Single crystal structure of fully dehydrated fully Tl+-exchanged zeolite Y, ∣Tl71∣[Si121Al71O384]-FAU

Microporous and Mesoporous Materials 94 (2006) 313–319 www.elsevier.com/locate/micromeso

Single crystal structure of fully dehydrated fully Tl+-exchanged zeolite Y, jTl71j[Si121Al71O384]-FAU Gyoung Hwa Jeong a, Young Mi Lee a, Yang Kim a

a,*

, David E.W. Vaughan b, Karl Seff

c,*

Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, South Korea b Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA c Department of Chemistry, University of Hawaii, Honolulu, HI 96822, USA Accepted 17 January 2006 Available online 24 May 2006

Abstract ˚ ), has been determined by The structure of fully dehydrated, fully Tl+-exchanged zeolite Y, jTl71j[Si121Al71O384]-FAU (a = 24.948(2) A single-crystal X-ray diffraction techniques in the cubic space group Fd  3m at 21(1) C. An aqueous exchange solution 0.05 M in Tl+ with pH = 7.2 was allowed to flow past the crystal for 7 days. The resulting colorless crystal became black after being dehydrated at 400 C and 2 · 106 Torr for 2 days. Diffraction data were then gathered and were refined using all 702 unique reflections to the final error indices (based upon the 365 reflections for which Fo > 4r(Fo)) R1 = 0.058 and wR2 = 0.130. In this structure, Tl+ ions occupy four crystal˚ lographic sites. About 30.4(6) Tl+ ions fill or nearly fill site I 0 in the sodalite cavities on 3-fold axes opposite double 6-rings; each is 1.37 A ˚ ˚ from the plane of three oxygens (Tl–O = 2.568(17) A, O–Tl–O = 93.9(5)). Each is also 3.673(4) A from three (perhaps sometimes only ˚ from the plane of two) equivalent site-I 0 Tl+ ions. Thirty-two Tl+ ions fill site II opposite single 6-rings in the supercage; each is 1.63 A ˚ , O–Tl–O = 87.9(6)). The remaining Tl+ ions are located at two nonequivalent sites III 0 with occuthree oxygens (Tl–O = 2.724(15) A pancies of 5.7(8) and 3.0(4) per unit cell; each coordinates to three framework oxygens. It appears that TlOH was imbibed during aqueous ion exchange at pH = 7.2, and that it decomposed upon vacuum dehydration at 400 C to deposit as Tl2O (black) on the surface of the zeolite crystal.  2006 Elsevier Inc. All rights reserved. Keywords: Structure; Zeolite Y; Faujasite; Thallium; Ion exchange; Dehydrated; Imbibition

1. Introduction Zeolite Y is important in applications such as sorption, ion exchange, and catalysis [1,2]. Its structure is best described by the space group Fd  3m. Its framework is isomorphous with that of zeolite X and the mineral faujasite (FAU), as previously determined by X-ray [3,4] and neutron [5] diffraction. McMurray et al. suggested that proton-loaded zeolites, prepared from fully dehydrated zeolites and gaseous anhydrous Brønsted acids, could be used to synthesize zeolite

*

Corresponding authors. Tel.: +1 808 956 7665; fax: +1 808 956 5908. E-mail addresses: [email protected] (Y. Kim), seff@hawaii.edu (K. Seff). 1387-1811/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.01.023

semiconductors. Adsorption-induced 23Na MAS NMR chemical shifts, far-IR Na+ and Tl+ translatory mode frequency shifts, and Tl+ luminescence quenching effects were chosen as probes of the cation–anion interactions in these materials [6]. Recently, the structure of fully dehydrated fully K+exchanged zeolite Y, jK71j[Si121Al71O384]-FAU, was determined by single-crystal X-ray diffraction methods [7]. The 71 K+ ions per unit cell were found to be distributed over a surprisingly large number of sites, eight. Dehydrated jK92j[Si100Al92O384]-FAU [8] also showed this degree of complexity. In contrast, the fully dehydrated structures of jTl92j-FAU [9], jNa92j-FAU [10], and jNa56j-FAU [5], were all much simpler. Large single crystals of zeolite Y, jNa71j[Si121Al71O384]FAU or jNa71j-FAU, recently became available [11]. This

