R.J. Kalbasi et al. / Catalysis Communications 11 (2010) 1109–1115
1111
the N-alkylation of pyridine ring. The broad bands located at 515 to
750 cm−1 can be ascribed to the Mn–O and Mn–O–Mn vibrations of
MnO4, respectively [21,22].
The specific area and the pore size of mesoporous silica SBA-15,
P4VP/SBA-15 and P4MVPMnO4/SBA-15 had been calculated using
Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH)
methods, respectively (Table 1). It was clear that P4VP/SBA-15 and
P4MVPMnO4/SBA-15 exhibit a smaller specific area, pore size and
volume in comparison with those of pure SBA-15, which might be
due to the presence of polymer on the surface of the SBA-15 [17].
Although these textural properties were smaller than those found
for mesoporous silica SBA-15, P4MVPMnO4/SBA-15 had still meso-
porous form and did not block the pores of the SBA-15, therefore, it
was suitable to act as an oxidant catalyst in oxidation reaction.
Fig. 3 gave the scanning electron microscopy (SEM) photographs
of SBA-15, P4VP/SBA-15 and P4MVPMnO4/SBA-15 composite. It was
obvious that before hybridization the surface of SBA-15 was
somewhat coarse and irregular, whereas after hybridization with
P4VP, the surface of SBA-15 becomes more smooth (Fig. 3.b). In
addition, Fig. 3.c shows some changes in the surface of catalyst after
the quaternization of pyridine group and exchange of iodide ion with
MnO−4
.
3.2. Effect of reaction conditions on the benzyl alcohol oxidation
The oxidation of benzyl alcohol was chosen as a model reaction to
test the catalytic activity of the poly (4-methyl vinylpyridinium
permanganate)/SBA-15 for oxidation of alcohols.
The oxidation of 0.4 mmol benzyl alcohol was carried out with
several common organic solvents using 0.12 g of the catalyst
(Table 2). As can be seen, toluene and 1,2-dichloromethane were
not suitable solvents for this catalytic system, because their polarity
were not enough high. Water as solvent was weakly active and gained
no yield and selectivity. The mass transfer is probably retarded
because water may form a film layer on the catalyst surface, and thus
the possibility of benzyl alcohol diffusing to active sites was reduced
[23,24]. On the other hand, by using acetonitrile and also THF as the
reaction solvent, the catalyst led to the good yield and selectivity
during the oxidative reaction, among which acetonitrile was the best
one with 100% selectivity at 80% yield. It was likely that the catalyst
remains coated by aprotic solvent and benzyl alcohol and benzalde-
hyde diffused better through aprotic solvent than through water.
Interestingly, as can be seen, the solvent-free reaction gave 82% yield
with 100% selectivity to benzaldehyde. So, the solvent-free condition
was chosen in other experiments.
Effect of reaction temperature on the progress of benzyl alcohol
oxidation was studied in the temperature range of 25–80 °C by
employing 0.12 g of the catalyst for oxidation of 0.4 mmol benzyl
alcohol at solvent-free condition. The percentage of yield increased
from 48% to 82% with the increase in temperature from 25 °C to 80 °C,
without any substantial loss of reaction selectivity (100%). This
reaction was classified as very slow reaction, kinetically controlled for
which reaction temperature influences the progress of the reaction
significantly [25].
Fig. 2. FTIR spectra of (a) mesoporous silica SBA-15, (b) P4VP, (c) P4VP/SBA-15,
(d) P4MVPI/SBA-15, (e) P4MVPMnO4(21.2%)/SBA-15, (f) P4MVPMnO4(35.58%)/SBA-15,
(g) P4MVPMnO4(45.45%)/SBA-15, (h) P4MVPMnO4(52.65%)/SBA-15, (i) P4MVPMnO4
(58.16%)/SBA-15, (j) P4MVPMnO4(63.47%)/SBA-15 and (k) P4MVPMnO4(67.61%)/
SBA-15.
1602 cm−1 was the stretching vibration absorption of C–N bond and the
bands at 1558, 1496 and 1415 cm−1 were attributed to the stretching
vibration absorption of C=C bond [19,20]. In the spectra of P4MVPI/
SBA-15 and P4MVPMnO4/SBA-15, a peak at 1640 cm−1 was ascribed to
the C–N (CH3-pyridinium) bond absorption. This observation confirmed
The effect of MnO−4 amount on oxidation reaction was investigated
using 0.12 g of the catalyst and 0.4 mmol of benzyl alcohol. The results
showed that by increasing the amount of MnO−4 , the percentage of yield
also was increased (Table 3). With further increase in the MnO4−
amount, the percentage of yield remained the same. Moreover, low
or high MnO−4 content did not have any effect on the selectivity
(100%). In addition, the activities of SBA-15, P4VP/SBA-15, and P4MVPI/
SBA-15 as catalyst in the oxidation of benzyl alcohol were investigated.
The synthesized materials did not show any activity for the oxidation
of benzyl alcohol to produce benzaldehyde.
Table 1
Surface data of MnO4-supported poly (4-methyl vinylpyridine)/SBA-15.
Sample
BET surface area Pore volume Pore diameter
(m2 g−1
)
(cm3 g−1
)
(nm)
Mesoporous silica SBA-15
P4VP/SBA-15
P4MVPMnO4(63.47%)/SBA-15
1430
1180
807
1.9
1.51
1.03
9.9
9.42
8.89
In order to study the amount of Mn leaching, we measured amount
of Mn in the solution after the reaction by Atomic Absorption. The Mn