Role of Gel Aging in Zeolite Crystallization - ACS Symposium Series

Chapter 9

Role of Gel Aging in Zeolite Crystallization

Downloaded by UNIV OF MISSOURI COLUMBIA on April 16, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0398.ch009

A. Katović, B. Subotić, I. Šmit, Lj. A. Despotović, and M. Ćurić Ruder Bošković Institute, P.O. Box 1016, 41001 Zagreb, Croatia, Yugoslavia

The tetragonal form of zeolite Ρ crystallized as the first crystalline phase and subsequently transformed into the cubic form of zeolite Ρ when freshly prepared gel was heated at 80°C, while zeolite X and the cubic form of zeolite Ρ crystallized simultaneously from gels aged at 25°C for 1 day and more. The increase in the ageing time shortened the induction periods of zeolite X and zeolite Ρ and increased the yield of zeolite X crystallized, respectively. The effects observed were explained by the formation of particles of quasicrystalline zeolite Ρ and zeolite X inside the gel during the ageing at ambient temperature and by the growth of particles of quasicrystalline phase during the crystallization step. I t i s w e l l known t h a t t h e low-temperature a g e i n g o f a l u m i n o s i l i c a t e g e l p r e c u r s o r markedly i n f l u e n c e s t h e c o u r s e o f z e o l i t e c r y s t a l l i z a t i o n a t t h e a p p r o p r i a t e temperature (1-10). The p r i m a r y e f f e c t s o f the g e l a g e i n g a r e t h e s h o r t e n i n g o f t h e i n d u c t i o n p e r i o d and t h e a c c e l e r a t i o n o f t h e c r y s t a l l i z a t i o n p r o c e s s ( 1 - 5 ) , but i n some cases the g e l ageing a l s o i n f l u e n c e s the type(s) o f z e o l i t e ( s ) formed (1,6,7,10). Thus, i n many s y n t h e s e s t h e g e l a g e i n g (8-11) o r t h e a d d i t i o n o f t h e " c r y s t a l d i r e c t i o n a g e n t " (aged, X-ray amorphous a l u m i n o s i l i c a t e g e l ) (7,12-14) i s a n e c e s s a r y s t e p needed f o r t h e o b t a i n i n g o f t h e d e s i r e d type o f z e o l i t e a t t h e d e s i r e d r e a c t i o n r a t e . I t i s w e l l known t h a t z e o l i t e s o f type NaP c o - c r y s t a l l i z e w i t h f a u j a s i t e s (15,16). The t y p i c a l r e a c t i o n sequence under t h e a p p r o p r i a t e s y n t h e s i s c o n d i t i o n i s (17): a m o r p h o u s — * f a u j a s i t e — » gismondine type Na-P. However, i n some c a s e s , z e o l i t e Na-P appears as t h e f i r s t c r y s t a l l i n e phase when f r e s h l y p r e p a r e d g e l has been heated a t t h e a p p r o p r i a t e temperature (15,18); i n t h e s e c a s e s , f a u j a s i t e can be c r y s t a l l i z e d e i t h e r by a d d i n g t h e seed c r y s t a l s i n t o t h e f r e s h l y p r e p a r e d g e l (6,13,18) o r by a g e i n g t h e g e l a t ambient temperature p r i o r t o t h e c r y s t a l l i z a t i o n a t t h e

0097-6156/89/0398-0124$06.00/0 o 1989 American Chemical Society In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


9. KATOVICETAL.


125

appropriate temperature (6,8,19). While the influence of the gel ageing on the k i n e t i c s of c r y s t a l l i z a t i o n can generally be explained by the increase i n the number of nuclei formed i n the gel phase or/and i n the l i q u i d phase during the ageing (2-6), the explanation of the effect of the gel ageing on the type of z e o l i t e formed requires the study of chemical and s t r u c t u r a l changes i n both the l i q u i d phase and the s o l i d phase of the reaction mixture during the ageing and the c r y s t a l l i z a t i o n , respectively. Thus, the objective of t h i s work i s to investigate the chemical and s t r u c t u r a l changes in the s o l i d and the l i q u i d phase of the reaction mixture during i t s ageing as well as during the c r y s t a l l i z a t i o n process, i n order to explain why z e o l i t e X c r y s t a l l i z e s only from the aged gel of given composition. The influence of the gel ageing on the c r y s t a l l i z a t i o n rates of z e o l i t e s X and Ρ w i l l also be discussed. Experimental Aluminosilicate gels, having a molar composition 4.24 Na2Û · A I 2 O 3 ' 3.56 Si02*230.6 H2O, were prepared by slow pouring of 150 ml of diluted water-glass solution (1.715 molar S1O2, 0.65 molar N a 2 Û ) , thermostated at 25°C, into a p l a s t i c beaker containing 150 ml of vigorously s t i r r e d sodium aluminate solution (0.482 molar A I 2 O 3 , 1.39 molar Na2Û) thermostated at 2 5 ° C . The aluminosilicate gels were aged at 25°C for given times ( t = 0 to 10 dyas). After ageing for a predetermined time t , the gel was poured into a s t a i n l e s s - s t e e l reaction vessel and heated to c r y s t a l l i z a t i o n temperature ( 8 0 ° C ) . The moment the gel was added into the preheated reaction vessel was taken as the zero time ( t = 0) of the c r y s t a l l i z a t i o n process. The reaction mixture was s t i r r e d with a teflon-coated magnetic bar driven by a magnetic s t i r r e r . At ageing times t a = 0 to 10 days before the c r y s t a l l i z a t i o n process ( t = 0 at 2 5 ° C ) , as well as the times t after the beginning of the c r y s t a l l i z a t i o n process (at 80°C), aliquots of the reaction mixture were drawn o f f and centrifuged i n order to separate the s o l i d from the l i q u i d phase and to stop the c r y s t a l l i z a t i o n process, respectively. Aliquots of the clear l i q u i d phase above the sediment were used to measure S i and Al concentrations by atomic absorption spectrometry (Perkin-Elmer atomic absorption spectrometer, model 3030B) and for the determination of the degree of S i polycondensation i n the l i q u i d phase by molybdate method (20). The s o l i d phase, after having been washed and dried i n the dessicator at room temperature up to the constant weight, was used for analyses. The fractions f of g e l , f p of z e o l i t e Na-Pc and fx of z e o l i t e X i n the powdered s o l i d samples were taken by P h i l i p s diffractometer with C U K - P C radiation i n the region of Bragg*s angles 2Θ· = 10° - 46°. The weight fractions were calculated by the mixing method (21) using the measured integral i n t e n s i t y of the amorphous maximum (2Φ = 17° - 39°) and the sharp maximum corresponding to the d i f f r a c t i o n from (533) c r y s t a l l a t t i c e planes of z e o l i t e X, as well as the sharp maximum corresponding to the d i f f r a c t i o n from (310) c r y s t a l l a t t i c e planes of z e o l i t e Na-Pc. The average values of c r y s t a l l i t e size were determined by the i n t e g r a l width of the d i f f r a c t i o n maximums corresponding to the a

