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Currently, the industrial production of adipic acid uses nitric acid oxidation of cyclohexanol or a cyclohexanol/cyclohexanone mixture. The nitrous oxide emission from this process measurably contributes to global warming and ozone depletion. Therefore, the development of an adipic acid production process that is less damaging to the environment is an important subject in chemical research. Cyclohexene can now be oxidized directly to colorless crystalline adipic acid with aqueous 30 percent hydrogen peroxide under organic solvent- and halide-free conditions, which could provide an ideal solution to this serious problem.
Adipic acid is an important chemical, whose production is necessary for the manufacture of nylon-6,6, which is used in carpet fibers, upholstery, tire reinforcements, auto parts, apparel, and other products. Most industrial processes of adipic acid production use nitric acid oxidation of cyclohexanol, cyclohexanone, or both, which are accessible from benzene (1). The worldwide industrial adipic acid production of up to 2.2 million metric tons per year (1, 2) relies on a refined technology that minimizes the emission of nitrous oxide (N^sub 2^O) (3), an inevitable stoichiometric waste (2) that is commonly thought to cause global warming and ozone depletion (4) as well as acid rain and smog. Despite the efficient recovery or recycling of N^sub 2^O, ~400,000 metric tons are still emitted each year, which corresponds to 5 to 8% of the worldwide anthropogenic emission of N^sub 2^O. Therefore, the development of environmentally conscious practical procedures for the oxidation of six-carbon feedstocks is highly desirable, particularly for the medium- to large-scale synthesis of adipic acid and its derivatives (5).
Aqueous H^sub 2^O^sub 2^ is an ideal clean oxidant if the oxidation reaction is achieved with a H202 concentration of <60%. The use, storage, and transportation of higher concentrations of H^sub 2^O^sub 2^ are not desirable for safety reasons (6). We recently developed practical oxidation methods with aqueous 30% H^sub 2^O^sub 2^ in the presence of small amounts of Na^sub 2^WO^sub 4^ and [CH^sub 3^(n-C^sub 8^H^sub 17^)^sub 3^N]HSO^sub 4^ as a phasetransfer catalyst (PTC) (7, 8). This aqueous, organic biphasic reaction enables the highyield epoxidation of olefins, either terminal or internal (7), as well as the oxidation of primary and secondary alcohols (8) under conditions that are entirely free of organic solvents and halides. The high...