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Preparation of ZSM-5 thin film on cordierite honeycomb by solid state in situ crystallization
Microporous and Mesoporous Materials 46 (2001) 249±255
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Preparation of ZSM-5 thin ®lm on cordierite honeycomb by solid state in situ crystallization C.D. Madhusoodana 1, R.N. Das, Y. Kameshima, A. Yasumori, K. Okada * Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, Japan Received 11 November 2000; received in revised form 11 April 2001; accepted 11 April 2001
Abstract Thin ®lms of ZSM-5 zeolite were prepared on porous cordierite honeycombs by a solid state in situ crystallization method and were characterized by XRD, FTIR, SEM and nitrogen gas adsorption. The zeolite ®lms were formed by coating the zeolite precursor sol on the cordierite honeycomb substrate and gelling it in the surface pores of the substrate. This gel was then converted into ZSM-5 by heating the coated substrate in an autoclave without adding solution. The amount of coating was increased by forming an interfacial layer of microporous silica on the cordierite substrate by acid leaching. The maximum speci®c surface area of the honeycomb coated with zeolite ®lm was 249 m2 /g. Recoating the substrate with zeolite ®lm was eective not only in increasing the amount of zeolite ®lm and its surface area but also in reducing the macropores in the honeycomb surface. The zeolite ®lm was found to retain its porous properties up to the calcining temperature of 900°C. The zeolite ®lm is considered to be formed by a vapor transport mechanism initiated by the water included in the gel. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Zeolite ®lm; ZSM-5; Solid state in situ crystallization; Cordierite; Honeycomb substrate
1. Introduction The ZSM-5 families of high silica zeolites have high shape selectivity for molecular sorption and diusion [1] and are thus widely used as HC adsorbents, deNOx catalysts (with Cu/Ce/Pt exchanged) etc. [2,3]. They are generally synthesized under hydrothermal conditions, crystallizing from a solution containing SiO2 , Al2 O3 and Na2 O
*
Corresponding author. E-mail addresses: [email protected] (K. Okada), [email protected] (C.D. Madhusoodana). 1 Present address: Ceramic Technological Institute, BHEL, EPD, P.B. No. 1245, Bangalore 560 012, India.
sources optionally with templates or seeds. Various raw materials have been used for synthesizing ZSM-5 and other high silica zeolites [4]. Recently, there has been a trend to prepare zeolites in the solid state. ZSM-5 was synthesized from dried amorphous aluminosilicate gels in vapors of ethylenediamine, triethylamine, and water [5±7]. Based on in situ microscopic observation of the formation of MFI zeolites in the presence of steam by Sano et al. [8], a dry gel could be converted into zeolite in the dry state [9]. In this method, dry gels are placed in an autoclave where they come into contact with steam to form zeolite. In another vapor phase transport method, dry powder containing an aluminosilicate precursor and NH4 F was converted into ZSM-5 [10]. In a solid state
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 3 0 4 - 3
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C.D. Madhusoodana et al. / Microporous and Mesoporous Materials 46 (2001) 249±255
method, direct crystallization of aluminosilicate gels to silicalite and ZSM-5 occurred by heating in glass ampoules at 130±150°C for 18±24 h [11,12]. Many similar methods have been developed to prepare thin zeolite ®lms. The dierent methods for forming thin ®lms and membranes including dry methods have been reviewed by Mizukami [13]. The dry method involves coating the precursor solution on the substrate and crystallizing it in the presence of vapors of templating reagents and water in a special autoclave [14±16]. Most of the above coatings or ®lms were deposited on porous alumina supports to form membranes. For the automotive application as hydrocarbon (HC) adsorbers for cold start emission control and for deNOx catalysts, ZSM-5 should preferably be coated on honeycomb monoliths [2,3]. High catalytic surface area and good mechanical strength to withstand road vibrations are important criteria for the coated honeycombs, which also have good HC trapping eciency, good catalytic activity and high thermal durability of the zeolite layer. They are prepared either by washcoating the zeolite particles on to the substrate [17,18] or by in situ crystallization through wet hydrothermal methods [19,20]. The adhesion of the coating may be stronger in the wet hydrothermal synthesis but the mechanical strength of the honeycomb may be less due to chemical attack by the strongly alkaline solution. There are some methods in which honeycomb monoliths are prepared by extruding zeolite-containing or zeolite-forming materials [21]. In a different approach, the zeolite ®lm is formed on a sintered clay honeycomb by leaching the free silica phase and subsequent crystallizing it on the surface [22,23]. In this wet hydrothermal process, prolonged (up to 28 day) treatment was necessary for the formation of the zeolite ®lm and the surface area obtained was about 110 m2 /g. In another method, cordierite honeycombs were coated with zeolite forming solution then crystallized in an autoclave containing water [24]. The objective of the present study is to prepare a continuous thin ®lm of zeolite on the cordierite honeycomb substrate without immersing it in the precursor sol or using additional water in the autoclave during zeolitization. This is in order to
