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Weak Ag+–Ag+ bonding in zeolite X. Crystal structures of Ag92Si100Al92O384 hydrated and fully dehydrated in flowing oxygen
Microporous and Mesoporous Materials 41 (2000) 49±59
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Weak Ag ±Ag bonding in zeolite X. Crystal structures of Ag92Si100Al92O384 hydrated and fully dehydrated in ¯owing oxygen Seok Hee Lee a, Yang Kim a,1, Karl Se b,* b
a Department of Chemistry, Pusan National University, Pusan 609-735, South Korea Department of Chemistry, University of Hawaii at Manoa, 2545 The Mall, Honolulu, HI 96822-2275, USA
Received 1 February 2000; accepted 26 June 2000
Abstract and the other fully deTwo crystal structures of fully Ag -exchanged zeolite X, one hydrated (a 24:996(4) A) have been determined by single-crystal X-ray diraction techniques in the cubic space hydrated (a 25:200(4) A), group Fd 3 at 21(1)°C. Each initial Na92 ±X crystal was ion exchanged in a ¯owing stream of 0.05 M aqueous AgNO3 . The second crystal was dehydrated at 360°C for two days in a ¯owing stream of oxygen gas (790 Torr) followed by evacuation at 400°C and 2 10ÿ6 Torr for 2 h. Their structures were re®ned to the ®nal error indexes R1 =R2 0:088=0:104 with 216 re¯ections, and R1 =R2 0:047=0:041 with 312 re¯ections, respectively, for which Ag ±Ag interactions. In the hydrated crystal, 92 Ag ions I > 3r I. Both structures show weakly attractive 3.0±3.3 A were found at seven crystallographic sites: 16 ®ll site I at the centers of the double six-rings, 16 at site I0 in the sodalite to those at site I, 32 ®ll site II in the supercages, and 28 cavities opposite double six-rings bond weakly (3.045(3) A) occupy four dierent III0 sites. Some H2 O molecules were found at two dierent 3-fold axis sites: 16 coordinate to site I0 Ag ions in the sodalite cavities, and 32 coordinate to site II Ag ions in the supercage. In the dehydrated crystal, Ag ions or atoms were found at eight crystallographic sites: three Ag ions are at site I, 26 Ag ions and six Ag0 atoms are at two I0 sites in the sodalite cavities ®lling site I0 , 32 Ag ions ®ll site II as in crystal 1, two Ag0 atoms are on 2-fold axes in the sodalite cavities, and 23 Ag ions occupy three dierent III0 sites. The 26 Ag ions at site I0 bond weakly in pairs Three linear Ag clusters per unit cell with atoms at sites I0 , I, and I0 , respectively, lie along 3-fold axes, and (3.224(3) A). 3 two bent 168(2)° Ag2 3 clusters per unit cell are in the sodalite cavities. It remains possible, considering Ag±Ag and Ag± O distances, that no Ag0 atoms have formed, that the product is (Ag )92 ±X, and that the bonding in the clusters, both of which would then be Ag3 3 , is due to additional Ag ±Ag interactions. Ó 2000 Elsevier Science B.V. All rights reserved. 2 Keywords: Zeolite X; Structure; Silver; Clusters; Ag2 2 ; Ag3 ; Ag3
1. Introduction *
Corresponding author. Fax: +1-808-956-5908. E-mail address: kse@gold.chem.hawaii.edu (K. Se). 1 Also corresponding author.
Small clusters of the group VIII transition metals, Ni, Pd, and Pt, have been extensively studied within zeolites because of their pronounced
1387-1811/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 0 ) 0 0 2 7 0 - 5
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S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
catalytic activity [1±3]. Related elements of lesser catalytic importance include rhodium, iridium, and silver. Silver-exchanged zeolites have been well studied in recent years, both for their own importance and as models for other systems. Beyer and Jacobs studied the kinetics and mechanisms of silver-cluster formation in Ag forms of zeolites treated with hydrogen gas [4,5]. For zeolite Ag±Y [1], two reduction mechanisms were established: one at low temperature and another at high temperature (T > 157°C). At low temperatures, the regeneration of centers for hydrogen activation is the rate-determining step of the silver ion reduction; at higher temperatures, the process of Ag ion migration is rate determining. Upon attempted dehydration, some Ag ions in zeolite A are reduced by water oxygens or oxygens of the zeolite framework to form neutral Ag6 silver clusters [6,7]. Each octahedral hexasilver cluster lies within a cube of eight Ag ions, each of which is nearly in the plane of a six-oxygen ring. The concentration of silver clusters within the zeolite has been found to depend upon the dehydration time and temperature. (Ag6 )0 stabilized by coordination to eight Ag ions may also be viewed as (Ag14 )8 whose atoms are arranged approximately as one unit cell of face-centered cubic (cubic closest packing). Schoonheydt and Leeman [8] used e.s.r. to folclusters upon H2 relow the formation of Agn 6 duction of zeolite Ag, M±A (M Na , K , Cs , and Ca2 ) over the temperature range ÿ15 to 25°C. They found that the diusion of the cation with the highest activation energy determines the rate of Ag 6 cluster formation. Thus, in Ag, Na±A the rate depends on the mobility of the Ag cation. Its activation energy, determined from the temperature dependence of the initial rate of Ag 6 cluster formation, was established to be in the range 47±63 kJ/mol, for samples with three, four, and six Ag ions per unit cell. Schoonheydt and Leeman also recorded the e.s.r. spectra of Ag, Na± X [8]. After the same pretreatment and reduction procedures as was used for Ag, Na±A, and no Agn 6 clusters were observed. The redox reactions of Ag±Y with oxygen and hydrogen were followed by powder X-ray dirac-
tion techniques [9]. Oxygen treatment of dehydrated Ag±Y under vacuum at 600°C, which had already formed reduced linear Ag2 clusters by 3 autoreduction [10], caused Ag ions to migrate into the sodalite units and the D6Rs from general positions in the supercages. The reaction for autoreduction may be viewed as (1) partial decomposition of the Ag2 O component of the silver aluminosilicate (Ag2 O)46 (Al2 O3 )46 (SiO2 )100 , (2) 4Ag 2H2 O ! 4H 4Ag O2 or (3) 4Ag 2OHÿ ! 2H 4Ag O2 . (The hydroxide ions in (3) are from the dissociation of water; this equation is written to indicate that bent Ag ±OHÿ ±Ag clusters or cyclic (Ag )3 (OHÿ )3 clusters [11] in sodalite units may be the immediate precursors of the Ag atoms and clusters with atoxygen molecules.) Linear Ag2 3 oms at sites I0 , I, and I0 , respectively, formed along the 3-fold axes. After hydrogen reduction at 75°C, a considerable increase in occupancy at site I0 , partly at the expense of site I, was observed. Nikola and Nadezda [12] studied the kinetics and mechanism of silver-cluster formation in zeolite Ag±X by diuse re¯ectance spectroscopy. The oxidation of the zeolite Ag±X samples was performed in ¯owing O2 after dehydration at 300°C and 512°C. They found that the 410 nm band, used for kinetic investigations, is associated with the silver clusters, most probably the linear Ag2 3 cluster at sites I0 ±I±I0 , that formed in some of the small cages of the zeolite. Beyer et al. reported that polynuclear cations of mean or approximate composition Ag 3 form upon partial reduction of dehydrated fully Ag exchanged zeolite Y [4]. Hydrogen uptake by silver and Ag chabazite produced Ag2 4 3 clusters according to e.p.r. measurements. Subsequently, the silver clusters (Ag3 ) and (Ag3 )2 were identi®ed in the large cavity of dehydrated Ag6 Na6 ±A treated with 50 Torr of H2 at room temperature [13]. This work was done to see whether fully dehydrated, fully Ag -exchanged zeolite X could be prepared. If so, its cation distribution would be learned. Dehydration was done in a ¯owing stream of oxygen gas to inhibit both autoreduction and reaction with leaving H2 O molecules. Repeated
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
attempts to study Ag±X by dehydration under vacuum had given complex results with substantial Ag reduction and some crystal damage [14]. Some Ag ions in zeolite X were expected to be reduced upon dehydration even in an O2 atmosphere [22], so it was hoped that the stoichiometry, positions, and geometries of new silver clusters within the cavities of zeolite X would be learned. For comparison, the structure of the fully hydrated material was also determined. 2. Experimental section Large single crystals of sodium zeolite X, stoichiometry Na92 Si100 Al92 O384 , were prepared in St. Petersburg, Russia [15]. Each of two single crystals, colorless octahedra about 0.20 mm in crosssection, was lodged in a ®ne pyrex capillary. An exchange solution of 0.05 M aqueous AgNO3 (99.9999% purity, Aldrich Chem. Co.) was allowed to ¯ow past each crystal at a velocity of 10 mm/s for three days. Since the exchange of Ag for Na has been shown to be facile and complete after milder treatment [16±18], complete exchange was assured in this case. The ®rst crystal was evacuated at 25°C and 1 10ÿ3 Torr for 2 h to remove excess moisture. It was then removed from the vacuum line and its capillary tip was opened to the atmosphere. Two days were allowed for rehydration before data collection began on the resulting colorless crystal. The second crystal was dehydrated at 360°C for two days in a ¯owing stream of oxygen gas (P 790 Torr) followed by evacuation at 400°C and 2 10ÿ6 Torr for 2 h. Care was taken (a bank of zeolite beads in series evacuated in situ) to prevent polar molecules from downstream parts of the vacuum system that had not been baked out from reaching the crystal. After cooling to room temperature, the second crystal, still under vacuum and pale brown in color, was sealed in its capillary by torch. The space group Fd 3 was used for both crystals. This choice is supported by (a) the systematic absences, (b) the low Si/Al ratio, which in turn requires, at least in the short range, alternation of Si and Al, and (c) the observation that these crystals, like all other crystals from the same batch that
51
have not suered minor damage due to chemical treatment such as acidity, do not have intensity symmetry across (1 1 0) and therefore lack that mirror plane. This choice is also supported by the ®nal results: The Al±O bonds are all longer than the Si±O bonds. Diraction data were collected with an automated Enraf-Nonius four-circle computer-controlled CAD-4 diractometer equipped with a pulse-height analyzer and a graphite monochromator, using MoKa radiation (Ka1 , k 0:70930 The unit cell constant at Ka2 , k 0:71359 A). A, 21(1)°C determined by least-squares re®nement of 25 intense re¯ections for which 14° < 2h < 24° are for crystal 1 and 25.200(4) A for a 24:996(4) A crystal 2. The x ÿ 2h scan technique was used. The data were collected using variable scan speeds. Most re¯ections were observed at slow scan speeds, ranging between 0.24 and 0.34° minÿ1 in x. The intensities of three re¯ections in diverse regions of reciprocal space were recorded every 3 h to monitor crystal and instrument stability. Only small random ¯uctuations of these check re¯ections were noted during the course of data collection. All unique re¯ections in the positive octant of an Fcentered unit cell for which 2h < 50 , l > h, and k > h were examined. The raw data were corrected for Lorentz and polarization eects, including that due to incident beam monochromatization, and the resultant estimated standard deviations were assigned to each re¯ection by the computer programs G E N E S I S and 2 P R O C E S S , respectively. Of the 1267 re¯ections examined for both crystals 1 and 2, only the 216 and 312 re¯ections, respectively, for which I > 3r I were used in subsequent structure determinations. An absorption correction (lR 0:37 and qcal 2:271 g cmÿ3 for crystal 1, and lR 0:36 and qcal 2:216 g cmÿ3 for crystal 2) was made empirically using a w scan for each crystal [19]. The calculated transmission coecients ranged from 0.982 to 0.989 for crystal 1, and from 0.984
2 Calculations were performed with structure determination package programs, M O L E N , Enraf-Nonius, Netherlands, 1990.
