VOLUME 20, NUMBER 2
MARCH/APRIL 2006
© Copyright 2006 American Chemical Society
Articles Promotional Effects of Al2O3 Addition to Co/SiO2 Catalysts for Fischer-Tropsch Synthesis Yi Zhang,† Satoshi Nagamori,† Sukamon Hinchiranan,‡ Tharapong Vitidsant,‡ and Noritatsu Tsubaki*,† Department of Applied Chemistry, School of Engineering, UniVersity of Toyama, Gofuku 3190, Toyama 930-8555, Japan, and Department of Chemical Technology, Faculty of Science, Chulalongkorn UniVersity, Bangkok 10330, Thailand ReceiVed July 19, 2005. ReVised Manuscript ReceiVed January 19, 2006
The addition of a small amount of Al2O3 to silica-supported cobalt catalysts significantly increased the dispersion of cobalt and Co-metallic surface area, resulting in the remarkable enhancement of the FischerTropsch synthesis (FTS) activity in the slurry-phase reaction. The addition of Al2O3 adjusted the interaction between cobalt and the silica support quite well, realizing the favored dispersion and reduction degree of supported cobalt and leading to high catalytic activity in FTS. The properties of various catalysts were characterized by in situ DRIFT, XRD, TPR, N2 physisorption, and H2 chemisorption.
1. Introduction The environmental demands for clean fuels lead to an obvious trend in energy consumption from oil and coal to clean resources. The uses of natural gas have attracted interest because of their clean burn and abundant supply. Fischer-Tropsch synthesis (FTS) is a promising way to convert coal and natural gas to clean fuels and chemicals via syngas. Cobalt-based catalysts attract most of the current attention for the direct conversion of syngas in FTS because of their high activity, high selectivity for long-chain paraffins, and low water gas shift activity.1 Besides cobalt, some supports such as SiO2, Al2O3, and TiO2 are indispensable. The chemical and textural properties of the support influence the catalytic activity and product selectivity of Co catalysts via their modifications on the reducibility and dispersion of cobalt or the formation of wellfined phases. The synthesis of highly dispersed Co catalysts * To whom correspondence should be addressed. Tel./Fax: (81)-76-4456846. E-mail:
[email protected]. † University of Toyama. ‡ Chulalongkorn University. (1) Eglesia, E. Appl. Catal., A 1997, 161, 59.
requires strong interactions between the support and the Co precursor, but in turn such strong interactions generally lower the reducibility of such precursors.2 Alumina is one of the mostemployed supports for cobalt FTS catalysts because of its favorable mechanical properties and adjustable surface properties. Supports such as Al2O3 have significant support interaction, contributing to the formation of a smaller supported-metal cluster, whereas supports such as silica always yield a large supported-metal cluster size and high reduction degree. However, unlike the cobalt silica catalyst, Co/Al2O3 catalysts have limited reducibility because the support can interact with the active phase by the diffusion of the cobalt ion into the structure of alumina, forming cobalt aluminate to impede the reduction.3,4 Usually, noble metals are used as promoters of supported Co catalysts. Tsubakiet al. proposed the different functions of Ru, Pd, and Pt added into Co/SiO2 catalysts.2 Ru enriched on a cobalt surface promoted the reduction of Co catalysts, whereas Pt and (2) Tsubaki, N.; Sun, S.; Fujimoto, K. J. Catal. 2001, 199, 236. (3) Bessel, S. Appl. Catal., A 1993, 96, 253. (4) Qukaci, R.; Singleton, A. H.; Goodwin, J. G., Jr. Appl. Catal., A 1999, 186, 129.
10.1021/ef050218c CCC: $33.50 © 2006 American Chemical Society Published on Web 02/09/2006
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Table 1. Reaction Performances of Various Co/SiO2 Catalysts with Different Al2O3 Loading in Slurry-Phase FTSa catalyst
Al2O3 loading (wt %)
pore size of support (nm)
BET of support (m2/g)
CO conversion (%)
CH4 selectivity (%)
CO2 selectivity (%)
R
Q-10 5 Al2O3 10 Al2O3 15 Al2O3 Co-10 Al2O3b
0 5 10 15 10
10 10 11 11 11
283 272 270 262 257c
45 61 66 63 64
8.3 8.1 7.9 7.8 7.6
0.56 0.52 0.47 0.60 0.49
0.86 0.86 0.87 0.87 0.87
a Reaction conditions: T ) 513 K, P ) 1.0 MPa, H /CO ) 2, W/F ) 5 g-cat h mol-1; weight of catalyst ) 1 g; cobalt loading ) 10 wt %. b Prepared 2 by coimpregnation of alumina and cobalt. c BET of calcined catalyst.
