Molecular sieves vol 1 5 karge weitkamp vol 3 post synthesis modification i 2003

Plots of the corrected selectivity coefficient defined below; see Ion Exchange in Molecular Sieves by Conventional Techniques 3 Fig.. Phenomena which havereceived either considerable att

Preface to Volume 3

For many purposes, zeolites and related materials are not utilized in the thesized form Rather, they are only employed after an appropriate post-synthe-sis modification

as-syn-Undoubtedly, the classic procedure of zeolite treatment after synthesis is that

of ion exchange achieved through treatment of a suspension of the sized (or natural) zeolite powder (usually in the sodium or potassium form) in

as-synthe-an aqueous solution of a salt containing the cations to be introduced Starting inthe 1930s, this type of ion exchange has been extensively studied, not only as amethod of preparation, but also with respect to thermodynamics and kinetics.Application on an industrial scale is well developed and, because of its im-portance, ion exchange in zeolites has been reviewed several times Thus, thefirst chapter of Volume 3 of the series “Molecular Sieves – Science and Techno-

logy”, which was contributed by R.P Townsend and R Harjula, was able to focus

on the developments and advances made during the last decade It emphasizesthe need for improvement of theoretical approaches, utilization of the rapidlygrowing computational power, and the importance of acquiring reliable data asthe bases for progress in fundamental studies on conventional ion exchange.The more recent development of solid-state ion exchange and related modi-fication techniques such as reactive ion exchange between solid zeolite powdersand solid or gaseous compounds containing the cations we wish to introduce is

rather exhaustively dealt with in the subsequent chapter written by H.G Karge and H.K Beyer The concept of solid-state ion exchange is explained and con-

trasted to the conventional exchange process Experimental procedures as well

as techniques for monitoring the solid-state modification of zeolites are scribed in great detail and illustrated by a large number of investigated systems.Related methods of post-synthesis modification, possible mechanisms, and firstapproaches to study the kinetics of solid-state ion exchange are discussed.Post-synthesis modification of zeolites via alteration of the aluminum con-tent of the framework became a most important topic of zeolite chemistry when,

de-in the mid 1960s, the effect of stabilization through dealumde-ination was

discov-ered In Chapter 3, H.K Beyer contributes a systematic review on techniques

for the dealumination of zeolites by hydrothermal treatment or isomorphoussubstitution amended by a section on the reverse process, i.e., introduction ofaluminum into and removal of silicon from the framework

Methods of post-synthesis modification essentially different from those cussed in the first three chapters are based on the generation of extra-frame-

dis-work aggregates of metals (as presented in the chapter by P Gallezot), ionic sters (as described in the contribution by P.A Anderson), and oxides and sulfi- des (treated in the last chapter written by J Weitkamp et al.) One of the main

clu-motivations for studying the generation of such clusters inside the void volume

of zeolite structures originates, of course, from possible applications in catalysis.This is most evident in the case of metal cluster/zeolite systems which are suc-cessfully employed in heterogeneous catalysis of hydrogenation, hydrocracking,hydroisomerization, etc However, both ionic clusters and oxidic and sulfidicclusters hosted by the frameworks of zeolites are interestring candidates as cata-lysts for base-catalyzed, redox, photocatalyzed and perhaps other reactions Inview of cluster formation with zeolites as hosts, questions of size, location, dis-tribution, interaction with the framework, and stabilization of the active aggre-gates play a decisive role Thus, in all three contributions on clusters in zeolites,methods of their preparation as well as problems of their characterization andutilization as catalysts and photosensitive materials, as sensors, in optics, andelectronics are extensively dealt with These areas are still challenging for futureresarch and promising in view of potential applications

However, not all important phenomena of post-synthesis modification arecovered with the present six chapters of Volume 3 of the series ‘Molecular Sieves– Science and Technology’ Topics such as, for instance, ‘Incorporation of Dyesinto Molecular Sieves’, ‘Preparation of Ship-in-the-Bottle Systems’, ‘SecondarySynthesis in Zeolites’, ‘Pore Size Engineering’, ‘Modification of MesoporousMaterials’ are equally important and, to a large extent, presently subject to veryactive research and development Therefore, such topics will be dealt with in one

of the subsequent volumes under the title ‘Post-Synthesis Modification II’

Jens Weitkamp

1 Introduction . 2

1.1 The Importance of Ion Exchange Phenomena in Molecular Sieves 2 1.2 Origin and Nature of Ion Exchange Behaviour in Molecular Sieves 5 2 The Importance and Utility of Theoretical Approaches . 9

2.1 Preference, Uptake and Selectivity 9

2.2 Batch and Column Exchange Operations 13

2.3 Thermodynamic Parameters, Non-Ideality and the Prediction of Exchange Compositions 16

2.4 Kinetic Processes and the Prediction of Rates of Exchange 20

2.4.1 Hierarchical Model of Zeolite Particle or Pellet 21

2.4.2 Intraparticular Exchange Rate Processes 22

2.5 Trace Ion Exchange 24

2.6 Column Models 27

3 Experimental Approaches . 29

3.1 Practical Experiments 29

3.2 Pitfalls 30

3.2.1 Selectivity Reversal and Ion Sieving 31

3.2.2 Zeolite Hydrolysis Effects 32

3.2.3 Colloidal Solids in Suspension 36

4 Concluding Remarks . 38

References . 39

Ion Exchange in Molecular Sieves

by Conventional Techniques

Rodney P Townsend1, Risto Harjula2

1 Scientific Affairs, Royal Society of Chemistry, Burlington House, Piccadilly,

London W1J 0BA, UK; e-mail: townsendr@rsc.org

2 Laboratory of Radiochemistry, PO Box 55, 00014 University of Helsinki, Finland;

e-mail: risto.harjula@helsinki.fi

Dedicated to Professor Gerhard Ertl on the occasion of his 65 th birthday

Molecular Sieves, Vol 3

© Springer-Verlag Berlin Heidelberg 2002

Introduction

1.1

The Importance of Ion Exchange Phenomena in Molecular Sieves

Throughout the 1990s there was a decline in the number of fundamental studiescarried out on the ion exchange properties of zeolites and related materials Onehas only to examine the content of published conference proceedings on thesubject over the last 20 years to observe this trend: the situation has moved fromone where whole sessions were devoted to ion exchange studies, to one wherethe subject is subsumed into sessions covering other areas Part of this decline is

to be expected, as increased attention has been rightly paid to the intriguingpossibilities that can arise through the exploitation of newer alternative post-synthesis methodologies, many of which are discussed elsewhere in this volume.Nevertheless, the fact remains that conventional ion exchange techniquescontinue to be used routinely for post-synthesis modification during the prepa-ration of molecular sieves for major industrial applications Also, there are nowareas where molecular sieves find major application directly as ion exchangers

per se In this respect the situation has changed markedly since the early 1960s,

when Helfferich, in his classic book on ion exchange, could justifiably describezeolites “as ion exchangers they are of little practical importance” [1] Thesedirect applications are especially detergency [2–7] and also the removal ofnuclear waste [8–13] or other environmental pollutants [3] However, it is

generally a combination of properties of a particular zeolite in addition to its

ion exchange capability that has tipped the balance in favour of its use, rather

than any intrinsic superiority per se, which the zeolite may possess as an ion

exchanger

If, therefore, conventional ion exchange remains an important post-synthesispreparative technique, and the materials have in addition major direct applica-tions as ion exchangers, why have the number of fundamental studies decreased?

It is certainly not because ion exchange behaviour of molecular sieves is ciently well understood and predictable to render further fundamental researchstudies unnecessary Two causes are suggested to explain this decline:

suffi-1 Many theoretical treatments of the ion exchange reaction within zeolites(both equilibrium and kinetic) are obscure and complicated This has with-out doubt rendered inaccessible the real value of the work to those manyworkers who have a practical need to predict and control ion exchange be-haviour during the industrial exploitation of molecular sieves Althoughtheoretical understanding is important, it is easy to forget that the end pur-pose of such work should be to provide information and tools that the

chemical engineer or other user of the molecular sieve can apply simply and

effectively Obscurities in theoretical treatments mean that users often do notappreciate how basic theory can be used, not just to simplify the number ofmeasurements which need to be made, but also to predict and control be-haviour during application The theory should not be an end in itself!

2 The second cause is related to the first Even where the value of theory for theprediction and control of the behaviour of these materials has been recog-nised, the utility of these approaches has often been greatly reduced because

of the experimental methods which have been employed or by the poorexperimental data which have been available, or both Indeed, it is only com-paratively recently that a proper recognition has arisen concerning the num-ber of potential pitfalls and difficulties that can militate against the acquisi-tion of meaningful and accurate experimental data

A good example of this is the frequently studied Na/Ca-zeolite A system, whichhas received much attention because of its importance in detergency applica-tions Careful and detailed experimental studies over a period spanning some

20 years by different sets of workers [14–20] resulted in calculated values of thestandard free energy of exchange (kJ equiv–1*) which ranged from –0.59 [14] to–3.09 [17] Plots of the corrected selectivity coefficient (defined below; see

Ion Exchange in Molecular Sieves by Conventional Techniques 3

Fig 1. Plots of the logarithm of the corrected selectivity coefficient ln KG [cf K E

A/B in

Eq (7b)] as determined by different workers for the Na/Ca exchange in zeolite A E–Ca is the

equivalent fraction of calcium in the zeolite [(Eq (3b)] BRW Barrer, Rees and Ward [14];

A Ames [15]; WF Wolf and Furtig [16]; SW Sherry and Walton [17]; BR Barri and Rees [18]; WGC Wiers, Grosse and Cilley [19]; FT Franklin and Townsend [20] Taken from [8]

* Throughout this paper the term “equiv” denotes 1 mol of unit negative or positive charges.

