Bifunctionalized Metal Organic Frameworks, UiO-66-NO2-N (N = -NH2

Article pubs.acs.org/jced

Bifunctionalized Metal Organic Frameworks, UiO-66-NO2‑N (N = -NH2, -(OH)2, -(COOH)2), for Enhanced Adsorption and Selectivity of CO2 and N2 Zana Hassan Rada,† Hussein Rasool Abid,†,‡ Hongqi Sun,† and Shaobin Wang*,† †

Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia Department of Environmental Health, Applied Medical Science College, Karbala University, 56001 Karbala, Iraq

‡

ABSTRACT: Metal organic frameworks (MOFs), UiO-66-NO2 and UiO-66-NO2−N (N = -NH2, -(OH)2, -COOH)2), were synthesized under solvothemal conditions using different functionalized organic linkers. The synthesized samples were further activated by solvent exchange using methanol and chloroform. All samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and N2 adsorption/desorption. Gas adsorption of CO2 and N2 on all samples has been evaluated at 760 mmHg and different temperatures. It was found that UiO-66-NO2-NH2 by methanol or chloroform activation presented much higher CO2 adsorption than other samples and the adsorption capacities are around (87 and 74) cm3·g−1 on chloroform-activated and methanolactivated samples, respectively, at 273 K and 760 mmHg. In addition, high selectivity of CO2/N2 has been found on chloroformactivated UiO-66-NO2-NH2 giving a factor of 65 while the selectivity of CO2/N2 is only 40 for chloroform-activated UiO-66NO2.

1. INTRODUCTION Nowadays, the rising level of carbon dioxide in the atmosphere, which has mostly resulted from burning of fossil fuels such as coal, natural gas, and oil, has become one of the biggest problems worldwide. Therefore, reducing carbon dioxide emission from the burning processes by capture and storage has been proposed as one of the control strategies and thus has been studied by many researchers throughout the world using different technologies. Among these methods, adsorption technology using novel materials such as metal organic frameworks (MOFs) has been considered to be a very promising solution to controlling the CO2 emission from flue gases.1−4 MOFs are a new class of crystalline porous materials which are built via metal-oxide units joined by organic linkers through coordination bonds.5,6 These crystalline porous MOFs have been used in many applications such as catalysis, separation, drug delivery, gas storage, and gas adsorption due to their high specific surface areas and effective functional groups.7−9 Many MOFs have been synthesized in the past 10 years such as MOF-5, MOF-199, Mg-MOF-74, MOF-177, MIL-53(Al), and Zr-MOFs (UiO-66)10−14 and some of them showed very high © XXXX American Chemical Society

adsorption toward carbon dioxide capture due to high pore volume and surface area such as Mg-MOF-74.15 However, others, for example PCN-14, have shown good capacity in methane storage.16 Titanium oxide (TiO2) has been known as a photocatalyst,17,18 and thus, Ti-based MOFs (MIL-125) with high porosity have been synthesized and used for production of porous TiO2.1920 Recently, Zr-based MOFs (UiO-66) have been found to have good thermal stability and adsorption capacity toward CO2, CH4, and H2.13,21−23 More recently, MOFs with mixed linkers (mixMOFs) or different metals (MC-MOFs) have been synthesized and used for adsorption of CO2 and H2. For instance, Zn-, Al-, and Febased MOFs with different ligand linkers have been tested for adsorption of CO2 and H2.24−26 Zr-based mixMOFs were reported using different functional groups such as (Br-BDC) and (NH2-BDC).27 Chavan et al. prepared a series of mixed linker Zr-based MOFs using a mixture of BDC and NH2-BDC with different ratios.4 However, Zr-based mixMOFs with NO2Received: March 11, 2015 Accepted: June 1, 2015

