IONEXCHANGN IN MOLTEN SALTS
2885
as much as 10 kcal/mol lower than the 28.3 kcal/mol observed for compound I with a normal preexponential factor and at the same time have the stability observed by van Tamelen and Pappas.8 This leads to the conclusion that fluorine substitution of the second cyclobutene ring affords much less stabilization to the molecule than fluorine substitution on the first.
duce the exothermicity of the isomerization reaction less than in the case of a single cyclobutene ring. Similar observations can be made about the kinetic data. Chesick's tabulation' of available data shows that fluorine substitution of all hydrogens of cyclobutene and of 1,2-dimethylcyclobutene reduces the activation energy for the isomerization reaction to the butadiene derivative by 14.6 and 10.0 kcal/mol, respectively, while the preexponential factors remain constant within a power of 10. It is obvious that bicyclohexadiene could not have an activation energy
Acknowledgments. The author wishes to thank H. A. Huggins for carrying out the kinetic measurements and A. R. Taranko for operating the differential scanning calorimeter.
Ion Exchange in Molten Salts. I. The Ion-Exchange Properties of Sodium Zeolite A in Molten NaNO,; Exchange Reactions with Alkali Metal, Thallium, and Silver Cations by M. Liquornikl and Y. Marcus2 Israel Atomic Energy Commission, Soreq Nuclear Research Centre, Yavne, Israel
(Received February 6, 1968)
Cation-exchange equilibria between sodium zeolite A and molten mixtures of sodium nitrate and those of lithium, potassium, cesium, silver, or thallium were studied over substantial ranges of compositions of the exchanger a t 330' (350" for the potassium exchanges). In those cases (lithium, sodium, silver) where occlusion of nitrate in the a cage of the zeolite occurs, all of the cations can be exchanged, but in the other cases limited exchange only occurs. When extrapolated to trace concentrations in the sodium zeolite-sodium nitrate system, the selectivity order is Cs > T1 > Ag > K > Li > Na, but the positions of Cs, T1, and K are transposed to the right as their loading increases. The standard free energy change for complete exchange from sodium to silver zeolite in equilibrium with the pure molten salts a t 330" is AGO = -3230 cal/mol.
Introduction A considerable body of data describes the selective uptake of ions on ion exchangers in aqueous systems, but comparatively few results have been obtained in nonaqueous media, in general, and in molten salts, in particular. The highly ionic molten salts make possible the study of ion-exchange systems without the sometimes disturbing effects of solvents. The main difficulty in using molten-salt solutions for ion exchange is the high temperature involved, which makes organic resins useless. This difficulty can be circumvented by working with crystalline ion exchangers like zeolites, which provide a well-defined robust exchanger framework. Of the natural and synthetic zeolites available, the synthetic zeolite "Linde Molecular Sieve 4A" was chosen for the present study, because (a) it has a wellknown and defined structure, (b) its ion-exchange properties in aqueous solution have been more exten-
sively investigated than those of any of the other zeolites, and (c) it is readily available in uniform quality. Linde R/lolecular Sieve 4A is a sodium aluminum silicate, its components being SOz, AIOz-, and Na+ in the molar ratio 1: 1: 1. The aluminosilicate framework of the zeolite is based on Si04 and A104 tetrahedra, linked together to form a ring of eight oxygen atoms in the center of each face of the unit cell and an irregular ring of six oxygen atoms a t each corner of the threefold axis.3~~In the center of the unit cell is a large cavity 11.4 8 in diameter-the a cage-which is connected to (1) Taken from a part of the Ph.D. thesis aubmitted by M. L. to The Hebrew University, Jerusalem, 1967. (2). Department of Inorganic and Analytical Chemistry, The Hebrew University, Jerusalem, Israel. (3) D. W. Breck, W. G. Eversole, R. M. Milton, T. B. Rud, and T. L. Thomas, J. Am. Chem. SOC.,78, 5963 (1956). (4) R. M. Barrer and W. M. Meier, Trans. Faraday Soc., 54, 1079 (1958). Volume 72, Number 8
Auguat 1968
2886 the corresponding cavities in the six neighboring unit cells via the eight-membered rings, which form restricted openings 4.2 A in diameter. In addition, the large cavity is connected to eight small cavities, 6.6 8 in diameter-the p cages. The connection is produced by the six-membered rings, which form openings 2.0 8 in diameter. The chemical composition of one unit cell of the dehydrated sodium zeolite is 12Na. 12A102. 