Novel zeolite transformations: The template-mediated conversion of Cubic P zeolite to SSZ-13

PAP rR ; Novel zeolite transformations: The template-mediated conversion of Cubic P zeolite to SSZ-13 S.I. Zones and R.A. Van Nordstrand

Chevron Research Company, Richmond, California 94802, USA (Received 31 August 1987) A study of the synthesis of zeolite SSZ-13 (a high-silica chabazite) from sodium silicate solutions showed the transient formation of Cubic P zeolite. The N,N,N,-trimethyl ammonium-l-adamantane organocation converted Cubic P to the secondary zeolite product with an increase in SIO2/AI203 during the process, pH increase during the secondary zeolite formation was shown to be correlated with SiO2/AI203 increase in the product. The use of an ion-selective electrode for organocations and its application in kinetic analysis of zeolite crystallization are described. The advantages of a study of this zeolite system in the absence of a hydrogel phase are discussed in terms of mechanisms of zeolite crystallization. Keywords: Zeolite synthesis; Cubic P; chabazite; SSZ-13

INTRODUCTION During work on the synthesis of zeolites in the presence of adamantane derivatives, a new high-silica form of chabazite was discovered and designated SSZ-13. l The synthesis of SSZ-13 from sodiumsilicate solutions appears to involve as an intermediate material the lower silica zeolite Cubic P. The organic template molecule used in this work was the N,N,N-trimethylammonium d e r i v a t i v e o f 1Adamantanamine (Figure 1). This template was found intact within a significant portion of the pores of SSZ-13. The transformation of Cubic P to SSZ-13 was studied via three types of experiments: (1) Cubic P zeolite was grown from a gel phase in a system containing the organocation. The transformauon of P to SSZ-13 occurred even while P zeolite was still growing. (2) Cubic P zeolite was grown from a gel phase and in the absence of the organocation. The template was then added to bring about the transformation to SSZ-13. (3) Finally, the dry-powdered Cubic P was added to a sodium-silicate solution containing the organocation and SSZ-13 was synthesized directly from P. This novel hydrothermal synthesis points out the significance of the organocation as a template molecule in the sequence: (1) dissolution of Cubic P, (2) transport of aluminosilicate units, and (3) final crystallization of SSZ-13. Dwyer and Chu 2 showed that faujasite can be transformed in solution to omega in the presence of the tetramethylammonium cation. In the present study, a much more hydrophobic © 1988 Butterworth Publishers

166

ZEOLITES, 1988, Vol 8, May

cation was used and high dilution helped to eliminate a gel phase. A kinetic analysis of this zeolite transformation was carried out by following changes in pH, silica, alumina, sodium, and template concentrations for both the solids and solution. A new probe was introduced for following solution concentrations of the hydrophobic template molecule during zeolite crystallization. The template was progressively removed from solution and found within the cages of SSZ-13.

EXPERIMENTAL Banco "N" silicate (38.3% solids, SiO2/Na20 = 3.22) was used as the source of silica. A12(804)3.18 H 2 0 (J.T. Baker), t e t r a m e t h y l a m m o n i u m b r o m i d e (Aldrich), and tetrapropylammonium bromide (East® N (CH3) 3 1(~

