Synthesis optimization and structure analysis of the zincosilicate molecular sieve VPI-9

E L S E V I E R Microporous Materials 6 (1996) 295-309

MICROPOROUS MATERIALS

Synthesis optimization and structure analysis of the zincosilicate molecular sieve VPI-91

Lynne B. McCusker a,,, R.W. Grosse-Kunstleve a' 2,, Christian Baerlocher a, Masahito Yoshikawa b, Mark E. Davis b

" Laboratorium ffir Kristallographie, ETH-Zentrum, CH-8092 Zfirich, Switzerland b Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA

Received 26 February 1996; accepted 28 February 1996

Abstract

The synthesis of the zincosilicate molecular sieve VPI-9 has been optimized to produce a pure and highly crystalline material reproducibly. Its complex framework topology (structure type code VNI) has been determined in the space group P42/ncm (a=9.8946, c=36.8715 ,/k) from high resolution synchrotron powder diffraction data collected on an NH~-exchanged sample. Attempts to solve the structure by conventional methods or by exploiting anomalous scattering effects were unsuccessful. However, the application of a new approach to structure solution from powder data (FOCUS), which incorporates some of the principles used intuitively in model building into an automated structure determination procedure, generated the correct framework. With 7 T-sites, this framework topology is the most complex yet solved from powder diffraction data without manual intervention. The topology can be described in terms of two types of layers joined via isolated tetrahedra. One layer is a simple 4.82 net, and the other contains chains of pairs of [533] polyhedra. There is a 2-dimensional 8-ring channel system between the [533] polyhedra layers. Rietveld refinement of the structure of VPI-9 in the as-synthesized form (Rb44K4[Si96Zn24024o]' 48H20) required a doubling of the c-axis (a = 9.8837, c = 73.6505 ,~; V = 7195 ,~3), and a reduction of the symmetry to P41212. In addition to the 15 T-atoms and 30 oxygens in the framework, 9 Rb, 2 K and 3 H20 positions could be located within the 8-ring channels. Refinement of the 170 structural parameters using synchrotron data converged with Rwp =0.147 (P~xp =0.099) and Rr =0.069. The Zn atoms in the framework are completely ordered in layers. Each Zn is surrounded by four Si, and each Si by three Si and one Zn. The peaks in the 295i MAS NMR spectrum can be interpreted very convincingly in terms of the average Si-O-T angles found in the refinement. While the structure of VPI-9 has several features in common with those of the related zincosilicate molecular sieves VPI-7 and RUB-17, such as the framework density, 4.82 layers, 3-rings, and a subunit containing Zn, there are also some interesting differences, which might be related to the lower Zn concentration in VPI-9.

Keywords: VNI; VPI-9 synthesis; Zincosilicate; 295i MAS NMR; Synchrotron powder diffraction; Structure determination; Rietveld refinement

* Corresponding author. 1 Dedicated to Dr. Hellmut G. Karge on the occasion of his 65th birthday. 2present address: Howard Hughes Medical Institute, Yale University, New Haven, CT 06520-8114, USA.

0927-6513/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S0927-6513 (96)00015-6

296 L.B. McCusker et al./Microporous Materials 6 (1996) 295--309

1. In~oducfion

Several years ago, Brunner and Meier [1] showed that there is a correlation between the minimum ring size in a framework topology and the framework density. They suggested that syntheses tailored to produce 3-rings might produce very open framework structures. With this in mind, a series of zincosilicate syntheses were prepared by Annen [2,3], because it was known that some of the denser zincosilicates contain 3-rings (see Ref. [3]). By changing the Group I cation in the synthesis mixture from Na ÷ to Li ÷ to Rb ÷ to K +, he was able to prepare four new microporous materials (VPI-7, VPI-8, VPI-9 and VPI-10, respectively). The use of Cs ÷ gave a zincosilicate analog of analcime.

The structure of VPI-7 has since been solved [4, 5], and the code VSV has been assigned by the Structure Commission of the International Zeolite Association (IZA) to that framework topology. A natural analog, Gaultite, was reported shortly thereafter [6]. While the VSV topology does, in fact, contain 3-rings, the pore openings are only 8- and 9-rings, and the framework density at 17.1 T-atoms/1000 A 3 is rather high. Roehrig and Gies prepared a closely related zincosilicate, RUB-17, by using both Na ÷ and K ÷ ions in the synthesis mixture, and they were able to solve its structure by model building [7]. The topology, which has been assigned the structure type code RSN, has a projection in common with VSV. It also contains 3-rings, has 8- and 9-ring pore openings, and has a similar framework density (16.8 T-atoms/1000 A3).

VPI-8 proved to be more a high-silica than a zincosilicate material (i.e. little if any Zn was incorporated into the framework), and accordingly, its framework topology (VET) does not contain 3-rings [8]. The topology of VPI-10 has not yet been confirmed, but a promising structural model, which contains 3-rings but again with a relatively high framework density, has been pro- posed [9]. The determination of the structure of the other member of this series, VPI-9, is the subject of this investigation. Because the initial sample of VPI-9 appeared to contain an impurity, an optimization of the synthesis was necessary

before the powder diffraction data could be used for structure determination and refinement.

Initial experiments (XRD, 29Si MAS NMR) indicated that the VPI-9 structure was likely to be quite complex, so it was anticipated that structure determination using conventional methods would be difficult. We have a long-standing research program devoted to improving the methodology of structure determination from powder diffraction data, so the challenge offered by VPI-9 was most inviting. Since VPI-9 contains both Zn and Rb, two potential anomalous scatterers, the exploita- tion of anomalous scattering for structure solution could be explored. At the same time, an algorithm to incorporate the active use of chemical information into an automated structure determination procedure was under development, so the VPI-9 data also provided a true test example for that program.

