Fischer-Tropsch reaction over cobalt catalysts supported on zirconia-modified activated carbon

Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) 9 Elsevier B.V. All rights reserved.

349

Fischer-Tropsch reaction over cobalt catalysts supported on zirconia-modified activated carbon T. Wang, Y.-J. Ding*, J.-M. Xiong, W.-M. Chen, Z.-D. Pan, Y. Lu, L.-W. Lin Natural Gas Utilization and Applied Catalysis Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 # Zhongshan Rd., Dalian 116023, China

ABSTRACT An attractive Fischer-Tropsch catalyst was prepared using an activated carbon as carrier to support cobalt based catalysts. Zr promoted Co/AC catalysts remarkably enhanced the activity and the selectivity toward diesel distillates and lower the methane selectivity. This modification may be attributed to specific behavior of activated carbon with high surface area and the weak interaction between metallic cobalt active sites and activated carbon. It was emphasized that the pore size of activated carbon played a very important role in restricting the growth of carbon chain to wax. 1. INTRIDUCTION Supported cobalt based catalysts are proven to be excellent catalysts for Fischer-Tropsch (F-T) synthesis of long-chain paraffins [1,2]. The activity of cobalt based catalysts in F-T reaction was proposed to be proportional to the area of exposed metallic Co atoms [3]. A requirement for highly active Co based catalysts is therefore a high dispersion of the cobalt metal. Cobalt interact strongly with commonly used support materials, such as alumina and silica, yielding cobalt silicates and cobalt hydrosilicates species that are non-active species for CO hydrogenation in the case of silica [4,5]. In the present work we focus on zirconia that is reported to be a promoter for F-T reaction over Co/SiO2 [6,7], where the addition of Zr leads to improved an activity. An inert activated carbon (AC) with a high surface are employed as a carrier which has advantages of a high cobalt dispersion and non-interaction with cobalt.

* Corresponding Author: Email: [email protected] Phone: +86 411 4379143 Fax: +86 411 4379143

350

2. EXPERIMENTAL

2.1 Catalysts preparation All catalysts used in the study were prepared by aqueous incipiem wetness impregnation method. The activated carbon was sieved to 20-40 mesh and washed with distilled water for several times before being used. The activated carbon was first impregnated with a solution of Zr(NO3)4.5H20, followed by drying at room temperature and calcining at 393 K for 12 h, then was impregnated with Co(NO3) 2.6H20 solution followed by drying at 353k for 12h, and calcining at 393k for 16 h in a flow of nitrogen at 40ml/min.

2.2 CO Hydrogenation The testing apparatus consisted of a small fixed bed tubular reactor with an external heating system, which was made of stainless steel with 350 mm length, 8 mm inner diameter. The catalysts were in-situ reduced in a flow of H2 before the reaction. Then the syngas (H2/CO = 2) was fed into the catalyst bed and reacted for ca. 100 h under conditions of 523 K, 2.5 MPa and GHSV = 500h 1. The effluent passed through a high pressure gas-liquid separator and a low pressure gas-liquid separator in an ice-water bath. The liquid products and water including alcohols were collected by the separators. The aqueous solution containing oxygenates obtained was off-line analyzed by Varian CP-3800 gas chromatography with an FFAP column and FID detector, using 1-pentanol as an internal standard. C4-C26 hydrocarbons in the oil phase of F-T products were off-line analyzed on Varian CP-3800 with SE-54 capillary column and FID as a detector. The F-T wax if it was formed was off-line analyzed on Varian CP-3800 simulated-distillation with StarSD TM software. The tail gas was on-line analyzed by Varian CP-3800 GC with a Porapak QS column and TCD detector.

2.3 XRD Measurements An X-ray diffractometer (XRD D/max-ra) was used with monochoromatized Cu ka radiation and operated at 40kv and 30mA. The samples were scanned at a rate of 2.4 degree/min in the range 20=5-75 degrees.

2.4 Temperature-programmed reduction (TPR) TPR experiments were conducted to determine the reducibility of catalysts. It was performed on America Micromeritics Autochem 2910. Eighty milligrams of catalyst was placed in a quartz reactor and reduced by a 10% H2/Ar gas mixture in a flow rate of 50 ml/min. The temperature was ramped at 15 K/min and the hydrogen consumption was recorded with the thermal conductivity detector.

