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J. Phys. Chem. B 2002, 106, 264-270
Synthesis and Structure Determination by ZEFSAII of SSZ-55: A New High-Silica, Large-Pore Zeolite Minghong G. Wu and Michael W. Deem* Chemical Engineering Department, UniVersity of California, Los Angeles, California 90095-1592
Saleh A. Elomari, Ronald C. Medrud, Stacey I. Zones, Theo Maesen, Charles Kibby, Cong-Yan Chen, and Ignatius Y. Chan CheVron Research and Technology Company, Richmond, California 94802-0627 ReceiVed: June 25, 2001; In Final Form: October 22, 2001
The synthesis, structure solution, and characterization of the high-silica zeolite SSZ-55 are described. SSZ55 was synthesized under hydrothermal conditions using a [(1-(3-fluorophenyl)cyclopentyl)methyl]trimethylammonium cation as the structure-directing agent. The framework topology and symmetry of SSZ55 were determined by the Monte Carlo method ZEFSAII. Rietveld refinement of the X-ray powder diffraction data confirms the space group assignment of Cmc21. Transmission electron microscopy confirms the unit cell parameters and the topology of the structure. SSZ-55 contains one-dimensional pores circumscribed by 12 T-atom rings. The topology of SSZ-55 is that of the ATS framework, previously described for AlPO-based molecular sieves.
1. Introduction Zeolites have found widespread use as catalysts, molecular sieves, and ion exchangers. High-silica zeolites are of particular interest because of their importance in refining and petrochemical applications. A short list of applications is shown in Table 1. New high-silica zeolites continue to be discovered through the exploitation of systems of inorganic chemistry into which novel organo-cations are thrust in the hopes that they will effect special spatial features in the developing inorganic lattice.1,2 In virtually all instances involving high-silica zeolites, the crystallization involves complete filing of the void regions by an organo-cation guest molecule.3 Once an interesting structure has been obtained, the organo-cation guest molecule is thermally decomposed, leaving the ordered void regions of the molecular sieve material. The zeolitic material can subsequently be studied for catalysis or separation applications. Many of the eventual uses for novel molecular sieves have come about as a result of testing of applications in the absence of knowledge of the actual zeolite structure. Indeed, the structure solution can come as something of a surprise to the researchers using the material.4 In recent years, some wonderful computational developments have accelerated the rate at which novel structures can be elucidated from powder or electron diffraction data.4-8 In the present case, we have found a novel zeolite, designated as SSZ55, via use of a relatively asymmetric organo-cation guest molecule. We report here the synthesis and characterization of this material and its structure solution using the Monte Carlo method ZEFSAII.7 2. Experimental Section 2.1. Synthesis. The SSZ-55 zeolite was synthesized as a borosilicate zeolite using a [(1-(3-fluorophenyl)cyclopentyl)* Corresponding author: M. W. Deem. E-mail:
[email protected]. Fax: 310-267-0174.
Figure 1. Structure-directing molecule used in the SSZ-55 synthesis, a [(1-(3-fluorophenyl)cyclopentyl)methyl]trimethylammonium cation.
