalcollection.asme.org/energyresources/article/118/3/209/405909/Storage-of-Fuel-in-Hydrates-for-Natural-Gas
  67. https://www.jogmec.go.jp/english/news/release/release0110.html
  68. https://www.mh21japan.gr.jp/english/mh21s_project.html
  69. https://www.mdpi.com/1996-1073/10/6/828
  70. https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2022.990562/full
  71. https://www.ipcc.ch/report/ar5/wg1/
  72. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016rg000534
  73. https://www.nature.com/scitable/knowledge/library/methane-hydrates-and-contemporary-climate-change-24314790/
  74. https://pastglobalchanges.org/publications/pages-magazines/pages-magazine/137429
  75. https://tc.copernicus.org/articles/19/5023/2025/tc-19-5023-2025.pdf
  76. https://nautiluslive.org/blog/2024/06/19/deep-sea-biology-cold-seeps-and-gas-hydrates
  77. https://pubs.acs.org/doi/10.1021/acs.cgd.0c00855
  78. https://pmc.ncbi.nlm.nih.gov/articles/PMC7391661/
  79. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015GL065779
  80. https://www.sciencedirect.com/science/article/abs/pii/S0378381215302727
  81. https://netl.doe.gov/sites/default/files/netl-file/gas-hydrate-global-assessment-executive-summary.pdf
  82. https://www.energy.gov/sites/prod/files/2016/06/f33/FINAL_DOE%2520Assessment%2520of%2520SEAB%2520Report%2520on%2520Methane%2520Hydrates.pdf
  83. https://www.osti.gov/servlets/purl/1286804
  84. https://pubs.acs.org/doi/10.1021/acs.jpcc.2c04140
  85. https://publications.aston.ac.uk/id/eprint/46334/1/M_Aminnaji_et_al_CO2_Gas_hydrate_for_carbon_capture_and_storage_applications_part_1.pdf
  86. https://www.sciencedirect.com/science/article/abs/pii/S0009250911001783
  87. https://www.nature.com/articles/s41598-023-31669-7
  88. https://netl.doe.gov/sites/default/files/netl-file/JIP_Dallimore_Mallik.pdf
  89. https://www.pnnl.gov/main/publications/external/technical_reports/pnnl-17035.pdf
  90. https://www.sciencedirect.com/topics/materials-science/clathrasils
  91. https://www.nature.com/articles/ncomms1196
  92. https://www.nature.com/articles/s41598-018-30447-0
  93. https://www.degruyterbrill.com/document/doi/10.1515/9781501509698-020/html
  94. https://pubs.acs.org/doi/10.1021/ja00392a023
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