Eq 7b) naturally show a similar diversity but also differ from each other in curveshape and trends (Fig 1) These marked differences (particularly at the extrema

of the plots) were variously ascribed to experimental error [20], to variablequantities of non-exchangeable sodium in the materials employed [20] (thematerials differed in their source and in their method of preparation [14–20])

or to variable levels of hydronium exchange depending on the pH and other ditions used [20, 21]

con-Thus, even for this very important example, not only is some of the publishedtheoretical work difficult to interpret, but also experimental data from differentstudies are frequently incompatible and incomplete

It is essential therefore that a critical review of advances over the last decadeshould look at the developments in the context of the field as a whole This is ourintention here After a discussion of the origin, ubiquity and nature of ionexchange behaviour in molecular sieves, recent advances in the application ofthermodynamic and kinetic descriptions of the ion exchange process will

be described This will demonstrate some of the shortcomings of currentapproaches, together with the relative paucity of reliable literature data thatcan be applied easily and practically This whole topic has particular rele-vance to those industrial applications where zeolites are used directly as ionexchange materials and this will be exemplified throughout the chapter usingtwo main examples The first of these is the application of A- and P-type zeolites

as detergent builders, where the approach is to use a batch exchange approach

to remove hardness ions (especially calcium) as fast as is practicable beforethe indigenous water hardness harms the wash performance of the detergentproduct The second concerns the treatment of nuclear waste, where a variety

of higher silica zeolites have been employed using a continuous (column)process to remove, and subsequently store, high concentrations of monovalentand divalent radionuclides such as caesium and strontium For both thesemajor applications, in addition to selectivity, it is noteworthy that the systemsare normally multicomponent, that the kinetics of exchange are all im-portant and that the morphology of the exchanger material must be controlledcarefully

Post-synthesis modification comes into its own when preparing molecularsieves with desirable and exploitable properties other than those of ion ex-change, be they optical, magnetic, catalytic or adsorptive Here it is not directlythe thermodynamic and kinetic ion exchange properties that are of primeimportance but rather which experimental, preparative methods are most com-monly used Thus it is important to assess what are the most appropriate exper-imental methods of preparation, as well as to review the many pitfalls one canfall into which can subsequently give rise to very inaccurate and inadequateexperimental data These experimental problems can include frameworkhydrolysis, hydronium exchange, dealumination, the presence of key trace impu-rities, dissolution phenomena, carbonate and bicarbonate interference, colloidalphenomena, metal ion complex formation and cation hydrolysis

Having thus reviewed developments and advances over the last decade, thechapter concludes with some recommendations on directions and topics for thisarea of research in the future

Origin and Nature of Ion Exchange Behaviour in Molecular Sieves

Ion exchange is a characteristic property manifested by most molecular sieves

In essence, whenever isomorphous replacement of one cation by another of ferent charge occurs within an initially neutral crystalline framework such as apure silica molecular sieve, then a net electrical charge remains dispersed overthat framework This is neutralised through the presence, within the micropo-rous channels, of cations of opposite charge (often referred to as counterions)

dif-An example of this is seen in the introduction by direct synthesis of small tities of aluminium into the silicalite framework to give the material ZSM-5.Silicalite, the pure silica analogue of ZSM-5, is then seen to be just the end-mem-ber of a set of isomorphous microporous molecular sieves that exhibit ionexchange properties which are a function of the quantity and distribution ofaluminium atoms within the structurally similar frameworks In addition, sinceone can prepare, through post-synthesis modification of the framework com-position, a variety of other isomorphous metallosilicates and metal alumino-silicates, it is obvious that zeolites possessing ion exchange capabilities are acommon occurrence

quan-Pure aluminium phosphate molecular sieves are probably more commonthan are pure silica analogues of zeolites They resemble pure silica zeolites inthat they possess frameworks that are electrically neutral, but there is a signifi-cant difference between these two classes of inorganic solids In topologicalterms both are 4:2 connected nets of T:O atoms (“T” denoting tetrahedralframework and “O” denoting oxygen) From this it is obvious that it is onlyrequired for the T ion to have a charge of +4 for the connectivity of the net togive rise naturally to a neutral framework in concert with the oxide anions This

is fulfilled for pure silicalite In the case of ALPO molecular sieves the

require-ment is also fulfilled, but the 4:2 T:O net now comprises two types of strictly

alternating T-cations (aluminium and phosphorus, possessing respectively mal positive charges of 3 and 5) Providing the cations alternate strictly through-out the framework, the 4:2 Al,P :O net holds no overall charge; however, incontrast to a pure silica zeolite, where the formal charge at every atomic centre

for-is zero, within a pure AlPO the formal charge for-is not dfor-ispersed homogeneously,but changes from –1 at each aluminium to +1 at each phosphorus This greaterheterogeneity of charge distribution may in part explain the experimentalobservation that ALPOs frequently exhibit poorer thermal stability than do puresilica zeolites

For a particular ALPO molecular sieve to possess an ion exchange capacity as

an intrinsic property, it is necessary to prepare a material where some of the minium and/or phosphorus framework atoms have been replaced by otheratoms of different charge This can occur using for example silicon, to form theso-called SAPO materials, or with metals in addition or not to silicon, to formrespectively the so-called MeAPSO and MeAPO analogues However, it is impor-tant to note that although silicon could in principle replace either aluminium orphosphorus to give rise to positively or negatively charged SAPO molecularsieves, respectively, in practice only the latter process seems to occur, or another

alu-Ion Exchange in Molecular Sieves by Conventional Techniques 5

process in which two silicons replace one of each of aluminium and phosphorus,which gives rise to no net change in framework charge [22] In MeAPSOs, diva-lent or trivalent metal ions replace the aluminiums in the framework In this waythe charge imbalance is minimised as these isomorphous substitutions eithermake no difference to the overall framework charge (T3+ for Al3+) or onlyincrease it by one negative charge per substitution (e.g Mg2+for Al3+), a processanalogous to when aluminium replaces silicon in aluminosilicates [22].

Overall therefore, and in common with aluminosilicate zeolites, the norm isfor MeAPSOs and MeAPOs to possess cation exchange properties rather thanthe reverse In this respect, zeolites and ALPOs resemble many other classes ofion exchangers that are mineralogical in origin, such as the clay minerals Theseare layered materials where a cation exchange property can arise primarily fromisomorphous replacement of trivalent cations by divalent, or tetravalent cations

by trivalent ones, within the layers [23] However, there is a major exception:

these anionic exchangers are the double metal hydroxides, which are also

lay-ered structures but which exhibit a net positive charge across the lattice The

“parent” material here is the mixed Mg,Al hydroxide, commonly referred to ashydrotalcite It would be intriguing to understand better the conditions (if any)under which one might expect to synthesise microporous three-dimensionalframework structures which similarly have a net positive charge dispersed overthe lattice and hence an anion exchange capacity coupled with a molecular sievecapability

It is important to note that, up to this point, we have been considering the lite, ALPO, SAPO, etc., as being described adequately as a 4:2 T:O net This topo-logical description, which in general terms is, as Smith points out [24], nothingmore than a mathematical construct of the human brain, does neverthelessallow us to appreciate both the origin and magnitude of an ion exchange capac-ity arising from T-atoms being replaced by others of different charge However,this description is not sufficient to cover the observed differences in ion

zeo-exchange properties (i.e selectivity, kinetic rate, level of zeo-exchange) that may be

seen between various molecular sieves having similar exchange capacities Tounderstand these differences, one must not only examine more closely the topo-logical properties of the nets but also bring to bear structural considerations.Considering these topological properties in more detail, it is adequate at thispoint to take as read that all the T-atoms within the microporous net are joined

to each other by bridging oxygens One can therefore concentrate on the atoms only and describe molecular sieves in terms of four-connected three-dimensional (4-conn.3D) nets of T-atoms [25] that, in turn, can be derived fromappropriate 3-conn.2D nets [26] Considering the latter nets first, these differfrom one another in the ways the nodes (T-atoms) link to each other via net-works of polygons Any node can then be described by its “vertex symbol”, viz

T-by its surrounding polygons with the number of each type of similar polygonsurrounding the node being denoted by a superscript [26] Thus the simplestexample of a 3-conn.2D network (the hexagonal net) becomes a 63-net; a morecomplicated example could be the 4.6.12-net which forms the basis for thegmelinite structure [26] Note that all the nodes within each of these two sepa-rate examples are topologically equivalent This need not be the case For exam-

ple, consider the case of mordenite (Fig 2), which is derived from a (4.5.8)1(4.5.12)1(5212)1(5.8.12)1-net containing four topologically distinct types of T-atoms [24].

Similar considerations apply when one considers the 4-conn.3D nets thatconstitute molecular sieves Here it is often convenient to describe the structure

in terms of polyhedral units or cages, with the polyhedra described

topological-ly in terms of face symbols [25] (not to be confused with vertex symbols definedabove) Thus the face symbol for the familiar sodalite unit, which is geometri-cally a truncated octahedron, is 4668with all vertices geometrically and topolog-ically equivalent If these units are then linked together, for example eitherthrough their 4-windows or half their 6-windows, one forms respectively thezeolite A and faujasitic structures Both these structures possess cubic symme-try, with each structure comprising 26-hedral cages connected to each otherthroughout the microporous zeolite framework, but the vertices of the sodaliteunits are no longer all topologically equivalent For zeolite A the sodalite unitsenclose a cage which is the great rhombicuboctahedron (4126886) [25] whereasfor faujasite the cage is the so-called 26-hedron type II, denoted by the face sym-bol 4641264124[25]

Why are these matters significant when one considers the ion exchange erties of molecular sieves? The answer is that these topologically non-equivalentT-atoms combined with the overall structural properties of the three-dimen-sional microporous framework often give rise to several very different types oflocal environments which repeat themselves regularly throughout the crys-talline structure These different local environments, evidenced by solid stateNMR combined with X-ray crystallography [27], are distinct in themselves, dif-fering from each other sterically and electronically, and these differences will be

prop-Ion Exchange in Molecular Sieves by Conventional Techniques 7

Fig 2.Structure of mordenite viewed along the main 8-ring and 12-ring channels parallel to the c-axis Four topologically distinct types of T-atoms are observed within the 3-conn.2D (4.5.8) 1 (4.5.12) 1 (5 2 12) 1 (5.8.12) 1 net

manifested not only through their characteristic adsorptive and catalytic iour, but also through their ion exchange properties Formally, therefore, zeolitesmay be regarded as comprising a set of crystallographically distinct sublattices,each having characteristic selectivities for different exchanging cations, depend-ing on these local environments [28] The overall ion exchange behaviour of amolecular sieve can therefore be a subtle function of the structural and topo-logical properties combined An important combination of structural and topo-logical properties concerns the ordering of isomorphously substituted frame-work atoms [29]: this determines what fraction of the overall framework charge

behav-is found on each sublattice Other significant structural properties can be losses

in symmetry through restricted rotation [27], and whether the sites are ble to exchanging cations (i.e the sizes of the micropore channels allowingingress and egress of exchanging cations plus water)

accessi-A further point is worth emphasising: since site heterogeneity in a particularzeolite is manifested through such a set of crystallographically distinct sublat-tices, zeolites differ in this respect significantly from some other common class-

es of ion exchangers, such as the clay minerals or the resins Whereas in zeoliteswell-defined sites are repeated regularly through the crystalline matrix, in clayminerals and resins site heterogeneity is often manifested in terms of patches, orregions of the surface where the sorption energies are approximately constant[30] Thus a statistical thermodynamic model of ion exchange for clay mineralsand resins [30] can differ markedly in character from ones developed for zeolites[31, 32]