TABLE 1: Applications of High-silica Zeolites process
zeolite type
ref
isomerization dewaxing cumene synthesis ethylbenzene synthesis fluidized catalytic cracking xylene isomerization toluene disproportionation C9 aromatics transalkylation
SAPO-11 zeolite beta MCM-22 ZSM-5 ZSM-5 ZSM-5 mordenite
9 10-12 13 14 15 16 17
methyl]trimethylammonium cation, shown in Figure 1, as the structure-directing agent (SDA). Synthesis of the SDA and details of the synthesis of SSZ-55 are given in a separate publication devoted to that topic.18 Generally, SSZ-55 was synthesized by heating a mixture of a SiO2 source, sodium borate decahydrate, water, SDA (as an aqueous solution of its hydroxide form), and sodium hydroxide. The molar ratios were SiO2/Na ) 10, SiO2/OH- ) 3.6, SiO2/B ) 35, H2O/SiO2 ) 44, and SiO2/SDA ) 5. The heating was done at 160 °C with tumbling at 43 rpm. The synthesis described below is a representative example for making boron-SSZ-55. A mixture of 3 mmol of [(1-(3-fluorophenyl)cyclopentyl)methyl]trimethylammonium hydroxide (7.5 g of 0.4 M solution), 1.2 mmol of NaOH (1.2 g of 1.0 N aqueous solution), 3.3 g of
10.1021/jp012407b CCC: $22.00 © 2002 American Chemical Society Published on Web 12/19/2001
SSZ-55: A New High-Silica, Large-Pore Zeolite deionized water, and 0.08 g of sodium borate decahydrate (Na2B4O7‚10 H2O) was blended in a 23-mL Teflon liner. The mixture was stirred until the sodium borate was completely dissolved. To this solution, 0.9 g of CAB-O-SIL M-5 (SiO2) were added, and the mixture was thoroughly stirred. The Teflon liner containing the resulting gel was capped off, placed in a steel Parr reactor, and heated in an oven at 160 °C with tumbling at 43 rpm. The reaction was monitored by periodically checking the pH of the gel and looking for crystal growth using scanning electron microscopy (SEM). When the crystallization was completed, after heating for 12 days, the starting reaction gel mixture with a pH of 13.4 had been transformed into a biphasic mixture of a clear solution and a fine powder precipitate. The final pH was 12.3. Once the reaction was complete, the reaction mixture was filtered through a fritted-glass funnel. The obtained solids were rinsed several times with water and dried in air overnight. The powder was further dried in an oven at 120 °C for 2 h. The reaction yielded 0.85 g of SSZ-55. The structuredirecting agent was removed by slow calcination to 595 °C at 1 °C/min in an atmosphere of nitrogen with 2% oxygen bled in. 2.2. Analytical Methods. The SSZ-55 material used had a SiO2/B2O3 ratio of 31 as determined by elemental analysis. This translates to a boron weight percentage of about 1.1% for the anhydrous calcined material. X-ray diffraction (XRD) powder patterns were initially recorded on a Siemens D-500 instrument. Samples of both asmade and calcined SSZ-55 were examined. More detailed patterns were obtained at Beamline X7A at Brookhaven National Laboratory. The X7A sample was prepared in a 1.5-mmdiameter glass capillary tube that was sealed after being evacuated and heated to 350 °C. Diffraction patterns collected for the calcined SSZ-55 sample were used in the structure solution work. Scanning electron micrographs were recorded on a Hitachi S-570 instrument. Transmission electron microscope (TEM) work was carried out on a JEOL 2010 instrument at an accelerating voltage of 200 kV by dispersing the crystals on a continuous carbon film. Electron diffraction data were in complete agreement with the unit cell parameters determined from the X-ray diffraction data. A variety of adsorbates was studied in an effort to characterize the void volume of the new zeolite SSZ-55. It is important that the void volume that emerges be consistent with the eventual refined structure. An argon adsorption isotherm at 87.3 K was recorded on ASAP 2010 equipment from Micromeritics. The low-pressure dose was 2.00 cm3/g (STP) with a 15-s equilibration interval. The isotherm was analyzed using the density functional theory (DFT) formalism and parameters developed for activated carbon slits by Olivier (provided by