I n d . Eng. C h e m . Res. 1989, 28, 1411-1414
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GENERAL RESEARCH Synthesis of ZSM-5 Zeolite Using Silica from Rice Husk Ash Arun V. Rawtani and Musti S. Rao* Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208 016, India
K.V. G . K. Gokhale Department of Civil Engineering, Indian Institute of Technology, Kanpur 208 016, India
ZSM-5 zeolite has been synthesized from the Na-TPA cation system for the first time using silica from rice husk ash. In this study, rice husk ash was the only source of silica and alumina used in the synthesis. The molar ratio of Si02/A1203in the husk was 24.88, which was constant throughout. T h e synthesis has been carried out for temperatures ranging from 125 t o 200 "C and durations of synthesis from 6 t o 120 h with Na20/A1203and H20/A1203molar ratios ranging from 3.25 to 15.15 and 1081 to 2786, respectively. The effects of varying the composition of initial mixture, the alkalinity, and the time on synthesis were studied. The product a t each stage has been characterized by using X-ray diffraction analysis, infrared spectroscopy, differential scanning calorimetry, and electron microscopy. The zeolite ZSM-5 possesses a unique channel structure and is known for its unusually high silica/alumina ratio. The properties that make ZSM-5 suitable and important for industrial application are its exceptionally high degree of thermal and acid stability and high selectivity in certain catalytic conversions. The crystals have an idealized orthorhombic symmetry with the cell dimensions a = 20.1 A, b = 19.9 A, and c = 13.4 A (Kokotailo et al., 1978). ZSM-type zeolites were first developed by Mobil scientists (Argauer et al., 1972). Jacobs et al. (1981) have synthesized ZSM-5 starting from sodium silicate, tetraalkylammonium compounds, and aluminum sulfate in acid medium at 423 K for 3-6 days. Kulkarni et al. (1982) synthesized ZSM-5 in the temperature range 80-167 "C. The role of organic and inorganic cations during the zeolite crystallization has been summarized by several workers (Kerr and Kokotailo, 1961; Kokotailo and Meier, 1980; Rollmann, 1979; Kerr, 1981; Whyte and Dalla Betta, 1982). In this investigation, rice husk ash has been used as a source of silica and alumina, instead of the pure chemical sources used earlier. Since rice is the staple food in India, rice husk is available in considerable quantity. Rice husk on complete burning yields carbon-free white ash, which on analysis has been found to contain 88.86% SiOz, 6.4% A&O3,0.35% Fez03,and 1.16% alkalies. The rice husk ash had a loss on ignition of 3.23%. This silica, which can be extracted from rice husk ash by a suitable alkali such as sodium hydroxide, has been found to be very reactive for zeolite synthesis (Bajpai et al., 1981). Mordenite-type and NaX zeolites have been synthesized in the past using this source of silica (Bajpai et al., 1978,1981; Dalai et al., 1985).
Experimental Section Materials. The reactant materials used were pellets of sodium hydroxide (ARC Industries, Kanpur) and tetrapropylammonium (TPA)hydroxide (Fluka A, Chemische Fabrik). Rice husk obtained from a rice mill in Kanpur was burned in a muffle furnace a t 1000 "C for 10 h, and 0888-5885/89/2628-1411$01.50/0
the resultant carbon-free ash was ground in a ball mill to a -200-mesh size. Methods. Synthesis runs were carried out in a stainless steel (type 316) autoclave of 150-mL capacity a t autogenous pressure. Agitation was achieved by a Teflon-coated magnetic stirrer. The temperature of the reaction mixture in the autoclave was controlled by using an electronic temperature controller to an accuracy of fl "C. A predetermined amount of sodium hydroxide was added to demineralized water in order to prepare a sodium hydroxide solution. In another container, a stoichiometrically required amount of tetrapropylammonium hydroxide was added to the rice husk ash, yielding a viscous TPA-silicate-aluminate solution. The silica and alumina used in the reaction were only from rice husk ash. The two solutions were mixed, and the autoclave was quickly closed with its lid so as to prevent TPA from absorbing carbon dioxide gas from the atmosphere. The reaction vessel was maintained at the desired temperature for a predetermined time. At the conclusion of a run, the vessel was quenched immediately in cold water in order to stop the crystallization process. The solid products were filtered, washed, and dried overnight a t 120 "C. The synthesized samples were characterized by X-ray diffraction analysis using Seifert equipment fitted with a diffractometer. Ni-filtered Cu Ka radiation was used. For quantitative phase identification, the percentage crystallization was calculated with the summation of peak intensities method for peaks between 2 2 O and 25" (28). This range for the characteristic peaks of ZSM-5 was chosen as it avoided overlapping of peaks with each other. The standard ZSM-5 used for calibration was obtained from Mobil Research Laboratories. The crystalline compounds were further examined with infrared spectroscopy (Perkin-Elmer 521) using the KBr pellet technique. Thermal analyses were carried out by using a differential scanning calorimeter (DSC) with a heating rate of 8 OC/min using a-Alz03as a reference material. A scanning electron microscope (Philips 515) was used for the study of the 0 1989 American Chemical Society
1412 Ind. Eng. Chem. Res., Vol. 28, No. 9, 1989
*
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E
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X
60
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V
52
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V
4.
