Ruthenium nanoparticles prepared from ruthenium dioxide precursor: Highly active catalyst for hydrogenation of arenes under mild conditions

Article abstract of DOI:10.1016/j.molcata.2008.10.007

The hydrogenation of benzene and benzene derivatives was studied using Ru(0) nanoparticles prepared by a very simple method based on the in situ reduction of the commercially available precursor ruthenium dioxide under mild conditions (75 °C and hydrogen

Full text of DOI:10.1016/j.molcata.2008.10.007

Journal of Molecular Catalysis A: Chemical 298 (2009) 69–73
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Ruthenium nanoparticles prepared from ruthenium dioxide precursor:
Highly active catalyst for hydrogenation of arenes under mild conditions
Liane M. Rossi , Giovanna Machadob,c
a,∗
a
Institute of Chemistry, Universidade de São Paulo – USP, Av. Prof. Lineu Prestes 748, São Paulo 05508-000, SP, Brazil
Institute of Chemistry, Universidade Federal do Rio Grande do Sul, Av. Bento Gon c¸ alves 9500, Porto Alegre 91501-970, RS, Brazil
Chemical Engineering Department, Universidade de Caxias do Sul, Rua Francisco Getúlio Vargas 1130, Caxias do Sul 95070-560, RS, Brazil
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 May 2008
Received in revised form
The hydrogenation of benzene and benzene derivatives was studied using Ru(0) nanoparticles prepared by
a very simple method based on the in situ reduction of the commercially available precursor ruthenium
dioxide under mild conditions (75 C and hydrogen pressure 4 atm) in imidazolium ionic liquids. Total
turnovers (TTO) of 2700 mol/mol Ru were obtained for the conversion of benzene to cyclohexane under
solventless conditions and TTO of 1200 mol/mol Ru were observed under ionic liquid biphasic conditions.
When corrected for exposed ruthenium atoms, TTO values of 7940 (solventless) and 3530 (biphasic)
were calculated for benzene hydrogenation. These reaction rates are higher than those observed for Ru
nanoparticles prepared from decomposition of an organometallic precursor in similar conditions. The
presence of the partially hydrogenated product cyclohexene was also detected at low conversion rates.
© 2008 Elsevier B.V. All rights reserved.
◦
2
8 September 2008
Accepted 6 October 2008
Available online 14 October 2008
Keywords:
Nanoparticle
Ruthenium
Hydrogenation
Benzene
Ionic liquid
◦
1. Introduction
tions (75 C and hydrogen pressure 4 atm). It is worth to mention
the results of benzene hydrogenation by iridium(0) nanoparticles
Hydrogenation of benzene conventionally occurs under dras-
tic temperature and pressure conditions over metal-supported
stabilized by ionic liquid with total turnover (TTO) of 3509 in sol-
ventless mild reaction conditions (corrected for exposed atoms)
[6]. The hydrogenation of benzene by rhodium(0) nanoparticles
stabilized by ionic liquid under forcing conditions of 40 bar of
hydrogen attained the unprecedent TTO of 20,000. However, the
turnover frequency (TOF) of benzene hydrogenation by those cat-
alytic systems was not higher than 250 h , which necessitates long
reaction times. Stabilized aqueous colloidal suspensions of metal-
lic nanoparticles have also been reported as reusable catalyst for
arene hydrogenation in biphasic conditions (water/hydrocarbons)
[10,12,20].
More elegant is the use of ionic liquids to modulate product
selectivity based on different substrates, reaction intermediates
and products solubilities in the ionic liquid phase. Although the
selective hydrogenation of benzene to cyclohexene seems to be
a property characteristic of heterogeneous catalysts, the partially
hydrogenated product cyclohexene was observed during the hydro-
genation of benzene by Ru nanoparticles stabilized in ionic liquids
[3].
heterogeneous catalysts, such as Rh/Al O3 or Raney nickel [1].
