Encapsulating Photoluminescent Materials in Zeolites. Crystal

Article pubs.acs.org/JPCC

Encapsulating Photoluminescent Materials in Zeolites. Crystal Structure of Fully Dehydrated Zeolite Y (Si/Al = 1.69) Containing Eu3+ Cheol Woong Kim,† Ho-Cheol Kang,‡ Nam Ho Heo,*,† and Karl Seff§ †

Laboratory of Structural Chemistry, Department of Applied Chemistry, Kyungpook National University, Daegu 702-701, Korea Green Chemistry Division, Korea Research Institute of Chemical Technology, Yuseong, Daejon 305-600, Korea § Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, Hawaii 96822-2275, United States ‡

S Supporting Information *

ABSTRACT: Photoluminescent materials may acquire longterm stability and modified luminosity by encapsulation in zeolites. A single crystal of Eu,Tl−Y (FAU, Si/Al = 1.69) was prepared by Tl + -exchange of Na−Y (|Na 7 1 (H 2 O) x | [Si121Al71O384]) to give fully Tl+-exchanged zeolite Y, followed by Eu3+ exchange. It was then dehydrated at 623 K and 3 × 10−4 Pa. Its structure (Eu,Tl−Y, Eu17.3Tl19.0−Y, or | Eu4O44+3.3Eu3+4.1Tl+19.0H+26.4Al(OH)4−1.5|[Si121Al69.5H6.0O384]− FAU; no intact H2O molecules are present) was determined by single-crystal crystallography using synchrotron X-radiation. It was refined in the space group Fd3̅m (a = 24.915(1) Å) with all 1105 unique data to R1 = 0.059 (Fo > 4σ(Fo)) and R2 = 0.162. Eu3+ replaced 73% of the Tl+ ions during ion exchange. The 17.3 Eu3+ ions per unit cell occupy three crystallographically distinct cation sites: 0.6 ion is octahedral at site I, 3.6 ions are trigonal at site I′, and 13.2 ions, also octahedral, occupy a second site I′. The latter are members of Eu4O44+ clusters, tetrahedrally distorted cubes in 41% of the sodalite cavities; O−Eu−O = 69.8(9)°. Each of these Eu3+ ions bonds octahedrally to three oxide anions (2.414(9) Å) and to three oxygen atoms of the zeolite framework (2.518(5) Å). Some dealumination of the zeolite framework occurred to form Al(OH)4− at the centers of about 19% of the sodalite cavities. The 19 Tl+ ions per unit cell were found in the supercage, 16 at site II, 1 at site III, and 2 at site III′.

1. INTRODUCTION 1.1. Luminescence of Europium Compounds. Compounds containing rare-earth cations have been extensively studied as laser materials1,2 and phosphors.1,3 Among those cations, Eu3+ has received special attention because of its luminescence; it has a major emission band centered near 612 nm (red), one of the three primary colors: red, green, and blue (RGB). RGB photoluminescent materials (phosphors4 and photosensitive metal complexes5) are used in full color displays, e.g., computer monitors, TV sets, and hand-held devices.6−10 Long-term stability has been very difficult to achieve in these displays because the photoluminescent materials used are sensitive to their chemical environments. Encapsulating these materials within zeolites (nanohybrid systems) may allow these difficulties to be overcome.11−13 Recent reports search for ways to accomplish this encapsulation to attain long-term stability, perhaps with enhanced or otherwise modified photoluminescence.14−17 1.2. Europium in Zeolites. Zeolites are crystalline materials with nanosized channels and (sometimes) cavities that can host guest species.18 FAU zeolites ion-exchanged with rare earths and dehydrated are important catalysts in the petrochemical industry, partly because the rare-earth oxide/ hydroxide clusters that form in their sodalite cavities enhance © 2014 American Chemical Society

their hydrothermal stability (required, for example, during the decoking process) and their acidity.19−21 Accordingly, the zeolites and the clusters appear to be mutually stabilizing. In addition, such clusters sometimes have greatly enhanced luminescence intensities, primarily the result of a strong quantum-size confinement effect. This was seen for nanoclusters of EuS encapsulated in zeolite Y; the luminescence efficiencies of the bulk europium chalcogenides are very low.22 Furthermore, the interaction between the emitter and its host affects the transition properties of the emitter, so, for example, otherwise forbidden transitions may be observed.23 1.3. Structures of Europium-Exchanged Zeolites. 1.3.1. Single-Crystal Crystallography. The structures of fully and incompletely dehydrated Eu-exchanged zeolite A (LTA) were first determined crystallographically by Seff et al.24−26 They found the europium ions to be very coordinatively unsaturated in these structures: (1) three-coordinate nearly trigonal planar,24,25 (2) four-coordinate nearly tetrahedral,25 and (3) four-coordinate with all ligands at one side (opposite a four-oxygen ring).26 In hydrated Eu-exchanged LTA, they Received: February 21, 2014 Revised: April 17, 2014 Published: April 21, 2014 11014

dx.doi.org/10.1021/jp501826x | J. Phys. Chem. C 2014, 118, 11014−11025

The Journal of Physical Chemistry C

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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)x| [Si121Al71O384]−FAU (Tl71−Y or Tl−Y).38 This was done by dynamic ion exchange (the flow method) with 0.1 M aqueous (pH 6.4) thallous acetate (Tl(C2H3O2), 99.999%, Aldrich) at 294 K.39 It was then Eu3+-exchanged by the flow method at 353 K with 0.05 M aqueous (pH 5.0) Eu(NO3)3 (Eu(NO3)3·6H2O, 99.9%, Strem Chemicals). The resulting colorless Eu,Tl−Y crystal was cautiously vacuum dehydrated at 623 K (a heating rate of 25 K/h was used) and 3 × 10−4 Pa for 2 days. After being cooled to room temperature, the crystal, now dark brown and still under vacuum, was sealed off from the vacuum system and isolated in its capillary with a small torch. Additional details are given in Table 1.

