A very simple method to synthesize nano-sized manganese oxide: An efficient catalyst for water oxidation and epoxidation of olefins

Article abstract of DOI:10.1039/c2dt30553d

Nano-sized particles of manganese oxides have been prepared by a very simple and cheap process using a decomposing aqueous solution of manganese nitrate at 100 °C. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction spectrometry have been used to characterize the phase and the morphology of the manganese oxide. The nano-sized manganese oxide shows efficient catalytic activity toward water oxidation and the epoxidation of olefins in the presence of cerium(iv) ammonium nitrate and hydrogen peroxide, respectively.

Full text of DOI:10.1039/c2dt30553d

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PAPER
Avery simple method to synthesize nano-sized manganese oxide: an efficient
catalyst for water oxidation and epoxidation of olefins†
Mohammad Mahdi Najafpour,*a,b Fahimeh Rahimi,a Mojtaba Amini,*c Sara Nayeria and
Mojtaba Bagherzadehd
Received 8th March 2012, Accepted 4th July 2012
DOI: 10.1039/c2dt30553d
Nano-sized particles of manganese oxides have been prepared by a very simple and cheap process using a
decomposing aqueous solution of manganese nitrate at 100 °C. Scanning electron microscopy,
transmission electron microscopy and X-ray diffraction spectrometry have been used to characterize the
phase and the morphology of the manganese oxide. The nano-sized manganese oxide shows efficient
catalytic activity toward water oxidation and the epoxidation of olefins in the presence of cerium(IV)
ammonium nitrate and hydrogen peroxide, respectively.
In this regard, the synthesis of manganese oxide-nanomater-
1. Introduction
ials has attracted considerable attention.10 Currently, there are
Manganese oxides are not only earth abundant and low cost but
also environmentally friendly. Thus, they are particularly attrac-
tive for use as catalyst materials.1 Manganese oxides were
reported to be useful, versatile, and environmentally friendly cat-
alysts for important reactions and have been used extensively for
the oxidation of a variety of molecules, especially for water oxi-
dation,2 the deep oxidation of ethylene3 and methane,4 the
decomposition of nitrogen oxides,5 ethylbenzene6 and carbon
monoxide7 and the epoxidation of olefins.8
A nanoscale compound is defined as a particle with a size in
the range of 1 to 100 nm (102 to 107 atoms) from zero (0 D) to
three dimensions (3 D), which could exhibit unique physiochem-
ical properties as compared with the bulk compound. Interest-
ingly, it was reported that the nanophase transition metal oxides
show large thermodynamically driven shifts in oxidation–
reduction equilibria.9 The size effect could change the redox
potential of nano-sized transition metal oxides and may result in
the improved catalytic activity of these compounds as compared
to bulk manganese oxides. For example, Navrotsky et al.
reported that cobalt(II) oxide nanoparticles smaller than 8 nm,
when dropped into water, evolved hydrogen.9 Such spontaneous
oxidation of cobalt(II) and reduction of water is not seen for
larger particles.9
many methods for the synthesis of manganese oxide nanoparti-
cles. Among them, solvothermal,11,12 polymeric-precursor13 and
surfactant-mediated routes14–16 are quite popular.
Metal oxide decomposition is also an interesting and feasible
way to prepare small metal oxide particles. Decomposition of
manganese nitrate depends on some factors such as heating rate,
moisture content of the atmosphere or manganese nitrate and
different decomposition temperatures, mechanisms, and inter-
mediates are reported for the reaction.17 In air or oxygen manga-
nese nitrate decomposes up to 200–220 °C.17 In a moist
atmosphere the decomposition of the nitrates can occur at lower
temperatures and more interestingly, hydrated manganese nitrate
has been reported to decompose at lower temperatures than the
dehydrated one.17 These materials show good catalytic activity
toward the oxidation of different compounds.17 Here, we report
the synthesis of nano-sized particles of manganese oxide by a
very simple and cheap process, using the decomposition of an
aqueous solution of manganese nitrate at 100 °C. The compound
shows efficient catalytic activity toward water oxidation and the
epoxidation of olefins in the presence of cerium(IV) ammonium
nitrate and hydrogen peroxide, respectively.
2. Experimental
aDepartment of Chemistry, Institute for Advanced Studies in Basic
Sciences (IASBS), Zanjan, 45137-66731, Iran.
2.1 Materials
All reagents and solvents were purchased from commercial
sources and were used without further purification.
E-mail: mmnajafpour@iasbs.ac.ir; Tel: (+98) 421 2278900
bCenter of Climate Change and Global Warming, Institute for Advanced
Studies in Basic Sciences (IASBS), Zanjan, 45137-66731, Iran
cDepartment of Chemistry, Faculty of Science, University of Maragheh,
Golshahr, P.O. Box: 55181-83111731, Maragheh, Iran.
