4140
Ind. Eng. Chem. Res. 2010, 49, 4140–4148
Cobalt Fischer-Tropsch Catalysts Using Nickel Promoter as a Rhenium Substitute to Suppress Deactivation Erling Rytter,*,† Torild H. Skagseth,† Sigrid Eri,† and Anja O. Sjåstad‡,§ Statoil Research Center Trondheim, N-7005 Trondheim, Norway, and SINTEF Materials and Chemistry, N-0314 Oslo, Norway
Catalysts with cobalt as the Fischer-Tropsch (FT) metal and nickel and/or rhenium as promoters on alumina of different pore sizes, a stabilized alumina, silica, or titania as supports were investigated for activity, selectivity, and deactivation up to 800 h of operation in a fixed-bed reactor. From the observed selectivities, there is no indication that nickel as a promoter with a loading up to 5 wt % influences the selectivities to higher hydrocarbons for low-temperature ( {Ni(drying) + Co/Re} > {Co/Re} > {Co/Re + Ni} > {Co} Here a + sign signifies two impregnation steps. Except for one case as specified, impregnation is followed by drying and calcination. Some catalysts were scaled-up to kilogram amounts using small pilot-scale equipment comprising a Z-blade mixer and controlled fluid-bed calcination. The relative activity of a 12Co/ 2Ni/0.5Re wt% catalyst on AluminateLPV was found to be 0.84 in the fixed-bed compared to 0.65 for the laboratory method as reported in Table 3. Similarly, the activity of a 20Co/5Ni/0.5Re wt% catalyst on AluminaMPV increased from 1.77 to 1.87 (1.07 g hydrocarbons/g catalyst*h). In both cases the selectivity was unaltered. Catalyst Stability. As discussed above, when nickel substitutes cobalt in the catalyst formulation, and rhenium is present as a promoter, the activity drops. From Figure 1a, it can be deduced that this statement depends on the TOS as the nickel containing catalyst is remarkably stable. Actually there is a moderate increase in activity during this limited test and one can imagine the activity curves crossing at some point. A similar
4146
Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010
Figure 8. Deactivation profiles for selected catalysts on the AluminaMPV support showing the effect of promoters.
Figure 9. Extended deactivation profiles for catalysts with 20 wt % Co on AluminaMPV support. Green: 5 wt % Ni as promoter. Red: 0.5 wt % Re as promoter.
stability is found when the catalyst does not contain rhenium, but in this case the pure cobalt catalyst has a low activity already from the start. Further comparisons when nickel and/or rhenium are added as promoters are given in Figure 8. Adding just rhenium improves the activity level, but the deactivation rate is maintained. The catalyst with nickel as promoter is remarkably stable and possessing an activity on par with the rhenium promoted catalyst after ca. 100 h TOS. Adding both promoters enhances the activity further. Now we see that the typical activation period caused by nickel is present, but more importantly, the catalyst is unusually stable although some deactivation can be detected. Similar stabilizing effect of nickel as a promoter is found for all the other supports as well and we may therefore regard this as a universal modification of cobalt FT-catalysis. Unfortunately, the deactivation profiles cannot be extended for prolonged times as one often might assume by adopting an exponential profile. Some catalysts with and without nickel promoter were investigated for a period of up to 800 h TOS. These catalysts were also tested in separate runs with added steam to the feed in order to accelerate deactivation by simulating an elevated conversion of 75-80%. The most distinct feature of the results shown in Figure 9 is evidently the effect of water added after 100 h that accelerates the deactivation significantly, consistent with previous reports.3 At a first glance there is little difference between the rhenium and the nickel promoted catalysts, at least after a few hundred hours TOS. Looking at the runs under dry conditions, i.e., 45-50% conversion, the previously noted stability of the nickel containing catalyst disappears after ca. 150 h TOS and a normal deactivation is apparent. With addition of water there is a sudden drop in activity for the catalyst with nickel. Sometimes such a drop is reported for rhenium promoted catalysts on alumina as well, but this is not seen in the current experiment. Note that for extended TOS the curves cross and the nickel containing catalyst exhibits a suppressed deactivation rate.
