Efficient and highly selective aqueous oxidation of sulfides to sulfoxides at room temperature catalysed by supported iron oxide nanoparticles on SBA-15

Article abstract of DOI:10.1002/adsc.201100149

The present manuscript describes a simple, efficient and environmentally friendly room temperature aqueous catalytic approach to the selective preparation of sulfoxides from sulfides utilising low-loaded supported iron oxide nanoparticles and aqueous hydr

Full text of DOI:10.1002/adsc.201100149

FULL PAPERS
DOI: 10.1002/adsc.201100149
Efficient and Highly Selective Aqueous Oxidation of Sulfides to
Sulfoxides at Room Temperature Catalysed by Supported Iron
Oxide Nanoparticles on SBA-15
a,
a
b
c,
Fatemeh Rajabi, * Sareh Naserian, Ana Primo, and Rafael Luque *
a
Department of Science, Payame Noor University, PO Box: 19395-4697 Tehran, Iran
Fax : (+98)-281-334-4081; phone: (+98)-281-333-6366; e-mail: f_rajabi@pnu.ac.ir
Instituto de Tecnologia Quimica UPV-CSIC- Universidad Politecnica de Valencia, Av. de los Naranjos s/n, 46022
b
Valencia, Spain
c
Departamento de Quꢀmica Orgꢁnica, Universidad de Cꢂrdoba, Campus de Rabanales, Edificio Marie Curie (C-3), Ctra
Nnal Iva, KM 396, 14014 Cordoba, Spain
Fax : (+34)-95-721-2066; e-mail: q62alsor@uco.es
Received: March 3, 2011; Revised: April 15, 2011; Published online: July 22, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100149.
Abstract: The present manuscript describes a simple, metal leaching was observed during the reaction.
efficient and environmentally friendly room temper- This is thus the first selective aqueous oxidation of
ature aqueous catalytic approach to the selective sulfides using an iron-based heterogeneous system.
preparation of sulfoxides from sulfides utilising low-
loaded supported iron oxide nanoparticles and aque-
ous hydrogen peroxide as oxidant. Materials could Keywords: aqueous chemistry; heterogeneous cataly-
be easily recovered from the reaction mixture and sis; room temperature reactions; SBA-15; sulfide ox-
reused ten times without any loss in activity and no idation; supported iron oxide nanoparticles
[
10,11]
Introduction
ries
as well as intermediates playing key roles in
[12,13]
the activation of enzymes.
Supported metal catalysts have received a great deal
Several catalytic systems have been reported to be
[
1,2]
of attention in recent years
and indeed have been active in the oxidation of sulfides. These range from
[
14–20]
[21–23]
proved highly successful and versatile in several com- metal-containing
and metal-free protocols
to
[
3]
[15]
mercial scale processes. Mesoporous silica type ma- biocatalysts using various oxidants including H O2
2
terials, including SBA-15 and MCM-41, have been and O2.
some of the most common supports which emerged as
Nevertheless, despite considerable progress in the
tuneable, high surface and stable alternative materials field of sulfide oxidation, the selective synthesis of
to be decorated with a wide range of nanoparticles sulfoxides is still a major challenge as most protocols
from metals (e.g., Au, Pd, Pt) to metal oxides (e.g., involve overoxidation of sulfoxides to sulfones. Some
[1,4–6]
CuO, Fe O ).
These materials have largely found others have also inherent drawbacks including the
2
3
applications in heterogeneous catalysis ranging from need of a catalysts excess (typically homogeneous
[
1,4–7]
[16,18,24]
redox (hydrogenations and oxidations) processes
metal complexes
) and/or reagents, the use of
[
1,7a,8]
to CÀC and CÀheteroatom couplings
and acid- toxic solvents as well as lack of good activities and/or
selectivities (good yields are only obtained after long
The selective transformation of sulfides to sulfox- reaction times and harsh reaction conditions).
ides generates high-added value compounds as start- Furthermore, heterogeneous systems reported for this
[
9]
catalysed processes.
