Six Single-Crystal Structures Showing the Dehydration, Deamination

Ghyung Hwa Kim and Heung Soo Lee. Pohang Accelerator Laboratory, POSTECH, Pohang 790-784, Korea. Karl Seff*. Department of Chemistry, UniVersity of ...
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18294

J. Phys. Chem. C 2007, 111, 18294-18306

Six Single-Crystal Structures Showing the Dehydration, Deamination, Dealumination, and Decomposition of NH4+-Exchanged Zeolite Y (FAU) with Increasing Evacuation Temperature. Identification of a Lewis Acid Site Woo Taik Lim* and Sung Man Seo Department of Applied Chemistry, Andong National UniVersity, Andong 760-749, Korea

Ghyung Hwa Kim and Heung Soo Lee Pohang Accelerator Laboratory, POSTECH, Pohang 790-784, Korea

Karl Seff* Department of Chemistry, UniVersity of Hawaii at Manoa, Honolulu, Hawaii 96822-2275 ReceiVed: June 1, 2007; In Final Form: September 18, 2007

Single crystals of zeolite Y, |Na71|[Si121Al71O384]-FAU, with diameters up to 0.20 mm were grown. They were ion exchanged to generate |(NH4)64Na5K2|[Si121Al71O384]-FAU. Crystals 1 through 6, respectively, were vacuum dehydrated at 323, 373, 423, 473, 523, and 573 K, and without reexposure to the atmosphere, their structures were determined crystallographically at 294 K using synchrotron X-radiation. Their compositions (best integers) are seen to be respectively |(NH4)59H5Na5K2(H2O)3|[Si121Al71O384]-FAU (incomplete dehydration and partial deamination), |(NH4)31H33Na5K2|[Si121Al71O384]-FAU (further deamination), |(NH4)2H62Na5K2|[Si121Al71O384]-FAU (nearly complete deamination), |H61Na5K2(Al2O)0.5|[Si122Al70O384]-FAU (onset of dealumination and framework reconstruction), |H53Na5K2(Al2O)2|[Si124Al68O384]-FAU (further dealumination and reconstruction), and |H51Na5K2(Al2O)2.5|[Si124Al68O384]-FAU. The extent of deamination of NH4+ increases with increasing evacuation temperature until it is complete in crystal 4. In crystal 1, 59 NH4+ ions per unit cell are found at two crystallographically distinct positions: 30 at site I′ (in sodalite cavities opposite D6Rs) and 29 at site II (in supercages opposite S6Rs). Three water molecules coordinate to Na+ ions. In crystal 2, 17 NH4+ ions per unit cell are found at site I′ and 13 at site II. In crystal 3, only 2 NH4+ ions at site II are found. The extent of dealumination of the zeolite framework, seen in crystals 4, 5, and 6, also increases with temperature: 1, 4, and 5 Al3+ ions per unit cell, respectively, are found at site I′ recessed slightly into D6Rs. Each Al3+ ion coordinates trigonally (O-Al-O ) 119.3°) at ca. 1.85 Å to three framework oxygens. Pairs of these Al3+ ions are each bridged by a nonframework oxide ion at the center of a D6R to give linear Al-O-Al groups (Al-O ) 1.62 Å). These Al3+ ions are Lewis-acid sites and should be catalytically active when accessible. Five additional crystals vacuum dehydrated at 623 K and above showed no crystallinity, due presumably to further dealumination and dehydration (loss of H+ and framework oxygen).

1. Introduction Zeolite Y is a synthetic faujasite (FAU) with 1.5 e Si/Al e 3.0. It was first synthesized in its sodium form in 1964 by Union Carbide.1 Intended for use as sorbent, it eventually found use as the heterogeneous catalyst for the fluidized catalytic cracking (FCC) of hydrocarbons. Zeolite Y has a wide range of industrial applications as an ion exchanger, a sorption agent, a molecular sieve, and a catalyst due primarily to its excellent structural stability, large and accessible pore volume, high activity, high resistance to nitrogen compounds, and high regenerability.2 Especially important industrial processes in which zeolite Y in its hydrogen form (H-Y) is used as a catalyst are isomerization and hydrocracking, one of the most important processes for producing valuable petroleum products from crude oil. The single-crystal structures of several NH4+-exchanged A (LTA) and X (FAU with Si/Al < 1.5) zeolites have been reported.3-6 McCusker et al. investigated fully dehydrated, fully * To whom correspondence should be addressed.

NH4+-exchanged zeolite A by single-crystal X-ray diffraction methods and reported the positions of the NH4+ ions.3 Their single crystal was prepared by the flow method using 1.0 M NH4NO3 for 72 h at 298 K, followed by evacuation at 298 K and 10-5 Torr; this resulted in the complete dehydration of NH4-A with no NH4+ decomposition. Lee et al. studied the structures of NH4+, hydrolyzed-Cu2+ forms of zeolite A to find the Cu2+ positions.4 (They had added ammonia to their ionexchange solutions to avoid crystal damage from the protons arising from the hydrolysis of Cu2+.) Patalinghug et al. reported a structure of Ni2+- and NH4+-exchanged zeolite A.5 Zhen et al. prepared anhydrous NH4+-exchanged zeolite X, allowed it to react with HgCl2 vapor at 388 K, and determined the resulting structure.6 H-Y has both Bronsted and Lewis acid sites. The former appear easily from the deamination (loss of NH3) of NH4+ ions in the NH4+-exchanged zeolite; the latter are provided by aluminum ions that leave the zeolite framework (dealumination) by steaming at higher temperatures. Because of its vast

10.1021/jp0742721 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/13/2007

Six Single-Crystal Structures of Zeolite Y

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18295

Figure 1. SEM images (a) magnified 50×, (b) magnified 350×, (c) intentionally broken to expose a fresh surface for EDS analysis, and (d) EDS spectrum of the large single crystals of zeolite Y used in this work.

importance as a catalyst, very much work has been done over the years to learn the nature and strength of the acid sites in H-Y zeolites,7 the effect of framework Al content on H-Y acidity and cracking activity,8 the coordination of aluminum and silicon in H-Y zeolites,9-12 the influence of coke formation on the conversion of hydrocarbons on H-Y zeolites,13 the role of H-Y mesopores in the hydrocracking of heavy oils,14 the best conditions for the dealumination of NH4-Y,15 and its catalytic activity for various reactions.16-22 So far, however, a detailed structural study of the deamination and dealumination processes has not been reported. This work was done to observe crystallographically the dehydration, deamination, and dealumination processes in NH4+exchanged zeolite Y without steaming as a function of evacuation (vacuum calcination) temperature. The positions of the NH4+ and nonframework Al3+ ions would be seen. Future work to determine the positions of sorbate molecules in dehydrated (activated) H-Y might reveal the structures of reaction intermediates. 2. Experimental Section 2.1. Synthesis of Large Single Crystals of Zeolite Y (FAU). Colorless single crystals of sodium zeolite Y, stoichiometry Na71Si121Al71O384, with diameters up to 0.20 mm were synthesized from gels of composition 3.58SiO2:2.08NaAlO2:7.59NaOH: 455H2O:5.06TEA:1.23TCl.23 A starting gel was prepared from fumed silica (99.8%, Sigma), sodium aluminate (technical, Wako), sodium hydroxide (96%, Wako), triethanolamine (TEA, 99+%, Acros), bis(2-hydroxyethyl)dimethylammonium chloride (TCl, 99%, Acros), and distilled water. First, a silica slurry was prepared by placing 0.58 g of fumed silica in 10 g of distilled water in a 30-mL PTFE beaker. A suspension was prepared by shaking in an orbital shaker at 200 rpm for 10 min. In a 250-mL PTFE beaker, 11.51 g of sodium

hydroxide was dissolved in 170.4 g of distilled water, and 6.47 g of sodium aluminate was added. The resulting solution was filtered through a 0.2-µm membrane filter (PTFE syringe, Whatman). After adding 15.19 g of TEA and 4.19 g of TCl to the filtered sodium aluminate solution, it was filtered two times through 0.2-µm membrane filters. These filtering steps were done to minimize the number of particles that might seed crystallization; if there were too many, the resulting crystals would be too small. Finally, the latter solution was added to the former slowly; the mixture was a very viscous gel. These steps were all done at 294 K. This gel was put in a 30-mL PTFE bottle which was placed in a convection oven at 368 K for 18 days. The product was filtered, washed with distilled water 10 times, and dried at 323 K for 2 days. The product was characterized by optical microscopy, powder XRD, SEM-EDS, and ICP/MS analysis. SEM and powder XRD showed that the octahedral products were large faujasite-type single crystals with diameters up to 0.15-0.25 mm and that the polycrystalline spherical impurities were gismondine (see Figure 1, panels a and b). Microscopic examination showed that the single crystals were transparent and colorless. Ten of these were intentionally broken to expose fresh surfaces and were attached to carbon-attach tape for analysis (see Figure 1c). SEM images were taken using a JSM-6300 scanning electron microscope, and atomic concentrations were measured with an energy dispersive X-ray spectrometer (EDS), both at 294 K and 1 × 10-7 Torr (see Figure 1d). Each crystal was analyzed three times; the resulting Si/Al ratio was 1.69(3). This was confirmed crystallographically using a fully dehydrated, fully Tl+exchanged single crystal;24 the number of Tl+ ions per unit cell was 71.1(5) which corresponds to Si/Al ) 1.70(1). Tl+ was selected for this experiment because of its large scattering factor for X-rays, its ease of quantitative ion exchange into zeolites, and its easy distinguishability from Na+.