10
V
36 28 c0 ZSM-5 ZSM-5rAnolcimo X Analcime
(TPA,Na)20
20 A1203
50
Figure 1. Ternary diagram showing the reaction mixture composition in Na-TPA system for ZSM-5 formation after 48 h a t 175 "C.
c 20 .0 A
-
11
N --'
45
40
35
30 25 20 2Wdegrees)
15
10
Figure 3. X-ray pattern of synthesized ZSM-5: (a) for 12 h, (b) for 48 h, (c) for 120 h.
12 13 14 15 16 OH/H20 x lOOO(Na0H voriotion)
0
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ul
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L
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20 22 OH/H20 x 1000(TPAOH voriotion) 14
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Figure 2. Effect of hydroxyl ions on crystallization of ZSM-5 a t 175 "C for 48 h with H20/Si02 = 52.1.
morphology of the crystallized zeolites.
Results and Discussion Effect of Starting Composition. The synthesis has been carried out with a Si02/A1203molar ratio of 24.88 and (TPA)20/A1203,Na20/A1203,and H20/A1203molar ratios ranging from 3.73 to 18.53, 3.25 to 15.15, and 1081 to 2786 respectively, with the temperature and time varied in the ranges 125 to 200 "C and 6 to 120 h, respectively. For zeolite ZSM-5 synthesized at 175 "C for 48 h, the stability field as a function of the composition for the system Si02-A1203-(TPA,Na)z0 is indicated in Figure 1. It can be seen from the plot that the mole percent of Si02 should vary from 70% to 80% and that for (TPA,Na)20 from 20% to 35% so as to yield pure ZSM-6 in the final product. Effect of Alkalinity. The change in alkali concentration in the starting gel causes a change in the rate of nucleation (Figure 2 ) . With NaOH as the only reactant
50
45
40
35
30 25 20 2 B(degreas)
15
10
5
Figure 4. X-ray diffraction pattern of synthesized H-ZSM-5 after (a) 12 h, (b) 48 h, and (c) 120 h.
that is varied, it was found that the percent crystallinity decreases as the hydroxide ion concentration increases (Figure 2a). It has been noticed that the nucleation time increased with an increase in OH. The percent crystallization, when (TPA)OH was varied in the reaction mix, increases initially with an increase in hydroxide ion concentration (Figure 2b). When the OH/H20 molar ratio exceeds 0.016, the percent crystallization starts decreasing with an increase in hydroxide ion concentration. Thus, an increase in hydroxide ion concentration can shorten the induction period, while higher alkalinity beyond a certain value will prolong the nucleation time. There exists an optimum alkalinity for nucleation in the synthesis of ZSM-5. Similar observations have also been made by Chao et al. (1981). Effect of Time. For studying the effect of time on the synthesis of ZSM-5, the batch composition of 24.88SiO2-AIzO3.4.47(TPA)20.3.40Na20.1296H20 at 175 C was chosen. The percent of crystallized ZSM-5 as a function of time exhibited an induction period of about
Ind. Eng. Chem. Res., Vol. 28, No. 9, 1989 1413 improves in its intensity. The intense peaks a t 20.8 and 26.65 (28) are those of unreacted quartz along with ZSM-5. The characteristic peak at about 23.65 (28) becomes more distinct with synthesis time. The lattice parameters were calculated to be a = 20.08 A, b = 19.97 A, and c = 13.41 A and are in accordance with the reported values of Kokotailo et al. (1978). The X-ray patterns for the corresponding hydrogen forms of these samples prepared in the present study are presented in Figure 4. The strongest peak at 23.10 (20) decreases in its intensity and those at 7.85 and 8.80 increase in their intensities. Infrared Spectra and Thermal Pattern. The infrared spectra for H-ZSM-5 in the present case are typical of ZSM-5 material with absorption bands at 1225 (sh), 1100 (vs), 800 (mw), 550 (m), and 450 (s) cm-l (Coudurier et al., 1982). Synthetic zeolites can be interpreted in terms of the spectra in the 1400-300-~m-~ region (Flanigen et al., 1971). The thermal pattern for synthesized ZSM-5, obtained on a DSC, has exhibited a strong exothermic effect at about 721 K, attributed to the oxidative decomposition of organic cation occluded in the zeolite framework. This is confirmed by an earlier work (Prasada Rao, 1985). Pattern of the Product Crystallization. The scanning electron micrographs (Figure 5) for the different species obtained at different time intervals reveal characteristic patterns of different zeolite species. H-ZSM-5 crystals are seen in association with rodlike gismondine zeolite (Figure 5a). The ZSM-5 crystals, which are orthorhombic and have the form of pickets, doubly terminated at one end, are seen in Figure 5b. A well-developed crystal of H-ZSM-5 after 120 h of synthesis in association with crushed crystals (due to agitation) is presented in Figure 5c. Registry No. NaOH, 1310-73-2; TPA, 4499-86-9.
