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Designing a Capsule Catalyst and Its Application for Direct Synthesis of Middle Isoparaffins Jingjiang He,† Yoshiharu Yoneyama,† Bolian Xu,‡ Norikazu Nishiyama,§ and Noritatsu Tsubaki*,† Department of Applied Chemistry, School of Engineering, Toyama University, Gofuku 3190, Toyama 930-8555, Japan, Department of Chemistry, Nanjing University, Nanjing 210093, China, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Osaka 560-8531, Japan Received November 12, 2004. In Final Form: January 12, 2005 A catalyst in the form of a capsule catalyst was prepared by coating HZSM5 membrane on a preshaped Co/SiO2 catalyst pellet. The capsule catalyst with HZSM5 membrane exhibited excellent selectivity for light hydrocarbon synthesis, especially for isoparaffin synthesis from syngas (CO + H2). Long-chain hydrocarbon formation was totally suppressed by the zeolite membrane. The modification of membrane and core catalyst significantly improved the catalytic properties of these new kinds of capsule catalysts.
Introduction The Fischer-Tropsch synthesis (FTS) reaction, CO + H2 ) hydrocarbons + H2O, was found by Fischer and Tropsch in 1925.1 It can produce liquid fuels such as gasoline and diesel oil from coal or natural gas. As more and more natural gas resources are found in the world presently, the FTS process becomes more promising.2 Furthermore, as the technique of producing syngas (CO + H2) from biomass is being developed, synthetic liquid fuel can be obtained from biomass via FTS.3 The FTS products are almost normal hydrocarbons, either olefins or paraffins, and the product selectivity follows the ASF distribution. The advantages of FTS hydrocarbons are their high n-paraffin content, high cetane number as diesel fuel, and sulfur-free, aroma-free, nitrogen-free properties, especially on cobalt-based catalysts.4-7 The production of hydrocarbons rich in isoparaffins, alkylates, has gained much attention because of its high octane numbers if used as synthetic gasoline. Several groups have tried to make isoparaffins by utilizing FTS catalyst, which is metal dispersed on acidic zeolite or other acidic supports.8 But these catalysts performed with very low conversion due to a rather low reduction degree. We have recently reported that when a mechanical mixture of zeolite and normal FTS catalyst, Co/SiO2, was used in a single- or dual-step FTS reaction, the formation of short-chain isoparaffins was enhanced while the formation of longer hydrocarbons was suppressed.9 * To whom correspondence should be addressed. E-mail: tsubaki@ eng.toyama-u.ac.jp. † Toyama University. ‡ Nanjing University. § Osaka University. (1) Olive, H. G.; Olive, S. The Chemistry of the Catalyzed Hydrogenation of Carbon Monoxide; Springer-Verlag: Tokyo, 1984; p 144. (2) Wegrzyn, J. E.; Mahajan, D.; Gurevich, M. Catal. Today 1999, 50, 97. (3) Chum, H. L. R.; Overend, P. Fuel Process. Technol. 2001, 71, 187. (4) Iglesia, E.; Soled, S. L.; Fiato, R. A. J. Catal. 1992, 137, 212. (5) Johnson, B. G.; Bartholomew, C. H.; Goodman, D. W. J. Catal. 1991, 128, 231. (6) Schanke, D.; Vada, S.; Blekkan, E. A.; Hilman, A. M.; Hoff, A.; Holmen, A. J. Catal. 1995, 156, 85. (7) Van Der Laan, G. P.; Beenackers, A. Catal. Rev. Sci. Eng. 1999, 41, 255. (8) Chen, Y. W.; H. T. Tang; Goodwin, J. G., Jr. J. Catal. 1983, 83, 415. Nijs, H. H.; Jacobs, P. A. J. Catal. 1980, 66, 401. Jacobs, P. A. Catalysis by Zeolites; Imelik, B., Naccache, C., Vedrine, J. C., Eds.; Elsevier: Amsterdam, 1980; p 293.
Figure 1. A schematic image of the capsule catalyst role in the FTS reaction.
