Synthesis and characterization of phi-type zeolites LZ-276 and LZ-277: faulted members of the ABC-D6R family of zeolites

Synthesis and characterization of phi-type zeolites LZ-276 and LZ-277: faulted members of the ABC-D6R family of zeolites

Microporous and Mesoporous Materials 30 (1999) 335–346 Synthesis and characterization of phi-type zeolites LZ-276 and LZ-277: faulted members of the ...

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Microporous and Mesoporous Materials 30 (1999) 335–346

Synthesis and characterization of phi-type zeolites LZ-276 and LZ-277: faulted members of the ABC-D6R family of zeolites G.W. Skeels a,1, M. Sears a, C.A. Bateman a,2, N.K. McGuire c,3, E.M. Flanigen a, M. Kumar b, R.M. Kirchner b, * a UOP LLC, Tarrytown Technical Center, Tarrytown, NY 10591, USA b Manhattan College/College of Mt. St. Vincent, Joined Departments of Chemistry and Biochemistry, Bronx, NY 10471, USA c Union Carbide Chemicals and Plastics Co., Inc., Tarrytown, NY, USA Received 9 March 1998; received in revised form 3 October 1998; accepted 27 January 1999 This paper is dedicated to the memory of Gary Skeels.

Abstract A new zeolite, LZ-276, was synthesized in an organic (TEAOH ) system by varying the crystallization temperature in the procedure used by Jacobs and Martens for the synthesis of zeolite phi. LZ-276 (with SiO /Al O =7.8) is more 2 2 3 siliceous than phi. Another silicon-rich zeolite, LZ-277 (SiO /Al O =6.6), was synthesized in a totally inorganic 2 2 3 system. The similar chemical and physical properties of LZ-276 and LZ-277 are compared with those of zeolite phi described by Grose and Flanigen, and others. TEM [100] selected area diffraction patterns of LZ-277 can be indexed ˚ . Twin spots and considerable streaking parallel to {00l} indicate on a hexagonal unit cell with a=13.8 and c=15 A mirror faulting along c. High resolution images on selected crystals of LZ-277 show that the most closely spaced ˚ . The bulk X-ray sample of LZ-276 is less faulted. A close match between mirror faults occur approximately every 18 A the experimental synchrotron X-ray powder diffraction pattern of LZ-276 and one simulated by the DIFFaX program (with faulting probability=10%) indicates that the structures of these materials can be described as a chabazite (CHA) topology with faulting along c, the stacking direction in these ABC double six-ring (D6R) materials. The distribution of interior cages, including new larger cages that result from faulting, is presented. © 1999 Elsevier Science B.V. All rights reserved. Keywords: DIFFaX simulations of faulting in ABC double-six-ring (D6R) materials; Distribution of cha, gme and aft cages; SEM and TEM micrographs; Structure of phi-type zeolites LZ-276 and LZ-277; Synchrotron X-ray powder diffraction

* Corresponding author. E-mail address: [email protected] (R.M. Kirchner) 1 Deceased. 2 Present address: Saint-Gobain Norton, Northboro R&D Center, Northboro, MA 01532-1545, USA. 3 Present address: American Chemical Society, 1155 Sixteenth Street NW, Washington, DC 20036, USA. 1387-1811/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 04 5 - 1

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1. Introduction Zeolite phi, first synthesized by Grose and Flanigen [1], was characterized as a large-pore zeolite because it adsorbed neopentane ˚ ) and perfluorobutylam(kinetic diameter=6.2 A ˚ ). The synthesis procedure used an acidine (10.2 A extracted, calcined chabazite mineral as the silica and alumina source, with tetramethylammonium hydroxide as the organic base. The phi product had an SiO /Al O ratio of 4.6, with a range of 2 2 3 4–7 claimed in the patent. Zeolite phi has a distinctive X-ray powder diffraction pattern, containing broad and sharp peaks. Jacobs and Martens [2] inadvertently made zeolite phi in their attempt to synthesize ZSM-20. Their catalytic data suggested that their product was a large pore material [3]. Li et al. [4] reported synthesizing phi using tetramethylammonium hydroxide and waterglass. Franco et al. [5] synthesized zeolite phi using a gel similar to that of Jacobs and Martens, with the addition of K+. Lobo et al. [6 ] altered the recipes of Jacobs and Martens, and also of Franco et al., in attempts to synthesize zeolite phi. Lillerud et al. [7] synthesized phi-type zeolites from inorganic gel systems. They characterized these zeolites as members of the ABC double-six-ring (D6R) structure family that have faulting in the AABBCC stacking sequence of chabazite. The new zeolite LZ-276 was synthesized by modifying the synthesis gel of Jacobs and Martens, and by increasing the crystallization temperature to 125 from 100°C [8]. Subsequently, a zeolite with similar properties [9], LZ-277, was synthesized in a totally inorganic system [10]. The chemical and physical properties of LZ-276 and LZ-277 are presented here and compared to the products described as zeolite phi by others [1–10]. The material from the Grose and Flanigen patent is further characterized. A detailed description of the structure of LZ-276 is presented.

