Oxidative dehydrogenation of thiols to disulfides at room temperature using silica supported iron oxide as an efficient solid catalyst

Article abstract of DOI:10.1039/c6ra19832e

Selective transformation of thiols to disulfides by means of oxidative dehydrogenation has been described using silica supported iron oxide under base- and solvent-free reaction conditions at room temperature in an open atmosphere. The easiness of catalyst preparation, green reaction conditions, easy separation of the formed products and catalyst from the reaction mixture, and recyclability of the catalyst, are the most attractive facets of our synthetic procedure which, being ecofriendly, will find immense applications in academic and industrial sectors.

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Oxidative dehydrogenation of thiols to disulfides at room temperature using silica
supported iron oxide as an efficient solid catalyst
Susmita Paul,*a and Sk. Manirul Islam*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Selective transformation of thiols to disulfides by means of water refining. Supported or modified iron oxides catalyzed
oxidative dehydrogention has been described using silica reactions are one of the attracting alternatives for green
supported iron oxide under base and solvent free reaction chemical synthesis3 because of the facilities associated with
condition at room temperature in open atmosphere. The easiness heterogeneous catalyst system and environmental concern of
of catalyst preparation, green reaction condition, easy separation non toxic iron oxide.4 Iron and iron oxides are abundant in
technique of the formed products and catalyst from the reaction nature and iron is one of the most versatile transition metals
mixture, recyclability of the catalyst are the most attractive facets which act as important red-ox centre in biological systems and
of our synthetic procedure which being ecofriendly will find in many chemical transformations.5 On the other hand,
immense applications in academic and industrial sections.
disulfides are key intermediates in a broad range of organic
syntheses. Organic compounds containing disulfides play
crucial roles in chemistry and biology6 e.g. disulfide bond
formation is important in peptides7 and bioactive molecules.8
Moreover, thiols can be conveniently protected as disulfides
and can be regenerated by cleavage of the S–S bond.9 Due to
the low bond energy of the sulfur–sulfur bond in disulfides,
thiols can be easily over-oxidized giving thiosulfinates,
thiosulfonates, and sulfonic acids and hence revisions are fixed
on their selective oxidation to disulfides without over
oxidation which has been exhaustively investigated over the
years.10
Among the several methods of preparing disulfides,11 most
straightforward methodology involves oxidative coupling of
thiols to disulfides.12 In recent times, cleaner protocols have
been described by A. Talla et al12a under photocatalytic
condition using acetonitrile or ethanol as solvent and
equivalent amount of TMEDA; and under solvent-free
conditions by Montazerozohori et al.12f Instead of significant
efforts being made so far, most of these protocols are
suffering from use of expensive and/ lethal reagents, organic
or inorganic bases, longer reaction time, strong oxidizing
agents or flow of molecular oxygen.11,12 Among some
approaches involving iron/iron oxide as heterogeneous
catalysts for oxidative coupling of thiols to disulfides,12d (cross ref
45) H. Garcia et al. described Fe-BTC as catalyst for the selective
preparation of disulfides in acetonitrile solvent at 70o C with
the help external oxygen flow. Although, their method did
work effectively in several cases; moderate yields (60–70%)
were obtained with aliphatic thiols and with longer reaction
time. More recent approach using SBA-15 supported iron
Introduction
Molecular alteration is a prime dynamic force for chemical
research and present approaches focus emergent awareness
of the environmental impact of chemical production. The
development of clean synthetic methodologies can put
forward new doorway to gentle and cost-effective processes.
Central aspects of research toward more sustainable processes
for chemical synthesis involve replacements of heavy and toxic
metal catalysts, organic solvents as reaction media,
development of strategy to achieve higher atom economy,
escaping from strong basic reaction conditions, and energy
saving procedures.1 Oxidative dehydrogenations catalyzed by
various metal oxides including iron oxides are well known in
literature and attract considerable attention over the years in
the arena of industrial and academic research2 because
naturally occurring iron oxides are ubiquitously found in the
environment and are considered to be non-toxic to the
environment in general. Iron oxides also find extensive usage
as magnetic storage, medication, chemical industries and in
a.D. S. Kothari post doctoral fellow, Department of Chemistry, University of Kalyani,
Nadia, West Bengal, India. Fax: +91 33 2582 8282; Tel: +919474197728; E-mail:
susmitapaul2007@rediffmail.com.
b.Professor, Department of Chemistry, University of Kalyani, Nadia, West Bengal,
India. Fax: +91 33 2582 8282; Tel: +91 33 2582 8750; E-mail:
manir65@rediffmail.com.