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work was done to see if they were suitable for detailed crystallographic study. That they may have contained an organic template was a cause for concern. This work was also initiated with the hope that fully Tl+-exchanged zeolite Y, jTl71j-FAU, could readily be prepared from jNa71j-FAU. If so, it would be dehydrated and its structure would be determined by single-crystal X-ray diffraction techniques. All cations should readily be located because Tl+ (atomic number = 81) has a large scattering factor for X-rays. Its cation distribution could be compared with that in jK71jFAU [7] and jTl92j-FAU [9]. 2. Experimental Large single crystals of zeolite Y, jNa71j[Si121Al71O384]FAU, were prepared by David Vaughan et al. [11]. Numerous repetitions of this method have established that crystals larger than 50 lm in size are the exception. Most reactions yield FAU crystals in the 10–50 lm range and are fully crystallized within 6–15 days. This indicates that uncontrolled nucleation sources are the major inhibitors to the formation of larger crystals. A large single crystal of jNa71j-FAU, a colorless octahedron about 0.2 mm in cross-section, was lodged in a fine Pyrex capillary. An aqueous 0.05 M TlNO3 (Aldrich, 99.999%) ion-exchange solution with pH = 7.2 was allowed to flow past the crystal for 7 days. The capillary containing the still colorless crystal was attached to a vacuum system, and the crystal was cautiously dehydrated by gradually increasing its temperature (ca. 27 C/h) under dynamic vacuum to 400 C. Finally, the system was maintained at 400 C and 2 · 106 Torr for 2 days. During this process, the crystal was protected from rehydration from the adjacent (not baked out) sections of the vacuum system by an in-series 16 cm tube of zeolite 5A (jCa,Naj[Si12Al12O48]LTA) beads that had been fully dehydrated in situ. The resulting crystal was black; it was sealed under vacuum in its capillary by torch. Diffraction data were collected with an automated Enraf–Nonius four-circle computer-controlled CAD-4 diffractometer equipped with a pulse-height analyzer and a graphite monochromator, using Mo radiation. All unique reflections in the positive octant of an F-centered unit cell for which 2h < 50, l P h, and l P k were recorded. The cubic space group, Fd  3m, appropriate for zeolite Y [7,12], was used throughout this work. Equivalent reflections in this doubly redundant data set were merged. Calculations were performed with XCAD4 (LP corrections) [13] and with the structure determination program package, SHELX-97 [14]. The cubic unit cell constant at 21(1) C, determined by least-squares refinement of 25 intense reflec˚ . An tions for which 14 < 2h < 22, is a = 24.948(2) A 1 absorption correction [l = 18.8 mm ] was made empirically using a w scan [15]. This correction had little effect on the final error (R) indices. Additional experimental data is given in Table 1. Other details are the same as previously reported [16].

Table 1 Experimental and structure refinement data Crystal color Ion exchange T (C)/t (days) Data collection T (C) Scan technique ˚) Radiation (Mo Ka) k1/k2 (A ˚) Unit cell constant, a (A 2h range for a (deg) No. of reflections for a 2h range in data collection (deg) No. of unique reflections (m) No. of reflections with Fo > 4r(Fo) No. of parameters (s) Data/parameter ratio (m/s) Weighting parameters: a/b R1a/wR2b Fo > 4r(Fo) R1a/wR2b(all data) Goodness-of-fitc (all data) P P a R1 ¼ h jF o  jF c jj= F o . i1=2 P P b wR2 ¼ wðF 2o  F 2c Þ2 = wðF 2o Þ2 . hP i1=2 c 2 2 2 Goodness-of-fit ¼ wðF o  F c Þ =ðm  sÞ .

