HOWARD S. SHERRY
1158
7'17'2
AAE* = R7'2
- Ti
Kz
In Ki
where indexes 1 and 2 are related to the two temperatures. Applying eq 16 to the data on tolueneS at 40 and loo", the values of AAE* for the chlorinations turn out to be 1.2 and 2.8 kcal/mole going from toluene to benzyl chloride and benzylidene chloride, respectively. Mean values have been obtained from the data of Table 11, considering K12 and K24 for the chlorination
of CH, groups; K28, K45, and Kss for the attack on CH&l groups, and finally K n for the CHCl2 group. The results for AAE* are 1.2 (average value) and 1.9 kcal/mole going from the attack on CHBto the one on CHzCl and CHC12, respectively. These figures are quite significant with respect to the low values of the activation energies for chlorine atom attack on aliphatic C-H bonds, ranging between 0 and 4 kcal/ mole. l 3 Acknowledgment. We are indebted to the Italian Consiglio Nazionale delle Ricerche for financial aid.
The Ion-Exchange Properties of Zeolites. I. Univalent Ion Exchange in Synthetic Faujasite
by Howard S. Sherry Research Department, Socony Mobil Oil Company, Inc., Paulsboro, New Jersey
(Received October 18, 1966)
Ion-exchange isotherms describing the exchange of Li, K, Rb, Cs, Ag, and TI(1) ions into the Linde Na-X type of synthetic faujasite are presented. All the alkali metal ionexchange isotherms except the one for Li-Na exchange are sigmoidal and show selectivity reversals. The initial selectivity series is in the order Ag >> T1 > CS 2 Rb > K > Na > Li. Above approximately 40% replacement of sodium ions the selectivity series becomes Ag >> T1 > Na > K > Rb 2 Cs > Li. Evidence is also presented to demonstrate that 16 out of 85 sodium ions per unit cell are not replaceable by Rb and Cs ions in Na-X. Ion-exchange isotherms for the exchange of Li, K, Rb, Cs, Ag, Tl(I), and NH4 ions into the Linde Na-Y type of synthetic faujasite are also presented. These isotherms demonstrate that 16 out of the 50 sodium ions per unit cell cannot be replaced by Rb, Cs, TI(I), and NH4 ions. Furthermore, Rb-Na and Cs-Na ion exchange in zeolite Y gives nonsigmoidal isotherms which are compared to the sigmoidal isotherms found for these pairs of ions in zeolite X. I n addition, Na-X is much more selective for Ag than is Na-Y. From these data and the structural data for these zeolites it is concluded that there is much ion binding in Na-X and little ion binding in Na-Y.
The ion-exchange properties of zeolites are of interest for several reasons. The pore size of these materials amroaches. and in some cases is smaller than, the she Of most monatomic ions' and the patterns exhibited by these materials are quite varied. For A
example, the synthetic zeolite Linde A exhibits the selectivity pattern' Na > K > Rb > Li > Cs, and
L
The Journal of Physkd Che?nhtry
(1) R. M. Barrer, L. V. C . Reas, and D. (London), ~ 2 7 3 180 , (1963).
J. Ward, Proc.
Roy. SOC.
