Iron catalysts precipitation type

Catalyst composition also depends on the type of reactor used. Fixed-bed iron catalysts are prepared by precipitation and have a high surface area. A silica support is commonly used with added alumina to prevent sintering. Catalysts for fluidized-bed application must be more attrition-resistant. Iron catalysts produced by fusion best satisfy this requirement. The resulting catalyst has a low specific surface area, requiring higher operating temperature. Copper, another additive used in the preparation of precipitated iron catalysts, does not affect product selectivity, but enhances the reducibility of iron. Lower reduction temperature is beneficial in that it causes less sintering. 

Different Precipitation Catalysts with a NH3 Type Fused Iron Catalyst). 302... 

Schwarzheide—experiments with iron catalysts (comparison of different precipitation catalysts with a Nil 3 type fused iron catalyst). In view of the favorable results obtained by the Coal Research Institute in Miilheim (Fischer Pichler, 1937), industrial companies started experiments on medium-pressure synthesis with iron catalysts. 

Iron-based catalysts are used in both LTFT and HTFT process mode. Precipitated iron catalysts, used in fixed-bed or slurry reactors for the production of waxes, are prepared by precipitation and have a high surface area. A sihca support is commonly used with added alumina to prevent sintering. HTFT catalysts for fluidized bed apphcations must be more resistant to attrition. Fused iron catalysts, prepared by fusion, satisfy this requirement (Olah and Molnar, 2003). For both types of iron-based catalysts, the basicity of the surface is of vital importance. The probability of chain growth increases with alkali promotion in the order Li, Na, K, and Rb (Dry, 2002), as alkalis tend to increase the strength of CO chemisorption and enhance its decomposition to C and O atoms. Due to the high price o Rb, K is used in practice as a promoter for iron catalysts. Copper is also typically added to enhance the reduction of iron oxide to metallic iron during the catalyst pretreatment step (Adesina, 1996). Under steady state FT conditions, the Fe catalyst consists of a mixture of iron carbides and reoxidized Fe304 phase, active for the WGS reaction (Adesina, 1996 Davis, 2003). 

It has been shown that surface basicity of iron-based catalysts greatly affects probability of chain growth.5 7 Alkali metal incorporation promotes the formation of higher carbon number molecules. As described in Dry s review,7 iron-based catalysts are prepared by precipitation techniques and promoted with Cu, K20, and Si02 (typical composition 5 g of K20, 5 g of Cu, and 25 g of Si02 per lOOg of Fe). This type of catalyst is well suited for heavy liquid fuels produced from a low-temperature... 

Each of the catalysts listed earlier in the hydroformylation reaction are found to be catalysts for the reduction of nitrobenzene to aniline the rhodium, osmium, and ruthenium species are particularly eflFective although with osmium higher temperatures must be used. (A. F. M. Igbal previously has reported that several derivatives of rhodium, including Rh6(CO)i6 can act as catalysts for the reduction of nitrobenzene to aniline using CO + H2O (19).) The catalysts listed are also much sturdier than was found in the case of iron carbonyl, and in the experiments listed in Table III the total amount of nitrobenzene and catalyst in a 1000 molar ratio was added to the reaction at the outset. In no case was there observed any precipitate of metal oxides or carbonates, and presumably much higher catalytic turnover numbers could be realized if the reaction were run in a continuous-type reactor. 

Preparation. The catalyst used at Sasol is fairly similar (except for a few changes) to the catalyst that was developed by Ruhrchemie ((7), chapter 7, (20)). Scrap iron, together with copper metal, is dissolved in nitric acid, and the hydrated oxides are precipitated by the addition of a sodium carbonate solution. The type of oxide, the porosity and specific surface area depend on various factors such as the concentrations of the solutions, the precipitating temperature, and the final pH. The precipitate is washed and reslurried with water, and the appropriate amount of potassium water glass is added to yield the required amount... 

The sensitivity to pH is another aspect not always considered. Homogeneous Fenton processes need to operate in a narrow pH range (2.5. 0) and this is a main process issue, as it requires the addition of large amounts of salts to the wastewater for pH control. The use of solid Fenton-type catalysts allows, in principle, operation at natural pH, without the need to adjust the pH, with better process economics. In addition, solid Fenton-type catalysts avoid the need for precipitation of iron and remove the iron sludges after treatment. This is a relevant process issue, especially when the operation is a pre-treatment before further stages such as biological treatments. 

---