18296 J. Phys. Chem. C, Vol. 111, No. 49, 2007

Lim et al.

TABLE 1: Summary of Experimental and Crystallographic Data crystal 1 crystal cross-section (mm) ion exchange T (K) ion exchange for K+ (day, mL) ion exchange for NH4+ (day, mL) dehydration T (K) crystal color data collection T (K) space group, Z X-ray source wavelength (Å) unit cell constant, a (Å) 2θ range in data collection (deg) total reflections no. of unique reflections, m no. of reflections with Fo > 4σ(Fo) no. of variables, s data/parameter ratio, m/s weighting parameters, a/b initial error indices R1/R2 (Fo > 4σ(Fo))a final error indices R1/R2 (Fo > 4σ(Fo))a R1/R2 (all intensities)b goodness-of-fitc

crystal 2

crystal 3

crystal 4

crystal 5

crystal 6

0.18 0.20 0.19 0.20 353 353 353 353 4, 300 4, 300 4, 300 4, 300 4, 300 4, 300 4, 300 4, 300 323 373 423 473 pale yellow pale yellow pale yellow pale yellow 294(1) 294(1) 294(1) 294(1) Fd3hm, 1 Fd3hm, 1 Fd3hm, 1 Fd3hm, 1 Pohang Light Source, Beamline 4A MXW (PLS, 4A MXW BL) 0.76999 0.76999 0.76999 0.76999 24.9562(4) 24.9010(3) 24.8920(3) 24.9037(4) 60.55 60.57 60.60 60.57 87,660 90,279 86,729 88,821 922 912 913 911 835 745 786 808 57 53 49 49 16.2 17.2 18.6 18.6 0.068/139.2 0.068/80.3 0.053/115.6 0.055/97.9

0.20 353 4, 300 4, 300 523 yellow 294(1) Fd3hm, 1

0.19 353 4, 300 4, 300 573 dark yellow 294(1) Fd3hm, 1

0.76999 24.8859(5) 60.61 88,024 909 754 51 17.8 0.065/84.2

0.76999 24.8622(8) 60.52 101,305 881 644 51 17.3 0.079/70.7

0.134/0.506

0.089/0.382

0.076/0.389

0.074/0.399

0.074/0.378

0.081/0.369

0.041/0.153 0.044/0.153 1.23

0.038/0.134 0.046/0.135 1.13

0.042/0.136 0.046/0.137 1.21

0.040/0.135 0.044/0.136 1.21

0.041/0.145 0.048/0.145 1.20

0.046/0.158 0.061/0.161 1.13

a R1 ) Σ|Fo - |Fc||/ΣFo and R2 ) [Σw(Fo2 - Fc2)2/Σw(Fo2)2]1/2; R1 and R2 are calculated using only the 835, 745, 786, 808, 754, and 644 reflections for which Fo > 4σ(Fo). b R1 and R2 are calculated using all unique reflections measured. c Goodness-of-fit ) (Σw(Fo2 - Fc2)2/(m - s))1/2, where m and s are the number of unique reflections and variables, respectively.

The crystal structure of another of these single crystals, of fully dehydrated Na71Si121Al71O384, was also studied.25 Because of the low scattering power and occupancy parameters for Na+, some parameters could not be refined freely. 2.2. Ion-Exchange of Zeolite Y (FAU). Because the complete NH4+-exchange of Na-Y is difficult to achieve, complete K+-exchange was done first, to be followed by NH4+exchange of K-Y. Crystals of hydrated |K71|[Si121Al71O384]FAU (or K71-Y) were prepared by static ion-exchange of |Na71|[Si121Al71O384]-FAU (or Na71-Y) with aqueous 0.05 M KNO3 (Aldrich, 99.999%, 10.2 ppm Na, 0.6 ppm B, 0.2 ppm Ca), pH ) 5.9.26 This was done by mixing 20 mg of hydrated Na-Y with 15 mL of 0.05 M KNO3, a 6-fold excess. The mixture was then stirred on an orbital shaker for 2 h at 348 K. This was repeated 20 times with fresh KNO3 solution. The product was then filtered and oven-dried at 323 K for 1 day. To prepare NH4-Y from K-Y, these crystals were treated with 15 mL of 0.1 M NH4C2H3O2, a 15-fold excess, pH ) 6.9 (Aldrich, 99.999% NH4C2H3O2, 4.4 ppm Na, 0.7 ppm Li, 0.2 ppm Zn, 0.1 ppm Mg),6 and the mixture was stirred as before. This procedure was repeated 20 times with fresh NH4C2H3O2 solution. The product was oven-dried as before. The crystals remained colorless and transparent throughout. 2.3. Evacuation of Single Crystals. Eleven of these crystals, clear colorless octahedra each about 0.20 mm in cross-section (see Table 1), were lodged in separate fine Pyrex capillaries and were cautiously dehydrated by gradually increasing their temperatures (ca. 25 K/h) under dynamic vacuum to 323, 373, 423, 473, 523, 573, 623, 673, 723, 773, and 823 K, respectively, followed by 2 days at temperature and 1 × 10-6 Torr. While these conditions were maintained, the hot contiguous downstream lengths of the vacuum system, including a sequential 17-cm U-tube of zeolite 5A beads fully activated in situ, were allowed to cool to ambient temperature to prevent the movement of water molecules from more distant parts of the vacuum system to each crystal. Still under vacuum in their capillaries, the crystals were then allowed to cool and were sealed in their capillaries and removed from the vacuum line by torch. The

crystals dehydrated at temperatures up to 573 K were various shades of yellow as described in Table 1. The remaining five crystals, those dehydrated at 623 K and above, were black and had lost their single-crystal diffraction patterns; they appeared to be amorphous. 2.4. X-ray Diffraction Work. X-ray diffraction data for the six single crystals dehydrated at 323-573 K were collected at 294(1) K using an ADSC Quantum210 detector at Beamline 4A MXW at The Pohang Light Source. Crystal evaluation and data collection were done using λ ) 0.76999 Å radiation with a detector-to-crystal distance of 6.0 cm. Preliminary cell constants and an orientation matrix were determined from 36 sets of frames collected at scan intervals of 5° with an exposure time of 1 s per frame. The basic scale file was prepared using the program HKL2000.27 The reflections were successfully indexed by the automated indexing routine of the DENZO program.27 About 90 000 reflections (see Table 1) were harvested for each crystal by collecting 72 sets of frames with 5° scans and an exposure time of 1 s per frame. These highly redundant data sets were corrected for Lorentz and polarization effects; negligible corrections for crystal decay were also applied. The space group Fd3hm was determined by the program XPREP.28 A summary of the experimental and crystallographic data is presented in Table 1. 2.5. Analysis for Na+ and K+. Later in this work, to test the crystallographic indications that Na+ and K+ are present in the crystals studied, 30 single crystals from the original synthesis batch, ion-exchanged as described in section 2.2, were dissolved in a solution of 10 mL of H3PO4 (68.5%, Merck), 3 mL of HCl (37%, Merck), and 0.5 mL of HF (48%, Merck). ICP/MS (Perkin-Elmer, Elan DRC-e) indicated that the Na/K ratio was 2.29. By ratio with the Al content, 3.4 Na+ and 1.5 K+ ions per unit cell are indicated. These measurements are in reasonable agreement with the more accurate crystallographic results. The Na+ and K+ content of these crystals was something we had sought to avoid, and their presence is a surprise. It is due either to incomplete K+ exchange, perhaps due to the presence of the template cation (bis(2-hydroxyethyl)dimethylammonium),

Six Single-Crystal Structures of Zeolite Y

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18297

TABLE 2: Initial Steps of Structure Refinementa occupancyb at step 1c 2 3 4 5 6 7 8 1c 2 3 4 5 6 7 1c 2 3 4 5 6

N(I′)

N(II)

K(I)

Na(I′)

Na(II or II′)

Al

Ow

O(5)

