Crystal Structure of MCM-70: A Microporous Material with High

was determined from synchrotron powder diffraction data with the program FOCUS. .... Zeolithe mit sehr großen Poren als Bindeglied zwischen mikro...
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J. Phys. Chem. B 2005, 109, 13891-13898

13891

Crystal Structure of MCM-70: A Microporous Material with High Framework Density Douglas L. Dorset* and Gordon J. Kennedy Corporate Strategic Research, ExxonMobil Research and Engineering Company, 1545 Route 22 East, Annandale, New Jersey 08801 ReceiVed: January 13, 2005; In Final Form: April 6, 2005

The crystal structure of the borosilicate MCM-70 (siliceous framework formula Si12O24) was determined from synchrotron powder diffraction data with the program FOCUS. The framework crystallizes in space group Pmn21, where a ) 13.663, b ) 4.779, c ) 8.723 Å, and forms 1D ellipsoidal 10-ring channels with the following dimensions: 5.0 × 3.1 Å. Rietveld refinement of the model against synchrotron powder data from solvated material gives Rwp ) 0.15, R(F2) ) 0.11. In addition to the four tetrahedral sites and seven framework oxygens, one potassium position is found during this refinement. Because of the unreasonable geometry of five putative extraframework oxygen sites, another synchrotron pattern was obtained from a dehydrated specimen. A refinement in space group P1n1 (removing the mirror operation of Pmn21), where a ) 13.670, b ) 4.781, c ) 8.687 Å, and β ) 90.24°, verified that the previous framework geometry is preserved as well as the potassium position. One extraframework oxygen was located that would yield a reasonable K-O distance. The existence of potassium is verified by electron energy dispersive spectroscopic measurements as well as quantitative elemental analysis. (There are approximately 2.35 K sites per 12 Si in the unit cell.) It is likely that the constricted channels occlude KOH. 11B and 29Si MAS NMR measurements indicate a framework SiO2/B2O3 of approximately 40:1, which is consistent with a wavelength dispersive spectroscopic measurement. The silicate framework density is 2.10 gm/cm3. The resulting framework density for T sites, 21.1, is unusually high for a zeolite, just below the value for paracelsian (21.4) or scapolite (21.8), each of which also has a smallest four-ring loop. The 1H f 29Si CP MAS measurements suggest sample heterogeneity, that is, a portion of the sample that is strongly coupled to hydrogen and efficiently cross polarizes and another portion that does not.

Introduction One major objective in the synthesis of microporous materials is to construct a porous space that will support dimensionally constrained catalytic conversions.1 Various organic structure directing agents (SDA) have been employed commonly in the search for new zeolites with the intent that the dimensions of microporous cavities or channels will somehow be dictated by the outlines of these agents. A class of reagents with N,N,N′,N′tetraaklylbicyclo(2.2.2)octane2,3:5,6-dipyrrolidium cations had been used to prepare MCM-68, a zeolite with straight 12-ring channels,2 for example. By altering the alkyl substitution of the SDA cation to dipropyl or diisopropyl, a crystalline material, termed MCM-70, was synthesized.3 The crystal structure of its calcined form is presented in this communication. Materials and Methods Zeolite. MCM-70 is a borosilicate prepared as described in a recent patent.3 The rigid polycyclic diquarternary directing agents used in this synthesis, based on N,N,N′,N′-tetraalkyl bicyclo[2.2.2.]octane-2,3:5,6-dipyrolidinium diiodide and an unsaturated variant, are also described in the patent. As will be apparent in the structure analysis, it is important to realize that KOH is a principal ingredient of the synthesis mixture (20 wt %) with colloidal silicon (30 wt %) and boric acid (4 wt %) (see example 7 of ref 3). Powder X-ray Diffraction. Debye-Scherrer X-ray diffraction measurements (Figure 1a) were made on a calcined MCM* Corresponding author. E-mail: [email protected].

