[acs symposium series] zeolite synthesis volume 398 || role of gel aging in zeolite crystallization

Chapter 9

Role of Gel Aging in Zeolite Crystallization

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 well known that the low-temperature ageing of a l u m i n o s i l i c a t e g e l precursor markedly in f l u e n c e s the course of z e o l i t e c r y s t a l l i z a t i o n at the appropriate temperature (1-10). The primary e f f e c t s of the g e l ageing are the shortening of the induction period and the a c c e l e r a t i o n of the c r y s t a l l i z a t i o n process (1-5), but i n some cases the g e l ageing also influences the type(s) of z e o l i t e ( s ) formed (1,6,7,10).

Thus, i n many syntheses the ge l ageing (8-11) or the a d d i t i o n of the " c r y s t a l d i r e c t i o n agent" (aged, X-ray amorphous aluminos i l i c a t e gel) (7,12-14) i s a necessary step needed f o r the obtaining of the desired type of z e o l i t e at the desired r e a c t i o n r a t e .

I t i s well known that z e o l i t e s of type NaP c o - c r y s t a l l i z e with 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 the appropriate synthesis c o n d i t i o n i s (17): amorphous—* f a u j a s i t e — » gismondine type Na-P. However, i n some cases, z e o l i t e Na-P appears as the 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 prepared g e l has been heated at the appropriate temperature (15,18); i n these cases, 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 adding the seed c r y s t a l s i n t o the f r e s h l y prepared g e l (6,13,18) or by ageing the ge l at ambient temperature p r i o r to the c r y s t a l l i z a t i o n at the

0097-6156/89/0398-0124$06.00/0 o 1989 American Chemical Society

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

9. KATOVICETAL. Role of Gel Aging in Zeolite Crystallization 125

appropriate temperature (6,8,19). While the influence of the gel ageing on the kinetics of crystallization can generally be explained by the increase in the number of nuclei formed in the gel phase or/and in the liquid phase during the ageing (2-6), the explanation of the effect of the gel ageing on the type of zeolite formed requires the study of chemical and structural changes in both the liquid phase and the solid phase of the reaction mixture during the ageing and the crystallization, respectively. Thus, the objective of this work i s to investigate the chemical and structural changes in the solid and the liquid phase of the reaction mixture during i t s ageing as well as during the crystallization process, in order to explain why zeolite X crystallizes only from the aged gel of given composition. The influence of the gel ageing on the crystallization rates of zeolites X and Ρ wil 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 Na2Û), thermostated at 25°C, into a plastic beaker containing 150 ml of vigorously stirred sodium aluminate solution (0.482 molar A I 2 O 3 , 1.39 molar Na2Û) thermostated at 25 ° C . The aluminosilicate gels were aged at 25°C for given times ( t a = 0 to 10 dyas). After ageing for a predetermined time t a , the gel was poured into a stainless-steel reaction vessel and heated to crystallization temperature ( 8 0 ° C ) .

The moment the gel was added into the preheated reaction vessel was taken as the zero time ( t c = 0) of the crystallization process. The reaction mixture was stirred with a teflon-coated magnetic bar driven by a magnetic stirr e r . At ageing times ta = 0 to 10 days before the crystallization process ( t c = 0 at 2 5 ° C ) , as well as the times t c after the beginning of the crystallization process (at 8 0 ° C ) , aliquots of the reaction mixture were drawn off and centrifuged in order to separate the solid from the liquid phase and to stop the crystallization process, respectively.

Aliquots of the clear liquid phase above the sediment were used to measure Si and Al concentrations by atomic absorption spectrometry (Perkin-Elmer atomic absorption spectrometer, model 3030B) and for the determination of the degree of Si poly-condensation in the liquid phase by molybdate method (20).

The solid phase, after having been washed and dried in the dessicator at room temperature up to the constant weight, was used for analyses.

The fractions f Q of gel, fp c of zeolite Na-Pc and fx of zeolite X in the powdered solid samples were taken by Philips diffractometer with C U K - P C radiation in the region of Bragg*s angles 2Θ· = 10° - 46°. The weight fractions were calculated by the mixing method (21) using the measured integral intensity of the amorphous maximum (2Φ = 17° - 39°) and the sharp maximum corresponding to the diffraction from (533) crystal lattice planes of zeolite X, as well as the sharp maximum corresponding to the diffraction from (310) crystal lattice planes of zeolite Na-Pc.

