CH4 Selectivity by

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High CO2 Adsorption Capacity and CO2/CH4 Selectivity by Nanocomposites of MOF-199 Samira Salehi and Mansoor Anbia* Research Laboratory of Nanoporous Materials, Faculty of Chemistry, Iran University of Science and Technology, Farjam Street, Narmak, P.O. Box 16846-13114, Tehran, Iran ABSTRACT: A nanocomposite of multiwalled carbon nanotubes (MWCNTs) and the metal−organic framework MOF-199, denoted as CNT@MOF-199, has been synthesized. It was prepared by the incorporation of multiwalled carbon nanotubes into MOF-199, which exhibits an increased micropore volume, enhanced gas storage capacity, and improved stability compared to MOF-199. The adsorption capacities of gas molecules in MOF-199 and CNT@MOF-199 were found to decrease with increasing temperature, indicating the exothermic nature of the adsorption process. Novel functionalized adsorbents were synthesized by impregnation of various amounts (10, 20, and 30 wt %) of piperazine (PZ) on synthetic CNT@MOF-199. Although the surface area, pore size, and pore volume decreased after modification, CNT@MOF-199/PZ exhibited a higher adsorption capacity and selectivity than CNT@MOF-199 at the pressure under study because of the existence of a great affinity between CO2 molecules as a Lewis acid and the basic amine sites on CNT@MOF-199/PZ. The physicochemical properties of the adsorbents were characterized by N2 adsorption/desorption isotherms, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The highest selectivity was observed at 298 K for CNT@MOF-199/30PZ.

1. INTRODUCTION Gaseous carbon dioxide (CO2) and methane (CH4) are major contributors to the accumulation of greenhouse gases and, consequently, to increased global warming.1,2 Most of the CO2 emissions into the atmosphere result from the combustion of fossil fuels. The burning of fossil fuels supplies the world with 81% of its commercial energy and releases 30 × 10 12 kg of CO2 annually.3 The concentration of CO2 in Earth’s atmosphere is currently 390 ppm, a rise of around 110 ppm since the beginning of the Industrial Revolution.4 So far, the most common method for CO2 capture has been gas absorption, with monoethanolamine (MEA) as the currently applied solvent.5 However, this technique has some disadvantages, such as high degradation rates of the solvent as a result of its reaction with NOx and SO2, equipment corrosion in the presence of oxygen and other impurities, toxicity, and flow problems caused by viscosity. Also problematic are the need to use operating temperatures of less that 50 °C to reduce the substantial volatility of the solvent, the limited cycling capacity, and the greater amount of energy required for regeneration.6 As a result of these constraints, several other separation techniques, such as adsorption using solids, cryogenic distillation, and membrane purification, have been studied.7,8 Among gas separation technologies, adsorption-based separation has become a key gas separation tool in industry owing to its inherent simplicity, low cost, ease of control, and high energy efficiency.9,10 To date, numerous solid sorbents have been reviewed worldwide for CO2 adsorption. By appropriate design, faster CO2 sorption kinetics, higher CO2 working capacity and selectivity, and lower energy requirements for regeneration can be achieved.11,12 Porous materials such as zeolites, metal−organic frameworks (MOFs), mesoporous silica, and carbon materials offer a wide © XXXX American Chemical Society