ION-EXCHANGE PROPERTIES OF ZEOLITES
clinoptilolite, phillipsite, erionite, and the Norton Company’s synthetic mordenite all exhibit the selectivity pattern2 Cs > I< > Na. The latter selectivity series is the usual selectivity series observed in most commercially available cation-exchange resins in dilute solution. However, both these selectivity patterns were predicted by E i ~ e n m a n . ~ Synthetic faujasite is of particular interest because this zeolite has two independent, though interconnecting, three-dimensional networks of ca~ities.~fjCations are located in both networks of cavities.’ One network consists of large cavities, sometimes called supercages, which have a diameter of about 13 A. They are linked in a tetrahedral, diamond-type lattice by sharing rings of 12 tetrahedra.’ These rings have a free diameter of about 8 A.’ The other network is formed by linking smaller cages, called sodalite cages,’ in a tetrahedral, diamondlike lattice, through adjoining rings of 6 tetrahedra, thus forming secondary cavities which are hexagonal prisms, between sodalite cages. The composition of the synthetic zeolite Linde 13-X, which is the synthetic faujasite used in this work, is given by the anhydrous unit cell formula Nas5[(AlOs)@(Si02)107]. There are 85 sodium ions in a unit cell. Sixteen are located either in the hexagonal prisms, 1 in each of the 16 prisms per unit cell17or in the sodalite cages, 2 in each of the 8 sodalite cages per unit cell.* I t will be seen later that, for ion-exchange purposes, the disagreement is not important. Thirty-two of the sodium ions are located in the large cavities, very nearly in the plane of the rings of 6 tetrahedra which are shared by and, therefore, connect the supercages and the sodalite cages. The remaining 37 cations are either in several of many crystallographically equivalent positions or are in constant motion in the large cavities and cannot be located by X-ray techniques. The composition of the synthetic zeolite Na-Y used in this work is given by the anhydrous unit cell formula Na50[(A102)50(Si02)142]. In this zeolite 16 cations per unit cell are located in the hexagonal prisms or sodalite cages.8 The rest are located in the large cages. The Si02-A102 framework of Linde Y is almost identical in structure with that of Linde X. Study of synthetic faujasite provides an opportunity t o vary the ion-exchange capacity via the isomorphous substitution Of for NaA1oz the ‘Onfiguration of the crystalline aluminosilicate backbone remains essentially constant. In an amorphous ion exchanger, which might be typified by Dowex-50, this is equivalent to Varying the exchange Capacity at constant CIOSSlinking, polymer configuration, and water content. The crystalline system has the advantage that it is
1159
amenable to structural analysis, whereas the amorphous systems are not. Faujasite is also unique because it has the most open framework of all known zeolite structures. The hydrated zeolite contains 264 molecules of H2O per unit cell.9 Thus, the average number of HzO molecules per Na+ ion is about 3 in Na-X. This value is 3.1 for the Li form of X and smoothly decreases, as the alkali metal series is descended, to 2 for CS-X.~ The internal molality is about 19 for Na-X. In Na-Y the water content is essentially the same as in Na-X. lo There are approximately 5 water molecules per univalent ion and the internal molality is about 11. The cations and water in the large cages may be thought of as a concentrated salt solution. Evidence that the ions in the supercages are hydrated has been enumerated by Baul.8 and will not be presented here. We will just add the additional statement that this zeolite does obey the Donnan membrane equilibrium. l1 The unusual and interesting feature of this system is that sites are located in the different networks of cavities. The sites in the large cages might be expected to exhibit the same selectivity series as that exhibited by commercially available resinous-type exchangers such as Dowex-50 in dilute or moderately concentrated solutions3because of the higher water content and more open structure, whereas the sites in the network of small cavities might be expected to exhibit a different selectivity series more characteristic of less open zeolites. Polyfunctionality is built into this crystal.
Experimental Section All reagents used were Baker Analyzed reagent grade with the exception of RbCl and CsCl of 99.9% purity which were purchased from the Kawecki Chemical Co. The Na-X used was Linde 13-X Lot No. 262. An average of three analyses is given in Table I on an anhydrous basis. (2) L. L. Ames, Jr., A m . Minerologist, 49, 127 (1964). (3) F. Helfferich, “Ion Exchange,” McGraw-Hill Book Co., Inc., New York, N. Y., 1962. (4) G. Eisenman, Bwphys. J . , 2 , 259 (1962). (5) G. Bergerhoff, H. Koyama, and W. Nowacki, Ezperientk, 12,
418 (1956). (6) G. Bereerhoff, W. H. Baur, and W. Nowacki, Neues. Jahr. Mineral. Monatsh., 193 (1958). (7) L.Broussard and D. P. Shoemaker, J . Am. Chem. SOC.,8 2 , 1041 (1960).
w* Baury An* Minerobgist* 49* 697 (Iga). (9) R. M. Barrer and G. C . Bratt, J. Phys. Chen. Solids, 12, 130 (1959). (10) Determined by the author and many others in these laboratories* (11) R. M.Barrer and A. S. Walker, Trans. Faraday SOC.,60, 171 (1964). Volume 70, Number 4 April 1966
HOWARD S. SHERRY
1160
t .-A
A
a
a
N
N
0
0.2
0.6
0.8
1.0
Figure 3. The ion-exchange isotherm for the Rb-Na-X system at 0.1 total normality and 25": 0, Rbs+ Naz+; 0, Nas+ Rbz+.
