Support effect in hydrogenation of methyl benzoate over supported manganese oxide catalysts

Article abstract of DOI:10.1016/S1381-1169(03)00380-7

Manganese oxide supported on MgO, γ-Al₂O₃, SiO₂, ZrO₂, TiO₂ and SiO₂-Al ₂O₃ catalysts were prepared. The effect of support on their catalytic behavior for hydrogenation of methyl benzoate to benzaldehyde was studied. The formation of toluene is suppressed on the supported catalysts due to the dilution of oxygen vacancies on the catalyst surface. The benzaldehyde yield of the supported catalysts follows the trend Mn/γ-Al₂O₃ > Mn/TiO₂ > Mn/ZrO ₂ > Mn/SiO₂-Al₂O₃ > Mn/SiO₂ > Mn/MgO. XRD measurements show that the Mn nitrate precursor is essentially transformed to highly dispersed MnO₂ on the supports at calcination and subsequently to MnO under reaction conditions with an exception of Mn/MgO. TPR and XPS analyses suggest that a strong interaction between manganese oxide and the γ-Al₂O₃ support plays a positive role in the hydrogenation reaction.

Full text of DOI:10.1016/S1381-1169(03)00380-7

Journal of Molecular Catalysis A: Chemical 203 (2003) 299–306
Support effect in hydrogenation of methyl benzoate
over supported manganese oxide catalysts
∗
Aimin Chen, Hualong Xu, Yinghong Yue, Weiming Hua, Wei Shen, Zi Gao
Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200433, PR China
Received 5 January 2003; accepted 14 April 2003
Abstract
Manganese oxide supported on MgO, ␥-Al2O3, SiO2, ZrO2, TiO2 and SiO2-Al2O3 catalysts were prepared. The effect of
supportontheircatalyticbehaviorforhydrogenationofmethylbenzoatetobenzaldehydewasstudied. Theformationoftoluene
is suppressed on the supported catalysts due to the dilution of oxygen vacancies on the catalyst surface. The benzaldehyde
yield of the supported catalysts follows the trend Mn/␥-Al2O3 > Mn/TiO2 > Mn/ZrO2 > Mn/SiO2-Al2O3 > Mn/SiO2 >
Mn/MgO. XRD measurements show that the Mn nitrate precursor is essentially transformed to highly dispersed MnO2 on
the supports at calcination and subsequently to MnO under reaction conditions with an exception of Mn/MgO. TPR and XPS
analyses suggest that a strong interaction between manganese oxide and the ␥-Al2O3 support plays a positive role in the
hydrogenation reaction.
©
2003 Elsevier Science B.V. All rights reserved.
Keywords: Methyl benzoate; Hydrogenation; Benzaldehyde; Manganese oxide catalyst; Support effect
1
. Introduction
esters to corresponding aldehydes is a more preferable
method of synthesizing aldehydes from the point view
of green technology. Benzaldehyde was produced via
this method with a high yield using chromium modi-
fied zirconia catalyst by Mitsubishi Chemicals in 1988
[5]. Later on, a large number of metal oxides, includ-
ing alkali earth oxides, transition metal oxides and rare
earth oxides, have been suggested for use as catalysts
for the selective hydrogenation of benzoic acid and
methyl benzoate [6–17].
Among these metal oxide catalysts, manganese ox-
ide has shown good activity and selectivity to ben-
zaldehyde at rather low temperature [11–16]. In the
patent literature [11], the use of activated alumina sup-
port enhances the catalytic activity of manganese ox-
ide. However, from the patent role of the individual
oxides as catalyst or catalyst support in the hydrogena-
tion reaction is unclear.
Aromatic aldehydes are important intermediates in
fine chemical industries, manufacturing pharmaceuti-
cals, agrochemicals, perfumes and flavors [1]. Various
processes have been used for the synthesis of aromatic
aldehydes, e.g. partial oxidation of alkyl aromatics [2],
halogenation of alkyl aromatics followed by hydrol-
ysis [3], or halogenation of aromatic acids followed
by hydrolysis, the so-called Rosenmund reduction [4].
