Ion Exchange in Zeolites - ACS Publications

16

I o n E x c h a n g e in Z e o l i t e s

ADRIEN CREMERS

Downloaded by UNIV OF CINCINNATI on February 18, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch016

Centrum voor Oppervlaktescheikunde en Colloidale Scheikunde, Katholieke Universiteit Leuven, de Croylaan 42, B-3030 Heverlee, Belgium

ABSTRACT This paper reviews advances in zeolite ion exchange which have taken place since the Second International Conference and focuses attention on some remaining problem areas. Ion exchange in zeolites A, X, Y, mordenite and chabazite is covered. The review is in three sections: thermodynamic aspects, equilibria and kinetics.

Introduction The changing of the nature of the cations in a zeolite structure is a relatively simple task and a variety of homoionic or mixed ionic forms can be prepared using alkali- and alkaline earth- or transition metal- and rare earth ions. In some cases, the exchange reaction may go to completion or fail to do so, depending on the nature of the ion, the zeolite and the temperature. The fact that such changes in ionic composition may produce some marked changes in properties such as thermal stability Q), sieving (2), sorptive (3) (4) and catalytic functions (5) is from the purely practical point of view, one of the most important aspects of zeolite chemistry. Thermodynamic Formalism For Ion Exchange Reactions In contrast to ion exchange materials such as clays and organic resins, it is a distinctive feature of many zeolites, as established by X-ray diffraction, that the exchangeable cations may take positions in widely different crystallographic environments. The number of possible sites generally exceeds the crystal charge deficit which in turn may lead to a pattern of charge neutralization which is characteristic for a given ion or group of ions and which is generally quite sensitive to the state of hydration of the sample. It is also known that differences between two cationic forms of the zeolite are not limited to characteristic site preferences of the ions; significant differences may also occur with regard to the total number of cations and solvent molecules which may be localized and in the distribution of the solvent over the various possible sites. Of course, one may rightfully ignore the foregoing complications in deriving 179

In Molecular Sieves—II; Katzer, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

MOLECULAR

180

SIEVES—II

the overall free energy effect for some arbitrary ion exchange reaction and apply a standard thermodynamic procedure (7) to the equilibrium (bar refers to zeolite phase) Z

Z Z

Z

- B Β

a

A

+

Z

B

Z

A

Α

Z

Z

- A Α

B

+

B

Β

Ζ λ Z

A

(1)

The overall thermodynamic equilibrium constant K is defined as a

Z

K

m

=

a

Z


Z

B

A

Z

A

Β

m

Z

A f

B

Z

B A A ?B z

Z

B f A A

7

B

(2)

B A

in which the zeolite phase composition is defined on the equivalent fraction scale (represented by the capitals A , B) and the bulk solution composition specified as A* B ' rnolal scale; f / f β and 7 β / 7 Α represent the ion activity coefficient ratios in the two phases and z , ζβ refer to the ionic valences. Equation (2) implies the following expression for the free energy content of an adsorbed ion (omitting valence sign for simplicity) m

m

o

n

t

n

e

A

A

MA

=

"A

RT 1n A f

A

( 3 )

in which the excess free energy, resulting from changes in composition is specified a s f = exp ( u ^ / R T ) . However, in applying such an overall approach to a heterogeneous material, one must be aware that the "overall" activity coefficients may have very limited bearing on non-ideality effects "per se", such as site-site interactions, but are carrying the heavy burden of the site heterogeneity of the material as well. These coefficients serve a very limited purpose in terms of gaining a better understanding of the effect of charge heterogeneity on the ion exchange behavior of zeolites. An attempt to relate the overall equilibrium constant to the properties of the individual site groups was made by Barrer and Klinowski who considered the exchanger to contain a number of site groups (8). The sites within each group are taken as equivalent but differing from the sites in the other groups. The exchange reaction in each particular group may therefore be specified in terms of a charac teristic equilibrium constant Kj, i.e. the analog of equation (2) for the i group. A

t n

Z

A

Z

B m

i

A

Z

Z

, A, B A < i> T f

B

B

Kj =

(4) Z

Bj

z

A m

B

A

, B (fj)

Z v

Z

A

B 7 A

Consequently, the overall standard free energy effect is related to the free energy terms for the η groups of sites through the equation

AG° =

η Σ XjAGj

or

K

a

η Χ: = π Kj

(5)


16.