[22,25,26]
[
27,28]
ing materials and intermediates (which can subse- process either involve complex systems
or are
quently transformed into valuable pharmaceuticals, unable to efficiently work under aqueous condi-
[
17,23,29–31]
agrochemicals and related compounds), granting tions.
access to industrially and biologically important syn- The development of a simple and efficient hetero-
thetic organic compounds including chiral auxilia- geneous catalytic methodology that can efficiently
2060
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Adv. Synth. Catal. 2011, 353, 2060 – 2066

Efficient and Highly Selective Aqueous Oxidation of Sulfides to Sulfoxides at Room Temperature
promote a mild aqueous oxidation of sulfides to sulf-
oxides (either at room temperature or under micro-
wave irradiation with short reaction times and low
temperatures) with a green oxidant (e.g., hydrogen
peroxide or molecular oxygen) without any contami-
nation by sulfones will be a major achievement in oxi-
dation chemistry. Another important point to be con-
sidered is the use of transition metals and particularly
the widely available, cheap and non-toxic iron-based
heterogeneous catalysts, which have not been report-
ed to date in the oxidation of sulfides.
In our continuing recent endeavours devoted to the
preparation of environmentally friendly and reusable
catalysts (e.g., supported nanoparticles in porous ma-
terials) in view of their applications in heterogeneous
catalysis, we report here the use of highly efficient
and active Fe ACHTUNGTRENNUNG( III) oxide nanoparticles supported on
Figure 1. XRD pattern of Fe/SBA-15 material.
SBA-15 in the chemoselective oxidation of sulfides to
sulfoxides at room temperature using H O as a green
2
2
oxidant. This catalyst has been proved to be highly
versatile, active and reusable under the investigated
reaction conditions.
Results and Discussion
The search for an effective simple and efficient cata-
lytic system suitable for the synthesis of sulfoxides
from sulfides prompted us to design alternative heter-
ogeneous catalysts based on iron-supported nanopar-
ticles on silica-type materials. These catalysts were
previously reported to have excellent activities in a
[
32]
range of oxidations including alcohols and alkenes
as well as acid-catalysed processes (e.g., alkyla-
[9]
tions). Therefore, a similar system was devised and
synthesised for the selective aqueous oxidation of sul-
fides to sulfoxides.
The material, characterised by a series of tech-
niques including XRD, SEM, TEM and XPS, was
found to be mostly composed of supported Fe O
2
3
nanoparticles (hematite phase) on the SBA-15 materi-
al, which exhibited a well ordered hexagonal structure
[
33]
typical of SBA-15 materials (Figure 1). Metal oxide
nanoparticle sizes were ca. 5–7 nm, with an excellent
homogeneous dispersion of the Fe oxide NP on the
support (Figure 2). Fe content was around 0.5–
0
.7 wt%, similar to that reported for related materials
[9,32]
from our group.
Fe species were mostly hematite
(
Fe O ) as concluded from wide angle XRD (not
2 3
shown) and XPS peaks at B.E. 712 and 724.1 eV, typi-
[
34,35]
cal of iron ACHTUNGTRENNUNG( III) oxide.
A preliminary screening of the Fe/SBA-15 material
in the model oxidation reaction of methyl phenyl sul-
1
2
fide (Scheme 1, R =Ph; R =Me) under different
conditions and solvents has been included in Table 1.
Figure 2. TEM micrograph of Fe/SBA-15 (top). Image of the
Reactions were initially performed at room tempera- material at a higher magnification (bottom) in which several
ture (r.t.) using 30% aqueous H O as green oxidant. iron oxide NPs of ca. 6–7 nm can be clearly observed.
2
2
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Fatemeh Rajabi et al.
FULL PAPERS
room temperature) afforded a quantitative isolated
yield of methylphenyl sulfoxide.