However, all these technologies have their own draw-
backs, such as low selectivity, low yield and produc-
tion of a large amount of harmful wastes. The direct
hydrogenation of carboxylic acids or carboxylic acid
∗
Corresponding author. Tel.: +86-21-65642792;
fax: +86-21-65641740.
E-mail address: zigao@fudan.edu.cn (Z. Gao).
1
381-1169/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1381-1169(03)00380-7

3
00
A. Chen et al. / Journal of Molecular Catalysis A: Chemical 203 (2003) 299–306
In this work, the catalytic properties of a wide va-
(10%) and argon (90%). The catalyst was pretreated
in N2 flow at 300 C for 3 h. The temperature was
◦
riety of bulk metal oxides and oxide supported man-
ganese oxide catalysts in hydrogenation of methyl
benzoate are studied. The catalysts are characterized
by X-ray diffraction (XRD), temperature-programmed
reduction (TPR), X-ray photoelectron spectroscopy
◦
increased from 100 to 600 C with a ramp rate of
◦
−1
10 C min . A thermal conductivity detector was
used to monitor the hydrogen consumed during TPR
course. CuO was used as a standard sample for the
calibration of hydrogen consumption.
(
XPS) and N2 adsorption methods. The activity and
selectivity of the catalysts are correlated with the char-
acterization results to gain a better insight into the ef-
fect of the oxide supports on this type of catalyst.
2.3. Activity measurement
The catalytic test was carried out in a fixed bed reac-
tor with 600 mg of catalyst in the form of 20–40 mesh
◦
2
. Experimental
particles. The reaction was performed at 410 C and
ambient pressure. Methyl benzoate (MB) was pumped
2
.1. Preparation of catalysts
to a vaporizer to mix with hydrogen (GHSV =
−
1
7
00 h ), and the mixture (H2/MB = 20/1 vol.%)
◦
was preheated to 210 C before entering the reactor.
Supported manganese oxide catalysts were prepared
◦
Before the test, the catalyst was reduced at 260 C
by impregnation method. The supports used were
commercial ␥-Al2O3, SiO2, SiO2-Al2O3 (Si/Al = 1),
TiO2 (anatase), ZrO2 (tetragonal), and MgO of chemi-
cal pure grade. Aqueous solution of Mn(NO3)2·4H2O
was applied to the dried supports until incipient wet-
◦
◦
for 2 h, at 360 C for 3 h and, finally, at 420 C for
−
1
2 h in a H flow (30 ml min ). The reaction products
2
were condensed and analyzed by means of a GC–MS
(Finnigen Voyager) or a GC (HP 5890) equipped with
a 30 m SE-54 capillary column and a flame ionization
◦
ness, followed by drying at 120 C for 3 h and calcin-
ing at 450 C for 3 h in air. For comparison purposes,
◦
◦
detector. The injector temperature was 260 C, and
bulk MnO2, Mn2O3, and Mn3O4 were prepared ac-
cording to the procedures in the literature [18]. MnO2
was prepared by calcining Mn(OH)2 at 500 C for 10 h
the column temperature was increased from 80 to
260 C with a ramp rate of 15 C min .
◦
◦
−1
◦
in air. Mn2O3 was prepared by heating MnCO3·xH2O
◦
◦
in air consecutively for 3 h at 120 C, 8 h at 240 C
3. Results and discussion
◦
and, finally, 8 h at 550 C. Mn3O4 was prepared by
◦
calcining MnCO3·xH2O at 1000 C for 8 h in air.
