Effects of promoters on catalytic performance of Fe-Co/SiO 2 catalyst for Fischer-Tropsch synthesis

Transcription

1 Journal of Natural Gas Chemistry 18(2009) Effects of promoters on catalytic performance of Fe-Co/SiO 2 catalyst for Fischer-Tropsch synthesis Xiangdong Ma 1, Qiwen Sun 2, Weiyong Ying 3, Dingye Fang 3 1. Chemical Engineering & Pharmaceutics College, Henan University of Science and Technology, Luoyang , Henan, China; 2. Shanghai Yankuang Energy RD Co.Ltd, Shanghai , China; 3. School of Chemical Engineering, East China University of Science and Technology, Shanghai , China [ Received March 3, 2009; Revised April 28, 2009; Available online August 13, 2009 ] Abstract 2%Fe10%Co/SiO 2 catalysts with different potassium or zirconium loadings were prepared by aqueous incipient wetness impregnation and tested in Fischer-Tropsch synthesis in a flow reactor, using H 2 /CO = 1.6 (molar ratio) in the feed, under the condition of an overall pressure of 1 MPa, GHSV of 600 h 1 and temperature of 503 K. The zirconium and potassium promoters remarkably influenced hydrocarbon distribution of the products. CO conversion increased on the catalysts with the increase of zirconium loadings, which indicated that zirconium enhanced the activity of iron-cobalt catalysts. Low potassium loadings also enhanced the activity of the catalysts. However, high potassium loading made CO conversion on the catalysts decrease and weakened the secondary hydrogenations. The characterizations of catalysts were obtained by BET, XRD and TPR. The fresh catalysts revealed the presence of a Co 3 O 4 phase, and the spinel phase of Fe-Co alloy and CoO was formed on the used catalysts. Key words Fischer-Tropsch synthesis; bimetallic; iron; cobalt; promoter; potassium; zirconium 1. Introduction Fischer-Tropsch synthesis (FTS) is a heterogeneously catalyzed polymerization process which converts syngas (CO and H 2 ) into hydrocarbons. Iron and cobalt are traditional FTS catalysts, and the modification of these two catalysts has been investigated widely [1 7]. Fe-based catalysts lead to more olefinic products and lower CH 4 selectivity than Co-based catalysts, while Co-based catalysts are typically more active than Fe-based catalysts though they require lower reaction temperature [8]. When iron and cobalt are used together, reports [9 11] have indicated that they do not simply give the additive properties of the individual metals. Promoters, such as potassium, are known to enhance the production selectivity on Fischer-Tropsch catalysts because of the donation of electrons from promoter to iron. Iron is chosen as the active metal for its ability to adsorb CO faster than H 2, however iron deactivates more easily due to surface segregation of graphitic layers. It has been reported that potassium was added as a promoter to the catalyst and to suppress the formation of carbon nanowires. Feller et al. [12] studied the effect of zirconium addition to the catalyst formulation of Co/SiO 2, and found that the strong interaction between silica and cobalt was reduced and a somewhat weaker cobalt-zirconium interaction was observed with increasing zirconium content. The potassium promoter inhibits the reduction of Co/SiO 2 [13,14]. However, effects of promoters (potassium, zirconium) on catalytic performance of Fe-Co/SiO 2 catalyst for FTS have not been extensively studied. In our study, 2%Fe10%Co/SiO 2 catalysts with different potassium and zirconium loadings were prepared by the wetness impregnation. We wish to simply illustrate effects of different contents of potassium and zirconium on FTS activity and hydrocarbon distribution of 2%Fe10%Co/SiO 2. At the same time, the characterizations of catalysts were investigated by BET surface area, average pore volume, pore size distributions, X-ray diffraction (XRD) and temperature programmed reduction (TPR). Corresponding author. Tel: ; Fax: ; wying@ecust.edu.cn This work was supported by the Doctoral Foundation (NO ) and Scholastic Foundation of Henan University of Science and Technology Copyright 2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: /s (08)

