Crystal Structures of Vacuum-Dehydrated Ni2+

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J. Phys. Chem. C 2009, 113, 5164–5181

Crystal Structures of Vacuum-Dehydrated Ni2+-Exchanged Zeolite Y (FAU, Si/Al ) 1.69) Containing Three-Coordinate Ni2+, Ni8O4 · xH2O8+, x e 4, Clusters with Near Cubic Ni4O4 Cores, and H+ Cheol Woong Kim, Ki Jin Jung, and Nam Ho Heo* Laboratory of Structural Chemistry, Department of Applied Chemistry, Kyungpook National UniVersity, Daegu, 702-701 Korea

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

Karl Seff* Department of Chemistry, UniVersity of Hawaii, 2545 The Mall, Honolulu, Hawaii 96822-2275 ReceiVed: December 9, 2008; ReVised Manuscript ReceiVed: January 20, 2009

Five single crystals of vacuum-dehydrated Ni2+-exchanged zeolite Y (Ni-Y) were variously prepared by the exchange of Na-Y or K-Y (Na71- or K71-Si121Al71O384, Si/Al ) 1.69) with Ni2+ using flowing aqueous 0.05 M Ni(NO3)2 at 294 or 353 K, followed by vacuum dehydration at 2.0 × 10-6 Torr and 623 or 723 K. Their crystal structures and chemical compositions were determined using synchrotron X-radiation and energy dispersive X-ray (EDX) analyses to give Nin-Y, where 25.3 e n e 34.1 per unit cell: |NiuMVHw[Ni8O4 · xH2O]y[Al(OH)4]z|[Si121+zAl71-zO384]-FAU, where 21.4 e u e 24.2, M ) Na and/or Ca, 3.3 e V e 13.1, 9.5 e w e 21.6, 2 e x e 4, 0.3 e y e 3.2, and 0 e z e 2.5. In each of the five crystal structures (space group Fd3jm; mean a ) 24.47 Å), Ni2+ is found at sites I, I′, a second I′, II′, and sometimes II; unexpectedly, Na+ and Ca2+ are found at another site II. Two extra-framework oxygen positions (Oe), one in the sodalite cavity and the other nearby in the supercage, are seen on 3-fold axes in all crystals. They bond to Al3+ and Ni2+ ions in the sodalite cavities to form Al(OeH)4- and Ni8(Oe)4 · xH2Oe8+ clusters with Ni4(Oe)4 cores, and with framework oxygens (Of) to give trigonal bipyramidal Ni(Of)3(Oe)22+ and trigonal pyramidal Ni(Of)3Oe2+. At site I (centers of double 6-rings) in the Na-Y crystal that was Ni2+-exchanged at 294 K, ∼14 Ni2+ ions per unit cell each coordinate octahedrally to six framework oxygen atoms. The Ni2+ ions at the first I′ site are 3-coordinate near planar in all crystals. The Ni2+ ions at the second I′ site are members of the Ni4O4 cores, tetrahedrally distorted cubes with Ni-O ) 2.199(13) Å and O-Ni-O ) 79.6(9)° in a representative crystal; these Ni2+ ions are distorted octahedral with three Oe’s of the cluster and three Of’s. The Ni2+ ions at sites II′ and II are 3-, 4-, or 5-coordinate. Al(OH)4- from framework dealumination centers some sodalite cavities in four of the five crystals; their number increased with both ion-exchange and dehydration temperatures, suggesting that dealumination occurred during both processes. The number of Ni2+ ions per unit cell increases with Ni2+-exchange temperature and is greater with K-Y than with Na-Y, perhaps because the larger K+ ions are more loosely held. The leaving cation affects the Ni2+ distribution over the available sites, perhaps via the level of Ni2+-exchange. Both a greater degree of Ni2+-exchange and a higher dehydration temperature cause more Ni8O4 · xH2O8+ clusters to form, leaving fewer Ni2+ ions at sites I and II. As more Ni8O4 · xH2O8+ formed, more H+ ions were produced. Some H+ and some 3- and 4-coordinate Ni2+ ions are easily accessible for catalysis. 1. Introduction 1.1. Catalysis. Transition metal cations and their oxides in nanodimensional porous media play key roles in various catalytic reactions. To understand why, it is important to know their structural and chemical characteristics in these media. This understanding can foster the development of more advanced catalytic systems that may bring enormous benefit to mankind. The catalytic reductions of the environmental pollutants NOx and SOx have drawn much attention for several decades. Because these molecules are small, size selectivity is desired to maximize efficiency and to avoid unwanted products. For each process, a

specific zeolite is usually selected as the host material because of its unique structural properties. Ni2+-exchanged zeolites (Ni-Y) are frequently employed not only for the reduction of NOx1-4 but also for the hydrogenation5 and oxidation6-9 of CO. This is due to the advantageous combination of the catalytic activity of Ni2+ (or its oxide, hydroxide, or H+ from hydrolysis) and the shape selectivity of the zeolite. To better understand these catalysts and to improve their activity and selectivity, a moderate amount of crystallographic work has been done on Ni-Y.10-15 Ni2+ positions and possible coordination environments including residual water molecules and their fragments (OH- or O2-) have been reported.

10.1021/jp810846x CCC: $40.75  2009 American Chemical Society Published on Web 03/11/2009

Crystal Structures of Vacuum-Dehydrated Ni-Y This information is critical in identifying the active sites (by seeing which features of the structure are accessible to the reactants), in judging the stability of transition states, and in rationalizing the shape selectivity for specific products. 1.2. Structure. The positions of the Ni2+ ions in a Ni2+exchanged natural faujasite single crystal (Ni24-FAU, Ni24Si134Al58O384 (incomplete formula), Si/Al ) 2.31) vacuum dehydrated at 673 K were first determined crystallographically by Olson.10,11 He reported that Ni2+ ions occupied about twothirds of the sites I (double 6-ring, D6R, hexagonal prism; 10.6 of the 16 per unit cell), with each forming a nearly perfect octahedron with six D6R framework oxygen atoms. The remaining Ni2+ ions were distributed among four other crystallographically distinct positions: site I′ (3.1 Ni2+ ions per unit cell), another site I′ (refined as 5.8 oxygen atoms but discussed as 1.9 Ni2+ ions), site II (6.4 Ni2+ ions), and site II′ (1.9 Ni2+ ions). The Ni2+ ions at the second I′ position (0.081, 0.081, 0.081) were too far (2.8 Å) from the nearest framework oxygen atoms to bind to them. Accordingly, he suggested that these Ni2+ ions bonded to extra-framework oxygen atoms near site II′ (1.9 per unit cell at (0.161, 0.161, 0.161)). Each of these, described as residual water molecules which had not been removed by vacuum dehydration at 673 K, bonded also to a Ni2+ ion at site II′ (Ni(II′)-O ) 2.0 Å) on a different 3-fold axis near the center of a single 6-ring (S6R) in the same sodalite cavity. This was supported by the similar occupancies (ca. 1.9 atoms) at each of those three positions, Ni(I′), O, and Ni(II′). Dempsey and Olson reported some linear relationships between the occupancy of the site II′ extra-framework oxygen atoms and the occupancies of some divalent cations (Ca2+,10,16 Sr2+,10 and Ni2+ 11) at sites I, I′, and II′ without offering a chemical explanation.12 Apparent anomalies in the distribution of Ni2+ ions among the cation sites were attributed to the small Ni2+ ionic radius and the large framework Si/Al ratio (2.43). However, the Ni2+ ions at the second site I′ still looked odd because each had only one strong bond (2.0 Å to an extraframework oxygen atom) and three very long approaches, 2.8 Å, to framework oxygen atoms. It was perhaps for this reason that this Ni2+ position was refined as oxygen atoms. Using powder X-ray diffraction (XRD), the influence of thermal treatment and exchange level on the positions of the Ni2+ ions in dehydrated Ni-Y zeolites (Ni10,14,or19Si136Al56O384 (incomplete formula), Si/Al ) 2.42) was studied by Gallezot and Imelik.13 They found that the Ni2+ positions and their populations depended both on the degree of Ni2+-exchange and the dehydration conditions. As more water molecules were removed by dehydration at temperatures up to 873 K, more Ni2+ ions moved into the D6Rs (site I). The maximum Ni2+ population at site I reached 12 per unit cell, in agreement with Dempsey and Olson’s reasoning for faujasite-type zeolites with Si/Al > 2.0.12 Overall, Ni2+ ions were located at four different sites, I, I′, a second I′, and II′. All Na+ ions were found at site II, and residual water molecules were again found near site II′. In most of their structures, because the occupancies of these water molecules were close to three times those of the second site-I′ Ni2+ ions, they concluded that each of these Ni2+ ions is again bonded weakly to three framework oxygen atoms (Ni-O3 ) 2.6 Å) and strongly to three residual water molecules near site II′ (Ni-O ) 2.2 Å) to give a severely distorted 6-coordinate octahedral geometry, rather than the distorted tetrahedral geometry suggested by Olson.11 These extra-framework oxygen atoms were removed by dehydration at a relatively low temperature, 573 K, contrary to Olson’s observation, but some Ni2+ ions remained at the second site-I′. Upon dehydration at

J. Phys. Chem. C, Vol. 113, No. 13, 2009 5165 873 K, the Ni2+ positions at I′, the second I′, and II′, and the oxygen position near site II′, were all depopulated as the number of Ni2+ ions at site I climbed to ∼12 per unit cell.13 They noted that these 12 Ni2+ ions would not be accessible to sorbed molecules. Although results from powder diffraction are often less reliable than those from single-crystal work, the additional extraframework oxygen atoms that Gallezot and Imelik found near site II′, as compared with Olson’s results, should not be dismissed. Perhaps they are due to differing Ni2+-exchange levels. Gallezot and Imelik had shown clearly that the distribution of Ni2+ ions among the two I′ sites and site II′, and of the extra-framework oxygen atoms near site II′, depended on the Ni2+-exchange level.13 Apparently the second site-I′ Ni2+ ions interact strongly with (bond to) these extra-framework oxygen atoms. Still, a sensible explanation for the dehydration behavior of the extra-framework oxygen atoms, and of the geometry about the second site-I′ Ni2+ position, remained lacking. Other structural studies of Ni-Y powders,17 including that reported by Couves et al.,18 could not resolve this dilemma. On the basis of extended X-ray absorption fine structure (EXAFS) and electron spectroscopy for chemical analysis (ESCA) studies of dehydrated alkali-treated Ni-Y, Sano and co-workers reported the presence of oligomeric clusters of nickel atoms each with 3.5 oxygen neighbors at 2.08 Å and 3.0 second nearest nickel neighbors at 2.99 Å.7,9 They suggested that extraframework nickel oxides had formed within the zeolite. These nickel-oxide clusters were shown to be at least partly responsible for the high catalytic activity found in CO oxidation.6 In another EXAFS study, this time of a Ni-Y sample that had been vacuum dehydrated at 573 K, Dooryhee et al. reported the presence of one extra-framework oxygen atom 1.87 Å from Ni2+ and three at 2.02 Å,14 confirming that a simple vacuum dehydration process was sufficient to form some sort of nickel oxide structure within the zeolite. These methods did not allow the placements of these ill-defined clusters within the zeolite to be determined. More recently, the partial structures of fully dehydrated Ni2+exchanged zeolite Y (Ni30Na7Cl12-Y, Si/Al ) 2.49, prepared by the solid-state ion-exchange of Na-Y with NiCl2) and of its D2O sorption complex were studied by pulsed-neutron diffraction.15 In Ni30Na7Cl12-Y, the 30 Ni2+ ions occupied 4 crystallographic sites: 4 were at site I; 18 were again found at two I′ sites, 2.09(1) Å and 2.50(2) Å from three framework oxygens; and 8 were at site II′, 2.13(1) Å from framework oxygens. In addition, 12 Cl- ions per unit cell were found near site II′ where extra-framework oxygen atoms had been found in dehydrated Ni-Y; each bridged between two nonequivalent site-I′ Ni2+ ions (Ni-Cl ) 2.60(1) Å and 2.73(4) Å). Six of the eight sodalite units per unit cell contained a (NiClNiClNi)4+ cluster. The remaining two contained four Ni2+ ions tetrahedrally arranged at site II′. In Ni30Na7Cl12-Y · nD2O, each Ni2+ ion at site I′ coordinated octahedrally to three framework oxygens and either to three terminal D2O molecules or to one terminal and two bridging D2O molecules. Each site-III′ Ni2+ ion coordinated to four framework oxygens in a distorted square-planar manner. 1.3. Objectives. In this work, we attempted to maximize the number of Ni2+ ions, and therefore the number of extraframework oxygens with which they associate, in single crystals of zeolite Y to be able to characterize them well crystallographically. Zeolite Y with Si/Al ) 1.69 (Na-Y, Na71Si121Al71O384) was used as the starting material. (Crystals available to us with higher Si/Al values,19-21 more like the zeolite Y material commonly used in catalytic applications, were

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TABLE 1: Experimental Conditions and Crystallographic Data crystal cross section (mm) ion-exchange for K+ (T (K), days, mL) washing with DI water (T (K), days, mL) ion-exchange for Ni2+ (T (K), days, mL) dehydration (T (K), days, Torr) temperature for data collection (T (K)) X-ray source wavelength (Å) space group, no. unit cell constant, a (Å) maximum 2θ for data collection (deg) no. of unique reflections measured (m) no. of reflections (Fo > 4σ(Fo)) no. of variables (s) data/parameter ratio (m/s) weighting parameters (a/b) final error indices R1b R2c goodness of fitd

(1) Ni25.3-Y

(2) Ni28.5-Y

(3) Ni27.5-Y

(4) Ni33-Y

(5) Ni34.1-Y

0.15 294, 3, 15 623, 2, 2 × 10-6 294(1) PLS(4A MXW)a 0.7700 Fd3jm, 227 24.459(1) 60.61 868 849 64 13.3 0.0531/140.71

0.15 353, 5, 19 623, 2, 2 × 10-6 294(1) PLS(4A MXW)a 0.7699 Fd3jm, 227 24.501(1) 60.72 880 794 66 12.0 0.1320/114.75

