Science in China Series B: Chemistry © 2009

www.scichina.com chem.scichina.com www.springerlink.com

SCIENCE IN CHINA PRESS

Springer

Facile synthesis of small crystal ZSM-5 zeolite by acid-catalyzed hydrolysis of tetraethylorthosilicate WU YaJing, REN XiaoQian, LU YouDong & WANG Jun† State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China

Small crystal zeolites ZSM-5 with sizes of 150－300 nm were synthesized using the colloidal silicate precursors as the silica source created by the acid-catalyzed hydrolysis of tetraethylorthosilicate with tetrapropylammonium bromide as the structure-directing agent within a short crystallization time of 20－35 h. The precursors and final products were detected by XRD, SEM, ICP and DLS. hydrolysis, hydrothermal crystal growth, small crystal ZSM-5, tetraethylorthosilicate

The synthesis of small crystal zeolites attracts great attention because of the valuable applications in catalysis, separation, and optics[1,2]. For the synthesis of small or nano-sized MFI-structured ZSM-5 zeolites, the clear solution synthesis[3], two-stage synthesis[4], solid-liquid separation[5], seeding[1], template-assisted method[6], and fluoride method[7] have been developed. However, the above methods always lead to pure silicalite or ZSM-5 with high SiO2/Al2O3 ratios with long crystallization time and expensive structure-directing agents (SDA)[1]. Tetraethylorthosilicate (TEOS) is widely used as the silica source for zeolite synthesis. Conventionally, silica precursors are obtained through the base-catalyzed hydrolysis of TEOS before the onset of the crystallization of the synthesis gel[8]. Nevertheless, TEOS can be catalytically hydrolyzed in an acid medium with high efficiency. In fact, the reactivity of TEOS hydrolysis and resulting silica precursors in an acid solution are significantly different from those in a base solution[9]. However, the involvement of acid-catalyzed hydrolysis of TEOS has never attracted much attention in the synthesis of zeolitic materials. Very recently, we reported the rapid synthesis of MCM-22 zeolite via the acidcatalyzed hydrolysis of TEOS[10]. In the current work, based on the acid-catalyzed hydrolysis of TEOS, the stick-shaped small crystal ZSM-5 with the low SiO2/Al2O3 ratio was synthesized in a short crystalliza-

tion time and with inexpensive tetrapropylammonium bromide (TPABr) as SDA.

1 Experimental 10.0 g TEOS was mixed with 32.5 g deionized water in a beaker. Sulfuric acid was slowly dropped into the above mixture under vigorous stirring (pH=1.0) at 290 K for 20 h. Then 1.2 g TPABr and 0.75 g Al2(SO4)3·18H2O were added to the above mixture. After 10 min of stirring, the hydro-gel with pH of 10 was obtained by adding 0.8 g NaOH. The final molar composition of the synthesis gel was 8.8Na2O︰40SiO2︰Al2O3︰1600H2O. Finally, the synthesis gel with or without 0.3 g seed (commercial ZSM-5 crystals, supplied by the Zeolite Factory of Nankai University, China) was transferred to a Teflon-lined stainless steel autoclave, followed by the hydrothermal crystallization for 5－35 h at 453 K. The products were separated by centrifugation, washed with deionized water and air-dried at 273 K. The obtained as-synthesized samples were calcined at 823 K for 5 h in Received November 10, 2008; accepted December 10, 2008 doi: 10.1007/s11426-009-0090-3 † Corresponding author (email: [email protected]) Supported by the National Natural Science Foundation of China (Grant No. 20776069), Key Natural Science Foundation for Universities of Jiangsu Province (Grant No. 06KJA53012), and Program for Changjiang Scholars and Innovative Research Team in University (Grant No. PCSIRT 0732)

Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 549-551

air stream to give the as-calcined ZSM-5. The products were detected by X-ray powder diffractometer (XRD, Bruker D8 ADVANCE, CuKα radiation, λ=0.1542 nm, 40 kV, 20 mA), scanning electron microscopy (SEM, FEI QUANTA 200) and inductively coupled plasmaatomic emission spectrometry (ICP-AES, Perkin Elmer Optima 2000). The silica precursors from TEOS hydrolysis were measured by dynamic light scattering (DLS, Malven ZetaSizer-3000, scattering angle 90°, He-Ne laser with 3 mW output power at 633 nm wavelength).