The heterogeneous nature of the proposed method-
ology was subsequently investigated. With this pur-
pose, the catalyst was filtered off after different times
(
typically half way through the reaction) and the fil-
Scheme 1. Selective aqueous oxidation of sulfides to sulfox-
ides at room temperature.
trate and reused catalyst were separately allowed to
react further.
The results included in Table 2 clearly point to het-
erogeneous catalysis taking place in the protocol as
the filtration liquor reacted significantly more slowly
Table 1. Screening of solvents and reaction conditions in the
oxidation of methylphenyl sulfide.
[a]
(
at a similar rate to the reaction performed in the ab-
Entry
Solvent
Cat. (mol%)
t [h]
Yield [%] sence of the catalyst) as compared to the reused ma-
terial at different times (Table 2, entries 1 and 2).
AAS analysis of the final reaction mixture (upon
blank
SBA-15
H O
10
10
2.5
2
5
5
5
2
2
2
<10
99
2
H O
2
completion after the end of the first cycle) confirmed
that almost no iron could be detected in the solution
phase (<2 ppm Fe).
For practical applications of heterogeneous systems,
the lifetime of the catalyst and its recovery and exten-
sion of reusability are very important factors. We
therefore devised a set of experiments to recover and
reutilise the Fe/SBA-15 catalyst in the model oxida-
tion process. All reactions were carried out under sim-
1
2
3
4
5
6
7
8
9
EtOH
–
99
45
30
58
99
99
99
99
<10
MeOH
toluene
n-hexane
10
10
10
10
10
10
10
5
2
CH Cl2
H O
2
H O
2
H O
2
H O
2
2
[
a]
Reaction conditions: 1 mmol methylphenyl sulfide, ilar conditions.
0
.5 mmol aqueous H O (30% v/v), 1 mL solvent, Fe/
To our delight, the supported catalyst was highly re-
usable under the investigated reaction conditions, pre-
serving almost unaltered its initial catalytic activity
2
2
SBA-15 as catalyst, room temperature.
Blank runs (in the absence of catalyst) gave no yields after ten uses (Table 3). No significant quantity of Fe
of products even after 10 h reaction times (Table 1, (<3 ppm) was found in solution as determined by
entry 1). In comparison, quantitative yields to methyl ICP, in good agreement with AAS results of the fil-
phenyl sulfoxide could be obtained in different sol- trate previously obtained in this work.
vents (EtOH, MeOH and H O) after 2–2.5 h reaction
2
times.
The efficiency of the supported catalyst was primar-
ily found to be affected by the nature of the solvent Table 2. Leaching studies of the Fe/SBA-15 catalyst in the
[a]
and the quantity of the catalyst used in the reaction. oxidation of methylphenyl sulfide.
Polar solvents (alcohols, water) were found to be par-
ticularly suitable for the oxidation reaction as com-
Entry Reaction
Time
Yield
[
min] [%]
pared to organic solvents, which is probably related to
the better mixing with the aqueous hydrogen peroxide
utilised as oxidant in the process (organic solvents
will, in contrast, lead to a clear biphasic system). The
possibility to use water as reaction medium is a re-
markable advantage of the proposed methodology
which greatly increases its green credentials. Aqueous
phase reactions were indeed proved to provide the
best yields in the reaction, despite the very low solu-
bility of organic sulfides in water.
1
Fe/SBA-15 (normal reaction)
30
40
35
65
Fe/SBA-15 (recovered after 30 min, 30
further reacted with added fresh sub- 60
strates)
Filtrate (upon filtration after 30 min 30+30 42
reaction, further reacted with added
fresh substrates)
2
Fe/SBA-15 (normal reaction)
Fe/SBA-15 (recovered catalyst after 30
90
78
32
70
9
0 min, further reacted with added
90
Overoxidation is often observed in the oxidation of
sulfides. Interestingly, no overoxidation to the corre-
sponding sulfone was observed under the investigated
conditions. This is believed to be due to the mild con-
ditions utilised in our proposed methodology (room
temoperature, 2–5 h).
fresh substrates)
120
>98
Filtrate (upon filtration after 90 min 90+90 79
reaction, further reacted with added
fresh substrates
[a]
The catalyst was typically filtered off from the reacting
mixture at different times (Fe/SBA-15, normal reaction)
and then the filtrate and the recovered catalyst were sep-
arately left to react after the addition of fresh substrates.
