Alkyl sulfonated diphosphines-stabilized ruthenium nanoparticles as efficient nanocatalysts in hydrogenation reactions in biphasic media

Article abstract of DOI:10.1016/j.cattod.2011.09.012

The organometallic synthesis of ruthenium nanoparticles stabilized by water-soluble alkyl sulfonated diphosphines as ligands is described for the first time. After isolation, the so-obtained nanoparticles could be easily dispersed into water giving rise to stable aqueous colloidal solutions without precipitation over the course of several months. The catalytic behaviour of these aqueous colloidal solutions has been investigated in the hydrogenation of unsaturated substrates (tetradecene, styrene and acetophenone) in biphasic liquid-liquid conditions, showing interesting results in terms of reactivity. Interestingly, small structural differences in the backbone of the diphosphine ligands influence the catalytic activity of these nanocatalysts. In addition, preliminary tests of recycling showed promising results with neither loss of activity or significant precipitation.

Full text of DOI:10.1016/j.cattod.2011.09.012

Catalysis Today 183 (2012) 34–41
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Alkyl sulfonated diphosphines-stabilized ruthenium nanoparticles as efficient
nanocatalysts in hydrogenation reactions in biphasic media
M. Guerreroa,b, A. Roucouxc,d, A. Denicourt-Nowickic,d, H. Bricout , E. Monflier , V. Collièrea,b,
K. Fajerwerg , K. Philippot
e
e
a,b
a,b,∗
a
CNRS; LCC (Laboratoire de Chimie de Coordination); 205, Route de Narbonne, F-31077 Toulouse, France
Université de Toulouse; UPS, INPT; LCC; F-31077 Toulouse, France
b
c
Ecole Normale Supérieure de Chimie de Rennes, CNRS, UMR6226, Avenue du Général Leclerc, CS 50837, F-35708 Rennes Cedex 7, France
Université européenne de Bretagne, France
UMR CNRS 8181; Université d‘Artois, Faculté des Sciences Jean Perrin, Rue Jean Souvraz, SP 18, F-62307 Lens Cedex, France
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 July 2011
Received in revised form 8 September 2011
Accepted 11 September 2011
Available online 4 October 2011
The organometallic synthesis of ruthenium nanoparticles stabilized by water-soluble alkyl sulfonated
diphosphines as ligands is described for the first time. After isolation, the so-obtained nanoparticles
could be easily dispersed into water giving rise to stable aqueous colloidal solutions without precipitation
over the course of several months. The catalytic behaviour of these aqueous colloidal solutions has been
investigated in the hydrogenation of unsaturated substrates (tetradecene, styrene and acetophenone) in
biphasic liquid–liquid conditions, showing interesting results in terms of reactivity. Interestingly, small
structural differences in the backbone of the diphosphine ligands influence the catalytic activity of these
nanocatalysts. In addition, preliminary tests of recycling showed promising results with neither loss of
activity or significant precipitation.
Keywords:
Sulfonated diphosphine
Ruthenium
Nanoparticles
Biphasic catalysis
Hydrogenation
Water
© 2011 Elsevier B.V. All rights reserved.
1
. Introduction
The synthesis of well-controlled nanometric size metal species
metal catalysts from substrates and products is a key-point for
industrial applications, it is necessary to develop simple methods
for the preparation of appropriate metal nanocatalysts according
to the reaction conditions [3].
has received a considerable attention over the past decades due
to their attractive physical and chemical properties that lead to
numerous applications in various fields [1]. Among them, catalysis
is a tremendous active research domain where metal nanoparticles
are extensively used by the international scientific community [2].
