Studies on the crystallization of a novel, large-pore, high-silica molecular sieve, NCL-1

Studies on the crystallization of a novel, large-pore, high-silica molecular sieve, NCL-1 K. Ramesh Reddy, Veda Ramaswamy, R. Kumar, and A.V. Ramaswamy National Chemical Laboratory, Pune, India The synthesis of a novel, large-pore, high-silica molecular sieve, NCL-1, is described in detail. A comparison of the XRD powder patterns of NCL-1 and other known structures revealed that zeolite NCL-1 is a new phase. However, zeolite SSZ-31, a Chevron material, and NCL-1 have some common features and may belong to a common family. Adsorption measurements (o-xylene = 7.4 and mesitylene = 4.8 wt% at P/Po = 0.5, T = 298 K) rank NCL-1 as a large-pore zeolite. The synthesis has been carried out using fumed silica, aluminum sulfate (optional), and hexamethylene bis(triethylammonium bromide) (R 2.) as an organic template under hydrothermal conditions in alkaline medium. The kinetics of crystallization has been studied using X-ray powder diffraction, scanning electron microscopy (SEM), and framework i.r. spectroscopy. The influence of various synthesis parameters such as the Si/AI and Na+/Si molar ratios, alkalinity (OH-/Si), concentration of the organic template (R2+/Si), the H20/Si ratio, and the temperature on the crystallization time, phase purity, and yield of crystalline product has been investigated. Keywords: Novel large-pore zeolite; synthesis; crystallization kinetics of NCL-1; influence of synthesis parameters

INTRODUCTION

EXPERIMENTAL

High-silica, large-pore zeolites are of special interest in the field of adsorption and catalysis. Recently, we reported the synthesis of a novel, large-pore highsilica zeolite, designated as NCL-1, which can be obtained with Si/A1 ratios from 20 to o0.1 The adsorption and catalytic properties of NCL- 1 are characteristic of zeolites with a pore opening constituted of 12-membered tetrahedra. The effective pore diameter of NCL-1, as suggested from the adsorption measurements ~ (mesitylene: 4.8 wt% at P/Po -- 0.5 and at T = 298 K) and catalytic test reactions 1 (viz., constraint index 2 [CI = 1.0] and m-xylene conversion 3--6 [P/O = 1.0, I/D = 10]) seems to be close to that of mordenite.~ Additionally, our recent work on the characterization of NCL-1 through the spaciousness index 7 (SI = 6.6) and refined constraint index s (CI* = 1.5) clearly rank NCL-1 as a large-pore microporous material. ~'9 The corresponding values of SI and CI* for mordenite are around 7.5 and 1.8, respectively.7,s In this paper, we report further details on the synthesis and the kinetics of crystallization of NCL-1 molecular sieves.

The hydrothermal syntheses were carried out using 100 ml capacity stainless-steel autoclaves under agitation conditions. Before use, the autoclaves were thoroughly cleaned with 35% HF to minimize the seeding effect of residual crystalline products. The raw materials used in the synthesis were fumed silica (Sigma, USA, S-5005, 99.9%), A12(SO4)3-16 H20 (98%), sodium hydroxide (99% AR), and an organic template, hexamethylene bis(triethylammonium bromide) (prepared in the laboratory). The following molar composition range of the reaction mixtures was chosen for this study: Si/Al = 20 - infinity; R2+/Si = 0.00-0.1; Na+/Si = 0.25-0.5; OH-/Si -- 0.12-0.25; and H20/Si = 33-70. In a typical preparation, the following procedure was followed: 4.5 g of silica was slurried in 30 g of water before adding to it a solution of 0.75 g of NaOH in 10 g of water and allowed to be stirred for 45 min at 298 K. To the above mixture, a solution of 1.71 g of organic template, R 2+, in 15 g H20 was added and stirred for 10 min and the remaining water (15.2 g) was then added. For the preparation of aluminum-containing NCL-1, the aluminum source was added before the addition of the organic template (R2+). The resultant reaction mixture was stirred for 1 h. The desired OH-/Si molar ratio was adjusted by adding an appropriate amount of sulfuric acid. The crystallization was conducted at 443 + 2 K in a rotating (60 r.p.m.) container placed in

Address reprint requests to Dr. Ramaswamy at the National Chemical Laboratory, Pune 411 008, India. Received 24 November 1992; accepted 13 January 1994 (~ 1994 Butterworth-Heinemann 326

