Fe─Co Synergism in the Heterogeneous Catalytic Conversion of Cyclohexane to KA Oil

The oxidation of cyclohexane is a crucial bulk process to produce cyclohexanol and cyclohexanone (KA oil), used for the synthesis of adipic acid and further polyamide fibers and plastics production. The industrial process is carried out at high temperature in the presence of a cobalt-based homogeneous catalyst, which no longer meets the sustainability requirements imposed by the increase of environmental concerns. This research aimed to investigate the potential of Co-based heterogeneous catalysts, where Co is coupled with Fe. Fe has been selected considering its ability of decomposing hydroperoxides along with its oxidation-reduction properties, which can help the homolytic cleavage of the cyclohexyl hydroperoxide intermediate in the cyclohexane oxidation. Heterogeneous Fe─Co catalysts supported on alumina have been thus synthesized by three different methods, i.e., sol-immobilization (SOL), wetness impregnation (IW), and inverse microemulsion (ME). Fe- and Co- and the corresponding Fe1Co1/Al2O3 catalysts prepared by different preparation methods resulted in different physical and morphological features and in turn in a different catalytic activity and selectivity toward cyclohexane oxidation. In particular, the synthesis by inverse microemulsion technique resulted in the highest metal dispersion, where Co was in contact with Fe. The highest dispersion of Co resulted in the highest activity, where the 2 wt% Co/γ-Al2O3-ME was the most active catalyst, producing 5.1 mmol of KA oil after 5 h of reaction with a selectivity of 96.0%; on the other hand, the presence of Fe allowed to increase the selectivity, as the 2 wt% Fe1Co1/γ-Al2O3-ME catalyst produced 4.7 mmol of KA oil after 5 h of reaction with a selectivity of 98.8%.