10s EXCHANGE O F SYNTIHETIC ZEOLITES
IN
257 5
VARIOUS ALCOHOLS
-.
Ion Exchange of Synthetic Zeolites in Various Alcohols
by P. C. Huang,' A. Mizany, and J. L. Pauley Department of Chemistry, Kansas State C,dlege of Pittsburg, Pittsburg, Kansas
(Received April 6,1064)
Selectivity coefficients have been obtained for the potassium-sodium, lithium-sodium, silver-sodium, and calcium-sodium exchanges on Linde Molecular Sieve 13-X, in water, methanol, ethanol, isopropyl alcohol, n-propyl alcohol, and isobutyl alcohol. Rate data and enthalpies of exchange have been obtained for several of these systems. Tracer techniques were used. Variation of the logarithm of the selectivity coefficients with the reciprocal of the dielectric constant was noted, but the lack of linearity of the relationship suggests specific solvent effects are also important. Rate data and enthalpy data are in general agreement with that obtained for organic exchangers in aqueous and mixed aqueous niedia in that particle diffusion is apparently rate determining, and enthalpies are of the order of 1 kcal./mole. Rates decrease with decreasing dielectric constant of the media. Solvent uptake studies showed the anticipated correlation with selectivities except for the potassium-sodium exchange.
Experimental
radioactive sodium-22 was carrier free and was used as, obtained from Suclear Engineering Corp. Exchanger. The exchanger used was Linde Molecu-, lar Sieve 13-X in l/&. pellets provided in the sodium, form. The capacity of the resin was determined by complete conversion to the silver form followed by di-. gestion and titration of the silver with potassium thio-. cyanate. Capacities per dry gram of exchanger were calculated froin the experimentally obtained value for silver. Capacities in niequiv./dry g. were 3.39, 4.77, 5.15, 4.42, and 4.83 for the silver, sodium, lithium, potassium, and calcium exchangers, respectively. Samples of the various salt f o r m of the exchanger wen: spiked with carrier-free sodium-22 (activity 300,000700,000 counts/min./g.), dried at 600' under vacuunl, and stored over PzOs until used. Equilibrium Systems. Weighed samples of approximately 1 g. of the spiked exchanger in the salt form were transferred to glass serum bottles and permitted
Materials. Reagent grade methanol and ethanol were purified by treatment with magnesium activated with iodine and were maintained essentially anhydrous. The n-propyl, isopropyl, and n-butyl alcohols were reagent grade and were used without further purification. The potassium thiocyanate, silver nitrate, calcium nitrate, and lithium chloride used were reagent grade and were dried under vacuum before use. The
(1) Taken in part from the dissertation of P. C. Huang to the Graduate School of Kansas State College of Pittsburg in partial fulfillinent of the requirements for the Master of Science degree. (2) R. G. Fessler and H. A. Strobel, J . Phys. Chem., 67, 2562 (1963). (3) T. Kressmnn and J. Kitchener, J. Chem. Soc., 1190, 1211 (1949). (4) R. Goble and H . Strobel, J . P h y s . Chem., 60, 513 (1956). (5) D. D. Bonner and J. C. hloorefield, ibid., 58, 555 (1954). (6) T. Sakaki and H. Knkihaiin. Iiagclkv, 23, 471 (1953). (7) R . LM. Bnrrer and J. S. Haitt, J . Cham. soc., 4641 (1954).
Introduction Nonaqueous solvents have received comparatively little attention as ion-exchange media until rather re~ e n t l y . ~ - ' Nonaqueous media are attractive, however, in that they offer an opportunity to observe the manner in which both dielectric constant and specific medium effects influence ionic interaction. As solvents in the present study, a series of alcohols have been investigated since they show a reasonably regular change in dielectric constant and other physical and chemical properties with chain length and provide moderate salt solubilities. An inorganic niolecular sieve type exchanger was chosen since the rigid structure prevents swelling, providing nearly constant pore size in all solvent media studied. This allows for reasonably rapid attainment of equilibria even in pure nonaqueous solvents.
