Energy & Fuels 2008, 22, 1897–1901
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Gasoline-Range Hydrocarbon Synthesis over Cobalt-Based Fischer–Tropsch Catalysts Supported on SiO2/HZSM-5 Yuping Li,†,‡,§ Tiejun Wang,*,†,‡ Chuangzhi Wu,†,‡ Yongxing Lv,†,‡,§ and Noritatsu Tsubaki| Key Laboratory of Renewable Energy and Natural Gas Hydrate, Chinese Academy of Science, Guangzhou 510640, P. R. China; Guangzhou Institute of Energy ConVersion, Chinese Academy of Science, Guangzhou 510640, P. R. China; Graduate School of the Chinese Academy of Science, Beijing 100039, P. R. China; and Department of Applied Chemistry, School of Engineering, UniVersity of Toyama, Gofuku 3190, Toyama 930-8555, Japan ReceiVed October 22, 2007. ReVised Manuscript ReceiVed February 13, 2008
Two types of SiO2 with different mesopore size and HZSM-5 zeolite were used to prepare hybrid supported cobalt-based catalysts. The textual and structural properties of the catalysts were studied using N2 physisorption, X-ray diffraction (XRD), and H2 temperature-programmed reduction (TPR) techniques. Fischer–Tropsch synthesis (FTS) performances of the catalysts were carried out in a fixed-bed reactor. The combination effects of the meso- and micropores of the supports as well as the interaction between supports and cobalt particles on FTS activity are discussed. The results indicate that the catalyst supported on the tailor-made SiO2 and HZSM-5 hybrid maintained both meso- and micropore pores during the preparation process without HZSM-5 particles agglomerating. The mesopores provided quick mass transfer channels, while the micropores contributed to high metal dispersion and accelerated hydrocracking/hydroisomerization reaction rate. High CO conversion of 83.9% and selectivity to gasoline-range hydrocarbons (C5-C12) of 55%, including more than 10% isoparaffins, were achieved simultaneously on this type of catalyst.
1. Introduction Fischer–Tropsch synthesis (FTS) is an industrialized process to produce a wide range of hydrocarbons from syngas, derived from coal, natural gas, tar, biomass et al.1 Cobalt-based catalyst is one of the most promising catalysts for synthesizing longchain hydrocarbons summarized by Khodakov.2 To obtain branched hydrocarbons, especially high-octane clean gasoline, many efforts were done on modifying FTS catalysts to break the limit of hydrocarbon Anderson-Schultz-Flory (ASF) distribution law. It has been proved the catalytic supports play important roles on both the structure and performance of cobalt catalysts. Some mesoporous supports as MCM-41 and SBA3 were investigated besides conventional supports (SiO2, Al2O3, and others).4,5 Contemporary researches proved that the spatial shape-selectivity effect of some zeolites could change FT synthesis mechanism.6,7 Hydrocracking and hydroisomerization * To whom correspondence should be addressed. Telephone: +86-2087057751. Fax: +86-20-87057737. E-mail:
[email protected]. † Key Laboratory of Renewable Energy and Natural Gas Hydrate. ‡ Guangzhou Institute of Energy Conversion. § Graduate School of the Chinese Academy of Science. | University of Toyama. (1) Zwart, R. W. R.; Boerrigter, H.; van der Drift, A. Energy Fuels 2006, 20, 2192–2197. (2) Khodakov, A. Y.; Chu, W.; Fongarland, P. Chem. ReV. 2007, 107, 1692–1744. (3) Khodakov, A. Y.; Zholobenko, V. L.; Bechara, R.; Durand, D. Microporous Mesoporous Mater. 2005, 79, 29–39. (4) Ngamcharussrivichai, C.; Liu, X. H.; Li, X. H.; Vitidsant, T.; Fujimoto, K. Fuel 2007, 86, 50–59. (5) Xu, D. Y.; Duan, H. M.; Li, W. Z.; Xu, H. Y. Energy Fuels 2006, 20, 955–958. (6) Li, X. H.; Asami, K.; Luo, M. F.; Michiki, K.; Tsubaki, N.; Fujimoto, K. Catal. Today 2003, 84, 59–65. (7) Jothimurugesan, K.; Gangwal, S. K. Ind. Eng. Chem. Res. 1998, 37, 1181–1188.
