Oxidation of p-cresol to p-hydroxybenzaldehyde with molecular oxygen in the presence of CuMn-oxide heterogeneous catalyst

Article abstract of DOI:10.1002/adsc.200303226

A high-yield synthesis of p-hydroxybenzaldehyde from p-cresol and molecular oxygen was achieved over a CuMn-oxide supported carbon catalyst. The reaction parameters such as pressure, stirring speed, reaction temperature, solvent, and the amount of sodium hydroxide in the reaction media were optimized. As a result, a high conversion of p-cresol (99%) and a high selectivity to p-hydroxybenzaldehyde (96%) were realized at the same time. Catalyst separation and recycling tests clearly showed that the reaction proceeded on the heterogeneous catalyst but not on dissolved species.

Full text of DOI:10.1002/adsc.200303226

FULL PAPERS
Oxidation of p-Cresol to p-Hydroxybenzaldehyde with Molecular
Oxygen in the Presence of CuMn-Oxide Heterogeneous Catalyst
Feng Wang, Guanyu Yang, Wei Zhang, Wenhai Wu, Jie Xu*
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan
Road, Dalian, 116023, P. R. China
Fax: (þ86)-41184379245, e-mail: xujie@dicp.ac.cn
Received: October 9, 2003; Accepted: March 26, 2004
Abstract: A high-yield synthesis of p-hydroxybenzal- hyde (96%) were realized at the same time. Catalyst
dehyde from p-cresol and molecular oxygen was separation and recycling tests clearly showed that
achieved over a CuMn-oxide supported carbon cata- the reaction proceeded on the heterogeneous catalyst
lyst. The reaction parameters such as pressure, stirring but not on dissolved species.
speed, reaction temperature, solvent, and the amount
of sodium hydroxide in the reaction media were opti-
mized. As a result, a high conversion of p-cresol Keywords: copper; p-cresol; heterogeneous catalysis;
(
99%) and a high selectivity to p-hydroxybenzalde- p-hydroxybenzaldehyde; manganese
Introduction
dissolved into the reaction media, and the dissolved spe-
[18,19]
cies homogeneously catalyzed the reaction.
Recent-
Hydroxybenzaldehydes are useful raw materials. For ex- ly, encapsulated cobalt-Schiff base complexes have been
ample, p-hydroxybenzaldehyde is used as an intermedi- employed as a catalyst in the catalytic oxidation of p-cre-
[
20]
ate for making dyes, pharmaceuticals, as well as being a sol to p-hydroxybenzaldehyde. This study was an in-
textile auxiliary, and o-hydroxybenzaldehyde (salicylal- teresting attempt to immobilize homogeneous catalysts
[
1,2]
dehyde) is used in perfumery. This may be a reason on solid supports for facilitating catalyst separation, but
why many papers have dealt with the synthesis of hy- leaching of the active components was unavoidable in
droxybenzaldehydes. The formationofo- and p-hydroxy- this case.
benzaldehyde using the Reimer±Tiemann reaction from
Cobalt has been known as one of the favorable constit-
[
3±5]
phenol and chloroform was a classical example.
p- uents for a catalyst in selective oxidation of hydrocar-
[
21,22]
Hydroxybenzaldehyde was also produced from phenol bons for a long time.
In fact, we have reported that
and HCN by the Gattermann reaction. Industrially, p- cobalt supported on carbon was a highly active and se-
hydroxybenzaldehyde was synthesized by side-chain lective catalyst in the catalytic oxidation of o-cresol
[
23]
chlorination of p-cresol and by subsequent saponifica- with molecular oxygen to form salicylaldehyde. How-
[
6]
tion of the resulting dichloromethyl group. The draw- ever, when the cobalt catalyst was adopted in the oxida-
backs of these methods are the employment and the gen- tion of p-cresol, the selectivity for p-hydroxybenzalde-
eration of poisonous chemicals.
hyde was relatively low. In this report, we present high
Direct oxyfunctionalization of cresols at their benzylic and selective copper-manganese binary metal oxide cat-
positions provides another method for synthesizing hy- alysts for the oxidation of p-cresol by oxygen gas to p-hy-
[
7±12]
droxybenzaldehydes.
a homogeneous catalyst, Sharma et al. observed the for- ports,
mation of p-hydroxybenzaldehyde from p-cresol with tion of hydrocarbons using molecular oxygen, although
0% conversion and 59% selectivity in methanol as sol- copper is found in various metalloproteins, especially in
Using cobaltous chloride as droxybenzaldehyde (Scheme 1). Except for a few re-
[24,25]
copper has never been used in catalytic oxida-
9
[
13]
vent and in the presence of sodium hydroxide. How- enzymes implicated in the binding of molecular oxy-
ever, its application was restricted due to intricacy in cat- gen. Manganese oxide profoundly improved the se-
[
24]
alyst separation and reuse. Solid heterogeneous cata- lectivity for p-hydroxybenzaldehyde of copper oxide,
[
27]
lysts have the advantage of easy recovery and reuse, as we have reported in preliminary results.
[
14±16]
and are readily amenable to continuous processing.
Accordingly, zeolite catalysts such as CoAPO-5 and
[
17]
CoAPO-11 were applied to the oxyfunctionalization.
Unfortunately, it was found that these CoAPOs were
Adv. Synth. Catal. 2004, 346, 633±638
DOI: 10.1002/adsc.200303226
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
633

