Cobalt catalysts supported, selectivity

Whereas the effect of water on deactivation and on the overall activity of the FTS varies with the support, similar effects of water on the selectivity is reported for all catalysts, to a certain degree independent of the support, promoter and conditions. The effect can be summarized as an increase in C5 + selectivity, a decrease in methane selectivity, and in some instances a weak enhancement of the C02 selectivity is observed. Fig. 4 illustrates the effect on the C5 + and methane selectivity of adding water to cobalt catalysts supported on alumina, silica and titania, and both unpromoted and Re-promoted catalysts are shown. At the outset these selectivities are strong functions of the conversion, the C5 + selectivity increasing and the methane decreasing with increasing conversion, as illustrated by the trendlines in the figures. The points for methane are below, and C5 + -selectivity is above the line when water is added. Similar results were reported by many authors for alumina-supported catalysts,16-19 23 30 silica-supported catalysts,30 37 46-48 and titania-supported catalysts.19 30... 

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

Yu, Z., Borg, 0., Chen, D., Enger, B. C., Frpseth, V., Rytter, E., Wigum, H., and Holmen, A. 2006. Carbon nanofiber supported cobalt catalysts for Fischer-Tropsch synthesis with high activity and selectivity. Catalysis Letters 109 43 -7. 

Fig. 4 Effect of added water on the C5 + selectivity (filled symbols) and CH4 selectivity (open symbols) as a function of CO conversion at different conditions for Co/A1203 (A), CoRe/Al203 (B), Co/Si02 (C), CoRe/Si02 (D), Co/Ti02 (E), and CoRe/Ti02 (F). Before water addition ( , ), 20% water added ( , O), 33% water added (A, A) and after water addition ( , O).19 Reprinted from Journal of Catalysis, Vol. 231, S. Storsaeter, 0. Borg, E. A. Blekkan and A. Holmen, Study of the effect of water on Fischer-Tropsch synthesis over supported cobalt catalysts, pp. 405-419. Copyright (2005), with permission from Elsevier.

X. Huang and C. B. Roberts, Selective Fischer-Tropsch synthesis over an A1203 supported cobalt catalyst in supercritical hexane, Fuel Process Technol., 2003, 83, 81-99. 

Cobalt-based catalysts are effective in the ethanol reformation to hydrogen. Many oxides have been used to prepare supported cobalt catalysts of low cobalt content (circa 1 wt%) by impregnation from a solution of Co2(CO)8 catalysts were used in the ethanol reformation as prepared [156]. The performance of the catalysts in the steam reforming of ethanol was related with the presence, under reaction conditions, of metallic (ferromagnetic) cobalt particles and oxidized cobalt species. An easy exchange between small metallic cobalt particles and oxidized cobalt species was found. Comparison of Co/ZnO catalysts prepared from Co2(CO)8 or from nitrate precursor indicated that the catalyst prepared from the carbonyl precursor was highly stable and more selective for the production of CO-free hydrogen... 

While there have been much activity in the literature addressing Fe, Ru and Ni F-T catalysts, the largest body of papers and patents in the last three decades have dealt with Co-based F-T catalysts in attempts to make more active catalysts with high wax selectivities. It is, however, remarkable to notice that modern Co F-T catalysts are still very similar to the ones prepared by Fischer and co-workers i.e., they consist of promoted cobalt particles supported on a metal oxide and most of, if not all, Co-based F-T catalyst compositions contain the following components ... 

Indeed, eobalt and a promoter metal may form an integral metal particle deposited on the support oxide, altering the electronic properties of the surface cobalt metal atoms (Figure 4C). Depending on the promoter element added to the Co cluster, alloying might lead to an increased catalyst activity, selectivity, as well as stability. 

The Fischer-Tropsch (FT) synthesis leads to a broad range of products, i.e. hydrocarbons, alcohols, acids, esters, etc. The increasingly stringent regulations on the sulfur and aromatics content in fuels are the reasons for renewed interest in this reaction [1]. More efficient catalysts are required to improve FT activity and selectivity to the desired products. Cobalt catalysts have been found to be most suitable for the production of higher hydrocarbons [2]. Optimization of the metal function (Co, Fe, Ru, Mo) in supported FT catalysts has been studied in a large number of papers [3-6]. 

On the other hand, it is known that catalyst support exerts a great influence on the catalytic properties of the metallic particles deposited on it during the carbon dioxide reforming of methane. So for a given metal, catalytic activities can be changed [5], product selectivities modified [6] and carbon deposition resistivity altered [7]. Also the addition of certain promoters can improve the catalytic behavior of a given supported metal catalyst. In particular we have shown the benefit of the MgO addition to cobalt catalyst [ 8, 9]. 

A homogeneous catalytic solution to the alcohol inhibition problem (see the discussion under Uncatalyzed chain reactions of the oxidation of alcohol intermediates, above) does not appear to have been found. However, the presence of a heterogeneous oxidative dehydrogenation catalyst has been reported to be effective in the direct oxidation of alcohols to carbonyls and acids [109, 110]. The mechanism probably involves preliminaiy heterogeneous (oxidative) dehydrogenation of carbinols to carbonyls. If the carbonyl is an aldehyde, it is readily converted to the acid. Platinum, palladium, ruthenium, rhodium, and iridium catalysts, supported on carbon, are reported to be active and selective catalysts for the purpose [109]. Promoters such as cobalt and cadmium have been reported to be effective additives. 

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