70 sample in a capillary (λ ) 1.2201 Å) at the ExxonMobil beamline X10B at the Brookhaven National Laboratory. The 2θ scan range encompassed 8-60° in steps of 0.01°. Choices of appropriate unit cell dimensions and space group symmetry were suggested after the patterns were indexed with programs embedded in the MDI Jade software package. Using Le Bail equipartitioning of reflections,4 we extracted intensity data from the powder pattern with the program GSAS,5 after the background function and peak profiles were simulated adequately. Later, another synchrotron powder pattern (Figure 1b) was obtained (λ ) 0.8808 Å) from a sample that had been dehydrated by heating at 300 °C under house vacuum. (The 2.0-mm quartz sample capillary was then sealed.) The 2θ scan range used encompassed 5-50° in steps of 0.005°. Note that the ratio of low angle to medium angle peak heights changes somewhat as the sample is dehydrated. Electron Diffraction. Transmission electron diffraction measurements were carried out at 200 kV with a JEOL JEM2010 electron microscope. Zeolite samples were first crushed to fine powders with a mortar and pestle, then suspended in acetone to be dispersed by ultrasonication. Drops of the fine particle suspension were dried onto carbon-film-covered 300mesh copper electron microscope grids. Diffraction patterns were recorded on Kodak SO-163 electron microscope film developed in Kodak HRP developer. Diffraction pattern spacings were calibrated against a gold powder diffraction standard for measurement of reciprocal spacings. Intensity data were extracted from patterns digitized on a flat-bed scanner via the ELD6 software.

10.1021/jp0580219 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/06/2005

13892 J. Phys. Chem. B, Vol. 109, No. 29, 2005

Dorset and Kennedy

Figure 1. (a) Synchrotron powder pattern from calcined, hydrated MCM-70. (b) Synchrotron powder pattern from dehydrated sample.

Structure Determination and Refinement. Various attempts to solve the crystal structure from the powder pattern were made using conventional direct methods (EXPO7), simulated annealing (ZEFSA8), or a unique approach that recognizes potential silicate

frameworks (FOCUS9). When the model was identified, it was refined against the powder diffraction profile using GSAS. Structural models were visualized with the appropriate program in the Accelrys Cerius2 package.

Crystal Structure of MCM-70

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Figure 2. MCM-70: (a) Electron diffraction (h0l ) pattern. (b) Electron diffraction (hk0) pattern.

Multinuclear Solid-State NMR. 11B MAS, 29Si MAS, and CPMAS NMR spectra were recorded on a Varian InfinityPlus-500 spectrometer operating at 11.7 T (1H 499.2 MHz) corresponding to Larmor frequencies of 160.16 and 99.18 MHz for 11B and 29Si, respectively. The 29Si MAS (Bloch decay) and CPMAS NMR were recorded using a 7.5-mm Varian probe at spinning speeds of 4 and 3.5 kHz, respectively. The MAS spectra were recorded using a π/2 pulse and a repetition delay of 60 s. The 1H f 29Si CPMAS NMR were performed using a contact time of 3.5 ms, optimized on a sample of octakis(trimethylsiloxy)silesquioxane, also known as Q8M8, and a repetition delay of 3s. 1H dipolar decoupling was used during data acquisition. The 11B MAS NMR spectra were recorded on samples loaded in 4-mm (o.d.) MAS PENCIL rotors and spinning at the magic angle at rates of about 10 kHz with 1H decoupling during data acquisition, 0.7 µs π/6 pulses, a 0.5 s pulse delay, and 1200 scans were collected. Quantitative 1H MAS NMR experiments were performed on a Bruker AMX360 spectrometer at 8.4 T (1H 360 MHz) on a dried sample. The 29Si and 1H chemical shifts are referenced with respect to TMS (δSi ) 0.0 ppm, δH ) 0.0 ppm). The 11B chemical shifts are referenced with respect to an external solution of BF3(OEt)2 (δB ) 0.0 ppm). 29Si

