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使用碳气凝胶 做模板合成ZSM5和Y 介孔Template synthesis and characterization of mesoporous zeolites


Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 75–80

Template synthesis and characterization of mesoporous zeolites
Yousheng Tao a , H. Kanoh a,b , Y. Hanzawa c

, K. Kaneko a,b,?
a b

Material Science, Graduate School of Science and Technology, 1-33 Yayoi, Inage, Chiba 263-8522, Japan Center for Frontier Electronics and Photonics, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan c Faculty of Engineering, Chiba Institute of Technology, Chiba 275-0023, Japan Available online 28 May 2004

Abstract Well-crystalline ZSM-5 and zeolite Y having uniform mesopores were synthesized with template route method using carbon aerogels of different mesoporosities. These mesoporous zeolites were characterized with X-ray diffraction, FT-IR spectroscopy, ?eld emission scanning electron microscopy, and N2 adsorption. ZSM-5 having mesopore volume of 0.2–1 cm3 g?1 was obtained. Mesoporous ZSM-5 synthesized by heating at 423 K for 96 h was well-crystalline, which had mesopores of 11 nm in width. Mesoporous zeolite Y was well-crystalline, of which mesopore volume and width were 1.37 cm3 g?1 and 10 nm, respectively. ? 2004 Elsevier B.V. All rights reserved.
Keywords: Mesopore; Zeolite; Synthesis; Characterization; Carbon aerogel

1. Introduction With unique chemical properties and ability to couple with the size- or shape-selectivity of guest molecules, crystalline microporous solid zeolite molecular sieves have been widely studied in catalysis as well as in separation and puri?cation ?elds [1–5]. Although the well-described micropores of zeolites have excellent potential for chemical functions, it is a problem that their intricate pore and channel systems in the molecular size ranging from 0.3 to ?1.5 nm impose diffusion limitations on the reaction rate as well as a high backpressure on ?ow systems [6,7]. The controlled reduction of the crystal sizes of zeolites has been attempted as a means to solve the problem on the micropore diffusion [8–12]. However, none of these attempts have produced an easy means of controlling the crystal size. Moreover, as those synthesized zeolite crystals with sizes from 8 nm to 0.8 m, ?ltration of the small zeolite crystals is dif?cult due to their colloidal properties. For active catalysts, particularly for heterogeneous catalysts, fast mass transfer of the reactants and products to and from the catalytic sites is required; mesoporous zeolite materials, upgrading the performances of mesoporous molecular sieves and zeolites, are thus of great interest. Several dual templatCorresponding author. Tel.: +81-43-290-2779; fax: +81-43-290-2788. E-mail address: kaneko@pchem2.s.chiba-u.ac.jp (K. Kaneko). 0927-7757/$ – see front matter ? 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.04.013
?

ing methods have been proposed recently for the preparation of such solids. They include macrotemplating with carbon black particles for preparing ZSM-5 of a wide pore size distribution of 10–100 nm and with monodispersed polystyrene (PS) spheres for macroporous silicates (250 nm average diameter) [13,14], and nanocasting with colloid-imprinted carbons as templates for preparing nanosized ZSM-5 crystals with some interparticle mesopores [15]. Another approache proposed by Kaliaguine et al. show the preparation of a new type of material with semi-crystalline zeolitic mesopore walls based on a templated solid-state secondary crystallization of zeolites, starting from amorphous SBA-15 and the synthesis of zeolite coated mesoporous aluminosilicates [16]. More recently, there has been active development in the template synthesis for the preparation of new mesostructured materials [17–22]. Ryoo et al. prepared ordered mesoporous carbon, which is an inverse replica templated from mesoporous silica by a nanocasting route [17–20], and the synthesized ordered mesoporous carbon such as CMK-3 has been successfully employed as further template to achieve the reversible replication of ordered mesoporous silica [23,24]. Our studies show that a novel, simple, and reproducible method for donating mesopore channels to zeolite is of template route using carbon aerogels as template. Carbon aerogels are known to have uniform mesopores and is obtainable in a monolithic form [25–29]. Kaneko et al.

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and Tamon et al. demonstrated that pore-controlled carbon aerogels could be tailor-made by design and showed how to evaluate the porosity [26–29]. Recently, we reported the preparation of ZSM-5 monolith of uniform mesoporous channels by the templating method using a carbon aerogel of uniform mesopores in short communication [30]. This article describes mesoporous zeolites using the template of carbon aerogels of different mesopore structures.