Black 21/7 21 h–2h 0.70930/0.71359 24.948(2) 14–22 25 3 < 2h < 50 702 365 49 14.3 0.1353/1132 0.058/0.130 0.109/0.176 1.13

3. Structure determination Full-matrix least-squares refinement [14] was done on F2 using all data. Initial refinement began in the space group Fd 3m using the atomic parameters of the zeolite framework [(Si,Al), O(1), O(2), O(3), and O(4)] of jZn43j[Si137Al55O384]-FAU [17]. Since silicon and aluminum are indistinguishable in this space group, only the average species (Si,Al) is considered in this work. Initial isotropic refinement of (Si,Al) and the framework oxygens converged with R1 = 0.60 and wR2 = 0.88. These error indices are defined in footnotes to Table 1. A Fourier difference electron-density function yielded the positions of the Tl+ ions at Tl(1) (0.070, 0.070, 0.070) and Tl(2) (0.250, 0.250, 0.250). Isotropic refinement of the framework atoms, Tl(1), and Tl(2) converged to R1 = 0.069 and wR2 = 0.167. A subsequent difference Fourier function revealed a peak at (0.120, 0.141, 0.400) which was stable in least-squares refinement. Isotropic refinement of the framework atoms, Tl(1), Tl(2), and Tl(4) (see Table 2) converged to R1 = 0.063 and wR2 = 0.157. The fourth Tl+ ion position, Tl(3), was found on the ensuing Fourier function at (0.060, 0.075, 0.412). Isotropic refinement of the framework atoms and the four Tl+ positions converged to R1 = 0.062 and wR2 = 0.136. The occupancy numbers at Tl(i), i = 1, 4 had refined to the values shown in Table 2. The occupancy at Tl(2) was then fixed at 32, the maximum value at this position. The cationic charge then sums to 71.1+ per unit cell, in agreement with the ca. 71 expected from MAS-NMR analysis of the zeolite framework [11]. Finally, anisotropic least-squares refinement of all positions, except Tl(3) and Tl(4) which continued to be refined isotropically, with selected occupancy values fixed as shown in Table 2, converged to R1 = 0.058 and wR2 = 0.130.

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315

Table 2 Positional, thermal, and occupancy parametersa Atom

(Si,Al) O(1) O(2) O(3) O(4) Tl(1) Tl(2) Tl(3) Tl(4)

Site

Wyc. Pos.

x

y

z

U11 or Uisob

U22

U33

U12

U13

U23

I0 II III 0 III 0

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

535(2) 1041(4) 8(5) 765(5) 1787(5) 729(1) 2528(1) 603(18) 1220(20)

1253(2) 1041(4) 8(5) 765(5) 1787(5) 729(1) 2528(1) 791(20) 1429(20)

362(2) 0 1436(6) 299(7) 3217(7) 729(1) 2528(1) 4188(17) 4088(17)

175(29) 253(63) 164(52) 125(60) 234(66) 309(9) 339(6) 956(220) 351(220)

136(28) 253(63) 164(52) 125(60) 234(66) 309(9) 339(6)

122(23) 182(95) 350(101) 376(121) 205(103) 309(9) 339(6)

33(23) 110(84) 21(95) 31(81) 29(92) 23(7) 17(8)

30(21) 131(66) 34(67) 3(63) 75(62) 23(7) 17(8)

28(20) 131(66) 34(67) 3(63) 75(62) 23(7) 17(8)

Occupancyc Varied

Fixed 96 96 96 96 96

30.4(6) 32.3(6) 5.7(8) 3.0(4) P ¼ 71:4

32 P

¼ 71:1 ˚ , space group Fd 3m with origin at center of symmetry. Positional and anisotropic thermal parameters are given ·104. Numbers in a = 24.948(3) A parentheses are the esds in the units of the least significant digit given for the corresponding parameter. b The anisotropic temperature factor = exp[(2p2/a2) (U11h2 + U22k2 + U33l2 + U12hk + U13hl + U23kl)]. c Occupancy factors are given as the number of atoms or ions per unit cell. a