observed distorted seven-coordinate capped-trigonal-prismatic europium ions.27 Although that work was done with Eu2+ (an unstable oxidation state in aqueous solution), the oxidation state of Eu in those crystals could have been 3+ or mixed. In related work, tetrahedrally distorted La4O44+ cubes were seen in fully La3+-exchanged, fully dehydrated zeolite X.28 A tetrahedron of four La3+ ions at site I′, interpenetrated by a tetrahedron of four oxide ions, is found in each sodalite cavity; each La3+ ion also coordinates to three framework oxygens. The La4O44+ units are interconnected by oxide ions at their La3+ vertices to give a neutral La2O3 continuum. 1.3.2. EXAFS. An extended X-ray absorption fine structure (EXAFS) study of Eu3+-exchanged zeolite Y showed that the Eu3+ ions changed their positions within the zeolite as the temperature was increased.29 They reported that little change was seen upon heating to 373 K, but heating to 473 K led to the migration of Eu3+ cations into the sodalite cavities where, as partially hydrated ions, each coordinates to three 6-ring framework oxygen atoms. 1.3.3. Spectroscopy. Ozin et al. used far-infrared and laserinduced fluorescence spectroscopy to learn that the Eu3+ ions in dehydrated Eu3+-exchanged zeolite Y occupy three major sites; they proposed that these were sites I, I′, and II. They concluded that dehydration had caused some europium ions previously in the supercages to migrate into the sodalite cavities.30 Similarly, Lee et al. used time-resolved Eu3+ luminescence spectroscopy to study the binding sites and the migration of the Eu3+ cations in Eu−Y. They reported that Eu3+ ions, initially hydrated at undetermined sites in the supercage, pass through site II′ to migrate to sites I and I′ as dehydration proceeds with increasing calcination temperature. They found that Eu luminescence from site II′ decays rapidly, with an exponential decay constant of 150 μs; luminescence decay for the Eu3+ ions at sites I′ and I was 4 times slower.31 1.4. Luminescence Studies of Other EuropiumExchanged Zeolites. Li and co-workers reported that the emission color of Eu3+-exchanged zeolite L crystals annealed at 923 K was dependent on the solvent used in the washing step of their preparation.32 Wu et al. sintered Eu3+-exchanged zeolite X at various temperatures and were able to prepare relatively pure red, blue, and white phosphors.33 Chen and co-workers studied the structure and luminescence of Eu2O3 nanoparticles in MCM-41.34 1.5. Objectives of This Work. To better understand the photoluminescent properties of Eu3+-exchanged zeolites, we sought to learn the positions and detailed coordination environments of the Eu3+ ions in fully dehydrated zeolite Y. To be able to characterize them well crystallographically, we sought to maximize the number of Eu3+ ions in the zeolite. To help to accomplish that, a high Eu3+-exchange temperature, 353 K, was chosen. Also, Eu3+ exchange would be done with Tl−Y, fully Tl+-exchanged zeolite Y; Tl+ ions could be more readily identified crystallographically than Na+ because (1) their ionic size is much greater than that of Eu3+ and (2) they have much more scattering power than Na+. High-quality diffraction data would be obtained using synchrotron X-radiation.

Table 1. Experimental Conditions and Crystallographic Data crystal cross-section (mm) ion exchange with Tl+, T (K), t (day), V (mL), pH ion exchange with Eu3+, T (K), t (day), V (mL), pH dehydration, T (K), t (day), P (Pa) crystal color T (K) for data collection X-ray source wavelength (Å) space group unit cell constant, a (Å) detector detector to crystal distance (mm) maximum 2θ for data collection (deg) no. of unique reflns measured, m no. of reflns (Fo > 4σ(Fo)) no. of variables, s raio of no. of data to no. of params, m/s weighting params, a, b final error indices R1b R2c Rint Rσ goodness of fitd

0.12 294, 2, 10, 6.4 353, 2, 10, 5.0 623, 2, 3 × 10−4 dark brown 294(1) PLS(2D SMC)a 0.8000 Fd3̅m, No. 227 24.915(1) ADSC Quantum 210 CCD 62 68.09 1105 955 63 15.2 0.0920, 169.4 0.059 0.162 0.014 0.015 1.087

Beamline 2D SMC at the Pohang Light Source, Korea. bR1 = ∑|Fo − |Fc∥/∑Fo; R1 is calculated using only those reflections for which Fo > 4σ(Fo). cR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2 is calculated using all unique reflections measured. dGoodness-of-fit = (∑w(Fo2 − Fc2)2/(m − s))1/2. a

2.2. X-ray Diffraction. X-ray diffraction data for this crystal were collected at 294(1) K. A preliminary cell constant and an orientation matrix were determined from 36 sets of frames collected at scan intervals of 5° with an exposure time of 1 s per frame. The basic data file was prepared using the program HKL2000.40 The reflections were successfully indexed by the automated indexing routine of the DENZO program.40 About 60 000 reflections were harvested by collecting 72 sets of frames with a 5° scan and an exposure time of 1 s per frame. This highly redundant data set was corrected for Lorentz and polarization effects; a negligible correction for crystal decay was also applied. An empirical absorption correction (μl = 7.46

2. EXPERIMENTAL SECTION 2.1. Ion Exchange and Dehydration. 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 al.35,36 using the synthetic method of Vaughan et al.37 A 11015



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Figure 1. EDX spectra of partially hydrated |Eu4O44+3.3Eu3+4.1Tl+19.0H+26.4Al(OH)4−1.5|[Si121Al69.5H6.0O384]−FAU.

mm−1) was made. The space group Fd3̅m, standard for zeolite Y, was determined by the program XPREP.41 A summary of the experimental and crystallographic data is presented in Table 1. 2.3. SEM-EDX Analysis. After diffraction data collection, the crystal was removed from its capillary (exposed to the atmosphere) and attached to a sample holder with a silver paste for scanning electron microscopy energy-dispersive X-ray (SEM-EDX) analysis. The composition of the crystal was determined using a Versa 3D FIB (focused ion beam) within an Ametek EDX spectrometer and a FE (field emission) scanning electron microscope at 294 K and 1 × 10−3 Pa with a beam energy of 20 keV. The SEM-EDX results (Figure 1) are in general agreement with the composition determined crystallographically (Table 2).

as extraframework atoms is given in Table 2. At this point, after step 14 in Table 3, 17.50 Eu3+, 21.08 Tl+, and 1.4 Al3+ ions had been located per unit cell. The dealumination process per aluminum atom may be viewed as the reaction of the original zeolite framework, Si121Al71O38471− per unit cell, with four water molecules to give a dealuminated framework, Si121Al70H4O38470−, and an aluminate ion, Al(OH)4−. An Al3+ ion in the zeolite framework has been replaced at its original position by a nest of four H+ ions, as occurs in acid environments.43,44 Alternatively, zeolite framework reconstruction may have occurred to give a smaller crystal (fewer unit cells) with a framework richer in silicon, such as Si122Al70O38470− per unit cell.43,44 The former process (the formation of H+ nests) has been assumed in this study. In the following presentation and only for the purpose of exposition, only Al3+ ions and not AlO4 groups have been abstracted from the zeolite framework. Accordingly, the net reaction that occurred per unit cell upon aqueous Eu3+ exchange and subsequent vacuum dehydration can, at this stage of structure determination, be written as

Table 2. Crystal Composition by Crystallographic (SXRD) and SEM-EDX Analyses element Si Al Eu Tl O

no. of concn no. of concn no. of concn no. of concn no. of concn

SXRD b

atoms (atom %)c atomsb (atom %)c atomsb (atom %)c atomsb (atom %)c atomsb (atom %)c

