Highly efficient selective oxidation of sulfides to sulfoxides by montmorillonite-immobilized metalloporphyrins in the presence of molecular oxygen

Article abstract of DOI:10.1016/j.catcom.2014.04.021

Highly efficient and selective oxidation of sulfides to sulfoxides with dioxygen catalyzed by cationic meso-tetrakis (1-methyl-4-pyridyl) (TM4PyP) metalloporphyrins immobilized into montmorillonite (MT) interlayer was achieved. Manganese (II) porphyrin (MnTM4PyP-MT) presented excellent activity for the oxidation of sulfides under ambient conditions. In the model oxidation, thioanisole was converted completely and the selectivity towards sulfoxide was up to 95%. This catalyst also showed high activity and selectivity for the most sulfides. The catalyst could be reused consecutively five times without significant loss of activity.

Full text of DOI:10.1016/j.catcom.2014.04.021

Catalysis Communications 53 (2014) 29–32
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Highly efficient selective oxidation of sulfides to sulfoxides by
montmorillonite-immobilized metalloporphyrins in the presence of
molecular oxygen
Xian-Tai Zhou, Hong-Bing Ji ⁎
School of Chemistry and Chemical Engineering, Sun Yat-sen University, 510275 Guangzhou, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly efficient and selective oxidation of sulfides to sulfoxides with dioxygen catalyzed by cationic meso-tetrakis
Received 26 November 2013
Received in revised form 18 April 2014
Accepted 28 April 2014
(
1-methyl-4-pyridyl) (TM4PyP) metalloporphyrins immobilized into montmorillonite (MT) interlayer was
achieved. Manganese (II) porphyrin (MnTM4PyP-MT) presented excellent activity for the oxidation of sulfides
under ambient conditions. In the model oxidation, thioanisole was converted completely and the selectivity
towards sulfoxide was up to 95%. This catalyst also showed high activity and selectivity for the most sulfides.
The catalyst could be reused consecutively five times without significant loss of activity.
Available online 5 May 2014
Keywords:
©
2014 Elsevier B.V. All rights reserved.
Metalloporphyrins
Montmorillonite
Sulfide
Sulfoxide
1
. Introduction
Due to the large specific surface area, chemical and mechanical stabil-
ity, layered structure and high cation exchange capacity, montmorillon-
ite is preferable to be used as the support for preparing heterogeneous
metalloporphyrins [18]. Although the montmorillonite immobilized
metalloporphyrins have been applied in the oxidation of cyclohexene
and 2-mercaptoethanol [19–21], the selective oxidation of sulfide cata-
lyzed by montmorillonite-immobilized metalloporphyrins with
dioxygen is still unknown. In continuation of our ongoing research on
the metalloporphyrin-based oxidation of sulfides [22,23], we report
herein an efficient and highly selective oxidation of sulfides to sulfoxides
catalyzed by montmorillonite-supported metalloporphyrins catalysts
under ambient conditions. The heterogeneous catalyst showed excellent
activity and could be reused consecutively five times without significant
loss of activity.
The selective oxidation of organic sulfides to sulfoxides without any
over-oxidation to sulfones is a challenging research interest in synthetic
organic chemistry, partly because of the importance of sulfoxides as in-
termediates in biologically active compounds [1–3]. Among all methods
described so far, the oxidation of sulfides by various metal catalysts is
one of the most attractive routes [4–6]. Metalloporphyrins have been
used as cytochrome P-450 models and have been found to be highly
efficient homogeneous catalyst for sulfide oxidation in the presence of
various oxidants e.g. NaOCl, oxone and hydrogen peroxide [5–12]. How-
ever, decomposing easily and difficulty of recovery is the main weak-
ness, which limit their practical applications in both synthetic
chemistry and industrial processes.
Great efforts have been made to immobilize metalloporphyrins onto
supports to promote stability in the catalytic oxidations [13–15].
Polystyrene-bound manganese (III) porphyrins were applied in the se-
lective oxidation of sulfides by using sodium periodate as oxidant [16].
Recently, silica supported manganese (III) porphyrins were prepared
and used as efficient heterogeneous catalysts for biomimetic oxidation
of sulfides with sodium periodate [17]. From economic and environ-
mental viewpoints, immobilizing metalloporphyrins onto one kind of
expensive, stable support is gathering more interest.
2. Experimental
2.1. Materials and instruments
Sulfides were of analytical grade and purchased from Alfa Aesar with-
out further purification unless indicated. Pyrrole and isobutyraldehyde
were redistilled before use. Other solvents were all of analytical grade.
Montmorillonite-supported metalloporphyrin catalysts were prepared
as previously reported, and then characterized by diffuse reflectance
UV spectra, infrared, X-ray diffraction, nitrogen adsorption isotherm
and scanning electron microscopy [19].
⁎
Corresponding author. Tel.: +86 20 84113658; fax: +86 20 84113654.
E-mail address: jihb@mail.sysu.edu.cn (H.-B. Ji).
http://dx.doi.org/10.1016/j.catcom.2014.04.021
566-7367/© 2014 Elsevier B.V. All rights reserved.
1

