Olefin hydrogenation by ruthenium nanoparticles in ionic liquid media: Does size matter?

Article abstract of DOI:10.1016/j.jcat.2010.07.018

Tailor-made and size-controlled ruthenium nanoparticles, RuNPs, of three distinct sizes between 1 and 3 nm are generated from the decomposition of (η⁴-1,5-cyclooctadiene)(η⁶-1,3,5-cyclooctatriene) ruthenium(0) [Ru(COD)(COT)], under H₂ in 1-butyl-3-methylimidazolium bis(trifluoromethanesulphonyl)imide, C₁C₄ImNTf ₂, by simply varying experimental conditions. Catalytic hydrogenation of 1,3-cyclohexadiene, CYD, and cyclohexene, CYE, in C₁C ₄ImNTf₂, has been used as a probe for the relationship between size and catalytic performance (activity and selectivity) of RuNPs. To allow comparison between different reactions, all catalytic reaction mixtures were diligently prepared in order that the parameters such as substrate/catalyst and substrate/ionic liquid ratio, and therefore, viscosity and mass transport factors remained constant. It was found that the catalytic activity increases with the NP size, while high selectivity is only observed with the smaller NPs. In addition, the studied RuNPs exhibit a high level of recyclability with neither loss of activity nor significant agglomeration.

Full text of DOI:10.1016/j.jcat.2010.07.018

Journal of Catalysis 275 (2010) 99–107
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Olefin hydrogenation by ruthenium nanoparticles in ionic liquid media: Does
size matter?
a
a,*
a
a
b
Paul S. Campbell , Catherine C. Santini , François Bayard , Yves Chauvin , Vincent Collière ,
c
c
d
Ajda Podgoršek , Margarida F. Costa Gomes , Jacinto Sá
a
Université de Lyon, Institut de Chimie de Lyon, UMR 5265 CNRS-Université de Lyon-ESCPE Lyon, C2P2, Equipe Chimie Organométallique de Surface,
ESCPE 43 Boulevard du 11 Novembre 1918, F-69616 Villeurbanne, France
Laboratoire de Chimie de Coordination, UMR CNRS, 205, route de Narbonne, 31077-Toulouse cedex 04, France
Laboratoire de Thermodynamique des Solutions et des Polymères, Université Blaise Pascal, Clermont-Ferrand, 24 Avenue des Landais, 63177 Aubière, France
CenTACat, School of Chemistry and Chemical Engineering, Queen’s University, Stranmillis Road, Belfast, Northern Ireland BT9 5AG, UK
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Tailor-made and size-controlled ruthenium nanoparticles, RuNPs, of three distinct sizes between 1 and
Received 1 June 2010
Revised 19 July 2010
Accepted 21 July 2010
4
6
3
nm are generated from the decomposition of
(
g
-1,5-cyclooctadiene)(
g -1,3,5-cyclooctatriene)
ruthenium(0) [Ru(COD)(COT)], under H
2
in 1-butyl-3-methylimidazolium bis(trifluoromethanesulpho-
nyl)imide, C
hexadiene, CYD, and cyclohexene, CYE, in C
1
C
4
ImNTf
2
, by simply varying experimental conditions. Catalytic hydrogenation of 1,3-cyclo-
ImNTf , has been used as a probe for the relationship
C
1 4
2
between size and catalytic performance (activity and selectivity) of RuNPs. To allow comparison between
different reactions, all catalytic reaction mixtures were diligently prepared in order that the parameters
such as substrate/catalyst and substrate/ionic liquid ratio, and therefore, viscosity and mass transport
factors remained constant. It was found that the catalytic activity increases with the NP size, while high
selectivity is only observed with the smaller NPs. In addition, the studied RuNPs exhibit a high level of
recyclability with neither loss of activity nor significant agglomeration.
Keywords:
Nanoparticles
Ruthenium
Ionic liquids
Size effect
Selective hydrogenation
Ó 2010 Elsevier Inc. All rights reserved.
1
. Introduction
Transition-metal nanoparticles (NPs) of 1–10 nm in size exhibit
preferentially dissolved in polar domains and non-polar
compounds in non-polar ones [15]. The non-polar organometallic
complex, Ru(COD)(COT), is expected to be concentrated in the
non-polar domains of ILs. Therefore, the phenomenon of crystal
growth is controlled by the local concentration of Ru(COD)(COT)
and consequently limited to the size of the non-polar domains.
These play the role of nanoreactors in which the size of ruthenium
nanoparticles generated in situ can be controlled [13,16,17].
Using NPs formed in situ in ILs directly in catalysis offers the
opportunity to exploit the distinct physicochemical and solvation
properties of these media, resulting in unique activities and selec-
tivities [12,18–21].
