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Encapsulating Photoluminescent Materials in Zeolites. II. Crystal Structure of Fully Dehydrated Ce H O -Y (Si/Al = 1.69) Containing CeO , CeOH , Ce , and H 21

46

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Cheol Woong Kim, Ho Cheol Kang, Nam Ho Heo, and Karl Seff J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08373 • Publication Date (Web): 01 Oct 2015 Downloaded from http://pubs.acs.org on October 8, 2015

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The Journal of Physical Chemistry

Encapsulating Photoluminescent Materials in Zeolites. II. Crystal Structure of Fully Dehydrated Ce21H46O18-Y (Si/Al = 1.69) Containing Ce4O44+, CeOH2+, Ce3+, and H+

Cheol Woong Kim,† Ho-Cheol Kang,‡ Nam Ho Heo,*,† and Karl Seff§

†

Laboratory of Structural Chemistry,

Department of Applied Chemistry, School of Applied Chemical Engineering, Kyungpook National University, Daegu 41566, Korea ‡

Green Chemistry Division, Korea Research Institute of Chemical Technology, Yuseong, Daejon 34114, Korea §

Department of Chemistry, University of Hawaii,

2545 The Mall, Honolulu, Hawaii 96822-2275, U. S. A.

ACS Paragon Plus Environment


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(ABSTRACT)

Encapsulating photoluminescent centers within zeolites may give them long-term stability and enhanced luminosity. A single crystal of zeolite Na–Y (FAU, Si/Al = 1.69) was fully exchanged with Tl+, then exchanged with Ce3+, and dehydrated at 623 K and 2 × 10-4 Pa to give |Ce4O44+3.5CeOH2+4.6Ce3+2.1H+41.7|[Si121Al71O384]–FAU. Its structure was determined crystallographically (space group Fd 3 m, a = 24.909(1) Å) with synchrotron X-radiation and was refined using all 1236 unique data to R1 = 0.067 (Fo > 4σ(Fo)) and R2 = 0.247. The 20.5 Ce3+ ions per unit cell occupy four crystallographically distinct cation sites: 0.5 are octahedral at site I, 1.6 are trigonal at site I', 13.8 are octahedral at a second site I', and 4.6 are tetrahedral at site II. The 13.8 Ce3+ ions at the second site I' are members of Ce4O44+ clusters, tetrahedrally distorted cubes occupying 43% of the sodalite cavities. Each of these Ce3+ ions bonds octahedrally to three oxygen atoms of the zeolite framework (2.482(7) Å) and to three extraframework oxide ions (2.521(11) Å). Those at site II are 4-coordinate; each bonds to three oxide atoms of the zeolite framework (2.420(7) Å) and to one hydroxide ion (2.06(14) Å). The UV photoluminescence spectrum of this material has a broad emission band between 320 nm and 400 nm, peaking at 347 nm.

Keywords: zeolite Y, luminescence, single crystal, cerium oxide clusters, rare earth, lanthanide

2 ACS Paragon Plus Environment

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1. INTRODUCTION 1.1. Luminescence of Cerium Doped Materials. Materials doped with cerium have been used as detectors for ionizing radiation, scintillators to detect elementary particles, UV emitters, activators for energy transfer, and phosphors.1-3 Rare earth cations in general are important activators in phosphors because of their emission characteristics in the UV and visible regions, and Ce3+ in particular has a sharp high intensity blue emission. Because blue is one of the three primary colors, red, green, and blue (RGB), Ce3+ is heavily used. RGB photoluminescent materials such as phosphors4 and photosensitive metal complexes5 are used in full color displays (computer monitors, TV sets, and hand-held devices6-10). Long term stability has been difficult to achieve in these displays because the photoluminescent materials used are often sensitive to their chemical environments. Encapsulating these materials within zeolites may allow these difficulties to be overcome.1113

Recent work has focused on ways to accomplish this encapsulation to give zeolite

phosphors with long-term stability and enhanced or modified photoluminescence.14-18 1.2. Cerium Exchanged Zeolite Y (FAU). Ce3+ exchanged zeolite Y is used for the selective oxidation of hydrocarbons,19-20 for the conversion of lactic acid to acrylic acid (a popular raw material for the manufacture of chemical products),21 and for desulfurization.22 Mixed rare earth exchanged zeolite Y is used as fluid catalytic cracking (FCC) catalyst in the oil refining industry.23-24 Both the hydrothermal stability and the acidity of these catalysts are increased by the REO (rare earth oxide) "oxide/hydroxide" clusters generated in the sodalite cavities upon calcination or dehydration.19, 25 The zeolites and the clusters are mutually



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stabilizing. 1.3. Structures of Cerium Exchanged Zeolites X and Y. 1.3.1. Crystallography. The structures of hydrated and dehydrated Ce-exchanged FAU were first determined crystallographically by Olson et al. using single crystals of the mineral faujasite.26 They reported that the Ce3+ ions in hydrated Ce-FAU occupy 6-ring sites in the supercage. Upon dehydration under vacuum at 623 K, all Ce3+ ions move into the sodalite cavity where each Ce3+ ion is coordinated to three framework oxygen atoms and "up to three water oxygens." Similarly, the locations of the cerium ions in Ce-FAU zeolites were studied using powder X-ray diffraction.27-28 Hunter et al. studied Ce-X and found that the cerium ions moved from the supercages into sodalite cavities upon dehydration in the atmosphere at 813 K.27 They also reported that some Ce3+ ions had been oxidized to Ce4+, and that an oxide ion at the center of some sodalite cavities was surrounded tetrahedrally by four Ce3+ ions. Nery et al. reported that the Ce3+ ions in Ce,Na-Y migrate into the sodalite cavities as Na+ ions move into the supercages after heating to 773 K in air.28 Aluminum ions, presumably aluminate generated by dealumination, were also found in the sodalite cavities. 1.3.2. EXAFS.