Figure 1 Organocation used in SSZ-13 crystallization

Conversion o f Cubic P zeolite to SSZ-13: S.I. Zones and R.A. Van Nordstrand

man) were all used as received. T h e a d a m a n t a n e template was p r e p a r e d as described in Ref. 1. Zeolite syntheses were carried out at 135°C as described in example 8 o f Ref. 1 except that the a l u m i n u m concentration was doubled with the increased acidity accounted for with N a O H . Reactant ratios are O H - / SiO2 >- 0.9, H20/SiO2 ---- 35, and SIO2/A1203 = 32. p H m e a s u r e m e n t s were m a d e using an O r i o n Ross combination electrode with a 701A meter, calibrating with p H 7, 10, and 12.5 buffers. All m e a s u r e m e n t s were m a d e at 23 ° + 2°C. T h e ion-selective electrodes were obtained f r o m Prof. H. Freiser at the University o f Arizona and were calibrated using an O r i o n Double J u n c t i o n r e f e r e n c e electrode (Ag/AgC1, model no. 90-0200) with the O r i o n 701A meter. Measurements were m a d e at 23 ° + 2°C. T h e conversion e x p e r i m e n t s were r u n using conditions in Table 4 but using only 0.05 g o f organic template per r u n with 0.5 g Cubic P. For the SSZ-13 template, this gave an initial concentration o f 10 -2 molar, and in the case o f T P A Br, - 1.2 × 10 -2 M. Different electrodes were used for each organic salt, and a calibration curve was obtained for each. This is the reason that the initial mV o u t p u t at To is not quite the same for each electrode, although their sensitivities are about equal (slopes are both 55 + 2 mV). Scanning electron micrographs were taken on a Hitachi S-570 i n s t r u m e n t with a m o u n t e d Polaroid camera. T h e working beam distance was typically 15 m m with an o p e r a t i n g voltage o f 12 keV and 30 ° tilt. Samples were m o u n t e d on a l u m i n u m stubs using double-sided sticky tape and then coated u n d e r vacuum with gold-palladium b e f o r e being analyzed in the microscope. Both carbon-13 and silicon-29 spectra were r u n on a B r u k e r CXP 300 pulsed n.m.r, spectrometer. ~3C spectra were r u n at 75.48 MHz (~/2)c = (~/2)H = 4.2 ItS. T h e upfield peak in a d a m a n t a n e (28.7 p p m f r o m TMS) was used as an external reference. T h e magic angle spinning (MAS) speed was 3400 Hz with the angle adjusted using KBr spinning echoes and e m p l o y i n g B~N3-Kelvin F rotors. In the 29Si crosspolarization experiments, a f r e q u e n c y o f 59.621 MHz was used (~/2)si = (~/2)H = 5.5 kts (pulse widths). T h e H a r t m a n - H a h n mixing time was 5 ms. H e x a m e t h y l cyclotrisiloxane was used as external r e f e r e n c e ( - 9 . 8 p p m f r o m TMS). M A S speed was 3100 Hz. KBr and the B3Na-Kelvin F r o t o r were used as described above. Analyses o f sodium, a l u m i n u m , and silicon were p e r f o r m e d on an ARL Model 3400 direct reading s p e c t r o p h o t o m e t e r with an ICP source, as previously described. 3

RESULTS The formation of Cubic P zeolite in absence of organic cation T h e initial reactants were mixed to f o r m a clear solution o f composition 15 Na20-A12Oa.32 SiO2.1100 H 2 0 . This solution at r o o m t e m p e r a t u r e trans-

f o r m e d gradually into a gel. When this solution was held at 135°C for several days, Cubic P zeolite 4 formed. Samples were taken over the course o f the Cubic P synthesis at 135°C and p H m e a s u r e d at room temperature. Once the P formation began, there was essentially no change in p H or mass o f product observed over several days. Table I shows the relative a m o u n t o f Cubic P zeolite p r o d u c e d as a function o f time, as well as the mass and p H data. A l u m i n u m was the limiting reagent. It was all t r a n s f o r m e d to product, and the final SiO2/AI20:~ values o f 5 - 6 d e m o n s t r a t e that less than one-fifth o f the available silica was used and that silica e n r i c h m e n t in the p r o d u c t with time was not occurring. T h e s e results are in accord with results recently summarized by Lechert. 5 In the absence o f organocation, Cubic P f o r m e d when the value o f O H - / S i O 2 was greater than 0.9. At lower O H - / S i O 2 values m o r d e n i t e formed. T h e reactants that p r o d u c e d Cubic P at 130°-140°C gave considerable analcime at 160°C or higher. (For a discussion o f the crystallization field relationship o f these three zeolites, see Ref. 6.)