3.1. Catalytic behavior of various oxides
2
.2. Catalyst characterization
The conversion of methyl benzoate and product dis-
tribution on various metal oxides after reacting for 6 h
at 410 C are listed in Table 1. The metal oxides of the
◦
XRD patterns were recorded on a Siemens D8 Ad-
vance X-ray diffractometer (Cu K␣ radiation, 40 kV,
main groups, such as MgO, ␥-Al2O3, SiO2-Al2O3 and
SiO2, are inactive for the reaction, whereas the transi-
tion metal oxides with some redox properties, such as
TiO2, ZrO2 and manganese oxides, show rather high
conversions of methyl benzoate (66–99%) and mod-
erate selectivities to benzaldehyde (58–68%). ZrO2
gives the highest yield of benzaldehyde among the ox-
ides. The catalytic performance of different forms of
manganese oxide in the reaction is similar, since these
oxides are unstable in hydrogen at elevated tempera-
ture and they all reduce to MnO after catalyst pretreat-
◦
−1
4
0 mA) with a scan speed of 8 min and a scan
◦
range of 10–80 . The surface areas of the oxides and
the supported catalysts were measured by nitrogen
◦
adsorption at −196 C using a Micromeritics ASAP
2
000 instrument and calculated by BET method. XPS
spectra were obtained with a Perkin-Elmer PHI 5000
ESCA system using Mg K␣ radiation at 14 kV and
2
50 W. The vacuum of the specimen chamber was
−
7
better than 5×10 Pa, and C1s at 284.6 eV was taken
as reference. TPR experiments were carried out on
a Micromeritics ASAP 2900 instrument using 25 mg
◦
ment at 420 C in hydrogen. Fig. 1 shows the XRD
patterns of the as-synthesized manganese oxides and
−
1
catalyst under a gas flow (40 ml min ) of hydrogen

A. Chen et al. / Journal of Molecular Catalysis A: Chemical 203 (2003) 299–306
301
Table 1
Hydrogenation of methyl benzoate over various metal oxides
2
−1
Catalyst
Conversion (%)
Selectivity (%)
BA yield (%)
Surface area (m g
)
BA
B
T
BOH
ME
Others
MgO
SiO2
SiO2-Al2O3
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
4.4
23.3
28.4
26.7
20.9
–
–
–
–
–
–
–
–
2.3
1.1
0.6
0.6
0.5
–
–
–
–
18.4
4.9
10
12.8
12.3
–
–
–
–
41
67.1
51
57.2
62.4
41.8
316
347
110
107
91
7.7
16.9
1.1
␥-Al2O3
–
TiO2
ZrO2
MnO2
Mn2O3
Mn3O4
66
62
10.9
0.6
–
–
0.4
2
98.4
86.9
98.9
98.4
68.3
58.7
58.2
63.4
1.8
2.3
1.7
2.9
Reaction conditions: 600 mg catalyst; 410 C; H2 GHSV = 700 h−1; H2/methyl benzoate = 20/1 vol.%. BA: benzaldehyde; B: benzene; T:
◦
toluene; BOH: benzylalcohol; ME: methylbenzyl ether.
◦
the oxides reduced at 420 C. The diffraction patterns
of the reduced oxides are typical of MnO.
len type reaction [19]. The initial step of the reaction
is the adsorption of the benzoate molecule on an oxy-
gen vacancy of the oxide catalyst, which results in the
break of one C–O bond. Hydrogen from the nearby
OH group hydrogenates the fragment to benzaldehyde,
and the deposited oxygen is subsequently removed by
hydrogen in the reaction mixture. The high activity
of the manganese oxides implies that they are more
reducible than the other oxides and a higher concen-
tration of oxygen vacancy exists on the surface of the
It is interesting to note that the catalytic activity and
selectivity of the manganese oxides are very similar
to those of ZrO2 although their specific surface areas
are much lower. Selective hydrogenation of methyl
benzoate to benzaldehyde is a Mars and van Kreve-
◦
oxides after prereduction at 420 C. However, the high
concentration of oxygen vacancy may render a loss of
both the oxygen atoms in the benzoate molecule and
subsequently toluene is formed, leading to a reduction
in selectivity to benzaldehyde [17].
