Ion Exchange in Mordenite BY RICHARD M. BARRER" AND JACEK KLINOWSKI Physical Chemistry Laboratories, Chemistry Department, Imperial College, London SW7 2AY Received 9th May, 1974 Ion-exchange isotherms have been measured for synthetic mordenite involving the cation pairs Na+ +Cs+, NH: +K+, NH; +Na+, NH: +Li+, 2NH: +Ca2+, 2NH: S r 2 + and 2NH: +Ba2+. In a11 cases except those involving Ca2+ and Sr2+ reversible exchange was effected. The values of the thermodynamic equilibrium constant, Ka, and the standard free energy of exchange, AGe, were calculated for the reversible pairs. For additional combinations of the reversible ionic pairs, Ka and A G e were obtained using the triangle rule. The thermodynamic affinity sequence is Cs+ > K+ > NH; > Na+ > Ba2+ > Li+. The thermodynamic affinity sequence among monovalent alkali metal ions established in this work for mordenite was found to be repeated for other zeolites, provided exchanges which do not go to completion are normalised by considering the equilibrium only in terms of the exchangeable fraction of the ions.Despite the interest in synthetic mordenite, its ready availability and its siliceous, acid-stable nature there have been few measurements of cation exchange in this mineral,'-9 and of these studies only one (using a natural mordenite) presented any isotherm^.^ Measurements were therefore made of exchange isotherms in mordenite involving alkali and alkaline-earth metal cations and the NH: cation. The dimensions of the orthorhombic unit cell of mordenite are lo a = 18.13 A, b = 20.49 and c = 7.52 A, and the idealised unit cell content is Na8[A18Si40096]* 24H20.The main channels in the structure are parallel with (001) and have free dimensions of about 6.7 x 7.0 A. They are lined on both sides by side pockets having entrances of -3.9 A free diameter and are directed along (010). The pockets lining adjacent channels are displaced by $c with respect to each other. A pocket in one channel is linked to each of two pockets belonging to an adjacent channel by shared, distorted, 8-ring windows of free dimension -2.8 A. A sodium ion located in each of these $-rings accounts for half the exchangeable cations in the zeolite. EXPERIMENTAL Synthetic sodium mordenite (sodium Zeolon) was supplied by the Norton Co. The crystals were washed several times with distilled water and were stored over saturated calcium nitrate (relative humidity -56 %).Analysis by standard procedures gave the unit cell composition Thus the ratio Si02/A1203 was 10.53. ment with concentrated NH4CI. The unit cell composition of the product was Na7. 5 6fA17. 6 7si40. 3 6FeO. 0 2 7 0 9 61 *23*6H20* NH4-mordenite was prepared in quantity from some of the Na-form by prolonged treat- (NH4)7.4 6[A17. 67si40.3 6FeO. 0 2 7 0 9 61-l 9-5H20. Either the Na- or the NH,-form, as appropriate, was then used to determine the exchange isotherms of Na++Cs+, NH1; +K+, NH t +Na+, NH 1; $Li+, 2NH +Ca2+, 2NH 1; +Sr2+ and 2NHz+Ba2+. Equilibria were normally measured at 25°C with constant solution 2362R. M . BARRER AND J . KLINOWSKI 2363 concentrations of 0.05 equiv.dm-". Weighed amounts of the homoionic zeolite were equilibrated in 60 cm3 polypropylene bottles with solutions containing known proportions of the competing ions. For each point the bottles were rotated for 10 days for uni-univalent exchanges and 21 days for uni-divalent exchanges. The equilibrium compositions of the supernatant solutions were determined for the metal ions by atomic absorption or emission spectrophotometry and the NHZ by the Kjeldahl method. This was also used to find the NHZ content of the crystals. ANALYSIS OF ISOTHERMS The exchange reaction for ions A Z ~ + and B z ~ + is ZA and ZB are the valencies of the ions of A and B and the subscripts s and c refer to solution and crystalline zeolite respectively. The modified selectivity (or Kielland) quotient is then given by where A , and B, are the equivalent cation fractions of A and B in the zeolite, rnt and rn: are the molalities of A and B in solution and YA and YB are the activity coefficients of ions A and B in the mixed solution, Their ratio, r, was evaluated from the formula (AX) ZA(ZB + Zx)/Zx (BX) ZB(ZA + Zx)/Zx = yiA/y? = [YBX 1 ICYAX 1 (3) using the values of the mean activity coefficients of salts AX and BX in the mixed solution, y(Z5) and y(&, evaluated by the method introduced by Glueckauf.l 1 In eqn (3) Z, is the valence of the common anion X. The plots of log& against A, were then fitted by an appropriate polynomial in A,, using a least squares method. The activity coefficients, f, of individual cations and the thermodynamic equilibrium constant, K,, were then obtained by integration using the expressions : log fp = 0.4343(2, - ZA)B, - B, log K, + (4) log K , = 0.4343(Z,-ZA)+ log K , dA,.s: Linear, quadratic and cubic equations of the form logK, = C*+C1A,+C,A,2+. . . (7) where Co, C1 and C2 are coefficients, were each used to fit the experimental curves. The error of fit in each case is reflected in an R-factor already defined.13 The polynomial giving the lowest R was chosen, Although a higher order polynomial will fit a given number of points better than a lower order one, it does not follow that R is lower for the higher order polynomial because the order of the polynomial is one of the factors influencing R. The best fit equations were quadratic or cubic.TRIANGLE RULE I N GENERAL FORM From the values of K, determined as above [eqn (6)j values of K, may be derived for exchange equilibria additional to those actually measured. Let K:9A, KtPC and K:*A denote the thermodynamic equilibrium constants and AGgA, AGEc and AGgA the corresponding standard free energies per equivalent of reaction for reaction (1) and for the respective reactions Z,CF + + Z,BfB + + Z , BFB + + Z,C,Zc +2364 ION EXCHANGE IN MORDENITE and ZCAfA+ 4- z,c,Zc+ + z,cfc+ 4- ZCA:A+. Then per equivalent of reaction (l), (8) and (9) give AGgA = AGgA-AGgc where with similar relations for AGEA and AG&. Thus from eqn (10) and (11) one obtains (12) K ~ , A = (K~,A)ZC/ZBI(K~,C)ZA!ZB as the general form of the triangle rule. If Ka for any two of the equilibria in eqn (l), (8) and (9) has been measured then the third can be evaluated from eqn (12).RESULTS AND DISCUSSION The experimental isotherms are shown in fig. 1 and 2, and were analysed according to the procedures outlined above. 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 equivalent fraction of entering ion in crystal FIG. 1 .-Ion-exchange isotherms in mordenite. (a) Na+ +NH; ; (b) NH; +Li+ ; (c) NHf +K+ ; (a) Na++Cs+. In all isotherms open circles denote forward isotherm points and filled circles denote reverse isotherm points, all at 25°C and total solution concentration 0.05 equiv. dm-3. In (d) the iso- therm obtained for a natural mordenite is shown as the dashed line. EQUILIBRIUM CONSTANTS AND STANDARD FREE ENERGIES OF EXCHANGE The values of K, and AGe are given in table 1.The experimentally measured equilibria in the table are marked with asterisks. The thermodynamic affinity sequence is Csf > K+ > NHB > Na+ > Ba2+ > Lif.R . M. BARRER A N D J . KLINOWSKI 2365 equivalent fraction of entering ion in crystal FIG. 2.-Ion-exchange isotherms in mordenite. (a) 2NHi +Ca2+ ; (6) 2NHi +Sr2+ ; (c) 2NH: + Ba2+. Open circles denote forward isotherm points and the filled circle in (c) denotes a reverse isotherm point at 25°C and total solution concentration 0.05 equiv. dm-3. In addition in (a) A refers to exchange at 80°C and total solution concentration 0.05 equiv. dm-3, Cl refers to exchange at 80°C and total solution concentration 0.5 equiv. dm-3. TABLE 1 .-THERMODYNAMIC EQUILIBRIUM CONSTANTS AND STANDARD FREE ENERGIES FOR EXCHANGES AT 25°C exchan ye K, AG*/kJ equiv.