Electronic Supplementary Information (ESI) available: [UV, IR, TGA, SEM analysis
of support and catalyst, and scanned copy of 1H and 13C NMR spectra of selected
disulfides]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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oxide nanoparticles for disulfide synthesis involves use of H2O2 between 500-600 nm (Fig 1.b) whereas, silica gel G does not
in acetonitrile solvent at room temperature.12g But the scope show any characteristic peaks in this region (Fig. 1.a).
of this procedure is limited to fluoro thiophenol as the reaction
needs 0oC temperature; inertness of aliphatic thiols to undergo
0.2
self coupling or coupling with aromatic thiols to produce
unsymmetrical disulfides. Thus
a very careful survey of
0.0
-0.2
-0.4
-0.6
-0.8
literature reports encourages us to invent general and
straightforward procedure for the synthesis of disulfides from
thiols without over oxidation following the principles of green
chemistry. Herein, we report an efficient method for
synthesizing disulfides from thiols using silica supported iron
oxide catalyst at room temperature without the need of any
solvent, external oxygen flow or the presence of highly
oxidizing agents and bases (Scheme 1).
Scheme 1 Silica supported iron oxide catalyzed disulfide synthesis from thiols.
300
400
500
600
700
800
Wavelength(nm)
Figure 1.a; DRS-UV analysis of silica gel G.
R
S
S
R
R
SH
(72-95)%
Silica supported iron oxide
(1-8) h, (27-30)o
C
0.8
0.6
R = Aromatic, Heteroaromatic, Aliphatic, Alicyclic
0.4
Results and discussions
0.2
In continuation of searching for better synthetic methodology
using silica or silica supported iron catalysts;13 we have
prepared silica supported iron oxide within a very short time
which then employed to establish a general procedure for
conversion of thiols selectively and efficiently to corresponding
disulfides for which 99% atom efficiency and a high TON/TOF
(8000/1143h-1; Table 1, entry 7) is achieved easily.
0.0
-0.2
-0.4
300
400
500
600
700
800
Wavelength(nm)
We have prepared a series of catalysts by varying the
addition of an accurately weighed amount of ferric chloride to 1
g of silica (MERK, Silica Gel G for tlc) in a 100 mL round
bottomed flask and then 6 mL acetone is added to the flask. The
acetone is then evaporated under continuous stirring of the
mixture at 35o C. After complete evaporation of acetone, the
flask (containing yellow solid mass) is heated at 250o C for three
hour in an oil bath under continuous magnetic stirring in open
atmosphere. The yellow solid mass turns to deep amber colour.
The solid catalyst Silica supported iron oxide thus prepared is
then employed for selective transformation of thiols to
disulfides. The series of catalysts with decreasing iron content is
designated as Catalyst A (Prepared by mixing 1 mmol FeCl3 with
1g silica gel G); Catalyst B (Prepared by mixing 0.5 mmol FeCl3
with 1g silica); Catalyst C (Prepared by mixing 0.25 mmol FeCl3
with 1g silica); Catalyst D (Prepared by mixing 0.1 mmol FeCl3
with 1g silica).
Figure 1.b; DRS-UV analysis of Catalyst B.
The FT-IR study of silica Gel G (Fig. 2.a) and catalyst B (Fig. 2.b)
does not show much distinctive difference in between and
both spectra contain O-H stretching at 3400 cm-1, and at 582
cm-1 for Si-O stretching.
57.8
55
50
462.61
503.03
496.64
716.62
45
40
701.71 582.00 487.14
756.80
572.32 481.51
475.54
613.76
595.33
35
30
1636.05
3462.63
803.95
673.67
%T 25
20
15
10
5
1097.22
0
-5.4
The silica supported iron oxide prepared by the thermal
decomposition of ferric chloride is then characterized by solid
state UV and FT-IR spectroscopy and TGA analysis and SEM
analysis.
