Efficient palladium and ruthenium nanocatalysts stabilized by phosphine functionalized ionic liquid for selective hydrogenation

Article abstract of DOI:10.1039/c5ra01893e

Pd and Ru nanoparticles were synthesized in ionic liquid by using tri(m-sulfonyl)triphenyl phosphine 1-butyl-2,3-dimethyl-imidazolium salt ([BMMIM]₃[tppt]) as a stabilizing agent. The well-dispersed Pd and Ru NPs with mean diameters of 2.4 nm and 1.7 nm were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It was demonstrated that [BMMIM]₃[tppt] stabilized Pd and Ru NPs displayed high activity and excellent selectivity in the hydrogenation of functionalized olefins, aromatic nitro compounds and aromatic aldehydes. The Pd and Ru NPs showed better catalytic performance than corresponding commercially available Pd/C and Ru/C catalysts. The present catalytic system could be easily reused at least six times without significant decrease in activity and selectivity.

Full text of DOI:10.1039/c5ra01893e

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Efficient palladium and ruthenium nanocatalysts
stabilized by phosphine functionalized ionic liquid
for selective hydrogenation
Cite this: RSC Adv., 2015, 5, 34622
*
Zhifeng Wu and Heyan Jiang
Pd and Ru nanoparticles were synthesized in ionic liquid by using tri(m-sulfonyl)triphenyl phosphine
1-butyl-2,3-dimethyl-imidazolium salt ([BMMIM]3[tppt]) as a stabilizing agent. The well-dispersed Pd and
Ru NPs with mean diameters of 2.4 nm and 1.7 nm were characterized by transmission electron
microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It was
demonstrated that [BMMIM]3[tppt] stabilized Pd and Ru NPs displayed high activity and excellent
selectivity in the hydrogenation of functionalized olefins, aromatic nitro compounds and aromatic
aldehydes. The Pd and Ru NPs showed better catalytic performance than corresponding commercially
available Pd/C and Ru/C catalysts. The present catalytic system could be easily reused at least six times
without significant decrease in activity and selectivity.
Received 30th January 2015
Accepted 8th April 2015
DOI: 10.1039/c5ra01893e
www.rsc.org/advances
visible bulk species, and activity decrease obviously.18–20 Func-
tionalized ionic liquids which can provide stronger stability
Introduction
than simple ionic liquids are designed to act as efficient stabi-
lizers for NPs.18 Recently, transition-metal NPs stabilized by
N-containing functionalized ionic liquids showed satisfactory
performance. Zhao et al.21 used ionic-liquid-like copolymer
stabilized rhodium to catalyse the hydrogenation of benzene
and other arenes, high reaction rates and conversions were
obtained. Dyson et al.22 reported polyvinyl pyrrolidone stabi-
lized rhodium nanoparticles were highly soluble in hydroxyl-
functionalized ionic liquids, providing an effective and highly
stable catalytic system for biphasic hydrogenation. While
Dupont23 found functionalized ionic liquid containing nitrile
groups stabilized Ru NPs displayed unusual selectivity towards
the hydrogenation of nitrile-containing aromatic compounds.
In this work, Pd and Ru NPs stabilized by novel phosphine-
functionalized ionic liquid (PFIL) with characteristic of phos-
phine ligand as the anion were originally utilized as efficient
catalysts in the chemoselective hydrogenation of functionalized
olens, aromatic nitro compounds and aromatic aldehydes.
The well-dispersed Pd and Ru NPs were characterized by TEM,
XRD and XPS. The Pd and Ru NPs showed better catalytic
performance than corresponding commercially available Pd/C
and Ru/C catalysts.
The application of transition-metal nanoparticles (NPs) has
attracted great interest because these NPs have acted as highly
efficient catalysts in both scientic research and industrial
application in recent years.1–3 It is well-known that the high
catalytic activity of nanocatalysts is attributed to their large
surface-to-volume ratios and quantum size effects.4 However,
nanocatalysts are thermodynamically not stable because of
their high excess surface energy, therefore they have a tendency
to form aggregates and/or agglomerates, which lead to catalytic
activity decrease. Stabilizing agents,5–11 such as polymers,
quaternary ammonium salts, surfactants and polyoxoanions,
can form a protective layer around the surface of NPs through
electrostatic stabilization, steric protection and coordination
stabilization to maintain high reactivity of transition-metal NPs
in various reactions.12,13
Ionic liquids which have good thermal stability, low satu-
rated vapour pressure and variable physical and chemical
properties are deemed to be ideal media for various trans-
formations.14 In recent years, the number of reports on ionic
liquids as solvents and/or stabilizers to prepare NPs catalysts in
various reactions, such as hydrogenation,15 C–C coupling16 and
oxidation,17 has been growing. However, traditional simple
ionic liquids could not provide effective stabilization, especially
in catalytic recycle and severe reaction conditions, NPs form
Result and discussion
We chose 1-butyl-2, 3-dimethyl imidazolium hexa-
uorophosphate ([BMMIM]PF6, Scheme 1) rather than 1-butyl-
3-methylimidazolium hexauorophosphate ([BMIM]PF6) as
the reaction media, mainly because N-heterocyclic carbene
might form by C-2 deprotonation or oxidative addition of the
Key Laboratory of Catalysis Science and Technology of Chongqing Education
Commission, Chongqing Key Laboratory of Catalysis and Functional Organic
Molecules, College of Environmental and Biological Engineering, Chongqing
Technology and Business University, Chongqing 400067, China. E-mail: orgjiang@
163.com; Fax: +86-23-62769652
34622 | RSC Adv., 2015, 5, 34622–34629
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Scheme 1 The structures of [BMMIM]PF6 and [BMMIM]3[tppt].