were considered to take place on the acidic surface of the zeolites.8,9 Bimodal pore supports (ZrO2-SiO2, Al2O3-SiO2) were reported by Tsubaki.10 Multifunctional capsule catalysts were also designed by coating HZSM-5 zeolite membrane onto the surface of the preshaped Co/SiO2 pellets.11 Co/HZSM-5 showed high selectivity to gasoline synthesis, but the sharp pore channel of HZSM-5 and strong interaction between Co particles and zeolites resulted in low CO conversion and undesirable aromatic products.4,12 Although the space-confined effect of supporting materials can be expected, few studies focused on it, especially for the hybrid supported catalysts. In this study, two types of mesopore SiO2 (9 and 40 nm) and HZSM-5 zeolite were used to prepare cobalt-based catalysts. The objective of this work was to evaluate the combination effect of hybrid support (SiO2 and HZSM-5) and the interaction between the supports and Co particles which affect FTS catalytic activity. 2. Experimental Section 2.1. Catalyst Preparation. 20 wt % of SiO2/HZSM-5 supported catalysts was prepared by incipient wetness impregnating cobalt nitrate aqueous solution over two types of SiO2 (SG-1:BET surface area of 101 m2/g, pore volume of 0.58 mL/g, pore diameter of 40 nm; SG-2:BET surface area of 354 m2/g, pore volume of 0.93 mL/ g, pore diameter of 9 nm), and HZSM-5 (BET surface area of 416 (8) Fernandes, F. A. N. Ind. Eng. Chem. Res. 2006, 45, 1047–1057. (9) Yoneyama, Y.; He, J. J.; Morii, Y.; Azuma, S.; Tsubaki, N. Catal. Today 2005, 104, 37–40. (10) Shinoda, M.; Zhang, Y.; Yoneyama, Y.; Hasegawa, K.; Tsubaki, N. Fuel Process. Technol. 2004, 86, 73–85. (11) He, J. J.; Liu, Z. L.; Yoneyama, Y.; Nishiyama, N.; Tsubaki, N. Chem.-Eur. J. 2006, 12, 8296–8304. (12) Calleja, G.; de Lucas, A.; van Grieken, R. Appl. Catal. 1991, 68, 11–29.
10.1021/ef700625z CCC: $40.75 2008 American Chemical Society Published on Web 04/09/2008
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Table 1. Textural Properties of the Cobalt-Based Catalysts
catalysts Co/SG-1 Co/SG-2 Co/HZSM-5 Co/SG-1/ HZSM-5 Co/SG-2/ HZSM-5
micropore mesopore micropore pore size (nm) area volume volume BET (cm3/g) (cm3/g) BJH HK (m2/g) (m2/g) 95 300 216 113 356
172.7 63 76
0.40 0.63 0.30 0.60
0.08 0.03 0.03
35.2 7.5 4, 30.2 6.8
0.56 0.72 0.63
m2/g, pore volume of 0.21 mL/g, pore diameter of 0.5 nm). The NH3 uptakes of pure HZSM-5, SG-1, and SG-2 were 0.27, 0.02, and 0.05 mmol/g, respectively, following the same procedure of NH3-TPD mentioned by He.11 The pellet sizes of SiO2 and HZSM-5 were 74-246 µm. The hybrid support was prepared by mechanically mixing SiO2(SG-1 or SG-2) and HZSM-5 with the weight ratio of 4:1 before impregnation. The catalyst precursors were dried at 393 K for 12 h and then calcined in air from room temperature to 673 K with the ramping rate of 2 K/min, keeping at 673 K for 1 h. All samples were under 15 MPa pressure and sieved to desired pellet size (20-40 mesh). 2.2. Catalysts Characterization. The surface area, pore volume, and pore size distributions were determined by N2 physisorption using Micromeritics ASAP-2010 automated system. The meso- and micropore distributions were described by the BJH and HK model, respectively. Powder X-ray diffraction (XRD) patterns of the catalysts were obtained with a Rigaku D/max-rC X-ray diffractometer using Cu KR radiation operated at 40 kV, 50 mA. The catalysts were in the oxide state, and the average sizes of Co3O4 were calculated by Scherrer equation at the 2θ ) 44.7°. H2 temperature-programmed reduction (H2-TPR) studies were carried out in a quartz tube reactor. 