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J. Iran. Chem. Soc., Vol. 6, No. 1, March 2009, pp. 187-194. JOURNAL OF THE Iranian Chemical Society

Preparation and Characterization of Nanosized ZSM-5 Zeolite Using Kaolin and Investigation of Kaolin Content, Crystallization Time and Temperature Changes on

the Size and Crystallinity of Products

M. Khatamiana,* and M. Irania

aInorganic Chemistry Department, Faculty of Chemistry, University of Tabriz, C.P. 51664, Tabriz, Iran

(Received 4 September 2007, Accepted 20 May 2008)

Nano sized ZSM-5 zeolite samples were synthesized successively from kaolin clay as alumina source having a large amount of quartz (39%) and silicic acid as silica source by hydrothermal treatment with NaOH in the presence of tetrapropylammonium hydroxide as a template. Then the effect of kaolin content, crystallization temperature and time on the size and crystallinity of the products were investigated. The prepared samples were characterized using XRD, SEM, EDS and FT-IR techniques. The results showed that the synthesized ZSM-5 zeolite samples were almost pure and their crystallization was almost complete. The average particle size, as determined by Debye-Scherrer equation, was in the range of 20-42 nm. Increasing kaolin content on crystal size was more effective than increase in crystallization temperature and time. Additional evidences for the nano sized ZSM-5 zeolite were the asymmetric stretch vibration band at 1225 cm-1 in the FT-IR spectra and TEM images. The scanning electron micrographs of the synthesized zeolites showed that they are spherical shape crystals. Keywords: Nano zeolite, ZSM-5, Kaolin, Zeolite crystallization, Crystallinity INTRODUCTION ZSM-5, first prepared by Argauer and Landolt in 1971 [1], is a medium pore channel (~6 A۫) zeolite with three-dimensional channels defined by 10 member rings. Due to its unique channel structure, thermal stability, acidity and shape-selective property, ZSM-5 has been used as sorbents and catalysis. It can be applied to petrochemical processing, fine chemical production, and liquid-gas separation [2,3]. Thus, increasing attention has been devoted to the synthesis, properties and applications of ZSM-5 zeolite. Crystalline zeolites may be divided into three general classes based on their Si/Al ratio. Low-silica Zeolites are defined as having 1 < Si/Al < 2, while intermediate Si/Al *Corresponding author. E-mail: mKhatamian@yahoo.com

zeolites contain 2 < Si/Al < 5 and high-silica zeolites with Si/Al > 5 were synthesized [4]. As the Si/Al ratio increases, the properties of the zeolite are significantly altered. The pathway of the crystallization process as well as the morphology and singular properties (crystal size and distribution) of the MFI-type zeolite are influenced by different variables, including the silicon and aluminum source [5], the template/silicon ratio, the nature of the cations present in the synthesis medium [5,6,7], the alkalinity [8,9,10,11], etc. Thus, it is well established that an increase in the SiO2/Al2O3 and OH/SiO2 molar ratios as well as a decrease in the H2O/SiO2 molar ratio in the initial synthesis solution leads to a decrease in the final particle size [12]. In this research we used kaolin as alumina source and investigated the effect of changing kaolin content, crystallization time and temperature on the size and

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crystallinity of the nano-sized ZSM-5.

EXPERIMENTAL Materials and Procedure The alumina source used was kaolin from Zonouze Co. Iran, the silica source used was silicic acid (Merck). The alkaline source was sodium hydroxide pellets (Merck), template source used was 40% solution of tetrapropylammonium hydroxide in water (TPAOH, Merck) and boric acid (Merck). Nano sized zeolite was prepared according to a modified hydrothermal procedure which was reported previously by Miller for micro-sized ZSM-5 zeolite [13]. In this research kaolin was heated at 600 °C and a metastable phase referred to as metakaolin or dehydrated kaolin was obtained [14]. To synthesize the zeolite, 5 g of silicic acid was added to different amounts (1.0, 0.5 and 0.25 g) of metakaolin (70.05% SiO2 and 18.44% Al2O3). Then 1.6 ml of TPAOH and 20 ml distilled water were mixed for one hour in a mixer. A solution of 0.42 g H3BO3 in 2.5 ml water was then added to the above mixture along with a NaOH solution (0.33 g in 2.5 ml water), and mixing continued for one more hour. The mixture was placed in a stainless steel reactor with internal Teflon vessel and heated at autogenous pressure at different crystallization temperatures and times. Finally, it was calcined in air at 500 ºC for 8 h (yield 76%). The products were identified by XRD, FT-IR, TEM and SEM methods.

