Changes in the Sieving Action and Thermal Stability of Zeolite A produced by Ion-exchange BY T. TAKAISHI" Institute for Atomic Energy, Rikkyo (St. Paul's) University, Yokosuka, Japan 240-01 AND Y. YATSURUGI, A. YUSA AND T. KURATOMI Komatsu Electronic Metals Co., Shinomiya, Hiratsuka City, Japan 254 Received 8th April, 1974 Na-A and K-A zeolites have been ion-exchanged with Ca2+ and Zn'f. The molecular sieving properties of the ion-exchanged zeolites were measured as functions of their composition. Abrupt changes in sieving action occur at 75 % exchange in the (K, Ca)-A series, and at 75 and 83 % exchange in the (K, Zn)-A series, as measured with chemically unreactive molecules. The sieving action of (K, Zn)-A zeolite for some polar molecules and unsaturated hydrocarbons changes gradually with its composition.The thermal stability of melocular sieve 3A is greatly improved by introducing a large number of divalent cations into this K-A zeolite. These properties are discussed in terms of site-selectivities of the exchangeable cations. From these, are derived diagrams of cation distribution on three kinds of site in the crystal. The ideal composition of Na-A zeolite, per unit cell, is Na 2( A102) ( SO2) *xH, 0. * According to recent X-ray structural analysis studies of dehydrated Na-A zeolite,2 three kinds of site are available to the twelve sodium ions. The first is near the centre of the six-membered oxygen ring and called the P-site, since the six-membered oxygen ring constitutes a window for the B-cage. The second is near the centre of the eight- membered oxygen ring which lies on a {loo) face of the unit cube. This site is named the a-site, since the eight-membered oxygen ring is a window for the central cavity (a-cage).The third is in the central cavity, near the centre of the four-membered oxygen ring, the y-site. Each of the (100) faces of the unit cube contains four a-sites, but each face contains only one cation ; that is, three of the twelve a-sites in a unit cell are occupied by Na+. On the other hand, the eight p-sites in a unit cell are occupied by eight Na+ ions. The twelfth Na+ ion in the unit cell occupies one of the twelve y-sites, to which the affinity of a cation is very weak. Sites a and B are preserved in hydrated zeolite A, although their coordinates are slightly changed.In contrast, y-sites cannot be defined in the hydrated crystal; that is, the position of the twelfth Na+ cannot be 10cated.~ The molecular sieving property of a dehydrated zeolite A is primarily determined by the kind of ion occupying the a-sites. Rees and Berry4 studied the blocking effect of pre-sorbed NH3 which is localized upon a cation on an a-site, and concluded that K+ in (Na, K)-A Ions on a-sites partially block the window aperture to the a-cage. 1-4 9798 SIEVING ACTION OF ZEOLITE A preferentially occupies the a-site. However, most existing data on ion-exchanged zeolite A concern those containing Na+ ions. In view of the difference in site select- ivities of Na+ and K+, more systematic studies of the distribution of cations on a-sites are desirable to obtain insight into the chemistry of zeolite A.In the present work, we have studied the differences between Na-A and K-A zeolites when they are ion- exchanged with divalent cations such as calcium and zinc ions. EXPERIMENTAL CHEMICAL ANALYSIS Zeolite A was dissolved in 2 equiv. dnr3 HCI, and A1 and Si contents were determined by the usual method^.^ The alkali metal ion content was measured with an atomic absorption spectrometer. The content of divalent cation in the ion-exchanging solution was determined by ethylenediaminetetra-acetic acid titration, while that in the ion-exchanged zeolite was, in most cases, calculated from the contents in the solution and material balance. In some cases, however, direct analysis was performed with an atomic absorption spectrometer.Radio- active tracers were also used to obtain greater accuracies for low concentrations of the species in solution. Tracers were obtained by irradiating reagents with the TRIGA-I1 reactor of our institute. MATERIAL The starting material was Linde molecular sieve 4A (lot. no. 480017) powder. After the standard treatment of the sample,6 its composition was determined. The sum of the com- ponents amounted to 98.8 % of the original sample. The formula was 0.94Na20-1.00 Al2O3=1.86 Si02*4.50 H20. The Na20/AI2O3 ratio of less than 1 may be ascribed partially to protons, which had replaced alkali metal ions and were not analyzed. The composition of a real crystal of Na-A zeolite differs from that of an ideal one and is 6~ 1 in our crystal. All chemical reagents used were S.P.grade and common adsorbate gases contained in glass cylinders (Takachiho Chemical Co.) had nominal purities better than 99.9 % as determined by mass spectroscopic analysis. Less common gases, such as SiH4, PH3, ASH,, and B2H6, were obtained from the Nihon Oxygen Co., and were of the special high purity used in the semiconductor industry. Na12(A102)12(Si02)12(NaA102)~ with 0 < 6 < 1.6 ADSORPTION MEASUREMENTS