2. Synthesis

Three methods of preparation were investigated during the optimization of the synthesis of VPI-9. One of these (A) has been reported previously by Annen and Davis [2,3], another (B) by Camblor and Davis [10], and the third (C) is new. In each case, the products were collected by vacuum filtra- tion, washed with distilled water and dried in air at room temperature.

VPI-9(A) was synthesized from a reaction mixture of molar composition 1SiO2:l.18RbOH: 0.08TEAOH:0.04ZnO:22H20. A starting mixture was prepared by combining 2.57 g SiO2 (Syloid63), 0.13g ZnO (Aldrich), 12.12g distilled water, 10.45 g RbOH (50 wt.-% solution, Aldrich) and 1.17 g TEAOH (40 wt.-% solution, Aldrich) and was stirred for more than 2 h. The resulting mixture was then charged into a 45 ml Teflon-lined stainless steel autoclave (Parr) and heated under autogenous pressure and static conditions for 3 days at 473 K. The product contained an uniden- tified impurity in the form of black particles, which did not change color after calcination in air at 873 K. The diffraction pattern of the material was similar to that reported by Annen, but had an extra peak around 35°20 (Fig. la).

L.B. MeCusker et al./Microporous Materials 6 (1996) 295-309 297

(b) . . . . . t ~

to ~ ~o 40 50 Two-Theta(degree)

Fig. 1. X-ray powder diffraction patterns of (a) VPI-9(A), (b) the brown phase in VPI-9(B), (c) the white phase in VPI-9(B), and (d) VPI-9(C).

Since a pure phase was needed for the structure analysis, the method of Camblor and Davis was attempted. VPI-9(B) was synthesized from a reaction mixture of molar composition 1SiQ:0.9- RbOH:0.08TEAOH:0.039Zn (CH3COO)2:22H20. A reaction mixture was formulated as follows: 12.91 g RbOH and 19.93 g distilled water were mixed, and then 0.60g Zn(CH3COO)2.2HzO (Aldrich) was added. After stirring this mixture for 10 min, 4.20g SiO2 (Syloid63) and 2.06g TEAOH were added and the stirring continued for 2 h. The resulting mixture was charged into a 45 ml Teflon-lined stainless steel autoclave and heated under autogenous pressure and static conditions for 4 days at 473 K. The product in this case consisted of two distinct phases, one brown (Figs. lb and 2a) and one white (Figs. lc and 2b). The brown phase contained cubic analcime, and the white one appeared to be pure VPI-9. However, it was difficult to separate the phases completely, so a different synthesis procedure was attempted.

On the assumption that the high concentration of Rb was causing the black impurity in synthesis method (A) and the analcime crystals in method (B), part of the RbOH in the synthesis mixture was replaced by KOH. VPI-9(C) was synthesized from a reaction mixture of molar composition 1SiO2:0.60RbOH:0.30KOH:0.08TEAOH:0.04ZnO:

m

Fig. 2. Scanning electron micrographs of (a) the brown phase in VPI-9(B), (b) the white phase in VPI-9(B), and (c) VPI-9(C),

22H20. A reaction mixture was prepared by combining 2.57 g SiO2 (Syloid63), 0.13 g ZnO, 14.71g distilled water, 5.28 g RbOH, 0.71 g KOH (EM)

298 L.B. McCusker et al./Microporous Materials 6 (1996) 295-309

and 1.17 g TEAOH and stirring for more than 2 h. The resulting mixture was charged into a 45 ml Teflon-lined stainless steel autoclave and heated under autogenous pressure and static conditions for 3 days at 473 K. The X-ray diffraction pattern (Fig. ld) and the 298i MAS NMR spectrum (Fig. 3) of the product were very similar to those reported by Annen, and the SEM image (Fig. 2c) indicated that it was a single phase. No colored impurity was observed, and the reproducibility of the method proved to be very good.

3. Sample characterization

3.1. X-ray diffraction

Room temperature X-ray powder diffraction patterns were recorded on a Scintag XDS 2000 laboratory diffractometer (Bragg-Brentano geometry, flat-plate sample, liquid nitrogen cooled ger- manium detector, CuKct radiation, 2 = 1.54184 A).

(b)~ (a)

! ! I !

-85 -90 -95 -100

ppm

Fig. 3. 29Si MAS NMR spectrum for VPI-9(C): (a) deconvo- luted peaks, (b) simulated spectrum, (c) experimental spectrum.

3.2. Scanning electron microscopy (SEM)

SEM images were recorded on a Camscan 2-LV scanning electron microscope operating with an accelerating voltage of 15 kV.

3.3. 29Si MAS NMR

Solid-state NMR spectroscopy was performed using a Bruker AM 300 spectrometer equipped with a high-power assembly for solids. Samples were packed into 7 mm ZrO 2 rotors and spun in air. 29Si (59.63 Mhz) spectra were obtained using the magic angle spinning (MAS) technique at a spinning rate of 3 kHz, pulse widths of 4/~s, and a pulse interval of 5 s. The chemical shifts were referred to tetramethylsilane.

3.4. Chemical analysis

Two chemical analyses, performed at Galbraith Lab, Knoxville, TN, USA, of the VPI-9(C) sample yielded surprisingly different results (wt.-%): (1) 17.9Si:2.47K:ll.51Zn:22.62 Rb, and (2) 21.19Si: 3.05K:12.05Zn:24.45Rb. These analyses yield the chemical formulas (based on a unit cell with 120 T-sites): (1) Rb43Kg[Si94Zn26024o]" 128H20 and (2) RbasKlo[Si96Zn24024o].48H20. Assuming a Si:Zn ratio of 4 from interpretation of the 29Si NMR spectrum shown in Fig. 3 using the methods of Camblor and Davis [10] and there- fore a total of 48 non-framework cations, and taking into account the water content indicated by thermogravimetric analysis, an idealized composition Rb38_43 Ks_lo[ Si96Zn24024o] • 48H20 was formulated.