An extended X-ray absorption fine structure (EXAFS) study of Ce3+-

exchanged zeolite Y showed that hydrated Ce3+ ions are located within the supercages in hydrated Ce-Y, and that dehydration at 573 K causes most of them to move to multiple sites in the sodalite cavities where, as partially hydrated ions, each Ce3+ ion coordinates to three 6-ring framework oxygen atoms and to some water molecules.29


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1.3.3. Spectroscopy. Piscina et al. used vibrational spectroscopy to determine the nature and location of the cerium species in Ce3+-loaded Y zeolites heated in air.30 From their Raman results, they proposed that nanosized CeO2 particles were present in Ce-Y. From an analysis of the OH stretching region in their FTIR study, they concluded that Ce3+ ions associate with hydroxyl groups in the sodalite cavities of Ce-Y. Similarly Lee and Rees using inductively coupled plasma spectroscopy (ICPS), reported that the Ce3+ ions initially located in the supercages of Ce-Y begin to migrate to the sodalite cavities upon dehydration in air at about 333 K.31 The total number of Ce3+ ions in the sodalite cavities reached a maximum of 18 per unit cell. The hydroxycerium species that formed at temperatures up to 523 K had completely converted to oxycerium complexes at 573 K. Chen et al., using 129Xe NMR, concluded that hydrated Ce3+ cations in the supercages of Ce3+-Y all migrate to the sodalite cavities and the hexagonal prisms upon vacuum dehydration.32 1.4. Luminescence Studies of Ce3+-exchanged FAU Zeolites.

Kynast and co-workers

studied the UV emission and reflectance/absorbance spectra of zeolites X and Y prepared with various degrees of Ce3+ doping. They concluded that the luminescence efficiency depends on the sites that the Ce3+ ions (the emitters) occupy and their populations at each site,33

each site having its own photoluminescent properties.

Therefore, both the

interactions of the Ce3+ ions with the zeolite and the degree of Ce3+ exchange affect the luminescence properties of the zeolite. These interactions may allow otherwise forbidden



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transitions to be observed.34 Kynast et al. also reported that the photoluminescence properties in the UV and green spectral emission ranges of zeolite X doped with both Ce3+ and Tb3+ were dependent on the Ce3+ concentration and the Ce3+/Tb3+ ratio.35 Yan et al. studied the luminescent properties of Ce3+ exchanged zeolite Na-Y and reported that it exhibits its characteristic luminescence in the ultraviolet region in Ce,Na-Y, Ce,Y,Na-Y, Ce,Y,Tb,Na-Y, Ce,Zn,Na-Y, and Ce,Zn,Na-Y.36 The luminescence of the rare earth ions in zeolites has been reviewed.37 1.5. Objectives of This Work. To better understand the photoluminescent properties of Ce3+-exchanged zeolites, we sought to learn the positions and detailed coordination environments of the Ce3+ ions in fully dehydrated zeolite Y. To be able to characterize them well crystallographically, we wished to maximize the number of Ce3+ ions in the zeolite. After we observed that the exchange of Ce3+ into both Na-Y and K-Y at 294 K was incomplete, we tried again with Tl-Y, fully Tl+-exchanged zeolite Y, at 353 K. (If the exchange was again incomplete, the residual Tl+ ions, because of their ionic size and scattering power, could be readily identified crystallographically.) High quality diffraction data would be obtained by using single crystal methods with synchrotron X-radiation.

2. EXPERIMENTAL SECTION 2.1. Preparation of Ce21H46O18-Y. 2.1.1.

Single

Crystal.

Large

colorless

crystals

of

sodium

zeolite

Y

(|Na71(H2O)x|[Si121Al71O384]–FAU, Na71–Y, or Na–Y; Si/Al = 1.69) were prepared by Lim et


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al.38-39 using the synthetic method of Vaughan et al.40 A single crystal of Na–Y, a colorless octahedron 0.12 mm in cross-section, was lodged in a fine Pyrex capillary. This crystal was then Tl+-exchanged to give (|Tl71(H2O)y|[Si121Al71O384]–FAU, Tl71–Y, or Tl–Y).41-42 This was done by the flow method using 0.1 M aqueous (pH = 6.4) thallous acetate (TlC2H3O2, 99.999%, Aldrich) at 294 K. It was then Ce3+-exchanged by the flow method at 353 K with 0.05 M aqueous (pH = 4.8) Ce(NO3)3 (Ce(NO3)3·6H2O, 99.999%, Aldrich). The resulting colorless crystal was cautiously vacuum dehydrated (a heating rate of 25 K/h was used) and maintained at 623 K and 2 x 10-4 Pa for two days. After cooling to room temperature, the crystal, now dark brown (discussed in Section 6.3), was sealed off from the vacuum system and isolated in its capillary under vacuum with a small torch. Additional details are given in Table 1. 2.1.2. Powder. A powder sample was prepared by the batch method. Na-Y powder (1 g, Aldrich, Si/Al ca. 2.5) was stirred in 100 mL of 0.1 M TlC2H3O2 (a two-fold excess) at 294 K for 24 h. This was repeated two times with fresh solution. The resulting powder (Tl-Y) was stirred in a solution of 100 mL of 0.1 M Ce(NO3)3 (Ce(NO3)3·6H2O, 99.9%, Aldrich) (a 7.5-fold excess) at 294 K for 5 h. This was repeated three times with fresh solution. About 0.07 g of the product was placed in a thin walled Pyrex tube (2 mm × 15 mm) and was dehydrated under the same conditions used to dehydrate the single crystal. After being allowed to cool to room temperature, it was sealed off under vacuum. It was ivory white. 2.2. X-ray Diffraction.

Diffraction intensities for the single crystal were collected with



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synchrotron X-radiation via a silicon (111) double crystal monochromator. About 75,000 reflections were harvested by collecting 72 sets of frames with a 5o scan and an exposure time of 1 s per frame. The basic data file was prepared using the program HKL2000.43 The reflections were successfully indexed by the automated indexing routine of the DENZO program.43 This highly redundant data set was corrected for Lorentz and polarization effects; a negligible correction for crystal decay was also applied. The space group Fd 3 m, standard for zeolite Y, was determined by the program XPREP.44 Additional experimental and crystallographic data are presented in Table 1. 2.3. SEM-EDX Analysis.