30
X.-T. Zhou, H.-B. Ji / Catalysis Communications 53 (2014) 29–32
2
.2. Aerobic oxidation of sulfide
high valent metal-oxo intermediate is more easily reduced to initial va-
lence. In addition, the BET surface area of MnTM4PyP, CoTM4PyP and
FeTM4PyP was 83 m /g, 67 m /g and 62 m /g, respectively. Therefore,
the catalytic performance for this heterogeneous catalyst is positive
related with surface area.
2
2
2
A 20 mL Schlenk flask was charged with sulfide (1 mmol), catalyst
(
0.015 g), toluene (4 mL), isobutyraldehyde (3 mmol), 0.2 mmol naph-
thalene (inert internal standard) and then the mixture was stirred at
0 °C with a O balloon (1 atm). The product was analyzed by GC
8
2
(
Shimadzu GC-2010 plus) equipped with a flame ionization detector
(
FID) (conditions of GC: Rtx-5 capillary column, 30 m × 0.25 mm ×
3.2. Effect of catalyst amount on the aerobic oxidation of thioanisole
0
.25 μm, over temperature of 110 °C) and GC–MS (Shimadzu GCMS-
QP2010 plus) equipped with Rxi-5 ms capillary column (30 m ×
The influence of MnTM4PyP-MT catalyst amount on the aerobic ox-
idation of thioanisole in the presence of molecular oxygen and
isobutyraldehyde was summarized in Fig. 2.
0.25 mm × 0.25 μm).
For the recycling experiments, the catalyst was recovered by centri-
fugation and first washed with water (5 mL) for 5 times, and then with
acetonitrile (10 mL) to remove the occluded reactants and products.
After washing, the acetonitrile solution was checked for the existence
of reactants/products using gas chromatography. The recycled catalysts
were then dried in vacuum at 80 °C for 6 h and re-used using the same
experimental conditions as described above. In recycling experiments,
the loss of catalyst was compensated in the next catalytic oxidation.
As shown in Fig. 2, only 4% thioanisole was converted in blank exper-
iment as the reaction was conducted for 45 min with 87% selectivity to-
wards sulfone. The conversion of thioanisole was considerably
enhanced when the heterogeneous manganese porphyrin catalyst was
used. The MnTM4PyP-MT catalyst crucial in the reaction could be dem-
onstrated from the fact that the conversion of thioanisole was up to 40%
−
3
even if the amount of catalyst was only 0.005 g (0.23 × 10
mmol). As
depicted clearly in Fig. 2, the reaction rate increased with the increasing
catalyst contents. Thioanisole could be converted completely when the
3
. Results and discussion
−
3
amount of catalyst was 0.015 g (MnTM4PyP: 0.7 × 10
mmol), in
3
.1. Catalysis of the various supported metalloporphyrins
which the selectivity of sulfoxide was up to 95%. However, as the
amount of catalyst was increased continually, the selectivity towards
sulfoxide declined apparently. It indicated that the excess catalyst
promote the over-oxidation of sulfoxide.
With thioanisole as model compound, the effects of various
montmorillonite-supported metalloporphyrin catalysts on the oxida-
tion in the presence of dioxygen and isobutyraldehyde have been inves-
tigated. The loaded amounts of the MnTM4PyP, CoTM4PyP and
