A critical review of the kinetics and selectivity of the Fischer–Tropsch synthesis (FTS) is given. The focus is on reaction mechanisms and kinetics of the water–gas shift and Fischer–Tropsch (FT) reactions. New developments in the product selectivity as well as the overall kinetics are reviewed. It is concluded that the development of rate equations for the FTS should be based on realistic mechanistic schemes. Qualitatively, there is agreement that the product distribution is affected by the occurrence of secondary reactions (hydrogenation, isomerization, reinsertion, and hydrogenolysis). At high CO and H2O pressures, the most important secondary reaction is readsorption of olefins, resulting in initiation of chain growth processes. Secondary hydrogenation of α-olefins may occur and depends on the catalytic system and the process conditions. The rates of the secondary reactions increase exponentially with chain length. Much controversy exists about whether these chain-length dependencies stem from differe...

Kinetics and Selectivity of the Fischer–Tropsch Synthesis: A Literature Review

Lower olefins are key building blocks for the manufacture of plastics, cosmetics, and drugs. Traditionally, olefins with two to four carbons are produced by steam cracking of crude oil-derived naphtha, but there is a pressing need for alternative feedstocks and processes in view of supply limitations and of environmental issues. Although the Fischer-Tropsch synthesis has long offered a means to convert coal, biomass, and natural gas into hydrocarbon derivatives through the intermediacy of synthesis gas (a mixture of molecular hydrogen and carbon monoxide), selectivity toward lower olefins tends to be low. We report on the conversion of synthesis gas to C(2) through C(4) olefins with selectivity up to 60 weight percent, using catalysts that constitute iron nanoparticles (promoted by sulfur plus sodium) homogeneously dispersed on weakly interactive α-alumina or carbon nanofiber supports.

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Supported Iron Nanoparticles as Catalysts for Sustainable Production of Lower Olefins

Iron-based Fischer–Tropsch catalysts, which are applied in the conversion of CO and H2 into longer hydrocarbon chains, are historically amongst the most intensively studied systems in heterogeneous catalysis. Despite this, fundamental understanding of the complex and dynamic chemistry of the iron–carbon–oxygen system and its implications for the rapid deactivation of the iron-based catalysts is still a developing field. Fischer–Tropsch catalysis is characterized by its multidisciplinary nature and therefore deals with a wide variety of fundamental chemical and physical problems. This critical review will summarize the current state of knowledge of the underlying mechanisms for the activation and eventual deactivation of iron-based Fischer–Tropsch catalysts and suggest systematic approaches for relating chemical identity to performance in next generation iron-based catalyst systems (210 references).

The renaissance of iron-based Fischer-Tropsch synthesis: on the multifaceted catalyst deactivation behaviour.

The reduction of various iron oxides in hydrogen and carbon monoxide atmospheres has been investigated by temperature programmed reduction (TPR H2 and TPR CO ), thermo-gravimetric and differential temperature analysis (TG-DTA-MS), and conventional and “ in situ ” XRD methods Five different compounds of iron oxides were characterized: hematite α-Fe 2 O 3 , goethite α-FeOOH, ferrihydrite Fe 5 HO 8 ·4H 2 O, magnetite Fe 3 O 4 and wustite FeO In the case of iron oxide-hydroxides, goethite and ferrihydrite, the reduction process takes place after accompanying dehydration below 300 °C Instead of the commonly accepted two-stage reduction of hematite, 3 α-Fe 2 O 3  → 2 Fe 3 O 4  → 6 Fe, three-stage mechanism 3Fe 2 O 3  → 2Fe 3 O 4  → 6FeO → 6Fe is postulated especially when temperature of reduction overlaps 570 °C Up to this temperature the postulated mechanism may also involve disproportionation reaction, 3Fe 2+  ⇌ 2Fe 3+  + Fe, occurring at both the atomic scale on two-dimensional interface border Fe 3 O 4 /Fe or stoichiometrically equivalent and thermally induced, above 250 °C, phase transformation—wustite disproportionation to magnetite and metallic iron, 4FeO ⇌ Fe 3 O 4  + Fe Above 570 °C, the appearance of wustite phase, as an intermediate of hematite reduction in hydrogen, was experimentally confirmed by “ in situ ” XRD method In the case of FeO–H 2 system, instead of one-step simple reduction FeO → Fe, a much more complex two-step pathway FeO → Fe 3 O 4  → Fe up to 570 °C or even the entire sequence of three-step process FeO → Fe 3 O 4  → FeO → Fe up to 880 °C should be reconsidered as a result of the accompanying FeO disproportionation wustite ⇌ magnetite + iron manifesting its role above 150 °C and occurring independently on the kind of atmosphere—inert argon or reductive hydrogen or carbon monoxide The disproportionation reaction of FeO does not consume hydrogen and occurs above 200 °C much easier than FeO reduction in hydrogen above 350 °C The main reason seems to result from different mechanistic pathways of disproportionation and reduction reactions The disproportionation reaction wustite ⇌ magnetite + iron makes simple wustite reduction FeO → Fe a much more complicated process In the case of thermodynamically forced FeO disproportionation, the oxygen sub-lattice, a closely packed cubic network, does not change during wustite → magnetite transformation, but the formation of metallic iron phase requires temperature activated diffusion of iron atoms into the region of inter-phase FeO/Fe 3 O 4  Depending on TPR H2 conditions (heating rate, velocity and hydrogen concentration), the complete reduction of hematite into metallic iron phase can be accomplished at a relatively low temperature, below 380 °C Although the reduction behavior is analogical for all examined iron oxides, it is strongly influenced by their size, crystallinity and the conditions of reduction

