Biomimetic oxidation of organosulfur compounds with hydrogen peroxide catalyzed by manganese porphyrins

Article abstract of DOI:10.1016/j.apcata.2012.06.044

A biomimetic and environmentally benign approach, with potential application in the oxidative desulfurization procedure for several organosulfur compounds (thioanisol, diphenylsulfide, benzothiophene, 2-methylbenzothiophene, 3-methylbenzothiophene, benzothiophene-2-methanol and dibenzothiophene), is presented. The current methodology involves manganese porphyrins as catalysts, which are well-known biomimetic models of cytochrome P450 enzymes, and hydrogen peroxide as the oxygen source. [Mn(TDCPP)Cl] and [Mn(TPFPP)Cl], the manganese porphyrin complexes used in this study, proved to be very efficient catalysts, affording high conversions of all the substrates tested into the corresponding sulfones. The conversion of benzothiophene reaches 99.9% in 90 min, whereas the conversion of dibenzothiophene attains 99.9% after 120 min of reaction, both for a catalyst/substrate molar ratio of 150. The substituted benzothiophenes give rise to similar results, being the best conversions obtained for a catalyst/substrate molar ratio of 150. The oxidation of a model fuel (solution of benzothiophene, 3-methylbenzothiophene, 2-methylbenzothiophene, and dibenzothiophene in hexane) was performed using hydrogen peroxide and [Mn(TDCPP)Cl] as catalyst, achieving total conversion into the corresponding sulfones.

Full text of DOI:10.1016/j.apcata.2012.06.044

Applied Catalysis A: General 439–440 (2012) 51–56
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Biomimetic oxidation of organosulfur compounds with hydrogen peroxide
catalyzed by manganese porphyrins
a
a,∗
a
b
c
a,∗∗
S.M.G. Pires , M.M.Q. Simões , I.C.M.S. Santos , S.L.H. Rebelo , M.M. Pereira , M.G.P.M.S. Neves
,
a
J.A.S. Cavaleiro
QOPNA, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
REQUIMTE, Chemistry and Biochemistry Department, Faculty of Sciences, University of Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 May 2012
Received in revised form 21 June 2012
Accepted 22 June 2012
Available online 3 July 2012
A biomimetic and environmentally benign approach, with potential application in the oxidative desulfu-
rization procedure for several organosulfur compounds (thioanisol, diphenylsulfide, benzothiophene,
2
-methylbenzothiophene, 3-methylbenzothiophene, benzothiophene-2-methanol and dibenzothio-
phene), is presented. The current methodology involves manganese porphyrins as catalysts, which are
well-known biomimetic models of cytochrome P450 enzymes, and hydrogen peroxide as the oxygen
source. [Mn(TDCPP)Cl] and [Mn(TPFPP)Cl], the manganese porphyrin complexes used in this study,
proved to be very efficient catalysts, affording high conversions of all the substrates tested into the corre-
sponding sulfones. The conversion of benzothiophene reaches 99.9% in 90 min, whereas the conversion
of dibenzothiophene attains 99.9% after 120 min of reaction, both for a catalyst/substrate molar ratio of
150. The substituted benzothiophenes give rise to similar results, being the best conversions obtained
for a catalyst/substrate molar ratio of 150. The oxidation of a model fuel (solution of benzothiophene,
Keywords:
Oxidative desulfurization
Organosulfur compounds
Manganese
Porphyrins
Hydrogen peroxide
3
-methylbenzothiophene, 2-methylbenzothiophene, and dibenzothiophene in hexane) was performed
using hydrogen peroxide and [Mn(TDCPP)Cl] as catalyst, achieving total conversion into the correspond-
ing sulfones.
