Crystallization kinetics of zeolite TON

Crystallization kinetics of zeolite TON

Crystallization kinetics of zeolite TON Francesco Di Renzo, Frangoise RemouC, Pascale ,Massiani, Franqois Frangois Fi eras Laboratoire $”e Chimie ...

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Crystallization

kinetics

of zeolite

TON

Francesco Di Renzo, Frangoise RemouC, Pascale ,Massiani, Franqois Frangois Fi eras Laboratoire $”e Chimie Or anique Physique et Cine’tique Chimi ue Appliqukes, CNRS, Ecole Nationale l uptfrieure de Chimie, Montpellier, P rance Thierry Des Couri&res Centre de Recherche ELF-France,

St. Symphorien

Fajula,

and

URA 418 du

d’Ozon, France

Crystallization kinetics have been studied for zeolites of structure TON synthesized in the presence of diethanolamine, triethylenetetramine, or methanol. The composition of the solid and liquid fraction and the morphology of gel and crystals have been monitored throughout the syntheses. Under hydrothermal conditions, dissolution of the parent gel and precipitation around it of a secondary gel occur. The experimental conditions (concentration of electrolyte, dilution of the system, nature of the organic agent) modify the texture of the precipitate and the concentrations of silicon and aluminum in solution. Preferential heterogeneous nucleation of the zeolite takes place over the organophilic surface of the secondary gel. In the last phase of the synthesis, the accumulation in the liquid fraction of silicoaluminate species unsuitable for TON growth can bring to the coating of the crystals by an amorphous layer. Keywords:

Zeolite

TON;

synthesis;

gel aging;

precursors;

heterogeneous

INTRODUCTION The extension to the zeolite field of the usual kinetic laws of crystallization encounters some experimental difficulties due to the complexity of the synthesis media. The harder tasks are related to the polyphasic nature of the synthesis gel and the superposition of nucleation and growth. The retention of solution in the porosity of the gel and the incomplete settling of sol particles may hinder the evaluation of the composition of the solid and liquid phases. Variations of composition between core and rim of the same crystal add to the difficulties of measuring the level of supersaturation throughout the synthesis. Moreover, the distribution of crystal size is usually difficult to evaluate, due to the presence of amorphous solid and the agglomeration of the crystals. In some cases,these shortcomings may be mastered by proper design of the synthesis conditions. Useful tools are the separation of nucleation and growth by using assisted nucleation methods and the depolymerization of the silicate species in solution through the choice of an alkaline-rich, aluminumpoor medium.’ These methods cannot be used in every zeolite synthesis, and so far they have been used

Address reprint requests to Dr. Di Renzo at the Laboratoire de Chimie Organique Physique et Cinetique Chimique Appliquees, URA 418 du CNRS, Ecole-Nationale Superieure de Chimie, 8 rue de I’Ecole Normate, 34053 Montoeltier Cedex 1, France. Received 20 August 1990; accepted 19 November 1990

@ 1991 Butterworth-Heinemann

nucleation;

crystal

growth;

kinetics

to correlate growth rates and supersaturation, failing when nucleation rate was concerned. A more generally applicable kinetic treatment is based on population balance models that allow the crystallization parameters to be evaluated without any actual measurement of supersaturation.“’ In these methods, a set of equations representing the mass flows between the phases of the system is tested for consistency with the experimental crystallization curve and the best-fitting kinetic parameters are calculated. Under the assumption of concentration equilibrium between the solution and a solid phase acting as a reservoir of reagents, the mass conservation equations can be reduced to a single kinetic law formally similar to the solid-state Avrami equation.4-6 This assumption has been shown to be valid when the source of reagents was a metastable zeolite or a very aluminic gel. Nevertheless, the population balance method finds its limits in the correct modeling of nucleation rates and lags.’ Some researchers have tried to obtain mechanistical information from anomalous nucleation lags, using them as a key to discriminate between liquid-phase homogeneous nucleation and nucleation b rearrangement of the amorphous solid phase.%’ CY All the experiments cited concerned aluminumrich zeolites. Indeed, more silicic systems appear harder test grounds for kinetic models. The wider distribution of polymeric silica units in solution can complicate the correlations between supersaturation and elemental concentrations. The presence of a less

ZEOLITES,

1991, Vol 1 I, July/August

539

Crystallization Table

kinetics

1 Experimental

of zeolite

TON:

F. Di Renzo

et

al.

conditions

Experiment’

Alltet

orgltet

ort3

Na/tet

OH-/Si02

1 2 3 4 5 6 7 8 9

0.036 0.036 0.035 0.036 0.028 0.036 0.036 0.036 0.036

1.07 1.07 1.06 0.62 0.67 0.99 0.62 1.07 8.33

DEA DEA DEA DEA DEA TETA TETA TETA MeOH

1 .oo 1 .oo 1.03 0.17 0.11 0.98 1.01 0.13 0.57

0.07 0.07 0.07 0.07 0.06 0.08 0.07 0.08 0.04

tet = Si + Al; org

= organic

agent;

DEA

= diethanolamine;