R1

R2

0.1343 0.1021 0.0627 0.0550 0.0447 0.0431 0.0420 0.0414

0.5055 0.2966 0.1907 0.1859 0.1633 0.1572 0.1528 0.1530

0.0889 0.0795 0.0665 0.0522 0.0421 0.0392 0.0382

0.3818 0.2505 0.2185 0.1686 0.1465 0.1366 0.1341

0.0757 0.0548 0.0451 0.0423 0.0418 0.0415

0.3890 0.1675 0.1474 0.1366 0.1377 0.1362

0.7(3)

0.0736 0.0490 0.0425 0.0409 0.0403 0.0402

0.3986 0.1584 0.1466 0.1385 0.1357 0.1347

1.9(5)

0.0742 0.0528 0.0460 0.0448 0.0423 0.0416

0.3782 0.1746 0.1533 0.1469 0.1456 0.1466

2.4(8)

0.0806 0.0600 0.0511 0.0489 0.0471 0.0467

0.3689 0.1988 0.1721 0.1644 0.1597 0.1588

Crystal 1, |(NH4)59H5Na5K2(H2O)3|[Si121Al71O384]-FAU 29.6(13) 28.7(12) 31.3(10) 30.6(10) 30.9(10) 30.7(11)

27.1(13) 32.2(9) 32.5(9) 30.0(8) 29.6(8) 29.0(8) 28.8(9)

1.7(2) 1.6(2) 1.5(2) 1.5(2) 1.5(2)

3.6(4) 3.8(4) 3.6(4) 3.6(4)

1.5(5) 1.5(4) 1.5(5)

2.5(7) 2.7(8)

Crystal 2, |(NH4)31H33Na5K2|[Si121Al71O384]-FAU 17.3(15) 20.0(11) 16.8(9) 17.0(9) 17.6(10)

15.0(7) 14.9(6) 13.2(7) 13.5(7)

1.7(3) 4.3(3) 1.9(2) 1.7(1) 1.7(1) 1.7(1)

3.4(4) 3.8(4) 3.2(4)

1.9(4) 1.8(4)

Crystal 3, |(NH4)2H62Na5K2|[Si121Al71O384]-FAU

1.7(6) 1.8(6)

1c 2 3 4 5 6 1c 2 3 4 5 6 1c 2 3 4 5 6 a

2.2(2) 2.1(1) 2.3(1) 2.3(1) 2.3(1)

3.2(4) 3.5(4) 3.4(4) 3.5(4)

2.3(4) 1.9(4) 1.8(4)

Crystal 4, |H61Na5K2(Al2O)0.5|[Si122Al70O384]-FAUd 2.4(1) 2.3(1) 2.4(1) 2.3(1) 1.99(21)

2.7(4) 3.1(4) 3.1(4) 2.8(4)

1.9(4) 2.0(4) 2.3(4)

1.3(4) 1.4(5)

Crystal 5, |H53Na5K2(Al2O)2|[Si124Al68O384]-FAUd 2.6(2) 2.3(2) 2.4(2) 2.4(1) 1.6(3)

3.3(7) 3.2(7) 3.2(7)

2.1(5) 2.1(5)

4.4(5) 5.1(5) 3.0(5) 3.9(10)

Crystal 6, |H51Na5K2(Al2O)2.5|[Si124Al68O384]-FAUd 3.1(2) 2.5(2) 2.6(2) 2.7(2) 1.6(5)

2.4(6) 2.0(4) 2.0(4)

3.1(6) 3.1(6) +

+

6.0(6) 6.4(6) 3.2(6) 4.9(16) b

Isotropic temperature factors were used for all NH4 and Na positions except in the last step. The occupancy is given as the number of ions per unit cell at each position. c Only the atoms of the zeolite framework were included in the initial structure model. d Some loss of structural (“chemical”) water is possible to give fewer H+ ions and framework oxygen atoms, and perhaps fewer unit cells according to eq 2.

or to exchange of Na+ into the crystals from the ammonium acetate solution. The latter seems more likely because we had previously prepared fully K+-exchanged zeolite Y without difficulty.26 Because it is difficult to remove all Na+ ions in Na-Y by ion exchange, commercial H-Y catalysts often have several Na+ ions per unit cell. See, for example, refs 29 and 30. 3. Structure Determination Full-matrix least-squares refinement using SHELXL9731 was done on Fo2 using all data for each crystal. Each refinement began with the atomic parameters of the framework atoms [(Si,Al), O(1), O(2), O(3), and O(4)] in dehydrated |K71|[Si121Al71O384]-FAU.26 Each initial refinement used anisotropic thermal parameters and converged to the error indices given in Table 1. See Table 2 for the steps of structure determination

and refinement as new atomic positions were found on successive difference Fourier electron-density functions. Because crystal 1 had been ion exchanged with NH4+, it was expected that this cation would predominate; indeed the bond lengths from the predominant cation positions to framework oxygens supported this assignment (ionic radii for NH4+, K+, and Na+ are 1.43, 1.33, and 0.97 Å, respectively32). When a few cations were seen with a much shorter apparent ionic radius, with distances to framework oxygens similar to those seen many times before for Na+, they were identified as Na+, a common impurity cation. When cations were seen at site I, these were identified as K+ because (1) their radii were appropriate, (2) K+ had been introduced earlier to this crystal, and (3) K+ ions prefer site I,26 in contrast to Na+ ions which do not.25,33 In support of these assignments, the most popular sites for 1+ cations, sites I′ and II, are filled in crystal 1, and the refined

18298 J. Phys. Chem. C, Vol. 111, No. 49, 2007 occupancies for all cations provide the correct charge to balance that of the anionic zeolite framework. Toward the end of structure determination, the occupancy at K(I) was seen to be too large in crystals 4, 5, and 6 as compared to those in crystals 1, 2, and 3. Also the thermal parameters at K(I) increased, becoming unlike those in crystals 1, 2, and 3 and unlike those in dehydrated K-Y.26 The introduction of O(5), to share electron density at site I with K(I), with the O(5) occupancy constrained to be half of that at Al, allowed the K+ contents of crystals 4, 5, and 6 (see Table 2) and their thermal parameters (see Table 3) to become normal. The penultimate Fourier functions for crystals 5 and 6 each revealed one additional peak at O(3b), at (-0.0174, 0.0722, 0.0722) and (-0.0166, 0.0733, 0.0733) with heights of 0.57 and 0.61 eÅ-3, respectively. Recognizing that these might be framework oxygens coordinating to the nonframework Al3+ ions, the occupancy at O(3b) was constrained to be three times that at Al, and the occupancy at O(3a) was constrained to be 96 minus that at O(3b). The occupancy at O(3b) refined nicely to 12.9(10) and 16.3(4) for crystals 5 and 6, respectively. Because of its very small extraframework aluminum content, O(3) could not be resolved into O(3a) and O(3b) in crystal 4. The final cycles of refinement were done with anisotropic temperature factors for all positions except for O(3b) and O(5) and with the final refined weighting-scheme parameters (see Table 1). The final error indices R1 and R2 are given in Table 1. The largest peaks on the final difference Fourier function were not included in the final model either because they were too far from framework oxygen atoms to be cations or because their peak heights were negligible. All shifts in the final cycles of refinement were less than 0.1% of their corresponding estimated standard deviations. The final structural parameters are given in Table 3. Selected interatomic distances and angles are given in Table 4. Fixed weights were used initially; the final weights were assigned using the formula w ) 1/[σ2(Fo2) + (aP)2 + bP] where P ) [Max(Fo2,0) + 2Fc2]/3; a and b were refined parameters. Atomic scattering factors for N0, Na+, O-, and (Si,Al)1.82+ were used.34,35 The function describing (Si,Al)1.82+ is the weighted mean of the Si4+, Si0, Al3+, and Al0 functions assuming half formal charges. All scattering factors were modified to account for anomalous dispersion.36,37 Structures 2, 3, 4, and 5 were determined a second time, each using a second crystal prepared like the first. These second determinations were done to check reproducability; only the results for each first crystal are presented in this report except in Table 5. No significant differences were seen between a first and a second crystal structure. 4. Description of the Structures 4.1. Framework Structure. The framework structure of faujasite is characterized by the double 6-ring (D6R, hexagonal prism), the sodalite cavity (a cuboctahedron), and the supercage (see Figure 2). Each unit cell has 8 supercages, 8 sodalite cavities, 16 D6Rs, 16 12-rings, and 32 single 6-rings (S6Rs). The exchangeable cations, which balance the negative charge of the faujasite framework, usually occupy some or all of the sites shown with Roman numerals in Figure 2. The maximum occupancies at the cation sites I, I′, II, II′, III, and III′ in faujasite are 16, 32, 32, 32, 48, and (in Fd3hm) 192, respectively. Further description is available.39-41 4.2. Structure of Crystal 1: Partially Dehydrated NH4+Exchanged Zeolite Y. After vacuum dehydration at 323 K, only about 5 of the 64 NH4+ ions per unit cell had deaminated,

