ARTICLE pubs.acs.org/JPCC
Crystallographic Verification that Copper(II) Coordinates to Four of the Oxygen Atoms of Zeolite 6-Rings. Two Single-Crystal Structures of Fully Dehydrated, Largely Cu2+-Exchanged Zeolite Y (FAU, Si/Al = 1.56) Sung Man Seo and Woo Taik Lim* Department of Applied Chemistry, Andong National University, Andong 760-749, Korea and
Karl Seff* Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822-2275, United States
bS Supporting Information ABSTRACT: Two single crystals of fully dehydrated, mostly Cu2+-exchanged zeolite Y were prepared by the exchange of Na75Y (|Na75|[Si117Al75O384]-FAU, Si/Al = 1.56) with an aqueous stream 0.05 M in Cu(NO3)2, pH = 4.2, at 294 K for 18 and 24 h, respectively, followed by vacuum dehydration at 673 K and 1 106 Torr. Their crystal structures were determined by synchrotron X-ray diffraction techniques in the cubic space group Fd3m at 100(1) K and were refined to the final error indices R1/wR2 = 0.050/0.165 and 0.050/0.163, respectively, for |Cu32.6Na5.3H4.5|[Si117Al75O384]-FAU and |Cu33.0Na3.9H5.1|[Si117Al75O384]-FAU, respectively. Cu2+ ions occupy the 6-ring sites I0 and II, preferring I0 ; Na+ ions also occupy sites I0 and II; neither site is filled. All Cu2+ ions in these structures coordinate not only to three trigonally arranged oxygen atoms of their 6-rings, but also to a fourth oxygen atom of those 6-rings to achieve planar, severely distorted square, 4-coordination. This result is in agreement with DFT calculations by Pierloot et al. and Berthomieu et al. With increasing Cu2+-exchange time, both the Cu2+ and H+ contents of the zeolite increase.
desolvated Cu2+ ion-exchanged zeolite A crystals which were prepared in a closed vessel at 373 K from ammoniacal aqueous Cu(NO3)2 solution.3 Later Lee et al. determined the structures of NH4+, hydrolyzed-Cu2+ forms of zeolite A prepared from ammonia/water solution.4 Ho et al. attempted Cu2+ exchange into single crystals of zeolite A using solutions of CuCl2 and CuSO4 in zeolitically dry acetonitrile and of CuCl2 in zeolitically dry methanol; under these conditions, no ion exchange occurred, none at all.10 (These solvents were dried by contact with fully dehydrated zeolite beads; the entire experiment was done in an anhydrous manner without exposure to the atmosphere.10) 1.2. Catalysis Using Cu2+-Exchanged Zeolites. Cu2+-exchanged zeolite Y catalyzes the reduction of nitric oxide,5 the oxidation of hydrocarbons,1116 and the cracking of cumene.17 Neveu et al. showed that Cu2+-exchanged zeolite Y catalysts are very efficient for the selective catalytic reduction (SCR) of NO by ammonia in the presence of oxygen.5 Seiyama et al. studied the activities of zeolite Y ion exchanged with 12 transition metals for the oxidation of propylene and ethylene.11,12 The catalytic activity of Cu2+-exchanged zeolites X
1. INTRODUCTION Zeolite Y is a synthetic material with a faujasite (FAU) structure and, by definition, a Si/Al ratio greater than 1.5. It is used in a wide range of applications in catalysis, sorption, and ion exchange. It may find application in molecular electronics, quantum-confined semiconductors, and gas storage.1,2 1.1. Ion Exchange of Cu2+ into Zeolites. The results of ion exchange from aqueous solution are usually not simple. Often only a fraction of the original cations can be replaced, and attempts to overcome this may reveal a relatively sharp upper limit to exchange. If the exchanging cation hydrolyzes in aqueous solution, then the H+ concentrations may increase by orders of magnitude, encouraging H+ (H3O+) exchange which, for lowsilica aluminosilicate zeolites, can lead to zeolite framework modification, damage, destruction, or dissolution. Zeolites with H+ content can participate in acid catalysis, donating protons to reactants; this is a vital step in oil cracking operations. Cu2+-exchanged zeolites can be prepared either from aqueous solution38 or by solid-state reaction.9 Single crystals of zeolite A (LTA) have been seen under the microscope to dissolve in a few minutes at Cu2+ concentrations as low as 5 103 M.4 To avoid crystal damage from the protons arising from the hydrolysis of Cu2+, Lee and Seff investigated the structures of four differently r 2011 American Chemical Society
Received: October 3, 2011 Revised: November 16, 2011 Published: November 17, 2011 963
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were at sites IA0 , IB0 , and II0 with occupancies of 17.6(4), 4.6(2), and 5.2(2) per unit cell, respectively, and Cu2+O distances of 2.079(4), 2.53(2), and 2.219(3) Å; 11.4(2) Na+ ions were found at site II. Site I was vacant, contrary to the report on natural faujasite.7 To explain the deficiency of positive charge, the authors suggested the presence of 5.5 unlocated [CuOD]+ groups per unit cell, possibly in the supercages. The structures of partially dehydrated Cu2+-exchanged zeolite Y (Si/Al = 2.5), prepared by NH4+ exchange followed by solidstate ion exchange (SSIE) with CuCl2, and its D2O sorption complex were incompletely determined by pulsed neutron powder diffraction.9 In Cu24Na5H17Cl15Y, 24 Cu2+ ions occupied three crystallographic sites: 5.3(4) ions per unit cell were found at site I at the centers of double 6-rings, 7.1(6) occupied site I0 in the sodalite unit, and, surprisingly, 10.1(12) were at site III (incorrectly called III0 ) in the supercage. Each siteIII Cu2+ ion coordinated to four framework oxygens in a distorted square-planar manner. The possible presence of a fifth ligand, perhaps H2O, OH, or Cl, that might coordinate to Cu(III) to give a distorted square pyramid was explored; it was not found. In Cu24Na5H17Cl15Y 3 xD2O, 4.0(4) Cu2+ ions were found per unit cell at site I0 , 5.7(5) at 12-ring centers, and again most, 13.2(9), were at site III. Each site-III Cu2+ ion bonded to framework oxygen atoms in a distorted square-planar manner as in Cu24Na5H17Cl15Y. Compared to many other cationic forms of zeolites X and Y, it was unusual to see occupation at site III, to see it be so high, and to see square-planar coordination. Cu2+ appears to have selected this unusual site to achieve this coordination. 1.4. Calculated Local Structures for Cu2+ in Zeolite Y. Density functional theory (DFT) calculations have been reported for Cu2+ in zeolite Y.2023 As discussed in Section 6.5, some of the calculated results are in good agreement with those reported here. Similar calculations have been reported for Cu2+ in MCM-22 (MWW).24 1.5. Objective. This study was done to determine the positions of Cu2+ ions in fully dehydrated zeolite Y more precisely than had been done before. The effect of ion-exchange time on the extent of exchange might also be seen. In future work, crystals prepared in this way might be used to determine the structures of sorption complexes of Cu2+ within zeolite Y.