TETA

soluble amorphous solid can interfere with the assumption of concentration equilibrium between solution and gel during the growth of the crystalline phase. Moreover, the evidence for heterogeneous nucleation at the gel-solution interface in the synthesis media of zeolite MFI (Refs. 11 and 12) has not yet been properly considered from a kinetic point of view. Hence, to verify to what extent the proposed models apply to the crystallization of high-silica zeolites, we studied the evolution of the synthesis medium during the formation of zeolites of the TON structure type (corresponding to Theta- 1, ZSM-22, ISI- 1, Nu- 10, KZ-2)‘Ii in various synthesis conditions. Zeolite TON appeared to represent a good test ground for modification of experimental conditions, the preparation being known in the presence of organic agents as different as polyalkylene polyamines, I” diethanolami~~e, ” and methanol. “’

EXPERIMENTAL Crystallization experiments have been carried out in a 0.5 liter stainless-steel reactor equipped with an anchor-shaped stirrer and a sampling outlet allowing specimens to be withdrawn without altering the synthesis conditions. The system was designed so as to avoid accumulation of material inside sampling pipe and valves in the course of the reaction.” The reactor was heated by an electrical furnace that was regulated through a thermocouple immersed in the reaction medium. In all experiments, the heating rate was 8”C/min and the stirring rate 300 r.p.m. Table 1 reports the stoichiometry of the reagent mixtures and the temperature of the experiments. Experiments in the presence of cliethanolamine or triethylenetetramine were carried out in the temperature range 160 to 170°C dilution ratio HzO/SiOz between 17 and 40, and ionic strength between 0.1 and 1 Na/(Si + Al), while the alkalinity was kept neat 0.07 OH-/Si02 and the Si/AI ratio near 27. A comparison experiment was carried out in the presence of methanol (experiment 9). The reagents were silica sol (Cecasol 30, SiO:! 25%, Na 0.2%, Al 60 ppm, pH 8.8, grain size 12-18 nm), sodium aluminate (Carlo Erba RLE), aluminum sulfate (18 HPO, Prolabo Rectapur), diethanolamine and triethylenetetramine (Prolabo), methanol (Carlo Erba

540 ZEOLITES, 1991, Vol 11, July/August

H20/Si02 18 18 35 17 25 19 18 40 17

TV3 160 170 170 160 170 160 170 170 160

= triethylenetetramine

RPE 99%), sodium hydroxide and sodium chloride (Prolabo RP Normapur), and deionized water. 13C n.m.r. of the triethylenetetramine batch used showed a fraction of branched isomers and traces of alcohol groups. The reagents were mixed under stirring in the following order: alkaline solution, aluminate, organic agent, silica sol, and, when required, aqueous chloride solution. The mixture was stirred for 12 h at room temperature before the beginning of the synthesis. When the organic agent was methanol (experiment 9), a different procedure was used. Aluminum sulfate was dissolved in 3 M sulfuric acid. Silica sol was added to an aqueous solution containing the amount of alkaline hydroxide needed to neutralize sulfuric acid and give the required alkalinity. Aluminic and silicic solutions were slowly mixed, with stirring, methanol was added, and the mixture was stirred for 20 min at room temperature before the beginning of the synthesis. Samples of 10 ml were periodically withdrawn from the autoclave during crystallization. The slurry was collected in 100 ml of cold deionized water to separate the solid and liquid fractions.lx The solid fraction was recovered by filtration. The clear solution, containing the diluted mother liquor, was preserved for elemental analysis. The solid was washed with deionized water up to pH 9 and dried at 70°C in air. X-ray powder diffraction (CGR Theta 60 diffractometer, CuKa radiation) was used to identify the phases present in the solid fraction. The degree of crystallinity of the specimen was evaluated by comparison of the area of selected peaks with that of physical mixtures of the amorphous phase withdrawn at the beginning of the synthesis and well-crystallized zeolite. Texture of* the amorphous gel and size and aspect ratio (length/width) of the crystals of zeolite were determined by scanning electron microscopy (Cambridge SlOO instrument, resolving power 7 nm). Kinetic parameters were calculated according to the Avrami equation ‘..‘.“‘: a = 1 - exp(-/
(1) This formula

was

Crystallization

time

kinetics

(hrs)

TON:

time

Figure 1 Crystallization curves obtained in the presence of different organic agents. Experiments 1 [diethanolamine (011, 6 [triethylenetetramine, (O)] and 9 [methanol (+)I

cx = kt”

of zeolite

(2)

proposed by Subotic and Graovac’ in order to better model - although in an empirical way - the last part of the crystallization curve. Indeed, the two curves are superposed in their first part - the more interesting one from the point of view of the study of nucleation - but the assumption of constant supersaturation of the Subotic model brings it to overestimate the final rate of crystallization. In the calculation of the best-fitting parameters, a nonlinear regression program searching for the minimum standard deviation between experimental and calculated values of (Y was used.

Figure 3 periments

Crystallization 2 [HPO/SiOZ

F. Di Renzo

et al.

