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Synthesis, crystal structure and photocatalytic performance of polyoxometalate K13[Gd(GeW11O39)2]·34H2O
Accepted Manuscript Synthesis, crystal structure and photocatalytic performance of polyoxometalate K13[Gd(GeW11O39)2]·34H2O Haibin Liu, Lin Bai, Limei Ai, Wenshuang Dai, Danfeng Zhang, Qingyin Wu, Renchun Zhang PII:
S1002-0721(18)30507-6
DOI:
https://doi.org/10.1016/j.jre.2018.08.014
Reference:
JRE 304
To appear in:
Journal of Rare Earths
Received Date: 20 June 2018 Revised Date:
10 August 2018
Accepted Date: 15 August 2018
Please cite this article as: Liu H, Bai L, Ai L, Dai W, Zhang D, Wu Q, Zhang R, Synthesis, crystal structure and photocatalytic performance of polyoxometalate K13[Gd(GeW11O39)2]·34H2O, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2018.08.014. 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.
ACCEPTED MANUSCRIPT
Synthesis, crystal structure and photocatalytic performance of polyoxometalate K13[Gd(GeW11O39)2]·34H2O
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Haibin Liu,1 Lin Bai,1 Limei Ai,1 Wenshuang Dai,1 Danfeng Zhang,1 Qingyin Wu,1,2,* Renchun Zhang3 (1. School of Biomedical & Chemical Engineering, Liaoning Institute of Science and Technology, Benxi 117004, China; 2. Department of Chemistry, Zhejiang University, Hangzhou 310027, China; 3. College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang 455000, China)
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Abstract: A polyoxometalate K13[Gd(GeW11O39)2]·34H2O was synthesized and characterized by elemental analysis, IR spectroscopy, UV spectra, XRD and thermal gravimetric analysis. X-ray single-crystal structural analysis indicats that the title compound crystallizes in the triclinic crystal system. The heteropolyanion of [Gd(GeW11O39)2]13- consists of two [GeW11O39]8– vacant Keggin moieties linked via Gd3+. Photocatalytic performance of K13[Gd(GeW11O39)2]·34H2O in photodegradation of X-3B was also studied. When the reaction time is 180 min, the decoloring rate reaches 71.19 %. With the decrease of pH, the decolorization rate increases gradually.
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Keywords: polyoxometalate; Keggin structure; crystal structure; photocatalytic performance; Rare earths 1. Introduction
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The polyoxometalates (POMs) with Keggin structure are widely used in catalysis, biomedicine and material sciencefor their unique reducing oxide, acid, antivirus, antitumor and other excellent performance[1-5]. Keggin heteropoly anions can be degraded to obtain vacant 11 tungsten, which can form 1:1 structure and 1:2 structure coordinated with transition metal or rare earth ions. Vacant heteropolyanion can be coordinated with rare earth ion through 4 to 8 dentate ligands to form mixed heteropoly complexes. Because of their heat resistance, oxidation resistance, antiviral, vacant polyoxometalates have important applications in fields such as catalysis, functional materials, analytical chemistry and pharmaceutical chemistry[6–10].
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The wastewater of textile dye draining into the river without treatment would bring out serious pollution of water resource and threatening of ecology environment and human health. The photocatalytic method has shown to be efficient for degradation and mineralization of various organic pollutants in water at room temperature and normal pressure[11]. Heteropoly acid has been widely used in degradation as a photocatalyst[12-14], however, to our best knowledge, there are few reports about the photocatalytic activities of vacantpolyoxometalates. In this paper, a single crystal of vacant polyoxometalate containing Gd was obtained by simple ion exchange method. Its crystal structure was measured and analyzed, and its photocatalytic performance was studied. The results show that this POM is a good photocatalyst for X3B degradation. 2 Experimental
2.1 Materials and general methods
Foundation item: Projects supported by the Doctoral Scientific Research Foundation of Liaoning Province (20170520417), the Liaoning Provincial Natural Science Foundation of China (201602404) and the Zhejiang Provincial Natural Science Foundation of China (LY18B010001). Corresponding author: Q.Y. WU (E-mail: [email protected], [email protected];
ACCEPTED MANUSCRIPT Tel: +86-24-45861233 or +86-571-88914042). Gd(NO3)3·6H2O (99%) and GeO2 (99%) were purchased from Beijing Ouhe Technology Co. Ltd. All chemicals were of analytical grade and used without further purification. 2.2 Preparation of K13[Gd(GeW11O39)2]·34H2O
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H4GeW12O40 (2.88 g, 1 mmol) was dissolved in 20 mL water. Next,10 mL of 0.05 mol/L Gd(NO3)3 was added to the solution.The mixture was adjusted to pH=4.7 by addition of potassium acetate (4 mol/L) with vigorous stirring at 80 ºC water bath for 2 h. The homogeneous clear solution was filtered and left to evaporate at room temperature. Then, white bulk-like crystal was obtained and collected for X-ray diffraction. Anal. aclcd forK13[Gd(GeW11O39)2]·34H2O(Abbr. Gd(GeW11)2):Gd 2.34; Ge 2.16; W 60.22. Found: Gd 2.38; Ge 2.13; W 60.57. IR (KBr pellet): 945 (s), 881 (s), 821 (s), 760 (m), 709 (w), 534 (m).
