OXIDATION REACTIONS WITH IN-SITU GENERATED OXIDANTS

The main achievements and future prospects of in situ oxidations are d iscussed, focusing on the advantages and limits of the technique. This is based on hydrogen peroxide, peracids, metal peroxo and metal oxo s pecies, generated in situ by oxygen and a reducing agent: hydrogen, ca rbon monoxide, metallic iron or zinc, hydrides, aldehydes and other or ganic reductants. Hydrogen peroxide and hydrogen, respectively, are pr eferred reagents for the inherent cleanliness of their use, producing only water as the byproduct. Examples are the epoxidation of propylene by air and alkylated anthrahydroquinones, catalysed by titanium silic alite (TS-1), and the hydroxylation of alkanes and aromatics on TS-1 a nd on other heterogeneous catalysts loaded with noble metals. The halo genation of phenol with hydrogen/oxygen/halogenidric acid mixtures on Pd/TS-1, has also been reported. Carbon monoxide was used to replace h ydrogen in in situ oxidations occurring at higher temperatures. Reduci ng agents other than hydrogen and carbon monoxide lead to the formatio n of more than stoichiometric amounts of coproducts, which add complex ity to the overall process for their separation and recycle/disposal. In the in situ oxidations by Gif(III/IV) systems and by aldehyde/oxyge n mixtures, large amounts of metallic wastes and carboxylic acid are c o-produced, respectively, hindering their application in bulk chemical s production. Future developments might arise from the design of super ior catalysts both for the in situ generation of hydrogen peroxide or peroxidic species from oxygen/hydrogen mixtures and for its subsequent efficient use. Oxygen/carbon monoxide and nitrous oxide can replace h ydrogen/oxygen in oxidations at progressively higher temperatures, alb eit no in situ oxidation with N2O as yet has been reported. The genera l features of hydrogen peroxide and nitrous oxide are briefly compared and discussed. (C) 1998 Elsevier Science B.V. All rights reserved.