Direct synthesis of zeolite Y with large particle size

International Journal of Inorganic Materials 3 (2001) 773–780

Direct synthesis of zeolite Y with large particle size q Samia Ferchiche, Juliusz Warzywoda, Albert Sacco Jr.* Center for Advanced Microgravity Materials Processing, Department of Chemical Engineering, 147 Snell Engineering Center, Northeastern University, Boston, MA 02115 -5000, USA

Abstract Large zeolite Y particles have been synthesized directly from gels (4.76Na 2 O: 1.0Al 2 O 3 : xSiO 2 : 454H 2 O: 5TEA, x55.25–8.75) aged at |293 K before static heating at 368 K. Product purity, size, and Si /Al ratio of zeolite Y depended on the gel composition and aging time. Two-day aging of gels (x55.25 and 7.0) resulted in nearly pure zeolite Y (,5–10 wt.% zeolite Pt ) with Si /Al51.7 and 1.9, and maximum size of 95 and 120 mm, respectively. Sieving of products grown from gels with x58.75 aged for 7 days resulted in nearly pure zeolite Y in the |50–125 mm particle size range. The Si /Al ratio of all zeolite Y grown from gels with x58.75 did not change with aging time (2–20 days) and was 2.1.  2001 Elsevier Science Ltd. All rights reserved. Keywords: A. zeolites; A. inorganic materials; A. microporous materials; B. hydrothermal synthesis; B. chemical synthesis; B. crystal growth

1. Introduction Zeolite Y with the faujasite (FAU) framework and Si /Al.1.5 [1] is widely used as a component of fluid catalytic cracking (FCC) catalysts [2,3]. Commercial zeolite Y-based FCC catalyst particles (microspheres up to |150–200 mm in diameter, containing up to 40% zeolite) are obtained by spray drying the mixtures made of zeolite powder dispersed in a suitable inorganic oxide matrix [2,4,5]. Other available techniques involve partial, in situ crystallization of zeolite Y aggregates within pre-formed clay-derived particles, which are hydrothermally treated in the absence of an external aqueous phase [6], in water [7], or in aqueous sodium hydroxide solutions [8,9], containing metakaolin powder [10] or seeds [11], if necessary. These techniques result in catalyst particles containing up to 75% of zeolite Y. Literature on the synthesis of large, binderfree (i.e. containing less than |10% of binder) aggregate particles or single crystals of zeolite Y with size suitable for fluidized-bed applications is limited. McDaniel et al. [12,13] synthesized zeolite Y aggregates in a particle size q This paper was originally presented at the Second International Conference on Inorganic Materials, 13–16 September 2000, University of California, Santa Barbara, CA, USA. *Corresponding author. Tel.: 11-617-373-7910; fax: 11-617-3732209. E-mail address: [email protected] (A. Sacco Jr.).

range of 50–200 mm. Spray-dried and calcined microspheres, made of aqueous slurry of dehydrated kaolin clay and sodium silicate solution, were hydrothermally treated (with aging) in sodium hydroxide solution. Vesely [14] crystallized faujasite within pre-formed 0.02–1600 mm silica spheres, hydrothermally treated in aqueous solutions containing sodium aluminate, sodium silicate, and sodium hydroxide. Sanders and Laurent [15] obtained 65–80 mm aggregate particles of zeolite Y by hydrothermally treating pre-formed seed-containing aluminosilicate gel particles in water. By careful selection of the composition, reactants, and method of preparation, Khatami et al. [16] synthesized single crystals of zeolite Y directly from aluminosilicate gels. These crystals had sizes up to 40 mm and were contaminated by large (70–80%) quantities of zeolites R and S. Methods for the synthesis of large faujasite-type zeolite X (Si /Al#1.5) crystals with size up to 500 mm have been developed using triethanolamine (TEA) as an aluminum-complexing agent [17]. Zeolite Y crystals with sizes up to 40 mm have been grown using the method utilizing TEA but no experimental details were given [18]. Recently, zeolite X crystals with Si /Al¯1.4 and diameters up to 340 mm have been synthesized from TEA-containing gels with a substantially higher Si /Al gel ratio than those previously reported [19]. In this method, the silica is introduced in the form of an aqueous suspension. This makes it possible to add larger amounts of silica in order to prepare relatively silica-rich synthesis mixtures, which are