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2.3 Photocatalytic tests
1.4Instructions and reagents
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The photocatalytic procedure was performed as follows: 100 mL solution containing X3B (6.5×10–5 mol/L) and Gd(GeW11)2 (5.0×10–4 mol/L) was stirred for 0.5 h in the dark. Then, the mixture was stirred continuously under ultraviolet (UV) irradiation from a 125 W high pressure Hg lamp. At 20, 40, 60, 80, 100, 120, 150 and 180 min, 3 mL of the sample was taken from the beaker.
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IR spectra were recorded on a WQF-510a FT-IR spectrometer with KBr pellets in 4000–400 cm–1 region. XRD measurement was carried out using D/max-rB X-ray powder diffraction apparatus (λ = 0.15418 nm). TG-DTA measurement used a 6300 thermogravimetric analyzer in flowing N2 with a heating rate of 10 ºC/min. UV absorption spectra was obtained using a TU-1901 spectrophotometer. Gd, Ge and W were determined by ICP-AES Acros-EOP inductively coupled plasma (Spectro, German). 2.5 X-ray structure determination
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Suitable single crystal of Gd(GeW11)2 was selected under a pllarizing microscope and fixed with epoxy cement on fine gass fibers at 150 K. X-ray single-crystal data for Gd(GeW11)2 was collected on a D8 QUEST (Bruker) diffractometer with Mo Kα (λ=0.71073nm) at 150 K in the range of 2.384º <θ< 26.054º. Data processing was accomplished with the RAXWISH processing program. The structure was solved by direct methods and refined by full-matrix least squares on F2 using the SHELXL 97 software[15]. All the non-hydrogen atoms were refined anisotropically. A summary of crystal data and structural refinements Gd(GeW11)2 is provided in Table 1. Selected bond distances and angles are listed in Tables 2 and 3.
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Table 1Crystallographic data ofK13[Gd(GeW11O39)2]·34H2O
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GdGe2K13O112W22H68 6715.43 triclinic 150 P-1
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2.22503(11) 2.23643 (11) 2.61328(14) 99.9603(18) 113.0518(16) 109.6920(16) 10.5450(9) 2 4.187 25.701 11596 0.1 mm×0.1 mm×0.1 mm 2.223, 26.077 –27 ≤ h ≤ 27; –27 ≤ k ≤ 27; –32 ≤ l ≤ 32 41458, 33441, Rint = 0.0431 33441/216/2713 1.071 R1 = 0.0607, wR2 = 0.1688 R1 = 0.0732, wR2 = 0.1800 3.660×10–6, –2.913×10–6
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Empirical formula Formula weight Crystal system Temperature (K) Space group Unit cell parameters a/nm b/nm c/nm α/(°) β/(°) γ/(°) Volume/nm3 Z Density(calculated)/(g/cm3) Absorption coefficient/mm–1 F(000) Crystal size θ/(º) Limiting indices
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Reflections collected unique Data /restraints /parameters Goodness-of-fit on F2 Final R indices [I>2σ(I)] R indices (all data) Largest diff-peak and hole/(e/nm3)
Table 2 Selected bond distances
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Bond Gd1—O47 Gd1—O113 Gd1—O8 Gd1—O16 Gd1—O25 Gd1—O27 Gd1—O23 Gd1—O3
Lengths (nm) 0.2371(11) 0.2387(12) 0.2388(11) 0.2391(11) 0.2405(11) 0.2415(11) 0.2427(11) 0.244(1)
Bond W6—O8 W6—O93 W6—O48 W6—O59 W6—O17 W10—O93 W10—O119 W10—O91
Lengths (nm) 0.1769(10) 0.1904(10) 0.2010(11) 0.205(1) 0.2278(11) 0.1927(10) 0.1971(11) 0.2094(11)
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Table 3 Selected bond angles (°) for K13[Gd(GeW11O39)2]·34H2O Bond angles/(°)
Bond
Bond angles/(°)
O47—Gd1—O113
79.2(4)
O42—W1—O91
100.2(5)
O47—Gd1—O8 O47—Gd1—O16 O113—Gd1—O16 O61—Ge1—O74
75.7(4) 138.4(4) 118.2(4) 110.3(5)
O42—W1—O34 O34—W1—O90 O42—W1—O69 O61—Ge1—O17
102.9(5) 85.6(5) 97.8(5) 110.4(5)
3. Results and discussion
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3.1 Description of crystal structure
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Bond
Reaction of Gd(NO3)3 and H4GeW12O40·nH2O was carefully adjusted to 4.7 with CH3COOK (4 mol/L) in the molar ratio of 1.0:2.0 (Gd(NO3)3: H4GeW12O40) at 80 ºC to yield Gd(GeW11)2, and the reaction can be expressed as follows:
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Gd(NO3)3·6H2O + 2H4GeW12O40·nH2O+ 20CH3COOK + (28–n) H2O→ K13Gd(GeW11O39)2·34H2O + 2K2WO4 + 3KNO3 + 20CH3COOH (1) X-ray single crystal diffraction analysis reveals that the structural unit of Gd(GeW11)2 consists of two
[GeW11O39]8–
cation
(Fig.