C-2–H bond of 1,3-dialkylimidazolium ionic liquids, resulting
in the poison of the nanocatalysts and drastic decrease of the
catalytic activity and selectivity.24,25
Fig. 2 XRD pattern of Pd-1.
The Pd NPs stabilized by [BMMIM]3[tppt] (Scheme 1) were
prepared by hydrogenation reduction of Pd(OAc)2 in [BMMIM]PF6.
A black powder could be isolated from the obtained Pd NPs by
adding acetone and then centrifuging (5000 rpm, 5 min). Washed
three times with acetone and dried under reduced pressure. The
isolated black powder (Pd-1) was characterized by TEM, XRD and
XPS methods.
TEM was used to conrm the formation of Pd particles and
observe dispersal. TEM images and distribution histograms of
nearly spherical Pd NPs were displayed in Fig. 1, the observation
indicated that these particles were well dispersive with an
average diameter of 2.4 nm in fresh Pd-1. Furthermore, the Pd
NPs aer six recycles of styrene hydrogenation were also well-
dispersed with an average diameter of 2.76 nm. Size distribu-
tion histograms were obtained on a basis of the measurement of
300 particles.
The surface characteristics of Pd-1 were investigated by XPS
(Fig. 3). Pd 3d5/2 and Pd 3d3/2 signals, with binding energies of
335.8 eV and 341.2 eV, were observed respectively. In addition,
the main peak of Pd 3d5/2 shied 0.8 eV higher in binding
energy compared to the specimens of the giant clusters of
Pd(0) (335.0 eV). The XPS observation was consistent with
previous literature.26 We deduced there had a tendency that the
electron clouds transferred from Pd(0) particles surface to
[BMMIM]3[tppt].27 On the other hand, the Pd 3d spectrum
indicated the presence of two chemical states of Pd at the
nanoparticle surface with distinct binding energies; the main
contribution was related to Pd(0) (Pd–Pd bonds, Pd5/2 at
335.8 eV) and the other corresponding to Pd–F bonds (Pd5/2
at 337.3 eV).28
In short, the results of TEM, XRD and XPS indicated that
Pd(II) species were completely reduced to Pd(0) NPs by molec-
ular hydrogenation.
The XRD pattern of Pd-1 (Fig. 2) conrmed that the presence
of crystalline Pd(0). The most representative reections of Pd(0)
were indexed as face-centered cubic (fcc) structure. The Bragg
reections at 40.16ꢀ, 46.68ꢀ and 68.30ꢀ, corresponded to the
indexed planes of the crystals of Pd(0) (111), (200) and (220).
The effect of [BMMIM]3[tppt] in catalyst is noteworthy. Due
to the strong coordination capacity of the anion of
[BMMIM]3[tppt], Pd NPs were well dispersed and highly stable
in catalytic hydrogenation. On the basis of these results, it was
deduced that the [P(C6H4-m-SO3)3ꢁ] ions formed a layer around
the surface of the Pd NPs, leading to a sphere of negative charge,
and then the cation of [BMMIM]3[tppt] became arranged as an
outer layer for charge conservation (Fig. 4). Recently, several
groups have demonstrated ionic liquids possess self-organized
structures, which can create an external layer around the
surface of the metal NPs to protect them from aggregation.29–31
Fig. 1 TEM images of Pd-1 before (up) and after (down) six recycles of
styrene hydrogenation.
Fig. 3 X-ray photoelectron spectra of Pd-1.
RSC Adv., 2015, 5, 34622–34629 | 34623
This journal is © The Royal Society of Chemistry 2015

View Article Online
RSC Advances
Paper
the hydrogenation, diethyl ether was added to extract products,
and then the catalysts were washed with diethyl ether three
times and further treated under vacuum for the next run. It was
found that the catalyst Pd-1 could be reused at least six times
without signicant decrease in activity. However, the activity of
Pd-2 diminished dramatically aer the third cycle. The recy-
clability results demonstrated the remarkable performance of
Pd-1 catalyst and the strong stabilization effect of
[BMMIM]3[tppt] to Pd NPs.
