Measurement and prediction of Ca–Na ion-exchange equilibrium inmaximum aluminium P (MAP), a zeolite with the GIS frameworktopology

Christopher J. Adams, Abraham Araya, Karen J. Cunningham, Kevin R. Franklin* andIan F. WhiteUnilever Research Port Sunlight L aboratory, Quarry Road East, Bebington, W irral, UK L 633JW

Ion-exchange equilibrium studies have been carried out on the system CaÈNa-zeolite MAP (maximum aluminium P), and com-parison made with those reported previously for CaÈNa-zeolite A. Over much of the exchange system, zeolite MAP shows greaterselectivity for calcium than does zeolite A. The high selectivity for calcium shown by MAP and the unusual Z-shaped selectivityplot are attributed to the formation of coexisting sodium- and calcium-rich zeolite phases. The selectivity coefficient at anyKGgiven calcium loading is invariant with solution total normality over at least the range 0.025 to 0.4 equiv. dm~3, and predictionprocedures dependent on this requirement have been tested successfully.

The use of zeolites as builders (water softeners) in fabricwashing products is now widespread. The original choice ofzeolite for this application was zeolite A because of its highion-exchange capacity, good selectivity for calcium, acceptableCaÈNa ion-exchange kinetics, but mainly because of its easeof manufacture and cost.1 The study of CaÈNa ion exchangein zeolite A has consequently been studied extensively over thelast 30 years.2h11 Recently, however, an alternative zeolite hasbeen designed, developed and commercialised, speciÐcally as adetergent builder. This material, zeolite MAP, is a P-typezeolite (GIS framework topology) with an Si : Al ratio of 1 : 1,which has been shown to possess many advantages overzeolite A with respect to its use in modern detergents.12,13 Adetailed study of ion exchange equilibria in this zeolite has,however, not until now been reported.

Investigation of the ion-exchange properties of zeolites overa broad range of experimental conditions has been consider-ably aided by the development of predictive procedures basedon rigorous thermodynamic approaches.9,14 Such proceduresallow the ready comparison of equilibrium data obtainedunder very di†erent solution-phase conditions, such as solu-tion total normality and counter-ion type. The predictivemodels have, however, only been tested successfully with rela-tively few systems, such as CaÈNa-A,6,9 and their applicabilityto a broad range of systems is still in some doubt.15h17

In this paper, the unusual ion-exchange behaviour encoun-tered in the CaÈNa-MAP system is described, and the testingof some of the equilibria prediction procedures with thisimportant system is reported. Throughout the paper compari-son is made with the more widely studied CaÈNa-A system.

Basic theory and prediction proceduresFor a binary system containing ions and the ionAZA` BZB`exchange reaction can be written as

ZA BzZB`] ZBAsZA`] ZA BsZB`] ZBAzZA` (1)

where s and z indicate the solution and zeolite phases, respec-tively. The equilibrium constant is then deÐned as

Ka \aAzZB

aBzZA

aBsZA

aAsZB(I)

where and are the activities of ions A and B in theaAz aBzzeolite and and are the activities of ions A and B inaAs aBssolution. Expanding this in the normal way gives

Ka \x(Az)ZB

x(Bz)ZA

mBZA

mAZBCF\ KG F (II)

Where and are the equivalent fractions of ions Ax(Az) x(Bz)and B in the zeolite, and are the molarities of ions AmA mBand B in solution, and C and F are corrections for solution-phase and zeolite-phase non-ideality18 and may be evaluatedby the approaches of Glueckauf19 and Gaines and Thomas,20respectively. can be evaluated using8,18,20Ka

ln Ka \ (ZB [ ZA) ]P0

1ln KG d x(Az) (III)

and the standard Gibbs energy of exchange calculated from

*G0\ [(RT /zA zB) ln Ka (IV)

The selectivity coefficient, is a particularly useful quantityKG ,in that it can give a measure of the zeolite selectivity for ion Aat any given loading, that is independent of solution-phaseconditions such as counter-ion type and solution total nor-mality Implicit in this is that(nt).

