Chemistry Reference
In-Depth Information
Scheme 14.11
worldwide total. Cyclohexene is oxidised by H
2
O
2
in
the presence of NaWO
4
and a phase-transfer catalyst
([CH
3
(
n
-C
8
H
17
)
3
N]HSO
4
) to give adipic acid in 90%
isolated yield upon heating at 75-90°C for 8 h [51].
The proposed mechanism, based on extensive inves-
tigation of the reaction of HPM-catalysed oxidation
of olefins, is shown in Fig. 14.4. The catalytically
active species is probably the peroxytungstate
complex
2
(Scheme 14.13). The process is not limited
to cyclohexene. Substituted cyclohexenes and
cyclopentene also underwent catalytic oxidation in
similar yields.
More recently, the process was improved by elim-
inating the phase-transfer agent and adding a biden-
tate organic acid. This modification forms adipic acid
with 97% selectivity at 100% conversion [52]. The
most effective organic acid appears to be oxalic acid,
which forms complex
3
(Scheme 14.13).
Related oxidations also have been reported. An
oxidation of cyclohexane to a mixture of cyclohexa-
nol and cyclohexanone using oxygen in the presence
of an HPM has been described [53]. Direct catalytic
oxidation of cyclohexane using
N
-hydroxyphthalim-
ide, Mn(acac)
2
and Co(OAc)
2
as a catalyst mixture
also has been reported to form adipic acid in 73%
selectivity at 73% conversion [54].
Dimethylcarbonate is receiving attention as a new
green chemical intermediate. A wide range of appli-
cations for dimethylcarbonate is known [55] but the
primary interest in this material for the chemical
Scheme 14.12
components in water, but many organic compounds
exhibit low or zero water solubility, limiting the use
of aqueous systems. Efforts have been made to over-
come this problem through the design of new ligands
for the metal catalyst. Immobilisation of Rh catalysts
in a sol-gel matrix has been shown to promote
propene hydroformylation with yields and selectivi-
ties similar to the commercial system [46]. Modifi-
cation of the Rh catalyst by replacement of a TPPTS
ligand with PPh
3
has been used to increase the sol-
ubility of the catalyst in the organic phase and
improve the reactivity of those substrates exhibiting
only slight water miscibility [47].
Heteropolymetallates (HPMs) have seen increas-
ing interest as robust, homogeneous catalysts for the
activation of oxidants such as O
2
or H
2
O
2
[48]. A new
route for the production of adipic acid is based on
homogeneous catalysis. Adipic acid is a material pro-
duced in huge amounts (2.2 ¥ 10
6
t year
-1
) via nitric
acid oxidation of cyclohexanol or cyclohexanone,
both of which come from benzene [49]. It is a
primary constituent in the production of Nylon-6,6,
used in carpets, upholstery, tyres and other materi-
als [50], a new adipic acid process would have a
significant environmental impact. The current pro-
duction emits 4 ¥ 10
5
t year
-1
of N
2
O into the atmos-
phere, which is approximately 5-8% of the
