Cobalt porphyrins immobilized on polymer microspheres as catalysts for the oxidation of 2-naphthol to 2-hydroxy-1,4-naphthoquinone by molecular oxygen

Article abstract of DOI:10.1007/s10562-011-0703-2

Three types of porous polymer microspheres immobilized with cobalt porphyrins appending p-H, p-Cl and p-NO₂ phenyl substituents (designated as CoPP-GMA/MMA, CoCPP-GMA/MMA and CoNPP-GMA/MMA, respectively) were prepared. Their catalytic activities on the oxidation of 2-naphthol to 2-hydroxy-1,4-naphthoquinone by molecular oxygen were investigated in alkaline methanol. The experimental results showed that the porous microsphere supported cobalt porphyrin catalysts could effectively activate molecular oxygen, and 2-naphthol was selectively oxidized to 2-hydroxy-1,4-naphthoquinone with high conversion in alkaline methanol. A phenomenon of distance-dependent catalytic activity was observed and a critical distance of 3.8 nm between porphyrins was determined for the porous polymer microsphere supported catalyst. More interestingly, the activity of the recycled catalyst increased gradually with the increased times of reuse. These results may be helpful in designing highly efficient metalloporphyrin catalysts. Graphical Abstract: The catalytic oxidation of 2-naphthol to 2-hydroxy-1,4-naphthoquinone (HNQ) by molecular oxygen was performed in alkaline methanol using supported cobaltporphyrin as catalyst.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-011-0703-2

Catal Lett (2011) 141:1808–1813
DOI 10.1007/s10562-011-0703-2
Cobalt Porphyrins Immobilized on Polymer Microspheres
as Catalysts for the Oxidation of 2-naphthol
to 2-hydroxy-1,4-naphthoquinone by Molecular Oxygen
•
Jing Zhao Bao-jiao Gao Yong-kuan Gong
•
Received: 14 July 2011 / Accepted: 10 September 2011 / Published online: 18 October 2011
Ó Springer Science+Business Media, LLC 2011
Abstract Three types of porous polymer microspheres
immobilized with cobalt porphyrins appending p–H, p-Cl
and p-NO2 phenyl substituents (designated as CoPP-GMA/
MMA, CoCPP-GMA/MMA and CoNPP-GMA/MMA,
respectively) were prepared. Their catalytic activities on
the oxidation of 2-naphthol to 2-hydroxy-1,4-naphthoqui-
none by molecular oxygen were investigated in alkaline
methanol. The experimental results showed that the porous
microsphere supported cobalt porphyrin catalysts could
effectively activate molecular oxygen, and 2-naphthol was
selectively oxidized to 2-hydroxy-1,4-naphthoquinone with
high conversion in alkaline methanol. A phenomenon of
distance-dependent catalytic activity was observed and a
critical distance of 3.8 nm between porphyrins was deter-
mined for the porous polymer microsphere supported cat-
alyst. More interestingly, the activity of the recycled
catalyst increased gradually with the increased times of
reuse. These results may be helpful in designing highly
efficient metalloporphyrin catalysts.
1 Introduction
2-hydroxy-1,4-naphthoquinone (HNQ, Lawsone) is the
main ingredient found in henna, a herb which has excellent
dyeing properties used traditionally for centuries in Asia.
Nowadays, HNQ and its derivatives are also exploited as
potential anti-cancer drugs [1–4], redox intermediates in
bioelectrochemical fuel-cell [5, 6] and skin care products
[7, 8], besides their worldwide applications as a major
ingredient in many hair dyes and hair-care products. As one
of the most important chemicals, the complication and low
efficiency of separation and purification of HNQ from the
plants is not satisfied. High efficient chemical synthesis
will be a good choice.
HNQ appears as both an electron donor and receptor, the
chemical synthesis of HNQ from 2-naphthol is challenged
by delicated oxidation with high selectivity. Therefore,
selective catalytic oxidation of naphthol is one of the best
ways for preparation of HNQ [7]. By mimicking biological
enzyme, metalloporphyrins have shown excellent catalytic
activity and selectivity for oxidation of hydrocarbons [9].