2
Recently, soluble transition metal nanoparticles stabilized in ionic
liquids, polyoxoanions, quaternary ammonium salts or soluble
polymers, have shown enhanced lifetime and activity in benzene
hydrogenation under mild reaction conditions [2–15]. Ionic liq-
uids are outstanding solvents for the synthesis and stabilization
of metal nanoparticles of 2–3 nm particle size and allow easy
product recovery and catalyst recycling. The catalyst-containing
ionic liquid forms a biphasic reaction mixture with both sub-
strate and hydrogenated products that can be easily separated
by simple decantation. Hydrogenation of olefins and arenes cat-
alyzed by metal nanoparticles soluble in ionic liquids has received
increasing attention [3,6,11,13,15–19]. Rhodium(0), iridium(0), plat-
inum(0) and ruthenium(0) nanoparticles of 2–3 nm diameter with
narrow size distribution have been synthesized in 1-n-butyl-3-
methylimidazolium ionic liquids by reduction with molecular
hydrogen of metal complexes [6,16] or controlled decomposition
of organometallic compounds [3,13] under mild reaction condi-
−
1
We have previously prepared ruthenium(0) nanoparticles by
reduction of the corresponding commercially available ruthenium
◦
dioxide in 1-n-butyl-3-methylimidazolium ionic liquids at 75 C
∗ Corresponding author. Tel.: +55 11 30912181; fax: +55 11 38155579.
E-mail address: lrossi@iq.usp.br (L.M. Rossi).
and 4 atm hydrogen pressure. The resulting ruthenium(0) nanopar-
ticles, characterized by XRD, TEM and XPS, are a highly active and
1381-1169/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2008.10.007

70
L.M. Rossi, G. Machado / Journal of Molecular Catalysis A: Chemical 298 (2009) 69–73
re-usable catalyst for hydrogenation of 1-hexene under mild reac-
tion conditions with TTO up to 110,000 (based on total Ru atoms)
and 320,000 (based on exposed Ru atoms) [18]. We also argue that
the use of the commercially available catalyst precursor ruthenium
oxide is very advantageous compared to the organometallic pre-
cursor Ru(COD)(COT) usually required for the preparation of Ru
nanoparticles by reduction in mild conditions with H2 [3,4]. The
preparation of that compound is laborious, giving low yield and
the compound itself is very sensitive to air and requires very care-
in the H2 reservoir as a function of time. H2 uptake was mea-
sured at 1 min intervals with a Huba Control pressure transducer
interfaced via a Novus Field Logger converter to a computer. The
pressure versus time data were collected by the FieldChart Novus
Software, stored as a data file and exported to MicroCal Origin 7.0 for
hydrogenation rates calculations. The products were separated by
decantation and the organic phase was analyzed by GC and GC–MS.
The Ru(0) nanoparticles were isolated by precipitation with acetone
and centrifugation (3500 rpm) for 3 min. The solid was washed with
acetone (3 × 15 mL) and dried under reduced pressure. The samples
thus obtained were prepared for TEM analysis.
◦
ful handling. Moreover, in the same reaction conditions (75 C and
4
atm hydrogen pressure), the RuCl precursor is not reduced to give
3
Ru(0) nanoparticles and requires the use of other reducing agents,
such as NaBH4 [17].
Partial hydrogenation of benzene to cyclohexene was performed
in a Fischer–Porter reactor under H2 constant pressure (4 atm) and
heating. In a typical experiment, RuO ·3H O (3 mg, 0.0225 mmol)
Because of these promising catalytic results on olefin hydro-
genation catalyzed by our Ru(0) nanoparticles, we now investigate
the use of Ru(0) nanoparticles prepared from the RuO2 precursor
for catalytic hydrogenation of benzene and benzene derivatives.
Preliminary results concerning selective hydrogenation of benzene
will also be given.
2
2
was dispersed in 1 mL of room temperature ionic liquid (BMI·BF )
4
and 5.85 g (75 mmol) of benzene was added to the reactor under
inert atmosphere. The reactor was submitted to vacuum, placed in
◦
an oil bath at 75 C under stirring (700 rpm) and connected to the
hydrogen gas unit. The reaction was initiated by the admission of H2
gas of 4 atm (constant). The conversion and selectivity in cyclohex-
ene was estimated by GC analysis of aliquots collected in desired
time intervals.