Zeolite is a special material with unique pores and channels. The varied molecular diffusion rate in these pores and the shape selectivity, as well as acidic properties, make it widely used. Many studies on preparing zeolite membrane and its application for separation have already been reported.10 On the other hand, zeolite is also a good hydrocracking/hydroisomerization catalyst due to its acidic properties.11 In the present work, zeolite membrane was tailor-made coated on the FTS catalyst pellet, Co/ SiO2, and we call it a capsule catalyst. In the reaction, feed gas, CO + H2, diffused through zeolite membrane and arrived at the FTS catalyst. Then the hydrocarbons formed there and desorbed. When the hydrocarbons diffused into the zeolite membrane, all of them, in the form of normal hydrocarbons, could enter zeolite channels and must be cracked and isomerized by acidic sites inside zeolite channels. A schematic image is shown in Figure 1. For long-chain hydrocarbons, their low diffusion rate in zeolite membrane makes them stay in the membrane layer longer, having a higher possibility of isomerization and cracking reaction inside the membrane. Furthermore, compared to conventional membrane reactors, the catalyst designed above has larger membrane area per unit reactor volume. This kind of capsule catalyst is of great advantage in practical application, because membranes with large area and without pinholes or cracks are very difficult to (9) Li, X.; Asami, K.; Luo, M.; Michiki, K.; Tsubaki, N.; Fujimoto, K. Catal. Today 2003, 84, 59. (10) de Vos, R. M.; Verweij, H. Science 1998, 279, 1710. Nishiyama, N.; Miyamoto, M.; Egashira, Y.; Ueyama, K. Chem. Commun. 2001, 18, 1746. Lai, Z.; Bonlla, G.; Diaz, I.; Nery, J. G.; Sujaoti, K.; Amat, M. A.; Kokkoli, E.; Terasaki, O.; Thompson, R. W.; Tsapatsis, M. D.; Vlachos, G. Science 2003, 300, 456. (11) Feller, A.; Guzman, A.; Zuazo, I.; Lercher, J. A. J. Catal. 2004, 224, 80.
10.1021/la047217h CCC: $30.25 © 2005 American Chemical Society Published on Web 02/01/2005
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Figure 2. External surface of the Co/SiO2 pellet and the capsule catalyst pellet and the EDS analysis result: (A) Co/SiO2; (B) 2-Co/SiO2-zeolite; (C) Co/SiO2; (D) 2-Co/SiO2-zeolite.
prepare in most cases.12 This new kind of capsule catalyst is expected to have wide applications, if the combination of core catalyst and membrane catalyst is varied according to the target reaction. Experimental Section The conventional FTS catalyst was prepared by incipientwetness impregnation of an aqueous solution of Co(NO3)2‚6H2O and two kinds of silica support (Cariact Q-10, Fuji Silysia Co.; specific surface area, 323 m2‚g-1; pore volume, 1.03 mL‚g-1; pore diameter, 10 nm) whose pellet size was 0.85-1.7 and 0.38-0.50 mm, respectively. The catalyst precursors were dried in air at 393 K for 12 h and then calcined in air from room temperature to 673 K with a ramping rate of 2 K‚min-1 and kept at 673 K for 2 h. After calcination, the catalysts were cooled to room temperature in nitrogen. In zeolite membrane synthesis, distilled water and ethanol (Wako Pure Chemical Industries Ltd., 99.5%) were used for solutions. The templet was TPAOH (tetrapropylammonium hydroxide solution, Wako Pure Chemical Industries Ltd). Al and Si sources were Al(NO3)3‚9H2O (99.5%) and TEOS (tetraethyl ortho silicate; Wako Pure Chemical Industries Ltd.), respectively. TEOS/TPAOH/H2O/EtOH/Al(NO3)3 ) 1:0.25:60:4:0.025. First, TEOS, 10% TPAOH water solution, ethanol, and water were added in a 100 mL Teflon tank. Then Al(NO3)3‚9H2O was added to the mixture solution carefully and stirred at 333 K for 2 h until the lucid sol was formed. Continuously, the normal FTS catalyst was added in the sol, and the capped tank was put in hydrothermal synthesis equipment (DRM-420DA, Hiro Co., Japan), heated to 453 K, and run at 10 rpm with various duration times for crystallization. In this process, the zeolite membrane was coated on the surface of FTS catalyst pellets. Then, the coated catalyst was separated from the synthesis solution and dried at 393 K for 12 h, followed by calcination at 773 K for 5 h where the temperature rising rate was 1 K/min from 393 to 773 K. Thus, a capsule catalyst was obtained. The pure zeolite was (12) Nishiyama, N.; Ichioka, K.; Park, D. H.; Egashira, Y.; Ueyama, K.; Gora, L.; Zhu, W.; Kapteijn, F.; Moulijn, J. A. Ind. Eng. Chem. Res. 2004, 43, 1211.
