Thio-photocatalysis II: Synthesis of dioctylsulfide in the liquid phase by the photocatalytic addition of 1-octanethiol on 1-octene

Article abstract of DOI:10.1016/j.catcom.2010.05.023

Photocatalytic synthesis of dioctylsulfide has been performed at room temperature in liquid phase by addition of 1-octanethiol on 1-octene in contact with illuminated TiO₂. Under inert atmosphere, dioctylsulfide is selectively obtained whereas

Full text of DOI:10.1016/j.catcom.2010.05.023

Catalysis Communications 11 (2010) 1116–1119
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Catalysis Communications
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Thio-photocatalysis II: Synthesis of dioctylsulfide in the liquid phase by the
photocatalytic addition of 1-octanethiol on 1-octene
K. Schoumacker, M. Cattenot, C. Geantet ⁎, E. Puzenat, M. Lacroix, J.M. Herrmann
IRCELYON, UMR-CNRS 5256, Université Claude Bernard Lyon 1, 2 av A. Einstein, F-69626 Villeurbanne Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Photocatalytic synthesis of dioctylsulfide has been performed at room temperature in liquid phase by
Received 2 March 2010
Received in revised form 29 April 2010
Accepted 31 May 2010
2
addition of 1-octanethiol on 1-octene in contact with illuminated TiO . Under inert atmosphere,
dioctylsulfide is selectively obtained whereas in the presence of oxygen high selectivity in n-dioctylsulfide
has vanished. This study illustrates the potential of photocatalysis for the production of thio-compounds.
Available online 9 June 2010
©
2010 Elsevier B.V. All rights reserved.
Keywords:
Thiochemistry
Photocatalysis
Sulfides
Titania
Dioctylsulfide
1
. Introduction
formation of propan-1-thiol, which was not the product expected in a
conventional Markovnikov-type reaction nor in a heterogeneous
catalytic system [9]. The aim of this work is to explore further the
potentiality of thiophotocatalysis for the synthesis of organic sulfides. 1-
octene was chosen as the liquid olefin and 1-octanethiol as the sulfiding
agent. This reaction is usually very slow and requires severe conditions
In industry, thiochemistry is connected with the synthesis of several
compounds, such as mercaptans, sulfides, sulfones, thioacids, etc, which
are involved in many industrials fields. Among these compounds,
mercaptans constitute the most important group: they are used as
agrochemicals, additives in perfumes and cosmetics, as well as odorous
agents in natural gas distribution [1].They enable one to control the
propagation of some polymerization reactions and are present during
synthetic rubber vulcanization. Their preparationis often the first step of
the synthesis of other thio-compounds. Therefore, new synthesis
techniques of thiols are to be investigated. Mercarptans are also used
as intermediates for the synthesis of more complex thiocompounds i.e.
sulfides, di- or polysulfides industrially utilized as lubricants, antiwear
additives or extreme pressure additives [2], or sulfiding agents [3]. From
2 4
unless an acid catalyst like H SO is used but in the presence of free
radical initiators, reaction is facilitated [10]. Fig. 1 shows that the
reaction can lead to the formation of both linear and branched
dioctylsulfides. By-products, such as linear and branched dioctyl-
disulfides, can also be formed.
2. Experimental
The synthesis of dioctylsulfide was carried out at room temperature
in a batch photoreactor working at atmospheric pressure in the liquid
phase, described in [11]. The solution can be saturated by various gases
by means of a plunging tube. In this study, experiments were carried out
in a nitrogen flow, but also under air (5 mL/in both cases). The initial
concentrations of 1-octene and 1-octanethiol (from Aldrich) were close
2
an industrial point of view, mercaptans are prepared by addition of H S
to olefins or by thiolation of the corresponding alcohols [4,5]. Although
the interest of heterogeneous photocatalysis for functional group
transformations of organic compound is now well demonstrated for
many reactions [6,7], very few studies dealing with photocatalytic
synthesis of sulfur-containing products (thiocompounds) are reported
in the literature. Tada et al. reported the reduction of bis(2-dipyridyl)
disulfide to 2-mercaptopyridine by H
The photocatalytic synthesis of a mercaptan by direct reaction of an
olefin with H S has been also performed in the gas phase and in the
presence of TiO or CdS [9]. Both catalysts were highly selective in the
−1
to 10 mol/L for both reactants in order to insure a zero kinetic order
with a full surface coverage [11]. Decane (0.0968 mol/L) was added as
an internal standard and dodecane used as a solvent. The reaction
products were identified by GC-MS. Reactant and product concentra-
tions were measured by gas chromatography, using a HP 5890 equipped
with a HP 5 capillary column (30 m/0.32 mm) and a flame ionization
detector.
2 2 2
O using TiO or Ag-doped TiO [8].
2
2
The irradiation was provided by a 125 W medium-pressure mercury
lamp (Philips HPK 125), the IR emission of which was absorbed by a
circulating water cell. In order to minimize the photochemical route, a
⁎
Corresponding author.
E-mail address: christophe.geantet@ircelyon.univ-lyon1.fr (C. Geantet).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.05.023

