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: bqxu@mail.tsinghua.edu.cn

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.

References

[1] Larock R C. Comprehensive Organic Transformation: A Guide to

Functional Group Preparations. 2nd Ed. New York: Wiley/VCH,

1999

[2] Mallat T, Baiker A. Chem Rev, 2004, 104(6): 3037

[3] Firouzabadi H, Karimi B, Abbassi M. J Chem Res, Synop, 1999,

(3): 236

[4] Hirano M, Yakabe S, Chikamori H, Clark J H, Morimoto T. J

Chem Res, Synop, 1998, (6): 308

[5] Son Y Ch, Makwana V D, Howell A R, Suib S L. Angew Chem,

Int Ed, 2001, 40(22): 4280

[6] Makwana V D, Garces L J, Liu J, Cai J, Son Y Ch, Suib S L.

Catal Today, 2003, 85(2–4): 225

[7] Ding Y S, Shen X F, Sithambaram S, Gomez S, Kumar R,

Crisostomo V M B, Suib S L, Aindow M. Chem Mater, 2005,

17(21): 5382

[8] Tang X F, Huang X M, Shao J J, Liu J L, Li Y G, Xu Y D, Shen W

J. Chin J Catal, 2006, 27(2): 97

[9] Liu J, Makwana V, Cai J, Suib S L, Aindow M. J Phys Chem B,

2003, 107(35): 9185

[10] Chen X, Shen Y F, Suib S L, O’Young C L. Chem Mater, 2002,

14(2): 940

[11] Cheng F Y, Chen J, Gou X L, Shen P W. Adv Mater, 2005, 17(22):

2753

[11] Makwana V D, Son Y Ch, Howell A R, Suib S L. J Catal, 2002,

210(1): 46

[12] Malinger K A, Ding Y S, Sithambaram S, Espinal L, Gomez S,

Suib S L. J Catal, 2006, 239(2): 290

[13] Trawczynski J, Bielak B, Mista W. Appl Catal B, 2005, 55(4):

277

[14] Arena F, Torre T, Raimondo C, Parmaliana A. Phys Chem Chem

Phys, 2001, 3(10): 1911

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