Towards environmentally benign catalytic oxidation

Traditionally, oxidation in the liquid phase is not a “green” or environment-friendly method, when considering the stoichiometric use of inorganic oxidants and solvents. Catalysis provides the unique opportunity to make oxidation processes not only more atom-efficient but also more economic and less harmful for the environment. There are several requirements a selective catalytic oxidation process has to fulfill that it can be considered to be “green”. One of the most challenging is the use of molecular oxygen as an oxidant. This holds true for selective oxidations in the gas phase as well as in the liquid phase. Particularly in selective oxidations of relatively big and thermally sensitive molecules, as often encountered in fine chemical synthesis, liquid phase oxidation cannot be circumvented because the substrates cannot easily be evaporated and oxidized in the gas- or vapour-phase and the use of solvents may become inevitable. This often makes a green process difficult to be achieved. The question may even arise whether green oxidation in the liquid phase can be achieved at all.

We consider that the key characteristics of a green oxidation technology are the use of an easily recyclable catalyst, clean oxidants such as molecular oxygen or hydrogen peroxide without any additive such as co-reductant or co-oxidant, and high atom efficiency. While these requirements can in principle be achieved by homogeneous, heterogeneous or enzymatic catalysis, the use of a solid catalyst often has technical advantages concerning catalyst handling and separation. The aim of this themed issue is to report on new approaches towards environmentally benign oxidation catalysis with a special focus on solid-catalyzed aerobic oxidation in the liquid phase.

Oxidation in an organic solvent is probably rarely “green”, since the solvent itself can also take part in various redox reactions. A good solution – along with a few others – for a green technology, is oxidation in water, with this approach being repeatedly applied in this themed issue.

A typical feature of this field is the limited availability of solid catalysts, which can efficiently utilize molecular oxygen under the required mild conditions. A further difficulty is the normally narrow application range (strong substrate specificity) of solid catalysts and the related time consuming development and refinement of a catalyst system for the transformation of a specific substrate.

Several contributors to this issue focus on the development of new, sophisticated catalyst systems. Another focal point is the mechanistic understanding of the functioning of these catalysts, including theoretical calculations, kinetics and reaction engineering considerations. The development of new environment-friendly catalytic oxidation methods is a rapidly advancing area. The target of this issue is to give an idea of the present state-of-the-art by outstanding internationally recognized experts. While not all of the important aspects of this field could be covered in this themed issue, we feel that the authors have done an excellent job in presenting new promising developments. We thank all of them for their contribution and hope that the three reviews and ten experimental papers will generate great interest in academic and industrial research laboratories.

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