-' Na++ Cs+ 27.2* Na++K+ 7.28 Na+-+NHz 4.56* 2Na+--+Ba2+ 0.334 Na++Li+ 0.0539 Kf-+ Cs+ 3.74 K+-+NHi 0.627* K++ Li+ 0.007 40 2K+-+Ba2+ 0.006 29 Li++ Csf 505 2Li+--+Ba2+ 1 1 5 Li+-+NHi 84.8* NHi+Cs+ 5.97 2NHi-+Ba2+ 0.01 60" -8.19* - 3.76* 4.21 7.24 -4.92 3.27 1.1 6* 12.2 6.28 - 4.43 5.12* 2Cs+-+Ba2+ 0.000 440 9.58 * Ka and A G e measured experimentally.Other values are derived from the triangle rule. AFFINITY SEQUENCES I N ZEOLITES As the results in table 2 demonstrate, the thermodynamic affinity sequence among monovalent alkali metal ions established in this work for mordenite is repeated for some other zeolites, provided exchanges which do not go to completion are normal- ised l4 by considering the equilibrium only in terms of the fraction of the ions which is exchangeable. The larger alkali metal ions always show the greatest affinity and the smallest (Li) always shows the least affinity for the exchanger.There are a few irregularities in the table, but the trend is very clear. The zeolites considered are all so open that ion sieve effects of the kind that have been reported for analcite 2 5 or sodalite 26 are absent. The regularity of the results does not conform with the diverse2366 ION EXChANGB I N MORbENITE affinity sequences which Eisenman’s theory predicts,27 but does conform to the rule that smaller, more energetically solvated cations concentrate preferentially over larger, less energetically solvated ions in the aqueous solution, while within zeolite crystals of diverse structures the opposite is the case.TABLE 2.-THERMODYNAMIC AFFINITY SEQUENCES IN SOME ZEOLITES zeolite reference sequence mordeni te pliillipsite Na-P Na-X Na-X Na-Y zeolite A cha bazi te zeolite K-GI hectori te* this work 15 16 17 18 18, 19 20 21,22 23 24 Cs > K > Na > Li Cs > K > Rb > Na > Li Cs, Rb, K > Na > Li Rb > Cs > K > Na > Li Cs > Rb > Na > K > Li Cs > Rb 3 K > Na > Li Cs > Na > Rb > K > Li Cs > K > Rb > Na > Li Cs > K > Na > Li Cs > Rb > K > Na * For the clay mineral hectorite a mass action rather than the thermodynamic selectivity is given, based on shapes of forward isotherms only. Those exchanges in table 2 which do not go to completion have in some instances already been normalised by the various authors.Those for Na-X in ref. (1 8) have been normalised by us for the experimental exchange limits for Na+-+Rb+ of 65 % and for Na+-+Cs+ of 72 %. In ref. (18) the exchange limit for Na-X had been assumed for both Rb and Cs to be 82 %. Normalisation in ref. (20) was made for exchange limits Na++Rb+ of 76 % and for Na+--+Cs+ of 63 %. Originally these experimental ex- change limits had been disregarded when calculating K, and AG*, and as a result the affinity sequence in ref. (20) was Na > K > Rb > Li > Cs. Thus the regularity of the affinity sequences appears only after normalisation. Homoionic Ca-, Sr- and Ba-mordenites were not prepared. The largest equiva- lent cation fractions actually reached were 0.60, 0.62 and 0.84 for Ca2+, Sr2+ and Ba2+ respectively (compared with 0.72, 0.49 and 0.73 reported earlier ’).At least in part this behaviour is thought to be associated with the strong hydration of alkaline-earth metal ions, which makes it difficult for such ions to enter the side pockets in mordenite (see Introduction). This difficulty of access cannot be explained in terms of the radii of the anhydrous cations because Cs+, a much larger cation, readily replaces sodium [(fig. 1 (d)]. Because of uncertainty over the limiting extents of exchange [fig. 2(a) and (b)] the equilibria 2NH: +Ca2+ and 2NHi +Sr2+ were not submitted to thermodyn- amic analysis. However, for 2NHi +Ba2+ the form of the isotherm clearly suggests that full exchange could be effected [fig.2(c)] and this exchange was analysed accord- ingly. The exchange 2NHi +Ca2+ was also studied using more concentrated solutions. It was found that the selectivity at 80°C decreased when the concentration of the electrolyte was raised from 0.05 to 0.5 equiv. dm-3 [fig. 2(a)]. This behaviour