This interest mainly follows from the fact that metal nanoparticles
provide an efficient combination of heterogeneous and homoge-
neous catalysts advantages such as a high specific surface area
resulting in a high number of potential catalytic sites as well as
the possibility to tune their surface properties. In the particular
case of colloidal catalysis where metal nanoparticles are dispersed
in a solvent, the development of nanocatalysts active in various
reaction conditions such as in organic solvents, water, ionic liquids
or even supercritical fluids depending on the target reactions is of
high importance. Indeed, as the separation of expensive transition
Besides the C–C coupling reactions, metal nanoparticles are also
widely investigated in hydrogenation reactions [4]. Water-soluble
metal nanocatalysts are commonly used in the hydrogenation
of monocyclic arenes under mild conditions [5] and continue to
attract attention as potential “green” approaches due to increased
economic and environmental concerns [6]. Nevertheless, the
development of modified nanoparticles for total or partial arene
hydrogenations is still a challenging issue and remains an active
area of research since these reactions represent an important chem-
ical process for the synthesis of fine chemicals [7]. These later are
widely used for various applications from laboratory to industrial
scale process such as, for example, the obtaining of cyclohexane
for the production of nylon-6,6 [8]. Another interesting aspect lies
in the fact that the activity and the selectivity of nanocatalysts
can be tuned by the choice of suitable ligands for their stabi-
lization, as ligands can modulate the environment of the metal
surface through electronic/steric modifications. In the same way,
ligands can be used to adapt metal nanoparticles to chosen reac-
tion conditions as they can transfer them their own solubility
properties [9].
∗ Corresponding author at: CNRS; LCC (Laboratoire de Chimie de Coordination);
2
5
05, Route de Narbonne, F-31077 Toulouse, France. Tel.: +33 561333230; fax: +33
61553003.
E-mail address: karine.philippot@lcc-toulouse.fr (K. Philippot).
0
920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.09.012

M. Guerrero et al. / Catalysis Today 183 (2012) 34–41
35
As previously done for homogeneous catalysts, biphasic
liquid–liquid systems represent an attractive alternative for recov-
ering aqueous soluble catalysts after separation from the organic
phase containing products [10]. For that purpose, complexes con-
taining water-soluble phosphines as ligands [11] constitute an
interesting example of catalysts that are retained in the aqueous
phase, the most widespread ligand being the sodium salt of trisul-
fonated triphenylphosphine, which is industrially applied in the
hydroformylation of propene and butene [12] as well as in the
Suzuki coupling of non activated aryl bromides [13].
at 200 kV with a resolution point of 2.5 A˚ and equipped with X-
ray analysis PGT (light elements detection, resolution 135 eV). The
size distributions were determined through a manual analysis of
enlarged micrographs with Imagetool software to obtain a statis-
tical size distribution and a mean diameter. FFT treatments have
been carried out with Digital Micrograph Version 1.80.70.
2.3. Synthesis of alkyl sulfonated diphosphine-stabilized
ruthenium nanoparticles
In that context, and on the base of our previous works with phos-
phine derivatives [14] and 1,3,5-triaza-7-phosphaadamantane
In a typical reaction, 150 mg of [Ru(COD)(COT)] (0.476 mmol)
were introduced in a Fischer–Porter bottle and left in vacuum dur-
ing 0.5 h and cooled to 193 K. 150 mL of THF, containing 41 mg of L1
(
PTA) [15] as stabilizing ligands for the synthesis of metal nanopar-
ticles, we explored the use of alkyl sulfonated diphosphines as
water-soluble ligands to obtain ruthenium nanoparticles in aque-
ous phase. These latter are suitable nanocatalysts for their further
use in biphasic liquid–liquid hydrogenation reactions, the organic
phase being the substrate. To the best of our knowledge, there are
only a few examples using sulfonated phosphines as ligands for
metal nanoparticles stabilization [16]. In this work we report the
first one concerning ruthenium nanoparticles.
These Ru(0)NPs have been characterized by elemental analysis,
low and high resolution transmission electron microscopy (TEM,
HRTEM, EDX) and dynamic light scattering (DLS). As a function
of the backbone of the alkyl sulfonated diphosphine ligands and
thermodynamic parameters, the hydrogenation reactions of unsat-
urated compounds such as styrene, tetradecene and acetophenone
in biphasic liquid–liquid conditions have been investigated.
(
0.048 mmol, [L1]/[Ru] = 0.1) were then added. The Fischer Porter
bottle was heated to 298 K and then pressurized with 3 bars of
dihydrogen. After 18 h, a homogenous brown colloidal solution is
obtained. The volume of the solution was reduced to approximately
1
0 mL by solvent evaporation before its transfer onto a solution
of deoxygenated pentane (100 mL). A brown precipitate formed
which was filtered and dried in vacuum, giving rise to the nanopar-
ticles as a dark brown powder. The molar of [L]/[Ru] was varied
from 0.1 to 0.5 taking account previous results with diphenylphos-
phinobutane [14b]. In all cases, these ruthenium colloids were
found to be stable with time under argon atmosphere without
precipitation after a few weeks. Elemental Analysis results for the
different Ru(0)NPs prepared with the molar ratio [L]/[Ru] = 0.1 are:
L1 (39.1 wt% Ru); L2 (38.9 wt% Ru) L3 (38.6 wt% Ru).
