Cyclohexane selective oxidation

The selective oxidations of the terminal positions of -alkanes are an example of substrate-shape selectivity. Product-shape selectivity has been used to enhance the selectivity of the type IIaRH oxidation of cyclohexane [66-68], For example, oxidation of cyclohexane at 373 K for 8 hr using FeAlPO-31 (pore aperture 5.4 A) as a catalyst resulted in 2.5% conversion to a mixture which contained 55.3% of adipic acid and 37.3% of a mixture of cyclohexanol and cyclohexanone [68]. In contrast, oxidation under identical conditions using FeAlPO-5 (pore aperture 7.3 A) resulted in only 9.2% of adipic acid and 89.5%... 

The selective oxidation of saturated hydrocarbons is a reaction of high industrial importance. Besides a variety of other oxidants, hydrogen peroxide as a very clean oxidant has also been used for these purposes . As an example, in 1989 Moiseev and coworkers reported on the vanadium(V)-catalyzed oxidation of cyclohexane with hydrogen peroxide (Scheme 146) . When the reaction was carried out in acetic acid cyclohexanol and cyclohexanone were formed, bnt conversions were very poor and did not exceed 13%. Employing CF3COOH as solvent, complete conversions could be obtained within 5 min-ntes. Here, cyclohexyl trifluoroacetate was the main product (85% of the products formed) resulting from the reaction of cyclohexanol (the primary product of the oxidation) with CF3COOH. 

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. 

The influence of various physical parameters was studied in the case of the oxidation of cyclohexane selected as a model reaction. Since the main product is cyclohexanone, its rate of formation was chosen as representative of the photocatalytic activity of the system. 

Although cyclohexane oxidation dominates the market, because of cheaper raw materials, the hydrogenation of phenol remains competitive, offering better selectivity with fewer environmental and safety problems. In addition, this process allows efficient valorization of phenol-rich wastes from coal industries. Recently built plants make use of this technology, as reported by the engineering group Aker-Kvaerner (www.kvaerner.com, 2004). The availability of low-price phenol is the most important element for profitability. Besides the well-known cumene process, a promising route is the selective oxidation of benzene with N20 on iron-modified ZSM-5 catalyst [12]. In this way, the price of phenol may become independent of the market of acetone. 

The active oxidant was proposed to be a Ru(V)=0 species and access of benzene towards the Ru=0 bond is facilitated by the flat structure of the salicyldiimine ligand (see Fig. 8). This catalytic system was also applied to the epoxidation of stilbene, C-H bond activation of cyclohexane or cyclohexene and the oxidation of tetrahydrofuran to y-butyrolactone [37]. We conclude however, that a suitable and catalytic system for the selective oxidation of benzene to phenol has not yet been forthcoming. 

In the cyclohexane oxidation route cyclohexane is oxidized with air at 125-126°C and 8-15 bar in the liquid phase using Co or Mn naphthanates as the catalyst. This affords a mixture of cyclohexanol and cyclohexanone via a classical free radical autoxidation mechanism. Cyclohexane conversion is limited to 10-12% in order to minimize by-product formation via further oxidation. The selectivity to cyclohexanol/cyclohexanone is 80-85%. 

Recently the Co/Mn/N-hydroxyphthalimide (NHPI) systems of Ishii have been added to the list of aerobic oxidations of hydrocarbons, including both aromatic side chains and alkanes. For example, toluene was oxidized to benzoic acid at 25°C [125] and cyclohexane afforded adipic acid in 73% selectivity at 73% conversion [126], see Fig. 4.46. A related system, employing N-hydroxysac-charine, instead of NHPI was reported for the selective oxidation of large ring cycloalkanes [127]. 

The entrapped complexes are known to catalyze selective oxidation or hydrogenation reactions, depending mainly on the complexed transition metal cation [4, 82, 84]. Recently, two exciting examples have been published describing the synthesis of adipic acid from cyclohexene [93] or even from cyclohexane [94], respectively (cf Figure 6). 

