Coverage of this Specialist Periodical Report includes all major areas of heterogeneous catalysis. Examples of topics include experimental methods, acid/base catalysis, materials synthesis, environmental catalysis, and syngas conversion.
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Catalysis Volume 19
A Review of Recent Literature
By J.J. Spivey, K.M. Dooley
The Royal Society of ChemistryCopyright © 2006 The Royal Society of Chemistry
All rights reserved.
Promotion Effects in Co-based Fischer-Tropsch Catalysis
BY FERNANDO MORALES AND BERT M. WECKHUYSEN Department of Inorganic Chemistry and Catalysis, Utrecht University, Debye Institute, Sorbonnelaan 16, Utrecht 3584 CA, The Netherlands
1 General Introduction
1.1 Fischer-Tropsch Synthesis. – Franz Fischer, head of the Max-Planck Institut fur Kohlenforschung in Miilheim (Germany) and Hans Tropsch, a co-worker of Fischer and professor of chemistry in Prague (Czech Republic), Miilheim (Germany) and Chicago (Illinois, USA), discovered in 1922 a catalytic reaction between CO and H2, which yields mixtures of higher alkanes and alkenes. This invention made it possible for Germany to produce fuels from its coal reserves and by 1938 9 Fischer–Tropsch (F–T) plants were in operation making use of, e.g, cobalt-based F–T catalysts. The expansion of these plants stopped around 1940, but existing plants continued to operate during World War II. It is worthwhile to notice that in 1944, Japan was operating 3 F–T plants based on coal reserves. Whilst being a major scientific as well as a technical success, the F–T process could not compete economically with the refining process of crude oil, becoming important starting from the 1950s. All this coincided with major discoveries of oil fields in the Middle East and consequently the price of crude oil dropped. Although a new F–T plant was built in Brownsville (Texas, USA) in 1950, the sharp increase in the price of methane caused the plant to shut down. Thus, due to bad economics F–T technology became of little importance for the industrial world after World War II and no new F–T plants were constructed. An exception was South-Africa, which started making fuels and chemicals from gasified coal based on the F–T process a half century ago due to embargoes initiated by the country's apartheid policies. Till today, South Africa's Sasol (South African Coal, Oil and Gas Corporation, Ltd.), building its first commercial F–T plant in 1955, is known as a major player in this field.
It is remarkable to notice that there is today a renewed interest in F–T technology mainly due to:
(i) The rising costs of crude oil. For some time now, the oil prices are well above $50 per barrel.
(ii) The drive to supply environmentally friendly automotive fuels, more in particularly, the production of synthetic sulphur-free diesel, especially interesting for the European car fleet.
(iii) The commercialisation of otherwise unmarketable natural gas at remote locations. CO2 emission regulations will certainly lead in the future to a ban on natural gas flaring near crude oil production wells.
This all has led to the recent decisions on major investments by big petrochemical companies, such as Shell and ExxonMobil, to built large scale F–T plants in Qatar. This will result in an important shift from crude oil to natural gas as feedstock for the production of fuels and chemicals in the decades to come. Industry projections estimate that by 2020 5% of the production of chemicals could be based on F–T technology with methane instead of crude oil refining operations. All this is especially promising in view of the long-term reserves of coal, which are estimated to be more than 20 times that of crude oil and coal is still used as the carbon source at the largest and economically successful F–T complex, namely the plants Sasol One to Three near Sasolburg in South Africa. A picture of a Sasol Fischer–Tropsch plant is shown in Figure 1.