agrees qualitatively with an earlier study 28 and with the quantitative treatment 29 showing how the selectivity for the ion of higher valence increases with dilution of the aqueous solution. However, this increase is less than expected, which is a further indication that the low-concentration isotherm has not reached equilibrium. Also, for a given electrolyte concentration the isotherm for exchange of NH; by Ca2+ was, as found in earlier equilibrium studies on various zeolites,26 little changed when the temperature was altered from 25 to 80°C. Other notable features are the very low selectivity forR .M. BARRER AND J. KLINOWSKI 2367 Li+[fig. 1(6)] and the extremely good selectivity for Cs+ [fig. l(d)]. The selectivity based on the mass action quotient for Na-i-+Cs+ is greater in the synthetic mordenite than is that recorded by Rao and Rees for the natural mineral from Nova Scotia [fig. Wl. R. M. Barrer, J. Chem. SOC., 1948, 2158. L. L. Ames Jr., Amer. Mineral., 1961, 46, 11 20. H. Takahashi and Y . Nishimura, Seisan-Kenkyu, 1968, 20,466. D. B. Hawkins, Materials Res. Bull., 1967, 2, 1021. G. Lenzi and A. Pozzuoli, Rend. Accad. Sci. Fis. Mat. Naples, 1969, 36, 235. V. A. Chumakov, V. I. Gorshkov, A.M. Tolmachev and V. A. Fedorov, Vestnik Moskov Uniu., 1969, 24, 22. N. F. Erinolenko, L. N. Malashevich, S. A. Levina and A. A. Prokopovich, Vestsi Akad. Nawk Belarus. S.S.R., Ser. Khim. Navuk, 1968, 5 , 18. L. V. C. Rees and A. Rao, Trans. Faraday SOC., 1966, 62, 2103. A. Rao and L. V. C . Rees, Trans. Faraday SOC., 1966, 62,2505. l o W. M. Meier, 2. Krist., 1961, 115, 439. E. Glueckauf, Nature, 1949, 163,414. l2 G. L. Gaines and H. C . Thomas, J. Chem. Phys., 1953, 21(4), 714. l 3 B. M. Munday, Ph.D. Thesis (University of London, 1967). l4 R. M. Barrer, J. Klinowski and H. S . Sherry, J.C.S. Faraday II, 1973, 69, 1669. l 5 R. M. Barrer and B. M. Munday, J. Chern. SOC. A, 1971, 2904. l 6 R. M. Barrer and B. M. Munday, J. Chem. SOC. A, 1971, 2909. l7 R. M. Barrer, L.V. C. Rees and M. Shamsuzzoha, J. Inorg. Nuclear Chem., 1966, 28, 629. l 9 R. M. Barrer, J. A. Davies and L. V. C. Rees, J. Inorg. Nuclear Chem., 1968, 30, 3333. 2o R. M. Barrer, L. V. C . Rees and D. J. Ward, Proc. Roy. SOC. A, 1963, 273, 180. 21 R. M. Barrer and D. C. Sammon, J. Chem. Soc., 1955, 2838. 2 2 R. M. Barrer, J. A. Davies and L. V. C. Rees, J . Inorg. Nuclear Cheni., 1969, 31, 219. 2 3 R. M. Barrer and J. Klinowski, J.C.S. Faraday 1, 1972, 68, 1956. 24 R. M. Barrer and D. L. Jones, J. Chem. SOC. A, 1971, 503. 2 5 R. M. Barrer and D. C. Sammon. J. C12ein. Suc., 1956, 675. 26 R. M. Barrer and J. D. Falconer, Proc. Roy. Sor. A, 1956. 235, 227. ’’ G. Eisenman, Biuphys. J., 1962, 2(2) Suppl., 259. ’’ H. C. Subba Rao and M. M. David, A.I. Cheni. Eng.J., 1957, 3, 187. 29 R. M. Barrsr and J. Klinowski, J.C.S. Faraday I, 1974, 70, 2080. H. S. Sherry, J. Phys. Chem., 1966,70, 1158. Ion Exchange in Mordenite BY RICHARD M. BARRER" AND JACEK KLINOWSKI Physical Chemistry Laboratories, Chemistry Department, Imperial College, London SW7 2AY Received 9th May, 1974 Ion-exchange isotherms have been measured for synthetic mordenite involving the cation pairs Na+ +Cs+, NH: +K+, NH; +Na+, NH: +Li+, 2NH: +Ca2+, 2NH: S r 2 + and 2NH: +Ba2+. In a11 cases except those involving Ca2+ and Sr2+ reversible exchange was effected. The values of the thermodynamic equilibrium constant, Ka, and the standard free energy of exchange, AGe, were calculated for the reversible pairs. For additional combinations of the reversible ionic pairs, Ka and A G e were obtained using the triangle rule.The thermodynamic affinity sequence is Cs+ > K+ > NH; > Na+ > Ba2+ > Li+. The thermodynamic affinity sequence among monovalent alkali metal ions established in this work for mordenite was found to be repeated for other zeolites, provided exchanges which do not go to completion are normalised by considering the equilibrium only in terms of the exchangeable fraction of the ions. Despite the interest in synthetic mordenite, its ready availability and its siliceous, acid-stable nature there have been few measurements of cation exchange in this mineral,'-9 and of these studies only one (using a natural mordenite) presented any isotherm^.