Zn+ Cations, Probable Tl4Zn12 and Tl6 Clusters, and Zeolite

The products of the reaction of Zn vapor with fully dehydrated fully Tl+-exchanged zeolite X (Tl92Si100Al92O384 per unit cell) at 450 °C (crystal 1, ...
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J. Phys. Chem. B 2000, 104, 515-525

515

Zn+ Cations, Probable Tl4Zn12 and Tl6 Clusters, and Zeolite Desilication (Less Likely Dealumination): Crystallographic Study of the Incomplete Reaction of Zn Vapor with Tl+-Exchanged Zeolite X Shenyan Zhen, Donghan Bae, and Karl Seff* Department of Chemistry, UniVersity of Hawaii, 2545 The Mall, Honolulu, Hawaii 96822 ReceiVed: August 31, 1999; In Final Form: NoVember 17, 1999

The products of the reaction of Zn vapor with fully dehydrated fully Tl+-exchanged zeolite X (Tl92Si100Al92O384 per unit cell) at 450 °C (crystal 1, a0 ) 24.825(3) Å, Tl+62Zn+30Si100Al92O384‚2Tl0,15Zn0) and 500 °C (crystal 2, a0 ) 24.950(4) Å, Tl+18Zn2+46Si98Al94O384‚4SiO44-) have been studied by single-crystal X-ray diffraction in the cubic space groups Fd3hm and Fd3h, respectively. In crystal 1, 30 Tl+ ions were reduced by Zn(g) and replaced by Zn+ ions (18 are found at site I′ and 12 at site II′). This is the first report of the monatomic cation Zn+. Some of the product Tl atoms and additional Zn(g) atoms were captured by some of the remaining site-II Tl+ ions possibly to form (sparsely) Tl+4Zn012Tl04 clusters of symmetry 4h3m at supercage centers. This cluster has a central Tl4 tetrahedron surrounded by 12 Zn atoms, surrounded by four Tl+ ions (a third shell), all in a cubic closest-packed arrangement. Other Tl+ ions have captured Zn atoms, perhaps to form smaller clusters. The Tl+ populations at the III′ sites were unaffected by this reaction. In crystal 2, the desired redox reaction of Zn(g) with Tl+ has gone most of the way to completion. Almost all Tl+ ions at sites I′ and II, and 12 of the 28 at III′ sites, have been replaced by Zn2+ at sites I′, II′, and II. An orthosilicate ion (aluminate less likely) is found at the centers of half of the sodalite units. Each orthosilicate oxygen bridges between a Zn2+ ion at site I′ (2.11 Å) and another at site II′ (2.17 Å) and lies in its Zn2+, Zn2+, Si4+ plane. A formally neutral octahedral Tl6 cluster, supported by coordination to Zn2+ cations, appears to center some sodalite cavities.

1. Introduction The exchangeable cations in zeolites have received a great deal of attention in the scientific literature. The thermal stability, sorption parameters, and catalytic properties of zeolites all depend on the type and number of exchangeable cations and their distribution over the available sites. It has not been possible to prepare fully and stoichiometrically Zn2+-exchanged zeolite X, i.e., Zn46Si100Al92O384, by conventional aqueous methods; in all cases, about 54 Zn2+ ions were found per unit cell after exhaustive ion exchange.1-3 Work with incomplete Zn2+ exchange has been reported.4-6 In this work, anhydrous solvent-free ion exchange of Zn2+ for Tl+ was attempted (1) to avoid this overexchange, caused by the hydrolysis of Zn2+ ions,1-3 and (2) to avoid the subsequent loss of aluminate and reconstruction of the zeolite framework upon dehydration.2 An original purpose for the preparation of fully Tl+exchanged zeolite X7 involved its anticipated use as a starting material for the introduction by redox reaction of cations of elements more electropositive than Tl. Fully Cd2+-exchanged zeolite X was prepared for the same reason8 and is being used for this purpose.9 NH4+-X can sometimes be used for this purpose also.10 By reacting the vapors of metals or of other selected volatile compounds with these readily prepared forms of zeolite X, new ion-exchange compositions not accessible by conventional aqueous methods can be prepared. Parallel work in progress involves the reaction of Cd2+exchanged zeolite X with Zn vapor. When a limited amount of * To whom correspondence should be addressed. E-mail: kseff@ gold.chem.hawaii.edu.

Zn vapor was used, Zn atoms were simply sorbed onto Cd-X without redox reaction.11 The further introduction of Zn vapor into Cd46-X led to the desired reduction and removal as Cd(g) of all Cd2+ ions, but with the retention of additional atoms of Zn.9 In this work, fully dehydrated Tl92-X was exposed to Zn vapor with the hope of preparing either stoichiometrically Zn2+exchanged zeolite X or its Zn0 sorption complex. The stoichiometric ion exchange of Tl+ into zeolite X is facile,7 and it would be easy to distinguish Tl+ from Zn2+ in the product because of the large difference in their atomic scattering factors (81 e- for Tl and 30 e- for Zn) and ionic radii12 (Tl+, 1.47 Å and Zn2+, 0.74 Å). 2. Experimental Section 2.1. Crystal Preparation. Large single crystals of sodium zeolite X of stoichiometry Na92Si100Al92O384 per unit cell were prepared in Leningrad, now St. Petersburg, Russia.13 One of these, a colorless octahedron about 0.12 mm in cross section, was lodged in a fine Pyrex capillary. Aqueous 0.1 M TlNO3 (City Chemical Corp., NY, purified, code T7260) was allowed to flow past the crystal at a velocity of 20 mm/s for 2 days; this procedure using TlNO3 from the same source had led successfully to Tl92-X.7 The crystal was transferred to another capillary that contained small particles of Zn metal (Baker Analyzed Reagent, 99.9%, Pb 0.010%, Fe 0.003%) near its tip. The capillary containing the crystal and Zn metal was attached to a vacuum system. Both were dehydrated by increasing the temperature (ca. 15 °C/h) to 450 °C at a pressure of 1 × 10-5 Torr. However, the temperature increased quickly to 70 °C in

10.1021/jp993093j CCC: $19.00 © 2000 American Chemical Society Published on Web 12/31/1999

