Nanolayered manganese-calcium oxide as an efficient catalyst toward organic sulfide oxidation

Article abstract of DOI:10.1039/c4ra00008k

We for the first time report that nanolayered Mn-Ca oxide in the presence of H₂O₂ is an efficient catalyst toward sulfide oxidation to sulfoxide. We characterized the catalyst by DLS, UV-Vis, diffuse reflectance infrared Fourier transform spectroscopy, SEM, TEM and HRTEM. We also considered different parameters of the sulfide-oxidation reaction.

Full text of DOI:10.1039/c4ra00008k

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Nanolayered manganese–calcium oxide as an
efficient catalyst toward organic sulfide oxidation†
Cite this: RSC Adv., 2014, 4, 10851
a
bc
Hadi Naslhajian,ad Emad Aminib
*
*
Mojtaba Amini, Mohammad Mahdi Najafpour,
and S. Morteza F. Farniad
Received 1st January 2014
Accepted 5th February 2014
We for the first time report that nanolayered Mn–Ca oxide in the presence of H2O2 is an efficient catalyst
toward sulfide oxidation to sulfoxide. We characterized the catalyst by DLS, UV-Vis, diffuse reflectance
infrared Fourier transform spectroscopy, SEM, TEM and HRTEM. We also considered different parameters
of the sulfide-oxidation reaction.
DOI: 10.1039/c4ra00008k
www.rsc.org/advances
Organic sulfoxides are among the most important compounds
to synthesize various chemically and biologically signicant
molecules.1 Among different methods, the oxidation of
suldes is a common method for preparing sulfoxides.2 There
are several reagents for this conversion but many of them have
the problem of over oxidation of sulfoxides to sulfones.1,2
Different solvents, oxidants and catalysts are used in
improving the procedure.1,2 Mn compounds are among the
most attractive inorganic materials not only because of their
physical and chemical properties but also because they are low
cost and environmentally friendly.3 It was reported that ethyl
phenylsulde (2 mmol) was allowed to react with oxygen/piv-
alaldehyde (1 eq.) in peruoro-2-butyltetrahydrofuran (2.5 mL,
Materials and methods
All reagents and solvents were purchased from commercial
sources (Merck or Sigma-Aldrich) and were used without further
purication. MIR spectra of KBr pellets of compounds were
recorded on a Bruker vector 22 in the range between 400 and
4000 cmꢁ1. Diffuse reectance infrared Fourier transform
spectroscopy were recorded on Perkin Elmer-Spectrum one.
TEM and SEM were carried out with JEOL JEM 2010 Fas TEM
operating at 200 kV and LEO 1430VP, respectively. The X-ray
powder patterns were recorded with a Bruker, D8 ADVANCE
(Germany) diffractometer (Cu-Ka radiation). Mn atomic
absorption spectroscopy (AAS) was performed on an Atomic
Absorption Spectrometer Varian SpectrAA 110. Prior to analysis,
the oxide (10.0 mg metal) was added to 1 mL of concentrated
nitric acid and H2O2, le 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.
ꢀ
25 C) in the presence of Mn(OAc)3$2H2O (2 mol%) and led to
a mixture of ethylphenylsulfoxide and ethylphenylsulfone
(67 : 33) in 1 h.4 Asymmetric oxidation of suldes was also
examined by using (salen)Mn(III) complexes as catalyst and was
found to show high asymmetric induction.5 Mn oxides are
cheap and environmentally friendly compounds and used
extensively as bulk, supported, nano-sized with large surface
area and colloidal forms. Mn oxides are efficient catalysts for
oxidation of both organic6 and inorganic compounds such as
carbon monoxide,7 nitrogen oxides,8 and water.9 Recently, we
used layered Mn oxides as efficient catalysts for oxidation of
water,10 alcohols11 and alkenes.12 Here, we report nanolayered
Mn oxides as efficient catalysts toward organic sulde oxida-
tion to sulfoxide.
Synthesis of compound
The compound was synthesized by a previously reported
method.13 Briey, it was synthesized by mixing a solution
contain
CaCl2$4H2O
(2.70
mmol,
0.494
g)
and
Mn(CH3COO)2$4H2O (5.6 mmol, 1372 mg) in water (5 mL) and a
solution of KMnO4 (2.4 mmol, 379 mg) and KOH (17.0 g) in 100
mL water. The obtained suspension was ltered and washed
with distilled water (3 L) before being allowed to dry for 12 h at
60 ꢀC in an oven. Then the solid was heated to different
temperatures (100–400 ꢀC) for 10 h in air to obtain a brown
powder. Yield: >%90; at 400 ꢀC the oxide could be formulated as
Ca0.16MnO2$2H2O.
aDepartment of Chemistry, Faculty of Science, University of Maragheh, Golshahr, P.O.
