Detailed Determination of the Tl+ Positions in Zeolite Tl

J. Phys. Chem. C 2009, 113, 19937–19956

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Detailed Determination of the Tl+ Positions in Zeolite Tl-ZSM-5. Single-Crystal Structures of Fully Dehydrated Tl-ZSM-5 and H-ZSM-5 (MFI, Si/Al ) 29). Additional Evidence for a Nonrandom Distribution of Framework Aluminum Nam Ho Heo,* Cheol Woong Kim, and Hyeok Jeong Kwon Laboratory of Structural Chemistry, Department of Applied Chemistry, Kyungpook National UniVersity, Daegu, 702-701 Korea

Ghyung Hwa Kim Pohang Accelerator Laboratory, Pohang, 790-784 Korea

Seok Han Kim and Suk Bong Hong Department of Chemical Engineering and School of EnVironmental Science and Engineering, POSTECH, Pohang, 790-784 Korea

Karl Seff* Department of Chemistry, UniVersity of Hawaii at Manoa, Honolulu, Hawaii 96822-2275 ReceiVed: July 1, 2009; ReVised Manuscript ReceiVed: September 11, 2009

The structures of the fully exchanged, fully dehydrated zeolites Tl-ZSM-5 (a ) 20.064(1), b ) 19.946(1), c ) 13.416(1) Å) and H-ZSM-5 (a ) 20.079(1), b ) 19.948(1), c ) 13.419(1) Å) have been determined crystallographically using modestly twinned single crystals and synchrotron X-radiation. They were refined in the orthorhombic space group Pnma to R1 ) 0.071 and 0.066 with 3962 and 4278 reflections, respectively, for which Fo > 4σ(Fo). The geometry of the zeolite framework in both crystals (MFI, Al3.2Si92.8O192, Si/Al ) 29) is very similar to that in earlier determinations. The ZSM-5 framework and Tl+ positions are described using the four nonequivalent partially overlapping 10-rings (R), the two channel systems (C), the channel intersection volume, and the cove of the ZC. The latter two comprise the conventionally defined cavity. The zigzag (sinusoidal) channel (ZC) passes through R1, R3, and R4; the straight channel (SC) passes through R2. A total of 3.3(2) Tl+ ions per unit cell were found scattered among 18 crystallographically distinct positions. Based on their positions near R1, R2, and R3, these positions may be classified into three groups (Gn, n ) 1-3), each with two general cation sites (Sn and Sn′). Of the 18 Tl+ positions, six are in the intersection volume (at S2′ in G2), 11 are elsewhere in the ZC (in G1 and G3), and one is elsewhere in the SC (at S2) just outside the intersection volume. Among the six general cation sites, S1′ at the mouth of the cove has the highest Tl+ population, 28% of the total. Of the 18 Tl+ positions, 14 bond strongly to framework oxygen atoms with 2.68(4) < Tl-O < 2.87(5) Å (Tl+ + O2- radii ) 2.79 Å); three other shortest distances range upward to 2.97(5) Å; one at 3.18(10) Å appears to be an unresolved position. The framework sites (T) that Al atoms preferentially occupy are identified from Tl+ occupancies and Tl-O bond distances. Using Tl-O < 2.70 Å, the order is T10 (most preferred) > T6. The preference of Al for the T10 site remains when longer, less reliable but more comprehensive, Tl-O bond length criteria are used. Using Tl-O < 2.80 Å, the order is T10 > T6 ∼ T9 > T5 > T4 > T2 > T12 > T7 > T11, and with Tl-O < 2.90 Å, the preference order includes all 12 T sites: T10 > T3 > T2 > T12 > T6 ∼ T9 > T5 > T4 > T8 > T7 > T11 > T1. T10 is a member of R4, the 10-ring that is common to the intersection volume and the cove. 1. Introduction Zeolites are crystalline, microporous aluminosilicates that are now well established as molecular sieves and catalysts in the petrochemical and refining industry. Catalysis occurs at acidic (Brønsted and Lewis), basic (lone pair), metal ion, or reduced metal sites within a zeolite; in a bifunctional catalyst, two or more of these features are present. The choice of zeolite framework allows two additional essential features, pore dimensions and topology, to be optimized. * To whom correspondence should be addressed. E-mail: [email protected] (N.H.H.); [email protected] (K.S.).

The highly siliceous zeolite ZSM-5 (MFI) is an outstanding catalyst in petrochemical processing,1 in the conversions of various organic compounds,2-5 and in emission control processes such as NO decomposition6,7 and selective catalytic reduction (SCR) of NOx.8-12 Its catalytic properties, originally based only on Brønsted acid sites (H+ ions or “OH” groups) and the unique shape selectivity of its moderately sized channel systems (10rings), have been dramatically extended by increasing the Al content of its framework, thus allowing useful numbers of various charge compensating guest cations with different catalytic activities to be ion exchanged into the zeolite.3,4,13-16 A knowledge of the positions of the acidic/catalytic centers in

10.1021/jp9061913 CCC: $40.75  2009 American Chemical Society Published on Web 10/26/2009

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this zeolite, which should be close to the positions that guest cations (including H+) would select, is needed to understand specific catalytic reactions and variations among catalytic preparations. More specifically, a knowledge of the exact positions of the extraframework cations and the precise geometry around them in the zeolite is needed to understand the formation and stabilization of specific reaction intermediates and, therefore, the products of a catalytic reaction. The local structures and environments of the catalytic centers in variously ion-exchanged ZSM-5 zeolites have been extensively studied by spectroscopic techniques such as X-ray absorption spectroscopy (XAS) which includes the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopies,4,5,10,17,18 electron spin resonance (ESR),16,19,20 X-ray photoelectron spectroscopy (XPS),16 infrared spectroscopy (IR),16,21-23 Raman spectroscopy,23 and nuclear magnetic resonance (NMR).23 The structures of ZSM-524-28 and silicalite29,30 have been determined with and without the structure-directing templates tetrapropylammonium (TPA) and TPA fluoride (TPAF).27-30 Both zeolites have MFI topology and may be considered to be the same, differing only in aluminum content. Silicalite has negligible to no aluminum in its composition, and ZSM-5 has little; Si/Al in ZSM-5 is rarely less than 10. It is expected that the extraframework cations (e.g., H+, Na+, and Tl+) within ZSM-5 would occupy sites that would be close to the anionic [AlO4/2]- groups rather than to [SiO4/2]0, and thus would be indicative of the Al positions and, in turn, of the locations and geometry of the catalytic centers. The actual positions that these cations choose would also depend, of course, on their chemical nature and size. To maximize the signal, the crystallographic determination of the cation positions is best done using exchange cations with a high scattering power to change ratio. To maximize their number, a ZSM-5 material with a relatively high Al content is desirable. Finally, single-crystal crystallography generally provides more detailed results than powder diffraction. Suitably large single crystals with relatively high framework Al contents have been difficult to obtain, possibly because of increased nucleation rates due to Al3+-containing species in the synthesis solution.31 Accordingly, the crystallographic results previously reported for variously ion-exchanged ZSM-5 zeolites and described in the next three paragraphs were not learned using single crystals; only polycrystalline materials were used. With the intention of determining the extraframework cation positions, the Na+,32 K+,32 Rb+,32 Tl+,32,33 Cs+,34-38 Ni2+,39 and Cu+/Cu2+ 40 positions in the corresponding cation-exchanged ZSM-5 structures were determined. Only one to five cation positions were found in each, suggesting that Al3+ was not distributed randomly over the 12 T sites. Unfortunately, these results were generally marginal and inconsistent except for the results described in the next paragraph. Lin et al.,34 Olson et al.,35 and Mentzen et al.36-38 determined the Cs+ positions in Cs-ZSM-5. Lin found only one Cs+ position in both hydrated and dehydrated Cs-ZSM-5 (Cs3.56Si92.44Al3.56O192, Si/Al ) 26) and reported that it was near a 4-ring (near T9), near the intersection of the two (straight and sinusoidal) channels.34 However, Olson later reported three crystallographically distinct Cs+ positions in dehydrated Cs-ZSM-5 (Cs5.8Si90.2Al5.8O192, Si/Al ) 15.6), all different from that reported by Lin, close to the T12, T7, and T10 tetrahedra.35 Finally in more recent work, Mentzen36-38 reported the temperature dependence of the Cs+ positions in hydrated and dehydrated MFI; two of Mentzen’s five Cs+ positions were

Heo et al. essentially the same as those of Olson, two others amounted to a resolution of Olson’s third position into two, and the fifth was new. These results indicated that Al atoms prefer sites T12, T7, T10, and T5. Huddersman et al., again using powder diffraction and the Rietveld refinement method, reported three unique Tl+ positions in hydrated Tl-ZSM-5 (Tl3.4Si92.6Al3.4O192, Si/Al ) 27).33 All interacted, more or less, with 10-ring oxygen atoms at the intersections between the two channel systems and with water molecules that completed the coordination spheres of the Tl+ ions.33 However, only one of them had a reasonable interaction with a framework oxygen atom, 2.79(1) Å, in agreement with the sum of the ionic radii of Tl+ and O2-, 1.47 + 1.32 ) 2.79 Å.41,42 This oxygen atom, O8, bridges between T8 and T9. The other Tl-O distances were much longer, ranging from 3.12 to 3.45 Å. Finally, in his most recent work, Mentzen reported five Tl+ positions in dehydrated Tl-ZSM-5 (Si/Al ) 13).32 Only one of these positions, which held only 9% of the total number of Tl+ ions, was a reasonable distance, 2.71(2) Å, from the nearest framework oxygen atom; the shortest Tl-O distances for the other four Tl+ positions, 3.07(2), 3.18(2), 3.18(2), and 3.22(2) Å, were substantially longer than the sum of the corresponding ionic radii, 2.79 Å.41,42 A number of theoretical studies,43-47 including ab initio quantum mechanical calculations, have been performed to learn the siting preferences of Al or HO(Al) among the 12 T sites. This work did not yield the simple result that Al would selectively occupy a single one of the 12 T sites. Rather it predicted that Al would occupy multiple T sites with preferences for some, and the results were scattered depending on the techniques used. Some of these studies showed that the energy difference between the most and least favorable sites is only ca. 3 kcal/mol.43 Such rather small energy differences suggest that the Al siting is kinetically controlled during the crystallization process rather than energetically driven. Other theoretical studies, including calculations using density functional theory (DFT) alone48 and DFT combined with quantum mechanics and interatomic potential functions,49 predicted as expected that monovalent and some divalent cations would associate most closely with oxygen atoms bound to framework aluminum atoms. However, most importantly, none of these studies found a strong energetic driving force for Al ordering among MFI’s T sites, nor therefore for the ordering of cations, nor in turn for the localization of catalytic activity within the zeolite. Similar conclusions were reached experimentally in 27Al magic angle spinning (MAS) and multiple quantum (MQ) MAS NMR investigations using ZSM-5 zeolites with different Al contents.50 The situation described in the preceding paragraphs for ZSM-5 is remarkably similar to that in TS-1, where Ti4+ occupies framework T sites at low concentrations in the otherwise silicate MFI framework: experimental determinations do not agree, and theoretical work shows that the energy of substitution of Ti for Si is nearly the same for all 12 T sites.51 In this study, we sought to determine crystallographically the exchangeable cation positions in fully dehydrated ZSM-5. Available to us were moderate-sized modestly twinned single crystals of ZSM-5 with moderate Al content, Si/Al ) 29, and synchrotron X-radiation. Recognizing that the density of exchangeable cations would be low and that this would make them difficult to locate, H-ZSM-5 was ion-exchanged with Tl+ to maximize their scattering power. The crystal structure of H-ZSM-5 was also determined for comparison with that of Tl-ZSM-5 and with previous reports. With these high-quality

Tl+ Positions in Zeolite Tl-ZSM-5

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Figure 2. Representative EDX spectrum of Tl-ZSM-5. The unlabeled peaks are standard for the instrument and procedure used: near 0 keV, instrumental; near 0.3 keV, carbon contamination; near 2.1 keV, platinum; near 2.62 keV, possibly chlorine. Figure 1. Scanning electron micrograph of a typical TPA-ZSM-5 single crystal (scale bar ) 10 µm). The twin morphology can easily be seen.

data, we hoped to determine the positions of the low occupancy extraframework cations more accurately than had heretofore been possible. From this a reliable preference order, if any, for Al among the 12 T positions might be inferred. 2. Experimental Section 2.1. Synthesis and Characterization of TPA-ZSM-5. Colorless single crystals of TPA-ZSM-5, TPA3.2Al3.2Si92.8O192 · xH2O (Si/Al ) 29), were synthesized as twin tabloids by a modification of the procedures developed by Guth et al.52 The reagents used for their synthesis were tetrapropylammonium bromide (TPABr, 98%, Aldrich), NH4F (98%, Aldrich), Al(NO3)3 · 9H2O (98%, Junsei), fumed silica (Aerosil 200, Degussa), and deionized water. The molar composition of the final synthesis mixture was 2TPABr · 10NH4F · 0.5Al2O3 · 20SiO2 · 500H2O. After stirring overnight at room temperature, the synthesis mixture was charged into Teflonlined 45-mL autoclaves and then heated at 200 °C for 28 days without stirring under autogenous pressure. The solid product was recovered by filtration, washed repeatedly with water, and then dried overnight at room temperature. Scanning electron micrographs (SEM, JEOL JSM-6300) of the ZSM-5 product show that it consists of well-faceted, coffinshaped crystals approximately 80 × 20 × 20 µm in size (see Figure 1). Scanning electron microscopy-energy dispersive X-ray (SEM-EDX) measurements (Koriba EDX-3500 within a Hitachi S-4300) on two crystals (see Figure 2) showed Si/Al ) 27.5 at a crystal surface and 31.3 at a freshly broken surface; the mean 29.4 corresponds to 3.2 Tl+ ions per unit cell, in excellent agreement with the crystallographic result, 3.3(2) (vide infra). In poorer agreement is the Si/Al ratio of 25 (3.7 Al per unit cell) obtained for a bulk sample by inductively coupled plasma (ICP) and wet analyses for Al and Si in the Analytical Laboratory at the Korea Institute of Science and Technology (KIST). Thermogravimetric and differential thermal analyses (TA Instruments SDT 2960) showed that these crystals contain about four TPA cations per unit cell. As can be seen in Figure 1, the morphology of each twinned crystal suggests that at least one axis of one twin component is parallel to another axis of the second twin component. (This is discussed further in paragraph 2 of section 3.) A similar morphology was observed in silicalite29 and in [Li-Si-O]-MFI.53 2.2. Calcination, Ion Exchange, and Dehydration. Crystals of H-ZSM-5 were prepared by burning the encapsulated TPAs out of TPA-ZSM-5 with oxygen gas (99%, Korea Standard