516 J. Phys. Chem. B, Vol. 104, No. 3, 2000 about 5 min at the initial stage because of the overheating of the oven used. The system was maintained at these conditions for 2 days as Zn vapor (vapor pressure ) 0.4 Torr at 450 °C)14 flowed past the crystal into a dynamic vacuum. The hot contiguous downstream lengths of the vacuum system, including a sequential U-tube of beads of zeolite 3A fully activated in situ, were then cooled to ambient temperature to prevent the movement to the crystal of water molecules from more distant parts of the vacuum system that had not been baked out. The crystal was then slowly cooled to 24 °C in 24 h. The resulting lustrous black crystal (crystal 1) was sealed off in its capillary by torch. A large deposit of Zn was observed on the cooler glass downstream from the crystal, indicating that a substantial quantity of Zn(g) had flowed past the crystal. This work was repeated at 500 °C (vapor pressure of Zn ) 1.7 Torr)14 with another 0.12 mm crystal without oven mishap. Again the product was black and lustrous (crystal 2). The vapor pressures of Tl at 450 and 500 °C, 4 × 10-5 and 3 × 10-4 Torr,15 respectively, are high enough for the product Tl metal to leave the zeolite unless held by complexation. 2.2. X-ray Data Collection. The reflection conditions (hkl: h + k, k + l, l + h ) 2n; 0kl: k + l ) 4n) indicate that the space group is either Fd3h or Fd3hm. Fd3h was initially chosen for both crystals because most crystals from this synthesis batch, regardless of subsequent chemical treatment, have been refined successfully in Fd3h.16 Their diffraction data refined to error indexes lower in Fd3h than in Fd3hm, with mean Si-O distances correctly ca. 0.10 Å shorter than the mean Al-O distances.16 This is in agreement with Loewenstein’s rule17 which requires alternation of Si and Al atoms for crystals with Si/Al ) 1, and requires that also, at least in the short range, for these crystals whose Si/Al ratio is 1.09. An inspection of the diffraction intensity data from these crystals generally shows strong intensity inequalities for some hkl/khl reflection pairs, indicating the absence of the mirror plane of Fd3hm, hence Fd3h. Fd3h, justified by the reasons just given, was used for crystal 2. The diffraction data refined successfully to error indexes in Fd3h lower than in Fd3hm, with a mean Si-O distance, 1.623 Å, reasonably less than the mean Al-O distance, 1.719 Å (see Table 3). For crystal 1, however, Fd3h was rejected and the space group Fd3hm was used. In least-squares refinement, only a negligible difference (ca. 0.002 Å) was seen between the mean Si-O and Al-O bond lengths, and the error indexes did not increase when the space group was changed to Fd3hm. The erasure of the ca. 0.10 Å difference between mean Si-O and mean Al-O indicates that the Si,Al composition at the Si position is essentially the same as that at the Al position, and therefore the same as that of the entire crystal; long range Si,Al ordering has been lost. This can occur most easily by the establishment of antidomain structure.18 The unit cell constants at 23 °C, a0 ) 24.825(3) Å for crystal 1 and 24.950(4) Å for crystal 2, were determined by leastsquares refinement of 24 and 26 intense reflections, respectively, for which 10° < 2θ < 26°. All unique reflections in the positive octants of an F-centered unit cell for which 3° < 2θ < 55° were recorded at 23 °C. Background intensity was counted at each end of a scan range for a time equal to one-half the scan time. The intensities of three reflections in diverse regions of reciprocal space were recorded every 97 reflections to monitor crystal and instrument stability. Only small random fluctuations of these check reflections were observed.

Zhen et al. Standard deviations were assigned to individual reflections by σ(I) ) ω(CT + B1 + B2)1/2 where CT is the total integrated count, B1 and B2 are the background counts, and I is the intensity. Lorentz, polarization, and profile corrections were made. Empirical absorption corrections were not made; they do little to improve the data for a regular octahedron.7,19 For crystal 1, of the 4790 reflections gathered, 900 were unique (Fd3hm); Fo > 4σ(Fo) for 268 of these. For crystal 2, of the 4765 reflections gathered, 1498 were unique (Fd3h); Fo > 4σ(Fo) for 482 of these. Molybdenum KR radiation (KR1 ) 0.70930 Å; KR2 ) 0.71359 Å) was used for both crystals. 3. Structure Determination 3.1. Crystal 1 (Reaction at 450 °C). Full-matrix least-squares refinement20 was done on F2 using all unique reflections. It was initiated with the atomic positions of the framework atoms [(Si,Al), O(1), O(2), O(3), and O(4)] in the structure of dehydrated Zn2+-exchanged zeolite X.2 Since the SiO4 and AlO4 tetrahedra are indistinguishable in the space group Fd3hm, only the average species, (Si,Al), is considered. Fixed weights of a ) 0.10 and b ) 0 were used initially; the final weights were assigned using the formula w ) q/[σ2(Fo2) + (aP)2 + bP + d + e sin(θ)] where P ) fFo2 + (1 - f)Fc2, to give w ) 1/[σ2(Fo2) + (aP)2 + bP] where P ) (Fo2 + 2Fc2)/3, with a and b as refined parameters. R1 ) ∑(|Fo - |Fc||)/∑Fo was calculated using the 268 reflections for which Fo > 4σ(Fo) and wR2 ) {∑[w(Fo2 - Fc2)2]/∑(Fo2)2}1/2 is based on Fo2 and was calculated using all 900 unique reflections examined. Isotropic refinement of the framework atoms converged to R1/wR2 ) 0.58/0.89. A difference electron density Fourier function revealed four large peaks at (0.080, 0.080, 0.080), (0.256, 0.256, 0.256), (0.204, 0.204, 0.204), and (0.049, 0.049, 0.049). The first two had reasonable Tl+-O2- distances to the zeolite framework; the second two showed reasonable Zn2+O2- distances. Isotropic refinement treating them as Tl+ ions at Tl(I′) and Tl(II) and as Zn2+ ions at Zn(II′) and Zn(I′), respectively, converged quickly to R1/wR2 ) 0.17/0.48. An ensuing difference Fourier function revealed a strong peak at (0.139, 0.107, 0.413), a III′ cation position, with reasonable Tl-O distances to framework oxygens. It was refined as Tl(IIIA) to R1/wR2 ) 0.13/0.36. Anisotropic refinement of all the nonframework cation positions converged to R1/wR2 ) 0.12/ 0.34. Another three peaks were found at (0.070, 0.072, 0.414), (0.426, 0.426, 0.426), and (0.171, 0.085, 0.503). Refinement including them as Tl(3*), Tl(A), and Zn(A), respectively, led to R1/wR2 ) 0.108/0.294. Anisotropic refinement at Tl(3*) gave unreasonable principal mean-square atomic displacements of 0.930, 0.196, and 0.104 Å2, suggesting that it should be split into two positions. Refinement with the two suggested positions, Tl(IIIB), (0.086, 0.088, 0.429), and Tl(IIIC), (0.064, 0.067, 0.416), converged to R1/wR2 ) 0.102/0.289. At this stage, the largest peak on the difference Fourier function, 1.4 eÅ-3 in height, was at site I (0, 0, 0). Refinement including it as Zn(I) converged to R1/wR2 ) 0.100/0.288 with an occupancy of 1.4(5). This peak should not be oxygen because (1) the total charge would not be balanced, (2) this position is 2.36(6) Å from the Zn cations at site I′, somewhat longer than expected Zn-O distances, (3) the Zn(I′)-O(3) bond has not lengthened as it should have if Zn(I′)-O-Zn(I′) bonding existed, and (4) a similar atom-to-cation interaction was observed for Tl atoms and Zn2+ ions in crystal 2 (see section 4.3.) and for Zn atoms and Cd2+ ions in Cd46-X‚20Zn.11 In the final refinement, the occupancies at Tl(I′) and Zn(I′), which had refined freely to 13.2 and 22.0, were constrained to

Incomplete Reaction of Zn Vapor with Tl+-Exchanged Zeolite X

J. Phys. Chem. B, Vol. 104, No. 3, 2000 517

TABLE 1: Positional,a Thermal,a and Occupancy Parameters atom

Wyc. pos.

