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The substitution of germanium for silicon in AST-type zeolite
Solid State Sciences 5 (2003) 1421–1433 www.elsevier.com/locate/ssscie
The substitution of germanium for silicon in AST-type zeolite Yingxia Wang b , Jiaqing Song c , Hermann Gies a,∗ a Institute for Geology, Mineralogy and Geophysics, Ruhr-University Bochum, 44780 Bochum, Germany b College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China c Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
Received 29 May 2003; received in revised form 28 July 2003; accepted 1 September 2003
Abstract The substitution of silicon by germanium in the AST zeolite framework type, [Sin Ge40−n O80 ]∗4(SDA+ F− ) expressed as unit cell content in its cubic F-centered symmetry, has been studied. Three different kinds of templates, dimethyldiethylammonium, dimethyldiisopropylammonium and isopropyltrimethylammonium cations, were used in the hydrothermal synthesis process in fluoride medium. The products were identified with XRD, MAS NMR, SEM and thermal analysis. The analysis of the X-ray powder diagrams shows that AST crystallizes in different space group symmetries depending on the nature of the SDA and the degree of Ge-substitution. The resonance signals of 19 F in MAS NMR experiments for the pure Si- and Ge-end members are at −38.2 and ∼ −15 ppm, respectively, indicating that the F− -anion is located as co-template in the double-four-ring (D4R) of the tetrahedral framework. This is confirmed by Rietveld analysis of powder diffraction data of the pure Ge-end member. The peak splitting of the 19 F NMR signal in pure GeO2 AST-type material is related to the displacement of F− location inside the D4R. Two more distinct signals at ∼ −8 and ∼ −19 ppm, respectively, are observed for X-ray pure AST-samples of intermediate compositions and assigned to fluoride in D4R built of 4[GeO4 ]- and 4[SiO4 ]tetrahedra (4Ge, 4Si) and to (2Ge, 6Si)-D4R, respectively. An ordered distribution of Ge in the AST-framework is proposed for cubic AST with compositions around Si/Ge = 1.5–1 by correlating the intensities of 19 F NMR signals and the results from chemical analysis. This model is further confirmed by the quantitative analyses of the corresponding 29 Si MAS NMR spectra. 2003 Elsevier SAS. All rights reserved. Keywords: Substitution of Ge for Si; AST zeolite; Hydrothermal synthesis; 19 F MAS NMR; Dimethyldiethylammonium; Dimethyldiisopropylammonium; Isopropyltrimethylammonium
1. Introduction Recently, considerable interest has been shown in the substitution of silicon by germanium in zeolites [1–21]. New framework materials have been synthesized in the system SiO2 /GeO2 making use of the high flexibility of the Ge– O–Ge bond angle. On the other hand, the substitution of Si by Ge in Ti-silicate zeolites has been reported as a useful way to adjust the acidic strength to meet the demands of the epoxidation reaction of organic alkenes by H2 O2 [9]. Considering the crystal chemical relationship between silicates and germanates, the silicate analogues in the germanate systems are considered as prototypes for highpressure silicates. They have been studied for a long time as model systems for lower crust and upper mantel high * Corresponding author.
E-mail address: [email protected] (H. Gies). 1293-2558/$ – see front matter 2003 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2003.09.003
pressure minerals [22]. There is a wide similarity in chemical and physical properties such as polymorphism, structure types, chemical bonding, etc. Both GeO2 and SiO2 crystallize in the framework types of rutile, quartz and cristobalite, respectively. However, there are also significant differences between the GeO2 and SiO2 phases. The bond length Si–O for 4-coordinated Si is typically between 1.60 and 1.63 Å, whereas the Ge–O distance ranges from 1.70 to 1.80 Å. As a consequence, the Ge–O–Ge bond angle (∼ 120–130◦) is smaller than that of Si–O–Si (∼ 140–145◦). Therefore, the incorporation of germanium on T-sites in zeolites leads to structural diversity with new framework architectures [16,19]. In the past, several germanate analogues of silicate zeolites have been obtained for known framework types such as ABW, ANA, AST, GIS, SOD, RHO, etc. [13,21,23]. Solid solutions of GeO2 and SiO2 crystallizing in known silicate framework types also have been synthesized, such as the structures of MFI [24], ISV [8] and, fi-
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nally, BEC [7,10], a zeolite framework type related to zeolite beta. These examples are quite interesting for the fundamental understanding of the modification of zeolite frameworks by the incorporation of germanium on T-sites. Recently, the formation of new zeolite framework types built only by [GeO4 ]-tetrahedra has been studied intensively. As an example, the framework type ASV with a 4-connected germanate structure is given [16]. For a complete survey of new GeO2 structures see the atlas of zeolite framework types at: http://www.iza-structure.org. In the present paper, the substitution of silicon by germanium in AST-type zeolites has been investigated. The AST-framework is built of double-four-ring cages of [TO4]tetrahedra (D4R) linked by additional [TO4 ]-units and is one of the simplest structures among the zeolites framework types. It has been synthesized as pure AlPO4 -, pure silicaand pure germania-framework [16,25,26]. Therefore, it is a good model structure for systematic studies of the substitution of silicon by germanium on T-sites. An interesting feature of this structure is the location of the F− anion inside the D4R. For the analysis of the local structure around the F− anions, NMR experiments have proven to be very useful. The 19 F-signal is at −38 ppm in pure SiO2 -AST as was first reported by Caullet et al. [25]. The low field shift of the resonance frequency as compared with F− in zeolite channels (∼ −110 ppm) has been explained by the small volume of the D4R-cage. Since then, the behavior of F− inside SiO2 -D4R’s has been studied frequently [27–31]. Klock and co-workers showed that the chemical shift of the 19 F NMR signal is correlated with the free volume of the cage the anion is occupying [32]. The pure GeO2 analogue of AST was first synthesized by Li and Yaghi [16] in fluoride medium using DABCO (1,4-diazabicyclo-[2,2,2]-octane) as structure directing agent (SDA). However, the authors did not report details on the location of the F− anion inside the GeO2 framework. Instead, they suggested that the OH− anions might occupy these positions. Later, Villaescusa et al. [33] successfully synthesized the fluoride-containing allgermania D4R anionic building unit as zeolite precursors, [Ge8 O12 (OH)8 F]− . They showed that the germania-cube is a suitable host for the F− anion. The resonance signal of the F− anion inside the pure germania-cube is low-field shifted with respect to the corresponding F− in the SiO2 -cube and is at ∼ −14 ppm. Substituting Ge for Si in the synthesis of ITQ-7, Blasco et al. [8] found that the incorporation of Ge promoted the crystallization of ITQ-7. In this zeolite framework D4R units are present as well. Intermediate compositions with different Si/Ge ratios were synthesized for ITQ-7, however, no pure GeO2 end member was reported. For the increase of the crystallization rate the preferential substitution of Si by Ge in the D4R building unit was held responsible. In the 19 F NMR spectrum of ITQ-7 two more signals were found at −8 and −19 ppm, respectively, in addition to the signal corresponding to F− inside the pure SiO2 -cube. The 19 F-NMR signals at −8 and −19 ppm were assigned as for F− inside the (3Ge, 5Si)-D4R and as for (2Ge, 6Si)-D4R
or (7Ge, 1Si)-D4R, respectively [8]. A careful study of ITQ17 with different Si/Ge ratios [10] confirmed the preferential location of Ge in the D4R until 4 T-sites are occupied by Ge. 19 F MAS NMR revealed two additional signals at −20 and −7 ppm, respectively. The latter is suggested to correspond to the resonance of a fluoride ion in the D4R(4Ge, 4Si). However, the interpretation of the assignment is still under discussion. In this paper we report on the synthesis and structure analysis of both, Si- and Ge-end members with ASTtype framework as well as on materials with intermediate compositions which were made with different SDA. The compounds have been studied systematically by the X-ray diffraction (unit cell refinements and structure analysis), solid state NMR (19 F, 29 Si, 13 C), thermal analysis (TG-DTA) and electron microscopy (SEM). 2. Experimental 2.1. Synthesis 2.1.1. SDAs Three different structure directing agents, dimethyldiethylammonium (SDA1), dimethyldiisopropylammonium (SDA2) and isopropyltrimethylammonium (SDA3) cations, were used in the syntheses of AST in fluoride media. Dimethyldiethylammonium hydroxide solution is commercially available (Fluka, 20 wt% water solution). SDA2 and SDA3 were synthesized by methylation of commercially available amines. A typical procedure is described below for the synthesis of SDA3. Isopropyltrimethylammonium iodide was obtained by reaction of isopropylmethylamine with methyl iodide using ethanol as solvent. The reaction was carried out at 40– 50 ◦ C in a water bath for several hours under stirring. Potassium bicarbonate was added as a neutralizing agent. Then the final mixture was filtered and the solid was washed with ethanol. For the crystallization of the product, the solvent was evaporated from the combined solutions. The solid was recrystallized in acetone as solvent and dried at room temperature. 1 H and 13 C NMR, elemental analysis, and MS spectra were used to identify the product as isopropyltrimethylammonium iodide. The iodide was exchanged to the hydroxide by reacting it with Ag2 O in aqueous solution. 2.1.2. Zeolites Appropriate amounts of amorphous SiO2 (Aldrich, 98%), amorphous GeO2 (Aldrich, 99.99%), SDA solution and HF solution were mixed and stirred until a uniform gel was obtained. Then the water content was controlled by evaporation at 70 ◦ C in an oven. The typical composition of the starting gel was: (1 − x)SiO2 :xGeO2 :0.5SDA:0.5HF:yH2O with x = 0.0–1.0 and y = 2–16. The mixture was transferred into a Teflon lined stainless steel autoclave and kept at the corresponding reaction temperature (150, 175 ◦ C) with a
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15–30 rpm rotating speed for 15–30 days. After quenching in cold water, the solid products in the autoclave were separated by filtration or centrifugation, washed with distilled water and dried at 75 ◦ C overnight.
calculations for the structure refinement were carried out with the F ULLPROF suite including F ULLPROF 2 K [41] and G FOUR [42].
2.2. Characterization
3. Results and discussion
2.2.1. X-ray powder diffraction The powder diffraction data were collected on a Siemens D-5000 diffractometer in modified Debye–Scherrer geometry, using Cu-Kα1 radiation (λ = 1.54059 Å), a capillary sample holder and a PSD detector (5◦ 2θ window).