Gas Co.) at 623 K for 2 days in a Pyrex capillary connected to a vacuum line. One crystal of H-ZSM-5 was dehydrated at 653 K and 1 × 10-6 Torr for 4 days. Another crystal of H-ZSM-5 was ion-exchanged with 0.05 M aqueous thallous acetate (99.99%, Aldrich, pH 6.4) by the dynamic flow method and dehydrated as above. After cooling to room temperature, the two crystals, still under vacuum, were isolated in their capillaries by sealing both ends with a small torch. The H-ZSM-5 and Tl-ZSM-5 crystals were both slightly brownish before dehydration; the latter became a little darker after dehydration. Additional details are given in Table 1. 2.3. X-ray Diffraction Work. X-ray diffraction data were collected at 294(1) K on an ADSC Quantum210 detector at Beamline 4A MXW of The Pohang Light Source (PLS), Korea. The basic data files were generated by the program HKL2000 including the indexing program DENZO with the orthorhombic space group P222, although the tetragonal and monoclinic systems were also suggested with lower probabilities. Tetragonal space groups are often emulated by twinned orthorhombic systems when a and b are approximately equal in length.54 Other details regarding the procedures for data collection and manipulation are well described in a previous report.55 For dehydrated Tl- and H-ZSM-5, respectively, 64 and 82 reflections violated the systematic absences of the orthorhombic space group Pnma (No. 62, k + l ) 2n for 0kl and h ) 2n for hk0), none of them with particularly strong intensity. Furthermore, they would not violate the absence if their Miller indices h and k were switched, indicating that the a and b axes of the smaller second domain of the twinned crystal are parallel to the b and a axes of the other main domain, respectively. This, together with the observed violations of the systematic absences, suggests that the crystals used in this work are nonmerohedral twins whose reciprocal lattices do not overlap exactly so that only some of the reflections are affected by the twinning.54,56 The orthorhombic space group Pnma was therefore obvious for the main domains of the twinned crystals and was used throughout this work, with consideration given to the second domain by including it in the calculations of Fc2 (see section 3). Finally, an additional refinement of the framework atoms in the lower space group Pn21a (No. 33) was not successful, as experienced earlier by van Koningsveld et al.,28 although it had been reported using a rather poor quality ZSM-5 crystal synthesized with an additional reagent for crystal growth.31 A summary of the experimental and crystallographic data is presented in Table 1. Affirming that both crystals were fully dehydrated, their crystal structures refined to relatively low error indices and the final difference Fourier functions were relatively featureless.

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TABLE 1: Experimental Conditions and Crystallographic Data (a) Tl-ZSM-5 crystal size (µm) calcination with oxygen T (K), t (days), P (Torr) Tl+ ion exchange concn (M), T (K), t (days), V (mL) dehydration T (K), t (days), P (Torr) data collection T (K) X-ray source wavelength (Å) space group, No. unit cell parameters a (Å) b (Å) c (Å) max 2θ for data collection (deg) no. of unique reflections measured, m no. of reflections with Fo > 4σ(Fo), m′ no. of variables, s data/parameter ratio, m′/s weighting parameters, a/b BASF parameter, kb final error indices R1c R2d goodness of fite

(b) H-ZSM-5

20 × 20 × 80

20 × 20 × 80

673, 2, 100

673, 2, 100

0.05, 294, 60, 5

-

623, 2, 1 × 10-6 294(1) PLS(4A MXW BL)a 0.7500 Pnma, 62

623, 2, 1 × 10-6 294(1) PLS(4A MXW BL)a 0.7000 Pnma, 62

20.064(1) 19.946(1) 13.416(1) 60.78 6528 3962 417 9.50 0.1775/9.9434 0.102

20.079(1) 19.948(1) 13.419(1) 60.62 8658 4278 333 12.8 0.1886/0.4702 0.115

0.071 0.311 1.26

0.066 0.322 1.19

a Beamline 4A MXW of the Pohang Light Source, Korea. b A refined parameter for the fractional contribution of the second domain of the twinned crystal (see section 3). c R1 ) ∑|Fo - |Fc||∑Fo is calculated using only the reflections for which Fo > 4σ(Fo). d R2 ) [∑w(Fo2 - Fc2)2/ ∑w(Fo2)2]1/2 is calculated using all unique reflections measured. e Goodness of fit ) (∑w(Fo2 - Fc2)2/(m′ - s))1/2.

Also, these structures were each determined multiple times using several independently dehydrated crystals (four Tl-ZSM-5 and nine H-ZSM-5 crystals) and the results, although they showed some variation, were essentially the same. The structures reported here are those with the largest data sets. All 13 crystals were from the same synthesis batch. 3. Structure Determination Full-matrix least-squares refinement (SHELXL97)57 on F2 was initiated with the atomic parameters of the framework atoms, Ti, i ) 1-12, and Oi, i ) 1-26, in dehydrated Cs-ZSM-5.35 The initial refinements for dehydrated Tl- and H-ZSM-5 using isotropic thermal parameters for all positions quickly converged to the error indices R1 ) 0.151 and 0.134 (see Table 1, footnotes) with the 3900 and 4207 reflections for which Fo > 4σ(Fo), respectively, without using the unique reflections from the second domains of the twinned crystals. Subsequent refinements with anisotropic thermal parameters for all T and O positions converged to the error indices R1 ) 0.131 and 0.108, respectively. In subsequent stages of structure determination, the “twin refinement method” implemented in SHELXL9754 allowed all diffraction data to be used. From the success of these refinements, it was clear that, for each twinned crystal, the a and b axes of one twin component are parallel to the b and a axes of the other, respectively. Also, the violations of the systematic absences (paragraph 2 of section 2.3) (also seen for a silicalite precursor with the same topology29) suggested that Fo,hkl2 is the sum of those from the two components: the Fo,hkl2 of one component and Fo,khl2 of the other. This was also strongly indicated in the lists of “most disagreeable” reflections in the previous refinements (done without considering the twinning); the Fo’s tended to be much larger than Fc’s. In this method, therefore, (Fc2)* was calculated as (osf)2∑ki(Fc,i)2, where “osf”

is the overall scale factor, ki is the fractional contribution of the twin domain i (∑ki ) 1), and Fc,i is the calculated structure factor of the corresponding domain i.58,59 This was done by introducing the command lines “TWIN” with a transform matrix (010 100 00-1) and “BASF” with a refined parameter for the fractional contribution (ki) of the second domain.54 This kind of twinning and diffraction behavior is frequently encountered for orthorhombic systems with a and b approximately equal,54 as is the case in this work. When the unique reflections from the second domains were included, the error indices fell sharply to R1 ) 0.095 and 0.066 for dehydrated Tl- and H-ZSM-5 with 3962 and 4278 reflections, respectively. The refinements resulted in ki values of 0.102 and 0.115 for Tl-ZSM-5 and H-ZSM-5, respectively (see Table 1), indicating a ca. 11/89 ) 12% contribution of the second domains to the diffraction intensities of the first ones. This value was approximately the same for all crystals examined. For Tl-ZSM-5, Tl+ ions with reasonable peak heights and geometry with respect to framework oxygen atoms were found in difference Fourier functions. These were introduced stepwise into subsequent refinements. All had fractional occupancies. They were included in the model only if they were stable in least-squares refinement, had acceptable geometry, and allowed the error indices to decrease, even if some occupancy values were significant only at the 2σ level. A total of 18 Tl+ positions were found; their contributions to the error indices upon inclusion are tabulated in Table 2. The final cycles of refinement were performed with anisotropic thermal parameters and fixed occupancies for all framework atoms and isotropic thermal parameters and varied occupancies for all Tl+ ions. It converged with R1 ) 0.071 calculated using the 3962 reflections for which Fo > 4σ(Fo) and R2 ) 0.311 calculated using all 6528 unique reflections measured (see Table 1, footnotes). The final structural parameters are given in Table 3. Selected interatomic distances

0.08 0.03 0.03 0.04 0.11 0.11 0.10 0.10 0.11 0.16 0.08 0.08 0.08 0.08 0.07 0.29 0.32 0.32 0.23 0.26 0.26 0.33 0.32 0.27 0.25 0.16 0.18 0.18 0.22 0.21 0.14 0.19 0.19 0.18 0.18 0.18 0.19 0.20 0.18 0.19 0.18 0.19 0.18 0.18 0.18 0.18 0.15 0.28 0.26 0.23 0.15 0.15 0.15 0.20 0.21 0.70 0.66 0.14 0.15 0.16 0.17 0.16 0.16 0.16 0.16 0.06 0.06 0.07 0.06 0.06 0.07 0.08 0.08 0.07 0.08 0.28 0.28 0.30 0.30 0.32 0.34 0.33 0.37 0.38 0.42 0.40 0.11 0.10 0.10 0.11 0.12 0.15 0.16 0.15 0.15 0.15 0.15 0.09 0.02 0.13 0.12 0.12 0.12 0.14 0.15 0.15 0.14 0.14 0.05 0.05 0.06 0.05 0.07 0.10 0.09 0.09 0.11 0.11 0.10 0.09 0.20 0.20 0.22 0.26 0.31 0.34 0.28 0.26 0.33 0.38 0.38 0.47 0.44 0.28 0.23 0.24 0.11 0.33 0.31 0.30 0.23 0.22 0.27 0.22 0.22 0.18 0.18 1 2 3 4 5e 6f 7 8g 9 10 11 12 13 14 15h

a The estimated standard deviation for each value at Tl(i) is about the same as that given in Table 3. b Defined in footnotes to Table 1. c For all unique measured data. d The initial model consisted of the anisotropically refined atoms of the ZSM-5 framework. e The Tl(4), Tl(5), and Tl(6) positions, which were very close, had to be introduced simultaneously. f Tl(7) and Tl(8) were included together. g Tl(10), Tl(11), and Tl(13) were included together. h Weighting system parameters were refined; their final values are given in Table 1.

0.31 0.31

0.1445 0.1436 0.1430 0.1427 0.1385 0.1310 0.1303 0.1266 0.1259 0.1234 0.1223 0.1212 0.1212 0.1208 0.1203 0.0945 0.0935 0.0928 0.0923 0.0887 0.0820 0.0809 0.0776 0.0766 0.0743 0.0735 0.0725 0.0725 0.0719 0.0711 0.04 0.04 0.04

R1c R1 Tl(18) Tl(15) Tl(14) Tl(13) Tl(12) Tl(11) Tl(10) Tl(9) Tl(8) Tl(7) Tl(6) Tl(5) Tl(4) Tl(3) Tl(2) Tl(1)

d

step/atom

number of Tl+ ions per unit cella

TABLE 2: Steps of Structure Determination As Nonframework Atomic Positions Were Found in Dehydrated Tl-ZSM-5


Tl(17) Tl(16)

error indicesb


and angles are given in Table 4, and all Tl-O distances less than 3.5 Å are given in Table 5. Fixed weights were used initially. The final weights was assigned using the formula w ) 1/[σ2(Fo2) + (aP)2 + bP], where P ) [max(Fo2,0) + 2Fc2]/3, with a and b as refined parameters (see Table 1). Atomic scattering factors for Si, O-, and Tl+ were used.60,61 All scattering factors were modified to account for anomalous dispersion.62,63 In the last cycles of least-squares refinement, all shifts were less than 0.1% of their corresponding estimated standard deviations (esd’s). 4. Detailed Description of the ZSM-5 Framework ZSM-5 (MFI) has two 10-ring channel systems, the zigzag channels (ZC) along the a axis and the straight channels (SC) along b (see Figure 3). The ZCs are sometimes called the sinusoidal channels. These two channels intersect at right angles to form the intersection volume (see the center of box A in Figure 3a, Figure 4a, and the spherical bottom portion (ca. 75%) of Figure 5). This intersection volume together with a small portion of the ZC, hereafter called the cove (top portion (ca. 25%) of Figure 5), constitutes ZSM-5’s cavity (box A of Figure 3a and Figure 5) as defined by the International Zeolite Association.64 Unlike many zeolites (e.g., LTA, FAU, RHO), ZSM-5 does not have an obvious cavity and can readily be discussed without the use of this term. The numbering scheme used in this work for the T and O atoms is that of Olson et al.25 This is also true for the extraframework cation site designations (section 5.2.1.2). The numbering of the rings, Rn, follows from them. 4.1. Ten-Rings Define the Two Channel Systems. A pair of crystallographically unique and compositionally independent 10-rings, R1 and R3, repeated by a 21 axis along a define the ZC (see either box A or B in Figure 3a). A third crystallographically unique 10-ring, R2, is composed mostly of the components of R1 and R3 (see Figure 3a or 5). Two -T7-T4-T5-T6-T9- sequences related by a mirror plane (at y ) 0.25) form R1 (Figures 4b and 5), and two -T10-T1T2-T3-T12- sequences, related by the same mirror plane, form R3 (Figures 4a,b and 5). R1 and R3 face each other, although they are not close to being parallel (see box A in Figure 3a and Figure 5), in a somewhat eclipsed manner as they propagate along a (the ZC). Both are at the surface of the cavity (Figure 5). By the 21 operation along a, R3 and R1 again face each other, this time in an almost parallel manner to complete the ZC (see Figure 3b and box B in Figure 3a). In R2, the tetrahedra are -T1-T2-T3-T12-T11-T7-T8T9-T6-T5-. R2’s are repeated by mirror planes perpendicular to b (Figure 4a) and lie at the surface of the intersection volume and the cavity (Figure 5). Such a pair of R2’s related by an inversion center complete the SC (see that connection in Figures 3c and 4c). Among the 10 tetrahedra of R2, T7 and -T9-T6T5- are shared with R1 and -T1-T2-T3-T12- is shared with R3 (see Figure 5). 4.2. R1, R2, and R3 Connect To Form Additional Rings, Channels, and Cavities. Two m-related O21 oxygen atoms bridge between R1 and R3 to form a fourth 10-ring, R4 (see Figure 5). R4 is composed of two -T10-T1-T5-T6-T9sequences related by the mirror plane between them. Like R1 and R3, the ZC also passes through R4. Although R4 is crystallograpically unique, its constituent T and O atoms are not: -T5-T6-T9- is part of R1, -T10-T1- is part of R3, and -T5-T6-T9-T1- is part of R2. The two m-related R2s, far from parallel as can be seen in Figure 4a or 5, with three additional framework oxygen atoms,