1466-6049 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S1466-6049( 01 )00046-0

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necessary to synthesize zeolite Y. This method was utilized to synthesize high-purity faujasite-type materials with Si / Al.1.4, and to examine the feasibility of synthesizing zeolite Y with particle size suitable for fluidized-bed applications directly from gels in the absence of preformed precursor particles.

2. Experimental Zeolite Y was synthesized from gels with the composition 4.76Na 2 O: 1.0Al 2 O 3 : xSiO 2 : 454H 2 O: 5TEA, where x55.25, 7.0, and 8.75, by mixing aqueous silica slurries and triethanolamine (TEA)-containing aqueous sodium aluminate solutions. The procedure used was that developed to prepare large zeolite X crystals [19]. The silica slurries were prepared in 30-ml polyethylene (HDPE) bottles by suspending 1.45 g (x58.75) of Cab-OSil M-5 fumed silica (Cabot) in 10 g of deionized water (resistivity.18 MV cm). The sodium aluminate ‘stock’ solution was prepared in a 250-ml HDPE bottle by dissolving 6.468 g of sodium aluminate powder (technical, EM Science) in hot solution of 11.508 g of sodium hydroxide (pellets, 971%, Acros Organics) in 170.016 g of deionized water. After cooling, the solution was filtered through a 0.2-mm filter membrane (Gelman) before addition of 15.186 g of TEA (991%, Acros Organics) per 100 g of the filtered solution. The silica slurries and TEAcontaining sodium aluminate stock solution were left at room temperature (|293 K) for 1 day before 15.468 g of the stock solution was combined with the silica slurries in 30-ml HDPE bottles. These gels were sealed, hand-shaken for 15–20 s, and statically aged at room temperature before being heated statically in a convection oven at 368 K. The crystallizations took typically 2–3 weeks. The products were filtered, washed with deionized water, and dried for 24 h at 353 K before analysis. Zeolite Y in the |75–115 and |105–125 mm (projected area particle diameter [20]) range was obtained by wet sieving of as-synthesized products (after drying) using 200, 150, and 100 mesh sieves (US Sieve Series). The morphology of zeolite Y and the particle size distributions (PSDs) of the sieved products were determined using a Jeol JSM-840 scanning electron microscope (SEM). Samples for the SEM analysis were coated with Au–Pd film in an evaporator. The PSDs of the sieved samples were developed by visually (SEM) analyzing |200 particles. The estimated relative error in size determination using SEM did not exceed 65%. The PSDs of as-synthesized products (i.e. before sieving) were measured on an API Aerosizer LD (TSI, Inc., Particle Instruments /Amherst) equipped with an API Aero-Disperser dry powder dispersion system. For these measurements, the density of zeolite Y was assumed to be 1920 kg m 23 [21]. The PSD measurements performed on the Aerosizer were reproducible (relative error in size determination less than 65%). The comparison of the