FeTM4PyP [meso-tetrakis (1-methyl-4-pyridyl) (TM4PyP) manganese,
cobalt and iron porphyrins] was 4.7 × 10
and 3.5 × 10
CoTM4PyP and FeTM4PyP was 15 mg, 18 mg and 20 mg based on the
same mole of each catalyst, 0.7 × 10−3 mmol. The profiles for the yields
of sulfoxide and sulfone in the oxidation of thioanisole catalyzed by dif-
ferent metalloporphyrins with molecular oxygen were presented in
Fig. 1.
To illustrate whether the selectivity of sulfoxides is influenced by the
increasing amount of catalyst, as a comparison, we carried out the ho-
mogeneous oxidation of sulfides with the same mole of metal center.
It was also observed that the larger amount of catalyst was unfavorable
the selectivity towards sulfoxides (selectivity of sulfoxide was 79% with
−
5
−5
mol/g, 3.8 × 10
mol/g
−
5
mol/g, respectively. Hence, the weight of MnTM4PyP,
−
3
MnTM4PyP 0.94 × 10
mmol, while 99% selectivity of sulfoxide was
−3
obtained with 0.7 × 10 mmol MnTM4PyP catalyst). The increasing
amount of catalyst has little influence on the conversion of sulfides
(Fig. 2). Therefore, the effect of mass transport limitation on the selec-
tivity towards sulfoxides could be excluded.
As shown in Fig. 1, the catalytic activity of the metalloporphyrins is
dependent on the nature of their central ions. The results show that
montmorillonite-supported manganese porphyrins are considerably
more selective than cobalt and iron porphyrin catalysts for the aerobic
oxidation. The catalytic activity of different metalloporphyrins is
probably influenced by their electric potential and the stability of differ-
ent valences of metal atoms. During the catalytic oxidation reaction, the
central metal ion of metalloporphyrin undergoes a transition from high-
spin states to low-spin states and finishes electron-transfer by the
change of its own valences. For catalyst with higher redox potential,
3.3. Effect of temperature on the aerobic oxidation of thioanisole
The effect of reaction temperature on the conversion of thioanisole
were studied, the results were presented in Fig. 3. The conversion of
thioanisole increased greatly with the rising temperature from 50 °C
to 80 °C, then kept unchanged at temperature over 80 °C. When the ox-
idation was conducted under 80 °C, an excellent selectivity towards
sulfoxide was obtained. The selectivity towards sulfoxide decreased
1
00
1
00
100
80
1
00
MnTM4PyP-MT
CoTM4PyP-MT
FeTM4PyP-MT
80
80
60
40
20
0
8
6
4
2
0
0
0
0
0
60
6
4
2
0
0
0
0
Conv. of sulfide
Select. of sulfoxide
Select. of sulfone
40
20
0
0
10
20
30
40
50
60
0
5
10
15
20
Time/min
Amount of catalyst/mg
Fig. 1. Profiles of the conversion and yield of sulfoxide for the thioanisole catalyzed by various
Fig. 2. Effect of the amount of catalyst on the selective aerobic oxidation of thioanisole
montmorillonite-supported metalloporphyrins, thioanisole (1 mmol), isobutylaldehyde
catalyzed by MnTM4PyP-MT, thioanisole (1 mmol), isobutylaldehyde (3 mmol), toluene
(4 mL), O balloon (1 atm), 80 °C, 45 min.
2
3 mmol), toluene (4 mL), catalyst (0.7 × 10− mmol), O
3
balloon (1 atm), 80 °C.
(
2