2
.4. Quantification of hydrides at the surface of ruthenium
2
. Materials and methods
nanoparticles
2
.1. Reagents and general procedures
The quantification of hydrogen atoms adsorbed onto the sur-
face of Ru nanoparticles by GC analyses was carried out on
aqueous colloidal solutions following a previously described pro-
cedure [14b]. On each fresh colloidal solution, 5 equivalents of
olefin (2-norbornene), previously filtered through alumina, were
added. Samples were taken from the solutions for GC analysis
and estimation of the norbornene conversion into norbornane. The
measurement of the amount of alkane formed by GC analysis allows
to determine the necessary quantity of hydrogen atoms for reduc-
ing the alkene, and further to calculate the H/Rus ratio considering
the nanoparticles mean sizes.
All operations concerning nanoparticles syntheses were
carried out in Schlenck or Fischer–Porter glassware using
vacuum-argon lines techniques or in glove-box under
argon atmosphere. The organometallic complex used as pre-
cursor, (1,5-cyclooctadiene)(1,3,5-cyclooctatriene)ruthenium
0) complex
Nanomeps-Toulouse.
a
(
([Ru(COD)(COT)])
Sulfonated
was
purchased
from
(1,4-bis
diphosphines
[
(di-m-sulfonatophenyl) phosphino]butane (L1), 1,4-bis [(di-
m-sulfonatophenyl)phosphino]propane (L2), and 1,4-bis
(di-m-sulfonatophenyl)phosphino]ethane (L3)) used as stabilizing
[
For that study, gas chromatography was performed using
ligands were synthesized following a published procedure [17].
Solvents were dried and distilled before use: tetrahydrofu-
rane (THF) over sodium-benzophenone and pentane over calcium
hydride. All reagents and solvents were degassed before use by
means of three freeze–pump–thaw cycles. Water was distilled
twice by conventional method before use to prepare nanoparti-
cles suspensions. Tetradedecene, styrene and acetophenone used
as substrates in catalysis were purchased from Acros Organics or
Sigma–Alfa Aesar and used without further purification.
a
HP 5890 Series II Gas Chromatograph with
a SGE BP1
non polar 100% dimethyl polysiloxane capillary column of
5
at 40 C and a ramp of 8 C min until 250 C. Conversions were
determined as following: the peak area of the norbornane divided
by the sum of the peak area of norbornene and norbornane.
0 m × 0.32 mm × 0.25 ␮m. The method used consists of 15 min.
◦
◦
−1
◦
2
.5. Catalytic hydrogenation reactions
For styrene and tetradecene hydrogenation reactions gas chro-
2
.2. Characterization techniques
matography analyses were performed using a Carlo Erba GC 6000
with an FID detector equipped with a Factor Four column (30 m,
0.25 mm i.d.). Parameters were as follows: initial temperature,
Samples for TEM/HRTEM analyses were prepared by slow
◦ ◦ −1
40 C; initial time, 10 min; ramp, 10 C min ; final temperature,
evaporation of a drop of crude colloidal solution deposited onto
holey carbon-covered copper grids under argon (in glove-box) for
THF solutions and under air for aqueous solutions. Transmission
Electron Microscopy at low (TEM) and high resolution (HRTEM)
analyses were performed at the “Service Commun de Microscopie
Electronique de l’Université Paul Sabatier” (UPS-TEMSCAN). TEM
images were obtained using a JEOL 1011 electron microscope oper-
ating at 100 kV with resolution point of 4.5 A˚ . HRTEM observations
were carried out with a JEOL JEM 2010 electron microscope working
◦
◦
80 C; final time, 30 min; injector temperature, 220 C; detector
◦
temperature, 250 C. For acetophenone hydrogenation reactions
analysis a Trace GC Ultra (ThermoFisher) with a FID detector
equipped with a TR5 column (30 m, 0.25 mm i.d.). Selectivities are
stated as following: the peak area of the selected product (styrene,
ethylbenzene, ethylcyclohexane, tetradecane, phenylethan-1-ol,
cyclohexylethan-1-ol) divided by the sum of the peak area of all
the products formed.