Aerosizer (geometric diameter(equivalent volume spherical particle diameter) and the SEM (projected area particle diameter) PSDs of the sieved products showed that the Aerosizer undersized the zeolite Y particles (e.g. mode of the PSDs was undersized by |20–25%). The SEM images showed that the nearly spherical particles in the sieved products had a very rough surface (Fig. 4). Such particles can be expected to accelerate to higher velocities compared with smooth spherical particles of equal volume, which are used to calibrate API Aerosizers [22]. This results in shorter times-of-flight through the measurement zone, which are interpreted as smaller particles. The Aerosizer PSDs were, therefore, used only for comparative purposes to show the relative change of the size of the zeolite Y products grown under different conditions of gel composition and aging time. As-synthesized and sieved products were examined using X-ray powder diffraction (XRD) for phase identification, determination of the zeolite Y lattice parameter, and estimation of the impurity level of zeolite Pt . XRD data were collected with a step increment of 0.028 2u and a count time of 5 s / step on a Bruker D5005 u :2u Bragg-Brentano diffractometer using Cu K a radiation. To minimize preferred orientation, specimens for the XRD analysis were ground to a fine powder. Prior to grinding, the crystals were equilibrated for more than 72 h in a 75% RH constant-humidity chamber. Refinement of the lattice parameter of zeolite Y was accomplished by the least squares method using a lanthanum hexaboride (NIST 660) external reference standard to correct the measured peak positions. A second method to determine the lattice parameter of the zeolite Y samples was the standardized ASTM method for the faujasite-type zeolites [23]. The values of the lattice parameter of the same sample determined using these two methods were identical within the error limits of each method (,0.002 nm). The content of zeolite Y in as-synthesized and sieved products was estimated visually (SEM). The Si /Al ratio in zeolite Y was estimated as an average of the values calculated from the correlations between the unit cell parameter and the Si /Al ratio in the synthetic Na-faujasites [24,25]. For comparative purposes the measurement of the Si /Al ratio in the products with less than 5–10 wt.% of zeolite Pt was performed using atomic absorption spectrophotometry (AAS) — (Perkin-Elmer 3100). A small quantity of solids (|0.04 g) was dissolved in a mixture of HF and HCl. Acid digestion of solids took several hours, after which the solutions were diluted to the required concentration ranges for analysis. The relative error in the Si /Al ratio determination by AAS did not exceed 68%.

3. Results and discussion The results of these zeolite Y syntheses are shown in Table 1. All gels investigated (x55.25, 7.0, and 8.75) which were used in syntheses without aging produced

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Table 1 The results of synthesis of large zeolite Y particles (gel composition: 4.76Na 2 O: 1.0Al 2 O 3 : xSiO 2 : 454H 2 O: 5TEA, aging temperature |293 K, crystallization temperature 368 K) Gel SiO 2 content, x (mol)

Aging time (days)

Product

Impurity

Amount of zeolite Y in the product (wt.%)

Max. size of zeolite Y (mm)a

Zeolites Pt , R, S, FERe Zeolite Pt Zeolite Pt Zeolites Pt , R, S, FERe Zeolite Pt Zeolite Pt Zeolites Pt , R, S, FERe Zeolite Pt Zeolite Pt Zeolite Pt Zeolite Pt Zeolite Pt Zeolite Pt Zeolite Pt

,5–10

N /Af

.90–95 .90–95 ,5–10

95 b 65 b N /Af

.90–95 .90–95 ,5–10

120 85 N /Af

¯50 ¯75–80 ¯75–80 .90–95 .90–95 .90–95 .90–95

150 130 125 115 125 110 80

5.25

0

As synthesized

5.25 5.25 7.0

2 7 0

As synthesized As synthesized As synthesized

7.0 7.0 8.75

2 7 0

As synthesized As synthesized As synthesized

8.75 8.75 8.75 8.75 8.75 8.75 8.75

2 5 7 7 7 10 20

As synthesized As synthesized As synthesized Sieved c Sieved d As synthesized As synthesized

a

Projected area particle diameter (SEM analysis). Equivalent volume spherical particle diameter (SEM analysis). c Sieved through 150 and 200 mesh sieves. d Sieved through 100 and 150 mesh sieves. e Ferrierite. f Not measured. b

