CH₄ Adsorption in Wet Metal-Organic Frameworks under Gas Hydrate Formation Conditions Using A Large Reactor

Abstract

Nanoporous materials, such as metal-organic frameworks (MOFs), are renowned for their high selectivity as gas adsorbents due to their specific surface area, nanoporosity, and active surface chemistry. A significant challenge for their widespread application is reduced gas uptake in wet conditions, attributed to competitive adsorption between gas and water. Recent studies of gas adsorption in wet materials have typically used small amounts of powdered porous materials (in the milligram range) within very small reactors (1–5 mL). This leaves a gap in knowledge about gas adsorption behaviors in larger reactors and with increased MOF sample sizes (to the gram scale). Additionally, there has been a notable absence of experimental research on MOFs heavily saturated with water. In this study, we aimed to fill the gaps in our understanding of gas adsorption in wet conditions by measuring CH₄ adsorption in MOFs. To do this, we used larger MOF samples (in grams) and a large-volume reactor. Our selection of commercially available MOFs, including HKUST-1, ZIF-8, MOF-303, and activated carbon, was based on their widespread application, available previous research, and differences in hydrophobicity. Using a volumetric approach, we measured high-pressure isotherms (at T = 274.15 K) to compare the moles of gas adsorbed under both dry and wet conditions across different MOFs and weights. The experimental results indicate that water decreases total CH₄ adsorption in MOFs, with a more pronounced decrease in hydrophilic MOFs compared to hydrophobic ones at lower pressures. However, hydrophilic MOFs exhibited stepped isotherms at higher pressures, suggesting water converts to hydrate, positively impacting total gas uptake. In contrast, the hydrophobic ZIF-8 did not promote hydrate formation due to particle aggregation in the presence of water, leading to a loss of surface area and surface charge. This study highlights the additional challenges associated with hydrate-MOF synergy when experiments are scaled up and larger sample sizes are used. Future studies should consider using monolith or pellet forms of MOFs to address the limitations of powdered MOFs in scale-up studies.

1. Introduction

Gas hydrates represent an ice-like crystalline phase between ice and water, formed under high-pressure and low-temperature conditions. The gas hydrates are formed when water molecules create a cage-like structure through hydrogen bonding, with stability provided by guest molecules such as gaseous CH₄ (methane) and CO₂ (carbon dioxide). The system’s thermodynamics, kinetics, and stability are influenced by guest molecules and water’s ability to form hydrogen-bonded cages [1]. The formation of gas hydrates is a stochastic phenomenon characterized by low water-to-hydrate conversion rates and slow formation kinetics. As a result, various improvements have been proposed, including using chemical promoters, mechanical stirring, or employing porous materials to enhance the contact area between gas and liquid. Research on gas hydrate formation using porous materials has been summarized elsewhere [2,3,4]. Recent advances suggest the potential of hydrate formation in nanospaces as well as on the outer surfaces of nanoporous materials [5,6,7]. Nanoporous materials include metal-organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs), and activated carbon.

Methane (CH₄) gas has a significantly lower volumetric energy density (0.0107 MJ/L) compared to traditional petroleum fuels, such as gasoline (34.2 MJ/L), which reduces its viability for commercial-scale applications [8]. The volumetric energy density can be enhanced through compression or liquefaction processes [9]. Adsorption is viewed as a promising technology for the high-density gas storage of CH₄ under mild conditions, requiring only a single compression step [10]. Traditionally, adsorption-based technologies have relied on zeolite and carbon materials due to their straightforward synthesis process on a commercial scale. Nanoporous materials, such as metal-organic frameworks (MOFs), are recognized as effective gas adsorbents through physisorption, typically under high temperatures and moderate pressure. Key factors influencing this process include the MOFs’ surface area, surface chemistry (including hydrophobic or hydrophilic characteristics), and pore size. The development of metal-organic frameworks (MOFs) with potentially enhanced adsorption capacities and lower energy requirements for regeneration has attracted attention from the scientific community. MOF development allows for the precise control of pore size, geometry, and functionality [11]. HKUST-1 is among the most studied MOFs for CO₂ and CH₄ adsorption, as its open metal sites exhibit robust interactions with gas molecules [12].

A significant challenge in physisorption is the presence of water vapor or humidity in the incoming gas stream, which competes with the gas for adsorption sites on the MOFs’ surface, thereby reducing the total gas adsorption capacity. Prolonged exposure to water vapor can also lead to MOF degradation, resulting in their structural instability [13]. The main reasons include hydrolysis, irreversible changes in MOF structure caused by water adsorption and desorption, structural modification, etc. [14,15]. MOF water stability can be classified into two categories: thermodynamic stability and kinetic stability. Thermodynamic stability involves metal-ligand bond strength and the inertia of metal clusters, while kinetic stability involves the spatial effects of ligands and hydrophobicity [16].

Gas adsorption is measured using isotherms [17], employing volumetric or gravimetric techniques. High-pressure and low-temperature conditions enhance gas adsorption on nanoporous materials [18]. In addition to these operating conditions, certain materials exhibit breathing effects that produce stepped isotherms, thereby increasing total adsorption in nanoporous materials. It is desirable to have stepped isotherms at high pressure and low temperature to improve the process [19]. However, stepped isotherms are limited to certain nanoporous materials, also known as breathing/flexible adsorbents [15,16,18,19]. A few studies of MOF-gas hydrate combined systems have also confirmed the presence of stepped isotherms [7,20,21]. The presence of stepped isotherms also highlights the utility of studying gas hydrates and MOF systems. These isotherms are recorded for small amounts (in mg), smaller reactors, and low water/solid ratios. Further investigation is needed to scale up the study for larger material sizes. Figure 1 provides three different variations in isotherms from three different situations.

Converting water into gas hydrate crystals offers several benefits, including: (1) Introducing a phase change from water to hydrates in nanoporous materials. (2) Prolonging the chemical and water stability of metal-organic frameworks (MOFs) by converting water into hydrate crystals. (3) Producing stepped isotherms and increasing the gas adsorption value. (4) Saving energy through low-temperature-based hydrate formation. Due to these advantages, researchers have explored gas hydrates and nanomaterial systems for pure gases like CH₄ [6,22], CO₂, and H₂ [23]. Multiple review papers [2,5] have summarized the key advancements in this field and discussed the role of nanoporous material properties, such as surface area, pore size, and hydrophobicity.

