# Microporous MOF-5@AC and Cu-BDC@AC Composite Materials for Methane Storage in ANG Technology

Authors: Niloufar Yadavar Nikravesh; Mojtaba Beygzadeh; Mehrdad Adl
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Novel composites (MOF-5 and Cu-BDC) based on economically activated carbon have been developed to investigate methane adsorption capacity for ANG applications. The composites were synthesized by adding MOFs precursor in two different weight percent (10% and 40%) to commercial activated carbon under solvothermal conditions (110-120°C). The synthesized adsorbents were characterized by FT-IR, XRD, SEM, EDS, and BET techniques to gather information about their crystallinity, morphology, and specific surface area. A methane uptake measurement system based on the volumetric method was made to obtain methane adsorption capacity on each composite. Then, the amount of methane adsorption for each one was calculated, and the experimental data were compared with different isotherm adsorption models appropriate for gas adsorption, including Langmuir, Freundlich, and Dubinin-Radushkevich. This comparison showed that the best fitting belonged to the Langmuir isotherm model. Also, stability and kinetic studies were done for two MOF-540% @AC and Cu-BDC40% @AC composites. In the kinetics study, experimental data were compared and analyzed in terms of pseudo-first order, pseudo-second order, and intraparticle diffusion models. The kinetics study showed that methane adsorption happens very fast on the synthesized adsorbents. The amount of methane adsorption at 35 bar and room temperature for pure activated carbon was specified as 4.32 mmol/g. The 10% and 40% of Cu-BDC with activated carbon are 5.59 mmol/g and 6.85 mmol/g, respectively. The capacity of MOF-510% @AC and MOF-540% @AC composites were obtained at 5.9 mmol/g and 7.3 mmol/g, respectively. Increasing methane uptake (70%) was obtained by adding 40 wt.% of MOF-5 to commercial activated carbon.


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## Body

## 1. Introduction

A worldwide increase in gaseous fuels has increased the energy demand-supply for industries today. During the following decades, natural gas demand is expected to rise, as estimates predict it will reach over 5000 billion cubic meters by 2040 [1]. Furthermore, increasing the level of CO2 in the atmosphere, mainly from fossil fuels, contributes to global warming and acts as an unwelcome molecule. Although natural gas (NG) is a fossil fuel source of global warming, it still has some advantages compared to petroleum and coal. Due to the highest hydrogen-to-carbon ratio (octane number=107.5), the combustion of methane, which is the major constituent of natural gas, produces the smallest amount of CO2 for each unit of released heat [2]. There are three essential methods for the storage of natural gas that differ from each other on a gas-state basis: CNG (Compressed Natural Gas), LNG (Liquefied Natural Gas), and ANG (Adsorbed Natural Gas). In the ANG method, natural gas is stored on porous materials at ambient temperature and relatively lower pressure (<40 bar) [3]. The quantity of adsorption capacity is different for each adsorbent. The Department of Energy (DOE) has issued >0.5 gCH4/gsorbent as an appropriate natural gas adsorption capacity at a reasonable pressure and temperature range (298 K and 35 bar) for a promising adsorbent. [4]. Generating an adsorbent with high gas adsorption capacity and low cost of production is one of the main challenges facing the development of the ANG method [3].The adsorption process can be used as a promising technology for both gas capture and storage. So far, several types of porous adsorbents have been developed, such as zeolites, metal-organic frameworks, porous polymers, porous silica, and porous carbons [3, 5]. Among the porous materials, activated carbon and zeolites have been the most studied microporous materials used as a sorbent to store gases such as methane [4] and hydrogen [6]. Several studies have been performed on the usage of activated carbon for methane adsorption, such as Brazilian coconut shells modified by H3PO4 [7], activated carbons from coal/zinc chloride [8], and rice husk (AC-RH). [9]. The performance of prepared activated carbon was compared with the commercial sample (SRD-21) [10]. The cost of adsorbents in the development of a cost-effective methane storage system is one of the major challenges in large-scale ANG storage; so, increasing the capacity of inexpensive commercial carbon adsorbents with new advanced materials such as MOFs can be a new way to commercialize [3].In recent years, metal-organic frameworks (MOFs) have been considered an advanced porous material in gas adsorption. MOFs have a very high specific surface area, tunable pore size, and large accessible pore volume. MOFs are assembled by the coordination chemistry of metal ions or their clusters and organic linkers and are a useful porous crystalline material in many industries [11]. Compared with other porous materials such as activated carbon, silica, and zeolites, MOFs have better potential as adsorbents, including tunable pores, rich active sites, and selectable functional groups [12]. The first measurement of methane uptake by porous MOFs can be dated to 1977 by Kondo et al. [13]. Kondo et al. [13] used the weighted method to measure the adsorption rate. The adsorption rate was reported at 30 bar, and the ambient temperature of 52 cm3/g [14]. Subsequently, in 2002, Eddaoudi et al. examined the methane uptake capacity of the isoreticular MOFs material. The methane adsorption capacity was reported for IR-MOF-6 and IR-MOF-1 at a temperature of 298 K and pressure of 35 bar 155 cm3/cm3 (STP) and 135 cm3/cm3 (STP), respectively [15]. In addition, methane storage capability in HKUST-1 has been studied by several researchers, and its adsorption capacity was reported as 3160 cm3/cm3(STP) [16]. Yet, MOFs have some characteristics known as obstacles to using their full potential. A large number of MOFs have been reported for methane storage. Their methane adsorption capacities are often higher than the best-activated carbons. MOF-5 and Cu-BDC are well-studied MOFs containing metal ions (copper and zinc) as the inorganic component and terephthalate ligands (BDC) as the inexpensive petrochemical organic binder coordinated based on 2D layers connected by hydrogen bonds within the bulk crystal. IRMOF-1 (MOF-5) has Zn4O clusters (the primary adsorption site) and a ZnO2 site, which makes it a promising adsorbent for methane adsorption at low temperatures [17]. Also, studies on methane adsorption with high capacity at ambient temperature for Cu-BDC MOF have been reported [18].Most MOFs are nonelectricity conductive and not sturdy against water/vapor, high temperature, and robust electron beams. They were found in powder with weak mechanical strength, and the configurations are not optimal for subsequent processibility [19–21]. The low density of atoms in MOFs results in a high diffusion coefficient and affects the final capacity of adsorption [11, 22, 23]. There are some ways to manage the challenges mentioned. One of them is building MOF-based composites [24, 25]. Among them, MOF-carbon composites (a combination of metal-organic frameworks and carbon-based materials) have received much attention due to their unique, extraordinary characteristics such as high mechanical and elastic strength, chemical and thermal robustness, low weight, low toxicity, and sometimes low cost improves MOFs weaknesses [25–27]. Many studies have investigated MOF-carbon composites. In recent years, most of the studies in this field have focused on MOF-Go (graphite or graphene oxide) composite [24, 28, 29]. Generally, in most of the previous studies about the composite of MOFs and activated carbon materials, the base of the composite was constituted MOF, and activated carbon was added to the composite as the secondary material [12]. Furthermore, the thermal stability, mechanical strength, and resistance to the humidity of the MOFs were improved by compositing them with carbon-based materials as a protective shield [12].The present work investigated the incorporation of microporous Cu-BDC and MOF-5 framework into commercial activated carbon and their effect on the volumetric methane storage capacity. We prepared MOF-5@AC and Cu-BDC@AC by varying the amount of MOF from 10 to 40 wt.% through a one-pot and straightforward solvothermal method. Then, the methane uptake was measured on the synthesized adsorbents by a volumetric method at 298 K and up to 35 bar. Moreover, the SRK equation of state was used to obtain the amount of adsorption, and isotherm curves were plotted and fitted.


## 2. Experiment

### 2.1. Materials

Zn(NO3)2.4H2O, Cu(NO3)2.3H2O, terephthalic acid, N,N-dimethylformamide (DMF), and dichloromethane were purchased from Merck Co. Commercial activated carbon (815 m2 g-1) was obtained from Caware Int’l Corp (Taiwan). Autoclaving reagent bottle 100 ml, 250 ml with GL45 thread blue cap was prepared from Simax®.