Wyc. position
Site
ÿ533(3) ÿ534(3) ÿ1071(6) ÿ24(7) ÿ321(7) ÿ726(6) 0 703(2) 2406(1) 2767(28) 3480(40) 2463(22) 1801(31) 1587(20) 2947(34)
x
1241(4) 361(3) ÿ63(8) ÿ27(7) 700(7) 777(6) 0 703(2) 2406(1) 1952(28) 1916(40) 1723(21) 3639(30) 1587(20) 2947(34)
y
358(3) 1241(4) 1058(6) 1451(6) 687(7) 1742(6) 0 703(2) 2406(1) 4080(29) 4503(39) 4315(21) 4337(31) 1587(20) 2947(34)
z
61(15) 28(16) 120(49) 111(44) 123(49) 92(43) 293(15) 317(21) 380(11) 1140(246) 1447(373) 1140(172) 1127(274) 784(341) 1902d
U11 b or Uiso
293(15) 317(21) 380(11)
U22
293(15) 317(21) 380(11)
U33
138(18) ÿ38(27) 152(15)
U12
138(18) ÿ38(27) 152(15)
U13
138(18) ÿ38(27) 152(15)
U23
16.2(2) 16.6(3) 31.4(3) 7.5(8) 6.2(6) 9.5(7) 6.5(7) 15.4(32) 28.6(22)
Varied
Occupancyc
96 96 96 96 96 96 16 16 32 7 6 9 6 16 32
Fixed
units of the least signi®cant digit given for the corresponding parameter. b The anisotropic temperature factor expÿ2p2 aÿ2 U11 h2 U22 k 2 U33 l2 U12 hk U13 hl U23 kl. c Occupancy factors are given as the number of atoms or ions per unit cell. d This parameter showed oscillatory behavior in least-squares re®nement. It was ®xed at a mean value.
(b) Crystal 2, dehydrated Ag92 ±X Si 96(g) ÿ551(2) 1242(3) 360(2) 125(25) 125(25) 79(25) 11(36) 14(26) ÿ37(37) 96 Al 96(g) ÿ543(2) 351(3) 1248(2) 125(23) 160(26) 197(25) ÿ1(25) 23(35) 21(37) 96 O(1) 96(g) ÿ1049(4) 4(5) 1083(4) 245(80) 248(76) 313(94) ÿ1(69) 9(55) 48(72) 96 O(2) 96(g) ÿ28(4) ÿ14(4) 1421(4) 138(63) 77(59) 98(70) 20(56) 44(55) 30(56) 96 O(3) 96(g) ÿ348(5) 712(5) 743(5) 547(96) 120(81) 185(79) ÿ69(69) 147(73) ÿ140(67) 96 O(4) 96(g) ÿ701(5) 732(5) 1734(5) 197(71) 251(75) 224(71) 6(72) ÿ91(71) ÿ149(66) 96 Ag(1) 16(c) I 0 0 0 329(57) 329(57) 329(57) 11(66) 11(66) 11(66) 3.2(1) 3 Ag(2) 32(e) I0 370(2) 370(2) 370(2) 1016(12) 1016(12) 1016(12) 854(14) 854(14) 854(14) 26.5(2) 26 Ag(3) 32(e) I0 744(6) 744(6) 744(6) 1204(101) 6.2(2) 6 Ag(4) 32(e) II 2219(1) 2219(1) 2219(1) 576(8) 576(8) 576(8) 369(11) 369(11) 369(11) 33.5(2) 32 Ag(5) 48(f) 1250 1250 2008(15) 469(152) 2.4(2) 2 Ag(6) 96(g) III0 1008(4) 1496(5) 3943(5) 879(94) 798(100) 679(87) 438(76) ÿ506(62) ÿ297(74) 16.2(3) 16 Ag(7) 96(g) III0 2992(25) 1532(24) 4483(25) 722(215) 3.4(4) 4 2385(19) 1821(18) 4161(18) 266(114) 3.2(3) 3 Ag(8) 96(g) III0 a origin at center of symmetry. Positional and anisotropic thermal parameters are given in 104 scale. Numbers in parentheses are the e.s.d.s in the Space group Fd3;
(a) Crystal 1, hydrated Ag92 ±X Si 96(g) Al 96(g) O(1) 96(g) O(2) 96(g) O(3) 96(g) O(4) 96(g) Ag(1) 16(c) I Ag(3) 32(e) I0 Ag(4) 32(e) II Ag(6) 96(g) III0 Ag(7) 96(g) III0 Ag(8) 96(g) III0 Ag(9) 96(g) III0 O(5) 32(e) O(6) 32(e)
Atom
Table 1 Positional, thermal, and occupancy parametersa
52 S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
53
to 0.987 for crystal 2. This correction had little eect on the ®nal error indexes. Other details are same as previously reported [20].
values. The ®nal dierence Fourier function was featureless.
3. Structure determination
Full-matrix least-squares re®nement of dehydrated Ag92 ±X was initiated with the atomic parameters of the framework atoms [Si, Al, O(1), O(2), O(3), and O(4)] in hydrated Ag92 ±X. Initial isotropic re®nement of the framework atoms converged to R1 0:43 and R2 0:51. A dierence Fourier function revealed three large peaks at (0.22, 0.22, 0.22), (0.038, 0.038, 0.038), and (0.0, 0.0, 0.0) with heights of 14.0, 9.3, ÿ3 , respectively. Isotropic re®nement and 8.4 eA including them as Ag(4), Ag(2), and Ag(1), respectively, converged to R1 0:21 and R2 0:24 with occupancies of 33.5(2), 26.5(2), and 3.2(1), respectively. A subsequent dierence Fourier synthesis showed three peaks at Ag(3) (0.07, 0.07, 0.07), Ag(6) (0.40, 0.10, 0.15), and Ag(8) (0.42, 0.24, 0.18). Anisotropic re®nement of the framework atoms and isotropic re®nement of all silver positions converged to R1 0:119 and R2 0:113. On an ensuing dierence Fourier function, peaks appeared at Ag(5) (0.125, 0.125, 0.200) and Ag(7) (0.45, 0.30, 0.15). Anisotropic re®nement of the framework atoms, Ag(4), and Ag(6), and isotropic re®nement of the remaining silver positions converged to R1 0:047 and R2 0:041 (Table 1). The ®nal dierence Fourier function was featureless. Atomic scattering factors for Si, Al, Oÿ , and Ag were used for both structures [22,23]. Atomic scattering factors were modi®ed to account for anomalous dispersion [24]. The ®nal structural parameters and selected interatomic distances and angles are presented in Tables 1 and 2.