121 19.2 71 11.2 17.33 2.7 19.03 3.0 403.2d 63.8

SEM-EDXa 22

|Tl+71|[Si121Al 71O384 ] + 17.50Eu 3 + + x H 2O

13

(hydrolyzed by coordination to H+and Al3 +)

3.8

+ y H+ (additional H+ ions coexchanging from solution

3.7

with Eu 3 +) → |Eu 3 +17.50Tl+21.08Al3 +1.4H 2O (hydrolyzed)x H+ y|[Si121Al 69.6O384 ] + 49.92Tl+

58

a The zeolite crystal can be expected to have suffered some decomposition under the action of the electron beam. This can be a significant source of error. bNumber of atoms per unit cell. cAtomic percentage of the element. dA total of 384 framework and 19.2 nonframework oxygen atoms. Additional oxygen atoms should have been present in the somewhat hydrated crystal studied by SEM-EDX.

(1)

The hydrolyzed H2O includes the O5 position (oxygen atoms bound to Eu3+ and Al3+), any H+ ions that might be associated with them, the H+ ions that would be in nests,43,44 and any H+ ions that would bond to framework oxygen atoms. To balance the anionic charge of the zeolite framework per unit cell, 71− in Na71−Y and Tl71−Y but (71−) − (1.4 × 3+) = 75.2− in Si121Al69.6O384 (the zeolite framework without 1.4 Al3+ ions), the sum of the extraframework charges in the product zeolite should be 75.2+. (The H+ ions in nests are counted as extraframework atoms and charges in the above calculation.) The sum of the extraframework charges is therefore

3. STRUCTURE DETERMINATION Full-matrix least-squares refinements (SHELXL97)42 were done on F2 using all 1105 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.38 The initial refinement with isotropic thermal parameters for all framework atoms converged to the high error indices R1 = 0.57 and R2 = 0.88 (step 1 of Table 3). The detailed progress of structure determination as subsequent peaks were found on difference Fourier functions and identified

(17.50 × 3+) + (21.08 × 1+) + (1.4 × 3+) + (y × 1+) = 75.2+

(2)

and thus y = −2.58, a small negative value. Although the value of y cannot be negative, its departure from zero may be insignificant considering the esds of the occupancies involved 11016



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Table 3. Steps of Structure Determination as Extraframework Atomic Positions Were Found no. of ions or atoms per unit cella no. of steps/atom

Eu1

Eu11

Eu12

Tl2

14.6(4) 11.90(18) 12.00(21) 13.21(23) 13.50(23) 13.43(23) 13.43(22) 12.65(20) 12.73(20) 13.06(18) 13.12(17) 13.24(17) 13.17(17) 13.21(16)

16.2(10) 16.0(5) 16.3(3) 16.15(24) 16.05(22) 16.19(19) 16.29(19) 16.17(19) 15.77(17) 15.71(16) 15.98(14) 15.96(13) 15.91(12) 15.86(12) 15.87(12)

Tl3

error indicesb Tl31

Al

O5

R1

R2

19.1(12) 21.8(13) 25.3(13) 26.4(12) 22.5(12) 20.4(11) 19.17(22)

0.57 0.36 0.17 0.10 0.091 0.085 0.084 0.088 0.086 0.074 0.070 0.063 0.0590 0.0576 0.0587 0.0587

0.88 0.78 0.56 0.36 0.312 0.295 0.257 0.253 0.252 0.217 0.205 0.180 0.1682 0.1616 0.1621 0.1622

c

1 2 3 4 5d 6e 7f 8g 9 10 11 12 13h 14i 15j 16k

0.43(14) 0.41(11) 0.37(10) 0.63(12) 0.72(11) 0.70(11) 0.58(10) 0.57(10)

4.62(18) 4.40(16) 3.34(18) 3.41(19) 3.41(18) 3.38(18) 3.56(16) 3.71(16) 3.62(15) 3.55(14) 3.56(14) 3.55(14) 3.55(14)

1.30(20) 1.63(20) 1.75(20) 1.70(19) 1.15(13) 1.11(13)

3.06(29) 3.40(29) 3.47(29) 2.09(13) 2.05(13)

1.4(3) 1.4(3) 1.49(24)

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 refined anisotropically. eEu11, Eu12, and Tl2 were refined anisotropically. fA two-parameter weighting system was applied. gThe extinction parameter correction, EXTI, was applied. hEu1, Tl3, and Tl31 were refined anisotropically. iThe thermal parameter at Al was fixed. jAn occupancy constraint requiring that the charges of the Eu3+ and Tl+ ions sum to 71+ was applied. kThe occupancy of the extraframework oxygens at O5 was constrained to be equal to the occupancy at Eu12 + (4 × the occupancy at Al). a b

(see the next paragraph). It is, however, clear that y is not likely to have a positive value, so it can be concluded that few if any H+ ions have exchanged into the zeolite (independent of the hydrolysis of H2O) during the Eu3+-exchange process despite the (mild) acidity of that exchange solution (pH 5.0). It is also clear that least-squares refinement has yielded somewhat too many nonframework cations. It was anticipated that the problem lay with the lowoccupancy positions Tl3 and Tl31. The esds of their occupancies were large, and their thermal parameters were very large, U11 = U22 = 0.24 Å2 at Tl3 and U11 = U22 = 0.38 Å2 at Tl31. Occupancy and thermal parameters at an atomic position often correlate highly and positively in least-squares refinement, so it appeared that both were refining to values that were too high at Tl3 and Tl31. The correlation coefficient for U11 and the occupancy at Tl3 was +0.68, and the corresponding coefficient at Tl31 was +0.66. Thus, it could be anticipated that the largest changes in occupancy that might result would be decreased occupancy at Tl3 and Tl31. Assuming that y = 0, and because the thermal parameter at Al was fixed (so its occupancy is not subject to further change), eq 2 becomes

Accordingly, the constraint that the sum of the charges at the Eu3+, Tl+, and Al3+ positions sum to 75.2+ was applied (step 15 in Table 3); the fundamental error index, R2, did not increase. Indeed, as expected, the largest changes in occupancy were seen at the Tl3 and Tl31 positions (compare steps 14 and 15 in Table 3). The oxygen atoms at O5 are members of both the Eu4O4 clusters (section 5.2.2.2) and the AlO4 groups (section 5.2.3); these two groups are independent. Thus, the occupancy at O5 should be equal to the occupancy at Eu12 + (4 × the occupancy at Al). This was supported by least-squares refinement (step 15 in Table 3). When this constraint was applied (step 16 in Table 3), both error indexes, R1 and R2, remained unchanged. Refinements applying this constraint and the previous one (step 15 in Table 3) were performed alternately until both were satisfied. The final structural parameters are presented in Table 4. Because of the silicon and aluminum disorder in the zeolite framework, which extends to all framework oxygen positions and is substantially compounded by the local distortions induced by the partially occupied nonframework cation positions, careful consideration was given to the acceptability of the structural parameters, especially the thermal and occupancy parameters, that emerged at each stage of structure determination and least-squares refinement. 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. For the same reasons, the bond lengths and angles reported are correct only for averaged atom positions; they are often inaccurate for low-occupancy positions. 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 Eu3+, Tl+, O−, and T1.82+ were used.38,45,46 The function describing T1.82+ is the weighted (for

