Mechanism of Zeolite A Synthesis - ACS Symposium Series (ACS

17

M e c h a n i s m of Z e o l i t e A

Synthesis

C. L. ANGELL and W. H. FLANK

Downloaded by MONASH UNIV on December 7, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch017

Union Carbide Corp., Molecular Sieve Department, Tarrytown Technical Center, Tarrytown, N.Y. 10591

ABSTRACT Experiments u t i l i z i n g several different characterization techniques as a function of time, including chemical analyses, Raman spectra, x-ray diffraction, sorption and particle size measurements, have been performed to determine the mechanistic pathway in 4A synthesis. The evidence supports a mechanism i n volving formation and subsequent dissolution of an amorphous aluminosilicate intermediate, with solution transport from the gel to the growth surface of the c r y s t a l l i t e .

Introduction Two different zeolite synthesis mechanisms have been discussed in the recent literature. McNicol £ t à l « ( i > D argue in favor of a solid phase transformation mechanism and Flanigen(3) discusses surface diffusion in the absence of substantial liquid transport, while Sand fit a l . ( 4 , 5 J , Kacirek and Lechert(6), and Zhdanov(7J present evidence in support of a solution transport mechanism. To aid in resolving this c o n f l i c t , a variety of techniques were applied to the study of the 4A synthesis system as a function of time. Experimental Portions of a 4A gel synthesis formulation were placed in sample tubes which were withdrawn from constant temperature baths after aging at 25°C or crystallization at 96°C for various times, and centrifuged while hot in an insulated holder. The reaction rate in this series of unagitated experiments was relatively slow. The gel was formed by mixing solutions of sodium s i l i c a t e and sodium aluminate, and utilized ratios of Na20/Al2Û3, Si02/Al2Û3 and H2O/AI2O3 of 1.96, 1.98 and 83.0, respectively. The liquid and solid phases were promptly scanned in a J a r r e l l 194 In Molecular Sieves—II; Katzer, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


17.

ANGELL

A N D FLANK

Mechanism

of Zeolite A

Synthesis

195

Ash Model 25-300 laser Raman spectrometer. Several of the samples were re-scanned after standing for some time and showed no marked changes. Band assignments were made on the basis of data reported in the literature for the aluminate ion(8,9), silica species (IQ,U) and zeolite A(ll). A similar 4A gel synthesis formulation master batch was prepared and samples were withdrawn as a function of time during the course of the reaction. The reaction rate in this series of agitated experiments was moderately fast. Solid and liquid phases were separated by hot centrifugation in an insulated holder, and the solids were washed with weighed portions of water. The various sample fractions were weighed and subjected to chemical analysis. Material balance closures, with one exception, were better than 96% and averaged 97.0%. Another 4A gel synthesis formulation master batch was prepared, with samples also being withdrawn as a function of time during the course of the reaction. The gel was formed by mixing solutions of sodium silicate and sodium aluminate, and utilized ratios of Na20/Al2Û3, Si02/Al2Û3 and Η 2 Ο / Α Ι 2 Ο 3 of 1.97, 1.88 and 63.2, respectively. In this case, the solid product samples were evaluated for degree of crystal unity by x-ray powder diffraction via comparison with a 4A standard, O2 adsorption capacity at 100 torr and 90 K, water content removable by 350°C vacuum activation after humidity equilibration at 50% relative humidity, and mean equivalent spherical diameter as determined by a Sedigraph 5000 Particle Size Analyzer. The reaction rate in this series of experiments was fairly rapid, reflecting the use of a lesser amount of water in the formulation. Results The data obtained from the Raman spectra are summarized in Table I and Figure 1. These results, particularly during the crystallization step, can be contrasted with those reported in the literature by McNicol and co-workers( 1_ Z), who observed no changes with time other than the appearance of zeolite A. The terms silica and silicon-containing species are used in a general sense. While the observed band positions correspond to reported values for oxygenated silicon species, it is recognized that the degree of ordering in these materials is variable. The spectra show that conversion to framework silica occurs from precursor material. This may consist of localized concentrations of silica gel formed in the initial mixing of reactants, or by precipitation from a supersaturated solution, or an aluminosilicate gel with a variable degree of cross-linking. Analysis of liquid and solid phases shows, as illustrated in Figure 2, that the ratio of alumina in the liquid phase to that in the solid phase is relatively high during aging, and decreases during crystallization at elevated temperature. The silica to alumina ratio in the liquid phase is seen to increase sharply 9

In Molecular Sieves—II; Katzer, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

MOLECULAR

TABLE I

LASER R A M A N SPECTROSCOPY DATA AS A FUNCTION OF TIME DURING ZEOLITE A SYNTHESIS AGING

CRYST.


