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Two octamolybdate-based coordination polymers with high photocatalytic activities under visible light irradiation
Inorganic Chemistry Communications 86 (2017) 90–93
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Two octamolybdate-based coordination polymers with high photocatalytic activities under visible light irradiation Wei-Qiu Kan a, Shi-Zheng Wen b,⁎, Yuan-Chun He c,⁎ a b c
Jiangsu Province Key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huaian 223300, PR China Jiangsu Province Key Laboratory of Modern Measurement Technology and Intellige, School of Physics and Electronic Electrical Engineering, Huaiyin Normal University, Huaian 223300, PR China College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, PR China
a r t i c l e
i n f o
Article history: Received 29 August 2017 Received in revised form 24 September 2017 Accepted 26 September 2017 Available online 28 September 2017 Keywords: Coordination polymer Octamolybdate 2-aminopyrazine Band gap Photocatalysis
a b s t r a c t Two coordination polymers based on octamolybdate and 2-aminopyrazine, namely, [CuI2(HL)2(L)(β[Mo8O26])]·3H2O (1) and [(H2L)2(β-[Mo8O26])]·4H2O (2) (L = 2-aminopyrazine), have been synthesized hydrothermally. The structures of the compounds were determined by single-crystal X-ray diffraction analyses and further characterized by infrared spectra, elemental analyses, powder X-ray diffraction analyses and thermogravimetric analyses. Compound 1 displays a double chain structure, which is extended by the intermolecular N\\H⋯O hydrogen-bonding interactions to generate a 3D supramolecular architecture. Compound 2 exhibits a discrete structure, which is connected by the intermolecular O\\H⋯O hydrogen-bonding interactions to yield a 3D supramolecular architecture. Compounds 1 and 2 display small band gaps of 1.89 and 1.66 eV, respectively. Moreover, compounds 1 and 2 exhibit high photocatalytic activities for degradation of methylene blue under visible light irradiation. © 2017 Elsevier B.V. All rights reserved.
In recent years, with the development of industrialization and urbanization, water pollution have become more and more serious [1,2]. The significant amounts of organic pollutions, particularly the soluble organic pollutions in water are difficult to remove by the conventional water treatment technologies [3]. Photocatalytic degradation of organic pollution is a green technology for the treatment of environmental pollution, which can decompose organic pollutions into carbon dioxide, water and simple acids [3]. The light sources of photocatalytic processes include UV light and visible light. Up to now, most of the reported photocatalytic materials are UV light response. However, the ratio of the UV light in the sunlight is only 4–5%, whereas that of the visible light is approximately 40% [4]. In order to utilize sunlight effectively, the development of photocatalysts capable of responding to visible light is in high demand. Coordination polymers are compounds with infinite networks constructed by central metals and organic ligands via coordination bonds or weak chemical bonds interactions [5]. They have similar electronic structures with the traditional photocatalyst TiO2 and can be used for the photocatalytic degradation of organic pollutions, photocatalytic water reduction, photocatalytic CO2 reduction and so on [6–8]. Results from documents indicate that the absorption band of a photocatalyst can expand from UV light to visible light region by decreasing the
⁎ Corresponding authors. E-mail addresses: [email protected] (S.-Z. Wen), [email protected] (Y.-C. He).
https://doi.org/10.1016/j.inoche.2017.09.031 1387-7003/© 2017 Elsevier B.V. All rights reserved.
band gap [9,10]. The band gaps of coordination polymers can be decreased by introducing NH2-containing organic ligands into the compounds [9–14]. For example, coordination polymer MIL-125 constructed by the Ti(IV) ion and 1,4- benzenedicarboxylic acid has a band gap of 3.6 eV. When the 1,4-benzenedicarboxylic acid was replaced by 2amino-1,4-benzenedicarboxylic acid, the resultant coordination polymer MIL-125-NH2 has the same host framework with MIL-125, but its band gap decreased to 2.6 eV, and its absorption band expanded from UV light to visible light region [9]. On the other hand, coordination polymers constructed from the octamolybdate and NH2-containing ligand display diverse physicochemical properties such as electrochemical, second nonlinear optic and photochromism [15–18]. Moreover, this kind of coordination polymer has many potential applications in the fields such as catalysis and adsorption [19,20]. These fascinating properties and potential applications further encourage us to construct coordination polymers from the octamolybdate and NH2-containing ligand. In this work, two coordination polymers with small band gaps and high photocatalytic activities under visible light were successfully constructed from the NH2-containing ligand 2-aminopyrazine and octamolybdate. The structures of the compounds were determined by single-crystal X-ray diffraction analyses [21] and characterized by infrared spectra (IR), elemental analyses, powder X-ray diffraction (PXRD) analyses and thermogravimetric analyses (TGA). Moreover, the optical band gaps and photocatalytic behaviors of the compounds have been investigated under visible light.