avoid weakening the substrate and to obtain a good zeolite±substrate interface by in situ zeolitization by simple autoclaving. Although a similar method has already been reported by Milestone et al. [25], they did not succeed in forming a continuous ®lm on their substrate. We therefore used dierent type of substrates and also dierent starting materials for the preparation of the precursor gel to obtain a continuous thin ®lm of zeolite. The concept of the present method is to crystallize the zeolite using only the water present in the optimized precursor gel, exploiting the thin layer of microporous silica formed on the honeycombs to provide a strong interface. We refer to this method as solid state in situ crystallization (SSIC) and report here the successful preparation of ZSM-5 continuous thin ®lms on cordierite honeycomb substrates, which can be a candidate HC adsorber for controlling emissions from automobiles. 2. Experimental work 2.1. Preparation of zeolite ®lm The substrates used were cordierite (2MgO 2Al2 O3 5SiO2 ) honeycombs having a cell density of 400 cells per square inch (cpsi) and wall thickness of 0.17 mm, prepared at the Ceramic Technological Institute, BHEL, Banglore, India. The open porosity was 35% and more than 90% of the pores were between 0.5 and 5 lm radius as measured by mercury porosimeter. Small (1 1 1 cm3 ) samples were cut from the honeycomb monolith and used as substrates. These honeycomb samples were treated with 20% H2 SO4 at 90°C for 3 and 6 h to form a microporous silica layer on the honeycomb surfaces. Three types of substrate samples (unleached, 3 h leached and 6 h leached) were thus obtained. All these samples were outgased in the vacuum at 100°C for one day before coating. The zeolite precursor solution was prepared from tetraethylorthosilicate (Wako chemicals, Japan), tetrapropylammonium hydroxide (10%, Wako chemicals, Japan) and aluminum tri-secbutoxide (Soekawa chemicals, Japan). They were well mixed by stirring for 2 h at room temperature
C.D. Madhusoodana et al. / Microporous and Mesoporous Materials 46 (2001) 249±255
to produce a composition with a molar ratio of SiO2 : Al2 O3 : TPAOH 100:1:10. The substrate samples were dipped in the precursor solution for about 30 min. The samples were then removed from the solution and the excess solution on the surface was removed by careful air blowing. These samples were then heated in an airtight Te¯on vessel at 80°C for 2 h to convert the sol absorbed on the substrates to a gel. The dipping and gelling steps were repeated three times to produce a continuous coating layer on the substrates. The gelled-coated substrates were then aged overnight. Crystallization was carried out in a Te¯on lined autoclave (25 ml) at 150°C for 24 h without addition of water. After autoclaving, the samples were washed with distilled water in an ultrasonic bath for 30 min to remove any loosely adhering coating especially in the corners of the honeycomb. After drying at 100°C for 1 day, the samples were calcined at 600°C for 2 h to remove the template ions.
251
electron microscope (SEM; S-2050, Hitachi, Japan). 3. Results and discussion 3.1. Synthesis of ZSM-5 thin ®lm Fig. 1 shows XRD patterns of coated and uncoated cordierite, con®rming the formation of ZSM-5 on the cordierite surface. The intensity ratio of the strongest peaks of ZSM-5 and cordierite was used as a relative measure of the amount of ZSM-5. It can be seen from Fig. 1 and Table 1 that the leached samples showed more
2.2. Characterization and testing The formation of zeolite was con®rmed by powder X-ray diraction (XRD; Geiger¯ex, Rigaku, Japan) using powder samples obtained by grinding the coated honeycomb. The porous properties were determined from N2 gas adsorption and desorption isotherms measured at 77 K using an Autosorb-I instrument (Quanta Chrome, USA). The speci®c surface area (SSA) was calculated by the BET method, the pore size distribution (PSD) was calculated by the BJH method and the pore volume was obtained from the maximum adsorption at a relative pressure of 0.999. The microstructure of the zeolite ®lm on the honeycomb substrate was observed using a scanning
Fig. 1. XRD patterns of cordierite honeycombs before coating (a) and after coating on (b) unleached sample, (c) 3 h leached sample and (d) 6 h leached sample (Z: ZSM-5, C: cordierite).
Table 1 Properties of zeolite ®lm formed on unleached and leached honeycombs Treatment of honeycombs Unleached Leached for 3 h Leached for 6 h
% Weight gain
XRD peak ratio
Speci®c surface area (m2 /g)
Pore volume (ml/g)
After coating
After recoating
After coating
After recoating
Before
After coating
After recoating
Before
After coating
After recoating
9.3 12.6 16.9
± 16.9 21.2
0.1 0.26 1.02
± 0.53 1.14
0.5 81 227
49 110 239
± 120 249
0.008 0.055 0.136
0.078 0.113 0.212
± 0.147 0.238
252
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zeolite formation. The microporous silica layer formed in the leached cordierite honeycomb samples enhanced the coating of the sol and the formation of ZSM-5. The crystallization of ZSM-5 is thought to occur by a vapor transport mechanism [6]. It is however, initiated by the presence of water molecules and hydroxides in the gel which react with the silicates and assemble on the TPA
templates. Compared with conventional wet hydrothermal synthesis, the amounts of zeolite precursors and steam in the present SSIC method are much lower. The size of ZSM-5 crystals as shown in Fig. 2 was 2±3 lm, which is smaller than formed by the conventional method and also by a similar SSIC method [25]. This may be related to the less water content of the present method,
Fig. 2. SEM photographs of zeolite ®lms on honeycombs: (a) cross section of honeycomb with zeolite ®lm, (b) zeolite ®lm and substrate interface, (c) zeolite crystals on the honeycomb substrate, (d) honeycomb surface before coating, (e) honeycomb surface after coating and (f) honeycomb surface after recoating.
