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Cite this: DOI: 10.1039/c4cc06648k Received 23rd August 2014, Accepted 29th September 2014
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A porous metal–organic framework with –COOH groups for highly efficient pollutant removal† Qi Zhang,a Jiancan Yu,a Jianfeng Cai,a Ruijing Song,a Yuanjing Cui,a Yu Yang,a Banglin Chen*abc and Guodong Qian*a
DOI: 10.1039/c4cc06648k www.rsc.org/chemcomm
A new metal–organic framework with –COOH groups has been realized and demonstrates strong interactions with methylene blue and thus the complete removal of methylene blue from aqueous solution.
Endocrine disrupting chemicals (EDCs) are classified as any exogenous substances that cause adverse health effects in an intact organism, or lead to changes in endocrine function.1 In fact, even trace amounts of EDCs (1–1000 ng L1) can lead to adverse effects on the reproductive, neurological, and immune systems of aquatic organisms and humans.1,2 The release of EDCs in the aquatic environment has been an emerging challenge.3 Complete elimination of EDCs is essential prior to wastewater discharge or drinking water distribution; however conventional operation treatments may not produce suitable removal efficiencies.4 Metal–organic frameworks (MOFs), which can be easily selfassembled from metal ions/clusters with organic linkers, might provide solutions for EDC removal given the fact that they have large surface areas, adjustable pore sizes, and controllable/ functional pore surfaces. To make use of these unique features, MOFs have been widely explored in gas storage and separation,5 luminescence,6 catalysis,7 and biomedicine;8 though they are less developed for pollutant adsorption/removal.9 MOFs and their composites previously examined for the removal of methylene blue do not have strong recognition sites, limiting their removal capacities for methylene blue.10 The organic backbone of methylene blue can be considered as a cationic species, so if MOFs can have enough space to encapsulate
methylene blue, while the specific sites within such MOFs can have strong interactions with methylene blue, we should be able to construct suitable MOFs for the highly efficient removal of methylene blue. With this in mind, we design and synthesize a new organic linker (Fig. 1b) to construct a microporous MOF. The large organic backbone of this organic linker enforces the construction of large pore spaces to encapsulate methylene blue molecules, while the free –COOH groups on the pore surfaces generate strong interactions with the methylene blue molecules. To our surprise, the resulting MOF (ZJU-24 series) not only exhibits the highest reported methylene blue removal capacity of 902 mg g1, but can also completely remove methylene blue from its aqueous solution. The ZJU-24 series are iso-structural MOFs of the well-known NOTT-101 constructed from a copper paddle wheel Cu2(COO)4 with organic linkers of tetracarboxylates with NbO topologies.11
a
State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, Department of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China. E-mail: [email protected] b Department of Chemistry, University of Texas at San Antonio, San Antonio, TX 78249, USA. E-mail: [email protected] c Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 22254, Saudi Arabia † Electronic supplementary information (ESI) available. CCDC 1018379. For ESI and crystallographic data in CIF or other electronic formats see DOI: 10.1039/ c4cc06648k
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Fig. 1 The introduction of functional –COOH groups into the iso-structural (a) NOTT-101 forms (b) ZJU-24-0.89; the X-ray crystal structure of ZJU-240.89 shows (c) a spherical-like cage of about 13.8 Å and (d) a shuttle-shaped cage of about 14 Å in diameter (Cu, blue; C: grey; O: red); (e) N2 sorption isotherms of NOTT-101, ZJU-24-0.5 and ZJU-24-0.89 at 77 K demonstrating the effect of immobilized –COOH groups on their porosities (solid symbols: adsorption, open symbols: desorption).