Before the chemoselective hydrogenation of olens, we
chose hexane and cyclohexene as substrates to test the catalytic
performance of Pd-1. The results showed that complete
conversion could be achieved in 8 h and 5 h respectively. We
further investigated chemoselective hydrogenation of various
olens (Table 1). Pd-1 dispersed in [BMMIM]PF6 showed
excellent chemoselectivity in the hydrogenation of various
functionalized olens. The fact that styrene and ethyl acrylate
(Table 1, entries 1–2) exhibited higher activity than hexane and
cyclohexene indicated the catalytic hydrogenation was appar-
ently affected by the steric effect and the electronic effect.
Furthermore, 4-phenyl-3-buten-2-one and 1, 5-cyclooctadiene
(Table 1, entries 3–4), could be completely converted into the
corresponding saturated C–C single-bond compounds with the
aromatic ring and carbonyl group remained.
Selective hydrogenation of substituted nitrobenzene to cor-
responding aniline is still a signicant reaction and challenge
for chemists, since aniline and its derivatives are important
intermediates for dyes, polyurethanes, pharmaceuticals, explo-
sives and agrochemicals.33,34 As in Table 2, Pd-1 catalytic system
was very active and exclusively selective towards hydrogenation
of the nitro group (Table 2, entry 1). While commercial Pd/C
catalyst acted as the heterogeneous catalyst, chemoselectivity
to aniline was 98% (Table 2, entry 2). Additionally, Pd-2 showed
an unsatisfactory conversion of 80% and 95% chemoselectivity
to aniline (Table 2, entry 3). Pd-2 catalytic species formed black
particles visible to naked eyes aer the hydrogenation. The
results testied that traditional ionic liquid ([BMMIM]PF6)
could not provide effective stabilization for Pd NPs. The scope of
the substrates was also tested. The aromatic nitro compounds
Fig. 4 The PFIL ([BMMIM]3[tppt]) stabilized/modified Pd nanoparticles.
Initially, we chose styrene as a model substrate to explore the
catalytic performance of Pd-1. Fig. 5 showed the effect of the
conversion versus time. The hydrogenation of styrene produced
only ethylbenzene in the test. It was found that the conversion
increased linearly with time, showing no induction period,
proving that this catalyst did not convert into other catalytically
active species. Furthermore, in order to ascertain the involve-
ment of metallic Pd in the hydrogenation reactions, mercury-
poisoning experiments were run as they could selectively
poison metal nanoparticles, by forming an amalgam with
mercury, to help distinguish between homogeneous and
heterogeneous catalysts.32 An excess of Hg0 (300 equiv.) was
added to the reaction mixture aer 2 h (about 60% conversion)
and then the catalytic system was performed under standard
hydrogenation conditions. The catalytic activity was completely
suppressed and no catalytic activity was observed even though
the catalytic system had been vigorously stirred with Hg0 for
another 3 h. All the results above strongly support proposal that
the reaction progresses under heterogeneous catalysis rather
than homogeneous catalysis.
In order to investigate the recyclability of catalysts and the
stabilization effect of BMMIM]3[tppt] to Pd NPs, both Pd-1 and
Pd NPs with no addition of [BMMIM]3[tppt] in the preparation
(Pd-2) were tested in the hydrogenation of styrene (Fig. 6). Aer
Fig. 6 Recyclability of Pd-1 and Pd-2 in hydrogenation of styrene.
Fig. 5 Reaction profile for the hydrogenation of styrene and Hg0 catalyst Pd (2.5 ꢂ 10ꢁ3 mmol), substrate/Pd ¼ 250, [BMMIM]PF6 (1 mL),
poisoning test of Pd-1. Catalyst Pd-1 (2.5 ꢂ 10ꢁ3 mmol), substrate/ 1 MPa initial hydrogen pressure, 45 ꢀC. The hydrogenation of styrene
Pd ¼ 250, [BMMIM]PF6 (1 mL), 1 MPa initial hydrogen pressure, 45 ꢀC. produced only ethylbenzene in the test.