The final difference function was featureless except for a ˚ 3. With the peak at (0.052, 0.052, 0.052) of height 2.0 eA hope that Tl(1) might be resolved into two or more positions for 6-rings with different numbers of Al atoms, this peak was refined by least-squares to 0.060, 0.060, 0.060 with an occupancy of 1.7 Tl+ ions per unit cell. However, ˚ , far less than the sum its distance to O(3) was 2.32(7) A ˚ [18]. Also, 1.7 is of the corresponding ionic radii, 2.79 A far fewer than the number of Tl+ ions that might be expected for three-Al3+ 6-rings (7.4 per unit cell if all 6rings had either three or two Al3+ ions; more if some have one or zero Al3+ ions). In addition, the nearly spherical thermal ellipsoid at Tl(1) does not suggest that this resolution is achievable. Accordingly, this peak was not considered further. It appears to be primarily a residue of the thermal ellipsoid at Tl(1), indicating that this motion is noticeably anharmonic along the 3-fold axis. It appears that the swing of each Tl(1) cation into the sodalite cavity is shortened by the repulsion it encounters from other Tl(1) ˚ ). This resolution might cations (Tl(1)  Tl(1) = 3.673(4) A also have been seen at Tl(2), but its thermal ellipsoid is also not elongated, and no residual peaks were seen near it. There is no indication in the final structure that the cation, bis(2-hydroxyethyl)dimethylammonium, used to template the growth of the large single crystal used in this work, is present. Either it was not present initially, or it was replaced during the intensive Tl+-exchange procedure. All crystallographic calculations were done using SHELX-97 [14]. Fixed weights were used initially; the final weights were assigned using the formula w ¼ q=½r2 ðF 2o Þþ ðaP 2 Þ þ bP þ d þ e sinðhÞ, where p ¼ fF 2o þ ð1  f ÞF 2c to give w ¼ q=½r2 ðF 2o Þ þ ðaP 2 Þ þ bP , where p ¼ ðF 2o þ 2F 2c Þ= 3, with a and b as refinable parameters (see Table 1). Additional refinement data are given in Table 1. Atomic scattering factors [19,20] for (Si,Al), O, and Tl+ were used. All scattering factors were modified to account for anomalous dispersion [21]. The final structural parameters are given in Table 2, and selected interatomic distances and angles are presented in Table 3.

Table 3 ˚ ) and angles (deg)a Selected interatomic distances (A (Si,Al)–O(1) (Si,Al)–O(2) (Si,Al)–O(3) (Si,Al)–O(4) Mean

1.638(7) 1.670(7) 1.686(8) 1.659(8) 1.663

Tl(1)–O(3) Tl(2)–O(2) Tl(3)–O(1) Tl(3)–O(4) Tl(4)–O(1) Tl(4)–O(4)

2.568(17) 2.724(15) 2.77(5) 2.44(4) 3.03(3) 2.74(5),3.09(4)

Tl(1)  Tl(1) Tl(2)  Tl(3) Tl(2)  Tl(4)

3.673(4) 4.90(2) 5.24(2)

O(1)–(Si,Al)–O(2) O(1)–(Si,Al)–O(3) O(1)–(Si,Al)–O(4) O(2)–(Si,Al)–O(3) O(2)–(Si,Al)–O(4) O(3)–(Si,Al)–O(4) (Si,Al)–O(1)–(S,Al) (Si,Al)–O(2)–(Si,Al) (Si,Al)–O(3)–(Si,Al) (Si,Al)–O(4)–(Si,Al)

113.0(7) 111.4(7) 109.9(8) 104.5(8) 106.7(8) 111.1(9) 143.2(11) 142.6(11) 137.7(11) 140.8(13)

O(3)–Tl(1)–O(3) O(2)–Tl(2)–O(2) O(1)–Tl(3)–O(4) O(4)–Tl(4)–O(4)

93.9(5) 87.9(6) 62.0(10) 112.6(5)

a

Numbers in parentheses are estimated standard deviations in the units of the least significant digit given for the corresponding value.