The stoichiometry of the F–T process can be derived from the following two reactions, the polymerization reaction to produce hydrocarbon chains (1), and the water-gas shift reaction (2):
CO + 2 H2 -> -(-CH2-)- + H2O (1)
CO + H2O <-> H2 + CO2 (2)
The overall stoichiometry in case reaction (2) is completely driven to the right is:
2CO + H2 -> -(-CH2-)- + CO2 (3)
With ΔH227 = -204.8 kJ, while the maximum attainable yield is 208.5 g of alkenes CnH2n per Nm3 of a mixture of 2 CO and H2 for complete conversion., The CO/H2 is usually called synthesis gas, or in short syngas. The production of syngas, either by partial oxidation or steam reforming, can account for over 60% of the total cost of the F–T complex since the gasification process is highly endothermic and therefore a high-energy input is required. It should also be clear that the carbon source used, being it either coal or natural gas, is available at low cost, while the gasification of methane is much more efficient than that of coal since coal simply has a much lower hydrogen content. The syngas produced is then fed into a F–T reactor, which converts it into a paraffin wax that is subsequently hydrocracked to make a variety of chemicals, at present mostly diesel, but also some naphtha, lubricants and gases. A scheme of the F–T reaction process, including syngas production and hydrocracking of the wax, is given in Figure 2.
The F–T reaction involves the following main steps at the catalyst surface:
(i) The adsorption and maybe dissociation of CO;
(ii) The adsorption and dissociation of H2;
(iii) Surface reactions leading to alkyl chains, which may terminate by the addition or elimination of hydrogen, giving rise to either paraffin or olefin formation.
(iv) Desorption of the final hydrocarbon products, which can be considered as the primary products of the F–T process.
(vi) Secondary reactions taking place on the primary hydrocarbon products formed due to, e.g., olefin readsorption followed by hydrogenation or chain growth reinitiation.
Various detailed mechanisms have been proposed and this matter still remains a controversial issue in the literature.
Some of the scientific questions that arise are:
(i) Does the adsorbed CO molecule first dissociate into chemisorbed carbon and oxygen atoms? The chemisorbed carbon formed can then be hydrogenated to surface methyl and methylene groups in subsequent steps. Chain growth occurs by stepwise addition of Ci monomers to a surface alkyl group.
(ii) Is the adsorbed CO molecule hydrogenated to a CHO or HCOH species, which inserts in the growing hydrocarbon chain?
(iii) Is CO directly inserted in the growing chain and then subsequently hydrogenated?
It should be clear that a discussion on the F–T mechanism is beyond the scope of this paper and we refer the interested reader to several review papers on this topic. In this respect, it is noteworthy to mention the excellent updates by Dry on the challenges and technological implementations of Co F–T synthesis.
The overall selectivity of the F–T process is intimately related to the production of methane, which is not economic, since the back conversion to syngas encounters, severe thermal and yield penalties. Consequently, substantial research effors have been devoted to decrease the methane production by adjusting the catalyst composition. It is generally considered that the choice of the catalyst material is central to the F–T process. The latest generation of F–T catalysts are based on cobalt and the cobalt nanoparticles are usually supported on an oxide support, mostly silica, alumina and titania, while some promoters are added to the catalyst material in order to enhance the Co dispersion, e.g., some noble metals. Other metal oxide promoters are often added to the catalysts to improve the F–T selectivity, e.g., decreasing the methane production. Reducing the amount of promoter, especially in the case of noble metals, as well as the amount of cobalt are ways to reduce the catalyst production costs and it may be of no wonder that large research efforts in both academia and industrial laboratories have focused on finding the best performing, durable, but still cheap F–T catalyst formulation. Almost every industrial player in the F–T field has its own catalyst formulation, and is – as expected – very secretive about their exact composition of matter in the catalyst materials applied in pilot and/or industrial plants. The choice of the catalyst material is also related to the type of reactor used. In this respect, it is relevant to mention that Shell and BP use fixed bed reactors, whereas Sasol/Chevron and Exxon Mobil make use of slurry phase reactors. The latter plants require the continuous addition of catalyst material.