Zeolite crystallization in the presence of the organic template W h e n the same synthesis reaction was carried out in t h e p r e s e n c e of the quarternized 1A d a m a n t a n a m i n e cation (Q1-Ad) at an organic/SiO2 ratio o f 0.13, zeolite P was f o r m e d first. P was then c o n v e r t e d to zeolite SSZ-13 over several additional days at 135°C. T h e SSZ-13 zeolite has the c r y s t a l structure o f chabazite. T h e XRD pattern o f SSZ-13 is listed in Table 2 in the as-synthesized form. Parameters o f the r h o m b o h e d r a l unit cell for one particular sample are listed in Table 3 and are c o m p a r e d with c o r r e s p o n d i n g values for h y d r a t e d calcium chabazite r e p o r t e d by Smith et al. v SSZ-13 was f o r m e d at considerably higher SiO2/ AI20~ ratios than known synthetic or naturally occurring chabazites. 8 Although possessing cavities large e n o u g h to a c c o m m o d a t e the organocation, the zeolite is a small-pore variety. 9 T h e formation o f the SSZ-13 zeolite and the disappearance o f the initial product, Cubic P, was m e a s u r e d by XRD o f the solid product. Changes in both the liquid and solid products were followed as well.

Table 1 XRD data for synthesis of Cubic P zeolite at 135°C Time (h)

Phases detected by XRD

Grams product

Final pH

20 40 60 72 96 120

Amorphous 22% Cubic P 74% Cubic P 89% Cubic P 100% Cubic P 100% Cubic P

1.45 1.35 1.33 1.36 1.30 1.37

12.20 12.17 12.10 12.25 12.24 12.15

ZEOLITES, 1988, Vol 8, May

167

Conversion of Cubic P zeolite to SSZ-13: S.I. Zones and R.A. Van Nordstrand Table 2

XRD lines for the SSZ-13 zeolite

Table 3 Comparison of rhombohedral u.c. parameters for SSZ-13 and Ca chabazite (Ref. 7)

29

d (~)

I/Io

9.70 13.03 14.10

9.120 6.790 6.280

39 12 12

16.35 17.88

5.420 4.960

19.21 21.08 22.15 22.65 23.25 25.07 26.20 28.00 31.08

4.620 4.210 4.017 3.926 3.826 3.552 3.401 3.187 2.877 2.858 2.814 2.741 2.576 2.483

54 26 2

31.30

31.80 32.67 34.82 36.17

Zeolite

a (A)

Alpha

Vol/cell (A z)

SSZ-13 as prepared SSZ-13 calcined (550°C)

9.291 9.269

93.92 94.33

Ca-chabazite

9.420

94.47

796 789 828

The sequence in Figure 2 shows how the XRD patterns developed over time. Although the phase SSZ-1:3 was first detectable simultaneously with Cubic P, the latter increased more rapidly at the expense of amorphous material. Finally, Cubic P was consumed in the second transformation leading to SSZ-13. These phase changes were clearly seen by scanning electron microscopy. In Figure 3a, at 40 h of reaction, Cubic P was the predominant product in a mixture of Cubic P and SSZ-13. T h e P is characterized by the aggregates exhibiting a ball-of-yarn appearance. They are 10-15 ~t in diameter. Granulation of some of the spherical surfaces is the crystallizing SSZ-13. Some amorphous material can be seen. With time, regular polyhedra began to develop from the early aggregates (Figure 3b). At 73 h, SSZ-13 is the major phase. Highly developed spherical aggregates can be seen as well as the hollowed-out and broken shells.

100 5 6

6 36 22 5 59 19 2

2 5 6

Sequential zeolite product transformation When the reaction mixture was heated for about 1 day at 135°C, the product obtained was an amorphous aluminosilicate solid. Identical mixtures heated for longer periods of time showed replacement of the amorphous solid by aluminosilicate zeolites.