Pd dispersed uniformly in the form of Pt-Co or Pd-Co alloys enhanced the dispersion and scarcely affected the reducibility of Co catalysts. Although the noble metals can provide the promotional effects for FTS catalysts, their application is limited because of their high cost. As a result, the addition of metal oxides as promoters is a preferable way to improve the FTS catalyst because of their cheap price and available supply. The present work investigated the Al2O3 promotional effects on Co/SiO2 catalysts. It is considered that the addition of Al2O3 would adjust the interaction between cobalt and silica support quite well, because of the heteroatom bond between alumina and silica, realizing the favored dispersion and reduction degree of supported cobalt and leading to high catalytic activity in the FTS. This kind of catalyst was tested in a semibatch slurryphase FTS reaction and its properties were characterized by in situ DRIFT, XRD, TPR, and H2 chemisorption. 2. Experimental Section The alumina-modified silica supports were prepared by incipient wetness impregnation of commercially available silica gel (Cariact Q-10, Fuji Silysia Chemicals, Ltd., specific surface area ) 283 m2 g-1, pore volume ) 1.22 mL g-1, pellet size ) 75-500 µm, and mean pore diameter ) 10 nm) with an aqueous solution of aluminum nitrate. After impregnation, the samples were dried at 393 K for 12 h and then calcined at 673 K for 2 h. The loading amounts of Al2O3 were 5, 10, and 15 wt %. The aqueous solution of cobalt nitrate was further impregnated onto the obtained supports by the incipient wetness method. The loading of cobalt was 10 wt % for all catalysts. The catalysts were dried at 393 K for 12 h and calcined at 673 K for 2 h. The calcined samples were then reduced with H2 at 673 K for 10 h followed by passivation with 1% O2 in N2. The coimpregnation catalyst was prepared by the same procedure as that given above. The aqueous solution of aluminum nitrate and cobalt nitrate was impregnated onto silica support Q-10 together by the incipient wetness method. The loading of alumina and cobalt was 10 wt % each. The FTS reaction was carried out in a semibatch slurry-phase reactor with an inner volume of 80 mL. The passivated catalyst (1.0 g) was loaded in the reactor with 20 mL of liquid medium (n-hexadecane). The reaction conditions were P(total) ) 1.0 MPa, T ) 513 K, CO/H2 ) 1/2, W/F(CO+H2+Ar) ) 5 g cat. h mol-1. The effluent gas from the reactor was analyzed by on-line gas chromatography. A thermal conductivity detector (TCD) was used to analyze gaseous products (CO, CO2, and CH4). Light hydrocarbons (C1-C5) were on-line analyzed by a flame ionization detector (FID) with a Porapak-Q column. The analyses of hydrocarbons dissolved in the solvent and cooled in the trap were carried out with FID with a silicon SE-30 column. Temperature-programmed reduction (TPR) experiments were carried out in a quartz tube reactor using 0.2 g of calcined catalysts. The reducing gas, a mixture of 5.1% H2 diluted by N2, was fed via a mass flow controller at 50 mL min-1, and the temperature was increased at a rate of 8 K min-1 from 323 to 1073 K. The effluent of the reactor passed through a 5A molecular sieve trap to remove produced water before reaching TCD. The hydrogen consumption was calculated from 323 to 1073 K, and the reduction degree, which