(17.50 × 3+) + z ((number of Tl+ ions per unit cell) × 1+) + (1.4 × 3+) = 75.2+

(3)

and z = 18.50. (The charges of the water molecules hydrolyzed by Eu3+ would sum to zero.) Subtracting the number of Tl+ ions at Tl2, 15.91, leaves 18.50 − 15.91 = 2.59 ions at Tl3 and Tl31. It had been 1.70 + 3.47 (see Table 3, step 14) = 5.17. As a check to see if such a large change is acceptable, the difference divided by the combined esds of the occupancies at Tl3, Tl31, and Al in step 14 in Table 3 was calculated. It is (5.17 − 2.59)/0.46 = 5.6 esds from that determined crystallographically. This is judged to be acceptable. 11017


U

Positional parameters × 105 and thermal parameters × 104 are given. Numbers in parentheses are the estimated standard deviations in the units of the least significant figure given for the corresponding parameter. bThe anisotropic temperature factor is exp[−2π2a−2(U11h2 + U22k2 + U33l2 + 2U12hk + 2U13hl + 2U23kl)]. cOccupancy factors are given as the number of atoms or ions per unit cell. dExactly, by symmetry. eThese occupancies were constrained so that the charges of the Eu3+ and Tl+ ions sum to 71+. fThe thermal parameter at Al was fixed.

I I′ I′ II III III′

192(i) 96(h) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 48( f) 96(g) 32(e) 8(a) T O1 O2 O3 O4 Eu1 Eu11 Eu12 Tl2 Tl3 Tl31 O5 Al

Si/Al = 1.69) mean of the Si4+, Si0, Al3+, and Al0 functions. All scattering factors were modified to account for anomalous dispersion.47,48 Other crystallographic details are given in Table 1.

4. BRIEF DESCRIPTION OF THE FAU FRAMEWORK AND CATION SITES The framework structure of zeolite Y, a synthetic analogue of the naturally occurring mineral faujasite (FAU), is characterized by the double 6-ring (D6R), the sodalite cavity (a cuboctahedron), and the supercage (see Figure 2). Each unit

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

cell has 8 supercages, 8 sodalite cavities, 16 D6Rs, 16 12-rings, and 32 single 6-rings (S6Rs). The exchangeable cations that balance the negative charge of the FAU framework usually occupy some or all of the sites shown with Roman numerals in Figure 2. The maximum occupancies at the cation sites I, I′, II, II′, III, and III′ in FAU are 16, 32, 32, 32, 48, and (in Fd3̅m) 192, respectively. Further detailed descriptions are available.49−51

5. RESULTS 5.1. Framework Geometry. The mean T−O bond length (1.654 Å; see Table 5) in Eu,Tl−Y is between the Si−O (1.61 Å) and Al−O (1.74 Å) distances in Ca-LSX (FAU),52 appropriately closer to the Si−O distance. It is almost the same as that in K71−Y,35 1.655 Å, and very close to that in Tl71−Y,38 1.663 Å, indicating that the Eu,Tl−Y framework is not very distorted. Still, local distortions may be expected, especially near cations of higher charge. Among the four T−O−T angles, T−O3−T is the smallest (139.2°, Table 5) because all of the Eu3+ ions are at sites I and I′ where they bond to O3, pulling the oxygen atoms inward toward the centers of their 6-rings. The T−O3−T angle is indicative of the degree of distortion that Eu3+ induces in the zeolite framework as a result of its high charge. 5.2. Extraframework Ions: Eu3+, Tl+, Al3+, and O2−. Eu3+ ions are distributed over three crystallographically distinct positions. They occupy site I (Eu1) in the D6Rs and two I′ sites (Eu11 and Eu12) in the sodalite cavities. Tl+ ions are also distributed over three sites, II, III, and III′, all in the supercages. Extraframework oxygen atoms (O5) were found opposite S6Rs

a

0.57(10) 3.55(14) 13.21(16) 15.87(12) 1.11(13)e 2.05(13)e 19.17(22) 1.49(24)

192 96 96 96 96

fixed varied U12 U13

8(3) −108(13) −173(17) −17(14) −12(13) 110(93) −46(11) 75(9) −24(2) 0d −75(115)

U23

−42(3) −108(13) −173(17) −17(14) −12(13) 110(93) −46(11) 75(9) −24(2) 0d −75(115)

U33

129(5) 459(24) 652(33) 592(28) 402(21) 688(132) 288(17) 588(9) 493(4) 301(108) 298(81)

U22

145(5) 469(15) 493(17) 404(14) 425(14) 688(132) 288(17) 588(9) 493(4) 1975(320) 2513(336) 164(5) 469(15) 493(17) 404(14) 425(14) 688(132) 288(17) 588(9) 493(4) 1975(320) 2513(336) 931(62) 1000f 3627(4) 0d 14102(24) 3057(20) 32135(18) 0d 5167(17) 6994(9) 25543(2) 41309(98) 42101(80) 16491(57) 12500d 12584(4) 10375(14) −19(15) −7743(12) 17888(14) 0d 5167(17) 6994(9) 25543(2) 12500d 18704(160) 16419(57) 12500d −5359(4) −10375(14) −19(15) −7743(12) 17888(14) 0d 5167(17) 6994(9) 25543(2) 12500d 18704(160) 16419(57) 12500d