As a consequence of all these factors combined, both the equilibrium andkinetic aspects of selectivity and uptake of ions within molecular sieves canrarely be understood in a straightforward manner Phenomena which havereceived either considerable attention in recent years or deserve further studyinclude the so-called “ion sieve effect”, behaviour of high silica materials, theeffects that framework flexibility can have on selectivity and rates of exchange,multicomponent ion exchange, prediction of exchange equilibria, and the possi-bility of inducing phase transitions within zeolites through ion exchange Many

of these are considered further below

So far we have considered topological and internal structural factors whichgive the molecular sieve particular ion exchange properties However, an ionexchange capacity can also be manifested which is not an intrinsic property ofthe material The source of this property is unsatisfied valencies occurring at thetermination of the crystal edges and faces, or at faults within the crystallinestructure In formal terms, the origin of this is topological, in that this inciden-tal and secondary property arises from disruptions in the net at interfaces, sur-faces and faults, but the nature and extent of this incidental property dependsessentially on structural and morphological characteristics For the former, wecan take as an example an ion exchange capacity arising either from the pres-ence of silanol groups [33, 34], or from hydroxyl groups attached to aluminiumatoms situated at the surface [35] In clay minerals, as much as a fifth of the totalexchange capacity may arise from such sources whereas in the case of zeolitesthe contribution of such incidental (or secondary) ion exchange properties isusually small compared to the intrinsic, or primary source The exception here

can be high silica zeolites [35, 36], whose overall ion exchange properties havereceived considerable attention over the last decade [37–42].

Interestingly, the external morphology can also be an important factor indetermining ion exchange behaviour of molecular sieves The crystal habit, theaverage crystallite size, the distribution of crystallite sizes and the properties ofaggregates of crystallites can all affect the magnitude of secondary ion exchangecharacteristics, since these can alter significantly the surface to volume aspectratio and hence the number of external surface sites available [35] Also, thekinetic properties may depend on these morphological characteristics, asinstanced by recent studies on a highly aluminous form of zeolite P [6, 7]

2

The Importance and Utility of Theoretical Approaches

When a zeolite in (say) the sodium-exchanged form is suspended in a solutioncomprising a mixture of different cations and anions, two properties of thematerial are brought into sharp focus The first of these concerns which types ofcations are “preferred” over sodium or each other by the zeolite This property iscommonly referred to as the selectivity of a given form of zeolite for anothercation, but there are so many definitions of “selectivity” that the term “prefer-ence” may be better used for the present The second key property to which one’sattention is drawn, and which is separate from selectivity (however defined), isthe rate at which the mixture of cations achieves its equilibrium distributionbetween the exchanging phases (viz., the electrolyte solution and the sublatticeswithin the zeolite)

2.1

Preference, Uptake and Selectivity

The preference manifested by a molecular sieve for a particular cation is

strong-ly dependent not onstrong-ly on the character of the material under examination, butalso on the conditions of the system as a whole (viz., temperature, perhaps pres-sure, composition of exchanger and solution phases, pH, nature of solvent, etc.).Given a comprehensive definition of these conditions, the preference of a givenform of zeolite for a given cation will then be invariant for that set of conditionsbecause it is essentially an equilibrium property of the system However, it isimportant to define clearly what is meant by “preference” There are numerousselectivity coefficients defined in the literature and, on occasion, “selectivitycoefficient” is confused with “separation factor”, a function whose value doesdepend strongly on the total ion concentration in solution Similarly,“uptake” or

“loading” is often confused with “capacity” To distinguish these terms, a fewbasic definitions are required

Considering as an example a binary exchange involving cations A (valency

z A ) and B (valency z B), the reaction equation is usually written as:

Ion Exchange in Molecular Sieves by Conventional Techniques 9

where the overbars denote the exchanger phase The preference displayed by thezeolite for one ion over another is then described by a selectivity coefficient,which is just a mass action quotient According to the choice of concentrationunits, a series of these selectivity coefficients may be defined which differnumerically from one another:

corre-icant water content k X

A/Bhas been used most extensively for studies on zeolites

The selectivity coefficients given in Eq (2) can be used to derive more mental equilibrium properties of the system, such as the standard thermody-

funda-namic functions describing the exchange reaction (viz DG q , DH q , DS q), providedone has information on the nature and extent of all activity corrections for non-ideality However, the key point to note is that having defined the referencestates, by contrast with a selectivity coefficient, these standard thermodynamicfunctions are independent of exchanger composition since they refer by defini-tion to a reaction between components which move from one set of specific,defined standard states to another The magnitudes and signs of these standard

functions therefore give no immediate information whatsoever on the actual

preference which a zeolite may display for a particular ion under a given set ofexperimental conditions This point, obvious to the thermodynamicist, hasoften been missed, and effort has been invested uselessly in attempting to relatecalculated values of standard thermodynamic functions to mechanistic theories

of exchange under real conditions This has resulted in work being publishedthat is of little practical utility, if not plainly wrong The issue of misunder-standing and consequently misusing thermodynamic data in this manner isexpanded elegantly by McGlashan [44]

The selectivity of a particular molecular sieve for a given ion as a function ofexchanger composition is normally measured from an ion exchange isotherm,which is an isonormal [45], isothermal and reversible plot of equilibrium distri-butions of ions between the solution and zeolite phases It is emphasised that it

is only valid to calculate selectivity coefficients, and derived thermodynamicdata, from isotherms which are reversible (that is, the forward and reverseisotherms coincide within experimental uncertainty) The types of isotherms,

and the causes for the shapes observed, are discussed elsewhere [45] However,two isotherm types, which are particularly characteristic of molecular sieves(although not uniquely so), are shown in Figs 3 and 4.

Figure 3 shows isotherms for which only partial exchange for the incomingcation occurs The isotherm plots enable one to distinguish clearly variousbasic definitions Taking, for example, a constant level of exchange or uptake for

an incoming ion (e g E–A= 0.5, then for this given uptake, the selectivity cient can vary from low to high values (cf the two depicted curves) The abscis-

coeffi-sa of the isotherm ranges from E A = 0 to E A = 1; values of E–Aare determined

by dividing the uptake by the ion exchange capacity, which is the number ofexchange sites of unit charge per unit quantity of exchanger (defined as con-

Ion Exchange in Molecular Sieves by Conventional Techniques 11

Fig 3. Examples of ion exchange isotherms exhibiting both unselective and selective iour towards the incoming ion A (curves are respectively convex and concave with respect to the ordinate) Clear limits to exchange are also observed which are lower than those expected

behav-on the basis of the theoretical exchange capacity of the zeolite The arrows depict reversible behaviour

Fig 4.Example of an ion exchange isotherm showing non-reversibility of exchange within a plateau region, characteristic of phase separation and coexistence of two phases over the com- position range corresponding to hysteretic behaviour

venient – see comments on this above) However, Fig 3 shows curves which are

asymptotic to values of E–A< 1, demonstrating that the maximum uptake (orloading) under specified experimental conditions for the incoming cation can

be less than what would be expected from the value of the ion exchange ity The cause of this may be due to inadequate experimental rigour, especiallyduring batch exchange experiments (see Sects 2.2 and 3.2.1 for further discus-sion); however, genuine “ion sieve” or “volume steric” effects can also operate as

capac-a consequence of the crystcapac-alline capac-and microporous ncapac-ature of moleculcapac-ar sieves.Ion sieving, known for a long time and commonly observed in zeolites, ariseswhen part of the microporous channel network within the molecular sieve isinaccessible to the incoming exchanging cations simply because their ionicdiameters exceed the free diameters of the windows through which they mustpass [46] The “volume steric” effect is less common, and arises when thecations have free access to the microporous voids and channels within thecrystal but nevertheless the size of the incoming ions is such that the channelsare completely filled before 100 % exchange for the incoming ion can beachieved [47]

Over the last decade, during a series of studies on high silica zeolites ing ZSM-5, ZSM-11 and EU1-1, another possible cause for partial exchange hasbeen identified Although full exchange of hydronium ion for sodium wasobserved by Chu and Dwyer for a range of high silica zeolites [37], and ion sieveeffects were identified by the same workers to explain partial exchange withsome organic-substituted ammonium cations in ZSM-5 [39], Matthews and Reesfound more complex behaviour with alkaline earth and rare earth cations inZSM-5 [38] Univalent cations exchanged to 100% but this was not the case formultivalent cations Part of the explanation for the significantly lower maximumloadings found with multivalent cations (especially Ca2+and La3+) was ascribed

includ-to the distribution of the relatively low number of aluminium ainclud-toms in theframework, which could make it difficult for multivalent cations to neutraliseeffectively widely spaced negative charges on the framework [38] To test thishypothesis, McAleer, Rees and Nowak [40] carried out a series of Monte-Carlosimulations which implied that the charge on divalent cations could only be sat-isfied adequately by aluminium atoms within the framework which were spacedapart by < 0.12 nm More recently, similar experimental and theoretical studieswere carried out on zeolite EU-1, where analogous behaviour to ZSM-5 wasobserved, although cut-off values for exchange were much higher in EU-1 [41].Topological and structural differences between ZSM-5 and EU-1 were proposed

as explanations for this different behaviour [41] (see the earlier discussion inSect 1.2)