0.15 294, 2, 15 294, 1, 4 294, 8, 15 723, 2, 2 × 10-6 294(1) PLS(4A MXW)a 0.7700 Fd3jm, 227 24.528(1) 60.69 867 739 64 11.5 0.1175/83.373

0.15 294, 5, 13.5 294, 1, 1 353, 3, 14 623, 2, 2 × 10-6 294(1) PLS(4A MXW)a 0.7699 Fd3jm, 227 24.466(1) 60.59 796 537 65 8.3 0.1318/76.949

0.15 294, 2, 17 294, 1, 5 353, 3, 16.5 723, 2, 2 × 10-6 294(1) PLS(4A MXW)a 0.7000 Fd3jm, 227 24.406(1) 56.55 827 494 64 7.7 0.1007/187.21

0.0439 0.1305 1.245

0.0624 0.2066 1.136

0.0656 0.2108 1.110

0.0726 0.2281 1.123

0.0664 0.2356 1.086

a Beamline 4A MXW of the Pohang Light Source, Korea. b R1 ) Σ|Fo - |Fc|/ΣFo; R1is calculated using only those reflections for which Fo > 4σ(Fo). c R2 ) [Σw(Fo2 - Fc2)2/Σw(Fo2)2]1/2 is calculated using all unique reflections measured. d Goodness of fit ) [Σw(Fo2 - Fc2)2/(m - s)]1/2.

too small for single-crystal crystallography.) It was hoped that the framework of this zeolite would be more stable upon Ni2+exchange and dehydration than that of Na-X with Si/Al ) 1.09, which had sustained considerable damage upon exchange with Ni2+,22 Co2+,23 and many other cations,24 sometimes not until subsequent dehydration. To maximize the degree of Ni2+exchange and to learn the effect of temperature, Ni2+-exchange would be done at two different temperatures, 293 and 353 K. Also, in addition to Na-Y, Ni2+-exchange would be done with K-Y, fully K+-exchanged zeolite Y;19 perhaps, the more loosely bound K+ ions would be more easily replaced by Ni2+ than Na+. If the Ni2+-exchange of K-Y were incomplete, the residual K+ ions could be more readily identified crystallographically than Na+ because of their larger ionic size and greater scattering power. High quality diffraction data would be obtained using synchrotron X-ray radiation. 2. Experimental Section 2.1. Synthesis. Large colorless single crystals of sodium zeolite Y (|Na71(H2O)x|[Si121Al71O384]-FAU, Na71-Y · xH2O, Na71-Y, or Na-Y; Si/Al ) 1.69) were prepared by Lim and co-workers19,20 using the synthetic method of Vaughan et al.21 Each of five single crystals, colorless octahedra about 0.15 mm in cross section, was lodged in its own fine Pyrex capillary. Pale green crystals of hydrated Ni2+-exchanged zeolite Y (Ni-Y) were prepared by the dynamic (flow) ion-exchange of these crystals in their capillaries at either 294 or 353 K with aqueous 0.05 M Ni(NO3)2 solution7,25 (pH ) 6.2; Aldrich 99.999% Ni(NO3)2 · 6H2O, 6.6 ppm Hg, 5.0 ppm Na, 1.8 ppm Mg, 1.3 ppm Cu, 0.7 ppm Mn, 0.4 ppm Fe; deionized water). Two of these Ni-Y crystals were cautiously vacuum dehydrated (a heating rate of +25 K/h was used) to give crystals 1 and 2; see Table 1 for nomenclature and detailed experimental conditions. While these conditions were maintained, the hot contiguous downstream lengths of the vacuum system, including a Pyrex U-tube of zeolite 4A beads fully activated in situ, were cooled to ambient temperature to prevent the later movement of water molecules to the crystal from more distant parts of the vacuum system (that had not been baked out).10,19 Finally,

crystals 1 and 2, both now black, were allowed to cool and were sealed under vacuum in their capillaries by torch. Before Ni2+-exchange, the remaining three crystals of Na-Y were first fully K+-exchanged. This was done in their capillaries using aqueous 0.05 M KNO3 (Aldrich 99.999% KNO3, 10.2 ppm Na, 0.6 ppm B, 0.2 ppm Ca) at 294 K for 5 days.19 The resulting colorless crystals, hydrated K71-Y (K-Y),19 were thoroughly washed with flowing deionized water at 294 K for 2 days or more (see Table 1). One was exposed to a flowing stream of aqueous 0.05 M Ni(NO3)2 (described above) for about 3 days at 294 K (later to become crystal 3). The remaining two crystals (later to become crystals 4 and 5) were similarly exchanged, but at 353 K.7,25 All three crystals (hydrated Ni-Ys) were now pale green. They were dehydrated at 2 × 10-6 Torr for about 2 days (see Table 1) at 623 K (crystal 4) and 723 K (crystals 3 and 5) as above. All three crystals, now black as above, were sealed under vacuum in their capillaries. 2.2. Diffraction. X-ray diffraction data for the resulting crystals were collected at 294(1) K on an ADSC Quantum210 detector at Beamline 4A MXW of The Pohang Light Source. Crystal evaluation and data collection were done with a detectorto-crystal distance of 60 mm. Preliminary cell constants and an orientation matrix for each crystal 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 by using the program HKL2000.26 The reflections were successfully indexed by the automated indexing routine of the DENZO program.26 About 90 000 reflections were harvested for each crystal by collecting 72 sets of frames with a 5° scan and an exposure time of 1 s per frame. These highly redundant data sets were corrected for Lorentz and polarization effects; negligible corrections for crystal decay were also applied. The space group Fd3jm was determined by the program XPREP.27 A summary of the experimental and crystallographic data is presented in Table 1. 2.3. SEM-EDX Analysis. After diffraction data collection, the crystals were removed from their capillaries (exposed to the atmosphere) for energy dispersive X-ray (EDX) analysis using an EDX spectrometer within a scanning electron micro-

Crystal Structures of Vacuum-Dehydrated Ni-Y

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Figure 1. Representative EDX spectra of Ni-Y crystals. Upper, crystal 1; lower, crystal 4.

TABLE 2: Compositionsa of Crystals 1-5 by Crystallographic (SXRD)b and SEM-EDXc Analyses 1 crystal no./element Si Al Ni Na Ca

d

no. at %e no.d at %e no.d at %e no.d at %e no.d at %e

SXRD 121 52.5 71 30.8 25.3 11.0 13.1 5.7 -

2 EDX 50.8 30.0 13.2 5.9 -

SXRD 121 52.2 71 30.6 28.5 12.3 11.2 4.8 -

3 EDX 49.9 30.7 14.7 4.6 -

SXRD 121 54.0 71 31.7 27.5 12.3 4.6 2.1

4 EDX 52.2 31.0 10.1 2.2 4.4

SXRD 121 52.7 71 31.0 33.0 14.4 4.4 1.9

5 EDX 52.1 34.6 9.1 1.2 3.0

SXRD 121 52.7 71 31.0 34.1 14.9 3.3 1.4

EDX 52.6 32.8 12.5 1.2 0.8

a Composition calculated without oxygen atoms. b Composition determined by least-squares refinement of single-crystal X-ray diffraction data. c Composition determined by energy dispersive X-ray spectroscopy using a scanning electron microscope. d Number of atoms per unit cell. e Atomic percent of the element.

scope (SEM). Each was intentionally broken to expose a fresh surface and was attached to a piece of carbon attach tape. The composition of each crystal was determined using an Oxford INCA X-Sight EDX spectrometer within a JSM6480LV-SEM instrument at 294 K and 1 × 10-5 Torr. The resulting spectra are shown in Figure 1. A comparison of the compositions as determined by crystallographic and SEM-EDX analyses is presented in Table 2. 3. Structure Determination 3.1. Initial Procedures Common to All Five Crystals. Fullmatrix least-squares refinement (SHELXL97)28 was done on F2 using only those reflections for which Fo2 > 4σ(Fo2) for each crystal. Each refinement began with the atomic parameters of just the framework atoms ((Si,Al), O1, O2, O3, and O4) in K71-Y.19 Using isotropic thermal parameters, the initial refine-

ments converged nicely to the high error indices (defined in a footnote to Table 1) R1 ) 0.44, 0.37, 0.35, 0.47, and 0.46, and R2 ) 0.83, 0.80, 0.78, 0.78, and 0.78, respectively, for the five crystals. The progress of structure determination as subsequent peaks were found on difference Fourier functions and identified as nonframework atoms is given in Table 3. The near final refinements with anisotropic thermal parameters for Ni1 and all framework atoms (including the O3 and O3′ positions; see next paragraph) led to convergence with R1 ) 0.045, 0.063, 0.066, 0.073, and 0.067, and R2 ) 0.132, 0.207, 0.213, 0.228, and 0.245, respectively. In all five structures, some D6Rs have a Ni2+ ion (Ni1) at their centers (site I). When Ni1 is present in a given D6R, neither adjacent I′ site should be occupied; this would give unnecessarily short Ni2+-Ni2+ distances (see Figure 2). Therefore, because of the relatively high occupancies observed, some of the


Kim et al.

TABLE 3: Steps of Structure Determination As Nonframework Atomic Positions Were Found in Dehydrated Ni2+-Y error indicesi

number of ions or atoms per unit cell step/atom

Ni1

crystal 1, Ni25.3-Y 1a 2b 14.6(4) 3b 13.8(2) 4b 13.8(1) 5b 13.8(1) 6b 13.7(1) 7b 13.8(1) 8b 13.9(1) 9b 13.9(1) 10b,c 13.8(1) 11b-e 13.8(1) crystal 2, Ni28.5-Y 1a 2b 9.9(4) 3b 9.2(3) 4b 10.0(2) 5b 9.7(2) 6b 10.4(2) 10.6(2) 7b 8b 10.5(2) 9b 10.5(2) 10b 10.9(2) 11b,c 10.4(1) 12b-d 10.4(1) 13b-e 10.4(1) 14b-e,g 10.4(1) 15b-e,g,h 10.4(1) crystal 3, Ni27.5-Y 1a 2b 7.9(4) 3b 6.0(3) 4b 5.6(2) 5b 6.4(2) 6b 5.7(1) 7b 6.6(1) 8b 6.7(1) 9b 7.0(1) 10b 7.1(1) 11b,c 6.9(1) 12b-d 7.1(1) 13b-e 6.9(1) 14b-e,g 7.0(1) 15b-e,g 7.0(1) crystal 4, Ni33-Y 1a 2b 7.2(7) 3b 6.4(4) 4b 7.5(3) 5b 5.3(2) 6b 6.9(2) 6.3(2) 7b 8b 6.3(2) 9b 6.3(2) 10b 6.5(2) 11b,c 6.3(2) 12b-d 6.3(2) 13b-e 6.3(2) 14b-f 6.4(2)

Ni1′1

1.1(4) 1.1(3) 1.1(3)

2.3(4) 1.8(4) 2.1(4) 1.6(3) 4.0(4) 4.0(4) 3.9(4) 3.8(4) 3.7(4)

5.6(6) 5.4(5) 5.8(5) 4.9(4) 4.9(4) 4.7(3) 4.8(3) 4.7(3) 4.6(3) 4.7(3)

3.2(5) 3.3(5) 3.4(4) 3.6(6) 3.4(4) 3.4(4) 3.4(4) 3.1(4)

Ni1′2

1.5(2) 1.5(2) 1.6(2) 2.0(2) 1.1(4) 1.1(3) 1.1(3)

4.7(3) 5.7(3) 4.6(3) 4.9(3) 5.0(3) 5.0(3) 5.3(3) 5.3(3) 5.3(3) 5.3(2) 5.4(2)

7.7(4) 4.3(5) 5.3(4) 5.2(4) 5.1(4) 5.1(3) 5.1(3) 5.1(3) 5.1(3) 5.4(3) 5.4(3)

12.3(5) 12.6(4) 11.8(3) 9.4(4) 9.4(4) 9.2(4) 9.3(4) 9.3(4) 9.2(4) 9.2(4) 9.8(4)

Ni2′

4.8(4) 4.3(5) 4.1(6) 4.2(5) 4.2(4) 4.3(4)

11.1(7) 6.3(5) 8.5(4) 8.2(4) 8.5(3) 7.6(4) 7.5(4) 6.1(5) 5.5(4) 5.5(4) 5.6(4) 5.3(2) 5.4(2)

10.7(5) 7.3(5) 9.4(4) 8.6(3) 8.6(3) 8.7(3) 6.5(4) 6.0(4) 6.0(4) 6.0(4) 6.0(4) 5.4(3) 5.4(3)

9.4(16) 7.7(23) 8.4(21) 8.3(21) 7.5(23) 13.7(3)

Ni2

6.8(3) 7.0(3) 4.6(5) 4.9(5) 5.0(4) 5.0(4) 5.0(4) 5.0(4)

5.7(4) 4.8(3) 6.0(3) 6.1(3) 3.5(5) 3.6(5) 3.3(5) 3.5(4) 3.5(4) 3.5(4) 3.6(4) 3.6(4)

5.2(4) 3.9(3) 5.0(3) 5.6(3) 5.8(3) 4.3(4) 4.3(4) 4.8(4) 4.2(4) 4.6(4) 5.0(3) 5.0(3)

12.7(7) 12.7(5) 11.8(4) 13.9(3) 13.5(3) 13.6(3) 4.4(15) 6.1(23) 5.4(21) 5.4(21) 6.2(23) -

Ni22

13.4(4) 7.4(3) 7.5(3) 5.2(5) 5.0(4) 4.9(4) 4.9(4) 4.8(4) -

3.5(5) 3.5(5) 3.7(4) 3.9(4) -

3.4(4) 2.8(4) 2.7(3) -

4.3(4) 3.9(3) 4.592) 4.5(2) 4.1(2) 2.8(6) 2.9(5) -

remaining D6Rs must host two Ni2+ ions at I′ sites. Because there are two I′ sites, three combinations are possible. Additional D6Rs are likely to host only one Ni2+ ion at one of the I′ sites, and it is possible that some D6Rs do not host any Ni2+ ions. Accordingly, the O3 position, to which all of these Ni2+ ions of partial occupancy bond, must be a mean of many closely arrayed positions. Furthermore, the average O3 position

