- Email: [email protected]
Influence of metal ions on the selective catalytic oxidation properties of isostructural MOFs
Accepted Manuscript Research paper Influence of metal ions on the selective catalytic oxidation properties of isostructural MOFs Yu Han, Yuanyuan Li, Xiaoxiao Wang, Yansong Li, Haitao Xu, Siyan Chen, Zhen-liang Xu PII: DOI: Reference:
S0020-1693(17)31122-2 https://doi.org/10.1016/j.ica.2017.11.008 ICA 17978
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
19 July 2017 10 October 2017 3 November 2017
Please cite this article as: Y. Han, Y. Li, X. Wang, Y. Li, H. Xu, S. Chen, Z-l. Xu, Influence of metal ions on the selective catalytic oxidation properties of isostructural MOFs, Inorganica Chimica Acta (2017), doi: https://doi.org/ 10.1016/j.ica.2017.11.008
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of metal ions on the selective catalytic oxidation properties of isostructural MOFs
Yu Han, Yuanyuan Li , Xiaoxiao Wang, Yansong Li, Haitao Xu∗ , Siyan Chen, Zhen-liang Xu
State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Lab, Chemical Engineering Research Center, East China University of Science and Technology (ECUST), 130 Meilong Road, Shanghai 200237, China. RECEIVED DATE (automatically inserted by publisher); E-mail: [email protected](H. X.) Tel: 86-021-64252989
Abstract: Two new metal−organic frameworks (MOFs) [Co(AIA)(3-bpdh), Mn(AIA)(3-bpdh)], featuring 2-dimensional layer network topologies are successfully assembled by the solvent diffusion method as CoⅡ and MnⅡ isostructural MOFs, hereafter, 1Co and 1Mn. Moreover, a series of micro-crystals of these two kinds of MOFs with different morphologies and sizes are also obtained by adjusting the synthesis systems. Interestingly, one MOF exhibits selective catalytic oxidation properties for the degradation of malachite green dye, whereas the other MOF shows corresponding properties for methylene blue. In addition, a detailed possible oxidation catalytic mechanism is proposed. This work highlights that MOFs are functional materials for selective dye degradation and demonstrates that metal ion replacement is an efficient way to regulate MOFs for deserving applications.
Keywords: Metal-organic frameworks; Crystal structure; Microcrystals; Selective catalysis; Organic dyes
1
1. Introduction Metal−organic frameworks (MOFs), a class of materials prepared by the self-assembly of transition metals and functionalized organic linkers, have attracted significant research attention[1-7]. The applications of MOF materials are based on their structural design[8,9]. Since their network topologies can be rationally designed, MOFs exhibit various excellent characteristics compared with other porous materials, such as high porosity, large specific surface area, and low solid density[10-12]. In general, the topologies of MOFs can be regulated by the structure of the ligands, solvent system, metal-to-ligand ratio, etc[13,14]. Among these strategies, the selection of the metal ion remains a key factor for determining the crystal structures, morphologies, and properties of MOFs. Furthermore, MOFs are excellent candidates for the construction of functional microparticles, which can create new opportunities for their application. It is necessary to investigate how to successfully integrate MOFs into functional devices and regulate their growth at the microscale and nanoscale further in-depth. Organic dyes are regarded as serious pollutants due to their non-biodegradability, toxicity, and harmful effects for water[15] all through the world. The removal of organic pollutants has become an urgent and important issue. To date, various approaches, including physical, chemical, and biological treatment, have been developed to treat waste products. Because of the generally poor selectivity of current technologies for removing specific organic dye wastes, it is essential to develop highly active and selective materials for their degradation. Previous studies have shown that MOFs are green catalysts for the degradation of organic dyes[16]. However, MOFs with high selectivity for the degradation of organic dyes are currently limited. Herein, we synthesized two new MOFs, 1Co and 1Mn, by selecting Co and Mn, respectively, as the metal center while using 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene (3-bpdh) and 5-aminoisophthalic acid (AIA) as ligands in both cases. The results showed that 1Co and 1Mn were isostructural and possess two-dimensional (2D)-layer network topologies. In addition, 1Co exhibits selective oxidation−degradation of malachite green (MG), whereas 1Mn exhibits corresponding performance for methylene blue (MB).