W.-Q. Kan et al. / Inorganic Chemistry Communications 86 (2017) 90–93
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Fig. 1. (a) View of the coordination environments of Cu(I) ions in compound 1. Symmetry codes: #1 −x + 2, −y, −z + 1. (b) View of the chain formed by the Cu(I) ions and β-[Mo8O26]4− anions in compound 1.
Compounds 1 and 2 were synthesized under hydrothermal conditions [22]. The sample of compound 1 was obtained as brick-red bulk crystals mixed with unknown powders. These crystals of 1 were separated by hand picking. The sample of compound 2 was obtained as pale yellow crystals in a single phase. Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the triclinic space group P-1. The asymmetric unit of 1 contains two Cu(I) ions, one L ligand, two protonated L ligands (HL), one β-[Mo8O26]4− anion and three lattice water molecules. As shown in Fig. 1a, Cu1 is three-coordinated by one oxygen atom from one β[Mo8O26]4− anion and two nitrogen atoms from one L ligand and one HL cation in a T-shaped coordination geometry. Cu2 is four-coordinated by two oxygen atoms from one β-[Mo8O26]4− anion and two nitrogen atoms from one L ligand and one HL cation in a tetrahedral coordination geometry. The Cu(I)-N and Cu(I)-O bond lengths are in the normal ranges of 1.878(12)-1.965(8) Å and 2.267(4)-2.417(15) Å as other Cu(I)-containing coordination polymers [23–26]. Each β-[Mo8O26]4− anion links three Cu(I) ions to form a chain (Fig. 1b). The chains are further connected by the L ligands to generate a double chain. The HL ligands act as terminal ligands, further coordinate to the Cu(I) ions (Fig. 2a). The chains are further extended by the intermolecular N\\H⋯O hydrogen-bonding interactions to form a 3D supramolecular architecture (Fig. 2b). Compound 2 crystallizes in the monoclinic space group P21/n. The asymmetric unit of 2 contains one H2L, half a β-[Mo8O26]4− anion and two lattice water molecules. As shown in Fig. 3a, two H2L, one β[Mo8O26]4− anion and four lattice water molecules generate a discrete structure, which is extended by the intermolecular O\\H ⋯ O hydrogen-bonding interactions to form a 3D supramolecular architecture (Fig. 3b).
TGA were carried out for the compounds to determine the thermal stabilities of compounds 1 and 2. The TGA curves of compounds 1 and 2 are shown in Fig. S1. For compound 1, The first weight loss of 2.91% from 85 to 168 °C is attributed to the loss of three lattice water molecules (calcd 3.27%). The second gradually weight loss starts at 263 °C, and ends at 518 °C. Compound 2 undergoes dehydration from 45 to 161 °C (obsd 4.11%, calcd 4.97%). There is no further weight loss from 161 to 208 °C. After 208 °C, the organic components start to decompose and do not stop until 500 °C. To obtain the band gaps (Eg) of compounds 1 and 2, the diffuse reflectivity spectra of the compounds were measured. The plots of absorption (α/S) data versus energy E are shown in Fig.4 (α/S = (1 − R)2/2R (R, α and S are reflectivity, absorption coefficient and scattering coefficient)) [27]. As shown in the figure, The band gap is the energy of the intersection point between the E axis at α/S = 0 and the tangent of the maximum adsorption edge in the plot [27]. The band gaps of compounds 1 and 2 are 1.89 and 1.66 eV, respectively. These values are lower than most of the reported octamolybdate-based coordination polymers which do not include NH2-containing ligands [28–32]. The photocatalytic activities of compounds 1 and 2 have been investigated at room temperature. Photocatalytic experiments were performed in conventional processes as reported previously [33]. 200 mL methylene blue (MB) (5 × 10−5 mol/L−1) solution containing 30 mg samples of 1 or 2 and 2 mL H2O2 (30%) was stirred in the dark for about 1 h to reach the equilibrium between adsorption and desorption before irradiation. The mixture was then stirred under a 250 W Xe lamp continuously. Every 15 min, 4 mL of liquid was taken out of the reactor and centrifugated to remove suspended catalyst particles. The liquid was then used for spectroscopic measurement on the UV–vis spectrometer. The concentration of MB was estimated by the
Fig. 2. (a) View of the chain of compound 1. (b) View of the 3D supramolecular archiecture of compound 1.
