rate of tl+ exchange into single crystals of zeolite x
TRANSCRIPT
Rate of Tl� exchange into single crystals of zeolite X
Lin Zhu, Karl Se� *
Department of Chemistry, University of Hawaii at Manoa, 2545 The Mall, Honolulu, HI 96822-2275, USA
Received 16 September 1999; received in revised form 1 January 2000; accepted 16 March 2000
Abstract
A hydrated single crystal of zeolite X with unit-cell composition Na92Si100Al92O384, exclusive of water molecules, 0.14
mm in cross-section, was ion exchanged at 23°C with ¯owing 0.1 M aqueous TlNO3 for 90 s. Its structure, determined
crystallographically, showed that 74(4) of the 92 Na� ions per unit cell had been replaced by Tl�. A second crystal was
treated similarly for 900 s; this time 83(1) Tl� ions were found. Kim et al. had found 91(1) Tl� ions after
7:0 days � 6:0 � 105 s [Y. Kim, Y.W. Han, K. Se�, Zeolites 18 (1997) 325]. These results can be ®t relatively well to a
simple fourth-order rate law with k � 6:25 � 10ÿ7 (No. of Na� per unit cell)ÿ3 sÿ1. For crystals of such size, which are
important for X-ray crystallography, days may not be enough time to reach an ion-exchange end point by these
methods for some ions. Ó 2000 Elsevier Science B.V. All rights reserved.
Keywords: Rate; Ion exchange; Thallium; Zeolite X; Sodium
1. Introduction
Single-crystal crystallography has given themost detailed and reliable structural informationavailable for zeolites. To prepare such crystals forstudy, aqueous ion exchange is often done underconditions that encourage complete exchange, suchas ¯owing exchange solution, long exchange times,and elevated temperatures. Ion-exchange experi-ments on single crystals of zeolite X are routinelycarried out in this laboratory and others by ¯owmethods for one to two days, sometimes longer, toensure complete exchange. A single crystal islodged in a ®ne Pyrex capillary, and an ion-exchange solution is allowed to ¯ow past it, e�ec-
tively refreshing the solution in contact with thecrystal about a hundred times per second.
This work was done to examine the adequacy ofthese procedures for large single crystals. The ex-tent of exchange of Tl� into zeolite X is measuredas a function of time by determining the completecrystal structures of three fully dehydrated singlecrystals.
Tl� was chosen to replace the Na� ions thatwere initially in the zeolite because it was believedthat complete and stoichiometric Tl� exchangecould be accomplished in zeolite X without com-plication, and because the very high X-ray scat-tering factor of Tl� would allow the number of Tl�
ions per unit cell to be counted reliably after eachstructure was determined crystallographically. Inaddition, the large di�erence between the ionicradius of Tl� (1.47 �A) and Na� (0.97 �A) [1], andthat between their atomic scattering factors (80 e
Microporous and Mesoporous Materials 39 (2000) 187±193
www.elsevier.nl/locate/micromeso
* Corresponding author. Fax: +1-808-956-5908.
E-mail address: kse�@gold.chem.hawaii.edu (K. Se�).
1387-1811/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S13 8 7-1 8 11 (0 0 )0 01 9 5- 5
for Tl� and 10 e for Na�), would allow them to beeasily distinguished.
The kinetics of ion exchange in zeolites has beenstudied extensively [2±5], and some progress hasbeen made with this complex problem. The rate-controlling step of ion exchange involves theso-called particle di�usion process: the di�usionof cations, water molecules, or cation±watercomplexes through the zeolite framework [6]. Akinetics analysis of exchange rates can be con-structed via appropriate solutions to Fickian dif-fusion laws. An activation energy can be calculatedfor the di�usion process which represents the en-ergy barrier to ion movement, and it can be usedto suggest which route an ion may take througha zeolite framework, when alternative routes exist.It can also indicate whether the movement is likelyto be that of a hydrated or unhydrated species.Sometimes exchange-rate plots can be related tothe cation sites contributing to the process. Thismodel predicts that the rate of ion exchange isinversely proportional to the square of the crystalradius [7].