1.2 Scope of the Review Paper. – From the above reasoning it is clear that over the past decades a large number of studies have been reported on supported cobalt F–T catalysts. All these studies indicate that the number of available surface cobalt metal atoms determines the catalyst activity and attempts to enhance the catalytic activity have been focusing on two interconnected issues: (1) to reduce the cobalt-support oxide interaction and (2) to enhance the number of accessible cobalt atoms available for F–T reaction. It has been shown that the number of catalytically active cobalt atoms as well as their selectivity can be largely enhanced by the addition of small amounts of various elements, called promoters, to the catalyst material. The exact role of these promoters – as is the case for many other heterogeneous catalysts as well – remains often, however, unclear.
The aim of this review paper is to give an extensive overview of the different promoters used to develop new or improved Co-based F–T catalysts. Special attention is directed towards a more fundamental understanding of the effect of the different promoter elements on the catalytically active Co particles. Due to the extensive open and patent literature, we have mainly included research publications of the last two decades in our review paper. In addition, we will limit ourselves to catalyst formulations composed of oxide supports, excluding the use of other interesting and promising support materials, such as, e.g., carbon nanofibers studied by the group of de Jong.
The paper starts with an introduction in F–T catalysis, including some recent developments in gas-to-liquid technologies and an overview of the main F–T catalyst compositions. In a second part, we will focus on the effect of promoter elements on Co-based F–T catalysis. A classification for the different modes of promotion effects will be proposed and each promoter element reported in literature will be accordingly evaluated. The obtained insights have led to guidelines to design improved Co-based F–T catalysts. A third part will deal with some highlights on the literature of Mn-promoted F–T catalysts and a comparison between supported and unsupported Mn-promoted Fe-, Ru- and Co-based F–T catalysts will be made. It will be shown that many advanced characterization techniques, including spectroscopy and microscopy, are necessary to reveal physicochemical insights in this complex catalytic system. The paper ends with some concluding remarks and a look into the future.
2 Fischer–Tropsch Catalysis
2.1 Gas-to-Liquid Technology, Economic Impact and its Relevance to Society. – At present, the main commercial interest in F–T is the production of high quality sulfur-free synthetic diesel fuels from natural gas, currently being flared at crude oil production wells. This renewed interest in F–T synthesis has not just only come about as a result of the abundant supply of natural gas, but also because of the global development of fuel supplies and environmental regulations to improve air quality in cities around the world. While the concept of a hydrogen fuel economy remains an important option for the more distant future, synthetic diesel is being promoted by the fuel industry as the most viable next step towards the creation of a sustainable transport industry. Some advantages of synthetic diesel are:
Low content of suphur and aromatic compounds
High cetane number
Low particulate formation
Low NOx and CO emission
At the same time, increased efficiencies in the F–T process and the ability – based on past experience – to build large-scale plants to capture the economies of scale have made the F–T gas-to-liquid (GTL) technology attractive and competitive with the current crude oil refinery industries.
It has been estimated that F–T GTL should be viable at crude oil prices of about $20 per barrel. For some time now the oil price has been well above $50 per barrel (more recently it has even topped above $70 per barrel), making it a very appealing technology for countries, having huge reserves of natural gas, but little local market for it and no major pipeline infrastructure to ship it to larger economies. Alternatively, such countries could crack ethane or propane to make ethylene or propylene and further convert it into polyethylene or polypropylene, which can then be shipped to more heavily populated areas in the world. All this holds for the Middle East countries and, e.g., Saudi Arabia is known to heavily invest in propane dehydrogenation plants and polypropylene production facilities, while Qatar is focusing on F–T GTL activities. These activities are concentrated near Ras Laffan in Qatar's northern gas field, holding 9% of the world's proven gas resources. Table 1 gives a summary of the currently operated and recently announced F–T plants based on natural gas, together with the expected production levels and the industrial companies and countries involved. Industry projections suggest that by 2020 the total GTL capacity in the world could reach more than 1 x 106 bpd.