The average values of crysta l l i t e size were determined by the integral width of the diffraction maximums corresponding to the

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126 ZEOLITE SYNTHESIS

diffraction from (642) and (533) crystal lattice planes of zeolite X and by the integral width of the diffraction maximums corresponding to the diffraction from (310) and (211) crystal lattice planes of zeolite Na-Pc, using the Scherrer formula (22).

Si and Al contents in the solid samples were determined by measuring of Si and Al concentrations in the solutions obtained by the dissolution of the solids (75 mg) in 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 solid 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 crystallization of zeolite X (Figure 1A) and zeolite Na-Pc (Figure 1B), respectively, at 80°C from the gels aged at 25°C for 1, 3, 5, 7 and 10 days. In a l l cases, zeolite X appears as the f i r s t crystalline phase, thereafter zeolite Na-Pc co-crystallizes with zeolite X. After the maximal yield of zeolite X crystallized has been attained, the fraction f^ of zeolite X slowly decreases as the consequence of the spontaneous transformation of zeolite X into more stable zeolite Na-Pc (17). The induction periods of both zeolite X and zeolite Na-Pc shortens and the maximal yield of zeolite X increases, respectively, with the increased time of gel ageing. A l l kinetics of zeolite X and zeolite Na-Pc, respectively, can be mathematically expressed by the simple kinetic equation (5,23-26),

f = Κ · t q (1) ζ c

during the interval of the increasing crystallization rate. Here, f z i s the mass fraction of zeolite crystallized at the crystallization time t c , and Κ and q are constants for given experimental conditions. The numerical values of the constants Κ and q, calculated by the log f z versus log t c plots (25) using the corresponding experimental data from Figure 1, are listed in Table I.

Table I. Numerical values of the constants Κ and q which correspond to the kinetics of crystallization of zeolite X and zeolite Na-Pc, respectively, from aluminosilicate gels aged for various times t a

Time of gel Zeolite X Zeolite Na-Pc ageing (t Q/d) Κ q Κ q 1 - - 7.84 E-9 9.86 3 - - 2.78 E-7 7.83 5 6.34 E-4 4.03 2.21 E-6 7.27 7 1.28 E-3 4.36 - -10 1.35 E-3 4.69 - -

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KATOVIC ET AL. Role of Gel Aging in Zeolite Crystallization 127

Figure 1. Kinetics of crystallization of zeolite X (Figure A) and zeolite Na-Pc (Figure B) at 80°C, from the aluminosilicate gels aged for t Q = 1 d (• ), t Q = 3 d (a), t Q = 5 d ( Δ ), t Q = 7 d (·) and t Q = 10 d (o) at 25°C. f x and fp c are mass fractions of zeolite X and zeolite Na-Pc crystallized at crystallization time t c . Solid curves represent the kinetics of crystallization, calculated by Equation (1) and the corresponding values of the constants Κ and q from Table I. D

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There are three typical groups of the crystallization kinetics: I. zeolite Na-Pc i s the dominant product of the crystallization from the gels aged for 0 to 3 days, II. the mixture of approximately same amounts of zeolite X and zeolite Na-Pc crystallizes from the gel aged for 5 days (see scanning-electron micrograph in Figure 5) and III. zeolite X i s the dominant product of the crystallization from the gels aged for 7 and 10 days. The scanning-electron micrograph in the Figure 5 shows that the microcrystals of zeolite X can be clearly distinguished from the typical spherulites of zeolite Ρ in the f i n a l product of the crystallization.