Lim et al. leaving H+ behind. This is inferred by difference: the total number of cations found per unit cell, 66.1(6), is less than 71, the number required to balance the negative charge of the zeolite framework. (It is also possible, but unlikely, that a minor amount of H3O+ exchange had occurred to give H+ upon heating.) Two site-I′ positions, N(I′) and Na(I′), are occupied by 30 NH4+ and 2 Na+ ions, respectively, per unit cell. At two sites II, 29 NH4+ and 3 Na+ ions are found. In this way, each 6-ring in the structure hosts a cation, and sites I′ and II are filled. About two K+ ions per unit cell are at the centers of the D6Rs, at site I. Altogether, 59 NH4+ ions are distributed among two equipoints, five Na+ ions are also at two equipoints, and two K+ ions occupy one. Three nonframework oxygens lie deeper in the supercage; each associates with a site-II Na+ ion (see Tables 3 and 4). For the NH4+, Na+, and K+ ions at N(I′), Na(I′), and K(I) to fit into the 16 D6Rs per unit cell, the arrangements shown in Figure 3, panels a-c, must be present. Other arrangements with unnecessarily short intercationic distances, such as a D6R with two Na(I′) cations, are presumably avoided. A degree of cation crowding as seen in |K71|[Si121Al71O384]-FAU26 can be seen in Figure 3a: K(I)‚‚‚N(I′) ) 3.380(12) Å. The anisotropic thermal parameters at N(I′) give no indication that the N(I′) position seen in Figure 3a is different from that in Figure 3b, as might be expected due to intercationic repulsion. The 30 N(I′) and 2 Na(I′) ions each bond to three O(3)s at 2.754(8) and 2.309(22) Å, respectively. These distances are equal to the sum of the conventional ionic radii32 of NH4+ and O2-, 1.43 + 1.32 (respectively) ) 2.75 Å and Na+ and O2-, 0.97 + 1.32 (respectively) ) 2.29 Å (see Figure 4a). Each NH4+ ion at N(I′) is 1.62 Å from the (111) plane of the three O(3) framework oxygens to which it is bound (see Table 6). The corresponding distance for each Na(I′) cation is 0.60 Å. (NH4+ always extends further than Na+ from these (111) planes because its effective radius is so much larger.) The 29 NH4+ ions at N(II) and 3 Na+ ions at Na(II) are in the supercage (see Figure 5a). These ions are 1.72 and 0.55 Å, respectively, from the (111) plane of three O(2) framework oxygens to which each is bound (see Table 6). These NH4+ and Na+ ions are 2.814(6) and 2.297(8) Å, respectively, from their nearest neighbors, the O(2) framework oxygens (see Table 4), in close agreement with the sums of the corresponding radii,32 2.75 and 2.29 Å. It may be estimated, assuming that NH4+ is tetrahedral, that N-H is 1.04 Å, and that the N-H‚‚‚O plane contains the 3-fold axis, that the mean H‚‚‚O distance is about 1.77 Å and the mean N-H‚‚‚O angle is about 161 °. However, the NH4+ ions are likely to twist about the 3-fold axis so they can nestle closer to the anionic zeolite, as was seen for H3O+ by neutron diffraction.42 Each Na+ ion at Na(II) coordinates also to one non-framework oxygen, presumably a water molecule, Ow, at 2.55(10) Å to complete a near tetrahedron. This distance is insignificantly longer than the sum of the conventional ionic radii32 of Na+ and O2-, 2.29 Å. Each Ow lies far inside the supercage, 3.09 Å from the (111) plane of the three O(2) framework oxygens to which Na(II) bonds (see Table 6 and Figure 5a). The two K+ ions at K(I) are at the octahedral site I (see Figure 3a). Each coordinates to six O(3) oxygen atoms of a D6R at a distance of 2.844(3) Å, again a little longer than the sum of the conventional ionic radii32 of K+ and O2-, 1.33 + 1.32 (respectively) ) 2.65 Å, as was seen in |K71|[Si121Al71O384]-FAU.26 The distributions of NH4+, Na+, and K+ ions over sites are summarized in Table 5.

Six Single-Crystal Structures of Zeolite Y

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18299

TABLE 3: Positional, Thermal, and Occupancy Parametersa occupancyc atom

Wyckoff cation position site

Si,Al O(1) O(2) O(3) O(4) K(I) Na(I′) Na(II) N(I′) N(II) Ow

192(i) 96(h) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 32(e) 96(g)

Si,Al O(1) O(2) O(3) O(4) K(I) Na(I′) Na(II) N(I′) N(II)

192(i) 96(h) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 32(e)

Si,Al O(1) O(2) O(3) O(4) K(I) Na(I′) Na(II) N(II)

192(i) 96(h) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e)

Si,Al O(1) O(2) O(3) O(4) K(I) Na(I′) Na(II) Al O(5)

192(i) 96(h) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 16(c)

Si,Al O(1) O(2) O(3a) O(3b) O(4) K(I) Na(I′) Na(II’) Al O(5)

192(i) 96(h) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 16(c)

Si,Al O(1) O(2) O(3a) O(3b) O(4) K(I) Na(I′) Na(II′) Al O(5)

192(i) 96(h) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 16(c)

z

U11b

x

y

U22

U33

U23

U13

U12

initial

1256(1) 0 -24(1) 773(1) 708(1) 0 547(20) 2285(7) 782(3) 2556(2) 2874(23)

Crystal 1, |(NH4)59H5Na5K2(H2O)3|[Si121Al71O384]-FAU 361(1) 173(5) 154(4) 138(4) -28(2) 1047(1) 310(11) 334(17) 310(11) -92(9) 1429(1) 304(10) 304(10) 272(16) -29(9) 773(1) 367(19) 319(11) 319(11) 122(14) 1793(1) 274(16) 303(11) 303(11) -152(13) 0 203(70) 203(70) 203(70) 6(48) 547(20) 201(261) 201(261) 201(261) 114(196) 2285(7) 209(86) 209(86) 209(86) 92(68) 782(3) 449(33) 449(33) 449(33) 13(29) 2556(2) 377(28) 377(28) 377(28) 16(22) 2874(24) 438(301) 438(301) 438(301) 220(267)

6(2) -25(14) -29(9) 7(10) -3(10) 6(48) 114(196) 92(68) 13(29) 16(22) 220(267)

-27(3) -92(9) 132(13) 7(10) 3(10) 6(48) 114(196) 92(68) 13(29) 16(22) 220(267)

192 96 96 96 96

I I′ II I′ II II

-539(1) -1047(1) -24(1) -321(1) -711(1) 0 547(20) 2285(7) 782(3) 2556(2) 2874(23)

1253(1) 0 -25(1) 743(1) 736(1) 0 531(6) 2278(12) 780(5) 2565(4)

Crystal 2, |(NH4)31H33Na5K2|[Si121Al71O384]-FAU 362(1) 242(4) 206(4) 196(4) -36(3) 1049(1) 409(11) 416(17) 409(11) -117(9) 1437(1) 390(10) 390(10) 311(15) -35(9) 743(1) 481(19) 431(12) 431(12) 164(14) 1764(1) 332(16) 369(10) 369(10) -168(12) 0 238(55) 238(55) 238(55) 12(38) 531(6) 168(87) 168(87) 168(87) 15(56) 2278(12) 336(174) 336(174) 336(174) 17(130) 780(5) 634(72) 634(72) 634(72) 170(68) 2565(4) 438(52) 438(52) 438(52) 55(45)

7(2) -57(13) -35(9) 29(11) -41(9) 12(38) 15(56) 17(130) 170(68) 55(45)

-35(3) -117(9) 152(13) 29(11) 41(9) 12(38) 15(56) 17(130) 170(68) 55(45)

192 96 96 96 96

I I′ II I′ II

-535(1) -1049(1) -25(1) -314(1) -714(1) 0 531(6) 2278(12) 780(5) 2565(4)

1249(1) 0 -33(1) 709(1) 762(1) 0 527(6) 2259(13) 2586(34)

Crystal 3, |(NH4)2H62Na5K2|[Si121Al71O384]-FAU 363(1) 254(4) 217(4) 210(4) -45(3) 1063(1) 431(12) 446(19) 431(12) -122(10) 1436(1) 407(11) 407(11) 307(16) -25(10) 709(1) 571(22) 380(12) 380(12) 100(14) 1738(1) 333(16) 325(10) 325(10) -120(12) 0 264(45) 264(45) 264(45) 4(31) 527(6) 247(81) 247(81) 247(81) 44(58) 2259(13) 328(176) 328(176) 328(176) 14(131) 2586(34) 366(438) 366(438) 366(438) 446(432)

12(3) -80(15) -25(10) 81(12) -42(9) 4(31) 44(58) 14(131) 446(432)

-35(3) -122(10) 159(14) 81(12) 42(9) 4(31) 44(58) 14(131) 446(432)