3.1. Hydrated Ag92 ±X Full-matrix least-squares re®nement of hydrated Ag92 ±X was initiated with the atomic parameters of the framework atoms [Si, Al, O(1), O(2), O(3), and O(4)] in dehydrated Tl92 ±X [21]. Initial isotropic re®nement of the framework Patoms jFo ÿ converged to an unweighted R1 index, P jFP Fo , of 0.52 and a weighted R index, c k= 2 P w Fo ÿ jFc j2 = wFo2 1=2 , of 0.60. A dierence Fourier function revealed three large peaks, at (0.0, 0.0, 0.0), (0.24, 0.24, 0.24), and (0.07, 0.07, 0.07) with heights of 25, 21, and 14 ÿ3 , respectively. Re®nement including them as eA Ag ions at Ag(1), Ag(4), and Ag(3), respectively, with isotropic temperature factors converged to R1 0:122 and R2 0:156. A subsequent dierence Fourier synthesis showed a peak at Ag(7), ÿ3 and another at (0.45, 0.35, 0.19), of height 4.6 eA ÿ3 . SiAg(8), (0.42, 0.24, 0.17), of height 4.1 eA multaneous re®nement of positional and isotropic thermal parameters for the framework atoms and Ag ions at Ag(1), Ag(3), Ag(4), Ag(7), and Ag(8) converged to the error indexes R1 0:113 and R2 0:128. On an ensuing dierence Fourier function, four peaks appeared: at Ag(6), (0.39, 0.29, 0.20), height ÿ3 ; ÿ3 ; Ag(9), (0.43, 0.18, 0.36), height 1.6 eA 1.9 eA ÿ3 O(5), (0.29, 0.29, 0.29), height 1.2 eA ; and O(6), ÿ3 . Simultaneous (0.16, 0.16, 0.16), height 1.1 eA positional and isotropic thermal parameter re®nement of all atoms with non-framework occupancy numbers varying converged to the error indexes R1 0:081 and R2 0:098. With Ag(1), Ag(3), and Ag(4) re®ning anisotropically, the error indexes become R1 0:075 and R2 0:093 (Table 1). The occupancy numbers at the silver and water oxygen positions were ®xed as shown there. The ®nal error indexes converged to R1 0:088 and R2 0:104. Surely the many unlocated water molecules contribute to these relatively high R
3.2. Dehydrated Ag92 ±X
4. Discussion 4.1. Description of zeolite X Zeolite X is a synthetic Al-rich analog of the naturally occurring mineral faujasite (FAU, see Fig. 1). The 14-hedron with 24 vertexes known as
54
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
Table 2 and angles (deg)a Selected bond distances (A) Si±O(1) Si±O(2) Si±O(3) Si±O(4) mean Al±O(1) Al±O(2) Al±O(3) Al±O(4) mean Ag(1)±O(3) Ag(2)±O(3) Ag(3)±O(3) Ag(4)±O(2) Ag(5)±O(3) Ag(6)±O(1) Ag(7)±O(1) Ag(8)±O(1) Ag(9)±O(1) Ag(1)±Ag(3) Ag(2)±Ag(2) Ag(2)±Ag(5) Ag(3)±O(5) Ag(4)±O(6) O(5)±O(5)
Crystal 1
Crystal 2
1.638(18) 1.670(20) 1.667(20) 1.640(19) 1.654 1.689(19) 1.685(20) 1.707(20) 1.698(19) 1.695 2.581(17)
1.601(12) 1.650(12) 1.643(14) 1.604(14) 1.625 1.696(13) 1.735(12) 1.729(14) 1.717(14) 1.719 2.733(12) 2.206(12) 2.753(12) 2.273(10) 2.83(3) 2.702(16) 2.31(6) 2.45(5)
2.561(17) 2.425(16) 2.65(7) 2.72(9) 2.50(6) 2.58(8) 3.045(3) 2.373(10) 2.42(6) 2.61(4)
3.260(9) 3.224(3) 3.156(5)
Crystal 1
Crystal 2
O(1)±Si±O(2) O(1)±Si±O(3) O(1)±Si±O(4) O(2)±Si±O(3) O(2)±Si±O(4) O(3)±Si±O(4) O(1)±Al±O(2) O(1)±Al±O(3) O(1)±Al±O(4) O(2)±Al±O(3) O(2)±Al±O(4) O(3)±Al±O(4) Si±O(1)±Al Si±O(2)±Al Si±O(3)±Al Si±O(4)±Al
112.3(9) 108.0(9) 106.4(9) 106.2(6) 109.1(7) 109.6(6) 111.8(9) 107.3(9) 107.8(9) 106.6(6) 108.0(6) 109.3(6) 135.8(11) 140.4(11) 134.7(11) 144.8(11)
113.3(6) 110.0(7) 109.0(7) 105.9(6) 108.7(7) 109.7(6) 112.9(6) 108.5(7) 111.7(6) 106.4(6) 107.7(6) 109.6(6) 136.8(8) 143.8(7) 140.2(8) 149.5(8)
O(3)±Ag(1)±O(3) O(3)±Ag(2)±O(3) O(3)±Ag(3)±O(3) O(2)±Ag(4)±O(2) O(3)±Ag(5)±O(3) O(1)±Ag(6)±O(4) O(1)±Ag(7)±O(4) O(1)±Ag(8)±O(4) O(1)±Ag(9)±O(4) O(3)±Ag(1)±Ag(3) Ag(3)±Ag(1)±Ag(3) Ag(2)±Ag(5)±Ag(2)
91.1(3)
91.3(3) 118.2(3) 87.9(4) 118.5(4) 82.5(15) 62.3(4) 63.5(16) 66.2(10)
110.1(5) 103.1(4) 51.8(13) 55.6(19) 54.4(11) 57.5(16)
126.2(3) 180b 168.1(15)
a Numbers in parentheses are estimated standard deviations in the units of the least signi®cant digit given for the corresponding values. b Exactly 180° by symmetry.
the sodalite cavity or b-cage may be viewed as its principal building block. The sodalite cavities are connected tetrahedrally at six-rings by bridging oxygens to give double six-rings (D6Rs, hexagonal prisms) and, concomitantly, to give an interconnected set of even larger cavities (supercages) accessible in three dimensions through 12-ring (24membered) windows. The Si and Al atoms occupy the vertexes of these polyhedra. The oxygen atoms lie approximately midway between each pair of Si and Al atoms, but are displaced from those points to give near tetrahedral angles about Si and Al. Single six-rings (S6Rs) are shared by sodalite and supercages, and may be viewed as the entrances to the sodalite units. Each unit cell has eight sodalite units, eight supercages, 16 D6Rs, 16 12-rings, and
32 S6Rs. The entire structure may be viewed as that of diamond with sodalite units replacing carbon atoms and six bridging oxygens replacing each C±C bond. Exchangeable cations that balance the negative charge of the aluminosilicate framework are found within the zeoliteÕs cavities. They are usually found at the following general sites shown in Fig. 1: site I at the center of a D6R, site I0 in the sodalite cavity on the opposite side of one of the D6RÕs six-rings from site I, II0 inside the sodalite cavity near a S6R, II in the supercage adjacent to a S6R, III in the supercage opposite a four-ring between two 12-rings, III0 somewhat or substantially distant from III but otherwise near the wall of the supercage or the edge of a 12-ring.
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
Fig. 1. Framework structure of zeolite X. Near the center of each line segment is an oxygen atom. The dierent oxygen atoms are indicated by the numbers 1 to 4. Silicon and aluminum atoms alternate the tetrahedral intersections, except that Si substitutes for about 4% of the Al atoms. Extraframework cation positions are labeled with Roman numerals.
4.2. Hydrated Ag92 ±X (Ag92 (H2 O)32 ±X) Ag ions are found at seven crystallographic sites and H2 O molecules are found at two. As expected, there are no Ag0 atoms in this structure. The 16 Ag ions at Ag(1) ®ll site I at the centers of the D6Rs (Fig. 2). The octahedral Ag(1)±O(3) is exactly the same as the distance, 2.581(18) A, sum of the ionic radii of Ag and O2ÿ , 1:26 [25], so site I is able to accommodate 1:32 2:58 A Ag ions nicely.
55
The 16 Ag ions at Ag(3) are at site I0 , in the sodalite cavities opposite D6Rs (Fig. 2). This is a 32-fold position, but it is only about half occupied. Each Ag ion lies relatively far inside the sodalite from the plane of the three O(3) cavity, 1.99 A framework oxygens of the D6R to which it is bound. The Ag(3)±O(3) distance is 2.561(17) A, similar to the sum of the ionic radii, 2.58 A. Each Ag ion at Ag(3) is coordinated in a very distorted manner by three framework oxygens at O(3) and one non-framework oxygen at (2.561(17) A) (Fig. 2). O(5) (2.373(10) A) The distance between Ag(1) and Ag(3) is almost the same as the 3.058(1) A 3.045(3) A, distance found for an Ag ±Ag interaction in an organic complex of AgNO3 [26]. Shorter distances have been ranging from 2.655(2) to 3.002(1) A reported for similar Ag ±Ag interactions in six Ag complexes [27±32]. This has been reviewed and discussed by Jansen [33] for Ag ions in oxides, salts, and coordination complexes. Apparently site I0 accepts Ag ions until all site I Ag ions are accommodated: thus site I0 has reached an occupancy limit when it is only half-full. At Ag(4) (site II), 32 Ag ions are each recessed into the supercage from a S6R plane 1.031(5) A at O(2) (Fig. 3). This 32-fold position is also fully occupied. Each of these cations is coordinated in a nearly tetrahedral manner to three O(2) frame and one non-framework oxygens at 2.425(16) A Both distances work oxygen, O(6), at 2.42(6) A.
Fig. 2. (a) Stereoview of a sodalite cavity with two attached D6Rs in hydrated Ag92 ±X. Two Ag ions at Ag(1) (site I), two Ag ions at Ag(3) (site I0 ) and two H2 O molecules at O(5) are shown. The Ag ±Ag interactions between Ag(1) and Ag(3) can be seen. All sodalite is too long for this to be a signi®cant bonding interaction. The water cavities have this arrangement. The Ag(3)±Ag(3) distance, 3.87 A, Ellipsoids of 20% probability are shown. (b) The weak 3.045(3) A Ag Ag bond molecules at O(5) hydrogen bond at 2.61(4) A. through a 6-ring of a D6R.
56
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
Fig. 3. Stereoview of a supercage of hydrated Ag92 ±X. Four Ag ions at Ag(4) (site II), one Ag ion at Ag(6) (site III0 ), one Ag ion at Ag(7) (site III0 ), one Ag ion at Ag(8) (site III0 ), one Ag ion at Ag(9) (site III0 ) and four H2 O molecules at O(6) are shown. About 50% of the supercages have this arrangement. Ellipsoids of 20% probability are shown.
are a little shorter than the sum of the corre sponding ionic radii, 2.58 A. The occupancies at Ag(6), Ag(7), Ag(8), and Ag(9) (all III0 sites) are 7, 6, 9, and 6 per unit cell, respectively (Fig. 3). The Ag(6)±O(1), Ag(7)±O(1), Ag(8)±O(1), and Ag(9)±O(1) distances are 2.65(7), respectively (Table 2.72(9), 2.50(6), and 2.58(8) A, 2). These distances are all near the sum of the ionic These four III0 sites, each a 96-fold radii, 2.58 A. position, are sparsely occupied by a total of only 28 Ag ions. 4.3. Dehydrated Ag92 ±X Ag and Ag0 cannot be distinguished by X-ray crystallography because their scattering powers are virtually identical. They are also not easily distinguished by their approach distances to oxygen in this structure, perhaps because of charge delocalization. Because the existence of silver atoms or reduced cationic clusters has previously been established by e.s.r. and other spectroscopies in similar systems, the clusters seen here are as sumed to be reduced, from 3Ag to Ag2 3 and Ag3 . Silver atoms and ions are found at eight dierent crystallographic sites. Only three Ag ions at Ag(1) lie at site I at the centers of the D6Rs (Fig. 4): thus the D6Rs are only sparsely occupied (three ions per 16 sites per unit cell). Each Ag ion at Ag(1) is coordinated by the six O(3) oxygen atoms somewhat of its D6R at distances of 2.733(12) A, longer than the sum of the ionic radii of Ag and
This distance is likely to be virtual: O2ÿ , 2.58 A. the O(3) position is an average which most accurately describes the 13 D6Rs per unit cell that do not contain Ag ions at their centers. The Ag(2) position is at site I0 , on the 3-fold axis in the sodalite unit opposite D6Rs (Fig. 4). This 32-fold position is occupied by 26 Ag ions. Each is essentially in the plane of the three O(3) framework oxygens of the D6R to which it is bound (see Table 3). The Ag(2)±O(3) distance is 2.206(12) A, much shorter than the sum of the corresponding ionic radii. This indicates that Ag(2)±O(3) bonds are quite covalent, as had been seen for Ag in zeolite A [6,7]. The occupancy numbers at site I (Ag(1)) and site I0 (Ag(2)), 3.0 and 26.0, respectively, indicate that the D6Rs are ®lled: ((no. at I) (no. at I0 ))=2 must not exceed 16 to avoid an Of the intercationic approach distance of 1.62 A. 16 D6Rs per unit cell, three have a Ag ion at their center and 13 have two Ag ions in their six-ring faces at Ag(2). The Ag(2)±Ag(2) distance, 3.224(3) indicates that a weak but signi®cant Ag ±Ag A, interaction [27±32], discussed above for hydrated Ag92 ±X, occurs also in 13 of the 16 D6Rs of this crystal. Note in Figs. 4 and 5 that the Ag ions at Ag(2) do not extend away from their D6Rs to avoid a Ag ±Ag repulsion, as is commonly seen for other cations. Six Ag species at Ag(3) are located at site I0 in the sodalite cavity opposite the D6Rs. Each spe into the sodalite cies at Ag(3) is recessed 1.00 A cavity. This 32-fold position is occupied by only
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
57
Fig. 4. (a) Stereoview of a sodalite cavity with two attached D6Rs in dehydrated Ag92 ±X. One Ag ion at Ag(1) (site I), four Ag ions at Ag(2) (site I0 ) and two Ag0 atoms at Ag(3) (site I0 ) are shown. The Ag ±Ag interaction between Ag ions at Ag(2) can be seen. About 75% of the sodalite cavities may have this arrangement. Ellipsoids of 20% probability are shown. (b) The Ag 3 cluster in nearly the same orientation as shown in (a).