More excitingly, selective catalytic oxidations of naphthol
to HNQ by hydrogen peroxide and by molecular oxygen
under air pressure in an alkaline solution over iron por-
phyrin catalysts have been realized [7, 10]. By using
molecular oxygen as the oxidant and a supported metal-
loporphyrin as the catalyst, the economical and environ-
mental benefits will be greatly significant in synthesizing
HNQ. However, the reported method showed only a 16.9%
conversion and in low concentration level when methanol
was used as the solvent. Definitely, it is necessary to
investigate the reaction system for high yield in high
concentration level.
Keywords Cobalt porphyrin Á Naphthol Á
Catalytic oxidation Á 2-Hydroxy-1,4-naphthoquinone Á
Dioxygen
J. Zhao Á Y. Gong (&)
College of Chemistry and Materials Science, Northwest
University, Xi’an 710069, People’s Republic of China
e-mail: gongyk@nwu.edu.cn
B. Gao (&)
Department of Chemical Engineering, North University
of China, Taiyuan 030051, People’s Republic of China
e-mail: gaobaojiao@126.com
In this study, a series of polymer microsphere supported
cobalt porphyrins were prepared and their catalytic
123

Catalysts for the Oxidation of 2-naphthol
1809
properties on the oxidation of 2-naphthol to HNQ by
molecular oxygen in alkaline methanol was investigated.
Interestingly, the cobalt porphyrins not only increased the
yield of HNQ by 80%, the catalytic activity also increased
gradually with the reused times.
The benzaldehyde groups covalently attached on the
microspheres were further reacted with pyrrole and benz-
aldehyde or its derivatives in solution by Adler reaction.
Thus, the synchronic synthesis and immobilization of
phenyl porphyrins on the microspheres (RPP-GMA/MMA)
were performed successfully. The reactions of the surface
functionalization were shown in scheme 1 and the detailed
procedures of the preparation were reported elsewhere
[12]. Via the coordination reaction between the porphyrins
on the GMA/MMA microspheres and cobalt ions in solu-
tion, polymer-immobilized cobalt phenyl porphyrin CoPP-
GMA/MMA, cobalt chlorphenyl porphyrin CoCPP-GMA/
MMA and cobalt nitrobenzophenone porphyrin CoNPP-
GMA/MMA were prepared according to previous study
[11]. The immobilized density of cobalt porphyrins (lmol/
g) was determined by atomic absorption spectroscopy via
the cobalt content in the polymer microspheres. The
chemical structures were shown in Scheme 2. A typical
SEM image of the CoPP-GMA/MMA microspheres was
shown in Fig. 1.
2 Experimental
2.1 Materials and Instruments
2-Naphthol purchased from Tianjin Damao Chemical
Reagents Factory, 2-hydroxy-1,4-naphthoquinone (HNQ)
purchased from Alfa Aesar Company, methanol purchased
from Tianjin Chemical Reagent Co. Ltd., sodium hydrox-
ide (NaOH) purchased from Beijing Chemical Plant, all
reagents used were analytical reagent grade.
The instruments used in this study were as follows:
Thermo SOLAAR atomic absorption spectrometer (AAS,
Thermo Company, USA), HP 6890 gas chromatograph
(GC, Agilent company, USA). UV-2602 spectrophotome-
ter (Shanghai unique company), X-5-type micro melting
point apparatus (Gongyi Instrument Co. Ltd. China), LEO
438 VP scanning electron microscope (SEM, LEO Electron
Microscopy Ltd.).
2.3 Catalytic Oxidation of 2-Naphthol
A certain amount of the cobalt porphyrin catalyst was
soaked in 30 mL methanol for 12 h, then 0.80 g 2-naphthol
and 5.0 g NaOH were added and dissolved in the methanol.
The 2-naphthol was oxidized by flowing oxygen at 0 °C
and the catalytic reaction was continued for 2.5 h. After
filtering out the solid catalyst, the filtrate was diluted with
30 mL distilled water and precipitated with hydrochloric
acid. Bright orange precipitate was obtained by filtration,
washing and drying in vacuum. The melting point of the
product is 190–192 °C. The selectivity and yield of the
catalytic reactions were quantitatively analyzed by gas
chromatography and UV–Vis techniques [7].
2.2 Polymer Microsphere Supported Cobaltporphyrin
Catalysts
Crosslinked copolymer microspheres GMA/MMA of
glycidyl methacrylate (GMA) and methyl methacrylate
(MMA) were prepared in a suspension polymerization
system according to our previous report [11]. p-hydroxyl
benzaldehyde (HBA) was bound onto the microspheres via
ring-opening reaction of the epoxy groups on GMA/MMA.