2. Carbon aerogels 2.1. Preparation of carbon aerogels The carbon aerogel templates were prepared from the pryolysis of resorcinol-formaldehyde aerogels, which were synthesized from a sol–gel polymerization process of resorcinol and formaldehyde according to the method proposed by Pekala et al. [25–29]. The starting solutions were prepared from resorcinol (C6 H4 (OH)2 , min.99.0%, Wako), formaldehyde solution (HCHO, 36.0–38.0%, Wako), sodium carbonate (Na2 CO3 , min.99.5%, Wako), and ion-exchanged water. The glass vials containing the reaction solution were sealed and cured for 1 day at room temperature, and afterwards placed in an oven for 1 day at 323 K and successively 3 days at 363 K. Then the resorcinol–formaldehyde gels were washed with 0.125% tri?uoroacetic acid (CF3 COOH, min.98.0%, Wako). Multiple exchanges with fresh acetone were carried out to remove the residual water from resorcinol-formaldehyde gels. The carbon aerogels were obtained by the supercritical drying of resorcinol–formaldehyde gels with CO2 , of which the critical temperature and pressure are 304.1 K and 7.4 MPa, subsequently followed by pyrolysis at 1323 K in a nitrogen atmosphere. The carbon aerogel porosity was controlled by varying the reactant concentration of the starting solution. The carbon aerogel templates are denoted as CA1 and CA2 . Here, the subscripts of 1 and 2 indicate the molar ratios of resorcinol to formaldehyde in the starting solution. 2.2. Porosity of carbon aerogels Low-temperature nitrogen adsorption measurement is a common means for characterization of nanostructured materials. Adsorption/desorption isotherms of N2 at 77 K on carbon aerogels have a clear hysteresis loop of type H1, as shown in Fig. 1, which is caused by the agglomerate structure of uniform spherical particles. As the adsorption and desorption branches of the loop are almost vertical for CA1 , the mesopores are quite uniform. The subtracting pore effect (SPE) analysis [31,32] and the Dollimore–Heal (DH) method [33,34] of the N2 adsorption isotherms showed that the surface area of CA1 was 1330 m2 g?1 , mesopore volume was 3.15 cm3 g?1 (the micropore volume was 0.19 m2 g?1 ), and mesopore size was 23 nm (micropore size was 0.64 nm). The surface area of CA2 was 1034 m2 g?1 , mesopore volume 3. Mesoporous zeolite synthesis 3.1. Mesoporous zeolite ZSM-5 synthesis The agents used for synthetic reaction were tetraethyl orthosilicate (TEOS, min.95.0%, Wako), aluminum isopropoxide (Al(iPrO)3 , min.95.0%, Wako), tetrapropylammonium bromide (TPABr, min.98.0%, Wako), sodium hydroxide pellets (NaOH, min.96.0%, Wako), and ionexchanged water. The reaction mixture with composition (molar basis) 10 Na2 O:200 SiO2 :Al2 O3 :20 TPABr:16,000 H2 O was prepared by combining the mixture of TEOS and Al(iPrO)3 with each clear solution of TPABr, H2 O, and NaOH. After 0.5 h of
Fig. 1. Adsorption isotherms of nitrogen on carbon aerogels at 77 K. ( ): CA1 , ( ): CA2 .

Fig. 2. Mesopore size distributions of carbon aerogels. (a) CA1 and (b) CA2.

was 1.87 cm3 g?1 (the micropore volume was 0.15 cm3 g?1 ), and mesopore size was 15 nm (micropore size was 0.60 nm). The mesopore size distributions of carbon aerogels derived from the adsorption branch of isotherms using the Dollimore and Heal method are shown in Fig. 2. Accordingly, carbon aerogels can supply nanopore structures suitable for the template synthesis. Also, the mesopore size can be controlled by changing the molar ratio of reactants.

