Cobalt-based Fischer-Tropsch synthesis catalysts

In this work, a detailed kinetic model for the Fischer-Tropsch synthesis (FTS) has been developed. Based on the analysis of the literature data concerning the FT reaction mechanism and on the results we obtained from chemical enrichment experiments, we have first defined a detailed FT mechanism for a cobalt-based catalyst, explaining the synthesis of each product through the evolution of adsorbed reaction intermediates. Moreover, appropriate rate laws have been attributed to each reaction step and the resulting kinetic scheme fitted to a comprehensive set of FT data describing the effect of process conditions on catalyst activity and selectivity in the range of process conditions typical of industrial operations. 

Figure 8.17. Hydrocarbon distribution of the products formed by Fischer-Tropsch synthesis over cobalt-based catalysts and by additional hydrocracking, illustrating how a two-stage concept enables optimization of diesel fuel yield. [Adapted from S.T. Sie,...

Krishnamoorthy, S., Tu, M., Ojeda, M. P., Pinna, D., and Iglesia, E. 2002. An investigation of the effects of water on rate and selectivity for the Fischer-Tropsch synthesis on cobalt-based catalysts. J. Catal. 211 422-33. 

An Overview of Reported Claims of Bulk Cobalt Carbide Being Observed after/when Performing Fischer-Tropsch Synthesis over Supported Cobalt-Based Catalysts... 

Moodley, D. J., van de Loosdrecht, J., Saib, A. M., Overett, M. J., Datye, A. K., and Niemantsverdriet, J. W. 2009. Carbon deposition as a deactivation mechanism of cobalt-based Fischer-Tropsch synthesis catalysts under reahstic conditions. Appl. Catal. A, 354 102-10. 

Zhan, X., Arcuri, K., Huang, R., Agee, K., Engman, J., and Robota, H. J. 2004. Regeneration of cobalt-based slurry catalysts for Fischer-Tropsch synthesis. Prep. Pap.-Am. Chem. Soc. Div. Pet. Chem. 49 179-81. 

Iglesia, E. 1997. Design, synthesis, and use of cobalt-based Fischer-Tropsch synthesis catalysts. Appl. Catal. 161 59-78. 

Jacobs, G., Das, T.K., Li, J., Luo, M., Patterson, P.M., and B.H. Davis. 2007. Fischer-Tropsch synthesis Influence of support on the impact of co-fed water for cobalt-based catalysts. In Fischer-Tropsch synthesis Catalysts and catalysis, ed. B.H. Davis and M.L. Occelli, 217-53 Amsterdam, The Netherlands Elsevier. 

Detailed Kinetic Study and Modeling of the Fischer-Tropsch Synthesis over a State-of-the-Art Cobalt-Based Catalyst... 

The data available for heterogeneous Fischer-Tropsch catalysts indicate that with cobalt-based catalysts the rate of the water gas-shift reaction is very slow under the synthesis conditions (5). Thus, water is formed together with the hydrocarbon products [Eq. (14)]. The iron-based catalysts show some shift activity, but even with these catalysts, considerable quantities of water are produced. 

S. Krishnamoorthy, M. Tu, M. P. Ojeda, D. Pinna and E. Iglesia, An Investigation of the Effects of Water on Rate and Selectivity for the Fischer-Tropsch synthesis on Cobalt-Based Catalysts, J. Catal., 2002, 211, 422-433. 

N. O. Elbashir, P. Dutta, A. Manivannan, M. S. Seehra and C. B. Roberts, Impact of cobalt-based catalyst characteristics on the performance of conventional gas-phase and supercritical-phase Fischer-Tropsch synthesis, Appl. Catal. A, 2005, 285, 169-180. 

Continuing interest in cobalt catalysts used in the Fischer-Tropsch synthesis has led to the proposal of new methods of catalyst preparation that could determine the selectivity of the catalysts obtained. In this context, a highly selective material to produce C5+ hydrocarbons using a plasma-based method and carbonyl precursors has been prepared [147]. 

Fischer-Tropsch synthesis making use of cobalt-based catalysts is a hotly persued scientific topic in the catalysis community since it offers an interesting and economically viable route for the conversion of e.g. natural gas to sulphur-free diesel fuels. As a result, major oil companies have recently announced to implement this technology and major investments are under way to build large Fischer-Tropsch plants based on cobalt-based catalysts in e.g. Qatar. Promoters have shown to be crucial to alter the catalytic properties of these catalyst systems in a positive way. For this reason, almost every chemical element of the periodic table has been evaluated in the open literature for its potential beneficial effects on the activity, selectivity and stability of supported cobalt nanoparticles. 

Summarizing, there are still many scientific challenges and major opportunities for the catalysis community in the field of cobalt-based Fischer-Tropsch synthesis to design improved or totally new catalyst systems. However, such improvements require a profound knowledge of the promoted catalyst material. In this respect, detailed physicochemical insights in the cobalt-support, cobalt-promoter and support-support interfacial chemistry are of paramount importance. Advanced synthesis methods and characterization tools giving structural and electronic information of both the cobalt and the support element under reaction conditions should be developed to achieve this goal. 

Shell Gas B.V. has constructed a 1987 mVd (12,500 bbl/d) Fischer-Tropsch plant in Malaysia, start-up occurring in 1994. The Shell Middle Distillate Synthesis (SMDS) process, as it is called, uses natural gas as the feedstock to fixed-bed reactors containing cobalt-based catalyst. The heavy hydrocarbons from the Fischer-Tropsch reactors are converted to distillate fuels by hydrocracking and hydroisomerization. The quality of the products is very high, the diesel fuel having a cetane number in excess of 75. 

Support modification has been reported earlier in the open literature [5,6,7,8,9]. Zirconia modification of silica supports was used to prevent the formation of unreducible cobalt-silicates [5]. Zr, Ce, Hf, or U modification of titania supports was reported to prevent the formation of cobalt-titanates during regeneration [6]. To increase the porosity of titania supports, they were modified with small amounts of binders, e.g. silica, alumina or zirconia [7]. Lanthanum oxide promotion of alumina was reported to be beneficial for improved production of products with higher boiling points [8], and zirconia modification of alumina supports was carried out to decrease the interaction of cobalt with alumina [9]. All these modified supports were either used for fixed bed cobalt based Fischer-Tropsch synthesis catalysts or they were used for slurry phase cobalt catalysts, but not tested under realistic Fischer-Tropsch synthesis conditions in large scale slurry bed reactors. 

Fischer-Tropsch synthesis influence of support on the impact of co-fed water for cobalt-based catalysts... 

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