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W.-Q. Kan et al. / Inorganic Chemistry Communications 86 (2017) 90–93
Fig. 3. (a) View of the discrete structure of compound 2. (b) View of the 3D supramolecular archiecture of compound 2.
Fig. 4. Plots of absorption (α/S) data versus energy E for compounds 1 and 2.
absorbance at 665 nm. As shown in Fig. 5, the absorbance of MB solution decreased clearly with the increasing of irradiation time. The plots of concentrations (C/C0) versus irradiation times (t) of compounds 1 and 2 are shown in Fig. 6. It can be seen from the figure that the degradation rates of MB solutions without any catalyst and H2O2 after irradiation by visible light for 75 min and 120 min are 1% and 2%, respectively.
However, the degradation rates of the MB solutions without any catalyst but containing H2O2 after irradiation by visible light for 75 min and 120 min are 34% and 45%, respectively. The degradation rates of the MB solutions employing compounds 1 and 2 as photocatalysts increase from 34% to 97% for 1 and 58% for 2 after 75 min of irradiation. After 120 min of irradiation, the degradation rates increased to 100% and
Fig. 5. Absorption spectra of the MB solution during the decomposition reaction with the presence of compounds 1 or 2 and H2O2.
W.-Q. Kan et al. / Inorganic Chemistry Communications 86 (2017) 90–93
Fig. 6. Photocatalytic decomposition rates C/C0 versus t plots of MB solution.
82% for compounds 1 and 2. The results indicate that the photocatalytic activities of compounds 1 and 2 are higher than many of the reported coordination polymers [34–36]. The PXRD patterns of 1 and 2 after photocatalytic processes are nearly identical to those of the original compounds (Fig. S2). The recovered photocatalysts were reused for the subsequent cycles of photocatalytic experiments under the same conditions. The results indicate that the degradation rates of the MB solutions employing compounds 1 and 2 as photocatalysts did not change obviously even after five cycles of operations (Fig.S3). Therefore, compounds 1 and 2 may be good candidates for the photocatalytic degradation of organic pollution MB. In conclusion, two coordination polymers based on octamolybdate and the NH2-containing ligand 2-aminopyrazine have been constructed under hydrothermal conditions. The two compounds display 3D supramolecular architectures. They have small band gaps and high photocatalytic activities for degradation of methylene blue solution under visible light irradiation. Acknowledgements This work was supported by the National Natural Science Foundation of China (21401063, 21403081 and 21601104) and the Natural Science Foundation of Jiangsu Province (BK20140452 and BK20140453). Appendix A. Supplementary data CCDC 1570610-1570611 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac. uk/ data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi: https://doi.org/10.1016/j.inoche. 2017.09.031. References [1] Q. Wang, Z.M. Yang, Environ. Pollut. 218 (2016) 358–365. [2] H.L. Kong, H.F. Wu, Bioresour. Technol. 99 (2008) 7886–7891. [3] R.G. El-sharkawy, A.S.B. El-din, S.E.H. Etaiw, Spectrochim. Acta A 79 (2011) 1969–1975. [4] M. Pelaez, N.T. Nolanb, S.C. Pillai, Appl. Catal. B 125 (2012) 331–349. [5] A.Y. Robin, K.M. Fromm, Coord. Chem. Rev. 250 (2006) 2127–2157. [6] D.R. Sun, Y.H. Fu, W.J. Liu, L. Ye, D.K. Wang, L. Yang, X.Z. Fu, Z.H. Li, Chem. Eur. J. 19 (2013) 14279–14285. [7] W.M. Liao, J.H. Zhang, Y.J. Hou, H.P. Wang, M. Pan, Inorg. Chem. Commun. 73 (2016) 80–89.