ZEOLITES, 1994, Vol 14. June

Crystallization

an oven. After crystallization, the solid was filtered, washed with water, and dried at 393 K. The yield of fully crystalline solid was invariably in the range of 90-95 wt%. Samples A-E (Si/Al = 23, 84, 156, 257, and infinity, respectively) have thus been prepared. The organic additive, hexamethylene bis(triethylammonium bromide) has been synthesized by quarternizing triethylamine with 1,6-dibromohexane (in the molar ratio of 2) in presence of acetone as the solvent. Typically, to a mixture of 25 g of triethylamine in 20 g of acetone, 30 g of 1,6-dibromohexane (Aldrich, > 99%), and 50 g of acetone were added and the homogeneous mixture was refluxed for 10 h. After completion of the reaction, a white (diquaternary) salt was formed in > 90% yield (m.p. 22O”C), which was separated from the reaction mixture and dried in vacuum before use in the synthesis. The X-ray powder diffraction patterns were recorded on a Rigaku, (D Max III VC Model) instrument, using Ni-filtered CuKor radiation and a graphite crystal monochromator. A step-scan of 0.02” 26 and a counting time of 10 s at each step were employed while the sample was rotated during the scan. Silicon was used as an internal standard. To determine the relative crystallinity of the samples, the XRD pattern was recorded between 20 = 17” and 25.5”. The samples were further characterized by framework i.r. (Perkin-Elmer, Model 22 1) spectroscopy and scanning electron microscopy (JEOL, Model JSM 5200) techniques. The chemical analyses of the samples were performed by the XRF (Rigaku, 3070 Model) technique using standard reference materials.

RESULTS AND DISCUSSION Characterization Figure I depicts the X-ray powder diffraction profiles of the products crystallized at different intervals of time (curves a-d refer to crystallization times of 19, 31, 36, and 48 h, respectively) and demonstrates the evolution of the crystalline material from the Al-free gel mixture of the following molar composition (sample E):

R*+/Si = 0.05; OH-/Si = 0.25; H20/Si = 52

= 0.25; Na+/Si

The fully crystalline material was obtained after 48 h at the crystallization temperature of 443 K (Figure 1, curve d). The crystallinity of the samples, collected at different intervals of time (Figure 1, curves a-c), was 7, 28 and 73%, respectively, with reference to the fully crystalline (100%) sample (curve d), calculated from the ratio of the integrated intensities of 20 lines between 17” and 25.5”. During crystallization of NCL-1 samples, all the major lines grow simultaneously and the relative intensity of all the major peaks remain unchanged even when the crystallization period is further extended (after the complete crystallization). The fully crystalline samples of NCL- 1 exhibited essentially the same XRD pattern with similar relative intensities.

i? ”

of NCL-1:

K. R. Reddy

et a/.

f-_r a

L 5

10

15

20

25

30

2e’

Figure 1 The evolution of the NCL-1 phase as seen from X-ray powder diffraction profiles of samples withdrawn at different intervals of time. Curves a-d refer to samples withdrawn at 19, 31,36, and 46 h, corresponding to 8,28,73; and 100% crystalline material, respectively. Si/AI = m, l?“/Si = 0.05, OH-/Si = 0.25, Na+/Si = 0.25, H*O/Si = 52, and temperature = 443 K.

2 depicts the XRD patterns of fully crystalline calcined NCL- 1 samples having different WA1 molar ratios (samples A-E). The XRD pattern of assynthesized sample E is also included in Figure 2. The XRD profiles are similar. The scanning electron micrographs of samples B, D, and E (Figure 3) show that the morphology of NCL-1 is either needles-like (samples B and D) or oblonged, probably closely packed bundle of needles (sample E, Al-free Si-NCL1). The shape and size of the NCL-1 crystals probably depend on their WA1 ratios. However, the XRD patterns and their indexing using orthorhomic symmetry suggest that NCL-1 (with varying WA1 ratios) crystallizes as a single phase. In addition, all NCL-1 samples are thermally stable at least up to 1173 K. To determine whether NCL-1 is really a novel zeolite, we made a comparison of the XRD patterns of NCL-1 with some similar zeolites, particularly SSZ-3 1 (synthesized using N, N, N-trimethyl-8-ammonium tricycle [5.2.1.O2s6] decane as an organic additive) by Chevron researchers.” The XRD patterns (line diagrams from 28”ld 8, and I/lo values) of NCL-1, SSZ-31,‘” ZSM-12,” and ZSM-48’* are depicted in Figure 4. To check the possibility of the presence of SSZ-3 1, ZSM-12, or ZSM-48 in NCL-1, either individually or along with some other crystalline phase, we analyzed the data critically. Considering the possibility of a minor shift in 28” values due to different experimental and instrumental conditions for the XRD data (Table I), we analyzed the XRD patterns in terms of the difference between 20” values of the two strongest lines (A 26) in the 26 region of 7-9” (I) and in the region of 20.5-23.5” (II), because all the four zeolites exhibit very strong lines in these two regions. If the position as well as the A26 (I) and A28 (II) Figure

ZEOLITES,

1994, Vol 14, June

327

’

Crystaflization of NCL-I : K.R. Reddy et al.