FULL PAPERS
Feng Wang et al.
Scheme 1. Catalyticoxidation of p-cresol to p-hydroxyben-
zaldehyde.
Results and Discussion
The catalytic performances of different catalysts in the
oxidation of p-cresol by molecular oxygen are compared
in Table 1. Data were taken at the optimum experimen-
tal conditions, which were determined as described in
Scheme 2. The process of catalytic oxidation of p-cresol.
the following sections. Scheme 2 indicates the process highest activity, followed by Mn/carbon although similar
of oxidation of p-cresol. In addition to the desired prod- high selectivities to p-hydroxybenzaldehyde were ob-
uct, p-hydroxybenzaldehyde (HBA), by-products such served on both Co/carbon and Mn/carbon catalysts.
as p-hydroxybenzoica ci d (HZ), p-hydroxybenzylmeth- Cu/carbon showed to be inferior in both activity and se-
yl ether (HBME), 4-hydroxy-3-methoxybenzyl alcohol lectivity. However, the addition of manganese into the
(
(
(
HMBA) and p-hydroxybenzoicai cd methyl ester
copper catalyst remarkably improved not only the activ-
HZME) were observed. p-Hydroxybenzyl alcohol ity but also the selectivity for p-hydroxybenzaldehyde.
[
21]
HA)was not detected. These products can be classi- The CuCo/carbon catalyst, which was reported to be ac-
fied into four groups; one benzyl alcohol-derived com- tive and selective in the catalytic oxidation of o-cresol to
pound of p-hydroxybenzylmethyl ether, p-hydroxyben- salicylaldehyde, was less selective for p-hydroxybenzal-
zaldehyde, two benzoic acid-derived compounds of p- dehyde production than the CuMn catalyst. The maxi-
hydroxybenzoica ci d and p-hydroxylbenzoica ci d meth-
yl ester, and one ring proton substituted compound of 4- Cu/Mn¼4 when the amount of Cu on the active carbon
hydroxy-3-methoxybenzyl alcohol. support increased from 1.9 to 7.4 wt % while the amount
mal yield of p-hydroxybenzaldehyde was observed at
As seen in Table 1, the reaction did not proceed in the of Mn remained constant at 1.4 wt %. The best catalyst
absence of catalyst although autocatalysis by the reactor of the CuMn/carbon series (Cu:Mn¼4:1) will be used
[
28]
wall is often recognized in oxidation reactions. In the hereafter for optimization of the experimental condi-
single component catalysts, Co/carbon exhibited the tions.
[a]
Table 1. Catalyticoxidation of p-cresol by molecular oxygen in the liquid phase.
Catalyst
Molar ratio
Metal wt % in C support
Conversion (mol %)
Product distributions (mol %)
[
b]
[c]
HBA
BZ
Others
[
d]
-
±
±
±
±
4: 1
5: 1
4: 1
2: 1
1: 1
±
0
13
19
0
10
61
55
52
89
96
95
54
0
0
75
11
12
2
1
0
2
22
[e]
Cu
Mn
Co
1.3
2.2
5.0
10.2
8.7
7.4
4.8
3.5
15 (3)
28 (6)
33 (6)
46 (8)
10 (1)
4 (1)
38
[
f]
CuCo
100
100
99
98
98
CuMn
CuMn
CuMn
CuMn
3 (1)
24 (5)
[
a]
Reaction conditions: catalyst¼0.6 g, p-cresol¼16.0 g, solvent (methanol)¼50 mL, sodium hydroxide¼23.0 g, pressure¼
0
.3 MPa, temperature¼348 K, time¼3 h, stirring speed¼700 r.p.m.
[
[
[
[
[
b]
c]
d]
e]
f]
p-Hydroxybenzaldehyde.
p-Hydroxybenzoica ci d.
No catalyst.
Selectivity for p-hydroxybenzoica ci d methyl ester in parenthesis.
[23]
From Ref.
634
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2004, 346, 633±638