(hrs)

curves at different conentrations. 18 (VII and 3 [H20/Si02 35 (011

Ex-

RESULTS Crystallization

kinetics

Complete crystallization of zeolite TON occurred in less than 48 h throughout the experimental field. The influence of the synthesis conditions on the kinetics is depicted in Figures l-4. The experimental values of crystallinity are given, together with the crystallinity versus time curves calculated by Equation (1). The corresponding best-fitting parameters k and n for all experiments are reported in Table 2, together with the standard deviation o between calculated and experimental values. Before examining the results, it can be useful to

1

time (hrsl Figure 2 Crystallization periments 1 (160°C (O)] diethanolamine. 6 [16O”C of triethylenetetramine

curves at different temperatures. Exand 2 [17O”C (V)] in the presence of (O)], and 7 [17O”C (VII in the presence

time Figure 4 Experiments

Crystallization 1 [Na/(Si

(hrsl

curves at different ionic strengths. + Al) 1 .O (011 and 4 [Na/(Si + Al) 0.17 (O)]

ZEOLITES,

1991, Vol 11, July/August

541

Crystallization Table

kinetics

2 Kinetic

of zeolite

TON:

F. Di Renzo

et al.

parameters k

n

Experiment

4.1 6.3 11.0 4.8 12.0 3.8 10.0 20.0 3.8

1 2 3 4 5 6 7 8 9

1.05.10 4.6 4.5 3.1 3.0 . 7.0 2.8 2.6 . 7.3

0 5 10 6 10 ‘* 1om7 10.” 1om5 10 ’ lo-l7 1om6

0.017 0.021 0.017 0.015 0.062 0.010 0.012 0.030 0.045

n ad k of the equation In(1 - (Y) = kT”, where t is in hours and (Y is the crystalline fraction; o = standard deviation between measured LX values and corresponding calculated values; reported values of n for experiments 3, 5, and 8 correspond to the lower limit of the field of n values for which o is lower than the expected experimental error

of t h;il, ;iccortlin~ 10 the theory time shoi~lcl homogeneous nucleation, lhe’intluctic,n IX inversely proportion:ll IO the nLlc-ktion r:tte.‘)” As a consequence, the exponent )I ;ippe;irs especially relevant ;IS ;I guide to the mtio between mte of growth mid rate of‘ rluclextion: The fitster the crystallmition ;intl the Ionger rhe intluction time, the higher the v;iliie of ~1. The influence of’ the I~~~I~IIY ot’ the organic ;lgen’ is represented in FjqIr(, I. ‘I‘he c.l.).st;illi/.~ilic,n r;ile increases in the order nieth:iiiol < tli~thanol~~mine < 11-iethylencletI.~imine. Possibly the lower :ilk:Jinil), required by the synthesis in the presence of‘ methmol (experiment 9) slows tlou~n [he cl-?,sr~llliz~ltion. AI 1GO”<:, ;I[ the higher le\rels of‘ concent~~;ition 2nd ionic strength tested, the \~~iliir of‘ the espone~it t/ is ;il\v:i~;s ne;ir 4, wh;ite\fer the org;lnic ;igeill tised. ~l‘his \2lue Is p;irticul:irlJ, rele\x~ir, col.l-esl”)l”liiig (0 the lhresholtl for antoc:it:ilylic iiiic.lr~ilic)n.!‘-‘” It c’;IIl I)e OliSer\‘ed that only experiments (i ;intl 9 ;irc silri;ited helow Lhe threshold \2llie. When the len~per;il~ii-e is r;iisecl f’ronl 160 to 17OY:. the cry.st;~lliz;ltiorl r;tce increxes. as showetl in /;;qr~, 2. The exponent II 21~0 incre:tses. ‘l’his result cl;issic;illy corresponds with ;in iilc2-exe of‘ the r:ilio I)etween rememI)el-

Figure 5 experiment

542

From 6

left to right:

ZFCIUTFS,

799i,

VO/

(a) cut of the

‘i 7, J~//v/August

sample

at 3 h, experiment

growth rate and nucleation rate.” The superposition of nucleation ancl growth and the difficulty of an assessment of the size clistribution over the whole crystal population prevent any absolute value of activation energy to he evaluated. More physical parameters affect the crystallization kinetics. A twofold dilution of the synthesis medium significantly lengthens the induction time, as showed in Figtrw 3. As a consequence, the value of )I is strongly increased. The influence of the ionic strength is represented in Ficg~~w 4. The addition of sodium chloride to the synthesis medium increases the crystallization rate. The corresponding decrease of the exponent )I suggests that nucleation is more affected thm is growth. Texture of gel and zeolite Scanning electron niicroscopy allows the evolution of the morphology of the solicl to be followed. The starting point is representecl hy mmrphous particles from 5 to 100 p in size forniecl by coalescence of the grains of silica sol. Their evolution in the synthesis conditions cm he followed hy examining some cuts of solid samples enil>edded in resin, xs representecf in Figrrw 50 and 6. The former compact particles clivide into ;~n outer’ rim md ;I shrinking inner core (Fj, 1 p dependiilg on the s),tithesis conclitions (Fjgxrw &I :~ntl 70). In tl le experiments iu \\hich the gr;iili si/,e is larger, the p;irfic-les 21-e h~-tllv ol)ser\f;il,le ;is individiml entities. ‘I‘he gr2ins 2i.e tlee$!, coalescetl

1; (b) cut of the

sample at 4 h, experiment 6; (c) final product,

Crystallization Table

3 Characterization

of the

nucleation

PH

(mmolll)

12.6 12.6 12.2 12.6 12.3 12.0 12.0 12.6 12.8

50 60 15 110 50 20 60 80

of zeolite

TON:

et al.

