MOF-GO Hybrid Nanocomposite Adsorbents for Methane Storage

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MOF-GO Hybrid Nanocomposite Adsorbents for Methane Storage Qasim Al-Naddaf, Mana Al-Mansour, Harshul Thakkar, and Fateme Rezaei Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03638 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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Industrial & Engineering Chemistry Research

MOF-GO Hybrid Nanocomposite Adsorbents for Methane Storage Qasim Al-Naddaf, Mana Al-Mansour, Harshul Thakkar, Fateme Rezaei* Department of Chemical & Biochemical Engineering, Missouri University of Science and Technology, 1101 N State Street, Rolla, MO, 65409, United States Abstract In this study, the storage of methane in nanocomposite adsorbents comprising of metalorganic framework (MOF) and graphene oxide (GO) was investigated. Three different sets of MOF-GO nanocomposites comprising of HKUST-1 and pristine GO, reduced GO (rGO), and carboxyl-functionalized GO (fGO) were developed by solvothermal method and their methane storage characteristics were assessed through high-pressure methane adsorption measurements. The formation of MOF-GO nanocomposites was confirmed by XRD, FTIR, XPS, SEM, and TEM. All three types of nanocomposites exhibited higher surface area and porosity than the pristine MOF. Our results indicated that MOF@rGO nanocomposite with 10 wt% rGO exhibited the best performance with a methane deliverable capacity of 193 cm3(STP)/cm3 in the pressure range of 5.8-65 bar and at room temperature which was approximately 30% higher than that of pristine HKUST-1 with deliverable capacity of 149 cm3(STP)/cm3. Moreover, the methane deliverable capacity of MOF@GO and MOF@fGO were found to be 181 and 162 cm3(STP)/cm3, respectively. The findings of this study demonstrate the synergistic effect of graphene oxide on methane storage performance of MOF-GO nanocomposites. Keywords: MOF, GO, Nanocomposite, Methane storage, Adsorption 1. Introduction The growing increase in energy consumption of transportation by a factor of ~ 4%/year has sparked intense interest in the use of natural gas (NG) as a potential alternative fuel for vehicular applications over the past decades.1 Compared to gasoline, methane, the primary component of 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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NG, has a relatively low volumetric energy density (VED). Therefore, it is necessary to densify NG to improve its energy storage density and achieve an acceptable vehicle range between refueling. The two common methods of NG storage currently used are liquefaction at -162 °C and compression to 250 bar to produce liquefied natural gas (LNG) and compressed natural gas (CNG), respectively.2,3 Despite gaining popularity in the United States and several European countries, LNG and CNG have several drawbacks such as storage in expensive cryogenic vessels and the use of heavy, thick-walled cylindrical storage tanks and multi-stage compressors to achieve a reasonable energy density. Moreover, the VED of CNG and LNG are only 27% and 64% of that of gasoline (34.2 MJ L-1), respectively.3,4 To address the limitations associated with LNG and CNG processes, development of safe, efficient, and cost-effective NG storage alternatives has received considerable interest. In particular, the use of porous adsorbents as potential storage media for methane in the form of adsorbed natural gas (ANG) has been suggested as a safer, simpler, and more energy-efficient platform for advancement of NG vehicular systems.3,5–8 Several classes of materials have been investigated as potential ANG adsorbents including carbon-based materials,9–13 MOFs,14–17 covalent-organic frameworks (COFs),18–23 and porous polymer networks (PPNs).24–27 Owing to their high surface area, pore volume, and tunable surface chemistry, MOFs have received considerable attention as promising candidates for methane storage. MOFs are able to store appreciable amounts of NG at moderately low pressures (less than 100 bar). This class of materials is generally considered to have the potential to improve methane deliverable capacity by physisorption through achieving a favorable balance of large surface area and pore volume with high-affinity gas adsorption sites.1 The maximum attainable deliverable capacity was obtained by several MOFs with rigid structure, such as HKUST-1, UTSA-76, MOF-519, NU-

