Chemical Engineering Journal 225 (2013) 686–694
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Highly stable hierarchical ZSM-5 zeolite with intra- and inter-crystalline porous structures Haixiang Tao, Hong Yang, Xiaohui Liu, Jiawen Ren, Yanqin Wang ⇑, Guanzhong Lu ⇑ Shanghai Key Laboratory of Functional Materials Chemistry, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Hierarchical ZSM-5 nanocrystals with
meso- and macropores are synthesized. Each ZSM-5 nanocrystal contains intra-crystalline mesopores generated by PEG. The aggregation of ZSM-5 nanocrystals produces intercrystalline macropores. Hierarchical zeolites with high crystallinity are achieved in only 12 h at 160 °C. Hierarchical ZSM-5 shows higher catalytic activities than conventional ZSM-5.
a r t i c l e
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Article history: Received 7 January 2013 Received in revised form 8 March 2013 Accepted 23 March 2013 Available online 6 April 2013 Keywords: Hierarchical zeolite Polyethylene glycol Steam-assisted conversion Separation
a b s t r a c t Highly stable hierarchical ZSM-5 zeolites with intra-crystalline mesopores and inter-crystalline macropores have been synthesized via a steam-assisted conversion in the presence of polyethylene glycol. XRD, SEM, TEM, N2 sorption and mercury intrusion measurement revealed that the irregular intra-crystal mesopores (2–10 nm) in each zeolite nanocrystal were generated by the removal of encapsulated PEG, and the inter-crystalline macropores (50–90 nm) were formed by the aggregation of the well crystallized ZSM-5 nanocrystals with a small size (50–200 nm). The ZSM-5 zeolite nanocrystals were achieved by a dense-gel with high TPA+/SiO2, highly concentrated TPA+ can induce a burst in nucleation rate during the first step of steam-assisted conversion, which makes the dry gel easy to convert into zeolite nanocrystals, PEG arrested in the gel can be easily encapsulated by zeolite crystals during the steam-assisted conversion. Finally, these nanocrystals containing intra-crystalline mesopores aggregate into the bulky materials with an inter-crystalline macroporous network. In addition, the mesopore volume and external surface areas of Hier-ZSM-5 can be easily adjusted by changing the amount of PEG. Catalytic tests showed that Hier-ZSM-5 had a similar activity to Nano-ZSM-5, which must be collected by high-speed centrifugation; while had a higher activity than Con-ZSM-5, especially for the reactions involving large molecules. The high catalytic activity and easy separation from liquid mixture of Hier-ZSM-5 would facilitate its practical applications in chemical industry. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Zeolites are widely used as catalysts or catalyst supports in a variety of applications in the refining and (petro)chemical ⇑ Corresponding authors. Tel.: +86 21 64253824; fax: +86 21 64252923. E-mail addresses:
[email protected] (Y. Wang),
[email protected] (G. Lu).