Box. 55181-83111731, Maragheh, Iran. E-mail: mamini@maragheh.ac.ir; Tel: +98
935 4171381
bDepartment of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS),
Zanjan, 45137-66731, Iran. E-mail: mmnajafpour@iasbs.ac.ir; Tel: +98 241 415 3201
cCenter of Climate Change and Global Warming, Institute forAdvanced Studies in Basic
Sciences (IASBS), Zanjan, 45137-66731, Iran
Oxidation of suldes catalyzed by Ca0.16MnO2$2H2O
To a solution of sulde (0.5 mmol) and catalyst (10.0 mg) in
CH3CN (1.0 mL), H2O2 (1.5 mmol) as oxidant was added. For the
analysis of products, aer stirring at room temperature for 2 h,
dDepartment of Chemistry, University of Tehran, Tehran, Iran
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra00008k
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the solution was subjected to ether extraction (3 ꢂ 10 mL), and
the extract was also concentrated down to 1.0 mL by distillation
in a rotary evaporator at room temperature. Then, a sample
(2 mL) was taken from the solution and analyzed by GC. The
retention times of the peaks were compared with those of
commercial standards, and chloro benzene was used as an
internal standard for GC yield calculation.
Results and discussion
The procedure to synthesize of the nanolayered Mn oxides was
simple, green, easily and cheap by the reaction of KMnO4,
Mn(II)(OAc)2 and CaCl2 in the presence of KOH. In reectance
infrared Fourier transform spectra and FTIR spectra of oxide
show a broad band at ꢃ3200–3500 cmꢁ1 related to antisym-
metric and symmetric O–H stretchings and at ꢃ1630 cmꢁ1
related to H–O–H bending are observed.13 The peaks at ꢃ700
cmꢁ1 are related to Mn–O–Mn bonds (Fig. S1, ESI†).13
XRD data for the compound is of very poor resolution and
the weak peak in 2q ꢃ11 related (00l) in layered Mn oxides,
reveals the absence of long-range order of the layer stacking
(Fig. S2, ESI†).13
Using extended-range X-ray absorption spectroscopy (XAS) at
the K-edges of both Ca and Mn, two different Ca-containing
motifs were identied. One of the motifs was the formation of
Mn3CaO4 cubes while the other Ca(II) ions probably inter-
connected oxide-layer fragments (Fig. 1).14
SEM images from the catalyst show aggregated nanoparticles
from <100 nm. In TEM images, layers of Mn oxide are clearly
detected and showed that the nano-compounds observed by
SEM contain nanosheets (Fig. 2 and S3, ESI†). Only a few short-
range orders among layers can be observed in the sample by
Fig. 2 SEM micrographs (a), TEM (b) and HRTEM (c and d) images of
nano-size Mn–Ca oxide prepared at 400 ꢀC.
HRTEM. Such layered Mn oxides without any long-range order
among layers are rare. DLS shows that particles are in the range
of 100–200 nm (Fig. S4, ESI†). Comparing with SEM (<100 nm),
Fig. 1 A motif for Mn–Ca cubic in layered Mn oxide (a). Surface area of
layered Mn oxide (Ca(II) ions are omitted for clarification) (b).
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we found aggregation of particles in solution. Peak at ꢃ400 nm
A series of various types of structurally diverse suldes were
in solid state UV-Vis spectrum of oxide is related to Mn(III)/ subjected to the oxidation reaction using Ca0.16MnO2$2H2O as
Mn(IV) ions in the structure (Fig. S5, ESI†).
catalyst and H2O2 as oxidant (Table 2). Arylalkyl (Table 2, entries
1 and 2), arylbenzyl (Table 2, entry 3), dibenzyl (Table 2, entry 4),
diaryl (Table 2, entry 5) and dialkyl (Table 2, entries 6–8) suldes
underwent clean and the selectivity for MPS oxidation to sulf-
oxide under air, is high (89–100%). Signicantly, the very good
conversions of substrates depending on the nature of the
sulde, in the range of 51–100% were obtained for all cases. It
was observed that aromatic suldes undergo oxidation reac-
tions can be oxidized to sulfoxide more easily than aliphatic
substrates. These results may indicate that the MnV]O inter-
mediate can efficiently transfer oxygen atom to electron-rich
Catalytic performance of Ca0.16MnO2$2H2O
In order to evaluate the catalytic activities of Ca0.16MnO2$2H2O
for oxidation reactions, the reactions were optimized according
to the oxidation of methyl phenyl sulde (MPS) through the
investigation of the inuence factors of oxidation, such as the
various solvents and oxidants, and the amount of catalyst and
oxidant.
