Direct Oxidation of Cyclohexane with Air

Compared to the traditional technology, the direct oxidation of cyclohexane with air or oxygen, also called the One Step AA process, should lower the total investment cost. This is due to the following differences  

Many patents and papers [16-22] describe catalysts and process configurations for the single-step aerobial oxidation of cyclohexane to AA, catalyzed by homogeneous 

Many companies have studied the optimization of catalyst composition and process conditions in order to improve the performance of the reaction and the economics ofthe process. In the Gulf process, the reaction is carried out at 90-100 °C, with a Co(III) acetate catalyst and acetic acid as the solvent [17]. The molar selectivity is around 70-75%, for a cyclohexane conversion that can be as high as 80-85%. The high concentration of Co(III) acetate used also favors the direct reaction ofthe cation with cyclohexane, generating the cydohexyl radical. In fact, in Gulf patents the reaction is reported to occur in a critical amount of Co(III) (25-150 mmoles per mole of cyclohexane). The catalyst is activated during the initial induction period, and water is also added in the initial stage to enhance the selectivity to AA, but the rate of production decreases because the induction period increases. 

In Amoco patents [18b], the addition of controlled amounts of water after the initiation ofthe oxidation reaction is claimed to be a tool to obtain a better yield to AA. The best yield achieved was 88% (based on the identifiable compounds) at 98% cyclohexane conversion, with a Co(II) acetate catalyst, an acetic add solvent, at 95 °C and 70 atm air pressure. It is reported that water, if present during the induction period, depletes the concentration of free radicals in the absence of water, the yield was remarkably lower. These results are comparable to those attained by the air /nitric acid two-step oxidation of cydohexane. 

In patents issued by Redox Technologies, oxidation is reported to occur more efficiently if high concentrations of cydohexane are used (AcOH/cydohexane weight ratio between 50 50 and 15 85, instead of80 20), and if the conversion ofthe reactant is kept below 30%, instead of more than 70% [19]. A low concentration of catalyst is recommended. An 88% AA selectivity at 21% cydohexane conversion is obtained in an acetic add solvent with a Co(OAc)2 catalyst (cydohexane/Co ratio higher than 150), 

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