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
substituents at the meso- and/or ␤-pyrrolic positions [4,5]. These
enhanced porphyrin complexes proved to be highly efficient and
much more resistant to oxidative conditions, affording excellent
conversions in several oxidation reactions of an enormous plethora
of substrates using several oxygen donors. Chloro[5,10,15,20-
tetrakis(2,6-dichlorophenyl)porphyrinato]manganese(III), abbrev-
iated as [Mn(TDCPP)Cl] or (I), and chloro [5,10,15,20-tetra-
kis(pentafluorophenyl)porphyrinato]manganese(III), [Mn(TPFPP)
Cl] or (II), are good examples of high efficient porphyrin based
catalysts, which have been tested in the oxidation of a range of
substrates [6–8].
Several oxygenase enzymes are known to selectively produce
sulfones, sulfoxides, and even chiral sulfoxides, under very mild
conditions [9,10]. Since metalloporphyrins can be considered as the
best models of cytochrome P450 monooxygenases, sulfoxidation
catalyzed by metalloporphyrins has been explored, almost exclu-
sively with sulfides as substrates [11–17].
The growing concern with problems associated to a sustain-
able development and the urgent need for clean technologies had
indubitably contributed to the development and dissemination
of catalytic processes, namely those related with oxidative trans-
formations. In this context, metalloporphyrins, perhaps the best
understood and well studied biomimetic or bio-inspired catalysts,
have emerged due to their ability to mimic the catalytic activity of
the cytochrome P450 enzymes in the presence of several oxygen
donors [1,2]. Since the first report by Groves et al. [3], concerning
the use of an iron complex of tetraphenylporphyrin (TPP) as catalyst
in the oxidation of alkanes and alkenes, huge improvements have
been observed. The amount of work meanwhile disclosed, regard-
ing mainly the use of manganese and iron porphyrin complexes,
led to the development of the so-called second and third genera-
tion of porphyrin based catalysts, containing electron-withdrawing
Nowadays, the negative impact induced by the presence of
sulfur-containing compounds (thiols, sulfides, disulfides, and thio-
phenes) in petroleum products is well established, both from
industrial and environmental reasons [18,19]. Firstly, they are
responsible for the poisoning of the catalysts and for the corro-
sion of parts of the internal combustion engines in petrochemical
∗
Corresponding author. Tel.: +351 234370713; fax: +351 234370084.
Corresponding author. Tel.: +351 234370710; fax: +351 234370084.
E-mail addresses: msimoes@ua.pt (M.M.Q. Simões),
∗
∗
gneves@ua.pt (M.G.P.M.S. Neves).
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.06.044

5
2
S.M.G. Pires et al. / Applied Catalysis A: General 439–440 (2012) 51–56
Scheme 1.
Scheme 2.
industries. Secondly, the SOx emissions from their combustion pol-
lute air, water, and soil, being harmful to health, besides promoting
acid rains [20,21]. Therefore, deep desulfurization of liquid fuels has
become a worldwide challenge [19,22]. In recent years, oxidative
desulfurization (ODS) has become a promising and emerging alter-
native to the conventional hydrodesulfurization (HDS) technology
used by most of the oil refineries all over the world [21,23,24].
Besides being arduous and very costly, HDS presents limited perfor-
mance and do not allow the achievement of ultralow sulfur levels
an easy, high efficient, green, and sustainable procedure to oxidize
sulfides (1–2), benzothiophenes (3–6), and dibenzothiophene (7).
Taking into account the amount of work performed in our lab-
oratory in the field of oxidative catalysis with metalloporphyrins
[
60–70], the methodology, as pointed out in Scheme 3, consists
on the use of CH CN as the reaction solvent, ammonium acetate
3
as the co-catalyst, and H O2 as the oxidant. This one is progres-
2
sively added to the reaction mixture, at each 15 min, in aliquots
corresponding to half-substrate amount. All the reactions were
performed at room temperature, in the absence of light and were
followed by GC–FID. The conversion values presented in Table 1
were determined based on the GC–FID results and chlorobenzene
was used as the internal standard.
