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Journal of Chemistry
Volume 2013, Article ID 639409, 6 pages
http://dx.doi.org/10.1155/2013/639409
Research Article

Synthesis, Structure, and Characterization of Keggin-Type Germanate

1School of Chemical Engineering, Changchun University of Technology, Changchun 130012, China
2College of Chemistry, Jilin University, Changchun 130023, China

Received 13 June 2012; Revised 20 September 2012; Accepted 18 October 2012

Academic Editor: Cengiz Soykan

Copyright © 2013 Ya-feng Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

A novel Keggin-type germanate, (NH4)9[Ge7O14F3]3·1.75H2O (I), is hydrothermally synthesized and structurally characterized by X-ray single-crystal diffraction, elemental analysis, XRD, and TG. (I) is tetragonal system with space group I4/mmm and unit cell: (4) Å, (5) Å, (6) Å3, , , , for 2576 reflections with (Fo). Ge7O14F3 entry is defined as the cluster including one octahedron, two edge-sharing triganol bipyramids, and four tetrahedra. Every Ge7O14F3 entry links adjacent four Ge7O14F3 entries by four tetrahedra. Twelve Ge7O14F3 entries construct a cage with all octahedra of Ge7O14F3 pointing inside, which can be simplified into Keggin-type cage through Ge7O14F3 as the node. The solvent experiment proves that (I) is stable in the water and sensitive to base and acid. The result of XRD shows that the structural water of (I) is easily lost to drop the crystalline. The thermal study indicates that the Keggin-type cage of (I) begins to partly collapse at 200°C and finally changes into GeO2.

1. Introduction

Over the two decades, more efforts have been focused on synthesis of germanates because germanates could not only form the zeolites or molecular sieves [1, 2] but also achieve more openness than silicates owing to smaller rings and lower framework density based on the flexible Ge–O–Ge of ~130° [35]. It is hard for germanium to condense itself into zeolite due to the limitation of synthesis method. Only several examples have been found as BEC, ASV, and UOZ [68], and more instances can be accessible by germanium and the other tetrahedral elements—B, Al, Ga, Si, and so on [5, 917]. As a result of the large atomic radii of germanium conforms higher five- and six-coordination rather than four-coordination, the clusters comprised of the mixed coordinations give rise to large porous and extra porous frameworks [1823]. The mixed 4-, 5- and 6-coordination Ge7O14F3 cluster is discussed in detail as robust building unit of 2D and 3D nets [2427]. In recently reported tubular germanate [22], [(C5N2H14)4(C5N2H13)(H2O)4] [Ge7O12O4/2(OH)F2] [Ge7O12O5/2(OH)F]2[GeO2/2(OH)2], twelve Ge7O14F3 clusters form a cage which mimics the Keggin structure with respect to Ge7O14F3 cluster as the node. Interestingly, this Keggin-type germanate cage possesses 5.3 Å aperture and 8.3 Å cavity, which is bigger than Keggin-type POM- . In this work, we have aimed to crystallize Keggin-type germanate, (NH4)9[Ge7O14F3]3·1.75H2O—CCUT-8 (denoted as the Changchun University of Technology) and furthermore studied the solvent and thermal stabilities of Keggin-type cage.

2. Experimental Section

2.1. Materials and Instrument

All of the reagents were of analytical grade and used as received. The deionized water was used in all the experiments.

The infrared (IR) spectra were recorded within the 400~4000 cm−1 region on a BRUKER Vertex 70 FTIR spectrometer using KBr pellets. The elemental analyses were performed on a PerkinElmer 2400 element analyzer. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX PC2200 diffractometer for Cu Kα radiation (  Å), with a scan speed of 5°/min−1. The thermal gravimetric analyses (TG) were performed on Pyris Diamond TG/DTA instrument used in an atmospheric environment with a heating rate of 10°C/min.

2.2. Synthesis

The colorless crystals of (NH4)9[Ge7O14F3]3·1.75H2O were obtained under solvothermal condition. GeO2 (0.25 g) was firstly dispersed in H2O (1 mL). Then pyridine (4 mL), 2-methylpiperazine (0.95 g), and hydrofluric acid (0.5 mL) were successively added under vigorous stirring. The clear solution with molar ratio of 1 GeO2 : 4 (2-methylpiperazine) : 50 pyridine : 58.3 H2O : 4.8 HF was stirring for 4 hours and then it was transferred into 15 mL stainless steel autoclave and heated at 438 K for 14 days. After naturally cooled to room temperature, colorless product was collected as a single phase. All the crystals were washed by water and alcohol. The resultant crystals were dried naturally before single-crystal X-ray diffraction. H158F36Ge84N36O175 (10245.60): H 1.54, N 4.92; found H 1.74, N 5.32.

2.3. Spectra of FTIR

The peaks at 3254 and 1590 cm−1 were attributed to the stretching and bending vibrations of ; the peaks at 3446 and 1454 cm−1 were assigned to stretching and bending vibrations of H2O; the peaks at 860, 831, 578, 460 cm−1 were due to vibrations of Ge–O or Ge–F.