1(a)),
13
K+ and
34
K13[Gd(GeW11O39)2]·34H2O. The heteropolyanion [GeW11O39]
8–
crystal
water
molecules,
i.e.
exhibits the well-known Keggin
structure, which consists of 11WO6 octahedra and one GeO4 tetrahedron. The central GeO4 tetrahedron shares its oxygens with three {W3O13} groups, each of which is made up of three edge-sharing WO6
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octahedra. W3O13 subunits are joined by corner-sharing oxygens, which can be divided into four groups according to their coordination in [GeW11O39]8–: Od terminal oxygens connect to one W, Ob share corners between two W3O13 units, Oc connect edge-sharing WO6 octahedra in the same W3O13 unit, and Oa connect the Ge and three W. The Ge1-O distances are 0.1725(10)–0.1757(11) nm. Relevant W–O bond distances in the anion can be classified into three groups: W–Od 0.1721–0.1743(11) nm, W–Ob,c
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0.1857(10)–0.2087(10) nm and W–Oa 0.2203(10)–0.2342(11) nm, indicating that the polyanion maintains the basic Keggin structure[16]. The Gd3+ cation is in the middle of two vacant heteropolyanions [GeW11O39]8– and the sandwich configuration was formed by Gd3+linked with two
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vacant heteropolyanions through binding with O atoms (Fig. 1(b)). The rare earth Gd3+ ion is connected with eight O atoms by the quadrilateral anti-prism coordination mode (Fig. 1(c)), and each of vacant anions [GeW11O39]8– provides 4 oxygen atoms. The Gd–O bond is within the range of 2.366–2.438 nm.
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Fig.1 Molicular structure of [Gd(GeW11O39]2]13– represented by ball-and-stick model (a), polyhedral models (b), and coordination mode of Gd3+ in polydedral model (c)
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3.2. X-ray powder diffraction
Fig. 2XRD patterns of Gd(GeW11)2
(1) Experimental dest; (2) the single crystal data simulation map In order to test the purity of Gd(GeW11)2, the XRD test is carried out, and the XRD pattern of Gd(GeW11)2 is shown in Fig. 2. It can be seen from Fig. 2 that the location of the diffraction peak of the composite sample is basically consistent with the theoretical simulation map, indicating that the obtained Gd(GeW11)2 has a high purity. 3.3 IR spectra IR spectrum is a useful method for investigation of the structure information of polyoxometalate.
ACCEPTED MANUSCRIPT There are four kinds of oxygen atoms in [XW12O40]n–. The XW4 tetrahedron is located in the center of the twelve MO6 octahedra, which can be split into four groups of three edge-shared octahedra, M3O13. The oxygen atoms in [XW12O40]n– can be classified into four groups: Oa, Ob, Oc and Od, each separately representing the inner oxygen, corner-sharing oxygen, edge-sharing oxygen and terminal oxygen[17]. Fig. 3(a) represents the IR spectrum of H4GeW12O40 and Fig. 3(b) represents the IR spectrum of Gd(GeW11)2. As seen from Fig. 3, the whole red shift occurs for Gd(GeW11)2(b) in contrast to H4GeW12O40 (a). The vas(W–Od) vibrational frequency (946 cm–1) has a red shift of 33 cm-1, the
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possible major reason may be that the Gd3+ has stronger interaction to the W–Od bond, reducing the W–Od bond force constant, and leading to decrease in the W–Od vibration frequency[18]. The νas(W–Ob–W) frequency shows a red shift of 25 cm–1, while νas(W–Oc–W) frequency shows two peaks at 759 and 711 cm–1. The νas(Ge–O) frequency at 534 cm–1 is almost equal to that of H4GeW12O40. The similarity of vibration bands demonstrates that the polyoxometalate Gd(GeW11)2 has the Keggin
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structure as its parent acid H4GeW12O40 does[19].
Fig.3 IR spectra of H4GeW12O40(a) and Gd(GeW11)2(b) 2.4 UV spectrum
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The UV spectrum is very helpful for estimating the electronic properties of the metal ions. As shown in Fig. 4, two absorption bands are identified in the UV spectrum of Gd(GeW11)2. The intense peak at 193 nm belongs to the charge-transfer from terminal oxygen to metal atoms (Od→M). The relatively weak band at 253 nm is ascribed to the charge transfer from bridge oxygen to metal atoms (Ob/Oc→M)[20]. It indicates that these two absorption bands are the characteristic bands of Keggin-type heteropolyanion.
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Fig.4 The UV spectrum of Gd(GeW11)2
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3.5 Thermogravimetric analysis
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Fig. 5 shows the TG curve of title compound. The TG curve of Gd(GeW11)2exhibits one weight loss in the temperature range 10–231 ºC, due to the loss of crystallized water (TG=4.33%).
Fig.5TG curve of Gd(GeW11)2
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3.6 Photocatalytic activities of Gd(GeW11)2 in photodegradationof X-3B As one of the most important performances of POMs, the photocatalytic degradation of X3B was investigated by using POMs as the photocatalyst. As displayed in Fig.6, along with the UV-light irradiation time, the characteristic absorption peak in the range of 450–600 nm in UV spectra of X3B is reduced gradually with Gd(GeW11)2as the photocatalyst. While, with the increase of pH, the absorption peak is enhanced gradually.