samples containing less than 5–10 wt.% of the faujasitetype material. Products obtained in these cases consisted mostly of zeolites Pt , R, S, and Ferrierite. Aging of the same gels at |293 K before syntheses at 368 K resulted in the products containing a mixture of zeolites Pt and Y (with much increased content of the latter). The amount of zeolite Pt in the form of spherical polycrystalline particles (diameters generally larger than 100–110 mm) depended on the aging time and on the gel composition used. Gels with x55.25 and x57.0, aged for at least 2 days, resulted in the products nearly free of zeolite Pt (,5–10 wt.%). Gels with x58.75, aged for 2 days, resulted in larger amounts of zeolite Pt (¯50 wt.%). Aging of gels with x58.75 for longer times (.2 days) resulted in improved purity of the zeolite Y products. When these gels were aged for 5–7 days the resulting products contained |20–25 wt.% of zeolite Pt . If aging lasted at least 10 days, less than 5–10 wt.% of zeolite Pt formed. The observation that aging results in products with reduced impurity levels is consistent with other observations showing aging a beneficial step for the formation of a high purity zeolite Y [26]. Fig. 1 illustrates the morphology of zeolite Y obtained from different gel compositions at different aging times. Intergrown octahedrons, characteristic of the faujasite morphology, were observed in all products grown from aged gels. As shown in the figure, degree of crystal intergrowth depended strongly on the gel composition used in syntheses. Aging time employed did not significantly affect degree of crystal intergrowth in zeolite Y grown

from the same gel composition. Gels with x55.25 resulted in zeolite Y crystals with much less intergrowth than gels with the higher silica content. Gels with x58.75 produced nearly spherical particles of zeolite Y. The SEM analysis revealed (Table 1) that the maximum size of zeolite Y particles grown from gels aged for the same amount of time increased with the increasing content of silica in the starting gel. Also (Table 1), the maximum size of the zeolite Y particles grown from gels with the same batch composition decreased with the increasing aging time. The Aerosizer PSDs of the products obtained after 2-day aging are shown in Fig. 2a. As shown in the figure, the PSDs were bimodal with peaks at smaller (|5–10 mm) and larger (|70–100 mm) geometric diameters. The peaks centered near 5–10 mm are due to the presence of small zeolite Y and zeolite Pt particles, the presence of very small amounts of an amorphous material [19], and / or due to the break up of the larger particles (especially of zeolite Pt ) during pneumatic transport in the Aerosizer. The peaks centered near 70–100 mm are due to large particles of zeolites Y and Pt . In agreement with the SEM observations (Table 1), the maximum size of the product analyzed by the Aerosizer increased with the increasing amount of silica in the starting batch (at constant aging time). In addition, the peaks at |70–100 mm (i.e. the population of large particles) shifted to larger sizes with increasing amount of silica in the starting gel. The Aerosizer PSDs of the products grown from gels with x58.75 aged for different times are illustrated in Fig. 2b.

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Fig. 1. SEM images of zeolite Y particles grown from gels 4.76Na 2 O: 1.0Al 2 O 3 : xSiO 2 : 454H 2 O: 5TEA (aging temperature |293 K, crystallization temperature 368 K) with (a) x55.25 (2-day aging), (b) x55.25 (7-day aging), (c) x57.0 (2-day aging), (d) x57.0 (7-day aging), (e) x58.75 (2-day aging), (f) x58.75 (10-day aging).

S. Ferchiche et al. / International Journal of Inorganic Materials 3 (2001) 773 – 780

Fig. 2. Particle size distributions (Aerosizer) of products grown from gels 4.76Na 2 O: 1.0Al 2 O 3 : xSiO 2 : 454H 2 O: 5TEA (aging temperature |293 K, crystallization temperature 368 K) with (a) different compositions (x) aged for the same amount of time, (b) the same composition aged for different amounts of time.

As shown in the figure, the maximum and the mean size of products (in populations of large particles) grown from the same gel composition decreased with the increasing aging time, consistent with the SEM observations shown in Table 1. This agrees qualitatively with the results of aging of the faujasite-type zeolite X-producing reaction mixtures [27], and with the theoretical considerations of aging [28]. Table 2 shows the results of the lattice parameter determination of the zeolite Y products. As shown in the table, the lattice parameter decreased with the increasing silica content (increasing Si /Al ratio) in the starting gel.