K. Schoumacker et al. / Catalysis Communications 11 (2010) 1116–1119
1117
3.3. Photocatalytic reaction in air
The results obtained in air are quite different from those observed
in nitrogen (Fig. 3). The initial rate is 10 times lower (1.2×10− mol/s
g). The 1-octene conversion is only 57.5% after 300 min and the main
products are the linear and the branched dioctyl-disulfide (selectiv-
ities equal to 18% and 6%, respectively). An additional oxygenated
compound was identified as thio-octyl octanoate (9% selectivity). The
carbon balance in the liquid phase only accounted for 50% of the initial
number of carbon atoms. The presence of small amounts of
unidentified products in the remaining solution but principally the
6
2 x 2
CO , SO in the gas phase and H O involved by mineralization should
Fig. 1. Reaction scheme of the addition of octan-1thiol on octene.
be taken into account for a total mass balance determination.
Corning 0–52 filter (λN340 nm) was used. The radiant flux (or
intensity) of the incident photons passing through the filters was
equal to 3.3 mW/cm , as measured by an Oriel-70260 radiometer.
4. Discussion
2
The catalyst used in this study was titania P25 from Degussa (non
porous, 75% anatase, 25% rutile, mean particle diameter: ca.30 nm,
specific surface area: 50 m /g). 100 mg of this solid were loaded in the
4.1. Catalyst
2
First, concerning the photocatalyst, one could have chosen a
sulfide catalyst such as CdS as in ref. [9]. However, it was shown that
titania is much more efficient than CdS, even in thio-photocatalysis.
Actually, this is due to the ability of titania's surface to become
sulfurized.
reactor and kept in suspension in the reactant mixture by magnetic
stirring.
3
. Results
For instance, it was experimentally demonstrated that H
interacts with TiO . It is dissociatively adsorbed on the defects of Ti
110) surface [12] or rutile and anatase [13] or under reducing
2
S
2
3
.1. Photochemistry and blank tests
(
atmospheres such as hydrotreating conditions [14,15].
It was first checked that none of the reactants tested alone, under
At room temperature, titania can get sulfurized under UV-
illumination by anion exchanged. It is known that UV-irradiated
irradiation and in the presence of decane, dodecane and TiO
2
, were
converted and that TiO was inactive when the reaction was
2
1
6 2−
titania can exchange its natural
O
surface anions with gaseous
performed without UV-irradiation. Under UV-irradiation without
catalyst, conversions of only 3% and 2% were obtained after 300 min
UV-irradiation under a flow of nitrogen or air, respectively, showing
that the photochemical route is negligible in the chosen conditions.
18
2
O (refs ) according to the reaction:
1
8
16 2−
16 2−
18
16
O2
þ
O
→
O
þ
O ¼
O
ðgÞ
ð1Þ
ðgÞ
ðsurfaceÞ
ðsurfaceÞ
In presence of H
2
S, the exchange of 6O2−(surface) by S2− is possible,
1
2
−
3
.2. Photocatalytic reaction in a neutral atmosphere (nitrogen)
owing to the lability of O surface anions:
2
−
2
−
Fig. 2 shows the transformation of the reactants and the
O
þ H S→S
þ H O
ð2Þ
ðsurfaceÞ
2
ðsurfaceÞ
2
formation of the products under nitrogen atmosphere. The initial
−
5
In the present case, titania's surface can be sulfurized by S atoms
from 1-octanethiol:
rate is 1.15×10 mol/s g and the conversion rate of 1-1-octene is
0% after 300 min. The main product is the linear dioctylsulfide, with
9
a selectivity of 96% (with respect to 1-octanethiol conversion) and in
agreement with the similar temporal conversions of both reactant
and of dioctylsulfide production. Trace amounts of the branched
dioctylsulfide and of linear and branched dioctyl-disulfides could
also be detected.
2
−
2
−
O
ðsurfaceÞ þ C8H17SH→S ðsurfaceÞ þ C8H17OH
ð3Þ
The photo-assisted self-sulfurization of titania creates sulfide sites
for adsorption of sulfur-containing reactants (thio-compounds).
Fig. 2. Photocatalytic addition of octanethiol on octene under nitrogen atmosphere (L :
Fig. 3. Liquid phase photocatalytic reactions of octanethiol with octene in presence of
linear, B : branched).
air.