Results Although orthorhombic unit cells with similar dimensions (e.g., 13.64 × 4.75 × 8.72 Å) were suggested after indexing the powder pattern by MDI Jade, it was not possible to distinguish likely choices of space group by the figures of merit provided. Electron diffraction data from individual microcrystals were evaluated to determine likely zonal symmetries. Most commonly, a rectangular pattern with dimensions 8.57(27) × 4.74(1) Å was observed (Figure 2b). Allowing for secondary scattering, there is a suggested glide operation along the longest axis. Another primitive rectangular pattern was observed with dimensions 13.37(14) × 4.41(32) Å. Very rarely observed was a c-centered rectangular pattern (Figure 2a) with dimensions 13.4 × 8.65 Å, but the identification of this zonal symmetry was crucial for the space group determination. Comparing these electron diffraction results to the choices from the indexed powder X-ray data, only two orthorhombic space groups10 were

suggested: Pmmn (59) and Pmn21 (31). (Again, allowing for secondary scattering perturbations, this would account for the observed e. d. zonal symmetries.) Attempts at crystal structure determination by EXPO7 and ZEFSA8 were unsuccessful, although an overall istotropic temperature factor can be obtained easily from the former program package via a Wilson plot. A model that accounted for the low angle intensities of the powder pattern and the positions of observed major reflections was found using the program FOCUS9 in space group Pmn21. The derived model is a densely packed framework with 1D compressed 10ring channels (Figure 3). A DLS refinement11 demonstrated the innate stability of the framework (R ) 0.0042). This procedure 2 minimizes the function Fw ) ∑m,nw2j [Doj - Dm,n j ] , involving prescribed and observed Si-O bond distances. The reported figure of merit was defined R ) (∑jwj[Doj o 2 1/2 2 Dm,n j ] /∑jwj[Dj ] ) . From the appearance of patterns from as-synthesized and calcined MCM-70, each compared to the predicted pattern, it was clear that the proposed channel in the framework model could not be empty. This is because the lowest angle intensities in the observed pattern are weaker with respect to the midangle reflections than those predicted for the framework structure. (This conclusion could be reached readily, for example, from a statistical overview of zeolite powder XRD patterns compiled by Treacy et al.,12 but also in early powder diffraction studies of zeolites,13 depending on whether extraframework atoms are found in the channels.) Starting with T-site and oxygen positions for this framework after DLS refinement, a Rietveld refinement was begun using GSAS.5 Initial electron density maps, after refinement of peak profile and background based on the framework model, revealed a strong peak that improved the fit of the model to the observed pattern. Initially incorporated as oxygen, refinement on site occupancy approximately doubled its weight. Selecting a cation that would correspond to this increased weight, a potassium position was proposed. The proposed K+ site was assigned a unitary occupancy; this improved relevant figures of merit upon its refinement. Its presence was verified subsequently by energy dispersive spectroscopy (EDS), carried out on a scanning electron microscope, as well as elemental analysis that found 0.195 K and 0.004 Na per Si atom. Given 12 Si atoms in the

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Dorset and Kennedy TABLE 1: Atomic Coordinates for MCM-70 Model (Hydrated, Sample) atom

x/a

y/b

z/c

Uiso

Si1 Si2 Si3 Si4 O5 O6 O7 O8 O9 O10 O11

0.0 0.203(1) 0.207(1) 0.0 0.0 0.0 0.903(1) 0.278(1) 0.212(1) 0.221(1) 0.098(1)

0.599(3) 0.144(2) 0.372(2) 0.080(3) 0.410(3) 0.918(3) 0.506(3) 0.886(3) 0.393(3) 0.262(4) 0.993(3)

0.314(2) 0.576(2) 0.235(2) 0.518(2) 0.470(2) 0.355(2) 0.218(2) 0.616(2) 0.702(2) 0.408(2) 0.609(2)

0.028 0.028 0.028 0.028 0.020 0.020 0.020 0.020 0.020 0.020 0.020

extraframework atoms atom

x/a

y/b

z/c

occ.

Uiso

K12

0.5

0.448(3)

0.293(3)

0.96

0.024

TABLE 2: Bond Distances (Å) and Angles (deg) for Hydrated MCM-70

Figure 3. Structural model for MCM-70, including K+ counterion.