Reduction behavior of iron oxides in hydrogen and carbon monoxide atmospheres

Iron carbide nanoparticles have long been considered to have great potential in new energy conversion, nanomagnets, and nanomedicines. However, the conventional relatively harsh synthetic conditions of iron carbide hindered its wide applications. In this article, we present a facile wet-chemical route for the synthesis of Hagg iron carbide (Fe5C2) nanoparticles, in which bromide was found to be the key inducing agent for the conversion of Fe(CO)5 to Fe5C2 in the synthetic process. Furthermore, the as-synthesized Fe5C2 nanoparticles were applied in the Fischer–Tropsch synthesis (FTS) and exhibited intrinsic catalytic activity in FTS, demonstrating that Fe5C2 is an active phase for FTS. Compared with a conventional reduced-hematite catalyst, the Fe5C2 nanoparticles showed enhanced catalytic performance in terms of CO conversion and product selectivity.

Fe5C2 nanoparticles: a facile bromide-induced synthesis and as an active phase for Fischer-Tropsch synthesis.

Activation studies with a precipitated iron catalyst for Fischer-Tropsch synthesis. I. Characterization studies

This paper reports on the influence of silica and alumina binders (supports) on the activity and product selectivity of precipitated iron catalyst for Fischer-Tropsch synthesis (FTS) that was studied in a fixed bed reactor at 1.5-3.0 MPa and 220-250{degrees} C, using synthesis gas with H{sub 2}/CO = 1 M feed ratio. Both the FTS and the water gas shift activity decreased with increasing support content. The catalyst deactivation rate decreased with catalysts containing 24 and 100 g of SiO{sub 2} per 100 g of Fe. Secondary reaction (olefin hydrogenation and isomerization) increased with increasing silica content. Hydrocarbon selectivities improved (less gaseous hydrocarbons) with the addition of support, except for the catalyst containing 24 SiO{sub 2}/100Fe. The results were interpreted in terms of interactions between potassium and/or iron with supports.

Binder/support effects on the activity and selectivity of iron catalysts in the Fischer-Tropsch synthesis

The authors discuss the effect of reductant type (CO, H/sub 2/, or H/sub 2//CO = 0.68), activation temperature (250, 280, and 310{sup 0}C) duration (8 and 24 h), and pressure (0.1 and 1.48 MPa) on the activity and selectivity of a promoted precipitated iron catalyst (100Fe/3Cu/0.2{Kappa}) studied in a fixed bed reactor. It was found that activation parameters have a strong effect on the catalyst behavior during Fischer-Tropsch (FT) synthesis at 250{sup 0}C, 1.48 MPa, 2 nL/g of catalyst/h, and H/sub 2//CO = 1. Activations in CO led to catalysts that had higher initial activity and better selectivity (less methane and other light hydrocarbons) than H/sub 2/ activated catalysts. However, the activity of CO-reduced catalysts declined with time on stream, whereas the activity of H/sub 2/-reduced catalysts remained constant or increased during 120 h of testing. Activation in CO at 280{sup 0}C and 0.1 MPa for 24 h was the most desirable on the basis of overall catalyst activity, selectivity, and stability.

Chiuping Li

Papers

Activation studies with a precipitated iron catalyst for Fischer-Tropsch synthesis. I. Characterization studies

Binder/support effects on the activity and selectivity of iron catalysts in the Fischer-Tropsch synthesis

Activation studies with a promoted precipitated iron Fischer-Tropsch catalyst