Cyclohexane oxidation products

If an excess of solid triphenylphosphine is added to a solution of alkane (for example, cyclohexane) oxidation products 10-20 min before the GC analysis, the resulting chromatogram differs drastically from that of a sample not subjected to the reduction with triphenylphosphine. After the reduction, the cyclohexanol peak rises markedly while the intensity of the cyclohexanone peak decreases. The sum of alcohol and ketone concentrations in the reduced sample is approximately equal to the total concentration of products in the solution untreated with triphenylphosphine. These results can be explained by the fact that the mixture of products of the reaction under discussion contains cyclohexyl hydroperoxide as the main component. 

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. 

Caprolactam [105-60-2] (2-oxohexamethyleiiiiriiQe, liexaliydro-2J -a2epin-2-one) is one of the most widely used chemical intermediates. However, almost all of the aimual production of 3.0 x 10 t is consumed as the monomer for nylon-6 fibers and plastics (see Fibers survey Polyamides, plastics). Cyclohexanone, which is the most common organic precursor of caprolactam, is made from benzene by either phenol hydrogenation or cyclohexane oxidation (see Cyclohexanoland cyclohexanone). Reaction with ammonia-derived hydroxjlamine forms cyclohexanone oxime, which undergoes molecular rearrangement to the seven-membered ring S-caprolactam. 

Dutch State Mines (Stamicarbon). Vapor-phase, catalytic hydrogenation of phenol to cyclohexanone over palladium on alumina, Hcensed by Stamicarbon, the engineering subsidiary of DSM, gives a 95% yield at high conversion plus an additional 3% by dehydrogenation of coproduct cyclohexanol over a copper catalyst. Cyclohexane oxidation, an alternative route to cyclohexanone, is used in the United States and in Asia by DSM. A cyclohexane vapor-cloud explosion occurred in 1975 at a co-owned DSM plant in Flixborough, UK (12) the plant was rebuilt but later closed. In addition to the conventional Raschig process for hydroxylamine, DSM has developed a hydroxylamine phosphate—oxime (HPO) process for cyclohexanone oxime no by-product ammonium sulfate is produced. Catalytic ammonia oxidation is followed by absorption of NO in a buffered aqueous phosphoric acid... 

The oxidation of cyclohexane to a mixture of cyclohexanol and cyclohexanone, known as KA-od (ketone—alcohol, cyclohexanone—cyclohexanol cmde mixture), is used for most production (1). The earlier technology that used an oxidation catalyst such as cobalt naphthenate at 180—250°C at low conversions (2) has been improved. Cyclohexanol can be obtained through a boric acid-catalyzed cyclohexane oxidation at 140—180°C with up to 10% conversion (3). Unreacted cyclohexane is recycled and the product mixture is separated by vacuum distillation. The hydrogenation of phenol to a mixture of cyclohexanol and cyclohexanone is usually carried out at elevated temperatures and pressure ia either the Hquid (4) or ia the vapor phase (5) catalyzed by nickel. 

The alternative route involves the air oxidation of cyclohexane and proceeds via the production of a mixture of cyclohexanol and cyclohexanone often known as KA oil. It was in the cyclohexane oxidation section of the caprolactam plant of Nypro Ltd that the huge explosion occurred at Flixborough, England in 1974. 

A solution of the quinoxaline 1-oxide (4 mmol) in cyclohexane was degassed by boiling and passing N2 through and irradiated with a medium-pressure water-cooled Hg lamp (Hanau TQ 150), equipped with a Pyrex filter, until conversion was complete. The solvent was evaporated in vacuo at 20 C and the residue was extracted with a small amount of cyclohexane. The product was deposited on strong cooling. Attempted chromatography resulted in the formation of AfW-diacylbenzene-l,2-diamines. 

The production of alcohols by the catalytic hydrogenation of carboxylic acids in gas-liquid-particle operation has been described. The process may be based on fixed-bed or on slurry-bed operation. It may be used, for example, for the production of hexane-1,6-diol by the reduction of an aqueous solution of adipic acid, and for the production of a mixture of hexane-1,6-diol, pentane-1,5-diol, and butane-1,4-diol by the reduction of a reaction mixture resulting from cyclohexane oxidation (CIO). 

Another reaction which enjoys much commercial interest is the production of adipic acid from cyclohexane oxidation. The two types of processes that are employed commercially are outlined in Figure 24. 

Inspired by Gif or GoAgg type chemistry [77], iron carboxylates were investigated for the oxidation of cyclohexane, recently. For example, Schmid and coworkers showed that a hexanuclear iron /t-nitrobenzoate [Fe603(0H) (p-N02C6H4C00)n(dmf)4] with an unprecedented [Fe6 03(p3-0)(p2-0H)] " core is the most active catalyst [86]. In the oxidation of cyclohexane with only 0.3 mol% of the hexanuclear iron complex, total yields up to 30% of the corresponding alcohol and ketone were achieved with 50% H2O2 (5.5-8 equiv.) as terminal oxidant. The ratio of the obtained products was between 1 1 and 1 1.5 and suggests a Haber-Weiss radical chain mechanism [87, 88] or a cyclohexyl hydroperoxide as primary oxidation product. 