4. Data collection for structure determination and refinement

The first attempts to solve the structure of VPI-9 were performed on data collected from a sample provided by Michael Annen. Since the material contained both Zn and Rb, it appeared to be an ideal candidate for anomalous scattering experiments, so high-resolution powder diffraction data were collected at three different wavelengths (near

L.B. McCusker et al./Microporous Materials 6 (1996) 295-309 299

the Zn absorption edge, near the Rb edge and in between the two) on Beamline X7A [11] at the NSLS synchrotron facility in Brookhaven, NY. As hoped, small but significant intensity differences could be seen in the three diffraction patterns (Fig. 4).

Although most of the peaks in these patterns could be indexed using a tetragonal unit cell with a~9.9 and c,~37A, a number of smaller peaks required that the c dimension be doubled. Even then, a few small peaks remained unindexed. These were assumed to belong to an impurity phase, but it could not be identified conclusively. In an attempt to reduce the strong contribution of the non-framework Rb ÷ ions to the diffraction pattern, a sample was exchanged with NH~- and data were collected on a laboratory Stoe diffractometer. Although the intensities changed signifi- cantly with this treatment, the indexing problem remained. Attempts to solve the structure (assuming space groups consistent with the extinction symbol P-c-) using any of these datasets by Patterson or direct methods, by using the intensity differences in the anomalous scattering experiments, or by a new method called FOCUS, which

was under development at the time (see below), were unsuccessful.

Meanwhile, the synthesis conditions had been optimized as described in the previous section, so a sample of VPI-9 (C) could be investigated. High- resolution powder diffraction data were collected on both the as-synthesized material and an NHf-exchanged sample on the Swiss-Norwegian Beamline (SNBL) at the European Synchrotron Radiation Facility (ESRF) in Grenoble. The data for the NH~-exchanged sample could be indexed completely using a tetragonal unit cell with a = 9.8946 and c=36.8715A, and the systematic absences suggested the extinction symbol Pnc- rather than P-c-, and hence the space group P42/ncm, which had not been considered previously. The data for the as-synthesized material, however, could only be indexed completely if c was doubled (Fig. 5). Details for all data collections are given in Table 1.

5. Structure determination

A new method for solving zeolite-like structures from powder diffraction data [9] was applied suc-

c

Normalized 29

Fig. 4. Synchrotron powder diffraction patterns collected on the VPI-9 sample provided by Michael Annen (a) near the Zn absorption edge (2= 1.2831 ,~,), (b) away from both the Zn and the Rb absorption edges (2= 1.0198 .~), and (c) near the Rb absorption edge (2 =0.8164 ,/~). The 20 scale has been normalized to 2= 1.0 ]k to facilitate comparison.


l =odd I I I I I I I HI I I I I I I I I I I I I I I I I

1 I I I I II I I I II

/ - -even I I I II I III I I I I III I I I II I II I I I I I III IIII I I I I I I I I I IIII I I I I I

1 ' 0 ' 1 ' 1 ' I '2 ' 13 ' I '4 ' I '5 ' 16 ' I '7 ' 18 I '9 20"20

Fig. 5. A small section of the diffraction pattern of as-synthesized VPI-9 showing some of the peaks reqmring the doubling in the c direction (*). Reflections with even l indices (lower tick marks) could be indexed on the smaller cell, but those with odd ones (upper tick marks) require the larger unit cell.

cessfully to the data from the NH~--exchanged VPI-9(C) sample using the space group P42 /ncm

and the smaller unit cell. The program (FOCUS) will be described in detail elsewhere [12], but briefly, a large number of electron density maps are generated using random starting phases and intensities extracted from the powder diffraction pattern. These are then subjected to a Fourier recycling procedure combined with a specialized topology search routine. The latter involves a search (ca. 50 peaks in the asymmetric unit are considered) for a 3-dimensional, 4-connected framework structure consistent with typical interatomic distances and angles. In this way, the information in the powder diffraction pattern is supplemented by chemical and structural information specific to zeolitic structures, and both are used actively in an automated structure determination procedure.

The framework topology (Fig. 6) that emerged from FOCUS as the most probable one has 7 T-sites in the asymmetric unit. The framework can be described in terms of two types of layers joined via isolated tetrahedra. One layer (A) is a simple 4.82 net with an U U D D U U D D arrangement of tetrahedra in the 8-rings, but the other (B) is

somewhat more complicated. In the latter, pairs of [533] polyhedra sharing common 3-rings are joined to adjacent pairs via edges to form sinusoi- dal chains. These, in turn, are connected via oxygen bridges to neighboring chains displaced by half a period. The 8-rings formed between the chains are oval in shape with free dimensions of ca. 2.3 x4.3 ,~, so the channel system is effectively blocked in the c-direction. There is, however, a two-dimensional, 8-ring channel system between these layers.

