J. Phys. Chem. B 1998, 102, 17-23
17
Crystal Structure of Indium-Exchanged Zeolite A Containing Sorbed Disulfur Nam Ho Heo,* Seok Han Kim, Hee Cheul Choi, and Sung Wook Jung Department of Industrial Chemistry, Kyungpook National UniVersity, Taegu 702-701, Korea
Karl Seff* Department of Chemistry, UniVersity of Hawaii at Manoa, Honolulu, Hawaii 96822-2275 ReceiVed: May 7, 1997; In Final Form: September 12, 1997X
Molecules of S2 are sorbed by dehydrated fully indium exchanged zeolite A from S(g) at 623 K. The crystal structure of dehydrated In8Si12Al12O48‚(In)0.75(S2) (R1 ) 0.053, R2 ) 0.050, and a ) 12.090(2) Å) has been studied by single-crystal X-ray diffraction methods at 294 K using the space group Pm3hm. The complex structural results are interpreted as follows. Each unit cell contains eight indium cations, 0.75 indium atoms, and two sulfur atoms. Six In ions per unit cell are found at four nonequivalent 3-fold axis equipoints: two In+ ions and 0.5 In3+ ions lie opposite six-rings in the large cavity, and three In2+ ions and 0.5 In+ ions lie opposite six-rings in the sodalite unit. Two In+ ions per unit cell are found at eight-ring positions, off the plane. Three-quarters of an indium per unit cell, as near-neutral atoms associated with In2+ cations, are found at the centers of the sodalite units. The distances of In+, In2+, and In3+ to the nearest framework oxygens are ca. 2.55, 2.37, and 2.26 Å, respectively. The structure may be viewed as having two kinds of “unit cells.” Unit cell 1 (In8-A‚(In)(S2), 75%) contains the (In5)8+ cluster (four In2+ ions tetrahedrally arranged about an In0 in the sodalite unit (In2+-O ) 2.37(1) Å and In0-In2+ ) 2.75(2) Å)). Unit cell 2 (In8-A‚(S2), 25%) has two In3+ ions. In each large cavity, one atom of a disulfur molecule (S-S ) 2.13(13) Å) associates with three framework oxygens at 3.11(11) Å, and the other sulfur atom bridges between two In+ ions at 3.26(3) and 3.37(19) Å.
Introduction The sorption of elemental sulfur onto nanoporous materials, including zeolites, has been a subject of interest to many material scientists because of the unusual and interesting chemistry of nanoclusters of sulfur and sulfur compounds.1-8 The reversible sorption isotherms for sulfur onto Ca2+exchanged zeolite A (Ca-A) and the sodium form of zeolite X (Na-X) show that the sulfur complexes of these zeolites are relatively stable (the heats of sorption on Ca-A and Na-X are -25 and -31 kcal/mol S, respectively).9 The crystal structures of sulfur sorption complexes of zeolite 4A (Na-A)10 and partially Co2+-exchanged zeolite A (Co,Na-A)11 both contain S8 rings, stabilized by association with exchangeable cations and framework oxygens in the large cavity. The structure of zeolite Na-X with sorbed tellurium, in which tellurium ions or atoms in the sodalite unit interact with Na+ ions, and tellurium ions in the large cavity approach framework oxygens, was reported by D. H. Olson.12 Various sulfide nanoclusters in zeolite cavities have unusual optical properties. These are due to the quantum size effect and to the regular three-dimensional arrangements which the zeolite structure requires. ZnS and CdS clusters prepared in zeolite Y show an unusual blue shift in their absorption spectra, as compared to the bulk materials.4 The absorption spectra of clusters of CdS and PbS synthesized in the cavities of zeolite Y and zeolite M are greatly shifted to the high-energy side and have weaker oscillatory strength compared to the bulk.4,5 Similarly, Laponite, porous Vycor glass,6 zeolite X, and sodalite X
Abstract published in AdVance ACS Abstracts, December 1, 1997.