351 3. RESULTS AND DISCUSSION Fig. 1 gives the TPR profiles of the un-promoted and Zr-promoted Co/AC catalysts. There were three major peaks at 473K, 650K and a broad one between 723K and 978K (maximum at 810K) in the TPR profile for un-promoted catalyst. The first small shoulder peak was due to the complete decomposition of the Co(NO3)2 precursor and its intensity was greatly diminished by prolonged calcination [8]. The second peak was essentially due to Co304 which was reduced in two steps Co304 ~ Co2+-+ Co o [9], while the latter broad one was due to the methane formation at 723K-978K, which resulting from the carbon support gasification [ 10]. The TPR profiles of CoZr/AC catalysts were basically comparable to those of Co/AC catalysts, however, some small changes were observed in the case of CoZr/AC catalysts, the H2 consumption peak attributed to cobalt oxide reduction was shifted to lower temperature. This might be explained due to the weak interaction between high dispersion ZrO2 and cobalt oxides on the surface of activate carbon. It was found that almost no change appeared when Zr loadings increased, indicating that high Zr loading seems to have no significant effect on Co/AC catalysts. Fig. 2 shows XRD patterns of Co/AC, CoZr/AC catalysts before and after F-T reaction. It could be seen that almost no diffraction peak was observed after the Co/AC and CoZr/AC samples were reduced at 673K for 4h, probably due to the formation of highly dispersed metallic cobalt or the formation of an amorphous cobalt phase (Fig. 2a and Fig.2c). While the XRD pattern of used Co/AC showed some apparent diffraction peaks, and the major peak of the cobalt metal phase was identified as the fcc form Co, with some hcp form Co. It was clear that tb ~ ( iffraction peaks became weaker with the addition of Zr into

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Co/AC catalyst, indicating the well-dispersed metallic cobalt particles (Fig. 2d). It was worth note that the existence of the Zr promoter on the surface of activated carbon hindered the aggregation of cobalt species even in the presence of water formed during Fischer-Tropsch reaction. Activity data obtained at T=523K, P=2.5MPa and H2/CO=2 are presented in Fig. 3, where the catalyst activity is depicted as a function of the cobalt loading for three different zirconia loadings and compared to un-promoted catalysts. For the un-promoted samples the activity increased sharply with cobalt loading, while the promotion effect was not so distinctively evident for the promoted cobalt catalysts. It was found that zirconia exhibited a distinguished promotion effect on the CO conversion for low cobalt loadings. The promotion with zirconia also influenced the product selectivity. The yield of the C5+ fraction of the hydrocarbon product increased for all of the zirconia promoted samples compared to the un-promoted catalysts (Fig. 4). It was interesting to note that the activated carbon supported cobalt catalysts displayed a novel property: very little fraction of F-T products was heavy oil parts (C25+), more than 60 wt. % in the liquid oil phase was diesel fuel (see Fig. 8),

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and the methane selectivity went down with time on stream in the first hundred hours. Duration test showed little change in the activity and Hydro-carbon distribution with time on stream after the first hundred hours. It is apparent that CO conversion increased with the reaction temperature and pressure over 15Co2Zr/AC (Fig. 5, and Fig. 6), and the optimized temperature and pressure were 503-513 K and 2.5-3.0 MPa. However, the CO conversion and C5+ selectivity decreased with GHSV (Fig. 7). Fig. 8 shows the Anderson-Schulz-Flory distribution over Co/AC with different Co loadings. It was interesting to find that the selectivity towards C10-C20 paraffins was gradually decreased with the increase of the cobalt loading. The selectivity towards C~0-C20 hydrocarbons in liquid products decreased from 62% over 7 wt. % Co/AC to 38% over 20 wt. % Co/AC catalysts, while the selectivity of C5-C9 hydrocarbons increased from 36% to 53% when the Co loading increased from 7 wt. % to 20 wt. %. As we know, the ratio of the cobalt active sites that located in the pores of the carrier and the others that located on the outer surface of the carrier for low cobalt loading catalyst was higher than that for high cobalt loading catalyst. These results implied that the pore structure, especially the pore size, could play a very important role in restricting growth of carbon chain during F-T reaction. More evidences from meso-pore material with different pore size from 4.0 to 8.0 nm, such as SBA-15 supported cobalt catalysts with or without promoters, confirmed powerfully the roles of the pore structure of the carrier in restricting the growth of carbon chain during F-T reaction, which will be published elsewhere. Therefore, the distribution of paraffin products of high cobalt loading was similar to that of conventional cobalt catalyst with c~ value of ca. 0.78, while there apparently were two c~ values in the curve of log(Wn/n) versus the carbon number n in the case of low cobalt loading catalyst. Duration test (more than 1000 hours) results confirmed

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