Most studies of MOFs’ application in improving gas hydrate technology have focused on CH₄ hydrate formation. Available studies have shown that many MOFs, including MIL-53 [24,25], MIL-100 [26], ZIF-8 [25,26], HKUST-1 [6,25], Cr-based MOF-1, and Y-shaped MOF-5 [7] have been covered by recent studies. ZIF-8 and HKUST-1 have been the most extensively studied MOFs in gas hydrate research. HKUST-1 is a hydrophilic MOF, while ZIF-8 is a hydrophobic MOF. ZIF-8 and HKUST-1 are also different in terms of thermal conductivity (HKUST-1 = 0.44–0.73 W/(m K) [27], ZIF-8 = 0.32 W/(m K) [28]). Higher thermal conductivity supports faster heat dissipation during hydrate formation.

In the case of MIL-53, CH₄ hydrates form in meso- and macroporous spaces and interparticle spaces, with no hydrate formation in micropores [24]. Recent studies have indicated that hydrophilic MOFs like MIL-100 (Fe) impede hydrate formation when exposed to moisture, leading to a decrease in gas density at the water-MOF interface. Conversely, hydrophobic MOFs such as ZIF-8 facilitate gas hydrate formation [26]. Experimental data suggest that hydrophilic surfaces obstruct gas hydrate nucleation due to disrupted water structures within the pores. In contrast, dissolved gas molecules accumulate on hydrophobic surfaces, forming surface nanobubbles due to hydrophobic attraction.

Current experimental studies on gas adsorption in wet nanoporous materials under hydrate formation conditions involve two approaches. In the first approach, high-pressure differential calorimetry is used to study the water-to-hydrate conversion under high pressure (80–100 bars). For example, recent studies using HKUST-1 [6] and ZIF-8 [22] showed that hydrates predominantly formed on the outer surface of the MOFs, with the water/solid ratio controlling the spread of water over the surface. In another set of studies, a high-pressure volumetric isotherm method was used to measure gas adsorption (in mmol/g) in wet nanoporous materials. This method allowed researchers to compare and calculate the positive contribution of hydrate formation to total gas mole adsorption under different water saturations but at constant material weight [7,26].

However, small sample sizes limit the conclusions drawn from these studies, and there is a lack of understanding of scale-up processes. Additionally, there is a notable lack of experimental research on heavily water-saturated MOFs. To address these gaps, we investigated the CH₄ gas adsorption in wet MOFs and wet activated carbon material under hydrate formation conditions (T = 274.15 K and up to 80 bars) using a large-volume reactor and larger sample sizes. We used commercially available MOFs, activated carbon on a gram scale, and recorded high-pressure isotherms. This study aimed to confirm the hydrate formation (stepped isotherms) when the material was used in large quantities in powdered form. Furthermore, we explored how the degree of wetness affects gas adsorption and the role of material properties in influencing adsorption behavior.

2. Materials and Methods

2.1. Setup and Materials

All nanoporous materials used in this study were procured commercially, and their essential characteristics are provided in

Table 1. Commercial available data information on MOFs/nanoporous materials.

Sample Type	HKUST-1	ZIF-8	MOF-303	NuChar Carbon
BET Surface area (m2/g) *	641	1550	1133	1366
Adsorption energy *** for CH4 (kJ/mol)	15–30 [29]	18.7–44.7 [30,31]	19 [32]	12–14 [33]
Density (g/cm3)	0.88 [34]	0.95 [35]	0.40 **	0.40
Procurement	Sigma Aldrich	Sigma Aldrich	NovoMOF	Nuchar
Purity	99%	99%	99%	99%

* BET surface area is taken from the material data sheet provided by the supplier. Similar values are also reported in the literature. ** Reported by the supplier. *** Reported in the literature.

. There were three different MOFs (ZIF-8, HKUST-1, and MOF-303) and one activated carbon (Nuchar AC). ZIF-8 and NuChar AC are hydrophobic nanomaterials, while HKUST-1 and MOF-303 are hydrophilic. The difference in hydrophilicity was measured through the total water uptake/water isotherms method [29]. The total water uptake for HKUST-1 and MOF-303 was also measured in this study.

ZIF-8 (Basolite^® Z1200) and HKUST-1 (Basolite^® C 300) were procured from Sigma-Aldrich (St. Louis, MO, USA). MOF-303 (3,5-Pyrazoledicarboxylic acid monohydrate) was procured from novoMOF (Zofingen, Switzerland), and activated carbon (AC) was procured from NuChar (Logroño, Spain). Before the experiments, the MOFs were heated to 120 °C in a vacuum oven for 24 h to eliminate impurities. Ultrapure water from Merck Millipore wet the MOFs and activated carbon in situ during the experiments. The wetting procedure involved adding a measured amount of water (in grams) into the dry MOFs using a syringe needle in the reactor without removing the samples. The R_w value was subsequently increased for the next experiment without the samples being removed.

The selection of materials was based on commercial availability, the difference in hydrophobicity, previous literature availability, and water stability. HKUST-1 and ZIF-8 were studied previously for gas hydrate systems, and their studies were summarized in our last review paper [5]. MOF-303 is used for atmospheric water harvesting and has demonstrated high stability in water-rich conditions [36,37,38]. MOF-303 could act as a replacement material for HKUST-1. Nuchar carbon is activated carbon. There are multiple studies on ZIF-8 [39,40,41] and activated carbons (also known as porous carbon) [42] are known to be highly hydrophobic, and various researchers have previously investigated them in hydrate formation studies.

HKUST-1 (Cu₃(BTC)₂) is comprised of copper nodes linked by 1,3,5-benzenetricarboxylic acid and bimodal pores [43]. Some of the key advantages of HKUST-1 for CH₄ adsorption are its large internal pore diameter, high thermal conductivity, and supportive surface chemistry, which could lead to high methane uptake. [44,45] and water adsorption [46]. The water-to-gas contact area increases because of higher wettability. Copper at metal nodes, which helps remove the local heat of hydrate formation, could account for HKUST-1’s high thermal conductivity.

ZIF-8 is a zeolitic imidazolate framework (ZIF), which falls under the subclass of metal-organic frameworks (MOFs). ZIFs consist of inorganic (metal ions) and organic (imidazolate linkers) components, forming a framework of tetrahedral structural units, similar to those found in zeolites [47]. ZIFs are isostructural and are known for having a surface area greater than 1000 m² per gram, a pore volume of approximately 0.6 cm³ per gram, and an internal pore diameter of 1.2 nm [48,49].

MOF-303 is synthesized using the ligand 3,5-pyrazoledicarboxylic acid (PDC) and exhibits a pore-limiting diameter (PLD) of 5.78 Å [50]. MOF-303 features a three-dimensional framework with one-dimensional (1D) rhombic channels (open space of ∼0.6 nm) along the axis. The clusters and ligands endow the 1D channels with hydrophilic sites. The high pore volume and the hydrophilic nature of the framework translated into impressive water capacity. Furthermore, fast sorption kinetics were observed and attributed to the 1D hydrophilic channels, which created a favorable situation for forming well-defined water cluster structures [51]. MOF-303 is an extremely effective MOF for water adsorption (water harvesting) and displays both a hydrophilic and a water-stable nature. No previous studies are available where MOF-303 has been used to study hydrate-MOF synergy or CH₄ adsorption (dry or wet cases). Therefore, MOF-303 is selected in this study as an alternative to HKUST-1 due to its enhanced water stability.

The quantity across the experiments differed due to the original sample availability. We tried to maximize the quantity of MOF in our experiments; however, commercially available MOFs come in different quantities and prices, and due to density differences, varying numbers of MOFs were added. We focused on keeping the dead weight the same across all experiments, and the gas adsorption value was normalized to weight.

The high-pressure adsorption and desorption experiments on dry and wet solid materials were conducted using the apparatus depicted in Figure 2. This system includes a high-pressure reactor, a cylinder with a piston, and a Vindum pump. The reactor, with a volume of 210 cm³ and a maximum working pressure of 150 bar, is connected to a bath system for temperature control. Dry material is placed in the reactor, followed by the addition of water at different R_w values to achieve the desired wetness. The cylinder with a movable piston separates the gas portion, which is connected to the reactor, from the water portion, which is linked to the Vindum pump. This setup allows for precise control and measurement of gas adsorption and desorption in dry and wet solid materials.

The Vindum pump maintains the water pressure and piston position to regulate the gas pressure in the reactor, facilitating adsorption and desorption experiments through stepwise pressurization and depressurization. A thermometer at the reactor’s bottom measures the in situ temperature, with measurement uncertainties of ±0.1 K for temperature and ±0.5 bar for pressure. The system continuously records changes in pressure, temperature, and volume. Glass beds and glass cotton are placed in the reactor after the material to reduce dead volume. Studies on samples and blank reactors were performed to measure and eliminate the contribution of the dead volume.

SEM images (Section 3.5.2) were obtained using a HITACHI S4800 field emission microscope at 2 kV in secondary and backscattered electron modes. X-ray diffraction analysis (Section 3.5.3) of the crystalline powder samples was performed using a Bruker D8-Advance Diffractometer with Cu Kα radiation, measuring from 3° to 50° (2θ) with a step size of 0.02°. XRD diffraction was collected for the materials of interest, HKUST-1 and MOF 303, as they were shown to have gas hydrate formation. Before analysis, samples were dried at 373.13 K until their mass became constant.

H₂O sorption isotherms at 298 K (Section 3.3.4) were obtained using Anton-Paar’s VSTAR vapor adsorption equipment. Samples were outgassed under vacuum at 423 K for 24 h before the adsorption measurements.

2.2. Procedure and Data Processing

2.2.1. Gas Adsorption and Desorption Experiment

Dry material with a known weight (in grams) is placed in a large reactor, followed by a metal mesh to prevent mixing with glass beads and glass cotton, which minimizes dead volume. The reactor is then sealed and leak-tested. Before the CH₄ gas adsorption experiment, a vacuum removes any air pockets.

In the first gas injection stage, methane gas is introduced into the reactor until the pressure reaches 20 bar, which is maintained for two hours. Initially, the piston is filled with gas at 20 bar and then connected to the reactor. The piston movement is balanced through a water pump to ensure the reactor reaches 20 bar, with the water side of the piston in equilibrium with the gas side. The total water volume is measured, equating to the amount of gas injected into the reactor containing the material.

Subsequently, the pressure inside the reactor is increased stepwise using the water pump, starting at 20 bar and increasing in 5-bar increments. At each step, the pressure is constant for two hours to allow the system to reach equilibrium. The total water volume injected into the piston is calculated and adjusted with the piston volume to determine the total volume of gas used. The experiment concludes when equilibrium is achieved at 80 bars, taking approximately 42 h. During this time, the change in the total volume of gas on the MOF side is measured to calculate the total moles of gas adsorbed.

For each material, the experiment is conducted first in the dry and wet states. After the dry case, water is added in known quantities to progressively increase water/solid ratios (R_w) of 0.5, 1, 1.5, 2, and so on. The same material with the exact same weight is used throughout all individual experiments (dry and wet). The weight of the material varies between different materials due to commercial availability, so R_w is adjusted accordingly, and the moles of gas adsorbed are reported per gram of material for accurate comparisons.

The reactor is maintained at isothermal conditions (274.15 K) using a water bath, with temperature variations of ±0.25 °C due to room temperature changes over the experiment’s long duration. Gas desorption experiments are performed at the end of selected adsorption experiments. Once the reactor is pressurized to 80 bar, the experiment undergoes gradual depressurization using stepwise methods with the same pressure steps and duration.

2.2.2. Data Processing Gas Calculations

The customized high-pressure reactor was utilized to measure the total volume of CH₄ gas adsorbed, with the reactor being filled with various MOFs and subjected to different water-to-solid ratios. The temperature within the reactor was maintained at 274.15 K, which falls within the hydrate formation zone, to ensure a high driving force for adsorption. Considering the known weights and densities of the materials used, including the weights of the glass beads, the space inside the reactor—including the voids within the MOFs—was measured. The reactor was connected to a gas piston. The gas piston is controlled by the water pump. The pressure inside the gas piston is covered by the water pump. The total gas volume available in a combined piston and reactor is calculated at any given pressure using Equation (1).

$${V_{gas}=V}_{piston}-V_{water}+V_{chamber}$$

(1)

where V_piston is the total volume of the piston (equal to 1200 cm³ in this experiment), V_water is the volume in the piston occupied by water, V_chamber is the volume inside the high-pressure reactor occupied by gas, and V_gas is the total volume available for CH₄ gas across the piston and reactor, all corresponding to a specific pressure value and R_w value. The volume occupied by the moving part inside the piston and the line connecting the piston and the chamber is neglected. Then, to accurately calculate the amount of CH₄ available in the gas piston at the start of the experiment at 20 bar with a known volume, it is converted to the number of moles using Equation (2):

$$n_{{CH}_4,total}={\displaystyle \frac{P{V}_{piston}}{ZRT}}$$

(2)

where$n_{{CH}_4,total}$ is the total number of CH₄ available in the overall system, P is a specific pressure value, T is the temperature measured simultaneously with the pressure and volume value, and Z_i is the compressibility factor of CH₄ gas at the given pressure P_i and temperature T_i. The Z_i is calculated using the Benedict-Webb-Rubin-Starling equation of state. R is the universal gas constant, 8.314 J·mol⁻¹·K⁻¹.

