Is it possible to implement N-hydroxyphthalimide homogeneous catalysis for industrial applications? A case study of cumene aerobic oxidation

BACKGROUND
In spite of its efficiency and versatility, examples of the use of N-hydroxyphthalimide (NHPI) as catalyst for oxidations at industrial scale are scant, mainly due to the lack of methodologies for the recovery and reuse of the catalyst. This paper reports a step by step investigation of the aerobic oxidation of cumene (CU), in order to verify the usability of NHPI catalysis in industrial processes.

RESULTS
CU is oxidized to the corresponding cumyl hydroperoxide (CHP) at 70°C in the presence of a polar co-solvent and 1% of NHPI with good conversion (>30%) and almost complete selectivity (>90%), without requiring the use of additional initiators. The catalyst is recovered at the end of the oxidation process by removal of polar co-solvent, with consequent precipitation of NHPI. The resulting filtered mixture is treated with non-basic supports, which are capable of adsorbing the residual NHPI by physical interaction. Finally, the adsorbing solids can be regenerated by washing with a polar solvent, allowing the recovery of residual catalyst.

CONCLUSION
We provide evidence for the feasibility of the use of NHPI catalysis for industrial applications by investigating the case study of CU oxidation. The process can be performed under mild conditions without requiring the use of sacrificial initiators. The effect of temperature and composition in the NHPI precipitation step is investigated. The performances of basic and non-basic adsorbing beds during the recovery of residual NHPI are compared. The results obtained indicate that the recovery and recycling of the unaltered catalyst is possible without affecting, at the same time, the nature of cumyl hydroperoxide.