2 ACS Paragon Plus Environment

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111, MOF-905, and soc-MOF, and with flexible framework such as Co(bdp).15,28–30 Despite significant progress, this class of materials falls short of the capacity targets. Among other microporous adsorbents, carbon-based materials are considered suitable candidates for methane storage, as a result of their low weight, high stability, and environmentally-friendly nature.9–13 In particular, the tunable structure of graphene oxide (GO) allows for selective adsorption of light gases.31–33 It has been shown that with proper inter-layer spacing, graphene could potentially enhance the methane storage capacity.34–36 According to the U.S. department of energy (DOE), the targeted deliverable capacity for NG adsorbents should be 315 cm3 (STP)/cm3 at 25 °C.37 Here, deliverable capacity is defined as the difference between methane capacities at 65 and 5.8 bar. A recent computational screening of thousands of materials suggests, however that this target is unattainable, mainly due to fundamental limits arising from thermodynamic or material design constraints.38,39 One strategy to overcome the design barrier is to develop hybrid materials that exhibit storage capacity higher than their individual components.40,41 Hybrid nanocomposites based on MOFs and graphene enable integration of the unique properties of the two constituents, thus allowing the design of advanced materials with properties not possessed by either component. A number of studies have reported the incorporation of GO into various porous materials such as MOFs. Bandosz and coworkers have extensively evaluated MOF-graphite oxide composites for adsorption of small molecule gases at ambient conditions.42–48 The suggested model for such composites was based on the alternation of GO sheets with layers of MOF via linkage between epoxy groups from GO and metal oxide from MOF framework. Petit et al.45,48 developed several HKUST-1/graphite oxide composites and evaluated their performance for adsorption of NH3, H2S, NO2, and CO2 under ambient conditions. On the basis of the obtained results, the authors reported different



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trends in the performance of HKUST-1 and the composites as reactive adsorbents of NH3, H2S, NO2 and physical adsorbents of CO2. Most recently, Zhao et al.46,47 reported the development of composites of HKUST-1/graphite oxide modified with urea as CO2 adsorbents with improved uptake capacity. Loh and co-workers49–51 intercalated MOF-5 between benzoic acidfunctionalized GO sheets and investigated its properties for oxygen reduction reaction. On the basis of the obtained results, the authors suggested that by functionalizing the basal planes of GO with a high density of carboxylic groups and controlling the length of the carboxylic linker group, a greater degree of tunability in terms of pore size and structural motif can be achieved. In another study, ZIF-8/GO nanocomposites was developed and evaluated for CO2 capture.52 Most recently, Chen et al.53 synthesized MOF-505@GO composites and tested them for separation of CO2/CH4 and CO2/N2 pairs. The authors reported high adsorption selectivity and moisture stability for the composites. In another investigation, Li et al.,54 used HKUST-1/GO composite for adsorption of methylene blue from aqueous solutions. Through performing a set of equilibrium and dynamic adsorption measurements, the authors attributed the increase in adsorption capacity and stability in water solution to the addition of GO which imparts excellent hydrophilicity to the composite. Motivated by the above studies and taking into account that the incorporation of graphene is an effective way to adjust the properties of MOFs for enhancement of gas storage capacity, 50,55–57

we developed MOF-GO nanocomposites by incorporating graphene oxide (GO), reduced

graphene oxide (rGO), and graphene oxide functionalized with benzoic acid (fGO) into HKUST1. Our hypothesis was that incorporation of graphene oxide sheets into the MOF structure can enhance the porosity as a result of creation of extra pores and also the methane interaction with the adsorbent since GO itself has affinity toward methane molecules. For HKUST-1, the 4 ACS Paragon Plus Environment

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intercalation is typically achieved via monodendate epoxy or hydroxyl groups on the basal plane of GO layers. Nevertheless, we employed various forms of graphene oxide in development of nanocomposites to evaluate the effect of bridging linkers on MOF-GO properties and performance by reducing GO and further functionalizing with benzoic acid to become extended by phenyl carboxylic groups. After synthesis and evaluation of the nanocomposites, their methane adsorption behavior was assessed by high pressure adsorption measurements at room temperature to determine their deliverable capacity. 2. Experimental Section 2.1.

Materials Graphene oxide was purchased from Goographene. The other chemicals required for the

synthesis of the MOF-GO nanocomposites including sodium carbonate (Na2CO3), sodium borohydride (NaBH4), diazonium salt, benzoic acid, sodium dodecylbenzen-sulfonate (SDBS), N, N’-dimethylformamide (DMF), Cu(NO3).5H2O, and 1,2,5-benzenetricarboxylic acid (BTC) were all of ACS grade and purchased from Sigma-Aldrich. All UHP grade gases used in this work were purchased from Airgas. 2.2.