4000.0
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
400.0
cm-1
Figure 2.a; FT-IR analysis of silica gel G.
DRS-UV analysis in the range of 200-800 nm shows distinct
difference between the silica gel G and catalyst B. Catalyst B
shows three absorbance peaks viz. a sharp peak around 250
nm; another peak around 350 nm, and a broad peak in
2 | J. Name., 2012, 00, 1-3
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53.2
50
557.54
545.67
614.07
594.83 483.87
45
1635.93
808.63
473.07
460.83
673.82
3436.63
40
35
30
25
20
15
%T
Figure 4. SEM analysis of Catalyst B
1153.81
1096.85
10
8.0
4000.0
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
450.0
Table 1. A comparative study of oxidative dehydrogenation of thiophenol (2 mmol) to
diphenyl disulfide under different conditions at room temperature.
cm-1
Figure 2.b; FT-IR analysis of Catalyst B.
The thermal stability of the catalyst B in comparison to the
support silica gel G is examined by the TG/DTA analysis (Fig. 3).
The study is carried out in the range of (25-500)⁰ C with 10⁰ C/
min. heating rate. The initial steady decrease of the line graph
of TG/DTA spectrum of silica Gel G (Fig. 3.a) and catalyst B (Fig.
3.b) may be due to the loss of water present in the silica matrix
and the absence of any sharp changes in the range of (200-
250) ⁰C for catalyst B (Fig. 3.b) signifies the continuity of the
same phase present in the catalyst system B.
Entry
Catalyst
Amount of
catalyst
Time
Conv.
(%)a
1
2
Nil
Nil
24h
5
7
silica
250 mg
24 h
3
4
Cat. A
Cat. B
250 mg
500 mg
40 min.
50 min.
100
100
5
6
7
8
Cat. B
Cat. C
Cat. D
Cat. A#
250 mg
250 mg
250 mg
250 mg
1 h
2h 30 min.
7 h
100
100
100
100
100
98
96
94
92
50 min.
Cat. B#
Cat. C#
Cat. D#
Fe2O3
250 mg
250 mg
250 mg
20 mg
1h 20 min.
2h 40 min.
9 h
100b
100
100
85
100
200
300
400
500
9
0
10
11
12
Temperature ( C)
30 h
Figure 3.a; TG/DTA analysis of silica gel G.
a; % conversions were checked by HPLC using MeOH as solvent. b; Reaction
carried out by using commercially available Fe2O3; Cat. A# (1 mmol Fe2O3 mixed
with 1g silica); Cat. B# (0.5 mmol Fe2O3 mixed with 1g silica); Cat. C# (0.25 mmol
Fe2O3 mixed with 1g silica); Cat. D# (0.1 mmol Fe2O3 mixed with 1g silica).
100
98
96
94
92
90
88
We have started our study of synthesizing disulfides from
thiols taking thiophenol as the model substrate to optimize the
reaction condition. The results are summarized in table 1. We
have observed the formation of disulfide only in trace amount
under neat condition without any catalyst or support, or in
presence of support without catalyst (Table 1, entry 1 and 2
respectively). The reaction proceeds to full conversion to
disulfide with 250 mg of catalyst A within very short reaction
time without any requirement of base or solvent (Table 1,
entry 3). The time needed for complete conversion increases
with decreased catalyst loading as expected (Table 1, entries 3-
7). The fact that silica supported iron oxide catalyst (prepared
by thermal pyrolysis of FeCl3) shows comparable activity14 with
commercially available iron oxide (mixed by mechanical
grinding in a mortar pestle with silica (MERK, Silica Gel G for
tlc)) (Table 1, entries 8-11) emphasizes that Fe2O3 moiety is
100
200
300
400
500
Temperature (0C)
Figure 3.b; TG/DTA analysis of Catalyst B.
SEM analysis of catalyst B is performed and the pictures are
given in Fig. 4. (Plots are included in the Electronic
Supplementary Information).