(dKG/dnt)x(Az) \ 0 (V)

This assumption is expected to apply where salt imbibitionand hydrolysis/hydronium exchange are insigniÐcant.9,14,15Eqn. (II) and (V) together provide the basis for a procedure topredict the equilibrium position for an ion-exchange reactionover a wide range of solution conditions from one set ofexperimental data.9,14 Taking the deÐnition of given inKGeqn. (II) and substituting solution-phase equivalent fractionsfor the solution molarities such that

mA \ x(As)nt/ZAand

mB\ x(Bs)nt/ZB (VI)

and rearranging gives

KGx(Bz)ZA

x(Az)ZB\

x(Bs)ZA

x(As)ZB

C

Q(VII)

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where

Q\ZBZA

ZAZBnt(ZB~ZA) (VIII)

If now as a function of is known from one set ofKG x(Az)experimental data, then by Ðxing a value of the left-x(Az),hand side of eqn. (VII) can be evaluated. If we now select avalue of the right-hand side of eqn. (VII) can also be evalu-nt ,ated. Since C is, however, a function of these valuesx(As),cannot be evaluated directly but must instead be obtained byiteration.9,14 Employing this procedure the equilibriumsolution-phase composition for our chosen zeolite loading(equivalent fraction) and solution total normality can be pre-dicted.14 Alternatively the solution-phase composition can beÐxed, thus allowing direct evaluation of the right-hand side ofeqn. (VII). and can then be determined by iteration.KG x(Az)In this case we can predict the zeolite-phase composition for agiven solution-phase composition and total normality.9,15,21It is this latter approach that has been used in the currentwork.

For any system of interest it is essential to establish Ðrstthat eqn. (V) holds, before attempting to predict changes inthe ion-exchange position with total solution normality. Infact very few systems have been checked thoroughly in thisway. For the systems CaÈNa-A and MgÈNa-A eqn. (V) hasbeen shown to hold for total normalities in the range 0.4 to0.005 equiv. dm~3,6,9 and also for MgÈNa-X, CdÈNa-X andCdÈK-X in the range 0.025 to 0.4 equiv. dm~3.15,21 Ithowever fails over all total normality ranges for MgÈNa-Yand Harjula et al.16 also showed that, for theMgÈNH4-Y.15systems CaÈNa-X and CaÈNa-Y, eqn. (V) is applicable at totalnormalities above ca. 0.01 equiv. dm~3, but fails signiÐcantlyat lower concentrations. To the authorsÏ knowledge the appli-cability of eqn. (V) has not been tested with any combinationof cations with P-type zeolites, or any of the natural or syn-thetic zeolites with Ñexible framework structures similar toMAP.

ExperimentalA commercial detergent grade sample of Zeolite Na-MAP(Doucil A24) was obtained from the CrosÐeld Group. Asdescribed elsewhere,13 the zeolite consists of small crystallites(ca. 50 nm) assembled into micrometre-sized particles. NospeciÐc pretreatment was carried out on the zeolite exceptthat the material was equilibrated with water vapour oversaturated KCl solution for a week before use.(aW\ 0.843)Chemical composition of the zeolite, obtained by X-Ray Ñuo-rescence and thermogravimetric analysis, is given in Table 1.The equilibrium isotherms were constructed by contactingbetween 0.1 and 0.5 g of hydrated zeolite with 50 cm3 of solu-tion containing appropriate quantities of sodium and calciumnitrate in sealed polypropylene bottles. Total solution normal-ities employed were 0.4, 0.1, 0.05, 0.025 and 0.01 equiv. dm~3.The suspensions were shaken continuously for 7 days at 25 ¡C.The suspensions were separated by centrifugation, and afterwashing the zeolite three times with water it was dissolved ina mixture (by volume) of 25% concentrated nitric acid and

Table 1 Chemical analysis of zeolite Na-MAP and unit-cell com-position

SiO2 32.4%Al2O3 28.2%Na2O 17.7%H2O 21.7%Si/Al 0.97Na/Al 1.03

unit cell composition, Si7.9 Al8.1Na8.1O16 É 14.3H2O É 0.3NaOH

75% water. Calcium and sodium were determined in both thesolution and zeolite phases by inductively coupled plasmaspectroscopy. The aluminium content of the zeolite phase wasalso determined by the same method. Reversibility(replacement of Ca ions in the zeolite with Na ions) was inves-tigated by the wet method detailed elsewhere.22 Equilibriumconcentrations of each ion in solution were calculated fromthe experimental data in terms of equivalent fractions on thebasis that

x(Caz) ] x(Naz) \ 1and

x(Cas) ] x(Nas) \ 1

The data were thus adjusted for any hydronium ion exchangewhich may have occurred.8 Mass balance calculation sug-gested, in general, that \5% of the calcium and sodium ionsin the zeolite were replaced by hydronium.