Results of the cyclohexane oxidation tests are shown in Table 41.4. Mono-oxygenated products are cyclohexanone, cyclohexanol and cyclohexyl hydroperoxide. Cu and Cr were very active, but subsequent tests showed considerable leaching for both metals, whereas Co-Si-TUD-1 did not show ai r leaching. Tests with different Co loadings indicate that the lowest Co concentration has the best conversion and ketone selectivity. Isolated cobalt species are most efficient for the conversion of cyclohexane, as agglomeration of Co reduces... 

The reaction of ions with peroxyl radicals appears also in the composition of the oxidation products, especially at the early stages of oxidation. For example, the only primary oxidation product of cyclohexane autoxidation is hydroperoxide the other products, in particular, alcohol and ketone, appear later as the decomposition products of hydroperoxide. In the presence of stearates of metals such as cobalt, iron, and manganese, all three products (ROOH, ROH, and ketone) appear immediately with the beginning of oxidation, and in the initial period (when ROOH decomposition is insignificant) they are formed in parallel with a constant rate [5,6]. The ratio of the rates of their formation is determined by the catalyst. The reason for this behavior is evidently related to the fast reaction of R02 with the... 

Very fast reactions of R02 and ROOH with tin(II) and the fast reaction of RO with cyclohexane result in the formation of cyclohexanol as the main oxidation product. 

Figure 11.2 shows a cyclohexane oxidation reactor. The further oxidation of the ketone and alcohol to adipic acid is very complex but occurs in good yield, 94%, despite some succinic and glutaric acid by-products being formed because the adipic acid can be preferentially crystallized and centrifuged. 

In any case, the initial reagents must be oxidized in the presence of the reaction products, and it is always best if the concentration of products can be increased because it corresponds to a better conversion rate per run—e.g., cyclohexane oxidation must be done in the presence of cyclohexanol and cyclohexanone—and recent advances have increased the amount of conversion per run from 8 to 15% without any loss of selectivity. 

Cyclohexane oxidation has been studied in supercritical carbon dioxide medium for homogenizing the initial reaction mixture to produce cyclohexanone and cyclohex-anol as the chief reaction products. Conversions obtained are low compared to the liquid phase oxidation because of dilute concentrations of the reactants. Cyclohexanone is more selectively formed and favored by both pressure and temperature. Results suggest that the reaction in supercritical CO2 medium can be gready manipulated (Srinivas and Mukhopadhyay, 1994). 

Decalin. Decalin was chosen to study the reactivities and selectivities of and a-(5 disubstituted cyclohexane ring as well as the influence of the adjonction of a second saturated Cf, ring. The cis- and trans-isomers were photoxidized in the same conditions. The cis-isomer was found at least 10 times more reactive with a selectivity of 81% in mild oxidation products and 19% in CO2. The following relative selectivities for the main mild oxidation products (2-decalone, with small amounts of 1-decalone and 2-decalol (decahydro-2-naphtol) were found ... 

Cvclohexene. This molecule was chosen to compare its reactivity with that of cyclohexane and to determine the influence of a double bond in the C6 ring, The main oxidation products with their relative selectivities are given in the following scheme ... 

This information can be used to deduce die elemental composition of a compound. For example, the oxidation of 1,2-diazacyclohexane was carried out in cyclohexane. A product was isolated and was found to have a molecular ion of m jc = 84. At this point the experimenter realized that both the expected product and the reaction solvent have a molecular weight of 84. 

Studies on cyclohexane oxidation at Monash University were started in the year 1965 (see Wild, 4). Initial reaction studies indicated that the reaction rate was high. Wild (4) presented a theoretical analysis of the reaction by idealizing it as a consecutive reaction. It was found that diffusion limitation reduces selectivity towards the intermediate product. Selectivity could be enhanced by minimizing the contact time between the gas and the liquid, and also decreasing the partial pressure of the reactive gas. For systems not under diffusion control, these two factors will not have any effect on the reaction course. 

An important goal is, therefore, to develop effective methods for catalytic oxidations with dioxygen, under mild conditions in the liquid phase. Two substrates which are often chosen as models for alkane oxidations are cyclohexane and adamantane. Cyclohexane is of immense industrial importance as its oxidation products - cyclohexanone and adipic acid - are the raw materials for the manufacture of nylon-6 and nylon-6,6. Adamantane is an interesting substrate as the ratio of oxidation at the secondary versus the tertiary C-H bonds is used as a measure of radical versus nonradical oxidation pathways. Industrial processes for the oxidation of cyclohexane, to a mixture of cyclohexanol and cyclohexanone, generally involve low conversions (under 10%). Even at such low conversions, selectivities are modest (70-80%) and substantial amounts of overoxidation products, mostly dicarboxylic acids, are formed. 

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