a

c

c

c

Q

c

In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

126

ZEOLITE SYNTHESIS


d i f f r a c t i o n from (642) and (533) c r y s t a l l a t t i c e planes of z e o l i t e X and by the integral width of the d i f f r a c t i o n maximums corresponding to the d i f f r a c t i o n from (310) and (211) c r y s t a l l a t t i c e planes of z e o l i t e Na-Pc, using the Scherrer formula (22). Si and Al contents i n the s o l i d samples were determined by measuring of S i and Al concentrations i n the solutions obtained by the dissolution of the solids (75 mg) i n 50 ml of 2N HC1. Scanning-electron micrographs of the samples were taken with a Jeol JSM-940 scanning-electron microscope. Infrared transmission spectra of the s o l i d samples were measured by the KBr wafer technique. Spectra were recorded on a Perkin-Elmer infrared spectrometer, model 580 B. Results and discussion Figure 1 shows the kinetics of the c r y s t a l l i z a t i o n of z e o l i t e X (Figure 1A) and z e o l i t e Na-Pc (Figure 1B), respectively, at 80°C from the gels aged at 25°C f o r 1, 3, 5, 7 and 10 days. In a l l cases, z e o l i t e X appears as the f i r s t c r y s t a l l i n e phase, thereafter z e o l i t e Na-Pc c o - c r y s t a l l i z e s with z e o l i t e X. After the maximal y i e l d of z e o l i t e X c r y s t a l l i z e d has been attained, the f r a c t i o n f^ of z e o l i t e X slowly decreases as the consequence of the spontaneous transformation of z e o l i t e X into more stable z e o l i t e Na-Pc (17). The induction periods of both z e o l i t e X and z e o l i t e Na-Pc shortens and the maximal y i e l d of z e o l i t e X increases, respectively, with the increased time of gel ageing. A l l k i n e t i c s of z e o l i t e X and z e o l i t e Na-Pc, respectively, can be mathematically expressed by the simple k i n e t i c equation (5,23-26), f

ζ

= Κ · t c

q

(1)

during the i n t e r v a l of the increasing c r y s t a l l i z a t i o n rate. Here, f i s the mass f r a c t i o n of z e o l i t e c r y s t a l l i z e d at the c r y s t a l l i z a t i o n time t , and Κ and q are constants f o r given experimental conditions. The numerical values of the constants Κ and q, calculated by the log f versus log t plots (25) using the corresponding experimental data from Figure 1, are l i s t e d i n Table I. z

c

z

c

Table I. Numerical values of the constants Κ and q which correspond to the k i n e t i c s of c r y s t a l l i z a t i o n of z e o l i t e X and z e o l i t e Na-Pc, respectively, from aluminosilicate gels aged f o r various times t a

Time of gel ageing (t /d) Q

1 3 5 7 10

Zeolite X Κ

6.34

E-4 1.28 E-3 1.35 E-3

q

-4.03 4.36 4.69

Zeolite Na-Pc Κ

q

7.84 E-9 2.78 E-7 2.21 E-6

9.86 7.83 7.27

-


-


KATOVIC ET AL.