12Si0,. One formula weight of this unit cell is taken as equivalent to 1 mol of exchanger. Callahan and Kay6 have recently reviewed the earlier work on ion exchange in molten salts, which concentrated mainly on chromatographic separations, but discussed also the preparation of certain exchanged forms of natural zeolites. Ion-exchange selectivity for ions at tracer concentrations was studied in several recent investigations. Platek and Marinsky6 determined the selectivity of the lithium form of the zeolite A for Na+, Rb+, and Cs+ in molten LiCl. Ames7 investigated the effect of temperature on the distribution of Cs+ and Na+ between the natural zeolites erionite, mordenite, and clinoptilolite and molten LiN03 and RbN03. Callahan and KayS investigated the exchange of alkali metal and alkaline earth ions with the felspatoid sodalite and the natural zeolites heulandite, analcite, natrolite, and especially chabazite in molten lithium, sodium, and rubidium nitrates. The ion-exchange properties of zirconium phosphate in fused alkali nitrate mixtures were studied by Alberti and coworkers* and by Tikavyi and G l o c h o ~ a . ~ The principal aim of the present work is to investigate systematically the ion-exchange process when substantial exchange occurs in the system and the physicalchemical characteristics of the ion-exchanger phase. The liquid phase is a molten alkaline nitrate solution having known physical-chemical properties, as used also in some of the studies cited above. Preliminary results on the exchange of silver and thallium ions between zeolite A and molten sodium nitrate have been reported by the present authors.1°
Experimental Section Materials. All of the salts used were of reagent grade and were oven dried at 110" for a t least 24 hr. The very hygroscopic LiN03 was dried by melting, bubbling dry nitrogen through the melt, and filtering it through a porous glass filter. The material was then handled in a drybox. The Linde Molecular Sieve 4A exchanger was obtained in powder form from Linde Air Products Ltd. The exchanger was first sieved through a 200-mesh sieve and then sedimented in water in a 100-cm column. The main fraction was collected4 and analyzed. The ratios SiOz/A1203 = 1.94 and Na20/Alz03 = 0.965 were found,lO in good agreement with the composition expected from the formula of the unit cell. The material was dried at 100" and then kept open to the air to The Journal of Physical Chemistry
M. LIQUORNIK AND Y. MARCUS ensure equilibrium with the atmosphere and therefore constant weight. Apparatus. The experiments were conducted either in a high-temperature oven or in a molten-salt bath. The oven was fitted with stronger heating elements and better insulation than ordinary drying ovens have, allowing heating up to 500": Pyrex test tubes with their proper charge of zeolite and nitrate were inserted in steel tubes connected to a frame inside the oven and agitated by a shaker from outside. The temperature in the oven was controlled to within 5-7", but the variations inside the test tubes were only 1-2") owing to the equilibrating effect of the steel tubes. The molten-salt bath consisted of an aluminum container charged with equimolar NaN03-KN03 and fitted with three heating elements-two of which could be regulated manually and the third controlled by a contact thermometer. The temperature was maintained constant within 0.2-0.3". Pyrex test tubes, with their proper charge, were suspended in the bath, and mixing was effected by a motor-driven Pyrex stirrer. Equilibration Procedure. The test tube containing weighed quantities of hydrated exchanger and NaN03 was brought to the desired temperature in the oven or the molten-salt bath. This treatment causes the water in the zeolite to be replaced by occluded sodium nitrate."J,l1 After thermal and chemical equilibrium had been reached, weighed quantities of the nitrate of the counterions were added and mixed for 8-16 hr. Experiments with radioactive tracers (IYaZ2,Agllom),as well as measurements of the change of weight of the zeolite (in the case of T1+) and emf measurements (in the case of Ag+), showed that equilibrium was reached after 1-2 hr. After equilibration, the molten salt was filtered off by drawing it through a Pyrex tube fitted a t one end with a porous-glass filter. The salt was solidified by rapid cooling, in order to prevent any changes in the homogeneity of the melt, and was then ground, and an aliquot was taken for cation analysis. The zeolite phase was rinsed several times with water to dissolve away the adhering nitrate, after which it was dried in an oven a t 110" and analyzed for cations and silica content. The rinsing operation was carried out rapidly, with the aid of a suction filter, in order to avoid disturbing the equilibrium. Leaching tests showed that the ( 5 ) C. M. Callahan and M. A. Kay, J . Inorg. Nucl. Chem., 28, 233
(1966);C. M. Callahan, ibid., 28, 2743 (1966). (6) W. A. Platek and J. A. Marinsky, J . Phya. Chem., 65, 2118 (1961). (7) L. L. Ames, Am. Mineralogist, 46, 1120 (1961). (8) G. Alberti, S.Alluli, and A. Conte, J . Chromatog., 24, 148 (1966). (9) V. I. Tikavyi and N. P. Glochova, Inorg. Materials, 1, 1386 (1965). (10) M. Liquornik and Y . Marcus, Israel AEC Report IA-810 (1963). (11) M . Liquornik and Y. Marcus, Israel J. Chem., 6, 115 (1968).