and Y for the oxidation of propylene was studied by gas chromatography and electron spin resonance by Kevan et al.13 They identified Cu2+ as the active species and reported that it migrated from the inaccessible site I (in the double 6-ring) to the accessible site II where propylene coordinated to it prior to reaction. Ben Taarit and Naccache reported that Cu2+-exchanged zeolite Y exhibits Br€onsted acidity at a low activation temperature, and that the oxidizing properties of Cu2+-exchanged zeolite Y could be attributed to the cupric ions, while those of a reduced sample were due to true Lewis acid sites.14 Tsuruya et al. studied the gas-phase oxidative dehydration of ethanol over Cu2+-exchanged zeolite Y to give acetaldehyde; the dependence of the catalytic activity on the Cu2+ content showed the active site to be the Cu2+ ions themselves.15 Kuwada et al. investigated the vapor-phase oxidation of benzyl alcohol catalyzed by Cu2+, Na+Y in a flow system with reaction temperatures between 573 to 663 K and proposed a mechanism for this oxidation.16 Takahashi et al. examined the cumene cracking activity of Cu2+-exchanged synthetic faujasite zeolites; they reported that the activity is greater when the Cu2+ exchange is done with CuCl2 (aq) than it is with Cu(NO3)2 (aq).17 Kaushik and Ravindranathan observed 90% selectivity for the liquid-phase hydration of acrylonitrile to acrylamide using Cu2+-exchanged zeolite Y.18 1.3. Structures of Cu2+-Exchanged Zeolites. X-ray diffraction work on Cu2+-exchanged zeolite Y was first done by powder methods by Gallezot et al.6 Cu2+ ions were found at sites I and I0 , and Na+ at site II, in fully dehydrated Cu16Na24Y and Cu12Na5H27Y. Upon the introduction of organic molecules, the Cu2+ ions migrated to the supercages, presumably to interact with the sorbed molecules. The structure of fully Cu2+-exchanged zeolite A, prepared by a static method using 0.1 M Cu(NH3)42+ and desolvated at 773 K for 120 h, was determined by single-crystal X-ray diffraction techniques in the cubic space group Pm3m.3 All Cu2+ ions were found on 3-fold axes in the 6-membered rings. Each Cu2+ ion coordinated to three oxygen atoms at a distance of 2.14(4) Å and extended slightly into the large cavity; the OCu2+O angle was 119°. In 1975, Maxwell et al. investigated the crystal structures of hydrated and dehydrated Cu2+-exchanged natural faujasite (Si/Al = 2.42) by single-crystal X-ray diffraction techniques.7 Only 6.3(2) Cu2+ ions per unit cell were located with certainty in the hydrated structure, at site I0 , too far (Cu2+O = 2.58 Å) from the zeolite framework to be bonded directly to it. The dehydrated crystal was prepared by evacuation at 423 K and 105 Torr for 20 h. This temperature appears to have been too low to achieve complete dehydration. In this (partially) dehydrated form, 25.1 Cu2+ ions, where 28 would be needed per unit cell for charge balance, were found at sites I, I0 , II0 , II, and III. These were reported to coordinate to framework oxygens as follows (only the shortest distances are given): Cu(I)O(3) = 2.68(2) Å, Cu(I0 A)O(3) = 2.12(1) Å, Cu(I0 B)O(3) = 2.56(1) Å, Cu(II0 )O(2) = 2.29(9) Å, Cu(IIA)O(2) = 2.22(1) Å, Cu (IIB)O(2) = 2.51(10) Å, and Cu(III)O(1) = 2.16(7) Å, respectively. All shortest Cu2+-O distances found in this report (Section 5.1) are shorter than all seven of those distances, consistent with the higher Al content and therefore higher polarity of the zeolite Y used here. The simplicity seen here indicates a problem with that previous report, as its principle author has long ago acknowledged.19 The structure of fully dehydrated partially Cu2+-exchanged zeolite Y (Si/Al = 2.1), Cu33Na12Si130Al62O384, was determined by high resolution neutron diffraction from a powder sample.8 The Cu2+ ions
2. EXPERIMENTAL SECTION 2.1. Ion Exchange and Dehydration. Large, clear, colorless, octahedral single crystals of sodium zeolite Y, |Na75|[Si117Al75O384]FAU (Si/Al = 1.56), with diameters up to 0.32 mm were prepared by Lim et al.25 by the method of Sacco et al.26 Two of these were lodged in Pyrex capillaries, one in each. Ion exchange was done by the dynamic (flow) method using aqueous 0.05 M Cu(NO3)2 (Aldrich, 99.999%, Fe 2.5 ppm, Cr 1.9 ppm, Ni 1.2 ppm, Ag 0.3 ppm, Zn 0.3 ppm, Mn 0.2 ppm, Al 0.1 ppm, Mo 0.1 ppm, Pb 0.1 ppm). The pH of this solution was 4.2, a low value chosen to minimize the possible uptake of CuOH+ in excess of H+ by the zeolite; it is believed that this excess CuOH+ leads upon dehydration to the formation of Cu+. This solution was allowed to flow past crystals 1 and 2 for 18 and 24 h, respectively, at 294 K. Each capillary containing its clear pale greenish crystal was then attached to a vacuum system. Under dynamic vacuum, the temperature was cautiously raised at a heating rate of 10 K/h to 423 K and maintained at that temperature for 5 h. It was raised further (12.5 K/h) to 673 K and was fully dehydrated at this temperature 964
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Table 1. Summary of Experimental and Crystallographic Data crystal 1
crystal 2
crystal cross-section (mm)
0.32
0.32
ion exchange with Cu2+ T (K), t (h)
294, 18
294, 24
crystal color before dehydration
pale green
pale green
dehydration T (K)
673
673
crystal color after dehydration
yellowish green
yellowish green
data collection T (K)
100(1)
100(1)
space group, Z
Fd3m, 1
Fd3m, 1
X-ray source wavelength (Å)
Pohang Light Source (PLS) (Beamline 6B MXI) 0.90000
0.90000
Unit cell constant, a (Å)
24.605(1)
24.583(1)
2θ range in data collection (deg)
60.69
60.75
total reflections harvested
37,906
36,612
no. of unique reflections, m
573
569 508
no. of reflections with Fo > 4σ(Fo)
529
no. of variables, s
67
67
data/parameter ratio, m/s weighting parameters, a/b
8.6 0.074/140.3
8.5 0.067/120.3
R1/wR2 (Fo > 4σ(Fo))a
0.050/0.165
0.050/0.163
R1/wR2 (all intensities)b
0.053/0.174
0.054/0.179
goodness-of-fitc
1.23
1.28
final error indices
R1 = Σ|Fo|Fc||/ΣFo and wR2 = [Σw(Fo2 Fc2)2/Σw(Fo2)2]1/2; R1 and wR2 are calculated using only the reflections for which Fo > 4σ(Fo). b R1 and wR2 are calculated using all unique reflections measured. c Goodness-of-fit = [Σw(Fo2 Fc2)2/(m s)]1/2, where m is the number of unique reflections and s is the number of variables, respectively. a
and a dynamic vacuum of 1 106 Torr for 2 days. While these conditions were maintained, the hot contiguous downstream lengths of the vacuum system, including a sequential 17-cm U-tube of zeolite 5A beads fully activated in situ, were allowed to cool to ambient temperature to prevent the movement of water molecules from more distant parts of the vacuum system to each crystal. Still under vacuum in their capillaries, the crystals were only then allowed to cool to room temperature and were sealed by torch in their capillaries. Microscopic examination showed that both crystals had become yellowish green. 2.2. X-ray Diffraction. Synchrotron X-ray diffraction data were collected for the two crystals at 100(1) K using an ADSC Quantum210 detector at the Pohang Light Source. Crystal evaluation and data collection were done with a detector-to-crystal distance of 60 mm. Preliminary cell constants and an orientation matrix were determined from 72 sets of frames collected at scan intervals of 5° with an exposure time of 1 s per frame. The basic scale file was prepared using the HKL2000 program.27 The reflections were successfully indexed by the automated indexing routine of the DENZO program.27 The diffraction data were harvested by collecting 72 sets of frames with 5° scans with an exposure time of 1 s per frame. These highly redundant data sets were corrected for Lorentz and polarization effects; a very small correction for crystal decay was also applied. The space group Fd3m was determined by the program XPREP.28 A summary of the experimental and crystallographic data is presented in Table 1.
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 tetrahedral positions, although it is expected that Loewenstein’s (ref 29) and Dempsey’s (ref 39) rules would be obeyed. Extraframework cation positions are labeled with Roman numerals.