### 2.2. Synthesis of MOF-5 and MOF-5@AC Composite

MOF-5 was synthesized by the previously reported procedures [30]. For a typical synthesis, Zn(NO3)2.4H2O (1.45 g, 6 mmol) and terephthalic acid (0.33 g, 2 mmol) were mixed in 80 ml DMF. The resulting mixture was stirred for 20 minutes and then transferred to a glass reagent bottle heated for 7 h at 100°C under solvothermal conditions. Then, the white precipitate obtained was washed with the predried DMF three times and immersed in CH2Cl2. The solvent was exchanged daily for 24 h, exchanged once every 8 h, and was finally dried at 60°C under a high vacuum for 12 h. Similarly, MOF-5@AC composite was synthesized by adding activated carbon to the terephthalic acid solution. The MOF-5@AC composites with different MOF-5 ratios (10 and 40%) were synthesized by adjusting the amount of activated carbon, and they are referred to as MOF-510% @AC/and MOF-540% @AC in the rest of this paper.


### 2.3. Synthesis of Cu-BDC and Cu-BDC@AC Composite

Cu-BDC was synthesized as previously reported [31]. In a typical preparation, a mixture of Cu(NO3)2.3H2O (1.45 g, 6 mmol), terephthalic acid (1 g, 6 mmol), and DMF (75 ml) were stirred for about 20 min to get a clear solution, then transferred to a glass reagent bottle heated at 110°C for 36 h. The blue precipitate was obtained and washed with DMF several times, and it was dried in an oven at 220°C for 24 h. The Cu-BDC@AC composite was prepared by adding activated carbon to well-dissolved MOF precursors in the same procedure described for Cu-BDC. The obtained sample was washed with DMF several times at 24 h and finally dried in an oven at 220°C for 24 h. Samples with Cu-BDC weight ratios of 10%, and 40% were synthesized by changing the amount of activated carbon, and the samples are referred to as Cu-BDC10% @AC and Cu-BDC40% @AC after here.


### 2.4. Characterization

Fourier-transform infrared (FT-IR) spectra were obtained at room temperature on a Brucker V33 instrument for the chemical structure and binding properties of activated carbon MOF and composite. X-ray powder diffraction patterns were carried out by an X’pert MPD Philips DW371 with Cu Kα radiation (λ=1.5406 Å). The morphology of prepared samples was also observed by scanning electron microscopy (SEM, TESCAN VEGA-XMU). Surface area and pore size analyses were made from an analyzer (Belsorp-Mini II, Gemini 2375 (Bel Japan Inc.)) using nitrogen adsorption measurements at liquid nitrogen temperature (77 K). Firstly, each sample was degassed under the vacuum (10-5 Torr) at 150°C for 6 h to remove any entrapped guest molecules.


### 2.5. Design of Experimental

In order to obtain the adsorption capacity of methane on different synthesized adsorbents, the static volumetric method was used. For this purpose, a volumetric-based apparatus was made. As is shown in Figure1, this apparatus consists of a reference cell with a specified volume and a sample cell used to measure the methane uptake. The apparatus was placed in a constant-temperature water bath for the isothermal system could maintain a constant temperature. In the inlet line, two main valves were considered for the injection of methane gas for measuring the adsorption and helium gas as the inert gas for determining the amount of dead volume. This apparatus was equipped with a vacuum pump and a heater to discharge residual gas in the whole apparatus. A line that was directed to the atmosphere was considered for gas venting. The operating condition for this apparatus is 35 bar at ambient temperature. A pressure transducer was mounted between two cells to measure the pressure of the reference cell before and after gas injection to the adsorption cell.Figure 1
Schematic diagram of the experimental setup.


### 2.6. Adsorbent Test in Operating Conditions

After preparing the measuring apparatus, the following procedure was used to collect the experimental adsorption capacity for synthesized adsorbents.At first, a weighted adsorbent was introduced into the adsorption cell. Before beginning the measurements, the adsorbents were degassed at about 150°C under vacuum pressure for about 4 hours. The adsorption cell was cooled down to the ambient temperature to measure the amount of gas adsorbed. Later, the pressure regulator on the methane cylinder was fixed on 5 bar, gas was injected into the reference cell, and the inlet valve was closed. The valve between the two cells was opened in the next step, and the pressure drop after receiving to equilibrium condition was recorded. The condition of equilibrium was the condition at which the pressure in the sample cell stabilized. This observation confirmed that methane adsorption could occur when the adsorbent and methane came into contact. The mentioned step was repeated for different pressures up to 35 bar to achieve the isotherm. The pressure drops, which were observed in these steps, refer to adsorption and pressure drop caused by void volume and pipelines. Therefore, to determine the void volume of the adsorbents, a known amount of helium (as an inert gas that cannot be adsorbed by adsorbent) was injected in the next step, and a pressure drop was recorded.The void volume of the cell at equilibrium was determined from measured values of temperature, pressure, and amount of helium injected into the cell by using the following equation:(1)V2=P1He/Z1HeP2He/Z2HeV1,where V2 represents the volume of the sample cell and other valves and connections after helium introducing to the sample cell and V1 represents the volume before opening the valve between two cells. Also, P1He, Z1He, and P2He, Z2He shows the pressure and compressibility factor of helium in the reference cell before and after the equilibrium, respectively.
(2)Vvoid=V2−V1.The helium void volume (Vvoid) includes all the volumes of the cell section, exclusive of the adsorbent volume that is impenetrable to helium. Thus, the amount of mole of gases before the equilibrium state was obtained from the sum of gas molecules in the reference cell, the connections and gas molecules in the sample cell are as follows:The mole of gas in the reference cell(3)n1=P1V1Z1RT,where P1 represents the pressure, V1 represents the volume, Z1 shows the compressibility factor of gas injection to the reference cell, and R and T show gas constant and temperature, respectively.The mole of gas in voids of sample cell(4)n1′=P1′VvoidZ1′RT,where P’1 and Z’1 represent the equilibrium pressure and compressibility factor from previous gas injection stage.Therefore, the mole of gas before gas injection to the sample cell equals the sum ofn1 and n’1.
(5)ntotal=n1+n1′.After opening the valve between the sample and the reference cell, the gases from the reference cell were in contact with the adsorbent. After the equilibrium state, the mole of gas was obtained from the following equation:(6)n2=P2V2Z2RT.It should be mentioned that in all the equations,Z1 and Z2 represent the compressibility factor of methane in the reference cell before and after equilibrium, respectively, and P2 shows the pressure of methane in the reference cell after equilibrium. Moreover, to determine Z-factor, the SRK equation was used as follows:
(7)Z3−Z2+A−B−B2.Z−A.B=0,where
(8)A=a.PR2.T2,B=b.PR.T,a=ac.α,ac=0.42747R2.Tc2Pc,α1/2=0.48+1.574ϖ−0.176ϖ2,b=0.08664R.TcPc,where Tc, Pc, and ɷ are critical temperature, critical pressure, and acentric factor for methane gas.Therefore, the amount of adsorbed gas in one stage is equal to(9)Δnadsorbed=n1+n1′−n2.The procedure was repeated to increase the pressure of methane incrementally. Finally, the estimate of the total amount of gas adsorbed,nads at the ith step was calculated from the following equation:
(10)nads=Δnads1+Δnads2+⋯⋯+Δnadsi.Also, to determine the adsorbent’s capacity, the amount of gas adsorbed was divided into the weight of the adsorbent as follows:(11)q=nabsorbedWabsorbent.


## 2.1. Materials

Zn(NO3)2.4H2O, Cu(NO3)2.3H2O, terephthalic acid, N,N-dimethylformamide (DMF), and dichloromethane were purchased from Merck Co. Commercial activated carbon (815 m2 g-1) was obtained from Caware Int’l Corp (Taiwan). Autoclaving reagent bottle 100 ml, 250 ml with GL45 thread blue cap was prepared from Simax®.


