The partial hydrogenation of benzene to cyclohexene by nanoscale ruthenium catalysts in imidazolium ionic liquids

Article abstract of DOI:10.1002/chem.200305765

The controlled decomposition of an Ru⁰ organometallic precursor dispersed in 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI·PF₆), tetrafluoroborate (BMI·BF₄) or trifluoromethane sulfonate (BMI·CF₃SO₃) ionic liquids with H₂ represents a simple and efficient method for the generation of Ru⁰ nanoparticles. TEM analysis of these nanoparticles shows the formation of superstructures with diameters of ≈57 nm that contain dispersed Ru⁰ nanoparticles with diameters of 2.6 ± 0.4 run. These nanoparticles dispersed in the ionic liquids are efficient multiphase catalysts for the hydrogenation of alkenes and benzene under mild reaction conditions (4 atm, 75°C). The ternary diagram (benzene/cyclohexene/ BMI·PF₆) indicated a maximum of 1% cyclohexene concentration in BMI·PF₆, which is attained with 4% benzene in the ionic phase. This solubility difference in the ionic liquid can be used for the extraction of cyclohexene during benzene hydrogenation by Ru catalysts suspended in BMI·PF₆. Selectivities of up to 39% in cyclohexene can be attained at very low benzene conversion. Although the maximum yield of 2% in cyclohexene is too low for technical applications, it represents a rare example of partial hydrogenation of benzene by soluble transition-metal nanoparticles.

Full text of DOI:10.1002/chem.200305765

FULL PAPER
The Partial Hydrogenation of Benzene to Cyclohexene by Nanoscale
Ruthenium Catalysts in Imidazolium Ionic Liquids
[a]
[a]
[a]
Edson T.Silveira, Alexandre P.Umpierre, Liane M.Rossi, Giovanna Machado,[a]
Jonder Morais,[b] Gabriel V.Soares, Israel J.R.Baumvol,
Paulo F.P.Fichtner,
Sergio R.Teixeira,
[b]
[b]
[b]
and Jairton Dupont*[a]
[c]
Abstract: The controlled decomposi-
tion of an Ru0 organometallic pre-
cursor dispersed in 1-n-butyl-3-meth-
2.6Æ0.4 nm. These nanoparticles dis-
persed in the ionic liquids are efficient
multiphase catalysts for the hydrogena-
tion of alkenes and benzene under
mild reaction conditions (4 atm, 758C).
The ternary diagram (benzene/cyclo-
hexene/BMI¥PF6) indicated a maximum
of 1% cyclohexene concentration in
BMI¥PF6, which is attained with 4%
benzene in the ionic phase. This solu-
bility difference in the ionic liquid can
be used for the extraction of cyclo-
hexene during benzene hydrogenation
by Ru catalysts suspended in BMI¥PF6.
Selectivities of up to 39% in cyclo-
hexene can be attained at very low
benzene conversion. Although the
maximum yield of 2% in cyclohexene
is too low for technical applications, it
represents a rare example of partial hy-
drogenation of benzene by soluble
transition-metal nanoparticles.
