Oxidation of Cyclohexane/Cyclohexanone Mixture with Oxygen as Alternative Method of Adipic Acid Synthesis

Abstract

Herein, an alternative method for adipic acid (AA) synthesis of industrial importance has been reported. The proposed novel method involves the one-step, solvent-free oxidation of a cyclohexane/cyclohexanone (CH/CH=O) mixture, with a cheap oxidizing agent such as O2 or air under mild conditions in the presence of N-hydroxyphtalimide (NHPI) and transition metals as catalysts. It has been showed that CH/CH=O mixture under applied mild conditions oxidized faster than CH and CH=O separately. This was due to the greater oxidizability of CH=O compared to CH. The peroxyl radicals formed by CH=O oxidation initiated the oxidation of the less reactive CH. Additionally, CH=O increased the polarity of the reaction mixture, promoting the solubility of NHPI. The influence of type and amount of catalyst, cyclohexane to cyclohexanone ratio, temperature, time, type of oxidizing agent on the composition of CH/CH=O oxidation products have been reported.

Selective Oxidation of Cyclohexanone to Adipic Acid Using Molecular Oxygen in the Presence of Alkyl Nitrites and Transition Metals as Catalysts

“…Then, as a result of oxidation, reduction, and disproportionation, the isopentyl radical transformed into IPN [ 70 ]. The exact course of AA formation from 2-carbonylcyclohexyl radical was presented in our previous paper [ 21 ].…”

“…The reaction is mainly carried out in acetic acid (AcOH) as a solvent and in the presence of transition metal salts or complexes [20]. In order to initiate the process, small amounts of cyclohexanone (C-ON) are introduced [21,22]. Additionally, reports have utilized oxidative organocatalysts such as N-hydroxyphthalimide (NHPI) [23][24][25][26][27][28][29] and but-2-one (MEK) [30].…”

Oxidative Cleavage of C−C Bonds in Non‐aromatic Oxygenates with Molecular Oxygen for Synthesis of Carboxylic Acids

“…Oxidation with nitric acid can give very high selectivity to adipic acid such as 95 %, [3,68] while oxidation systems using O 2 give higher selectivity to by-products such as glutaric acid (C5 dicarboxylic acid) and succinic acid (C4 dicarboxylic acid) (Table 6). [68][69][70][71][72][73][74][75] Oxidation of cyclohexanone with O 2 is a radical reaction, and there are many by-products with small amount in addition to major products (adipic acid, glutaric acid and succinic acid). Some works reported the sum of selectivity to these three dicarboxylic acids as 100 %, but these values are probably significantly overestimated.…”

“…Such values have very high risk of overestimation, and we do not include these papers in Table 6. The direct oxidation of cyclohexane to adipic acid with radical-based oxidation system has been also investigated; [72,76] however, the problem of the overestimation of selectivity is further severer. The selectivity is often calculated by the ratio to the sum of detected products because the loss of cyclohexane by evaporation is difficult to be estimated.…”