MIL-101(Cr) composite

Journal of CO₂ Utilization 22 (2017) 238–249

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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou

Synthesis, characterization, and CO2 breakthrough adsorption of a novel MWCNT/MIL-101(Cr) composite Naef A.A. Qasema,b, Najam U. Qadira,c, Rached Ben-Mansoura,b, Syed A.M. Saida,b,c, a b c

MARK

⁎

Mechanical Engineering Department, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia King Abdulaziz City for Science and Technology (KACST), Carbon Capture and Sequestration Technology Innovation Center, KFUPM, Dhahran 31261, Saudi Arabia Center of Research Excellence in Renewable Energy, KFUPM, Dhahran 31261, Saudi Arabia

A R T I C L E I N F O

A B S T R A C T

Keywords: Adsorption MIL-101(Cr) MWCNT Characterization Breakthrough Carbon Capture

Carbon dioxide is quantitatively the most significant component which facilitates in the global warming, it is utmost necessary to suppress the rapid increase of CO2 concentration in the atmosphere by means of Carbon Capture and Storage (CCS). Metal-organic framework (MOF) MOFs has recently been developed with the primary objective of CO2 capture from flue gases with minimum energy penalties. MIL-101(Cr) holds the supreme importance amongst such types of MOFs owing to its extraordinary thermal and hydro stability. In this study, a novel composite composed of multi-walled carbon nanotubes (MWCNTs) incorporated in a MIL-101(Cr) framework has been synthesized using a molecular-level mixing process in order to enhance the thermal properties of the base framework as well as augment the CO2 adsorption capacity and separation. The synthesized and activated MWCNT/MIL-101(Cr) composites have been characterized for degree of crystallinity, microstructure, thermal stability, and CO2 and N2 equilibrium adsorption capacity. Actual dynamic behavior of adsorption breakthrough tests have been conducted under ambient conditions (297 K and 101.325 kPa) to address the real adsorptive behavior and to quantify the level of the CO2 adsorption capacity and breakpoint enhancements. Results demonstrate that a substantial improvement in the CO2 adsorption capacity and breakpoint over pristine MIL-101(Cr) is achievable with the 2 wt% MWCNT/MIL-101(Cr) composite with measured optimal enhancements of about 35.94% and 32.11%, respectively.

1. Introduction In the recent decade, environmental pollution is being identified as the most significant factor in the context of environmental health and safety. In this scenario, the major portion of the environmental pollution arises from fossil fuel burning processes in the form of greenhouse gases, including carbon dioxide, nitrogen oxide, and methane. These gases play a continuous and a vital role in facilitating universal environmental hazards like global warming, shore floods, atmospheric heat waves, land droughts, and destruction of cold-marine life. Moreover, the continuous change in climatic conditions are also expected to reduce the world’s gross domestic product by about 5–20% [1]. The increase of the atmospheric temperature was measured to be about 0.74% in the last century and is predicted to reach to about 6.4% at the end of the current century [1]. In this context, carbon dioxide holds the most significant proportion of the flue gases being released into the atmosphere from various sources [2]. Therefore, a considerable amount of effort has been made by scientists, institutions, countries, and environmental organizations to minimize and control the amount ⁎

of CO2 emission in the atmosphere. The main source of CO2 emission are considered to be the fossil fuel combustion processes whereby the fossil fuel appears to be the most dominating and pollution-free source of electricity on a global scale and therefore plays a vital role for a comfortable and sustainable lifestyle. Therefore, the only currently available feasible solution to continue fossil fuel utilization to meet energy demands with minimized CO2 emission is carbon capture and storage, with the secondary aim of mitigating the global climate change. The on-going research in the field of Carbon Capture and Storage (CCS) is gaining momentum every day. A vast majority of researchers have already investigated CO2 separation and storage, using both experimental and simulation methods, with the primary objective of developing novel adsorption materials or adsorbents for this purpose [3]. The foremost advantage of using adsorption as a means of CO2 separation is the ease of regeneration of the adsorbent material by applying heat and/or decreasing the operating pressure [4]. Activated carbons and zeolites are currently the most commonly exploited adsorbents in the context of research based on CO2 separation and storage.

Corresponding author at: Mechanical Engineering Department, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran 31261, Saudi Arabia. E-mail address: [email protected] (S.A.M. Said).

http://dx.doi.org/10.1016/j.jcou.2017.10.015 Received 4 December 2016; Received in revised form 17 July 2017; Accepted 16 October 2017 2212-9820/ © 2017 Elsevier Ltd. All rights reserved.