Na

Ca

Al

O5

4.8(10) 5.1(10) 6.3(10) 6.3(10)

13.1(11)

0.6(3) 0.4(3) 0.9(4) 0.9(4) 0.9(4) 0.9(4) 1.0(4)

10.8(11) 10.8(11) 10.9(11) 11.2(13)

4.2(5) 4.3(5) 4.6(5) 4.6(5)

4.4(8) 4.4(8) 4.4(9)

1.3(4) 1.5(4) 1.5(3) 1.5(3) 1.4(3) 1.6(3) 1.6(3) 1.6(4)

0.5(3) 0.593) 0.9(4) 0.8(4) 0.8(3) 0.8(3) 0.8(4)

13.3(15) 15.6(15) 14.7(14) 15.3(15) 13.0(13) 9.6(10) 9.6(10) 9.6(10) 9.7(10) 9.7(10)

10.4(12) 10.0(12) 7.9(10) 7.9(9) 9.3(9) 8.2(8) 9.3(9) 9.4(9) 11.8(11)

16.2(13) 20.5(14) 20.9(14) 20.6(13) 21.3(13) 21.2(12) 21.1(12) 21.2(12) 21.2(12)

O6/O6′

R1

R2

5.2(8) 5.3(8) 5.5(8) 5.4(7) 5.4(7)

0.4378 0.1830 0.1074 0.0675 0.0609 0.0544 0.0515 0.0490 0.0473 0.0451 0.0439

0.8302 0.6000 0.2783 0.1888 0.1720 0.1609 0.1516 0.1435 0.1377 0.1322 0.1305

7.0(12) 10.1(12) 10.2(12) 10.1(12) 10.4(11) 5.4(2)/3.6(4)

0.3737 0.2058 0.1660 0.1392 0.1042 0.0866 0.0798 0.0741 0.0731 0.0714 0.0633 0.0635 0.0633 0.0631 0.0624

0.7973 0.5893 0.5493 0.4707 0.3743 0.2586 0.2384 0.2324 0.2306 0.2259 0.2072 0.2086 0.2081 0.2081 0.2066

4.5(11) 5.0(9) 3.4(8) 4.3(9) 4.5(9) 4.6(9)

0.3506 0.2495 0.1926 0.1671 0.1256 0.1090 0.0875 0.0847 0.0747 0.0724 0.0661 0.0667 0.0662 0.0663 0.0656

0.7750 0.6324 0.5678 0.5117 0.4258 0.3881 0.2694 0.2622 0.2351 0.2283 0.2127 0.2180 0.2135 0.2135 0.2108

5.7(20) 4.6(18) 4.5(20) 4.5(19) 4.8(20)

0.4660 0.4184 0.2479 0.1505 0.1326 0.1039 0.0832 0.0815 0.0772 0.0739 0.0728 0.0731 0.0731 0.0726

0.7773 0.7199 0.6278 0.4478 0.3901 0.3378 0.2533 0.2511 0.2357 0.2317 0.2276 0.2294 0.2294 0.2281

(-0.06006, -0.06006, 0.03119) did not make satisfactory distances to any Ni2+ ions: it was somewhat to very far from all of the Ni2+ ions at sites I and I′ (Ni1-O3 ) 2.213(3) Å, Ni1′1-O3 ) 2.18(6) Å, and Ni1′2-O3 ) 2.53(4) Å in crystal 1, for example). For comparison, the sum of the conventional ionic radii of Ni2+ and O2-, 0.69 + 1.32 Å, respectively, is 2.01 Å.29 The rather long Ni1-O3 distance in the D6Rs at site



TABLE 3: Continued error indicesi

number of ions or atoms per unit cell step/atom

Ni1

crystal 5, Ni34.1-Y 1a 2b 6.8(6) 3b 7.2(5) 4b 4.3(3) 5b 4.1(2) 6b 4.2(2) 7b 4.7(2) 8b 4.8(2) 9b 4.8(2) 10b 4.8(2) 11b,c 4.8(2) 12b-d 4.9(2) 13b-e 4.8(4) 14b-f 4.9(2) 15b-g 4.9(2)

Ni1′1

3.5(5) 4.2(6) 4.3(6) 4.3(5) 4.2(5) 4.3(5) 4.3(5) 3.6(6) 3.8(4)

Ni1′2

11.8(5) 12.3(5) 11.5(4) 12.1(5) 11.9(5) 12.0(5) 12.0(5) 12.1(5) 12.1(5) 12.1(5) 13.0(5) 12.7(3)

Ni2′

4.6(16) 3.5(16) 3.4(16) 3.4(15) 3.9(38) 12.6(3) 12.7(3)

Ni2

14.2(9) 13.9(4) 14.5(4) 14.5(4) 12.1(3) 12.6(3) 8.0(16) 8.9(15) 9.2(15) 9.1(15) 8.7(37) -

Ni22

3.6(4) 4.1(3) 4.193) 3.9(3) 2.1(6) 2.1(6) -

Na

Ca

3.3(9) 3.2(8) 3.2(9) 3.3(9)

Al

2.3(7) 2.1(5) 2.0(5) 2.3(6) 2.5(7) 2.6(7) 2.3(6) 2.5(6)

O5

27.2(22) 22.0(18) 24.2(15) 23.1(15) 22.8(14) 22.9(13) 22.6(13) 22.9(13) 22.8(13) 23.1(13) 22.7(13)

O6/O6′

R1

R2

6.9(21) 6.5(20) 6.1(21) 6.2(19) 6.1(21) 5.9(20)

0.4575 0.3689 0.2613 0.1556 0.1182 0.1100 0.0848 0.0803 0.0733 0.0695 0.0673 0.0675 0.0675 0.0665 0.0664

0.7817 0.7492 0.6740 0.4738 0.4212 0.3868 0.2766 0.2637 0.2616 0.2476 0.2454 0.2479 0.2473 0.2374 0.2356

a Only the atoms of the zeolite framework were present in the initial structure model. They were all refined isotropically. b Framework atoms were allowed to refine anisotropically. c The framework oxygens at O3 are divided into two positions, O3 and O3′. d The Ni2+ ions at Ni22, site II, were ultimately refined as Na+ in crystals 1 and 2 and Ca2+ in crystals 3-5. The Ca2+ position holds both Ca2+ and Na+ ions. e Ni1 in all crystals and Ni2′ in crystal 5 were refined anisotropically. f Anisotropic thermal parameters were used for Ni2′ (Ni2′ and Ni2 are combined into a single position) in crystals 4 and 5, Ni1′2 in crystal 4, and Ni1′1 and Ni1′2 in crystal 5. g Constrained occupancies at Ni1′2 and Ni2′ and fixed occupancies for extra-framework oxygens at O5 are used in crystals 3 and 5. h O6 is divided into O6 and O6′; the occupancies at Ni1′2, Ni2′, and O6 are constrained to be equal; the occupancies at Ni2 and O6′ are constrained to be equal. i Defined in footnotes to Table 1.

Figure 2. 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 1-4. 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 rule65 would be obeyed. Extra-framework cation positions are labeled with Roman numerals or the letter U.

I may be attributed to the high coordination number at Ni1 and to the rigidity of the zeolite framework, and the Ni1′1-O3 distance might somehow be explained, but the Ni1′2-O3 distance is unreasonably too long. An additional difference-Fourier peak, very close to O3 and like O3 bonded to two T (Si or Al) atoms, was observed in all five crystals. Its inclusion allows a better description of the actual geometry about Ni1′1 and especially Ni1′2. The occupancy at O3 was divided between O3 and the new O3′ position, with Ni1 bonded to the oxygens at O3 (those at O3′ are too distant) and the Ni2+ ions at sites I′ bonded to the O3′ position (those at O3 are too distant). The resulting bond lengths and angles were all more reasonable. Our excellent synchrotron diffraction data nicely supported this separation for all five crystals, including anisotropic refinement at both O3 and O3′ (see Table 4); a further separation was not possible.

The coordination geometry about the positions that would ultimately be named Na or Ca was seen to be three-coordinate pyramidal with M-O distances too long to be Ni-O in all structures (see Table 5). This caused us to suspect that this electron density was due to an unexpected Na+ impurity. At this point, SEM-EDX analysis was done, confirming the presence of Na+ impurities in all five crystals and revealing the presence of Ca2+ impurities in crystals 3-5 (see Figure 1 and Table 2). Na+ and Ca2+ are nearly the same size29 and therefore are likely to be at similar positions; they were refined as a single mixed position in crystals 3-5. This position was refined to a reasonable geometry for all five crystals (see Table 5). In crystals 3-5, it was refined as Ca2+ ions at Ca because SEM-EDX analysis (Table 2) indicated that the calcium content dominated. 3.2. Procedures Unique to Each Crystal. 3.2.1. Crystal 1. The anisotropic refinement of Ni2′, Ni2, and Na showed no significant anisotropy nor improvement in error indices. The occupancy at Ni1′2 suggested that a small number of Ni4O4 cores, 1.1/4 ) 0.3 per unit cell (seen better (with greater occupancy) in the other four structures), are present (see Tables 6 and 7). About 10 extra-framework oxygen atoms (5(1) at O5 and 5.4(7) at O6) appeared to coordinate axially to the Ni2+ ions at S6R sites, either at Ni2′ or at Ni2 (virtually ca. 0.3 Å apart). (The combined Ni2′/Ni2 position when refined anisotropically yielded an unreasonably long thermal parameter and unreasonable distances to O5 and O6 (1.73(6) and 2.30(6) Å), so these two positions were refined with isotropic thermal parameters as tabulated in Table 4.) The Ni-O distances indicate that the ∼4.3(4) Ni2+ ions per unit cell at Ni2′ each coordinate to O5 and O6 (Ni2′-O5 ) 1.89(6) Å and Ni2′-O6 ) 2.10(6) Å); approximately 1.1 of these could also be members of (Ni4O4)Ni4 · 4H2O8+ clusters (see section 4.3.3.3 and Table 7). Ni2 coordinates neither to O5 (Ni2-O5 ) 2.45(7) Å is too long) nor to O6 (Ni2-O6 ) 1.54(6) Å is too short). Including their coordination to three S6R oxygen atoms, Ni2′ is therefore 5-coordinate trigonal bipyramidal and Ni2 is only 3-coordinate (see Table 7).


Kim et al.

TABLE 4: Positional, Thermal, and Occupancy Parametersa occupancyc atoms

Wyckoff position

crystal 1, T(Si,Al) O1 O2 O3 O3′ O4 Ni1 Ni1′1 Ni1′2 Ni2′ Ni2 Na O5 O6

Ni25.3-Y 192(i) 96(h) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 32(e) 32(e) 32(e) 32(e)

crystal 2, T(Si,Al) O1 O2 O3 O3′ O4 Ni1 Ni1′1 Ni1′2 Ni2′ Ni2 Na Al O5 O6 O6′

Ni28.5-Y 192(i) 96(h) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 32(e) 32(e) 8(a) 32(e) 32(e) 32(e)

crystal 3, T(Si,Al) O1 O2 O3 O3′ O4 Ni1 Ni1′1 Ni1′2 Ni2′ Ni2 Ca Al O5 O6

Ni27.5-Y 192(i) 96(h) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 32(e) 32(e) 8(a) 32(e) 32(e)

crystal 4, T(Si,Al) O1 O2 O3 O3′ O4 Ni1 Ni1′1 Ni1′2 Ni2′ Ca Al O5 O6

Ni33-Y 192(i) 96(h) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 32(e) 8(a) 32(e) 32(e)

site

x

y

z

I I′ I′ II′ II II II′ II

-5302(3) -1086(1) -33(1) -594(1) -720(13) 1667(1) 0d 566(23) 707(10) 2118(7) 2251(9) 2378(5) 1671(13) 2614(13)

12194(3) 1086(1) -33(1) -594(1) -720(13) 1667(1) 0d 566(23) 707(10) 2118(7) 2251(9) 2378(5) 1671(13) 2614(13)

3552(3) 0d 1467(1) 313(2) 257(29) 3155(2) 0d 566(23) 707(10) 2118(7) 2251(9) 2378(5) 1671(13) 2614(13)

I I′ I′ II′ II II U II′ II II

-5259(4) -1075(2) -30(1) -595(2) -742(4) 1681(1) 0d 530(10) 744(3) 2135(4) 2273(10) 2410(6) 1250d 1664(8) 2557(28) 2645(27)

12247(4) 1075(2) -30(1) -595(2) -742(4) 1681(1) 0d 530(10) 744(3) 2135(4) 2273(10) 2410(6) 1250d 1664(8) 2557(28) 2645(27)

I I′ I′ II′ II II U II′ II

-5265(5) -1060(2) -22(1) -607(4) -724(4) 1698(2) 0d 570(5) 759(3) 2063(3) 2204(7) 2370(6) 1250d 1656(8) 2563(20)

I I′ I′ II′ II U II′ II

-5159(6) -1036(2) -14(2) -601(7 -734(6) 1690(2) 0d 553(13) 755(3) 2113(1) 2430(9) 1250d 1664(5) 2558(29)

U11 or Uisob

U22

U33

U23

U13

U12

82(5) 338(11) 357(11) 203(16) 235(137) 308(11) 192(5) 621(198) 235(119) 437(41) 379(36) 224(32) 925(248) 594(167)

11(4) 338(11) 357(11) 203(16) 235(137) 308(11) 192(5)

53(5) 342(17) 229(15) 253(18) 1237(446) 496(21) 192(5)

-33(3) -96(9) 11(9) 8(12) 232(190) -28(11) -14(3)

-14(3) -96(9) 11(9) 8(12) 232(190) -28(11) -14(3)

-3(3) 51(13) 95(13) -59(16) 152(178) 105(13) -14(3)

3582(4) 0d 1473(2) 319(3) 228(10) 3166(2) 0d 530(10) 744(3) 2135(4) 2273(10) 2410(6) 1250d 1664(8) 2557(28) 2645(27)