MOF-GO Nanocomposite Synthesis To prepare rGO, the as-purchased GO was reduced according to the procedure reported

elsewhere.50 Briefly, an appropriate amount of GO was added into DI water, then sonicated for 1 h to completely disperse the GO sheets in water. Subsequently, 5% sodium carbonate solution was added to the dispersed GO to balance the pH level to 10, followed by heating to 90 ºC for 9 h. In the next step, NaBH4 was added to the solution and maintained at 80 ºC in an oil bath for 3 h. Finally, the product was filtered and washed with DI water. For the synthesis of fGO, an appropriate amount of rGO was dispersed in 1 wt% aqueous SDBS surfactant and then sonicated 5 ACS Paragon Plus Environment


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for 1 h. Next, the diazonium salt solution was added to the rGO/SDBS solution in an ice bath under constant stirring for 4 h. After that, the reaction was stirred for 4 h at room temperature. The product was then filtered and washed immensely with DMF, ethanol, and acetone.50 HKUST-1 was synthesized according to a standard procedure described by Zhou et al.58 A one-pot synthesis method was used to prepare the nanocomposites, in a similar way to that of pristine HKUST-1. Initially, 10 wt% of GO, rGO, and fGO was added to the mixture of HKSUT-1 precursors before hydrothermal synthesis. The mixtures were then heated to 110 ºC for 20 h, after which they were cooled down to room temperature, filtered and washed extensively with dichloromethane. Lastly, the resulting products were dried under vacuum at 150 ºC for 24 h. This is common synthesis procedure for MOF-GO composites that has been widely used by other researchers.42,45,50,51 The nanocomposites were denoted as HKUST-1@GO, HKUST-1@rGo, and HKUST-1@fGO. It is noteworthy to point out here that all syntheses were subjected to the same exact procedure to avoid any variability in the materials properties due to the synthesis conditions. 2.3.

MOF-GO Nanocomposite Characterization To determine the successful reduction and functionalization of GO, Fourier transform

infrared (FTIR) and Raman spectra of the MOF@GO nancomposites were obtained using a Nicolet-FTIR Model 750 and Nicolet-Raman Model 550 spectrometers, respectively. The FTIR analysis was also employed to confirm the formation of MOF-GO nanocomposites. The X-ray diffraction (XRD) measurements were carried out to determine the crystallinity of the MOF crystals in the nanocomposites using a PANalytical X’Pert multipurpose X-ray diffractometer with a scan step size of 0.02 °/step at the rate of 137.2 s/step. The textural properties of the nanocomposites were evaluated by N2 physisorption tests at 77 K and in the pressure range of 0 6 ACS Paragon Plus Environment

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to 1 bar on a Micromeritics (3Flex) instrument. Prior to the measurements, the samples were degassed under vacuum for 6 h at 150 ºC to drive off the pre-adsorbed gases. The BrunauerEmmet-Teller (BET) and non-local density functional theory (NLDFT) methods were used to estimate the surface area and pore size distribution (PSD), respectively. Field-emission scanning electron microscopy (SEM, Hitachi Model S4700) was utilized to examine the structural morphology of the obtained monoliths. High-resolution transmission electron microscopy (HRTEM) images were obtained on a JEOL JEM-2100 operated at 200 kV. Prior to taking images, the samples were dispersed in ethanol and then collected using copper grids covered with carbon film. To measure the elemental composition of the samples, X-ray photoelectron spectroscopy (XPS) analysis was performed by a Kratos Axis 165 XPS. The thermal stability of the MOF-GO samples was also assessed using TGA (Q500, TA Instruments). The temperature was varied from 25 to 800 °C at the rate of 20 °C/min. 2.4.

Methane Adsorption Measurements The high-pressure adsorption isotherms were collected from 0 to 65 bar on a BELSORP-

HP adsorption apparatus at room temperature. Prior to the measurements, the samples were regenerated in-situ under vacuum at 150 ºC for 6 h. The gravimetric excess adsorption nexc(P,T) measured by the instrument was converted to total adsorption ntot(P,T) using the following equation:

ntot ( P, T )  nexc ( P, T )   gas ( P, T )V p

(1)

where ρgas(P,T) and Vp are respectively, bulk density of methane and pore volume of the adsorbent. The later can be determined from the nitrogen physisorption isotherms at 77 K while the former can be determined from NIST database.59 To obtain the volumetric isotherms, the



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gravimetric isotherms were multiplied by the crystallographic bulk density of the adsorbents that was estimated by the the following equation: Bulk density 

1

(2)

1/ d skl  V p

where dskl is skeletal density of the adsorbent which was estimated directly from BELSORP-HP instrument. 3. Results and Discussion 3.1.