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Since the last one and a half decade, carbon nanotubes (CNTs) have also attracted a considerable attention of research groups engaged in carbon capture and sequestration in view of the chemical affinity exhibited by CNTs towards CO2 uptake and selectivity [24–28]. More specifically, the attachment of amino- functionalities to the sidewalls of CNTs has resulted in a considerable enhancement in the intrinsic CO2 adsorption capacity measured for CNTs [29–32]. Su et. al [33] have studied the effect of functionalization of CNTs with 3-aminopropyltriethoxysilane (APTES) groups on the CO2 uptake and selectivity. They found that the co-existence of moisture increased the CO2 adsorption capacity measured for APTES-functionalized CNTs. The CO2 adsorption capacity was measured to be about 2.59 mol/kg at 293 K for APTESfunctionalized CNTs which clearly confirms functionalization to be an effective tool for improving the intrinsic CO2 adsorption potential of CNTs. Incorporating CNT and lithium ions with MOF Cu3(btc)2 resulted in improving the CO2 capacity by about 305% compared with those of the base adsorbent (Cu3(btc)2) [34]. Also, the adsorption capacity of CO2 at high pressure and room temperature (10 bar and 298 K) was improved by adding MWCNT to MIL101 [35]. A considerable number of research attempts based on CO2 capture and separation have been conducted so far in terms of breakthrough, pressure swing adsorption and temperature swing adsorption which include both experimental method and numerically validated simulations [36–50]. The maximum amount of research focus has, however, been invested on the lab-scale development of novel adsorbent materials designed for achieving high CO2 capture capacity as well as selectivity. In this context, the poor thermal conductivity exhibited by a vast majority of these newly-synthesized adsorbents has been experienced as a major obstacle in improving the CO2 capture capacity of these materials. More specifically, a very limited number of research attempts have focused on the improvement of CO2 capture and/or separation via improving the thermal properties of the base adsorbent. In this context, the current study aims at investigating the effects of incorporating multi-walled carbon nanotubes (MWCNTs) inside MIL101(Cr) with the aim of improving the thermal properties of the framework and investigating the influence of MWCNT addition on the CO2 uptake and adsorption breakpoint of the resulting MWCNT/MIL101(Cr) composites. The synthesized and activated MWCNT/MIL101(Cr) composites have been characterized for degree of crystallinity, microstructure, thermal stability, CO2 adsorption isotherms, and CO2 breakthrough characteristics. The effects of MWCNT addition on the CO2 uptake and breakthrough capacity have been documented, and the most favorable proportion of MWCNTs inside MIL-101(Cr) resulting in the most optimum combination of these two characteristics has been proposed.

More specifically, zeolites have been researched to a larger extent than activated carbons for carbon capture and hydrogen storage in applications involving relatively lower operating pressures [5,6], whereas carbon based materials, including activated carbons, have been preferred over zeolites for high pressure applications [6,7]. However, the noticeable advantages of carbon-based materials over zeolite-based adsorbents include cost-effectiveness, stability towards exposure to water vapor, lower energy required for regeneration due to lower heat of adsorption, and ease of production on a commercial scale [8]. Polyethylenimine (PEI)-impregnated millimeter-sized mesoporous carbon spheres have been developed and studied for CO2 post-combustion capture [9]. The maximum CO2 uptake was measured to be about 163.4 mg/g at 0.15 bar and 75 °C, which however was observed to decline during the cyclic adsorption/desorption experiment. The advantage of zeolite-based adsorbents over activated carbons is the relatively higher CO2 adsorption capacity, especially at lower adsorption pressures. However, the CO2 uptake is greatly reduced in case of CO2/H2O mixture as and requires significantly higher regeneration energy [10,11]. A novel class of mesoporous materials have been discovered almost two decades ago which are referred to as metal-organic frameworks (MOFs) in accordance with an organic portion (linker) and an inorganic constituent (metal ion clusters) coexisting in the same structure [12]. In the context of CO2 adsorption, MOF-2 appeared as the first candidate to be evaluated for CO2 uptake as well as selectivity [12]. The highest CO2 uptake was, however, reported for MOF-177 as 1470 mg/g at 35 bar [13]. In the subsequent years, a vast majority of MOFs have been synthesized by research communities worldwide with the aim of designing the most optimum framework topology in order to maximize the CO2 uptake as well as selectivity simultaneously. More specifically, a total of about 54,341 MOFs designed for these applications have been recorded in the Cambridge Structure Database before July 2015 [14]. A certain class of MOFs incorporating functionalized and open metal sites have shown high separation efficiency at ambient pressure like HKUST-1, Mg-MOF-74 and NH2MIL-53 (Al) [15]. With regards to maximum uptake, a few MOFs have shown a reasonably high CO2 adsorption capacity such as CPM-5, MIL-53(Al), UMCM-150 and Ni-STA12, while others have been evaluated for comparatively lower uptakes like MOF-5 and MOF-177 [15]. A nickel-based MOF, Ni/DOBDC, was also investigated for CO2 capture and was measured to exhibit a fairly high adsorption capacity [16]. MIL-101(Cr) is a chromium(III) terephthalate and is characterized by a higher specific surface area as well as water-stability than a wide variety of MOFs reported in literature [17]. In addition, it has been reported to show a strong resistance towards contaminants contained in flue gases such as H2O, NO and SO2, a relatively lower energy required for regeneration, and ease of commercial production [17]. These features have helped to prioritize the use of MIL-101(Cr) as an attractive adsorbent material for CO2 capture and storage [17]. A recent investigation aimed at improving the CO2 uptake and separation efficiency of MIL-101(Cr) by incorporation of polyethyleneimine (PEI) in the framework resulting in PEI/MIL-101(Cr) composites. The results demonstrated a relatively lower adsorption capacity of CO2 for PEI/ MIL-101(Cr) composites compared to that measured for pristine MIL101(Cr); however, the selectivity of CO2 over N2 was significantly improved [18]. Functionalizing Mg-DOBDC (MOF-74) with ethylene diamine (ED) improved both CO2 adsorption capacity and recycling stability under low CO2 partial pressures [19]. Supported Layered Double Hydroxides by Graphene Oxide exhibited an increase in the CO2 adsorption capacity by about 60% more than the pure Layered Double Hydroxides [20]. N-doped zeolite-templated carbon could adsorb about 4.4 mmol/g of pure CO2 at ambient conditions (1 bar and 298 K) [21]. Poly(allylamine)–silica composites showed a good capacity for CO2 separation from flue gases and air [22]. Incorporating titanium with DMS-TN showed almost twice CO2 adsorption capacity and more stability than those of pristine adsorbent (DMS-TN) [23].

