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Ind. Eng. Chem. Res. 2009, 48, 3228–3233
RESEARCH NOTES Deactivation by Filamentous Carbon Formation on Co/Aluminum Phosphate during Fischer-Tropsch Synthesis Jong Wook Bae, Seung-Moon Kim, Seon-Ju Park, P.S. Sai Prasad, Yun-Jo Lee, and Ki-Won Jun* Petroleum Displacement Technology Research Center, Korea Research Institute of Chemical Technology (KRICT), P.O. Box 107 Yuseong, Daejeon 305-600, Korea
Amorphous aluminum phosphate (AlPO4)-supported cobalt catalysts, with and without Ru promotion, were evaluated to study CO hydrogenation activity during Fischer-Tropsch synthesis (FTS). The Ru-promoted catalyst (RuCo/AlPO4) exhibited higher CO conversion and C8+ selectivity than that of the unpromoted catalyst (Co/AlPO4) and Co/Al2O3 catalyst. Interestingly, filamentous carbon was formed, even at the mild FTS reaction conditions (T ) 220-240 °C and P ) 2.0 MPa), on Co/AlPO4, and it eventually accelerated catalyst deactivation. The presence of an Ru promoter restricted the formation of filamentous carbon and enhanced catalyst stability. 1. Introduction Because of the fast depletion of crude oil reserves and the environmental impact of the petroleum-derived products, Fischer-Tropsch synthesis (FTS) is gaining more and more importance1 to produce fuels from alternate sources. Cobalt catalysts deposited on various supports such as Al2O3, TiO2, and SiO2 and promoted by different noble metals (such as ruthenium, rhenium, and platinum) have been shown to exhibit high activity and selectivity to linear paraffins, possess high resistance toward deactivation, and also exhibit low activity for the water-gas shift (WGS) reaction in the FTS reaction. The reactivity of these catalysts is mainly dependent on the dispersion and reducibility of the Co species. A moderate interaction of cobalt with the support is found to be favorable for achieving high activity and selectivity2-4 with reduced deactivation rate. Recently, amorphous aluminum phosphate (AlPO4) has been receiving considerable attention as a support for a variety of catalysts,5,6 because of its high surface area, large pore size, and specific interaction with the supported species.7,8 In addition, the phosphorus modification of hydrotreating catalysts is widely investigated by many researchers and their positive effects are related to the stabilization of active species by suppressing the metal-support interaction, because of the strong infinity between Al and P, resulting in enhancement of the reducibility of the active metal species.9 However, its application in cobaltbased Fischer-Tropsch catalysts is scarce. Therefore, we undertook a series of studies involving this AlPO4 support. Furthermore, filamentous carbons are extensively used for structural reinforcement applications, as catalyst supports and adsorbents.10 Various carbon-containing gases (CH4, C2H4, C2H2, and CO) can be used for the production of these filaments in the presence of metals of the iron group and their alloys. Normally, for the decomposition of the organic compounds into carbon filaments, temperatures of the order of 500-675 °C are required. Although several attempts have been made to prepare filamentous carbon at lower temperatures with limited success, the facile method for filamentous carbon formation under mild * To whom correspondence should be addressed. Tel.: +82-42-8607671. Fax: +82-42-860-7388. E-mail address:
[email protected].
