Rhodium nanoparticles stabilized with phosphine functionalized imidazolium ionic liquids as recyclable arene hydrogenation catalysts

Article abstract of DOI:10.1016/j.cattod.2011.09.016

Rhodium nanoparticles (Rh NPs) stabilized by phosphine functionalized ionic liquids (FILs) were prepared in [BDMI]NTf₂ (BDMI = 1-butyl-2,3-dimethylimidazolium, NTf₂ = bis(trifluoromethanesulfonyl) imide) using H₂(g) (4 bar) as a reducer. Rh(allyl)₃ was used as a "salt-free" Rh NP precursor and allowed to enhance the stability of the Rh NPs compared to the usual RhCl₃ precursor. The synthesized FIL stabilized Rh NPs proved to be active biphasic catalysts for the hydrogenation of toluene, styrene and xylenes under mild conditions (75°C, 40 bar H₂(g), 3 h). The impact on activity of the length of the spacer between the phosphine function and the ionic liquid moiety in the FIL was studied. The Rh NPs stabilized by FILs showed higher catalytic activity and recyclability than NPs synthesized in the absence of a stabilizer and more stable than the system employing triphenylphosphine (PPh₃) as a stabilizer. The size of the stabilized Rh NPs was measured to be around 2 nm by TEM, while those produced in the absence of a FIL stabilizer formed only aggregates.

Full text of DOI:10.1016/j.cattod.2011.09.016

Catalysis Today 183 (2012) 96–100
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Rhodium nanoparticles stabilized with phosphine functionalized imidazolium
ionic liquids as recyclable arene hydrogenation catalysts
∗
Samantha A. Stratton, Kylie L. Luska, Audrey Moores
Department of Chemistry, McGill University, Otto Maass Chemistry Building, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2011
Received in revised form
Rhodium nanoparticles (Rh NPs) stabilized by phosphine functionalized ionic liquids (FILs) were prepared
in [BDMI]NTf2 (BDMI = 1-butyl-2,3-dimethylimidazolium, NTf2 = bis(trifluoromethanesulfonyl)imide)
using H2(g) (4 bar) as a reducer. Rh(allyl)3 was used as a “salt-free” Rh NP precursor and allowed to
enhance the stability of the Rh NPs compared to the usual RhCl3 precursor. The synthesized FIL stabilized
Rh NPs proved to be active biphasic catalysts for the hydrogenation of toluene, styrene and xylenes under
1
4 September 2011
Accepted 14 September 2011
Available online 7 October 2011
◦
mild conditions (75 C, 40 bar H2(g), 3 h). The impact on activity of the length of the spacer between the
phosphine function and the ionic liquid moiety in the FIL was studied. The Rh NPs stabilized by FILs
showed higher catalytic activity and recyclability than NPs synthesized in the absence of a stabilizer and
more stable than the system employing triphenylphosphine (PPh3) as a stabilizer. The size of the sta-
bilized Rh NPs was measured to be around 2 nm by TEM, while those produced in the absence of a FIL
stabilizer formed only aggregates.
Keywords:
Rhodium
Nanoparticles
Biphasic catalysis
Hydrogenation
Functionalized ionic liquids
Arene
© 2011 Elsevier B.V. All rights reserved.
1
. Introduction
Transition metal nanoparticles (NPs) have gained increasing
These biphasic systems allow the products to be extracted from
the IL phase containing the catalytic particles and permits easy
recoverability of the catalyst [17,18]. Recently, several groups have
looked into the impact of alterations of the IL parameters (i.e. N-
alkyl chain length, counter anion volume) and have shown them
to influence the properties of the resulting NPs [19–22]. How-
ever, under catalytic conditions, these IL stabilized NPs have been
observed to aggregate [21,23–26] as the displacement of the IL
from the particle surface can occur facilely [27–29]. Several groups
[22,30,31] have been developing functionalized ionic liquids (FILs)
to increase the stabilization of catalytically active NPs in ILs to
produce more robust recyclable catalysts. FILs offer electrostatic
stabilization to NPs in a similar fashion to traditional ILs, while
also providing covalent stabilization through an additional chem-
ical moiety able to bind to the NP surface (i.e. bipyridine [22,28],
phosphine [32], thiol [33]). For instance, the groups of Dyson and
Roucoux have developed bipyridine ligands functionalized with
an imidazolium head group to stabilize Rh NPs used as arene
hydrogenation catalysts [22,28]. The group of Dyson studied the
effect of the spacer between the bipyridine group and the imida-
zolium cation and found that a longer chain spacer had a beneficial
effect on catalysis. Recently, our group synthesized phosphine
FILs based on an imidazolium head group, varied in the N-alkyl
chain length and the nature of the counter anion, as stabiliz-
ers for Pd NPs. The catalytic activity of these Pd NPs was also
dependent on the N-alkyl chain length and the counter anion;
interest in recent years due to their applications in such areas as
catalysis, sensing and nanoelectronics [1]. These chemical species
feature properties specific to their nanometric dimension and thus
considerable research effort was dedicated to their stabilization in
order to extend their lifetime [2]. In the context of catalysis, this
additional stabilization influences the activity and the recyclabil-
ity of the particles as well as the selectivity of the products. This
stabilization can be provided by traditional organic ligands (i.e.
phosphines or pyridines) [3–5], polymers [6–8] or solid supports
[
9–11].
Ionic liquids (ILs) have come into the spotlight as interesting
systems for NP stabilization as they act as both solvent and lig-
and. DLVO theory has been used as a first approximation to explain
the colloidal stabilization provided by the cationic head group and
counter anion of the IL [12–14]. In addition, imidazolium ILs are
known to feature hydrophobic and hydrophilic domains through-
out the liquid [12,13,15], which help control growth during NP
synthesis [15,16]. Numerous transition metal NPs have been syn-
thesized in imidazolium ILs and they exhibit high catalytic activity.
∗ Corresponding author. Tel.: +1 514 398 4654; fax: +1 514 398 3797.
E-mail address: audrey.moores@mcgill.ca (A. Moores).
0
920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.09.016

