마이크로파를 이용한 무기물질의 합성에 관한 연구

김대성, 장종산, 김지만, 이철위, 박상언*

한국화학연구소, 분자활성공학연구실

Synthesis of Inorganic Materials by using Microwave Irradiation Technique

D. S. Kim, J.-S. Chang, J. M. Kim, C. W. Lee, and S.-E. Park*

Catalysis Center for Molecular Engineering, KRICT, Taejeon 305-600, Korea

INTRODUCTION

Since the progress of synthesis on the molecular sieves containing pores and cavities of molecular dimensions, many researches have been studied on the catalysis, separation technology, and biomaterials engineering. Recently, there has been paid attention to the utilization of microwave heating technique in the fields of inorganic and organic synthesis. Among them the microwave-assisted synthesis of zeolites is considered to be a new promising field of zeolite research due to several fascinating advantages compared with conventional hydrothermal method. The advantages of microwave-induced heating include homogeneous heating throughout a reaction vessel, resulting in more homogeneous nucleation and shorter crystallization times [1]. The possibility in the use of microwave energy for the crystallization of materials such as zeolites was suggested from the observation of Roussy et al. that microwave can be used to desorb water from a zeolite [2]. Mobils scientists firstly claimed that microwave energy was successfully used in the crystallization of zeolites with employing a heat transfer agent which is sympathetic to microwave [3].  It has been reported that several porous materials were synthesized in very short crystallization time by microwave heating of their precursor gels [3-7]. Recently, Park and coworkers previously reported that rapid formation mechanism for MCM-41 material  under microwave conditions is directly monitored by fluorescence and ESR spectroscopy [8]. In this work, we report the rapid preparation of various kinds of inorganic materials such as zeolite Y, ZSM-5, MCM-41, VSB-1 (Versailes/Santa Babara-1), micro-mesoporous composite material (MCM-41/ZSM-5), and layered double hydroxides (LDH) under microwave irraidation.

EXPERIMENTAL

Various kinds of molecular sieves and layered double hydroxide materials ( zeolite Y, ZSM-5, MCM-41, VSB-1, micro-mesoporous material, and LDH) were synthesized under microwave irradiation following the procedures described elsewhere [5,7,8,12]. The detailed conditions of microwave syntheses for these materials were listed in Table 1 compared with those of hydrothermal syntheses, respectively. In case of microwave synthesis, the mixture was heated with microwave for 20 - 180 min at 70 ∼ 165 ℃ in a CEM microwave oven (MDS-2000) by using 50 - 100 % of the maximum power of the oven. After this step, the solid products were collected by filtration and dried in air for 1 day. For comparison, the syntheses by hydrothermal heating were carried out using autoclave system with the same gel compositions as the microwave-assisted syntheses following the conventional synthetic procedures. The dried products were collected and analyzed. Synthesized inorganic materials were characterized by X-ray diffractometer (Rigaku 2155D6, Ni-filtered Cu K□ radiation with □=1.5418 Å). Scanning electron microscopy (SEM) was performed with a JEOL scanning electron microscope (model JSM 840).

Table 1. Synthesis of Various Molecular Sieve and Clay Materials under Microwave Irradiaion and Hydrothermal Heating.

Materials	Synthetic conditions
	Microwave (300-600 watt)	Hydrothermal Heating
Zeolite NaY	150 oC for 30 min	100 oC for 1d
ZSM-5	165 oC for 2 h 30 min	165 oC for 2 d
MCM-41	120-100 oC for 40 min	100 oC for 2 d
NiPO(VSB-1)	165 oC for 3 h	180 oC for 2 d
MCM-41/ZSM-5	165 oC for 1-3 h	100 oC for 2-5 d
LDH(Mg/Al, Ni/Al)	70 oC for 30 min	70 oC for 1 d

RESULTS AND DISCUSSION

Figure 1 shows XRD patterns of various molecular sieve materials prepared by microwave irradiation. Figure 1a and 1c indicate that ZSM-5 and NaY are formed as single phases in the presence conditions. Figure 1b exhibits four well defined peaks, which can be assigned to hexagonal mesoporous MCM-41. A large-pore zeolitic nickel(II) phosphate, VSB-1, is successfully prepared as shown in Figure 1d. Recently, Cheetham and coworkers reported that the undimensional pore system is delineated by 24 NiO6 and PO4 polyhedra and has a free diameter of approximately 9Å [9]. The micro-mesoporous composite materials (MCM-41/ZSM-5) are synthesized with two-step treatment using two different kinds of templates such as TPABr and MTAB. SEM of the calcined MCM-41/Silicalite-1 are shown in Fig. 2. With the increase of microwave irradiation time, the morphologies shows transition from mesopore materials to the fine small particles of round-like type of ZSM-5 having ca. 3-10 □m size of cubic phase surrounding with small mesoporous particle.  Park and coworkers previously published that the relative portion and properties for microphase and mesophase of the final products in terms of changing microwave irradiation time were confirmed by XRD, SEM (not shown) [7]. XRD patterns in Figure 3 show layered double hydroxides of (a) [Mg6Al2(OH)16]CO3□4H2O and (b) [Ni6Al2(OH)16]CO3□4H2O. The synthesis of LDHs carried out at pH 10 in the carbonate form. Especially, the ion exchange reactions of LDHs were greatly accelerated by microwave irradiation.

Figure 1. XRD patterns of (a) ZSM-5, (b) MCM-41 (c) NaY, and (d) VSB-1 prepared by microwave irradiation.

In case of microwave method, the available time for the formation of the same structure is much shorter than hydrothermal method. A discussion of the microwave effects in synthesis of zeolite and zeolitic materials have been given by several authors [6,10,11]. The short crystallization time was ascribed to relatively fast dissolution of the gel upon microwave irradiation as compared to conventional hydrothermal heating [6]. The shorter synthesis time by microwave heating could be explained by two different mechanisms, i.e., the rapid heat-up of the sample and superheating by a better heat transfer which results in rapid and sufficient heating of the synthesis mixture [10,11]. Recently, our group has reported that fast formation of MCM-41 upon microwave irradiation is ascribed to the microwave-susceptible head groups of surfactant molecules in addition to fast dissolution of the precursor gel [8]. Therefore, it seemed that microwave heating not only increases the rate of crystallization but also directs the crystallization mechanism.

Figure 3. XRD Patterns of layered double hydroxides of (a) [Mg6Al2(OH)16]CO3□4H2O and (b) [Ni6Al2(OH)16]CO3□4H2O.

Figure 2. SEM image of MCM-41/ZSM-5 composite material at 165 oC for 3 h under microwave irradiation.

REFERENCES

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[2]  G. Roussy and P. J. Chenot, J. Phys. Chem., 85, 2199 (1981)

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[9]  N. Guillou, Q. Gao, M. Nogues, R. E. Morris, M. Hervieu, G. Ferey, and A. K. Cheetham, C. R. Acad. Sci. Paris, t. 2, Serie II c, p 387, 1999.

[10] D. M. P. Mingos and D. R. Baghurst, Chem. Soc. Rev., 20, 1 (1991).

[11] P. M. Slangen, J. C. Jansen, H. van Bekkum, Microporous Mater., 9, 259          (1997).

[12] Park et al., manuscript in preparation