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[8] L.Y. Zhang, S.Y. Yin, M. Pan, W.M. Liao, J.H. Zhang, H.P. Wang, C.Y. Su, J. Mater. Chem. A 5 (2017) 9807–9814. [9] C.H. Hendon, D. Tiana, M. Fontecave, C. Sanchez, L. D'arras, C. Sassoye, L. Rozes, C. Mellot-Draznieks, A. Walsh, J. Am. Chem. Soc. 135 (2013) 10942–10945. [10] Y.H. Fu, D.R. Sun, Y.J. Chen, R.K. Huang, Z.X. Ding, X.Z. Fu, Z.H. Li, Angew. Chem. Int. Ed. 51 (2012) 3364–3367. [11] S.N. Kim, J. Kim, H.Y. Kim, H.Y. Cho, W.S. Ahn, Catal. Today 204 (2013) 85–93. [12] L.J. Shen, S.J. Liang, W.M. Wu, R.W. Liang, L. Wu, Dalton Trans. 42 (2013) 13649–13657. [13] Y. Horiuchi, T. Toyao, M. Saito, K. Mochizuki, M. Iwata, H. Higashimura, M. Anpo, M. Matsuoka, J. Phys. Chem. C 116 (2012) 20848–20853. [14] J.L. Long, S.B. Wang, Z.X. Ding, S.C. Wang, Y.G. Zhou, L. Huang, X.X. Wang, Chem. Commun. 48 (2012) 11656–11658. [15] A. Harchanil, A. Haddad, J. Clust. Sci. 26 (2015) 1773–1785. [16] W.S. Zhang, J.J. Gong, L.L. Zhang, Y.Y. Yang, Y. Liu, H.C. Zhang, G.J. Zhang, H. Dong, H.L. Hu, F.F. Zhao, Z.H. Kang, Dalton Trans. 42 (2013) 1760–1769. [17] T.R. Veltman, A.K. Stover, A.N. Sarjeant, K.M. Ok, P.S. Halasyamani, A.J. Norquist, Inorg. Chem. 45 (2006) 5529–5537. [18] J.M. Gutierrez-Zorrilla, T. Yamase, M. Sugeta, Acta Cryst C50 (1994) 196–198. [19] H.C. Gao, J.H. Yu, J. Du, H.L. Niu, J. Wang, X.J. Song, W.X. Zhang, M.J. Jia, J. Clust. Sci. 25 (2014) 1263–1272. [20] C. Dey, R. Das, P. Pachfule, P. Poddar, R. Banerjee, Cryst. Growth Des. 11 (2011) 139–146. [21] Crystallographic data of compounds 1 and 2 were recorded on a Bruker SMART APEX II diffractometer with graphite-monochromated Mo Kα radiation (λ = .71073 Å) at 293 K. Absorption corrections were applied using multi-scan technique. The hydrogen atoms of water molecules in compound 1 were not included in the model, and those of compound 2 are located from difference Fourier maps and refined as riding atoms with d(O-H) = 0.846–0.852 Å and Uiso = 1.5Ueq(O). Crystal data for 1: C12H23Cu2Mo8N9O29, M = 1651.99, triclinic, space group P-1, T = 293(2) K, a = 9.6353(13) Å, b = 9.9770(14) Å, c = 19.664(3) Å, α = 87.414(2)°, β = 80.443(2)°, γ = 87.037(2)°, V = 1860.3(4) Å3, Z = 2, Dc = 2.949 g/cm3, a total of 7340 reflections were collected, of which 7315 were unique, final R1 = 0.0335 for I N 2σ(I), wR2 = 0.1018 for all data, GOOF = 1.054. Crystal data for 2: C8H22Mo8N6O30, M = 1449.84, monoclinic, space group P21/n, T = 293(2) K, a = 8.309(7) Å, b = 16.935(13) Å, c = 11.657(10) Å, β = 92.701(16)°, V = 1639(2) Å3, Z = 2, Dc = 2.939 g/cm3, a total of 7337 reflections were collected, of which 2985 were unique, final R1 = 0.0323 for I N 2σ(I), wR2 = 0.0777 for all data, GOOF = 1.078. [22] Synthesis of compound 1: (NH4)6Mo7O24 ∙4H2O (0.24 g, 0.2 mmol), CuCl2 ∙2H2O (0.034 g, 0.2 mmol), L (0.019 g,0.2 mmol) and 10 mL water was placed in a 15 mL Teflon reactor. Then the pH of the mixture was adjusted to about 1 with 1 M HCl. The reactants were then heated at 120 °C for 3 days. After cooling to room temperature at a rate of 10 °C/h, brick-red crystals of compound 1 were collected in a 34% yield. The final pH value of the reaction system was about 2. Anal. Calcd for C12H23Cu2Mo8N9O29 (Mr = 1651.99): C, 8.72; H, 1.39; N, 7.63. Found: C, 8.61; H, 1.43; N, 7.72%. IR data (KBr, cm−1): 3530 (m), 3380 (m), 3213 (m), 3102 (m), 3021 (m), 2764 (m), 1671 (s), 1619 (m), 1529 (w), 1422 (m), 1307 (w), 1226 (w), 952 (s), 909 (s), 850 (s), 705 (s), 649 (s), 516 (s). Synthesis of compound 2: (NH4)6Mo7O24 ∙4H2O (0.24 g, 0.2 mmol), L (0.019 g,0.2 mmol) and 10 mL water was placed in a 15 mL Teflon reactor. Then the pH of the mixture was adjusted to about 1 with 1 M HCl. The reactants were then heated at 120 °C for 3 