3
6
M. Guerrero et al. / Catalysis Today 183 (2012) 34–41
Scheme 1. Synthesis of alkyl sulfonated diphosphine-stabilized Ru(0)NPs.
Table 1
2.5.1. Atmospheric hydrogenation reactions
Mean diameters of the RuNPs according to the ligand and the [L]/[Ru] molar ratio.
A 25 mL round bottom flask, charged with 10 mL of the water
colloidal suspension of Ru(0)NPs stabilized with sulfonated diphos-
phines L1, L2 or L3 and the appropriate amount of the organic
substrate (styrene, tetradecene, acetophenone; 0.45, 1 and 0.45 mL,
respectively) providing a molar ratio [Substrate]/[Metal] = 100, was
connected to a gas burette (500 mL) and a flask to balance pressure.
Then, the system was filled with hydrogen (PH2 = 1 bar) and the mix-
ture was magnetically stirred at 1500 rpm. Samples were removed
from time to time (1 h, 20 h and 40 h) to monitor the reaction by
gas chromatography in previously mentioned conditions.
RuNPs systems
[L]/[Ru] molar
ratio
Mean size in
THF, nm (SD)
Mean size in
H2O, nm (SD)
Ru/L1
0.1
0.2
1.51 (0.18)
1.27 (0.15)
1.25 (0.16)
1.48 (0.19)
1.26 (0.14)
1.23 (0.17)
1.50 (0.19)
1.23 (0.16)
1.23 (0.14)
1.50 (0.20)
1.29 (0.18)
1.26 (0.21)
1.50 (0.21)
1.24 (0.20)
1.23 (0.24)
1.52 (0.23)
1.24 (0.22)
1.27 (0.21)
0.5
Ru/L2
Ru/L3
0.1
0
0
.2
.5
0.1
0
0
.2
.5
2.5.2. High pressure hydrogenation reactions
The stainless steel autoclave was charged with the aque-
Whatever the alkyl diphosphine used, the mean size of the par-
ticles tends to slightly decrease with the increase of the [L]/[metal]
molar ratio from 0.1 to 0.2 but does not change in the presence
of more ligand ([L]/[Ru] = 0.5). In addition, no significant difference
in the mean sizes of the particles depending on the ligand used for
their stabilization was observed although small changes are present
in their backbones. In consequence, the key-parameter for the con-
trol of the mean size of the particles seems to be more related to
the [L]/[metal] molar ratio than the alkyl chain between the two
phosphorus atoms (see L, Scheme 1).
ous suspension of the sulfonated diphosphine-stabilized Ru(0)
nanoparticles and a magnetic stirrer. The investigated substrate
was added into the autoclave and dihydrogen was admitted to the
system at constant pressure (up to 10 bar). Samples were removed
from time to time (1 h, 20 h and 40 h) to monitor the reaction by
gas chromatography in previously mentioned conditions.
3
. Results and discussion
3
.1. Synthesis and characterization of alkyl sulfonated
Afterwards, the nanoparticles were isolated as a black solid
from the THF solution by precipitation with pentane addition, dried
under vacuum, and easily redispersed into water leading to stable
aqueous colloidal solutions for at least up to three months. The
transfer into water did not induce any relevant change in disper-
sion and in their mean diameters as confirmed by TEM analysis
(Fig. 2, Table 1).
High-resolution transmission electron microscopy (HRTEM)
analysis performed from colloidal solutions in water revealed that
the Ru nanoparticles are well-crystallized. They are single crystals
with reticular planes extending over the entire particle as pre-
sented in Fig. 3 for the Ru/L1 system with a mean size of 1.5 nm
as previously determined by TEM. The obtained images show
some nanoparticles which are well-oriented and appear crystalline
(Fig. 3) and for which we could measure through Fast Fourier Trans-
form (FFT) the lattice parameters. The measured lattice parameters
appear very close to the theoretical ones expected for the hcp
structure of ruthenium bulk. Moreover, Energy-dispersive X-ray
spectroscopy (EDX) analysis shows a characteristic pattern of
metallic ruthenium.