127


Figure 1. Kinetics of c r y s t a l l i z a t i o n of z e o l i t e X (Figure A) and z e o l i t e Na-Pc (Figure B) at 80°C, from the aluminosilicate gels aged f o r t = 1 d (• ), t = 3 d (a), t = 5 d ( Δ ), t = 7 d (·) and t = 10 d (o) at 25°C. f and f p are mass fractions of z e o l i t e X and zeolite Na-Pc c r y s t a l l i z e d at c r y s t a l l i z a t i o n time t . Solid curves represent the kinetics of c r y s t a l l i z a t i o n , calculated by Equation (1) and the corresponding values of the constants Κ and q from Table I. Q

Q

Q

Q

x

c

c


Q


128

ZEOLITE SYNTHESIS

There are three t y p i c a l groups of the c r y s t a l l i z a t i o n k i n e t i c s : I. z e o l i t e Na-Pc i s the dominant product of the c r y s t a l l i z a t i o n from the gels aged for 0 to 3 days, I I . the mixture of approximately same amounts of z e o l i t e X and z e o l i t e Na-Pc c r y s t a l l i z e s from the gel aged for 5 days (see scanning-electron micrograph i n Figure 5) and I I I . z e o l i t e X i s the dominant product of the c r y s t a l l i z a t i o n from the gels aged for 7 and 10 days. The scanning-electron micro graph i n the Figure 5 shows that the microcrystals of z e o l i t e X can be c l e a r l y distinguished from the t y p i c a l spherulites of z e o l i t e Ρ in the f i n a l product of the c r y s t a l l i z a t i o n . Figures 2 - 4 show the changes i n chemical characteristics of the l i q u i d and the s o l i d phase, respectively, during the c r y s t a l l i z a t i o n from the gels aged for t = 1 d (group I., see Figure 2), for t : 5 d (group I I . , see Figure 3) and for t = 10 d (group I I I . , see Figure 4). In a l l analyzed cases, the concentration of both s i l i c o n and aluminum i n the l i q u i d phase increase l i t t l e from t = 0 up to t ^ 1 h as the consequence of the increase i n the s o l u b i l i t y of the gel with increasing temperature (heating from 25°C up to 80°C). After the c r y s t a l l i z a t i o n temperature has been reached, the concentration C ^ ( L ) of the aluminum i n the l i q u i d phase keeps constant during the main part of the c r y s t a l l i z a t i o n process while the concentration Cgj(L) of the s i l i c o n i n the l i q u i d phase keeps constant during the induction period of the c r y s t a l l i z a t i o n only. The g e l - c r y s t a l transformation i s followed by the increase i n the concentration C 5 J ( L ) up to the maximal value C5j(L) and by the simultaneous decrease i n the S i / A l molar r a t i o i n the s o l i d phase, respectively. Hence, i t i s reasonable to assume that the increase i n the concentration of s i l i c o n i n the l i q u i d phase, during the c r y s t a l l i z a t i o n , i s the consequence of the releasing of soluble s i l i c a t e species from the s o l i d phase due to the lower S i / A l r a t i o i n the c r y s t a l l i n e phase(s) ( C S i / A l J 1.305) than i n the starting aluminosilicate gels ( [ S i / A l ] ~ 1.4), as shown i n Figures 2C-4C. At the c r y s t a l l i z a t i o n time when about 70 % of the gel has been transformed into the c r y s t a l l i n e phase(s), the rate of c r y s t a l l i z a t i o n starts to deccelerate simultaneously with the sudden decrease i n the concentrations of both s i l i c o n and aluminum i n the l i q u i d phase. This i s probably the consequence of the decrease i n the p a r t i c l e growth rate(s) (see Figure 6) caused by the decrease i n the solute concentration at the time when the rate of deposition of the soluble species from the l i q u i d phase onto the surfaces of the growing p a r t i c l e s becomes higher than the rate of feeding of the solution with new soluble species (5,26). The chemical changes i n the l i q u i d phase and the s o l i d phase, respectively, c l e a r l y indicate that the c r y s t a l l i z a t i o n of z e o l i t e X and z e o l i t e Na-Pc, respectively, from gels i s a solution-mediated transformation process i n which the amorphous phase i s a precursor for s i l i c a t e , aluminate and/or aluminosilicate species needed for the growth of the c r y s t a l l i n e phase(s) (2,16,19,23-26). Table I I . shows that the concentrations Cgj and of s i l i c o n and aluminum i n the l i q u i d phase of the gel as well as the molar r a t i o [ S i / A l ] of s i l i c o n and aluminum i n the s o l i d phase of the gel keeps approximately constant during the ageing at 25°C, indicating that the chemical equilibrium between the s o l i d and the l i q u i d phase of the gel has been attained for to ^ 1 d. After the a

Q

a

c

c

M G X

s

s

s



KATOVIC ET AL.


0

2 t

4 c

129

6

(h)

Figure 2. The changes i n : A. mass fractions f^ of z e o l i t e X ( o ) , f p of z e o l i t e Na-Pc (·) and f^ of g e l (Δ), B. concentrations C^^(L) of aluminum (·) and CgjiL) of s i l i c o n (o) i n the l i q u i d phase and C. molar r a t i o [ S i / A l ] i n the s o l i d phase ( o ) , with the c r y s t a l l i z a t i o n time t , during the c r y s t a l l i z a t i o n from the gel aged for t = 1 d. c

s

c

a

0

2 t

c

4 (h)

Figure 3. The changes i n : A. mass fractions f of z e o l i t e X ( o ) , f p of z e o l i t e Na-Pc (·) and fç of gel (Δ), B. concentrations CAJ(L) of aluminum (·) and CgjCD of s i l i c o n (o) i n the l i q u i d phase and C. molar r a t i o [ S i / A l J i n the s o l i d phase ( o ) , with the c r y s t a l l i z a t i o n time t , during the c r y s t a l l i z a t i o n from the gel aged for t = 5 d. x

c

s

c

a



130

ZEOLITE SYNTHESIS

t

(h)

Figure 4. The changes i n : A. mass fractions f of z e o l i t e X (o), f p of z e o l i t e Na-Pc (·) and fç of gel ( Δ ) , Β. concentrations CA[(L) of aluminum (·) and C $ j ( L ) of s i l i c o n (o) i n the l i q u i d phase and C. molar r a t i o [ S i / A l ] i n the s o l i d phase (o), with the c r y s t a l l i z a t i o n time t , during the c r y s t a l l i z a t i o n from the gel aged for t = 10 d. x

c

s

c

Q



KATOVIC ET AL.