Characterization of MOF-GO Nanocomposites The Raman and FTIR spectra of bare GO, rGO, and fGO are presented in Figure 1a and

1b, respectively. As shown in Figure 1a, two bands were observed in the spectra of GO and its modified counterparts; the first bad at 1320 cm-1, known as D band, is attributed to the presence of disordered regions in the GO, whereas the second peak at 1570 cm-1, known as G band, is related to the in-phase vibration of the GO lattice. It has been shown that the ratio of the intensity of these two peaks (ID/IG) indicates the extent of the defects in the graphene surface after modifications.46 A relatively similar intensity ratios for all three forms of graphene oxide revealed no drastic changes in the chemistry of the GO sheets during reduction and functionalization. Considering the FTIR spectrum of rGO in Figure 1b, the reduction in the intensity of the peaks at 1112, 1605, and 1730 cm-1 which were related to epoxy groups (C-O-C), carboxylic acid and carbonyl (aromatic) groups, respectively, provides direct evidence for successful reduction of GO sheets by sodium borohydride.42–45,49–51 The intensity of the transmittance peak at 1082 cm-1 in the FTIR spectrum of fGO, which was attributed to C-O bond, increased significantly for fGO in comparison to bare GO, illustrating the successful COOH 8 ACS Paragon Plus Environment

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functionalization. Furthermore, the confirmation of the nanocomposites is provided by the FTIR spectra shown in Figure 1c. The spectra of the parent MOF and the nanocomposites appeared to be quite similar. The asymmetric stretching of the carboxylate group present in the organic linker (BTC) was observed at ~1576-1646 cm-1; whereas the symmetric stretching of the carboxylate groups in (BTC) was detected at ~1375-1448 cm-1. Moreover, the bands observed in the 5001300 cm-1 region were assigned to the out-of-plane vibration of the organic linker BTC. The presence of GO characteristic peaks in the nanocomposites indicated that the structure was preserved and that the addition of GO, rGO, and fGO did not disturb the structure of the parent MOF, as previously reported.60–62



(b)

(a)

G band

D band

Transmittance (%)

Intensity (a.u.)

fGO

fGO

rGO

GO

rGO GO 1050

1200

1350

1500

1650

1800

1950

3500

3000

Raman Shift (cm-1)

2500

2000

Wavenumber (cm-1)

1500

1000

500

(c)

HKUST-1@fGO

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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HKUST-1@rGO

HKUST-1@GO

HKUST-1

3500

3000

2500

2000

1500

Wavenumber (cm-1)

1000

500

Figure 1. (a) Raman and (b) FTIR spectra for GO, rGO, and fGO, and (c) FTIR spectra for HKUST-1, HKUST-1@GO, HKUST-1@rGO, and HKUST-1@fGO.

Figure 2 illustrates the XRD patterns of the parent HKUST-1 and the nanocomposite adsorbents. The diffraction peaks of HKUST-1@GO, HKUST-1@rGO, and HKUST-1@fGO were found to be consistent with the peaks observed for pristine HKUST-1, demonstrating that the well-defined MOF structure was preserved and that the presence of GO, rGO, and fGO did not interfere with the reaction of metal cluster (Cu) and the organic linker (BTC) to form the MOF during solvothermal synthesis. In addition, the well-defined higher order diffraction peaks of the nanocomposites and the parent HKUST-1 were in agreement with the previously reported


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literature data,45,54 indicating successful synthesis of MOF-GO nanocomposites. It should be noted here that the intensity of diffraction peaks for HKUST-1 increased with the addition of rGO. The high intensity observed for HKUST-1@rGO could be attributed to higher degree of MOF crystallization. In contrast, the incorporation of fGO resulted in an apparent broadening of the major XRD peaks which could be due to the formation of smaller sized grains. However, the intensity of the peaks in the HKUST-1@GO nanocomposites decreased in comparison to the parent MOF which could be ascribed to the constraints that GO imposed on the degree of freedom and the growth of the MOF crystals, as a result of having more surface functional groups that can interfere with crystal growth during MOF formation.

HKUST-1@fGO Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HKUST-1@rGO HKUST-1@GO

HKUST-1 10

20

30

40

50

 

Figure 2. The XRD patterns for HKUST-1, HKUST-1@GO, HKUST-1@rGO, and HKUST1@fGO.