synthesized by the same method without adding the Co/SiO2 catalyst, as a blank experiment. The morphology and surface component analysis of the Co/ SiO2 catalyst and the prepared capsule catalysts were investigated with a scanning electron microscope equipped with an EDX attachment (JEOL JSM-6360LV, 15-20 kV, 1.0 nA). The catalyst was precoated with Pt before characterization. The FTS reaction was conducted under pressurized conditions, 1.0 MPa, 533 K, by using a flow-type fixed reactor. Before reaction, the catalyst was reduced in flowing hydrogen at 80 mL/min at 673 K for 10 h and at last cooled to 353 K in nitrogen. The catalyst amount was 0.5 g on the Co/SiO2 base, and the H2/CO ratio of the feed gas was 2. During the reaction, effluent gas released from the reactor was analyzed by on-line gas chromatography using an active charcoal column equipped with a thermal conductivity detector (TCD). The hydrocarbons were also analyzed on-line using a capillary column (J&W Scientific GSAlumina, 30 m) equipped with a hydrogen flame ionization detector (FID). A trap with concentrated sulfuric acid was attached to the system for adsorbing the olefins. The olefin hydrocarbons were calculated from difference of FID peaks after the olefins were absorbed by the concentrated sulfuric acid.
Results and Discussion The morphology of the Co/SiO2 FTS catalyst and the obtained capsule catalysts is shown in Figure 2. The image of a capsule catalyst in Figure 2B suggested that the HZSM5 crystal formed on the surface of the Co/SiO2 catalyst pellet, while no crystals were observed on the Co/SiO2 pellet surface as in Figure 2A. The EDS plane analysis results confirmed the formation of zeolite membrane on the Co/SiO2 pellet because there was Al X-ray signal on the capsule catalysts but not on the Co/SiO2 catalyst as shown in Figure 2C,D. Furthermore, no cobalt was detected on the capsule catalyst, indicating the zeolite membrane was perfect. Figure 3 shows a cross-sectional view of the prepared capsule catalyst pellet. The zeolite membrane can be clearly distinguished from the cobalt
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Figure 3. A cross-sectional image of one capsule catalyst (2-Co/SiO2-zeolite) and the intensity of Si KR and Al KR X-ray signals from the EDX line scan indicated in the image.
catalyst supported on silica because of the quite different morphology. The thickness of the zeolite of the membrane was measured as about 10 µm. The intensity of Si KR and Al KR X-ray signals from an EDX line scan is also shown in Figure 3. The intensity of Al KR X-rays increased dramatically at the zeolite membrane layer, while at the center of the catalyst, the Al KR X-ray intensity was near to zero. The Si signal intensity changed according to its content in the Co/SiO2 FTS catalyst and HZSM5. The SiO2/ Al2O3 ratio of the membrane of the capsule catalyst was 48.
The catalytic properties of the catalysts were studied with a high-pressure flow-type fixed bed reactor. Co/SiO2 catalyst only and its mechanical mixture with zeolite (20 wt %) were operated in the same conditions as a comparison. All the capsule catalysts gave similar CO conversion, which was slightly lower than mixture catalyst or normal FTS catalyst Co/SiO2, probably because the zeolite membrane slowed the diffusion rate of CO and H2 as shown in Table 1. Also, methane selectivities increased when the membrane was coated and increased when the zeolite coating amount increased. The low diffusion efficiency of CO and H2 led to a high H2/CO ratio in the interior part of the catalyst pellet, which might increase methane selectivity,13 because H2 diffuses more quickly than CO, especially inside small pores or channels. The CO2 in FTS reaction is mainly from WGS (water gas shift) reaction, and its selectivity hardly changed. The hydrocarbon distributions of conventional FT catalyst Co/SiO2 and the mixture catalyst (Co/SiO2 + zeolite) are compared in Figure 4A,B. The mixture catalyst gives a narrower distribution than the Co/SiO2 catalyst, because the waxy product migrating from the conventional FTS catalyst to the surface of zeolite catalysts was subject to secondary isomerization and hydrocracking, forming lighter hydrocarbons containing isoparaffins. The sequential isomerization and hydrocracking reaction on zeolite reduced the selectivity of long-chain paraffins and remarkably enhanced the selectivity of light isoparaffins. Different from the random behavior in the mechanical mixture catalyst, all products, in the form of straight chains, from Co/SiO2 core FTS catalyst must enter zeolite channels and diffuse through the zeolite membrane in capsule catalysts, which ensures that all waxy products receive secondary reactions inside zeolite membrane. The hydrocarbon diffusion rate in zeolite membrane depends
