Chemistry Reference
In-Depth Information
and Fenton's reagent chemistry in that both go
through an intermediate in which H
2
O
2
or its anion
is ligated to the metal. In fact, in scrupulously anhy-
drous conditions, iron salts with H
2
O
2
exhibit epox-
idation and regioselective hydroxylation typical of
oxenes but inconsistent with hydroxyl radical
chemistry [44].
The most important oxene chemistry driven by
peroxygens is that of the metalloporphyrins, espe-
cially iron and manganese. The latter group function
with a co-catalyst such as imidazole to carry out
oxygen-transfer and insertion reactions, notably
epoxidation [45], tertiary amine oxidation [46] and
hydroxylation [47]. The reactions can be carried out
in two-phase systems with a phase-transfer catalyst
or in single phase with a co-solvent for the H
2
O
2
, cat-
alyst and substrate. These systems are mimics of
peroxidase enzymes, including cytochrome P-450,
which use iron porphyrin oxene chemistry [48].
Although metal catalysts are involved, the levels of
these catalysts are often very small and recovery can
be simple. Unfortunately, the catalyst lifetime often
is limited owing to self-attack by the oxene and to
some radical side reactions. Appreciable work has
been directed towards supported forms of metallo-
porphyrins, where site isolation is expected to
improve the stability to oxidation.
small amounts (typically about 2%) of titanium
substituted for silicon in the zeolite framework. The
resulting material catalyses a wide range of oxida-
tions, including hydroxylation of alkanes [51] and
of phenol [52], oxidation of ammonia to hydro-
xylamine [53] (itself usable as an intermediate in
e-caprolactam production [54]), epoxidation of
alkenes [55] and some other olefin oxidations
involving further reaction of the initial epoxide
product [56,57]. Applications are, however, limited
by the size of the zeolite channels (5.5 Å), in that
substrate must pass down these in order to reach the
active sites. Intense work on analogues of TS-1 using
larger pore zeolites is currently in progress [58],
which will be discussed in more detail in Sections 3
and 4. This heterogeneous system is an outstanding
example of technology leading to relatively simple
process plant and conforming to the 'atom utilisa-
tion' principle.
The peroxo complexes of vanadium have not, by
comparison with the other three elements cited,
been used extensively. The ease of the redox step,
V(V)-V(IV), introduces a mixture of two-electron
and one-electron character into vanadium-peroxo
chemistry, which in the case of epoxidation leads to
side reactions of substrate and products [26,49].
The reported peroxo chemistry of molybdenum
and tungsten is extensive [59]. Many of the peroxo
complexes can be isolated and used stoichiometri-
cally as oxidants in organic solvents. However, such
isolation can be hazardous and the stoichiometric
use on a large scale would require a high metal
inventory, even though recycling is relatively easy.
Of more industrial relevance is the catalytic use of
these complexes, usually in a two-phase system with
a phase-transfer catalyst. In such a system, the
peroxo complex is formed in the aqueous phase
using H
2
O
2
and a catalytic amount of the metal(VI)
compound and then the peroxo complex is taken by
the phase-transfer agent into the organic phase in
which the substrate is dissolved. After oxidation, the
oxo complex is regenerated in the aqueous phase.
Such systems epoxidise olefins and oxidise organic
nitrogen and sulfur centres. Simple molybdate and
tungstate salts can be the metal source in these reac-
tions. However, more powerful systems can be
formed by the addition of phosphate or arsenate, or
by the use of pre-formed polyoxometallate species
such as the 'Keggin'
dodeca
-tungstophosphate ion
(Fig. 11.6) [60].
Metal-peroxo and hydroperoxo systems
This is the largest and most important of the oxidant
classes within this category. Metal-peroxo complexes
are formed rapidly in water over a wide pH range
from a range of d
0
metal compounds, mainly of
groups IVb, Vb and VIb, and notably Ti(IV), V(V),
Mo(VI) and W(VI). These complexes are elec-
trophilic in nature and many of them have oxygen
transfer properties. Their re-formation from the
parent compound and H
2
O
2
is rapid enough for them
to be used catalytically in oxidation systems. As such,
they can be regarded as inorganic catalytic analogues
of peracids, which immediately implies a large num-
ber of potential applications in oxidation [49].
Titanium complexes in aqueous solution are used
to determine H
2
O
2
in analytical chemistry but are not
very active as oxidants. There is, however, a very
important heterogeneous catalyst system believed
to be based on Ti-peroxo chemistry. This is titanium
silicalite or TS-1, discovered by Enichem [50]—
a silicalite with the ZSM-5 structure containing