Optimised conditions for the model reaction
(
1 mol% catalyst, 2 h reaction, aqueous medium,
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Adv. Synth. Catal. 2011, 353, 2060 – 2066

Efficient and Highly Selective Aqueous Oxidation of Sulfides to Sulfoxides at Room Temperature
Table 3. Reuses of Fe/SBA-15 in the selective oxidation of generally obtained for Fe/SBA-15 after 2–4 h reaction
[
a]
methylphenyl sulfide.
at room temperature, with the exception of moderate
to good yields (ca. 58–75%) observed for the particu-
lar case of aliphatic sulfides (Table 4, entries 13–15).
Activities in the systems were particularly reduced
Run no.
1
2
3
4
5
6
7
8
Yield [%]
99
99
98
99
99
99
97
95
Time [min] 120 120 120 120 120 150 150 180 when electron-withdrawing groups (EWG) were di-
[
a]
rectly bonded to the sulfur atom (Table 4, entries 9
and 10). The oxidation of other aryl methyl sulfides
also proceeded with high yields (Table 4, entries 5 and
Reaction conditions: 2.5 mmol sulfide, 0.5 mmol H O ,
2
2
1
mol% catalyst, 1 mL water, room temperature.
6). However, bulky sulfides including benzyl tolyl sul-
Having proved the activity and stability of the sup- fide were more slowly oxidized (Table 4, entry 11)
ported iron oxide nanoparticles in the model reaction, and needed longer reaction times (360 min and
the scope of the protocol was subsequently extended above) to reach quantitative conversion to the sulfox-
to a range of the sulfides as shown in Table 4. Excel- ide product under the optimised reaction conditions.
lent conversions and selectivities to sulfoxides were Sulfoxides were selectively obtained in all cases (>
9
9%) in the most selective and mild protocol to date
reported for the oxidation of sulfides. Furthermore,
the efficiency of H O was found to be remarkably
Table 4. Selective oxidation of a range of sulfides using Fe/
[a]
SBA-15.
2
2
high for the reported oxidation in most cases, reach-
ing values >90% for the majority of the investigated
substrates (e.g., >95% for methyl phenyl sulfide).
Only for the more challenging compounds (e.g.,
linear sulfides and thiophene), was the H O efficien-
2
2
cy reduced (50–70%).
TOF values for the different reactions and sub-
strates were in the 70 (for methyl phenyl sulfide as
À1
substrate) to 8 h range (for thiophene as substrate)
Most substrates had TOF values ranging from 70 to
À1
3
5 h (Table 4, entries 1 to 12). As comparison, some
of the most relevant systems reported to date have
been included in Table 5. These results clearly point
out the efficiency of the proposed methodology
(
Table 5, last entry) in both activity, selectivity and re-
cyclability of the Fe/SBA-15 catalyst as compared to
literature reports involving several homogeneous and
heterogeneous systems under various conditions
[
16–18,23,24,27–31]
(
Table 5, entries 1–10).
Conclusions
A supported iron oxide nanoparticles SBA-15 catalyst
was found to be an efficient and easily recoverable
catalyst in the aqueous selective oxidation of sulfides
to sulfoxides using hydrogen peroxide. The protocol
featuring an easy work-up, simplicity and mild reac-
tion conditions as well as high selectivity toward sulf-
oxides is highly advantageous compared to alternative
reported methodologies. We envisage the extension of
this highly efficient system to related aqueous phase
transformations including oxidations of biomass-de-
rived platform molecules (e.g., sugars and polyols) to
high-added value chemicals, studies in this direction
are currently underway in our laboratories.