2. Experimental section Synthesis of the single-crystals of 1Co and 1Mn: a mixture of Co(NO3)2·6H2O (0.075 mmol, 0.0218 g), AIA(0.075 mmol, 0.0135 g), NaOH (0.083 mmol, 0.0033 g) and 4mL deionized water was stirred to obtain a pink homogeneous solution of Co-AIA. A solution of 3-bpdh (0.075 mmol, 0.0178 g) in methanol (4 mL) was then carefully layered onto the Co-AIA solution through 2mL buffer solution (H2O and MeOH mixture, v/v =1:1) in a test tube. The tube was sealed and left undisturbed at room temperature. After two weeks, the light-pink sheet crystals of 1Co were obtained (Fig. S1), which were suitable for single X-ray diffraction analysis. The yellow 2
block crystals of 1Mn were synthesized using the same procedure, but with MnCl2·4H2O (0.075 mmol, 0.0218 g) instead of Co(NO3)2· 6H2 O. Elemental analysis results are as follows: calcd for 1Co: C22H19CoN5O4: C, 55.46; H, 3.99; N, 14.71%; Found: C, 54.88; H, 3.58; N, 14.56%. Calcd for 1Mn: C22H19 MnN5O4: C, 55.94; H, 4.03; N, 14.83%; Found: C, 54.72; H, 3.72; N, 14.26%. For the micro-crystals of 1Co and 1Mn synthesis, the above solution of M-AIA (M = Co or Mn) was mixed with the ligand solution directly. Then the mixture was stirred for 2 h and the precipitate was filtered and air-dried. In addition, other solvents (ethanol, propanol, ether, etc.) and surfactants (op-10, tween-40, span-80, etc.) were selected so as to controllably synthesize micro-crystals of 1Co and 1Mn. As a result, a series of crystals with different morphologies and sizes were obtained.
3. Results and discussion
Fig. 1. The coordinate structure of M(AIA)(3-bpdh) (M=Co or Mn): (a) The coordination environment around the M(II) ions, (b) perspective view showing the 2D chain structure formed by the metal ion and AIA ligands, and (c) perspective view showing the layer structure formed by the 2D chain structure linked to the 3-bpdh ligands.
Single crystals of 1Co and 1Mn were synthesized using the solvent diffusion method. The detailed synthesis and elemental analysis results are given in the Supplementary Information. The crystals of 1Co and 1Mn were stable in air and insoluble in common organic solvents. In the IR spectra of the complexes, the strong characteristic peak of AIA at 1631 cm−1 shifted to lower frequencies (1Co, 1546 cm−1; 1Mn, 1587 cm−1), which was attributed to the coordination between the carboxyl groups and the metal ions (Fig. S2). The expected strong characteristic peaks of C=N were observed in the spectra of both 1Co (1616 cm−1) and 1Mn (1618 cm−1), indicating the stretching vibration of the 3-bpdh ligands. In addition, the TGA curves showed that 1Co has no significant weight loss up to 380 °C (Fig. S4). Meanwhile, for 1Mn, a weight loss of 5% was observed from room 3
temperature to 84 °C due to the removal of adsorbed water. Moreover, the dehydrated framework of 1Mn was also stable up to 380 °C. At this temperature, both crystals simultaneously decomposed. A sharp weight loss of 46.8% was found for 1Co in the temperature range of 380−494 °C, which resulted from the loss of the 3-bpdh ligands (calcd: 50%). In addition, a 40.1% weight loss was observed for 1Mn from 380 to 564 °C because of the decomposition of the ligands (calcd: 38%). The BET surface areas of 1Co (Fig. S5) and 1Mn (Fig. S6) were 20 and 14 m2/g, respectively. Single-crystal X-ray structural analysis revealed that MOFs 1Co and 1Mn are isostructural only differing in the metal ion (Fig. S7-S8). As shown in Fig. 1, both of the MOFs have layered network topologies and crystallize in the triclinic P-1 space group with a Z value of 2. Their unit cell dimensions were also similar (for 1Co: a = 10.056(10) Å, b = 11.294(11) Å, c = 11.572(18) Å; for 1Mn: a = 10.169(11) Å, b = 11.243(13) Å, c = 11.66(2) Å). In their frameworks, only one crystallographically independent M(II) ion (M=Co or Mn) was located in the hexacoordinate environment. Each M(II) ion was linked to four oxygen atoms from three AIA ligands and to two nitrogen atoms from two 3-bpdh ligands. As shown, every M(II) ion was chelated by one AIA ligand and bridged by the other AIA ligand to produce a 2D [M(AIA)]n chain. Then, the 2D chains (occupying the axial positions) were linked by the 3-bpdh ligands (occupying the equatorial positions) to construct a layered network topology. In
Fig. 2. SEM images of microcrystals of 1Co (a1-a9) and 1Mn (b1-b9) synthesized under different conditions. 1Co: (a1) methanol−water; (a2) ethanol−water; (a3) n-propanol−water; (a4) methanol/ether−water; (a5) methanol/(op-10)−water; (a6) methanol/ether/(op-10)−water; (a7) methanol/(tween-40)−water; (a8) methanol/(tween-60)−water; (a9) methanol/(span-80)−water. 1Mn: (b1) methanol−water; (b2) n-propanol−water; (b3) n-butanol/ether−water; (b4) ethanol/ether−water; (b5) methanol/ether−water; (b6) methanol/(op-10)−water; (b7) methanol/ether/(op-10)−water; (b8) methanol/(tween-60)−water; (b9)methanol/(span-80)−water.