Currently, there are two F–T plants operating on offshore methane. The first one is the Shell Bintuli plant in Malaysia, which produces 15000 barrels per day. The second one is the Moss Bay plant (PetroSA) located in South Africa. Recently, Sasol/Chevron, ExxonMobil and Shell announced major investments in F–T GTL plants. In addition, there are many small (mainly for local markets) and large (mainly for export) project proposals for F–T GTL projects on the table. Most of the large project proposals are in the Middle East (Qatar), while the other envisaged projects are in Russia, Australia, Argentina, Egypt, Iran, Bolivia, Brazil, Indonesia, Malaysia and Trinidad. Especially, Russia is expected to have significant long-term potential for F–T GTL technology taking into account the huge country's gas reserves.
2.2 Fischer–Tropsch Catalysts. – It is well known that all Group VIII transition metals are active for F–T synthesis. However, the only F–T catalysts, which have sufficient CO hydrogenation activity for commercial application, are composed of Ni, Co, Fe or Ru as the active metal phase. These metals are orders-of-magnitude more active than the other Group VIII metals and some characteristics of Ni-, Fe-, Co- and Ru-based F–T catalysts are summarized in Table 2.
The exact choice of the active F–T metal to be used in a particular catalyst formulation depends on a number of parameters, including the source of carbon used for making syngas, the price of the active element and the end products wanted. F–T catalysts for the conversion of syngas made from a carbon-rich source, such as coal, are usually based on Fe. This is due to the high WGS activity of Fe, as given in reaction (2), so that less hydrogen is required and oxygen exits the reactor in the form of carbon dioxide. There are, however, new environmental considerations such as the greenhouse effect, which may preclude the future use of Fe precisely due to its high WGS activity. In the case of syngas production from hydrogen-rich carbon sources, such as natural gas, the preferred catalysts due to their lower WGS activities are based on Co or Ru.
Nickel F–T catalysts, due to an easy dissociation of CO, possess too much hydrogenation activity, unfortunately, resulting in high yields of methane. At elevated pressure, Ni tends to form nickel carbonyl compounds (highly toxic), and the active component of the catalyst is lost from the F–T reactor. In addition, with increasing reaction temperature the selectivity changes to mainly methane with Ni. This tendency is also observed with Co- and Ru-based catalysts. Instead, with Fe, the selectivity towards methane remains low even at high reaction temperatures. Ru is the most active F–T element working at the lowest reaction temperature of, e.g., only 150°C, very high molecular weight products have been isolated. However, the very low availability and as a consequence the high cost of Ru makes the use of this element in large-scale industrial F–T applications questionable.
This leaves Co and Fe as the most appropriate elements to prepare commercially interesting F–T catalysts and both systems have their own advantages and disadvantages. It is important to notice that Co is 3 times more active than Fe in F–T while its price is over 250 times more expensive. Because of the relatively low cost of Fe, fresh catalyst material can be added on-line to fluidizied bed reactors, and this practice results in long runs at high conversion levels. This luxury cannot be afforded for the more expensive Co F–T catalysts and so, it is vital that the minimum amount of Co is employed, while maintaining its high activity and long effective catalyst life. Co-based catalysts are preferred for the production of paraffins, as they give the highest yields for high molecular weight hydrocarbons from a relatively clean feedstock, and produce much less oxygenates than Fe catalysts. This is due to a higher hydrogenation activity of Co compared to Fe. On the other hand, if linear olefins are wanted as the end product, it is better to employ Fe-based F-T catalysts because there is less secondary hydrogenation of the primary formed olefins. However, Fe-based catalysts are known to produce aromatics and other non-paraffins, such as oxygenated compounds, as by-products
Excerpted from Catalysis Volume 19 by J.J. Spivey, K.M. Dooley. Copyright © 2006 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Meet the Author
Professor Spivey is the McLaurin Shivers Professor of Chemical Engineering at Louisiana State University and Director of the DOE Energy Frontier Research Center at LSU. Professor Spivey's research interests include the application of the principles of heterogeneous catalysis to catalytic combustion, control of sulfur and nitrogen oxides from combustion processes, acid/base catalysis (e.g., for condensation reactions), hydrocarbon synthesis, and the study of catalyst deactivation.
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