Metalloporphyrin-catalyzed aerobic oxidation of 2-methoxy-4-methylphenol as a route to vanillin

Article abstract of DOI:10.1016/j.molcata.2013.03.004

We report the use of simple metalloporphyrins (T(p-Cl)PPMnCl, T(p-Cl)PPCo, T(p-Cl)PPFeCl, T(p-Cl)PPCu or [T(p-Cl)PPFe]₂O) as a catalyst for the direct oxidation of 2-methoxy-4-methylphenol to vanillin with molecular oxygen under mild conditions. The research results showed that the type of metalloporphyrin used, catalyst loading, temperature, reaction time, the amount of NaOH, solvent, the amount of solvent and the flow rate of oxygen influenced the conversion of 2-methoxy-4-methylphenol and the selectivity of vanillin. Under the optimal conditions, the conversion of 2-methoxy-4-methylphenol was up to 87% and the selectivity of vanillin reached 74%. A possible mechanism was also proposed for the present oxidation.

Full text of DOI:10.1016/j.molcata.2013.03.004

Journal of Molecular Catalysis A: Chemical 373 (2013) 121–126
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Metalloporphyrin-catalyzed aerobic oxidation of 2-methoxy-4-methylphenol
as a route to vanillin
Qing Jiang, Wenbing Sheng, Xiangdong Guo, Jie Tang, Cancheng Guo∗
College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the use of simple metalloporphyrins (T(p-Cl)PPMnCl, T(p-Cl)PPCo, T(p-Cl)PPFeCl, T(p-Cl)PPCu
or [T(p-Cl)PPFe]2O) as a catalyst for the direct oxidation of 2-methoxy-4-methylphenol to vanillin with
molecular oxygen under mild conditions. The research results showed that the type of metalloporphyrin
used, catalyst loading, temperature, reaction time, the amount of NaOH, solvent, the amount of solvent
and the flow rate of oxygen influenced the conversion of 2-methoxy-4-methylphenol and the selectivity
of vanillin. Under the optimal conditions, the conversion of 2-methoxy-4-methylphenol was up to 87%
and the selectivity of vanillin reached 74%. A possible mechanism was also proposed for the present
oxidation.
Received 21 September 2012
Received in revised form 1 March 2013
Accepted 10 March 2013
Available online 16 March 2013
Keywords:
Vanillin
Metalloporphyrins
2-Methoxy-4-methylphenol
Oxidation
© 2013 Elsevier B.V. All rights reserved.
Molecular oxygen
1. Introduction
oxidation methods which can be performed under safe and mild
conditions.
Vanillin (4-hydroxy-3-methoxybenzaldehyde) is one of the
most important flavors and has attracted considerable synthetic
interest [1] owing to its importance in various fields; it is exten-
sively used as fragrance in food preparations [1,2], intermediate
in the production of antifoaming agents, herbicides or drugs [3,4],
ingredient of household products such as air-fresheners and floor
polishes, and, because of its antioxidant and antimicrobial prop-
erties [5,6], also as food preservative [6–9]. Traditionally, vanillin
is mainly obtained by chemical methods from lignin, coniferin,
the glucoside of coniferyl alcohol, guaiacol, or eugenol [1,10–12].
However, these methods have drawbacks such as multiple steps,
relatively high costs and serious environmental pollution due to
the generation of excessive wastes. More recently, the produc-
tion of vanillin via biotechnological methods has been extensively
investigated [1,13–25]. While all of these methods are useful in
its own right, each suffers from one or more limitations includ-
ing long production time, difficult purification, relatively high costs
and necessity of selected strains of microorganisms. Besides, there
are few reports on the homogeneously or heterogeneously cat-
alytic direct oxidation of 2-methoxy-4-methylphenol by oxygen
to vanillin [26–30]. Although great advances have been made, it
is still highly desirable to develop efficient and selective catalytic
In our previous studies, metalloporphyrins exhibited high cat-
alytic performance for oxidation of hydrocarbons with molecular
oxygen [31–50]. As a continuation of our research involving
metalloporphyrins-catalyzed oxidations with dioxygen, we now
report a novel catalytic system for the direct oxidation of 2-
methoxy-4-methylphenol to vanillin by using molecular oxygen
(1 atm) as oxidant and metalloporphyrin as the catalyst under mild
conditions. The optimum conditions of the process were obtained
through investigating the effects of various reaction parameters
such as the type of metalloporphyrin used, catalyst loading, tem-
perature, reaction time, the amount of NaOH, solvent, the amount
of solvent and the flow rate of oxygen on the conversion of 2-
methoxy-4-methylphenol, and selectivity of vanillin. Furthermore,
a possible mechanism for the observed catalytic oxidation reaction
was also proposed.