ylimidazolium
(BMI¥PF6), tetrafluoroborate (BMI¥BF4)
or trifluoromethane sulfonate
(BMI¥CF3SO3) ionic liquids with H2
represents simple and efficient
hexafluorophosphate
a
method for the generation of Ru0
nanoparticles. TEM analysis of these
nanoparticles shows the formation
of superstructures with diameters of
ꢀ57 nm that contain dispersed Ru0
nanoparticles with diameters of
Keywords: biphasic
hydrogenation
catalysis
ionic liquids
¥
¥
¥
nanoparticles ¥ ruthenium
Introduction
sis.[4] Moreover, in these multiphase processes, primary prod-
ucts can be extracted during the reaction to modulate the
product selectivity (playing with different substrates and re-
action products solubility with the catalyst-containing
phase). Indeed, this approach can constitute a suitable
method to avoid consecutive reactions of primary products
and it has been exploited to some extent in aqueous-phase
catalytic processes. For example, the selectivity in primary
amines was successfully controlled in aqueous-phase palladi-
um-catalysed telomerisation of butadiene with ammonia in
which the formation of secondary amines was reduced to
less than 2%.[5] More importantly, the selective hydrogena-
tion of benzene to cyclohexene can be successfully achieved
by the use of nonsupported ultrafine ruthenium catalysts
suspended in an aqueous phase, usually in the presence of
additives.[6,7] Surprisingly, this method has been rarely ex-
ploited in multiphase ionic liquid catalysis. In only two cases
was the reaction selectivity (hydrogenation of dienes to
monoenes) attributable to differences of substrate and prod-
uct solubility in the ionic liquid, catalytic phase.[8] Moreover,
it is well-known that the solubility of organic compounds in
1,3-dialkylimidazolium ionic liquids can be modulated by
simple changes in N-alkyl imidazolium substituents and/or
in the anion. In particular, benzene is highly soluble in the
1-n-butyl-3-methylimidazolium hexafluorophosphate ionic
Multiphase transition-metal catalysis can combine advantag-
es of both classical homogeneous and heterogeneous proc-
esses, such as catalyst recycling, product separation, mild re-
action conditions and modulation of catalyst properties.[1]
Indeed, these properties are nowadays largely explored in
both academia and industry mainly by the use of aqueous[2]
and ionic liquid[3] phase organometallic and colloidal cataly-
[a] M.Sc. Chem. E. T. Silveira, Dipl. Chem. Eng. A. P. Umpierre,
Dr. L. M. Rossi, Dr. G. Machado, Prof. Dr. J. Dupont
Laboratory of Molecular Catalysis
Institute of Chemistry, UFRGS, Av. Bento GonÁalves
9500 Porto Alegre 91501-970 RS (Brazil)
Fax : (+55)5133167304
E-mail: dupont@iq.ufrgs.br
[b] Prof. Dr. J. Morais, Dipl. Phys. G. V. Soares,
Prof. Dr. I. J. R. Baumvol, Prof. Dr. S. R. Teixeira
Institute of Physics, UFRGS, Av. Bento GonÁalves
9500 Porto Alegre 91501-970 RS (Brazil)
[c] Prof. Dr. P. F. P. Fichtner
Department of Metallurgy, UFRGS, Av. Bento GonÁalves
9500 Porto Alegre 91501-970 RS (Brazil)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author. This information
includes XRD and XPS spectra and TEM micrographs of the Ru
nanoparticles.
3734
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/chem.200305765
Chem. Eur. J. 2004, 10, 3734 3740

3734 3740
liquid at room temperature, whereas alkenes and alkanes
are sparingly soluble.[9] Therefore, imidazolium ionic liquids
could constitute an alternative media for the selective hy-
drogenation of benzene to cyclohexene. It is also reasonable
to assume that the ionic liquid can act as a modifier of the
metal catalyst, thus repelling the cyclohexene formed and
decreasing the re-adsorption and further hydrogenation to
cyclohexane. Herein we report our results concerning the
partial hydrogenation of benzene by supported and nonsup-
ported ruthenium catalysts in the ionic liquid 1-n-butyl-3-
methylimidazolium hexafluorophosphate (BMI¥PF6). Fur-
thermore, we have also determined the ternary diagram of
benzene/cyclohexene/BMI¥PF6 ionic liquid and investigated
the formation and stabilisation of nanoscale Ru0 particles in
the ionic liquid.
Figure 1. Benzene and cyclohexane contents in the ionic phase at 758C in
a benzene/cyclohexane/BMI¥PF6 mixture.
Results and Discussion
Benzene/cyclohexene/BMI¥PF6 ternary mixture: The room-
temperature solubilities of benzene and cyclohexene in
BMI¥PF6 are 37 and 6 wt%, respectively.[9,10] However, in
order to determine the benzene and cyclohexene contents in
the ionic liquid phase under the catalytic reaction condi-
tions, we investigated the different compositions of benzene,
cyclohexene and cyclohexane in the organic and ionic
phases at 758C by gravimetric methods. The results are pre-
sented in Figures 1 3.