Adsorption measurements were made with a McBain type quartz spring balance which had a sensitivity of 0.5 m 8-l and an accuracy of +O.l mg. Prior to adsorption experiments, adsorbents were baked at 400°C for 5 h under a vacuum pressure lower than 1 x N m-2. RESULTS K-A AND Na-A ZEOLITES EXCHANGED WITH Zn2+ The ion exchange was carried out at 80+0.5"C with solutions of various composi- tions all of 0.2 total metal ion normality.Solutions of ZnCl, + NaCl and ZnC1, + KCl were used for Na-A and K-A, respectively. Although the reaction proceeded very rapidly,' the exchange was carried out for 20 h in order to obtain a true equilibrium. The forward exchange (Na+ or K+-+Zn2+) and the reverse exchange (Zn2++Na+ or K+) were measured to check the attainment of true equilibrium and the reversibility of the exchange reaction. After the reaction, the zeolite was filtered off and the Na, K and Zn contents in the solid and liquid phases were determined. The ion-exchange isotherms obtained are shown in fig. 1, in which (Zn), and (Zn), denote the equivalentT. TAKAISHI, Y . YATSURUGI, A. YUSA AND T.KURATOMI 99 fraction of zinc ion (2[Zn]/(2[Zn] + "a]) or 2[Zn]/(2[Zn] + [K])) in zeolite and in liquid, respectively. The isotherms show a steep rise up to 66 % exchange, and then become rather flat. The isotherm of (K, Zn)-A has a distinct step, while that of (Na, Zn)-A has the usual form. This difference may be ascribed, in part, to the differ- ence in the site-selectivities of potassium and sodium ions. After heating the zinc- exchanged zeolites in air at 400"C, their sorptive capacities and X-ray diffraction patterns were compared with those of unheated ones, and no change was introduced by such treatment. This indicates that the zinoexchanged zeolites are stable, in contrast to the earlier conclusion of Barrer and Meier.6 FIG. 1.-The ion-exchange isotherms for zeolite A at 0.2 totaf metal ion normality Zn/Na-A system ; (0) Zn/K-A system.and 80°C: (A) First, the sorptive abilities of the above zeolites for non-polar gases were studied. The rate of sorption was not studied in detail, and only the amounts of sorption at equilibrium are discussed herein. Sorption was measured at fixed temperatures and pressures for : nitrogen and argon at - 195°C ; carbon dioxide, n-butane, silane and diborane at 0°C. Curves of amount sorbed against composition of zeolite are shown in fig. 2, in which a distinct difference between the two series (Zn, Na)-A and (Zn, K)-A is apparent. The curves for the (Zn, Na)-A series for nitrogen and n-butane sorption show steep rises at 33 % exchange in a manner similar to (Ca, Na)-A systems.l* In the (Zn, K)-A series, on the other hand, the curve for n-butane rises sharply at 75 % exchange, and those for nitrogen and argon at 83 %.The window size of (Zn, K)-A zeolite with a composition 0.75 < (Zn), .c 0.85 may be larger than that of Na-A zeolite but smaller than that of Ca-A, since the Zn-exchanged zeolites sorb n-butane but do not sorb argon or nitrogen. Hence, we shall tentatively call these 4.5A. We must not conclude, from the above sieving action of 4SA, that the molecular diameters of argon and nitrogen are larger than that of n-butane, because the cation on the a-site can be displaced from its equilibrium position to widen the window for a visiting gaseous molecule. Such cation displacement, however, has an activation energy and does not easily occur at liquid nitrogen temperatures at which sorption of nitrogen and argon was measured.8 The steep rise is also seen in diborane adsorption at 83 % exchange in the (Zn, K)-A series, at 0°C.This result is reasonable, since diborane has a bridged structure and its molecular diameter is larger than that of ethane.100 SIEVING ACTION OF ZEOLITE A 0 0.2- 0.4 0.6 6.8 1.0 - 0 0.2 0.4 0.6 0.8 1.0 (4 (b) znz FIG. 2.-(a) Adsorption of non-polar gases on (Zn, K)-A zeolites. (A) nitrogen, 200 Torr, - 195°C ; (m argon, 50 Torr, - 195°C ; (0) carbon dioxide, 210 Torr, 0°C ; (e) n-butane, 200 Torr, 0°C ; (0) monosilane, 160 Tom, 0°C ; (m) diborane, 50 Torr, 0°C. (6) Adsorption of non-polar gases on (Zn, Na)-A zeolites. (A) nitrogen, 200 Torr, - 195°C ; (0) carbon dioxide, 210 Torr, 0°C ; (e) n-butane, 220 Torr, 0°C ; (0) monosilane, 160 Torr, 0°C.1 Torr = (101 325/760) N m-2. 0.20 0 c) .- c( 0.15 B c1 (d 0 (d .C) c, =!! 0.10 d 0 0 a .- c, .C( 0.05 .* c) a 5 4 0 znz FIG. 3.