variety of structures and compositions that are suitable for the adsorption of various gases.13−15 Developments in porous materials have been achieved through advances in MOFs and mesoporous materials, which are among the most important and fastest growing groups of porous materials.16,17 MOFs are crystalline organic−inorganic hybrid structures composed of metal ions or clusters and multidentate organic linkers that are linked together to form extended porous frameworks.18 The wide selection of possible linker and metal units and the possibility of modifying MOFs after synthesis creates a very high number of possible structures.19 Indeed, with respect to the selection of the organic ligand and metal, networks with well-defined porous structures, volumes, and sizes can be synthesized.20 Some attractive motifs include brickwall, rectangular grid, bilayer, herringbone, ladder, diamondoid, honeycomb, and octahedral geometries. The majority of MOFs have three-dimensional structures that enclose uniform pores that are interconnected to make ordered frameworks of channels. After removal of the guest species, the integrity of these pores and channels can be maintained. The residual voids in the threedimensional structures then can adsorb other guest molecules. So far, MOFs have found a wide variety of applications, including gas adsorption/storage, separation, sensing, drug delivery, and catalysis.21 Their potential for use increases every year because of the simple modification of MOFs, which makes them an outstanding group of materials for many possible applications. Recently, a wide variety of MOFs have been synthesized and tested for CO2 adsorption, such as those with high-porosity Received: February 2, 2017 Revised: March 26, 2017 Published: March 29, 2017 A

DOI: 10.1021/acs.energyfuels.6b03347 Energy Fuels XXXX, XXX, XXX−XXX

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synthesize a hybrid nanocomposite, denoted as CNT@MOF199. The results obtained indicate that the equilibrium uptake of gas molecules increased after the MOF was functionalized with the MWCNTs. We found that the adsorption capacity and selectivity of CNT@MOF-199 for CO2 over CH4 could be maximized through the introduction of amine groups to selectively interact with CO2. Novel functionalized adsorbents were synthesized by the impregnation of various amounts (10, 20, and 30 wt %) of PZ on synthetic CNT@MOF-199. N2 adsorption/desorption isotherms, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) were used to characterize the structures of the prepared adsorbents. It was found that loading PZ into CNT@MOF-199 can significantly enhance the selective CO2 adsorption capacity at ambient temperature and low pressure. After amine modification, the amine groups become effective chemisorption centers for the adsorption of higher amounts of CO2.

frameworks (MOF-5 and MOF-177), hexagonally packed cylindrical channels (MOF-74), square channels (MOF-2), functional-group-modified pores (IRMOF-3), and pores with exposed metal sites (Cu-BTC).22−25 Among the investigated MOFs, MOF-199 (also called Cu-BTC or HKUST-1), which has the general formula Cu3(BTC)2(H2O)3, has been reported to be a highly efficient porous adsorbent for gas adsorption.26,27 MOF199 is constructed from dimer Cu paddle wheels connected to 1,3,5-benzenetricarboxylate units and was structurally characterized by Chui et al.28 for the first time.29 Wang et al.30 previously improved the synthesis process for the scaled-up production of MOF-199 and carried out adsorptive separation studies for some common gases. MOF-199 presents several advantages: high chemical stability, large surface area, open metal sites with Lewis acidity, simple preparation method (i.e., anhydrous conditions are not required), and easily obtained and inexpensive precursors.31,32 Furthermore, the as-synthesized form contains bound solvent molecules (ethanol or H2O) on the axial coordination sites of every Cu2+ metal center, which can be subsequently eliminated in a vacuum at high temperatures to generate open binding sites for guest molecules such as CO2.33 The principal defects of MOFs are their air and moisture sensitivities, ability to reversibly bind water, and poor chemical and thermal stabilities.34 Therefore, the modification of MOFs for higher adsorption capacities and selectivities is difficult. The preparation of hybrid materials combining MOFs and other functional materials such as carbonaceous materials [carbon nanotubes (CNTs), carbon nanofibers, graphite oxide, and graphene oxide] has been proposed as a solution to overcome the disadvantages of MOFs and to expand their field of applications.35 The porous structure of MOF materials requires their modification to integrate with carbon-based materials that can consequently improve and extend MOF-based applications. In particular, the incorporation of CNTs into MOFs can yield better crystals and improve the composite performance as a result of the unusual mechanical, thermoconductive, electroconductive, and hydrophobic properties of the CNTs.36 Intermixing CNTs with MOFs leads to the appearance of unique chemical and physical properties, namely, good dispersibility and high micropore volume, which are associated with increasing the rapidity of adsorption and strongly changing the kinetics of the underlying process.37 The CO2 adsorption selectivities and capacities of MOFs have been found to be low at atmospheric pressure because of the weak interactions between the framework and CO2 gas. Therefore, it is very important to improve the capacity and selectivity of MOF materials by combining them with appropriate materials. The adsorption capacities and selectivities of MOFs for CO2 over other gases can be maximized by introducing polarizing groups into the pore space to selectively interact with CO2. The modification of adsorbents with amines is an effective method for this purpose.38 Both the capacity and selectivity for CO2 adsorption can be considerably improved by using piperazine (PZ) to modify the surface of an MOF. PZ is a cyclic diamine that has a high capacity to absorb CO2 and has been studied as a promoter for amine systems because it provides the highest absorption rate of all alkanolamines.39 The major advantages of PZ are its high resistances to oxidation and thermal degradation.40 The present report examines the adsorption of CO2 and CH4 gases and the CO2/CH4 selectivity on MOF-199 at 298, 323, and 348 K using a volumetric apparatus. We also incorporated multiwalled carbon nanotubes (MWCNTs) into MOF-199 to

