Fused Fe catalyst

Results for the Synthol entrained-bed process (16) are plotted in Figure 8. The available C to C15 data follow the conventional Flory plot with a equal to 0.7. The Synthol process uses a fused Fe catalyst of low surface area and porosity and operates at high temperatures ( 590K). The products in the reactor are mainly gaseous, wax formation is minimal, and the pellet pore structure remains free of liquid products therefore, diffusion-enhanced a-olefin readsorption is much less likely than in the ARGE process. Whereas the product selectivity in the ARGE process is altered by diffusion-enhanced a-olefin readsorption, that in the Synthol process is not. 

Similarly, amination of diols with ammonia and hydrogen leads to heterocyclic compounds via the amino alcohol intermediate. A promoted fused Fe catalyst afforded 93% 2,5-dimethylpyrrolidine from 2,5-hexanediol [11]. 

Cu catalysts can be used in case of short-chain olefins, while nitrided fused Fe catalysts have proven to yield high selectivities to oxygenates (39,98). 

Plot of pMp° - p) against p/p° (r is expressed in cm (stp)). (1) Unpromoted Fe catalyst (2) AljOj-promoted Fe catalyst (3) AI2O3-KjO-promoted Fe catalyst (4) fused copper catalyst (5) chromium oxide gel (6) silica gel. (Courtesy Brunauer, Emmett and Teller.)... 

Some of these same experiments have been done using 10% Fe/Al203 rather than the fused iron catalyst (53). Figure 22 shows the result of a switch from H2 to 10% CO in H2 over a freshly reduced catalyst. Here a large initial rate of methane formation is observed and water does not appear until most of the initial peak has passed. The probable explanation for the presence of the CHi peak is that water produced by methanation is adsorbed on the initially dry y-Al203 support (100 m2/g). Thus the iron remains briefly in a relatively reduced state. For the CCI catalyst the AI2O3 promoter is not sufficient to prevent the water from rising quickly as shown in Fig. 19. The H/0 ratio on the surface is reduced, and carburization occurs more rapidly than methanation, as for the unsupported catalyst. 

Linear a-olefins together with linear paraffins are the main primary products. On Fe the olefin content in the fraction of linear hydrocarbons for small carbon numbers was found to be about 80% (Fig. 4), which is very close to their primary selectivity [6]. This can be due to the high potassium loading, which suppresses the secondary reactions of the olefins. With increasing CO2 content a slight increase of the olefin content is observed. This can be due to the increasing amount of water formed from the reaction with CO2 instead of CO. The effect of added water on the olefin selectivity for a potassium promoted fused iron catalyst has been reported earlier by Satterfield [7]. With increasing CO2 concentration in the reaction gas on Co no more olefins were present in the products. 

Schulz and El Deen made a very detailed study of the product distribution obtained with precipitated and fused alkalized-Fe catalysts. They worked with pressures of ca. 20 atm, at temperatures of about 220 °C and with fixed as well as entrained solid reactors. One of their very interesting results is shown in Figure 1. We observe there that the distributions of hydrocarbons and of alcohols show a great similarity. 

The possible steps of Fischer-Tropsch (FT) reaction and its catalysts (Fe, Co, Ru, Ni) represent a very complicated systemThe catalysts usually need a formation or self-organisation , meaning that the full activity will only be reached after a certain period. This means that for Fe-based catalysts, a part of the initial Fe oxide is transformed into iron carbide. This was investigated as early as 1948 by the tracer method.A fused iron catalyst was carbided with The synthesis product from CO/H2 = 1 1 reactant contained 10-15% labelled molecules, almost independently of the reaction conditions, even in repeated runs, indicating the minor role of carbide incorporation into hydrocarbons. The formation of a Fe-Al-Cu catalyst at 523 K and various H2/CO ratios required 100 to 2000 minutes. The yield of retained carbon decreased gradually, while the FT yield increased more abruptly after this period. [ 1... 

Table indicates that only a fraction of the C label from ethene was incorporated into chain growth products on Co and Fe catalysts. About the same amount of C (31%) was found in FT products formed with 1[ C]-1-propene, but only 18% when l-[ C]-l-hexadecene was applied.I About 50% of radioactive ethene gave methane, and 50% chain growth products on a Co catalyst, I the specific activity of higher products being practically constant between C4 and C32. Less than 10% of C from labelled ethene was incorporated into C10-C17 products, but the incorporation from C-ethanol was 60-80 times higher on a fused iron catalyst.The difference... 