-Adsorption of polar gases and unsaturated hydrocarbons on (Zn, K)-A zeolites. (A) ammonia, 20 Torr, 0°C ; (& phosphine, 21 Torr, 0°C ; (W) arsine, 10 Torr, 0°C ; (0) but-1-ene, 50 Torr, 0°C ; (a) trans-but-2-ene, 50 Torr, 0°C ; (El) cis-but-Zene, 50 Torr, 0°C.T . TAKAISHI, Y . YATSURUGI, A . YUSA AND T . KURATOMI 101 Next, sorptive abilities for unsaturated hydrocarbons and polar gases were studied. Curves of the sorbed amount against composition of zeolite are shown in fig. 3. The curves have rather gentle slopes in comparison with those in fig. 2.In these cases molecules can be sorbed by zeolites whose window apertures are expected to be smal- ler than the molecular diameters. It is probable that the temporary displacement of the ion on an a-site is facilitated by the polarity or unsaturated bond of the visiting molecule. K-A AND Na-A ZEOLITES EXCHANGED WITH Ca2+ Ion-exchange isotherms and sorption properties of these zeolites are shown in fig. 4 and 5. The difference between the (K, Ca)-A and (Na, Ca)-A series is also apparent. The ion-exchange isotherm rises steeply up to 33 % exchange in the (Ca, Na)-A series, and in the (Ca, K)-A series up to 66 % exchange, both having no plateau. The change in the window size occurs at 33 % exchange in the (Ca, Na)-A series and at 75 % exchange in the (Ca, K)-A series, but never at 83 % exchange.The difference between the (K, Ca)-A and (Na, Ca)-A series may be attributable to a difference between the site-selectivities of potassium and sodium ions, and the existence of the 4.5A phase depends on the nature of the exchanging divalent cations. FIG. 4.4011-exchange isotherms zeolite A at 0.2 total metal ion normality and at 80°C : (0) Ca/K-A system : (0) Ca/Na-A system. THERMAL STABILITY OF ION-EXCHANGED MOLECULAR SIEVE 3 A Generally zeolite A is thermally not so stable as other zeolites, say, X or Y. Commercial molecular sieve 3A is used as a powerful sorbent for dehydrating organic vapours and oils. In the reactivation process, it is unavoidable that the mol- ecular sieve 3A is heated in an atmosphere of steam generated from itself, thus losing its sorptive ability more easily than in vacuum if the temperature is as high as 350°C.9 On the other hand, it is well-known that the (Na, Ca)-A zeolite becomes thermally more stable.It may be that thermally weak points in zeolite A are alkali metal ions or protons on y-site. Such weak points disappear if 17 % of the K+ is ion-exchanged102 SIEVING ACTION OF ZEOLITE A with CaZ+.I0 According to the results in the preceding section, K-A zeolite retains its sieving character as 3A, even if 66 % of its K+ is replaced by divalent cations. Hence, an investigation of the thermal stability of K-A, heavily ion-exchanged with divalent cations, promised to be interesting. " 0 0.2 0.4 0.6 0.8 1.0 Caz FIG. 5.-Adsorption of non-polar gases on (Ca, K)-A zeolites.(0) carbon dioxide, 200 Torr, 0°C ; (a) oxygen, 160 Torr, - 183°C ; (A) nitrogen, 200 Torr, - 195°C ; (0) n-butane, 200 Torr, 0°C ; (El) methane, 90 Torr, - 183°C ; (0) monosilane, 160 Torr, 0°C. FIG. 6.-The rate of adsorption of water by A zeolites in pellet form ; (A) KI2-A, untreated ; (E) K,,-A, treated ; (0) (Ca3K6)-A, untreated ; (0) (CasKs)-A, treated.T . TAKAISHI, Y . YATSURUGI, A . YUSA A N D T . KURATOMI 103 Thermal treatment was carried out at 450°C by passing air containing 25 Torr (3340 N m-2) water through a zeolite column for about 100 h. The thermal stabilities of the zeolities were checked by measuring and comparing the rate and equilibrium amount of sorption of water vapour at O’C, for untreated and treated samples. The results obtained are shown in fig.6. It is concluded that the thermal stability of K-A zeolite is appreciably improved by introducing a large number of divalent cations in place of the potassium ions occupying P-sites. DISCUSSION Let us consider the site-selectivity of alkali metal ions in dehydrated A zeolites. When Na-A zeolite is ion-exchanged with lithium, n-hexane is sorbed at about 66 % exchange.ll This is interpreted as follows : up to this composition, a-sites are occupied by three Na+ ions and the introduced Li+ ions occupy P- or y-sites selectively. In other words, the affinity of Li+ ions to P-sites is stronger than that of Na+ ions, for dehydrated samples.l1. l2 From X-ray structural analysis, Ca2+ and Na+ ions in dehydrated Ca,Na,-A occupy exlusively @-sites.Hence Ca2+ and Na+ prefer the P-site to the a-site. On the other hand, when Na-A is ion-exchanged with potassium, the (4A43A) transition takes place at 15-25 % exchange.’. Also, when K-A zeolite is ion-exchanged with zinc or calcium, the transition occurs at about 75 % exchange. This means that K+ selectively occupies the a-site and reduces the size of the aperture to the a-cage ; thus, the affinity of K+ to the a-site is stronger than to the P-site. The above site-selectivities of cations are qualitatively tabulated in table 1. This table is deduced from a limited number of two components systems, and gives no knowledge on such systems as (Li, Zn, Ca)-A or (Li, Na, Ca)-A which have not yet been studied. TABLE AF AFFINITY OF CATIONS TO THE THREE KINDS OF SITE IN DEHYDRATED ZEOLITE A Li+ ++ ++++ +(?I Na+ ++ +++ + K+ +++ ++ + ++++ -(?) Ca2+ - Zn2+ + ++++ -(?) cation u-site &site y-site The magnitude of the affinity is qualitatively expressed by the number of + .The sign - means no affinity, and (?) “probably but not verified”. A cation on a given site interacts with the charged anion-framework and other cations via long-range coulombic forces. Hence, its affinity to a site is a function of the composition of the crystal. The determination of such a composition dependence may be carried out, in principle, by structural analysis, but