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Synthesis of highly siliceous ZSM-5 zeolite using silica from rice husk ash
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Microporous and Mesoporous Materials 115 (2008) 189–196 www.elsevier.com/locate/micromeso
Synthesis of highly siliceous ZSM-5 zeolite using silica from rice husk ash K. Kordatos a,*, S. Gavela a, A. Ntziouni a, K.N. Pistiolas b, A. Kyritsi b, V. Kasselouri-Rigopoulou a a
School of Chemical Engineering, National Technical University of Athens, 9 Heroon Polytechniou Street, 15780 Zografou, Greece b ‘‘Agrino’’, EV.GE. Pistiolas S.A., National Road 20, 30100 Agrinio, Greece Received 22 July 2007; accepted 19 December 2007 Available online 19 February 2008
Abstract In this study, a simple synthetic route is demonstrated towards the efficient production of ZSM-5 zeolite. Crystalline rice husk ash (RHA) was utilized as an alternative silica source for the synthesis of ZSM-5. The reaction of RHA with the organic template TPABr at low temperature and under atmospheric pressure led to the successful transformation of the crystalline ash to ZSM-5. The synthesis was studied as a function of time and the products at each stage were characterized using a variety of analytical techniques, including Xray diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), thermogravimetry-differential thermogravimetry analyses (TG-DTG), scanning electron microscopy (SEM) and electron dispersion X-ray analysis (EDX). Porosimetry measurements revealed that the product is a microporous material having a high specific surface area of 397 m2 g–1. Ó 2008 Elsevier Inc. All rights reserved. Keywords: ZSM-5 zeolite; Synthesis; Low temperature; Atmospheric pressure; RHA
1. Introduction Rice husk is a major by-product of the rice-processing industries and like most of the other rural biomass materials (e.g., sugar, cane leaf, corn leaf) is recognized as a potential source for energy generation from gasification or incineration processes. The burning of rice husk in air results in the formation of RHA with a content in SiO2 that varies from 85% to 98% depending on the burning conditions, the furnace type, the rice variety, the rice husk moisture content, the climate and the geographic area [1]. Some small amounts of inorganic impurities are always present in the ash along with unburned carbon. The unburned carbon can be removed from the ash by further heating treatments at high temperatures, but this usually leads to the crystallization of the amorphous silica to cristobalite and/or tridy-
*
Corresponding author. Tel.: +30 210 7723100; fax: +30 210 7723188. E-mail address: [email protected] (K. Kordatos).
1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.12.032
mite [2]. The crystallization of the contained silica of the ash also occurs when the burning conditions of husk are uncontrolled [3]. This crystallization is a disadvantage towards the preparation of silicon based materials, because the silica ash is rendered inactive in its crystalline form [3,4]. RHA can be used as an alternative cheap source of amorphous silica for the production of silicon based materials with industrial and technological interests [5–9]. Among the various utilizations of RHA, there is a significant interest in its use in the preparation of zeolites due to the widespread industrial use of zeolites in separation processes as sorbents [10], as well as in catalytic refinery and petrochemical processes [11]. In 1972 Mobil Co. reported the synthesis of the high silica zeolite of type ZSM-5 [12]. ZSM-5 is a medium pore zeolite formed by ten-membered rings which possesses a pore dimension of 0.54–0.56 nm. Its unique pore structure has excellent shape selectivity, while the ability to develop
190
K. Kordatos et al. / Microporous and Mesoporous Materials 115 (2008) 189–196
internal acidity makes it an interesting material for catalyzing organic reactions. Since its discovery, extensive work has been carried out on the synthesis and applications of ZSM-5. High crystalline ZSM-5 can be synthesized within a period of about 168 h in autoclave at 120–180 °C under autogenous pressures. The autoclavation time can be reduced to 4–6 h under high pressures and temperatures (about 40–60 atm and 230–250 °C) [13]. The possibility of synthesizing ZSM-5 at temperatures between 90–100 °C and atmospheric pressure has also been reported [14–19]. These methods make available the study of the crystallization mechanism since they require a long induction period. Among the several templates used for the synthesis of ZSM-5, the most common is tetrapropyl ammonium bromide (TPABr). In all the above mentioned studies on the synthesis of ZSM-5, the starting materials for silica and alumina were from pure chemical sources. Efforts have also been made to use RHA as a source of silica for the synthesis of ZSM-5 with the autoclave process [3,20–22]. In these works, however, the silica prior to the synthesis of the zeolites was prepared in the amorphous form, either after extraction with suitable alkali solutions or under controlled burning of husks. In the present study, we have utilized crystalline silica of RHA for synthesizing highly siliceous zeolite of type ZSM-5 under atmospheric pressure for the first time. The synthesis under atmospheric conditions is advantageous compared with hydrothermal treatments at high pressures due to the lower cost of energy and setup simplicity required, especially when mass production of ZSM-5 is needed for the different applications.
