C O M M U N I C A T I O N
The selective aerobic oxidation of methylaromatics to benzaldehydes
using a unique combination of two heterogeneous catalysts
Fatemeh Rajabi, James H Clark,* Babak Karimi* and Duncan J Macquarrie*b
a
b
c,d
a
Department of Chemistry, Sharif University of Technology, PO Box 11365-9516, Tehran, Iran
Clean Technology Centre, Department of Chemistry, University of York, Heslington, York,
b
YO10 5DD, UK
c
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
PO Box 45195-1159, Gava Zang, Zanjan, Iran
Institute for Fundamental Research (IPM), Farmanieh, PO Box 19395-553, Tehran, Iran
d
Received 24th December 2004, Accepted 24th December 2004
First published as an Advance Article on the web 21st January 2005
A unique combination of a supported cobalt complex
and the first example of supported NHPI in acetic acid
gives a surprisingly stable heterogeneous catalytic system
for the selective aerobic oxidation of methylaromatics to
benzaldehydes at atmospheric pressure.
The selective oxidation of methylaromatics is an area of consid-
1
erable industrial importance and academic interest. Achieving
this with only air as the source of oxygen is particularly challeng-
ing and environmentally desirable since it avoids the resource-
inefficient use of stoichiometric oxidants and the consequential
hazardous waste. One of the more recent promising devel-
opments in this direction is the use of N-hydroxyphthalimide
2
Scheme 1
.75 mmol g . The supported cobalt(II) catalyst is also built up
from AMPS using the method we described previously to give
a silica–Schiff base–Co(II) complex (Scheme 2).
−
1
0
(
NHPI) as an oxidation promoter typically alongside a cobalt(II)
4
3
catalyst. The NHPI has a marked effect on the rate of reactions
including the oxidation of methylaromatics but the reactions are
rarely selective, give benzoic acids as the major products and
4
the catalyst and promoter are not easily recoverable. We have
previously reported cobalt(II) and other redox metal centres
immobilised in chemically modified mesoporous silicas as
5,6
catalysts for reactions including oxidations. When used in their
normal particulate form in conventional stirred tank reactions,
they can easily be recovered (by filtration or decantation) thus
simplifying the work-up and reducing waste. In an attempt to
extend this logic to the promoter NHPI we have now prepared
what we believe to be the first active form of immobilised NHPI
also using a mesoporous silica support. More remarkably, these
two solids act co-operatively yet also heterogeneously to enhance
the rate of, and give excellent selectivity in, the aerial oxidations
of methylaromatics when used in acetic acid – a medium which,
due to its high polarity, would not normally be considered for
supported reagent chemistry.
Scheme 2
We initially screened the activity of the supported NHPI
using toluene as the substrate. Using an atmospheric pressure
of oxygen, the best conditions proved to be supported Co(II)
(0.25 mol%, 0.125 g), supported NHPI (5 mol%, 0.375 g)
The supported NHPI is built up from aminopropylsilica
3
(
AMPS) which is prepared by our sol-gel method using a mix-
substrate (5 mmol) in AcOH (15 cm ) in a baffled flask using
ture of aminopropyl(trimethoxy)silane and tetraethoxysilane
a high shear mixer. This reproducibly gave benzaldehyde as the
only detectable product (by GC). The yield and selectivity of any
reaction in the absence of either of the supported reagents or the
AcOH was significantly worse (the supported Co(II) alone gives
small amounts of benzoic acid only while the use of MeCN at
in aqueous acetonitrile using n-dodecylamine as the neutral
7
templating agent. The NHPI source is prepared by reacting
1
,2,4-benzenetricarboxylic anhydride, 1, (pretreated by refluxing
in xylene with a Dean–Stark trap to convert any residual triacid
to the anhydride) with NH
◦
2
OH·HCl in pyridine to make the
80 C gives only 5% benzaldehyde compared to 12% using AcOH
8
phthalimide, 2. A solution of this in dry THF is then reacted
with the AMPS at room temperature to give the supported
reagent, 3 (Scheme 1).
under the same conditions). The product yield increased with
◦
increasing temperature up to 100 C but higher temperatures
led to a reduction in rate consistent with O availability being
2
The recovered solid is thoroughly washed with methanol,
or becoming rate limiting. The catalyst and promoter can be
removed together from the reaction mixture by filtration and
then washed with hot AcOH and reused with fresh substrate to
give only a small reduction in the rate of reaction and no loss in
selectivity (Table 1).
The reaction passed the ‘hot filtration test’ (i.e. the reaction
stopped after the solids were removed from the hot mixture)
and we were unable to detect cobalt in the reaction liquor. We
aqueous oxalic acid, aqueous NaHCO , water and finally
3
9
hot methanol in a Soxhlet apparatus. The material was
1
3
characterised by C MAS NMR and DRIFT spectroscopy
−
1
(
characteristic C=O stretching bond at 1710 cm ). Porosimetry
2
−1
gives a BET surface area of 241 m g and a pore volume
3
−1
2
−1
of 0.458 cm g (the AMPS precursor shows 287 m g and
3
−1
0
.543 cm g ). Thermal analysis gives an organic loading of
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 7 2 5 – 7 2 6
7 2 5
Rajabi, Fatemeh
