Transition-metal complexes for liquid-phase catalytic oxidation: some aspects of industrial reactions and of emerging technologies

Abstract

This review presents recent developments in some important industrial processes. For homolytic systems, the similarities and specificities of the transformation of cyclohexane into adipic acid (via the formation and isolation of the cyclohexanol–cyclohexanone mixture) and of the process for the preparation of terephthalic acid (by oxidation of p-xylene) are analysed from a prospective viewpoint. Innovations in heterolytic chemistry are also presented. For example, the epoxidation of propene currently performed by the Arco–Lyondell method, which uses a molybdenum catalyst and tert-butyl hydroperoxide, is in competition with other systems involving recycling of the co-product, or the use of hydrogen peroxide as oxygen donor. Access to diphenols, from phenol and H₂O₂, is becoming more selective. New oxoperoxo complexes of titanium, rhenium, tungsten and molybdenum are presented with their potential for catalytic oxidation. Achievements of our group in the development of catalysts for C–C bond cleavage by dioxygen, and for epoxidations by H₂O₂ are also given. Biomimetic oxidations and systems without metal catalysts feature among the innovations. These novel systems could compete with transition-metal-based chemistry for fine chemicals, but industry increasingly focuses research and development on fully inorganic systems, and there is little impact on the large scale of catalytic complexes with organic ligands.