U11 or Uisob z y x cation site Wyckoff position atom position

Table 4. Positional, Thermal, and Occupancy Parametersa

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−41(3) −26(19) 191(21) 15(17) 124(18) 110(93) −46(11) 75(9) −24(2) −102(342) 424(365)

occupancyc


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I (Eu1, a 16-fold position; see Figure 3). Each of these ions coordinates octahedrally to the six O3 framework oxygen atoms of its D6R at a distance (virtual) of 2.833(5) Å. This distance is noticeably longer than the sum of the conventional ionic radii of Eu3+ and O2−, 2.27 Å (0.95 + 1.32 Å, respectively).53 Surely the Eu3+ ion at Eu1 has pulled its six O3 oxygens closer to it than 2.833(5) Å; the O3 position has likely refined near the most populous of its various unresolved positions, depending on its several possible coordination situations. Eu1 lies on an inversion center; its coordination is mildly distorted octahedral (see Table 5). 5.2.2. Trigonal and Octahedral Eu3+ Ions at the I′ Sites. Eu3+ ions were found at both Eu11 and Eu12. The sum of the occupancies at these positions is 16.76(2) Eu3+ ions per unit cell (see Tables 4 and 6), so the 32-fold site I′ is approximately half filled. The Eu1···Eu11 distance is only 2.230(7) Å, and the Eu1··· Eu12 distance is only 3.018(4) Å. Intercationic electrostatic repulsion should be severe for Eu3+ ions at these distances. However, the observed occupancies indicate that they can readily be avoided. If a site I is occupied, the two I′ positions of that same D6R should not be. Accordingly, (16 − 0.57) × 2 = 30.86 site I′ positions should be available for Eu3+ ions per unit cell, more than enough to accommodate the 16.76 Eu3+ ions at the two I′ sites. 5.2.2.1. Trigonal Eu3+ Ions at the First Site I′. Eu11 is 2.241(5) Å from three O3 framework oxygens (Table 5). This distance is about the same as the sum of the conventional radii of Eu3+ and O2−, 2.270 Å.53 Eu11 bonds to no other atoms; a Eu3+ ion at Eu12 cannot coexist in the same 6-ring because this would give an impossibly short Eu11···Eu12 distance (0.789(5) Å), and the nearest other atom, O5, is too far away (3.051(8) Å) to bond (see Figure 4), and even this distance is readily avoided. The Eu3+ ions at Eu11 are therefore simply 3coordinate trigonal, nearly trigonal planar: the O3−Eu11−O3 angles are 116.21(13)°. 5.2.2.2. Octahedral Eu3+ Ions at the Second Site I′ (Members of Eu4O4 Clusters). Eu12 is 2.518(5) Å from three O3 framework oxygens. This is appreciably longer than the sum of the conventional radii of Eu3+ and O2−, 2.270 Å,53 suggesting each ion at Eu12 makes other important bonds and has a coordination number substantially greater than 3. Indeed, Eu12 is also 2.414(9) Å from three O5 extraframework oxygen atoms (see Figure 5). Thus, each Eu12 ion has a somewhat distorted

Table 5. Selected Interatomic Distances (Å) and Angles (deg)a distances T−O1 T−O2 T−O3 T−O4 mean

1.6376(18) 1.6548(17) 1.6837(20) 1.6420(17) 1.654

Eu1−O3 Eu11−O3 Eu12−O3 Eu12−O5 Tl2−O2 Tl3−O4 Tl31−O4 Tl31−O1 Al−O5

2.833(5)b 2.241(5) 2.518(5) 2.414(9) 2.856(6) 2.971(19) 2.500(21)b 3.094(13) 1.691(24)

O5···O2

3.085(17)

angles O1−T−O2 O1−T−O3 O1−T−O4 O2−T−O3 O2−T−O4 O3−T−O4 mean

112.79(18) 110.82(19) 110.89(22) 102.96(27) 108.85(24) 110.26(23) 109.43

T−O1−T T−O2−T T−O3−T T−O4−T mean

144.8(3) 146.2(3) 139.2(3) 141.1(3) 142.8

O3−Eu1−O3

84.40(15), 95.60(15), 180c 116.21(13) 98.16(16) 69.8(9) 94.5(4), 160.6(6) 84.65(16) 79.4(6) 56.6(3) 112.8(7) 109.47d 107.0(6)

O3−Eu11−O3 O3−Eu12−O3 O5−Eu12−O5 O3−Eu12−O5 O2−Tl2−O2 O4−Tl3−O4 O1−Tl31−O4 O1−Tl31−O1 O5−Al−O5 Eu12−O5−Eu12

a

The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding value. b These distances are not expected to be accurate. The cations at these low-occupancy positions are likely to have induced severe local distortions to the zeolite framework. cExact value by symmetry. dThe tetrahedral angle.

(near cation site II′) in the sodalite cavity. The occupancy at O5 is a simple function of the occupancies at Eu12 and Al, indicating that O5 is an averaged position that participates in two secondary structures, one involving Eu12 and the other involving Al. 5.2.1. Octahedral Eu3+ Ions at Site I. Per unit cell, 0.57(10) Eu3+ ions, relatively few, were found at the centers of D6Rs, site

Figure 3. Stereoview of a double 6-ring (D6R) with an octahedral Eu3+ ion at its center. The zeolite Y framework is drawn with open bonds between oxygens and T atoms. The coordination about Eu3+ is indicated by solid lines. Ellipsoids of 40% probability are shown. 11019



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Table 6. Distribution and Occupancies of Nonframework Species and Charge Budget cation site

atom position

chemical speciesa

CNb

M−Oc (Å)

rd (Å)

occupancye

occ. × charge

Ions I I′ I′

Eu1 Eu11 Eu12 ∑Eu

Eu3+ Eu3+ Eu3+

6 3 6

2.833(5) 2.241(5) 2.414(9)

1.513f 0.921 1.094

0.57(10) 3.55(14) 13.21(16) 17.33(24)

1.71 10.65 39.63

II III III′

Tl2 Tl3 Tl31 ∑Tl

Tl+ Tl+ Tl+

3 2 3

2.856(6) 2.971(19) 2.500(21)

1.536 1.651 1.180f

15.87(12) 1.11(13) 2.05(13) 19.03(22)

15.87 1.11 2.05

II′g U

O5 Al

O2− Al3+

4 4

U U

Eu12, O5 Al, O5

Eu4O44+ Al(OH)4−

19.17(22)h 1.49(24) ∑charges H+(required)j

−38.4i 4.47 37.1 38.4

Clusters 3.30(4) 1.49(24)

13.2 −1.49

a

Chemical formulas of nonframework species. bCoordination numbers including three framework oxygen atoms (not shown in the formula) when present. cShortest Eu−O and Tl−O bond lengths. dRadii of Eu3+ and Tl+ ions obtained by subtracting 1.32 Å (the radius of the oxide ion, ref 52) from the shortest Eu−O and Tl−O bond lengths. eOccupancy given as the number of ions and atoms per unit cell. fThese distances are not expected to be accurate. The cations at these low-occupancy positions are likely to have induced severe local distortions to the zeolite framework. gNot a cation, but near cation site II′. hIncludes the oxide ions in Al(OH)4− and Eu4O44+. iRounded insignificantly upward for internal consistency. j Number of H+ ions per unit cell required to balance the negative charge, (71−) − (1.49 × 3+) = 75.47−, of the zeolite framework, Si121Al69.5O384 (Si, Al, and O atoms only): 75.5 − 37.1 = 38.4.