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They can be easily synthesized from Cu(NO3)2 and a mixture of H4TPTC ([1,1 0 :4 0 ,100 -terphenyl]-3,300 ,5,500 -tetracarboxylic acid) and H6TPHC ([1,10 :4 0 ,100 -terphenyl]-20 ,3,300 ,5,50 ,500 -hexacarboxylic acid) of different ratios (Fig. S1, ESI†). Based on the content of H6TPHC in a mixture of H4TPTC/H6TPHC, the resulting MOFs were termed as ZJU-24-0.25, ZJU-24-0.5, ZJU-24-0.75 and ZJU-24-0.89, respectively. As shown in Fig. S1 (ESI†), these MOFs have the same structure as NOTT-101 if the ratios of H4TPTC:H6TPHC are within 1:8. The content and the inclusion of H6TPHC in ZJU-24-0.25, ZJU-24-0.5, ZJU-24-0.75 and ZJU-24-0.89 were further confirmed by NMR analysis after the MOF crystals were dissolved in DCl/ DMSO-D6 (Fig. S1b and S4, ESI†). N2 adsorption isotherms at 77 K for the two samples named ZJU-24-0.5 and ZJU-24-0.89 were measured to determine their permanent porosities and these were compared with NOTT-101. As expected, the incorporation of the –COOH groups decreases the pore spaces and thus the N2 uptake accordingly (Fig. 1e). ZJU-24-0.5 and ZJU-24-0.89 have lower BET surfaces of 1700 and 1189 m2 g1, respectively, than NOTT-101 of 2316 m2 g1.12 The existence of the –COOH groups on the pore surfaces was further established by single crystal structure determination of ZJU-24-0.89. As shown in Fig. 1, ZJU-24-0.89 has the same basic structure as NOTT-101 except that some of the organic linkers of TPTC have been replaced by H2TPHC. The pore spaces are about 13.8–14.0 Å in diameter (according to the space-filling models). Because the –COOH groups are located in the framework voids of ZJU-24-0.89 while the pore channels are large enough to encapsulate methylene blue molecules, its performance for the removal of methylene blue (MB) from aqueous solution was examined. As shown in Fig. 2, after small crystals of ZJU-24-0.89 (5 mg) were immersed in a 50 mL aqueous solution of MB (5 ppm), the concentration of MB in solution decreased by 50% in 5 minutes. After 12 h, MB can be completely removed from the aqueous solution in which no MB can be detected in UV spectra. This is significantly superior to NOTT-101: after 12 h, only very small amount of about 5% MB can be removed. Once the MB has been completely removed by ZJU-24-0.89 from the aqueous solution after 12 h, the solution becomes colourless and clear (Fig. 2b). It is reasonable to speculate that the –COOH groups within the pore surfaces of ZJU-24-0.89 play the most important role in the removal of MB. To further confirm this claim, the ZJU-24 series with different amounts of H6TPHC ZJU-24-0.25, ZJU-24-0.5, ZJU-24-0.75 and ZJU-24-0.89 were compared for their performance to remove MB. As shown in Fig. 3, the MOF with the higher content of H6TPHC exhibited the highest adsorption speed and uptake for MB. The adsorption isotherms were collected accordingly for ZJU-24-0.89 and NOTT-101 (Fig. S6, ESI†). The amount of adsorbed MB over ZJU-24-0.89 after 12 h is 902 mg g1, which is the highest reported among any porous materials for MB removal so far (Table S2, ESI†).10b,13 To further study the microscopic mechanism of the host– guest interaction, the vibrational properties of the two states before and after MB adsorption in DMF were investigated by Raman (Fig. S7, ESI†) and FT-IR (Fig. S8, ESI†) spectroscopy. The assignment of the MOFs vibrational modes is a necessary
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Fig. 2 (a) UV spectra of an aqueous MB solution during an adsorption test with ZJU-24-0.89; (b) photographs of the removal of MB in aqueous solution with ZJU-24-0.89 over time; (c) UV spectra of an aqueous MB solution during an adsorption test with NOTT-101; (d) photographs of the removal of MB in aqueous solution with NOTT-101 over time.
Fig. 3 Effect of the H6TPHC content in the ZJU-24 series for the removal of MB.