When comparing the catalytic activity of differently sized nano-
particles, it is important to maintain constant all other possible
variables. Indeed, differences in catalytic activity could have a
physicochemical origin [22,23], resulting from peculiar solvation
phenomena including specific interactions between the IL and
physicochemical properties intermediate to those of the smallest
element from which they can be composed and those of the bulk
material [1–3]. In catalysis, the performance (activity and selectiv-
ity) of NPs is often said to be related to their size, as this controls
the number of corner, edge and face atoms available for adsorption
and activation of substrates [4–9]. However, the synthesis of nano-
particles (NPs) with a controlled size in the range of 1–10 nm in
order to corroborate this theory is still a challenging issue [10].
The use of ionic liquids (ILs) in NP synthesis has recently be-
come a popular route. The major advantage is that stabilising addi-
tives such as ligands, polymers and supports are not required. Also,
we can tune the IL moieties and reaction conditions to obtain
monodisperse and catalytically active NPs of controlled size
[
7,11–13]. This is because imidazolium-based ILs exhibit a 3-D
organisation in the liquid state due to an extended hydrogen-bond
network of ionic channels, coexisting with non-polar domains cre-
ated by the grouping of lipophilic alkyl chains. Consequently, ILs
present specific solvation properties [14]. Polar substrates are
the substrate (H bonds, cation-p) [24–26], mass transfer factors
(viscosity, diffusivity) [22,27], and effects attributed to the highly
structured nature of ILs [14,28,29].
It has been shown that it is possible to obtain ruthenium nano-
particles, RuNPs, of differing sizes from the decomposition of Ru
(
1 4 2
COD)(COT) in C C ImNTf , by simply varying the experimental
*
Corresponding author. Fax: +33 472431795.
E-mail address: santini@cpe.fr (C.C. Santini).
conditions [16]. In this work, we use the catalytic hydrogenation
0
021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.07.018

1
00
P.S. Campbell et al. / Journal of Catalysis 275 (2010) 99–107
of 1,3-cyclohexadiene, CYD, and cyclohexene, CYE, as probes for
the relationship between size and catalytic performance of tailor-
made and size-controlled RuNPs, generated in the ionic liquid, all
other physicochemical variables being constant.
carbon). Binding energies were determined with an accuracy of
±0.2 eV. The data were analysed using Casa-XPS (v 2.3.13) employ-
ing a Shirley background subtraction prior to fitting and a peak
shape with a combination of Gaussian and Lorentzian (30%
Lorentzian). High-resolution spectra were acquired in the region
of Ru 3p as the Ru 3d region overlaps with the C 2p region of the
residual ionic liquid.
2
. Experimental
2.1. Materials and methods
2.5. Preparation of catalytic experiments
All operations were performed in the strict absence of oxygen
and water under a purified argon atmosphere using glovebox (Jac-
omex or MBraun) or vacuum-line (Schlenk) techniques. The ionic
In Table 1 are collected all data concerning the studied hydroge-
nation reaction. In columns 1–3, size and dispersion values of
(
0
Ru ), (Ru25) and (Ru50) are reported.
1 4 2
liquid, C C ImNTf [30], and the complex, [Ru(COD)(COT)] [31],
Each solution of NP was produced from the decomposition of
were synthesised as reported. The halide content of the ionic liquid
was under 200 ppm (E.A.) and water under 5 ppm (limit of Karl
Fischer titration). Elemental analyses were performed at the CNRS
Central Analysis Department of Solaize.
ꢀ1
4
2
3.0 mmol L solution of Ru(COD)(COT) as described in Section
.2 and shown in column 4.
For each different temperature of decomposition, a different
size of NP is obtained: at 0 °C, 1.1 nm (Ru
0
); at 25 °C, 2.3 nm
Ru25); and at 50 °C, 2.9 nm (Ru50), shown in columns 1 and 2.
The value of dispersion, D, describing the ratio of surface sites
to total number of ruthenium atoms, varies with the NP size
1
-Methylimidazole (>99%) was purchased from Aldrich and dis-
(
tilled prior to use. Chlorobutane (>99%, Aldrich) and lithium bis(tri-
fluoromethanesulfonyl)imide (Solvionic) were used without
further purification.