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ZEOLITES, 1988, Vol 8, M a y

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Conversion of Cubic P zeolite to SSZ-13: S.I. Zones and R.A. Van Nordstrand

SEQUENTIAL

ZEOLITE SYNTHESIS

AT 135°C

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Figure 3 Changesin SEM during Cubic P-to-SSZ-13 transformation

T h e latter belong to the dissolving Cubic P. In some instances, the SSZ-13 can be seen growing and attached to the last of the Cubic P shell. Finally (Figure 3c), the highly regular intergrowths, characteristic of SSZ-13, were all that could be seen. By 168 h, SSZ-13 is the only product and the 15-1~ aggregates are very uniform in their morphology. As can be seen at higher magnification (Figure 3d), the crystallites are aggregates of interpenetrating cubic geometry. We found that the formation of the SSZ-13 is aluminum dependent. Also, because all of the aluminum in the reaction was always found in the solids, the SSZ-13 must grow at the expense of the Cubic P.

Mass balance during phase transformations While morphological transformations occurred, important changes in the chemical composition of the products were also observed. The most noticeable phenomenon was that total product mass increased as SSZ-13 began to grow. Figure 4 shows the relative mass increase in the product with respect to time.

Unity is the product mass attained by the formation of Cubic P when the reaction was carried out in the absence of the organocation. The increase in mass accompanied the SSZ-13 formation and was due to incorporation of additional silica and organocation as. the structure grew. The SiO2/Al2Os values of products increased linearly with product mass. The pronounced rise in SiO2/Al2Os corresponded to the time when SSZ-13 was forming. The SIO2/A1203 values almost doubled from initial (Cubic P) to final product.

p H profile of the reaction While the product mass and SiO2/A1,_,O3 ratio increased, the pH of the solution (measured at room temperature) also indicated changes in the chemistry of the system. Figure 5 shows pH values for reaction solutions with and without organocation heated to 135°C over time. In the first 40 h, the pH dropped slightly. This corresponded to amorphous aluminosilicate precipitation. Once zeolites were the only

ZEOLITES, 1988, Vol 8, May

169

Conversion of Cubic P zeolite to SSZ-13: S.I. Zones and R.A. Van Nordstrand 100

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Hours at 135°C

Figure 4 Change in product mass during Cubic P-to-SSZ-13 transformation

products observed, in the presence of the organocation, the pH steadily rose as the product distribution shifted toward SSZ-13 zeolite. In fact, the plateau at higher pH values corresponded to the period of nearly complete crystallization to the higher silica zeolite, SSZ-13. The increase in pH coincided with the increase in mass and SIO2/A1203 values and with the decrease of NazO/Al203 in the solid phase.

Uptake of organocations The presence of the organocation is responsible for the transformation to SSZ-13 zeolite and the content of hydrocarbon in the solids produced is directly related to the amount of SSZ-13. Figure 6 verifies the proportionality between this carbon content and percent SSZ-13. The C/N ratios found in the solids are close to the value of 13 expected for the intact template. As organocations were incorporated into the growing SSZ-13 structure, they could have counterbalanced aluminate framework sites. However, more than half of the aluminate anionic sites remained counterbalanced by sodium cations. In the typical SSZ-13 product formed here, SIO2/A1203 = I0. There are, on average, four aluminum tetrahedral

sites per cavity (20 T atoms) in this chabazite structure. Geometric constraints limit the filling of the cavities to one organocation each. Some of the alkali cations can be accommodated by sites in the double six-rings (hexagonal prisms) in the structure, but to the extent that sodium cations are in the large cavities, organocations may be excluded. In this reaction, once 100% crystallinity was achieved, the organocations counterbalanced only about one-fourth of the aluminate sites. This indicated that there was probably less than maximal filling of cavities by the organocation. On the other hand, if only one organocation is accommodated per cavity at SIO2/A1203 = 10, t h e m a x i m u m organocation/aluminate ratio will be below 0.50. Consequently, more than half of the cavities that could theoretically be filled by the organocation were occupied. In addition, not all the organic cation available in solution was used during crystallization. Figure 7 shows that early in the reaction the Na/A1 ratio was greater than 1 in the solids. There was more than enough sodium to counterbalance aluminate anions. At 40 h of reaction, as SSZ-13 was starting to form, the Na/AI value was unity and dropped below 1 as SSZ-13 continued to form at the expense of P zeolite. An inverse correlation was observed between the carbon (or nitrogen) content of the solids and the Na/AI ratio. A good closure was obtained for (Na + + organocation)/Al at 100% crystallization.