[
a]
Reaction conditions: 1 mmol sulfide, 1 mol% catalyst,
.5 mmol H O , 1 mL water, room temperature.
0
2
2
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Fatemeh Rajabi et al.
FULL PAPERS
Table 5. Comparison of literature reports on the selective oxidation of methyl phenyl sulfide under various conditions.
Ref. Sulfide
Catalyst
H O
Solvent
Temp. Time
Yield [%]
Uses
2
2
(
equiv.)
[8C]
Homogeneous catalysts non-reusable
sulfoxide sulfone
Na WO , C H PO H ,
and PTC
[
[
[
16]
18]
24]
2
4
6
5
3
2
1
.1
–
0
9 h
93
98
56
7
–
–
–
–
–
VO(acac) /ligand
A
H
U
G
R
N
U
G
1.1
1
CH Cl
r.t.
r.t.
24 h
5 h
2
2
2
1
% DMF in
Mn-salen complexes
water
Heterogeneous catalysts in organic solvents
4
reaction
cycles with-
out loss in
activity
[
[
[
[
[
30]
17]
23]
29]
31]
Si-V10-2 gel
1
MeOH
r.t.
20 min 94
90 min 82
120 min 57
60 min 97
60 min >99
99
-
8
reaction
silica-based tungstate
interphase
cycles with-
out loss in
activity
3
CH Cl :MeOH r.t.
2
2
2
2
NaBO ·4H O,
3 2
Na CO ·3H O silica,
2
3
2
2,
–
3
CH Cl
r.t.
r.t.
40
–
–
2
2
sulfuric acid, KBr, wet
SiO (50% w/w)
2
6
reaction
0%
cycles with-
out loss in
activity
HAuCl ·4H O
CH OH
3
4
2
3
H O
2
2
none (when 4 reaction
adding only cycles with-
MCM-Mo
1.5
CH CN
3
1
equiv of
out loss in
activity
oxidant)
Heterogeneous catalysts in aqueous solvents
5
reaction
cycles with-
out activity
loss
[
[
27]
28]
VO
A
H
U
G
E
N
N
(acac) -resin
1.1
H O
r.t.
r.t.
r.t.l
15 min >99
30 min 88
–
2
2
6
reaction
tungstate-exchanged
Mg-Al-LDH catalyst
cycles with-
out activity
loss
1
H O
2
12
–
1
0 reaction
This
work
cycles with-
out activity
loss
Fe/SBA-15
0.5
H O
2
120 min >99
Experimental Section
slowly added into the solution. The molar composition of
the mixture was 0.9 TEOS:0.1 APTES:6.1 HCl:0.017
Preparation of SBA-15-NH2
P123:165 H O. The resulting mixture was stirred at 408C for
2
Aminopropyl-functionalised SBA-15 materials (denoted as
20 h and then aged at 908C under static condition for 24 h.
The solid product was recovered by filtration and dried at
room temperature overnight. The template was removed
from the material by refluxing in excess ethanol for 24 h. Fi-
nally, the material was filtered, washed several times with
water and ethanol, and dried at 508C.
SBA-NH ) were synthesised according to the procedure de-
2
[36]
scribed by Wang et al. Pluronic 123 (4 g) was dissolved in
25 g of 2.0M HCl solution at room temperature. After
1
TEOS had been added, the resultant solution was equili-
brated at 408C for pre-hydrolysis, and then APTES was
2064
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ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2060 – 2066

Efficient and Highly Selective Aqueous Oxidation of Sulfides to Sulfoxides at Room Temperature
Preparation of Supported Iron Oxide Nanoparticles
on SBA-15 Supports
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Adv. Synth. Catal. 2011, 353, 2060 – 2066
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ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2060 – 2066