A

DOI: 10.1021/acs.jced.5b00229 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Figure 1. (a) XRD patterns of UiO-66-NO2 and UiO-66-NO2-N before activation. (b) XRD patterns of UiO-66-NO2-N after activation by methanol or chloroform.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals including zirconium chloride (ZrCl4, 99.9%), N,N-dimethylformamide (DMF, C3H7NO, 98%), chloroform (analytic grade, CHCl3 99%), methanol (analytic grade, CH3OH, 99%), 2-nitroterephthalic acid (H2BDC-NO2, ≥ 99%), 2-aminoterephthalic acids (H2BDCNH2, 99%), 2,5-dihydroxyterephthalic acid (H2BDC-OH, 98%), and 2,5-furandicarboxylic acid (H2FDA, C6H4O5 97%) were supplied by Sigma−Aldrich without further purification. 2.2. Synthesis of UiO-66−NO2 and UiO-66-NO2-N [N = -NH2, -(OH)2, -(COOH)2]. UiO-66-NO2 was synthesized based on the previous report with some modifications.28,29 Generally, 6.3 mmol of ZrCl4 and 5.8 mmol of H2BDC-NO2 were mixed together in 86 mL of DMF for 25 min, and then the homogeneous mixture was placed in a Parr PTFE-lined

BDC ligand and other linkers such as -(OH)2 or -(COOH)2 have not been experimentally reported. In this investigation, we synthesized nitro-functionalized Zr-MOF (UiO-66-NO2) and mixed Zr-MOFs, UiO-66-NO2-N [N = -NH2, -(OH)2, -(COOH)2] and tested their sorption performance in CO2 and N2. UiO-66-NO2 has a good capacity in carbon dioxide adsorption; however, the capacity is similar to UiO-66. Due to the high polarizability of the NO2 group, it will not significantly influence the functionality of a second linker in MOFs. This work aims to investigate the effect of mixed linkers on the enhanced adsorption and selectivity of UiO-66-NO2. The effects of a second linker on UiO-66 structure and gas adsorption were elucidated. B



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Figure 2. TGA analysis profiles of UiO-66-NO2 and UiO-66-NO2-N (a) before and (b) after activation.

digestion vessel of 125 mL. The digestion vessel was put in an oven preheated to 393 K for 24 h. After that, the autoclave was cooled to room temperature and uniform white crystals were collected by vacuum filtration and washed several times using DMF. UiO-66-NO 2-NH 2 was synthesized according to the preceding procedure. Typically, 0.552 mmol of H2BDC-NH2 was mixed with 5.8 mmol of H2BDC-NO2 and 6.3 mmol of ZrCl4 in 86 mL of DMF for 40 min, and then the homogeneous mixture was placed in a Parr PTFE-lined digestion vessel of 125 mL, which was put in an oven preheated to 393 K for 24 h. After the autoclave was cooled to room temperature, a greenish yellow crystalline product was extracted from the solution by vacuum filtration and washed several times using DMF.

UiO-66-NO2-(OH)2 was also synthesized according to the above procedure. First, 0.77 mmol H2BDC-(OH)2 was mixed with 5.8 mmol of H2BDC-NO2 and 6.3 mmol of ZrCl4 in 86 mL of DMF for 40 min, and then the homogeneous mixture was placed in a Parr PTFE-lined digestion vessel of 125 mL. After heating at 393 K for 24 h, the autoclave was cooled to room temperature and a greenish yellow crystalline product was obtained by vacuum filtration and washed several times using DMF. Finally, UiO-66-NO2-(COOH)2 was synthesized similarly using the preceding procedure. For preparation, 1.281 mmol of H2FDA was mixed with 5.8 mmol of H2BDC-NO2 and 6.3 mmol of ZrCl4 in 86 mL of DMF for 40 min, and then the homogeneous mixture was placed in a Parr PTFE-lined C



Article

Figure 3. (a) FTIR spectra of UiO-66-NO2 and UiO-66-NO2-N before and after activation. (b) Magnification part for UiO-66-NO2 and UiO-66NO2-N samples.