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TABLE 3: Positional, Thermal, and Occupancy Parametersa

atoms

Wyckoff position

occupancyc x

y

z

U11 or Uisob

U22

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14 O15 O16 O17 O18 O19 O20 O21 O22 O23 O24 O25 O26 Tl(1) Tl(2) Tl(3) Tl(4) Tl(5) Tl(6) Tl(7) Tl(8) Tl(9) Tl(10) Tl(11) Tl(12) Tl(13) Tl(14) Tl(15) Tl(16) Tl(17) Tl(18)

8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 4c(m) 4c(m) 4c(m) 4c(m) 4c(m) 8d(1) 4c(m) 8d(1) 8d(1) 8d(1) 4c(m) 8d(1) 4c(m) 8d(1) 8d(1) 8d(1) 8d(1) 4c(m) 8d(1) 8d(1) 4c(m) 8d(1)

42241(9) 30471(10) 28055(9) 12263(9) 7107(9) 18226(9) 42177(9) 30471(10) 27519(9) 12031(9) 6880(9) 18353(9) 36791(30) 30487(34) 20140(30) 10056(35) 11043(24) 23888(32) 36861(32) 30648(34) 19775(29) 9276(38) 11135(30) 23795(32) 30677(50) 7803(31) 42044(31) 41132(34) 40246(36) 18690(32) 18833(34) 19409(37) -568(26) -795(29) 42450(51) 18771(47) 28555(41) 11000(41) 1948(21) 3524(28) 2672(33) 4066(26) 4378(19) 4611(19) 4412(26) 4395(13) 3730(23) 5262(16) 5354(18) 4136(21) 5042(29) 4220(29) 4321(18) 5124(30) 5154(29) 2478(51)

5684(10) 2903(9) 6234(9) 6398(9) 2829(9) 5841(10) -17220(9) -12944(9) -17253(9) -17340(9) -12979(9) -17257(9) 5733(41) 6175(32) 5906(39) 6399(30) 5364(31) 5210(42) -15893(41) -15724(40) -15333(31) -16430(37) -15788(35) -15295(42) -4948(33) -5104(29) 12686(29) -281(31) -13343(29) 12993(28) -2(31) -13032(28) 4888(29) -14786(33) -25000d -25000d -25000d -25000d 7500d 1927(26) 7500d 1511(26) 1127(21) 785(22) 7500d 7943(12) 7500d 2230(17) 1949(21) 6985(23) 2143(25) 7500d 7221(30) 7777(43) 7500d 7058(48)

-34357(14) -19891(15) 2315(14) 2114(14) -19057(14) -33791(14) -33180(14) -19207(14) 2557(14) 2241(14) -18860(15) -32644(14) -25774(50) -8982(43) 2932(57) -9317(41) -28611(37) -25562(48) -24767(48) -8044(44) 2453(49) -8867(46) -28096(43) -23712(50) -18992(66) -17853(50) -40289(49) -42161(44) -43189(44) -39143(45) -41975(46) -42741(45) -20615(45) -21036(45) -35835(69) -35541(65) 5174(57) 5557(52) 4346(27) 8823(35) 4741(45) 8892(32) 9064(23) 9118(21) 4356(37) 4061(17) 2078(28) 6100(24) 6412(28) 1916(29) 6828(45) 9163(31) 9179(25) 8811(45) 8784(41) 3447(85)

(a) Tl-ZSM-5 130(8) 153(8) 184(9) 177(9) 173(9) 150(10) 171(9) 161(10) 141(8) 130(8) 156(9) 157(9) 144(9) 124(8) 193(10) 174(9) 183(9) 122(8) 183(9) 108(8) 144(9) 132(8) 188(9) 122(8) 243(31) 776(53) 542(41) 403(37) 151(30) 698(52) 588(42) 338(36) 161(23) 412(31) 278(32) 841(56) 278(35) 680(49) 386(38) 658(46) 202(29) 278(29) 685(50) 462(42) 296(33) 502(39) 234(35) 825(56) 1320(87) 157(33) 452(37) 163(25) 359(36) 199(27) 549(43) 257(31) 540(42) 267(31) 453(39) 208(27) 555(43) 295(33) 606(46) 240(29) 189(25) 386(33) 196(30) 463(37) 603(64) 117(36) 546(58) 101(34) 366(46) 145(34) 441(49) 101(33) 1409(206) 2853(326) 1237(296) 945(211) 688(159) 1692(164) 740(215) 406(90) 1765(247) 580(121) 772(150) 1276(203) 2288(323) 495(195) 433(148) 166(254) 392(218) 4270(931)

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 O1 O2 O3 O4 O5 O6 O7

8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1)

42251(8) 30552(9) 27995(9) 12274(9) 7129(9) 18360(8) 42227(8) 30576(10) 27506(9) 12035(9) 6927(9) 18467(9) 36960(30) 30616(29) 20124(28) 9978(32) 11191(24) 24016(29) 37016(31)

5670(9) 2906(9) 6225(9) 6400(8) 2808(8) 5862(9) -17207(9) -12909(9) -17265(9) -17333(8) -12982(8) -17278(9) 5631(35) 6110(28) 5910(37) 6226(26) 5429(28) 5319(37) -15685(36)

-34113(13) -19537(14) 2627(13) 2333(13) -18827(13) -33475(14) -32937(13) -18964(13) 2734(13) 2478(13) -18643(14) -32348(13) -25248(47) -8567(4) 3014(57) -8933(38) -28284(37) -25206(47) -24389(46)

(b) H-ZSM-5 143(7) 169(8) 208(9) 196(9) 196(9) 185(9) 189(8) 174(9) 162(8) 160(8) 173(8) 192(9) 161(8) 147(8) 211(9) 198(8) 203(8) 137(8) 203(8) 130(8) 153(8) 166(8) 217(8) 147(8) 357(31) 796(47) 507(36) 426(34) 199(29) 865(56) 668(41) 335(32) 223(23) 459(31) 302(29) 825(51) 355(33) 759(44)

U33

U23

U13

U12

fixed

182(10) 190(10) 169(10) 150(9) 152(9) 188(10) 196(9) 167(10) 171(9) 172(9) 188(10) 217(10) 392(38) 220(31) 660(47) 205(31) 221(27) 346(37) 417(40) 273(34) 616(40) 317(36) 298(32) 414(39) 676(55) 557(40) 518(39) 371(37) 254(32) 365(35) 371(35) 309(34) 375(32) 399(35) 503(55) 449(51) 318(44) 198(38)

9(7) 4(6) -23(7) -27(6) -3(6) 25(7) 0(6) -2(6) 20(7) 3(6) 13(6) -10(6) -73(34) -40(23) -52(37) -14(23) 65(23) 17(34) -76(33) 180(30) -26(27) 165(29) -134(27) -74(35) -6(31) 23(27) 124(25) -118(25) 71(22) 97(21) -120(25) 82(22) -19(24) 30(26) 0 0 0 0

-16(6) 15(7) -14(6) 22(6) 32(7) 55(6) 0(6) 36(8) -25(7) -5(7) -3(7) 47(7) 163(26) 68(25) -25(28) -102(26) 93(19) -58(26) 145(28) 100(26) 27(27) -231(32) 80(23) -88(27) 7(50) 19(28) -171(28) -82(28) -68(27) 183(27) 82(29) 86(29) 61(22) 17(25) -53(43) 196(40) -50(32) -8(30)

2(7) 11(7) 13(6) -3(6) -3(6) -3(6) 5(6) -5(7) -25(6) 18(6) 9(6) -11(6) -40(34) -150(31) 41(31) -134(28) 6(22) 93(35) 51(32) 48(33) -9(23) -85(36) 45(28) -110(34) -52(43) 14(25) 6(23) -76(27) 74(26) -30(24) 102(29) -76(28) 125(24) -89(24) 0 0 0 0

8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 4 4 4 4

varied

0.18(2) 0.44(5) 0.09(2) 0.14(3) 0.15(3) 0.40(4) 0.08(2) 0.16(2) 0.21(3) 0.18(3) 0.20(4) 0.21(3) 0.32(5) 0.07(3) 0.11(3) 0.04(2) 0.04(2) 0.31(6) 195(8) 205(9) 174(8) 154(8) 143(8) 209(8) 197(8) 164(8) 168(8) 174(8) 176(8) 196(8) 413(34) 227(24) 713(49) 199(24) 255(24) 462(38) 402(36)

3(7) 12(7) -23(7) -30(7) -2(6) 29(7) 2(7) -8(6) 11(7) 6(7) 8(7) -15(7) -44(35) -68(23) -108(41) -38(24) 120(24) 27(38) -109(34)

-22(6) 21(8) -22(6) 16(6) 28(7) 69(6) 9(6) 44(8) -20(6) 8(7) 0(7) 43(7) 261(27) 75(24) -21(29) -117(25) 83(19) -129(27) 220(30)

8(7) 8(7) 10(7) 3(6) 2(7) -2(7) 12(6) -7(7) -16(7) 24(7) -3(6) -14(7) -55(35) -68(28) 37(34) -64(27) -44(24) 169(33) 73(33)

8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8



TABLE 3: Continued occupancyc atoms

Wyckoff position

x

y

z

U11 or Uisob

U22

U33

U23

U13

U12

fixed

O8 O9 O10 O11 O12 O13 O14 O15 O16 O17 O18 O19 O20 O21 O22 O23 O24 O25 O26

8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 8d(1) 4c(m) 4c(m) 4c(m) 4c(m)

30707(30) 19768(26) 9210(33) 11311(27) 24046(30) 30773(42) 7851(30) 41931(29) 40907(33) 40177(33) 18808(29) 19051(31) 19437(34) -487(22) -672(25) 42304(48) 18877(42) 28539(41) 11030(39)

-15611(35) -15373(29) -16448(32) -15767(31) -15458(40) -5012(30) -5122(26) 12697(26) -229(27) -13295(25) 12970(26) 49(28) -13022(25) 4934(28) -14855(30) -25000d -25000d -25000d -25000d

-7805(42) 2629(46) -8562(44) -27761(43) -24493(49) -18725(65) -17767(47) -39852(48) -41787(44) -42927(41) -38797(44) -41591(46) -42406(43) -20571(39) -20881(41) -35466(62) -35283(60) 5347(56) 5785(53)

408(35) 194(25) 633(43) 267(28) 291(32) 1267(77) 522(37) 429(35) 691(45) 591(40) 435(34) 582(41) 701(26) 156(21) 180(25) 763(66) 543(52) 467(47) 470(47)

661(40) 339(27) 523(40) 509(34) 914(53) 157(30) 172(25) 208(25) 239(29) 252(28) 263(26) 296(30) 222(26) 453(32) 495(34) 110(33) 113(31) 126(33) 98(31)

302(29) 605(38) 305(30) 375(32) 458(39) 797(56) 578(37) 567(37) 422(33) 269(29) 408(32) 445(34) 338(30) 333(28) 367(32) 415(46) 432(44) 359(42) 273(387)

186(29) -22(28) 188(29) -152(28) -196(38) 66(30) -8(26) 162(25) -115(26) 89(22) 115(23) -128(25) 100(22) -26(24) 36(25) 0 0 0 0

94(27) 6(27) -198(30) 79(23) -140(29) -76(49) -26(29) -203(30) -187(32) -94(27) 184(32) 84(31) 59(31) 66(19) 31(23) -60(45) 189(39) -47(35) 37(32)

62(31) -16(23) -18(33) 20(27) -92(34) 3(38) -16(25) 18(23) -45(27) 67(24) -40(24) 111(28) -107(27) 87(23) -109(23) 0 0 0 0

8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 4 4 4 4

varied

a Positional parameters of framework atoms × 105 are given, while others and thermal parameters × 104 are given. Numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. b The anisotropic temperature factor is exp[-2π2a-2(U11h2 + U22k2 + U33l2 + 2U12hk + 2U13hl + 2U23kl)]. c Occupancy factors are given as the number of atoms or ions per unit cell. d Exact value by symmetry.

