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whether the complexes are in the shell wall or core or are
present throughout the structure. As estimated from measure-
ments taken by atomic absorption spectroscopy (AAS), the
loading amount of CuPcS in the MCs was 3.05 wt% (Cu
tively formed in the reaction carried out at both temperatures.
The solvent effect in terms of conversion of trans-stilbene fol-
lows the order chloroform>DMF>o-xylene>n-propanol. The
solvent polarity is not clearly reflected in the trend of trans-stil-
bene conversion. A highly polar solvent competes with the re-
actant to interact with the hydrophilic silica surface of the cata-
lyst, thereby resulting in lower conversion of trans-stilbene. On
the other hand, the least polar solvents allow better solubility
of the reactant, but the interaction of the reactant with the
catalyst surface is diminished. Hence, medium polar solvents
were optimum for higher conversion.
ꢁ
0.20 wt%). From thermogravimetric analysis (TGA), we found
the total organic content to be 18 wt%, which corresponds to
the amount of PAA and CuPcS present in the CuPcS@MC sam-
ples (see Figure S4 in the Supporting Information).
The synthesized materials were further characterized by
FTIR, micro-Raman, and UV/Vis spectroscopic analysis. In the
FTIR spectrum, the vibrational bands at n˜ =1462 and
ꢀ
1
1
418 cm observed in both neat CuPcS and CuPcS@MC sam-
On the other hand, the blank reaction without the catalyst
under similar conditions showed a conversion of 2.3% only
(see Table S2 in the Supporting Information). The MCs without
CuPcS did not show any appreciable activity either, thus indi-
cating that the encapsulated CuPcS as present in CuPcS@MC
are not only accessible to the reactants for reaction, but are
also responsible for the observed catalytic activities in the ep-
oxidation of trans-stilbene. Interestingly, when we used an
equivalent amount of neat CuPcS as in CuPcS@MC as a catalyst
under similar conditions, the conversion was only 7% (see
Table S2 in the Supporting Information). This outcome clearly
suggests that the microcapsule structure offers a suitable envi-
ronment for better dispersity of CuPcS. Moreover, it is well rec-
ognized that the spatial confinement of catalytic systems in
appropriate matrices can lead to enhancement of catalytic ac-
ples are attributable to the stretching modes of C=N and C=C
of the phthalocyanine ring, respectively (see Figure S5 in the
[
16]
Supporting Information). The confocal micro-Raman spectra
of neat CuPcS and CuPcS@MC display a typical intense band at
ꢀ
1
n˜ =1525 cm , which is assignable to a C ꢀC vibration in
b
b
phthalocyanine (see Figure S6 and Table S1 in the Supporting
[
17]
ꢀ1
Information). The Raman signal at n˜ =1441 cm represents
the CꢀN and CꢀC of pyrrole moiety of the phthalocyanine
unit.
The UV/Vis diffusion reflectance spectra of solid samples of
MC, CuPcS@MC, and neat CuPcS are shown in Figure 2b.
Phthalocyanine complexes exhibit characteristic Q bands in the
visible region due to p!p* transitions from HOMO–LUMO
and B bands (Soret band) in the UV region due to deeper p!
[
18]
[20]
LUMO transitions. The position and splitting of the Q bands
are generally assigned to monomeric and dimeric/oligomeric
tivity.
The surrounding environment of the phthalocyanine com-
plexes are known to influence their monomeric and oligomeric
[
19]
forms. Although the monomeric form absorbs at around l=
00 nm, this band blue-shifts with di-/oligomerization. The
[19]
7
forms. Although the monomeric forms have been reported
to be the active species in catalytic reactions, these complexes
often tend to dimerize/oligomerize, thereby resulting in de-
neat CuPcS has Q bands centered at l=689 and 614 nm. The
encapsulated CuPcS in the MCs also exhibited two Q bands at
l=687 and 612 nm, thus indicating the presence of both
mono- and oligomeric forms of the complex in the MCs. The
CuPcS@MC also has B bands centered at l=328 and 420 nm.
Moreover, the similarity in the absorption positions for both
the encapsulated CuPcS and neat sample suggests that encap-
sulation by interaction with polyamine does not perturb the
phthalocyanine p system.
[8a,21]
creased activities.
We further analyzed the UV/Vis spectra
of the catalyst and the neat complex (see Figure S7a,b in the
Supporting Information). From the corresponding deconvolut-
ed spectra, it can be seen that ratio R (ratio of the absorbance
due to monomeric and oligomeric CuPcS species) in the
CuPcS@MC moiety is higher than that in neat CuPcS. Moreover,
the interaction of CuPcS with polyamine aggregates as moni-
tored during the microcapsule assembly process also shows
stabilization of the monomeric CuPcS species (see Figure S7c
in the Supporting Information). Thus, the higher amount of
monomeric form in the microcapsules indicates that the ionic
interaction amongst CuPcS and the embedded polyamines
with the assembled silica nanoparticles in the microcapsule
structure allows separation of the oligomerized forms of the
complex. It is also known that heating of phthalocyanine can
We examined the catalytic activity of CuPcS@MCs in epoxi-
dation reactions. Table 1 shows the catalytic performance of
CuPcS@MC for the epoxidation of trans-stilbene carried out in
different solvents at two temperatures. The GC and GC-MS re-
sults show that trans-stilbene oxide is the only product selec-
[
a]
Table 1. Epoxidation of trans-stilbene in different solvents.
[22]
lead to stabilization of the monomeric form. Therefore, we
[
b]
further examined the possibility of increasing the monomeric
form by heating the catalysts at various temperatures. As ob-
served from our experiments, the R ratio is increased with an
increase in the heating temperature (see Figure S7d,e in the
Supporting Information). When the heat-treated CuPcS@MC
catalysts were used in the epoxidation reaction, the activities
increased to conversions of 60 and 65%, respectively. This out-
come further supports the observed enhancement in activity
Entry
Solvent
Conversion [%]
7
08C
908C
1
2
3
4
DMF
20
50
10
<1
39
87
14
<1
CHCl
3
o-xylene
n-propanol
[a] Reactions were carried out with
(0.5 mmol), CuPcS@MC catalyst (5 mg), Na
(1.5 mmol) in water (70%) for 24 h. [b] GC conversion.
a
solvent (5 mL), trans-stilbene
CO (1.5 mmol), and TBHP
2
3
Chem. Eur. J. 2014, 20, 8453 – 8457
8455
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim