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Chinese Chemical Letters 23 (2012) 355–358 www.elsevier.com/locate/cclet

Synthesis and structure of a pure inorganic polyoxo-metalate-based porous framework
Hai Hong Wu, Zhi Ming Zhang, En Bo Wang *
Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Changchun 130024, China Received 26 October 2011 Available online 26 January 2012

Abstract A pure inorganic porous framework based on the tungstoferrate [FeW12O40]5?, Fe(H2O)6H[Na6FeW12O40]2?44H2O (1) was obtained by the conventional aqueous solution method and characterized by elemental analysis, TG, FT-IR, UV–vis spectroscopy. Single-crystal X-ray diffraction analyses reveal that compound 1 crystallizes in the space group Fm-3m, which is composed of a porous inorganic framework [Na6FeW12O40]n with two kinds of pores A and B, accommodating Fe(H2O)6 units in pore A, which was observed rarely in the pure inorganic framework. # 2012 En Bo Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Polyoxometalate; Tungstoferrate; Framework; Porous materials

Crystalline porous materials, such as zeolites and metal-organic frameworks (MOFs), have been attracting considerable interest [1,2]. In the past decades, numerous MOF materials have been obtained by reaction of the organic linkers with metal cations or metal clusters [3]. They have attracted signi?cant interest in part owing to their potential applications in gas storage [4], molecular separations [5], and heterogeneous catalysis [6] and magnetism [7]. Polyoxometalates (POMs), as a unique class of metal–oxide clusters, have many properties that make them attractive for applications in catalysis, biology, magnetism, optics, and medicine [8]. Impressive studies on linking of these well de?ned metal–oxygen building blocks by the metal-organic linkers to generate related extended structures have been performed, in which metal ions or metal clusters are connected by organic ligands to induce one-, two-, and three dimension frameworks [9]. Recently, with the introduction of POM guests, MOFs have been ?ne-tuned for more specialized applications such as heterogeneous catalysis [10]. But, until now, only a limited number of pure inorganic porous frameworks have ever been reported, indicating that the preparation of such compounds is still a currently great challenge [11]. In this paper, an inorganic porous framework based on the tungstoferrate with two types of channels A and B has been obtained, Fe(H2O)6H[Na6FeW12O40]2?44H2O (1).

* Corresponding author. E-mail address: wangeb889@nenu.edu.cn (E.B. Wang). 1001-8417/$ – see front matter # 2012 En Bo Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2012.01.013

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Table 1 Crystal data and structure re?nement for 1. Compound 1 Empirical formula Formula mass Temperature [K] ? Wavelength [A] Crystal system Space group ? a [A] ? V [A3]
a b

Compound 1 Z Dcalcd. [g cm–3] m [mm–1] F(000) Data/restraints/parameters Goodness-of-?t on F2 R1 a [I > 2s(I)] wR2 b 4 4.004 24.529 12244 718/84/56 1.088 0.0521 0.1632

H101Fe3Na6O130W24 6899.70 293(2) K 0.71073 Cubic Fm-3m 22.5360 11445(3)

R1 = SjjFoj ? jFcjj/SjFoj. wR2 = S[w(Fo2 ? Fc2)2]/S[w(Fo2)2]1/2.