F, Di Renzo

medium

[Sil Experiment

kinetics

Gel grain size (11) 0.010 0.005 0.004 0.002 0.005 0.021 0.006 0.006

0.10 0.08 0.2 0.02 0.05 0.6 0.4 0.07 0.5

pH measured just before heating; silicon concentration in solution and organic molar fraction of the solid fraction measured on the last sample before detection of zeolite crystals; texture of the gel withdrawn after 1 h at the reaction temperature evaluated by scanning electron microscopy

and appear ;IS eml~etlc~etl in ;I glass>, strrface. ‘l‘lle sim of the a~no~-~~hous gmins ilf‘ter 1 II of’ synthesis is reported in TcrOl~ 3 f’or all experiments. The main steps of the gel-to-zeolite p;lth in experiments in which the grain size of the gel ;q>pro;iches, respectively, the upper and the ~OHXY- end 01 the sc~ale are represented in the I;;grrrc:\ 6 and 7, respecti\,el\.. Figyre 60 represents the ~~~~~orphous phse f’e:tttrring big p:irticles and high1J. \,itrified surf’bces fi)mletl during very

experiment selectively

6. on

the

Small esteriial

q3tals

of‘ surface

molite of‘

aplw;ir the

l~ollo\~

(Fipw hi)). Oiice the descrikl is achieved,, the crystAs appwr iis with radial s)~mmetr~~ (1;ipw aggregated in clusters 6~). This tmttern, which fe:itures surf’& niicle;ition and “hedgehog-sh~ll,etl” crystA clusters, is f’ollo\\~ecl also in experiment 9, whose finA product is depicted spheres

previously

crystallization

Figure 7 Experiment right (b) and bottom final product it1 Figuw

hd.

5. Upper left left (c): sample

(a): sample at 3 h; upper at 22 h; bottom right (d):

Oiil~~c.sl~ri-iii~e1lts

lo ~111 exponent

of

6 ailcI

I;illetic.

tllv

9 (-(,~.i.esl)oiitlrtl

k:qLi;ltioii

(1)

hnvei-

tli;iil

4.

‘l‘he c\~olutioii of’the solitl f‘r;icIioii is depicted iii I;;,quj7 7. ;I5 ;I11 cs;llllplc fi)llo\vetl Iq thr csperinimts ~vhose pres~i~tul

the

phous phase is depicted phous

finest

of‘

thy

5

p;‘““‘”

s~Yx~II~~;II-~~

gel

siii;lll-grain

;iiiioi--

forinetl in the first steps of’the sytlthesis in I;j,qrrt-0 irr. A \~itrific;itioii of. the miior-

surface

gi-atltr;illy

taks

2if‘ter the detectioii (:rystals appew then phase

‘l‘lle

p;irticles.

01’rsp~riment

(Fjguu

i/l),

pl;~ce.

cotltinuing

of‘ the fii.st cr\‘st;ils as eml~etltletl mrl

the

ii\

s;11ma

the

fe;ltures

also

of Leolite. a1norphoi1s are

present

in ~11 c;t\,ities ;lccessihle to ohser\,;ttion (I;jgrrj.r, 7~). The final product is ti~rmetl Iq mtangled 1-~111tl01111~~ oriented c-r)W;ils (Fj,qrI7 7fI). The iiioi-l~hoIogi~~i~ e\.olution of‘ experiment 2, ;1 s)%eni fe:ituriiig 2 slightly coiirser seconclar)~ gel. is depicted iii F1(,uw 8. ‘i-he freshly deposed seconclar~ gel

Figure 6 Upper left (a): sample at 2 h, experiment 6; upper right (b): sample at 7 h, experiment 6; bottom left (c): final product, experiment 6; bottom right (d): final product, experiment 9

is

granlil;ir

vitrifies

(Figrrw

at

outer

the

(Fjguw

8~1).

S/J). ?‘he surface

crystals,

enll~etltlecl

scattered

clusters

first of

in (F;guw

the

but

its

cr@s the

surface

gel.

both

amorphous SC).

The

rapidl)!

of‘zeolite

appea1 as

isolated

surface, crystals

and of

the

as final

product are ra~iclo~iily oriented ;incl present iI marked heterogeneit), of size (Figuw Sd). Final crystal size for all the experiments are refeaturported in Tcibl~ -t. I ii the use of experiments ing hulk nucleation and randondy oriented crystals, 21 significant size heterogeneity cm be observed, the bigger crystals being located ilt the inner surface of the hollow spheres described above. In this c:ise, the crystal size &ported in Tab/~ 4 refers to the crystals observecl ;lt the outer surface of the hollow spheres. The reaction variables (Tr161~ I) affect the texture of gels (Table 3) and crystals (Table 4). Dilution of the

ZEOLITES,

1991, Vol 11, July/August

543

Crystallization

kinetics

of zeolite

Figure 8 Experiment 2. Upper right (b): sample at 3 h; bottom right (d): final product

TON:

F. LX Renzo

left (a): sample left (c): sample

et al.

