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ELSEVIER

Quantitative relations of the batchcomposition and the Si/AI ratio in theproduct of zeolites

H. Lechert, P. Staelin , and C. KuntzInstitute of Physical Chemistq, Universityof Hamburg, Hamburg, Germany

An analysis of the correlation of the Si/AI ratio of faujasites with the composition of the solutionphase from which they were crystallized gives with good accuracy an equation

(Si/AI)prOdUCt= 1 + b . (ISiO~ l/IOH-I),o,,~ i,, (Al 1

where b is a constant that can be taken from the experiments. For a series of faujasites b = 2 canbe observed.

The batch composition can be described formally by

NaAIO, : n (Na,H, _ ,SiO,) : p H,O.

With some simplifying assump tions it can be concluded that

(A21

(Si/AI) ,,roduct = (b + n-d . Mb + n . n-t) (A31

where n is the SVA I ratio andm the excess alkalinity (NaOH - NaAIO,)/SiO, of the batch.From a comparison with a large num ber of experimental data it can be seen that Equation (A3)

describes the SVAI ratios for an extended range of batch compositions and different zeolites withgood accuracy, allowing a reliable prediction of the composition of the final product from theparameters of the batch composition. Deviations observed for low SVAI ratios can be corrected

by an additional empirical term 0.3/n - 0.3. The validity of the equation and its limits are testedalso for data from the literature, including unusual batch compositions.

Keywords: Zeolite; crystallization; Si/Al-ratio

INTRODUCTION

Zeolites are produced in large amounts for variousfields of industrial application. Large quan tities of Yzeolites are used in cracking and hydrocracking cata-lysts; A and X zeolites are used as ion exchangers inlaundry detergents and for sorption and separationprocesses. Others, such as ZSM -5, are applied in more

special catalytic processes where an improved selectivityis demanded. The current knowledge and a largeamount of data about the crystallization of zeolites havebeen summa rized in the books of Barrer and Breck2and more recently in various articles in a series of booksby Occelli and Robson, Szostak, and Van Bekkum etal. Further reviews can be obtained from the roceed-ings of the International Zeolite LConferences.

Address reprint requests to Prof. Lechert at the Institute of Phys-ical Chemistry; University of Hamburg, Bundestrasse 45,D-20146 Hamburg, Germany.Received 29 May 1995; accepted 27 July 1995

For application in industrial proce sses generalbhighly reproducible materials with respect to the com-position and other parameters such as particle size arewanted. Therefore, a thorough understanding of themech anisms of the crystallization is interesting not onhfor a deeper insight into the principles of zeolite crys-tallization but also for industrial application.

In reality, crystallization is done mostly according tocarefully tested recipies that are more or less well dc-scribed in the open and the patent literature and whichare often a matter of extensive additional experimen-tation in the course of the final establishment of thl:applied process. Generally, zeolites crystallize in thepresence of a gel phase that is dissolved during crystal-lization, supporting the growing crystal with the matt.-rial necessary for growth. The gel and the solutionphases are often regarded to be in some kind of dla-namic equilibrium at least during the growth process

It is now generally accepted that crystallization pro-ceeds via the solution phase and not by a direct tranrformation of the gel.

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In earlier investigations we have studied extensivelythe kinetics of growth and the composition of Y zeolitesin their dependence on batch composition.12-5 Forstudies of zeolite crystallization, faujasites are very con-venient. The faujasite structure is cubic, and only oneconstant of linear growth has to be taken into accou ntfor the kinetic studies. Faujasite crystallizes mostly in

octahedra and has a comparatively low tendency fortwinning and intergrowth. In the range of Si/Al ratiosof about 1.1 to about 3.4, which can be obtained bydirect synthesis, a wide range of rate constants of lineargrowth can be observed. This means that general infor-mation about the mech anisms of zeolite crystallizationcan be obtained, especially regarding the dependenceof rate contents on composition.

As has been discussed in detail in several pa-pers 12-14,18-21 nucleation of faujasite does not occur be-low an alkali concentration of about 2 mol of NaO H/liter. Batche s of this alkali concentration give a compo-sition of the crystallizing faujasites of only about 1.4-1.5. For application in cata lysts , higher Si/Al ratios are

needed. It is well known that for the higher Si/Al ratiosthe alkali concentration mus t be lowered to a levelwhere only nucleation of P zeolite takes place. Forlower a lkalinities, Y zeolite can be crystallized only ifnuclei are added or otherwise created in the batch.This offers the chance to study the processes of growthand nucleation separately with great accuracy.