this is extremely tedious and difficult. Molecular sieving properties of ion-exchanged zeolites provide clues to the problem, but their effectiveness is limited. Here, we construct diagrams of distribution of cations on sites, mainly from sorption data.Some parts of the diagrams are supported by X-ray data, but others are more or less speculative. Furthermore, there exists some descrepancy between sorption and X-ray data. For instance, Rees and Berry’s sorption data are interpreted by the model where one a-site per unit cell of Na-A zeolite is occupied by a pair of Naf ions, although X-ray data deny such a model. This discrepancy may be attributed to a difference in states of the two samples, say, degrees of dehydration or amount of H30+ which had replaced Na+. The following diagrams refer to completely dehydrated zeolites with ideal1 04 SIEVING ACTION OF ZEOLITE A compositions. Such diagrams may be used as a guide-line for future structural studies.The ion distribution diagram for (Na, Ca)-A zeolites is shown in fig. 7(a). For the (K, Ca)-A zeolites, there are no X-ray structural data, and therefore only the sieving characteristics can be used to deduce the provisional distribution diagram of cations, as shown in fig. 7(b). The fifth and sixth Ca2+ may be arranged in two ways, which are shown by broken and dotted lines. A choice between these can not be made from data on the sieving characteristics, but should be possible from X-ray structural analysis. 2 1 8 I_ 6 4 2 0 - 2 1 8 - 6 4 2 0 0 2 4 6 8 1 0 1 2 0 2 4 6 8 1 0 1 2 no. of Na+ exchanged no. of K+ exchanged (4 (6) FIG. 7.-(a) The distribution of cations in the (Ca, Na)-A series. V indicates vacancy. (6) The dis- tribution of cations in the (Ca,K)-A series.V indicates vacancy. m +a .r( z 2 1 s 6 4 2 0 0 2 4 6 8 1 0 1 2 no. of I(+ exchanged FIG. 8.-The distribution of cations in the (Zn, K)-A series. V indicates vacancy. The series of (K, &)-A zeolites is unique in that the 4.5 A phase appears. A plausible explanation for such a window of intermediate size is as follows. The ionic radius of Zn2+ (7.4 x 10-l1 m) is smaller than that of Na+ (9.5 x 10-l1 m) and of K+ (1.33 x 10-lo m).I4 Hence, if Zn2+ occupies the a-site, the aperture in the window to the a-cage will be larger than in those occupied by K+ or Na+, but smaller than the aperture in the vacant window. Thus, the fifth Zn2+ may occupy an mite.T . TAKAISHI, Y . YATSURUGI, A . YUSA A N D T . KURATOMI 105 Further verification of the position of the fifth Zn2+ ion may be made by X-ray structural analysis and application of the theory of per~olation,~ which are future problems.The site potential experienced by a cation depends on the composition of the zeolite, that is, the curvature and the depth of the potential surface are functions of the composition. If the curvature is gentle, the displacement of a cation is easily induced by a visiting molecule. The abnormal sieving properties, seen in fig. 3, may be ascribed to such a situation. The gentle curvature brings about a large Debye- Waller factor in the X-ray diffraction pattern. For (K, Zn)-A zeolite, the verification of the present model is under way by Seff.ls The characteristic properties of (Zn, K)-A, given in fig. 3, are of practical use in the separation of some gas mixtures.We succeeded, for example, in ultra-high purifica- tion of SiH4 containing a trace of PH3.16 Detailed results will be published in a separate paper. A possible distribution diagram is shown in fig. 8. The authors thank Messrs. Y. Kaneko and K. Itabashi for their assistance in chemical analysis and Prof. G. S. Lehman for language correction. This work was partially supported by a Grant-in-Aid for Research from the Ministry of Education of the Japanese Government. D. W. Breck, W. G. Eversole, R. M. Milton and T. B. Reed, J. Amer. Chem. Suc., 1956, 78, 5963. R. Y. Yanagida, A. A. Amaro and K. Seff, J. Phys. Chern., 1973, 77, 805. V. Gramlich and W. M. Meier, 2. Krist., 1971, 133, 134. L. V. C. Rees and T. Berry, Pruc. Cunf.Molecular Sieves (SOC. Chem. Ind., London, 1968), p. 149. F. P. Treadwell and W. T. Hall, Analytical Chemistry, vol. I1 (Wiley, New York, 9th edn., 1959), p. 409. R. M. Barrer and W. H. Meier, Trans. Faruday Suc., 1958, 54, 1074; 1959, 55, 130. H. Hoinkins and H. W. Levi, 2. Naturforsch. A , 1967, 22, 226; 1968, 23, 813; 1969,24, 1672; Pruc. Cunf. MoZecular Sieves (SOC. Chem. Ind, London, 1968), p. 339. N. Nagase, Sekiyu Gakkai Shi, 1970, 14, 101 (in Japanese). * D. W. Breck and J. V. Smith, Sci. Amer., 1959,200, 85. lo C. R. Allenbach and F. M. O’Conner, U. S. Pat. 3,506,593/1970. l 1 P. Colline and R. Wey, Cumpt. rend., 1970, 270, 1069. l 2 R. M. Barrer, L. V. C. Rees and D. J. Ward, Proc. Roy SOC. A, 1963, 273, 180. l3 K. Seff and D. P. Shoemaker, Acta Cryst., 1967, 22, 162.l4 L. Pauling, The Nature of the Chemical Bond (Cornell University Press, 3rd edn., 1970). l6 T. Takasihi, A. Yusa and Y. Yatsurugi, Pruc. 3rd Int. Con$ Molecular Sieves (Leuven University K. Seff, personal communication. Press, 1973), p. 