3.2. Hydrogenation on supported manganese
oxide catalysts
Manganese oxide catalysts supported on different
oxides were prepared and tested. XRD patterns of the
supported catalysts before and after reaction are illus-
◦
trated in Fig. 2. After calcination at 450 C, Mn(NO3)2
is decomposed to form MnO2 species on the oxide
supports. Under reaction conditions, MnO2 is reduced
to MnO, and the latter is the active component for
the reaction. The diffraction peaks of both MnO2 and
MnO on the catalysts are very low and broad, which
signals a rather high dispersion of the manganese ox-
ides on the supports. The Mn/MgO catalyst is an ex-
ception. A stable complex oxide Mg6MnO8 is formed
Fig. 1. XRD patterns of manganese oxides before and after reac-
tion: (a) MnO2 before reaction; (b) Mn2O3 before reaction; (c)
Mn3O4 before reaction; (d) MnO2 after reaction; (e) Mn2O3 after
reaction; (f) Mn3O4 after reaction.

3
02
A. Chen et al. / Journal of Molecular Catalysis A: Chemical 203 (2003) 299–306
◦
when calcining at 450 C, and unlike all the other cat-
alysts it is irreducible during reaction.
The activity and selectivity of the supported man-
ganese oxide catalysts for the hydrogenation of methyl
benzoate are listed in Table 2. The yield of benzalde-
hyde increases insignificantly as the manganese oxide
loading is increased from 10 to 20%. The selectivity
to toluene of all the supported catalysts is suppressed,
which may be explained by the dilution of the oxygen
vacancies on the catalyst surface after supporting. In
accordance to the catalytic activities of the supported
manganese oxide catalysts, they can be divided into
three categories. For most of the catalysts, such as
Mn/TiO2, Mn/ZrO2, Mn/SiO2 and Mn/SiO2-Al2O3,
their catalytic activity is likely an additive property of
the two oxides. However, a marked “positive” support
effect is observed in the case of Mn/␥-Al2O3. The con-
version of methyl benzoate and selectivity to benzalde-
hyde on Mn/␥-Al2O3 catalyst are unexpectedly high,
although ␥-Al2O3 itself is inactive for the reaction.
The yields of benzaldehyde on 10% Mn/␥-Al2O3 and
20% Mn/␥-Al2O3 are 73.6 and 74.0%, respectively,
which are evidently higher than those on all the other
catalysts. On the contrary, the activity of Mn/MgO is
much lower than those of the other catalysts, imply-
ing the existence of a “negative” support effect. From
the XRD results in Fig. 2, such a negative support ef-
fect is probably associated with the formation of the
irreducible complex oxide Mg MnO8 under reaction
6
conditions.
For getting an even clearer picture of the interac-
tion between the oxides, the hydrogenation reaction
was carried out on mechanical mixtures of man-
ganese oxide with the oxide supports as well. The
reaction results are listed in Table 3. The activity and
selectivity of the mechanical mixtures of the oxides
are similar to those of the supported catalysts except
Mn + ␥-Al2O3 and Mn + MgO. The catalytic activity
of Mn + ␥-Al2O3 is two times less than that of the
supported Mn/␥-Al2O3 catalyst, whereas the catalytic
activity of Mn + MgO is 1.5 times higher than that of
the supported Mn/MgO catalyst. Since the interaction
between two oxides in mechanical mixtures is usually
weaker than that in catalysts prepared by an impreg-
nation method, the above observation further confirms
that the positive or negative support effect of ␥-Al2O3
or MgO originates from the strong interaction be-
tween manganese oxide and these two oxide supports.
Fig. 2. XRD patterns of supported manganese oxide catalysts
A) before and (B) after reaction: (a) 10% Mn/SiO2-Al2O3; (b)
0% Mn/␥-Al2O3; (c) 10% Mn/SiO2; (d) 10% Mn/TiO2; (e) 10%
Mn/ZrO2; (f) 10% Mn/MgO ((᭿) MnO2; (᭹) MnO).
