SHORT COMMUNICATION
CHINESE JOURNAL OF CATALYSIS
Volume 28, Issue 12, December 2007
Online English edition of the Chinese language journal
Cite this article as: Chin J Catal, 2007, 28(12): 1025–1027.
Received date: 2007-05-21.
* Corresponding author. Tel: +86-01-62792122; E-mail: [email protected]
Foundation item: Supported by the National Natural Science Foundation of China (20590362).
Copyright © 2007, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
Amorphous Manganese Oxide for Catalytic Aerobic Oxidation of Benzyl Alcohol
HU Jing1,2, SUN Keqiang1, HE Daiping1,2, XU Boqing1,*1 Innovative Catalysis Program, Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua
University, Beijing 100084, China 2 College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, Sichuan, China
Abstract: The effect of calcination temperature on the textual structure and catalytic properties of amorphous MnOx for the liquid-phase
aerobic oxidation of benzyl alcohol was studied. The amorphous nature of the synthesized MnOx was retained when the calcination tem-
perature was lower than 400ºC, and calcination temperatures higher than 500ºC led to the transformation of amorphous MnOx to crystalline
OMS-2 and Mn2O3. Catalytic reaction studies revealed that the amorphous MnOx exhibited higher mass specific activity than OMS-2,
Mn2O3, and -MnO2, and MnOx calcined at 110ºC showed the highest activity. An inverse linear correlation between the onset reduction
temperature in H2-TPR and the mass specific activity of the various manganese oxides was observed. These results suggest that the re-
ducibility of manganese oxide is the key factor responsible for its activity for the aerobic oxidation of benzyl alcohol.
Key words: amorphous manganese oxide; benzyl alcohol; selective oxidation; reducibility
The oxidation of an alcohol to the corresponding carbonyl
compound is one of the most important reactions for the syn-
thesis of fine chemicals and intermediates. Transition metal
oxides are traditionally used as stoichiometric oxidants for this
reaction, which generate large amounts of toxic wastes [1].
Recently, the catalytic aerobic oxidation of alcohols by mo-
lecular oxygen has thus received significant attention [2].
Manganese oxides have long been used as stoichiometric
oxidants for alcohol oxidation to aldehydes or ketones [3,4].
The recent works by Suib’s group [5–7] revealed that a crys-
talline microporous manganese oxide known as cryptomelane
octahedral molecular sieves (OMS-2) is an efficient catalyst for
the aerobic oxidation of an alcohol in the liquid phase. Since
amorphous MnOx is the precursor for the synthesis of OMS-2
[5–7], the effect of calcination temperature on the textural
structure and catalytic properties of amorphous MnOx for the
liquid phase aerobic oxidation of benzyl alcohol was explored
in the present work.
The amorphous manganese oxide was synthesized by the
oxidation of manganese sulfate with potassium permanganate
in an acidic medium at room temperature. Specifically, 7.75 ml
of MnSO4 solution (1.7 mol/L) was added dropwise to 25 ml of
KMnO4 solution (0.4 mol/L) under vigorous stirring, followed
by the addition of 5 ml of nitric acid solution (0.6 mol/L). The
precipitate was aged at room temperature for 24 h, then filtered,
washed four times with deionized water, and dried at 110ºC for
10 h. The solid sample obtained was then calcined in flowing
air (60 ml/min) at different temperatures and designated as
MnOx-t, where t refers to the temperature of drying/calcination.
For comparison, OMS-2 prepared according to the method of
Suib et al. [5] and commercial MnO2 ( -MnO2, Sinopharm
Chemical Reagent Beijing Co., Ltd.) were also used as cata-
lysts.
The catalytic oxidation of benzyl alcohol was carried out in a
round-bottom flask with a condenser and magnetic stirrer.
Typically, 50 mg catalyst was used for the oxidation of 1 mmol
benzyl alcohol in 10 ml toluene solvent at 110ºC. The reaction
was stopped at a selected time by cooling the reactor in an
ice–water bath. The reaction products were analyzed by a HP
4890 GC with a HP-5 capillary column and FID detector using
benzene as an internal standard.