Figure 4. Stereoview of a sodalite cavity with an Al(OH)4− ion, a Eu3+ ion at site I′ (Eu11), and a Tl+ ion at site II (Tl2). For simplicity, the hydrogen atoms are not shown. Al(OH)4− is held in place and stabilized by O5···O2 hydrogen bonds. These would be shorter than given in Table 4 and more linear if O5 is somewhat off its 3-fold axis position. The Al−O bonds in Al(OH)4− and the coordination bonds about Eu3+ and Tl+ are indicated by solid lines. See the caption to Figure 2 for other details.

Figure 5. Stereoview of a sodalite cavity with a Eu4O44+ cluster. It is held firmly in place and stabilized by 12 Eu3+−O (Eu12−O3) bonds. See the caption to Figure 2 for other details.

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1.691(24) Å, and O5−Al−O5 angle, tetrahedral by symmetry (Table 5). These AlO4 groups (Al(OH)4−, see section 6.1) could reasonably have been produced by the dealumination of the zeolite framework during Eu3+ exchange (due to the low pH) and/or subsequent dehydration. 5.2.4. All Tl+ Ions Are in the Supercage. The 19.03(22) Tl+ ions per unit cell are distributed among three cation sites, site II (Tl2), site III (Tl3), and site III′ (Tl31) (see Table 4). Per unit cell, 15.87(12) Tl+ ions are at Tl2. They lie on 3-fold axes opposite S6Rs in the supercages. Each coordinates trigonally at 2.856(6) Å to three O2 framework oxygen atoms of a S6R and is recessed ca. 1.79 Å into the supercage from its O2 plane (see Figure 7 and Tables 5 and 7). The Tl2−

octahedral geometry (see Table 5) with three 2.518(5) Å bonds to O3 framework oxygens and three 2.414(9) Å bonds to O5 extraframework oxygens. Eu12 and O5 constitute a tetrahedrally distorted (Eu12)4(O5)4 cube with reasonable angles at Eu12 and O5 (see Figures 5 and 6). Eu12 reasonably makes shorter bonds to

Table 7. Displacements (Å) of Atoms from 6-Ring Planesa atom

cation site

at O3a

Eu1 Eu11 Eu12 Tl2

I I′ I′ II

−1.79 0.44 1.23

at O2b

Figure 6. The Eu4O44+ cluster is tetrahedrally distorted; its point symmetry is Td. A cube is drawn with faint lines at the Eu12 positions to illustrate the magnitude of the distortion. Each Eu12 is pulled outward to coordinate to three O3 framework oxygen atoms as shown in Figure 4. See the captions to Figures 2 and 4 for other details.

a

the extraframework oxygens (O5), each of which bonds to two other Eu3+ ions, than to framework oxygens (O3), each of which bonds to two Si4+ ions (or to one Si4+ and one Al3+ ion); these two Tn+ ions would draw more bonding electron density from O3 than two Eu3+ ions would draw from O5 because the Tn+ ions are smaller and more highly charged. Accordingly, O3 should be less negative and a poorer ligand to Eu3+ than O5. This assignment of O5 oxygens to the Eu4O4 units does not deplete the occupancy at O5. The remaining O5 oxygens can be assigned to Al−O5 bonds with reasonable bond lengths (see section 5.2.3), so O5 (like O3 in section 5.2.1) may be viewed as an averaged position. This is supported by the large thermal parameters at O5 (see Table 4). Attempts to resolve the two kinds of oxygen atoms at O5 were unsuccessful. In the Eu4O4 unit, the four Eu3+ ions at Eu12 are tetrahedrally arranged, each opposite a D6R (to which it bonds). Its four oxygen atoms (at O5) are also tetrahedrally arranged, each opposite a S6R of the same sodalite cavity. The topological cube formed by these two interpenetrating tetrahedra has point symmetry Td. 5.2.3. AlO4 at Site U. At the very centers of 1.49(24) sodalite cavities per unit cell, Al3+ ions were found (see Figure 4 and Tables 4 and 6). The O5 position readily provides four tetrahedrally arranged extraframework oxygen atoms to each Al3+ ion with an entirely suitable Al−O5 bond length,

O2 distance, 2.856(6) Å, is comparable to the sum (2.79 Å) of the conventional radii of Tl+ and O2− (1.47 and 1.32 Å,53 respectively) as well as that found in dehydrated Tl71−Y at this position,38 2.724(15) Å. The O2−Tl2−O2 bond angle is 84.65(16) °, rather small because the large Tl+ ion does not fit into an S6R. The remaining Tl+ ions are at Tl3 and Tl31 in the supercage with occupancies of 1.11(13) and 2.05(13) per unit cell, respectively (see Figure 7). The Tl3−O4 distance, 2.971(19) Å, is somewhat longer than the sum of the conventional radii of Tl+ and O2−, 2.79 Å. The Tl31−O4 and Tl31−O1 distances, 2.500(21) and 3.094(13) Å, respectively, are similar to those found at site III′ positions in dehydrated Tl71−FAU,38 Tl92− FAU,54 Cd24.5Tl43−FAU,55 Sr8.5Tl75−FAU,56 Pd18Tl56−FAU,57 and Pd21Tl50−FAU.57 The very short Tl31−O4 distances, 2.500(21) Å, may be a consequence of the low coordination number, 3, and the irregular coordination at this site. It may also be inaccurate because only an averaged O4 position has been determined; the positions of the 2.05(13) O4 atoms that are approached by Tl31 atoms may be modified by this interaction. The thermal parameters at Tl3 and Tl31 are quite

1.80

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

Figure 7. Stereoview of a representative supercage with Tl+ ions at sites II (Tl2), III (Tl3), and III′ (Tl31). See the caption to Figure 2 for other details. 11021



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large (see Table 4) and realistic (see Figure 7) for ions whose coordinating ligands are all toward one side.

