Rapid and Mass Production of Porous Materials Using a Continuous Microwave Equipment

Rapid and Mass Production of Porous Materials Using a Continuous Microwave Equipment Dae Sung Kim, Ji Man Kim, Jong-San Chang, and Sang-Eon Park* Catalysis Center for Molecular Engineering, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107, Yusung, Taejon 305-600, Korea A continuous microwave equipment (CME) has been successfully developed in order to achieve an extremely rapid and mass production for ZSM-5 and NaY zeolite. A precursor mixture for synthesis of ZSM-5 was prepared by mixing aluminosilicate gel with a nanoseed solution obtained under microwave irradiation, and pumped continuously into the CME. Duration time in the CME was 5 min to accomplish the crystallization of ZSM-5 under microwave irradiation. In the case of NaY zeolite, the precursor gel without nanoseeds was also introduced into the CME and crystallization time was within 30 min. Results from X-ray diffraction patterns and scanning electron microscopy indicate that the structural properties of ZSM-5 and NaY zeolite thus obtained are similar to those of the materials obtained using batch-type microwave instrument and by conventional hydrothermal synthesis. Advantages and applications of the CME will be also discussed along with microwave effect. 1. INTRODUCTION Microwave effect has received much attention in inorganic and organic syntheses and reactions. Beneficial effects of microwave environment are known to be rapid heat-up time, rapid reaction, superheating, suppression of undesired phases and so on, compared with hydrothermal method [1-7]. Some representative examples are as follows: the highpermeance NaA zeolite membrane by utilizing homogeneous nucleation [8], the rapid chemical synthesis by superheating of the solvent [9], the effective decomposition of NO x by plasma-assisted catalysis induced by microwave [10], etc. Especially, microwave-assisted syntheses of porous inorganic solid such zeolites and mesoporous molecular sieves [1,2,5-8,11-19] are considered to be a new promising field in materials research due to several fascinating advantages, compared with conventional

hydrothermal synthesis [2]. Several researchers [1,2] have reported synthesis of porous materials and clamed the rapid, mass and economically lucrative synthesis of the porous materials. However, a considerable mass production of zeolite synthesis was not feasible with batch-type system since the more energy and higher powder is required to irradiate the larger amounts of reactants. Rapid crystallization of zeolites under microwave irradiation makes it possible to introduce a continuous flow system for mass production of zeolites. In this work, we demonstrate a rational design and development of continuous microwave equipment (CME) for the synthesis of porous materials such as ZSM-5 and NaY [19]. Microwave effects on the formation of porous materials are also discussed. 2. EXPERIMENTAL Preparation of ZSM-5 nanoseed solution Microwave preparation for nanosized ZSM-5 was carried out in a CEM microwave oven (MARS-5) using tetrapropylammonium hydroxide (TPAOH; 25 %) as templating agent and tetraethylorthosilicate (TEOS) as silica source. The microwave power was programmed in percent increments to control the rate of heating in the range of 25-100% of the maximum power of the oven (Wmax = 1200 watts, frequency = 2.45 GHz, Pmax = 800 psi). The fiber optic probe with a type of phosphor sensor was used for temperature control of microwave oven. A typical gel composition was SiO 2 : TPAOH : H 2 O = 1 : 0.2 : 20. TEOS and TPAOH were mixed homogeneously into distilled water with aging. Thereafter this mixture was heated at 70-80 o C for 1 h in order to eliminate ethanol produced by hydrolysis of TEOS. Subsequently, weight loss was supplied by addition of doubly distilled water to the reaction mixture. The resulting mixture was loaded in a microwave oven equipped with a Teflon autoclave and then irradiated at 165 o C for 10 min under 300-600 W. The resulting mixture was used as a nanoseed solution for the synthesis of ZSM-5 without separation of solid product. For characterization of nanoseed, a potion of the solid product was isolated by filtering or centrifuging, washing with doubly distilled water, and drying in an oven at 100 o C for 10 h. Continuous synthesis of ZSM-5 and NaY under microwave irradiation A continuous microwave equipment (CME) consisted of a microwave oven, which radiates continuous or pulse multimode type having microwave power range of 0-650 watt, equipped with a perfluoroalkoxy (PFA) teflon tube as microwave inert material [19]. The teflon tube was connected with high-pressure slurry pump, pressure gauge at the inlet end and outlet end, and a cooling jacket around the outlet tube. Temperature and pressure of