The number of moles of CH₄ available in the system will always be the same if it is ensured that there is no leaking. Consequently, the amount of CH₄ inside the chamber is calculated with Equation (3):

$$n_{{CH}_{4,}reactor}=n_{{CH}_4,total}\ast \left({\displaystyle \frac{V_{chamber, MOF}}{V_{gas}}}\right)$$

(3)

where $n_{{CH}_4,reactor}$ is the amount of CH₄ inside the chamber corresponding to any specific pressure value. The $n_{{CH}_4}$ value includes the CH₄ inside the pores of the MOFs as well as the dead volume. The V_gas value is achieved when it is assumed that the gas and piston reach equilibrium within the piston and pressure-driven adsorption is completed. The higher the adsorption, the higher the drop in V_gas. Typically, V_gas is sensitive to volume change on account of the gas adsorbed and the change in the dead volume; thus, V_gas would depend on the adsorptive behavior of different materials and different dead volumes on account of varying mass and R_w values.

To normalize moles of CH₄ in a filled reactor, the $n_{{CH}_4,reactor}$ value is divided by the sample weight (in grams) by using Equation (4)

$$n_{{CH}_4,MOF}={\displaystyle \frac{n_{{CH}_4,reactor}}{m_{MOF}}}$$

(4)

where $n_{{CH}_4,MOF}$ is the moles of CH₄ inside the MOF-filled reactor, and mMOF is the weight of the MOF sample available inside the reactor. Finally, an isotherm ranging from 0 bar to 80 bar for all R_w cases of MOFs is created to qualitatively understand the comparison. The isotherm is plotted with pressure values on the x-axis and corresponding $n_{{CH}_4,MOF}$ values on the y-axis.

Desorption experiments were performed in a few cases to confirm the working capacity. A potential material for CH₄ storage should achieve a high working capacity, meaning it must deliver the maximum amount of gas at a certain low pressure before refueling.

3. Results

The stepwise pressure build-up methodology was used to create high-pressure isotherms to analyze water/MOF interactions under isothermal conditions.

3.1. Reproducibility of the Experiment

The dry ZIF-8 adsorption experiment was repeated to ensure the reproducibility of our results using the proposed method for two different sample amounts.

Table 2. CH₄ gas adsorbed on ZIF-8 in two different experiments. (** Experiment was terminated at P = 70 bars).

	CH4 Amount Adsorbed (Reactor Volume = 210 mL)
	Exp.1#	Exp.2#
Temp (°C)	274.15 K	274.15 K
Weight (g)	15	6.3
Pressure (bars)	(mmol/g)	(mmol/g)
0	0.00	0.00
20	24.03	22.92
25	28.72	27.54
30	32.74	31.87
35	36.47	36.01
40	39.91	39.59
45	42.85	43.17
50	45.66	46.35
55	48.49	49.24
60	51.26	52.09
65	53.35	54.98
70	55.53	57.60
75	XX **	59.99
80	XX **	62.28

presents the mmol/g of CH₄ adsorbed on dry ZIF-8 in two distinct experiments with different initial weights. The results confirm the reproducibility of our findings using this method. Consequently, the same method was used to compare gas adsorption values across various materials with different degrees of wetness.

Samples placed in the reactor were not removed, and water was added to the system without removing the samples. The absolute value of gas adsorbed per gram was calculated by subtracting the blank value from the measured excess adsorption value. This analysis provides a qualitative assessment of the adsorption efficiency at high pressure.

The results of this study depend on the analytical technique used. For example, the number of gas molecules adsorbed was determined based on a stepwise increase in pressure, assuming that a steady state was achieved after maintaining constant pressure for two hours. We also performed desorption experiments on ZIF-8, discussed in Section 3.4.

3.2. Gas Adsorption for Dry Nanoporous Materials

The pore size, surface area, and kinetic diameter of the gases control gas adsorption in dry MOFs. The water kinetic diameter is equal to 0.265 nm, the H₂ kinetic diameter is 0.289 nm, the CO₂ kinetic diameter is 0.33 nm, and the CH₄ kinetic diameter is 0.38 nm [52]. These kinetic diameters are much smaller than the pore size of nanoporous materials (0.7 to 1.2 nm). We selected four nanoporous materials, and the total CH₄ gas adsorbed/gram of material is measured and mentioned in

Table 3. CH₄ gas adsorbed on dry MOF at 274.15 K.

	CH4 Amount Adsorbed (mmol/g) (RW= 0)
Temp (K)	274.15	274.15	274.15	274.15
Weight (g)	6.3	42.1	20.5	17.3
Pressure (bar)	ZIF-8	NuChar-AC	HKUST-1	MOF-303
0	0.00	0.00	0.0	0.0
20	22.92	2.15	5.1	7.2
25	27.54	2.58	5.7	8.5
30	31.87	3.16	6.0	9.7
35	36.01	3.66	6.2	10.8
40	39.59	4.13	6.8	11.7
45	43.17	4.60	7.5	12.6
50	46.35	5.08	7.7	13.3
55	49.24	5.52	8.4	14.0
60	52.09	6.01	9.1	14.7
65	54.98	6.49		15.3
70	57.60	6.78		15.9
75	59.99	6.81		16.0
80	62.28	6.81		16.0

.

In the dry case, we found that CH₄ gas adsorption among MOFs was highest in ZIF-8, whereas, among hydrophilic MOFs, MOF-303 exhibited a higher CH₄ adsorption capacity than HKUST-1. Zeolitic imidazolate frameworks (ZIFs), a subgroup of metal-organic frameworks, exhibit a zeolitic structure due to coordination between transition metal ions (inorganic) and imidazolate ligands (organic), such that the framework is composed of tetrahedral structural units similar to zeolites. [47]. ZIFs are known for their large surface area and high porosity and are chemically and thermally stable. ZIF-8 is the most common among different ZIF variants and is thus a popular candidate to capture and store CO₂ [53]. ZIF-8 applications for CH₄ adsorption have been limited [54]. ZIF-8 exhibits a higher CH₄ gas adsorption capacity than other traditional MOFs, which confirms that ZIF-8 has an expandable framework. This framework further expands at low temperatures and high pressures. [55]. ZIF-8 shows lower gas uptake at pressures below 25 bar; however, its expandable network increases the total gas uptake.

Other researchers have also observed that the higher CH4 adsorption capacity of ZIF-8 surpasses that of HKUST-1. Generally, HKUST-1 has a higher CH4 adsorption capacity than other MOFs due to its open metal site. However, neither HKUST-1 nor MOF-303 exhibits any expandable structure under the influence of high pressure. Our study also shows that MOF-303 has a higher CH4 adsorption capacity (1.6 times at P = 60 bar) as compared to HKUST-1, which can be attributed to higher BET surface area availability for MOF-303. The total adsorption amount increases due to an increase in pressure or a lowering in temperature. In one of the similar studies, the CH₄ adsorption value in HKUST-1 was measured for the dry case at 303 K and 315 K [56].