Sherry [8] studied univalent ion (Li�, K�, Rb�,Cs�, Ag�, and Tl�) exchange in microcrystallinesamples of zeolites X and Y at 25°C. He foundthat all Na� ions in zeolite X with unit-cell com-position Na85Si107Al85O384 were replaced with Tl�
ions within 24 h. The high selectivity of zeolite Xfor Tl� over Na�, seen throughout the composi-tion range, was attributed to the high polarizabil-ity of Tl�.
Zeolite X, an aluminum-rich synthetic analog ofthe naturally occurring mineral faujasite (Fig. 1), isan ideal material for studying ion exchange in ze-olites because of its relatively open framework.Exchangeable cations, which balance the negativecharge of the aluminosilicate framework, are usu-ally found at the following sites shown in Fig. 1:site I at the center of the D6R, site I0 in the sodalitecavity on the opposite side of either of the D6RÕssix rings from site I, site II0 inside the sodalite cavitynear a S6R, site II at the center of the S6R or dis-placed from this point into a supercage, site III inthe supercage on a 2-fold axis opposite a four ringbetween two 12 rings, and various III0 sites some-what or substantially distant from III but otherwisenear the inner walls of the supercages or the edges
of 12 rings. Exchangeable cations select siteswithin the zeolite that best balance its charge withconventional cation-to-oxygen bond lengths. Inaddition, cations with speci®c coordination re-quirements will seek to satisfy them.
2. Experimental section
2.1. Sample preparation
Large single crystals of sodium zeolite X ofstoichiometry Na92Si100Al92O384 per unit cell wereprepared in Russia [9]. Two of these, colorlessoctahedra about 0.14 mm in cross-section, werelodged in ®ne Pyrex capillaries. Crystal 1 was ionexchanged with ¯owing aqueous 0.1 M TlNO3 for90 s (for experimental details, see Table 1). Crystal
Fig. 1. A stylized drawing of the framework structure of zeolite
X. The cuboctahedron (8� 6 � 14 hedron with 8� 3 � 24
vertices) known as the sodalite cavity or b cage may be viewed
as the principal building block of zeolite X. These sodalite units
are connected tetrahedrally at six rings by bridging oxygens to
give double six rings (D6Rs, hexagonal prisms) and, concomi-
tantly, an interconnected set of even larger cavities (supercages)
accessible in three dimensions through 12-ring (24-membered)
windows. The Si and Al atoms occupy the vertices of these
polyhedra. The oxygen atoms lie approximately midway be-
tween 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 and supercages,
and may be viewed as the entrances to the sodalite units. Each
unit cell has eight sodalite units, eight supercages, 16 D6Rs, 16
12 rings, and 32 S6Rs. Extra-framework cation sites are labeled
with Roman numerals.
188 L. Zhu, K. Se� / Microporous and Mesoporous Materials 39 (2000) 187±193
2 was similarly ion-exchanged for 900 s at the same¯ow rate. The amounts of exchange solution usedfor crystals 1 and 2 contained a 30-fold and a 300-fold excess of Tl� with respect to complete ex-change, respectively.
Both crystals were then dehydrated at 400°Cand 1 � 10ÿ5 Torr for 48 h. While these conditionswere maintained, the hot contiguous downstreamlengths of the vacuum system, including a se-quential U-tube of zeolite 5A beads fully activatedin situ, were allowed to cool to ambient tempera-ture to prevent the movement of water moleculesfrom more distant parts of the vacuum system tothe crystal. Still, under dynamic vacuum in itscapillary, each crystal was then allowed to cool,and was sealed in its capillary and removed from
the vacuum line by torch. Each crystal had re-mained colorless.
2.2. Crystallographic work
The re¯ection conditions (h� k; k � l; l� h �2n; 0k l: k � l � 4n) indicate that the space groupis either Fd�3 or Fd�3m. Fd�3 was chosen because (a)the low Si/Al ratio requires by LoewensteinÕs rule[10], at least in the short range, alternation of Siand Al, (b) these crystals, like most or all crystalsfrom the same batch [11], do not have intensitysymmetry across (1 1 0) and therefore lack thatmirror plane, and (c) the di�raction data fromthese crystals have re®ned successfully to errorindices lower than with Fd�3m, with Si±O distancesreasonably less than Al±O (mean Si±O � 1.60�A and mean Al±O � 1:75 �A for crystal 1; thecorresponding values are 1.64 and 1.71 �A forcrystal 2).