unit cell, this accounts for a total of 2.35 K atoms. With the full occupancy of two K atoms found in the Rietveld refinement, a fractional amount remains elsewhere in the channel structure. Wavelength dispersive spectroscopy (WDS) on the scanning electron microscope also detected a small amount of boron (between 0.5 and 1.0 wt %) in the sample. (NMR measurements suggest 0.82 wt %.) During the course of the refinement, five putative, additional extraframework atoms were detected in difference electron density maps. These were first screened with the structure-visualizing program in Cerius2 to verify that the peaks were initially not too close to framework atoms. During the refinement, soft restraints were imposed on framework atomic bonding parameters; this set the variation of tetrahedral distances: dSi-O ) 1.61(3); dO-O ) 2.65(6) Å. The refined unit cell dimensions were a ) 13.6628(6), b ) 4.7788(3), and c ) 8.7227(5) Å. Atomic coordinates of the refined structure are listed in Table 1 and framework bond distances and angles in Table 2. The average dSi-O is 1.62(2) Å and values range from 1.57 to 1.65 Å, well within the distances found in other zeolite structure determinations.14-16 The average O-Si-O angle is 109.4 ( 3.5°, close to an ideal tetrahedral angle. Aside from one outlying value at 100.7°, all O-Si-O bond angles lie within the range found in typical zeolite structure determinations.15,16 The average Si-O-Si angle is 139.2 ( 13.0°, where the expected17 value is 140°. Most extraframework atoms satisfied the criterion that they should be at least 2.7 Å away from framework oxygens. Others, however, refined to positions that were unreasonably close to framework atoms. Such occasional, meaningless geometrical values are not unusual, even in single-crystal X-ray determinations of hydrated zeolites containing cations.14,18 However, another refinement was carried out against data from a dehydrated sample to better characterize

Si1-O5 Si1-O6 Si1-O7 Si2-O8 Si2-O9 Si2-O10 Si2-O11 Si3-O7 Si3-O8 Si3-O9 Si3-O10 Si4-O5 Si4-O6 Si4-O11 Si1-O5-Si4 Si1-O6-Si4 Si1-O7-Si3 Si2-O8-Si3 Si2-O9-Si3 Si2-O10-Si3 Si2-O11-Si4

1.63 1.57 1.63 1.65 1.63 1.59 1.63 1.64 1.62 1.60 1.61 1.63 1.62 1.61 138.2 131.7 143.6 141.5 133.3 163.8 122.0

O5-Si1-O6 O5-Si1-O7 O6-Si1-O7 O7-Si1-O7 O8-Si2-O9 O8-Si2-O10 O8-Si2-O11 O9-Si2-O10 O9-Si2-O11 O10-Si2-O11 O7-Si3-O8 O7-Si3-O9 O7-Si3-O10 O8-Si3-O9 O8-Si3-O10 O9-Si3-O10 O5-Si4-O6 O5-Si4-O11 O6-Si4-O11 O11-Si4-O11

110.4 106.2 112.4 108.9 110.9 111.1 100.7 110.6 105.9 117.2 110.8 110.2 109.0 109.2 109.6 108.1 103.8 112.0 108.1 112.2

the contents of the pore channel. Although the best results were found after relaxation of the mirror operation in Pmn21, yielding,10 monoclinic space group P1n1, the framework geometry remained virtually unchanged (Table 3) with average Si-O bond distances (Table 4) of 1.63(3) Å. Average angles were O-Si-O: 109.5 ( 3.7°; Si-O-Si: 139.4 ( 15.6°. Refined unit cell dimensions were a ) 13.6997(6), b ) 4.7809(3), c ) 8.6866(4) Å, and β ) 90.24(2)°, revealing the small deviation from the orthorhombic angle. The potassium site (Table 3) is near the position found in the hydrated structure (Table 1) and at about the same fractional occupancy. An additional extraframework oxygen position (O13) was identified (Table 3) that is close (2.64 Å) to the potassium site. Moreover, the potassium lies similarly close to framework oxygens: O5 (2.84 Å), O9a (3.02 Å), and O11a (2.88 Å). All are typical distances expected, for example, in a KOH lattice.19 The oxygen O13 makes a rather short contact to Si1 (2.5 Å), indicating that this position is not quite accurately placed, as also indicated by the relatively high-temperature factor. With the very small concentration for boron in the framework indicated above, it is difficult to rationalize the large presence of potassium in this material. From the 0.82 wt % detected by solid-state NMR, 0.6 mol B would be present in the framework compared to >2.0 mol K+ in the channels. Because, for example, similarly dense (see below) feldspathoids often contain ion pairs within the