have been used to host small particles of CdS;7 the optical properties of these materials are studied as a function of Cd2+ concentration and are discussed in terms of particle size. InP synthesized in mordenite also shows such a quantum size effect with a larger energy gap between valence and conduction bands.8 Various compounds of indium are semiconductors,13-15 but to date these have not been introduced into zeolites in high concentration to give three-dimensionally arrayed quantum dots. Toward this end, chemically reactive cations or clusters of In have recently been introduced into zeolites as precursors for preparing such nanoclusters.16 Elements of interest in this regard include In,8,16 Ga,17,18 and Te,19,20 which have been introduced into zeolites by various chemical treatments. To date, however, the chemistry which would lead to the conversion of these atoms to nanoclusters of binary compounds has not been explored. In this work, elemental sulfur was exposed to fully indiumexchanged zeolite A (In in various oxidation states) and its crystal structure was determined. This was done to explore the chemistry of In0, In+, and In2+ in zeolite A and to study the sorption properties of sulfur onto indium zeolites. It was hoped that indium would react with sulfur to generate cationic indiumsulfur nanoclusters with novel optical properties within zeolite A. Experimental Section Crystals of zeolite A (Na-A) were prepared by Kokotailo and Charnell.21 Colorless single crystals, about 80 µm on each edge, of fully Tl+-exchanged zeolite A, Tl12Si12Al12O48 (Tl-A), were prepared by dynamic ion-exchange (flow method) of Na-A with
S1089-5647(97)01538-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/01/1998
18 J. Phys. Chem. B, Vol. 102, No. 1, 1998 an aqueous solution of 0.1 M thallium acetate (pH ) 6.4, Aldrich Chem. Co., 99.99%). Several Tl-A crystals were completely dehydrated (623 K and 1 × 10-6 Torr for 2 d) and were brought into contact with In0 (Samchun Chem. Co., 99.99%) in fine Pyrex capillaries at 623 K for 4 d. This was achieved as described before,16 by condensing In0 around the crystal whose temperature was somewhat lower than that of the metal in a coaxially connected heating oven. Although the vapor pressure of In(l) is very low at 623 K (ca. 2.92 × 10-10 N/m2 ) 2.19 × 10-12 Torr),22 droplets of In were seen to form very close to the zeolite crystals. It is possible that In2O, from the decomposition of surface In2O3 on the original In sample, mediated the transport of In to the surface of the zeolite.16 Except for this reversed temperature gradient during the reaction period, other experimental procedures for the reaction were similar in detail to those previously described for solvent free ion-exchange.23-25 The color of the resulting crystals, still under vacuum in their capillaries, was pale yellow. Upon exposure to air, they became black. One of these black crystals, after being washed with deionized water (still black even after the washing), was again dehydrated as before. The crystal, now In11-A,16 was again colorless and transparent. This crystal, still protected from the atmosphere in its capillary, became red after exposure to dry sulfur (Aldrich Chem. Co., 99.998%) at 623 K for 5 d. The cubic space group Pm3hm (no systematic absences) was used throughout this work for reasons discussed previously.26,27 A CAD4/Turbo diffractometer equipped with a rotating anode generator and a graphite monochromator was used for preliminary experiments and for the subsequent data collection of diffraction intensities, all at 21 °C. Molybdenum radiation was used for all experiments (KR1, λ ) 0.70930 Å; KR2, λ ) 0.71359 Å). The unit cell constant, a ) 12.090(2) Å at 21(2) °C, was determined by a least-squares treatment of 15 intense reflections for which 20 < 2θ < 30°. Each reflection was scanned by the θ-2θ method at a constant scan speed of 0.5 deg/min in 2θ with a scan width of (0.63 + 0.60(tan θ))°. Background intensity was counted at each end of a scan range for a time equal to half the scan time. The intensities of three reflections in diverse regions of reciprocal space were recorded every 3 h to monitor crystal and instrumental stability. Only small random fluctuations of these check reflections were observed during the course of data collection. The intensities at 842 lattice points, a unique set (h e k e l) for 2θ < 70°, were recorded. Intensities were calculated as Iraw ) ATN(C - RB)/NPI, where C ) total count, R ) ratio of scan time to total background counting time (1.0), B ) total background count, NPI ) ratio of fastest possible scan rate to scan rate for each measurement, and ATN ) attenuation factor, respectively. The observed structure factor amplitude of each reflection (Fo) was then obtained as the square root of Iraw after correction for Lorentz-polarization (Lp); the contribution of the monochromator crystal was calculated assuming it to be half-perfect and half-mosaic in character. Standard deviations (σ(Fo)) of observed structure factors were assigned to individual reflections by the formula (σ2(I) + (pFo2))1/2/2Fo, where σ(I) is the standard deviation, based on counting statistics, of Iraw. The value p ) 0.04 was found to be appropriate for the instrumentation used. An absorption correction (µ ) 3.25 mm-1)28 was judged to be unimportant for the crystal, since semiempirical ψ-scans showed only negligible fluctuations for several reflections. Of the unique reflections examined, only those for which the net count I exceeded 3σ(I) were used in structure solution and refinement;
Heo et al. TABLE 1: Experimental Conditions and Crystallographic Data dehydration, temperature & duration indium reaction, temperature & duration hydration, temperature & duration (with deionized water) redehydration, temperature & duration sulfur sorption, temperature & duration no. of reflections with I > 3σ(I), m no. of variables, s unit cell parameter, Å final error indexes, R1a and R2b goodness of fitc
350 °C, 2 days 350 °C, 4 days 25 °C, 1 day 350 °C, 2 days 350 °C, 5 days 277 48 12.090(2) 0.053 and 0.050 1.38
a R1 ) ∑|Fo - |Fc||/∑Fo. b R2 ) (∑w(Fo - |Fc|)2/∑wFo2)1/2. c Goodness of fit ) (∑w (Fo - |Fc|)2/(m - s))1/2.