E-mail: mamini@maragheh.ac.ir
2.2 Water oxidation
dChemistry Department, Sharif University of Technology, PO Box
11155-3516, Tehran, Iran
Oxygen evolution from aqueous solutions in the presence of
(NH4)2Ce(NO3)6 (Ce(IV)) was measured using an HQ40d port-
able dissolved oxygen meter connected to an oxygen monitor
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt30553d
11026 | Dalton Trans., 2012, 41, 11026–11031
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with a digital readout. The reactor was maintained at 25.0 °C in
a water bath. In a typical run, the instrument readout was cali-
brated against air-saturated distilled water, or stirred continually
with a magnetic stirrer in the air-tight reactor. After ensuring a
constant baseline reading, the water in the reactor was replaced
with Ce(IV) solution. Without catalyst, Ce(IV) was stable in this
condition and oxygen evolution was not observed. After deaera-
tion of the Ce(IV) solution with argon, manganese oxides as
several small particles were added, and oxygen evolution was
recorded with the oxygen meter under stirring (Fig. S1†). The
formation of oxygen was followed, and oxygen formation rates
per manganese site were obtained from linear fits of the data.
2.3 Synthesis of compounds
Fig. 1 XRD patterns of the obtained sample. Blue stars show the
peaks for α-Mn2O3 (JCPDS 41-1442).
We used a new, very easy and cheap method to obtain nanoparti-
cles of manganese oxide, which is based on decomposition of
manganese nitrate in moderate temperature.
In the IR spectra of the compound, a broad band at
∼3200–3500 cm−1, related to antisymmetric and symmetric
O–H stretchings, and at ∼1630 cm−1, related to H–O–H
bending, are observed (Fig. S3†).10 The intensities of these
peaks reduced in higher temperature. The absorption bands
Hot water (5.0 mL, 95 °C) was added to manganese nitrate
(Mn(NO3)2·4H2O) (39.8 mmol, 10.0 g). After 10 min of mag-
netic stirring at room temperature, the solution was heated to
100 °C for 24 h in an oven to obtain a viscous liquid. The black
particles were obtained by adding water (10 mL) to the black
viscous liquid and centrifugation of the resulting solution. The
black particles were washed with water (10 mL, two times) and
dried at 100 °C.
characteristic for a MnO6 core in the region 400–500 cm−1
,
assigned to the stretching vibrations of Mn–O bonds in manga-
nese oxide, was also observed in the FTIR spectra of these
compounds.10
Scanning Electron Microscopy (SEM) images allowed us to
determine that the compound consists of particles from <100 nm
in size (Fig. 2 and Fig. S4†). Transmission Electron Microscopy
(TEM) pictures allowed us to determine that the compound con-
sists of amorphous and also crystals of manganese oxide (Fig. 2
and Fig. S5†). The BET test showed that the surface of this com-
pound is 33.49 m2 g−1 (Fig. S6 and S7†).
2.4 Characterization
MIR spectra of KBr pellets of compounds were recorded on a
Bruker vector 22 in the range between 400 and 4000 cm−1
.
TEM and SEM were carried out with Philips CM120 and LEO
1430VP, respectively. The X-ray powder patterns were recorded
with a Bruker, D8 ADVANCE (Germany) diffractometer (Cu-Kα
radiation). Manganese atomic absorption spectroscopy (AAS)
was performed on an Atomic Absorbtion Spectrometer Varian
Spectr AA 110. Prior to analysis, the oxide (2.0 mg oxide) were
added to 1 mL of concentrated nitric acid and H2O2, which was
left at room temperature for at least 1 h to ensure that the oxides
were completely dissolved. The solutions were then diluted to
25.0 mL and analyzed by AAS. The products of the oxidation of
olefins were determined and analyzed by using a HP Agilent
6890 gas chromatograph equipped with a HP-5 capillary column
(phenyl methyl siloxane 30 m × 320 lm × 0.25 lm) and a flame-
ionization detector.