2. Experimental work methodology For the synthesis of MIL-101(Cr), the method proposed by Férey et al. [51] has been adopted. Briefly, 4 g chromium nitrate nonahydrate (Cr(NO3)3.9H2O), 1.66 g 1,3 benzenedicarboxylic acid (BDC) and 47.4 ml de-ionized water were added to a 125 ml Teflon-liner which was sealed inside a stainless steel autoclave and kept at 220 °C for 8 h. The autoclave was cooled slowly to room temperature, after which the light green solid was recovered using centrifugation at 8000 RPM for 45 min. In order to remove the guest molecules, the as-synthesized MIL101(Cr) was washed twice with 90 ml deionized water and further purified 5 times using an 80% aqueous solution of ethanol, till the decanted solvent following centrifugation became completely colorless. The green solid was then immersed in 30 mM aqueous NH4F solution and stirred at 60 °C for 10 h (1 g:150 ml). The suspension was centrifuged, after which the solid was washed 5 times with deionized water at 60 °C. The green solid was then washed three times with 70 ml DMF, and 5 times with 75 ml deionized water, and finally dried in air at 75 °C for 2 days and 95 °C for 2 days. The first step involved in the synthesis of MWCNT/MIL-101(Cr) 239

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TEM samples were prepared by placing drops of the as-synthesized MOF, suspended in ethanol on carbon-coated 200 mesh Cu grids. Images were collected using an accelerating voltage of 120 kV.

composites is the acid-functionalization of MWCNTs in order to attach negatively charged carboxyl (COOH−) groups on their sidewalls. Typically, 3 g MWCNTs (Cheap Tubes Inc., Cambridgeport, VT, U.S.A) were first dispersed in 200 ml concentrated HNO3 using ultrasonication. The mixture was then transferred to a 250 ml round bottom flask equipped with a condenser and was refluxed at 120 °C for 48 h. After cooling to room temperature, the mixture was diluted with 500 ml deionized water and then vacuum filtered through a 2.5 μm polymeric membrane. The filtered cake was washed repeatedly with deionized water till the pH of the filtrate reached approximately 5. The filtered cake was dried at 80 °C in air for 24 h and ground into a fine powder. For the synthesis of MWCNT/MIL-101(Cr) composites, 4 g Cr (NO3)3.9H2O and a pre-determined amount of functionalized MWCNTs (60 mg, 120 mg, 180 mg, and 240 mg) were mixed thoroughly in solidstate until a uniform color of the mixture was achieved. 5 ml of deionized water was then added periodically to the mixture and the resulting paste was ultrasonicated till the water was completely vaporized. The dried paste, along with 1.66 g BDC and 47.4 ml de-ionized water, were then transferred completely to a 125 ml Teflon-lined autoclave which was kept at 220 °C for 8 h. The post-synthesis activation procedure was exactly the same as the one adopted for unmodified MIL101(Cr). The in-situ synthesis of MWCNT/MIL-101(Cr) composites, starting with a single representative MWCNT, is summarized schematically in Fig. 1. The weight fractions of MWCNTs in each of the synthesized MWCNT/MIL-101(Cr) composites were determined using Elemental Analysis (EA). Sample designations were based according to the weight percentage of MWCNTs in MIL-101(Cr) as: MIL-101(Cr), 2 wt% MWCNT/MIL-101(Cr), 4 wt% MWCNT/MIL-101(Cr), 6 wt% MWCNT/ MIL-101(Cr), and 8 wt% MWCNT/MIL-101(Cr).

2.2. Powder X-ray diffraction (PXRD) analysis PXRD analyses were performed using a Rigaku UltimaIV Multipurpose diffractometer equipped with Ni-filtered Cu Kα radiation. Samples were packed densely in a 0.5 mm deep well on a zero-background holder. Programmable divergence slits were used to illuminate a constant length of the samples (8 mm), thus preserving the constant volume assumption. The operating power of the diffractometer was set at 45 kV and 40 mA, and the diffraction data were collected between 3–50° (2θ) with a total scan time of 3 h. PXRD patterns for the residues recovered after thermogravimetric analysis, as well as the MOF samples subjected to cyclic water ad-/desorption tests, were obtained on a Bruker D8 Advance Multipurpose diffractometer equipped with a Nifiltered Cu Kα radiation. The operating power of the diffractometer was set at 35 kV and 25 mA, and the diffraction data were collected between 10 and 60° (2θ) with a step-size of 0.02046°, and 0.25 s per step. 2.3. Fourier transform infrared spectroscopy (FTIR) FTIR spectra were recorded with KBr pellets on a Thermo Fisher Scientific Nicolet 6700 FTIR spectrometer. 2.4. Thermogravimetric analysis (TGA) TGA was carried out on a TA Q500 thermal analyser under air flow with a heating rate of 5 °C min−1. Powdered samples were heated overnight at 120 °C to remove moisture prior to performing TGA measurements.

2.1. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM)

2.5. Gas physisorption measurements SEM was performed on a TESCAN LYRA3 FEG microscope. SEM samples were prepared by placing drops of the as-synthesized MOF suspended in acetone on Cu tapes. The solvent was allowed to evaporate before the images were obtained at an accelerating voltage of 5 kV and a working distance of 5–6 mm with a standard secondary electron detector. TEM was performed on a JEOL JEM-2100F/HR microscope.

