Metal–Organic Frameworks and Gas Hydrate Synergy: Comparison
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Metal–organic frameworks consist of metal nodes and organic linkers connected by coordination bonds to form programmable modular structures that are symmetric and have tunable properties. Metal–organic frameworks, also known as microporous or nanoporous materials, provide a large pore volume and surface area suitable for capturing, separating and storing gases through physisorption mechanisms. However, water and water interactions within the nanopores, open metal sites, coordination bonds and surface make metal–organic framework usage in water-based technologies an exciting research topic. Water-based gas hydrate technology could be potential technology that can take advantage of MOF tunable properties, such as a large surface area and a high pore volume, to improve its efficiency and formation mechanism.

  • metal–organic framework
  • nano confinement
  • phase equilibrium
  • Gas capture and storage
  • Gas adsorption
  • Gas hydrates

1. Metal–Organic Frameworks (MOF)–Hydrate Synergy: Current State of the Art

Hydrate–MOF synergies have been investigated mainly in CH4 gas storage and in CO2 gas storage and capture, and the initial results are promising. CH4 gas is a major component of natural gas, and higher natural gas consumption is promoted due to lower CO2 emissions and higher octane values compared with petroleum or coal consumption [49][1]. Currently, natural gas is transported either in a compressed form or by liquefaction under difficult conditions (high pressures and very low temperatures) because natural gases have a low density, low boiling point, high critical pressure and high diffusivity, and their transportation and storage consume excessive energy [50][2]. Therefore, hydrates provide an alternative medium for storage and transportation under moderate conditions and in safer environments [51,52][3][4]. When CH4 gas is stored in hydrates, the storage capacity varies between 160 and 180 v/v, and the CH4 gas molecules do not fill all hydrate cages. Therefore, different methods (stirring, promoters and porous media) are being tested to improve kinetics and storage capacity at the laboratory scale [53][5]. Research on the synergy between hydrates and MOFs is limited to CH4 and CO2 hydrates, and no such studies exist for the storage of hydrogen hydrates in MOFs. However, recent publications using activated carbon have confirmed that nanomaterials can positively accelerate hydrogen hydrates in confined spaces at lower pressures and can enable higher conversion of water to hydrates [54][6].

1.1. MOF Material Characterization

The characterization of the process of hydrate formation and dissociation and its structure in a confined space is challenging due to the limitations of visualization techniques (size of the confined space < 100 nm) and the possibility of a phase change in the host structure or the main water phase [55][7]. Visualization techniques, such as magnetic resonance imaging (MRI) and X-ray computed tomography (CT), are not always effective due to their lower resolutions (7.5 µm) and inability to distinguish hydrates from the ice phase. Therefore, alternative techniques, such as the high-pressure adsorption technique, could be sufficient for pre-wetted material [55][7]. Inelastic neutron scattering (INS) at low temperatures (about 4 K) and in the presence of deuterated water could also be used to identify hydrogen atoms in gas hydrates, either in bulk or in a confined nano space, such as metal–organic frameworks or the cavities of nanoporous carbons [56,57][8][9]. Synchrotron X-ray diffraction (SXRD) is another technique that offers additional advantages over single-crystal X-ray diffraction due to its high intensity and associated shorter sampling time. Some of the first methods for characterizing CH4 hydrates in confined spaces using SXRD show a decrease in unit cell parameters in microporous and mesoporous carbons [58][10]. Other SXRD results confirmed the thermal stability of CH4 and CO2 hydrates in confined spaces [59][11]. Raman and 13C NMR spectroscopy techniques are other potential techniques that could be used to characterize gas hydrates at the nanoscale. However, sufficient research is still needed in this direction because there are few studies on this topic [60,61][12][13]. Scanning electron microscopy (SEM) has been used to investigate crystal morphology and stability before and after the MOF and hydrate system went through a heating and cooling cycle [62][14]. Recently, differential scanning calorimetry has been used to study the conversion of water to hydrates in nanoporous materials, such as MOFs, zeolites, porous organic cages, etc. [62,63,64][14][15][16].
Although the above techniques are well known and influential for the characterization of hydrates in confined spaces, the sample size is in the range of mg or less. To confirm the phenomenon on an industrial scale, a larger sample size (in grams) is required for laboratory experiments. In these cases, additional challenges, such as heat and mass transfer, can pose a much greater problem than those with smaller sample sizes. Unfortunately, there are very few examples of combined hydrate–MOF studies that focus on large MOF sample sizes to confirm and compare results due to upscaling. In one recent study [65][17], the authors argued that a packed column system is the best choice to study MOF–hydrate synergies upon upscaling. Therefore, they tested the synergy of HKUST-1 and CH4 hydrate in a packed column configuration (60 cm3) using chenille fabric and found that HKUST-1, when spread over the fabric, effectively reduces induction time, showing reproducible results and improving recyclability.