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This indicates less incorporation of Al atoms in the faujasite framework, because the Al–O bond length in tetrahedral groups is larger than the Si–O bond length [29]. Presented in Table 2 are also the Si /Al ratios in the zeolite Y products estimated from their lattice parameters [24,25]. These results (Si /Al.1.5) indicate that zeolite Y was synthesized in all cases. Aging time had no effect on the zeolite Y unit cell parameter (Si /Al ratio), as illustrated in Table 2 for products grown from gels with x58.75 aged for 2–20 days. The Si /Al ratios in the zeolite Y products determined from the XRD analysis (Table 2) are plotted in Fig. 3 as a function of the Si /Al ratios in the starting gels. The Si /Al ratio in zeolite Y obtained in this study grown from gels with x510.5 (a 0 5 2.472 nm, Si /Al¯2.3) is also plotted in Fig. 3. As shown in the figure, in the composition range investigated a linear relationship was observed between the Si /Al ratio in the starting gel and in the zeolite Y product. The efficiency for crystallization of siliceous faujasite material observed in the present study was lower than that observed in other zeolite Y syntheses [30]. This could be due to a higher alkalinity of starting gels used in the present study, which favors the Al incorporation in the zeolite framework [31,32]. The data in Fig. 3 indicate the possibility of increasing the Si /Al ratio above 2.1 in zeolite Y obtained from batches with x . 8.75. The difficulties encountered in the preparation of homogeneous gels with x510.5 (due to their increasing viscosity) made formation of the high quality (purity and particle size) zeolite Y products with Si /Al¯2.3 difficult. Even if more successful, such preparations would probably require significantly

Table 2 The lattice parameter and the Si /Al ratio of the zeolite Y products (gel composition: 4.76Na 2 O: 1.0Al 2 O 3 : xSiO 2 : 454H 2 O: 5TEA, aging temperature |293 K, crystallization temperature 368 K) Gel SiO 2 content, x (mol)

Aging time (days)

Lattice parameter, a 0 (nm)a

Si /Al a

Si /Al b

5.25 7.0 8.75 8.75 8.75 8.75 8.75

2 2 2 5 7 10 20

2.482 2.478 2.475 2.475 2.476 c 2.475 2.475

1.760.1 1.960.1 2.160.1 2.160.1 2.160.1 c 2.160.1 2.160.1

1.660.1 1.860.1 N /Ad N /Ad 2.260.2 c N /Ad N /Ad

a

From the XRD analysis, absolute error ,0.002 nm. From the AAS analysis. c Sieved through 150 and 200 mesh sieves. d Not measured. b

Fig. 3. The Si /Al ratio in the zeolite Y product as a function of the Si /Al ratio in the starting gel (formulation 4.76Na 2 O: 1.0Al 2 O 3 : 5.25– 10.5SiO 2 : 454H 2 O: 5TEA, aging temperature |293 K, crystallization temperature 368 K).

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more siliceous gels to synthesize zeolite Y with the Si /Al ratio substantially higher than 2.3. This conjecture is supported by data on the synthesis of siliceous zeolite Y, which indicate that the efficiency for crystallization of the siliceous material decreases dramatically with the increasing silica content in the starting gel [30]. Instead of the

higher silica content in starting gels, the use of templating / structuring agents [32–35] or reduction of the pH of the synthesis batch [31] could be explored to grow more siliceous large zeolite Y. The SEM observations of products grown from gels with x58.75 aged for 7 days showed that the majority of

Fig. 4. SEM images of zeolite Y products grown from gels 4.76Na 2 O: 1.0Al 2 O 3 : 8.75SiO 2 : 454H 2 O: 5TEA aged for 7 days and sieved through (a) 150 and 200 mesh sieves, (b) 100 and 150 mesh sieves (aging temperature |293 K, crystallization temperature 368 K).