compounds 1Co and 1Mn, the M(II)−O bond lengths varied from 2.004(4) to 2.285(3) Å (1Co) and from 2.102 (4) 4
to 2.324(3) Å (1Mn); the corresponding M(II)−N bond lengths were from 2.167(3) to 2.168(3) Å (1Co) and from 2.280(4) to 2.285(4) Å (1Mn), respectively. Moreover, the N/O−M(II)−O/N angles varied from 59.18 to 175.34° (1Co) and from 57.29 to 173.28° (1Mn) (Table S2-S6). Various synthesis systems were applied to regulate the morphologies and sizes of the microcrystals of 1Co and 1Mn. The purities of 1Co and 1Mn synthesized in these different solvent systems were verified by PXRD and were in accordance with the simulated patterns (Fig. S9-S10). As can obviously be seen (Fig. 2), the microcrystals of these two samples showed different morphologies and sizes under the same conditions. When the crystals were synthesized in a water−methanol system, 1Co exhibited a block-like morphology with different sizes, whereas 1Mn showed a uniform rod-like morphology with lengths of 15 µm (Fig. 2, a1 and b1). When the crystals were synthesized in a methanol/ether−water system, the crystals of 1Co also formed block-like aggregates but smaller in size. In addition, the crystals of 1Mn changed to block-like aggregates (Fig. 2, a4 and b5). When the crystals were synthesized in a methanol/ether/surfactant−water system, the crystals of 1Co (Fig. 2, a5-a9) agglomerated into microspheres comprising flakes. In contrast, the microcrystals of 1Mn formed micro-rods but with smaller sizes of about 10 µm (Fig. 2, b6-b9). The results discussed above indicated that the nature of the materials played an important role in determining the morphologies and sizes of microcrystals. Moreover, the synthesis system, including the reaction solvents and surfactant, influenced the morphologies of microcrystals. 1Co exhibited block-like aggregates with different sizes in an alcohol−water system. However, the morphology tended to become microspherical as surfactants were added to the system. For 1Mn, the more hydrophilic the synthesis system, the greater was the proportion of rod-like microcrystals. However, block-like aggregates instead of rod-like ones were obtained when ether was added to the system. The various morphologies and particle sizes of 1Co and 1Mn synthesized under different systems can be explained from the view point of growth kinetics. The growth rates differ along different crystallographic directions. As discussed above, these two MOFs showed different compositions and morphologies, which might affect their catalytic performance. To reveal the catalytic oxidation properties of 1Co and 1Mn, the catalytic degradation of dyes was chosen as a model reaction. In this experiment, MG, MB, orange G (OG), rhodamine B (RhB), and methyl orange (MO) were chosen as typical organic pollutant targets (SchemeS1). In a typical experiment, a suspension containing 1Co or 1Mn catalyst (30 mg) and 125 mL of a 12 ppm dye solution was stirred for 180 min. For RhB, OG, and MO, 1Co and 1Mn did not show any catalytic activity (Fig. 3). In comparison, 1Co degraded 19% of MG, whereas 1Mn degraded 21% of MB. This result indicated the highly selective degradation properties of these two 5
Fig. 3. Degradation of dyes using 1Co (left) and 1Mn (right) crystals. (a) methyl orange, (b) orange G, (c) malachite green, (d) methylene blue, and (e) rhodamine B. The initial concentration of all the dyes was 12 ppm.
crystals. Therefore, although the different compositions of the MOFs resulted in different degradation properties, both of the MOFs may be useful for waste water treatment. In addition, the degradation performances of crystal 1Co and 1Mn were enhanced by increasing the amount of catalyst per volume unit in the solution, which could provide more active sites (Fig. S12-S13). After the catalytic degradation was complete, the crystals could be easily retrieved by filtration and reused. The morphologies of the recovered crystals remained the same, except for the appearance of some cracks due to the stirring process (Fig. S1). The IR (Fig. S3) and PXRD (Fig. S11) patterns of the recovered crystals were all in good agreement with those of the original crystals, and there was no evidence of the presence of newly adsorbed products. Specially, some characteristic peaks of IR patterns for dyes were not found in that of catalysts, indicating that there seemed no adsorption of dyes for catalysts (Fig. S14-15). What is more, in order to demonstrate that the degradation of the dye is due to the catalytic oxidation, we conducted a comparative test under nitrogen protection conditions. As the results indicated, there are almost no degradation effect can be detected under anaerobic conditions (Fig. S16-17). According to our previous study, iminium organic dyes, which contains C=N group, can be catalytically degraded. While RhB cannot be degraded due to its steric hindrance. MO and OG are azo-compounds .Although the N=N group in MO and OG is more easily to break, the energy which 1Co and 1Zn acquired is just accordance with the energy that break C=N group needed in MG and MB respectively. The coordinated Co and Mn have different energy to break the C=N bond. From the structure of MG and MB, π electronic conjugate of MG is greater than that of MB. while coordinated hexacoordinate Co posses more energy than hexacoordinate Mn, so 1Co and 1MB can degradate MG and MB respectively but not RhB, OG and MO.