^ Measurements were therefore made of exchange isotherms in mordenite involving alkali and alkaline-earth metal cations and the NH: cation.The dimensions of the orthorhombic unit cell of mordenite are lo a = 18.13 A, b = 20.49 and c = 7.52 A, and the idealised unit cell content is Na8[A18Si40096]* 24H20. The main channels in the structure are parallel with (001) and have free dimensions of about 6.7 x 7.0 A. They are lined on both sides by side pockets having entrances of -3.9 A free diameter and are directed along (010). The pockets lining adjacent channels are displaced by $c with respect to each other. A pocket in one channel is linked to each of two pockets belonging to an adjacent channel by shared, distorted, 8-ring windows of free dimension -2.8 A. A sodium ion located in each of these $-rings accounts for half the exchangeable cations in the zeolite.EXPERIMENTAL Synthetic sodium mordenite (sodium Zeolon) was supplied by the Norton Co. The crystals were washed several times with distilled water and were stored over saturated calcium nitrate (relative humidity -56 %). Analysis by standard procedures gave the unit cell composition Thus the ratio Si02/A1203 was 10.53. ment with concentrated NH4CI. The unit cell composition of the product was Na7. 5 6fA17. 6 7si40. 3 6FeO. 0 2 7 0 9 61 *23*6H20* NH4-mordenite was prepared in quantity from some of the Na-form by prolonged treat- (NH4)7.4 6[A17. 67si40.3 6FeO. 0 2 7 0 9 61-l 9-5H20. Either the Na- or the NH,-form, as appropriate, was then used to determine the exchange isotherms of Na++Cs+, NH1; +K+, NH t +Na+, NH 1; $Li+, 2NH +Ca2+, 2NH 1; +Sr2+ and 2NHz+Ba2+.Equilibria were normally measured at 25°C with constant solution 2362R. M . BARRER AND J . KLINOWSKI 2363 concentrations of 0.05 equiv. dm-". Weighed amounts of the homoionic zeolite were equilibrated in 60 cm3 polypropylene bottles with solutions containing known proportions of the competing ions. For each point the bottles were rotated for 10 days for uni-univalent exchanges and 21 days for uni-divalent exchanges. The equilibrium compositions of the supernatant solutions were determined for the metal ions by atomic absorption or emission spectrophotometry and the NHZ by the Kjeldahl method. This was also used to find the NHZ content of the crystals. ANALYSIS OF ISOTHERMS The exchange reaction for ions A Z ~ + and B z ~ + is ZA and ZB are the valencies of the ions of A and B and the subscripts s and c refer to solution and crystalline zeolite respectively.The modified selectivity (or Kielland) quotient is then given by where A , and B, are the equivalent cation fractions of A and B in the zeolite, rnt and rn: are the molalities of A and B in solution and YA and YB are the activity coefficients of ions A and B in the mixed solution, Their ratio, r, was evaluated from the formula (AX) ZA(ZB + Zx)/Zx (BX) ZB(ZA + Zx)/Zx = yiA/y? = [YBX 1 ICYAX 1 (3) using the values of the mean activity coefficients of salts AX and BX in the mixed solution, y(Z5) and y(&, evaluated by the method introduced by Glueckauf. l 1 In eqn (3) Z, is the valence of the common anion X. The plots of log& against A, were then fitted by an appropriate polynomial in A,, using a least squares method.The activity coefficients, f, of individual cations and the thermodynamic equilibrium constant, K,, were then obtained by integration using the expressions : log fp = 0.4343(2, - ZA)B, - B, log K, + (4) log K , = 0.4343(Z,-ZA)+ log K , dA,. s: Linear, quadratic and cubic equations of the form logK, = C*+C1A,+C,A,2+. . . (7) where Co, C1 and C2 are coefficients, were each