CREMERS

Ion Exchange

in Zeolites

181

in which Xj represents the equivalent fraction of all cations in group i. These rela tions allow the overall selectivity coefficient K , to be related to the selectivity coefficients within the various site groups according to the equation c

n

z

[IXjBi K

c

=

A

/ z

B

i (K ) c

B

Z

]

—

B (6)

η [Σ X; Bj] 1


1 / 2

Z

A

Using various combinations of Sj and Kj and simple composition dependences of the ion activity coefficient ratios within each group i.e. f ^ / f Ç (or synonymously taking K{* a simple function of the composition in the ith set) the authors were able to successfully predict Kielland plots which agreed closely with experimental data. One of the crucial points in this approach seems to be the assignment of a characteristic non-ideality pattern to each site group. Implicit in such a choice is the existence of the analog of eqn(3) for each group of sites, i.e.

>ή

=

+

RT 1n Aj f^

(7)

ir\ which f^ is some function of the composition in the i th set and for which f A "* 1asAj ~* 1. More specifically, the excess chemical potential of an ion in the i tn set is defined exclusively in terms of the composition within this set. It is however quite possible that the excess free energy function of a particular ion-site combination is very sensitive to composition changes in other sets and nearly inde pendent of the composition of the sites belonging to its own group: for example, one may imagine the case of a given type of site surrounded by sites belonging to another group. It would therefore appear that the interaction of a given group of sites on the exchange phenomenon in another site group through this influence on the activity coefficients in this other group, as invoked by Barrer, Klinowski and Sherry (9), is hard to reconcile with the fact that such effects, however likely, are outside the scope of the definitions; in other words, the definition of the activity coefficient contains no provision for such interactions. Consequently, it would seem that the thermodynamic constants Kj for the various site groups are not to be considered as thermodynamic quantities "sensu stricto" but rather as a quantitative measure of the relative affinities of a pair of ions for a given site group. A characteristic feature of many ion exchange reactions in zeolites is the failure to proceed to completion. Barrer and co-workers (23), followed by Sherry (10), introduced a normalization procedure which amounts to expressing the com position of the zeolite sites which are accessible to both exchanging ions with reference to the maximum exchange level for the ingoing cation. Thermodynamic quantities for this limited exchange process are then obtained by standard proce dures. In some cases the maximum exchange level is temperature dependent. For this reason, and on the grounds that the Barrer method might fail to take into account a possible effect of unexchanged ions and also because redistributions may occur between small cages and supercages, Vansant and Uytterhoeven (12) presen ted an alternative normalization procedure. This method however was criticized by Barrer, Klinowski and Sherry (9) and shown to contain some inaccuracies. In



182

MOLECULAR

SIEVES—II

the same paper, three possible cases of incomplete exchange were envisaged. The first case, in which no thermodynamic treatment is justified is a pseudo-equilibrium which shows a time dependent drift in the ion distribution. The second case corre sponds to a temperature-independent maximum exchange level which is indepen dent of the time scale used. The third case is identical to the second but the maximum exchange level changes reversibly with the temperature. Several practical cases were discussed and some doubt was expressed whether case 3 is actually ever encountered. When measuring ion exchange distributions between ions of different charge, it is well known that the ion of higher charge is more selectively adsorbed with increasing dilution of the liquid phase, i.e. the electrovalence effect. This is one of the main reasons for carrying out the exchange reaction at a constant total normal ity, c . The quantitative calculation of an isotherm at some arbitrary value of co from a known isotherm at c , including the effect of solution concentration on activity coefficient ratios, was worked out by Barrer and Klinowski (13). It was shown that, when working at relatively low c values, the terms pertaining to water activity effects (7) can generally be ignored in zeolites. Perhaps, it is appro priate in this context to recall the thermodynamic relation between selectivity coefficient and water activity, as derived by Laudelout and Thomas (14,15): Q