The selective oxidation of cyclohexane to cyclohexanol and cyclohexanone has been studied by Suo and co-workers [395,396]. This reaction is a key process in the chemical industry, for the oxidation products of cyclohexane, via cyclohexanol and cyclohexanone, are important intermediates in the manufacture of nylon-6 and nylon-66 polymers and are also used as solvents for lacquers, shellacs and varnishes as well as stabilisers and homogenisers for soaps and S3mthetic detergent emulsions. 

TABLE 1. Selective oxidation of cyclohexane with molecular oxygen over MnAPO-5 in different solvents. ... 

Cyclohexane is obtained either by the hydrogenation of benzene, or from the naphtha fraction in small amounts. Its oxidation to the KA Oil dates back to 1893 and was first industrialized by DuPont in the early 1940s. Oxidation is catalyzed by Co or Mn organic salts (e.g., naphthenate), at between 150 and 180 °C and 10-20 atm. Indeed, this reaction is a two-step process (an oxidation and a deperoxidation step), and two variants are currently in use [2,3]. The oxidation step can be performed with or without a catalyst. The deperoxidation step always uses a catalyst (Co(II) or NaOH). The overall performance of both variants is almost identical, although the selectivity in the individual steps may be different. For example, in a first reactor, cyclohexane is oxidized to cyclohexylhydroperoxide the concentration of the latter is optimised by carrying out the oxidation in passivated reactors and in the absence of transition metal complexes, in order to avoid the decomposition of the hydroperoxide. In fact, the synthesis of the hydroperoxide is the rate-limiting step of the process, and, on the other hand, alcohol and ketone are more reactive than cyclohexane. The decomposition of the hydroperoxide is then carried out in a second reactor, in which the catalyst amount and reaction conditions are optimised, thus allowing the Ol/One ratio to be controlled. 

Selective Oxidation of Cyclohexane over Rare Earth Exchanged Zeolite Y... 

We have shown that cyclohexane can be selectively converted to cyclohexanone. In this reaction, the intermediate (cyclohexanol) serves as the electron source. What is important is that this selective oxidation was achieved in a conventional batch reactor. 

Although La-ZSM-5 has been reported as a selective catalyst in the oxidation of phenol with N20 to give / -hydroquinone with a selectivity of 82.1% (Deng et al. 1997), Ce-containing microporous materials have been the most studied catalysts in redox processes. Thus, Ce-Y zeolite has been used in the complete oxidation of methylene chloride in air (Chatteijee and Greene 1991). However, Ce-modified zeolites can also be used in selective oxidation reactions. This is the case for the selective oxidation of /7-xylene at 130°C on Ce-containing Mordenite (Hasimoto et al. 1997) or the selective oxidation of cyclohexane on Ce-exchange Y (Pires et al. 1997). Recently, it has been... 

Pires, E.L., M. Wallau and U. Schuchardt, 1997, Selective oxidation of cyclohexane over rare earth exchanged zeolite Y, in 3rd World Congr. on Oxidation Catalysis, eds R.K. Grasselli, S.T. Oyama, A.M. Gaflhey and J.E. Lyons, Vol. 110 of Studies in Surface Science and Catalysis (Elsevier, Amsterdam) pp. 1025-1027. 

The oxidation of various hydrocarbons such as n-octane, cyclohexane, toluene, xylenes and trimethyl benzenes over two vanadium silicate molecular sieves, one a medium pore VS-2 and the other, a novei, iarge pore V-NCL-1, in presence of aqueous HjOj has been studied. These reactions were carried out in batch reactors at 358-373 K using acetonitrile as the solvent. The activation of the primary carbon atoms in addition to the preferred secondary ones in n-octane oxidation and oxidation of the methyl substituents in addition to aromatic hydroxyiation of alkyl aromatics distinguish vanadium silicates from titanium silicates. The vanadium silicates are also very active in the secondary oxidation of alcohols to the respective carbonyl compounds. V-NCL-1 is active in the oxidation of bulkier hydrocarbons wherein the medium pore VS-2 shows negligible activity. Thus, vanadium silicate molecular sieves offer the advantage of catalysing selective oxidation reactions in a shape selective manner. 

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