2. EXPERIMENTAL SECTION 2.1. Materials. Benzene-1,3,5-tricarboxylic acid (H3BTC, 95%) and PZ (99%) were obtained from Aldrich. Copper(II) nitrate trihydrate [Cu(NO3)2·3H2O, 99.99%], ethanol, and dimethylformamide (DMF) were supplied by Fluka. All reagents were of analytical grade and were used without further purification. 2.2. Preparation of MOF-199. For the preparation of copper(II) benzene-1,3,5-tricarboxylate (Cu3BTC2), 0.51 g of Cu(NO3)2·3H2O and 0.353 g of H3BTC were added to 6 mL of solvent consisting of equal parts of deionized water and ethanol, and the mixture was stirred. The resultant slurry was poured into a Teflon-lined stainless autoclave and placed in an oven at 85 °C under static conditions to yield a porous material. After 24 h, the autoclave was removed from the oven and allowed to cool to room temperature, and the obtained blue crystals were recovered by filtration. The resulting precipitate was washed with a mixture of deionized water and ethanol repeatedly and dried under a vacuum at 80 °C. 2.3. Preparation of CNT@MOF-199 Nanocomposite. CNT@ MOF-199 was synthesized as follows: First, the MWCNTs were purified by being soaked in a mixture of sulfuric acid and nitric acid (1:3) for 24 h at 80 °C with stirring, recovered by filtration, washed with deionized water, and dried at 70 °C to produce purified MWCNTs. Then, the purified MWCNTs were added in situ during the synthesis of MOF-199 along with the raw materials to prepare CNT@MOF-199 nanocomposite MOF material. For the synthesis of 10 wt % MWCNT incorporated MOF-199, 0.05 g of purified MWCNTs was dispersed in 5 mL of DMF, and then this mixture was mixed with a solution of Cu(NO3)2·3H2O (0.353 g), H3BTC (0.51 g), and 6 mL of solvent (DMF, ethanol, and deionized water). The suspension was stirred at 25 °C, transferred into a stainless-steel autoclave, and maintained at 85 °C under static conditions for 24 h for further condensation. Finally, the obtained precipitate was recovered by filtration, washed two times with acetone/boiling deionized water solution, and dried under a vacuum at 100 °C for 4 h. 2.4. Preparation of CNT@MOF-199/PZ Nanocomposite. PZ was introduced into the CNT@MOF-199 nanocomposite by the wet impregnation method. In this procedure, dry CNT@MOF-199 was impregnated with 10, 20, or 30 wt % PZ. For the 10 wt % sample, 0.05 g of PZ was dissolved in 5 mL of ethanol under stirring for about 15 min, and then 0.5 g of CNT@MOF-199 was added to the mixture. The resultant solution was mixed in a 50 mL beaker at 500 rpm employing a magnetic bar on a stirring hot plate for 2 h. The top of the beaker was sealed with parafilm. After that, the solid product was dried in an oven at 60 °C for 90 min to completely remove the solvent. The procedure was repeated for the preparation of 20 and 30 wt % samples with 0.1 and 0.15 g, respectively, of PZ. Finally, the obtained samples are denoted as CNT@MOF-199/10PZ, CNT@MOF-199/20PZ, and CNT@MOF199/30PZ for 10, 20, and 30 wt % PZ, respectively. B