2. Experimental
2.1. General reagents and equipments
All solvents and reagents were obtained commercially and
used without further purification unless indicated otherwise. Sim-
ple metalloporphyrins (T(p-Cl)PPMnCl, T(p-Cl)PPCo, T(p-Cl)PPFeCl,
T(p-Cl)PPCu or [T(p-Cl)PPFe]2O) were synthesized according to the
literature methods [51,52]. MS spectra were recorded on a Shi-
madzu OP-2010 GC-MS and an Agilent 1100 LC-MS. UV–Vis spectra
∗
Corresponding author. Tel.: +86 731 8821314; fax: +86 731 8821667.
E-mail address: ccguo@hnu.edu.cn (C. Guo).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.03.004

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Q. Jiang et al. / Journal of Molecular Catalysis A: Chemical 373 (2013) 121–126
OH
OH
OH
OH
OH
H3CO
H3CO
H3CO
+
H3CO
+
H3CO
+
other
Metalloporphyrins
CH3OH, NaOH, 338 K
+
O2
+
products
CHO
CH2OH
COOH
4
CH2OCH3
5
1
2
3
Scheme 1. Aerobic oxidation of 2-methoxy-4-methylphenol catalyzed by metalloporphyrins.
were obtained on a ShimadzuUV-2450 spectrometer. LC analysis
was performed on an Agilent 1100.
not only related to the bond energy between metal and oxygen,
but also related to the redox potential of metal ions. This is also
consistent with our previous report [54] that the catalytic activity
of metalloporphyrin is related to the electronic arrangement of
central metal: the catalytic activity of metalloporphyrins with high
spin arrangement is much higher than that of metalloporphyrins
with low spin arrangement and with antiferromagnetic coupling.
As shown in Fig. 1, T(p-Cl)PPMnCl will be selected as the best
catalyst for optimization of the experimental conditions.
2.2. Typical procedure for oxidation of
2-methoxy-4-methylphenol using molecular oxygen catalyzed by
metalloporphyrins
To a 100 mL three-neck round bottom flask equipped with a
magnetic stirring bar, condenser, and an oxygen gas inlet was added
2-methoxy-4-methylphenol (4.14 g, 30 mmol), catalyst (7.2 mg),
sodium hydroxide (6 g, 0.15 mol) and methanol (20 mL). Oxygen
(0.1 MPa) was continuously introduced into the flask at flow rate
of 0.1 L min−1 while the temperature was kept at 338 K (the boiling
point of methanol). After the reaction was completed, the mixture
was cooled to room temperature and the products were quantita-
tively analyzed by HPLC
3.2. Effect of catalyst loading
T(p-Cl)PPMnCl presented the highest selectivity and yield of 2,
so we investigated the effect of the amount of T(p-Cl)PPMnCl on the
oxidation of 1. From Fig. 2, it was observed that the conversion of 1
and the selectivity of 2 increased with the increase of the amount of
T(p-Cl)PPMnCl until its amount reached 7.2 mg. The conversion of
1 was almost unchanged when the amount of T(p-Cl)PPMnCl was
increased from 7.2 mg to 10 mg, while the selectivity of 2 decreased.
Surprisingly, further increase of the catalyst amount led to lower 1
conversion (72% when 15 mg of T(p-Cl)PPMnCl was used, 53% when
20 mg of T(p-Cl)PPMnCl was used). This observation is consistent
with our earlier results that the oxidation reaction can be inhibited
at high concentration of metalloporphyrin [34,41,43]. In addition
to the desired product 1, by-products such as vanillyl alcohol (3),
vanillic acid (4) and 4-hydroxy-3-methoxybenzyl methyl ether (5)
were observed. It was found that the selectivity of 3, 4 and 5
almost remained unchanged with the increase of the amount of T(p-
Cl)PPMnCl. Besides the above products, a black tarry product was
formed in the reaction process. Similar phenomena also observed
by Chaudhari in the liquid phase catalytic oxidation of p-cresol
[55]. The tarry product was obviously observed in the absence of
2.3. Products analysis
The oxidation products were identified by GC–MS, LC–MS, by
HPLC co-injection of commercially available authentic samples, and
quantitated by Agilent 1100 HPLC equipped with a UV (280 nm)
detector and a Zorbax SB-C8 (4.6 mm × 250 mm) column. Eluent
consisting of 35% CH3OH and 65% phosphoric acid buffer (pH = 2.5)
with a flow rate of 0.6 mL min−1 was used. Vanillin, vanillyl alcohol,
vanillic acid and 4-hydroxy-3-methoxybenzyl methyl ether were
determined by the internal standard method using benzoic acid as
the internal standard.
