Synthesis research of nanosized ZSM-5 zeolites in the absence of organic template

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Abstract

Nanosized ZSM-5 zeolite was successfully synthesized at 453 K for 24 h after agitating for a duration of 5 h with great speed and 24 h aging in the synthesis sol with the molar composition of 12Na2O:100SiO2:2Al2O3:2500H2O. X-ray diffraction confirmed ZSM-5 crystal phase, transmission electron microscopy showed ZSM-5 zeolite crystal sizes are about 15 nm. Fourier transform infrared analysis further showed that nanosized zeolite has pentacycle of typical absorption summit of ZSM-5 at 546 cm−1. The results clearly show that Na+ cation plays a role of structure directing as well as a role of charge balancing agent. In addition, it is noticed that when the SiO2/Al2O3 molar ratio is 50, higher or lower Na2O content seems unable to obtain pure ZSM-5. Otherwise, as the SiO2/Al2O3 molar ratio decreases to 30 or lower than it, the crystallization tends to direct toward the formation of mordenite. The results show that synthesis temperature and time, concentration, agitation and aging are very important in preparing nanosized ZSM-5 zeolite.

Introduction

Nanosized zeolite offers outstanding potential for molecular recognition at the subnanometer level and the ability to operate at high temperatures (Davis, 2002, Kostas et al., 2004). Although the templating effect of TPA+ cation is excellent, it can cause many problems such as poison-producing and high production cost, the contamination of waste water by organic template agent, air pollution arising from thermal decomposition of organic template agent, and coke deposit due to incomplete decomposition. In some fields of the unsuitable handling in high temperature need to use nanosized zeolite as assembly component, it easily occurs as nonreversible polymerization in the course of thermal decomposition of organic template agent and losing assembly performance. In order to overcome these detrimental problems, many researchers have made great effort to try to synthesize ZSM-5 in the absence of organic template (Li et al., 2003, Stefan et al., 2003). Grose and Flanigen (1981) first reported the synthesis of organic-free ZSM-5 with/without seeds after 68–72 h of reaction at 200 °C. Schwieger et al. (1989) also reported the synthesis of organic-free ZSM-5 using narrow SiO2/Al2O3 molar ratio of 30–50 and reaction time of 36–72 h. Kim et al. (Shin et al., 2004) synthesized ZSM-5 zeolite with size of 5–6 μm in the absence of organic template within a short reaction time under stirring at 200 rpm and 463 K. Though a significant amount of researches are carried out on the synthesis of organic-free ZSM-5, hardly any result has been satisfactory. In this research program, nanosized ZSM-5 zeolite was successfully synthesized in the absence of organic template by hydrothermal method and was characterized with XRD, SEM, TEM, EDS and FTIR.

Section snippets

The synthesis of ZSM-5 zeolite

Template-free ZSM-5 zeolite was prepared from a synthesis mixture by sodium hydroxide (NaOH > 96.0 wt.%, A.R. Shanghai Chemical Reagent Co. Ltd.) and sodium aluminates (NaAlO2 L.R. Shanghai Chemical Reagent Co.) in deionized water in a beaker. The contents of the beaker were carefully mixed and silica sol (25 wt.%, Shanghai Second Reagent Factory) was slowly added to the above mixture under stirring at 300 rpm. The molar composition of synthesis sol is xNa2O:100SiO2:yAl2O3:zH2O. For example, a

Effect of stirring time

The XRD patterns of organic-free ZSM-5 zeolite was obtained at 453 K at different time from 12Na2O:100SiO2:2Al2O3:2500H2O after different stirring time which was shown in Fig. 1. This figure clearly shows that the crystal products have no specific ZSM-5 zeolite phase but amorphous if they were hydrothermally synthesized immediately for 10 h without stirring or when the stirring time is less than 5 h. When the stirring time is more than 5 h, the obtained results are better. The specific peaks of

Conclusion

Nanosized ZSM-5 zeolite was successfully synthesized at 453 K for 24 h after stirring for 5 h and aging for 24 h in the synthesis mixture with the molar composition of 12Na2O:100SiO2:2Al2O3:2500 H2O. Na+ cation plays a structure directing agent role in place of organic template. It can enhance the nucleation and help to form nanosized ZSM-5 zeolite crystals. In addition, it is noticed that when the SiO2/Al2O3 molar ratio is 50, pure ZSM-5 cannot be obtained, no matter Na2O content is higher or

Acknowledgements

This work is supported by the Specialized Research Fund for the Doctoral Program of High Education (SRFDP) (No. B20020288015). The authors acknowledge Chemistry and Engineering School, Nanjing University of Science and Technology, for financially supporting and Jingdezhen Ceramic Institute, for TEM and supporting.

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