## 2.2. Synthesis of MOF-5 and MOF-5@AC Composite

MOF-5 was synthesized by the previously reported procedures [30]. For a typical synthesis, Zn(NO3)2.4H2O (1.45 g, 6 mmol) and terephthalic acid (0.33 g, 2 mmol) were mixed in 80 ml DMF. The resulting mixture was stirred for 20 minutes and then transferred to a glass reagent bottle heated for 7 h at 100°C under solvothermal conditions. Then, the white precipitate obtained was washed with the predried DMF three times and immersed in CH2Cl2. The solvent was exchanged daily for 24 h, exchanged once every 8 h, and was finally dried at 60°C under a high vacuum for 12 h. Similarly, MOF-5@AC composite was synthesized by adding activated carbon to the terephthalic acid solution. The MOF-5@AC composites with different MOF-5 ratios (10 and 40%) were synthesized by adjusting the amount of activated carbon, and they are referred to as MOF-510% @AC/and MOF-540% @AC in the rest of this paper.


## 2.3. Synthesis of Cu-BDC and Cu-BDC@AC Composite

Cu-BDC was synthesized as previously reported [31]. In a typical preparation, a mixture of Cu(NO3)2.3H2O (1.45 g, 6 mmol), terephthalic acid (1 g, 6 mmol), and DMF (75 ml) were stirred for about 20 min to get a clear solution, then transferred to a glass reagent bottle heated at 110°C for 36 h. The blue precipitate was obtained and washed with DMF several times, and it was dried in an oven at 220°C for 24 h. The Cu-BDC@AC composite was prepared by adding activated carbon to well-dissolved MOF precursors in the same procedure described for Cu-BDC. The obtained sample was washed with DMF several times at 24 h and finally dried in an oven at 220°C for 24 h. Samples with Cu-BDC weight ratios of 10%, and 40% were synthesized by changing the amount of activated carbon, and the samples are referred to as Cu-BDC10% @AC and Cu-BDC40% @AC after here.


## 2.4. Characterization

Fourier-transform infrared (FT-IR) spectra were obtained at room temperature on a Brucker V33 instrument for the chemical structure and binding properties of activated carbon MOF and composite. X-ray powder diffraction patterns were carried out by an X’pert MPD Philips DW371 with Cu Kα radiation (λ=1.5406 Å). The morphology of prepared samples was also observed by scanning electron microscopy (SEM, TESCAN VEGA-XMU). Surface area and pore size analyses were made from an analyzer (Belsorp-Mini II, Gemini 2375 (Bel Japan Inc.)) using nitrogen adsorption measurements at liquid nitrogen temperature (77 K). Firstly, each sample was degassed under the vacuum (10-5 Torr) at 150°C for 6 h to remove any entrapped guest molecules.


## 2.5. Design of Experimental

In order to obtain the adsorption capacity of methane on different synthesized adsorbents, the static volumetric method was used. For this purpose, a volumetric-based apparatus was made. As is shown in Figure1, this apparatus consists of a reference cell with a specified volume and a sample cell used to measure the methane uptake. The apparatus was placed in a constant-temperature water bath for the isothermal system could maintain a constant temperature. In the inlet line, two main valves were considered for the injection of methane gas for measuring the adsorption and helium gas as the inert gas for determining the amount of dead volume. This apparatus was equipped with a vacuum pump and a heater to discharge residual gas in the whole apparatus. A line that was directed to the atmosphere was considered for gas venting. The operating condition for this apparatus is 35 bar at ambient temperature. A pressure transducer was mounted between two cells to measure the pressure of the reference cell before and after gas injection to the adsorption cell.Figure 1
Schematic diagram of the experimental setup.


## 2.6. Adsorbent Test in Operating Conditions

After preparing the measuring apparatus, the following procedure was used to collect the experimental adsorption capacity for synthesized adsorbents.At first, a weighted adsorbent was introduced into the adsorption cell. Before beginning the measurements, the adsorbents were degassed at about 150°C under vacuum pressure for about 4 hours. The adsorption cell was cooled down to the ambient temperature to measure the amount of gas adsorbed. Later, the pressure regulator on the methane cylinder was fixed on 5 bar, gas was injected into the reference cell, and the inlet valve was closed. The valve between the two cells was opened in the next step, and the pressure drop after receiving to equilibrium condition was recorded. The condition of equilibrium was the condition at which the pressure in the sample cell stabilized. This observation confirmed that methane adsorption could occur when the adsorbent and methane came into contact. The mentioned step was repeated for different pressures up to 35 bar to achieve the isotherm. The pressure drops, which were observed in these steps, refer to adsorption and pressure drop caused by void volume and pipelines. Therefore, to determine the void volume of the adsorbents, a known amount of helium (as an inert gas that cannot be adsorbed by adsorbent) was injected in the next step, and a pressure drop was recorded.The void volume of the cell at equilibrium was determined from measured values of temperature, pressure, and amount of helium injected into the cell by using the following equation:(1)V2=P1He/Z1HeP2He/Z2HeV1,where V2 represents the volume of the sample cell and other valves and connections after helium introducing to the sample cell and V1 represents the volume before opening the valve between two cells. Also, P1He, Z1He, and P2He, Z2He shows the pressure and compressibility factor of helium in the reference cell before and after the equilibrium, respectively.
(2)Vvoid=V2−V1.The helium void volume (Vvoid) includes all the volumes of the cell section, exclusive of the adsorbent volume that is impenetrable to helium. Thus, the amount of mole of gases before the equilibrium state was obtained from the sum of gas molecules in the reference cell, the connections and gas molecules in the sample cell are as follows:The mole of gas in the reference cell(3)n1=P1V1Z1RT,where P1 represents the pressure, V1 represents the volume, Z1 shows the compressibility factor of gas injection to the reference cell, and R and T show gas constant and temperature, respectively.The mole of gas in voids of sample cell(4)n1′=P1′VvoidZ1′RT,where P’1 and Z’1 represent the equilibrium pressure and compressibility factor from previous gas injection stage.Therefore, the mole of gas before gas injection to the sample cell equals the sum ofn1 and n’1.
(5)ntotal=n1+n1′.After opening the valve between the sample and the reference cell, the gases from the reference cell were in contact with the adsorbent. After the equilibrium state, the mole of gas was obtained from the following equation:(6)n2=P2V2Z2RT.It should be mentioned that in all the equations,Z1 and Z2 represent the compressibility factor of methane in the reference cell before and after equilibrium, respectively, and P2 shows the pressure of methane in the reference cell after equilibrium. Moreover, to determine Z-factor, the SRK equation was used as follows:
(7)Z3−Z2+A−B−B2.Z−A.B=0,where
(8)A=a.PR2.T2,B=b.PR.T,a=ac.α,ac=0.42747R2.Tc2Pc,α1/2=0.48+1.574ϖ−0.176ϖ2,b=0.08664R.TcPc,where Tc, Pc, and ɷ are critical temperature, critical pressure, and acentric factor for methane gas.Therefore, the amount of adsorbed gas in one stage is equal to(9)Δnadsorbed=n1+n1′−n2.The procedure was repeated to increase the pressure of methane incrementally. Finally, the estimate of the total amount of gas adsorbed,nads at the ith step was calculated from the following equation:
(10)nads=Δnads1+Δnads2+⋯⋯+Δnadsi.Also, to determine the adsorbent’s capacity, the amount of gas adsorbed was divided into the weight of the adsorbent as follows:(11)q=nabsorbedWabsorbent.


## 3. Results and Discussions

The schematic illustration of the formation of MOF-5 and Cu-BDC on the activated carbon resulting in MOF-5@AC and Cu-BDC@AC composites is shown in Figure2. The reaction conditions are elementary using a facile one-pot solvothermal method.Figure 2
Schematic diagram of synthesizing MOF-5@AC and Cu-BDC@AC composite.


### 3.1. Characterization

#### 3.1.1. XRD Patterns

Figures3(a)–3(d) compare the XRD (X-ray Diffraction) patterns of MOF-5, MOF-5@AC, Cu-BDC, and Cu-BDC@AC. Figure 3(a) represents the XRD pattern of the MOF-5 with high intensity of the characteristic peaks (2θ=6.8°, 9.6°, 13.6°, and 24.7°), which are consistent with the results reported in the literature, confirming the successful formation of MOF-5. Figure 3(b) shows the formation of MOf-5 in the composite based on activated carbon, which confirms the successful formation of the MOF-5@AC composite because the amorphous carbon does not have a specific crystallinity. Furthermore, a small amount of zinc species impurity is also visible in the phase that was probably entrapped in the framework MOF-5 [30, 32]. Figure 3(c) represents the characteristic peaks of Cu-BDC, which appeared at 2θ=10.1° and 12.1°, representing (001) and (020) planes of crystal structure, respectively, are matched well with XRD patterns, which have been reported before with no impurity. Consequently, The XRD patterns of Cu-BDC@AC (Figure 3(d)) composites show the same diffraction patterns as Cu-BDC, confirming that the incorporation of activated carbon to Cu-BDC MOFs follows those reported in the literature [33, 34].Figure 3
Powder X-ray diffraction patterns of (a) MOF-5, (b) MOF-5@AC composite, (c) Cu-BDC, and (d) Cu-BDC@AC composite.