L~~~~

~

values), we may conclude that these two materials are not the same. However, it is quite likely that they may belong to the same family, like ZSM-5 and ZSM-11 or FAU, EMT, ZSM-20, etc. It may be recalled h e r e that NCL- 1 and SSZ-31 have been synthesized with different organic additives. T h e indexing o f the X-ray p o w d e r diffraction pattern o f the calcined Si-NCL-1 (sample E) is listed in Table 3. T h e observed interplanar d spacings were

(f,)

i /

I

10

1

20

I

30

z#

4

I0

50

Figure 2 X-ray diffraction patterns of NCL-1 samples A-E. Curves a-e represent the calcined samples with Si/AI (molar ratio) of 23, 84, 156, 257, and /> 3000. Curve f refers to as-synthesized sample E.

values o f these strong peaks are similar, then there is a possibility that the materials are similar. T h e values for both the criteria are listed in Table 2. T h e value (in 20 °) o f A20(I) increases in the o r d e r : 0.74 (NCL-1) ~< 0.79 (SSZ-31) < 1.19 (ZSM-48) < 1.40 (ZSM-12) and the value (in 20 °) o f A20(II) also follows a similar trend: 1.30 (NCL-1) = 1.29 (SSZ-31) < 1.75 (ZSM-48) < 2.34 (ZSM-12). Further, the relative intensities o f the major peaks follow a different trend in all the samples. On the basis o f above discussion, the presence o f ZSM-12 and ZSM-48 in NCL-1 can safely be ruled out. However, NCL-1 and SSZ-31 are perhaps related structures. Although the XRD patterns o f NCL-1 and SSZ-31 show quite some similarity, there are some lines that are not c o m m o n (Figure 4 and Table I). For example, the XRD peaks at 20 = 20.53 °, 23.03 °, and 31.09 ° with I/Io = 12, 7, and 8, respectively, o f NCL- 1 are absent in that o f SSZ-31. Similarly, the peaks at 20 = 14.44 °, 20.35 °, and 30.97 ° with I/Io = 14, 13, and 7, respectively, exhibited by SSZ-31 are absent in the pattern o f NCL-1. Apart from these differences, the relative I/Io values o f most o f the lines (both strong and weak) o f NCL-1 and SSZ-31 are quite different. Since the information available on SSZ-31 is limited ~3 (to only d and I/Io

328

ZEOLITES, 1994, Vol 14, J u n e

Figure 3 Scanning electron micrographs of samples with Si/AI (molar ratio) of (a) 84, (b) 257, and (c) /> 3000.

Crysta/lization of NCL-I : K.R. Reddy et al. 100

1 zeolites

Table ZSM- 48

80

Comparison

NCL-1 40

,I

0

10o

6C 40 ZO

I,,, II

,11,

Ill

c 10C

SSZ-31

8C 6C 4C i 2O

C IO0

26 °

-

100

.

8O

~ ~

I/Io

7.45

ZSM-12

.I

I

I I

Ii II ......

I

NCL-1

6O 40

SSZ-31

20 °

6.17

8.19 . . . 10.95 . 12.30 . . 14.55 14.80 . . 16.10 16.33 17.61 . 18.57 . . 20.53 . 21.17 21.58

. . . .

. . .

. .

. . . .

of X-ray powder

diffraction data of four

ZSM-12

I/Io

28°

5.05 6.09

2 27

. .

17 . 13 . . . 3 . 1 . . 2 2 . . 2 1 4 . 30 . . 12 . 52 12

7,39

96

8.18 . . . 10.34 10.81 . 12.20 . . 14.44 14.83 . . 15.99 . 17.51 . 18.45 . . 20.35 . 21.09 21.51

43

52 7 . . 35 . 9 7 5 -

22.38 23.76 . . 24.83 25.18 . 26.18 26.78 27.68 28.18 28.93

.

1 6 2

14 9

1 .

. 5 6

13 64 4

7.42 7.62 7.92 8.82 9.09 9.90 10.34 . 11.90 12.49 13.28 13.89 14.70 15.11 15.56 15.90 . 17.72 17.87 18.39 18.67 18.87 19.94 20.74 21.66

ZSM-48

I/Io . .

. . 27 10 10 35 5 2 2

.