Figure 1. The ion-exchange isotherm for the Li-Na-X system at 0.1 total normality and 25": 0, Lis+ Naz+; A, Nas+ Lizf.
+
0.4
'Rb
LI
+
+
+
3
Y
N
N
sCS
Figure 2. The ion-exchange isotherm for the K-Na-X system at 0.1 total normality and 25": 0, Ks+ Naz+; A, Nay+ Kz+.
Figure 4. The ion-exchange isotherm for the Cs-Na-X system at 0.1 total normality and 25": 0, CSS+ Naz+; n, Nas+ CSZ+.
+
+
+
+
~
Table I : Chemical Analyses of Zeolites Used in This Study Zeolite
Na-X Wt % Millimoles/gram Na-Y Wt % Millimoles/gram
Si02
AlnOa
NarO
46.8 7.79
32.2 3.16
21.0 3.38
65.6 10.92
20.9 2.05
12.1 1.96
The atom ratio of Na/A1 in Na-X was 1.07 as received, probably due to NaOH occluded during synthesis, and the SiOZ/A1203 mole ratio was 2.46 f 0.02. The Journal of Physical Chemistry
Batches of zeolite were washed with deionized water until the atom ratio of Na/A1 was 1.00 0.02 and then stored in desiccators over saturated NH&1 solution to maintain constant moisture content. When constant weight was reached, the moisture content was determined in duplicate by calcining samples for 5 hr at 815". The same procedure of washing and storing over saturated NH&l solution was used when other ionexchanged forms of the zeolite were prepared. The average of three analyses of the Na-Y used is also given in Table I. The atom ratio of Na/Al was 0.955 f 0.045, and the SiO2/Al2O3mole ratio was 5.33. This zeolite was contacted four times for 2 hr each
*
ION-EXCHANGE PROPERTIES OF ZEOLITES
1161
was determined by obtaining a complete chemical analysis of both phases, cation analysis of both phases, or cation analysis of the solution phase. In the latter case, the chemical analysis of the zeolite phase was calculated from the solution analyses before and after exchange and from the initial Naf and H20 content of the zeolite phase. Selected ion-exchanged samples of zeolite were submitted to X-ray analysis to ensure that the crystal structure was retained after exchange.
8 0.6 N
.,g "0
Results and Discussion 0.2
0.6
0.4
0.8
1.0
sAo Figure 5. The ion-exchange isotherm for the Ag-Na-X system a t 0.1 total normality and 25": 0, Ags+ Naz+.
+
I-
N
0 0
0.2
0.4
0.6
0.8
1.0
'TI
Figure 6. The ion-exchange isotherm for the T1-Na-X system st 0.1 total normality and 25": 0, Tis+ Nazf.
+
with 1.0 M NaCl solution and then stored in desiccators as described above for Na-X. Both zeolites had negligible amounts of crystalline impurities or amorphous material. Equilibrations were performed in polyethylene bottles to which were added the appropriate quantities of zeolite and solution. The amounts of zeolite and solution used were measured by weighing the bottles before and after each addition. The bottles were agitated in a water bath for 24 hr. The temperature of the water bath was maintained at 25 f 0.1". Sodium isotope exchange showed that equilibrium was reached in less than 1 hr. After equilibration the phases were rapidly separated by filtration. The extent of exchange
The ion-exchange isotherms for the Li-Na-X, K-Na-X, Rb-Na-X, Cs-Na-X, Ag-Na-X, and TlNa-X systems at 25" and 0.1 total normality are shown in Figures 1 through 6. I n these figures Z L ~ZK, , etc. represent the ratio of the equivalents of the ion of interest to the total equivalents of cations in the zeolite, or to the gram-atoms of aluminum in the zeolite. This is the equivalent fraction of the ion of interest. The concentrations in the solution, S L ~SK, , etc., are also expressed in equivalent fractions. The K-Na-X, Rb-Na-X, and Cs-Na-X isotherms show selectivity reversals. This is due to the fact that the univalent cations are located in three different kinds of crystallographic sites, each of which should exhibit different preferences for alkali metal ions. Let us assume, in the case of K+, Rb+, and Cs+ exchange of Na-X, that the order of exchange of the sites is: (1) Exchange of the 37 cations per unit cell in the large cages which cannot be located by X-ray techniques. These ions probably are present in the large cages as hydrated ions.* (2) Exchange of the 32 cations per unit cell located near the rings of 6 tetrahedra which interconnect the supercages and the sodalite cages.7 (3) Exchange of the 16 cations per unit cell which are located in either the hexagonal prisms7or the sodalite cages.8 The 37 sites which do not bind ions tightly and whose counterions probably exist as hydrated ions should prefer all of the alkali metal ions, except Lif, to Naf from consideration of hydrated ionic radiil2 and coulombic interactions between the hydrated counterions and the anionic sites. Thus, the selectivity series Ag >> T1 > Cs > Rb > K > Na > Li is encountered below about 40% exchange. At the other two sets of cation positions, consideration of the lattice oxygencounterion internuclear distances7 leads to the conclusion that there are no water molecules interposed between the fixed anionic sites and the counterions The selectivity of these sites for the counterions is (12) E.