3. DESCRIPTION OF THE ZEOLITE Y FRAMEWORK AND EXCHANGEABLE CATION SITES The framework structure of zeolite Y (FAU) is characterized by the double 6-ring (D6R, hexagonal prism), the sodalite cavity (a cuboctahedron), and the supercage (see Figure 1). Each unit
cell has 8 supercages, 8 sodalite cavities, 16 D6Rs (32 6-rings), 16 12-rings, and 32 single 6-rings (S6Rs). 965
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966
21.8(4)
21.3(4)
21.0(5) 20.7(7)
21.0(5)
21.2(4)
9f
10g
11h 12i
13j
14k
21.5(6)
21.6(5)
21.6(5)
21.1(4)
21.2(3)
21.2(4)
20.9(4) 21.0(4)
7d
8e
9f
10g
11h
12i
13j 14k
12.0(2) 12.0(2)
12.1(2)
12.3(2)
12.3(2)
12.4(3)
12.2(4)
12.4(4)
12.2(4) 12.3(3)
2.1(10) 2.0(9)
1.0(11)
1.0(8)
1.3(11)
1.7(13)
1.8(14)
1.9(4) 1.9(4)
1.4(5)
1.4(5)
1.7(6)
1.8(9)
2.1(10)
1.7(9)
22.2(3) 21.6(5)
5 6
1.9(16)
22.0(3)
4c
12.7(2)
22.0(3)
3 96 96
96
96
96
15.3(8)
2
1.6(9) 1.4(8)
3.2(4)
3.2(4)
3.0(6) 2.9(6)
3.2(7)
3.6(9)
3.8(10)
96
1.6(13)
2.1(9)
2.6(12)
2.7(13) 3.5(19)
1.7(12)
1.8(11)
1.9(12)
1b
12.7(3)
11.4(2)
11.4(2)
11.6(2) 11.4(2)
11.5(2)
11.5(3)
11.4(4)
3.4(10)
96
96
39.1(6) 39.0(6)
38.7(6)
37.8(5)
38.1(6)
35.8(8)
55.8(8)
65.5(8)
40.5(6)
40.9(6)
40.3(6) 41.1(7)
40.1(6)
45.4(8)
45.1(8)
54.8(8)
O(2a)
O(2c)
O(3)
21.2(4)
21.0(5)
21.0(5) 20.7(7)
21.3(4)
16.0(12)
20.0(27)
96
96 96
96
96
96
96
30.5(8)
36.0(5) 36.0(5)
36.2(5)
37.0(5)
36.8(6)
37.1(10)
18.1(30)
20.9(4) 21.0(4)
21.2(4)
21.2(3)
21.1(4)
23.1(12)
22.1(29)
96
96
96
96
96
96
96
96 96
96
96
96
96
crystal 2, |Cu33.0Na3.9H5.1|[Si117Al75O384]-FAU
34.3(5)
34.1(5)
34.7(6) 34.2(6)
34.6(6)
34.5(10)
30.9(28)
41.2(8)
96
96
96 96
96
96
crystal 1, |Cu32.6Na5.3H4.5|[Si117Al75O384]-FAU
O(2b)
occupancya at
75.0(4)
74.8(4)
O(3a)
21.0(4)
21.2(4)
O(3b)
96
96
96
96
96 96
96
96
96
96
96
96
96
96
96
96
96 96
96
96
O(4)
84.0(2) 84.0(2)
83.9(2)
83.7(2)
84.6(2)
84.6(2)
84.4(2) 84.6(2)
O(4a)
12.0(2) 12.0(2)
12.1(2)
12.3(2)
11.4(2)
11.4(2)
11.6(2) 11.4(2)
O(4b)
0.0501 0.0498
0.0503
0.0515
0.0556
0.0679
0.0681
0.0701
0.0758 0.0744
0.0774
0.087
0.32
0.45
0.0504
0.0499
0.0506 0.0501
0.0547
0.0675
0.0684
0.0734
0.0748
0.0763
0.089 0.080
0.28
0.45
R1
0.1668 0.1634
0.1676
0.1683
0.1721
0.1904
0.1911
0.1947
0.2213 0.2201
0.2254
0.235
0.77
0.84
0.1652
0.1652
0.1686 0.1662
0.1710
0.1935
0.1972
0.2120
0.2187
0.2244
0.233 0.230
0.74
0.83
wR2
The occupancy is given as the number of ions per unit cell. b Only the atoms of zeolite framework were included in the initial structure model. They were all refined anisotropically. c Cu(I0 ) and Cu(II) were refined anisotropically. d O(2) was divided into two positions, O(2a) and O(2b). Both O(2a) and O(2b) were refined isotropically. e The framework oxygens at O(2a) and O(2b) are divided into three positions, O(2a), O(2b), and O(2c). All three O(2)s were refined isotropically. f The occupancies of Cu(II) and O(2b) were constrained to the ratio 1: 3. Both O(2a) and O(2c) were refined isotropically and O(2b) was refined anisotropically. g Cu(I0 ) was refined off the 3-fold axis because its environment no longer had symmetry 3. The occupancies of Cu(I0 ) and O(2c) were constrained to be equal. h The framework oxygens at O(4) are divided into two positions, O(4a) and O(4b). O(4a) was refined anisotropically and O(4b) was refined isotropically with constraint that its occupancy equals that at Cu(II). i Cu(II) was refined off the 3-fold axis because its environment no longer had symmetry 3. j The thermal parameters at Na(I0 ) and Na(II) were fixed at 0.03 Å2 in both structures because they were refining to unreasonably small values. k O(3) was divided into two positions, O(3a) and O(3b) because the O(3)s were no longer equivalent, now that Cu(I0 ) had refined to a position off the 3-fold axis. O(3a) was refined anisotropically and O(3b) was refined isotropically with the constraint that its occupancy equals that at Cu(I0 ).
a
21.8(5)
8e
11.6(4)
2.0(14)
21.8(5)
3.0(10)
3.1(10)
7d
11.5(4)
1.8(12)
21.8(5)
11.5(4)
22.3(3)
6
96 96
5
12.3(3) 12.3(3)
22.2(3) 22.1(3)
3 4c
O(2)
96
Na(II)
15.8(7)
Na(I0 )
2
Cu(II)
96
Cu(I0 )
1b
step
Table 2. Initial Steps of Structure Refinement
The Journal of Physical Chemistry C ARTICLE
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967
II
I0
II
I0
II
II I0
I0
cation site
0 87(5)
1037(2) 87(5)
2410(21)
574(34)
2163(3)
371(2)
638(10)
2410(21)
574(34)
2163(3)
371(2)
638(10)
774(2)
733(36)
226(46) 774(2)
71(8) 708(11)
71(8) 277(15)
34(6)
1246(1)
529(1)
34(6)
2409(12)
2164(4) 574(32)
372(2)
633(10)
2409(12)
2164(4) 574(32)
372(2)
633(10)
774(2)
729(30)
264(46) 774(2)
72(7)
89(4) 38(6) 706(8)
1035(2)
72(7)
89(4) 38(6)
529(1)
271(13)
1245(1) 0
y
x
U11b or Uiso U22
300d
293(50) 300d
318(26)
312(88)
625(27)
480(230)
1019(117)
512(72)
442(49) 389(61)
679(22)
543(11)
452(10)
293(50)
318(26)
625(27)
682(60)
389(61)
756(37)
2410(21)
574(34)
2294(5)
521(2)
2962(15)
3208(3)
733(36)
1177(11) 708(11)
1505(11)
1490(9)
1037(2)
362(1)
512(11)
497(10)
93(6)
2(6)
151(36)
74(22)
18(28)
95(62)
54(70)
62(28)
69(25)
50(20)
239(33)
95(62)
127(83)
169(20)
79(6)
53(27)
49(18)
245(31)
79(50)
131(76)
177(19)
76(5)
U12
96
192
96
192
fixed
12.0(2)
21.0(4)
12.0(2)
84.0(2)
21.0(4)
21.0(4) 75.0(4)
36.0(5)
39.0(6)
3.2(4)
11.4(2) 2.1(9)
21.2(4)
11.4(2)
84.6(2)
21.2(4)
74.8(4)
21.2(4)
40.5(6) 34.3(5)
varied
1.9(4)
151(36)
74(22)
18(28)
19(54)
54(70)
169(20)
124(43)
70(21)
21(27)
79(50)
16(66)
56(28)
8(6)
U13
2.0(9)
246(71)
351(42)
819(51)
740(81)
813(146)
725(23)
124(43)
70(21)
21(27)
37(55)
16(66)
177(19)
91(5)
U23
300d
302(45)
365(29)
679(29)
740(81)
476(67)
830(39)
252(83)
312(40)
778(47)
682(60)
775(145)
679(22)
440(10)
U33
300d
302(45)
365(29)
353(91)
679(29)
516(171)
571(76) 1061(175)
476(67)
500(58)
725(23)
611(12)
crystal 2, |Cu33.0Na3.9H5.1|[Si117Al75O384]-FAU
2409(12)
2288(7) 574(32)
520(2)
2962(15)
3205(3)
729(30)
706(8)
1184(10)
1498(8) 1500(10)
1035(2)
363(1)
crystal 1, |Cu32.6Na5.3H4.5|[Si117Al75O384]-FAU
z
occupancyc
Positional 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. b The anisotropic temperature factor is exp[2π2a2(U11h2 + U22k2 + U33l2 + 2U23kl + 2U13hl + 2U12hk)]. c The occupancy is given as the number of atoms or ions per unit cell. d The isotropic thermal parameters at Na(I0 ) and Na(II) were fixed.