diphosphine-stabilized ruthenium nanoparticles
The
pared by hydrogenation of the organometallic precursor
Ru(COD)(COT)] (COD = 1,5-cyclooctadiene; COT = 1,3,5-
ruthenium
nanoparticles
(RuNPs)
were
pre-
[
cyclooctatriene) in THF at room temperature (Scheme 1) in
the presence of alkyl sulfonated diphosphine ligands (1,4-
bis [(di-m-sulfonatophenyl) phosphino]butane (L1), 1,4-bis
[
[
(di-m-sulfonatophenyl)phosphino]propane (L2) and 1,4-bis
(di-m-sulfonatophenyl)phosphino]ethane (L3)) in different [lig-
and]/[metal] molar ratio (0.1, 0.2 or 0.5) following a previously
described procedure [18]. The choice of the [ligand]/[metal] molar
ratios was made on the basis of previous results obtained with
alkyl diphosphines [14b].
All the Ru(0)NPs obtained in this way were found to be stable
for at least three weeks in THF, demonstrating the efficiency of the
three alkyl sulfonated diphosphines in stabilizing the Ru(0) nan-
oclusters. Analysis of the THF colloidal solutions by transmission
electron microscopy (TEM) revealed the presence of small parti-
cles (Fig. 1) with a mean size in the range 1.2–1.5 nm (Table 1),
which are well-dispersed on the grid.
In order to study the mean size of the Ru nanoparticles directly
in solution, dynamic light scattering experiments (DLS) of aqueous

M. Guerrero et al. / Catalysis Today 183 (2012) 34–41
37
Fig. 1. TEM images of Ru(0)NPs stabilized with L1 (a), L2 (b) and L3 (c) in the [L]/[Ru] ratio of 0.1 from THF colloidal solutions.
colloidal solutions have been performed [19]. DLS experiments are
used for the accurate and fast size measurement of hydrodynamic
diameter and aggregation effects of nanoparticles samples. These
are important parameters for the latter application of the NPs
in liquid phase catalysis. The hydrodynamic diameter takes into
consideration the metallic core but also the shell of ligand present
around the nanoparticles. It is thus a good method to know if the
ligand is present at the surface of the particles and if they are
agglomerated or not in solution. The obtained results show a good
correlation between the values obtained by TEM (after evaporation
of a drop of the colloidal solution on the grid) and DLS (directly in
solution) as presented in Fig. 4 for the Ru/L1 system. These results
indicate that the nanoparticles are isolated and do not agglomerate
in solution. In addition, they show that the ligand is present under
the form of a shell surrounding the metallic core. The presented
system displays a mean size of 1.8 nm by DLS against 1.5 nm by
TEM (see Table 1), which allows us to estimate the ligand shell
thickness to ca. 0.15 nm.
of norbornene. This reaction allows the titration of surface hydrides
because no extra hydrogen is added to the reaction mixture [14b].
The measurement of the amount of alkane formed by GC analy-
sis allows to determine the necessary quantity of hydrogen atoms
for reducing the alkene, and further to calculate the H/Rus ratio
considering the nanoparticles mean sizes. This methodology was
applied for aqueous colloidal solutions and revealed 1.6, 1.1, and
1.0 hydrogen atom per surface ruthenium atom for the Ru/L1,
Ru/L2, and Ru/L3 nanoparticles, respectively. These values indicate
that the nanoparticles are able to activate dihydrogen. They also
show a trend L1 > L2 ≈ L3 and a significant difference for the Ru/L1
nanoparticles system which have more hydrides at their surface.
It appears thus that a longer alkyl chain is more favorable for the
presence of a higher quantity of hydrides at the Ru(0)NPs surface.
In summary of this chapter, various alkyl sulfonated
diphosphine-stabilized Ru(0) NPs were reproducibly prepared.