Ru
s
and is given for each size of NP in column 3. Using this and the
ꢀ1
known concentration of ruthenium (43.0 mmol L ), it is possible
to calculate for each size of catalyst the concentration of Ru (col-
umn 5). For example, for Ru (1.1 nm), we have a dispersion of 82%
and therefore a Ru concentration of 35.2 mmol L
2
.2. Catalyst synthesis
s
4-1,5-cyclooctadiene)(
g
6-1,3,5-cyclooctatri-
0
A
solution of
ene)ruthenium(0) Ru(COD)(COT) (43 mmol L ) in the ionic liquid
-butyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide
ImNTf ) was transferred under argon to a glass autoclave,
(
g
ꢀ1
s
.
ꢀ
1
To prepare the catalytic mixtures, we must have the same num-
ber of catalyst sites. For this reason, we dilute the most concen-
1
(
C
1
C
4
2
trated solutions, i.e. Ru
0
and Ru25, to match the least
the temperature of which was controlled with aid of a thermostatic
bath (0, 25, 50 or 75 °C). Once stabilised, the argon atmosphere was
evacuated and replaced with molecular hydrogen (4 bar) without
stirring. The yellow solution turned black over time (up to 3 days)
as RuNPs were generated releasing cyclooctane (COA) as the only
by-product. The resulting solutions were treated under dynamic
concentrated, i.e. Ru50, by the addition of the appropriate amount
of pure IL.
ꢀ1
ꢀ5
Ru50 has a concentration of Ru
s
of 18.5 mmol L . A 5-mL sam-
ple of this solution therefore contains 9.52 ꢁ 10 mol of Ru
s
.
ꢀ1
Ru25 has a concentration of Ru
s
of 22.9 mmol L . A 3.74-mL
sample of this solution is therefore taken, containing
vacuum during a period of 6 h to remove all H
2
and cyclooctane.
ꢀ5
9
.52 ꢁ 10 mol of Ru
s
, and then diluted with 1.26 mL of pure IL
The black solutions could then be stored under argon atmosphere
with long-term stability (at least 6 months – no precipitation, coa-
lescence or agglomeration – verified by TEM).
to make a 5-mL solution.
Ru has a concentration of Ru
ꢀ1
ꢀ5
0
s
of 35.2 mmol L . A 2.58-mL
sample of this solution therefore contains 9.52 ꢁ 10 mol of Ru
s
diluted with 2.42 mL of pure IL to make a 5-mL solution (columns
–8).
To each of these 5-mL catalyst/IL solutions, weighing 7.0 g (col-
2
.3. Determination of particle size by TEM
6
Transmission electron microscopy (TEM) experiments were
umn 9), is added 0.78 g of CYD (column 10). This gives catalytic
mixtures with constant substrate/catalyst ratios of 105 (column
1) and constant substrate/IL ratios of 0.59 (column 12) ensuring
performed directly in the IL media. A thin film of RuNP solution
in IL was deposited on a carbon film supported by a copper grid.
Conventional TEM micrographs were obtained at the Centre Tech-
nologique des Microstructures, Université Claude Bernard Lyon 1,
Villeurbanne, France, using a Philips 120 CX electron microscope
with acceleration voltage of 120 kV. Size distribution histograms
were constructed from the measurement of at least 200 different
nanoparticles assuming a near spherical shape and random orien-
tation. High-resolution electron micrographs were obtained at the
1
identical viscosities and a single phase.
Using solutions prepared as described, the reaction is carried
out in parallel in several 0.5-mL batches under 1.2 bars of pure
molecular hydrogen, which are stirred and heated with the aid of
a thermostatic carousel, to ensure identical reaction conditions.
2
.6. Catalytic tests
‘
‘TEMSCAN” centre of the Université Paul Sabatier Toulouse 3, Tou-
louse, France, using a JEOL JEM 200CX electron microscope with
acceleration voltage of 200 kV.
Catalytic solutions were made as described in Section 2.5 in a
glove box and left stirring for 12 h in a closed system to ensure
homogeneity. Aliquots of 0.5 mL were transferred to identical
Schlenk tubes containing cross-shaped magnetic stirrer bars. The
argon atmosphere was removed, and the solution was degassed
in vacuo whilst cooling in liquid nitrogen (ꢀ196 °C). For reactions
at 30 °C, six of these Schlenk tubes were placed in a thermostatic
carousel to ensure identical temperature and stirring conditions.
After 30 min, when the temperature had stabilised, the Schlenk
2.4. XPS
X-ray photoelectron spectroscopy was performed in a Kratos
Axis Ultra DLD spectrometer, using a monochromated Al K X-
a
ray with a pass energy of 20 eV and a coaxial charge neutraliser.