3. Results and discussion
2-methoxy-4-methylphenol (1) was efficiently oxidized by
dioxygen under mild conditions in the presence of metallopor-
phyrins (shown in Scheme 1). Based on LC, GC–MS and LC–MS
analysis, the major product was vanillin (2). In most cases, the
by-products were vanillyl alcohol (3), vanillic acid (4), 4-hydroxy-
3-methoxybenzyl methyl ether (5) and tarry material.
100
Conversion of 1
Selectivity for 2
Yield of 2
80
3.1. Effect of different metalloporphyrins
60
40
20
The performance of different metalloporphyrins (T(p-
Cl)PPMnCl,
T(p-Cl)PPCo,
T(p-Cl)PPFeCl,
T(p-Cl)PPCu
or
[T(p-Cl)PPFe]2O) for 2-methoxy-4-methylphenol oxidation
was studied under the same reaction conditions. The results are
shown in Fig. 1. It could be seen that 1 was just oxidized with 16%
conversion in the absence of metalloporphyrin catalyst. On the
other hand, it was found that various metalloporphyrins presented
different catalytic activities in this reaction system. T(p-Cl)PPFeCl
showed much higher conversion than T(p-Cl)PPMnCl. Other
metalloporphyrins (T(p-Cl)PPCo, T(p-Cl)PPCu or [T(p-Cl)PPFe]2O)
exhibited rather low activity under the same conditions. However,
T(p-Cl)PPMnCl was superior to other metalloporphyrins for selec-
tivity and yield of 2. The catalytic reactivity differences between
catalyst systems might be related to the redox electric potential
and the stability of different valences of the metal atoms [53].
Similar results were also reported by Ye et al. [53] that the catalytic
activity of different metal ions in the liquid phase oxidation was
0
O
Cl
Mn
Cl
Fe
]
lyst
cata
2
Fe
PPCu
PPCo
)
)
l
PP
PP
PP
)
)
)
-C
-Cl
p
p
No
-Cl
-Cl
p
-Cl
p
T(
T(
p
(
T(
T(
T
[
Catalysts
Fig. 1. Catalyst performance over different catalysts. Reaction conditions: 2-
methoxy-4-methylphenol = 4.14 g, catalyst = 7.5 mg, solvent (methanol) = 20 mL,
sodium hydroxide = 6.0 g, temperature = 338 K, time = 3.5 h, O2 pressure = 0.1 MPa,
O2 flow rate = 0.1 L min−1
.

Q. Jiang et al. / Journal of Molecular Catalysis A: Chemical 373 (2013) 121–126
123
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
Conversion of
Selectivity for
Selectivity for
Selectivity for
Selectivity for
1
Conversion of
1
2
Selectivity for
Selectivity for
Selectivity for
Selectivity for
2
50
40
30
20
10
0
5
3
4
5
3
4
Selectivity for others
Selectivity for others
0
5
10
15
20
0
1
2
3
4
5
6
T(p-Cl)PPMnCl (mg)
Time (h)
Fig. 2. Effect of catalyst loading on catalytic performance. Reaction condi-
tions: 2-methoxy-4-methylphenol = 4.14 g, solvent (methanol) = 20 mL, sodium
hydroxide = 6.0 g, temperature = 338 K, time = 3.5 h, O2 pressure = 0.1 MPa, O2 flow
Fig. 4. Effect of the reaction time on catalytic performance. Reaction con-
ditions: 2-methoxy-4-methylphenol = 4.14 g, T(p-Cl)PPMnCl = 7.5 mg, solvent
(methanol) = 20 mL, sodium hydroxide = 6.0 g, temperature = 338 K, O2 pres-
rate = 0.1 L min−1
.
sure = 0.1 MPa, O2 flow rate = 0.1 L min−1
.
3.4. Effect of reaction time
catalyst. Some studies on the analysis of the tarry product are cur-
rently under investigation.