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the zeolite Y particles are in the |70–125 mm diameter range. Most of the particles of zeolite Pt were either smaller (|5–60 mm) or larger (|130–200 mm) than zeolite Y particles. These products were sieved to obtain fractions in the 74–105 and 105–149 mm nominal size range. The SEM images of the sieved products and the PSDs based on such images are shown in Figs. 4 and 5, respectively. As illustrated in Fig. 4, the sieved products consisted almost exclusively of zeolite Y. Calculations based on the analysis of the SEM images indicated that the sieved products contained ,5–10 wt.% of zeolite Pt (Table 1). As expected, the majority of the zeolite Y particles in the sieved products was in the |75–125 mm range (Fig. 5). The number of particles smaller than |75 mm was low. These (mostly zeolite Y) particles were located on the surface of the large zeolite Y particles and were not removed by sieving probably due to the electrostatic force between smaller and larger particles. The particles larger than |125 mm were generally due to zeolite Pt . The XRD analysis of the products grown from gels with x58.75 aged for 7 days was performed before and after sieving. As-synthesized products (before sieving) exhibited peaks characteristic of both zeolites Y and Pt . The peaks due to zeolite Pt virtually disappeared after sieving, indicating that it effectively removed the impurity from the zeolite Y product. This confirmed the very small amounts of zeolite Pt in the sieved products calculated from the SEM images. Comparison of the XRD patterns of the

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sieved products grown from gels with x58.75 aged for 7 days, and of as-synthesized products grown from gels with x55.25 and 7.0, aged for 2 days, confirmed the visual estimation that no more than 5–10 wt.% of zeolite Pt contaminated these products. The results of the AAS analysis performed on the nearly pure (,5–10 wt.% of zeolite Pt ) zeolite Y products are shown in Table 2. Comparison of the Si /Al ratio determined using the AAS and XRD methods showed good agreement. These results confirm that large particles of zeolite Y have been grown directly from TEA-containing aluminosilicate gels in the absence of pre-formed precursor particles.

4. Conclusions Large particles of zeolite Y with sizes up to 150 mm were synthesized directly from TEA-containing aluminosilicate gels (4.76Na 2 O: 1.0Al 2 O 3 : xSiO 2 : 454H 2 O: 5TEA) in the absence of pre-formed precursor particles. Products crystallized from gels with x55.25, 7.0, and 8.75 without the room temperature aging step contained only small amounts (,5–10 wt.%) of the faujasitetype material. Gel aging resulted in the products containing much higher amounts (up to 90–95 wt.%) of zeolite Y particles in the form of intergrown crystals. Purity of as-synthesized product as well as the size, Si /Al ratio and degree of crystal intergrowth in the zeolite Y particles depended on the starting gel composition and the aging time employed. Purity of zeolite Y product increased with the increasing gel aging time. The Si /Al ratio of zeolite Y increased linearly from 1.7 to 2.1 with the increasing gel silica content but was found to be independent on the gel aging time. Degree of crystal intergrowth in zeolite Y particles increased with the increasing gel silica content. The mean and maximum size of large zeolite Y particles decreased with the increasing aging time (at constant composition), and increased with the increasing content of silica in the starting gel (at constant aging time). Zeolite Y (Si /Al52.1) particles in the fluidized-bed size range (diameters¯50–125 mm) were obtained by sieving as synthesized products grown from gels with x58.75 aged for 7 days.

Acknowledgements The authors acknowledge the financial support of NASA.

Fig. 5. Particle size distributions (SEM) of zeolite Y products grown from gels 4.76Na 2 O: 1.0Al 2 O 3 : 8.75SiO 2 : 454H 2 O: 5TEA aged for 7 days and sieved though (a) 150 and 200 mesh sieves, (b) 100 and 150 mesh sieves (aging temperature |293 K, crystallization temperature 368 K).

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