Nucleation mus t be achieved by X seeds or by appro-priate nucleation gels&*l that were first described byMcDaniel and co-workers.22,2s Later discussions ofthese gels can be found by Kasahara et a1.24 The pro-cedure of creating faujasite nuclei in the batch hasbeen an alyzed by Ginter et a1.25 using n.m.r. method s.Many have been published about the composition of

the silicate solutions and the ossible building units ofthe zeolites grown from Bit.* Investigators have sub-sequently tried to conceptualize the structure of thecrystallizing zeolite and the silicate species found in thesolution phase. Until now, only qualitative relationsamong the batch composition, the solution phase, thecomposition of the final product, and the rate of crys-tallization could be obtained. The aim of this study wasto attempt to find some quantitative relations amongthe parameters characterizing the batch compositionand the solution phase and the Si/Al ratio of the finalproduct. The obtained relation will also be checked forzeolites other than faujasite. Finally, it should be men-tioned that Y zeolites are still used in mu ch larger quan-tities than any other zeolite in the cracking processes sothat an improvement of faujasite synthesis is still inter-esting from a comm ercial point of view.

EXPERIMENTAL

The batches for the faujasite syntheses were generallyprepared by mixing two solutions, one containing thealumina and the other the silica source. As aluminasource Na-aluminate was used which was prepared fromaluminum hydroxide (Merck, Al (OH) s) dissolved in anNaOH solution prepared from NaOH beads (Merck,analysis grade). One thousand g of solution contained

2.5 mol o f Al(OH), and 5 mol o f NaOH . For the ex-periments of ser-ies 4 and 5 the aluminate was made ofcomm ercial AlCl, (Merck) and NaOH . An equivalentamount of AlCl, was used, and the NaOH concentra-tion was adjusted. The silica source was comm ercialwater glass (density 1.37) with 273.5 g (4.6 mol) of SiO,and 83.0 g (1.3 mol) in a 1,000-g solution.

To obtain comparable conditions, in the crystallizingbatches the composition was chosen so that the totalconcentration of the solid remained constant at

50 g of (AlO,- + n. SiO,)/l,OOO g of H,O. (1)

Generally, the compositions can be described by

NaAlO, : IZ (Na,H,-,SiO,) : p H,O. (2)

On the whole, five series of experiments were carriedout, varying n and m in the reaction gels. In series 1 ,crystallization of X-type zeolites was performed withoutseeds or nucleation gel (see also Ref. 16). In series 2,crystallization of Y-type zeolites was done with the addi-tion of seeds (see also Refs. 13 and 14). In series 3-5,

crystallization of Y-type zeolites was done with the addi-tion of nucleation gel. The compositions of the gelswere similar to those of the gels in Refs. 12 and 13.

For the preparation of the batches for crystallizationthe solutions were mixed at room temperature in 100-or 300-ml polyethylene bottles under stirring, wherethe silicate w as added to the aluminate and NaOH so-lution. The stirring was continued for about 2 h forhomogenization. For the experiments of series 3-5 thehomogenization was done by shaking the reaction mix-tures. In preliminary experiments it was shown that thiskind of homogenization did not cause formation of anyP nuclei. Before heating to 88C seeds were added tothe gels of series 2. Into the gels of series 3-5 1 ml of

nucleation gel/loo-ml reaction gel was added. The ho-mogenization was then continued for another 30 min.The batche s were left in the bottles for crystallization.The batches of series 1 were heated to 88C immed i-ately after homogenization. The NaOH content ofthese batches was above 2 mol/liter.

The seeds were prepared from b atches with the com-position

NaAlO, : n (Na,H,-,SiO,) : p H,O

where the parameters where chosen in the ranges n =2-5, m = 1.4-4.4, p = 180-450, which is characteristic forthe crystallization of X-type zeolites. Preferably thecomposition

NaAlO, : 2 (Na,SiO,) : 200 H,O

was used for the preparation of the nuclei. The reactionmixture was stirred for at least 8 h at ambient temper-ature before heating to 88C for crystallization. Thisensures a rather homogeneous nucleation. After about5 h of crystallization, nuclei with an average radius of1.8 pm with a rather narrow particle size distributionwere obtained (see also Ref. 17). The nucleation gelhad the composition

NaAlO, : 7.5 (Na,H,SiO,) : 153 H,O.