The solubility of benzene in BMI¥PF6 at 758C is lower
than that determined at room temperature.[9] It is clear from
the data in Figures 1 and 2that the maximum concentration
Figure 2. Benzene and cyclohexene contents in the ionic phase at 758C in
a benzene/cyclohexene/BMI¥PF6.
Abstract in Portuguese: A decomposiÁào controlada de com-
plexos organometµlicos de Ru0 dispersos nos lÌquidos iÙnicos
hexafluorofosfato de 1-n-butil-3-metilimidazÛlio (BMI¥PF6),
tetrafluoroborato de 1-n-butil-3-metilimidazÛlio (BMI¥BF4)
ou triflato de 1-n-butil-3-metilimidazÛlio (BMI¥CF3SO3) na
presenÁa de H2 representam um mÿtodo simples e eficiente
para a geraÁào de nanopartÌculas de Ru0. Anµlises de micros-
copia eletrÙnica de transmissào indicam a formaÁào de supe-
restruturas com tamanho aproximado de 57 nm formadas
por nanopartÌculas dispersas de Ru0 com di‚metro entre
2.6Æ0.4 nm. Estas nanopartÌculas dispersas em lÌquidos iÙni-
cos sào catalisadores multifase eficientes para a hidrogenaÁào
de alquenos e benzeno em condiÁes reacionais suaves
(4 atm, 758C).
O diagrama ternµrio de (benzeno/ciclo-
-hexeno/BMI¥PF6) indica uma concentraÁào mµxima de 1%
de ciclo-hexeno em BMI¥PF6, atingida com 4% de benzeno
na fase iÙnica. Estas diferenÁas de solubilidade no lÌquido
iÙnico podem ser exploradas para a extraÁào de ciclo-hexeno
durante a hidrogenaÁào de benzeno por catalisadores de Ru
dispersos em BMI¥PF6. Seletividades superiores a 39% em
ciclo-hexeno podem ser obtidas a baixas converses benzeno.
Embora o rendimento mµximo de 2% em ciclo-hexeno ÿ
bastante baixo para uma aplicaÁào prµtica, este caso repre-
senta um exemplo raro de hidrogenaÁào parcial de benzeno
por nanopartÌculas solÇveis de metais de transiÁào.
Figure 3. Cyclohexane and cyclohexene contents in the ionic phase at
758C in a cyclohexene/cyclohexane/BMI¥PF6.
of cyclohexene (~1%) in the ionic liquid phase is attained
when the benzene concentration is ꢀ4%. Moreover, with
benzene concentrations greater than 34%, the content of cy-
clohexene is negligible in the ionic liquid phase.
Preparation and characterisation of nanoscale Ru0 particles:
Among the various methods available for the generation of
Ru0 nanoparticles,[11] the controlled decomposition of orga-
Chem. Eur. J. 2004, 10, 3734 3740
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3735

J. Dupont et al.
FULL PAPER
nometallic compounds developed by Chaudret is one of the
most simple and effective.[12] Thus, Ru nanoparticles of a
controlled size were easily prepared by the controlled de-
composition of [Ru(cod)(cot)] (cod=1,5-cyclooctadiene,
cot=1,3,5-cyclooctatriene), dissolved in pure alcohols or al-
cohol/THF mixture, with molecular hydrogen.[12] In this con-
text, imidazolium ionic liquids[8,13] (shown here) provide a
unique medium for the preparation and stabilisation of Ir0,
Rh0, Pd0 and Pt0 particles with small diameters (2 3 nm) and
a narrow size distribution.[14] Therefore, it was of interest to
check whether these liquids could
provide a suitable environment for
the syntheses and stabilisation of
Ru0 nanoparticles by means of Chau-
dret×s approach.