132(6) 480(16) 375(14) 163(19) 247(47) 390(16) 222(9) 1260(156) 447(32) 377(28) 369(51) 219(46) 855(432) 610(115) 416(185) 476(333)

42(6) 480(16) 375(14) 168(19) 247(47) 390(16) 222(9)

75(6) 378(23) 292(20) 300(30) 1014(153) 597(29) 222(9)

-30(3) -111(12) 15(11) -2(18) 27(62) 41(15) -7(5)

-12(3) -111(12) 15(11) -2(18) 27(62) 41(15) -7(5)

-17(3) -66(21) 89(17) -90(22) -55(56) 147(19) -7(5)

12323(4) 1060(2) -22(1) -607(4) -724(4) 1698(2) 0d 570(5) 759(3) 2063(3) 2204(7) 2370(6) 1250d 1656(8) 2563(20)

3602(4) 0d 1465(2) 340(7) 228(8) 3177(2) 0d 570(5) 759(3) 2063(3) 2204(7) 2370(6) 1250d 1656(8) 2563(20)

394(8) 819(21) 645(17) 350(36) 643(59) 658(19) 369(12) 865(61) 707(41) 471(26) 667(48) 409(60) 1044(271) 1008(90) 701(223)

269(7) 819(21) 645(17) 350(36) 643(59) 658(19) 369(12)

295(7) 604(28) 601(26) 502(60) 1068(132) 887(37) 369(12)

-50(4) -14216) -9(16) -86(31) 48(56) -6(19) -12(8)

-24(4) -142(16) -9(16) -86(31) 48(56) -6(19) -12(8)

-32(4) -137(26) 108(22) -85(39) 103(65) 206(24) -12(8)

12322(6) -1036(2) -14(2) -601(7) -734(6) 1690(2) 0d 553(13) 755(3) 2113(1) 2430(9) 1250d 1664(5) 2558(29)

3657(6) 0d 1483(6) 349(13) 199(10) 3198(3) 0d 553(13) 755(3) 2113(1) 2430(9) 1250d 1664(5) 2558(29)

417(11) 880(30) 651(23) 428(70) 647(76) 625(24) 402(21) 955(141) 617(35) 682(20) 371(91) 813(475) 995(93) 352(199)

263(10) 880(30) 651(23) 428(70) 647(76) 625(24) 402(21)

292(10) 623(38) 606(37) 605(133) 967(159) 933(49) 402(21)

-44(6) -78(19) 68(20) -35(57) 177(69) 36(25) -22(12)

-28(6) -78(19) 68(20) -35(57) 177(69) 36(25) -22(12)

-35(6) -329(37) 70(28) -53(74) 194(84) 150(30) -22(12)

617(35) 682(20)

617(35) 682(20)

53(25) 116(15)

53(25) 116(15)

53(25) 116(15)

3.2.2. Crystal 2. After all extra-framework atomic positions were identified (Table 3, crystal 2, step 13), it was seen that the occupancies at Ni1′2 and Ni2′ were essentially the same, 5.3(3) and 5.6(4), respectively. This strongly suggested that the Ni4O4 core (Ni1′2 and O5 in the sodalite unit, see section 4.3.3.3) should be extended to include Ni2′ to become (Ni4O4)Ni4 via additional bonds between the O5s of the Ni4O4 core and the Ni2′s. This was confirmed by a refinement with a

varied

constrained

fixed 192 96 96 84 12 96

13.8(1) 1.1(3) 1.1(3) 4.3(4) 5.0(4) 13.1(11) 6.3(10) 5.4(7) 192 96 96 62 34 96 10.4(1) 3.9(4) 5.3(3) 5.6(4) 3.5(4) 10.8(11) 0.9(4) 9.6(10) 10.1(12)

10.4(1) 3.7(4) 5.4(2)e 5.4(2)e 3.6(4)e 11.2(13) 1.0(4) 9.7(10) 5.4(2)e 3.6(4)e 192 96 96 42 54 96

6.9(1) 4.7(3) 5.1(3) 6.0(4) 4.6(4) 4.3(5) 1.6(3) 9.3(9) 4.3(9)

7.0(1) 4.7(3) 5.4(3)e 5.4(3)e 5.0(3) 4.6(5) 1.6(4) 11.8(11)f 4.6(9) 192 96 96 38 58 96

6.4(2) 3.1(4) 9.8(4) 13.7(3) 4.4(9) 0.8(4) 21.2(12) 4.8(20)

constraint between the occupancies at these two Ni positions (Ni1′2 and Ni2′) which yielded unchanged R values (Table 3, crystal 2, line 14) and a very reasonable O5-Ni2′ distance, 2.00(4) Å. It was expected that the extra-framework oxygen atoms at O6 would coordinate to Ni2′, extending the (Ni4O4)Ni4 clusters further to (Ni4O4)Ni4O4 (see section 4.3.3.3), but the Ni2′-O6 distance (1.32(6) Å) was too short. Because there are two kinds of Ni2+ ions around the S6Rs (Ni2′ and Ni2), there



TABLE 4: Continued occupancyc atoms

Wyckoff position site

crystal 5, Ni34.1-Y T(Si,Al) 192(i) O1 96(h) O2 96(g) O3 96(g) O3′ 96(g) O4 96(g) Ni1 16(c) Ni1′1 32(e) Ni1′2 32(e) Ni2′ 32(e) Ca 32(e) Al 8(a) O5 32(e) O6 32(e)

I I′ I′ II′ II U II′ II

x

y

z

-5137(9) 12352(7) 3667(8) -1026(3) 1026(3) 0d -13(2) -13(2) 1493(3) -593(9) -593(9) 329(20) -743(5) -743(5) 196(10) 1701(2) 1701(2) 3201(3) 0d 0d 0d 526(12) 526(12) 526(12) 764(3) 764(3) 764(3) 2124(2) 2124(2) 2124(2) 2447(16) 2447(16) 2447(16) 1250d 1250d 1250d 1658(6) 1658(6) 1658(6) 2604(29) 2604(29) 2604(29)

U11 or Uisob

U22

U33

U23

U13

U12

varied

493(14) 350(13) 367(12) -51(9) -25(9) -34(8) 933(37) 933(37) 778(52) -93(27) -93(27) -301(46) 719(28) 719(28) 667(47) 24(26) 24(26) 94(36) 426(105) 426(105) 786(221) -74(97) -74(97) -124(116) 658(73) 658(73) 1085(155) 101(77) 101(77) 178(83) 686(30) 686(30) 987(60) 6(30) 6(30) 145(38) 400(32) 400(32) 400(32) -31(21) -31(21) -31(21) 4.9(2) 1478(207) 1478(207) 1478(207) -240(167) -240(167) -240(167) 3.6(6) 770(26) 770(26) 770(26) 55(26) 55(26) 55(26) 13.0(5) 794(28) 794(28) 794(28) 165(25) 165(25) 165(25) 12.6(3) 510(171) 3.2(9) 1644(465) 2.3(6) 961(65) 23.1(13) 507(240) 6.1(21)

constrained fixed 192 96 96 29 67 96 4.9(2) 3.8(4) 12.7(3)e 12.7(3)e 3.3(9) 2.5(6) 22.7(13)f 5.9(20)

a Positional and thermal parameters ×104 are given except for the T(Si,Al) positions which are given as ×105. 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[-2π2a-2(U11h2 + U22k2 + U33l2 + 2U12hk + 2U13hl + 2U23kl)]. c Occupancy factors are given as the number of atoms or ions per unit cell. d Exactly, by symmetry. e These occupancies were constrained in the final least-squares refinement. f The occupancy of the extra-framework oxygens at O5 was fixed.

should be two kinds of O6 oxygen atoms in the supercage. To test this proposition, O6 was divided into two positions, O6 and O6′, and the occupancy at each position was constrained to be equal to that of (Ni4O4)Ni4 and Ni2, respectively. This led to the lowest R1 value (see Table 3, crystal 2, line 15) and the best geometry (Ni2′-O6 ) 1.80(12) Å and Ni2-O6′ ) 1.57(13) Å) with a retention of the total occupancy at the O6/ O6′ positions (see Tables 4 and 5). Moreover, the occupancy at O5, 9.7(10), is very close to the sum of that at O6 and 4 times that at Al (5.4 + 4 × 1.0 ) 9.4). Thus, it appears that there are 5.4/4 (Ni4O4)Ni4O4 clusters per unit cell in the sodalite units (and extending into the supercages) of crystal 2, and 3.6 4-coordinate near trigonal-pyramidal Ni2+ ions at Ni2 (see Table 7). 3.2.3. Crystal 3. Once again, the occupancies at Ni1′2 and Ni2′ were nearly the same after free refinement. When these two values were constrained to be equal (supposition of the (Ni4O4)Ni4 cluster), R1 increased only slightly (0.001; Table 3, crystal 3, steps 13 and 14). Furthermore, the occupancy at O6 was also close to that of Ni1′2 (and Ni2′), indicative of a (Ni4O4)Ni4O4 cluster. However, because some terminal oxygen atoms (O6 in the supercage) might have been lost to vacuum dehydration at 723 K, the occupancy at O6 was not constrained; it converged to 3.4(7) per (Ni4O4)Ni4O4 unit, or (Ni4O4)Ni4Ox, where x ) 3 or 4. Finally, a refinement with the occupancy at O5 fixed to be the sum of that at Ni1′2 (equal to that at Ni2′; for (Ni4O4)Ni4Ox) and 4 times that at Al (for Al(OH)4-) resulted in the lowest R1 value (see Table 3) and better geometry (see Table 5). The resulting geometry about Ni2′ indicates that it is Ni2′ rather than Ni2 that bonds to O6 (Ni2′-O6 ) 2.12(8) Å, Ni2-O6 ) 1.53(9) Å) and O5 (Ni2′-O5 ) 1.73(3) Å and Ni2-O5 ) 2.33(4) Å). The Ni2′-O5 distance is still observed to be too short, probably because O5 is an averaged position: most O5s bond to Al. Thus, there are 5.4/4 ) 1.35 (Ni4O4)Ni4Ox clusters (containing 5-coordinate Ni2′) and 5.0 3-coordinate Ni2+ ions at Ni2 per unit cell (see Tables 6 and 7). 3.2.4. Crystal 4. As with the other crystals, Ni1 had been refined anisotripically. Attempts to refine Ni1′1 and Ni1′2 anisotropically were unsuccessful; large estimated standard deviations (esds) were seen, and the error indices did not decline. The short distance between the isotropically refined Ni2′ and Ni2 positions (virtually only 0.35 Å apart) and the somewhat poor geometry they made with extra-framework oxygen atoms

(Ni2′-O5 ) 1.76(5) Å and Ni2′-O6 ) 2.05(13) Å; Ni2-O5 ) 2.11(8) Å and Ni2-O6 ) 1.70(14) Å) suggested that these two Ni2+ positions should be refined anisotropically as a single position, Ni2′. This led to more acceptable distances, Ni2′-O5 ) 1.91(2) Å and Ni2′-O6 ) 1.88(12) Å. A subsequent refinement with this combined position (Ni2′) and the highly populated Ni1′2, both refined anisotropically, led to lowered error indices (Table 3, crystal 4, step 14). The occupancies found at Ni1′2 (9.8(4)), O5 (21.2(12)), Ni2′ (13.7(3)), and O6 (4.8(20)), together with the consideration that the terminal oxygen position, O6, might not be fully occupied after vacuum dehydration, indicate that each unit cell of crystal 4 contains ∼9.8/4 ) 2.45 Ni8O4 · xH2O8+ clusters and ∼3.9 4or 5-coordinate Ni2+ ions at Ni2′, each with O5 and perhaps O6 at axial positions. Together with those in Al(OH)4-, the occupancy at O5 necessary for this arrangement must be ∼9.8 + 3.9 + 4 × 0.8 ) 16.9, comparable to the refined occupancy at O5, 21.2(12) (see Tables 4, 6, and 7). 3.2.5. Crystal 5. After all Ni positions were refined anisotropically, several additional refinements were carried out to identify nickel-oxide clusters within and extending out of some sodalite cavities (see Table 3). First of all, neither Ni2′ nor Ni2 showed suitable bond lengths to O5 and O6 nor appropriate occupancies for the expected 3- or 5-coordinate geometries. Because they were only ∼0.3 Å apart (virtually), we attempted to refine them as a single anisotropic position (Ni2′). This was done with a small decrease in R1 (Table 3, crystal 5, step 14), and it yielded much better distances (Ni2′-O5 ) 1.97(2) Å and Ni2′-O6 ) 2.03(12) Å). The occupancies found at Ni1′2 (13.0(5)), the combined position at Ni2′ (12.6(3)), and O5 after accounting for Al(OH)4-, (23(1) - 4 × 2.3(6) ) 14(3)), are all about the same; that at O6 is still much smaller (6(2)). This indicates that (Ni4O4)Ni4 clusters are present (Ni1′2, Ni2′, and O5) in some sodalite units with partial occupancy of terminal O6 oxygens ((Ni4O4)Ni4Ox, x ∼ 2). Introducing the constraint that the occupancies at Ni1′2 and Ni2′ be equal resulted in the lowest error indices (Table 3, crystal 5, step 15) and excellent geometry (see Table 5). The result, therefore, indicates that each unit cell of crystal 5 contains 12.7/4 ) 3.2 (Ni4O4)Ni4Ox clusters, x ) 5.9(20)/4 ) ∼1.5 (see Tables 4, 6, and 7). Extensive efforts to distinguish the two kinds of O5s, one in Al(OH)4- and the other as a member of the Ni4O4 core of (Ni4O4)Ni4Ox, were unsuccessful. Finally, all Ni2+ positions were


Kim et al.