(600 to 4000) cm−1 with a resolution of 4 cm−1 by using an attenuated total reflectance (ATR) technique. A thermogravimetric analysis (TGA) instrument (TGA/DSC1 STARe system, METTLER-TOLEDO) was used to determine the thermal stability of all samples. About (10 to 20) mg samples were loaded in an alumina pan, and then argon gas was introduced into the furnace at a flow rate of 20 mL·min−1 and a heating rate of 10 K·min−1 from (308 to 1173) K. The crystalline structure of samples was measured by an XRD diffractometer (Diffractometer D8 Advance-Bruker XS) with Cu Kα radiation (λ = 1.5406 Å). The morphologies of the samples were determined by a SEM machine (Zeiss NEON 40 EsB Cross-Beam) for particle size and shape of crystals. A Micromeritics Tristar 3000 analyzer was used to determine the surface area and pore size of each sample. All samples were

digestion vessel of 125 mL for heating at 393 K for 24 h. A white crystalline product was extracted from the solution by vacuum filtration and washed several times using DMF. 2.3. Activation of UiO-66-NO2 and UiO-66-NO2-N. Activation of UiO-66-NO2 and UiO-66-NO2-N was carried out using the method as previously reported.30,31 About 0.5 g of each sample was immersed separately in 50 mL of methanol or chloroform solution for 5 days, and then the solids were filtered, washed several times using the solvents, and dried in an oven at 353 K for 12 h. Finally, these materials were heated under vacuum at 463 K overnight. 2.4. Characterization of UiO-66-NO2 and UiO-66-NO2N. The surface functional groups on samples, UiO-66-NO2 and UiO-66-NO2-N, were checked by FTIR spectra (a PerkinElmer 100 FT-IR spectrometer). The spectrum was scanned from D



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Figure 4. SEM images of (a) UiO-66-NO2, (b) UiO-66-NO2-NH2, (c) UiO-66-NO2-(OH)2, and (d) UiO-66-NO2-(COOH)2.

degassed at 463 K under vacuum overnight before analysis, and then N2 adsorption at 77 K was carried out. Surface areas were calculated using BET method, pore volumes were obtained using the adsorption at p/p0 = 0.99, and pore size distributions were calculated using the BJH method. 2.5. Adsorption Study of CO2 and N2. The adsorption isotherm of pure CO2 (99.999%) on each sample was measured by a Micromeritics instrument (Gemini I-2360) at temperatures of (273 and 296) K and low pressure up to 760 mmHg. First, about 0.14 g of sample was loaded in a sample tube and activated by a Vacprep 061 degasser at 463 K under vacuum for 1 day. After the degassing process, analysis was carried out on the Gemini I-2360. In addition, N2 adsorption isotherm at 273 K and low pressure up to 760 mmHg was also evaluated by the Gemini I-2360 instrument.

Table 1. Porous Structure of Samples, UiO-66-NO2 and UiO-66-NO2-N Activated by Methanol and Chloroform sample UiO-66-NO2 UiO-66-NO2-NH2 UiO-66-NO2-(OH)2 UiO-66-NO2-(COOH)2 a

SBET (m2·g−1) a

771, 868,a 732,a 462,a

b

704 729b 704b 397b

pore size (nm) a

4.4, 1.7,a 1.6,a 1.6,a

b

4.6 1.8b 1.6b 1.9b

pore vol (m3·g−1) 0.69,a 0.65b 0.039,a 0.038b 0.037,a 0.037b 0.047,a 0.025b

Methanol activation. bChloroform activation.

TGA profiles of all samples are shown in Figure 2. It is indicated that all all as-synthesized samples (Figure 2a) show an initial weight loss of 8 %, relating to moisture (water) and free solvent removal, and then remain thermally stable up to 423 K. However, it can be seen from Figure 2b that thermal stability of UiO-66-NO2 after activation processes by methanol and chloroform remains up to 673 K. The coordinated bonds between linkers and metal ions were broken down at 823 K. In addition, it has been shown that more removal of unreacted linkers and coordinated solvent can be achieved by methanol activation.32 Also, samples UiO-66-NO2-N after activation processes showed similar steps of weight losses with different ratios, and their thermal stabilities almost remain up to 670 K (Figure 2b). From the weight loss, the amounts of functionalized organic linkers (UiO-66-NO2-N [N = -NH2, -(OH)2, -(COOH)2], which have been coordinated to construct the structure of the samples, were calculated to be (34, 33, 34, and 34) %, respectively.