Tetrahedrally distorted cubic groups are commonly found in the sodalite cavities of zeolites. They have been seen in Cd2+exchanged,63 Ni2+-exchanged,59 and In3+-exchanged39,71 zeolite Y (FAU), in Co2+-exchanged,49 Zn2+-exchanged,72 Pb2+exchanged,73 and La3+-exchanged28 zeolite X (FAU), and in Tl+-exchanged,74 Cd2+-exchanged,75 and Pb2+-exchanged76−78 zeolite A (LTA). With Cd2+ 63 and Zn2+,72 the near cubic Cd4O4 (in Cd8O48+) and Zn4(OH)44+ clusters formed upon evacuation at various temperatures, while [Pb4(OH)44+] forms directly upon Pb2+ exchange at ambient temperature.73 With La3+, a La2O3 continuum composed of La4O4 subunits bridged at their La3+ vertices by oxide ions was seen.28 With Ni2+, the concentration of Ni8O4·xH2O8+ clusters (with Ni4O4 cores) increased with increasing Ni2+ exchange and dehydration.59 With In3+, it was reported that In4(OH)48+ formed when In−Y was allowed to react with air/oxygen39 and that In4O44+ formed when In−Y reacted with NO(g) at 623 K.71 The H+ ions in In4(OH)48+ 39 appear to have been assigned incorrectly for reasons discussed in the second paragraph of section 6.1. It appears that the clusters in both structures are the same, In4O44+. 6.4. Net Reaction per Unit Cell. The balanced reactions 4 and 5 below are written without esds and contain a somewhat excessive number of significant figures. Recall that, for the purposes of exposition, only Al3+ ions and not AlO4 groups have been abstracted from the zeolite framework in the proceeding presentation of the dealumination process (section 3, third paragraph). Therefore, to form the 3.30 Eu4O44+ clusters and the 1.49 Al(OH)4− groups, (3.30 × 4) + (1.49 × 4) = 19.2 oxide ions are needed. Accordingly, x in reaction 1 must be 19.2. Thus, the net reaction per unit cell can be written as

6. DISCUSSION 6.1. H+ Ions and Their Assignment. About 38.4 H+ ions, not found in this work, in part because hydrogen atoms scatter X-rays too weakly, must be present per unit cell to achieve charge balance (see Table 6). They should bond to the oxide ions in this structure. However, there are four kinds of oxide ions: those in the framework that had previously bonded to Al3+ ions, those in the AlO4 and Eu4O4 groups, and those of the zeolite framework. They should bond first to the most negative oxide ions, those with the least adequate coordination (the fewest cations withdrawing electron density), and then to others in order of increasing coordination until all 38.4 H+ ions have been placed. The charges, numbers, and chemical natures of these cations were also considered, but this served only to further support the following result. The oxide ions of the AlO4 groups and the oxide ions that had coordinated to the Al3+ ions that left the framework have the lowest coordination number, 1. A H+ ion is assigned to each of these, a total of 12.0 H+ ions. The remaining 26.4 H+ ions are assigned to those oxygen atoms of the zeolite framework that do not coordinate to Eu3+ or Tl+; their coordination number is 2. Thus, because they are already 3-coordinate, H+ ions are not assigned to oxygen atoms of the Eu4O4 groups nor to oxygen atoms of the zeolite framework that coordinate to Eu3+ or Tl+. Thus, the AlO 4 groups are identified to be the orthoaluminate anions Al(OH)4−, and Eu4O4 is Eu4O44+. 6.2. Dealumination to Form Al(OH)4−. This structure shows the small degree of framework dealumination, 1.49(24) per unit cell. It may have occurred during the Eu3+-exchange process or vacuum dehydration at 623 K or both. In Ni2+exchanged zeolite X (Si/Al = 1.09), dealumination was seen upon ion exchange and evacuation at 296 K; 1.9(5) orthoaluminate ions were found per unit cell.58 In Ni2+exchanged zeolite Y (Si/Al = 1.69) prepared at pH 6.2, the number of Al(OH)4− ions increased with both increasing Ni2+exchange level and dehydration temperature from 0 to 2.5 per unit cell.59 In another study of zeolite Y (Si/Al = 1.59) Ni2+exchanged at pH 4.9, ca. 1.1(4) orthoaluminate ions per unit cell were found at the centers of sodalite cavities, whether the dehydration was partial (done at 294 K) or complete (done at 673 K).60 In the latter structures each AlO45− group bonds to four Ni2+ ions at site II to form a Ni4AlO43+ cluster.60 As in this study, the orthoaluminate ions in the above studies were all found at the centers of sodalite cavities. 6.3. Eu4O44+ Clusters. Of the 8 sodalite cavities per unit cell, 3.30(4) contain tetrahedrally distorted cubic Eu4O44+ clusters (see Figure 6). In these clusters, each Eu3+ ion bonds to three extraframework oxide ions and each oxide ion bonds to three Eu3+ ions, all at 2.414(9) Å. O5−Eu12−O5 is 69.8(9)°, and the Eu12−O5−Eu12 angle is 107.1(6)°. Thus, it can be seen that each Eu3+ ion is pulled sharply outward from its Eu4O44+ “cube” to coordinate to three O3 framework oxygen atoms. The anionic sodalite cavity often hosts and stabilizes cationic clusters such as Eu4O44+. Examples are Na43+,61 Zn56+,62 cycloZn68+,62 Cd8O48+,63 Pb2S2+,64 and Ni8O4·xH2O8+ 59 in zeolite Y (FAU), S44+,65 In57+,66 and Pb8O4n+ 67,68 in zeolite X (FAU), and In57+,69 Cd6S44+,70 Cd2Na2S4+,70 and Cd2O2+ 70 in zeolite A (LTA); a brief review is available.65

|Tl+71|[Si121Al 71O384 ] + 17.3Eu 3 + + 19.2H 2O → |Eu 3 +17.3Tl+19.0Al3 +1.5H+32.4O2 −19.2 |[Si121Al 69.5H6.0O384 ] + 52Tl+

(4)

which rearranges to |Tl+71|[Si121Al 71O384 ] + 17.3Eu 3 + + 19.2H 2O → |Eu4O4 4 +3.3Eu 3 + 4.1Tl+19.0Al(OH)4 −1.5H+26.4|[Si121Al 69.5H6.0O384 ] + 52Tl+

(5)

Note that the hydrolysis of water to give Al(OH)4− is normal: H2O → H+ and OH−. With regard to Eu4O44+, it is complete: H2O → 2H+ + O2−. 6.5. Implications Regarding Luminescence. In this structure, Eu3+ ions were found at three nonequivalent sites, site I and two I′ sites. The point symmetry at site I is 3̅m (3̅2/ m, D3d), which has a center of symmetry. The point symmetry at each site I′ is 3m (C3v), which is not centric. Such sites, one I site and two I′ sites, have been seen directly by far-IR spectroscopy,30 and they were suggested by time-resolved Eu luminescence spectroscopy.31 In these three environments the Eu3+ ions may be expected to luminesce differently; the luminescence peaks of the europium ions located at sites I and I′ are around 579 and 613 nm (higher intensity), respectively.30 Even the Eu3+ ions at the two I′ sites may be expected to luminesce differently because they have different polarizing environments; the Eu3+ ion at site I′ is simply 3-coordinate trigonal (nearly trigonal planar), while the Eu3+ ion at the second site I′ has a distorted octahedral geometry. 11022



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The pH, time, and temperature of Eu3+ exchange and the time and temperature of the subsequent dehydration, if dehydration is done at all, can be varied to alter the relative populations of the ions and clusters within the zeolite, allowing the photoluminescent behavior of a Eu3+-exchanged zeolite to be optimized for a specific application.

