Photon Factory Activity Report 2018 #36 (2019)

BL-5A/2018G551

Disproportionation of Elemental Sulfur in Zeolite. Crystal Structure of Zeolite LTA
Containing Two New Ions: Perthiosulfite, S42-, and Trisulfur Cation, S32+
Hyeon Seung Lim1, Joon Young Kim1, Gi Beom Park1, So Yeon Yoon1, Nam Ho Heo1,*,and Karl Seff2
1
Department of Applied Chemistry, School of Applied Chemical Engineering, Kyungpook National
University, Daegu, 41566, Korea
2
Department of Chemistry, University of Hawaii, 2545 The Mall, Honolulu, Hawaii, 96822, U. S. A.
1 Introduction
Elements have been observed to disproportionate
electronically into cations and anions upon sorption intozeolites.[1,2] Because anhydrous polar zeolites are polar
media, this is akin to the dissociation of salts when theydissolve in polar solvents. Perhaps NO, an odd molecule,disproportionates into NO+ and NO- upon sorption into
polar zeolites.[3] Zeolites A (LTA) and X (FAU) are
examples of polar zeolites; they have very anionicframeworks due to their high aluminum contents and, tobalance that negative charge, a high density of
exchangeable cations. Here we report a third example ofthe disproportionation of an element upon sorption into azeolite, sulfur sorbed at a high temperature into
anhydrous zeolite A.
This work was undertaken simply to find the twelfth
Na+ ion in a sulfur sorption complex of Na 12-A;
Na12Si12Al12O48 is its unit cell formula. Its structure,reported in 1972, was found to contain two S8 rings perunit cell, but only 11 Na+ ions could be found.[4] Now,
with the availability of higher quality diffraction data, wewished to find that twelfth ion. We anticipated that it
would be at the center of the large cavity, sandwiched
between two parallel (non-planar) S8 rings.
2 Experiment
A tube of sulfur flakes was attached as a side arm to a
Pyrex reaction vessel. Another arm to this vessel ended
with the capillary containing a hydrated Na-A single
crystal. The complete vessel was then attached to a
vacuum line for dehydration. After the crystal wasdehydrated under vacuum at 623 K, the temperature of
the sulfur-containing side arm was increased to 623 K andthe sulfur was distilled in situ under dynamic vacuum.
The crystal at 623 K was then exposed to 20 Pa of S(g)for 48 hours. It was then maintained at 333 K to remove
any excess S from near or inside the crystal.
Synchrotron X-ray diffraction intensities for the single
crystal were measured at the Photon Factory (PF),Tsukuba, Japan. The space group Pm 3 m (a = 12.192(1)Å ), appropriate for zeolite A unless high precision isachievable, was used. Its crystal structure was refinedwith all unique data to the final error index R1 = 0.075 forthe 679 unique reflections for which Fo > 4σ(Fo).

3 Results and Discussion
Elemental sulfur disproportionated electronically upon
sorption into anhydrous Na12-A at 623 K. The net reaction,and also the net reaction per 12.2- Å unit cell, is:
7S → S42- + S32+.
The perthiosulfite anion, S42-, the perthio form of SO32-,
is in the large cavity. It bonds to one 6-ring and three 8ring Na+ ions (S-Na = 2.83(6) Å , 2.80(5) Å , and 2.43(5)Å ). It appears to be the first example of a branchedpolysulfide ion. Its geometry, S-S = 2.16(5) Å and S-S-S= 105.4(22)˚, is in agreement with that calculated, 2.08 Åand 108.2˚(Figure 1).[5] These calculations also indicatethat the perthiosulfite ion is only about 10 kJ/mol lessstable than the n-tetrasulfide ion,[5] suggesting that it
could readily form in an environment that a zeolite mightprovide.
The electron deficient trisulfur cation, S32+ (S-S =2.11(8) Å and S-S-S = 102.0(14)˚), bonds to framework
oxygen atoms in the sodalite cavity (S-O = 2.64(7) Å )(Figure 1).
Twelve Na+ ions are found per unit cell at five
crystallographic sites similar to, but different from, thosein anhydrous Na-A. Eleven are near 6- and 8-rings, andthe twelfth lies opposite a 4-ring in the sodalite cavity.
A mixture of sulfur species is present in the vapor
phase at 623 K; S2 and S3 should easily be able to enterthe zeolite structure. The reaction proceeded until thesodalite cavities (one per unit cell) became full with S32+.

Fig. 1: The Perthiosulfite anion, S42+, and the trisulfur
cation, S32+, in their coordination environments.
Thetwoprevious
observationsof
thedisproportionation of an element in a zeolite involved 3coordinate Cd2+ ions.[1,2] These ions satisfied theirPhoton Factory Activity Report 2018 #36 (2019)

coordinative unsaturation by making strong bonds tosulfide or iodide ions, and this was viewed as theprinciple driving force for those reactions. In this work, azeolite in its Na+ form is seen to be capable ofdisproportionating a substrate.
Thus electronic
disproportionation appears to be a general property ofpolar zeolites. It may be an important step in someintrazeolitic reaction mechanisms.
Acknowledgement
We gratefully acknowledge the Photon Factory, HighEnergy Accelerator Research Organization, KEK,Tsukuba, Japan for the use of their synchrotron facilities
including diffractometer and computing systems.
References[1] M. K. Song, Y. Kim; K. Seff, J. Phys. Chem. B 107,3117, (2003).
[2] M. K. Song, E. Y. Choi, Y. Kim; K. Seff, J. Phys.
Chem. B 107, 10709, (2003).
[3] C.-C. Chao, J. H. Lunsford, J. Am. Chem. Soc, 93,6794, (1971).
[4] K. Seff, J. Phys. Chem. 76, 2601, (1972).
[5] R. Steudel, Y. Steudel, Chem. Eur. J. 107, 10709,(2003).
* nhheo@knu.ac.kr