Cobalt catalysts, heterogeneous

FIGURE 9.26 Fischer-Tropsch synthesis vs. Oxo synthesis on cobalt catalysts. The thermodynamically controlled shift from heterogeneous to homogeneous catalysis. 

Raney predicted that many other metal catalysts could be prepared with this technique, but he did not investigate them [8], Copper and cobalt catalysts were soon reported by others [4,5], These catalysts were not nearly as active as Raney s nickel catalyst and therefore have not been as popular industrially however they offer some advantages such as improved selectivity for some reactions. Skeletal iron, ruthenium and others have also been prepared [9-13], Wainwright [14,15] provides two brief overviews of skeletal catalysts, in particular skeletal copper, for heterogeneous reactions. Table 5.1 presents a list of different skeletal metal catalysts and some of the reactions that are catalyzed by them. 

The hydroformylation of alkenes was accidentally discovered by Roelen while he was studying the Fischer-Tropsch reaction (syn-gas conversion to liquid fuels) with a heterogeneous cobalt catalyst in the late thirties. In a mechanistic experiment Roelen studied whether alkenes were intermediates in the "Aufbau" process of syn-gas (from coal, Germany 1938) to fuel. He found that alkenes were converted to aldehydes or alcohols containing one more carbon atom. It took more than a decade before the reaction was taken further, but now it was the conversion of petrochemical hydrocarbons into oxygenates that was desired. It was discovered that the reaction was not catalysed by the supported cobalt but in fact by HCo(CO)4 which was formed in the liquid state. 

Fell also described the hydroformylation of fatty acids with heterogenized cobalt carbonyl and rhodium carbonyl catalysts [37]. The products of the reaction with polyunsaturated fatty acids were, depending on the catalyst metal, poly- or monoformyl products. The catalyst carrier was a silicate matrix with tertiary phosphine ligands and cobalt or rhodium carbonyl precursors on the surface. The cobalt catalyst was applied at 160-180°C and gave mostly monofunctionalized fatty acid chains. With linoleic acid mixtures, the corresponding rhodium catalyst gave mono- and diformyl derivatives. Therefore, the rhodium catalyst was more feasible for polyfunctionalized oleocompounds. The reaction was completed in a batch experiment over 10 h at 100 bar and 140°C rhodium leaching was lower than 1 ppm. 

In the first experiment, no carbon monoxide was present. The reduction proceeded smoothly the calculated pressure drop was secured and butanol was isolated as the reaction product. This experiment demonstrated the activity of the reduced metallic cobalt catalyst for the usual type of heterogeneous catalysis. 

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 synthesis of cyclohexanone, which is an intermediate in the manufacture of nylon 6 and nylon 6,6 is an important industrial process [1], One of the major current routes for the synthesis of cyclohexanone is the liquid-phase autoxidation of cyclohexane at 125-160 °C and 10 bar followed by the selective decomposition of the intermediate cyclohexyl hydroperoxide, using a soluble cobalt catalyst, to a mixture of cyclohexanol and cyclohexanone [2]. These severe conditions are necessary due to the low reactivity of cyclohexane towards autoxidation. Due to the high reactivity of the products in the autoxidation step conversions must be kept low (<10%) [3,4]. Heterogeneous catalysts potentially offer several advantages over their homogeneous counterparts, for example, ease of recovery and recycling and enhanced stability. Recently we found that chromium substituted aluminophosphate-5 and chromium substituted silicalite-1 (CrS-1) are active, selective and recyclable catalysts for the decomposition of cyclohexyl hydroperoxide to cyclohexanone [5j. 

A heterogeneous cobalt catalyst was employed for arylations of styrene (2) and two acrylates with aryl iodides. Generally, isolated yields were significantly lower than those observed for heterogeneous nickel catalysts [24]. Further, a silica-supported poly-y-aminopropylsilane cobalt(II) complex was reported as a highly active and stereoselective catalyst for Mizoroki-Heck-type reactions of styrene (2) and acrylic acid (16) using aryl iodides [23,25]. 

Heterogenous additives such as molecular sieves or preabsorbtion of the cobalt catalyst onto charcoal" have also been employed to accelerate the PKR. As with other additives, the current view is that these additives promote the dissociation of, or trap, CO to provide the reactive catalyst or increase the concentration of the alkene coordinated intermediate 1-3. 

HCo(CO)4 can be prepared directly from Co2(CO)g under hydroformylation conditions [11]. Alternatively, other precursors, particularly tvater-soluble salts such as Co(OAc)2, Co(HCOO)2, or Co(ethylhexanoate)2, have been suggested for technical scale processes. These Co salts are reduced to Co" under the effect of H2. The catalyst formation can be accelerated by the addition of aqueous non-miscible alcohols such as 2-ethylhexanol or isononanol [12]. The generation from water-soluble Co " salts is especially useful for the preparation of cobalt catalysts anchored on heterogeneous surfaces [13]. 

The addition of other metals to the heterogeneously cobalt-catalyzed reaction can have a beneficial effect on hydroformylation. For example, small amounts of ruthenium added to a carbon-supported cobalt catalyst (Co/AC) increased activity as well as Hb selectivity [64]. The effect was rationalized by the high dispersion and reducibUity of supported cobalt. When ruthenium was added, small particles of an unbalanced alloy were formed. These particles keep more CO in a nondisso-ciative state and lower the surface hydrogen pressure. This was in contrast to the related but uniformly distributed Pt-Co or Pd-Co alloys. Activity and regioselectivity increased with increased Ru loading. 

Similar yields were obtained with a heterogenized cobalt catalyst prepared by copol5nneiization of Co(AAEMA)2 [2-(acetoacetoxy)ethyl methacrylate 1-)] with A,A-dimethylacrylamide and MA -methylenebis-(acrylamide). ... 

Robert D.A., Geoffroy G.L. Compounds with heteronuclear bonds between transition metals. In Comprehensive Organometallic Chemistry, Wilkinson G., Stone F.G.A., Abel E.W. Eds., Perga-mon, Oxford, UK, 1982 Vol. 6, pp. 821-877 and references therein Rogovin, M., Neumann, R. Silicate xerogels containing cobalt as heterogeneous catalysts for the side-chain oxidation of alkyl aromatic compounds with tert-butyl hydroperoxide. J. Mol. Catal. A Chem. 1999 138 315-318... 

---