1118
K. Schoumacker et al. / Catalysis Communications 11 (2010) 1116–1119
4
.2. Photocatalytic addition of 1-octanethiol on 1-octene under nitrogen
which, by addition of one H° radical produced in reaction (9), generates
the branched dioctylsulfide with a methyl group in α-position with
In the absence of oxygen, both reactants, 1-octene and n-
respect to S atom.
octanethiol, chemisorb at titania's surface, independently of UV-
irradiation. Similarly to alcohols [16], thiols can chemisorb dissocia-
tively into thiolate anions and protons, owing to the amphoteric
properties of titania's surface sites
C H –S–CHðC H –CH2 ðadsÞ þH →C H –S–CHðCH3Þ–C H
13ðadsÞ
ð13Þ
Another very limited parallel reaction is the dimerization of
octylthiolate radical forming trace amounts of n-dioctyl-disulfide:
8
17
6
13Þ
˚
˚
8
17
6
−
þ
C H17–SH
→C H17–S
þ H
ð4Þ
8
ðliquidÞ
8
ðadsÞ
ðadsÞ
2
C H –S ðadsÞ→C H –S–S–C H
ð14Þ
8
17
˚
8
17
8
17
Under UV-illumination, the fundamental activation process cre-
ates photo-electrons and holes:
2
In summary, as for the addition of H S on propene, photocatalysis is
presently very selective in the addition of a thiol on an olefin in liquid
phase with the sulfur atom linked to the terminal carbon atom of the
latter.
−
þ
ðTiO Þ þ hν→e þ h
ð5Þ
2
Since titania is an n-type semiconductor, i.e. with an excess of
electrons, the (precious) minority charge carriers are the holes which
must not recombine with excess electrons (conduction electrons+
photoelectrons):
4.3. Photocatalysis in the presence of oxygen (air)
−
þ
The above selective thio-addition has also been performed in the
e þ h →NðneutralcenterÞ þ heat
ð6Þ
presence of air instead of a neutral inert atmosphere (nitrogen),
mainly to check possible perturbations due to oxygen. Indeed, as
expected, these perturbations were very important concerning both
titania's activity and reaction selectivities. The photocatalytic oxida-
tion of sulfur compounds has been extensively studied in order to
perform odour abatement or pesticides mineralization or decompo-
sition of chemical warfare agents [17–19].
Recombination reaction (6) corresponds to a degrading energy
loss [16]. Presently, holes h spontaneously react with thiolate anions
produced by reaction (4)
+
−
þ
C H17–S
þ h →C H17–S
ð7Þ
8
ðadsÞ
8
˚
ðadsÞ
Reaction (7), which corresponds to a charge neutralization
reaction, is substantially exothermic and generates energetic and
nucleophilic thio-radicals. Because of the co-adsorption of 1-octene,
octyl-thiolate radicals preferentially react with 1-octene since 1-
octene coverage at the surface of titania is important owing to its high
concentration:
4
.3.1. Activity
There is a dramatic drop in thio-photocatalysis by one order of
magnitude. This is easily understandable since titania returns to its own
nature i.e. that of an oxide catalyst, working very efficiently in oxidation
reactions in the presence of oxygen or air. However, since there is no
water in the system, the oxidation reactions remain mild in contrast with
the degrading total oxidation induced by OH° radicals photogenerated
by water oxidation [20]. The decrease in activity observed by a factor of
C H
þ C H
17–S˚ðadsÞ
→C H –S– C H