C.D. Madhusoodana et al. / Microporous and Mesoporous Materials 46 (2001) 249±255
though many other factors may also contribute to this eect. It was also observed that the volume ratio of the sample and autoclave was very crucial to the crystallization of the zeolite. When a large autoclave (100 ml) was used, ZSM-5 did not form, indicating that the optimum autogenious pressure of vapor formed by the water in the gel in¯uences the kinetics of zeolitization. ZSM-5 ®lm can thus be formed in a simple autoclaving step by the proper selection of the volume of the autoclave and the optimum water content in the precursor gel by the SSIC method. 3.2. Eect of substrate properties Table 1 shows the properties of the zeolite ®lms formed on unleached and leached (3 and 6 h in 20% H2 SO4 at 90°C) cordierite honeycombs. The weight increase of the three substrates after coating was 9.3±16.9 wt.%, with a further increase of the 6 h leached substrate to 21.2 wt.% after coating. Since the weight gain increased with longer substrate leaching times, it was concluded that the acid treatment is eective in increasing the amount of zeolite coating. The increase in surface area and pore volume after coating is attributed to the formation of zeolite ®lm. The amount of zeolite formed increased with higher surface area of the substrates (more leached), as evidenced by the XRD intensity ratios of zeolite and cordierite zeolite cordierite (I 501 =I 100 ). An increase in the ZSM-5 peak heights is observed in the leached samples. The XRD results are compatible with the weight gains of the samples (Table 1). The surface areas of the coated ZSM-5 ®lms were evaluated from the gains of weight and surface area of the samples by coating. The surface area value corresponded to zeolite calculated ranged from 300 to 360 m2 /g and were in good agreement with that of ZSM-5 powder (350 m2 /g) prepared separately. This indicates that the weight gain by the coating is fully attributed to the formation of coated ZSM-5 zeolite grains. Fig. 3 shows the PSDs of the three dierent zeolite coated honeycombs. An apparent dierence in the PSD between the unleached and leached samples was observed. The PSD of the unleached
253
Fig. 3. PSDs of zeolite ®lms on unleached and leached honeycombs.
sample showed a peak at 6 nm in pore radius, while the PSD of the leached samples showed micropores smaller than 1 nm and mesopores of 2 nm radius. The micropores can be attributed with the pores in the leached substrates and formed zeolites and the mesopores with the spaces between the zeolite crystals. There are also some larger meso and macropores, which may be due to agglomeration of the zeolite crystals. Fig. 4 shows the PSDs of the 3 h leached honeycombs before and after coating. Before coating, the samples contained only micropores of about 1 nm radius corresponding to the microporous silica phase in the surface of honeycomb produced by selective leaching. After coating, the micropores
Fig. 4. Comparison of PSDs of zeolite ®lms on 3 h leached sample; before coating, after coating and after recoating.
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C.D. Madhusoodana et al. / Microporous and Mesoporous Materials 46 (2001) 249±255
were still seen, together with mesopores between the zeolite crystals in the coating. The same trend was also observed in the 6 h leached samples. The presence of the interface layer of microporous silica facilitates the formation of zeolite ®lm, both increasing its quantity due to its high surface area and microporosity as with a conventional wash coating. Since the microporous silica in the surface of the leached substrates is considered to react partially during the zeolitization, the resulting zeolite crystals are expected to form a strong interface with the cordierite as a result of their participation in the zeolitization. 3.3. Eect of recoating The coated samples (leached substrates with zeolite ®lms) were recoated by the same procedure using a fresh precursor solution. The aim was to increase the amount and continuity of the zeolite ®lm and to reduce the macroporosity, which may enhance the mechanical strength of the honeycomb. The porous properties before and after recoating the leached samples are listed in Table 1. In the both leached samples (3 and 6 h leached) recoating increased the SSA and pore volume. An almost linear increase in the SSA and pore volume is observed between the uncoated, coated and recoated samples. This is due to the linear increase in the amount of zeolite formation (Table 1) which is also evident from the increase in the XRD peak ratio. Fig. 4 shows the PSDs of the 3 h leached honeycombs samples before coating, after coating and after recoating. The same trend was observed in the 6 h leached samples. In the both samples, the recoating resulted in the reduction of the larger pores in the honeycomb, in which the macropores were largely ®lled by the newly formed zeolite. The recoated samples contained mainly micropores smaller than 1 nm and mesopores of 2 nm radius. Thus, a more uniform and continuous ®lm of zeolite was obtained by repeated coating. 3.4. Thermal stability of zeolite ®lm The thermal stability of the 3 h leached sample was tested by heating at 800°C for 2 h and at 900°C for 6 h and the 6 h leached sample
Table 2 Thermal stability of zeolite ®lms formed on honeycombs Sample
Heat treatment
Speci®c surface area (m2 /g)
Pore volume (ml/g)
XRD peak ratioa
