Synthesis and Characterization of Fe/SUZ-4 Zeolite
S. Turapana, P. Kongkachuichayb, P. Worathanakula* aDepartment of Chemical Engineering, Faculty of Engineering,
King Mongkut’s University of Technology North Bangkok, Bangkok, 10800, Thailand bDepartment of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok, 10900, Thailand
Elsevier use only: Received 30 September 2011; Revised 10 November 2011; Accepted 25 November 2011.
Zeolites are crystalline, hydrated aluminosilicates are formed in nature or can be synthesized. The zeolites framework of [SiO4]4- and [AlO4]5- tetrahedral, bonded together via the oxygen atoms. Hence, they are generating a high internal surface area available for adsorption and catalytic processes. The properties of zeolites are molecular sieves which make them particularly suitable for use as catalysts. They are well-defined uniform pore and crystal structure, temperature stability and easy ion-exchange method.
SUZ-4 is a new type of zeolite [1-4]. It is a medium pore zeolite consisting of a three dimensional pore system having straight ten-membered channels intersected by two eight-membered channels on its topology. Most catalysts for NOx reduction application, zeolites are used as support catalyst on metals
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ion- exchanged (e.g. Cu, Ag, Fe, Co, Pd, Pt) for improved activity for lean-NOx reduction [5-8]. Most attention has been given to the ZSM-5 catalyst [5-10]. However, deactivation of ZSM-5 catalyst was attributed to dealumination of the zeolite framework [4,5]. Up to now, SUZ-4 zeolite has been reported to great potential catalytic properties, high thermal stability and resistance to organic solvents [1-5]. Therefore, it is interested to synthesize SUZ-4 and Fe/SUZ-4 zeolites before using them as catalysts in NOx reduction.
The aims of this research were to synthesized SUZ-4 zeolite, and Fe(II) loading in the SUZ-4 zeolite by conventional ion-exchange method were investigated. Different Fe/SUZ-4 loadings and pristine SUZ-4 zeolites were prepared and investigated the physico-chemical properties. The characterization has been performed with atomic absorption spectroscopy (AAS), X-ray powder diffraction (XRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS). With the characterization on their physico-chemical properties discussed here, analysis the potential of the Fe-loaded SUZ-4 catalyst for NOx reduction is reserved for future work.
2. Experimental
2.1 Materials
The materials used to synthesize SUZ-4 zeolite are as follows: potassium hydroxide (85 wt.% KOH, Merck) for potassium source, aluminum powder (93 wt.% Al, Himedia) for aluminum source, tetraethylammoniumhydroxide (35 wt.% TEAOH, Aldrich) for the template, silica sol (Ludox AS-40 colloidal silica 40 wt.% SiO2 Aldrich) for silica source, and iron(II)chloride (99 wt.% FeCl2.4H2O, Merck) for iron loading.
2.2 SUZ-4 synthesis
The K/SUZ-4 zeolite was synthesized by sol-gel method under hydrothermal reaction following molar ratio 21.2SiO2 : Al2O3 : 7.9KOH : 2.6TEAOH : 498.6H2O reported in our previous study and others [4]. Firstly, aluminum solution was prepared by potassium hydroxide dissolved in distilled water and added aluminum powder under stirring for 24 hours or until the Al was completely dissolved. The gel was prepared by mixing of tetraethylammoniumhydroxide, silica sol and distilled water under stirring for 2 hours, and then the two solutions were mixed slowly with stirring for 3 hours, formed as gel. The gel was transferred to an autoclave (Parr Model 4561, USA) at 150 C for 4 days under autogenous pressure. The product was filtered, washed until pH < 9 and dried at 120 C for 4 hours. Finally, the product was calcined at 550 C for 4 hours in air.
2.3 Ion-exchange of K/SUZ-4
Fe(II)-exchanged K/SUZ-4 zeolite was prepared by conventional ion-exchange method with different weight percentages:1, 3, 5, 8 and 10. In this research, Fe/SUZ-4 was obtained from 1 g K/SUZ-4 and mixed solution 100 ml with stirring at room temperature for 24 hours. During the ion-exchange, it was performed in flowing Ar 20 ml/min, in order to prevent oxidation of Fe(II) to Fe(III). The Fe-exchanged zeolite was filtered and washed with deionized water to eliminate chlorine (detected with 0.1 M silver nitrate solution) before drying at 120 C overnight, then calcined at 350 C for 6 hours in air.
Analysis of the solution as well as of dissolved zeolites after the ion-exchange was performed by atomic absorption spectrometer (AAS Varian, AA280FS, USA). The zeolite powderd were determined by X-ray diffraction (XRD, Philips PW 1830/40, Netherlands). The functional group and the surface chemical bonding were observed by fourier transform infrared spectrometer (FT-IR, Perkin Elmer Spectrum One, USA). The morphology, structure and element of zeolite were analyzed by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS Jeol, JSM-5600 LV, USA).
3. Results and Discussion
3.1 AAS analysis of solution
The Fe(II) with K/SUZ-4 zeolite was ion-exchanged which was checked by AAS analysis of the solution after the completed ion-exchange. The results from AAS analysis confirmed that the ion-exchange occurred between Fe loading with potassium (K) from SUZ-4 zeolite, shown in Fig. 1. The effect of Fe loaded increasing, concentration of potassium in the solution after the ion-exchange was increased by the amount of wt.% Fe loading. The optimum Fe loading was at 5% wt., the reaction rate of concentration of potassium was constant. The iron contents in the zeolite were increased when Fe loading increased according to the percentage of Fe exchanged level increased (see in Table 1).