at 1 h; upper at 4 h; bottom

and the concentrations of silicon and aluminum in the liquid phase are given. The corresponding crystallization curve is also drawn for sake of comparison. The solid withdrawn from the reaction medium heated at the reaction temperature is composed of a sodium-rich, diethanolamine-poor amorphous phase with an aluminum content corresponding to the stoichiometry of the reagents. Throughout the induction period, the solid aluminum and sodium contents slowly decrease. A simultaneous decrease of silicon concentration and increase in aluminum concentration in the liquid phase can be observed. Sometime after the detection of the first zeolite crystals, a sudden change in composition occurs. The sodium content of the solid fraction rapidly decreases and the diethanolamine content increases until the composition of the final zeolite is reached. The final composition is already attained when the crystalline fraction is not greater than 25%. During the first part of the crystal growth, silicon concentration in solution continues to decrease. Once the crystalline fraction goes beyond lo%, a different trend prevails: Both silicon and aluminum concentrations increase. Some of the experiments followed a slightly different pattern. In the case of the experiment 6, differing only in the use of triethylenetetramine as a

reaction medium (see experiments 2 and 3), addition of salt (see experiments 1 and 4), or use of triethylenetetramine instead of diethanolamine (see experiments 1 and 6) results in an increase in the gel grain size, an increase in crystal size, and a decrease in the aspect ratio. An increase in temperature from 160 to 170°C results in a decrease in crystal size when diethanolamine is used (see experiments 1 and 2) and in an increase in crystal size in the presence of triethylenetetramine (see experiments 6 and 7). The synthesis in the presence of methanol (experiment 9) was quite peculiar. In this case, an extended initial vitrification of the gel grains corresponded to the formation of a big number of needle-shaped crystals.

8f 1 0.05

Elemental composition The evolution of the composition of the synthesis medium before and during crystallization is represented in Figure 9 for experiment 2, chosen as a representative example. The mole fractions of aluminum, sodium, and organic agent in the solid fraction Table

4 Product

0

characterization

Experiment

Alltet

orgltet

Na/tet

1 2 3 4 5 6 7 8 9

0.027 0.027 0.028 0.037 0.028 0.036 0.035 0.023 0.041

0.026 0.026 0.023 0.031 0.033 0.027 0.030 0.025 0.034

0.020 0.020 0.023 0.019 0.005 0.025 0.035 0.004 0.038

tet = Si + Al; org

544

ZEOLITES,

_ 0.2

= organic

agent;

aspect

Crystal size (p) 1.5 1 .O 1.3 0.6 0.6 2.0 2.7 3.4 1.5 ratio

1991, Vol 71, July/August

x x x x x x x x x

0.20 0.15 0.20 0.07 0.05 0.4 0.5 0.3 0.10

Aspect ratio 7.5 6.7 6.0 8.6 12 5.0 5.4 11 15

= length/width

0

. 0 A

A oA

0

A

I

0.0 5

10 time

(hrs)

Figure 9 Composition of the synthesis system vs time, experiment 2. Top: solid fraction, crystallinity curve and mole fraction (referred to Si + Al) for aluminum (A), diethanolamine (VI, and sodium (M). Bottom: liquid fraction, concentration of silicon (0) and aluminum (A)

Crystallization

a05 0 > * C .F 2 1

50

.-+5 : t 01 z E

.L” z

0.00

0

0.2 \ 6 0.1 -2

\ 2 E

100 0

0

.Y,

z

0

OL 0

Boa AMM

,A

10

0.0

I

kinetics

of zeolite

TON:

F. Di Renzo

et aI.

ate composition data are available for experiment 9. From the data of the Table 3. it can be argued that, as a general rule, the lower the silicon concentration in the liquid phase, the higher the grain size of the amorphous solid. The correlations established above between experimental conditions and grain size of the gel appear to be also valid when silicon concentration is concerned. Moreover, an increase in the synthesis temperature from 160 to 170°C significantly lowers the initial organic content of the amorphous solid. The elemental composition of all final solids is reported in Table 4. When these data are compared to the stoichiometry of the reagent mixture (Table I), some observations are possible. Variation of the temperature from 160 to 170°C or twofold dilution of the reaction system do not seem to affect the S/Al ratio. On the other hand, the selectivity of incorporation of aluminum in the zeolite lattice decreases at higher electrolyte concentration (compare experiments 1 and 4 and experiments 3 and 5). In this respect, the methanol system (experiment 9) presents peculiar behavior. In this case, the final S/AI ratio is lower than the initial one, in spite of the significant amount of salt added. A more efficient incorporation of aluminum in the presence of alcohols has been already observed during the crystallization of other zeolitic materials.‘”

20 time

(hrs)

100 -

Figure 10 Composition of the synthesis system vs time, experiment 6. Top: solid fraction, crystallinity curve and mole fraction (referred to Si + Al) for aluminum (A), triethylenetetramine (V), and sodium (W). Bottom: liquid fraction, concentration of silicon (0) and aluminum (A)

substitute of diethanolamine, the solid phase featured throughout all the synthesis has the same content of organic agent and nearly the same content of aluminum as that of the final zeolite (Figure 10). In all the experiments, the sodium content decreased during the crystallization. The evolution of the silicon and aluminum concentrations in the liquid fraction for experiment 6 followed a pattern similar to the one reported for experiment 2, with the exception of the aluminum concentration, which remained below the detection threshold until zeolite crystallinity went beyond 25%. The affinity of the ethanolamines for the A13+ ion22 may account for the higher concentration of aluminum in solution in the experiments in the presence of diethanolamine. An important difference with respect to the behavior exemplified by experiment 2 was featured by experiments 5 and 8, carried out in diluted conditions at low ionic strength (Figure II). In these experiments, after the nucleation of the zeolite, concentrations of silicon and aluminum in the liquid phase steadily decreased. The concentration of silicon in the liquid phase just before detection of the first crystals of zeolite and the mole fraction of organic agent in the solid fraction at the same moment are given in Table 3. No intermedi-

n

I \ 2 E

'

100

1: .-

9

A

A 0

n

A

0 A

t A

Ln

0

0 0

I

I

10

20

O O

0.0

time (hrs) Figure 11 Composition of the synthesis system vs. time, experiment 5. Top: solid fraction, crystallinity curve and mole fraction (referred to Si + Al) for aluminum (A), diethanolamine (V), and sodium (W). Bottom: liquid fraction, concentration of silicon (0). and aluminum (A)

ZEOLITES,

1991, Vol 11, July/August

545

Crystallization

kinetics

of zeolite

TON:

F. Di Renzo

et al.

