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COMMENTS Comment on “Synthesis of Fully Dehydrated Fully Zn2+-Exchanged Zeolite Y and Its Crystal Structure Determined by Pulsed-Neutron Diffraction”. Cationic Zinc Clusters Formally Containing Zn(I) in the Sodalite Cavities of Zeolite Y (FAU) Karl Seff* Department of Chemistry, UniVersity of Hawaii, Honolulu, Hawaii 96822 ReceiVed: January 12, 2005 Although the crystallographic work in the above report1 on Zn-Y (FAU) was done correctly, the results were interpretted incorrectly. Rather than being a structure in which all of the zinc ions are monionic Zn2+, the structure has a high Zn+ content and contains clusters of Zn+, or mixed clusters of Zn2+ and Zn+, or both, all in its sodalite cavities. This error occurred because the pulsed-neutron powder diffraction data used could not support the independent refinement of the occupancy and thermal parameters of the zinc ions. Accordingly some assumption was needed to complete the structure. Because Zn+ was unknown in the solid state in 1992, the assumption that the zinc cations were all Zn2+ seemed safe. Since then, however, monatomic Zn+ has been found in two zeolite materials.2,3 Both were prepared by treating a zeolite sample with Zn(g), as we had done in our 1992 report.1 In the first of these two reports,2 we treated fully dehydrated, fully Tl+-exchanged zeolite X (FAU) with Zn(g), and, using singlecrystal crystallography, we reported the presence of Zn+. Its radius, 0.88 Å, was the same as the value that Linus Pauling had calculated4 for Zn+. Because it agreed with the values that we had reported in 1992 for Zn-Y, 0.86 and 0.91 Å,1 it appeared that the zinc cations in Zn-Y were also, at least predominantly, Zn+. For comparison, the conventional ionic radius of Zn2+ is 0.74 Å.5 In 2003, Zn+ was reported for the second time, this time by esr, in the chabazite cages of SAPOCHA, a silicoaluminophosphate zeolite whose H+-form had been treated with Zn(g).3 For comparison, to prepare Zn-Y in our 1992 report,1 we had treated H-Y with Zn(g). Because of these two reports,2,3 we analyzed our original Zn-Y sample1 by electron-microprobe methods using a CAMECA SX50 instrument.6 This analysis accurately confirmed the Si/Al ratio in Na55Si137Al55O384, the initial zeolite composition exclusive of water molecules. It also showed that 43(1) zinc ions were present per unit cell, substantially more than the 27.5 Zn2+ ions that would be needed to balance the 55- anionic charge of the zeolite framework. This could have occurred by a reaction whose simplest form would be Zn2+ + Zn(g) f 2Zn+ (following 2H+ + Zn(g) f H2(g) + Zn2+). Using the value of 43 zinc ions per unit cell, and the need for a total charge of 55+, it may be concluded formally that there are 31 Zn+ and 12 Zn2+ cations per unit cell; some charge delocalization can * E-mail:
[email protected] be expected. The net reaction per unit cell was therefore
H55 - Y + 43 Zn(g) f Zn+31 Zn2+12 - Y + 27.5 H2(g) Dividing the number of zinc ions per unit cell, 43, by the number of sodalite cavities per unit cell, 8, yields an average value of 5.4 zinc cations per sodalite cavity. Because 5.4 is greater than 4.0, there must be some 3.14(2)-Å1 (a mean value) Zn-Zn contacts. For the same reason, some sodalite cavities must have six or more zinc cations, and, because there is no alternative within the sodalite cavity, these six or more must be bound together with 3.14-Å bonds. For comparison, the (shorter) bond length in zinc metal is 2.66 Å. The sample was also analyzed by wavelength dispersive X-ray fluorescence spectroscopy.7 This was inappropriate because it involved heating the sample to 1350 °C, at which temperature any residual Zn+ could have disproportionated and Zn(g) could have been lost. This analysis, done without standards, yielded a unit-cell composition of Zn36Si140Al52O384, exclusive of the