#### 3.1.2. SEM and EDS

The texture of the as-synthesized of MOF-5, MOF-5@AC, Cu-BDC, and Cu-BDC@AC composite can be observed in scanning electron microscopy (SEM) images presented in Figure4, respectively. The SEM image (Figure 4(a)) shows that the MOF-5 particles are uniform with a regular cubic with an average size of about 30-40 μm [32]. Figure 4(c) shows that the Cu-BDC MOF particles are uniform with well-formed cubic microcrystals, and their average edge length is about 9.3 μm. Their morphology is compatible with the previous results that were achieved from hydrothermal synthesis [35]. Comparing the SEM images of MOF-5@AC and Cu-BDC@AC composite (Figures 4(b) and 4(d)) with primary MOFs exhibit the successful formation of MOf-5 and Cu-BDC based on activated carbon. Furthermore, the EDS (Energy Dispersive Spectroscopy) spectra in Figures 4(e) and 4(f) show that MOF-5 and Cu-BDC have been successfully introduced in activated carbon composites resulting in zinc and copper element presence in EDS analysis.Figure 4
SEM images of (a) MOF-5, (b) MOF-5@AC composite, (c) Cu-BDC, and (d) Cu-BDC@AC composite and EDS spectra of (e) MOF-5@AC and (f) Cu-BDC@AC composite.
(a)(b)(c)(d)(e)(f)


#### 3.1.3. FT-IR Results

FT-IR spectra, as shown in Figure5, are obtained to gain more detailed information about the structure of the synthesized composites. The IR spectrum of the original activated carbon shows no significant bands, but after the formation of the composite, all peaks of MOF absorption spectra appear. In the FT-IR spectrum of MOF-5@AC, two absorption bands, 2948.2 and 2870.7 cm-1, are assigned to C-H stretching vibrations of the methylene group of remaining DMF in the composition of synthesized MOF. The vibration bands appearing in 1700–1400 cm-1 are assigned to the carboxylic functionality of the 1,4-benzenedicarboxylate. The strong peak appearing at 1758.1 cm-1 can be attributed to the C=O stretching vibration of the carboxylate group of MOF-5. All dedicated absorption peaks of Cu-BDC are observed in the infrared spectrum of the Cu-BDC@AC composite. The sharp absorption peaks at 1602 and 1398 cm-1 belong to the carboxylate groups’ asymmetric and symmetric stretching modes, respectively. The bands at 1512 and 743 cm-1 are assigned to the vibrations of the phenyl ring. The vibration bands at 468 and 556 cm-1 can be assigned to the Cu–O stretching vibration [30, 36]. All these points illustrate that MOF-5 and Cu (BDC) had developed. SEM results also support this assumption.Figure 5
FT-IR spectra of activated carbon, MOF-5@AC, and Cu-BDC@AC composite.


#### 3.1.4. N2 Adsorption–Desorption Isotherms Analysis

Figure6 shows the N2 adsorption-desorption isotherms at 77 K and pore size distributions obtained by the MP-plot method for various samples with the structural parameters listed in Table 1. According to the IUPAC classification, the isotherms obtained for all samples are of type I, which is typical for microporous materials. The isotherm of AC and MOF-5 belongs to type I in the IUPAC classification, which is a characteristic of microporous materials. Cu-BDC revealed the isotherm type I with H2 hysteresis, which is related to microporous material. The hysteresis loop observed at high relative pressure implies the existence of some mesopores. The pore size distributions from the MP method of as-synthesized MOFs and composites are shown in Figures 6(c) and 6(d), which are about 0.6 nm specific to microporous materials.Figure 6
N2 adsorption-desorption isotherms of (a) MOF-5, Cu-BDC, (b) AC, MOF-5@AC, and Cu-BDC@AC, (c) and (d) pore size distribution of MOF-5, Cu-BDC, and composites obtained by MP-plot.
(a)(b)(c)(d)Table 1
Textural characteristics of samples.
SamplesBET surface areaMean pore diameter (nm)Total pore volume (cm3 g-1)BET (m2 g-1)Langmuir (m2 g-1)Commercial AC8159201.670.342MOF-57608471.670.317Cu-BDC6337402.620.416MOF-5@AC711866.32.000.355Cu-BDC@AC657.27662.530.416


### 3.2. Test of Adsorbents

This section examines and discusses methane uptake on the synthesized adsorbents. Figure7 demonstrates the CH4 uptake capacities as a function of pressure for pure adsorbents. Also, the amount of methane uptake on the composite of 10 and 40 weight percent of MOF-5 and Cu-BDC based on activated carbon compared with commercial activated carbon has been in Figure 8. The capacity of CH4 in adsorbents is usually proportional to their surface areas. Despite the higher surface area in the commercial activated carbon, a lower methane uptake has occurred compared with the synthesized MOFs. MOFs have higher CH4 uptake than conventional porous materials such as carbon and zeolites. The mechanism of the enhanced uptake is due to the existing three sites: the center of the cluster, pore cages of the phenyl rings, and unsaturated metal ion active sites located at the center of the cavity. van der Waals contact and partial π-HC interactions can significantly interact between methane and adsorption centers [37–39]. A clear result of increasing the amount of MOF used in the composite is that it significantly increases the CH4 uptake. By comparing the amount of gas uptake on the different adsorbents, this result is obtained that MOF-5 has the highest capacity of methane uptake than other composites and activated carbon. However, adding MOFs to the composite indicated a positive effect in increasing methane uptake.Figure 7
CH4 uptake of the AC, MOF-5, and Cu-BDC adsorbents (experimental data and Langmuir’s model).Figure 8
CH4 uptake of the MOF-540% @AC, MOF-510% @AC, Cu-BDC40% @AC, Cu-BDC10% @AC, and AC adsorbents (experimental data and Langmuir’s model).Table2 compares the amount of methane uptake in the synthesized adsorbents in this study with other adsorbents reported in the literature. To have a better comparison, the sorbents are selected from the articles having the same test conditions, including pressure and temperature. This comparison shows that the amount of gas adsorption on a specific adsorbent may be different, and it is due to the different conditions of synthesis and tests that are taken on the adsorbents.Table 2
Comparison of the methane adsorption capacity of the synthesized adsorbents with others.
MOFsFramework density Dc (g/cm3)BET surface area (m2/g)Total uptakeConditions pressure (bar)/temperature (K)References(cm3/cm3)(cm3/g)(mmol/g)MOF-9050.549349014526411.7835/298[40]LIFM-820.92216241962149.5535/298[40]NU-1250.589328618231514.0635/298[40]AC10.51060.3——5.1135/298[10]AC20.58363.3——3.0635/298[10]AC30.391177.7——7.9235/298[10]AC40.491050.8——5.1835/298[10]LIFM-830.9171715192—9.3635/298[41]MOF-50.59187013522810.1836/298[14]IR-MOF10.59—128.29217.459.7135/298[42]MOF-50.61—1352219.8735/298[15]MOF-50.593—1101858.2835/298[43]CMK-30.87950—117.335.235/298[44]Cu-BDC—624——3.835/298[45]Cu-BDC—603——11.5835/293[46]MIL-53(Cu)/CNT—112313.7235/298[47]Mn(HCO2)2/graphite—2804.241/298[48]MOF-540% @AC—711——7.335/298This studyMOF-510% @AC————5.935/298This studyCu-BDC40% @AC—657——6.8535/298This studyCu-BDC10% @AC————5.5935/298This studyPure activated carbon—815——4.3235/298This studyMOF-5—760——8.1835/298This studyCu-BDC—633——7.7435/298This study


