(C H3)2 N H2) (Ga Ge3 O8)

Hydrothermal Synthesis and Structural Characterization of Zeolite-like Structures Based on Gallium and Aluminum Germanates Xianhui Bu, Pingyun Feng, Thurman E. Gier, Dongyuan Zhao, and Galen D. Stucky* Contribution from the Department of Chemistry, UniVersity of California, Santa Barbara, California 93106 ReceiVed August 24, 1998 Abstract: While extensive commercial success has been achieved through the use of organic molecules in the synthesis of aluminosilicate zeolites, the synthesis of the germanium-based zeolite structures using organic structure-directing agents is a largely unexplored area. Here we report a novel class of germanate zeolite structures prepared with inorganic cations or organic amines as structure-directing agents. These new materials possess five 4-connected 3-dimensional topologies with large (12-ring), medium (10-ring), small (8-ring), or ultrasmall (6-ring) pores. They have a variety of chemical compositions such as various Ge to Ga (or Al) ratios and exhibit interesting structural features such as helical chains and odd-membered rings. UCSB-15GaGe and UCSB-15AlGe are the first germanate-based structures with 5-rings. UCSB-7 refers to a collection of isostructural, large-pore zeolite-like structures constructed by the cross-linking of helical ribbons. UCSB- 3GaGe is the only known T3+/T4+ (T refers to tetrahedral atoms) based zeolite structure with the net 38 topology. This structure is unusual because of its very high framework charge density unsurpassed by other amine- directed zeolite structures. GaGe-SOD2 and AlGe-SOD2 (sodalite analogues) have an unusually low framework symmetry for a sodalite structure. The cubic GaGe-SOD1 (or AlGe-SOD1) and the triclinic GaGe-SOD2 (or AlGe-SOD2) are ideal examples of the compositional and structural control of the inorganic framework by structure-directing amines. GaGe-ANA1 and GaGe-ANA2, two gallogermanate analcime analogues, are noncentrosymmetric, unlike many other analcime structures. Introduction Zeolites are open-framework aluminosilicates constructed from corner-sharing Al and Si oxygen tetrahedra.1 Over the past several decades, zeolites have been extensively studied because of their utility in commercial processes such as gas separation and petrochemical-based catalysis.2 Of fundamental importance is the synthesis of zeolitic materials with novel catalytic properties. This has led to an everincreasing interest in preparing zeolitic materials with new framework topologies or chemical compositions.3 The discovery of a large family of aluminophosphate-based zeolite-type materials in the early 1980s has generated a widespread interest in non-aluminosilicate-based microporous materials.4-7 Recently, a large number of phosphate-based zeolite-type structures with high framework charge densities were reported.8,9 Here, we seek to expand the methodology developed in the synthesis of the highly charged phosphates to the germanium system. Our interest in developing germanium- based zeolite-like structures is not limited to the expansion of known zeolite structures into a new compositional domain. It is also important to understand the essential factors in the zeolite synthesis and to develop a synthesis strategy to prepare zeolite structures with previously unknown framework topologies. While a large number of zeolite structures are possible on the basis of topological considerations, only a small fraction of these have been synthesized.10 This is in part due to the restrictive chemical factors, which favor some particular framework topologies for given chemical compositions.11 Thus changing chemical factors by working in novel compositional domains should provide a more feasible route to new framework structures. Germanates are of particular interest because they have metal-oxygen bond distances significantly greater than those in silicate-based materials. The large T-O distances in ger- manates lead to smaller T-O-T angles. We