3.1. Effects of SDA and Si/Ge ratio on the formation of AST-type structure
2.2.2. NMR spectroscopy MAS NMR spectroscopy was performed on a Bruker ASX 400 spectrometer with conventional Bruker probes. The 19 F NMR was measured under different spinning speeds (5000–12000 Hz) to identify the isotropic signals. Variable—temperature experiments for 19 F were run in the temperature range between RT and 150 ◦ C with the standard Bruker variable temperature (VT) unit. Both 29 Si NMR and 1 H–29 Si CP NMR were performed in order to identify the connectivity of Si. 1 H and 13 C spectra were recorded to confirm that the SDA molecules were intact. TMS was used as the standard for 1 H, 13 C and 29 Si and NaF for 19 F. The intensities of 19 F NMR signals were calculated by simulating the spectra with the program DMFIT [34]. 2.2.3. Chemical analyses As-synthesized samples were dissolved using Na2 O2 and diluted for ICP analyses. The contents of Si and Ge were measured with a Philips PU 7000 ICP-AES-Spectrometer. 2.2.4. Morphology The morphology of the crystallites was studied with a scanning electron microscope (SEM) (LEO-1530 Gemini). Furthermore, we studied the homogeneity and purity of the sample by scanning the morphologies of a representative mass of particles carefully. 2.2.5. Thermal analysis Thermal properties were recorded on a Bähr STA 503 thermal analyzer from room temperature to 560 ◦ C with a heating rate of 10 ◦ C min−1 using a mixture of nitrogen and air as flow gases. 2.2.6. X-ray data processing The raw data from X-ray diffraction were processed with P OWDER C ELL [35] and converted to the formats suitable for the programs W INPLOTR [36] and P OWDER X [37]. T REOR 90 [38] was used for indexing the X-ray powder patterns. The subsequent refinements of the unit cell parameters were also carried out with T REOR 90. A partial structure model was obtained by the direct method program EXPO [39,40] which was used for the structure analysis. The structure was completed using Fourier techniques. All
The decisive influence of structure directing agents on the formation of zeolites and, in particular, electro-neutral silica zeolites has been recognized and discussed in the past in many articles [43,44]. However, the detailed understanding of the templating effect of the SDA during crystal nucleation is still poorly understood. Therefore, the knowledge about the function of SDAs on the formation of a zeolite framework type is helpful in elucidating the fundamental mechanisms. In the present work, three different structure directing agents, dimethyldiethylammonium (SDA1), dimethyldiisopropylammonium (SDA2) and isopropyltrimethylammonium (SDA3) cations were used in hydrothermal syntheses in fluoride medium. The SDA’s are rather similar in size and shape, and almost identical in their chemical properties. In a wide range of Si/Ge ratios they allowed for the formation of the AST framework type structures, [Sin Ge40−n O80 ]∗4(SDA+ F− ) for the cubic unit cell, [Sin Ge20−n O40 ]∗2(SDA+ F− ) for the tetragonal unit cell and [Sin Ge30−n O60 ]∗3(SDA+ F− ) for the hexagonal unit cell. For convenience, the general framework formula, Six Ge1−x O2 , will be used in the following discussion. The details of the synthesis conditions and their results are summarized in Table 1. SEM images for all samples synthesized show that the morphology is octahedral (Fig. 1), which is related to the highest possible cubic (Aristotype-)symmetry of the material. This is the typical morphology of AST. Though the particle size varies from sample to sample and depends upon the SDA, most of the samples are well-crystalline. The analysis of the powder XRD diagrams shows that the AST framework type crystallizes as a function of Si/Ge ratio and in different symmetries depending on the SDA and the degree of Ge-substitution. Whereas for SDA1 and SDA3 the ASTframework, [Six Ge1−x O2 ], has been realized from x = 0 to x = 1, for SDA2 none of the end members has been obtained. The pure GeO2 -end member containing SDA1 crystallizes in tetragonal symmetry, the silica framework of its corresponding pure SiO2 -end member, however, is seriously distorted and could not be indexed in its as-synthesized form. The tetragonal unit cell parameters presented in Table 1 were obtained after calcination at temperatures as high as 940 ◦ C. The AST-samples obtained with SDA2 are cubic for intermediate compositions and trigonal for the highly Sisubstituted framework. For SDA3, the pure GeO2 - and SiO2 end member as well crystallize in tetragonal symmetry. The GeO2 -end member is obtained in high yield.
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Table 1 Summary of AST phases synthesized Sample
Phase(s)*
SiO2 content
Lattice parameters a (Å)
c (Å)
V (Å3 )
AST-T AST-T AST-T AST-T AST-T AST-T AST-T
9.271(1) 9.373(1) 9.370(1) 9.369(1) 9.358(1) 9.359(1) 9.342(1)
14.349(1) 14.039(1) 13.999(1) 13.957(1) 13.892(1) 13.895(1) 13.753(1)
1233.3(2) 1233.4(2) 1229.1(2) 1225.0(2) 1216.8(2) 1217.3(2) 1200.5(2)
AST
9.262(2)
13.505(3)
1158.5(4)
AST-T AST-T
9.464(1) 9.476(1)
14.117(1) 14.035(1)
1264.5(2) 1260.3(2)
13.550(2) 13.517(1) 13.468(1) 9.514(1)
23.091(8)
2488.0(6) 2469.8(3) 2442.8(3) 1810.3(7)
9.371(1) 9.353(1)
14.154(1) 14.095(1)
1243.1(2) 1233.1(2)
9.326(1) 9.311(1) 9.297(1) 9.284(1) 9.238(1)
13.937(1) 13.863(1) 13.779(1) 13.716(1) 13.460(1)
1212.2(2) 1202.0(2) 1190.9(2) 1182.3(2) 1148.8(2)
Initial
Sample
SDA1-1 SDA1-2 SDA1-3 SDA1-4 SDA1-5 SDA1-6 SDA1-7 SDA1-8 SDA1-9 SDA1-10**
0.0 0.111 0.143 0.20 0.25 0.333 0.5 0.667 0.8 1.0
0.0 0.177 0.219 0.279 0.368 0.394 0.571
SDA2-1 SDA2-2 SDA2-3 SDA2-4 SDA2-5 SDA2-6 SDA2-7
0.11 0.2 0.4 0.5 0.6 0.75 0.8
0.300 0.352 0.468 0.493 0.6 0.647 0.729
AST-C AST-C AST-C AST-TH
SDA3-1 SDA3-2 SDA3-3 SDA3-4 SDA3-5 SDA3-6 SDA3-7 SDA3-8
0.0 0.111 0.200 0.333 0.5 0.67 0.8 1.00
0.0 0.13 0.217 0.337 0.486 0.637 0.788 1.0
AST-T1 AST-T1s AST-T1s + 2s AST-T2s AST-T2s AST-T2s AST-T2s + 2 AST-T2
* AST-T for tetragonal symmetry (details for [SDA3]: T1: tetragonal phase 1, all GeO ; T1s: tetragonal phase 1, solid solution; T2: tetragonal phase 2, all 2 SiO2 ; T2s: tetragonal phase 2, solid solution), AST-C for cubic symmetry and AST-H for hexagonal symmetry. ** The cell parameters refer to the product after calcination at 940 ◦ C.
Fig. 1. Scanning electron micrograph of typical AST materials with octahedral morphology.
The increase of the volume of the unit cell can be assigned to the substitution of Si by Ge on T-sites of the silicate framework. The largest unit cell volumes are observed for the all Ge-AST end members. The correlation of the unit cell parameters with the Si-content of the three series are shown in Figs. 2–5, respectively. As can be seen in the diagrams the substitution of Si by Ge in AST framework type materials, [Six Ge1−x O2 ], is
not a uniform process expressed in a continuous variation of the unit cell volume. For the samples synthesized using SDA1, the pure GeO2 -form shows a discontinuity in lattice parameters and unit cell volume compared with the more Sirich samples. Its a unit cell parameter shrinks dramatically while the c parameter expands more than expected, resulting in a unit cell volume almost equal to the value for the 15 mol% Si (equivalent to n = 6 per unit cell) (Figs. 2 and 3).
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Fig. 2. Correlation of the unit cell volume with Si content in materials synthesized with SDA1.
Fig. 3. Correlation of the unit cell parameters with Si content in tetragonal AST-type materials synthesized with SDA1.
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Fig. 5. Correlation of the unit cell volume with Si content in AST-type materials synthesized with SDA2 (2: Double volume of tetragonal unit cell; ": Volume of cubic unit cell; Q: Volume of hexagonal unit cell multified by 4/3).
However, with increasing Si-content from x ∼ 0.15–0.6, the change of cell parameters and unit cell volume shows a linear correlation with the Si-concentration suggesting a statistical substitution of Ge by Si. For SDA3, a linear correlation between unit cell parameters, unit cell volume and Si-content from x = 0.0 to ∼ 0.8 is observed (Fig. 4). However, careful inspection of the powder XRD diagrams of samples made in the high Ge-region (0.15 < x(Si) < 0.30) and in the high Si-region (0.8 < x(Si) < 1) showed that mixtures of phases had been formed. On the Ge-rich side of the phase diagram a miscibility gap for the Si/Ge substitution in AST framework type materials is observed in the compositional range of ∼ (0.15−0.3). The miscibility gap in the low-Si range divides the phase diagram in two parts, one which is narrow and close to the all Ge-end member, the other covers the intermediate compositions from x = 0.3 to 0.8 for Si. Beyond x = 0.8 only allsilica AST type material has been obtained leading to the miscibility gap on the high Si-side of the phase diagram. For the AST-samples obtained with SDA2, the variation of unit cell parameters with the composition is regular but not linear (Fig. 5). This finding in combination with the fact that no pure end-member has been synthesized for SDA2 indicate a complex substitution pattern, which will be discussed later in combination with the corresponding MAS NMR spectra. 3.2. Rietveld refinement of the powder XRD data of the as-synthesized sample with all GeO2 AST type framework
Fig. 4. Correlation of the unit cell volume with Si content for materials synthesized with SDA3. I: Solid solution 1; II: I + III; III: Solid solution 2; IV: III + SiO2 -AST.
