In the Laboratory
Self-Assembly of Microporous Thiogermanate Frameworks
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Mark J. MacLachlan, Robert W. J. Scott, Geoffrey A. Ozin,* and Douglas F. McIntosh Department of Chemistry, University of Toronto, Toronto, ON M5S 3H6, Canada; *
[email protected] Advanced materials are everywhere—Si in microchips, TiN in drill bits, composites in automobiles—yet solid-state chemistry is often neglected in undergraduate inorganic chemistry laboratories. This arises from a perception that solid-state chemistry is a crude and primitive field where materials are made by “shake-and-bake” techniques, with more emphasis on serendipity and persistence than on creativity and design. As many preparations can require extreme conditions (e.g., high temperatures and pressures) and long reaction times and offer little synthetic experience to the student, they are frequently omitted from undergraduate programs. While these characteristics certainly apply to many solid-state reactions, advances in materials chemistry suggest that a molecular approach to building solid-state structures can be effective. Background Microporous materials (solids with pore diameters less than ca. 20 Å) are an important class of materials and are the subject of many modern research initiatives (1). These 2-D and 3-D framework structures have been constructed from nearly every element in the periodic table, the most important class of microporous materials being the naturally occurring and synthetic zeolites (2). Zeolites are used in many industrial processes, such as oil refining, and catalytic applications. Molecular sieves, as porous solids such as zeolites are called, find utility in shape-selective catalysis, molecular separation, and chemical sensing. Their pores normally contain water molecules that can be removed upon heating under vacuum. As dehydrated sieves are extremely hygroscopic, they are used commercially as drying agents for solvents. The wide range of applications for microporous materials, from adsorbates and detergent additives to the isomerization of xylenes and the cleanup of radioactive isotopes in nuclear disasters, are well documented (3, 4). This experiment, the cumulation of our efforts to introduce third-year undergraduate students to modern techniques of solid-state synthesis and characterization, demonstrates the ability of a tetradentate thiogermanate ligand, [Ge4S10]4᎑ (Fig. 1), to direct the formation of three-dimensional, openframework structures containing transition metals (5). These S
Figure 1. The structure of the tetradentate adamantanoid [Ge 4 S 10] 4᎑ cluster drawn with thermal ellipsoids at the 25% probability level.
Ge S
S S Ge
Ge S
S
S Ge S S
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thiogermanate clusters, T4Ge4S10 (T is a charge-balancing cation), were first identified nearly 30 years ago by Krebs (6 ). They resemble adamantane and are isostructural with P4O10. Experimental Procedure In the first step of the experiment, the water-soluble precursor, T4Ge4S10, is made from the elements in a one-step hydrothermal synthesis. TMA4Ge4S10 (TMA = tetramethylammonium) is obtained as a light yellow aqueous solution and, by precipitation with acetone, can be isolated as a fine white powder in yields exceeding 80%. In this part of the experiment, students are introduced to hydrothermal synthesis, a common solid-state chemistry technique that has been used to form a wide variety of materials. In the second step of the experiment, microporous framework materials with the composition TMA2MGe4S10 (M = Fe, Co, and Zn) are prepared by the stoichiometric reaction of the precursor thiogermanate and transition metal salts of Fe(II), Co(II) and Zn(II). These metal ions were chosen because the kinetics of the reaction allow the preparation and isolation of the framework materials within a single four-hour laboratory period. By using EDTA (ethylenediaminetetraacetate) as a complexing agent for Zn(II) and tartrate for Co(II), students can observe the effects of metal complexation in the reaction. This self-assembly route to microporous metal germanium sulfides was discovered in 1994 by Yaghi and coworkers, who recognized the ability of this large tetradentate adamantanoid ligand to direct the assembly of solid-state framework structures (7 ). Interestingly, Yaghi had found a convenient route to materials previously patented by Universal Oil Products, in which GeS2 was combined with transition metal salts under hydrothermal conditions (8). The ability to divide the synthesis into two steps enhances the control of reaction progress, making crystal growth and compositional control easier. The frameworks for Fe(II), Co(II), and Zn(II) with TMA and Cs cations are isomorphous with TMA2MnGe4S10 (9, 10). In the final step of the experiment, careful acidification of the precursor thiogermanate produces a crystalline polymorph of GeS2 with an intriguing structure (11) in which [Ge4S10]4᎑ clusters are linked together into two interpenetrating diamond networks (where each C in diamond is replaced by Ge4S6S4/2) of the clusters. It is believed that the reaction proceeds through a polycondensation mechanism in which the [Ge4S10]4᎑ clusters link together to form a 3-D framework of δ-GeS2, previously isolated only under high-temperature, high-pressure conditions. •CAUTION: The hydrothermal synthesis of TMA4Ge4S10 creates trace amounts of undesired substances such as trimethylamine, dimethyl sulfide, and hydrogen sulfide. Thus all of the reactions must be performed in a fume hood. Students are required to wear gloves. None of the glassware used should be removed from the fume hood until it has been
Journal of Chemical Education • Vol. 77 No. 5 May 2000 • JChemEd.chem.wisc.edu
In the Laboratory
Figure 2. Raman spectra of (top) TMA 4 Ge 4S 10 and (bottom) TMA2MnGe4S10.
