J. Phys. Chem. 1994,98, 3796-3800
3796
Crystal Structures of Fully Dehydrated Cd(I1)-Exchanged Zeolite A and of Its Cadmium Sorption Complex Containing Cd2+, Cd+, Cdz2+,and CdzO Se Bok Jang, U n Sik Kim, and Yang Kim' Department of Chemistry, Pusan National University, Pusan 609- 735, South Korea Karl Seff * Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, Hawaii 96822-2275 Received: August 9, 1993; In Final Form: January 4 , 1994'
The crystal structures of Cd6-A evacuated at 2 X 10" Torr and 750 OC ( a = 12.216(1) A) and of its cadmium sorption complex Cdll-A ( a = 12.127( 1) A) have been determined by single-crystalx-ray diffraction techniques in the cubic space group Pmgm at 21( 1) OC. Their structures were refined to the final error indices, R1 = 0.055 and R , = 0.067 with 191 reflections and R1 = 0.072 and R, = 0.064 with 197 reflections, respectively, for which I > 3a(I). In dehydrated Cda-A, six Cd2+ ions are found at two different 3-fold-axis sites near 6-oxygen-ring centers. Four Cd2+ ions are each recessed 0.50 A into the sodalite cavity from the (1 11) plane of the three oxygens to which it is bound, and the other two extend 0.28 A into the large cavity from such planes. To prepare Cdll-A, fully dehydrated Cdz+-exchanged zeolite A was exposed at 550 O C to 40 Torr of cadmium metal vapor and, during slow cooling, to lower pressures at lower temperatures. In this structure, three Cd2+ ions lie near 6-ring centers where each coordinates to three framework oxygens at 2.27(1) A. Six Cd+ ions are found at three crystallographic sites: three occupy 6-ring sites in the sodalite unit (Cd-0 = 2.68( 1) A); two form (Cd2)2+ which associates with an &oxygen ring (Cd-Cd = 2.35(1) A, Cd-0 = 2.50(2) A); and one Cd+ ion lies at a 6-ring site in the large cavity (Cd-0 = 2.41(1) A). Two Cd atoms per unit cell are found deep in the large cavity far from the zeolite framework: they bond at a distance of 2.97(3) A. This dicadmium molecule is stabilized by interaction with a 6-ring Cd2+ion and an &ring Cd22+ ion. The sorption of cadmium metal vapor into zeolite A has caused three of the six Cd2+ ions to be reduced to Cd+. In addition, two cadmium atoms were sorbed per unit cell to form Cd2+.Cd2°.Cd22+, a ( C ~ S )cluster ~ + of low symmetry in the large cavity.
Introduction
Experimental Section
Complete dehydration of fully CdZ+-exchangedzeolite A had not been achieved. Cdb-A evacuated at 500 OC and 2 X 10-6 Torr for 2 days contains three HzO molecules per unit cell,lv2 and temperatures as high as 700 OC had not been found to be sufficient to remove all water.3 Some cations in zeolites are easily reduced. For example, the Ni2+ ions in zeolite Y can be reduced to Ni+ by sodium vapor and to the metallic state by hydrogen.4 Cu2+ in Cu,Na-Y is reduced to Cu+ by treatment with carbon monoxide at elevated temperatures and further reduction by hydrogen gas gives CUO.~The dipositive cations of the relatively volatile elements Hg, Cd, and Zn can be removed as atoms from zeolite X by heating in hydrogen.6 Ag+ ions in zeolite Acan also be reduced by heating,',* by reaction with reducing agents:,'' or by sorption of metal vapors.11-12 The formation of Cdnm+,m < 2n, was considered another possible, and equally interesting, outcome of such a reduction experiment. The existence of Cd,Z+ and Cd42+species had been postulated on the basis of Raman measurements.I3 Similar sodium species, with a single-electronfully delocalizedover several Na+ ions, have been identified using ESR techniques in zeolite X (Na54+a ~ ~ d N a ~ ~ + ) l ~ a n d z e (Na43+).15 o l i t e Y Whendehydrated zeolite A is exposed to cesium or rubidium vapor, cationic clusters like Cs43+,Rb43+,and Rb64+have been identified.16~'~Such metal clusters dispersed within zeolites have application in catalysis.18 This work was done to learn whether fully dehydrated, fully Cd2+-exchanged zeolite A could be prepared and, if so, to determine its structure. It was also hoped that dehydrated Cd6-A would react with (sorb) cadmium metal vapor, to yield reduced species and perhaps to form clusters within the zeolite.