GeO2 with AST-type framework structure crystallizes with an I-centered tetragonal unit cell in space group I 4/m. The initial structure model with two germanium atoms, two oxygen atoms and the fluorine atom was obtained from powder XRD data using the direct methods program EXPO [39,40]. The atomic positions were in agreement
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and 141◦ ) [25], respectively. This indicates reduced strain in the framework structure of the germanate analogue, but still more tension than in most of the other germanates. The analysis is supported by the observation reported in the literature that the substitution of Si by Ge in zeolites is most successful in framework structures containing D4Runits such as BEC and ISV [8,10]. The increased flexibility of the Ge-substituted D4R-units leads to thermodynamically more stable products [7,8,19]. The fluorine anions have been located in the center of the D4R with a large, isotropic displacement factor. This is due to the fact that the volume of GeO2 -D4R is larger by ∼ 15% (d(Ge–Ge) = 3.183 Å), than that of the corresponding SiO2 -D4R (d(Si–Si) = 3.036 Å). 1 H and 13 C solid state MAS NMR experiments confirm the identity and the dynamic disorder of the SDA dimethyldiethylammonium cation inside the octadecahedral cage. The averaged positions of the centers of electron density have been located by difference-Fourier techniques in the course of the structure refinement. The N-atom of the ammonium cation is located in the center of the cage, the C-atoms are statically and dynamically disordered as suggested by 13 C NMR experiments and the smeared out electron density map calculated from the diffraction data. Since no information concerning the substitution of Si by Ge can be obtained by an in depth crystallographic analysis of the SDA and its disorder and since the most important information, the chemical identity of the SDA, has been obtained by NMR, the treatment of the dynamic disorder and partial occupancy of the SDA has been simplified. In the course of the Rietveld analysis the scattering contribution of the SDA was accounted for by refining the displacement parameters and the site occupancy factors of C-atoms on electron density maxima. The final result of the refinement is summarized in Table 3 and reflects ∼ 100% occupancy of the octadecahedral cage by the SDA.
Table 2 Crystallographic data, experimental conditions for X-ray data collection and results of the Rietveld analysis of GeO2 -AST-type material synthesized with SDA1, F Formula Crystal system Space group a (Å) b (Å) c (Å) Z Wavelength Range 2θ (deg) Step width 2θ (deg) Number of points Number of restraints Number of structural variables Peak profile Background correction RF (%) RBragg (%) Rp (%) Rwp (%) χ2 Distance restraints d(Ge–Ge) d(Ge–O) d(O–O) Weight for restraints
[Ge10 O20 F](C6 H16 N) Tetragonal I 4/m (No. 87) 9.271(1) 9.271(1) 14.349(1) 2 1.54059 Å (Cu-Kα1 ) 5–95 0.0078 11580 15 24 Thomson–Cox–Hastings Linear interpolation of background points 3.44 3.9 3.13 4.35 3.56 3.15(5) 1.735(20) 2.85(5) 1/esd
with the AST framework listed in the Atlas of Zeolite Framework types. The missing atoms were located using Fourier techniques. The subsequent Rietveld refinement was performed with soft constraints on Ge–Ge, Ge–O and O–O distances using 1/esd as weighting scheme. The details of the structure refinement, the atomic parameters and the final results of the Rietveld analysis are presented in Tables 2 and 3. Selected bond lengths and angles are summarized in Table 4. Fig. 6 shows the experimental, simulated and difference powder pattern after convergence. It can be seen that the GeO2 AST type framework is isostructural with the SiO2 analogue. The Ge–O-bond lengths range from d = 1.694 to 1.762 Å, which is typical for germanates. Bond angles (Ge–O–Ge) are ∼ 137.4◦ and 133.84◦ and 134.22◦ for the D4R unit. They are a slightly larger than usual for germanates, however, less tight than the corresponding values in the all silica system (148◦
3.3. Thermal analysis The TG-DTA analysis shows that the GeO2 -framework collapses to an amorphous phase at T ∼ 500–600 ◦ C as the SDA decomposes and is driven out. In a second step the amorphous phase is transformed to quartz-type GeO2 at 700 ◦ C. The weight loss of 8% in the temperature range from 500 to 600 ◦ C indicates the decomposition of the organic
Table 3 Atomic positions, temperature factors and occupancies for GeO2 -AST[SDA1,F] Atom Ge1 Ge2 O1 O2 O3 F1 N1 C1 C2
x
y
z
0.44471(8) 0.5 0.40278(47) 0.30961(40) 0.30535(51) 0.5 0.0 0.339(2) 0.571(15)
0.26411(8) 0.0 0.11730(39) 0.38517(41) −0.08158(68) 0.5 0.0 0.528(5) 0.535(8)
0.38951(4) 0.25 0.31919(25) 0.35946(26) 0.0 0.5 0.5 0.0 1.137(1)
Biso 0.55(4) 0.55(4) 2.3(2) 2.3(2) 2.3(2) 3.5(8) 16(1) 20(1) 20(1)
Multiplicity
Occupancy
1.0 0.25 1.0 1.0 0.5 0.125 0.125 0.5 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.895(2) 0.46(2)
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Fig. 6. Observed, calculated and difference plots for Rietveld refinement of GeO2 -AST[SDA1]. Table 4 Selected bond lengths and angles of GeO2 -AST[SDA1,F] d(Atom1–Atom2)
Distance (Å)
d(Ge1–Ge1) d(Ge1–Ge1) d(Ge1–Ge2) d(Ge1–O1) d(Ge1–O2) d(Ge1–O2) d(Ge1–O3) d(Ge2–O1) d(F–Ge1) d(F–O2) d(F–O3)
3.171(1) 3.171(1) 3.204(1) 1.738(4) 1.736(4) 1.689(4) 1.729(3) 1.727(4) 2.750(1) 2.884(4) 2.930(5)
Atom1–Atom2–Atom3
Angle (deg)
(Ge1)–(O1)–(Ge2) (Ge1)–(O2)–(Ge1) (Ge1)–(O3)–(Ge1)
135.3(2) 136.1(2) 133.1(1)
SDA and corresponds to an almost fully SDA-occupied cavity (theoretical: 8.6%). 3.4. Solid state MAS NMR 19 F and 29 Si experiments The chemical shift of 19 F MAS NMR signals gives valuable information on the chemical and geometrical environment of the co-templating anion within the D4R-unit. In the AST type framework, the F− inside the D4R is tightly constrained yielding low field shifted 19 F signals in the NMR
experiment. In present study, the 19 F resonance signal for the all silica end member is at −38.2 ppm (Fig. 7a) which is consistent with the data from [25–31]. For the all germania end members, two signals of 19 F are observed at ∼ −16 and −14 ppm at room temperature for samples synthesized with SDA1 and SDA3, respectively (Figs. 7b and 8). This is similar to values observed by Villaescusa et al. [33] in their study of the precursor type D4R anionic building unit, [Ge8 O12 (OH)8 F]− . In the 19 F spectrum of the AST type material synthesized with SDA1, the signal at around −16 ppm is split into two peaks at −15.5 and −16.5 ppm, respectively. The shape of 19 F signal at −14 ppm from GeO2 SDA3 is also slightly asymmetric. The fluoride anion might be displaced from the geometrical center of the cube, since the volume of GeO2 -D4R is slightly larger than that of the SiO2 -D4R thus leading to two, symmetrically inequivalent fluoride sites. According to a simulation calculation by Villaescusa and co-workers, there are two nonequivalent sites for the fluoride anion in GeO2 -D4R with almost equal energy [33], one with the fluoride anion displaced towards one face of the cube and the other with a displacement towards the corner. Though they pointed out the possible splitting of the 19 F NMR signal, it was not observed experimentally. In order to confirm the models proposed above, in situ variable temperature X-ray diffraction and NMR studies were carried out for the sample made with SDA1. A pair of peaks with indices (103) and (211) in tetragonal symmetry was inspected with powder XRD. Upon phase transition to the cubic sys-
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Table 5 Comparison of the results of calculations for different models of Si/Ge-order on T-sites (CSi ) in samples [SDA2] and [SDA3] with those from chemical analysis (CA). Numbers in bold show best coincidence of the model with the result of the chemical analysis 19 F NMR signals
SDA2-2 SDA2-3 SDA2-4 SDA2-5 SDA2-6 SDA2-7 SDA3-4 SDA3-5 SDA3-6 SDA3-7
−37.8 −38.0 −38.4 −38.3 −38.2
−18.6 −18.4 −18.5 −19.1 −19.4 −19.2 −19.2
−9.4 −8.1 −7.7 −7.3 −7.6 −8.0 −10.0 −9.1 −8.7 −8.8
nC −12.7 −12.3 −12.4 −12.2 −12.3 −11.8 −11.5
Fig. 7. Comparison of 19 F NMR spectra of SiO2 - and GeO2 -AST (spinning sidebands marked ∗). a: SiO2-AST: −38.2 ppm; b: GeO2-AST[SDA3]: −13.9 ppm.
tem they should combine into one peak with index (113). As can be seen from Fig. 9, the (211) peak shifts to lower angles, the (103) peak shifts slightly to higher angles, however, up to 440 ◦ C the phase change is not completed and the coalescence of the diffraction peaks is not observed. At higher temperatures the decomposition of the SDA sets in
0.848 0.864 0.929 0.965 1.28 1.753 0.693 0.984 1.245 2.216
CSi
CA
Static
T2 = Si
T2 = Ge
0.459 0.464 0.482 0.491 0.561 0.637 0.409 0.496 0.555 0.689
0.567 0.571 0.586 0.593 0.649 0.710 0.528 0.597 0.644 0.751
0.367 0.371 0.386 0.393 0.449 0.510 0.327 0.397 0.444 0.551
0.352 0.468 0.493 0.600 0.647 0.729 0.337 0.486 0.637 0.788
which limits the temperature range of the experiment. The fact that the shift to lower diffraction angles is dominating is explained with the thermal expansion of the material indicating reduction of tension. The result of the variable temperature 19 F MAS NMR experiments in the temperature range from RT to 150 ◦ C clearly shows the coalescence of the split signal into one at T = 100 ◦ C together with a shift of the signal to lower field (Fig. 8). This clearly demonstrates that at temperatures below the displacive phase transition of the GeO2 -framework from tetragonal to cubic symmetry, the peak splitting in the 19 F NMR spectrum disappears indicating the thermally activated, dynamical averaging of two Fsites inside the GeO2 -D4R at RT into one at T > 100 ◦ C, at least on the NMR time scale. For the AST type material of intermediate degree of Si/Ge-substitution, two more 19 F NMR signals, at least, are observed for X-ray pure phases. The signals at ∼ −8 and −19 ppm (Figs. 10 and 11) show up at distinct resonances. In addition, another signal appears as a sharp peak in the range between −14 and −11 ppm in GeO2 rich regime. Depending on the composition of the sample it gradually shifts to lower field with increasing Si-content. The fact that the 19 F chemical shifts in D4R of AST with intermediate composition are at lower field than in the pure all-GeO2 AST-type material and also shifts to lower field with increasing Si-content is unexpected and makes the assignment of the signals ambiguous, at least without further confirmation. Fortunately, the intensities of the signals at ∼ −8 and −19 ppm, vary systematically with sample composition, e.g., the signal at −19 ppm appears as the first signal for low Ge-concentration, increases to a maximum and disappears for low Si-concentrations as shown in Fig. 11. The main signal for the solid solution material is the signal at ∼ −8 ppm. It is strongest at ratios Si/Ge close to unity and is almost the only 19 F− signal for the sample made with SDA2 with Si/Ge = 1. In order to interpret the 19 F NMR spectra quantitatively, the integrated intensities have been measured and used for the analysis of the Si–Ge distribution in the framework structure. Then, we can calculate the Si/Ge ratio of the D4R and/or the total Si/Ge ratio in AST from the MAS
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Fig. 9. Variable—temperature XRD measurement of GeO2 -AST[SDA1].
equations: nCA = (Si/Ge)D4R =
nIF(nSi)
(8 − n)IF(nSi) ,
where nCA : ratio of Si/Ge in D4R, n: number of Si atoms in the D4R and IF(nSi) : relative intensity of the 19 F resonance for a F(nSi, (8−n)Ge) cage. For a statistical distribution of Ge on T1 and T2 sites, the (Si/Ge) ratio in the AST-type framework structure equals to that in the D4R, e.g., nTA = (Si/Ge)total = (Si/Ge)D4R . For Ge atoms located only on T1 sites, leading to T2 sites occupied by Si atoms only, the relation between (Si/Ge)total and (Si/Ge)D4R can be calculated as nTA = (Si/Ge)total = (5/4)(Si/Ge)D4R + 1/4. In the opposite case, for all T2 sites which are occupied by Ge atoms only, the following equation is obtained: nTA = (Si/Ge)total = 4(Si/Ge)D4R / (Si/Ge)D4R + 5 .