2 Results and discussion Figure 1 shows XRD patterns for the as-synthesized powders obtained by sulfuric acid-catalyzed hydrolysis of TEOS with/without adding the ZSM-5 crystal seed. For the seeded synthesis in Figure 1(a), the diffraction peaks for ZSM-5 were observed after only 5 h of crystallization, and the full crystallization of ZSM-5 was found after 20 h of crystallization. Figure 1(b) demonstrates that ZSM-5 was fully crystallized within 35 h without adding any seeds. It is well known that seeding can shorten the induction time and accelerate the crystallization[11]. Consistent with previous observations, the crystallization time was shortened from 35 h to 20 h by seeding in this synthesis.

Figure 1 XRD patterns of as-synthesized samples synthesized by sulfuric acid-catalyzed hydrolysis of TEOS at different crystallization time. (a) With adding the ZSM-5 seed, (b) without adding the ZSM-5 seed.

Figure 2(a)－(d) illustrates SEM images of the Z S M - 5 s a mp l e s s y n t h e s i z e d b y t h e s u l f u r i c acid-catalyzed hydrolysis of TEOS. The particle size from the seeded synthesis was about 5－7 μm (Figure 2(a)), which was composed of closely packed 150－200 550

nm small-sized ZSM-5 crystals (Figure 2(b)). Similarly, the synthesis without seeding resulted in small-sized ZSM-5 crystals of about 300 nm (Figure 2(d)), which assembled into larger spherical particles of around 10 μm (Figure 2(c)). If we prolonged the crystallization time, the size of formed small crystal ZSM-5 kept constant. Considering that the alumina species can be well dissolved in a strong acid media, another synthesis was carried out by co-hydrolyzing alumina source with TEOS in sulfuric acid solution for 20 h. The result is shown in Figure 2(e). The stick-shaped crystals with small sizes of around 100 nm were clearly observed, and they aggregated into micron-sized particles (SiO2/Al2O3 =27 in the solid product). For comparison, ZSM-5 was also synthesized through the sodium hydroxide-cata-

Figure 2 SEM images of the ZSM-5 samples synthesized by the sulfuric acid-catalyzed and sodium hydroxide base-catalyzed hydrolysis of TEOS. (a) and (b) Sulfuric acid-catalyzed hydrolysis of TEOS with adding the ZSM-5 seed and crystallization for 24 h; (c) and (d) sulfuric acid-catalyzed hydrolysis of TEOS without adding ZSM-5 seed and with crystallization for 35 h; (e) the co-hydrolyzing alumina source with TEOS in sulfuric acid solution with adding the ZSM-5 seed and with crystallization for 24 h; (f) sodium hydroxide base-catalyzed hydrolysis of TEOS with adding the ZSM-5 seed and crystallization for 24 h.

WU YaJing et al. Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 549-551

lyzed hydrolysis of TEOS with other conditions the same as those in Figure 2(a). The tablet-like ZSM-5 crystals with the size of about 2 μm were observed (Figure 2(f)). Even if Na2SO4 was added into this sodium hydroxide-catalyzed system, the similar micron-sized ZSM-5 crystals were produced. Previous reports have shown a clear difference between the acid- and base-catalyzed hydrolyses of TEOS[12,13], i.e., at pH < 3, hydrolysis and condensation lead to small, and cage-like units, and at pH > 3.5, larger particles are formed. Figure 3 gives the DLS results for the two clear solution samples prepared from the sulfuric acid- and sodium hydroxide-catalyzed hydrolysis of TEOS. For acid-catalyzed sample, the created amorphous silicate colloidal particles showed a particle size range of 100－200 nm, with a mean diameter of 157.5 nm, while for base-catalyzed one, the colloidal particles grew to 500－800 nm, with a mean diameter of 457.2 nm. Obviously, the sizes of precursor units for base-catalyzed system are much larger than the size of the acid-catalyzed one. Therefore, the formation of small-sized crystals of ZSM-5 in this work may be relative to the smaller size of amorphous silicate colloidal particles created by acid-catalyzed hydrolysis of TEOS. This supposition is consistent with the observation by Mintova et al.[14], i.e., the sizes of the crystalline MFI nanoparticles corresponded to the sizes of the amorphous silica particles formed in the precursor mixtures. Nevertheless, the formation of the final zeolite may depend on not only the precursor particle size but also the complicated structure of polymerized silica in the hy1 2 3