TABLE 5: Selected Interatomic Distances (Å) and Angles (deg)a (1) Ni25.3-Y

(3) Ni27.5-Y

mean of 1-3

(4) Ni33-Y

(5) Ni34.1-Y

mean of 4 and 5

mean, overall

T-O1 T-O2 T-O3 (weighted) T-O3 T-O3′ T-O4 mean

1.6464(16) 1.6577(14) 1.713 1.721(3) 1.65(3) 1.6301(10) 1.662

1.6474(23) 1.6587(18) 1.708 1.724(3) 1.678(11) 1.6256(14) 1.660

1.634(3) 1.6543(20) 1.705 1.712(9) 1.699(10) 1.6172(16) 1.653

1.628(3) 1.657(3) 1.700 1.697(16) 1.703(12) 1.6232(24) 1.652

1.620(3) 1.658(3) 1.706 1.722(23) 1.699(12) 1.617(3) 1.650

1.635 1.657 1.71 1.72 1.686 1.623 1.655

Ni1-O3 Ni1′1-O3′ Ni1′2-O3′ Ni1′2-O5 O5 · · · O2 Ni2′-O2 Ni2′-O5 Ni2′-O6/O6′ Ni2-O2 Ni2-O5 Ni2-O6/O6′ Na-O2 Ca-O2 Al-O5

2.193(5) 2.08(8) 2.36(7) 2.40(4) 3.02(4) 2.143(5) 1.89(6) 2.10(6) 2.150(6) 2.46(7) 1.54(6) 2.291(8) -

2.205(6) 2.00(3) 2.380(25) 2.278(17) 3.037(24) 2.122(4) 2.00(4) 1.80(12)/2.16(12) 2.153(9) 2.59(5) 1.21(13)/1.57(13) 2.334(12) 1.76(3)

2.263(17) 2.027(22) 2.425(22) 2.218(18) 3.042(24) 2.165(6) 1.73(3) 2.12(9) 2.122(5) 2.33(4) 1.53(9) 2.282(10) 1.72(3)

2.25(3) 1.94(3) 2.335(24) 2.241(13) 2.976(17) 2.076(6) 1.902(22) 1.89(12) 2.337(20) 1.755(22)

2.20(4) 1.92(3) 2.344(25) 2.199(13) 2.978(18) 2.042(7) 1.972(24) 2.03(12) 2.34(3) 1.723(23)

2.22 1.99 2.37 2.27 3.01 2.110 1.90 2.02 2.14 2.46 1.47 2.31 2.32 1.74

O1-T-O2 O1-T-O3 (weighted) O1-T-O3 O1-T-O3′ O1-T-O4 O2-T-O3 (weighted) O2-T-O3 O2-T-O3′ O2-T-O4 O3-T-O4 (weighted) O3-T-O4 O3′-T-O4 mean

112.12(12) 106.65 105.00(18) 118.2(21) 110.91(16) 106.93 107.01(20) 106.4(21) 107.60(16) 112.51 114.21(19) 100.6(15) 109.5

111.95(16) 108.75 102.69(24) 119.8(7) 110.55(23) 106.8 107.9(3) 105.2(7) 107.48(21) 110.9 116.3(3) 100.8(6) 109.4

112.41(18) 109.0 99.9(4) 116.0(5) 111.3(3) 105.3 108.1(6) 103.2(6) 107.82(23) 110.6 117.3(5) 105.4(6) 109.4

112.30(23) 108.0 96.1(7) 115.8(6) 110.9(3) 105.9 110.4(11) 103.0(8) 108.9(3) 110.3 117.8(9) 105.4(7) 109

112.4(3) 109.3 95.3(10) 115.3(6) 111.4(4) 105.4 109.4(16) 103.6(8) 107.8(4) 110.3 120.3(11) 105.9(7) 109

112.2 108.3 99.8 117.0 111.0 106.1 108.6 104.3 107.9 110.9 117.2 103.6 109.4

T-O1-T T-O2-T T-O3-T (weighted) T-O3-T T-O3′-T T-O4-T mean

127.24(21) 134.95(20) 121.6 120.5(3) 129.3(41) 158.1(3) 135.5

130.4(3) 134.8(3) 123.1 121.0(4) 126.9(14) 155.6(4) 136.0

135.0(4) 136.8(3) 125.0 124.2(11) 125.7(13) 153.4(4) 137.6

139.7(6) 134.0(4) 123.7 124.1(20) 123.4(15) 148.1(5) 136.4

142.4(6) 132.8(5) 123.0 121.0(27) 123.8(15) 146.9(6) 136.3

134.9 134.7 123 122 125 152.4 136.3

O3-Ni1-O3

88.68(19)/91.32(19)/ 180.0b 108.5(30) 91.6(20) 74.9(22) 96.3(12)/168.7(24) 119.16 118.6(3) 96.8(7) 83.2(7) 118.0(5) 81.7(10) 98.3(10) 107.1(5) 103.3(17) 115.1(14)

88.18(23)/91.82(23)/ 180.0b 114.6(10) 89.8(6) 78.1(12) 95.8(6)/172.1(9) 119.26(13) 95.0(5) 85.0(4)/85.0(4) 116.6(7) 79.2(11) 100.8(11)/100.8(11) 103.4(7) 109.47c 100.7(9) 117.2(8)

87.0(5)/93.0(5)/ 180.0b 109.0(7) 85.8(7) 78.8(12) 97.6(6)/175.3(10) 115.7(3) 102.1(4) 77.9(4) 119.5(2) 86.1(8) 93.9(8) 106.9(6) 109.47c 100.2(10) 117.6(8)

86.1(9)/93.9(9)/ 180.0b 112.3(15) 87.4(8) 79.5(8) 96.5(5)/174.7(8) 118.64(9) 96.75(22) 83.25(21) 99.6(11) 109.47c 99.6(7) 118.1(5)

87.3(15)/92.7(15)/ 180.0b 115.6(11) 87.5(7) 79.6(9) 96.4(5)/174.6(8) 118.99(10) 95.8(3) 84.2(3) 97.6(19) 109.47c 99.6(7) 118.2(6)

87.5/92.5/ 180.0b 112 88.4 78 96.5/173

O3′-Ni1′1-O3′ O3′-Ni1′2-O3′ O5-Ni1′2-O5 O3′-Ni1′2-O5 O5-H · · · O2 O2-Ni2′-O2 O2-Ni2′-O5 O2-Ni2′-O6/O6′ O2-Ni2-O2 O2-Ni2-O5 O2-Ni2-O6/O6′ O2-Na-O2 O2-Ca-O2 O5-Al-O5 Ni1′2-O5-Ni1′2 Ni1′2-O5-Ni2′ c

(2) Ni28.5-Y

3.03 2.14 1.87 2.05

118 98 82 118

2.977 2.059 1.94 1.96

118.82 96.28 83.73

118.2 97.3 82.7 118.0 82.3 97.7 105.3 101.4 109.47c 101 117

a The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding value. b Exact value by symmetry. The tetrahedral angle.

refined anisotropically with the occupancies at Ni1′2 and Ni2′ constrained to be equal and a fixed occupancy at O5 equal to the sum of values needed for the (Ni4O4)Ni4Ox and Al(OH)4groups (12.7 + 4 × 2.5 ) 22.7; Table 3, crystal 5, step 15, and Table 4). 3.3. Final Refinements and Other Crystallographic Details. The final cycles of refinement were carried out with some constrained occupancies at Ni1′2 and Ni2′ for the observed (Ni4O4)Ni4Ox8+ clusters in some crystals and fixed occupancies

at some O5 positions as indicated in Tables 3 and 4. Crystals treated more harshly (see Table 1) refined to higher R values, perhaps due to crystal damage. In the final cycle of least-squares refinement for each crystal, all shifts were less than 0.1% of their corresponding esds. The final structural parameters are presented in Table 4, and selected interatomic distances and angles are given in Table 5. The distributions and plausible arrangements of extra-framework species are tabulated in Tables 6 and 7.



TABLE 6: Distribution and Occupanciesa of Extra-Framework Speciesb in Vacuum Dehydrated Ni-Y Ni positions crystal exch. dehyd. varied/ no. M+ -Yc T (K)d T (K) conste

Ni1

Ni1′1

Ni1′2

Ni2′

Σ

Ni2

Na

Ca

-

O5, calch

O6/O6′g

4 × Al + Ni1′2 + Ni2′

O5f

Al

25.3 13.1(11) -

O positions

6.3(10)

Σ

1

Na-Y

294

623

varied

13.8(1) 1.1(3) 1.1(3)

4.3(4)

5.0(4)

2

Na-Y

353

623

varied conste

10.4(1) 3.9(4) 5.3(3) 10.4(1) 3.7(4) 5.4(2)e

5.6(4) 5.4(2)e

3.5(4) 28.7 10.8(11) 3.6(4)e 28.5 11.2(13) -

3

K-Y

294

723

varied conste

6.9(1) 7.0(1)

4.7(3) 5.1(3) 4.7(3) 5.4(3)e

6.0(4) 5.4(3)e

4.6(4) 5.0(3)

27.3 27.5 -

4.3(5) 1.6(3) 9.3(9) 4.3(9) 4.6(5) 1.6(4) 11.8(11) i 4.6(9)

13.6(9) 11.5(9) 16.4(11) 11.8(11)

4

K-Y

353

623

varied

6.4(2)

3.1(4) 9.8(4)

13.7(3)

-

33.0 -

4.4(9) 0.8(4) 21.2(12)

4.8(20)

26.0(12) 16.9(12)

5

K-Y

353

723

varied conste

4.9(2) 4.9(2)

3.6(6) 13.0(5) 12.6(3) 3.8(4) 12.7(3)e 12.7(3)e

34.1 34.1 -

3.2(9) 2.3(6) 23.1(13) 3.3(9) 2.5(6) 22.7(13)i

6.1(21) 5.9(20)e

29.2(13) 22.2(13) 28.6(13) 22.7(13)

0.9(4) 9.6(10) 1.0(4) 9.7(10)

5.4(7)

11.7(10) 4.3(10)

10.1(12) 19.7(10) 8.9(10) 5.4(2)e/3.6(4)e 18.7(10) 9.4(10)

a Number of atoms per unit cell. b Ni2+, Na+, Ca2+, Al3+, and extra-framework oxygens. c Ni2+-exchange was done on this zeolite. d Temperature for Ni2+-exchange. Constrained. f Extra-framework oxygen atoms near cation site II′ in the sodalite cavity. g Extra-framework oxygen atoms in the supercage. h Calculated total occupancies for O5 as members of three secondary structures: Al(OH)4- (4 × Al), Ni4O4 core of Ni8O4 · xH2O8+ (Ni1′2), and 5-coordinate Ni2+ (Ni2′, see Table 7). The values in this column may be compared with the observed values in the O5 column. i This occupancy was fixed in the final refinement. e

TABLE 7: Arrangements and Numbers of Extra-Framework Species in Ni-Y crystal no./ EF speciesa Ni and O

EF positionsa Ni1 Ni1′1 Ni1′2, O5, Ni2′, O6d x, in [(Ni4O4)Ni4 · xH2O]8+ Σ Ni1′2 Ni2′/O5, O6 Σ Ni2′ Ni2/O6

sites

chemical speciesb 2+

CN of Mn+ c

I I′ I′ and II′

Ni Ni2+ (Ni4O4)Ni4 · xH2O8+

6 3 6 and 5

II′

[O-Ni-O]2+ e Ni2+

5 3

II

[Ni-O]2+ e Ni2+

4 3

Σ Ni2 Σ Ni

1

2

3

13.8(1) 1.1(3) 0.3(4) 8(4) 1.1(3) 3.2(10)

10.4(1) 3.7(4) 1.4(2) 4.0(4) 5.4(2)

4.3(4)

5.4(2) 3.6(4)

4

5

7.0(1) 4.7(3) 1.4(3) 3.4(9) 5.4(3)

6.4(2) 3.1(4) 2.5(4) 2(2) 9.8(4) 3.9(20)

4.9(2) 3.8(4) 3.2(3) 2(2) 12.7(3)

5.4(3)

13.7(3)

12.7(3)

5.0(4) 5.0(4) 25.3(4)

3.6(4) 28.5(4)

5.0(3) 5.0(3) 27.5(3)

0.0 33.0(4)

0.0 34.1(4)

1.6(4)

0.8(4)

2.5(6)

4.6(5)

4.4(9)

3.3(9)

Al and O

Al, O5

U

[Al(OH)4]-

4

0.0

1.0(4)

Na

Na

II

Na+

3

13.1(11)

11.2(13)

Ca

Ca

II

Ca2+

3

O

O5

near II′

O2-/OH-/H2O

O6

near II

H2O

ΣO5f observed ΣO6g ΣO6h observed calcd observed

4.3(10) 6.3(10) 3.2(7) 5.4(7) 5.4(7) 9.7(10) 11.7(10)

9.4(10) 9.7(10) 3.6(4) 9.0(4) 9.0(4) 18.4(10) 18.7(10)

11.8(11) 11.8(11) 0.0(9) 4.6(9) 4.6(9) 16.4(11) 16.4(11)

16.9(12) 21.2(12) 3.9(20) 8.7(20) 4.8(20) 17.8(20) 26.0(20)

22.7(13) 22.7(13) 0.0(20) 5.9(20) 5.9(20) 28.6(20) 28.6(20)

calcdi

9.5(11)

13.6(13)

17.6(11)

15.8(12)

21.6(13)

H a

ΣO

O2-/OH-/H2O

H (required)

H+

b

c

2+

3+

+

2+

Extra-framework. Chemical formulas of extra-framework species. Coordination numbers of Ni , Al , Na , and Ca , including three framework oxygen atoms (not shown in the formula) when present. d Esds are estimated using only those of the Ni2+ ions (Ni1′2 and Ni2′). e This group may have a lesser charge if some of the coordinated water molecules have dissociated. f The sum of the O5s in (Ni4O4)Ni4 · xH2O8+, [O-Ni-O]2+, and Al(OH)4-. g O6s in [O-Ni-O]2+ and [Ni-O]2+. h O6s in (Ni4O4)Ni4 · xH2O8+, [O-Ni-O]2+, and [Ni-O]2+. i 71 - [2 × (Ni1 + Ni1′1 + Ni1′2 + Ni2′ + Ni2 + Ca) + 1 × (Na)] + 8(Ni4O4)Ni4 · xH2O8+). Additional H+ ions will be present if some of the coordinated water molecules have dissociated.

