Influence of Gel Ageing on Zeolite Nucleation Processes

P.J. Grobet et al. (Editors) /1nnovation in Zeolite Materials Science © Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

107

INFLUENCE OF GEL AGEING ON ZEOLITE NUCLEATION PROCESSES J. BRONIC, B. SUBOTIC, 1. :;f,:IT and LJ. A. OESPOTOVIC Ruder Boskovic Institute, P.D.Box 1016, 41001 Zagreb (Yugoslavia) ABSTRACT The changes in fraction of zeoli te crystallized, concentrations of silicon and aluminum in the liquid phase, crystal size and the number of microcfi:stals in final products were measured during the crystallization of zeolite A at 80 C from aluminosilicate gels aged at 25 0C for various time (0 to 25 days). Analysis of the results obtained have shown that the increase in the crystallization rate at constant crystal growth rate is caused by the increase in the number of nuclei with the gel ageing. The effects observed are discussed in relation to the internal and external recrystallization of the gel during its ageing. INTRODUCTION There are many indications that the ageing of aluminosilicate gels precursors markedly influences the rate of zeolites crystallization (refs. 1-6). In the study of zeolite A crystallization from the gels aged at ambient temperature for 0, I and 2 days, Zhdanov and Samulevich found that the time of ageing does not influence the rate of linear crystal growth, whereas the duration of crystallization at 90

0C

and

the size of crystals in final products decrease with the time of ageing (ref. 3). Therefore, the authors concluded that the development of nucleation at ambient temperature is the only reason for a decrease in duration of the crystallization after ageing of the gel. Similar conclusion has been made by Thomson et aI., with the assumption that the ageing step results in the formation of viable nuclei which effectively lie dormant until the temperature is raised (ref. 7). The objective of this work is to investigate the particle forming processes during the gel ageing and during the crystallization of zeolite A from aluminosilicate gels aged at 25°C for given times. EXPERIMENTAL Aluminosilicate gels were prepared by pouring diluted water-glass solution thermostated at 25 0C, into a vigorously stirred sodium aluminate solution thermostated at 25°C. The aluminosilicate gels of 2.04 Na 0 'AI 0 1.9 510 ' 2 12 H 0 2 2 2 3' 2 batch composition were aged at 25°C for given times (t = 0 to 25 days). After a ageing for a predetermined time t 50 ml of gel was poured into a stainless-steei a, reaction vessel containing 100 ml 2.5 M NaOH solution preheated at the crystallization temperature (BOoC). At times after the beginning of the crystallization (the moment when the suspension of gel was added into the reaction vessel was taken as zero time of the crystallization process, t

= 0), aliquots of the reaction mixture c were drawn off and centrifuged in order to stop the crystallization process and

108

separate the solid from the liquid phase, respectively. Aliquots of the clear liquid phase above the sediments were used to measure Na, Si and Al concentrations by atomic absorption spectrometry, while the solid phase (mixture of gel and zeolite A) 0C)

was, after washing and drying (24 h at 105

used for the determination of

zeolite A fractions by powder X-ray diffractometry, as well as for scanning-electron microscopy. The change in particle size during the crystallization was determined from scanning-electron micrographs by the method proposed by Zhdanov (ref. 8). Parts of the wet solids obtained at the end of the crystallization (pure zeolite A) were redispersed in double distilled water by ultrasonic waves and used for particle size analysis (M-III Disc Centrifuge with Sedimentometer, Joyce-Loebl). The number of particles in the mass unit of samples was calculated from the corresponding particle size distribution curves by the mathematical procedures described earlier (ref. 9). RESUL TS AND DISCUSSION Generally, it is well known that the kinetics of most gel-zeolite and zeolite-zeolite transformations during the main part of the transformation process, can be mathematically expressed by the simple kinetic equation (refs. 8-12): f

c

= K·

t

q

(I)

c

Here, fc is the mass fraction of the stable form of zeolite, t

is the transformation c (crystallization) time, and K and q are constants for given experimental conditions.