131


Figure 5. Scanning-electron micrograph of the s o l i d sample drawn off the reaction mixture at the end of the c r y s t a l l i z a t i o n process from the gel aged for t = 5 d; t = 6.5 h, f^ = 0.43, f = 0.57. Q

c

P c

4 t

5 c

6

(h)

Figure 6. The changes i n the average size of the c r y s t a l l i t e of z e o l i t e X (solid curves) and i n the average size L p of zeolite Na-Pc (dashed curves) with the c r y s t a l l i z a t i o n time t , during the c r y s t a l l i z a t i o n from gels aged for 1 (•), 3 (*), 5 U ) , 7 (·) and 10 (o) days. c

c


ZEOLITE SYNTHESIS

132

c r y s t a l l i z a t i o n temperature has been reached, the concentrations of both s i l i c o n and aluminum i n the_liquid phase_slowly increase, thereafter their average values Cgj(L) and C ^ ( L ) keep approximately constant during the induction period of the c r y s t a l l i z a t i o n or even, during the main part of the c r y s t a l l i z a t i o n process (for aluminum) (see Figures 2-4), and are not markedly influenced by the gel ageing, as shown i n Table I I I . Also, the maximum value C s j ( ) of the s i l i c o n concentration i n the l i q u i d phase, attained during the c r y s t a l l i z a t i o n , i s not markedly influences by the ageing of the gel, as shown i n Figures 2B-4B and i n Table I I I . L


m a x

Table I I . Characteristics of the aluminosilicate gels of 4.24 Na 0-Al 0 -3.56 Si0 - 230.6 H 0 batch composition aged for various times t at 25°C 2

2

3

2

2

a

t /d

C5j/mol

Q

1 2

3

dm"

C /mol Al

0.203 0.203 0.213

3

0.209 0.199

5 7 10

[Si/Al]

3

dm"

0.0150 0.0141

1.42

1.22

1.39

0.0137

1.40 1.45

1.11 1.23 1.27

1.43

1.27

0.0145 0.0142 0.0144

0.186

1

R/min"

s

1.41

Table III. Chemical composition of the l i q u i d phase during the c r y s t a l l i z a t i o n from the gels aged for various times t Q

C^(L)/mol

t /d

dm"

3

C^j(L)/mol

3

dm"

c

L

r

a

o

0.0210

0.224 0.221

3

0.0199 0.0162

0.237

0.267

5

0.0190

0.223

7 10

0.0170

0.216

0.255 0.262

0.0160

0.203

0.239

1 2

s i

(

W 0.251 0.262

a

1

d m

~

3

The measuring of the degree of polycondensation of s i l i c a t e anions in the l i q u i d phases of the gels aged fro 1, 2, 3, 5, 7 and 10 days, as well as i n the l i q u i d phases of the c r y s t a l l i z i n g systems, has shown that i n a l l the cases the logarithm In UR, of the percentage of S i 0 unreacted with molybdic acid i s a linear function of the reaction time t , with the slopes R = d(ln UR)/dt between 1.11 min~1 and 1.27 min-1 (see Table I I I . ) . It has been appreciated from the results obtained, that the l i q u i d phase of the reaction mixture contains a mixture of monomeric s i l i c a t e species (60 % - 80 % ) , dimeric s i l i c a t e species (20 % - 40 %) and possibly, low-"molecular" aluminosilicate species that give monosilicic acid and d i s i l i c i c acid, respectively, i n an a c i d i c degradation (20,27,28). Figure 6 shows that the average growth rate of z e o l i t e X (£gjX) = d L / d t » 3 x 1 0 " cm/h) and of z e o l i t e Na-Pc (K (Pc) = d L p / d t « 1.6x10*6 cm/h), respectively, i s constant during the 2

5

x

c

c

g

c



9.