3 h leached honeycomb
Untreated After 800°C/2 h After 900°C/2 h
120 119
0.147 0.091
0.53 ±
104
0.088
0.32
Untreated After 900°C/2 h
249 241
0.238 0.210
1.14 1.47
6 h leached honeycomb
a zeolite cordierite I501 =I100 :
was heated at 900°C for 20 h. No changes were observed in the XRD patterns before and after heating. The thermally treated samples also showed zeolite peaks, con®rming that the zeolite coating was stable up to 900°C. Table 2 shows the properties of the samples before and after heat treatment. The reduction in the SSA, pore volume and XRD peak ratio was much less in both samples, and there was no change in the PSDs after heat treatment. Here, the XRD peak ratio in the 6 h leached sample increased after heating, but the reason for this unusual result is unclear at present. 3.5. Microstructure of zeolite ®lm Fig. 2 shows SEM photographs of honeycombs with zeolite ®lm. The cross section of the honeycomb cell shown in Fig. 2(a) shows that the ®lm is continuous, with a thickness of 10±20 lm (Fig. 2(b)). Fig. 2(c) indicates that the ®lm consists of uniform 2 to 3 lm zeolite crystals. This crystal size is much smaller than that reported by Milestone et al. [25] (10 lm) even using a similar SSIC method. The present success in obtaining a continuous ZSM-5 ®lm on cordierite honeycomb by the SSIC method can be attributed to the formation of these small crystals, and to the higher precursor gel loading on the substrates. These factors can be optimized by an appropriate choice of starting materials for the precursor gel, and by pre-conditioning the porous substrate, especially by leaching it to increase its surface area. Fig. 2(d) shows the macroporous honeycomb substrate
C.D. Madhusoodana et al. / Microporous and Mesoporous Materials 46 (2001) 249±255
surface (3 h leached) with some micropores due to the acid leaching. After the formation of the zeolite ®lm, the macropores are completely covered by zeolite crystals (Fig. 2(e)), but the surface contains mesopores between the formed zeolite crystals. After recoating, these spaces are further ®lled and the dense packing of the formed crystals results in a more continuous zeolite ®lm as shown in Fig. 2(f). These observations are consistent with the PSD shown in Fig. 4. The surface of the zeolite ®lm was relatively ¯at because of the uniformity of the ZSM-5 zeolite crystals. 4. Conclusions Continuous thin ®lms of ZSM-5 zeolite were successfully produced on porous cordierite honeycombs by SSIC. The in situ crystallization of ZSM-5 zeolite from a precursor gel on the porous honeycomb substrates is promoted in the solid state by the small amount of water present in the gel. The formation of microporous silica on the cordierite honeycomb by selective acid leaching treatment enhances the formation of zeolite. Recoating treatments increase the formation of zeolite and its SSA and also reduce the larger pores in the honeycombs, improving the packing density of zeolite crystals in the coated ®lms. The zeolite ®lm is thermally stable up to 900°C and maintains its original porous properties. This method is not only simple and easy compared with the conventional wet hydrothermal synthesis methods but also produces a better zeolite±substrate interface by the in situ formation of zeolite ®lm on acid treated cordierite honeycombs, suggesting its usefulness for automotive applications. Acknowledgements CDM wishes to thank UNESCO/Monbusho for the research fellowship and CTI, BHEL/EPD,
255
India for the sponsorship. We are grateful to Dr. K.J.D. MacKenzie of New Zealand Institute for Industrial Research and Development for proof reading and editing of the manuscript. References [1] T. Bein, Chem. Mater. 8 (1996) 1636. [2] M.J. Heimrich, L.R. Smith, J. Kitowski, SAE 920847 (1992) 954. [3] W. Held, A. Konig, T. Richter, L. Puppe, SAE 900496 (1990) 209. [4] P.A. Jacobs, J.A. Martens, Synthesis of High Silica Aluminosilicate Zeolites, Elsevier, Amsterdam, 1987. [5] W. Xu, J. Dong, J. Li, F. Wu, J. Chem. Soc. Chem. Commun. (1990) 755. [6] M. Kim, H. Li, M.E. Davis, Micropor. Mater. 1 (1993) 191. [7] M. Matsukata, N. Nishiyama, K. Ueyama, Micropor. Mater. 1 (1993) 219. [8] T. Sano, Y. Kiyozumi, F. Mizukami, A. Iwasaki, M. Ito, M. Watanabe, Micropor. Mater. 1 (1993) 353. [9] P.R.H.P. Rao, M. Matsukata, Chem. Commun. (1996) 1441. [10] R. Altho, S. Reitmaier, W. Schmidt, B. Zibrowius, K.K. Unger, F. Schuth, Stud. Surf. Sci. Catal. 98 (1995) 36. [11] S. Shimizu, Y. Kiyozumi, F. Mizukami, Chem. Lett. (1996) 403. [12] S. Shimizu, Y. Kiyozumi, F. Mizukami, Japanese Patent JP 08319112 (1996). [13] F. Mizukami, Stud. Surf. Sci. Catal. 125 (1999) 1. [14] M. Matsukata, J. Chem. Soc. Chem. Commun. (1994) 339. [15] M. Matsukata, Stud. Surf. Sci. Catal. 84 (1994) 1183. [16] Y.H. Ma, S. Xiang, US Patent 5258339 (1993). [17] K. Kamiyama, Japanese Patent JP 4224109 (1992). [18] Z.S. Rak, H.J. Verginga, React. Kinet. Catal. Lett. 60 (1007) 303. [19] I.M. Lachman, M.D. Patil, US Patent 4800187 (1889). [20] V.J. Peterus, US Patent 5871650 (1999). [21] Englehard Min. Chem., British Patent GB 2017520 (1979). [22] S. Komarneni, H. Katsuki, S. Furuta, J. Mater. Chem. 8 (1998) 2327. [23] S. Komarneni, H. Katsuki, S. Furuta, Japanese Patent JP 11171668 (1999). [24] R. Aiello, F. Crea, F. Testa, A.S. Gattuso, Stud. Surf. Sci. Catal. 125 (1999) 29. [25] N.B. Milestone, F. Mizukami, Y. Kiyozumi, K. Maeda, S. Niwa, 12th Int. Zeolite Conf., Materials Research Society, 1999, p. 1833.