step for determining the interaction between the framework and guest molecules. In the Raman spectroscopy, in the 1800– 900 cm1 range, the spectrum is dominated by modes associated with the organic part of the MOF.14 The band at 1608 cm1 is assigned to the phenyl mode n8a[(CQC) stretching mode of ligand] that only appears in the Raman spectrum because it has parity under Ci symmetry.15 A comparison of the two states’ Raman spectra indicates that the n8a(CQC) mode of the benzene ring shifts from 1608 to 1623 cm1 after the MB molecules were introduced into the pores. In the FT-IR spectra, two bands at 1566 cm1 and 1373 cm1 assigned to n(CQC) drift to 1581 cm1 and 1380 cm1, respectively. Such changes might be attributed to the p–p interactions between the framework and the MB molecules. The changes for COO stretching are also obvious. In the Raman spectroscopy, the bands at 1545 cm1 and 1430 cm1 are due to the nas (COO) and ns (COO) units. After MB is absorbed, nas (COO) vanishes and ns (COO) drifts to 1441 cm1. In the FT-IR spectra, the –COO
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Fig. 4 (a) Crystal structure of ZJU-24-0.89; (b) crystal structure of ZJU-24-0.89*MB with simulated MB molecules inside the pores.
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of 902 mg g1 and complete removal after 12 h. This is attributed to the immobilized –COOH groups on the pore surfaces and their strong interactions with the methylene blue molecules. This new strategy will facilitate the extensive research on functional porous MOFs for EDC extraction, and lead to some practically useful materials for these important applications in the near future. We would like to thank Zhi Li of the State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249 for her help in the molecular simulation studies and the National Natural Science Foundation of China (No. 51010002, 51272229 and 51272231), the Zhejiang Provincial Natural Science Foundation of China (No. LR13E020001) and the Welch Foundation (AX-1730).
Notes and references group is characterized by the in phase or out of phase vibrations of the two equivalent C–O bonds, leading to the symmetric and antisymmetric stretch modes. nas (COO) is at 1623 cm1, and the symmetric stretch mode of ns (COO) should be weaker than the band at 1623 cm1. Since the band at 1373 cm1 is attributed to the benzene ring mode reference to IR spectra of ligands (Fig. S9, ESI†) and reference assignments,16 we assign ns (COO) at 1433 cm1. After MB adsorption, nas (COO) drifts to 1631 cm1, and no obvious drift was observed for ns (COO). This is because IR spectra are sensitive to antisymmetric stretch modes. The transformation of the –COO vibration mode is due to the electrostatic interactions between the anionic framework and cationic MB molecules. Single-crystal X-ray diffraction was carried out to characterize the crystal structure change before and after MB being absorbed in DMF (Fig. 4). The inclusion of MB in ZJU-24-0.89 was revealed by NMR analysis after the MOF crystals were dissolved in DCl/ DMSO-D6. The NMR analysis revealed that the MB molecules absorbed by the crystals were in 0.4 equivalent amounts of the ligands (Fig. S5, ESI†). The N2 sorption isotherm indicates that ZJU-24-0.89*MB has a very low porosity with a BET of 36 m2 g1 (Fig. S13, ESI†). Although a squeeze operation can reduce the effect of a solvent in pores, the structure may change in the process of modelling disordered electron density for a framework with many unordered guest molecules, so we consider the data without a squeeze operation more reliable. Single-crystal X-ray diffraction indicates that the intersection angle between plane 1 (Fig. S10, ESI†), determined by the middle benzene ring, and plane 2 (Fig. S10, ESI†), determined by the other two benzene rings, changes to 58.051 compared with the 45.621 before MB adsorption. This 12.431 rotation may be an adjustment caused by the electrostatic attraction