The base pressure in the analysis chamber was better than
ꢀ
8
5
ꢁ 10 Pa. XPS spectra of Ru3p, C1s, Si2p and O1s levels were
tubes were opened to 1.2 bars of H
was isolated and opened to air, releasing the H
2
. After t minutes, a Schlenk tube
atmosphere thus
measured at a normal angle with respect to the plane of the sur-
face. High-resolution spectra were corrected for charging effects
by assigning a value of 284.6 eV to the C1s peak (adventitious
2
quenching the reaction. The solution was entirely dissolved in
10 mL of acetonitrile containing a 1 M concentration of toluene.

P.S. Campbell et al. / Journal of Catalysis 275 (2010) 99–107
101
Table 1
Calculations for the composition of the catalytic systems. Column 1 – name of catalyst, column 2 – average RuNP diameter measured by TEM, column 3 – calculated dispersion,
column 4 – initial Ru(COD)(COT) concentration, column 5 – consequent Ru concentration, column 6 – volume of IL/RuNP solution, column 7 – volume of pure IL added, column 8
consequent number of moles of Ru in the 5 mL mixture, column 9 – mass of IL, column 10 – mass of substrate, column 11 – substrate/catalyst ratio, column 12 – substrate/IL
s
–
s
ratio.
1
2
d
3
D
4
5
[Ru
6
7
8
9
10
11
CYD/Ru
s
12
CYD/IL
[Ru] (mmol Lꢀ1)
] (mmol L
ꢀ1
)
Vol. IL–RuNP
solution (mL)
Vol. IL pure (mL) Ru
/10 (mol) m (IL/g)
ꢀ5
m (CYD/g)
s
s
(nm) (%)
Ru
Ru25 2.3
Ru50 2.9
0
1.1
82
53
43
43.0
43.0
43.0
35.2
22.9
18.5
2.42
3.74
5.00
2.58
1.26
–
9.52
9.52
9.52
7.0
7.0
7.0
0.78
0.78
0.78
105
105
105
0.59
0.59
0.59
The composition of the mixture was determined by gas-phase
chromatography using toluene as the internal standard.
CYD may be partially hydrogenated with high selectivity by molec-
ular catalysts due to the reduced miscibility of cyclohexene (CYE)
in the medium [27,32,33]. Full hydrogenation would lead to cyclo-
hexane (CYA), Scheme 1.
2.7. Product quantification
3.2. Solubility
The products were quantitatively analysed by gas chromatog-
raphy on a HP-6890 chromatograph equipped with a flame ioni-
sation detector (FID) and a HP-1 (crosslinked methylsiloxane)
The solubilities of the substrate and potential products may
play an important role in the activity and/or selectivity of the sys-
tem. For example, selective hydrogenation of butadiene to butenes
has been performed by Dupont’s group in ionic liquids due to the
difference in solubility of the partially hydrogenated product
column (L: 30 m, int: 0.32 mm, film thickness: 0.25
injector and detector temperature was 270 °C, and the injection
volume was 1 L. The programme was as follows: initial temper-
ature 70 °C for 13.5 min; ramp 40 °C/min to 250 °C, hold 2 min.
lm). The
l
[
34]. The same group has also described the possibility of extract-
ing cyclohexene during benzene hydrogenation using this solubil-
ity difference [35]. For this reason, solubilities of CYD, CYE and CYA
are measured. It is found that the solubility of the hydrogenated
products (6 ± 1% wt – CYE, 4 ± 1% wt – CYA) is much lower than
that of CYD (12 ± 2% wt); therefore, the medium may tend to a
biphasic system during the course of the reaction. As a result, the
collection of aliquots from a single batch would render inaccurate
results. Consequently, each point recorded in this work corre-
sponds to a separate experiment, quenched after time t by opening
the reaction vessel to air, thus releasing the hydrogen and dissolv-
ing the catalytic system entirely in a 1 M solution of toluene in ace-
tonitrile for gas-phase chromatography.
2
.8. Density and viscosity
The mixtures of IL and CYD at different compositions were pre-
pared gravimetrically following the procedure already described
[
(
27]. The viscosity of the mixture was measured at 298.15 K
controlled to within ±0.005 K and measured with the accuracy
better than ±0.05 K) using a rolling-ball viscometer from Anton
Paar, model AMVn [27]. The overall uncertainty of the viscosity
is estimated as ±2.0%. The densities of the mixtures, necessary to
calculate the viscosities, were measured in an Anton Paar vibrating
tube densimeter model 512 P, at 298.15 K (measured by a cali-
brated PRT with an accuracy of ±0.02 K). The overall uncertainty
of the density is estimated as ±0.01%.