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Fig.6 The spectral changes recorded during X3B photodegradation by Gd(GeW11)2 at reaction time of 0, 20, 40, 60, 90, 120, 150 and 180 min, respectively.
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(a) pH = 1, (b) pH = 2, (c) pH = 3, (d) pH = 4
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The absorption peak of 450-600 nm belongs to the −N=N− and naphthalene ring conjugated system in the X3B structure. It can be deduced that the azo-conjugated color system of X3B molecular is gradually destroyed and decomposed with prolonging the photocatalytic degradation time[21]. In order to estimate the photocatalytic activity of POMs, the decoloring rate of X3B was calculated according to the change before and after reaction of the absorbance. The decoloring rate (DC) was calculated using the following formula: DC= [(A0–At)/A0] ×100%
(2)
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where A0 is the absorbance of Gd(GeW11)2 before illumination, and At is the absorbance of Gd(GeW11)2 at time t after illumination. As seen from Fig. 7, it can be observed that DC was increased significantly with prolonging the UV-light irradiation time at different pH values. Gd(GeW11)2 exhibits satisfactory photcatalytic activity at pH = 1. When the irradiation time is 180 min, DC can even reach 71.19%, which indicats that X3B is degraded prominently. With the decrease of pH, DC increases gradually.
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Fig.7 The changes of DCforGd(GeW11)2 at different pH
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Also, from the digital photograph of Fig. 8, the color of X3B is faded remarkably after the photocatalytic reaction at pH = 1.
Fig. 8 The comparisonof color changes before and after X3B degradation at pH = 1
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4. Conclusions In summary, a polyoxometalate K13[Gd(GeW11O39)2]·34H2O was synthesized and characterized by X-ray single crystal diffraction, IR and UV. Its structure unit was analyzed in detail. This polyoxometalate shows outstanding photocatalytic property for degradation of X3B at pH = 1. The POMs of K13[Gd(GeW11O39)2]·34H2O has potential applications in wastewater purification and other environmental aspects. References
[1] Sadeghi O, Zakharov LN, Nyman M. Aqueous formation and manipulation of the iron-oxo Keggin ion. Science. 2015;347:1359. [2] Xie ZR, Wu H, Wu QY, Ai LM. Synthesis and performance of solid proton conductor molybdovanadosilicic acid. RSC Adv. 2018;8:13984. [3] Wang J, Chen S, Liu H, Li WK. Magnetic and thermosensitive luminescent nanocomposites based on Fe3O4/SiO2/poly (N-isopropylacrylamide)/lanthanide-polyoxometalates and their controllable luminescence properties. J Rare Earths. 2018;36:733. [4] Chen J, Ai LM, Feng W, Xiong DQ, Liu Y, Cai WM. Preparation and photochromism of nanocomposite thin film based on polyoxometalate and polyethyleneglycol. Mater Lett. 2007;61: 5247. [5] Lee S, Fiene A, Li W, Hanck T, Brylev KA, Fedorov VE, et al. Polyoxometalates-Potent and selective ecto-nucleotidase inhibitors. Biochem Pharmacol. 2015;93:171.
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[6] Artetxe B, Reinoso S, Felices LS, Lezama L, Gutiérrez-Zorrilla JM, García JA, et al. Cation-Directed Dimeric versus Tetrameric Assemblies of Lanthanide-Stabilized Dilacunary Keggin Tungstogermanates. Chem Eur J. 2014;20:12144. [7] Wang XH, Liu JF, Li JX, Yang Y, Liu JY, Li B, et al. Synthesis and antitumor activity of cyclopentadienyltitanium substituted polyoxotungstate [CoW11O39(CpTi)]7– (Cp=η5-C5H5). J Inorg Biochem. 2003;94:279. [8] Girardi M, Blanchard S, Griveau S, Simon P, Fontecave M, Bedioui F, et al. Electro-assisted reduction of CO2 to CO and formaldehyde by (TOA)6[α-SiW11O39Co(_)]Polyoxometalate. Eur J Inorg Chem. 2015, 3642.
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[9] Zou XM, Chen QS, Ke XX, Lei Q, Zhao ZM, Liu XL, et al. PW11Cu/TiO2 film photocatalyst: preparation and Visible photocatalytic performance. Chin J Inorg Chem (in Chin.), 2015;31:2037. [10] Suzuki K, Jeong J, Yamaguchi K, Mizuno N. Photoredox catalysis for oxygenation/deoxygenation between sulfides and sulfoxides by visible-light-responsive polyoxometalates. New J Chem. 2016;40: 1014. [11] Hanifehpour Y, Hamnabard N, Homami B, Joo SW, Min BK, Jung JH. A novel visible-light Nd-doped CdTe photocatalyst for degradation of Reactive Red 43: Synthesis, characterization, and photocatalytic properties. J Rare Earths. 2016;34:45.