(Si,Al) O(1) O(2) O(3) O(4) Tl(I′) Zn(I′) Tl(II) Zn(II′) Tl(IIIA) Tl(IIIB) Tl(IIIC) Tl(A) Zn(A) Zn(I)

192(i) 96(h) 96(g) 96(g) 96(g) 32(e) 32(e) 32(e) 32(e) 96(g) 96(g) 96(g) 32(e) 96(g) 16(c)

Si Al O(1) O(2) O(3) O(4) Zn(I′) Zn(II′) Tl(IIIA) Tl(IIIB) Tl(I′) Tl(II) Zn(II) Tl(A) Si(1) O(5)

96(g) 96(g) 96(g) 96(g) 96(g) 96(g) 32(e) 32(e) 96(g) 96(g) 32(e) 32(e) 32(e) 48(f) 8(a) 96(g)

site

I′ I′ II II′ III′ III′ III′

I′ II′ III′ III′ I′ II II

U11 or Uiso

x

y

z

U22

U33

-525(3) 0 -7(8) -254(8) 739(6) 738(1) 547(14) 2531(1) 2030(6) 1403(7) 534(20) 856(18) 4225(8) 1691(17) 0

369(3) -1026(6) -7(8) 735(6) 739(6) 738(1) 547(14) 2531(1) 2030(6) 1097(7) 534(19) 856(18) 4225(8) 809(17) 0

1248(3) 1026(6) 1482(8) 735(6) 3241(9) 738(1) 547(14) 2531(1) 2030(6) 4125(5) 4096(23) 4268(18) 4225(8) 5035(25) 0

(a) Reaction at 450 °C (Crystal 1) 255(17) 560(71) 563(65) 406(62) 455(62) 310(24) 310(24) 310(24) 1217(118) 1217(118) 1217(118) 391(12) 391(12) 391(12) 556(78) 556(78) 556(78) 1768(223) 1768(223) 438(84) 617(270) 1939(293) 422(140) 1439(382) 538(472)

-507(1f) -526(1f) -1038(3) 5(4) -239(4) -714(3) 563(2) 2017(1) 1100(6) 700(22) 766(17) 2467(22) 2304(17) 1250 1250 788(19)

1248(1f) 371(1f) 15(3) -8(4) 728(4) 791(4) 563(2) 2017(1) 1354(8) 791(26) 766(17) 2467(22) 2304(17) 1250 1250 781(19)

359(1f) 1234(1f) 981(3) 1491(3) 694(4) 1742(4) 563(2) 2017(1) 4117(4) 4201(9) 766(17) 2467(22) 2304(17) 365(25) 1250 1330(19)

(b) Reaction at 500 °C (Crystal 2) 361(20) 226(16) 298(19) 276(19) 228(18) 181(17) 559(63) 376(53) 429(56) 529(56) 479(55) 289(49) 737(72) 402(58) 488(62) 295(50) 618(66) 480(61) 500(21) 500(21) 500(21) 268(15) 268(15) 268(15) 1412(200) 1581(189) 268(45) 5076(742) 6034(969) 820(144) 328(175) 327(228) 382(157) 300g 252(64) 432(142)

U12

U13

U23

-63(18) 556(156) -9(13) 249(78) -403(83)

-63(18) 556(156) -9(13) 249(78) 403(83)

-63(18) 556(156) -9(13) 249(78) -464(222)

-32(18) 5(15) -222(48) 161(48) 3(52) 14(48) -70(18) 29(12) 245(99) 2104(661)

1(16) -36(48) -33(53) 31(46) -98(53) 20(46) -70(18) 29(12) -196(59) 1102(219)

-52(17) -21(17) -63(47) -44(45) 145(50) -195(51) -70(18) 29(12) 84(60) 1121(247)

occupancyb varied fixed

13.7(6)d 18.3(6)d 19.9(3)d 12.1(3)d 13.5(7) 3.5(7) 9.9(12) 2.2(2) 12.3(2) 1.4(5)

192c 96c 96c 96c 96c 14e 18e 20e 12e 14e 4e 10e 2e 13e 2e

22.5(5) 18.8(3) 9.0(4) 7.0(5) 0.7(2) 0.6(2) 2.5(5) 0.5(1) 4.1(2)h 16.4(9)h

96c 96c 96c 96c 96c 96c 23e 19e 9e 7e 1e 1e 3e 1e 4e 16e

Positional and thermal parameters are given ×104. Numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. The anisotropic temperature factor equals exp[-2π2a-2(h2U11 + k2U22 + l2U33 + 2hkU12 + 2hlU13 + 2klU23)]. b Occupancy factors are given as the number of atoms or ions per unit cell. c This occupancy was fixed in least-squares refinement. d The sum of the occupancies at Tl(1′) and Zn(1′) and the sum of the occupancies at Tl(2) and Zn(2′) were both constrained to sum to 32. e These integral occupancies were not fixed in least-squares refinement. They are used to facilitate the discussion. f The esd’s of the coordinates of Si and Al are all approximately 1.3 × 10-4. g The thermal parameter of Tl(A) was fixed in the least-squares refinement. h The occupancies at Si(1) and O(5) were constrained to Si/O ) 1/4. a

sum to 32, the maximum total occupancy at these positions. The occupancies at Tl(II) and Zn(II′), which had refined freely to 20.4 and 13.6, were similarly constrained to sum to 32. An attempt to refine the zeolite framework atoms anisotropically resulted in nonpositive definite thermal parameters at some oxygens with no improvement in the error indexes, so the framework was left isotropically refined. Refined weights were used in the final refinement for which R1/wR2 ) 0.099/0.229. In the final cycle of least-squares refinement, all shifts in atomic parameters were less than 0.1% of their corresponding standard deviations. The largest maximum/minimum in the final difference function was 1.4/-1.3 eÅ-3. The two largest peaks were both unstable in least-squares refinement. The goodnessof-fit ) {∑[w(Fo2 - Fc2)2]/(n - p)}1/2 ) 0.88; the number of unique reflections, n, is 900, and the number of parameters, p, is 54. Atomic scattering factors for Si, Al, O, Tl, and Zn were used.21 The form factor for (Si,Al) was that of Si diminished by a factor of 0.966 to give the correct number of electrons for a Si/Al framework ratio of 100/92. All scattering factors were modified to account for anomalous dispersion.22,23 The final structural parameters are presented in Table 1a, and selected interatomic distances and angles are in Table 2. 3.2. Crystal 2 (Reaction at 500 °C). The same procedure was used to determine the structure of crystal 2. The refinement was initiated with the atomic parameters of the framework atoms [Si, Al, O(1), O(2), O(3), O(4)] from Olson’s determination of

the structure of dehydrated Na-X.24 R1 was calculated using 482 reflections, and wR2 was calculated using all 1498 unique reflections examined. Isotropic refinement of the framework atoms converged to R1/wR2 ) 0.45/0.80. A difference electron density function revealed two large peaks at (0.056, 0.056, 0.056) and (0.200, 0.200, 0.200) with reasonable Zn2+-O2- distances to framework oxygens. Isotropic refinement treating them as Zn2+ ions at Zn(I′) and Zn(II′) converged quickly to R1/wR2 ) 0.21/0.52. An ensuing difference function revealed three strong peaks at (0.125, 0.125, 0.125), (0.110, 0.130, 0.410), and (0.239, 0.239, 0.239). Refinement including them as Si(1), Tl(IIIA), and Zn(II) led to R1/wR2 ) 0.14/0.36. Full anisotropic refinement, except for Si(1) and Zn(II) which refined isotropically, converged to R1/wR2 ) 0.12/0.32. Another strong peak was found at (0.061, 0.067, 0.413). It refined isotropically as Tl+ ions at Tl(IIIB) to R1/wR2 ) 0.109/ 0.284. Anisotropic refinement and further splitting of this position was not successful. Two more peaks found at (0.079, 0.079, 0.079) and (0.254, 0.254, 0.254) were refined isotropically as Tl+ ions at Tl(I′) and Tl(II). At convergence R1/wR2 ) 0.104/ 0.274. The next difference Fourier function revealed two more peaks at (0.082, 0.079, 0.132) and (0.034, 0.125, 0.125) that refined as O(5) and Tl(A) to R1/wR2 ) 0.098/0.262. The occupancies at Si(1) and O(5), which had refined freely to 4.5 and 13.8, respectively, were constrained to Si/O ) 1/4.