Volume 68, Number 0 September, 196.4
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P. C. HUANG,A. MIZANY,AND J. L. PAULEY
Table I : Concentration Selectivity Coefficient and Solvent Uptake Data for the Exchange of Various Ions for Sodium on Linde Molecular Sieve 13-X in a Series of Alcohols* K
Solvent
Water
(D
K'~a
78.7) Methanol (D = 31.66) Ethanol (D = 23.56) n-Propyl alcohol ( D = 19.8) Isopropyl alcohol (D = 17.9) Isobutyl alcohol ( D = 17.1) =
Solv. uptakeb KMN,
Solv. uptake
K'~a Solv. uptake
K'~a Solv. uptake KM~a Solv. uptake
KM~a S o h . uptake
1.10 6.54 0.96 2.80 0.70 1.90 0.50 1.39 0.19 1.37 0.19 1.15
Li
8 . 4 x 10-2 7.80 4.2 X 3.00 3 . 9 x 10-2 2.15 1 . 5 X loM2 1.45 1.3 x 1.37 1.1 x 10-2 1.17
Sodium in tracer quantities, all salf solutions 0.05 M , temperature 30 d e n t of exchanger in M forms.
to equilibrate with the pure solvent for 2 hr. Twentyfive milliliters of a 0.05 M alcohol solution of the appropriate salt corresponding to the exchanger cation was then added and the mixture was magnetically stirred a t constant temperature until equilibrium was attained. Serum caps for the bottles were used to exclude moisture and to permit withdrawal of samples for analysis. For certain exchange systems, selectivity coefficients were determined at both 30 and 60' to permit an estimate of heats of reaction. In all other cases the temperature was maintained a t 30 f 0.02'. Analytical Procedures. When equilibrium had been attained, as shown by constant activity of tracer in the exchange solution, samples of the solution phase were removed and activities determined using a scintillation counter. A total activity balance was run in several instances to determine that there were no losses of activity due to adsorption. From the activity of the solution phase, the concentration of exchanging salt in the solution phase, initial activity of the exchanger, and its capacity, distribution coefficients and equilibrium concentration selectivity coefficients were obtained. Selectivities. Selectivities calculated corresponded in each case to the exchange (l/u)M'+ Na*R = ( l / u ) YIR Na*+. Selectivity coefficients were calculated according to
+
+
K M ~=*
(counts/min./g. NA*R) (mMut)l'' (mequiv./g. MR) ""(counts/min./ml. Na* +)
The molarity of the metal salt solution and the capacity of the resin were assumed to remain constant. The activity of the resin phase was the original value corrected for the activity in the equilibrium solution phase. The Journal of Physical Chemistry
j=
0.02".
Exchanging Species, M------Ca
Ag
0.44 7.17 4 . 5 x 10-3 2.19 1 . 1 x 10-3 1.98
45.9 4.14 18.1 1.76 6.34 1.18
,..
...
...
...
Na
... 6.97
... 2.86
...
2.04
...
...
* Solvent uptake in terms of moles of solvent/equiv-
Rate Determinations. To determine rates of reaction the same procedure was used as for equilibrium studies except that samples were withdrawn periodically, centrifuged, and the aliquots counted to determine tracer activity in the solution phase. Solvent Uptake Measurements. A weighed amount of the exchanger in the appropriate salt form was allowed to equilibrate with each solvent studied, was filtered, and was dried superficially by blotting. The blotted resin was then weighed and dried a t 600' to constant weight. The solvent uptake was then computed from the differences in weight and reported as moles of solvent per equivalent of resin.