2 2 Xiangdong Ma et al./ Journal of Natural Gas Chemistry Vol. 18 No Experimental 2.1. Materials The catalysts were prepared by incipient wetness impregnation. Silica gel was crushed and sieved to mesh particles as catalyst support, then calcined at 673 K in an oven for 6 h. Zirconium nitrate solution was dissolved in deionized water and the support pellets was added to the solution. After impregnation, the mixture was aged at room temperature for more than 24 h, and then was filtrated; the precipitate obtained was dried at atmospheric pressure at 353 K for 8 h, and subsequently was calcined at 673 K for 6 h. Iron (III) nitrate and cobalt (II) nitrate were dissolved in deionized water and the precipitate after calcination was added to the solution above. The mixture was aged at room temperature for more than 24 h, and then was filtrated; the precipitate obtained was dried at atmospheric pressure at 353 K for 8 h, and subsequently was calcined at 673 K for 6 h. Finally, 2%Fe10%Co0%Zr/SiO 2, 2%Fe10%Co1%Zr/SiO 2, 2%Fe10%Co5%Zr/SiO 2 and 2%Fe10%Co15%Zr/SiO 2 catalysts (by mass) were obtained. For simplification, the catalysts was defined as 2Fe10Co, 2Fe10Co1Zr, 2Fe10Co5Zr, 2Fe10Co15Zr, respectively. Potassium nitrate, Iron (III) nitrate and cobalt (II) nitrate were dissolved in deionized water and the support pellets were added to the solution. The mixture was aged at room temperature for more than 24 h, and then was filtrated; the precipitate obtained was dried at atmospheric pressure at 353 K for 8 h, and subsequently was calcined at 673 K for 6 h. Finally, 2%Fe10%Co0%K/SiO 2, 2%Fe10%Co0.05%K/SiO 2, 2%Fe10%Co0.1%K/SiO 2 and 2%Fe10%Co5%K/SiO 2 catalysts (by mass) were obtained. The catalysts refined as 2Fe10Co, 2Fe10Co0.05K, 2Fe10Co0.1K and 2Fe10Co5K Characterization BET surface area, average pore volume and pore size distributions of the samples were measured with ASAP-2010 Micromeritics instrument. XRD patterns were obtained in a Rigaku D-max 2200 diffractometer using monochromatic Cu K α radiation scintillation detector. The spectrum between 2θ = 15 o and 2θ = 90 o was recorded at 40 kv, 30 ma, using a step size of 0.02 o. TPR experiments were performed on a AutochemII 2920 Micromeritics instrument. About 0.16 g catalyst was loaded into the quartz reactor. A mixture of 10% H 2 /Ar was followed over the freshly prepared catalyst. The temperature was increased from 312 K to 1000 K at a rate of 5 K/min. H 2 O etc. formed during TPR were removed by a cold trap; the effluent gas was monitored by a TCD Catalyst test CO hydrogenation experiments were performed in a down-flow fixed bed reactor (i.d. 12 mm). The catalyst (2 ml, mesh) was loaded into the isothermal zone of the reactor. The temperature was measured and controlled by a E type thermocouple in the middle of catalyst bed. The catalyst was reduced in a flow of H 2 (90 NTP ml/min) at 0.5 MPa by raising the temperature to K at a rate of 0.5 K/min and holding at the final temperature for 6 h. After reduction procedures, the reactor was cooled down to 453 K under H 2 and pressurized to the required pressure 1.5 MPa. The synthesis gas was then adjusted feed gas flow rate. About 5% nitrogen was added