Figure 3. PXRD patterns of (top) TMA4Ge4S 10 precursor and (bottom) TMA2FeGe4S10.
rinsed with dilute nitric acid to remove the residual organics and amines. Uncovered samples may not be taken out of the fume hood.
Ge–S stretching modes. As routine use of a Raman spectrometer was unavailable in our laboratory, spectra of the materials were provided. Students also ran IR spectra of pressed KBr discs of each of their materials. From the IR, they could easily observe C–N stretching and C–H bending from the template and confirmed the absence of complexing agents in the final product.
Analysis
Vibrational Spectroscopy Raman spectroscopy is a useful technique for examining the metal germanium sulfide framework as well as the cluster precursor. One can obtain valuable information about the organic template and determine the symmetry of the components. In addition, impurities such as sulfur can be identified by their distinctive vibrational modes. Perhaps the main advantages of Raman over IR spectroscopy, though, are (i) the types of samples employed (aqueous solutions or solid samples in glass capillaries) and (ii) the accessibility of vibrational modes below 400 cm᎑1. The theory of Raman spectroscopy has been clearly explained in several publications (12). The Raman spectra of the germanium sulfide materials are useful as a diagnostic tool to detect the presence of the [Ge4S10]4᎑ cluster and the formation of the solid framework (13). In the spectra (Fig. 2) there are two groups of vibrational modes situated within the 340–470 cm᎑1 and 150–230 cm᎑1 regions attributed to terminal Ge–S and bridging Ge–S stretching modes, respectively. Students were given Ge–S bond lengths in the adamantanoid cluster (7 ) and asked to speculate on the origin of each group of peaks. We also asked them to discuss changes in the modes attributed to the organic cation and to predict (approximately) where M–S modes would be found (on the basis of the reduced mass for M–S). The M–S modes are often located in the window between the two groups of
Powder X-ray Diffraction (PXRD) It is often difficult to obtain single crystals of solid-state compounds, microporous materials in particular, suitable for X-ray structural determination. PXRD produces a onedimensional “fingerprint” of the materials that contains similar information and is thus an important technique for students to learn (3). From analysis of the peaks in a diffraction pattern, the space group and unit cell dimensions can often be determined. Figure 3 shows the typical PXRD patterns generated by the precursor and framework materials. In some systems, particularly TMA2ZnGe4S10 prepared without a complexing agent, particle size or crystallinity effects lead to substantial broadening of the diffraction peaks. The use of EDTA as a complexing agent permits the controlled release of Zn(II) to the reaction, resulting in fewer nucleation centers. This leads to improved crystallinity and larger particle size. The corresponding PXRD pattern shows significantly narrower peaks, as seen in Figure 4. Similar effects are seen in the complexation of Co(II) with tartrate. One indication of the increased particle size is a noticeable decrease in the time needed to centrifuge samples prepared with a complexing agent present. Students were given a series of exercises on PXRD, using the basic equations of diffraction (e.g., the Bragg equation). They
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In the Laboratory
end of a lab period. The autoclave is then heated for 18 hours and cooled sometime before the students return for their next lab. In a 4-hour lab period, the precursor can be isolated and all the framework syntheses can be performed. The products are left in a desiccator to dry until the next lab period. Students are then required to submit a sample of their choice for PXRD analysis and to obtain an IR spectrum of each framework (as KBr pellets) during the next lab. In total, the experiment requires ca. 6 hours in the laboratory to complete. W
Supplemental Material
Supplemental material for this article is available in this issue of JCE Online. Literature Cited
Figure 4. PXRD patterns of TMA2ZnGe4S10 prepared (top) without and (bottom) with complexing agent.