3.2 Water oxidation
Photosystem II, located in the thylakoid membrane of plants,
algae, and cyanobacteria, is an enzyme that oxidizes water to
oxygen. The generated protons drive ATP synthase and the elec-
trons provide the reducing equivalents that ultimately lead to
carbon dioxide fixation.18,19 The site of water oxidation to O2 is
known as the water oxidizing complex (WOC) and is a
Mn4O5Ca cluster.19,20
In this structure, metal ions, one calcium and four manganese
ions, are bridged by five oxygen atoms. Four water molecules
were found also in this structure and two of them are suggested
as the substrates for water oxidation.20 To design an efficient
water oxidizing complex for artificial photosynthesis, to evolve
hydrogen efficiently in a sustainable manner, we may learn and
use wisely the knowledge about water oxidation and the water
oxidizing complex in the natural system.21 To synthesize a water
oxidizing catalyst, manganese compounds have been considered
not only because manganese has been used by nature to oxidize
water but also because manganese is cheap and environmentally
friendly. Manganese oxides have been reported as heterogeneous
catalysts for water oxidation by some groups.2
3. Results and discussion
3.1 Characterization
The X-ray diffraction (XRD) pattern of the as-prepared manga-
nese oxide was of poor resolution (Fig. 1). However, the peaks
observed at 2θ = 23°, 33°, 38°, 45°, 49°, 54°, and 65° could be
indexed to a cubic cell (JCPDS 41-1442), belonging to the
α-Mn2O3 (Fig. 1 and Fig. S2†). A small amount of ε-MnO2
could also be detected (Fig. 2S†). Thus, the compound contains
Mn2O3, some ε-MnO2 and amorphous manganese oxide.
The water oxidation experiment by the prepared compound in
the presence of cerium(IV) ammonium nitrate (Ce(IV)), a non-oxo
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Fig. 3 The relation between the rate of water oxidation and T−1 over
the range of 10–35 °C.
Fig. 4 Water oxidation of an aqueous solution of 0.045–0.91 M Ce(IV)
(40 mL) at 25.0 °C in the presence of nano-sized manganese oxides
(1.25 mg) prepared at 100 °C.
Membrane-inlet mass spectrometry (MIMS) showed the
oxygen evolution of the reactions of Ce(IV) is a “real water oxi-
dation” and both oxygen atoms of the O2 originate from the
water.22 In the temperature range of 15–35 °C, the values for ko
(oxygen evolution) increased progressively (Fig. 3).
A plot of −ln ko (oxygen evolution) versus T−1 was linear
and gave ΔH = +96.8 ( 0.10) kJ mol−1 and ΔS = +292.8
( 0.1) J mol−1 as apparent activation parameters for the water
oxidation reaction of the nano-sized manganese oxide. The ΔH
for water oxidation is higher than the related parameter for
layered manganese oxides.23 It may show that an open structure
or other ions (Ca(II),22 Zn(II)23 and Al(III)23) in layered manga-
nese oxides could reduce the activation energy for water oxi-
dation. To study the effect of concentration of Ce(IV), reactions
were done with different concentrations of Ce(IV), keeping all
other factors constant as shown in Fig. 4. The rate of oxygen
evolution increases with an increase in the concentration of
Ce(IV). This increase of the rate of oxygen evolution in a low
concentration of Ce(IV) is linear in the range of 0.01–0.1 M of
Ce(IV). However, in a high concentration of Ce(IV), the slope of
the increase of the rate of oxygen evolution becomes shallower
Fig. 2 SEM images of manganese nitrate solution after 2 h (a) and
24 h heating at 100 °C (b). TEM image of the nano-sized manganese
oxide prepared by decomposition of manganese nitrate solution at
100 °C for 24 h.
transfer and a strong one-electron oxidant, was performed and
the formation of oxygen was followed. The oxygen formation
rates per manganese site were obtained from linear fits of the data.
11028 | Dalton Trans., 2012, 41, 11026–11031
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Table 1 Epoxidation of olefins catalyzed by manganese oxidea
Conversionb Selectivityc
Entry Substrate
Product
(%)
(%)
1
91
93
2
78
95
3
4
100
47
100
100
5
6
43
33
100
100
Fig. 5 The relation between calcined temperature and water oxidizing
activity of the oxide nano-sized manganese oxides prepared at
100–600 °C.
7
8
17
11
100
100
a Reaction conditions: catalyst (0.05 mmol), CH3CN (1 mL), alkene
(0.5 mmol), NaHCO3 (0.1 mmol), H2O2 (2 mmol), time = 4 h, at room
temperature. b The GC conversion (%) are measured relative to the
starting olefin. c Selectivity to epoxide = (epoxide%/(epoxide% +
aldehyde%)) × 100.
(Fig. 4). The increase in the rate of oxygen evolution with the
increase of oxide concentration is also linear. In other words,
under high concentrations of Ce(IV) (>0.1), the derived rate law
is first order for the catalyst and zero order for Ce(IV).
Nano-sized manganese oxides prepared at 100–600 °C
showed efficient water oxidizing activity in the presence of
Ce(IV) (Fig. 5). In contrast to the layered manganese oxides,23 the
temperature in the range of 100–600 °C is not an important
factor for the water oxidizing activity of this compound (Fig. 5).
Interestingly, after adding the catalyst the reduction of Ce(IV) and
catalyst (0.005 mmol) in CH3CN (1.0 mL) was added H2O2
(2.0 mmol) as oxidant. After stirring at room temperature for
4 h, for the products analysis, the solution was subjected to ether
extraction (3 × 10 mL), and the extract was also concentrated
down to 0.5 mL by distillation in a rotary evaporator at room
temperature. Then, a sample (2 μL) was taken from the solution
and analyzed by GC. The retention times of the peaks were com-
pared with those of commercial standards, and chlorobenzene
was used as an internal standard for GC yield calculation.