(
1

A. Chen et al. / Journal of Molecular Catalysis A: Chemical 203 (2003) 299–306
303
Table 2
Hydrogenation of methyl benzoate over supported manganese oxide catalysts
2
−1
Catalyst
Conversion (%)
Selectivity (%)
BA yield (%)
Surface area (m g
)
BA
B
T
BOH
ME
Others
1
1
1
1
1
1
2
2
2
2
2
2
0% Mn/MgO
0% Mn/SiO2
0% Mn/SiO2-Al2O3
0% Mn/TiO2
0% Mn/␥-Al2O3
0% Mn/ZrO2
0% Mn/MgO
0% Mn/SiO2
0% Mn/SiO2-Al2O3
0% Mn/TiO2
0% Mn/␥-Al2O3
0% Mn/ZrO2
8.2
18.0
22.7
76.2
91.2
98.3
11.2
31.9
41.2
82.5
90.9
99.2
45.0
82.6
83.5
88.0
80.8
63.7
78.9
83.1
86.4
84.1
81.4
66.2
4.4
7.7
1.8
0.8
0.5
2.4
4.4
2.0
0.9
1.1
0.2
2
2.5
–
–
–
–
–
–
0.5
2.0
–
–
–
0.6
0.5
1.7
–
17
5.3
13.6
7.2
5.0
24.0
13.0
12.9
8.8
1.1
8.6
3.7
14.8
18.9
67.1
73.6
62.6
8.8
26.5
35.6
69.4
74.0
65.7
91.8
267
145
75.4
73.9
47.5
64
203
115
45.6
78.1
32.7
4.4
1.1
3
5.3
7.5
3.7
2.4
3.3
6
0.5
6.4
2.4
–
0.6
–
0.2
5.8
1.3
6.7
13.4
17.2
Reaction conditions: 600 mg catalyst; 410 C; H2 GHSV = 700 h−1; H2/methyl benzoate = 20/1 vol.%.
◦
The specific surface areas of the supported cata-
lysts were measured by N2 adsorption method and
given in Table 2 as well. Obviously, there is no di-
rect correlation between the catalytic activity and
specific surface area of the catalysts, suggesting
that besides the textural properties of the catalysts
the dispersion and chemical state of manganese ox-
ide on the surface of the catalysts probably play a
more important role in this selective hydrogenation
reaction.
For both series of catalysts, the conversion of
methyl benzoate is increased with reaction tempera-
ture, whereas the selectivity to benzaldehyde increases
when the reaction temperature is increased from 350
◦
to 370 C and then decreases as the reaction temper-
ature is further increased. The highest benzaldhyde
◦
yield is attained at 410 C. The lower selectivity at
◦
350 C is due to the increased concentration of heavier
aromatics such as phenyltoluene and diphenylmethane
in the product, whereas the reduction in selectivity
at higher temperatures is mainly associated with the
increased concentration of toluene in the product. In
the whole temperature region, the ␥-Al2O3 supported
catalyst is more selective to benzaldehyde than the
unsupported manganese oxide catalyst due to the
suppression of the formation of toluene and heavier
aromatics. The selectivity and yield of benzaldehyde
3
.3. Effect of temperature and H2
The activity and selectivity of MnO2 and 10%
Mn/␥-Al2O3 catalysts for the hydrogenation reac-
tion at different reaction temperatures and H2/methyl
benzoate ratios are listed in Tables 4 and 5.
Table 3
Hydrogenation of methyl benzoate over mixtures of oxides
Catalyst
Conversion (%)
Selectivity (%)
BA yield (%)
BA
B
T
BOH
ME
Others
1
1
1
1
1
1
0% Mn + MgO
0% Mn + SiO2
12.0
16.3
21.5
45.4
73.8
99.0
81.8
82.7
69.4
83.9
81.6
62.0
0.6
3.9
3.7
0.4
0.8
0.3
1.4
2.1
2.8
0.6
3.0
–
–
–
0.5
0.7
–
0.6
1.3
–
1.8
0.4
–
9.6
10.0
18.5
12.8
4.5
9.8
13.5
14.9
38.1
60.2
61.4
0% Mn + SiO2-Al2O3
0% Mn + ␥-Al2O3
0% Mn + TiO2
0% Mn + ZrO2
20.7
18.0
Reaction conditions: 600 mg catalyst; 410 C; H2 GHSV = 700 h−1; H2/methyl benzoate = 20/1 vol.%.
