Cobalt-based catalysts, Fischer-Tropsch

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... 

Xiong, J., Ding, Y., Wang, T., Yan, L., Chen, W., Zhu, H., and Lu, Y. 2005. The formation of Co2C species in activated carbon supported cobalt-based catalysts and its impact on Fischer-Tropsch reaction. Catal. Lett. 102 265-69. 

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... 

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. 

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. 

The earliest theory, advanced by Fischer and Tropsch in 1926 (84), proposed that the reaction proceeded via formation of intermediate metal carbides which react on the catalyst surface to form methylene groups. It was then suggested that these methylene groups polymerize on the surface to form hydrocarbon chains, which desorb as saturated and unsaturated hydrocarbons. In 1939 Craxford and Rideal expanded the carbide theory, proposing (85), for cobalt-based catalysts, the following reaction sequence ... 

E. van Steen and H. Schulz, Polymerisation kinetics of the Fischer-Tropsch CO hydrogenation using iron and cobalt based catalysts, Appl. Catal. A, 1999, 186, 309-320. 

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. 

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. 

Catalysts of commercial significance are either iron-based or cobalt-based. Iron-based catalysts are typically not supported, whereas cobalt-based catalysts are usually supported on alumina, silica, or a similar material. The three-phase low-temperature Fischer-Tropsch (LTFT) technology can be operated in either... 

GTSC [Gas To SynCrude] A process for converting natural gas to a synthetic crude oil, which may be mixed with natural crude oil and used in conventional oil refineries. Based on Fischer-Tropsch technology, but using a proprietary slurry bubble column reactor with a promoted cobalt catalyst. Developed by Syncrude Technology, Pittsburgh, PA, in the 1990s. 

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. 

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

Fischer-Tropsch catalysts are sensitive to poisoning by sulfur-containing compounds. The sensitivity of the catalyst to sulfnr is greater for cobalt-based catalysts than for their iron connterparts. 

Modeling Fischer-Tropsch Product Distribution of a Cobalt-based Catalyst in Different Reaction Media... 

This paper discusses research efforts towards the prediction of hydrocarbon product distribution for the Fischer-Tropsch synthesis (FTS) on a cobalt-based catalyst using a micro-kinetic model taken fiom the literature. In the first part of the study, a MATLAB code has been developed which uses the Genetic Algorithm Toolbox to estimate parameter values for the kinetic model. The second part of the study describes an ongoing experimental campaign to validate the model predictions of the fixed-bed reactor FTS product distribution in both conventional (gas phase) and non-conventional (near-critical and supercritical phase) reaction media. 

Equation (2.7) represents the low-temperature version of the Fischer-Tropsch synthesis using a cobalt-based catalyst at H2/CO ratios of 1.3-1.7. The reaction takes place at 200-260 °C and 20-40 bar producing a broad spectrum of straight-chain olefins and paraffins free from aromatics and hetero-atoms. Hydrogenation is typically used as further upgrading step requiring additional hydrogen. The final products are used as solvents, waxes, or fractions of kerosene or diesel fuel [3,27,28]. 

Cobalt-based catalysts have foimd several applications in different reactions, such as Fischer-Tropsch synthesis and natural gas reforming, among others [1, 2]. In all cases, the particle size is an important feature to take into account when one aims to get very efficient catalysts for different purposes. In Fischer-Tropsch synthesis, cobalt-based catalysts are recognized as a cotiunercially attractive option. Cobalt shows high activity and selectivity for long-chain hydrocarbons, lower water gas shift reaction activity than iron and has lower price in comparison to noble metals such as mthenimn [3]. 

Several cobalt compounds are widely used as oxidation catalysts. Cobalt-based catalysts are also important in some industrial process such as the Fischer-Tropsch process [9] and the hydroformylation of alkenes [10]. Also, the cross-coupling reactions promoted by this metal have been recently highlighted [11]. Conversely, cobalt is not a suitable metal catalyst for the O-H addition to alkynes, alkenes, and nitriles. 

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