The influence of reaction time on catalytic performance was
investigated and the results are shown in Fig. 4. It was found that
the selectivity of 3, 4 and 5 almost kept unchanged and the con-
version gradually increased during the oxidation of 1 from 0.5 h
to 6 h. The selectivity of 2 increased with the reaction time and
reached the maximum at about 3.5 h. The reaction speed sharply
increased during 0.5–2.5 h, which matched with the chain propaga-
tion process in the radical oxidation. Moreover, if the oxidation was
continuingly preceded to 6 h or longer reaction time, the selectiv-
ity of 2 drastically decreased due to formation of side products, viz.,
vanillic acid (4) and tarry product, in higher amount. The increase
in reaction time facilitated deep oxidation of 2 to vanillic acid (4)
and tarry product [55], leading to a decrease in selectivity of 2. So
3.5 h is considered as a preferable reaction time. From Fig. 4, it was
also observed that the selectivity of 2 linearly correlated with the
reaction time during 0.5–2.5 h, thus displaying a zero-order reac-
tion characteristic in the reaction kinetics. This also agrees with
the kinetic relationship in the reaction catalyzed by an enzyme
[56]. After 2.5 h, this linear relationship deviated gradually. This
phenomenon is consistent with our earlier reports [57–59].
3.3. Effect of temperature
The effect of temperature on both conversion of 1 and selectiv-
ity of 2 was studied in the temperature range of 328–343 K, and the
results are presented in Fig. 3. It was shown that the selectivity of 3,
4 and 5 almost kept unchanged and the conversion was increased
from 70% to 89% with the reaction temperature increasing. The
selectivity for 2 increased from 66 to 74% with the increase of tem-
perature from 328 K to 338 K. After that, selectivity to 2 decreased
drastically due to excessive formation of tar. When the tempera-
ture was 338 K, an 87% conversion and 74% selectivity of 2 were
obtained in the presence of T(p-Cl)PPMnCl. So the temperature of
338 K has been demonstrated to be satisfactory.
100
90
3.5. Effect of the amount of NaOH
80
70
It has been reported in the literature [60–62] that sodium
hydroxide was crucial for oxidation at the benzylic position of
phenols for two reasons: firstly, it reacts with the hydroxyl of 1
and prevents oxidation on the benzene ring; secondly, it is gen-
erally believed that acid media lead to hypo oxidation and base
media are beneficial for selective oxidation. The reaction did not
proceed at all without sodium hydroxide. Hence, it was important
to study the effect of the amount of NaOH on catalytic perfor-
mance. In Fig. 5, the conversion of 1 increased from 61 to 87%
with increase in NaOH to 1 mole ratio from 3 to 5, and it sharply
decreased with further increase in mole ratio to 6. One possible rea-
son could be that the mass transfer resistance of the overall reaction
system increased when a higher amount of NaOH was employed.
To support this, on one hand, the solubility of NaOH in methanol
was measured through acid–base titration using phenolphthalein
as indicator; NaOH solubility at 1 atmosphere in 1 mL solvent at
338 K is 0.32 g. This showed that the viscosity of the overall reaction
60
Conversion of 1
Selectivity for 2
50
Selectivity for 5
Selectivity for 3
40
Selectivity for 4
Selectivity for others
30
20
10
0
325
330
335
340
345
Temperature (K)
Fig. 3. Effect of temperature on catalytic performance. Reaction condi-
tions: 2-methoxy-4-methylphenol = 4.14 g, T(p-Cl)PPMnCl = 7.5 mg, solvent
(methanol) = 20 mL, sodium hydroxide = 6.0 g, time = 3.5 h, O2 pressure = 0.1 MPa,
O2 flow rate = 0.1 L min−1
.

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Q. Jiang et al. / Journal of Molecular Catalysis A: Chemical 373 (2013) 121–126
100
100
80
60
40
20
0
Conversion of 1
Selectivity for 2
Conversion of 1
Selectivity for 2
95
90
85
80
75
70
65
60
0
1
2
3
4
5
6
8
10
12
14
16
18
20
22
24
26
mole ratio of NaOH to1
Amount of methanol (mL)
Fig. 5. Effect of mole ratio of NaOH to
conditions: 2-methoxy-4-methylphenol = 4.14 g, T(p-Cl)PPMnCl = 7.5 mg, solvent
1 on catalytic performance. Reaction
Fig. 7. Effect of the amount of methanol on catalytic performance. Reaction
conditions: 2-methoxy-4-methylphenol = 4.14 g, T(p-Cl)PPMnCl = 7.5 mg, sodium
hydroxide = 6.0 g, time = 3.5 h, temperature = 338 K, O2 pressure = 0.1 MPa, O2 flow
(methanol) = 20 mL, time = 3.5 h, temperature = 338 K, O2 pressure = 0.1 MPa, O2 flow
rate = 0.1 L min−1
.
rate = 0.1 L min−1
.