246. Changes in the Sieving Action and Thermal Stability of Zeolite A produced by Ion-exchange BY T. TAKAISHI" Institute for Atomic Energy, Rikkyo (St. Paul's) University, Yokosuka, Japan 240-01 AND Y. YATSURUGI, A. YUSA AND T. KURATOMI Komatsu Electronic Metals Co., Shinomiya, Hiratsuka City, Japan 254 Received 8th April, 1974 Na-A and K-A zeolites have been ion-exchanged with Ca2+ and Zn'f. The molecular sieving properties of the ion-exchanged zeolites were measured as functions of their composition.Abrupt changes in sieving action occur at 75 % exchange in the (K, Ca)-A series, and at 75 and 83 % exchange in the (K, Zn)-A series, as measured with chemically unreactive molecules. The sieving action of (K, Zn)-A zeolite for some polar molecules and unsaturated hydrocarbons changes gradually with its composition. The thermal stability of melocular sieve 3A is greatly improved by introducing a large number of divalent cations into this K-A zeolite. These properties are discussed in terms of site-selectivities of the exchangeable cations. From these, are derived diagrams of cation distribution on three kinds of site in the crystal. The ideal composition of Na-A zeolite, per unit cell, is Na 2( A102) ( SO2) *xH, 0. * According to recent X-ray structural analysis studies of dehydrated Na-A zeolite,2 three kinds of site are available to the twelve sodium ions.The first is near the centre of the six-membered oxygen ring and called the P-site, since the six-membered oxygen ring constitutes a window for the B-cage. The second is near the centre of the eight- membered oxygen ring which lies on a {loo) face of the unit cube. This site is named the a-site, since the eight-membered oxygen ring is a window for the central cavity (a-cage). The third is in the central cavity, near the centre of the four-membered oxygen ring, the y-site. Each of the (100) faces of the unit cube contains four a-sites, but each face contains only one cation ; that is, three of the twelve a-sites in a unit cell are occupied by Na+. On the other hand, the eight p-sites in a unit cell are occupied by eight Na+ ions.The twelfth Na+ ion in the unit cell occupies one of the twelve y-sites, to which the affinity of a cation is very weak. Sites a and B are preserved in hydrated zeolite A, although their coordinates are slightly changed. In contrast, y-sites cannot be defined in the hydrated crystal; that is, the position of the twelfth Na+ cannot be 10cated.~ The molecular sieving property of a dehydrated zeolite A is primarily determined by the kind of ion occupying the a-sites. Rees and Berry4 studied the blocking effect of pre-sorbed NH3 which is localized upon a cation on an a-site, and concluded that K+ in (Na, K)-A Ions on a-sites partially block the window aperture to the a-cage. 1-4 9798 SIEVING ACTION OF ZEOLITE A preferentially occupies the a-site.However, most existing data on ion-exchanged zeolite A concern those containing Na+ ions. In view of the difference in site select- ivities of Na+ and K+, more systematic studies of the distribution of cations on a-sites are desirable to obtain insight into the chemistry of zeolite A. In the present work, we have studied the differences between Na-A and K-A zeolites when they are ion- exchanged with divalent cations such as calcium and zinc ions. EXPERIMENTAL CHEMICAL ANALYSIS Zeolite A was dissolved in 2 equiv. dnr3 HCI, and A1 and Si contents were determined by the usual method^.^ The alkali metal ion content was measured with an atomic absorption spectrometer. The content of divalent cation in the ion-exchanging solution was determined by ethylenediaminetetra-acetic acid titration, while that in the ion-exchanged zeolite was, in most cases, calculated from the contents in the solution and material balance.In some cases, however, direct analysis was performed with an atomic absorption spectrometer. Radio- active tracers were also used to obtain greater accuracies for low concentrations of the species in solution. Tracers were obtained by irradiating reagents with the TRIGA-I1 reactor of our institute. MATERIAL The starting material was Linde molecular sieve 4A (lot. no. 480017) powder. After the standard treatment of the sample,6 its composition was determined. The sum of the com- ponents amounted to 98.8 % of the original sample. The formula was 0.94Na20-1.00 Al2O3=1.86 Si02*4.50 H20.The Na20/AI2O3 ratio of less than 1 may be ascribed partially to protons, which had replaced alkali metal ions and were not analyzed. The composition of a real crystal of Na-A zeolite differs from that of an ideal one and is 6~ 1 in our crystal. All chemical reagents used were S.P. grade and common adsorbate gases contained in glass cylinders (Takachiho Chemical Co.) had nominal purities better than 99.9 % as determined by mass spectroscopic analysis. Less common gases, such as SiH4, PH3, ASH,, and B2H6, were obtained from the Nihon Oxygen Co., and were of the special high purity used in the semiconductor industry. Na12(A102)12(Si02)12(NaA102)~ with 0 < 6 < 1.6 ADSORPTION MEASUREMENTS