Table I. shows the numerical values of K and q calculated by the log fc vs, log t c plots using the experimental data from Fig. 2 (all fc values lower than 0.8 and the corresponding crystallization times were used in the calculations; in all cases, the linear correlation coefficient was higher than 0.99). Kinetic analysis of the solutionTABLE I Numerical values of the constants K and q, which correspond to the kinetics of crystallization of zeolite A from aluminosilicate gels aged for various times. Time of gel ageing t (days) a 0 I 2 3 7

K 4.64 2.30 4.26 4.84 2.36

q

E-5 E-4 E-4 E-4 E-3

Time of gel ageing t (days) a 9 II 14 17 25

6.22 5.35 5.10 5.25 4.84

-mediated transformations (crystallizations)

-dt~zeolites

K 4.06 1.29 3.80 7.08 2.13

q

E-3 E-2 E-2 E-2 E-I

4.85 4.45 3.90 3.98 3.60

has shown that the change in

the mass fraction fc' of the stable phase during the transformation at constant growth rate dD/rYr = K

g

(K

g

is the constant of the linear growth rate, D is the

crystal size and 'T is the time at which crystals start to grow) can be expressed as (refs. II, 12):

109

f

c

f (I) + f (II)

c

c

where fc (I) is the mass fraction of the stable phase formed by the growth of the constant number N of t: ,e particles (nuclei) present in the system at t = 0 (I.e, o c heteronuclei formed at the very start of the transformation process or/and the seed crystals present or added in the system at t

= 0; in the further text, such particles c will be marked as particles (nuclei) -I), f (II) is the mass fraction of the stable c phase formed by the growth of particles (nuclei) generated during the transformation

process at 0"; T,.; t

(in the further text, such particles will be marked as particles c (nuclei l-Il), G is the geometrical shape factor of growing parttcles.p is the specific

density of the stable phase, dN'T is the differential number of particles-II formed within a differential time d'T during the drystallization (transformation) process (by homogenous, secondary or/and any other time-consumed nucleation process at constant supersaturation), 'T is the time at which particles-II appear and start to grow (T = t

= 0 for particles-I) and mc (te ) = constant is the mass of the stable c phase at the end of the transformation (crystallization) process (t ~ t ,f = I). c e c Previously it has been shown (refs. 8, II, 12) that for dN'T /d'T = K = constant n (homogenous nucleation at constant supersaturation) the maximum value of q can be

4. In contrast, in the case of gel-crystal transformations (Le, crystallization of zeolites from gels) exponent q is very often greater than 4 which means that the production rate of particles-II increases during the crystallization process, ("autocatalytic nucleation") (ref. 8). Fig. 18 shows that the growth rate of zeolite A particles is constant (K = 2.4 7x10- 4 cm h-I) and independent of the gel age, as g the consequence of the constancy in the solute concentrations during the main part of the crystallization process (ref. 12). The comparison of Figs. IA and 18 shows that the kinetics of crystallization of zeolite A from variously aged gels start to deviate from eq. (I) at the crystallization times at which the growth rates begin to decrease as the consequence of the decrease in the solute concentrations (see eq. (5) in ref. 12). Therefore, in accordance with eqs, (I) and (2), the numerical values of the constants K and q and their changes with the gel ageing (see Table I) indicate that: (I) gel ageing influences only the nucleation processes and not the growth rate of zeolite A particles, (Il) the decrease in q with the gel ageing is the consequence of the decrease in the ratio [number of particles-II / number of particles-I] during gel ageing and (iii) thE!_ rate dN'T / d'T, of the production of particles-II increases with the crystallization time t

(q>3) at constant growth rate. c Zhdanov (ref. 8) explained such an increase in the particle production rate dN'T /drr, postulating "... that not only the aluminosilicate blocks formed in the liquid phase but

also the similar blocks with ordered structure occurring in the gel skeleton can be nuclei of crystals. The number of such blocks passing into solution and coming out

110

_
~Q8 ...J

~U6

N

u,

°U4 z

°GO.2
e:

0 12

E 10 ~ 8 ~

Vi ...J

;:!

