Mechanisms of Synthesizing Pseudomorphic Zeolite Particulates

18

M e c h a n i s m s of S y n t h e s i z i n g P s e u d o m o r p h i c Z e o l i t e Particulates

Using

High

Concentration Gradients

ANIL K. PATEL and L. B. SAND

Downloaded by MONASH UNIV on November 1, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch018

Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, Mass. 01609

ABSTRACT A hypothesis is proposed for the mechanisms of gelation and crystallization which occur during the reaction between activated alumina or sodium aluminate particles and sodium s i l i c a t e solu tions to produce pseudomorphic zeolite A, zeolite Β and zeolite HS particulates containing up to 100% zeolite with rapid sorption rates.

Introduction The objective of this work was to make a systematic study on direct syntheses of zeolite A, B, and HS (hydroxysodalite) par ticulates pseudomorphic after a suitable reactant particle using high concentration gradients in the synthesis system. This had been accomplished using sodium aluminate particles as precursors in a previous study on faujasite-type zeolites (1), in which i t was found that sodium salt additions were effective in promoting the mechanism. The f i r s t phase of this study, therefore, was to determine the effect of salt additions on the kinetics of crys tallization. If successful syntheses were achieved, an examina tion of the mechanisms involved in the process of pseudomorphic transformations was to be made. A review of the patent l i t e r a ture on reacting preformed shapes in-situ to zeolites is given by Breck (2), and a summary of the work in the U.S.S.R. is given by M i r s k i i _ £ t . à j . (3). These were also pseudomorphic transformations, starting generally with a silica-alumina particle. This study reports on the use of sodium aluminate or activated alumina as the particle precursor and the effects of synthesis parameters on the pseudomorphic conversion. The term pseudomorphic conversion is used in the broad sense, as with petrified wood, to describe the process of in-situ replacement of one species by another while retaining the size and shape of the original. 207 In Molecular Sieves—II; Katzer, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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Zeolites A, B, and HS were selected for the study, as they are closely related phases in the soda-alumina-silica-water sys tem to the faujasite-type zeolites on which similar successful studies had been made in our laboratories. Experimental The materials used were sodium aluminate particles (Nalco, l.lNa20-Al20 -3H20), activated alumina (ALCOA F - l , Αΐ2θ3·0.4Η2θ), sodium silicate solution (Phila. Quartz, 0.3Na20-Si02-7.3H20), distilled water, and reagent grade NaOH, NaCl, NaF, NaBr, andNal. Syntheses were made in 15ml capacity low carbon stainless steel vessels placed in controlled drying ovens with pressure de veloped autogenously. Using synthesis information summarized by Breck (4), the conditions for zeolite synthesis were determined by conventional techniques to obtain 100% yield of each phase as polycrystalline powder prior to the particulation study. The effects of anion additions on the kinetics of zeolite A crystallization then were determined. The order of mixing of reactants was found to be critical to achieve pseudomorphic conversion of the precursor particles: 1) the solution of distilled water, sodium hydroxide and sodium silicate, 2) sized sodium aluminate or alumina par ticles, and 3) salt. After reaction, the product was washed with distilled water on a suction-filtered Buchner funnel to near pH7 and then dried at 40-60°C. The samples were analyzed quantita tively by X-ray powder diffraction using a Philips model 3000 diffractometer with monochromatic Cu radiation. Morphology was determined with the use of a Jeolco U-3 scan ning electron microscope for which the samples first received a Au-Pd coating. Rates of adsorption were obtained with a sorption balance designed and constructed in our laboratories (5). Rates of gelation of the sodium aluminate particles were determined by leaching out the unreacted aluminate and of the activated alumina particles by following the decrease in boehmite content by XRD analysis. Induction period was defined as the time required for zeolite crystals to be detected by XRD analysis.


3

Results The starting batch composition used to study the effect of salt additions on zeolite A crystallization was 1.6Na20-Al2031.4Si02-96H20 reacted at 100°C. Examples of the effect of sodium salt additions to this batch composition on the reaction rates are given in Figure 1: without salt addition and with addition of 2 moles of NaF, Nal, NaCl or NaBr per mole of AI2O3 at 100°C. Additional data on this effect are given by Patel (6.). It was found that for the reaction condition selected, sodium salt addi tions inhibited the rate of crystallization for zeolite A.

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

18.