Adsorption measurements were made with a McBain type quartz spring balance which had a sensitivity of 0.5 m 8-l and an accuracy of +O.l mg.Prior to adsorption experiments, adsorbents were baked at 400°C for 5 h under a vacuum pressure lower than 1 x N m-2. RESULTS K-A AND Na-A ZEOLITES EXCHANGED WITH Zn2+ The ion exchange was carried out at 80+0.5"C with solutions of various composi- tions all of 0.2 total metal ion normality. Solutions of ZnCl, + NaCl and ZnC1, + KCl were used for Na-A and K-A, respectively. Although the reaction proceeded very rapidly,' the exchange was carried out for 20 h in order to obtain a true equilibrium. The forward exchange (Na+ or K+-+Zn2+) and the reverse exchange (Zn2++Na+ or K+) were measured to check the attainment of true equilibrium and the reversibility of the exchange reaction. After the reaction, the zeolite was filtered off and the Na, K and Zn contents in the solid and liquid phases were determined.The ion-exchange isotherms obtained are shown in fig. 1, in which (Zn), and (Zn), denote the equivalentT. TAKAISHI, Y . YATSURUGI, A. YUSA AND T. KURATOMI 99 fraction of zinc ion (2[Zn]/(2[Zn] + "a]) or 2[Zn]/(2[Zn] + [K])) in zeolite and in liquid, respectively. The isotherms show a steep rise up to 66 % exchange, and then become rather flat. The isotherm of (K, Zn)-A has a distinct step, while that of (Na, Zn)-A has the usual form. This difference may be ascribed, in part, to the differ- ence in the site-selectivities of potassium and sodium ions. After heating the zinc- exchanged zeolites in air at 400"C, their sorptive capacities and X-ray diffraction patterns were compared with those of unheated ones, and no change was introduced by such treatment.This indicates that the zinoexchanged zeolites are stable, in contrast to the earlier conclusion of Barrer and Meier.6 FIG. 1.-The ion-exchange isotherms for zeolite A at 0.2 totaf metal ion normality Zn/Na-A system ; (0) Zn/K-A system. and 80°C: (A) First, the sorptive abilities of the above zeolites for non-polar gases were studied. The rate of sorption was not studied in detail, and only the amounts of sorption at equilibrium are discussed herein. Sorption was measured at fixed temperatures and pressures for : nitrogen and argon at - 195°C ; carbon dioxide, n-butane, silane and diborane at 0°C. Curves of amount sorbed against composition of zeolite are shown in fig.2, in which a distinct difference between the two series (Zn, Na)-A and (Zn, K)-A is apparent. The curves for the (Zn, Na)-A series for nitrogen and n-butane sorption show steep rises at 33 % exchange in a manner similar to (Ca, Na)-A systems.l* In the (Zn, K)-A series, on the other hand, the curve for n-butane rises sharply at 75 % exchange, and those for nitrogen and argon at 83 %. The window size of (Zn, K)-A zeolite with a composition 0.75 < (Zn), .c 0.85 may be larger than that of Na-A zeolite but smaller than that of Ca-A, since the Zn-exchanged zeolites sorb n-butane but do not sorb argon or nitrogen. Hence, we shall tentatively call these 4.5A. We must not conclude, from the above sieving action of 4SA, that the molecular diameters of argon and nitrogen are larger than that of n-butane, because the cation on the a-site can be displaced from its equilibrium position to widen the window for a visiting gaseous molecule.Such cation displacement, however, has an activation energy and does not easily occur at liquid nitrogen temperatures at which sorption of nitrogen and argon was measured.8 The steep rise is also seen in diborane adsorption at 83 % exchange in the (Zn, K)-A series, at 0°C. This result is reasonable, since diborane has a bridged structure and its molecular diameter is larger than that of ethane.100 SIEVING ACTION OF ZEOLITE A 0 0.2- 0.4 0.6 6.8 1.0 - 0 0.2 0.4 0.6 0.8 1.0 (4 (b) znz FIG. 2.-(a) Adsorption of non-polar gases on (Zn, K)-A zeolites. (A) nitrogen, 200 Torr, - 195°C ; (m argon, 50 Torr, - 195°C ; (0) carbon dioxide, 210 Torr, 0°C ; (e) n-butane, 200 Torr, 0°C ; (0) monosilane, 160 Tom, 0°C ; (m) diborane, 50 Torr, 0°C.(6) Adsorption of non-polar gases on (Zn, Na)-A zeolites. (A) nitrogen, 200 Torr, - 195°C ; (0) carbon dioxide, 210 Torr, 0°C ; (e) n-butane, 220 Torr, 0°C ; (0) monosilane, 160 Torr, 0°C. 1 Torr = (101 325/760) N m-2. 0.20 0 c) .- c( 0.15 B c1 (d 0 (d .C) c, =!! 0.10 d 0 0 a .- c, .C( 0.05 .* c) a 5 4 0 znz FIG. 3.