Table 3 of atoms from six-ring oxygen planes Deviations (A) At O(2)a Ag(4) At O(3)b Ag(1) Ag(2) Ag(3)
Crystal 1
Crystal 2
1.031(5)
0.276(2)
ÿ1.542(3)
ÿ1.625(1) 0.021(2) 1.595(8)
1.509(6)
a
A positive deviation indicates that the atom lies in the supercage. b A positive deviation indicates that the atom lies in the sodalite cavity. A negative deviation indicates that the atom lies in a D6R.
Fig. 5. (a) Stereoview of a sodalite cavity with two attached D6Rs in dehydrated Ag92 ±X. Six Ag ions at Ag(2) (site I0 ) and one Ag0 atom at Ag(5) in the sodalite cavity are shown. This Ag2 3 cluster (Ag(2)±Ag(5)±Ag(2)) is in the sodalite cavity. The Ag ±Ag interaction between Ag ions at Ag(2) can be seen. About 25% of the sodalite cavities may have this arrangement. Ellipsoids of 20% probability are shown. (b) The Ag2 3 cluster in nearly the same orientation as shown in (a).
58
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
Fig. 6. Stereoview of a supercage in dehydrated Ag92 ±X. Four Ag ions at Ag(4) (site II), two Ag ions at Ag(6) (site III0 ), one Ag ion at Ag(7) (site III0 ) and one Ag ion at Ag(8) (site III0 ) are shown. Ag(7) and Ag(8) cannot coexist in the same supercage: only Ag(7) is present in about 50% of the supercages, only Ag(8) is present in about 37.5% of the supercages, and the remainder have neither. Ellipsoids of 20% probability are shown.
six Ag species (Fig. 4). The distance between Ag(3) and its nearest framework oxygens at O(3), is almost the same as the distance 2.753(12) A, between a Ag atom and a framework oxygen, 2.78 in previous studies [6,7]. Therefore, this species A, appears to be more like a Ag0 atom than a Ag is similar ion. The Ag ±Ag0 distance, 3.260(9) A, in Ag±A [6,7] to that between Ag and Ag0 , 3.31 A in Ag±Y [1]. This Ag(1)±Ag(3) interand 3.24 A action cannot be avoided and the linear Ag 3 subcluster, Ag(3)±Ag(1)±Ag(3), must exist (Fig. 4), three per unit cell. Where do the silver atoms come from? In a crystallographic study of zeolite A [7,34], Ag ions are found to be reduced by the following reactions that occurred within the sodalite units: 3
Ag3 H2 O3 ! Ag Ag2 H2 O
2
2H2 O
which is the penultimate step of the dehydration process, followed by Ag2 H2 O
2
! 2H 2Ag 1=2O2
A similar process could be occurring in zeolite X. Two Ag(5) atoms at (0.125, 0.125, 0.2008) lie on 2-fold axes in two sodalite cavities. The distance between Ag(5) and its two nearest framework ox similar to the 2.78 ygens at O(3) is long, 2.83(3) A, distance between a Ag atom and two framework A oxygens in zeolite A [6,7]. Therefore, this species appears to be more like a Ag0 atom than a Ag is an apion. The Ag ±Ag0 distance, 3.156(5) A, propriate Ag ±Ag0 distance (vide supra). To pro-
vide coordination to each silver atom at Ag(5) at its position of symmetry, two Ag(2) ions should be near, and bent Ag2 subclusters, Ag(2)±Ag(5)± 3 Ag(2) 168.1(15)°, should exist (see Fig. 5). The 32 Ag ions at Ag(4) ®ll site II (Fig. 6). into the supercage from Each is recessed 0.73 A the S6R plane at O(2). The Ag(4)±O(2) distance is much like Ag(2)±O(3) quite short, 2.273(10) A, and the Ag ion to framework oxygen distance, in the structure of partially decomposed 2.25 A, Ag12 ±A [6,7]. The occupancies at Ag(6), Ag(7), and Ag(8) (all III0 sites) are 16, 4, and 3, respectively, per unit cell (Fig. 6). The Ag(6)±O(1), Ag(7)±O(1), and Ag(8)± O(1) distances are 2.702(16), 2.31(6), and 2.45(5) respectively. These distances are all near the A, These three III0 sum of the ionic radii, 2.58 A. positions, each of order 96, are sparsely occupied by a total of 23 Ag ions. The Ag-atom production seen in crystal 2 is relatively small. When fully Ag -exchanged zeolites are dehydrated in the absence of oxygen, more silver atoms are produced and, necessarily, more oxygen is lost from zeolite framework [14,34]. Such structures have less crystallinity and are often de®cient in Ag due to Ag atom migration out of the zeolite [14]. Crystallographic work on such systems has yielded less de®nitive results [14]. 4.4. Additional concerns It could be that Ag 92 ±X, fully dehydrated zeolite X with all Ag in the 1 oxidation state, formed
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
at 360°C in ¯owing oxygen. The removal of O2 (g) by evacuation at 400°C (see Section 2) may then have led to the formation of eight Ag atoms per unit cell by the reaction, involving framework oxygens, 8Ag 4O2ÿ ! 8Ag0 2O2 , resulting in 2 the formation of three Ag 3 and two Ag3 clusters. Cooling to room temperature in ¯owing oxygen before evacuation might have avoided this decomposition to preserve Ag 92 ±X as the ®nal product. Alternatively, the ®nal dehydrated structure 2 might actually be Ag 92 ±X with no Ag3 nor Ag3 clusters. The variability of the Ag±O bond length in chemistry makes the assignment of the formal charge to Ag (1 or 0 ) dicult. If it is Ag 92 ±X, there would be more Ag ±Ag interactions than presented above. In support of this is the striking similarity between the bond lengths in the pro2 cluster posed Ag 3 cluster (3.260 A), the Ag3 (3.156 A), and the Ag ±Ag interaction (3.224 A) (Table 2). Arguing against this possibility is the occupancy at I0 in crystal 1 which indicates that Ag3 3 has not formed [33]. 5. Summary In partially hydrated Ag92 ±X, all Ag cations are found at sites I, I0 , II, and four III0 sites. A H2 O molecule coordinates to each Ag ion at Ag(3) (site I) and Ag(4) (site II) to complete a tetrahedral coordination geometry about Ag . Dehydration of Ag92 ±X under ¯owing O2 gas at 360°C, followed by evacuation at 400°C, has apparently produced reduced trisilver clusters. Comparisons with previous work suggest that the linear clusters that extend along 3-fold axes through double six-rings are Ag 3 and that the slightly bent clusters in sodalite cavities are Ag2 3 . In both structures, weakly attractive Ag ±Ag interactions are seen. Acknowledgements The authors wish to acknowledge the ®nancial support of the Korea Research Foundation made in the program year of 1998 (Project No. 1998-15D00146).