Fig. 8. Variable—temperature 19 F MAS NMR for sample GeO2 AST[SDA1] (spinning sidebands marked ∗).
NMR data. These NMR-analytical results can be compared with the results from chemical analysis. Using the approach similar to that applied for the calculation of Si/Al ratio in zeolites, the mean Si/Ge ratio in the double four ring can be expressed by the following set of
Apparently, the Si content can be calculated as CSi = Si/(Si + Ge) = nTA /(nTA + 1) in AST-type framework. We now assume that the broad peak at around −8 ppm is due to fluoride inside an ordered (4Si, 4Ge)-D4R and the peak at −19 ppm originates from fluoride in the (6Si, 2Ge)D4R. As already has been mentioned, the 19 F NMR signal at ca. −12 ppm changes its position with the Ge-content. Since it only shows up for high Ge-contents we assigned it to (nSi, (8 − n)Ge)-D4R with n < 4. Since their intensities are low and the peak areas are small, it is assumed in our calculation to arise from F− in (2Si, 6Ge)-D4R. Using the models introduced above for the analysis of the Si/Ge distribution and ordering, the results obtained from the extraction of intensities of the 19 F NMR signals, the results from elemental chemical analyses, and the theoretical Si/Ge ratio and the Si–Ge distribution based on the models and
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Fig. 10. 19 F spectra of Ge-substituted tetragonal AST-type materials [SDA3] (spinning sidebands marked ∗). a: SiO2 /GeO2 ≈ 1:7; b: SiO2 /GeO2 ≈ 1:2; c: SiO2 /GeO2 ≈ 1:1; d: SiO2 /GeO2 ≈ 1.5.
derived as a function of composition have been compared and are summarized in Table 5. The results obtained from the simulation of the different models showing the best agreement with the chemical analysis are emphasized in bold. In general the agreement of the data for the Si/Ge ratios and the Si–Ge distribution calculated for specific models of order with results from chemical and NMR analyses is excellent and different from a statistical distribution. This suggests an ordered substitution of Si by Ge on T-sites, obviously, following a strict, Loewenstein’s rule related Ge–Ge-avoidance pattern for Si/Ge > 1. As can be seen from the distribution of D4R units of different composition the structure preferentially contains (4Si, 4Ge)D4R and minimizes high silica and high germania D4R units to reach a most stable configuration. This coincides with an optimized host–guest fit. In particular for materials obtained with SDA2 the systematic changes in the 19 F NMR spectra with the composition of the host framework
are apparent. The analysis shows that for sample SDA2-4, a balanced distribution of Si and Ge atoms on T1 and T2 sites can be reached (Si/Ge = 1). The spectrum exhibits only one dominant set of 19 F NMR signals indicating an almost homogenous compositions and the Si/Ge distribution expected for this stoichiometry. For sample SDA2-5 which is higher in Si-content, the (4Si, 4Ge)-D4R cages are dominant and, in addition, most of T2 is occupied by Si atoms. For selected samples of AST-type materials synthesized with SDA3, the analysis of the 19 F NMR signals, the chemical analysis and the model based simulation of Si content and Si/Ge distribution is also shown in Table 5. The sample with Si/Ge ratio of 1 also reveals a most balanced distribution of Ge atoms between T1 and T2 sites, which is similar to that of SDA2. However, as the steric conformation of SDA3 is different from SDA2, the symmetry of the host framework is distorted and no longer cubic but tetragonal. The coexistence of (8Si)-D4Rs, (6Si, 2Ge)-D4Rs, (4Si,4Ge)-D4Rs
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Table 6 NMR resonance frequencies for Si atoms with different connectivity as derived from the Si/Ge-ordering model
Fig. 11. 19 F NMR spectra of Ge-substituted cubic AST-type materials [SDA2] (spinning sidebands marked ∗). a: SiO2 /GeO2 = 1; b: SiO2 / GeO2 = 3/2.
and ((n)Si, (8 − n)Ge with n < 4)-D4Rs with alterating composition as evidenced in 19 F NMR spectra complies with the occurrence of a solid solution. Comparing the line width of the NMR signals of 19 F assigned to the different D4Renvironments it is striking that the narrowest line width is observed for the all Si- and all Ge-materials. The broadest line width occurs for the molar ratio Si/Ge = 1. This indicates that a variation of geometries and/or compositions exist in the structure and that strict elemental ordering is not achieved in the samples investigated. These Si/Ge distribution patterns should also be reflected in the corresponding 29 Si NMR spectra. In the AST-type framework structure, each T1 site is connected to three other T1 sites in the same D4R unit and to one T2 atom. The formation of an alternative distribution of (4Ge, 4Si)-D4R results in the environment around Si1 consisting of three Ge atoms in T1 sites and one atom in T2 site, marked as Si1(3Ge, 1T2). T2 might be occupied by Si or Ge. For the sample with the ratio of Si/Ge = 1 assuming a distribution of Si and Ge in a strict alternative way, there should be
T site
Connectivity
Si1
4Si 3Si, 1Ge 2Si, 2Ge 1Si, 3Ge 4Ge
Chemical shift (ppm) −108 −105.5 −103 −100.5 −98
Si2
4Si 3Si, 1Ge 2Si, 2Ge 1Si, 3Ge 4Ge
−114 −111.5 −109 −106.5 −104
only two signals corresponding to Si1(4Ge) and Si2(4Ge), respectively. If all atoms on T2 are taken by silicon atoms, there should be a strong Si1(3Ge, 1Si) resonance as well as two additional smaller signals corresponding to Si2(4Ge) and Si2(4Si), respectively. For the pure SiO2 AST-type framework from SDA3, there are two main signals at −108 and −114 ppm in the 29 Si MAS NMR spectra corresponding to Si on T1 and T2, respectively. The third weak signal at ∼ 100 ppm relates to residual Si1–OH defects as relict from the synthesis. From the analysis of the 29 Si MAS NMR spectra from all samples of this series, the lowest field signal at −98 ppm can be assigned unambiguously to Si(4Ge) species on T1. Using this signal as reference, the chemical shifts for the NMR signals of all other possible Si-environment on T1 and T2 have been deduced. The complete set of signals is listed in Table 6. The 29 Si MAS NMR spectrum of the sample with Si/Ge ratio of 1 (sample SDA2-4 (Fig. 12a) shows a complex signal indicating a mixture of environments for Si atoms). The simulation of the 29 Si NMR spectrum (Fig. 12b) resolves four peaks at −106.2 (57%), −108.6 (21%), −112.9 (15%) and −117.4 (7%) ppm, respectively, which are assigned to the resonances of 29 Si for Si1(4Ge), Si1(3Ge, 1Si), Si2(4Ge) and Si2(4Si), respectively. For sample SDA2-5, three peaks at −108(strong), −112(weak) and −117(weak) ppm, respectively, have been measured (Fig. 12c), which correspond to Si environments of Si1(3Ge, 1Si), Si2(4Ge) and Si2(4Si), respectively. The analysis suggests that an alternating ordering of Si and Ge atoms in the AST-type framework is strongly preferred. In Fig. 13a a projection of the framework structure along the cubic a with Si and Ge in strict alternation on T-sites is shown. It is worth pointing out that in the Si-rich region of the phase diagram, Ge preferentially substitutes Si in T1 (D4R-site) (Fig. 13b). This is clearly supported by analyses of the 19 F NMR spectra and has already been indicated in [10]. In the Ge-rich region for samples obtained with SDA2, the changes of composition are accompanied by changes of symmetry, which makes the correlation between the composition and the 29 Si NMR spectrum rather complex. No unambiguous quantitative assignment to models of
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Fig. 13. Proposed ordering of Ge and Si on T-site in the AST structure. Small gray: Si atom; big gray: F− ion; black: Ge atom. a: Cubic AST with Si/Ge = 1; b: Cubic AST with Si/Ge = 3/2.
ordering could be made. However it should be noted that all 29 Si MAS NMR spectra are in agreement with the interpretation of the corresponding 19 F spectra.
4. Conclusion AST type zeolites have been synthesized as solid solution series in the system GeO2 –SiO2 /SDA with three new template molecules. Each SDA has a characteristic synthesis field. The substitution of Si by Ge follows a typical pattern which has been analyzed quantitatively with 19 F SS MAS NMR. Ge is first inserted in the D4R-unit ordering in a Lowenstein avoidance-rule like pattern. From the 19 F NMR spectra it is concluded that 6:2, 4:4 and 2:6 Si/Ge ratios are preferentially tuned in together with the all-Si and all-Ge D4R. The study shows that Ge can be substituted into silicate frameworks in a wide compositional range, however, in an ordered fashion. The D4R-unit plays a prominent role for the insertion of Ge and relates to the ISV, BEC and other, Ge-substituted zeolite framework types. Most likely, D4R units in the tetrahedral framework are favored for the substitution with Ge. Ge substitution in silicates might be the crystal chemical key to the synthesis of D4R-containing tetrahedral frameworks and represents the experimental answer for the σ -operation concept [45,46], i.e., the introduction of a D4R instead of a 4-membered ring using a mirror plane as symmetry element, for the derivation of hypothetical frameworks.
References
Fig. 12. 29 Si MAS NMR spectra of cubic AST-type materials [SDA2]. a: Si/Ge = 1; b: Result of the simulation calculation for pattern a: peak positions/ppm (from left to right): −106: Si1(4Ge); −108: Si(3Ge,1Si); −112: Si2(4Ge); −117: Si2(4Si); c: Si/Ge = 1.5.