Table 1 shows the textual structure data of manganese oxides
calcined at different temperatures and those of OMS-2 and
-MnO2. The samples with a calcination temperature lower
HU Jing et al. / Chinese Journal of Catalysis, 2007, 28(12): 1025–1027
than 400 C retained the amorphous structure. N2 adsorption/
desorption characterization showed that the BET surface area
of the amorphous MnOx decreased with the calcination tem-
perature. This is due to the decreased number of micropores in
the samples, as indicated by the increased pore diameter and
decreased pore volume. However, for MnOx-500 and
MnOx-600, strong diffraction peaks appear at 2 = 12.6 , 17.9 ,
28.6 , 37.4 , 41.8 , 49.7 , and 60.1 (OMS-2 [8,9]) and 2 =
23.1 , 33.0 , 38.2 , 45.2 , 55.2 , and 65.7 (Mn2O3 [10]), in-
dicating the transformation of amorphous MnOx to crystalline
cryptomelane (OMS-2) and Mn2O3 at higher calcination tem-
peratures. The BET surface area and the pore volume of the
samples calcined at 500 or 600 C decreased drastically, which
is in line with the changes of the crystalline structure.
Table 2 shows the results of catalytic oxidation of benzyl
alcohol over the various manganese oxides. All the tested
catalysts showed 100% selectivity to benzyl aldehyde. Special
attention was paid to detecting by-products, and any
by-product higher than 0.1% in the mixture can be detected.
Amorphous MnOx-110 showed the highest benzyl alcohol
conversion of 72.7% with a reaction time of 1.5 h, whereas the
conversions on crystalline OMS-2 and commercial -MnO2
were only 44.0% and 39.2%, respectively. The mass specific
activity of the amorphous MnOx-110 was 1.6 times that of
OMS-2 and 1.9 times that of -MnO2.
Since manganese oxides are widely used as stoichiometric
oxidants [1] according to the redox mechanism for alcohol
oxidation over manganese oxides [5,11–13], the oxygen spe-
cies in the MnOx can also stoichiometrically oxidize benzyl
alcohol to form benzyl aldehyde. A distinction between the role
played by the present amorphous MnOx as a stoichiometric
oxidant and a catalyst should be made first. A blank experiment
under inert Ar that used the MnOx as a stoichiometric oxidant
to oxidize benzyl alcohol was performed. The result revealed
that one gram of MnOx can oxidize 6.4 mmol of benzyl alcohol,
which is significantly less than the 14.5 mmol/g (row 1 in
Table 2) for the experiment performed in air. Additional ex-
periments in air that tripled the amount of benzyl alcohol (from
1 to 3 mmol) and extended the reaction time (from 1.5 to 10 h)
resulted in an even larger value (47.0 mmol/g, row 2 in Table 2)
of converted benzyl alcohol. These results strongly indicated
that the MnOx acted as a catalyst for the oxidation reaction
between benzyl alcohol and molecular oxygen. Further evi-
dence for the catalytic role of the amorphous MnOx was ob-
tained by the dependence of benzyl alcohol conversion on the
oxygen pressure. The conversion of benzyl alcohol was dou-
bled when the partial pressure of oxygen was increased from 21
to 101 kPa.
Table 2 also shows the effect of the calcination temperature
on the activity of the various MnOx. The mass specific activity
of amorphous MnOx decreased gradually from 9.7 to 8.1
mmol/(g·h) with increasing calcination temperature from 110
to 400 C. However, when the calcination temperature was
increased to 500 and 600 C (which transformed amorphous
MnOx to crystalline OMS-2 and Mn2O3), the mass specific
activity dropped sharply to 3.2 (MnOx-500) and 2.4 mmol/(g·h)
(MnOx-600). These results further proved that amorphous
MnOx has a higher activity than crystalline manganese oxides.