It was prepared by dissolving 21.8 g of NaAlO, (Merck,

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often mentioned in the literature and have been stud-ied extensively.

In Ref. 53 it is shown that the aluminate content inthe solution increases strongly for %/Al ratio below 1.0,which are usually not applied in zeolite crystalliza tion.In several papers it has been shown that the gel con-tains an amount of sodium corresponding to the alu-minum content.33-g From the formula the OH con-centration in the solution is given by

IOH-I = n.m . 55/p. (3)

This OH content has been checke d by pH measure-men ts and by titration with HCl. This gives the sum ofthe excess NaOH and also the silicate present asNa[Si(OH),O-] or as Na,[Si(OH),022-].

The values of the titration and the SiO, content areslightly higher than those obtained from Equation (3)from the parameters of the batch composition. Thismay be due to a partial hydrolyzation of the aluminatein the gel during the period of homogenization, whichis not taken into account in Equation (3).

The silicate in the solution is dependent on the Si/Alratio IZ of the batch and the Si/Al ratio of the gel phase.The mecha nisms leading to formation of a well agedgel may not be much different from those leading tozeolite formation. Therefore should be

@i/W prod = W/ W gel (4)

From Refs. 33-39 and 53 and also from studies done inour laboratory it can be seen that this is only a roughapproximation which will nevertheless prove to behighly useful in the following. From Equation (4) itfollows that

pure) in 375.2 g of water. A second solution was pre-pared from 420 g of waterglass (density 1.37, Merck),3.8 g of silicic acid (Fallungskieselsiure, Merck), and115.6 g of NaOH in 100 g of water.

Both solutions were mixed under cooling and stirredfor 1 h. A clear liquid formed which was aged at roomtemperature for at least 2 days before use. Gels pre-pared in this way are active for several weeks . For crys-tallization, about 1 ml of nucleation gel for 100 ml ofsynthesis gel was added. In the experiments of series3-5 1 ml of this gel was added to 100 ml of the reactionmixture after homogenization of the mixture. The ho-mogenization was then carried on 0.5 h. When the crys-tallization was finished the samples were filtered,wash ed to near neutra lity, dried, and equilibrated oversaturated NaCl solution.

For analysis of the solution phase after the gel for-mation, both were separated by centrifugation. To ob-tain the SiO, co ntent a defined part was dried,weighed, fumed with hydrofluoric acid, and weighedagain. There was always only a little Al,O, remaining,

indicating that only very little aluminate goes into thesolution phase. In the later experiments the silicate andthe aluminate were determined directly b y ICP-AESanalysis. The aluminum could not be detected by titra-tion with EDTA.

The OH concentration in the liquid phases was ob-tained by direct pH measurem ent and also by titrationwith hydroch loric acid. Calculation of the alkali con-centration will be discussed later.

For kinetic studies, the bottles were taken from theoven, shaken, and 2 ml of the crystallizing mixture wasremoved by a pipette. The bottle was then put back intothe oven. The synthesis of mordenite and L followedprocedures described in Ref. 51. The synthesis of zeo-

lite W is described in Ref. 52. The Dhase co mDositionand the crystallinity of all sample; were che:ked byX-ray diffraction. The composition of the samples wasanalyzed b y X-ray fluorescence spectroscopy and by

ISiO,l,,, = [(Si/Al),,, ,, - (Si/Al),,,] . 55.5/p (5)

orISiO ,l,,, = [ (Si/Al),,,,, - (Si/Al)prudl 55.5/p =

[n- (Si/Al) prodl x23/p. (6)EDfi analysis.