The first step in the physisorption measurements for CO2 and N2 is the sample degassing in order to remove any guest molecules within the pores of each material. Typically, 50–200 mg of each sample was transferred to pre-weighed empty sample cell with a 9 mm diameter and degassing was conducted at 150 °C under vacuum for about 17 h on Fig. 1. In-situ synthesis of MWCNT/MIL-101(Cr) nanocomposite, starting with a single representative MWCNT: (a) MWCNT carboxylation, (b) Dissociation of hydrated Cr-salt in deionized water, (c) Sonication-assisted electrostatic attraction of negatively charged carboxyl groups on MWCNT and Cr3+ ions in aqueous solution, (d) Hydrothermal synthesis of MIL-101(Cr) crystals on the surface of MWCNT.

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Fig. 2. Schematic diagram of CO2/N2 adsorption breakthrough setup.

The CO2 adsorption capacity of the MWCNT/MIL-101(Cr) composite is evaluated using the following equation:

an Autosorb degasser equipped with a turbo molecular vacuum pump and controlled heat jackets. After the completion of the degassing step, the equilibrium gas physisorption isotherms were measured for each gas at the designated temperatures. More specifically, the Brunauer–Emmett–Teller (BET) specific surface area (SBET), average pore radius, and total pore volume were evaluated from the N2 adsorption isotherms at 77 K. Moreover, equilibrium adsorption isotherms for CO2 at different temperatures (273, 298 and 313 K) and for N2 at ambient temperature (298 K) were recorded. The heat of adsorption for CO2 was evaluated using the adsorption isotherms measured at 273, 298 and 313 K in accordance with the Clausius-Clapeyron equation.

qco2 =

Q F C0 m

t

∫0

⎛1 − C(t) Q(t) ⎞ dt − εV C0 C0 Q F ⎠ m ⎝

⎜

⎟

(1)

where qco2 (mmol/g) represents the CO2 uptake, m is the mass of an adsorbent (kg), QF and Q (t) (m3/s) are the input and output volumetric flowrates, C0 and C(t) (mol/m3) are the influent and effluent CO2 concentrations, t (s) is the time, ε is the bed porosity, and V (m3) is the bed volume. 3. Results and discussion

2.6. Breakthrough experiments of binary gas mixture (CO2 + N2) 3.1. Powder X-ray diffraction (PXRD) analysis A dynamic CO2/N2 breakthrough setup was constructed to separate CO2 from a CO2/N2 mixture (representing a flue gas) as shown in Fig. 2. The home-made system is composed of a fixed adsorbent bed column (Inner diameter = 4 mm, Outer diameter = 6 mm and Length = 15 cm) which is filled with the MIL-101(Cr)/MWCNT composite (about 0.3-0.5 g), CO2 and N2 cylinders, two gas regulators with dual pressure gauges and output control valves, two mass flow controllers (one calibrated for CO2 flow and the other calibrated for N2), two check valves, bypass line (for calibrating the mass spectrometer from the input feed gas), bourdon absolute pressure, mass spectrometer (to analyze the output concentration of effluent gases from the bed), heater jacket and vacuum pump (for regeneration purpose), and interconnecting stainless steel valves and tubes to control and regulate the flow of carrier gas within the system. The first step in the operation of breakthrough setup involves the degassing of the activated MWCNT/MIL-101(Cr) composite sample at about 423 K under vacuum for 20 h to remove any guest molecules trapped inside the pores of MIL-101(Cr) framework. The samples were powders with an average particle size about 0.2-0.4 mm. The breakthrough experiments were conducted under ambient conditions (297 K and 101.3 kPa). The flow rate of the feed gas which was a mixture of 20% CO2 and 80% N2 was kept constant at 10 sccm. The full breakthrough capacity of CO2 and N2 was measured by evaluating the ratio of compositions of the downstream gas and the feed gas.

Fig. 3 shows the PXRD profiles of MWCNT/MIL-101(Cr) composite with various weight fractions of MWCNTs. The PXRD profile of acidtreated MWCNTs has also been added as the benchmark. It can be seen that the PXRD pattern of MIL-101(Cr) is in good agreement with the simulated pattern and the one reported in literature for similar method used for synthesis [52]. The incorporation of MWCNTs does not result in any noticeable peak shift or decrease in the crystallinity of the framework, as all the characteristic peaks representative of the MIL101(Cr) structure can also be observed in the patterns shown for each category of MWCNT/MIL-101(Cr) composite. Hence, it can be concluded that the incorporation of MWCNTs up to 8 wt% using an in-situ synthesis method preserves the characteristic lattice structure of the MIL-101(Cr) framework. 3.2. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) Fig. 4 shows the SEM micrographs of as-synthesized MIL-101(Cr) and MWCNT/MIL-101(Cr) composites. The octahedral-shaped crystals characteristic of the MIL-101(Cr) lattice are evident in the figure. The points of location of MWCNTs have been indicated by the use of red arrows. It can be seen that the incorporation of MWCNTs preserves the crystal morphology characteristic of the MIL-101(Cr) lattice. The 241

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and the 8 wt% MWCNT/MIL-101(Cr) composite. Since the electron beam used in TEM transmits through the thickness of the sample, the cross-section of one of the octahedral crystals of MIL-101(Cr) can be clearly seen in Fig. 5(a). Moreover, TEM can also be used to reveal the micropores within the MIL-101(Cr) framework represented by the bright-colored spots uniformly distributed across the entire cross-section of the crystal. The mean dimension of these spots was found to be equal to 3.1 nm, which is in excellent agreement with the average pore diameter of the smaller and larger cages of the MIL-101(Cr) framework [53]. The inset shown in Fig. 5(a) is the selected area diffraction pattern which clearly reveals bright spots arranged in the form concentric rings indicating the periodicity of the lattice structure of as-synthesized MIL101(Cr) crystals. Fig. 5(b) re-confirms the implantation of MWCNTs in the MOF matrix which can be clearly seen as transparent tubular structures spread across the octahedral-shaped crystallites of MIL101(Cr).