Q

G

δ 1n Κ ( ) = δ 1n a w

n£

.

n

B

(8)

A

in which A is the ingoing ion and nfi, n^ represent the water contents of the monoionic zeolites. This equation is based on the reasonable assumption that the water content of zeolites is independent of ionic strength. It was successfully tested in the case of clays (18) but no such tests were made in zeolites, for which pronounced differences in water content may occur, for example between NaX and CsX (1(>). An even better case would be the KY and alkylammonium Y zeo lites (17) in which the water content decreases from 240 in KY to values ranging between 125 and 100 water molecules/u.c. for the , Cj, c mixed K-alkyl ammonium forms. 3

Ion Exchange Equilibria Alkali and Alkaline Earth Metal Ion Exchange in Zeolites X and Y . Rather few new data have appeared on the ion exchange behavior of X and Y zeolites since the earlier review by Sherry (6). The study by Wolf and co-workers (19) on the Na/Ca equilibrium in NaX is in essential agreement with the work of Sherry (20). In contrast to Ca, it was shown by Wolf (19) that the maximum exchange level for Mg ions was only 60%. More recently, Lai and Rees (21) reinvestigated the adsorption of alkali and alkaline earth metal ions. As expected, Rb and Cs failed to displace all the Na ions: in zeolite Y , 25 out of 68 ions/u.c. could not be displaced. This number is intermediate between the values found by Sherry (22) and Barrer, Davies and Rees (23), i.e. 16 ions in NaY and 32 in NaX could not be displaced. In view of the known occupancies of Na in hydrated NaX (24), the reasons for incomplete exchange are not merely a question of steric effects. In contrast the data on Ba (21) in X and Y fitted exactly with the exchange limit of 68 and 82% in NaY and NaX, corresponding to the 16 Na ions/u.c. in the small cages. Of course, the



16.