The data for the system CaÈNa-zeolite A, which are usedfor comparative purposes throughout this paper, werepublished previously by Franklin and Townsend8,9 and are ingood agreement with several other data sets for the samesystem.3,5,6,8

Results and DiscussionIon-exchange isotherms and selectivity data

The CaÈNa-MAP ion-exchange isotherm obtained at 298 Kand a solution total normality of 0.1 equiv. dm~3 is given inFig. 1. The selectivity for calcium over sodium is seen to bevery high over most (99%]) of the isotherm. The reversepoints tend to suggest that the exchange is thermodynamicallyreversible although the shape of the isotherm makes trueassessment difficult. For comparison, an isotherm measured atthe same temperature and total normality for the systemCaÈNa-A is also shown (taken from ref. 8). The much greaterselectivity for calcium shown by MAP at high calcium load-ings is clearly seen. Although not apparent from Fig. 1, asemi-logarithmic representation of the isotherms (Fig. 2)shows that at low calcium loadings, although in both zeolitesthe absolute selectivity for calcium is high, zeolite A is moreselective for Ca than is zeolite MAP. The reversibility of theCaÈNa-MAP system is also more clearly demonstrated in thisrepresentation of the isotherm.

The selectivity plots [ln vs. derived from the 0.1KG x(Caz)]equiv. dm~3 data for both CaÈNa-MAP and CaÈNa-A areshown in Fig. 3. The solid lines through the experimental data

Fig. 1 CaÈNa ion-exchange isotherms measured at 0.1 equiv. dm~3and 298 K. Zeolite MAP forward points, (*) reverse points ;())zeolite A (data from ref. 8) (]) forward points, reverse points.(K)

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Fig. 2 Semi-logarithmic plot of the CaÈNa ion-exchange isotherms.Details as Fig. 1.

were obtained by Ðtting the data with polynomials of the form

ln KG \ K(1)] K(2)x(Caz) ] K(3)[x(Caz)]2

] K(4)[x(Caz)]3 (IX)

The polynomial coefficients are given in Table 2, and wereused in the calculation of the standard Gibbs energies ofexchange and in the equilibrium prediction procedure to cal-culate values of at any value of While a singleKG x(Caz).polynomial could be easily used to Ðt the zeolite A data, thecomplex shape of the selectivity plot for zeolite MAP was notamenable to similar treatment. Two separate polynomialscovering the range to 0.97 and to 1x(Caz) \ 0 x(Caz) \ 0.97were therefore used for this system.

Fig. 3 Selectivity plot for CaÈNa-MAP and CaÈNa-A. Details asFig. 1.

Fig. 3 shows quite clearly the greater selectivity of zeolite Afor calcium at low zeolite loadings and the[x(Caz) \ 0.25],much greater selectivity of MAP at higher loadings. The plotfor zeolite A is typical of that found for many exchangesystems reported in the literature ; viz. ln decreasesKGsmoothly with increasing calcium loading. The selectivity plotfor CaÈNa-MAP is much more unusual and is Z shaped. Thisshape of plot has been interpreted previously in terms ofstructural framework changes and the presence of twocoexisting zeolite phases.23,24 Using similar arguments thecurrent plot may be interpreted as follows. In the compositionregion to ca. 0.1 calcium ions are homogeneouslyx(Caz) \ 0distributed within essentially an Na-MAP framework. In theregion ca. 0.97 to 1 sodium ions are likewise distributedx(Caz)within a Ca-MAP framework. Both these regions show similarbehaviour to that shown across all compositions for zeolite A;ln and hence selectivity for calcium, decreases withKG ,calcium loading. In the composition region 0.1 tox(Caz) \ ca.ca. 0.97 the selectivity for calcium increases with calciumloading. Here cooperative binding of calcium is thought tooccur, resulting in the formation of a calcium-rich and asodium-rich phase or domain. If these two phases were totallyimmiscible the solution-phase composition would be constantover this entire region and only the relative amounts of thetwo coexisting zeolite phases would change. In the region