Their characterization indicates small sizes, well-crystallized nan-
oclusters as well as the presence of hydrides at their surface. More
importantly, it is obvious that the alkyl sulfonated diphosphine
ligands allow their easy dissolution into water giving rise to stable
aqueous colloidal solutions at least up to several months. For these
As ruthenium nanoparticles are prepared under dihydrogen
atmosphere the presence of hydrides at their surface can be
expected. This was confirmed by the hydrogenation model reaction

3
8
M. Guerrero et al. / Catalysis Today 183 (2012) 34–41
Fig. 2. TEM images of Ru(0)NPs stabilized with L1 (a), L2 (b) and L3 (c) in the [L]/[Ru] molar ratio of 0.1 from aqueous colloidal solutions.
reasons, these new ruthenium nanoparticles provide potential
nanocatalysts for hydrogenation reactions. That is why they were
further investigated in hydrogenation of unsaturated substrates in
biphasic liquid–liquid conditions.
Whatever the ligand L1, L2 and L3 used, similar selectivities
were obtained after a reaction time of 20 h with approximatively
a ratio 50/50 in EB/EC (Table 2; entries 2, 5 and 8) and 100% EC
selectivity in 40 h (Table 2, entries 3, 6 and 9) with 100% styrene
conversion. In contrast, the influence of the alkyl chain length (n = 4,
3 or 2 carbon atoms between the two phosphorus atoms for L1, L2,
L3, respectively) was significantly observed after 1 h. In these later
conditions, the best catalytic results were obtained for the longer
alkyl chain (L1) and the reactivity decreases in the following order:
L1 > L2 > L3 (Table 2, entries 1, 4 and 7). As all the Ru(0)NPs dis-
play similar mean sizes, the observed differences in their catalytic
properties can be correlated to the difference in the flexibility of
the alkylchain. A higher flexibility is expected when the number of
carbon atoms increases which could induce an easier diffusion of
the substrate towards the metal surface. As a consequence of the
flexibility of the ligands, a dynamic coordination mode of the alkyl
sulfonated diphosphine at the surface of the particle in aqueous
solution could be also considered. Mono- or bi-dentate coordina-
tion or a mixture of the two modes could take place on Ru(0)NPs.
When a monodentate coordination mode is plausible, particularly
with the more flexible L1 protective agent, the steric hindrance
decreases on the surface of the particles and therefore access of
3.2. Catalytic hydrogenation reactions
3.2.1. Styrene hydrogenation under atmospheric hydrogen
pressure
In a first approach, performances of L1, L2 and L3 sulfonated
diphosphine-stabilized Ru(0) nanoparticles used as nanocatalysts
were evaluated in the hydrogenation of styrene as a pertinent
model substrate due to an easily reducible exo C C double bond
and an aromatic ring. Table 2 reports the catalytic data obtained
◦
at 20 C under H2 at atmospheric pressure (molar ratio [L]/[Ru] of
0
.1). The influence of the alkyl chain length of the ligand in the dif-
ferent Ru(0)NPs used as nanocatalysts on the conversion of styrene
ST) and the selectivity in ethylbenzene (EB)/ethylcyclohexane (EC)
(
was studied. The selectivity of the reaction was evaluated accord-
ing to the EB vs. EC ratios. It is noteworthy that at the chosen molar
ratio [L]/[Ru] of 0.1, all Ru(0)NPs have a mean diameter of 1.5 nm
in water (Table 1).

M. Guerrero et al. / Catalysis Today 183 (2012) 34–41
39
Fig. 4. DLS analysis of Ru(0)NPs stabilized with L1 ([L1]/[Ru] = 0.1).
in flexibility of the ligand at the surface of the particles at higher
temperature.
3.2.2. Styrene hydrogenation under 10 bar hydrogen pressure
and preliminary recycling tests
In a second set of experiments, the influence of the hydrogen
pressure on the conversion and the selectivity in ethylbenzene (EB)
vs. ethylcyclohexane (EC) ratios was studied according to the choice
of the sulfonated diphosphine L1, L2 and L3 (Table 3) in a molar ratio
Fig. 3. HRTEM micrograph of Ru(0)NPs stabilized with L1 ([L1]/[Ru] = 0.1).
[
L]/[Ru] of 0.1. In all cases, a total conversion of the styrene into EB
the substrate increases providing a better reactivity and a more
efficient hydrogenation at the beginning of the reaction. Finally,
the more lipophilic character induced by a longer alkyl chain could
also increase the contact with the substrate and therefore favor the
catalysis.
and EC mixture was obtained in 2 h in comparison with 20 h under
milder conditions (1 bar H2 and 20 C, Table 2). Independently to
◦
the choice of the sulfonated diphosphine L1, L2 or L3, the hydro-
gen pressure dramatically impacts the selectivity properties of the
catalytic systems. The best performances in terms of selectivity is
again achieved with the sulfonated diphosphine L1, with 97% of EC
in 1 h (Table 3, entry 13) against 90 and 50% for L2 and L3 systems
(Table 3, entries 17 and 19).
