Iron-based catalyst

Recent commercialization efforts have focused on improved activity synthesis catalysts, which allow ammonia synthesis to be conducted at significantly lower pressures and temperatures. Catalyst manufacturers have focused on enhancing the activity of the iron-based catalyst through the use of promoters (23). 

Sasol Fischer-Tropsch Process. 1-Propanol is one of the products from Sasol s Fischer-Tropsch process (7). Coal (qv) is gasified ia Lurgi reactors to produce synthesis gas (H2/CO). After separation from gas Hquids and purification, the synthesis gas is fed iato the Sasol Synthol plant where it is entrained with a powdered iron-based catalyst within the fluid-bed reactors. The exothermic Fischer-Tropsch reaction produces a mixture of hydrocarbons (qv) and oxygenates. The condensation products from the process consist of hydrocarbon Hquids and an aqueous stream that contains a mixture of ketones (qv) and alcohols. The ketones and alcohols are recovered and most of the alcohols are used for the blending of high octane gasoline. Some of the alcohol streams are further purified by distillation to yield pure 1-propanol and ethanol ia a multiunit plant, which has a total capacity of 25,000-30,000 t/yr (see Coal conversion processes, gasification). 

Ammonia Synthesis. Ammonia (qv) has been manufactured by the hydrogenolysis of the N-N bond in nitrogen over an iron-based catalyst since the beginning of the twentieth century (46) ... 

Sasol uses both fixed-bed reactors and transported fluidized-bed reactors to convert synthesis gas to hydrocarbons. The multitubular, water-cooled fixed-bed reactors were designed by Lurgi and Ruhrchemie, whereas the newer fluidized-bed reactors scaled up from a pilot unit by Kellogg are now known as Sasol Synthol reactors. The two reactor types use different iron-based catalysts and give different product distributions. 

Fischer Tropsch technology is best exemplified by the SASOL projects in South Africa. After coal is gasified to a synthesis gas mixture, it is purified in a rectisol unit. The purified gas mixture is reacted in a synthol unit over an iron-based catalyst. The main products are gasoline, diesel fuel, and jet fuels. By-products are ethylene, propylene, alpha olefins, sulfur, phenol, and ammonia which are used for the production of downstream chemicals. 

When the iron-based catalyst 66 was used, a high level of enantiomeric excess in the cycloadditions between cyclopentadiene (18) and a,/i-unsaturated aldehydes [65] was observed. The cycloadditions were carried out in the presence of 2,6-di-t-butylpyridine (Scheme 3.15) which was added to scavenge residual acid impurities. 

Until recently only few examples on asymmetric epoxidation using iron-based catalysts were reported in the literature (Scheme 6) [42-44]. With [Fe(BPMCN) (CF3S03)2] 10, 58% of the epoxide with 12% ee was obtained in the oxidation of frans-2-heptene [42]. 

The design sketched above is an elaborate version of the so-called Kellogg Advanced Ammonia Process (KAAP) in which iron-based catalysts are used in the first bed, and ruthenium-based catalysts, which bind nitrogen more weakly, are used in the second, third and fourth beds [T.A. Czuppon, S.A. Knez, R.W. Schneider and G. Woroberts, Ammonia Plant Safety Relat. Pacil. 34 (1994) 236]. 

Figure 1. NHj concentration in the reactor effluent gas using a total flow of 40 Nml/min with Pnj Phs - 1 / 3 at atmospheric pressure. Traces A-E in fig.lA (from bottom to top) were obtained with Ru/AljOj, CS-RU/AI2O3, Ru/MgO, a multiply promoted iron-based catalyst, and Cs-Ru/MgO. The corresponding NH3 equilibrium concentration is displayed as dashed line. Traces A-C in fig.IB (from bottom to top) were obtained with Ru/MgO, K-Ru/MgO, and Cs-Ru/MgO.

Rao, V. U. S., Stiegel, G. J., Cinquegrane, G. J., and Srivastava, R. D. 1992. Iron-based catalysts for slurry-phase Fischer-Tropsch process Technology review. Fuel Process. Technol. 30 83-107. 

Machocki, A. 1991. Formation of carbonaceous deposit and its effect on carbon monoxide hydrogenation on iron-based catalysts. Appl. Catal. 70 237-52. 