34624 | RSC Adv., 2015, 5, 34622–34629
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Table 1 Chemoselective hydrogenation of various olefins by nanocatalyst Pd-1a
Entry
Substrate
Product
t. hꢁ1
Conv. (%)b
Sel. (%)
1
2
3
4
Styrene
Ethyl acrylate
4-Phenyl-3-buten-2-one
1,5-Cyclooctadiene
Ethylbenzene
4
4
15
15
100
99
100
100
100
100
100
100
Ethyl propionate
4-Phenyl-2-butanone
Cyclooctane
a
Reaction conditions: Pd-1 (2.5 ꢂ 10ꢁ3 mmol), substrate/Pd ¼ 250, [BMMIM]PF6 (1 mL), 1 MPa initial hydrogen pressure, 45 ꢀC. b GC yield.
bearing different substituent groups (–CH3, –Cl, –F, –C]O) to 100% (Table 3, entry 3). The conversion of commercial Pd/C
(Table 2, entries 4–10) did not show signicant inuence on the catalyst also increased to 93% (Table 3, entry 4). However, Pd-2
activity and selectivity during the hydrogenation. Furthermore, showed unsatised conversion of just 40% (Table 3, entry 5).
dehalogenation was not detected in the process of hydrogena- Some representative examples are listed in Table 3 for the che-
tion. However, considering substrates with a methoxy substit- moselective hydrogenation of aromatic aldehydes catalyzed by
uent, the activity was sharply decreased (Table 2, entries 11–12). Pd-1 in a mixture solvent of H2O and [BMMIM]PF6. The catalytic
The results revealed that Pd-1 showed promising activity for system showed good activity and selectivity to corresponding
chemoselective hydrogenation of nitrobenzene and its deriva- aromatic alcohols (Table 3, entries 6–10).
tives, which was not apparently affected by the steric effect, but
possibly by the electric effect.
Based on the obvious difference in catalytic activity during
the chemoselective hydrogenation of olens and aromatic
Aromatic alcohols are important intermediates in pharma- aldehydes (Table 1 and 3), the hydrogenation intermediates are
ceuticals, avors, fragrances, chemical intermediates and proposed and shown in Scheme 2. In comparison with the
photographic chemicals.35 We further applied Pd-1 in the chal- chemoselective hydrogenation of olens (Scheme 2A), the che-
lenging chemoselective hydrogenation of aromatic aldehydes. moselective hydrogenation of C]O in aromatic aldehydes
Benzaldehyde was selected as the model substrate and relatively (Scheme 2B) is less thermodynamically preferable. So, the
low conversion of 27.5% was obtained (Table 3, entry 1). The olens exhibited much higher hydrogenation activity than
conversion of commercial catalyst Pd/C was 24% (Table 3, entry aromatic aldehydes. Additionally, the results in Table 3 have
2). Water is an attractive alternative to traditional organic clearly shown that water plays a promotional role in the
solvents, because it is cheap, readily available, nontoxic, non- hydrogenation of aromatic aldehydes to aromatic alcohols. We
ammable and safe to environment. When water was intro- speculate that C]O not only interacts with palladium, but it
duced to catalytic system, the activity of Pd-1 sharply increased also forms the hydrogen bond with water (Scheme 2C). As a
Table 2 Chemoselective hydrogenation of aromatic nitro compounds by Pd-1a
Entry
Substrate
Product
T (h)
Conv. (%)b
Sel. (%)
1
Nitrobenzene
Nitrobenzene
Nitrobenzene
p-Nitrotoluene
Aniline
Aniline
Aniline
p-Touidine
m-Toluidine
p-Chloroaniline
o-Chloroaniline
p-Fluoroaniline
o-Aminoacetophenone
m-Aminoacetophenone
p-Anisidine
12
12
12
12
12
8
10
12
12
12
12
12
100
100
80
100
92
100
100
97
100
100
38.4
22.4
100
98
95
2c
3d
4
100
100
100
100
100
100
100
100
100
5
6
7
8
m-Nitrotoluene
p-Chloronitrobenzene
o-Chloronitrobenzene
p-Fluoronitrobenzene
o-Nitroacetophenone
m-Nitroacetophenone
p-Nitroanisole
9
10
11
12
o-Nitroanisole
o-Anisidine
a
Reaction conditions: catalyst Pd (2.5 ꢂ 10ꢁ3 mmol), substrate/Pd ¼ 200, [BMMIM]PF6 (1 mL), 5 MPa initial hydrogen pressure, 50 ꢀC. b GC yield.
c
10% Pd/C (2.66 mg). d Pd NPs with no addition of [BMMIM]3[tppt] in the preparation (Pd-2).
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 34622–34629 | 34625

View Article Online
RSC Advances
Paper
Table 3 Chemoselective hydrogenation of aromatic aldehydes catalyzed by Pd-1a
Entry
Substrate
Co-solventb
Product
Time (h)
Con. (%)c
Sel. (%)
1
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
—
—
Phenylmethanol
Phenylmethanol
Phenylmethanol
Phenylmethanol
Phenylmethanol
p-Tolylmethanol
p-Chlorobenzylalcohol
m-Chlorobenzylalcohol
o-Chlorobenzylalcohol
o-Methoxybenzylalcohol
19
19
19
19
19
40
45
48
48
48
27.5
24
100
93
40
100
99
85.1
97
78.5
100
100
100
100
100
100
100
100
100
100
2d
3
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
4d
5e
6
7
8
Benzaldehyde
p-Methylbenzaldehyde
p-Chlorobenzaldehyde
m-Chlorobenzaldehyde
o-Chlorobenzaldehyde
o-Methoxybenzaldehyde
9
10
a
Reaction condition: catalyst Pd (2.5 ꢂ 10ꢁ3 mmol), substrate/Pd ¼ 1 : 50, [BMMIM]PF6 (1 mL), 5 MPa initial hydrogen pressure, 50 ꢀC. b co-solvent
(1 mL) was introduced. c GC yield. d 10% Pd/C (2.66 mg). e Pd NPs with no addition of [BMMIM]3[tppt] in the preparation (Pd-2).
