Cobalt Fischer-Tropsch catalysts, preparation

Very recently Geus and co-workers [44, 45] have applied another method based on chemical complexes. This is the complex cyanide method to prepare both monocomponent (Fe or Co) and multicomponent Fischer-Tropsch catalysts. A large range of insoluble complex cyanides are known in which many metals can be combined, e.g. iron(n) hexacyanide and iron(m) hexacyanide can be combined with iron ions, but also with nickel, cobalt, copper, and zinc ions. Soluble complex ions of molybdenum(iv) which can produce insoluble complexes with metal cations are also known. Deposition precipitation (Section A.2.2.1.5) can be performed by injection of a solution of a soluble cyanide complex of one of the desired metals into a suspension of a suitable support in a solution of a simple salt of the other desired metal. By adjusting the cation composition of the simple salt solution, with a same cyanide, it is possible to adjust the composition of the precursor from a monometallic oxide (the case when the metallic cation is identical to that contained in the complex) to oxides containing one or several foreign elements. 

Another application that has been made of surface area measurements has to do with the preparation of Fischer-Tropsch catalysts. Anderson (41) and his coworkers at the Bureau of Mines are in the process of comparing the surface areas of the reduced and unreduced cobalt and iron oxide catalysts with the activities of the final catalysts. The work is in too early a stage of development to permit any final conclusions to be dra vn because many factors other than surface area may enter into the performance of Fischer-Tropsch catalysts. Nevertheless, the surface area measurements will permit the control of one of the variables that otherwise would have remained an unknown complicating factor. 

Product Selectivity. Tables III and IV summarize the product distributions for these catalysts. In Table III the kieselguhr-supported catalysts are compared. From this it is to be noted that the two most active catalysts for CO conversion are, unfortunately, highly selective towards methanation and hence are poor candidates for meeting the desired objective of producing C2-C4 olefins. The Co-Mn catalysts, however, are comparable with the modified Fischer-Tropsch catalyst (Run No. 19), cobalt-copper-thoria, which was prepared as an alkali-free... 

Even more spectacular results in terms of the increasing importance of nanocatalysis for bulk industrial processes have recently been reported by Kuipers and de Jong [32, 33]. By dispersing metallic cobalt nanoparticles of specific sizes on inert carbon nanofibers the authors were able to prepare a new nano-type Fischer-Tropsch catalyst. A combination of X-ray absorption spectroscopy, electron microscopy, and other methods has revealed that zerovalent cobalt particles are the true active centers which convert CO and H2 into hydrocarbons and water. Further, a profound size effect on activity, selectivity, and durability was observed. Via careful pressure-size correlations, Kuipers and de Jong have found that or cobalt particles of 6 or 8nm are the optimum size for Fischer-Tropsch catalysis. The Fischer-Tropsch process (invented in 1925 at the Kaiser-Wilhelm-Institute for... 

DP based on ligand removal has gained renewed attention recently as preparation of cobalt-based Fischer-Tropsch catalysts was successfully done for several support materials. 

Bezemer, G. L., Radstake, P.B., Koot, V., van Dillen, A. J., Geus, J. W., and de Jong, K. P. 2006. Preparation of Fischer-Tropsch cobalt catalysts supported on carbon nanofibers and silica using homogeneous deposition-precipitation. Journal of Catalysis 237 291-302. 

Kraum, M., and Baems, M. 1999. Fischer-Tropsch synthesis The influence of various cobalt compounds applied in the preparation of supported cobalt catalysts on their performance. Appl. Catal. A Gen. 186 189-200. 

Fischer-Tropsch synthesis could be "tailored by the use of iron, cobalt and ruthenium carbonyl complexes deposited on faujasite Y-type zeolite as starting materials for the preparation of catalysts. Short chain hydrocarbons, i.e. in the C-j-Cq range are obtained. It appears that the formation and the stabilization of small metallic aggregates into the zeolite supercage are the prerequisite to induce a chain length limitation in the hydrocondensation of carbon monoxide. However, the control of this selectivity through either a definite particle size of the metal or a shape selectivity of the zeolite is still a matter of speculation. Further work is needed to solve this dilemna. 

Continuing interest in cobalt catalysts used in the Fischer-Tropsch synthesis has led to the proposal of new methods of catalyst preparation that could determine the selectivity of the catalysts obtained. In this context, a highly selective material to produce C5+ hydrocarbons using a plasma-based method and carbonyl precursors has been prepared [147]. 

It was demonstrated that the production of clean waxy products (i.e. free of any cobalt contamination) during large scale slurry phase Fischer-Tropsch synthesis runs, was successfully effected with cobalt catalysts that were prepared on modified supports (i.e. supports displaying inhibited dissolution behaviour in aqueous environments). As an example, the silicon modification of alumina supports was discussed in detail. 

It has been shown that the dispersion of cobalt on various supports varies with the surface area and nature of the support. In the as-prepared calcined state the cobalt exhibits three reduction phases. Characteristics of the catalysts have been used to explain their Fischer-Tropsch activity. 

Nickel, cobalt, and iron catalysts are cmnmonly used for the Fischer-Tropsch s thesis. Nickel catalysts have been prepared by precipitation from a nitrate solution with potassium carbonate in the presence of thoria and kieselguhr in the proportions lOONiilSThOzilOO kieselguhr. It is not desirable to employ nickel catalysts at low temperatures and elevated pressures because the formation of nickel carbonyl is excessive. In the temperature range of 170-220°C at. low pressures, both liquid and gaseous products are obtained. As the temperature is increased to 300-350°C and the pressure increased to 300-400 psi, nickel catalysts produce only methane. Thus, these catal nsts can be used for making a gas from coal comparable in heating value to natural gas. 

Metallic cobalt is the active material for Fischer-Tropsch synthesis. Catalyst preparation involves impregnation of a metal preeursor on the support, drying, calcination, and finally, in order to transform the inactive oxide into the metallic state, the catalyst is reduced in situ prior to use. 

The power law kinetic equation could be a simplified form of a mechanistic scheme. A summary of some of the reported reaction orders for the partial pressure of hydrogen and carbon monoxide which have been obtained from power law fits by different groups are listed in Table 9. The partial pressure dependencies vary rather widely. The power law fits were obtained for different cobalt catalysts prepared using different supports and methods. The data in Table 9 show that there is not one best power law equation that would provide a good fit for all cobalt catalysts. Brotz [10], Yang et al. [12] and Pannell et al. [13] defined the Fischer-Tropsch rate as the moles of hydrogen plus carbon monoxide converted per time per mass of catalyst (r g+Hj) Wang... 

For the synthesis of higher hydrocarbons by Fischer-Tropsch, cobalt and iron are the most used metals. Under the form of trivalent cations, they have close ionic radii, which allow their crystallization in an ABO3 structure with La in the A sites. The final goal is the formation after reduction (partial or total) of an efficient catalyst for hydrocarbons synthesis. The most simple combination is to synthesize mixed La(Co cFei )03 perovskites. Bedel et al. [36] studied the preparation, by a sol-gel-like method, of these perovskites over the whole range of... 

TI Non-Flory product distributions in Fischer - Tropsch synthesis catalyzed by ruthenium, cobalt. and iron KW Fischer Tropsch synthesis hydrocarbon distribution. Flory kinetics carbon monoxide hydrogenation, chain growth carbon monoxide hydrogenation, ruthenium catalyst carbon monoxide hydrogenation, cobalt iron catalyst hydrocarbon distribution IT Hydrocarbons, preparation... 

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