I/Io

20 °

. 3 3 1 2 -

. . 7.48 8.67 . 12.30 12.84 14.51 15.11 15.78

74 29 7 3 -

-

7 20 4 -

2 5 3 14 11 6 100 8

21.09 21.77

82 9

14 67 5 9 7 1 16 20 11 10

22.26 22.84 23.77 24.57 24.78 26.43 27.75

8 100 3 3

3

-

2 1 3

29.06 . . . . 31.36 . . . .

-

5 3 4 5

.

.

2o

0

iI

,

I

I

,

Ill

,,

.l

2'o 2

e°

A comparison of the XRD patterns (line diagrams 4 s i m u l a t e d f r o m 28 a n d I/Io v a l u e s ) o f N C L - 1 , S S Z - 3 1 , Z S M - 1 2 , and Z S M - 4 8 . Figure

corrected with respect to silicon. T h e relative intensities (I/Io) were obtained after smoothening, backg r o u n d correction, and Ket2 stripping. T h e indexing was p e r f o r m e d by t r i a l - a n d - e r r o r m e t h o d using P D P l l software (University o f Trieste, Italy) and an automatic indexing p r o g r a m by Visser 14 (modified, latest version). After indexing the lines, the unit cell p a r a m e t e r s were refined, employing two different software packages (PDP l l and H O C T ) for leastsquare fit. T h e crystal symmetry was e x a m i n e d critically and was f o u n d to fit into o r t h o r h o m b i c symmetry with unit cell dimensions a = 1.185 nm, b = 0.838 nm, and c = 2.862 nm. T h e d values calculated on the basis o f the above unit cell dimensions c o m p a r e reasonably well with those observed experimentally (Table 3). On the o t h e r hand, a similar attempt for indexing all the d values o f SSZ-31 (as available f r o m Ref. 13) could not fit this material into an o r t h o r h o m bic symmetry. F r a m e w o r k i.r. T h e f r a m e w o r k i.r. spectra o f NCL-1 samples are shown in Figure 5. Curves a - d r e f e r to the same set o f

22.47 23.03 . . . . 24.90 -

. . 26.25 26.82 28.01 29.09 -

3

-

29.31 29.79 31.09 31.59

3 7 -

-

-

32.73 32.95

-

8 1 3 4

100 1

14 4 14 6 2 1 3 3

-

29.16 . 29.81 29.97 30.97 . 32.29 . 32.83 .

.

. 3 3 7

.

. 3 .

. 3

.

.

22.32 23.08 23.71 23.97 24.37 25.10 25.54 26.27 26.51 27.86 28.22 28.43 28.97 29.16 . . . . . . . . .

. . . .

. . .

.

. .

.

. .

4

4 4

4 -

14

2 Comparison of A26 values of the most intense XRD lines of the four zeolites

Table

Zeolite NCL-1 SSZ-31 ZSM-48 ZSM-12

Criterion I A20 = 7-9 ° 8.198.18 8.67 8.82 -

7.45 7.39 7.48 7.42

= = = =

0.74 0.79 1.19 1.40

C r i t e r i o n II A26 = 20.5-23.5 ° 22.4722.38 22.84 23.08 -

21.17 21.09 21.09 20.74

ZEOLITES, 1994, Vol 14, June

= = -=

1.30 1.29 1.75 2.34

329

Crystallization of NCL-I: K.R. Reddy et al. Table 3 Indexing of calcined Si-NCL-1 from the X-ray powder diffraction profile a d (expt.) nm

I/Io

hk/

d (calcd.) nm

1.431 1.186 1.079

100 17 13

002 100 101

1.430 1.185 1.094

0.807 0.719 0.608 0.598 0.550 0.542 0.503 0.477 0.432 0.419 0.411 0.395 0.386 0.357 0.339 0.332 0,318 0.307 0,304 0,299 0,287 0.283 0.273 0.272 0.268 0.262 0.259

3 1 2 2 2 1 4 30 12 52 12 52 7 35 9 7 5 3 3 7 8 1 3 4 1 1 1

011 004 104 200 202 014 203 006 213 020 205 120 107 008 216 222 009 109 126 019 218 403 404 028 413 133 0011

0.805 0.715 0.612 0.592 0.547 0.544 0.503 0.477 0.431 0.419 0.411 0.395 0.386 0.358 0.339 0.333 0.318 0.307 0.304 0.297 0.287 0.283 0.274 0.272 0.268 0.262 0.260