Significantly few studies have compared activated carbon’s CH₄ gas adsorption behavior with MOFs at hydrate formation temperatures and higher temperatures [57]. Activated carbon is known for its high surface area and excellent gravimetric capacity, but it has a lower density than MOF materials like ZIF-8 and HKUST-1. The density of activated carbon is 0.6 g/cm³, which refers to its packing density, whereas, for MOF crystals, it is their crystallographic density. This density difference affects the volumetric adsorption capacity, resulting in lower CH₄ adsorption for activated carbon than MOFs such as HKUST-1 and ZIF-8 [57].

However, using the gravimetric method for adsorption calculation at hydrate formation temperatures is challenging due to the risk of water condensation on the balance, which can affect the measurement values. Therefore, the volumetric approach is preferred.

3.3. Gas Absorption Behavior for Wet Nanoporous Materials

As described in the methodology, water was introduced into the material in controlled quantities without disturbing its placement in the cell. The water-to-solid ratio (R_w) was gradually increased from 0.5 to 2. This study examines the role of water in gas adsorption behavior using three different MOFs and one activated carbon, all procured from commercial vendors. Since most of the MOFs used in this study had pore sizes smaller than 1.2 nm, it was believed that hydrate formation would mostly occur on the surface of the MOFs and activated carbon, making hydrophobicity, surface area, and surface chemistry significant factors.

Figure 3 presents the high-pressure isotherms by connecting data from key pressure points for four nanoporous materials at various R_w values in dry and wet conditions. Generally, the presence of water leads to lower gas adsorption up to a certain pressure. For example, the decrease in CH₄ adsorption was more significant in hydrophilic MOFs (see Figure 3A,B) than in hydrophobic MOFs.

3.3.1. CH₄ Gas Adsorption in Wet Hydrophobic Nanoporous Materials

Table 4. CH₄ gas adsorbed on wet hydrophobic MOFs (Different R_w).

	CH4 Amount Adsorbed in Wet Hydrophobic Material (mmol/g)
T (K)	274.15					274.15
W (g)	42.2	42.2	42.2	42.2	42.2	6.3	6.3	6.3
Rw=	0	0.5	1	1.5	2	0	0.5	1.0
P (bar)	NuChar AC (mmol/g)					ZIF-8 (mmol/g)
0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
20	2.1	1.7	1.2	0.7	0.3	22.9	22.5	22.0
25	2.6	2.0	1.5	0.9	0.4	27.5	26.9	26.3
30	3.2	2.5	1.8	1.1	0.4	31.9	31.0	30.4
35	3.7	2.9	2.1	1.3	0.5	36.0	35.3	34.7
40	4.1	3.3	2.4	1.5	0.6	39.6	38.8	38.3
45	4.6	3.7	2.6	1.7	0.7	43.2	42.4	41.9
50	5.1	4.1	2.9	1.9	0.8	46.4	45.4	45.0
55	5.5	4.4	3.2	2.1	0.9	49.2	48.3	48.0
60	6.0	4.8	3.5	2.2	1.0	52.1	51.2	50.9
65	6.5	5.2	3.8	2.4	1.0	55.0	53.8	53.7
70	6.8	5.6	4.0	2.6	1.1	57.6	56.3	56.2
75	6.8	5.7	4.3	2.8	1.2	60.0	58.6	58.6
80	6.8	6.1	4.6	3.0	1.3	62.3	60.8	60.9

provides information concerning the high-pressure isotherm data for the hydrophobic ZIF-8 and Nuchar-activated carbon. For ZIF-8, the R_w values used were 0, 0.5, and 1, while for activated carbon, the R_w values used were 0, 0.5, 1, 1.5, and 2. Water was gradually added in situ and was not pre-adsorbed or impregnated into these materials. The isothermal temperature was kept constant at T = 274.15 K. The sample weight for activated carbon was 42.2 g, while the sample weight for ZIF-8 was 6.3 g. The CH₄ mmol adsorbed were normalized based on each material’s weight, to compare performance. In the dry case, all materials showed type 1 isotherms per the IUPAC classification. Figure 3C,D shows plot data from Table 4.

ZIF-8, in the dry case, is a better CH₄ adsorbent than activated carbon. Activated carbon is known to be a better CO₂ adsorbent than a CH₄ adsorbent. The study [58] compared the sorption behavior of CO₂ and CH₄ on activated carbon in the presence of water, highlighting the significant impact of the pore dimension on CO₂ sorption. The results show that CO₂, being condensable, has a higher sorption capacity than CH₄ due to better access to and interaction with larger pores. This difference in sorption behavior is linked to the physical states of CO₂ and CH₄. The study also examined the correlation between the water/solid ratio (R_w) and gas sorption. For CH₄, an increase in R_w (from 0 to 2) leads to lower adsorption, followed by a higher sorption capacity (for R_w > 2), with the highest quantity of fixed methane being 17.5 mmol/g at an R_w of 2.43. However, this does not represent the equilibrium capacity, as some water may remain in pores that are too small for hydrate formation. In contrast, CO₂ sorption capacity is less affected by R_w, with the highest CO₂ storage capacity being nearly the same across different water ratios (R_w > 0). The study suggests that pore size and water content significantly impact the enthalpy changes of hydrate formation, with smaller pores resulting in larger enthalpy changes. Overall, the sorption capacity for both gases is determined by the utilization of pore volume and the possibility of condensation or dissolution in water.

CH₄ gas adsorption behavior in Nuchar-activated carbon aligns with the above conclusion. In Figure 3, we can see that as we increase the water quantity up to R_w = 2, CH₄ gas adsorption decreases, confirming the previously observed behavior where CH₄ adsorption decreased for Rw to be lower than 2 and hydrate formation was triggered at R_w > 2 [58].

CH₄ adsorption in wet activated carbon is controlled by the wetting ratio (R_w). One study [59,60] explored how saturating the pore volumes of activated carbons with water affects the formation of methane hydrates under specific temperature and pressure conditions (2 °C and up to 8 MPa). It revealed that an optimal wetting ratio close to 1 led to the most efficient methane hydrate formation, with the presence of water facilitating the process at lower pressures equivalent to those in bulk water. However, thoroughly saturating the pores resulted in poorer methane storage performance. The research [60] also examined the role of sodium dodecyl sulfate (SDS) as an additive, finding that while it lowered the methane pressure required for hydrate formation, it did not increase the final amount stored. This study identified a gap in understanding the precise role of microporosity and pore accessibility in the methane storage process. Researchers have also used nanoparticle-coated activated carbon to reduce the wetting ratio, achieving a lower induction time and higher water-to-hydrate conversion. The study [61] found that Ag-NP@AC significantly enhances external nucleation kinetics, especially under a low driving force and a large wetting ratio, leading to a reduction in induction time of up to 96.88%. Another study [62] proposed that porous surfaces significantly reduce induction time for hydrate formation, with surface defects and inner pore convection providing numerous nucleation sites. Hydrate fiber morphology, observed under milder conditions, grows from inner pores and has diameters dependent on pore size.