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Figure 1. Setup for adsorption capacity tests. 2.5. Characterization. The MOF-199 samples were characterized by a variety of conventional techniques. The surface features and morphologies of the samples were investigated by scanning electron microscopy (SEM PHILIPS XL30). The sorbents were coated with gold to improve their conductivity before scanning. The Fourier transform infrared spectra of the unmodified and modified samples were measured on a DIGILAB FTS 7000 instrument in attenuated-total-reflection (ATR) mode using a diamond module. The crystallinity of the MOF materials was checked by X-ray diffraction (XRD) using a Philips 1830 diffractometer with Cu Kα radiation, operated at 40 kV and 20 mA. The diffraction patterns were obtained in the 2θ range of 5−40° with a 2θ step size of 0.018° and a step time of 1 s. To characterize the textural properties of the sorbents such as pore volume, pore size, and specific surface area, N2 adsorption−desorption isotherms were obtained at 77 K using a Micromeritics model ASAP 2010 sorptometer. To determine the specific surface areas (SBET) of the porous solids, the Brunauer−Emmett−Teller (BET) method was used. The pore volume was determined based on the amount of N2 adsorbed within the pores of the material, and the pore size distribution was estimated from the adsorption branch of the isotherm by the Barrett− Joyner−Halenda (BJH) method. Thermogravimetric analysis was used to determine the thermal stability of the samples and was carried out from room temperature to 800 °C using a Mettler Toledo 851 TGA/DTA analyzer at a heating rate of 10 °C/min under an argon atmosphere. 2.6. Gas Adsorption Measurements. To evaluate gas−solid equilibrium data, an accurate and simple version of a volumetric apparatus was constructed. A schematic diagram of the apparatus used is shown in Figure 1. At first, the inside of the adsorption cell (HP vessel) was loaded and attached to the system. Then, the system was tested with an inert helium flow to ensure that none of the connections were leaking. Helium was used to sweep out the existing gas in the system. To degas the system, all of the valves except valves 3 and 4 were closed, and the system was evacuated for 1.5 h at a heating temperature of 200 or 100 °C for CNT@MOF-199 or CNT@MOF-199/PZ, respectively. Following degassing, the system was allowed to cool to room temperature. To perform an adsorption test, valves 2 and 3 were opened, and the other valves were closed. The pressure of the HP vessel decreased because of some dead volume and some gas adsorption. The portion of the dead volume was obtained by He tests and subtracted from the total pressure change. Finally, the exact pressure decrease resulting from gas adsorption was obtained.

3. RESULTS AND DISCUSSION 3.1. Structural and Textural Properties of MOF-199 Adsorbents. The textural properties of the synthesized MOF199 samples were analyzed by N2 adsorption/desorption at 77 K; the results are shown in Figure 2. The adsorption isotherms

Figure 2. Adsorption−desorption isotherms of nitrogen at 77 K on MOF-199 samples.

correspond to type I according to the IUPAC classification, which is characteristic of microporous materials.41 The pore textural properties of the five samples of MOF-199 are reported in Table 1; these data indicate that the modification of the microporous materials caused a partial deformation of the Table 1. Textural Properties Determined by Nitrogen Adsorption−Desorption Experiments at 77 K

C

adsorbent

BET surface area (m2 g−1)

BJH pore volume (cm3 g−1)

t-plot micropore volume (cm3 g−1)

average pore size (nm)

MOF-199 CNT@MOF-199 CNT@MOF-199/10PZ CNT@MOF-199/20PZ CNT@MOF-199/30PZ

1370 1280 1205 1155 1110

0.57 0.49 0.45 0.42 0.39

0.48 0.73 0.66 0.62 0.59

1.67 1.42 1.30 1.22 1.17


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Figure 3. FTIR spectra of (a) MOF-199, (b) CNT@MOF-199, (c) CNT@MOF-199/10PZ, (d) CNT@MOF-199/20PZ, and (e) CNT@MOF-199/ 30PZ.