R. Nightingale, Jr., J. Phy8. Chem., 63, 1381
(1959).
Volume YO, Number .4 April 1986
HOWARD S. SHERRY
1162
the net result of the opposing effects of the free energy due to coulombic interactions between the partially dehydrated counterion and the negatively charged lattice of and the free energy of partial ion dehydration. On this basis, the selectivity series for the 32-fold set of sites located in the center, or slightly displaced from the center, of the rings of 6 tetrahedra separating the supercages from the sodalite cages which is observed in the region of 50% exchange, Ag >> T1 > Na > K > Rb 2 Cs > Li, is entirely reasonable. It is one of the 11 selectivity series predicted by Eisenmans4 The selectivity for alkali metal ions exhibited by the set of 16 sites, located either in the hexagonal prisms or in the sodalite cages, cannot be interpreted solely on the basis of equilibrium selectivity for the following reason. It has not been possible to prepare pure Rb-X and Cs-X even when the exchange equilibria have been forced by contacting RbSCN and CsSCN with Ag-X at 22’. This result should be expected because the crystal radii of Rb+ and Cs+ are appreciably greater than the 2.4-A diameter openings to the network of small cavities.’3 This result is also consistent with the report that Cs+ and Rb+ penetrate sodalite with great difficulty and that only partial exchange was attained after 5 days of exchange of the Ag form of basic sodalite with RbCl and CsCl at 85’.14 This crystalline aluminosilicate is composed wholly of sodalite cages, and an ion must diffuse through rings of 6 tetrahedra about 2.4 A in diameter in order to penetrate it. It is for this reason that we believe the isotherms for the RbNa-X and Cs-Na-X systems should terminate a t the point S = 1, 2 = 0.82, which corresponds to an Xtype zeolite with 16 out of 85 Na+ left in the unit cell. If we reject consideration of Cs+ and R b + selectivity in the network of small cavities for steric reasons, the equilibrium selectivity exhibited for the remaining 5 ions by the sites in the small cavities is Ag > TI > Na > K > Li. High selectivity for Ag+ and T1+ exhibited by Linde X over the whole course of the exchange reaction is demonstrated in Figures 5 and 6. This is consistent with the high polarizability of Ag+ and T1+ and indicates strong binding by all types of anionic sites. We would like to stress the importance of properly expressing ionic concentrations and standard states in zeolites. We have used units of equivalent fractions and have calculated these fractions by calculating the ratio of the equivalents of the ion in question in the zeolite to the equivalents of total cations in the zeolite. This latter quantity was always equal to the gram-atoms of aluminum in the zeolite. This ratio is not synonymous with the ratio of the equivalents The J O U T of ~ Physical Chemistry
.-4 N
’Li
Figure 7. The ion-exchange isotherm for the Li-Na-Y system at 0.1 total normality and 25’: 0, Lis+ Naz+; 0,Nas+ Liz+.
+
+
1.01
0.8
-
0.6
-
Y
N
0
0.2
0.4
0.6
0.0
Figure 8. The ion-exchange isotherm for the K-Na-Y system a t 0.1 total normality and 25’: 0, Ks+ Naz+; 0 , Nas+ K z f .
+
+
of the ion in question to the equivalents of exchange capacity which the zeolite has for that ion if the experimentally determined exchange capacity is not equal to the number of anionic AlOz- sites. When the latter method of calculating equivalent fractions is used, the exchange capacity should be reported so that it is made obvious that a point a t S = 1, Z = 1 does not represent a completely exchanged zeolite. This becomes important when the free energy of exchange is calculated. (13) T. Moellor, “Inorganic Chemistry,” John Wiley and Sons, Inc., New York, N. Y., 1952. (14) R. M. Barrer and J. D. Falconer, Proc. Roy. SOC.(London), A236, 227 (1956).