~Orthorhombic s y m m e t r y with a = 1.185 nm, b = 0.838 nm,

and c = 2.862 nm

samples as described in Figure 1 (sample E, crystallized at different intervals of time). Based on the literature values,15 the observed absorption bands are tentatively assigned to external asymmetric stretch (1230 cm-X), internal stretch (1098 cm-l), external symmetric stretch (793 cm-l), T - O - T and O - T - O bending vibrations (610 cm-l), double ring (548 cm-l), and T - O bending vibrations of TO4 tetrahedra (468 cm -1) (T = Si or AI), respectively. The intensity of the bands at 610 and 548 cm -I increases with increase in crystallinity of the samples. Figure 6 depicts the i.r. crystallinity (calculated by comparing the ratios of the intensities of i.r. bands of different samples at wavenumbers 548-468 cm -1 with that of the 100% crystalline sample) vs. XRD crystallinity of the same samples. The i.r. crystallinity is found to be always higher than the XRD crystallinity except in the case of the fully crystalline material. Difference in the size of the crystals, particularly in the early stages of crystallization, makes this observation common to the synthesis of many zeolites.

2~

o

:E tY I--

I

I

1OOO

500

WAVENUMBER

Figure 5 Framework i.r. spectra of samples withdrawn at different intervals of time during the crystallization of Si-NCL-1 (sample E). Legend for curves a - d as given in Figure I.

larger are either excluded or adsorbed with difficulty, thus providing insight into the size of the pore openings in the zeolite. The adsorption of five sorbate molecules of varying critical diameters on NCL-1 samples (A-E) with different Si/A1 (gel and product) ratios is listed in Table 4. The amount of uptake of H20 increases with a decrease in Si/AI ratio. Uptake of water increases with A1 content and is indicative of the relative hydrophobicity/hydrophilicity of the zeolite. From the adsorption volume of mesitylene (critical diameter = 0.78 nm) (4.8 wt%) in the NCL-I sample (the corresponding values for ZSM-12 and MOR are 2.6 and 7.2 wt%, respectively) suggest that NCL-1 belongs to the large-pore category with pore openings made up of 12-membered ring tetrahedra. IOC

80

60

=" 20

Adsorption studies One of the distinguishing properties of molecular sieves is their ability to discriminate among molecules of different sizes. Those molecules whose critical diameter is smaller than the dimensions of the pore openings are easily adsorbed, whereas those that are

330

ZEOLITES, 1994, Vol 14, June

(cn~ t )

°a

2'0 XRD

,'o

I

CRYSTALLINITY

do

,oo

(%)

Figure 6 XRD crystallinity (integrated intensities of all peaks) vs. i.r. (the ratio of intensity of 548/468 cm -1 bands) crystallinity of Si-NCL-1 (sample E).

Crystallization of NCL-I: K.R. Reddy et al.

Table 4 Physicochemical properties of NCL-1 m o l e c u l a r sieves

Sample

Si/AI

Si/AI in

in gel

product

n-Hexane

Cyciohexane

o- Xyl ene

Mesitylene

Water

20 80 150 250 =

23 84 166 257 > 3000

6.70 6.84 6.91 6.89 6.96

6.36 6.45 6.43 6.49 6.50

7.24 7.32 7.22 7.25 7.40

4.68 4.72 4.80 4.69 4.77

8.76 5.20 4.62 4.12 3.77

A B C D E

Sorption capacity (wt%) a

a Gravimetric adsorption at 298 K and at P/Po = 0.5

The sorption data also provide information about the secondary cage structure. Almost similar sorption capacities of all the NCL-1 samples (Table 4) for n-hexane and o-xylene indicate that all of the pore volume accessible to n-hexane is also accessible to 0-xylene. C r y s t a l l i z a t i o n kinetics

Effect of Si/Al ratio The effect of Si/AI ratio on the rate of crystallization of NCL-1 is shown in Figure 7. Curves a-e represent the crystallization from reaction mixtures of Si/AI = 20, 80, 150, 250, and infinity, respectively. With an increase in AI content in the reaction mixture, the rate of crystallization decreases. This is a common observation in the synthesis of high-silica molecular sieves. 16--2° The incorporation of aluminum (vis-a-vis Si) in a growing silica network is a difficult process. 16'2° As growth occurs and A13+ ions become incorporated, the charge-compensating cations (Na + or organic cation) must be included in the zeolite. Such a requirement can also slow down the crystallization process compared to the crystallization of Al-free silicate species. However, NCL-1 can be crystallized in a fairly large range of Si/A1 ratio of 20 and above. Attempts to prepare higher Al-containing NCL-1 (Si/AI < 20) were not successful under these conditions. 100

80 >-

I- 60 Z _J .J

~ 4o re

20

%-

Y

50

I

[

1(30 150 200 250 CRYSTALLIZATION TIME (h)

I

300

I

350

Figure 7 The influence of the Si/AI ratio on the crystallization of NCL-1. Curves a - e refer to Si/AI = 20, 80, 150, 250, and o% respectively. R2+/Si = 0.05, O H - / S i = 0.12, Na+/Si = 0.25, H20/Si -- 52, and t e m p e r a t u r e = 443 K.