Four additional single crystals were prepared as described in

Section 2.1.1. Two were dark brown, one was pale brown, and the fourth was colorless. Each was removed from its capillary (exposed to the atmosphere (rehydrated) without change of color) and attached to a sample holder with carbon attach tape for SEM-EDX (scanning electron microscopy energy dispersive X-ray) analysis. Their compositions were determined using a Horiba X-MAX N50 EDX spectrometer within a Hitachi SU8220-SR FE-SEM (field emission scanning electron microscope) at 294 K and 9 × 10-4 Pa. 2.4. UV Luminescence. The UV photoluminescence of the Ce3+ exchanged powder was studied with an Agilent Technologies Cary Eclips fluorescence spectrometer with a xenon flash lamp.

3. Structure Determination Full-matrix least-squares refinements (SHELXL2013)45 were done on F2 using all 1236


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unique reflections measured.

They were initiated with the atomic parameters of the

framework atoms (T (Si and Al, disordered), O1, O2, O3, and O4) in dehydrated Tl71–Y.41 The initial refinement with isotropic thermal parameters for all framework atoms converged to the high error indices R1 = 0.49 and R2 = 0.86 (step 1 of Table 2). The detailed progress of structure determination as subsequent peaks were found on difference Fourier functions and identified as extraframework atoms is given in Table 2. The largest peak, 0.8 e/Å, on the final difference Fourier function was at the center of the sodalite cavity. An aluminum atom of an aluminate ion leached from the zeolite framework is often found at this position. Its occupancy, however, refined to a small and insignificant value, 0.5(2) Al atoms per unit cell, so it was not considered further. The final structural parameters are presented in Table 3. The silicon and aluminum disorder in the zeolite framework extends to all framework oxygen positions and is substantially compounded by the local distortions induced by the partially occupied non-framework cation positions. Because of this, the results reported here are less reliable than those reported for ordered crystals where each position is fully occupied, as is usually the case in crystallography. Also, the bond lengths and angles reported are correct only for averaged atom positions; they are often inaccurate for low occupancy positions. A somewhat more detailed discussion is available.46 The final weights were assigned using the formula w = 1/[σ2(Fo2) + (aP)2 + bP] where P = [Max(Fo2,0) + 2Fc2]/3, and a and b are refined parameters (Table 1). Atomic scattering factors for Ce0, Tl0, O–, and T1.82+ were used.41, 47-48 The function



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describing T1.82+ is the weighted (for Si/Al = 1.69) mean of the Si4+, Si0, Al3+, and Al0 functions. All scattering factors were modified to account for anomalous dispersion.49-50 Other crystallographic details are given in Table 1.

4. Brief Description of the FAU Framework and Cation Sites The framework structure of zeolite Y is characterized by the double 6-ring (D6R), the sodalite cavity, and the supercage (see Figure 1). Each unit cell has 8 supercages, 8 sodalite cavities, 16 D6Rs, 16 12-rings, and 32 single 6-rings (S6Rs).

The exchangeable

extraframework cations that balance the negative charge of the FAU framework usually occupy some or all of the sites noted with Roman numerals in Figure 1. The maximum occupancies at the cation sites, I, I’, II, II’, III, and III’ in FAU are 16, 32, 32, 32, 48, and (in Fd 3 m) 192, respectively. Somewhat more detailed descriptions are available.51-53

5. Description of the Structure 5.1. Framework Geometry.

The mean T–O bond length (1.653 Å, see Table 4) in

Ce21H46O18–Y is between the Si–O (1.61 Å) and Al–O (1.74 Å) distances found in both dehydrated Ca-LSX (FAU)54 and hydrated Na-LTA,55 appropriately closer to the Si–O distance. It is about the same as that in K71–Y,38 1.655 Å, and close to that in Tl71–Y,41 1.663 Å, indicating that the Ce21H46O18–Y framework is not very distorted. Still, local distortions may be expected, especially near the Ce3+ cations. Among the four T–O–T angles, T–O3–T is the smallest (139.2(4)°, Table 4) because


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most of the Ce3+ ions, 15.9 of 20.5 per unit cell, are at sites I and I' where they bond to O3, pulling those oxygen atoms inward toward the centers of their 6-rings. 5.2. Extraframework Ions: Ce3+ and O2-. The Ce3+ ions are distributed over four equipoints. They occupy site I (Ce1) in the D6Rs, two sites I' (Ce11 and Ce12) in the sodalite cavities opposite D6Rs, and site II (Ce2) opposite S6Rs in the supercages. Two extraframework oxygen positions were found opposite S6Rs: O5 is in the sodalite cavity (near cation site II’) and O6 is in the supercage (near cation site II). 5.2.1. Octahedral Ce3+ Ions at Site I. Per unit cell, 0.51(7) Ce3+ ions, relatively few, were found at site I (Ce1, a 16-fold position, see Figure 2). Each lies on an inversion center and coordinates octahedrally to the six O3 framework oxygen atoms of its D6R. The Ce1-O3 distance, 2.842(5) Å, is noticeably longer than the sum of the conventional ionic radii of Ce3+ and O2–, 2.35 Å (1.034 + 1.32 Å, respectively).56 It is also much longer than the other Ce3+–O distances in this structure (see Table 4).

Surely this 2.842(5) Å distance is

inaccurate; each Ce3+ ion at Ce1 must have pulled its six O3 oxygen atoms much closer to it. It is clear that the O3 position has refined near the most populous of its various unresolved positions, close to that in an unoccupied D6R. 5.2.2. Trigonal and Octahedral Ce3+ Ions at the I’ Sites. Ce3+ ions were found at two I' sites, Ce11 and Ce12. The Ce11···Ce1 distance is only 2.35(6) Å, Ce12···Ce1 is only 2.926(9) Å, and the shortest Ce11···Ce12 distance, 0.58(5) Å, is even less. Although intercationic electrostatic repulsion would be severe for Ce3+ ions at these distances, the low occupancies observed readily allow these approaches to be avoided. The sum of the occupancies at site I