2. Experimental 2.1. LZ-276 — organic synthesis The following reagents were used to prepare the aluminosilicate gel: tetraethylorthosilicate

(98%, Aldrich), sodium aluminate (27.45% H O, 2 Matheson, Coleman & Bell ), tetraethylammonium hydroxide ( TEAOH 40%, Southwestern Analytical ), sodium hydroxide (Fisher) and deionized water. The molar gel composition was as follows: 15.5SiO :Al O :1.2 Na O:7.3 ( TEA) O: 2 2 3 2 2 434 H O. (Jacobs and Martens used the 2 following gel composition for phi: 20.0 SiO : 2 Al O :1.1 Na O:9.3 ( TEA) O:558 H O) 2 3 2 2 2 A silicate solution was prepared by slowly hydrolyzing the tetraethylorthosilicate in the tetraethylammonium hydroxide. This solution was heated to remove the ethanol formed by hydrolysis and then cooled. An aluminate solution was prepared by dissolving sodium hydroxide and sodium aluminate in deionized water. The aluminate solution was added with stirring to the silicate solution. The resulting clear solution was aged for 2 days at ambient temperature. The aged solution was then placed in sealed, polytetrafluoroethylene-lined stainless steel pressure vessels and crystallized without agitation for 1–21 days. One batch was crystallized at 100°C, and another (LZ-276) at 125°C. After crystallization, the samples were filtered, washed with deionized water, and dried in ambient air. Selected product samples were calcined in flowing air to remove the organic material, in preparation for further testing or ammonium exchange. The standard ammonium exchange method was used: a slurry was refluxed for 1 h with five times the stoichiometric quantity of NH Cl. Each sample 4 was ion exchanged three times, then washed well with deionized water until the filtrate tested negative for chloride. The ammonium-exchanged products were dried in ambient air. 2.2. LZ-277 — inorganic synthesis The aluminosilicate gel was prepared from silica sol (Ludox LS-30, DuPont) and a solution of aluminum trihydrate and sodium hydroxide ( Fisher) in deionized water. The molar gel composition for LZ-277 was as follows: 8 SiO :Al O :1.6 Na O:256 H O. 2 2 3 2 2 The aluminate solution was added to the silica sol and the resulting gel was mixed well to homogenize the gel. This gel was crystallized without

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agitation for 47 days at 100°C in polytetrafluoroethylene bottles. The resulting solids were filtered, washed with deionized water and dried in ambient air. Selected products were ammonium exchanged and characterized using similar methods to the organic synthesis above. There was no need to calcine these samples, since no organic materials were used in the synthesis.

Samples were carbon coated to prevent charging under the beam, then examined in a JEOL 2000 FX operated at 200 kV. High resolution micrographs of LZ-277 were recorded at 100 kX to minimize the electron dose to the samples. SEM samples were prepared by placing a small amount of powder on a SEM stub. The samples were gold coated prior to examination in a Cambridge Stereoscan S-250 operated at 20 kV.

3. Characterization

3.3. NMR

The products of the 100°C organic synthesis, the 125°C organic synthesis (LZ-276), and the inorganic synthesis (LZ-277) were characterized by IR spectroscopy, chemical and thermal analysis, McBain–Bakr adsorption, X-ray powder diffraction, TEM, SEM and NMR. In the UOP Tarrytown laboratories, framework-region IR spectra were obtained by FT-IR spectroscopy using KBr pellets, and elemental analysis was determined by ICP.