◦

3
04
A. Chen et al. / Journal of Molecular Catalysis A: Chemical 203 (2003) 299–306
Table 4
Hydrogenation of methyl benzoate over MnO2 and Mn/␥-Al2O3 catalyst under different temperatures
◦
Catalyst
T ( C)
Conversion (%)
Selectivity (%)
BA yield (%)
BA
B
T
BOH
ME
Others
MnO2
350
23.0
38.4
74.7
86.9
92.0
78.2
87.6
69.9
58.7
48.4
–
–
0.3
–
0.5
2.5
5.0
2.9
3.4
2.3
2.4
–
–
–
0.6
1.4
14.3
5.5
9.7
10.0
17.3
18.0
33.6
52.2
58.7
46.8
3
3
4
4
70
90
10
30
4.0
16.7
28.4
30.0
1
0%Mn/␥-Al2O3
350
13.6
24.7
52.8
91.2
92.3
82.2
91.2
90.7
80.8
71.8
–
2.4
1.4
0.7
5.3
11.8
0.6
2.0
3.5
6.4
5.9
–
–
–
2.0
2.1
14.8
4.4
4.5
5.0
7.9
11.2
22.5
47.9
73.6
66.3
3
3
4
4
70
90
10
30
1.0
0.6
0.5
0.5
Reaction conditions: 600 mg catalyst; 410 C; H2 GHSV = 700 h−1; H2/methyl benzoate = 20/1 vol.%.
◦
Table 5
Hydrogenation of methyl benzoate over 10% Mn/␥-Al2O3 catalyst at different H2/methyl benzoate ratios
H2:MB
Conversion (%)
Selectivity (%)
BA yield (%)
BA
B
T
BOH
ME
Others
5
:1
26.5
69.8
91.2
80.1
89.4
81.6
80.8
86.8
2.0
1.3
0.5
0.4
1.1
5.0
5.3
2.0
3
–
–
2.0
2.1
4.5
5.0
5.0
2.0
23.7
56.9
73.6
69.5
1
2
3
0:1
0:1
0:1
7.1
6.4
6.7
◦
−1
Reaction conditions: 600 mg catalyst; 410 C; H2 GHSV = 700 h
.
◦
on 10% Mn/␥-Al2O3 catalyst at 410 C reach 80.8 and
7
3.6%, respectively.
The ratio of H2 and methyl benzoate in the reactant
affects the activity and selectivity of 10% Mn/␥-Al2O3
as well. The highest yield of benzaldehyde is attained
at a H2/methyl benzoate ratio of 20:1. Further increas-
ing the H2/methyl benzoate ratio improves the selec-
tivity to benzaldehyde but reduces the conversion of
methyl benzoate.
3
.4. TPR and XPS results
The TPR profiles of the supported manganese oxide
catalysts are illustrated in Fig. 3. The 10% Mn/MgO
◦
catalyst is irreducible before 600 C due to the for-
mation of the complex oxide. All the other supported
catalysts show a two-step reduction. The two reduc-
tion peaks on the profile correspond to subsequent re-
duction of MnO2 to Mn2O3/Mn3O4 and MnO. The
reduction peak temperatures and the total H2 con-
Fig. 3. TPR profiles of various supported manganese oxide cata-
lysts: (a) 10% Mn/MgO; (b) 10% Mn/SiO ; (c) 10% Mn/␥-Al O ;
2
2
3
(d) 10% Mn/ZrO2; (e) 10% Mn/TiO2; (f) 10% Mn/SiO2-Al2O3.

A. Chen et al. / Journal of Molecular Catalysis A: Chemical 203 (2003) 299–306
305
Table 6
TPR data for supported manganese oxide catalysts
◦
Catalyst
T ( C)
H2 consumption
O/Mn)
(
I
II
1
1
1
1
1
0% Mn/ZrO2
0% Mn/SiO2-Al2O3
0% Mn/SiO2
0% Mn/TiO2
0% Mn/␥-Al2O3
323
360
328
375
370
394
409
434
434
467
0.89
0.90
0.87
0.91
0.95
sumption expressed as O/M ratio are summarized in
Table 6.
The O/Mn values in Table 6 indicate the presence
of a small fraction of Mn2O3 phase along with MnO2
phase on all the supported catalysts before reduction.