DISCUSSION Gel chkmisk-y Zeolite TON forms in a medium in which dissolution and precipitation of different amorphous materials are taking place. The gel particles formed at room temperature by coalescence of the grains of silica sol incorporate most of the aluminum available in the system. At reaction temperature, this primary gel gradually dissolves, giving rise to a less aluminic and less sodic material formed of bigger grains. The secondary gel forms a gradually thickening layer around the aggregates of the parent gel. The decrease in the concentration of dissolved silicic species during the formation of the secondary gel confirms that a more stable material is formed. The gradual increase of the organic content of the solid indicates that the process involves the formation of a less hydrophilic material. The appearance of the crystalline phase seems to speed up the evolution of the composition of the solid. Before the crystallinity has reached 25%, the sodium content of the solid fraction passes from one cation per 20 silicon or aluminum tetrahedra to one per 40, just short of the charge balance of the aluminate anions. At the same time, the organic agent content passes from one molecule per 200 tetrahedra to one per 40, nearly the same amount contained in the final zeolite. These changes of composition are not completely accounted for by the amount of zeolite formed. The occurrence of a simultaneous evolution of the amorphous solid is also testified to by the rapid progress of the surface vitrification of the secondary gel. The concentration of silicic species in the liquid phase continues to decrease during the first part of the zeolite crystallization. This phenomenon can account for the glassy appearance assumed by the amorphous solid. When supersaturation is inadequate to assure the formation of new gel particles, the deposition of silicic species continues on the available surface. Because the solubility of silica is lower at the contact neck between particles,24 the deposition is concentrated there until the grains coalesce in a continuous surface. In some experiments, a highly vitrified gel surface is observed from the beginning of the synthesis. The silicon concentration in the liquid phase is correspondingly low. When the higher level of ionic strength and the use of triethylenetetramine instead of diethanolamine are coupled (experiment 6), the biggest and more vitrified gel particles are formed. Moreover, the organic content in the solid is already at the higher final level from the heating of the reaction mixture. It seems then that in such experimental conditions the aging of the gel is nearly complete from the first steps of the synthesis.

Zeolite

nucleation

Experiment 6 features an exponent lower than 4 in Equation (1). As a consequence, the nucleation rate of

546

ZEOLITES,

1991, Vol II, July/August

the zeolite reaches a maximum at the beginning of the synthesis,“.‘“.‘” and the ratio between induction time and crystallization rate is the lowest of all experiments. It is tempting to correlate the early nucleation with the presence of a vitrified organophilic surface from the first steps of the synthesis. Some morphological observations seem consistent with this hypothesis. The aggregates of zeolite crystals conserve the shape of the hollow spheres of amorphous material. The retention of morphology can be easily accounted for by assuming that the crystals formed on neighboring portions of the gel entangle during their growth. Preferential nucleation is always observed on the outer surface of the amorphous aggregates. This feature is patent whether crystal formation occurs only on the surface lapped by stirred solution (Figure 66) or zeolite forms over the whole thickness of the amorphous layer with bigger crystal size nearer to the inner surface. This result seems hard to explain if homogeneous nucleation is postulated. The solution wetting the inner surface of the secondary gel nutshells is still in the presence of the more soluble primary gel; hence, it should present a concentration higher than the solution surrounding the gel particles. The homogeneous nucleation rate is proportional to a high power of supersaturation2s*26; nucleation should therefore be expected to occur preferentially at the inner surface of the hollow spheres. A better consistency may be assured if some influence of the nature of the amorphous surface on the location of the stable nuclei is admitted. Shells of secondary gel form inward, starting from an external layer (Figure 5), then their outer surface features the older gel formed in hydrothermal conditions. Moreover, wetting by a stirred solution equalizes concentrations on the external surfaces, accelerating gel aging.24 The outer surface of the shell is, hence, the first part of the gel to reach the final step of its evolution, and there zeolite nucleation preferentially occurs. Heterogeneous nucleation is far from a new con- ’ cept in zeolite synthesis.“~‘2*27*28 In a general way, the formation of stable nuclei in every crystallizing system is made easier by the local increases in concentration near any surface interacting with the solute.2g*30 In the case of the present study, the nature of the substrate also exerts a significant influence. A decrease of the activation energy of nucleation when the surface of the more aged, organic-rich gel becomes available can account for the preferential location of the crystallization and the variation of the nucleation lag.