Figure 10. Relationship between methane and selectivity to higher hydrocarbons.
The origin of the effect of nickel on deactivation will at this point be somewhat speculative. Three main deactivation mechanisms have been put forward; sintering, reoxidation, and coking. Sintering is found to take place only during the initial hours of reaction and is therefore disregarded.1-4 We have seen that nickel facilitates reduction and, therefore, may suppress reoxidation by hydrogen spillover. Spillover may also reduce coking or formation of carbon rich hydrocarbon deposits. Any nickel decoration on the cobalt surface may also hinder graphitic carbon to form. Thus, there appear to be valid arguments for the observed nickel stabilization during the first 100-200 h TOS, but we cannot in the present investigation per se distinguish between the mechanisms. However, there is solid evidence from literature that coking plays a major role during deactivation of cobalt FT-catalysts when operated under industrial conditions.4 The catalyst used in this Sasol based work, platinum promoted cobalt on alumina, also has similarities with the present ones. We therefore favor coking as the main deactivation mechanism and have used coking to rationalize the nickel behavior under Conclusions. In later phases of operation the proximity of the curves is somewhat fortuitous as the activity level depends on the actual promoter loading. It is noteworthy, however, that after loosing its initial stabilization effect nickel still lifts the activity well above an unpromoted catalyst system. Mechanistic Implications. It is not the intention of the present work to discuss details of the FT-reaction mechanism. However, some implications for further studies can be found. The observed increase in selectivity with rhenium promotion means that the light gas make is reduced. It is often assumed that methane is formed on sites separate from chain growth, but examining Figure 10 there is a reasonable relationship between the two selectivity parameters when incorporating all data in the present work (R2 ) 0.88). It is significant that no type of catalyst stands out in the diagram, e.g., a type of support or in particular the inclusion of nickel. The relationship between methane and C5+ seems to be a general property for most cobalt based Fischer-Tropsch catalysts at given process conditions, e.g. using the data of different cobalt particle sizes.6 The implication is that rhenium has a direct influence on the chain growth probability, the R-value. A likely explanation therefore is that rhenium reduces the chain termination rate for a given cobalt surface area. Presently no established termination mechanism exists for cobalt Fischer-Tropsch polymerization, but most primary products clearly are R-olefins presumably formed by β-hydrogen elimination.28 According to such a model, rhenium hinders abstraction of this β-hydrogen atom. One possibility is that termination takes place on dedicated sites that are influenced by the Re promoter. Such termination sites clearly also depend on the type of support (Figure 2). If the termination sites are located at or close to the periphery of the cobalt clusters,
Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010