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magnetically stirring, the mixture was transferred to a cell containing carbon aeogels, which were evacuated there at 383 K and 1 mPa for 2 h prior to use. In this way, the carbon aerogels were immersed in the mixture and the zeolite precursors were introduced into the mesopores of carbon aerogels. Then they were put in a PTFE cell, introduced into a stainless steel autoclave, and heated in an oven at 423 K for 72 h mainly. After the hydrothermal synthesis, the autoclave was cooled to room temperature and the monolithic carbon aerogels containing zeolites were separated. The carbon aerogels as well as any organics were removed by pyrolysis in an oxygen ?ow diluted with argon at 823 K for 18 h with an electronic muf?e furnace after washing and drying. Finally, the mesoporous ZSM-5 monolith was obtained. The synthesized mesoporous ZSM-5 is denoted using the sort of carbon aerogel and crystalline state. As mesoporous ZSM-5 samples using CA1 were heated in the oven for 72 and 96 h, both samples were distinguished using [L] and [H] such as meso-ZSM5-CA1 L. Here meso-ZSM5-CA1 L and meso-ZSM5-CA1 H were obtained by heating at 423 K for 72 and 96 h, respectively. The template of meso-ZSM5-CA2 is CA2 . The zeolite ZSM-5 was synthesized for comparison. 3.2. Mesoporous zeolite Y synthesis The agents used for synthetic reaction were aluminum isopropoxide (Al(iPrO)3 , min.95.0%, Wako), silica sol (SiO2 , 30%, Aldrich), tetramethylammonium hydroxide pentahydrate (TMAOH·E5H2 O, ICN), sodium hydroxide pellets (NaOH, min.96.0%, Wako), and ion-exchanged water. The synthesis of zeolite Y (NaY) used as the standard in this study was performed from a clear aqueous solution with a mole composition of 1.00 Al2 O3 :4.35 SiO2 :2.39 (TMA)2 O:0.065 Na2 O:248.00 H2 O at 373 K. For the synthesis of mesoporous zeolite Y, the reaction mixture with the same composition as that for the synthesis of zeolite Y was transferred to a reaction cell containing carbon aerogel template, which was evacuated at 383 K and 1 mPa for 2 h prior to use; the zeolite precursor was introduced into the mesopores of carbon aerogels. Then the reaction cell was transferred into a stainless steel autoclave and heated in an oven at 373 K for different time up to 216 h. After the hydrothermal synthesis the monolithic carbon aerogels containing zeolites were separated. The carbon aerogel template was removed by ?ring in an oxygen ?ow at 803 K for 18 h after washing and drying. Finally, the mesoporous zeolite Y was obtained. As only CA1 was used as the template, the mesoporous zeolite Y is denoted as meso-NaY-CA1 .

Fig. 3. Adsorption isotherms of nitrogen on ZSM-5 and mesoporous ZSM-5 at 77 K. ( ): meso-ZSM5-CA2 , ( ): meso-ZSM5-CA1 [L], ( ): meso-ZSM5-CA1 [H], ( ): ZSM-5.

4. Mesoporous zeolites 4.1. Mesoporous ZSM-5 The N2 adsorption isotherm of ZSM-5, which is shown in Fig. 3 as a standard, basically belongs to IUPAC type I. The

predominant adsorption ?nishes below P/P0 = 0.02, which is characteristic of uniform microporous solids. Fig. 3 also shows the N2 adsorption isotherms of mesoporous ZSM-5 at 77 K. All N2 adsorption isotherms of meso-ZSM5-CA2 , meso-ZSM5-CA1 L, and meso-ZSM5-CA1 H have a steep uptake below P/P0 = 0.02 and hysteresis loop from P/P0 = 0.6 to about P/P0 = 1. Hence the co-existence of micropores and mesopores is suggested by the N2 adsorption isotherm. The mesopore size distributions (PSDs) of meso-ZSM5-CA1 L and meso-ZSM5-CA1 H determined from Dollimore–Heal (DH) method were very narrow with the maximum at ca. 8 nm and ca. 11 nm, and a width at half height of only ca. 4 nm and ca. 3 nm, respectively. Only the DH mesopore size distribution of meso-ZSM5-CA2 showed the mesopore widths were widely distributed in the range of 10–50 nm. The additional evidence is obtained by the following scanning electron microscopic (SEM) images in Fig. 4, although these images are diffused due to the insulative nature of zeolites. The Saito–Foley method, which is useful for the analysis of micropores, was used to analyze the micropore size distibution (PSD) of ZSM-5 and mesoporous ZSM-5. The micropores of both the samples are 0.51 nm in width. The micropore size of 0.51 nm is comparable to the value known for the crystallographic aperture of the 10-membered ring zeolite ZSM-5 [1,35]. The pore structural parameters are given in Table 1. The micropore volumes of meso-ZSM5-CA1 L and meso-ZSM5-CA2 are only 0.09 cm3 g?1 , being much smaller than that of ZSM-5. The mesopore volumes of meso-ZSM5-CA1 L and meso-ZSM5-CA2 are considerably great, which is still smaller than the mesopore volume of carbon aerogels. The micropore volumes of ZSM-5 and meso-ZSM5-CA1 H almost agree with each other. The mesopore volume of meso-ZSM5-CA1 H was 0.2 cm3 g?1 . The micropore size and volume of zeolite originate from the intrinsic crystalline structure. Since the meso-ZSM5-CA1 L and meso-ZSM5-CA2 have the same crystallinity as described below, they should have the same micropore volume.