O25, O23, and O24 that lie on that mirror plane, form a pair of 6-rings (see the bottom left sides of Figure 4a or 5). These two 6-rings, -T9-T8-T7-T7-T8-T9- and -T7-T11-T12T12-T11-T7- at the surface of the cavity, share an edge at O23 and are fairly planar. Three additional 6-rings bridge between these m-related R2s outside the intersection volume (see top of Figure 5). Two are an m-related -T11-T10-T1T5-T4-T7- pair which are bridged by O23 and O26 to give -T7-T11-T10-T10-T11-T7-, the third 6-ring, between them. These three 6-rings are considerably more distorted than those in the first set; they form the cove. This cove may provide a more suitable environment for extraframework cations, and, indeed, has attracted a large fraction of the Tl+ ions in Tl-ZSM-5 (see paragraph 2 of section 5.2.2.1), but at its mouth rather than within its volume. These two sets of 6-rings, together with R1, R3, and the two m-related R2s, complete the cavity (Figure 5).64 Finally, together with the cavity, additional connections between R1 and R3, consisting of one 6-ring, six 5-rings, and one 4-ring, complete the ZC (see Figure 4b and box B in Figure 3a). With the intersection volume, the connections between R2 and its inversion-related R2, each consisting of four 5-rings and two 6-rings (see Figure 4a,c) complete the SC. 4.3. Planarity and Ellipticity of the 10-Rings. Unlike the rings that define the windows and channels in LTA, FAU, and RHO, the 10-rings in ZSM-5 are neither round nor planar (see Figures 3 and 4). The ellipticity of these rings is discussed in section 5.1.3. 5. Results and Discussion 5.1. Framework Geometry of Tl- and H-ZSM-5. All of the atomic parameters of the framework atoms in both structures (Table 3) are similar to those reported earlier for ZSM-5 and silicalite.25-30 Even though the crystals studied are (1) not as large as some of those studied before and (2) modestly twinned, the precision of the framework geometry is high enough for detailed differences to be seen among the T and O atoms. In fact, all parameters in these results are substantially more precise than those reported by Lermer et al.26 (see Table 6) using a

much larger (260 × 130 × 120 µm) untwinned single crystal (Si/Al ) 11.9) of ZSM-5 (TPA-ZSM-5 with no postsynthesis treatment; the TPA cations were not found). They are almost as precise as the parameters in TPA-ZSM-5 (Si/Al ) 299) by Van Koningsveld et al. (see Table 6), who studied an even larger single crystal (230 × 200 × 150 µm) at similar resolution (2θmax ca. 60.0°, λ ) 0.710 69 Å) (see Table 1).28 5.1.1. T-O Bond Lengths. The T-O bond lengths in both structures affirm their siliceous nature. The mean T-O bond lengths for the 12 T tetrahedra lie in the narrow ranges 1.590(6)-1.601(6) Å for Tl-ZSM-5 and 1.586(6)-1.600(6) Å for H-ZSM-5, essentially the same in the two crystals (see Table 4). Appropriately, they are slightly longer than those (1.584-1.591 Å) in TPA-ZSM-5 with a very low Al content (Si/Al ) 299).28 Individual Ti-Oi bond lengths range somewhat more widely, from 1.572(6) to 1.611(6) Å. The overall mean T-O bond length for both structures is 1.594 Å (1.596 and 1.593 Å for Tl- and H-ZSM-5, respectively). 5.1.2. T-O-T Bond Angles and Framework Flexibility. The T-O-T angles are essentially the same in Tl- and H-ZSM-5; only the angle at O16 seems to be significantly different (see Table 4 or Figure 6). This is a measure of the inflexibility of the ZSM-5 framework. Its flexibility should be more apparent in these angles than in the much more inflexible O-T-O angles (all very close to tetrahedral) and T-O bond lengths. Nonetheless, although none of the T-O bond lengths differed by more than 3σ (see Table 4), some small structural changes were observed upon replacement of H+ by Tl+. Seven of the 12 T sites in Tl-ZSM-5, T2, T3, T6, T7, T8, T11, and T12, have two Cartesian coordinates that differ by more than 3σ of their corresponding esd’s from those in H-ZSM-5 (Table 3). The unit cell constants of dehydrated Tl- and H-ZSM-5 also indicate a small change: the a values differ significantly, but only by less than 0.1%. The T-O-T angles in both structures are compared in Figure 6 with those previously reported for dehydrated Cs-ZSM-5,34 TPA-ZSM-5,26 and TPA-silicalite.29 It is apparent that most T-O-T angles in Tl- and H-ZSM-5 do not range as widely as the corresponding angles in TPA-silicalite.29 This is also

19944


Heo et al.

TABLE 4: Selected Bond Lengths (Å) and Angles (deg)a bond lengths

Tl-ZSM-5

H-ZSM-5

angles

Tl-ZSM-5

H-ZSM-5

T1-O1 T1-O15 T1-O16 T1-O21 mean

1.588(6) 1.608(6) 1.600(6) 1.597(5) 1.598(6)

1.595(6) 1.601(5) 1.587(6) 1.595(5) 1.595(6)

T2-O1 T2-O2 T2-O6 T2-O13 mean

1.597(6) 1.603(6) 1.592(6) 1.571(7) 1.591(7)

1.593(6) 1.605(6) 1.591(6) 1.584(6) 1.593(6)


1.592(6) 1.592(6) 1.588(6) 1.593(6) 1.591(6)

1.592(6) 1.583(6) 1.588(6) 1.596(5) 1.590(6)


1.587(6) 1.596(6) 1.595(6) 1.603(6) 1.595(6)

1.582(6) 1.581(5) 1.596(6) 1.593(5) 1.588(6)


1.601(6) 1.588(5) 1.597(6) 1.608(5) 1.599(6)

1.598(5) 1.597(5) 1.595(5) 1.604(5) 1.599(5)


1.603(5) 1.589(6) 1.600(6) 1.606(6) 1.600(6)

1.601(5) 1.592(6) 1.590(5) 1.597(6) 1.595(6)

T7-O7 T7-O17 T7-O22 T7-O23 mean

1.575(6) 1.597(6) 1.595(6) 1.593(3) 1.590(6)

1.582(6) 1.605(5) 1.586(5) 1.591(2) 1.591(6)


1.596(6) 1.598(6) 1.600(6) 1.596(7) 1.598(7)

1.584(6) 1.592(6) 1.590(6) 1.576(6) 1.586(6)


1.584(6) 1.600(6) 1.594(6) 1.598(3) 1.594(6)

1.588(6) 1.599(3) 1.604(5) 1.596(3) 1.597(6)

T10-O9 T10-O10 T10-O15 T10-O26 mean

1.605(6) 1.600(6) 1.592(6) 1.605(3) 1.601(6)

1.601(5) 1.596(6) 1.597(5) 1.605(3) 1.600(6)

T11-O10

1.582(6)

1.587(6)

O1-T1-O15 O1-T1-O16 O1-T1-O21 O15-T1-O16 O15-T1-O21 O16-T1-O21 mean O1-T2-O2 O1-T2-O6 O1-T2-O13 O2-T2-O6 O2-T2-O13 O6-T2-O13 mean O2-T3-O3 O2-T3-O19 O2-T3-O20 O3-T3-O19 O3-T3-O20 O19-T3-O20 mean O3-T4-O4 O3-T4-O16 O3-T4-O17 O4-T4-O16 O4-T4-O17 O16-T4-O17 mean O4-T5-O5 O4-T5-O14 O4-T5-O21 O5-T5-O14 O5-T5-O21 O14-T5-O21 mean O5-T6-O6 O5-T6-O18 O5-T6-O19 O6-T6-O18 O6-T6-O19 O18-T6-O19 mean O7-T7-O17 O7-T7-O22 O7-T7-O23 O17-T7-O22 O17-T7-O23 O22-T7-O23 mean O7-T8-O8 O7-T8-O12 O7-T8-O13 O8-T8-O12 O8-T8-O13 O12-T8-O13 mean O8-T9-O9 O8-T9-O18 O8-T9-O25 O9-T9-O18 O9-T9-O25 O18-T9-O25 mean O9-T10-O10 O9-T10-O15 O9-T10-O26 O10-T10-O15 O10-T10-O26 O15-T10-O26 mean O10-T11-O11

109.7(4) 112.5(4) 108.7(4) 108.6(4) 108.4(3) 109.0(3) 109.5(4) 107.8(4) 108.7(4) 111.7(5) 108.7(4) 109.6(4) 110.3(5) 109.5(5) 110.8(4) 109.4(4) 107.8(3) 109.6(4) 109.4(4) 109.9(4) 109.5(4) 110.0(4) 110.2(4) 109.8(4) 110.1(3) 106.9(4) 109.7(4) 109.5(4) 109.5(3) 109.0(3) 110.3(4) 110.7(3) 106.9(3) 110.5(3) 109.5(4) 109.7(3) 107.5(3) 108.8(4) 109.9(4) 111.3(4) 109.6(4) 109.5(4) 110.9(4) 107.1(4) 110.3(5) 111.4(4) 107.0(4) 110.2(5) 109.5(5) 107.0(4) 110.4(4) 110.9(5) 110.5(4) 109.2(5) 108.8(5) 109.5(5) 109.3(4) 109.6(4) 109.4(4) 110.0(3) 111.1(4) 107.5(4) 109.5(4) 108.8(4) 109.9(3) 110.9(4) 110.1(4) 108.8(4) 108.3(4) 109.5(4) 109.9(4)

109.7(4) 111.6(4) 108.3(3) 109.3(3) 107.8(3) 110.1(3) 109.5(4) 107.4(3) 109.4(4) 110.5(4) 108.9(3) 109.5(4) 111.0(4) 109.5(4) 111.1(4) 109.1(3) 107.4(3) 109.0(4) 110.0(4) 110.2(3) 109.5(4) 110.2(4) 108.8(4) 109.8(4) 109.8(3) 108.1(3) 110.2(3) 109.5(4) 109.7(3) 108.5(3) 110.5(3) 110.5(3) 106.5(3) 111.2(3) 109.5(3) 109.5(3) 107.1(3) 109.6(3) 109.5(4) 111.4(3) 109.7(3) 109.5(4) 110.0(4) 107.7(3) 110.4(5) 110.9(3) 107.4(4) 110.4(4) 109.5(5) 107.5(4) 110.3(4) 109.8(4) 110.2(4) 108.6(4) 110.5(4) 109.5(4) 109.7(3) 109.5(4) 110.1(4) 109.2(3) 110.9(4) 107.5(3) 109.5(4) 109.2(3) 109.5(3) 110.6(4) 110.9(4) 108.5(4) 108.1(3) 109.5(4) 109.9(4)



TABLE 4: Continued bond lengths

Tl-ZSM-5

H-ZSM-5

angles

Tl-ZSM-5

H-ZSM-5

T11-O11 T11-O14 T11-O22 mean

1.606(6) 1.587(6) 1.608(6) 1.596(6)

1.606(6) 1.583(6) 1.599(5) 1.594(6)

T12-O11 T12-O12 T12-O20 T12-O24 mean

1.599(6) 1.574(6) 1.609(6) 1.595(3) 1.594(6)

1.592(5) 1.580(6) 1.606(6) 1.592(3) 1.593(6)

O10-T11-O14 O10-T11-O22 O11-T11-O14 O11-T11-O22 O14-T11-O22 mean O11-T12-O12 O11-T12-O20 O11-T12-O24 O12-T12-O20 O12-T12-O24 O20-T12-O24 mean

108.8(4) 110.3(4) 110.4(4) 106.9(3) 110.4(4) 109.5(4) 109.0(4) 110.2(4) 108.6(4) 110.3(4) 111.7(5) 107.2(4) 109.5(5)

109.5(4) 109.5(3) 109.6(3) 107.4(3) 110.9(3) 109.5(4) 109.8(3) 109.5(4) 109.0(4) 110.7(4) 110.5(5) 107.3(3) 109.5(5)

angles

Tl-ZSM-5

H-ZSM-5

angles

Tl-ZSM-5

H-ZSM-5

T1-O1-T2 T2-O2-T3 T3-O3-T4 T4-O4-T5 T5-O5-T6 T2-O6-T6 T7-O7-T8 T8-O8-T9 T9-O9-T10 T10-O10-T11 T11-O11-T12 T8-O12-T12 T8-O13-T2

154.6(5) 150.7(4) 170.9(6) 152.4(4) 144.7(4) 161.4(5) 160.2(5) 154.1(5) 151.7(4) 160.7(5) 147.2(4) 165.0(5) 173.8(7)

153.9(5) 150.3(4) 172.2(6) 155.2(4) 145.4(4) 160.5(5) 160.1(5) 154.4(4) 152.2(4) 160.4(5) 148.7(4) 165.7(5) 174.1(6)

T5-O14-T11 T1-O15-T10 T1-O16-T4 T4-O17-T7 T6-O18-T9 T3-O19-T6 T3-O20-T12 T1-O21-T5 T7-O22-T11 T7-O23-T7 T12-O24-T12 T9-O25-T9 T10-O26-T10

164.1(5) 146.2(4) 160.6(5) 145.6(4) 144.8(4) 158.1(5) 147.0(5) 146.8(4) 148.3(4) 153.9(7) 151.1(6) 150.4(6) 144.4(5)

164.9(5) 146.9(4) 164.0(5) 146.4(4) 145.1(4) 159.8(5) 147.0(4) 147.0(4) 150.0(4) 155.3(6) 150.7(6) 150.4(5) 144.6(5)

Tl-Ob

Tl-ZSM-5

Tl-Ob

Tl-ZSM-5

Tl-Ob

Tl-ZSM-5

Tl(1)-O24 Tl(1)-O20 Tl(2)-O2 Tl(2)-O20 Tl(3)-O24 Tl(3)-O20 Tl(4)-O2 Tl(4)-O1 Tl(5)-O1 Tl(5)-O2 Tl(5)-O21 Tl(6)-O21 Tl(6)-O1

2.82(4) 3.021(23) 2.81(5) 2.99(5) 2.79(6) 3.10(4) 2.72(5) 2.83(5) 2.84(3) 2.86(4) 3.18(3) 2.90(3) 2.97(3)


3.15(4) 2.79(5) 3.03(3) 2.71(3) 2.94(3) 2.73(4) 2.99(3) 2.87(3) 2.88(3) 2.74(3) 2.99(4) 3.15(4) 2.68(4)


2.94(4) 2.68(5) 2.94(5) 2.97(5) 3.05(4) 3.29(5) 2.83(4) 2.85(4) 2.72(7) 2.76(7) 2.97(5) 3.07(4) 3.18(10)

a The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. b All Tl-O distances shorter than 3.20 Å.

true for high-Al TPA-ZSM-5 (Si/Al ) 11.9)26 with respect to TPA-silicalite.29 This indicates that Al atoms, substituting for Si, allow the ZSM-5 framework to relax a little, to become more “rounded” as T-O-T angles move toward minimum energy values. A possible explanation might be that the Al-O bond, longer than Si-O, allows the AlO4 tetrahedra to be more flexible than SiO4. It is also possible that Al atoms substitute for those Si atoms whose Si-O bonds are stretched, thus relieving strain (see the final paragraph of section 5.3.4 for further discussion). 5.1.3. Ellipticity of 10-Rings. The ellipticity (ε) of a 10-ring has been defined to be the ratio of the longest and shortest of the five diagonal O · · · O approach distances.32 The ellipticities of R1, R2, R3, and R4 in Tl-ZSM-5 (this work) are therefore ε1 ) (O23 · · · O25/O17 · · · O18) ) 1.05, ε2 ) (O1 · · · O7/ O5 · · · O11) ) 1.16, ε3 ) (O15 · · · O20/O1 · · · O2) ) 1.04, and ε4 ) (O25 · · · O26/O15 · · · O18) ) 1.02, respectively. The corresponding values in H-ZSM-5, 1.08, 1.13, 1.03, and 1.02, are similar. R2 is therefore the most elliptical of the four 10rings and the SC is more oval in its cross section than the ZC (compare Figure 3b,c).