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[12] Jin HX, Wu QY, Pang WQ. Photocatalytic degradation of textile dye X-3B using polyoxometalate-TiO2 hybrid materials. J Hazard Mater. 2007;141:123. [13] Sha J, Yang X, Sun L, Li S, Sheng N, Li JS. Construction and catalytic activities of polyoxometalate-based compound containinghelical chains. J Coord Chem. 2017;70:392. [14] Wang X, Lin L, Yu XW, Liu JH, Lin HY, Liu GC. Two organic-inorganic hybrids constructed from metal/ttb segments and different polyoxometalates: Syntheses, structures and multifunctional catalytic properties. Polyhedron. 2018;141:25.
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[16] Wang WD, Li XX, Fang WH, Yang GY. Hydrothermal synthesis and structural characterization of a new keggin-type tungstogermanate containing heterometallic 3d–4f cubane clusters. J Clust Sci. 2011;22:87. [17] Cai HX, Wu XF, Wu QY, Yan WF. Synthesis and high proton conductive performance of a quaternary vanadomolybdotungstosilicic heteropoly acid. Dalton Trans. 2016;45:14238. [18] Zhao Q L, Han RJ, Li JL, Du XD, Wang JP. Synthesis and crystal structure of a 1D chainlike rare earth coordinated tungstogermenate:[(CH3)4N]1.50H3.50[Gd(GeW11O39)(-H2O)2]·2.5H2O. J Rare Earths. 2009;27:177.
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[19] Zhang BS, Wu CS, Qiu JP, Li YX, Liu ZX. α-Keggin tungstogermanate supported transition metal complexes: hydrothermal synthesis, characterization, and magnetismofCu2(phen)4(GeW12O40) and [Ni2(bpy)4(H2O)2(GeW12O40)]·2H2O. J Coord Chem. 2014;67:787. [20] Ding Y, Gao Q, Li GX, Zhang HP, Wang JM, Yan L, et al. Selective epoxidation of cyclohexene to cyclohexene oxide catalyzed by Keggin-type heteropoly compounds using anhydrous urea–hydrogen peroxide as oxidizing reagent and acetonitrile as the solvent. J Mol Catal A: Chem. 2004;218:161. [21] Li KB, Zhao F, Wei H, Zhang T, Wang Q. Photocatalytic reductive degradation of acid red 3R by posphotungstic acid. Chem J Chin Univ (in Chin.). 2011;32:1812.
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Graphical abstract: A polyoxometalate K13[Gd(GeW11O39)2]·34H2O was synthesized and characterized. The heteropolyanion of [Gd(GeW11O39)2]13– consists of two [GeW11O39]8– vacant Keggin moieties linked via Gd3+. Photocatalytic performance of K13[Gd(GeW11O39)2]·34H2O in photodegradation of X-3B was
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decrease of pH, the decolorization rate increases gradually.
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also studied. When the reaction time is 180 min, the decoloring rate reaches 71.19 %. With the
S1002-0721(18)30507-6
DOI:
https://doi.org/10.1016/j.jre.2018.08.014
Reference:
JRE 304
To appear in:
Journal of Rare Earths
Received Date: 20 June 2018 Revised Date:
10 August 2018
Accepted Date: 15 August 2018
Please cite this article as: Liu H, Bai L, Ai L, Dai W, Zhang D, Wu Q, Zhang R, Synthesis, crystal structure and photocatalytic performance of polyoxometalate K13[Gd(GeW11O39)2]·34H2O, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2018.08.014. 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.
ACCEPTED MANUSCRIPT
Synthesis, crystal structure and photocatalytic performance of polyoxometalate K13[Gd(GeW11O39)2]·34H2O
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Haibin Liu,1 Lin Bai,1 Limei Ai,1 Wenshuang Dai,1 Danfeng Zhang,1 Qingyin Wu,1,2,* Renchun Zhang3 (1. School of Biomedical & Chemical Engineering, Liaoning Institute of Science and Technology, Benxi 117004, China; 2. Department of Chemistry, Zhejiang University, Hangzhou 310027, China; 3. College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang 455000, China)
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Abstract: A polyoxometalate K13[Gd(GeW11O39)2]·34H2O was synthesized and characterized by elemental analysis, IR spectroscopy, UV spectra, XRD and thermal gravimetric analysis. X-ray single-crystal structural analysis indicats that the title compound crystallizes in the triclinic crystal system. The heteropolyanion of [Gd(GeW11O39)2]13- consists of two [GeW11O39]8– vacant Keggin moieties linked via Gd3+. Photocatalytic performance of K13[Gd(GeW11O39)2]·34H2O in photodegradation of X-3B was also studied. When the reaction time is 180 min, the decoloring rate reaches 71.19 %. With the decrease of pH, the decolorization rate increases gradually.
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Keywords: polyoxometalate; Keggin structure; crystal structure; photocatalytic performance; Rare earths 1. Introduction
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The polyoxometalates (POMs) with Keggin structure are widely used in catalysis, biomedicine and material sciencefor their unique reducing oxide, acid, antivirus, antitumor and other excellent performance[1-5]. Keggin heteropoly anions can be degraded to obtain vacant 11 tungsten, which can form 1:1 structure and 1:2 structure coordinated with transition metal or rare earth ions. Vacant heteropolyanion can be coordinated with rare earth ion through 4 to 8 dentate ligands to form mixed heteropoly complexes. Because of their heat resistance, oxidation resistance, antiviral, vacant polyoxometalates have important applications in fields such as catalysis, functional materials, analytical chemistry and pharmaceutical chemistry[6–10].