1.2. MOF–Hydrate Synergy for CH4 Storage

Most studies on the application of MOFs to improve gas hydrate technology are related to the formation of CH4 hydrates. Used MOFs can be divided into hydrophobic and hydrophilic MOFs. Water adsorption isotherms are generally used to characterize MOFs as hydrophobic and hydrophilic [66][18]. Studied MOF–hydrate synergies include Al metals (MIL-53), Fe based (MIL-100), Cr (MIL-101) and Cu metals (HKUST-1), and Cr-based MOF-1 and Y-shaped MOF-5 are used. MIL-53 and HKUST-1 are widely studied MOFs for gas adsorption due to their higher hydrothermal stability [67,68][19][20]. The water adsorption isotherm suggests that MIL-101 and MIL-100 (Fe) are the most stable MOFs in water as hydrophilic mesoporous compounds and compared with HKUST-1, and zeolitic imidazolate frameworks (ZIF-8 and ZIF-67) are characterized as hydrophobic MOFs [66][18]. In one study, amine-functionalized UiO-66 was used to perform flow assurance studies of natural gas hydrates.
Another common difference among MOFs used so far is the difference in their thermal conductivity. MOFs are known to have low thermal conductivity and are considered thermal insulators (thermal conductivity < 2 Wm−1 K−1 at room temperature) [69[21][22][23][24],70,71,72], and their thermal conductivity further decreases in the presence of adsorbates, such as liquids (water, ethanol and methanol) and gases (CH4 or CO2) [69][21]. (For example, the following are the thermal conductivities of selected MOFs: HKUST-1 = 0.44–0.73 W/(m K) [69][21], ZIF-8 = 0.32 W/(m K) [71][23], CH4-Cr-soc-MOF-1 = 1.3 W/(m K) [70][22] and CH4-Y-shp-MOF-5 = 0.32 W/m K [72][24]). HKUST- 1 has a relatively higher thermal conductivity, which facilitates local heat dissipation during hydrate formation and creates a suitable environment for hydrate growth. Available research also indicates that, in the presence of hydrates and ice, the overall thermal conductivity of the system improves [73[25][26],74], and the difference in the conductivity of the porous material also affects the dissociation rate of the hydrates [75][27].
Zeolitic imidazolate frameworks (ZIF) are a subclass of organometallic frameworks that contain metal ions (inorganic) and imidazolate linkers (organic) and form a framework of tetrahedral structural units, similar to zeolites [83][28]. The main difference between ZIF-8 and ZIF-67 is the different metal ions (Zn in ZIF-8 and Co in ZIF-67). ZIF-8 has been extensively tested for CH4 storage in hydrates due to its high methane absorption capacity (∼8 mmol/g at 80 bar) [77,84][29][30] and its hydrophobic properties, which increase the stability of ZIF in the presence of water [85][31] and under high pressures (up to 800 bar) [84][30]. ZIF is mainly a microporous MOF (pore size < 1.2 nm), whereas other common MOFs also have a sufficient amount of meso- and macropores. Available research results indicate that ZIF-8 has higher gas uptake than other available MOFs, such as HKUST-1 and MIL-53, in both the dry and wet states, which is due to its large surface area and hydrophobic nature [77][29].
Although ZIF’s framework exhibits low water adsorption (0.2 mmol/g–0.5 mmol/g under various relative pressures) [86[32][33],87], it has been reported that ZIFs have a flexible structure, and the pore windows expand under high pressures (3.4 Å) to allow the diffusion and transport of CH4 gas molecules (3.8 Å). This is referred to as the “gate opening phenomenon” [88][34] and shows that the adsorption of CH4 molecules under high-pressure conditions increases from 1.8 mmol/g (at 10 bar) to 8.5 mmol/g at 100 bar [77][29], confirming why ZIF exhibits higher gas uptake under both dry and wet conditions. In another study [63][15], the performance of microporous ZIF-8 and ZIF-67 was compared in CH4 hydrate formation, and no significant difference was found due to different metal sites (Zn and Co). Both ZIFs showed increased gas uptake under hydrate formation conditions, and hydrate formation occurred in the interstitial space between MOF crystals, which was attributed to the hydrophobic nature of the pores, enhanced gas diffusion through the crystals due to pore expansions and high gas adsorption in the inner pore walls.