6
A probable mechanism for the degradation of organic dyes over 1Co and 1Mn has been proposed[17-19] (Fig. S18). First, we propose that an oxygenated intermediary species I is generated by the coordination of oxygen to the metal center along with the corresponding expansion of the coordination number. As 1Co (or 1Mn) attracts dioxygen, the coordination between oxygen and the M(II) ion becomes increasingly weak, whereas the oxygen of the dioxygen becomes more strongly bound to the M(II) ion. This interaction between 1Co (or 1Mn) and dioxygen yields the oxygenated intermediary species I, whose active transition-state complex can catalytically oxidize the C=N or C=C groups of organic dyes. The second step involves a nucleophilic attack of the unsaturated substrate on species I followed by oxygen transfer, leading to the formation of species II. In the final step, the M(II)−O bond is ruptured, oxidizing the dye and regenerating the catalyst.
4. Conclusion In summary, two isostructural microcrystal MOFs, [M(AIA)(3-bpdh)] (M=Co or Mn), have been synthesized using slow diffusion reaction. It was demonstrated that these two precipitation/MOFs is a suitable approach for the fabrication of pre-synthesized units into functional micro-and nanocrystals, exhibiting excellent conversion and selectivity in the catalytic degradation of MG by 1Co and MB by 1Mn. The high flexibility of this strategy and the effective catalysis will certainly be an efficient way to produce novel coordination polymers for useful applications. It will also enhance the applications in other fields.
Associated content Supporting Information Available: The structural data of the single crystals in CIF format, packing view of the complex, and room-temperature IR spectra of the complexes are available free of charge.
Acknowledgments We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21371058) and the Ministry of Education (A200-C-1101). This work was supported in part by East China University of Science & Technology (YA0157117) and the Key Laboratory for Ultrafine Materials of the Ministry of Education (ECUST).
References 1.
H.C. Zhou, J. R. Long and O. M. Yaghi, Chem. Rev., 112 (2012) 673.
2.
X. Yan, M. Wang, T. R. Cook, M. Zhang, M. L. Saha, Z. Zhou, X. Li, F. Huang and P. J. Stang, J. Am. Chem. Soc. 138 (2016) 4580-4588.
3.
S. L. Li, Y. Q. Lan, H. Sakurai and Q. Xu, Chemistry., 18 (2012) 16302-16309.
4.
J. Yi, H. Li, L. Jiang, K. Zhang and D. Chen, Chem. Commun., 51 (2015) 10162. 7
5.
D. Dang, P. Wu, C. He, Z. Xie and C. Duan, J. Am. Chem. Soc., 132 (2010) 14321-14323.
6.
Y.-J. Zhang, T. Liu, S. Kanegawa and O. Sato, J. Am. Chem. Soc., 132 (2009) 912-913.
7.
X.-Y. Wang, Z.-M. Wang and S. Gao, Chem. Commun., (2007) 1127.
8.
H. Bunzen, M. Grzywa, M. Hambach, S. Spirkl and D. Volkmer, Crystal Growth & Design, 16 (2016) 3190-3197.
9.
X. Zhang, Z. Zhang, J. Boissonnault and S. M. Cohen, Chem. Commun. (Camb.), 52 (2016) 8585-8588.
10.
P. Horcajada, F. Salles, S. Wuttke, T. Devic, D. Heurtaux, G. Maurin, A. Vimont, M. Daturi, O. David, E. Magnier, N. Stock, Y. Filinchuk, D. Popov, C. Riekel, G. Ferey and C. Serre, J. Am. Chem. Soc., 133 (2011) 17839-17847.
11. 12.
G.-X. Liu, H. Xu, H. Zhou, S. Nishihara and X.-M. Ren, CrystEngComm., 14 (2012) 1856. H. Q. Xu, J. Hu, D. Wang, Z. Li, Q. Zhang, Y. Luo, S. H. Yu and H. L. Jiang, J. Am. Chem. Soc., 137 ( 2015) 13440-13443.
13.
C. Liu, C. Zeng, T. Y. Luo, A. D. Merg, R. Jin and N. L. Rosi, J. Am. Chem. Soc., 138 (2016) 12045-12048.
14.
N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O'Keeffe and O. M. Yaghi, Science., 300 (2003) 1127.
15.
M. Liang and J. Chen, Chem. Soc. Rev., 42 (2013) 3453.
16.
T. Zhang and W. Lin, Chem. Soc. Rev., 43 (2014) 5982.
17.
Q. Meng, J. Ye, Z.-l. Xu and H. Xu, Mater. Lett., 116 (2014) 378.
18.
S. El-Korso, I. Khaldi, S. Bedrane, A. Choukchou-Braham, F. Thibault-Starzyk and R. Bachir, J. Mol. Catal. A: Chem., 3942 (2014) 89.
19.
K. Brown, S. Zolezzi, P. Aguirre, D. Venegas-Yazigi, V. Paredes-García, R. Baggio, M. A. Novak and E. Spodine, Dalton Transactions., (2009), 1422.