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Preparation of ZSM-5 thin ®lm on cordierite honeycomb by solid state in situ crystallization C.D. Madhusoodana 1, R.N. Das, Y. Kameshima, A. Yasumori, K. Okada * Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, Japan Received 11 November 2000; received in revised form 11 April 2001; accepted 11 April 2001
Abstract Thin ®lms of ZSM-5 zeolite were prepared on porous cordierite honeycombs by a solid state in situ crystallization method and were characterized by XRD, FTIR, SEM and nitrogen gas adsorption. The zeolite ®lms were formed by coating the zeolite precursor sol on the cordierite honeycomb substrate and gelling it in the surface pores of the substrate. This gel was then converted into ZSM-5 by heating the coated substrate in an autoclave without adding solution. The amount of coating was increased by forming an interfacial layer of microporous silica on the cordierite substrate by acid leaching. The maximum speci®c surface area of the honeycomb coated with zeolite ®lm was 249 m2 /g. Recoating the substrate with zeolite ®lm was eective not only in increasing the amount of zeolite ®lm and its surface area but also in reducing the macropores in the honeycomb surface. The zeolite ®lm was found to retain its porous properties up to the calcining temperature of 900°C. The zeolite ®lm is considered to be formed by a vapor transport mechanism initiated by the water included in the gel. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Zeolite ®lm; ZSM-5; Solid state in situ crystallization; Cordierite; Honeycomb substrate
1. Introduction The ZSM-5 families of high silica zeolites have high shape selectivity for molecular sorption and diusion [1] and are thus widely used as HC adsorbents, deNOx catalysts (with Cu/Ce/Pt exchanged) etc. [2,3]. They are generally synthesized under hydrothermal conditions, crystallizing from a solution containing SiO2 , Al2 O3 and Na2 O
*
Corresponding author. E-mail addresses: [email protected] (K. Okada), [email protected] (C.D. Madhusoodana). 1 Present address: Ceramic Technological Institute, BHEL, EPD, P.B. No. 1245, Bangalore 560 012, India.
sources optionally with templates or seeds. Various raw materials have been used for synthesizing ZSM-5 and other high silica zeolites [4]. Recently, there has been a trend to prepare zeolites in the solid state. ZSM-5 was synthesized from dried amorphous aluminosilicate gels in vapors of ethylenediamine, triethylamine, and water [5±7]. Based on in situ microscopic observation of the formation of MFI zeolites in the presence of steam by Sano et al. [8], a dry gel could be converted into zeolite in the dry state [9]. In this method, dry gels are placed in an autoclave where they come into contact with steam to form zeolite. In another vapor phase transport method, dry powder containing an aluminosilicate precursor and NH4 F was converted into ZSM-5 [10]. In a solid state
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 3 0 4 - 3
250
C.D. Madhusoodana et al. / Microporous and Mesoporous Materials 46 (2001) 249±255
method, direct crystallization of aluminosilicate gels to silicalite and ZSM-5 occurred by heating in glass ampoules at 130±150°C for 18±24 h [11,12]. Many similar methods have been developed to prepare thin zeolite ®lms. The dierent methods for forming thin ®lms and membranes including dry methods have been reviewed by Mizukami [13]. The dry method involves coating the precursor solution on the substrate and crystallizing it in the presence of vapors of templating reagents and water in a special autoclave [14±16]. Most of the above coatings or ®lms were deposited on porous alumina supports to form membranes. For the automotive application as hydrocarbon (HC) adsorbers for cold start emission control and for deNOx catalysts, ZSM-5 should preferably be coated on honeycomb monoliths [2,3]. High catalytic surface area and good mechanical strength to withstand road vibrations are important criteria for the coated honeycombs, which also have good HC trapping eciency, good catalytic activity and high thermal durability of the zeolite layer. They are prepared either by washcoating the zeolite particles on to the substrate [17,18] or by in situ crystallization through wet hydrothermal methods [19,20]. The adhesion of the coating may be stronger in the wet hydrothermal synthesis but the mechanical strength of the honeycomb may be less due to chemical attack by the strongly alkaline solution. There are some methods in which honeycomb monoliths are prepared by extruding zeolite-containing or zeolite-forming materials [21]. In a different approach, the zeolite ®lm is formed on a sintered clay honeycomb by leaching the free silica phase and subsequent crystallizing it on the surface [22,23]. In this wet hydrothermal process, prolonged (up to 28 day) treatment was necessary for the formation of the zeolite ®lm and the surface area obtained was about 110 m2 /g. In another method, cordierite honeycombs were coated with zeolite forming solution then crystallized in an autoclave containing water [24]. The objective of the present study is to prepare a continuous thin ®lm of zeolite on the cordierite honeycomb substrate without immersing it in the precursor sol or using additional water in the autoclave during zeolitization. This is in order to
