Amorphous Manganese Oxide for Catalytic Aerobic Oxidation of Benzyl Alcohol

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.

SHORT COMMUNICATION

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 temperature 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 Ȗ-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 reducibility 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 synthesis 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 molecular 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 crystalline 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 (Ȗ-MnO2, Sinopharm Chemical Reagent Beijing Co., Ltd.) were also used as catalysts. 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 Ȗ-MnO2. The samples with a calcination temperature lower

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.

HU Jing et al. / Chinese Journal of Catalysis, 2007, 28(12): 1025–1027

Table1 BET surface area, pore volume, average pore diameter, and crystal structure of various manganese oxides Catalyst

BET surface Pore volume Average pore area (m2/g)

(cm3/g)

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

Ȗ-MnO2

43

0.23

9.61

Ȗ-MnO2

than 400qC retained the amorphous structure. N2 adsorption/ desorption characterization showed that the BET surface area of the amorphous MnOx decreased with the calcination temperature. 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 2ș = 12.6q, 17.9q, 28.6q, 37.4q, 41.8q, 49.7q, and 60.1q (OMS-2 [8,9]) and 2ș = 23.1q, 33.0q, 38.2q, 45.2q, 55.2q, and 65.7q (Mn2O3 [10]), indicating the transformation of amorphous MnOx to crystalline cryptomelane (OMS-2) and Mn2O3 at higher calcination temperatures. The BET surface area and the pore volume of the samples calcined at 500 or 600qC 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 Table 2 Benzyl alcohol oxidation over manganese oxidesa Catalyst MnOx-110 MnOx-110d

a

Time (h)

Conversion

Activity b

(%)

(mmol/(g·h))

1.5 10

tonsetc/ºC

72.7

9.7

78.0

4.7

148 —

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

Ȗ-MnO2

1.5

39.2

5.2

232

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.

conversions on crystalline OMS-2 and commercial Ȗ-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 Ȗ-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 species 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 experiments 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 evidence for the catalytic role of the amorphous MnOx was obtained by the dependence of benzyl alcohol conversion on the oxygen pressure. The conversion of benzyl alcohol was doubled 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 400qC. However, when the calcination temperature was increased to 500 and 600qC (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 400qC, exhibited TPR curves with a similar shape showing sharp peaks at 270 and 345qC. When the calcination temperature was increased to 500qC, the reduction peak at 270qC disappeared, and two peaks at 335 and 368qC were observed. For comparison, the OMS-2 showed a H2 consumption peak at 340qC, and the commercial Ȗ-MnO2 showed two peaks at 320 and 425qC. 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 temperature increased with the calcination temperature of the MnOx. MnOx-110 had an onset reduction temperature of 145qC,

HU Jing et al. / Chinese Journal of Catalysis, 2007, 28(12): 1025–1027

controlling their activity for the catalytic oxidation of benzyl alcohol.

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whereas the sample calcined at 400qC had an onset reduction temperature of 187qC. A calcination temperature higher than 500qC, however, led to the onset reduction temperature being increased drastically to above 275qC. A correlation between the mass specific activity and the onset reduction temperature of the various MnOx was made, and an inverse linear relationship was observed. That is, the lower the catalytic activity, the higher the onset reduction temperature. The data of the OMS-2 and Ȗ-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 temperature 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, Ȗ-MnO2, and Mn2O3). The reducibility of the manganese oxide is a key factor in

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