2. Experimental
2.1. Materials and instrumentation
2.3. Hg(0) poisoning test
RuO2 hydrate was purchased from Aldrich Chem. Co. By
The catalytic reactions were carried out in the same standard
hydrogenation conditions, except for the addition of elemental Hg
1.4 g, 300 equiv.) to the reaction mixture at 10% conversion of
benzene to cyclohexane. The reaction was monitored by the fall
in hydrogen pressure in the H2 reservoir as a function of time
before and after the addition of Hg. The products were separated
by decantation of the nanoparticles and the organic phase was ana-
lyzed by GC and GC–MS.
following previously reported procedures [21], 1-n-butyl-3-
methylimidazolium ionic liquids BMI·PF , BMI·BF and BMI·CF SO
6
4
3
3
(
−
+
were prepared. The absence of Cl was verified by an Ag test and
the water content (<0.1%, v/v) was checked by cyclic voltamme-
try of an authentic sample and after the addition of water [22].
All manipulations were carried out using Schlenk techniques. Gas
chromatographic analyses were performed on a Shimadzu GC 14B
gas chromatograph equipped with a 30-m capillary column with
a dimethylpolysiloxane stationary phase, with the parameters set
as follows—initial temperature: 50 C, initial time: 5 min, ramp:
0 C min , final temperature: 250 C, final time: 5 min, injector
and detector temperature: 250 C, injection volume: 2 ␮L. Gas chro-
2
.4. CS2 poisoning test
◦
◦
◦
−1
1
The catalytic reactions were carried out under the same stan-
◦
dard hydrogenation conditions, except for the addition of 0.5 equiv.
of CS2 (1.7 mg) dissolved in an additional 1.17 g of benzene to the
reaction mixture at 10% of conversion. The reaction was monitored
by the fall in hydrogen pressure in the H2 reservoir as a function of
matographic analyses for detection of cyclohexene were performed
on a Varian Star 3400 gas chromatograph equipped with Petro-
col DH 100 m × 0.25 mm × 0.5 ␮m column with the parameters set
◦
as follows—initial temperature: 100 C, initial time: 20 min, ramp:
time before and after the addition of CS . The products were sep-
◦ −1 ◦
2
2
0 C min , final temperature: 250 C, final time: 10 min, injec-
arated by decantation of the nanoparticles and the organic phase
was analyzed by GC and GC–MS.
◦
◦
tor temperature: 250 C, detector temperature: 250 C, injection
volume: 2 ␮L.
Transmission electron microscopy (TEM) micrographs were
taken on a JEM-2010 microscope operating at an accelerating volt-
age of 200 kV. Samples for TEM observations were prepared by
placing a thin film of the ionic liquid containing the ruthenium
nanoparticles in a holey carbon grid. The metal particle size distri-
butionwasestimatedfromthemeasurementofabout200particles,
assuming a spherical shape, found in an arbitrary chosen area in
enlarged micrographs.