avoid weakening the substrate and to obtain a good zeolite±substrate interface by in situ zeolitization by simple autoclaving. Although a similar method has already been reported by Milestone et al. [25], they did not succeed in forming a continuous ®lm on their substrate. We therefore used dierent type of substrates and also dierent starting materials for the preparation of the precursor gel to obtain a continuous thin ®lm of zeolite. The concept of the present method is to crystallize the zeolite using only the water present in the optimized precursor gel, exploiting the thin layer of microporous silica formed on the honeycombs to provide a strong interface. We refer to this method as solid state in situ crystallization (SSIC) and report here the successful preparation of ZSM-5 continuous thin ®lms on cordierite honeycomb substrates, which can be a candidate HC adsorber for controlling emissions from automobiles. 2. Experimental work 2.1. Preparation of zeolite ®lm The substrates used were cordierite (2MgO 2Al2 O3 5SiO2 ) honeycombs having a cell density of 400 cells per square inch (cpsi) and wall thickness of 0.17 mm, prepared at the Ceramic Technological Institute, BHEL, Banglore, India. The open porosity was 35% and more than 90% of the pores were between 0.5 and 5 lm radius as measured by mercury porosimeter. Small (1 1 1 cm3 ) samples were cut from the honeycomb monolith and used as substrates. These honeycomb samples were treated with 20% H2 SO4 at 90°C for 3 and 6 h to form a microporous silica layer on the honeycomb surfaces. Three types of substrate samples (unleached, 3 h leached and 6 h leached) were thus obtained. All these samples were outgased in the vacuum at 100°C for one day before coating. The zeolite precursor solution was prepared from tetraethylorthosilicate (Wako chemicals, Japan), tetrapropylammonium hydroxide (10%, Wako chemicals, Japan) and aluminum tri-secbutoxide (Soekawa chemicals, Japan). They were well mixed by stirring for 2 h at room temperature
C.D. Madhusoodana et al. / Microporous and Mesoporous Materials 46 (2001) 249±255
to produce a composition with a molar ratio of SiO2 : Al2 O3 : TPAOH 100:1:10. The substrate samples were dipped in the precursor solution for about 30 min. The samples were then removed from the solution and the excess solution on the surface was removed by careful air blowing. These samples were then heated in an airtight Te¯on vessel at 80°C for 2 h to convert the sol absorbed on the substrates to a gel. The dipping and gelling steps were repeated three times to produce a continuous coating layer on the substrates. The gelled-coated substrates were then aged overnight. Crystallization was carried out in a Te¯on lined autoclave (25 ml) at 150°C for 24 h without addition of water. After autoclaving, the samples were washed with distilled water in an ultrasonic bath for 30 min to remove any loosely adhering coating especially in the corners of the honeycomb. After drying at 100°C for 1 day, the samples were calcined at 600°C for 2 h to remove the template ions.
251
electron microscope (SEM; S-2050, Hitachi, Japan). 3. Results and discussion 3.1. Synthesis of ZSM-5 thin ®lm Fig. 1 shows XRD patterns of coated and uncoated cordierite, con®rming the formation of ZSM-5 on the cordierite surface. The intensity ratio of the strongest peaks of ZSM-5 and cordierite was used as a relative measure of the amount of ZSM-5. It can be seen from Fig. 1 and Table 1 that the leached samples showed more
2.2. Characterization and testing The formation of zeolite was con®rmed by powder X-ray diraction (XRD; Geiger¯ex, Rigaku, Japan) using powder samples obtained by grinding the coated honeycomb. The porous properties were determined from N2 gas adsorption and desorption isotherms measured at 77 K using an Autosorb-I instrument (Quanta Chrome, USA). The speci®c surface area (SSA) was calculated by the BET method, the pore size distribution (PSD) was calculated by the BJH method and the pore volume was obtained from the maximum adsorption at a relative pressure of 0.999. The microstructure of the zeolite ®lm on the honeycomb substrate was observed using a scanning
Fig. 1. XRD patterns of cordierite honeycombs before coating (a) and after coating on (b) unleached sample, (c) 3 h leached sample and (d) 6 h leached sample (Z: ZSM-5, C: cordierite).
Table 1 Properties of zeolite ®lm formed on unleached and leached honeycombs Treatment of honeycombs Unleached Leached for 3 h Leached for 6 h
% Weight gain
XRD peak ratio
Speci®c surface area (m2 /g)
Pore volume (ml/g)
After coating
After recoating
After coating
After recoating
Before
After coating
After recoating
Before
After coating
After recoating
9.3 12.6 16.9
± 16.9 21.2
0.1 0.26 1.02
± 0.53 1.14
0.5 81 227
49 110 239
± 120 249
0.008 0.055 0.136
0.078 0.113 0.212
± 0.147 0.238
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zeolite formation. The microporous silica layer formed in the leached cordierite honeycomb samples enhanced the coating of the sol and the formation of ZSM-5. The crystallization of ZSM-5 is thought to occur by a vapor transport mechanism [6]. It is however, initiated by the presence of water molecules and hydroxides in the gel which react with the silicates and assemble on the TPA
templates. Compared with conventional wet hydrothermal synthesis, the amounts of zeolite precursors and steam in the present SSIC method are much lower. The size of ZSM-5 crystals as shown in Fig. 2 was 2±3 lm, which is smaller than formed by the conventional method and also by a similar SSIC method [25]. This may be related to the less water content of the present method,
Fig. 2. SEM photographs of zeolite ®lms on honeycombs: (a) cross section of honeycomb with zeolite ®lm, (b) zeolite ®lm and substrate interface, (c) zeolite crystals on the honeycomb substrate, (d) honeycomb surface before coating, (e) honeycomb surface after coating and (f) honeycomb surface after recoating.