Scheme 1 Surface reaction
process for the cobalt
porphyrin-immobilization
GMA/
MMA
GMA/
MMA
CH
HO
CHO
CH2
CH
OH
+
CH2
O
CHO
O
4
1)
N
2)
3
CHO
Lactic acid
3)
NH N
N HN
GMA/
MMA
CH
CH2
O
OH
123

1810
J. Zhao et al.
3.0
2.5
2.0
1.5
1.0
0.5
0.0
R
A
B
A Product mixture
B 2-Naphthol
C HNQ
N
N
N
N
GMA/
MMA
O
CH CH2
OH
Co
R
R
452nm
C
200 250 300 350 400 450 500 550 600
/nm
R=H
R=Cl
CoPP-GMA/MMA
CoCPP-GMA/MMA
Fig. 2 UV-visible spectra of 2-naphthol, HNQ and product mixture
R=NO2 CoNPP-GMA/MMA
Scheme 2 Chemical structures of three kinds of cobalt porphyrin-
immobilized microspheres
32
CoCPP-GMA/MMA
Without cat.
28
24
20
16
12
8
4
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
t/h
Fig. 3 Variation of HNQ yield with time. (Amount of CoCPP-GMA/
MMA: 0. 163 g; Density of CoCPP: 64.49 lmol/g; Temperature:
0 °C)
Fig. 1 Scanning electron micrograph of CoPP-GMA/MMA
microspheres
3.2 Catalytic Activity of the Immobilized Cobalt
Porphyrins
3 Results and Discussion
The oxidation reaction of 2-naphthol was conducted in
alkaline methanol solvent at 0 8C with a fixed oxygen flow
rate under atmospheric pressure. The variation of HNQ
yield with reaction time with or without catalyst CoCPP-
GMA/MMA is shown in Fig. 3.
3.1 UV–visible Absorption Spectra of the Reactant
and Product
The UV–visible spectra of 2-naphthol, HNQ and product
mixture are showed in Fig. 2. The spectrum of HNQ shows
a wide peak at 452 nm besides its UV absorption at 231
and 268 nm. The absorption wavelength of 2-naphthol is
less than 400 nm. Furthermore, the possible peroxidation
products of 2-naphthol, naphthalic acids have no absorp-
tion over 400 nm. As the GC analysis does not detect other
naphthoquinone product in the reaction, the spectrum of the
product mixture is simply a superposition of 2-naphthol
and HNQ spectra. Therefore, the absorbance value of the
product at 452 nm is used for determining the concentra-
tion of HNQ. Thus, the conversion percentage of the
reactant or yield of the reaction is calculated and the
activity of the catalysts is evaluated.
The oxidation reaction of 2-naphthol to HNQ by
molecular oxygen was very difficult without catalyst. With
the addition of 0.163 g CoCPP-GMA/MMA microspheres
to the same reaction system, HNQ was produced easily and
its concentration increased in 2.5 h. GC analysis and UV–
visible measurement showed that the collected products
were mainly HNQ with a selectivity no less than 99%. The
results suggest that the polymer microsphere supported
cobalt porphyrin CoCPP-GMA/MMA can activate molec-
ular oxygen effectively in alkaline methanol. As HNQ is
unstable at high temperature [13], low reaction temperature
is preferable for maintaining high selectivity. However,
prolonged reaction could induce further oxidation of the
123

Catalysts for the Oxidation of 2-naphthol
1811
HNQ and produce final product of naphthalic acids [14].
Therefore, the catalytic reaction should be stopped in 2.5 h
for high yield and 99% selectivity.
combines the advantages of high activity, high selectivity
(C99% for HNQ), a clean oxidant, mild conditions, as well
as recycled catalysts.
3.3 Effect of Various Factors on Catalytic Activity
3.3.2 Influence of the Porphyrin Ring Substituents
3.3.1 Effect of Sodium Hydroxide Amount
Three catalysts CoPP-GMA/MMA (66.18 lmol/g), CoC-
PP-GMA/MMA (64.49 lmol/g) and CoNPP-GMA/MMA
(64.25 lmol/g) with similar immobilization density of
cobalt porphyrins were selected for studying the effect of
the porphyrin ring substituents. The catalytic oxidation
reactions were conducted according to the steps described
in Sect. 2.3 under the same conditions. The variations of
HNQ yields with reaction time were shown in Fig. 5.