3
. Results and discussion
We have previously reported that Ru(0) nanoparticles of small
size and narrow size distribution can be easily prepared by in
situ H2 reduction of RuO ·3H O dissolved in room temperature
2
2
◦
ionic liquids, under mild reaction conditions (75 C and hydro-
gen pressure 4 atm) [18]. The Ru(0) nanoparticles stabilized by
1
-n-butyl-3-methylimidazolium hexafluorophosphate ionic liquid
(
BMI·PF ) are very stable in the biphasic and recyclable catalytic
6
2
.2. Nanoparticles preparation and hydrogenation experiments
systems for the hydrogenation of 1-hexene. In an extension of that
work, we investigate the formation of Ru(0) nanoparticles and their
catalytic performance in the hydrogenation of benzene and other
aromatic compounds in solventless and ionic liquid biphasic con-
ditions. For solventless hydrogenation of benzene, RuO2 hydrate
powder was added to the reactor containing benzene (molar ratio
1/667) under inert atmosphere, and the system was subjected to
hydrogen pressure and heating. After 42 min we observed >99% of
In a typical experiment, RuO ·3H O (3 mg, 0.0225 mmol) and
2
2
1.17 g of benzene (15 mmol) were added to a Fischer–Porter reactor
under inert atmosphere. In biphasic experiments, the RuO was dis-
2
persed in 1 mL of room temperature ionic liquid (BMI·PF , BMI·BF
6
4
or BMI·CF SO ) prior to the addition of the substrate. The reactor
3
3
◦
was submitted to vacuum, placed in an oil bath at 75 C under stir-
ring (700 rpm) and connected to the hydrogen gas reservoir. The
reaction was initiated by the admission of H2 gas of 4 atm (con-
stant). The reaction was monitored by the fall in hydrogen pressure
−
1
conversion to cyclohexane corresponding to a TOF = 953 h (based
on total Ru). This turnover frequency is about 10 times higher than
that observed for Ru(0) nanoparticles prepared from Ru(COD)(COT)

L.M. Rossi, G. Machado / Journal of Molecular Catalysis A: Chemical 298 (2009) 69–73
71
Table 1
Hydrogenation of arenes by RuO2 hydrate pre-catalyst.
a
Entry
Substrate
Product
Stabilizing agent
Time (h)
Conv.b (%)
TOFc (h
−1
)
1
2
3
4
5
6
7
8
Benzene
Benzene
Benzene
Benzene
Toluene
Toluene
Styrene
p-Xylene
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
Methylcyclohexane
Methylcyclohexane
Ethylcyclohexane
1,4-Dimethylcyclohexane
None (solventless)
BMI·PF6
0.7
13
20
12
1.2
20
4
100
96
46
40
100
54
953 (2803)
49 (144)
15 (44)
22 (65)
556 (1635)
18 (53)
BMI·BF4
BMI·CF3SO3
None (solventless)
BMI·PF6
None (solventless)
None (solventless)
100
97d
168 (494)
150 (441)
4.3
a
◦
Conditions: catalyst/substrate ratio = 1/667, temperature = 75 C, hydrogen pressure = 4 atm.
Measured by GC.
b
c
Catalytic turnover frequency: moles of substrate transformed per mole of catalyst per hour (total metal) and in parentheses TOF corrected for exposed metal.
cis/trans 3:2.
d
in similar conditions [3]. Under biphasic ionic liquid conditions,
slower reaction rates were obtained (Table 1, entries 2–4), and only
The Ru(0) nanoparticles dispersed in ionic liquid or in solvent-
less conditions are also efficient catalysts for the hydrogenation of
other aromatic compounds (Table 1, entries 5–8). It is worth to note
the reaction performed in BMI·PF reached 96% of conversion in
6
13 h. The turnover frequency of our catalyst is about two times that
that the hydrogenation of toluene to methylcyclohexene occurred
◦
observed for Ru(0) nanoparticles prepared from the Ru(COD)(COT)
precursor in ionic liquids [3].
under mild conditions (75 C, 4 atm H ) with a turnover number
2
of 667 mol of substrate converted per mol of metal in 70 min of
−
1
−1
Further experiments under identical hydrogenation conditions,
but with catalyst to substrate molar ratio of 1/1330, were performed
to investigate the catalyst lifetime in solventless and BMI·PF6
biphasic conditions. After each reaction in biphasic conditions, the
hydrogenated product was easily separated by simple decantation
and the ionic liquid phase containing the catalyst could be reused
in successive reactions. Under solventless conditions, the hydro-
genated product was distilled from the mixture and the resulting
powder could be reused in successive reactions. TTO of 2700 was
obtained in solventless conditions after three successive runs (the
reaction reached >99% conversion in 3 h (first cycle) and 4.6 h (sec-
ond cycle) and only 10% conversion after 12 h in the third cycle),
when deactivation of the catalyst had occurred. Under BMI·PF6
biphasic conditions, total turnovers of 1200 were observed with
reaction (not optimized) which gives a TOF of 556 h or 1635 h
(corrected for exposed Ru(0) atoms). This is an outstanding result
if we consider that one of the most active catalysts for benzene
hydrogenation reported elsewhere [11] hydrogenates toluene with
−
1
a TOF of 158 h under forcing conditions (40 atm H ) [15]. The Ru
2
nanoparticles prepared from Ru(COD)(COT) precursor [3] hydro-
−
1
genates toluene with TOF of 45 h under the same conditions used
in this study.