In order to understand the effect of catalyst on the catalytic
reactivity, the rst, MPS oxidation reaction was carried out
without catalyst. Results indicated that in the absence of cata-
lyst, trace amount of product was generated in presence of H2O2
as an oxidant. Various oxidants including TBHP, H2O2, NaClO
and molecular oxygen were tested for the oxidation of MPS over
Ca0.16MnO2$2H2O at room temperature. Among these oxidants,
molecular oxygen and H2O2 were much more efficient for the
oxidation of MPS. The amount of H2O2 could also signicantly
affect the conversion and selectivity for MPS oxidation to sulf-
a
Table 2 Oxidation of sulfides catalyzed by Ca0.16MnO2$2H2O/H2O2
Entry Substrate
Conversionb (%) Selectivityc (%)
1
2
83
85
95
91
oxide (selectivity: sulfoxide/(sulfoxide
+
sulfone)
ꢂ
100)
(Table 1). When the amount of H2O2 was increased from 0.5 to
1.5 mmol, the conversion of MPS increased drastically from 47
to 85%. With a further increasing of H2O2 to 2.0 mmol, the
selectivity for MPS oxidation to sulfoxide decreases from 92 to
70%. The conversion of MPS monotonously increases with the
addition of catalyst from 5 to 10 mg. When the amount of
catalyst is increased to 20 mg, the selectivity for MPS oxidation
to sulfoxide reduces from 95 to 70%. Hence, the amount of
catalyst can enhance the reaction rate for the selectivity for MPS
oxidation to sulfoxide. In the next step, a series of reactions was
investigated by performing the model reaction in different
solvents such as dichloromethane, methanol, acetonitrile,
benzene, acetone and n-hexane. Among the solvents examined,
acetonitrile was found to be the best for this protocol (Table 1).
3
4
99
91
89
100
5
68
98
6
7
8
CH3CH2–S–CH2CH3
CH3(CH2)2–S–(CH2)2CH3
CH3(CH2)7–S–(CH2)7CH3
51
55
52
100
100
100
a
The molar ratios for substrate : oxidant are 1 : 3. The reactions were
performed in CH3CN (1 mL) under air at room temperature within 2
b
h. The GC yield (%) are measured relative to the starting sulde.
c
Selectivity to sulfoxide ¼ (sulfoxide/(sulfoxide + sulfone)) ꢂ 100.
Table 1 The effect of various conditions in the oxidation of MPS by Ca0.16MnO2$2H2O
Entry
Catalyst amount (mg)
Oxidant
Oxidant amount (mmol)
Solvent
Conversiona (%)
Selectivityb (%)
1
2
3
4
5
6
7
8
—
20
20
20
20
20
20
20
15
10
5
10
10
10
10
10
H2O2
H2O2
TBHP
NaClO
O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
0.5
0.5
0.5
0.5
Bubbled
1.0
1.5
2.0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3OH
CH2Cl2
C6H6
Trace
47
45
28
97
63
85
88
83
83
62
75
21
29
33
8
—
100
94
100
41
99
92
70
93
95
99
96
100
100
100
100
9
10
11
12
13
14
15
16
CH3COCH3
n-Hexane
a
The GC conversion (%) are measured relative to the starting substrate. b Selectivity to sulfoxide ¼ (sulfoxide/(sulfoxide + sulfone)) ꢂ 100.
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ꢄ Both Mn(III)/Mn(IV) ions are stable in the structure that the
factor helps to conversion of Mn(III) and Mn(IV) in the catalytic
cycle.15
Conclusions
In conclusions, we introduced layered Mn–Ca oxide as efficient
catalyst with high selectivity for MPS oxidation to sulfoxide in
the presence of H2O2. The catalyst is stable and could be recy-
cled ve times with no signicant loss in activity.
Fig. 3 Recycling studies of the catalyst Ca0.16MnO2$2H2O in the
oxidation of MPS. Condition is similar to Table 2.
Acknowledgements
This article is dedicated to the memory of Dr Warwick Hillier who
died on January 10, 2014. The authors are grateful to Institute for
Advanced Studies in Basic Sciences, the National Elite Founda-
tion and University of Maragheh for nancial support.
suldes than electron-poor one. The highest and the lowest
conversions were obtained for dibenzyl sulde (100%) and
diethyl sulde (51%), respectively (Table 2, entries 4 and 6).
The recovery and recyclability of Ca0.16MnO2$2H2O have
been also examined for the oxidation of MPS at room temper-
ature for 2 h and the results have been shown in Fig. 3. The
catalyst was separated aer each run by ltration, washed 2–3
times with water and diethyl ether repeatedly, and then dried at
50 ꢀC for half an hour. The catalyst could be recycled ve times
with no signicant loss in activity. During reusability studies
h run provided ꢃ79% conversion and 95% selectivity.
In order to investigate catalyst leaching in the oxidation of
MPS as a model reaction, the reaction was stopped at half the
reaction time (1 h) and the solid catalyst was completely sepa-
rated from solution. The rest of the reaction mixture (without
catalyst) was allowed to stir for another period of half the
reaction time. As seen in Fig. 4, no amount of MPS was
produced aer catalyst separation. Aer separation of the
catalyst, the reaction mixture was analyzed by AAS and no Mn
ions were detected. These results show that Ca0.16MnO2$2H2O
is truly heterogeneous and catalyst leaching is negligible under
these conditions.
Notes and references
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The probable factors of Mn–Ca oxide for activity toward
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ꢄ The catalyst has a layered structure with considerable
thermodynamic stability.15
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active sites regarding layered structure. In other words, in
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Fig. 4 Leaching experiment; the blue line shows the reaction without
Ca0.16MnO2$2H2O and the red line shows the reaction with
Ca0.16MnO2$2H2O.
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