was a reduction percentage of supported cobalt, was calculated by assuming stoichiometric conversion of Co3O4 to metallic Co. Chemisorption experiments were carried out in a static volumetric glass high-vacuum system (QUANTACHROME Autosorb-1, Yuasa Ionics). Research-grade gases (H2, 99.9995%; CO, 99.99%; Takachiho Co.) were used without further purification. Before adsorption of H2, the catalysts, which were previously reduced by H2 and passivated, were treated in H2 at 673 K for 1 h, followed by evacuation. H2 adsorption isotherms were measured at 373 K. It was reported that H2 chemisorption at 373 K was most reliable for the silica-supported cobalt catalyst.5,6 The calculation of the metalliccobalt surface area, dispersion, and average crystalline diameter of the supported cobalt are described elsewhere.7 The BET surface area was determined by an adsorption method (Quantachrome Autosorb-1, Yuasa Ionics), where nitrogen was used as adsorbent. An X-ray diffractometer (XRD, Rigaku, RINT2000) was used to detect the supported cobalt crystalline size of the passivated catalysts. The crystalline average size was calculated by L ) Kλ/∆(2θ)cos θ0, where L is the crystalline size, K is a constant (K ) 0.9-1.1), λ is the wavelength of the X-ray (Cu KR ) 0.154 nm), and ∆(2θ) is the width of the peak at half-height. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectra were collected on a Nexus 470 FT-IR spectrometer equipped with a diffuse reflectance attachment and a MCT detector. The catalyst powder (14 mg) was contained in a diffuse reflectance infrared cell with a ZnSe window, which can work at high temperature and high pressure. In situ absorbance spectra were collected using 32 scans at a resolution of 2 cm-1. Before the adsorption of CO, the passivated catalyst sample was treated in situ in a He stream flowing at 50 mL min-1 at 298 K for 20 min, 473 K for 1 h, and 673 K for 20 min; it was then reduced in a H2 stream flowing at 50 cm3 min-1 at 673 K and atmospheric pressure for 30 min and then at 0.5 MPa for 1 h. The catalyst sample was swept by He at 673 K for 1 h; the catalyst sample was then cooled in He to 298 K for 1 h. The spectra were obtained at room temperature after the exposure of the catalyst to CO/He at a ratio of 5/10 flowing at a total rate of 20 mL min-1 for 1 h, followed by sweeping in helium for 20 min.
3. Results and Discussion 3.1 Reaction Performances. To investigate the promotional role of the added alumina, alumina-modified Co/SiO2 was applied to a liquid-phase Fischer-Tropsch synthesis reaction. Such a liquid-phase Fischer-Tropsch synthesis reaction has advantages in temperature control, wax extraction, and catalyst lifetime extension, compared to the common gas-phase reaction.8 As shown in Table 1, Al2O3-promoted catalyst with 10 wt % alumina loading showed a highest CO conversion level of 66% in the liquid-phase FTS reaction, and the selectivity of CH4 and CO2 were slightly lower than those of the unpromoted Q-10 catalyst. On the other hand, the catalysts promoted by 5 or 15 wt % alumina loading exhibited a highly catalytic activity similar (5) Zowtiak, J. M.; Bartholomew, C. H. J. Catal. 1983, 83, 107. (6) Reuel, R. C.; Bartholomew, C. H. J. Catal. 1984, 85, 63. (7) Sun, S.; Tsubaki, N.; Fujimoto, K. Appl. Catal., A 2000, 202, 121. (8) Fan, L.; Yokota, K.; Fujimoto, K. Top. Catal. 1995, 22, 67.