between the exposed carboxyl group and MB. After MB adsorption, the exposed –COOH groups are stretched, the angle of O–C–O changes from 121.8471 to 137.2751, the length of CQO changes from 1.0759 Å to 1.2423 Å, and the length of C–O changes from 1.1781 Å to 1.3040 Å. Molecular simulation studies further confirm the interaction of the carboxylic acid groups with the MB molecules (Fig. S15, ESI†). In summary, we designed and synthesized a unique microporous metal–organic framework for the efficient removal of methylene blue from aqueous solution with the highest reported adsorbed amount
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1 S. A. Snyder, P. Westerhoff, Y. Yoon and D. L. Sedlak, Environ. Eng. Sci., 2003, 20, 449. ¨nge, T. H. Hutchinson, C. P. Croudace, F. Siegmund, 2 R. La H. Schweinfurth, P. Hampe, G. H. Panter and J. P. Sumpter, Environ. Toxicol. Chem., 2001, 20, 1216. 3 (a) M. J. Benotti, R. A. Trenholm, B. J. Vanderford, J. C. Holady, B. D. Stanford and S. A. Snyder, Environ. Sci. Technol., 2008, 43, 597; (b) D. W. Kolpin, E. T. Furlong, M. T. Meyer, E. M. Thurman, S. D. Zaugg, L. B. Barber and H. T. Buxton, Environ. Sci. Technol., 2002, 36, 1202. 4 (a) A. C. Johnson and J. P. Sumpter, Environ. Sci. Technol., 2001, 35, 4697; (b) P. Westerhoff, Y. Yoon, S. Snyder and E. Wert, Environ. Sci. Technol., 2005, 39, 6649. 5 (a) P. Nugent, Y. Belmabkhout, S. D. Burd, A. J. Cairns, R. Luebke, K. Forrest, T. Pham, S. Ma, B. Space, L. Wojtas, M. Eddaoudi and M. J. Zaworotko, Nature, 2013, 495, 80; (b) Q. Lin, T. Wu, S.-T. Zheng, X. Bu and P. Feng, J. Am. Chem. Soc., 2011, 134, 784; (c) P.-Q. Liao, D.-D. Zhou, A.-X. Zhu, L. Jiang, R.-B. Lin, J.-P. Zhang and X.-M. Chen, J. Am. Chem. Soc., 2012, 134, 17380; (d) D.-X. Xue, A. J. Cairns, Y. Belmabkhout, L. Wojtas, Y. Liu, M. H. Alkordi and M. Eddaoudi, J. Am. Chem. Soc., 2013, 135, 7660; (e) H. Wu, Q. Gong, D. H. Olson and J. Li, Chem. Rev., 2012, 112, 836; ( f ) Y. He, W. Zhou, T. Yildirim and B. Chen, Energy Environ. Sci., 2013, 6, 2735; ( g) F. Luo, C. B. Fan, M. B. Luo, X. L. Wu, Y. Zhu, S. Z. Pu, W. Y. Xu and G. C. Guo, Angew. Chem., Int. Ed., 2014, 53, 9298; (h) F. Luo, M. S. Wang, M. B. Luo, G. M. Sun, Y. M. Song, P. X. Li and G. C. Guo, Chem. Commun., 2012, 48, 5989. 6 (a) W. J. Rieter, K. M. L. Taylor and W. Lin, J. Am. Chem. Soc., 2007, 129, 9852; (b) B. Chen, S. Xiang and G. Qian, Acc. Chem. Res., 2010, 43, 1115; (c) H.-C. Zhou, J. R. Long and O. M. Yaghi, Chem. Rev., 2012, 112, 673; (d) Y. Cui, Y. Yue, G. Qian and B. Chen, Chem. Rev., 2012, 112, 1126; (e) K. A. White, D. A. Chengelis, K. A. Gogick, J. Stehman, N. L. Rosi and S. Petoud, J. Am. Chem. Soc., 2009, 131, 18069. 7 (a) Q. Fang, S. Gu, J. Zheng, Z. Zhuang, S. Qiu and Y. Yan, Angew. Chem., Int. Ed., 2014, 53, 2878; (b) J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen and J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450; (c) Q.-L. Zhu and Q. Xu, Chem. Soc. Rev., 2014, 43, 5468. 8 (a) P. Horcajada, R. Gref, T. Baati, P. K. Allan, G. Maurin, ´rey, R. E. Morris and C. Serre, Chem. Rev., 2011, P. Couvreur, G. Fe 112, 1232; (b) R. V. Thaner, I. Eryazici, O. K. Farha, C. A. Mirkin and S. T. Nguyen, Chem. Sci., 2014, 5, 1091; (c) K. Khaletskaya, J. Reboul, M. Meilikhov, M. Nakahama, S. Diring, M. Tsujimoto, S. Isoda, F. Kim, K.-i. Kamei, R. A. Fischer, S. Kitagawa and S. Furukawa, J. Am. Chem. Soc., 2013, 135, 10998. 9 (a) N. A. Khan, Z. Hasan and S. H. Jhung, J. Hazard. Mater., 2013, 244–245, 444; (b) E. Haque, J. E. Lee, I. T. Jang, Y. K. Hwang, J.-S. Chang, J. Jegal and S. H. Jhung, J. Hazard. Mater., 2010, 181, 535; (c) S.-H. Huo and X.-P. Yan, J. Mater. Chem., 2012, 22, 7449; (d) X. Li, H. Xu, F. Kong and R. Wang, Angew. Chem., Int. Ed., 2013, 52, 13769; (e) S.-T. Zheng, X. Zhao, S. Lau, A. Fuhr, P. Feng and X. Bu, J. Am. Chem. Soc., 2013, 135, 10270; ( f ) M. Maes, S. Schouteden, L. Alaerts, D. Depla and D. E. De Vos, Phys. Chem. Chem. Phys., 2011, 13, 5587.