6. Rietveld refinement

A preliminary Rietveld refinement of the data from the NH~--exchanged sample indicated that the topology was likely to be correct [9]. This refinement was not pursued because the ion exchange appeared to be incomplete, and because the location of NH~- in the presence of H20 is difficult. It was felt that a refinement of the as-synthesized material with its heavier non-framework cations would provide more definitive structural information. Of particular interest were (1) whether or not the Zn atoms were ordered in the


Table 1 Data collection details

VPI-9 from Michael Annen

As-synthesized sample Synchrotron facility Beamline X7A at NSLS

Wavelength Zn edge 1.28306 A Rb edge 0.81638 .~ Off edge 1.01985 Analyzer crystal Ge 220 Sample oscillating flat plate Step size 0.01°20 Time per step 10-15 s

NH~-exchanged sample Diffractometer Stoe STADI Wavelength CuKcq Sample 0.3 mm capillary 20 range 3-90 ° Effective step size 0.02°20 Total data collection time 96 h

VPI-9 (synthesis method C) As-synthesized sample Synchrotron facility SNBL at ESRF Wavelength 0.99995 .~ Diffractometer geometry Debye-Scherrer Sample rotating 0.5 mm capillary 20 range 2.5-60 ° Step size 0.01°20 Time per step 15-17 s

NH~-exchanged sample Synchrotron facility SNBL at ESRF Wavelength 0.94734 Diffractometer geometry Debye-Scherrer Sample rotating 0.5 mm capillary 20 range 2-55 ° Step size 0.01°20 Time per step 15 s

20 range 3.5-70 ° 2-50 ° 2.5-60 °

framework, (2) where the non-framework Rb + and K + ions were located, and (3) the reason for the doubling of the unit cell in the as-synthesized form. The model of the framework structure from the preliminary refinement of the NH~--exchanged form in the smaller unit cell was used as a starting point.

Since the diffraction pattern of the as-synthesized material could only be indexed by doubling the c-axis (see Fig. 5), the 42 screw axis in the smaller unit cell had to be changed to a 41 axis, and this led to the selection of the space

group P41212 for the refinement. The 7 T-a tom and 12 oxygen positions from the refinement of the NH~-exchanged material were used to gener- ate the 15 T-a tom and 30 oxygen positions required by the new cell and space group. Rietveld refinement was initiated using DLS-optimized coordinates for these atoms [13] assuming a random distribution of Zn over the 15 T-a tom sites. Appropriate geometric restraints were placed on the (Si ,Zn)-O bond distances and O- (S i ,Zn) -O and (Si ,Zn)-O-(Si ,Zn) angles.

Initial profile and unit cell parameters were obtained by performing a model-free whole- pattern refinement using the program E X T R A C [14]. Then a series of difference Fourier maps, based on iteratively improved structural models, were generated in order to locate the non-framework species in the channels. A number of promising positions could be found, but even with these included in the model, the error indices Rv and Rwp could not be coaxed below 0.167 and 0.453, respectively.

Attempts to locate the Zn atoms in the framework by refining the T-site occupancy parameters were unsuccessful, so a partial ordering of the Zn atoms consistent with chemical analysis was introduced. In view of related structures, it was rea- soned that the Zn atoms would probably be associated with 3-rings, so all nine such T-sites were assigned population parameters and geometric restraints corresponding to 2/3 Si and 1/3 Zn occupancy and the other six were assumed to be pure Si. Further refinement with this model reduced the RF and Rwp to 0.110 and 0.274, respectively, and it became clear that three of the nine T-sites were pure Zn and the other six pure Si. This Zn ordering scheme was then introduced into the model and the geometric restraints adjusted accordingly (see Table 2).

All non-framework species were removed at this point, so a new series of difference Fourier maps could be generated starting with the new framework model. In all, nine Rb ÷, two K ÷ and three H 2 0 positions, representing 44 Rb ÷, 4 K ÷ and 12 H 2 0 per unit cell, were found and added, step by step, to the model. Since all three species can be expected to bond to framework oxygens at similar distances, it is difficult to distinguish them

302 LB. McCusker et aL /Microporous Materials 6 (1996) 295-309

A

B

A

B

Fig. 6. A projection of the VNI framework topology down [ 110] with the positions of layers A and B shown (left). Layer B at z ~ 1/2 is rotated 90 ° with respect to those at zm0 and zm 1. Similarly, layer A at z~3/4 is rotated 90 ° with respect to that at z g 1/4. Layer A (4.82 net) and layer B (containing chains of [533] polyhedra) are shown individually on the right.

Table 2 Crystallographic data for as-synthesized VPI-9

Unit cell

Space group a (A) c (A) Refinement Standard peak for peak shape function (hkl, °20) 104, 6.58 Step size for Durbin-Watson statistics (°20) 0.09 Peak range (number of F W H M ) 16 Number of observations 5584 Number of contributing reflections 1906 Number of geometric "observations" 180

Si-O prescribed: 1.61 ( 1 ) .~ 48 Z n - O prescribed: 1.95(1) .~ 12 O-S i -O prescribed: 109.5(10) ° 72 O - Z n - O prescribed: 109.5(10) ° 18 Si -O-Si prescribed: 145 (8)° 18 S i -O-Zn prescribed: 123(8) ° 12

Number of structural parameters 170 Number of profile parameters 8 /~xp = [ ( N - P1 - P2)/~wy2(obs)] 1/2 0.099 Rwp = {~w[y(obs) -y(calc)]2/~wy2(obs)} t/2 0.147 RF = ~[F(obs) - F(calc) l /~F(obs) 0.069

P41212 9.8837(1) 73.6505(6)

except by electron density and chemical analysis considerations. Occupancy parameters were fixed at values reflecting stoichiometrically reasonable numbers of atoms in the unit cell that were consistent with the approximate electron density and

that accounted for positions that cannot be occu- pied simultaneously (e.g. Rb3/Ow3, Rb3/Rb8, Rb6/K6, Rb7/K7 and Rb8/Ow8). K positions were assumed to be nearer to framework oxygens than Rb ones. The relative weight of the geometric restraints on the framework atoms with respect to the X-ray data was gradually reduced during the course of refinement, and in the final cycles could be set to 1.0. Final refinement of this model converged with the error indices RF =0.069 and Rwp = 0.147 (Roxp = 0.099).