Al2O3 Addition to Catalysts for Fisher-Tropsch Synthesis
to that of the 10 wt % alumina-promoted catalyst, with higher CO conversion levels, 61 and 63%, respectively, than that of the Q-10 catalyst. The Q-10 catalyst shows the lowest CO conversion rate, 45%, in this study. It was considered that because of the weak interaction between the silica support and supported cobalt, the Q-10 catalyst formed larger-sized cobalt particles, leading to a lower cobalt surface area and resulting in lower catalytic activity. For alumina-promoted catalysts, because of significant support interaction, the added alumina contributed to the formation of small supported cobalt particles; this results in a high dispersion of supported cobalt, which would contribute to high catalytic activity of this kind of catalysts in the FTS reaction. The 10 wt % alumina-promoted catalyst prepared by coimpregnation of alumina and cobalt showed a reaction performance similar to that of the catalyst prepared by successive impregnation of alumina and cobalt, indicating that the impregnation method did not influence the reaction performance of alumina-promoted catalysts in this study. 3.2 Characterization of Catalysts. The activities and selectivities of the FTS catalysts are markedly dependent on their structures. The properties of various supports, including alumina-modified supports, are compared in Table 1. The BET surface area of alumina-modified supports decreased from 283 m2/g for silica Q-10 to 262 m2/g for 15 wt % alumina-loaded support, with increased alumina loading. Because the decreased BET surface area of alumina-modified silica supports would result in a decreasing dispersion of supported cobalt, it was considered that high dispersion of the supported cobalt should be relative to the chemical effects of added alumina. On the other hand, the selectivities of the FTS catalysts are markedly dependent on their pore structures.9 As shown in Table 1, the pore size of various supports is almost the same, indicating that the added alumina did not change the pore structure of silica support Q-10. On the basis of these findings, the pore size of alumina-promoted catalysts could not influence the products selectivity. It was considered that the heteroatom effects of alumina and silica resulted in a lower CH4 selectivity of aluminapromoted catalysts as well as a slightly higher carbon-chaingrowth probability. The activity of cobalt catalyst in FTS depends solely on the number of active sites located on the surface of crystalline metal formed by reduction. The number of active sites is determined by the Co particle size, loading amount, and reduction degree.9 The supported-cobalt particle size of various catalysts was determined by XRD and H2 chemisorption. The XRD patterns of various passivated catalysts are shown in Figure 1. The cobalt crystalline size of the Q-10 catalyst was the largest, because of lighter interaction between the supported cobalt and silica support. Because of the addition of alumina, the supported-cobalt particle size of alumina-promoted catalysts was smaller than that of Q-10. And the cobalt particle size of alumina-promoted catalysts decreased with increased alumina loading, according to H2 chemisorption data, as compared in Table 2. It was observed that support interactions on cobalt oxide species from alumina were stronger than those from silica,10 which could form smaller supported-cobalt particles. On the basis of this finding, it is considered that because of the stronger chemical effects of the alumina structure, the alumina-promoted catalysts formed supported-cobalt particles that were smaller in size than those of the silica Q-10 catalyst, resulting in a higher supported-cobalt dispersion, as illustrated in Table 2, even though their BET (9) Fan, L.; Yokota, K.; Fujimoto, K. AIChE J. 1992, 38, 1639. (10) Jacobs, G.; Das, T. K.; Zhang, Y. Q.; Li, J. L.; Racoillet, G.; Davis, B. H. Appl. Catal., A 2002, 233, 263.
Energy & Fuels, Vol. 20, No. 2, 2006 419
Figure 1. XRD patterns of various passivated catalysts. (1) Q-10 catalyst. Alumina-promoted catalyst with alumina loading of (2) 5, (3) 10, and (4) 15 wt %. (5) Coimpregnation catalysts with Al2O3 loading of 10 wt %.
surface area was decreased. On the other hand, the CH4 selectivity of alumina-promoted catalysts was lower than that of the Q-10 catalyst, even though the small cobalt particle size would lead to high CH4 selectivity. On the basis of these findings, it is proven that chemical effects of added alumina improve the dispersion of the supported cobalt and decrease the CH4 selectivity. The reduction performance of various catalysts was determined by temperature-programmed reduction (TPR). In TPR spectra of various catalysts, as shown in Figure 2, two peaks exist for the Q-10 catalyst, located at 571 and 646 K. The two peaks have been identified as the conversion of Co3+ to Co2+ followed by the conversion of Co2+ to Co, and the broad region above 800 K indicates the existence of several species reduced at approximately the same temperature.11 For alumina-promoted catalysts, a sharp and low-temperature peak appears at 595 K, and another broad peak, centered at 720 K, distributes from 620 to 1000 K, which should be attributed to the reduction of various Co species, including the species with different particle sizes. Comparing the reduction degrees of various catalysts, which were calculated by TPR data from 353 to 1073 K, we found that the Q-10 catalyst showed the best reducibility, 73%, because it has the largest supported-cobalt particle and lighter interaction with the silica support, as shown in Table 2. For alumina-promoted catalysts, because of their small supportedcobalt particles, the reduction degree was lower than that of the Q-10 catalyst and decreased from 62 to 52% with increased alumina loading, because of the strong interaction between supported cobalt and added alumina. The very low reduction degree would result in a lower surface area, even though the dispersion of supported cobalt was high, because of the large amount of unreduced supported cobalt. Concerning the cobalt surface area, however, alumina-promoted catalysts formed a larger metallic-cobalt surface area than did the Q-10 catalyst, as exhibited in Table 2; this indicates that this kind of catalyst kept a relatively high reduction degree, contributing to a reduction in the highly dispersed supported-cobalt particles resulting in a high cobalt surface area. It was considered that the large-size cobalt particles or those located at the silica surface (11) Rosynek, M. P.; Polansky, C. A. Appl. Catal., A 1991, 73, 97.