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though many other factors may also contribute to this eect. It was also observed that the volume ratio of the sample and autoclave was very crucial to the crystallization of the zeolite. When a large autoclave (100 ml) was used, ZSM-5 did not form, indicating that the optimum autogenious pressure of vapor formed by the water in the gel in¯uences the kinetics of zeolitization. ZSM-5 ®lm can thus be formed in a simple autoclaving step by the proper selection of the volume of the autoclave and the optimum water content in the precursor gel by the SSIC method. 3.2. Eect of substrate properties Table 1 shows the properties of the zeolite ®lms formed on unleached and leached (3 and 6 h in 20% H2 SO4 at 90°C) cordierite honeycombs. The weight increase of the three substrates after coating was 9.3±16.9 wt.%, with a further increase of the 6 h leached substrate to 21.2 wt.% after coating. Since the weight gain increased with longer substrate leaching times, it was concluded that the acid treatment is eective in increasing the amount of zeolite coating. The increase in surface area and pore volume after coating is attributed to the formation of zeolite ®lm. The amount of zeolite formed increased with higher surface area of the substrates (more leached), as evidenced by the XRD intensity ratios of zeolite and cordierite zeolite cordierite (I 501 =I 100 ). An increase in the ZSM-5 peak heights is observed in the leached samples. The XRD results are compatible with the weight gains of the samples (Table 1). The surface areas of the coated ZSM-5 ®lms were evaluated from the gains of weight and surface area of the samples by coating. The surface area value corresponded to zeolite calculated ranged from 300 to 360 m2 /g and were in good agreement with that of ZSM-5 powder (350 m2 /g) prepared separately. This indicates that the weight gain by the coating is fully attributed to the formation of coated ZSM-5 zeolite grains. Fig. 3 shows the PSDs of the three dierent zeolite coated honeycombs. An apparent dierence in the PSD between the unleached and leached samples was observed. The PSD of the unleached
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Fig. 3. PSDs of zeolite ®lms on unleached and leached honeycombs.
sample showed a peak at 6 nm in pore radius, while the PSD of the leached samples showed micropores smaller than 1 nm and mesopores of 2 nm radius. The micropores can be attributed with the pores in the leached substrates and formed zeolites and the mesopores with the spaces between the zeolite crystals. There are also some larger meso and macropores, which may be due to agglomeration of the zeolite crystals. Fig. 4 shows the PSDs of the 3 h leached honeycombs before and after coating. Before coating, the samples contained only micropores of about 1 nm radius corresponding to the microporous silica phase in the surface of honeycomb produced by selective leaching. After coating, the micropores
Fig. 4. Comparison of PSDs of zeolite ®lms on 3 h leached sample; before coating, after coating and after recoating.
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were still seen, together with mesopores between the zeolite crystals in the coating. The same trend was also observed in the 6 h leached samples. The presence of the interface layer of microporous silica facilitates the formation of zeolite ®lm, both increasing its quantity due to its high surface area and microporosity as with a conventional wash coating. Since the microporous silica in the surface of the leached substrates is considered to react partially during the zeolitization, the resulting zeolite crystals are expected to form a strong interface with the cordierite as a result of their participation in the zeolitization. 3.3. Eect of recoating The coated samples (leached substrates with zeolite ®lms) were recoated by the same procedure using a fresh precursor solution. The aim was to increase the amount and continuity of the zeolite ®lm and to reduce the macroporosity, which may enhance the mechanical strength of the honeycomb. The porous properties before and after recoating the leached samples are listed in Table 1. In the both leached samples (3 and 6 h leached) recoating increased the SSA and pore volume. An almost linear increase in the SSA and pore volume is observed between the uncoated, coated and recoated samples. This is due to the linear increase in the amount of zeolite formation (Table 1) which is also evident from the increase in the XRD peak ratio. Fig. 4 shows the PSDs of the 3 h leached honeycombs samples before coating, after coating and after recoating. The same trend was observed in the 6 h leached samples. In the both samples, the recoating resulted in the reduction of the larger pores in the honeycomb, in which the macropores were largely ®lled by the newly formed zeolite. The recoated samples contained mainly micropores smaller than 1 nm and mesopores of 2 nm radius. Thus, a more uniform and continuous ®lm of zeolite was obtained by repeated coating. 3.4. Thermal stability of zeolite ®lm The thermal stability of the 3 h leached sample was tested by heating at 800°C for 2 h and at 900°C for 6 h and the 6 h leached sample
Table 2 Thermal stability of zeolite ®lms formed on honeycombs Sample
Heat treatment
Speci®c surface area (m2 /g)
Pore volume (ml/g)
XRD peak ratioa
3 h leached honeycomb
Untreated After 800°C/2 h After 900°C/2 h