this amounted to 277 reflections. The experimental data are summarized in Table 1. Structure Determination Full-matrix least-squares refinement29 began with the atomic parameters of the framework atoms [(Si, Al), O(1), O(2), and O(3)] in Tl-A30 and In ions at In(1) opposite six-rings in the large cavity (0.243, 0.243, 0.243), a most popular site for In cations in In11-A.16 Refinement with isotropic thermal parameters converged to the error indices R1 ) 0.45 and R2 ) 0.53 with an occupancy of 1.68 at In(1). Introducing In(2) isotropically at a peak (0.131, 0.131, 0.131) from a difference Fourier function further reduced the error indices to R1 ) 0.27 and R2 ) 0.31 with resulting occupancies of 1.55 and 3.58 at In(1) and In(2), respectively. Isotropic refinement including In(6), found in a subsequent difference function at the origin (center of sodalite unit), converged to R1 ) 0.12 and R2 ) 0.17 with occupancies of 1.55, 3.33, and 0.61 at In(1), In(2), and In(6), respectively. Another peak in the large cavity at In(8) (0.208, 0.208, 0.208) was stable in subsequent refinement; the error indices decreased to R1 ) 0.11 and R2 ) 0.14 with occupancies of 2.90, 3.67, 0.84, and 0.80 at In(1), In(2), In(6), and In(8), respectively. Inclusion of In(4) (0.036, 0.424, 0.472) further reduced the error indices to R1 ) 0.079 and R2 ) 0.076 with occupancies of 2.32, 3.46, 2.08, 0.81, and 0.41 for In(i), i ) 1, 2, 4, 6, and 8, respectively. Another peak was found in the ensuing difference Fourier function at In(3) (0.112, 0.112, 0.112), near In(2) on a 3-fold axis in the sodalite unit. Refinement including In(3) reduced the error indices to R1 ) 0.076 and R2 ) 0.075 with occupancies of 2.31, 3.20, 0.27, 2.08, 0.81, and 0.41 for In(i), i ) 1, 2, 3, 4, 6, and 8, respectively. Refinements with two other peaks, refined isotropically as sulfurs at S(1) (0.242, 0.361, 0.361) and S(2) (0.300, 0.300, 0.5), converged to R1 ) 0.072 and R2 ) 0.071 with resulting occupancies of 2.31, 3.21, 0.24, 2.11, 0.79, 0.45, 1.32, and 0.83 for In(i), i ) 1, 2, 3, 4, 6, and 8, S(1) and S(2), respectively. Although one of the sulfur positions (S(2)) is similar to that of an indium atom in In11-A,16 it is assigned to S0 because its occupancy in refinement is more rational (0.83 S vs 0.34 In). A subsequent refinement with anisotropic thermal parameters for all framework atoms and the indium species at In(i) ) 1, 2, 4, 6, and 8 and isotropic thermal parameters for all sulfur atoms and the indium ion at In(3) in this model converged to the error indices R1 ) 0.051 and R2 ) 0.048 with occupancies of 1.98, 3.26, 0.20, 2.13, 0.77, 0.65, 1.33, and 0.97 for In(i) and S(i) as above. Sulfur atoms (S0) in the large cavity might aggregate to form various stable molecules, such as S2, S3, or S8. During the above refinements, the shortest S(1)-S(2) distance ranged from 1.9 to 2.3 Å with no such distances available to symmetry-related
Crystal Structure of Indium-Exchanged Zeolite A
J. Phys. Chem. B, Vol. 102, No. 1, 1998 19
TABLE 2: Positional, Thermal, and Occupancy Parametersa occupancyc (Si,Al) O(1) O(2) O(3) In(1)f In(2) In(3) In(4) In(6) In(8) S(1) S(2)
Wyckoff position
x
y
z