S.A. Stratton et al. / Catalysis Today 183 (2012) 96–100
97
Fig. 1. Synthesis of Rh NPs in [BDMI]NTf2.
although, the catalytic activity decreased as the N-alkyl chain
2.2. Catalyst preparation
length increased [32].
Herein, we report the employment of these phosphine FILs
towards the synthesis of Rh NPs and their use as recyclable
biphasic arene hydrogenation catalysts. These systems featured
high activity under reasonably mild conditions for the hydro-
genation of toluene, styrene and various xylenes. In our study
we compared the influence of the chain length spacer on the
activity and proved that FILs are superior stabilizers compared
to using only the IL solvent in terms of activity and recyclability,
respectively. We also demonstrated that PPh3 is a good stabi-
lizer, except that it leached out of the IL and contaminated the
extracted product. We also selected two Rh precursors for this
Rh NPs were synthesized by mixing Rh(allyl) (0.05 mmol) with
FIL 1, 2 or PPh3 (0.05 mmol) in [BDMI]NTf2 (1 mL) to afford 1:Rh,
3
2:Rh and PPh :Rh respectively. 1:Rh* was synthesized by mix-
3
ing RhCl ·3H O (0.05 mmol) with FIL 1 (0.05 mmol) in [BDMI]NTf
3
2
2
(1 mL). none:Rh was synthesized by mixing Rh(allyl) (0.05 mmol)
3
and [BDMI]NTf2 (1 mL). All the above mentioned systems were
stirred for 1 h under argon and then exposed to a constant pressure
◦
of H (g) (4 bar) for 18 h at 50 C (Fig. 1).
2
2
.3. Catalysis
study: Rh(allyl)3 and RhCl . While RhCl3 has been traditionally
3
The NP sample was prepared as outlined above and charged
used to generate Rh NPs, Rh(allyl)3 was introduced by Chaudret’s
group as a “salt-free” precursor. Under H2 pressure, Rh(allyl)3
with the arene substrate (5.0 mmol, 100 equiv.). The reaction mix-
◦
ture was stirred under a constant pressure H (g) (40 bar) at 75 C
2
0
decomposes into Rh and volatiles, leaving no impurity at the NP
for 3 h (Fig. 3). The reactor was cooled to below room tem-
perature and depressurized slowly. The catalysis products were
extracted with pentane (3 mL × 8 mL) and analyzed by GC (Table 1).
Each sample was dried for at least 1 h in vacuo between catalytic
runs.
surface [17].
2
. Experimental
2.1. General
2.4. CS2 poisoning test
All syntheses were carried out under an inert atmosphere.
Rh(allyl)3 [34], [BDMI]NTf2 [32], FIL 1 [35] and FIL 2 [32]
were prepared following literature procedures. All substrates
and solvents were purchased from commercial sources and used
as received. Nuclear magnetic resonance (NMR) spectra were
The NP sample was prepared as outlined above and charged with
CS2 (1.4 ␮L, 0.5 equiv.) and toluene (5.0 mmol) [36]. The reaction
mixture was stirred under a constant pressure H2(g) (40 bar) at
◦
75 C for 3 h (Fig. 3). The reactor was cooled to below room tem-
taken using a 200 MHz Varian Mercury spectrometer. The 31
P
perature and depressurized slowly. The catalysis products were
NMR spectra were calibrated using 85% H PO . Transmission
electron microscopy (TEM) images were taken using a JEOL
JEM-2100F microscope operating at 200 kV with a resolution of
extracted with pentane (3 mL × 8 mL).
3
4
2.5. Catalyst characterization
0.1 nm. High-pressure experiments were performed using a 25 mL
glass vessel in a Parr instruments 5000 Series Multiple Reactor
System.
The Rh NPs were characterized by TEM using a JEOL JEM-
100F microscope operating at 200 kV with a resolution of
2
Table 1
TEM characterization of Rh NPs before catalysis, after 1 catalytic cycle and after 10 catalytic cycles.
Entry
Rh NP
NP size (nm)
Before catalysis
After cycle 1
After cycle 10
1
2
3
4
5
none:Rh
1:Rh
2:Rh
PPh3:Rh
1:Rh*
Clusters of small particles
2.4 ± 0.6
2.3 ± 0.8
Aggregates
1.5 ± 0.4
2.3 ± 0.5
2.0 ± 0.5
1.7 ± 0.3
1.9 ± 0.5
1.5 ± 0.4
Clusters of small particles
Aggregates
Aggregates
Aggregates
Very few particles