Figures 2 - 4 show the changes in chemical characteristics of the liquid and the solid phase, respectively, during the crystallization from the gels aged for t a = 1 d (group I., see Figure 2), for t Q : 5 d (group II., see Figure 3) and for t a = 10 d (group III., see Figure 4). In a l l analyzed cases, the concentration of both silicon and aluminum in the liquid phase increase l i t t l e from t c = 0 up to t c ^ 1 h as the consequence of the increase in the solubility of the gel with increasing temperature (heating from 25°C up to 80°C). After the crystallization temperature has been reached, the concentration C ^ ( L ) of the aluminum in the liquid phase keeps constant during the main part of the crystallization process while the concentration Cgj(L) of the silicon in the liquid phase keeps constant during the induction period of the crystallization only. The gel-crystal transformation is followed by the increase in the concentration C 5 J (L ) up to the maximal value C 5 j ( L ) M G X and by the simultaneous decrease in the Si/Al molar ratio in the solid phase, respectively. Hence, i t i s reasonable to assume that the increase in the concentration of s i l i c o n in the liquid phase, during the crystallization, i s the consequence of the releasing of soluble s i l i c a t e species from the solid phase due to the lower Si/Al ratio in the crystalline phase(s) (CSi/AlJ s 1.305) than in the starting aluminosilicate gels ([Si/Al] s ~ 1.4), as shown in Figures 2C-4C. At the crystallization time when about 70 % of the gel has been transformed into the crystalline phase(s), the rate of crystallization starts to deccelerate simultaneously with the sudden decrease in the concentrations of both silicon and aluminum in the liquid phase. This is probably the consequence of the decrease in the particle growth rate(s) (see Figure 6) caused by the decrease in the solute concentration at the time when the rate of deposition of the soluble species from the liquid phase onto the surfaces of the growing particles becomes higher than the rate of feeding of the solution with new soluble species (5,26). The chemical changes in the liquid phase and the solid phase, respectively, clearly indicate that the crystallization of zeolite X and zeolite Na-Pc, respectively, from gels is a solution-mediated transformation process in 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 crystalline phase(s) (2,16,19,23-26).

Table II. shows that the concentrations Cgj and of silic o n and aluminum in the liquid phase of the gel as well as the molar ratio [ S i / A l ] s of sili c o n and aluminum in the solid phase of the gel keeps approximately constant during the ageing at 25°C, indicating that the chemical equilibrium between the solid and the liquid phase of the gel has been attained for to ^ 1 d. After the

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0 2 4 6 t (h) c

Figure 2. The changes in: A. mass fractions f^ of zeolite X (o), fp c of zeolite Na-Pc (·) and f^ of gel (Δ), B. concentrations C^^(L) of aluminum (·) and CgjiL) of silicon (o) in the liquid phase and C. molar ratio [ S i / A l ] s in the solid phase (o), with the crystallization time t c , during the crystallization from the gel aged for t a = 1 d.

0 2 4 t (h) c

Figure 3. The changes in: A. mass fractions f x of zeolite X (o), fp c of zeolite Na-Pc (·) and fç of gel (Δ), B. concentrations C A J ( L ) of aluminum (·) and CgjCD of silicon (o) in the liquid phase and C. molar ratio [Si/AlJ s in the solid phase (o), with the crystallization time t c , during the crystallization from the gel aged for t a = 5 d.

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t (h)

Figure 4. The changes in: A. mass fractions f x of zeolite X (o), fp c of zeolite Na-Pc (·) and fç of gel ( Δ ) , Β. concentrations C A [ ( L ) of aluminum (·) and C $ j ( L ) of silicon (o) in the liquid phase and C. molar ratio [ S i / A l ] s in the solid phase (o), with the crystallization time t c , during the crystallization from the gel aged for t Q = 10 d.

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Figure 5. Scanning-electron micrograph of the solid sample drawn off the reaction mixture at the end of the crystallization process from the gel aged for t Q = 5 d; t c = 6.5 h, f^ = 0.43, f P c = 0.57.

4 5 6 t (h) c

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

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crystallization temperature has been reached, the concentrations of both silicon and aluminum in the_liquid phase_slowly increase, thereafter their average values Cgj(L) and C^(L) keep approximately constant during the induction period of the crystallization or even, during the main part of the crystallization process (for aluminum) (see Figures 2-4), and are not markedly influenced by the gel ageing, as shown in Table III. Also, the maximum value C s j ( L ) m a x

of the silicon concentration in the liquid phase, attained during the crystallization, is not markedly influences by the ageing of the gel, as shown in Figures 2B-4B and in Table III.