were expected to calculate unit cell edge lengths from examination of indexed peaks and to convert 2θ data to d-spacing data (with the Bragg equation). Further exercises involved indexing several unknown peaks utilizing the knowledge that (i) lower {hkl} reflections occur at higher d-spacings and (ii) systematic absences for each space group are known from the International Tables of X-ray Crystallography (13). There are many opportunities for modifying the experiment to meet physical characterization techniques available in the lab. For example, solid-state NMR, EPR, Mössbauer, nearIR, and reflectance UV–vis spectroscopies, thermogravimetric analysis (TGA), and density measurements can be useful characterization tools. Students may examine the pyramidal morphology of the crystals under an optical microscope. The syntheses may be modified using Cs+ as the counter-cation by replacing TMAOH with CsOH in the experimental procedures (9b). Suggested Time Frame The hydrothermal autoclave for the production of the tetramer can be loaded and sealed in ca. 30 minutes at the
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1. Bowes, C. L.; Ozin, G. A. Adv. Mater. 1996, 8, 13–28. 2. Breck, D. W. Zeolite Molecular Sieves; Wiley: Toronto, 1974. 3. Smart, L; Moore, E. Solid State Chemistry: An Introduction, 2nd ed.; Chapman & Hall: New York, 1995; Chapters 2 and 7 and references therein. 4. Smoot, A. L.; Lindquist, D. A. J. Chem. Educ. 1997, 74, 569– 570. Balkus, K. J.; Ly, K. T. J. Chem. Educ. 1991, 68, 875– 877. Copperthwaite, R. G.; Hutchings, G. J.; van der Riet, M. J. Chem. Educ. 1986, 63, 632–634. Bibby, D. M.; Copperthwaite, R. G.; Hutchings, G. J.; Johnston, P.; Orchard, S. W. J. Chem. Educ. 1986, 63, 634–637. 5. McIntosh, D. F. Laboratory Manual, Third Year Inorganic Chemistry; University of Toronto: Toronto, 1997. 6. Krebs, B.; Pohl, S. Z. Naturforsch. B 1971, 26b, 853–854. 7. Yaghi, O. M.; Sun, Z.; Richardson, D. A.; Groy, T. L. J. Am. Chem. Soc. 1994, 116, 807–808. 8. Bedard, R. L.; Vail, L. D.; Wilson, S. T.; Flanigen, E. M. Crystalline Microporous Metal Sulfide Compositions; U.S. Patent 4,880,761, 1989. Bedard, R. L.; Wilson, S. T.; Vail, L. D.; Bennett, J. M.; Flanigen, E. M. In Zeolites: Facts, Figures, Future; Jacobs, P. A.; van Santen, R. A., Eds.; Elsevier: Amsterdam, 1989; pp 375–387. 9. (a) Bowes, C. L.; Huynh, W. U.; Kirkby, S. J.; Malek, A.; Ozin, G. A.; Petrov, S.; Twardowski, M.; Young, D.; Bedard, R. L.; Broach, R. Chem. Mater. 1996, 8, 2147–2152. (b) Bowes, C. L.; Lough, A. J.; Malek, A.; Ozin, G. A.; Petrov, S.; Young, D. Chem. Ber. 1996, 129, 283–287. 10. Achak, O.; Pivan, J. Y.; Maunaye, M.; Loüer, M.; Loüer, D. J. Solid State Chem. 1996, 121, 473–478. 11. MacLachlan M. J.; Petrov, S.; Bedard, R. L.; Manners, I.; Ozin, G. A. Angew. Chem., Int. Ed. Engl. 1998, 37, 2075–2079. 12. Harris, D. C.; Bertolucci, M. D. Symmetry and Spectroscopy; Dover: New York, 1989. 13. International Tables for X-ray Crystallography; Henry, N. F. M.; Lonsdale, K., Eds.; Kynoch Press: Birmingham, 1971.
Journal of Chemical Education • Vol. 77 No. 5 May 2000 • JChemEd.chem.wisc.edu