KATOVIC ET AK


133

c r y s t a l l i z a t i o n process and independent on the gel ageing. The independence of the growth rates Kg(X) and Kg(Pc) on the increase i n the s i l i c o n concentration during the c r y s t a l l i z a t i o n indicates t'.iat the excess of s i l i c o n , produced i n the l i q u i d phase during the dissolution of the gel and i t s p a r t i a l transformation into c r y s t a l l i n e products with lower S i / A l molar r a t i o , exists i n the form of "inactive" monosilicic and d i s i l i c i c acids (8) and hence, does not participate i n the reactions at the surfaces of the growing c r y s t a l s (Katovic, Α.; Subotic, B.; Smit, I.; Despotovic, L j . A. Zeolites, i n press). Thus, i t i s reasonable to assume that the growth process takes place by the reaction of low-"molecular" aluminosilicate species from the l i q u i d phase at the surfaces of growing z e o l i t e p a r t i c l e s and that the aluminum concentration (in the form of "reactive" aluminosilicate species) i s the determining factor of the growth rate of the p a r t i c l e s of z e o l i t e X and z e o l i t e Na-Pc, respectively. Now, the constancy of the c r y s t a l growth rate of both z e o l i t e X and z e o l i t e Na-Pc indicates that the shortening of the induction periods of the c r y s t a l l i z a t i o n of both z e o l i t e s i s the consequence of the increase i n the number of nuclei during the gel ageing. The simultaneous c r y s t a l l i z a t i o n of z e o l i t e X and z e o l i t e Na-Pc under almost i d e n t i c a l chemical conditions indicates that the nuclei of z e o l i t e X and the nuclei of z e o l i t e Na-Pc exist as separate e n t i t i e s . The analysis of the c r y s t a l l i z a t i o n k i n e t i c s of z e o l i t e X and z e o l i t e Na-Pc, respectively, shows that the numerical value of the exponent q i n Equation (1) i s greater than 4 i n a l l k i n e t i c s (see Table I.), which means that the nucleation rate increases during the c r y s t a l l i z a t i o n process. The changes i n the c r y s t a l l i z a t i o n k i n e t i c s with the gel ageing, at constant chemical composition of the reaction mixture, indicate that the s t r u c t u r a l changes i n the s o l i d phase of the gel, during i t s ageing, should be responsible for the effects observed. Figure 7 shows that the X-ray diffractogram of the s o l i d phase of f r e s h l y prepared gel (0-0) exhibits the amorphous maximum at the 2& angle corresponding to the d i f f r a c t i o n from (310) c r y s t a l l a t t i c e planes of z e o l i t e Na-Pc (strongest X-ray d i f f r a c t i o n maximum of z e o l i t e Na-Pc; see Figure 1 i n : Katovic, Α.; Subotic, B.; Smit, I.; Despotovic, L j . A. Zeolites, i n press). Figure 8 shows that the IR spectra of the s o l i d phase of freshly prepared gel (spectra a), as well as the IR spectra of the s o l i d phases of gels aged for 5 d (spectra b) and for 10 d (spectra c) have a broad band with the maximum at 600 cm~1, indicating the presence of D4R secondary building units of z e o l i t e Na-Pc (29). These findings lead to the assumption that the mixing of s i l i c a t e and aluminate solutions produces the predominantly amorphous aluminosilicate gel containing a number ( N ) p of very small p a r t i c l e s of q u a s i c r y s t a l l i n e phase having a structure close to the structure of the cubic modifica tion of z e o l i t e P. Such p a r t i c l e s of the q u s i c r y s t a l l i n e phase, probybly formed by the polycondensation processes inside the gel matrix during i t s p r e c i p i t a t i o n , can be potential nuclei (nuclei-II) for the c r y s t a l l i z a t i o n of z e o l i t e Na-Pc. At the same time (during the gel precipitation) a number (N )p = N(ht) of nuclei (nuclei-I) i s assumed to be formed i n the l i q u i d phase by the heterogeneous nucleation, catalyzed by the presence of the active centers at the Q

c

Q

c


134

ZEOLITE SYNTHESIS

306 nm


w c Q) C

u

-

2* ( )

F i g u r e 7. X-ray d i f f r a c t o g r a m s ( r e l a t i v e i n t e n s i t i e s I versus Bragg*s a n g l e s 2& and v e r s u s X-ray d i f f r a c t i o n s p a c i n g s d) o f the s o l i d phase o f t h e g e l s aged a t 25°C f o r t = 0 (0-0), t = 2 d (2-0), and t = 10 d (10-0) and o f t h e same g e l s heated a t 80°C f o r t = 4 h ( 0 - 4 ) , t = 3 h (2-3) and t = 1 h (10-1). r

a

a

Q

c

I

I

c

1 1

1800

I

1

I

I

1

I

I

1000

I

I

I

c

I

I

I

I I

600

wave number (cm

)

F i g u r e 8. IR s p e c t r a o f t h e s o l i d phase o f t h e g e l s aged a t 25°C f o r t = 0 ( a ) , t = 5 d ( b ) , t = 10 d ( c ) as w e l l as t h e IR s p e c t r a o f z e o l i t e X (d) and z e o l i t e Na-Pc ( e ) . a

Q

a


9. KATOVIC ET AL.

135


impurities always present i n the l i q u i d phase (30). Hence, after the gel has been precipitated, the system contains a number ( N ) p = N(ht) of nuclei-I distributed through the l i q u i d phase and a number ( N ) p of nuclei-II distributed inside the s o l i d phase of the g e l . Since the positions of the amorphous maximums and t h e i r i n t e n s i t i e s do not change during the induction period (see Figure 7 and Figure 1 in Katovic, Α.; Subotic, B.; Smit, I.; Despotovic, L j . Zeolites, in press), i t i s reasonable to conclude that the p a r t i c l e s of quasic r y s t a l l i n e phase (nuclei-II), distributed inside the gel matrix, cannot grow, or their growth i s considerably deccelerated due to the slow material transport inside the gel matrix i n comparison with the rate of material transport i n the l i q i d phase. Such conclusion i s in accordance with Kacirek's and Lechert's finding (18) that the growth of c r y s t a l l i n e p a r t i c l e s inside the gel matrix i s blocked considerably and that they can grow only i n f u l l contact with the solution phase. On the other hand, the s h i f t i n the position of the amorphous maximum i n the X-ray diffractograms of the s o l i d phase of variously aged gels toward lower X-ray d i f f r a c t i o n spacings (see Figure 7), indicates that structural changes take place i n the s o l i d phase of the gel during i t s ageing. At t h i s moment, we do not know the fine mechanism of these changes, but on the basis of the the Raman spectroscopic study of the ageing of the gel prepared for the c r y s t a l l i z a t i o n of z e o l i t e Y (31), i t i s reasonable to assume that the structural changes obseved are the consequence of the slow formation of six-membered aluminosilicate rings, their ordering into sodalite cages and the possible formation of p a r t i c l e s of quasic r y s t a l l i n e phase (nuclei-II) with the structure near to the structure of faujasite, inside the gel matrix. Q