have found that these geometric factors have an important effect on the types of structures that are favored under particular synthesis condi- tions.12 Another advantage of the germanate system over other non-silicate compositions is that the germanate-based materials are capable of forming both even- and odd-membered rings; thus many more framework topologies are accessible. The exploration of such a compositional domain has the potential to uncover new zeolite-like structures including 3-ring structures with low framework densities.13 Even in the early days of the zeolite synthesis, there were some efforts aimed at substituting Al and/or Si with other (1) Meier, W. M.; Olson, D. H.; Baerlocher, Ch. Atlas of Zeolite Structure Types; Elsevier: Boston, MA, 1996. (2) Breck, D. W. Zeolite Molecular SieVes; Wiley: New York, 1974. (3) Occelli, M. L.; Kessler, H. Synthesis of Porous Materials, Zeolites, Clays, and Nanostructures; Marcel Dekker: New York, 1997. (4) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146-1147. (5) Flanigen, E. M. In Studies in Surface Science and Catalysis; van Bekkum, H., Flanigen, E. M., Jansen, J. C., Eds.; Elsevier: Amsterdam, 1991, Vol. 58, pp 13-34. (6) Khan, M. I.; Meyer, L. M.; Haushalter, R. C.; Schweitzer, A. L.; Zubieta, J.; Dye, J. L. Chem. Mater. 1996, 8, 43-53. (7) Suib, S. T. Curr. Opin. Solid State Mater. Sci. 1998, 3, 63-70. (8) Feng, P.; Bu, X.; Stucky, G. D. Nature 1997, 388, 735-741. (9) Bu, X.; Feng, P.; Stucky, G. D. Science 1997, 278, 2080-2085. (10) Smith, J. V. Chem. ReV. 1988, 88, 149-182. (11) Brunner, G. O. Zeolites 1993, 13, 88-91. (12) Gier, T. E.; Bu, X.; Feng, P.; Stucky, G. D. Nature 1998, 395, 154- 157. (13) Bu, X.; Feng, P.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 11204-11205. 13389J. Am. Chem. Soc. 1998, 120, 13389-13397 10.1021/ja983042k CCC: $15.00 © 1998 American Chemical Society Published on Web 12/30/1998 tetrahedral elements such as Ga and Ge. The early synthesis of Ge4+-based phases was performed in a purely inorganic system similar to that used for the synthesis of aluminosilicate zeolites, and a few phases were claimed.14 In fact, compared to the case of silicate or phosphate structures, much less is known about germanium-based zeolite structures. In gallogermanates, only single-crystal structures of hydrated sodalite and anhydrous natrolite analogues (Na3[GaGeO4]3â4H2O and RbGa2Ge3O10) have been reported.15,16 The high pressure and high-temperature (800 bar, 750 °C) synthesis conditions for the natrolite analogue are, however, unsuitable for the synthesis of more open zeolite- like structures. Here we report a large family of germanate zeolite structures denoted as UCSB-15GaGe, UCSB-15AlGe, UCSB-7GaGe, UCSB-7AlGe, UCSB-3GaGe, GaGe-SOD1, AlGe-SOD1, GaGe- SOD2, AlGe-SOD2, GaGe-ANA1, and GaGe-ANA2. These materials possess various Ge to Ga (or Al) ratios (1, 1.5, 2, 3, 5, etc.), demonstrating the diversity of chemical compositions that can be achieved in such a system. With the exception of the analcime analogues, these new structures were synthesized with organic molecules as structure-directing agents. A variety of amines (primary, secondary, tertiary, or quarternary; linear, branched, or cyclic) have been successfully used in the crystal growth of germanate 4-connected, 3D structures. It is noteworthy that the Ga3+ cations in structures reported here are strictly tetrahedrally coordinated, in contrast with many gallophosphates in which Ga3+ usually has a higher coordination number,17 a configuration that in general is less desirable in the synthesis of open-framework structures. Prior to this work, every zeolite-type structure could be prepared as either a silicate or a phosphate, or both. On such a basis, zeolite-type structures have been classified into three groups by the International Zeolite Association.1 The work reported here creates a new group of zeolite-type structures because neither UCSB-7 nor UCSB-15 has been prepared