Several peaks 1.0 e- /A in height were observed in the final difference Fourier map, but all were located too close to atoms in the structural model to be chemically sensible. The fact that the map is not flat is a reflection of the fact that not all of the H20 positions could be located. Unfortunately, the size of the structure, the presence of heavy scatterers like Rb, and the likelihood that the H20 molecules are disordered probably preclude further interpretation of the map.

The XRS-82 package of programs [15] was used throughout. A cylindrical sample absorption correction (#R~0.7) was applied to the data to compensate for the significant X-ray absorption of this material [16]. Neutral scattering factors were used for all atoms. Crystallographic data are sum-

LB. McCusker et al. / Microporous Materials 6 (1996) 295-309 303

marized in Table 2. The esd's for the atomic coordinates given in Table 3 were calculated using every ninth data point, in accordance with the recom- mendation of Hill and Madson [17] that the Durbin-Watson d-statistic be used to calculate the appropriate step size for error analysis in the final cycle of refinement. Selected interatomic distances and angles are listed in Table 4. The Zn ordering and the T-atom labelling scheme are shown in Fig. 7, and the location of the Rb ÷ ions within the channels in Fig. 8. The final observed and calculated powder patterns are given in Fig. 9.

7. Discussion

With 7 T-atoms in the asymmetric unit, the VPI-9 framework topology is the most complex zeolite-like structure yet solved from powder diffraction data using an automated procedure. The more complex topologies have all been determined either from single crystal data or by manual intervention and model building. By incorporating the active use of some model-building principles into the structure determination procedure, the FOCUS algorithms allow some of the limitations imposed by reflection overlap in a powder diffraction pattern to be overcome and the complexity of structures that can be handled to be increased.

The refinement of the structure of as-synthesized VPI-9 required that the already large topological unit cell be doubled. As a result of this symmetry

reduction, the 19 positions describing the framework topology had to be increased to 45. With the non-framework atoms added to the model, this translates into 170 structural parameters, and the structure thereby qualifies as one of the most complex yet refined using the Rietveld method. The high quality of the synchrotron powder diffraction data made the refinement of this huge structure possible.

The topology of VPI-9 described in the Structure Determination section has been assigned the structure type code VNI. The Zn atoms in the framework are all associated with 3-rings, and are arranged in layers (Fig. 7). Those at Zn7 and Znl2 are the isolated tetrahedra connecting layers A and B, and those at Znl5 are located within layer B. The latter occupy the tips of the 3-rings in the [533] polyhedra that point up or down the c-axis. No two Zn atoms share common oxygens, and each Si atom shares only one oxygen with a Zn atom. That is, each Zn is surrounded by four Si atoms and each Si by three Si and one Zn. This very well-balanced ordering scheme is also possible in the topological space group P42/ncm with the smaller unit cell, so the very long 73.7 .~ c-axis cannot be attributed to Zn ordering.

The reason for the doubling of the unit cell must be sought elsewhere. A careful examination of the atomic coordinates of the framework atoms revealed that 6 of the 15 T-atoms are displaced by more than 0.2 ]~ and 19 of the 30 framework oxygens by more than 0.5 ,~ from equivalent posi-

Table 3 Positional, thermal and occupancy parameters for as-synthesized VPI-9 a

Atom Wyckoff position x y z Occupancy b Uiso (~2)

Sil 8b -0.056(4) 0.141(5) 0.0465(5) 8.0 0.004(4) c Si2 8b 0.124(5) 0.366(5) 0.0334(4) 8.0 0.004(4) o Si3 8b 0.348(4) 0.542(4) 0.0458(6) 8.0 0.004(4) c Si4 8b 0.646(4) 0.451 (4) 0.0453(5) 8.0 0.004(4) c Si5 8b 0.710(5) 0.229(5) 0.0195(4) 8.0 0.004(4) c Si6 8b 0.918(4) 0.594(4) 0.0369(4) 8.0 0.004(4) ¢ Zn7 8b 0.769(4) 0.270(4) 0.0786(3) 8.0 0.010(3) d Si8 8b 0.852(4) 0.376(6) 0.1170(5) 8.0 0.004(4) c Si9 8b 0.640(4) 0.156(7) 0.1137(5) 8.0 0.004(4) ~ Sil0 8b 0.359(4) 0.145(6) 0.1302(5) 8.0 0.004(4) c Sil 1 8b 0.136(4) 0.352(7) 0.1343(5) 8.0 0.004(4) o

(Continued overleaf)


Table 3 (contmued)

Atom Wyckoffposition x y z Occupancy b Uiso (~x2)