Results Equilibrium results for the several exchanges are shown in Table I. The results confirm the dependence of the logarithm of the selectivity coefficients on the reciprocal of the dielectric constant of the exchange med i u m . * ~The ~ relationship is only roughly linear, suggesting that specific solvent effects may play a significant part in selectivity. Values of selectivity coefficient compare favorably with those obtained for other inorganic exchangers.' The reversal of order of selectivity for the potassium-sodium exchange is unusual but similar effects have been noted by Materova and coworkersQ for the ammonium-potassium exchange in acetone-water systems. This reversal could be accounted for by changes in activity coefficient. Relative activity coefficients of sodium and potassium show a (8) A. T. Davydov and R. F. Skolbionok, Zh. Fiz. K h i m . , 32, 1703 (1958). (9) .A. Materova, Zh. L. Verts, and G. P. Grinberg, Z h . Obshch. Khzm., 24, 953 (1954).
2577
IONEXCHANGE OF SYNTHETIC ZEOLITES IN VARIOUS ALCOHOLS
reversal of relative magnitude in going from water to methanol according to the calculations of Oiwa.'O Materova has accounted fur the ammonium-potassium reversal of selectivity in acetone-water mixtures on this basis using activity coefficients calculated by the method of Vinogradov." The lithium-sodium, calcium-sodium, and silversodium exchanges showed no corresponding reversal of selectivity with solvent change and selectivities varied in general as anticipated with changes in dielectric constant. There is rather a large change in selectivity coefficient for the calcium-sodium exchange in going from water to methanol which may well be a reflection of the rather large difference in solvent uptake in these two media. The reversal of order of selectivity for the calcium-sodium exchange in all solvents for this exchanger compared to the organic exchangers presumably reflects differences in exchanger structure. The molecular sieve exchangers, having a rigid structure, underwent no observable volume change witlh change in solvent but, as shown in Table I, the solvent uptake did show a significant dependence on the nature of the solvent as well as the ionic form of the exchanger. With the exception of the potassium-sodium exchange, selectivity coefficients correlated in the anticipated manner with Lhe amount of solvent uptake, the exchanger Favoring the less solvated ion. Due to the ditferences in charge type, no such simple correlation could be anticipated for the calcium-sodium exchange. Exchange enthalpies for the potassium-sodium SYEItem were calculated from exchange data at 30 and 60' using the simple integrated form of the ClausiusClapeyron equation. The values are shown in Table 11. These calculated enthalpies are in agreement with the observed values of 1-2 kcal. generally reported for other exchange system^.^^'^ Again, a reversal of seleativities for this exchange in water was noted with increased temperature. This reversal may reflect changes in ion solvation or activity with temperature, analogous to those occurring in going from water to a less polar solvent. The kinetics of the potassium-sodium and silversodium system in water, methanol, and ethanol, anld the calcium-sodium system in water were investigated. Plots of Bt us. t were made for each of these exchangers from tables presented by Reichenbergi3 following the derivation given by Boyd, Adamson, and Rllyers.14 In all cases linear relationships were attained up to SO-SO% completion. The derivation of the Bt function used is based on different boundary conditions than in the present investigation; but, following the derivation of Barrer, the final expression is appropriate, presuming that the rate of diffusion of the ion replacing sodium is
Table I1 : Concentration Selectivity Coefficients for the Potassium-Sodiurn Exchange as a Function of Temperature and Solvent, and Calculated Enthalpies of Exchange Water, KK~,
Temp. 30' Temp. 60' AH%&, cal.
1.10 0.96 -993
Solvent Methanol,
Ethanol,
KKNa
K'IN,
0.96 0.75 -1614
0.70 0.61 -859
rate determining. In the early stages the rate follows fairly well the square root dependence predicted for l 4 The spherical diffusion into a semiinfinite case appears to represent results better, however. Mass action equations derived for the particular boundary conditions were also tested and found to be unsuitable, from which it was concluded that particle diffusion is the more probable rate-controlling process, as would be anticipated. I n order to give an idea of reaction times involved the times required for 50% and 75% attainment of equilibrium are shown in Table 111.
Table 111 : Kinetic Data for the Exchange of Various Metal Ions for Sodium in Water, Methanol, and Ethanol Solutions a t 30' Metal ion
K+ K+ Kf Ag Ag + Ag + Ca2f +
tQ.10,
t0.71~
Solvent
min.
rnin.
