Ind. Eng. Chem. Res. 2004, 43, 2391-2398
2391
Fischer-Tropsch Synthesis over Activated-Carbon-Supported Cobalt Catalysts: Effect of Co Loading and Promoters on Catalyst Performance Wen-Ping Ma,* Yun-Jie Ding, and Li-Wu Lin State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China
The effects of Co loading and K, Ce, and Zr promoters on initial activities and selectivities of activated-carbon-supported cobalt (Co/AC) Fischer-Tropsch synthesis (FTS) catalysts, were studied under conditions of 240 °C, 2.4 MPa, 650 h-1, and H2/CO ) 2:1 in a fixed-bed reactor. The reduction and metal dispersion properties of the catalysts were investigated through temperature-programmed reduction (TPR) and CO chemisorption. Detectable hydrocarbons up to C20 were formed on the Co/AC catalysts. Co loading affected initial syngas conversion and CH4 selectivity in the following order: 20%Co > 15%Co > 10%Co > 7%Co, whereas it had an opposite effect on content of C12-C20 in the liquid organic product. K, Ce, and Zr promoters remarkably changed the initial catalytic performance of the Co/AC catalysts. Addition of K to the Co/AC catalyst significantly decreased the FTS activity and CH4 selectivity, but increased the water gas shift (WGS) activity. Addition of Ce improved the Co/AC catalyst activity, accompanied by high CH4 selectivity. Neither CO2 nor CH4 selectivity changed greatly on the Zr-promoted Co/AC catalyst even though the Zr promoter increased the FTS activity. The TPR and CO adsorption studies showed that K, Ce, and Zr promoters improved Co dispersion and interaction between Co oxide and the AC surface, which might be the reasons for the subsequent catalyst performance, especially for the Ce- and Zr-promoted catalysts. The TPR study also showed that the reduction steps of the reductively decomposed Co/AC catalysts by pure H2 are Co3O4 f CoO f Co. 1. Introduction The Fischer-Tropsch synthesis (FTS), which is regarded as a clean route for the conversion of coal/natural gas to motor fuels, has received considerable attention since it was discovered in 1923. Over the past decades, extensive efforts have been made to develop FTS catalysts with high activity and high C5+ selectivity.1-8 Two kinds of “high alpha” FTS catalysts, i.e., iron- and cobalt-based catalysts, have been successfully developed and employed in FTS plants in Sasol and Malaysia, respectively.2,4,5 High-quality diesel fuels are produced after further treatments (hydro-cracking or isomerization) of high-molecular FTS products (i.e., wax).1-5 Catalytic synthesis of light hydrocarbons (C1-C6) from syngas also has been studied by many investigators.9-19 Molecular sieve, silicalite, and activated-carbon (AC)supported iron and cobalt catalysts, and muticomponent catalysts were reported. Production of hydrocarbons with a moderate carbon number range, e.g. C1-C20, via the FTS reaction is of great significance in the C1 chemistry field. Researchers have employed both novel FTS technologies and new porosity FTS catalysts to limit hydrocarbon chain growth to C20. Syntroleum used a periodic pulsing of hydrogen technique to cut hydrocarbon chain growth to C20; as a result, C10-C20 yield was obtained maximally on Co-ZrO2/SiO2 catalysts.20 Recently, van Steen et al.
published their finding of synthesis of C1-C15 hydrocarbons on carbon-nanotube-supported iron catalysts.21 But activity of the catalyst was quite low and declined quickly (from 45% to 15%) within 50 h of testing. In the last twenty years, AC has been widely used as a heterogeneous catalyst support, but it was not commonly applied in the FTS field.22-26 Vannice et al. were pioneers to use activated carbon to make supported FTS catalysts.16-18,27 They found that only CH4 and C2-C4 olefins were produced selectively from syngas on activated-carbon-supported iron-manganese catalysts.16,17 Ma et al. have studied the FTS with activated-carbonsupported iron catalysts in a slurry-phase reactor under conditions of 300 °C, 3.0 MPa, 1000 h-1, and H2/CO ) 2.28,29 It was found that catalyst activity was increased with increase in reduction temperatures between 350 and 420 °C, and gas and liquid hydrocarbons were formed on the catalysts with a C5+ weight percentage of about 35%. In this paper, we report a study of AC-supported cobalt catalysts for the FTS reaction in a fixed-bed reactor. A series of Co/AC catalysts were prepared for an attempt to direct synthesis of C1-C20 hydrocarbons from syngas through the FTS route. The effects of Co loading and promoters of K, Ce, and Zr on the catalyst behaviors were studied by the FTS reaction in a fixedbed reactor, TPR, and CO chemisorption. 2. Experimental Section
* To whom correspondence should be addressed. Present address: Department of Chemical Engineering, West Virginia University, Morgantown, WV 26506-6102. E-mail:
[email protected].