1.3. MOF–Hydrate Synergy for CO2 Storage and/or Separation

Although CH4 and CO2 hydrates both form sI hydrates, the ability of CO2 hydrate to stabilize cages is better than that of CH4 hydrate because the molecular size of CO2 (5.12 Å) is larger than that of CH4 (4.36 Å), which also leads to differences in their thermodynamic stability [89,90][35][36]. Of the common gases that form hydrates, such as N2, CO2, CH4 and O2, CO2 forms the most stable hydrate. Therefore, it is more thermodynamically stable and has faster formation kinetics. This difference is used to capture and separate CO2 from gas mixtures containing CO2, e.g., CO2/N2 mixtures [91][37] or CO2/H2 mixtures [92][38]. Although the thermodynamic driving force to separate CO2 gas from the gas mixture is strong, the method suffers from low separation efficiency and is limited by operating conditions. Most work on the synergy between hydrates and MOF is limited to CH4 hydrates, and minimal research is available for pure CO2 gas and CO2 gas separation and storage from a mixture.
Furthermore, it was also found that CO2 molecules tend to settle in small hydrophobic pores at low pressures and in large pores at higher pressures. The authors also suggested that CO2 storage occurs in the pore space partially surrounded by water molecules because the chemical potential of CO2 in water-saturated HKUST-1 is negative enough to replace water because of it having a higher quadrupole moment than that of H2 or N2. Therefore, the authors argued that the different gas storage capacities of HKUST-1 can be used for CO2 gas separation from gas mixtures, such as CO2 + N2 and CO2 + H2.
There are very few studies on the efficiency of the capture/sequestration of CO2 from CO2/N2 or CO2/H2 mixtures using the synergy of gas hydrates and MOFs. It has been argued and experimentally demonstrated that microporosity and mesoporosity play a more important role in enhancing hydrate formation than the pure bulk phase. Therefore, further studies should be carried out on the MOFs with microporosity and mesoporosity to investigate CO2 capture and storage under gas hydrate formation conditions [47][39].