Nevertheless, this small amount of dispersed Mn2O3
species is not detected in XRD measurement. The re-
duction peak temperatures of Mn/␥-Al2O3 are higher
than those of the other supported catalysts, suggesting
the occurrence of a stronger interaction between the
manganese oxide and the ␥-Al2O3 support.
Fig. 4. XPS spectra of the supported catalysts reduced in hydrogen:
(a) 3% Mn/ZrO2; (b) 3% Mn/SiO2; (c) 3% Mn/␥-Al2O3.
nitrate precursor. This may explain the cause of the
variations in the structure or electronic state of the
manganese oxide active species on the catalysts and
ultimately the differences in catalytic behavior.
XPS spectra of some representative reduced cata-
lysts with a Mn loading 3 wt.% were recorded instead
of the supported catalysts with Mn loadings of 10 and
4. Conclusions
2
0 wt.%, because the sensitivity of the measurement
of electron binding energy shift related to surface in-
teractions is decreased with increasing loading [20].
In the literature [20–22], it has been reported that the
shifts in binding energy of surface metal oxides on
alumina substrate reflect the interaction of the metal
oxides with the alumina environment. The Mn 2p3/2
The catalytic performance of bulk oxides and sup-
ported manganese oxide catalysts for hydrogenation
of methyl benzoate to benzaldehyde were studied.
Transition metal oxides, such as ZrO2, TiO2 and
manganese oxides with redox properties are active
for the reaction, whereas oxides of the main groups,
such as MgO, ␥-Al2O3, SiO2 and SiO2-Al2O3,
are inactive. ␥-Al2O3 is a good support for this
type of catalyst. The selectivity to benzaldehyde on
Mn/␥-Al2O3 catalyst is improved considerably in
comparison to the bulk manganese oxides due to
suppression of the formation of toluene and other
heavier aromatics. The benzaldehyde yield of the
supported manganese oxide catalysts follows the or-
der of Mn/␥-Al2O3 > Mn/TiO2 > Mn/ZrO2 >
Mn/SiO2-Al2O3 > Mn/SiO2 > Mn/MgO. Under
our reaction conditions, the conversion of methyl ben-
zoate, selectivity to benzaldehyde and benzaldehyde
yield on 10% Mn/␥-Al2O3 catalyst are 91.2, 80.8 and
73.6%, respectively.
electron binding energies of Mn2 compounds are in
the range of 640.6–641.7 eV [20]. The binding energy
of the Mn 2p3/2 electrons of the reduced Mn/␥-Al2O3
catalyst in Fig. 4 is obviously higher than those ob-
served for the other supported catalysts, which is again
in agreement with the suggestion on a stronger interac-
tion between the surface Mn species and the ␥-Al2O3
support.
In the preparation of supported catalysts, the sup-
port pH at the point of zero charge (PZC) may in-
fluence the molecular structure of the surface active
species [23,24]. The pH values of the supports used in
this work decrease in the order of MgO > ␥-Al2O3 >
TiO2 ∼ ZrO2 > SiO2 > SiO2-Al2O3. The higher pH
values of MgO and ␥-Al2O3 could lead to a stronger
interaction between the support and the acidic Mn
+
XRD measurements show that manganese oxide on
the supported catalysts is reduced to highly dispersed

3
06
A. Chen et al. / Journal of Molecular Catalysis A: Chemical 203 (2003) 299–306
MnO under reaction conditions with the exception
of Mn/MgO, in which an irreducible complex ox-
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ide Mg MnO8 is formed. TPR and XPS analyses re-
6
veal the occurrence of a strong interaction between
the ␥-Al2O3 support and the surface manganese ox-
ide, that may explain the positive support effect of
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Acknowledgements
1
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14] Y. Yang, H. Xu, W. Shen, Y. Yang, J. Chen, Y. Zhou, Y.
Xiang, Chin. J. Catal. 20 (1999) 530.
We gratefully acknowledge the financial support for
this work by the Chinese Major State Basic Research
Development Program (Grant 2000077507) and the
Foundation for University Key Teachers by the Chi-
nese Ministry of Education.
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