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The wastewater of textile dye draining into the river without treatment would bring out serious pollution of water resource and threatening of ecology environment and human health. The photocatalytic method has shown to be efficient for degradation and mineralization of various organic pollutants in water at room temperature and normal pressure[11]. Heteropoly acid has been widely used in degradation as a photocatalyst[12-14], however, to our best knowledge, there are few reports about the photocatalytic activities of vacantpolyoxometalates. In this paper, a single crystal of vacant polyoxometalate containing Gd was obtained by simple ion exchange method. Its crystal structure was measured and analyzed, and its photocatalytic performance was studied. The results show that this POM is a good photocatalyst for X3B degradation. 2 Experimental
2.1 Materials and general methods
Foundation item: Projects supported by the Doctoral Scientific Research Foundation of Liaoning Province (20170520417), the Liaoning Provincial Natural Science Foundation of China (201602404) and the Zhejiang Provincial Natural Science Foundation of China (LY18B010001). Corresponding author: Q.Y. WU (E-mail: [email protected], [email protected];
ACCEPTED MANUSCRIPT Tel: +86-24-45861233 or +86-571-88914042). Gd(NO3)3·6H2O (99%) and GeO2 (99%) were purchased from Beijing Ouhe Technology Co. Ltd. All chemicals were of analytical grade and used without further purification. 2.2 Preparation of K13[Gd(GeW11O39)2]·34H2O
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H4GeW12O40 (2.88 g, 1 mmol) was dissolved in 20 mL water. Next,10 mL of 0.05 mol/L Gd(NO3)3 was added to the solution.The mixture was adjusted to pH=4.7 by addition of potassium acetate (4 mol/L) with vigorous stirring at 80 ºC water bath for 2 h. The homogeneous clear solution was filtered and left to evaporate at room temperature. Then, white bulk-like crystal was obtained and collected for X-ray diffraction. Anal. aclcd forK13[Gd(GeW11O39)2]·34H2O(Abbr. Gd(GeW11)2):Gd 2.34; Ge 2.16; W 60.22. Found: Gd 2.38; Ge 2.13; W 60.57. IR (KBr pellet): 945 (s), 881 (s), 821 (s), 760 (m), 709 (w), 534 (m).
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2.3 Photocatalytic tests
1.4Instructions and reagents
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The photocatalytic procedure was performed as follows: 100 mL solution containing X3B (6.5×10–5 mol/L) and Gd(GeW11)2 (5.0×10–4 mol/L) was stirred for 0.5 h in the dark. Then, the mixture was stirred continuously under ultraviolet (UV) irradiation from a 125 W high pressure Hg lamp. At 20, 40, 60, 80, 100, 120, 150 and 180 min, 3 mL of the sample was taken from the beaker.
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IR spectra were recorded on a WQF-510a FT-IR spectrometer with KBr pellets in 4000–400 cm–1 region. XRD measurement was carried out using D/max-rB X-ray powder diffraction apparatus (λ = 0.15418 nm). TG-DTA measurement used a 6300 thermogravimetric analyzer in flowing N2 with a heating rate of 10 ºC/min. UV absorption spectra was obtained using a TU-1901 spectrophotometer. Gd, Ge and W were determined by ICP-AES Acros-EOP inductively coupled plasma (Spectro, German). 2.5 X-ray structure determination
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Suitable single crystal of Gd(GeW11)2 was selected under a pllarizing microscope and fixed with epoxy cement on fine gass fibers at 150 K. X-ray single-crystal data for Gd(GeW11)2 was collected on a D8 QUEST (Bruker) diffractometer with Mo Kα (λ=0.71073nm) at 150 K in the range of 2.384º <θ< 26.054º. Data processing was accomplished with the RAXWISH processing program. The structure was solved by direct methods and refined by full-matrix least squares on F2 using the SHELXL 97 software[15]. All the non-hydrogen atoms were refined anisotropically. A summary of crystal data and structural refinements Gd(GeW11)2 is provided in Table 1. Selected bond distances and angles are listed in Tables 2 and 3.