Crystal Structure of MCM-70

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TABLE 3: Atomic Coordinates for Dehydrated MCM-70 atom

x/a

y/b

z/c

Uiso

Si1 Si2a Si2b Si3a Si3b Si4 O5 O6 O7a O7b O8a O8b O9a O9b O10a O10b O11a O11b

0.003(1) 0.201(1) -0.210(1) 0.200(1) -0.214(1) 0.009(1) 0.015(2) 0.012(2) 0.899(1) -0.911(1) 0.283(1) -0.280(1) 0.212(1) -0.230(1) 0.210(1) -0.240(1) 0.098(1) -0.098(1)

0.591(2) 0.130(3) 0.134(3) 0.370(3) 0.361(3) 0.071(2) 0.418(2) 0.922(2) 0.483(4) 0.483(4) 0.884(4) 0.860(4) 0.388(4) 0.364(4) 0.240(5) 0.233(5) -0.011(4) 0.030(4)

0.307(1) 0.566(1) 0.564(1) 0.219(1) 0.217(1) 0.506(1) 0.472(1) 0.337(1) 0.232(2) 0.193(2) 0.592(2) 0.591(2) 0.683(2) 0.693(2) 0.390(1) 0.388(1) 0.613(2) 0.586(2)

0.0009 0.0009 0.0009 0.0009 0.0009 0.0009 0.0020 0.0020 0.0020 0.0020 0.0020 0.0020 0.0020 0.0020 0.0020 0.0020 0.0020 0.0020

extraframework atoms atom

x/a

y/b

z/c

Uiso

occ.

K12 O13

0.510(2) O.507(5)

0.456(2) 0.173(6)

0.292(1) 0.552(4)

0.0026 0.1026

0.95 1.00

TABLE 4: Bond Distances (Å) and Angles (deg) for Dehydrated MCM-70 Si1-O5 Si1-O6 Si1-O7a Si1-O7b Si2a-O8a Si2a-O9a Si2a-O10a Si2a-O11a Si2b-O8b Si2b-O9b Si2b-O10b Si2b-O11b Si3a-O7b Si3a-O8b Si3a-O9b Si3a-O10a Si3b-O7a Si3b-O8a Si3b-O9a Si3b-O10b Si4-O5 Si4-O6 Si4-O11a Si4-O11b

1.66 1.61 1.65 1.62 1.64 1.61 1.62 1.62 1.64 1.59 1.64 1.63 1.63 1.58 1.60 1.62 1.65 1.59 1.60 1.65 1.69 1.64 1.59 1.64

O5-Si1-O6 O5-Si1-O7a O5-Si1-O7b O6-Si1-O7a O6-Si1-O7b O7a-Si1-O7b O8a-Si2a-O9a O8a-Si2a-O10a O8a-Si2a-O11a O9a-Si2a-O10a O9a-Si2a-O11a O10a-Si2a-O11a O8b-Si2b-O9b O8b-Si2b-O10b O8b-Si2b-O11b O9b-Si2b-O10b O10b-Si2b-O11b O7b-Si3a-O8b O7b-Si3a-O9b O7b-Si3a-O10a O8b-Si3a-O9b O8b-Si3a-O10a O9b-Si3a-O10a O7a-Si3b-O8a

109.9 105.4 107.4 115.7 111.0 107.0 113.1 107.9 104.3 109.9 104.5 117.2 110.6 103.1 106.7 114.5 114.9 107.2 105.9 109.2 110.3 111.2 112.7 109.7

O7a-Si3b-O9a O7a-Si3b-O10b O8a-Si3b-O9a O8a-Si3b-O10b O9a-Si3b-O10b O5-Si4-O6 O5-Si4-O11a O5-Si4-O11b O6-Si4-O11a O6-Si4-O11b O11a-Si4-O11b

109.9 105.1 114.4 109.4 107.8 105.5 108.8 103.5 113.4 110.4 114.4

Si1-O5-Si4 Si1-O6-Si4 Si1-O7a-Si3b Si1-O7b-Si3a Si2a-O8a-Si3b Si2b-O8b-Si3a Si2a-O9a-Si3b Si2b-O9b-Si3a Si2a-O10a-Si3a Si2b-O10b-Si3b Si2a-O11a-Si4 Si2b-O11b-Si4