0.1 to ca. 0.7 this does appear to be the case (Fig.x(Caz) \ ca.2), however in the region 0.7 to ca. 0.97 a signiÐ-x(Caz)\ ca.cant increase in the associated solution calcium concentrationis observed. It thus appears that the immiscible two-phaseregion is smaller than suggested by the selectivity plot (Fig. 3).In the region where the solution-phase concentration rises it ispossible that two partially miscible phases exist. Alternatively,it may be a single-phase region in which the zeolite has eitherthe same structural form as the pure calcium zeolite or has amodiÐed framework structure. X-Ray di†raction studiescarried out to investigate the suggested phase behaviour willbe reported elsewhere. The existence of phase-miscibility gapscaused by ion exchange are not uncommon and are oftenassociated with zeolites showing considerable framework Ñex-ibility.23,25h29 The most noteworthy example in the presentcontext is that of an immiscibility gap extending over 20È40%of the zeolite composition observed by X-ray di†raction withCa and Na in high-silica zeolite P (Si/Al[ 1.6).26 No formalisotherm data were, however, reported in this study and it is,therefore, not known whether the unusual selectivity behav-iour shown by MAP is also exhibited by higher Si : Al ratioP-type zeolites.

Also shown in Fig. 3 are the reverse ion-exchange points forMAP. These conÐrm that the system is, indeed, thermody-namically reversible and hence that a thermodynamic treat-ment to obtain the equilibrium constant and the standardKaGibbs energy of exchange is justiÐable. Values calculated forMAP and A are given in Table 3, and show that MAP has amuch greater overall selectivity for calcium over sodium thandoes zeolite A.

Equilibrium prediction procedures and isotherm predictions

Fig. 4 shows a series of selectivity plots for the system CaÈNa-MAP measured at total normalities in the range 0.01 to 0.4equiv. dm~3. Over the range 0.025 to 0.4 equiv. dm~3 theselectivity coefficient for a given calcium loading is seen toKG

Table 2 Polynomial coefficients expressing ln as a function ofKG x(Caz)

Zeolite x(Caz) K(1) K(2) K(3) K(4) K(5) K(6)

MAP 0È0.97 5.5629 [45.151 280.41 [647.50 650.60 [236.96MAP 0.97È1 [6241.4 5318.6 8806.1 [7884.9

A (ref. 8) 0È1 6.8614 [15.556 23.132 [14.048

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Table 3 Equilibrium constants and standard Gibbs energy ofexchange for CaÈNa-MAP and -A

Ka *G2980 /kJ equiv.~1

CaÈNa-MAP 62.9 [5.13CaÈNa-A 9.83 [2.83a

a Ref. 8.

be essentially invariant. Some deviation does however occurat 0.01 equiv. dm~3, particularly at low calcium loadings. Thereason for this is unclear. Harjula et al.16,17 considered severalpossible explanations for reduced ion-exchange selectivity atlow ionic strengths, but came to no Ðrm conclusion. In thepresent case the apparent reduced selectivity for calcium at0.01 equiv. dm~3 is, however, probably an artefact caused byexperimental error in the determination of the calcium con-centration in the solution phase, caused by contaminationfrom the glassware and polyalkene equipment employed, orby the incomplete separation of calcium-rich colloidal zeoliteÐnes from the solution phase ; in the region where the devi-ation is most apparent, concentrations of calcium in solutionof the order of 2.5 ] 10~6 M were obtained, but these wouldneed to have been ten times lower for the calculated values of

to have agreed with those obtained at higher total normal-KGities.Despite the applicability of eqn. (VI) to the CaÈNa-MAP