A comparison of the ε2 values reported for variously ionexchanged ZSM-5 with different framework compositions32,36 suggests that they depend on the guest cations and their degree of exchange. The reported ε2’s range from 1.15 to 1.22 for M-ZSM-5, where M ) large cations such as K+, Rb+, Tl+, and Cs+, but they are close to 1.0 with smaller cations such as Li+ and Na+.32 Note that ε2 for H-ZSM-5 (1.13) is much larger than that found with Li+ and Na+.32 It is much less, however, close to 1.0 with substantial variation, in four perdeuterobenzene complexes of H-ZSM-5.32 The ε2 of Tl-ZSM-5 (1.16) agrees with that reported for a similarly prepared powder sample (1.15).32 5.2. Tl+ Positions in Tl-ZSM-5. 5.2.1. Classification of Tl+ Positions into Groups, Sites, and Subsites. 5.2.1.1. Groups of Tl+ Positions. The 18 Tl+ positions found in this work are assigned to three cation site groups Gn, n ) 1-3, about the correspondingly labeled three 10-rings Rn, n ) 1-3 (see Table 7). G1 holds the most Tl+ ions, 1.36(10) per unit cell, and G2 and G3 have similar populations, 0.95(9) and 1.02(8) per unit cell, respectively. To better describe their

19946


TABLE 5: All Tl-O Distances Less Than 3.5 Å in Tl-ZSM-5a distance (Å) Tl(1)-O24 Tl(1)-O20b

2.82(4) 3.021(23)

Tl(2)-O2 Tl(2)-O20 Tl(2)-O1

2.81(5) 2.99(5) 3.30(5)

Tl(3)-O24 Tl(3)-O20b Tl(4)-O2 Tl(4)-O1 Tl(4)-O20 Tl(5)-O1 Tl(5)-O2 Tl(5)-O21 Tl(5)-O20

2.79(6) 3.10(4) 2.72(5) 2.83(5) 3.21(4) 2.84(3) 2.86(4) 3.18(3) 3.48(4)

Tl(6)-O21 Tl(6)-O1 Tl(6)-O2 Tl(6)-O5

2.90(3) 2.97(3) 3.15(4) 3.47(4)

Tl(7)-O23 Tl(7)-O17b Tl(7)-O26

2.79(5) 3.03(3) 3.39(5)

Tl(8)-O17 Tl(8)-O4 Tl(8)-O15 Tl(8)-O23

2.71(3) 2.94(3) 3.22(3) 3.29(3)

Tl(9)-O25 Tl(9)-O18b

2.73(4) 2.99(3)

distance (Å) Tl(10)-O15 Tl(10)-O26 Tl(10)-O17

2.87(3) 2.88(3) 3.31(3)

Tl(11)-O15 Tl(11)-O4 Tl(11)-O21 Tl(11)-O17 Tl(11)-O26 Tl(12)-O18 Tl(12)-O5 Tl(12)-O25 Tl(13)-O15 Tl(13)-O26 Tl(13)-O21

2.74(3) 2.99(4) 3.15(4) 3.31(4) 3.32(4) 2.68(4) 2.94(4) 3.34(4) 2.68(5) 2.94(5) 3.32(5)

Tl(14)-O8b Tl(14)-O7 Tl(14)-O25

2.97(5) 3.05(4) 3.29(5)

Tl(15)-O8c Tl(15)-O7c Tl(15)-O22 Tl(15)-O25

2.83(4)/3.48(5) 2.85(4)/3.49(5) 3.38(4) 3.49(4)

Tl(16)-O11c Tl(16)-O22c

2.72(7)/3.38(7) 2.76(7)/3.48(7)

Tl(17)-O11b Tl(17)-O22 Tl(17)-O24

2.97(5) 3.07(4) 3.47(6)

Tl(18)-O2 Tl(18)-O6

3.18(10) 3.36(10)

a The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. b Because this Tl+ position lies on a mirror plane, this distance appears twice, to two equivalent oxygen atoms. c Two different Tl-O distances to two symmetry equivalent oxygen atoms.

locations with respect to their corresponding 10-rings, each of these three groups is further divided into two cation sites Sn and Sn′ in section 5.2.1.2. This follows established nomenclature; however, because of the resolution achieved in this report, each of these “sites” is home to one to six actual Tl+ positions. In turn, each of these six cation sites is subdivided in section 5.2.1.3 into two or three subsites, each of which is home to zero to four actual Tl+ positions. 5.2.1.2. Sites (Subgroups) of Tl+ Positions. For each cation site group Gn, the two cation sites Sn and Sn′ are distinguished by their locations about the corresponding 10-rings, Rn, as tabulated in Table 7. The Tl+ positions on or nearly on the Rn planes are assigned to Sn; those that are near but substantially further from that plane are assigned to Sn′. Similar notation, but without such detailed definition because fewer exchangeable cation positions were found, was used to describe the locations of cations in variously ion-exchanged powder samples of ZSM-5.32,35,36,39 The Sn′ sites require additional definition to avoid spatial overlap among the three Sn′ sites. This is due to the ring stackings and to the sharing of segments by some of the Rn’s. For example, the region designated S2′ (the volume between the m-related R2’s) overlaps with those of S1′ and S3′ (see Figures 4a, 5, and 7a). Because this S2′ region is, by the above

Heo et al. definition, in the intersection volume, it partially overlaps with S1′ and S3′. This entire region, together with the volume between the i-related R2’s, is here defined to be S2′ in the main body of the SC. The region between R1 and R3 at their parallel stacking (see box B of Figure 3a and Figures 4b and 7a), which could be considered either S1′ or S3′ by the original definition, is here defined to be S3′. These two additional definitions of specific regions for S2′ and S3′ limits the S1′ region to the volume defined by the three consecutive 6-rings (the cove) at the eclipsed stacking of R1 and R3 at the top of the cavity in Figures 5 and 7. The 18 Tl+ positions are tabulated by group (Gn, n ) 1-3) and cation site (Sn and Sn′ for each Gn) in Table 7. Seven Tl+ positions (two at S1 and five at S1′) are in cation group G1. Another seven (one at S2 and six at S2′) are in G2, and four (two at S3 and two at S3′) are in G3. All Tl+ positions are shown in Figure 7, together with their cation site designations, S1, S1′, S2, S2′, S3, and S3′. 5.2.1.3. Subsites of Tl+ Positions. Finally, to provide a more detailed description of the coordination situations of the Tl+ ions, each of the cation sites Sn and Sn′ is further divided into two or three subsites to indicate which atoms (O and T) of secondary 10-rings are closest (see Table 7). If a Tl+ ion at Sn is close to its Rn plane and interacts with a part of Rn (a set of O and T atoms) that is shared with another 10-ring, Rm, the subsite for that Tl+ ion is named Snm. Considering the shared O and T atoms among the three 10rings, therefore, S1 can be divided into S11 and S12, S2 into S21, S22, and S23, and S3 into S32 and S33. For example, the Tl+ ions at subsites Snm (n * m) are close to a set of O and T atoms that is shared by the two 10-rings, Rn and Rm, but they are closer to the Rn plane. On the other hand, those at subsites Snn interact only with the O and T atoms of the Rn 10-ring. In this way, the Tl+ ions at cation site S1′ are assigned to two subsites: S1′1 for those interacting only with the oxygen atoms of R1 and S1′3 for those interacting with framework atoms that are also part of ring R3 (see Table 7). The Tl+ ions at S2′ are assigned to three subsites, S2′1, S2′2, and S2′3, for those close to the oxygen atoms common to R2 and R1, those close only to the oxygen atoms of R2 (between m- and i-related R2s), and those close to the oxygen atoms common to R2 and R3, respectively. Similarly, the Tl+ ions at S3′ are in subsites either S3′1 or S3′2. 5.2.2. Tl+ Positions and Occupancies. It can be seen in Table 4 that 14 of the 18 Tl+-to-nearest-oxygen bond lengths lie in the narrow range between 2.68(4) and 2.87(3) Å. Considering their esd’s, these 14 can be considered to be essentially the same, and the same as the sum of the Tl+ and O2- radii,41,42 2.79 Å. At somewhat longer distances, 2.90(3)-2.97(5) Å, are Tl(6), Tl(14), and Tl(17). Remaining is Tl(18), which at 3.18(10) Å is unacceptably far from its nearest framework oxygen. This is discussed in paragraphs 2 and 3 of section 5.2.2.3. 5.2.2.1. Tl+ Ions in Group 1. Per unit cell, 0.42(4) Tl+ ions were found at subsite S12, 0.21(3) at Tl(9), and 0.21(3) at Tl(12) (see Table 7). Both Tl+ positions are near the middle of the ring segment that R1 and R2 share. The shortest distance between Tl(9) and Tl(12), 1.33 Å, is impossibly short, indicating that it is virtual; these ions can readily be distributed within their equipoints so as to avoid this contact. Tl(9) lies on a mirror plane and is nearly on the R1 plane, while Tl(12) is somewhat off both of those planes. Because Tl(12) is nearly on the R1 plane, it is considered to be at subsite S12 (see Figure 8 and Table 7). Their bond lengths to the closest framework oxygen



Figure 3. (a) Two unit cells (dashed lines) of the framework structure of Tl-ZSM-5 viewed along the b axis. Box A highlights the cavity64 formed by the three 10-rings (R1, R2, and R3) around the straight channel (SC), and box B shows the relationship between R1 and R3 in defining the zigzag channel (ZC). Three crystallographically unique 10-rings, R1, R2, and R3, are labeled. R1 is highlighted with large orange atoms, and R3 is highlighted with large red atoms. The R2 10-ring is perpendicular to the viewing axis, b, and defines the SC. (b) A segment of the ZC viewed along its sinusoidal curvature at x ≈ 0.5. R1 and R3 are shown. (c) A segment of the SC viewed approximately along b. Two R2 rings related by an m ⊥ b are shown. They are far from parallel as can better be seen in Figure 4a.

atoms, 2.73(4) Å for Tl(9)-O25 and 2.68(4) Å for Tl(12)-O18, appear to be a little shorter than the sum of the corresponding ionic radii (2.79 Å ) rTl+ (1.47 Å) + rO2- (1.32 Å))41,42 and are among the shortest Tl-O distances in Tl-ZSM-5 (see Table 4). Tl(9) and Tl(12) are strongly held (see Figure 8). Per unit cell, a total of 0.94(8) Tl+ ions are found at Tl(7), Tl(8), Tl(10), Tl(11), and Tl(13) (see Figure 8), at site S1′ (between R1 and R3, in the cove most easily seen at the top of the cavity64 in Figure 5). They are distributed among the S1′1 and S1′3 subsites according to their interactions with R1 and R3 (see Table 7). At subsite S1′3, a total of 0.70(8) Tl+ ions per unit cell were found at Tl(10), Tl(11), and Tl(13) (0.18(4), 0.20(4), and 0.32(5) Tl+ ions, respectively). Fewer Tl+ ions were found at subsite S1′1: 0.08(2) at Tl(7) and 0.16(2) at Tl(8). They are in the ZC between R1 and R3, closer to R1. Their bond lengths to the closest framework oxygen atoms (see Table 4) are all close to the sum of the corresponding ionic radii, 2.79

Å, indicative of strong Tl-O bonds. They are also comparable to Tl-O bond lengths previously seen in zeolites.65-67 Of the five positions reported by Mentzen in a powder sample of Tl-ZSM-5,32 one (Tl(1) at S1′, his nomenclature, unreasonably far (3.18(2) Å) from its nearest framework oxygen atom) is nearly at the center of four Tl+ positions at S1′, Tl(i), i ) 7, 8, 10, and 11, of which have all good shortest Tl-O bond lengths (see Table 4). Similarly, of the three positions reported by Olson for Cs+ ions in dehydrated Cs-ZSM-5,35 one (Cs(1) at S1′, again unreasonably far (3.43(4) Å) from its nearest framework oxygen atom when it is compared to the sum of the corresponding ionic radii,41,42 2.99 Å ) rCs+ (1.67 Å) + rO2- (1.32 Å)) is also close to the middle of these four Tl+ positions. Thus it appears that Mentzen’s Tl(1) and Olson’s Cs(1) have each been resolved in this work into these four positions.

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Figure 4. Stereoviews of three segments of the framework structure of ZSM-5 in dehydrated Tl-ZSM-5. The connections between the 10-rings can be seen. (a) The intersection volume (symmetry m, the spherical volume in box A of Figure 3a). (b) The part of the ZC that is not in the intersection volume, symmetry m. The connection between adjacent R1 and R3 rings (box B of Figure 3a) can be seen. The volume between R1 and R4 is the cove, symmetry m. The intersection volume plus the cove has been defined by the IZA to be the cavity (Figure 5), symmetry m.64 (c) The part of the SC that is not in the intersection volume, symmetry i. The connection between two R2 rings is shown. The entire ZSM-5 framework may be generated by bringing these three segments together so that identical R2, R3, and R4 rings coalesce to become shared. Ellipsoids of 50% probability are shown.

S1′ must therefore be one of the most popular cation sites in ZSM-5, at least for large monopositive cations such as Tl+ and Cs+. Furthermore, one of the two positions found for Ni2+ in dehydrated Ni-ZSM-539 is at S1′, where it bonds very strongly to two R1 oxygen atoms (Ni(1)-O13 ) 2.23 Å). It appears that small and highly charged cations like Ni2+ prefer to be in the cove where they can achieve a relatively adequate first coordination sphere. Additional determinations of the positions of other first row transition metal cations in dehydrated ZSM-5 are in progress in this laboratory.