CREMERS

Ion Exchange

in Zeolites

183

exchange process can be activated at higher temperature, and complete exchange can be achieved for Ca and Ba-ions as shown by Sherry (20). One of the main difficulties in understanding the peculiar behavior of X and Y zeolites is to account for the fact that in both cases, 16-17 ions per u.c. are local ized in the small cages (24) (25) and that all the remaining ions can sometimes not be displaced, either by bulky ions which, for steric reasons, cannot penetrate the small cages, or by ions such as calcium for which no steric reasons can be invoked. Usually, the strong hydration of divalent ions and the necessity of a partial dehy dration is thought to be associated with difficulty of displacing the residual Na-ions. Another hypothesis to account for limited exchange to levels which are incon sistent with cation localization studies is that the ingoing ions may lead to a rearrangement of the other ions in the zeolite (26), which may lead to a different type of charge neutralization. In the absence of appropriate structural data, none of the foregoing hypotheses could be submitted to a quantitative test. Some recent structural data on monoionic and biionic hydrated zeolites provide strong indica tions that ion exchange reactions are accompanied with a rearrangement of the ions in the zeolite. Table I shows a comparison of cation localizations, as obtained from X-ray diffraction in NaX, NaY, KX, KY and CaY (24) (26) (27). These data show that for NaX and KX, the pattern is fairly similar, i.e. about 9 ions per u.c. in Sj, 7-8 ions/u.c. in S|' and 23-24 in S\\. In zeolite Y however, some very distinct differences occur: the total number of ions in the small cages is 15-17, nearly exclusively localized in Sj'; however, contrary to predictions made by Smith (28) on the basis of the absence of six-rings with 3 ΑΙ-atoms followed by Sherry (10) (11) on the absence of ion siting in the supercages in Y zeolite, a significant number of ions is found on Sj|, 20 in K Y , and 10 in NaY, and 3 in CaY. Pronounced differences are seen in the localization of water molecules: in NaY, 13 water mole cules are localized in S\\, whereas in K Y , none were localized in this site; in CaY, 31 water molecules were localized in S\\, »-P localized in S|\ and were thought to be coordinated to the calcium ions. One of the common features in most ion exchange work in zeolites is that the selectivity is mostly referred to the Na ion, the Na form being generally used as starting material. Mortier, Costenoble and Uytterhoeven (27) have shown that the calcium exchange levels which may be reached, using room temperature conditions, are very much dependent upon the nature of the outgoing cation. Using the same treatment, .6M CaCI for 6 days at room temperature, the calcium saturations were 68% in NaY and 90% in KY. These data show that properties of the ingoing cation are not the sole factor controlling the exchange levels at a given temperature. From these data one may attempt a rough estimate of the ion selectivity of Κ to Na ions, as expressed with reference to the calcium ion. It would appear that the Na ion is strongly preferred ( K n _ k — 20), which is in contrast with the equilibrium data by Sherry (10) and Barrer, Davies and Rees (23) which point to a very slight prefer ence of K: K-Na — ^ -5- This would indicate that, when using the Na form of the zeolite, a pseudo- or metastable equilibrium is reached. The comparison of ion localization in NaCaY and KCaY (27), also shown in Table I, is also revealing. The data show that, when using NaY, no calcium is detected in Sf, whereas the water occupancy in the NaCaY is roughly identical to NaY (about 1 H 0 / N a ion in Sf|). In contrast, when using the K Y , 10.6 Ca ions are found in S\ which is accompanied by a strong increase in water occupancy in S|'|, roughly identical to CaY, i.e. 3 H 0 molecules per Ca ion. It would appear that partial dehydration to account for the difficulty could not be invoked as an important factor to account for the limited e

a

b

o

u

t 3

er C

a

i o n

2

3

2

2


184

M O L E C U L A R SIEVES—Π

exchange of ions such as calcium. The authors point out that, in addition to a partial dehydration, the calcium ion must displace a stable Na-hydrate structure which is non-existent in K Y , forming a stable hydrate structure itself, i.e. being coordinated to three 0 oxygens and three water molecules in the sodalite cage. A further indication that ion redistributions may occur in the course of an ion exchange reaction is shown in an X-ray study on K-alkylammonium zeolite Y (29, 30). It was shown that the exchange of Κ by alkylammonium ions leads to a pro gressive decrease of the water content in K Y , a process which was accompanied with a Κ occupancy increase in S| from 1.3 to 5.4 and from 13.3 to 16.5 in S\ in the case of propylammonium, which corresponds to a total occupancy of nearly 22 ions/u.c. for the small cages. At the same time, the Κ occupancy in S| | de creased from 20 ions/u.c. in KY to about 15 in K-propylammonium Y zeolite. The significant fact however is that such a distribution is nearly identical to the one found for dehydrated KY (30), an observation which lead the authors to interpret the shift of Κ ions to the small cages as resulting from a decrease in hydration level. The foregoing data indicte that, in general, the relative occupancies of the various sites are interdependent and that the hydration level is a very important factor in the pattern of charge neutralization. In the case of monoionic zeolites, the quantitative relations between the relative occupancies of the various sites and the energy differences between these sites were shown to be related quantitatively through a Maxwell-Boltzman expression for the case of Κ zeolites (31 ).