The reducibility of the various MnOx was then studied by
H2-TPR technique. As shown in Fig. 1, amorphous MnOx, i.e.
samples with calcination temperature less than 400 C, exhib-
ited TPR curves with a similar shape showing sharp peaks at
270 and 345 C. When the calcination temperature was in-
creased to 500 C, the reduction peak at 270 C disappeared, and
two peaks at 335 and 368 C were observed. For comparison,
the OMS-2 showed a H2 consumption peak at 340 C, and the
commercial -MnO2 showed two peaks at 320 and 425 C. The
assignment of these reduction peaks need further work since
the reduction of manganese oxides is strongly affected by their
crystalline structure, crystallinity, and surface area [14].
The onset reduction temperatures of various MnOx in
H2-TPR are also listed in Table 2. The onset reduction tem-
perature increased with the calcination temperature of the
MnOx. MnOx-110 had an onset reduction temperature of 145 C,
Table 1 BET surface area, pore volume, average pore diameter, and
crystal structure of various manganese oxides
CatalystBET surface
area (m2/g)
Pore volume
(cm3/g)
Average pore
radius (nm) Crystal phase
MnOx-110 186 0.22 1.99 amorphous
MnOx-200 181 0.22 1.99 amorphous
MnOx-300 125 0.18 2.32 amorphous
MnOx-400 95 0.13 2.45 amorphous
MnOx-500 23 0.08 4.44 OMS-2, Mn2O3
MnOx-600 12 0.03 2.67 OMS-2, Mn2O3
OMS-2 61 0.36 4.42 cryptomelane
-MnO2 43 0.23 9.61 -MnO2
Table 2 Benzyl alcohol oxidation over manganese oxidesa
Catalyst Time (h) Conversion
(%)
Activity b
(mmol/(g·h)) tonset
c/ºC
MnOx-110 1.5 72.7 9.7 148
MnOx-110d 10 78.0 4.7 —
MnOx-200 1.5 68.3 9.1 164
MnOx-300 1.5 66.9 8.9 170
MnOx-400 1.5 60.9 8.1 187
MnOx-500 1.5 24.1 3.2 275
MnOx-600 1.5 17.7 2.4 280
OMS-2 1.5 44.0 5.9 225
-MnO2 1.5 39.2 5.2 232
a Reaction conditions: 50 mg catalyst, 1 mmol benzyl alcohol in 10 ml
toluene, 110ºC, 1.5 h, atmospheric pressure.
b Mass specific activity.
c Onset reduction temperature in H2-TPR.
d 3 mmol benzyl alcohol, 10 h.
HU Jing et al. / Chinese Journal of Catalysis, 2007, 28(12): 1025–1027
whereas the sample calcined at 400 C had an onset reduction
temperature of 187 C. A calcination temperature higher than
500 C, however, led to the onset reduction temperature being
increased drastically to above 275 C. A correlation between
the mass specific activity and the onset reduction temperature
of the various MnOx was made, and an inverse linear rela-
tionship was observed. That is, the lower the catalytic activity,
the higher the onset reduction temperature. The data of the
OMS-2 and -MnO2 also obeyed the same trend. These results
clearly suggested that the onset reduction temperature is the
key factor controlling the activity of MnOx for the catalytic
oxidation of benzyl alcohol. The lower onset reduction tem-
perature allowed a more facile supply of the oxygen species
needed for the reaction, which accounts for the higher activity
of amorphous MnOx compared to the crystalline MnOx.
In summary, amorphous MnOx were found to exhibit higher
activity than crystalline MnOx (OMS-2, -MnO2, and Mn2O3).
The reducibility of the manganese oxide is a key factor in
controlling their activity for the catalytic oxidation of benzyl
alcohol.
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Fig. 1 H2-TPR profiles of various manganese oxides
(1) MnOx-110, (2) MnOx-200, (3) MnOx-300, (4) MnOx-400,
(5) MnOx-500, (6) MnOx-600, (7) OMS-2, (8) -MnO2