Fixed weights were used initially for each crystal. The final weights were assigned using the formula w ) 1/[σ2(Fo2) + (aP)2 + bP], where P ) [max(Fo2,0) + 2Fc2]/3, with a and b as refined parameters (see Table 1). Atomic scattering factors for Ni2+, Na+, Ca2+, K+, O-, and (Si,Al)1.82+ were used.19,30,31 The function describing (Si,Al)1.82+ is the weighted mean of the Si4+, Si0, Al3+, and Al0 functions (Si/Al ) 1.69). All scattering factors were modified to account for anomalous dispersion.32,33

4. Description of the Structures 4.1. 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 cell has 8 supercages, 8 sodalite cavities, 16 D6Rs, 16 12-rings,

5174 J. Phys. Chem. C, Vol. 113, No. 13, 2009 and 32 single 6-rings (S6Rs). The exchangeable cations, which 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 Fd3jm) 192, respectively. Further detailed descriptions are available.23,34 4.2. Framework Geometry. The mean T-O bond length in the five crystal structures (1.655 Å, see Table 5) is appropriately between the standard Si-O (1.61 Å) and Al-O (1.72 Å) distances. It is the same as that reported for K-Y (Si/ Al ) 1.69 as in this work),19 1.655 Å. However, a monotonic decrease in the mean T-O bond length upon increase in the degree of Ni2+-exchange is found: from 1.662 Å in crystal 1 (Ni25-Y) to 1.652 and 1.650 Å in crystals 4 (Ni33-Y) and 5 (Ni34-Y), respectively (see Table 5). Moreover, among the four T-Oi bonds, i ) 1-4, the weighted mean T-O3 (T-O3 and T-O3′ as assigned in section 3) bond length, 1.706 Å, is noticeably longer than the others: T-O1 ) 1.635 Å, T-O2 ) 1.657 Å, and T-O4 ) 1.623 Å (see Table 5). This difference was observed to a smaller degree in dehydrated Ni-Y (Si/Al ) 2.31),11 and it is almost absent in dehydrated K-Y (1.665 Å vs 1.647, 1.660, and 1.647 Å, respectively, see Table 5).19 This is seen because most of the Ni2+ ions coordinate to O3 or O3′ in each structure. It indicates the degree of distortion that Ni2+ induces in the zeolite framework as a result of its higher charge, much smaller size,29 and ability to bond much more covalently to oxygen than K+. Ion-exchange with Ni2+ and subsequent dehydration has substantially distorted the zeolite Y framework. The mean T-O-T angle (136.3°) (137.9° for Olson’s Ni-Y11) is much smaller than that in K-Y, 143.9° 19 (Si/Al ) 1.69 as in this work). This is another measure of the ability of Ni2+ to distort the zeolite framework. T-O3-T (weighted mean for O3 and O3′) is the smallest (123.1°); the others are 134.9°, 134.7°, and 152.4°, respectively (see Table 5), again because most Ni2+ ions (more than 60% in each structure, see Tables 4 and 6) are at I and I′ sites where they bond to O3 or O3′. The mean O-T-O angle in the five Ni-Y crystals studied (109.4°, essentially the tetrahedral angle) is unaffected, essentially the same as that in dehydrated K-Y (109.5°).19 4.3. Extra-Framework Atoms: Cations (Ni2+, Al3+, Na+, and Ca2+) and Oxygen. In all five crystal structures, Ni2+ ions are distributed over four crystallographic equipoints. They occupy sites I (Ni1), I′ (Ni1′1), another I′ (Ni1′2), and II′ (Ni2′). In the first three structures, Ni2+ ions are also found at site II (Ni2) (see Tables 4 and 6). Na+ ions that either remained in the zeolite or were concentrated in the zeolite during the ionexchange processes, and Ca2+ ions that selectively exchanged into the zeolite, were all found at another site II (Na and Ca). All five structures have two extra-framework oxygen positions, O5 and O6. The occupancies at O5 are a function of those at Ni1′2, Al, and Ni2′ (see Tables 6 and 7). The occupancies at O6 are less than or equal to those at Ni2′ or Ni2 (see Table 7). These relationships indicate the presence of several secondary structures involving Ni2+, Al3+, and extra-framework oxygen. 4.3.1. Octahedral Ni2+ Ions at Site I. Ni1 is octahedral in all five structures; the Ni1-O3 distances range between 2.205(6) and 2.263(17) Å with a mean value of 2.222 Å (see Table 5). With increasing Ni2+-exchange and dehydration temperature (from 623 to 723 K, crystals 2-5, see Tables 4, 6, and 7), these site-I occupancies range downward monotonically from 10.4 to 4.9 per unit cell. Ni2+ therefore increasingly prefers other sites in zeolite Y, consistent with the formation of secondary structures.

Kim et al.

Figure 3. Stereoview of a double 6-ring (D6R) with an octahedral Ni2+ ion at its center in crystal 1. The zeolite Y framework is drawn with open bonds between oxygens and T (Si,Al) atoms. The coordination about Ni2+ is indicated by solid lines. Ellipsoids of 20% probability are shown.

About 14 Ni2+ ions per unit cell were found at site I (Ni1, a 16-fold position) in crystal 1. Actually, most of the Ni2+ ions in that structure, 13.8(1) of the ∼25.3 per unit cell, occupy site I. Each Ni1 ion coordinates to the six O3 framework oxygen atoms of its D6R at a distance of 2.193(5) Å, which is somewhat longer than the sum of the conventional ionic radii of Ni2+ and O2-, 2.01 Å.29 Ni1 is almost perfectly octahedral with O3-Ni1-O3 bond angles of 88.7(2)°, 91.3(2)°, and 180° (exactly; Ni1 lies on an inversion center) (see Figure 3). Using powder diffraction data, Couves et al. similarly found 14.8(3) Ni2+ ions at this site in a Ni-Y catalyst that had been dehydrated in air at 673 K.18 Thus, site I appears to be the preferred site for Ni2+ in Ni-Y dehydrated at temperatures ∼50 K or more below 723 K, or ∼150 K above 723 K (see section 1.2, paragraph 2), but not during the final stages of dehydration. In the window between ∼673 and ∼873 K, some or all site-I Ni2+ ions relocate to the sodalite cavities to form symmetric clusters with oxygen atoms of the remaining water molecules. 4.3.2. Al(OH)4- at Site U. At the very centers of about 1.0-2.5 sodalite cavities per unit cell in crystals 2-5, Al3+ ions were found (see Tables 4, 6, and 7). The O5 position readily provides four tetrahedrally arranged extra-framework oxygen atoms to them with an entirely suitable mean Al-O5 bond length of 1.740(23) Å (see Table 5 and Figure 4). We judge this to be the tetrahydrogenated orthoaluminate ion, Al(OH)4-, held in place by 12 O5 · · · O2 hydrogen bonds (3.01 Å) to the zeolite framework. Alternatively, fewer shorter more linear hydrogen bonds could have pulled O5 somewhat off an averaged 3-fold axis position. These orthoaluminate anions could reasonably have been produced by the dealumination of the zeolite framework during Ni2+-exchange and subsequent dehydration (see section 5.1). This assignment of O5 oxygens to Al(OH)4- does not deplete the occupancy at O5. Looking ahead, it can be seen that the remaining occupancy can be assigned to Ni2+-O bonds with reasonable bond lengths, so O5 maybe viewed as widely averaged position. This is supported by the large thermal parameters at O5, larger than those at O6 in all five structures (see Table 4). This is further supported by the esds of the occupancy parameters which are larger than those at O6 in all five structures (see Table 4), and these esds are likely to have been underestimated in refinement. After subtracting the number of O5 oxygen atoms bound to Al3+ from those observed crystallographically (see Tables 4 and 6), 6.3(10), 5.7(10),



Figure 4. Stereoview of a sodalite cavity with Al(OH)4- and Ni2+ ions at sites I′ (Ni1′1) and II′ (Ni2′) in crystal 2. Al(OH)4- is held in place and stabilized by 12 OH- · · · O (O5 · · · O2) hydrogen bonds which, for simplicity, are not shown. These may be fewer 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 Ni2+ are indicated by solid lines. See the caption to Figure 3 for other details.

5.4(11), 18.0(12), and 12.7(13) O5s remain per unit cell in crystals 1-5, respectively. 4.3.3. Trigonal and Octahedral Ni2+ Ions at the I′ Sites. Ni2+ ions were found at both Ni1′1 and Ni1′2 in all five crystals. The sum of the occupancies at these two positions reached a maximum of 16.5 Ni2+ ions per unit cell in crystal 5 (see Tables 4, 6, and 7). As mentioned briefly in section 3.1, paragraph 2, when Ni1 is present in a given D6R, the adjacent I′ sites, Ni1′1 and Ni1′2, should not be occupied; otherwise, unnecessarily short and unmitigated Ni2+-Ni2+ distances would result: Ni1-Ni1′1 ) 2.46 Å and Ni1-Ni1′2 ) 3.01 Å for crystal 1, a representative crystal. The approximately two D6Rs per unit cell in crystal 1 that do not host a Ni2+ ion at Ni1 may each accommodate two Ni2+ ions at I′ sites. Of these four positions, only about two are occupied with one Ni2+ ion at Ni1′1 and the other at Ni1′2 (see Tables 4, 6, and 7) in crystal 1. Again in crystals 2-5, only half or somewhat more than half of the available I′ sites are occupied. 4.3.3.1. Trigonal Ni2+ Ions at the First Site I′. Ni1′1 is 1.993 Å from three framework oxygens at O3′ or 2.184 Å from three at O3. (These values are means for the five crystals; see Table 5.) The first distance is essentially the same as the sum of the conventional radii of Ni2+ and O2-, 2.01 Å,29 and the second is somewhat longer. Ni1′1 makes no other acceptable approaches to other atoms; a Ni2+ ion at Ni1′2 cannot coexist in the same 6-ring because this would give an impossibly short Ni1′1 · · · Ni1′2 distance (0.94 Å), and the nearest other atom, O5, is too far away (2.91 Å) to bond. Therefore, Ni1′1 is 3-coordinate with three direct bonds either to three O3′ atoms or to three O3s. That it has this inadequate coordination is further supported by its large thermal parameter which, for all five crystals, is larger than that at the other Ni1′ position, the 6-coordinate Ni1′2 (see section 4.3.3.2 and Table 4). For this low coordination, the shorter Ni1′1-O3′ bond (1.993 Å) is appropriate, so we may conclude that Ni1′1 bonds to O3′. The mean O3′-Ni1′1-O3′ angle (110.9°, Table 5) is consistent with trigonal Ni1′1; the O3-Ni1′1-O3 angle (96.1°) would have been less acceptable. 4.3.3.2. Octahedral Ni2+ Ions at the Second Site I′ are Members of Ni4O4 Cores. Ni1′2 is either 2.37 Å from three framework oxygens at O3′ or 2.69 Å (mean values) from three at O3. Considering the sum of the conventional radii of Ni2+ and O2-, 2.01 Å,29 the first distance is long and the second unacceptable. Ni1′2 therefore, like Ni1′1, bonds to three O3′ oxygens but at a substantially longer distance, suggesting that it makes other important bonds and has a coordination number substantially greater than three. Indeed, Ni1′2 is 2.27 Å from

Figure 5. The Ni8O4 · 4H2O8+ cluster has a mildly distorted cubic Ni4O4 core and point symmetry Td. A cube is drawn at the Ni1′2 positions to illustrate the magnitude of the distortion. Each Ni1′2 is pulled outward to coordinate to three O3′ oxygens as shown in Figure 6. The terminal oxygen atoms (O6s) may be water molecules and/or hydroxide anions; some of them may be missing in crystals with higher degrees of dehydration. See the captions to Figures 3 and 4 for other details.

three O5 extra-framework oxygen atoms. In turn, each O5 is this distance from three Ni2+ ions at Ni1′2. This constitutes a (Ni1′2)4(O5)4 near cube with reasonable angles at Ni1′2 (79°) and O5 (100°). Ni1′2 reasonably makes shorter bonds to extraframework oxygens (O5), each of which bonds to two other Ni2+ ions, than to framework oxygens (O3′), each of which bonds to two Si4+ ions (or to one Si4+ and one Al3+ ion), because the former (O5) are more negative. As the occupancies at Ni1′2 increased in the five crystals studied, those at O5 followed monotonically, consistent with the formation of Ni4O4. In the Ni4O4 unit, the four Ni2+ ions (at Ni1′2) 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 (without bonding) of the same sodalite cavity. The near cube formed by two interpenetrating tetrahedra has point symmetry Td (see the central unit in Figure 5). Thus, each Ni1′2 ion has a somewhat distorted octahedral geometry with three 2.37 Å bonds to three O3′ framework oxygens and three 2.27 Å bonds to three O5 extra-framework oxygens (see Figure 6). The bond angles are close to octahedral: O′3-Ni1′2-O3′ ) 88°, O5-Ni1′2-O5 ) 79°, and O3′-Ni1′2-O5 ) 96° and 174° (again mean values).


Kim et al.

Figure 6. Stereoview of a Ni8O4 · 4H2O8+ cluster centered within, and extending out of, a sodalite unit in crystal 2. Bonds between Ni2+ and extra-framework oxygens are indicated by heavy solid lines. Those between Ni2+ and framework oxygens are shown as light solid lines. See the captions to Figures 3 and 5 for other details.

Figure 7. Stereoview of a representative third kind of sodalite cavity (not containing an Al(OH)4- nor a Ni8O4 · 4H2O8+ group) with Ni2+ ions at sites I′ (Ni1′1), II′ (Ni2′), and II (Ni2) and Na+ or Ca2+ ions at Ca (site II) in crystal 3. See the caption to Figure 3 for other details.