Non-fused iron catalysts have been studied earlier. The famous Uhde catalyst was KAl (Fe(CN)6), which was used, to be applied in industry. It was abandoned because of its poor stability, and up to now there are still reports about its modifications. Intermetallic compound and alloy catalysts, such as LaNij, FeTi, Fe2Ce and FeZr etc., were also expected to be prospective, but until now they have not been put into practice. In 1970s, the well-known electron donor-acceptor (EDA) catalysts, e.g., phthalocyanine iron-alkali metal, molysite — graphite — potassium and ferrocene-activated carbon-potassium catalyst systems, were found to have the ability to synthesize ammonia under mild conditions in the laboratory. Unfortunately, their activities declined rapidly in the experiments of scale-up. The application of EDA catalysts in industry turned to be a visionary. Therefore, replacement of fused iron catalyst is not an easy thing for a very long time. 

College of Chemical Engineering investigated fused iron catalysts containing cobalt oxide and found that the activity increased significantly. Work of ICI in UK was noticeable for cobalt containing catalysts, and patented the Fe-Co catalysts. 

In 1986, Liu et at found that the iron catalyst with wiistite as the precursor has extremely high ammonia synthesis activity and rapid reduction rate, which led to the invention of wiistite (Fei xO) based catalyst for ammonia synthesis. The relationship between the activity and the iron oxides (Fe304, FeO and Fe203) and their mixtures in the precursor were studied systematically, and a hump type curve was found between the activity and the ratio (Fe +/Fe +). It was speculated that the monophase of iron oxide phase in the precursor is an essential condition for high activity of the catalyst and a uniform distribution of iron oxide phase and promoters is a key to make a better performance of catalyst. The hump type curve was interpreted by the ratio of phase compositions in the precursor, that is, the activity change of the fused iron catalyst depends essentially on the molecule ratio of different iron oxides but not on the atomic ratio of Fe + and Fe +, or Fe +/Fe +, in the precursor under certain promoters. Thus we found that Fei xO based catalyst with wiistite phase structure (Fei xO, 0.04 < x < 0.10) for ammonia synthesis has the highest activity among all the fused iron catalysts for ammonia synthesis. 

During their initial studies about the airmionia synthesis catalysts, Bosch et alA found that the catalysts obtained from the reduction of natural magnetite are better than the catalysts from other iron compoimds. Almquist et alA studied the relation between the activity of iron catalyst and the oxidization degree before its reduction and found that those catalysts, of which the ratio of Fe +/Fe + is closer to be 0.5 and compositions closer to magnetite, has the highest activity. Bridger et alA further studied the fused iron catalysts promoted with binary promoters AI2O3-K2O,... 

In other words, the activity of fused iron catalysts with iron oxides as a precusor relates to not only the content of FeO, but also, more importantly, to its crystal structure of wiistite. When the Fe +/Fe + ratio is smaller than one, although the content of FeO increases the activity decreases, because the crystal structure of wiistite is not yet formed. When the Fe +/Fe + ratio is smaller than 3.15 where the catalyst precursor begins to come to an incomplete structure of wiistite, the activity increases and surpasses strikingly that of the traditional catalyst with Fe +/Fe + at about 0.5. After the Fe +/Fe + reaches five, catalyst precursor forms a complete wiistite structure, while the fused iron catalysts shows its highest activities. Both the activity and reduction behavior are enhanced significantly compared to that of the traditional catalysts. 

The above-mentioned results indicate that, in the reduced fused iron catalysts with the total Fe contents of 91%-92%, there are only 0.54%-0.73% of Fe atoms exposed on the catalyst surface, while the ones that are larger than 99% still need to be imbeded in the inner of body catalysts or covered by promoters. Therefore, absolute great parts of Fe atoms in fused iron catalysts cannot bring into playing role in the catalytic reaction and in fact only play a role as carrier for transfer electron. If more of Fe atoms are exposed on the catalyst surface, then there are possibly the further improvements of the activities of fused iron catalysts. 

After the fused iron catalysts with Fe304 or Fei xO as precursor are reduced, in which the oxygen anions are removed out and the iron cations form the a-Fe crystallites with body centered cubic lattice structures as shown in Fig. 3.76. Both... 

Presently, the Fe-based ammonia synthesis catalysts used in industry are produced by traditional molten method, and are also known as fused iron catalysts. The advantages of molten method include simple processes, easy operation, low cost and excellent performance of product. Even though there are some differences in the chemical composition and catalytic performance of various types of fused iron catalysts, their manufacture process is mainly the same as follow. 12... 

In Chapter 3, the authors proposed the single-phase principle of the preparation of fused iron catalyst. It is clearly pointed out that high catalytic activity can be achieved when wiistite or magnetite phase exists separately in the catalyst. When wiistite and magnetite coexist in the catalyst, the catal3dic activity is always low. In the FeO phase region, the activity of the catalyst decreases due to the formation of a new phase a-Fe in the case of f > 11. 

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