The scanning electron micrographs of NCL-1 samples shown in Figure 3 (micrographs a, b, and c refer to samples with Si/A1 molar ratio of 84, 257, and infinity [samples B, D, and El, respectively) indicate the absence of amorphous material in the samples. When the amount of aluminum increases in the sample, the morphology of the crystals changes (from oblong-shaped to needle-shaped) although the XRD patterns of these samples are similar. Almost no oblong-shaped crystals are present in the sample with Si/AI molar ratio of 84 (Figure 3a). In the silica polymorph of NCL- 1 (Si-NCL- 1), most of the crystals are oblong-shaped.

Influence of OH-/Si The alkalinity of the reaction mixture is one of the major factors that govern the nucleation as well as the crystallization processes during zeolite synthesis. Hence, the O H - ion concentration (OH-/Si molar ratio) was adjusted by adding an appropriate amount of sulfuric acid and keeping the Na + ion concentration the same (Si/Na + = 4). Figure 8 shows the effect of OH-/Si on the nucleation and crystallization of the silica polymorph of NCL- 1. At lower O H - concentration (OH-/Si = 0.12), both the nucleation and the subsequent crystallization were very slow (curve a), though fully crystalline material was obtained after 76 h. An increase in the OH-/Si ratio from 0.12 to 0.2 enhanced the rate of crystallization considerably, leading to fully crystalline material after 60 h (curve b). At OH-/Si = 0.25, nucleation and crystallization rates were further accelerated (curve c) and fully crystalline material was obtained after 48 h. This is in accordance with the general observation that the rate of crystallization increases with the concentration of the free base (OH-), a mineralizing agent, in the synthesis of high-silica zeolitesY 6'18--2° An increase in the OH-/Si ratio results in the increased solubility of the reactants at higher alkalinity. This leads to enhanced supersaturation of the mother liquor and, hence, a decrease in nucleation and crystallization time. In the synthesis of aluminosilicates (Si/A1 = 20, 80, and 150), however, an increase in the alkali concentration, i.e., OH-/Si ratio of 0.12-0.20 or more, resulted in a mixture of NCL-1 and other crystalline impurities (like ZSM-5). This observation suggests that there exists an optimum range of O H - / SiO2 ( - 0.12) molar ratio for successful synthesis of aluminosilicate NCL-1, as for other high-silica zeolites. ]9

ZEOLITES, 1994, Vol 14, June 331

Crystallization of NCL-I : K.R. Reddy et al.

,o0

'°f f/o/ t

~- 6o

'

~- 401-

///

1

-'°f ,/l/--~ 60

.

~

20

2O O 0 O0

20

40

60

80

Figure 8 The influence of concentration of alkali on the crystallization of Si-NCL-I. Curves a-c refer to O H - / S i = 0.12, 0.2, and 0.25, respectively. Si/AI = % R2+/Si = 0.05, Na+/Si = 0.25, H20/Si = 52, and temperature = 443 K.

The change in the pH of the mother liquor during the course of crystallization is shown in Figure 9 (curves a-c). During the synthesis of high-silica aluminosilicate, EU-1 (ZSM-50), Casci and Lowe 21 showed that a large increase in the pH of the mother liquor during the crystallization suggested the formation of a stable and highly crystalline material• This is explained on the basis that during synthesis of highsilica zeolites the incorporation of SiOz into the framework leads to an increase in the [free base]/ [SiO~] ratio (i.e., [M20-AI203]/[SiO2] ) for the remaining reaction mixture, thus enhancing the pH.21'~2- However, the incorporation of AlOe- does not influence the pH significantly because of the simultaneous incorporation of charge-compensating cation M20 as well as SiO2 (as there are no adjacent A104 tetrahedra). This does not change the [M20--AlzO3]/[SiO2] ratio significantly in the remaining solution. 21"22 However, an insignificant change or decrease in the value of pH indicates either the redissolution of the less metastable phase formed initially or the presence of amorphous material or I

I

1,.G 11"~

11.2q

zb

4b

6b

8b

lOO

CRYSTALLIZATION TIME (h)

Figure 9 The change in pH during the crystallization of NCL-1. Legend for curves a-c, as given in Figure 8.