Ul

~

u

6 4

2 0

o

1

2

3

CRYSTALLIZATION TIME, tc (h)

4

Fig. I. The change in fraction f of zeolite A (Fig. IA.) and size L of largest crystals of zeolite A (Fi~ lB.) during the crystallization from gels aged for t = 3 d (Ii), t = 9 d (e) and t = 17 d (0). Solid curves in Fig. fA. are calculat~d by eq. (I) and 8orresponding numerical values of K and q (see Table I.). at the surface of gel particles for a unit of time must increase with increasing dissolution rate of the gel skeleton during the autocatalytic stage of crystallization". This means that the dissolution of gel in hot alkaline media should produce both soluble alurriinosilicate species and less soluble crystalline particles (nuclei}-II from the dissoloved amount of gel. The number of such produced "nuclei'' is proportional to the amount of gel dissolved during the zeolite crystalization process. Later, the existance of very small particles of crystalline phase, distributed inside predominantly amorphous gel structure was also observed experimantally (refs. 13, 14). Following this Zhdanov's idea, under the assumption that the growth of crystalline particles surrounded by the gel is considerably blocked and that they can grow only in contact with the solution phase (refs. 4, 14), we derived previously the kinetic equation of zeolite crystallization from gels (ref. 15). Under the real assumption that at relatively low supersaturation (5 ~ I. 78) the rate of homogenous nucleation of zeolite A is negligable, the kinetics of zeolite A crystallization, from gels can be expressed as (ref. i 5):

111

fA

= fA(I)

3 3

= GQ NoKgt/mA(te)(I

+ fA(II)

3) 3/(I K t - K t a c a c

~ K . t cq

(3)

where No has the same meaning as in eq. (2), "J being the number of crystalline particles (nuclei)-II released from the mass of gel needed for the crystallization of a unit mass of zeolite and B = 6/(q+l)(q+2)(q+3) for the size independent crystal 3 growth. The numerical values of the constants K = G Q K /m t ) and K = A( 3 age a Gl? K , listed in Table 2 as functions of gel ageing, were calculated by the gB"1 procedure described earlier (ref. 15). Fig. 2 shows that the fractions fA calculated by eq. (3) agree very well with the measured fractions during the main part of the TABLE 2 Numerical values of K Time of gel ageing t (days) a

o

and K

K /h- 3 0

2.40 3.20 4.10 5.00 1.04

0 I

2 3 7

E-3 E-3 E-3 E-3 E-2

a

as functions of time t

K / h- 3 a 6.20 6.70 7.30 8.50 I.38

a

of gel ageing.

Time of gel ageing t (days) a

E-3 E-3 E-3 E-3 E-2

K /h- 3 0

9 II 14 17 25

1.42 2.50 5.00 8.08 2.03

E-2 E-2 E-2 E-2 E-I

K /h- 3 a 2.06 3.00 3.50 6.13 7.97


l1J 10 •

I--

-.J

00.8 l1J

N

u, 0.6

o zO.t. o

00.2

« e= 0 L...J._---.l.-_L---.L_...L---L_--I----J'---'---'---::--

o

1

2

3

4

CRYSTALLIZATION TIME, t c (h) 0C

Fig. 2. Kinetics of crystallization of zeolite A at 80 from gels aged for (A) 0, (B) I, (C) 2, (0) 3, (E) 7, (F) 9, (G) II, (I) 14, (J) 17 and (K) 25 days. The solid curves were calculated by eq. (3) using the corresponding numerical values of the constants K and K (see Table 2). a o