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Cite this: DOI: 10.1039/c4cc06648k Received 23rd August 2014, Accepted 29th September 2014
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A porous metal–organic framework with –COOH groups for highly efficient pollutant removal† Qi Zhang,a Jiancan Yu,a Jianfeng Cai,a Ruijing Song,a Yuanjing Cui,a Yu Yang,a Banglin Chen*abc and Guodong Qian*a
DOI: 10.1039/c4cc06648k www.rsc.org/chemcomm
A new metal–organic framework with –COOH groups has been realized and demonstrates strong interactions with methylene blue and thus the complete removal of methylene blue from aqueous solution.
Endocrine disrupting chemicals (EDCs) are classified as any exogenous substances that cause adverse health effects in an intact organism, or lead to changes in endocrine function.1 In fact, even trace amounts of EDCs (1–1000 ng L1) can lead to adverse effects on the reproductive, neurological, and immune systems of aquatic organisms and humans.1,2 The release of EDCs in the aquatic environment has been an emerging challenge.3 Complete elimination of EDCs is essential prior to wastewater discharge or drinking water distribution; however conventional operation treatments may not produce suitable removal efficiencies.4 Metal–organic frameworks (MOFs), which can be easily selfassembled from metal ions/clusters with organic linkers, might provide solutions for EDC removal given the fact that they have large surface areas, adjustable pore sizes, and controllable/ functional pore surfaces. To make use of these unique features, MOFs have been widely explored in gas storage and separation,5 luminescence,6 catalysis,7 and biomedicine;8 though they are less developed for pollutant adsorption/removal.9 MOFs and their composites previously examined for the removal of methylene blue do not have strong recognition sites, limiting their removal capacities for methylene blue.10 The organic backbone of methylene blue can be considered as a cationic species, so if MOFs can have enough space to encapsulate
methylene blue, while the specific sites within such MOFs can have strong interactions with methylene blue, we should be able to construct suitable MOFs for the highly efficient removal of methylene blue. With this in mind, we design and synthesize a new organic linker (Fig. 1b) to construct a microporous MOF. The large organic backbone of this organic linker enforces the construction of large pore spaces to encapsulate methylene blue molecules, while the free –COOH groups on the pore surfaces generate strong interactions with the methylene blue molecules. To our surprise, the resulting MOF (ZJU-24 series) not only exhibits the highest reported methylene blue removal capacity of 902 mg g1, but can also completely remove methylene blue from its aqueous solution. The ZJU-24 series are iso-structural MOFs of the well-known NOTT-101 constructed from a copper paddle wheel Cu2(COO)4 with organic linkers of tetracarboxylates with NbO topologies.11
a
State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and Applications, Department of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China. E-mail: [email protected] b Department of Chemistry, University of Texas at San Antonio, San Antonio, TX 78249, USA. E-mail: [email protected] c Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 22254, Saudi Arabia † Electronic supplementary information (ESI) available. CCDC 1018379. For ESI and crystallographic data in CIF or other electronic formats see DOI: 10.1039/ c4cc06648k
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Fig. 1 The introduction of functional –COOH groups into the iso-structural (a) NOTT-101 forms (b) ZJU-24-0.89; the X-ray crystal structure of ZJU-240.89 shows (c) a spherical-like cage of about 13.8 Å and (d) a shuttle-shaped cage of about 14 Å in diameter (Cu, blue; C: grey; O: red); (e) N2 sorption isotherms of NOTT-101, ZJU-24-0.5 and ZJU-24-0.89 at 77 K demonstrating the effect of immobilized –COOH groups on their porosities (solid symbols: adsorption, open symbols: desorption).