### 3.3. Adsorption Kinetic Studies

To investigate the correlation of methane adsorption and predict the rate-limiting step in MOF-540% @AC and Cu-BDC40% @AC, the gathered experimental data are fitted to two regular models, pseudo-first-order and pseudo-second-order models. The reason for selecting these models is their simplicity in describing the interaction rate in the solid-gas adsorption process. The equation of the pseudo-first-order model is defined as follows:
(12)qt=qe1−ek1t,where qe and qt (mmol/g) represent the amount of methane gas adsorbed at equilibrium and at time t (min). Also, k1 (1/min) is the constant rate of adsorption.When pressure increases at a constant temperature,k1 is increased. It means that at higher pressures, the methane uptake will be decreased. Otherwise, the required time for reaching the equilibrium state will be increased.Also, pseudo-second-order was another kinetic model which is used in this study with the following equation:(13)qt=qe2k2t1+qtk2t,where qe and qt (mmol/g) represent the amount of methane gas adsorbed at equilibrium and at time t (min), also, k2 (g/mmol.min) is the constant rate of the adsorption.According to the result of the pseudo-second-order model, when pressure increases,k2 will be increased. It represents a low repulsion in methane molecules, which increases diffusivity [49]. Also, by comparison of R2 values of pseudo-first-order and second-order in Table 3, it seems that the amount of R2 values of the pseudo-first-order is relatively higher than the second-order model. The pseudo-first-order model better matches theoretical and experimental data [50]. On the other hand, since pseudo-first-order and pseudo-second-order cannot show the diffusion mechanism, the extracted results from kinetic models have been analyzed by the intraparticle diffusion model [51]. The intraparticle diffusion equation is defined as the following equation:
(14)qt=kpt0.5+C.Table 3
Kinetic parameters of methane adsorption on the synthesized adsorbents.
SamplePressure (bar)1st order2nd orderIntraparticle diffusionk1 (min-1)R2k2R2ktCCu-BDC40% @AC1019.345170.999826.122240.99740.657353.230762024.643290.999828.61970.998821.008313.928653534.993250.999727.652850.99891.041484.506MOF-540% @AC1022.616930.9986710.345060.998610.745863.446282024.239230.9983713.948490.995120.934514.354723528.89760.9999717.219310.999090.995.08543In Equation (14), kp is defined as the intraparticle diffusion rate constant (mmol.g-1 min-0.5) and t represents time (min).In Figure9 the plot of methane adsorption versus t0.5 is shown. According to the gathered data, the methane adsorption process has high-speed kinetics, and the adsorption reaches the equilibrium state in the early minutes. Equation (14) is a linear equation with the intercept C, and the value of zero for the intercept shows that intraparticle diffusion is the only controller of the adsorption process. When the amount of C is not zero, according to Table 3, it means that the adsorption process is based on the pseudo-first-order kinetic model and intraparticle diffusion [51].Figure 9
Methane adsorption onto (a) Cu-BDC40% @AC and (b) MOF-540% @AC vs. t0.5.
(a)(b)


### 3.4. Adsorption Isotherm Studies

The adsorption isotherms can describe the equilibrium performance of adsorbents in an isotherm process. Many factors, including adsorbents, physical properties of adsorbate, and adsorption process condition, affect adsorption isotherm [42].Also, adsorption isotherm models explain the interaction between adsorbate fluid and synthesized adsorbents. The extracted data from the adsorption experiment are fitted with three isotherm models, Langmuir, Freundlich, and Dubinin-Radushkevich equilibrium isotherm models. The isotherm constants are reported in Table4. These isotherm models are used to investigate adsorption conditions, including homogeneous/heterogeneous and monolayer/multilayer. Many different isotherm models can be used to investigate the experimental data; however, these models are selected since they are more appropriate for the gas adsorption process than others [52, 53].Table 4
Isotherm adsorption parameters of methane on the synthesized adsorbents.
Isotherm modelParametersAdsorbentMOF-5Cu-BDCMOF-540% @ACMOF-510% @ACCu-BDC40% @ACCu-BDC10% @ACACLangmuirb0.236480.182850.142660.135770.120860.129880.07714qm9.111929.117058.857477.128968.682676.803666.29136R20.998640.998860.999010.99760.995290.995610.99178Freundlichkf3.243572.801052.255531.766791.907821.617480.89296n3.672023.375412.8989712.8575512.6438942.7794762.117881R20.971460.976470.938820.980950.923190.985630.92945Dubinin-Radushkevichqm8.385827.955137.650616.08717.309385.758724.71858k0.013510.015650.018690.018870.020920.019260.02697R20.993670.994710.984830.994790.969120.995210.96889The Langmuir adsorption isotherm model is a straightforward and appropriate isotherm model used for homogeneous surfaces of sorbent to explain monolayer and chemical adsorption in the gas-solid phase. This model is the most common isotherm model to investigate methane and carbon dioxide of activated carbon [52, 54]. According to this isotherm model, the surface of a solid adsorbent is a completely uniform and homogeneous surface composed of one type of material, and there is no preferred place for adsorption on its surface. All places have the same priority for adsorption. Also, each site does not adsorb more than one molecule; in such a situation, a single layer of molecules will be adsorbed on the solid surface [55]. The equation of the Langmuir isotherm model is defined as follows:
(15)qe=bP1+bPqm,where qm shows the monolayer uptake capacity (mmol/g), b is the affinity coefficient constant (1/bar), qe represents the amount of gas uptake on the adsorbent (mmol/g), and P is the adsorbed gas pressure (bar). The isotherm constants are shown in Table 4.The affinity coefficient constant (b) indicates the extent of interaction between the adsorbate and the surface. Therefore, the highest value in MOF-5 indicates that CH4 has the strongest affinity toward the MOF-5 surface. Also, for evaluating an adsorbent’s adsorption capacity, Langmuir’s predicted qm is valuable. Experimentally, measured CH4 adsorption isotherms for MOF-5 show good compatibility with the single-site Langmuir’s model [56].Freundlich’s adsorption isotherm model is suitable for predicting adsorption on heterogeneous surfaces and explaining nonideal processes and multilayer surfaces [52]. Therefore, it could be a good complementary model addition to the Langmuir isotherm model to better investigate heterogeneous and multilayer sites. The main difference between the Langmuir and Freundlich models is that the Freundlich model is applicable for multilayer adsorption [57]. Freundlich’s equation has been shown in the following equation:
(16)qe=kfP1/n,where kf is the Freundlich isotherm constant (mmol/g) and n is the adsorption intensity and is a heterogeneity factor.Dubinin-Radushkevich’s (D-R) adsorption isotherm model is a semiempirical equation for multilayer surfaces. Initially, the application of this model was for subcritical vapors on micropore materials. Now, it is used to determine chemical and physical gas adsorption in microporous materials [52]. The equation of the D-R model is as follows:
(17)qe=qmexp−kε2,⟶ε=RTlnPsP.In the equation,Ps and P are the saturation vapor pressure and the adsorbate equilibrium pressure, respectively. Also, k value represents the activity coefficient (mol2/kJ2), R is the gas constant, and qm is the maximum amount of adsorption that occurs in monolayer adsorption (mmol/g).The correlation coefficients (R2) values of all isotherm models represent which equation is more appropriate for the synthesized adsorbents. The higher R2 value, the more well-suited an isotherm model is to describe the methane uptake process. By comparing this value in different models, the Langmuir isotherm is the best isotherm model to explain the CH4 uptake process. Besides, since methane is a nonpolar gas, it tends to be adsorbed on homogeneous surfaces. The Langmuir isotherm model’s application for homogeneous materials and R2 values confirm the homogeneity of the synthesized adsorbents.