8
Research Highlights: Synthesis and structure of isostructural metal–organic framework materials 1Co and 1Mn Selective catalysis Catalytic properties of metal–organic frameworks controlled by metal ions
9
S0020-1693(17)31122-2 https://doi.org/10.1016/j.ica.2017.11.008 ICA 17978
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
19 July 2017 10 October 2017 3 November 2017
Please cite this article as: Y. Han, Y. Li, X. Wang, Y. Li, H. Xu, S. Chen, Z-l. Xu, Influence of metal ions on the selective catalytic oxidation properties of isostructural MOFs, Inorganica Chimica Acta (2017), doi: https://doi.org/ 10.1016/j.ica.2017.11.008
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of metal ions on the selective catalytic oxidation properties of isostructural MOFs
Yu Han, Yuanyuan Li , Xiaoxiao Wang, Yansong Li, Haitao Xu∗ , Siyan Chen, Zhen-liang Xu
State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Lab, Chemical Engineering Research Center, East China University of Science and Technology (ECUST), 130 Meilong Road, Shanghai 200237, China. RECEIVED DATE (automatically inserted by publisher); E-mail: [email protected](H. X.) Tel: 86-021-64252989
Abstract: Two new metal−organic frameworks (MOFs) [Co(AIA)(3-bpdh), Mn(AIA)(3-bpdh)], featuring 2-dimensional layer network topologies are successfully assembled by the solvent diffusion method as CoⅡ and MnⅡ isostructural MOFs, hereafter, 1Co and 1Mn. Moreover, a series of micro-crystals of these two kinds of MOFs with different morphologies and sizes are also obtained by adjusting the synthesis systems. Interestingly, one MOF exhibits selective catalytic oxidation properties for the degradation of malachite green dye, whereas the other MOF shows corresponding properties for methylene blue. In addition, a detailed possible oxidation catalytic mechanism is proposed. This work highlights that MOFs are functional materials for selective dye degradation and demonstrates that metal ion replacement is an efficient way to regulate MOFs for deserving applications.
Keywords: Metal-organic frameworks; Crystal structure; Microcrystals; Selective catalysis; Organic dyes
1
1. Introduction Metal−organic frameworks (MOFs), a class of materials prepared by the self-assembly of transition metals and functionalized organic linkers, have attracted significant research attention[1-7]. The applications of MOF materials are based on their structural design[8,9]. Since their network topologies can be rationally designed, MOFs exhibit various excellent characteristics compared with other porous materials, such as high porosity, large specific surface area, and low solid density[10-12]. In general, the topologies of MOFs can be regulated by the structure of the ligands, solvent system, metal-to-ligand ratio, etc[13,14]. Among these strategies, the selection of the metal ion remains a key factor for determining the crystal structures, morphologies, and properties of MOFs. Furthermore, MOFs are excellent candidates for the construction of functional microparticles, which can create new opportunities for their application. It is necessary to investigate how to successfully integrate MOFs into functional devices and regulate their growth at the microscale and nanoscale further in-depth. Organic dyes are regarded as serious pollutants due to their non-biodegradability, toxicity, and harmful effects for water[15] all through the world. The removal of organic pollutants has become an urgent and important issue. To date, various approaches, including physical, chemical, and biological treatment, have been developed to treat waste products. Because of the generally poor selectivity of current technologies for removing specific organic dye wastes, it is essential to develop highly active and selective materials for their degradation. Previous studies have shown that MOFs are green catalysts for the degradation of organic dyes[16]. However, MOFs with high selectivity for the degradation of organic dyes are currently limited. Herein, we synthesized two new MOFs, 1Co and 1Mn, by selecting Co and Mn, respectively, as the metal center while using 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene (3-bpdh) and 5-aminoisophthalic acid (AIA) as ligands in both cases. The results showed that 1Co and 1Mn were isostructural and possess two-dimensional (2D)-layer network topologies. In addition, 1Co exhibits selective oxidation−degradation of malachite green (MG), whereas 1Mn exhibits corresponding performance for methylene blue (MB).
2. Experimental section Synthesis of the single-crystals of 1Co and 1Mn: a mixture of Co(NO3)2·6H2O (0.075 mmol, 0.0218 g), AIA(0.075 mmol, 0.0135 g), NaOH (0.083 mmol, 0.0033 g) and 4mL deionized water was stirred to obtain a pink homogeneous solution of Co-AIA. A solution of 3-bpdh (0.075 mmol, 0.0178 g) in methanol (4 mL) was then carefully layered onto the Co-AIA solution through 2mL buffer solution (H2O and MeOH mixture, v/v =1:1) in a test tube. The tube was sealed and left undisturbed at room temperature. After two weeks, the light-pink sheet crystals of 1Co were obtained (Fig. S1), which were suitable for single X-ray diffraction analysis. The yellow 2
block crystals of 1Mn were synthesized using the same procedure, but with MnCl2·4H2O (0.075 mmol, 0.0218 g) instead of Co(NO3)2· 6H2 O. Elemental analysis results are as follows: calcd for 1Co: C22H19CoN5O4: C, 55.46; H, 3.99; N, 14.71%; Found: C, 54.88; H, 3.58; N, 14.56%. Calcd for 1Mn: C22H19 MnN5O4: C, 55.94; H, 4.03; N, 14.83%; Found: C, 54.72; H, 3.72; N, 14.26%. For the micro-crystals of 1Co and 1Mn synthesis, the above solution of M-AIA (M = Co or Mn) was mixed with the ligand solution directly. Then the mixture was stirred for 2 h and the precipitate was filtered and air-dried. In addition, other solvents (ethanol, propanol, ether, etc.) and surfactants (op-10, tween-40, span-80, etc.) were selected so as to controllably synthesize micro-crystals of 1Co and 1Mn. As a result, a series of crystals with different morphologies and sizes were obtained.