332

ZEOLITES, 1994, Vol 14, June

Figure 10 The influence of template concentration on the crystallization. Curves a--e refer to R2+/Si ratios of 0.1, 0.05, 0.033, 0.02, and 0.00, respectively. Si/AI = oo, O H - / S i = 0.25, Na+/Si = 0.25, H20/Si = 52, and temperature = 443 K.

both. Depending on the OH-/Si ratio, the initial pH of the reaction mixture varies. At higher ratios, the initial pH is higher. However, the change in pH, i.e., ApH, after the completion of crystallization is almost similar, i.e., 0.5-0.62 for all the three preparations of Si-NCL-1.

Influence of the organic compound (R 2+) Figure 10 depicts the effect of the concentration of organic template (R2+/Si molar ratios) on the nucleation and crystallization of the silica polymorph (curves a-e refer to molar ratios of 0.1, 0.05, 0.033, 0.02, and 0.00, respectively). There is an optimum range of the R2+/Si molar ratio (viz., 0.05-0.033, represented by curves b and c, respectively) below which either the crystallization is incomplete (curve d) or practically absent (curve e) due to an insufficient amount of the template. Above the optimum range, the nucleation becomes slower and the fully crystalline material is obtained after 3 days (curve a), compared to 2 d for the composition represented by curves b and c. Requirement of an optimum concentration of template is a common observation in the synthesis of high-silica zeolites. 16,is. 19 The effect of changing the H20/Si molar ratio in the initial reaction mixture on the crystallization of silica polymorph of NCL-1 is depicted in Figure 11. The rate of crystallization decreases marginally with dilution. A faster rate of crystallization and nucleation, obtained in the concentrated system, can be rationalized if it is considered that with a decreasing amount of water in the reaction mixture the concentration of reactants and, therefore, pH increase, leading to higher supersaturation of the mother liquor. 19 This has the same effect as an increase in the OH-/Si ratio discussed above.

b

11'8

11"00

120

Influence of H20/Si

i

C 12'0

~ 40 60 80 100 CRYSTALLIZATION TIME (h)

100

CRYSTALLIZATION TIME (h)

I

20

Influence of Na +/Si ratio The experiments were done by adding a calculated amount of NaC1 to the gel system while keeping the OH-/Si ratio constant at 0.25. Figure 12 depicts the

Crystallization of NCL-I : K.R. Reddy et al. 100

100

8O 80 >,I-- 6 0

~ 6o F

.J

J

t-.- 4 0 u) >t~ u

4o

20

20

0

0

20

40

60

CRYSTALLIZATION

80

100

TIME (h)

Figure 11 The influence of dilution on the crystallization. Curves a - e refer to H20/Si ratios of 33, 52, and 70, respectively. Si/AI = 0% R2+/Si = 0.05, O H - / S i = 0.25, Na+/Si = 0.25, and temperature = 443 K.

influence of Na + concentration on the nucleation and crystallization of silica polymorph of NCL-1. At Na+/ Si ratios of 0.33 and 0.5, it is observed that much before the complete crystallization of NCL-I dense phases like 0~-quartz or crystabolite start to crystallize in the system at the expense of NCL-1. At Na+/Si = 0.5, the formation of the dense phases starts after I0 h and the maximum crystallinity after 10 h was 20% (curve a). At Na+/Si = 0.33, the formation of dense phases starts after about 40 h (curve b). For the preparation of silica polymorph of NCL-1, the optimum Na + ion concentration in the gel corresponds to a Na+/Si ratio of ~< 0.25, when the OH-/Si ratio is also optimally at 0.25 (curve c).

Influence of crystallization temperature In all the above-mentioned studies, the crystallization was done at 443 K. Figure 13 depicts the influ-

100

8O

6O _1 .J

~ 40 >-

0(;

~20 40 60 CRYSTALLIZATION

80 100 TIME (h)

Figure 12 The influence of Na + concentration on the crystallization of NCL-1. Curves a - c refer to Na+/Si = 0.5, 0.33, and 0.25, respectively. Si/AI = 0% R2+/Si = 0.05, O H - / S i = 0.25, H20/Si = 52, and temperature = 443 K.