flowing reaction mixtures, controlled by microwave power, were monitored with thermosensors and pressure gauges, respectively. In order to prevent boiling of the reaction mixture, in-line pressure was regulated with nitrogen gas at optimum temperature for zeolite preparation. ZSM-5 was synthesized as follows: a precursor inorganic solution was prepared by mixing colloidal silica sol (Ludox HS40, 40 wt% SiO 2 ), sodium aluminate, sodium hydroxide and doubly distilled water. A typical molar composition was 1 SiO 2 : 0.033Al 2 O 3 : 0.25NaOH : 55.6H 2 O. ZSM-5 nanoseed solution prepared above was added to the aluminosilicate mixture. The nanoseed solution contained ZSM-5 less than 100 nm in size, of which amount was calculated as 5 wt% based on the total amount of SiO 2. TPAOH in the nanoseed solution was used as the templating agent and no additional organic template was introduced. This resulting precursor gel was stirred at room temperature for 30min. The resulting mixture was pumped continuously into CME fitted with teflon tube, and irradiated at 165 o C for 5-20 min under 100-250 W of microwave power. The resulting solid product was isolated by filtering, washing with deionized water, and drying in air at 100 o C for 10 h. In the case of NaY zeolite, a typical gel composition was 1 SiO 2 : 0.1 Al 2 O 3 : 0.1 NaOH : 55.6 H 2 O. NaY zeolite was synthesized without addition of nanoseed solution at150 o C for 30 min with the similar way to the synthesis of ZSM-5. X-ray powder diffraction (XRD) patterns were obtained on a Rigaku diffractometer using Cu Kα radiation. Scanning electron microscopy (SEM) was performed with a JEOL scanning electron microscope (model JSM 840). 3. RESULTS AND DISCUSSION 3.1. Preparation of ZSM-5 nanoseed solution Figure 1 shows the XRD pattern and SEM image of ZSM-5 nanoseed synthesized under microwave irradiation at 165 o C for 10 min. This material exhibits characteristics of the MFI structure with orthorhombic symmetry as shown in Figure 1a. SEM image in Figure 2b shows that the nanosized ZSM-5 exhibits very uniform spherical morphology having less than 100 nm particle sizes. The average particle size is 83 nm as determined by using dynamic light scattering (not shown data). Uniformity and particle size of ZSM-5 are controllable by (i) a source of particulate silica; (ii) an amount of organic templating agent; (iii) an alkali source: and (iv) aging time and stirring speed. Moreover microwave irradiation results in the formation of more uniform sized nanoseed particles than those from hydrothermal heating due to fast, homogeneous heating, formation of active water molecules, the gel dissolves quickly, and finally simultaneous nucleation of zeolites [7.11,12].

3.2. Rapid synthesis of ZSM-5 using batch-type microwave oven Figure 2 shows crystallization times of ZSM-5 synthesized using batch-type microwave Intensity/ a.u. 10 20 30 40 50 2 theta/ degree (a) (b) Figure 1. XRD pattern (a) and SEM image (b) of nanosized ZSM-5 material synthesized under microwave irradiation at 165 o C for 10 min. oven (Figure 2a and 2b) and conventional hydrothermal oven (Figure 2c). The crystallization time of microwave environment is dramatically reduced in comparison with that of Crystallinity/ % 100 80 60 40 20 (a) (b) 0 0 1 2 3 4 20 40 Time/ h Figure 2. Relationship between crystallinity and crystallization time of ZSM-5 synthesized at 165 o C in (a) batch-type microwave oven with 5 wt% nanoseed solution, (b) batch-type microwave oven without nanoseed, and (c) conventional hydrothermal oven. (c) hydrothermal method. The crystallization time for ZSM- 5 is extremely reduced within 5 min in the presence of nanoseed as shown in Figure 2a. Preparation method of seed solution will be mentioned at following section in detail. These phenomena by microwave environment can be deducted as rapid heat-up effect, nucleation crystallization, homogeneous and local superheating, etc. [2-4].

Previously, Jansen et al. have reported that an essential difference between conventional and microwave heating is the enhancement of the Brownian motion and the rotation dynamics of the water molecules [7]. In case of the rotational motion, far more hydrogen bridges of water molecules are destroyed resulting in so-called active water molecules. The active water molecules have a higher potential compared to the hydrogen-bonded water molecules to dissolve gel because the lone pairs and OH groups of the active water molecules are available to attack gel bondings [7]. However, it was suggested that the rearrangement of the synthesis mixture to yield nuclei was the bottleneck in a microwave synthesis [16]. A nucleationrelated bottleneck in microwave zeolite synthesis was also demonstrated by Cundy et al. and proved the remarkable synergic effects between microwave heating and the addition of seed crystals in zeolite synthesis, similar with the present result (Figure 2a). This rapid crystallization of zeolite makes it possible to design a continuous system for the synthesis. 3.3. Synthesis of ZSM-5 and NaY zeolite using continuous microwave equipment Figure 3 shows XRD patterns of ZSM-5 prepared from the mixture containing 5 wt% nanoseed solution using CME. All XRD patterns in Figure 3 illustrates that high-quality ZSM-5 can be continuously obtained under the present CME conditions. The pressure was controlled to100 psi by nitrogen, a feeding rate was 12.6 ml/min and a microwave power of 250 watt. Here, molar ratio of total silica/ TPA template is about 57. Although our CME is (c) Intensity/ a.u. (b) (a) 10 20 30 40 50 2theta/ degree Figure 4. SEM image of ZSM-5 prepared at 165 o C for 5 min under a continuous microwave equipment. Figure 3. XRD patterns of ZSM-5 prepared under a continuous microwave equipment at 165 o C for (a) 5 min, (b) 10 min, and (c) 20 min.