Thus, the treatment of a yellow
À
À
Figure 4. XPS surface survey of the Ru nanoparticles (Ru Ru and Ru O
BE region) before and after sputtering with Ar+.
suspension of [Ru(cod)(cot)] in 1-n-
butyl-3-methylimidazolium
hexa-
fluorophosphate (BMI¥PF6), tetra-
fluoroborate (BMI¥BF4) or trifluoro-
The ruthenium nanoparticles were also examined by
transmission electron microscopy (TEM). TEM observations
show the formation of spherical superstructures of Ru parti-
cles with a regular size of 57Æ8 nm (Figure 5a and b).
methane sulfonate (BMI¥CF3SO3) ionic liquids with molecu-
lar hydrogen (4 atm) at 758C for 18 h affords a black solu-
tion. The black solid material isolated from this solution by
centrifugation was washed with acetone and dried under re-
duced pressure.
XRD powder analysis indicated that the solid consists of
metal particles of hexagonal close packed (hcp) ruthenium.
The Bragg reflections corresponding to crystalline rutheni-
um particles were observed at 2q=37.76, 42.05, 43.43, 57.78,
68.17, 77.80, 83.48 and 84.388, which correspond to the in-
dexed planes of the (hcp) crystals of Ru0: (100), (002),
(101), (102), (110), (103), (112) and (201), respectively.
The most representative reflections of Ru0 were indexed as
hexagonal with unit cell parameters a=2.7487 and c=
4.2937 ä. The mean diameter of the Ru particles, calculated
with Scherrer×s equation, is ꢀ2.5 nm, which is in good
agreement with the TEM results (see later). Moreover,
energy dispersion spectrometry (EDS) indicates the pres-
ence of Ru0 and selected area diffraction (SAD) produces
ring patterns that can be fitted to a simulation based on Ru0
parameters (see the Supporting Information). Note that all
the samples display the same spectrum, independent of the
ionic liquid used (BMI¥PF6, BMI¥BF4 of BMI¥CF3SO3) for
their preparation.
However, X-ray photoelectron spectroscopy (XPS) of a
sample shows oxidised ruthenium and oxygen peaks that in-
dicate the presence of a passivated surface layer. However,
the Ru O components disappear almost completely after
sputtering with Ar+; this shows that only the external sur-
face ruthenium atoms were oxidised (Figure 4). The same
behaviour was observed by Chaudret for Ru0 nanoparticles
prepared in the presence of pure alcohols or alcohol/THF
mixtures.[12] Moreover, the ease with which the surface
atoms are oxidised is a general trend for ruthenium parti-
cles.[15] It is interesting to note that only the peaks corre-
sponding to carbon (support), ruthenium and oxygen were
Figure 5. Images of the Ru nanoparticles prepared in BMI¥PF6 showing
a) the superstructures (bar=50 nm), b) inside the spherical superstruc-
tures (bar=25 nm), and c,d) the corresponding histograms.
A mean diameter of 2.6Æ0.4 nm for the Ru0 nanoparti-
cles present inside the superstructures was estimated from
ensembles of 150 particles found in arbitrary chosen areas of
the enlarged micrographs of five superstructures. Figure 5c
and d show the obtained particle size distributions, which
can be fitted reasonably well to a Gaussian curve. The same
type of superstructure was found for Ru particles prepared
in a MeOH/THF mixture; however, the size of the Ru nano-
particles could be not determined on account of their close
proximity.[12]
3
observed. Note that the Ru 3d = peak appears to be more in-
2
tense owing to the superposition with the carbon 1s signal
(Figure 4).
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chem. Eur. J. 2004, 10, 3734 3740

Partial Hydrogenation of Benzene
3734 3740
Hydrogenation of olefins and arenes: We first investigated
the catalytic properties of these nanoparticles re-dispersed
in the ionic liquids as a stable black ™solution∫ or in solvent-
less conditions with respect to the hydrogenation of alkenes
and benzene (Table 1).[16,17]
The data in Table 1 indicate that the Ru nanoparticles act
as a relatively highly active catalyst for both multiphase and
solventless hydrogenation of 1-hexene, cyclohexene and 2,3-
dimethyl-2-butene (see entries 1 8) under mild reaction con-
ditions. The Ru0 nanoparticles dispersed in the ionic liquid
seem to be quite stable under the reaction conditions, and
the recovered ionic liquid catalytic solution (Table 1,
entry 2) could be re-used at least 8 times without any signifi-
cant changes in the catalytic activity for the 1-hexene hydro-
genation. Moreover, TEM analysis of the particles embed-
ded in the ionic liquid after benzene hydrogenation shows
the same average size (2.6Æ0.4 nm) and size distribution
(Figure 6).