ZIF-8 is known to have a pore size of around 1.2 nm, which is nearly equal to the size of gas hydrate crystals [5]. However, it has not been confirmed whether gas hydrates can form inside the micropores of ZIF-8. Cosco et al. proposed that hydrates form on the external surface, while Mu et al. [63] suggested that CH₄ hydrates could form inside the pores. Monte Carlo and molecular dynamics simulations have indicated that phase separation between CH₄ and water molecules prevents hydrates from forming in the ZIF-8 cavities. [64]. Generally, the hydrophobic surface of ZIF-8 facilitates the formation of hydrogen bonds in interfacial water by inducing ordered structures, and this interfacial water promotes the contact between the hydrate cage and the ZIF-8 surface [64].

In our study, when ZIF-8 was used in large quantities, an increase in water content (R_w) did not drastically reduce the CH₄ adsorption, nor did we see stepped isotherms (refer to Figure 3C). It is well known that ZIF-8 shows low recyclability and aggregates in the presence of water, which causes a reduction in specific surface area and resistance to surface charge [65]. This aggregation is more pronounced in large sample sizes (in grams) than in small ones, which could be one reason why no hydrate formation was observed. Another reason is that water was added to the sample in situ in our method, whereas in other studies, hydrophobic materials are impregnated with water and water is pre-adsorbed in the material—a time-consuming process different from our method.

3.3.2. CH₄ Gas Adsorption in Wet Hydrophilic Nanoporous Materials

To better understand CH₄ adsorption at high pressure and its connection with the properties of hydrophilic microporous materials, we created adsorption isotherms for HKUST-1 and MOF 303 at a temperature of 274.15 K. The pressure was increased stepwise, significantly above the stability pressure of the bulk case, to ensure that hydrate formation would occur.

Table 5. CH₄ gas adsorbed on wet hydrophilic MOFs (different R_w).

	CH4 Amount Adsorbed in Wet and Hydrophobic Material (mmol/g)
T (K)	274.15			274.15
W (g)	20.5	20.5	20.5	17.3	17.3	17.3
Rw=	0	0.85	2.36	0	1	1.5
P (bar)	HKUST-1 (mmol/g)			MOF-303 (mmol/g)
0	0.0	0.0	0.0	0.0	0.0	0.0
20	5.1	4.9	3.5	7.2	6.3	5.9
25	5.7	5.5	4.2	8.5	7.6	7.1
30	6.0	6.0	5.3	9.7	8.8	8.3
35	6.2	6.6	7.2	10.8	10.2	9.9
40	6.8	7.5	9.2	11.7	11.8	11.7
45	7.5	8.3	11.1	12.6	13.0	13.0
50	7.7	9.1	14.2	13.3	14.0	14.1
55	8.4	9.8	18.5	14.0	15.0	15.2
60	9.1	10.6	24.6	14.7	15.9	16.1
65				15.3	16.8	17.0
70				15.9	17.6	17.9
75				16.0	18.4	18.8
80				16.0	19.0	19.6

includes the data from the high-pressure CH₄ adsorption in the presence of HKUST-1 and MOF-303. HKUST-1 and MOF-303 are known hydrophilic MOFs with different degrees of water uptake. For HKUST-1, Rw varied from 0 to 2.36, while for MOF 303, Rw varied from 0 to 1.5.

Table 5 data are plotted in Figure 3A,B. Figure 3A,B shows the presence of stepped isotherms for HKUST-1 and MOF-303 due to water-to-hydrate conversion. Hydrophilic MOFs showed stepped isotherms, indicating the gas hydrate formation that leads to a total cumulative adsorption higher than in the dry case. For our study, we identified the crossover pressure at which gas adsorption for wet MOF is nearly equal to that in dry MOF. For our study, the crossover pressure was recorded between 20 and 40 bars. This stepped isotherm phenomenon depends on the surface chemistry (hydrophobic/hydrophilic) and the degree of wetness (R_w). Since the hydrophilic materials used in our research had pore sizes smaller than 1.2 nm (the size of hydrate crystals), hydrates would only form on the outer surface of the MOFs and interparticle space. Previous studies using hydrophilic MOFs have shown that high-pressure isotherms demonstrate a drastic decrease in CH₄ adsorption in the hydrophilic material MIL-100 when water is present [26]. No hydrate formation was observed as R_w increased from 0 to 0.56, but a stepped isotherm was observed around 40 bar for R_w = 1.1.

All microporous compounds have pore diameters larger than the water molecule; however, water molecule penetration into ZIF-8 is unsuccessful due to its hydrophobic surface chemistry [66,67]. In contrast, HKUST-1 and MOF-303 are hydrophilic and should be able to accommodate water into their pores [31,68]. Due to differences in hydrophobicity, the amount of water added to the materials could lead to different water distributions within the pores and on the surface. These water loadings (R_w) controlled the degree of water dispersion. HKUST-1 has an internal pore diameter equal to 1.4 nm and 1.0 nm [41], causing the faster diffusion of water molecules into HKUST-1. At higher R_w, HKUST-1 pores become supersaturated with water. Even though pore sizes are not big enough to fit CH₄ hydrate crystals, a higher R_w in HKUST-1 may lead to higher gas solubility in a confined pore space. Thus, at R_w = 2.36, gas adsorption contrition would come from gas hydrate formed at the outer surface and CH₄ gas over solubility in confined pore water.

For HKUST-1, the difference between the stepped isotherm (from wet MOF) and the normal isotherm (from dry MOF) increased after the crossover. This highlights higher water-to-hydrate conversion at higher pressure. The crossover pressure for HKUST-1 was 32 bar, similar to the CH₄ hydrate stability pressure for bulk water. This indicates that, in the case of MOF-303, the deviation between the dry and wet isotherms after the crossover pressure point did not expand rapidly, possibly due to incomplete hydrate formation and uneven water distribution through the MOF-303 powder. MOF-303, also known for its exceptional water-harvesting capabilities and stability [36], along with HKUST-1, exhibits high hydrophilicity but differs in water loading capacity, which may influence its behaviors toward hydrate formation. For MOF-303, a crossover pressure was observed at P = 40 bar. Comparing MOF 303 and HKUST-1 data, crossover isotherm occurrences are not probabilistic and indicate a competitive adsorption phenomenon between H₂O and CH₄. One improvement for future studies could be to use a MOF-water premix sample to ensure pores where hydrates are formed.