518 J. Phys. Chem. B, Vol. 104, No. 3, 2000 TABLE 2: Selected Interatomic Distances (Å) and Angles (deg)a (Crystal 1)

Zhen et al. TABLE 3: Selected Interatomic Distances (Å) and Angles (deg)a (Crystal 2)

(a) Framework

(a) Framework

(Si,Al)-O(1) (Si,Al)-O(2) (Si,Al)-O(3) (Si,Al)-O(4)

1.648(8) 1.701(9) 1.712(11) 1.668(11)

O(1)-(Si,Al)-O(2) O(1)-(Si,Al)-O(3) O(1)-(Si,Al)-O(4) O(2)-(Si,Al)-O(3) O(2)-(Si,Al)-O(4) O(3)-(Si,Al)-O(4) (Si,Al)-O(1)-(Si,Al) (Si,Al)-O(2)-(Si,Al) (Si,Al)-O(3)-(Si,Al) (Si,Al)-O(4)-(Si,Al)

112.6(10) 110.4(10) 108.8(11) 104.5(10) 106.7(10) 114.0(11) 145.2(15) 136.1(14) 129.9(13) 138.2(17)

(b) Framework and Nonframework Tl(II)-O(2) 2.619(20) (three)b Tl(I′)-O(3) 2.474(21) (three) Tl(IIIA)-O(4) 2.899(22) (two) Tl(IIIA)-O(1) 3.122(17) (two) Tl(IIIB)-O(4) 2.25(6) Tl(IIIB)-O(1) 3.05(3) (two) Tl(IIIC)-O(4) 2.60(5) Tl(IIIC)-O(1) 3.22(3) (two) Zn(II′)-O(2) 2.17(3) Zn(I′)-O(3) 2.10(3) Zn(A)-Tl(II) 2.75(6) Tl(A)-Tl(A) 3.35(6) (three) Tl(A)-Zn(A) 2.94(5) (six) Zn(A)-Zn(A) 2.73(11) (two) 3.10(14) Zn(I)-Zn(I′) 2.36(6) Tl(I′)-Tl(I′) 3.615(11) Zn(I′)-Zn(II′) 3.71(3) Zn(I′)-Tl(I′) 4.32(5) Zn(II′)-Tl(II) 2.163(24) Zn(II′)-Tl(I′) 3.361(19) O(2)-Tl(II)-O(2) O(3)-Tl(I′)-O(3) O(2)-Zn(II′)-O(2) O(3)-Zn(I′)-O(3) Zn(A)-Tl(II)-Zn(A) Zn(A)-Zn(A)-Zn(A)

87.3(8) 89.7(7) 113.1(7) 112.0(15) 59.6(21) 60c

a Numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. b Each first atom bonds to this many second atoms. c Exactly, by symmetry.

Anisotropic refinement of Tl(IIIB) with this constraint converged to R1/wR2 ) 0.095/0.256. The splitting of the Tl(IIIB) position was not successful, perhaps because its occupancy is relatively low. The refinement of cations at III′ positions, especially Tl+, often yields very large thermal parameters indicative of disorder or shallow minima.7,25 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 electron density function was almost featureless, indicating that all atoms were accounted for. The largest peak, 0.8 eÅ-3 at (-0.121, 0.049, 0.050), was too close to framework oxygens and refined to an insignificant occupancy. The goodness-of-fit is 0.87; the number of unique reflections, n, is 1498, and the number of parameters, p, is 100. The final structural parameters are presented in Table 1b, and selected interatomic distances and angles are in Table 3. 4. Structure Descriptions 4.1. Structure of Zeolite X. Zeolite X is an aluminum-rich synthetic analogue of the naturally occurring mineral faujasite.

Si-O(1) Si-O(2) Si-O(3) Si-O(4)

1.587(8) 1.631(9) 1.674(9) 1.599(9) Mean Si-O 1.623

Al-O(1) Al-O(2) Al-O(3) Al-O(4)

1.672(8) 1.742(9) 1.759(10) 1.701(9) Mean Al-O 1.719

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

111.7(5) 109.4(5) 111.0(5) 104.5(5) 108.2(5) 111.8(5) 115.3(5) 106.8(5) 113.2(5) 104.2(5) 105.5(5) 111.6(5) 146.9(6) 133.0(5) 127.4(6) 144.0(6)

(b) Framework and Nonframework Zn(I′)-O(3) 2.059(11) (three)b Zn(I′)-O(5) 2.05(4) Zn(II′)-O(2) 2.145(9) (three) Zn(II′)-O(5) 2.01(5) Tl(IIIA)-O(4) 2.794(17), 2.872(19) Tl(IIIA)-O(1) 3.080(15) Tl(IIIB)-O(1) 2.98(3) Tl(IIIB)-O(4) 2.462(22) Tl(I′)-O(3) 2.50(5) (three) Tl(II)-O(2) 2.43(6) (three) Zn(II)-O(2) 2.135(22) (three) Si(1)-O(5) 1.65(5) (four) Tl(A)-Zn(I′) 2.460(13) Tl(A)-Zn(II′) 2.709(8) Tl(A)-O(3) 2.41(4) Tl(A)-Tl(A) 3.11(9) O(3)-Zn(I′)-O(3) O(2)-Zn(II′)-O(2) O(3)-Tl(I′)-O(3) O(2)-Tl(II)-O(2) O(2)-Zn(II)-O(2) O(5)-Si(1)-O(5) Si(1)-O(5)-Zn(I′) Si(1)-O(5)-Zn(II′) Zn(I′)-O(5)-Zn(II′)

108.2(3) 111.7(3) 84(3) 92(3) 112.4(17) 92(4) (two) 119.6(4) (four) 105.3(23) 128.7(24) 126.0(23)

a Numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. b Each first atom bonds to this many second atoms.

The 14-hedron with 24 vertexes known as the sodalite cavity or β cage (see Figure 1) may be viewed as its principal building block. These sodalite cages are connected tetrahedrally at sixrings by bridging oxygens to give double six-rings (D6Rs, hexagonal prisms) and concomitantly to give an interconnected set of even larger cavities (supercages, R cages) accessible in three dimensions through 12-ring (24-membered) 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 units and supercages, and may be viewed

Incomplete Reaction of Zn Vapor with Tl+-Exchanged Zeolite X

Figure 1. Stylized drawing of the framework structure of zeolite X. Near the center of each line segment is an oxygen atom. The different oxygen atoms are indicated by the numbers 1-4. Silicon and aluminum alternate at the tetrahedral interactions, except that Si substitutes for about 4% of the Al’s. Extraframework cation sites are labeled with Roman numerals.

as entrances to the sodalite cavities. Each unit cell has eight sodalite units, eight supercages, 16 D6Rs, 16 12-rings, and 32 S6Rs. Exchangeable cations that balance the negative charge of the aluminosilicate framework are found within the zeolite’s cavities. They are usually found at the following general sites shown in Figure 1: I at the center of a D6R; I′ in the sodalite cavity on the opposite side of either of the D6R’s six-rings from site I; II′ inside the sodalite cavity near a S6R; II in the supercage adjacent to a S6R; III in the supercage on a 2-fold axis opposite a four-ring between two 12-rings; III′ somewhat or substantially distant from III but otherwise near the inner walls of the supercage or the edges of 12-rings. 4.2. Structure of Tl92-X. In fully dehydrated fully Tl+exchanged zeolite X,7 92 Tl+ ions are located at four different crystallographic sites in space group Fd3h. In all, 32 fill site I′ with Tl(I′)-O(3) ) 2.59(1) Å, and another 32 fill site II with Tl(II)-O(2) ) 2.68(1) Å. About 16 Tl+ ions are found at a III′ site with two Tl-O(4) distances of 2.81(2) and 2.87(2) Å, and the remaining 12 occupy another III′ site with Tl-O(4) ) 2.38-