3

Transition Metal Ion Exchange in Zeolite X, Y , A and Mordenite. In spite of the importance of transition metal ion exchanged zeolites as potential catalysts, the interest in the ion exchange thermodynamics of these ions in zeolites remains rather limited (19) (21 ) (32-40). Some of the well known features common to the behaviour of alkaline earth metal ions are also found in the case of transition metal ions: sigmoidal-shaped isotherm, incomplete exchange and temperature-dependent maximum exchange levels. Consequently, interpretations rely heavily on earlier views, especially since there are practically no data available on the localization of these ions in hydrated systems. The maximum exchange levels show some rather large variations which can, in part, be accounted for by differences in experimental conditons such as equili brium time and concentration of the saturating solutions. It appears that only in rare cases (21) are maximum exchange levels obtained which are consistent with the number of sodium ions known to be localized in the large cages. In zeolite X, complete exchange seems possible with the ions Cu, Cd and Zn (19) (37) (40), although lower values have been reported for Cu (21) and Zn (21) (38). The values for other ions cover the range .75 - .85 for Co (37) (38), .70 - .80 for Ni (35) (37) (38) and .80 for Mn (35). The maximum limits in Y zeolite vary within even wider ranges: .85 -1 for Cu (37) (42), .75 - .95 for Zn (36) (37) (40), .6 for Cd (36), .70 - .75 for Ni (35) (36) (37), .65 - .80 for Co (35) (36) (37) and .78 for Mn (35). In the case of zeolite A complete exchange is possible for Cd and Zn and a maxi mum of about .8 is obtained for Co and Ni (32) (33). In Mordenite, the maximum exchange levels range from .45 to .50. The diversity in these data is surprising in view of the nearly identical values of the Pauling radii (about .7 Â except for Cd for which the value is .97). Therefore, the reasons for incomplete exchange should not be ascribed exclusively to steric effects or partial dehydration, particularly in view of the fact that ions which are so closely similar in dimensions behave so drastically different, and that the Cd ion which has the largest radius seems to pene-



16.

CREMERS

Ion Exchange in Zeolites

185

trate relatively easy in the small cages. The shape of the ion exchange isotherms is similar to what is found for alka line earth metal ions. Some typical examples are shown in figures 1 and 2. Judging from the foregoing data on maximum exchange levels, it appears that some of the ions do take positions in the small cages. It is tempting to adhere to the interpretation which is used for other bivalent ions (6): the sites in the small cages prefer Na ions to divalent ions and the inability of divalent cations do dis place Na ions from the sodalite cages results from an unfavorable exchange equilibrium (23), particularly in the case of zeolite Y. The data on calcium exchange in KY and NaY (26) have shown that the ability to take positions in the small cages is not exclusively related to the properties of the divalent ion but is very much dependent on the nature of the outgoing cation. A rather different point of view was expressed by Maes and Cremers (34, 37, 42) in ascribing a preference of bivalent ions for the small cage sites. Such a view is based on the finding that at a low overall loading of the zeolite, the relative occu pancy of small cage sites exceeds the value in the supercages. In some cases, and for reasons which are probably kinetic in origin, complete exchange may not be achieved within reasonable time limits. Preference of (some) transition metal ions for small cage sites is likely to have a direct influence on the exchange phenomena in the supercages. That such interactions do occur has been demonstrated by Schoonheydt and De Wilde (43) and Schoonheydt and Velghe (44) for dehydrated zeolites. They showed that the filling of the small cages with divalent ions such as Ca or Cu, i.e. putting a high positive charge density in the small cages, increases the potential energy of the cations in*S|| and consequently leads to a decrease in the activation energy for migratior. This effect is more pronounced in the case of Cu, a finding which agrees with the fact that Ca prefers site I (45) and Cu site Γ (46). The oxygen atoms of the hexagonal prism provide a better shielding effect for S|| sites. Additional evidence may be found in hydrated systems. Maxwell and de Boer (42) have shown that in hydrated Cu faujasite 6.3 ions/u.c. were found in Sj; no other Cu ions were localized. In view of the theoretical argument by Mortier (31 ) connecting site occupancy numbers and site energy differences, it would follow that in hydrated faujasite, site Γ is by far the energetically most favorable site for copper ions. It is not unlikely that a similar interpretation may even hold for calcium ions. Relying on the calcium occupancies shown in Table I for CaY, it is seen from the ratio of S|/S|| occupancies, that Sj is energetically the most favored site. The selectivity values among the transition metal ions at low loading increases in the order Ni < Co < Zn < Cu < Cd in X, Y and A (32, 33, 37, 38). This is the sequence reported by Barrer and Townsend (39) for mordenite. In view of the fact that in zeolite X, Y and A , different normalization factors, i.e. different stan dard states, were used, we are somewhat reluctant to compare Δ G° values since a more or less pronounced involvement of small cage sites may be implied, it is perhaps more important that this selectivity sequence coincides with the order found by Ahrland for the relative concentration of inner sphere complexes of these ions with sulfate anions (47). It is also relevant that Ni and Co ions, which seem most reluctant to penetrate the "small cages, have been shown to exist as fully hydrated octahedrally coordinated species in hydrated zeolite X, Y and A (48-50). Upon dehydration these ions pass through an intermediate near-tetrahedral stage and more finally to the hexagonal prisms (51) where they return to an octahedral coordination.