3.3. Fourier transform infrared spectroscopy (FTIR) Fig. 6 shows the FTIR spectra for MIL-101(Cr) as well as MWCNT/ MIL-101(Cr) composites. For MIL-101(Cr), the vibrational band observed around 1625 cm−1 indicates the surface-adsorbed water molecules [54]. The bands evidenced around 1404 cm−1 are assigned to the symmetric OeCeO vibrations, indicating the existence of carboxylate groups within the framework. The bands observed between 600 and 1600 cm−1 are attributed to phenyl groups which primarily incorporate the C]C stretching at 1508 cm−1, and the CeH deformation at 1159, 1018, 885 and 751 cm−1, along with a weak signal observed at 3070 cm−1. The narrow and weak bands observed around 749 and 1017 cm−1 are assigned to δ (CeH) and γ (CeH) vibrations of the aromatic rings respectively. The weak bands present within the region of 400–700 cm−1 are attributed to the in-plane and the out-of-plane bending vibrations of eCOOe groups [55]. A signal of medium strength

Fig. 3. PXRD patterns for MIL-101(Cr) and MWCNT/MIL-101(Cr) composites.

MWCNTs can also be seen implanted in the surrounding MIL-101(Cr) matrix, which is a consequence of the molecular-level interaction of the COOH− groups on the sidewalls of MWCNTs and Cr3+ ions in aqueous solution during the in-situ synthesis of the composite. Moreover, the MWCNTs can be clearly seen to be decorated with the MIL-101(Cr) crystals along the portion protruding out of the surrounding matrix as shown in Fig. 4(b), which is also a confirmation of the strong interfacial bond formed between the MWCNTs and the MIL-101(Cr) crystals. Fig. 5 shows the TEM micrographs of as-synthesized MIL-101(Cr)

Fig. 4. SEM micrographs of MWCNT/MIL-101(Cr) composites with various weight fractions of MWCNTs – (a) 0 wt% (b) 2 wt%, (c) 4 wt%, (d) 6 wt% and (e) 8 wt% (Red arrows indicate points of location of MWCNTs). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. TEM micrographs of (a) MIL-101(Cr), and (b) 8 wt% MWCNT/MIL-101(Cr) composite.

Fig. 6. FTIR spectra for MIL-101(Cr) and MWCNT/ MIL-101(Cr) composites.

around 577 cm−1 corresponds to Cr–O vibrations characteristic of the SBUs of the MIL-101(Cr) framework, while the peak observed in the region of 1560–1650 cm−1 is attributable to the vibration of C]O group. Finally, the broad peak spread between 2900 and 3700 cm−1 can be attributed to the water of crystallization and/or the OeH vibration of the carbonyl group. In case of MWCNT/MIL-101(Cr) composites, the peak observed at 3445 cm−1 is attributed to the Oe H stretch within the carboxyl groups (CeOH and O]CeOH) present on the acid treated MWCNTs as shown in Fig. 6. The weakening of the broad peak between 2900 and 3700 cm−1 observed in the spectra of 4–8 wt% MWCNT/MIL-101(Cr) composites indicates the increased hydrophobicity of the framework with increase in the weight fraction of MWCNTs. The absence of any extraneous peaks other than those characteristic of the MIL-101(Cr) framework in the spectra of the composites indicates that the incorporation of MWCNTs using the in-situ synthesis method does not introduce any additional functional groups which are not associated with the intrinsic chemistry of the framework.

350 °C represents the removal of water molecules from the middle-sized framework cages (d = 29 Å) [58]. The final weight loss above 350 °C corresponds to the decomposition of the benzenedicarboxylic acid linker. It can be further observed from Fig. 7(a) that MIL-101-C1, 4 wt% MWCNT/MIL-101(Cr) composite, and 6 wt% MWCNT/MIL-101(Cr) composite show relatively higher hydrophobicity than MIL-101(Cr) as signified by the comparatively lower weight losses observed between 30 and 100 °C, while the 8 wt% MWCNT/MIL-101(Cr) composite depicts lower hydrophobicity than MIL-101(Cr) owing to possible agglomeration of MWCNTs within the framework. Finally, it can be seen that 2 wt% and 4 wt% MWCNT/MIL-101(Cr) composites both show higher thermal stability than MIL-101, while the 8 wt% MWCNT/MIL-101(Cr) composite show comparatively lower thermal stability. In contrast, the TGA conducted in the air atmosphere reveals that all the composite samples show lower hydrophobicity than MIL-101(Cr) for the entire temperature range between 30 and 100 °C, but also lower thermal stability than MIL-101(Cr) for the entire temperature range between 30 and 600 °C. Hence, for applications involving service temperatures above 300–350 °C, the use of 2 and 4 wt% MWCNT/MIL-101(Cr) composites is preferable over MIL-101(Cr) if the primary atmospheric medium is N2; however, none of the MWCNT- incorporated materials can be used in place of MIL-101(Cr) above this temperature range in applications necessitating the use of air atmosphere during service.