Crystals of zeolite 4A were prepared by a modification of Charnell's method.I9 Two single crystals about 85 pm on an edge were selected and lodged in separate fine quartz capillaries. An exchange solution of C d ( N 0 3 ) ~and Cd(02CCH& in the mole ratio of 1:1with a total concentration of 0.05 M was allowed to flow past each crystal at a velocity of approximately 1.5 cm/s for 3 days. Each crystal was washed by continuing this procedure using distilled water at 80 OC for 1 h.2 Each crystal, still in its fine quartz capillary, was attached to a vacuum system and was cautiously dehydrated by gradually increasing its temperature (ca. 25 OC/h) to 750 OC at a constant pressure of 2 X 10-6 Torr. Finally, the system was maintained at this state for 48 h. After cooling to room temperature, one crystal (crystal l), colorless, still under vacuum, was sealed in its capillary by torch. Cd metal was distilled from a sidearm into the glass tubing above the capillary containing the second crystal. This arrangement of distilled Cd and crystal was sealed, still under vacuum, from the main vacuum line. Finally, the crystal was exposed to 40 Torr of Cd metal vapor by heating this vessel to 550 OC. The temperature was slowly decreased to 25 OC over a period of 24 h, and then the crystal, still under vacuum, was sealed in its capillary with a torch. Microscopic examination showed that the crystal (crystal 2) had become red.
* Abstract published in Advance
ACS Abstracts, March 1, 1994.
X-ray Data Collection The cubic space group Pmjm (no systematic absences) was used instead of FmJc throughout this work for the reasons discussed previously by Seff and Mellum and references therein.20.21 Diffraction data were collected with an automated EnrafNonius four-circle computer-controlled CAD-4 diffractometer equipped with a pulse-height analyzer and a graphite mono-
0022-3654/94/2098-3796%04.50/0 @ 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3797
Cd6-A and Its Cd Sorption Complex
TABLE 1: Positional, Thermal, and Occupancy Parameters. Wyckoff Bi,or atom position X Y z 81l b 822 833 Crystal 1: Fully Dehydrated Cd6-A (S1,Al) O(1) O(2) O(3)
CdU)
24(k) 12(h) 12(i) 24(m) 8(g)
W2)
8(g)
(Si,AI)
24(k) 12(h) 12(i) 24(m)
O(1) O(2) O(3)
CdU) Cd(2)
8(g) 8(g)
Cd(3)
6(f)
C44)
8(g)
Cd(5)
24(m)
Cd(6)
8(g)
0 0 0
1119(6) 1967(5) 1595(3)
0 0 0 1133(6) 2139(4) 1147(3) 2570(5) 3360(20) 490(30) 2340(10)
1833(5) 2020(20) 2940(10) 1119(6) 1967(5) 1595(3)
812
3683(4) 22(3) 22(3) 28(3) 0 5000 50(20) 50(20) 40(20) 0 2940(10) 0 40(10) 31(8) 31(8) 3261(9) 37(6) 37(6) 50(10) lO(20) 1967(5) 33(3) 33(3) 33(3) 28(8) 1595(3) 42(2) 42(2) 42(2) 41(5) Crystal 2: Cdll-A, a Cadmium Sorption Complex of C&-A 1823(3) 3696(4) 21(3) 23(3) 14(3) 0 2070(20) 5000 180(20) 60(20) lO(10) 0 2995(9) 2995(9) 28(8) 70(10) 28(8) 0 1133(6) 3360(10) 46(6) 46(6) llO(10) 50(20) 2139(4) 2139(4) 47(2) 47(2) 47(2) 57(6) 1147(3) 1147(3) 44(2) 44(2) 44(2) 60(6) 5000 5000 23(2)e 3360(20) 3360(20) ll(1) 4410(20) 4410(20) 16(2) 2340(10) 2340(10) 5.6(7)
813
823
0 0 0 lO(10) 28(8) 41(5)
3(7) 0 50(20) lO(10) 28(8) 41(5)
0 0 0 lO(20) 57(6) 60(6)
4(6) 0 -0(20) lO(20) 57(6) 60(6)
occupancy varied fixed
2.11(4) 4.27(4)
24.od 12.0 12.0 24.0 2.0 4.0
3.17(3) 3.16(3) 1.05(6) 1.23(6) 2.42(7) 0.82(4)
24.P 12.0 12.0 24.0 3.0 3.0 1.0 1.0 2.0 1.0
Positional and anisotropic thermal parameters are given X lo4. Numbers in parentheses are the esd's in the units of the least significant digit given + 812hk + &,hZ + 823kr)l. Occupancy factors for the correspondingparameter. The anisotropic temperature factor = exp[-(811h2 + &k2 + are given as the number of atoms or ions per unit cell. Occupancy for (Si) = 12; occupancy for (AI) = 12. e Isotropic thermal parameters in units of
'42.