MOLECULAR

186

SIEVES—Π

0.8

+

0.6

0.4


O. 2 Journal of the Chemical Society, Faraday Transactions I Figure 1. Ion exchange isotherm for Cu in NaX and NaY at 25°C (37)

0.2

0.4

0.8

0.6

Su

+ 2

C

Journal of the Chemical Society, Faraday Transactions I Figure 2. Ion exchange isotherm for Zn in NaX and NaY at 45°C, 25°C, 5°C (from top to bottom) ( 37 )

0.2

0.4

0.6

0.8


I.O


16.

CREMERS

Ion Exchange

in Zeolites

187

The foregoing shows that the thermal history of transition metal exchanged zeolites determines the ion distribution in that high temperature conditions leave these ions in the small cages where they seem irreversibly locked upon rehydration (21). A similar process may occur when submitting fully hydrated zeolites to a higher temperature (34,41); upon returning to a lower temperature, part of the ions which moved to the sodalite cages, no longer participate in the ion exchange equilibrium. In studying ion exchange equilibria of the ions it is strictly necessary not to submit the hydrated samples to any higher temperature prior to equilibrium studies at a lower temperature. Only under these conditions can reversibility be ensured. An attempt to resolve ion exchange isotherms in terms of more than one group of sites was made by Gallei, Eisenbach and Ahmed (35). They showed that the isotherms for Na-Co, Na-Ni and Na-Mn in the supercages of X and Y zeolite could be described on the basis of a model of two groups of sites with a character istic selectivity coefficient. A similar attempt was made by Costenoble and Maes (52) and Maes and Cremers (53) in a combined X-ray and ion exchange study of silver ions in zeolite Y. They showed that the Ag-Na exchange in zeolite Y could be described in four types of sites I, Γ, II and U (unlocalized) in which Ag showed the highest selectivity for site I. The Ag-Na exchange in the presence of a large excess of Cs ions was shown to be confined to the small cages and could be described in terms of two groups of site I and Γ. The important fact was that the presence of Cs ions in the supercages had a lowering effect on the silver selectivity for site I. More significantly, the occupancy of silver ions in S| was reduced by a factor of 2 which again demonstrates the possibility of interactions between site groups: These data show that, as already emphasized by Sherry, an entering ion does not necessarity take the leaving ion's place in the crystal. Ion Exchange Equilibria in Chabazite and Mordenite. Barrer and Klinowski (54) have studied the exchange of alkali- and alkaline earth metal ions in a high framework charge chabazite-type zeolite (55) and compared its behavior with that found earlier for natural chabazite (56). Except for Cs, the alkali metal ions exchange readily; the exchange for the divalent cations is slow although full ex change is achieved. The incomplete exchange of Cs is explained in terms of electro static repulsion due to its large size. The slow rate of exchange of the ions Ca, Sr, Ba is interpreted on the basis of the tightly held hydration sheath. The thermody namic affinity sequence is Cs> K > Na> Li and the differences in affinity between a given pair of ions is larger in the zeolite of lower charge density, as is found in X and Y zeolite (23). The effect is explained in terms of a dielectric theory based on the consideration that the local dielectric constant in a given cationic form of the zeolite of higher charge density exceeds the value of the zeo lite of lower charge. Perhaps some care should be taken in generalizing this result to other silicates. For example, the opposite effect is found in the case of a series of isostructural montmorillonites of different charge density (57) which had been prepared by a method based on the Hofmann Klemen effect (58). The ion exchange adsorption of alkali and alkaline earth metal ions was studied in synthetic mordenite by Barrer and Klinowski (59). The thermodynamic affinity sequence for monovalent cations is Cs > Na > L i , which is typical for some other zeolites, provided that normalization methods are used in case of incomplete exchange. The extent of exchange which could be achieved for divalent ions is