3. Results and discussion
Fig. 1. The coordinate structure of M(AIA)(3-bpdh) (M=Co or Mn): (a) The coordination environment around the M(II) ions, (b) perspective view showing the 2D chain structure formed by the metal ion and AIA ligands, and (c) perspective view showing the layer structure formed by the 2D chain structure linked to the 3-bpdh ligands.
Single crystals of 1Co and 1Mn were synthesized using the solvent diffusion method. The detailed synthesis and elemental analysis results are given in the Supplementary Information. The crystals of 1Co and 1Mn were stable in air and insoluble in common organic solvents. In the IR spectra of the complexes, the strong characteristic peak of AIA at 1631 cm−1 shifted to lower frequencies (1Co, 1546 cm−1; 1Mn, 1587 cm−1), which was attributed to the coordination between the carboxyl groups and the metal ions (Fig. S2). The expected strong characteristic peaks of C=N were observed in the spectra of both 1Co (1616 cm−1) and 1Mn (1618 cm−1), indicating the stretching vibration of the 3-bpdh ligands. In addition, the TGA curves showed that 1Co has no significant weight loss up to 380 °C (Fig. S4). Meanwhile, for 1Mn, a weight loss of 5% was observed from room 3
temperature to 84 °C due to the removal of adsorbed water. Moreover, the dehydrated framework of 1Mn was also stable up to 380 °C. At this temperature, both crystals simultaneously decomposed. A sharp weight loss of 46.8% was found for 1Co in the temperature range of 380−494 °C, which resulted from the loss of the 3-bpdh ligands (calcd: 50%). In addition, a 40.1% weight loss was observed for 1Mn from 380 to 564 °C because of the decomposition of the ligands (calcd: 38%). The BET surface areas of 1Co (Fig. S5) and 1Mn (Fig. S6) were 20 and 14 m2/g, respectively. Single-crystal X-ray structural analysis revealed that MOFs 1Co and 1Mn are isostructural only differing in the metal ion (Fig. S7-S8). As shown in Fig. 1, both of the MOFs have layered network topologies and crystallize in the triclinic P-1 space group with a Z value of 2. Their unit cell dimensions were also similar (for 1Co: a = 10.056(10) Å, b = 11.294(11) Å, c = 11.572(18) Å; for 1Mn: a = 10.169(11) Å, b = 11.243(13) Å, c = 11.66(2) Å). In their frameworks, only one crystallographically independent M(II) ion (M=Co or Mn) was located in the hexacoordinate environment. Each M(II) ion was linked to four oxygen atoms from three AIA ligands and to two nitrogen atoms from two 3-bpdh ligands. As shown, every M(II) ion was chelated by one AIA ligand and bridged by the other AIA ligand to produce a 2D [M(AIA)]n chain. Then, the 2D chains (occupying the axial positions) were linked by the 3-bpdh ligands (occupying the equatorial positions) to construct a layered network topology. In
Fig. 2. SEM images of microcrystals of 1Co (a1-a9) and 1Mn (b1-b9) synthesized under different conditions. 1Co: (a1) methanol−water; (a2) ethanol−water; (a3) n-propanol−water; (a4) methanol/ether−water; (a5) methanol/(op-10)−water; (a6) methanol/ether/(op-10)−water; (a7) methanol/(tween-40)−water; (a8) methanol/(tween-60)−water; (a9) methanol/(span-80)−water. 1Mn: (b1) methanol−water; (b2) n-propanol−water; (b3) n-butanol/ether−water; (b4) ethanol/ether−water; (b5) methanol/ether−water; (b6) methanol/(op-10)−water; (b7) methanol/ether/(op-10)−water; (b8) methanol/(tween-60)−water; (b9)methanol/(span-80)−water.