chromator, using Mo Ka radiation (Ka1, X = 0.709 30 A, Kaz, X = 0.713 59 A). Theunitcellconstantsat21(1) OC,determined by least-squares refinement of 25 intense reflections for which 18O < 20 < 25O, are a = 12.216(1) A for crystal 1 and a = 12.127(1) A for crystal 2. For each crystal, reflections from two intensity-equivalent regions of reciprocal space (hkl, 0 Ih I k 5 1 and lhk, 0 I 1 Ih Ik ) were examined. The intensities were measured using the 0-20 scan technique over a scan width of (0.80 0.344 tan 0 ) O in w. The data were collected using variable scan speeds. Most reflections were observed at slow scan speeds, ranging between 0.24 and 0.33 deg/min-l in w. The intensities of three reflections in diverse regions of reciprocal space were recorded every 3 h to monitor crystal and X-ray source stability. Only small, random fluctuations of these check reflections were noted during the course of data collection. For each region of reciprocal space, the intensities of all lattice points for which 20 < 70° were recorded. Only those for which Z > 3 4 ) were used for structure solution and refinement. This amounted to 191 of the 862 reflections examined for crystal 1, and 197 of the 849 reflections for crystal 2. The intensities were corrected for Lorentz and polarization effects; the reduced intensities were merged and the resultant estimated standard deviations were assigned to each averaged reflection by the computer programs PAINT and WEIGHT.22 For the first crystal, r R = 0.090, pal = 1.408 g/cm3, and F(000) = 990; for the second the corresponding values are 0.15, 2.406 g/cm3, and 1230. The absorption correction was judged to be negligible for both crystals and was not applied.23
The thermal ellipsoid of the Cd2+ position had become very elongated, suggesting the presence of two nonequivalent Cd2+ ions at this position. Also, the Fourier function showed this peak to be resolvable into two peaks, which were refined at x = 0.16 and x = 0.20 on the 3-fold axes. Occupancyrefinement converged at 2.1 l(4) and 4.27(4), respectively. Thesevalues were reset and fixed at 2.0 and 4.0 Cd2+ions at Cd( 1) and Cd(2), respectively, because the cationic charge should not exceed +12 per P m j m unit cell and because the occupancies of the heaviest atoms correlate with the scale factor. Anisotropic refinement of the framework atoms and Cd2+ ions at Cd( 1) and Cd(2) converged to R1 = 0.055 and R, = 0.067 (see Table 1). In the final cycle of least-squares refinement, all shifts in atomic parameters were less than 0.1% of their corresponding standard deviations. The final difference function was featureless except for a peak at (0.0, 0.0, 0.0) of height 2.6(13) e A-3. This peak refined as one A1 atom with B = 28 A2 or 0.5 A1 with B = 13 A2. Also, the 0.50-A deviation of the four Cd2+ ions at Cd(2) into the sodaliteunitfromthe [1,1,1] planeatO(3) suggeststhatoxygens may be present in the sodalite cavity. A search for oxygens, not found on a difference function, bonded to this A1 position yielded about two oxygens at about (0.0,0.096,0.096) with B = 16 A2. Altogether, this lowered the error indices sharply to R1 = 0.043 and R, = 0.036 and may indicate the presence of an aluminate species in the sodalite cavity. The A1-0 and Cd-0 distances are all reasonable. In the second structure herein reported, no evidence for such a cluster was seen. The product of the treatment of Cd6-A dehydrated at 750 OC with Rb(g)25or Cs(g)26yielded structures very similar to those of dehydrated Na12-A16.27treated with Rb(g) or Cs(g). This Structure Determination argues that