E-2 E-2 E-2 E-2 E-2

112

crystallization processes, thus indicating that the crystallization of zeolite A from variously aged gels takes place by the process described above (simultaneous growth of constant number No of particles-I present in the system at t = 0 and the c oC). particles-II released from the gel during the crystallization of zeolite A at 90 Since eq. (3) is derived for the constant growth rate, the deviation of the calculated fA values (solid curves in Fig. 2) from the measured values for fA > 0.8 are the consequence of the decrease in the growth rates at the end of the transformation processes, as shown in Fig. lB. On the basis of the proposed model of zeolite crystallization, it has been shown earlier that the number N by the growth of nuclei present in the system at t

c

=0

o

of particles-I formed

and the number N

a

=

of particles-II formed by the growth of nuclei released from the gel during the crystallization ("autocatalytic nucleation"), both contained in I g of zeolite A at the end of the crystallization process can be calculated as (ref. 15): N = a 3. K /G'~'K3 and N = "l = K /G'~' B· K The numerical values of Nand N , o g a a g o a calculated in the described way using the numerical values G = I (cubes), l? = 3, 2 g cm- and K = 2.47xI0- 4 cm h- I (see Fig. IB), as well as their sums N g c No + Na, are presented in Table 3 as functions of the gel age and compared with

the measured total numbers N of zeolite A particles contained in I g of the e products obtained at the end of the crystallization processes from gels aged for the

corresponding times tao TABLE 3 Numbers N0' N ="'}, N N + N and Ne of zeolite A particles contained in a a c a I g of final products of crystallization from the gels aged for given times t a Time of gel ageing t (days) a 0 I 2 3 7 9 II 14 17 25

N

N ="( a

0

7.96 1.06 1.36 1.55 3.45 4.71 8.29 1.66 2.68 6.72

E7 E8 E8 E8 E8 E8 E8 E9 E9 E9

1.88 1.44 1.42 1.76 2.38 3.58 4.34 3.86 7.05 7.49

EIO EIO EIO EIO EIO EIO EIO EIO EIO EIO

N

N

c

1.89 1.45 1.43 1.78 2.42 3.63 4.42 4.03 7.32 8.16

EIO EIO EIO EIO EIO EIO EIO EIO EIO EIO

2.08 1.46 1.20 2.04 2.84 2,82

e EIO EIO EIO EIO EIO EIO

4.08 EIO 6.29 [10 9.08 EIO

The increase of N with the age of gel indicates that the formation of the crystala line phase inside gel structure is a time-consumed process (refs. 3, 15) which can be explained by the simultaneous formation of both distorted (Si,AI,O)-network and simplest structural blocks (for example single and double 4 and 6 membered rings of (5i-0)- and (AI-O)-tetrahedra) by polymerization-depolymerization processes during

113

the gel formation and ageing (ref. 8). On the other hand, the increase in number No of particles-I with gel ageing, under the assumption that the number of impurities in the liquid phase is limited and constant, and that the heterogeneous nucleation catalysed by the presence of the impurities is a fast process (ref. 16), is a somewhat surprising result. However, this effect can be explained by the recrystallization of gel (dissolution of small particles and the growth of large ones) via Ostwald's ripening, during its ageing at ambient temperature (ref. 3). Therefore, the number 7 N = N(ht) +N(a), where N(ht) = N (for t = 0) = 7.96x10 is the number of o 0 a particles-I formed by heterogeneous nucleation in the liquid phase during the precipitation of gel and N(a) is the number of particles-I formed by the growth of nuclei released from the gel during its recrystallization at ambient temperature. The very good agreement between measured total number N of particles contained in e I g of zeolite A and the calculated values N (see Table 3) shows that the proposed c mechanism of zeolite A particles formation is quite reasonable. The decrease in the value of q (see Table I) Is the consequence of the decrease in N

a IN0 ratio with

gel ageing, as stated earlier. Fig. 3 shows the change in fractions fA (I) and fA (II) of zeolite A formed by the growth of particles-I (dashed curve) and particles-II


-to ... <{

wO.S

I-

5°·6 w N04

LL'

0

z o

i= U <{ OC

LL

02

0

,

_._._.-. hOI) "

~

,, , ," ,., , " .,. .'.'

.i->:

o 1 2 CRYSTALLIZATION TIME, t c (h)

L-..I--_ _I - -_ _l - -_ _' - - _ - ' - ' - -

o

I

1 234 PARTICLE DIAMETER t 0 (urn)

Fig. 3 Change in fractions fA (r)

Fig. 4 Particle size distribution by

('iashed curve) and f (II) (dotted curve) during the cr~tallization of zeolite A from gel aged for 17 d. The solid curve represents the crystallization kinetics calculated by eq. (3), fA = fAW + fAW).

mass of zeolite A obtained at the end of the crystallization process from qel aged for 17 d. m is the mass d percentage of particles with the corresponding diameter D.