Di�raction data were collected at 23(1)°C withan automated Siemens P3 four-circle computer-controlled di�ractometer equipped with a pulse-height analyzer and a graphite monochromator.The intensities of three re¯ections in diverse re-gions of reciprocal space were recorded after every97 re¯ections to monitor crystal and instrumentstability. Only small random ¯uctuations of thesecheck re¯ections were observed. Absorption cor-rections were not made.
2.3. Structure solution
Full-matrix least-squares re®nements were doneon F 2
o using all data. It was initiated for eachstructure with the atomic positions of the frame-work atoms [Si, Al, O(1), O(2), O(3), and O(4)] ofdehydrated ``Tl92-X'' [12]. (Tl92-X was reported ashaving that ideal composition; the number of Tl�
ions actually found was 90.8(10), insigni®cantlydi�erent from 92.) Fixed weights were used ini-tially; the ®nal weights were assigned using the for-mula w � q=�r2�F 2
o � � �aP �2 � bP � d � e sin�h��where P � �F 2
o � 2F 2c �=3, with a and b as re®ned
parameters. Atomic scattering factors for Si, Al,O, Tl, and Na were used for both structures [13],and corrections for anomalous scattering were
Table 1
Summary of experimental data
Crystal 1 Crystal 2
Crystal cross-section (mm) 0.14 0.14
Ion exchange time (s) 90 900
Ion exchange T (°C) 23 23
Volume of solution used (ml) 0.01 0.10
Mean ¯ow ratea (cm/s) 1.0 1.0
Data collection T (°C) 23 23
Scan technique h±2h h±2hScan rate (degree/min) 6.0 6.0
Radiation (MoKa)
k1 (�A) 0.70930 0.70930
k2 (�A) 0.71359 0.71359
Unit cell constants, a0 (�A) 25.054(7) 25.071(3)
2h range for a0 (deg) 14±19 14±19
No. of re¯ections for a0 20 20
2h range in data collection (deg) 3±50 3±50
No. of re¯ections gathered 3743 3720
No. of unique re¯ections (m) 1178 1178
No. of re¯ections �F0 > 4r�F0�� 121 326
No. of parameters (s) 80 84
Merging R (all re¯ections) 0.46 0.23
R1b 0.074 0.049
wR2c 0.350 0.087
Goodness of ®td 0.54 0.61
a Flow rate� (volume of exchange solution)(time)ÿ1 ((area
of cross-section of capillary) ) (cross-section of crystal))ÿ1.b R1 �
P jFo ÿ jFcjj=P
Fo is calculated using re¯ections with
Fo > 4r�Fo�.c wR2 � f
Pw�F 2
o ÿ F 2c �2=
Pw�F 2
o �2g1=2is calculated using all
re¯ections.d Goodness of fit � fPw�F 2
o ÿ F 2c �2=�mÿ s�g1=2
is calculated
using all re¯ections.
L. Zhu, K. Se� / Microporous and Mesoporous Materials 39 (2000) 187±193 189
Ta
ble
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783(1
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)20.5
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(II)
32
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35
1(2
6)
2351(2
6)
2351(2
6)
24(9
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(7)
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II)
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52
7(2
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III)
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III
12
50
1250
4077(2
3)
79(2
5)
5.6
(23)
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III0
)9
6(g
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I06
87
(16
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5)
4196(1
4)
186(2
2)
18.4
(14)
Tl(
III00 )
96
(g)
III0
10
70(2
5)
1426(2
4)
4106(1
6)
60(2
1)
8.2
(25)
(b
)C
ryst
al
2,
Tl 8
3N
a9S
i 100A
l 92O
384
Si
96
(g)
ÿ52
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)15(4
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)15(3
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)0(3
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)96
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06
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)96
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6(g
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9(7
)27(7
)1440(4
)34(7
)26(7
)16(6
)20(9
)19(9
)12(9
)96
O(3
)9
6(g
)ÿ2
82�5�
743(6
)718(6
)39(1
0)
24(9
)36(1
0)
7(8
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6(g
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738(7
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00
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52
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)29(5
)29(5
)2(1
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)25.8
(2)
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)9
6(g
)II
I06
91
(10
)699(1
0)
4192(5
)348(3
5)
350(3
4)
42(8
)126(2
5)
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4�
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(6)
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III00 )
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(g)
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11
10(4
)1353(7
)4073(3
)63(1
1)
149(1
5)
19(4
)ÿ8�9�
2(4
)30(6
)16.2
(4)
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190 L. Zhu, K. Se� / Microporous and Mesoporous Materials 39 (2000) 187±193
made [14]. Final values and additional details aregiven in Table 1. The ®nal structural parametersare presented in Table 2.