The 29Si NMR spectra were run on a Bruker MSL-400 high resolution solid state NMR operating at 9.4 T or 400.13 MHz for protons. The 29Si spectra were taken in Bloch decay (single pulse) mode at 79.494 MHz, using a 3.5 ms pulse (60° pulse), a 90 s recycle delay and collecting 8192 data points with a sweep width of 251.6 ppm or 20 kHz. The data were transformed using an apodization of 40 Hz (0.5 ppm). The sample was spun at the magic angle with a rotation rate of 4076 Hz. Spectra were processed using the Bruker program DISMSL running on an ASPECT 3000 minicomputer.

3.1. X-ray diffraction Synchrotron powder X-ray diffraction (PXRD) data were collected for a sample of LZ-276 using the powder diffractometer at beamline X7A of the National Synchrotron Light Source (NSLS ) at Brookhaven National Laboratory. This instrument was equipped with Si [111] and Ge [220] crystal monochromators, a KevexA solid state detector, and two 16 mm slits. The receiving slit measured 16 mm×1.5 mm. The wavelength of the incident radiation, determined using a silicon standard, was ˚ . The sample was contained in a 0.69417(4) A capillary tube, which was oscillated during the diffraction scan. Data were collected using a step size of 0.005°. A counting time of 4 s per step was used in the range from 2H=2.0 to 22.0°, and an 8 s count time was used in the range from 22.005 to 27.0°. 3.2. Electron microscopy Samples were prepared for TEM by embedding the powder in LR White@ resin and microtoming thin sections that were collected on copper grids.

4. Results and discussion The crystallization temperature plays an important role in determining the SiO /Al O ratio of 2 2 3 the final product of the organic synthesis. After 14 days of crystallization, the product of the 100°C organic synthesis had a SiO /Al O ratio of 5.0, 2 2 3 while the product synthesized at 125°C, denoted LZ-276, had a SiO /Al O ratio of 7.8. The product 2 2 3 of the inorganic system, denoted LZ-277, typically had a SiO /Al O ratio of 6.6. Table 1 compares 2 2 3 the chemical compositions of these three products with those of similar materials described in the literature. Franco et al. observed a range of SiO /Al O 2 2 3 in their products, and attributed the formation of the more siliceous products to an increase in the crystallization temperature and a decrease in the K+ concentration of the synthesis gel. No chemical composition was given for the Jacobs and Martens or the Lobo et al. materials. The organic cation/

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Table 1 Chemical composition Organic synthesis product (100°C )

SiO /Al O 2 2 3 Na O/Al O 2 2 3 K O/Al O 2 2 3 ( TMA) O/Al O 2 2 3 ( TEA) O/Al O 2 2 3 M+/Al Oa 2 3

5.0 0.90 – – 0.08 0.98

LZ-276 (125°C )

7.8 0.72 – – 0.26 0.98

LZ-277 (100°C )

6.6 0.97 – – – 0.97

Zeolite phi (100°C )

4.6 0.99 – 0.03 – 1.02

Franco and Perez-Pariente [5] (100°C )

(120°C )

4.1–4.2 0.54–0.60 0.20–0.23 – 0.04 0.81–0.84

5.1–6.3 0.45–0.75 0.04–0.39 – 0.10–0.16 0.86–1.0

Lillerud et al. [7] (100°C )

4.0–4.46 0.68 0.32 – – 1.0

a Where M+ equals the sum R N++Na++K+. 4

alumina ratio for all of the zeolite phi products synthesized at 100°C was lower than that for the products synthesized at 120 or 125°C, indicating that an increase in the SiO /Al O ratio corres2 2 3 ponds to an increase in the organic content of the zeolite. Framework-region IR spectra (not shown) show an asymmetric stretch band at 1027 cm−1 for the 100°C organic synthesis product, 1040 cm−1 for LZ-276, 1045 cm−1 for LZ-277, and 1026 cm−1 for zeolite phi. The higher frequency of the bands for LZ-276 and LZ-277 is indicative of the higher SiO /Al O content in these materials ( Table 1). 2 2 3 Fig. 1 shows the X-ray powder diffraction patterns of the 100°C organic synthesis product, LZ-276, LZ-277, and the zeolite phi synthesized by Grose and Flanigen. The d-spacings of all of these materials are similar, but there are some differences in the relative intensities of the peaks. Each pattern displays both broad and sharp peaks. The X-ray powder diffraction pattern for zeolite phi may contain impurity peaks attributable to erionite. These peaks are marked with an asterisk. By comparison, the X-ray powder diffraction patterns reported by Li et al. and Franco et al. for their zeolite phi materials show much sharper peaks, and there are also differences in their relative intensities. The sharp reflections of these two materials differentiate them from the products of Grose and Flanigen and the materials of this study. Jacobs and Martens do not report any X-ray powder diffraction pattern for their material. Lobo et al., following the procedure of Franco et al., produced a material containing d-spacings not

