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

Article (PDF Available)inDalton Transactions 41(36):11026-31 · August 2012with242 Reads
DOI: 10.1039/c2dt30553d · Source: PubMed
Abstract
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.

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PAPER
A very simple method to synthesize nano-sized manganese oxide: an efcient
catalyst for water oxidation and epoxidation of olens
Mohammad Mahdi Najafpour,*
a,b
Fahimeh Rahimi,
a
Mojtaba Amini,*
c
Sara Nayeri
a
and
Mojtaba Bagherzadeh
d
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 efcient
catalytic activity toward water oxidation and the epoxidation of olens in the presence of cerium(
IV)
ammonium nitrate and hydrogen peroxide, respectively.
1. Introduction
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 ethylene
3
and methane,
4
the
decomposition of nitrogen oxides,
5
ethylbenzene
6
and carbon
monoxide
7
and the epoxidation of olens.
8
A nanoscale compound is dened as a particle with a size in
the range of 1 to 100 nm (10
2
to 10
7
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
In this regard, the synthesis of manganese oxide-nanomater-
ials has attracted considerable attention.
10
Currently, there are
many methods for the synthesis of manganese oxide nanoparti-
cles. Among them, solvothermal,
11,12
polymeric-precursor
13
and
surfactant-mediated routes
1416
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 200220 °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 compo unds.
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 efcient catalytic activity toward water oxidation and the
epoxidation of olens in the presence of cerium(
IV) ammonium
nitrate and hydrogen peroxide, respectively.
2. Experimental
2.1 Materials
All reagents and solvents were purchased from commercial
sources and were used without further purication.
2.2 Water oxidation
Oxygen evolution from aqueous solutions in the presence of
(NH
4
)
2
Ce(NO
3
)
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
a
Department of Chemistry, Institute for Advanced Studies in Basic
Sciences (IASBS), Zanjan, 45137-66731, Iran.
E-mail: mmnajafpour@iasbs.ac.ir; Tel: (+98) 421 2278900
b
Center of Climate Change and Global Warming, Institute for Advanced
Studies in Basic Sciences (IASBS), Zanjan, 45137-66731, Iran
c
Department of Chemistry, Faculty of Science, University of Maragheh,
Golshahr, P.O. Box: 55181-83111731, Maragheh, Iran.
E-mail: mamini@maragheh.ac.ir
d
Chemistry Department, Sharif University of Technology, PO Box
11155-3516, Tehran, Iran
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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 ts of the data.
2.3 Synthesis of compounds
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.
Hot water (5.0 mL, 95 °C) was added to manganese nitrate
(Mn(NO
3
)
2
·4H
2
O) (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.
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 H
2
O
2
, 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
olens 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 ame-
ionization detector.
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-Mn
2
O
3
(Fig. 1 and Fig. S2). A small amount of ε-MnO
2
could also be detected (Fig. 2S). Thus, the compound contains
Mn
2
O
3
, some ε-MnO
2
and amorphous manganese oxide.
In the IR spectra of the compound, a broad band at
32003500 cm
1
, related to antisymmetric and symmetric
OH stretchings, and at 1630 cm
1
, related to HOH
bending, are observed (Fig. S3).
10
The intensities of these
peaks reduced in higher temperature. The absorption bands
characteristic for a MnO
6
core in the region 400500 cm
1
,
assigned to the stretching vibrations of MnO 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 m
2
g
1
(Fig. S6 and S7).
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 xation.
18,19
The site of water oxidation to O
2
is
known as the water oxidizing complex (WOC) and is a
Mn
4
O
5
Ca cluster.
19,20
In this structure, metal ions, one calcium and four manganese
ions, are bridged by ve 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 efcient
water oxidizing complex for articial photosynthesis, to evolve
hydrogen efciently 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
The water oxidation experiment by the prepared compound in
the presence of cerium(
IV) ammonium nitrate (Ce(IV)), a non-oxo
Fig. 1 XRD patterns of the obtained sample. Blue stars show the
peaks for α-Mn
2
O
3
(JCPDS 41-1442).
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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 ts of the data.
Membrane-inlet mass spectrometry (MIMS) showed the
oxygen evolution of the reactions of Ce(
IV)isareal water oxi-
dation and both oxygen atoms of the O
2
originate from the
water.
22
In the temperature range of 1535 °C, the values for k
o
(oxygen evolution) increased progressively (Fig. 3).
A plot of ln k
o
(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 0.010.1 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. 4 Water oxidation of an aqueous solution of 0.0450.91 M Ce(IV)
(40 mL) at 25.0 °C in the presence of nano-sized manganese oxides
(1.25 mg) prepared at 100 °C.