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They can be easily synthesized from Cu(NO3)2 and a mixture of H4TPTC ([1,1 0 :4 0 ,100 -terphenyl]-3,300 ,5,500 -tetracarboxylic acid) and H6TPHC ([1,10 :4 0 ,100 -terphenyl]-20 ,3,300 ,5,50 ,500 -hexacarboxylic acid) of different ratios (Fig. S1, ESI†). Based on the content of H6TPHC in a mixture of H4TPTC/H6TPHC, the resulting MOFs were termed as ZJU-24-0.25, ZJU-24-0.5, ZJU-24-0.75 and ZJU-24-0.89, respectively. As shown in Fig. S1 (ESI†), these MOFs have the same structure as NOTT-101 if the ratios of H4TPTC:H6TPHC are within 1:8. The content and the inclusion of H6TPHC in ZJU-24-0.25, ZJU-24-0.5, ZJU-24-0.75 and ZJU-24-0.89 were further confirmed by NMR analysis after the MOF crystals were dissolved in DCl/ DMSO-D6 (Fig. S1b and S4, ESI†). N2 adsorption isotherms at 77 K for the two samples named ZJU-24-0.5 and ZJU-24-0.89 were measured to determine their permanent porosities and these were compared with NOTT-101. As expected, the incorporation of the –COOH groups decreases the pore spaces and thus the N2 uptake accordingly (Fig. 1e). ZJU-24-0.5 and ZJU-24-0.89 have lower BET surfaces of 1700 and 1189 m2 g1, respectively, than NOTT-101 of 2316 m2 g1.12 The existence of the –COOH groups on the pore surfaces was further established by single crystal structure determination of ZJU-24-0.89. As shown in Fig. 1, ZJU-24-0.89 has the same basic structure as NOTT-101 except that some of the organic linkers of TPTC have been replaced by H2TPHC. The pore spaces are about 13.8–14.0 Å in diameter (according to the space-filling models). Because the –COOH groups are located in the framework voids of ZJU-24-0.89 while the pore channels are large enough to encapsulate methylene blue molecules, its performance for the removal of methylene blue (MB) from aqueous solution was examined. As shown in Fig. 2, after small crystals of ZJU-24-0.89 (5 mg) were immersed in a 50 mL aqueous solution of MB (5 ppm), the concentration of MB in solution decreased by 50% in 5 minutes. After 12 h, MB can be completely removed from the aqueous solution in which no MB can be detected in UV spectra. This is significantly superior to NOTT-101: after 12 h, only very small amount of about 5% MB can be removed. Once the MB has been completely removed by ZJU-24-0.89 from the aqueous solution after 12 h, the solution becomes colourless and clear (Fig. 2b). It is reasonable to speculate that the –COOH groups within the pore surfaces of ZJU-24-0.89 play the most important role in the removal of MB. To further confirm this claim, the ZJU-24 series with different amounts of H6TPHC ZJU-24-0.25, ZJU-24-0.5, ZJU-24-0.75 and ZJU-24-0.89 were compared for their performance to remove MB. As shown in Fig. 3, the MOF with the higher content of H6TPHC exhibited the highest adsorption speed and uptake for MB. The adsorption isotherms were collected accordingly for ZJU-24-0.89 and NOTT-101 (Fig. S6, ESI†). The amount of adsorbed MB over ZJU-24-0.89 after 12 h is 902 mg g1, which is the highest reported among any porous materials for MB removal so far (Table S2, ESI†).10b,13 To further study the microscopic mechanism of the host– guest interaction, the vibrational properties of the two states before and after MB adsorption in DMF were investigated by Raman (Fig. S7, ESI†) and FT-IR (Fig. S8, ESI†) spectroscopy. The assignment of the MOFs vibrational modes is a necessary
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Fig. 2 (a) UV spectra of an aqueous MB solution during an adsorption test with ZJU-24-0.89; (b) photographs of the removal of MB in aqueous solution with ZJU-24-0.89 over time; (c) UV spectra of an aqueous MB solution during an adsorption test with NOTT-101; (d) photographs of the removal of MB in aqueous solution with NOTT-101 over time.