present in either LZ-276, LZ-277, zeolite phi of Grose and Flanigen, or the zeolite phi synthesized by Franco et al. The diffraction patterns obtained by Lillerud et al. are similar to those in Fig. 1, but there are large differences in the peak intensities. Table 2 compares the adsorption capacities of LZ-276 and LZ-277 with that of zeolite phi and the Li et al. materials. Jacobs and Martens, and Lillerud et al. did not report adsorption capacities. LZ-276 adsorbed n-butane, but no significant amount of isobutane, and no neopentane. This indicates that LZ-276 is a small- to medium-pore zeolite. In contrast, the patent for zeolite phi claims adsorption of neopentane and perfluorobutylamine, making it a large-pore zeolite. LZ-276 has an O adsorption capacity of 21.0 wt.%, giving a 2 calculated micropore volume of 0.18 cm3 g−1. This is in agreement with Franco et al., who determined a micropore volume of 0.19 cm3 g−1 for their zeolite phi using N adsorption. However, Lobo 2 et al. reported nitrogen adsorption isotherms that show no micropore volume. There was some adsorption at high partial pressures, indicative of a mesoporous material. Lobo et al. followed the zeolite phi synthesis of Franco et al. and produced a material with a SiO /Al O of 5.06. We have 2 2 3 found from thermal stability studies on synthesis products with a SiO /Al O ratio of 5.0 that there 2 2 3 is almost complete crystal collapse at 600°C with negligible O adsorption. Since Lobo et al. calcined 2 their samples at 600°C to remove any organic material, the absence of micropore volume could result from crystal collapse. DTA traces in air (not shown) for the 100°C

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Fig. 1. Comparison of in-house X-ray powder diffraction patterns. The patterns are, from top to bottom, of LZ-277, the 100°C organic synthesis product, LZ-276, and zeolite phi synthesized by Grose and Flanigen. Peaks marked with an asterisk (1) in the zeolite phi pattern might be impurity peaks attributable to erionite. (l=Cu Ka: for all patterns.) Table 2 Adsorption capacities (wt.%)

O 2 O 2 HO 2 n-butane isobutane neopentane

Pressure (Torr)

Temperature (°C )

LZ-276

LZ-277

Zeolite phi

Li et al. [4]

100 750 4.6 700 700 500

−183 −183 25 25 25 25

21.0 – 20.6 6.3 0.2 0.0

21.5 – 23.9 – – –

12.1 18.4 – 8.1a 2.7a 3.5a

– – 13.6 13.2 – –

a Measured at 750 Torr.

organic synthesis product and for LZ-276 show two distinct thermal events. An endothermic peak between 25–300°C corresponds to the loss of water. An exothermic peak between 350–450°C corresponds to loss of the organic. The exothermic peak is centered at about 400°C for the 100°C organic synthesis product and centered at about 425°C for LZ-276. This is in close agreement with Franco et al., who reported an exothermic peak centered at about 425°C which was attributed to the combustion of the TEA+ cation. LZ-276 shows two additional small exothermic peaks at about 500 and about 580°C.

The scanning electron micrographs (SEMs) of the 100°C organic synthesis product, LZ-276, and LZ-277 are shown in Fig. 2. The 100°C organic synthesis product has intersecting disc-shaped particles with an average particle size of 5 mm, while LZ-276 has disc-shaped particles with substantially more intergrowths and an average particle size of 4–5 mm. The SEM of LZ-277 shows a range of crystal morphologies and sizes. The smaller crystallites resemble those of the 100°C organic synthesis product, while the very large crystallites approach the morphology of the LZ-276. Crystallite sizes range from 1 to 7 mm in LZ-277. This variation in

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crystallite size can be attributed to the effects of the long crystallization time on the rate of nucleation and crystal growth. The SEMs reported by Jacobs and Martens are similar to those for LZ-276, but the crystallites are spherical. All of the SEMs show the presence of only a single phase, with no evidence of amorphous debris. The SEMs for the product of Lobo et al., synthesized using the Jacobs and Martens procedure, showed a material with a phi-type crystal morphology along with a second dissimilar material with a spherical morphology. The SEMs for the product of Lobo et al., synthesized using the Franco et al. procedure, show a third morphology. This supports the conclusion of Lobo et al. that their products are physical mixtures of chabazite, offretite and phillipsite. (a)