compounds 1Co and 1Mn, the M(II)−O bond lengths varied from 2.004(4) to 2.285(3) Å (1Co) and from 2.102 (4) 4
to 2.324(3) Å (1Mn); the corresponding M(II)−N bond lengths were from 2.167(3) to 2.168(3) Å (1Co) and from 2.280(4) to 2.285(4) Å (1Mn), respectively. Moreover, the N/O−M(II)−O/N angles varied from 59.18 to 175.34° (1Co) and from 57.29 to 173.28° (1Mn) (Table S2-S6). Various synthesis systems were applied to regulate the morphologies and sizes of the microcrystals of 1Co and 1Mn. The purities of 1Co and 1Mn synthesized in these different solvent systems were verified by PXRD and were in accordance with the simulated patterns (Fig. S9-S10). As can obviously be seen (Fig. 2), the microcrystals of these two samples showed different morphologies and sizes under the same conditions. When the crystals were synthesized in a water−methanol system, 1Co exhibited a block-like morphology with different sizes, whereas 1Mn showed a uniform rod-like morphology with lengths of 15 µm (Fig. 2, a1 and b1). When the crystals were synthesized in a methanol/ether−water system, the crystals of 1Co also formed block-like aggregates but smaller in size. In addition, the crystals of 1Mn changed to block-like aggregates (Fig. 2, a4 and b5). When the crystals were synthesized in a methanol/ether/surfactant−water system, the crystals of 1Co (Fig. 2, a5-a9) agglomerated into microspheres comprising flakes. In contrast, the microcrystals of 1Mn formed micro-rods but with smaller sizes of about 10 µm (Fig. 2, b6-b9). The results discussed above indicated that the nature of the materials played an important role in determining the morphologies and sizes of microcrystals. Moreover, the synthesis system, including the reaction solvents and surfactant, influenced the morphologies of microcrystals. 1Co exhibited block-like aggregates with different sizes in an alcohol−water system. However, the morphology tended to become microspherical as surfactants were added to the system. For 1Mn, the more hydrophilic the synthesis system, the greater was the proportion of rod-like microcrystals. However, block-like aggregates instead of rod-like ones were obtained when ether was added to the system. The various morphologies and particle sizes of 1Co and 1Mn synthesized under different systems can be explained from the view point of growth kinetics. The growth rates differ along different crystallographic directions. As discussed above, these two MOFs showed different compositions and morphologies, which might affect their catalytic performance. To reveal the catalytic oxidation properties of 1Co and 1Mn, the catalytic degradation of dyes was chosen as a model reaction. In this experiment, MG, MB, orange G (OG), rhodamine B (RhB), and methyl orange (MO) were chosen as typical organic pollutant targets (SchemeS1). In a typical experiment, a suspension containing 1Co or 1Mn catalyst (30 mg) and 125 mL of a 12 ppm dye solution was stirred for 180 min. For RhB, OG, and MO, 1Co and 1Mn did not show any catalytic activity (Fig. 3). In comparison, 1Co degraded 19% of MG, whereas 1Mn degraded 21% of MB. This result indicated the highly selective degradation properties of these two 5
Fig. 3. Degradation of dyes using 1Co (left) and 1Mn (right) crystals. (a) methyl orange, (b) orange G, (c) malachite green, (d) methylene blue, and (e) rhodamine B. The initial concentration of all the dyes was 12 ppm.
crystals. Therefore, although the different compositions of the MOFs resulted in different degradation properties, both of the MOFs may be useful for waste water treatment. In addition, the degradation performances of crystal 1Co and 1Mn were enhanced by increasing the amount of catalyst per volume unit in the solution, which could provide more active sites (Fig. S12-S13). After the catalytic degradation was complete, the crystals could be easily retrieved by filtration and reused. The morphologies of the recovered crystals remained the same, except for the appearance of some cracks due to the stirring process (Fig. S1). The IR (Fig. S3) and PXRD (Fig. S11) patterns of the recovered crystals were all in good agreement with those of the original crystals, and there was no evidence of the presence of newly adsorbed products. Specially, some characteristic peaks of IR patterns for dyes were not found in that of catalysts, indicating that there seemed no adsorption of dyes for catalysts (Fig. S14-15). What is more, in order to demonstrate that the degradation of the dye is due to the catalytic oxidation, we conducted a comparative test under nitrogen protection conditions. As the results indicated, there are almost no degradation effect can be detected under anaerobic conditions (Fig. S16-17). According to our previous study, iminium organic dyes, which contains C=N group, can be catalytically degraded. While RhB cannot be degraded due to its steric hindrance. MO and OG are azo-compounds .Although the N=N group in MO and OG is more easily to break, the energy which 1Co and 1Zn acquired is just accordance with the energy that break C=N group needed in MG and MB respectively. The coordinated Co and Mn have different energy to break the C=N bond. From the structure of MG and MB, π electronic conjugate of MG is greater than that of MB. while coordinated hexacoordinate Co posses more energy than hexacoordinate Mn, so 1Co and 1MB can degradate MG and MB respectively but not RhB, OG and MO.
