Chemistry Letters 2002
213
relative pressure of 0.65–0.85, which is a characteristic of
capillary condensation within uniform mesopores.10 Pore sizes
be very low and terephthalic acid was not detected on cobalt
acetate contrary to cobalt bromide. The bromide ion acts as a
promoter as well as free radical initiator. It was reported that the
addition of bromide ion and manganese to the cobalt catalyst, its
activity was increased by 16 times and long induction periods
2
ꢂ1
are 8.2 nm and 8.1 nm and BET surface areas are 684 m ꢁg to
2
ꢂ1
4
54 m ꢁg for the SBA-15-COOH and CoSBA-15, respectively.
These indicate that the immobilization of cobalt does not affect on
the pore structures of the SBA-15 material and cause the decrease
of surface area that is consistent with cobalt loading. TG-DTA
studies on the SBA-15-COOH and the CoSBA-15 reveal that the
organic moieties begin to be decomposed above 573 K, which
means the catalysts are stable at reaction temperature. IR spectra
1
were absent. In the recent times there were some reports
regarding non-bromide systems, but peroxides were used for the
4
substitution of bromide ion. It seems that it is impossible to
eliminate the bromide ion activator from the homogeneous
catalytic system unless and otherwise substituted by the free
ꢂ1
ꢂ1
4
show CN stretch band at 2248 cm for SBA-15-CN material and
C¼O absorption band at 1711 cm for SBA-15-COOH material.
radical initiators (peroxides). Although the bromide ion is the
most important component of the oxidation catalyst, a non-
bromide CoSBA-15 solid catalyst was equally active when
compared to the bromide systems under the specified reaction
conditions. Our results reveal that neither the bromide ion nor the
free radical initiator is necessary for the oxidation of p-xylene. It
should be noted that the catalytic activity of the CoSBA-15
catalyst (Run No. 2) is comparable with the existing industrial
homogeneous catalyst (Run No. 3) at the low pressure.
However, at higher temperature, the catalytic activity of
CoSBA-15 (Run No. 7) was inferior to that of CoBr2/Mn(OAC)2
(Run No. 8), which is probably due tothe loss of pyridine from the
catalyst at elevated temperatures, as confirmed by DTA and CHN
elemental analysis. The catalyst can be easily rejuvenated by the
addition of pyridine. However, different approaches are under
investigation to stabilize the CoSBA-15 catalyst even at elevated
temperatures.
In conclusion, highly active CoSBA-15 solid catalyst for the
oxidation of p-xylene in the absence of solvent exhibited an
excellent performance even at the low pressure and also its
activity is very high even without a solvent or bromide. The
results suggest that there is a bright possibility to develop a
bromide-free and solvent-free catalytic system for the liquid
phase oxidation of alkylaromatics.
This indicates that CN groups are completely converted to COOH
by oxidation using sulfuric acid, which can be also confirmed by
13C MAS solid NMR experiments.
The remarkable activity of the CoSBA-15 catalyst can be
seen from the Table 1. The CoSBA-15 catalyst (Run Nos. 1 and
) has shown considerable activity in the absence of solvent.
4
Although the conversions were lower than the homogeneous
systems (Run Nos. 3 and 5), it showed very high turnover
numbers considering the amount of reactant used. A key factor in
achieving higher catalytic performance of a CoSBA-15 material
is due to stabilization of Co(III) on the support. It is reported that
the higher oxidation state of Co is desirable for the high activity
of the oxidation catalyst.11 Initially the cobalt ion exists as Co(II)
in cobalt nitrate, the oxidation state of which is raised to Co(III)
during immobilization in the support and the higher oxidation
5
state is stabilized by pyridine ligands. The comparison of the
oxidations using cobalt bromide (Run No. 5) and cobalt acetate
(Run No. 6) at 403 K and 20 atm elucidates the role of bromide
ion in the homogeneous system. The conversion was observed to
Table 1. Oxidations of p-xylene with the heterogenized catalyst
and soluble catalysts.
aꢀ
2a
3b
aꢀ
5c
6d
7a
8b
Run No.
Temp./K 403 403 403 403 403 403 463 463
1
4
References
1
2
W. Partenheimer, Catal. Today, 23, 69 (1995).
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Time/h
Ptotal/atm
PO2/at
16 16 16
2
3 3
20 20 20 20 20
3
3
3
2
2
m
1
1
1
5
5
5
5
5
3
4
M. Harustiak, M. Hronec, and J. Ilawky, J. Mol. Catal., 53,
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Conv./mol% 27.4 95.1 98.8 50.6 88.1 9.0 95.2 94.0
Yield=mol%
TA
TPA
20.1 62.6 76.6 36.1 57.3 3.6 76.6 3.8
0.1 12.6 12.6 5.3 19.1 14.2 82.7
7.0 1.0 1.1 4.1 2.5 3.8 0.4 0.1
0.5 0.1 3.6 0.2
0
5
6
B. K. Das and J. H. Clark, Chem. Commun., 2000, 605.
Q. Huo, R. Leon, P. M. Petroff, and G. D. Stucky, Science,
268, 1324 (1995).
TALD
MBAL
TPAD
MBAC
Others
0
0
0
0
7
D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F.
Chmelka, and G. D. Stucky, Science, 279, 548 (1998).
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0.1 10.7 1.1 0.5 0.1 1.1 1.0 0.6
0.2 2.0 0.7 0.2 0.2 0.5 0.2 0.1
0
8
9
5.9 6.7 1.2 9.0
0
2.7 6.8
ꢀ
Reaction conditions: p-xylene ¼ 48:6 mmol ( 244 mmol for
a
solvent-free system), acetic acid (25.26 g), CoSBA-15 (0.2 g
cat. ¼ 0:255 mmol of Co), CoBr2/Mn(OAc)2, (0.541 mmol/
b
10 B. L. Newalkar, J. Olanrewaju, and S. Komarneni, Chem.
Mater., 13, 552 (2001).
11 R. Raja, G. Sankar, and J. M. Thomas, J. Am. Chem. Soc.,
121, 11926 (1999).
c
d
0
(
.647 mmol),
CoBr2
0.541 mmol). TA: p-toluic acid, TPA: terephthalic acid,
(0.541 mmol),
Co(OAc)2
TALD: p-tolualdehyde, MBAL: p-methylbenzyl alcohol,
TPAD: terephthaldehyde, MBAC: p-methylbenzyl acetate.