2. MOF–Hydrate Synergy: Physical Properties

2.1. Confinement–Compression Effect in Nanopores

Nanopores experience both a confinement effect and a supercompression effect. The result of these two effects can control hydrate nucleation, growth kinetics and the conversion of water to hydrates, as well as formation pressure.
The existence of the compression effect in nanopores has been confirmed in several studies. Fujimori et al. [94][40] observed the formation of a monoatomic sulfur chain with exceptional conductivity properties, which could be due to the presence of ultra-high pressures (above 90 GPa) with one-dimensional carbon nanotubes under ambient pressures. Another phase change within nanotubes was successfully achieved when Urita et al. achieved the phase transition from B1 to B2 in KI crystals, a transition process that generally occurs above 1.9 GPa under bulk conditions [95][41]. Therefore, the high compression effect in nanopores can be used for phase changes or for the growth of other crystalline structures, such as ice or gas hydrates [22][42]. Studies on the pronounced compression effect during hydrate formation have not been adequately explored and therefore require further investigation.
The confinement effect occurs in pores whose sizes correspond to the lateral dimension of a few molecular diameters and affects the physicochemical properties of the liquid phase. In nanopores, the interactions (repulsive and attractive interactions) between the liquid–pore interface and the gas–liquid interface develop adsorption potential and control the properties of molecules or atoms. Due to the confinement effect, the liquid phase in the pores can undergo a phase transition compared with the liquid phase under bulk conditions [96][43]. Furthermore, the interface between the molecule and the surface is generally controlled by the surface curvature, so a curved surface has a van der Waals energy that is eight times larger than that of a flat surface [97][44]. Therefore, entrapped molecules exhibit various properties such as super mobility, strong physisorption and a high heat of adsorption [98][45].
To investigate the confinement effect, CH4 and C3H8 hydrates were formed in silica gel pores (radius 7 nm) [99][46]. It was found that the experimental formation pressure in the pores was higher than that in the bulk phase. Due to the confinement effect, the water activity decreases, the required hydrate formation pressure increases and the freezing temperature of water decreases [100][47]. In the case of extremely low water activity and an extreme decrease in the freezing temperature of water, the water in these nanopores can reach a supercooled state. The increase in thermodynamic stability pressure in nanopores compared with the main phase is also known as thermodynamic inhibition. Some key factors that may contribute to the enhancement of thermodynamic inhibition are the metastable state of the fluid, surface heterogeneities and pore interconnection effects [101][48]. The inhibition effect in a confined space could depend on the surface properties of the pores and the pore size and geometry [76][49]. As the pores become smaller, the water activity in the pore space becomes weaker, and the inhibition effect becomes stronger due to geometric constraints in the nanopores [102,103,104][50][51][52].
In another study conducted on confined water in the cavities of activated carbon under atmospheric pressure, the trapped water did not freeze in the micropores and did not undergo a phase transition from liquid to solid. However, water did freeze in large micropores and mesopores, albeit at a much lower freezing point. Moreover, at high pressures and in a CH4 gas environment, freezing (CH4 hydrate formation) was observed in confined micropores at higher temperatures than those in previous cases [105][53].
Thus, the authors hypothesize that there is competition between the compression and confinement effects and that the dominant mechanism controls gas hydrate nucleation and growth kinetics. It is also possible that different dominant mechanisms exist in different pore spaces of the same material. If the confining effect dominates, one would expect lower water to hydrate conversion, a high-required formation pressure and a long nucleation time. On the other hand, there are also experimental studies that confirm the presence of a dominant compression effect. For example, a recent study on hydrogen hydrates in nanopores showed a 30% lower formation pressure than that in the bulk phase and almost 100% conversion of water to hydrates using activated carbon [54][6].

2.2. Pore Size Effect

One of the main conclusions from the available studies and reviews is that nanomaterial surface chemistry and pore size control and influence hydrate formation in confined spaces [106][54]. Pore size controls water intrusions and extrusions, water properties in confined spaces, the hydrate formation mechanism and hydrate phase behavior [47,107][39][55]. Micropores with a diameter of <1.2 nm do not provide a sufficient internal volume for hydrate cages to assemble during hydrate nucleation [108][56], although some studies have discussed the possibility of defective nano hydrates [109][57]. Due to the lack of advanced characterization techniques, it is difficult to quantify hydrates and the formation mechanism in micropores [22][42]. It would not be wrong to conclude that microporosity is not exploited for hydrate formation. However, micropores can be exploited for gas adsorption due to their high gas solubility [110][58].
On the other hand, mesoporosity facilitates water intrusion by capillary condensation [111][59] and generates the gas–liquid interface in addition to the liquid–solid interface [112][60]. It has also been found that hydrates form first in macroporous pores and then in mesoporous pores at higher pressures [110][58]. The difference in pore size also controls hydrate stability and dissociation temperatures. Hydrates that form in larger pores have a higher dissociation temperature [113][61]. As pore size decreases, the ability to form hydrates within the pores decreases, and hydrates are more likely to form in the interparticle space and on the outer surface.
Experimental results show that hydrate crystals do not form in wetted micropores, and hydrate dissociation is similar to that in the bulk phase. In wetted mesopore nanomaterials (diameter > 6.2 nm), the hydrate forms in the pores at higher pressure and lower temperature conditions, whereas in wetted macropore nanomaterials (diameter > 100 nm), there is no effect of pore space on the hydrate stability line [77][29].
Pore diameter also affects the amount of interfacial water trapped in the pore space. For example, interfacial water at a distance of 0.5 nm from the surface was found to have different structural properties compared with that of the bulk phase (e.g., loss of hydrogen bonding and no freezing on cooling) [114][62]. Therefore, it is difficult to form hydrates in microporous materials with a pore diameter of <1 nm, because the interfacial water has different structural properties.
Currently, the formation of hydrates in nanomaterials appears to be most promising if the mesopores can be optimized. Therefore, nanomaterials with dominant mesopores have shown higher conversion of water to hydrates compared with other microporous compounds. For example, up to 95% conversion of water to hydrates has been observed in activated carbon with well-developed mesoporosity [115,116][63][64]. In another study, the CH4 adsorption capacity in a water-saturated activated model porous carbon increased from 18% (pore size = 0.8 nm) to 102% and to 170% (pore size 10 nm–25 nm) when the pores become mesopores [58][10]. This experimental evidence confirms the positive role of mesoporous carbon and suggests that metal–organic frameworks with well-defined mesoporosity can be beneficial, whereas microporosity can support methane adsorption at the inner pore surface.