IONEXCHANGE IN MOLTEN SALTS equilibrium was indeed not disturbed by this washing procedure, presumably because the occluded and solidified NaNOa effectively blocks the channels and cages of the zeolite.11 Analytical Procedure. The composition of the melt was determined from an aqueous solution of the ground solidified salt. Na+, K+, and Li+ were determined by flame spectrophotometry; T1+ was determined by oxidation to Tla+with bromine, followed by iodometric titration. In the zeolite silica was determined according to the Lawrence-Smith method by fusing with CaC03; after precipitation of the SiOz, the solution remaining was analyzed for the cations by the methods mentioned above. Because of the labor involved, the solid phase was analyzed only in the case of the exchanges involving K+, Cs+, and T1+ ions. For the Li+ and Ag+ exchange the loading of the counterion in the zeolite was calculated from the composition of the melt. From the si1ica:alkali ratio obtained, the composition was determined by assuming that the molar ratio Si02:A10zis always 1:l. The concentration of the Ag+ ions in the exchange reaction between the zeolite and (Na,Ag)N03 melts was determined in situ while the reaction was in progress by measuring the emf of the cell AgolAgN03,NaN031IAgNOa,NaN03, zeolite/ Ago.lz,la The zeolitic water was determined by heating the zeolite at 850" for a few hours; under these conditions, the structure of the zeolite collapses and the zeolitic water is released.
2887
Figure 1. The ion-exchange isotherm for the system NaA and molten solutions of (Na,Ag)NOa ( 0 )and (Na,Li)NOs (A)at 330'.
mol fraction N of KN03, CsNO, or T1NO3 in melt
Results The unit cell of sodium zeolite, Na12(AlSi04)12, taken as 1 mol of zeolite, will henceforth be called Na12A. As reported elsewhere by us," Na12A in contact with molten sodium nitrate gives an occlusion compound of the composition Naz2[(A1Si04)12(N03)~~].Not all of the 22 sodium ions of the occlusion compound are available for exchange by all of the cations investigated, but it is on this number that the calculations of the mole fractions in the zeolite are based: RM = a ~ / 2 2 . 0 , where f i is~ the number of moles of cations M per mol of zeolite. This expression was adopted because (a) it was not always possible to determine the actual ionexchange capacity of the zeolite for a given counterion and (b) comparison between different exchanges is thus facilitated. The ion exchange between the sodium zeolite and solutions of lithium, cesium, thallium, and silver nitrates in molten NaN03 were investigated a t 330". The concentration of the minor component was varied in the mole fraction range from or to about 0.2 (Figures 1 and 2). Solutions of potassium nitrate in molten sodium nitrate were investigated at 350" in the range from very dilute solutions up to pure potassium nitrate. The variation of the selectivity coefficient K with the
Figure 2. The ion-exchange isotherm for the system NaA and molten solutions of (Na,Cs)NOa (0)and (Na,Tl)NOa (A) a t 330' and (Na,K)NOa ( X ) a t 350'.
composition of the zeolite is given in Figures 3 and 4. The selectivity coefficient is defined as
The mole fraction N in the molten salt phase is defined according to Ternkin's model. Figure 3 shows that up i to 0.5,log K is constant. After the to flAgor f l ~ equal exchange of 12 sodium ions for silver ions, the selectivity coefficient increases, approaching its final value asymptotically. All the 22 sodium ions in the zeolite are exchangeable for silver ions and presumably for lithium ions, too. On the other hand, the exchange between sodium zeolite and potassium, cesium, and thallium ions is characterized by: (a) a limit to the number of sodium ions which can be exchanged by these cations-hence the observed plateau in the exchange (12).M. Liquornik and Y. Marcus, Israel AEC Report IA-968 (1964). (13) M. Liquornik and Y. Marcus, Proceedings of the 16th Meeting of the Comite International de Thermodynamique et de Cinetique Electrochimiques, London, 1963; Israel AEC Report IA-941 (1964).