### 3.5. Regenerability of the Adsorbents

Two adsorbents MOF-540% @AC and Cu-BDC40% @AC are selected for further investigation of the reusability and stability. Figure 10(a) shows that the adsorption capacity of the adsorbents was compared in three cycles, and Figure 10(b) is shown the adsorption index of each adsorbent in each cycle in comparison with the highest capacity of each adsorption. By repeating the test three times and according to the adsorption index, Cu-BDC40% @AC showed better regenerability than MOF-540% @AC. The capacity of Cu-BDC40% @AC in the second cycle reduced to 0.97 percent of the first cycle, although MOF-540% @AC reached 0.92 percent of the maximum capacity. Also, in the last cycle, the capacity of MOF-540% @AC is reduced to 0.85 percent of the first cycle, and Cu-BDC40% @AC was reduced to 0.89 percent.Figure 10
(a) Adsorption capacity of three cycles of MOF-540% @AC and Cu-BDC40% @AC (b) Adsorption index performance of MOF-540% @AC and Cu-BDC40% @AC.
(a)(b)


## 3.1. Characterization

### 3.1.1. XRD Patterns

Figures3(a)–3(d) compare the XRD (X-ray Diffraction) patterns of MOF-5, MOF-5@AC, Cu-BDC, and Cu-BDC@AC. Figure 3(a) represents the XRD pattern of the MOF-5 with high intensity of the characteristic peaks (2θ=6.8°, 9.6°, 13.6°, and 24.7°), which are consistent with the results reported in the literature, confirming the successful formation of MOF-5. Figure 3(b) shows the formation of MOf-5 in the composite based on activated carbon, which confirms the successful formation of the MOF-5@AC composite because the amorphous carbon does not have a specific crystallinity. Furthermore, a small amount of zinc species impurity is also visible in the phase that was probably entrapped in the framework MOF-5 [30, 32]. Figure 3(c) represents the characteristic peaks of Cu-BDC, which appeared at 2θ=10.1° and 12.1°, representing (001) and (020) planes of crystal structure, respectively, are matched well with XRD patterns, which have been reported before with no impurity. Consequently, The XRD patterns of Cu-BDC@AC (Figure 3(d)) composites show the same diffraction patterns as Cu-BDC, confirming that the incorporation of activated carbon to Cu-BDC MOFs follows those reported in the literature [33, 34].Figure 3
Powder X-ray diffraction patterns of (a) MOF-5, (b) MOF-5@AC composite, (c) Cu-BDC, and (d) Cu-BDC@AC composite.


### 3.1.2. SEM and EDS

The texture of the as-synthesized of MOF-5, MOF-5@AC, Cu-BDC, and Cu-BDC@AC composite can be observed in scanning electron microscopy (SEM) images presented in Figure4, respectively. The SEM image (Figure 4(a)) shows that the MOF-5 particles are uniform with a regular cubic with an average size of about 30-40 μm [32]. Figure 4(c) shows that the Cu-BDC MOF particles are uniform with well-formed cubic microcrystals, and their average edge length is about 9.3 μm. Their morphology is compatible with the previous results that were achieved from hydrothermal synthesis [35]. Comparing the SEM images of MOF-5@AC and Cu-BDC@AC composite (Figures 4(b) and 4(d)) with primary MOFs exhibit the successful formation of MOf-5 and Cu-BDC based on activated carbon. Furthermore, the EDS (Energy Dispersive Spectroscopy) spectra in Figures 4(e) and 4(f) show that MOF-5 and Cu-BDC have been successfully introduced in activated carbon composites resulting in zinc and copper element presence in EDS analysis.Figure 4
SEM images of (a) MOF-5, (b) MOF-5@AC composite, (c) Cu-BDC, and (d) Cu-BDC@AC composite and EDS spectra of (e) MOF-5@AC and (f) Cu-BDC@AC composite.
(a)(b)(c)(d)(e)(f)


### 3.1.3. FT-IR Results

FT-IR spectra, as shown in Figure5, are obtained to gain more detailed information about the structure of the synthesized composites. The IR spectrum of the original activated carbon shows no significant bands, but after the formation of the composite, all peaks of MOF absorption spectra appear. In the FT-IR spectrum of MOF-5@AC, two absorption bands, 2948.2 and 2870.7 cm-1, are assigned to C-H stretching vibrations of the methylene group of remaining DMF in the composition of synthesized MOF. The vibration bands appearing in 1700–1400 cm-1 are assigned to the carboxylic functionality of the 1,4-benzenedicarboxylate. The strong peak appearing at 1758.1 cm-1 can be attributed to the C=O stretching vibration of the carboxylate group of MOF-5. All dedicated absorption peaks of Cu-BDC are observed in the infrared spectrum of the Cu-BDC@AC composite. The sharp absorption peaks at 1602 and 1398 cm-1 belong to the carboxylate groups’ asymmetric and symmetric stretching modes, respectively. The bands at 1512 and 743 cm-1 are assigned to the vibrations of the phenyl ring. The vibration bands at 468 and 556 cm-1 can be assigned to the Cu–O stretching vibration [30, 36]. All these points illustrate that MOF-5 and Cu (BDC) had developed. SEM results also support this assumption.Figure 5
FT-IR spectra of activated carbon, MOF-5@AC, and Cu-BDC@AC composite.


### 3.1.4. N2 Adsorption–Desorption Isotherms Analysis

Figure6 shows the N2 adsorption-desorption isotherms at 77 K and pore size distributions obtained by the MP-plot method for various samples with the structural parameters listed in Table 1. According to the IUPAC classification, the isotherms obtained for all samples are of type I, which is typical for microporous materials. The isotherm of AC and MOF-5 belongs to type I in the IUPAC classification, which is a characteristic of microporous materials. Cu-BDC revealed the isotherm type I with H2 hysteresis, which is related to microporous material. The hysteresis loop observed at high relative pressure implies the existence of some mesopores. The pore size distributions from the MP method of as-synthesized MOFs and composites are shown in Figures 6(c) and 6(d), which are about 0.6 nm specific to microporous materials.Figure 6
N2 adsorption-desorption isotherms of (a) MOF-5, Cu-BDC, (b) AC, MOF-5@AC, and Cu-BDC@AC, (c) and (d) pore size distribution of MOF-5, Cu-BDC, and composites obtained by MP-plot.
(a)(b)(c)(d)Table 1
Textural characteristics of samples.
SamplesBET surface areaMean pore diameter (nm)Total pore volume (cm3 g-1)BET (m2 g-1)Langmuir (m2 g-1)Commercial AC8159201.670.342MOF-57608471.670.317Cu-BDC6337402.620.416MOF-5@AC711866.32.000.355Cu-BDC@AC657.27662.530.416


## 3.1.1. XRD Patterns

Figures3(a)–3(d) compare the XRD (X-ray Diffraction) patterns of MOF-5, MOF-5@AC, Cu-BDC, and Cu-BDC@AC. Figure 3(a) represents the XRD pattern of the MOF-5 with high intensity of the characteristic peaks (2θ=6.8°, 9.6°, 13.6°, and 24.7°), which are consistent with the results reported in the literature, confirming the successful formation of MOF-5. Figure 3(b) shows the formation of MOf-5 in the composite based on activated carbon, which confirms the successful formation of the MOF-5@AC composite because the amorphous carbon does not have a specific crystallinity. Furthermore, a small amount of zinc species impurity is also visible in the phase that was probably entrapped in the framework MOF-5 [30, 32]. Figure 3(c) represents the characteristic peaks of Cu-BDC, which appeared at 2θ=10.1° and 12.1°, representing (001) and (020) planes of crystal structure, respectively, are matched well with XRD patterns, which have been reported before with no impurity. Consequently, The XRD patterns of Cu-BDC@AC (Figure 3(d)) composites show the same diffraction patterns as Cu-BDC, confirming that the incorporation of activated carbon to Cu-BDC MOFs follows those reported in the literature [33, 34].Figure 3
Powder X-ray diffraction patterns of (a) MOF-5, (b) MOF-5@AC composite, (c) Cu-BDC, and (d) Cu-BDC@AC composite.