129.4 124.9 161.4 134.6 129.8 139.5 138.8 138.3 169.7 153.2 113.6 139.9

TABLE 5: Coordination Sequence and Vertex Symbols for Calcined MCM-70 atom

coordination sequence

vertex symbol

Si1 Si2 Si3 Si4

4 10 20 38 61 84 115 162 199 226 4 11 22 39 61 88 118 155 199 245 4 11 22 39 61 88 118 155 199 245 4 10 20 38 61 84 115 162 199 226

4 63 4 63 62 106 4 6 3 6 6 6 2 62 4 6 3 6 6 6 2 62 4 63 4 63 62 106

frameworks,20 evidence given here suggests the presence of occluded KOH. The coordination sequence21 and vertex symbols22 are given in Table 5. Note that, topologically, there are only two unique T sites (i.e., the material is pseudocentrosymmetric). Average bond geometries, however, indicate that the topologically equivalent T1T2 and T3T4 pairs in the orthorhombic framework are dimensionally distinct in that the standard deviations of bond distances around the mean tetrahedral value are different (109.4 ( 4.4° vs 109.3 ( 2.4°, respectively). Three zonal views of the model with potassium are shown in Figure 3.

The final figures of merit from the Rietveld refinement of powder data from the hydrated sample are: Rwp ) 0.1515, R(F2) ) 0.1072 with the match of the model profile to the pattern shown in Figure 4. These R-factor values lie within the range listed for published zeolite structures based on powder diffraction data.12 The fit of the final model (Figure 3) to the experimental pattern from hydrated material was somewhat difficult because of the preferential broadening of h0l reflections (identified by the indexing program). No obvious reason could be found for the broadening from inspection of electron diffraction patterns (e. g., “shape transform” effects23). Attempts to model strain broadening in the Rietveld refinement were also unsuccessful. The fit of the model to experimental data was evaluated further with the most commonly observed 0kl electron diffraction intensities. (These were obtained from the thinnest crystalline projections. Scanning electron microscopy indicated that the preparation contained just one crystal habit, thin laths.) When the potassium position is included, RI ) 0.34 and RF ) 0.40, where R ) (∑||Fobs| - k|Fcalcd||)/∑|Fobs| is weighted by k (I, F) depending on ∑Iobs ) k∑Icalcd or ∑Fobs ) k∑Fcalcd, respectively. The latter value, RF, is lowered to 0.17 after a correction for secondary scattering.23,24 The model without potassium does not give a satisfactory match to the observed intensity distribution. Finally, the simplest extraframework model (Table 3) refined against powder data from an anhydrous sample gives Rwp ) 0.1650, R(F2) ) 0.2000. Although these figures of merit can be lowered further with other extraframework species, the distorted geometrical results do now allow these to be accepted as actual atomic sites. The 29Si MAS and CPMAS spectra of the calcined MCM70 sample in this study are shown in Figure 6. The three common peaks at δSi ) -94.9, - 97.6, and -102.1 ppm have the relative intensities of 1:2:2. This ratio is independent of relaxation delay. These peaks in the CPMAS spectrum are unusually sharp for a borosilicate, suggesting that there is a high degree of local order and a nonrandom distribution of B in the crystalline framework. However, the peaks are broader in the MAS spectrum, hinting at local disorder or sample heterogeneity. The additional sharp peak observed at δSi ) -106.8 ppm is consistent with a quartz impurity although none is detected by XRD. As expected for quartz, the peak at δSi ) -106.8 ppm is not observed in the CPMAS spectrum. This is consistent with a hydrogen deficient, dense phase silica. Another interesting aspect of the CPMAS NMR spectrum is that only the sharp peaks at δSi ) -94.9, - 97.6, and -102.1 ppm are detected. This suggests sample heterogeneity, that is, a portion of the sample that is strongly coupled to hydrogen and efficiently cross polarizes and another portion that does not. The 11B MAS spectrum of the calcined MCM-70 sample in this study is shown in Figure 7. The peak at δB ) -1.3 ppm is associated with Td B in the MCM-70 framework. This sample also shows a small shoulder at δB ) -2.6 ppm. This suggests a possible impurity, although XRD analysis indicates it is pure, or a slightly different local B environment in the crystalline lattice. The total B content, as determined from comparison with a known B-ZSM-5 standard, for this sample is 0.82 wt %. Wavelength dispersive spectroscopic (WDS) measurements on this sample also indicate that a small amount of B (0.5-1.0 wt %) is present, in good agreement with the NMR measurement. The 0.82 wt % Td B measured by 11B MAS NMR translates to a framework SiO2/B2O3 ratio of approximately 42:1. Assuming this B is indeed all in the MCM-70 framework, one should be able to interpret the corresponding 29Si MAS NMR spectrum accordingly. The peaks at δSi ) -94.9, -97.6, and