5. Structure

(b)

The SEMs in Fig. 2 show the presence of a single phase. Thermal treatments show that, as crystallinity is lost with increasing temperature, all peaks in the pattern disappear simultaneously. The 29Si MAS NMR spectra of LZ-277 has peaks at −109.3, −103.6, −98.0, and −93.0 ppm, assignable to Si(0Al ), Si(1Al ), Si(2Al ) and Si(3Al ), respectively. These peaks are sharp, consistent with LZ-277 being a single phase. All evidence indicates zeolites LZ-276 and LZ-277 are single phase materials. The organic-containing LZ-276 was sensitive to the TEM beam and went amorphous before images could be recorded. However, the inorganic LZ-277 gave high resolution TEM images. The TEM [100] selected area diffraction patterns (SADPs) of LZ-277 can be indexed on a hexagonal unit cell ˚ . The (001) spacing is with a=13.8 and c=15 A ˚ . Twin spots deduced from the (003) spacing at 5 A and considerable streaking parallel to {00l} indi-

Fig. 2. Scanning electron micrographs showing morphology obtained for various synthesis conditions. Each figure has a different scale. (a) The 100°C organic synthesis product after 14 days at 100°C. (b) The LZ-276 product after 14 days at 125°C. (c) The LZ-277 product after 54 days at 100°C. (c)

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Fig. 3. The TEM [100] selected area diffraction patterns (SADPs) of LZ-277. Twin spots and considerable streaking parallel to {00l} indicate faulting along c due to mirror planes.

cate mirror-plane faulting along c ( Fig. 3). Several high-resolution images of [100] show closely spaced mirror-planes with an average distance ˚ (Fig. 4). These between faults of about 18 A dimensions are consistent with those obtained from indexing [11] all lines in the synchrotron X-ray powder diffraction pattern using a hexagonal cell ˚ . However, the with a=13.697 and c=18.567 A figures of merit for the PXRD indexing are very poor. The discrepancy in the lengths of the c axis indicates there is no well-defined repeat along c in this material. The broad peaks in the powder X-ray diffraction pattern of LZ-276 suggests the structure is disordered. Attempts to model the disorder using Reitveld techniques were unsatisfactory. However, the PXRD pattern of LZ-276 was successfully simulated using the DIFFaX technique [12]. An all-silica model was presumed using idealized atom positions and cell coordinates taken from the structure of chabazite [13]. Cell parameter a was determined from indexing the sharp peaks in the PXRD pattern, while c is an average value for similar chabazite-type (CHA) structures. A distance least-squares (DLS ) refinement [14] of this

model in space group R3: m with cell parameters ˚ produced idealized a=13.697 and c=15.10 A ˚ and Si– atomic coordinates, with Si–O=1.628 A O–Si=145°. These atomic coordinates were used in the DIFFaX program to simulate the powder pattern of LZ-276. Two types of layers were defined from the idealized atom coordinates. One layer type is ‘stack up’ (AB¬BC¬CA) and the other is ‘stack down’ (CB¬BA¬AC ). A, B and C refer to the six-ring layer types possible in the ABC six-ring family of materials. Both layer types, ˚ thick, consist of two single each approximately 5 A six-ring layers joined by appropriate oxygen bridges. The best simulated powder pattern calculated by the DIFFaX program using this model for LZ-276 was obtained with a faulting probability of 0.10 (10% random faulting). The simulated pattern compares well with the experimental synchrotron X-ray powder pattern of as-synthesized LZ-276 ( Fig. 5), except for some differences in peak intensity. Particle size effects, aluminum atoms in the framework, water molecules and sodium or organic cations in the cavities were not included in the simulation and are probably responsible for the differences between the simu-