59
References [1] P. Gallezot, Catal. Rev. Sci. Engng. 20 (1979) 121. [2] I.E. Maxwell, Adv. Chem. Catal. 31 (1982) 2. [3] R. Beer, G. Calzaferri, J. Li, B. Waldeck, Coord. Chem. Rev. 111 (1991) 193. [4] H. Beyer, P.A. Jacobs, J.B. Uytterhoeven, J. Chem. Soc. Faraday Trans. 72 (1976) 674. [5] H. Beyer, P.A. Jacobs, Metal Microstructures in Zeolites, Studies in Surface Science and Catalysis, vol. 12, Elsevier, Amsterdam, 1982, p. 95. [6] Y. Kim, K. Se, J. Am. Chem. Soc. 99 (1977) 7055. [7] Y. Kim, K. Se, J. Am. Chem. Soc. 100 (1978) 6989. [8] R.A. Schoonheydt, H. Leeman, J. Phys. Chem. 93 (1989) 2048. [9] L.R. Gellens, W.J. Mortier, J.B. Uytterhoeven, Zeolites 1 (1981) 85. [10] L. Kevan, N. Narayana, ACS Symp. Ser. 218 (1983) 283. [11] Y. Kim, K. Se, J. Phys. Chem. 100 (1978) 1071. [12] D.C. Nikola, A.P. Nadezda, Zeolites 14 (1994) 35. [13] Y. Kim, K. Se, Bull. Kor. Chem. Soc. 5 (1984) 135. [14] S.J. Choi, S.H. Lee, Y. Kim, unpublished results. [15] V.N. Bogomolov, V.P. Petranovskii, Zeolites 6 (1986) 418. [16] D.W. Breck, W.G. Eversole, R.M. Milton, T.B. Reed, T.L. Thomas, J. Am. Chem. Soc. 78 (1956) 5963. [17] H.S. Sherry, J. Phys. Chem. 71 (1967) 1457. [18] M. Nitta, K. Aomura, S. Matsumoto, J. Catal. 35 (1974) 317. [19] International Tables for X-ray Crystallography, vol. II, Kynoch Press, Birmingham, England, 1974, p. 302. [20] J.H. Kwon, S.B. Jang, Y. Kim, K. Se, J. Phys. Chem. 100 (1996) 13720. [21] Y. Kim, Y.W. Han, K. Se, Zeolites 18 (1997) 325. [22] D.T. Cromer, Acta Crystallogr. 18 (1965) 17. [23] International Tables for X-ray Crystallography, vol. IV, Kynoch Press, Birmingham, England, 1974, p. 73. [24] International Tables for X-ray Crystallography, vol. IV, Kynoch Press, Birmingham, England, 1974, p. 149. [25] Handbook of Chemistry and Physics, 70th ed., The Chemical Rubber Co., Cleveland, Ohio, 1989/1990, p. F-187. [26] M.A. Romero, J.M. Salas, M. Quiros, J. Molina, J. Mole. Struct. 354 (1995) 189. [27] N.C. Baenziger, A.W. Stuss, Inorg. Chem. 15 (1976) 1807. [28] C. Gagnon, J. Hubert, R. Rivest, A.L. Beauchamp, Inorg. Chem. 16 (1977) 2469. [29] F.A. Cotton, X. Feng, M. Matusz, R. Poli, J. Am. Chem. Soc. 110 (1988) 7077. [30] M. Munakata, M. Maekawa, S. Kitayama, M. Adachi, H. Masuda, Inorg. Chim. Acta 167 (1990) 181. [31] D. Fenske, G. Baum, A. Zinn, K. Dehnicke, Z. Naturforsch. Teil B 45 (1990) 1273. [32] F. Olbrich, B. Zimmer, M. Kastner, C. von Schlabrendorn, G. Vetter, G.Z. Klar, Naturforsch. Teil B 47 (1992) 1571. [33] M. Jansen, Angew. Chem. Int. Ed. Engl. 26 (1987) 1098. [34] T. Sun, K. Se, Chem. Rev. 94 (1994) 857.
www.elsevier.nl/locate/micromeso
Weak Ag ±Ag bonding in zeolite X. Crystal structures of Ag92Si100Al92O384 hydrated and fully dehydrated in ¯owing oxygen Seok Hee Lee a, Yang Kim a,1, Karl Se b,* b
a Department of Chemistry, Pusan National University, Pusan 609-735, South Korea Department of Chemistry, University of Hawaii at Manoa, 2545 The Mall, Honolulu, HI 96822-2275, USA
Received 1 February 2000; accepted 26 June 2000
Abstract and the other fully deTwo crystal structures of fully Ag -exchanged zeolite X, one hydrated (a 24:996(4) A) have been determined by single-crystal X-ray diraction techniques in the cubic space hydrated (a 25:200(4) A), group Fd 3 at 21(1)°C. Each initial Na92 ±X crystal was ion exchanged in a ¯owing stream of 0.05 M aqueous AgNO3 . The second crystal was dehydrated at 360°C for two days in a ¯owing stream of oxygen gas (790 Torr) followed by evacuation at 400°C and 2 10ÿ6 Torr for 2 h. Their structures were re®ned to the ®nal error indexes R1 =R2 0:088=0:104 with 216 re¯ections, and R1 =R2 0:047=0:041 with 312 re¯ections, respectively, for which Ag ±Ag interactions. In the hydrated crystal, 92 Ag ions I > 3r I. Both structures show weakly attractive 3.0±3.3 A were found at seven crystallographic sites: 16 ®ll site I at the centers of the double six-rings, 16 at site I0 in the sodalite to those at site I, 32 ®ll site II in the supercages, and 28 cavities opposite double six-rings bond weakly (3.045(3) A) occupy four dierent III0 sites. Some H2 O molecules were found at two dierent 3-fold axis sites: 16 coordinate to site I0 Ag ions in the sodalite cavities, and 32 coordinate to site II Ag ions in the supercage. In the dehydrated crystal, Ag ions or atoms were found at eight crystallographic sites: three Ag ions are at site I, 26 Ag ions and six Ag0 atoms are at two I0 sites in the sodalite cavities ®lling site I0 , 32 Ag ions ®ll site II as in crystal 1, two Ag0 atoms are on 2-fold axes in the sodalite cavities, and 23 Ag ions occupy three dierent III0 sites. The 26 Ag ions at site I0 bond weakly in pairs Three linear Ag clusters per unit cell with atoms at sites I0 , I, and I0 , respectively, lie along 3-fold axes, and (3.224(3) A). 3 two bent 168(2)° Ag2 3 clusters per unit cell are in the sodalite cavities. It remains possible, considering Ag±Ag and Ag± O distances, that no Ag0 atoms have formed, that the product is (Ag )92 ±X, and that the bonding in the clusters, both of which would then be Ag3 3 , is due to additional Ag ±Ag interactions. Ó 2000 Elsevier Science B.V. All rights reserved. 2 Keywords: Zeolite X; Structure; Silver; Clusters; Ag2 2 ; Ag3 ; Ag3
1. Introduction *
Corresponding author. Fax: +1-808-956-5908. E-mail address: kse@gold.chem.hawaii.edu (K. Se). 1 Also corresponding author.
Small clusters of the group VIII transition metals, Ni, Pd, and Pt, have been extensively studied within zeolites because of their pronounced
1387-1811/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 0 ) 0 0 2 7 0 - 5
50
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
catalytic activity [1±3]. Related elements of lesser catalytic importance include rhodium, iridium, and silver. Silver-exchanged zeolites have been well studied in recent years, both for their own importance and as models for other systems. Beyer and Jacobs studied the kinetics and mechanisms of silver-cluster formation in Ag forms of zeolites treated with hydrogen gas [4,5]. For zeolite Ag±Y [1], two reduction mechanisms were established: one at low temperature and another at high temperature (T > 157°C). At low temperatures, the regeneration of centers for hydrogen activation is the rate-determining step of the silver ion reduction; at higher temperatures, the process of Ag ion migration is rate determining. Upon attempted dehydration, some Ag ions in zeolite A are reduced by water oxygens or oxygens of the zeolite framework to form neutral Ag6 silver clusters [6,7]. Each octahedral hexasilver cluster lies within a cube of eight Ag ions, each of which is nearly in the plane of a six-oxygen ring. The concentration of silver clusters within the zeolite has been found to depend upon the dehydration time and temperature. (Ag6 )0 stabilized by coordination to eight Ag ions may also be viewed as (Ag14 )8 whose atoms are arranged approximately as one unit cell of face-centered cubic (cubic closest packing). Schoonheydt and Leeman [8] used e.s.r. to folclusters upon H2 relow the formation of Agn 6 duction of zeolite Ag, M±A (M Na , K , Cs , and Ca2 ) over the temperature range ÿ15 to 25°C. They found that the diusion of the cation with the highest activation energy determines the rate of Ag 6 cluster formation. Thus, in Ag, Na±A the rate depends on the mobility of the Ag cation. Its activation energy, determined from the temperature dependence of the initial rate of Ag 6 cluster formation, was established to be in the range 47±63 kJ/mol, for samples with three, four, and six Ag ions per unit cell. Schoonheydt and Leeman also recorded the e.s.r. spectra of Ag, Na± X [8]. After the same pretreatment and reduction procedures as was used for Ag, Na±A, and no Agn 6 clusters were observed. The redox reactions of Ag±Y with oxygen and hydrogen were followed by powder X-ray dirac-
tion techniques [9]. Oxygen treatment of dehydrated Ag±Y under vacuum at 600°C, which had already formed reduced linear Ag2 clusters by 3 autoreduction [10], caused Ag ions to migrate into the sodalite units and the D6Rs from general positions in the supercages. The reaction for autoreduction may be viewed as (1) partial decomposition of the Ag2 O component of the silver aluminosilicate (Ag2 O)46 (Al2 O3 )46 (SiO2 )100 , (2) 4Ag 2H2 O ! 4H 4Ag O2 or (3) 4Ag 2OHÿ ! 2H 4Ag O2 . (The hydroxide ions in (3) are from the dissociation of water; this equation is written to indicate that bent Ag ±OHÿ ±Ag clusters or cyclic (Ag )3 (OHÿ )3 clusters [11] in sodalite units may be the immediate precursors of the Ag atoms and clusters with atoxygen molecules.) Linear Ag2 3 oms at sites I0 , I, and I0 , respectively, formed along the 3-fold axes. After hydrogen reduction at 75°C, a considerable increase in occupancy at site I0 , partly at the expense of site I, was observed. Nikola and Nadezda [12] studied the kinetics and mechanism of silver-cluster formation in zeolite Ag±X by diuse re¯ectance spectroscopy. The oxidation of the zeolite Ag±X samples was performed in ¯owing O2 after dehydration at 300°C and 512°C. They found that the 410 nm band, used for kinetic investigations, is associated with the silver clusters, most probably the linear Ag2 3 cluster at sites I0 ±I±I0 , that formed in some of the small cages of the zeolite. Beyer et al. reported that polynuclear cations of mean or approximate composition Ag 3 form upon partial reduction of dehydrated fully Ag exchanged zeolite Y [4]. Hydrogen uptake by silver and Ag chabazite produced Ag2 4 3 clusters according to e.p.r. measurements. Subsequently, the silver clusters (Ag3 ) and (Ag3 )2 were identi®ed in the large cavity of dehydrated Ag6 Na6 ±A treated with 50 Torr of H2 at room temperature [13]. This work was done to see whether fully dehydrated, fully Ag -exchanged zeolite X could be prepared. If so, its cation distribution would be learned. Dehydration was done in a ¯owing stream of oxygen gas to inhibit both autoreduction and reaction with leaving H2 O molecules. Repeated