Figure 5 indicates that all the cobalt porphyrins have
obvious catalytic effect on the oxidation of 2-naphthol to
HNQ by molecular oxygen. All the HNQ yields increased
rapidly with time and reached their maxima in the same
time. However, the maxima yields are different only due to
the substitute groups, since the polymer microspheres
of the catalysts are exactly identical. By comparison,
the catalytic activities of the immobilized cobaltporphyrins
are in the order of CoPP-GMA/MMA [ CoCPP-GMA/
MMA [ CoNPP-GMA/MMA. Obviously, the effects of
the substitute groups on the activity are phenyl [ chloro-
phenyl [ nitrophenyl. The influence of substitute groups
on the catalytic activity of CoRPP is an effect of electron
repelling or attracting on the central metal ion in the large
conjugated porphyrin. According to the literature [13],
electron repelling or electron attracting groups on the
contrapuntal position of phenyl and the porphyrin macro-
cycle are located at the same conjugated system, which
exclude or withdraw electron from the central metal ion of
porphyrin through conjugating and inducing. The forma-
tion and stability of catalytic intermediate called high-
valence metalloporphyrin are affected, so that the same
Sodium hydroxide amount or concentration plays an
important role in the oxidation reactions. Although NaOH
is not a catalyst itself, but it can adjust the redox potentials
[10]. Therefore, sodium hydroxide can speed up the reac-
tion rate. The dependence of HNQ yield on NaOH amount
in a reaction system was shown in Fig. 4.
The quickly enhanced yield of HNQ with the increasing
amount of NaOH suggests that alkaline medium can
increase the reaction rate, reduce the redox potentials in the
catalytic reaction. The most suitable amount of NaOH in
CoCPP-GMA/MMA catalytic reaction system is 5 g in
30 mL methanol. Both the value and trend are similar to
that oxidized by hydrogen peroxide using metalloporphyrin
as catalysts [7], suggesting that the polymer microsphere
supported CoCPP-GMA/MMA may have the same cata-
lytic mechanism with the free metalloporphyrins for oxi-
dizing 2-naphthol to HNQ.
The microsphere supported CoCPP-GMA/MMA is not
only easy for recycled use, but also has high catalytic
activity. In optimized conditions, the immobilized CoCPP-
GMA/MMA catalyst can produce a yield of 28 * 31%,
which is five times higher than that catalyzed by free
TPPCo in similar conditions [7], and 80% increase com-
pared to 16.9% conversion using TMOPPFeCl as catalyst
[10]. Furthermore, the present catalytic reaction employs
molecular oxygen as the clean oxidant, not peroxides.
Therefore, the present synthesis system provides a green
strategy for the production of high purity HNQ, which
30
25
20
15
10
5
28
24
20
16
12
CoPP-GMA/MMA
8
CoCPP-GMA/MMA
CoNPP-GMA/MMA
4
0
1
2
3
4
5
6
7
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Amount of NaOH/g
t/h
Fig. 4 Variation of HNQ yield with the amount of NaOH. (Amount
of CoCPP-GMA/MMA: 0.163 g; Density of CoCPP: 64.49 lmol/g;
Temperature: 0 °C)
Fig. 5 Variation of HNQ yield with time using different catalysts.
(Amount of immobilized catalyst: 0.163 g; Amount of NaOH: 5.0 g;
Temperature: 0 °C)
123

1812
J. Zhao et al.
kind of central metalloporphyrin ligands show different
catalytic activity. The excluded electron in the porphyrin
ring can be pushed to the center Co by conjugated effect,
reducing the redox potentials of forming high-valence
cobaltporphyrin, which makes it easy to be oxidized and
form the catalytic intermediates of high-valence cob-
altporphyrin and keep stable [15].