3.4. Thermogravimetric analysis (TGA) Fig. 7 shows the TGA curves for MIL-101(Cr) and MWCNT/MIL101(Cr) composites in both nitrogen and air atmospheres. It can be seen that, for the curves measured in N2 atmosphere, all the samples are stable within the range of 300–350 °C. The TGA curves exhibit three major weight losses spread between 30 and 600 °C. An initial weight loss between 30 and 100 °C corresponds to the removal of guest gas/ water molecules trapped inside the large cages within the framework (d = 34 Å) [56,57]. The second weight loss spread between 100 and

3.5. Adsorption equilibrium isotherms of carbon dioxide and nitrogen The N2 physisorption isotherms for MIL-101(Cr) and the MWCNT/ 243

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Fig. 7. TGA of MIL-101(Cr) and MWCNT/MIL-101(Cr) composites in (a) nitrogen and (b) air.

measured earlier for CO2. In other words, all the samples have been noticed to exhibit preferential selectivity of CO2 over N2. Fig. 12 depicts the variation of heat of adsorption for CO2 and N2, Qst, against the instantaneous uptake for MIL-101(Cr) and MWCNT/ MIL-101(Cr) composites. For MIL-101(Cr), the Qst values are observed to exhibit a more or less linear correlation with the instantaneous CO2 uptake as shown in Fig. 12. In contrast, the MWCNT/MIL-101(Cr) composites, after a steep initial increase in Qst (CO2) till an uptake of almost 0.2 mmol/g, result in a more or less constant value of Qst of about 23 kJ/mol for all values of instantaneous uptake higher than 0.2 mmol/g. In a nutshell, the pristine MIL-101(Cr) results in increasingly higher heat of adsorption values for CO2 than MWCNT/MIL101(Cr) composites with steadily increasing values of the instantaneous uptake when the Clausius-Clapyeron equation is employed for the heat of adsorption calculation. For N2, the adsorption heat values are almost less than the half values of those of CO2.

Table 1 Pores characterization of the MWCNT/MIL-101(Cr) composites for N2 at 77 K. Characterizations

SBET (m2/ g)

Pore volume (cc/g)

Average pore radius (Å)

MIL-101(Cr) 2 wt% MWCNT/MIL-101(Cr) 4 wt% MWCNT/MIL-101(Cr) 6 wt% MWCNT/MIL-101(Cr) 8 wt% MWCNT/MIL-101(Cr)

3745 3146 4004 3307 2446

1.95 1.63 2.07 1.77 1.26

10.5 10.4 10.3 10.7 10.3

MIL-101(Cr) composites have been measured at 77 K. Table 1 lists the important porosity-related parameters evaluated from the N2 adsorption/desorption data for MIL-101(Cr) and each of the four MWCNT/ MIL-101(Cr) composites. The highest BET surface area was measured for 4 wt% MWCNT/MIL-101(Cr) of about 4004 m2/g, followed by pristine MIL-101(Cr) which showed almost 3750 m2/g. This value of the pristine adsorbent is close to the reported values for MIL-101(Cr) [54,56]. The lowest BET surface area was evaluated for 8 wt% MWCNT/MIL-101(Cr) composite showing almost 35% lower surface area than pristine MIL-101(Cr). In contrast, the highest total pore volume, at a relative pressure of P/P0 = 0.95, was measured for the 4 wt. % MWCNT/MIL-101(Cr) composite of about 2.1 cc/g, which is an improvement of 5.9% over pristine MIL101(Cr), while the three remaining composites exhibited lower pore volume values. The average pore size measured in terms of diameter was determined to be almost the same for all the samples around 20.9 Å. Hence, it can be deduced from the data shown in Table 1 that the addition of MWCNTs does not result in a well-defined trend concerning its influence on the porosity-related parameters evaluated for the MWCNT/MIL-101(Cr) composites. The CO2 adsorption/desorption isotherms for MIL-101(Cr) and the MWCNT/MIL-101(Cr) composites, measured at 273, 298 and 313 K, are shown in Figs. 8–10. It is obvious that the adsorption uptake increases more or less linearly with increasing adsorption pressure. However, as expected, an increase in the measurement temperature shows an adverse effect on the recorded uptakes for each material. As obvious from Figs. 8–10, the highest CO2 uptake has been measured for the 2 wt.% MWCNT/MIL-101(Cr) composite at 273 K. The pristine MIL-101(Cr) resulted in the second highest uptake, followed by 4, 6, and 8 wt% MWCNT/MIL-101(Cr) composites respectively. For instance, the uptake amounts recorded at 298 K and 20 kPa are observed to be about 1.2, 1, 0.65, 0.58, and 0.51 mmol/g for 2, 4, 6, and 8 wt% MWCNT/MIL101(Cr) composites, respectively. The N2 adsorption isotherms for MIL-101(Cr) and MWCNT/MIL101(Cr) composites, measured at 298 K, are displayed in Fig. 11. It is evident that a loading of 2 wt.% MWCNTs in MIL-101(Cr) exhibits the largest uptake amount, followed by pristine MIL101(Cr), 4 wt.%, 6 wt. %, and 8 wt.% MWCNT/MIL-101(Cr) composites, respectively. It is worth mentioning here that the same sequence was previously observed regarding CO2 uptake at 298 K, except the fact that the maximum uptake measured for N2 is observed to be significantly smaller than that

