Chemistry Reference
In-Depth Information
OH
H
2
O
2
tungstate
CO
2
H
CO
2
H
oleic acid
9,10-dihydroxystearic acid
OH
O
2
/ NHPI
Co(acac)
3
HO
2
C
CO
2
H
CO
2
H
nonanoic (pelargonic) acid
Fig. 11.23
Oxidative cleavage of oleic
acid using hydrogen peroxide and
oxygen.
azelaic acid
ates in the oxidation, they themselves are cleaved by
much the same range of catalysts [211].
H
2
O
2
WO
3
/ SiO
2
(/TiO
2
)
O
O
Aliphatic hydrocarbon oxidations
Fig. 11.24
Oxidative cleavage of cyclopentene to
glutaraldehyde.
Saturated aliphatic hydrocarbons are oxidised to
alcohols (predominantly secondary and tertiary) and
ketones by a variety of catalysed peroxygen systems,
although the reactions are rarely selective enough
to be useful commercially. Activity and product pat-
terns often are investigated in the course of charac-
terising catalysts such as TS-1 and its 'offshoots'
already encountered. For example, a kinetic and
product study of
n
-hexane oxidation over Ti cata-
lysts, includes a literature review on partial oxida-
tion [212].
The use of membranes for this reaction is worthy
of note as a means of solving a common problem
in H
2
O
2
oxidations—lack of mutual solubility be-
tween hydrophobic substrate and hydrophilic
oxidant. Kaliaguine's group has studied
n
-hexane
oxyfunctionalisation over TS-1-containing catalytic
PDMS membranes [213], avoiding the use of a co-
solvent to bring H
2
O
2
and alkane into solution by
contacting on opposite sides of the membrane. Prod-
ucts are alcohols and ketones, with generally low
conversions in the experiments quoted.
A mixed oxygen/H
2
O
2
system that has received
much attention recently comprises vanadium(V) and
heterocyclic acids such as pyrazinecarboxylic acid in
carboxylic acid solvent [106,107]. Mizuno also has
reported [214] a system comprising trifluoroace-
tic anhydride and H
2
O
2
with the polyoxometallate
H
4
PVMo
11
O
40
as catalyst precursor. The activity of
these systems appears comparable with Fenton's
reagent, with even methane being attacked—once
again, selectivity can be the main issue.
In a lot of cases, metal-peroxo catalysts alone can
be sufficient to cleave olefins if the appropriate
excess of H
2
O
2
is used [205]. Cleavage of cyclopen-
tene to glutaraldehyde can be achieved with H
2
O
2
in
t
-butanol solvent over heterogeneous catalysts com-
prising tungstic oxide (WO
3
), supported on sol-gel-
prepared silica [206] or on mixed TiO
2
/SiO
2
[207]
(Fig. 11.24). In both cases the support was post-
impregnated with ammonium tungstate. It seems
possible that at least some of the tungsten may be
solubilised as a peroxo complex at some stage during
the reaction, although it may reprecipitate if all the
peroxide is consumed.
Heteropolyacids catalyse this and other olefin
cleavage reactions [208]. Polyoxometallates have
been reported for olefin cleavage: for example, the
Keggin structures Mo
12
P, W
12
P and W
6
Mo
6
P have
been used with H
2
O
2
, 2,3-dimethylbut-2-ene gives
acetone, cyclohexene gives adipic acid, 1-octene
gives heptanoic acid, styrene and stilbene both give
benzoic acid and 1-methylcyclohexene gives 6-
ketoheptanoic acid, all in at least 90% yields [209].
Once again, there are doubts that the intact complex
is the active catalyst. This reaction has been reviewed
recently with respect to oleic acid cleavage [210]. Not
surprisingly, because diols are common intermedi-