T I M E , H R S . T I M E , HRS.

BAND POSITION, C M "

B A N D INTENSITY 1

AND SHAPE

SPECIES

LIQUID PHASE

0

0

1

0

2

0

STRONG, SHARP

ALUMINATE

400-500.700400

VERY WEAK

SILICA (?)

UNCHANGED

UNCHANGED

UNCHANGED

MEDIUM. SHARP

ALUMINATE

WEAK

SILICA (?)

WEAK

ALUMINATE

400-500.700-800

WEAK

SILICA (?)

620

3

0

4

0

-

0.5

1

620

0.5

2

450,880 620

"

0.5

3

UNCHANGED

UNCHANGED

UNCHANGED

0.5

4

620

VERY WEAK

ALUMINATE

WEAK, BROAD

SILICA (?)

405

WEAK

SILICA

450

STRONG, BROAD

SILICA

WEAK. BROAD UNCHANGED

SILICA UNCHANGED

400-500,700400

SOLID P H A S E

0

0

1

0

2

0

3

0

4

0

0.5

1

800 UNCHANGED

" " 450

STRONG, BROAD

SILICA

800

WEAK, BROAD

SILICA

0.5

2

UNCHANGED

0.5

3

490

0.5

4

450,800 490 340. 700,1040

UNCHANGED

UNCHANGED

NEW SHOULDER

ZEOLITE A

WEAK, BROAD

SILICA

STRONG, SHARP

ZEOLITE A

WEAK

ZEOLITE A


SIEVES-

ANGELL


17.

ι

ι

I ι 1000

Figure 1.

ι ι cm

1

1

ι

I 500

ι

ι

of Zeolite

I 200

ι

ι

A

I ι 1000

197

Synthesis

ι ι cm 1

ι

I 500

ι

ι—I 200

Raman spectra of solid and liquid phases in zeolite A synthesis

A G I N G HEAT-UP

0

Mechanism

AND FLANK

CRYSTALLIZATION

2 3 TIME, Hours

4

I

5

Figure 2. Composition of Gel 4A synthesis system as a function of time



198

MOLECULAR

SIEVES—Π

after aging, with the increase continuing during the crystalliza tion period. This is indicative of the increase in liquid phase silica concentration at higher temperature as well as the deple tion of alumina in the liquid phase as crystallization proceeds. The silica to alumina ratio in the solid phase is seen to drop sharply during heat-up, and more gradually thereafter, approaching the ideal value of two as the reaction proceeds toward completion. Significant compositional changes as a function of time are thus clearly established. The fractions of the synthesis composition found in the sep arated solid phase, as a function of time during the synthesis reaction, are shown in Figure 3. The plotted data are normalized and are derived from material balance data which had closures averaging 97.0%. The changes with time that are seen in the plot indicate significant mass transport between the solid and liquid phases during the reaction. A ternary mole fraction composition diagram for the solid phase, shown in Figure 4, can be used to illustrate the distinc tive compositional changes occurring along the reaction trajec tory, as the reaction proceeds through the several synthesis stages. Total initial solids are shown at Point (1). The initially formed amorphous sodium aluminosilicate, shown at Point (2), briefly becomes depleted in soda and enriched in silica content, then steadily is depleted in silica and enriched in soda and alumina content as the zeolite forms. The mean equivalent spherical diameter is plotted in Figure 5 as a function of time through the heat-up and crystallization periods(12). All of the samples were ultrasonically dispersed. The amount of agitation required to achieve a stable size distri bution decreased significantly as crystallization approached com pletion. During the aging period, however, the mean particle size is both small and unstable with respect to degree of agita tion. Beyond this point, the amorphous particles are more firmly cross-linked and are relatively large, but the minimum mean dia meter in the series is found after 30 minutes at 96°C. This corresponds to about 35% conversion to zeolite, as indicated in Figures 6 and 7, and shows that some dissolution of the amorphous gel particles takes place prior to crystal growth. The increase with time of x-ray diffraction intensity and oxygen adsorption capacity for the product, indicative of the degree of crystal1inity, parallel each other very closely, as shown in Figures 6 and 7, and quickly reach maximum values(12). Figure 7 also shows that the amorphous solid is indeed hydrous and is undergoing change prior to the appearance of detectable crystal unity. Discussion Several papers in recent years have attempted to distinguish between direct solid phase transformation of gel to crystalline material(1*2), or re-ordering of gel to an ordered crystalline


ANGELL

AND FLANK

Mechanism

of Zeolite

A

Synthesis


17.