lab-scale in the present work, ZSM-5 material can be obtained over 100g per hour and ZSM- 5 can be produced as the same quality for 24 hours. SEM image in Figure 4 indicates ZSM-5 particles (sample in Figure 3a) exhibit regular and agglomerate round-like morphology (< 300 nm) that can be controlled by the size of nanoseed crystal, an amount of organic template, a synthesis time and so on. These structural properties are very similar to those of ZSM-5 obtained by the batch-type microwave oven at 165 o C (650 watt) for 5 min from the same reaction mixture. Intensity/ a.u. Figure 5 shows XRD patterns of NaY zeolite prepared using CME and conventional hydrothermal oven. CME results in the formation of high quality of NaY zeolite within 30 min. Particle morphology of NaY thus obtained are similar with that of NaY zeolite synthesized by hydrothermal heating. However, the longer crystallization time at CME gives mixture of NaY zeolite and P zeolite. The results suggest that the CME is believed to be very effective for the synthesis of porous materials. Thus, the manufacturing process of CME provides the following advantages: (a) the reaction time is further shortened by several to tens of minutes for crystallization, compared to the conventional hydrothermal reaction requiring a prolonged time, (b) the continuous manufacturing and collection processes of CME can give access to mass-scale production of inorganic crystals with relatively small facility, compared to the conventional batch-type hydrothermal or microwave synthesis, and (c) less amount of organic templating material can be required during the manufacture of molecular sieve. 5 10 15 20 25 30 35 40 45 50 2theta/ degree Figure 5. XRD patterns of NaY zeolites obtained (a) with continuous microwave equipment at 150 o C for 30 min and (b) by conventional hydrothermal synthesis at 100 o C for 1 d. (b) (a) Microwave effects on synthesis of the porous materials may be explained by two different mechanisms, i.e., rapid heat-up of the reaction mixture and superheating by a better heat transfer which results in rapid and sufficient heating of the synthesis mixture [15]. 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 [11]. We speculate that the extremely fast synthesis may be give rise to selective microwave adsorption of particle surface having the microwave susceptible hydroxyl groups and of its surface mediated TPA organic templates or of its boundary layer associated water molecules, as an even more significant microwave effect. 4. CONCLUSIONS This study demonstrates that a rapid and lucrative mass production of porous materials such as ZSM-5 and NaY is successfully achieved using a continuous microwave equipment (CME). Especially, ZSM-5 synthesis was accomplished within 5 min in the presence of nanoseed. Synthesis of NaY zeolite also produced within 30 min under CME. The results show that the structural properties of the zeolites thus obtained are similar to those of the materials obtained using batch-type microwave oven and by conventional hydrothermal synthesis. ACKNOWLEDGMENTS We appreciate the Korean Ministry of Science and Technology (Institutional Research Program, KK-0005-F0) for supporting this work. REFERENCES 1. P. Chu and F.G. Dwyer, US Patent No. 4 778 666 (1988). 2. C.S. Cundy, Collect. Czech. Chem. Commun. 63 (1998) 1699. 3. S. A. Galema, Chem. Soc. Rev., 26 (1997) 233. 4. C. Gabriel, S. Gabriel, E. H. Grant, B. S. J. Halstead, D. M. P. Mingos, Chemical Society Reviews, 27 (1998) 213. 5. A. Arafat, J.C. Jansen, A.R. Ebaid, and H. van Bekkum, Zeolites, 13 (1993) 162. 6. S.-E. Park, D. S. Kim, J.-S. Chang, and W. Y. Kim, Catal. Today, 44 (1998) 301. 7. J.C. Jansen, A. Arafat, A.K. Barakat and H. van Bekkum, in M.L. Occelli and H.E. Robson (Eds.), Synthesis of Microporous Materials, Van Nostrand Reinhold, Vol. 2, New York, 1992, p. 507. 8. X.C. Xu, W.S. Yang, J. Liu, and L.W. Lin, Adv. Mater., 12(3) (2000) 195. 9. R. Gedye, K. Westaway, and F. Smith, in D.E. Clark, D.C. Folz, S.J. Oda (Eds.), Microwave: Theory and Application in Materials Processing III, Ceram. Trans. 59, Am.

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