Oxidation of p-Cresol to p-Hydroxybenzaldehyde
FULL PAPERS
The amount of the CuMn/carbon catalyst was set to veloped heat and for improving the selectivity for p-hy-
.6 g since the catalytic performance such as conversion, droxybenzaldehyde. A stirring speed of 700 r.p.m. was
0
product selectivity did not change beyond 0.2 g at the ex- used in the following experiments because of experi-
perimental conditions of 16.0 g of p-cresol, 50 mL of mental limitations of our system.
methanol, 23.0 g of sodium hydroxide, 0.3 MPa oxygen
pressure, 348 K temperature, 3 h sampling time, and ranging from 333 K to 353 K on the catalytic perform-
00 r.p.m. stirring speed. ance. As seen, the conversion increased with reaction
Figure 1 shows the time course of catalytic performan- temperature while the selectivity peaked at 348 K. At
Figure 3 shows the effect of reaction temperature
7
ces in the p-cresol oxidation on the CuMn/carbon cata- a temperature lower than 348 K, the selectivity for the
lyst at 0.1 MPa, where a portion of the reaction media desired product, p-hydroxybenzaldehyde, was lowered
can be easily withdrawn for the analysis. The yield of due to the formation of p-hydroxybenzyl methyl ether.
p-hydroxybenzaldehyde increased with time and On the other hand, at a temperate higher than 348 K,
reached a plateau at 90% after 13±14 h reaction time. formation of p-hydroxybenzoic acid became apprecia-
As shown in Table 1, a similar yield of 91% was observed ble. These results may suggest the consecutive oxidation
after 3 h reaction time at 0.3 MPa. It is clear from these of p-cresol via p-hydroxybenzyl alcohol, p-hydroxyben-
results that the reaction at 0.3 MPa proceeds more than zaldehyde and finally p-hydroxybenzoica ic d. So the
3 times faster than that at 0.1 MPa. The higher reaction temperature of 348 K has been demonstrated to be sat-
rate at 0.3 MPa, compared with that at 0.1 MPa, can be isfactory.
easily rationalized from the increase in oxygen solubili-
The influence of solvent is summarized in Table 2. No
ty, according to Henry×s law. An oxygen pressure of product was observed in water except for tar. When
0
.3 MPa was used in the following experiments because methanol and water were mixed in the volume ratio of
of experimental limitations of our system. 1:1 and employed as solvent, the conversion of p-cresol
The catalytic oxidation of p-cresol involves three and the selectivity for p-hydroxybenzaldehyde in-
phases: gas, liquid and solid. Then, mass transfer might creased to 92% and 61%, respectively. Methanol among
be the rate-determining step. Figure 2 shows the effect the four solvents tested was the best solvent in the cata-
of the stirring speed on the activity and the selectivity. lyticoxidation of p-cresol. One of the reasons for the
It can be seen that the conversion and the selectivity in- high yields of p-hydroxybenzaldehyde in methanol and
creased with stirring speed. When the stirring speed ethanol should come from the higher solubility of oxy-
reached 700 r.p.m., the 95% yield of p-hydroxybenzal- gen in methanol and ethanol than that in water; oxygen
dehyde was realized. It is considered that the overall re- solubilities at 1 atmosphere in 1 mL solvent at 293 K are
action may be affected by gas transferring from bubbles 0.031, 0.220 and 0.256 mL for water, ethanol and meth-
[
29]
to the liquid phase and by removal of products from the anol, respectively.
In the course of mass transfer,
catalyst surface. Moreover, because that the reaction is methanol is helpful for p-cresol to approach the catalyst
exothermic, it is rather difficult to control the reaction surface and for p-hydroxybenzaldehyde to diffuse into
temperature without stirring. On the other hand, in-
creasing the stirring speed was helpful for quenching de-
Figure 2. Effect of stirring speed on the catalytic activity Re-
Figure 1. Time course of the reaction at 0.1 MPa. Reaction
conditions: catalyst (CuMn/carbon, Cu:Mn¼4:1)¼0.6 g, p-
cresol¼16.0 g, solvent (methanol)¼50 mL, sodium hydrox-
ide¼23.0 g, temperature¼348 K, stirring speed¼700 r.p.m.
action conditions: catalyst (CuMn/carbon, Cu:Mn¼4:1)¼
0.6 g, p-cresol¼16.0 g, solvent (methanol)¼50 mL, sodium
hydroxide¼23.0 g, pressure¼0.3 MPa, temperature¼348 K,
time¼3 h.
Adv. Synth. Catal. 2004, 346, 633±638
asc.wiley-vch.de
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
635

FULL PAPERS
Feng Wang et al.
Figure 3. Influence of reaction temperature on catalytic per-
formance. Reaction conditions: catalyst (CuMn/carbon,
Cu:Mn¼4:1)¼0.6 g, p-cresol¼16.0 g, solvent (meth-
anol)¼50 mL, sodium hydroxide¼23.0 g, pressure¼
Figure 4. Influence of the amount of solvent. Reaction condi-
tions: catalyst (CuMn/carbon, Cu:Mn¼4:1)¼0.6 g, p-
cresol¼16.0 g, sodium hydroxide¼23.0 g, pressure¼
0.3 MPa, temperature¼348 K, time¼3 h, stirring speed¼
0
.3 MPa, time¼3 h, stirring speed¼700 r.p.m.
7
00 r.p.m.
the solvent. Otherwise water, as one of the oxidation
products, offers resistance to the forward reaction kinet- dium hydroxide. Figure 5 shows the effect of the molar
[30,31]
ically.
ratio of NaOH to p-cresol on the conversion and the se-
The influence of the amount of methanol was investi- lectivity for p-hydroxybenzaldehyde. A maximum selec-
gated to minimize solvent use. The results are depicted tivity for p-hydroxybenzaldehyde was obtained at the
in Figure 4. The yield of p-hydroxybenzaldehyde in- molar ratio 4:1 while p-cresol conversion was more
creased to 87% from 51% while the methanol amount than 95% at molar ratios higher than 2. With further in-
varied from 40 to 70 mL. More methanol could help to creases of the NaOH to p-cresol ratio, the selectivity de-
achieve better temperature control and is beneficial clined sharply.
for product diffusion. Another reason for the higher
After 9 h of reaction at 348 K and 0.1 MPa, the CuMn
yield at higher solvent volume may be the increase of catalyst was separated on a B¸chner funnel at 348 K and
the oxygen supply due to the use of a larger amount of the reaction was continued using the filtrate under the
solvent and lower NaOH concentration at the higher same conditions, in a similar manner as in the litera-
amount of methanol. Both are beneficial for increasing
the amount of oxygen in solution.
It has been reported in the literature
[
32,33]
that sodium
hydroxide was essential for oxidation at the benzylicpo-
sition of phenols. The desired product p-hydroxybenzal-
dehyde was not observed in an experiment without so-
[a]
Table 2. Influence of solvent in the oxidation of p-cresol.
[
b]
Solvent
Conversion
mol %)
Yield of HBA
(mol %)
(
H O
17
92
95
98
0
61
71
96
2
[c]
CH OHþH O (1: 1)
3
2
CH CH OH
3
2
CH OH
3
[
a]
Reaction conditions: catalyst (CuMn/carbon, Cu: Mn¼
: 1)¼0.6 g, p-cresol¼16.0 g, solvent¼50 mL, sodium hy- Figure 5. Influence of the molar ratio of NaOH to p-cresol.
4
droxide¼23.0 g, pressure¼0.3 MPa, temperature¼348 K, Reaction conditions: catalyst (CuMn/carbon, Cu:Mn¼
time¼3 h, stirring speed¼700 r.p.m.
p-Hydroxybenzaldehyde.
Volume ratio.
4:1)¼0.6 g, p-cresol¼16.0 g, solvent (methanol)¼50 mL,
pressure¼0.3 MPa, temperature¼348 K, time¼3 h, stirring
speed¼700 r.p.m.
[
[
b]
c]
636
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2004, 346, 633±638