Micromeritics),19,20 using the Saito Foley adaptation of the HorvathKawazoe formalism21 and using the conventional t-plot method.22 Analogous measurements were made with nitrogen as the adsorbate on a Digisorb system. The vapor-phase hydrocarbon adsorption experiments were conducted at room temperature using a Cahn C-2000 balance coupled to a computer via an ATI-Cahn digital interface. The adsorbates studied were n-hexane, cyclohexane, 2,2-dimethylbutane, and 1,3,5-triisopropylbenzene. The vapor of the adsorbate was delivered from the liquid phase. The relative vapor pressure P/P0 was maintained at ∼0.3 by controlling the temperature of the liquid adsorbate using a cooling circulator. Here, P is the saturation vapor pressure of the adsorbate at the temperature of the adsorbate, and P0 is the saturation vapor pressure of the
J. Phys. Chem. B, Vol. 106, No. 2, 2002 265 adsorbate at the temperature of the zeolite. Prior to the adsorption experiments, the calcined zeolites were degassed at 350 °C in a vacuum of ∼10-3 Torr for 5 h. The adsorption capacities are reported in milliliters of liquid per gram of dry zeolite, assuming that the adsorbed adsorbate has the same density as the bulk liquid. Data for 1,3,5-triisopropylbenzene adsorption were collected after 5 days, whereas data for the adsorptions of other adsorbates were collected after 5 h. A detailed discussion of hydrocarbon adsorption experiments is reported elsewhere.23 3. Results and Discussion 3.1. Template Structure. The successful SDA for generating SSZ-55, shown in Figure 1, arises from the reduction of the precursor nitrile to the corresponding primary amine, which is then quaternized. A few related molecules will also work.18 A key feature of those molecules is that the SDA contains elements of both structural rigidity and conformational flexibility. We have previously shown that very flexible molecules do well in high-silica zeolite synthesis in generating ZSM-5 (MFI topology) and ZSM-12 (MTW), depending on the length of the molecules. Organo-cations with floppy rings up to C8 are particularly able to achieve favorable conformations in MTW, and Davis and co-workers have also shown that piperidine units, linked by flexible methylene bridge units, can also have a high selectivity for this zeolite.24 Knowing this body of work, we have moved increasingly toward rigid polycyclic molecules, with the result that several new zeolites have been found, many organized on a “cage-centered” basis.25 A number of Diels-Alder and cycloaddition-based polycyclic cations have proven quite useful for crystallizing the novel 10-ring SSZ-35, with large, undulating cavities. We searched for a route around this synthesis sink. The template in Figure 1 combines a rigid portion in the substituted phenyl ring with an easily solvated trimethylammonium group. These features might prove key in the drive to the solvation of larger organo-cations.26 The bridging substituent, the cyclopentyl ring, has some conformational flexibility and packs well into void regions in the 6-8 Å range. If the cyclopentyl group is removed, a benzyl derivative of trimethylammonium cation results. This benzyl derivative has proven successful in generating zeolite structures such as MTW. 3.2. XRD Characterization. The XRD data for SSZ-55 were taken on the calcined sample placed in a glass capillary, heated to 350 °C, evacuated, and then sealed. The raw data peaks were fit using the procedure developed by Finger, Cox, and Jephcoat.27 The resulting 2θ peak values were used as input to three commonly used powder diffraction pattern indexing programs: ITO,28 TREOR99,29 and DICVOL.30 All three programs selected the same unit cell. The figures of merit (FOM) for the selected unit cell were M20 ) 19131 and F20 ) 576.32 These high values for FOM compare favorably with those published by Cernik;33 note that higher-symmetry and simpler structures are more likely to give higher FOM values than are lower-symmetry structures. The unit cell is monoclinic and has cell parameters a ) 12.421 Å, b ) 5.080 Å, c ) 12.419 Å, and β ) 117.3° (V ) 696.34 Å3). The only possible space group extinctions are 0k0, k ) 2n. The most likely space group is P21/m. Other possibilities are P21, P2/m, P2, Pm, P1h, and P1. There is a geometrically equivalent orthorhombic unit cell (see Table 2) that has the unit cell parameters a ) 12.956 Å, b ) 21.194 Å, and c ) 5.080 Å (V ) 1394.91 Å3). The experimental parameters obtained show a good correlation with the previosly reported diffraction data for MAPO-36 (ATS).34,35 3.3. Structure Solution with ZEFSAII. The framework structure of SSZ-55 was determined using the Monte Carlo
