CsBxGe6–xO12 (x = 1): A Zeolite Sodalite-Type Borogermanate with a

Feb 20, 2017 - Its framework shows the highest atomic ratios of Ge/B in reported borogermanates and leads to a new type of zeolite sodalite-type net...
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CsBxGe6−xO12 (x = 1): A Zeolite Sodalite-Type Borogermanate with a High Ge/B Ratio by Partial Boron Substitution Rui Pan,† Jian-Wen Cheng,*,‡ Bai-Feng Yang,† and Guo-Yu Yang*,†,§ †

MOE Key Laboratory of Cluster Science, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China ‡ Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, Zhejiang 321004, China § Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Department of Chemistry, Shantou University, Shantou, Guangdong 515063, China S Supporting Information *

porosity of zeolites and usually act as new optical materials with second-harmonic-generation response due to the distorted planar borate rings in their structures. Notably, different Ge/B ratios lead to different connectivity fashions and structural features. The synthesis of whole new types of BGOs is challenging and scientifically interesting. It is known that the partial replacement of Ge atoms in tetrahedral positions by a small amount of Si atoms can modify the pore size and shape and further improve the physical properties of germanates.18 A notable example is quasi-zeolite PKU-15 (Ge/Si = 11),19 which contains a unique tribridging O2− anion in the cagelike (Ge,Si)12O31 building unit and exhibits 3D 12 × 10 × 7R multipore channels. PKU-15 shows a good adsorption affinity toward CO2. It is expected that the partial boron substitution may lead to higher Ge/B ratios and direct the formation of new zeolites different from the reported silicogermanate because of their different charge and ionic radius. The network of BGOs is sensitive to experimental conditions such as the synthetic methods and size of alkali ions. We have obtained several open-framework BGOs under solvothermal conditions using small alkali metals (Na+ and K+).13a,16b Recently, Mao and co-workers systematically investigated the BGOs in the Cs−Ge−B−O system and prepared Cs2GeB4O9 and CsGeB3O7 via high-temperature solid-state reaction.20 Herein, we utilize the larger Cs+ ions as templates to make a new partially boron-substituted BGO, CsBxGe6−xO12 (1; x = 1), which exhibits a zeolite sodalite (SOD)-type net with the highest atomic ratios of Ge/B in reported BGOs.12 Colorless block crystals of compound 1 were obtained by the solvothermal reaction of H3BO3, GeO2, CsCl, HF, pyridine, and H2O at 200 °C for 7 days.21 The phase purity of 1 was confirmed by the agreement between the experimental and simulated powder X-ray diffraction (PXRD) patterns (Figure 1a). The diffuse-reflectance UV/vis spectrum indicates that compound 1 has a UV cutoff edge lower than 200 nm, and the optical absorption data derived from the reflectance show that the band gap is 4.89 eV (Figure 1b), indicating that 1 may be a potential wide-band-gap semiconductor. Thermogravimetric analysis curve shows a straight line, and no weight loss was observed

ABSTRACT: The partial replacement of Ge atoms in tetrahedral positions by a small number of B atoms leads to a new microporous borogermanate, CsBxGe6−xO12 (x = 1), under solvothermal conditions. Its framework shows the highest atomic ratios of Ge/B in reported borogermanates and leads to a new type of zeolite sodalite-type net.

T

he structures of zeolites composed of corner-sharing TO4 tetrahedra (T = Si, P, Al, etc.) with well-defined channels or cavities, more than 230 zeolitic framework types, have been classified and approved by the Commission of the International Zeolite Association.1,2 Aluminosilicate and aluminophosphate molecular sieves are the most well-known such materials and have been widely used in adsorption, catalysis, and ion-exchange processes.3,4 Recently, great effort has been focused on the new microporous materials with framework building elements other than Si and P. For example, a number of zeolite-type aluminoborates have been obtained by replacing SiO4 tetrahedra with oxoboron clusters, which exhibit different 4-connected net topology with channels from 7-ring (7R) to 24R.5−7 As the analogue of Si, Ge has a similar outer electronic configuration and larger ionic radius, which may adopt more coordination geometries, such as GeO4 tetrahedra, GeO5 trigonal bipyramids, and GeO6 octahedra, and tend to form various larger clusters of Ge7, Ge8, Ge9, and Ge10.8−11 These clusters can be used as a TO4 substituent in zeolite to construct crystalline mesoporous materials with extra-large pores and low framework density.9−11 Borogermanates (BGOs) are a new class of open-framework inorganic materials that combine the borate anionic groups with germanate groups.12 The major structure types of these openframework BGOs are the strict alternation of oxoboron clusters and GeO4 tetrahedra and resulted in the formation of borate-rich open frameworks.13−15 Notable examples are AnGeB4O9 (A = organic amines or inorganic cations; n = 1 or 2) with various types of 4-connected zeolitic structures due to the different linkage modes of B4O9 and GeO4.13 Germanate-rich open frameworks are also known;16,17 NH4(Ge3BO8) consists of {Ge6O18}n chains and is further linked via BO4 tetrahedra to form a 3D open framework with intersecting channel systems including 1D 10R channels.17 These BGOs maintain the uniform © XXXX American Chemical Society