In our studies, we found that an increase in the water amount in ZIF-8 did not lead to a drastic reduction in CH₄ adsorption values. However, for HKUST-1 and MOF-303, an increase in R_w led to a lower CH₄ adsorption value in wet than dry conditions at lower pressure. This underscores the need to synthesize and develop novel materials that can rapidly nucleate hydrates at lower driving forces. Our findings demonstrate that MOF makes hydrate crystallization less random and initiates hydrate formation without mechanical energy within 6–8 h. Further improvements in kinetics could be achieved by adding promoter-rich water, such as amino acids or surfactants.

3.3.3. Experimental Methodology, Key Assumption, and the Way Forward

The differences between present and earlier studies include the preparation of water-wet samples and measurement methodology. In this study, the wetness of the MOF is increased in situ within a reactor by gradually adding water after each experiment. In other studies, samples are pre-saturated with water before being placed in the reactor. Pre-saturating the material with water is time-consuming but effectively distributes water homogeneously on the surface and within the pores of the MOFs. Another key difference is the sample size. In similar studies, smaller sample sizes make it easier to achieve uniform water distribution, whereas larger samples present more challenges in this regard. Thus, the large sample size and the method of water introduction are expected to influence the isotherms and represent a limitation of this work. Nevertheless, the current experiments highlight unique challenges encountered when scaling up the study of gas adsorption on water-wet MOFs under high-pressure, low-temperature conditions.

Available research indicates the formation of hydrates on the hydrophobic surfaces of MOFs [20] and nanomaterials such as activated carbon. However, our studies did not observe any step-up isotherms for hydrophobic materials. For ZIF-8, we believe this is due to our method of introducing water into the samples. In previous studies, the materials were pre-adsorbed with water, whereas in our case, water was added in situ, resulting in poorer water distribution across the hydrophobic materials. Another drawback of using large samples and placing water directly into the sample is the risk of aggregation of hydrophobic nanoparticles in the presence of water. Aggregation leads to larger particle sizes, causing decreased charge transfer and a reduced interfacial area, which prevents hydrate formation [65] Our findings underscore the critical need for sufficient water distribution among hydrophobic materials to support hydrate formation. Future research should focus on this key aspect.

Due to the lack of standard high-pressure volumetric or gravimetric devices in our laboratory, our approach is limited by the assumption that equilibrium is established after 2 h at each pressure value. Based on our in-house setup and approach, this assumption affects the isotherms and results in less steep crossovers than those reported by other authors. Increasing the equilibrium time beyond 2 h would improve the quality of the isotherms, making them steeper and enhancing overall gas uptake. We recommend that future studies investigate the effect of equilibrium time on isotherm behavior to overcome this limitation.

As a result, we classify our analysis as qualitative and refrain from direct comparisons with other available and accurate data. The primary goal of our study was to produce high-pressure isotherms for large sample sizes in a large reactor volume to compare the performance of different MOFs and confirm the presence of stepped isotherms. Our method, while not aiming for direct comparison, offers a potential technique as a quick screening tool for qualitative analysis when more accurate isotherm equipment is unavailable. We believe this method could be a reliable and practical solution for quickly screening material performance in the case of large sample sizes, replicating a scale-up study. We recommend additional studies using volumetric and gravimetric methods to confirm results for scale-up studies, further reinforcing the practicality of our proposed method.

3.3.4. Effect of Material Properties on Gas Adsorption

Two main properties researched in this work were surface area and degree of hydrophilicity. One key challenge associated with large volumes is uniform water distribution across the material. With large volumes, we encounter mass transfer and heat transfer issues.

Our studies indicate that hydrate formation is more likely to occur in hydrophilic materials than in hydrophobic materials within a large reactor when the sample is used in large quantities. We conducted water adsorption experiments to compare the degree of hydrophilicity between MOF-303 and HKUST-1. Figure 4 shows the H₂O isotherms at 298 K, with HKUST-1 displaying a type 2 isotherm and MOF-303 displaying a type 4 isotherm. The results show that MOF-303 is slightly more hydrophilic than HKUST-1. The total water uptake value for MOF-303 is equal to 25 mmol/g, while for HKUST-1 the value is equal to 20 mmol/g. Water filled the pores of MOF-303 at a relatively lower relative pressure (≈0.2) compared to HKUST-1. Above this relative pressure, the isotherms show a sharp increase in water adsorbed. This increase is associated with the capillary condensation of H2O molecules in the inner cavities of the MOFs. Due to the difference in their response to water uptake, MOF 303 had higher a crossover pressure than HKUST-1 due to more water-saturated inter-particle space that shifted hydrate formation at higher pressure.

3.4. Desorption Experiments

Figure 5 shows the desorption plot in selected cases. Desorption experiments were performed on a few materials for selected R_w cases to continue the adsorption experiments. In previous studies, hydrate dissociation experiments were conducted on wetted porous materials in excess water [69]. Experiments with potential hydrate formation and/or higher R_w were selected for desorption experiments. It is evident through desorption experiments that, for a given R_w case of a MOF, the adsorbed CH₄ amount corresponding to a pressure value is higher than that of the adsorption values for the same case. This seems plausible because it should be difficult for CH₄ molecules to leave the MOFs after getting trapped inside their pores. Desorption studies indicate the regeneration of the material for the next cycle.

3.5. Material Stability and Characterization

3.5.1. Powdered Material Visualization before and after the Experiment

During the experiments, sample pictures were taken after different stages of the experiment to demonstrate the change in material color and texture (refer to Figure 6).

Colors and texture do not appear to change much; however, MOFs in the presence of water are observed to lump together. A certain degree of lumping was visible in all MOFs; however, the effect of lumping on MOF-hydrate synergy was much more visible and negatively impacted ZIF-8 as no hydrate formation was observed.

The authors believe the powdered form is not the most effective material for scale-up studies, where a large sample size is needed. In such cases, MOFs should be used in pellet forms [70] or monolith forms [71,72]. MOF-hydrate synergy studies using MOFs in pellet or monolith forms are not yet available, and future research should be conducted in this direction. Examples of monolith and pellet forms for gas adsorption can be referred to elsewhere [73,74]. One research article [70] investigated the physical and chemical properties of four metal-organic frameworks (MOFs) before and after pelletization, a process important for industrial applications. The study found that ZIF-8 maintained its structure well during pelletization, while HKUST-1 and UiO-66 experienced reversible structural changes when exposed to water. In contrast, ZIF-67’s structure was irreversibly damaged by pelletization. These findings suggest that while some MOFs can withstand the pelletization process, others may require specific conditions or treatments to maintain their functionality.