(8) Moreno Castro, C.; Ruiz Delgado, M. C.; Hernandez, V.; Shirota, Y.; Casado, J.; Lopez Navarrete, J. T. Vibrational Spectroscopic Features of a Novel Family of Amorphous Molecular Materials Containing an Oligothiophene Moiety as Color-Tunable Emitting Materials. J. Phys. Chem. B 2002, 106, 7163−7170. (9) Jin, R.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Photoinduced Conversion of Silver Nanospheres to Nanoprisms. Science 2001, 294, 1901−1903. (10) Chen, W.; Joly, A. G.; Malm, J.-O.; Bovin, J.-O.; Wang, S. FullColor Emission and Temperature Dependence of the Luminescence in Poly-P-phenylene ethynylene−ZnS/Mn2+ Composite Particles. J. Phys. Chem. B 2003, 107, 6544−6551. (11) Wada, Y.; Okubo, T.; Ryo, M.; Nakazawa, T.; Hasegawa, Y.; Yanagida, S. High Efficiency Near-IR Emission of Nd(III) Based on Low-Vibrational Environment in Cages of Nanosized Zeolites. J. Am. Chem. Soc. 2000, 122, 8583−8584. (12) Wang, L.; Yang, C.; Tan, W. Dual-Luminophore-Doped Silica Nanoparticles for Multiplexed Signaling. Nano Lett. 2005, 5, 37−43. (13) Chen, W.; Sammynaiken, R.; Huang, Y. Photoluminescence and Photostimulated Luminescence of Tb3+ and Eu3+ in Zeolite-Y. J. Appl. Phys. 2000, 88, 1424−1431. (14) Yang, X.; Tiam, T. S.; Yu, X.; Demir, H. V.; Sun, X. W. Europium(II)-Doped Microporous Zeolite Derivatives with Enhanced Photoluminescence by Isolating Active Luminescence Centers. ACS Appl. Mater. Interfaces 2011, 3, 4431−4436. (15) Aoyama, M.; Hayakawa, T.; Honda, S.; Iwamoto, Y. Development of Zeolite-Derived Novel Aluminosilicate Phosphors. J. Lumin. 2012, 132, 2603−2607. (16) Wen, T.; Zhang, W.; Hu, X.; He, L.; Li, H. Insight into the Luminescence Behavior of Europium(III) β-Diketonate Complexes Encapsulated in Zeolite L Crystals. ChemPlusChem. 2013, 78, 438− 442. (17) Rong gui, S.; Imakita, K.; Fujii, M.; Hayashi, S. Photosensitization of Europium Ions by Silver Clusters in Zeolite. Opt. Mater. 2014, 36, 916−920. (18) Wada, Y.; Sato, M.; Tsukahara, Y. Fine Control of Red-GreenBlue Photoluminescence in Zeolites Incorporated with Rare-Earth Ions and a Photosensitizer. Angew. Chem., Int. Ed. 2006, 45, 1925− 1928. (19) Ruthven, D. M.; Kaul, B. K. Adsorption of n-Hexane and Intermediate Molecular Weight Aromatic Hydrocarbons on LaY Zeolite. Ind. Eng. Chem. Res. 1996, 35, 2060−2064. (20) Venuto, P. B.; Hamilton, L. A.; Landis, P. S.; Wise, J. J. Organic Reactions Catalyzed by Crystalline Aluminosilicates: I. Alkylation Reactions. J. Catal. 1966, 5, 81−98. (21) Choudary, B. M.; Matusek, K.; Bogyay, I.; Guczi, L. The Effect of Lanthanum Promoter on the Selectivity of Pd/Zeolite-X in Methanol Synthesis. J. Catal. 1990, 122, 320−329. (22) Chen, W.; Zhang, X. H.; Huang, Y. Luminescence Enhancement of EuS Nanoclusters in Zeolite. Appl. Phys. Lett. 2000, 76, 2328−2330. (23) Bhargava, R. N. Doped Nanocrystalline MaterialsPhysics and Applications. J. Lumin. 1996, 70, 85−94. (24) Firor, R. L.; Seff, K. Near-Zero-Coordinate, Three-Coordinate, and Four-Coordinate Europium(II). Bonding Effects Involving Europium(II) Valence Orbitals. Crystal Structure of Dehydrated Near-Fully Europium(II)-Exchanged Zeolite A. J. Am. Chem. Soc. 1977, 99, 7059−7061. (25) Firor, R. L. Zero Coordinate Cations and Europium (IV): Crystal Structures of Novel Coordination Complexes within Zeolite A. Ph.D. Thesis, University of Hawaii, 1978. (26) Yanagida, R. Y.; Amaro, A. A.; Seff, K. Redetermination of the Crystal Structure of Dehydrated Zeolite 4A. J. Phys. Chem. 1973, 77, 805−809. (27) Firor, R. L.; Seff, K. Seven-Coordinate Distorted C3v-Capped Trigonal-Prismatic Europium(II). Crystal Structure of Hydrated Europium(II)-Exchanged Sodium Zeolite A, Eu5Na2-A. Inorg. Chem. 1978, 17, 2144−2148. (28) Park, H. S.; Seff, K. Crystal Structures of Fully La3+-Exchanged Zeolite X: An Intrazeolitic La2O3 Continuum, Hexagonal Planar and

7. CONCLUSIONS Eu3+ exchange replaced 73% of the Tl+ ions in Tl71−Y. The resulting 17.3 Eu3+ ions per unit cell occupy three crystallographically distinct cation sites: per unit cell, 0.6 ion is at site I, 3.6 ions are at site I′, and 13.2 ions are at a second site I′. The latter are members of Eu4O44+ clusters, tetrahedrally distorted cubes in 41% of the sodalite cavities. These Eu3+ ions are octahedral; each bonds to three oxide ions (2.414(9) Å) and to three oxygen atoms of the zeolite framework (2.518(5) Å). Some dealumination of the zeolite framework occurred to form Al(OH)4− at the centers of 19% of the sodalite cavities. All of the 19 Tl+ ions that were not replaced by Eu3+ were found in the supercages: 16 at site II, 1 at site III, and 2 at site III′.

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ASSOCIATED CONTENT

S Supporting Information *

Observed and calculated structure factors for Eu,Tl−Y. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82 53 950 5589. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of the staff of beamline 2D SMC of the Pohang Light Source, Korea, for their diffractometer and computing facilities. This work was supported by the Korean National Research Foundation, Ministry of Education, Science, and Technology, primarily by Grant No. NRF-2011-355-C00044, but also by Grant No. NRF-2011-0006652.

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REFERENCES

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