industries, due to their high surface area, high acidity and good (hydro)thermal stability [1–3]. Notwithstanding the positive effect of the presence of micropores (pore size 5 lm) and nanocrystallined ZSM-5 (Nano-ZSM-5, 50–200 nm) (The XRD patterns, N2 sorption isotherms and SEM images are shown in Figs. S1 and S2, respectively). Test reactions used included alkylation of toluene with benzyl chloride (Reaction 1), aldol condensation of benzaldehyde with n-butyl alcohol (Reaction 2) and acetalization of cyclohexanone with methanol (Reaction 3). The reaction results are summarized in Table 2. As expected, all HierZSM-5 and Nano-ZSM-5 show similar conversion for all the reactions. Compared with Con-ZSM-5, in the reactions (1) and (2), which involve large molecules, Hier-ZSM-5 and Nano-ZSM-5 showed higher activity than Con-ZSM-5. For reaction (1), the conversion of toluene over Hier-ZSM-5 and Nano-ZSM-5 was between 70% and 76%, which was nearly 3 times higher than that of ConZSM-5 (26.5%). The most active catalyst for the reaction was Hier-ZSM-5-2 with the highest external surface and mesopore volume. The same phenomena were observed in the reaction (2) for all ZSM-5 zeolites. The temperature-programmed desorption of ammonia for all the samples was measured to characterize the acidity of ZSM-5 zeolites (H+ form) (Fig. S3), the total amount of acid sites characterized by the TPD of NH3 did not correlate with
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the catalytic activity. Although, the details of the acidity and distribution of these acid sites in zeolites is not clear now, it is reasonable to assume that the improved mass transfer of Hier-ZSM-5 and Nano-ZSM-5 is one of important factors responsible for the different catalytic activity, owing to the presence of mesopores. Fig. 8 shows the time-conversion plots of toluene (A) and benzaldehyde (B) in which Hier-ZSM-5-1.5 and Hier-ZSM-5-2 demonstrate a higher activity than those of Hier-ZSM-5-0, Hier-ZSM-5-1, NanoZSM-5 and much higher than Con-ZSM-5. For reaction (1), the Hier-ZSM-5-1.5 and Hier-ZSM-5-2 lead to near 70% conversion within 1.5 h, while the conversion of toluene is about 55% on Hier-ZSM-5-0, Hier-ZSM-5-1, Nano-ZSM-5, which means the reaction rate is much higher on Hier-ZSM-5-1.5 and Hier-ZSM-5-2 than those on Hier-ZSM-5-0, Hier-ZSM-5-1 and Nano-ZSM-5. When the reaction proceeds for 3 h, the toluene conversion tends to plateau on Hier-ZSM-5 and Nano-ZSM-5. Similar phenomena are observed in reaction (2). These results indicate that the presence of intercrystalline macropores (voids between nanocrystals) dramatically improves mass transport; in addition, the introduction of intracrystalline mesopores (open) can further improve mass transport and thereby leads to a remarkable catalytic activity. As shown in the TEM images, the meso- and marcropores in Hier-ZSM-5-1.5 and Hier-ZSM-5-2 are interconnected and open, while the mesopores in Hier-ZSM-5-1 are closed, which results into the higher catalytic activity of Hier-ZSM-5-1.5 and Hier-ZSM-5-2 than that of Hier-ZSM-5-1. In addition, reaction (3) involving small molecules was conducted to further confirm this aspect. In this case, it turns out that the catalytic activity was very similar for all of ZSM-5 catalysts. Therefore, the high catalytic activity of Hier-ZSM-5 in all the cases may be a result of the formation of zeolite nanocrystals and the presence of intra- and inter-crystalline pores, which will shorten the diffusion path and improve accessibility of active sites. Considering the morphology and size of the bulky-particles for easy separation by filtration and superior catalytic property in acid catalyzed reaction, Hier-ZSM-5 has greater potential for industrial applications than Nano-ZSM-5 and Con-ZSM-5.
4. Conclusion Mechanically stable bulky hierarchical zeolite particles (HierZSM-5) with intra- and inter-crystalline porous structure have been successfully synthesized in the presence of PEG via a steam-assisted method. Such bulky hierarchical zeolite particle turns out to be composed of zeolite nanocrystals with a size of 50–200 nm, resulting into inter-crystalline macropores (50–90 nm), and there are many intra-cryatlline mesopores (2–10 nm) generated by the removal of encapsulated PEG existed in each zeolite nanocrystal. Importantly, a complete conversion into such hierarchical Hier-ZSM-5 was achieved in only 12 h at 160 °C. Furthermore, the catalytic tests show that Hier-ZSM-5 has a higher activity than Con-ZSM-5, especially for the reaction involving large molecules. However, Nano-ZSM-5 has a similar activity to Hier-ZSM-5, but need high-speed centrifugation for collection. The high catalytic activity and easy separation from liquid mixture of Hier-ZSM-5 facilitate its practical application in chemical industry.
Acknowledgements This project was supported financially by the National Basic Research Program of China (No. 2010CB732306), the National Natural Science Foundation of China (Nos. 20973058, 21101063, and 21273071), and the Fundamental Research Funds for the Central Universities.
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