Volume 7.9, Number 8 August 1968
M. LIQUORNIK AND Y. MARCUS
2888
lnji =
(mi
- 1) In K(fNaNOs/fAgNOs) K(fNaNOs/fAgNOs) d n i (3)
,
where a is the activity and f is the rational activity coefficient in the melt. The values found were Kth = 15 and AGO = -3.2 kcal/mol, at 330". These values are estimated to be correct to within about 10%. Values for the activity coefficients fNa and j~~ are presented in Table I.
,
o,5pf.l I 0
1.0
0.2 0.4 0.6 Q0 mol fraction of Ag or Li in zeolite
Figure 3. The selectivity coefficient for the system NaA and molten solutions of (Na,Ag)NOs ( 0 )and (Na,Li)NOa (A)at 330°, as a function of zeolite composition.
-
-
-
Table I: Mean Rational Activity Coefficientsin the Zeolite NAS:
0.00
0.05 0.10 0.20 0.30 0.40 0.50
0.60 0.70 0.80
0.90 1.oo
7"a 1.00 0.88 1.00 1.22 1.44 1.87 2.51 3.51 5.35 9.42 22.7
7A8
1.31 1.48 2.13 3.06 3.38 3.86 4.49 5.25 6.32 7.96 1.00
-
-
-
I
0
0.2 mol froction
R
0.4
0.6
I
I
0.0
I
of K ,Cs or TI in zeolite
Ix)
Figure 4. The selectivity coefficient for the system NaA and molten solutions of (Na,Cs)NOa (0)and (Na,Tl)NOa (A) at 330" and (Na,K)NOB ( X ) at 350°, as a function of zeolite composition.
isotherm (Figure 2) and the steep decrease in log K as the exchange proceeds (Figure 4); (b) no significant change in the number of occluded sodium nitrate molecules, when the sodium of the zeolite is exchanged for + ~ ~ at the bigger cesium or thallium ions, f i ~ remaining 20.4 A 1.1and 22.0 f 1.7, respectively. The Nu+-Ag+ Exchange. From the exchange results and the activities in the (Ag,Na)NO3 system determined by us previously, l3 the mean rational activity coefficient fi in the zeolite and the thermodynamic constant for the exchange reaction were found by graphical integration, using Ekedahl's14 expressions
The Nu+-K+ Exchange. The experiments were conducted a t 350" so that the results (Figures 2 and 4) could be compared with those for the exchange between potassium zeolite and sodium ions reported elsewhere.l5 The ion-exchange capacity for potassium ions was determined by treating a sample of occluded zeolite Nazz(A1SiO~)~~(N03)l~ with molten potassium nitrate. Not more than 11mol of K+/mol of zeolite could be introduced. The results of 25 analyses of the zeolite after equilibration showed that the number of cations per mole of zeolite remains unchanged at 22 to within f6%.
Discussion The zeolite A has the property of forming occlusion compounds with some molten nitrates. We have shownl0P1lthat the lithium, sodium, and silver zeolites readily occlude lithium, sodium, and silver nitrates, respectively, but no occluded form of the potassium zeolite has been observed. The presence or absence of an occluded form of the zeolite has a profound influence on the exchange. This is clearly seen in Figures 3 and 4, which show that the exchange reactions with the silver and lithium ions proceed quite differently from those with potassium, cesium, and thallium ions. In the latter case, the plateau in the exchange isotherm appears after a loading (14) E. Ekedahl, E. Hogfeldt, and L. G. Sillh, Acta Chem. Scand.,
4, 556 (1960).
and The Journal of Physical Chemistry
(15) M. Liquornik and Y . Marcus, to be submitted for publication.