the present crystal of Cd6-A is simply stoichiometrically exchanged and fully dehydrated. Also in these structures, Cd6-A Dehydrated at 750 OC. Full-matrix least-squares there is no peak at the origin to suggest the presence of an refinement was initiated with the atomic parameters of the aluminate species. framework atoms [(Si,Al), 0(1), 0(2), and 0(3)] of Cd6-A Cd6-A Treated with Cd Vapor at 550 O C . Full-matrix leastvacuum dehydrated at 450 0C.24Anisotropic refinement of the refinement was initiated with the atomic parameters of ) framework atoms converged to an R1 index ((CIF,- ~ F c ~ ~ ) / ~ F osquares the framework atoms [(Si,Al), 0(1), 0(2), and 0(3)] of crystal of 0.46 and a weighted R, index ((Ew(F, - IFcl)2/CwFo2)1~2) of 1. Anisotropic refinement of the framework atoms converged to 0.55. R I= 0.47 and R, = 0.55. The initial difference Fourier function revealed one large peak The initial difference Fourier function revealed two large 3-foldat (0.166, 0.166, 0.166) of height 9.5(2) e A-3. It refined by axis peaks at (0.222,0.222,0.222) and (0.1 16,0.116,0.116) with least-squares analysis as six Cd2+ ions to a reasonable isotropic heights of 17.5(2) and 15.4(2) e A-3, respectively. Anisotropic thermal parameter. Anisotropic refinement of the framework refinement of the framework atoms and these Cd species at atoms and Cd2+ions lowered the error indices to R1 = 0.095 and R, = 0.085. Cd(1) andatCd(2)convergedtoR1 =0.144andRW=0.196(see
+
3798 The Journal of Physical Chemistry, Vol. 98, No. 14, 1994
d
“‘I
Jang et al.
t
IT
f
d
4L
L
Figure 1. Stereoview of the sodalite unit of fully. dehydrated Cd6-A. Two CdZ+ions at Cd(1) and four CdZ+ ions at Cd(2) are shown. Ellipsoids of 20% probability are used.
Table 1). Full-matrix least-squares refinement converged to the occupancies Cd(1) = 3.17(3) and Cd(2) = 3.16(3). A difference Fourier synthesis revealed three more peaks: (0.244,0.5,0.5), height 2.3(3)e.&-3;(0.332,0.332,0.332), height 2.8(3) eA-’;and (0.01,0.43,0.43), height 2.2(2) eA-3. Inclusion of these peaks as Cd(3), Cd(4), and Cd(5) allowed the error indices toconvergeto R1 = 0.071 and R, = 0.061 withoccupancies of 1.05(6), 1.23(6), and 2.42(7), respectively (see Table 1). A subsequent difference Fourier function revealed a peak of height 1.6(2) e A-3 at (0.237, 0.237, 0.237). Its inclusion as Cd(6) lowered the error indices to R1 = 0.070 and R, = 0.060 (see Table 1) with an occupancy of 0.82(4). The occupancies obtained were reset, usually only by small amounts to the nearest integers, and fixed as shown in Table 1. Another consideration involved total cationic charge, which needed to sum to +12, or nearly so, per unit cell to balance the charge of the zeolite framework. All shifts in the final cycles of least-squares refinement were less than 0.05% of their corresponding standard deviations. The final error indices converged to R1 = 0.072 and R, = 0.064. The final difference function was featureless except for a peak at (0.5,0.5,0.5) of height 2.5(8) e A-3; this peak refined to insignificant occupancy. The full-matrix least-squares program used minimized Cw(Fothe weight ( w ) of an observation was the reciprocal square of u(Fo),its standard deviation. Atomic scattering f a ~ t o r s for ~ 8 Cd2+, ~ ~ ~ CdO, 0, and (Si,A1)1.75+were used. The function describing (Si,A1)1.7S+is the mean of the Sio, Si4+,A10, and A13+ functions. All scattering factors were modified to account for anomalous dispersion.30 The final structural parameters and selected interatomic distances and angles are presented in Tables 1 and 2, respectively.