Figure 4 shows a type of isotherm shape that is seen with crystalline ionexchangers such as molecular sieves and clay minerals, but is nevertheless rela-tively uncommon The shape resembles the type II vapour adsorption isotherm

of the Brunauer classification, having a clear “plateau” region and inflexionpoint An example is the Na/K exchange in zeolite P [48] that was found to bereversible over the whole range of equivalent fraction of potassium in the crys-

tal (E – K) Zeolite P has the gismondine-type structure (GIS [49]) More

common-ly, isotherms of this type are found to be partially irreversible in the plateau

region, resulting in a hysteresis loop between the forward and reverse isotherms(Fig 4) Examples of such hysteretic behaviour include the Na/K and Na/Liexchanges in zeolite K-F [50], which is a framework structure isotype of eding-tonite EDI [49], and the Sr/Na exchange in zeolite X [51] Isotherms of this type(whether fully reversible or not) are characteristic of systems where the process

of exchanging one cation for another induces structural distortions and changes

in the molecular sieve framework, resulting in the end-members of the exchange

(E–A = 0 and E–A= 1, respectively) being different phases If the framework is ible and consequently the required structural transformation can occur readily,the plateau region (where the two phases coexist) will be reversible This is thesituation observed for the Na/K exchange in zeolite P [48] which has long beenrecognised as a material which exists as several structural varieties [52] depend-ing on ion exchange form and level of hydration [53] and which is recognised ashaving an unusually flexible framework [49, 52]

flex-When a hysteresis loop occurs, this corresponds to a situation where the members of the exchange exhibit limited mutual solid solubility; in other words,over this region of the isotherm two separate phases coexist Barrer and Kli-nowski considered the conditions under which phase separation may be expect-

end-ed to occur in a statistical thermodynamic treatment involving an interaction

energy for entering ions wAA/kT [31] When this term is sufficiently negative, sothat the cations segregate rather than form a homogeneous phase, they showedthat conditions could arise under which a physical mixture of two A- and B-typecrystals has a lower free energy than the homogeneous A/B phase [31] If inaddition the nuclei of the A-rich phase grow within the B-rich “parent” phasematrix then two positive free energy terms are involved in the exchange process.These are a strain free energy resulting from the misfit between the new grow-ing phase within the old, and an interfacial free energy These tend “to delay theappearance of the new phase beyond the true equilibrium points for forwardand reverse reactions” [31] This is the proposed explanation for the hystereticbehaviour seen in systems such as the Na/K and Na/Li exchanges in K-F [50] orthe Sr/Na exchange in X [51], and contrasts with P [48, 53] This has significancefor the use of a high aluminium analogue of P in detergency [6, 7] This materi-

al, named “maximum aluminium P” (MAP), has the gismondine frameworkstructure of zeolite P but with a Si/Al ratio of unity [6] The unusually flexibleframework [49, 52] is reported to lead to cooperative calcium binding, as well as

to unusual water adsorption/desorption properties that enhance bleach stability[6, 7] These properties, combined with superior kinetic behaviour, result in amaterial that reduces water hardness much more effectively than zeolite A (sic,[6, 7, 45])

2.2

Batch and Column Exchange Operations

Practically all industrial ion exchange applications, except the use of zeolites indetergency, involve column operations (e.g the removal of radionuclides fromnuclear waste effluents) However, basic studies of ion exchange equilibria areusually carried out using the batch method

Ion Exchange in Molecular Sieves by Conventional Techniques 13

It is instructive at this point to compare these two techniques by consideringthe conversion of a zeolite from one ionic form (B) to another (A) as shown in

Eq (1) and using the selectivity coefficient k A/Bdefined in Eq (2)

In batch ion exchange, a given amount m of zeolite in the B-form is

contact-ed with a given volume v of a salt solution of ion A At equilibrium, the ions are

distributed between the solid and solution phase according to:

c– A z B c A z B

5= kA/B

c– B z A c B z A

The progress of the reaction is illustrated in Fig 5 for two univalent cations

(z A = z B = 1) assuming a constant selectivity coefficient k A/B= 10 and an ionexchange capacity of 4 mequiv g–1 It is clear that it is difficult to obtain a highdegree of conversion by a single batch equilibration In this example, 430 cm3of0.1 equiv dm–3solution of ion A is required for 99% conversion This is almost

an 11-fold excess even though the exchange equilibrium operates in favour ofions A

In zeolites strong selectivity reversals are often observed and this makes itvery difficult to obtain a high conversion to the required ionic form This prob-lem is discussed in more detail in Sect 3.2.1 Here, conversion will be discussed

in qualitative terms The solution concentrations of A and B [Eq (4)] can bewritten as:

and

where Q is the ion exchange capacity (equiv kg–1), V/m is the solution volume

(dm3) to zeolite mass (kg) ratio in the batch equilibration and cA(o)is the initial

Fig 5. Batch exchange: loading of ion A in zeolite (solid curve) and concentration of A in tion (broken curve) as a function of solution volume when contacting 1 g of zeolite in B-form

solu-batchwise with 0.1 g equiv –1 solution of A Selectivity coefficient kA/B and exchange capacity Q

have been given values of 10 and 4.0 mequiv g –1 , respectively

Solution volume (ml)

concentration of A (equiv dm–3) in the solution To obtain a high conversion to

the A-form in a single equilibration, k A/B and c A must be high and c Bmust be low

c Bcan be made low by using a large volume of solution per unit mass of zeolite

(maximum value of c B = Q/(V/m)) (Eq 5) c Acan be made large by using a high

initial concentration of A and large V/m ratios (Eq 6).

In column exchange, a solution of ion A is passed through a column that

con-tains a given quantity (m) of zeolite This process is illustrated in Fig 6 using the

same parameters as in Fig 5 for the batch exchange In column exchange, theconversion to the A-form proceeds much more easily, as ion B is constantlyremoved from the system However, ion A is not homogeneously distributed inthe bed, but is first taken up by material near the column inlet and the conver-sion proceeds in the direction of solution flow When most of the zeolite has

been converted to the A-form, ion A starts to emerge from the column and c A

tends to the value of the feed concentration, when the column has become pletely exhausted

com-The important point to note is that by contrast with batch exchange, far lesssolution is needed for full conversion In the example of Fig 6, only 50 cm3of0.1 equiv dm–3solution is required for every gram of zeolite to achieve 99% con-version This is only a 25% excess

Figures 5 and 6 represent highly idealised cases and serve here only todescribe qualitatively the differences between batch and column exchanges Inrealistic situations, the selectivity coefficient decreases with increasing loading

of A in the zeolite (see Fig 1) This means that an even higher excess of A must

be used under real conditions In addition, in column exchange, the rate ofexchange reaction often tends to decrease at high loadings, which lowers thegradients of the loading and concentration curves (Fig 5) and increases thesolution volume needed for full conversion

Pure synthetic zeolites are fine powders that are usually unsuitable for umn operation Therefore, batch methods are used for the study of ion exchange

col-Ion Exchange in Molecular Sieves by Conventional Techniques 15

Effluent volume (ml)

Fig 6.Column exchange: average loading of ion A in zeolite (solid curve) and concentration

of A in outlet solution (broken curve) as a function of solution volume passed through the

column Mass of zeolite bed 1 g, inlet solution pure A at 0.1 equiv dm –3concentration kA/Band

Q as in Fig 5

equilibria Granular zeolite exchangers that are suitable for column work aremanufactured by using suitable binders (e.g clay, silica, alumina) and care must

be taken in extrapolating data obtained from batch experiments to columnoperation

2.3

Thermodynamic Parameters, Non-Ideality and the Prediction

of Exchange Compositions

To derive thermodynamic parameters of ion exchange, the normal procedure is

to correct for solution phase non-ideality first by deriving a corrected selectivitycoefficient in which concentrations within the external solution are replaced byactivities The means by which this may be done, for binary or multicomponentsystems, is described elsewhere [54, 55] The corrected selectivity coefficients

A/B is identical to the function K Gshown in Fig 1 and taken from [20]

The thermodynamic equilibrium constant K ais then obtained by integratingthe appropriate form of the Gibbs-Duhem equation to give as correspondingexpressions for Eqs (7a) and (7b), respectively, the following:

where D is the water activity term [56, 57] D is normally ignored on the

assump-tion its magnitude is small; however, it should be noted that for the most monly employed formulation, corresponding to Eq (8b) and after Gaines and

com-Thomas [58], D π 0 when the system is behaving ideally if z A π z Bbut rather

equates to (z A – z B) [56, 59] This must follow since, when the system is behavingideally, the values of all the activity coefficients are by definition unity for all

compositions and hence K a = K X

A/B = K E A/B= constant [56, 57] since

from a principle put forward some time ago [60], viz., that because D is small and

changes little with zeolite composition, and providing salt imbibition is ble (which is true for relatively dilute electrolyte solutions [61]), then for a giv-

negligi-en zeolite composition, the ratios of activity coefficinegligi-ents f i ,g iwill hardly change

in value as the total concentration of electrolyte in the external solution ischanged [56, 60, 62] Providing these assumptions hold, then taking as an exam-ple a binary exchange process, from Eqs (8b) and (9), it follows that [62]:

where the subscripted E–Ain parentheses indicates that the values of the

correct-ed selectivity coefficient and the rational activity coefficients refer to a

particu-lar composition E–B , E–Aand must be invariant since all the terms of the hand side are constant or hardly change when the total concentration of theexternal electrolyte solution is changed The details of the methods which must

right-be employed to predict selectivity trends are descriright-bed elsewhere [62]; the

important point to note is that if the above assumptions hold then for successful

predictions it is only required to evaluate the appropriate corrected selectivitycoefficient as a function of zeolite phase composition and to have an accurate

knowledge of the solution phase activity coefficient g [54, 55, 62] For binary

exchanges, this approach has been used to test a variety of systems over the lastdecade, including exchanges involving Pb/Na, Pb/NH4, Cd/Na and Cd/NH4equilibria in clinoptilolite, ferrierite and mordenite [63–65] using different co-anions (chloride, nitrate and perchlorate [62, 66]) as well as the Cd/Na-X andCd/K-X systems [67], with a high level of predictive success [62, 67] Recently, arelated model has been used with good accuracy for the prediction of K/Na andCa/Na equilibria over a wide range of total ionic concentrations in solution fornatural clinoptilolite [68] Successful predictions were also achieved for theCa/Na, Ca/Mg and Mg/Na systems in zeolite A [18, 20, 69]; however, for Mg/Naand Mg/NH4exchanges in a range of faujasites [70], predictions failed badly insome cases The failures were attributed at the time to salt imbibition, but fur-ther detailed experimental studies involving hydronium exchange in the Ca/Na-

X, Ca/Na-Y, Cs/Na-MOR and Cs/K-MOR systems [71–75] have shown that thesituation is in reality much less straightforward Failures in predictive methods,particularly at trace levels of exchange, cannot be attributed simply to hydroly-sis, hydronium exchange or salt imbibition despite earlier suggestions to thiseffect [70, 76] An important factor appears to be the presence of colloid-sizezeolite particles [74] These matters are discussed further in Sect 3.2.3

To apply the same prediction procedure as that described above for ternary

or multicomponent exchanges, it is helpful to derive analogous equations tothose shown in Eqs (8), (9) and (10) for binary exchange For ternary exchange,this was done by Fletcher and Townsend [77] and this approach was used topredict compositions for Na/Ca/Mg-A [20, 69], Na/K/Cd-X [67] and Na/NH4/Mg-X,Y ternary equilibria [70] For the first two of these systems, ternaryexchange equilibria were predicted successfully but for the Na/NH4/Mg-X,Y sys-tems, the procedure failed for the higher silica Y materials, as for the corre-sponding conjugate binary exchanges [70] In parallel with these studies, the

Ion Exchange in Molecular Sieves by Conventional Techniques 17

model of ternary ion exchange in zeolites [77] was compared with other modelspublished in the literature for clay minerals and resins [78, 79] and a furtherdetailed study [80] came to the conclusion that these other approaches wereappropriate under certain specified conditions [80] for the prediction of ex-change equilibria in zeolites.