ION-EXCHANGE PROPERTIES OF ZEOLITES
1163
n a N
.r 0.6
I N
0.4
IR o*2 0
I
t f
I 0.2
0
I
I
0.4
0.6
I 0.8
02
0 1.0
o
02
0.6
OB
1.0
S
'Rb
NH4
Figure 9. The ion-exchange isotherm for the Rb-Na-Y system at 0.1 total normality and 25': 0 , Rbs+ Naz+; A, Nas+ Rbz+.
+
+
a4
Figure 11. The ion-exchange isotherm for the NH4-Na-Y system a t 0.1 t80talnormality and 25': 0 , NHls+ Naz+; A, Nas+ N&+.
+
+
B N
Figure 10. The ion-exchange isotherm for the Cs-Na-Y system a t 0.1 total normality and 25': 0, Cssf Naz+; A, Nas+ CSZ+.
Figure 12. The ion-exchange isotherm for the Ag-Na-Y system at 0.1 total normality and 25': 0, Ags+ f Naz+.
At this point in our work we realized that, if our interpretation of the sigmoidal isotherms which describe alkali metal ion exchange in Linde X is correct, the shape of the isotherms in Linde Y might be predicted. We reasoned that in Linde Y, the lower ion-exchange capacity and lower internal molality might lead to less ion binding. The sites in the large cages of Linde Y might all prefer Rb+, Cs+, NH4+, and K + to Na+ and Na+ to Li+, a selectivity series characteristic of hydrated ions in the exchanger phase. Thus, nonsigmoidal isotherms should characterize the ion-exchange properties of the large cages. The isotherms in Figures 7 to 13 demonstrate that
over the first 68% of exchange the selectivity series is T 1 > Ag > Cs > Rb > NH4 > K > Na > Li. Nost important to note is the nonsigmoidal nature of the Cs-Na-Y, Rb-Na-Y, NH4-Na-Y, and TI-Na-Y isotherms and the fact that, from the shapes of these isotherms, we can safely say that 32% of the Na+ ions, or 16 per unit cell, are not replaceable by these 4 cations at 25'. The Linde-Y systems can be interpreted as follows. (1) The nonsigmoidal shape of the isotherms below the 68% exchange level indicates no site heterogeneity in the large cages. (2) The termination of the Rb, Cs, NH4, and T1-
+
+
Volum 70, Number 6 A M 1086
HOWARD S. SHERRY
1164
0
02
os
OA
0.8
1.0
s*..I Figure 13. The ion-exchange isotherm for the T1-Na-Y system a.t 0.1 total normality and 25': 0, Tis+ Naz+.
+
Na-Y isotherms at the point S = 1, 2 = 0.68 indicates these ions are too large to penetrate the sodalite cages. (3) The sodalite cages can be penetrated by Li+, Na+, Ag+, and K + indicating that the rings of six Si-AIOl tetrahedra which are the windows between the large cages and the sodalite cages have an effective diameter, for the purpose of univalent ion exchange, of between 2.66 and 2.88 A at 25" if Pauling's crystal radii of the cations are used. (4) The sites in the network of small cavities exhibit a selectivity pattern typical of a surface which binds ions tightly: Ag > Na > K > Li. This last statement is consistent with the fact that there are 16 univalent cations and 32 water moIecules per unit cell in the network of small cavities.* The cation-lattice oxygen internuclear distance also indicates direct coordination of the cation to the fixed negative chargesson the surface of the zeolite. The isotherms for the Ag-Na-X and Ag-Na-Y systems shown in Figures 5 and 11 support these conclusions. The preference of the sodium-faujasite crystal for Ag+ over Na+ is sharply reduced as NaAlO2 is replaced by SiOz (going from Linde X to Linde Y). This change is to be expected, if the binding of d g + by the anionic surface decreases in the same direction and is consistent with the decrease in site density a t constant water content in the unit cell. We can summarize by stating that the 16 cations in the network of small cavities in Linde X and Linde Y form ion pairs with the fixed negative charges of the aluminosilicate backbone. The cations in the large cages, which comprise the majority, behave differently in Linde X and Y. In Linde X the majority of these cations are tightly bound to the fixed negative charges The Journal of Physieal Chemistry