24(k) 12(h) 12(i) 24(m) 8(g) 8(g) 8(g) 48(n) 1(a) 8(g) 24(m) 48(n)
0 0 0 1126(4) 2550(3) 1314(1) 1181(23) 348(14) 0 2237(18) 2560(77) 2912(82)
1830(2) 2028(10) 3015(6) 1126(4) 2550(3) 1314(1) 1181(23) 4287(9) 0 2237(18) 3248(64) 3191(86)
3679(2) 5000e 3015(6) 3253(7) 2550(3) 1314(1) 1181(23) 4723(14) 0 2237(18) 3248(64) 4854(86)
U11 or
Uisob
17(1) 139(12) 45(6) 29(3) 27(1) 6(1) 123(18) 140(20) 11(1) 61(6) 96(42) 72(28)
U22
U33
U12
U13
U23
fixed
varied
12(1) 52(8) 17(3) 29(3) 27(1) 6(1)
12(1) 6(5) 17(3) 77(5) 27(1) 6(1)
0 0 0 11(3) -7(2) 3(1)
0 0 0 -20(3) -7(2) 3(1)
2(1) 0 10(4) -20(3) -7(2) 3(1)
45(7) 11(1) 61(1)
92(14) 11(1) 61(1)
-41(8) 0 53(7)
24d 12 12 24 2 3 0.5 2 0.75 0.5 1 1
2.00(2) 3.27(2) 0.20(4) 2.12(4) 0.77(1) 0.63(3) 1.25(20) 1.02(14)
12(10) 0 53(7)
44(7) 0 53(7)
a Positional parameters × 104 and thermal parameters × 103 are given. Numbers in parentheses are the estimated standard deviations in the units of the least significant figure given for the corresponding parameter. The anisotropic temperature factor is exp[-2π2a-2 (U11h2 + U22k2 + U33l2 + 2U12hk + 2U13hl + 2U23kl)]. b Isotropic thermal parameter in units of Å2. c Occupancy factors are given as the number of atoms or ions per unit cell. d Occupancy for (Si) ) 12, occupancy for (Al) ) 12. e Exactly 0.5 by symmetry. f Notation chosen to correspond to the In positions in In10A‚In, ref 16.
sulfur atoms. This suggested the existence of S2 molecules in the large cavities. Furthermore, because S(2) approaches, in addition to In(1), the general position In(4), S(2) should also be at a general position. Therefore, a refinement was attempted with S(2) at (0.291, 0.320, 0.485). It converged to the error indices R1 ) 0.051 and R2 ) 0.048 with the (varied) occupancies for the In(i) and S(i) positions as given in Table 2. The final cycles of refinement were carried out with occupancies fixed at the values given in Table 2. This model converged to the final error indices R1 ) 0.053 and R2 ) 0.050. The final difference Fourier function was featureless. Finally, the small sulfur thermal parameters suggested that more than two sulfur atoms might exist per unit cell. Additional refinements with occupancies of 2.0 at S(1) and S(2), with other occupancies fixed as above, converged to the final error indices R1 ) 0.053 and R2 ) 0.051. However, this model was dismissed for the following reasons: (1) in the least-squares output, the occupancies at S(1) and S(2) are both nearer to 1.0 than 2.0; (2) the error indices R1 and R2 increased; and (3) the thermal parameters (Uiso) at S(1) and S(2) are increased from 0.10 and 0.08 Å2 to 0.33 and 0.24 Å2, respectively, perhaps unreasonably large values. In the last cycle of least-squares refinement, all shifts were less than 1% of their corresponding esd’s. Final structural parameters are presented in Table 2, and interatomic distances and angles are given in Table 3. All atomic scattering factors were taken from the International Tables for X-ray Crystallography.31,32 Atomic scattering factors for In3+, In2+, In+, In0, S0, and (Si, Al)1.75+ were used. Since the cubic space group Pm3hm does not distinguish between Al and Si positions, the scattering factor of (Si, Al)1.75+, the mean of the Si0, Si4+, Al0, and