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
attempts to study Ag±X by dehydration under vacuum had given complex results with substantial Ag reduction and some crystal damage [14]. Some Ag ions in zeolite X were expected to be reduced upon dehydration even in an O2 atmosphere [22], so it was hoped that the stoichiometry, positions, and geometries of new silver clusters within the cavities of zeolite X would be learned. For comparison, the structure of the fully hydrated material was also determined. 2. Experimental section Large single crystals of sodium zeolite X, stoichiometry Na92 Si100 Al92 O384 , were prepared in St. Petersburg, Russia [15]. Each of two single crystals, colorless octahedra about 0.20 mm in crosssection, was lodged in a ®ne pyrex capillary. An exchange solution of 0.05 M aqueous AgNO3 (99.9999% purity, Aldrich Chem. Co.) was allowed to ¯ow past each crystal at a velocity of 10 mm/s for three days. Since the exchange of Ag for Na has been shown to be facile and complete after milder treatment [16±18], complete exchange was assured in this case. The ®rst crystal was evacuated at 25°C and 1 10ÿ3 Torr for 2 h to remove excess moisture. It was then removed from the vacuum line and its capillary tip was opened to the atmosphere. Two days were allowed for rehydration before data collection began on the resulting colorless crystal. The second crystal was dehydrated at 360°C for two days in a ¯owing stream of oxygen gas (P 790 Torr) followed by evacuation at 400°C and 2 10ÿ6 Torr for 2 h. Care was taken (a bank of zeolite beads in series evacuated in situ) to prevent polar molecules from downstream parts of the vacuum system that had not been baked out from reaching the crystal. After cooling to room temperature, the second crystal, still under vacuum and pale brown in color, was sealed in its capillary by torch. The space group Fd 3 was used for both crystals. This choice is supported by (a) the systematic absences, (b) the low Si/Al ratio, which in turn requires, at least in the short range, alternation of Si and Al, and (c) the observation that these crystals, like all other crystals from the same batch that
51
have not suered minor damage due to chemical treatment such as acidity, do not have intensity symmetry across (1 1 0) and therefore lack that mirror plane. This choice is also supported by the ®nal results: The Al±O bonds are all longer than the Si±O bonds. Diraction data were collected with an automated Enraf-Nonius four-circle computer-controlled CAD-4 diractometer equipped with a pulse-height analyzer and a graphite monochromator, using MoKa radiation (Ka1 , k 0:70930 The unit cell constant at Ka2 , k 0:71359 A). A, 21(1)°C determined by least-squares re®nement of 25 intense re¯ections for which 14° < 2h < 24° are for crystal 1 and 25.200(4) A for a 24:996(4) A crystal 2. The x ÿ 2h scan technique was used. The data were collected using variable scan speeds. Most re¯ections were observed at slow scan speeds, ranging between 0.24 and 0.34° minÿ1 in x. The intensities of three re¯ections in diverse regions of reciprocal space were recorded every 3 h to monitor crystal and instrument stability. Only small random ¯uctuations of these check re¯ections were noted during the course of data collection. All unique re¯ections in the positive octant of an Fcentered unit cell for which 2h < 50 , l > h, and k > h were examined. The raw data were corrected for Lorentz and polarization eects, including that due to incident beam monochromatization, and the resultant estimated standard deviations were assigned to each re¯ection by the computer programs G E N E S I S and 2 P R O C E S S , respectively. Of the 1267 re¯ections examined for both crystals 1 and 2, only the 216 and 312 re¯ections, respectively, for which I > 3r I were used in subsequent structure determinations. An absorption correction (lR 0:37 and qcal 2:271 g cmÿ3 for crystal 1, and lR 0:36 and qcal 2:216 g cmÿ3 for crystal 2) was made empirically using a w scan for each crystal [19]. The calculated transmission coecients ranged from 0.982 to 0.989 for crystal 1, and from 0.984
2 Calculations were performed with structure determination package programs, M O L E N , Enraf-Nonius, Netherlands, 1990.
Wyc. position
Site
ÿ533(3) ÿ534(3) ÿ1071(6) ÿ24(7) ÿ321(7) ÿ726(6) 0 703(2) 2406(1) 2767(28) 3480(40) 2463(22) 1801(31) 1587(20) 2947(34)
x
1241(4) 361(3) ÿ63(8) ÿ27(7) 700(7) 777(6) 0 703(2) 2406(1) 1952(28) 1916(40) 1723(21) 3639(30) 1587(20) 2947(34)
y
358(3) 1241(4) 1058(6) 1451(6) 687(7) 1742(6) 0 703(2) 2406(1) 4080(29) 4503(39) 4315(21) 4337(31) 1587(20) 2947(34)
z
61(15) 28(16) 120(49) 111(44) 123(49) 92(43) 293(15) 317(21) 380(11) 1140(246) 1447(373) 1140(172) 1127(274) 784(341) 1902d
U11 b or Uiso
293(15) 317(21) 380(11)
U22
293(15) 317(21) 380(11)
U33
138(18) ÿ38(27) 152(15)
U12
138(18) ÿ38(27) 152(15)
U13
138(18) ÿ38(27) 152(15)
U23
16.2(2) 16.6(3) 31.4(3) 7.5(8) 6.2(6) 9.5(7) 6.5(7) 15.4(32) 28.6(22)
Varied
Occupancyc
96 96 96 96 96 96 16 16 32 7 6 9 6 16 32
Fixed
units of the least signi®cant digit given for the corresponding parameter. b The anisotropic temperature factor expÿ2p2 aÿ2 U11 h2 U22 k 2 U33 l2 U12 hk U13 hl U23 kl. c Occupancy factors are given as the number of atoms or ions per unit cell. d This parameter showed oscillatory behavior in least-squares re®nement. It was ®xed at a mean value.
(b) Crystal 2, dehydrated Ag92 ±X Si 96(g) ÿ551(2) 1242(3) 360(2) 125(25) 125(25) 79(25) 11(36) 14(26) ÿ37(37) 96 Al 96(g) ÿ543(2) 351(3) 1248(2) 125(23) 160(26) 197(25) ÿ1(25) 23(35) 21(37) 96 O(1) 96(g) ÿ1049(4) 4(5) 1083(4) 245(80) 248(76) 313(94) ÿ1(69) 9(55) 48(72) 96 O(2) 96(g) ÿ28(4) ÿ14(4) 1421(4) 138(63) 77(59) 98(70) 20(56) 44(55) 30(56) 96 O(3) 96(g) ÿ348(5) 712(5) 743(5) 547(96) 120(81) 185(79) ÿ69(69) 147(73) ÿ140(67) 96 O(4) 96(g) ÿ701(5) 732(5) 1734(5) 197(71) 251(75) 224(71) 6(72) ÿ91(71) ÿ149(66) 96 Ag(1) 16(c) I 0 0 0 329(57) 329(57) 329(57) 11(66) 11(66) 11(66) 3.2(1) 3 Ag(2) 32(e) I0 370(2) 370(2) 370(2) 1016(12) 1016(12) 1016(12) 854(14) 854(14) 854(14) 26.5(2) 26 Ag(3) 32(e) I0 744(6) 744(6) 744(6) 1204(101) 6.2(2) 6 Ag(4) 32(e) II 2219(1) 2219(1) 2219(1) 576(8) 576(8) 576(8) 369(11) 369(11) 369(11) 33.5(2) 32 Ag(5) 48(f) 1250 1250 2008(15) 469(152) 2.4(2) 2 Ag(6) 96(g) III0 1008(4) 1496(5) 3943(5) 879(94) 798(100) 679(87) 438(76) ÿ506(62) ÿ297(74) 16.2(3) 16 Ag(7) 96(g) III0 2992(25) 1532(24) 4483(25) 722(215) 3.4(4) 4 2385(19) 1821(18) 4161(18) 266(114) 3.2(3) 3 Ag(8) 96(g) III0 a origin at center of symmetry. Positional and anisotropic thermal parameters are given in 104 scale. Numbers in parentheses are the e.s.d.s in the Space group Fd3;
(a) Crystal 1, hydrated Ag92 ±X Si 96(g) Al 96(g) O(1) 96(g) O(2) 96(g) O(3) 96(g) O(4) 96(g) Ag(1) 16(c) I Ag(3) 32(e) I0 Ag(4) 32(e) II Ag(6) 96(g) III0 Ag(7) 96(g) III0 Ag(8) 96(g) III0 Ag(9) 96(g) III0 O(5) 32(e) O(6) 32(e)
Atom
Table 1 Positional, thermal, and occupancy parametersa
52 S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
53
to 0.987 for crystal 2. This correction had little eect on the ®nal error indexes. Other details are same as previously reported [20].
values. The ®nal dierence Fourier function was featureless.
3. Structure determination
Full-matrix least-squares re®nement of dehydrated Ag92 ±X was initiated with the atomic parameters of the framework atoms [Si, Al, O(1), O(2), O(3), and O(4)] in hydrated Ag92 ±X. Initial isotropic re®nement of the framework atoms converged to R1 0:43 and R2 0:51. A dierence Fourier function revealed three large peaks at (0.22, 0.22, 0.22), (0.038, 0.038, 0.038), and (0.0, 0.0, 0.0) with heights of 14.0, 9.3, ÿ3 , respectively. Isotropic re®nement and 8.4 eA including them as Ag(4), Ag(2), and Ag(1), respectively, converged to R1 0:21 and R2 0:24 with occupancies of 33.5(2), 26.5(2), and 3.2(1), respectively. A subsequent dierence Fourier synthesis showed three peaks at Ag(3) (0.07, 0.07, 0.07), Ag(6) (0.40, 0.10, 0.15), and Ag(8) (0.42, 0.24, 0.18). Anisotropic re®nement of the framework atoms and isotropic re®nement of all silver positions converged to R1 0:119 and R2 0:113. On an ensuing dierence Fourier function, peaks appeared at Ag(5) (0.125, 0.125, 0.200) and Ag(7) (0.45, 0.30, 0.15). Anisotropic re®nement of the framework atoms, Ag(4), and Ag(6), and isotropic re®nement of the remaining silver positions converged to R1 0:047 and R2 0:041 (Table 1). The ®nal dierence Fourier function was featureless. Atomic scattering factors for Si, Al, Oÿ , and Ag were used for both structures [22,23]. Atomic scattering factors were modi®ed to account for anomalous dispersion [24]. The ®nal structural parameters and selected interatomic distances and angles are presented in Tables 1 and 2.