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and the two I' sites is 15.7(4), indicating that each of the 16 D6Rs per unit cell has just one Ce3+ ion, either at site I, the first site I', or the second. Thus the much longer I'···I' distances across the D6Rs, 4.70 Å (Ce11···Ce11), 5.85 Å (Ce12···Ce12), and 5.27 Å (Ce11···Ce12), are also avoided. 5.2.2.1. Trigonal Ce3+ Ions at the First Site I’. Ce11 is 2.276(16) Å from three O3 framework oxygens and the O3–Ce11–O3 angles are 114.0(13)° (see Figure 3). This distance is about the same as the sum of the conventional radii of Ce3+ and O2–, 2.35 Å,56 perhaps a little shorter because of its low coordination number. The nearest other atom, O5, is too far away (2.97(5) Å) to bond, and this distance is readily avoided. The possibility that a hydroxide ion might be coordinating to Ce11 was considered, but the short Ce11–O3 bond lengths (2.276(16) Å) and the closeness of Ce11 to the O3 plane (0.56 Å, Table 5) argue against that. (The occupancy at this possible oxygen position would be too low, we judge, for it to be found in this work.) 5.2.2.2. Octahedral Ce3+ Ions (Members of Ce4O4 Clusters) at the Second Site I’. Ce12 is 2.482(7) Å from three O3 framework oxygens. This is appreciably longer than the sum of the conventional radii of Ce3+ and O2–, 2.35 Å,56 and longer than Ce11-O3, 2.276(16) Å. It is also much further from its 6-ring plane, 1.13 Å, than the ions at Cell are from theirs (Table 5). This suggests that each ion at Ce12 participates in further bonding and has a coordination number greater than three. Indeed, Ce12 is also 2.520(11) Å from three O5 extraframework oxygen atoms (see Figures 4 and 5). Thus, each Ce12 ion has a somewhat distorted octahedral geometry (Table 4) with three 2.482(7) Å bonds to O3 framework


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oxygens and three 2.520(11) Å bonds to O5 extraframework oxygens.

Ce12 and O5

constitute a tetrahedrally distorted (Ce12)4(O5)4 cube with reasonable angles at Ce12 and

O5 (Table 4). In the Ce4O4 unit, the four Ce3+ ions at Ce12 are tetrahedrally arranged, each bonding to a 6-ring of a D6R. Its four oxygen atoms (at O5) are also tetrahedrally arranged, each opposite a S6R of the same sodalite cavity. Each Ce3+ ion is pulled sharply outward from its Ce4O44+ “cube” to coordinate to three O3 framework oxygen atoms (Figure 4). The topological cube formed by these two interpenetrating tetrahedra has point symmetry 3 m (Td). 5.2.3. 4-coordinate Ce3+ Ions at Site II. Ce3+ ions were also found in S6Rs in the supercages at site II (Ce2). The occupancy here is relatively high, 4.57(10) Ce3+ ions per unit cell. Ce2 is 2.420(7) Å from three O2 framework oxygens (Table 4), longer than the sum of the conventional radii of Ce3+ and O2–, 2.35 Å56, and noticeably longer than Ce11-O3, 2.276(16) Å. Also Ce2 extends further from its 6-ring plane (0.93 Å, Table 5) than Ce11 (0.56 Å). As with Ce12 in Section 5.2.2.2, this geometry suggests that the ions at Ce2 participate in further bonding. The extraframework oxygen atoms at O6 in the supercage are within bonding distance of Ce2 on the same 3-fold axes (Ce2-O6 = 2.06(14) Å, see Figure 3) and are judged (Section 5.3) to represent hydroxide ions. The Ce2-O6 distance is insignificantly less than the sum of the ionic radii, 2.35 Å, but it is reasonable that this bond would be shorter because O6 (OH-) is a terminal hydroxide atom (group). Thus each ion at Ce2 is 4coordinate tetrahedral.



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5.3. Assignment of H+ Positions. Many H+ ions, about 46.3 per unit cell (see Table 6), must be present per unit cell to achieve charge balance. They were not found in this work, in part because hydrogen atoms scatter X-rays too weakly. They should bond to the oxide ions in this structure. However, there are three kinds of oxide ions, those of the zeolite framework, those in the Ce4O4 groups, and those in the CeO (Ce2-O6) groups. The H+ ions should prefer to bond to the most negative oxide ions, those least heavily coordinated by cations with high charges, and then to others in order of increasing coordination until all have been placed.

The charges, numbers, and the chemical natures of these cations were all

considered. On this basis, 4.57(10) H+ ions should first bond to O6 atoms; CeO then becomes CeOH2+. The remaining H+ ions are all expected to bond to oxygen atoms of AlO-Si groups in the zeolite framework. None are expected to bond to oxygen atoms of the Ce4O4 groups. Thus Ce4O4 is Ce4O44+, and the dissociation of the water molecules that yielded the oxide ions in Ce4O44+ was complete: H2O 2H+ + O2-.

6. Discussion 6.1. The Ce4O44+ Clusters. Of the eight sodalite cavities per unit cell, 3.450(10) contain Ce4O44+ clusters, tetrahedrally distorted cubes (Figures 4 and 5). Of the 20.5 Ce3+ ions per unit cell, 13.8 = 4 × 3.450 are members of these clusters. Lanthanide M4O44+ clusters had been seen at the same position in fully La3+-exchanged, fully dehydrated zeolite X57 and in partially Eu3+-exchanged, fully dehydrated zeolite Y.46 In the La-X structure each sodalite cavity contains a La4O44+ ion. These are connected by


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oxide ions at their La3+ vertices to give a neutral three-dimensional La2O3 continuum. In Eu-Y Eu4O44+ clusters occupy 41% of the sodalite cavities. Ce4O44+, La4O44+, and Eu4O44+ are all tetrahedrally distorted cubes of symmetry 3 m (Td). Furthermore, they are all very similarly distorted; their O3-M-O3 angles are 71.0(7)°, 71.2(4)°, and 69.8(9)°, respectively. In older work Olson et al. reported a value of 64.0(3)° for the corresponding Ce4O4 groups in vacuum dehydrated Ce-FAU.26 The anionic sodalite cavity often hosts and stabilizes cationic clusters.58-69 A brief review is available.63 In particular, tetrahedrally distorted M4O4n+ groups are frequently found in the sodalite cavities of zeolites. Examples are Cd4O4 (in Cd8O48+),60 Ni4O4 (in Ni8O4·xH2O8+),62 and In4O44+ 70 in zeolite Y (FAU); Zn4(OH)44+ 71 and Pb4(OH)44+