3. RESULTS AND DISCUSSION Figure 1 shows XRD patterns of all samples of UiO-66-NO2 and UiO-66-NO2-N, before and after activation. The profiles of the as-synthesized and activated (methanol and chloroform) samples are similar to the pattern of UiO-66 as reported in the previous studies.28 This suggests that the second linker can strongly be coordinated with metal ions to form the same crystalline structure of UiO-66 without producing other structured MOFs. However, it can be seen that the peak intensities of chloroform-activated samples is higher than those of samples activated by methanol. This indicates that activation process by chloroform can be stronger than methanol for DMF solvent exchange. E



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Figure 5. N2 adsorption/desorption isotherms of UiO-66-NO2 and UiO-66-NO2-N: (a) methanol activation and (b) chloroform activation.

1655 cm−1 nearly disappeared after activation. This demonstrates that a good exchange efficiency of chloroform or methanol to DMF and BDC could be achieved during activation and heating, which leads to removal of most of the noncoordinated linkers (H2BDC-NO2, H2FDA, H2BDC-OH,

Figure 3a shows FTIR spectra of all samples, UiO-66-NO2 and UiO-66-NO2-N, as-synthesized and after activation by solvent exchange in chloroform and methanol. On assynthesized samples, carboxyl group from aromatic carboxylic acid was observed at 1655 cm−1.33−35 However, the peak at F



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Figure 6. CO2 adsorption on UiO-66-NO2 and UiO-66-NO2-N samples at different temperatures: methanol-activated samples at (a) 296 K and (b) 273 K and chloroform-activated samples at (c) 296 K and (d) 273 K.

linker will only produce additional functional groups. The morphological features of modified samples were different from the morphology of Zr-MOF (UiO-66) because the modified samples were synthesized at a different molar ratio. Porous structures of UiO-66-NO2 and UiO-66-NO2-N samples are presented in Table 1. It can be seen that a second linker in bifunctional UiO-66 has a different effect on surface area. Addition of -NH2 results in a higher surface area while addition of -OH and -COOH reduces the surface area. It can also be seen that samples activated by methanol show higher BET surface area than the ones activated by chloroform. Among all of the samples, UiO-66-NO2-NH2 activated by methanol showed the highest BET surface area at 868 m2·g−1. In addition, the pore size and pore volume of UiO-66-NO2-N generally showed less values than those of UiO-66-NO2. Figure 5 shows N2 adsorption/desorption isotherms of UiO66-NO2 and UiO-66-NO2-N samples (activated by methanol (a) and chloroform (b), respectively). It can be seen that the adsorption and desorption isotherms were overlapped in chloroform-activated samples, which can refer to dominating microporous structure without a hysteresis. Moreover, it can be seen that UiO-66-NO2 presented higher N2 adsorption than mixed linker MOFs, UiO-66-NO2-N. Also, UiO-66-NO2 activated by methanol or chloroform showed a hysteresis at higher p/p0 = 0.75 to 0.95, which indicated the presence of larger pores.

and H2BDC-NH2). Furthermore, all COOH groups in the linkers are well coordinated with metal centers, and there is no COOH functional group in UiO-66-NO2-(COOH)2. Chloroform activation can also help to maintain the crystallinity of synthesized samples, confirming XRD results. In addition, asymmetrical and symmetrical stretching vibrations of the functional group of NO2 on all samples can be seen at (1536 and 1389) cm−1, respectively.36 The C−O bond of the C−OH group of carboxylic acid can also be recognized at around 1400 cm−1 for each sample.30 Coordinating metal center with linkers (CO in carboxylates) occurs at the region of (1540 to 1600) cm−1 through the deprotonating process. Figure 3b shows magnification of the spectrum of samples in the range of (2400 to 4000) cm−1. The peaks appearing at (3367 and 3475) cm−1 are related to the primary amino of functional groups of amino (N−H) on samples UiO-66-NO2-NH2. The peak at 3350 is related to the O−H functional group in coordinated UiO-66NO2-(OH)2. The presence of these functional groups on UiO66-NO2-N samples indicates that the second ligand has been added on these samples as no such a peak can be seen on UiO66-NO2. Figure 4 shows SEM images of UiO-66-NO2 and UiO-66NO2-N. The SEM images indicate that triangular particles of UiO-66-NO2 and UiO-66-NO2-N were obtained with similar particle size, suggesting that the addition of a second linker will not induce structural and morphological changes. The second G