_ ,For the system

RESULTS AND DISCUSSION

Derivation of a general relation between theSi /Al ratio of zeolite products and theparameters of batch composition

As a basis for the derivation of a general relationbetween the Si/Al ratio of the product and the param-eters of the solution phase and the batch, data from ourearlier expe riments were used .*- The relative con-centrations of the different compon ents of the batchcan be described formally by

NaAlO, : n (Na,H,_,SiO,) : 1, H,O.

in which zeolite A , faujasite, NaPl, sodalite, mordenite,and analcime are crystallizing, a great deal of data hasbeen gathered in Refs . 12-15 connecting the Si/Al ra-tio in the final prod uct with the Si/Al ratio and theexcess alkalinity in the batch. The studies have beenextended with ions other than Na in the batch, leadingto a series of other zeolites52 similar studies have

been reported by Zhdanov.j4In Ref. 15 the attempt has been made to obtain a

comm on function for the composition of zeolites by thedevelopment of a general model of the crystallizationprocess. It is well known that the Si/Al ratio of the finalproducts in all zeolites increases w ith decreasing excessalkalinity m. Plotting the (Si/Al),,, , of various zeolitesagainst m for a larger number of batches, a correlationof the form

(Si/Al),,,,, = 1 + d/m (7)

may be suspected, where the constant d can be ob-tained by a fit to the experimental values. Similar re-

NaAlO, : n (Na,H,-,SiO,) : p H,O.

The total alkali concentration is finally given by theexcess alkalinity m, the Si/Al ratio n, and the watercontent p, which are adjusted in the batch. The low Alconcentrations in the solution show that after mixingthe silicate and the aluminate compon ents almost all ofthe aluminate vanishes in the gel. Similar results are

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suits have been obtained by Zhdanovs4 for an extendednumber of zeolite types.

The general experience, expressed with some ap-proximation by Equation (7)) indicates that for the es-tablishment of the %/Al ratio of the zeolites a pH-dependent equilibrium should be involved in the crys-tallization process. In our papers,16 the assumption of

such an equilibrium at the surface of the growing zeo-lite

=Si-OH + OH- u =Si-O- + H,O (8)

was discussed in detail. Assum ing further that the alu-minate from the solution is added preferably to thecharged silicate groups =Si-O- at the surface, for thesteady state of the incorporation of both silicate andaluminate species, the Si/Al ratio of the product

@i/N prod= 1 + u/lO H-I (9)

can be obtained, where IOH- is the hydroxyl concen-tration in the solution phase. Inspection of the data ofour earlier papers sho ws that the Si/Al ratio of thebatch has a distinct influence on the Si/Al ratio at leastfor the low silica X zeolites. This dependence is obvi-ously not described by the reported model leading toEquation (9).

Plotting the Si/Al ratios of the products versus therelation at the right side of the Equation s (7) and (9) itcan be seen that the correlation is rather poor. A muchbetter correlation can be obtained taking

@VAUprod 1 + b . ISiO,I,,,/IOH-I (10)instead of Equation (9). Gene rally, the ratioISiO,l/IOH-I determines the distribution of different silicatespecies in the solution phase.265

In Figure 1 the data from Refs. 12-16 are related byEquation (10). Using Equation (10) and the estima-tions of ISiO,l,,, and IOH- from Equations (3) and (6),one obtains

WAl) prod= 1 + b. [n- (Si/Al),,, , ,]/n. m (11)

Pi/W prod . n. m= n. m+ b. [n- (Si/Al),,,,l(12)

y=a+bx

a = 0.89b = 1.92

carr.caeff. = 0.993

0,o 0,3 0,5 0,8 14 1.3

SiOZ/NaOH in solution

Figure 1 Dependence of the Si/AI ratio of faujasites on the ratioISiO,I/IOH-1 in the solution phase.

W/W,,,d o(b+n.m)=n.m+b.nor

CWAl),,,d = (b+ m) . n/(b+ n.m).

From Figure 1 follows

(13)

b= 1.92 = 2 (14)

which g ives; as an approximate of the (Si/Al),,,, de-pendence on the n and m ,

WAl) prod =(2+m).n/(2+n.m). (15)

The importance of Equation (13) and the approxima-tion of Equation (15) is that the Si/Al ratio of the prod-uct can be calculated directly from param eters n and mof the batch composition, without referring to the datain the solution phase.

These relations can be tested easily by experiment.The parameters in the solution phase are difficult or atleast unconvenient to control in routine work. Equa-tion (13) contains, besides n and m, only the parameter

b, which has been fitted from Figure 1 to the concentra-tions of SiO, and O H- in the solution phase.