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active catalytic phase in our catalyst system. The importance the catalytically active centres remain free for further reaction.
of silica as support13 in this study is revealed by the fact that These amazing observations excel importance of silica as an
85% conversion of thiophenol to corresponding disulfide indispensable support for this reaction. With the optimization
requires as long as 30h when we employ commercially data in our hand, we have chosen the reaction with 250 mg of
available iron oxide to the reaction under neat condition the supported catalyst B (0.5 mmol FeCl3 supported on 1g
(Table 1, entry 12), while, our system took only 1h for silica) to convert 2 mmol of the thiophenol to corresponding
complete conversion (Table 1, entry 5). This result may be due disulfide as the model one. Table 2 is clearly showing the
to the coating of iron oxide particle by the formed diphenyl efficiency and cleanliness of our procedure in comparison to
disulfide thereby inhibiting the chances of showing any further some recent developments of methodology for preparing
catalytic activity under neat catalytic condition. But the disulfides from thiols by photocatalytic method without metal
situation is different for Silica supported iron oxide system catalyst (Table 2, entry 1); solid state method (Table 2, entry 2)
where the formed diphenyl disulfide gets distributed over a and iron catalyzed systems (Table 2, entries 3-7).
large surface area provided by the mesoporous support and
Table 2; Comparative table of the present catalyst system with the other reported catalysts.
Entry
1
Catalyst/oxidant
Eosin Y
Catalyst/oxidant
Amount
Additive
Solvent
Ethanol
RSH
Temp.
Time
Yield%
93-99
Reference
Ref12a
1 mol%
TMEDA (1
equiv.
occasional
adding)24 W
CFL, air
Aromatic,
heteroarmatic,
aliphatic
Not
mentioned
16 h
2
3
Moist sodium periodate
1mmol
100 mg
1 mol%
--
--
Aromatic,
heteroarmatic,
benzyl,
Rt
(1.5-5) min.
(1-20) h
70-96
55-91
Ref12f
aliphatic
Fe(BTC)/External oxygen
flow
--
Acetonitrile
(4 mL)
Aromatic,
heteroarmatic,
benzyl,
70o C
Refcross ref no.45
of Ref. 11(a)
aliphatic
4
5
Fe-NPS@SBA-15/H2O2
Fe(TPP)Cl/UHP
Nano Fe2O3
--
--
Acetonitrile
(3 mL)
Aromatic,
heteroarmatic,
benzyl
25o C
0o C
(15-360) min.
(10 -90) min.
70-99
60-91
Ref12g
Ref12e
0.02
mol/1mmol
Methanol (10
mL)
Aromatic,
heteroarmatic,
benzyl
6
7
8
80 mg
Ionic liquid
Methanol
DMF
Aromatic,
aliphatic
60o C
(10-75) min.
(0.5-3) h
(1-8) h
45-99
100
Ref12i
Ref12h
Cobalt–iron magnetic
composites
0.1 wt%
--
--
Aromatic
(25-50)o C
(27-30)o C
Silica supported iron
oxide
250 mg (0.1
mol%)
--
Aromatic,
heteroarmatic,
benzyl,
(72-95)
Our
procedure
aliphatic
We explore the scopes and limitations of our reaction protocol
by employing different thiols and the observations are
summarized in table 3. Ortho- and para- substituted aromatic
thiophenols including heteroaromatic thiol (pyridyl) result in
very good yield of the corresponding disulfides exclusively (82-
95 %) within a very short reaction time (1-5) h (Table 3, entries
1-5; except in the case of 2-napthalene thiophenol Table 3,
the disulfides in case of heteroaromatic thiols, moderate yield
with benzyl thiol and low yield with thioacetic acid.
Depending on the fact that iron oxide can be involved in redox
reaction in heterogeneous condition or on solid surface14,15 we
propose
the
plausible
mechanism
for
oxidative
dehydrogenation as depicted in scheme 2. In the first step the
acidic hydrogen of –SH gets attached to one of the doubly
bonded oxygen of iron oxide through the lone pair of electron
entry
4 and 2-pyridyl thiol). The methoxy substituted
thiophenols (both ortho and para) require comparatively
longer reaction time (8 h) to produce very good yield of
disulfides (Table 3, entry 6, 7). Halogenated thiols also produce
very good yield within a reasonable time period (Table 3,
entries 8, 9). Alicyclic thiol viz. cyclohexylthiol, and other
aliphatic straight chain thiols namely pentane and heptane
thiol also produce excellent yield of the corresponding
disulfides (Table 3, entries 11-13). We have isolated disulfide
of thioglycolic acid in very good yield and in this case product
has been separated after esterification of carboxylic group
with ethanol in one pot reaction procedure. Here, worth
mentioning is that Garcia et al. reported only good yields of
thereby weakening the strength of the –S—H bond [E]. The
electron deficiency thereby formed on the iron centre can be
partially meet up by the electron donating capability of
sulphur of thiophenol group
weakening of –S—H bond. In the next step S atom of another
thiol attacks the S atom of the catalytically bonded thiol [
which ultimately forms the desired disulfide bond through [
[F] which causes further
G]
H
].