A recent criticism of the ternary exchange model [81], on the basis that theequations could have simply been built up from the conjugate binary systems(obviously true), overlooks the main point If one uses the conjugate binary sys-tems it is necessary to use a model-based approach to predict activity coeffi-cients for the multicomponent exchange equilibrium in the zeolite and the pres-ence of sublattices within the zeolite framework can make this more difficult to

do than for clay minerals and resins (Sect 1.2) [80] The ternary exchange

mod-el of Fletcher and Townsend [77] does not require one to measure at all the ity coefficients, let alone predict them for multicomponent systems from binarydata, using some model All that is required is knowledge of the ternary correct-

activ-ed selectivity coefficients that are obtainactiv-ed by integrating the appropriateGibbs-Duhem equations over the ternary composition surface [77] in analogywith the binary approach pioneered by Gaines and Thomas [58] However,acquiring sufficient data for a ternary system is a difficult and time-consumingexercise [20, 67, 70, 82] and simpler approaches can prove quite adequate pro-vided one validates some of the predictions made [83] Thus, another model,developed originally for clay minerals [84], has been shown after minor revision

to work well for ternary anion [85] and cation [86] exchanges in organic resinsand has even been extended successfully to a five-component zeolitic system(Sr/Cs/Ca/Mg/Na equilibria in chabazite) [87] This system is very important inthe field of nuclear waste treatment [87]

Accurate prediction is similarly much needed for detergent applications [2, 7,

18, 69] The level and nature of “hardness” in household water varies

extensive-ly from one location to another, as do the conditions under which consumersexpect effective laundering to occur (e.g temperature) Thus accurate selectivitydata (i.e isotherms and selectivity plots as a function of loading), and reliablepredictive models that are simple to use, are important, since it would clearly beimpossible to measure directly the performance of a given “builder” zeolite forall conceivable situations Successful predictions have been achieved for thebinary Na/Ca-A, Na/Mg-A and Ca/Mg-A systems [2, 18, 69] as well as for the cor-responding ternary system [2, 69] Similar successful predictions were recentlyachieved also for zeolite MAP [7] once the original iterative procedures ofFranklin and Townsend [69] had been modified appropriately Figures 7 and 8show examples of such successful predictions in A, for both the binary andternary cases

Occasionally, isotherms of binary and multicomponent exchanges aredescribed using various empirical adsorption equations These cannot be usedfor the prediction of multicomponent equilibria [88] In fact, a closer inspection

of these equations reveals that they have no in-built facility for true prediction(i.e for the calculation of equilibria over ranges of different total solutionconcentrations for heterovalent exchanges) Thus these equations are useful

in describing the observed isotherm in a mathematical form but the only

pre-Ion Exchange in Molecular Sieves by Conventional Techniques 19

Fig 7 a, b Predicted isotherms and experimental points for a the Na/Ca-A system and b the

Na/Mg-A system Solid lines are predicted isotherms; experimental points are measured at

normalities of 0.025, 0.10 and 0.4 equiv dm –3, shown respectively as solid triangles, circles and

squares Taken from [69]

a

b

diction these equations can give is the interpolation of the isotherm underone given set of experimental conditions With such limited utility, these empir-ical approaches are not recommended for the “prediction” of ion exchange equi-libria.

2.4

Kinetic Processes and the Prediction of Rates of Exchange

In direct applications involving zeolites as ion exchangers, it is not normally thecase that the system is allowed to reach equilibrium In batch operations (e.g indetergency) the time available may be such that the exchange process is inter-rupted long before equilibrium is reached Similarly, in column operations (e.g.effluent purification), when the system is operating under steady-state condi-tions, the balance between throughput of liquid and time of exchange means

Fig 8. Ternary experimental and predicted points for the Na/Ca/Mg-A system at a normality

of 0.4 equiv dm –3 Measured solution and zeolite phase equilibrium compositions are shown

as unfilled stars and filled squares, respectively The predicted zeolite phase at 0.4 equiv dm–3

is shown as an unfilled circle while the filled circle represents experimental validations at

0.4 equiv dm –3 Taken from [69]

that the system is frequently operating under non-equilibrium conditions.Knowledge of the kinetics of the multicomponent exchange processes (i.e all ofthe reaction rates, diffusive mechanisms and hydrodynamic processes whichcontribute to the overall rates of exchange of all of the different types of ionsinvolved) is therefore of key importance if one is to be able to predict and con-trol behaviour Unfortunately, this is easier said than done The kinetics of ionexchange processes in zeolites are extremely complicated even when one focus-

es on just one mechanistic process [45]; only recently, it was rightly stated thatthe “picture presented in the literature for diffusion in zeolites is confusing, con-flicting and/or inconsistent with theory” [89] Space permits only a briefoverview of the current state of affairs and this is presented here using a hierar-chical model [90] for the zeolite particle or pellet Much more detail is given else-where [45]

2.4.1

Hierarchical Model of Zeolite Particle or Pellet

Whether one is considering an agglomerate of aggregated zeolite crystallites, or

a pellet, a hierarchical model [89, 90] allows one to distinguish the differenttransport and/or rate processes which operate at different length scales.The highest level is concerned with the macroparticle or pellet itself; and thekey issue here is whether transport of ions through the fluid film which encom-passes the macroparticle is rate-controlling or not That this process can be rate-controlling has been recognised for a long time, being favoured by a low con-centration of exchanging ions in solution and a small mean particle size; how-ever, it is known that the hydrodynamic regime pertaining can affect its influ-ence markedly, with high levels of agitation (such as are achieved at highimpeller speeds in a batch reactor [89]) rendering relatively insignificant anymass transfer resistance through the boundary film The mechanical integrity ofthe macroparticle can also be very important Taking detergent powder particles

as an example [which can comprise agglomerates of (primary) zeolite talline particles held together by means of adhesive, viscoelastic surfactantbridges], these are designed to break up under shear and/or other hydrodynam-

crys-ic regimes that are imposed as part of the wash cycle On breaking up and persing, some of these dispersed smaller particles may find themselves inregions of low agitation and consequently the rate of removal of hardness ionsfrom the wash liquor can be slower than desired due to the onset of film diffu-sion control

dis-Generally, however, the aim is to avoid conditions leading to film diffusioncontrol This means that the focus is shifted towards transport processes thatoccur at the intermediate level (that is, in the mesopores and macropores with-

in the macroparticle or pellet itself) and those which occur at the smallestdimensional level (viz., in the very micropores of the molecular sieve) [45,89] Within the mesopores and macropores between the primary zeolite crystal-lites transport will be dominated by molecular and ionic intercrystalline dif-fusion possibly coupled to surface diffusion processes, while, in the zeolitemicropores themselves, intracrystalline diffusion occurs, also possibly coupled

Ion Exchange in Molecular Sieves by Conventional Techniques 21

with specific exchange rates associated with the different zeolite sublattices[91, 92].

The overall observed kinetics of exchange is of course the result of all theabove-described mechanisms working in concert [45, 89] To cope with the com-plexities of the system, a simple approach one may adopt is the homogeneousdiffusion model, which assumes that the behaviour of each distinct diffusingspecies within the macroparticle may be described in terms of a single solid-phase “effective diffusivity” [89] More sophisticated approaches include theheterogeneous diffusion models, where the macropore and micropore diffusionprocesses are described separately and are then assumed in different mathe-matical treatments either to occur in series or in parallel [45, 89]

In practice, to date, most research activity has focused on the intraparticulardiffusion which takes place in the zeolite micropores themselves, on the ques-tionable assumption that these processes are normally the rate-controlling ones

2.4.2

Intraparticular Exchange Rate Processes

Our understanding of the processes which govern the rates of ion exchangewithin the micropores of molecular sieves has advanced little over the lastdecade, yet the imperative to be able to control and manipulate these ratesremains as strong as ever To summarise the current situation it is necessary first

to emphasise some basic principles and then to define certain terms and cients

coeffi-To begin, it is important to distinguish the intrinsic dynamic nature of thesystem from the kinetic processes we actually observe during an ion exchangereaction An obvious yet important point to remember is that even afterexchange equilibrium has been attained, the equilibrium is a dynamic one Thustransport of all exchangeable cations and of the solvent molecules continues but

after equilibrium has been reached there are no net changes in the relative

dis-tribution of species between, and hence concentrations in, phases with time.This dynamic character is readily verified by adding to the equilibrated system

a trace amount of a radioactive isotope of one of the cation types into (say) thesolution phase of the system and then observing the rate at which isotopicexchange between the two phases takes place The isotopic exchange processmay include as a rate-determining step an intracrystalline exchange process [91,92] but it is also certainly a transport process, which is described in terms of a

self-diffusion coefficient D* AA [93] Self-diffusion coefficients D* AA and D* BB,which can change markedly with temperature [45] or as the equilibrium con-centrations of different cations within the zeolite are altered [45, 94], should be

sharply distinguished from the exchange diffusion coefficient D AB [95] D AB

describes the kinetics of the A/B exchange process, that is, the observed rates ofchange of concentrations of ions A and B within each phase as a function of timeand as the system moves to equilibrium

Consider therefore a binary A/B exchange between the zeolite and externalsolution, which is not initially at equilibrium On mixing the two phases, the Aand B cations, which will almost certainly possess different ionic radii and pos-

sibly charge, will begin to move in their respective directions of negative ical potential gradient in order to equalise their respective chemical potentialswithin all phases in the system However, the mobilities of the two cation types

chem-A and B are likely to be different, which means that the more mobile cation typewill tend to build its concentration, and hence lower its concentration gradient,faster than the other If this process were to continue unchecked, charge separa-tion within each phase and between the phases would occur, with a concomitantelectrical potential gradient In practice, of course, the electrical potential gradi-ent that forms as charge separation takes place does not build, but rather acts toslow the faster moving cations and speed the slower ones Thus it is not adequate

to consider only the chemical potential gradients The net flux J Aof (say) the exchanging species is actually described by:

A-J A = – D AB [grad c– A – (z A c– A F/RT) grad V] (11)

where F is the Faraday constant and V the electrical potential An expression for

D ABhas been derived by Barrer and Rees using an irreversible thermodynamicapproach The form of this is complicated but, if cross-coefficients otherthan those due to the electrical potential gradient are assumed to be negligible,then [96]:

tion of ionic concentrations, but also because it is a function of both D* AAand

D* BB,which we have already noted vary with exchanger composition [45]

Sec-ondly, DABis a function of the non-ideality of the zeolite [data for which can beobtained, as we saw earlier, from the activity coefficients described in Eq (10)].One may expect therefore that to describe adequately the kinetic behaviour ofeven a binary exchange process in a molecular sieve would be a very complicat-

ed task

To validate this and other similar models, it is necessary to solve, using priate boundary conditions, the differential equations describing overall thetransient diffusion process for each ion, of the general form:

Sr/Ca-was taken into account The effect on DABof taking non-ideality into account was

even more dramatic, with a discontinuity appearing in the plot of ∂D /∂c–

Ion Exchange in Molecular Sieves by Conventional Techniques 23

(Fig 10) Even allowing for this non-ideal behaviour, prediction of exchangerates was still poor [95] Other similar studies, including the measurement of

∂D* ii /∂c– ifunctions, are described elsewhere [45, 94]

It is unfortunate that we are still not able to rationalise adequately the ics of ion exchange in zeolites, let alone manipulate rate processes For example,

kinet-in realistic detergency applications, the issue can be of prime importance, skinet-incethe contact time of zeolite suspended in the wash solution is usually shorter thanthe time required to attain the equilibrium state Elsewhere in this chapter thestrong effects that crystallite size and mesoporosity can have on kinetic rates hasbeen emphasised (Sects 1.2 and 2.4.1); it is precisely these properties which areidentified as being key (in addition to cooperative calcium binding) for thesuperior performance of zeolite MAP as a builder [6, 7]

For zeolite A, binary and ternary kinetic measurements of the Na/Ca/Mgexchange have been undertaken [21] in addition to equilibrium studies For theternary system, the inhibiting effect of magnesium on the uptake of calcium ionswas clearly demonstrated (Fig 11) [21]

2.5

Trace Ion Exchange

In the preceding sections ion exchange processes involving large changes in thechemical composition of the solution and zeolite phase have been discussed.Under these circumstances, attention has to be paid to the changes in the value

of the selectivity coefficient with composition In the case of exchange of traceions for the ion present at much higher concentrations (described henceforth asthe “bulk ion”), the situation is somewhat different This brings us to the otherimportant area of zeolite application, viz., the purification of nuclear waste efflu-

Fig 9 a, b. Radial concentration distributions at various fractional attainments of equilibrium

for the Ca/Sr exchange in chabazite for a the ideal exchanger and b the non-ideal exchanger.

The continuous lines represent the Ca/Sr exchange and the broken lines the reverse process.

Taken with permission from [95]

Ion Exchange in Molecular Sieves by Conventional Techniques 25

Fig 10. Variation of the exchange diffusion coefficient DABas a function of equivalent function

of strontium (CSr ) for the Ca/Sr exchange in chabazite Taken with permission from [95]

sys-ents This always involves column ion exchange What is of interest in this

appli-cation is the capacity (Qv) of the ion exchanger, in terms of the solution volume (V) that can be treated with a given amount (m) of ion exchanger The maximum (saturation) value of this capacity (Q v,max) is unambiguously given by the distri-

bution coefficient K Dof the radionuclide In general, the distribution coefficient

K Dof ion A is defined by the equilibrium ratio:

The distribution coefficient is determined by two factors, selectivity and ionexchange capacity Let us consider here uni-univalent exchange for clarity

Inserting c– B = Q – c– A (where Q = ion exchange capacity) into k A/Bin Eq (2) and

combining the resulting expression with Eq (15) gives for K D

It can be seen that when c B /k A/B  cA(i.e when the ion A is a radioactive traceion), then the capacity of the ion exchanger is independent of the concentration

of the trace ion A in solution but depends only on Q and CB.

Let us consider removal of radioactive Cs ions (e.g.137Cs) from a waste tion containing sodium salts As an example, the chemical concentration of137Cs

solu-in solution correspondsolu-ing to an activity concentration of 1 µCi dm–3(which is

typical in low-active waste) is 8 ¥ 10–11mol dm–3 Selectivity coefficients kCs/Naare typically in the order of 10–100 in zeolites Thus, unless the Na concentra-tion in solution is very low ([Na]  10–8mol dm–3), the K D(and volumetriccapacity) of the exchanger is independent of the concentration of caesium in thesolution and the familiar relationship is obtained from Eq (16) (B = Na, A = Cs)

for K D, in the logarithmic form

In other words, the K Dof the trace caesium ion is inversely proportional to theconcentration of the macro-ion (sodium) in the solution The selectivity coeffi-cient can be assumed to be constant in this case as the loading of caesium in the

exchanger is very low In general, the logarithmic equation for K Dis [75]

log K D = (1/z B ) log (k A/B Q z A ) – (z A /z B )log c T (18)

where C Tis the total concentration of exchanging ions in solution (mol dm–3)

Thus, plotting log K Dagainst the logarithm of the bulk ion concentration yields

a straight line with a slope of –(z A /z B) In experiments this equation is used to

determine the selectivity coefficient kA/B, which is obtained from the intercept of

the linear plot A linear plot also confirms the stoichiometry of the exchange

reaction over the concentration range of interest However, quite often the log K D

plots are linear in the more concentrated solution of B only In dilute solutions,

leveling-off of the log K Dplot is often observed This can be rationalised when it

is kept in mind that Eq (18) is valid for the free cations with the charges cated at the equilibrium concentrations of the cations Many meaningless datahave been produced when this point has been forgotten This issue is discussedfurther in Sect 3.2.2

indi-Very few data can be found in the literature for the exchange of trace ions inthe presence of more than one type of bulk ion Harjula et al [75] have studiedexchange of134Cs in mordenite and in mixed salt solutions of sodium and potas-

sium It was found that at a given constant total concentration C T (cNa+ c k) of

solution, K Dof134Cs was a linear function of the potassium loading in the lite The equilibria were treated using appropriate pseudobinary selectivity co-

zeo-efficients From the linear dependence of log K Da more simple treatment canalso be obtained, i.e.:

log K D = E–––Nalog (kCs/NaQ/C T ) + E––––K log (kCs/KQ/C T) (19)

where kCs/Naand kCs/Kare the (limiting) binary selectivity coefficients for trace

Cs exchange in pure Na- and K-forms of the zeolite There are too few data to

conclude whether the form of Eq (19) is generally valid for the calculation oftrace ion distribution coefficients in the presence of two or more types of bulkions If Eq (19) were always valid the following equation would apply for trace

ion A in the presence of M other different ions present in much higher

concen-trations:

M

i = 1

where the k A/iare the selectivity coefficients of the trace ion A in the pure i-forms

of the zeolite In order to use Eqs (19) and (20), additional models for the diction of bulk ion concentrations need to be used (see Sect 2.4)

pre-2.6

Column Models

With a few exceptions, industrial applications of zeolites involve column tion in feeds containing more than two counterions In general, therefore, theprediction of column performance involves the prediction of multicomponentequilibria and kinetics under dynamic flow conditions Considering the com-plexity and diversity of these models (see Sects 2.3 and 2.4), it is obviousthat simplifications and approximations need to be made for practical columnmodelling For engineering purposes, the most popular approach for columnmodelling is the “linear driving force – effective plate” concept [97]

opera-Consider again the removal of a radionuclide from a solution as an example

of column exchange By definition (Eq 15), K Ddescribes the equilibrium bution of ions between the zeolite and the solution However, at the same time,

distri-it is a measure of the equilibrium distribution of the solution volume and lite mass Thus the total column capacity can be calculated from Eq (16) or (18)for a simple binary system The volume that can be treated with the column con-

zeo-taining m kg of zeolite is equal to the area above the breakthrough curve (Fig 12), and can then be calculated from V = mK D However, in the purification

of radioactive effluents, it is necessary to discontinue operation, and change to afresh column, immediately when the radioactive ion first starts to emerge fromthe column The volume at which the breakthrough of the radionuclide com-

Ion Exchange in Molecular Sieves by Conventional Techniques 27

mences is called the breakthrough capacity (Q BT) of the column The through capacity is usually defined in terms of some chosen level of break-through (e.g at 1%) The efficiency of the column can then be measured by thedegree of column utilisation, which is the ratio of breakthrough capacity to the

break-maximum capacity (Q BT /K D)

According to the plate concept, the number of “transfer units” or “effective

plates” (N) is a measure of the column efficiency Increasing N makes the

break-through curve steeper and thus improves the degree of column utilisation(Fig 12) The number of effective plates can be calculated from fundamentaldata Thus for film-diffusion controlled exchange:

where s is the column length, d is the particle diameter, u ois the linear flow rate

and D fis the diffusion coefficient For particle diffusion controlled exchange:

A and B in Eqs (21) and (22) are empirical factors Both mechanisms may

con-tribute simultaneously to the overall exchange kinetics but can be taken intoaccount in an appropriate model [98]

It can be seen from Eqs (21) and (22) that the degree of column utilisationand the breakthrough capacity increase when the zeolite grain size is decreasedand the solution flow rate is decreased (Fig 12) In the limit, the kinetic perfor-mance is determined by the magnitude of the diffusion coefficients

The plate approach has been used in the development and operation of theprocess for the purification of the highly radioactive solutions that arose in theaccident in the Three Mile Island nuclear power station [99] In this process,

a mixed zeolite bed (Linde IE-96 and A51) was used for the removal of137Csand90Sr

Fig 12.Schematic representation of the effect of plate number N on the breakthrough curve

in column exchange

Experimental Approaches

It has been emphasised already that accurate and reliable data are essential inthe construction of adequate ion exchange models for the industrial applica-tions of zeolite and other ion exchangers In this section we will discuss ionexchange experimentation and its utility for industrial applications We alsodiscuss major pitfalls that may lead to unreliable results Although industrialapplications always involve more than two exchanging ions, seeing trends in theoverall equilibria under these circumstances may be difficult Therefore, in thissection, only binary ion exchange equilibria are considered in order to keep themajor issues in focus