Parameter b is important for crystallization but hasno simple mea ning. A parameter in the solution whichis proportional to the ratioISiO,I/IOH-I is the concen-tration of the undissociated silicic acid Si(OH),. Thepossibility of such an interpretation demands furtherinvestigation and careful analysis of the data of the sil-icate species in the solution. Further, it may be ex-pected that b is dependent on temperature and possiblyon the presence of templates or complexing agents.

In Figure 2 the results of crystallization experimentswith nucleation gels (series 3-5) are compared with thedata from o ur earlier papersi2-15 (series 1 and 2). It can

be seen that the observed Si/Al ratios can be predictedwell from Equation (15) within the limits of experimen-tal error. Looking at the data from the earlier experi-men ts (series 1 and 2) in which care had been taken forexactly uniform conditions of gel aging and crystalliza-tion, it can be seen that for low Si/Al ratios the devia-tions are comparatively larger than for the samples withthe higher Si/Al ratios. Fo r a further ana lysis, th e re-gion in question is plotted sepa rately in Figure 3, wheresamples with different Si/Al ratios n of the batch arecharacterized by different symb ols. Th ere is a system -atic variation of the Si/Al ratio in the batches.

Empirically, all data can be fitted by a function

(si/AI) prod =(2+m).n/(2+n.m)+0.3/n-0.3(16)

which is demonstrated in Figures 4 and 5 for the regionof series 1 and the extended region of series 2.

Figure Gshow s a comparison of the data obtained withfaujasite with some data for mordenite and zeolites Land W. Additionally, in this figure some data obtainedby Zhdanov are included in which chabazite and eri-onite were obtained besides Y. Equation (15) describesalso the data in the extended range of Si/Al ratios withsatisfying a ccuracy. The points for mordenite increasemore steeply, which can be explained by a slightlylarger value of 6.

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4

( . *. . l I

. . l series I

a

3-T---I i--

.t l. . .

series 2ea . series 3

.r

s1 ;p**

0 series 4CJJ 2 --.---&L-8.

7. series5

:.JF*

1,.1 2 3 4

(2 + m)*n/(2 + n*m)

Figure 2 Dependence of the Si/AI ratio of faujasites from dif-ferent series of experiments on (2 + m) . n/(2 + n. m) accordingto Equat ion (15). For details see text.

Comparison with some data from the literatureStudies of the compositions of the zeolites from com-

pletely inorganic batches have been reported in earlierliterature. Extensive studies in the system

NaAlO, : n (Na,H,-,SiO,) : p H,O.

have been reported by Zhdanov.53 In this paper a thor-ough review of the early literature can be found. Be-sides the batch composition in the paper ofZhdanov the composition of the gel direc tly a fter themixing, a detailed composition of the liquid phase, andthe composition of the final products of the crys talliza-tion are given.

The Si/ Al ratios n and the excess alkalinities m in

batches of the described composition are varied withinlarge regions. The S i/A l ratio varied from 0.33 to 25.Excess alkalinity m varied from 25 to 0.7. Values of nbelow 1.0 and values of m above 5.0 are normal ly notused for the crysta llization of zeolites. As mentioned,

Si/AI in botch

- 1.4, 1.5

- 2.0

- 3.0

- 4.0

- 5.0

l,ol..f....~....l....i ,,,.l ,o 1,l 12 1,3 1,4 1,5

(2 + m)*n/(2 + n-m)

Figure 3 Dependence of the Si/AI ratio of faujasites from dif-ferent series of experiments on (2 + m) n/(2 + n ml accordingto Equation (15) in the range of low Si/AI ratios. The Si/AI ratiosn in the batch are demonstrated by different symbols.

.9/A/ ratio of different zeolites: H. Lecher? et al.

1 o I,1 12 1,3 1,4 1312+m)*W + n*m) +0.3/n - 0.3

Si/Al in batch

+ 1.4,1.5

. 2.0

. 3.0

0 4.0. 5.0

Figure 4 Dependence of the SVAI ratio of faujasites on the cor-rected function (2 + m) . n/(2 + n . m) + 0.3/n - 0.3 according toEquation (16) in the range of low Si/AI ratios. The Si/AI ratios nin the batch are demonstrated by different symbols.

some of these data are included in Figure 6.Excess al-kalinity m for chabazite and erionite of these samplesconsists of NaOH and KOH, as is usual for the synthesisof these zeolite types.

