IOK-EXCHANGE: SmEiCTrvIm
OF THE
SYNTHETIC ZEOLITE LINDEA
85
e Selectivity of the Synthetic, Zeolite Linde A in
. €3. Barrett*lb and J. A. Marinsky Department of Chemistry, State University of N e w Y o r k at Buffalo, Buffalo, N e w York
14214
(Received A p r i l I S , 2070)
Publacation costs borne completely by T h e J o u r n a l of Physical Chemistry
The selectivity coefficients for the ion exchange of cesium with sodium A zeolite and cesium and sodium with potassium A zeolite have been determined in nonaqueous and mixed media. The exchanging ions were a t radioactive tracer level concentrations so that the fraction of the macroion of the zeolite was essentially unity in both the zeolite and external liquid phase. The dramatic increase in ion selectivity a t very high adcoholie content of the liquid phase could only be qualitatively examined and mas attributed to the significant solva tion of the zeolite by the alcohol in the alcohol-rich media. Quantitative interpretat'ion. of the data obtained in the mixed media prior to this sharp increase in selectivity was attempted by assuming that the xeolite phase may be treated as a highly concentrated electrolyte solution having only one diffusible ion and as a crosslinked. network which exerts a pressure on the internal solution phase. The special properties of t,he zeolite, rigidity, resistance to electrolyte incursion, and high selectivity for water, were believed to permit recourse to t,his model in mixed media over an. alcohol-water weight per cent composition ranging frorn 0 to 7OYb. Correlation of experimental and calculated selectivity points was sufficiently good to support -the validity of this approa.ch.
Introduction I n recent years, several theoretical approaches to an understanding of ion-exchange phenomena in aqueous solution have met with considerable success. It seemed appropriate, then, to extend the limits of investigation to test the applicability of the models proposed for aqueous solutions in nonaqueous and mixed media. Studies for this purpose were limited to uni-univalent exchange of the aP1i:tli cations in anhydrous methanol, ethanol, and ethylene glycol as well as water-alcohol solutions. The sg nthetic zeolite I h d e A was selected as the ion exc:hangt:r because its structure and ionexchange behavior in aqueous solutions have been well characterized.
Experimental Section NaA zeolite was kindly supplied by the Linde Division of the TJnion Carbide Corp. Its preparation for use and the synthesis of the potassium form have been described elsewhere.? TQ prepare the sodium zeolite for use in nonaqueous media, it was heated to 500" under vacuum .for 1'2 hr. No further weight loss was discernible upon ignition in Pyrex tubes. It was not feasible to dehydrate KR similarly. The KA zeolite has bean found to undergo subtle irreversible structural modifications iupon prolonged heating. Instead, this form of the zeolite was exhaustively rinsed with methanol and then placed under vacuum at room temperature overnight The final product was 92 wt % matrix, 7 wt o/o methanol, and 11 wt % HzO. For experiments in which wateer was added to alcohols fully hydrated zeolites were ernployed.
To facilitate the ion-exchange selectivity studies in nonaqueous media,, Fisher Certified methanol and 200 proof ethanol from E'ubliclier Industries were distilled over Linde 5-A molecular sieves and collected under nitrogen prior to their use. Water content of the product mas below the limits of detection by Karl Fischer titration. The ethanol and methanol were usually used as received when employed in mixed media. They contained /RT of In K N and In ( I & f / I x ) at each solvent composition yield values of -2 In ( y + ~ ~ x / y k ~ in~ the x ) different mixed media. The results of this computation presented in an earlier publication16are relisted in Table VI for the convenience of the reader. The increasing deviation between the mean molal activity coefficient of K I and NaI with methanol content that is observed is reasonable. As Ihe dielectric of the solution medium decreases with larger methanol content, differences in the ion-pair formation capability of Na+ and IT+ are expected to be enhanced Kith the
+
+
(15) R. B. Barrett, J. A. Marinsky, and P. Pavelich, “The Properties of Linde A in Aqueous, Non-Aqueous and Mixed Media,” accepted for Symposium Volume “Second International Conference on Molecular Sieve Zeolites,” September 1970 t o be puhlished in
“Advances in Chemistry Series.”