◦
The best Ru/L1 nanocatalyst was also investigated at 50 C to
evaluate the influence of the temperature on the catalytic per-
formances (Table 2, entries 10–12). The ratio ST/EB significantly
increases from 25/75 to 10/90 in 1 h according to the increase of
Finally, more drastic reaction conditions such as an increase of
◦
◦
◦
the temperature from 20 C to 50 C and the ethylcyclohexane for-
mation (18%) is already observed after a reaction time of 2 h. Finally,
the styrene was completely converted in ethylcyclohexane (EC) in
temperature to 50 C (Table 2, entry 12) or of hydrogen pressure
up to 10 bar (Table 3, entry 14) similarly modify the behaviour of
the catalytic suspensions allowing total reduction of the aromatic
ring in respectively 3 and 2 h without visible formation of aggre-
gates. Consequently, the successive hydrogenation of styrene was
preliminary carried out with L1-stabilized ruthenium nanoparti-
cles (Table 3, entries 15 and 16). No loss of activity was observed
3
h (Table 2, entry 12) in comparison with 40 h at room temperature
(
Table 2, entry 3). These experiments display the significant influ-
ence of the temperature on the catalytic performances in terms of
conversion and selectivity which could be attributed to the increase
Table 2
a
Styrene hydrogenation under atmospheric hydrogen pressure catalyzed by alkyl sulfonated diphosphine-stabilized Ru(0)NPs: influence of the ligand.
.
◦
Entry
Sulfonated diphosphine L
T ( C)
Time (h)
1
20
40
1
20
40
1
20
40
Products selectivity (%)
ST EB
25 75
EC
1
2
3
4
5
6
7
8
9
L1
L1
L1
L2
L2
L2
L3
L3
L3
20
20
20
20
20
20
20
20
20
0
0
0
45
3
55
97
1
53
98
1
59
0
0
40
47
2
75
0
0
24
41
1
59
99
10
11
12
L1
L1
L1
50
50
50
1
2
3
10
0
0
90
82
0
0
18
100
a
Reaction conditions: [L]/[Ru] = 0.1; ruthenium (3.9 × 10−5 mol), styrene (3.9 × 10−3 mol), 1 bar H2, water (10 mL).

4
0
M. Guerrero et al. / Catalysis Today 183 (2012) 34–41
Table 3
Styrene hydrogenation under 10 bar H2 catalyzed by sulfonated diphosphine-stabilized Ru(0)NPs: influence of the sulfonated diphosphine ligand (L) on the EB/EC ratio.a
Entry
Sulfonated diphosphine L
Time (h)
Conversion (%)
Products selectivity (%)
EB
EC
97
99
100
100
90
100
50
99
13
14
15
16
17
18
19
20
L1
L1
L1
L1
L2
L2
L3
L3
1
2
2
2
1
2
1
2
100
100
100
100
100
100
100
100
3
1
0
0
10
0
b
c
50
1
a
b
c
Reaction conditions: [L]/[Ru] = 0.1, ruthenium (3.9 × 10 mol), styrene (3.9 × 10−3 mol), RT, water (10 mL)
−5
Recycling of 14.
Recycling of 15.
Table 4
Styrene hydrogenation under atmospheric hydrogen pressure catalyzed by sulfonated diphosphine-stabilized Ru(0)NPs: influence of the [L3]/[Ru] ratio on the catalytic
a
activity.
Entry
[L3]/[Ru] molar ratio
Time (h)
Conversion (%)
Products selectivity (%)
EB
EC
21
22
23
24
25
26
0.1
0.1
0.2
0.2
0.5
0.5
1
20
1
20
1
25
100
26
100
26
24
41
26
74
24
84
1
59
0
26
2
20
100
16
a
Reaction conditions: Ruthenium (3.9 × 10−5 mol), Styrene (3.9 × 10−3 mol), RT, 1 bar H2, water (10 mL).
during the second and third runs with the conversion of styrene into
can be explained by a limited access of the aromatic substrate
to the nanoparticles surface due to increased sterical hindrance
in the presence of more ligand. However, as the ruthenium col-
loidal suspensions are always stable after 20 h of reaction, we could
presume a complete conversion of styrene into ethylcyclohexane
for longer reaction times. Thus, a molar ratio of 0.1 seems to be a
good compromise between stabilisation and catalytic activity of the
particles.