PATEL

A N D SAND

Pseudomorphic

Zeolite

Particulates

209

Assuming that the formation of stable nuclei is an energeti cally activated process, and as the nucleation process is ratedetermining during the induction period, the apparent activation energy for nucleation, E , was determined by the relation n

d ]" Ο Λ Ο

d (1/t)

= LΕ /R K

n'

where θ is the induction time (7). Table I lists the apparent activation energies for nucleation in the system without salt addition and with salt additions. Downloaded by MONASH UNIV on November 1, 2014 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch018

Table I.

Apparent activation energies for nucleation of Zeolite A from batch composition 1.6Na?0-Al?03-l.4Si0?-96H?0 with and without salt additions

Salt addition to batch composition 2NaF 2NaCl 2NaBr 2NaI

Temp. Range °C 10-100 80-120 60-100 60-100 80-100

En kcal/gmole 10.5 13.7 11.1 11.1 17.3

Pseudomorphic conversions were made with salt additions, which previously had been found effective in promoting faujasitetype particulation by this mechanism, but in the systems chosen for zeolites A, B, and HS, the best results were obtained with no salt additions. Figure 2 is a scanning electron micrograph of zeolite A particulates crystallized from a batch composition 3.2Na20-Al203-l.05St02~60.5H20 at 65°C using activated alumina as the precursor particle. As can be seen in Figure 2 (top), zeo lite A particulates are formed in the 500ym to lOOOym size range. Figure 2 (bottom) shows the surface of one of these particulates in which the individual crystals can be seen. The crystals are twinned cubes and in the 5ym and 6ym size range. Figure 3 (top) is a scanning electron micrograph of zeolite Β particulates in the 200ym to 1000ym size range synthesized using sodium aluminate particles as precursors. Figure 3 (bottom) shows the surface of these particulates in which small spheroid crystals of zeolite Β in the 2ym to 3ym size range are visible. Figure 4 (top) is a scanning electron micrograph of zeolite HS particulates in the 400-500ym size range synthesized from batch composition 4.5Na20-Al20 -3.6Si02-60.8H20 at 105°C. Figure 4 (bottom) shows the individual crystals in the 0.1-0.3ym size range. Figure 5 gives the crystallization curves resulting in the particulates of zeolites A, B, and HS shown in Figures 2, 3 and 4, respectively. Table II summarizes the batch composition and 3


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210

Figure 1. Crystallization curves for zeolite A from overall batch composi tion 1.6 Na O-Al O -1.4 Si0 -96 H 0-X salt at 100°C. Δ, no salt; •, X = 2 NaF; • , X = 2 Nal; A, X = 2 NaBr; ·, X — 2 NaCl g

2

s

SIEVES—II

2

2

0

4

2

Time

6

8

(Hours)

Figure 2. (left) Scanning electron micrograph of pseudomorphic zeolite A particulates from the overall batch composition 3.21 Na O-AUO -1.05 SiO -60.5 H 0 at 65°C. (right) Scanning electron micrograph of surface of the zeolite A particulate of Figure 2. 2

s

2

2



18.

PATEL

A N D SAND

Pseudomorphic

Zeolite

211

Particulates

Figure 3. (left) Scanning electron micrograph of pseudomorphic zeolite Β particulates from the overall batch composition 12.33 Na O-Al O -20 SiO -223.05 H 0 at 105°C. (right) Scanning electron micrograph of surface of the zeolite Β particulates. 2

2

s

2

2

Figure 4. (left) Scanning electron micrograph of pseudomorphic zeolite H S particulates from the overall batch composition 4.45 Na O-Al O -3.64 SiO -60.83 H0 at 105°C. (right) Scanning electron micrograph of surface of the zeolite HS particules. t

2

s

2

2


212

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the size range of the particulates and individual crystals ob tained. Table III lists the induction periods and crystalliza tion rates. Table II.

Size range of particulates and individual crystals in the particulates of zeolites Α. Β and HS formed from certain batch compositions.

Batch Composition (Na 0/Al 0 /Si0 /H 0

type

Π . 4/1/3.00/169.8 3.2/1/1.05/35 12.3/1/ 20 /223 4.5/1/3.6/60.8

A A Β HS

2


Zeolite

Table III.

2

3

2

2

Crystals ( m)

(μΐϋ)

u

1-3 100-500 500-1000 4-6 200-1000 2-3 400-500 0.1-0.3

Typical induction periods and crystallization rates for zeolites A, Β and HS particulates

Batch Composition

Temp. Zeolite Induction Cryst.