Table II. Characteristics of the aluminosilicate gels of 4.24 Na 20-Al 20 3-3.56 Si0 2- 230.6 H20 batch composition aged for various times t a at 25°C

t Q/d C5j/mol dm"3 C A l/mol dm"3 [Si/Al] s R/min"1

1 0 . 2 0 3 0.0150 1.42 1.22

2 0 . 2 0 3 0.0141 1.39 1.11

3 0 . 2 1 3 0 . 0 1 3 7 1.40 1 .23

5 0.209 0.0145 1.45 1.27

7 0.199 0.0142 1.43 1.27

10 0 . 1 8 6 0.0144 1.41

Table III. Chemical composition of the liquid phase during the crystallization from the gels aged for various times t Q

t a/d C^(L)/mol dm" 3 C^j(L)/mol dm"3 c s i ( L W r a o 1 d m ~ 3

1 0 . 0 2 1 0 0.224 0.251 2 0 . 0 1 9 9 0.221 0.262 3 0 . 0 1 6 2 0 . 2 3 7 0 . 2 6 7

5 0.0190 0 . 2 2 3 0.255 7 0 . 0 1 7 0 0.216 0.262

10 0 . 0 1 6 0 0 . 2 0 3 0 . 2 3 9

The measuring of the degree of polycondensation of s i l i c a t e anions in the liquid phases of the gels aged fro 1, 2, 3, 5, 7 and 10 days, as well as in the liquid phases of the crystallizing systems, has shown that in a l l the cases the logarithm In UR, of the percentage of Si0 2 unreacted with molybdic acid is 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 III.). It has been appreciated from the results obtained, that the liquid phase of the reaction mixture contains a mixture of monomeric sil 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, in an acidic degradation (20,27,28).

Figure 6 shows that the average growth rate of zeolite X (£gjX) = dL x/dt c »3x10" 5 cm/h) and of zeolite Na-Pc (K g(Pc) = dLp c/dt c « 1.6x10*6 cm/h), respectively, is constant during the

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9. KATOVIC ET AK Role of Gel Aging in Zeolite Crystallization 133

crystallization process and independent on the gel ageing. The independence of the growth rates Kg(X) and Kg(Pc) on the increase in the silicon concentration during the crystallization indicates t'.iat the excess of silicon, produced in the liquid phase during the dissolution of the gel and i t s partial transformation into crystalline products with lower Si/Al molar ratio, exists in the form of "inactive" monosilicic and d i s i l i c i c acids (8) and hence, does not participate in the reactions at the surfaces of the growing crystals (Katovic, Α.; Subotic, B.; Smit, I.; Despotovic, L j . A. Zeolites, in 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 liquid phase at the surfaces of growing zeolite particles and that the aluminum concentration (in the form of "reactive" aluminosilicate species) is the determining factor of the growth rate of the particles of zeolite X and zeolite Na-Pc, respectively.

Now, the constancy of the crystal growth rate of both zeolite X and zeolite Na-Pc indicates that the shortening of the induction periods of the crystallization of both zeolites is the consequence of the increase in the number of nuclei during the gel ageing. The simultaneous crystallization of zeolite X and zeolite Na-Pc under almost identical chemical conditions indicates that the nuclei of zeolite X and the nuclei of zeolite Na-Pc exist as separate entities. The analysis of the crystallization kinetics of zeolite X and zeolite Na-Pc, respectively, shows that the numerical value of the exponent q in Equation (1) i s greater than 4 in a l l kinetics (see Table I.), which means that the nucleation rate increases during the crystallization process. The changes in the crystallization kinetics with the gel ageing, at constant chemical composition of the reaction mixture, indicate that the structural changes in the solid 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 solid phase of freshly prepared gel (0-0) exhibits the amorphous maximum at the 2& angle corresponding to the diffraction from (310) crystal lattice planes of zeolite Na-Pc (strongest X-ray diffraction maximum of zeolite Na-Pc; see Figure 1 in: Katovic, Α.; Subotic, B.; Smit, I.; Despotovic, L j . A. Zeolites, in press). Figure 8 shows that the IR spectra of the solid phase of freshly prepared gel (spectra a), as well as the IR spectra of the solid 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 zeolite 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 Q)p c of very small particles of quasicrystalline phase having a structure close to the structure of the cubic modification of zeolite P. Such particles of the qusicrystalline phase, probybly formed by the polycondensation processes inside the gel matrix during i t s precipitation, can be potential nuclei (nuclei-II) for the crystallization of zeolite Na-Pc. At the same time (during the gel precipitation) a number (N Q)p c = N(ht) of nuclei (nuclei-I) is assumed to be formed in the liquid phase by the heterogeneous nucleation, catalyzed by the presence of the active centers at the

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w c Q) C

306 nm

- 2* ( u)

Figure 7. X-ray diffractograms ( r e l a t i v e i n t e n s i t i e s I r versus Bragg*s angles 2& and versus X-ray d i f f r a c t i o n spacings d) of the s o l i d phase of the gel s aged at 25°C f o r t a = 0 (0-0), t a = 2 d (2-0), and t Q = 10 d (10-0) and of the same gels heated at 80°C f o r t c = 4 h (0-4), t c = 3 h (2-3) and t c = 1 h (10-1).