120 119
0.147 0.091
0.53 ±
104
0.088
0.32
Untreated After 900°C/2 h
249 241
0.238 0.210
1.14 1.47
6 h leached honeycomb
a zeolite cordierite I501 =I100 :
was heated at 900°C for 20 h. No changes were observed in the XRD patterns before and after heating. The thermally treated samples also showed zeolite peaks, con®rming that the zeolite coating was stable up to 900°C. Table 2 shows the properties of the samples before and after heat treatment. The reduction in the SSA, pore volume and XRD peak ratio was much less in both samples, and there was no change in the PSDs after heat treatment. Here, the XRD peak ratio in the 6 h leached sample increased after heating, but the reason for this unusual result is unclear at present. 3.5. Microstructure of zeolite ®lm Fig. 2 shows SEM photographs of honeycombs with zeolite ®lm. The cross section of the honeycomb cell shown in Fig. 2(a) shows that the ®lm is continuous, with a thickness of 10±20 lm (Fig. 2(b)). Fig. 2(c) indicates that the ®lm consists of uniform 2 to 3 lm zeolite crystals. This crystal size is much smaller than that reported by Milestone et al. [25] (10 lm) even using a similar SSIC method. The present success in obtaining a continuous ZSM-5 ®lm on cordierite honeycomb by the SSIC method can be attributed to the formation of these small crystals, and to the higher precursor gel loading on the substrates. These factors can be optimized by an appropriate choice of starting materials for the precursor gel, and by pre-conditioning the porous substrate, especially by leaching it to increase its surface area. Fig. 2(d) shows the macroporous honeycomb substrate
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surface (3 h leached) with some micropores due to the acid leaching. After the formation of the zeolite ®lm, the macropores are completely covered by zeolite crystals (Fig. 2(e)), but the surface contains mesopores between the formed zeolite crystals. After recoating, these spaces are further ®lled and the dense packing of the formed crystals results in a more continuous zeolite ®lm as shown in Fig. 2(f). These observations are consistent with the PSD shown in Fig. 4. The surface of the zeolite ®lm was relatively ¯at because of the uniformity of the ZSM-5 zeolite crystals. 4. Conclusions Continuous thin ®lms of ZSM-5 zeolite were successfully produced on porous cordierite honeycombs by SSIC. The in situ crystallization of ZSM-5 zeolite from a precursor gel on the porous honeycomb substrates is promoted in the solid state by the small amount of water present in the gel. The formation of microporous silica on the cordierite honeycomb by selective acid leaching treatment enhances the formation of zeolite. Recoating treatments increase the formation of zeolite and its SSA and also reduce the larger pores in the honeycombs, improving the packing density of zeolite crystals in the coated ®lms. The zeolite ®lm is thermally stable up to 900°C and maintains its original porous properties. This method is not only simple and easy compared with the conventional wet hydrothermal synthesis methods but also produces a better zeolite±substrate interface by the in situ formation of zeolite ®lm on acid treated cordierite honeycombs, suggesting its usefulness for automotive applications. Acknowledgements CDM wishes to thank UNESCO/Monbusho for the research fellowship and CTI, BHEL/EPD,
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India for the sponsorship. We are grateful to Dr. K.J.D. MacKenzie of New Zealand Institute for Industrial Research and Development for proof reading and editing of the manuscript. References [1] T. Bein, Chem. Mater. 8 (1996) 1636. [2] M.J. Heimrich, L.R. Smith, J. Kitowski, SAE 920847 (1992) 954. [3] W. Held, A. Konig, T. Richter, L. Puppe, SAE 900496 (1990) 209. [4] P.A. Jacobs, J.A. Martens, Synthesis of High Silica Aluminosilicate Zeolites, Elsevier, Amsterdam, 1987. [5] W. Xu, J. Dong, J. Li, F. Wu, J. Chem. Soc. Chem. Commun. (1990) 755. [6] M. Kim, H. Li, M.E. Davis, Micropor. Mater. 1 (1993) 191. [7] M. Matsukata, N. Nishiyama, K. Ueyama, Micropor. Mater. 1 (1993) 219. [8] T. Sano, Y. Kiyozumi, F. Mizukami, A. Iwasaki, M. Ito, M. Watanabe, Micropor. Mater. 1 (1993) 353. [9] P.R.H.P. Rao, M. Matsukata, Chem. Commun. (1996) 1441. [10] R. Altho, S. Reitmaier, W. Schmidt, B. Zibrowius, K.K. Unger, F. Schuth, Stud. Surf. Sci. Catal. 98 (1995) 36. [11] S. Shimizu, Y. Kiyozumi, F. Mizukami, Chem. Lett. (1996) 403. [12] S. Shimizu, Y. Kiyozumi, F. Mizukami, Japanese Patent JP 08319112 (1996). [13] F. Mizukami, Stud. Surf. Sci. Catal. 125 (1999) 1. [14] M. Matsukata, J. Chem. Soc. Chem. Commun. (1994) 339. [15] M. Matsukata, Stud. Surf. Sci. Catal. 84 (1994) 1183. [16] Y.H. Ma, S. Xiang, US Patent 5258339 (1993). [17] K. Kamiyama, Japanese Patent JP 4224109 (1992). [18] Z.S. Rak, H.J. Verginga, React. Kinet. Catal. Lett. 60 (1007) 303. [19] I.M. Lachman, M.D. Patil, US Patent 4800187 (1889). [20] V.J. Peterus, US Patent 5871650 (1999). [21] Englehard Min. Chem., British Patent GB 2017520 (1979). [22] S. Komarneni, H. Katsuki, S. Furuta, J. Mater. Chem. 8 (1998) 2327. [23] S. Komarneni, H. Katsuki, S. Furuta, Japanese Patent JP 11171668 (1999). [24] R. Aiello, F. Crea, F. Testa, A.S. Gattuso, Stud. Surf. Sci. Catal. 125 (1999) 29. [25] N.B. Milestone, F. Mizukami, Y. Kiyozumi, K. Maeda, S. Niwa, 12th Int. Zeolite Conf., Materials Research Society, 1999, p. 1833.