3.1. Hydrated Ag92 ±X Full-matrix least-squares re®nement of hydrated Ag92 ±X was initiated with the atomic parameters of the framework atoms [Si, Al, O(1), O(2), O(3), and O(4)] in dehydrated Tl92 ±X [21]. Initial isotropic re®nement of the framework Patoms jFo ÿ converged to an unweighted R1 index, P jFP Fo , of 0.52 and a weighted R index, c k= 2 P w Fo ÿ jFc j2 = wFo2 1=2 , of 0.60. A dierence Fourier function revealed three large peaks, at (0.0, 0.0, 0.0), (0.24, 0.24, 0.24), and (0.07, 0.07, 0.07) with heights of 25, 21, and 14 ÿ3 , respectively. Re®nement including them as eA Ag ions at Ag(1), Ag(4), and Ag(3), respectively, with isotropic temperature factors converged to R1 0:122 and R2 0:156. A subsequent dierence Fourier synthesis showed a peak at Ag(7), ÿ3 and another at (0.45, 0.35, 0.19), of height 4.6 eA ÿ3 . SiAg(8), (0.42, 0.24, 0.17), of height 4.1 eA multaneous re®nement of positional and isotropic thermal parameters for the framework atoms and Ag ions at Ag(1), Ag(3), Ag(4), Ag(7), and Ag(8) converged to the error indexes R1 0:113 and R2 0:128. On an ensuing dierence Fourier function, four peaks appeared: at Ag(6), (0.39, 0.29, 0.20), height ÿ3 ; ÿ3 ; Ag(9), (0.43, 0.18, 0.36), height 1.6 eA 1.9 eA ÿ3 O(5), (0.29, 0.29, 0.29), height 1.2 eA ; and O(6), ÿ3 . Simultaneous (0.16, 0.16, 0.16), height 1.1 eA positional and isotropic thermal parameter re®nement of all atoms with non-framework occupancy numbers varying converged to the error indexes R1 0:081 and R2 0:098. With Ag(1), Ag(3), and Ag(4) re®ning anisotropically, the error indexes become R1 0:075 and R2 0:093 (Table 1). The occupancy numbers at the silver and water oxygen positions were ®xed as shown there. The ®nal error indexes converged to R1 0:088 and R2 0:104. Surely the many unlocated water molecules contribute to these relatively high R
3.2. Dehydrated Ag92 ±X
4. Discussion 4.1. Description of zeolite X Zeolite X is a synthetic Al-rich analog of the naturally occurring mineral faujasite (FAU, see Fig. 1). The 14-hedron with 24 vertexes known as
54
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
Table 2 and angles (deg)a Selected bond distances (A) Si±O(1) Si±O(2) Si±O(3) Si±O(4) mean Al±O(1) Al±O(2) Al±O(3) Al±O(4) mean Ag(1)±O(3) Ag(2)±O(3) Ag(3)±O(3) Ag(4)±O(2) Ag(5)±O(3) Ag(6)±O(1) Ag(7)±O(1) Ag(8)±O(1) Ag(9)±O(1) Ag(1)±Ag(3) Ag(2)±Ag(2) Ag(2)±Ag(5) Ag(3)±O(5) Ag(4)±O(6) O(5)±O(5)
Crystal 1
Crystal 2
1.638(18) 1.670(20) 1.667(20) 1.640(19) 1.654 1.689(19) 1.685(20) 1.707(20) 1.698(19) 1.695 2.581(17)
1.601(12) 1.650(12) 1.643(14) 1.604(14) 1.625 1.696(13) 1.735(12) 1.729(14) 1.717(14) 1.719 2.733(12) 2.206(12) 2.753(12) 2.273(10) 2.83(3) 2.702(16) 2.31(6) 2.45(5)
2.561(17) 2.425(16) 2.65(7) 2.72(9) 2.50(6) 2.58(8) 3.045(3) 2.373(10) 2.42(6) 2.61(4)
3.260(9) 3.224(3) 3.156(5)
Crystal 1
Crystal 2
O(1)±Si±O(2) O(1)±Si±O(3) O(1)±Si±O(4) O(2)±Si±O(3) O(2)±Si±O(4) O(3)±Si±O(4) O(1)±Al±O(2) O(1)±Al±O(3) O(1)±Al±O(4) O(2)±Al±O(3) O(2)±Al±O(4) O(3)±Al±O(4) Si±O(1)±Al Si±O(2)±Al Si±O(3)±Al Si±O(4)±Al
112.3(9) 108.0(9) 106.4(9) 106.2(6) 109.1(7) 109.6(6) 111.8(9) 107.3(9) 107.8(9) 106.6(6) 108.0(6) 109.3(6) 135.8(11) 140.4(11) 134.7(11) 144.8(11)
113.3(6) 110.0(7) 109.0(7) 105.9(6) 108.7(7) 109.7(6) 112.9(6) 108.5(7) 111.7(6) 106.4(6) 107.7(6) 109.6(6) 136.8(8) 143.8(7) 140.2(8) 149.5(8)
O(3)±Ag(1)±O(3) O(3)±Ag(2)±O(3) O(3)±Ag(3)±O(3) O(2)±Ag(4)±O(2) O(3)±Ag(5)±O(3) O(1)±Ag(6)±O(4) O(1)±Ag(7)±O(4) O(1)±Ag(8)±O(4) O(1)±Ag(9)±O(4) O(3)±Ag(1)±Ag(3) Ag(3)±Ag(1)±Ag(3) Ag(2)±Ag(5)±Ag(2)
91.1(3)
91.3(3) 118.2(3) 87.9(4) 118.5(4) 82.5(15) 62.3(4) 63.5(16) 66.2(10)
110.1(5) 103.1(4) 51.8(13) 55.6(19) 54.4(11) 57.5(16)
126.2(3) 180b 168.1(15)
a Numbers in parentheses are estimated standard deviations in the units of the least signi®cant digit given for the corresponding values. b Exactly 180° by symmetry.
the sodalite cavity or b-cage may be viewed as its principal building block. The sodalite cavities are connected tetrahedrally at six-rings by bridging oxygens to give double six-rings (D6Rs, hexagonal prisms) and, concomitantly, to give an interconnected set of even larger cavities (supercages) accessible in three dimensions through 12-ring (24membered) windows. The Si and Al atoms occupy the vertexes of these polyhedra. The oxygen atoms lie approximately midway between each pair of Si and Al atoms, but are displaced from those points to give near tetrahedral angles about Si and Al. Single six-rings (S6Rs) are shared by sodalite and supercages, and may be viewed as the entrances to the sodalite units. Each unit cell has eight sodalite units, eight supercages, 16 D6Rs, 16 12-rings, and
32 S6Rs. The entire structure may be viewed as that of diamond with sodalite units replacing carbon atoms and six bridging oxygens replacing each C±C bond. Exchangeable cations that balance the negative charge of the aluminosilicate framework are found within the zeoliteÕs cavities. They are usually found at the following general sites shown in Fig. 1: site I at the center of a D6R, site I0 in the sodalite cavity on the opposite side of one of the D6RÕs six-rings from site I, II0 inside the sodalite cavity near a S6R, II in the supercage adjacent to a S6R, III in the supercage opposite a four-ring between two 12-rings, III0 somewhat or substantially distant from III but otherwise near the wall of the supercage or the edge of a 12-ring.
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
Fig. 1. Framework structure of zeolite X. Near the center of each line segment is an oxygen atom. The dierent oxygen atoms are indicated by the numbers 1 to 4. Silicon and aluminum atoms alternate the tetrahedral intersections, except that Si substitutes for about 4% of the Al atoms. Extraframework cation positions are labeled with Roman numerals.
4.2. Hydrated Ag92 ±X (Ag92 (H2 O)32 ±X) Ag ions are found at seven crystallographic sites and H2 O molecules are found at two. As expected, there are no Ag0 atoms in this structure. The 16 Ag ions at Ag(1) ®ll site I at the centers of the D6Rs (Fig. 2). The octahedral Ag(1)±O(3) is exactly the same as the distance, 2.581(18) A, sum of the ionic radii of Ag and O2ÿ , 1:26 [25], so site I is able to accommodate 1:32 2:58 A Ag ions nicely.
55
The 16 Ag ions at Ag(3) are at site I0 , in the sodalite cavities opposite D6Rs (Fig. 2). This is a 32-fold position, but it is only about half occupied. Each Ag ion lies relatively far inside the sodalite from the plane of the three O(3) cavity, 1.99 A framework oxygens of the D6R to which it is bound. The Ag(3)±O(3) distance is 2.561(17) A, similar to the sum of the ionic radii, 2.58 A. Each Ag ion at Ag(3) is coordinated in a very distorted manner by three framework oxygens at O(3) and one non-framework oxygen at (2.561(17) A) (Fig. 2). O(5) (2.373(10) A) The distance between Ag(1) and Ag(3) is almost the same as the 3.058(1) A 3.045(3) A, distance found for an Ag ±Ag interaction in an organic complex of AgNO3 [26]. Shorter distances have been ranging from 2.655(2) to 3.002(1) A reported for similar Ag ±Ag interactions in six Ag complexes [27±32]. This has been reviewed and discussed by Jansen [33] for Ag ions in oxides, salts, and coordination complexes. Apparently site I0 accepts Ag ions until all site I Ag ions are accommodated: thus site I0 has reached an occupancy limit when it is only half-full. At Ag(4) (site II), 32 Ag ions are each recessed into the supercage from a S6R plane 1.031(5) A at O(2) (Fig. 3). This 32-fold position is also fully occupied. Each of these cations is coordinated in a nearly tetrahedral manner to three O(2) frame and one non-framework oxygens at 2.425(16) A Both distances work oxygen, O(6), at 2.42(6) A.
Fig. 2. (a) Stereoview of a sodalite cavity with two attached D6Rs in hydrated Ag92 ±X. Two Ag ions at Ag(1) (site I), two Ag ions at Ag(3) (site I0 ) and two H2 O molecules at O(5) are shown. The Ag ±Ag interactions between Ag(1) and Ag(3) can be seen. All sodalite is too long for this to be a signi®cant bonding interaction. The water cavities have this arrangement. The Ag(3)±Ag(3) distance, 3.87 A, Ellipsoids of 20% probability are shown. (b) The weak 3.045(3) A Ag Ag bond molecules at O(5) hydrogen bond at 2.61(4) A. through a 6-ring of a D6R.