used to fit the experimental curves. The error of fit in each case is reflected in an R-factor already defined.13 The polynomial giving the lowest R was chosen, Although a higher order polynomial will fit a given number of points better than a lower order one, it does not follow that R is lower for the higher order polynomial because the order of the polynomial is one of the factors influencing R.The best fit equations were quadratic or cubic. TRIANGLE RULE I N GENERAL FORM From the values of K, determined as above [eqn (6)j values of K, may be derived for exchange equilibria additional to those actually measured. Let K:9A, KtPC and K:*A denote the thermodynamic equilibrium constants and AGgA, AGEc and AGgA the corresponding standard free energies per equivalent of reaction for reaction (1) and for the respective reactions Z,CF + + Z,BfB + + Z , BFB + + Z,C,Zc +2364 ION EXCHANGE IN MORDENITE and ZCAfA+ 4- z,c,Zc+ + z,cfc+ 4- ZCA:A+. Then per equivalent of reaction (l), (8) and (9) give AGgA = AGgA-AGgc where with similar relations for AGEA and AG&.Thus from eqn (10) and (11) one obtains (12) K ~ , A = (K~,A)ZC/ZBI(K~,C)ZA!ZB as the general form of the triangle rule. If Ka for any two of the equilibria in eqn (l), (8) and (9) has been measured then the third can be evaluated from eqn (12). RESULTS AND DISCUSSION The experimental isotherms are shown in fig. 1 and 2, and were analysed according to the procedures outlined above. 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 equivalent fraction of entering ion in crystal FIG. 1 .-Ion-exchange isotherms in mordenite. (a) Na+ +NH; ; (b) NH; +Li+ ; (c) NHf +K+ ; (a) Na++Cs+. In all isotherms open circles denote forward isotherm points and filled circles denote reverse isotherm points, all at 25°C and total solution concentration 0.05 equiv. dm-3. In (d) the iso- therm obtained for a natural mordenite is shown as the dashed line.EQUILIBRIUM CONSTANTS AND STANDARD FREE ENERGIES OF EXCHANGE The values of K, and AGe are given in table 1. The experimentally measured equilibria in the table are marked with asterisks. The thermodynamic affinity sequence is Csf > K+ > NHB > Na+ > Ba2+ > Lif.R . M. BARRER A N D J . KLINOWSKI 2365 equivalent fraction of entering ion in crystal FIG. 2.-Ion-exchange isotherms in mordenite. (a) 2NHi +Ca2+ ; (6) 2NHi +Sr2+ ; (c) 2NH: + Ba2+. Open circles denote forward isotherm points and the filled circle in (c) denotes a reverse isotherm point at 25°C and total solution concentration 0.05 equiv. dm-3. In addition in (a) A refers to exchange at 80°C and total solution concentration 0.05 equiv.dm-3, Cl refers to exchange at 80°C and total solution concentration 0.5 equiv. dm-3. TABLE 1 .-THERMODYNAMIC EQUILIBRIUM CONSTANTS AND STANDARD FREE ENERGIES FOR EXCHANGES AT 25°C exchan ye K, AG*/kJ equiv.-' Na++ Cs+ 27.2* Na++K+ 7.28 Na+-+NHz 4.56* 2Na+--+Ba2+ 0.334 Na++Li+ 0.0539 Kf-+ Cs+ 3.74 K+-+NHi 0.627* K++ Li+ 0.007 40 2K+-+Ba2+ 0.006 29 Li++ Csf 505 2Li+--+Ba2+ 1 1 5 Li+-+NHi 84.8* NHi+Cs+ 5.97 2NHi-+Ba2+ 0.01 60" -8.19* - 3.76* 4.21 7.24 -4.92 3.27 1.1 6* 12.2 6.28 - 4.43 5.12* 2Cs+-+Ba2+ 0.000 440 9.58 * Ka and A G e measured experimentally. Other values are derived from the triangle rule. AFFINITY SEQUENCES I N ZEOLITES As the results in table 2 demonstrate, the thermodynamic affinity sequence among monovalent alkali metal ions established in this work for mordenite is repeated for some other zeolites, provided exchanges which do not go to completion are normal- ised l4 by considering the equilibrium only in terms of the fraction of the ions which is exchangeable.The larger alkali metal ions always show the greatest affinity and the smallest (Li) always shows the least affinity for the exchanger. There are a few irregularities in the table, but the trend is very clear. The zeolites considered are all so open that ion sieve effects of the kind that have been reported for analcite 2 5 or sodalite 26 are absent. The regularity of the results does not conform with the diverse2366 ION EXChANGB I N MORbENITE affinity sequences which Eisenman’s theory predicts,27 but does conform to the rule that smaller, more energetically solvated cations concentrate preferentially over larger, less energetically solvated ions in the aqueous solution, while within zeolite crystals of diverse structures the opposite is the case.TABLE 2.