There are basically two major reasons for studying ion exchange equilibria.The first of these is concerned with understanding selectivity and its causes Forthese studies, correlations are sought between the properties of exchangingcations (e.g cation size, charge, acidity, etc.) and the ion exchanger (charge den-sity, pore diameter, acidity, etc.) with the aim of predicting the magnitude of theselectivity In the case of zeolites, where strong decreases in selectivity are oftenobserved (see, for example, Fig 1), it is also of great interest to predict how selec-tivity changes with loading To date, no useful and general theory has beendeveloped for these predictions, in zeolites or indeed in any other ion exchangematerials

Secondly, for application-oriented studies, selectivity data are measured inorder to predict the performance of the zeolite under given operational condi-tions (ion concentration, temperature, contact time, etc.) using appropriate ther-modynamic or kinetic approaches and hence to choose and optimise the oper-ating conditions for the application in question

3.1

Practical Experiments

It should be obvious by now that one of the key tasks in ion exchange ments is the accurate determination of the selectivity coefficient In principle,this is straightforward: the zeolite, initially in the B-form, must be equilibrated

experi-in solution usexperi-ing an experi-increasexperi-ing ratio cA /c B and, after equilibrium has beenattained, the concentrations of A and B in the zeolite and also in the solution aremeasured

This can be done either batch-wise, or, in the case of the granular zeolites, umn-wise Both techniques should give in principle an identical result Howev-

col-er, there is one important difference between the two techniques In columnexchange, the equilibrium concentrations of A and B in the solution at equilib-rium will be known beforehand since these will be equal to the initial concen-trations of A and B in the feed solution Because of this, it is a relatively simplematter to decide the initial conditions for the experiments in order to determinethe selectivity coefficient as a function of the loading For instance, isonormal(e.g 0.1 g equiv–1) solution mixtures of A and B may be prepared, containingprogressively increasing amounts of A (1%, 5%, 10%, 20%, …, 90%) Each of

Ion Exchange in Molecular Sieves by Conventional Techniques 29

these solutions is pumped through the column until the outlet concentrations of

A and B are equal to those in the inlet Equilibrium concentrations of A and B inthe zeolite are then determined by direct analysis of the solid, or by analysing asolution containing the dissolved zeolite

In batch ion exchange, the equilibrium concentrations of A and B in the tion will not be known beforehand These will depend on the experimental con-ditions (total solution concentration, ion exchange capacity, and the solution

solu-volume to zeolite mass ratio V/m) This makes it difficult to decide on the best

initial conditions for the experiments Commonly, the zeolite is converted wise from the B-form to the A-form by successive equilibrations in solution mix-tures of A and B (low conversion) or in solutions of pure A (high conversion).Only some rough guidelines are available for choosing the initial conditions forthe experiment Thus, after the first measurements have been evaluated, it isoften necessary to carry out further experiments in order to fill in gaps in thedistribution of the data points across the isotherm

step-In general, the advantage of batch equilibration is that the experimental ratus is simple so that a large number of experiments can be carried out in par-allel using a minimal amount of solution and zeolite In column experiments,one column “run” is required for each selectivity measurement and a large num-ber of solution concentration measurements have to be carried out to check thatequilibrium has been finally attained Run times can be very long, especiallywhen the equilibrium is unfavourable at higher loadings or when the uptake of

appa-trace ions is being studied For instance, determination of a K D value of20,000 cm3g–1requires that at least 40,000 cm3of solution is passed through a 1-g zeolite bed Such an experiment may take several months By the batchmethod, the same information can be obtained by carrying out the experiment

in a 20-cm3plastic bottle in just 1 week It is clear that the batch method is thepreferred option when large quantities of materials are to be assessed in paral-lel, or when multicomponent equilibria are to be studied In finely divided zeo-lites the batch method may be anyway the only alternative

3.2

Pitfalls

One might infer from the above that the measurement of zeolite ion exchangeselectivities is simple In practice, several factors may interfere which maydistort the result As a consequence, in theoretical work, understanding theselectivity data may become impossible as one tries to rationalise these distor-tions without knowing their origin In application-oriented work, a com-pletely wrong picture may be obtained about the performance and utility of agiven zeolite due to these problems In the following these problems are exam-ined

Selectivity Reversal and Ion Sieving

Many ion exchanges in zeolites involve incomplete exchange so that some of theions (usually Na) originally present in the zeolite are not exchangeable for theincoming cations This may arise from ion sieving, volume steric effects or fromvery low framework charge densities (see Sect 2.1) Often divalent cations orlarge cations are partially excluded In these cases it is also common to observevery strong selectivity decreases with increasing loading of the incoming cation

A strong selectivity reversal is also common in exchanges that have nearly gone

to completion Due to this selectivity decrease it is very difficult to convert azeolite from one ionic form to another even though there should be no steric orother hindrance to 100% exchange In addition, it may be very difficult to detectwhether a genuine saturation, or maximum loading, has been achieved, sincewhen the selectivity becomes low, very small changes take place in the ion con-centrations even when the exchange is pushed forward by large increases in theamount of incoming cation

Accurate determination of maximum exchange level is very important,since in the determination of the selectivity coefficient one should consideronly those ions that are exchangeable [100] Non-exchangeable cations areobviously not formally involved in the equilibrium, so their presence need not

be taken into account directly, although of course their effects may be mademanifest indirectly in the values of the activity coefficients of the exchangingions [100] The use of the correct value for the maximum exchange limit isespecially vital in the determination of the thermodynamic quantities of theexchange reaction (i e the thermodynamic equilibrium constant or the ionicactivity coefficients in the zeolite), since the determination of these quantities

involves integrating the appropriate selectivity coefficient from E–A = O to 1

(Eq 8) and this scale and the magnitude of the selectivity coefficient strongly

depend on the choice of the maximum exchange level fmax B arrer, Davies

and Rees [101] demonstrated the great effect of the choice of the fmaxon the

magnitude and variation of the selectivity coefficient with E–A A major aim

of many fundamental ion exchange studies in the zeolites has been the nalisation of the selectivity gradient, since this reflects the non-ideality ofthe zeolite phase It is clear that any attempts to do this require a very reliable

ratio-value for fmax In addition one should not compare systems from which

ther-modynamic parameters have been derived using different values of fmax, sincethe reference states of the systems are different and therefore are not directlycomparable

As a general rule, conversion of a zeolite from one ionic form to another inone single batch equilibration is difficult, even when the zeolite is selective forthe incoming cation Considering also the common selectivity reversal exhibit-

ed by most zeolites, conversion by a single equilibration becomes a practicalimpossibility in most cases

Despite this, in many studies in the past, only single equilibrations or at best

a few successive equilibrations have been carried out to measure maximalexchange levels in zeolites It is doubtful whether these results and the selectivity

Ion Exchange in Molecular Sieves by Conventional Techniques 31

plots derived from these maximum exchanges are correct It is also obvious thatthe inaccuracies in determining the maximum exchange contribute strongly tothe frequently observed high levels of scatter in zeolite selectivity data Forinstance, zeolites NaX and NaY appear to have very different selectivities for cal-cium ions at room temperature, when the maximum exchange level of 68%,determined by a single equilibration, is used for CaNaY [102] However, whenthe exchange is pushed to a higher level by using 8–12 successive equilibrations,

an 85% exchange level is obtained and the pattern of selectivity in X and Y for

Ca starts to appear very similar [72], as one would expect intuitively for the twoisomorphous zeolites

Another obvious point, easily overlooked, is that in some cases impurities

in the salt solutions may cause the exchange to appear not to go to completion

If at high loadings of the incoming ion A the selectivity coefficient leads to avalue of (say) 0.01 and the impurity level of B in A within the salt used is (say)0.1%, it can easily be shown that no matter how many successive equilibrationsare carried out, only about 90% conversion to the A-form will be obtained It

is therefore very important to use reagents of very high purity for the ments

experi-3.2.2

Zeolite Hydrolysis Effects

Hydrolysis of zeolites gives rise to a range of “impurity” species in both the tion and zeolite phases, which may interfere with the study of binary metalcation exchange Up to now, most zeolite ion exchange research has been carriedout using initially the sodium forms of the zeolites In this form, zeolites hydro-lyse by taking up hydronium ions from the water, viz.,

be seen in the increase of the selectivity coefficients for H3O+/Na+exchange withthe increasing aluminium content of faujasite zeolites [73–75] Zeolite hydroly-sis also leads to several secondary phenomena First, since the zeolite imparts analkaline reaction to the water imbibed in the pores, carbon dioxide is picked upfrom the air When the zeolite is then immersed in water, carbonate and bicar-bonate ions are released into the solution Secondly, hydroxyl ions released intothe solution enhance the dissolution of silica and alumina from the zeoliteframework into the solution [73] As a consequence, the following electroneu-trality condition can be found to hold in pure water after it has been contactedwith a zeolite such as NaX [73]:

[H3O+] + [Na+] = [OH–] + [HCO–] +2 [CO32–] + [Al(OH)4]

Ion Exchange in Molecular Sieves by Conventional Techniques 33

Fig 13.Diagrammatic representation of F [F = (mNa+ mNH4)/mAl ] as a function of equivalent

fraction of sodium (E–Na in four faujasitic zeolites with an Si/Al ratio which increased in the

order X < Y2 < Y3 < Y4 The fractional level of hydronium exchange at any composition E–Na,

ENH4is given by (1–F) Taken from [100]

These reactions may have several effects on the binary metal cation exchangeprocess that is primarily under study These may be summarised as:

1 mass-action effect of hydronium ion exchange on the binary metal cationequilibrium;

2 association of metal cations with bicarbonate, carbonate, silicate and nate ions in the solution; and

alumi-3 cation hydrolysis (i.e association of metal cations with the OH–produced byzeolite hydrolysis and/or precipitation of metal hydroxides)