2. Experimental 2.1. Materials Rice husk ash, pellets of sodium hydroxide (fluka), tetrapropylammonium bromide (TPABr) (fluka) and distilled water were used as the starting materials in the initial mixture for the synthesis of ZSM-5 zeolite [19]. The silica and alumina source used for the experiments was a RHA obtained from a Greek rice mill (AGRINO) after burning rice husk during the process of product manufacture. The as received RHA was further heat treated at 700 °C for 5 h, for increasing the silica content by reducing the amount of unburned carbon. The molar ratio of SiO2/ Al2O3 in RHA was 978. Table 1 Chemical composition of RHA before and after heat treatment at 700 °C for 5 h Oxides
RHA (wt%) as received
RHA (wt%) after heat treatment
SiO2 Al2O3 Fe2O3 K2O CaO P2O5 MgO Na2O MnO TiO2 ZnO Loss on ignition
76.66 0.07 0.04 2.47 1.07 0.86 0.65 0.29 0.13 0.01 0.01 17.74
92.28 0.16 0.08 2.86 1.33 0.97 0.77 0.33 0.22 0.02 0.01 0.92
*
* Cristobalite
RHA at 1000oC for 5h
o Tridymite
* o o
*
*
* *
* *
Intensity
RHA at 1000oC for 2h
RHA at 700oC for 5h
RHA
10
15
20
25
30
35
40
45
50
2θ Fig. 1. XRD patterns of untreated RHA, heat treated RHA at 700 °C for 5 h, heat treated RHA at 1000 °C for 2h and heat treated RHA at 1000 °C for 5 h.
K. Kordatos et al. / Microporous and Mesoporous Materials 115 (2008) 189–196
2.2. Methods
191
RHA (3.01 g) was predispersed in distilled water (25 mL) containing NaOH (2.5 g) with stirring. The whole system was introduced in an oil bath and left to reflux at an oil bath temperature of 110 °C for 24 h. An aqueous solution of tetrapropylammonium bromide (TPABr) prepared separately by mixing TPABr (2.33 g) and distilled water
Synthesis runs were carried out using the same initial molar composition of the starting materials, 12NaOH: 30SiO2:0.03Al2O3:6TPABr:1800H2O, according to the following procedure.
Fig. 2. SEM images of heat treated RHA at 700 °C for 5 h (a) 200, (b) 800. Scale bars correspond to (a) 200 lm and (b) 50 lm.
(313)
(033)
(051) (012) (031) (022)
(111)
(200)
(501)
(011)
* cristobalite
11 days
*
Intensity
10 days
9 days
8 days
7 days
6 days 10
20
30
40
50
2θ Fig. 3. XRD patterns of the obtained materials after 6, 7, 8, 9, 10 and 11 days of reaction.
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K. Kordatos et al. / Microporous and Mesoporous Materials 115 (2008) 189–196
(25 mL) was slowly added to the predispersed RHA solution under stirring and the whole mixture was continued to reflux at 110 °C. Samples were collected for runs from 1 to 11 days. At the end of each run, the reaction mixture was centrifuged and the solid phase was washed several times with distilled water and dried at 110 °C for 12 h. The obtained materials were then calcined with a heating rate of 10 °C/min at 550 °C for 5 h to remove the organic template.
were measured by the Brunauer, Emmet and Teller (BET) method using nitrogen adsorption–desorption isotherms at 196 °C on a Quantachrone Autosorb-1 instrument. Samples were outgassed at 300 °C for 24 h prior to measurements. The measurements were performed after removal of the organic template. 3. Results and discussions 3.1. Formation of ZSM-5 zeolite crystals
2.3. Characterization Quantitative chemical analyses of RHA were accomplished by XRF (XRF, ARL Advant XP). XRD measurements were performed with a Siemens D5000 difractometer ˚ ). The morphologies of the samples (Cu Ka, k = 1.5406 A were studied by SEM after gold coating using a FEI Quanta 200 instrument operating at 30 keV and equipped with an EDAX detector. Infrared spectra were recorded on a Spectrum One Perkin Elmer spectrometer using the KBr pellet technique. The samples for FT-IR measurements were prepared by a KBr/sample weight ratio of 100/0.04. TG-DTG analyses were carried out in air at a heating rate of 10°C/min from 25 °C to 800 °C on a Mettler Toledo TGA/SDTA851e instrument. In addition, the specific surface areas of RHA, intermediates and final product
RHA was used as the silica and alumina source in the synthesis of ZSM-5. For increasing the silica ash content by decreasing the coexisting amount of unburned carbon, three different heat treatments were applied to the supplied RHA. Particularly, RHA was heat treated at 700 °C for 5 h, 1000 °C for 2 h and 1000 °C for 5 h, respectively. Fig. 1 shows the XRD patterns of the resulting ashes along with the untreated RHA. The analysis of the patterns shows that the supplied RHA presents its crystalline phase in the form of cristobalite. The presence of cristobalite is indicated by diffraction peaks at 2h, 21.9°, 28.5°, 31.5° and 36.3°. When RHA is heat treated at 700 °C for 5 h, it retains its initial cristobalite phase. However, when the RHA is heat treated at 1000 °C for 2 and 5 h, its crystalline phase is partially changed as indicated in the XRD patterns with the appear-
6 days
Derivative weight (%/oC)
7 days
8 days
9 days
10 days
11days
100
200
300
400
500
600
700
800
Temperature (oC) Fig. 4. DTG thermodiagrams of the obtained products after 6, 7, 8, 9, 10, and 11 days of reaction.