this would rationalize these effects on the selectivity. At this point, however, we do not exclude other electronic or structural effects. Alternatively, the chain propagation rate increases, but at this point we find such an explanation less likely. The rate limiting step is normally assumed to be in the formation of the monomer for chain propagation, and with a constant TOF, the propagation rate is given. The olefin to paraffin ratio, o/p, particularly for the light olefins C3-C6, is found to be a sensitive parameter for selectivity and diffusivity variations in low-temperature FT-synthesis.6,28,32 However, plotting the C3 o/p ratio for all catalysts in Tables 1-3 as a function of C5+ reveals no trend. The main variation between 1.9 and 2.6 is as expected. As no trend can be seen for variations in nickel content, this is further evidence that nickel does not impact the hydrogenation properties of the working catalyst. There are two outliers, silica with nickel showing a very low o/p of 1.5 and titania showing exceptionally high values of 2.9 and 3.0. The latter is presumably a consequence of an open support structure with large pores minimizing secondary hydrogenation of olefins. On silica the implication is that nickel promotes secondary hydrogenation, but this is a single observation that calls for confirmation. Conclusions From the observed selectivities there is no indication that nickel as a promoter with a loading up to 5 wt % influences the selectivities to higher hydrocarbons for low-temperature cobalt FT-synthesis. No hydrogenation activity of nickel influencing the reaction can be seen, neither from methane make, C5+ selectivity nor from the olefin to paraffin ratio of light products. This conclusion is valid for a range of catalyst carrier materials, prep aration methods, use of rhenium as additional promoter and for standard as well as simulated high conversion conditions. In other words, it appears that nickel as a promoter does not affect the propagation or termination mechanisms or rates. Rhenium, on the other hand, promotes the formation of higher hydrocarbons for all conditions and systems investigated. Nickel as a promoter up to 5 wt % has, however, a profound impact on the catalytic activity. Both the start of run activity, steady-state level and deactivation rate are influenced. The freshly reduced catalyst is nearly inactive, but the activity increases rapidly to a steady state level significantly higher than for (nickel) unpromoted catalysts. However, adding nickel to the catalyst in a final impregnation step has a retarding effect. It appears that nickel can substitute rhenium as a reduction and activity promoter. The steady-state activity is maintained constant for a prolonged time, but deactivation starts after ca. 100 h TOS. We propose the following model for the activity behavior. After coimpregnation, calcination and reduction a nickelcobalt alloy is formed that is nearly inactive due to too small cobalt domains, following the known structure sensitivity of these catalysts. Exposure to synthesis gas causes partly segregation of nickel and cobalt on the metal surface, increasing the TOF to the expected level, but simultaneously suppressing deactivation by suppressing the formation of coke precursors (and/or hindering reoxidation of cobalt). After a given TOS nickel has segregated nearly completely from cobalt and normal deactivation starts to occur. Note that nickel cannot substitute cobalt as a low-temperature FT-catalysts for wax production as the activity is inferior and the selectivity is reduced slightly.
4147
Acknowledgment Thanks are due to Asbjørn Lindvåg and Rune Myrstad of SINTEF, Trondheim, for running the fixed-bed test rig. The authors appreciate co-operation through the InGAP project (Innovative Natural Gas Processes and Products) on deactivation of FT-catalysts and valuable discussions with Øyvind Borg, Statoil. GTL.F1 AG is acknowledged for releasing the material for publication. Literature Cited (1) Bartholomew, C. H. Mechanisms of Catalyst Deactivation. Appl. Catal., A 2001, 212, 17. (2) Jacobs, G.; Patterson, P. M.; Zhang, Y.; Das, T. K.; Li, J.; Davis, B. H. Fischer-Tropsch Synthesis: Deactivation of Noble Metal-Promoted Co/Al2O3. Catalyst Appl. Catal. A 2002, 233, 215. (3) Blekkan, E. A.; Borg, Ø.; Frøseth, V.; Holmen, A. Fischer-Tropsch synthesis on cobalt catalysts: the effect of water. Catalysis; The Royal Society: Cambridge, U.K., 2007, 13. (4) Moodley, D. J.; van de Loosdrecht, J.; Saib, A. M.; Overett, M. J.; Datye, A. K.; Niemantsverdriet, J. W. Carbon Deposition as a Deactivation Mechanism of Cobalt-based Fischer-Tropsch Synthesis Catalysts under Realistic Conditions. Appl. Catal., A 2009, 354, 102. (5) Bezemer, G. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Oosterbeek, H.; Holewijn, J. E.; Xu, X.; Kapteijn, F.; van Dillen, A. J.; de Jong, K. P. Cobalt Particle Size Effects in the Fischer-Tropsch Reaction Studied with Carbon Nanofiber Supported Catalysts. J. Am. Chem. Soc. 2006, 128, 3956. (6) Borg, Ø.; Dietzel, P. D. C.; Spjelkavik, A. I.; Tveten, E. Z.; Walmsley, J. C.; Eri, S.; Holmen, A.; Rytter, E. Fischer-Tropsch Synthesis: Cobalt Particle Size and Support Effects on Intrinsic Activity and Product Distribution. J. Catal. 2008, 259, 161. (7) Borg, Ø.; Walmsley, J. C.; Dehghan, R.; Tanem, B. S.; Blekkan, E. A.; Eri, S.; Rytter, E.; Holmen, H. Electron Microscopy Study of γ-Al2O3 Supported Cobalt Fischer-Tropsch Catalysts. Catal. Lett. 2008, 126, 224. (8) Arsland, I.; Walmsley, J.; Rytter, E.; Bergene, E.; Midgley, P. Toward Three-Dimensional Nano-Engineering of Heterogeneous Catalysts. J. Am. Chem. Soc. 2008, 130, 5716. (9) Jacobs, G.; Das, T. K.; Patterson, P. M.; Li, J.; Sanches, L.; Davis, B. H. Fischer-Tropsch Synthesis: XAFS Studies of the Effect of Water on a Pt-Promoted Co/Al2O3 Catalyst. Appl. Catal., A 2003, 247, 335. (10) Schanke, D.; Hilmen, A. M.; Bergene, E.; Kinnari, K.; Rytter, E.; Ådnanes, E.; Holmen, A. Study of the Deactivation Mechanism of Al2O3Supported Cobalt Fischer-Tropsch Catalyst. Catal. Lett. 1995, 34, 269. (11) Lee, W. H.; Bartholomew, C. H. Multiple Reaction States in CO Hydrogenation on Alumina-Supported Cobalt Catalysts. J. Catal. 1989, 120, 256. (12) Rytter, E.; Eri, S.; Schanke, D. Fischer-Tropsch Catalysts. Patent Application: International Publication Number WO04/043596 A2, 2004. (13) Eri, S.; Rytter, E. Cobalt and Rhenium Containing Fischer-Tropsch Catalysts. Patent Application: International Publication Number WO06/ 010936 A1, 2006. (14) Borg, Ø.; Eri, S.; Storsæter, S.; Blekkan, E. A.; Wigum, H.; Rytter, E.; Holmen, A. Fischer-Tropsch Synthesis over γ-Alumina-Supported Cobalt Catalysts: Effect of Support Variables. J. Catal. 2007, 248, 89. (15) Saib, A. M.; Claeys, M.; van Steen, E. Silica Supported Cobalt Fischer-Tropsch Catalysts: Effect of Pore Diameter of Support. Catal. Today 2002, 71, 395. (16) Eri, S.; Kinnari, K. J.; Schanke, D.; Hilmen, A.-M. Fischer-Tropsch Catalyst with Low Surface Area Alumina, It’s Preparation and Use Thereof. Patent application: International Puiblication Number WO 02/47816 A1, 2002. (17) Schanke, D.; Eri, S.; Rytter, E.; Aaserud, C.; Hilmen, A.-M.; Lindvåg, O. A.; Bergene, E.; Holmen, A. Fischer-Tropsch Synthesis on Cobalt Catalysts Supported on Different Aluminas. Stud. Surf. Sci. Catal. 2004, 147, 301. (18) Rytter, E.; Skagseth, T. H.; Wigum H.; Sincadu, N. Enhanced strength of alumina based Co Fischer-Tropsch catalyst. Patent application: International Publication Number WO 05/072861 A1, 2005. (19) Mauldin, C. H. Cobalt Catalysts for the Conversion of Methanol to Hydrocarbons and for Fischer-Tropsch Synthesis. Patent: United States Patent 4,568,663, 1986. (20) Eri, S.; Goodwin, J.; Marcelin, G.; Riis, T. Catalyst for production of hydrocarbons. United States Patent 4.801.573, 1987. (21) Mauldin, C. H.; Varnado, D. E. Rhenium as a Promoter of TitaniaSupported Cobalt Fischer-Tropsch Catalysts. Stud. Surf. Sci. Catal. 2001, 136, 417.