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The substitution of germanium for silicon in AST-type zeolite Yingxia Wang b , Jiaqing Song c , Hermann Gies a,∗ a Institute for Geology, Mineralogy and Geophysics, Ruhr-University Bochum, 44780 Bochum, Germany b College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China c Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
Received 29 May 2003; received in revised form 28 July 2003; accepted 1 September 2003
Abstract The substitution of silicon by germanium in the AST zeolite framework type, [Sin Ge40−n O80 ]∗4(SDA+ F− ) expressed as unit cell content in its cubic F-centered symmetry, has been studied. Three different kinds of templates, dimethyldiethylammonium, dimethyldiisopropylammonium and isopropyltrimethylammonium cations, were used in the hydrothermal synthesis process in fluoride medium. The products were identified with XRD, MAS NMR, SEM and thermal analysis. The analysis of the X-ray powder diagrams shows that AST crystallizes in different space group symmetries depending on the nature of the SDA and the degree of Ge-substitution. The resonance signals of 19 F in MAS NMR experiments for the pure Si- and Ge-end members are at −38.2 and ∼ −15 ppm, respectively, indicating that the F− -anion is located as co-template in the double-four-ring (D4R) of the tetrahedral framework. This is confirmed by Rietveld analysis of powder diffraction data of the pure Ge-end member. The peak splitting of the 19 F NMR signal in pure GeO2 AST-type material is related to the displacement of F− location inside the D4R. Two more distinct signals at ∼ −8 and ∼ −19 ppm, respectively, are observed for X-ray pure AST-samples of intermediate compositions and assigned to fluoride in D4R built of 4[GeO4 ]- and 4[SiO4 ]tetrahedra (4Ge, 4Si) and to (2Ge, 6Si)-D4R, respectively. An ordered distribution of Ge in the AST-framework is proposed for cubic AST with compositions around Si/Ge = 1.5–1 by correlating the intensities of 19 F NMR signals and the results from chemical analysis. This model is further confirmed by the quantitative analyses of the corresponding 29 Si MAS NMR spectra. 2003 Elsevier SAS. All rights reserved. Keywords: Substitution of Ge for Si; AST zeolite; Hydrothermal synthesis; 19 F MAS NMR; Dimethyldiethylammonium; Dimethyldiisopropylammonium; Isopropyltrimethylammonium
1. Introduction Recently, considerable interest has been shown in the substitution of silicon by germanium in zeolites [1–21]. New framework materials have been synthesized in the system SiO2 /GeO2 making use of the high flexibility of the Ge– O–Ge bond angle. On the other hand, the substitution of Si by Ge in Ti-silicate zeolites has been reported as a useful way to adjust the acidic strength to meet the demands of the epoxidation reaction of organic alkenes by H2 O2 [9]. Considering the crystal chemical relationship between silicates and germanates, the silicate analogues in the germanate systems are considered as prototypes for highpressure silicates. They have been studied for a long time as model systems for lower crust and upper mantel high * Corresponding author.
E-mail address: [email protected] (H. Gies). 1293-2558/$ – see front matter 2003 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2003.09.003
pressure minerals [22]. There is a wide similarity in chemical and physical properties such as polymorphism, structure types, chemical bonding, etc. Both GeO2 and SiO2 crystallize in the framework types of rutile, quartz and cristobalite, respectively. However, there are also significant differences between the GeO2 and SiO2 phases. The bond length Si–O for 4-coordinated Si is typically between 1.60 and 1.63 Å, whereas the Ge–O distance ranges from 1.70 to 1.80 Å. As a consequence, the Ge–O–Ge bond angle (∼ 120–130◦) is smaller than that of Si–O–Si (∼ 140–145◦). Therefore, the incorporation of germanium on T-sites in zeolites leads to structural diversity with new framework architectures [16,19]. In the past, several germanate analogues of silicate zeolites have been obtained for known framework types such as ABW, ANA, AST, GIS, SOD, RHO, etc. [13,21,23]. Solid solutions of GeO2 and SiO2 crystallizing in known silicate framework types also have been synthesized, such as the structures of MFI [24], ISV [8] and, fi-
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nally, BEC [7,10], a zeolite framework type related to zeolite beta. These examples are quite interesting for the fundamental understanding of the modification of zeolite frameworks by the incorporation of germanium on T-sites. Recently, the formation of new zeolite framework types built only by [GeO4 ]-tetrahedra has been studied intensively. As an example, the framework type ASV with a 4-connected germanate structure is given [16]. For a complete survey of new GeO2 structures see the atlas of zeolite framework types at: http://www.iza-structure.org. In the present paper, the substitution of silicon by germanium in AST-type zeolites has been investigated. The AST-framework is built of double-four-ring cages of [TO4]tetrahedra (D4R) linked by additional [TO4 ]-units and is one of the simplest structures among the zeolites framework types. It has been synthesized as pure AlPO4 -, pure silicaand pure germania-framework [16,25,26]. Therefore, it is a good model structure for systematic studies of the substitution of silicon by germanium on T-sites. An interesting feature of this structure is the location of the F− anion inside the D4R. For the analysis of the local structure around the F− anions, NMR experiments have proven to be very useful. The 19 F-signal is at −38 ppm in pure SiO2 -AST as was first reported by Caullet et al. [25]. The low field shift of the resonance frequency as compared with F− in zeolite channels (∼ −110 ppm) has been explained by the small volume of the D4R-cage. Since then, the behavior of F− inside SiO2 -D4R’s has been studied frequently [27–31]. Klock and co-workers showed that the chemical shift of the 19 F NMR signal is correlated with the free volume of the cage the anion is occupying [32]. The pure GeO2 analogue of AST was first synthesized by Li and Yaghi [16] in fluoride medium using DABCO (1,4-diazabicyclo-[2,2,2]-octane) as structure directing agent (SDA). However, the authors did not report details on the location of the F− anion inside the GeO2 framework. Instead, they suggested that the OH− anions might occupy these positions. Later, Villaescusa et al. [33] successfully synthesized the fluoride-containing allgermania D4R anionic building unit as zeolite precursors, [Ge8 O12 (OH)8 F]− . They showed that the germania-cube is a suitable host for the F− anion. The resonance signal of the F− anion inside the pure germania-cube is low-field shifted with respect to the corresponding F− in the SiO2 -cube and is at ∼ −14 ppm. Substituting Ge for Si in the synthesis of ITQ-7, Blasco et al. [8] found that the incorporation of Ge promoted the crystallization of ITQ-7. In this zeolite framework D4R units are present as well. Intermediate compositions with different Si/Ge ratios were synthesized for ITQ-7, however, no pure GeO2 end member was reported. For the increase of the crystallization rate the preferential substitution of Si by Ge in the D4R building unit was held responsible. In the 19 F NMR spectrum of ITQ-7 two more signals were found at −8 and −19 ppm, respectively, in addition to the signal corresponding to F− inside the pure SiO2 -cube. The 19 F-NMR signals at −8 and −19 ppm were assigned as for F− inside the (3Ge, 5Si)-D4R and as for (2Ge, 6Si)-D4R
or (7Ge, 1Si)-D4R, respectively [8]. A careful study of ITQ17 with different Si/Ge ratios [10] confirmed the preferential location of Ge in the D4R until 4 T-sites are occupied by Ge. 19 F MAS NMR revealed two additional signals at −20 and −7 ppm, respectively. The latter is suggested to correspond to the resonance of a fluoride ion in the D4R(4Ge, 4Si). However, the interpretation of the assignment is still under discussion. In this paper we report on the synthesis and structure analysis of both, Si- and Ge-end members with ASTtype framework as well as on materials with intermediate compositions which were made with different SDA. The compounds have been studied systematically by the X-ray diffraction (unit cell refinements and structure analysis), solid state NMR (19 F, 29 Si, 13 C), thermal analysis (TG-DTA) and electron microscopy (SEM). 2. Experimental 2.1. Synthesis 2.1.1. SDAs Three different structure directing agents, dimethyldiethylammonium (SDA1), dimethyldiisopropylammonium (SDA2) and isopropyltrimethylammonium (SDA3) cations, were used in the syntheses of AST in fluoride media. Dimethyldiethylammonium hydroxide solution is commercially available (Fluka, 20 wt% water solution). SDA2 and SDA3 were synthesized by methylation of commercially available amines. A typical procedure is described below for the synthesis of SDA3. Isopropyltrimethylammonium iodide was obtained by reaction of isopropylmethylamine with methyl iodide using ethanol as solvent. The reaction was carried out at 40– 50 ◦ C in a water bath for several hours under stirring. Potassium bicarbonate was added as a neutralizing agent. Then the final mixture was filtered and the solid was washed with ethanol. For the crystallization of the product, the solvent was evaporated from the combined solutions. The solid was recrystallized in acetone as solvent and dried at room temperature. 1 H and 13 C NMR, elemental analysis, and MS spectra were used to identify the product as isopropyltrimethylammonium iodide. The iodide was exchanged to the hydroxide by reacting it with Ag2 O in aqueous solution. 2.1.2. Zeolites Appropriate amounts of amorphous SiO2 (Aldrich, 98%), amorphous GeO2 (Aldrich, 99.99%), SDA solution and HF solution were mixed and stirred until a uniform gel was obtained. Then the water content was controlled by evaporation at 70 ◦ C in an oven. The typical composition of the starting gel was: (1 − x)SiO2 :xGeO2 :0.5SDA:0.5HF:yH2O with x = 0.0–1.0 and y = 2–16. The mixture was transferred into a Teflon lined stainless steel autoclave and kept at the corresponding reaction temperature (150, 175 ◦ C) with a
Y. Wang et al. / Solid State Sciences 5 (2003) 1421–1433
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15–30 rpm rotating speed for 15–30 days. After quenching in cold water, the solid products in the autoclave were separated by filtration or centrifugation, washed with distilled water and dried at 75 ◦ C overnight.
calculations for the structure refinement were carried out with the F ULLPROF suite including F ULLPROF 2 K [41] and G FOUR [42].
2.2. Characterization
3. Results and discussion
2.2.1. X-ray powder diffraction The powder diffraction data were collected on a Siemens D-5000 diffractometer in modified Debye–Scherrer geometry, using Cu-Kα1 radiation (λ = 1.54059 Å), a capillary sample holder and a PSD detector (5◦ 2θ window).