300 particles. These particles displayed a monomodal size
distribution and the average diameter was 1.7 nm. In compar-
ison with Pd NPs, Ru NPs were signicantly smaller. This
phenomenon could be explained that Pd NPs can easily form
aggregation, especially in severe reaction conditions.18
XRD analysis indicated that the solid consists of metal
particles of hexagonal close packed (hcp) ruthenium (Fig. 8).
The Bragg reections at 38.41ꢀ, 42.03ꢀ, 43.79ꢀ, 58.13ꢀ, 69.06ꢀ,
78.44ꢀ corresponded to the indexed planes of (hcp) crystals of
Ru (0): (100), (002), (101), (102), (110), (103) respectively. XPS
analysis revealed the surface composition of Ru NPs. Ru 3d5/2
and 3d3/2 signals with binding energies of 280.0 eV and 284.8 eV
were observed, which was consistent with Ru(0).36
We selected nitrobenzene as standard substrate to explore
the performance of Ru-1 catalyst. The results revealed that Ru-
1 nanocatalyst showed high activity and chemoselectivity in
the hydrogenation of nitrobenzene (Table 4, entry 1). However,
the catalytic activity of commercial Ru/C catalyst was only 49%
(Table 4, entry 2). When there was no addition of
[BMMIM]3[tppt] in the preparation of Ru NPs (Ru-2), the Ru-2
catalyst displayed a very poor conversion of only 4.6% (Table 4,
entry 3). Ru-2 species formed black particles visible to naked
eyes aer the hydrogenation. The above results demonstrated
Scheme 2 Proposed intermediates for olefins and aromatic aldehydes
hydrogenation catalyzed by Pd-1.
result, the hydrogen bond between C]O and water improves
the activity of Pd-1 for the hydrogenation of aromatic aldehydes
to aromatic alcohols.
On the basis of the above results, we examined the use of Ru
NPs as active species and extended the application scope of PFIL
stabilized transition-metal NPs catalytic system. The Ru nano-
catalyst was prepared by hydrogenation reduction of RuO2
ꢀ
hydrate in [BMMIM]PF6 at 75 C for 4 h. A black powder could
be isolated from the obtained Ru NPs by adding acetone and
then centrifuging (5000 rpm, 5 min). Washed three times with
acetone and dried under reduced pressure. The obtained black
species (Ru-1) were characterized by TEM, XRD and XPS
methods.
TEM analysis was displayed in Fig. 7, the metal particle size
distribution was estimated from the measurement of about
Fig. 7 TEM image of Ru-1.
Fig. 8 X-ray diffraction pattern of isolated Ru-1.
34626 | RSC Adv., 2015, 5, 34622–34629
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Table 4 Chemoselective hydrogenation of aromatic nitro compounds by Ru-1a
Entry
Substrate
Product
T (h)
Conv. (%)b
Sel. (%)
1
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Aniline
Aniline
Aniline
Aniline
p-Toluidine
m-Toluidine
p-Chloroaniline
o-Chloroaniline
p-Fluoroaniline
p-Anisidine
o-Anisidine
o-Aminoacetophenone
1.5
1.5
1.5
2.5
2
4
4
4
4
100
49
4.6
100
100
100
100
100
100
100
100
100
100
100
100
2c
3d
4e
5
100
100
100
100
100
100
100
100
100
p-Nitrotoluene
6
7
8
9
10
11
12
m-Nitrotoluene
p-Chloronitrobenzene
o-Chloronitrobenzene
p-Fluoronitrobenzene
p-Nitroanisole
4
4
4
o-Nitroanisole
o-Nitroacetophenone
a
Reaction conditions: catalyst Ru (17.75 ꢂ 10ꢁ3 mmol), [BMMIM]PF6 (1 ml), substrate/Ru ¼ 200, inner hydrogen pressure 5 MPa, 60 ꢀC. b GC yield.
c
Ru/C (5% wt, 36 mg). d No addition of [BMMIM]3[tppt] in preparation of Ru NPs. e Catalyst Ru (2.5 ꢂ 10ꢁ3 mmol), 50 ꢀC.