56
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
Fig. 3. Stereoview of a supercage of hydrated Ag92 ±X. Four Ag ions at Ag(4) (site II), one Ag ion at Ag(6) (site III0 ), one Ag ion at Ag(7) (site III0 ), one Ag ion at Ag(8) (site III0 ), one Ag ion at Ag(9) (site III0 ) and four H2 O molecules at O(6) are shown. About 50% of the supercages have this arrangement. Ellipsoids of 20% probability are shown.
are a little shorter than the sum of the corre sponding ionic radii, 2.58 A. The occupancies at Ag(6), Ag(7), Ag(8), and Ag(9) (all III0 sites) are 7, 6, 9, and 6 per unit cell, respectively (Fig. 3). The Ag(6)±O(1), Ag(7)±O(1), Ag(8)±O(1), and Ag(9)±O(1) distances are 2.65(7), respectively (Table 2.72(9), 2.50(6), and 2.58(8) A, 2). These distances are all near the sum of the ionic These four III0 sites, each a 96-fold radii, 2.58 A. position, are sparsely occupied by a total of only 28 Ag ions. 4.3. Dehydrated Ag92 ±X Ag and Ag0 cannot be distinguished by X-ray crystallography because their scattering powers are virtually identical. They are also not easily distinguished by their approach distances to oxygen in this structure, perhaps because of charge delocalization. Because the existence of silver atoms or reduced cationic clusters has previously been established by e.s.r. and other spectroscopies in similar systems, the clusters seen here are as sumed to be reduced, from 3Ag to Ag2 3 and Ag3 . Silver atoms and ions are found at eight dierent crystallographic sites. Only three Ag ions at Ag(1) lie at site I at the centers of the D6Rs (Fig. 4): thus the D6Rs are only sparsely occupied (three ions per 16 sites per unit cell). Each Ag ion at Ag(1) is coordinated by the six O(3) oxygen atoms somewhat of its D6R at distances of 2.733(12) A, longer than the sum of the ionic radii of Ag and
This distance is likely to be virtual: O2ÿ , 2.58 A. the O(3) position is an average which most accurately describes the 13 D6Rs per unit cell that do not contain Ag ions at their centers. The Ag(2) position is at site I0 , on the 3-fold axis in the sodalite unit opposite D6Rs (Fig. 4). This 32-fold position is occupied by 26 Ag ions. Each is essentially in the plane of the three O(3) framework oxygens of the D6R to which it is bound (see Table 3). The Ag(2)±O(3) distance is 2.206(12) A, much shorter than the sum of the corresponding ionic radii. This indicates that Ag(2)±O(3) bonds are quite covalent, as had been seen for Ag in zeolite A [6,7]. The occupancy numbers at site I (Ag(1)) and site I0 (Ag(2)), 3.0 and 26.0, respectively, indicate that the D6Rs are ®lled: ((no. at I) (no. at I0 ))=2 must not exceed 16 to avoid an Of the intercationic approach distance of 1.62 A. 16 D6Rs per unit cell, three have a Ag ion at their center and 13 have two Ag ions in their six-ring faces at Ag(2). The Ag(2)±Ag(2) distance, 3.224(3) indicates that a weak but signi®cant Ag ±Ag A, interaction [27±32], discussed above for hydrated Ag92 ±X, occurs also in 13 of the 16 D6Rs of this crystal. Note in Figs. 4 and 5 that the Ag ions at Ag(2) do not extend away from their D6Rs to avoid a Ag ±Ag repulsion, as is commonly seen for other cations. Six Ag species at Ag(3) are located at site I0 in the sodalite cavity opposite the D6Rs. Each spe into the sodalite cies at Ag(3) is recessed 1.00 A cavity. This 32-fold position is occupied by only
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
57
Fig. 4. (a) Stereoview of a sodalite cavity with two attached D6Rs in dehydrated Ag92 ±X. One Ag ion at Ag(1) (site I), four Ag ions at Ag(2) (site I0 ) and two Ag0 atoms at Ag(3) (site I0 ) are shown. The Ag ±Ag interaction between Ag ions at Ag(2) can be seen. About 75% of the sodalite cavities may have this arrangement. Ellipsoids of 20% probability are shown. (b) The Ag 3 cluster in nearly the same orientation as shown in (a).
Table 3 of atoms from six-ring oxygen planes Deviations (A) At O(2)a Ag(4) At O(3)b Ag(1) Ag(2) Ag(3)
Crystal 1
Crystal 2
1.031(5)
0.276(2)
ÿ1.542(3)
ÿ1.625(1) 0.021(2) 1.595(8)
1.509(6)
a
A positive deviation indicates that the atom lies in the supercage. b A positive deviation indicates that the atom lies in the sodalite cavity. A negative deviation indicates that the atom lies in a D6R.
Fig. 5. (a) Stereoview of a sodalite cavity with two attached D6Rs in dehydrated Ag92 ±X. Six Ag ions at Ag(2) (site I0 ) and one Ag0 atom at Ag(5) in the sodalite cavity are shown. This Ag2 3 cluster (Ag(2)±Ag(5)±Ag(2)) is in the sodalite cavity. The Ag ±Ag interaction between Ag ions at Ag(2) can be seen. About 25% of the sodalite cavities may have this arrangement. Ellipsoids of 20% probability are shown. (b) The Ag2 3 cluster in nearly the same orientation as shown in (a).
58
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
Fig. 6. Stereoview of a supercage in dehydrated Ag92 ±X. Four Ag ions at Ag(4) (site II), two Ag ions at Ag(6) (site III0 ), one Ag ion at Ag(7) (site III0 ) and one Ag ion at Ag(8) (site III0 ) are shown. Ag(7) and Ag(8) cannot coexist in the same supercage: only Ag(7) is present in about 50% of the supercages, only Ag(8) is present in about 37.5% of the supercages, and the remainder have neither. Ellipsoids of 20% probability are shown.
six Ag species (Fig. 4). The distance between Ag(3) and its nearest framework oxygens at O(3), is almost the same as the distance 2.753(12) A, between a Ag atom and a framework oxygen, 2.78 in previous studies [6,7]. Therefore, this species A, appears to be more like a Ag0 atom than a Ag is similar ion. The Ag ±Ag0 distance, 3.260(9) A, in Ag±A [6,7] to that between Ag and Ag0 , 3.31 A in Ag±Y [1]. This Ag(1)±Ag(3) interand 3.24 A action cannot be avoided and the linear Ag 3 subcluster, Ag(3)±Ag(1)±Ag(3), must exist (Fig. 4), three per unit cell. Where do the silver atoms come from? In a crystallographic study of zeolite A [7,34], Ag ions are found to be reduced by the following reactions that occurred within the sodalite units: 3
Ag3 H2 O3 ! Ag Ag2 H2 O
2
2H2 O
which is the penultimate step of the dehydration process, followed by Ag2 H2 O
2
! 2H 2Ag 1=2O2
A similar process could be occurring in zeolite X. Two Ag(5) atoms at (0.125, 0.125, 0.2008) lie on 2-fold axes in two sodalite cavities. The distance between Ag(5) and its two nearest framework ox similar to the 2.78 ygens at O(3) is long, 2.83(3) A, distance between a Ag atom and two framework A oxygens in zeolite A [6,7]. Therefore, this species appears to be more like a Ag0 atom than a Ag is an apion. The Ag ±Ag0 distance, 3.156(5) A, propriate Ag ±Ag0 distance (vide supra). To pro-
vide coordination to each silver atom at Ag(5) at its position of symmetry, two Ag(2) ions should be near, and bent Ag2 subclusters, Ag(2)±Ag(5)± 3 Ag(2) 168.1(15)°, should exist (see Fig. 5). The 32 Ag ions at Ag(4) ®ll site II (Fig. 6). into the supercage from Each is recessed 0.73 A the S6R plane at O(2). The Ag(4)±O(2) distance is much like Ag(2)±O(3) quite short, 2.273(10) A, and the Ag ion to framework oxygen distance, in the structure of partially decomposed 2.25 A, Ag12 ±A [6,7]. The occupancies at Ag(6), Ag(7), and Ag(8) (all III0 sites) are 16, 4, and 3, respectively, per unit cell (Fig. 6). The Ag(6)±O(1), Ag(7)±O(1), and Ag(8)± O(1) distances are 2.702(16), 2.31(6), and 2.45(5) respectively. These distances are all near the A, These three III0 sum of the ionic radii, 2.58 A. positions, each of order 96, are sparsely occupied by a total of 23 Ag ions. The Ag-atom production seen in crystal 2 is relatively small. When fully Ag -exchanged zeolites are dehydrated in the absence of oxygen, more silver atoms are produced and, necessarily, more oxygen is lost from zeolite framework [14,34]. Such structures have less crystallinity and are often de®cient in Ag due to Ag atom migration out of the zeolite [14]. Crystallographic work on such systems has yielded less de®nitive results [14]. 4.4. Additional concerns It could be that Ag 92 ±X, fully dehydrated zeolite X with all Ag in the 1 oxidation state, formed
S.H. Lee et al. / Microporous and Mesoporous Materials 41 (2000) 49±59
at 360°C in ¯owing oxygen. The removal of O2 (g) by evacuation at 400°C (see Section 2) may then have led to the formation of eight Ag atoms per unit cell by the reaction, involving framework oxygens, 8Ag 4O2ÿ ! 8Ag0 2O2 , resulting in 2 the formation of three Ag 3 and two Ag3 clusters. Cooling to room temperature in ¯owing oxygen before evacuation might have avoided this decomposition to preserve Ag 92 ±X as the ®nal product. Alternatively, the ®nal dehydrated structure 2 might actually be Ag 92 ±X with no Ag3 nor Ag3 clusters. The variability of the Ag±O bond length in chemistry makes the assignment of the formal charge to Ag (1 or 0 ) dicult. If it is Ag 92 ±X, there would be more Ag ±Ag interactions than presented above. In support of this is the striking similarity between the bond lengths in the pro2 cluster posed Ag 3 cluster (3.260 A), the Ag3 (3.156 A), and the Ag ±Ag interaction (3.224 A) (Table 2). Arguing against this possibility is the occupancy at I0 in crystal 1 which indicates that Ag3 3 has not formed [33]. 5. Summary In partially hydrated Ag92 ±X, all Ag cations are found at sites I, I0 , II, and four III0 sites. A H2 O molecule coordinates to each Ag ion at Ag(3) (site I) and Ag(4) (site II) to complete a tetrahedral coordination geometry about Ag . Dehydration of Ag92 ±X under ¯owing O2 gas at 360°C, followed by evacuation at 400°C, has apparently produced reduced trisilver clusters. Comparisons with previous work suggest that the linear clusters that extend along 3-fold axes through double six-rings are Ag 3 and that the slightly bent clusters in sodalite cavities are Ag2 3 . In both structures, weakly attractive Ag ±Ag interactions are seen. Acknowledgements The authors wish to acknowledge the ®nancial support of the Korea Research Foundation made in the program year of 1998 (Project No. 1998-15D00146).
59
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