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Table 2. Properties of Co/SiO2 Catalysts with Different Al2O3 Loadinga catalyst
Al2O3 loading (wt %)
Q-10 5 Al2O3 10 Al2O3 15 Al2O3 Co-10 Al2O3b
0 5 10 15 10
Co particle size (nm) XRD H2c 18 15 10 9 13
22 16 13 10 15
reduction degreed (%)
Co dispersione (%)
TOFf (s-1)
Co surface areag (m2/g)
73 62 57 52 60
5.2 7.0 7.4 7.8 7.1
0.13 0.16 0.17 0.17 0.17
2.3 2.8 3.1 2.8 2.9
a Cobalt loading was 10 wt % on the basis of total catalyst weight for all catalysts. b Prepared by coimpregnation of alumina and cobalt. c Calculated from H2 chemisorption at 373 K. d Calculated by TPR from 323 to 1073 K. e Determined by H2 chemisorption. f On the basis of H2 total uptake determined by H2 chemisorption. g Determined by H2 chemisorption.
Figure 3. In situ DRIFT spectra of various catalysts. (1) Q-10 catalyst. (2) Alumina-promoted catalyst with 10 wt % alumina loading. (3) Coimpregnation catalyst with 10 wt % Al2O3 loading. Figure 2. TPR profiles of various calcined catalysts. (1) Q-10 catalyst. Alumina-promoted catalyst with alumina loading of (2) 5, (3) 10, and (4) 15 wt %. (5) Coimpregnation catalysts with Al2O3 loading of 10 wt %.
were easier to be reduced, which activated hydrogen after reduction, and the activated hydrogen reduced the small cobalt oxide particles by a spill-over effect, contributing to the high reducibility of supported cobalt.12 On the other hand, it was considered that the coexisting silica could influence the reactivity of the alumina surface, improving the reducibility of the supported cobalt on the alumina surface. The TOF of various catalysts was calculated. As shown in Table 2, the TOF of alumina-promoted catalysts was higher than that of the silica Q-10 catalyst, indicating that the addition of alumina promoted the site activity of supported cobalt in this kind of catalyst. In situ DRIFT spectra of the adsorbed CO for the reduced catalysts are compared in Figure 3. Many studies report one band in the 2000-2050 cm-1 range after CO adsorption. This band has been attributed to linearly bonded CO molecules with surface cobalt atoms.13 In our investigation, this adsorbed species manifested itself by bands at 2034, 2038, and 2038 cm-1 for the Q-10 catalyst, 10 wt % alumina-promoted catalyst, and 10 wt % alumina-promoted catalyst prepared by coimpregnation, respectively. In general, the blue shift of this band indicated that the CO band became strong if the CO adsorbed onto small cobalt. The band of CO adsorbed on Co metal in a linear mode shifted from 2034 cm-1 for the Q-10 catalyst to 2038 cm-1 for (12) Zhang, Y.; Koike, M.; Tsubaki, N. Catal. Lett. 2005, 99, 193. (13) Mohana Rao, K.; Scarano, D.; Spoto, G.; Zecchina, A. Surf. Sci. 1988, 204, 319.