0.15 g of catalyst was heated in a flow of 5% H2/95% N2 (30 mL/min) from 373 to 1023 K at a heating rate of 6 K/min. Hydrogen consumption was monitored by the change of thermal conductivity of the effluent gas stream. 2.3. FTS Reaction System and Operation Procedure. FTS reaction was carried out in a stainless steel fixed bed reactor fed by the syngas with H2/CO ratio of 2 and 4.5% N2 as internal standard, and 2.5% CO2 to suppress water gas shift reaction. In each FTS run, 1.0 g of catalyst was activated in situ at 673 K for 8 h with 5% H2/95% N2 gas flow at atmosphere pressure. After activation, the reactor was cooled to 453 K and purged with the syngas. The flow rate of the syngas was adjusted by the Brooks 5850E mass flow controller. The typical operation conditions were T ) 523 K, P ) 2.0 MPa, W/F ) 12 gcat h mol-1. The effluent gas passed through a cyclone separator and a cold trap placed between the reactor exit and back-pressure regulator to condense liquid products. The tail gas flow rate was monitored by a mass flow meter. 2.4. Analysis of the Products. Tail gas was analyzed by two online gas chromatographs. CO, N2, CH4, and CO2 were analyzed by carbon-sieve column with TCD detector. Gaseous hydrocarbons (C1-C5) were analyzed by Porapak Q column with FID detector. After each interval of reaction, liquid product in the traps was collected and analyzed by a FID GC equipped with SE-30 capillary column. N2 contained in the feed gas was used to determine the CO conversion, the selectivity to CO2 and hydrocarbons. The selectivity distribution to hydrocarbons was calculated on the basis of carbon number.
3. Results and Discussion 3.1. Textural Properties of the Catalysts. Textural properties of the catalysts are shown in Table 1 and Figure 1. The BET surface area, pore volume, and pore size decreased when SG-1, SG-2, and HZSM-5 were used solely to prepare Co/SG1, Co/SG-2, and Co/HZSM-5. But the pore distribution patterns (Figure 1) indicate that the main pore structures were not destroyed during the preparation process. Because of the large surface area and small pore volume of HZSM-5, the BET surface area, micropore surface area and micropore volume of
Co/SG-1/HZSM-5 and Co /SG-2/ HZSM-5 prepared on SiO2 and HZSM-5 hybrids all slightly increased compared with Co/ SG-1 and Co/SG-2, respectively. BJH mesopore model of Co/ SG-1/HZSM-5 (Figure 1a) shows a small peak at about 4 nm besides the peak of 35 nm similar with Co/SG-1. But there is only a shoulder peak at 4 nm to Co/SG-2/HZSM-5. This difference can only be explained from the textural difference of SG-1 and SG-2: (a) the particles of HZSM-5 formed new pores of about 4 nm through dehydration and aggregation happened on the surface acidic centers of HZSM-5 and hydroxyl groups of SiO2 during impregnation and calcinations process;13 (b) SG-2 with large surface area dispersed HZSM-5 particles evenly and almost no HZSM-5 clusters formed. So Co/SG-2/ HZSM-5 exhibited BJH pore pattern with Co/SG-2 except for slightly decreased mesopore surface area and volume, and increased micropore surface area of 76 m2/g. But the small surface area (101 m2/g) of SG-1 made HZSM-5 particles dehydrate and aggregate more easily than SG-2. The decreased mesopore volume and surface area of Co/SG-1/HZSM-5 indicated further that the newly formed HZSM-5 clusters might block some mesopore pores of SG-1, which affected FTS activity explained below. 