266 J. Phys. Chem. B, Vol. 106, No. 2, 2002 TABLE 2: Refined Crystallographic Data space group a b c wavelength profile range Rwp Rp
Cmc21 12.953 23(13) Å 21.185 10(36) Å 5.078 398(8) Å 1.199 63 Å 2.035-50° 10.86% 7.98%
method ZEFSAII.7 This is a direct, real-space method that requires only the powder diffraction data, the indexed cell constants, and the symmetry. The basic idea of ZEFSAII is to use Monte Carlo moves to effectively minimize a zeolite figure of merit.5,6 This figure of merit is constructed in a way that takes into account both the observed powder diffraction data and the geometrical features common to all zeolites, including the favored T-T distances and T-T-T angles, where T represents the Si or Al atoms.6 The figure of merit is a function of the cell parameters, the symmetry, and the positions of the unique T atoms. During the computation, we keep fixed the cell parameters, the space group symmetry, and the number of unique T atoms. The only variables are the positions of the T atoms in the unit cell. By definition, the global minimum of the figure of merit should correspond to the structure of the zeolite sample. In general, one starts from a random configuration of T atoms and seeks the minimum of the figure of merit by moving the T atoms suitably. Powerful Monte Carlo methods such as biased Monte Carlo and parallel tempering7,36 significantly enhance the ability of the program to locate the true minimum when there are more than 6 unique T atoms in the framework. This method has been tested on 118 publicly known zeolite structures and has succeeded in all cases. Roughly a dozen new zeolite structures have been solved by the Monte Carlo method. It has also been applied to solve the structure of the layered silicate MCM-47.37 The space groups P21/m, P21, P2/m, P2, Pm, and P1 were examined in the monoclinic cell. All space groups were searched via the simulated annealing approach except the low-symmetry P1, in which the more powerful parallel tempering7 approach was used instead. A range of values of 14-20 T atoms/1000 Å3 was tested for each space group. This range covers the typical densities of almost all zeolites. For each density, 10 independent runs with different initial configurations were carried out to ensure effective sampling. The best structure was found in the space groups P21/m, P21, and P1 (see Figure 3). The number of T atoms in the monoclinic cell was determined to be 12, corresponding to a framework density of 17.22 T atoms/1000 Å3. It turns out that the structure has three pairs of T atoms that are topologically identical. This implies that there is a higher-symmetry setting with only three unique T atoms. Inspection of the T-atom positions shows that the highersymmetry setting is the space group Cmc21 (space group symmetry number 36).38 Simulation runs in the space group Cmc21, indeed, lead to the same structure. The structure solution provides the positions of the Si atoms. The oxygen atoms are easily added to the middle of each pair of adjacent Si atoms. The DLS-76 program was used for an initial refinement of the structure.39,40 The resulting all-atom structure was used as a starting point for further refinement. 3.4. Rietveld Refinement. The Rietveld refinement41 of the SSZ-55 structure was performed in the space group Cmc21. Initial coordinates for the framework atoms were taken from the DLS refinement of the ZEFSAII structure solution. A cosine Fourier series with 10 terms was used as the background function for fitting. The pseudo-Voigt function of Thompson