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meso-ZSM5-CA1 . On the other hand, the SEM image of meso-ZSM5-CA2 exhibits nonuniform mesopores of 10–50 nm in width. The crystal structures of ZSM-5 and mesoporous ZSM-5 were examined using X-ray diffraction and FT-IR spectroscopy. The results were in agreement with our previous ?ndings [30]. The X-ray diffraction pattern of ZSM-5 basically coincides with that of mesoporous ZSM-5 [36–38]. However, the peak broadening was different from one sample to another with mesoporous ZSM-5 having slight line broadening of the diffraction peaks. The FT-IR spectra of the framework absorption region (1500–300 cm?1 ) of mesoporous ZSM-5, in which absorption bands were observed at 1224 (shoulder), 1150–1050 (strong), 795 (weak), 550 (weak to medium), and 455 (strong) cm?1 , agrees with the absorption peaks of characteristic MFI-type zeolites as reported previously [39,40]. The optical density ratio of the 550 and 450 cm?1 bands in the IR spectrum, which is proposed by Jacobs [39], was taken as a measure of crystallinity. Those for meso-ZSM5-CA1 L and meso-ZSM5-CA2 are 0.57 and those for meso-ZSM5-CA2 H and ZSM-5 are 0.80 and 0.81, respectively. As the optical density ratio of all pure pentasil samples in the literature is 0.8 [39,40], the IR examination shows that meso-ZSM5-CA1 H has a highly crystalline frame structure, whereas meso-ZSM5-CA1 L and meso-ZSM5-CA2 are less-crystalline ZSM-5. 4.2. Mesoporous NaY
Fig. 4. Field emission scanning electron micrographs of mesoporous ZSM-5. (a) Meso-ZSM5-CA1 and (b) meso-ZSM5-CA2 .

Actually, meso-ZSM5-CA1 H and ZSM5 have almost the same crystallinity, and thereby they have roughly the same micropore volume. The mesopore width and volume of the mesoporous zeolites are closely related to the frame structure of the carbon aerogel template. Carbon aerogel CA2 has much larger pore-wall volume and less mesopore volume than CA1 does; the carbon aerogel CA2 template can donate much larger mesopore volume to the mesoporous ZSM-5. The SEM images of meso-ZSM5-CA2 and meso-ZSM5CA1 L are shown in Fig. 4. Relatively uniform mesopores of ca. 10 nm in width can be seen in the SEM image of
Table 1 Pore structural parameters of the samples Samples SBET (cm2 g?1 ) Microporosity Volume Meso-ZSM5-CA1 [L] Meso-ZSM5-CA2 Meso-ZSM5-CA1 [H] ZSM-5 Meso-NaY-CA1 NaY 646 CA1 CA2 227 241 385 395 581 646 1330 1034 0.09 0.09 0.15 0.17 0.21 0.29 0.19 0.15 (cm3 g?1 )

Fig. 5 shows X-ray diffraction patterns of NaY and mesoporous NaY prepared using the template of carbon aerogel CA1 by ripening at 373 K for 60 and 216 h. The ripening for 60 h gives a diffuse X-ray diffraction pattern, while a highly crystalline pattern of FAU zeolite is obtained by ripening for 216 h for both of NaY without the template and mesoporous NaY. A further evidence for the FAU structure of the mesoporous zeolites is provided by IR spectroscopic examinations, as shown in Fig. 6 The IR spectra of samples have the bands at approximately 570 and 470 cm?1 . The band at 470 cm?1 is assigned to the structure-insensitive T–O bending modes for tetrahedral TO4 units (T = Si or Al), while the band at 570 cm?1 is associated with the FAU structure [9,41]. The spectrum of the sample heated for 60 h has very

