A novel immobilised cobalt(III) oxidation catalyst†
Birinchi K. Das‡ and James H. Clark*
Clean Technology Centre, Department of Chemistry, University of York, York, UK YO10 5DD.
E-mail: jhc1@york.ac.uk
Received (in Liverpool, UK) 19th January 2000, Accepted 25th February 2000
Table 1 Physical data for the supported Co(III) complex
A complex form of cobalt(III) has been successfully im-
mobilised on a chemically modified silica and proven to be
an active catalyst for the selective oxidation of alkylaro-
matics using air as the source of oxygen and in the absence
of solvent.
HMS–Co(III) complex
HMS–CO2H
BET surface area/m2 g21
Pore volume/cm3 g21
Co+Si ratio (EDAX)
528
1041
0.688
—
0
—
0.315
1+7
0.68
580
Cobalt salts are widely used as catalysts in the liquid phase
oxidation of alkylaromatics.1 Reactions are normally carried out
in acetic acid as solvent and in the presence of a promoter such
as a bromide. It has been reported that under such reaction
conditions, the aerial oxidation of Co(II) to Co(III) is a slow
process2 although this can be accelerated by the addition of
bases such as pyridine which stabilises the higher oxidation
state of the metal.3 The heterogenisation of catalysts and their
use in the oxidation of neat substrates are subjects of
considerable interest especially in these environmentally con-
scious days when solvent losses and catalyst losses on
separation can lead to unacceptable levels of waste.4 Several
heterogeneous metal catalysts have been designed for aerial
oxidation reactions5 but limited activities and questionable
stabilities6 have delayed their commercial exploitation. It
seemed to us that if we could design a heterogeneous system
that favours and stabilises Co(III), this should enhance rates of
reaction where the Co(II) to Co(III) step was rate limiting and
should increase catalyst stability owing to the relative substitu-
tional inertness of Co(III). Here, we report our preliminary
results from designing such a catalyst that can indeed be
successfully used to catalyse the aerial oxidation of alkylaro-
matics in the absence of solvents.
The novel catalyst is based on a chemically modified porous
silica.7,8 Cyanoethyl-silica was prepared from the sol–gel
reaction of tetraethylorthosilicate (196 mmol) and cyanoethyl-
triethoxysilane (98 mmol) in aqueous ethanol in the presence of
N-dodecylamine as the templating agent. The resulting precip-
itate was washed with boiling ethanol to remove the template
and then treated with hot aqueous H2SO4 so as to hydrolyse the
CN group to CO2H.9 The resulting silica-(CH2)2CO2H material
was washed with water and ethanol and then dried at 110 °C
before use as a support for the Co(III) complex. Pyridine (40
mmol) was added dropwise to a mixture of the silica material
(5g), Co(NO3)2·6H2O (20 mmol), and sodium acetate (40
mmol) in water (100 ml). Complexation of the cobalt to the
support only occurs on addition of the pyridine as witnessed by
the purple colour of the solid particles. After adding more water
(50 ml), the mixture was heated to boiling and dilute hydrogen
peroxide (80 mmol) was slowly added over 8 h. This resulted in
the apparent oxidation of the immobilised cobalt with a change
in colour of the solid particles from purple to olive green. It is
necessary to treat the supported metal complex with peroxide so
as to form the immobilised Co(III). This solid was separated by
filtration, thoroughly washed with water and acetone and then
dried at 110 °C.
Co loading (AAS)/mmol g21
lmax(UV–VIS)/nm
(Table 1). Atomic absorption analysis of the digested material
gives a cobalt loading of 0.68 mmol g21. Optical and scanning
electron microscopic studies on the material reveal a homoge-
neous mass with elemental analysis (EDAX) carried out at
randomly selected spots showing a consistent Co+Si ratio of ca.
1+7. This suggests an evenly distributed cobalt complex rather
than a simple mixture of a cobalt complex with the silica. The
BET isotherms for the supported Co(III) complex and the
intermediate silica-(CH2)2CO2H material have almost identical
appearances and suggest a mixture of micropores and meso-
pores. The BJH pore size distributions of these materials show
ca. 65 and 80%, respectively, of the pore volumes are due to
pores with diameters of < 6 nm. The surface area is also reduced
on going from the intermediate supported carboxylic acid to the
final supported metal complex (from 1041 to 528 m2 g21
)
although it is still very high compared to more traditional
supported reagents.10,11 These results are consistent with a
reasonably homogeneous build-up in the surface species as the
metal complex is formed. The powder X-ray diffraction pattern
of the supported Co(III) material shows a weak peak at < 2° with
the strongest peak at 2q ca. 22.5° consistent with a lack of long
range order as expected for materials of this type.8,10 Diffuse
reflectance FTIR (DRIFTS) of the final material reveals bands
characteristic of surface bound CO22, pyridine and acetate. The
difference in the wavenumbers of the symmetric and asym-
metric stretching vibrations of the acetato ligand carboxyl group
(125 cm21) is consistent with these ligands bonding in a
bridging fashion. The diffuse reflectance UV–VIS spectrum of
the final material shows an ill-defined absorption maximum at
ca. 580 nm in contrast to the strong band at 545 nm for the
material before treatment with hydrogen peroxide. This is
consistent with a change from Co(II) to Co(III) on peroxide
treatment. Furthermore, by carrying out a similar complex
preparation but in the absence of the support material, the final
cobalt complex, believed to contain the cation [Co3(m3-O)(m-
O2CR)6(py)3]+ 12 (ClO4– as counter anion) gives an olive green
complex with a very similar UV–VIS spectrum. The measured
C+N ratio of 12+1 in the final material is consistent with the
presence of a similar complex rather than a simple species
containing two pyridines and two carboxylato ligands per cobalt
atom (for which the C+N ratio would be 7+1)
The supported cobalt(III) reagent was tested as a catalyst in
the aerial oxidation of neat ethylbenzene, chlorotoluene and
toluene as representative alkylbenzenes (Table 2). Reactions
were carried out at atmospheric pressure with 400 ml of
substrate and 0.8 g of the catalyst in a baffled glass reactor with
a overhead stirrer operating at 700 rpm, an air feed at 400 ml
min21, chilled water condensers and twin Dean–Stark sepa-
rators to remove the water produced during the reactions.
Reactions could be monitored both by the amount of water
While the precise structure of the surface-bound cobalt
complex is as yet unknown we have carried out a number of
analytical studies to gain information on the cobalt loading and
distribution, the material structure and the surface species
† UK Patent application, 1999.
‡ Current address: Department of Chemistry, Gauhati University, Guwa-
hati 781 014, India.
DOI: 10.1039/b000535p
Chem. Commun., 2000, 605–606
This journal is © The Royal Society of Chemistry 2000
605