days. After cooling to room temperature at a rate of 10 °C/h, yellow crystals of compound 2 were collected in a 38% yield. The final pH value of the reaction system was about 2. Anal. Calcd for C8H22Mo8N6O30 (Mr = 1449.84): C, 6.62; H, 1.52; N, 5.79. Found: C, 6.71; H, 1.43; N, 5.72%. IR data (KBr, cm−1): 3527 (s), 3376 (s), 3203 (s), 3102 (s), 3014 (s), 2773 (s), 2053 (w), 1968 (w), 1872 (w), 1762 (w), 1669 (s), 1522 (m), 1421 (s), 1316 (s), 1223 (s), 1180 (w), 1046 (m), 902 (s), 848 (s), 700 (s). [23] Y.H. Luo, Z.L. Lang, X.X. Lu, W.W. Ma, Y. Xu, H. Zhang, Inorg. Chem. Commun. 72 (2016) 13–16. [24] Y. Zhang, K. Chen, H.T. Fan, Inorg. Chem. Commun. 38 (2013) 47–49. [25] T.H. Huang, H. Yang, G. Yang, S.L. Zhu, C.L. Zhang, Inorg. Chim. Acta 455 (2017) 1–8. [26] C. Dai, X.Y. Zhou, X.M. Jing, G.H. Li, Q.S. Huo, Y.L. Liu, Inorg. Chem. Commun. 52 (2015) 69–72. [27] V. Coué, R. Dessapt, M. Bujoli-Doeuff, M. Evain, S. Jobic, J. Solid State Chem. 179 (2006) 3615–3627. [28] W.Q. Kan, J. Yang, Y.Y. Liu, J.F. Ma, Dalton Trans. 41 (2012) 11062–11073. [29] W.Q. Kan, J. Yang, Y.Y. Liu, J.F. Ma, Inorg. Chem. 51 (2012) 11266–11278. [30] W.Q. Kan, S.Z. Wen, Y.H. Kan, H.Y. Hu, S.Y. Niu, X.Y. Zhang, Synth. Met. 198 (2014) 51–58. [31] H.Y. Zang, Y.Q. Lan, G.S. Yang, X.L. Wang, K.Z. Shao, G.J. Xu, Z.M. Su, CrystEngComm 12 (2010) 434–445. [32] H.Y. Zang, Y.Q. Lan, S.L. Li, G.S. Yang, K.Z. Shao, X.L. Wang, L.K. Yan, Z.M. Su, Dalton Trans. 40 (2011) 3176–3182. [33] W.Q. Kan, B. Liu, J. Yang, Y.Y. Liu, J.F. Ma, Cryst. Growth Des. 12 (2012) 2288–2298. [34] L. Liu, D.Q. Wu, B. Zhao, X. Han, J. Wu, H.W. Hou, Y.T. Fan, Dalton Trans. 44 (2015) 1406–1411. [35] Y.P. Wu, X.Q. Wu, J.F. Wang, J. Zhao, W.W. Dong, D.S. Li, Q.C. Zhang, Cryst. Growth Des. 16 (2016) 2309–2316. [36] M. Song, B. Mu, R.D. Huang, J. Solid State Chem. 246 (2017) 1–7.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
Two octamolybdate-based coordination polymers with high photocatalytic activities under visible light irradiation Wei-Qiu Kan a, Shi-Zheng Wen b,⁎, Yuan-Chun He c,⁎ a b c
Jiangsu Province Key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry and Chemical Engineering, Huaiyin Normal University, Huaian 223300, PR China Jiangsu Province Key Laboratory of Modern Measurement Technology and Intellige, School of Physics and Electronic Electrical Engineering, Huaiyin Normal University, Huaian 223300, PR China College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, PR China
a r t i c l e
i n f o
Article history: Received 29 August 2017 Received in revised form 24 September 2017 Accepted 26 September 2017 Available online 28 September 2017 Keywords: Coordination polymer Octamolybdate 2-aminopyrazine Band gap Photocatalysis
a b s t r a c t Two coordination polymers based on octamolybdate and 2-aminopyrazine, namely, [CuI2(HL)2(L)(β[Mo8O26])]·3H2O (1) and [(H2L)2(β-[Mo8O26])]·4H2O (2) (L = 2-aminopyrazine), have been synthesized hydrothermally. The structures of the compounds were determined by single-crystal X-ray diffraction analyses and further characterized by infrared spectra, elemental analyses, powder X-ray diffraction analyses and thermogravimetric analyses. Compound 1 displays a double chain structure, which is extended by the intermolecular N\\H⋯O hydrogen-bonding interactions to generate a 3D supramolecular architecture. Compound 2 exhibits a discrete structure, which is connected by the intermolecular O\\H⋯O hydrogen-bonding interactions to yield a 3D supramolecular architecture. Compounds 1 and 2 display small band gaps of 1.89 and 1.66 eV, respectively. Moreover, compounds 1 and 2 exhibit high photocatalytic activities for degradation of methylene blue under visible light irradiation. © 2017 Elsevier B.V. All rights reserved.