Znl2 8b 0.276(4) 0.235(4) 0.1698(3) 8.0 0.010(3) a Sil3 8b 0.440(4) 0.363(4) 0.2026(5) 8.0 0.004(4) c Sil4 8b 0.287(5) 0.291(5) 0.2295(4) 8.0 0.004(4) c Znl5 8b 0.000(3) 0.503(3) 0.2469(3) 8.0 0.010(3) a O1 8b 0.082(5) 0.216(6) 0.0400(8) 8.0 0.025(6)' 02 8b -0.084(5) 0.171(7) 0.0676(6) 8.0 0.025(6) ~ 03 86 -0.041(5) -0.018(5) 0.0435(8) 8.0 0.025(6) ° 04 8b -0.177(5) 0.181(6) 0.0336(7) 8.0 0.025(6) ° 05 8b 0.279(6) 0.399(5) 0.0401(8) 8.0 0.025(6) e 06 8b 0.022(8) 0.475(8) 0.0420(6) 8.0 0.025(6) ~ 07 8b 0.111(7) 0.377(6) 0.0110(4) 8.0 0.025(6)' 08 8b 0.507(4) 0.531(6) 0.0461(9) 8.0 0.025(6) ° 09 8b 0.654(7) 0.375(5) 0.0262(7) 8.0 0.025(6) e O10 8b 0.773(5) 0.553(7) 0.0458(9) 8.0 0.025(6) e Ol l 8b 0.768(9) 0.241(9) -0.0002(5) 8.0 0.025(6) e O12 8b 0.593(6) 0.118(6) 0.0197(10) 8.0 0.025(6) e O13 8b 0.912(7) 0.619(6) 0.0143(4) 8.0 0.025(6) ~ O14 8b 0.967(7) 0.730(5) 0.0465(8) 8.0 0.025(6) ~ O15 8b 0.642(6) 0.349(6) 0.0623(7) 8.0 0.025(6) ~ O16 8b 0.858(6) 0.391(6) 0.0951(5) 8.0 0.025(6)" O17 8b 0.662(5) 0.141(6) 0.0926(6) 8.0 0.025(6)" O18 8b 0.734(7) 0.273(9) 0.1227(6) 8.0 0.025(6) ~ O19 8b 0.680(6) 0.020(8) 0.1239(10) 8.0 0.025(6) ~ 020 8b 0.485(5) 0.194(9) 0.1179(8) 8.0 0.025(6) e O21 8b 0.237(7) 0.244(9) 0.1254(6) 8.0 0.025(6) e 022 8b 0.389(6) 0.146(7) 0.1516(5) 8.0 0.025(6)" 023 8b 0.314(6) -0.002(8) 0.1226(12) 8.0 0.025(6)" 024 8b 0.134(5) 0.340(7) 0.1562(5) 8.0 0.025(6)" 025 8b -0.009(5) 0.321(9) 0.1253(9) 8.0 0.025(6)' 026 8b 0.368(7) 0.381(5) 0.1832(7) 8.0 0.025(6)' 027 8b 0.203(6) 0.093(6) 0.1849(7) 8.0 0.025(6) e 028 8b 0.333(6) 0.367(7) 0.2184(8) 8.0 0.025(6) ~ 029 8b 0.190(7) 0.149(6) 0.2193(9) 8.0 0.025(6) ° 030 8b 0.082(6) 0.378(6) 0.2304(10) 8.0 0.025(6) # Rbl 8b 0.003(5) 0.008(6) 0.1198(4) 7.0 0.037(2) f Rb2 8b 0.169(6) 0.362(6) 0.0826(7) 7.0 0.037(2) f Rb3 4a 0.417(6) 0.417 0 3.0 0.037(2) f Ow3 4a 0.417 0.417 0 1.0 0.037(2) f Rb4 4a 0.079(4) 0.078 0 4.0 0.037(2) f Rb5 8b 0.649(6) 0.843(6) 0.0398(6) 7.0 0.037(2) f Rb6 8b 0.407(5) 0.133(5) 0.0495(6) 3.0 0.037(2) r K6 8b 0.328(38) 0.135(41) 0.0355(40) 2.0 0.037(2) f Rb7 8b 0.839(7) 0.691(7) 0.0869(7) 6.0 0.037(2) f K7 8b 0.863(41) 0.613(40) 0.0777(44) 2.0 0.037(2) t Rb8 4a 0.622(9) 0.622 0 1.0 0.037(2) e Ow8 4a 0.622 0.622 0 3.0 0.037(2) t Rb9 8b 0.538(10) 0.569(8) 0.0889(12) 3.0 0.037(2) f Owl 8b 0.569(19) 0.904(16) 0.0775(18) 10.0 g 0.037(2) f

a Numbers in parentheses are the esd's in the units of the least significant digit given. b Occupancy parameters are given in terms of atoms per unit cell. " f Thermal parameters with the same superscript were constrained to be equal. s Occupancy increased from 8.0 to 10.0 to approximate the electron density of the associated H atoms.


Table 4 Selected interatomic distances (,~) and angles (°) for as-synthesized VPI-9

Framework

Si-O O-Zn-O minimum 1.57 ( 6 ) minimum 100 ( 3 ) maximum 1.68 (5) maximum 116(3) average 1.60 aver age 109.5

Zn-O Si-O-Si minimum 1.91 ( 7 ) minimum 130 ( 4 ) maximum 2.01 (6) maximum 174(4) average 1.94 average 147.4

O-Si-O Si-O-Zn minimum 104 ( 4 ) minimum 122 ( 4 ) maximum 114(4) maximum 150 (4) average 109.4 average 130.0

Non-framework species R b l - O18 3.35(10) Rb6- 030 3.35(8)

O19 3.21 (8) Owl 3.46(16) 020 3.23 (10) K6 1.29(33) O21 3.31(10) 023 3.08(8) K6-- O1 2.58(38) 025 3.12(10) 05 2.68(41 )

07 3.47(30) Rb2- 02 3.32(8) O12 2.87(37)

05 3.33(8) 028 3.10(40) O16 3.22(9) 030 2.93(40) O21 3.43(7) 022 3.10(9) Rb7- O10 3.39(8) 027 2.91 (8) O14 3.25(8)

O16 3.03(9) Rb3- 05 3.26(6) (x 2) O18 3.17(7) (Ow3) 07 3.15(8) (x 2) O19 3.35(9)

09 3.06(8) (x 2) 022 2.85(9) 029 3.39(7) (x 2) Owl 3.47(19) Rb8 2.87(8) K7 1.05(37) (Ow8)

K7- O6 3.36(36) Rb4- O1 3.24(6) (x 2) O10 2.58(34)

07 3.07(6) (x 2) O14 2.77(35) Ol l 3.46(9) (x 2) O16 2.54(38) 028 3.24(7) (x 2) 022 2.91(39)

Rb5- 03 3.37(8) Rb8- 09 3.13(5) (x 2) 08 3.42(8) (Ow8) O13 3.05(11)(× 2) O10 3.15(9) O12 3.14(9) Rb9- 08 3.19(11) O14 3.37(9) O15 3.11(10) 030 2.75(8) 024 3.19(11) Owl 2.95(14) 027 2.97(11)