It is also clear from the data in Table 1 that the reactions
performed under solventless conditions are faster than those
performed with the particles dispersed in the ionic liquids,
particularly for the cyclohexene and benzene hydrogena-
Figure 6. TEM micrograph of ruthenium nanoparticles embedded in
BMI¥BF4 after hydrogenation of benzene under biphasic standard condi-
tions (bar=10 nm). Histogram illustrating the particle size distribution.
Table 1. Hydrogenation of alkenes and arenes by Ru0 nanoparticles under multiphase and solventless condi-
It is also worth noting that
the Ru nanoparticles are only
marginally active for the hy-
tions (758C and 4 atm, constant pressure, substrate/Ru=500).
[b]
Entry
Medium
Substrate
t[h]
Conversion[%]
TON[a]
TOF[hÀ1
]
1
solventless
1-hexene
1-hexene
0.7
0.6
>99
>99
>99
>99
>99
>99
76
90
30
73
50
>99
>99
>99
>99
<1
500
500
500
500
500
500
380
450
150
365
240
250
250
250
250
714
833
1000
1000
100
62
316
82
9
20
14
125
45
39
drogenation
of
anisole
2BMI¥BF
4
3
BMI¥PF6
1-hexene
0.5
(Table 1, entry 16) under the
standard reaction conditions
(4 atm, 758C).
4
5
6
7
8
9
10
11
solventless
BMI¥BF4
BMI¥PF6
solventless
solventless
BMI¥BF4
cyclohexene
cyclohexene
cyclohexene
2,3-dimethyl-2-butene
benzene
0.5
5.0
8.0
1.2
Partial hydrogenation of ben-
zene to cyclohexene: The cata-
lytic tests for the partial hydro-
genation of benzene to cyclo-
hexene were performed with
Ru nanoparticles and with a
supported Ru catalyst (Ru/
Al2O3, 5%, Strem) under sol-
ventless and multiphase condi-
tions (entries 8 11, Table 2).
5.5
benzene
benzene
17.3
18.5
17.5
2.0
5.6
6.4
BMI¥PF6
BMI¥CF3SO3
benzene
[c]
12solventless
benzene
solventless
solventless
solventless
solventless
13
14
15
16
toluene[c]
isopropylbenzene[c]
tert-butylbenzene[c]
anisole
14.1
18
18
[a] Turnover number TON=mol of hydrogenated product/mol of Ru. [b] Turnover frequency TOF=TON/h.
[c] Arene/Ru=250.
tions (compare entry 4 with 5 and 6, and entry 8 with 9 11,
Table 1). This difference is attributed to the typical multi-
phase conditions of the reactions performed in the ionic liq-
uids; this can be a mass-transfer-controlled process.[18]
The catalytic activity attained under either solventless or
multiphase conditions for this reaction is far superior to that
of Ru nanoparticles prepared in alcohols/THF mixture. A
TON (TON=turnover number) of 48 was obtained in the
benzene hydrogenation after 6 days at 808C and 5 atm of
H2.[12b] The hydrogenation of alkyl benzenes catalysed by
the Ru nanoparticles in solventless conditions (arene/Ru=
250, 758C, 4 atm) is sensitive to the to steric bulk of the
alkyl group (Table 1, entries 12 14 and Figure 7). The initial
TOF for 20% arene conversion was 122, 55, 43 and 35 hÀ1
Figure 7. Compared hydrogenation of arenes under solventless conditions
by Ru0 nanoparticles [4 atm of H2 (constant pressure) at 758C, arene/
Ru=250].
for the benzene, toluene, isopropylbenzene and tert-butyl-
benzene hydrogenation, respectively.
Chem. Eur. J. 2004, 10, 3734 3740
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J. Dupont et al.