a
32(e)
96(g)
O(2b)
Na(II)
96(g)
O(2a)
32(e)
96(h)
O(1)
Na(I0 )
192(i)
Si,Al
96(g)
32(e)
Na(II)
Cu(II)
96(g) 32(e)
Cu(II) Na(I0 )
96(g)
96(g)
Cu(I0 )
Cu(I0 )
96(g)
O(4b)
96(g)
96(g)
O(4a)
O(4b)
96(g)
O(3b)
96(g)
96(g)
O(3a)
O(4a)
96(g)
O(2c)
96(g)
96(g) 96(g)
O(2a) O(2b)
O(3b)
96(h)
O(1)
96(g) 96(g)
192(i)
Si,Al
O(2c) O(3a)
Wyckoff position
atom
Table 3. Positional, Thermal, And Occupancy Parametersa
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Table 4. Continued
Table 4. Selected Interatomic Distances (Å) and Angles (deg)a crystal 1
crystal 2
(Si,Al)O(1)
1.6179(21)
1.6178(22)
(Si,Al)O(2a) (Si,Al)O(2b)
1.671(7) 1.724(10)
1.661(8) 1.723(10)
(Si,Al)O(2c)
1.650(9)
1.647(9)
(Si,Al)O(3a)
1.693(15)
1.688(14)
(Si,Al)O(3b)
1.72(5)
1.73(5)
(Si,Al)O(4a)
1.6154(23)
1.6173(24) 1.669(13)
(Si,Al)O(4b)
1.676(14)
mean (Si,Al)-Ob
1.656
1.655
Cu(I0 )O(3a) Cu(I0 )O(3b)
1.84(3) 2.31(12)
1.85(4) 2.23(9)
Cu(I0 )-O(2c)
1.94(3)
1.92(3)
Cu(II)O(2a)
2.079(20)
2.084(22)
Cu(II)O(2b)
1.842(24), 2.20(3)
1.831(24), 2.21(3)
Cu(II)O(2c)
2.52c
2.53c
Cu(II)O(4b)
1.95(4)
1.95(4)
Na(I0 )O(3a)
2.13(6)
2.14(7)
Na(I0 )O(2c) Na(II)O(2a)
2.30(4)c 2.33(3)
2.29(5)c 2.34(4)
Na(II)O(2b) O(1)(Si,Al)O(2a)
2.25(4) 104.6(5) 118.8(6)
118.6(7)
115.0(9)
114.9(9)
O(1)(Si,Al)O(3a)
108.1(10)
108.1(14)
O(1)(Si,Al)O(3b)
111(3)
113(5)
O(1)(Si,Al)O(4a) O(1)(Si,Al)O(4b)
111.9(3) 125.9(12)
111.4(3) 125.9(13)
O(2a)(Si,Al)O(3a)
112.2(12)
111.5(7)
O(2a)(Si,Al)O(3b)
113(4)
110(3)
O(2b)(Si,Al)O(3a)
102.2(13)
103.4(14)
O(2b)(Si,Al)O(3b)
103(4)
100(3)
O(2c)(Si,Al)O(3a)
78.8(14)
79.1(13)
O(2c)(Si,Al)O(3b)
80(4)
77(3)
O(2a)(Si,Al)O(4a) O(2a)(Si,Al)O(4b)
108.5(7) 81.9(14)
109.3(8) 83.0(14)
O(2b)(Si,Al)O(4a)
103.9(8)
104.0(8)
O(2c)(Si,Al)O(4a)
125.4(9) 99.6(14) 111.5(10)
110.9(13)
O(3b)(Si,Al)O(4a)
107(4)
109(4)
O(3a)(Si,Al)O(4b) O(3b)(Si,Al)O(4b)
118.9(15) 115(4)
118.2(19) 114(5)
100.4(14)
(Si,Al)-O(1)-(Si,Al)
142.8(4)
142.5(4)
(Si,Al)O(2a)(Si,Al)
136.3(12)
137.6(14)
(Si,Al)O(2b)(Si,Al)
128.2(14)
127.9(14)
(Si,Al)O(2c)(Si,Al)
140.3(17)
140.1(17)
(Si,Al)O(3a)(Si,Al)
129.6(21)
130.9(21)
(Si,Al)O(3b)(Si,Al)
131(7)
126(6)
(Si,Al)O(4a)(Si,Al) (Si,Al)O(4b)(Si,Al)
147.7(5) 135.5(23)
147.1(5) 136.7(23)
O(3a)Cu(I0 )O(3a)
134.9(6)
135.0(8)
O(3a)Cu(I0 )O(3b)
111.6(3)
111.7(4)
O(2b)Cu(II)O(2b)
111.6(7)d, 130.8(11)e
111.3(5)d, 131.7(9)e
67.7(6), 176.3(13)
67.9(5), 176.0(13)
O(3a)Na(I0 )O(3a)
106(4)
106(4)
O(2a)Na(II)O(2a)
109.2(16)
109(3)
Cu(I0 ) 3 3 3 Cu(I0 )f Cu(I0 ) 3 3 3 Na(I0 )f
3.63
3.63
4.23
4.25
4. STRUCTURE DETERMINATION Full-matrix least-squares refinement (SHELXL97)32 was done on Fo2 using all data for each crystal. Each refinement was initiated with the atomic parameters of the framework atoms [(Si,Al), O(1), O(2), O(3), and O(4)] in dehydrated |Tl75|[Si117Al75O384]-FAU.25 Each initial refinement used anisotropic thermal parameters and converged to the high error indices (given in steps 1 of Table 2) R1/wR2 = 0.45/0.83 and 0.45/0.84 for crystals 1 and 2, respectively. For each crystal, the initial difference Fourier function revealed two large peaks at Cu(I0 ) and Cu(II). The further steps of structure determination and refinement as new atomic positions were found on successive difference Fourier electron-density functions are shown in Table 2. 4.1. Considerations Unique to O(2). When structure determination seemed complete, an additional difference Fourier peak 0.71 eÅ3 in height was seen at (0.0091, 0.0091, 0.1053) in crystal 1 (0.67 eÅ3 at (0.0099, 0.0099, 0.1056) in crystal 2). This peak was very close to O(2) and, like O(2), bonded to the same two T (Si or Al) atoms. This indicated that a minor O(2) peak, O(2b), is near the major O(2) peak which will henceforth be called O(2a). O(2b) has the lower occupancy and is closer to Cu(II), indicating that the Cu2+ ions (small with relatively high charge) at Cu(II), 11.5 in crystal 1 and 12.3 in crystal 2 (see steps 6 in Table 2(a) and (b)), are pulling 3 11.5 = 34.5 and 3 12.3 = 36.9 O(2b) atoms, respectively, inward toward the centers of their S6Rs, leaving 96 - 34.5 = 61.5 and 96 - 36.9 = 59.1, respectively, at the majority position, O(2a). (Na+ is larger and has a smaller charge; it is too large to fit into a 6-ring, so it does not lie in any 6-ring plane and has no need to pull its 6-ring oxygens inward to achieve adequate coordination, much like the majority of the S6Rs which do not host a cation at all.)
77.0(14)
O(2c)(Si,Al)O(4b)
68.5(3) 165.4(6)
The exchangeable cations, which balance the negative charge of the zeolite Y framework, usually occupy some or all of the sites shown with Roman numerals in Figure 1. The maximum occupancies at the cation sites I, I0 , II, II0 , and III in zeolite Y are 16, 32, 32, 32, and 48, respectively. Site III0 in zeolite Y studied using space group Fd3m is a 192-fold position. Further description is available.30,31 A discussion of the pervasive disorder present in this structure, some intrinsic to zeolite Y and some due to Cu2+ exchange and dehydration, is presented in Section 6.1.
126.2(10)
O(3a)(Si,Al)O(4a)
68.7(3) 167.1(5)
The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. Because of the wide-ranging disorder present in these structures, it is expected that many of these esds are underestimated. b Weighted for occupancy. c It is expected that this approach is virtual. d Involving one short and one long Cu(II)-O(2b) bond. e Involving two short Cu(II)-O(2b) bonds. f Nonbonding approach distances.
2.23(5)
O(1)(Si,Al)O(2c)
76.9(14)
a
crystal 2
O(3a)Cu(I )O(2c) O(3b)Cu(I0 )O(2c) O(2b)Cu(II)O(4b)
104.7(6)
O(1)(Si,Al)O(2b)
O(2b)(Si,Al)O(4b)
crystal 1 0
968
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Figure 3. Stereoview of a representive sodalite unit in crystals 1 and 2 is shown in (a); (b) is a simpler stereoview in the same orientation. The coordinates plotted are those of crystal 2. See the caption to Figure 2 for other details. Figure 2. Stereoviews of two of the ways that Cu2+ and Na+ ions can occupy double 6-rings (D6Rs) in both crystals. Two Cu2+ ions must be present as shown in (a) in at least 5.0(5) of the 16 D6Rs in crystal 1, and at least 4.9(5) in crystal 2. Other combinations involving empty 6-rings must be present, and D6Rs with two Na+ ions are possible. Cu(I0 ) 3 3 3 Cu(I0 ) = 3.63 Å in both crystals 1 and 2 as shown in (a), and Cu(I0 ) 3 3 3 Na(I0 ) = 4.23 and 4.25 Å, respectively, as shown in (b). The zeolite Y framework is drawn with heavy bonds. The coordination of the exchangeable cations to oxygens of the zeolite framework is indicated by light bonds. The coordinates plotted are those of crystal 2. Ellipsoids of 25% probability are shown.