192 96 96 96 96

I I’ II II

-532(1) -1063(1) -33(1) -321(1) -706(1) 0 527(6) 2259(13) 2586(34)

1249(1) 0 -34(1) 708(1) 762(1) 0 520(1) 2250(15) 322(27) 0

Crystal 4, |H61Na5K2(Al2O)0.5|[Si122Al70O384]-FAUf 363(1) 261(4) 225(4) 217(4) -42(2) 1066(1) 429(11) 440(17) 429(11) -117(9) 1434(1) 402(10) 402(10) 325(15) -23(9) 708(1) 565(21) 390(11) 390(11) 104(13) 1738(1) 340(15) 336(9) 336(9) -119(11) 0 218(49) 218(51) 218(49) 4(30) 520(1) 325(135) 325(135) 325(135) 110(104) 2250(15) 841(256) 841(256) 841(256) -123(187) 322(27) 1052(499) 1052(499) 1052(499) -284(363) 0 200e

11(2) -85(14) -23(9) 79(11) -44(8) 4(30) 110(104) -123(187) -284(363)

-33(3) -117(9) 150(13) 79(11) 44(8) 4(30) 110(104) -123(187) -284(363)

192 96 96 96 96

I I′ II I′ I

-532(1) -1066(1) -34(1) -326(1) -705(1) 0 520(1) 2250(15) 322(27) 0

1248(1) 0 -33(1) 701(3) 722(15) 766(1) 0 566(32) 2144(31) 375(28) 0

Crystal 5, |H53Na5K2(Al2O)2|[Si124Al68O384]-FAUf 363(1) 314(5) 276(5) 267(4) -44(3) 1065(1) 494(12) 507(20) 494(12) -120(10) 1435(1) 457(12) 457(12) 374(17) -3(11) 701(3) 541(42) 466(16) 466(16) 112(19) 722(15) 200e 1734(1) 392(17) 392(17) 389(10) -120(13) 0 276(69) 276(69) 276(69) 26(44) 566(32) 319(308) 319(308) 319(308) -73(257) 2144(31) 1874(661) 1874(661) 1874(661) 1040(660) 375(28) 800(318) 800(318) 800(318) 389(311) 0 200e

14(3) -91(16) -3(11) 46(25)

-33(3) -120(10) 159(15) 46(25)

192 96 96

-45(10) 26(44) -73(257) 1040(660) 389(311)

45(10) 26(44) -73(257) 1040(660) 389(311)

96

I I′ II I′ I

-531(1) -1065(1) -33(1) -342(2) -207(16) -703(1) 0 566(32) 2144(31) 375(28) 0

1249(1) 0 -33(1) 700(1) 716(9) 768(1) 0 557(20) 2087(25) 379(36) 0

Crystal 6, |H51Na5K2(Al2O)2.5|[Si124Al68O384]-FAUf 363(1) 412(6) 372(6) 362(6) -45(4) 1065(1) 583(15) 626(25) 583(15) -113(13) 1436(1) 543(14) 543(14) 467(21) -10(13) 700(1) 544(40) 588(20) 588(20) 116(22) 716(9) 200e 1732(1) 486(21) 480(13) 480(13) -123(16) 0 511(101) 511(101) 511(101) 4(81) 557(20) 320(319) 320(319) 320(319) -190(237) 2087(25) 1082(398) 1082(398) 1082(398) 653(388) 379(36) 1052(452) 1052(452) 1052(452) 470(448) 0 200e

17(3) -96(19) -10(13) 24(25)

-38(4) -113(13) 168(18) 24(25)

192 96 96

-41(12) 4(81) -190(237) 653(388) 470(448)

41(12) 4(81) -190(237) 653(388) 470(448)

96

I I′ II′ I′ I

-530(1) -1065(1) -33(1) -348(2) -181(10) -701(1) 0 557(20) 2087(25) 379(36) 0

varied

fixedd

1.5(2) 1.5(5) 3.6(4) 30.7(11) 28.8(9) 2.7(8)

2 2 3 30 29 3

1.7(1) 3.2(4) 1.8(4) 17.6(10) 13.5(7)

2 3 2 17 13

2.3(1) 3.5(4) 1.8(4) 1.8(6)

2 3 2 2

1.99(21) 2.8(4) 2.3(4) 1.4(5) 0.7(3)

2 3 2 1 0.5

83.1(10) 12.9(10)

84 12

1.6(3) 2.1(5) 3.0(6) 3.8(10) 1.9(5)

2 2 3 4 2

79.7(4) 16.3(4)

81 15

1.6(4) 3.1(6) 2.3(5) 4.8(14) 2.4(7)

2 3 2 5 2.5

a Positional parameters ×104 and thermal parameters ×104 are given. Numbers in parentheses are the estimated standard deviations in the units of the least significant figure given for the corresponding parameter. b The anisotropic temperature factor is exp[-2π2a-2(U11h2 + U22k2 + U33l2 + 2U23kl + 2U13hl + 2U12hk)]. c Occupancy factors are given as the number of atoms or ions per unit cell. d These integral values were used only in the presentation of this work, to facilitate readability. e To achieve convergence, the isotropic thermal parameters at O(3b) and O(5) were fixed. f Some loss of structural (“chemical”) water is possible to give fewer H+ ions and framework oxygen atoms, and perhaps fewer unit cells according to eq 2.

18300 J. Phys. Chem. C, Vol. 111, No. 49, 2007

Lim et al.

TABLE 4: Selected Interatomic Distances (Å) and Angles (deg)a crystal 1

crystal 2

crystal 3

crystal 4

crystal 5

evacuation T

323 K

373 K

423 K

473 K

523 K

573 K

(Si,Al)-O(1) (Si,Al)-O(2) (Si,Al)-O(3) (Si,Al)-O(3a) (Si,Al)-O(3b) (Si,Al)-O(4) mean (Si,Al)-O K(I)-O(3) K(I)-O(3a) Na(I′)-O(3) Na(I′)-O(3a) Na(II)-O(2) Na(II′)-O(2) Na(II)-Ow N(I′)-O(3) N(II)-O(2) Al-O(3) Al-O(3a) Al-O(3b) Al-O(5) O(1)-(Si,Al)-O(2) O(1)-(Si,Al)-O(3) O(1)-(Si,Al)-O(3a) O(1)-(Si,Al)-O(4) O(2)-(Si,Al)-O(3) O(2)-(Si,Al)-O(3a) O(2)-(Si,Al)-O(4) O(3)-(Si,Al)-O(4) (Si,Al)-O(1)-(Si,Al) (Si,Al)-O(2)-(Si,Al) (Si,Al)-O(3)-(Si,Al) (Si,Al)-O(3a)-(Si,Al) (Si,Al)-O(3b)-(Si,Al) (Si,Al)-O(4)-(Si,Al) O(3)-K(I)-O(3) O(3a)-K(I)-O(3a) O(3)-Na(I′)-O(3) O(3a)-Na(I′)-O(3a) O(2)-Na(II)-O(2) O(2)-Na(II′)-O(2) O(2)-Na(II)-O(5) O(3)-N(I′)-O(3) O(2)-N(II)-O(2) O(3)-Al-O(3) O(3b)-Al-O(3b) O(3)-Al-O(5) O(3b)-Al-O(5)

1.6395(13) 1.6620(12) 1.6757(13)

1.6449(12) 1.6582(11) 1.6771(14)

1.6672(15) 1.6518(12) 1.6805(15)

1.6711(14) 1.6513(11) 1.6787(13)

1.6697(16) 1.6507(12)

1.6702(18) 1.6476(14)

1.6690(26) 1.780(21) 1.6276(11) 1.6580(17)

1.6636(23) 1.810(14) 1.6243(13) 1.6576(16)

2.610(9)

2.607(7)

2.31(6)

2.31(3)

2.234(7)

2.256(17)

2.121(8) 1.89(5) 1.61(12) 110.52(13)

2.130(10) 1.83(3) 1.63(16) 110.54(15)

105.92(23) 109.12(15)

105.28(22) 109.23(18)

108.59(21) 110.87(15) 111.71(23) 136.75(24) 144.87(22)

109.14(21) 110.99(17) 111.52(28) 136.65(28) 144.84(25)

138.0(5) 122.2(24) 147.74(21)

138.7(4) 118.7(15) 148.10(25)

1.6509(12) 1.6570(13) 2.844(3)

1.6400(11) 1.6551(12) 2.732(3)

1.6301(11) 1.6574(13) 2.620(3)

1.6296(10) 1.6577(12) 2.622(3)

2.309(22)

2.233(8)

2.205(9)

2.208(13)

2.297(8)

2.269(12)

2.264(10)

2.265(11)

2.55(10) 2.754(8) 2.814(6)

2.728(14) 2.814(12)

crystal 6

2.87(9) 2.109(7)