## 3.1.2. SEM and EDS

The texture of the as-synthesized of MOF-5, MOF-5@AC, Cu-BDC, and Cu-BDC@AC composite can be observed in scanning electron microscopy (SEM) images presented in Figure4, respectively. The SEM image (Figure 4(a)) shows that the MOF-5 particles are uniform with a regular cubic with an average size of about 30-40 μm [32]. Figure 4(c) shows that the Cu-BDC MOF particles are uniform with well-formed cubic microcrystals, and their average edge length is about 9.3 μm. Their morphology is compatible with the previous results that were achieved from hydrothermal synthesis [35]. Comparing the SEM images of MOF-5@AC and Cu-BDC@AC composite (Figures 4(b) and 4(d)) with primary MOFs exhibit the successful formation of MOf-5 and Cu-BDC based on activated carbon. Furthermore, the EDS (Energy Dispersive Spectroscopy) spectra in Figures 4(e) and 4(f) show that MOF-5 and Cu-BDC have been successfully introduced in activated carbon composites resulting in zinc and copper element presence in EDS analysis.Figure 4
SEM images of (a) MOF-5, (b) MOF-5@AC composite, (c) Cu-BDC, and (d) Cu-BDC@AC composite and EDS spectra of (e) MOF-5@AC and (f) Cu-BDC@AC composite.
(a)(b)(c)(d)(e)(f)


## 3.1.3. FT-IR Results

FT-IR spectra, as shown in Figure5, are obtained to gain more detailed information about the structure of the synthesized composites. The IR spectrum of the original activated carbon shows no significant bands, but after the formation of the composite, all peaks of MOF absorption spectra appear. In the FT-IR spectrum of MOF-5@AC, two absorption bands, 2948.2 and 2870.7 cm-1, are assigned to C-H stretching vibrations of the methylene group of remaining DMF in the composition of synthesized MOF. The vibration bands appearing in 1700–1400 cm-1 are assigned to the carboxylic functionality of the 1,4-benzenedicarboxylate. The strong peak appearing at 1758.1 cm-1 can be attributed to the C=O stretching vibration of the carboxylate group of MOF-5. All dedicated absorption peaks of Cu-BDC are observed in the infrared spectrum of the Cu-BDC@AC composite. The sharp absorption peaks at 1602 and 1398 cm-1 belong to the carboxylate groups’ asymmetric and symmetric stretching modes, respectively. The bands at 1512 and 743 cm-1 are assigned to the vibrations of the phenyl ring. The vibration bands at 468 and 556 cm-1 can be assigned to the Cu–O stretching vibration [30, 36]. All these points illustrate that MOF-5 and Cu (BDC) had developed. SEM results also support this assumption.Figure 5
FT-IR spectra of activated carbon, MOF-5@AC, and Cu-BDC@AC composite.


## 3.1.4. N2 Adsorption–Desorption Isotherms Analysis

Figure6 shows the N2 adsorption-desorption isotherms at 77 K and pore size distributions obtained by the MP-plot method for various samples with the structural parameters listed in Table 1. According to the IUPAC classification, the isotherms obtained for all samples are of type I, which is typical for microporous materials. The isotherm of AC and MOF-5 belongs to type I in the IUPAC classification, which is a characteristic of microporous materials. Cu-BDC revealed the isotherm type I with H2 hysteresis, which is related to microporous material. The hysteresis loop observed at high relative pressure implies the existence of some mesopores. The pore size distributions from the MP method of as-synthesized MOFs and composites are shown in Figures 6(c) and 6(d), which are about 0.6 nm specific to microporous materials.Figure 6
N2 adsorption-desorption isotherms of (a) MOF-5, Cu-BDC, (b) AC, MOF-5@AC, and Cu-BDC@AC, (c) and (d) pore size distribution of MOF-5, Cu-BDC, and composites obtained by MP-plot.
(a)(b)(c)(d)Table 1
Textural characteristics of samples.
SamplesBET surface areaMean pore diameter (nm)Total pore volume (cm3 g-1)BET (m2 g-1)Langmuir (m2 g-1)Commercial AC8159201.670.342MOF-57608471.670.317Cu-BDC6337402.620.416MOF-5@AC711866.32.000.355Cu-BDC@AC657.27662.530.416


## 3.2. Test of Adsorbents

This section examines and discusses methane uptake on the synthesized adsorbents. Figure7 demonstrates the CH4 uptake capacities as a function of pressure for pure adsorbents. Also, the amount of methane uptake on the composite of 10 and 40 weight percent of MOF-5 and Cu-BDC based on activated carbon compared with commercial activated carbon has been in Figure 8. The capacity of CH4 in adsorbents is usually proportional to their surface areas. Despite the higher surface area in the commercial activated carbon, a lower methane uptake has occurred compared with the synthesized MOFs. MOFs have higher CH4 uptake than conventional porous materials such as carbon and zeolites. The mechanism of the enhanced uptake is due to the existing three sites: the center of the cluster, pore cages of the phenyl rings, and unsaturated metal ion active sites located at the center of the cavity. van der Waals contact and partial π-HC interactions can significantly interact between methane and adsorption centers [37–39]. A clear result of increasing the amount of MOF used in the composite is that it significantly increases the CH4 uptake. By comparing the amount of gas uptake on the different adsorbents, this result is obtained that MOF-5 has the highest capacity of methane uptake than other composites and activated carbon. However, adding MOFs to the composite indicated a positive effect in increasing methane uptake.Figure 7
CH4 uptake of the AC, MOF-5, and Cu-BDC adsorbents (experimental data and Langmuir’s model).Figure 8
CH4 uptake of the MOF-540% @AC, MOF-510% @AC, Cu-BDC40% @AC, Cu-BDC10% @AC, and AC adsorbents (experimental data and Langmuir’s model).Table2 compares the amount of methane uptake in the synthesized adsorbents in this study with other adsorbents reported in the literature. To have a better comparison, the sorbents are selected from the articles having the same test conditions, including pressure and temperature. This comparison shows that the amount of gas adsorption on a specific adsorbent may be different, and it is due to the different conditions of synthesis and tests that are taken on the adsorbents.Table 2
Comparison of the methane adsorption capacity of the synthesized adsorbents with others.
MOFsFramework density Dc (g/cm3)BET surface area (m2/g)Total uptakeConditions pressure (bar)/temperature (K)References(cm3/cm3)(cm3/g)(mmol/g)MOF-9050.549349014526411.7835/298[40]LIFM-820.92216241962149.5535/298[40]NU-1250.589328618231514.0635/298[40]AC10.51060.3——5.1135/298[10]AC20.58363.3——3.0635/298[10]AC30.391177.7——7.9235/298[10]AC40.491050.8——5.1835/298[10]LIFM-830.9171715192—9.3635/298[41]MOF-50.59187013522810.1836/298[14]IR-MOF10.59—128.29217.459.7135/298[42]MOF-50.61—1352219.8735/298[15]MOF-50.593—1101858.2835/298[43]CMK-30.87950—117.335.235/298[44]Cu-BDC—624——3.835/298[45]Cu-BDC—603——11.5835/293[46]MIL-53(Cu)/CNT—112313.7235/298[47]Mn(HCO2)2/graphite—2804.241/298[48]MOF-540% @AC—711——7.335/298This studyMOF-510% @AC————5.935/298This studyCu-BDC40% @AC—657——6.8535/298This studyCu-BDC10% @AC————5.5935/298This studyPure activated carbon—815——4.3235/298This studyMOF-5—760——8.1835/298This studyCu-BDC—633——7.7435/298This study


## 3.3. Adsorption Kinetic Studies

To investigate the correlation of methane adsorption and predict the rate-limiting step in MOF-540% @AC and Cu-BDC40% @AC, the gathered experimental data are fitted to two regular models, pseudo-first-order and pseudo-second-order models. The reason for selecting these models is their simplicity in describing the interaction rate in the solid-gas adsorption process. The equation of the pseudo-first-order model is defined as follows:
(12)qt=qe1−ek1t,where qe and qt (mmol/g) represent the amount of methane gas adsorbed at equilibrium and at time t (min). Also, k1 (1/min) is the constant rate of adsorption.When pressure increases at a constant temperature,k1 is increased. It means that at higher pressures, the methane uptake will be decreased. Otherwise, the required time for reaching the equilibrium state will be increased.Also, pseudo-second-order was another kinetic model which is used in this study with the following equation:(13)qt=qe2k2t1+qtk2t,where qe and qt (mmol/g) represent the amount of methane gas adsorbed at equilibrium and at time t (min), also, k2 (g/mmol.min) is the constant rate of the adsorption.According to the result of the pseudo-second-order model, when pressure increases,k2 will be increased. It represents a low repulsion in methane molecules, which increases diffusivity [49]. Also, by comparison of R2 values of pseudo-first-order and second-order in Table 3, it seems that the amount of R2 values of the pseudo-first-order is relatively higher than the second-order model. The pseudo-first-order model better matches theoretical and experimental data [50]. On the other hand, since pseudo-first-order and pseudo-second-order cannot show the diffusion mechanism, the extracted results from kinetic models have been analyzed by the intraparticle diffusion model [51]. The intraparticle diffusion equation is defined as the following equation:
(14)qt=kpt0.5+C.Table 3
Kinetic parameters of methane adsorption on the synthesized adsorbents.
SamplePressure (bar)1st order2nd orderIntraparticle diffusionk1 (min-1)R2k2R2ktCCu-BDC40% @AC1019.345170.999826.122240.99740.657353.230762024.643290.999828.61970.998821.008313.928653534.993250.999727.652850.99891.041484.506MOF-540% @AC1022.616930.9986710.345060.998610.745863.446282024.239230.9983713.948490.995120.934514.354723528.89760.9999717.219310.999090.995.08543In Equation (14), kp is defined as the intraparticle diffusion rate constant (mmol.g-1 min-0.5) and t represents time (min).In Figure9 the plot of methane adsorption versus t0.5 is shown. According to the gathered data, the methane adsorption process has high-speed kinetics, and the adsorption reaches the equilibrium state in the early minutes. Equation (14) is a linear equation with the intercept C, and the value of zero for the intercept shows that intraparticle diffusion is the only controller of the adsorption process. When the amount of C is not zero, according to Table 3, it means that the adsorption process is based on the pseudo-first-order kinetic model and intraparticle diffusion [51].Figure 9
Methane adsorption onto (a) Cu-BDC40% @AC and (b) MOF-540% @AC vs. t0.5.
(a)(b)