Figure 3 shows TGA profiles of the nanocomposites and the parent MOF. The main turning points of all samples are quite similar. As can been observed, three noticeable weight losses are evident for all samples. The first weight loss below 100 ºC corresponds to the removal of moisture and other guest molecules from the framework, while the second weight loss starting at about 290 ºC could be ascribed to the removal of chemisorbed (attached) water in the MOF 11 ACS Paragon Plus Environment


structure. The most prominent weight loss appearing in the temperature range of 358-363 ºC is due to the decomposition of the BTC units. For HKUST-1, a weight loss of 42% was observed in the range of 300 to 800 ºC, whereas for HKUST-1@GO, HKUST-1@rGO, and HKUST1@fGO, the weight loss was found to be 44, 37, and 38%, respectively in the same temperature range. Moreover, from Figure 3, it can be seen that the third weight loss shifted to slightly higher temperatures for HKUST-1@rGO (363 ºC) and HKUST-1@fGO (361 ºC) compared to the parent MOF (358 ºC) and HKUST-1@GO (358 ºC) indicating that the thermal stability of HKUST-1@rGO and HKUST-1@fGO was slightly enhanced compared to that of the parent MOF. It has been previously shown that the bare GO decomposes at ~ 210 °C,53 however, the weight loss due to GO was not noticeable for our nanocomposites mainly because of its relatively low content in comparison to the MOF. 110

HKUST-1@fGO HKUST-1@rGO HKUST-1@GO HKUST-1

100 90

Weight Loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 70 60 50 40 30 100

200

300

400

500

600

700

800

Temperature (°C)

Figure 3. TGA profiles for HKUST-1, HKUST-1@GO, HKUST-1@rGO, and HKUST-1@fGO.

To detect different elements on the surface of nanocomposites, the XPS analysis was performed and the results are presented in Figure 4 and Table 1. From this figure, the elements 12 ACS Paragon Plus Environment

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detected by XPS were Cu, C, O, and H. The characteristic peak of C(1s) observed at 285 eV revealed a carbon content of 42.98, 44.51, and 46.06% for HKUST-1@GO, HKUST-1@rGO, and HKUST-1@fGO, respectively, whereas, HKUST-1 exhibited a C(1s) content of 36.11%. On the contrary, the copper content was found to be lower for the nanocomposites than for the bare MOF. The increase in the carbon content and decrease in the copper content of the hybrid nanocomposites relative to the parent MOF further indicated that the graphene sheets were successfully incorporated into the MOF structure. Moreover, the elemental analysis showed oxygen content values for nanocomposites were smaller than that for HKUST-1. Notably, while the carbon content of the rGO and fGO based nanocomposites were found to be higher than that of GO based materials, the opposite was observed for oxygen content. O(1s) C(1s) Cu(2p)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cu Cu

O(2s) Cu(3s)

Cu

HKUST-1@fGO

HKUST-1@rGO

HKUST-1@GO

HKUST-1 1000

800

600

400

200

0

Binding Energy (eV)

Figure 4. XPS profiles of MOF-GO nanocomposites and the parent HKUST-1.

Table 1. XPS elemental composition of MOF-GO nanocomposites and HKUST-1. Adsorbent

Mass Composition [%] O

C

Cu

HKUST-1

32.1

36.1

31.8

HKUST-1@GO

27.8

43.0

29.3 13

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HKUST-1@rGO

25.4

44.5

30.1

HKUST-1@fGO

28.2

46.1

25.8

Figure 5 shows the N2 physisorption isotherms and PSD curves obtained at 77 K for the bare HKUST-1 and the corresponding nanocomposites. Overall, as demonstrated in Figure 5a, the isotherms were comparable in shape and both the nanocomposites and the parent MOF displayed type I isotherm without hysteresis loop, indicating the predominant microporous nature of the obtained adsorbents. The nanocomposites exhibited higher nitrogen uptake than the bare MOF as a result of the enhanced porosity upon incorporation of graphene sheets into the MOF structure. This enhancement was more pronounced for nanocomposites consisted of rGO and fGO than the bare GO. The microporous character of the nanocomposites was also evident from PSD profiles presented in Figure 5b. According to the NLDFT-cylindrical analysis, the nanocomposites possessed 1.9 nm sized micropores. 500


450

(a)

400

350

300

250 0.0

0.2

0.4

0.6

0.8

1.0

0.035

dV/dW Pore Volume (cm3/g.nm)

Quantity Adsorbed (cm3/g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.030 0.025

(b)

0.020 0.015 0.010 0.005 0.000 2

3

4

5

6

7

8

9

10

Pore Width (nm)

Relative Pressure (P/P0)

Figure 5. (a) N2 physisorption isotherms and (b) PSD curves for HKUST-1, HKUST-1@GO, HKUST-1@rGO, and HKUST-1@fGO.