FULL PAPER
Table 2. Partial hydrogenation of benzene by Ru0 nanoparticles and supported Ru/Al2O3 catalysts under ionic
liquid and solventless conditions (758C, constant hydrogen pressure, benzene/Ru=1500).
but, in these cases, the
BMI¥PF6 can act as a reaction
modifier (similar to the process
in water).[6] It is worth noting
that the selectivity attained in
cyclohexenes for the hydroge-
nation of alkyl benzenes
(Figure 7 and entries 13 15,
Entry
Catalyst/Medium
P[atm]
t[h]
Conversion[%]
Selectivity[%][a]
TON[b]
1
[Ru0]n/BMI¥PF6
0]n/BMI¥PF6
[Ru0]n
4
4
4
4
6
6
6
6
6
6
6
6
6
6
6
6
6
6
2.0
4.5
1.0
2.7
0.4
0.7
2.9
1.0
10
22
9
19
27
8
15
7
4
2
30
1
<1
<1
30
21
11
15
9
150
330
135
285
2[Ru
3
4
5
6
7
8
9
10
11
12Ru/Al
13
14
15
16
17
18
[Ru0]n
Ru/Al2O3
[Ru0]n
120
2
90
[Ru0]n
17
6
55
Table 1) was less than 2%,
Ru/Al2O3
even at very low arene conver-
sions. This is probably related
to the hydrogenation rate of
these arenes, which are very
low compared to the benzene.
The selectivity in cyclohex-
[Ru0]n/BMI¥PF6
[Ru0]n/BMI¥PF6
[Ru0]n/BMI¥PF6
2O3/BMI¥PF6
Ru/Al2O3/BMI¥PF6
Ru/Al2O3/BMI¥PF6
Ru/Al2O3/H2O
Ru/Al2O3/H2O
[Ru0]n/H2O
[Ru0]n/H2O
1.22
34
4.5
27.7
4.1
5.5
21.2
0.7
1.5
0.5
1.217
7
15
3
6
11
7
105
225
45
90
5
165
105
2
120
2
<1
<1
1
ene attained in the benzene
17
8
55
hydrogenation catalysed by
Ru0 nanoparticles in BMI¥PF6
<1
55
is reasonable (up to 39% at
[a] Selectivity in cyclohexene. [b] Turnover number TON=mol of hydrogenated products (cyclohexene and cy-
clohexane)/mol of Ru.
1%
benzene
conversion).
However, the cyclohexene
yields achieved so far (2%)
It is clear that for both Ru catalysts the best cyclohexene
selectivity was attained in the reactions performed in the
presence of the ionic liquid (compare entries 1 and 2with 3
and 4). It is also evident that in BMI¥PF6 the best cyclohex-
ene selectivity is achieved with the Ru0 nanoparticles com-
pared to the supported Ru catalysts at any benzene conver-
sion (compare 9 11 with 12 14, Table 2). This result is in
agreement with what was observed earlier, namely, that
nonsupported Ru catalysts are more effective than support-
ed ones.
are too low for technical application (compared, for exam-
ple, to the industrial nonsupported Ru catalysts suspended
in aqueous phase that can reach 60% yield).[19] Nevertheless,
these values of cyclohexene selectivity and yield are similar
to those observed for the gas-phase hydrogenation of ben-
zene catalysed by supported Ru catalysts.[7] Catalysts based
on soluble transition-metal nanoparticles that promote the
selective reduction of benzene to cyclohexene are very rare.
For example, cyclohexenes were observed in the hydrogena-
tion of sterically bulky arenes, such as 1,2,4,5-tetramethyl-
benzene, by Rh0 nanoparticles in an aqueous biphasic
system[20] and in the partial hydrogenation of anisole by the
use of polyoxoanions and tetrabutylammonium-stabilised
Rh0 nanoclusters (an initial selectivity of 30% in 1-methoxy-
cyclohexene was observed at low anisole conversions).[21]
It is also interesting to note that the addition of water to
the ionic phase causes the partial decomposition of the imi-
dazolium ionic liquid,[14] and the selectivity in cyclohexene
drops to ꢀ1% at 3% benzene conversion.