3.3. Viscosity
2.9. Solubility
Thermophysical properties of the reaction medium such as den-
sity and viscosity may also influence the catalytic performance. We
have recently demonstrated that reaction kinetics in IL media are
highly dependent on the mobility of molecules [27]. Consequently,
identical concentrations of substrate must be used in each case in
order to maintain constant viscosity and eliminate effects due to
mass transport. Furthermore, knowledge of the viscosity is very
important from engineering point of view as it plays a major role
in stirring, mixing and pumping processes. The densities and vis-
To measure the solubility, 1 mL of the substrate was stirred
with the ionic liquid in a closed system at 298.15 K for 12 h and
then left to settle for a further 2 h. A 0.1-mL sample of the ionic
liquid phase was weighed, and its composition was determined
by GC using the procedure described in Section 2.6. Tests were
repeated four times for each substrate to guarantee reproducibility.
3
. Results and discussion
1 4 2
cosities of the pure C C ImNTf and those of the mixtures with
CYD were measured at different molar ratio CYD/IL (R) at 25 °C
and atmospheric pressure. The results are presented in Table 2.
As can be seen, the viscosity of the mixtures of CYD in IL varies
greatly with the concentration of CYD. From the Stokes–Einstein
3
.1. Choice of substrate
The substrate investigated is the conjugated diene, 1,3-cyclo-
hexadiene (CYD). It has been shown that in ionic liquid media,
Table 2
Density,
q, and viscosity, g, of CYD–IL mixtures of different compositions. xIL = molar
fraction of IL, R = molar ratio CYD/IL.
(g cm 3
ꢀ
)
g (m Pa s)
H2
R
xIL
q
+
0
0
0
0
0
0
.000
.100
.200
.300
.397
.498
1.000
0.909
0.833
0.769
0.716
0.667
1.4376 ± 0.0001
1.4202 ± 0.0001
1.3999 ± 0.0001
1.3874 ± 0.0003
1.3718 ± 0.0001
1.3597 ± 0.0001
48.5 ± 0.4
44 ± 1
37.0 ± 0.4
33.3 ± 0.3
31.0 ± 0.3
24.8 ± 0.3
CYD
CYE
CYA
Scheme 1. 1,3-Cyclohexadiene and its hydrogenation products.

1
02
P.S. Campbell et al. / Journal of Catalysis 275 (2010) 99–107
160
relation, the diffusion coefficient, D, which reflects to the mobility
of molecules, varies inversely with g,
1
1
1
40
20
00
T = 0 °C
kT
D ¼ 6
ð1Þ
T = 25 °C
T = 50 °C
T = 75 °C
pgr
s
3
.4. Catalyst characterisation
8
6
4
0
0
0
It has been previously demonstrated that the size of RuNPs
generated from the decomposition of [Ru(COD)(COT)] under H
36], may be governed by the degree of self-organisation of the
2
[
imidazolium-based ionic liquid in which they are formed: the
more structured the ionic liquid, the smaller the size [16]. Follow-
ing previously described methods, RuNPs are synthesised at 0 °C,
20
0
2
5 °C, 50 °C and 75 °C in an attempt to obtain a selection of mono-
1
2
3
4
5
disperse sizes of RuNP in the same IL.
Size/ nm
3
.4.1. TEM
Fig. 2. Comparative size distribution histograms for RuNPs prepared in C
at different temperatures.
1
C
Im NTf
4 2
Analysis of the suspensions obtained by transition electron
microscopy allows the determination of the sizes generated:
.1 ± 0.2 nm, 2.3 ± 0.3 nm, 2.9 ± 0.4 nm and 3.1 ± 0.7 nm, for RuNPs
generated at 0 °C (Ru ), 25 °C (Ru25), 50 °C (Ru50) and 75 °C (Ru75),
interplanar distances match with the hcp crystalline phase of Ru
see Supplementary information). For Ru , only a larger NP of
ꢂ2 nm is observed by HREM, probably due to the difficulty in
observing the smallest NPs with limited contrast although may
be indicative of a lower degree of crystallinity in very small RuNPs,
as already observed by reverse Monte Carlo simulations.[37].
1
(
0
0
respectively, Fig. 1. As can be seen from the TEM image of Ru75 and
the consequent size distribution histogram Fig. 2, the size of these
NPs does not vary significantly compared to those of Ru50 although
a poorer size control (wider distribution) is apparent. For this rea-
son, these NPs are not used in catalytic tests. High-resolution elec-
tron microscopy reveals the crystalline nature of the RuNPs formed
through elucidation of the crystal planes. The Fourier transform
images of the HREM have been exploited and indicate that the
3.4.2. XPS
In order to establish the oxidation state of the RuNPs, X-ray
photoelectron spectroscopy (XPS) is performed. Due to the weak
0
Fig. 1. Transition electron micrograph of RuNPs and high-resolution electron micrograph examples showing crystallinity for Ru (top left), Ru25 (top right), Ru50 (bottom left)
and Ru75 (bottom right).