[
25,26], mostly due to the presence of the so-called refractory S-
containing aromatic compounds, predominantly alkyl-substituted
benzothiophenes (BTs) and dibenzothiophenes (DBTs). In contrast,
ODS not only allows moderate process conditions, without employ-
ing hydrogen consumption, but it is also efficient in the treatment
of BTs and DBTs [20]. The oxidation of these thiophenic compounds
to the corresponding sulfoxides and sulfones, more polar products,
permits an easier separation from the fuel product by extraction,
distillation, decomposition, or adsorption on activated carbon, alu-
mina or silica [20,27–31]. Furthermore, sulfoxides and sulfones
have received much attention since they are known as interme-
diates of biologically significant molecules, ligands in asymmetric
catalysis, and oxo-transfer reagents [32–34].
For both catalysts, (I) and (II), and for all the substrates tested,
the resultant sulfones were obtained as the only products at the
end of the oxidation reactions, corresponding to moderate to high
substrate conversion values, depending on the catalyst and on the
substrate/catalyst molar ratio. To confirm that metalloporphyrins
are in fact catalyzing the oxidation of substrates (1–7), blank exper-
iments were performed, without catalyst, for all the substrates,
and no significant conversions were registered (always <5%). In the
GC–FID and GC–MS analyses for all the substrates, two products
were detected at the beginning of the reactions. The peaks with
lower retention times were identified by GC–MS as the resulting
sulfoxides, whereas the peaks with higher retention times were
attributed to the corresponding sulfones. As exemplified for 3-
methylbenzothiophene (Fig. 1), after the initial 30 min of reaction
the sulfoxide decreases until its total disappearance whereas the
sulfone increases until the end of the reaction. So, the manganese
porphyrin catalyzed oxidation reactions seem to involve a two step
process: first the oxidation to the sulfoxide, followed by its oxida-
tion to the respective sulfone.
Many oxidants have been tested in several ODS methodologies
such as peroxyacids, NaIO , MnO , CrO , SeO , t-butyl hydroper-
4
2
3
2
oxide, cumene hydroperoxide, PhIO, O3 and O , being H O the
2
2
2
most commonly used, by efficiency and environment protection
reasons [35,36]. So, using H O as oxidant, several catalytic systems
2
2
have been reported, namely heterogeneous and homogeneous sys-
tems [36–49], organic acid catalysts [50–52], and polyoxometalates
[
53–57]. However, the narrow applicability to a limited number of
BTs and DBTs, observed for several of these procedures, associated
with the necessity to meet more and more stringent environmental
regulations, makes the development of alternative ODS processes
a matter of crucial significance.
There are some reports concerning the use of tetrapyrrolic
macrocycles such as porphyrins [11–17], phthalocyanines [58],
and corroles [59] in oxidative desulfurization, involving the cor-
responding Fe(III), Mn(III) or Ru(III) complexes, and oxidants such
as O , H O , PhIO and Oxone (as TBA or K salts). Now we dis-
2
2
2
close the high potentiality of manganese porphyrins (I) and (II)
Scheme 2) as catalysts in S-oxidation of sulfides, and more sig-
(
nificantly, of benzothiophenes and dibenzothiophene, under very
mild conditions. As far as we know, this is the first application of
manganese porphyrins in the oxidation of recalcitrant dibenzoth-
iophene, benzothiophene and alkyl-substituted benzothiophenes,
using hydrogen peroxide as oxidant.
2
. Results and discussion
The results obtained for the oxidation of substrates (1–7)
(
(
Scheme 1) with H O using porphyrin catalysts [Mn(TDCPP)Cl]
I) and [Mn(TPFPP)Cl] (II) (Scheme 2) show that the approach is
2 2
Fig. 1. 3-Methylbenzothiophene (4) oxidation reaction profile with H O2 in the
presence of catalyst (I) for a sub/cat molar ratio of 150.