K. Kordatos et al. / Microporous and Mesoporous Materials 115 (2008) 189–196
ance of new diffraction peaks corresponding to the tridymite phase. Consequently, we chose to utilize the heat treated RHA at 700 °C for 5 h as silica source for the ZSM-5 zeolite preparation, because this treatment did not change the cristobalite phase of the ash, but increased its silica content from 76.66% to 92.28%. The glass content was 51% of total silica content of RHA and it was determined according to EN 196-2. In all cases, the heat treatment changed the color of the ash from black to gray due to decreasing amount of unburned carbon. The chemical compositions of RHA before and after thermal treatment were determined by X-ray fluorescence (XRF) and are summarized in Table 1. The loss on ignition, which corresponds to the removal of moisture and the coexisting unburned carbon, was determined by heating certain quantities of RHA samples at 975 ± 25 °C in air according to the EN 196-2 and prEN 450-1. The morphology of the heat treated RHA was studied by scanning electron microscopy (SEM). Fig. 2 shows SEM images of RHA after thermal heating at 700 °C. The ash retains the serrated structure of rice husk and consists mainly of fragments of loose flakes with a skeleton like inner structure. The solids obtained from the synthesis runs after calcination at 550 °C were characterized using X-ray diffraction
193
technique. The XRD patterns collected for runs from 6 to 11 days are presented in Fig. 3. Samples obtained for synthesis runs from 1 to 5 days exhibit similar XRD patterns with the sample obtained after 6 days of reaction, which consists of crystalline silica in the form of cristobalite. The XRD patterns corroborate that ZSM-5 is not obtained within 6 days under the present experimental conditions. However, with an increase of synthesis time the characteristic peaks of ZSM-5 become distinct. The diffraction peaks that appear after 7 days of reaction correspond to the characteristic peaks of ZSM-5 zeolite. The d and the hkl values obtained from the XRD analysis are in close agreement with the reported data of ZSM-5 zeolite [23]. The characteristic peaks of unreacted cristobalite coexist along with those of ZSM-5 and as the synthesis time increases their intensity decrease dramatically. 3.2. Thermal analysis Fig. 4 illustrates the DTG results of the samples obtained for different reaction runs. All measurements were carried out under an air atmosphere. Two peaks were observed in the DTG curve of the obtained product after 6 days of reaction. Similar thermal behavior was observed for the products obtained for less than 6 days of reaction
6 days
7 days
Transmittance (%)
8 days
9 days
10 days
11 days
546 1232 1400
1200
1000
Wavenumber
800
600
400
(cm-1)
Fig. 5. FT-IR spectra of the obtained products after 6, 7, 8, 9, 10 and 11 days of reaction.
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K. Kordatos et al. / Microporous and Mesoporous Materials 115 (2008) 189–196
Fig. 6. SEM images of the obtained products after (a) 4, (b) 5, (c) 6, (d) 7, (e) 8, (f) 9, (g) 10 and (h) 11 days of reaction. Scale bars correspond to 5 lm and all images have a magnification of 10,000.
K. Kordatos et al. / Microporous and Mesoporous Materials 115 (2008) 189–196
3.3. FT-IR Infrared spectroscopy was also used to follow the reaction process. Fig. 5 presents the FT-IR spectra of the samples obtained for different reaction runs. According to the Flanigen–Khatami–Szymanski correlation [25], the vibrational modes near 1100, 800 and 450 cm–1 are assigned to internal vibrations of Si, AlO4 tetrahedra and are also observed in silica, quartz and cristobalite, while the vibrational modes near 550 and 1230 cm–1 are due to the doublerings tetrahedra vibration and to the asymmetric stretching of Si and AlO4 tetrahedra in the zeolite framework, respectively. By examining the FT-IR spectra of the samples, it can be seen that the characteristic vibrational modes of the ZSM-5 zeolite framework are only appeared after 7 days of reaction time, in agreement with the XRD and DTG data. Particularly, the evidence for the zeolite formation comes from the appearance of the modes at 546 and 1232 cm–1 in the spectra of all the obtained samples for reaction runs from 7 to 11 days. As it is observed from the spectra, upon increasing of reaction time the zeolite’s framework bands become more pronounced due to the increasing amount of the zeolite in the samples. Besides those bands, all samples also show a small band at 620 cm–1, corresponding to the characteristic vibration of cristobalite. 3.4. Morphology Fig. 6 shows SEM images of the solids obtained for reaction runs from 4 to 11 days. As we can see from the images, for reaction runs less than 7 days the morphologies
of the solids consist of large fragments. With increasing of reaction time, the size of those fragments becomes smaller, for example, from about 20 lm for reaction time of 4 days (Fig. 6a) to about 5 lm for 6 days of reaction (Fig. 6c). Additionally, the reduced in size fragments are begun to be covered by agglomerates of silica particles (Fig. 6b and c). In agreement with the XRD, FT-IR and DTG data, ZSM-5 crystals are formed after 7 days of reaction (Fig. 6d). This sudden transformation might be explained taking into consideration the proposed crystal growth mechanism by Flanigen [26] in which, the amorphous gel is converted to crystals through the solid phase transformation mechanism. As the reaction proceeds further, the crystal growth continues resulting to the typical morphology of the ZSM-5 zeolite crystals in the form of prisms (Fig. 6e– h). The electron dispersion X-ray analysis (EDX) of the material obtained after 11 days of reaction shows as it was expected that it consists of Si, O and Al. 3.5. Nitrogen porosimetry For following the undergone changes into the textural evolution of the samples, N2 adsorption–desorption isotherms were measured at –196 °C over calcined samples prepared at different synthesis times. Fig. 7 shows the nitrogen isotherms of RHA and the samples produced after 3, 7, 10 and 11 days of reaction. The isotherms corresponding to RHA indicate that it is mainly non/macroporous, a large amount of the nitrogen adsorption being observed at high P/P0. Similar results are observed for the sample obtained after 3 days of reaction. As the reaction time increases and the formation of zeolite proceeds, the adsorption isotherms after 7, 10 and 11 days of reaction reveal the microporosity
300 RHA 3 days
250
7 days 10 days 11 days
200
V (cm3/g)
not shown in Fig. 4. In all these reaction runs, the samples consist of silica in the form of cristobalite as it was shown by the XRD analysis. The DTG pattern showed a peak bellow 100 °C which results from dehydration and a second peak at around 530 °C related to the combustion of unburned carbon [24]. The carbon content in the samples comes from the burning process of rice husk in which carbon is trapped as an impurity in the silica ash structure. For synthesis runs from 7 days or more, where the formation of ZSM-5 zeolite proceeds, the samples show four distinct peaks of weight loss. Along with the earlier mentioned peaks, two new peaks appear. A third peak at around 220 °C due to loosely bound TPA+ cations [18] and a fourth peak with a strong exothermic effect at around 430 °C due to the strongly bound TPA+ cations in the zeolite framework [18], which neutralize negative charges from Al species in the ZSM-5 framework. As it can be seen, by the evolution of the strong exothermic peak, both the amount and the decomposition temperature of the strongly bound TPA+ cation increased with increasing of reaction time, revealing the development of the zeolite framework in the products. It can be also noted that in the temperature range of 600–800°C there is not any weight loss, confirming the thermal stability of MFI phase.