111.28(13) 111.10(15)

111.23(12) 109.39(14)

110.57(13) 107.59(16)

1.39(11) 110.46(12) 107.45(14)

110.17(15) 106.90(16)

109.83(14) 106.70(15)

109.07(15) 107.27(16)

108.87(14) 107.50(15)

107.05(16) 110.21(17) 142.45(23) 145.77(22) 141.06(23)

108.89(14) 110.75(16) 141.25(21) 144.41(19) 138.50(22)

110.65(15) 111.65(16) 137.29(24) 144.76(21) 136.26(22)

110.86(14) 111.65(15) 136.67(22) 145.26(20) 136.76(20)

141.51(21) 85.53(11)b

143.96(19) 85.92(11)b

147.05(21) 87.55(12)b

147.25(19) 87.98(11)b

113.5(17)

112.9(6)

110.6(6)

111.1(10)

114.6(5)

115.3(9)

117.0(8)

117.6(9)

87.81(13)b

103.7(7) 89.1(3) 86.75(24)

86.1(6) 85.9(4)

87.64(15)b

105.32(22)

106.1(21)

119.8(5)

117.7(14)

119.3(11)

119.2(15)

84.6(33) 117.6(16) 99.1(3) 95(4)

95(5)

a

The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. b The supplement of this angle is also seen. The trans angles are all 180° by symmetry.

4.3. Structures of Crystals 2 and 3: Fully Dehydrated, Partially Deaminated NH4+-Exchanged Zeolite Y. When crystal 2 was evacuated at 373 K, about 33 of the 64 NH4+ ions per unit cell deaminated to become H+. When crystal 3 was evacuated at 423 K, nearly all, 62 of the 64, NH4+ ions had deaminated. In both of these structures, as in crystal 1, about five Na+ ions were found at two equipoints and two K+ ions at a third, at site I. Thirty-one NH4+ ions were found at two equipoints in crystal 2, but the two NH4+ ions remaining in crystal 3 were both at one equipoint. Two site-I′ positions, N(I′) and Na(I′), are occupied by 18 NH4+ and 3 Na+ ions, respectively, per unit cell in crystal 2 (see Figure 4b). At site I′ in crystal 3, only the three Na(I′) ions remain per unit cell. At site II, 13 NH4+ and 2 Na+ ions are found in crystal 2 (see Figure 5b); in crystal 3 these numbers are 2 and 2. Thus the 32-fold I′ and II positions are only partially occupied by NH4+ and Na+ in these structures. No water remains.

The total number of cations found per unit cell in crystals 2 and 3, 37.8(5) and 9.4(4), respectively, is far less than 71, the number required to balance the negative charge of the zeolite framework. The difference is attributed to H+ ions presumed to be present within the zeolite. They could not be found by X-ray diffraction because their scattering factors are too small. In crystal 2, the N(I′) and Na(I′) ions inside the sodalite cavities are 1.68 and 0.61 Å, respectively, from the (111) plane of the three O(3) framework oxygens to which each is bound (see Table 6). The N(I′)-O(3) and Na(I′)-O(3) distances are 2.728(14) and 2.233(8) Å, respectively, about same as the sum of the corresponding conventional ionic radii,32 2.75 and 2.29 Å (see Figure 4b). In crystal 3, the Na(I′) ions are 0.70 Å from the (111) plane of three O(3) framework oxygens (see Table 6). The Na(I′)-O(3) distance, 2.205(9) Å, is again close to the sum of the corresponding radii,32 2.29 Å.

Six Single-Crystal Structures of Zeolite Y

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18301

TABLE 5: Distributions of Nonframework Ions (NH4+, Na+, K+, Al3+, and O2-) over Sitesa crystal

1

2

3

4

5

6

evacuation temperature

323 K

373 K

423 K

473 K

523 K

573 K

structure determination

first

first

secondb

first

30.7(11) 28.8(9) 59.5(14) 1.5(5) 3.6(4)

17.6(10) 13.5(7) 31.1(12) 3.2(4) 1.8(4)

12.5(10) 13.5(7) 26.0(12) 3.3(4) 1.4(3)

1.8(6) 1.8(6) 3.5(4) 1.8(4)

2.3(7) 2.3(7) 3.3(4) 1.8(4)

5.1(6) 1.5(2)

5.0(6) 1.7(1)

4.7(5) 2.0(1)

5.3(6) 2.3(1)

5.1(6) 2.3(1)

0.041 0.153

0.038 0.134

0.041 0.140

0.042 0.136

0.040 0.137

cation NH4+ Na+

ΣNH4+ f

ΣNa+

K+ Al3+ O2Fo > 4σ(Fo)

site I′d IIe I′d IIe II′g Ic I′d Ic R1 R2

secondb

first

secondb

2.8(4) 2.3(4)

2.0(4) 3.0(4)

5.1(6) 1.99(21) 1.4(5) 0.7(3) 0.040 0.135

5.0(6) 1.9(2) 1.4(3) 0.72(17) 0.042 0.136

first

secondb

first

2.1(5)

1.9(7)

3.1(6)

3.0(6) 5.1(9) 1.6(3) 3.8(10) 1.9(5) 0.041 0.145

3.3(6) 5.2(9) 1.9(2) 3.9(7) 1.9(4) 0.044 0.151

2.3(5) 5.4(8) 1.6(4) 4.8(14) 2.4(7) 0.046 0.158

a Exchangeable cations that balance the negative charge of the aluminosilicate framework are found within the zeolite’s cavities (see Figure 2). The structure was determined a second time using a second similarly prepared crystal for comparison. These results are presented only in this table. c Site I is at the center of a D6R. d Site I′ is generally in the sodalite cavity opposite one of the D6R’s 6-rings; Al3+, however, is just inside a D6R. e Site II is in the supercage adjacent to a S6R. f About five NH4+ ions were deaminated at 323 K. g Site II′ is in the sodalite unit adjacent to a S6R.

b

Figure 2. Stylized drawing of the framework structure of zeolite Y. Near the center of the each line segment is an oxygen atom. The nonequivalent oxygen atoms are indicated by the numbers 1-4. There is no evidence in this work of any ordering of the silicon and aluminum atoms among the tetrahedral positions, although it is expected that Loewenstein’s rule (ref 38) would be obeyed. Extraframework cation positions are labeled with Roman numerals.

At site II in crystals 2 and 3, each NH4+ ion lies far inside the supercage, 1.74 and 1.77 Å, respectively, from the (111) plane of the three O(2) framework oxygens to which it is bound (see Table 6). Each of the Na+ ions is 0.50 and 0.40 Å, respectively, from its (111) plane. The NH4+ and Na+ ions are 2.814(12) and 2.269(12) Å for crystal 2 and 2.87(9) and 2.264(10) Å for crystal 3, respectively, from their nearest neighbors, the O(2) framework oxygens (see Table 4), in close agreement with the sums of the radii,32 2.75 and 2.29 Å, respectively. Two K+ ions at K(I) again occupy the octahedral sites I in crystals 2 and 3 (see Figure 3(d)). Each K(I) ion coordinates to the six O(3) oxygen atoms of its D6R at distances of 2.732(3) and 2.620(3) Å for crystals 2 and 3, respectively, near the sum of the corresponding conventional ionic radii,32 2.65 Å. As deamination proceeds, the K(I)-O(3) distances decrease to become close to the sum of the corresponding radii32 (see Table

4). The two sparsely occupied arrangements of Na+ and NH4+ ions around D6Rs shown in Figure 3, panels e and f, become possible as the number of NH4+ ions decreases. 4.4. Structures of Crystals 4, 5, and 6: Fully Dehydrated, Fully Deaminated, and Partially Dealuminated Zeolite Y. After evacuation at 473, 523, and 573 K, no NH4+ ions remained. Rather, in this temperature range, progressive dealumination of the zeolite framework was seen. With Al3+ ions at site I′ and oxide ions at the centers of the D6Rs in these crystals, 0.5, 2.0, and 2.5 linear Al3+-O2--Al3+ groups, respectively, are present per unit cell. Al3+ in a 6-ring would be expected to distort that ring appreciably, pulling three framework oxygens close. This was seen in crystals 5 and 6 where O(3) could be resolved into O(3a) and O(3b) (see Figure 6). (The Al3+ content of crystal 4 was too small to allow this resolution.) One, four, and five Al3+ ions per unit cell are found at site I′ in crystals 4, 5, and 6, respectively. (The values given below for crystal 4 are less reliable because of low occupancy and unresolved O(3).) Respectively for the three crystals, each coordinates in a trigonal pyramidal manner to three O(3b) framework oxygens at 2.109(7), 1.89(5), and 1.83(3) Å and to one O(5) oxide ion at the center of a D6R at 1.39(11), 1.61(12), and 1.63(16) Å. The mean Al-O(3b) bond length, 1.86 Å, is longer than 1.72 Å, the mean Al-O bond length found in zeolites, because the 6-ring is resisting extreme distortion. The mean Al-O(5) bond length, 1.62 Å, is appropriately shorter than 1.72 Å. The Al3+ ions are nearly in the plane of the O(3) framework oxygens (see Figure 3g); they are recessed 0.10, 0.16, and 0.17 Å into the D6R from that plane for crystals 4, 5, and 6, respectively (see Table 6). The O(3)-Al-O(3) or O(3b)Al-O(3b) bond angles are 117.6(16)o, 119.3(11)o, and 119.2(15)o for crystals 4, 5, and 6, respectively, nearly trigonal planar (see Figure 6). In all three crystals, as before, five Na+ ions occupy two equipoints and two K+ ions occupy one, site I. At site I′, Na(I′) has an occupancy of three (crystals 4 and 6) or two (crystal 5) per unit cell. At site II, two Na+ ions (crystals 4 and 6) or three (crystal 5) are found per unit cell. All these positions are sparsely occupied, of the order of 10%. Their placements may be seen in Figures 3 (panels d, e, and f), 4 (panels b and c), and 5 (panel