3.1. Effects of SDA and Si/Ge ratio on the formation of AST-type structure
2.2.2. NMR spectroscopy MAS NMR spectroscopy was performed on a Bruker ASX 400 spectrometer with conventional Bruker probes. The 19 F NMR was measured under different spinning speeds (5000–12000 Hz) to identify the isotropic signals. Variable—temperature experiments for 19 F were run in the temperature range between RT and 150 ◦ C with the standard Bruker variable temperature (VT) unit. Both 29 Si NMR and 1 H–29 Si CP NMR were performed in order to identify the connectivity of Si. 1 H and 13 C spectra were recorded to confirm that the SDA molecules were intact. TMS was used as the standard for 1 H, 13 C and 29 Si and NaF for 19 F. The intensities of 19 F NMR signals were calculated by simulating the spectra with the program DMFIT [34]. 2.2.3. Chemical analyses As-synthesized samples were dissolved using Na2 O2 and diluted for ICP analyses. The contents of Si and Ge were measured with a Philips PU 7000 ICP-AES-Spectrometer. 2.2.4. Morphology The morphology of the crystallites was studied with a scanning electron microscope (SEM) (LEO-1530 Gemini). Furthermore, we studied the homogeneity and purity of the sample by scanning the morphologies of a representative mass of particles carefully. 2.2.5. Thermal analysis Thermal properties were recorded on a Bähr STA 503 thermal analyzer from room temperature to 560 ◦ C with a heating rate of 10 ◦ C min−1 using a mixture of nitrogen and air as flow gases. 2.2.6. X-ray data processing The raw data from X-ray diffraction were processed with P OWDER C ELL [35] and converted to the formats suitable for the programs W INPLOTR [36] and P OWDER X [37]. T REOR 90 [38] was used for indexing the X-ray powder patterns. The subsequent refinements of the unit cell parameters were also carried out with T REOR 90. A partial structure model was obtained by the direct method program EXPO [39,40] which was used for the structure analysis. The structure was completed using Fourier techniques. All
The decisive influence of structure directing agents on the formation of zeolites and, in particular, electro-neutral silica zeolites has been recognized and discussed in the past in many articles [43,44]. However, the detailed understanding of the templating effect of the SDA during crystal nucleation is still poorly understood. Therefore, the knowledge about the function of SDAs on the formation of a zeolite framework type is helpful in elucidating the fundamental mechanisms. In the present work, three different structure directing agents, dimethyldiethylammonium (SDA1), dimethyldiisopropylammonium (SDA2) and isopropyltrimethylammonium (SDA3) cations were used in hydrothermal syntheses in fluoride medium. The SDA’s are rather similar in size and shape, and almost identical in their chemical properties. In a wide range of Si/Ge ratios they allowed for the formation of the AST framework type structures, [Sin Ge40−n O80 ]∗4(SDA+ F− ) for the cubic unit cell, [Sin Ge20−n O40 ]∗2(SDA+ F− ) for the tetragonal unit cell and [Sin Ge30−n O60 ]∗3(SDA+ F− ) for the hexagonal unit cell. For convenience, the general framework formula, Six Ge1−x O2 , will be used in the following discussion. The details of the synthesis conditions and their results are summarized in Table 1. SEM images for all samples synthesized show that the morphology is octahedral (Fig. 1), which is related to the highest possible cubic (Aristotype-)symmetry of the material. This is the typical morphology of AST. Though the particle size varies from sample to sample and depends upon the SDA, most of the samples are well-crystalline. The analysis of the powder XRD diagrams shows that the AST framework type crystallizes as a function of Si/Ge ratio and in different symmetries depending on the SDA and the degree of Ge-substitution. Whereas for SDA1 and SDA3 the ASTframework, [Six Ge1−x O2 ], has been realized from x = 0 to x = 1, for SDA2 none of the end members has been obtained. The pure GeO2 -end member containing SDA1 crystallizes in tetragonal symmetry, the silica framework of its corresponding pure SiO2 -end member, however, is seriously distorted and could not be indexed in its as-synthesized form. The tetragonal unit cell parameters presented in Table 1 were obtained after calcination at temperatures as high as 940 ◦ C. The AST-samples obtained with SDA2 are cubic for intermediate compositions and trigonal for the highly Sisubstituted framework. For SDA3, the pure GeO2 - and SiO2 end member as well crystallize in tetragonal symmetry. The GeO2 -end member is obtained in high yield.
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Table 1 Summary of AST phases synthesized Sample
Phase(s)*
SiO2 content
Lattice parameters a (Å)
c (Å)
V (Å3 )
AST-T AST-T AST-T AST-T AST-T AST-T AST-T
9.271(1) 9.373(1) 9.370(1) 9.369(1) 9.358(1) 9.359(1) 9.342(1)
14.349(1) 14.039(1) 13.999(1) 13.957(1) 13.892(1) 13.895(1) 13.753(1)
1233.3(2) 1233.4(2) 1229.1(2) 1225.0(2) 1216.8(2) 1217.3(2) 1200.5(2)
AST
9.262(2)
13.505(3)
1158.5(4)
AST-T AST-T
9.464(1) 9.476(1)
14.117(1) 14.035(1)
1264.5(2) 1260.3(2)
13.550(2) 13.517(1) 13.468(1) 9.514(1)
23.091(8)
2488.0(6) 2469.8(3) 2442.8(3) 1810.3(7)
9.371(1) 9.353(1)
14.154(1) 14.095(1)
1243.1(2) 1233.1(2)
9.326(1) 9.311(1) 9.297(1) 9.284(1) 9.238(1)
13.937(1) 13.863(1) 13.779(1) 13.716(1) 13.460(1)
1212.2(2) 1202.0(2) 1190.9(2) 1182.3(2) 1148.8(2)
Initial
Sample
SDA1-1 SDA1-2 SDA1-3 SDA1-4 SDA1-5 SDA1-6 SDA1-7 SDA1-8 SDA1-9 SDA1-10**
0.0 0.111 0.143 0.20 0.25 0.333 0.5 0.667 0.8 1.0
0.0 0.177 0.219 0.279 0.368 0.394 0.571
SDA2-1 SDA2-2 SDA2-3 SDA2-4 SDA2-5 SDA2-6 SDA2-7
0.11 0.2 0.4 0.5 0.6 0.75 0.8
0.300 0.352 0.468 0.493 0.6 0.647 0.729
AST-C AST-C AST-C AST-TH
SDA3-1 SDA3-2 SDA3-3 SDA3-4 SDA3-5 SDA3-6 SDA3-7 SDA3-8
0.0 0.111 0.200 0.333 0.5 0.67 0.8 1.00
0.0 0.13 0.217 0.337 0.486 0.637 0.788 1.0
AST-T1 AST-T1s AST-T1s + 2s AST-T2s AST-T2s AST-T2s AST-T2s + 2 AST-T2
* AST-T for tetragonal symmetry (details for [SDA3]: T1: tetragonal phase 1, all GeO ; T1s: tetragonal phase 1, solid solution; T2: tetragonal phase 2, all 2 SiO2 ; T2s: tetragonal phase 2, solid solution), AST-C for cubic symmetry and AST-H for hexagonal symmetry. ** The cell parameters refer to the product after calcination at 940 ◦ C.
Fig. 1. Scanning electron micrograph of typical AST materials with octahedral morphology.
The increase of the volume of the unit cell can be assigned to the substitution of Si by Ge on T-sites of the silicate framework. The largest unit cell volumes are observed for the all Ge-AST end members. The correlation of the unit cell parameters with the Si-content of the three series are shown in Figs. 2–5, respectively. As can be seen in the diagrams the substitution of Si by Ge in AST framework type materials, [Six Ge1−x O2 ], is
not a uniform process expressed in a continuous variation of the unit cell volume. For the samples synthesized using SDA1, the pure GeO2 -form shows a discontinuity in lattice parameters and unit cell volume compared with the more Sirich samples. Its a unit cell parameter shrinks dramatically while the c parameter expands more than expected, resulting in a unit cell volume almost equal to the value for the 15 mol% Si (equivalent to n = 6 per unit cell) (Figs. 2 and 3).
Y. Wang et al. / Solid State Sciences 5 (2003) 1421–1433
Fig. 2. Correlation of the unit cell volume with Si content in materials synthesized with SDA1.
Fig. 3. Correlation of the unit cell parameters with Si content in tetragonal AST-type materials synthesized with SDA1.
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Fig. 5. Correlation of the unit cell volume with Si content in AST-type materials synthesized with SDA2 (2: Double volume of tetragonal unit cell; ": Volume of cubic unit cell; Q: Volume of hexagonal unit cell multified by 4/3).
However, with increasing Si-content from x ∼ 0.15–0.6, the change of cell parameters and unit cell volume shows a linear correlation with the Si-concentration suggesting a statistical substitution of Ge by Si. For SDA3, a linear correlation between unit cell parameters, unit cell volume and Si-content from x = 0.0 to ∼ 0.8 is observed (Fig. 4). However, careful inspection of the powder XRD diagrams of samples made in the high Ge-region (0.15 < x(Si) < 0.30) and in the high Si-region (0.8 < x(Si) < 1) showed that mixtures of phases had been formed. On the Ge-rich side of the phase diagram a miscibility gap for the Si/Ge substitution in AST framework type materials is observed in the compositional range of ∼ (0.15−0.3). The miscibility gap in the low-Si range divides the phase diagram in two parts, one which is narrow and close to the all Ge-end member, the other covers the intermediate compositions from x = 0.3 to 0.8 for Si. Beyond x = 0.8 only allsilica AST type material has been obtained leading to the miscibility gap on the high Si-side of the phase diagram. For the AST-samples obtained with SDA2, the variation of unit cell parameters with the composition is regular but not linear (Fig. 5). This finding in combination with the fact that no pure end-member has been synthesized for SDA2 indicate a complex substitution pattern, which will be discussed later in combination with the corresponding MAS NMR spectra. 3.2. Rietveld refinement of the powder XRD data of the as-synthesized sample with all GeO2 AST type framework
Fig. 4. Correlation of the unit cell volume with Si content for materials synthesized with SDA3. I: Solid solution 1; II: I + III; III: Solid solution 2; IV: III + SiO2 -AST.