as a silicate or phosphate. Experimental Section Hydrothermal Synthesis. (a) UCSB-7. UCSB-7 can have several distinct crystal morphologies even from a single synthesis batch (Figure 1). Crystals of different shapes have been studied by single-crystal X-ray diffraction and are confirmed to have identical structures. A typical procedure for the synthesis of gallogermanate UCSB-7 is given below. To a polypropylene bottle were added 0.730 g of GeO2, 6.462 g of H2O, 7.32 g of tris(2-aminoethyl)amine, 0.551 g of Ga(NO3)3âxH2O, and 7.079 g of ethylene glycol. The mixture was then heated at 180 (14) Barrer, R. M.; Baynham, J. W.; Bultitude, F. W.; Meier, W. M. J. Chem. Soc., Chem. Commun. 1959, 195-208. (15) Nenoff, T. M.; Harrison, W. T. A.; Gier, T. E.; Keder, N. L.; Zaremba, C. M.; Srdanov, V. I.; Nicol, J. M.; Stucky, G. D. Inorg. Chem. 1994, 33, 2472-2480. (16) Klaska, K. H.; Jarchow, O. Z. Kristallogr. 1960, 113, 430-444. (17) Martens, J. A.; Jacobs, P. A. In Studies in Surface Science and Catalysis; Jansen, J. C., Stocker, M., Karge, H. G., Weitkamp, J., Eds.; Elsevier: Amsterdam, 1994; Vol 85, pp 653-685. Figure 1. SEM pictures of gallogermanate UCSB-7 synthesized with N-(2-aminoethyl)-1,3-propanediamine showing various crystal morphologies. 13390 J. Am. Chem. Soc., Vol. 120, No. 51, 1998 Bu et al. °C for 8 days in a Teflon-coated steel autoclave. The product was recovered by filtration and washed with deionized water. The X-ray powder pattern (Figure 2) could be indexed using a body-centered cubic cell, and no impurity lines were detected. (b) UCSB-15GaGe. To a polypropylene bottle were added 0.454 g of GeO2, 8.389 g of hexamethyleneimine, 1.531 g of piperazine hexahydrate, 0.244 g of Ga(NO3)3âxH2O, and 2.258 g of ethylene glycol. The mixture was then heated at 180 °C for 8 days in a Teflon-coated steel autoclave. The product was recovered by filtration and washed with deionized water. Crystals of UCSB-15GaGe had a distinct crystal morphology showing the orthorhombic symmetry (Figure 3). Two other phases were UCSB-7GaGe and a piperazine-templated germanate with some Ge4+ sites in nontetrahedral coordinations.18 (c) UCSB-3GaGe and GaGe-SOD2. To a polypropylene bottle were added 0.497 g of GeO2, 5.813 g of H2O, 6.58 g of ethylenediamine, 0.748 g of Ga(NO3)3âxH2O, and 6.912 g of ethylene glycol. The mixture was then heated at 180 °C for 8 days in a Teflon-coated steel autoclave. The product was recovered by filtration and washed with deionized water. Three different phases could be identified on the basis of the crystal morphology and single-crystal X-ray diffraction. Crystals of UCSB-3GaGe were thin-needle-shaped whereas crystals of GaGe-SOD2 were thick-plate-shaped. Crystals of UCSB-7GaGe did not have a well- defined shape and were a minor phase. (d) GaGe-SOD1. GaGe-SOD1 was prepared by mixing GeO2 (1.051 g), 25% aqueous tetramethylammonium hydroxide (2.840 g), Ga(NO3)3â xH2O (0.472 g), and ethylene glycol (4.258 g). The mixture was then heated at 180 °C for 16 days in a Teflon-coated steel autoclave. The product was recovered by filtration and washed with deionized water. All crystals had a cubic morphology. (e) GaGe-ANA1. GaGe-ANA1 was prepared by mixing GeO2 (0.824 g), 1,4-diaminobutane (3.07 g), Ga(NO3)3âxH2O (0.439 g), and ethylene glycol (0.405 g). The mixture was then heated at 180 °C for 8 days in a Teflon-coated steel autoclave. The product was recovered by filtration and washed with deionized water. Two distinct phases could be recognized on the basis of the crystal morphology and single-crystal X-ray diffraction. UCSB-7GaGe was the major phase, and crystals were in the form of thick needles (Figure 1). Crystals of GaGe-ANA1 were bowl-shaped. (f) GaGe-ANA2. To a polypropylene bottle were added 50% aqueous CsOH (15.64 g) and a mixture of Ga2O3 (0.375 g) and GeO2- (0.858 g). After an overnight soak at 100 °C, all solids were in solution. Sequential additions of concentrated HNO3 (up to 3.737 g) were then