6
A probable mechanism for the degradation of organic dyes over 1Co and 1Mn has been proposed[17-19] (Fig. S18). First, we propose that an oxygenated intermediary species I is generated by the coordination of oxygen to the metal center along with the corresponding expansion of the coordination number. As 1Co (or 1Mn) attracts dioxygen, the coordination between oxygen and the M(II) ion becomes increasingly weak, whereas the oxygen of the dioxygen becomes more strongly bound to the M(II) ion. This interaction between 1Co (or 1Mn) and dioxygen yields the oxygenated intermediary species I, whose active transition-state complex can catalytically oxidize the C=N or C=C groups of organic dyes. The second step involves a nucleophilic attack of the unsaturated substrate on species I followed by oxygen transfer, leading to the formation of species II. In the final step, the M(II)−O bond is ruptured, oxidizing the dye and regenerating the catalyst.
4. Conclusion In summary, two isostructural microcrystal MOFs, [M(AIA)(3-bpdh)] (M=Co or Mn), have been synthesized using slow diffusion reaction. It was demonstrated that these two precipitation/MOFs is a suitable approach for the fabrication of pre-synthesized units into functional micro-and nanocrystals, exhibiting excellent conversion and selectivity in the catalytic degradation of MG by 1Co and MB by 1Mn. The high flexibility of this strategy and the effective catalysis will certainly be an efficient way to produce novel coordination polymers for useful applications. It will also enhance the applications in other fields.
Associated content Supporting Information Available: The structural data of the single crystals in CIF format, packing view of the complex, and room-temperature IR spectra of the complexes are available free of charge.
Acknowledgments We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21371058) and the Ministry of Education (A200-C-1101). This work was supported in part by East China University of Science & Technology (YA0157117) and the Key Laboratory for Ultrafine Materials of the Ministry of Education (ECUST).
References 1.
H.C. Zhou, J. R. Long and O. M. Yaghi, Chem. Rev., 112 (2012) 673.
2.
X. Yan, M. Wang, T. R. Cook, M. Zhang, M. L. Saha, Z. Zhou, X. Li, F. Huang and P. J. Stang, J. Am. Chem. Soc. 138 (2016) 4580-4588.
3.
S. L. Li, Y. Q. Lan, H. Sakurai and Q. Xu, Chemistry., 18 (2012) 16302-16309.
4.
J. Yi, H. Li, L. Jiang, K. Zhang and D. Chen, Chem. Commun., 51 (2015) 10162. 7
5.
D. Dang, P. Wu, C. He, Z. Xie and C. Duan, J. Am. Chem. Soc., 132 (2010) 14321-14323.
6.
Y.-J. Zhang, T. Liu, S. Kanegawa and O. Sato, J. Am. Chem. Soc., 132 (2009) 912-913.
7.
X.-Y. Wang, Z.-M. Wang and S. Gao, Chem. Commun., (2007) 1127.
8.
H. Bunzen, M. Grzywa, M. Hambach, S. Spirkl and D. Volkmer, Crystal Growth & Design, 16 (2016) 3190-3197.
9.
X. Zhang, Z. Zhang, J. Boissonnault and S. M. Cohen, Chem. Commun. (Camb.), 52 (2016) 8585-8588.
10.
P. Horcajada, F. Salles, S. Wuttke, T. Devic, D. Heurtaux, G. Maurin, A. Vimont, M. Daturi, O. David, E. Magnier, N. Stock, Y. Filinchuk, D. Popov, C. Riekel, G. Ferey and C. Serre, J. Am. Chem. Soc., 133 (2011) 17839-17847.
11. 12.
G.-X. Liu, H. Xu, H. Zhou, S. Nishihara and X.-M. Ren, CrystEngComm., 14 (2012) 1856. H. Q. Xu, J. Hu, D. Wang, Z. Li, Q. Zhang, Y. Luo, S. H. Yu and H. L. Jiang, J. Am. Chem. Soc., 137 ( 2015) 13440-13443.
13.
C. Liu, C. Zeng, T. Y. Luo, A. D. Merg, R. Jin and N. L. Rosi, J. Am. Chem. Soc., 138 (2016) 12045-12048.
14.
N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O'Keeffe and O. M. Yaghi, Science., 300 (2003) 1127.
15.
M. Liang and J. Chen, Chem. Soc. Rev., 42 (2013) 3453.
16.
T. Zhang and W. Lin, Chem. Soc. Rev., 43 (2014) 5982.
17.
Q. Meng, J. Ye, Z.-l. Xu and H. Xu, Mater. Lett., 116 (2014) 378.
18.
S. El-Korso, I. Khaldi, S. Bedrane, A. Choukchou-Braham, F. Thibault-Starzyk and R. Bachir, J. Mol. Catal. A: Chem., 3942 (2014) 89.
19.
K. Brown, S. Zolezzi, P. Aguirre, D. Venegas-Yazigi, V. Paredes-García, R. Baggio, M. A. Novak and E. Spodine, Dalton Transactions., (2009), 1422.
8
Research Highlights: Synthesis and structure of isostructural metal–organic framework materials 1Co and 1Mn Selective catalysis Catalytic properties of metal–organic frameworks controlled by metal ions
9