GeO2 with AST-type framework structure crystallizes with an I-centered tetragonal unit cell in space group I 4/m. The initial structure model with two germanium atoms, two oxygen atoms and the fluorine atom was obtained from powder XRD data using the direct methods program EXPO [39,40]. The atomic positions were in agreement
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and 141◦ ) [25], respectively. This indicates reduced strain in the framework structure of the germanate analogue, but still more tension than in most of the other germanates. The analysis is supported by the observation reported in the literature that the substitution of Si by Ge in zeolites is most successful in framework structures containing D4Runits such as BEC and ISV [8,10]. The increased flexibility of the Ge-substituted D4R-units leads to thermodynamically more stable products [7,8,19]. The fluorine anions have been located in the center of the D4R with a large, isotropic displacement factor. This is due to the fact that the volume of GeO2 -D4R is larger by ∼ 15% (d(Ge–Ge) = 3.183 Å), than that of the corresponding SiO2 -D4R (d(Si–Si) = 3.036 Å). 1 H and 13 C solid state MAS NMR experiments confirm the identity and the dynamic disorder of the SDA dimethyldiethylammonium cation inside the octadecahedral cage. The averaged positions of the centers of electron density have been located by difference-Fourier techniques in the course of the structure refinement. The N-atom of the ammonium cation is located in the center of the cage, the C-atoms are statically and dynamically disordered as suggested by 13 C NMR experiments and the smeared out electron density map calculated from the diffraction data. Since no information concerning the substitution of Si by Ge can be obtained by an in depth crystallographic analysis of the SDA and its disorder and since the most important information, the chemical identity of the SDA, has been obtained by NMR, the treatment of the dynamic disorder and partial occupancy of the SDA has been simplified. In the course of the Rietveld analysis the scattering contribution of the SDA was accounted for by refining the displacement parameters and the site occupancy factors of C-atoms on electron density maxima. The final result of the refinement is summarized in Table 3 and reflects ∼ 100% occupancy of the octadecahedral cage by the SDA.
Table 2 Crystallographic data, experimental conditions for X-ray data collection and results of the Rietveld analysis of GeO2 -AST-type material synthesized with SDA1, F Formula Crystal system Space group a (Å) b (Å) c (Å) Z Wavelength Range 2θ (deg) Step width 2θ (deg) Number of points Number of restraints Number of structural variables Peak profile Background correction RF (%) RBragg (%) Rp (%) Rwp (%) χ2 Distance restraints d(Ge–Ge) d(Ge–O) d(O–O) Weight for restraints
[Ge10 O20 F](C6 H16 N) Tetragonal I 4/m (No. 87) 9.271(1) 9.271(1) 14.349(1) 2 1.54059 Å (Cu-Kα1 ) 5–95 0.0078 11580 15 24 Thomson–Cox–Hastings Linear interpolation of background points 3.44 3.9 3.13 4.35 3.56 3.15(5) 1.735(20) 2.85(5) 1/esd
with the AST framework listed in the Atlas of Zeolite Framework types. The missing atoms were located using Fourier techniques. The subsequent Rietveld refinement was performed with soft constraints on Ge–Ge, Ge–O and O–O distances using 1/esd as weighting scheme. The details of the structure refinement, the atomic parameters and the final results of the Rietveld analysis are presented in Tables 2 and 3. Selected bond lengths and angles are summarized in Table 4. Fig. 6 shows the experimental, simulated and difference powder pattern after convergence. It can be seen that the GeO2 AST type framework is isostructural with the SiO2 analogue. The Ge–O-bond lengths range from d = 1.694 to 1.762 Å, which is typical for germanates. Bond angles (Ge–O–Ge) are ∼ 137.4◦ and 133.84◦ and 134.22◦ for the D4R unit. They are a slightly larger than usual for germanates, however, less tight than the corresponding values in the all silica system (148◦
3.3. Thermal analysis The TG-DTA analysis shows that the GeO2 -framework collapses to an amorphous phase at T ∼ 500–600 ◦ C as the SDA decomposes and is driven out. In a second step the amorphous phase is transformed to quartz-type GeO2 at 700 ◦ C. The weight loss of 8% in the temperature range from 500 to 600 ◦ C indicates the decomposition of the organic
Table 3 Atomic positions, temperature factors and occupancies for GeO2 -AST[SDA1,F] Atom Ge1 Ge2 O1 O2 O3 F1 N1 C1 C2
x
y
z
0.44471(8) 0.5 0.40278(47) 0.30961(40) 0.30535(51) 0.5 0.0 0.339(2) 0.571(15)
0.26411(8) 0.0 0.11730(39) 0.38517(41) −0.08158(68) 0.5 0.0 0.528(5) 0.535(8)
0.38951(4) 0.25 0.31919(25) 0.35946(26) 0.0 0.5 0.5 0.0 1.137(1)
Biso 0.55(4) 0.55(4) 2.3(2) 2.3(2) 2.3(2) 3.5(8) 16(1) 20(1) 20(1)
Multiplicity
Occupancy
1.0 0.25 1.0 1.0 0.5 0.125 0.125 0.5 1.0
1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.895(2) 0.46(2)
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Fig. 6. Observed, calculated and difference plots for Rietveld refinement of GeO2 -AST[SDA1]. Table 4 Selected bond lengths and angles of GeO2 -AST[SDA1,F] d(Atom1–Atom2)
Distance (Å)
d(Ge1–Ge1) d(Ge1–Ge1) d(Ge1–Ge2) d(Ge1–O1) d(Ge1–O2) d(Ge1–O2) d(Ge1–O3) d(Ge2–O1) d(F–Ge1) d(F–O2) d(F–O3)
3.171(1) 3.171(1) 3.204(1) 1.738(4) 1.736(4) 1.689(4) 1.729(3) 1.727(4) 2.750(1) 2.884(4) 2.930(5)
Atom1–Atom2–Atom3
Angle (deg)
(Ge1)–(O1)–(Ge2) (Ge1)–(O2)–(Ge1) (Ge1)–(O3)–(Ge1)
135.3(2) 136.1(2) 133.1(1)
SDA and corresponds to an almost fully SDA-occupied cavity (theoretical: 8.6%). 3.4. Solid state MAS NMR 19 F and 29 Si experiments The chemical shift of 19 F MAS NMR signals gives valuable information on the chemical and geometrical environment of the co-templating anion within the D4R-unit. In the AST type framework, the F− inside the D4R is tightly constrained yielding low field shifted 19 F signals in the NMR
experiment. In present study, the 19 F resonance signal for the all silica end member is at −38.2 ppm (Fig. 7a) which is consistent with the data from [25–31]. For the all germania end members, two signals of 19 F are observed at ∼ −16 and −14 ppm at room temperature for samples synthesized with SDA1 and SDA3, respectively (Figs. 7b and 8). This is similar to values observed by Villaescusa et al. [33] in their study of the precursor type D4R anionic building unit, [Ge8 O12 (OH)8 F]− . In the 19 F spectrum of the AST type material synthesized with SDA1, the signal at around −16 ppm is split into two peaks at −15.5 and −16.5 ppm, respectively. The shape of 19 F signal at −14 ppm from GeO2 SDA3 is also slightly asymmetric. The fluoride anion might be displaced from the geometrical center of the cube, since the volume of GeO2 -D4R is slightly larger than that of the SiO2 -D4R thus leading to two, symmetrically inequivalent fluoride sites. According to a simulation calculation by Villaescusa and co-workers, there are two nonequivalent sites for the fluoride anion in GeO2 -D4R with almost equal energy [33], one with the fluoride anion displaced towards one face of the cube and the other with a displacement towards the corner. Though they pointed out the possible splitting of the 19 F NMR signal, it was not observed experimentally. In order to confirm the models proposed above, in situ variable temperature X-ray diffraction and NMR studies were carried out for the sample made with SDA1. A pair of peaks with indices (103) and (211) in tetragonal symmetry was inspected with powder XRD. Upon phase transition to the cubic sys-
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Table 5 Comparison of the results of calculations for different models of Si/Ge-order on T-sites (CSi ) in samples [SDA2] and [SDA3] with those from chemical analysis (CA). Numbers in bold show best coincidence of the model with the result of the chemical analysis 19 F NMR signals
SDA2-2 SDA2-3 SDA2-4 SDA2-5 SDA2-6 SDA2-7 SDA3-4 SDA3-5 SDA3-6 SDA3-7
−37.8 −38.0 −38.4 −38.3 −38.2
−18.6 −18.4 −18.5 −19.1 −19.4 −19.2 −19.2
−9.4 −8.1 −7.7 −7.3 −7.6 −8.0 −10.0 −9.1 −8.7 −8.8
nC −12.7 −12.3 −12.4 −12.2 −12.3 −11.8 −11.5
Fig. 7. Comparison of 19 F NMR spectra of SiO2 - and GeO2 -AST (spinning sidebands marked ∗). a: SiO2-AST: −38.2 ppm; b: GeO2-AST[SDA3]: −13.9 ppm.
tem they should combine into one peak with index (113). As can be seen from Fig. 9, the (211) peak shifts to lower angles, the (103) peak shifts slightly to higher angles, however, up to 440 ◦ C the phase change is not completed and the coalescence of the diffraction peaks is not observed. At higher temperatures the decomposition of the SDA sets in
0.848 0.864 0.929 0.965 1.28 1.753 0.693 0.984 1.245 2.216
CSi
CA
Static
T2 = Si
T2 = Ge
0.459 0.464 0.482 0.491 0.561 0.637 0.409 0.496 0.555 0.689
0.567 0.571 0.586 0.593 0.649 0.710 0.528 0.597 0.644 0.751
0.367 0.371 0.386 0.393 0.449 0.510 0.327 0.397 0.444 0.551
0.352 0.468 0.493 0.600 0.647 0.729 0.337 0.486 0.637 0.788
which limits the temperature range of the experiment. The fact that the shift to lower diffraction angles is dominating is explained with the thermal expansion of the material indicating reduction of tension. The result of the variable temperature 19 F MAS NMR experiments in the temperature range from RT to 150 ◦ C clearly shows the coalescence of the split signal into one at T = 100 ◦ C together with a shift of the signal to lower field (Fig. 8). This clearly demonstrates that at temperatures below the displacive phase transition of the GeO2 -framework from tetragonal to cubic symmetry, the peak splitting in the 19 F NMR spectrum disappears indicating the thermally activated, dynamical averaging of two Fsites inside the GeO2 -D4R at RT into one at T > 100 ◦ C, at least on the NMR time scale. For the AST type material of intermediate degree of Si/Ge-substitution, two more 19 F NMR signals, at least, are observed for X-ray pure phases. The signals at ∼ −8 and −19 ppm (Figs. 10 and 11) show up at distinct resonances. In addition, another signal appears as a sharp peak in the range between −14 and −11 ppm in GeO2 rich regime. Depending on the composition of the sample it gradually shifts to lower field with increasing Si-content. The fact that the 19 F chemical shifts in D4R of AST with intermediate composition are at lower field than in the pure all-GeO2 AST-type material and also shifts to lower field with increasing Si-content is unexpected and makes the assignment of the signals ambiguous, at least without further confirmation. Fortunately, the intensities of the signals at ∼ −8 and −19 ppm, vary systematically with sample composition, e.g., the signal at −19 ppm appears as the first signal for low Ge-concentration, increases to a maximum and disappears for low Si-concentrations as shown in Fig. 11. The main signal for the solid solution material is the signal at ∼ −8 ppm. It is strongest at ratios Si/Ge close to unity and is almost the only 19 F− signal for the sample made with SDA2 with Si/Ge = 1. In order to interpret the 19 F NMR spectra quantitatively, the integrated intensities have been measured and used for the analysis of the Si–Ge distribution in the framework structure. Then, we can calculate the Si/Ge ratio of the D4R and/or the total Si/Ge ratio in AST from the MAS
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Fig. 9. Variable—temperature XRD measurement of GeO2 -AST[SDA1].
equations: nCA = (Si/Ge)D4R =
nIF(nSi)
(8 − n)IF(nSi) ,
where nCA : ratio of Si/Ge in D4R, n: number of Si atoms in the D4R and IF(nSi) : relative intensity of the 19 F resonance for a F(nSi, (8−n)Ge) cage. For a statistical distribution of Ge on T1 and T2 sites, the (Si/Ge) ratio in the AST-type framework structure equals to that in the D4R, e.g., nTA = (Si/Ge)total = (Si/Ge)D4R . For Ge atoms located only on T1 sites, leading to T2 sites occupied by Si atoms only, the relation between (Si/Ge)total and (Si/Ge)D4R can be calculated as nTA = (Si/Ge)total = (5/4)(Si/Ge)D4R + 1/4. In the opposite case, for all T2 sites which are occupied by Ge atoms only, the following equation is obtained: nTA = (Si/Ge)total = 4(Si/Ge)D4R / (Si/Ge)D4R + 5 .