3.6. Effect of solvent
system increased with the increase of the amount of NaOH, which
in turn affected mass transfer of the overall reaction, resulting in a
decrease in conversion of 1 and selectivity of 2 at the molar ratio
6:1. On the other hand, the influence of the stirring speed on the
activity and the selectivity was investigated. The conversion and
the selectivity increased with stirring speed (75% conversion and
67% selectivity of 2 were realized when the stirring speed reached
600 r.p.m., 82% conversion and 70% selectivity of 2 were realized
when the stirring speed reached 700 r.p.m., 87% conversion and
74% selectivity of 2 were realized when the stirring speed reached
800 r.p.m.). This indicated that mass transfer resistance was impor-
tant in the reaction system. Further experiments for ascertaining
the importance of mass transfer resistance is in progress for kinetic
studies. Moreover, a maximum selectivity for 2 was obtained at the
molar ratio 5:1. With further increases of the NaOH to 1 ratio, the
selectivity decreased.
To evaluate the suitability of a solvent for 1 oxidation, three com-
mon solvents (water, ethanol, and methanol) were screened. As
can be seen from Fig. 6, the conversion of 1 was markedly affected,
while the selectivity of 2 was dramatically altered for water and
ethanol as oxidation solvents. A trace product was observed in
water except for tar. When methanol and water were mixed in the
volume ratio of 1:1 and employed as solvent, 14% conversion of 1
and 56% selectivity of 2 were obtained. Among the solvents exam-
ined, methanol was the best solvent in the catalytic oxidation of 1.
One of the reasons for the high conversion of 1 and the high selec-
tivity of 2 in methanol should come from the higher solubility of
oxygen in methanol than that in water and ethanol; oxygen solubil-
ities at 1 atmosphere in 1 mL solvent at 293 K are 0.256, 0.220 and
0.031 mL for methanol, ethanol and water, respectively [63]. More-
over, water, as one of the oxidation products, offers resistance to
the forward reaction kinetically [64,65].
100
80
100
Conversion of
Selectivity for
1
90
80
70
60
50
40
30
20
10
0
2
Yield of
2
60
40
Conversion of 1
Selectivity for 2
20
0
H2O
CH3OH + H2 O
CH3CH2OH
CH3OH
-0.02 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16
flow rate of oxygen (L/min)
Solvents
Fig. 8. Effect of the flow rate of oxygen on catalytic performance. Reaction
conditions: 2-methoxy-4-methylphenol = 4.14 g, T(p-Cl)PPMnCl = 7.5 mg, solvent
(methanol) = 20 mL, sodium hydroxide = 6.0 g, time = 3.5 h, temperature = 338 K, O2
pressure = 0.1 MPa.
Fig. 6. Oxidation of benzyl alcohol in different solvents. Reaction conditions: 2-
methoxy-4-methylphenol = 4.14 g, T(p-Cl)PPMnCl = 7.5 mg, solvent = 20 mL, sodium
hydroxide = 6.0 g, time = 3.5 h, temperature = 338 K, O2 pressure = 0.1 MPa, O2 flow
rate = 0.1 L min−1
.

Q. Jiang et al. / Journal of Molecular Catalysis A: Chemical 373 (2013) 121–126
125
O2
O-(H+ Na+)
O-(H+ Na+)
OCH3
O
H3CO
H3CO
SET
O2
Highly active
intermidiate
T(p-Cl)PPMnCl
CH3
CH2
CH2OO
O-(H+ Na+)
H3CO
O-(H+ Na+)
O-(H+ Na+)
OCH3
H3CO
CH3
CH3
CH2OOH
NaOH
OH
O-(H+ Na+)
OCH3
O-(H+ Na+)
OCH3
H3CO
CH3
CH2OH
CHO
[O]
CH3OH
O-(H+ Na+)
OCH3
O-(H+ Na+)
OCH3
COOH
CH2OCH3
Scheme 2. Proposed reaction mechanism.