Figure 4. XRD patterns of (a) MOF-199, (b) CNT@MOF-199, (c) CNT@MOF-199/10PZ, (d) CNT@MOF-199/20PZ, and (e) CNT@MOF-199/ 30PZ.

BTC. After MWCNT incorporation (curve b), two peaks appeared at 2370 and 3450 cm−1 that are associated with multiwalled carbon nanotubes. The FTIR spectrum of CNT@ MOF-199 shows the presence of −OH and −COOH functional groups on the surface of the MWCNTs. It is also important to note that MWCNT incorporation did not disturb the MOF-199 crystal structure. As follows from Figure 3, the FTIR spectra of CNT@MOF-199/PZ revealed a N−H functional group at about 1400−1700 cm−1. The XRD patterns of the MOF-199, CNT@MOF-199, and CNT@MOF-199/PZ samples are compared in Figure 4. Characteristic peaks of MOF-199 (at 2θ = 6.9°, 9.5°, 11.6°, 13.4°, 17.5°, 19.0°) appear in the XRD patterns. It is clear that the diffraction pattern of the CNT@MOF-199 is in good agreement with that of pristine MOF-199, indicating the existence of the well-defined framework units in the synthesized materials, confirming that MWCNT incorporation did not destroy the MOF-199 crystal structure. As can be seen, no obvious peaks of MWCNTs can be observed in the spectra of the nanocomposites because of the incorporation of only low percentages of MWCNTs. It is clear from Figure 4 that the CNT@MOF-199/PZ crystalline structure was well retained after

ordered micropore structure of the microporous materials, resulting in a decreased surface area, pore size, and total pore volume. The decrease in the total pore volume might be a result of the decrease in the pore width and pore blocking by the MWCNTs in the pores of MOF-199. However, the micropore volume increased as a result of the formation of additional micropores upon MWCNT incorporation in the CNT@MOF199 nanocomposite material, which was later confirmed by gas sorption studies. As shown in Figure 2 and Table 1, the N2 uptakes of CNT@MOF-199 decreased after PZ had been loaded. The diminished surface area, pore size, and pore volume of CNT@MOF-199 upon PZ impregnation provide a strong indication that PZ was successfully introduced into the channels of the support. The IR spectra of the samples before and after impregnation are shown in Figure 3. The complete vibrational spectrum (curve a) was in good agreement with the published data on MOF199.42 The appearance of a broad band at 2700−3500 cm−1 indicated the presence of −OH groups and water in the structure of the material. The bands at 1370 and 1450 cm−1 and at 1590 and 1645 cm−1 correspond to the symmetric and asymmetric stretching vibrations, respectively, of the carboxylate groups in D


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Figure 5. SEM images of (a) MOF-199, (b,c) CNT@MOF-199, (d) CNT@MOF-199/10PZ, (e) CNT@MOF-199/20PZ, and (f) CNT@MOF-199/ 30PZ. E


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Figure 6. TGA plots of (a) PZ, (b) MOF-199, (c) CNT@MOF-199/30PZ, (d) CNT@MOF-199/20PZ, (e) CNT@MOF-199/30PZ, and (f) CNT@ MOF-199.