In recent years, with the development of industrialization and urbanization, water pollution have become more and more serious [1,2]. The significant amounts of organic pollutions, particularly the soluble organic pollutions in water are difficult to remove by the conventional water treatment technologies [3]. Photocatalytic degradation of organic pollution is a green technology for the treatment of environmental pollution, which can decompose organic pollutions into carbon dioxide, water and simple acids [3]. The light sources of photocatalytic processes include UV light and visible light. Up to now, most of the reported photocatalytic materials are UV light response. However, the ratio of the UV light in the sunlight is only 4–5%, whereas that of the visible light is approximately 40% [4]. In order to utilize sunlight effectively, the development of photocatalysts capable of responding to visible light is in high demand. Coordination polymers are compounds with infinite networks constructed by central metals and organic ligands via coordination bonds or weak chemical bonds interactions [5]. They have similar electronic structures with the traditional photocatalyst TiO2 and can be used for the photocatalytic degradation of organic pollutions, photocatalytic water reduction, photocatalytic CO2 reduction and so on [6–8]. Results from documents indicate that the absorption band of a photocatalyst can expand from UV light to visible light region by decreasing the
⁎ Corresponding authors. E-mail addresses: [email protected] (S.-Z. Wen), [email protected] (Y.-C. He).
https://doi.org/10.1016/j.inoche.2017.09.031 1387-7003/© 2017 Elsevier B.V. All rights reserved.
band gap [9,10]. The band gaps of coordination polymers can be decreased by introducing NH2-containing organic ligands into the compounds [9–14]. For example, coordination polymer MIL-125 constructed by the Ti(IV) ion and 1,4- benzenedicarboxylic acid has a band gap of 3.6 eV. When the 1,4-benzenedicarboxylic acid was replaced by 2amino-1,4-benzenedicarboxylic acid, the resultant coordination polymer MIL-125-NH2 has the same host framework with MIL-125, but its band gap decreased to 2.6 eV, and its absorption band expanded from UV light to visible light region [9]. On the other hand, coordination polymers constructed from the octamolybdate and NH2-containing ligand display diverse physicochemical properties such as electrochemical, second nonlinear optic and photochromism [15–18]. Moreover, this kind of coordination polymer has many potential applications in the fields such as catalysis and adsorption [19,20]. These fascinating properties and potential applications further encourage us to construct coordination polymers from the octamolybdate and NH2-containing ligand. In this work, two coordination polymers with small band gaps and high photocatalytic activities under visible light were successfully constructed from the NH2-containing ligand 2-aminopyrazine and octamolybdate. The structures of the compounds were determined by single-crystal X-ray diffraction analyses [21] and characterized by infrared spectra (IR), elemental analyses, powder X-ray diffraction (PXRD) analyses and thermogravimetric analyses (TGA). Moreover, the optical band gaps and photocatalytic behaviors of the compounds have been investigated under visible light.
W.-Q. Kan et al. / Inorganic Chemistry Communications 86 (2017) 90–93
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Fig. 1. (a) View of the coordination environments of Cu(I) ions in compound 1. Symmetry codes: #1 −x + 2, −y, −z + 1. (b) View of the chain formed by the Cu(I) ions and β-[Mo8O26]4− anions in compound 1.
Compounds 1 and 2 were synthesized under hydrothermal conditions [22]. The sample of compound 1 was obtained as brick-red bulk crystals mixed with unknown powders. These crystals of 1 were separated by hand picking. The sample of compound 2 was obtained as pale yellow crystals in a single phase. Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the triclinic space group P-1. The asymmetric unit of 1 contains two Cu(I) ions, one L ligand, two protonated L ligands (HL), one β-[Mo8O26]4− anion and three lattice water molecules. As shown in Fig. 1a, Cu1 is three-coordinated by one oxygen atom from one β[Mo8O26]4− anion and two nitrogen atoms from one L ligand and one HL cation in a T-shaped coordination geometry. Cu2 is four-coordinated by two oxygen atoms from one β-[Mo8O26]4− anion and two nitrogen atoms from one L ligand and one HL cation in a tetrahedral coordination geometry. The Cu(I)-N and Cu(I)-O bond lengths are in the normal ranges of 1.878(12)-1.965(8) Å and 2.267(4)-2.417(15) Å as other Cu(I)-containing coordination polymers [23–26]. Each β-[Mo8O26]4− anion links three Cu(I) ions to form a chain (Fig. 1b). The chains are further connected by the L ligands to generate a double chain. The HL ligands act as terminal ligands, further coordinate to the Cu(I) ions (Fig. 2a). The chains are further extended by the intermolecular N\\H⋯O hydrogen-bonding interactions to form a 3D supramolecular architecture (Fig. 2b). Compound 2 crystallizes in the monoclinic space group P21/n. The asymmetric unit of 2 contains one H2L, half a β-[Mo8O26]4− anion and two lattice water molecules. As shown in Fig. 3a, two H2L, one β[Mo8O26]4− anion and four lattice water molecules generate a discrete structure, which is extended by the intermolecular O\\H ⋯ O hydrogen-bonding interactions to form a 3D supramolecular architecture (Fig. 3b).