P.S. Campbell et al. / Journal of Catalysis 275 (2010) 99–107
103
Ru 3p1/2
Ru 3p3/2
Ru 3p1/2
Ru 3p3/2
Ru 3p3/2
Ru 3p1/2
490
480
470
460
450 490
480
470
460
450 490
480
470
460
450
Binding Energy / eV
Binding Energy / eV
Binding Energy / eV
2 0
Fig. 3. XPS spectra of the Ru 3p region after NP filtration onto SiO , experimental data and fitted peaks. Left Ru , middle Ru25 and right Ru50.
1
00%
concentration of the solution and the penetration limit of the
X-rays in the solution (maximum depth of 10 nm), no peaks corre-
sponding to Ru binding energies are observed when the analyses
are carried out directly on the RuNP/IL solutions. Samples are
therefore prepared by filtering the RuNPs onto silica under inert
atmosphere and eliminating as much IL as possible. The resulting
spectra of the Ru 3p region are depicted in Fig. 3. It is clear that
in each case, fine peaks are observed, indicating the presence of
only one Ru species. The low 3p3/2 binding energy observed in each
case, 460.3 eV, and doublet separation of 22.2 eV correspond clo-
sely to metallic zero-valent ruthenium, often reported with a
9
0%
0%
8
70%
6
5
4
0%
0%
0%
30%
2
0%
0%
1
3p3/2 binding energy of around 461 eV [38]. The small difference
0%
may be attributed to the presence of small crystallites, which tend
to exhibit lower binding energies than bulk metal. Indeed, as re-
cently shown for AuNPs [39], the d band narrows with decreasing
particle size and shifts towards the Fermi level.
0
0.5
1
1.5
2
2.5
3
3.5
Diameter / nm
Fig. 4. Curve of dispersion D against mean diameter of crystalline hcp RuNPs.
3
s
.5. Ru concentration
in several 0.5-mL batches under 1.2 bars of pure molecular hydro-
gen, which are stirred and heated with the aid of a thermostatic
carousel, to ensure identical reaction conditions.
Maintaining constant the initial ratio of substrate to catalyst is
imperative. In NP catalysis, as in heterogeneous catalysis, only the
atoms at the surface (Ru ) take part in reaction. The dispersion (D)
presents the ratio between surface atoms, Ru , and the total num-
ber of atoms, Ru , (D = Ru /Ru ) and varies with the size of the NPs,
s
s
3.7. Catalytic activity
T
s
T
smaller particles of course having a larger percentage of surface
atoms. The different dispersion values must therefore be taken into
account for each size of nanoparticle formed.
Ruthenium is known to exhibit a hexagonal close-packed crys-
tal structure, with the following lattice parameters: a: 270.59 pm,
It can be seen in Table 3 (Experiments 1–3) that the largest NPs
2.9 nm) are the most active in the hydrogenation of CYD. In a sim-
(
ilar fashion, the three sizes of RuNP are tested in the catalytic
hydrogenation of cyclohexene (CYE) to cyclohexane (CYA) to
establish whether a difference in activity is also apparent in the
case of a monoene. The results tabulated in Table 4 show that no
substantial difference in activity is observed unlike the case of
the conjugated diene CYD.
b: 270.59 pm, c: 428.15 pm, a: 90°, b: 90°, c: 120° [40]. Using these
parameters, SYBYL software can be applied to extrapolate the lat-
tice until the measured diameters in order to model the structure
of the different size NPs, assuming crystallinity. It is seen that crys-
talline hexagonal close-packed RuNPs would adopt a truncated
hexagonal bipyramid form, with two symmetric hexagonal faces
In the case of in the catalytic hydrogenation of cyclohexene
CYE) to cyclohexane (CYA), the TOF value for Ru is comparable
0
(
to that reported by Roucoux’s group with RuNPs (2.5 nm,
TOF = 34) in water in the presence of cyclodextrins [42], although
clearly here the experimental conditions are entirely different.
According to literature results, the catalytic activity of NPs
depends on their size and generally reaches a maximum for those
of around 3 nm [43,44]. Here, only a size effect on activity was
observed for the case of CYD. The fact that the hydrogenation of
CYD is faster with larger NPs can be related to two factors:
(
0 0 0 1) and 12 irregular and uneven trapezoid faces (1 0 (ꢀ1) 1)
[
41]. From these findings, a curve of D with respect to diameter
can be plotted and then used to estimate D for each size of nano-
particle, Fig. 4.