4. Discussion 4.1. Framework structure The framework structure of FAU is characterized by double 6-rings (D6Rs, hexagonal prisms), sodalite cavities (cubooctahedra), and supercages (see Fig. 1). Each unit cell has 8 supercages, 8 sodalite cavities, 16 D6Rs, 16 12-rings, and 32 single 6-rings (S6Rs). The exchangeable cations, which balance the negative charge of the zeolite framework, usually occupy some or all the sites shown with Roman numerals in Fig. 1. The maximum occupancies at the cation sites I, I 0 , II, II 0 , III, and III 0 in FAU are 16, 32, 32, 32, 48, and 96 or 192, respectively. Further description is available [22–25]. 4.2. Tl+ positions In this structure, Tl+ ions are found at four different crystallographic sites. Site II, a 32-fold position, is filled.

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G.H. Jeong et al. / Microporous and Mesoporous Materials 94 (2006) 313–319 Table 4 ˚ ) of Tl+ ions from 6-ring planes Deviation (A Position At O(3) At O(2) a b

Fig. 1. A stylized drawing of the framework structure of FAU (includes zeolites X and Y). Near the center of each line segment is an oxygen atom. The different oxygen atoms are indicated by the numbers 1–4. Each tetrahedral site is occupied by a Si or an Al atom with no long range order. Extraframework cation positions are labeled with Roman numerals.

Site I 0 , another 32-fold position, may not quite be full (30.4(6) out of 32). The remaining 8.7(8) Tl+ ions are distributed among two III 0 sites: 5.7(8) at Tl(3) and 3.0(4) at Tl(4). Site I, an often used cation site in faujasite structures, is not used by Tl+. In fact, Tl+ has never been found at site I, not in jTl92j-FAU [9], jCd24.5Tl43j-FAU [26], jSr8.5Tl75jFAU [27], jPd18Tl56j-FAU [28], nor jPd21Tl50j-FAU [28]. Four Tl(1) cations at site I 0 are arranged tetrahedrally in ˚ (see each sodalite cavity with Tl(1)  Tl(1) = 3.673(4) A Fig. 2). (Some sodalite cavities may have only three Tl(1) ions.) Each Tl+ ion lies relatively far inside the sodalite cav˚ from the plane of the three O(3) framework oxyity, 1.37 A

Fig. 2. Stereoview of a sodalite cavity with an attached D6R in jTl71jFAU. Four Tl+ ions at Tl(1) are arranged tetrahedrally at site I 0 . Four Tl+ ions at Tl(2) are arranged tetrahedrally at site II. All (or nearly all) D6Rs host two Tl(1) cations at site I 0 as shown. Ellipsoids of 20% probability are used.

a

Tl(1) Tl(2)b

Site 0

I II

Displacement 1.37 1.63

Tl(1) is in the sodalite cavity. Tl(2) is in the supercage.