Al3+ scattering functions, was used. Scattering factors of In2+ and In+ were calculated from those of In3+ and In0 as follows, (2In3+ + In0)/3 and (In3+ + 2In0)/3, respectively. All scattering factors were modified to account for anomalous dispersion.33,34 Results and Discussion The complex crystallographic results are interpreted as follows. The structure per unit cell is In8-A‚(In)0.75(S2), with 8.75 indium atoms or ions distributed over six crystallographic positions. The oxidation state at each indium position is identified on the basis of its approach distance to framework
TABLE 3: Selected Interatomic Distances (Å) and Angles (deg)a distance
angle
(Si,Al)-O(1) (Si,Al)-O(2) (Si,Al)-O(3)
1.615(4) 1.643(8) 1.686(6)
O(1)-(Si,Al)-O(2) O(1)-(Si,Al)-O(3) O(2)-(Si,Al)-O(3) O(3)-(Si,Al)-O(3)
110.7(6) 112.2(5) 106.9(3) 107.7(4)
In(1)-O(3) In(1)-O(2) In(2)-O(3) In(2)-O(2) In(3)-O(3) In(3)-O(2) In(4)-O(2) In(4)-O(1) In(6)-O(3) In(6)-O(2) In(8)-O(3) In(8)-O(2) S(1)-O(3) S(1)-O(2) S(2)-O(3) S(2)-O(1)
2.578(6) 3.183(5) 2.366(11) 3.314(5) 2.51(5) 3.45(3) 2.61(6) 2.78(6) 4.378(9) 5.155(6) 2.262(23) 3.01(3) 3.10(11) 3.12(12) 3.83(9) 3.80(13)
(Si,Al)-O(1)-(Si,Al) (Si,Al)-O(2)-(Si,Al) (Si,Al)-O(3)-(Si,Al) O(3)-In(1)-O(3) O(2)-In(1)-O(2) O(3)-In(2)-O(3) O(2)-In(2)-O(2) O(3)-In(3)-O(3) O(2)-In(3)-O(2) O(1)-In(4)-O(2) O(3)-In(8)-O(3) O(2)-In(8)-O(2)
162.9(11) 148.5(4) 136.2(6) 89.7(3) 108.1(2) 100.4(2) 102.1(1) 93.0(12) 96.9(12) 59.5(14) 107.0(16) 117.6(7)
In(2)-In(6) In(3)-In(6) In(1)-S(2) In(4)-S(1) In(4)-S(2) In(8)-S(2) S(1)-S(2)
2.751(2) 2.474(29) 3.26(3) 3.45(11)c 3.37(19) 3.47(6)c 2.13(13)
In(2)-In(6)-In(2)
109.47b
In(4)-S(1)-S(2) In(1)-S(2)-In(4) In(1)-S(2)-S(1) In(4)-S(2)-S(1)
93(6) 91(3) 145(7) 106(5)
a The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameters. b The tetrahedral angle, by symmetry. c This contact is avoided in the arrangements shown in the figures.
atoms and sulfur atoms (vide infra), with the general expectation that indium ions furthest from framework oxygens should have the smallest cationic charges and with the requirement of electrical neutrality as was done in In11-A.16 (The latter consideration was also used to select the fixed occupancies in Table 2.) Six In’s per unit cell are found at four nonequivalent 3-fold axis equipoints: two In+ ions at In(1) and 0.5 In3+ ions at In(8) lie opposite six-rings in the large cavity, three In2+ ions at In(2) and 0.5 In+ ions at In(3) lie opposite six-rings in the sodalite unit. Two In+ ions per unit cell are found at In(4) near an eight-ring plane. Three-quarters of an indium per unit cell, near-neutral atoms associated with indium cations, are found at the centers of the sodalite units at In(6).
20 J. Phys. Chem. B, Vol. 102, No. 1, 1998
Heo et al.
TABLE 4: Deviations of Atoms (Å) from the (111) Plane at O(3)a cation
charge
distance
In(1) In(2) In(3) In(6) In(8)
+1 +2 +1 0 +3
1.50 -1.09b -1.37b -3.84b,c 0.84
a The more positive the Inn+ ion, the more closely it approaches the plane of the O(3) oxygens. b A negative deviation indicates that the ion lies on the same side of the plane as the origin, i.e., inside the sodalite unit. c Located at the origin, at the center of the sodalite unit.