TGA were carried out for the compounds to determine the thermal stabilities of compounds 1 and 2. The TGA curves of compounds 1 and 2 are shown in Fig. S1. For compound 1, The first weight loss of 2.91% from 85 to 168 °C is attributed to the loss of three lattice water molecules (calcd 3.27%). The second gradually weight loss starts at 263 °C, and ends at 518 °C. Compound 2 undergoes dehydration from 45 to 161 °C (obsd 4.11%, calcd 4.97%). There is no further weight loss from 161 to 208 °C. After 208 °C, the organic components start to decompose and do not stop until 500 °C. To obtain the band gaps (Eg) of compounds 1 and 2, the diffuse reflectivity spectra of the compounds were measured. The plots of absorption (α/S) data versus energy E are shown in Fig.4 (α/S = (1 − R)2/2R (R, α and S are reflectivity, absorption coefficient and scattering coefficient)) [27]. As shown in the figure, The band gap is the energy of the intersection point between the E axis at α/S = 0 and the tangent of the maximum adsorption edge in the plot [27]. The band gaps of compounds 1 and 2 are 1.89 and 1.66 eV, respectively. These values are lower than most of the reported octamolybdate-based coordination polymers which do not include NH2-containing ligands [28–32]. The photocatalytic activities of compounds 1 and 2 have been investigated at room temperature. Photocatalytic experiments were performed in conventional processes as reported previously [33]. 200 mL methylene blue (MB) (5 × 10−5 mol/L−1) solution containing 30 mg samples of 1 or 2 and 2 mL H2O2 (30%) was stirred in the dark for about 1 h to reach the equilibrium between adsorption and desorption before irradiation. The mixture was then stirred under a 250 W Xe lamp continuously. Every 15 min, 4 mL of liquid was taken out of the reactor and centrifugated to remove suspended catalyst particles. The liquid was then used for spectroscopic measurement on the UV–vis spectrometer. The concentration of MB was estimated by the
Fig. 2. (a) View of the chain of compound 1. (b) View of the 3D supramolecular archiecture of compound 1.
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Fig. 3. (a) View of the discrete structure of compound 2. (b) View of the 3D supramolecular archiecture of compound 2.
Fig. 4. Plots of absorption (α/S) data versus energy E for compounds 1 and 2.
absorbance at 665 nm. As shown in Fig. 5, the absorbance of MB solution decreased clearly with the increasing of irradiation time. The plots of concentrations (C/C0) versus irradiation times (t) of compounds 1 and 2 are shown in Fig. 6. It can be seen from the figure that the degradation rates of MB solutions without any catalyst and H2O2 after irradiation by visible light for 75 min and 120 min are 1% and 2%, respectively.
However, the degradation rates of the MB solutions without any catalyst but containing H2O2 after irradiation by visible light for 75 min and 120 min are 34% and 45%, respectively. The degradation rates of the MB solutions employing compounds 1 and 2 as photocatalysts increase from 34% to 97% for 1 and 58% for 2 after 75 min of irradiation. After 120 min of irradiation, the degradation rates increased to 100% and
Fig. 5. Absorption spectra of the MB solution during the decomposition reaction with the presence of compounds 1 or 2 and H2O2.
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Fig. 6. Photocatalytic decomposition rates C/C0 versus t plots of MB solution.