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Table 1Crystallographic data ofK13[Gd(GeW11O39)2]·34H2O
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GdGe2K13O112W22H68 6715.43 triclinic 150 P-1
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2.22503(11) 2.23643 (11) 2.61328(14) 99.9603(18) 113.0518(16) 109.6920(16) 10.5450(9) 2 4.187 25.701 11596 0.1 mm×0.1 mm×0.1 mm 2.223, 26.077 –27 ≤ h ≤ 27; –27 ≤ k ≤ 27; –32 ≤ l ≤ 32 41458, 33441, Rint = 0.0431 33441/216/2713 1.071 R1 = 0.0607, wR2 = 0.1688 R1 = 0.0732, wR2 = 0.1800 3.660×10–6, –2.913×10–6
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Empirical formula Formula weight Crystal system Temperature (K) Space group Unit cell parameters a/nm b/nm c/nm α/(°) β/(°) γ/(°) Volume/nm3 Z Density(calculated)/(g/cm3) Absorption coefficient/mm–1 F(000) Crystal size θ/(º) Limiting indices
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Reflections collected unique Data /restraints /parameters Goodness-of-fit on F2 Final R indices [I>2σ(I)] R indices (all data) Largest diff-peak and hole/(e/nm3)
Table 2 Selected bond distances
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Bond Gd1—O47 Gd1—O113 Gd1—O8 Gd1—O16 Gd1—O25 Gd1—O27 Gd1—O23 Gd1—O3
Lengths (nm) 0.2371(11) 0.2387(12) 0.2388(11) 0.2391(11) 0.2405(11) 0.2415(11) 0.2427(11) 0.244(1)
Bond W6—O8 W6—O93 W6—O48 W6—O59 W6—O17 W10—O93 W10—O119 W10—O91
Lengths (nm) 0.1769(10) 0.1904(10) 0.2010(11) 0.205(1) 0.2278(11) 0.1927(10) 0.1971(11) 0.2094(11)
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Table 3 Selected bond angles (°) for K13[Gd(GeW11O39)2]·34H2O Bond angles/(°)
Bond
Bond angles/(°)
O47—Gd1—O113
79.2(4)
O42—W1—O91
100.2(5)
O47—Gd1—O8 O47—Gd1—O16 O113—Gd1—O16 O61—Ge1—O74
75.7(4) 138.4(4) 118.2(4) 110.3(5)
O42—W1—O34 O34—W1—O90 O42—W1—O69 O61—Ge1—O17
102.9(5) 85.6(5) 97.8(5) 110.4(5)
3. Results and discussion
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3.1 Description of crystal structure
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Bond
Reaction of Gd(NO3)3 and H4GeW12O40·nH2O was carefully adjusted to 4.7 with CH3COOK (4 mol/L) in the molar ratio of 1.0:2.0 (Gd(NO3)3: H4GeW12O40) at 80 ºC to yield Gd(GeW11)2, and the reaction can be expressed as follows:
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Gd(NO3)3·6H2O + 2H4GeW12O40·nH2O+ 20CH3COOK + (28–n) H2O→ K13Gd(GeW11O39)2·34H2O + 2K2WO4 + 3KNO3 + 20CH3COOH (1) X-ray single crystal diffraction analysis reveals that the structural unit of Gd(GeW11)2 consists of two
[GeW11O39]8–
cation
(Fig.
1(a)),
13
K+ and
34
K13[Gd(GeW11O39)2]·34H2O. The heteropolyanion [GeW11O39]
8–
crystal
water
molecules,
i.e.
exhibits the well-known Keggin
structure, which consists of 11WO6 octahedra and one GeO4 tetrahedron. The central GeO4 tetrahedron shares its oxygens with three {W3O13} groups, each of which is made up of three edge-sharing WO6
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octahedra. W3O13 subunits are joined by corner-sharing oxygens, which can be divided into four groups according to their coordination in [GeW11O39]8–: Od terminal oxygens connect to one W, Ob share corners between two W3O13 units, Oc connect edge-sharing WO6 octahedra in the same W3O13 unit, and Oa connect the Ge and three W. The Ge1-O distances are 0.1725(10)–0.1757(11) nm. Relevant W–O bond distances in the anion can be classified into three groups: W–Od 0.1721–0.1743(11) nm, W–Ob,c
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0.1857(10)–0.2087(10) nm and W–Oa 0.2203(10)–0.2342(11) nm, indicating that the polyanion maintains the basic Keggin structure[16]. The Gd3+ cation is in the middle of two vacant heteropolyanions [GeW11O39]8– and the sandwich configuration was formed by Gd3+linked with two
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vacant heteropolyanions through binding with O atoms (Fig. 1(b)). The rare earth Gd3+ ion is connected with eight O atoms by the quadrilateral anti-prism coordination mode (Fig. 1(c)), and each of vacant anions [GeW11O39]8– provides 4 oxygen atoms. The Gd–O bond is within the range of 2.366–2.438 nm.
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Fig.1 Molicular structure of [Gd(GeW11O39]2]13– represented by ball-and-stick model (a), polyhedral models (b), and coordination mode of Gd3+ in polydedral model (c)
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3.2. X-ray powder diffraction
Fig. 2XRD patterns of Gd(GeW11)2
(1) Experimental dest; (2) the single crystal data simulation map In order to test the purity of Gd(GeW11)2, the XRD test is carried out, and the XRD pattern of Gd(GeW11)2 is shown in Fig. 2. It can be seen from Fig. 2 that the location of the diffraction peak of the composite sample is basically consistent with the theoretical simulation map, indicating that the obtained Gd(GeW11)2 has a high purity. 3.3 IR spectra IR spectrum is a useful method for investigation of the structure information of polyoxometalate.