that Ru-1 nanocatalyst was more efficient than commercial
Ru/C catalyst in chemoselective hydrogenation of nitroben-
zene in [BMMIM]PF6 (Table 4, entries 1 vs. 2). The catalytic
performance of Ru-2 revealed the strong stabilization effect of
BMMIM]3[tppt] to Ru NPs (Table 4, entries 1 vs. 3). Since the
percentage of Ru in Ru-1 catalyst is higher than the percentage
of Pd in Pd-1 catalyst (Table 4, entry 1 vs. Table 2, entry 1), it is
necessary to clarify whether the high catalytic activity of Ru-1 is
owing to the high concentration of active metal. Under the
same reaction conditions with Pd-1 in Table 2, Ru catalyst with
the same mole loading to Pd-1 was also tested in the hydro-
genation of nitrobenzene (Table 4, entry 4). Nitrobenzene
could be completely hydrogenated to aniline in 2.5 h. However,
12 hours was needed for Pd-1 to achieve the complete hydro-
genation (Table 2, entry 1). Substituted nitrobenzenes were
also investigated over Ru-1 in [BMMIM]PF6. The substrates
were completely converted to corresponding anilines within
4 h (Table 4, entries 5–12). The results revealed that Ru-1
showed promising activity for chemoselective hydrogenation
of nitrobenzene and its derivatives, which was not apparently
affected by the steric effect and the electric effect.
Experimental section
Materials
Palladium acetate, ruthenium dioxide hydrate, 10% Pd/C, 5%
Ru/C, and triphenylphosphine were purchased from Adamas
Reagent Co. Ltd., various substrates and all other materials were
obtained from Aladdin. The purity of H2, N2 and air were 99.99%.
All manipulations involving air sensitive materials were carried
out by using standard Schlenk line techniques under an atmo-
sphere of nitrogen and all solvents were dried by standard
methods prior to use. [BMMIM]PF6 was prepared according to
previously reported procedure.37 Tris(3-sulfophenyl)phosphine
trisodium salt (Na3[TPPTS]) was synthesized according to litera-
ture.38 The phosphine functionalized ionic liquid [BMMIM]3[tppt]
was synthesized according to the literature.29
Preparation of Pd-1
A mixture of 2.5 mmol Lꢁ1 Pd(OAc)2 solution in acetone (1 mL),
2.3 mg [BMMIM]3[tppt] and [BMMIM]PF6 (1 mL) was stirred at
room temperature in a stainless steel autoclave (50 mL) for
several minutes, then the solvent was evaporated under
vacuum, followed by reduction with molecular hydrogen
ꢀ
(0.2 MPa) at 60 C in an oil bath for 20 min which afforded a
Conclusion
dark brownish solution that was used directly for the hydroge-
nation. The percentage of Pd in Pd-1 was 0.26& wt/wt.
In summary, highly dispersed Pd and Ru NPs were prepared in
[BMMIM]PF6 by using PFIL ([BMMIM]3[tppt]) as the stabilizer.
These metal NPs catalysts showed excellent catalytic activity and
chemoselectivity in the challenging hydrogenation of alkenes,
Preparation of Pd-2
aromatic nitro compounds and aromatic aldehydes. These Pd A mixture of 2.5 mmol Lꢁ1 Pd(OAc)2 solution in acetone (1 mL),
and Ru NPs showed better catalytic performance than [BMMIM]PF6 (1 mL) were stirred at room temperature in a
commercially available Pd/C and Ru/C catalysts. The present stainless steel autoclave (50 mL) for several minutes, then the
catalytic system could be easily reused at least six times without solvent was evaporated under vacuum, followed by reduction
ꢀ
signicant decrease in activity and selectivity. Additional work with molecular hydrogen (0.2 MPa) at 60 C in an oil bath for
is currently in progress in this and related areas.
20 min which afforded a dark brownish solution that was used
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 34622–34629 | 34627

View Article Online
RSC Advances
Paper
directly for the hydrogenation. The percentage of Pd in Pd-2 was reaction mixture, the reaction system was reestablished in the
ꢀ
0.26& wt/wt.
same conditions (1 MPa, 45 C), aer stirring for another 2 h,
the products were extracted with diethyl ether and analysed by
GC method.
Preparation of Ru-1
In a typical experiment the precursor RuO2 hydrate (3 mg,
0.0225 mmol), [BMMIM]3[tppt] (16.3 mg, 0.0225 mmol) was
dispersed in [BMMIM]PF6 (1 mL) in a stainless steel autoclave
(50 mL), then the ask was purged three times with H2, which
was pressurized to 4 atm with H2 and heated to 75 ꢀC in an oil
bath. Aer stirring for 4 h, the reactor was cooled to ambient
temperature and carefully vented. A dark solution was obtained
that was used directly for the hydrogenation. The percentage of
Ru in Ru-1 was 1.7& wt/wt.