3.2. Structure Property of the Catalysts. Table 2 clearly shows the dispersion and reduction degree of the catalysts are affected by the supports with different textural property. It seems the high dispersion and small Co particle of Co/SG-2 and Co/ SG-2/ HZSM-5 resulted from the large surface area of SG-2. But HZSM-5, the largest surface area, did not lead to high cobalt dispersion of Co/HZSM-5. van Steen et al.14 found that the silanol groups of SiO2 could form bonds with the water ligands of the cobalt complex, which lead to the formation possibility of surface cobalt silicate. In this paper, when HZSM-5 was used solely as the support, the separate hydroxides (silanols) and acidic centers of HZSM-5 enhanced the formation of cobalt silicate, confirmed by the XRD patterns in Figure 2. Furthermore, the dispersion of Co particles was suppressed, so the largest Co3O4 clusters formed and the surface area decreased to almost half of HZSM-5 on Co/HZSM-5, while when SiO2/ HZSM-5 hybrids were used, SiO2 decreased the acidity of HZSM-5 and the strong interaction between HZSM-5 and Co particles. The enhanced surface areas of Co/SG-1/HZSM-5 and Co/SG-2/HZSM-5 resulted in higher dispersion degree and smaller Co particle than that of Co/SG-1 and Co/SG-2. The reduction performance of Co/SG-1, Co/SG-2, and Co/ SG-2/HZSM-5 presents two peaks as shown in Figure 3. As mentioned in other literature,15 the first peak, between 540 and 630 K, is most likely identified as the reduction of large Co3O4 particles to Co2+. The following peak is the conversion of Co2+ to Co, summarized in Table 2. The lower temperature and more intensive peaks of H2-TPR profile of Co/SG-1 indicated easily reduction than Co/SG-2. The results can be deduced from larger Co particles and weaker interaction between SiO2 and Co particles.16 For Co/HZSM-5, the broad peak at about 790 K was considered as the coupled effect of reduction of Co oxide particles and cobalt silicate species, and for Co/SG-1/HZSM-5 there was only one wide peak from 590 to 750 K and high (13) Zhang, Y.; Shinoda, M.; Tsubaki, N. Catal. Today 2004, 93–95, 55–63. (14) van Steen, E.; Sewell, G. S.; Makhothe, R. A.; Micklethwaite, C.; Manstein, H.; de Lange, M.; O’Connor, C. T. J. Catal. 1996, 162, 220– 229. (15) Saib, A. M.; Borgna, A.; van de Loosdrecht, J.; van Berge, P. J.; Geus, J. W.; Niemantsverdriet, J. W. J. Catal. 2006, 239, 326–329. (16) van de Water, L. G. A.; Bezemer, G. L.; Bergwerff, J. A.; VersluijsHelder, M.; Weckhuysen, B. M.; de Jong, K. P. J. Catal. 2006, 242, 287– 298.
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Energy & Fuels, Vol. 22, No. 3, 2008 1899
Figure 1. Pore size distributions of the cobalt-based catalysts: (a) BJH mesopore model and (b) HK micropore model. Table 2. Structure Property of the Catalysts catalysts
XRD Co particle sizea (nm)
reduction degreeb (%)
TPR peak (K)
Co dispersionc (%)
CO conversion (%)
Co/SG-1 Co/SG-2 Co/HZSM-5 Co/SG-1/ HZSM-5 Co/SG-2/ HZSM-5
24.5 18.8 44.5 20.6 15.8
72.10 44.7 52.6 45.3 66.2
573, 666 593, 748 663, 790 731 592, 688
3.72 4.89 2.48 4.01 5.67
86.7 41.4 60.2 53.7 83.9
a
Calculated by dv(Co0) ) 0.75d(Co3O4). b Determined by TPR from 373 to 850 K. c Determined by H2 chemisorption.
Figure 2. XRD patterns of the cobalt-based catalysts (0, Co3O4; O, cobalt silicate).