2.1. Catalyst Preparation. The Co/AC catalysts were prepared by an incipient wetness coimpregnation (CI) technique. Almond AC (20-40 mesh) purchased
10.1021/ie034116q CCC: $27.50 © 2004 American Chemical Society Published on Web 04/15/2004
2392 Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 Table 1. CO Uptakes and Turnover Frequencies on Co/AC Catalysts
no.
catalyst
I II III IV V VI VII
7:93 Co/Ac 10:90 Co/AC 15:85 Co/AC 20:80 Co/AC 15:2:83 Co/K/AC 15:2:83 Co/Ce/AC 15:2:83 Co/Zr/AC
metal prep. CO uptake, dispersion TOFc, a µmol/gcat (D)b, % h-1 meth. CI CI CI CI CI CI CI
75.2 83.9 76.6
6.3 4.9 3.0
172.1 164.5 148.0
6.8 6.5 5.8
5.17 5.15 5.14 4.62
a CI, co-impregnation. b Calculation based on CO/Co(s) ) 1:1. TOF, turnover frequency ) rates of CO and H2 converted (mol/ h)/moles of active Co in the catalysts.
c
QS column (3 m × 2 mm); the liquid aqueous product was analyzed by another Varian 3800 GC equipped with a PEG-20M capillary (30 m × 0.25 mm) column. The Co/AC catalysts were pretreated with H2 at 400 °C, 0.5 MPa, and 1000 h-1 for 16 h. After reduction, the flow was switched to Helium and the reactor temperature was cooled to 200 °C, and the reactor system was pressurized to 2.4 MPa. Helium was then cut off and syngas (H2/CO ) 2.0) was introduced at a gas space velocity of 650 h-1. Finally, reaction temperature was adjusted gradually to 240 °C. The screening tests discussed in this paper lasted 21-46 h. In this paper, syngas conversion and selectivity (CH4, C2-C4, and CO2) were defined as follows
Syngas conversion (%) ) 100 × (mass of syngas consumed/total mass of inlet syngas) CH4 selectivity (mol %) ) 100 × (mole number of CH4 produced/ (mole number of inlet CO mole number of outlet CO mole number of CO2 produced))
Figure 1. Schematic diagram of a fixed-bed reactor system.
from Beijin Guanghua timber factory was used as catalyst support. The received almond AC was calcined at 300 °C in stationary air for 5 h to remove water and impurities. The BET surface area of the calcined AC is 934.9 m2/g and total pore volume is 0.36 cm3/g.29 The metal (Co, Zr, K, and Ce) nitrate solutions with the desired metal concentrations were impregnated on a certain amount of AC support under vacuum condition. Water on the wet Co/AC catalyst was then evaporated at room temperature for a week. Finally, the Co/AC catalyst was dried in stationary air at 110-120 °C for 8 h. Table 1 lists all the catalysts used in this study. The number before each element represents weight percentage of the element in the catalysts. 2.2. Experimental Setup and Operation Procedures. The catalytic properties of the Co/AC catalysts were screened in a fixed-bed stainless steel reactor of 14-mm i.d. The Co/AC catalyst (2-3 g) was diluted by a volume ratio of 1:4 with the same-sized porcelain beans before they were charged into the reactor. The schematic of the reactor setup is illustrated in Figure 1. The flow rate of feed gas was adjusted with a mass flow meter. The feed gas (H2 or H2/CO) passed through four purification traps to remove trace oxygen, sulfur, moisture, and iron carbonyl. The measured feed gas was preheated in a heating trap and then was introduced into the fixed-bed reactor. After leaving the reactor, exit gases successively passed through a high-pressure trap (120 °C), a backpressure controller, and a low-pressure trap. Liquid product was collected in the high-pressure and low-pressure ice traps. Noncondensable gaseous (tail-gas) entered into a waste trap or a mass balance trap. Finally the tail-gas passed through a wet test meter or went to a GC for on-line analysis. The FTS liquid organic product and tail-gas (CO, H2, CO2, C1C8) were analyzed using a Varian 3800 GC equipped with a SE-54 capillary (30 m × 0.25 mm) and a Porapak