3. Cation placements
The cation placements in crystals 1, 2, and``Tl92-X'' are given in Table 3.
3.1. Crystal 1, Tl74Na18Si100Al92O384
About 74 Tl� ions are located per unit cell at®ve crystallographic sites. Sites I0 and II are oc-cupied by 20.5(5) and 19.7(7) Tl� ions, respec-tively. Their coordinates are essentially the same asthose seen in the structure of ``Tl92-X'' [12]. Twosite-III0 positions, similar to the ones found in``Tl92-X'', are occupied by 18.4(14) and 8.2(25) Tl�
ions, respectively. The remaining 5.6(23) Tl� ionsare found at a site-III position. Na� ions occupytwo positions in this structure: 6.0(2) at site I and11.8(7) at site II.
3.2. Crystal 2, Tl83Na9Si100Al92O384
About 83 Tl� ions are found per unit cell. Theyoccupy the four crystallographic sites found in
dehydrated ``Tl92-X'' [12]. The occupancies at siteI0 and II increased from 20.5(5) and 19.7(7) incrystal 1 to 24.8(2) and 25.8(2), respectively. Twodi�erent site-III0 positions are each occupied byabout 16 Tl� ions. Of the ca. nine Na� ions re-quired to balance the negative charges of theframework, only ca. three were located at site I.The other ca. six are likely to be at site II tocomplete the ®lling of that site.
4. Discussion
Quite a short time was needed to replace mostof the Na� ions in these large crystals of zeolite X.As the exchange proceeded, the zeolite cavitiesbecome more and more ``crowded'' because of thelarge size of the Tl� ions; this may a�ect the ex-change rates. Perhaps problematically, Na� ionsbegan to occupy site I, the least accessible site fromwhich Na� ions may not be easily removed.
In both structures, Na� ions are found at site I,an empty position in the structure of dehydratedNa92-X [15]. This agrees with the observation ofParise et al., in their study of K� ion exchange intoNa-LSX: as K� ions replace the Na� ions at site I0,Na� ions begin to populate site I [16].
The results of this work can be ®t relatively wellto a simple fourth-order rate law (correlation co-e�cient� 0.998) (Fig. 2) with k � 6:25 � 10ÿ7 (#of Na�/u.c.)ÿ3 sÿ1. The results of Kim et al. (ionexchange for 7 days � 6:0 � 105 s, 90.8(10) Tl�
ions per unit cell)12 also ®t this rate law (Table 4).However, the esdÕs of the total Tl� occupancies inthe three crystal structures (penultimate line ofTable 3) do not allow the conclusion that thisprocess is simple fourth order. The actual mecha-nism of ion exchange must be much more complexthan this formal rate law suggests. Nevertheless, itsuggests that very long exchange times may beneeded to achieve levels of ion exchange near thecomplete ion-exchange limit. More points (morecrystal structures) at additional exchange times areneeded to establish the rate law.