Figure 7. Adsorption isotherms of CO2 and CH4 on (a) MOF-199 and (b) CNT@MOF-199 at different temperatures (298, 323, and 348 K).

the loading of PZ. In addition, the intensity of the characteristic peaks of CNT@MOF-199 decreased with PZ loading, which can be ascribed to the fairly strong binding of PZ to CNT@MOF199. To observe the surface morphology, images of the MOF-199 materials were taken with a scanning electron microscope after gold deposition. The SEM images of the synthesized MOF-199 products are shown in Figure 5. The crystals of MOF-199 in the SEM images had a double-sided pyramidal form with a width of about 10−20 μm. Figure 5b,c shows images of the MWCNTs on the MOF-199 surface. MWCNTs with a tubular structure formed a three-dimensional network on the MOF surface. The SEM images clearly show that very little PZ could be observed on the external surface of the adsorbent, suggesting that most of the loaded PZ dispersed into the pores of the modified CNT@MOF199. To some extent, the image also suggests that the MOF structure was preserved after the loading of PZ. The thermal stabilities of the as-prepared samples were analyzed by TGA, and representative results are presented in Figure 6. As shown in curve b, the curve for MOF-199 is consistent with prominent weight-loss steps. The weight-loss region between 30 and 100 °C indicates the vaporization of water and guest molecules residing in the pores. After that, a smaller step was seen from 100 to 200 °C related to loss of the embedded solvent. At temperatures between 300 and 350 °C, a sharp weight

loss can be observed from the curve, which reflects the collapse of the structure of MOF-199 and the burying of the organic linker. Finally, after decomposition, at 350 °C, the residual material could be related to the formation of copper oxides, metallic copper, or residual carbon. It can also be observed that the thermal stability of MOF-199 increased upon incorporation of MWCNTs, compared with that of MOF-199 without MWCNTs. Apparently, the enhancement of stability is due to the incorporation of the MWCNTs into the MOF. As a result of the hydrophobic nature of carbon nanotubes, the amount of adsorbed water molecules is decreased after MWCNT incorporation, and the weight loss between 30 and 100 °C is seen to be reduced. It can be seen that the TG curve for the PZcontaining shows only a weight-loss step and decomposes at 78.99 °C. Interestingly, when PZ was loaded into CNT@MOF199, the weight loss of PZ took place at higher temperatures. The volatility of PZ was reduced as a result of adsorption onto the CNT@MOF-199 surface. Apparently, the enhancement of stability is due to the strong interactions between the amine group and the CNT@MOF-199 framework. 3.2. Adsorption Capacities and Selectivities of MOF199 and CNT@MOF-199 Nanocomposites. The MOF-199 framework has been studied extensively for CO2 capture because of its high capacity for CO2 at room temperature and at low partial pressures. The single-component adsorption behaviors of F


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microporous adsorbents. It is known that the adsorption temperature is an essential factor that can significantly affect the adsorption capacities of various adsorbents. The adsorption isotherm curves reveal the conventional behavior showing the effect of temperature on the gas adsorption capacity. It can be clearly seen that higher adsorption capacities were found at lower temperatures for both adsorbents (MOF-199 and CNT@MOF199), which is consistent with exothermic physical adsorption. This can be explained by the fact that, at higher temperatures, the kinetic energies of the molecules of both the gas and the solid, given by the Boltzmann equation, are higher. This increase in kinetic energy causes a corresponding increase in interaction between the molecules of the gas and the solid at the interface that lead to interaction. Therefore, there is a resulting reduction in the effective area of the solid available for adsorption. The single-component isotherm is a tool for calculating the ideal selectivity of the adsorbent for CO2 against CH4. The effect of the nanocomposite on the adsorption selectivity at different pressures is shown in Figure 8. The adsorption selectivity of gaseous CO2 over gaseous CH4 was determined using the equation43,44