3.6. Catalysis mixture preparation
In Table 2 (Section 2.6) are collected all data concerning the
studied hydrogenation reaction. The experimental conditions are
established to ensure identical concentrations of Ru (column 8)
and substrate (columns 9 and 10), permitting as a result both a
constant substrate/catalyst ratio (column 11) and a constant sub-
strate/IL ratio (column 12) hence constant viscosity. Using solu-
tions prepared as described, the reaction is carried out in parallel
(1) Larger NPs present the appropriate number of neighbouring
s
surface sites to facilitate the
gated system [45].
p-bond activation of the conju-
(2) Through this
p-bonding activation similar to benzene, 1,3-
cyclohexadiene would lose part of its resonance energy
and react more readily [41].

1
04
P.S. Campbell et al. / Journal of Catalysis 275 (2010) 99–107
Table 3
Conversion, selectivity, turnover number and turnover frequency for the hydrogenation of CYD at 30 °C. Experiments 1–3 using 1.2 bar H
2
2
. Experiments 4 and 5 using 4 bar H .
ꢀ
Experiment number
Catalyst (nm)
Ru (1.1)
Ru25 (2.3)
Ru50 (2.9)
Pressure H
2
(bar)
Conversion at 90 min (%)
Selectivity CYE (%)
TON
TOF (h 1
)
Size after catalysis (nm)
1
2
3
4
5
0
1.2
1.2
1.2
4.0
4.0
66 ± 5
75 ± 5
83 ± 5
57 ± 5
73 ± 5
97
86
80
92
80
70 ± 5
79 ± 5
87 ± 5
59 ± 5
77 ± 5
46 ± 3
53 ± 3
58 ± 3
40 ± 3
51 ± 3
1.3 ± 0.4
2.1 ± 0.5
2.7 ± 0.5
–
Ru
0
(1.1)
Ru50 (2.9)
–
Table 4
Conversion, turnover number and turnover frequency for the hydrogenation of CYE at
the hydrogenation of 1,3-cyclohexadiene on metallic surfaces
46,47], and thus, rapid consecutive hydrogenation of both double
[
3
2
0 °C under 1.2 bar H .
bonds leading to the fully hydrogenated CYA may be envisaged. In
Fig. 6 are represented simplified SYBYL models of CYD molecules
coordinating to the surface of perfectly crystalline RuNPs of calcu-
lated average diameter 1.3 nm and 2.8 nm. This illustrates nicely
the greater facility of planar coordination to faces of the larger
NPs and could explain the lower selectivity of the larger RuNPs de-
spite identical reaction conditions. Likewise, in the hydrogenation
of 1,3-butadiene or 1-hexyne, the selectivity of small NPs towards
Experiment
number
Catalyst
(nm)
Conversion at 90 min
(%)
TON
TOF
(h 1)
ꢀ
6
7
8
Ru
0
(1.1)
61 ± 5
64 ± 5
67 ± 5
64 ± 5 43 ± 3
67 ± 5 45 ± 3
70 ± 5 47 ± 3
Ru25 (2.3)
Ru50 (2.9)
The coordination of monoenes such as CYE does not necessitate
large surfaces, explaining the less pronounced size effect in this
case.
1
-butene or 1-hexene versus butane or hexane is still higher than
that of larger NPs [45–48]. In this work, in the hydrogenation of
CYD, the selectivity in CYE versus CYA drops from 97% to 80% when
the RuNP size increases from 1.1 to 2.9 nm.
Our hypothesis is based on idealised particle shape, which is not
likely to exist in reality. Nonetheless, it is widely accepted that
large NPs are more likely to present larger open facets where pla-
3
.8. Catalytic selectivity
In the hydrogenation of CYD, CYE is obtained as the major prod-
uct. Interestingly, the selectivity for CYE diminishes with increas-
ing NP size. Indeed, for Ru , selectivity for CYE is 100% at low
conversion and only slightly diminishes at high conversion (97%).
In contrast, for Ru50, the hydrogenation is unselective even at
low conversion, Fig. 5.
Assuming highly crystalline particles with hcp structure, a par-
ticle of diameter 1.1 nm would have the vast majority of catalytic
surface atoms occupying vertex or edge positions. Such vertex
nar
p-coordination of diene substrates can occur, whereas small
0
NPs are often reported to be amorphous, therefore presenting no
open facets, making this planar coordination even less likely [41].
In studies of CO hydrogenation on RhNPs, the difference in reac-
tivity with size was related to the increasing probability of finding
step sites with increasing NP size [49–51]. However, for RuNPs of
less than 3 nm, as reported here, calculations have shown that such
step sites are not likely to exist [52].