13.00 In the Presence of • //Organo-Cation

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I 60

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0.5(]

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Hoursat 135"C

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Figure 5 pH profile for the synthesis of SSZ-13 at 135°C

170

ZEOLITES, 1988, Vol 8, May

Figure 7 Decrease in NazO/AI203 ratio with increasing SSZ-13 formation

Conversion of Cubic P zeolite to SSZ-13: S.I. Zones and R.A. Van Nordstrand

Aluminate dependence

Table 4

Reference has already been made above to the fact that aluminate was the limiting reagent in the synthesis of SSZ-13 zeolite or Cubic P. Figure 8 shows that the product mass of SSZ-13 obtained at the completion of synthesis was proportional to the aluminum content of the synthesis mix. When all other components were held constant, the yield was a direct function of the quantity of A1~(SO4)3.18 H20. Again, no AI +3 was detected in solution at any point during the synthesis. From the slope of the line in Figure 8, and the measured values of Na20 and organic in the solid, a value of SIO2/A1203 = 9.5 can be derived. This is in agreement with the measured SIO2/A1203 of about 10 obtained for SSZ- 13. With the synthesis reaction carried out at pH 12-13, in the absence of alumina, ~° silica has high solubility. 1].12 Aluminum is required to produce first the gel and, subsequently, the Cubic P zeolite. As has been shown, the organocation is not necessary for Cubic P formation. It remains a spectator in solution during the earliest organization of aluminate ion into aluminosilicate oligomers and gel, and then into Cubic P zeolite.

Zeolite transformation starting with Cubic P zeolite

Secondary zeolite formation Dried Cubic P could also be transformed to SSZ-13 by heating it as a slurry with the organocation, sodium silicate, and enough NaOH to mimic the pH o f the hydrothermal synthesis. Table 4 shows the data for the Cubic P and for SiO2/AI20~, pH, and mass comparisons for the SSZ-13 zeolite obtained at 130°C after several days of reaction. Starting from dried Cubic P removed the gel-forming step in the zeolite transformation: gel ~ P ~ SSZ-13. 16

T ¢}

i ~J 8 u

Zeolite pH Relative mass SiO2/AI203 Na20/AI203

Initial state

Final state

Cubic P 12,35 1.00 6.00 1.03

SSZ-13 13.10 2.20 15.00 0.60

a Reaction conditions: 1.00 g adamantane derivative, 0.5 g Cubic P zeolite, 10 g H20, 5.0 g of "N" silicate as silica source and 0.30 g 50% NaOH. Six days at 130°C at 30 rpm tumbling

Another experiment was to synthesize the P zeolite in solution without the organocation, then add the organocation and transform P to SSZ-13. The reaction for P was run several days (see Table I) until the P had crystallized. Adding the organocation and reheating for several more days produced a product that was SSZ-13 with a small amount of P. This conversion of Cubic P was not limited to formation of SSZ-13. When the above conversion reaction was carried out using the tetramethylammonium (TMA) ion, instead of Q1-Ad, sodalite was produced (see Table 5). TMA is a known template for sodalite. 13 Once again, the product formed in the presence of the organocation showed an increased pH, mass, and SIO2/A1203 ratio.