195
150
100
50
0 0
0.1
0.2
0.3
0.4
0.5
0. 6
0.7
0.8
0.9
1
P/Po Fig. 7. Nitrogen adsorption–desorption isotherms at –196 °C of RHA and calcined samples after 3, 7, 10 and 11 days of reaction.
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K. Kordatos et al. / Microporous and Mesoporous Materials 115 (2008) 189–196
Table 2 Textural parameters from N2 adsorption–desorption isotherms on calcined samples at 550 °C Sample
SBET (m2/g)
Micropore volume (cm3/g)
Size of micropores (nm)
RHA 3 Days 7 Days 10 Days 11 Days
28 26 306 369 397
– – 0.109 0.131 0.140
– – 0.55 0.60 0.60
Pore Volume (cc/gÅ)
40 35
7 days
30
10 days
25
11 days
verted RHA to ZSM-5 was studied by N2 porosimetry and showed that starting with a non/macroporous material with a specific surface area of 28 m2 g–1, a microporous material with a high specific surface area of 397 m2 g–1 could be obtained. Acknowledgment The present work was supported from the 05-DSVEPRO-06-2005 program of General Secretariat for Research and Technology and is part-financed from the European Fund of Regional Growth. We would like to thank the Greek rice mill ‘‘Agrino”, EV.GE. Pistiolas S.A., for the RHA supply. References
20 15 10 5 0 0
2
4
6
8
10 12
14 16 18 20 22 24 26 28
Pore radius (Å) Fig. 8. Pore size distribution of calcined samples after 7, 10 and 11 days of reaction.
characteristics of the resulting samples, since they consist of sharp knees at P/P0 lower than 0.1 due to filling of microporous. The BET surface area, micropore volume and size of microporous of the examined samples are summarized in Table 2. As it can be seen, the increasing transformation of RHA into ZSM-5 with time results in increasing microporosity giving rise to a final product with a specific surface area of 397 m2 g–1 and a micropore volume of 0.14 cm3 g–1. The mean size of micropores in the zeolite was estimated to about 0.6 nm as shown in Fig. 8. 4. Conclusions It is demonstrated how an industrial by-product can be converted to a high value added product by using a simple inexpensive method. A powder consisted of 92% of silica was prepared after calcination of RHA at 700 °C for 5 h and it was then utilized as an alternative silica source for the production of ZSM-5 zeolite under atmospheric pressure. The reaction was studied as a function of time and the products were characterized at each stage using several analytical techniques. XRD, TG-DTG, FT-IR and SEM analyses revealed the transformation of RHA to ZSM-5 after 7 days of reaction. The textural evolution of the con-
[1] F.C. Lanning, J. Agric. Food Chem. 11 (1963) 435. [2] R.V. Krishnarao, J. Subrahmanyam, T. Jagadish Kumar, J. Eur. Ceram. Soc. 21 (2001) 99. [3] H. Hamdan, M.N.M. Muhid, S. Endud, E. Listiorini, Z. Ramli, J. Non-Cryst. Solids 211 (1997) 126. [4] J. Paya´, J. Monzo´, M.V. Borrachero, A. Mellado, L.M. Ordon˜ez, Cem. Concr. Res. 31 (2001) 227. [5] R.V. Krishnarao, J. Eur. Ceram. Soc. 12 (1993) 395. [6] I.A. Rahman, F.L. Riley, J. Eur. Ceram. Soc. 5 (1989) 11. [7] J.A. Amick, J. Electrochem. Soc. 129 (1982) 864. [8] L.P. Hunt, J.P. Dismukes, J.A. Amick, A. Schei, K. Larsen, J. Electrochem. Soc. 131 (1984) 1683. [9] A. Karera, S. Nargis, S. Patel, M. Patel, J. Sci. Ind. Res. 45 (1986) 441. [10] M.F.M. Post, Stud. Surf. Sci. Catal. 58 (1991) 391. [11] S.T. Sie, Stud. Surf. Sci. Catal. 85 (1994) 587. [12] R.J. Argauer, G.R. Landolt, US Patent 3702886, Mobil Co., 1972. [13] S.J. Kulkarni, P. Srinivasu, N. Narender, K.V. Raghavan, Catal. Commun. 