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Figure 8. Selectivity of CO2/N2 on UiO-66-NO2 and UiO-66-NO2-N at varying pressures and 273 K: (a) methanol-activated samples and (b) chloroform-activated samples.

Figure 7. N2 adsorption on UiO-66-NO2 and UiO-66-NO2-N samples at 273 K: (a) methanol-activated samples and (b) chloroformactivated samples.

66-NO2-(OH)2 have almost the same N2 adsorption capacity, which is around 6.2 cm3·g−1. But, the MOFs by chloroform activation process showed different N2 adsorption capacity. Single linker UiO-66-NO2 presented the highest N2 adsorption than UiO-66-NO2-N by mixed linkers. Furthermore, all samples presented higher CO2 adsorption than that of N2, due to the greater quadrupolar moment of CO2 (13.4·1040 cm2) than N2 (4.7·1040 cm2) and greater polarizability of CO2 (26.3·1025 cm3) compared to N2 (17.7·1025 cm3).39 Figure 8 shows the theoretical selectivity factor of CO2 over N2 (CO2/N2) based on the static adsorption capacities of CO2 and N2 at 273 K as defined by

Figure 6 shows CO2 adsorption isotherms at two temperatures ((273 and 296) K) on all samples (activated by methanol and chloroform). In general, methanol-activated samples presented higher CO2 adsorption than chloroformactivated samples. It is noted that methanol activation has produced a higher surface area of the samples and affinity of the structure for adsorption of CO2. Figure 6 also indicates that, except for UiO-66-NO2-(COOH)2, other mixed linker MOFs [UiO-66-NO2-NH2 and UiO-66-NO2-(OH)2] have higher CO2 adsorption capacity than UiO-66-NO2 from both activation processes. Moreover, methanol-activated UiO-66-NO2-NH2 samples were showing the highest CO2 adsorption capacities, giving (87.2 and 64.9) cm3·g−1 at (273 and 296) K, respectively. NH2 and OH functional groups can provide more CO2 adsorption sites35,37,38 and increase the affinity toward CO2.12 COOH functional group, however, could lose H+, making acidic COO− group and reducing CO2 uptake. Figure 7 shows N2 adsorption capacity on different samples activated by chloroform and methanol at a temperature of 273 K. It can be seen that UiO-66-NO2 samples activated by methanol present higher N2 capacities than those using chloroform activation. This is attributed to methanol activation having improved the removal efficiency and most of unreacted solvent, DMF, being removed from sample pores. Moreover, methanol-activated UiO-66-NO2, UiO-66-NO2-NH2, and UiO-

selectivity =

X1/X 2 Y1/Y2

where Xi indicates the mole fraction of component i in the adsorbed phase and Yi indicates the mole fraction of component i in the bulk gas phase. It can be seen that UiO-66-NO2-NH2 samples display the best separation factor for carbon dioxide over N2 whereas UiO66-NO2-(COOH)2 has the lowest of separation factors. The selectivity factor of UiO-66-NO2-NH2 is the highest because it has the larger pore size after UiO-66-NO2. Moreover, N2 molecules are smaller than that of CO2 molecules. Therefore, they can diffuse through the pores faster than CO2. However, the affinity of the functional group (NH2) toward capturing of H


work work work 0.0082,a 0.0069b 0.0093,a 0.0104b 0.0082,a 0.0089b 0.0003 0.0061 0.0059 0.0046 0.0032 0.0050