Additional experiments were carried out in order to character-
ize our nanocluster catalyst in more detail and to establish whether
the present system works as a homogeneous or as a heterogeneous
metal particle catalyst [2,5,24,25]: (i) the observation of a sigmoidal
curve for the H2 consumption as a function of time during the
hydrogenation of benzene is the first evidence for the in situ forma-
tion of the true catalyst by H2 reduction of RuO2 precatalyst prior
to the hydrogenation of the substrate—sigmoidal kinetic curves for
hydrogenation reaction have been attributed to transition metal
nanocluster catalysis; (ii) transmission electron microscopy analy-
sis performed directly in the ionic liquid catalyst containing phase
detected the presence of ruthenium nanoparticles. After a hydro-
genation reaction performed following the standard conditions, the
product was evaporated to dryness and the ionic liquid phase, con-
taining the ruthenium nanoparticles, was placed as a thin film in
a holey carbon grid and immediately analyzed. The TEM photog-
raphy obtained in the ionic phase 1-n-butyl-3-methylimidazolium
83% conversion to cyclohexane in 17 h (first run) and 10% con-
version in 15 h (second run). The catalyst deactivation occurred
before the third cycle both in solventless and in ionic liquid biphasic
conditions. We expected to find in this study a more pronounced
difference in the stability of the Ru(0) nanoparticles in solventless
and in ionic liquid conditions, especially because particles aggre-
gation is less probable in the ionic liquid phase. As reported in our
previous work [18], the same catalyst was reused for up to 4 and 17
successive cycles in solventless and BMI·PF6 biphasic conditions,
respectively, during hydrogenation of 1-hexene. By approximat-
ing the Ru(0) average nanoparticles (2.6 ± 0.4 nm) as close-packed
clusters with the same density as bulk Ru metal, it is possible to
estimate, by means of “magic numbers” approach [23], that roughly
tetrafluoroborate (BMI·BF ) is shown in Fig. 1. An average diameter
4
of 2.6 ± 0.4 nm was estimated from the measurement of about 200
particles diameters, assuming spherical shape, found in an arbitrary
chosen area in enlarged microphotographs.
3
4% of the Ru atoms should be on the surface; 22–30 Å Ru nanoclus-
ters correspond to 6 (923 atm)–9 shell (2869 atm) nanoclusters
with ca. 39–28% of metal atoms on the surface, respectively. Consid-
ering that only the Ru atoms on the surface are available for catalysis
in the nanocluster, the total turnovers corrected for exposed Ru
atoms reach the values of 7940 in solventless and 3530 in bipha-
sic reactions. The catalytic lifetime values presented here are far
superior to that of other soluble ruthenium nanoparticle catalysts
previously reported by us [3] in solventless (TON = 450 after 5.5 h at
(iii) Poisoning tests with elemental Hg were examined as it is a
well-known metal-heterogeneous catalyst poison through efficient
adsorption on the surface of transition metal(0) catalysts [24]. In
the hydrogenation of benzene in BMI·PF6 performed under stan-
dard conditions, excess Hg0 (300 equiv.) was added to the reaction
mixture after about 10% conversion and the catalytic activity was
suppressed in a few minutes; (iv) fractional poisoning experiments
were examined by addition of 0.5 equiv. of CS2 to the reaction mix-
ture after about 10% conversion of benzene under the standard
BMI·PF6 biphasic hydrogenation conditions. The catalytic activity
was immediately suppressed (Fig. 2). The poisoning test with CS2
suggests that we have an heterogeneous catalyst with only a frac-
tion of metal atoms on the surface and available for catalysis, since
the catalyst was completely poisoned with <1 equiv. of added CS2.