72

in

zeolite X (FAU); and Cd4(OH)44+ 73 and Pb4O4n+ 74 in zeolite A (LTA). 6.2. Net Reaction per Unit Cell. The balanced reactions 1 and 2 below are written without esds and contain a somewhat excessive number of significant figures. To form the 3.45 Ce4O44+ clusters and the 4.57 CeOH2+ groups, 18.37 = (3.45 × 4) + (4.57 × 1) oxide ions are needed. Therefore, of the 46.30 H+ ions per unit cell needed for charge balance (Table 6), 36.74 = 2 × 18.37 must come from water molecules and only 46.30 - 36.74 = 9.56 H+ ions exchanged into the zeolite during the Ce3+-exchange process. Thus the complete reaction per unit cell for Ce3+ exchange followed by dehydration under vacuum at 623 K can be written as |Tl+71|[Si121Al71O384] + 20.48Ce3+ + 18.37H2O + 9.56H+ |Ce3+20.48H+46.30O2–18.37|[Si121Al71O384] + 71Tl+


(1)


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To the precision of this work, and more informatively, this reaction is better written as |Tl+71|[Si121Al71O384] + 20.5Ce3+ + 18.4H2O + 9.5H+ |Ce4O44+3.45CeOH2+4.6Ce3+2.1H+41.7|[Si121Al71O384] + 71Tl+

(2)

The zeolite product of Reaction 2 has Ce3+ ions in three forms, predominantly as Ce4O44+. This suggests that Reaction 2 had not gone to completion due to a limiting reagent. If more water had been available to the crystal as it was being heated (if the dehydration had been done less cautiously or in air), the following reaction might have occurred. |Tl+71|[Si121Al71O384] + 20.5Ce3+ + 20.5H2O + 9.5H+ |Ce4O44+5.125H+50.5|[Si121Al71O384] + 71Tl+

(3)

By Reaction 3, the formula of the product would be much simpler. All Ce3+ ions would have reacted to form Ce4O44+ clusters, and more H+ ions would have been produced. Dehydrating zeolites by heating in the atmosphere (common practice in industry) would have made this water available, but it would also have provided oxygen for the oxidation of Ce3+ to Ce4+.27 6.3. SEM-EDX Results. The SEM-EDX results (Table 7) vary widely, in large part due to crystal decomposition in the electron beam, but are generally supportive of the composition determined crystallographically. No thallium was found in the dark brown single crystal studied crystallographically, and no thallium could be detected in the colorless and pale brown single crystals analyzed by SEM-EDX (Figure 6 and Table 7). Thus the brown color of some of the crystals is attributed to a separate phase deposited on their surfaces. Apparently, for some crystals,


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small amounts of TlC2H3O2 had remained in the ion exchange apparatus as the Ce3+ exchange was done and remained on the crystal surfaces afterwards. As the temperature was increased during subsequent vacuum dehydration, the TlC2H3O2 decomposed to give Tl2O3 (brown) and perhaps Tl2O (black). 6.4. Luminescence Properties of the Powder. The UV photoluminescence spectrum of the dehydrated powder consists of a broad emission band between 320 and 400 nm, peaking at 347 nm, for an excitation wavelength of 296 nm (Figure 7). It is due to the 4f05d1 → 4f15d0 transition in Ce3+. This transition is from the excited state, 2D3/2, to either the 2F7/2 state or the 2F5/2 ground state; these cannot be distinguished directly.36 Although the dehydration of the powder caused many water molecules bound to Ce3+ to be lost, the emission peak position at 347 nm is almost the same as it was in hydrated Ce,Na-Y, 350 nm.36-37 Suib et al. suggested that most dehydrated rare earth exchanged zeolites show extremely broadened emission spectra due to the loss of water molecules and the movement of Ce3+ ions into the sodalite cavities.37 The luminescence results may not be directly related to the structure reported because the powder (Si/Al ca. 2.5) whose optical properties were measured has a lower Al3+ content, and therefore a lower exchangeable cation content (the cerium species and H+), than the single crystal studied (Si/Al = 1.69). The Ce3+ species in the powder and their relative abundances may be substantially different from those in the single crystal. The relative populations of the ions and clusters within a zeolite can be modified by the ion exchange conditions (pH, time and temperature of exchange) and post treatments such



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as time and temperature of dehydration. This may allow the photoluminescent properties of a Ce3+ exchanged zeolite to be modified. 6.5.

Comparison with Previous Reports of Structure. This work affirms the several

previous reports that there is a substantial presence of Ce3+ ions in the sodalite cavities of dehydrated Ce3+ exchanged zeolite Y26-32. Also in agreement with the current findings, Chen et al. found Ce3+ ions in D6Rs.32 Additional comparisons are difficult to make because of differences in sample preparation. The initial zeolite samples used for Ce3+ exchange differed in framework composition (Si/Al ratio) and in their cationic forms (Na-Y, NH4-Y, H-Y, and (here) Tl-Y). The pH of the Ce3+ exchange solution, the manner and extent of the Ce3+ exchange, and the conditions of dehydration also differed; each is important. Nonetheless, Reaction (3), if it is correct, indicates that all Ce3+ ions should be in the sodalite cavities, none in the supercages, in highly Ce3+-exchanged zeolites X and Y that were dehydrated in air, as has been reported.27-28 Olson et al. who dehydrated their samples under vacuum, also reported that all Ce3+ ions were in sodalite cavities26; the conditions under which their samples were dehydrated may have allowed enough water to have been present for this to happen. Similarly, Chen et al. found that all Ce3+ ions had left the supercages to occupy sodalite and D6R positions.32

7. SUMMARY All Tl+ ions in Tl71–Y were replaced by Ce3+ (87%) and H+ (13%) during Ce3+ exchange


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from aqueous solution at pH = 4.8. After dehydration under vacuum at 623 K, 20.5 Ce3+ ions per unit cell were found at four crystallographically distinct cation sites, 0.5 at site I, 1.6 at site I', 13.8 at a second site I', and 4.6 at site II. Those at the second site I' are members of Ce4O44+ clusters, tetrahedrally distorted cubes occupying 43% of the sodalite cavities. These Ce3+ ions are octahedral; each bonds to three oxygen atoms of the zeolite framework (2.482(7) Å) and to three extraframework oxide ions (2.521(11) Å). Those at site II are 4-coordinate; each bonds to three oxygen atoms of the zeolite framework (2.420(7) Å) and to one hydroxide ion (2.06(14) Å). The UV photoluminescence spectrum of this zeolite has a broad emission band between 320 nm and 400 nm, peaking at 347 nm for an excitation wavelength of 296 nm.