(Na 0/Al 0o/Si0 /H 0) ?

Particulates

2

9

ό

ά

9

Particle

°C

type

ά

3.21/1/1.05/60.5 12.3/1/ 20 /223 4.45/1/3.64/60.6

a.a. s.a. s.a.

100 105 105

A Β HS

Period, Rate hrs. %/hr 1.5 1 1.5

13.0 40.0 14.7

a.a. = activated alumina s.a. = sodium aluminate Depending on the system, the rates of gelation varied con siderably. For example, the overall rate of gelation for zeo lite A was 10%/hr as compared to 10%/min for zeolite B. Figure 6 shows a typical rate curve obtained during gelation of sodium aluminate particles in a system producing zeolite A particulates. Figure 7 shows two results of a scanning electron micro scopic study of the gel particles as they crystallize into zeo lite A from the overall batch composition 1.6Na20-Al203-l.4Si0296H20-NaCl at 100°C. After about 1 hour at 100°C, the gel par ticles degrade, and holes develop on the surface of the particles as can be seen in Figure 7 (top). This differential dissolution and breaking down of the particle can lead to the unusual tubular configuration of the gel as is shown in Figure 7 (bottom). This phenomenon is noted when the system does not produce strong par ticulate pseudomorphs but produces either weak particulates or dispersed polycrystalline powder. Figure 8 gives a plot of grams of S02 adsorbed per 100 grams of adsorbent against time at 25°C and 80mmHg pressure. The rate


PATEL

A N D SAND

Pseudomorphic

Zeolite

213

Particulates


100

10

Time

20

15

(Hours)

Figure 5. Crystallization curves re sulting in pseudomorphic zeolite A, B, and HS particulates. Batch com positions and temperatures are listed in Table 3; · , zeolite HS; O, zeolite Β; Δ, zeolite A.

ο

60

)

/

ο

30 ο

15

ο

/

Figure 6. Gel formation rate for the overall batch composition 3.21 Na 0Al O -1.05 SiO -60.50 H 0, at room temperature using sodium aluminate as the precursor particle 2

2

2

Time

3

4

(Hours)

3

2

2



214

MOLECULAR

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Figure 7. (left) Scanning electron micrograph of a gel particle from the batch composi tion 1.6 Na 0-Al 0 -1A Si0 -96 H 0-2 NaCl after 1 hr at 100°C. (right) Scanning electron micrograph of a gel particle from the same batch composition after 2V2 hr at 100°C. 2

2

s

2

2

16

^

12

ο (Λ

bf

8

Ο CO

Figure 8. Rates of SO adsorption on pseudomorphic zeolite A particulates (O) and on zeolite A extrudates (A) at 25°C and 80 mmHg pressure g

2

4 Time

6

8

( Minutes )

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

18.

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A N D SAND

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Zeolite

Particulates

215


of S02 adsorption on zeolite A particulates was 6 wt%/min. as compared with 4 wt%/min. for commercial extrudates averaged over the first 2 min. The adsorption capacity after ten minutes for zeolite A particulates was 14 wt% as compared to 10 wt% for the extrudates, due to the higher zeolite content in the particulates. Results on the flat plate crushing strength of these particulates showed that large particulates of zeolite A had a 0.5% decrease in flat-plate crushing strength after a solution life test as compared to a 13% decrease for the commercial zeolite A extrudates. This is significant in that ion exchanges can be performed without physical deterioration. Discussion Pseudomorphic zeolite particulates with rapid sorption rates can be obtained over a wide particle size range by direct synthesis using reactant particles which have a high concentration gradient relative to the solution. It was found that to obtain particulates by this method with the desired properties of size distribution, dry and wet strength, and rapid sorption rates, much experimentation is required for each zeolite type and for each different condition for crystallization of a given zeolite^ The complex dynamic systems are d i f f i cult to delineate, but they are reproducible once the conditions are determined. Although further research is needed to fully understand the process mechanisms, the initial studies indicate that the batch composition must be sufficiently viscous to inhibit nutrient transfer among particles to control rates of gelation, nucleation, and crystallization. Conditions which did not produce the desired pseudomorphic conversions showed a phenomenon that provides an insight into the mechanism. The gel particles developed vent holes on the surface; and in some cases in an intermediate stage of crystallization, a tubular configuration of the gel developed on the surface as is shown in Figure 7 (bottom). The 5ym particles are zeolite A crystals. It is proposed that when the gel particles in these non-optimum systems were exposed to the temperature of crystallization, the contained fluid phase vented at the surface. The venting fluids developed the cylindrical gel structure. This breaking down of the particle is prevented if viscosity of the surrounding solution is high enough to contain the vapors and the precursor particle is strong enough to allow the reaction to proceed to completion. Salt added to increase the solution viscosity did not promote the mechanism in these synthesis systems, but the study did demonstrate that the synthesis of pseudomorphic zeolite particulates by this method is not necessarily dependent on salt additions. This raises the possibility that better pseudomorphic faujasitetype particulates might be achieved in a system without salt additions than had been obtained in the previous study.