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The surface area (SBET), pore volume (Vp), and average pore with (dp) parameters estimated from nitrogen isotherms are summarized in Table 2. Overall, it can be observed that the surface area and porosity of the MOF-GO nanocomposites were higher than those of the parent MOF. This could be attributed to the formation of new pores at the interface between the oxygencontaining functional groups on the GO surface and the copper from the HKUST-1 units. The surface area and pore volume of the parent HKUST-1 were found to be 1137 m2/g and 0.6 cm3/g, respectively. These values were somewhat intermediate compared to those reported in the literature.46,54 The incorporation of rGO resulted in the highest increase in both SBET and Vp compared to GO and fGO with the values of 1271 m2/g and 0.66 cm3/g, respectively for HKUST-1@rGO, followed by 1263 m2/g and 0.67 cm3/g for HKUST-1@fGO and 1259 m2/g and 0.65 cm3/g for HKUST-1@GO. These results imply that post-modification of the GO has little impact on porosity of the nanocomposites. Li et al.54 reported SBET of 575 and 1214 m2/g and Vp of 0.39 and 0.61 cm3/g for bare HKUST-1 and HKUST-1/GO composite, respectively. The textural properties of the pristine GO included in Table 2 reflect the less porous nature and surface area of this component than the bare MOF. The graphene oxide loading and bulk density of the materials are also presented in this table. Table 2. Textural properties of the nanocomposites and the bare HKUST-1.

HKUST-1

SBET [m2/g] 1137

Vp [cm3/g] 0.60

dp [nm] 1.4

GO loading [wt%] -

Bulk density (ρ) [g/cm3] 1.04

HKUST-1@GO

1259

0.65

1.9

10

1.10

HKUST-1@rGO

1271

0.66

1.9

10

0.95

HKUST-1@fGO

1263

0.67

1.9

10

0.98

GO

34

0.03

2.0

100

-

Adsorbent



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The surface morphology of the bare HKUST-1 and HKUST-1@rGO with the best textural properties were evaluated by SEM and the corresponding images are shown in Figure 6a-b. As clearly evident, while the bare MOF exhibited a smooth facet surface, the composite displayed a lacelike structure in the vicinity of GO layers embedded within MOF crystals. Such surface topology has been previously reported for MOF-graphite oxide composites in the literature.46,47 In fact, the enhancement in porosity and surface area of the composites noted earlier is due to the new pores formed in the MOF-GO composites which are visible in Figure 6b. It should be pointed out that a similar surface morphology was observed for the other nanocomposites (HKUST-1@GO and HKUST-1@fGO). The TEM images shown in Figure 6c-d were used to examine the microstructure of the HKUST-1@rGO. The faceted MOF crystals with the attached GO sheets can be observed in Figure 6c, while the magnified image in Figure 6d displays the parallel GO sheets within the composite structure. These images clearly confirm the occurrence of intercalation of GO in the MOF structure for this nanocomposite. Nevertheless, it should be pointed out here that the size of MOF crystals in the nanocomposites was not uniform and intercalation gave rise to a wide range of MOF crystal sizes.


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Figure 6. (a) SEM image of (a) bare HKUST-1, (b) SEM image of HKUST-1@rGO, and (c-d) TEM images of HKUST-1@rGO. 3.2.

Methane Adsorption Experiments The total volumetric CH4 adsorption isotherms for the parent MOF and the nanocomposites

obtained at 25 °C and in the pressure range of 0-65 bar are illustrated in Figure 7a. It is apparent from these isotherms that even though the CH4 uptake over HKUST-1@GO and HKUST1@rGO nanocomposites was similar to that over bare HKUST-1 at low pressures, HKUST1@GO, and HKUST-1@rGO outperformed the pristine MOF at higher pressures, giving rise to a much higher deliverable capacity in the pressure range investigated. In contrast, HKUST1@fGO displayed only a slight enhancement in CH4 uptake relative to HKUST-1 over the entire pressure range. At 65 bar, the total CH4 uptake reached 217, 247, 270, and 220 cm3(STP)/cm3 for HKUST-1, HUKST-1@GO, HUKST-1@rGO, and HUKST-1@fGO, respectively. Considering 17 ACS Paragon Plus Environment