Figure 4.

Synthesis reaction trajectory


199


200 MOLECULAR


SIEVES—II


ANGELL

Mechanism

AND FLANK

0

1

2

of Zeolite A

3

4

5

Synthesis

6

TIME, Hours

Figure 7. Wt % equilibrium H 0 content and 0 sorption capacity as a function of time during synthesis 2

2



202

MOLECULAR

SIEVES—II

state via surface diffusion(3), and crystallization via liquid phase mass transport^, 5,6^7j as the operative mechanism in zeolite synthesis. We Fave employed a number of different techniques to characterize the liquid and solid phases of the reacting zeolite A synthesis system as a function of time along the reaction coordinate. An attempt has been made to adequately sample the system at a number of points, covering the several stages of the synthesis process, to help identify the mechanistic route involved. The Raman data establish that, during the aging step conducted at ambient temperature, the liquid phase is rich in aluminate ion and the solid phase is rich in silica gel. The observed band intensities remain relatively unchanged during this period, as noted in Table I. The analytical data shown in Figure 2 are in good general agreement with this, but it can be noted that a more detailed examination of the solid phase composition reveals that small changes are indeed occurring during the aging period, as evidenced by the data in Figure 4. Also indicative of a change in the system during aging are the data in Figure 3, which suggest that an ambient temperature quasi-equilibrium solubility limit for the amorphous sodium aluminosilicate gel is being approached. Such a compositional plateau is common to a number of aluminosilicate systems(7,13). During the period in which the reaction mixture is being heated from ambient temperature to 96°C, a number of very significant changes are observed to occur. It can be noted, however, that the x-ray diffraction intensity and oxygen sorption capacity characteristic of zeolite A are absent during this period, as shown in Figures 6 and 7. The hydrous nature of the solids present during heat-up is apparent in Figure 7. Alumina is transferred from the liquid phase to the solid phase and silica goes into the liquid phase, as can be seen in Figures 2 and 4. This findinq is analogous to that reported earlier by Schwochow and Heinze(14). There is a net increase in the solid to liquid ratio in the system (see Figure 3), and at the same time the average size of the gel particles is seen in Figure 5 to decrease. This indicates that material is being transferred between the liquid and solid as new solubility product relationships are established as a function of temperature. There is the further indication, supported by the data of Zhdanov(^) and of Aiello and coworkers (15_, 16), that the initially formed sodium aluminosilicate gel is converted via solution transport to an apparently amorphous aluminosilicate intermediate. It is this latter material which is converted to crystalline zeolite via dissolution by the basic medium. The conditions to which a gel or aluminosilicate intermediate are exposed must be controlled to provide the necessary liquid phase composition for formation of the desired crystalline phase(17). Zhdanov has recently demonstrated this point very well(18)". He showed the influence of gel aging on crystallization



17.