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Figure 3. Stereoviews of the seven ways that cations occupy double 6-rings (D6Rs) in crystals 1-6. Of the 16 D6Rs per unit cell in crystal 1, two are occupied as shown in (a), 12 as shown in (b), and two as shown in (c). The N(I′)‚‚‚K(I) distance in (a) is 3.380(12) Å, N(I′)‚‚‚N(I′) in (b) is 6.761(20) Å, and N(I′)‚‚‚Na(I′) in (c) is 5.746(21) Å. Of the 16 D6Rs per unit cell in crystals 2-6, two are occupied as shown in (d). The arrangement (e) is also possible in crystals 2 and 3. Crystals 4, 5, and 6 have Al3+-O2--Al3+ groups on 3-fold axes in D6Rs as shown in (g); the coordinates of crystal 5 are drawn here. The zeolite Y framework is drawn with heavy bonds. The coordination of the exchangeable cations to oxygens of the zeolite framework are indicated by light bonds. Ellipsoids of 25% probability are shown.

c). The geometry about them is reasonable (see Tables 4 and 6), much like that described for crystals 1, 2, and 3. 5. Discussion Largely NH4+-exchanged zeolite Y single crystals were evacuated at various temperatures, and their structures were

determined by X-ray diffraction methods. As the temperature increased, dehydration, deamination, dealumination, and decomposition were seen, in that order. All crystals showed deamination of NH4+, the extent increasing with increasing evacuation temperature until it was complete in crystal 4. After evacuation at 323 K, a small amount of deamination was seen

Six Single-Crystal Structures of Zeolite Y

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18303 TABLE 6: Displacements of Atoms (Å) from Six-Ring Planesa position

site

displacement

Crystal 1, |(NH4)59H5Na5K2(H2O)3|[Si121Al71O384]-FAU at O(3) K(I) I 1.77 Na(I′) I′ 0.60 N(I′) I′ 1.62 at O(2) Na(II) II 0.55 N(II) II 1.72 Ow II 3.09 Crystal 2, |(NH4)31H33Na5K2|[Si121Al71O384]-FAU K(I) I 1.68 Na(I′) I′ 0.61 N(I′) I′ 1.68 at O(2) Na(II) II 0.50 N(II) II 1.74 at O(3)

Crystal 3, |(NH4)2H62Na5K2|[Si121Al71O384]-FAU K(I) I 1.58 Na(I′) I′ 0.70 at O(2) Na(II) II 0.40 N(II) II 1.77 at O(3)

Crystal 4, |H61Na5K2(Al2O)0.5|[Si122Al70O384]-FAUb K(I) I 1.57 Na(I′) I′ 0.61 Al I′ -0.10 at O(2) Na(II) II 0.36 at O(3)

Crystal 5, |H53Na5K2(Al2O)2|[Si124Al68O384]-FAUb K(I) I 1.52 Na(I′) I′ 0.92 at O(3b) Al I′ -0.16 at O(2) Na(II′) II′ -0.10 at O(3a)

Figure 4. Stereoviews of the three representative sodalite units in crystals 1-6; the coordinates of crystal 2 are drawn in (a) and (b) and crystal 4 in (c). See the caption to Figure 3 for other details.

and a few water molecules remain. This is nicely in accord with reports that deamination begins before dehydration is complete.43,44 (It is unlikely that appreciable H3O+ exchange had occurred and that it was these that had decomposed to give H+ ions.) The NH4+ ions are evenly distributed over sites I′ and II. In the crystals dehydrated at 373 and 423 K, only 30 and 2 NH4+ ions remain per unit cell, respectively. For the crystals dehydrated at 473, 523, and 573 K, deamination was complete and dealumination of the zeolite framework was seen, increasing with temperature. From the balanced reactions (see below), dealumination must be accompanied by the loss of framework oxygen which, with H+ ions, leaves as water, a second round of dehydration (loss of “chemical” water) that threatens the integrity of the zeolite structure. Finally, crystals evacuated at 623 K and above had sustained so much damage that they no longer showed single-crystal diffraction and their structures could not be determined. The deamination reaction is straightforward because the zeolite framework is not involved.

NH4+ f H+ + NH3(g)

(1)

The dealumination reaction is more complex. As an example, the reaction that occurs when crystal 3 is evacuated at higher temperature to become crystal 5 may be written as

196H64Na5K2Si121Al71O384 f 192H53.3333Na5.1042K2.0417 (Al2O)2Si123.5209Al68.4791O384 + 1152H2O(g) (2)

Crystal 6, |H51Na5K2(Al2O)2.5|[Si124Al68O384]-FAUb at O(3a) K(I) I 1.51 Na(I′) I′ 0.89 at O(3b) Al I′ -0.17 at O(2) Na(II′) II′ -0.34 a Site II is in the supercage; displacements from its 6-rings are given as positive. A negative deviation indicates that the atom is at site II′ and lies in a sodalite unit. Site I′ is near the plane of one 6-ring of a D6R; displacements into the sodalite unit are given as positive. A negative deviation indicates that the atom lies within a D6R. b Some loss of structural (“chemical”) water is possible to give fewer H+ ions and framework oxygen atoms, and perhaps fewer unit cells according to eq 2.

The formula for crystal 3 has been simplified by “allowing” its last two NH4+ ions to deaminate. The integers used in the formula of crystal 3 and for the Al2O content of crystal 5 are experimental and approximate. The coefficients indicate the number of unit cells. They decrease because we have assumed that the vacancies have been filled by Si4+ ions from adjacent unit cells (reconstruction).45 The experimental conditions have been such that any water that formed should have left the zeolite crystal relatively quickly. Most work done previously on the structure of zeolite H-Y prepared for use as a catalyst has involved steaming, where excess water was present (1) to coordinate to dealuminated Al3+ ions (mostly as OH- groups) and (2) to provide 4 H+ ions to each vacancy, leaving “hydroxyl nests”.46 At higher temperatures, as more water was lost, these hydroxyl nests collapsed (these vacancies disappeared) and the framework reconstructed itself.45 This assumption is weakly supported (1) by the high retention of crystallinity (see “no. of reflections with Fo > 4σ(Fo)” in Table 1) and (2) by the decrease in the unit cell edge length seen in crystals 4, 5, and 6 which would be expected to accompany an enrichment in the Si content of the zeolite framework (see Table 1 or Figure 7a). The zeolite is apparently not able to continue this process beyond crystal 6. High precision is given for the other components of eq 2 to

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Figure 7. (a) Unit cell constants as a function of evacuation temperature. (b) The number of NH4+ ions per unit cell as a function of evacuation temperature.

Figure 5. Stereoviews of the three representative supercages in crystals 1-6; the coordinates of crystals 1, 2, and 4 are drawn in (a), (b), and (c), respectively. See the caption to Figure 3 for other details.