3.7. Effect of the amount of methanol
radical inhibitor can quench substantially the oxidation reaction.
This showed that the oxidation probably proceeded in a radical
pathway. When the oxidation of vanillyl alcohol was performed
with this catalytic system, the reaction was very fast and the con-
version was more than 95% under the same conditions, but the
selectivity of 2 was less than 50%. According to the results that
we obtained here and the above, a direct pathway leading from
1 to 2 seemed likely. In addition, the oxidation did not occur in the
absence of molecular oxygen even with NaOH present (see above),
further indicating that molecular oxygen is the ultimate oxidant
rather than the oxygen contained in the OH− anion.
In metalloporphyrin/O2 system, the active species is recog-
nized to be high-valent metal-oxo species [67]. Here we have
not identified the short-lived intermediate derived from T(p-
Cl)PPMnCl, but the candidates are the manganese-oxo complexes
([PMnIV = O]). On the basis of these preliminary data and related
reports [31,32,68–71], a possible mechanism for the present pro-
cess is proposed as shown in Scheme 2. Under the reaction
conditions, T(p-Cl)PPMnCl react with O2 to give intermediate
[PMnIV = O]. The intermediate [PMnIV = O] readily abstracts one
electron from phenoxyl anion formed by 1 reacting with sodium
hydroxide to produce phenoxyl radical, which was further trans-
formed to benzyl radical through a single-electron transfer process.
The resulting benzyl radical being trapped by molecular oxygen
provides peroxy radical, which is eventually converted into prod-
ucts through hydroperoxide. Nevertheless, a detailed mechanism
on the present catalytic oxidation reaction remains to be fully inves-
tigated.
The influence of the amount of methanol on catalytic perfor-
mance was investigated and the results are depicted in Fig. 7. In
this figure, the conversion of 1 increased with the increase of the
amount of methanol. The selectivity of 2 increased from 66 to 74%
with increase in methanol from 10 mL to 20 mL, and it dramatically
decreased with further increase of the amount of methanol. One
plausible explanation was the increase of the oxygen supply due to
the use of a larger amount of methanol and lower NaOH concen-
tration at the higher amount of methanol. Moreover, the increase
in the amount of methanol resulted in deep oxidation of 2 to tarry.
3.8. Effect of the flow rate of oxygen
To assess the effect of the flow rate of oxygen on 1 oxidation, the
catalytic performance was investigated in 0–0.15 L min−1 oxygen
flow rates. As indicated in Fig. 8, no 1 oxidation was observed in
the absence of molecular oxygen. In Fig. 8, it could be clearly seen
that the best flow rate of oxygen is about 0.1 L min−1. When the flow
rate is below 0.1 L min−1, the conversion of 1 and the selectivity of
2 increased with the increase of the flow rate of oxygen. But when
the flow rate is higher than 0.1 L min−1, the conversion of 1 and
the selectivity of 2 are decreasing with the increasing of the flow
rate of oxygen. This indicated that the aerobic oxidation of 1 was
influenced by the diffusion rate of oxygen when the flow rate of
oxygen is small than 0.1 L min−1, and that the oxidation reaction
was controlled by the kinetic factors [47].
3.9. Mechanism of oxidation of 2-methoxy-4-methylphenol
4. Conclusion
Several control experiments were carried out to probe the reac-
tion mechanism. The oxidation of 1 in the presence of 0.1 or
1.2 equiv of hydroquinone [66] was examined. The addition of the
In summary, we have developed simple metalloporphyrins for
catalytic oxidation of 2-methoxy-4-methylphenol with molecu-
lar oxygen. The main oxidation product was vanillin. Under mild

126
Q. Jiang et al. / Journal of Molecular Catalysis A: Chemical 373 (2013) 121–126
conditions, this research realized 74% selectivity for vanillin at 87%
conversion of 2-methoxy-4-methylphenol. Further experiments
are underway to fully elucidate the reaction mechanism. Contin-
ued development of metalloporphyrin-based catalytic oxidation of
hydrocarbons also remains.
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Acknowledgement
This project was financially supported by The Chinese National
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1
The definition of conversion and selectivity are as follows: conversion = moles of
2-methoxy-4-methylphenol consumed/moles of 2-methoxy-4-methylphenol used
×100%; selectivity = moles of product/moles of 2-methoxy-4-methylphenol con-
sumed × 100%.