2.3. Hydrophobicity

MOFs are generally hydrophilic (>10,000 MOFs), and hydrophobic MOFs and MOF-derived composites are a small subclass of MOFs (<100 MOFs) [117][65]. The hydrophobicity of MOFs can be divided into surface hydrophobicity and internal pore hydrophobicity. The degree of hydrophobicity controls the competition between water adsorption and gas molecule adsorption. In the early days of MOF-based studies, hydrophobicity was largely overlooked. Today, however, hydrophobic MOFs are of great interest because hydrophobic MOFs are believed to improve mechanical and chemical stability, water stability, working efficiency, durability and reusability in various applications.
A recent study showed that, for MIL-100 (Fe) hydrophilic MOFs, the presence of moisture/water inhibits the hydrate formation process in the intrinsic porosity. The figure shows that the adsorption capacity, at a high pressure in the presence of water, was much lower than that of a dry sample because the water adsorbed and blocked the pores [57][9]. Using hydrophobic MOFs such as ZIF-8, it was found that wet and dry samples had similar adsorption isotherms up to a pressure of 30–40 bar (CH4 hydrate stability pressure); after that, the adsorption profile of the wet MOF increased dramatically, indicating the onset of gas hydrate formation. Hydrate formation occurred in the interparticle space, whereas the non-wettable microporosity only participated in the adsorption process by pure physisorption [57][9].
The above study suggests that hydrophobic MOFs are better suited for gas hydrate formation because they can reduce competition between water and CO2. It is also suggested that the hydrophobic surface does not disturb the ordering of water molecules required to form hydrate cages around the guest molecule. In contrast, the hydrophilic surface disturbs the tetrahedral arrangement of the water structure and reduces gas density at the interface between the hydrophilic surface and the water [106][54]. Experimental work has also shown that a hydrophilic surface inhibits the nucleation of gas hydrates [114,118][62][66] because the interfacial water near the hydrophilic surface experiences a significant loss of tetrahedral water caused by H-bonds between the phases [114,118][62][66]. It was also discovered that dissolved gas molecules tend to collect closer to the hydrophobic surface–water interface in the form of surface nanobubbles [119][67] due to hydrophobic attraction [120,121][68][69]. A detailed discussion of surface nanobubbles and their role in hydrate formation can be found in another review article [122][70].
In hydrophobic MOFs, CO2 adsorption can occur in physisorption and hydrate formation. In hydrophobic MOFs, hydrates form only in the interparticle space, so the total surface area and micropores are not used (physisorption only). The above results contradict the idea that water spreads much more on hydrophilic surfaces than it does on hydrophobic ones. Therefore, the gas–liquid interface is higher, but the low water activity of hydrate formation overrides the ability to spread water on hydrophilic surfaces. It is also pointed out that hydrophilic MOFs with high water content can saturate the nanopores with water, limiting gas diffusivity through the MOFs. Therefore, further research should be conducted to optimize the water content of hydrophilic but water-stable MOFs to take advantage of high water dispersion on the surface and the nano-confinement properties of water. Efforts should focus on developing water-stable MOFs with superior water dispersibility, high surface water activity and a high degree of nanoscale confined water.
Another study investigated hydrophobic and hydrophilic mesoporous carbon materials for confined methane hydrate formation [112][60]. The results show that hydrophobicity controls water mobility at the pore walls, which controls water conversion to hydrates. It was also found that confined water in hydrophobic mesopores is converted to hydrates, and liquid water is uniformly distributed in hydrophilic mesopores but is not converted to hydrates.
Nanopores are inherently hydrophobic due to their small size, allowing water to enter and exit under high pressure. It is unclear whether water intrusion/extrusion can occur under hydrate formation conditions (high pressure and low temperature) and whether it can be sustained during multiple cycles of formation and dissociation [123][71]. A preliminary hydrophobic MOF ZIF-8 with a pore size of 0.34 nm allows water molecule intrusion (0.29 nm) between 19.9 MPa and 22 MPa [124,125][72][73].
Recycling and the water stability of MOFs control the lifetime and frequency of the replacement of MOF materials to maintain profitable commercial operations [126][74]. Although MOFs have good thermal conductivity, many MOFs tend to decompose in humid environments because the bonds between metal and oxygen are unstable and decompose by hydrolysis [127][75]. Zeolites imidazolate frameworks (ZIFs) are a subclass of MOFs known for their water, acid, chemical and thermal stability [127][75]. Recent research on zeolite-based hydrate formation shows that zeolites form gas hydrates in their lattice structures [79][76]. It was found that the growth of hydrates occurs in the interstitial space between the pores because water cannot penetrate into the hydrophobic inner pores. The study showed a 45% improvement in CH4 gas storage capacity in hydrates.