3 (2002) 113. [14] W.J. Kim, M.C. Lee, D.T. Hayhurst, Micropor. Mesopor. Mater. 26 (1998) 133. [15] H.S. Oh, K.K. Kang, M.H. Kim, H.K. Rhee, Korean J. Chem. Eng. 18 (2001) 113. [16] K. Suzuki, Y. Kiyozumi, K. Matsuzaki, S. Shin, Appl. Catal. 42 (1988) 35. [17] K.H. Yi, S.K. Ihm, Micropor. Mater. 1 (1993) 115. [18] Y. Kiyozumi, S. Shin, Y.G. Shul, S.K. Ihm, K.K. Koo, Korean J. Chem. Eng. 13 (1996) 144. [19] K. Suzuki, Y. Kiyozumi, K. Matsuzaki, S. Shin, Appl. Catal. 35 (1987) 401. [20] A.V. Rawtani, M.S. Rao, K.V.G.K. Gokhale, Ind. Eng. Chem. Res. 28 (1989) 1411. [21] Z. Ramli, H. Bahruji, Malaysian J. Chem. 5 (2003) 48. [22] R.K. Vempati, R. Borade, R.S. Hegde, S. Komarneni, Micropor. Mesopor. Mater. 93 (2006) 134. [23] M.M.J. Treacy, J.B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, fourth ed., Elsevier, Amsterdam, 2001. [24] D.S. Chaudhary, M.C. Jollands, J. Appl. Polym. Sci. 93 (2004) 1. [25] E.M. Flanigen, H. Khatami, H.A. Shymanski, Adv. Chem. Ser. 101 (1971) 201. [26] E.M. Flanigen, Adv. Chem. Ser. 121 (1973) 119.
Microporous and Mesoporous Materials 115 (2008) 189–196 www.elsevier.com/locate/micromeso
Synthesis of highly siliceous ZSM-5 zeolite using silica from rice husk ash K. Kordatos a,*, S. Gavela a, A. Ntziouni a, K.N. Pistiolas b, A. Kyritsi b, V. Kasselouri-Rigopoulou a a
School of Chemical Engineering, National Technical University of Athens, 9 Heroon Polytechniou Street, 15780 Zografou, Greece b ‘‘Agrino’’, EV.GE. Pistiolas S.A., National Road 20, 30100 Agrinio, Greece Received 22 July 2007; accepted 19 December 2007 Available online 19 February 2008
Abstract In this study, a simple synthetic route is demonstrated towards the efficient production of ZSM-5 zeolite. Crystalline rice husk ash (RHA) was utilized as an alternative silica source for the synthesis of ZSM-5. The reaction of RHA with the organic template TPABr at low temperature and under atmospheric pressure led to the successful transformation of the crystalline ash to ZSM-5. The synthesis was studied as a function of time and the products at each stage were characterized using a variety of analytical techniques, including Xray diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), thermogravimetry-differential thermogravimetry analyses (TG-DTG), scanning electron microscopy (SEM) and electron dispersion X-ray analysis (EDX). Porosimetry measurements revealed that the product is a microporous material having a high specific surface area of 397 m2 g–1. Ó 2008 Elsevier Inc. All rights reserved. Keywords: ZSM-5 zeolite; Synthesis; Low temperature; Atmospheric pressure; RHA
1. Introduction Rice husk is a major by-product of the rice-processing industries and like most of the other rural biomass materials (e.g., sugar, cane leaf, corn leaf) is recognized as a potential source for energy generation from gasification or incineration processes. The burning of rice husk in air results in the formation of RHA with a content in SiO2 that varies from 85% to 98% depending on the burning conditions, the furnace type, the rice variety, the rice husk moisture content, the climate and the geographic area [1]. Some small amounts of inorganic impurities are always present in the ash along with unburned carbon. The unburned carbon can be removed from the ash by further heating treatments at high temperatures, but this usually leads to the crystallization of the amorphous silica to cristobalite and/or tridy-
*
Corresponding author. Tel.: +30 210 7723100; fax: +30 210 7723188. E-mail address: [email protected] (K. Kordatos).