EXCHAIVGE I N n/IOLTEN
ION
SALTS
of six or seven potassium or thallium ions and three or four cesium ions. Since no significant change in the occlusion could be detected, it is concluded that the limited exchange capacity of the zeolite for these cations is due to the steric difficulty encountered by the zeolite in accommodating the bigger cations. (The volume of Csf is 4.5 times greater than that of Na+; T1* is 3.3 times greater.) The selectivity sequence of sodium zeolite for different monovalent cations (Table 11) shows that (a) at extremely low loading the zeolite prefers the bigger and more polarizable ions to the smaller ones, with the exception of lithium, which, although smaller, is preferred t o the sodium ion and (b) there are transpositions in the selectivity series as the concentration of the counterion in the zeolite increases. The selectivity series of any exchanger is influenced by the properties of both phases, the liquid and solid. I n the systems studied here it is difficult to point to any specific properties of the liquid phase which might exert an influence. The binary solutions of the nitrates studied are, from the thermodynamic point of view, quite close to ideal solutions as defined by Temkin. Measurements of the thermodynamic excess functions (AHM, AGM) show16 that in general the values of these functions are less than 1 kcal/mol, and for low concentrations of the minor component, they are only a few tens or units of calories per mole. Moreover, there are no known phenomena of complexation or association in these binary mixtures. Table I1 : Selectivity Series as a Function of Zeolite Composition 1
- NN.
0.0 0 1 0.2
0.3 0.4
---
Series----------
Cs > TI > Ag > X > Li > Na Cs > T1 > Ag > Li = K > Na Ag > T1 > Li > Cs > K > Na Ag > Li > T1> Na > K Ag > Li > Na > K
It is generally agreed that in exchange reactions from aqueous solutions, the ions from the solution are dehydrated at least partly upon replacing the matrix cations.6*17 The selectivity series in aqueous solutions depends, then, on the ease of dehydration of the cations and on the interaction of the cation with the fixed ions in the exchanger itself. Platek and hIarinsky6 investigated the selectivity order of lithium zeolite for tracer concentrations of sodium, rubidium, and cesium ions, in 0.1-13.5 m aqueous solutions and in molten anhydrous lithium chloride. Obviously there are profound differences in the hydration of the tracer cation in the various cases, and this was indeed reflected in the variation of the magnitude of the selectivity coefficient.
2889 However, the selectivity sequence did not change, emphasizing that energy considerations inside the zeolite are the decisive factors which determine the order in the selectivity series. The preference of aluminosilicates for the bigger alkaline or alkaline earth cations observed in nature was interpreted by Pauling in terms of his electrostatic valence rule. l8 In this interpretation the negative charge resulting from the substitution of A1 for Si in the tetrahedral framework is equally distributed between the four oxygens surrounding the aluminum, so that there is in effect 1/4 charge on each oxygen. This model maj7 be used t o interpret the influence of the zeolite on the selectivity sequence. Stamireslg gave the positions of the different cations in the zeolite, albeit the nonoccluded one. The ions may be divided into two categories: (a) the small cations lithium and sodium, which are located inside the six-membered ring, and (b) cations larger than sodium, which are displaced toward the inside of the a cage where they screen the negative oxygen atoms from the occluded negative nitrate ions. The bigger the cation, the easier it will be for it to bridge the oxygens in order t o neutralize their charge and the better the screening effect, i e . , the lower the repulsion between the negative charges in the zeolite. The preference for lithium over sodium is more difficult to explain. It should be mentioned, however, that Platek and I\Iarinsky6 found that, in a lithium chloride melt at 660°, lithium zeolite showed a preference for sodium. This may be due either to the higher temperature or to the influence of the different anion of the melt. The transpositions in the selectivity series at higher loadings (Table 11) are due to the steep decrease in the selectivity for the bigger cations brought about by the steric effect mentioned before. The monovalent cations can thus be grouped into two categories: (a) those which are able to exchange all of the cations of the zeolite, including the occluded ones, and (b) those which can exchange only part of the cations of the zeolite. In the first group there is no transposition in the selectivity as the exchange proceeds, while in the second group the cations change places in the selectivity series owing t o steric hindrance.
Acknowledgments. The authors wish to thank the members of the analytical department for their help and Miss Nili Abel and Mr. L. Gefen for their technical assistance. The support of the Israel Atomic Energy Commission is gratefully acknowledged. (16) J. Lumsden, “Thermodynamics of Molten Salt Mixtures,” Academic Press, London, 1966, p 109. (17) H. S. Sherry and B. F. Walton, J . Phys. Chern., 71, 1457 (1967). (18) L. Pauling, “The Nature of the Chemical Bond,” Cornell University Press, Ithaca, N. Y . , 1960. (19) D. K.Stamires, J . Chem. Phys., 36, 3174 (1962). Volume 72, Number 8 August 1068