ION-EXCIIANGE WELECTIVT'B~I
OF THE
SYNTHETIC ZEOLITE LINDEA
Table V I : Coniputeztion of the Mean Molal Activity Coefficie it Ratio of Simple Electrolytes in Mixed Media
0 10 20
30 40 50 60 70
1.235 1.280
1.091 1.077 1.068 I.e37 1,009 0,970 0.9L'? 0 . 842
0.142 0.203 0.286 0.348 0.431 0.530 0.632 0,768
1.344 1.385 1.440 I . 500 I . 549 1.610
more highly polarizable iodide ion to yield the above result. The mean molal activity coefficient data that are published for the alkali chlorides16 facilitate a more meaningful prognosis of the osmotic pressure model. With the availability of these data it has been possible to evaluate the -2 In ( ~ * M x / ~ * N x )term of eq 4 by assuming that distortion of this ratio by cation-cation interaction is negligible a t the low ionic strength of the experirnents. A comparison of KNB1*((oalod) and K N ~ * €or ( ~ the ~ E( ~ A-KCI-Na ~ ~ ) *C1-MeOH-HzO system as a functifon of solvent composition is presented in Table '(III for this purpose. Good agreement between experiment and prediction is obtained until the weight per cent of alcohol exceeds 50%.
--___-Table V U : Prediction of Ion-Exchange Selectivihy in Mixed Media System KA-KC1-Na *Cl-CH30H-KIO;
wt
%
- 2 In
methanol
(Y+KC1/ Y+N&I)
0 10 20 30 40 50 60 70
0,0207 0.1058 0.1270 0.1904 0.3059 0.3114 0.6261 0,9168
mKc1
KITNa*(oalod)
2.41 2.54 2.53 2.64 2.88 2.80 3.68 4.63
=
0.1
KKNa*(exptl)
2.41 rrt 0 . 1 2.69 f 0 . 1 2.743~0.1 2.85 rrt 0 . 1 2.80 f 0.1 2.74&0.1 2.53 & 0 . 1 2.47 =!c 0.1
The apparent failure of the osmotic model in the higher alcohol-containing media may be a consequence (I) of more significant entry of the alcohol into the
89
zeolite phase than i s anticipated from extrapolation of the solvent selectivity data, (2) of the change of VM and VN in these alcohol-enriched mediti, or ( 3 ) of m o r in the values of y i ~ X and/or y*ti\'x that are published For the 60 and 70% alcohol by weight m d a . h' e first possibility is considered uiilikely since the selectivity behavior of K and Na" in ethanol (where the affinity of 1320 over alcohol for [be %;colitei, much greater) parallels almost exactly the behavior in methanol. The second possibilitv i s also uiireasonable since the apparent molal volnme of KCl has been diown to be constant m e r the solvent roniptiwtion range investigatedl* in this study. The regular decrea~rein the activity coefficient of KCI. with the alcoholic content of the mixed medium under investigation :~ppenssto be a linear function of the alcoholic content tat the solvent until 50% by weight alcohol conient is e r c e e d d Beyond this composition range the inemn rxiolsl activity coefficient of KC1 seems to drop xuuch l o o rapidly v,iLh increasing alcoholic content. This irwgular behavior or Y + K C ~mag indeed he indicative of PP"I"OK in the published values in the higher alcoholic media. In any case the correlation of' RJP"(~.I~,I) a,nd ~ b ~ * ( over ~ ~ ~a ~large i ) aIcohol-m.atw composiiion range is supportive of the osrnolje rrrcdel. It is suggested that the osmotic pressure spproach may be tis useful for the estimate of thermodynamic: properties of simple electrolyte mixtures ju mixed solvents as in aqueous media. At present this model cannot be usefully emplog7ed to interpret ion selectivity in the essentially pure nonaqueous solvent. Various parameters essential to the model are not yet available in the literature. IIowever, since ion seleet,ivitg is so sensitive to the solvent content of the exchanger it appears that! the most irnportant selectivity term in eq 4 must be Lm (Tw +/iN +>, A number of speculative approachez are presently under consideration to facilitate a quantitative i of the ion selectivity of the A xeoiiie in anhydrous media.
Acknozuledgment. Financial support through Contract No. hT(30-1)-2269 with the U. S.Atomic Energy Commission is gratefully acknowledgcd. The selectivity data for the KCl-Na*C1-MeOH-€~-120system were obtained by Mrs. P. Pavelich oE this laboratory.
(le) R. Parsons, "Handbook of Electrochemical Constants," Academic Press, New York, N. Y., 1959. (17) P. Pavelich and J. A. Marinsky, unpublished data, 1970. (18) H. A. Neidig, J . Chem. Educ., 42, 309 (1985).
Th.e Journal of Physical Chemistry, Vol, 76, ATo. 1, 1971