Wu et al. TABLE 3: Refined Atomic Position Parameters of SSZ-55 atom
x
y
z
Si1 Si2 Si3 O1 O2 O3 O4 O5 O6 O7
0.308 46(27) 0.115 32(28) 0.619 27(27) 0.188 6(4) 0.312 7(4) 0.500 0(0) 0.376 2(6) 0.148 2(6) 0.144 5(4) 0.500 0(0)
0.174 19(17) 0.741 12(19) 0.039 49(18) 0.195 54(26) 0.100 41(24) 0.060 9(4) 0.210 9(4) 0.310 1(4) 0.502 5(5) 0.265 6(4)
0.092 8(15) 0.595 7(14) 0.122 5(14) 0.097 9(21) 0.112 6(24) 0.107 8(27) 0.292 7(12) 0.292 5(12) 0.378 2(17) 0.606 0(30)
Uiso (Å2) multiplicity 0.080 0.080 0.080 0.065 0.065 0.065 0.065 0.065 0.065 0.065
8 8 8 8 8 4 8 8 8 4
et al.42 and the asymmetry correction described by Finger et al.27 were used to model the peak profiles. Atoms of the same element type were constrained to have the same isotropic thermal displacement parameters. Soft distance constraints were used to enforce the Si-O bond distances and the O-O distances about a tetrahedrally bonded Si atom. The average distances were chosen as 1.60 Å for Si-O and 2.60 Å for O-O. The standard deviations were chosen as 0.02 Å for Si-O and 0.01 Å for O-O. Table 3 shows the final atomic parameters obtained from the Rietveld refinement. The simulated XRD pattern closely matches the experimental data, as shown in Figure 2. The final agreement values for the refinement are Rp ) 7.98% and Rwp ) 10.86%. Table 2 provides a summary of the refinement details. The average bond distance, d(Si-O), is 1.60 Å, with a range of 1.54-1.67 Å. The average Si-O-Si angle is 145.3°, with a maximum of 155° and a minimum of 140°. The average O-Si-O angle is 109°, with a maximum of 118° and a minimum of 104.5°. The range of bond length and bond angle values is broader than is typical for all-silicon, single-crystal zeolites, both because of the presence of the boron and because of the imperfect localization of atomic positions by the Rietveld refinement. The structure of SSZ-55 viewed down the c axis (Figure 3) is composed of large pores circumscribed by 12 T atoms. The distance between the centers of the oxygen atoms across the pore is 9.22 Å along the a axis and 9.92 Å along the b axis. Subtracting the excluded volume of the oxygens (2 × 1.35 Å), we find pore dimensions of 6.52 Å along the a axis and 7.22 Å along the b axis. The SSZ-55 topology also contains zigzag ladders of 4-rings and 6-rings. The framework density is 17.22 T atoms/1000 Å3, and the total calculated density is 1.716 g/cm3. This value is somewhat greater than the 16.4 T atoms/1000 Å3 reported for the magnesium aluminophosphate MAPO-36. In the latter group of materials, the description of T atoms might be somewhat idealized in that it is not unusual for water molecules to induce Al or P ions to become five- or six-coordinate. This has been observed, for example, with VIP-5.43 3.5. SEM and TEM. Figure 4 shows the morphology and sizes of the SSZ-55 sample as determined by scanning electron microscopy. The transmission electron micrographs, shown in Figure 5, corroborate the observations of the SEM images. The high-magnification image in Figure 5 shows lattice fringes of approximately 12 Å perpendicular to the 12-ring pore, consistent with the determined value of the lattice constant a. 3.6. Argon Adsorption. SSZ-55 was examined for adsorption behavior after (1) calcination to remove the organo-cation guest, (2) NH4+ ion exchange to remove any residual alkali cations from the synthesis, and (3) recalcination to convert any cationic sites to protons. The samples were typically dried at elevated temperatures before experiments were run. Given the topology of the SSZ-55 structure, the micropore filling is surprisingly high, ranging from 0.15 to 0.18 cm3/g depending on the
SSZ-55: A New High-Silica, Large-Pore Zeolite
J. Phys. Chem. B, Vol. 106, No. 2, 2002 267
Figure 2. Experimental, simulated, and difference profiles for the powder XRD pattern of SSZ-55 (λ ) 1.199 63 Å).
Figure 3. (Top) Projections of the SSZ-55 framework obtained from ZEFSAII and (bottom) projections of the SSZ-55 framework structure obtained from Rietveld refinement. Note that ZEFSAII calculates the Si positions only.