In the case of Ru0 nanoparticles in BMI¥PF6, a maximum
of 39% selectivity at 1% benzene conversion was obtained.
This drops to 11% at 15% benzene conversion (Figure 8).
It is reasonable to assume that the cyclohexene selectivity
of hydrogenation processes in the ionic liquid phase for
both Ru catalysts investigated is not only a consequence of
the different cyclohexene/benzene solubility in BMI¥PF6,
Conclusion
We have shown that stable Ru0 nanoparticles can be easily
prepared by simple decomposition of [Ru(cod)(cot)] dis-
persed in imidazolium ionic liquids. The ternary diagram
(benzene/cyclohexene/BMI¥PF6) indicates that a maximum
of 1% cyclohexene concentration in BMI¥PF6 is attained at
a 4% benzene concentration in the ionic phase. This differ-
ence in solubility in the ionic liquid can be used for the ex-
traction of cyclohexene during the hydrogenation of ben-
zene by Ru catalysts suspended in BMI¥PF6. A selectivity of
<39% in cyclohexene can be attained at very low benzene
conversion by the use of nanoscale Ru particles. However,
the cyclohexene yield and selectivity achieved so far are too
low for technical applications and are much lower than
those obtained by Ru catalysts suspended in water.
Figure 8. Cyclohexene selectivity in the hydrogenation of benzene by Ru0
nanoparticles in BMI¥PF6 (6 atm of H2 at 758C).
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Chem. Eur. J. 2004, 10, 3734 3740

Partial Hydrogenation of Benzene
3734 3740
Transmission electron microscopy: Transmission electron microscopy
(TEM) observations and selected area diffraction patterns were taken on
a JEM-2010 microscope operating at an accelerating voltage of 200 kV.
Samples for TEM observations were prepared by placing a thin film of
the ruthenium nanoparticles dispersed in isopropanol in a holed carbon
grid. The size distribution of the metal particles was determined from the
measurement of ꢀ150 particle, assuming spherical shape, found inside
five superstructures.
Experimental Section
General methods: All reactions involving ruthenium compounds were
carried out under an argon atmosphere in oven-dried Schlenk tubes. The
ionic liquids BMI¥PF6, BMI¥BF4 and BMI¥CF3SO3 were prepared accord-
ing to known procedures[22] and dried over molecular sieves (4 ä). Their
purity was checked by an AgNO3 test, 1H and 31P NMR spectroscopy,
cyclic voltammetry (water (<0.1% v/v) and chloride content
(<1.8 mgLÀ1)).[23] Solvents and arenes were dried with appropriate
drying agents and distilled under argon prior to use. [Ru(cod)(cot)] was
prepared by reduction of RuCl3 with Zn in the presence of 1,5-cycloocta-
diene following a known procedure.[24] All other chemicals were pur-
chased from commercial sources and used without further purification.
NMR spectra were recorded on a Varian Inova 300 spectrometer. Mass
spectra were recorded on a GC/MS Shimadzu QP-5050 (EI, 70 eV). Gas
chromatography analyses were performed on a Hewlett-Packard5890 gas
chromatograph with a FID and 30 m capillary column with a dimethylpo-
lysiloxane stationary phase. The powder X-ray diffraction was performed
in a Philips X×Pert MRD diffractometer. The diffraction data were col-
lected at room temperature in a Bragg Brentano geometry with a curved
graphite crystal as the monochromator. The equipment was operated at
40 kV and 40 mA with a scan range between 208 to 1008. Transmission
electron microscopy (TEM) was performed on a JEOL 2010 microscope
operating at 200 kV and with a nominal resolution of 0.25 nm.
