Cobalt-based catalyst, fischer-Tropsch selectivity

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. 

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. 

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. 

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. 

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]. 

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]. 

Cobalt-based Fischer-Tropsch catalysts are the subject of continuing interest as large-scale Gas-to-Liquids plants come on line. Fernando Morales and Bert Weckhuysen (Utrecht University, the Netherlands) look specifically at the effects of various promoters for these catalysts, particularly Mn. The effect of these promoters in controlling the activity and selectivity of the overall reaction can be critical in the overall process economics. This chapter also looks at new spectroscopic techniques that have recently been used to study the effects of these promoters. 

Fischer-Tropsch activity, selectivity and deactivation data obtained in fixed bed reaction tests of Co/Si02 catalysts are summarized in Table 1. The turnover frequencies (TOFs) or site time yields based on H2 uptake and on rate measured after 20 hours of reaction agree within a factor of two with those reported for other cobalt catalysts [2, 3, 25-27]. CO conversion and methane selectivity versus time for Cab-O-Sil supported cobalt at both low and high space velocities are shown in Figure 1. It can be seen that at high conversion the catalyst deactivates rapidly while at low conversion the catalyst appears to be stable. The conversion is proportional to the water partial pressure thus water could be causing this deactivation. 

Supported cobalt is considered to be the most favourable catalytic material for the synthesis of long-chain hydrocarbons from natural gas-based synthesis gas because of its high activity, high selectivity for linear paraffins, low water-gas shift activity, and low price compared to noble metals. Common supports for Fischer-Tropsch catalysts include alumina, silica, and titania. 

Statoil has been involved in Fischer-Tropsch based GTL technology development since the mid 1980 s (Rytter et al.,1990). In order to maximize distillate production, a low temperature, cobalt catalyst based Fischer-Tropsch technology has been selected. A slurry bubble column reactor offers the best performance in terms of economy of scale, throughput and yield, but presents several technical challenges. A highly active and selective cobalt catalyst is needed and must be adapted to suit the requirements of the slurry reactor. Separation of wax from the slurry is another critical aspect of this technology. Statoil has developed a supported cobalt catalyst and a continuous filtration technique that forms the heart of the Fischer-Tropsch process. 

Cobalt-based low temperature Fischer—Tropsch catalysts, appHed at approximately 220 °C and 30 atm, are usually supported on high-surface-area Y-AI2O3 (150—200 m g ) and typically contain 15—30% weight of cobalt. To stabihze them and decrease selectivity to methane, these catalysts may contain small amounts of noble metal promoters (typically 0.05—0.1 wt% of ruthenium, rhodium, platinum, or palladium) or an oxide promoter (e.g., zir-conia, lanthana, cerium oxide, in concentrations of 1—10 wt%) (409). 

Based on the development of both catalysts and reactors [4, 5], the Fischer-Tropsch synthesis activity and selectivity of cobalt catalyst have increased as illustrated in Figure 1.1. The volume-based activity has increased by a factor of 10 going from 1940 at space time yield (STY) = 10 to 1990 at STY = 100, and another factor of 3 is expected to lead to STY = 300 by 2010. Most importantly, with increasing activity the catalysts displayed improved selectivities to higher hydrocarbons. 

In order to produce methanol the catalyst should only dissociate the hydrogen but leave the carbon monoxide intact. Metals such as copper (in practice promoted with ZnO) and palladium as well as several alloys based on noble group VIII metals fulfill these requirements. Iron, cobalt, nickel, and ruthenium, on the other hand, are active for the production of hydrocarbons, because in contrast to copper, these metals easily dissociate CO. Nickel is a selective catalyst for methane formation. Carbidic carbon formed on the surface of the catalyst is hydrogenated to methane. The oxygen atoms from dissociated CO react with CO to CO2 or with H-atoms to water. The conversion of CO and H2 to higher hydrocarbons (on Fe, Co, and Ru) is called the Fischer-Tropsch reaction. The Fischer-Tropsch process provides a way to produce liquid fuels from coal or natural gas. 

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