J. Phys. Chem. B, Vol. 104, No. 3, 2000 519 (4) Å and Tl-O(1) ) 2.95(4) and 3.08(4) Å. For comparison, the sum of the ionic radii12 of Tl+ and O2- is 1.47 + 1.32 ) 2.79 Å. 4.3. Structure of Crystal 1, Tl+62Zn+30Si100Al92O384‚2Tl0,15Zn0. As compared to the structure of fully dehydrated Tl92X, 18 of the 32 Tl+ ions at site I′ have been replaced by 18 Zn+ ions. Similarly, 12 of the 32 Tl+ ions at site II have been replaced by 12 Zn+ ions at the nearby site II′. Both site I′ and the S6Rs (sites II and II′) remain full. For electrical neutrality, the charge of each Zn ion must be 1+ because each has replaced a Tl+ ion. The net reaction is Tl+ + Zn0 f Tl0 + Zn+. The total occupancy at the III′ sites, 28 Tl+ ions, did not change, but the number of III′ sites increased from two to three. Tl+ ions are found at five equipoints in this structure. Each + Tl ion at Tl(I′) coordinates trigonally to three O(3) framework oxygens at 2.474(21) Å. These distances are a little shorter than those in fully dehydrated Tl92-X,7 2.59(1) Å, and both are less than the sum of the corresponding ionic radii,12 2.79 Å, presumably because the coordination number of these Tl+ ions is only three and because interthallium repulsions push them closer to their coordinating oxygens. Each Tl+ ion at Tl(II) coordinates trigonally to three O(2) framework oxygens at 2.619(20) Å, also possibly a little shorter than those in fully dehydrated Tl92-X,7 2.68(1) Å. The Tl+ ions at the three III′ sites, Tl(IIIA), Tl(IIIB), and Tl(IIIC), all coordinate to O(1) and O(4) framework oxygens (see Figure 2) with mean distances of 3.01 Å to four oxygens, 2.79 Å to three oxygens, and 3.01 Å to three oxygens, respectively (see Table 2). Zn+ ions are found at two equipoints. Each of the 18 Zn+ ions at site I′ coordinates trigonally to three O(3) framework oxygens at 2.10(3) Å. Each of the 12 Zn+ ions at site II′ coordinates trigonally to three O(2) framework oxygens at 2.17(3) Å. These distances are likely to be somewhat inaccurate because only average O(2) and O(3) oxygen positions have been determined; six-rings containing Tl+ ions are likely to be a little different from those containing Zn+. Nonetheless, these distances agree with 2.20 Å, the sum of the ionic radii of Zn+, 0.88 Å,12,28 and O2-, 1.32 Å;12 the average appears to be appropriately shorter than 2.20 Å because of the low coordination number, three, at Zn+. (For comparison, the Zn2+ radius is 0.74 Å.12) The repulsions among the Tl(I′) ions, which have the shortest intercationic contacts, 3.615(11) Å, can be minimized by placing no more than two Tl(I′) ions in each sodalite cavity (see Figure 3a). This leads to the following arrangements: six of the eight sodalite units per unit cell can each have two site I′ Tl+ ions, two site I′ Zn+ ions, two site II′ Zn+ ions, and two site II Tl+ ions outside the cage (Figure 3a); each of the two remaining

Figure 2. Stereoview of a supercage in crystal 1, Tl+62Zn+30Si100Al92O384‚2Tl0,15Zn0. Three Tl+ ions, one each at Tl(IIIA), Tl(IIIB), and Tl(IIIC), coordinate to four, three, and two framework oxygen atoms at various distances, respectively. This and subsequent figures were drawn with thermal ellipsoids of 20% probability using the program ORTEP3.26,27 In all drawings of crystal 1, the (Si,Al) position is labeled as alternating Si and Al atoms because of the short-range order which must exist.17 Most atom names have been abbreviated for the purposes of these drawings; for example, Tl(IIIA) has become Tl3a.

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Figure 3. (a) Stereoview of a sodalite cavity showing Tl+ ions at sites I′ and II and Zn+ ions at sites I′ and II′ in crystal 1. Each cation coordinates to three framework oxygens. 75% of the sodalite units have this arrangement. (b) Stereoview of a sodalite cavity showing Tl+ ions at site II and Zn+ ions at sites I′ and II′ in crystal 1. 25% of the sodalite units have this arrangement. See the Figure 2 caption for further details.

sodalite units can have one site I′ Tl+ ion, three site I′ Zn+ ions, and four site II Tl+ ions outside the cage (Figure 3b). These occupancies suggest that reaction of Zn with Tl92-X stopped not at a stoichiometric end point, but when the stream of Zn(g) stopped. About 13.7 zinc atoms at Zn(I) and Zn(A) and 2.2 thallium atoms at Tl(A) have been retained by complexation per unit cell. The 1.4 Zn atoms at site I each bridge between two Zn+ ions at site I′ at 2.36(6) Å, perhaps a little longer than the sum of the corresponding radii, 1.33 Å(Zn0)29 + 0.88 Å(Zn+)12,28 ) 2.21 Å. For this reason, the ions at Zn(I′) must be nonequivalent, some bind to Zn(I) but most do not; accordingly the 2.36 Å distance may not be accurate and the refined thermal parameter at Zn(I′) may be too large. About 6.6 of the Zn atoms at Zn(A) and the 2.2 Tl atoms per unit cell at Tl(A) have been trapped by about 2.2 of the Tl+ ions at site II in the supercage to form a fraction, 2.2/4 ) 0.55, of a possible Tl+4Zn012Tl04 cluster per unit cell (see Figure 4). This corresponds to one such cluster in about 15 supercages. The remaining ca. six Zn atoms have been captured by some of the remaining ca. 18 Tl+ ions at site II, perhaps to form smaller clusters. 4.4. Structure of Crystal 2, Tl+18Zn2+46Si98Al94O384‚4SiO44-. After reaction of Tl92-X with Zn vapor at 500 °C, only two or fewer of the original 64 Tl+ ions at sites I′ and II remain. Correspondingly, 42 Zn2+ ions per unit cell were found at these sites, 23 at site I′ and 19 at site II′ (see Figures 5 and 6). Each site-I′ Zn2+ ion coordinates trigonally to three O(3) framework oxygens at 2.059(11) Å; each site-II′ Zn2+ ions coordinates trigonally to three O(2) framework oxygens at 2.145(9) Å. The Zn(II′)-O(2) distances are somewhat longer than the sum of the ionic radii12 of Zn2+ and O2-, 0.74 + 1.32 ) 2.06 Å, because