the effect of bridging linkers, it appeared that reduced GO gave rise to much better promising results than bare GO and functionalized GO. Although volumetric methane uptake is more relevant to the practical application of methane storage, we estimated the gravimetric storage uptake as well. Figure 7b shows the total gravimetric CH4 adsorption isotherms estimated using the bulk density values (shown in Table 2) over the 0-65 pressure range. Despite high volumetric capacity, HKUST-1@GO exhibited smaller improvement in gravimetric uptake capacity owing to its higher bulk density (ρ = 1.099 g/cm3 versus ρ = 1.035 g/cm3 for HKUST-1). For HKUST-1@rGO, the total uptake reached 0.21 g/g at 65 bar which was 35% higher than that for the bare HKUST-1. Notably, HKUST-1@fGO showed 7% higher gravimetric capacity despite its similar volumetric uptake to the MOF owing to its lighter density (ρ = 0.984 g/cm3). It is noteworthy here that for each nanocomposite, two more graphene loadings (3 and 6 wt%) were used to synthesize the materials and their corresponding methane storage results are presented in Figure S1, Supporting Information. By increasing the graphene oxide content, the CH4 uptake showed improvement, however, the obtained capacities were lower than those obtained with 10 wt% for all GO, rGO, and fGO based nanocomposites. 300

(a)

0.20 (b)

250

0.18

200 150 100

HKUST-1 HKUST-1@Go HKUST-1@fGo HKUST-1@rGo

50

CH4 Total Uptake (g /g)

CH4 Total Uptake (cm3(STP)/cm3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.16 0.14 0.12 0.10 0.08

HKUST-1 HKUST-1@Go HKUST-1@fGo HKUST-1@rGo

0.06 0.04 0.02 0.00

0 0

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40

Pressure (bar)

50

60

70

0

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Figure 7. Total (a) volumetric and (b) gravimetric CH4 adsorption isotherms for MOF-GO nanocomposites and the parent HKUST-1 at 25 °C.

The corresponding excess, total, and deliverable capacities for the adsorbents are summarized in Table 3. HKUST-1@rGO exhibited the highest deliverable capacity of 193 cm3(STP)/cm3, followed by HKUST-1@GO with 181 cm3(STP)/cm3, while the HKUST-1@fGO exhibited the lowest deliverable capacity of 162 cm3(STP)/cm3 among the nanocomposites investigated here. Table 3. Methane adsorption characteristics of the nanocomposites and the bare HKUST-1. excess (5.8 bar)

total (5.8 bar)

excess (65 bar)

total (65 bar)

[cm3/g]

[cm3/cm3]

[cm3/g]

[cm3/cm3] [cm3/g]

HKUST-1

63

68

171

217

108

149

HKUST-1@GO

58

67

191

247

134

181

HKUST-1@rGO 78

77

242

270

165

193

HKUST-1@fGO 55

58

179

220

124

162

Adsorbent

deliverable capacity [cm3/cm3]

Ideally, any adsorbent material is required to not only have high total uptake capacity but also high deliverable capacity to be cost-effective for ANG applications. Figure 8 depicts the comparison of the attainable deliverable capacity versus total uptake for HKUST-1@rGO and several promising MOFs such as HKUST-1, UTSA-76, MOF-519, NU-111, MOF-905, socMOF and Co(bdp) that have been reported in the literature.15,28–30 Although the obtained capacity for our best performing MOF-GO nanocomposite (HKUST-1@rGO) is lower than the targeted DOE capacity, it is still among the highest reported capacities reported so far. As evident, the HKUST-1@rGO exhibited much better storage performance than our bare HKUST-1 which may 19 ACS Paragon Plus Environment


be attributed to the synergistic effect of rGO on the textural properties through creation of new pores, and adsorption of methane on graphene sheets as well. Moreover, the better performance of rGO nanocomposite than functionalized (fGO) and bare GO nanocomposites may be due to the presence of more surface defects in rGO sheets which enhances the uniformity of the MOF crystals during growth process. More in-depth analyses however need to be performed to thoroughly understand such behavior and verify this hypothesis. 220

CH4 Deliverable Capacity (cm3(STP)/cm3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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MOF-519 200

UTSA-76

MOF-905 Co(bdp)

180

HKUST-1@rGO NU-111

soc-MOF 160

HKUST-1 140

MOF-74 120 190

200

210

220

230

240

250

CH4 Total Uptake (cm

3

260

(STP)/cm

270 3

280

290

)

Figure 8. Total high pressure CH4 adsorption isotherm for different adsorbents.