Owl 3.44(18) Rb6- O1 3.38(7)

05 3.00(7) Owl- O17 2.74(17) O12 2.87(8) 024 2.43(18) O15 3.29(8)


Fig. 7. The structure of VPI-9 showing the ordering of the Zn atoms (highlighted in black) in the framework and the T-atom labelling scheme. The rotation of layers A and B resulting from the 41 axis is indicated.

tions in the smaller unit cell. This distortion is probably a result of interactions with the Rb ÷ and K ÷ ions within the channels of the molecular sieve. Apparently this distortion relaxes upon NH~-exchange. The cell doubling appears to be a concerted effect that cannot be attributed to any single feature of the structure. All of the observed superstructure reflections (/---odd) are small, but there are many of them (see Fig. 5), and this is a clear indication that the distortion is real.

Each Rb ÷ or K ÷ ion is associated with an 8-ring of the structure and coordinates to at least four framework oxygens (Table 4). Twenty-seven of the 30 oxygens (exceptions: 04, O17 and 026) interact directly with these cations. The locations of the Rb ÷ ions are shown in Fig. 8. Rbl is associated with the 8-ring in layer A; Rb2, Rb7 (K7) and Rb9 with the 8-ring channels parallel to [110] between layers A and B; Rb3, Rb4 and Rb8 with the 8-ring channels parallel to [110] within layer B; and Rb5 and Rb6 (K6) with the con- stricted 8-rings between the chains in layer B. The K6 and K7 positions are near those of Rb6 and Rb7, respectively, but make closer approaches to

the framework oxygens. The Rb ÷ ions in the 8-ring channels parallel to [110] are arranged in a zig-zag fashion between layers A and B, but are constrained by the framework to lie in a straight line within layer B.

Like VPI-7 and RUB-17, VPI-9 has a relatively high framework density (16.7 T-atoms/1000 .~3). The three framework topologies (VSV, RSN and VNI) also have other features in common. All have 4.82 layers with the same orientation of tetrahedra, all contain 3-rings, all have relatively small pore openings, and all have unit cells with one very long axis (VSV: I41/amd a=7.2, c= 39.9 ,~; RSN: C2/m a=7.2, b=40.6, c=7.3 .A, fl= 91.8°; VNI: P42/ncm a=9.9, c=36.8 A). The Zn atoms in all three structures are associated with 3-rings. Interestingly enough, in VPI-7 and RUB-17 the Zn atoms are located in the 4.82 layers, whereas those in VPI-9 are not. Nonetheless, two of the three Zn environments in VPI-9 (Zn7 and Znl2) are very similar to those in VPI-7 and RUB-17 in that they form the link between a 3-ring and a 4-ring in a spiro-type arrangement. The third Zn position (Znl5) in

LB. McCusker et al./Microporous Materials 6 (1996) 295-309 307

B

A

B

A

Fig. 8. The structure of VPI-9 showing the location of the Rb + ions within the channels of the framework. The positions of layers A and B are indicated as a guide.

VPI-9, however, is unique, as is the layer containing it. The framework structures of both VPI-7 and RUB-17 contain spiro-5 units (two 3-rings sharing a common node) with a Si in the central position and a Zn in each of the two 3-rings, while VPI-9 does not contain any such units. The Si:Zn ratio in both VPI-7 and RUB-17 is 3.5:1, while that for VPI-9 is 4:1, and this probably plays an important role in determining the structural sub- units formed.

The 29Si MAS NMR spectrum (Fig. 3) can be decomposed into 6 peaks with the relative intensities 1:5:1:3:1:1 (i.e. a total of 12). In the change from the topological unit cell with the space group

P42/ncm to the larger unit cell with lower symmetry, the five topologically distinct Si sites are split into (1) Si2 and Si6, (2) Si8 and Si9, (3) Sil0 and Sill, (4) Sil, Si3, Si4 and Sil3, and (5) Sil4 and SiS (see Fig. 7). The fact that six peaks are observed rather than five is a clear indication that the topological symmetry is broken, and the fact that their areas total 12 lends further support to the unit cell and space group selected for the structure refinement. The average T-O-T angle for each of the twelve Si sites (each of which is in a general position and has one neighboring Zn and three Si atoms) is given in Table 5. Using this as the only criterion for the interpretation of the peaks in the 29Si MAS NMR spectrum (assuming that the smaller the average T-O-T angle the higher the field), leads to the assignment of the peaks shown in Table 5. The first six Si sites are all associated with 3-rings, the next four with one 4-ring and two 5-rings, and the last two with one 4-ring and four 5-rings. SiS, Si9, Sil0 and Sil 1 are all in the 4 .8 2 layer and have very similar environments, so it is not surprising that they should have similar chemical shifts. The assignment appears to make chemical sense and is fully consistent with the refined structure.

It had been hoped initially that the structure could be solved by exploiting anomalous disper- sion effects as suggested, for example, by Prandl [18], but all attempts to do so failed. Now that the structure is known, this failure can be eval- uated. A comparison of the intensities calculated from the final coordinates listed in Table 3 for each of the three wavelengths used for data collection, shows that there are small but significant differences. In theory these differences could be used to advantage in a structure determination procedure. However, the intensities of individual reflections are often obscured by overlapping reflections in a powder diffraction pattern, so separate intensity differences cannot be measured with any accuracy. The error introduced by the equipartitioning of the intensities of overlapping reflections is of the same order of magnitude as the expected differences due to anomalous scattering effects [9], so it is unlikely that structures of this complexity can be solved using the approach suggested by Prandl. The problem is exacerbated


80000-

60000.