gens of the D6R to which it is bound (see Table 4). The ˚ , shorter than the Tl(1)–O(3) distances are 2.568(17) A sum of the corresponding ionic radii, 1.47 + 1.32 (respec˚ [18]. This indicates that each Tl+ ion coortively) = 2.79 A dinates strongly to its three O(3) oxygens, as would be expected by their low coordination number, three. ˚ ) within the sodalite unit Tl+  Tl+ repulsions (3.673(4) A should also contribute to this short distance. Perhaps these repulsions and the likely presence of 6-rings with fewer than two Al3+ ions cooperate to keep the Tl(1) position from being full. The O(3)–Tl(1)–O(3) bond angles, 93.9(5), are far from trigonal planar (see Table 3). Thirty-two Tl+ ions at Tl(2) fill site II; four Tl(2) cations are arranged tetrahedrally about each sodalite unit (see Fig. 2) and within each supercage (see Fig. 3). Each Tl(2) ˚ to three O(2) ion coordinates trigonally at 2.724(15) A ˚ into the framework oxygen and is recessed ca. 1.63 A supercage from its O(2) plane (see Table 4). The Tl(2)– ˚ , are longer than the Tl(1)– O(2) distances, 2.724(15) A ˚ O(3) distances, 2.568(17) A, but are still somewhat less than ˚ , as would be the sum of the conventional radii, 2.79 A expected because of their low coordination number, as with Tl(1). The O(2)–Tl(2)–O(2) bond angles are 87.9(6), again far from trigonal planar (see Table 3). The remaining Tl+ ions at Tl(3) and Tl(4) occupy two different 192-fold site III 0 positions in the supercages with occupancies of 5.7(8) and 3.0(4), respectively. The Tl(3)– O and Tl(4)–O bond distances (see Table 3) are all similar to those found in dehydrated jTl92j-FAU [9], jCd24.5Tl43jFAU [26], jSr8.5Tl75j-FAU [27], jPd18Tl56j-FAU [28], and jPd21Tl50j-FAU [28]. The very short Tl(3)–O(4) distance, ˚ , may be a consequence of the low coordination 2.44(4) A number, again three, and the irregular coordination at this site. It may also be somewhat inaccurate because only the

Fig. 3. Stereoview of a supercage. Four Tl+ ions at Tl(2) (site II) and one Tl+ ion at Tl(3) (site III 0 ) are shown. Ellipsoids of 20% probability are used.

G.H. Jeong et al. / Microporous and Mesoporous Materials 94 (2006) 313–319

317

Fig. 4. Stereoviews of a single 6-ring (S6R) with one of its adjacent 4-rings. (a) A Tl+ ion at Tl(2) lies at site II, and a Tl+ ion at Tl(3) lies at site III 0 . Tl(3) is ˚ from one O(4) oxygen atom and ca. 2.77 A ˚ from two O(1) oxygen atoms. (b) A Tl+ ion at Tl(2) lies at site II, and a Tl+ ion at Tl(4) lies at a ca. 2.44 A ˚ from two O(4) oxygen atoms and 3.03 A ˚ from one O(1) oxygen atom. Ellipsoids of 20% probability are used. second site III 0 . Tl(4) is ca. 2.74/3.09 A

average O(4) position has been determined; the positions of the 12 O(4)s that are approached by Tl(3)s may be modified by this interaction. In Fig. 4(a), an ion at Tl(2) and an adjacent ion at Tl(3) are shown. In Fig. 4(b), an ion at Tl(2) and an adjacent ion at Tl(4) are shown. The Tl(2)  Tl(3) and ˚ and 5.24(2) Tl(2)  Tl(4) distances in Fig. 4 are 4.90(2) A ˚ , respectively; because the Tl(2) position is fully occupied, A these distances are unavoidable. Finally, an inspection of Fig. 2 suggests that an oxide ion might be present at the center of the sodalite cavity where it would be tetrahedrally surrounded by four Tl+ ions at Tl(1). This might account for the high occupancy ˚ Tl+  Tl+ at this I 0 site, despite the multiple 3.673-A approaches that it requires (Table 3). However, no peak appears on the final difference Fourier function at that ˚ , would be position. Also, the Tl(1)–O distances, 3.15 A sharply longer than the sum of the conventional radii of ˚ [18]. Finally, considering that TlOH Tl+ and O2, 2.79 A imbibition occurred during Tl+-exchange at pH = 7.2 (see Section 4.3), the ca. eight additional oxide ions required per unit cell would in turn require about 16 additional Tl+ ions, a total of 87 Tl+ ions per unit cell; this is not seen. Oxide ions are therefore not at the centers of sodalite cavities, consistent with the observation that Tl2O has migrated out of this zeolite upon vacuum dehydration at 400 C. Smaller thermal parameters might have been expected for the heavy Tl+ ions, especially at the primary positions, Tl(1) and Tl(2). Surely these thermal ellipsoids include the disorder to be expected because of the non-equivalence of the 6-rings (three and fewer Al atoms) in zeolite Y. 4.3. Imbibition of TlOH, Cd(OH)2, and Ca(OH)2 It was observed crystallographically in zeolite A (LTA) that ion exchange with aqueous TlOH resulted in the pseudo-unit-cell composition jTl12(TlOH)(H2O)xj[Si12Al12O48]-LTA (colorless); this became jTl12j[Si12Al12O48]-LTA