216

MOLECULAR


Gelation Reactant

Gel

Particle

Q

\

Particle

/

Solution

Nucleation

: C r y s t a II i z a t i o n

Zeolite

Figure 9.

Particle

Nuclei

Of

Zeolite

Schematic sequence of proposed mechanism of forming pseudomorphic zeolite particulates


SIEVES—II


18.

patel and sand

Pseudomorphic Zeolite Particulates

217

The proposed sequence of gelation and crystallization that a reactant particle undergoes is diagrammed in Figure 9. In start ing with a high concentration gradient between the reactant par t i c l e and an alkali s i l i c a t e solution, a gel forms on the surface of the particle and progresses to the center to result in the formation of an intermediate gel particle pseudomorphic after the original particle. In other words, because of the high concentra tion gradient between the solution and the surface of the reactant particle, the s i l i c a t e solution diffuses into the reactant par t i c l e and forms the gel phase. When the reaction front reaches the center of the particle, the reactant particle is replaced com pletely by an intermediate gel particle pseudomorphic after the precursor. This contrasts with the usual synthesis method in which a solution of sodium aluminate is mixed simultaneously with a solution of sodium s i l i c a t e to precipitate dispersed gel par ticles. Those individual gel particles contain a solid and l i q uid phase which react to form dispersed zeolite crystals. How ever, with the technique of this study a coprecipitated gel is not formed instantaneously; and with a high concentration gradi ent between the aluminum and silicon sources, the particulation process can proceed. After exposing these pseudomorphic gel particles to the tem perature required for c r y s t a l l i z a t i o n , i t is proposed that a very large number of nuclei form simultaneously. Because of this simultaneous nucleation and with uniform crystal growth, the re sulting small intergrown crystals form aggregates which retain the same size and shape of the gel particles. It is these small, intergrown crystals which give strength to the final pseudomor phic zeolite particulates. The more rapid rate of adsorption on these zeolite A particu lates observed in a comparative test with extrudates indicates an additional advantage of this method to form particulates in either the fluidized or fixed bed size ranges. As the particulates could be formed by this technique on the f i r s t four zeolites chosen, i t appears l i k e l y that other alkali zeolite particulates can be produced by this method. Acknowledqments The Petroleum Research Fund, Grant No. 8866-AC3,7 and the Backlund Fund of Worcester Polytechnic Institute provided finan cial support. Dr. R.R. Biederman and Mr. George Schmidt are gratefully acknowledged for their assistance in using the scanning electron microscope in their laboratories. Literature Cited 1.

Dhanak, Β . , M.S. Thesis, Dept. of Chem. Eng., Worcester Polytechnic Institute, Worcester, MA, 1975.


218 2. 3.

4. 5.


6. 7.

MOLECULAR SIEVES—II Breck, D.W., Zeolite Molecular Sieves, John Wiley & Sons (1974) 736. M i r s k i i , Ya. V . , Aleksandrova, I.L., Budovskaya, L.V., Kosolapora, A . P . , Golovko, V . G . , Adsorbently, Ikh Poluch., Svoistua Primen., Tr. Uses. Soveshch. Adsorbentam, 3rd 1969 (Pub. 1971), 63-5 (Russ.), CA 77,52636W (1972). Breck, D.W., Zeolite Molecular Sieves, John Wiley & Sons (1974) 270. Keisling, C . A . , Hayhurst, D . T . , Sand, L . B . , I&EC Fundamentals, in press. Patel, A . K . , M.S. Thesis, Dept. of Chem. Eng., Worcester Polytechnic Institute, Worcester, MA (1976). Culfaz, A. and Sand, L . B . , Adv. in Chem. Series, 121 (1973) 144.


Mechanisms of Synthesizing Pseudomorphic Zeolite Particulates

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