Temperature-Programmed EXAFS/ XANES Characterization of the Impact of Cu and Alkali Promoters to Iron-Based Catalysts on the Carbide Formation Rate... 

CO can be converted into either hydrocarbon products and water (via FTS) or C02 and Fl2 via the water-gas shift (WGS) reaction. The reversible WGS reaction accompanies FTS over the iron-based catalyst only at high temperature conditions. The individual rates of FTS (rFTS) and the WGS reaction (rWGS) can be calculated from experimental results as rWGS = r(,and rFTS = rco-rc02, where rCo2 is the rate of C02 formation and rco is the rate of CO conversion. 

Ngantsoue-Hoc, W., Zhang, Y., O Brien, R.J., Luo, M., and Davis, B.H. 2002. Fischer-Tropsch synthesis Activity and selectivity for Group I alkali promoted iron-based catalysts. Appl. Catal. 236 77-89. 

Li, S., Krishnamoorthy, S., Li, A., Meitzner, G.D., and Iglesia, E. 2002. Promoted iron-based catalysts for the Fischer-Tropsch synthesis Design, synthesis, site densities, and catalytic properties. J. Catal. 206 202-17. 

A continuous cross-flow filtration process has been utilized to investigate the effectiveness in the separation of nano sized (3-5 nm) iron-based catalyst particles from simulated Fischer-Tropsch (FT) catalyst/wax slurry in a pilot-scale slurry bubble column reactor (SBCR). A prototype stainless steel cross-flow filtration module (nominal pore opening of 0.1 pm) was used. A series of cross-flow filtration experiments were initiated to study the effect of mono-olefins and aliphatic alcohol on the filtration flux and membrane performance. 1-hexadecene and 1-dodecanol were doped into activated iron catalyst slurry (with Polywax 500 and 655 as simulated FT wax) to evaluate the effect of their presence on filtration performance. The 1-hexadecene concentrations were varied from 5 to 25 wt% and 1-dodecanol concentrations were varied from 6 to 17 wt% to simulate a range of FT reactor slurries reported in literature. The addition of 1-dodecanol was found to decrease the permeation rate, while the addition of 1-hexadecene was found to have an insignificant or no effect on the permeation rate. 

Traditionally, iron-based catalysts have been used for FT synthesis when the syngas is coal derived, because of their activity in both FTS and WGS reactions. Complex mixtures of straight-chain paraffins, olefins, and oxygenate (in substantial proportions) compounds are known to be formed during iron-based FTS. Olefin selectivity of iron catalysts is typically greater than 50% of the hydrocarbon products at low carbon numbers, and more than 60% of the produced olefins are a-olefins.13 For iron-based catalysts, the olefin selectivity decreases asymptotically with increasing carbon number. 

The two important discoveries in the search for iron-based Fischer-Tropsch catalysts were (a) the finding that the addition of alkali yielded significant improvements in the activity and selectivity (to liquid products) of iron catalysts (15), and (b) the development of the medium-pressure synthesis (16). In 1943 a pilot plant was constructed at Schwarz-Leide in Germany for the comparative testing of iron-based catalysts. However, the outcome of World War II curtailed its activities. After 1945 many of the plants were destroyed and, for those remaining, recommencement of operation was forbidden for several years. Of the three plants restarted, the last at Bergkamen was closed in 1962. 

Iron-based catalysts have been used in all the plants constructed after the war, because (a) iron is considerably cheaper than cobalt, (b) iron systems are generally more stable, and (c) greater flexibility with regard to product distribution can be attained. With the exception of the SASOL complex, which will be dealt with in Section I,B, the only Fischer-Tropsch plant of any appreciable size constructed in the West since the... 

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. 

F-T Catalysts The patent literature is replete with recipes for the production of F-T catalysts, with most formulations being based on iron, cobalt, or ruthenium, typically with the addition of some pro-moter(s). Nickel is sometimes listed as a F-T catalyst, but nickel has too much hydrogenation activity and produces mainly methane. In practice, because of the cost of ruthenium, commercial plants use either cobalt-based or iron-based catalysts. Cobalt is usually deposited on a refractory oxide support, such as alumina, silica, titania, or zirconia. Iron is typically not supported and may be prepared by precipitation. 

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