In Figure 7 the data obtained applying Equation (15)are compared with data obtained from Equation (10)using the results of the analysis of the gel and liquidphases of the batch. To calculate the OH- concentra-tion the concentration of [AI(OH in the solution issubtracted from the values obtained according to Equa-tion (3). This correction is important only for the re-gion with n < 1.0. Outside this region the aluminateconcentrations in the liquid phase are negligible. Thesedata are demonstrated by the triangles in Figure 7.Fi-nally, the Si /Al ratio of the final product has been es-timated from the values obtained from the data aboutthe gel composition using Equation (11) with (S i/Al) .-,instead of (Si/Al) rrod. The resul ts are demonstrated %the asterisks in Figure 7. It can be seen that the result s areconsistent with Equation (10) as well as with Equation

+ series 1,2. series 3

611 2 3 4

(2 + m)*n/(2 + nm) + 0.3/n _ 0.3

Figure 5 Dependence of the Si/AI ratio of faujasites on the cor-rected function (2 + m) .n/(2 + n.m) + 0.3/n - 0.3 according toEquat ion (16) for different series of experiments.

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a-

7 ---

t 6-2 .

li. I

From an unseeded gel aged for 5 weeks and heated for

-.-. T-..-j--30 h, NaP with Si/Al = 2.87 is obtained, determined byMAS n.m.r. ICP-AES gives for the same sample Si/Al =

. 3.11. After 8 h of heating seeding and no aging yield-7--- + faujasite

Q 5 1. ---.~~~~--..--..( A mordenite

NaYwith Si/Al = 2.55. Aging for 1 day and heating for

e A * 6 h produce NaY Si/Al = 2.56 with ICP-AES. The values

.F I c .with the other methods are slightly lower. From two of

2 .

, -----I--; i zeolite L,W. the reported experiments amorphous products with

m 3: p ---- q ZDHANOV[53] higher Si/Al ratios were obtained. It should be men-

8 tioned that the batches have a comparatively low water---7--.-.--- content.

I Similar expe riments are reported in the paper of, , , , . . . . I. . . . , . . , , . . . Kasahara et al.

1 2 3 5 6 7 824 These authors analyzed the Toyo Soda

Process. They used a seed gel with the composition

Si/AI colculated

Figure 6 Dependence of the Si/AI ratio of different zeolites on(2 + m) . n/(2 + n. m) according to Equation (15).

(15) within the range of experime ntal error. In thefollowing some results from papers using unusual batch

compositions will be used to test our results.In the paper of Ginter et al. a batch of a nucleation

gel of the composition

NaAlO, : 5 (Na,,94H2.06Si04) : 90 H,O

was analyzed. A solid phase, precipitated after I day ofaging, was analyzed by MAS n .m.r., FT i.r., and ICP-AES,giving Si/Al ratios o f 1.96, 2.0, and 2.06, respectively.One day of aging and 1 day of heating gave a productwith Si/Al = 1.95. With Equation (15) a value of 1.69can be obtained, Equation (16) gives Si/Al = 1.45. Thisis because the Si/Al ratios of the amorphous gels andproducts of the early stages of crystallization are usuallyslightly higher if an excess of silicate is in the batch.

From these gels, synthesis gels have been prepared byneutralizing a part of the Na,O by H&SO ,. Further theresults of the crystallization for a batch composition

NaAlO, : 5 (Na,,,,Hs,,,SiO,) : 90 H,O

are reportedZ5 for different condition s of aging andseeded and unseeded batches. Equation (15) gives avalue of Si/Al = 2.86; from Equation (16)) Si/Al = 2.62 .

+ 2)*n /(Z i-* m )A 1 + Z*(SiO2/(NoOH-Al( )j* 1 + 2*] n (Si/AI$,, ]/ (n*m)

0 1 2 3 4S/AI calculated

Figure 7 Si/A l ratio for the samples, described in Ref. 53, re-lated to Equations (15) and (IO) using different information onthe composition of the solution phase.

NaAlO, : 5 (Na,,,,H,,,,SiO,) : 92 HZ0

yielding, after 6 h, a crystalline produc t with Si/Al = 1.3;after 24 h, a product with Si/Al = 1.15. Some times anamorphous product with Si/Al = 1.6 is obtained. Equa-tion (15) gives Si/Al = 1.49; after correction with Equa-tion (16) Si/Al = 1.25 is found. After mixing with the

nucleation gel the reaction mixture had a composition

NaAlO, : 5 (Na,.4H,,6Si04) : 92 H,O.