Each of these effects will now be examined in more detail, together with a sideration as to how they can be avoided or taken into account

con-Hydronium Ion Exchange Considerable quantities of hydronium ions can be

exchanged from water into the zeolite, when for example the zeolite is washedafter synthesis or even prior to ion exchange experiments Thus, before theexperiments are begun, the zeolite is likely already to be partially exchanged intothe hydronium form Preparing the zeolite in the pure sodium form may be dif-ficult, since hydronium ions can be picked up even from concentrated salt solu-tions of sodium [73] When the zeolite is in the process of being converted toanother ionic form, further hydronium ion exchange can take place in one direc-tion or the other [103] For instance, NH4+/Na+exchange in zeolites X and Y isaccompanied by significant hydronium exchange Initially, almost 20% of theexchange capacity of NaX (Si/Al = 1.26) was taken up by H3O+, and this amountdecreased steadily to about 12% upon conversion to the NH4+form (Fig 13) Incontrast, zeolite NaY (Si/Al = 2.47) contained no H3O+initially, but conversion

to the NH4+form was accompanied by H3O+exchange so that in the NH4+form,about 7% of the exchange capacity was taken up by the hydronium ions [103].The consequences of this can be profound In the past, it was common prac-tice for metal ion concentrations in the zeolite phase to be determined from the

changes in the corresponding concentrations in the solution phase It is obvious

that this can lead to significant error if significant hydronium exchange alsotakes place in parallel However, even if the metal ion concentrations are mea-sured in both phases, the calculated selectivity coefficient will not be that of thepure binary metal exchange if concomitant hydronium exchange occurs There-

fore, k A/Bvalues will not reflect the relative preference of the zeolite frameworkfor the two metal cations Because a three-component system is actuallyinvolved, relative preferences between the metal cations and hydronium ionswould be intrinsic in the selectivity coefficient and it is doubtful if this selectiv-ity coefficient could then be used for accurate prediction of the binary ion equi-libria For an accurate description of the binary and overall equilibria one may

be forced to use an appropriate ternary model (see Sect 2.3)

In general, one can detect hydronium ion exchange by measuring the balance

of the contents of aluminium and exchangeable metal cations in the zeolite[103] If all the aluminium in the zeolite is present in the framework in tetrahe-dral coordination, then the degree of hydronium exchange (DH) is given by

However, a minor fraction of non-framework aluminium may be present in thezeolite (e.g in the cation exchange sites) Framework aluminium (tetrahedral)and non-framework aluminium (octahedral) can be distinguished using 27AlNMR although it is very difficult to be quantitative [104].

It is obvious that often the interfering effect of zeolite hydrolysis and nium ion exchange cannot be avoided In general, hydronium ion exchange isfavoured in solutions of low salt concentration so one can try to minimise it bycarrying out the experiments in moderate salt concentrations

hydro-The acid/base nature of high alumina zeolites is in fact very similar to that ofweak-acid organic resins In these materials metal ion uptake depends strongly

on the solution pH [105] This can be seen for zeolites, too For instance, uptake

of caesium and strontium by chabazite or sodium A zeolite depends strongly onsolution pH (pH 2–10) at a constant sodium background of 0.1 equiv dm–3[106] The effects of hydronium ion exchange and solution pH on metal cationexchange in zeolites have been almost completely overlooked in past studies As

a consequence, there may be considerable systematic error in many publishedzeolite selectivity data, especially for high aluminium zeolites

Ion Association in the Solution Phase Ion exchange experiments are usually

carried out using anions (chloride, nitrate, perchlorate) that do not interactappreciably with the metal cations under study However, zeolite hydrolysisproduces many anionic species that tend to associate with metal ions (see Eq.24) For instance, when pure water is contacted with zeolite NaX, between

5 ¥ 10–4–5 ¥ 10–5mol dm–3soluble silica and alumina and between 1 ¥ 10–3–1 ¥

10–4mol dm–3of total carbonates can be found in solution [74], depending on

the value of V/m The pH of the solution contacted with NaX can become

mark-edly alkaline (pH 11.5–9.5) so precipitation of metal hydroxides (or carbonates,aluminosilicates) is also possible Even if no precipitation of metals takes place,ion association can have a marked effect on the observed selectivities of the zeo-

lites For example, consider the exchange of a divalent metal ion M2+for sodium

in a zeolite If it is assumed that the metal cation associates with a univalent

ligand L–to form the complex ion ML+, then this ion association can be

charac-terised by an association constant k1of the form

[ML+]

kl=

[M2+][L–]

Most analytical techniques employed in the measurements of metal ion

concen-trations in solution yield the total concentration of the metal [M] T, viz.,

Assuming that only the free metal cations are exchanged into the zeolite, the

observed selectivity coefficient [k M/Na (obs)] that one obtains from the ment for the exchange is thus:

but since [M] T = [M2+] (1 + k1[L–]), it follows that

Carbonate, bicarbonate and silicate ions form moderately strong complexeswith most metal ions For instance, the association constants of alkaline earthcations for carbonate and bicarbonate are in the range of 10–1000 [107, 108].Similarly, silicate ions complex readily with many metal ions, e.g with calcium

(k1= 1230) and magnesium (k1= 1.5 ¥ 104) [108] There seems to be no data onmetal ion association with aluminate ions but it is likely that ion association ismoderately strong here also

Cation Hydrolysis Many metal hydroxides have a low solubility in moderately

alkaline solutions arising from the hydrolysis of high alumina zeolites Forinstance, most transition metals and magnesium precipitate at pH 9–10 and atthis pH range carbonates are likely to precipitate other metals such as calcium,strontium and cadmium [109] Such precipitation phenomena can seriously dis-tort the measurements of ion exchange selectivities In addition, even when themetal concentrations are below the limits of hydroxide precipitation, hydrolysedspecies, such as MOH+, M(OH)2(aq) and M(OH)3, often form the majority of themetal species in solution For the determination of the ion exchange selectivitycoefficient, the concentration of free, non-hydrolysed metal cation should beknown If the concentrations of hydrolysed species are used, a too low value mayagain be obtained for the selectivity coefficient

3.2.3

Colloidal Solids in Suspension

Very fine colloidal particles in the nanometre size range may be left suspended

in solution when centrifugation or filtration is used for the phase separationoperation during the measurement of ion exchange equilibria in zeolites andother inorganic materials [75] Especially in the study of the ion exchange ofradionuclides, which can be present in very low concentrations, the presence ofcolloidal particles carrying the metal cation under study can bring large errors

in the determination of discrete metal cation concentrations in solution.Depending on the analytical technique used, metal ions associated with col-loidal particles may be indistinguishable from free metal ions For instance, inthe determination of the distribution coefficients of radionuclides (Eq 15), largeerrors may take place [75] In cases where the metal ion concentration in the col-

loidally suspended zeolite is much larger than that of the free metal ions, the

measurement of KD (cm3 g–1) yields just the reciprocal of the concentration(g cm–3) of suspended zeolite in the solution, instead of the ratio of metal con-centrations in the zeolite and solution phases [75]

The presence of colloidal zeolite particles in solution may also apparentlydecrease the selectivity of the zeolite for a given metal ion This problem isencountered especially when very low elemental concentrations, corresponding

to low degrees of loading, are measured by highly sensitive methods such as bythe use of radioactive tracers [75] or by atomic emission or absorption spec-trophotometry with plasma or graphite furnace atomisation Considering again

the exchange of a divalent metal cation M2+ for sodium as an example, the

observed selectivity coefficient k M/Na(obs) would be given by

[3M2+][Na+]2

k M/Na (obs) =

([M2+]+[M] c)[4Na+]2

where [M] c is the amount of metal M in the suspended colloids per unit volume

of solution Recent studies indicate that as much as 3 mg dm–3of suspended lite may be present in solution after centrifugation with a low-speed centrifuge(G = 2000) [75]

zeo-The results of several studies in the past have shown very low selectivities and

“strange” selectivity gradients compared to more recent studies For instance, inthe study of calcium, strontium and barium exchanges in zeolites NaX and NaY,selectivities were low for these ions at low degrees of loading and then increased,finally exhibiting maxima at high loadings [101, 110] In these experiments ini-tial metal concentrations were very low (and the corresponding equilibriumconcentrations even much lower) and cation loadings in the zeolite wereincreased by increasing the initial metal ion concentrations.Appearance of max-ima in the selectivity plots is difficult to rationalise in the absence of a phasechange (Sect 2.1) since one would expect that the most selective cation siteswould be occupied first so that the selectivity would steadily decrease withcation loading This common pattern has been observed in other studies,carried out in isonormal solutions at considerably higher concentrations(0.1–1.2 g equiv–1) for most alkali and alkaline earth cations in NaX and NaY[72, 102, 111] It is therefore likely that the observed low selectivities at low met-

al loadings and very low solution concentrations for NaX and NaY are due to ionassociation or suspended colloidal zeolite in the solution phase, since, at thesevery low solution concentrations, most of the metal ions in the solution mayhave been present as other species rather than as free metal cations Similardecreases of selectivity were observed for the calcium exchange in NaX and NaY

in dilute isonormal solutions (N T<0.1–0.001 equiv dm–3), but, in more trated solutions, the selectivity was independent of total ion concentration in

concen-solution (N T= 0.1–1.2 g equiv dm–3) [72]

Theoretically, after a correction is made to the selectivity coefficients (Eq 2)for solution phase non-ideality (Eq 7), the obtained corrected selectivity coeffi-cients are then independent of the total ion concentration in the solution at anygiven degree of ion loading in the zeolite, provided that no other parallel reac-

Ion Exchange in Molecular Sieves by Conventional Techniques 37

tions (salt imbibition, ion association) affecting the metal ion distributionbetween the zeolite and solution phase take place This means that one can inprinciple check whether the selectivity coefficient is unaffected by the above fac-tors by carrying out the determinations of selectivity in isonormal solutions ofdifferent total ion concentrations In general, these interfering phenomena arestrongest at low metal concentrations, when the metal ion concentrationsbecome comparable to or lower than the concentration of interfering species(e.g complexing ligands, suspended particles) In many cases these interfer-ences can be avoided by using sufficiently high total ion concentrations in solu-tion; however, in some cases, interferences may operate even in rather high con-

centrations (N T= 0.1–0.4 equiv dm–3) Additional interference in the

determi-nation of selectivity may be caused in concentrated solutions (N T>1 equiv dm–3)

Secondly, the relative neglect of detailed kinetic studies in recent years hasbeen noted Although our attempts to describe theoretically the rates of ionexchange (let alone predict them) remain simplistic (Sect 2.4), nevertheless itshould be obvious to the reader that most applications involving ion exchangeprocesses in molecular sieves are likely to be kinetically controlled Thereremain major theoretical and computational challenges in this area which willentail the utilisation of the current rapidly growing computational powertogether with increasingly sophisticated models to throw light on exchange rateprocesses, both at the atomistic and mesoscopic scales

Finally, computational approaches are obviously only as good as the mental studies which underpin and validate them Consequently, we hope thatthe growing awareness of the experimental pitfalls and complexities which canhinder the acquisition of reliable data [115] will encourage further fundamentalstudies on ion exchange processes, not only in zeolites, but especially in thealuminophosphate families of molecular sieves, where so much unexplored ter-ritory remains