ACCEPTED MANUSCRIPT There are four kinds of oxygen atoms in [XW12O40]n–. The XW4 tetrahedron is located in the center of the twelve MO6 octahedra, which can be split into four groups of three edge-shared octahedra, M3O13. The oxygen atoms in [XW12O40]n– can be classified into four groups: Oa, Ob, Oc and Od, each separately representing the inner oxygen, corner-sharing oxygen, edge-sharing oxygen and terminal oxygen[17]. Fig. 3(a) represents the IR spectrum of H4GeW12O40 and Fig. 3(b) represents the IR spectrum of Gd(GeW11)2. As seen from Fig. 3, the whole red shift occurs for Gd(GeW11)2(b) in contrast to H4GeW12O40 (a). The vas(W–Od) vibrational frequency (946 cm–1) has a red shift of 33 cm-1, the
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possible major reason may be that the Gd3+ has stronger interaction to the W–Od bond, reducing the W–Od bond force constant, and leading to decrease in the W–Od vibration frequency[18]. The νas(W–Ob–W) frequency shows a red shift of 25 cm–1, while νas(W–Oc–W) frequency shows two peaks at 759 and 711 cm–1. The νas(Ge–O) frequency at 534 cm–1 is almost equal to that of H4GeW12O40. The similarity of vibration bands demonstrates that the polyoxometalate Gd(GeW11)2 has the Keggin
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structure as its parent acid H4GeW12O40 does[19].
Fig.3 IR spectra of H4GeW12O40(a) and Gd(GeW11)2(b) 2.4 UV spectrum
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The UV spectrum is very helpful for estimating the electronic properties of the metal ions. As shown in Fig. 4, two absorption bands are identified in the UV spectrum of Gd(GeW11)2. The intense peak at 193 nm belongs to the charge-transfer from terminal oxygen to metal atoms (Od→M). The relatively weak band at 253 nm is ascribed to the charge transfer from bridge oxygen to metal atoms (Ob/Oc→M)[20]. It indicates that these two absorption bands are the characteristic bands of Keggin-type heteropolyanion.
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Fig.4 The UV spectrum of Gd(GeW11)2
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3.5 Thermogravimetric analysis
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Fig. 5 shows the TG curve of title compound. The TG curve of Gd(GeW11)2exhibits one weight loss in the temperature range 10–231 ºC, due to the loss of crystallized water (TG=4.33%).
Fig.5TG curve of Gd(GeW11)2
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3.6 Photocatalytic activities of Gd(GeW11)2 in photodegradationof X-3B As one of the most important performances of POMs, the photocatalytic degradation of X3B was investigated by using POMs as the photocatalyst. As displayed in Fig.6, along with the UV-light irradiation time, the characteristic absorption peak in the range of 450–600 nm in UV spectra of X3B is reduced gradually with Gd(GeW11)2as the photocatalyst. While, with the increase of pH, the absorption peak is enhanced gradually.
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Fig.6 The spectral changes recorded during X3B photodegradation by Gd(GeW11)2 at reaction time of 0, 20, 40, 60, 90, 120, 150 and 180 min, respectively.
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(a) pH = 1, (b) pH = 2, (c) pH = 3, (d) pH = 4
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The absorption peak of 450-600 nm belongs to the −N=N− and naphthalene ring conjugated system in the X3B structure. It can be deduced that the azo-conjugated color system of X3B molecular is gradually destroyed and decomposed with prolonging the photocatalytic degradation time[21]. In order to estimate the photocatalytic activity of POMs, the decoloring rate of X3B was calculated according to the change before and after reaction of the absorbance. The decoloring rate (DC) was calculated using the following formula: DC= [(A0–At)/A0] ×100%
(2)
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where A0 is the absorbance of Gd(GeW11)2 before illumination, and At is the absorbance of Gd(GeW11)2 at time t after illumination. As seen from Fig. 7, it can be observed that DC was increased significantly with prolonging the UV-light irradiation time at different pH values. Gd(GeW11)2 exhibits satisfactory photcatalytic activity at pH = 1. When the irradiation time is 180 min, DC can even reach 71.19%, which indicats that X3B is degraded prominently. With the decrease of pH, DC increases gradually.
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Fig.7 The changes of DCforGd(GeW11)2 at different pH
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Also, from the digital photograph of Fig. 8, the color of X3B is faded remarkably after the photocatalytic reaction at pH = 1.
Fig. 8 The comparisonof color changes before and after X3B degradation at pH = 1
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4. Conclusions In summary, a polyoxometalate K13[Gd(GeW11O39)2]·34H2O was synthesized and characterized by X-ray single crystal diffraction, IR and UV. Its structure unit was analyzed in detail. This polyoxometalate shows outstanding photocatalytic property for degradation of X3B at pH = 1. The POMs of K13[Gd(GeW11O39)2]·34H2O has potential applications in wastewater purification and other environmental aspects. References
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Graphical abstract: A polyoxometalate K13[Gd(GeW11O39)2]·34H2O was synthesized and characterized. The heteropolyanion of [Gd(GeW11O39)2]13– consists of two [GeW11O39]8– vacant Keggin moieties linked via Gd3+. Photocatalytic performance of K13[Gd(GeW11O39)2]·34H2O in photodegradation of X-3B was
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decrease of pH, the decolorization rate increases gradually.
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also studied. When the reaction time is 180 min, the decoloring rate reaches 71.19 %. With the