adsorbate and method used. The higher values were found under dynamic pore-filling conditions for argon.4 Using both the t-plot method and DFT formalism, values closer to 0.15 cm3/g were obtained for both nitrogen and argon. It is possible that, under some experimental conditions, when the higher filling values are obtained, we are seeing a denser, more solidlike density packing behavior for argon. Assuming that the framework density and adsorbate uptake are correlated, the value of 0.15 cm3/g provides additional confirmation of the structure, as this value is the same as the micropore volume of SSZ-24, and the framework density of 16.9 T atoms/1000 Å3 for the SSZ-24 framework is close to that of 17.22 T atoms/1000 Å3 for SSZ55. A comparison of MAPO-36 and AlPO-535 also shows that,
for the smaller adsorbates, there is an increased uptake in the ATS structure relative to AFI. These authors also note, in comparing the two materials, that they believe the ATS structure has “side pockets” along the channel. In the next section, we will see that the hydrocarbon uptake values also support a larger capacity for SSZ-55 over SSZ-24, and more experiments are needed in the sorption characterization of this zeolite. Finally, both the DFT and Saito-Foley formalisms yield a pore opening of 7 ( 1 Å, which is in agreement with the determined structure when the excluded volume of the oxygens is taken into account. 3.7. Hydrocarbon Adsorption. SSZ-55 was studied with vapor-phase hydrocarbon adsorption and compared with the following three groups of zeolites that possess one-dimensional channel systems but different pore sizes and shapes: (1) largepore zeolites with one-dimensional straight channels SSZ-24 and SSZ-31; (2) extra-large-pore zeolites with one-dimensional straight channels CIT-5, UTD-1, and VPI-5; and (3) large-pore zeolites with one-dimensional undulating channels SSZ-42 and SSZ-48. In addition, the zeolite NaY containing a threedimensional channel and supercage system was used as a reference. The results from the hydrocarbon adsorption experiments on these zeolites are shown in Table 4. The kinetic diameters of molecules of the adsorbates studied in this work (n-hexane, cyclohexane, 2,2-dimethylbutane, and 1,3,5-triisopropylbenzene) were obtained from reference 44 and are also listed in this table. For SSZ-55, the adsorption capacities are in the range 0.13-0.16 cm3/g for n-hexane, cyclohexane, and 2,2dimethylbutane, whereas a significant drop in adsorption capacity occurs between 2,2-dimethyl-butane and 1,3,5-triisopropylbenzene. The adsorption capacity of SSZ-55 for 1,3,5triisopropylbenzene ranges between those of SSZ-31/NaY/CIT5/UTD-1/VPI-5 and SSZ-24/-42/-48, implying that the pore opening in SSZ-55 consists of either 12- or 14-rings and providing another piece of experimental evidence in support of the determined structure reported in this work. 3.8. SSZ-55 and MAPO-36. Examination of the coordination sequence for SSZ-55 reveals that the framework topology of this material is of the ATS type.45 Materials of the ATS
268 J. Phys. Chem. B, Vol. 106, No. 2, 2002
Figure 4. Scanning electron micrographs of SSZ-55 crystallites taken at two different magnifications (2.5 × 103 and 1.0 × 104).
framework have been found in a variety of metal-substituted aluminophosphates.35 A pure-silica polymorph, however, has not yet been achieved. The overall structural features of SSZ-55 and MAPO-36 are similar. The density of 17.22 T atoms/Å3 for SSZ-55 is slighly larger than that of 16.4 T atoms/Å3 for MAPO-36. This is perhaps to be expected, as the Al-O bond is slightly longer than the Si-O bond, leading to a lower density for the metalsubstituted polymorph. It is useful to compare the finding here of the new, highsilica ATS phase with the previously described MAPO-36 and with subsequent AlPO-36 phases.46 In the latter syntheses, built around aluminophosphate chemistry, there is considerable intrusion of the AFI-type phases in the attempted syntheses. This might be in part because more water is associated with the void regions of these materials, and the organic molecules lend themselves more to pore-filling behavior rather than strong structure-directing activity. The kinetic factors for the reaction conditions play a larger role in the metal-substitued instances.
Wu et al.
Figure 5. Transmission electron micrographs of SSZ-55 crystallite (top) on a holey carbon grid and (bottom) at a magnification of 2.0 × 105 along the c axis. The large parallel channels are visible running diagonally in the figure.
