SYNTHESIS with activated cobalt

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. 

As a general phenomenon, observed already by Fischer and coworkers, activity and FT synthesis selectivity develop in the initial time of a run in a process of Formierung (formation)16—in modem terms self-organization and catalyst restructuring. In order to achieve high performance of synthesis with cobalt as catalyst, the temperature had to be raised slowly up to the temperature of steady-state conversion. A distinct thermodynamically controlled state of the Co surface, populated with reactants and intermediates, can be assumed. This state depends on temperature and particularly on CO partial pressure, and its catalytic nature changes with changing conditions. 

Rhodium and cobalt participate in several reactions which are of value in organic synthesis. Rhodium and cobalt are active catalysts for the reaction of alkenes with hydrogen and carbon monoxide to give aldehydes. This reaction is called hydro-... 

A three-site system for peptide synthesis around a cobalt(III) complex has been studied. Instead of a quadridentate ligand as used in the above experiments, Wu and Busch chose the tridentate ligand diethylenetriamine. The formation of dipeptide and tetrapeptide complexes is shown in Scheme 92.360 The ester carbonyl group in the 0-bonded amide intermediate (127) cannot be activated by coordination because it cannot reach the metal ion. Isomerization to the jV-bonded amide complex (128) occurs with base and enables coordination and therefore activation of the ester carbonyl group. 

Dautzenberg et al. (3) have determined the kinetics of the Fischer-Tropsch synthesis with ruthenium catalysts. The authors showed, that because the synthesis can be described by a consecutive mechanism, the non steady state behaviour of the catalyst can give information about the kinetics of the process. On ruthenium they found that not only the overall rate of hydrocarbon production per active site is small, but also that the rate constant of propagation is low. Hence, Dautzenberg et al. find that the low activity of Fischer-Tropsch catalysts is due to the low intrinsic activity of their sites. On the other hand, Rautavuoma (4) states that the low activity of cobalt catalysts is due to a small amount of active sites, the amount being much smaller than the number of adsorption sites measured. 

Figure 1. H2 and CO chemisorption, and catalytic activity for methanol synthesis by Cu/Zn0/AL203 catalysts with added cobalt.

Wide variations in the yield of tri-O-acetyl-D-glucal obtained by the conventional treatment of 2,3,4,6-tetra-O-acetyI-cr-D-gluco-pyranosyl bromide with zinc dust in aqueous acetic acid have been attributed to differences in the activity of the zinc.6 In some instances, pretreatment of inert samples of zinc with dilute hydrochloric acid rendered them active, but in no instance were copper or platinum salts useful promoters, as has been claimed.1 Treatment of 3,4,6-tri-O-acetyl-l,2-anhydro-D-glucopyranose with sodium cobalt tetracar-bonyl and carbon monoxide in ether gave tri-O-acetyl-D-glucal as the main product,8 but this does not offer a practical alternative synthesis of the compound. 

Medium-pressure synthesis with cobalt catalysts. Earlier publications (18) indicated that the synthesis of hydrocarbons ought to be carried out at atmospheric pressure. The formation of high molecular products seemed to be the reason for rapid inactivation of the catalysts in the case of superatmospheric experiments. The decline of catalyst activity could not be compensated by an increase of the temperature. At higher temperatures, the conversion of carbon monoxide and hydrogen... 

Figure 29 Molar activity of the hydrogenated synthesis product as a function of the carbon number In the cobalt normal pressure synthesis with the addition of ethene [ C] to the synthesis gas (O, n-paraffins , monomethyl-paraffins) (redrawn from Reference 74).

Similar data on the effect of pressure on the durability of iron catalysts are presented by Pichler (37, 74, 86). Iron catalysts usually are much less durable at atmospheric pressure than are cobalt catalysts. As with the cobalt catalyst it is possible, however, by frequent flushings with hydrogen to keep a carefully prepared and pretreated iron catalyst active at atmospheric pressure for 6-12 months (37). The pressure coefficient of the synthesis on iron catalysts is shown in Fig. 11, prepared by Pichler (37) from... 

Tris(propylenediamine)cobalt(III) salts were first synthesized from racemic pro-pylenediamine Later, the synthesis with the optically active base showed that the action of S(+)-pn on [CoCl2(R-pn ] ve a mixture of —)[Co(R-pn)j] and (-t-)[Co(S-pn)3] rather than [Co(R-pn>2(S-pn)p. After all the salts prov to be (+)[Co(S-pn)3]Br3 from the S-base and (—)[Co(R-pn)3]Br3 from... 

In some cases, the cobalt precursor tends to interact with the support. This interaction impedes the generation of active cobalt sites during reduction. Normally, it leaves a fraction of the cobalt chemically inactive. According to Jacobs et al. [1], the strength of the interaction for the three most common supports follows the order y-alumina > titania > silica. The presence of a promoter such as rhenium facilitates reduction of cobalt species interacting with the support [2-4]. However, cobalt is usually not completely reduced after the normal reduction procedures. The effect of rhenium for Fischer-Tropsch synthesis selectivity was recently described in detail by Storsaeter et al. [5]. It was concluded that presence of rhenium shifts the product distribution to heavier compounds, quantified by the C5+ selectivity. 

Catalytic activity of N2O decomposition on various thermally calcined catalysts are reported in Fig.8. The activity followed the order Co-Al-HT>Co-Pe-HT Co-Cr-HT. nie low activity of Co-Cr catalysts can be attributed to low activity of n-type Cr203. Generally for N2O decomposition p-t3T>e oxides are more active than n-tyro oxides owing to the cumulative adsorption in the former [26]. We presume that Cr s regates on the surface in Co-Cr catalysts, and thereby covering the active cobalt sites. Such segregation is also observed for Co-Fe-HT, althou the extent of is small [24]. In the synthesis of hydrocarbons on Co-Cr catalysts, it is reported that even with hi Co contents, a low activity is observed indicating that Co alone is not responsible for the activity [27]. 

Active under mild conditions, palladium catalysts are the more generally used they allow the stereospecific synthesis of vinylic esters [12]. Nickel is sometimes used, but only in polar solvents [15] cobalt is only active under phase transfer conditions combined with activation by irradiation [16]. 

The attempts to achieve enantioselective synthesis of cycloadduct compounds by using a chiral Lewis acid associated with achiral cobaltoxime resulted in moderate Another approach is to replace the dimethylglyoxime ligand set by salen ligands. Cobalt(ii) salen complexes synthesized in achiral form showed good results in Diels-Alder reactivity. In the chiral salen complex, the corresponding optically active dienylcobalt complexes 189 have been obtained by the reaction of optically active cobalt(ll) salen complexes 188 with allenic compounds (Equation (28)). 

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