to the synthesis gas, as an internal standard, to determine the CO conversion. Once the reaction temperature was reached, the reaction started, and proceeded for 12 h or so to ensure its stabilization. During the reaction, the effluent from the reactor passed through a hot trap kept at 458 K at the system pressure, and the waxy product was collected; then the stream from the hot trap passed through a cold trap kept at 273 K to condense liquid product as much as possible. H 2, CO, N 2, CO 2 and CH 4 in the tail gas from the cold trap were analyzed on line by GC900B with a carbosieve TDX-01 packed column equipped with a thermal conductivity detector. C 1 C 6 gaseous hydrocarbons were analyzed by a GC900C with 0.53 mm 35 m ATAl 2 O 3 /S capillary column, detected by an flame ionization detector. The two results were correlated by CH 4. Liquid products and wax were collected in a cold trap and a hot trap, respectively, and were separated using a 50 m OV-101 column employing a temperature program from 318 K to 603 K, then the products were analyzed using an FID. 3. Results and discussion 3.1. Characterization of the catalysts The results of textural parameters, BET surface area and pore volume of the fresh 2%Fe10%Co/SiO 2 catalysts with different zirconium loadings and potassium loadings are summarized in Table 1 and Table 2, respectively. And BJH pore size distribution of the 2%Fe10%Co/SiO 2 catalysts with different zirconium and potassium loadings is given in Figure 1 and Figure 2, respectively. It can be found that the surface area and the pore volume of the 2%Fe10%Co/SiO 2 catalysts with different Zr and K loadings decreased with the increase of the loading; and the pore size increased with the loadings (The BET area of SiO 2 support is m 2 /g, and its pore volume is 0.39 ml/g). Table 1. Textural properties of 2%Fe10%Co/SiO 2 catalysts with different zirconium loadings Catalyst S BET (m 2 /g) Pore volume (ml/g) 2Fe10Co Fe10Co1Zr Fe10Co5Zr Fe10Co15Zr In catalyst characterizations, diffraction patterns are mainly used to identify the crystallographic phase in the cata-

3 Journal of Natural Gas Chemistry Vol. 18 No lyst. The XRD patterns of the fresh 2%Fe10%Co/SiO 2 catalysts with different zirconium and potassium loadings are presented in Figure 3 and Figure 4, respectively. It is revealed that the Co 3 O 4 spinel phase has been formed on all the catalysts; and there was no Fe 2 O 3 phase. The spinel line of Fe 2 O 3 of the catalyst was probably strongly overshadowed by that of the Co 3 O 4 because of the presence of a little iron loading. Besides, no peaks attributed to ZrO 2 were observed. The reason possibly lied in the zirconium existed in the amorphous or highly dispersed form. Table 2. Textural properties of 2%Fe10%Co/SiO 2 catalysts with different potassium loadings Catalyst S BET (m 2 /g) Pore volume (ml/g) 2Fe10Co Fe10Co0.05K Fe10Co0.1K Fe10Co5K Figure 3. XRD patterns of the fresh Fe-Co-Zr/SiO 2 catalysts. (1) 2Fe10Co, Figure 4. XRD patterns of the fresh Fe-Co-K/SiO 2 catalysts. (1) 2Fe10Co, (2) 2Fe10Co0.05K, (3) 2Fe10Co0.1K, (4) 2Fe10Co5K Figure 1. BJH pore size distribution of the 2%Fe10%Co/SiO 2 catalysts with different zirconium loadings. (1) 2Fe10Co, (2) 2Fe10Co1Zr, (3) 2Fe10Co5Zr, (4) 2Fe10Co15Zr The XRD patterns of the used 2%Fe10%Co/SiO 2 catalysts with different zirconium and potassium loadings are presented in Figure 5 and Figure 6, respectively. It is revealed that the spinel phase of Fe-Co alloy and CoO was formed on all catalysts. Additionally, the crystallinity of the used catalysts became weaker compared with that of the fresh catalysts. Figure 2. BJH pore size distribution of the 2%Fe10%Co/SiO 2 catalysts with different potassium loadings. (1) 2Fe10Co, (2) 2Fe10Co0.05K, (3) 2Fe10Co0.1K Figure 5. XRD patterns of the used Fe-Co-Zr/SiO 2 catalysts. (1) 2Fe10Co,