Mesoporosity Diameter (nm) 0.51 0.51 0.51 0.51 0.75 0.75 0.64 0.60 Volume 0.34 0.98 0.2 – 1.37 – 3.15 1.87 (cm3 g?1 ) Diameter (nm) 8 10 11 – 10 – 23 15 ±2 ? 50 ±2 ±2 ±5 ±5

IR crystallinity (%)

57 57 80 81 – – – –

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Fig. 5. X-ray powder diffraction patterns of NaY and meso-NaY-CA1 . (a) NaY crystallized for 216 h, (b) meso-NaY-CA1 crystallized for 216 h, and (c) meso-NaY-CA1 crystallized for 60 h.

Fig. 7. Adsorption isotherms of nitrogen at 77 K on mesoporous NaY, NaY, and carbon aerogels. (a) CA1 , (b) meso-NaY-CA1 , and (c) NaY.

diffuse bands near 470 and However those peaks become sharp after 216 h of heating. Hence, mesoporous NaY prepared by ripening for 216 h is denoted meso-NaY-CA1 , of which pore structure will be described below. The nitrogen adsorption/desorption isotherms of mesoNaY-CA1 , NaY, and CA1 are shown in Fig. 7 The isotherm of NaY is of representative type I, indicating the presence of uniform micropores. The adsorption isortherm of meso-NaY-CA1 exhibits type IV behavior, showing a hysteresis loop above P/P0 = 0.6, which arises from the presence of mesopores. The pore size distribution of NaY and meso-NaY-CA1 was determined by use of Saito–Foly and Dollimore–Heal methods. The pore structural parameters derived from the nitrogen adsorption/desorption isotherms are also summarized in Table 1. The pore volume of NaY is attributed only to micropores and the PSD peak is at 0.75 nm. Meso-NaY-CA1 has both of micropores and mesopores, and their PSD peaks are at 0.75 and 10 nm. The micropore size of meso-NaY-CA1 is the

570 cm?1 .

same as that of NaY, being close to the free aperture of the main channel of zeolite Y [1,35]. The mesopore PSD of meso-NaY-CA1 is narrow, having a peak at 10 nm. Although meso-NaY-CA1 has the initial adsorption uptake same as NaY at lower relative pressure, the initial adsorption amount of meso-NaY-CA1 is a little smaller than that of NaY. It results that the micropore volume of meso-NaY-CA1 is 28% smaller than that of NaY. This suggests a partial collapse of micropores or blocking of micropores by carbon. The examination of TGA of meso-NaY-CA1 indicates the absence of carbon after removal of the template. Consequently, the partial collapse or defects should be the above reason. This point must be improved in the future study in the case of mesoporous NaY synthesis.

5. Conclusion The mesoporous zeolites have been synthesized with the template method using carbon aerogels of different mesoporosities. By varying the mesopore structures of carbon aerogel template and crystallization conditions of zeolites, mesoporisity-controlled mesoporous zeolites have been obtained. It is expected that these mesoporous zeolites can further extend the applications of zeolites, since they have remarkable bene?ts for the surface function. For example, micropores in zeolites provide size- or shape-selectivity for guest molecules, while mesopores provide easier access to the active sites in micropores. The mesoporous diffusion pathways of the mesoporous zeolites are expected to improve mass transfer and catalytic reaction ef?ciency and minimize channel blocking. Moreover, these mesoporous zeolites should be applied to catalysis for larger molecules which can not be freely accessed in micropores of zeolites. Detailed understanding of what extent the accessibility and diffusion are enhanced by donation of mesopores in the mesoporous zeolites must be studied in future.

Fig. 6. IR spectra of NaY and mesoporous NaY. (a) meso-naY-CA1 ripened for 60 h, (b) meso-NaY-CA1 ripened for 216 h, and (c) NaY.

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Acknowledgements We thank Dr. N. Uekawa for his assistance in X-ray diffraction measurements. This work was supported by a Grant-in-Aid for Scienti?c Research (S) of Japan Science Promotion Society. Y.T. is grateful to the Ministry of Education, Culture, Sports, Science and Technology for supporting Ph.D. research program.

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