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Fig. 4. High resolution images of [010] in LZ-277. The ‘herringbone pattern’ results from faulting. There is a range of distances observed between various fault planes. In regions exhibiting frequent faulting, the fault planes are observed about every 1.5 to 2 nm ˚ ). (the average value, 1.8 nm=18 A

lated and experimental powder pattern of the as-synthesized material. The in-house X-ray powder patterns ( Fig. 2) of the 100°C organic synthesis product, LZ-276, LZ-277, and zeolite phi resemble each other, and therefore these phi-type materials are expected to have similar probabilities for random faulting in the stacking sequence. Estimating the probability of faulting from TEM SADPs provides additional insight. There is a range of distances between the various fault planes observed in typical SADPs of LZ-277 (Fig. 4). Presuming the sub-structure in ˚ repeat, and estimating D6R materials has a 5 A

from regions of frequent faulting an average dis˚ , indicates a faulting tance between faults of 18 A probability of 0.28. The faulting probabilities determined from SADPs are not directly comparable with those determined from DIFFaX calculations. SADPs reveal local details (e.g. type of faulting) well, but because the amount of material examined is so small, global parameters (e.g. degree of faulting) are not necessarily well revealed. Sample selection is important. The X-ray diffraction pattern represents a bulk sample of LZ-276, while larger ‘good looking’ single crystals were chosen for the SADPs of LZ-277. In conclu-

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Fig. 5. Experimental and simulated powder diffraction patterns for LZ-276. (a) Experimental synchrotron X-ray powder diffraction pattern of as-synthesized LZ-276 plotted using the wavelength for Cu Ka: . The background (estimated by handpicked points) has been subtracted from the experimental pattern. (b) Simulated X-ray powder diffraction pattern (l= Cu Ka: ) using the DIFFaX program presuming 10% random faulting probability, an idealized SiO framework topology, and 2 Lorentzian instrumental broadening with 0.1° full width at halfmaximum peaks. No water, nor sodium or organic cations were included in the simulation.

sion, the faulting probability can be as high as 30% in highly faulted regions of individual crystallites, but the bulk samples have an average faulting probability of about 10%. The structures of LZ-276 and LZ-277 can be described as faulted chabazite [7,15]. The faulting in the stacking sequence of chabazite (AABBCC ) can be described by mirror planes in the double six-ring layers that are perpendicular to the hexagonal c axis (the stacking direction). The occurrence of a single fault between an AA, BB, or CC layer

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affects the topology by converting some cha cages into gme cages in every instance. In addition, aft [16 ] and larger cages may also form, depending upon the sequence of layers that follow. Fig. 6 illustrates how a fault in the stacking sequence after a CC D6R layer generates gme and aft cages producing a local AFX topology in the faulted structure, as found in the structure of SAPO-56 [17,18]. Fig. 7 illustrates how an additional fault after an AA (or BB) layer produces a localized aft topology, as found in the structure of AlPO-52 [16 ]. It is possible to form cages larger than an aft cage from two or more (n) successive stacking faults, but the probability [equal to (1−n)2 (0.10)n in LZ-276 ] [15] of forming larger cages rapidly diminishes as n increases. An alternative description is to consider the structures of LZ-276 and LZ-277 to be intergrowths of CHA and GME framework topologies. This intergrowth description is less informative because it suggests that only regions of CHA and GME topologies are present in the crystallites. It does not readily suggest the probability of obtaining cages larger than cha or gme cages, nor does it readily suggest the crystallites have regions with a variety of D6R sequences. The relative distribution (both number and type) of cages obtained for a given faulting probability can be determined by having the DIFFaX program list a typical sequence of layers presuming some probability of random faulting in the stacking sequence. In the case of LZ-276, 5000 layers were generated presuming a 10% random faulting probability. The sequence of calculated layers must be analyzed for the types of cages present in adjacent cage columns (the ‘asymmetric unit’) along c. The sequence of D6Rs producing various types of cages in adjacent columns is well known from the structures of chabazite, gmelinite, SAPO-56 and AlPO-52. Larger cages can be predicted from model building. The distribution of cages in LZ-276 is given in Fig. 8. Large cages completely encapsulated within the crystallite are of little importance in catalysis or adsorption. Of more interest is the type of pore openings on the surface of the crystallites. These allow access to the interior voids within the crystal-

Fig. 6. Stereo illustration of a stacking fault. A stacking fault produces a change of sequence of D6R layers during crystal growth along c. The fault can be represented by a mirror plane (shown as - - -) perpendicular to c. The c axis is vertical in this figure. Read this figure from bottom to top (the direction of crystal growth along c) to get proper mirror symmetry about fault planes. The local topology produced by the single fault in this illustration is found in the SAPO-56 structure.

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Fig. 7. Illustration of successive stacking faults. A stacking fault produces a change of sequence of D6R layers during crystal growth along c. The fault can be represented by a mirror plane (shown as - - -) perpendicular to c. The c axis is vertical in this figure. Read this figure from bottom to top (the direction of crystal growth along c) to get proper mirror symmetry about fault planes. The results are identical for an initial stacking fault after layer BB. The local topology produced by the two faults in this illustration is found in the AlPO-52 structure.