1
00% ethylcyclohexane in 2 h also, thus justifying the stability and
the reusability of the aqueous suspension of the alkyl sulfonated
diphosphine-stabilized Ru(0) nanoparticles.
3.2.3. Study of the influence of the [L]/[Ru] metal ratio on styrene
hydrogenation at atmospheric hydrogen pressure
To study the influence of the sterical hindrance at the surface of
the particles on the catalytic activity of the aqueous colloidal sus-
pension, various [L3]/[Ru] ratios (0.1, 0.2 and 0.5) were investigated
under atmospheric hydrogen pressure and up to 20 h of reaction
time (Table 4). Despite a lower reactivity (Table 2), nanocata-
lysts prepared with ligand L3 were chosen for this study as it
is the less flexible ligand thus limiting more the substrate diffu-
sion towards the active surface. Whatever the quantity of ligand
used for the synthesis of the nanocatalysts, a styrene conver-
sion of ca. 25% in ethylbenzene (EB) is observed after 1 h with
only traces of ethylcyclohexane (EC) (Table 4; entries 21, 23,
3.2.4. Tetradecene and acetophenone hydrogenation
Finally, preliminary catalytic investigations were performed in
the hydrogenation of other unsaturated model substrates contain-
ing a C C or C O bond for olefin and ketone respectively with the
aqueous suspension of the sulfonated diphosphine L1-stabilized
Ru(0) nanoparticles in a molar ratio [L1]/[Ru] of 0.1 (Table 5).
First, the tetradecene as an olefinic substrate was studied under
1 bar H . In opposition to the previously observed hydrogena-
2
tion of the exo C C bond of the styrene (75% after 1 h; entry 1,
Table 2), the presence of the long alkyl chain of the tetradecene
tends to decrease the olefin hydrogenation as only 7% of conver-
sion are observed in the same reaction time (Table 5, entry 27).
This result could be explained by the very low water-solubility
of the tetradecene substrate which limits the accessibility of the
reducible C C double bond to the active sites at the surface of
the particles which are in water. Moreover, the obtained satu-
rated tetradecane gradually forms an unsoluble phase in which
the substrate is more significantly soluble providing a two phase
2
5). However, all Ru(0) suspensions lead to a total conversion
of the substrate in mixtures of ethylbenzene/ethylcyclohexane
Table 4, entries 22, 24, 26) of various ratios in 20 h. Nevertheless,
(
when the [L3]/[Ru] ratio increases from 0.1 to 0.5, the propor-
tion of EC significantly decreases from 59 to 16%, respectively
(
Table 4, entries 22–26). According to the fact that the mean
size of the RuNPs does not pertinently change in the presence of
more diphosphine ligand (see Table 1, from 1.51 to 1.25 nm with
[L3]/[Ru] = 0.1 to [L3]/[Ru] = 0.2 or 0.5), the difference in selectivity
Table 5
a
Hydrogenation of tetradecene and acetophenone with L1-stabilized Ru(0) NPs.
Entry
Substrate
P (H2 bar)
Time (h)
Products selectivity (%)
27
28
29
Tetradecene
Tetradecene
Tetradecene
1
1
1
1
20
40
Tetradecane (7)
Tetradecane (78)
Tetradecane (100)
30
31
32
Acetophenone
Acetophenone
Acetophenone
1
1
10
2
20
2
Phenylethan-1-ol (18)
Phenylethan-1-ol (84)/cyclohexylethan-1-ol (15)
Cyclohexylethanol (100)
a
Reaction conditions: [L1]/[Ru] = 0.1, ruthenium (3.9 × 10−5 mol), substrate (3.9 × 10−3 mol), RT, water (10 mL).

M. Guerrero et al. / Catalysis Today 183 (2012) 34–41
41
mixture, which is not convenient for a conversion increase in the
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The authors thank L. Datas (LCC-CNRS and UPS-TEMSCAN) for
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