2.4. MOF Chemistry and Surface Functionalization

The surface chemistry of nanopore materials is expected to control growth kinetics. It has been proposed and experimentally proven that, when water is present in nanomaterials, thin water films develop at the water–solid interface, and hydrate nucleation starts at the solid–liquid interface rather than at the liquid–gas interface. The growth rate of the hydrate film is controlled by the mass-transfer-based driving force caused by the difference in methane saturation in the liquid phase at the gas–liquid interface and the liquid–solid interface [128,129,130][77][78][79]. It is argued that surface chemistry plays an important role because hydrate formation starts at the liquid–solid interface. For example, an experimental study [131][80] showed that activated carbon has improved growth kinetics and a shorter induction time (30 min) compared with nano-silica (84 min). This difference could be due to the interconnectivity of the pore space, which can facilitate the formation of crystal nuclei and can support hydrate growth during the transient hydrate formation and dissociation process [55][7].
There are few studies on the formation and dissociation of hydrates in the presence of MOFs, and there are no initial studies on the effects of MOF chemistry on hydrate formation. Since carbon-based nanomaterials (activated carbon, porous model carbon and multi-walled carbon nanotubes) have shown promising results for hydrate growth kinetics, the authors believe that carbon-based MOFs could be tested for hydrate formation and dissociation studies.
The kinetics of gas hydrate formation in a confined space depend on water–gas and water–solid surface interactions. The difference in induction time is the main difference between MOF materials and other microporous materials (carbon, silica or zeolite-based) in hydrate formation and dissociation. The difference in induction time is due to the different pore sizes and surface chemistry of the different materials [55][7]. Because MOFs are a rapidly developing field of research and new MOFs are discovered regularly, it is of great interest to thoroughly investigate the effects of MOF materials on formation and dissociation kinetics. The results of such a study could open the door to developing new MOF materials with a suitable surface chemistry, which is expected to optimize hydrate storage concerning specific gases, such as CH4, CO2 or H2.
In addition to controlling nucleation and growth kinetics by altering material chemistry, another means of control is using kinetic promoters in confined spaces. Kinetic promoters are a class of chemicals that, when added to water, affect kinetics without affecting thermodynamic stability. Common kinetic promoters include various surfactants [132][81] and amino acids. Their ability to promote kinetics has been tested in a porous sandy medium [133,134,135][82][83][84]. Few studies focus on the combined role of nanomaterials and kinetic promoters [136[85][86],137], which should be further explored. Another way to improve water activity and the interaction of water molecules at the water–solid interface is to change the surface chemistry via surface functionalization. When oxygen-containing surface groups are introduced into the pore surfaces of carbon nanomaterials, it was found that the formation of CH4 hydrates at lower pressure conditions (about 30–40 bar) was enhanced by promoting the formation of water clusters, leading to hydrate formation at lower pressures [138][87]. More detailed studies are needed, especially in the case of metal–organic frameworks, because surface functionalization is expected to enhance both physisorption and gas hydrate formation.

2.5. Water Properties in Confined Spaces

Water properties in confined spaces depend on pressure, water chemistry, the degree of water saturation, pore size, surface chemistry and surface properties. The liquid phase in nano-confined spaces is subject to the over-solubility phenomenon. Researchers have found that the solubility of nonpolar gases, such as CH4 and CO2, in supercooled water increases by almost 100 fold in a nano-confined space [56,139,140][8][88][89]. Research has confirmed that water in nanopores enters a supercooled state such that the melting temperature can be less than 50 K, depending on the pore size [141][90].
It is not known what the relative contributions of the above factors are. The factors depend on the water saturation in the pore space and the surface chemistry of the pore space. The solubility of gases in a solvent confined in nanopores lies between Henry’s law and Langmuir’s curve [144][91]. The over-solubility of gases in a nano space can be caused by two main mechanisms [145][92], mainly adsorption or entrapment. Which mechanism ultimately dominates depends on the predominant interaction between the gas–solid interface and liquid–solid interactions. In cases where liquid–solid interactions predominate, gas over-solubility is driven by confinement. In this case, over-solubility can be further increased by decreasing the water saturation relative to full saturation, forming a gas–liquid interface. It has been argued that over-solubility does not depend on the specific surface area but can be optimized by controlling the pore size, because an optimal pore size allows layering of the liquid without strong curvature effects within the nano-confinement [145][92]. Thus, it is possible to take advantage of microporosity due to over-solubility because microporosity in a metal–organic framework is not involved in forming hydrate crystals. Therefore, metal–organic frameworks with optimal microporosity and mesoporosity distributions, optimal pore sizes and partial water saturation levels can optimize the overall gas uptake due to hydrate formation and over-solubility. Thus, it should be further investigated.
A thermodynamic study of CO2 hydrates in MIL-53 [76][49] confirmed the nano-confinement effect on water present in nanopores. The hydrophilic MIL-53 exhibited micropores and meso/macropores due to a limitation in the laboratory synthesis method. When the pores were saturated with water and the hydrate phase diagram was plotted, it showed that two different hydrates were present (trapped and bulk hydrates), with the number of trapped hydrates in the meso/macropores being much higher than that of bulk hydrates. Due to CO2 over-solubility in micropores, a difference was observed in the formation and dissociation phase diagram. Based on the difference between the pore size and the hydrate unit size, it was assumed that hydrates cannot form in micropores and that CO2 is dissolved in water-saturated micropores via both physisorption and a confining effect. As a result, CO2 was dissolved in water-filled micropores, and CO2 hydrates formed in meso/macropores due to a weaker inhibition effect.