5.2.2.2. Tl+ Ions in Group 2. At seven positions within sites S2 and S2′ (see Table 7), 0.95(9) Tl+ ions were found per unit cell. Among the seven positions, only the one at Tl(6) is at S2; it is close to the ring segment shared with R3, so it is at subsite S23. The remaining six Tl+ positions are at site S2′, at subsites S2′2 and S2′3. The Tl+ ions at S2′3, Tl(4) and Tl(5), are near the R2 plane and are close to the oxygen atoms shared by R2 and R3 (see Figures 4a, 7, and 9). The remaining Tl+ ions, Tl(i), i ) 14-17, are at subsite S2′2, between the m-related R2s, so they interact only with R2 oxygen atoms.



Figure 5. Stereoview of the ZSM-5 cavity (as defined by the IZC),64 symmetry m, in dehydrated Tl-ZSM-5. This egg-shaped cavity is composed of the intersection (of the straight and zigzag channels) volume (at the large end of the cavity, about 75% of its volume) and the cove (at the narrow end of the cavity, actually a section of the ZC). See the Figure 4 caption for other details.

TABLE 6: Comparisons among Reported ZSM-5 Structures structure

samplea

H-ZSM-5 Tl-ZSM-5 TPA-ZSM-5 TPA-ZSM-5e TPA-ZSM-5 TPA-ZSM-5e TPA-silicalite Cs-ZSM-5 Cs-ZSM-5 Tl-ZSM-5

S S S S S S S P P P

Si/Al 25.4 25.4 299 11.9 25 86 ∞ 26 15.6 13

no. of unique reflections

R1

T-O rangeb (Å)

T-O-T rangeb (deg)

O-T-O rangeb (deg)

postsynthesis treatmentsc

reference

8658 6528 7944 10928 4871 1026f 6829

0.066 0.071 0.042 0.097 0.079 0.119 0.067

1.576(6)-1.606(6) 1.571(7)-1.609(6) 1.567(4)-1.605(4) 1.54(2)-1.62(2) 1.52(2)-1.64(2) 1.50(4)-1.67(3) 1.510(21)-1.689(21)

144.6(5)-174.1(6) 144.4(5)-173.8(7) 140.7-178.6d 144.2(9)-178.0(12) 142.3-177.6d 115.6-174.4d 134.9(12)-178.6(13) 137.5-177.2h 136.5-173.1d 143.9-175.6d

106.5(3)-111.6(4) 106.9(3)-112.5(4) 106.0(2)-112.0(2) 105.6(7)-113.7(8) 105.3-112.5d 74.2-125.9d 104.7-114.1d

B, D B, I, D N N N N N B, I, D B,i I, D B, I, D

this work this work 28 26 27 25 29 34 35 32

g

1.56-1.62d 1.56-1.61d

g

99.6-118.8d 106.8-111.3d

a Sample: S ) single crystal and P ) powder. b Minimum and maximum values among those reported or calculated. c Postsynthesis treatments: B ) organic templates (TPA) decomposed and removed by heating, sometimes in oxygen; D ) dehydration, I ) ion exchange; and N ) no postsynthesis treatment, no B, I, or D. d Calculated using the reported cell parameters and framework coordinates. e TPA was not found crystallographically. f For reflections with Fo g 3σ(Fo). g Neither these values nor the coordinates from which they could be calculated were given. h Reported without esd. i Assumed. The postsynthesis treatment for the preparation of the starting material (H-ZSM-5) was not given.

The 0.40(4) Tl+ ions per unit cell at Tl(6), at subsite S23, lie nearly on the R2 plane. They bond to the O2 and O21 framework oxygen atoms at 2.97(3) and 2.90(3) Å, respectively; these distances are among the longest in Tl-ZSM-5 and are noticeably longer than 2.79 Å, the sum of the Tl+ and O2- ionic radii.41,42 The two positions at subsite S2′3, Tl(4) and Tl(5), hold only 0.14(3) and 0.15(3) Tl+ ions per unit cell, respectively. They are in the intersection volume (between R2 and R3, see Figures 4a, 7, and 9), and bond strongly to framework oxygen atoms: Tl(4) to O2 at 2.72(5) Å and Tl(5) to O1 at 2.84(3) Å, respectively (Table 4). At subsite S2′2, a total of 0.26(5) Tl+ ions per unit cell are found at four positions, Tl(i), i ) 14-17, at the closest approach of two m-related R2’s (see Table 7 and Figure 9). These four Tl+ positions are clustered together, but they can be arranged within their equipoints to avoid close contacts. Per unit cell, 0.07(3) Tl+ ions at Tl(14) and 0.04(2) at Tl(17) are found on the mirror plane between the two R2’s; each interacts rather weakly with both an O8 and an O11 framework oxygen, both at 2.97(5) Å. On the other hand, the 0.11(3) and 0.04(2) Tl+ ions per unit cell at Tl(15) and Tl(16), respectively, found off the mirror plane in the cavity, bond strongly to the same framework oxygen atoms, O8 and O11, at 2.83(4) and 2.72(8) Å, respectively (Table 4). The Tl(2) and Tl(2′) positions (his notation) reported by Mentzen32 are close to the R2 plane. They are in the vicinity of

Tl(i), i ) 14-17, at subsite S2′2 but are more than ca. 1.3-1.8 Å from them, and are again excessively distant, 3.22 and 3.27 Å, respectively, from their nearest oxygen atoms. 5.2.2.3. Tl+ Ions in Group 3. Per unit cell 1.02(8) Tl+ ions were found in group 3, 0.62(5) at S3 and 0.40(6) at S3′ (see Table 7). At S3 were 0.18(2) Tl+ ions at Tl(1) and 0.44(5) at Tl(2). At S3′ were 0.09(2) Tl+ ions at Tl(3) and 0.31(6) at Tl(18) (see Figure 10 and Tables 3 and 7). The Tl+ ions at S3 are on or nearly on the R3 plane at S32 at the boundary of the SC. Those at S3′ are between the R1 and R3 planes and lie in neither (they are at S3′2). Again, these low occupancy Tl+ ions can readily be distributed among the four symmetry-related asymmetric units per unit cell to avoid impossibly close distances. The Tl+ ions at Tl(i), i ) 1, 2, and 3, bond strongly to the framework oxygen atoms O24, O2, and O24 at 2.82(4), 2.81(5), and 2.79(6) Å, respectively (see Figure 10 and Table 4). These bond lengths are essentially the same as the sum of the corresponding ionic radii, 2.79 Å.41,42 This confirms the assignments of electron density at these positions to Tl+. The Tl(18) position is too far, 3.18(10) Å, from its closest framework oxygen atoms, two O2’s, suggesting, as was noted in paragraph 3 of section 5.2.2.1 in regard to previous work,32,35 that Tl(18) is an unresolved position containing two or more actual Tl+ positions. Consistent with this

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Figure 6. T-O-T angles in H-ZSM-5, Tl-ZSM-5, Cs-ZSM-5, TPA-ZSM-5, and TPA-silicalite. Included in this figure are all of the ZSM-5 structures in space group Pnma for which T-O-T angles were given.

TABLE 7: Group, Site, and Subsite Populations of Tl+ Ions in Dehydrated Tl-ZSM-5 cation sitinga groups

sites S1

G1 S1′ S2 G2 S2′

occupanciesd subsites

10-ringsb

channelsc

S11 S12 S1′1 S1′3

R1 R1/R2 R1 R3

ZC ZC/SC ZC ZC

S21 S22 S23 S2′1 S2′2 S2′3

R1/R2 R2 R2/R3 R1/R2 R2 R2/R3

ZC/SC ZC/SC ZC/SC ZC/SC ZC/SC ZC/SC

Tl(i) ions

subsites

Tl(9), Tl(12) Tl(7), Tl(8) Tl(10), Tl(11), Tl(13)

0.42(4) 0.24(3) 0.70(8)

Tl(6)

0.40(4)

Tl(14), Tl(15), Tl(16), Tl(17) Tl(4), Tl(5)

0.26(7) 0.29(4)

sites

groups

group %

1.36(10)

41

0.95(9)

28

0.42(4) 0.94(9) 0.40(4) 0.55(8)

S32 R2/R3 ZC/SC Tl(1), Tl(2) 0.62(5) 0.62(5) S33 R3 ZC 1.02(8) 31 S3′1 R1 ZC S3′ 0.40(6) S3′2 R3 ZC Tl(3), Tl(18) 0.40(6) a Cation groups, sites, and subsites are defined in the text (see section 5.2.1). b Rings or parts of rings which have strongest interactions with the Tl+ ions. c Exposed to the specified channel system (ZC ) zigzag sinusoidal channel and SC ) straight channel). d Number of Tl+ ions per unit cell, or sums thereof. S3

G3

possibility, the thermal parameter at Tl(18) is the largest among the 18 Tl+ positions. Two of the Tl+ positions, Tl(3) and Tl(3′) (his notation), that Mentzen found in Tl-ZSM-532 are again somewhat close to Tl(i), i ) 1, 2, and 18, at S3. The one at Tl(3′) shows a

good 2.71(2) Å approach distance to its nearest oxygen atom and is only 0.49 Å from Tl(1) at S32. His Tl(3) position is far, ca. 1.3-1.5 Å, from our Tl(2) and Tl(18) positions, respectively. The Cs(3) position that Olson found at S3′ in dehydrated Cs-ZSM-535 is close to Tl(3) and Tl(18): it is



Figure 7. Stereoviews of the channels in Tl-ZSM-5 with all Tl+ positions including symmetry equivalents within the volume shown, and with their classification into cation sites (S1, S1′, S2, S2′, S3, and S3′, see Table 7). (a) Along the b axis (the SC), but only approximately so to minimize the number of overlapping atoms in this view; the a axis is vertical. Two intersection volumes, at the top right and lower left, are connected by the segment of the ZC that is not in the intersection volume. That segment and one intersection volume constitute half of the repeating unit of the ZC (one half-wave of the sinusoidal channel). (b) Along the a axis (the ZC); the b axis is vertical. For simplicity, only one intersection volume is shown. For ease of viewing, all Tl+ ions are drawn with small fixed isotropic thermal parameters (Uiso ) 0.01).

0.61 Å from Tl(3). However, his Cs(3)-O distance, 3.41(4) Å, was also much longer than the sum of corresponding ionic radii, 3.09 Å,41,42 suggesting that Cs(3) is probably also an average of two or more positions, perhaps like those found at Tl(18) in this work. Olson did not find any Cs+ ions at S3. Finally, the coordination environments and placements of some additional Tl+ ions are shown in Figure 11. 5.3. Tl+ Site Preferences Are Indicative of Al Site Preferences. MFI can have a pure silica composition. The replacement of a Si4+ ion in the MFI framework with an Al3+ ion introduces a deficiency of positive charge at that position and simultaneously the need for an extraframework cation to balance that charge. Because this extraframework cation obviously cannot be at the Al position, it may be expected that it would associate itself with one or two of the oxygen atoms in the Al ion’s coordination sphere. Thus the positions of the extraframework cations, if they can be determined, should allow the Al positions, otherwise essentially indistinguishable by X-ray diffraction (and perhaps by any other diffraction because of their low occupancy) from those of the Si ions, to be recognized.

There are 12 T sites in MFI. If the Al ions distribute themselves rather evenly among these 12 sites, as theoretical calculations43-49 and NMR measurements50 indicate they do, there should be at least 12 different extraframework cation positions. If a second of these Al ions is close enough to the first to affect the extraframework cation position nearest that first Al, the number of exchangeable cation positions can be much larger. Other extraframework cation positions which are governed by three or more Al ion positions can be envisioned, especially when the Al content is relatively high. Even if some of the 12 T sites are negligibly or not at all populated by Al, it is easy to see that ZSM-5 might well have many extraframework cation positions with non-negligible occupancies, e.g., the 18 found in this work or more. Not only should Tl+ populations be higher near AlO4/2groups, but also the Tl-O distances should be shorter because these oxygen atoms should bear a greater negative charge than those bound only to Si; Al3+ should draw less charge than Si4+ from them. Using Tl-O distances, two working procedures were employed. In the first, the oxygen atom, O, closest to each Tl+ position is identified. Thus either T atom, Tn or Tm, that that

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Figure 8. Stereoview of R1 and R4 with Tl+ ions at cation sites S1 and S1′ (at subsites S12, S1′1, and S1′3) in dehydrated Tl-ZSM-5. Only the framework T and O atoms that are close to Tl+ ions are labeled. Tl+ positions are identified only by their serial numbers (i). Heavy bonds are used for the zeolite framework. Interactions of Tl+ ions with framework oxygen atoms are indicated by light bonds. For ease of viewing, Uiso ) 0.01 for the Tl+ ions. The very short virtual (avoidable) distances shown are Tl(7)-Tl(8) ) 0.97 Å, Tl(7)-Tl(10) ) 1.04 Å, Tl(7)-Tl(11) ) 1.58 Å, Tl(7)-Tl(13) ) 2.06 Å, Tl(8)-Tl(10) ) 0.80/1.59 Å, Tl(8)-Tl(11) ) 0.84/2.14 Å, Tl(8)-Tl(13) ) 1.65/ 2.29 Å, Tl(10)-Tl(11) ) 0.72/1.70 Å, Tl(10)-Tl(13) ) 1.09/1.65 Å, Tl(11)-Tl(11) ) 2.20 Å, Tl(11)-Tl(13) ) 0.92/2.00 Å, and Tl(13)-Tl(13) ) 1.42 Å.