16ðadsÞ
ð8Þ
8
16ðadsÞ
8
8
17
˚
8
In parallel and simultaneously, the photo-electrons generated in
+
reaction (5) neutralize the protons H created in reaction (4) into H°
1
0 only concerns thiocatalysis since titania has become very active in
radicals, already detected in photocatalytic hydrogen production [16]
and even in oxidative total degradation of organic pollutants [17].
concurrentoxidation reactionsby oxygen. In particular, similarly to what
has been observed in propene oxidation [21], photoactivated oxygen is
able to attack the double bond of an olefin, presently 1-octene, forming
transient 1-octene oxide, which spontaneously decomposes into
heptanal and formaldehyde, both products being able to undergo
subsequent oxidations. In particular, formaldehyde is very reactive and
spontaneously oxidizes in HCOOH formic acid , the last product detected
−
þ
e þ H →H ðadsÞ
ð9Þ
˚
The radicals produced in reactions (8) and (9) recombine into the
final product:
C H –S– C H
þ H ðadsÞ→C H –S–C H
ð10Þ
2
before CO generation according to the photo-Kolbe reaction [22].
8
17
˚
8
16ðadsÞ
˚
8
17
8
17ðadsÞ
4
.2.1. Selectivity in inert atmosphere
As in mild oxidation reactions relative to fine chemistry, photo-
4.3.2. Selectivities
Because of concurrent oxidation reactions by oxygen, the high
selectivity in n-dioctylsulfide observed in inert atmosphere has
catalysis is presently highly selective in production of linear n-
dioctylsulfide.
vanished. Octane-thiol still dissociatively chemisorbs (Eq. (4)) and
8
This implies that C H17–S° radicals preferentially react with 1-
°
generates C
8
H
17–S radicals (Eq. (7)). However, since 1-octene is no
octene's double bond, adding on the less substituted carbon atom, in
line with a radical reaction. This can also be due to the mode of
adsorption of 1-octene at the surface of titania. Eq. (8) is to be written
as:
more available because it is mainly oxidized by photoactivated oxygen
as mentioned above, these radicals have a higher probability to dimerize
and recombine into n-dioctyl-disulfide as the main product (Eq. (14)).
Actually, reactions (7) and (14) correspond to a true oxidation
+
C H13–CH ¼ CH2ðadsÞ þ C H –S ðadsÞ→C H – CH–CH –S–C H
ð11Þ
reaction, induced by the positive photo-holes h , with the passage of
6
8
17
˚
6
13
˚
2
8
17
element S from the (-2) to the (-1) oxidation level.
thus accounting for the main formation of n-dioctylsulfide, with a
very high selectivity of 96%.
In a very limited concurrent and parallel reaction, the thiolate
radical can add on the non-terminal carbon atom:
Indeed, a limited formation of branched dioctyl-disulfide and of n-
dioctylsulfide could also be observed (Fig. 3), whereas branched
dioctylsulfide was only detected as ultra-trace amounts.
Eventually, an oxygenated intermediate was detected, identified as
thio-octyl octanoate. Such a result clearly indicates the competition
between oxo- and thio-photocatalysis when the latter is performed in
presence of oxygen.
C H13–CH ¼ CH2ðadsÞ þC H –S ðadsÞ→C H –S–CHðC H
6
8
17
˚
8
17
6
13Þ–CH2˚ðadsÞ
ð12Þ

K. Schoumacker et al. / Catalysis Communications 11 (2010) 1116–1119
1119
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