Sample preparation and XRD powder analysis: The thin 1.0 mm layer of
Ru powder was deposited into a small cavity on a glass substrate covered
with a Kapton tape. The diffraction pattern was obtained after subtrac-
tion of the powder spectrum from a background measured with a glass
substrate plus Kapton tape. The WAXD pattern confirmed crystalline
Ru0 by indexation of Bragg reflections obtained by a pseudo-Voigt pro-
file fitting using the FULLPROF code. The most representative reflec-
tions to Ru0 were indexed as hexagonal and the cell unit parameters ob-
tained were a=2.7487 and c=4.2937 ä. The Bragg reflections at 37.76,
42.05, 43.43, 57.78, 68.17, 77.80, 83.48 and 84.388 correspond to the in-
dexed planes of the (hcp) of Ru0 crystals: (100), (002), (101), (102),
(110), (103), (112) and (201). The mean diameter of the ruthenium par-
ticles was estimated to be 2.5 nm by means of the Debye Scherrer equa-
tion[25] and assuming spherical particles.
X-ray photoelectron spectroscopy (XPS): XPS was performed with MgK
radiation (hn=1253.6 eV). High-resolution scans were recorded with a
pass energy of 15 eV, an angular acceptance ofÆ48 and an entrance slit
of 2mm diameter with an Omicron EA125 hemispheric analyser. The
detection angle, q, of the photoelectrons with respect to the normal to
the sample surface (takeoff angle) was 458. Samples for analysis were
prepared by placing the Ru nanoparticles on a conducting carbon tape.
In addition to the C peaks (support), only Ru and O were observed (spe-
cial monitoring the presence of other possible elements such as chloride
or fluoride was not detected).
The synthesis of the nanoparticles and their hydrogenation reactions
were carried out in a modified Fischer Porter bottle immersed in a sili-
con oil bath and connected to a hydrogen tank. The drop in the hydrogen
pressure in the tank was monitored with a pressure transducer interfaced
through a Novus converter to a PC. The data was processed with Micro-
cal Origin 5.0 (adapted from reference [6k]). The temperature was main-
tained at 758C by a hot stirring plate connected to a digital controller
(ETS-D4IKA). Controlled stirring at 800 rpm was used (no ionic catalyt-
ic solution projection was observed). The catalyst/substrate ratio was cal-
culated from the initial quantity of [Ru0]n used.
Phase diagrams: The three ternary diagrams C6H6/C6H10/BMI¥PF6, C6H6/
C6H8/BMI¥PF6 and C6H8/C6H10/BMI¥PF6 were determined by a gravimet-
ric method. Known quantities of each component were added to a centri-
fuge tube, and the system was stirred at 758C for ꢀ30 min. The upper
phase was then separated from the system and its mass and composition
determined by GC (no significant amount of the ionic liquid in the or-
ganic phase was observed). The composition of the lower phases was de-
termined by component mass balance, namely, the lever-arm rule for bi-
phasic systems. Each experiment was repeated at least three times in
order to ensure the reproducibility of the method.
Formation and isolation of nanoparticles: In a typical experiment, a
Fischer Porter bottle containing a yellow suspension of [Ru(cod)(cot)]
(92.5 mg, 0.3 mmol) in one of the ionic liquids BMI¥PF6, BMI¥BF4 and
BMI¥CF3SO3, 7 mL) was treated with 4 atm of H2 at 758C to afford a
black solution after stirring for 18 h. The Ru nanoparticles were isolated
by centrifugation (3000 rpm) for 30 minutes, washed with water (5î
15 mL) and acetone (5î15 mL), and then dried under reduced pressure.
The Ru samples thus obtained were prepared for TEM, XPS and XRD
powder analysis, and for catalytic experiments (see below).
Hydrogenation reactions:
Liquid liquid biphasic: The olefins or benzene were added to the ionic
catalytic solution obtained by dispersion of the isolated Ru nanoparticles
(described above) in the ionic liquid (1 mL) and hydrogen was admitted
to the system at 4 or 6 atm (constant pressure). Samples for GC and GC-
MS analysis were also removed from time to time under H2. The reaction
mixture forms a typical two-phase system (the lower phase contained the
Ru nanoparticles in the ionic liquid and the upper phase the organic
products). The organic phase was separated by decantation or distillation,
and was then weighed and analysed by GC, GC-MS and 1H NMR spec-
troscopy.
Acknowledgments
We thank CNPq, FAPERGS, CTPETRO, CAPES and CENPES-Petro-
bras for financial support.
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Received: December 21, 2003
Published online: June 15, 2004
3740
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 3734 3740