made with overnight soaks at 100 °C. Recoveries of varying quantities of polycrystalline products (identified using X-ray powder diffraction as an analcime analogue) were made after each soak. Filtrates were saved for subsequent acidification. Finally, 0.109 g of concentrated HNO3 was added to the last filtrate, and the mixture was sealed in a Teflon pouch and heated to 170 °C for 4 days in an autoclave. The cooled recovered product consisted of cubic crystals of GaGe-ANA2 with a typical dimension of 120 ím. Elemental Analysis. Quantitative elemental analyses for selected materials were carried out on a Cameca SX-50 electron probe microanalyzer equipped with five wavelength-dispersive (WD) X-ray spectrometers and one energy-dispersive (ED) X-ray spectrometer. The analyses for different elements were simultaneously performed on different WD spectrometers. Calculated values (in mass percent based on the formula derived from the single-crystal structure analysis) and observed values in parentheses are as follows. UCSB-3GaGe: Ga, 29.4 (27.4); Ge, 30.6 (33.9). UCSB-7GaGe(methylamine): Ga, 15.6 (15.6); Ge, 48.7 (47.2). Single-Crystal X-ray Crystallography. A crystal of each sample was glued to a thin glass fiber with epoxy resin and mounted on a SMART CCD diffractometer equipped with a normal-focus, 2.4 kW sealed-tube X-ray source (Mo KR radiation, ì ) 0.71073 Å) operating at 50 kV and 40 mA. About 1.3 hemispheres of intensity data were collected in 1321 frames with ö scans (width of 0.30° and exposure time of 30 s per frame). The empirical absorption corrections were based on the equivalent reflections, and other possible effects such as absorption by the glass fiber were simultaneously corrected. The structures were solved by direct methods followed by successive difference Fourier methods. All calculations were performed using SHELXTL, and final full-matrix refinements were against F2. The crystallographic results are summarized in Tables 1 and 2, and some important bond distances and angles are given in Table 3. Results and Discussion We will first discuss the use of the geometrical data in the evaluation of the Ge/Ga ratio and the Ge and Ga site distribution in this new family of gallogermanates. The structural cor- respondence between two classes of body-centered cubic zeolite- type structures (the Im3hm group and the Ia3hd group) will then be discussed. Finally, new gallogermanate materials will be grouped and discussed together on the basis of their framework topological types. A summary of new aluminogermanate materi- als isostructural with gallogermanate structures is provided in Table 4. Ge/Ga and Al/Ge Ratios. Determining the Ge/Ga ratio is one of the most important aspects in the structural analysis of gallium germanates. The similarity in X-ray atomic scattering factor between Ge4+ and Ga3+ makes it difficult to distinguish Ge4+ and Ga3+ atomic sites in the crystal structure refinement. However, in some cases, it is possible to determine the distribution of Ga and Ge atoms by analyzing metal-oxygen (M-O) bond distances. The ideal bond distances for Ge-O and Ga-O are 1.74 and 1.82 Å, respectively.19 Thus, a comparison between refined M-O bond distances with these two ideal values can sometimes allow an unambiguous assignment of T-atom types to different (18) Bu, X.; Feng, P.; Stucky, G. D. Manuscript in preparation. (19) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767. Figure 2. X-ray powder diffraction pattern of gallogermanate UCSB-7 prepared in the presence of tris(2-aminoethylamine). Figure 3. SEM picture of a crystal of the gallogermanate UCSB-15. Zeolite-like Structures Based on Germanates J. Am. Chem. Soc., Vol. 120, No. 51, 1998 13391 T-atom sites. For example, in GaGe-SOD2 described below, the distribution of Ga and Ge cations among three unique tetrahedral atom sites can be determined on the basis of the average bond distances which are 1.754 Å (the Ge site), 1.752 Å (the Ge site), and 1.820 