Q

c

c

Now, assuming that the freshly prepared gel contains only the nuclei of z e o l i t e Ρ (no z e o l i t e X c r y s t a l l i z a t i o n ) (Katovic, Α.; Subotic, B.; Smit, I.; Despotovic, L j . A. Zeolites, i n press), i t s ageing at ambient temperature causes two e f f e c t s : ( i ) the formation of six-membered aluminosilicate rings and their ordering into quasic r y s t a l l i n e faujasite (nuclei-II of z e o l i t e X) and ( i i ) the r e c r y s t a l l i z a t i o n of the gel (the dissolution of small and the growth of large p a r t i c l e s of the gel) (2), releases the number N(a)p of p a r t i c l e s of quasicrystalline z e o l i t e Na-Pc and the number N(a)^ of quasicrystalline p a r t i c l e s of faujasite from the dissolved gel p a r t i c l e s . Since the c r y s t a l growth rate at ambient temperature i s assumed to be negligible i n comparison with the growth rate at c r y s t a l l i z a t i o n temperature, the p a r t i c l e s of the q u a s i c r y s t a l l i n e phase released from the gel during i t s ageing (and being i n the f u l l contact with the l i q u i d phase), become new nuclei-I, i . e . , (N )p = N(ht) + N(a)p and (Ν )χ = N(a)^at c r y s t a l l i z a t i o n time t = 0 for any ageing time t , where N(ht) i s the number of nuclei-I of z e o l i t e Na-Pc formed i n the l i q u i d phase by heterogeneous nucleation during the gel p r e c i p i t a t i o n ( ( N ) p = N(ht) for t = 0 and t = 0). Similar process of the increase i n the number of nuclei-I during the gel ageing was observed i n z e o l i t e A c r y s t a l l i z i n g system (5). The heating of the reaction mixture induces the growth of nuclei-I of both z e o l i t e s from the solution supersaturated with soluble aluminosilicate species. Since the growth rate of z e o l i t e X i s considerably greater than the growth rate of z e o l i t e Na-Pc (see Figure 6), z e o l i t e X appears as the f i r s t c r y s t a l l i n e phase. The c

0

c

0

c

a

0

c

a

c


c


136

ZEOLITE SYNTHESIS

starting growth of nuclei-I exhausts the soluble species from the l i q u i d phase causing the dissolution of a part of the gel i n order to keep the s o l i d - l i q u i d equilibrium. The p a r t i c l e s of quasic r y s t a l l i n e faujasite and z e o l i t e P, respectively (nuclei-II of both z e o l i t e s ) , which are released from the dissolved amount of the g e l , start to grow from the supersaturated solution. The increase i n the number of nuclei (that are i n the f u l l contact with the l i q u i d phase) accelerates the formation of the c r y s t a l l i n e phase and at the same time accelerates the rate of the gel dissolution and the rate of the releasing of new n u c l e i - I I . The consequence i s an "explosive" rate of the outcoming of nuclei-II from the gel and the increase i n the c r y s t a l l i z a t i o n rate during the autocatalytic stage of the c r y s t a l l i z a t i o n (5,24,25). The described c r y s t a l l i z a t i o n process can be mathematically expressed by Equation (1) with q 4 (5,24,25) or by the equivalent kinetic form: f

z

= W3 3/(1

" GÇnfl K t3) = K t£/(1 - K t )

G

3

t

3

G

a

(2)

a

e a r l i e r derived (25) on the basis of Zhdanov's idea on autocatalytic nucleation (2,24). Here, G i s the geometrical shape factor of z e o l i t e p a r t i c l e s , § i s the s p e c i f i c density of z e o l i t e formed, N i s the number of p a r t i c l e s - I (formed by the growth of nuclei-I), N = i s the number of p a r t i c l e s - I I (formed by the growth of the p a r t i c l e s of quasicrystalline phase released from the gel during the c r y s t a l l i z a t i o n process), both contained i n a unit mass of z e o l i t e formed at the end of the c r y s t a l l i z a t i o n process, Kg i s the constant of the linear growth rate of z e o l i t e p a r t i c l e s and fl = 6/(q+1)(q+2) (q+3). The numerical values of the constants K = G § N K 2 and K = GÇflN K , calculated by the procedure described e a r l i e r (25), as well as the ratios Ν / N = K / β K , are l i s t e d i n Table IV. as functions of time t of the gel ageing. 0