1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.12.032
mite [2]. The crystallization of the contained silica of the ash also occurs when the burning conditions of husk are uncontrolled [3]. This crystallization is a disadvantage towards the preparation of silicon based materials, because the silica ash is rendered inactive in its crystalline form [3,4]. RHA can be used as an alternative cheap source of amorphous silica for the production of silicon based materials with industrial and technological interests [5–9]. Among the various utilizations of RHA, there is a significant interest in its use in the preparation of zeolites due to the widespread industrial use of zeolites in separation processes as sorbents [10], as well as in catalytic refinery and petrochemical processes [11]. In 1972 Mobil Co. reported the synthesis of the high silica zeolite of type ZSM-5 [12]. ZSM-5 is a medium pore zeolite formed by ten-membered rings which possesses a pore dimension of 0.54–0.56 nm. Its unique pore structure has excellent shape selectivity, while the ability to develop
190
K. Kordatos et al. / Microporous and Mesoporous Materials 115 (2008) 189–196
internal acidity makes it an interesting material for catalyzing organic reactions. Since its discovery, extensive work has been carried out on the synthesis and applications of ZSM-5. High crystalline ZSM-5 can be synthesized within a period of about 168 h in autoclave at 120–180 °C under autogenous pressures. The autoclavation time can be reduced to 4–6 h under high pressures and temperatures (about 40–60 atm and 230–250 °C) [13]. The possibility of synthesizing ZSM-5 at temperatures between 90–100 °C and atmospheric pressure has also been reported [14–19]. These methods make available the study of the crystallization mechanism since they require a long induction period. Among the several templates used for the synthesis of ZSM-5, the most common is tetrapropyl ammonium bromide (TPABr). In all the above mentioned studies on the synthesis of ZSM-5, the starting materials for silica and alumina were from pure chemical sources. Efforts have also been made to use RHA as a source of silica for the synthesis of ZSM-5 with the autoclave process [3,20–22]. In these works, however, the silica prior to the synthesis of the zeolites was prepared in the amorphous form, either after extraction with suitable alkali solutions or under controlled burning of husks. In the present study, we have utilized crystalline silica of RHA for synthesizing highly siliceous zeolite of type ZSM-5 under atmospheric pressure for the first time. The synthesis under atmospheric conditions is advantageous compared with hydrothermal treatments at high pressures due to the lower cost of energy and setup simplicity required, especially when mass production of ZSM-5 is needed for the different applications.
2. Experimental 2.1. Materials Rice husk ash, pellets of sodium hydroxide (fluka), tetrapropylammonium bromide (TPABr) (fluka) and distilled water were used as the starting materials in the initial mixture for the synthesis of ZSM-5 zeolite [19]. The silica and alumina source used for the experiments was a RHA obtained from a Greek rice mill (AGRINO) after burning rice husk during the process of product manufacture. The as received RHA was further heat treated at 700 °C for 5 h, for increasing the silica content by reducing the amount of unburned carbon. The molar ratio of SiO2/ Al2O3 in RHA was 978. Table 1 Chemical composition of RHA before and after heat treatment at 700 °C for 5 h Oxides
RHA (wt%) as received
RHA (wt%) after heat treatment
SiO2 Al2O3 Fe2O3 K2O CaO P2O5 MgO Na2O MnO TiO2 ZnO Loss on ignition
76.66 0.07 0.04 2.47 1.07 0.86 0.65 0.29 0.13 0.01 0.01 17.74
92.28 0.16 0.08 2.86 1.33 0.97 0.77 0.33 0.22 0.02 0.01 0.92
*
* Cristobalite
RHA at 1000oC for 5h
o Tridymite
* o o
*
*
* *
* *
Intensity
RHA at 1000oC for 2h
RHA at 700oC for 5h
RHA
10
15
20
25
30
35
40
45
50
2θ Fig. 1. XRD patterns of untreated RHA, heat treated RHA at 700 °C for 5 h, heat treated RHA at 1000 °C for 2h and heat treated RHA at 1000 °C for 5 h.
K. Kordatos et al. / Microporous and Mesoporous Materials 115 (2008) 189–196
2.2. Methods
191
RHA (3.01 g) was predispersed in distilled water (25 mL) containing NaOH (2.5 g) with stirring. The whole system was introduced in an oil bath and left to reflux at an oil bath temperature of 110 °C for 24 h. An aqueous solution of tetrapropylammonium bromide (TPABr) prepared separately by mixing TPABr (2.33 g) and distilled water
Synthesis runs were carried out using the same initial molar composition of the starting materials, 12NaOH: 30SiO2:0.03Al2O3:6TPABr:1800H2O, according to the following procedure.
Fig. 2. SEM images of heat treated RHA at 700 °C for 5 h (a) 200, (b) 800. Scale bars correspond to (a) 200 lm and (b) 50 lm.
(313)
(033)
(051) (012) (031) (022)
(111)
(200)
(501)
(011)
* cristobalite
11 days
*
Intensity
10 days
9 days
8 days
7 days
6 days 10
20
30
40
50
2θ Fig. 3. XRD patterns of the obtained materials after 6, 7, 8, 9, 10 and 11 days of reaction.
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(25 mL) was slowly added to the predispersed RHA solution under stirring and the whole mixture was continued to reflux at 110 °C. Samples were collected for runs from 1 to 11 days. At the end of each run, the reaction mixture was centrifuged and the solid phase was washed several times with distilled water and dried at 110 °C for 12 h. The obtained materials were then calcined with a heating rate of 10 °C/min at 550 °C for 5 h to remove the organic template.