ASSOCIATED CONTENT Supporting Information Observed and calculated structure factors for Ce21H46O18–Y. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Tel: +82 53 950 5589. Fax: +82 53 950 6594. E-mail: [email protected] Notes The authors declare no competing financial interest.



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ACKNOWLEDGEMENT We gratefully acknowledge the Pohang Accelerator Laboratory at the Pohang Institute of Science and Technology (POSTECH) for the use of their diffractometer and computing facilities. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. NRF-2014R1A2A1A11054075).


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42. Kim, J. J.; Kim, C. W.; Heo, N. H.; Lim, W. T.; Seff, K., Tetrahydroxytetraindium(III) Nanoclusters, In4(OH)48+, in Air-Oxidized Fully In-Exchanged Zeolite Y (FAU, Si/Al = 1.69). Preparation and Crystal Structures of In−Y and In−Y[In4(OH)4]. J. Phys. Chem. C 2010, 114, 15741-15754. 43. Otwinowski, Z.; Minor, W., Methods Enzymol. 1997, 276, 307. 44. Bruker-AXS XPREP, Program for the Automatic Space Group Determination, version 6.12; Bruker AXS Inc.: Madison, WI, 2001. 45. Sheldrick, G. M. SHELXL97, Program for the Refinement of Crystal Structures, University of Gottingen: Gottingen, Germany, 1997. 46. Kim, C. W.; Kang, H.-C.; Heo, N. H.; Seff, K., Encapsulating Photoluminescent Materials in Zeolites. Crystal Structure of Fully Dehydrated Zeolite Y (Si/Al = 1.69) Containing Eu3+. J. Phys. Chem. C 2014, 118, 11014-11025. 47. Revised and Supplementary Tables to Volumes II and III. In International Tables for X-ray Crystallography, Ibers, J. A.; Hamilton, W. C., Eds. Kynoch Press: Birmingham, England, 1974; Vol. IV, p 71. 48. Doyle, P. A.; Turner, P. S., Relativistic Hartree-Fock X-ray and Electron Scattering Factors. Acta Crystallogr. Sect. A. 1968, 24, 390-397. 49. Cromer, D. T., Anomalous Dispersion Corrections Computed from Self-Consistent Field Relativistic Dirac-Slater Wave Functions. Acta Crystallogr. 1965, 18, 17-23. 50. Revised and Supplementary Tables to Volumes II and III. In International Tables for X-ray Crystallography, Ibers, J. A.; Hamilton, W. C., Eds. Kynoch Press: Birmingham, England, 1974; Vol. IV, p 148. 51. Bae, D.; Seff, K., Structures of Cobalt(II)-Exchanged Zeolite X. Microporous Mesoporous Mater. 1999, 33, 265-280. 52. Breck, D. W., Zeolite Molecular Sieves; John Wiley & Sons: New York, 1974, p 93. 53. Van Bekkum, H.; Flanigen, E. M.; Jacobs, P. A.; Jansen, J. C., Introduction to Zeolite Science and Practice; Elsevier: New York, 2001, p 44. 54. Vitale, G.; Bull, L. M.; Morris, R. E.; Cheetham, A. K.; Toby, B. H.; Coe, C. G.; Mac Dougall, J. E., Combined Neutron and X-ray Powder Diffraction Study of Zeolite Ca LSX and a 2H NMR Study of Its Complex with Benzene. J. Phys. Chem. 1995, 99, 16087-16092. 55. Fischer, R. X.; Sehovic, M.; Baur, W. H.; Paulmann, C.; Gesing, T. M., Crystal Structure and Morphology of Fully Hydrated Zeolite Na-A. Z. Kristallogr. - Crystal. Mater. 2012, 227, 438445. 56. In Handbook of Chemistry and Physics, 64th ed.; Lide, D. R., Ed. The Chemical Rubber Co.: Cleveland, OH, 1983; p F170. 57. Park, H. S.; Seff, K., Crystal Structures of Fully La3+-Exchanged Zeolite X: an Intrazeolitic La2O3 Continuum, Hexagonal Planar and Trigonally Monocapped Trigonal Prismatic Coordination. J. Phys. Chem. B 2000, 104, 2224-2236. 58. Armstrong, A. R.; Anderson, P. A.; Woodall, L. J.; Edwards, P. P., Structure of Na43+ in Sodium Zeolite Y. J. Am. Chem. Soc. 1995, 117, 9087-9088. 59. Seff, K., Cationic Zinc Clusters with Mean Formula Zn5.46.9+ in the Sodalite Cavities of Zeolite Y (FAU). Microporous Mesoporous Mater. 2005, 85, 351-354. 60. Lee, Y. M.; Jeong, G. H.; Kim, Y.; Seff, K., Single Crystal Structure of Fully Dehydrated, Excessively Cd2+-Exchanged Zeolite Y, ∣Cd27.5(Cd8O4)2∣[Si121Al71O384]-FAU, Containing Cd8O48+ Clusters. Microporous Mesoporous Mater. 2006, 88, 105-111. 61. Sun, T.; Seff, K., Pulsed-Neutron Powder-Diffraction Study of Lead Sulfide and Deuterium Ions in Zeolite Y. J. Phys. Chem. 1993, 97, 7719-7723. 62. Kim, C. W.; Jung, K. J.; Heo, N. H.; Kim, S. H.; Hong, S. B.; Seff, K., Crystal Structures of Vacuum-Dehydrated Ni2+-Exchanged Zeolite Y (FAU, Si/Al = 1.69) Containing ThreeCoordinate Ni2+, Ni8O4·xH2O8+, x ≤ 4, Clusters with Near Cubic Ni4O4 Cores, and H+. J. Phys. Chem. C 2009, 113, 5164-5181. 63. Song, M. K.; Kim, Y.; Seff, K., Disproportionation of an Element in a Zeolite. I. Crystal