After this model was refined to convergence, yet another difference Fourier peak appeared at (0.0141, 0.0141, 0.1564) with height of 0.50 eÅ3 in crystal 1, close to O(2b) and also bonded to the same two T atoms. At this time, the anisotropic thermal parameter at the nearest position, O(2b), was unreasonably elongated. Accordingly, this new peak was refined isotropically as O(2c) with the constraint that the occupancies at O(2a), O(2b), and O(2c) sum to 96. This resulted in a sharp decrease in the error indices (Table 2, step 8). Most surprisingly, O(2c) bonds to Cu(I0 ), indicating an extreme distortion of some D6R 6-rings. Its occupancy was almost equal to that at Cu(I0 ), suggesting that each Cu(I0 ) ion bonds to an O(2c) atom. The results for crystal 2 were similar (see Tables 3 and 4). 4.2. Considerations Unique to O(4). When structure determination again seemed complete, an additional difference Fourier peak 0.47 eÅ3 in height was seen at (0.0564, 0.0564, 0.2877) in crystal 1 (0.44 eÅ3 at (0.0572, 0.0572, 0.2866) in crystal 2). This peak was very close to O(4) and, like O(4), bonded to the same two T (Si or Al) atoms. This indicated that a minor O(4) peak, O(4b), is near the major O(4) peak which will henceforth be called O(4a). It was refined isotropically as O(4b) with the
Figure 4. Stereoviews. (a) A 6-ring of a D6R with a Cu2+ ion at site I0 , and (b) an S6R with a Cu2+ ion at site II. The coordinates plotted are those of crystal 2. See the caption to Figure 2 for other details.
constraints that the occupancies at O(4a) and O(4b) sum to 96. It was then seen that O(4b) bonds to Cu(II) with occupancies that were almost equal; this constraint was then applied. This indicates an extreme distortion of some S6Rs as was seen for Cu(I0 )-containing D6Rs. The results for crystal 2 were similar (see Tables 3 and 4). 969
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Table 5. Displacements of Atoms (Å) from 6-Ring Planes crystal 1 planes
atom a
(111) at O(3a)
site
0
Cu(I )
I
0
O(2c)
O(3a),O(3a),O(2c),a,b nearly (111) at O(3a)
Na(I0 )
I0
Cu(I0 )
I0
crystal 2
displacement 0.17
0.18
0.27
0.26
0.83
0.83
0.08
0.09
c
O(3a)
0.09
0.09
O(3b)
0.01
0.06
O(2a) O(2b)
0.17 0.53
0.17 0.53
(111) at O(2a)d
Na(II)
II
0.78
0.80
(111) at O(2b)d
Cu(II)
II
0.27
0.26
O(2b),O(2b),O(4b),d,e
Cu(II)
II
nearly (111) at O(2b)
0.14
0.14
O(2b)f O(2c)
0.07 0.42
0.08 0.44
O(4a)
0.89
0.89
Figure 5. Stereoview of a representive supercage in crystals 1 and 2; the coordinates plotted are those of crystal 2. See the caption to Figure 2 for other details.
After step 9 (Table 2), the Uij values of the anisotropic thermal parameter for Cu(I0 ) were negative in both structures, to give an oblate thermal ellipsoid. Cations with planar coordination usually have prolate thermal ellipsoids: their greatest motion is perpendicular to the plane. Thus Cu(I0 ), with its 4-coordinate planar environment (clearly not of symmetry 3), was allowed to refine off the 3-fold axis. After anisotropic refinement with the constraint that the occupancies at Cu(I0 ) and O(2c) to be equal, the Cu(I0 )O(2c) distance decreased to 1.96(3) Å in crystal 1 (1.94(2) Å in crystal 2). Thus the Cu(I0 )O(2c) distance became closer to the Cu(I0 )O(3a) distances, allowing the geometry at Cu(I0 ) to become more reasonable. Similarly, for the same reasons and to the same end, Cu(II) was allowed to refine off the 3-fold axis (steps 12 in Table 2). Fixed weights were used initially; the final weights were assigned using the formula w = 1/[σ2(Fo2) + (aP)2 + bP] where P = [Max(Fo2,0) + 2Fc2]/3; the refined parameters of a and b are given in Table 1. Atomic scattering factors for Cu2+, Na+, O, and (Si,Al)1.80+ were used.33,34 The function describing (Si, Al)1.80+ is the weighted (for composition) mean of the Si4+, Si0, Al3+, and Al0 functions, assuming half formal charges. All scattering factors were modified to account for anomalous dispersion.35,36 All shifts in the final cycles of refinement were less than 0.1% of their corresponding estimated standard deviations. The final error indices are given in Table 1. Structural parameters are given in Table 3, and selected interatomic distances and angles are given in Table 4.
a
A positive deviation indicates that the atom lies in a sodalite cavity. b The three oxygen atoms that bond most closely to Cu(I0 ) in a Cu(I0 )containing 6-ring. c The distant O(3a) atom. d A positive displacement indicates that the atom lies in a supercage. e The three oxygen atoms that bond most closely to Cu(II) in a Cu(II)-containing 6-ring. f The distant O(2b) atom.
4.3. Considerations Unique to O(3). When structure determination yet again seemed complete, an additional difference Fourier peak 0.19 eÅ3 in height was seen at (0.0201, 0.0801, 0.0801) in crystal 1 (0.19 eÅ3 at (0.0185, 0.0793, 0.0793) in crystal 2). This peak was very close to O(3) and, like O(3), bonded to the same two T (Si or Al) atoms. This indicated that a minor O(3) peak, O(3b), is near the major O(3) peak which will henceforth be called O(3a). It was refined isotropically as O(3b) with the constraints that the occupancies at O(3a) and O(3b) sum to 96. O(3b) is close to Cu(I0 ) and the occupancies were almost equal, so this constraint was applied. O(3b), like O(2c), bonds to Cu(I0 ), and contributes to the distortion of the Cu(I0 )containing D6Rs. The results for crystal 2 were similar (see Tables 3 and 4). 4.4. Remaining Considerations. After step 6 (Table 2), a subsequent difference Fourier function showed a peak at (0.125, 0.125, 0.125) with height 0.75 eÅ3 in crystal 1 (0.76 eÅ3 in crystal 2). It is at a very special position far from any other atom; with a fixed thermal parameter of 0.05 Å2, its occupancy refined to 0.3(4) atoms per unit cell (using an oxygen scattering factor) (0.4(4) in crystal 2). Accordingly, this peak was not included in the final model. Another difference Fourier peak was seen at (0, 0, 0) with height 0.50 eÅ3 in crystal 1 (0.48 eÅ3 in crystal 2). It was considered because it might represent oxide ions bridging linearly between pairs of Cu(I0 ) ions. It was refined isotropically with a thermal parameter equal to that of Cu(I0 ). In crystal 1, it led to an insignificant occupancy, 0.7(5) oxygen atoms per unit cell. A much larger value, 10.9, would be needed for all of the ions at Cu(I0 ) to participate in Cu2+OCu2+ groups.
5. DESCRIPTION OF THE STRUCTURES 5.1. Crystal 1, |Cu32.6Na5.3H4.5|[Si117Al75O384]-FAU. The two site-I0 positions, Cu(I0 ) and Na(I0 ), are occupied by 21.2(4) Cu2+ and 2.2(9) Na+ ions, respectively, per unit cell. Similarly, two sites II, Cu(II) and Na(II), hold 11.4(2) Cu2+ and 3.1(4) Na+ ions. Both sites I0 and II are of order 32, so neither is filled. Unexpectedly, the 21.2(4) Cu2+ ions at site I0 coordinate to four oxygen atoms of their 6-rings (6-rings of D6Rs) rather than the usual three (see Figures 2, 3, and 4(a)). Each is only 0.08 Å from the plane of the three atoms, O(3a), O(3a), and O(2c), that coordinate most closely to it (see Figure 4(a)). The two short Cu(I0 )O(3a) bond lengths, 1.84(3) Å, and the Cu(I0 )O(2c) distance, 1.94(3) Å, are somewhat shorter than the sum of the corresponding ionic radii of Cu2+ and O2, 2.04 Å,37 indicating strong bonds to the zeolite framework. The remaining Cu(I0 )O(3b) bond length is 2.31(12) Å with this O(3b) atom 970
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Table 6. Distribution and Occupanciesa of Cu2+, Na+, and H+ Ions in Fully Dehydrated Cu2+-Exchanged Zeolites Y Cu2+ positions crystal no.