Ion-selective electrode studies Recently, Freiser and Martin 14 introduced a coated-wire, ion-selective electrode (ISE) with good sensitivity for hydrophobic quaternary ammonium cations. Typically, a linear millivoh response over several decades of concentration was obtained with very little interference from alkali cations. Good sensitivity occurred in the 10-2-10 -5 M range. As the organocation disappeared during the formation of SSZ-13 zeolite, the ISE demonstrated a drop in concentration. Organocation concentrations were measured by analyzing the filtrates of samples after cooling to 25°C. The reactions were run with a starting concentration of the organocation at 10 -9 molar. Figure 9 shows the measured ISE output with respect to time when reactions were run starting with Cubic P zeolite as the aluminum source and using a dilute solution of the organocation (~ 10 -2 M) and a little silica and NaOH. As Cubic P was converted to SSZ-13, a steady drop in template concentration was observed. Within 20 h the organocation concentration decreased an order of magnitude, as indicated by Table 5

•

Conversion of Cubic P zeolite to SSZ-13 at 130°C a

Conversion of Cubic P zeolite to TMA-sodalite at 130°C a

4

O~ 0

I 8

I 16

I 24

I 32

Reactant Moles Ai2(SO4)3 • 18 H 2 0 x 10 .4

Figure 8 A l u m i n u m dependence for product yield in SSZ-13 synthesis

Zeolite pH Relative mass SiO2/AI2Oz Na20/AI203

Initial state

Final state

Cubic P 12.39 1.00 6.00 1.03

TMA-sodalite 12.83 2.00 13.00 0.56

a Same ratios as given in Table 4 except TMA is now used as the organocation

ZEOLITES, 1988, Vol 8, May

171

Conversion of Cubic P zeolite to SSZ-13: S.I. Zones and R.A. Van Nordstrand 250 o T P A BR • SSZ-13 Template

k\

200 -

o

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f50

0

I 35 Hours

I 70

Figure 9 Uptake of organocations during Cubic P transformation as measured by ISE

the change in millivoh output. (The slope of the ISE is usually - 55 mV/decade in the linear region.) Greater than 90% of the organocation was consumed. XRD showed the presence of SSZ-13 zeolite as well as the parent Cubic P. For comparison, the same experiment was run using an equally hydrophobic template that had been studied by Freiser and Martin. 15 This organocation, tetrapropylammonium (TPA), has a very high selectivity for crystallizing the high-silica ZSM-5 zeolite.* Under these reaction conditions, the TPA was unable to transform the Cubic P zeolite to any other structure* and no concentration drop was observed.

(Figure 6). The incorporation of the organocation was also demonstrated by n.m.r, studies. Figure 10 gives comparative C ~a n.m.r, spectra for the template molecule itself and for the synthesized zeolite. A very good structural correlation is observed for the template molecule within the zeolite and in its isolated, pure state. Another method of demonstrating the incorporation of the organocation was by using crosspolarization experiments for Si20 and H l (from the organocation as a rigid, fixed source of H z) nuclei. Figure 11 shows the spectra in such a crosspolarization treatment for the zeolite SSZ-13 as made. Upon removing the organocations by calcination at 550°C, the cross-polarization phenomena almost completely disappeared. Some signal was still observed, but at considerably lower intensity. Although the calcination procedure was carried out to remove the organocations from the zeolite cavities, there often was still a small amount of hydrocarbonaceous material left behind (typical carbon analysis, 0.1-0.2%). This residual material may have been able to provide enough proton cross-polarization to account for the observed spectrum. (Note that after treatment at 550°C, the zeolite itself remains intact as determined by XRD.) There was also some lattice relaxation observed by XRD as bulky organocations were removed from the large cavities within the structure.

DISCUSSION There is a clear picture of the bulk transformation of solids in the sequence: amorphous ~ Cubic P SSZ-13 zeolite. A corresponding change in the concentration of dissolved components could also be seen over the course of these transformations. How aluminosilicate units are transferred from P to tile organozeolite remains unknown. Most of the organocation was detected in solution by the ion-selective electrode before SSZ-13 formation. But the incorporation of the organocation into tile zeolite was irreversible. There was a good correlation between decrease in organocation concentration and increase in SSZ-I 3 crystallized. There are instances in which the organocation directs zeolite formation without becoming part of the final structure. This was demonstrated in studies of Losod formation carried out by Meier and Sieber using quarternary ammonium hydroxides, j6 They were unable to detect the incorporation of the large organocations into the small-pore zeolite structure. Here there is a good linear correlation of organocation uptake and crystallization of the SSZ-13 zeolite * The template can be used over a great temperature (100 ~ 200°C) and SiO2/AIzOz range, producing a pure ZSM-5. t TPA OH can be used in AIPO-5 synthesis under acidic conditions. Otherwise, ZSM-5 is the only other structure produced by this template molecule.