Figure 11. Stereoviews of the cavity (as defined by the IZC)64 in dehydrated Tl-ZSM-5 showing the coordination environments of some additional Tl+ ions, one or two from each of the six cation sites: (a) Tl(i), i ) 1, 5, 7, 9, and 14, (b) Tl(i), i ) 2, 4, 8, 12, and 15. The Tl+ populations are sufficiently low that all of the excessively short Tl+ · · · Tl+ approaches shown can be avoided. Heavy bonds are used for the zeolite framework. Light bonds show Tl+-O interactions. Ellipsoids of 50% probability are shown. Figure 9. Stereoview of two m-related R2 rings with Tl+ ions at cation sites S2 and S2′ (subsites S23, S2′2, and S2′3) in dehydrated Tl-ZSM5. The very short virtual (avoidable) distances shown are Tl(14)-Tl(15) ) 0.59 Å, Tl(16)-Tl(17) ) 0.56 Å, Tl(15)-Tl(15) ) 1.11 Å, Tl(16)-Tl(16) ) 1.10 Å, Tl(4)-Tl(5) ) 1.02 Å, and Tl(5)-Tl(6) ) 0.83 Å. See the caption to Figure 8 for other details.

Figure 10. Stereoview of ring R3 with Tl+ ions at cation sites S3 and S3′ (at subsites S32 and S3′2) in dehydrated Tl-ZSM-5. The virtual (avoidable) distances shown are Tl(1)-Tl(2) ) 1.64 Å, Tl(1)-Tl(3) ) 1.55 Å, and Tl(2)-Tl(3) ) 2.93 Å. See the caption to Figure 8 for other details.

O bridges between (Tn-O-Tm) may be an Al ion. To decide which, the second closest oxygen position to that Tl+ ion, with the requirement that it bonds either to Tn or Tm, is identified. If it is bound to Tn, then Tn may be Al. If it bonds to Tm, then Tm is implicated. This information is provided in Table 8. In

the second (simpler) working procedure, the Tl+-T distances are considered. In this it is assumed that the Tl+ ion should attempt to compensate for the deficiency of charge at Al by approaching it (its AlO4/2- unit) as closely as possible. Table 9 contains this information. 5.3.1. Dismissal of Possible Interferences. In both structures, some T-O-T angles are well above, and others well below, the range 150° < θ < 165° (see Table 4 or Figure 6), values commonly found in aluminosilicate zeolites. For example, the oxygen atoms at O3 and O13 have T-O-T angles that are larger than 170°. They should be less approachable by extraframework cations because of the two concomitant cation to T atom (cation to cation) approaches that would entail. On the other hand, the T-O-T angle at O26 is the smallest (T10-O26-T10 ) 144.4° in Tl-ZSM-5), so it should be more approachable by extraframework cations. Tl+ and H+ may both selectively approach framework oxygens for these reasons. Consistent with this expectation, higher Tl+ populations were observed near oxygen atoms with relatively smaller T-O-T angles in dehydrated Tl-ZSM-5 (see Table 10). However, it can be seen there that the Tl+ occupancies are far from monotonic with the T-O-T angles. They are clearly governed by other considerations, by Al site preferences it is proposed. If Al atoms partially occupied a particular T site, and all four of the T-O-T angles immediately about that T site were large, Tl+ ions might fail to approach any of the four oxygen atoms about that T atom for that reason, or at least not approach any very closely nor with sufficient occupancy. The degree of



TABLE 8: Nearest and Next Nearest Oxygens to Each Tl+ Position Tl(i)

Oia

Tl(i)-Oib

Ojc

Tl(i)-Ojb

Tid

occupancye

Tl(1) Tl(2) Tl(3) Tl(4) Tl(5) Tl(6) Tl(7) Tl(8) Tl(9) Tl(10) Tl(11) Tl(12) Tl(13) Tl(14) Tl(15) Tl(16) Tl(17) Tl(18)

O24 O2 O24 O2 O1 O21 O23 O17 O25 O15 O15 O18 O15 O8f O8 O11 O11f O2

2.82(4) 2.81(5) 2.79(6) 2.72(5) 2.84(3) 2.90(3) 2.79(5) 2.71(3) 2.73(4) 2.87(3) 2.74(3) 2.68(4) 2.68(5) 2.97(5) 2.83(4) 2.72(8) 2.97(5) 3.18(10)

O20f O20 O20f O1 O2 O1 O17f O4 O18f O26 O4 O5 O26 O7f O7 O22 O22f O6

3.02(2) 2.99(5) 3.10(4) 2.83(5) 2.86(4) 2.97(3) 3.03(3) 2.94(3) 2.99(2) 2.88(3) 2.99(4) 2.94(5) 2.99(4) 3.05(4) 2.85(4) 2.76(7) 3.07(4) 3.36(10)

T12 T3 T12 T2 T2 T1 T7 T4 T9 T10 T1g T6 T10 T8 T8 T11 T11 T2

0.18(2) 0.44(5) 0.09(2) 0.14(3) 0.15(3) 0.40(4) 0.08(2) 0.16(2) 0.21(3) 0.18(4) 0.20(4) 0.21(3) 0.32(5) 0.07(3) 0.11(3) 0.04(2) 0.04(2) 0.31(6)

a The framework oxygen atom closest to the Tl+ ion at Tl(i). Note that 11 of the 18 Tl+ positions make their closest approaches to only five of the 26 framework oxygen positions, and that the next closest approaches involve only seven additional oxygen positions. This is a measure of the selectivity that Tl+ shows toward the oxygen atoms of the ZSM-5 framework. b Tl-O bond lengths in Å. c The framework oxygen atom with the second shortest approach distance to the Tl+ ion at Tl(i). d The T atom that bonds to both Oi and Oj. e Number of Tl(i) per unit cell. f Two symmetry-related oxygen atoms. g -T1-O21-T5- bridges between O15 and O4. T1 is selected because Tl(11)-O15 is shorter than Tl(11)-O4.

preference of that T site for Al would then be underestimated. In Tl-ZSM-5, however, for each of 10 of the 12 T sites, the smallest T-O-T angle is 147.2° or less (see Table 11). This is only 2.8° or less larger than the smallest T-O-T angle, 144.4°, in the entire structure (see Table 4). Thus these 10 T sites each have at least one T-O-T angle that is sufficiently small to accept Tl+ ions with negligible hindrance. The smallest T-O-T angle about T2 is 150.7°, and that about T8 is 154.1°. An inspection of Table 11 suggests that the preference of Al for T8, and possibly T2 also, may be underestimated in this report. If the ZSM-5 framework were to provide to Tl+ ions particularly favorable coordination situations, the Tl+ occupancies should be higher at those places, leading to exaggerated Al preferences for T sites near them. However, with some exceptions, dehydrated zeolites generally do not provide adequate coordination situations to exchangeable cations. Dehydrated ZSM-5 is no exception: the coordination environments of all Tl+ positions are vastly inadequate (see Figures 8-11 and Table 5). Nonetheless, the cove (at the top of the cavity64 in Figure 5 and site S1′ in Figure 7) is a good place to look for a coordination situation that might be better than the rest; S1′ has the highest Tl+ population of the six sites (Sn and Sn′, n ) 1-3). The Tl+ positions at S1′ are shown most clearly in Figure 8. It can be seen in Table 3 that the occupancy at Tl(7) is the lowest of the five Tl+ positions at S1′ while that at Tl(13) is the highest, 0.08(2) and 0.32(5) Tl+ ions per unit cell, respectively. However, an examination of Table 5, with some support from Figure 8, shows that the Tl(7) position offers better coordination to Tl+ than Tl(13). We conclude that occupancies observed are not governed by opportunities for better coordination. If many or all oxygen atoms about a T site were inaccessible to Tl+, the presence of Al at that site would be underestimated or would not be seen. However, all 26 oxygen positions lie on the walls of the ZC, the SC, or both (see Figure 4) where Tl+ ions have easy access to them.

Tl+ ions might selectively approach those oxygens, among the 26 nonequivalent positions in the silicalite MFI structure, which are intrinsically the most electronegative. However, these differences can be expected to be about an order of magnitude smaller than that which results from the deficiency of positive charge that arises when Si4+ is replaced by Al3+. 5.3.2. Cation Groups Preferred by Tl+ Ions. Among the three cation groups, G1 shows the highest Tl+ population, 41% of the total number (see Table 7), while the other two groups are almost equally populated, 28% in G2 and 31% in G3. Therefore, the segment of the ZC that does not include the intersection volume (Figure 4b) holds, within its volume or close to its surface, 41 + 31 ) 72% (G1 + G3) of the Tl+ ions in Tl-ZSM-5; the remaining 28% at G2 are in the intersection volume (Figure 4a) (actually T1(6) at G2 is in the SC just outside this volume). If the Tl+ ions on the R1 and R3 planes (those at S1 and S3, respectively, see Table 7) are excluded from G1 and G3 because of their locations near the boundary of the SC, it is clear that G1 still has the highest Tl+ population (Table 7). In contrast, quite different population figures were reported earlier for a similarly prepared powder sample of Tl-ZSM-5: 27, 52, and 21% for groups G1, G2, and G3, respectively.32 5.3.3. Cation Sites and Subsites Preferred by Tl+ Ions. Of the two cation sites in group G1, S1′ has the higher Tl+ population: 0.94 Tl+ ions per unit cell at site S1′ compared to 0.42 at S1. Moreover, S1′ is the most populated of the six cation sites (see Table 7), so Tl+ ions prefer site S1′ (in the cove between R1 and R3, at the top right of Figure 4b and at the top of Figure 5). Among the two subsites of site S1′, Tl+ ions prefer S1′3 over S1′1; there are 0.70 Tl+ ions there compared to 0.24 at S1′1. The Tl+ ions at S1′3 interact with the R3 oxygen atoms O15 and O26. Both bond to T10, indicating that the Al population is highest at T10. Within G2, more Tl+ ions are located at cation site S2′ (in the intersection volume) than at S2 (on the R2 plane): 0.55 Tl+ ions at S2′ compared to 0.40 at S2. About half (0.26 out of 0.55) of Tl+ ions at site S2′ coordinate to the oxygen atoms of T atoms which belong only to R2 (at S2′2), while the other half (0.29) coordinate to those shared by R2 and R3 (at S2′3) (see Table 7 and Figure 9). Within G3, Tl+ ions prefer site S3 (on the R3 plane) to S3′: 0.62 Tl+ ions are there compared to 0.40 at S3′. All of these Tl+ ions approach oxygen atoms that coordinate to T atoms shared by R3 and R2 (at S32 and S3′2, respectively) (see Table 7 and Figure 10). 5.3.4. T Sites Preferred by Al. It has been frequently suggested35,43-50 that the Al ions are not distributed randomly among the T sites. On the basis of the observed T-O bond lengths (see section 5.1.1 and Table 4), however, no T site is obviously different from the others: there are no significant changes in geometry at any of the T sites that can be attributed to Al substitution. This is reasonable because only ca. 3.2 Al atoms per unit cell are distributed among the 12 T sites, each with 8-fold multiplicity, so none can be fully or even as much as half occupied by Al. Even if Al were to prefer some T sites, it would be outnumbered there by the Si ions and least-squares refinement would be expected to converge at the Si position. An indication of an inequality among the T-O bond lengths can, however, be seen at T10. It has the longest mean T-O bond length in both crystal structures (1.601 in Tl-ZSM-5 and 1.600 Å in H-ZSM-5, see Table 4), although, considering the esd’s, this is not significant. Moreover, the second longest T-O bond in TPA-silicalite was T10-O (Si1-O in their nomenclature).29

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TABLE 9: Nearest Tl+ Ions and Their Populations about T Atoms T atom T10 T11 T2 T6 T4 T5 T8 T9 T7 T12 T3 Tl

c

nearest

T · · · Tl+ a (Å)

occupancya,b

2nd nearest

T · · · Tl+ a (Å)

occupancya,b

3rd nearest

T · · · Tl+ a (Å)

occupancya,b

Tl(10) Tl(16) Tl(4) Tl(12) Tl(8) Tl(11) Tl(15) Tl(9) Tl(15) Tl(3) Tl(2) Tl(6)

3.32(3) 3.37(7) 3.39(5) 3.41(4) 3.45(3) 3.46(5) 3.48(4) 3.50(4) 3.50(4) 3.52(5) 3.52(5) 3.54(3)

0.18(4) 0.04(2) 0.14(3) 0.21(3) 0.16(2) 0.20(4) 0.11(3) 0.21(3) 0.11(3) 0.09(2) 0.44(5) 0.40(4)


3.40(5) 3.66(4) 3.45(3) 4.026(14) 3.82(4) 3.76(4) 3.67(4) 3.60(4) 3.51(5) 3.57(3) 3.57(5) 3.55(3)

0.32(5) 0.04(2) 0.15(3) 0.21(3) 0.20(4) 0.21(3) 0.07(3) 0.21(3) 0.08(2) 0.18(2) 0.14(3) 0.15(3)


3.53(3) 4.08(7) 3.57(5) 4.08(10) 4.090(22) 3.775(24) 4.18(5) 3.60(4) 3.53(6) 3.65(6) 3.66(3) 3.56(5)

0.20(4) 0.04(2) 0.44(5) 0.31(6) 0.08(2) 0.16(2) 0.11(3) 0.11(3) 0.04(2) 0.04(2) 0.15(3) 0.32(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 Number of Tl(i) per unit cell. c Note that T10 is approached most closely by Tl+ ions, with relatively high occupancies.