Two sulfur atoms per unit cell are found at two crystallographically distinct positions: the one at S(1) lies opposite a six-ring (near a 3-fold axis) in the large cavity, while the other at S(2) lies opposite a nearby four-ring in the large cavity. (a) Monopositive Indium Ions. The two indiums at In(1) are each 2.578(6) Å from three six-ring oxygens at O(3) (see Table 3). These indiums extend 1.50 Å into the large cavity from the (111) planes at O(3) (see Table 4). Considering the ionic radii of the framework oxygens to be 1.32 Å,35,36 the radii of these indiums must be ca. 2.58 - 1.32 ) 1.26 Å, almost identical with those, 1.25 Å, in In11-A.16 This is somewhat shorter than those found for In+ in various indium halides, which range from 1.32 to 1.51 Å.37,38 However, similarly shortened radii have been found for Tl+ ions in zeolites; ca. 1.30 vs ca. 1.47 Å in various thallous halides, respectively,30,39 probably due to the low coordination number of these ions in the dehydrated zeolite. An even smaller radius, 1.17 Å, was suggested for In+ by a theoretical calculation using the numerical Hartree-Fock (HF) method.40 Accordingly, the oxidation state of the indiums at In(1) appears to be +1. For the same reasons, the indium at In(3) is also judged to be an In+ ion. The radii of these indiums must be ca. 2.51 1.32 ) 1.19 Å. This is shorter than those found for In+ in its halides, and a little shorter than those in In11-A,16 but similar to that from the HF calculation.40 Two In+ ions at In(4) are each 2.61(6) Å from an O(2) eightring oxygen, showing a radius, 1.29 Å, very similar to that at In(1). This somewhat longer distance can probably be attributed to differences in environment: approach distances of monopositive cations to eight-ring oxygens are commonly longer than those to six-ring oxygens in zeolite A.23,24,30,35,39 (b) Di- and tri-positive indium ions. The three indiums per unit cell at In(2) are each 2.366(11) Å from three six-ring oxygens at O(3) (see Table 3). The radii of these indiums is ca. 2.37 - 1.32 ) 1.05 Å. This is sharply less than those of the In+ ions, indicating a higher oxidation state, probably In2+. The ionic radius of 1.05 Å for the In2+ at In(2) is somewhat longer than the average value, 0.95 Å, of the ionic radii of the adjacent cations in the periodic table, Cd2+ (0.97 Å) and Sn2+ (0.93 Å).36,41 This is probably because of the reduced cationic character at In(2) due to association with the In0 atoms at the center of the sodalite unit. These In2+ ions may have been produced by the disproportionation of In+, i.e. 2In+ f In2+ + In0, upon exposure of the crystal to the atmosphere just after reaction with In0.42 In2+ ions have an odd number of electrons and may be unstable unless they participate in clusters such as those at In(2). In2+ ions had been seen before only in fully indium-exchanged zeolite A16 and in some indium trihalide dimers43 such as [In2Cl6]2-. The half indium per unit cell at In(8) is found to be sharply closer than all others to framework oxygens. The radii of these
indiums is ca. 2.26 - 1.32 ) 0.94 Å. This is somewhat longer than those found for In3+, which range from 0.77 to 0.87 in various indium oxides and indium sulfide,44,45 but less than that found for In2+. Furthermore, assigning In(8) as In3+ completes charge balance with the zeolite framework: the sum of the charges of the indium cations becomes 12+. In3+, like In2+, may also have formed by disproportionation of In+. (c) Indium Atoms and Clusters. The location of indium at In(6), at the very center of the sodalite unit, is unambiguous, because no other chemically possible atom or ion, such as Al, O, or S, could account for the electron density (ca. 37 e-) found at this special position. The failure of In(6) to approach the negatively charged framework oxygens (the shortest approach, In(6)-O(3), is 4.378(9) Å) indicates that the In’s at In(6) are not cations, but atoms. As such, they may preferentially approach cations rather than framework oxygens. Indeed, they are reasonably close to the indium cations at In(2) and In(3) in the sodalite unit. These In0 atoms at In(6) might have been retained during the reaction between In0 and Tl-A, or might have been produced by disproportionation during later exposure to the atmosphere or to sulfur, or both. The possible In(6) to In(3) approach distance (ca. 2.47 Å) is unreasonably short when compared to 2.92 Å, the sum of the atomic radius