Figure 3. H2-TPR profiles of the cobalt-based catalysts.
level of H2 consumption rate lasting above 850 K. Also, the reducibility calculated by TPR data from 373 to 850 K was
Figure 4. Dependence of CO conversion on reaction time.
only 45.3%, which might result from the inefficiency of the H2 reduction due to partial blocked pores. The existence of micropore zeolite diffraction regions of 20°-30° of Co/SG-1/HZSM-5, Co/SG-2/HZSM-5, and Co/ HZSM-5 also confirmed the existence of micropores derived from HZSM-5 in these catalysts. But when HZSM-5 were doped on SG-1 or SG-2, the intensity of the zeolite peaks decreased, which meant that HZSM-5 dispersed over the SG-1 or SG-2 supports. This was more evidence that HZSM-5 particles either entered the channels or deposited on the surface of SiO2 to form multifunctional FTS catalyst. 3.3. CO Conversion of the Catalysts. The compromise effect of the reducibility and dispersion of the Co metal sites determined the CO conversion of the catalysts followed the order of Co/SG-1 > Co/SG-2/HZSM-5/ > Co/HZSM-5 > Co/ SG1/HZSM-5 > Co/SG-2 in Table 2, after about 8 h steady period, as showed in Figure 4. The CO conversion of Co/SG-1 (86.7%) was higher than that of Co/SG-2 (41.4%) because the larger Co3O4 particles derived from cobalt nitrate were easier to reduce to Co0 than smaller ones on SiO2 supports. In this study,
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Figure 5. Hydrocarbon selectivity distributions of FTS catalyzed by cobalt-based catalysts.
although the Co3O4 particles on HZSM-5 were the largest, CO conversion of Co/HZSM-5 was only 60.2% because the formation of hardly reducible cobalt silicate decreased the cobalt utilization rate. CO conversion dropped to 53.7% when the catalyst was supported on HZSM-5 and SG-1 hybrid. It can be deduced that the partially blocked mesopores by HZSM-5 clusters mentioned above raised the mass diffusion restriction and decreased FTS activity. Although Co/SG-2/HZSM-5 had smaller Co3O4 particle size than Co/SG-2, higher CO conversion of 83.9% was achieved. This phenomenon might be attributed to the coeffect of large amounts of Co metal sites from high reduction, dispersion degree and the mesopore structure, which favored mass diffusion of syngas or products. 3.4. Effect of SiO2/HZSM-5 Hybrid on Products Selectivity. The hydrocarbon distributions over the Co/SiO2 catalysts followed ASF model except for CH4, shown in Figure 5. Similar derivations have been observed in many other studies.17 But non-ASF distribution existed in Co/HZSM-5, which was divided into two stages at C11 due to its special pore structure. It was mentioned by Zhang that the large pores of the catalysts favored mass transport and chain growth of CH2 fragmemt.18 Thus, the mesopore catalysts of Co/SG-2 accelerated chain growth probability and showed relatively higher selectivity to heavy hydrocarbon (C16+) than micropore HZSM-5-containing catalysts. The higher selectivity to C1-C7 over Co/SG-1 than Co/SG-2 was due to higher CO conversion, which enhanced the primary and secondary reactant rate. So the high selectivity toward certain some light hydrocarbons as CH4 was obtained, which was as high as 30.7%; and the increased C5+ selectivity occurred mainly at the expense of CH2 chain growing,19 so the highest C24+ selectivity over Co/SG-2 was obtained. Meanwhile, the catalysts supported by HZSM-5 yielded more water. Water molecules, as Lewis base centers, adsorbed on Lewis acid centers of HZSM-5 and suppressed water gas shift reaction activity. So the selectivity to CO2 of Co/HZSM-5 and Co/SG-2/ HZSM-5 was low. But high water pressure resulted in the formation of cobalt silicate as mentioned by van de Loosdrecht,20 which decreased the FTS activity rapidly as showed in Figure 4 of Co/HZSM-5. (17) Elbashir, N. O.; Roberts, C. B. Ind. Eng. Chem. Res. 2005, 44, 505–521. (18) Zhang, Y.; Koike, M.; Tsubaki, N. Catal. Lett. 2005, 99, 193– 198. (19) Morales, F.; de Smit, E.; de Groot, F. M. F.; Visser, T.; Weckhuysen, B. M. J. Catal. 2007, 246, 91–99. (20) van de Loosdrecht, J.; Balzhinimaev, B.; Dalmon, J.-A.; Niemantsverdriet, J. W.; Tsybulya, S. V.; Saib, A. M.; van Berge, P. J.; Visagie, J. L. Catal. Today 2007, 123, 293–302.