Discussion Fully Dehydrated C&-A. In the crystal structure of vacuumdehydrated Cda-A, all six Cd2+ions associate with 6-ring oxygens on 3-fold axes. Two ions at Cd(1) and four at Cd(2) per unit cell are 2.16(1) and 2.20(1) A, respectively, from their respective three O(3)’s (see Figure 1). For comparison, the sum of the conventional ionic radii of Cd2+and 0 2 - is 2.29 A.30 These bonding distances between Cd2+and O(3) are similar to those in partially dehydrated Cd6-A.2,3 In that structure, the Cd-O(3) distance is 2.16( 1) A for three-coordinate Cd2+and 2.23( 1) A for each Cd2+ ion which coordinates further to a water molecule. The two Cd2+ ions a t Cd( 1) are recessed 0.28 A into a large cavity from the (1 11) plane at O(3) (Table 3) and the four Cd2+ ions at Cd(2) are correspondingly recessed 0.50 A into the sodalite cavity. Cd(2) ions arranged tetrahedrally within the sodalite unit and Cd( 1) ions occupying two of the remaining four 6-rings, is one of several arrangements which minimizes total intercadmium repulsions. If, further, these two groups lie one recessed into the sodalite unit and the other into the large cavity, as is observed,
TABLE 2 Selected Interatomic Distances (A) and Angles (deg). 1.62(1) 1.63(1) 1.70(1) 2.16(1) 2.20(1)
l.61( 1) 1.66(1) 1.66(1) 2.27(1) 2.68(1) 4.64(4) 3.82(2) 2.50(2) 2.98(2) 2.41(1) 2.57(1) 2.71(7) 2.97(3) 3.84(7) 2.35(3) 110.0(7) 109.7(6) 107.8(3) 108.7(3) 158(1) 151.8(5) 140.6(7) 114.2(2) 90.7(2) 112.8(3) 125.6(3)
115.7(8) 111.8(5) 105.0(4) 107.0(4) 164(1) 157.2(6) 137.0(7) 118.3(2) 114.9(2)
87.0(9)
98.5(9) * Numbers in parentheses are estimated standard deviations in the units of the least significant digit given for the corresponding value.
TABLE 3: Deviations of Atoms (A) from the (111) Plane at O(3P . . Cds-A Cdll-A O(2) 0.27 0.30 Cd(1) 0.28 0.56 Cd(2) -0.50 -1.53 Cd(4) 3.12 0.94
W6)
A negative deviation indicates that the atom lies on the same side of the plane as the origin.
these repulsions are slightly further diminished. Thus it is more favorable electrostatically that six Cd2+ions occupy two different 3-fold-axis positions as in the present structure (see Table 1). Cdll-A. After treating dehydrated Cd6-A with cadmium metal vapor, Cd species are found a t six different crystallographic sites. The distances between Cd species, Cd(i) (i = 1-6), and their closest framework oxygens are 2.27( l), 2.68( l), 4.64(4), 3.82(2), 2.50(2), and 2.41(1) A, respectively. For comparison, the sum of the conventional ionic radii of CdZ+ and 02-is 0.97 + 1.32 = 2.29 A31and that of Cd+ and 02-is 1.14 1.32 = 2.46
+
Cd6-A and Its Cd Sorption Complex
The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 3799
Figure 2. Stercoview of the sodalite unit of Cdll-A. Three Cd2+ions at Cd(1) and three Cd+ ions at Cd(2) are arranged in triangular fashions, and the one Cd+ ion per sodalite cavity at Cd(6) is also shown. Ellipsoids of 20% probability are used.
1 02
h1 02
Si
Si
01
01
Figure 3. Stereoview of the large cavity of Cdll-A. Three CdZ+ions at Cd(l), three Cd+ ions at Cd(2), and the Cds4+cluster (Cd(l)-Cd(4)Cd(3)-Cd(S)-Cd(S))are shown. Ellipsoids of 20% probability are used.
s1
Si
Figure 4. Stereoview of (Cdz)O (Cd(3) and Cd(4)), bound to Cd2+ (Cd(1)) and (Cdz)Z+ (both Cd(5)), in the large cavity. This is the Cd++cluster shown in Figure 3. Ellipsoids of 20% probability are used. A.3'832 Considering these distances, with the constraint that the cationic charge sums to + 12 per unit cell, the Cd species at Cd( 1) may be tentatively identified as Cd2+ions, those at Cd(2), Cd(5), and Cd(6) as Cd+ ions, and those at Cd(3) and Cd(4) as Cd atoms (see Table 1). (This assignment would be open to modification if required by close intercadmium contacts). At Cd( l), three Cd2+ ions are recessed 0.56 A into the large cavity from the (1 11) plane at O(3); each coordinates at 2.27(1) A to three oxygens at O(3) (Figure 2). The 114.2(2)O O(3)Cd( 1)-0(3) angle indicates a moderate deviation from trigonalplanar geometry. The six Cd+ ions are found at three different crystallographic sites (Figure 3). (1) Three Cd+ ions on 3-fold axes at Cd(2) are recessed 1.53 A into the sodalite unit. Each coordinates to three O(3) oxygens at 2.68(1) A. These two distances are relatively long and suggest that the Cd(2) ions may cluster in the sodalite cavity; intercadmium distances as short as 2.78 A are possible. However, to avoid close Cd( 1)-Cd(2) contacts and to distribute Cd2+ ions widely about the unit cell, such 2.78 A approaches should not occur. (2) Two Cd+ ions at Cd(5) associate with an 8-ring and are located off the 8-oxygen plane. Each approaches a frameworkO(2) oxygen at 2.50(2) 8, and t w o 0 ( 1)'s at 2.98(2)
A.