## 3.4. Adsorption Isotherm Studies

The adsorption isotherms can describe the equilibrium performance of adsorbents in an isotherm process. Many factors, including adsorbents, physical properties of adsorbate, and adsorption process condition, affect adsorption isotherm [42].Also, adsorption isotherm models explain the interaction between adsorbate fluid and synthesized adsorbents. The extracted data from the adsorption experiment are fitted with three isotherm models, Langmuir, Freundlich, and Dubinin-Radushkevich equilibrium isotherm models. The isotherm constants are reported in Table4. These isotherm models are used to investigate adsorption conditions, including homogeneous/heterogeneous and monolayer/multilayer. Many different isotherm models can be used to investigate the experimental data; however, these models are selected since they are more appropriate for the gas adsorption process than others [52, 53].Table 4
Isotherm adsorption parameters of methane on the synthesized adsorbents.
Isotherm modelParametersAdsorbentMOF-5Cu-BDCMOF-540% @ACMOF-510% @ACCu-BDC40% @ACCu-BDC10% @ACACLangmuirb0.236480.182850.142660.135770.120860.129880.07714qm9.111929.117058.857477.128968.682676.803666.29136R20.998640.998860.999010.99760.995290.995610.99178Freundlichkf3.243572.801052.255531.766791.907821.617480.89296n3.672023.375412.8989712.8575512.6438942.7794762.117881R20.971460.976470.938820.980950.923190.985630.92945Dubinin-Radushkevichqm8.385827.955137.650616.08717.309385.758724.71858k0.013510.015650.018690.018870.020920.019260.02697R20.993670.994710.984830.994790.969120.995210.96889The Langmuir adsorption isotherm model is a straightforward and appropriate isotherm model used for homogeneous surfaces of sorbent to explain monolayer and chemical adsorption in the gas-solid phase. This model is the most common isotherm model to investigate methane and carbon dioxide of activated carbon [52, 54]. According to this isotherm model, the surface of a solid adsorbent is a completely uniform and homogeneous surface composed of one type of material, and there is no preferred place for adsorption on its surface. All places have the same priority for adsorption. Also, each site does not adsorb more than one molecule; in such a situation, a single layer of molecules will be adsorbed on the solid surface [55]. The equation of the Langmuir isotherm model is defined as follows:
(15)qe=bP1+bPqm,where qm shows the monolayer uptake capacity (mmol/g), b is the affinity coefficient constant (1/bar), qe represents the amount of gas uptake on the adsorbent (mmol/g), and P is the adsorbed gas pressure (bar). The isotherm constants are shown in Table 4.The affinity coefficient constant (b) indicates the extent of interaction between the adsorbate and the surface. Therefore, the highest value in MOF-5 indicates that CH4 has the strongest affinity toward the MOF-5 surface. Also, for evaluating an adsorbent’s adsorption capacity, Langmuir’s predicted qm is valuable. Experimentally, measured CH4 adsorption isotherms for MOF-5 show good compatibility with the single-site Langmuir’s model [56].Freundlich’s adsorption isotherm model is suitable for predicting adsorption on heterogeneous surfaces and explaining nonideal processes and multilayer surfaces [52]. Therefore, it could be a good complementary model addition to the Langmuir isotherm model to better investigate heterogeneous and multilayer sites. The main difference between the Langmuir and Freundlich models is that the Freundlich model is applicable for multilayer adsorption [57]. Freundlich’s equation has been shown in the following equation:
(16)qe=kfP1/n,where kf is the Freundlich isotherm constant (mmol/g) and n is the adsorption intensity and is a heterogeneity factor.Dubinin-Radushkevich’s (D-R) adsorption isotherm model is a semiempirical equation for multilayer surfaces. Initially, the application of this model was for subcritical vapors on micropore materials. Now, it is used to determine chemical and physical gas adsorption in microporous materials [52]. The equation of the D-R model is as follows:
(17)qe=qmexp−kε2,⟶ε=RTlnPsP.In the equation,Ps and P are the saturation vapor pressure and the adsorbate equilibrium pressure, respectively. Also, k value represents the activity coefficient (mol2/kJ2), R is the gas constant, and qm is the maximum amount of adsorption that occurs in monolayer adsorption (mmol/g).The correlation coefficients (R2) values of all isotherm models represent which equation is more appropriate for the synthesized adsorbents. The higher R2 value, the more well-suited an isotherm model is to describe the methane uptake process. By comparing this value in different models, the Langmuir isotherm is the best isotherm model to explain the CH4 uptake process. Besides, since methane is a nonpolar gas, it tends to be adsorbed on homogeneous surfaces. The Langmuir isotherm model’s application for homogeneous materials and R2 values confirm the homogeneity of the synthesized adsorbents.


## 3.5. Regenerability of the Adsorbents

Two adsorbents MOF-540% @AC and Cu-BDC40% @AC are selected for further investigation of the reusability and stability. Figure 10(a) shows that the adsorption capacity of the adsorbents was compared in three cycles, and Figure 10(b) is shown the adsorption index of each adsorbent in each cycle in comparison with the highest capacity of each adsorption. By repeating the test three times and according to the adsorption index, Cu-BDC40% @AC showed better regenerability than MOF-540% @AC. The capacity of Cu-BDC40% @AC in the second cycle reduced to 0.97 percent of the first cycle, although MOF-540% @AC reached 0.92 percent of the maximum capacity. Also, in the last cycle, the capacity of MOF-540% @AC is reduced to 0.85 percent of the first cycle, and Cu-BDC40% @AC was reduced to 0.89 percent.Figure 10
(a) Adsorption capacity of three cycles of MOF-540% @AC and Cu-BDC40% @AC (b) Adsorption index performance of MOF-540% @AC and Cu-BDC40% @AC.
(a)(b)


## 4. Conclusions

MOF-5@AC and Cu-BDC@AC composites were prepared based on the commercial activated carbon. Since MOFs are not cost-effective, composites of MOFs based on commercial activated carbon could be effective strategies to reduce the cost of the ANG method. The result of experiments on different adsorbents was shown that among activated carbon and composites, MOF-540% @AC has the highest amount of methane uptake, equal to 7.3 mmol/g at 35 bar pressure and 298 K temperature. The capacity of MOF-5 was about 90% higher than the pure activated carbon, and by adding 40% MOF-5 to the composite, the methane uptake was increased 70%. Also, more studies, including regenerability and kinetic studies, were done on the composites (MOF-540% @AC and Cu-BDC40% @AC). These studies showed that the recovered adsorbents retain their original porous structure, and Cu-BDC40% @AC had better stability than MOF-540% @AC. Moreover, based on the kinetic studies of these two adsorbents in three different pressures, it was indicated that sorption kinetics happens very fast and is suitable for practical applications of methane adsorption. The good adjustment of experimental data with pseudo-first-order indicated that physisorption controls the methane adsorption process.



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