Cobalt catalysts distribution

Huang, X. W., Elbashir N. O., and Roberts, C. B. 2004. Supercritical solvent effects on hydrocarbon product distributions from Fischer-Tropsch synthesis over an alumina-supported cobalt catalyst. Industrial Engineering Chemistry Research 43 6369-81. 

FIGURE 11.3 Cobalt catalyst (for reaction conditions, see Figure 11.1) fraction distribution 1/distribution total fraction 13C incorporation. 

The product distribution frcm the Fischer-Tropsch reaction on 5 is shown in Table I. It is similar but not identical to that obtained over other cobalt catalysts (18-21,48, 49). The relatively low amount of methane production (73 mol T when compared with other metals and the abnormally low amount of ethane are typical (6). The distribution of hydrocarbons over other cobalt catalysts has been found to fit the Schulz-Flory equation [indicative of a polymerization-type process (6)]. The Schulz-Flory equation in logarithmic form is... 

These adducts are more active than the iron ones in the conversion of syngas. At 250°C, a higher yield of methane is observed (Table U) and carbon dioxide is produced in smaller amounts. Inspection of Table 5 summarizing the influence of the H2/CO ratio on products selectivity also indicates a higher production of saturated hydrocarbons. This behavior is typical for cobalt catalysts in F-T synthesis (j2,25). The chain-length distribution is similar to that observed for catalysts derived... 

X. Huang, N. O. Elbashir and C. B. Roberts, Supercritical Solvent Effects on Hydrocarbon Product Distributions from Fischer-Tropsch synthesis over an Alumina-Supported Cobalt Catalyst, Ind. Eng. Chem. Res., 2004, 43, 6369-6381. 

The structure-activity relationship for cobalt catalysts in the pyridine synthesis can be summarized in the following manner If the substituent R is a donor, the Co-NMR signals are shifted to higher field and the catalytic activity decreases. If R is an acceptor, the Co-NMR signal is shifted to lower field and the activity increases. Donor substituents are oriented orthogonal to complexed cod in the catalyst precursors acceptors are oriented parallel. The deformation of the spherical charge distribution about cobalt is also dependent on the nature of R. 

The concept of a (bound) formaldehyde intermediate in CO hydrogenation is supported by the work of Feder and Rathke (36) and Fahey (43). Experiments under H2/CO pressure at 182-220°C showed that paraformaldehyde and trioxane (which depolymerize to formaldehyde at reaction temperatures) are converted by the cobalt catalyst to the same products as those formed from H2/CO alone. The rate of product formation is faster than in comparable H2/CO-only experiments, and product distributions are different, apparently because secondary reactions are now less competitive. However, Rathke and Feder note that the formate/alcohol ratio is similar to that found in H2/CO-only reactions (36). Roth and Orchin have reported that monomeric formaldehyde reacts with HCo(CO)4 under 1 atm of CO at 0°C to form glycolaldehyde, an ethylene glycol precursor (75). The postulated steps in this process are shown in (19)—(21), in which complexes not observed but... 

Mechanism A is a generalised mechanism which was proposed for those metals where the frans-but-2-ene cis-but-2-ene ratio was around unity. This mechanism contains a variety of reversible steps which permit the conformational interconversion of the diadsorbed buta-1 3-diene. Consequently, the trans cis ratio will depend upon the relative rates of these reversible steps and the ratio may be much lower than would be expected if the relative surface concentrations of anti- and syn-diadsorbed buta-1 3-diene, species I and III, respectively, in Fig. 37, were similar to the relative amounts of anti- and syn-buta-1 3-diene in the gas phase. It was also suggested that the relative importance of the various steps in mechanism A may be different for different metals. Thus, for example, the type A behaviour of nickel and cobalt catalysts, as deduced from the butene distributions and a detailed examination of the butene AAprofiles [166], was... 

The liquid-phase oxidation of toluene with molecular oxygen is another example of a well established process (Table 4, entry 40). A cobalt catalyst is used in the process and the reaction proceeds via a free-radical chain mechanism. Heat of reaction is removed by external circulation of the reactor content and both bubble columns or stirred tanks are employed. It is important to note that air distribution is critical to prevent the danger of a runaway. Another example of direct oxidation is the commercial production of nitrobenzoic acid by oxidation of 4-nitrotoluene with oxygen (Table 4, entry 41). 