of the further coordination of the Zn2+ ions at Zn(II′) to orthosilicate ions (see later in this section and section 5.2.3.). About three Zn2+ ions are found at site II (see Figure 7); each coordinates trigonally to three O(2) framework oxygens at 2.135(22) Å. Less than one (0.6) Tl+ ion per unit cell remains at site I′; the same is seen at site II where 0.7 Tl+ ions remain unreacted (see Table 1b). Each of them coordinates to O(3) and O(2) framework oxygens (see Figure 7) as they did in crystal 1, with Tl(I′)-O(3) ) 2.50(5) Å and Tl(II)-O(2) ) 2.43(6) Å. Of the 28 Tl+ ions at the III′ sites, 16 remain, nine at Tl(IIIA) and seven at Tl(IIIB). They also continue to occupy their original positions (see Figure 7). About four orthosilicate ions (orthoaluminate less likely) are found per unit cell, one each in four of the eight sodalite units per unit cell (see Figure 6). The Si(1)-O(5) distances are 1.65(5) Å, and the angles range from 92(4)° to 119.6(4)°. These O(5) oxygens also each bridge between two Zn2+ ions at sites I′ and II′ at distances of 2.05(4) and 2.01(5) Å, respectively, with each O(5) lying exactly in the plane of Si(1), Zn(I′), and Zn(II′). The sum of the three angles at O(5) is 105.3° + 128.7° + 126.0° ) 360.0° (see Table 2). About 0.5 Tl atoms per unit cell at Tl(A) have been trapped in the sodalite units by Zn2+ ions at sites I′ and/or II′. Tl(A) is on a 2-fold axis. An environment to support this symmetry could be coordination by two Zn2+ ions at Zn(I′) (2.460(13) Å), or two Zn2+ ions at Zn(II′) (2.709(8) Å), or all four. Good bonding distances (3.11(9) Å) from Tl(A) to four other Tl(A) atoms are available, and these four additional Tl(A) atoms form a square with the same Tl(A)-Tl(A) distance. A regular octahedron can be readily completed, coordinated, and supported by four or

Incomplete Reaction of Zn Vapor with Tl+-Exchanged Zeolite X

J. Phys. Chem. B, Vol. 104, No. 3, 2000 521

Figure 4. (a) A possible Tl+4Zn012Tl04 cluster in a supercage in crystal 1. Four Tl+ ions, each at a Tl(II) position coordinating to three framework oxygens, trapped 12 Zn atoms (at Zn(A)), three at each Tl(II). Four Tl atoms at Tl(A) bind to each other; each also coordinates to six Zn atoms. Each Tl atom lies almost in the plane of the six Zn atoms. Besides its Tl(II) and Tl(A) coordination, each Zn atom at Zn(A) also binds to other three Zn atoms. (b) The same cluster as in (a) with the zeolite framework omitted for clarity. See the Figure 2 caption for further details.

Figure 5. (a) Stereoview of the possible Tl60Zn2+8 cluster in a sodalite cavity in crystal 2. The Zn2+ ions at Zn(I′) each coordinate to three O(3) framework oxygens; the Zn2+ ions at Zn(II′) each coordinate to three O(2) framework oxygens. In this drawing, each Tl(A) atom of the central octahedral Tl60 cluster coordinates to two Zn2+ ions at site I′ and two Zn2+ ions at site II′ and binds to four other Tl(A) atoms. It is possible that the Zn(I′) ions or the Zn(II′) ions do not participate in this cluster, which would then be Tl60Zn2+4. Each Tl atom also coordinates to two O(3) framework oxygens. (b) The same cluster as in (a) with the zeolite framework omitted for clarity. See the Figure 2 caption for further details.

eight Zn2+ ions. The latter is shown in Figure 5. One such Tl60Zn2+4 or Tl60Zn2+8 cluster can be found in about 100 sodalite units. 5. Discussion In both structures, all framework oxygens at O(2) and O(3) coordinate to Zn+, Zn2+, or Tl+ cations. Most oxygens at O(1) and O(4) do not coordinate to these cations, or do so at distances that are generally longer. Accordingly the T-O distances from Si, Al, and (Si,Al) to O(2) and O(3) are longer than those to

O(1) and O(4). See Tables 2 and 3. This effect is commonly seen. 5.1. Tl+62Zn+30-X‚2Tl0,15Zn0. The reaction of Zn(g) with + Tl ions has proceeded about one-third of the way to completion with the simple redox replacement of some Tl+ ions by Zn+ ions. Some of the Tl0 atoms produced, less than 10%, have been retained within the structure, and some Zn0 atoms have been sorbed without oxidation. 5.1.1. Zn+. There have been no previous reports of the monatomic cation Zn+. Its ionic radius was estimated by Pauling to be 0.88 Å.28

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Figure 6. (a) Stereoview of a sodalite unit containing a tetrahedral silicate ion, SiO44- in crystal 2. Each oxygen of this silicate ion coordinates to one Zn2+ ion at Zn(I′) and one Zn2+ ion at Zn(II′). The oxygens lie exactly in the plane of Si(1), Zn(I′), and Zn(II′). (b) The orthosilicate ion coordinated by eight Zn2+ ions with framework omitted. See the Figure 2 caption for further details.

Figure 7. Stereoview of a supercage in crystal 2. The Tl+ ions at Tl(IIIA) and Tl(IIIB) each coordinate two framework oxygens. The Zn2+ ion and Tl+ ion at Zn(II) and Tl(II) each coordinate to three O(2) framework oxygens. See the Figure 2 caption for further details.

However, the Zn22+ ion has been shown by Raman and other spectroscopic methods to be present in the yellow diamagnetic glass obtained by adding metallic zinc to molten ZnCl2 at 500700 °C.30-32 Zn22+ has not been seen crystallographically. The shortest possible Zn+-Zn+ approach in this structure, Zn(I′)-Zn(II′), is 3.71(3) Å, too long to be a Zn22+ bond. For comparison, the Cd+-Cd+ bond length has been observed crystallographically only twice; it is 2.35(4) Å in zeolite A (Cd(H2O)2+3Cd+3Cd22+1.5-A)33 and 2.576(1) Å in (Cd2)(AlCl4)2.34 A mean Hg+-Hg+ bond length is 2.51 Å.35 It is possible that Zn+ at Zn(II′) is interacting weakly with a + Tl ion at Tl(I′), although there is no structural indication that this is occurring. On the basis of the observed Cd+-Cd+ bond

lengths above, the estimated radius of Cd+, 1.14 Å,28 and the radii of Zn+ and Tl+, 0.8828 and 1.47 Å,9 respectively, a Zn+Tl+ two-electron bond length should be between 2.42 and 2.646 Å (0.88 + 1.47 + [(2.35 to 2.576) - 2(1.14)], more or less). However, Tl+ does not have an unpaired electron to contribute so only a one-electron bond is possible and a reasonable bond length might be somewhere in the 2.8-3.3 Å range. The shortest possible Zn+-Tl+ distances in this structure are 2.163(24) Å for Zn(II′)-Tl(II), 3.361(19) Å for Zn(II′)-Tl(I′), and 4.32(5) Å for Zn(I′)-Tl(I′), respectively. The first of these distances is far too short and can be avoided in the placement of these ions in their sparsely occupied equipoints; the last is far too long. Below zinc in the periodic table is cadmium. Monatomic