were measured by the Brunauer, Emmet and Teller (BET) method using nitrogen adsorption–desorption isotherms at 196 °C on a Quantachrone Autosorb-1 instrument. Samples were outgassed at 300 °C for 24 h prior to measurements. The measurements were performed after removal of the organic template. 3. Results and discussions 3.1. Formation of ZSM-5 zeolite crystals
2.3. Characterization Quantitative chemical analyses of RHA were accomplished by XRF (XRF, ARL Advant XP). XRD measurements were performed with a Siemens D5000 difractometer ˚ ). The morphologies of the samples (Cu Ka, k = 1.5406 A were studied by SEM after gold coating using a FEI Quanta 200 instrument operating at 30 keV and equipped with an EDAX detector. Infrared spectra were recorded on a Spectrum One Perkin Elmer spectrometer using the KBr pellet technique. The samples for FT-IR measurements were prepared by a KBr/sample weight ratio of 100/0.04. TG-DTG analyses were carried out in air at a heating rate of 10°C/min from 25 °C to 800 °C on a Mettler Toledo TGA/SDTA851e instrument. In addition, the specific surface areas of RHA, intermediates and final product
RHA was used as the silica and alumina source in the synthesis of ZSM-5. For increasing the silica ash content by decreasing the coexisting amount of unburned carbon, three different heat treatments were applied to the supplied RHA. Particularly, RHA was heat treated at 700 °C for 5 h, 1000 °C for 2 h and 1000 °C for 5 h, respectively. Fig. 1 shows the XRD patterns of the resulting ashes along with the untreated RHA. The analysis of the patterns shows that the supplied RHA presents its crystalline phase in the form of cristobalite. The presence of cristobalite is indicated by diffraction peaks at 2h, 21.9°, 28.5°, 31.5° and 36.3°. When RHA is heat treated at 700 °C for 5 h, it retains its initial cristobalite phase. However, when the RHA is heat treated at 1000 °C for 2 and 5 h, its crystalline phase is partially changed as indicated in the XRD patterns with the appear-
6 days
Derivative weight (%/oC)
7 days
8 days
9 days
10 days
11days
100
200
300
400
500
600
700
800
Temperature (oC) Fig. 4. DTG thermodiagrams of the obtained products after 6, 7, 8, 9, 10, and 11 days of reaction.
K. Kordatos et al. / Microporous and Mesoporous Materials 115 (2008) 189–196
ance of new diffraction peaks corresponding to the tridymite phase. Consequently, we chose to utilize the heat treated RHA at 700 °C for 5 h as silica source for the ZSM-5 zeolite preparation, because this treatment did not change the cristobalite phase of the ash, but increased its silica content from 76.66% to 92.28%. The glass content was 51% of total silica content of RHA and it was determined according to EN 196-2. In all cases, the heat treatment changed the color of the ash from black to gray due to decreasing amount of unburned carbon. The chemical compositions of RHA before and after thermal treatment were determined by X-ray fluorescence (XRF) and are summarized in Table 1. The loss on ignition, which corresponds to the removal of moisture and the coexisting unburned carbon, was determined by heating certain quantities of RHA samples at 975 ± 25 °C in air according to the EN 196-2 and prEN 450-1. The morphology of the heat treated RHA was studied by scanning electron microscopy (SEM). Fig. 2 shows SEM images of RHA after thermal heating at 700 °C. The ash retains the serrated structure of rice husk and consists mainly of fragments of loose flakes with a skeleton like inner structure. The solids obtained from the synthesis runs after calcination at 550 °C were characterized using X-ray diffraction
193
technique. The XRD patterns collected for runs from 6 to 11 days are presented in Fig. 3. Samples obtained for synthesis runs from 1 to 5 days exhibit similar XRD patterns with the sample obtained after 6 days of reaction, which consists of crystalline silica in the form of cristobalite. The XRD patterns corroborate that ZSM-5 is not obtained within 6 days under the present experimental conditions. However, with an increase of synthesis time the characteristic peaks of ZSM-5 become distinct. The diffraction peaks that appear after 7 days of reaction correspond to the characteristic peaks of ZSM-5 zeolite. The d and the hkl values obtained from the XRD analysis are in close agreement with the reported data of ZSM-5 zeolite [23]. The characteristic peaks of unreacted cristobalite coexist along with those of ZSM-5 and as the synthesis time increases their intensity decrease dramatically. 3.2. Thermal analysis Fig. 4 illustrates the DTG results of the samples obtained for different reaction runs. All measurements were carried out under an air atmosphere. Two peaks were observed in the DTG curve of the obtained product after 6 days of reaction. Similar thermal behavior was observed for the products obtained for less than 6 days of reaction
6 days
7 days
Transmittance (%)
8 days
9 days
10 days
11 days
546 1232 1400
1200
1000
Wavenumber
800
600
400
(cm-1)
Fig. 5. FT-IR spectra of the obtained products after 6, 7, 8, 9, 10 and 11 days of reaction.
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Fig. 6. SEM images of the obtained products after (a) 4, (b) 5, (c) 6, (d) 7, (e) 8, (f) 9, (g) 10 and (h) 11 days of reaction. Scale bars correspond to 5 lm and all images have a magnification of 10,000.