Characterization X-ray diffraction (XRD) patterns were measured on a D 500 Siemens X-ray diffractometer using monochromatic Cu Kα radiation (30-40 kV and 40-50 mA), and the relative crystallinity of ZSM-5 was calculated based on the intensity of the peaks of 2θ = 22-25. The morphology and crystalline size of the samples were examined under a transmission electron microscope (TEM) Philips CM 200 FEG (Field Emission gun) and scanning electron microscope (SEM, LEO 440i) using samples coated with an Au film Elemental analysis was carried out using link, ISIS-300, Oxford EDS (energy dispersion spectroscopy) detector. The FT-IR spectra were recorded using Bruker Model Tensor 27 equipment. Calculation of degree of crystallinity was carried out using the IR data as follows: degree of %crystallinity = peak area at 542/peak area at 450 cm-1 [15,16]. RESULTS AND DISCUSSION X-Ray Diffraction XRD pattern of synthesized zeolite is shown in Fig. 1. The peaks at 2θ = 7.96, 8.91, 14.00, 14.87, 20.92, 23.28, 23.97, 24.52, 26.66 confirmed ZSM-5 zeolite [17]. Figure 2a indicates XRD pattern of samples that crystallized by changing crystallization temperature from 120 ºC to 180 ºC. The crystallinity of as-synthesized sample, which was crystallized at 180 ºC temperature, showed the highest

Fig. 1. XRD pattern of ZSM-5 types obtained from weight ratio of kaolin/silicic acid = 10 × 10-2.

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crystallinity. Figure 2b indicates the XRD pattern of samples crystallized by changing crystallization time from 168 to 72 h. The crystallinity of as-synthesized sample, which was crystallized at 168 h, showed the highest crystallinity. Figure 2c indicates the XRD pattern of samples that crystallized by changing the amount of kaolin amount. The crystallinity of as-synthesized sample, which was crystallized at 168 h, 180 ºC and 20 × 10-2 Kaolin/silicic acid (%wt), is the highest. The effect of crystallization time, crystallization temperature and different amount of kaolin on average crystal size of as-synthesized samples are summarized in Tables 1, 2 and 3, respectively. By increasing the Si/Al ratio in zeolite, the crystal size of ZSM-5 became smaller. It could be identified from investigations which their results are gathered in Tables 1-3 that variation in

Si/Al ratio has more effect on crystal size. Average crystal size measured by Scherer's equation from XRD peak between 2θ = 7-10. D = K (λ/βcosθ) (Scherer's equation) (1)

K = 0.89 (constant), λ = 0.154 (nm) X-ray wavelength

β = the peak width at half of its height

θ = Scattering angle

D1 = 0.89 (0.154/4.98 × 10-3 × 0.998) = 27.57 (nm) 2θ ~ 7 D2 = 0.89 (0.154/7.5 × 10-3 × 0.997) = 18.32 (nm) 2θ ~ 9

Fig. 2. Effect of crystallization temperature (a), time (b) and amount of kaolin(c) on crystallinity of

as-synthesized samples

Table 1. Effect of Crystallization Time on Crystallinity, Average Crystal Size of as- Synthesized Samples

Formed phase Average crystal size (nm)a

Relative intensity of the peaks at 2θ = 22-25

Crystallization times (h)

ZSM-5 27.60 1666.370, 844.190, 550.560 168 ZSM-5 24.80 1103.084, 591.106, 388.940 144 ZSM-5 23.05 1066.940, 834.640, 538.390 120 ZSM-5 22.95 1030.997, 512.411, 333.376 72

aAverage crystal size measured by Scherre's equation for the peaks between 2θ = 7-10.

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D(average) = 22.94 (nm) for A/B (weight ratio of kaolin/silicic acid) = 5 ×10-2

FT-IR Figure 3 shows FT-IR spectra of synthesized ZSM-5. The findings emerge that the bands near 1080 cm-1 (internal asymmetric stretch), 790 cm-1 (external symmetric stretch), 542 cm-1 (double ring vibration) and 450 cm-1 (T-O bend) were presented for all samples, due to formation of only ZSM-5 phase. Additional evidence for the nano sized ZSM-5 zeolite was the asymmetric stretch vibration of the band at 1225 cm-1, which has been assigned to external linkages (between TO4 tetra hedral) and is a structure-sensitive IR band of ZSM-5 zeolite [16]. In the FT-IR spectra (Fig. 3) an absorption signal corresponding to the MFI (pentasil, S5R, SBU) phase skeletal vibration appears at 542 cm-1. This band, not observed in the amorphous silica, is indicative of the presence of MFI atomic ordering, even in the range of a few unit cells (<8 nm) [18,19]. Figure 4 shows FT-IR spectra of as-synthesized samples obtained by using different amount of kaolin (a), different crystallization time (b), and different crystallization temperature (c).