TABLE 8: Displacements of Atoms (Å) from 6-Ring Planesa at O3 crystal no. 1 2 3 4 5 mean

at O3′

at O2

Ni1

Ni1′1

Ni1′2

Ni2′

Ni2

Na

Cab

-1.24 -1.23 -1.24 -1.20 -1.21 -1.22

0.73 0.47 0.69 0.55 0.41 0.58

1.33 1.38 1.50 1.41 1.41 1.40

-0.25 -0.13 -0.33 -0.24 -0.21 -0.26

0.31 0.57 0.27 0.32

0.85 1.15 0.95

0.98 1.11 1.16 1.04

a The negative deviations given for Ni1 indicate that it lies within a D6R. Ni1′1 and Ni1′2 (I′ sites) are near the plane of one 6-ring of a D6R; displacements into the sodalite unit are given as positive. The negative deviations given for Ni2′ indicate that it is at site II′ and lies in a sodalite unit. Ni2, Na, and Ca are at sites II in the supercage; displacements from their 6-rings are given as positive. b Both Ca2+ and Na+ occupy this averaged position.

The presence of the Ni4O4 unit shown in Figures 5 and 6 is supported by the EXAFS studies of Sano et al.7 and Dooryhee et al.14 who reported the presence of “oligomeric nickel oxides” in Ni-Y.6,7 About each Ni2+ ion, they found 3.0-3.5 oxygens at 2.08 or 2.02 Å and 3.0 Ni2+ ions at 2.99 Å. In all five structures reported here, each Ni2+ ion at Ni1′2 has three equivalent O5 atoms at an average distance of 2.26 Å in its first coordination sphere and three symmetry-related Ni2+ ions in its second coordination sphere at 3.33 Å. The Ni4O4 cube is the repeating unit (not the conventional unit cell) in NiO(s). NiO(s) has the NaCl structure.35 Finally, while the Ni2+ ions at Ni1′1 extend 0.58 Å (a mean value) into the sodalite cage from their averaged O3′ planes, those at Ni1′2 extend much further, 1.40 Å (see Table 8). Correspondingly, the mean O3′-Ni2+-O3′ angles are 111° and 88°, respectively. This further supports the 3-coordinate geom-

etry about Ni1′1 (see Figures 4 and 7) and the octahedral geometry about Ni1′2 (see Figure 6). This assignment of bond lengths and angles about the Ni2+ ions at sites I and I′ has been possible only because the diffraction data have been of sufficient quality to allow the O3 framework oxygen position to be resolved into O3 and O3′ (see section 3.1). This was not done in previous studies.10-13,18 When the number of O5 oxygen atoms required for the Ni4O4 units is subtracted from those remaining at the end of section 4.3.2, 5.2(10), 0.3(10), 0.0(11), 8.2(12), and 0.0(13) O5s remained per unit cell of crystals 1-5, respectively. This indicates, at least for crystals 1 and 4, that some O5 oxygen atoms remain to be assigned (see sections 4.3.3.3 and 4.3.4). 4.3.3.3. Ni8O4 · xH2O8+ Clusters. With three Ni2+ ions coordinated to each O5 oxygen atom in Ni4O4, it would be unreasonable to expect this O5 to represent a water molecule.

Crystal Structures of Vacuum-Dehydrated Ni-Y As such, it should be very acidic, and the O5 oxygen would be 5-coordinate. It would be more reasonable to expect this O5 oxygen to represent a hydroxide or even an oxide anion, having dissociated one or two H+ ions, to give Ni-Y, entirely or in part, its acid-catalyst properties. If it is a hydroxide ion, this O5 oxygen would then be 4-coordinate tetrahedral with H+ on the same 3-fold axis to form (Ni4O4)H44+. Each O5 atom is a reasonable hydrogen-bonding distance from three O2 framework oxygen positions (3.01 Å, as were the same oxygen atoms at O5 in Al(OH)4-, see Figure 4). Alternatively, however, we see that O5 is the correct bonding distance, 1.97 Å, from a Ni2+ ion at site II′ (Ni2′) (see Table 5; this distance is most precisely seen in crystal 5). This distance is in close agreement with the sum of the conventional radii of Ni2+ and O2-, 2.01 Å.29 Ni2′ is near the same S6R as O5 and on the same 3-fold axis; it can complete a tetrahedral geometry about O5. Furthermore, the near equality of the occupancies at Ni1′2 and Ni2′ in crystals 2, 3, and 5 indicates that Ni2′ bonds to Ni4O4. Additional occupancies at Ni2′, beyond that which can bond to the Ni4O4 units, are observed in crystals 1 and 4 (see Tables 6 and 7), indicating the presence of other Ni species at Ni2′ (see section 4.3.4). These may bond to O5 oxygen atoms which are in similar excess (see the last paragraph of section 4.3.3.2). If the O5s of the Ni4O4 unit bond to Ni2′, these O5s should be oxide ions and the Ni4O4 unit would center a larger cluster, (Ni4O4)Ni48+, henceforth to be written as Ni8O48+. Each O5 oxide ion bonds tetrahedrally to four Ni2+ ions, three at Ni1′2 and one at Ni2′. Ni8O48+ is present in a fraction of the sodalite cavities of all five Ni-Y crystals studied (see Tables 6 and 7), with the possible exception of crystal 1 where its concentration is lowest. There (Ni4O4)H44+ (described in the opening paragraph of this subsection) might instead be present. To continue, some of the extra-framework oxygen atoms at O6 in the supercage are within bonding distance of Ni2′ on the same 3-fold axis (Ni2′-O6 ) 2.02(12) Å), suggesting that the Ni8O48+ units should be extended to give Ni8O48+(O)x clusters. The parameter x is introduced because the occupancy at O6 lags behind that of the rest of the cluster; some of these O6s appear to have been lost as a result of the high temperature vacuum dehydration (see section 4.3.4 and Tables 6 and 7). These O6s are likely to be acidic coordinated water molecules, bringing us to Ni8O4 · xH2O8+, but they may also be OH-s by dissociation. The anionic sodalite cavity often hosts and stabilizes cationic clusters such as Ni8O4 · xH2O8+. Examples are Na43+,36 Zn56+,37 cyclo-Zn68+,37 Cd8O48+,38 and Pb2S2+ 39 in zeolite Y (FAU), S44+,40 In57+,41 and Pb8O4n+ 42,43 in zeolite X (FAU), and In57+,44 Cd6S44+,45 Cd2Na2S4+,45 and Cd2O2+ 45 in zeolite A (LTA); a brief review is available.40 Hydroxide ions often accompany metal ions into zeolites upon ion-exchange, especially if the incoming cation can hydrolyze and its hydrolysis products are present in reasonable concentration in the exchange solution. They can be the precursors for the formation of various hydroxide or oxide clusters which may evolve further upon heating and/or vacuum dehydration. This has been seen when Tl+,46 Cd2+,47 and Pb2+ 48-50 were exchanged into zeolite A (LTA), Cd2+ into zeolite Y (FAU),38 and Co2+,23 Zn2+,51 Pb2+,52 and La3+ 53 into zeolite X (FAU). With Cd38 or Zn,51 the near cubic M4(O or OH)40 or 4+ clusters formed upon evacuation at various temperatures, while [Pb4(OH)4]4+ forms directly upon Pb2+-exchange at ambient temperature.52 For La, a La2O3 continuum composed of La4O4

J. Phys. Chem. C, Vol. 113, No. 13, 2009 5177 subunits (in the sodalite cavities like the Ni4O4 units observed in this work) bridged by oxide ions at their La3+ vertices was seen.53 Occupancies greater than needed to complete the Ni8O4 · xH2O8+ clusters were found at Ni2′ and O5 in crystals 1 and 4 (see Table 7). Somewhat less or equal values are observed at O6, while those at Ni2, Na, and Ca are independent of the number of Ni8O4 · xH2O8+ clusters (see sections 4.3.4 and 4.3.5). Although the presence of Ni8O4 · xH2O8+ could not be entirely confirmed in crystal 1 due to the low occupancy at Ni1′2 (and, therefore, at the Ni4O4 cores), they were clearly present in crystal 2 where it was possible to split O6 into two positions, O6 and O6′, O6 for the Ni8O4 · xH2O8+ clusters and O6′ for the remaining 4-coordinate Ni2+ ions at Ni2 (see section 3.2.2 and Table 7). The Ni8O4 · xH2O8+ clusters have higher occupancies and are unambiguously present in crystals 3-5. The residual Ni2+ ions at Ni2 in crystal 3 are trigonal (see Table 7), while those at Ni2′ in crystal 4 are trigonal bipyramidal. The values of x in the Ni8O4 · xH2O8+ clusters (see Table 7) are about 4.0, their maximum possible value, for crystals 1 and 2 which were dehydrated at the lower temperature (623 K). They are smaller in the remaining crystals, indicating that some terminal O6 atoms have been lost. This may be attributed to a higher degree of dehydration at the higher dehydration temperature (723 K) in crystals 3 and 5. 4.3.4. 3-, 4-, and 5-Coordinate Ni2+ Ions at Sites I′ and II. In crystals 1-3, two Ni2+ positions were found on 3-fold axes near the centers of the S6Rs, both close to their O2 planes, one at site II′ (Ni2′) and the other at site II (Ni2). On average, they extend 0.33 Å into the sodalite cavities and 0.20 Å into the supercages, respectively (see Table 8). Together, these positions host 9.3-10.4 Ni2+ ions per unit cell. The average Ni2+-O2 bond lengths, 2.110 and 2.142 Å, respectively, are somewhat longer than the sum of the conventional ionic radii of Ni2+ and O2- (2.01 Å29). The extra-framework oxygen atoms at O5 and O6, on 3-fold axes in the sodalite cavities and supercages, respectively, coordinate axially (O5-Ni2+-O6) to Ni2′ and/or Ni2. To decide which of these two Ni2+ positions has these axial ligands, let us consider (as before) the distances involved. In crystal 1, the O5-Ni2′ and Ni2′-O6 distances are 1.89(6) and 2.10(6) Å, respectively, while the O5-Ni2 and Ni2-O6 distances are 2.45(7) and 1.54(6) Å, respectively. Because the second pair of distances is unacceptable, O5 and O6 coordinate to Ni2′ in crystal 1, and the coordination geometry about Ni2′ is 5-coordinate trigonal bipyramidal, leaving Ni2 to be near 3-coordinate trigonal planar. Some of these O5-Ni2′-O6 groups are segments of Ni8O4 · xH2O8+ clusters (see section 4.3.3.3 and Table 7). From such distance considerations, it is concluded that O6 bonds to Ni2 in crystals 2 and 3 (see Tables 5 and 7). In crystal 4, at least 3.9 5-coordinate Ni2+ ions are present per unit cell at Ni2′ in addition to the 9.8/4 ) 2.45 Ni8O4 · xH2O8+ clusters in the sodalite units (see Table 7). This conclusion was reached because of the much larger occupancies observed at Ni2′ and O5 in crystal 4 as compared to those in crystals 1-3 (see Tables 4, 6, and 7). The maximum number of O6 atoms were assigned to Ni2′ in crystal 4, to give that position full 5-coordination, because the remainder, which must be assigned to Ni8O4 · xH2O8+, allowed that x value to have an acceptable value (not excessively large) as compared to the other four crystals, considering the experimental conditions of their preparation. In crystal 5, the condensation of the 5-coordinate Ni2+ the ions at Ni2′ is complete (see Table 7) to yield the maximum number of Ni8O4 · xH2O8+ clusters per unit cell


Kim et al.

Figure 8. Stereoview of a representative supercage with Ni2+ ions at sites II′ (Ni2′), II (Ni2), and Na+ or Ca2+ ions at Ca (site II) in crystal 3. See the caption to Figure 3 for other details.

observed in this work, 12.7/4 ) 3.2, with x ) 5.9(20)/3.2 ) ∼2. This was accomplished only in crystal 5 which had been prepared from K-Y at the higher exchange temperature (to give the highest degree of Ni2+-exchange) and had been dehydrated at the higher temperature. When the number of O5 oxygen atoms needed to complete the 5-coordinate Ni2+ the ions at Ni2′ are subtracted from those remaining at O5 (see the last paragraph of section 4.3.3.2 and Table 7), the number of O5 oxygens remaining are 2.0(10), 0.3(10), 0.0(11), 4.3(12), and 0.0(13) for crystals 1-5, respectively. These values may all be considered zero, including that for crystal 4, an outlier, indicating that all of the occupancy at O5 has been assigned. Ni2′ in crystal 4 is clearly an averaged position, and this has hampered our ability to apportion the oxygen atoms at O5 and O6 among the solitary Ni2′ position and Ni8O4 · xH2O8+ clusters. It is for this reason that some fields on line 7 of the body of Table 7 have been left blank. 4.3.5. Na+ and Ca2+ Ions at Site II in the Supercage. In crystals 1 and 2, Na+ ions were found at a site-II position (Na, see Tables 2 and 4). This shows that Ni2+-exchange was incomplete, perhaps due to the Na+ content of the Ni2+-exchange solution even though it was very small (see section 2.1). The mean Na-O2 bond length, 2.313 Å, is about the same with the sum (2.29 Å) of the conventional ionic radii of Na+ and O2(0.97 and 1.32 Å,29 respectively). In crystals 3-5, both Na+ and Ca2+ were found by EDX analysis. When the Na position was refined using a Na+ form factor, its occupancy became much larger than the EDX analysis (Table 2) allowed, indicating that the Na position must be occupied by both Na+ and Ca2+ in these three crystals. These two positions could not be resolved crystallographically, as might be expected because the Ca2+ radius29 is only ∼0.02 Å larger than that of Na+. The averaged position is named Ca in crystals 3-5 because Ca2+ predominated in the EDX analysis. Unless there was another source of which we are unaware, these Ca2+ ions must have been captured by (concentrated in) the zeolite from the aqueous ion-exchange solutions (see impurity levels in section 2, paragraph 2). Na+ ions were also concentrated in the K+-exchanged zeolite during the Ni2+-exchanges. The Na position in crystals 1 and 2 and the Ca position in crystals 3-5 extend 0.95 and 1.04 Å (means, see Table 8), respectively, into the supercage. 4.4. Arrangements of Extra-Framework Species in the Cages of Ni-Y. In crystals 2-5, the sodalite cavities have three different contents. Two of these, those with Al(OH)4- at their centers (see Figure 4) and those with Ni8O4 · xH2O8+ (see Figure 6), are well defined. The third type of sodalite cavity hosts varying numbers of Ni2+ ions at Ni1′1 and Ni2′ (see Figure 7).