The effect of temperature on methane uptake over HKUST-1@rGO is demonstrated in Figure 9a. Raising temperature from 25 to 40 °C led to 12% reduction in total methane uptake at 65 bar and further increase to 55 °C reduced the capacity to 222 cm3(STP)/cm3. Furthermore, single-site Langmuir isotherm model (equation S1, Supporting Information) was used to fit the high-pressure isotherms and as evident, the model could adequately fit the experimental data with R2 of 0.999. A similar trend was observed for HKUST-1@GO and HKUST-1@fGO materials, as can be seen in Figure S2, Supporting Information. To better assess the nature of 20 ACS Paragon Plus Environment

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interactions between methane and adsorbents, the isosteric heat of adsorption (Qst) for CH4 over three composites was directly measured by the BELSORP-HP instrument and the profiles as a function of CH4 loading are shown in Figure 9b. It should be mentioned here that the instrument uses excess loading to calculate the Qst, so to verify the estimations, we estimated the Qst values from the total loading values using Clausius-Clapeyron equation (S2, Supporting Information) and obtained the exact trend shown in Figure 9b. At zero-coverage, a maximum Qst of 18.7 kJ/mol was obtained for HKUST-1@rGO and it experienced a drop at higher CH4 loadings and reached 12.0 kJ/mol at 242 cm3(STP)/cm3. For HKUST-1@GO and HKUST@fGO, the estimated Qst values were smaller than those of HKUST-1@rGO over the pressure range studied, and the zero-coverage Qst values for these materials were found to be 15.8 and 14.5 kJ/mol, respectively. It has been shown that a cooperative interplay between Qst, the accessible surface area, porosity, and pore topology determines the methane storage or deliverable capacity in porous adsorbents.3 The optimal Qst values that lead to deliverable capacities greater than the DOE target value are not typically high (< 20 kJ/mol).38 To yield a high deliverable capacity, the affinity for methane in the adsorbent framework must be strong enough to store a large amount at the charging pressure, yet weak enough to release most of the methane at the discharge pressure.43 Although the heat effects could be substantial, the composites are expected to exhibit a relatively good heat transfer properties owing to relatively high heat capacity and thermal conductivity of GO.



300

20

(a)

HKUST-1@rGO

(b)

18

250

16

200

Qst (kJ/mol)

CH4 Total Uptake(cm3(STP)/cm3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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150 100

25 °C 40 °C 55 °C

50

2

R = 0.9993 R2 = 0.9996 R2 = 0.9997

14 12 10 8

HKUST-1@rGO HKUST-1@GO HKUST-1@fGO

6 4

0 0

10

20

30

40

Pressure (bar)

50

60

70

0

50

100

150

200

250


Figure 9. (a) Total high pressure CH4 adsorption isotherms at 25, 40, and 55 °C for HKUST1@rGO, and (b) isosteric heat of adsorption of CH4 over composites for zero-coverage uptake up to 240 cm3(STP)/cm3.

4. Conclusion In this study, we reported the development of a series of hybrid nanocomposites comprising of HKUST-1 and GO in different forms (bare, reduced, and COOH-functionalized) and the assessment of their performance in methane storage application. Demonstrating similar characteristics to pristine HKUST-1, the characterization measurements thoroughly revealed the successful formation of MOF-GO nanocomposites with an unaffected MOF formation within the GO layers. Both nanocomposites consisting of 10 wt% GO or rGO showed substantial enhancement in methane deliverable capacity and total uptake in comparison to the bare HKUST-1. It was also found that the functionalization of GO with carboxylic groups prior to composite formation does not lead to any noticeable improvement in storage performance of the composites and the MOF-fGO composites exhibited a similar methane storage performance to their parent MOF. In addition, the estimated heat of adsorption at zero coverage was 18.7 kJ/mol for MOF-rGO which is in the typical range for methane storage adsorbents. The findings


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reported in this investigation highlight the suitability of MOF-GO composites as ANG adsorbents for practical applications. Supporting Information The Supporting Information covers additional CH4 adsorption isotherms for MOF-GO nanocomposites with different GO content and at different temperatures, single-site Langmuir and Clausius-Clapeyron equations, and isotherm fitting parameters.

Author Information Corresponding Author *Email: [email protected] ORCID: 0000-0002-4214-4235

Acknowledgement The funding from Innovation at Missouri S&T (Miner Tank) is acknowledged. Q. AlNaddaf would also like to acknowledge The Higher Committee for Education Development in Iraq-HCED for financially supporting his PhD study. The authors thank Materials Research Center (MRC) of Missouri S&T for SEM, TEM, and XRD measurements. References (1)

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Graphical Abstract

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