I I 1 1 I 1 I I 1 I I I I I I I I I I

1800 1000 600 wave number (cm )

Figure 8. IR spectra of the s o l i d phase of the gel s aged at 25°C f o r t a = 0 (a), t Q = 5 d (b), t a = 10 d (c) as well as the IR spectra of z e o l i t e X (d) and z e o l i t e Na-Pc (e).

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9. KATOVIC ET AL. Role of Gel Aging in Zeolite Crystallization 135

impurities always present in the liquid phase (30). Hence, after the gel has been precipitated, the system contains a number (N Q)p c = N(ht) of nuclei-I distributed through the liquid phase and a number (N Q)p c of nuclei-II distributed inside the solid phase of the gel. Since the positions of the amorphous maximums and their intensities 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 particles of quasi-crystalline 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 in comparison with the rate of material transport in 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 crystalline particles inside the gel matrix is blocked considerably and that they can grow only in f u l l contact with the solution phase. On the other hand, the shift in the position of the amorphous maximum in the X-ray diffractograms of the solid phase of variously aged gels toward lower X-ray diffraction spacings (see Figure 7), indicates that structural changes take place in the solid phase of the gel during i t s ageing. At this 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 crystallization of zeolite 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 particles of quasi-crystalline phase (nuclei-II) with the structure near to the structure of faujasite, inside the gel matrix.

Now, assuming that the freshly prepared gel contains only the nuclei of zeolite Ρ (no zeolite X crystallization) (Katovic, Α.; Subotic, B.; Smit, I.; Despotovic, L j . A. Zeolites, in press), i t s ageing at ambient temperature causes two effects: (i) the formation of six-membered aluminosilicate rings and their ordering into quasi-crystalline faujasite (nuclei-II of zeolite X) and ( i i ) the recrystallization of the gel (the dissolution of small and the growth of large particles of the gel) (2), releases the number N(a)pc of particles of quasicrystalline zeolite Na-Pc and the number N(a)^ of quasicrystalline particles of faujasite from the dissolved gel particles. Since the crystal growth rate at ambient temperature is assumed to be negligible in comparison with the growth rate at crystallization temperature, the particles of the quasicrystalline phase released from the gel during i t s ageing (and being in the f u l l contact with the liquid phase), become new nuclei-I, i.e., (N 0)p c = N(ht) + N(a)pc and (Ν0)χ = N(a)^at crystallization time t c = 0 for any ageing time t a , where N(ht) i s the number of nuclei-I of zeolite Na-Pc formed in the liquid phase by heterogeneous nucleation during the gel precipitation ((N 0)p c = N(ht) for t a = 0 and t c = 0). Similar process of the increase in the number of nuclei-I during the gel ageing was observed in zeolite A crystallizing system (5).

The heating of the reaction mixture induces the growth of nuclei-I of both zeolites from the solution supersaturated with soluble aluminosilicate species. Since the growth rate of zeolite X is considerably greater than the growth rate of zeolite Na-Pc (see Figure 6), zeolite X appears as the f i r s t crystalline phase. The

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

f z = G W 3 t 3 / ( 1 " GÇnfl aK 3t3) = KGt£/(1 - K at 3) (2)

earlier derived (25) on the basis of Zhdanov's idea on autocatalytic nucleation (2,24). Here, G i s the geometrical shape factor of zeolite particles, § i s the specific density of zeolite formed, N 0 is the number of particles-I (formed by the growth of nuclei-I), N A = is the number of particles-II (formed by the growth of the particles of quasicrystalline phase released from the gel during the crystallization process), both contained in a unit mass of zeolite formed at the end of the crystallization process, Kg is the constant of the linear growth rate of zeolite particles and fl = 6/(q+1)(q+2) (q+3). The numerical values of the constants K 0 = G § N 0 K 2 and K a = GÇflN aK 3, calculated by the procedure described earlier (25), as well as the ratios Ν α/ N 0 = K a/β KQ, are listed in Table IV. as functions of time t Q of the gel ageing.