Another article [72] reviewed advancements in the fabrication and applications of 3D monoliths based on metal-organic frameworks (MOFs). It highlighted the transition from powdery MOFs to robust, reusable, and scalable MOF-monoliths, which offer improved mechanical stability and higher volumetric capacity. It emphasizes the potential of MOF-monoliths to overcome the limitations of MOF powders, such as fragility and poor processability, suggesting that MOF-monoliths are promising for commercial and clinical applications due to their enhanced properties and functionalities. The researchers also compared CH₄ adsorption capacity in activated carbon and MOFs in dry conditions. They found that the amount of stored CH₄ directly depends on the specific surface area, microporous volume, and the apparent density of the material [75]. Pellets or monolith forms provide different packing densities as compared to the powder forms, which will also affect gas adsorption, and the role of packing density has been highlighted elsewhere [57].

3.5.2. SEM Images Visualization

Samples were prepared by drying a diluted dispersion of the particles in methanol on a silicon wafer substrate. SEM images indicated the relatively larger size of HKUST-1 compared to MOF 303 and ZIF 8, thus confirming the lower reported surface area (600 m²/g) compared to ZIF-8 and MOF 303, which had much smaller crystals. HKUST-1 crystals retained a well-faceted bipyramid octahedral morphology after testing under hydrate formation conditions.

Figure 7 shows representative SEM images of the HKUST-1 sample before and after hydrate formation and dissociation studies. After hydrate formation, we did not see a considerable increase in small particles on the surface of the HKUST-1 crystals, confirming HKUST-1 stability in highly aqueous environments and under high-pressure situations. This confirms HKUST-1 stability in a highly aqueous environment, but with a shorter exposure duration (around six days). For example, in another publication, HKUST-1 was exposed to water over a period of 28 days [76] and small particles were visible on the face of HKUST-1 crystals, indicating the decomposition of material on account of water-induced instability.

In the case of ZIF-8, there are no remarkable changes in the morphology of the particles after a series of processes, including water saturation, adsorption, and desorption, which in turn suggests that the presence of water leads to agglomeration in the ZIF-8 particles, which could also lead to the smaller bed volume in an aqueous environment [63]. No visible agglomeration was observed for HKUST-1. Studies show that HKUST-1 and ZIF-8 retain crystallinity up to 100 MPa [77]. Thus, hydrate formation pressure is expected to cause no harm to HKUST-1 or ZIF-8 crystallinity.

3.5.3. XRD Diffraction Pattern

Power X-ray diffraction (XRD) was collected to investigate further and confirm whether there was any change in the structural characteristics of the hydrophilic MOF crystals. Figure 8 shows the powder XRD patterns for the MOF crystals before and after hydrate formation in the large-volume reactor. These images show that the crystals retained their well-faceted nature. Most of the MOFs used in this study have already been commercially developed; thus, research should be focused on scalability. No apparent change occurred in the XRD pattern between pre- and post-experiment MOF 303 samples after a minimum of three cycles of adsorption and desorption experiments and more than six days in an aqueous environment. HKUST-1 crystals showed some deviation that was previously recognized by other researchers [6]. The authors described this deviation on account of a change in the preferential plane exposure of the plane (222) over other crystallographic planes. The change in the intensity ratio before and after hydrate could be due to the higher concentration of crystallites [78]. Studies have focused on the water effect on HKUST-1 crystallinity, which showed changes in the 200/222 ratio and 220/222 ratio due to the densification of the HKUST-1 structure in the presence of water [79,80].

We did not perform N₂ adsorption isotherm on the residual/treated HKUST-1 sample after hydrate formation due to the availability of data in other publications [6]. These studies showed that pore volume decreased from 0.50 to 0.33 cm³/g, supporting the XRD observation and possible explanation behind the densification of HKUST-1 on account of water and under high pressure. Overall, high-pressure isotherms and XRD/SEM data suggest that MOF-303 is an excellent candidate to replace HKUST-1 MOF in hydrate-MOF-combined studies without compromising the total gas uptake at a lower wetting ratio.

3.6. Contribution to Sustainable Development

A recent article in Nature pointed out that seven chemical separation processes (difficult and energy intensive) that have the potential to change the world require extensive work on technologies based on either adsorption (solid porous materials) or absorption (liquid solvents) [81]. The MOF-based adsorption process is an energy-intensive process. Gas hydrate technology involves water crystallization under high gas pressures and low temperatures and is less energy-intensive than adsorption [82]. Combining MOF-based adsorption with gas-hydrate-based crystallization can reduce energy consumption in separation processes [5]. MOF and gas-hydrate technologies complement each other in separation processes. MOFs are selected based on their properties, such as pore size, water stability, surface area, and affinity for specific components, while gas hydrates rely on the different thermodynamic stability pressures of the hydrate-forming gases. Lower overall process temperatures would improve the stability of the MOFs.

The present work on gas adsorption in nanoporous materials, such as metal-organic frameworks (MOFs) and gas hydrates (a hybrid process), supports sustainable development by improving the efficiency of gas adsorbents, which is crucial for applications like carbon capture, natural gas purification, and hydrogen storage [83]. Such new approaches and hybrid technologies could help reduce greenhouse gas emissions and promote cleaner energy sources [84]. The research addresses the gap in understanding gas adsorption behaviors in real-world, large-scale applications using larger MOF samples and a large-volume reactor under wet conditions. This scalability is vital for implementing sustainable technologies globally. The key findings suggest that hydrophilic MOFs and gas hydrate crystallization combined can enhance gas uptake under wet conditions, making a hybrid approach more versatile in various humidity conditions. This versatility promotes the broader adoption of a MOF-gas hydrate-based hybrid approach, supporting global sustainability efforts. Finally, this work encourages future research into monolith or pellet forms of MOFs and gas hydrate interaction to overcome the limitations of powdered MOFs, paving the way for more robust and practical materials in large-scale applications and further advancing sustainable development.

4. Conclusions

In this research, we explored CH₄ adsorption in wet metal-organic frameworks (MOFs) using a large-volume reactor for the first time. High-pressure isotherms were measured to investigate the role of gas hydrates in enhancing total gas adsorption in the presence of hydrophobic and hydrophilic nanoporous materials. We identified a stepped isotherm for hydrophilic materials, where isotherms with water content (R_w > 0) diverged from the isotherms without water (R_w = 0). This breakthrough pressure depends on the amount of water (R_w) and the surface chemistry of the materials. HKUST-1 and MOF-303 exhibited stepped isotherms, crossover pressure, and chemical stability after oversaturation and multiple adsorption cycles. However, as reported previously by researchers, we observed a discrepancy in ZIF-8 between large and small sample sizes, with no hydrate formation observed in larger samples. This was due to the aggregation of ZIF nanoparticles in the presence of water, which reduced the surface area and increased surface charge resistance, affecting the gas/water interface and interactions. Wet-activated carbon did not show improved gas adsorption until R_w reached 2. Our lab-based experimental methodology sets the stage for future investigations under varied wet conditions, temperatures, and different material packing densities. This study’s findings will support the application of MOFs in large-scale reactors, enhance scalability, and facilitate the implementation of MOFs for large-scale adsorption studies.