3. MOF–Hydrate Synergy: Metal–Organic Framework Stability in Water

MOFs are known for their good thermal stability, but when they come into contact with moist gases, many MOFs decay. However, ZIF MOFs have shown considerable thermal and chemical stability [127,146][75][93]. The water stability of MOF is of particular importance because large amounts of water vapor are present in various industries (natural gas streams, air pollution, air separation, etc.) where MOFs are to be used. Water vapor is also present in industrial flue gases (up to 10%), an important area for implementing hydrate-based gas separation, capture and storage. Because hydrate technology is water-based, the interaction between MOFs and water would be critical in MOF selection, controlling MOF efficiency and the cost of the overall process. The degree of hydrophobicity controls the water stability of MOFs. However, even hydrophobic MOFs can become water-unstable when the water loading is very high and exceeds the critical limit, but this phenomenon is not yet fully understood [40][94]. Hydrophilic MOFs can exhibit hydrophobic nanopores even under high compression if the pores are not large enough. Therefore, water intrusion needs to be further investigated in the context of hydrate formation in nanopores (especially mesopores and macropores) under high pressure. In addition to the chemical and thermal stability of ZIF-based MOFs, surface functionalization, e.g., with amine (-NH2), has also had a positive effect on the stability of MOFs and has improved CO2 uptake due to the interactions between amine and CO2 [126][74].
The stability of MOFs in a highly aqueous environment should also be considered when selecting MOFs for hydrate technology. MOFs should be structurally stable in the presence of water and after several formation/dissociation cycles. Currently, most available MOFs are hydrophilic, so their ability to survive in a water-rich environment under high pressures and low temperatures needs further investigation. Hydrophilic MOFs control the amount of water available for gas hydrate formation and facilitate water intrusion into the nanopores, and they usually have a nano-confinement effect. Metal nodes on MOFs tend to be hydrolysed in water, so further studies on hydrolytically stable MOFs need to be conducted. However, it is known that the metal ion charge controls hydrolysis reactions. Therefore, not all hydrophilic MOFs are water-unstable. For example, MIL-53 (Al-based) is more hydrophilic than MIL-100 and MIL-101 (Fe-based) but is also more water-stable than MIL-100,101 (due to the higher charge of Al in MIL-53). Therefore, it is necessary to identify hydrophilic MOFs with higher metal ion charges at the node and their relative water stability and to compare their performance at high water saturation. Another practical implication is to identify MOFs with optimal pore size distributions that can utilize micropores for gas absorption and meso/macropores for hydrate formation at lower thermodynamic pressures and enhanced hydrate growth.
The effects of water and high temperatures on the chemical and thermal stability of MOFs have been extensively studied, and many MOFs tend to undergo chemical decomposition due to either high temperatures or high water content. However, the chemical and thermodynamic stability of MOFs under hydrate formation conditions (high pressure, low temperature and presence of water) has not yet been studied. Recent experimental studies have shown that MOFs are relatively stable at lower temperatures for extended periods. For example, images from SEM showed that HKUST-1 is stable and does not chemically decompose after undergoing several cycles of formation and dissociation (in high-pressure differential scanning calorimetry(µ-DSC)) [62][14]. The chemical decomposition of HKUST-1 starts at the crystal structure [147][95], and its degradation can be controlled and reversed with an ethanol treatment, which improved the longevity of HKUST-1 crystals [148,149][96][97].

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