water, and perhaps oxygen, that had entered the sample when it had been exposed to the atmosphere. Even this analysis, however, because 36 > 27.5, shows a reliable excess of zinc, supportive of a significant Zn+ content in the original sample. This reinterpretation of the structure resolves a number of awkward aspects of that initial report.1 Then we could not explain why the “Zn2+” ions were as far from their coordinating oxygen atoms (and as close to each other) as they were; if they were predominantly Zn+ ions instead, these distances would be correct (and reasonable, respectively). The following additional serious concerns were also satisfied: (1) Why were all of the “Zn2+” ions in the sodalite cavities? They should occupy more conventional sites in the double 6-rings (site I) and in the supercages (site II), as had many other dipositive cations, e.g., Cd2+,8 Mn2+,9 Ca2+,10 Mg2+,10 and Ba2+.10 They would then be much further apart, minimizing repulsions between cations. (2) Why were there two kinds of “Zn2+” cations in the sodalite cavities? All 27.5 cations could partially occupy whichever 32fold position (Zn(1) or Zn(2)) had the lower energy. (3) Why did the sodalite-cavity arrangement with the shorter “Zn2+” to “Zn2+” distance have the higher occupancy? Intercationic repulsion should have given the opposite result. (4) Why were the thermal parameters at the “Zn2+” positions anomalously low? Crystallographically, this suggests a greater occupancy and therefore the presence of Zn+ by the uptake of additional zinc atoms. The sodalite cavity, by providing multiple anionic coordination contacts, can stabilize cationic clusters. Many such clusters have been found there crystallographically, including Na43+,11 Na54+,12 S44+,13 and In57+;14 a longer listing is available.13 Similarly, this work shows sodalite cavities hosting reduced (compared to Zn2+) cationic zinc clusters. Their mean formula is Zn5.3756.875+. This explains why all of the Zn cations in the Zn-Y structure have condensed into its sodalite cavities with relatively short Zn-Zn contacts. Reasonable placements of these cations among the sodalite units, and hence the formulas of the cationic zinc clusters, may be proposed.15
10.1021/jp0580165 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/28/2005
Comments Note Added in Proof. Zn(I) has been found as Zn22+ in decamethyldizincocene.16,17 References and Notes (1) Peapples-Montgomery, P. B.; Seff, K. J. Phys. Chem. 1992, 92, 5962-5965. (2) Zhen, S.; Bae, D.; Seff, K. J. Phys. Chem. B 2000, 104, 515525. (3) Tian, Y.; Li, G.-D.; Chen, J.-S. J. Am. Chem. Soc. 2003, 125, 66226623. (4) Pauling, L. The Nature of Chemical Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960; p 514. (5) Handbook of Chemistry and Physics, 70th ed.; CRC Press: Cleveland, OH, 1989/90; p F-187. (6) Mr. Kent Ross performed this analysis at this university. (7) Ms. Heidi O ¨ sterholm of Fortum Oil and Gas (Finland) performed this analysis.
J. Phys. Chem. B, Vol. 109, No. 28, 2005 13841 (8) Kwon, J. H.; Jang, S. B.; Kim, Y.; Seff, K. J. Phys. Chem. 1996, 100, 13720-13724. (9) Jang, S. B.; Jeong, M. S.; Kim, Y.; Seff, K. J. Phys. Chem. B 1997, 101, 9041-9045. (10) Yeom, Y. H.; Jang, S. B.; Kim, Y.; Song, S. H.; Seff, K. J. Phys. Chem. B 1997, 101, 6914-6920. (11) Armstrong, A. R.; Anderson, P. A.; Woodall, L. J.; Edwards, P. P. J. Am. Chem. Soc. 1995, 117, 9087-9088. (12) Kim, Y.; Han, Y. W.; Seff, K. J. Phys. Chem. 1993, 97, 1266312664. (13) Song, M. K.; Kim, Y.; Seff, K. J. Phys. Chem. B 2003, 107, 31173123. (14) Heo, N. H.; Park, J. S.; Kim, Y. J.; Lim, W. T.; Jung, S. W.; Seff, K. J. Phys. Chem. B 2003, 107, 1120-1128. (15) Seff, K. Micropor. Mesopor. Mater. 2005, in press. (16) Resa, I.; Carmona, E.; Gutierrez-Puebla, E.; Monge, A. Science 2004, 305, 1136-1138. (17) Resa, I.; Carmona, E.; Gutierrez-Puebla, E.; Monge, A. Science 2004, 306, 411.