The effects of crystallization time, crystallization temperature, amount of kaolin on the ratio of the intensities of the peak near 542 (A) and 450 (B) cm-1 are given in Table 4. The results display that the ratio and consequently relative crystallinity increased in the following order: 72 < 120 < 144 < 168 h for different crystallization time, 120 < 140 < 160 < 180 ºC for different crystallization temperature, 5 × 10-2

< 10 × 10-2 < 20 × 10-2 (Kaolin/silicic acid %wt) for different amount of kaolin, which are in close agreement with the calculated results from XRD. SEM The scanning electron micrographs of zeolites having 20 × 10-2, 10 × 10-2, 5 × 10-2 (Kaolin/silicic acid %wt) (a), (b) and (c), respectively are shown in Fig. 5 which shows that these zeolites crystallize in spherical shape crystals [15]. Also EDS of sample b is shown in Fig. 6 that indicates the Si/Al ratio is about 58. TEM TEM image of zeolite having 5 × 10-2 kaolin/silicic acid %wt is shown in Fig. 7a, indicating less than 20 nm particle

Table 2. Effect of Crystallization Temperature on Crystallinity and Average Crystal Size of the as- Synthesized Samples

Formed phase Average crystal size (nm)a

Relative intensity of the peaks at 2θ = 22-25

Crystallization temperature (ºc)

ZSM-5 27.60 1666.37, 844.19, 550.56 180 ZSM-5 23.05 1536.72, 758.75, 518.64 160 ZSM-5 22.97 1118.97, 581.86, 363.66 140 ZSM-5 22.96 910.32, 455.16, 291.09 120

aAverage crystal size measured by Scherre's equation for the peaks between 2θ = 7-10. Table 3. Effect of Different Amount of Kaolin on Crystallinity and Average Crystal Size of the as- Synthesized Samples

Formed phase Average crystal size (nm)a

Relative intensity of the peaks at 2θ = 22-25

A/B (Weight ratio of kaolin/silicic acid)

ZSM-5 42.40 1666.37, 844.19, 550.5 20 × 10-2 ZSM-5 27.60 1497.12, 778.91, 515.9 10 × 10-2 ZSM-5 22.94 1422.15, 766.72, 513.2 5 × 10-2

aAverage crystal size measured by Scherrer's equation for the peaks between 2θ = 7-10.

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Fig. 3. FT-IR spectra of a nano sized ZSM-5 obtained from weight ratio of kaolin/silicic acid = 10 × 10-2.

Fig. 4. FT-IR spectra of as-synthesized samples considering the effect of kaolin/silicic acid ratio time and temperature.

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Table 4. Effect of the Crystallization Time, Crystallization Temperature and Amount of Kaolin on the Ratio of the Intensities of the Peaks Near 542 (A) and 450 (B) cm-1

A/B Ratio Kaolin/silicic

acid (%wt) A/B Ratio Temperature

(ºC) A/B Ratio Time (h)

0.98 20 × 10-2 0.98 180 0.98 168 0.92 10 × 10-2 0.93 160 0.86 144 0.85 5 × 10-2 0.91 140 0.85 120

0.89 120 0.83 72

(a)

(b)

(c)

Fig. 5. SEM images of the samples prepared by using different weight ratio of kaolin/silicic acid, where a = 20 × 10-2, b = 10 × 10-2 and c = 5 × 10-2.

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Fig. 6. EDS pattern of nano ZSM-5 obtained from weight ratio of kaolin/silicic acid = 10 × 10-2.

a b

Fig. 7. (a) TEM image of ZSM-5 zeolite with A/B (weight ratio of kaolin/silicic acid) = 5 × 10-2, (b)The TEM image of some well grown ZSM-5 crystals.

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size for ZSM-5 sample; however some well grown particles are observable in this sample (Fig. 7b). CONCLUSIONS ZSM-5 zeolite was successively synthesized and the effects of crystallization time, crystallization temperature and different amount of kaolin on zeolite crystallization have been investigated. XRD patterns of the samples that crystallized by different conditions of synthesis confirmed that ZSM-5 zeolite phase is formed. The crystallinity of as-synthesized sample, which crystallized at 168 h, 180 °C and 20 × 10-2 (weight ratio kaolin/silicic acid), is the highest. SEM images show that the samples are crystallized in spherical shape crystals and Si/Al ratio of the synthesized zeolites was determined about 58-60 by EDS method. IR Spectra confirmed that the samples, which were prepared by different condition of time, temperature and amount of kaolin, resulted in the ZSM-5 phase. ACKNOWLEDGEMENTS We are grateful to university of Tabriz Research Council for the financial support of this research also we thank the assistant Ali-Almasi and Mohamed-Bahrami for SEM micrographs and XRD tests. REFRENCES [1] R.J. Argauer, G.R. Landolt, US Patent 3,702,886 (1972). [2] C. Falamaki, M. Edrissi, M. Sohrabi, Zeolite 19 (1997)

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