Figure 4. FT synthesis product distribution on (A) Co/SiO2, (B) Co/SiO2 + zeolite-MX, (C) 2-Co/SiO2-zeolite, and (D) 1-Co/SiO2zeolite-S catalyst; H2/CO ) 2; 1.0 MPa; W/F ) 10 g‚h/mol based on Co/SiO2; 533 K. Table 1. FT Reaction Properties of Capsule Catalystsa sample
CO conversion (%)
CH4 selectivity (%)
CO2 selectivity (%)
isoparaffin/n-paraffin
zeolite coating amount (%)
Co/SiO2 Co/SiO2-zeolite-MXb 1-Co/SiO2-zeolitec 2-Co/SiO2-zeolite 7-Co/SiO2-zeolite 1-Co/SiO2-zeolite-Sd 2-Co/SiO2-zeolitee
98.4 93.6 83.6 85.5 86.1 91.5 80.1
15.7 16.9 22.7 31.3 37.4 24.3 29.9
10.6 8.0 9.95 10.2 7.0 10.4 6.5
0 0.49 0.37 0.73 1.88 1.21 0.51
20.0 mixed 11.5 17.2 24.3 29.1 17.2
a Reaction conditions: 533 K, 1.0 MPa, W/F ) 10 g‚h‚mol-1, H /CO ) 2. b MX in the name of sample means the physical mixture of Co/SiO 2 2 catalyst and zeolite whose zeolite additive is 20%. c Numbers in sample names represent crystallization time in days in zeolite synthesis. d S in the sample name means the catalyst was prepared from small Co/SiO pellets (0.38-0.50 mm), while the others were prepared from 2 large pellets (0.85-1.7 mm). e W/F ) 5 g‚h‚mol-1 and other reaction conditions were the same.
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on chain length. Consequently, long-chain compounds stayed in the zeolite membrane longer, which caused all the long-chain hydrocarbons to crack and isomerize. All capsule catalysts gave a very sharp hydrocarbon distribution that ended at C9-C10, while there were still some C13-C20 hydrocarbons in the products of the mechanical mixture catalyst. Figure 4C,D shows the hydrocarbon distribution for two typical capsule catalysts. It is suggested that the covering membrane had an excellent selectivity for short-chain hydrocarbons, inhibiting the long-chain hydrocarbon completely. In addition, the capsule catalyst produced much isoparaffins and olefins. The yield of isoparaffin depended on the zeolite membrane content, which is also listed in Table 1. With the increase of zeolite membrane content, the ratio of isoparaffin to n-paraffin (>C3) increased. With a similar zeolite content, the capsule catalyst (2-Co/SiO2-zeolite) produced rather more isoparaffins than the mixture catalyst. These findings indicate that zeolite membrane had higher secondary reaction efficiency than the mechanically mixed zeolite because the capsule catalyst avoided the random occurrence of the secondary reactions of FTS hydrocarbons in the mechanical mixture catalyst. Also compared with the 2-Co/SiO2-zeolite catalyst in Table 1, when contact time W/F changed from 10 to 5 g‚h‚mol-1, CO conversion decreased slightly due to the faster flow rate. Methane selectivity decreased and the ratio of isoparaffin to normal paraffin became lower, (13) Madon, R. J.; Iglesia, E. J. Catal. 1994, 149, 428.
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indicating the residence time of normal paraffins from the core FTS catalyst, inside zeolite membrane, was too short to receive enough isomerization and hydrocracking, for this consecutive reaction regime. CO2 selectivity was down because the amount of water formed was reduced, related to the decreased CO conversion. Moreover, the capsule catalyst prepared from small FTS silica support pellets which crystallized in 24 h (1-Co/ SiO2-zeolite-S) showed a higher isoparaffin/n-paraffin ratio than the catalyst prepared from large silica pellets in 48 h crystallization (2-Co/SiO2-zeolite) but a lower methane selectivity. It was implied that the pellet size of the core catalyst had a strong effect on the capsule catalyst properties. Modifying the membrane catalyst and the core catalyst or optimizing the reaction conditions might enhance the catalytic activity and selectivity of capsule catalysts. Further studies are needed soon. Furthermore, the typical chemical process can be represented as Cat1
Cat2
A 98 B 98 C Building catalyst Cat1 as a membrane on the surface of catalyst Cat2 pellets can be applied to prepare a lot of capsule catalysts. This process can realize the combination of these two sequential reactions coupled with the in situ reaction-separation effect. These new kinds of capsule catalysts can be applied to many fields of chemical processes. LA047217H