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.
Fig. 3 The relation between the rate of water oxidation and T
1
over
the range of 1035 °C.
11028 | Dalton Trans., 2012, 41, 1102611031 This journal is © The Royal Society of Chemistry 2012
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(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 rst order for the catalyst and zero order for Ce(
IV).
Nano-sized manganese oxides prepared at 100600 °C
showed efcient 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 100600 °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
MnO
4
formation in solution were also detected by UV-vis
(Fig. S8). More interestingly, after 72 h, a decrease in the con-
centration of the MnO
4
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 MnO
4
and
produce manganese oxide again. The reaction could be related to
a reduction of the concentration of MnO
4
after 72 h. MnO
4
formation from nano-manganese oxides, and not bulk, in the
presence of Ce(
IV) could be related to the thermodynamically
driven shifts in oxidationreduction equilibria for metal oxides.
9
3.3 Epoxidation of olens
Because of the epoxides versatility in preparing many chemical
intermediates, the epoxidation of olens is an important catalytic
reaction in the industry.
8,24
Usually, epoxides are produced by
the oxidation of olens with a stoichiometric amount of peracid
as oxidant.
25
Many manganese complexes were reported for the
epoxidation of olens 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 olens.
8
The nano-sized manga-
nese oxide shows efcient catalytic activity toward the epoxida-
tion of olens 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 olens.
25,26
3.3.1. General procedures for the epoxidation of olens. To
a solution of olens (0.5 mmol), NaHCO
3
(0.1 mmol) and
catalyst (0.005 mmol) in CH
3
CN (1.0 mL) was added H
2
O
2
(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 μ L) 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 calcul ation.
3.3.2. Catalytic epoxidation of various olens with H
2
O
2
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 CH
3
OHCH
2
Cl
2
. Our ndings showed that
CH
3
CN was a much more efcient solvent for the epoxidation
of styrene.
The nano-sized manganese oxide not only shows high activity
in the epoxidation of aromatic olens
25
but also mild activity in
the epoxidation of some non-aromatic olens (Table 1).
We thus used the manganese oxide catalyst for epoxidation of
several linear and cyclic olens (Table 1). The aromatic sub-
strates (styrene, α-methylstyrene and indene) were oxidized to
give the corresponding epoxides with 78100% conversion
(entries 13). Linear and cycl ic olens were less reactive in com-
parison to aromatic olens (entries 48).
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 ltered 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 rst reaction. Catalyst recycling studies show that
the conversion of styrene slowly decreased in the range of
86ca. 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, MoO
3
and CuO were
Fig. 5 The relation between calcined temperature and water oxidizing
activity of the oxide nano-sized manganese oxides prepared at
100600 °C.
Table 1 Epoxidation of olens catalyzed by manganese oxide
a
Entry Substrate Product
Conversion
b
(%)
Selectivity
c
(%)
1
91 93
2
78 95
3 100 100
4
47 100
5
43 100
6
33 100
7
17 100
8
11 100
a
Reaction conditions: catalyst (0.05 mmol), CH
3
CN (1 mL), alkene
(0.5 mmol), NaHCO
3
(0.1 mmol), H
2
O
2
(2 mmol), time = 4 h, at room
temperature.
b
The GC conversion (%) are measured relative to the
starting olen.
c
Selectivity to epoxide = (epoxide%/(epoxide% +
aldehyde%)) × 100.
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shown to be highly active for the selective epoxidation of styrene
to styrene oxide by tert-butyl hydroperoxide or H
2
O
2
but mild
activity or high peroxide decomposition is an associated problem
with these catalytic systems.
28
Based on the previous
reports,
25,26
a proposal for the mechanism is shown in
Scheme 1. HCO
4
(eqn (1)), an important active oxidant that is
more nucleophilic than H
2
O
2
, has been detected by
13
C MAS
NMR in this reaction.
H
2
O
2
þ HCO
3
Ð H
2
O þ HCO
4
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 efcient cata-
lysts for water oxidation in the presence of Ce(
IV). The nano-
sized manganese oxide showed high activity in the epoxidation
of aromatic olens and mild activity in the epoxidation of some
non-aromatic olens in the presence of H
2
O
2
and bicarbona te ions.
Acknowledgements
These authors are grateful to Institute for Advanced Studies in
Basic Sciences, University of Maragheh, and Sharif University
of Technology for nancial support.
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Fig. 6 Recycling studies of the nano-sized manganese oxide in the
reaction of epoxidation of styrene.
