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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
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