IE Timeb (h)
Na+ positions
Cu(I0 )
Cu(II)
Σ
Na(I0 )
Na(II)
Σ
H+ (required)c
% Cu2+ exchanged
1
18
21.2(4)
11.4(2)
32.6(5)
2.2(9)
3.1(4)
5.3(10)
4.5(11)
86.9
2
24
21.0(4)
12.0(2)
33.0(5)
2.0(9)
1.9(4)
3.9(10)
5.1(11)
88.0
Number of ions per unit cell. b Ion exchange time as contact between the crystal and the flowing Cu2+-exchange solution. c No. of H+ ions per unit cell required to balance the negative charge of the zeolite framework. d [ΣCu2+ ions/(75+/2)] 100. a
only 0.01 Å from the above plane (Table 5). Thus the coordination geometry about Cu(I0 ) is distorted square planar. The O(3a)Cu(I0 )O(3a) angle for the two short Cu(I0 )O(3a) distances is 134.9(6)°, far from 180°; the remaining two O(3a)Cu(I0 )O(3b) angles are 111.6(3)°. Similarly the two O(3a)Cu(I0 )O(2c) and O(3b)Cu(I0 )O(2c) angles involving short bonds are 68.7(3)° and 69.1(6)°, far from 90°. Thus the square-planar coordination about Cu(I0 ) is severely distorted. At site II, close to the S6R planes, 11.4(2) Cu2+ ions are found. Like Cu(I0 ), each coordinates to four oxygen atoms of its S6R instead of the usual three (see Figures 3, 4(b), and 5) Each is only 0.14 Å from the plane of the three atoms, O(2b), O(2b), and O(4b), that coordinate most closely to it (see Figure 4(b)). The two short Cu(II)O(2b) bond lengths, 1.842(24) Å, and the Cu(II)O(4b) distance, 1.95(4) Å, are somewhat shorter than the sum of the corresponding ionic radii of Cu2+ and O2‑, 2.04 Å,37 indicating strong bonds to the zeolite framework. The remaining Cu(II)O(2b) bond length is 2.20(3) Å. Thus the coordination geometry about Cu(II) is again distorted square planar. The O(2b)Cu(II)O(2b) angle for the two short Cu(II)O(2b) distances is 130.8(10)°, far from 180°; the remaining two O(2b)Cu(II)O(2b) angles are 111.6(7)°. Similarly the two O(2b)Cu(II)O(4b) angles involving short bonds are 67.7(6)°, far from 90°; that angle for the long Cu(II)O(2b) bond is straight 176.3(13)°. Thus the squareplanar coordination about Cu(II), like that about Cu(I0 ), is severely distorted. The remaining cations, 2.2(9) Na+ ions per unit cell at site I0 and 3.1(4) Na+ ions at site II, are shown in Figures 3 and 5. The occupancy at Na(I0 ) is not significant at the 3σ level, but both Na(I0 ) and Na(II) have refined well to chemically sensible positions. Each Na(I0 ) ion is within the sodalite cavity, 0.83 Å from the plane of three O(3a) framework oxygens (Na(I0 ) O(3a) = 2.13(6) Å). Na(II) extends 0.78 Å into the supercage from its three O(2a) plane (Na(II)O(2a) = 2.33(3) Å). Both distances are comparable to the sum of the conventional ionic radii, 2.29 Å.37 5.2. Crystal 2, |Cu33.0Na3.9H5.1|[Si117Al75O384]-FAU. As in crystal 1, 33.0(5) Cu2+ and 3.9(10) Na+ ions per unit cell are distributed among four crystallographic positions (see Table 3). At site I0 , 21.0(4) Cu2+ ions are found (Figures 2, 3, and 4(a)). Each bonds most closely to two O(3a) framework oxygens at 1.85(4) Å and to one O(2c) framework oxygen at 1.92(3) Å. As in crystal 1, each Cu(I0 ) also coordinates to third O(3b) framework oxygen atom at 2.23(9) Å. Each Cu2+ ion at Cu(I0 ) is only 0.09 Å from the O(3a), O(3a), and O(2c) plane (Table 5) and its coordination geometry is again square planar, severely distorted in the plane. At site II, 12.0(2) Cu2+ ions are found per unit cell (Figures 3, 4(b), and 5). Each again coordinates at 1.842(24), 2.20(3), and
1.95(4) Å to two short O(2b), one longer O(2b), and one O(4b) framework oxygen and is again 4-coordinate with distortedsquare planar geometry. As in crystal 1, each Cu(II) ion is nearly in its O(2b), O(2b), and O(4b) plane (Tables 5 and 6). As in crystal 1, the few Na+ ions occupy two crystallographic sites, I0 and II (see Figures 2(b), 3(a), and 5) with occupancies of 2.0(9) (again not significant at the 3σ level) and 1.9(4), respectively. Both are 3-coordinate with Na(I0 )O(3a) = 2.14(7) and Na(II)O(2a) = 2.34(4) Å, again comparable to the sum of the corresponding ionic radii, 2.29 Å.37
6. DISCUSSION The complete exchange of Cu2+ into zeolite Y was not achieved. Some Na+ ions remain and H3O+ has accompanied Cu2+ into the zeolite. With increasing contact time with the ionexchange solution, the Cu2+ content appears to have increased slightly from 32.6(5) to 33.0(5) per unit cell as the number of Na+ ions decreased from 5.3(10) to 3.9(10). Therefore the H+ content, observed by difference, must also have increased. In addition, the unit cell constant of the zeolite decreased with this higher level of Cu2+ exchange (see Table 1). This is generally seen in dehydrated zeolites with an increased number of more highly charged cations.38 6.1. Pervasive Structural Disorder. This structure shows a great deal of disorder. This can be seen in the thermal parameters (Table 3) which are irregular and far too large for a rigid structure studied at 100 K. First of all, the Si and Al atoms are disordered, although not completely according to Loewenstein’s Rule29 and, to a lesser degree, Dempsey’s Rule.39 Of the 192 T atoms per unit cell, 117 are Si, and the remaining 75 are Al. Because SiO bond lengths (ca. 1.61 Å) are shorter than those for AlO (ca. 1.73 Å), the entire zeolite framework is distorted in a manner that has not been accounted for crystallographically. Second, the Cu2+ ions have distorted the zeolite framework further by their placements in positions of partial occupancy, presumably in 6-rings that are richer in Al atoms and therefore more negative. With eight 6-rings and about four Cu2+ ions per sodalite cavity distributed among sites I0 and II, some Cu2+ ions must occupy adjacent 6-rings (one S6R and one 6-ring of a D6R). Third, the Cu2+ ions at Cu(I0 ) and Cu(II) have contributed heavily to the total distortion by being 4-coordinate (see Figure 4). Therefore, the disorder is indeed pervasive and only some sort of averaged structure has been determined. It follows that the esds given for the bond lengths and angles are generally underestimated, and many, especially those with low occupancy, may be inaccurate. The thermal parameters do not reasonably indicate thermal vibrational amplitudes. Still, the diffraction data have been strong enough to allow the O(2), O(3), and O(4) positions to be divided into three, two, and two positions, respectively. Perhaps this process could have been carried further, to resolve O(2b), for example, into two 971
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positions (it is clear in Figure 4(b) that the O(2b) oxygens are not all equivalent), but only with diminishing reliability. 6.2. Cu2+ Positions. At site I0 , 21.2(4) and 21.0(4) Cu2+ ions are found in crystals 1 and 2, respectively; 11.4(2) and 12.0(2) Cu2+ ions are at site II. Site I0 appears to be the preferred site for Cu2+ ions in zeolite Y. This was also seen in fully dehydrated partially Cu2+-exchanged zeolite Y powder; a total of 22.2(4) Cu2+ ions were found at sites IA0 and IB0 , while site II was occupied only by Na+ ions.8 For the first time, a cation in a zeolite 6-ring has been found to coordinate to four of the oxygen atoms of that ring, and therefore to be off its 3-fold axis. This stands in sharp contrast to the many cations (e.g., Na+, K+, Rb+, Cs+, NH4+, Mg2+, Ca2+, Sr2+, Ba2+, Cr2+, Mn2+, Co2+, Co3+, Ni2+, Zn+, Zn2+, La3+, Eu3+, Tl+, Pb2+, Hg+, Ga3+, Pd2+, Pd4+or 5+, Ag+, Cd+, Cd2+, In+, and In3+) that have been found on 3-fold axes coordinating trigonally to three 6-ring oxygen atoms in innumerable structures of LTA, FAU, and other zeolites with 6-rings. (In many of these structures, additional nonframework ligands coordinate to these cations.) An exception is Li+, which, because of its small size, small charge, and very low covalency, has been found at an off-3-fold-axis position.40,41 All Cu2+ ions in both fully dehydrated zeolite Y structures have achieved distorted square-planar coordination rather than accept trigonal coordination. This has imparted substantial strain to the zeolite framework as can be seen in the bond lengths and angles about both Cu2+ positions in both structures (Section 5 and Table 4). Six-coordinate octahedral is the favored geometry for Cu2+, with four equatorial ligands held tightly in a square-planar manner and two axial ligands at somewhat to substantially longer distances.42 (Tetrahedral is also a common coordination geometry for Cu2+, but it