system over a wide range of total normality, initial predictionsof equilibrium composition using the procedure described byFranklin and Townsend,9 met with limited success ; some pre-dictions agreed well with experimental measurements whileothers were a long way out. With CaÈNa-A and other systemswhere eqn. (V) was found to hold, this procedure had workedwell in all cases.9,15,21 The failure with the CaÈNa-MAPsystem was found to be a consequence of the iteration pro-cedure adopted to evaluate the left-hand side of eqn. (VII).Franklin and Townsend9 suggested limiting the possiblevalues of and which should be tested by imposing ax(Caz) KGstarting point governed by the value of and at thex(Caz) KGtotal normality of the measured experimental data (i.e. 0.1equiv. dm~3). The direction of the iteration was then deter-mined by a set of conditions linked to whether the prediction

was lower or higher than 0.1 equiv. dm~3. This procedurenthowever seeks only the Ðrst minimum in the di†erencebetween the two sides of eqn. (VII). For zeolite MAP, predic-tions failed because the Ðrst minimum found was often not theglobal minimum, a consequence of the unusual variation of

Fig. 4 Selectivity plots for CaÈNa-MAP measured at di†erent totalnormalities. (]) 0.4, 0.1, 0.05, (*) 0.024, (]) 0.01 equiv. dm~3.()) (K)

with The iteration procedure was therefore modi-KG x(Caz).Ðed to remove any conditions on the values tested ; values offrom 0 to 1 in steps of 0.001 were tested, and thenx(Caz)reÐnement down to 0.000 01 was carried out once the global

minimum had been identiÐed.Full predicted CaÈNa-MAP isotherms for solution total

normalities of 0.025 to 0.4 equiv. dm~3 obtained using thismodiÐed procedure, together with experimentally measuredpoints, are given in Fig. 5 and 6. The semi-logarithmic plot(Fig. 6) illustrates particularly well the quality of the predic-tions at all The predictions do not, however, reproducent s.exactly the region in which the solution-phase composition isinvariant with varying zeolite-phase composition, but theerrors are relatively small. The reason for this inaccuracy inthe predictions appears to emanate from the polynomialÐtting of as a function of which is not perfectKG x(Caz),owing to the complex shape (Fig. 3). Improvement in the pre-dictions should therefore be possible by employing a moresophisticated Ðtting procedure.

In Fig. 7 predictions made at 0.01 equiv. dm~3 are shown.The deviation of the predicted line from the experimental

Fig. 5 Predicted CaÈNa-MAP ion-exchange isotherms at di†erenttotal normalities. Solid lines are the predictions and the points areexperimental measurements made at total normalities of (]) 0.4, ())0.1, 0.05 and (*) 0.025 equiv. dm~3.(K)

Fig. 6 Semi-logarithmic plot of the predicted isotherms. Details asFig. 5.

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Fig. 7 Semi-logarithmic of the predicted CaÈNa-MAP ion-exchangeisotherm at 0.01 equiv. dm~3 together with experimentally measuredpoints

points is in line with the apparent failure of eqn. (VII) at thistotal normality as detailed above. Despite this, it should bepossible to make accurate predictions in the region x(Caz) [0.6.

ConclusionsZeolite MAP shows very high selectivity for calcium oversodium at all calcium loadings. Except at loadings of

it is much more selective for calcium than isx(Caz) \ 0.25,zeolite A. The high selectivity for calcium shown by MAP andthe unusual Z-shaped selectivity plot can be attributed to theformation of coexisting sodium-rich and calcium-rich phasesover much of the exchange system.

Tests have clearly shown that the CaÈNa-MAP system isamenable to thermodynamic treatment and that computa-tional procedures based on this type of treatment can be usedto make accurate predictions of ion-exchange equilibria over arange of solution total normalities. In a future publication itwill be shown how the prediction procedures, developed andtested here, can be used to compare in detail the e†ectivenessof zeolites MAP and A at removing calcium from solutionover a range of typical fabric washing conditions.

The authors acknowledge the assistance of Mr. C. Whittakerand Mrs. A. Rockli†e in the chemical analysis of the isothermsolutions. The authors would also like to thank the large

number of people within Unilever with whom we have hadvaluable discussions on this work.

References

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19 E. Glueckauf, Nature (L ondon), 1949, 163, 414.20 G. L. Gaines and H. G. Thomas, J. Phys. Chem., 1953, 21, 714.21 K. R. Franklin and R. P. Townsend, J. Chem. Soc., Faraday

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Paper 6/04919B; Received 12th July, 1996

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