Water Methanol Ethanol Water Methanol Ethanol Water
18 19 48 40 120 295 16
55 60 142a 120 330 85Om
49
Extrapolated data, both curves show positive deviation from the linear plot above about 60% attainment of equilibrium.
Both the potassium-sodium and silver-sodium exchange rates decreased with decreasing dielectric constant of the solvent. This decrease may be due to tlhe decrease in ionic mobility for potassium and silver ions (10) L. T. Oiwa, Sei. Rept. Tohoku Univ., First Ser., 41, 129 (1957). (11) G. 1 ' . Vinogrxdov, Progr. Chem., 8, 378 (1939). (12) G. E. Boyd, J. Schubert, and A. W. Adamson, J . A m . Chem. SOC.,69, 2818 (1947). (13) D. J. Riechenberg, ibid., 7 5 , 589 (1953). (14) G. E. Boyd, A. W. Adnmson, and L. S. Myers, Jr., ibid., 69, 2836 (1947). (15) R. M.Barrer, "Diffusion in and Through Solids," Cambridge University Press, London, 1941, p. 129.
Volume 68, Number 9
September; 1964
2578
R. A. HORNE,R. A. COURANT, B. R. MYERS,AND J. H. B. GEORGE
in going from water to methanol to ethanol,16 to the decrease in fractional pore volume of the exchanger due to the decreased solvent uptake in going from water to methanol to ethanol as discussed by Wheeler, Mackie, and others,17or to a combination of both factors as seems most probable.
Conclusion Molecular sieve type ion exchangers can be used effectively for exchange processes in conipletely nonaqueous media. Equilibrium times, although longer than for similar processes in water, are within usable
limits and certainly less than for the usual organic exchangers. Selectivity coefficients from this study indicate that in considering the effect of the solvent it is necessary to include specific solvent effects as well as changes in dielectric constant, presumably reflecting changes in ion-solvent as well as ion-ion interactions as the solvent media are changed. Further work is planned to investigate these specific solvent effects. (16) M. Barak and H. Hartley, z. Physik. Chem., 165, 290 (1933). (17) F. Helfferich, “Ion Exchange,” McGraw-Hill Book Co., New York, N. Y . , 1962, pp. 302 ff.
Ion-Exchange Equilibria at High Hydrostatic Pressures. The Hydrogen Ion-Potassium Ion and Hydrogen Ion-Strontium Ion Systems on Sulfonic Acid-Type Cation-Exchange Resins
by R. A. Horne, R. A. Courant, B. R. Myers, and J. H. B. George Arthur D. Little, Inc., Cambridge, Massachusetts
(Reeeited April 6 , 1964)
The application of hydrostatic pressure does not affect the K+-H+ exchange on a sulfonic acid-type cation-exchange resin but does affect the Sr2+-H+ exchange. As the pressure increases, the resin’s affinity for Sr2+ increases. The effect becomes more pronounced with increasing cross linking and is attributed to the ability of pressure to reduce the size of the hydration atmospheres of ions in solution, thereby increasing the charge density of the hydrated ions. The effect is relatively greater in the case of more heavily hydrated ions and is thus of greater importance in the case of Sr2+than of E(+ or H+.
Introduction The pressure dependence of ion-exchange equilibria is given by the relation1 (1)
RT where K is the thermodynamic equilibrium constant and AVO is the standard volume change of the total system, that is to say, the combined volume change of T
The Journal of Physical Chemistry
resin and solution. Although the specific voluine of the resin itself depends on the nature of the absorbed cation,Zthere is very little volume change of the total systen1 during ion exchange, and on this basis Helfferich’ has predicted that pressure should have little, if any,
(1) F. Helfferich, “Ion Exchange,” McGraw-Hill Book Co., IIIC., New York, N. Y., 1962, pp. 167-168. (2) H. P. Gregor, J . Am. Chem. Soc., 70, 1293 (1948).