Literature Cited
Figure 5. Scanning electron micrographs of H-ZSM-5 for a synthesis time of (a) 1 2 h (7380X), (b) 48 h (4500X), and (c) 120 h (9600X).
6 h followed by rapid crystallization, ultimately reaching an asymtotic value around 95% in about 21/2 days. This behavior is similar to the behavior reported earlier (Erdem and Sand, 1979). The X-ray diffraction patterns of ZSM-5 crystallized from a batch composition of 24.88SiO2.Al2O3-4.47(TPA)20-3.40Na20-1296H20 at 175 "C for 12,48, and 120 h, respectively, are indicated in Figure 3a-c. Initially the peak at 7.85 (28) is of greater intensity than that at 8.80 (28). But with an increase of synthesis time, the latter
Argauer, R. J.; Olson, D. H.; Landolt, G. R. Molecular Sieves. US Patent 3,702,886, 1972. Bajpai, P. K.; Rao, M. S.; Gokhale, K. V. G. K. Synthesis of Mordenite-type Zeolite. Ind. Eng. Chem. Prod. Res. Dev. 1978, 1 ?, 223. Bajpai, P. K.; Rao, M. S.; Gokhale, K. V. G. K. Synthesis of Mordenite-Type Zeolite Using Silica from Rice Husk Ash. Ind. Eng. Chem. Prod. Res. Dev. 1981,20, 721. Chao, K. J.; Tasi, T. C.; Chen, M.; Wang, I. Kinetic Studies on the Formation of Zeolite ZSM-5. J . Chem. SOC.,Faraday Trans. 1 1981, 77, 547. Coudurier, G.; Naccache, C.; Vedrine, J. C. Uses of I. R. Spectroscopy in Identifying ZSM Zeolite Structure. J . Chem. SOC.,Chem. Commun. 1982, 1413. Dalai, A. K.; Rao, M. S.; Gokhale, K. V. G. K. Synthesis of NaX Zeolite Using Silica from Rice Husk Ash. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 465. Erdem, A.; Sand, L. B. Crystallization and Metastable Phase Transformations of Zeolite ZSM-5 in the (TPA)20-Na20-K20A1203-Si02-H20 System. J. Catal. 1979, 60, 241. Flanigen, E. M.; Khatami, H.; Szymanski, H. A. Infrared Structural Studies of Zeolite Frameworks. Adv. Chem. Ser. 1971,101,201. Jacobs, P. A.; Bayer, H. K.; Valyon, J. Properties of the End Members in the Pentasil Family of Zeolites: Characterisation as Adsorbents. Zeolites 1981, 1, 161. Kerr, G. T. The Synthesis and Properties of Two Catalytically Important Zeolites. Catal. Rev. Sei. Eng. 1981, 23, 281. Kerr, G. T.; Kokotailo, G. T. Sodium Zeolite ZK-4, A Synthetic Crystalline Aluminosilicate. J. Am. Chem. SOC.1961, 83, 4675. Kokotailo, G. T.; Meier, W. M. Pentasil Family of High Silica Crystalline Materials. Spec. Pub1.-Chem. SOC.1980, 33, 133. Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M. Structure of Synthetic Zeolite ZSM-5. Nature 1978, 272, 437. Kulkarni, S. B.; Shiralkar, V. P.; Kolasthane, A. N.; Borade, R. B.; Ratnasamy, P. Studies in the Synthesis of ZSM-5 Zeolites. Zeolites 1982, 2, 313.
I n d . Eng. C h e m . Res. 1989, 28, 1414-1424
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Prasada Rao, T. S. R. Advances in Catalysis Science and Technology; Wiley Eastern Ltd.; New Delhi, India, 1985. Rollmann, L. D. Inorganic Compounds with Unusual Properties. Adu. Chem. Ser. 1979, No. 173. Whyte, T. E., Jr.; Dalla Betta, R. A. Zeolite Advances in the Chem-
ical and Fuel Industries: A Technical Perspective. Catal. Rev. Sci. Eng. 1982, 24, 567.