-Adsorption of polar gases and unsaturated hydrocarbons on (Zn, K)-A zeolites. (A) ammonia, 20 Torr, 0°C ; (& phosphine, 21 Torr, 0°C ; (W) arsine, 10 Torr, 0°C ; (0) but-1-ene, 50 Torr, 0°C ; (a) trans-but-2-ene, 50 Torr, 0°C ; (El) cis-but-Zene, 50 Torr, 0°C.T . TAKAISHI, Y .YATSURUGI, A . YUSA AND T . KURATOMI 101 Next, sorptive abilities for unsaturated hydrocarbons and polar gases were studied. Curves of the sorbed amount against composition of zeolite are shown in fig. 3. The curves have rather gentle slopes in comparison with those in fig. 2. In these cases molecules can be sorbed by zeolites whose window apertures are expected to be smal- ler than the molecular diameters. It is probable that the temporary displacement of the ion on an a-site is facilitated by the polarity or unsaturated bond of the visiting molecule. K-A AND Na-A ZEOLITES EXCHANGED WITH Ca2+ Ion-exchange isotherms and sorption properties of these zeolites are shown in fig. 4 and 5. The difference between the (K, Ca)-A and (Na, Ca)-A series is also apparent.The ion-exchange isotherm rises steeply up to 33 % exchange in the (Ca, Na)-A series, and in the (Ca, K)-A series up to 66 % exchange, both having no plateau. The change in the window size occurs at 33 % exchange in the (Ca, Na)-A series and at 75 % exchange in the (Ca, K)-A series, but never at 83 % exchange. The difference between the (K, Ca)-A and (Na, Ca)-A series may be attributable to a difference between the site-selectivities of potassium and sodium ions, and the existence of the 4.5A phase depends on the nature of the exchanging divalent cations. FIG. 4.4011-exchange isotherms zeolite A at 0.2 total metal ion normality and at 80°C : (0) Ca/K-A system : (0) Ca/Na-A system. THERMAL STABILITY OF ION-EXCHANGED MOLECULAR SIEVE 3 A Generally zeolite A is thermally not so stable as other zeolites, say, X or Y.Commercial molecular sieve 3A is used as a powerful sorbent for dehydrating organic vapours and oils. In the reactivation process, it is unavoidable that the mol- ecular sieve 3A is heated in an atmosphere of steam generated from itself, thus losing its sorptive ability more easily than in vacuum if the temperature is as high as 350°C.9 On the other hand, it is well-known that the (Na, Ca)-A zeolite becomes thermally more stable. It may be that thermally weak points in zeolite A are alkali metal ions or protons on y-site. Such weak points disappear if 17 % of the K+ is ion-exchanged102 SIEVING ACTION OF ZEOLITE A with CaZ+.I0 According to the results in the preceding section, K-A zeolite retains its sieving character as 3A, even if 66 % of its K+ is replaced by divalent cations.Hence, an investigation of the thermal stability of K-A, heavily ion-exchanged with divalent cations, promised to be interesting. " 0 0.2 0.4 0.6 0.8 1.0 Caz FIG. 5.-Adsorption of non-polar gases on (Ca, K)-A zeolites. (0) carbon dioxide, 200 Torr, 0°C ; (a) oxygen, 160 Torr, - 183°C ; (A) nitrogen, 200 Torr, - 195°C ; (0) n-butane, 200 Torr, 0°C ; (El) methane, 90 Torr, - 183°C ; (0) monosilane, 160 Torr, 0°C. FIG. 6.-The rate of adsorption of water by A zeolites in pellet form ; (A) KI2-A, untreated ; (E) K,,-A, treated ; (0) (Ca3K6)-A, untreated ; (0) (CasKs)-A, treated.T . TAKAISHI, Y . YATSURUGI, A . YUSA A N D T . KURATOMI 103 Thermal treatment was carried out at 450°C by passing air containing 25 Torr (3340 N m-2) water through a zeolite column for about 100 h.The thermal stabilities of the zeolities were checked by measuring and comparing the rate and equilibrium amount of sorption of water vapour at O’C, for untreated and treated samples. The results obtained are shown in fig. 6. It is concluded that the thermal stability of K-A zeolite is appreciably improved by introducing a large number of divalent cations in place of the potassium ions occupying P-sites. DISCUSSION Let us consider the site-selectivity of alkali metal ions in dehydrated A zeolites. When Na-A zeolite is ion-exchanged with lithium, n-hexane is sorbed at about 66 % exchange.ll This is interpreted as follows : up to this composition, a-sites are occupied by three Na+ ions and the introduced Li+ ions occupy P- or y-sites selectively. In other words, the affinity of Li+ ions to P-sites is stronger than that of Na+ ions, for dehydrated samples.l1.l2 From X-ray structural analysis, Ca2+ and Na+ ions in dehydrated Ca,Na,-A occupy exlusively @-sites. Hence Ca2+ and Na+ prefer the P-site to the a-site. On the other hand, when Na-A is ion-exchanged with potassium, the (4A43A) transition takes place at 15-25 % exchange.’. Also, when K-A zeolite is ion-exchanged with zinc or calcium, the transition occurs at about 75 % exchange. This means that K+ selectively occupies the a-site and reduces the size of the aperture to the a-cage ; thus, the affinity of K+ to the a-site is stronger than to the P-site. The above site-selectivities of cations are qualitatively tabulated in table 1.This table is deduced from a limited number of two components systems, and gives no knowledge on such systems as (Li, Zn, Ca)-A or (Li, Na, Ca)-A which have not yet been studied. TABLE AF AFFINITY OF CATIONS TO THE THREE KINDS OF SITE IN DEHYDRATED ZEOLITE A Li+ ++ ++++ +(?I Na+ ++ +++ + K+ +++ ++ + ++++ -(?) Ca2+ - Zn2+ + ++++ -(?) cation u-site &site y-site The magnitude of the affinity is qualitatively expressed by the number of + . The sign - means no affinity, and (?) “probably but not verified”. A cation on a given site interacts with the charged anion-framework and other cations via long-range coulombic forces. Hence, its affinity to a site is a function of the composition of the crystal. The determination of such a composition dependence may be carried out, in principle, by structural analysis, but