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3.3. FT-IR Infrared spectroscopy was also used to follow the reaction process. Fig. 5 presents the FT-IR spectra of the samples obtained for different reaction runs. According to the Flanigen–Khatami–Szymanski correlation [25], the vibrational modes near 1100, 800 and 450 cm–1 are assigned to internal vibrations of Si, AlO4 tetrahedra and are also observed in silica, quartz and cristobalite, while the vibrational modes near 550 and 1230 cm–1 are due to the doublerings tetrahedra vibration and to the asymmetric stretching of Si and AlO4 tetrahedra in the zeolite framework, respectively. By examining the FT-IR spectra of the samples, it can be seen that the characteristic vibrational modes of the ZSM-5 zeolite framework are only appeared after 7 days of reaction time, in agreement with the XRD and DTG data. Particularly, the evidence for the zeolite formation comes from the appearance of the modes at 546 and 1232 cm–1 in the spectra of all the obtained samples for reaction runs from 7 to 11 days. As it is observed from the spectra, upon increasing of reaction time the zeolite’s framework bands become more pronounced due to the increasing amount of the zeolite in the samples. Besides those bands, all samples also show a small band at 620 cm–1, corresponding to the characteristic vibration of cristobalite. 3.4. Morphology Fig. 6 shows SEM images of the solids obtained for reaction runs from 4 to 11 days. As we can see from the images, for reaction runs less than 7 days the morphologies
of the solids consist of large fragments. With increasing of reaction time, the size of those fragments becomes smaller, for example, from about 20 lm for reaction time of 4 days (Fig. 6a) to about 5 lm for 6 days of reaction (Fig. 6c). Additionally, the reduced in size fragments are begun to be covered by agglomerates of silica particles (Fig. 6b and c). In agreement with the XRD, FT-IR and DTG data, ZSM-5 crystals are formed after 7 days of reaction (Fig. 6d). This sudden transformation might be explained taking into consideration the proposed crystal growth mechanism by Flanigen [26] in which, the amorphous gel is converted to crystals through the solid phase transformation mechanism. As the reaction proceeds further, the crystal growth continues resulting to the typical morphology of the ZSM-5 zeolite crystals in the form of prisms (Fig. 6e– h). The electron dispersion X-ray analysis (EDX) of the material obtained after 11 days of reaction shows as it was expected that it consists of Si, O and Al. 3.5. Nitrogen porosimetry For following the undergone changes into the textural evolution of the samples, N2 adsorption–desorption isotherms were measured at –196 °C over calcined samples prepared at different synthesis times. Fig. 7 shows the nitrogen isotherms of RHA and the samples produced after 3, 7, 10 and 11 days of reaction. The isotherms corresponding to RHA indicate that it is mainly non/macroporous, a large amount of the nitrogen adsorption being observed at high P/P0. Similar results are observed for the sample obtained after 3 days of reaction. As the reaction time increases and the formation of zeolite proceeds, the adsorption isotherms after 7, 10 and 11 days of reaction reveal the microporosity
300 RHA 3 days
250
7 days 10 days 11 days
200
V (cm3/g)
not shown in Fig. 4. In all these reaction runs, the samples consist of silica in the form of cristobalite as it was shown by the XRD analysis. The DTG pattern showed a peak bellow 100 °C which results from dehydration and a second peak at around 530 °C related to the combustion of unburned carbon [24]. The carbon content in the samples comes from the burning process of rice husk in which carbon is trapped as an impurity in the silica ash structure. For synthesis runs from 7 days or more, where the formation of ZSM-5 zeolite proceeds, the samples show four distinct peaks of weight loss. Along with the earlier mentioned peaks, two new peaks appear. A third peak at around 220 °C due to loosely bound TPA+ cations [18] and a fourth peak with a strong exothermic effect at around 430 °C due to the strongly bound TPA+ cations in the zeolite framework [18], which neutralize negative charges from Al species in the ZSM-5 framework. As it can be seen, by the evolution of the strong exothermic peak, both the amount and the decomposition temperature of the strongly bound TPA+ cation increased with increasing of reaction time, revealing the development of the zeolite framework in the products. It can be also noted that in the temperature range of 600–800°C there is not any weight loss, confirming the thermal stability of MFI phase.
195
150
100
50
0 0
0.1
0.2
0.3
0.4
0.5
0. 6
0.7
0.8
0.9
1
P/Po Fig. 7. Nitrogen adsorption–desorption isotherms at –196 °C of RHA and calcined samples after 3, 7, 10 and 11 days of reaction.
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Table 2 Textural parameters from N2 adsorption–desorption isotherms on calcined samples at 550 °C Sample
SBET (m2/g)
Micropore volume (cm3/g)
Size of micropores (nm)
RHA 3 Days 7 Days 10 Days 11 Days
28 26 306 369 397
– – 0.109 0.131 0.140
– – 0.55 0.60 0.60
Pore Volume (cc/gÅ)
40 35
7 days
30
10 days
25
11 days
verted RHA to ZSM-5 was studied by N2 porosimetry and showed that starting with a non/macroporous material with a specific surface area of 28 m2 g–1, a microporous material with a high specific surface area of 397 m2 g–1 could be obtained. Acknowledgment The present work was supported from the 05-DSVEPRO-06-2005 program of General Secretariat for Research and Technology and is part-financed from the European Fund of Regional Growth. We would like to thank the Greek rice mill ‘‘Agrino”, EV.GE. Pistiolas S.A., for the RHA supply. References
20 15 10 5 0 0
2
4
6
8
10 12
14 16 18 20 22 24 26 28
Pore radius (Å) Fig. 8. Pore size distribution of calcined samples after 7, 10 and 11 days of reaction.
characteristics of the resulting samples, since they consist of sharp knees at P/P0 lower than 0.1 due to filling of microporous. The BET surface area, micropore volume and size of microporous of the examined samples are summarized in Table 2. As it can be seen, the increasing transformation of RHA into ZSM-5 with time results in increasing microporosity giving rise to a final product with a specific surface area of 397 m2 g–1 and a micropore volume of 0.14 cm3 g–1. The mean size of micropores in the zeolite was estimated to about 0.6 nm as shown in Fig. 8. 4. Conclusions It is demonstrated how an industrial by-product can be converted to a high value added product by using a simple inexpensive method. A powder consisted of 92% of silica was prepared after calcination of RHA at 700 °C for 5 h and it was then utilized as an alternative silica source for the production of ZSM-5 zeolite under atmospheric pressure. The reaction was studied as a function of time and the products were characterized at each stage using several analytical techniques. XRD, TG-DTG, FT-IR and SEM analyses revealed the transformation of RHA to ZSM-5 after 7 days of reaction. The textural evolution of the con-
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