of In0 (1.67 Å) and the ionic radius of In+ (ca. 1.25 Å); it can readily be avoided by not putting both In(6) and In(3) in the same sodalite units. However, the In(6)-In(2) distance (ca. 2.75 Å) is reasonable, comparable to 2.62 Å, the sum of the atomic radius of In0 and the ionic radius of In2+ (0.95 Å). The association of In0 at In(6) with In(2) explains the rather longer approach distances of In2+ at In(2) to the sixring oxygens, due to the reduced cationic character of In2+. This reasonable distance between In0 and In2+ suggests the possibility of having clusters such as (In2)2+, (In3)4+ (linear or bent), (In4)6+, or (In5)8+ (four In2+ ions tetrahedrally arranged in the sodalite unit with an In0 at their center). With the occupancy of 0.75 at In(6), the presence of a mix of the clusters (In2)2+, (In3)4+, or (In4) 6+ would require the sodalite units to have nonequivalent In2+ ions in the vicinity of In(2), as well as nonequivalent In0 atoms near the present In(6) position. This is not seen crystallographically and is not suggested by the small thermal parameters at both In(2) and In(6). The (In5)8+ cluster was found in fully indium-exchanged zeolite A,16 and the ratio of occupancies, In(2)/In(6), 3.0/0.75 ) 4.0 in this structure, is a constant for various structures in this system.16,46 These considerations strongly suggest that only the highly symmetric (In5)8+ cluster, with four In2+ ions at In(2) placed tetrahedrally about an In atom at In(6), is present in the sodalite units. (d) Sulfur Atoms and Sorption Sites. Disulfur molecules are stable species in S(g). They are sorbed from S(g) and stabilized among framework oxygens and In+ ions in the large cavities (see Figure 1). Each unit cell holds one S2 molecule with nonequivalent sulfurs. One at S(1) approaches an O(2) and two O(3)’s, and the other at S(2) approaches a six-ring In+ ion at In(1) and an eight-ring In+ ion at In(4). Because the ionic radius of S2- is essentially the same as the van der Waals radius of S0, and because some of the approach distances to the neighboring oxygens (oxides) are rather longer than those expected for a sulfur atom, the possibility of having S2- was also considered. It was dismissed because the formation of S2- or S22- would require, for charge balance, higher oxidation states than have been assigned to In ions (no other reducing agent was present), and because S22- should not approach framework oxygens at all for electrostatic reasons.
Crystal Structure of Indium-Exchanged Zeolite A
J. Phys. Chem. B, Vol. 102, No. 1, 1998 21
Figure 1. A stereoview of S2 (or (In2(S2))2+) in the large cavity, showing disulfur coordinating to two In+ ions and to three framework oxygens. The zeolite A framework is drawn with solid bonds between tetrahedrally coordinated (Si, Al) and oxygen atoms. Fine solid lines are used to indicate the bonds among the In ions and S atoms, and from them to framework oxygens. Ellipsoids 20% probability are shown.
TABLE 5: Distribution of In and S Species in Component “Unit Cells” of In8-A‚(In)0.75(S2) In and S species
position
charge
In8-A‚(In)(S2), unit cell 1 (75%) no. charge
In8-A‚(S2), unit cell 2 (25%) no. charge
no.
average charge
In(1) In(2) In(3) In(4) In(6) In(8) S(1) S(2) ∑(S) ∑(In)
opposite 6-ringa,b opposite 6-ringb,c opposite 6-ringb,c 8-ringa,d originc,e opposite 6-ringa,b opposite 6-ringa,f opposite 4-ringa
+1 +2 +1 +1 0 +3 0 0
(a) As Individual Ions or Atoms 2 +2 4 +8 0 0 2 +2 1 0 0 0 1 0 1 0 2 0 9 +12
2 0 2 2 0 2 1 1 2 8
+2 0 +2 +2 0 +6 0 0 0 +12
2 3 0.5 2 0.75 0.5 1 1 2 8.75
+2 +6 +0.5 +2 0 +1.5 0 0 0 +12
(In5)8+ (In2(S2))2+ In+ In+ In+ In3+ ∑(S) ∑(In)
(4In(2), In(6))c (In(1), In(4), S(1), S(2))a In(1)a,b In(3)b,c In(4)a,d In(8)a,b
+8 +2 +1 +1 +1 +3
(b) As Monatomic and Polyatomic Cations 1 +8 1 +2 1 +1 0 0 1 +1 0 0 2 0 9 +12
0 1 1 2 1 2 2 8
0 +2 +1 +2 +1 +6 0 +12
0.75 1 1 0.5 1 0.5 2 8.75
+6 +2 +1 +0.5 +1 +1.5 0 +12
a In the large cavity. b On 3-fold axes. c In the sodalite unit. d Off the 8-ring plane. e At the origin, at the center of the sodalite unit. f Off 3-fold axes.
Figure 2. A stereoview of a sodalite unit in In8-A‚(In)(S2) (unit cell 1), showing an (In5)8+ cluster in its center. The zeolite A framework is drawn with open bonds between tetrahedrally coordinated (Si, Al) and oxygen atoms. The bonds between the In ions and framework oxygens are indicated by fine solid lines. Bonds within the cluster are solid. Ellipsoids of 20% probability are shown.