Scheme 1 Proposed mechanism for the epoxidation of olens by
manganese oxide in the presence of H
2
O
2
and HCO
3
.
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    • "It has been proposed that a terminal Mn(V)@O undergoes a nucleophilic attack by a Ca(II) bound hydroxide or water ligand to form a Mn-bound hydroperoxide by Pecoraro and Brudvig's groups [9,10,22]. Mn oxides were reported as water-oxidizing catalysts for artificial photosynthetic systems232425262728293031323334353637383940 . Among Mn oxides, nanolayered Mn oxides with different organic or inorganic ions between layers are efficient and stable in water oxidation [41]. "
    [Show abstract] [Hide abstract] ABSTRACT: Here, we used a strategy to answer to the question that whether Ca(II) ion is specific for water oxidation or not? In the procedure, first we synthesized layered Mn oxides with K(I) between layers and then replaced K(I) by Ca(II), K(I), Mg(II), La(III) or Ni(II). We proposed that Ca(II), K(I), Mg(II), La(III) and Ni(II), between layers are important to form efficient water-oxidizing catalyst, but not specific in water oxidation. However, Cu(II) ions decrease water-oxidizing activity of layered Mn oxides. The result is important to find critical factors in water oxidation by low-cost and environmentally friendly nanolayered Mn oxides.
    Full-text · Article · Mar 2015
    • "The Mn 4 O 4 cubical core of k-MnO 2 was synthesized by the delithiation of nanocrystalline LiMn 2 O 4 with dilute nitric acid for the photochemical water oxida- tion [39]. Chemical water oxidation with different manganese oxide based catalysts was developed by Najafpour et al. [27,36,38,40414243444546474849505152. Similarly, Jiao et al. employed the photochemical water oxidation with acid stable MnO 2 [53] whereas amorphous manganese oxides for effective water oxidation were designed by Suib et al. [54]. "
    [Show abstract] [Hide abstract] ABSTRACT: Development of efficient bio-inspired water oxidation system with transition metal oxide catalyst has been considered as the one of the most challenging task in the recent years. As the oxygen evolving center of photosystem II consists of Mn4CaO5 cluster, most of the water oxidation study was converged to build up manganese oxide based catalysts. Here we report the synthesis of efficient artificial water oxidation catalysts by transferring the inactive manganese monooxide (MnO) under highly oxidizing conditions with ceric ammonium nitrate (CAN) and ozone (O3). MnO was partially oxidized to form mixed-valent manganese oxide (MnOx) with CAN whereas completely oxidized to mineral phase of ε-MnO2 (Akhtenskite) upon treatment of O3 in acidic solution, which we explore first time as a water oxidation catalyst. Chemical water oxidation, as well as the photochemical water oxidation in the presence of sacrificial electron acceptor and photosensitizer with the presented catalysts were carried out that followed the trends: MnOx>MnO2>MnO. Structural and activity correlation reveals that the presence of larger extent of Mn(III) in MnOx is the responsible factor for higher activity compared to MnO2. Mn(III) species in octahedral system with eg(1) configuration furnishes and facilitates the Mn-O and Mn-Mn bond enlargement with required structural flexibility and disorder in the manganese oxide structure which indeed facilitates water oxidation. Copyright © 2014 Elsevier B.V. All rights reserved.
    Full-text · Article · Dec 2014
    • "To explain the improved selectivities and the different effects of DMF and NaHCO 3 in the epoxidation mediated by CuO/nanotubes- 450, according to the literature [28,50], two possible mechanisms were put forward and illustrated inFig. 12. "
    [Show abstract] [Hide abstract] ABSTRACT: Cu-containing nanotubes with a large surface area and pore volume were prepared by using nanoscrolls derived from K4Nb6O17 as a support and a subsequent thermal transformation of Cu-containing nanoscrolls into Nb2O5 nanotubes. The method is facile and template-free. The catalytic performance of the resulted Cu-containing nanotubes was evaluated for styrene epoxidation in the presence of tert-butyl hydroperoxide (TBHP) and H2O2, respectively. It was found that Cu-containing nanotubes displayed a relative good catalytic performance with a styrene oxide (SO) selectivity of 46.9% by TBHP and a much higher SO selectivity of 94.6% by H2O2. Two possible mechanisms were put forward to explain the different catalytic behaviors in the two types of oxidation systems. Because of the thermal transformation of nanoscrolls into nanotubes, nanoscrolls may be a new kind of promising support for the design and assembly of novel heterogeneous catalysts.
    Full-text · Article · Apr 2014
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