-THERMODYNAMIC AFFINITY SEQUENCES IN SOME ZEOLITES zeolite reference sequence mordeni te pliillipsite Na-P Na-X Na-X Na-Y zeolite A cha bazi te zeolite K-GI hectori te* this work 15 16 17 18 18, 19 20 21,22 23 24 Cs > K > Na > Li Cs > K > Rb > Na > Li Cs, Rb, K > Na > Li Rb > Cs > K > Na > Li Cs > Rb > Na > K > Li Cs > Rb 3 K > Na > Li Cs > Na > Rb > K > Li Cs > K > Rb > Na > Li Cs > K > Na > Li Cs > Rb > K > Na * For the clay mineral hectorite a mass action rather than the thermodynamic selectivity is given, based on shapes of forward isotherms only.Those exchanges in table 2 which do not go to completion have in some instances already been normalised by the various authors. Those for Na-X in ref. (1 8) have been normalised by us for the experimental exchange limits for Na+-+Rb+ of 65 % and for Na+-+Cs+ of 72 %. In ref. (18) the exchange limit for Na-X had been assumed for both Rb and Cs to be 82 %. Normalisation in ref. (20) was made for exchange limits Na++Rb+ of 76 % and for Na+--+Cs+ of 63 %. Originally these experimental ex- change limits had been disregarded when calculating K, and AG*, and as a result the affinity sequence in ref. (20) was Na > K > Rb > Li > Cs. Thus the regularity of the affinity sequences appears only after normalisation. Homoionic Ca-, Sr- and Ba-mordenites were not prepared.The largest equiva- lent cation fractions actually reached were 0.60, 0.62 and 0.84 for Ca2+, Sr2+ and Ba2+ respectively (compared with 0.72, 0.49 and 0.73 reported earlier ’). At least in part this behaviour is thought to be associated with the strong hydration of alkaline-earth metal ions, which makes it difficult for such ions to enter the side pockets in mordenite (see Introduction). This difficulty of access cannot be explained in terms of the radii of the anhydrous cations because Cs+, a much larger cation, readily replaces sodium [(fig. 1 (d)]. Because of uncertainty over the limiting extents of exchange [fig. 2(a) and (b)] the equilibria 2NH: +Ca2+ and 2NHi +Sr2+ were not submitted to thermodyn- amic analysis. However, for 2NHi +Ba2+ the form of the isotherm clearly suggests that full exchange could be effected [fig.2(c)] and this exchange was analysed accord- ingly. The exchange 2NHi +Ca2+ was also studied using more concentrated solutions. It was found that the selectivity at 80°C decreased when the concentration of the electrolyte was raised from 0.05 to 0.5 equiv. dm-3 [fig. 2(a)]. This behaviour agrees qualitatively with an earlier study 28 and with the quantitative treatment 29 showing how the selectivity for the ion of higher valence increases with dilution of the aqueous solution. However, this increase is less than expected, which is a further indication that the low-concentration isotherm has not reached equilibrium.Also, for a given electrolyte concentration the isotherm for exchange of NH; by Ca2+ was, as found in earlier equilibrium studies on various zeolites,26 little changed when the temperature was altered from 25 to 80°C. Other notable features are the very low selectivity forR . M. BARRER AND J. KLINOWSKI 2367 Li+[fig. 1(6)] and the extremely good selectivity for Cs+ [fig. l(d)]. The selectivity based on the mass action quotient for Na-i-+Cs+ is greater in the synthetic mordenite than is that recorded by Rao and Rees for the natural mineral from Nova Scotia [fig. Wl. R. M. Barrer, J. Chem. SOC., 1948, 2158. L. L. Ames Jr., Amer. 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