(black) upon vacuum dehydration [29]. The one molecule of TlOH which had been imbibed per unit cell had decomposed upon vacuum dehydration, exited the zeolite structure, and deposited on the surface of the crystal as black Tl2O. It appears that the imbibition of TlOH occurred in the present work (jTl71j-FAU) also (extent undetermined), and that the imbibed TlOH again decomposed upon vacuum dehydration to deposit on the surface of the crystal. After that, only ca. 71 Tl+ ions remained per unit cell, the correct number needed to balance the zeolite’s framework charge. The gray color of fully dehydrated fully Tl+-exchanged zeolite X, jTl92j-FAU [9], indicates that little or no imbibition of TlOH occurred there. Also, the colors of jCd24.5Tl43j-FAU [26], jSr8.5Tl75j-FAU [27], jPd18Tl56jFAU [28], and jPd21Tl50j-FAU [28] were colorless, pale yellow, dark orange, and dark orange, respectively, indicating that no imbibition of TlOH occurred there. Still, although the ion-exchange procedures were similar for all of the above (the crystals were not washed free of any ion-exchange solution that might have remained), it remains possible that the black color of the crystal has arisen from the decomposition on the surface of unimbibed TlNO3 or TlOH. Cd2+-exchange into zeolite X (FAU) was not accompanied by imbibition; jCd46j-FAU was readily prepared from jNa92j-FAU by Cd2+-exchange at pH = 7.0 [30]. However, when a similarly prepared solution was used to Cd2+-exchange zeolite Y, jNa71j-FAU, eight molecules of Cd(OH)2 were imbibed per unit cell [31]. As with Tl+ exchange, the imbibition of metal hydroxide was seen only with zeolite Y (FAU). As in jCdj[Si121Al71O384]-FAU [31], jTlj[Si121Al71O384]FAU appears to be reluctant to accept the diminished number of cations that it has, as compared to jTlj[Si100Al92O384]-FAU, to locally balance the anionic charge of the zeolite. The ability of Tl+ to hydrolyze somewhat in aqueous solution allows zeolite Y (FAU) to keep its number of cations relatively high by hydroxide imbibition.

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Table 5 Comparison of jTl71j-FAU and jTl92j-FAU jTl71j[Si121Al71O384]-FAU

jTl92j[Si100Al92O384]-FAU

Si/Al ratio ˚) Cell constant, a (A Space group ˚) Framework bond distance (A

1.69 24.948(2) Fd 3m mean (Si,Al)–O

1.663

Framework bond angles (deg)

(Si,Al)–O(1)–(Si,Al) (Si,Al)–O(2)–(Si,Al) (Si,Al)–O(3)–(Si,Al) (Si,Al)–O(4)–(Si,Al) Site I Site I 0 Site II Site III 0

143.2(11) 142.6(11) 137.7(11) 140.8(13) – 30a 32 6a/3

1.09 25.043(1) Fd 3 Mean Si–O Mean Al–O Si–O(1)–Al Si–O(2)–Al Si–O(3)–Al Si–O(4)–Al Site I Site I 0 Site II Site III 0

Distribution of Tl+ ions

a

1.63 1.72 141.3(11) 140.9(9) 138.2(9) 142.9(10) – 32 32 16/12

Rounded to integers.