on the aluminosilicate backbone. X-Ray diffraction studies tell us that 32 of these cations are located near the center of the 32 rings of 6 A104-Si04 tetrahedra which are the windows between the large cavities and sodalite cages.? Some of the remaining 37 cations may also be bound to the zeolite backbone. In Linde Y the majority of cations in the large cavities are probably completely hydrated, and we can think of the salt NaAlO2 as being completely ionized. Support for this view is found in the study of electrolyte imbibement by Linde X and Y made by Barrer and Walker." Their data indicate that Na-X imbibes much more NaCl from NaCl solutions than does Ka-Y. Consideration of ion-exchange capacity alone makes this result anomalous. However, this apparent anomaly can be explained by assuming extensive cation binding in the Na-X crystal and little cation binding in the Na-Y crystal. As a result of the ion-exchange data, we predict that a determination of the complete crystal structure of hydrated Na-Y should show that the rings of 6 tetrahedra which are the windows between the large cages and the sodalite cages (a 32-fold set of sites) are empty of cations, or, if there are cations located near these rings, the univalent cation-lattice oxygen internuclear distances will be larger than in Linde X, large enough to support interposition of water molecules between the cation and the lattice oxygen atoms. In this latter case we are dealing with ionic atmosphere binding in the sense used by Strauss,l5 that is, the localization of hydrated cations by the electrostatic field of the negatively charged lattice. If our interpretation of all of the isotherms encountered in univalent exchange of Linde X and Y is correct, sigmoidal isotherms should also be encountered in Linde 4-A. In this zeolite, which has 12 sodium ions per unit cell, 4 ions cannot be located and are probably completely hydrated when the zeolite is fully hydrated, and 8 are located near the center of six-membered rings of tetrahedra. These latter rings are the windows between large and small cavities. The lattice oxygen-sodium internuclear distances16 indicate that the ions in the rings of 6 tetrahedra are bound to the surface through oxygen atoms. This distribution of ion sites is strikingly similar to that encountered in Linde 13-X;5-8 hence, we would predict sigmoidal Cs-Na, Rb-Na, and K-Na isotherms. Indeed, sigmoidal isotherms have been repor+ed for these systems in Linde A.' (15) U. P. Strauss and Y. P. Leung, J. Am. Chem. SOC.,87, 1476 (1965). (16) T. B. Reed and D. Breck, ibid., 78,5972 (1956).
w.
ION-EXCHANGE PROPERTIES OF ZEOLITES
-0.6 I
1165
1
1.0
- 0.8 - 1.0
.-
0
0.0 0.6
-1.2
0.4
- 1.4
0.2
J*z
-1.6
z
-2-ol \ -1.8
sxP
=
0.0 -0.2
(3
5
-0.4
-2.2
-
-2.4
-0.8
0.6
- 1.0 -1.2
0
0.2
0.4
0.6
0.8
1.0
'Rb
Figure 16. The rational selectivity coefficient for the Rb-Na-X system a t 0.1 total normality and 25' as a function of zeolite composition. 0.0 r
Y
z
cYs z
0.0 (3
s
*(
z
s
I
-0.2
- 0.4
-OB -0.8
t
Figure 17. The rational selectivity coefficient 3 for the Cs-Na-X system a t 0.1 total normality 0.2 0.4 0.6 0.8 1.0
0
*K Figure 15. The rational selectivity coefficient for the K-Na-X system at 0.1 total normality and 25" as a function of zeolite composition.
A most interesting feature of the faujasite series is that TI+ will replace all of the cations in Na-X but will not replace 16 cations per unit cell in Na-Y. This is illustrated in Figures 6 and 13. This difference in behavior of TI+ in the more siliceous form of synthetic faujasite is attributed to the lattice contraction which occurs when Si02 replaces NaAl02. We have observed
and 25" as a function of zeolite composition.
that the lattice parameter, ao, for this cubic unit cell can change by as much as 0.05 A. This lattice contraction must be sufficient to prevent Tl+ from penetrating the sodalite cages of Na-Y. We have calculated the free energy of exchange for the reaction Ms+ Naz+ MZ+ Nasi, by using the method of Gaines and Thomas.I7 A modified
+
(17) G. (1953).