Fig. 8. Variable—temperature 19 F MAS NMR for sample GeO2 AST[SDA1] (spinning sidebands marked ∗).
NMR data. These NMR-analytical results can be compared with the results from chemical analysis. Using the approach similar to that applied for the calculation of Si/Al ratio in zeolites, the mean Si/Ge ratio in the double four ring can be expressed by the following set of
Apparently, the Si content can be calculated as CSi = Si/(Si + Ge) = nTA /(nTA + 1) in AST-type framework. We now assume that the broad peak at around −8 ppm is due to fluoride inside an ordered (4Si, 4Ge)-D4R and the peak at −19 ppm originates from fluoride in the (6Si, 2Ge)D4R. As already has been mentioned, the 19 F NMR signal at ca. −12 ppm changes its position with the Ge-content. Since it only shows up for high Ge-contents we assigned it to (nSi, (8 − n)Ge)-D4R with n < 4. Since their intensities are low and the peak areas are small, it is assumed in our calculation to arise from F− in (2Si, 6Ge)-D4R. Using the models introduced above for the analysis of the Si/Ge distribution and ordering, the results obtained from the extraction of intensities of the 19 F NMR signals, the results from elemental chemical analyses, and the theoretical Si/Ge ratio and the Si–Ge distribution based on the models and
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Fig. 10. 19 F spectra of Ge-substituted tetragonal AST-type materials [SDA3] (spinning sidebands marked ∗). a: SiO2 /GeO2 ≈ 1:7; b: SiO2 /GeO2 ≈ 1:2; c: SiO2 /GeO2 ≈ 1:1; d: SiO2 /GeO2 ≈ 1.5.
derived as a function of composition have been compared and are summarized in Table 5. The results obtained from the simulation of the different models showing the best agreement with the chemical analysis are emphasized in bold. In general the agreement of the data for the Si/Ge ratios and the Si–Ge distribution calculated for specific models of order with results from chemical and NMR analyses is excellent and different from a statistical distribution. This suggests an ordered substitution of Si by Ge on T-sites, obviously, following a strict, Loewenstein’s rule related Ge–Ge-avoidance pattern for Si/Ge > 1. As can be seen from the distribution of D4R units of different composition the structure preferentially contains (4Si, 4Ge)D4R and minimizes high silica and high germania D4R units to reach a most stable configuration. This coincides with an optimized host–guest fit. In particular for materials obtained with SDA2 the systematic changes in the 19 F NMR spectra with the composition of the host framework
are apparent. The analysis shows that for sample SDA2-4, a balanced distribution of Si and Ge atoms on T1 and T2 sites can be reached (Si/Ge = 1). The spectrum exhibits only one dominant set of 19 F NMR signals indicating an almost homogenous compositions and the Si/Ge distribution expected for this stoichiometry. For sample SDA2-5 which is higher in Si-content, the (4Si, 4Ge)-D4R cages are dominant and, in addition, most of T2 is occupied by Si atoms. For selected samples of AST-type materials synthesized with SDA3, the analysis of the 19 F NMR signals, the chemical analysis and the model based simulation of Si content and Si/Ge distribution is also shown in Table 5. The sample with Si/Ge ratio of 1 also reveals a most balanced distribution of Ge atoms between T1 and T2 sites, which is similar to that of SDA2. However, as the steric conformation of SDA3 is different from SDA2, the symmetry of the host framework is distorted and no longer cubic but tetragonal. The coexistence of (8Si)-D4Rs, (6Si, 2Ge)-D4Rs, (4Si,4Ge)-D4Rs
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Table 6 NMR resonance frequencies for Si atoms with different connectivity as derived from the Si/Ge-ordering model
Fig. 11. 19 F NMR spectra of Ge-substituted cubic AST-type materials [SDA2] (spinning sidebands marked ∗). a: SiO2 /GeO2 = 1; b: SiO2 / GeO2 = 3/2.
and ((n)Si, (8 − n)Ge with n < 4)-D4Rs with alterating composition as evidenced in 19 F NMR spectra complies with the occurrence of a solid solution. Comparing the line width of the NMR signals of 19 F assigned to the different D4Renvironments it is striking that the narrowest line width is observed for the all Si- and all Ge-materials. The broadest line width occurs for the molar ratio Si/Ge = 1. This indicates that a variation of geometries and/or compositions exist in the structure and that strict elemental ordering is not achieved in the samples investigated. These Si/Ge distribution patterns should also be reflected in the corresponding 29 Si NMR spectra. In the AST-type framework structure, each T1 site is connected to three other T1 sites in the same D4R unit and to one T2 atom. The formation of an alternative distribution of (4Ge, 4Si)-D4R results in the environment around Si1 consisting of three Ge atoms in T1 sites and one atom in T2 site, marked as Si1(3Ge, 1T2). T2 might be occupied by Si or Ge. For the sample with the ratio of Si/Ge = 1 assuming a distribution of Si and Ge in a strict alternative way, there should be
T site
Connectivity
Si1
4Si 3Si, 1Ge 2Si, 2Ge 1Si, 3Ge 4Ge
Chemical shift (ppm) −108 −105.5 −103 −100.5 −98
Si2
4Si 3Si, 1Ge 2Si, 2Ge 1Si, 3Ge 4Ge
−114 −111.5 −109 −106.5 −104
only two signals corresponding to Si1(4Ge) and Si2(4Ge), respectively. If all atoms on T2 are taken by silicon atoms, there should be a strong Si1(3Ge, 1Si) resonance as well as two additional smaller signals corresponding to Si2(4Ge) and Si2(4Si), respectively. For the pure SiO2 AST-type framework from SDA3, there are two main signals at −108 and −114 ppm in the 29 Si MAS NMR spectra corresponding to Si on T1 and T2, respectively. The third weak signal at ∼ 100 ppm relates to residual Si1–OH defects as relict from the synthesis. From the analysis of the 29 Si MAS NMR spectra from all samples of this series, the lowest field signal at −98 ppm can be assigned unambiguously to Si(4Ge) species on T1. Using this signal as reference, the chemical shifts for the NMR signals of all other possible Si-environment on T1 and T2 have been deduced. The complete set of signals is listed in Table 6. The 29 Si MAS NMR spectrum of the sample with Si/Ge ratio of 1 (sample SDA2-4 (Fig. 12a) shows a complex signal indicating a mixture of environments for Si atoms). The simulation of the 29 Si NMR spectrum (Fig. 12b) resolves four peaks at −106.2 (57%), −108.6 (21%), −112.9 (15%) and −117.4 (7%) ppm, respectively, which are assigned to the resonances of 29 Si for Si1(4Ge), Si1(3Ge, 1Si), Si2(4Ge) and Si2(4Si), respectively. For sample SDA2-5, three peaks at −108(strong), −112(weak) and −117(weak) ppm, respectively, have been measured (Fig. 12c), which correspond to Si environments of Si1(3Ge, 1Si), Si2(4Ge) and Si2(4Si), respectively. The analysis suggests that an alternating ordering of Si and Ge atoms in the AST-type framework is strongly preferred. In Fig. 13a a projection of the framework structure along the cubic a with Si and Ge in strict alternation on T-sites is shown. It is worth pointing out that in the Si-rich region of the phase diagram, Ge preferentially substitutes Si in T1 (D4R-site) (Fig. 13b). This is clearly supported by analyses of the 19 F NMR spectra and has already been indicated in [10]. In the Ge-rich region for samples obtained with SDA2, the changes of composition are accompanied by changes of symmetry, which makes the correlation between the composition and the 29 Si NMR spectrum rather complex. No unambiguous quantitative assignment to models of
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Fig. 13. Proposed ordering of Ge and Si on T-site in the AST structure. Small gray: Si atom; big gray: F− ion; black: Ge atom. a: Cubic AST with Si/Ge = 1; b: Cubic AST with Si/Ge = 3/2.
ordering could be made. However it should be noted that all 29 Si MAS NMR spectra are in agreement with the interpretation of the corresponding 19 F spectra.
4. Conclusion AST type zeolites have been synthesized as solid solution series in the system GeO2 –SiO2 /SDA with three new template molecules. Each SDA has a characteristic synthesis field. The substitution of Si by Ge follows a typical pattern which has been analyzed quantitatively with 19 F SS MAS NMR. Ge is first inserted in the D4R-unit ordering in a Lowenstein avoidance-rule like pattern. From the 19 F NMR spectra it is concluded that 6:2, 4:4 and 2:6 Si/Ge ratios are preferentially tuned in together with the all-Si and all-Ge D4R. The study shows that Ge can be substituted into silicate frameworks in a wide compositional range, however, in an ordered fashion. The D4R-unit plays a prominent role for the insertion of Ge and relates to the ISV, BEC and other, Ge-substituted zeolite framework types. Most likely, D4R units in the tetrahedral framework are favored for the substitution with Ge. Ge substitution in silicates might be the crystal chemical key to the synthesis of D4R-containing tetrahedral frameworks and represents the experimental answer for the σ -operation concept [45,46], i.e., the introduction of a D4R instead of a 4-membered ring using a mirror plane as symmetry element, for the derivation of hypothetical frameworks.
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Fig. 12. 29 Si MAS NMR spectra of cubic AST-type materials [SDA2]. a: Si/Ge = 1; b: Result of the simulation calculation for pattern a: peak positions/ppm (from left to right): −106: Si1(4Ge); −108: Si(3Ge,1Si); −112: Si2(4Ge); −117: Si2(4Si); c: Si/Ge = 1.5.
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