◦ ◦
5 C, 4 atm H ) or biphasic (TON = 365 after 18.5 h at 75 C, 4 atm
2
7
H2 and BMI·PF ) conditions and by Chaudret and co-workers [4]
6
◦
(
TON = 410 after 14 h at 80 C, 20 atm H ). We also mention here that
2
the Ru nanoparticles were prepared by reduction of the commer-
cially available precursor, RuO ·3H O, which avoid the laborious
2
2
and expensive preparation of the very unstable organometallic pre-
cursor Ru(COD)(COT) used in the above-mentioned reports.

72
L.M. Rossi, G. Machado / Journal of Molecular Catalysis A: Chemical 298 (2009) 69–73
Fig. 2. Hydrogenation of benzene by ruthenium nanoparticles in BMI·PF6 bipha-
sic conditions with addition of CS2 (0.5 equiv.) and Hg0 (300 equiv.) at about 10%
conversion.
The influence of the temperature on cyclohexene selectivity
and reaction rates in the hydrogenation of benzene using our
Ru(0)/BMI·BF4 catalytic system was examined. The reaction rate
◦
rose by a factor of 15 with the temperature increase from 50 C to
◦
◦
7
5 C. The reaction rate diminished at 120 C, probably due to cata-
lyst deactivation and was null at 150 C and 180 C. The cyclohexene
◦
◦
selectivity was increased with the temperature increase from
◦
7
5 C (maximum selectivity of 53% at 0.2% benzene conversion)
◦
to 120 C (maximum selectivity of 65% at 0.3% benzene conver-
◦
sion). However, our Ru(0)/BMI·BF system deactivates at 150 C.
4
◦
In the experiments above 150 C, neither cyclohexane nor cyclo-
Fig. 1. (a) TEM of ruthenium nanoparticles in the ionic liquid BMI·BF4 after
hydrogenation of benzene under biphasic standard conditions and (b) histogram
illustrating the particle size distribution.
As an initial attempt to identify cyclohexene in the hydro-
genation of benzene, the reactions performed under standard
◦
hydrogenation conditions (catalyst/substrate = 1/667, 75 C and
4
atm hydrogen pressure) were stopped at low benzene conversion
and analyzed by GC. The results were 4% of cyclohexene selectivity
at 10% of benzene conversion in BMI·PF , 13% of cyclohexene selec-
6
tivity at 7% of benzene conversion in BMI·BF and 8% of cyclohexene
4
selectivity at 9% of benzene conversion for the catalytic hydro-
genations performed in BMI·CF SO . Cyclohexene selectivity was
3
3
expressed as the ratio of the formed cyclohexene and the benzene
converted to products. After these preliminary results, BMI·BF was
4
elected for further experiments. In order to identify cyclohexene
intermediate in the hydrogenation of benzene, the reaction condi-
tions were changed to keep the reaction sufficiently slow to permit
careful analysis of the intermediate at very low conversion rates.
As shown in Fig. 3, high cyclohexene selectivity was attained at a
very low benzene conversion (<2%).
Fig. 3. Selectivity to cyclohexene as
a function of benzene conversion at
120 ◦C (square), 75 ◦C (circle) and 50 ◦C (triangle). Conditions: catalyst/substrate
ratio = 1/3333, BMI·BF4, PH2 = 4 atm.

L.M. Rossi, G. Machado / Journal of Molecular Catalysis A: Chemical 298 (2009) 69–73
73
tion of cyclohexene and/or slow down its further hydrogenation to
cyclohexane.
Acknowledgments
We gratefully acknowledge the CENPES, CTPETRO-CNPq and
FAPERGS for financial support and fellowships. Also, we are par-
ticularly grateful to Prof. Jairton Dupont for the suggestions and
helpful discussions.
References
[1] R.L. Augustine, Heterogeneous Catalysis for the Synthetic Chemist, Marcel
Dekker, New York, 1996.