is not readily available in fully dehydrated zeolite Y.) In the limit as the axial bonds lengthen, the distinction between octahedral and square-planar coordination vanishes.42 This is seen in CuO (the mineral tenorite) and in many complexes of Cu2+.42 In tenorite at 293 K, Cu2+ bonds to four oxygen atoms with a mean CuO bond length of 1.956 Å (two at 1.9608(13) Å and two at 1.9509(26) Å) with OCuO angles of 95.72(4)o and 84.28(4)o.43 (Cu2+ lies on an inversion center so these five atoms are coplanar by symmetry.) Two axial oxygen atoms are far, 2.78 Å, from each Cu2+ ion.43,44 These 1.956-Å bond lengths, somewhat less than the sum of the conventional radii, 2.04 Å, can be compared to the four Cu(I0 )O distances (1.84(3), 1.84(3), 1.94(3), and 2.31(12) Å) and the four Cu(II)O distances (1.84(2), 1.84(2), 1.95(4), and 2.20(3) Å) in crystal 1 (at 100 K). The values for crystal 2 are very similar (Table 4). Square-planar Cu2+ had been seen before in partially dehydrated, Cu2+-exchanged zeolite Y (Si/Al = 2.5),9 but at a very different position (last paragraph of Section 1.3). In Cu24Na5H17Cl15Y, each of 10.1(12) Cu2+ ions at site III0 coordinated in this way to four framework oxygens; Cu2+ was 1.819(10) Å from two O(4) oxygens, 1.73(4) Å from one O(3) oxygen, and 2.46(4) Å from another O(3) oxygen. (These distances may be somewhat to quite inaccurate because of the disorder present in the structure.) Each of 13.2(9) Cu2+ ions at site III0 in Cu24Na5H17Cl15Y 3 xD2O coordinated in the same way to the same four framework oxygen atoms. Perhaps the 3-coordinate Cu2+ ions reported to be on 3-fold axes in those structures were actually 4-coordinate and off those axes as seen in this work, but the data collected (pulsed neutron diffraction data from zeolite powders) were inadequate to reveal this.
As stated in Section 1.1, large single crystals of zeolite A were seen to dissolve in dilute aqueous Cu(NO3)2.4 Earlier attempts to study Cu2+-exchanged structures of zeolite X (Si/Al = 1.09) had failed due to crystal damage. Ni2+-exchanged zeolite X could be studied (showed little or no crystal damage upon dehydration) although those materials had appreciable H+ content. The instability of Cu2+exchanged zeolites LTA and X (FAU with a low Si/Al ratio) could be due both to the dealuminating effect of H+ and to the extreme stress that Cu2+ imparts to the zeolite framework as it strives for 4-coordination in 6-rings (see Figure 4). 6.3. Na+ Positions. The Na+ ions at site I0 are 2.14(7) Å (a mean value) from three O(3a) framework oxygens. This appears to be shorter than the sum of the corresponding ionic radii of Na+ and O2‑, 0.97 + 1.32 = 2.29 Å,37 respectively, but it is in agreement with previous results, Na(I0 )O(3) = 2.205(9)45 and 2.207(13) Å25 in zeolite Y. It may be attributed to the low coordination number of Na+, three, in these dehydrated structures. The Na(II)O distances (Table 4) agree better with the sum of the ionic radii. 6.4. H+ Content and H3O+ Exchange. In crystals 1 and 2, the sum of the charges of the Cu2+ and Na+ cations were 70.5+ and 69.9+, respectively. To balance the charge of the zeolite framework, 75- per unit cell, H+ ions, which could not be found by X-ray diffraction because their scattering factors are too small, must be present in both crystals, about 4.5 in crystal 1 and 5.1 in crystal 2. This is consistent with the Br€onsted acidity seen by Ben Taarit and Naccache.14 It appears that 4.5 and 5.1 H3O+ ions, respectively, entered the zeolite per unit cell from the acidic (due in part to the hydrolysis of Cu2+) exchange solution (Section 2.1), and that they decomposed upon dehydration to give H+ ions. This would be net, exclusive of any CuOH+ and H3O+ hydrolysis pairs that might have exchanged into the zeolite (in addition to Cu2+) and could have been present in these crystals before dehydration. H+ and H3O+ ions have been found before in faujasite zeolites that were ion exchanged with first row transition metal cations from aqueous solution. In vacuum dehydrated Ni2+-exchanged zeolite Y (Si/Al = 1.69), all five crystals studied required H+ ions, from 9.5(11) to 21.6(13) per unit cell, for charge balance.38 In partially dehydrated Co2+-exchanged zeolite X, 18 H3O+ ions were found per unit cell at two equipoints, 10 at site I0 and 8 at site II0 .46 In partially dehydrated Ni2+-exchanged zeolite X, 46 H3O+ ions were found per unit cell, 24 at site II0 and 20 at site II.47 The pHs of these ion-exchange solutions were quite low due in part to the heavy hydrolysis of the transition metal ions in the solution, and likely in the zeolite also, to give high intrazeolitic concentrations of H3O+ ions. Anhydrous CuY (Cu,HY, Cu,H,NaY) has both Lewis and Br€onsted acid sites that can be active in catalytic applications. 6.5. Comparisons with DFT Calculations. After the work reported herein was completed, we learned from a reviewer that DFT calculations had been reported for Cu2+ in zeolite Y.2023 Some of the calculated geometries are remarkably similar to those shown here (see Figure 4). Square-planar Cu2+ was first found in zeolite Y at site III (incorrectly labeled III0 ) in a partially dehydrated sample of approximate unit cell composition Cu24Na4.5Cl15H17.5Si55Al137O384 exclusive of water molecules.9 Cu2+ occupies the central 4-ring of a three 4-ring ladder. This Cu2+ position is very close to that shown in the fifth of nine models calculated for sites III and III0 (all referred to as III).22 This result is also shown in the second of eight drawings in Figure 5 in a later reference.23 The calculated Cu2+O distances, proceeding cyclically around 972
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The Journal of Physical Chemistry C the square plane, are 1.94 Å, 2.00 Å, 1.95 Å, and 1.99 Å. The corresponding distances found crystallographically, 1.73(4) Å, 1.819(10) Å, 1.73(4) Å, and 1.819(10) Å, are unrealistically short. They are likely to be unreliable because the relevant framework oxygen positions, O(3) and O(4), made nonequivalent by the presence of Cu2+, could not be resolved with the pulsed-neutron data gathered from a powder sample. The calculation was done for a zeolite 4-ring with two Al atoms at opposite positions,22 while the crystallographic result may have been an average among 4-rings with two or fewer Al atoms.9 The bond lengths for the two 6-rings shown in Figure 4 are remarkably similar. Averaging them, and proceeding cyclically around the ring as above, they are 1.84, 1.94, 1.84, and 2.22 Å. Because of the excellent diffraction data, the high occupancies at the Cu2+ positions, and the agreement seen between the two independent 6-rings, these distances should be reliable. They may be compared to the corresponding calculated distances in the first of four drawings in Figure 3 in ref 22, 2.04, 2.01, 2.09, and 2.21 Å, and in the third20 (1.97, 2.07, 1.88, 2.37 Å), fourth22 (2.04, 1.94, 1.96, 2.65 Å), fifth20 (1.90, 2.08, 2.01, 2.19 Å), and sixth22 (1.91, 2.09, 1.92, 2.34 Å) drawings in Figure 5 in ref 23. The latter calculations were done for various numbers and placements of Al atoms in 6-rings: one Al,20 two Al meta,22 two Al meta,20 and three Al alternating,22 respectively. The crystallographic result is for some kind of average over the numbers and placements of Al atoms in the crystals studied. Peterson has calculated the distribution of Al atoms to be expected for zeolite Y at various Si/Al compositions.48 Interpolating to the Si/Al ratio of the crystals used in this work, 1.56, the percentages of S6Rs with one Al atom, with two Al atoms meta, with two Al atoms para, and with three Al atoms alternating is about 1%, 39%, 21%, and 39%, respectively. Note that the crystallographically determined bond lengths agree relatively well with those calculated for 6-rings with three Al atoms alternating,22 and with the values calculated by Pierloot et al.20 for a 6-ring with two Al atoms meta. The mean of those two calculations, which by Peterson’s calculation should describe 39% + 39% = 78% of the S6Rs in the two crystals studied and therefore be the position of the crystallographic least-squares minimum, is 1.905, 2.085, 1.965, and 2.265 Å. These distances trend similarly and compare relatively well with the mean of those found crystallographically, 1.841(4), 1.940(4), 1.841(4), and 2.22(3) Å.