The highly selective hydrogenation of 1,3-cyclohexadiene to
cyclohexene has also been performed with PdNPs in organic and
IL media [33,53,54]. The high selectivity results from the intrinsic
properties of Pd metal and its small NP size [55].
v 2
ruthenium atoms Ru , which under H atmosphere are ligated by
hydrides, may coordinate one C@C double bond of CYD. The prod-
uct of the subsequent hydrogenation is CYE, which must undergo a
second coordination to give the fully hydrogenated CYA. Similarly,
for a larger particle of average diameter 2.9 nm assuming high
crystallinity and an hcp structure, it is evident that most of the cat-
alytically active surface ruthenium atoms are found in facial posi-
3.9. Hydrogen effect
tions, Ru
observed by HREM, Fig. 1. Ru
the mechanism previously discussed, but due to the planar
arrangement of Ru , another mechanism may be envisaged involv-
f
. Indeed, here, such crystallinity has already been
According to the literature, the hydrogenation of olefins in ILs is
often biphasic in its nature [56–59], due to the poor solubility of
hydrogen and olefins in these media [60–63]. Therefore, the diffu-
f
may hydrogenate the olefin using
f
sion process of the substrate or H
hydrogenation.
2
may limit the rate of
ing the double coordination of the diene, as generally found during
To find out whether H
iments varying the H pressure are performed. Increasing H
sure to 4 bars in the case of Ru and Ru50 is seen to affect neither
2
is in fact a rate-limiting reagent, exper-
2
2
pres-
0
the activity nor selectivity in a substantial manner, Table 3, Exper-
iments 4 and 5. This is similar to results reported by Dupont’s
group who observed that the reaction rate does not depend on
the H
In parallel, it is generally reported that a higher H
tion should influence the activity of the large rather than small par-
ticles, as the H storage capacity is related to the particle volume;
2 1 4 4
pressures in C C ImBF [64].
2
concentra-
2
therefore, large particles may experience an increase in the avail-
ability of subsurface hydrogen [44].
In reality, little difference in activity is observed in either case.
This proves that the rate is not dictated by the availability and
adsorption of H , in agreement with observations of labile surface
2
hydrides on the NP surface [17], but by the mobility and absorption
of the substrate [26]. Indeed, it is highly likely that the surface of
the NPs is already saturated with adsorbed H
of the reaction [17].
2
at the temperature
Fig. 5. Selectivity for cyclohexene as a function of conversion for the three different
sizes of RuNPs.

P.S. Campbell et al. / Journal of Catalysis 275 (2010) 99–107
105
Fig. 6. SYBYL representations of CYD coordinated to the face of highly crystalline RuNPs of mean diameter 1.3 nm (left) and 2.8 nm (right).
0
Fig. 7. Transition electron micrographs of RuNPs after CYD hydrogenation for Ru (left), Ru25 (middle) and Ru50 (right).
Table 5
Recycling of the catalyst Ru
0
. Conversion, selectivity, turnover number and turnover frequency for the hydrogenation of CYD at 30 °C under 1.2 bar H
2
. Products removed under
vacuum after each run and analysed by GC.
TOF (h 1
ꢀ
)
Size after catalysis (nm)
Experiment number
Cycle
Conversion at 90 min (%)
Selectivity CYE (%)
TON
1
9
0
1
2
3
1st
66
73
69
68
64
65
97
95
94
89
86
86
70
76
73
71
67
68
46
51
49
47
45
45
1.3 ± 0.4
–
–
–
2nd
3rd
4th
5th
6th
1
1
1
1
–
1.8 ± 0.5
3.10. Recycling
0
the most selective catalyst, Ru , recycling experiments are per-
formed, by extracting in vacuo and quantifying the volatiles after
each 90-min run. More CYD is then added for hydrogenation. From
the results, Table 5 and Fig. 8, it can be seen that both the activity
and selectivity remain high after five recycles, diminishing only
slightly with each run. This small decrease of course is attributable
to the gradual coalescence of the NPs, leading to a diminution in
the number of active surface sites and larger, less selective NPs.
Indeed, TEM images obtained of the NPs after all recycling
TEM images of the reaction medium after hydrogenation, Fig. 7,
show that the average size measured (Table 3) does not differ
greatly from the original size, in accordance with the stability of
RuNPs in ILs under molecular hydrogen [17]; however, the size dis-
tribution is larger, probably as an effect of stirring [65].
The apparent resistance to coalescence of the NPs means that
they may be tested for their recyclability. Consequently, using

1
06
P.S. Campbell et al. / Journal of Catalysis 275 (2010) 99–107
5
5
4
4
3
3
2
5
0
5
0
5
0
5
100
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Supplementary data associated with this article can be found, in
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