[
[
2] J.A. Widegren, R.G. Finke, J. Mol. Catal. A: Chem. 191 (2003) 187–207.
3] E.T. Silveira, A.P. Umpierre, L.M. Rossi, G. Machado, J. Morais, G.V. Soares,
I.J.R. Baumvol, S.R. Teixeira, P.F.P. Fichtner, J. Dupont, Chem. Eur. J. 10 (2004)
3
734–3740.
4] K. Pelzer, O. Vidoni, K. Philippot, B. Chaudret, V. Colliere, Adv. Funct. Mater. 13
2003) 118–126.
5] J.A. Widegren, M.A. Bennett, R.G. Finke, J. Am. Chem. Soc. 125 (2003)
0301–10310.
6] G.S. Fonseca, A.P. Umpierre, P.F.P. Fichtner, S.R. Teixeira, J. Dupont, Chem. Eur. J.
(2003) 3263–3269.
7] K.S. Weddle, J.D. Aiken, R.G. Finke, J. Am. Chem. Soc. 120 (1998) 5653–5666.
[
[
[
[
Fig. 4. Selectivity to cyclohexene and initial rate of hydrogenation of benzene as a
function of hydrogen pressure at 75 C in BMI·BF4 (catalyst/substrate ratio = 1/3333).
(
◦
1
hexene was formed. Most 1,3-dialkylimidazolium ILs are stable
towards temperature and many organic and inorganic substances
9
[
26]. However, under certain reaction conditions, the cation and
[8] J.L. Pellegatta, C. Blandy, V. Collière, R. Choukroun, B. Chaudret, P. Cheng, K.
Philippot, J. Mol. Catal. A: Chem. 178 (2002) 55–61.
anion undergo degradation reactions and this might be the reason
for catalyst deactivation observed above 150 C [27]. On the other
hand, the deactivation of the catalytic system can also be due to the
poor solubility of the substrate in this phase. At 150 C the benzene
[9] X. Mu, D.G. Evans, Y. Kou, Catal. Lett. 97 (2004) 151–154.
◦
[
10] (a) A. Roucoux, J. Schulz, H. Patin, Adv. Synth. Catal. 354 (2003) 222–226;
(b) J. Schulz, A. Roucoux, H. Patin, Chem. Commun. (1999) 535–536;
(c) J. Schulz, A. Roucoux, H. Patin, Chem. Eur. J. 6 (2000) 618–624.
◦
[
11] X. Mu, J. Meng, Z.-C. Li, Y. Kou, J. Am. Chem. Soc. 127 (2005) 9694–9695.
is in the vapor phase, which decreases its miscibility in the ionic liq-
uid phase and, consequently, deactivates the catalytic system [28].
The influence of H2 pressure on the partial hydrogenation of
benzene over Ru(0)/BMI·BF4 catalytic system was also examined.
The plot of the initial rate of benzene conversion and the initial
selectivity to cyclohexene as a function of hydrogen pressure (Fig. 4)
showed the reaction rate being proportional to the pressure, which
meant that hydrogen mass transfer should govern the overall reac-
tion rate, and that the selectivity is not influenced by the hydrogen
pressure in the studied pressure range.
[
[
[
12] V. Mévellec, A. Roucoux, E. Ramirez, K. Philippot, B. Chaudret, Adv. Synth. Catal.
346 (2004) 72–76.
13] C.W. Scheeren, G. Machado, J. Dupont, P.F.P. Fichtner, S.R. Texeira, Inorg. Chem.
42 (2003) 4738–4742.
14] R.L.S. Ott, R.G. Finke, Coord. Chem. Rev. 251 (2007) 1075–1100.
[15] C. Zhao, H.-Z. Wang, N. Yan, C.-X. Xiao, X.-D. Mu, P.J. Dyson, Y. Kou, J. Catal. 250
2007) 33–40.
(
[
16] J. Dupont, G.S. Fonseca, A.P. Umpierre, P.F.P. Fichtner, S.R. Teixeira, J. Am. Chem.