Catalyst characterization
Transmission electron microscopy (TEM) images were obtained
with JEM 2010 transimission electron microscopy at 200 kV with
a point resolution of 0.23 nm. Samples for TEM were prepared
by dropping acetate solutions containing the nanoparticles
onto carbon-coated Cu grids. X-ray photoelectron spectroscopy
(XPS) measurements were performed on a Kratos XSAM 800
spectrometer. The X-ray diffraction (XRD) analysis was per-
formed in a D/MAX 2550 VB/PC using a graphite crystal as
monochromator. Products were analyzed by GC instrument
with an FID detector and HP-5 column (30 m ꢂ 0.25 mm).
Preparation of Ru-2
In a typical experiment the precursor RuO2 hydrate (3 mg,
0.0225 mmol), was dispersed in [BMMIM]PF6 (1 mL) in a
stainless steel autoclave (50 mL), then the ask was purged
three times with H2, which was pressurized to 4 atm with H2
Acknowledgements
ꢀ
This work was nancially supported by National Natural
Science Foundation of China (no. 21201184), Natural Science
Foundation Project of CQ (no. cstc2014jcyjA10105), Ministry of
Education of Chongqing (no. KJ1400601) and 100 leading
scientists promotion project of Chongqing.
and heated to 75 C in an oil bath. Aer stirring for 4 h, the
reactor was cooled to ambient temperature and carefully ven-
ted. A dark solution was obtained that was used directly for the
hydrogenation. The percentage of Ru in Ru-2 was 1.7& wt/wt.
Catalytic hydrogenation
Notes and references
All hydrogenation reactions were carried out in a 50 mL
stainless-steel high pressure reactor with a glass inlet. The
stainless-steel reactor containing previously prepared nano-
particles dispersed in ionic liquid [BMMIM]PF6 was charged
with appropriate substrate, then the reactor was ushed three
times with molecular hydrogen, H2 was charged to the desired
pressure and heated to optimal temperature and stirred for
given time in an oil bath. Aer the reaction, the reactor was
cooled to ambient temperature and carefully vented. The
products were extracted with diethyl ether three times and
analysed by GC method. For the recycling procedure, the
remained diethyl ether in catalytic system was evaporated under
vacuum and then the fresh substrate was added for the next
recycling in the same conditions. Additionally, it was found that
the amount of leaching from catalyst was negligible by ICP-AES
analysis aer the catalytic reaction.
1 H. J. Chen, H. W. Liu, W. S. Liao, H. B. Pan, C. M. Wai,
K. H. Chiu and J. F. Jen, Appl. Catal., B, 2012, 111–112, 402.
2 M. Pumera, S. Sanchez, I. Ichinose and J. Tang, Sens.
Actuators, B, 2007, 123, 1195.
3 C. X. Xiao, Z. P. Cai, T. W. Wang, Y. Kou and N. Yan, Angew.
Chem., Int. Ed., 2008, 47, 746.
4 O. A. Belyakova and Y. L. Slovokhotov, Russ. Chem. Bull.,
2003, 52, 2299.
5 C. Pan, K. Pelzer, K. Philippot, B. Chaudret, F. Dassenoy,
P. Lecante and M. J. Casanove, J. Am. Chem. Soc., 2001,
123, 7584.
6 J. L. Pellegatta, C. Blandy, V. Colliere, R. Choukroun,
B. Chaudret, P. Cheng and K. Philippot, J. Mol. Catal. A:
Chem., 2002, 178, 55.
7 J. Schulz, A. Roucoux and H. Patin, Chem. Commun., 1999,
535.
Hg(0) poisoning experiment
8 Y. M. A. Yamada, T. Arakawa, H. Hocke and Y. Uozumi,
Angew. Chem., Int. Ed., 2007, 46, 704.
The previously prepared Pd-1 dispersed in [BMMIM]PF6 was
charged with styrene (52.1 mg, 0.625 mmol) in a 50 mL
stainless-steel reactor. The system was ushed three times with
9 W. W. Zhu, H. M. Yang, Y. Y. Yu, H. Li, H. Li, B. Feng and
Z. S. Hou, Phys. Chem. Chem. Phys., 2011, 13, 13492.
H2; the reactor was placed in an oil bath at 45 ꢀC and in 1 MPa 10 A. Nowicki, V. L. Boulaire and A. Roucoux, Adv. Synth. Catal.,
H2 pressure. Aer stirring for 4 h, the reactor was cooled to
room temperature and depressurized carefully. The products 11 D. Astruc, F. Lu and J. R. Aranzaes, Angew. Chem., Int. Ed.,
were extracted three times by diethyl ether and analysed by GC. 2005, 44, 7852.