172

ZEOLITE& 1988, Vol 8, May

o..@.; Neat

t

80

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l

70

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=

10

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t

0

ppm Figure 10

C 13 n.m.r, spectra or organocation in SSZ-13 zeolite

Conversion of Cubic P zeolite to SSZ-13: S.I. Zones and R.A. Van Nordstrand

In high-silica zeolite crystallization, pH rise is detected toward the end of crystallization, l'-~ These crystallizations are carried out in hydrogel systems, rich in silica, as the solubility of polymeric silicates becomes poor as the pH is adjusted below 11 (Ref. 20). pH rise must occur when NaOH is released from the last of the remaining gel or silicates as they are incorporated into the zeolite structure. In some of the reactions described here, zeolite synthesis occurred with an increase in silica in the product and no hydrogel present. Here, also, a more continuous pH increase was correlated with zeolite crystallization. This type of data is not observed in the hydrogel, high-silica zeolite synthesis.

Calcined

CONCLUSIONS

Uncalclned ~//f'k j] - 0

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86

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-114

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ppm Figure 11

Si 29, H 1 cross-polarization n.m.r, spectra generated in the presence of the SSZ-13 template

pH increase can be an important measure of zeolite crystallization. The relationship of pH increase and zeolite crystallization has recently been discussed by Lowe et al. for high-silica zeolite (SiO2/AI20.~ > 50) syntheses. 17-19 These studies include an equilibrium model for reactant ratios versus expected pH change. In these systems, almost all of the pH change is observed during the final stages of crystallization. Although there are differences between this reaction system and those for high-silica zeolites, it is believed that in both cases pH rise is due to formation of siloxane bridges replacing silanol and the partially ionized SiO-Na entities. This accompanies transformation of both soluble silica and the gel into zeolite. This bridging reaction is shown below: 0

O

~

O / Si-O-

Na +

+ HO-Si/O 0---* O O ~ s i / O ~ s i / O XO + NaOH

O/

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This study has demonstrated the conversion of one zeolite phase in solution to a more stable phase mediated by an organocation. This transformation was accompanied by significant changes in product morphology, product chemical composition, and solute concentrations. Study of this system provided some insight into the role of" reactants and intermediates in zeolite synthesis. The determination of pH rise and concomitant increase in SiO2/Al~O3 and decrease in NagO/A1203 in the solid phase during the formation of SSZ-13 from Cubic P provided new data on the role of organocations in zeolite synthesis. The transformation of Cubic P zeolite was not simply restricted to the SSZ-13 system: Starting with Cubic P, sodalite could also be produced using the tetramethylammonium cation. Several other hydrophobic organocations were found to be capable of transforming Cubic P to higher silica zeolite species. The reaction course paralleled the studies discussed here, and these findings will be reported shortly.

ACKNOWLEDGEMENTS The authors are grateful for the technical support supplied by various members of the Chevron Research technical staff. In particular, 13C M A S n.m.r. and 29Si M A S n.m.r, cross-polarization experiments were carried out by Dr. Don Wilson. Messrs. Lun Teh Yuen and Louis Scampavia provided technical expertise and much useful discussion throughout this work. Ms. Hilda Shea provided technical assistance in the synthetic aspects of this work. We thank Prof. Henry Freiser of the University of Arizona for providing us with coated-wire electrodes for hydrophobic cation studies and for much useful discussion on this subject. Dr. J.N. Ziemer of Chevron Research also contributed in the electrochemical work. Finally, we thank the management of the Process Research Department of Chevron Research for continued support over the course of this work.

(1)

The higher the degree of silica incorporation into the growing structure, the greater the pH (increased net NaOH).

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