Å (the Ga site), respectively. In some cases, the structural refinement gives a M-O distance that is between the ideal Ga-O and Ge-O distances so that it is not possible to unambiguously distinguish Ga and Ge sites. A similar problem exists in aluminosilicate zeolites, in which it is often not possible to distinguish between Al3+ and Si4+ sites. However, the M-O distances in these gallogermanates can still be quite useful in evaluating the overall Ge/Ga ratio in the structure. For example, the difference in the Ge/Ga ratios between two sodalite analogues (GaGe-SOD1 and GaGe-SOD2) is clearly reflected in the average M-O bond distances. GaGe- SOD2 has a longer average M-O distance (1.775 Å) because of its lower Ge/Ga ratio (Ge/Ga ) 2). The higher Ge/Ga ratio (Ge/Ga ) 5) in GaGe-SOD1 leads to a significantly shorter M-O distance (1.755 Å). It is worth mentioning that the average M-O bond distance in aluminogermanates reported here is independent of Al/Ge ratios (Table 4) because the ideal bond distances for Ge-O and Al-O are both 1.74 Å.19 On the other hand, the X-ray atomic scattering factors of Ge4+ and Al3+ are significantly different to allow a fairly accurate estimate of Al/Ge ratios through occupancy refinement. For example, in AlGe-SOD1, the refined occupancy factors for Ge and Al are 0.2083 and 0.0417, respectively. This gives a Ge/Al ratio of 5.00, in perfect agreement with the Ge/Al ratio derived from the charge neutrality. T-Atom Site Selectivity. In gallogermanates, the distribution of Ge and Ga among unique tetrahedral atom sites can be highly selective. In addition to Loewenstein’s aluminum avoidance rule,20 which may be applicable to gallogermanates and thus disfavor Ga-O-Ga linkages, there are other chemical factors that could lead to a specific site selectivity. Such a site selectivity is a phenomenon occurring at the nucleation and crystallization stages and is related to the difference in local charge density of the inorganic framework. Specifically, the tetrahedral atom site selectivity is correlated with the orientation of organic molecules and the more negative region (i.e. Ga sites) should be close to the positively charged head groups of amine molecules (R- NH3+). Such an effect is illustrated by the orientation of organic molecules in the ethylenediamine sodalite analogue (GaGe- SOD2, Ge/Ga ) 2). The three shortest NâââO distances in GaGe- SOD2 are 2.858, 2.732, and 2.899 Å for N1-O1, N1-O3, and N1-O4, respectively. All three oxygen atoms are coordinated to the Ga site whereas only one of these O atoms is coordinated to the Ge1 site and two of these O atoms are connected to the (20) Loewenstein, W. Am. Mineral. 1954, 39, 92-96. Table 1. Summary of Crystal Data and Refinement Results for UCSB-3GaGe (Refined in Both Super- and Subcells), UCSB-7GaGe-dma, and UCSB-15GaGea-c UCSB-3GaGe UCSB-3GaGe UCSB-7GaGe-dma UCSB-15GaGe formula (R2)Ga2Ge2O8 (R2)Ga2Ge2O8 (R12)GaGe3O8 (R3)GaGe5O12 habit thin needle thin needle dodecahedron plate color clear translucent clear translucent clear translucent clear translucent size (ím3) 266 � 40 � 40 266 � 40 � 40 67 � 67 � 67 226 � 106 � 13 a (Å) 10.6245(1) 15.0254(2) 18.5356(2) 7.7308(3) b (Å) 10.6245(1) 15.0254(2) 18.5356(2) 13.3750(4) c (Å) 8.9822(2) 8.9822(2) 18.5356(2) 14.6126(5) V (Å3) 1013.91(3) 2027.85(6) 6368.3(1) 1510.93(9) Z, 2ımax (deg) 4, 50 8, 56 24, 56.32 4, 56.56 space group P42/n P42/n Ia3hd Pnma no. of tot./unique data 5178/898 12 459/2430 17 425/660 8927/1914 data with I > 2ó(I) 786 1633 647 1586 no. of parameters 83 122 38 129 R(F) (%) 4.27 7.37 3.79 3.27 Rw(F 2) (%) 11.4 14.1 6.80 7.09 GOF 1.27 1.31 1.37 1.05 a R(F) ) ∑jjFoj - jFcjj/∑jFoj with Fo > 4.0ó(F). b Rw(F2) ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]]1/2 with Fo > 4.0ó(F). c R2 ) [NH3CH2CH2NH3]2+; R12 ) (CH3)2NH; R3 ) monoprotonated piperazine. Table 2. Summary of Crystal Data and Refinement Results for Sodalite and Analcime Analoguesa-d GaGe-SOD1 GaGe-SOD2 GaGe-ANA1 GaGe-ANA2 f