CO2 and CH4 on MOF-199 and CNT@MOF-199 at three temperatures were studied, and that results are shown in Figure 7a,b. It can be seen that the gas adsorption capacity increased significantly with increasing pressure. MOF-199 forms large cavities and small octahedral cages that can be accessible to small molecules through small windows. At low pressures, gas molecules are preferentially adsorbed inside the octahedral cages, and only a few molecules adsorb in the vicinity of the metal of the framework. First, saturation of the octahedral cages with gas molecules occurs, followed by adsorption around the exposed organic linkers and metal sites. The octahedral cages and the region that separates these cages from the large ones represent the adsorption sites. At higher pressures, the large central cages become preferential adsorption sites when the octahedral cages are full. As can be seen, CO2 adsorption on the adsorbents is greater than CH4 adsorption. Because CO2 is a high-quadruplemoment gas molecule but CH4 has no quadruple moment at all, both the high microporosity and the content of metal sites in the MOF enhance CO2 adsorption, but CH4 adsorption relies solely on the microporosity. The properties of the adsorbate gases are listed in Table 2. An enhancement of the micropore volume in

⎛V ⎞ CO2 ⎟⎟ αCO2 /CH4 = ⎜⎜ ⎝ VCH4 ⎠ P , T

Table 2. Properties of Adsorbate Gases adsorbate

polarizability (×10−25 cm3)

dipole moment (×1018 esu cm)

quadrupole moment (×10−26 esu cm2)

CO2 CH4

26.5 26.0

0.00 0.00

4.30 0.00

(1)

where VCO2 and VCH4 are the volumes of gaseous CO2 and CH4, respectively, adsorbed at any given pressure P and temperature T.45 The CO2/CH4 selectivity is high because CH4 has no quadrupole moment, whereas CO2 does. 3.3. Adsorption Capacity and Selectivity of CNT@MOF199/PZ Nanocomposites. The adsorption capacities and selectivities of MOFs for CO2 over CH4 can be maximized by introducing polarizing groups into the pore space to selectively interact with CO2. The affinity of basic amine groups for acidic CO2 is well-known; therefore, it is promising to study the functionalization of open metal sites in MOFs with various amines to improve the CO2 capture properties of these materials. Several studies have shown that polyamine and diamine incorporated into the framework of MOFs can dramatically enhance the selective CO2 adsorption capacity at ambient temperature and low pressure.46,47 The modification of open metal sites in the metal−organic framework Cu-BTTri with the secondary amine N,N′-dimethylethylenediamine (mmen) was already reported.48 The CO2 and CH4 adsorption isotherms at 298 K of CNT@MOF-199/PZ nanocomposites are shown in

MOFs can be achieved through the incorporation of other microporous materials such as multiwalled carbon nanotubes into the MOFs, which also provide sufficient gas adsorption capacities. It is clear from the plots that CNT@MOF-199 has a higher adsorption capacity toward gas molecules than MOF-199. The nanocomposite prepared by incorporation of multiwalled carbon nanotubes into MOF-199 can exhibit an increased micropore volume, enhanced gas storage capacity, and improved stability, compared with MOF-199. CO2 adsorption can occur with or without the formation of chemical bonds. Molecules physisorb through a variety of physical interactions. For the physisorption of CO2, the interaction between the electric quadrupole moment and the electric field gradient usually dominates the interactions of CO2 with a solid surface. The isotherm behavior follows the type-I isotherm category according to the IUPAC adsorption isotherm classification, which indicates a monolayer adsorption mechanism, commonly applied to

Figure 8. Adsorption selectivities of CO2/CH4 on MOF-199 and CNT@MOF-199 at 298 K and different pressures. G


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Figure 9. Adsorption isotherms of CO2 and CH4 on CNT@MOF-199/PZ nanocomposites with different weight percentages of PZ (10, 20, and 30 wt %) at 298 K and different pressures.