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Dorset and Kennedy

Figure 4. Fit of model pattern to observed powder diffraction profile (hydrated sample).

-102.1 ppm fall into the range that are consistent with Si(OB)(OSi)3, Si(OSi)4, and Si(OSi)4, respectively. The relative peak areas of 1:2:2 equate to a framework SiO2/B2O3 ratio of approximately 40:1, in good agreement with the ratio predicted from B NMR and WDS measurements. The multiplicity in the Si(OSi)4 peaks is due to the inequivalent crystallographic sites in the structure. Using empirical correlation of chemical shifts with average T-O-T bond angles would suggest that the peak at δSi ) -97.6 ppm can be assigned to the sum of Si1 + Si4 and the peak at δSi ) -102.1 ppm can be assigned to Si2 + Si3. The 1H MAS NMR spectrum of this sample is given in Figure 8. The total H content in mmol/g for this sample is only 0.24. The peak at δH ) 3.15 ppm is presumably due to B-OH type species and those in the 0 to 2.5 ppm region are consistent with silanols. Discussion Using accepted van der Waals radii25,26 for framework atoms, a geometric representation of the MCM-70 pore (Figure 3) reveals that the elliptical 10-ring opening of the channel is approximately 5.0 × 3.1 Å, agreeing dimensionally with similar 10-ring channels1. There is no doubt that the extraframework potassium and oxygen sites must be included in the refinement

to account for the increased intensity of medium-resolution diffraction peaks. As stated above, diffraction from zeolites is often a study of contrast between framework and channels. When the channels are empty, the lowest angle reflections are generally the most intense but they become less so when material is inserted into the empty space. However, modeling of such channel sites, particularly water, can be difficult. The position of potassium in this material was rather clear from inspection of initial electron density maps. Its existence is verified via its detection by EDS and elemental analysis. However, it was initially difficult to understand why potassium is present in such high amounts if no significant cause for framework acidity is found as well as the absence of clearlydetected counterions in the channels. In keeping with the preparative procedures, EDS does not detect the presence of Cl- anions (or any other halides) in the channel. However, OHcould be present on the basis of the study of anhydrous material and, thus, we postulate that the channels contain occluded KOH. Oxygen positions, due to water or hydroxide ions, are often difficult to define within zeolite channels, even by single-crystal X-ray structure analysis. In some cases, known solvation sites cannot be found at all;27 in others, the occupancies are stated to be “statistical” to explain away very close oxygen positions,14,18 for example. The study of faujasite and related

Crystal Structure of MCM-70

J. Phys. Chem. B, Vol. 109, No. 29, 2005 13897

Figure 5. Rietveld refinement of anhydrous MCM-70.

Figure 6. 29Si MAS (top) and CPMAS (bottom) NMR spectra of calcined MCM-70.

Figure 7.

11

B MAS NMR spectrum of calcined MCM-70.