4 4 Xiangdong Ma et al./ Journal of Natural Gas Chemistry Vol. 18 No of fresh 2Fe10Co0.05K catalyst shifted toward the lower temperature, contrary to the TPR profiles of fresh 2Fe10Co0.1K catalyst and 2Fe10Co5K catalyst. It is indicated that addition of more potassium made Fe-Co/SiO 2 catalyst harder to reduce. Figure 6. XRD patterns of the used Fe-Co-K/SiO 2 catalysts. (1) 2Fe10Co, (2) 2Fe10Co0.05K, (3) 2Fe10Co0.1K, (4)2Fe10Co5K TPR technique can trace the reduction of the oxide phase and provide information about the interaction of metal-support and metal-metal. TPR profiles of the fresh 2%Fe10%Co/SiO 2 catalysts with different zirconium and potassium loadings are shown in Figure 7 and Figure 8, respectively. The peaks were consistent with threestep consecutive reduction of metal oxide phase to metal: α-fe 2 O 3 Fe 3 O 4 FeO Fe. Similarly the reduction of cobalt oxide (Co 3 O 4 ) preceeded through two reduction steps: Co 3 O 4 CoO Co 0 [9]. The TPR profiles of Fe-Co/SiO 2 were complex. The TPR profiles of fresh 2%Fe10%Co/SiO 2 catalysts with different zirconium and potassium loadings indicated three peaks, respectively. These were consistent with Duvenhage s result [9]. It was clear that the peaks of TPR profiles of fresh 2%Fe10%Co/SiO 2 catalysts with different zirconium loadings shifted toward the lower temperature with the increase of zirconium content, so the addition of zirconium made Fe-Co/SiO 2 catalyst easier to reduce. It is indicated that the interactions between the metal and support as well as iron-cobalt on the catalysts modified by Zr became weaker than those of 2Fe10Co catalyst. The same behaviors were observed by Feller et al. [12]. The peaks of TPR profile Figure 7. TPR profiles of the fresh Fe-Co-Zr/SiO 2 catalysts. (1) 2Fe10Co, Figure 8. TPR profiles of the fresh Fe-Co-K/SiO 2 catalysts. (1) 2Fe10Co, (2) 2Fe10Co0.05K, (3) 2Fe10Co0.1K, (4) 2Fe10Co5K 3.2. Catalyst test Catalytic results of zirconium and potassium promoted 2%Fe10%Co/SiO 2 catalysts are shown in Table 3. In the case of 2%Fe10%Co/SiO 2 catalysts, CO conversion increased with the increase of zirconium loading under the same reaction conditions. It was clear that zirconium enhanced the activity of Fe-Co/SiO 2 catalysts. Increasing the loading of zirconium caused a decrease in the interaction between the metal and the support, which favored the formation of Fe-support and Co-support compounds and increased the reducibility of 2%Fe10%Co/SiO 2 in the course of reduction. It was illustrated by the TPR profiles of fresh 2%Fe10%Co/SiO 2 catalysts with different zirconium loadings that in the profiles the peaks shifted toward lower temperatures. The CO 2 selectivity hardly changed and slightly increased with the increase of CO conversion. Among the zirconium modified catalysts, the C 5+ fraction of the 2Fe10Co1Zr catalyst reached the maximum; C 1 selectivity and the total C 2 C 4 fraction decreased to the minimum on the 2Fe10Co1Zr catalysts. It is indicated that the zirconium loading of 2Fe10Co1Zr catalyst was optimum. With increasing zirconium loadings, the olefin content in the total C 2 C 4 fraction passed the minimum. This indicates that secondary hydrogenation was strongly inhibited at high zirconium loadings [12]. The CO conversion of the 2Fe10Co0.05K catalyst reached the maximum for 2%Fe10%Co/SiO 2 catalysts with different potassium loadings under the same reaction conditions. Higher potassium loadings inhibited the activity of the catalyst. Van den Berg et al. [13,14] found that potassium interacts with both cobalt phase and SiO 2, and potassium promoter inhibits the reduction of the Co/SiO 2 accord-