3.6. Experimental adsorption breakthrough test for MIL-101(Cr) and MWCNT/MIL-101(Cr) composites In order to quantify the improvements of in CO2 adsorption capacity as well as breakpoint during CO2/N2 separation as a result of the incorporation MWCNTs inside MIL-101(Cr), CO2 breakthrough experiments have been performed. In a typical procedure, predetermined amounts of MIL-101(Cr) and MWCNT/MIL-101(Cr) composite samples are first transferred to a stainless steel tube (Length L = 14 cm, Inner diameter ϕ = 4 mm). All breakthrough experiments have been performed at ambient temperature of 297 K. The experimentally measured CO2 and N2 adsorption breakthrough curves for MIL-101(Cr) and MWCNT/MIL-101(Cr) composites are shown in Fig. 13. A typical volumetric flow rate measured at the bed outlet for MIL-101(Cr) is shown in Fig. 13(a). Fig. 13(a) indicates that there was no gas left the bed in the first period (< 0.92 min). The outlet flow rate values increased as the N2 appeared in the outlet. When both the CO2 and N2 have left the bed, the value of the flow rate increased till it reached the inlet flow rate value of 10 sccm. The outlet concentration ratios calculated each of these two gases have been plotted against the measurement time Fig. 13(b). In general, it was observed for all the tested samples that the concentration ratio evaluated for CO2 at the bed outlet keeps constant at zero for the first 2.5–3.4 min (Fig. 13(b)), whereas the concentration ratio for N2 increased up to about 1.3 owing to the absence of CO2 which was pre-adsorbed into the MWCNT/MIL101(Cr) composite adsorbent bed. Following the first 2.5-3.4 min of measurement time, the CO2 concentration ratio was observed to increase up to 1, whereas the concentration ratio of N2 was evaluated to gradually drop to a value close to 1. The optimal value of the breakpoint, which is defined as the time at which the concentration ratio at the bed outlet is evaluated to be less than 5%, was measured to be about 3.38 min for 6 wt.% MWCNT/MIL-101(Cr) composite as shown in Fig. 13(b). This is followed by the value measured for 8 wt.% MWCNT/ MIL-101(Cr) composite of about 3.36 min, and then by that measured 244

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Fig. 8. CO2 adsorption isotherms of MWCNT/MIL-101(Cr) composites at 273 K.

MIL-101(Cr) 2 wt % MWCNT/MIL-101(Cr) 4 wt % MWCNT/MIL-101(Cr) 6 wt % MWCNT/MIL-101(Cr) 8 wt % MWCNT/MIL-101(Cr)

5 4 3 2 1 0 0

20

40

60

80

100

the 4, 6 and 8 wt.% MWCNT/MIL-101(Cr) composites for which the corresponding improvements in adsorption capacity and breakpoint values over pristine MIL-101(Cr) have been evaluated to be 31.73% and 32.05%, 22.71% and 22.06%, and 10.07% and 4.71%, respectively. It is worth mentioning here that each of the four MWCNT/MIL-101(Cr) composites have already been characterized for lower values of heat of adsorption for CO2 in comparison with pristine MIL-101(Cr) as shown earlier in Fig. 12, which theoretically implies that each of these composites should not only exhibit higher CO2 uptake values than pristine MIL-101(Cr), but also require comparatively lower energy for regeneration process (recycling recovery). The observed enhancement in the CO2 adsorption capacity and breakpoint is primarily attributable to an improvement in the thermal properties of MIL-101(Cr) framework upon the incorporation of MWCNTs [59–61]. In a similar fashion, a MWCNT-incorporated 13X/ CaCl2 composite has been reported to show higher thermal conductivity and adsorption capacity values than those measured for 13X/CaCl2 and pure 13X [59,60]. More recently, a comparatively higher water-stability and adsorption capacity have also been recorded for MIL101-68 (Al) following the incorporation of MWCNT into the framework [61].

for pristine MIL-101(Cr) of about 3.2 min. However, since these breakpoint values correspond to a variable adsorbent mass in accordance with the added proportion of MWCNTs in each of the four composite samples, the re-calculated normalized optimal CO2 adsorption breakpoint for 2 wt% MWCNT/MIL-101(Cr) composite was observed to be about 8.91 min per gram of adsorbent. Accordingly, the corresponding values evaluated for MIL-101(Cr) and 4, 6, and 8 wt.% MWCNT/MIL-101(Cr) composites were recorded to be 6.7, 8.9, 8.2, and 7.1 min per gram of adsorbent, respectively. In order to evaluate the improvement in adsorption uptake by virtue of MWCNT incorporation in MIL-101(Cr), the adsorbed amounts of CO2 have been calculated from the experimental breakthrough curves using Eq. (1). The maximum CO2 uptake for MIL-101(Cr) calculated from the respective breakthrough curve was estimated to be about 0.80 mmol/g at 0.2 molar fraction of 10 sccm, 297 K, and 101 kPa. The maximum CO2 uptakes along with the adsorption breakpoint ratios for MIL101(Cr) as well as each of the four MWCNT/MIL-101(Cr) composites are displayed in Fig. 14. As evident, each of the four composites exhibit a substantial improvement over pristine MIL-101(Cr) with regards to both the adsorption capacity and the adsorption breakpoint ratio values. More specifically, the most optimum combination of adsorption capacity and breakpoint ratio value have been evaluated for 2 wt.% MWCNT/MIL-101(Cr) composite which has shown an improvement of 35.94% and 32.11% over pristine MIL-101(Cr) for adsorption capacity and breakpoint ratio, respectively. This pair of statistics is followed by

4. Conclusion A novel MWCNT/MIL-101(Cr) composite has been synthesized using a molecular level approach which involves in-situ incorporation of Fig. 9. CO2 adsorption isotherms of MWCNT/MIL-101(Cr) composites at 298 K.

MIL-101(Cr) 2 wt % MWCNT/MIL-101(Cr) 4 wt % MWCNT/MIL-101(Cr) 6 wt % MWCNT/MIL-101(Cr) 8 wt % MWCNT/MIL-101(Cr)

3.5 3 2.5 2 1.5 1 0.5 0 0

20

40

60

80

100

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Fig. 10. CO2 adsorption isotherms of MWCNT/MIL-101(Cr) composites at 313 K.

