Copper and manganese: two concordant partners in the catalytic
oxidation of p-cresol to p-hydroxybenzaldehyde
Feng Wang, Guan-yu Yang, Wei Zhang, Wen-hai Wu and Jie Xu*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P. O. Box 110, Dalian, 116023, P. R.
China. E-mail: xujie@dicp.ac.cn; Tel: 86-411-4379245
Received (in Cambridge, UK) 16th January 2003, Accepted 26th March 2003
First published as an Advance Article on the web 16th April 2003
Table 1 Catalytic oxidation of p-cresola
Copper and manganese were found to be two concordant
partners in the synthesis of p-hydroxybenzaldehyde from p-
cresol; under mild conditions, this research realised 95.6%
selectivity for p-hydroxybenzaldehyde at 98.5% conversion
of p-cresol.
Metal
Con-
Product distribution (mol%)
Molar weight
version
(mol%) PHBb
Catalysts
ratio
(wt%)
PHBAc
Others
—d
Cu/C
Mn/C
Co/C
—
—
—
—
4+1
5+1
4+1
2+1
1+1
—
0
0
9.5
0
0
Catalytic functionalization of the relatively inert hydrocarbon
1.3
2.2
5.0
10.2
8.7
7.4
4.8
3.5
12.8
18.7
37.9
99.5
99.9
98.5
97.7
98.0
11.8
21.9
27.2
38.4
8.6
2.6
1.8
19.2
78.7
16.9
18.0
9.3
2.7
1.8
C–H bond under mild conditions is one of the most desirable
61.2
54.8
52.3
88.7
95.6
94.9
54.4
1
reactions and remains a challenge for chemists. Employing p-
e
cresol as feedstock, researchers have developed several meth-
CuCo/C
CuMn/C
CuMn/C
CuMn/C
CuMn/C
a
ods to obtain p-hydroxybenzaldehyde. Besides the chlorination
route, homogeneous2 and heterogeneous3 catalysts are ex-
3.3
26.4
tensively used, among which cobalt is most favoured. We also
found that cobalt is an excellent catalyst for C–H bond
activation in the oxidation of o-cresol to salicylaldehyde.4
Reaction conditions: catalyst
=
0.6 g, p-cresol
=
16.0 g, solvent
(methanol) = 50 mL, sodium hydroxide = 23.0 g, pressure = 0.3 MPa,
However, when the catalyst (copper and cobalt impregnated on
activated carbon) was used in the oxidation of p-cresol, the
selectivity for p-hydroxybenzaldehyde was relatively low.
Generally cobalt oxides possess an extremely high oxidative
reactivity and manganese dioxide has a lower, although still
b
c
time
= 3 h, temperature = 348 K. p-Hydroxybenzaldehyde. p-
d
e
Hydroxybenzoic acid. No catalyst. Ref. 4.
5
rather high, activation ability. Considering the difference in the
exchange in molecular oxygen. Low activation energies result
in a high oxygen supply ability. The literature data5b on oxygen
isotopic exchange give the sequence of oxygen supply ability
as: Co O > MnO > CuO. The oxidation reaction requires a
3 4 2
reasonable activation match of the C–H and oxygen bond via
interaction with the catalyst active site. The electrophilic
two substrates, we chose manganese instead of cobalt in the
catalyst preparation to improve the selectivity for p-hydrox-
ybenzaldehyde in the oxidation of p-cresol. The results are quite
impressive.
To the best of our knowledge, there are no reports on the
heterogeneously catalysed oxidation of p-cresol to p-hydrox-
ybenzaldehyde over copper and manganese bimetallic oxide
2
2
oxygen (O , O ) tends to accumulate on the surface of the
2
cobalt catalyst, which enhances the probability of electrophilic
attack on the CNO bond of p-hydroxybenzaldehyde. Manganese
acting as an assistant to copper improves the oxygen exchange
between bulk phase and the catalyst surface by accelerating the
valence transformation of copper. The concentration of electro-
philic oxygen is kept at a relatively low level thus avoiding
oxidation of p-hydroxybenzaldehyde. Activation of the C–H
bond happens on active sites by virtue of hydrogen abstraction
from the methyl group of p-cresol. If one C–O bond is formed,
the product is alcohol, and if one C–H bond in the adsorbed
benzyl species is activated and dissociates, another C–O bond
will be formed and aldehyde is produced. During the oxidation
of p-cresol, copper and manganese work together well as two
concordant partners to realise the reasonable match of C–H and
oxygen bond activation.
The influence of reaction temperature on the conversion of p-
cresol was investigated from 333 to 353 K (Fig. 1). It can be
seen that the yield of p-hydroxybenzaldehyde peaks at 348 K. A
further increase in reaction temperature aggravates the oxida-
tion of p-hydroxybenzaldehyde to p-hydroxybenzoic acid. The
change of p-cresol, p-hydroxybenzaldehyde and p-hydrox-
ybenzoic acid with time was investigated (Fig. 2). It indicates
that the conversion of p-cresol and the yield of p-hydrox-
ybenzaldehyde improve rapidly up to 14 hours. The conversion
of p-hydroxybenzaldehyde to p-hydroxybenzoic acid is in-
hibited by the catalyst. A slight decrease in the yield of acid
occurs after a 9 h run due to the reaction of acid with methanol
to form methyl-p-hydroxybenzoate which was detected by GC-
MS analysis. Sodium hydroxide is essential for the reaction for
two reasons: firstly, it reacts with the hydroxyl of p-cresol and
prevents oxidation on the benzene ring; secondly, it is generally
2b
catalysts. Compared to organometallic catalysts and zeoli-
2c,3c
tes,
the CuMn/C catalyst is less demanding, economical in
preparation, and suitable for large-scale production. The
catalyst was prepared using a commercial activated carbon
2
21
(
specific surface area 1100 m g ) as support. The support was
impregnated with an aqueous solution of copper and/or
manganese nitrates using the incipient wetness method. After
impregnation, the samples were first dried at 393 K and then
calcined at 673 K in a vacuum quartz tube to afford the oxides.
The ratio and metal weight of the catalysts are listed in Table
1.
The catalytic oxidation reactions were carried out in a 600
mL capacity stirred autoclave (Parr Instruments, USA) under an
oxygen pressure of 0.3 MPa with analysis by liquid chromatog-
raphy (HPLC, Shimadzu liquid chromatograph, Model LC–9A,
equipped with a Chrompak C18 15 cm column, UV 254 nm).
Authenticated standard samples were used to determine the
identity of the products and for calibration. The total conver-
sion† and product distribution were evaluated with calibration
curves.
The performance of different catalysts for p-cresol oxidation
was studied under the same reaction conditions (Table 1). A
blank run without catalyst gave no products. Single component
catalysts exhibit lower activity and selectivity than bimetallic
catalysts; addition of cobalt increases the selectivity for acid
whilst introduction of manganese can remarkably improve the
selectivity for p-hydroxybenzaldehyde. Changing the weight
content of manganese in the CuMn/C catalysts has little affect
on the conversion of p-cresol. Variations in performance are
partly due to the difference in the activation energies of
1
172
CHEM. COMMUN., 2003, 1172–1173
This journal is © The Royal Society of Chemistry 2003
Wang, Feng