4148
Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010
(22) Hilmen, A.-M.; Schanke, D.; Holmen, A. TPR Study of the Mechanism of Rhenium Promotion of Alumina-Supported Cobalt FischerTropsch Catalysts. Catal. Lett. 1996, 38, 143. (23) Fan, L.; Yoshii, K.; Yan, S.; Zhou, J.; Fujimoto, K. SupercriticalPhase Process for Selective Synthesis of Wax from Syngas: Catalyst and Process Development. Catal. Today 1997, 36, 295. (24) Ishihara, T.; Horiouchi, N.; Inoue, T.; Eguchi, K.; Takiyta, Y.; Arai, H. Effect of Alloying on CO Hydrogenation Activity over SiO2-Supported Co-Ni Alloy Catalysts. J. Catal. 1992, 136, 232. (25) Rytter, E. Promoted Cobalt catalyst. Patent applications: International Publication No. WO 06/032907, 2002; US 20080255256-A1. (26) Rotan, M.; Tolchard, J.; Rytter, E.; Einarsrud, M.-A.; Grande, T. On the solid solution of the spinel phase in the system NiO-Al2O3. J. Sol. State Chem. 2009. in print, published on-line. (27) Eri, S.; Rytter, E.; Borg, Ø.; Antonini, A.; McCullock, C. Preparation of FT Catalysts. Patent application: WO 08/093073 A2, 2007. (28) Rytter, E.; Eri, S.; Skagseth, T. H.; Schanke, D.; Bergene, E.; Myrstad, R.; Lindvåg, A. Catalyst Particle Size of Cobalt/Rhenium on Porous Alumina and the Effect on Fischer-Tropsch Catalytic Performance. Ind. Eng. Chem. Res. 2007, 46, 9032. (29) Borg, Ø.; Hammer, N.; Eri, S.; Lindvåg, O. A.; Myrstad, R.; Blekkan, E. A.; Rønning, M.; Rytter, E.; Holmen, A. Fischer-Tropsch Synthesis over Un-Promoted and Re-Promoted γ-Al2O3 Supported Cobalt Catalysts with Different Pore Sizes. Catal. Today 2009, 142, 70. (30) Schanke, D.; Eri, S.; Rytter, E.; Aaserud, C.; Hilmen, A.-M.; Lindvåg, O. A.; Bergene, E.; Holmen, A. Fischer-Tropsch Synthesis on
Cobalt Catalysts Supported on Different Aluminas. Stud. Surf. Sci. Catal. 2004, 147, 301. (31) Bakken, V.; Swang, O.; Bergene, E.; Rytter, E. Bimetallic Cobalt/ Rhenium Systems: Preferred Position of Rhenium through an Interdisciplinary Approach. Catal. Lett. 2009. in print. (32) Shi, B.; Davis, B. H. Fischer-Tropsch Synthesis: The Paraffin to Olefin Ratio as a Function of Carbon Number. Catal. Today 2005, 106, 129. (33) Castner, D. G.; Watson, P. R.; Chan, I. Y. X-Ray Absorption Spectroscopy, X-Ray Photoelectron Spectroscopy, and Analytical Electron Microscopy Studies of Cobalt Catalysts. 2. Hydrogen Reduction Properties. J. Phys. Chem. 1990, 94, 819. (34) Twigg, M. W.; Richardson, J. T. Effect of Alumina Incorporation in Coprcipitated NiO-Al2O3 Catalysts. Appl. Catal., A 2000, 190, 61. (35) Rytter, E.; Schanke, D.; Eri, S.; Wigum, H.; Skagseth, T. H.; Bergene, E. Some QA and Optimization Issues During Development of the Statoil FT-Catalyst. CatCon2005, TCGR Pittsburg, CD. (36) Ducreax, O.; Rebours, B.; Lynch, J.; Roy-Auberger, M.; Bazin, D. Microstructure of Supported Cobalt Fischer-Tropsch Catalysts. Oil & Gas Sci. Tech.-ReV.IFP 2009, 64, 49.
ReceiVed for reView February 8, 2010 ReVised manuscript receiVed March 5, 2010 Accepted March 10, 2010 IE100308F