By comparison, we note that, as the reaction conditions were varied in synthesis experiments for high-silica SSZ-55, AFI was never observed as an impurity phase. SSZ-55 can be synthesized over a range of lattice substituion values using boron or aluminum, as well as when there is no substituion at all, i.e., for the pure silica phase. The same quaternary amonium cation (Figure 1) is used in all instances. The structure-directing capacity of these larger, partially hydrophobic cations seems to be much stronger for high-silica zeolite synthesis. In contrast, such cations have been observed to exhibit less structuredirecting capacity in the context of aluminophosphate-based syntheses. 3.9. SSZ-55 and SSZ-24. The reporting of the zeolite SSZ24 almost a decade ago47 was of particular interest in regard to the all-silica nature of the material that was devoid of 5-ring subunits. The entire substructure features a linking of 4- and 6-rings to generate a structure with 12-ring apertures. This
SSZ-55: A New High-Silica, Large-Pore Zeolite
J. Phys. Chem. B, Vol. 106, No. 2, 2002 269
TABLE 4: Adsorption Capacities for SSZ-55 and Some Other Zeolites with Four Different Adsorbates: (I) n-Hexane, (II) Cyclohexane, (III) 2,2-Dimethylbutane, and (IV) 1,3,5-Triisopropylbenzene adsorption capacity (cm3/g) adsorbate
KDa (Å)
SSZ-24
SSZ-31
SSZ-42
SSZ-48
SSZ-55
CIT-5
UTD-1
VPI-5b
NaY
I II III IV
4.4 6.0 6.2 8.5
0.101 0.114 0.128 0.011
0.111 0.091 0.100 0.061
0.205 0.192 0.195 0.015
0.106 0.062 0.041 0.018
0.158 0.128 0.135 0.033
0.092 0.090 0.093 0.041
0.121 0.111 0.119 0.111
0.198 0.156 0.148c 0.117
0.282 0.253 0.251 0.175
a
KD is the kinetic diameter. b Data from ref 50. c Adsorption capacity from dimethylpropane.
inorganic chemistry was previously much more common in structures with considerable aluminum framework substitution. Most high-silica zeolites, SiO2/Al2O3 > 30, tend to be characterized by a majority of the T atoms being present in 5-ring silicate subunits. At the time, there was some speculation as to the role of the SDA or template in the organization required to produce this inorganic structural anomaly. In the SSZ-24 work, the structure was originally produced by the use of an adamantyl derivative for the guest organic component in the synthesis. Now, the discovery of SSZ-55 has produced a zeolite structure with this same inorganic anomalys high silica but no 5-rings. In fact, the topology is quite similar to that of SSZ-24. On the other hand, considering the SDA used (Figure 1), it is difficult to see a relationship to the adamantyl guest, which possesses a higher symmetry. Eventually, it also came to be realized that a variety of less symmetric organic molucules could also be used to crystallize SSZ-2448 when different inorganic reactant contexts were supplied. These microcrystalline zeolite products are formally metastable in air at room temperature, and the factors that determine which nuclei will survive and propogate to a crystal remain a mystery. The net pore filling by the organo-cation SDA is higher in SSZ-55 than in SSZ-24 by about 15%, illustrating the different packing possiblities in SSZ-24 and SSZ-55. This greater packing ability is consistent with the higher hydrocarbon uptake observed for SSZ-55 relative to SSZ-24 (see Table 4). A suggestion has been made49 that the 4-rings in SSZ-55, which comprise part of the structure generating the 12-ring apertures, might be tilted back down the channel to a greater extent than in SSZ-24. This could provide for a larger net micropore region. Although the calculated framework densities for SSZ-24 and SSZ-55 are quite close, it might be useful to obtain experimental densities for each. This is one of the future experiments that will be carried out as we make a better comparison concerning the differences between these two 12-ring, high-silica zeolites. 4. Conclusion The synthesis and characterization of the novel high-silica zeolite SSZ-55 have been reported. The structure of SSZ-55 has been determined, and the material was shown to contain large, one-dimensional pores circumscribed by 12 T atoms. No 5-rings are present in the material. The space group is Cmc21, and Rietveld refinement of the synchrontron X-ray powder data gives refined unit cell parameters of a ) 12.953 23(13) Å, b ) 21.185 10(36) Å, and c ) 5.078 398(8) Å (V ) 1393.45 Å3). Electron diffraction and transmission electron microscopy confirm the unit cell parameters and topology of the structure. All of the bond distances and bond angles are within reasonable ranges for silicate materials. The structure solution is also consistent with the ATS topology previously described for the aluminophosphate-based MAPO-36 molecular sieve. The ease with which the structure was solved by powder diffraction data and the Monte Carlo method ZEFSAII7 is indicative of the power and generality of the approach.
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