Incomplete Reaction of Zn Vapor with Tl+-Exchanged Zeolite X three-coordinate Cd+ ions, like those of Zn+ found here, were found in the crystal structure of a cadmium sorption complex of Cd(II)-exchanged zeolite A,33 [Cd(H2O)2+]3Cd+3Cd22+1.5-A. The observed Cd+-O2- distance, 2.30(1) Å, is somewhat shorter than the sum of their corresponding ionic radii,12 1.14 + 1.32 ) 2.46 Å. The Zn+-O2- distances observed here, 2.10(3) and 2.17(3) Å, are also shorter than (but closer to) the sum of their corresponding ionic radii,12 0.88 + 1.32 ) 2.20 Å. In all three cases, the low coordination number, three, explains the shortness. Two reasons for the stabilization of Zn+ by zeolite X can be offered. First of all, 1+ cations balance the local anionic charge of the zeolite framework more evenly than 2+ cations can. The zeolite framework, with its 92- charge and 384 oxygens per unit cell, may be viewed as having a formal charge at each oxygen of 92-/384 ) 0.24-. Because cations in six-rings approach only three oxygens closely, those with a 1+ charge give a net local residual charge of (1+) + 3(0.24-) ) 0.28+; for Zn2+, this value would be 1.28+, further from neutral. Second, Zn+ fits into six-rings better than Zn2+. Na+ ions, which are present when zeolite X is synthesized, are the right size to give a relaxed zeolite. Ions much larger do not fit into six-rings and ions much smaller cause ring strain, so the zeolite should favor an oxidation state that fits. The estimated Zn+ radius, 0.88 Å,28 is closer than that of Zn2+, 0.74 Å,12 to the Na+ radius, 0.97 Å.12 The other available cation, Tl+, is too large to lie near the planes of six-rings; Zn+ can do that best. Only the first of these two arguments can be used to justify the formation of Cd+. Other unusual oxidation states of transition metals have been reported in zeolites. Ni+ ions have been prepared in zeolites A, X, Y, mordenite, and synthetic clinoptilolite.36-42 Pd+, Pd3+ ions have been reported in zeolite Y.43-45 Cr+ and Cr2+ have been seen in zeolites Y and mordenite.46 Among the first-row transition elements, the 1+ oxidation state is better known for cobalt than for any other element except copper.47 5.1.2. The Diatomic DipositiVe Group IIB Cations. Hg22+ is a stable cation commonly found in solids, melts, and in solution.31,32,35,48 In contrast, the Cd22+ ion has been seen crystallographically only twice in the solid state33,34 and in molten Cd(AlCl4)2 with added Cd.34 Zn22+ has been found only in the glass.30-32 Recent theoretical calculations, however, indicate that all M2X2 molecules (M ) Zn, Cd, Hg; X ) F, Cl, Br, I) should be stable in the gas phase49 and that Zn2X2 and Cd2X2 should be more stable than Hg2X2.50 5.1.3. The Possible Tl+4Zn012Tl04 Cluster. Atoms, not ions, are found at Zn(A) and Tl(A) in the supercages. These positions are neither conventional cation sites, nor close to zeolite framework oxygens as one would expect cations to be. The Zn(A) position is close only to Tl+ ions at site II at 2.75(6) Å, which matches the sum of the radii of a Zn atom and a Tl+ ion, 1.33 Å29 + 1.47 Å12 ) 2.80 Å, and the Tl(A) position, near the center of the supercage, is close only to three other Tl(A) positions and to six Zn0 positions at Zn(A). Each Tl(A) is 2.94(5) Å from six Zn(A) positions. This distance agrees relatively well with the sum of the corresponding radii of Zn and Tl atoms, 1.33 Å29 + 1.74 Å51 ) 3.07 Å. These positions are sparsely filled. These distances may largely be avoided by distributing the atoms widely within their equipoints. Still some physical support must be provided for the Tl(A) atoms near the supercage centers. Attractive highly symmetric clusters may exist which would be consistent with the experimental occupancies and which would utilize these entirely reasonable experimental bond distances. Four Tl+ ions

J. Phys. Chem. B, Vol. 104, No. 3, 2000 523 in the supercage could capture 12 Zn atoms and four Tl atoms to form a Tl+4Zn012Tl04 cluster (Figure 4). The Tl-Tl distances in this cluster are 3.35(6) Å, nearly the same as the Tl-Tl metal bond length,51 1.74 +1.74 ) 3.48 Å. Each Tl atom binds tightly to six Zn atoms and to three Tl atoms. In addition to its coordination to Tl+, each Zn atom bonds to three other Zn atoms, two at 2.73(11) Å and one at 3.10(14) Å. These distances can be compared with the Zn-Zn metal bond length,29 2.66 Å. The Tl+4Zn012Tl04 cluster can be seen in Figure 4b to be cubic closest packed, except for the distortions resulting from the difference in the size of Tl and Zn atoms. Because the refined occupancies at Zn(A) and Tl(A), 12.3 and 2.2 per unit cell, respectively, are not in the ratio required for Zn012Tl04 (12:4 ) 3:1), other Tl+ ions appear to have captured only Zn atoms to form smaller clusters, perhaps just Tl+Zn0. 5.1.4. Total Reaction. The total reaction of Zn vapor with Tl92-X, per unit cell, is

Tl92Si100Al92O384 + 45Zn(g) f Tl62Zn30Si100Al92O384‚2Tl0,15Zn0 + 28Tl(g) Only a small fraction of the Tl formed (two of the 30 atoms per unit cell) has remained in the zeolite. The remainder vaporized away (vapor pressure of Tl at 450 °C is 4 × 10-5 Torr15). A much higher occupancy of Tl+4Zn012Tl04 clusters should form if this synthesis were done in a closed system; full occupancy would then be possible. These clusters may be responsible for the black lustrous appearance of crystal 1. 5.2. Tl+18Zn2+46Si98Al94O384‚4SiO44-. The reaction of Zn(g) with Tl+ ions has proceeded about 80% of the way to completion. Zn is entirely in the 2+ oxidation state. Less than 1% of the Tl0 atoms produced have been retained within the structure. The zeolite framework has lost TO4n- ions to positions at sodalite-cavity centers; T appears to be Si, so n ) 4. 5.2.1. Formation of Zn2+ Ions. In all, 18 Tl+ ions and 46 Zn2+ ions were found per unit cell. If these ions are in their conventional oxidation states, the sum of the charges, 18 × (1+) + 46 × (2+) ) 110+, nicely balances the anionic charges of the zeolite framework and the orthosilicate ions, 94- (see next section) + 4 × (4-) ) 110-. The Zn+ ions formed in crystal 1 do not remain as the reaction of Tl92-X with Zn(g) proceeds toward completion. 5.2.2. Desilication (Less Likely Dealumination) of the Zeolite Framework. Half of the sodalite units in this structure contain a distorted tetrahedral TO4n- ion at their centers (T ) Si with n ) 4 or Al with n ) 5). Each coordinates to eight Zn2+ ions. The observed T-O distance, 1.65(5) Å, suggests that T ) Si. The sum of the ionic radii52 of four-coordinate Si4+ and threecoordinate O2- is 0.26 + 1.36 ) 1.62 Å. In Zn-X,9 also prepared by anhydrous redox ion exchange (of Cd46-X), the T-O distance, 1.60(5) Å, also suggests that T is at least largely Si. The anhydrous reaction of Zn(g) with zeolite X containing reducible cations appears to lead to desilication of the zeolite framework. In contrast, the sum of the ionic radii52 of four-coordinate 3+ Al and three-coordinate O2- is 0.39 + 1.36 ) 1.75 Å. Tetrahedral orthoaluminate ions, observed in the structure of dehydrated Zn2+-exchanged zeolite X2, had Al-O distances of 1.79(6) Å. That these were orthoaluminate and not orthosilicate ions has been verified by solid-state NMR.6 In Ni-X, also prepared by aqueous ion exchange, the T-O distance, 1.75(3) Å, suggests that T is Al.53 The dehydration of zeolite X

524 J. Phys. Chem. B, Vol. 104, No. 3, 2000 containing hydrolyzing cations appears to lead to dealumination of the zeolite framework. The generation of four extraframework Si4+ ions per unit cell without noticeable crystal damage suggests that the zeolite has reconstructed itself to give a composition richer in Al. This was seen in the structure of dehydrated Zn2+-exchanged zeolite X.2 As two SiO44- groups are lost from the framework per unit cell, other unit cells (composition Si96Al96O38496- assumed here for simplicity) “dissolve” to replenish the framework of each unit cell with two AlO45- groups and to give two more SiO44groups to sodalite-unit positions. The total reaction is approximately