+
L. Gaines and H. C. Thomaa, J. Chem. Phya.,
21, 714
Volume 70,Number 4 April 1966
HOWARD S. SHERRY
1166
NKNaM= ZMSNa ZN~SM
4 0.2
‘0
a4
0.6
0.0
1.0
0.8
1.0
z41
Figure 18. The rational selectivity coefficient for the Ag-Na-X system at 0.1 total normality and 25’ as a function of zeolite composition.
2.0 I .6
and is the mean molal activity coefficient. Use of this equation is justified because imbibement” of external electrolyte and changes in water activity’s are negligible at a total ionic strength of 0.1 (the total normality was always maintained at 0.1). Activity coefficient ratios were calculated using Harned’s ruleI6 by assuming that the interaction coefficient for one ion is the negative of that for the other ion. This procedure is justified because the change in the activity coefficient ratio with change in solution composition is small. The first integral in the free energy equation was graphically evaluated by calculating the area under the curve obbained by plotting the logarithm of the rational selectivity coefficient vs. ionic composition of the zeolite. These plots are shown in Figures 14 to 26. The rational selectivity coefficients for Rb and Cs ion exchange into Na-X and Na-Y and for Tl(1) and NH4 ion exchange in Na-Y were calculated from normalized ionexchange isotherms. The ordinate in the normalized
I .2 0.8
-
Oe4 0
t 0
0.2
0.4
0.6
Figure 19. The rational selectivity coefficient for the T1-Na-X system at 0.1 total normality and 25’ as a function of zeolite composition.
form of their equation, which neglects salt imbibement and changes in water activity, was used AFoT
-2.303RT X
-
2.4 0
0.2
0.6
0.4
0.8
1.0
‘Li
AFOzss = -1362 X
Figure 20. The rational selectivity coefficient for the Li-Na-Y system at 0.1 total normality and 25’ aa a function of zeolite composition.
I n this equation the rational selectivity coefficient for the exchange of the metal ion in question into Na-X, NK~aM is defined , as The Journal of Phyaical Chemistry
~~
~
(18) R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” Butterworth and Co. Ltd., London, 1959.
ION-EXCHANGE PROPERTIES OF ZEOLITES
1167
0.6
0.4
0.2 0
Y Z
d
o*0.1 2* 0
o
0.2
0.4
0.6
0.8 0
Z
ZNH4
0
OJ -0.2
Figure 24. The rational selectivity coefficient for the NHd-Na-Y system at 0.1 total normality and 25" as a function of zeolite composition.
-0.4
- 0.6
0
0.2
0.4
0.6
0.8
uz Y
=K
2
(3
Figure 21. The rational selectivity coefficient for the K-Na-Y system at 0.1 total normality and 25" as a function of zeolite composition.
0.8
-
0.6 .
0
O m
0 0
I 0.2
0
1
0.4
0.6
I 0.8
1.0
Figure 25. The rational selectivity coefficient for the Ag-Na-Y system at 0.1 total normality and 25" as a function of zeolite composition.
z
0.5
0
0.2
0.4-
0.6
0.8
1.0
'Rb
Figure 22. The rational selectivity coefficient for the Rb-Na-Y system a t 0.1 total normality and 25" as a funct,ion of zeolite composition.
(3
0'90
0.2
0.4
0.6
0.8
1.0
zTI
I
Q
Q
Figure 23. The rational selectivity coefficient for the Cs-Na-Y system at 0.1 total normality and 25" as a function of zeolite composition.