3.3.3 Effect of Cobaltporphyrin Density
CoCPP-GMA/MMA catalysts with different densities of
immobilized cobalt porphyrins were used in the oxidation
of 2-naphthol as described in Sect. 2.3. All the reactions
were conducted in the same conditions, except for the
weight of catalysts. The added weights of the catalysts with
different densities of immobilized cobalt porphyrins were
adjusted to ensure all the reactions contain same number of
cobalt porphyrins. Figure 6a showed the variation of HNQ
yield with immobilized density of cobalt porphyrins. Sur-
prisingly, the yield of HNQ decreases monotonously with
increasing density of the immobilized cobalt porphyrin,
suggesting a decrease in activity with increasing density of
the cobalt porphyrin. This phenomenon is similar with that
reported in ethylbenzene oxidation catalyzed by mangano-
porphyrin immobilized on poly (glycidyl methacrylate)
coated silica gel particles [16].
Fig. 6 Variation of HNQ yields with different immobilization
densities (a) and distances (b) of CoCPP in GMA/MMA micro-
spheres. (Amount of CoCPP-GMA/MMA: 0.163 g; Amount of
NaOH: 5.0 g; Temperature: 0 °C)
In order to understand this phenomenon, it is necessary
to obtain the average distance between the nearest por-
phyrin groups. However, the distance data calculated from
the surface area and group density of the solid sphere is
extremely small (*0.03 nm). Figure 1 shows clearly that
the polymer microsphere has porous structures both inside
and outside of the sphere. The size of the pores is around
micrometers, which definitely allows the cobalt porphyrin
immobilized and reacted with a reactant inside the sphere.
Considering this, we suppose that the cobalt porphyrins
have a homogenous distribution inside the sphere. Thus, an
average distance between the neighboring cobalt porphy-
rins is calculated by using the density of cobalt porphyrin
(lmol/g) and assuming the density of the 200 lm particles
is approximately 1.0 g/cm3.
porphyrin density is very possibly due to aggregate for-
mation of the cobalt porphyrins. Cobalt porphyrins
immobilized onto the porous microsphere GMA/MMA
should have certain freedom to move in a limited area due
to the flexible polymer chains. In this study, the critical
distance between cobalt porphyrin groups is less than
3.8 nm, suggesting a maximum free movement radius of
1.9 nm. Further insight work on the distance dependence
will benefit for exploiting high activity porphyrin catalyst.
3.3.4 Reuse Performance
The greatest advantage of immobilized catalyst is easy to
reuse. However, the activity of immobilized catalyst usu-
ally decreases with increasing reuse cycles. Interestingly,
Cobalt porphyrins immobilized on polymer microspheres
exhibit a slowly increased activity in repeated use on the
oxidation of 2-naphthol to 2-hydroxy-1,4-naphthoquinone
by molecular oxygen. As shown in Fig. 7, the yield of the
product HNQ increases gradually with the increase of
recycle number, indicating an improvement in activity
during the reuse. The reuse experiment was conducted
under the same conditions using the recycled catalyst,
which was filtrated, washed with methanol to remove
The influence of the distance between neighboring
cobalt porphyrins on the HNQ yield is plotted in Fig. 6b.
The catalytic reaction yield, which indicates the activity of
the catalysts, decreases sharply when the distance reduced
to less than 3.8 nm in the porous polymer microsphere.
This kind of critical distance seems significant for inves-
tigating polymer immobilized porphyrin catalyst, but has
not been discussed before.
Porphyrins are easily aggregated into dimers or
J-aggregate in confined environment, which exhibit dif-
ferent activity and selectivity than do monomeric species
[17–19]. The reduced catalytic activity of higher cobalt
123

Catalysts for the Oxidation of 2-naphthol
1813
33
oxidation of 2-naphthol to 2-hydroxy-1,4-naphthoquinone
by molecular oxygen in alkaline methanol. Interestingly, a
phenomenon of distance dependent catalytic activity was
observed and a critical distance of 3.8 nm between por-
phyrins was determined for the porous polymer micro-
sphere supported catalyst. Furthermore, the activity of the
recycled catalyst increased gradually with the increased
times of reuse. These results may be significant in
designing highly efficient metalloporphyrin catalysts.
32
31
30
29
Acknowledgments The authors are grateful to the Natural Science
Fund of Shanxi Province of China (No. 2008021013) for financial
support in this study.
0
1
2
3
4
5
6
Recycle Number
Fig. 7 Effect of recycle number on catalyst activity. (Density of
CoCPP: 33.94 lmol/g; Temperature: 0 °C; Reaction time of each
cycle: 2.5 h)
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