Soc. 124 (2002) 4228–4229.
[17] L.M. Rossi, J. Dupont, G. Machado, P.F.P. Fichtner, C. Radtke, I.J.R. Baumvol, S.R.
Teixeira, J. Braz. Chem. Soc. 15 (2004) 904–910.
[
18] L.M. Rossi, G. Machado, P.F.P. Fichtner, S.R. Teixeira, J. Dupont, Catal. Lett. 92
2004) 149–155.
(
[
19] J. Huang, T. Jiang, B. Han, H. Gao, Y. Chang, G. Zhao, W. Wu, Chem. Commun.
(2003) 1654–1655.
4
. Conclusion
[20] A. Nowicki, V. Le Boulaire, A. Roucoux, Adv. Synth. Catal. 349 (2007) 2326–
2
330.
In summary, the Ru(0) nanoparticles prepared by a very sim-
ple method based on H2 reduction of the commercially available
[
21] J. Dupont, P.A.Z. Suarez, C.S. Consorti, R.F. de Souza, Org. Synth. 79 (2002)
236–243.
22] B.K. Sweeny, D.G. Peters, Electrochem. Commun. 3 (2001) 712–715.
23] B.K. Teo, H. Zhang, in: D.L. Feldheim, C.A. Foss Jr. (Eds.), Metal Nanoparticles:
Synthesis, Characterization and Applications, Marcel Dekker, New York, 2002
(Chapter 3).
24] B.J. Hornstein, J.D. Aiken, R.G. Finke, Inorg. Chem. 41 (2002) 1625–1638;
J.A. Widegren, R.G. Finke, J. Mol. Catal. A: Chem. 198 (2003) 317–341;
P. Foley, R. DiCosimo, G.M. Whitesides, J. Am. Chem. Soc. 102 (1980) 6713–6725.
[25] L. Lin, R.G. Finke, Inorg. Chem. 33 (1994) 4891–4910.
26] J. Dupont, J. Spencer, Angew. Chem. Int. Ed. 43 (2004) 5296–5297.
27] J. Dupont, S.M. Silva, R.F. Souza, Catal. Lett. 77 (2001) 131–133.
[
[
precursor RuO ·3H O, which avoids the use of an organometallic
2
2
precursor, has proved to be an efficient catalyst for hydrogenation
of benzene and other benzene derivatives. We also demonstrated
by poisoning experiments and TEM that the catalyst behaves as
an heterogeneous metal nanoparticle catalyst. The partially hydro-
genated product cyclohexene was also detected in certain reaction
conditions. The maximum selectivity to cyclohexene (65% at 0.3%
[
[
[
[
◦
of benzene conversion) was reached at 120 C and 4 atm hydrogen
28] G.S. Fonseca, J.B. Domingos, F. Nome, J. Dupont, J. Mol. Catal. A: Chem. 248
pressure in the BMI·BF4 ionic liquid biphasic system. Although the
maximum yield of cyclohexene is too low for practical applications,
the use of reaction modifiers such as methanol [29], ZnSO4 [30] or
NaOH [31] has been suggested to enhance selectivity to cyclohexene
and will the subject of further investigation in our catalytic system.
Such modifiers to enhance the selectivity must stimulate desorp-
(2006) 10–16.
[
[
[
29] J. Struijk, J.J.F. Scholten, Appl. Catal. A: Gen. 82 (1992) 277–287.
30] S. Xie, M. Qiao, H. Li, W. Wang, J.F. Deng, Appl. Catal. A: Gen. 176 (1999) 129–134.
31] (a) S.-C. Hu, Y.-W. Chen, Ind. Eng. Chem. Res. 36 (1997) 5153;
(b) S.-C. Hu, Y.-W. Chen, Ind. Eng. Chem. Res. 40 (2001) 3127–3132;
(c) S.-C. Hu, Y.-W. Chen, Ind. Eng. Chem. Res. 40 (2001) 6099–6104;
(
d) S.-C. Hu, Y.-W. Chen, J. Chem. Technol. Biotechnol. 76 (2001) 954–958.