isotherms is the ratio of the equivalents of the ion of interest to the total equivalents of exchangeable cations. Thus, in the case of Rb and Cs ion exchange
Figure 26. The rational selectivity coefficient for the T1-Na-Y system at 0.1 total normality and 25" 89 a function of zeolite composition,
of Na-X, the total milliequivalents of exchangeable cations per gram of zeolite is 82% of the milliequivalents of Na per gram reported in Table I. In the case of Rb, Cs, Tl(I), and NHh ion exchange of Na-Y, the total milliequivalents of exchangeable cations per gram of zeolite is 68% of the milliequivalents of Na per gram of zeolite reported in Table I. The normalied isotherms all terminate at the point S = 1, 2 = 1. The standard free energies of the ion-exchange reactions, which are listed in Table 11, are, then, the differences between the free energy of the sodium form of the zeolite in pure water and the free energy of a zeolite in which all of the Na ions which can be replaced by a second ion have been replaced by that ion. This latter zeolite is also in pure water in its reference state. Volume 70,Number 4 April 1966
HOWARD S. SHERRY
1168
Table 11 : Standard Free Energy of Exchange of Na-X and Na-Y at 25" AF'sos,
Reaction
+ + + Cs+ + Na-X Ag+ + Na-X T1+ + Na-X Lif + Na-Y K + + Na-Y Rb+ + Na-Y Cs+ $. Na-Y NHa+ + Na-Y Ag+ + Na-Y T1+ + Na-Y
Li+ Na-X K + Na-X Rb+ Na-X
cal/equiv of zeolite
1600 f 200 1 4 0 f 10 140 f 10 87 f 5 -2520 f 20 - 1840 f 100 2700 f 200 -190 f 30 -1300 3= 30 -1460 f 100 -660 f 60 -1100 f 20 -1630 f 30
The only known thermodynamic data to which the data in Table I1 can be compared are the data of Ames2 on K f and Csf exchange of Na-X at 25". In this work, neither the exchange capacity of the zeolite nor the final state of the zeolite is reported for Cs+, and it is difficult to compare our data. However, the Cs-Na-X isotherm is extrapolated through the point S = 1, 2 = 1. If this point represents a pure Cs form of zeolite X, then we believe the extrapolation to be incorrect. If the composition in the zeolite is normalized so that the point S = 1, 2 = 1 represents the zeolite Cs0.82Nao.1&in equilibrium with pure 1.0 N CsC1, the extrapolation is correct. However, we report in Table I1 that the standard free energy of exchange 'of Na-X to C S ~ . ~ ~ N is ~O ~& 87. cal/equiv of Na-X using a normalized isotherm. Ames reports 600 cal. If we extrapolate our unnonnalized Cs-Na-X isotherm to the point S = 1, 2 = 1, we obtain close to 600 cal. Evidently, Ames has assumed that all of the sodium ions in. Na-X are replaceable by cesium ions and has extrapolated an unnormalized Cs-Na-X isotherm to thepointS= 1,Z= 1. This problem should not exist in the K-Na-X system. In Table 11,we report that the standard free energy of exchange of Na-X to K-X is 140 cal/equiv of zeolite; Ames reports -200 cal/mole of zeolite
The Journal of Physical Chemistry
(presumably per equivalent of zeolite). We cannot account for this difference, unless the difference in total normality is the cause. The fact that Ames obtained his isotherm at a total normality of 1.0 may mean that imbibement of external electrolyte occurred." Our ion-exchange isotherms do agree with the alkali metal ion selectivity series at the 50% exchange level reported by Barrer, Buser, and Grutter.lg
Conclusions The cation positions in synthetic faujasite are reasonably well defined, and we have interpreted selectivity reversals in terms of differing selectivities of the various types of sites for alkali metal ions. It is important to realize that all of the sites in a crystalline ion exchanger may not be accessible to all cations and that the ion-exchange capacity of zeolites can vary with the ingoing cation. Thus, extrapolation of an isotherm to 100% exchange is not always justifiable. Much confusion can result in the literature if ion-exchange capacities are not published along with the ion-exchange isotherms. We have found that we cannot put Rbf and Cs+ into the small cavities of Ag-X by using concentrated solutions of the SCN salts at room temperature and have concluded that these ions are sterically hindered from entering these cages. Our results in the Linde-Y systems verify this. The nonsigmoidal isotherms in the Rb-Na-Y and Cs-Na-Y systems and sigmoidal isotherms over the first 82% exchange in the Rb-Na-X and CsNa-X systems indicate extensive ion binding in the supercages of the Linde-X type of synthetic faujasite and little ion binding in the supercages of the LindeY type of synthetic faujasite.
Acknowledgments. The author wishes to express his appreciation to the Socony Mob2 Oil Co., Inc., for its support and encouragement of this work. In particular, he wishes to thank Dr. L. J. Reid and Mr. A. B. Schwartz for many helpful discussions and Mr. J. F. Charnel1 for his X-ray analyses of many zeolite samples. (19) R. M. Bmrer, W. Buser, and W. F. Grutter, Chim& (Asrsu), 9 , 118 (1955).