They may be placed within a single 8-ring to give a Cd(5)Cd(5) bond length, 2.35(3) A, which is exactly the same as that found for CdzZ+speciesin previous work2and is in good agreement with theoretical expectation^.^^ (3) One Cd+ ion at Cd(6) is recessed 0.97 A into a large cavity from the (1 11) plane at O(3) (see Table 3) where it coordinates to three O(3) framework oxygens at 2.41(1) A. The monoatomic Cd+ ion to framework oxygen approaches are longer (by ca. 0.14, 0.23, and 0.41 A) than the Cd2+ ion to framework distance, 2.27( 1) A, in general agreement with the difference between the ionic radii of Cd+ and Cd2+ (1.14-0.97 = 0.17A).31.32 TheactualCd2+-0(3) distances should be somewhat shorter than those observed, and the Cd+O(3)distances longer, because Cd2+ ions can pull 6-ring oxygens toward 6-ring centers, while Cd+ ions should be less able to cause ring deformation. Two nonequivalent cadmium atoms have been found in this structure, each at its own position deep in the large cavity (see Figure 3). One at Cd(3) extends 3.33 A into the large cavity from the (100) plane, and the other at Cd(4) is 3.12 A from the (1 11) plane at O(3) (Table 3). Only two Cd(3)-Cd(4) distances are possible. The shorter, 2.97(3) A, is the same as the bond length in Cd metal, 2.979 A;33 the longer is too long to be bonding.
3800 The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 The closest possible Cd(4)-Cdn+ distance, n = 1 or 2, Cd(4)Cd(6) = 2.15(2) A, is too short to be a Cd-Cd interaction and must be avoided. However, the Cd(4)-Cd( 1) distance, 2.57( 1) A, is reasonable for a CdZ+ ion to Cd atom interaction. The Cd atom at Cd(3) can coordinate (and should for stabilization) to one of the two 8-ring Cd+ ions (which form CdZ2+)at 2.71(7) A. This dicadmium molecule and its environment constitute a Cd54+ cluster of low symmetry (see Figure 4) with bond lengths, from one end to the other, of 2.57( l), 2.97(3), 2.71(7), and 2.35(3) A. This cadmiumcluster is stabilized by chargedelocalization within itself and by interaction with 6-ring and 8-ring oxygens. The existence of Cd22+ and Cd+ has been reported, but crystallographic information on these species was heretofore available only in one previous cadmium zeolite structure.* Irradiation of aqueous solutions of Cd2+produces highly unstable, strongly reducing monatomic Cd+ i0ns.3~Since Cd+ is not stable in aqueous solution, most attempts to study the univalent state have involved the dissolution of cadmium metal in molten cadmium dihalides.lj Cadmium(1) should be a strong reducing agent, whether monoor binuclear, and within a zeolite it may have unique chemical and physical properties.
Acknowledgment. The present studies were supported in part by a program of the Basic Research Institute, Ministry of Education, Korea, 1993, Project No. BSRI-93-306. Supplementary Material Available: Tables of observed and calculated structure factors with esd’s (6 pages). Ordering information is given on any current masthead page. References and Notes (1) A discussion of nomenclature is available: (a) Yanagida, R. Y.; Amaro, A. A,; Seff, K. J. Phys. Chem. 1973, 77,805-809. (b) Broussard, L.; Shoemaker, D. P. J. Am. Chem. SOC.1960,82,1041-1051. (c) Seff, K. Acc. Chem. Res. 1976.9, 121-128. (2) McCusker, L. 8.; Seff, K. J.Am. Chem. Soc. 1979,101,5235-5239. (3) McCusker, L. B.; Seff, K. J. Phys. Chem. 1980,84, 2827-2831. (4) Rabo, J. A.; Angel], C. L.; Kasai, P. H.; Schomaker, V. Discuss. Faraday SOC.1966,41, 328-349.
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