2.4. Thermal Stability

The results of thermal gravimetric analyses showed that the total weight loss was 15.3% from room temperature to 600°C, corresponding to structural water and decomposition of ammonia and fluoride (calculated value: 15.6%), respectively, (Figure 1). CCUT-8 experienced the two weight losses from room temperature to 600°C and gave rise to final residues (GeO2). The gradual part from 100°C to 250°C was assigned to structural water, and the sharp part from 300°C to 350°C was attributed to the decomposition of ammonia and fluoride.

639409.fig.001
Figure 1: TG analyses of I.

The crystalline and thermal stability were investigated through the XRD (Figure 2). The broad and weak peak of (I) in XRD pattern of room temperature indicated that the crystalline of (I) decreased owing to fast loss of the structure water. Furthermore, we made an attempt to fuse the separated Keggin-type cage of (I) by condensing the oxygen of adjacent cage to build the solid structure. However, the XRD of the samples treated at 200°C and 300°C for 3 hrs in the air showed that Keggin-type cage of (I) began to partly collapse at 200°C and basically changed into the GeO2 at 300°C, which consisted with the results of TG.

639409.fig.002
Figure 2: The XRD patterns of I. (a) simulated; (b) experimental; (c) treated at 200°C for 3 hrs; (d) treated at 300°C for 3 hrs.
2.5. X-Ray Single-Crystal Diffraction

X-ray diffraction data were collected at 293 K from a suitable single crystal sealed in glass capillary on a Rigaku R-AXIS RAPID diffractometer equipped with graphite-monochromatized Mo Kα radiation (  Å). The structure was solved by the direct-method routine of SHELXS-97 and refined by full-matrix least-squares on using SHELXL-97. The heavy atoms (Ge) were firstly located by the direct method, and then the light atoms (N, O, and F) were indentified from the different Fourier maps. All nonhydrogen atoms were refined anisotropically. CCDC 903319 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

A summary of experiment data and refinement parameters of I is given in Table 1. The atomic coordinates and selected bond distances and angles of I are given in Tables 2 and 3.

tab1
Table 1: The crystallographic data and structural refinement for I.
tab2
Table 2: Atomic coordinates ( ) and equivalent isotropic displacement parameters (Å2 ) for I.
tab3
Table 3: Selected bond lengths (Å) and bond angles (°) for I.
2.6. The Solvent Experiments

10 mg of I was added to 10 mL water, 10 mL base (1 M NaOH), and 10 mL acid (1 M HCl) overnight, respectively. The experimental results show that I was unsoluble in the water and soluble in the base and acid.

3. Results and Discussion

I may be designated to the Keggin-type cage of which the MO6 octahedron in POM- is replaced with Ge7O14F3 cluster. The mixed-coordination Ge7O14F3 cluster has been considerably studied [2427], which consists of one octahedron, two edge-sharing trigonal bipyramids, and four tetrahedra (Figure 3). The distances of Ge–O (1.703(9) Å~2.168(13) Å) and Ge–F (1.736(14) Å~1.797(15) Å) and bond angles of O–Ge–O(88.6(6)°~178.0(6)°) and Ge–O–Ge(87.5(7)°~ 136.3(3)°) are consistent to the reported results [24].

639409.fig.003
Figure 3: The Ge7O14F3 cluster containing one octahedron, two edge-sharing trigonal bipyramids, and four tetrahedra.

In Keggin-type anion cage, twelve MoO6 octahedra can be falled into 4 entities in which three edge-sharing MoO6 octahedra are integrated by an oxygen in PO4 tetrahedron. The Keggin cage is formed by vertex-sharingly connecting 4 entities, showing the molecular symmetry. In cage of I, twelve Ge7O14F3 clusters surround the cavity of 8.3 Å with two kinds of apertures defined as 6 MR and 12 MR. The larger 12 MR aperture gives rise to 5.3 Å pore. Like Keggin PMo12O403− anion, when three Ge7O14F3 clusters by which 6 MR aperture is surrounded are looked as an entity, I mimics the Keggin structure (Figure 4). In the case of JLG-5 in which the tetrahedral GeO2 units link the adjacent Keggin-type cage into the infinite structure, I becomes the 0D separated cage because the excessive HF impedes the formation of GeO2 unit.

fig4
Figure 4: (a) Keggin-type I with the cavity bearing two kinds of apertures defined as 6 MR and 12 MR; (b) the simplified I with Ge7O14F3 cluster as node.

4. Conclusions

Keggin-type I has been solvothermally obtained. The structural determination shows that in cage of I, twelve Ge7O14F3 clusters surround the cavity of 8.3 Å with two kinds of apertures defined as 6 MR and 12 MR. The thermal studies show that Keggin-type cage of I begins to partly collapse from 200°C to finally change into GeO2.

Acknowledgment

This work was supported by the Scientific Research Foundation for the Returned Overseas Team, Chinese Education Ministry.

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