Blue colour signifies developing bond and red one is for
breaking bond.
4 | J. Name., 2012, 00, 1-3
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Table 3 Selective conversion of thiols to disulfides using silica supported iron oxide,
Catalyst B.
Fe
S
O
R
Fe
O
H
O
Fe
O
Fe
O
O
Yield (%)a
95
[E]
Entry
1
Thiol
Time
1 h
Fe
S
O
R
Fe
H2O
O2
Fe
O
H
O
SH
[F]
Fe
O
O
O
SH
2
3
3 h
92
93
H
H
Fe
S
O
Fe
Fe
O
Fe
Me
O
H
O
H
O
O
R
S
S
R
S
S
R
H
Me
2.5 h
[H]
SH
[G]
R
R
R
S
H
Me
Scheme 2. Probable mechanism for aerobic oxidation of thiols to disulfides using silica
supported iron oxide.
SH
4
5
8 h
3 h
72
92
The advantage of silica supported iron oxide in this reaction is
examined by its recovery and reuse after the first run (Table 4)
taking thiophenol as the reactant. After completion of the
reaction, the solid mixture is centrifuged repeatedly with
dicholoromethane (2mL x 3). The dicholoromethane extract is
examined under AAS which shows bare existence of leached
iron. The solid is then dried in a hot air oven at 100o C for 2 h
and then the next batch of reaction is performed with the
recovered silica-supported iron oxide and found to be equally
active in catalyzing the process. Table 3 shows the results of six
consecutive recycle runs. The DRS-UV analysis and FT-IR
spectra of catalyst B after six recycle run does not show
difference in spectral pattern (Fig. 5a and 5b respectively) and
SEM analysis performed on the reused catalyst B (Fig. 5c)
shows that catalyst looses granularity in comparison to the
fresh catalyst (Fig. 4). The catalyst is air stable and can be used
after six months of preparation. The reaction is scaled up to 10
mmol of thiophenol with 250 mg of catalyst B which took 38 h
for 100% conversion to diphenyl disulfide.
NH2
SH
SH
6
7
8 h
8 h
82
83
MeO
OMe
SH
SH
8
9
3 h
5 h
86
87
F
SH
Cl
10
11
SH
2 h
3 h
86
85
SH
0.8
0.6
SH
12
13
14
15
5 h
7 h
8 h
8 h
88
85
SH
0.4
0.2
83b
82
HOOC
SH
0.0
-0.2
-0.4
N
SH
300
400
500
600
700
800
a; % yield represents yield after column chromatographic purification however,
solid disulfides could be obtained by simple filtration followed by evaporation of
the filtrate. b; Disulfide was isolated by esterification of the acid by ethanol after
completion of the reaction in one pot procedure.
Wavelength(nm)
Figure 5a. DRS-UV analysis of Catalyst B after six run.
In this reaction sequence iron oxide gets partially reduced to (-
-FeOH) which gets converted to iron oxide again by aerial
oxygen.
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74.6
70
discovery that disulfide bond can be prepared under solvent
free condition on oxide support at room temperature could
find a new window to investigate the mechanism of biological
evolution process where disulfide bond formation plays a key
role in peptide synthesis and determining tertiary and
quaternary structure of proteins.