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Table 1. Experimental Conditions and Crystallographic Dataa crystal cross-section (mm)

0.12

+

ion exchange with Tl , T (K), t (day), V (mL), pH 3+

294, 2, 10, 6.4

ion exchange with Ce , T (K), t (day), V (mL), pH

353, 2, 10, 4.8

dehydration, T (K), t (day), P (Pa)

623, 2, 2 x 10-4

crystal color

dark brown

temperature for data collection, T (K)

294(1)

X-ray source

PLS(2D SMC)b

wavelength (Å)

0.67000

space group, no. unit cell constant, a (Å)

Fd 3 m, 227 24.909(1)

Detector

ADSC Quantum 210 CCD

detector to crystal distance (mm)

63

maximum 2θ for data collection (deg)

59.08

no. of unique reflections measured, m

1236

no. of reflections with Fo > 4σ(Fo)

815

no. of variables, s

53

data/parameter ratio, m/s

23.3

weighting parameters, a, b

0.1241, 110.0

final error indices 0.067

R1 c

0.247

R2d goodness of fit

1.11

e

a

For the single crystal used for structure determination. bBeamline 2D SMC at the Pohang Light Source, Korea. cR1 = Σ|Fo - |Fc||/ΣFo; R1 is calculated using only those reflections for which Fo > 4σ(Fo). dR2 = [Σw(Fo2 -Fc2)2/Σw(Fo2)2]1/2 is calculated using all unique reflections measured. eGoodness-of-fit = (Σw(Fo2 - Fc2)2/(m-s))1/2.



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Table 2. Steps of Structure Determination as Extraframework Atomic Positions Were Found step/atom

number of ions or atoms per unit cella Ce1

Ce11

Ce12

Ce2

O5

1 2

13.9(3)

3 4 5d 6e

14.04(22) 14.06(20) 13.98(18) 14.87(17)

4.09(17) 4.12(17) 4.00(16) 4.43(14)

7f 8 9 10g

0.69(9) 0.65(8) 0.55(9)

1.7(3) 1.7(3)

15.33(15) 15.18(14) 13.7(3) 13.97(24)

11h 12i

0.57(8) 0.51(7)

1.8(5) 1.6(4)

13.6(5) 13.8(4)

O6

c

error indicesb R1 R2 0.49 0.86 0.18 0.54

10.6(13) 10.0(12) 10.1(10)

0.13 0.117 0.106 0.090

0.46 0.44 0.41 0.36

4.75(12) 4.77(12) 4.68(11) 4.61(11)

16.0(10) 18.3(10) 18.5(9) 13.97(24)

0.083 0.077 0.0668 0.0679

0.287 0.261 0.2415 0.2497

4.58(11) 4.57(10)

13.6(5) 13.8(4)

0.0677 0.0665

0.2506 0.2471

4.57(10)

Numbers in parentheses are the estimated standard deviations in the units of the least significant figure given for the corresponding parameter. Defined in footnotes to Table 1. cOnly the atoms of the zeolite framework were present in the initial structure model. They were all refined isotropically. dFramework atoms were allowed to refine anisotropically. eCe12 and Ce2 were refined anisotropically. fThe two-parameter weighting system was applied. gThe occupancy at O5 was constrained to equal the occupancy at Ce12. hCe1, Ce11, and O5 were refined anisotropically. iThe occupancy at O6 was constrained to equal the occupancy at Ce2. a b

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Table 3. Positional, Thermal, and Occupancy Parametersa atom Wyckoff cation position position site T O1 O2 O3 O4 Ce1 Ce11 Ce12 Ce2 O5 O6

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

I I' I' II

x 3613(5) 0d -13(18) 3084(23) 17943(15) 0d 5441(138) 6781(22) 23510(24) 16654(47) 28294(330)

y -5399(5) 10407(15) -13(18) -7768(15) 17943(15) 0d 5441(138) 6781(22) 23510(24) 16654(47) 28294(330)

z 12581(5) -10407(15) 14029(24) -7768(15) 32013(21) 0d 5441(138) 6781(22) 23510(24) 16654(47) 28294(330)

U11 or Uiso

b

287(6) 587(28) 658(20) 728(32) 598(18) 786(151) 389(81) 582(9) 987(33) 549(48) 1612(500)

U22 318(7) 618(18) 658(20) 579(17) 598(18) 786(151) 389(81) 582(9) 987(33) 549(48)

U33 313(7) 618(18) 836(37) 579(17) 593(29) 786(151) 389(81) 582(9) 987(33) 549(48)

U23 -39(4) -60(22) -182(21) 45(22) -11(16) 144(131) 43(53) 2(11) 408(33) 56(51)

U13 -45(4) -63(15) -182(21) -31(17) -11(16) 144(131) 43(53) 2(11) 408(33) 56(51)

U12 7(4) -63(15) 198(26) -31(17) 166(22) 144(131) 43(53) 2(11) 408(33) 56(51)

Occupancyc varied fixed 192 96 96 96 96 0.51(7) 1.6(4) 13.8(4)e 4.57(10)f 13.8(4)e 4.57(10) f

Positional parameters x 105 and thermal parameters x 104 are given. Numbers in parentheses are the estimated standard deviations in the units of the least significant figure given for the corresponding parameter. bThe 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. dExactly, by symmetry. eThese occupancies were constrained to be equal; Ce12 and O5 form the Ce4O44+ cluster. fThese occupancies were constrained to be equal; Ce2 and O6 form the CeOH2+ group. a

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Table 4. Selected Interatomic Distances (Å) and Angles (deg)a distances T–O1 T–O2 T–O3 T–O4 Mean

1.6307(19) 1.6570(20) 1.6855(23) 1.6375(21) 1.6527

Ce1–O3 Ce11–O3 Ce12–O3 Ce12–O5 Ce2–O2 Ce2–O6

2.842(5)b 2.276(16) 2.482(7) 2.520(11) 2.420(7) 2.06(14)