2

S.M.G. Pires et al. / Applied Catalysis A: General 439–440 (2012) 51–56
53
Table 1
Results obtained for the oxidation of substrates (1–7) with H2O2 catalyzed by manganese porphyrins (I) and (II).a
Substrate
Catalyst
Sub/cat molar ratio
300
Conversion (%)
Product
Time (min)
1
I
99.9
99.9
92.1
99.9
90
60
180
120
1
50
300
50
II
1
2
3
4
5
6
7
I
300
50
300
50
99.7
99.9
92.5
99.9
120
90
240
150
1
II
1
I
300
50
300
50
97.0
99.9
87.4
97.8
180
90
180
150
1
II
1
I
300
50
300
50
95.1
99.7
85.9
98.8
120
90
180
120
1
II
1
I
300
50
300
50
96.1
99.8
81.9
96.5
150
120
180
180
1
II
1
I
300
50
300
50
99.4
99.7
66.1
95.9
180
120
150
150
1
II
1
I
300
50
300
50
90.9
99.9
50.9
92.4
180
120
120
120
1
II
1
a
◦
The substrate (0.3 mmol) was dissolved in 2.0 mL of CH3CN and kept under magnetic stirring at 22–25 C in the presence of Mn–Porph (for sub/cat molar ratio of 150,
−3
−3
the catalyst amount was 2.0 × 10 mmol; for sub/cat molar ratio of 300, the catalyst amount was 1.0 × 10 mmol; for sub/cat molar of 600, the catalyst amount was
−
3
0
.5 × 10 mmol). The co-catalyst used was NH4CH3CO2 (0.12 mmol). The oxidant was progressively added at regular intervals of 15 min in small aliquots, each corresponding
to a half-substrate amount. The conversion values are the result of at least two essays.
As clearly evidenced by the results presented in Table 1 and
in Fig. 2, generally [Mn(TDCPP)Cl] affords the best results, giving
higher conversions in lower reaction times, under similar condi-
tions. The conversion values obtained after 90 min of reaction using
a substrate/catalyst molar ratio of 300 (Fig. 2i) or of 150 (Fig. 2ii),
also put in evidence the better performance of both catalysts in
the oxidation of sulfides (1) and (2), comparatively to the so-called
refractory substrates 3–7, probably due to the sulfur unshared elec-
trons involvement in the aromatic system of 3–7. Besides that,
in the presence of catalyst (I), a substituent at positions 2 or 3
of benzothiophene does not seem to affect the efficiency of the
reaction (substrates 4–6). In general, the substrates tested give rise
to lower conversions in the presence of catalyst (II), with special
emphasis on 6 and 7. As referred previously, this behavior can be
understood by considering the electron-withdrawing capabilities
of [Mn(TPFPP)Cl] relatively to [Mn(TDCPP)Cl], thus suggesting a
more difficult formation of the active oxo-species for [Mn(TPFPP)Cl]
The catalytic system was evaluated for the removal of refrac-
tory S-containing compounds in a model fuel, consisting on a
mixture of benzothiophene (3), 3-methylbenzothiophene (4), 2-
methylbenzothiophene (5), and dibenzothiophene (7) in hexane,
using the more efficient catalyst [Mn(TDCPP)Cl], as demonstrated
above. GC–MS analyses were used to follow the oxidation reactions
of the model fuel. The chromatograms obtained before (Fig. 3A) and
after (Fig. 3D) the catalytic oxidation of the model fuel were used
to confirm that all the S-containing compounds had been com-
pletely oxidized to the corresponding sulfones 10 (Rt = 10.23), 11
(Rt = 10.84), 12 (Rt = 11.93), and 14 (Rt = 15.68). After 60 min of reac-
tion (Fig. 3B) is possible to observe the four chromatographic peaks
corresponding to the substrates, four new peaks corresponding to
the sulfones and two additional peaks identified by GC–MS as the
sulfoxides (Rt = 10.04 for the benzothiophene oxide and Rt = 11.68
for the 2-methylbenzothiophene oxide). After 120 min of reaction
almost complete conversion of the model fuel is achieved, as can
be observed in Fig. 3C.