ANGELL

A N D FLANK

Mechanism

of Zeolite A

Synthesis

203

time for the reaction and on crystallite size, as well as evidence for solution transport and participation of the liquid phase in directing the course of the reaction, supporting the conclusion that liquid and solid phases are in dynamic equilibrium. The existence of a complex set of liquid-solid dynamic equilibria during both the aging and crystallization periods, and the influence of these on the rate and direction of the reaction are thus apparent. Concentration of silica and alumina species in the liquid phase is constrained by a solubility product limit for sodium aluminosilicate solids, which changes as a function of temperature. It appears likely that primary nucleation is preceded by the formation of a hydrous aluminosilicate intermediate which contributes to both crystal growth and secondary nucleation ( i . e . , that which occurs during crystal growth), while the initially formed gel continues to furnish "dissolved" species for crystal growth and further formation of aluminosilicate intermediate. In the elevated-temperature crystallization stage of the synthesis process, the Raman and chemical analysis data confirm that aluminate disappears from the liquid phase, while the concentration of silica species increases modestly. The fraction of the synthesis composition found in the solid phase, which dropped slightly during aging and then increased sharply upon heating the system to 96°C,*continues to increase slowly as a function of crystallization time. The silica to alumina ratio in the liquid phase, which rose sharply during heat-up, continues to increase as crystallization proceeds. The silica to alumina ratio in the solid phase, which was at a high level during aging and dropped sharply upon heating the system to crystallization temperature, continues to drop slowly during crystallization toward the ideal value of two for zeolite A. The solid phase soda to alumina ratio, derivable from Figure 4, is constant during crystallization, showing that, as aluminum is incorporated into the solid, a corresponding amount of sodium is associated with i t , apparently as an ordered moiety (possibly a hydrated ion pair), transported from the intermediate to the crystal growth surface. The mean particle size of the solids in the system drops as the system goes through heat-up and the initial portion of the crystallization period, at the same time that the solids to liquid ratio is increasing. The subsequent increase in mean particle size is not accompanied by a concomitant change in the solids to liquid ratio. It can be concluded from these observations that a hydrous sodium aluminosilicate intermediate, differing in composition from the initially formed gel, is formed during the heat-up period and that mass transport from this intermediate occurs through the liquid phase to the growth surface. The Raman data show that the concentration of non-framework silicon-containing species in the solid decreases with time and zeolite A, after an induction period, appears and increases with



204

MOLECULAR

SIEVES—II

time. There is a definite time lag between the beginning of aluminum disappearance from the liquid and the appearance of zeolite A in the solid phase. This is also borne out by the data in Figures 3 and 4. Wet chemical analysis indicates that s i l i c a , the dominant phase in the solid, is relatively unchanged in concentration during both aging and crystallization. However, unlike Raman spectroscopy, it cannot distinguish between amorphous silica gel and silica present in the zeolite framework. It is apparent, nonetheless, that a small steady-state concentration of a liquid phase silicon-containing species is present, and that it increases with temperature. Since the temperature coefficients for the solubility of the various species in the system are not equal, dynamic equilibria must be re-established during heat-up and the initial portion of the crystallization period, giving rise to the various changes observed. The evidence discussed in this work supports a synthesis reaction mechanism involving formation and subsequent dissolution of an amorphous sodium aluminosilicate intermediate, with solution transport from the gel to the growth surface of the nucleated zeolite crystallite. It is not clear whether ordered moieties formed in the system represent nucleation centers, or building blocks for crystal growth as proposed by Barrer(19) and by Breck(20). This may be difficult to determine because of overlap of the several reaction steps involved. However, these building blocks would not have to be in the form of discrete cage units, as assumed by McNicol and co-workers(2). Ciric concluded that the most likely building-block units were dimer or cyclic tetramer species with dinegative charge(21). It might be speculated that the weak bands observed in the Raman spectra for the liquid phase could be attributable to such species. It is apparent that dissolution of gel intermediate occurs during the course of crystallization, in agreement with the earlier results of Kerr(22) and Ciric(21), and that a direct gel transformation does not take place in the system. It might be argued that these differing views are not totally irreconcilable, since in most synthesis processes of practical interest, rather short transport distances obtain in the porous hydrogel network. It must be remembered, however, that depletion of local concentration zones in a diffusion-controlled process would require some longerrange mass transfer, as shown mathematically by Ciric(21). It has also been pointed out that the colloidal nature of some of the species involved may obscure the mechanistic picture to some degree(_3), but this would not obviate the need for liquid phase participation in the process, as noted by Kerr(22). While the mechanistic discussion originally presented a number of years ago(23) has been often mi s-interpreted(4,22), it was noted therein that "formation of zeolite nuclei requires the solubilization of silicate anions from the colloidal silica particles and a resulting interaction with the aluminate ion