188

MOLECULAR

SIEVES—II


.60 for Ca, .62 for Sr and .84 for Ba and is thought to be associated with strong hydration and the corresponding difficulties in entering the side pockets in this zeolite. Complex Ions in Zeolites. It is well known that transition metal ions may form stable complexes in dehydrated zeolites such as X and Y (60) (§1) (62) and that the adsorption of ligands may lead to a shift of metal ions from the small cavi ties to the supercages (63) (64) (65). Such cationic complexes may form in hydra ted zeolites and may be introduced by simple ion exchange methods (60) (67). Although some studies have been carried out in other silicates (68), very limited attention was given to these phenomena in zeolites (66) (69). In general it is seen that the adsorption behavior of such complexes in aluminosilicates is quite differ ent from the one found for the hydrated cations. The formation of adsorbed complexes may be characterized in quantitative thermodynamic terms, as is com monly done in aqueous solution. A preliminary treatment of the quantitative rela tions between adsorption behavior and excess stability was presented by Pleysier and Cremers (68). More recently, a more rigorous thermodynamic approach on these interrelations was presented by Maes, Marynen and Cremers (70). A systematic study of the stability of the ethylenediamine complexes of Cu, Zn, Ni and Cd in hydrated X and Y was carried out by Peigneur (66). It was shown that the one-complexes were stabilized by some two to three orders of magnitude whereas the two complexes were strongly destabilized, as inferred from the shift of the metal ions into the liquid phase at high ethylenediamine concentrations. Ion Exchange Kinetics Isotopic exchange kinetics were studied for Zn ions in zeolite X and Y by Dyer and Townsend (71 ) and in zeolite A by Radak, Gal and Salai (72). Dyer and Townsend showed that the self-diffusion of Zn ions in X and Y could be described by a simple equation relating to the case of self diffusion out of a sphere into a well stirred fluid. Two Y zeolites, with Si/A1 ratio 1.87 and 2.62, and X zeolite were studied at low and high temperature. In all cases the resulting τ vs t plots were linear. Except in the case of ZnX at high temperature, the Arrhenius plots were linear for all cases. A summary of the kinetic data is shown in Table II. These results were discussed in terms of differences in the ordering of the zeolitic water by the diffusing Zn ion. The activation energies for Zn diffusion were similar to these reported by Radak, Gal and Salai (72), who analyzed the self-diffusion of Zn in zeolite A on the basis of the Brown-Sherry-Krambeck (73) model of a fast diffusion process coupled with a slow first order exchange between mobile and sited cations. This model has been found adequate for Na self-diffusion in zeolite X and A (74) but no tests were made as yet on transition metal ions. The kinetic data are shown in Table III. A comparison of the data for the two particle sizes shows that, as re quired by the model, the diffusional frequency B, depends on the reciprocal of R , i.e. Β = π D / R . The rate constants for the slow intracrystalline exchange step, which should be independent of particle size, are seen to be systematically lower in the large particle size by a factor of about 2. A comparison of the relative rates for the two processes in ZnA and NaA is relevant. In zeolite A , the two rates are of a similar magnitude whereas in ZnA, the diffusional frequency Β exceeds the rate constant for the slow process by an order of magnitude. Although the two 2

2

2


16.