5 Journal of Natural Gas Chemistry Vol. 18 No ing to Co K-edge XANES spectra. The TPR profiles of the 2%Fe10%Co/SiO 2 catalysts with different potassium loadings agreed with the conclusion. CO 2 selectivity decreased with the increase of potassium loadings. O/P increased from 0.53 to 0.62 with the increase of potassium loadings. In other words, the olefin content in the total C 2 C 4 fraction increased with the increase of potassium loadings, and this indicated that the formation of secondary hydrogenations on K modified catalysts was harder than that on 2%Fe10%Co/SiO 2 catalysts modified by zirconium. Table 3. Catalytic result of the 2%Fe10%Co/SiO 2 catalysts with different zirconium or potassium loadings Catalyst CO conversion CO 2 selectivity Hydrocarbon distribution (%, by mass) (%) (mol%) C 1 C = 2 C 2 C = 3 C 3 C = 4 C 4 C 5+ Total C 2 C 4 O/P a 2Fe10Co Fe10Co1Zr Fe10Co5Zr Fe10Co15Zr Fe10Co0.05K Fe10Co0.1K Fe10Co5K trace a O/P, olefin/paraffin ratio in C 2 C 4 fraction (by mass); Reaction conditions: 503 K, 1.0 MPa, 600 h 1, H 2 /CO ratio of Conclusions 2%Fe10%Co/SiO 2 catalysts with different Zr and K loadings were prepared by impregnation method, the promoters (zirconium, potassium) remarkably influenced the hydrocarbon distribution of the products. Zirconium promoter caused weaker metal-metal and metal-support interactions, which made the catalyst easily be reduced and improved the catalytic properties. Low potassium loadings enhanced the activity of the 2%Fe10%Co/SiO 2 catalysts. However, high potassium loadings led to stronger metal-metal and metal-support interactions, which made the reduction of the catalysts harder and increased the secondary hydrogenations. Under the conditions employed during catalyst activation, surface and structural data obtained by BET, XRD and TPR. The XRD patterns of fresh 2%Fe10%Co/SiO 2 with different zirconium loadings and potassium loadings revealed that Co 3 O 4 spinel phase was formed on all catalysts. The spinel phase of Fe-Co alloy and CoO was formed on all catalysts. In the bimetallic cobalt-iron catalysts, the loadings of promoter appeared to influence the activity and hydrocarbon distribution of the catalysts. Acknowledgements We gratefully acknowledge the financial support of the Doctoral Foundation and Shanghai Yankuang Energy RD Co. Ltd. References [1] Curtis V, Nicolaides C P, Coville N J, Hildebrandt D, Glasser D. Catal Today, 1999, 49(1-3): 33 [2] Ma X D, Sun Q W, Cao F H, Ying W Y, Fang D Y. J Natur Gas Chem, 2006, 15(4): 335 [3] Sun S, Fujimoto K, Zhang Y, Tsubaki N. Catal Commun, 2003, 4(8): 361 [4] Dutta P, Elbashir N O, Manivannan A, Seehra M S, Roberts C B. Catal Lett, 2004, 98(4): 203 [5] Wu B S, Tian L, Xiang H W, Zhang Z X, Li Y W. Catal Lett, 2005, 102(3-4): 211 [6] Panpranot J, Goodwin J G, Sayari A. J Catal, 2002, 211(2): 530 [7] Xu J, Bartholomew C H, Sudweeks J, Eggett D L. Top Catal, 2003, 26(1-4): 55 [8] Li S Z, Krishnamoorthy S, Li A W, Meitzner G D, Iglesia E. J Catal, 2002, 206(2): 202 [9] Duvenhage D J, Coville N J. Appl Catal A: Gen, 1997, 153(1-2): 43 [10] Duvenhage D J, Coville N J. Appl Catal A: Gen, 2002, 233(1-2): 63 [11] Amelse J A, Schwartz L H, Butt J B. J Catal, 1981, 72(1): 95 [12] Feller A, Claeys M, van Steen E. J Catal, 1999, 185(1): 120 [13] van den Berg F R, Craje M W J, van der Kraan A M, Geus J W. Appl Catal A: Gen, 2003, 242(2): 403 [14] Huffman G P, Shan N, Zhao J M, Huggins F E, Hoost T E, Halvorsen S, Goodwin J G. J Catal, 1995, 151(1): 17