Alkanes cobalt catalysts

Cyclohexane, the six-carbon ring hydrocarbon with the molecular formula C6H12, is the most significant of the cyclic alkanes. Under ambient conditions it is a clear, volatile, highly flammable liquid. It is manufactured by the hydrogenation of benzene and is used primarily as a raw material for the synthesis of cyclohexanol and cyclohexanone through a liquid-phase oxidation with air in the presence of a dissolved cobalt catalyst. 

Scheme 4 shows a platinum catalyst 1 containing such a bis-SPO bidentate ligand anion, designed for the hydroformylation of ethylene and of 1-heptene, and various other, similarly built, platinum catalysts. Catalyst 1 has an activity comparable to that of the commercial cobalt catalysts that were used at the time and displays a higher selectivity for linear products than the cobalt-containing catalysts (66). Like the latter, the platinum complex exhibits hydrogenation activity to give, in part, alcohols in addition to aldehydes and also produces alkanes (an undesired reaction that implies a loss of feedstock). The catalysts are also active for isomerization, as are the cobalt complexes, and for internal heptene hydroformylation (Table 1), with formation of 60% linear products. 

It is likely that some of the mechanisms that we discussed are operational in parallel, and there are data that confirm this view. It is experimentally observed that the primary alkene alkane molar ratio seems to be in the range between 2 and 4. A value of 4 has been found for reaction on an iron-manganese catalyst operating at 550 K (107), on a precipitated cobalt catalyst operating at 463 K (108), and on a Ru/Si02 catalyst operating at 483 K (109). 

A higher carbon number olefin, 1-hexadecene, was also used as a labeled tracer molecule with the cobalt catalyst. As the carbon number increased from 2 to 16, the fraction of the alkene converted to the corresponding alkane increased, the extent of incorporation into higher products decreased (about 30 percent for ethene versus 6.3 percent for 1-hexadecene) and the fraction undergoing hydrogenolysis (or another type of splitting reaction) increased (4.3 percent for ethene versus 14.1 percent for 1-hexadecene). 

Industrially, the rhodium-catalyzed hydroformylation is normally operated at about 100°, at pressures up to 50 atm and in the presence of a large excess of added phosphorus ligand, it can be carried out in molten PPhs. Under these conditions, a terminal olefin can be converted in over 90% yield to linear aldehyde. By-products include branched aldehydes as well as small amounts of alkanes and isomerized olefins. Advantages over the more conventional cobalt catalysts include lower temperatures and pressures, higher ratios of linear to branched products, and less hydrogenation of aldehyde products to alcohols. 

Porphyrins were first introduced into clays in 1977 by the physical absorption of porphyrin molecules into montmor-illonite in aqueous solutions." The most common examples are the binding of tetracationic M(TMPyP) porphyrins, M = Co(II), Mn(III), Fe(III), into montmor-illonite clays. Co(TMPyP) was the first porphyrin to be intercalated into montmorillonite by ion exchange in acid solution. The interlayer distance expanded from 27 to 37 A upon intercalation. UV-visible studies revealed the retention of cobalt ions in the porphyrin molecules. Mansuy and coworkers have extended this approach and prepared the Mn-porphyrin intercalated materials. These solids are efficient alkene epoxidation and alkane hydroxylation catalysts." Additionally, the catalyst exhibited a marked shape selectivity in favor of small linear alkanes when compared to more bulky substrates. It was also shown that... 

Functionalization of alkanes under mild conditions catalyzed by coordination complexes is known. For example, much of the cyclohexanol and cyclohexanone used in the synthesis of adipic acid is produced by the oxidation of cyclohexane with a cobalt catalyst. Chapter 18, however, focuses on the t) pes of alkane functionalization that occur by the reactions described... 

The second generation processes use rhodium as the metal and the first ligand-modified process came on stream in 1974 (Celanese) and more were to follow in 1976 (Union Carbide Corporation) and in 1978 (Mitsubishi Chemical Corporation), aU using triphenylphosphine (tpp). The UCC process has been licensed to many other users and it is often referred to as the LPO process. Not only are rhodium catalysts much faster - which is translated into milder reaction conditions -, but also their feedstock utilization is much better than that of cobalt catalysts. For example, the cobalt-alkylphosphine catalyst may give as much as 10% of alkane... 

The resulting Fischer-Tropsch process led to both alkanes and alkenes from the reduction of carbon monoxide (CO) by hydrogen (H2) over a cobalt catalyst (which is a mixture of dicobalt octacarbonyl [Co2(CO)8] and cobalt tetracarbonyl hydride [HCo(CO)4] and which is used at temperatures over 120°C and pressures above 200 atm), that is,... 

On the other hand, Tilley et al. have reported a synthesis of a well-defined tris(tert-butoxy)siloxy-iron(lll) complex [13] as well as respective molecular siloxide complexes of cobalt [14] and copper [15], which appear to become precursors for their grafting onto silica and application as catalysts for oxidation of alkanes, alkenes and arenes by hydrogen peroxide. 

The calcined iron-grafted materials exhibit high selectivity as catalysts for oxidations of alkanes, alkenes and arenes with H2O2 as the oxidants [13a]. A similar method has been used by Tilley et al. to prepare a pseudotetrahedral (Co(II) [Co(4,4 -di Bu-bipy) OSi(0 Bu)3 2]) complex grafted onto the SBA-15 surface and subsequently use it in catalytic oxidation of alkylaromatic substrates with tert-butyl hydroperoxide [14]. Unfortunately, neither iron nor cobalt surface organometaUic compounds have been tested in the recycled catalytic system. 

The catalysis of the selective oxidation of alkanes is a commercially important process that utilizes cobalt carboxylate catalysts at elevated (165°C, 10 atm air) temperatures and pressures (98). Recently, it has been demonstrated that [Co(NCCH3)4][(PF6)2], prepared in situ from CoCl2 and AgPF6 in acetonitrile, was active in the selective oxidation of alkanes (adamantane and cyclohexane) under somewhat milder conditions (75°C, 3 atm air) (99). Further, under these milder conditions, the commercial catalyst system exhibited no measurable activity. Experiments were reported that indicated that the mechanism of the reaction involves a free radical chain mechanism in which the cobalt complex acts both as a chain initiator and as a hydroperoxide decomposition catalyst. 

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