Received: December 9, 2016

A

DOI: 10.1021/acs.inorgchem.6b03002 Inorg. Chem. XXXX, XXX, XXX−XXX

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occupied site, one Cs atom, and two O atoms (Figure 2a). The mixed position at Ge4+/B3+ is tetrahedrally coordinated by four

Figure 1. (a) Experimental and simulated PXRD patterns of 1. (b) UV/ vis absorption and optical diffuse-reflectance spectra of 1.

(Figure S1). Single-crystal X-ray diffraction reveals that 1 crystallizes in the trigonal space group R3̅ (No. 148) (Table 1). The asymmetric unit of 1 contains one Ge4+/B3+ mixed Table 1. X-ray Crystallographic Data for 1 empirical formula Mr cryst syst space group a (Å) c (Å) V (Å3) Z Dc (g cm−3) μ (mm−1) F(000) index ranges GOF collected reflns unique reflns (Rint) obsd reflns [I > 2σ(I)] refined param R1a/wR2b [I > 2σ(I)] R1a/wR2b (all data) largest difference peak/hole a R1 = ∑||F o | − |F c ||/∑|F o |. ∑[w(Fo2)2]}1/2.

Figure 2. (a) ORTEP plot of the Ge/BO4 unit and the Cs+ cation in 1. Symmetry codes: A, −1/3 − x + y, 1/3 − x, 1/3 + z; B, x − y, x, 1 − z. (b) Structures of the SOD-type cage. Color code: Ge/BO4 tetrahedra, green. (c) Framework structure of 1 viewed along the [001] direction. Color code: Ge/B, green; O, red.

BCsGe5O12 698.67 trigonal R3̅ 12.3298(7) 6.1561(6) 810.49(10) 3 4.294 17.129 948 3.82−25.18 1.078 768 313 (0.0509) 280 31 0.0459/0.1016 0.0513/0.1060 1.079/−1.135 b

O atoms. Structure determination revealed that the Ge4+/B3+ position is statistically occupied by Ge4+ (83.3%) and B3+ (16.6%) when refined with a deficiency model, can be confirmed by the Ge/B−O distances ranging from 1.691(8) to 1.706(7) Å (Table S1), which is between the known average values of 1.743 Å in GeO4 and 1.476 Å in BO4, and can be interpreted due to a mixed occupation Ge4+/B3+. The IR spectrum of 1 shows the presence of both GeO4 and BO4 tetrahedra in the structure (Figure S2). The strong peaks at 881 and 826 cm−1 can be attributed to the characteristic asymmetric Ge−O stretching of GeO4 tetrahedra while its symmetric stretching peaks are around 577 and 522 cm−1, and the band at 1056 cm−1 is associated with stretching vibrations of BO4 units. In addition, the energydispersive spectroscopy analysis results reveal that the Ge/Cs molar ratio is around 5.1:1, which is in good agreement with the result of structure refinement (Ge/Cs = 5:1).

wR2 = {∑[w(F o 2 − F c 2) 2 ]/ B

DOI: 10.1021/acs.inorgchem.6b03002 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

in progress for discovering new types of open-framework BGOs with different Ge/B ratios under solvothermal conditions.