C2-C4 selectivity (mol %) ) 100 × (mole number of C2-C4 produced/ (mole number of inlet CO mole number of outlet CO mole number of CO2 produced)) CO2 selectivity (%) ) 100 × (mole number of CO2 produced/ (mole number of inlet CO mole number of outlet CO) 2.3. Catalyst Characterization. The TPR experiments were performed using a Micromeritics Autochem 2910. A 10% H2/Ar gas mixture at a total flow rate of 50 mL/min was used during TPR runs. The sample (20 mg) was linearly heated from room temperature to 900 °C at 10 °C/min. A TCD device was used to monitor H2 consumption. CO chemisorption on Co/AC samples was also measured using the Autochem 2910. The sample (20 mg) was pretreated at 500 °C for 2 h with a H2 flow rate of 20 mL/min. After that, the sample surface was further swept by flowing He at 20 mL/min for 1 h to remove trace species. Finally, the sample in situ adsorbed CO in a diluted CO (5%CO/He) gas mixture at room temperature and atmospheric pressure. The CO concentration in efflux was measured by the TCD. 3. Results and Discussions 3.1. Effect of Co Loading. The changes of syngas conversion with time-on-stream (TOS) in four tests with different Co-loaded (7-20%) Co/AC catalysts are shown in Figure 2. The reaction conditions for all tests were 240 °C, 2.4 MPa, 650 h-1, and H2/CO ) 2.0. Syngas conversions were increased with TOS amounting to 31.8% at 13 h, 41.5% at 14 h, 58.4% at 7 h, and 70.2% at 16 h on the 7:93, 10:90, 15:85, and 20:80 Co/AC catalysts, respectively. After 7-16 h, syngas conversion either remained stable (10-20% Co/AC catalysts) or decreased slightly with TOS (7% Co/AC). Hence, the
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2393
Figure 2. Changes of syngas conversion with time on-stream on the unpromoted Co/AC catalysts (240 °C, 2.4 MPa, 650 h-1, and H2/CO ) 2.0).
catalyst activity was increased with cobalt loading between 7% and 20%. To illustrate the activity trend with cobalt loading, turnover frequency (TOF, h-1), which is defined as rate of converted CO and H2 (mol/h) per mole of active cobalt assuming that the cobalt-reducing degree of the each catalyst was the same (e.g., 100%), of each unpromoted catalyst was calculated and is listed in Table 1. It is noted that the cobalt in each unpromoted catalyst should not be completely reduced during pretreatments; however, the assumption of the same reduction degree (e.g., 100%) used for calculation of TOF values is acceptable because the measured Co dispersions of the Co/AC catalysts were very low (9.8%) and C2-C4 (>11.0%) in 21-46 h of testing. The effect trend of cobalt loading on CH4 selectivity (Figure 3a) was similar to that on syngas conversion, namely, the higher the cobalt loading, the higher the CH4 selectivity (18.5-23.8%, 15.1-18.4%, 11.1-15.9%, and 9.8-11.7% on the 20%, 15%, 10%, and 7%Co/AC catalysts, respectively). This might be a result of different cobalt particle sizes and/or different pore structures of the catalysts. CH4 selectivity of 22.1% at 8 h on the 20%Co/AC catalyst was higher than that between 16 and 30 h. This was either an artificial data or was caused by hydrogenation of additional active carbon species, which may be generated by oxygen-containing groups on the AC surface.31 The effect sequence of cobalt loading on C2-C4 hydrocarbon selectivity was different from that on the conversion and CH4 selectivity. The
Figure 3. Changes of CH4 selectivity (a) and C2-C4 selectivity (b) with time on-stream on the unpromoted Co/AC catalysts (240 °C, 2.4 MPa, 650 h-1, and H2/CO ) 2.0).
highest (17.4-19.6%) and lowest C2-C4 selectivities (10.8-13.8%) were found on the 7:93 and 15:85 Co/AC catalysts (Figure 3b). The effect of Co loading on composition of liquid organic products formed on the Co/AC catalysts is illustrated in Figure 4. Clearly, four GC traces show the detectable C4-C20 hydrocarbons in the liquid phase. N-paraffins and 1-olefins were the primary components in the liquid organic product. It is interesting that hydrocarbon distribution in the liquid organic phase was dependent upon cobalt loading. Liquid organic products formed on the 7:93 and 10:90 Co/AC catalysts contained more C12-C20 hydrocarbons (55.0 wt % in Figure 4a and 38.3 wt % in Figure 4b), whereas lower percentages of C12-C20 (26.8 wt % in Figure 4c and 28.6 wt % in Figure 4d) were observed in the liquid organic products formed on the 15:85 and 20:80 Co/AC catalysts. Therefore, more large molecular hydrocarbons (e.g., >C12) prefer to be formed on lower Co loaded catalysts (