Crystallization

kinetics

The value of the exponent n of the overall kinetic Equation (1) is a function of the ratio between nucleation lag and crystallization time. Values of the exponent n higher than 4 can be justified assuming that the nucleation rate increases during the crystal growth.g*‘O*‘g Experiments featuring nucleation selectivity on the outer surface of the amorphous

Crystallization Table 5 Kinetic nucleation and

parameters growth

Experiment 1 2 3 4 5 6 7 8 9

under

the

assumption

k

r=(h) 4.0 3.3 7.4 7.0 18.3 1.5 3.1 5.6 3.5

5.1 1.9 4.0 2.6 6.4 7.1 3.6 8.0 1.48.

. . . . . . . .

of separated

0 lo-4 10-Z 10-Z IO-“ lO-3 IO-’ IO-’ IO-’ 1O-4

0.014 0.031 0.002 0.010 0.066 0.008 0.027 0.029 0.051

k of the equation In(1 - LX) = -k(t - Pj3, where tare hours, (Y is the crystalline fraction, and t” is the best-fitting nucleation lag; CJ = standard deviation between measured a values and corresponding calculated values

shells were the only ones in which exponent n was lower than 4, consistent with a rapid decrease of nucleation rate once growth began. The experiments in which nucleation was spread over all the gel thickness featured exponents 11much higher than 4, corresponding to a sudden increase of the nucleation rate at the end of an induction lag. The kinetic modeling of such a common phenomenon as the induction time in zeolite synthesis is largely an unsolved prc)blenl.H One of the mechanisms that has been proposed”.‘” assumes that the nuclei, formed inside the amorphous solid during the induction lag, would be unable to grow due to the lack of any access to the feeding solution. Theit liberation by some dissolution of the gel at the beginning of the crystallization would rapidly increase the growth surface and the transformation rate. The application of a similar mechanism to the present results could be tempting. The delayed autocatalytic nucleation when crystals form throughout all the secondary gel seems consistent with an induction time correlated to a restricted access of the feeding solution. Nevertheless, some objections can be raised. The organic content of the solid during the induction period is quite low. It seems then difficult to accept that latent nucleation occurs in a confined medium, without any contribution of the organic agent from the liquid phase. On the other hand, do we have adequate evidence to justify the variation of the exponent II from 3.8 to 20 only on the basis of different nucleation kinetics of the zeolite? To answer this question, we can verify whether the present data fit a completely different hypothesis. If the nucleation lag would simply correspond to the aging time of the amorphous solid and all zeolite nuclei would form at the same time, the crystalline fraction a should follow the equation: (Y = 1 - exp[-k(t

- t.)“]

(3)

where t- is the induction time. The best-fitting values of k and t- for all crystallization curves are reported in

kinetics

of zeolite

TON:

F. Di Renzo

et al.

Table 5, together with the standard deviation between the measured crystallinity and the corresponding calculated values. Equation (3) fits the data with a deviation not significantly different from the fitting obtained when Equation (1) is used.

Some thoughts

on supersaturation

Direct br indirect knowledge of the level of supersaturation is needed for any modeling of crystal growth. All kinetic methods proposed assume supersaturation to be a function of the elemental concentrations in solution.16 Some of the present results suggest that this function can sometimes assume a non-straightforward form. Crystallization curves present the usual sigmoid shape featuring a final slowing-down rate notwithstanding that the concentrations of silicon and aluminum increase in the last part of the synthesis. As a consequence, not every species present in the liquid fraction of the late samples of each synthesis takes an even part in the zeolite growth. Under this assumption, the quite surprising increase of the concentrations can be tentatively explained. Once a significant amount of secondary gel has been dissolved in the process of zeolite formation, the primary gel, previously trapped in the core of the aggregates, comes again in contact with the solution. The dissolution of the more soluble primary gel raises the concentration of silicic and silicoaluminic species in the liquid fraction. Not every species features the suitable topology to compose the network of zeolite TON. Unlike many other zeolite syntheses, the alkalinity of the system is low. The rearrangement reactions of the silicic units are then expected to be slow, and the species unfit for the crystal growth can accumulate in the liquid phase. Anyway, this explanation can hold only if alkalinity is high enough to avoid the dep;j;ition of a tertiary gel from the liquid fraction: A high concentration of unfit species affects not only our evaluation of supersaturation but also directly the growth phenomena. The elementary step of adsorption of a silicoaluiiiiiiate unit on the surface of the crystal is probably not structure-sensitive. Unfit species can stick to the crystal and c~~~upy a fraction of its surface, hindering in this way any further growth until closer unfit

they have heen clesorl~ed or rearranged to a fitting with the crystal network. In other words, s ecies can act as ;I classical crystallization rate if tlieii poison,. R- slowing down the growth

adsorption

is reversible

surface with desorbecl. It the porosity

;W

can of

and completely

blocking

anlorphous layer if they are he rememherecl that the plugging the zeolite hy a surface anlorphous

the not

of

layer has been proposecl as an explanation of the shift towarcl higher temperatures of the decomposition of the organic agent in sonic sanlples of zeolite TON.:“’ Moreover, the neecl of acicl or basic washings as important

steps

in

seems to confirm synthesized solid common.

the by

ZEOLITES,

activation

of

the

zeolite’

.“‘.I



that the coating of the asan amorphous layer is not un-

1991, Vol 11, July/August

547

Crystallization

kinetics

of zeolite

TON:

F. Di Renzo

et al.