TABLE 10: T-O-T Bond Angles (deg) and Nearest Tl+ Ions Oi

T-Oi-Ta

nearest Tl+

occupancya,b at Tl+

Oi-Tl+ a (Å)

O26 O5 O18 O17 O15 O21 O20 O11 O22 O25 O2 O24 O9 O4 O23 O8 O1 O19 O7 O16 O10 O6 O14 O12 O3 O13

144.4(5) 144.7(4) 144.8(4) 145.6(4) 146.2(4) 146.8(4) 147.0(5) 147.2(4) 148.3(4) 150.4(6) 150.7(4) 151.1(6) 151.7(4) 152.4(4) 153.9(7) 154.1(5) 154.6(5) 158.1(5) 160.2(5) 160.6(5) 160.7(5) 161.4(5) 164.1(5) 165.0(5) 170.9(6) 173.8(7)

Tl(10) Tl(12)c Tl(12) Tl(8) Tl(13) Tl(6) Tl(2) Tl(16) Tl(16)c Tl(9) Tl(4) Tl(3) Tl(18)c Tl(8)c Tl(7) Tl(15) Tl(4)c Tl(6)c Tl(15)c Tl(8)c Tl(10)c Tl(18)d Tl(16)c Tl(3)c Tl(3)c Tl(6)c

0.18(3) 0.21(3) 0.21(3) 0.16(2) 0.32(5) 0.40(4) 0.44(5) 0.04(2) 0.04(2) 0.21(3) 0.14(3) 0.09(2) 0.31(6) 0.16(2) 0.08(2) 0.11(3) 0.14(3) 0.40(4) 0.11(3) 0.16(2) 0.18(3) 0.31(6) 0.04(2) 0.09(2) 0.09(2) 0.40(4)

2.88(3) 2.94(4) 2.68(4) 2.71(3) 2.68(5) 2.90(3) 2.99(5) 2.72(7) 2.76(7) 2.73(4) 2.72(5) 2.79(6) 4.53(11) 2.94(3) 2.79(5) 2.83(4) 2.83(5) 4.07(3) 2.85(4) 4.70(2) 3.77(3) 3.36(10) 4.56(8) 4.25(5) 3.93(2) 4.24(3)

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 occupancy is given as the number of Tl+ ions per unit cell. c This Tl+ position approaches another oxygen position more closely; see elsewhere in this table. These shorter approaches should be considered more seriously when concluding that Tl+ ions prefer to approach oxygen atoms with smaller T-O-T angles. d Tl(18) makes no closer approach than 3.36(10) Å to a framework oxygen. It is judged to be a composite position that could not be resolved into actual Tl+ positions. This is discussed in paragraph 3 of section 5.2.2.1 and in paragraphs 2 and 3 of section 5.2.2.3.

Although their results were less precise than ours, this suggests that elongated T10-O distances are natural to ZSM-5. Although the T10-O bond length argument may be dismissed as described above, the number of Tl+ ions about the oxygen atoms at O15 and O26, which bond directly to two symmetryrelated T10’s (O15-T10-O26-T10-O15), 0.18 + 0.32 ) 0.50, is the highest among the 12 T sites in Tl-ZSM-5 (see Table 8), and this indicates that there may be more Al atoms at T10 than at any other T site in the crystals used in this work.

TABLE 11: T-O-T Angles (deg) about the T Atoms in Tl-ZSM-5a T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12

146.2 150.7 147.0 145.6 144.7 144.7 145.6 154.1 144.8 144.4 147.2 147.0

146.8 154.6 150.7 152.4 146.8 144.8 148.3 160.2 150.4 146.2 148.3 147.2

154.6 161.4 158.1 160.6 152.4 158.1 153.9 165.0 151.7 151.7 160.7 151.1

160.6 173.8 170.9 170.9 164.1 161.4 160.2 173.8 154.1 160.7 164.1 165.0

a Each T atom bonds to four oxygen atoms, each of which bonds to only one other T atom. The four T-O-T angles about each T atom are given in increasing order. The oxygen and the other T atom corresponding to each angle may be identified from the angle values by using Table 4. Note that every T atom, with the exception of T2 and T8, participates in at least one T-O-T angle that is among the smallest in the structure. Therefore, if that first T atom was Al, Tl+ could readily approach an oxygen atom bound to it. Tl+ approaches to oxygens bound to T2 and T8 would be somewhat more hindered by T · · · Tl+ repulsions.

The energetically favored sites for Tl+ ions, as evidenced by higher populations, possibly supported by the shorter Tl-O bond lengths, should be indicative of the preferred Al positions in the ZSM-5 framework. However, it remains possible that some Tl+ positions are less able to approach framework oxygen atoms for steric reasons. Also, Tl+ ions on mirror planes (Wyckoff position 4c, see Table 3) should have smaller populations because they have only half as many equivalent positions. In group G1, the two Tl+ positions at subsite S12, Tl(9) and Tl(12), are ca. equally populated and have reasonable Tl-O bond lengths (2.73(4) and 2.68(4) Å in Tl(9)-O25-T9 and Tl(12)-O18-T6, respectively, see Table 8). Among the five Tl+ positions at site S1′, Tl(13), closest to T10, has the shortest Tl-O bond length (2.68(5) Å in Tl(13)-O15-T10). It also has the highest occupancy in G1, 0.32(5) Tl+ ions per unit cell. Also at S1′, Tl(10) approaches T10 at a slightly longer but reasonable Tl-O distance, 2.87(3) Å. The remaining Tl+ positions, Tl(7), Tl(8), and Tl(11) at S1′, close to the Ti atoms, i ) 7, 4, and 5, respectively, have much lower occupancies. Ignoring the small variation in the Tl-O distances in group G1, the preference for the locations of Tl+ ions at Tl(i) positions seems to be in the order Tl(13) > Tl(8) ∼ Tl(9) ∼Tl(10) ∼ Tl(11) ∼ Tl(12) > Tl(7), which leads, using Table 8, to T10 > T4 ∼ T5 ∼ T6 ∼ T9 > T7.



Figure 12. Relationships between the populations (as cumulative occupancies) of Tl+ ions and their T-O bond lengths at the 12 T sites of dehydrated Tl-ZSM-5.

In group G2, the preference order Tl(4) ∼ Tl(5) > Tl(15) > Tl(16) leads to T2 > T8 > T11. This is done without considering Tl(6), Tl(14), and Tl(17), whose Tl-O distances were somewhat longer. In group G3, the Tl+ ions at the S3 cation sites (Tl(1) and Tl(2) at subsite S32 and Tl(3) and Tl(18) at subsite S3′2) are close to a particular part of R3, -T3-T12-T12-T3-, which is shared with R2 but not with R1 (see Table 7 and Figures 5, 7, and 10). Among the two T sites, T3 and T12, which are close to the Tl+ ions at S3 and S3′, T3 has the higher Tl+ population, 0.44 vs 0.09 + 0.18 ) 0.27 (see Table 8), even though their Tl-O bond lengths are almost equal (2.81(5), 2.79(6), and 2.82(4) Å in Tl(2)-O2-T3, Tl(3)O24-T12, and Tl(1)-O24-T12, respectively. The Tl(18) position, by being closer to T3 than T12, supports this conclusion; it is given less consideration in this argument because of its long Tl(18)O2 distance. The preference order in group G3 is therefore clearly Tl(2) > Tl(1) > Tl(3), which leads to T3 > T12. Finally, the Tl(i) populations are shown in Figure 12 for five Tl-O bond length ranges. Considering the sum of the Tl+ and O2- ionic radii41,42 (2.79 Å) and the mean esd for a Tl-O bond length (0.06), Tl+ ions more than 3.00 Å from their nearest framework oxygen atoms are considered less seriously. When Tl-O bond lengths less than 2.90 Å are considered, Tl+ ions prefer T sites in the order T10 > T3 > T2 > T12 > T6 ∼ T9 > T5 > T4 > T8 > T7 > T11. When Tl-O bond lengths less than 2.80 Å (a more reliable distance criterion) are used, fewer T sites can be assigned because there are fewer such bond lengths; the order becomes T10 > T6 ∼ T9 > T5 > T4 > T2 > T12 > T7 > T11. If only those Tl-O bond lengths less than 2.70 Å (the most reliable distance criterion used) are used, it becomes T10 > T6. Finally, the least favorite site for Al substitution seems to be T1 because no Tl+ ions are closer than 3.00 Å from the oxygen atoms that bond to it. This in itself is evidence for site selectivity for Al in MFI. For all of the above bond length ranges, T10 has the highest concentration of Tl+ ions around it, indicating that it has the highest Al3+ content among the 12 T sites. The second most

popular T site for Al may be T6, while the least favorable is T1. The possibility of having a higher Al content at site T10 is weakly supported by the insignificantly longer T10-O bond lengths. Because an Al-O bond is longer than Si-O, Al atoms might be expected to selectively occupy those T sites which, intrinsic to MFI, have the longest T-O bond lengths. In the two structures reported here, the T10-O bonds are the longest, albeit insignificantly so, and in TPA-silicalite,29 a less precisely determined structure, T10-O was second longest. This would relieve strain (discussed in paragraph 2 of section 5.1.2). Indeed, the very rigidity of the MFI structure (see paragraph 1 of section 5.1.2) suggests the presence of strain. Because the O26 position bridges between the two T10 atoms (see Figure 4a or 5), two such strains would be simultaneously relieved by the replacement of Si with Al at either of those two positions. When that would occur, the bridging O26 atom would be somewhat displaced from its mirror plane. 6. Summary Relatively precise crystal structures of fully dehydrated Tland H-ZSM-5 (Si/Al ) 29) have been determined crystallographically using modestly twinned crystals and synchrotron X-radiation. The framework structure and Tl+ sites in ZSM-5 are described using the four unique, partially overlapping, 10rings (R), the two channel systems (C), the volume at their intersection, and the conventionally defined cavity (which includes the intersection volume). The Tl+ positions are described with reference to the framework structure and are classified into three cation groups: G1, G2, and G3. A total of 3.3(2) Tl+ ions per unit cell were found at 18 positions, nearly all with reasonable geometry and fractional occupancies. Of the 18 Tl+ positions, six are in the intersection volume (at S2′ in G2), 11 are elsewhere in the ZC (all in G1 and G3), and one is elsewhere in the SC (at S2 in G2), very near the intersection volume. Among the six cationic sites, site S1′ of group G1, at the mouth of the cove (see Figure 7), has the highest Tl+

19956


population. Of the 18 Tl+ positions, 14 are found to be strongly bonded to framework oxygen atoms with Tl-O bond lengths in the range of 2.68(4)-2.87(5) Å while three others range between 2.90(3) and 2.97(5) Å; one at 3.18(10) Å appears to be an unresolved position. The nonrandom nature of the Al distribution among the T sites of the ZSM-5 framework has been demonstrated by using Tl+ occupancy and Tl-O bond length criteria. With Tl-O bond lengths shorter than 2.70 Å, the preference order for the distribution of Al atoms at T sites is T10 (most preferred) > T6. The preferences for some T sites remained unchanged when longer, less reliable but more comprehensive, Tl-O bond length criteria are used. It becomes T10 > T6 ∼ T9 > T5 > T4 > T2 > T12 > T7 > T11 with Tl-O < 2.80 Å, and it is T10 > T3 > T2 > T12 > T6 ∼ T9 > T5 > T4 > T8 > T7 > T11 > T1 (least preferred) with Tl-O < 2.90 Å (least reliable bond length criterion). T10 is a member of R4, the 10-ring that is common to the intersection volume and the cove of the ZC (see Figure 5). It is not a member of R2 (the SC 10-ring). Because it has attracted a high population of Tl+ ions at short distances, it has the highest Al content. Acknowledgment. We gratefully acknowledge the support of The Pohang Accelerator Laboratory of POSTECH for their diffractometer and computing facilities. This work was supported by the Korea Science & Engineering Foundation (20090074154 and R0A-2007-000-20050-0). Supporting Information Available: Observed and calculated structure factors squared (Supporting Table 1) for Tl-ZSM-5 and H-ZSM-5. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Degnan, T. F.; Chitnis, F. K.; Schipper, P. H. Microporous Mesoporous Mater. 2000, 35, 245. (2) Pereira, C.; Kokotailo, G. T.; Gorte, R. J. J. Phys. Chem. 1991, 95, 705. (3) Li, B.; Li, S.; Li, N.; Chen, H.; Zhang, W.; Bao, X.; Lin, B. Microporous Mesoporous Mater. 2006, 88, 244. (4) Wei, L.; Yu, Y.; Meitzner, G. D.; Iglesia, E. J. Phys. Chem. B 2001, 105, 1176. (5) Zhang, Y.; Drake, I. J.; Briggs, D. N.; Bell, A. T. J. Catal. 2006, 244, 219. (6) Iwamoto, M.; Furukawa, H.; Kagawa, S. In New DeVelopments in Zeolite Science and Technology; Murakami, Y., Ijima, A., Ward, J. W., Eds.; Elsevier: New York, 1986; p 63. (7) Iwamoto, M.; Yahiro, H.; Mine, Y.; Kagawa, S. Chem. Lett. 1989, 213. (8) Bethke, K. A.; Li, C.; Kung, M. C.; Yang, B.; Kung, H. H. Catal. Lett. 1995, 31, 287. (9) Iwamoto, M.; Yahiro, H. Catal. Today 1994, 22, 5. (10) Liu, D.-J.; Robota, H. J. J. Phys. Chem. B 1999, 103, 2755. (11) Walker, A. P. Catal. Today 1995, 26, 107. (12) Kharas, K. C. C.; Liu, D.-J.; Robota, H. J. Catal. Today 1995, 26, 129. (13) Li, Y.; Armor, J. N. Appl. Catal., B 1993, 2, 239. (14) Yogo, K.; Ihara, M.; Terasaki, I.; Kikuchi, E. Chem. Lett. 1993, 229. (15) Kikuchi, E.; Yogo, K. Catal. Today 1994, 22, 73. (16) Sun, T.; Trudeau, M. L.; Ying, J. Y. J. Phys. Chem. 1996, 100, 13662. (17) Zhang, Y.; Drake, I. J.; Bell, A. T. Chem. Mater. 2006, 18, 2347. (18) Drake, I. J.; Zhang, Y.; Gilles, M. K.; Liu, C. N. T.; Nachimuthu, P.; Perera, R. C. C.; Wakita, H.; Bell, A. T. J. Phys. Chem. B 2006, 110, 11665. (19) Kucherov, A. V.; Slinkin, A. A.; Kondra’ev, D. A.; Bondarenko, T. N.; Rubomstein, A. M.; Minachev, K. M. Zeolites 1985, 5, 320. (20) Kucherov, A. V.; Gerlock, G. L.; Jen, H.-W.; Shelef, M. Catal. Today 1996, 27, 79. (21) Lei, G. D.; Adelman, B. J.; Sakany, J.; Sachtler, W. M. H. Appl. Catal., B 1995, 5, 245. (22) Valyon, J.; Hall, W. K. J. Phys. Chem. 1993, 97, 7054. (23) Lacheen, H. S.; Iglesia, E. Chem. Mater. 2007, 19, 1877.

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Detailed Determination of the Tl+ Positions in Zeolite Tl

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