A

0

0

3

a

a

α

0

a

Q

Q

Table IV. The change i n the numerical values of K , K and N / N with the time t of the gel ageing 0

Q

Q

z e o l i t e Na-•Pc

zeolite X t /d a

K /h-3 Q

1 3 5 7 10

2.35 5.14 6.52

-E-3 E-3 E-3

K /h-3 a

2.13 6.15 9.58

- E-3 E-3 E-3

Q

a

NQ/

-

32 50 72

N

0

K /h-3 Q

5.56 E-4 8.50 E-4 1.11 E-3

K /h"3 Q

3.09 3.67 3.92

V

E-3 E-3 E-3

ÏÏ

o

1533 775 464

-

-

Figures 2A-4A show that the fractions f^ and f p , calculated by Equation (2) and the corresponding values of the constants K and K from Table IV. (solid curves i n Figures 2A-4A) agree very well with the measured fraction during the autocatalytic stages of the c r y s t a l l i z a t i o n processes, thus indicating that the c r y s t a l l i z a t i o n of z e o l i t e X and z e o l i t e Na-Pc, respectively, from variously aged gels takes place by the mechanism described above. c

a

0


9. KATOVIC ET AL.

137

Role ofGel Aging in Zeolite Crystallization

Conclusions The analysis of the experimental data and the numerical values of the constants K and K and the ratios N /N for the c r y s t a l l i z a t i o n of both z e o l i t e X and z e o l i t e Na-Pc, leads to the following conclusions: The t o t a l number of nuclei of z e o l i t e Na-Pc (number of nuclei-I + number of nuclei-II = N + N = K / G § K 2 + K / § G β K ^ ; see Equation (2)), does not change, or s l i g h t l y decreases during the ageing of the gel. A part of quasicrystalline p a r t i c l e s of z e o l i t e Na-Pc (nuclei-II) releases from the gel and becomes nuclei-I during the gel ageing so that the r a t i o N /N f o r z e o l i t e Na-Pc_decreases with the gel ageing (see Table IV.). The high r a t i o s N / a for z e o l i t e Na-Pc indicate that only very small proportion of the p a r t i c l e s of quasicrystalline z e o l i t e Ρ has been released from the gel during i t s ageing, and this i s a possible reason f o r the long induction period of the c r y s t a l l i z a t i o n of z e o l i t e Na-Pc. The number of p a r t i c l e s of quasicrystalline faujasite (nuclei-II of z e o l i t e X) increases inside the gel matrix during the gel ageing. A part of the p a r t i c l e s of quasicrystalline faujasite releases from the gel during i t s ageing and becomes nuclei-I f o r the c r y s t a l l i z a tion of z e o l i t e X. The increase of the r a t i o N /N during the gel ageing indicates that the rate of the formation of the p a r t i c l e s of quasicrystalline faujasite inside the gel matrix i s greater than the rate of their outcoming from the gel during i t s ageing (recrystallization) . The absence of the IR band at 560 cm~1 (characteristic f o r D6R secondary building units of z e o l i t e X) (29) and the presence of the broad band with the maximum at 600 cnH (characteristic for D4R secondary building units of z e o l i t e P) (29) even i n the IR spectra of the s o l i d phase of the gel aged for 10 d (see Figure 8c), as well as the much lower values N /N f o r z e o l i t e X than the values N /N f o r z e o l i t e Na-Pc (see Table IV.), indicate that the t o t a l number of nuclei (nuclei-I + nuclei-II) of z e o l i t e Na-Pc i s much greater than the t o t a l number of nuclei of z e o l i t e X, f o r a l l the gels examined. The shortening of the induction periods of the c r y s t a l l i z a t i o n of both z e o l i t e X and z e o l i t e Na-Pc i s most probably the consequence of the increase i n the number of nuclei-I of both zeolites with the ageing of the g e l . The increase i n the y i e l d of z e o l i t e X c r y s t a l l i z e d , with the gel ageing, i s the consequence of the increase i n the t o t a l number of nuclei of z e o l i t e X at constant, or even decreasing t o t a l number of z e o l i t e P, during the gel ageing. The high yields of z e o l i t e X c r y s t a l l i z e d from the systems aged for 5 days and more, i n which the t o t a l number of nuclei of z e o l i t e Ρ i s considerably greater than the t o t a l number of nuclei of z e o l i t e X, are most probably influenced by the much greater growth rate of z e o l i t e X p a r t i c l e s r e l a t i v e to the growth rate of z e o l i t e Na-Pc p a r t i c l e s and by r e l a t i v e l y low N /N r a t i o for z e o l i t e X compared with the N /N r a t i o for z e o l i t e Ρ (see Table IV.). For i l l u s t r a t i o n , i t i s easy to calculate by Equation (2) and the data i n Table IV. that_in the case when the t o t a l number of nuclei of z e o l i t e Na-Pc (N /NQ = 464) would be 2000 times greater than the t o t a l number of nuclei of z e o l i t e Χ (N /N = 32), and when the growth rate of z e o l i t e 0

Q

0

Q

Q

Q

Q

Q

0

a


N

0

0

0

a

0

0

Q

a

Q

Q

0

0

a


a

138

ZEOLITE SYNTHESIS

X p a r t i c l e s would be 10 times higher than the growth rate of z e o l i t e Na-Pc p a r t i c l e s , 50 % of z e o l i t e X and 50 % of z e o l i t e Na-Pc would be formed at the end of the c r y s t a l l i z a t i o n process.