CONCLUSIONS The evolution of the solid phase of the synthesis gel triggers off the formation of zeolite TON. A mechanism of heterogeneous nucleation on the surface of the aged gel is able to account for the observed results. The way in which some kind of epitaxy can be established between an amorphous solid and a crystalline embryo remains an open investigation field. Concentration gradients across the gel particles and mutable dependence of the growth rates from elemental concentrations make questionable any evaluation of kinetic mechanisms based on the crystallization curve alone. In such conditions, the size distribution of the crystals is no more equivalent to the distribution of the nucleation time. The present population balance models appear then unable to deal with complex polyphasic systems. Better methods of quantitative sampling and analysis of the different phases of the synthesis medium appear as the first requirement in order to check more evoluted kinetic models.

ACKNOWLEDGEMENTS The authors are grateful to the “Service Central d’Analyse” of CNRS in Solaize for elemental analyses, to Roger Dutartre for the electron microscopy experiments, and to Marie-Agnes Nicolle and Didier Tichit for useful discussions.

7 8 9 10 11

12

13 14 15 16 17 18 19 20 21 22 23 24 25 26

REFERENCES

27

Fajula, F., Nicolas, S., Di Renzo, F., Gueguen, C. and Finueras, F., ACS Symp. Ser. 398, Am. Chem. Sot., Wishington, DC, 1989; p..493 Randolph. A.D. and Larson, M.A. Theorv of Particulate Procesies, Academic Press, New York, 197i Nyvlt, J., Siihnel, O., Matuchova, M. and Broul, M. The Kinetics of industrial Crystallization, Elsevier, Amsterdam 1985, p. 218 Thompson, R.W. and Dyer, A. Zeolites 1985,5,202, 292 Zhdanov, S.P. and Samulevich, N.N., in Proceedings of the 5th International Conference on Zeolites (Ed L.V.C. Rees) Heyden, London, 1980, p. 75 Subotic, B. and Graovac, A., in 5th lntemational Conference

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ZEOLITES,

1991, Vol 1 I, July/August

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33

on Zeolites Recent Progress Reports Discussion Naples, 7980 (Eds. R. Sersale, C. Colella and R. Aiello) p. 54 Katovic, A., Subotic, B., Smit, I. and Despotovic, L.A. Zeolites 1989, 9, 45 Warzywoda, J., Edelman, R.D. and Thompson, R.W. Zeolites 1989, 9, 187 Subotic, B. and Graovac, A., Studies in Surface Science Catalysis, Vol 24, Elsevier, Amsterdam, 1985, 199 Subotic, B. in ACS Symp. Ser. 398, Am. Chem. Sot., Washington; DC, 1989, p. 110 Jansen, J.C., Engelen, C.W.R. and van Bekkum, H. in ACS Symp. Ser. 398, Am. Chem, Sot., Washington, DC, 1989, p. 257 B. Nagy, J., Bodart, Ph., Collette, H., Fernandez, C., Gabelica, Z., Nastro, A. and Aiello, R. J. Chem. Sot., Faraday Trans. I 1989, 85, 2749 Meier, W.M. and Olson, D.H. At/as of Zeolite Structure Tvpes. 2nd Ed., Butterworths. London, 1987. D. 134 Hogan, Ph. J., Stewart, A. and Whittam, T.V; Eur. Pat. Appl. 65 400 (1982) Barri, S.A.I., Howard, Ph. and Telford, C.D. Eur. Pat. Appl. 57 049 (1982) Takatsu, K. and Kawata, N. Eur. Pat. Appl. 87 017 (1983) Remoue, F., Thesis, ENSCM, Montpellier, 1989 Iler, R.K. The Chemistry of Silica, Wiley, New York, 1979, p. 233 Sestak, J. and Berggren, G. Thermochim. Acta 1971, 2, 42 Heicklen, J. Colloid Formation and Growth, Academic Press, New York, 1976, p. 63 Freund, E.F. J. Crystal Growth 1976, 34, 11 Charnell, J.F. J. Crystal Growth 1971, 8, 291 Cruceanu, M., Popovici, E. and Inorga, N. An. Stiint. Univ. “A/. 1. Cuza” lasi, Sect. Ic 1971, 17, 241 Iler, R.K. The Chemistry of Silica, Wiley, New York, 1979, p. 227 Heicklen, J. Colloid Formation and Growth, Academic Press, New York, 1976, p. 57 Nyvlt, J., Sohnel, 0.. Matuchova, M. and Broul, M. The Kinetics of Industrial Crystallization, Elsevier, Amsterdam, 1985, p. 52 Aiello, R., Barrer, R.M. and Kerr, IS. Adv. Chem. Ser. 1971, 101,44 Culfaz, A. and Sand, L.B. Adv. Chem. Ser. 1973, 121, 140 Nyvlt, J. industrial Crystallization from Solutions, Butterworths, London, 1971, p. 42 Walton, A.G., in Nucleation (Ed. A.C. Zettlemoyer) Marcel Dekker, New York, 1969, p. 265 Henisch, H.K. Crystals in Gels and Liesegang Rings, Cambridge University Press, Cambridge, 1988, p.36 Nvvlt. J.. Sohnel. 0.. Matuchova. M. and Broul. M. The Kihetics bf lndus~rial~Crysta//izatidn, Elsevier, Amsterdam, 1985, p. 189 Di Renzo, F., Remoue, F., Massiani, P., Fajula, F. and Figueras, F. Thermochim. Acta 1988, 135, 359