17. angell and flank

Mechanism of Zeolite A Synthesis

205

present in the solution". "After the i n i t i a l gel formation", i t was further noted, "the room temperature aging equilibrates the heterogeneous gel with the solution"(24). A concise discussion of this can be found in the monograph on molecular sieves by Breck(25). It is clear that the "solid" gel phase contains a considerable amount of aqueous solution in which liquid transport can occur. Additional support for liquid phase involvement in zeolite crystal growth can be found upon examination of the zeolite A micrographs in Breck's monograph showing multiple layered growth steps on crystal faces(26_). This type of surface structure would be expected in growth from solution, both on theoretical grounds and as a result of observational experience(27_). Counter parts to a number of other observations of zeolite crystal habit can be found in crystals of various kinds grown from solution. Annealed metals seem to be the only corresponding structures formed via solid-state crystallization. It can be further noted that earlier Raman work(l_) was apparently not continued far enough into the crystallization period to observe the changes which are reported here. Although the later work(2) pursued the search for "cage-like building blocks" into the crystallization period without success, i t is not clear why the changes reported here were not observed, although i t is true that a highly dilute system was employed, compared to that employed here, and that no aging step was used. Finally, the possibility of crystallization occurring by an Ostwald ripening mechanism and surface diffusion is not supported by recent work on the ripening process by Kahlweit(28), who demonstrated that particle growth rate as a function of time approaches zero after going through a maximum, rather than approaching a constant rate. The lack of constancy found in the present work in solids to liquid r a t i o , and distribution of chemical species in the solid and liquid as a function of time, are not consistent with a ripening process as the dominant crystallization pathway. In summary, based on sampling of the synthesis system at a number of points covering the several stages of the process, and using a variety of characterization techniques, i t can be con cluded that the trajectories of the observed physical and chemical parameters in the system are incompatible with a solid phase transformation mechanism, but agree quite well with solution transport considerations. Literature Cited (1) (2) (3)

McNicol, B. D . , Pott, G. T., and Loos, K. R., J. Phys. Chem. (1972) 76, 3388. McNicol, B. D . , Pott, G. T., Loos, K . R . , and Mulder, Ν . , Advan. Chem. Ser. (1973) 121, 152. Flanigen, Ε. Μ., Advan. Chem. Ser. (1973) 121, 119.


206 (4) (5) (6) (7) (8)


(9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23)

(24) (25) (26) (27)

(28)

MOLECULAR SIEVES—II Cournoyer, R. Α., Kranich, W. L., and Sand, L. B . , J. Phys. Chem. (1975) 79, 1578. Culfaz, Α . , and Sand, L. B . , Advan. Chem. Ser. (1973) 121, 140. Kacirek, H . , and Lechert, H . , J. Phys. Chem. (1975) 79, 1589. Zhdanov, S. P . , Advan. Chem. Ser. (1971) 101, 20. Moolenaar, R. J., Evans, J. C., and McKeever, L. D . , J . Phys. Chem. (1970) 74, 3629. Glastonbury, J . R . , Chemistry and Industry (1969), 121. Fortnum, D . , and Edwards, J. O., J. Inorg. Nuclear Chem. (1956) 2, 264. Angell, C. L., J. Phys. Chem. (1973) 77, 222. Flank, W. H., and Hinchey, R. J., unpublished data. Flank, W. H., unpublished data. Schwochow, F. E., and Heinze, G. W., Advan. Chem. Ser. (1971) 101, 102. A i e l l o , R., Barrer, R. M . , and Kerr, I. S . , Advan. Chem. Ser. (1971) 101, 44. A i e l l o , R., C o l e l l a , C., and Sersale, R . , Advan. Chem. Ser. (1971) 101, 51. Flank, W. H., Advan. Chem. Ser. (1971) 101, 43. Zhdanov, S. P . , Proceedings of the Third Tnternational Conference on Molecular Sieves, Zurich (1973), p. 25. Barrer, R. Μ., Baynham, J. W., Bultitude, F. W., and Meier, W. M . , J. Chem. Soc. (1959), 195. Breck, D. W., J. Chem. Education (1964) 41, 678. C i r i c , J., J. Colloid & Interface S c i . (1968) 28, 315. Kerr, G. T., J. Phys. Chem. (1966) 70, 1047. Flanigen, Ε. Μ., and Breck, D. W., presented at the 137th Meeting of the American Chemical Society, Division of Inorganic Chemistry, Cleveland, Ohio, April 1960; Breck, D. W., and Flanigen, Ε. Μ., Molecular Sieves, Soc. Chem. Ind., London (1968), 47. Breck, D. W., personal communication. Breck, D. W., "Zeolite Molecular Sieves", Wiley, New York (1974), p. 338. I b i d . , p. 343. Elwell, D . , and Scheel, H. J., "Crystal Growth from HighTemperature Solutions", Academic Press, London (1975), Chapters 4 and 5. Kahlweit, M . , presented at the International Conference on Colloids and Surfaces, San Juan, Puerto Rico, June 1976.


Mechanism of Zeolite A Synthesis - ACS Symposium Series (ACS

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