82% for compounds 1 and 2. The results indicate that the photocatalytic activities of compounds 1 and 2 are higher than many of the reported coordination polymers [34–36]. The PXRD patterns of 1 and 2 after photocatalytic processes are nearly identical to those of the original compounds (Fig. S2). The recovered photocatalysts were reused for the subsequent cycles of photocatalytic experiments under the same conditions. The results indicate that the degradation rates of the MB solutions employing compounds 1 and 2 as photocatalysts did not change obviously even after five cycles of operations (Fig.S3). Therefore, compounds 1 and 2 may be good candidates for the photocatalytic degradation of organic pollution MB. In conclusion, two coordination polymers based on octamolybdate and the NH2-containing ligand 2-aminopyrazine have been constructed under hydrothermal conditions. The two compounds display 3D supramolecular architectures. They have small band gaps and high photocatalytic activities for degradation of methylene blue solution under visible light irradiation. Acknowledgements This work was supported by the National Natural Science Foundation of China (21401063, 21403081 and 21601104) and the Natural Science Foundation of Jiangsu Province (BK20140452 and BK20140453). Appendix A. Supplementary data CCDC 1570610-1570611 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac. uk/ data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi: https://doi.org/10.1016/j.inoche. 2017.09.031. References [1] Q. Wang, Z.M. Yang, Environ. Pollut. 218 (2016) 358–365. [2] H.L. Kong, H.F. Wu, Bioresour. Technol. 99 (2008) 7886–7891. [3] R.G. El-sharkawy, A.S.B. El-din, S.E.H. Etaiw, Spectrochim. Acta A 79 (2011) 1969–1975. [4] M. Pelaez, N.T. Nolanb, S.C. Pillai, Appl. Catal. B 125 (2012) 331–349. [5] A.Y. Robin, K.M. Fromm, Coord. Chem. Rev. 250 (2006) 2127–2157. [6] D.R. Sun, Y.H. Fu, W.J. Liu, L. Ye, D.K. Wang, L. Yang, X.Z. Fu, Z.H. Li, Chem. Eur. J. 19 (2013) 14279–14285. [7] W.M. Liao, J.H. Zhang, Y.J. Hou, H.P. Wang, M. Pan, Inorg. Chem. Commun. 73 (2016) 80–89.
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Crystal data for 1: C12H23Cu2Mo8N9O29, M = 1651.99, triclinic, space group P-1, T = 293(2) K, a = 9.6353(13) Å, b = 9.9770(14) Å, c = 19.664(3) Å, α = 87.414(2)°, β = 80.443(2)°, γ = 87.037(2)°, V = 1860.3(4) Å3, Z = 2, Dc = 2.949 g/cm3, a total of 7340 reflections were collected, of which 7315 were unique, final R1 = 0.0335 for I N 2σ(I), wR2 = 0.1018 for all data, GOOF = 1.054. Crystal data for 2: C8H22Mo8N6O30, M = 1449.84, monoclinic, space group P21/n, T = 293(2) K, a = 8.309(7) Å, b = 16.935(13) Å, c = 11.657(10) Å, β = 92.701(16)°, V = 1639(2) Å3, Z = 2, Dc = 2.939 g/cm3, a total of 7337 reflections were collected, of which 2985 were unique, final R1 = 0.0323 for I N 2σ(I), wR2 = 0.0777 for all data, GOOF = 1.078. [22] Synthesis of compound 1: (NH4)6Mo7O24 ∙4H2O (0.24 g, 0.2 mmol), CuCl2 ∙2H2O (0.034 g, 0.2 mmol), L (0.019 g,0.2 mmol) and 10 mL water was placed in a 15 mL Teflon reactor. Then the pH of the mixture was adjusted to about 1 with 1 M HCl. The reactants were then heated at 120 °C for 3 days. After cooling to room temperature at a rate of 10 °C/h, brick-red crystals of compound 1 were collected in a 34% yield. The final pH value of the reaction system was about 2. Anal. Calcd for C12H23Cu2Mo8N9O29 (Mr = 1651.99): C, 8.72; H, 1.39; N, 7.63. Found: C, 8.61; H, 1.43; N, 7.72%. IR data (KBr, cm−1): 3530 (m), 3380 (m), 3213 (m), 3102 (m), 3021 (m), 2764 (m), 1671 (s), 1619 (m), 1529 (w), 1422 (m), 1307 (w), 1226 (w), 952 (s), 909 (s), 850 (s), 705 (s), 649 (s), 516 (s). Synthesis of compound 2: (NH4)6Mo7O24 ∙4H2O (0.24 g, 0.2 mmol), L (0.019 g,0.2 mmol) and 10 mL water was placed in a 15 mL Teflon reactor. Then the pH of the mixture was adjusted to about 1 with 1 M HCl. The reactants were then heated at 120 °C for 3 days. After cooling to room temperature at a rate of 10 °C/h, yellow crystals of compound 2 were collected in a 38% yield. The final pH value of the reaction system was about 2. Anal. 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