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’ ASSOCIATED CONTENT
bS
Supporting Information. Observed and calculated structure factors squared with esds for both crystal structures. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: seff@hawaii.edu (K.S.);
[email protected] (W.T.L.).
’ ACKNOWLEDGMENT The authors wish to thank the staff at beamline 6B MXI of the Pohang Light Source, Korea, for assistance during data collection. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2011-0026300). ’ REFERENCES (1) Warzywoda, J.; Bac, N; Jansen, J. C.; Sacco, Jr., A. J. Cryst. Growth 2000, 220, 140, and references therein. (2) Warzywoda, J; Valcheva-Traykova, M; Rossetti, G. A., Jr.; Bac, N; Joesten, R.; Suib, S. L.; Sacco, A., Jr. J. Cryst. Growth 2000, 220, 150. (3) Lee, H. S.; Seff, K. J. Phys. Chem. 1981, 85, 397. (4) Lee, H. S.; Cruz, W. V.; Seff, K. J. Phys. Chem. 1982, 86, 3562. (5) Kieger, S.; Delahay, G.; Coq, B.; Neveu, B. J. Catal. 1999, 183, 267. (6) Gallezot, P.; Ben Taarit, Y.; Imelik, B. J. Catal. 1972, 26, 295. (7) Maxwell, I. E.; de Boer, J. J. J. Phys. Chem. 1975, 79, 1874. (8) Fowkes, A. J.; Ibberson, R. M.; Rosseinsky, M. J. Chem. Mater. 2002, 14, 590. (9) Haniffa, R. M.; Seff, K. Microporous Mesoporous Mater. 1998, 25, 137. (10) Ho, K.; Lee, H. S.; Leano, B. C.; Sun, T.; Seff, K. Zeolites 1995, 15, 377. (11) Mochida, I.; Hayata, S.; Kato, A.; Seiyama, T. J. Catal. 1969, 15, 314. (12) Mochida, I.; Hayata, S.; Kato, A.; Seiyama, T. J. Catal. 1971, 23, 31. (13) Yu, J.-S.; Lee, H.; Kevan, L. Stud. Surf. Sci. Catal. 1988, 38, 273. (14) Naccache, C. M.; Ben Taarit, Y. J. Catal. 1971, 22, 171. (15) Tsuruya, S.; Tsukamoto, M.; Watanabe, M.; Masai, M. J. Catal. 1985, 93, 303. (16) Tsuruya, S.; Okamoto, Y.; Kuwada, T. J. Catal. 1979, 56, 52. (17) Tsutsumi, K.; Fuji, S.; Takahashi, H. J. Catal. 1972, 24, 146. (18) Kaushik, V. K.; Ravindranathan, M. Zeolites 1992, 12, 415. (19) Maxwell, I. E. private communication, 1985. (20) Pierloot, K.; Delabie, A.; Groothaert, M. H.; Schoonheydt, R. A. Phys. Chem. Chem. Phys. 2001, 3, 2174. (21) Berthomieu, D.; Goursot, A.; Ducere, J.-M.; Delahay, G.; Coq, B.; Martinez, A. Stud. Surf. Sci. Catal. 2001, 135, 2618. (22) Berthomieu, D.; Ducere, J.-M.; Goursot, A. J. Phys. Chem. B 2002, 106, 7483. (23) Berthomieu, D.; Delahay, G. Catal. Rev. 2006, 48 (3), 269. (24) Milanesio, M.; Croce, G; Viterbo, D.; Pastore, H. O.; Mascarenhas, A.J.d.S.; Munsignatti, E.C.d.O.; Meda, L. J. Phys. Chem. A 2008, 112, 8403–8410. (25) Lim, W. T.; Seo, S. M.; Wang, L.; Lu, G. Q.; Seff, K. Microporous Mesoporous Mater. 2010, 129, 11. (26) Ferchiche, S.; Valcheva-Traykova, M.; Vaughan, D. E. W.; Warzywoda, J.; Sacco, A., Jr. J. Cryst. Growth 2001, 222, 801. (27) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307. (28) Bruker-AXS (ver. 6.12), XPREP, Program for the Automatic Space Group Determination; Bruker AXS Inc.: Madison, Wisconsin, USA, 2001.
7. CONCLUSIONS Two single crystals of largely Cu2+-exchanged zeolite Y were prepared from aqueous solution and dehydrated fully. Their crystal structures were determined by single-crystal X-ray diffraction techniques. Some Na+ ions remained after Cu2+ exchange, and H+ ions must be present for charge balance. In both structures, Cu2+ ions occupy sites I0 and II, preferring 0 I . The Cu2+ ions at both sites have planar, severely distorted square, 4-coordination; they coordinate not only to the three trigonally arranged oxygen atoms of their 6-rings, but also to a fourth oxygen atom of those 6-rings. A small number of Na+ ions occupy a second I0 site and a second site II in both crystals. With increasing Cu2+-exchange time, both the Cu2+ and H+ content of the zeolite increased as the Na+ content and the unit cell constant decreased. 973
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(29) Loewenstein, W. Am. Mineral. 1954, 39, 92. (30) Breck, D. W. Zeolite Molecular Sieves; John Wiley & Sons: New York, 1974, p 93. (31) Van Bekkum, H.; Flanigen, E. M.; Jacobs, P. A.; Jansen, J. C. Introductions to Zeolite Science and Practice; Elsevier: New York, 2001, p 44. (32) Sheldrick, G. M. SHELXL97, Program for the Refinement of Crystal Structures; University of G€ottingen: Germany, 1997. (33) Doyle, P. A.; Turner, P. S. Acta Crystallogr., Sect. A 1968, 24, 390. (34) International Tables for X-ray Crystallography; Ibers, J. A., Hamilton, W. C., Eds.; Kynoch Press: Birmingham, England, 1974; Vol. IV, pp 7198. (35) Cromer, D. T. Acta Crystallogr. 1965, 18, 17. (36) International Tables for X-ray Crystallography; Ibers, J. A., Hamilton, W. C., Eds.; Kynoch Press: Birmingham: England, 1974; Vol. IV, pp 148150. (37) Handbook of Chemistry and Physics, 70th ed., CRC Press: Cleveland, OH, 1989/1990; p F-187. (38) Kim, C. W.; Jung, K. J.; Heo, N. H.; Kim, S. H.; Hong, S. B.; Seff, K. J. Phys. Chem. C 2009, 113, 5164. (39) Dempsey, E. J. Phys. Chem. 1969, 73, 3660. (40) Plevert, J.; Di Renzo, F.; Fajula, F.; Chiari, G. J. Phys. Chem. B 1997, 101, 10340. (41) Feuerstein, M.; Lobo, R. F. Chem. Mater. 1998, 10, 2197. (42) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochman, M. Advanced Inorganic Chemistry, 6th ed.; Wiley-Interscience: New York, 1999; p 865. (43) Åsbrink, S.; Norrby, L.-J. Acta Crystallogr., Sect. B 1970, 26, 8. (44) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon Press: Oxford, 1986; p 1120. (45) Lim, W. T.; Seo, S. M.; Kim, G. H.; Lee, H. S.; Seff, K. J. Phys. Chem. C 2007, 111, 18294. (46) Bae, D.; Seff, K. Microporous Mesoporous Mater. 1999, 33, 265. (47) Bae, D.; Seff, K. Microporous Mesoporous Mater. 2000, 40, 219. (48) Peterson, B. K. J. Phys. Chem. B 1999, 103, 3145.
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