About 9.6 molecules of Ca(OH)2 were imbibed per unit cell when zeolite X (FAU) was Ca2+ exchanged at pH = 12.0 using aqueous Ca(OH)2 [32]. In this case, it is the high pH of the solution, rather than the ability of the cation to hydrolyze, that has facilitated the imbibition of metal hydroxide. 4.4. Comparisons with related structures This study indicates that all of the Na+ ions in zeolite Y, jNa71j-FAU, can readily be replaced by Tl+ ions. The unit ˚ compared to cell constant decreased to a = 24.948(2) A that in dehydrated zeolite X, jTl92j[Si100Al92O384]-FAU ˚ , a consequence of the increased silicon con[9], 25.043(1) A tent (see Table 5). As in jTl92j-FAU [9], Tl+ ions do not occupy site I, not because Tl+ is too large to do so, but because a more favor˚ able arrangement is possible; Cs+ ions (radius = 1.69 A ˚ [18]), [18]) are formally larger than Tl+ (radius = 1.47 A yet they have been found at site I [33,34]. It is clear that this is not a kinetic or sieving effect; because Tl+ ions are found in the sodalite units of zeolite A (LTA) [35,36] and Y (FAU) (this work), they must have entered through 6-rings. Mechanisms for this have been carefully discussed [34]. When the fully dehydrated structures of jTl71j-FAU and jK71j-FAU [7] are compared, it can be seen that jK71j-FAU ˚ is far more complex (see Table 6). K+ ions (radius = 1.33 A + [18]) occupy site I (D6R centers), but Tl ions do not. It would appear that Tl+ ions should fit very nicely into ˚ in this structure with D6Rs: compare site I–O(3) = 2.80 A + 2 ˚ [18]. The apparent the sum of the Tl and O radii, 2.79 A + 0 preference of Tl for site I , perhaps to maximize the number of Tl+ ions associated with D6Rs while avoiding site I–I 0 contacts, as demonstrated by its avoidance of site I [9,26– 28], causes the structures of jTl71j-FAU and jTl92j-FAU [9] to be much simpler than those of jK71j-FAU [7] and jK92jFAU [8]. Clearly, the K-FAU system is unique, apparently ˚ [18]) which a consequence of the K+ ionic radius (1.33 A + + ˚ ˚ [18]). lies between that of Na (0.97 A [18]) and Tl (1.47 A

Table 6 Site occupancies in three zeolite Y (FAU) structures (monopositive cation forms) Site

jK71j-FAUa

Ia Ib I0a I0b II 0 II III III 0 a 0 III P b

6 5 10 5

a b

30.3(3) 9.4(4) 2.0(7) 1.5(6) 69.2

jTl71j-FAU (this work)

jNa56j-FAUb 7

30.4(6)

18.5

32

32

5.7(8) 3.0(4) 71.1

57.5

Ref. [7]. Ref. [5].

5. Summary A single crystal of fully dehydrated, fully Tl+-exchanged zeolite Y, jTl71j-FAU, was prepared and its structure was determined by X-ray diffraction methods. The 71 Tl+ ions per unit cell are distributed over four crystallographic sites: 30.4(6) fill or nearly fill site I 0 , 32 fill site II, 5.7(8) sparsely occupy site III 0 , and 3.0(4) sparsely occupy another site III 0 . It appears that TlOH was imbibed during aqueous ion exchange at pH = 7.2, and that it decomposed upon vacuum dehydration at 400 C to deposit as Tl2O (black) on the surface of the zeolite crystal. Acknowledgment This work was supported by Brain Korea 21 Project, 2003. Appendix A. Supplementary data Tables of calculated structure factors squared, and observed structure factors squared with esds (12 pages).

G.H. Jeong et al. / Microporous and Mesoporous Materials 94 (2006) 313–319

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