Other than that with S(1), S(2) has its primary interactions with two In+ ions, one at In(1) opposite a six-ring and the other at In(4) on an adjacent eight-ring. The distances of S(2)-In(1) and S(2)-In(4) are 3.26(3) Å and 3.37(19) Å, respectively. These distances are somewhat longer than the sum of the van der Waals radius of sulfur and ionic radius of In+, 1.85 + 1.23 ) 3.08 Å. (A mean ionic radius found for In+ in this structure is (1.19 + 1.26 Å)/2 ) 1.23 Å). On the other hand, S(1) is 3.12(12) Å from an O(2) and 3.10(11) Å from two crystallographically equivalent O(3)’s. These distances are very similar to the sum of the van der Waals radius of sulfur and the ionic
radius of framework oxygen, ca. 1.85 + 1.32 ) 3.17 Å. The S(1)-S(2) molecule is therefore polarized δ+-δ-, respectively, in this environment. For this reason, the S(1) to In(4) distance (3.45(11) Å) is much longer than 3.08 Å (vide supra) and longer than the S(2)-In interactions. Similar interactions between sorbed Te and framework oxygens of zeolite X had been reported in the crystal structure of Na-X with sorbed Te.12 Furthermore, the In(1)-S(2)-In(4), In(1)-S(2)-S(1), and In(4)-S(2)-S(1) angles, 91(3)°, 145(7)°, and 106(5)°, respectively, are reasonable as is that of In(4)-S(1)-S(2), 93(6)°. The S(1)-S(2) bond length, 2.13(13) Å, agrees with that found in
22 J. Phys. Chem. B, Vol. 102, No. 1, 1998
Heo et al.
Figure 3. A stereoview of the large cavity of In8-A‚(In)(S2) (unit cell 1), showing disulfur coordinating to two In+ ions and to three framework oxygens. See the caption to Figure 1 for other details.
Figure 4. A stereoview of a sodalite unit in In8-A‚(S2) (unit cell 2). See the caption to Figure 2 for other details.
Figure 5. A stereoview of the large cavity of In8-A‚(S2) (unit cell 2), showing disulfur coordinating to two In+ ions and to three framework oxygens. See the caption to Figure 1 for other details.
monoclinic sulfur, 2.06 Å, and appears to be longer than that in S2(g), 1.89 Å,47 as might be expected due to the sorption. (e) Arrangement of Indium and Sulfur Species in In8-A‚(In)0.75(S2). The fractional occupancies observed indicate that more than one kind of “unit cell” must exist in this crystal. Furthermore, In(3) and In(6) are too close to coexist in the same sodalite unit. Therefore, two “unit cells” are used to describe the structure of In8-A‚(In)0.75(S2). 75% of the unit cells (sodalite units) contain (In5)8+ clusters (unit cell 1), and the remainder do not (unit cell 2). By completing each unit cell reasonably, avoiding contacts which are too short and distributing cationic charge most evenly, the formulas In8-A‚(In)(S2) for unit cell 1 and In8-A‚(S2) for unit cell 2 emerge. The placements of the various indiums, sulfurs, and their clusters in the two kinds of unit cells are shown in Table 5. In unit cell 1, four In2+ ions, four In+ ions, one In0 atom, and two S0 atoms are distributed over six crystallographically distinct positions. Stereoviews of the sodalite unit and large cavity of unit cell 1 are shown in Figures 2 and 3, respectively. Two In+ ions at In(1) are in the large cavity opposite six-rings. Four In2+ ions at In(2) are arranged tetrahedrally opposite six-
rings in the sodalite unit. One In0 atom at In(6) is at the center of this sodalite unit, forming an (In5)8+ cluster with the four In2+ ions at In(2). Two In+ ions at In(4) are near the centers of eight-rings. One sulfur atom at S(1) is in the large cavity off a 3-fold axis and opposite a six-ring, and the other sulfur atom (at S(2)) is in the large cavity opposite a four-ring. This unit cell has one (In5)8+ cluster in the sodalite cavity and one disulfur molecule in the large cavity. In unit cell 2, two In3+ ions, six In+ ions, and a disulfur molecule are distributed over six crystallographically distinct positions. Stereoviews of the sodalite unit and large cavity of unit cell 2 are shown in Figures 4 and 5, respectively. Two In+ ions at In(1) are in the large cavity opposite six-rings. Two In+ ions at In(3) are in the sodalite unit opposite six-rings. Two In+ ions at In(4) are near eight-rings. Two In3+ ions at In(8) are in the large cavity opposite six-rings. The disulfur molecule is the same as in unit cell 1. Acknowledgment. This work was supported by the Ministry of Education of Korea (No. 96-01-D-0048). N. H. Heo gratefully acknowledges the support of the Central Laboratory of Kyungpook National University for the diffractometer and computing facilities.
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