this is extremely tedious and difficult.Molecular sieving properties of ion-exchanged zeolites provide clues to the problem, but their effectiveness is limited. Here, we construct diagrams of distribution of cations on sites, mainly from sorption data. Some parts of the diagrams are supported by X-ray data, but others are more or less speculative. Furthermore, there exists some descrepancy between sorption and X-ray data. For instance, Rees and Berry’s sorption data are interpreted by the model where one a-site per unit cell of Na-A zeolite is occupied by a pair of Naf ions, although X-ray data deny such a model. This discrepancy may be attributed to a difference in states of the two samples, say, degrees of dehydration or amount of H30+ which had replaced Na+.The following diagrams refer to completely dehydrated zeolites with ideal1 04 SIEVING ACTION OF ZEOLITE A compositions. Such diagrams may be used as a guide-line for future structural studies. The ion distribution diagram for (Na, Ca)-A zeolites is shown in fig. 7(a). For the (K, Ca)-A zeolites, there are no X-ray structural data, and therefore only the sieving characteristics can be used to deduce the provisional distribution diagram of cations, as shown in fig. 7(b). The fifth and sixth Ca2+ may be arranged in two ways, which are shown by broken and dotted lines. A choice between these can not be made from data on the sieving characteristics, but should be possible from X-ray structural analysis.2 1 8 I_ 6 4 2 0 - 2 1 8 - 6 4 2 0 0 2 4 6 8 1 0 1 2 0 2 4 6 8 1 0 1 2 no. of Na+ exchanged no. of K+ exchanged (4 (6) FIG. 7.-(a) The distribution of cations in the (Ca, Na)-A series. V indicates vacancy. (6) The dis- tribution of cations in the (Ca,K)-A series. V indicates vacancy. m +a .r( z 2 1 s 6 4 2 0 0 2 4 6 8 1 0 1 2 no. of I(+ exchanged FIG. 8.-The distribution of cations in the (Zn, K)-A series. V indicates vacancy. The series of (K, &)-A zeolites is unique in that the 4.5 A phase appears. A plausible explanation for such a window of intermediate size is as follows. The ionic radius of Zn2+ (7.4 x 10-l1 m) is smaller than that of Na+ (9.5 x 10-l1 m) and of K+ (1.33 x 10-lo m).I4 Hence, if Zn2+ occupies the a-site, the aperture in the window to the a-cage will be larger than in those occupied by K+ or Na+, but smaller than the aperture in the vacant window.Thus, the fifth Zn2+ may occupy an mite.T . TAKAISHI, Y . YATSURUGI, A . YUSA A N D T . KURATOMI 105 Further verification of the position of the fifth Zn2+ ion may be made by X-ray structural analysis and application of the theory of per~olation,~ which are future problems. The site potential experienced by a cation depends on the composition of the zeolite, that is, the curvature and the depth of the potential surface are functions of the composition. If the curvature is gentle, the displacement of a cation is easily induced by a visiting molecule. The abnormal sieving properties, seen in fig. 3, may be ascribed to such a situation. The gentle curvature brings about a large Debye- Waller factor in the X-ray diffraction pattern. For (K, Zn)-A zeolite, the verification of the present model is under way by Seff.ls The characteristic properties of (Zn, K)-A, given in fig. 3, are of practical use in the separation of some gas mixtures. We succeeded, for example, in ultra-high purifica- tion of SiH4 containing a trace of PH3.16 Detailed results will be published in a separate paper. A possible distribution diagram is shown in fig. 8. The authors thank Messrs. Y. Kaneko and K. Itabashi for their assistance in chemical analysis and Prof. G. S. Lehman for language correction. This work was partially supported by a Grant-in-Aid for Research from the Ministry of Education of the Japanese Government. D. W. Breck, W. G. Eversole, R. M. Milton and T. B. Reed, J. Amer. Chem. Suc., 1956, 78, 5963. R. Y. Yanagida, A. A. Amaro and K. Seff, J. Phys. Chern., 1973, 77, 805. V. Gramlich and W. M. Meier, 2. Krist., 1971, 133, 134. L. V. C. Rees and T. Berry, Pruc. Cunf. Molecular Sieves (SOC. Chem. Ind., London, 1968), p. 149. F. P. Treadwell and W. T. Hall, Analytical Chemistry, vol. I1 (Wiley, New York, 9th edn., 1959), p. 409. R. M. Barrer and W. H. Meier, Trans. Faruday Suc., 1958, 54, 1074; 1959, 55, 130. H. Hoinkins and H. W. Levi, 2. Naturforsch. A , 1967, 22, 226; 1968, 23, 813; 1969,24, 1672; Pruc. Cunf. MoZecular Sieves (SOC. Chem. Ind, London, 1968), p. 339. N. Nagase, Sekiyu Gakkai Shi, 1970, 14, 101 (in Japanese). * D. W. Breck and J. V. Smith, Sci. Amer., 1959,200, 85. lo C. R. Allenbach and F. M. O’Conner, U. S. Pat. 3,506,593/1970. l 1 P. Colline and R. Wey, Cumpt. rend., 1970, 270, 1069. l 2 R. M. Barrer, L. V. C. Rees and D. J. Ward, Proc. Roy SOC. A, 1963, 273, 180. l3 K. Seff and D. P. Shoemaker, Acta Cryst., 1967, 22, 162. l4 L. Pauling, The Nature of the Chemical Bond (Cornell University Press, 3rd edn., 1970). l6 T. Takasihi, A. Yusa and Y. Yatsurugi, Pruc. 3rd Int. Con$ Molecular Sieves (Leuven University K. Seff, personal communication. Press, 1973), p. 246.