−
MnO4 formation in solution were also detected by UV-vis
(Fig. S8†). More interestingly, after ∼72 h, a decrease in the con-
−
centration of the MnO4 was observed (Fig. S8†) that could be
related to self-repair of the catalyst. As we reported before,23
a
small amount of Mn(II) (detected by EPR) dissolved under this
condition; these Mn(II) ions could react with MnO4 and
−
3.3.2. Catalytic epoxidation of various olefins with H2O2
over the nano-sized manganese oxide. In order to choose a suit-
able solvent, the oxidation of styrene was carried out in dichloro-
methane, chloroform, acetonitrile, acetone, methanol and a
(1 : 1) mixture of CH3OH–CH2Cl2. Our findings showed that
CH3CN was a much more efficient solvent for the epoxidation
of styrene.
produce manganese oxide again. The reaction could be related to
−
−
a reduction of the concentration of MnO4 after 72 h. MnO4
formation from nano-manganese oxides, and not bulk, in the
presence of Ce(IV) could be related to the thermodynamically
driven shifts in oxidation–reduction equilibria for metal oxides.9
The nano-sized manganese oxide not only shows high activity
in the epoxidation of aromatic olefins25 but also mild activity in
the epoxidation of some non-aromatic olefins (Table 1).
We thus used the manganese oxide catalyst for epoxidation of
several linear and cyclic olefins (Table 1). The aromatic sub-
strates (styrene, α-methylstyrene and indene) were oxidized to
give the corresponding epoxides with 78–100% conversion
(entries 1–3). Linear and cyclic olefins were less reactive in com-
parison to aromatic olefins (entries 4–8).
Also, the reusability of the catalyst was tested at room temp-
erature for 4 h and the results have been shown in Fig. 6.
In each reaction, the catalyst from the reaction was filtered off,
washed several times with water and diethyl ether repeatedly,
dried at 50 °C and reused for the next run under the same con-
ditions as the first reaction. Catalyst recycling studies show that
the conversion of styrene slowly decreased in the range of
86–ca. 91% after the sixth cycle and the selectivity of the
epoxide was kept partly constant around 91% during recycling
(Fig. 6). Nano-iron oxides,27 NiO, CoO, MoO3 and CuO were
3.3 Epoxidation of olefins
Because of the epoxides’ versatility in preparing many chemical
intermediates, the epoxidation of olefins is an important catalytic
reaction in the industry.8,24 Usually, epoxides are produced by
the oxidation of olefins with a stoichiometric amount of peracid
as oxidant.25 Many manganese complexes were reported for the
epoxidation of olefins using peracids and hydroperoxides as oxi-
dants.24 A few manganese oxides, as low-cost, easily synthesized
and environmentally friendly compounds, have been used as a
catalyst for the epoxidation of olefins.8 The nano-sized manga-
nese oxide shows efficient catalytic activity toward the epoxida-
tion of olefins in the presence of hydrogen peroxide as an
environmentally friendly and low-cost oxidant. Hydrogen per-
oxide in the bicarbonate solutions was shown to be a good
oxidant for the epoxidation of olefins.25,26
3.3.1. General procedures for the epoxidation of olefins. To
a solution of olefins (0.5 mmol), NaHCO3 (0.1 mmol) and
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Dalton Trans., 2012, 41, 11026–11031 | 11029

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Scheme 1 Proposed mechanism for the epoxidation of olefins by
−
manganese oxide in the presence of H2O2 and HCO3
.
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−
Scheme 1. HCO4 (eqn (1)), an important active oxidant that is
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ꢀ
ꢀ
H2O2 þ HCO3 Ð H2O þ HCO4
ð1Þ
4. Conclusion
Nano-sized manganese oxide, as a low-cost, easily synthesized
and environmentally friendly compound, was synthesized by a
very simple method in moderate temperature and without using
any organic compounds. It was characterized by SEM, XRD,
FTIR, AAS and TEM. These compounds act as efficient cata-
lysts for water oxidation in the presence of Ce(IV). The nano-
sized manganese oxide showed high activity in the epoxidation
of aromatic olefins and mild activity in the epoxidation of some
non-aromatic olefins in the presence of H2O2 and bicarbonate ions.
Acknowledgements
These authors are grateful to Institute for Advanced Studies in
Basic Sciences, University of Maragheh, and Sharif University
of Technology for financial support.
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11030 | Dalton Trans., 2012, 41, 11026–11031
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Dalton Trans., 2012, 41, 11026–11031 | 11031