Oxidation of p-Cresol to p-Hydroxybenzaldehyde
FULL PAPERS
[
34]
ture. This experiment was conducted to determine
whether the reaction was promoted by the heterogene-
ous catalyst or by a homogeneous catalyst of dissolved
species. In Figure 6, the conversion was plotted against
time-on-stream for the results before catalyst separation
and after catalyst separation, together with 18 h contin-
ter optimization of the reaction conditions, 99% con-
version of p-cresol and 96% selectivity for p-hydroxy-
benzaldehyde were achieved at 0.3 MPa oxygen pres-
sure, 348 K temperature, NaOH/p-cresol mole ratio
of 4, Cu/Mn mole ratio of 4 and in methanol as a sol-
vent.
uous experiment with catalyst as a reference experi- 2. The activity obeys almost first-order kinetics with re-
ment. The figure clearly shows that no reaction was ob-
served from the filtrate and so the reaction proceeds un-
der heterogeneous catalysis. Furthermore, the re-usabil-
ity of catalyst was tested. In this experiment, the oxida-
spect to oxygen concentration. We found a big differ-
ence in catalytic activity between water and methanol.
This result was also rationalized from the solubility of
oxygen in these solvents.
tion reaction was conducted at 348 K and 0.3 MPa in an 3. A large amount of alkali, NaOH, is needed for the re-
autoclave for 3 h and the catalyst was separated. The action to be highly active and selective.
separated catalyst was washed with deionized water to 4. The catalyst was re-usable after washing and drying.
neutrality and dried at 393 K for 3 hours. This catalyst
was applied for a 2nd experiment by adding fresh p-cre-
sol. Similarly a 3rd experiment was conducted. Results
were conversion¼100%, selectivity for p-hydroxy-
benzaldehyde¼93% for the 2nd experiment and con-
version¼100%, selectivity for p-hydroxybenzalde-
hyde¼88% for the 3rd experiment, showing a minor
drop in selectivity. When the catalysts were regenerated
under the same conditions as washing by methanol and
calcination at 673 K, the catalytic activity was complete-
ly recovered to conversion¼98% and selectivity¼94%,
indicating that there is a small amount of substances
strongly adsorbed on the catalyst surface.
The catalyst can be separated from the reaction mix-
ture by simple filtration. Furthermore, the filtrate af-
ter removing the catalyst did not show any apprecia-
ble reaction, indicating that the reaction was cata-
lyzed on the heterogeneous solid catalyst.
Experimental Section
Reagents
All chemicals were of analytical grade and were purchased
from commercial sources. Oxygen supplied in a high-pressure
cylinder was used without further treatment.
Conclusion
The results can be summarized as follows.
1
. Copper-manganese bimetallicoxide-supported on ac-
tive carbon was found to be effective in the catalytic
oxidation of p-cresol to p-hydroxybenzaldehyde. Af-
Preparation of Catalysts
Catalysts were prepared using a commercial activated carbon
ꢀ1
(
60±80 mesh, sorption capacity 1.4 mL g , specific surface
2
ꢀ1
area 1100 m g ) as support. The support was impregnated
with an aqueous solution of copper and/or manganese nitrates
according to the ratio and metal weight of the catalysts, 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.
Catalytic Oxidation of p-Cresol
The oxidation reactions were carried out in a stirred autoclave
(
Parr Instrument, 600 mL, Series 4560, USA) equipped with a
motor drive magnetically coupled to an internal turbine-type
impeller, a speedometer, a thermocouple, a pressure gauge
and a water-cooling coil. In a typical reaction, p-cresol
(
16.0 g, 0.15 mol), sodium hydroxide (23.0 g, 0.58 mol), metha-
nol (50 mL) and catalyst (0.6 g) were placed in the reactor and
were mixed with the impeller. The reactor was heated up to a
set temperature by an electric furnace. After the desired tem-
perature was reached, oxygen was charged to the reactor
through a dip tube. The pressure was kept constant by supply-
ing oxygen gas during the reaction. The reaction was terminat-
ed when the required time had elapsed.
Figure 6. Influence of catalyst separation. Reaction condi-
tions: catalyst (CuMn/carbon, Cu:Mn¼4:1)¼0.6 g, p-
cresol¼16.0 g, solvent (methanol)¼50 mL, sodium hydrox-
ide¼23.0 g, pressure¼0.1 MPa, temperature¼348 K, stir-
ring speed¼700 r.p.m.
Adv. Synth. Catal. 2004, 346, 633±638
asc.wiley-vch.de
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
637

FULL PAPERS
Feng Wang et al.
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Acknowledgements
This work is supported by the National Natural Science Founda-
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