The crystalline product is reported to have Si/Al = 2.75.Equation (15) gives Si/Al = 3.0; Equation (16) Si/Al =2.76.

Generally, it can be concluded from these data thatEquation (15) is less exact, if the water content of thebatches is low. This effect will be studied in a separatepaper.

In a paper of Senderov and Khitarov5 the geologicformation of natrolite and analcime has been simulatedfrom formally neutral batches of the composition

NaAlO, : 2.3 SiO,.

From Eq. (15) can be concluded that in this case boththe batch composition and the composition of theproduct should be equal. In the experiments for thelowest temperature of 423 K in accordance with thisargument an Si/Al ratio of 2.35 was found. For temper-atures up to 623 K the Si/Al ratio in the produc t de-creases to 1.85, indicating that in a refined theory thetemperature dependence of b in Equation (13) mu st betaken into account.

Finally, a paper of Collela et al. should be men -tioned in which several rare zeolites are crystallizedfrom batches of the composition

1.40 L&O : 2.11 M,O : f&O, : 2 SiO, : 130 H,O.

If M is Rb or Cs, cancrinite can be obtained with thecompositions

Li 5,05Rb1.24[A14,5Si07,6204) 4.63 H,O (Si/Al = 1.76)

Li Cs, , , ,2.88 [Alu4,7:iSi0,.(i204) 5.70 H,O (Si/Al = 1.6).

From Equation (15) S i/Al = 1 .O follows, which is obvi-ously different from the observed values. Again, thewater co ntent of the batches is rather low. The valueobtained from Equation (16) would be below 1 .0. Amore system atic review of the importance of Equation

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(15) for data from the literature will be given in a sep-arate paper.

Some remarks on limitations of the theoryIt can be seen that Equations (lo), (13), (15), and

(16) allow th e calculation of the %/Al ratio of differentzeolites from the composition of the solution phase or

the batch composition with good accuracy. Refine-men ts of the theory will be necessary to explain thedependence of the Si/Al ratio on the water content ofthe batch. In the present theory, the water content iseliminated in Equa tions (10) and (11) becau se the for-mation of the ratio is in terms of the silicate and theOH- content of the batch. This does not disturb thedescription of the syste m as long as the assump tions ofour theory are valid.

One of these assumptions is that the aluminate con-centration in the solution is low and that there isenough water in the batch so that all of the silicategiven by Equation (6) is present in the solution phase.If, on the other hand, for large amoun ts of water the gelphase is partially hydrolyzed, corrections will be necces-sary . Practica lly, these conditions are fulfilled in a rangeof the parameter p between about 100 and 500.

Changes in the solubility of the gel phase may be alsopresent if complexing agents are added to the batch.Thus it can be assumed that one e ffect of the templatesis an influence on the silicate concentration and thekind of silicate species in the solution phase. Refinedconsiderations for extreme water contents will be dis-cussed in a separate paper.

CONCLUSIONS

The Si/Al ratio of faujasites correlates with the concen-trations of the silicate and the OH- in the solutionphase with very good a ccuracy by the Equation (10)

(Si/W prod= 1 + b . ISiO,l,,,/IOH-I

where b is a constant that can be taken from the exper-iments. With some simplifying assumptions it can beconcluded that

W/N prod= (b+ m) . n/(b+ n. m )

with the Si/Al ratio n and the excess alkalinity

m = (NaOH - NaAlO,) /SiO,

in the batch.With b = 2 a wide range of batch compo sitions can be

related to the Si/Al ratio of the final produ ct with verygood accuracy. The constant b is certainly dependenton temperature and may have different values for otherzeolites such as ZSM-5. Deviations at low Si/Al ratioscan be corrected emp irically, obtaining Equation (16)

W/ W prod =(2+m).n/(2+n.m)+0.3/n-0.3.

The exactness of the fit is a good guideline for furtherdiscussions of a more detailed comm on mecha nism forthe crystallization process of zeolites.

.%/A/ ratio of different zeolites: H. Lecher? et a/.

ACKNOWLEDGEMENT

This work was supported by the Deutsche For-schungsgemeinschaft.

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