Figure 6. One 6-ring of a D6R in a partially dealuminated structure, crystal 6. (a) Not containing Al3+. (b) Containing an Al3+ ion of an Al3+-O2--Al3+ group. The Si,Al and O(2) positions have not been resolved, are the same in both figures, and should be somewhat incorrect in (b). Ellipsoids of 25% probability are shown.

show that such equations can be balanced and that water must be produced, six molecules per product unit cell. Such equations cannot be balanced with small integers. Other dealumination reactions may occur, for example to give the four-H+ (four hydroxyl) tetrahedral “nests” initially proposed by Kerr.46 In this process an Al3+ ion in the zeolite framework is replaced by four H+ ions. The reaction per unit cell would be (with the same crystal 3 composition used for reaction 2)

H64Na5K2Si121Al71O384 + 2H2O f H52Na5K2(Al2O)2Si121Al67(4H+)4O384 (3)

Reaction 3 cannot proceed independently of reaction 2. It requires water molecules from the destructive dehydration of the zeolite, reaction 2. Again, it is likely to be of limited importance in this work which, except for escaping water molecules, was quite anhydrous. The structure of a Lewis-acid site has been identified in H-Y (see Figures 3g and 6). Al3+ ions are nearly in the planes of both 6-rings of some D6Rs, presenting near trigonal planar faces to guest molecules that might approach from within either of the two adjacent sodalite cavities. This appears to be the first time that Al3+ has been found crystallographically at a conventional ionic site in a zeolite. However, it is not an isolated 3-coordinate cation; a bridging oxide ion at the center of each Al3+-containing D6R allows each Al3+ ion to be 4-coordinate (near trigonal pyramidal). These Lewis acid sites should be very stable because the linear Al-O-Al4+ group is protected in a robust setting, the D6R. Such a structure, linear Mn+-X2--Mn+, had been seen before in zeolite Y; it was Pb2+-S2--Pb2+.47 In zeolite X, it was seen twice as part of larger clusters: Pd-O-Pd in HO-Pd4+-O2-Pd4+-OH or O2--Pd5+-O2--Pd5+-O2-,48 and La3+-O2-La3+ in a lanthanum oxide continuum that extends throughout all D6Rs and sodalite cavities.49 By window-size arguments, only the smallest organic molecules should be able to reach the Al3+ Lewis acid site in the sodalite unit. Accordingly, this site may not be catalytically active in petrochemical operations. However, such arguments often fail, especially if moisture is present and all the more so at elevated temperatures. Zeolite chemistry, like all chemistry, is based on thermodynamics and reaction mechanisms; it cannot

Six Single-Crystal Structures of Zeolite Y be relied upon to reduce to matters of window size in a model that does not recognize such things as oxygen mobility, ring opening, and defect structure. This has been discussed at length.50,51 In addition, the inductive effects of this (Al2O)4+ group should very much enhance the acidity of adjacent sites. The presence of three-coordinate aluminum in faujasite-type zeolites was suggested by Larson et al.,9 Uytterhoeven et al.,10 and Kuhl et al.11,12 Because of the absence of an IR band at about 2900 cm-1, Uytterhoeven et al.10 reported planar threecoordinate aluminum and that the pyramidal three-coordinate Bronsted acid form was not present in substantial quantity. Three-coordinate Al3+ has never been seen crystallographically in any aluminosilicate structure. It is an unreasonably low coordination number for such a small 3+ cation. Grey et al.52 and Bae et al.53 observed tetrahedral extraframework aluminum crystallographically in the Zn2+-exchanged zeolites LSX and X, respectively. The monomeric tetrahedral aluminate ion (the orthoaluminate ion), AlO45-, exclusive of associated H+ ions, had been found in their powder and singlecrystal X-ray diffraction work, respectively, on these FAU-type zeolites. Zeolite H-Y, more accurately zeolite Na,H-Y, is generally steamed at about 873 K before it is used as an acid catalyst. This process releases Al3+ from the zeolite framework (ultimately creating Lewis acid sites) and stabilizes the zeolite (USY ) ultrastable Y).54 This material has been studied by NMR and crystallographic methods. Two signals were seen for 4-coordinate aluminum in steamed zeolite H-Y studied by 27Al MAS and MQMAS (solid-state NMR) at very high fields.29 The peak at 60.7 ppm is attributed to tetrahedral framework aluminum. The other, a broad peak at 60.4 ppm with much larger quadrupolar coupling, usually not resolved in lower-field MAS experiments, is recognized as another kind of 4-coordinate aluminum. In later work on non-hydrated samples of a very similar material, a signal at 60-70 ppm is attributed to framework aluminum and to “tetrahedrally coordinated aluminum atoms in neutral extraframework aluminum oxide clusters”.30 Earlier crystallographic work had found Al3+ ions at three sites, I′, II′, and II,45 all too far from oxygens of the zeolite framework to coordinate to them, so those Al3+ ions must be fully coordinated by non-framework oxygens. A very recent report using 27Al MAS NMR and X-ray diffraction from powders finds non-framework Al3+ at sites I′ and II′.55 In other NMR work, Deng et al. investigated aluminum species in three dealuminated zeolites (ultrastable HY, HZSM-5 (MFI), and mordenite (MOR)) using the 1H/27Al TRAPDOR method in combination with 27Al MAS NMR.56 They suggested that the 6.8 ppm signal in the 1H MAS spectra and the 54 ppm signal in the 27Al MAS spectra of the H-Y and mordenite zeolites were due to four-coordinate Al3+. Prins et al.57 assigned the signals at 0, 30, and 53 ppm in the 27Al NMR spectrum of HZSM-5 to 6-coordinate, highly distorted tetrahedral or 5-coordinate, and tetrahedral Al3+, respectively. In this report, the crystals studied have not been steamed. The extraframework Al3+ is not at any of the positions described above,45,55 but rather they coordinate to framework oxygens; they are at a site that does not have a designation, within the D6R between sites I and I′. There are no reports of highresolution NMR work on such a material. The Na+ ions have redistributed themselves among sites I′ and II as the evacuation temperature increased (see Table 5). This had been seen in copper-exchanged zeolite Y58 and nickelexchanged zeolite Y.59 It is assumed that the Na+ migration and relocation in zeolite Y are a minor consequence of other

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18305 TABLE 7: Si,Al-O Bond Lengths in Three Crystals, Åa Si,Al-O(1) Si,Al-O(2) Si,Al-O(3) Si,Al-O(4)

H59-faujasiteb

H62-Y (crystal 3)c

H61-Y (crystal 4)c

1.653(2) 1.634(1) 1.663(2) 1.623(2)

1.667(2) 1.652(1) 1.681(2) 1.630(1)

1.671(1) 1.651(1) 1.679(1) 1.630(1)

a Si,Al-O(1) and Si,Al-O(3) are longer than Si,Al-O(2) and Si,Al-O(4) for each crystal. This indicates that the two kinds of H+ ions in the structures (see ref 60 and references therein) are bonded to O(1) and O(3), at least predominantly. b Reference 60. H59Al59Si133O192 was the expected composition. c This work. Because the framework composition is richer in Al than that reported in ref 60, the (mean) Si,Al-O bonds are all longer.

structural changes. The K+ ions are always at site I where they fit, both in terms of size and coordination number. It had been established by IR and sorption experiments that two kinds of H+ ions are present within zeolite H-Y.60 When H+ binds to a framework oxygen, the two Si,Al-O bonds in which that oxygen participates lengthen. Therefore, the Si,AlO(n) distances, n ) 1-4, should indicate to which two of the four framework oxygen atoms the H+ ions bond, at least predominantly. This work reaffirms the classic result of Olson and Dempsey,60 who studied a natural crystal of faujasite in its H+ form (see Table 7); because Si,Al-O(1) and Si,Al-O(3) were the longest, they were able to conclude that H+ ions bind to O(1) and O(3). In this work, this result is best seen in crystals 3 and 4 (see Table 7) because they have the most H+ ions with the fewest other cations (NH4+ or Al2O4+), but it can also be seen in crystals 5 and 6 (see Table 4). It is now well-established directly by neutron diffraction that the D+ ions in D-Y bind, at least predominantly, to the O(1) and O(3) oxygens.61-64 Kuhl et al. studied the structural stability of Na,NH4-X (Si/ Al ) 1.26) and suggested that the faujasite structure with any Si/Al ratio is perfectly stable to calcinations of the ammonium form if not more than 32 protons are generated per unit cell.11 Zhu et al. observed that Na,H-X (Si/Al ) 1.09) containing between 33 and 56 H+ ions kept good crystallinity.42 Park et al.49 reported that a single crystal of fully La3+-exchanged zeolite X vacuum dehydrated at 673 K had 92 H+ ions per unit cell; this was possible because a La2O3 continuum (La32O48 per unit cell) filled all sodalite cavities and D6Rs, apparently thus providing great stability. They suggested that the protons all associated with oxygens of the anionic zeolite framework and were all in the supercages. This work shows that zeolite Y with Si/Al ) 1.70, with some alkali metal cations and 64 H+ ions per unit cell (at T ≈ 450 K), retains its crystallinity up to a temperature between 573 and 623 K in an anhydrous environment. This relatively low decomposition temperature can be attributed to the relatively high Al content of the single crystals used, as compared to zeolite Y with less Al and to commercial zeolite Y (USY) used in catalytic applications. Attempts to grow single crystals with lower Al content for diffraction experiments, to better represent the commercial material, had been unsuccessful. Acknowledgment. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2006-331-C00152). Supporting Information Available: Tables of observed and calculated structure factors squared with esds for crystals 1-6. This material is available free of charge via the Internet at http:// pubs.acs.org.

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