Ion Exchange

CREMERS

TABLE

189

in Zeolites

I. Ion Localization in hydrated zeolites NaX, KX, NaY, KY, CaY (26.8 ion/u.c.) CaNay (19.3 Ca ions/u.c) and CaKY (25 Ca ions/u.c.) Data are taken from references (24) (26) (27).

Site

KX

9 Na

8.9 Κ

8 Na 12 H 0

7.2 Κ

1

1

NaY

NaX

CaY

CaNaY

CaKY

9.7 Ca

17.3 Na

10.6 Ca

30.7 H 0

15.4 H 0

25.2 H 0

3.1 Ca

6.0 Ca

6.5 Ca

KY 1.3 Κ

17.3 Na

13.3 Κ


2

13.4 Η 0

II

26 H 0

II

24 Na 8 H2O

10.2 Na

23.2 Κ

20 Κ

2

2

2

2

2

TABLE II . Kinetic data for Zn isotopic exchange (71)

4.35.10 ~

1.87 Y

-36 " -12

2.88.10

1,87 Y

64 - 108

2.62 Y

-36 - -12

3.04.10"

TABLE III.

45

76.6

119.8

224.0

50.2

59.8

-82.5

82.0

87.5

127.3

51.5

53.8

-91.3

78.3

10

1

1.06.10"

10

Diffusion coefficients and rate constants for the self-diffusion of Zn in zeolite A(72) on the basis of the Brown-Sherry-Krambeck model (73).

Particle Temp radius (Mm) °K

30

35.6

2.59.10

75 - 106.5

2.62 Y

89.6

4

6

KJ/mole

J/°/mole

(KJ/mole)

m 2.sec*l

54.5 - 94.5

Χ

AG*

AS*

Temp, range (°C)

Zn-zeolite

B(TT D/R ) 2

2

sec "1

D m^ec

Parameters for diffusion process

1

1.87.10 •16 D - Ι Ο " ·

298

2.05.ΚΓ

313

8.11.10"

333

3.39.10"

3.09.10 -15

298

8.89.10"

1.82.10'

318

5.19.10"

1.05.10"

333

1.68.10-

3.44.10"

3

6

6

5

7

6

5

7.39.10 •16

16

15

15

Exchange rate k (sec )

Parameters for exchange rate

2.25.IO-

k =10 -

2

9 7

_1

7

Q

3

B/k

2

9.11

05

0

m sec"l

sec'l

6.92.10'

2

7

11.72 17.12

1.98.10"

7

Ε =67.1 ο

7.14.10"

8

Ε . = 56.6 k

12.45

KJ mol" 3.44.10"

KJ mol" 15.09

9.48.10 -7

17.72

1

7

1


MOLECULAR

190

SIEVES—II

processes remain coupled in ZnA, the diffusional movement is by far the faster process indicating the more pronounced siting in the case of Zn. The activation energy values for both processes are significantly higher than in NaA (ED=22.4 and Ek=42.5 KJ/mole). The activation energy for the slow exchange process is lower than for the diffusional process, although the expected order is found for the free energies of activation ( A G ° = 117 KJ/mole). This is related to the very large entropy loss for the exchange process (AS|< = -194 and A S Q = 27 J.K: mol ). The authors suggest that in the activated state the derealization of the Zn ion from the sites is accompanied by considerable water dipole ordering around the strongly hydrated Zn ion. Tracer diffusion studies of Na and Κ ions were made in homoionic and biionic chabazite by Duffy and Rees (75) using the Carman Haul equation (76). It was shown that, in the homoionic zeolite, the self-diffusion coefficient of K, Dj

Ion Exchange in Zeolites - ACS Publications

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