Figure 10. CO2/CH4 adsorption selectivities on CNT@MOF-199/30PZ at 298 K and different pressures.

samples plays a predominant role in CO2 adsorption. It was found that the CO2 adsorption capacity of CNT@MOF-199/PZ increased with increasing PZ loading and reached a maximum when the PZ loading was 30 wt %. The separation performances of the samples were calculated according to the reported equation. The selectivity of CO2 over CH4 was calculated at different pressures, and the results are shown in Figure 10. The mechanism of CH4 adsorption might be more physical, whereas the mechanism of CO2 adsorption on CNT@MOF-199/PZ could be more chemical. According to the surface interaction, CO2 molecules adsorb on the CNT@MOF199/PZ surface through the formation of ammonium carbamate. As can be seen from the data, the CNT@MOF-199/PZ samples exhibited better performance in CO2 adsorption than CNT@ MOF-199 but lower adsorption of CH4, suggesting a better potential for selective adsorption.

Figure 9. In the amine-based nanocomposite adsorbents, the diamine groups of PZ play a key role in CO2 adsorption, as the ad sorption is through the interaction beween amine groups and CO2. Therefore, the PZ loading can greatly affect the CO2 adsorption performance of the adsorbents. It can be seen that increasing the PZ loading enhances the CO2 adsorption uptake but reduces the CH4 adsorption uptake. This is because the amine in PZ-incorporated CNT@MOF-199 could ensure strong interactions between CO2 and the adsorbent, so that the CO2 molecules can be captured. In contrast, decreased CH 4 adsorption occurred because of the absence of enhanced CH4−adsorbent interactions. In this system, CNT@MOF199/PZ, the PZ was incorporated on the Cu2+ cation sites exposed on the framework pore surfaces. Because the PZ molecules were shorter than the distance between two adjacent metal sites, it was assumed that one amine from each PZ molecule was bound to a single metal site, whereas the other amine was free to interact with guest gas molecules. PZ was selected because of its relatively small size (ca. 3 Å in diameter), which minimizes its imposition on the pore space. From the above data, it is evident that the proposed chemisorption interaction takes place between CO2 and the free amine site of PZ. It should also be noted that, although the surface area of CNT@MOF-199/PZ was lower than that of CNT@MOF-199, as shown in Table 1, the CO2 equilibrium adsorption capacities of CNT@MOF-199/PZ were higher than those of the parent CNT@MOF-199. This suggests that the surface chemistry (or PZ loading) rather than surface area of the CNT@MOF-199/PZ

4. CONCLUSIONS Capturing CO2 from flue gases is currently a key issue in environmental protection. MOF-199 and multiwalled carbon nanotubes (MWCNTs) show enhanced stability toward ambient moisture and improve gas adsorption capacity. The increment in the gas adsorption capacity of CNT@MOF-199 can be attributed to the increase of the micropore volume of MOF199 upon MWCNT incorporation. Isotherm measurements (CO2 and CH4) were made for all samples at 298, 323, and 348 K using a volumetric apparatus. The amount of CO2 adsorbed was found to be significantly greater than the amount of CH4 H


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adsorbed because CO2 has a large quadrupole moment, which is the reason for its strong interaction with the adsorbent, whereas CH4 has no dipole or quadrupole moment. The adsorption capacity and selectivity of CNT@MOF-199 for CO2 over CH4 can be maximized by introducing amine groups to selectively interact with CO2. Adsorbent functionalization was incarried out by impregnation with different loadings of PZ (10, 20, and 30 wt %). An obvious increase was observed in the adsorption capacity for CO2 on all adsorbates when the percentage ratio of PZ was enhanced from 10 to 30 wt %. N2 adsorption/desorption isotherms, FTIR spectroscopy, XRD, SEM, and TGA were employed to characterize the structures of the prepared adsorbents. The increase in CO2 capture by CNT@MOF-199/ PZ is related to the existence of a great affinity between the amine sites on this adsorbent and the CO2 molecules. The results demonstrate that both physisorption and chemisorption play important roles in CO2 adsorption on CNT@MOF-199/PZ, whereas the physisorption process is dominant for CO 2 adsorption on CNT@MOF-199.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +98 21 77240516. Fax: +98 21 77491204. ORCID

Mansoor Anbia: 0000-0002-0180-5244 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the Research Council of Iran University of Science and Technology (Tehran) for financial support of this study.

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CH4 Selectivity by

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