4

5

6

7

8

Tosheva L, Valtchev V P. Nanozeolites: Synthesis, crystallization mechanism, and applications. Chem Mater, 2005, 17(10): 2494－2513 Davis M E. Ordered porous materials for emerging applications. Nature, 2002, 417: 813－821 Reding G, Mäurer T, Kraushaar-Czarnetzki B. Comparing synthesis routes to nano-crystalline zeolite ZSM-5. Micropor Mesopor Mater, 2003, 57(1): 83－92 Li Z J, Li S, Luo H M, Yan Y S. Effects of crystallinity in spin-on pure-silica-zeolite MFI low-dielectric-constant films. Adv Funct Mater, 2004, 14(10): 1019－1024 Naik S P, Chen J C, Chiang A S T. Synthesis of silicalite nanocrystals via the steaming of surfactant protected precursors. Micropor Mesopor Mater, 2002, 54(3): 293－303 Zhu G S, Qiu S L, Gao F F, Li D S, Li Y F, Wang R W, Gao B, Li B S, Guo Y G, Xu R R, Liu Z, Terasaki O. Template-assisted self-assembly of macro-micro bifunctional porous materials. J Mater Chem, 2001, 11(6): 1687－1693 Shiralkar V P, Joshi P N, Eapen M J, Rao B S. Synthesis of ZSM-5 with variable crystallite size and its influence on physicochemical properties. Zeolites, 1991(5), 11: 511－516 Ban T, Mitaku H, Suzuki C, Matsuba J, Ohya Y, Takahashi Y. Crys-

Figure 3 DLS data of colloidal samples prepared by catalytical hydrolysis of TEOS in acid and base media. The DFA (distribution function analysis) is displayed as scattering intensity per unweighted particle size class.

drolysis procedure. Therefore, the formation mechanism of small crystals of ZSM-5 based on the acid-catalyzed hydrolysis of TEOS remains to be further investigated.

3 Conclusions In this work, we report a novel method to synthesize stick-shaped small-sized crystals of ZSM-5 zeolite with the TEOS as silica source, aluminum sulfate as alumina source and the inexpensive TPABr as SDA. This can be achieved simply by altering the way for catalytically hydrolyzing TEOS from the traditional strong base to the present strong acid medium. Using this novel route, the synthesis of other zeolitic materials is in progress in this laboratory.

9

10

11

12

13

14

tallization and crystal morphology of silicalite-1 prepared from silica gel using different amines as a base. J Cryst Growth, 2005, 274(3-4): 594－602 Tan B, Rankin S E. Study of the effects of progressive changes in alkoxysilane structure on sol-gel reactivity. J Phys Chem B, 2006, 110(45): 22353－22364 Wu Y J, Ren X Q, Lu Y D, Wang J. Rapid synthesis of zeolite MCM-22 by acid-catalyzed hydrolysis of tetraethylorthosilicate. Mater Lett, 2008, 62(2): 317－319 Gonthier S, Thompson R W. Effects of seeding on zeolite crystallisation, and the growth behavior of seeds. Stud Surf Sci Catal, 1994, 85: 43－73 Sanchez J, McCormick A. Kinetic and thermodynamic study of the hydrolysis of silicon alkoxides in acidic alcohol solutions. J Phys Chem, 1992, 96(22): 8973－8979 Šefčík J, McCormick A V. Kinetic and thermodynamic issues in the early stages of sol-gel processes using silicon alkoxides. Catal Today, 1997, 35(3): 205－223 Mintova S, Valtchev V, Bein T. Formation of colloidal molecular sieves: Influence of silica precursor. Colloids and Surfaces A: Physicochem Eng Aspects, 2003, 217(1-3): 153－157

WU YaJing et al. Sci China Ser B-Chem | May 2009 | vol. 52 | no. 5 | 549-551

551