(dash-dotted curve), respectively (both calculated by eq. (3) and the corresponding data from Table 3) during the crystallization of zeolite A from gel aged for 17 d. It is interesting that in spite of the fact that the number N of particles-II is more a

114

then 26 times higher than the number No of particles-I, the fractions fA(I) of zeolite A formed by the growth of particles-I are greater than the fractions fA (II) of zeolite A formed by the growth of particles-II during the whole course of the crystallization process, fA (I)

= 0.57

and fA (II)

= 0.43

for fA (calculated)

= I.

This

effect is in accordance with Lechert's conclusion (ref. 4) that the growth of crystalline particles inside the gel matrix (in this case, particles-II) is blocked considerably and that they can grow only in full contact with the solution phase. The result shown in Fig. 3 (the same effect is observed for all kinetics with increasing influence of particles-II with the decrease in gel ageing time) indicates that two particle systems could be found in the system during the crystallization of zeolite A. The bimodal nature of particle size distribution by mass, measured using centrifugal sedimentation method at the end of the crystallization process from gel aged for 17 days confirms such conclusion, as can be seen in Fig. 4. Bimodal particle size distribution by mass is characteristic for all crystallizing systems examined, with better peaks separation for shorter ageing times t crystallization times t

a

and hence for the longer

c'

REFERENCES 1 2 3

F. Polak and A. Cichocki, Advan. Chem. Ser., 121 (1973) 209-216. F. Polak and E. Stobiecka, Prace Chem., 21 (1976) 291-307. S.P. Zhdanov and N.N. Samulevich, in L.V. Rees (Editor), Proc. Fifth Int. Cont. Zeolites, Naples, Italy, June 2-6, 1980, Heyden, London-Philadelphia-Rheine, 1980, pp. 75-84. 4 H. Lechert, in P.A. Jacobs et al. (Editors), Structure and Reactivity of Modified Zeolites, Elsevier, Amsterdam, 1984, pp. 107-123. 5 C.R. De Kimpe and H. Kodama, Clay Minerals, 19 (1984) 237 -242. 6 G. Seo, Hwahak Konghak, 23 (1985) 295-30 I. 7 R.W. Thompson, Personal Communication. 8 S.P. Zhdanov, Advan. Chem. Ser. 101 (1971) 20-43. 9 B. Subotic, N. Masie and 1. Smit, in B. Drfa], S. Hocevar and S. Pejovnik (Editors), Studies in Surface Science and Catalysis, Zeolites-Synthesis, Structure, Technology, Applications, Vol. 24, Elsevier, Amsterdam, 1985, pp. 207-214. 10 J. Ciric, J. Colloid Interface Sci., 28 (1968) 315-324. 11 B. Subotlc, D. Skr tlc, 1. Smit and L. Sekovanic, J. Cryst. Growth, 50 (J 980) 498-508. 12 B. Subotic and L. Sekovanic, J. Cryst. Growth, 75 (1986) 561-572. 13 L.A. Bursill and J.M. Thomas, in R. Sersale, C. Collela and R. Aiello (Editors), 5th Int. Conf. Zeolites - Recent Progress Reports and Discussion, Naples, Italy, June 2-6,1980, Giannini, Napoli, 1981, pp. 25-30. 14 Z: Gabelica, J.B. Nagy, G. Debras and E.G. Derouane, Acta Chim. Hung., 119 (1985) 275-284. 15 B. Subotlc and A. Graovac, in B. Drzaj, S. Hocevar and S. Pejovnik (Editors), Studies in Surface Science and Catalysis, Zeolites-Synthesis, Structure, Technology, Applications, Vol. 24, Elsevier, Amsterdam, 1985, pp. 199-206. 16 A. G. Walton, in A.C. Zettlemoyer (Editor), Nudeation, Marcel Dekker, New York, 1969, pp, 225-317.