40000.

20000.

O.

40000-

, , : . . . . i . . . . i . . . . i . . . . i , , |

5 10 15 20 25 3 0

30000.

20000

10000

0

. . . . i . . . . i . . . . i . . . . i . . . . i . . . . I 0

30 35 40 45 50 55 6 "20

Fig. 9. The observed (top), calculated (middle) and difference (bottom) profiles for the Rietveld refinement of as-synthesized VPI-9.

To show more detail, the scale for the second half of the pattern has been increased by a factor of two.

Table 5 Assignment of the 29Si M A S N M R peaks based on the average T - O - T angles

Chemical shift Peak area Si site Average Si-O-T angle

- 8 6 . 9 p p m 1 Si (2) 134.3 °

- 9 2 . 9 p p m 5 Si(10) 139.0 °

S i (9) 139.6 °

S i ( l l ) 140.1 ° S i (8) 141.4 °

S i (6) 141.5 °

- 9 4 . 5 p p m 1

- 9 5 . 9 p p m 3

Si(13) 142.3 °

Si (4) 143.3 ° Si (1) 145.8 °

Si (3) 148.5 °

- 9 8 . 2 p p m 1 Si(14) 150.6 °

- 9 9 . 9 p p m 1 Si (5) 151.4 °

in practice because errors due to the scaling of the profiles to one another, and the estimation of the background and of the absorption correction cannot be eliminated entirely.

8. Conclusions

The combination of an optimized synthesis, synchrotron powder diffraction data and a new approach to structure determination from powder data has allowed the complex structure of VPI-9 to be determined and refined. With 7 T-sites, this framework topology is the most complex yet solved from powder diffraction data using an automated procedure. The incorporation of chemical and structural information specific to zeolite structures into the structure determination process made this possible. Attempts to solve the structure by conventional methods or by exploiting anomalous scattering effects were unsuccessful. The structure


of VPI-9 in the as-synthesized form has 12 Si, 3 Zn, 30 O, 9 Rb, 2 K and 3 H20 positions in the asymmetric unit and requires a 7195 .~3 unit cell. This translates into 170 structural parameters, so the high quality of the synchrotron powder diffraction data was essential to the success of the structure refinement. The refinement itself illustrates the power of the Rietveld method for structure refinement using powder data. The 295i

MAS NMR spectrum can be interpreted very convincingly in terms of the Si-O-T angles found in the refinement, and this is a further indication that the results of the refinement are sound. While the structure of VPI-9 has several features in common with those of the related zincosilicate molecular sieves VPI-7 and RUB-17, such as the framework density, the presence of 4.82 layers and 3-rings, and a subunit containing Zn, there are also some interesting differences, which may be related to the lower concentration of Zn in VPI-9.

Acknowledgments

We thank Michael Annen for providing us with the first sample of VPI-9, and Drs. D.E. Cox, P. Pattison and A. Fitch for their assistance with the synchrotron data collections. We also thank the NSLS in Brookhaven, which is supported by the US Department of Energy, Division of Materials Science and Division of Chemical Sciences, and the SNBL at the ESRF in Grenoble for allowing us access to their synchrotron radiation facilities. This work was supported in part by the Swiss National Science Foundation.

References

[1] G.O. Brunner and W.M. Meier, Nature, 337 (1989) 146. [2] M.J. Annen, Ph.D. Thesis, VPI, Blacksburg, VA, 1992. [3] M.J. Annen and M.E. Davis, Microporous Mater., 1

(1993) 57. [4] M.J. Annen, M.E. Davis, J.B. Higgins and J.L. Schlenker,

J. Chem. Soc., Chem. Commun., (1991) 1175. [5] C. Roehrig, H. Gies and B. Marler, Zeolites, 14 (1994)

498. [6] T.S. Ercit and J. Vanvelthuizen, Can. Mineral., 32

(1994), 855. [7] C. Roehrig and H. Gies, Angew. Chem., Int. Ed. Engl.,

34 (1995) 63. [8] C.C. Freyhardt, J.E. Lewis, Jr., M.M. Helmkamp, S.

Khodabandeh, M. Yoshikawa, M.A. Camblor, R.F. Lobo, M. Tsapatsis, M. Pan, S.I. Zones and M.E. Davis, submitted.

[9] R.W. Grosse-Kunstleve, Ph.D. Thesis, ETH, Zurich, 1996. [10] M.A. Camblor and M.E. Davis, J. Phys. Chem., 98

(1994) 13151. [11] D.E. Cox, J.B. Hastings, L.P. Cardoso and L.W. Finger,

Mater. Sci. Forum, 9 (1986) 1. [12] R.W. Grosse-Kunstleve, L.B. McCusker and Ch.

Baerlocher, in preparation. [13] Ch. Baerlocher, A. Hepp and W.M. Meier, Distance Least

Squares Refinement Program DLS-76, Institut ftlr Kristallographie und Petrographic, ETH, Zurich, 1977.

[14] Ch. Baerlocher, FORTRAN Program for the Extraction of Integrated Intensities from a Powder Pattern, EXTRACT, Institut ftlr Kristallographie, ETH, Zurich, 1990.

[ 15] Ch. Baerlocher, X-Ray Rietveld System XRS-82, Institut ftir Kristallographie und Petrographic, ETH, Zurich, 1982.

[16] E.N. Maslen, in A.J.C. Wilson (Editor), International Tables for Crystallography, Kluwer, Dordrecht, 1995, p. 520-529.

[17] R.J. Hill and I.C. Madson, Powder Diffraction, 2 (1987) 146.

[18] W. Prandl, Acta Crystallogr. A46 (1990) 988.

Documents

Synthesis optimization and structure analysis of the zincosilicate molecular sieve VPI-9