Cyclohexane oxidation, vanadium

Although many variations of the cyclohexane oxidation step have been developed or evaluated, technology for conversion of the intermediate ketone—alcohol mixture to adipic acid is fundamentally the same as originally developed by Du Pont in the early 1940s (98,99). This step is accomplished by oxidation with 40—60% nitric acid in the presence of copper and vanadium catalysts. The reaction proceeds at high rate, and is quite exothermic. Yield of adipic acid is 92—96%, the major by-products being the shorter chain dicarboxytic acids, glutaric and succinic acids,and CO2. Nitric acid is reduced to a combination of NO2, NO, N2O, and N2. Since essentially all commercial adipic acid production arises from nitric acid oxidation, the trace impurities patterns ate similar in the products of most manufacturers. 

The second-step oxidation is normally by means of nitric acid, but catalytic air oxidation results in good yields of adipic acid. In recent practice, the refined first-step product of cyclohexane oxidation freed of unconverted hydrocarbon and a 50-60 per cent nitric acid solution containing copper-vanadium catalyst are separately and continuously fed to a jacketed reaction vessel at a ratio such that weight ratio of 100 per cent nitric acid to organic feed is between 2.5 and 6. The reaction mixture is rapidly recirculated through a tubular reactor at 60-80°C, and fresh feed is admitted to give about 5 min time of contact. Yields are improved by reheating the continuously withdrawn effluent stream to 95-100°C for a... 

The oxo-vanadium(rV) complexes [VOCl2 HOCH2C(pz)3 ] (5) and [VO(acac)2(Hpz)] (7), immobilized on a polydimethyl-siloxane (PDMS) membrane, act as supported catalysts for the cyclohexane oxidation (Scheme 22.2) using benzoyl peroxide (BPO), ferf-butyl hydroperoxide, mCPBA, hydrogen peroxide, or the urea-hydrogen peroxide adduct as oxidants (TONs up to 620) [5b]. The best results were obtained with the less polar mCPBA or BPO on account of the hydrophobic character of the membrane that favors their sorption. 

The scorpionate vanadium complexes [VCl3 HC(pz)3 ] (10) and [VCl3 S03C(pz)3 ] (15), which catalyze cyclohexane oxidation with H2O2 (Section 22.2.1,), also operate with dioxygen under solvent-free conditions. Cyclohexane is oxidized to cyclohexanol (the main product) and cyclohexanone (13% conversion), with a high selectivity, typically at the O2 pressure of 15 atm, at 140 °C, 18 h reaction time [6]. The reaction is further promoted (to 15% conversion) by pyrazinecarboxylic acid. The reactions proceed via radical mechanisms with possible involvement of both C-centered and 0-centered radicals. 

Keywords vanadium phosphate, exfoliation, dispersion on alumina, cyclohexane oxidation... 

The question about the competition between the homolytic and heterolytic catalytic decompositions of ROOH is strongly associated with the products of this decomposition. This can be exemplified by cyclohexyl hydroperoxide, whose decomposition affords cyclo-hexanol and cyclohexanone [5,6]. When decomposition is catalyzed by cobalt salts, cyclohex-anol prevails among the products ([alcohol] [ketone] > 1) because only homolysis of ROOH occurs under the action of the cobalt ions to form RO and R02 the first of them are mainly transformed into alcohol (in the reactions with RH and Co2+), and the second radicals are transformed into alcohol and ketone (ratio 1 1) due to the disproportionation (see Chapter 2). Heterolytic decomposition predominates in catalysis by chromium stearate (see above), and ketone prevails among the decomposition products (ratio [ketone] [alcohol] = 6 in the catalytic oxidation of cyclohexane at 393 K [81]). These ions, which can exist in more than two different oxidation states (chromium, vanadium, molybdenum), are prone to the heterolytic decomposition of ROOH, and this seems to be mutually related. 

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. 

SCHEME 146. Vanadium-catalyzed oxidation of cyclohexane with H2O2... 

Air oxidation of /i-butane to maleic anhydride is possible over vanadium phos(4tate and, remaiicably, a 60% selectivity is obtained at 85% conversion. In the gas phase oxidation, in conffast to the situation found in the liquid, n-allcanes are oxidized more rapidly than branched chain alkanes. This is because secondary radicals are more readily able to sustain a chain for branched alkanes the relatively stable tertiary radical is preferentially formed but fails to continue the chain process. Vanadium(V)/ manganese(II)/AcOH has been used as a catalyst for the autoxidation of cyclohexane to adipic acid, giving 25-30% yields after only 4 h. ... 

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. 

Oxidation of cyclohexane Compared to VS-2, V-NCL-1 is more active (TON = 7.3 vs. 2.0) in the oxidation of cyclohexane to cydohexanol and cyclohexanone (Table 2). The HjOj efficiency is also higher on V-NCL-1 (53.3 vs. 33.0). This superior performance may be partly due to the large-pore character and a lower concentration of vanadium in V-NCL-1. On both the catalysts, the mono-functional product selectivity is high (about 94-95%) and the cydohexanol to cydohexanone ratio is simiiar. The catalysts after separation from the products, washing and reactivation were found to be as active as the fresh, caicined samples. 

Most of the catalysts employed in the chemical technologies are heterogeneous. The chemical reaction takes place on surfaces, and the reactants are introduced as gases or liquids. Homogeneous catalysts, which are frequently metalloorganic molecules or clusters of molecules, also find wide and important applications in the chemical technologies [24]. Some of the important homogeneously catalyzed processes are listed in Table 7.44. Carbonylation, which involves the addition of CO and H2 to a C olefin to produce a + 1 acid, aldehyde, or alcohol, uses rhodium and cobalt complexes. Cobalt, copper, and palladium ions are used for the oxidation of ethylene to acetaldehyde and to acetic acid. Cobalt(II) acetate is used mostly for alkane oxidation to acids, especially butane. The air oxidation of cyclohexane to cyclohexanone and cyclohexanol is also carried out mostly with cobalt salts. Further oxidation to adipic acid uses copper(II) and vanadium(V) salts as catalysts. The... 

Nitric acid oxidation with 50-60 per cent acid of the hydrocarbon-free mixture of cyclohexanol and cyclohexanone, obtained from a first-step air oxidation of cyclohexane, in the presence of catalysts of dissolved salts of copper, vanadium, or manganese, results in commercial yields of adipic acid ... 

Figure X, 1. Kinetic curves of cyclohexy hydroperoxide (1) formation and its decomposition to yield cyclohexaiiol (2) and cyclohexanone (i) in the oxidation of cyclohexane fO.46 mol dm ) by the reagent O - II1O2 - vanadium complex - pyrazine-2-carboxylic acid in MeCN at 50 (top) and 70 C (bottom).

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