As shown in Figure 2b, 24 Ge/BO4 tetrahedra are alternately linked by 36 μ2-O bridges to form a truncated octahedral cage, which consists of eight large hexagon rings and six small rhombus rings. The diameter of the cage is 9.24 Å (Figure 2b). Each cage connects with eight nearest neighbors and gives rise to a 3D anionic framework (Figure 2c). The Cs+ ions are located in the center of each cage and interacted with O atoms from the BGO framework with the ionic Cs−O bonds varying from 3.273(7) to 3.309(7) Å (Figure S3 and Table S1). From the topological point of view, the framework of 1 can be described as a rare zeolite SOD-type net, in which Ge/BO4 tetrahedra act as 4-connected nodes (Figure 3). The framework density of 1, defined as the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b03002. Selected bond lengths, TGA, and IR for 1 and a view of the 3D open anionic framework (PDF) X-ray crystallographic files in CIF format for structure 1 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-W.C.). *E-mail: [email protected] (G.-Y.Y.). ORCID

Guo-Yu Yang: 0000-0002-0911-2805 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (NSFC; Grants 21571016 and 91122028) and the NSFC for Distinguished Young Scholars (Grant 20725101).

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REFERENCES

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Figure 3. View of the topology network in 1, in which the Ge/BO4 tetrahedra are shown as green nodes.

number of polyhedra per 1000 Å3, is 22.21, which is comparable to that of open-framework germanates ASU-7 (21.51) and ASU9 (19.97).22 The frameworks of ASU-7 and ASU-9 are constructed from corner-sharing GeO4 tetrahedra, forming different 4-connected nets with extended 1D channels and cages, respectively. The mixed tetrahedral Ge4+/B3+ positions are rare in BGOs. To the best of our knowledge, the only example is strontium borogermanate Sr3−x/2B2−xGe4+xO14 (x = 0.32), which was synthesized by high-temperature solid-state reaction and shows a Ca3Ga2Ge4O14-like structure,23 while the well-crystalline CsBxGe6−xO12 (x = 1) was made under solvothermal conditions. The partial replacement of Ge atoms in tetrahedral positions by a small number of B atoms leads to the highest Ge/B ratios in reported BGOs. The cagelike feature is often observed in germanate structures,24 but less appeared in reported BGOs.12 Note that a SOD-type cage has not been observed in openframework germanates. In summary, a new microporous BGO has been made under solvothermal conditions using a mixed solvent of pyridine and H2O. 1 exhibits the highest atomic ratios of Ge/B in BGOs by the partial boron substitution with mixed tetrahedral Ge4+/B3+ positions and leads to a zeolite SOD-type net. The IR spectrum of 1 clearly shows the presence of BO4 and GeO4 tetrahedra in the structure. We believe that the partial incorporation of boron into the tetrahedra in the Ge7, Ge8, Ge9, or Ge10 clusters could offer exciting opportunities for preparing novel crystalline BGOs with diverse structures and interesting properties. Further work is C

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Cs2GeB4O9: a New Second-Order Nonlinear-Optical Crystal. Inorg. Chem. 2013, 52, 5831−5837. (21) Synthesis of 1: GeO2 (0.104 g) was added to a mixture of hydrofluoric acid (1.0 mL) and H2O (1.0 mL) and stirred until GeO2 was dissolved completely, and then H3BO3 (0.185 g), CsCl (0.168 g), and pyridine (3.0 mL) were added to the solution and stirred for 1 h in the air. The resultant mixture was transferred to a Teflon-lined stainless steel vessel, heated to 200 °C for 7 days, and then cooled to room temperature. Colorless block crystals suitable for single-crystal X-ray diffraction were obtained by filtration, washed with distilled H2O, and dried in air (yield: 53% based on Ge). (22) Li, H.; Yaghi, O. M. Transformation of Germanium Dioxide to Microporous Germanate 4-Connected Nets. J. Am. Chem. Soc. 1998, 120, 10569−10570. (23) Petermüller, B.; Petschnig, L. L.; Wurst, K.; Heymann, G.; Huppertz, H. Synthesis and Characterization of the New Strontium Borogermanate Sr3−x/2B2−xGe4+xO14 (x = 0.32). Inorg. Chem. 2014, 53, 9722−9728. (24) Liang, J.; Su, J.; Wang, Y.; Chen, Y.; Zou, X.; Liao, F.; Lin, J.; Sun, J. A 3D 12-Ring Zeolite with Ordered 4-Ring Vacancies Occupied by (H2O)2 Dimers. Chem. - Eur. J. 2014, 20, 16097−16101.

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DOI: 10.1021/acs.inorgchem.6b03002 Inorg. Chem. XXXX, XXX, XXX−XXX