The number of sodalite units per unit cell with Ni8O4 · xH2O8+ and Al(OH)4- vary with the degree of Ni2+-exchange and the temperature of dehydration (see section 5.2 and Tables 4, 6, and 7). Crystal 1 is the same except that it has no Al(OH)4-. A representative supercage is shown in Figure 8. 5. Discussion Some extra-framework oxygen atoms are oxide ions, others are OH-, and the remainder are water molecules that have not been lost despite the relatively high vacuum-dehydration temperatures used. The core oxygen atoms (O5) in Ni8O4 · xH2O8+, each of which bonds tetrahedrally to four Ni2+ ions, are clearly oxide ions. The Al atoms and the O5s around them are readily recognized to be Al(OH)4-. Finally, the terminal oxygens (the O6 and some of the O5 oxygens) that coordinate to Ni2′ in Ni8O4 · xH2O8+, to Ni2′ in trigonal bipyramidal Ni2+, and to Ni2 in tetrahedral Ni2+ may be undissociated water molecules, OHions resulting from dissociation, or OH- ions that had entered the zeolite (together with H+) during the Ni2+-exchange process. Moderate concentrations of Ni-OH+ (3.2 × 10-5 M) were present in the ion-exchange solution (pH ) 6.2) due to the hydrolysis of aqueous Ni2+ (pKh ) 9.40). 5.1. Dealumination to Form Al(OH)4-. 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.54,55 The two major sources of H+ ions within the zeolite are direct exchange and the intrazeolitically enhanced hydrolysis of Ni2+, especially as a result of the formation of the oxide ions in Ni8O4 · xH2O8+. All crystals, except crystal 1 which had been treated most mildly (Ni2+-exchange from Na-Y at the lower temperature and dehydrated at the lower temperature), contained small numbers of Al(OH)4- ions (ca. 1.0-2.5 per unit cell) (see Tables 4, 6, and 7). Their concentration increased with increasing dehydration temperature, from ∼0 to 1.0 per unit cell for crystals 1 and 2 (prepared by Ni2+-exchange of Na-Y), and from ∼1.0 to 2.5 for crystals 3 to 5 (prepared from K-Y). While dealumination may occur during the Ni2+-exchange process, it certainly happens during dehydration (see Tables 4, 6, and 7). The maximum number of Al(OH)4- per unit cell, ∼2.5, was seen in crystal 5 which was treated most harshly (Ni2+-exchange from K-Y at the higher temperature followed by dehydration at the higher temperature).

Crystal Structures of Vacuum-Dehydrated Ni-Y 5.2. The Ni8O4 · xH2O8+ Clusters. Ni8O4 · xH2O8+ clusters with near cubic Ni4O4 cores at their centers were found in the sodalite cavities of four of the five crystals studied, and are likely to be present in fifth (crystal 1) also. Ni4O4 appears to be a little too small for the sodalite cavity; its Ni2+ ions are pulled outward to coordinate to framework oxygen atoms (see Figure 6). As the degrees of Ni2+-exchange and dehydration increase (the result of prior K+-exchange and higher temperatures of ionexchange and dehydration), the concentration of Ni8O4 · xH2O8+ clusters increases (see Tables 4, 6, and 7). The number of sodalite units per unit cell with Ni8O4 · xH2O8+ clusters was greatest in crystal 5 whose eight sodalite units hosted about 3.2 (see Tables 6 and 7). Incompletely defined “oligomeric nickel oxides” had been reported in calcined Ni-Y.7,8 Many inorganic and polymeric compounds containing Ni4O4 cubane cores have been reported,56 some of which are single-molecule magnets (SMMs). They have high-spin ground states and show uniaxial magnetic anisotropy.57-59 Ni8O4 · xH2O8+ may similarly have interesting magnetic properties. Gas-phase NixOy clusters can be produced by laser ablation.60,61 5.3. H+ Ions. To achieve charge balance, H+ ions, not found in this work, must be present in all five crystals. Some may have been introduced as hydronium ions upon Ni2+-exchange and may be present as H+ ions coordinated to framework oxygens. Others have arisen from the formation of Al(OH)4and Ni8O4 · xH2O8+ from Al3+, Ni2+, and H2O. This amounted to a maximum of ∼22 H+ ions per unit cell in crystal 5, a high concentration of Bronsted acid sites (see Table 7). Additional H+ ions, beyond the numbers given in Table 7, would be present if some of the water molecules that coordinate to Ni2+ have dissociated. The H+ ions that occupy former Al3+ sites in the zeolite framework as a result of dealumination should be unavailable for catalysis. Those that resulted from the formation of oxide ions in Ni8O4 · xH2O8+ should be readily available for catalysis and could have accelerated the dealumination process. 5.4. Some Ni2+ Ions Are in Position for Catalysis. The 3-coordinate Ni2+ ions seen at Ni1′1, Ni2′, and Ni2 (see Table 7) are easily accessible to guest molecules and are therefore in position to participate directly in catalysis as they seek to increase their coordination numbers to more conventional values. Such interactions within zeolites have been seen directly by Fourier transform infrared (FTIR) methods.62,63 These interactions may be expected to play a direct role in the hydrogenation5 and oxidation6-9 of CO. The same is true for the 4-coordinate trigonal pyramidal Ni2+ ions at Ni2′ which appear when x < 4 in Ni8O4 · xH2O8+ (when some O6 atoms shown in Figures 5 and 6 are lost). Additional Ni2+ ions, currently at apparently inaccessible sites such as site I within the zeolite, may move into the supercage upon the introduction of guest molecules and thus participate in catalysis. In work in progress, we see a massive movement of both Ni2+ and Co2+ ions into the supercages of the dehydrated zeolites Ni-Y and Co-Y, respectively, upon the introduction of NO(g). 5.5. Effects of Experimental Variables on Ni2+-Exchange Levels and Distribution. 5.5.1. Effect of Ion-Exchange Temperature. More Ni2+ ions exchanged into zeolite Y at 353 K than did at 294 K (see Tables 1, 6, and 7). The maximum Ni2+exchange levels observed were ∼34.1 and 27.5 Ni2+ ions per unit cell at the two temperatures, respectively. With Na-Y as the starting material (crystals 1 and 2), ∼3.2 more Ni2+ ions ion-exchanged into zeolite Y per unit cell at 353 K than at 294

J. Phys. Chem. C, Vol. 113, No. 13, 2009 5179 K. With K-Y as the starting material (crystals 3-5), ∼6.6 more Ni2+ ions entered at the higher temperature. This can be attributed to the greater mobility of all cations at the higher ion-exchange temperature. Also dependent on the ion-exchange temperature is the distribution of Ni2+ ions among the cation positions, although this may simply be a result of the higher degree of Ni2+exchange. Crystals 1 and 2 were prepared identically except for their ion-exchange temperatures, 294 and 353 K, respectively (see Tables 1 and 6). The same is true for crystals 3 and 5. A lower Ni2+ population at site I (Ni1) with corresponding increases at the two I′ sites (Ni1′1 and Ni1′2) is always observed at the higher temperature: ∼3.4 fewer Ni2+ ions at site I per unit cell for crystal 2 than crystal 1 and ∼2.0 fewer for crystal 5 than crystal 3. When the distribution of Ni2+ ions in crystals 1 and 2 is more carefully compared, it is seen that some Ni2+ ions from sites I and II have moved into the sodalite cavities at 353 K, resulting in increases in the Ni2+ populations at sites I′1, I′2, and II′ (see Table 6). A similar trend is seen when crystals 3 and 5 are compared. It is clear that hydrated Ni2+ ions have condensed to form additional Ni8O4 · xH2O8+ in crystals that had been ion-exchanged at the higher temperature. 5.5.2. Effect of Precursor Cation. More Ni2+ ions exchanged into K-Y than into Na-Y at a given ion-exchange temperature, ∼2.2 more Ni2+ ions per unit cell at 294 K (compare crystal 1 with crystal 3 in Table 6) and ∼4.5 or 5.6 more at 353 K (compare crystal 2 with crystals 4 and 5 in Table 6). This may be attributed to the greater mobility of the more loosely bound K+ ion as compared to Na+, as was the intent of the K+exchange step. The precursor cation also had a big effect on the distribution of Ni2+ ions among the cation sites (see Table 6). This also may simply result from the higher degree of Ni2+-exchange. When Ni2+-exchange was done using Na-Y (crystals 1 and 2), site I was most populated. When K-Y was used (crystals 3-5), the I′ sites had the highest population (and there were more Ni8O4 · xH2O8+ clusters). At the same ion-exchange temperature, 294 K, ∼7.4 more Ni2+ ions are found at site I in crystal 1 (from Na-Y) than in crystal 3 (from K-Y). Again, this may have occurred because the more loosely bound K+ ions are more easily replaced by Ni2+. K+ ions also appear to be hindering the entry of Ni2+ into D6Rs. 5.5.3. Effect of Dehydration Temperature. Crystals 4 and 5 were prepared identically except for their dehydration temperatures (see Tables 1 and 6). As would be expected, these two crystals have similar numbers of Ni2+ ions per unit cell, 33.0 and 34.1, respectively. However, about 0.8 more Ni8O4 · xH2O8+ clusters were found at the higher dehydration temperature. The greater hydrolysis at the higher dehydration temperature, to generate more NiOH+ groups within the zeolite, and the increased loss of water appear to be responsible for this condensation. More Al(OH)4- was produced at the higher dehydration temperature: the ∼1.0 per unit cell at 623 K (crystals 2 and 4) increased to 1.6 and 2.5 at 723 K (crystals 3 and 5, respectively). This may be attributed to the action of H+ ions on the zeolite framework. 5.5.4. Summary of Section 5.5. The total number of Ni2+ ions per unit cell depends on the temperature of Ni2+ ionexchange (ca. 25.3-27.5 at 294 K, and 28.5-34.1 at 353 K). The number of Ni2+ ions at site I depends primarily on the precursor cation (ca. 7.4 more Ni2+ ions at site I per unit cell with Na+ than with K+) but also on the ion-exchange temperature (from ca. 2.0 to 3.4 fewer at the higher temperature). The

5180 J. Phys. Chem. C, Vol. 113, No. 13, 2009 number of Ni8O4 · xH2O8+ clusters was generally higher in crystals prepared via K-Y at the higher ion-exchange temperature. As the number of clusters increases, fewer Ni2+ ions are seen at site I. 5.6. Processes That Occurred during Attempted Ni2+Exchange. Ni2+-exchange was done to replace the Na+ ions in Na71-Y and the K+ ions in K71-Y with Ni2+. The product, instead of simply being Ni35.5-Y, was substantially more complex. First of all, impurity cations from the high purity exchange solutions selectively exchanged into (were concentrated in) the zeolite; Na+ and Ca2+ were identified. In addition, for charge balance, H+ ions must have exchanged into crystals 1-3. Beyond that, some of the Ni2+ ions within the zeolite were seen to be hydrolyzed (were members of the condensed Ni8O4 · xH2O8+ clusters); this generated additional H+ ions. (Upon attempted Ni2+-exchange into zeolite X (FAU, Si/Al ) 1.09), this Bronsted acidity had led to crystal damage and destruction.22,64) Finally, the zeolite framework experienced some dealumination to generate Al(OH)4-. 6. Summary Five single crystals were prepared by ion-exchanging Na-Y and K-Y with Ni2+ at 294 or 353 K followed by vacuum dehydration at 623 or 723 K and 2.0 × 10-6 Torr. Their crystal structures and chemical compositions were determined using synchrotron X-ray crystallography and EDX analyses. Some residual or introduced Na+ and Ca2+ ions were found. In all five structures, Ni2+ ions occupy four crystallographically distinct cation sites, sites I, I′, another I′, and II′. In crystals 1-3, Ni2+ is also found at site II. Na+ and Ca2+ ions were found at another site II, and some Al(OH)4- ions occupy site U. Ni2+ ions at one of the two I′ sites, the one that is deeper within the sodalite cavities, are members of Ni8O4 · xH2O8+ clusters; these are centered within these cavities, with its water molecules extending out into the supercages. The Ni2+ ions at the other site I′ are trigonal. Some of the Ni2+ ions near the centers of S6Rs have residual water molecules coordinated axially to give trigonal pyramidal and trigonal bipyramidal Ni2+. At the centers of some D6Rs, Ni2+ coordinates octahedrally to framework oxygens. The Ni2+ ions in the Ni4O4 cores of the Ni8O4 · xH2O8+ clusters have distorted octahedral geometry with three framework oxygens and three extra-framework oxide ions. The Ni4O4 core is a mildly distorted cube, consisting of interpenetrating tetrahedra of Ni2+ and of O2-. The Na+ and Ca2+ ions each associate with three framework oxygen atoms at site II. The total number of Ni2+ ions per unit cell depends on the temperature of Ni2+-exchange (more at 353 K than at 294 K) and on the precursor cation used for ion-exchange (more with K-Y than with Na-Y). Higher degrees of Ni2+-exchange and dehydration lead to the increased condensation of Ni8O4 · xH2O8+ clusters from mononuclear Ni2+ ions at sites I and II. The concentration of orthoaluminate ions, Al(OH)4-, also increased with increasing Ni2+-exchange level and dehydration temperature. The number of H+ ions necessary to balance the charge of the zeolite increases monotonically with the number of Ni8O4 · xH2O8+ clusters. Up to 22 H+ ions can be present per unit cell, more if the axially coordinated water molecules have dissociated, to give a highly acidic zeolite. The greater the degree of Ni2+-exchange into zeolite Y, the more Bronsted acid sites produced. Some of these, and some Ni2+ ions, are in or very near to the supercages where they are immediately available for catalytic processes. Acknowledgment. We gratefully acknowledge the support of Dr. Kyung Hwa Kim and The Pohang Accelerator Laboratory

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Crystal Structures of Vacuum-Dehydrated Ni2+

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