1.02Tl92Si100Al92O384 + 46Zn(g) f Tl18Zn46Si98Al94O384‚4SiO44- + 76Tl(g)

Zhen et al. crystal 2, seems to have lost its long-range Si/Al ordering. A possible initial temperature jump to ca. 70 °C (experimental error, steaming) during the initial step of dehydration might have caused this. Otherwise the crystal may have been a minority member of the initial synthesis batch with symmetry Fd3hm, or the effect could conceivably be due to the uneven distortions that mixed ions at both the D6R and S6R sites give to their respective six-rings.56 Crystal 2 retained its Fd3h space group and therefore its longrange Si/Al ordering. Desilication itself acts to preserve Fd3h by moving the framework Si/Al ratio closer to 1:1; a 1:1 crystal must have space group Fd3h by Loewenstein’s rule.17 If additional Zn0 atoms reacted with the zeolite framework to extract more silicate, perhaps as Zn2SiO4 (unlocated) which might have left the zeolite, a 1:1 ratio might have been achieved. 6. Summary

5.2.3. The Possible Tl60Zn2+4 or 8 Cluster. Almost all of the product Tl(g) atoms have left the zeolite (vapor pressure of Tl at 500 °C ) 3 × 10-4 Torr).12 Only about 0.5 Tl atoms per unit cell were trapped by the Zn2+ ions at sites I′ and/or II′, possibly to form small concentrations of Tl60Zn2+4 or Tl60Zn2+8 clusters. In these clusters, four or eight Zn2+ ions lie on the 3-fold axes of the octahedral Tl6 molecule (see Figure 5). The Tl(A)-Tl(A) distances, 3.11(9) Å, appear to be shorter than the Tl-Tl metal bond length,51 1.74 + 1.74 ) 3.48 Å, because of the greater bond orders in a small cluster. The positive charge induced on the Tl6 cluster by coordination to Zn2+ ions explains the short distance, 2.41(4) Å, between Tl(A) and the O(3) framework oxygens. The Tl(A)-Zn(I′) and Tl(A)-Zn(II′) distances can be largely avoided by arranging all the Zn2+ ions (23 at site I′ and 19 at site II′) into seven of the eight sodalite units. However, because Tl(A) is at a special position, there should be a cluster that requires this. This cluster may be responsible for the lustrous black color of crystal 2. A geometrically equivalent cluster was seen in the sodalite unit of fully Ag+-exchanged zeolite A,54 where a Ag60 cluster is stabilized by coordination to eight Ag+ ions. 5.2.4. Residual Tl+ Ions at Sites I′ and II. Even though less than one Tl+ ion per unit cell was found at each of the sites I′ and II (see Table 1b), including these two positions in leastsquares refinement contributed significantly to the error indexes: R1/wR2 decreased from 0.109/0.284 to 0.104/0.274. In addition, their positional coordinates are very similar to those in crystal 1 and Tl92-X. They appear to be surviving residuals of the redox reaction. 5.3. Tl+ Ions at III′ Sites. The Tl+ ions at all III′ positions have very large thermal parameters (Table 1) as they did in Tl92-X.7 In Tl92-X, Tl+ ions are found at two different III′ sites, one close to site III near two O(1) and two O(4) oxygens, and the other near one O(4) and two O(1) oxygens. The latter Tl+ position could be resolved in crystal 1 (section 3.1.) to give two site-III′ positions, Tl(IIIB) and Tl(IIIC) (Figure 3). However, this was not possible for the corresponding Tl(IIIB) position in crystal 2. These large thermal parameters indicate crystallographically the absence of sharp minima at the III′ sites for the Tl+ ions. This is discussed in detail in work recently completed here.55 5.4. Space Group Results. It is unusual and noteworthy that two crystals from the same synthesis batch that have been subjected to similar chemical treatment should have different space groups. Surprisingly, crystal 1, which was treated somewhat less harshly (a lower pressure of Zn(g) at a lower temperature) than

A single crystal of fully dehydrated Tl92-X was allowed to react with flowing Zn(g) for 2 days at 450 °C. Another crystal was similarly treated at 500 °C. In neither case were all Tl+ ions reduced, but the extent of the reaction was much greater at 500 °C. At 450 °C, only some Tl+ ions originally at sites I′ and II were replaced by Zn+; the 28 Tl+ ions at III′ positions appeared unaffected. After treatment at 500 °C, almost no Tl+ ions were found at sites I′ and II; they have been replaced by Zn2+. Only 16 Tl+ ions remained at the III′ positions. Desilication (less likely dealumination) of the zeolite framework was observed. At 450 °C, some leaving Tl0 atoms and some Zn(g) atoms have been retained to form, with Tl+ ions, cationic clusters. At 500 °C, some leaving Tl0 atoms have been retained to form, with Zn2+ ions, cationic clusters. Supporting Information Available: Tables of observed and calculated structure factors squared with esd’s. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kim, Y.; Lee, S. H.; Seff, K. Unpublished work. (2) Bae, D.; Zhen, S.; Seff, K. J. Phys. Chem. B 1999, 103, 5631. (3) Bae, D.; Seff, K. Microporous Mesoporous Mater., in press. (4) Macs, A.; Cremers, A. J. Chem. Soc., Faraday Trans. 1 1975, 71, 265. (5) Wilkinson, A. P.; Cheetham, A. K.; Tang, S. C.; Peppart, W. J. J. Chem. Soc. Chem. Commun. 1992, 1485. (6) Ciraolo, M. F.; Norby, P.; Hanson, J. C.; Corbin, D. R.; Grey, C. P. J. Phys. Chem. B 1999, 103, 346. (7) Kim, Y.; Han, Y. W.; Seff, K. Zeolites 1997, 18, 325. (8) Kwon, J. H.; Jang, S. B.; Kim, Y.; Seff, K. J. Phys. Chem. 1996, 100, 13720. (9) Zhen, S. Ph.D. Thesis, University of Hawaii, Honolulu, HI, 1999. (10) Zhen, S.; Seff, K. J. Phys. Chem. B 1999, 103, 10409. (11) Zhen, S.; Seff, K. J. Phys. Chem. B 1999, 103, 6493. (12) Handbook of Chemistry and Physics, 72th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1991/1992; Section 12, p 8. (13) Bogomolov, V. N.; Petranovskii, V. P. Zeolites 1986, 6, 418. (14) Handbook of Chemistry and Physics, 76th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1995/1996; Section 6, p 111. (15) Selected Values of Thermodynamic Properties of Metals and Alloys; Hultgren, R. R., et al., Eds.; Wiley: New York, 1963; p 293. (16) Bae, D.; Seff, K. J. Phys. Chem. B, submitted for publication. (17) Loewenstein, W. Am. Mineral. 1954, 39, 92. (18) Peterson, B. K. J. Phys. Chem. B 1999, 103, 3145. (19) Park, H. S.; Seff, K. J. Phys. Chem. B, in press. (20) Sheldrick, G. M. SHELXL97, Programs for Crystal Structure Analysis (Release 97-2); Institu¨t fu¨r Anorganische Chemie der Universita¨t: Tammanstrasse 4, D-3400 Go¨ttingen, Germany, 1998. (21) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV, p 72. (22) Cromer, D. T. Acta Crystallogr. 1965, 18, 17. (23) Reference 21, p 149. (24) Olson, D. H. Zeolites 1995, 15, 439.

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