1. Experimental All chemicals were commercially purchased and used without further puri?cation. TG analyses were performed on a Perkin-ELMer TGA7 instrument in ?owing N2 with a heating rate of 10 8C/min. W and Fe elements were determined by a Leaman inductively coupled plasma (ICP) spectrometer; IR spectrum was recorded in the range of 400–4000 cm– 1 on an Alpha Centaurt FT/IR spectrophotometer with pressed KBr pellets. UV–vis absorption spectrum was obtained by using a 752 PC UV–vis spectrophotometer. 0.5 g FeCl3?6H2O (1.85 mmol) was added to a solution of 3.0 g Na2WO4?2H2O (9.1 mmol) in a mixture of 25 mL H2O and 15 mL 100% CH3COOH. The pH of the mixture was adjusted to ca. 2 with 2 mol/L hydrochloric acid. Then the mixture was heated to 90 8C and stirred for 9 h. After cooling to room temperature, the mixture was ?ltered and the ?ltrate kept at room temperature for slow evaporation. The yellow block crystals of 1 were isolated after 15 days (yield 33% based on W). Anal. Found (%): H, 1.25; Na, 3.92; Fe, 2.18; W, 62.70; Calcd. (%): H, 1.46; Na, 3.90; Fe, 2.40; W, 62.49. The crystal data of 1 was performed on a Rigaku R-AXIS RAPID IP diffractometer using graphite monochromatic ? Mo–Ka radiation (l = 0.71073 A) and IP technique. Suitable crystal was af?xed to the end of a glass ?ber using silicone grease and transferred to the goniostat. The structures were solved by the direct method and re?ned by the fullmatrix least-squares method on F2 using the SHELXTL-97 crystallographic software [12]. Further details of the X-ray structure analysis are given in Table 1. 2. Results and discussion The compound was obtained in a pH range of 1.6–2.0 by the conventional aqueous method. Polyoxoanion 1 consists of a a-Keggin anion [FeW12O40]5?, six Na+ cations, and a Fe(H2O)6 group. Compound 1 shows a high symmetry cubic space group Fm-3m. The framework of 1 adopts the a-Keggin anion [FeW12O40]5? secondary ? building unit (Fig. 1a). The central iron ion exhibits tetrahedral geometry with Fe–O distances of 1.8193 A, and the ? for W-O1, 1.941 peripheral W atoms exhibit {WO6} octahedral con?guration. The W–O distances are 2.207(7) A ? ? ? (5) A for W-O2, 1.9052(2) A for W-O3, and 1.761(11) A for W-O4. The a-Keggin anions as the nodes are connected + ? by 12 surrounding Na ions into a three-dimensional cubic network. The band lengths of Na–O are 2.932(16) A. One structural feature of 1 is that there are two kinds of pores (A and B) (Fig. 1a), with free diameters of ca. 9.18 and ? 6.76 A. Pore A accommodates a [Fe(H2O)6]3+ group in its cuboctahedral shape cage (Fig. 1d–f). Pores A and B are arranged alternately in the face-centered cubic lattice. The cubocathedral pore A is connected to the adjacent six pores B (Fig. 1g). The crystallization water molecules are located in the pores B. Thus compound 1 exhibits a multiple ? ? porous system, and the total potential solvent volume is 3268.6 A3, which corresponds to 28.6% (11445.0 A3) empty volume from the PLATON calculations [13]. Bound valence sum calculations [14] indicate that the iron center is FeIII ion, and all the W atoms are in the VI oxidation state. X-ray photoelectron spectroscopy (XPS) was performed to identify the oxidation states of W and Fe centers in compound 1. The XPS spectra exhibit two overlapped peaks at 35.2 eV and 37.3 eV in the energy region of W4f5/2, consistent with the WVI oxidation state [15] (Fig. 2a). Furthermore, the peak at 711.7 eV in the energy region of Fe2p ascribes to FeIII oxidation state [16] (Fig. 2b). All these

[(Fig._1)TD$IG]

H.H. Wu et al. / Chinese Chemical Letters 23 (2012) 355–358

357

Fig. 1. (a) Arrangement of the Keggin-type polyoxoanions, showing two kinds of channels; (b and c) the linking modes between the polyoxoanions and Na+ ions; (d–f) the cubic framework showing two kinds of channels and pore A accommodates a [Fe(H2O)6] group in its cuboctahedral shape cage; (g) pores A and B are arranged alternately in the face-centered cubic lattice.

[(Fig._2)TD$IG]

Fig. 2. (a and b) XPS spectra of 1 for W and Fe, respectively; (c) UV–vis spectrum of 1; (d) TG curve; and (e) IR spectrum of 1.

measurements are consistent with the BVS calculation results. The thermal stability of compound 1 was examined by the thermogravimetric analysis, which exhibits two continuous weight losses in the temperature range 55–398 8C. The weight loss of 15.0% corresponds to the loss of all lattice and coordinated water molecules (Calcd 13.1%) (Fig. 2d). In the UV spectrum of 1, the characteristic peaks of 1 are located at 235 nm and 284 nm, which could correspond to the Keggin structure [17] existing in the title compound (Fig. 2c). In the IR spectrum of 1, the broad peaks at 3421 cm?1 and peak at 1632 cm?1 are the characteristic vibration peaks of H2O; the peaks at 946 cm?1, 893 cm?1, 784 cm?1, and 412 cm?1 correspond to the characteristic peaks of W = Od, W–Ob–W, W–Oc–W, Fe–O of the Keggin polyoxoanion [18] in 1 (Fig. 2e). 3. Conclusion In summary, an inorganic porous framework was prepared by the conventional aqueous solution method. The structure exhibits a three-dimensional cubic network with two kinds of pores (A and B), which was rarely observed in the inorganic material chemistry. The further study on the catalytic ability of the porous material is ongoing in our group.

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H.H. Wu et al. / Chinese Chemical Letters 23 (2012) 355–358

Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21101022/91027002), Fundamental Research Funds for the Central Universities (No. 10QNJJ009), and the Program for Changjiang Scholars and Innovative Research Team in University. References
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