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.