Figure 8. 1H MAS NMR spectrum of calcined MCM-70.

synthetic analogues gives a historical perspective to this problem, revealing that the difficulty lies not only in the localization of water molecules but also, occasionally, cations. For example, in the crystal structure of the synthetic X-zeolite,13 only one-half of the Na+ cations could be found. A reinvestiga-

tion28 with single-crystal data also failed to find all of these positions. Moreover, there were also short interoxygen distances in the identified solvent, a result duplicated by the natural faujasite.29 Comparing refinements of synthetic X and Y zeolites having different Si/Al ratios, none of the water molecules were located specifically in the hydrated materials30 so that a phenomenological liquid scattering factor31 was required to account for this component. Another continuous scattering function was used to account for nonspecific K+ sites. There was also some shift of K+ cation sites for the dehydrated material.32 (Similar discrepancies exist in the postulation of putative “detrital” atomic sites, e.g., in MCM-2233 or even disordered SDA positions, e.g., in MCM-47.15 Solvation sites in natural zeolites, e.g., mutainite,14 have been similarly difficult to characterize.) Otherwise the framework structures in synthetic faujasite remained virtually identical for hydrated and anhydrous samples, a result seen again in this study of MCM-70. Because the refinements have not accounted for all of the K+ in the material (see above), there are obviously other extraframework atoms, even for the anhydrous material. In fact, the rather low isotropic temperature factors applied to framework atoms (Table 3) during the refinement of data in Figure 1b was an attempt to compensate for the deficiency of an incomplete extraframework structure to model the contrast between low and medium angle diffraction intensities. Clearly the missing cations and so forth cannot be modeled by atomic models at well-defined crystallographic sites and may require the use of phenomenological scattering functions as suggested earlier.30 From the framework structure, the calculated density is 2.10 gm/cm3. An unusual feature of the framework is its very high density (T/1000 Å3), that is, 21.1. Because the smallest loop structure comprises four T sites, this framework density is higher than all known zeolites, placing it in the boundary region between zeolites and dense framework materials.34 The paracelsian structure35,36 also has small four-rings and has a framework density of 21.4. The framework density of the feldspathoid, scapolite,37 is slightly larger (21.8).

13898 J. Phys. Chem. B, Vol. 109, No. 29, 2005 The positioning of boron in the framework is not yet understood. Because Rietveld refinements involve a refinement space with similar shallow minima, possible structural models were found in which a preferential siting of B would be closest to the potassium position, this indicated by a lowered occupancy factor for that T site (Si4). However, an attempt to build a framework model incorporating a B atom at this site did not allow a stable DLS refinement to be carried out. Obviously, the small amount of boron cannot account for the larger amount of potassium, that is, by creation of acid sites in the framework. Again, an OH- counterion appears to be occluded in the channels in an ion pair with K+. Although the organic SDA is known3 to be necessary for the synthesis of MCM-70, it is clear that it cannot be accommodated into the very small channel space. Hence its role is not fully understood. Given the tenacious inclusion of K+ that cannot be removed from the channels by NH4+ exchange, it may be that the cation itself has a templating function. Such issues will be the topic of further investigation. Acknowledgment. We thank Karl G. Strohmaier for stimulating discussions of this work, for obtaining the elemental analysis on the material and for dehydrating a sample to be used for collection of synchrotron powder diffraction data. Kirk D. Schmitt is also thanked for helpful discussions and his efforts to remove K+ by ammonia exchange. James Keenan and William Lamberti are thanked for SEM images as well as EDS and WDS data from the sample. Clarence Chase is gratefully acknowledged for recording the NMR spectra discussed in this report. S. S. Dhingra is thanked for preparing the material and obtaining the synchtrotron powder data from the hydrated sample and Steve Bennett for obtaining the pattern from the anhydrous sample. Wilfried Mortier is thanked for helpful discussions about the definition of extraframework species. References and Notes (1) Sjostak, R. S. Molecular SieVes, 2nd ed.; Blackie Academic & Professional: London, 1998. (2) Calabro, D. C.; Cheng, J. C.; Crane, R. A., Jr.; Kresge, C. T.; Dhingra, S. S.; Steckel, M. A.; Stern, D. L.; Weston, S. C. U.S. Patent 6,049,018, 2000. (3) Dhingra S. S.; Weston, S. C. U.S. Patent 6,656,268, 2003. (4) Le Bail, A.; Duroy, H.; Fourquet, J. L. Mater. Res. Bull. 1988, 23, 447. (5) Larsson, A. C.; von Dreele, R. B. General Structure Analysis System, GSAS; Los Alamos Laboratory: Los Alamos, NM, 1994.

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