FTS reaction conditions using syngas is another important finding, although it eventually accelerated catalyst deactivation. Literature reports suggest that there are many reasons for the catalyst deactivation during the FTS reaction, such as reoxidation by water, carbon deposition, and transformation to inactive cobalt species, etc.1 In the present investigation, we report one important reason for catalyst deactivation induced from the filamentous carbon formation on Co/AlPO4 catalyst at the mild FTS reaction conditions (T ) 220 - 240 °C and P ) 2.0 MPa) and the influence of Ru promoter in restricting filamentous carbon formation and eventually increasing the catalyst stability during FTS reaction. 2. Experimental Section Amorphous aluminum phosphate with a P/Al ratio of 0.9 was made via the coprecipitation method from Al(NO)3 · 6H2O and (NH4)2HPO4 at pH 8, followed by drying the hydrogel at 110 °C for 16 h and its calcination at 500 °C in air for 0.5 h. The 20 wt % Co catalyst (Co/AlPO4) and the 0.5 wt %-Ru-promoted Co/AlPO4 catalyst (RuCo/AlPO4) were prepared by sequential impregnation of AlPO4 with aqueous solutions of cobalt(II) nitrate and Ru(NO3)2(NO2)2, with drying and calcination being performed after each addition. For the comparison with Co/ AlPO4, a cobalt-supported γ-Al2O3 catalyst was also prepared via the same method (laboratory-made γ-Al2O3 was prepared by sol-gel method with a surface area of 350 m2/g).11 The Brunauer-Emmett-Teller (BET) surface area and the average pore size were obtained from nitrogen adsorption and desorption isotherms at -196 °C, using a constant-volume adsorption apparatus (Micromeritics, model ASAP-2400). The surface compositions of the Co catalysts were obtained using scanning electron microscopy (SEM) (JEOL, model SM6700F) and energy-dispersive spectroscopy (EDS). The catalysts used were characterized using transmission electron microscopy (TEM) (TECNAI, G2 instrument). The powder X-ray diffraction (XRD) patterns of samples were obtained with a Rigaku diffractometer, using Cu KR radiation to identify the crystalline phases of cobalt on Al2O3 and AlPO4. For the TPR experiments, the samples were first pretreated to remove adsorbed water and contaminants in a helium flow
10.1021/ie801956t CCC: $40.75 2009 American Chemical Society Published on Web 02/16/2009
Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 3229 Table 1. Physicochemical Properties and Cobalt Particle Size by BET, XRD, TPR, and H2 Chemisorption H2 Chemisorption BET
XRD
TPR
Particle Size (nm)
notation
surface area (m2/g)
pore volume (cm3/g)
average pore diameter (nm)
Co3O4 particle size (nm)
degree of reduction (%)a
dispersion
uncorrected
correctedb
Co/Al2O3 Co/AlPO4 RuCo/AlPO4
227.2 122.1 89.6
0.68 0.71 0.65
9.8 20.1 23.8
16.4 15.6 24.2
23.5 44.8 69.8
13.1 12.6 4.6
32.5 15.7 28.0
7.6 7.1 19.6
a The reduction degree is calculated from the TPR experiments by the following equation; (the amount of H2 consumption below 500 °C/total hydrogen consumption (mmol H2/g) × 100) b The particle size was corrected after considering the reduction degree.
Table 2. CO Conversion and Product Distribution during FTS Reactiona temperature (°C)
conversion of CO
turnover frequency, TOF (× 10-2)b
220 240
22.5 63.2
1.52 4.28
Product Distribution (C-mol%) C1
C2-C4
C5-C7
C8+
O/(O + P)c
14.1 17.3
15.0 13.0
58.7 51.0
34.8 14.3
14.1 16.5
14.9 17.9
54.3 45.0
40.8 29.1
10.9 15.1
12.4 14.2
61.6 53.4
20.8 13.4
Co/Al2O3 12.2 18.7 Co/AlPO4 220 240
31.4 57.4
1.03 1.87
220 240
29.3 72.3
1.71 4.22
16.7 20.6 RuCo/AlPO4 15.1 17.3
a The averaged steady-state values for CO conversion and product distribution at each temperature were obtained at 50-60 h on stream under the following reaction conditions; T ) 220-240 °C; P ) 2.0 MPa; and SV ) 2000 L/kgcat/h. b The unit of TOF (turn-over frequency) is converted CO molecules/surface Co atom/s. c The O/(O + P) value was calculated from the olefin content divided by total hydrocarbons (olefin + paraffin) in the range of C2-C4 hydrocarbons.
up to 350 °C. After cooling to 100 °C, the sample was reduced in a 5% H2/He mixture at a flow of 30 mL/min with a heating rate of 10 °C/min up to the 800 °C and kept at the same temperature for 0.5 h. The dispersion and cobalt particle size were measured by H2 chemisorption at 100 °C under static conditions (Micrometrics, model ASAP 2000). Prior to adsorption measurements, the sample (0.5 g) was reduced in situ at 400 °C for 12 h. Two successive isotherms were obtained and the difference of the two isotherms extrapolated to zero pressure was considered as the amount of chemisorbed hydrogen molecules. The metal dispersion and cobalt metal surface area were calculated with the assumption of H/Co metal’s stoichiometry of 1.0. The degree of reduction of the samples was calculated from the TPR experiments using the following equation: degree of reduction ) amount of H2 consumption below 500 °C total hydrogen consumption (mmol H2/g)
Figure 1. X-ray diffraction (XRD) patterns of calcined Fischer-Tropsch catalysts.
× 100
This equation assumes that the amount of reducible cobalt oxides at