Fig. 3 Effect of the H6TPHC content in the ZJU-24 series for the removal of MB.
step for determining the interaction between the framework and guest molecules. In the Raman spectroscopy, in the 1800– 900 cm1 range, the spectrum is dominated by modes associated with the organic part of the MOF.14 The band at 1608 cm1 is assigned to the phenyl mode n8a[(CQC) stretching mode of ligand] that only appears in the Raman spectrum because it has parity under Ci symmetry.15 A comparison of the two states’ Raman spectra indicates that the n8a(CQC) mode of the benzene ring shifts from 1608 to 1623 cm1 after the MB molecules were introduced into the pores. In the FT-IR spectra, two bands at 1566 cm1 and 1373 cm1 assigned to n(CQC) drift to 1581 cm1 and 1380 cm1, respectively. Such changes might be attributed to the p–p interactions between the framework and the MB molecules. The changes for COO stretching are also obvious. In the Raman spectroscopy, the bands at 1545 cm1 and 1430 cm1 are due to the nas (COO) and ns (COO) units. After MB is absorbed, nas (COO) vanishes and ns (COO) drifts to 1441 cm1. In the FT-IR spectra, the –COO
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Fig. 4 (a) Crystal structure of ZJU-24-0.89; (b) crystal structure of ZJU-24-0.89*MB with simulated MB molecules inside the pores.
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of 902 mg g1 and complete removal after 12 h. This is attributed to the immobilized –COOH groups on the pore surfaces and their strong interactions with the methylene blue molecules. This new strategy will facilitate the extensive research on functional porous MOFs for EDC extraction, and lead to some practically useful materials for these important applications in the near future. We would like to thank Zhi Li of the State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249 for her help in the molecular simulation studies and the National Natural Science Foundation of China (No. 51010002, 51272229 and 51272231), the Zhejiang Provincial Natural Science Foundation of China (No. LR13E020001) and the Welch Foundation (AX-1730).
Notes and references group is characterized by the in phase or out of phase vibrations of the two equivalent C–O bonds, leading to the symmetric and antisymmetric stretch modes. nas (COO) is at 1623 cm1, and the symmetric stretch mode of ns (COO) should be weaker than the band at 1623 cm1. Since the band at 1373 cm1 is attributed to the benzene ring mode reference to IR spectra of ligands (Fig. S9, ESI†) and reference assignments,16 we assign ns (COO) at 1433 cm1. After MB adsorption, nas (COO) drifts to 1631 cm1, and no obvious drift was observed for ns (COO). This is because IR spectra are sensitive to antisymmetric stretch modes. The transformation of the –COO vibration mode is due to the electrostatic interactions between the anionic framework and cationic MB molecules. Single-crystal X-ray diffraction was carried out to characterize the crystal structure change before and after MB being absorbed in DMF (Fig. 4). The inclusion of MB in ZJU-24-0.89 was revealed by NMR analysis after the MOF crystals were dissolved in DCl/ DMSO-D6. The NMR analysis revealed that the MB molecules absorbed by the crystals were in 0.4 equivalent amounts of the ligands (Fig. S5, ESI†). The N2 sorption isotherm indicates that ZJU-24-0.89*MB has a very low porosity with a BET of 36 m2 g1 (Fig. S13, ESI†). Although a squeeze operation can reduce the effect of a solvent in pores, the structure may change in the process of modelling disordered electron density for a framework with many unordered guest molecules, so we consider the data without a squeeze operation more reliable. Single-crystal X-ray diffraction indicates that the intersection angle between plane 1 (Fig. S10, ESI†), determined by the middle benzene ring, and plane 2 (Fig. S10, ESI†), determined by the other two benzene rings, changes to 58.051 compared with the 45.621 before MB adsorption. This 12.431 rotation may be an adjustment caused by the electrostatic attraction between the exposed carboxyl group and MB. After MB adsorption, the exposed –COOH groups are stretched, the angle of O–C–O changes from 121.8471 to 137.2751, the length of CQO changes from 1.0759 Å to 1.2423 Å, and the length of C–O changes from 1.1781 Å to 1.3040 Å. Molecular simulation studies further confirm the interaction of the carboxylic acid groups with the MB molecules (Fig. S15, ESI†). In summary, we designed and synthesized a unique microporous metal–organic framework for the efficient removal of methylene blue from aqueous solution with the highest reported adsorbed amount
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