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Figure 6. Comparison of product species over cobalt-based catalysts (left to right: C1-C4; C5-C12; C13-C16; C16+; CO2).
The main hydrocarbon species produced were straightchain paraffins. Lower selectivity to olefins of less than 5% in all catalysts was due to the high hydrogenation ability of cobalt-based catalyst.21 Co/HZSM-5 showed higher selectivity to isoparaffins and olefins. In this situation, HZSM-5 not only acted as a support, but had spatial shape-selectivity effect due to its special pore structure and high acidity. The small pore size of 0.5 nm prolonged mass contact time, which enhanced the re-adsorption ability of primary olefins and secondary hydrocracking and isomerization reactions; and its small pore size was the right one for gasoline-range hydrocarbon (C5-C12) to diffuse. So the selectivity to isoparaffins for Co/HZSM-5 was about 15%, the highest among the five catalysts, as illustrated in Figure 6. Co/SG1/HZSM-5 had meso- and micropore pore structures, yet the decreased mesopores made it exhibit similar hydrocarbon selectivity to its reference of Co/SG-1. But the selectivity to gasoline-range hydrocarbon of Co/SG-2/HZSM-5 was the highest of about 55% including more than 10% isoparaffins, and the C12+ selectivity was less than 15%. They were attributed to the coexistence of meso- and micropores in the catalyst. The former improved the diffusion rate of syngas and products; the latter dispersed active Co metal particles well and accelerated the secondary FTS reaction. Although Co/SG-2/HZSM-5 showed about 5% lower selectivity to isoparaffins than Co/HZSM-5 in gasoline-range hydrocarbons, it is considered as the proper catalyst due to its high and stable catalytic activity (Figure 4). 4. Conclusions The structural and textural properties of the supports and interaction between hybrid supports affect the numbers of active cobalt sites, and finally impact the catalytic activity. It seems that the high dispersion degree of cobalt particles resulted from the high surface area of the supports, but strong acidity of HZSM-5 decreases this tendency. When HZSM-5 is used along with relative large pore size SiO2 as hybrid support, as SG-1, there are newly formed pore structures by HZSM-5 clusters on the surface of SG-1 due to the small surface area and intense interaction between silanols and HZSM-5 acidic centers. The original mesopore pores (40 nm) of SG-1 might be partial blocked by HZSM-5 clusters. These (21) Yang, J.; Sun, Y. H.; Tang, Y.; Liu, Y.; Wang, H. L.; Tian, L.; Wang, H.; Zhang, Z. X.; Xiang, H. W.; Li, Y. W. J. Mol. Catal. A: Chem. 2006, 245, 26–36.
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decreased the FTS efficiency. The large surface area of proper pore size SG-2 disperses HZSM-5 well and reduces the acidity of HZSM-5 without mesopore structure destruction. The meso- and micropore structures of tailor-made Co/SG2/HZSM-5 combine the advantage of the quick diffusion rate and high shape selectivity to gasoline-range hydrocarbons production.
Energy & Fuels, Vol. 22, No. 3, 2008 1901 Acknowledgment. We gratefully acknowledge financial support from National Natural Science Foundation of China (Project No. 50506030), National Key Basic Research Program 973 Project founded by MOST of China (Project No. 2007CB210207), and Innovative Program of The Chinese Academy of Sciences (Project No. KGCX2-YW-306). EF700625Z