65
60
2923.14
500.47
458.92
55
50
45
614.15
595.12
674.49
1642.82
804.07
473.20
466.51
40
35
30
25
20
15
10
5
3454.93
%T
Acknowledgements
Susmita
Paul
thankfully
acknowledges UGC DS Kothari funding agency for financial
support (No. F.4-2/2006/(BSR)/CH/13-14/0075); Prof. Anilava
Kabiraj, Department of Zoology, University of Kalyani for
recording AAS spectra, Mominul Islam and Mita Halder for
recording spectroscopic data; SMI acknowledges Department
of Science and Technology, New Delhi, Govt. of India, (DST-
SERB), University grant Commission, New Delhi, Govt. of India,
(UGC), and Department of Science and Technology, Govt. of
West Bengal (DST-W.B.) for financial support. We gratefully
acknowledge DST and UGC Govt. of India for providing support
to the Department of Chemistry, University of Kalyani under
FIST, PURSE and SAP programme.
1099.99
-1.0
4000.0
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
450.0
cm-1
Figure 5.b; FT-IR analysis of Catalyst B after six recycle run.
Notes and references
‡
§
Experimental
Representative procedure for the synthesis of di-phenyl-di-
Figure 5c. SEM analysis of Catalyst B after sixth cycle.
sulfide: Silica supported iron oxide (Catalyst B, 250 mg) was
taken in a glass mortar and then 2 mmol of thiophenol was
added to the catalyst and mixed by a pestle. After mixing the
solid mass was transferred to a 25 mL round bottomed flask
fitted with a guard tube and a small magnet was introduced for
continuous magnetic stirring for 1 h at room temperature. After
the completion of the reaction (checked by tlc); the reaction
mixture was washed successively with dichloromethane (2mL x
3) and filtered through Whatmann 42 filter paper. The filtrate
was then evaporated to obtain white crystal of di-phenyl di-
sulfide. The product was then characterized by 1H, 13C NMR
spectral data and melting point was compared with literature
report.
Table 4 Recycling table with thiophenol as the reactant using catalyst B.
Entry
1
Cycle
0
Yield %a
95
2
3
4
5
6
7
1
2
3
4
5
6
92
94
94
92
92
91
a; Represents % yield after column chromatographic purification.
Table 2, Entry 1: 1H NMR (CDCl3, d ppm-1 relative to TMS, 300 MHz):
7.18–7.31 (6H, m); 7.48–7.50 (4H, d, J = 7.5 Hz); 13C NMR (CDCl3, d
ppm-1, 75 MHz): 127.1, 127.4, 129, 136.9. Melting point observed
60o C [(59-62)oC (lit.)]
Conclusions
The representative procedure for the synthesis of diphenyl disulfide
is followed for the preparation of other disulfides mentioned in
table 3.
In conclusion, atmospheric oxidation of thiols to disulfides can
be performed using silica supported iron oxide as the reusable
catalyst under very mild reaction condition. The catalyst is
effectively applicable for a very wide range of thiols including
aromatic, aliphatic, alicyclic, benzylic and heteroaromatic thiol
and easy separation of the reaction product from the reaction
mixture only by simple filtration makes the procedure
economically more viable than demonstrated by Fe(BTC) or
SBA 15 supported iron oxide catalyst.12d,g The reaction protocol
does not require any extra flow of oxygen or highly oxidizing
agent such as hydrogen peroxide or sodium periodate; the
condition does not require presence strong inorganic bases or
organic base like TMEDA. The catalyst precursor FeCl3 is very
cheap and the final catalyst is environmentally non hazardous.
This protocol being eco-friendly will find immense industrial
and academic applications. Finding out more insights of this
reaction mechanism are underway in our laboratory. The
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DOI: 10.1039/C6RA19832E
Graphical Abstract
Title: Oxidative dehydrogenation of thiols to disulfides at room temperature using silica supported
iron oxide as an efficient solid catalyst
Susmita Paul*a and S. M. Islam*b
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, West Bengal, India
Email: susmitapaul2007@rediffmail.com; manir65@rediffmail.com.
Selective transformation of thiols to disulfides by means of oxidative dehydrogention has been described using
silica supported iron oxide under base and solvent free reaction condition at room temperature in open atmosphere.
The easiness of catalyst preparation, green reaction condition, easy separation technique of the formed products and
catalyst from the reaction mixture, recyclability of the catalyst are the most attractive facets of our synthetic
procedure which being ecofriendly will find immense applications in academic and industrial sections.
SH
SH
SH
SH
N
SH
Silica supported iron oxide
(1-8) h, (27-30)o C
S
S
S
S
N
S
S
S
S
N
S
S