[
65].

5
4
S.M.G. Pires et al. / Applied Catalysis A: General 439–440 (2012) 51–56
Scheme 3.
3. Experimental
The syntheses of manganese porphyrin catalysts were accom-
plished according to already established procedures [71,72]. For the
catalytic studies, in a typical experiment, the substrate (0.3 mmol),
−
3
−3
−3
the catalyst (2.0 × 10 mmol, 1.0 × 10 mmol or 0.5 × 10 mmol,
depending on the sub/cat molar ratio employed, 150, 300 or 600,
respectively), the co-catalyst (ammonium acetate, 0.2 mmol) and
the internal standard (chlorobenzene, 0.3 mmol) were dissolved
in CH CN (2.0 mL). For the model fuel catalytic studies, the sub-
3
strates 3–5 and 7 (0.03 mmol each in hexane), the catalyst (I)
−
3
(
1.7 × 10 mmol in 475 ␮L of CH CN), and the co-catalyst (ammo-
3
nium acetate, 0.1 mmol) were dissolved in hexane for a total
volume of 2.0 mL. The reaction mixtures were kept under mag-
netic stirring and in the absence of light at 22–25 C. The oxidant,
◦
3
0% H O2 (w/w, aqueous solution), was diluted in CH CN (1:10)
2
3
and added to reaction in regular intervals of 15 min, in aliquots
corresponding to a half-substrate amount. The reactions were fol-
lowed by GC–FID and by GC–MS. For all the substrates, the resulting
sulfones were isolated by preparative TLC using CH Cl as elu-
2
2
ent and characterized by 1H NMR (please see the Supplementary
information for the analytical data of compounds 8–14). The
GC–FID analyses were performed on a Varian 3900 chromatograph
using helium as the carrier gas (30 cm/s). The GC–MS analyses were
performed on a Finnigan Trace GC/MS (Thermo Quest CE instru-
ments) using helium as the carrier gas (35 cm/s). In both cases fused
silica capillary DB-5 type columns (30 m, 0.25 mm i.d., 0.25 ␮m film
thickness) were used. The 1H NMR spectra were recorded on a
Bruker Avance 300 at 300.13 MHz, using CDCl3 as solvent and TMS
as the internal reference.
Fig. 2. Conversion values obtained for substrates (1–7) after 90 min of oxidation
reaction with H2O2, using (A) a sub/cat molar ratio of 300 and (B) a sub/cat molar
ratio of 150.
Relative Abundance
7
.23
1
1
1
1
00
7
.09
12.41
(
A)
5
.73
5
0
0
1
1.90
00
(B)
(C)
(D)
7
.09
11.68
10.82
1
0.23
12.41
15.66
5
.73
5
0
7.26
1
0.04
0
1
1
5.65
5.68
00
1
1.90
1
0.82
1
0.23
5
0
7
.09
12.41
11.93
0
00
1
0.84
1
0.23
5
0
0
0
2
4
6
8
10
12
14
16
18
Time (min)
Fig. 3. Typical GC–MS chromatograms illustrating the model fuel oxidation reaction profile with H2O2 in the presence of catalyst (I) for a sub/cat molar ratio of 150. (A)
Initial reaction mixture before the addition of H2O2; (B) after 60 min of reaction; (C) after 120 min of reaction and (D) after 180 min of reaction. The model fuel is a solution
of 3 (Rt = 5.73), 4 (Rt = 7.09), 5 (Rt = 7.23), and 7 (Rt = 12.41) in hexane (with 0.03 mmol each).

S.M.G. Pires et al. / Applied Catalysis A: General 439–440 (2012) 51–56
55
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I) and [Mn(TPFPP)Cl] (catalyst II) are able to efficiently oxidize
several organosulfur compounds, namely sulfides and refractory
S-containing aromatic compounds, such as benzothiophenes and
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-methylbenzothiophene, 2-methylbenzothiophene, and diben-
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