O5···O2

3.016(15)

angles O1–T–O2 O1–T–O3 O1–T–O4 O2–T–O3 O2–T–O4 O3–T–O4 mean

113.00(22) 111.37(22) 111.85(24) 102.3(3) 107.9(3) 110.0(3) 109.4

T–O1–T T–O2–T T–O3–T T–O4–T mean

144.4(4) 146.7(4) 139.2(4) 142.6(4) 143.2

O3–Ce1–O3 O3–Ce11–O3 O3–Ce12–O3 O5–Ce12–O5 O3–Ce12–O5 Ce12–O5–Ce12 O2–Ce2–O2 O2–Ce2–O6

84.52(17), 95.48(17), 180c 114.2(11) 100.7(3) 71.0(7) 92.4(3), 159.3(5) 106.1(5) 106.1(3) 112.6(3)

The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding value. bThis distance is believed to be inaccurate, as discussed in Section 5.2.1. cExact value by symmetry. a

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TABLE 5. Displacements (Å) of Atoms from 6-Ring Planesa atoms

cation sites

at O3a

Ce1

I

–1.79

Ce11

I'

0.56

Ce12

I'

1.13

Ce2

II

at O2b

0.93

Displacements into the sodalite cavity (from the 6-ring of a D6R) are given as positive. A negative displacement indicates that the atom lies within a D6R. bDisplacements into the supercage (from a S6R) are given as positive. a

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Table 6. Distribution and Occupancies of Nonframework Species and Charge Budget cation atom chemical occ. x b c d site

position

speciesa

CN

M-O , Å

r,Å

occupancy

charge

Ions I I'

Ce1 Ce11

Ce3+ Ce3+

6 3

2.842(5)e 2.276(16)

1.522e 0.956

0.51(7) 1.6(4)

1.53+ 4.80+

I'

Ce12

Ce3+

6

2.482(7)

1.162

13.8(4)

41.40+

II

Ce2

Ce3+

4

2.06(14)

0.74

4.57(10)

13.71+

Σ Ce II'f II

f

O5 O6

20.48 O22- g

O

3

13.8(4)

27.6–

2

4.57(10)

9.14–

Σ charges +

24.7+

H (required) Ui II

Ce12, O5 Ce2, O6

Groups Ce4O44+

h

46.3

3.450(10)

2+

4.57(10)

CeOH

Chemical formulae of nonframework species. bShortest Ce–O bond lengths or cluster. cRadii of Ce3+ ions obtained by subtracting 1.32 Å (the radius of the oxide ion, ref 56) from the shortest Ce– O bond lengths or cluster. dOccupancy given as the number of ions per unit cell. eThis distance is believed to be inaccurate. See Section 5.2.1. fAlthough not cations, O5 and O6 are in the vicinity of cation sites II' and II. gA H+ ion is assigned to this oxide ion, making it a hydroxide ion, see Section 5.3. hNumber of H+ ions per unit cell required to balance the negative charge, 71-, of the zeolite framework, Si121Al71O384. iAt the center of the sodalite cavity. a

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Table 7. Crystal Composition (Atomic %) by Crystallographic and SEMEDX Analyses SXRDa

SEM-EDXb crystal 1

crystal 2

crystal 3

crystal 4

dark brown

dark brown

dark brown

pale brown

colorless

Si

19.7

24.5

18.5

17.6

22.3

Al

11.5

15.2

12.4

11.8

14.2

O

65.4

49.1

64.9

67.5

56.8

Ce

3.3

4.6

3.9

3.2

6.7

color

Tl a

c

6.6

0.24 b

Single-crystal X-ray diffraction. The SEM-EDX results range widely. This is commonly seen for zeolite crystals. It is due, in part, to decomposition in the electron beam. cThe colors of the single crystals after dehydration. The four crystals analyzed by SEM-EDX did not change color when they were temporarily exposed to the atmosphere prior to analysis.

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(figure captions) Figure 1. Stylized drawing of the framework structure of zeolite Y. Near the center of the each line segment is an oxygen atom. The nonequivalent oxygen atoms are indicated by the numbers 1 to 4. There is no evidence in this work of any ordering of the silicon and aluminum atoms among the tetrahedra, although it is expected that Loewenstein’s rule would be obeyed. Extraframework cation positions are labeled with Roman numerals or the letter U. Figure 2. Stereoview of a double 6-ring (D6R) with an octahedral Ce3+ ion at its center. It is expected that this Ce3+ ion has pulled each of the six O3 atoms to which it bonds about 0.4 Å closer to it than is shown, significantly distorting this double 6-ring. See Section 5.2.1. The zeolite Y framework is drawn with open bonds between oxygen and T atoms. The coordination about Ce3+ is indicated by solid lines. Ellipsoids of 40% probability are shown. Figure 3. Stereoview of a sodalite cavity with Ce3+ ions at site I’ (Ce11) and site II (Ce2) and an extraframework oxygen atom (presumably a hydroxide ion) at O6. See the caption to Figure 2 for other details. Figure 4. Stereoview of a sodalite cavity with a Ce4O44+ cluster. Ce4O44+ is held firmly in place and stabilized by twelve Ce3+–O (Ce12–O3) bonds. See the caption to Figure 2 for other details. Figure 5. The Ce4O44+ cluster is tetrahedrally distorted; its point symmetry is 3 m (Td). Each Ce12 is pulled outward to coordinate to three O3 framework oxygen atoms as shown in Figure 4. A cube is drawn with faint lines at the Ce12 positions to illustrate the magnitude of the distortion. See the captions to Figures 2 and 4 for other details. Figure 6. SEM-EDX spectra of dehydrated Ce21H46O18-Y after exposure to the atmosphere. (a) crystal 1 (dark brown) (b) crystal 4 (colorless). Figure 7. The excitation spectrum is drawn in violet. The resulting emission spectrum from the dehydrated powder is drawn in blue.

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Figure 1. Kim, Kang, Heo, and Seff

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Note to editor: This stereoview is the correct size for clearest viewing, ca. 4 1/4 inches black to black. This width is dictated by the average distance between adult human eyes. If it is enlarged it will not be viewable in stereo. If necessary to save journal space, it may be reduced to as little as one full column width, ca. 3 1/8 inches black to black.

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(a)

(b)


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TOC art

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Encapsulating Photoluminescent Materials in ... - ACS Publications

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