X.-T. Zhou, H.-B. Ji / Catalysis Communications 53 (2014) 29–32
31
100
100
the conversion was 82% and 89% after much longer reaction time
entries 6–7), respectively. Comparing with thioanisole, methyl benzyl
(
8
6
4
2
0
0
0
0
0
80
60
40
20
0
sulfides could be converted under the same conditions with slightly lon-
ger reaction time (entry 8). Sulfoxidation of the linear chain di-n-butyl
sulfide smoothly proceeded with high conversion and selectivity
(
entry 9). In addition, the catalytic system exhibited specific selective
oxidation performance towards the sulfide contains hydroxyl group,
and no products from hydroxyl group oxidation were detected (entry
10).
Large scale thioanisole oxidation experiment was carried out. To a
stirring solution of toluene (40 mL), thioanisole (10 mmol),
isobutyraldehyde (30 mmol) and MnTM4PyP-MT catalyst (0.15 g)
were added in the presence of molecular oxygen. After reaction, the
crude products were purified via column chromatographic (silica gel,
eluting agent: Vacetic ester/Vpetroleum ether = 1/3) to obtain pure sulfoxide
(1.27 g, isolated yield 91%).
Conv. of thioanisole
Select. of sulfoxide
Select. of sulfone
50
60
70
80
90
o
Temperature/ C
Fig. 3. Effect of the temperature on the selective aerobic oxidation of thioanisole catalyzed
by MnTM4PyP-MT, thioanisole (1 mmol), MnTM4PyP-MT (0.7
−
3
×
10
mmol),
isobutylaldehyde (3 mmol), toluene (4 mL), O balloon (1 atm), 45 min.
2
3.5. Catalyst reuse and stability
while the temperature was over 80 °C. Evidently, sulfoxide could be
further oxidized to sulfone at higher temperature.
The stability of the MnTM4PyP-MT catalyst was monitored using
multiple sequential aerobic oxidation of thioanisole in the presence of
isobutyraldehyde. The catalyst was recovered by centrifugation, filtra-
tion, washed with acetonitrile and dried before using it in the subse-
quent run. The results are presented in Fig. 4.
3
.4. Aerobic oxidation of various sulfides catalyzed by MnTM4PyP-MT
catalyst
From Fig. 4, the catalyst was consecutively reused five times without
a significant loss of its activity. The leaching of the metalloporphyrins
from the support to the reaction medium was checked by taking
UV–vis spectra of the solution at the end of the reaction. No presence
of manganese porphyrins, characterized by the Soret band was ob-
served, indicating that the metalloporphyrins remain tightly bound to
the support during the oxidation reaction. The results indicated that
the supported catalyst was active in the oxidation and could be reused
without significant decrease in the catalytic activity and with almost
similar selectivity.
To evaluate the scope of the catalytic system, various sulfides were
subjected to the reaction system using only 0.015 g of montmorillonite-
supported manganese porphyrins (Table 1). As shown in Table 1, most
substrates could be smoothly selective converted to corresponding sulf-
oxides with high conversion rate and excellent selectivity.
It can be observed that the efficiency of the oxidation in this catalytic
system is related with the electronic property of substrates. Compared
with the electron-withdrawing groups at p-position of phenyl ring,
the electron-donating groups were more favorable to the conversion
of sulfides. It indicated that the electron-withdrawing effect retarded
the oxidation (entries 1–5). The influence of steric effects could be ob-
served. In the oxidation of diphenyl sulfide and isopropyl phenyl sulfide,
3.6. Plausible mechanism of MnTM4PyP-MT catalyzed oxidation of sulfide
As reported previously, the homogeneous oxidation catalyzed by
metalloporphyrin complexes plus aldehyde is considered to involve a
radical and high-valent metal intermediate mechanism. The feature of
radical-involved reaction could be informed from the facts that the
oxidation was subsequently quenched in the presence of a free radical
inhibitor. High-valent intermediate is generally accepted as the active
species for the oxidations catalyzed by metalloporphyrins [24–26].
The oxidation of sulfides was assumed to occur via reactive high-
valent metal oxo intermediates, which was generated from a series of
radical species in the presence of dioxygen and aldehyde.
Table 1
a
Selective aerobic oxidation of various sulfides catalyzed by MnTM4PyP-MT .
Entry
Substrate
Time (min)
Conv. (%)
Select. (%)
Sulfoxide
95
Sulfone
5
1
2
45
45
N99
N99
96
4
3
4
5
6
7
90
90
97
89
90
82
89
98
98
96
98
94
2
2
4
2
6
100
8
6
4
0
0
0
120
120
60
8
9
60
45
99
96
94
4
6
20
0
N99
0
1
2
3
4
5
6
10
60
90
89
11
Run
a
Sulfide (1 mmol), MnTM4PyP-MT (0.7 × 10−3 mmol), isobutylaldehyde (3 mmol),
balloon (1 atm), 80 °C.
Fig. 4. The stability and reuse of MnTM4PyP-MT catalyst in the aerobic oxidation of
thioanisole, ( ): conversion of thioanisole, ( ): selectivity towards epoxide.
toluene (4 mL), O
2

32
X.-T. Zhou, H.-B. Ji / Catalysis Communications 53 (2014) 29–32
Fig. 5. The plausible mechanism of sulfide oxidation catalyzed by MnTM4PyP-MT catalyst.
Therefore, in the oxidation of sulfides with dioxygen catalyzed by
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The authors thank the National Natural Science Foundation of China
(
nos. 21036009, 21176267 and 21376278) for providing financial sup-
port to this project.