4. CONCLUSIONS By solvothermal synthesis, multifunctional UiO-66-NO2-N [N = -NH2, - (OH)2, -(COOH)2] materials using two linkers were successfully prepared. These samples were activated by solvent exchange, methanol or chloroform. Methanol-activated samples (UiO-66-NO2-NH2) show higher CO2 adsorption capacity (87 cm3·g−1, 273 K, 1 bar) than chloroform-activated samples. Also, these multifunctional MOFs demonstrated good selectivity of CO2 over N2 such as (65 on UiO-66-NO2-NH2). It is concluded that UiO-66-NO2-NH2 with mixed linkers are potential materials for CO2 capture and CO2/N 2 gas separation.

■

6.0,a 4.84b (273 K, 1 bar) 4.3,a 4.13b (273 K, 1 bar) 6.3,a 5.3b (273 K, 1 bar) 1.1 (273 K, 1 bar) 5.8 (273 K, 1 bar) 4.5 (273 K,1 bar) 5 (273 K, 1 bar) 5.5 (273 K, 1 bar) 1.25 (273 K, 1 bar)

6.2,a 4.5b (273 K, 1 bar) 6 (273 K, 1 bar)

0.0072,a 0.0062b 0.0097

55.98,a 64.77b 51.1 49 48.12,a 54.48b 47.46,a 55.03b 45.31a 40.4b 34.3 19.9 19.6 18.7 17.3 14.3

this 40 39 this this this 41 40 42 40 40 43

work

N2 is much lower than CO2. Table 2 summarizes CO2 and N2 adsorption and selectivity on various MOFs with different surface areas. In general, UiO-66-NO2 in this work presented higher CO2 adsorption than other types of MOFs but similar capacity in N2 adsorption, thus giving higher selectivity of CO2 to N2. Bifunctionalized UiO-66-NO2-N presented even higher selectivity of CO2 to N2, demonstrating to be promising materials for CO2 adsorption and separation of mixture, CO2N2.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +61 8 92663776. Fax: +61 8 92662681. Funding

We acknowledge the Ministry of Higher Education and Minister of Natural Resources/Kurdistan regional government-Iraq for a Ph.D. scholarship. Notes

The authors declare no competing financial interest.

■ ■

0.097b 0.137b 0.084b

0.102b

Methanol-activated samples. bChloroform-activated samples. a

87.19,a 74.10b (273 K, 1 bar) 51.5 (273 K, 1 bar) 225 (278 K, 1 bar) 79.12,a 68.26b (273 K, 1 bar) 55.71,a 54.16b (273 K, 1 bar) 69.9,a 58.9b (273 K, 1 bar) 87.3 (273 K, 1 bar) 40.6 (273 K, 1 bar) 71 (273 K,1 bar) 37.6 (273 K, 1 bar) 55.0 (273 K, 1 bar) 17.5 (273 K, 1 bar) UiO-66-NO2-NH2 ZIF-78 Mg-MOF-74 UiO-66-NO2-(OH)2 UiO-66-NO2-(COOH)2 UiO-66-NO2 Cu3(BTB6−) ZIF 69 Al(OH)(NDC) ZIF-68 ZIF-70 Zn3(OH)(L)2.5(DMF)4

REFERENCES

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0.100,a 0.083 0.125 0.108,a 0.120,a 0.091,a 0.026 0.043 0.093 0.034 0.032 0.070

CO2 adsorption (cm3·g−1)

CO2 adsorption/SBET (cm3·cm−2)

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ACKNOWLEDGMENTS We thank Ms. Elaine Miller for SEM measurements.

MOFs

Table 2. Comparison of CO2 and N2 Adsorption on Various MOFs

N2 adsorption (cm3·g−1)

N2 adsorption/SBET (cm3·cm−2)

selectivity factor (CO2/N2)

ref


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Bifunctionalized Metal Organic Frameworks, UiO-66-NO2-N (N = -NH2

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