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

t a/d zeolite X zeolite Na-•Pc

t a/d KQ/h-3 Ka/h-3 N Q / N 0 KQ/h-3 KQ/h"3 V ÏÏo

1 - - - 5.56 E-4 3.09 E-3 1533 3 - - - 8.50 E-4 3.67 E-3 775 5 2.35 E-3 2.13 E-3 32 1.11 E-3 3.92 E-3 464 7 5.14 E-3 6.15 E-3 50 - -10 6.52 E-3 9.58 E-3 72 - -

Figures 2A-4A show that the fractions f^ and fp c, calculated by Equation (2) and the corresponding values of the constants K a and K 0 from Table IV. (solid curves in Figures 2A-4A) agree very well with the measured fraction during the autocatalytic stages of the crystallization processes, thus indicating that the crystallization of zeolite X and zeolite Na-Pc, respectively, from variously aged gels takes place by the mechanism described above.

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9. KATOVIC ET AL. Role ofGel Aging in Zeolite Crystallization 137

Conclusions

The analysis of the experimental data and the numerical values of the constants K 0 and K Q and the ratios N0/NQ for the crystallization of both zeolite X and zeolite Na-Pc, leads to the following conclusions:

The total number of nuclei of zeolite Na-Pc (number of nuclei-I + number of nuclei-II = N Q + N Q = K Q / G § K 2 + K Q / § G β K ^ ; see Equation (2)), does not change, or slightly decreases during the ageing of the gel. A part of quasicrystalline particles of zeolite Na-Pc (nuclei-II) releases from the gel and becomes nuclei-I during the gel ageing so that the ratio N0/Na for zeolite Na-Pc_decreases with the gel ageing (see Table IV.). The high ratios N 0/ Na for zeolite Na-Pc indicate that only very small proportion of the particles of quasicrystalline zeolite Ρ has been released from the gel during i t s ageing, and this i s a possible reason for the long induction period of the crystallization of zeolite Na-Pc.

The number of particles of quasicrystalline faujasite (nuclei-II of zeolite X) increases inside the gel matrix during the gel ageing. A part of the particles of quasicrystalline faujasite releases from the gel during i t s ageing and becomes nuclei-I for the crystallization of zeolite X. The increase of the ratio N0/Na during the gel ageing indicates that the rate of the formation of the particles of quasicrystalline faujasite inside the gel matrix i s greater than the rate of their outcoming from the gel during i t s ageing (re-crystallization) .

The absence of the IR band at 560 cm~1 (characteristic for D6R secondary building units of zeolite X) (29) and the presence of the broad band with the maximum at 600 cnH (characteristic for D4R secondary building units of zeolite P) (29) even in the IR spectra of the solid phase of the gel aged for 10 d (see Figure 8c), as well as the much lower values N0/Na for zeolite X than the values N0/Na for zeolite Na-Pc (see Table IV.), indicate that the total number of nuclei (nuclei-I + nuclei-II) of zeolite Na-Pc i s much greater than the total number of nuclei of zeolite X, for a l l the gels examined.

The shortening of the induction periods of the crystallization of both zeolite X and zeolite Na-Pc is most probably the consequence of the increase in the number of nuclei-I of both zeolites with the ageing of the gel.

The increase in the yield of zeolite X crystallized, with the gel ageing, is the consequence of the increase in the total number of nuclei of zeolite X at constant, or even decreasing total number of zeolite P, during the gel ageing.

The high yields of zeolite X crystallized from the systems aged for 5 days and more, in which the total number of nuclei of zeolite Ρ is considerably greater than the total number of nuclei of zeolite X, are most probably influenced by the much greater growth rate of zeolite X particles relative to the growth rate of zeolite Na-Pc particles and by relatively low N0/NQ ratio for zeolite X compared with the NQ/NQ ratio for zeolite Ρ (see Table IV.). For illustration, i t i s easy to calculate by Equation (2) and the data in Table IV. that_in the case when the total number of nuclei of zeolite Na-Pc ( N 0 / N Q = 464) would be 2000 times greater than the total number of nuclei of zeolite Χ (N0/Na = 32), and when the growth rate of zeolite

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X particles would be 10 times higher than the growth rate of zeolite Na-Pc particles, 50 % of zeolite X and 50 % of zeolite Na-Pc would be formed at the end of the crystallization process.

Literature Cited

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RECEIVED December 22, 1988

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[acs symposium series] zeolite synthesis volume 398 || role of gel aging in zeolite crystallization

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