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lites. At present the exact nature of the surface of crystallites is unknown. Presuming a very simple model where the surface of a crystallite resembles the wire-frame models used to illustrate the structure of the material, it is possible to estimate the distribution of the pores and the size of their openings on the surfaces of the material. The largest pore opening in the structure of a complete cha, gme, aft or larger cage is an eight-membered ring. However, on the surface of the crystallite normal to the c axis in chabazite, there are 12-ring openings that result from incomplete cha cages present on the surface. In chabazite these uncapped cha cages form shallow cavities on the surfaces of the crystal. However, in SAPO-56, AlPO-52, LZ-276 and LZ-277, the 12-ring pores that result from an incomplete capping of cages along c can allow access to deeper interior cavities (incomplete aft and larger cages)! For LZ-276, the percentage of deeper cavities accessible from the crystal face normal to c due to faulting is estimated to be not more than 8.7% (Fig. 8). This is consistent with the observation of negligible neopentane absorption in LZ-276 ( Table 2). Variations in amounts of faulting and crystal morphology (particularly the relative surface areas of faces perpendicular to c versus a and b) may affect sorption properties as the number of larger cavities accessible on the surfaces of the crystallites changes. Also, as particle size decreases, the amount of non-specific non-microporous sorption may increase as the ratio of exterior crystallite surface area to interior crystallite volume increases.

6. Conclusions

Fig. 8. Distribution of cages in LZ-276. The relative distribution of types of cages present in LZ-276 presuming a 10% random faulting probability between D6R layers.

The 100°C organic synthesis product, LZ-276, and LZ-277 show sorption properties characteristic of small- to medium-pore zeolites. The X-ray powder diffraction patterns of LZ-276, LZ-277, and the zeolite phi synthesized by Grose and Flanigen are similar, but the Grose and Flanigen material shows large-pore sorption properties. Jacobs and Martens probably synthesized the phi of Grose and Flanigen, but there is insufficient independent characterization for conclusive proof.

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The phi synthesized by Franco et al. using the Jacobs and Martens recipe has adsorption data that agrees with LZ-276 and LZ-277, but the Franco product shows relatively sharp X-ray powder diffraction peaks, atypical of phi-type zeolite materials. The product of Lillerud et al. appears to be a phi-type zeolite material. However, the Lobo et al. products are mixtures, not phitype zeolite materials. Phi-type zeolite materials can be synthesized under a broad range of synthesis conditions and gel compositions, in organic or inorganic systems. Products have a broad range of SiO /Al O ratios. 2 2 3 The synthesis temperature affects product morphology and compositions, as well as the sorption properties. Each phi-type zeolite material reported can have similar X-ray diffraction patterns but significantly different physical properties. LZ-276 and LZ-277 are best described as faulted ABC-D6R structure type materials, where the probability of random faulting in the stacking sequences of D6Rs shows a range of values. In LZ-277 some selected single crystals showed an approximately 28% probability of faulting, while the bulk sample of LZ-276 has on average 10% faulting. The faulting can be described by mirror planes in the double six-ring layers normal to the hexagonal c axis (the stacking direction). The occurrence of a single fault between an AA, BB, or CC layer affects the topology by converting some cha cages into gme cages in every instance. This fault may also form aft or larger cages, depending upon the sequence of layers and faults that follow. The faulting allows 12-ring openings on the crystal face perpendicular to c to provide access to deeper cavities within the crystal. In LZ-276 less than approximately 8.7% of these 12-ring openings could provide access to incomplete aft or larger cavities. Variations in amount of faulting, crystal morphology, and particle size can produce differing sorption properties. Phi-type zeolite materials have similar structures and can be considered to be members of the same faulted chabazite-gmelinite group of the ABC-D6R family. They presumably have varying degrees of faulting.

Acknowledgements Thanks are expressed to M.M.J. Treacy (NEC ), Robert W. Broach ( UOP), R. Lyle Patton ( UOP), and J. Ple´vert (MC ) for valuable discussion, to C.S. Blackwell ( UOP) for NMR spectra, to C. Angell ( Union Carbide) for FT-IR spectra, and to D.E. Cox (BNL) for assistance in the collection of the data at beamline X7-A at the National Synchrotron Light Source (Brookhaven National Laboratory), which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences.

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