There is now some concern that the amount oftime that can comfortably be spent doing an ion-exchange experiment with large single crystals,a day to a week, may be inadequate to achieve
Table 3
Cation distributions in crystals 1, 2, and ``Tl92-X''
Site Crystal 1 Crystal 2 ``Tl92-X''a
I 6.0(2) Na� 3.6(1) Na�
I0 20.5(5) Tl� 24.8(2) Tl� 31.8(4) Tl�
II 19.7(7) Tl� 25.8(2) Tl� 32.1(4) Tl�
11.8(7) Na�
III 5.6(23) Tl�
III0 18.4(14) Tl� 16.1(6) Tl� 16.3(5) Tl�
III0 0 8.2(25) Tl� 16.2(4) Tl� 10.6(6) Tl�
Total # of
Tl�74(4) 82.9(8) 90.8(10)b
Total # of
Na�17.8(7) 3.6(1)c 0d
a Ref. [12].b Not signi®cantly di�erent from 92, the ideal value at com-
plete exchange.c An additional ®ve or six Na� ions were not located. They
may be at site II, to ®ll that site.d No Na� ions were found crystallographically.
L. Zhu, K. Se� / Microporous and Mesoporous Materials 39 (2000) 187±193 191
complete exchange in some to many systems.Modi®ed or resourceful ion-exchange methodsinvolving high solution temperatures or concen-trations, the complete prior exchange of an inter-mediate cation, or some speci®c chemistry tosequester or destroy the leaving cation, may oftenbe necessary.
References
[1] D.R. Lide (Ed.), Handbook of Chemistry and Physics,
76th ed., CRC Press, Boca Raton, FL, 1995,
pp. 12±14.
[2] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York,
1974.
[3] A. Cremers, in: Proceedings of the Fourth International
Zeolite Conference, American Chemical Society, Washing-
ton, DC, 1977, p. 179.
[4] R.M. Barrer, in: Proceedings of the Fifth International
Zeolite Conference, Heyden, London, 1982, p. 273.
[5] H.S. Sherry, in: Supplementary Textbook for the Eleventh
International Zeolite Association Summer School, Taejon,
Korea, 1996, unpublished.
[6] A. Dyer, An Introduction to Zeolite Molecular Sieves,
Wiley, Chichester, 1988.
[7] F.G. Hel�erich, Ion Exchange, McGraw-Hill, New York,
1962.
[8] H.S. Sherry, J. Phys. Chem. 70 (1966) 1158.
[9] V.N. Bogolomov, V.P. Petranovskii, Zeolites 6 (1986)
418.
[10] W. Loewenstein, Am. Mineral. 39 (1954) 92.
[11] D. Bae, K. Se�, Meso. Micro. Mater., submitted for
publication.
[12] Y. Kim, Y.W. Han, K. Se�, Zeolites 18 (1997) 325.
[13] J.A. Ibers, W.C. Hamilton (Eds.), International Tables for
X-ray Crystallography, vol. 4, Kynoch Press, Birmingham,
England, 1974, p. 73.
Fig. 2. A linear relationship between (# of Na�ions/u.c.)ÿ3 and time shows that a simple fourth-order rate law ®ts the data relatively
well. Plots for third-order and ®fth-order rate laws (not shown) appear to be equally (and oppositely) incorrect. Uncertainty is in-
dicated by the size of the diamonds plotted.
Table 4
Extent of Tl� exchange vs. time
No. of Tl� ions per
unit cell
Time
0 0 s
74 90 sa
83 900 s � 15:0 mina
90 6:67� 104 s � 19 hb
91 5:33� 105 s � 6:2 daysb;c
91.5 4:27� 106 s � 49 daysb
91.9 5:33� 108 s � 17 yearsb
a Experimental data.b Calculated (extrapolated) from the fourth-order rate law.c In good agreement with 90.8(10) Tl� after 7.0 days ([12]).
Experimental conditions used in Ref. [12] that di�er from those
of this work: crystal cross-section � 0.25 mm; ¯ow rate � 0.5
cm sÿ1.
192 L. Zhu, K. Se� / Microporous and Mesoporous Materials 39 (2000) 187±193
[14] J.A. Ibers, W.C. Hamilton (Eds.), International Tables for
X-ray Crystallography, vol. 4, Kynoch Press, Birmingham,
England, 1974, p. 148.
[15] L. Zhu, K. Se�, J. Phys. Chem. B 103 (1999) 9512.
[16] Y. Lee, S.W. Carr, J.B. Parise, Chem. Mater. 10 (1998)
2561.
L. Zhu, K. Se� / Microporous and Mesoporous Materials 39 (2000) 187±193 193