Fig. 1. Concentration of potassium in the solution with different wt.% Fe/SUZ-4
Table 1. Chemical elemental analysis of zeolite
Sample Iron content (wt.%) Si/Al Fe/Al Fe exchange level (%)
1 wt.% Fe/SUZ-4 0.49 6.08 0.052 15.92
3 wt.% Fe/SUZ-4 0.48 6.02 0.051 15.27
5 wt.% Fe/SUZ-4 0.63 6.64 0.063 18.70
8 wt.% Fe/SUZ-4 0.98 6.27 0.091 27.20
10 wt.% Fe/SUZ-4 1.08 6.08 0.098 29.32
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3.2 XRD studies of zeolite
The XRD patterns of K/SUZ-4 and 10%wt.Fe/SUZ-4 are shown in Fig. 2. The XRD pattern of K/SUZ-4 zeolite is shown in Fig. 2a the in line well with the diffraction standard of K/SUZ-4 as shown 2 theta at 7.7, 11.8, 15.1, 23.5, 25, 26 and 28.5 [5]. A comparison of XRD patterns of K/SUZ-4 and Fe/SUZ-4, diffraction lines for iron oxide were not observed in the XRD pattern of Fe/SUZ-4. This is not unusual as the diffraction lines iron oxide are expected to be broadened and likely to be buried in the background noise of the XRD pattern. Furthermore, the iron content of Fe/SUZ-4 zeolite is very low (<2%, as detected from elemental analysis as shown in Table 1. The results were according to Chen et al. [11] that reported the diffraction lines of -Fe2O3 could not be observed upon incorporation in mesoporous silica when the iron content was less than 10%. From our calcinations temperature at 350 C of Fe/SUZ-4, the main phases should be found in the iron oxide were -Fe2O3 (maghemite) and -Fe2O3 (hematite). -Fe2O3 was found at 300 C and mixture of -Fe2O3 and -Fe2O3 were formed when the temperature was raised up to 400 C [12]. Fe2O3 will be transformed into -Fe2O3 completely while the temperature was above 500 C [12]. G. Yao et al. [13] reported to -Fe2O was stable and did not transform to -Fe2O3 at 150-355 C.
Fig. 2. The XRD patterns of a) K/SUZ-4 and b) 10%wt.Fe/SUZ-4
3.3 FT-IR spectrum
FT-IR spectrum in the region of 4000-400 cm-1 for K/SUZ-4 and Fe/SUZ-4 were presented in Fig. 3 The peaks at 3436 cm-1 and 1630 cm-1 are assigned to stretching Si-OH and bending vibration of water molecular (OHO) [12]. The peaks at 1203 cm-1and 1077 cm-1 are assigned to the complex of Si-O stretching [12,13]. The peak at 800 cm-1 is assigned to vibration of Al-O-Si [15]. The position of the Si-O bending vibration is located at 590 cm-1, 521 cm-1 and 450 cm-1 due to Si-O-Al and Si-O-Si [16]. The IR spectra of -Fe2O3 appear at 445, 460 and 635 cm-1 [12], no indications for -Fe2O3 or any other iron oxide were observed. Nonetheless, one also keep in mind that the amount of -Fe2O3 or -Fe2O3 if formed might be too small to be detected by the FTIR.
Fig. 3 FT-IR spectra of a) K/SUZ-4, b) 3%wt. Fe/SUZ-4, c) 5%wt. Fe/SUZ-4, d) 8%wt. Fe/SUZ-4 and e) 10%wt. Fe/SUZ-4
3.4 The morphology and chemical-elemental analysis of zeolite
SEM micrographs of Fe/SUZ-4 were taken as shown in Fig.4. The SEM images shown structures of Fe/SUZ-4 zeolites still have needle-shaped crystal. It was also found that the crystal of Fe/SUZ-4 was approximately 0.06 dia. x 0.62 μm length. Chemical element of Fe/SUZ-4 zeolites was analyzed by energy dispersive spectroscopy (EDS) at 10,000x of magnification presented in table 1. The results were shown lower iron content of loading of Fe in the Fe/SUZ-4 zeolites. The Fe-exchange level was calculated by 3 x (number of iron ions)/(number of aluminum ions) [9] and was in the Fe exchange level between 15.2 to 27.2 only. However, our method is according to Subbiah et al. [5] for low exchange level that it can be occurred for Fe loading.
Fig. 4. SEM images of a) 1%wt. Fe/SUZ-4, b) 3%wt. Fe/SUZ-4, c) 5%wt. Fe/SUZ-4, d) 8%wt. Fe/SUZ-4 and e) 10%wt.
Fe/SUZ-4
W a v e n u m b e r ( c m - 1 )1 0 0 02 0 0 03 0 0 04 0 0 0
% T
rans
mis
tanc
e3436
1630
1077800 521
5904501203
a) K/SUZ-4
b) 3% wt. Fe/SUZ-4
d) 8% wt. Fe/SUZ-4
c) 5% wt. Fe/SUZ-4
e) 10% wt. Fe/SUZ-4
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4. Conclusions
Fe/SUZ-4 zeolite was prepared by conventional ion-exchange method followed by washing and calcinations. Fe/SUZ-4 zeolite was low iron content. Phase of iron oxide, can not be identified exactly that it also observed of the temperature calcined. The Fe-exchanged with zeolite remains the same, have still needle-shaped crystal.
Acknowledgements
The authors gratefully acknowledge the financial support of Thailand Research Fund (TRF) under grant number MRG5280110. We also acknowledge the grant for new researcher 2011 (Patcharin Worathanakul) from the Science and Technology Research Institute King Mongkut’s University of Technology North Bangkok (STRI, KMUTNB), office of National Research Council of Thailand (NRCT), Graduate school and Department of Chemical Engineering King Mongkut’s University of Technology North Bangkok.
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