The remaining products in the catalytic system was evaporated 12 S. Sawoo, D. Srimani, P. Dutta, R. Lahiri and A. Sarkar,
under vacuum, then additional fresh styrene (52.1 mg, Tetrahedron, 2009, 65, 4367.
2007, 349, 2326.
0.625 mmol) were added into the reactor, the pressure was 13 K. L. Luska and A. Moores, Green Chem., 2012, 14, 1736.
reestablished to 1 MPa and heated to 45 ꢀC. Aer stirring for 2 h, 14 J. D. Scholten, B. C. Leal and J. Dupont, ACS Catal., 2012, 2,
the reaction was stopped and elemental Hg was added to the
184.
34628 | RSC Adv., 2015, 5, 34622–34629
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
¨
15 J. Julis, M. Holscher and W. Leitner, Green Chem., 2010, 12, 28 A. P. Umpierre, G. Machado, G. H. Fecher, J. Morais and
1634. J. Dupont, Adv. Synth. Catal., 2005, 347, 1404.
16 M. H. G. Prechtl, J. D. Scholten and J. Dupont, Molecules, 29 K. L. Luska and A. Moores, Green Chem., 2012, 14, 1736.
2010, 15, 3441.
30 C. W. Scheeren, G. Machado, S. R. Teixeira, J. Morais,
J. B. Domingos and J. Dupont, J. Phys. Chem. B, 2006, 110,
13011.
17 N. Jain, A. Kumar and M. S. C. Shive, Tetrahedron Lett., 2005,
46, 2599.
18 K. L. Luska and A. Moores, Adv. Synth. Catal., 2011, 353, 31 W. W. Zhu, Y. Y. Yu, H. M. Yang, H. Li, Y. X. Qiao, X. G. Zhao
3167. and Z. S. Hou, Chem.–Eur. J., 2013, 19, 2059.
19 C. W. Scheeren, G. M. achado, S. R. Teixeira, J. Morais, 32 B. J. Hornstein, J. D. Aiken and R. G. Finke, Inorg. Chem.,
J. B. Domingos and J. Dupont, J. Phys. Chem. B, 2006, 110,
13011.
20 X. Yuan, N. Yan, C. X. Xiao, C. N. Li, Z. F. Fei, Z. P. Cai, Y. Kou
and P. J. Dyson, Green Chem., 2010, 12, 228.
2002, 41, 1625.
33 Y. Hu, Y. Y. Yu, X. G. Zhao, H. M. Yang, B. Feng, H. Li,
Y. X. Qiao, L. Hua, Z. Y. Pan and Z. S. Hou, Sci. China:
Chem., 2010, 53, 1541.
21 Z. Chen, H. Z. Wang, N. Yan, C. X. Xiao, X. D. Mu, P. J. Dyson 34 G. Wienhofer, I. Sorribes, A. Bod-dien, F. Westerhaus,
and Y. Kou, J. Catal., 2007, 250, 33.
22 X. Yang, N. Yan, Z. F. Fei, R. M. C. Quesada, G. Laurenczy,
K. Junge, H. Junge, R. Llusar and M. Beller, J. Am. Chem.
Soc., 2011, 133, 12875.
L. K. Minsker, Y. Kou, Y. D. Li and P. J. Dyson, Inorg. 35 Y. H. Zhou, J. Liu, X. Y. Li, X. L. Pan and X. H. Bao, J. Nat. Gas
Chem., 2008, 47, 17. Chem., 2012, 21, 241.
23 M. H. G. Prechtl, J. D. Scholten and J. Dupont, J. Mol. Catal. 36 Y. H. Zhu, W. Effendi, L. P. S. Shirley, Z. Wang, C. Keith,
A: Chem., 2009, 313, 74.
24 G. S. Fonseca, J. B. Domingos, F. Nome and J. Dupont, J. Mol.
Catal. A: Chem., 2006, 248, 10.
25 L. S. Ott, S. Campbell, K. R. Seddon and R. G. Finke, Inorg.
Chem., 2007, 46, 10335.
26 A. Tressaud, S. Khairoun, H. Touhara and N. Z. Watanabe, Z.
Anorg. Allg. Chem., 1986, 540, 291.
J. A. Maguire, N. S. Hosmane and M. F. Hawthorne, J. Am.
Chem. Soc., 2007, 129, 6507.
37 J. G. Huddleston, H. D. Willauer, R. P. Swatloski, A. E. Visser
and R. D. Rogers, Chem. Commun., 1988, 1765.
38 W. A. Herrmann, G. P. Albanese, R. B. Manetsberger,
P. Lappe and H. Bahrmann, Angew. Chem., Int. Ed., 1995,
34, 811.
27 Y. Hu, H. M. Yang, Y. C. Zhang, Z. S. Hou, X. R. Wang,
Y. X. Qiao, H. Li, B. Feng and Q. F. Huang, Catal.
Commun., 2009, 10, 1903.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 34622–34629 | 34629