MIL-101(Cr) 2 wt % MWCNT/MIL-101(Cr) 4 wt % MWCNT/MIL-101(Cr) 6 wt % MWCNT/MIL-101(Cr) 8 wt % MWCNT/MIL-101(Cr)

2.5 2 1.5 1 0.5 0 0

20

40

60

80

100

Fig. 11. N2 adsorption isotherms of MWCNT/MIL-101(Cr) composites at 298 K.

MIL-101(Cr) 2 wt % MWCNT/MIL-101(Cr) 4 wt % MWCNT/MIL-101(Cr) 6 wt % MWCNT/MIL-101(Cr) 8 wt % MWCNT/MIL-101(Cr)

0.3 0.25 0.2 0.15 0.1 0.05 0 0

20

40

60

80

100

Fig. 12. CO2 and N2 heat of adsorption (Qst) for MWCNT/MIL-101(Cr) composites.

30 25 CO2

20 15

Q

MIL-101(Cr) 2 wt% MWCNT/MIL-101(Cr)

10

4 wt% MWCNT/MIL-101(Cr)

N2

6 wt% MWCNT/MIL-101(Cr)

5

8 wt% MWCNT/MIL-101(Cr) 0 0

0.5

1

1.5

2

preserved upon the incorporation of MWCNTs, and that the MWCNTs are properly implanted into the MOF crystals in accordance with the protocol proposed for the synthesis of MWCNT/MIL-101(Cr) composites. The powder X-ray diffraction patterns as well as the Fourier Transform Infrared spectra measured for each of the four composites do not include any extraneous peaks, noticeable peak shifts, or chemical functionalities indicating that the characteristic MIL-101(Cr) crystal

MWCNTs within the MIL-101(Cr) framework. The as-synthesized and activated MOF materials containing 0, 2, 4, 6, and 8 wt% MWCNTs have been characterized for degree of crystallinity, microstructure, thermal stability, intrinsic porosity, CO2 adsorption capacity and separation, and dynamic adsorption breakthrough characteristics. Preliminary characterization conducted on the sample materials indicates that the intrinsic morphology of the MIL-101(Cr) framework is 246

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Fig. 13. CO2/N2 breakthrough curves at 297 K and 101.3 kPa as: (a) typical outlet flow rate for MIL-101(Cr) sample, (b) concentration ratios of MWCNT/MIL-101(Cr) composites. The adsorbent masses are 0.475, 0.33, 0.32, 0.41, and 0.477 g for MIL-101(Cr), 2 wt% MWCNT/MIL-101(Cr), 4 wt% MWCNT/MIL-101(Cr), 6 wt% MWCNT/MIL-101(Cr), and 8 wt% MWCNT/MIL-101(Cr), respectively.

Outlet Ňowrate (sccm)

12 10 8 6 4 2 0 0

2

4

6

8

10

12

14

Time (min)

1.4 1.2 N2

1 0.8

MIL-101(Cr)

0.6

2 wt%MWCNT/MIL-101(Cr) 0.4

CO2

4 wt%MWCNT/MIL-101(Cr) 6 wt%MWCNT/MIL-101(Cr)

0.2

8 wt%MWCNT/MIL-101(Cr)

0 0

2

4

6

8

10

12

Fig. 14. Carbon dioxide adsorption capacity (cubic bars) and breakpoint (cylindrical bars) improvements (percent) for MWCNT/MIL-101(Cr) composites over pristine MIL-101(Cr) measured at 297 K and 1.013 bar (gas mixture pressure).

incorporation of MWCNTs resulting in surface area reduction of about 15.6%, 44.6%, 11.3% and 34.4% measured for 2, 4, 6, and 8 wt% MWCNT/MIL-101(Cr) composites, respectively. Equilibrium adsorption isotherms of CO2 measured at 273, 298, and 313 K, and N2 adsorption isotherms measured at 298 K confirm that the highest adsorption capacities for each of these two gases are exhibited by the 2 wt% MWCNT/MIL-101(Cr) composite, followed by the pristine MIL-101(Cr). The performance evaluation of the synthesized MWCNT/MIL101(Cr) composites has been achieved through the measurement of time-variant CO2 breakthrough curves, which have revealed a significant improvement in CO2 adsorption capacity as well as adsorption

lattice and chemical structure are unaffected by the incorporation of MWCNTs using the proposed method of synthesis. Finally, the thermogravimetric analysis conducted in nitrogen atmosphere suggests an MWCNT incorporation up to 4 wt% resulting in a higher thermal stability than pristine MIL-101(Cr) and a comparatively lower stability measured for composites containing higher MWCNT weight percentages. The porosity characterization data obtained from the nitrogen physiosorption isotherms measured at 77 K for the synthesized and activated MOF materials reveal the highest Brunauer–Emmett–Teller specific surface area evaluated for pristine MIL-101(Cr), with the 247

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breakpoint due to the incorporation of MWCNTs in the MIL-101(Cr) framework. The most optimum combination of these characteristics has been observed for an incorporation of 2 wt% MWCNTs in MIL-101(Cr) which has resulted in measured improvements of about 35.94% and 32.11% over pristine MIL-101(Cr) for CO2 adsorption capacity and breakpoint, respectively.

[24] [25]

[26]

Acknowledgments [27]

The technical and financial support received from King Abdulaziz City for Science and Technology (KACST), Carbon Capture and Sequestration Technology Innovation Center (CCS-TIC #32-753) under Project CCS10, and the Center of Research Excellence in Renewable Energy at King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Saudi Arabia, is highly acknowledged.

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