Received for review February 6 , 1989 Accepted May 2 , 1989
A Novel Fused Metal Anode Solid Electrolyte Fuel Cell for Direct Coal Gasification: A Steady-State Model Ioannis V. Yentekakis and Pablo G. Debenedetti* Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544-5263
Bruno Costa 80122 Naples, Italy
The simultaneous gasification of coal and the generation of electrical current in a high-temperature solid electrolyte fuel cell of the form C, 02,fused metal/Zr02 (8 mol % Y203)/Pt, O2 with a fused metal anode is a promising new concept. This process utilizes part of the Gibbs energy change of the coal gasification reaction to generate electricity with the high thermodynamic efficiency characteristic of fuel cells. We present a lumped parameter model that describes the steady-state behavior of this novel fuel cell and discuss the effect of operating conditions upon cell performance. T h e electrochemical cell reactor is capable of producing very high current and power densities and exhibits steady-state multiplicity over a wide range of operating conditions. In the last 10 years, there has been an increased interest in the utilization of solid electrolytes in heterogeneous catalysis (Vayenas, 1988). Zirconia-based solid electrolyte cells, for example, have found extended use in the investigation of the kinetics of heterogeneous oxidation reactions (Wagner, 1970; Vayenas et al., 1980; Stoukides and Vayenas, 1982a, 1983; Yentekakis et al., 1988; Metcalfe and Sundaresan, 1988) and, more recently, in the electrochemical modification of the catalytic activity of metals (Yentekakis and Vayenas, 1988; Vayenas, 1988; Vayenas et al., 1988; Bebelis and Vayenas, 1989; Neophytides and Vayenas, 1989). The latter application, in particular, has obvious technological significance and promises further use of these materials in heterogeneous catalytic processes. Yet another important application of solid electrolyte devices is their use in high-temperature fuel cells for the simultaneous production of chemicals and electricity. This mode of operation combines the concepts of a fuel cell and of a chemical reactor. Fuel cells can convert a significant portion of the Gibbs energy change of exothermic reactions into electricity rather than heat, their thermodynamic efficiency comparing favorably (Bockris and Reddy, 1970) with cyclic thermal power generation schemes which are limited by Carnot-type constraints. Another advantage of these devices is that they allow operation at temperatures of catalytic interest where activation polarization phenomena usually diminish or vanish. The operation of such zirconia-based solid electrolyte fuel cells has been investigated experimentally, up to the pilot plant scale (Weissbart and Ruka, 1962; Etsell and Flengas, 1971; Farr and Vayenas, 1980; Vayenas and Farr, 1980; Sigal and Vayenas, 1981; Stoukides and Vayenas, 1982b; Michaels and Vayenas, 1984a,b; Michaels et al., 1986; Manton, 1986; Kiratzis and Stoukides, 1987; Yentekakis and Vayenas, 1989) as well as theoretically (Debenedetti and Vayenas, 1983; Vayenas et al., 1985). A number of industrially important oxidations, such as the
* To whom
all correspondence should be addressed.
conversion of ammonia to nitric oxide (Vayenas and Farr, 1980; Sigal and Vayenas, 1981; Farr and Vayenas, 19801, the Andrussov process for the production of HCN (Kiratzis and Stoukides, 1987), the oxidative dehydrogenation of ethylbenzene to styrene (Michaels and Vayenas, 1984a,b) and l-butene to butadiene (Manton, 1986), the direct oxidation of H2Sto SOz (Yentekakis and Vayenas, 1989),and the ethylene epoxidation (Stoukides and Vayenas, 1982b), have been carried out in experimental cells. Solid electrolyte fuel cells, operating on H2 or CO as the fuel, have been constructed and tested for years (Weissbart and Ruka, 1962; Archer et al., 1965; Etsell and Flengas, 1971; Michaels et al., 1986). Comprehensive reviews of solid oxide fuel cells (Brown, 1986; Vayenas, 1988) and of the use of the fuel cells in electric utility power generation (Frickett, 1986) have appeared recently in the literature. The purpose of this paper is to present a new concept for the industrially important coal gasification process based on the use of solid electrolyte devices. This process allows, in principle, the clean, efficient cogeneration of carbon monoxide and electric power. The fuel cell modeled here is quite different from those previously described in the literature (Debenedetti and Vayenas, 1983) because it includes a fused metal bulk anode, instead of thin metallic catalyst films. T h e Process Coal gasification in molten metal baths is a relatively new process. Considerable effort, mostly in Japan (Nakajima et al., 1983), has been devoted to the investigation of the scientific and economic implications of this technology. Important technological advantages of this process include the ability to produce low-sulfur coal gas (consisting mainly of CO and H2) continuously and with high gasification efficiency and the potential for combining gasification with steel making through the use of excess gasification heat for ore smelting or scrap remelting. The economics of fused iron bath gasification can be made even more attractive if part of the Gibbs energy change of the
0888-5885/89/2628-1414$01.50/0 B 1989 American Chemical Society