In a recent study, R. Pettit et at. examined the validity of tire Fischer-Tropsch carbide mechanism, the Anderson-Emmett hydroxy carbene mechanism and the Pichlcr-Schulz mediaiiism [174. In a first experiment, the Schulz Flory distribution obtained by CO/H conversion over a cobalt catalyst in the absence and in the presence of CH N] was studied. It was found that addition of CHjN resulted in a signillcant increase of the propagation rate which is in favour of the assumption of methylene as a building block, as predicted by the carbide mechanism. Furthermore, the reaction was carried out using labeled CO (90% CO and 10% CO), H2. and CHjNj in variable ratios. The number of atoms in the propenc fraction was calculated according to the three... 

Under these conditions, the linear to branched aldehyde ratio for the hydroformylation of 1-octene was 1.9 1. Starting with 4-octene one still gets a 1.2 1 linear to branched ratio. Thus, one can start with a considerably less-expensive mixture of terminal and internal alkenes and get a product distribution favoring the linear aldehyde. The product distribution in Scheme 3 can be nicely explained by invoking facile alkene isomerization, with the fastest hydroformylation occurring for double bonds in the 1-position. Labeling studies have shown that alkene isomerization generally occurs without dissociation of the alkene from the cobalt catalyst. ... 

A value of c equal to 0.3, previously used to describe FT selectivity data on Ru catalysts (4), was also chosen here to describe the behavior of cobalt catalysts. This equation for hydrocarbon diffusion in melts reflects the strong influence of molecular size in reptation and entanglement models of transport in such systems (IJ6). Our model also requires the input of intrinsic values for jSn (given by the asymptotic j8r), jSo, j8r, and j8s, measured independently. After such parameters are specified, the model yields a non-Flory carbon number distribution of increasingly paraffinic hydrocarbons that agrees well with our experimental observations (Fig. 16). 

Fig. 18, Support effects on carbon number distribution for cobalt catalysts (473 K, H2/ CO = 2,1, 2000 kPa, 50-60% CO conversion).

In the heterogeneous catalysts this would be no more than a formal representation of the distribution of electronic charge in the active sites, but with soluble catalysts from (vr-Cj H5 [2 TiCl2 /AlMe2 Cl there is evidence from electrodialysis studies that the active species possesses a positive charge and probably has the structure (tt-CsHs )2Ti Me [27]. This type of structure is consistent with certain features of butadiene polymerization by soluble nickel and cobalt catalysts [28]. 

The behaviour of CO2 in Fischer-Tropsch synthesis was investigated using a promoted iron and a promoted cobalt catalyst. The decrease in yield of hydrocarbons is more pronounced on cobalt than on iron. The product distribution on iron remains nearly constant with increasing CO2 concentration, however on cobalt the selectivity to methane increases dramatically. 

Friedel, R.A. and Anderson, R.B. Composition of synthetic liquid fuels. I. Product distribution and analysis of C5-C8 paraffin isomers from cobalt catalyst. Journal of American Chemical Society, 1950, 72, 1212. 

The effect of Mn as promoter for CNF-supported cobalt catalysts was studied by impregnating a parent 9.5 wt% Co catalyst with different amounts of MnO (ranging from 0.03 to 1.1 wt%) [124]. It was found that manganese hindered cobalt reduction, the cobalt surface remaining more oxidic in character in the presence of Mn. The catalytic performance was affected differently in tests carried out at 1 and 20 bar. At atmospheric pressure, the chain growth probability increased, and the product distribution shifted toward olefinic products... 

Results were also obtained for the conversion of syngas containing C-labeled eth-ene or propene using a precipitated promoted iron catalyst. In addition, a fused iron catalyst was employed in a run with labeled ethene at 20 atm pressure. They found that the cracking reaction of ethene was of secondary importance with the iron catalyst, unlike the case with cobalt. The distribution of the synthesis products from C-ethene showed that about 50 percent of the transformation was to the C3 product the transformation to higher hydrocarbons decreased much quicker than for the cobalt normal pressure synthesis (Figure 33). With the addition of C-ethene the iso-paraffins had a lower activity than the normal paraffins this is consistent with the data for cobalt (Figure 34). 

By assuming the reaction scheme shown in Figure 48, transient differential equations were formulated for the fraction of in Cj, C2, and C3 products dependent on the time constants for the intermediates. A fit of the experimental curves to the derived equations gave the values of the time constants Xj, tj, T2, and T3. For the cobalt catalysts, the value of Xf, was 1 s while the values of Xj, X2, and X3 were dl the same (15 s) in conformity with the ASF distribution. Further, the values of the time constants were independent of the D2/CO ratio. 

Figure 3Cyd, Comparison of F-T gaseous product distributions for cobalt catalysts, carbon-atom % (H /CO = 3.0 for all runs, 7.8 atm (100 psig). (c) (left) Run No. 34. (d) (above) Run No. 35.

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