0

30 60 90 120 CRYSTALLIZATION TIME ( h )

150

Figure 13 The influence temperature on the crystallization of NCL-I. Curves a - c refer to crystallization at 423, 443, and 473 K, respectively. Si/AI = 0% R2+/Si = 0.05, O H - / S i = 0.12, Na+/Si = 0.25, and H20/Si = 52.

ence of temperature on nucleation and crystallization of silica polymorph of NCL-1 (sample E). Between 423 and 473 K, higher temperature enhances the rate of crystallization of NCL-1, as indeed observed in the synthesis of many high-silica zeolites. At all temperatures, fully crystalline materials were obtained. The apparent activation energies for nucleation and crystallization calculated from the Arrhenius plots are 69 and 51 kJ tool -l, respectively. CONCLUSIONS The following conclusions are drawn from the detailed studies on the kinetics of crystallization of the new, high-silica, large-pore zeolite, NCL-I: 1. NCL-1 can be readily synthesized using hexamethylene bis(triethylammonium bromide) as the organic template (R2+) in a wide range of Si/AI ratio (> 20), including the silica polymorph. A critical comparison of XRD pattern of NCL-1 with those of SSZ-31, ZSM-12, and ZSM-48 zeolites revealed that NCL-1 is a new phase. 2. There exists an optimum range of the R2+/Si ratio (0.05-0.03) and the OH-/Si ratio (0.12-0.25) for obtaining fully crystalline NCL-1 efficiently. 3. An increase in Si/A1 ratio in the reaction mixture and the crystallization temperature enhances the rate of crystallization. 4. As the OH-/Si ratio increases, the rate of nucleation and crystallization of silica polymorph of NCL-1 increases. 5. In the presence of an excess of Na + ions (Na+/Si > 0.25), dense phases like c~-quartz or crystabolite start to crystallize in the system. ACKNOWLEDGEMENTS

This work was partly funded by UNDP. The authors are thankful to Dr. P. Ratnasamy for useful discussions. R.K. thanks Dr. R.A. Mashelkar, Director, NCL

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for support under project KF-89/002 and K.R.R. thanks UGC, New Delhi, for a research fellowship. REFERENCES 1 Kumar, R., Reddy, K.R., Raj, A. and Ratnasamy, P. in Proceedings of the 9th International Zeolite Conference, Montreal, Canada, 1992, Paper A6 (Eds. R. von Ballmoos, J.B. Higgins and M.M.J. Treacy) Butterworth-Heinemann, Boston, 1993, Vol. I, p.189; Kumar, R., Reddy, K.R. and Ratnasamy, P. Ind. Pat. Appl. 766/DEL/91; EP Appl. 92 300 166.3 (1991); US Pat. 5 219 813 (1993) 2 Frilette, V.J., Haag, W.O. and Lago, R.M.J. Catal. 1981, 67, 218 3 Gnep, N.S., Tajeda, J. and Guisnet, M. Bull Soc. Chim. Ft. I 1982, 5 4 Martens J.A., Perez-Pariente, J., Sastre, E., Corma, A. and Jacobs, P.A. Appl. Catal. 1988, 45, 85 5 Kumar, R., Rao, G.N. and Ratnasamy, P. Stud. Surf. ScL CataL 1989, 48B, 1141 6 Dewing, J. J. Mol. Catal. 1986, 27, 25 7 Weitkamp, J., Ernst, S. and Kumar, R. Appl. CataL 1986, 27, 207 8 Martens, J.A., Tielen, M., Jacobs, P.A. and Weitkamp, J.

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Zeolites 1984, 4, 98 9 Kumar, R. and Reddy, K.R., submitted 10 Chevron Research Co. WP WO 90/04567 (1990) 11 Jacobs, P.A. and Martens, J.A. Stud. Surf. Sci. CataL 1987, 33, 317 12 Jacobs, P.A. and Martens, J.A, Stud. Surf. ScL Catal. 1987, 33, 39 13 Szostak, R. Handbook of Molecular Sieves, Van Nostrand Reinhold, New York, 1992, p. 454 14 Visser, J.W.J. Appl. Crystallogr. 1969, 2, 89 15 Flanigen, E.M., Khatami, H. and Szymanski, H.A. Adv. Chem. Ser. 1971, 101, 201 16 Ernst, S., Jacobs, P.A., Martens, J.A. and Weitkamp, J. Zeofites 1987, 7, 458 17 Jacobs, P.A. and Martens, J.A. Stud. Surf. ScL CataL 1987, 33, 58 18 Bhat, R.N. and Kumar, R. J. Chem. Tech. Biotech. 1990, 48, 453 19 Ernst, S., Kumar, R. and Weitkamp, J. in Zeolite Synthesis (Eds. M.L. Occelli and H.E, Robson) ACS Monograph 398, Am. Chem. Soc., Washington, DC, 1989, p. 560 20 Ghamami, L. and Sand, L.B. Zeolites 1983, 3, 155 21 Casci, J.L. and Lowe, B.M. Zeolites 1983, 3, 186 22 Szostak, R. Molecular Sieves, Principles of Synthesis and Identification, Reinhold, New York, 1989, p. 71