Hydrogenation, of CO to hydrocarbons

IT -cyclopentad ienyldicar bony 1 cobalt, CpCo(C0)2 This material is active in the hydrogenation of CO to saturated linear hydrocarbons and appears to retain its "homogeneous", mononuclear character during the course of its catalysis. 

Many of the catalysts which are usefiil in Fischer-Tropsch synthesis are also capable of catalyzing the hydrogenation of CO2 to hydrocarbons. Our structure-function studies have shown that it is possible to control the selectivity of CO2 hydrogenation by specific iron-based catalysts to generate yields of C5+ hydrocarbons that are comparable to those produced with conventional CO based... 

A potential way to use CO2 as the carbon source for the synthesis of organic compounds is the hydrogenation of CO2 via Fischer-Tropsch (FT) synthesis using a CO2 rich synthesis gas. Some indication can be found in the literature that the hydrogenation of CO2 to hydrocarbons proceeds via CO as an intermediate [1], which means that the catalyst must have a high activity for the reverse CO shift reaction (1) together with good properties for the FT reaction (2). 

The C-0 bond can be more easily activated when the CO molecule interacts with more than two metal atoms. Recently, the dissociative adsorption of carbon monoxide by polynuclear metal complexes, such as [(silox)2TaH2]2 (Eq. 57) [126, 127] and [(silox)2WCl]2 (Eq. 58) [126-129], and tetratungsten alkoxides [129] has been achieved. Hydrogenation of CO to give hydrocarbons promoted by metal clusters has been reviewed [130]. 

Metal carbonyl dusters (e.g. of Rh) can be used as precursors to form catalysts for reactions involving CO, induding the water gas shift reaction, alkene hy o-formylation, and CO hydrogenation. Although the catalysts exhibit some unusual selectivities, such as in the hydrogenation of CO to give non Schulz-Flory distributions in their hydrocarbon products, they are not highly active relative to some of the more conventional catalysts. The spedes in the zeolites that are formed from the cluster precursors and which are the actual catalytically active spedes have not yet been eluddated. 

Steam reforming of CH4 CH4 + H2O = CO + 3H2 NH3 synthesis from the elements Hydrogenation of CO and CO2 to form hydrocarbons (Fischer-Tropsch syndresis)... 

Carbonaceous species on metal surfaces can be formed as a result of interaction of metals with carbon monoxide or hydrocarbons. In the FTS, where CO and H2 are converted to various hydrocarbons, it is generally accepted that an elementary step in the reaction is the dissociation of CO to form surface carbidic carbon and oxygen.1 The latter is removed from the surface through the formation of gaseous H20 and C02 (mostly in the case of Fe catalysts). The surface carbon, if it remains in its carbidic form, is an intermediate in the FTS and can be hydrogenated to form hydrocarbons. However, the surface carbidic carbon may also be converted to other less reactive forms of carbon, which may build up over time and influence the activity of the catalyst.15... 

Figure 8. Differential tunneling spectrum of CO on rhodium/alumina heated to k20° K in hydrogen. Modes due to hydrocarbon are number 1 to 7 The hydrocarbon species is identified as an ethylidene moiety.

The metals known to be active for hydrogenating carbon monoxide to hydrocarbons and alcohols are Fe, Ni, Co and Ru. Rhodium is also receiving considerable attention by many research workers for the production of alcohols. The relative prices of these metals are given in Table I. 

Good evidence has been obtained that heterogeneous iron, ruthenium, cobalt, and nickel catalysts which convert synthesis gas to methane or higher alkanes (Fischer-Tropsch process) effect the initial dissociation of CO to a catalyst-bound carbide (8-13). The carbide is subsequently reduced by H2to a catalyst-bound methylidene, which under reaction conditions is either polymerized or further hydrogenated 13). This is essentially identical to the hydrocarbon synthesis mechanism advanced by Fischer and Tropsch in 1926 14). For these reactions, formyl intermediates seem all but excluded. 

The second mechanism is that the higher hydrocarbons are produced from the hydrogenation of CO, the dissociated product of C02 [66, 71], in a way similar to... 

Analysis of the tunneling spectra of the hydrocarbons formed by exposing the samples to H2 ( Fig. 14 ) showed two different species, one a formate like ion, the other an ethylidene ( CHCH3 ) species. The formate ion is not thought to be an active intermediate in hydrocarbon synthesis, but the ethylidene species may well be a catalytic intermediate. Kroeker, Kaska and Hansma were then able to suggest a reaction pathway for the hydrogenation of CO on a supported rhodium catalyst consistent with the formation of ethylidene as a catalytic intermediate. 

Catalytic hydrogenation of C02 is one of the most promising approaches to C02 fixation. The hydrogenation of C02 to CO, hydrocarbons, and... 

For any given catalytic reaction the active surface area is normally only a small fraction of the area of the active component (active phase). The term active sites is often applied to the sites effective for a particular heterogeneous catalytic reaction. The terms active site and active centre are often used as synonyms, but active centre may also be used to describe an ensemble of sites at which a catalytic reaction takes place. There is evidence that the centres required for some catalytic reactions are composed of a collection of several metal atoms (ensemble). This appears to be the case for such reactions as, for example, hydrogenolysis, hydrogenation of CO, and certain deuterium-exchange processes with hydrocarbons. 

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