Literature

Cited

1. Polak, F.; Cichocki, A. In Molecular Sieves; Meier, W. M.; Uytterhoeven, J. B. Eds.; Advances in Chemistry Series No. 121; American Chemical Society: Washington, DC, 1973; p 209. 2. Zhdanov, S. P.; Samulevich, Ν. N. Proc. 5th Int. Conf. Zeolites, 1980, p 75. 3. Lechert, H. In Structure and Reactivity of Modified Zeolites; Jacobs, P. Α., Ed.; Elsevier: Amsterdam, 1984; p 107. 4. Seo, G. Hwahak Konghak 1985, 23 295. 5. Bronić, J.; Subotić, B.; Smit, I.; Despotović, Lj. A. In Innovation in Zeolite Materials Science; Grobet, P. J . ; Mortier, W. J.; Vansant, E. F.; Schulz-Ekloff, G., Eds.; Studies in Surface Science and Catalysis No. 37; Elsevier: Amsterdam, 1988; p 107. 6. Kasahara, S.; Itabashi, K.; Igawa, K. In New Developments in Zeolite Science Tachnology; Murakami, Y.; Iijima, Α.; Ward, J. W., Eds.; Proc. 7th Int. Conf. Zeolites; Kodansha Ltd.:Tokyo, 1976; p 185. 7. Nicolas, S.; Massiani, P.; Vera Pacheco, M.; Fajula, F.; Figueras, F. In Innovation in Zeolite Material Science; Grobet, P. J.; Mortier, W. J.; Vansant, E. F.; Schulz-Ekloff, G., Eds.; Studies in Surface Science and Catalysis No. 37; Elsevier: Amsterdam, 1988; p 115. 8. Dewaele, N.; Bodart, P.; Gabelica, Z.; Nagy, J. B. Acta Chimica Hungarica 1985, 119, 233. 9. Cichocki, A. J. Chem. Soc. Faraday Trans. 1 1985, 81, 1297. 10. Kühl, G. H. Zeolites 1987, 7, 451. 11. Occelli, M. L.; Pollack, S. S.; Sanders, J. V. In Innovation in Zeolite Material Science; Grobet, P. J . ; Mortier, W. J.; Vansant E. F.; Schulz-Ekloff, G., Eds.; Studies in Surface Science and Catalysis No. 37; Elsevier: Amsterdam, 1988; p 45. 12. Albers, E. W.; Vaughan, D. E. W. U. S. Patent 3 947 482, 1976. 13. Ruren, X.; Jianmin, Z. Chem. J. Chin. Univ. 1982, 25, 349. 14. Qinhua, X.; Shulin, B.; Jialu, D. Scientia Sinica 1982, 25, 349. 15. Kerr, G. T. J. Phys. Chem. 1968, 72, 1385. 16. Freund, E. F. J. Cryst. Growth 1976, 34, 11. 17. Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982; Chapter 6, 6. 18. Kacirek, H.; Lechert, H. J. Phys. Chem. 1975, 79, 1589. 19. Fahlke, B.; Starke, P.; Wieker, W.; Wendlandt, K. P. Zeolites 1987, 7, 209. 20. Thilo, E.; Wieker, W.; Stade, H. Ζ. anorg. allg. Chem. 1965, 340, 260. 21. Zevin, L. S.; Zavyalova, L. L. Kolichestvenniy Rendgenographcheskiy prazoviy analiz; Nedra: Moskva, 1974; p 37. 22. Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures; Wiley: New York, 1954; p 491. 23. Ciric, J. J. Colloid Interface Sci. 1986, 28, 315.



9. KATOVIC ET AL.

Role ofGel Aging in Zeolite Crystallization

139

24. Zhdanov, S. P. In Molecular Sieve Zeolites-I; Gould, R. F.; Ed.; Advances in Chemistry Series No. 101; American Chemical Society: Washington, DC, 1971; p 20. 25. Subotić, B.; Graovac, A. In Zeolites: Synthesis, Structure, Technology and Application; Držaj, B.; Hočevar, S.; Pejovnik, S., Eds.; Studies in Surface Science and Catalysis No. 24; Elsevier: Amsterdam, 1985; p 199. 26. Subotić, B.; Sekovanić, L. J. Cryst. Growth 1986, 75, 561. 27. O'Connor, T. L. J. Phys. Chem. 1961, 65, 1. 28. Wieker, W.; Fahlke, B. In Zeolites: Synthesis, Structure, Technology and Application; Držaj, B.; Hočevar, S.; Pejovnik, S., Eds.; Studies in Surface Science and Catalysis No. 24; Elsevier: Amsterdam, 1985; p 161. 29. Flanigen, Ε. M.; Szymanski, Η. Α.; Khatami, H. In Molecular Sieve Zeolites-I; Gould, R. F., Ed.; Advances in Chemistry Series No. 101; American Chemical Society: Washington, DC, 1971; p 201. 30. Walton, A. G. In Nucleation; Zettlemoyer, A. C., Ed.; Marcel Dekker Inc.: New York, 1969; p 225. 31. Dutta, P. K.; Shieh, D. C.; Puri, M. J. Phys. Chem. 1978, 91, 2332. RECEIVED December 22, 1988


Role of Gel Aging in Zeolite Crystallization - ACS Symposium Series

Recommend Documents