the alumina-promoted catalyst here, as the cobalt particle size of the alumina-promoted catalyst was smaller than that of Q-10 catalysts, according to the data in Table 2, which indicates that the addition of alumina promoted the dispersion of the supported cobalt. The 2059 cm-1 shoulder peak can be assigned to the surface carbonyl species, which readily happened on corner sites of the cobalt metal, and the bands of 1934 cm-1 are attributed to the bridge-type CO adsorbed on cobalt metal particles.14,15 For the Q-10 catalyst, the peak of bridged adsorbed CO was weaker than the peak of linear adsorbed CO. For alumina-promoted catalysts, the peak of the bridged adsorbed CO was stronger than the peak of the linearly adsorbed CO. Especially, the peak of the bridged adsorbed CO was broader than that of the Q-10 catalyst, indicating that various bridged adsorbed CO species, including more-active bridged adsorbed CO species, were formed on the alumina-promoted catalyst. It was considered that the addition of alumina in the alumina-promoted catalyst might form an active interface with Co, which was responsible to some extent for the enhancement in Co activity. Two kinds of alumina-promoted catalysts prepared by different impregnation methods showed similar peaks in DRIFT spectra, indicating that the impregnation method of added alumina and cobalt did not influence the properties of the obtained catalysts. As pointed out, the bridged adsorbed CO was much more active than linearly adsorbed CO.16 The high activity of alumina-promoted (14) Reuel, R. C.; Bartholomew, C. H. J. Catal. 1984, 85, 63. (15) Chen, Y. G.; Tomishige, K.; Yokoyama, K.; Fujimoto, K. Appl. Catal., A 1997, 165, 335. (16) Iglesia, E.; Soled, S. L.; Fiato, R. A.; Via, G. H. J. Catal. 1993, 143, 345.
Al2O3 Addition to Catalysts for Fisher-Tropsch Synthesis
catalysts could be attributed to the increase in bridge-type adsorbed CO, which easily dissociated to carbon and oxygen, contributing to the higher reaction rate of FTS. Compared to the silica Q-10 catalyst, the alumina-promoted catalyst had higher CO conversion, higher metal dispersion, a larger metallic cobalt surface area, and the highest TOF, as listed in Tables 1 and 2. On the other hand, a stronger and broader bridged adsorbed CO peak, as shown in Figure 3, indicated that CO adsorbed on the alumina-promoted catalysts was more reactive than that on the silica Q-10 catalyst, resulting in the highest TOF in Table 2. All of these factors are considered to be due to the heteroatom structure in the alumina-promoted catalysts, determining its higher activity. It should be noted that supported cobalt on an Al2O3 support, which has a surface area and pore size similar to those of the silica Q-10 support, with a loading amount of 10 wt %, exhibited a lower activity of 40% CO conversion under the same reaction conditions. It seems that synergic effects of coexisting Al2O3 and SiO2 appeared on the Al2O3-modified Co/SiO2 FTS catalyst. A very strong interaction between cobalt and alumina severely lowered the reduction degree of the Co/Al2O3 catalyst, whereas a very light interaction between cobalt and SiO2 enlarged the size of the cobalt particle too much, decreasing the metallic cobalt surface area of the Co/SiO2 catalyst. Only in the Al2O3-modified Co/ SiO2 FTS catalyst was the interaction among cobalt, silica, and alumina well-adjusted to ensure simultaneously increased dispersion, reduction degree, and metallic surface area.
Energy & Fuels, Vol. 20, No. 2, 2006 421
Conclusions The addition of Al2O3 onto Co/SiO2 significantly improved the catalytic activity of FTS by increasing the dispersion of supported cobalt on a SiO2 support without significantly obstructing the reducibility of the supported cobalt. It is considered that the addition of Al2O3 modified the surface properties of the silica support, such as increasing the interaction between the supported cobalt and the support. This achieves the favored dispersion and keeps the reduction degree of supported cobalt high because of the synergic effects of coexisting Al2O3 and SiO2, leading to high catalytic activity in FTS. The high activity of alumina-promoted catalysts could be attributed to the increase in bridge-type adsorbed CO, which was easily dissociated to carbon and oxygen, contributing to the higher reaction rate of FTS. The impregnation method of added alumina and cobalt did not influence the reaction performance and properties of obtained catalysts. The addition of alumina to Co/SiO2 catalyst promoted the catalytic activity because of the heteroatom, synergic effects of alumina and silica. Acknowledgment. Financial support from the Royal Golden Jubilee of Thailand Research Fund is greatly appreciated by S.H. and T.V. EF050218C