9
8
S.A. Stratton et al. / Catalysis Today 183 (2012) 96–100
Fig. 2. TEM images of Rh NPs: (A) none:Rh, (B) 1:Rh, and (C) PPh3:Rh.
0.1 nm. Sample preparation involved dissolving 2 drops of the
Rh NP solution in ∼1.5 mL of MeCN followed by the deposi-
tion of the NPs on a 400 mesh carbon-coated copper grid. Size
distributions were calculated by measuring >100 particles per
sample.
Fig. 3. Arene hydrogenation catalyzed by Rh NPs.
3
. Results and discussion
3.2. Toluene hydrogenation and catalyst stability under catalysic
conditions
3.1. Nanoparticle synthesis and characterization
Toluene was chosen as a model substrate [28] to test the activity
The synthesis of Rh NPs 1:Rh and 2:Rh was achieved by reduc-
tion of the metal precursor Rh(allyl)3 in the presence of one molar
of the various synthesized Rh NPs (Fig. 3). The reaction conditions
chosen are relatively mild and on par with reported studies for this
equivalent of FIL 1 or 2 in an IL solvent [BDMI]NTf , under an atmo-
◦
2
difficult reaction: 40 bar of H (g) for 3 h at 75 C [22,28,38]. After
2
◦
sphere of 4 bar of H (g) (Fig. 1). The mixture was heated at 50 C for
2
each run, the product was extracted with pentane and the IL con-
taining the NPs could be reused after being dried for at least 1 h
in vacuo.
As can be seen in Table 2, the overall catalytic activity of the
tested NPs differed dramatically depending on the nature of the
stabilizing species. none:Rh (Table 2, entry 1) initially showed high
yields for the conversion of toluene to methylcyclohexane. After
run 2, the activity became inconsistent varying randomly between
1
, 4, 10 or 18 h. Short reaction times provided clusters of ill-defined
particles, while 18 h afforded well-dispersed Rh NPs. For compari-
son, Rh NPs were also synthesized in [BDMI]NTf , in the absence of
2
any additional stabilizer (none:Rh) or employing PPh as stabilizer
3
(
PPh :Rh). At last, we tested the influence of the rhodium precur-
3
sor, by exposing RhCl ·3H O to the same reaction conditions, in the
3
2
presence of FIL 1 to afford 1:Rh*.
TEM analysis was employed to characterize the obtained NPs
and determine their mean diameter (Table 1). In the absence of
4
0% and 90% in any given run. 1:Rh and 2:Rh (entries 2 and 3)
had to go through an induction period of 3–4 cycles, after which
both showed excellent activity, reaching complete conversion by
the third or fourth cycle. 1:Rh showed slightly better activity than
:Rh with quantitative conversion very consistently after cycle
. These results are comparable to the activities reported by the
Dyson group [22] with bipyridine FILs under similar conditions,
proving that phosphine FILs are good stabilizers in this context.
In their studies on the effect of the spacer chain length between
the bipyridine and the imidazolium moieties on catalytic activ-
ity, they reported a much higher activity for the longer chain FIL
compared to the shorter chain FIL because of electronic and steric
effects. We previously showed in our group the opposite effect on
phosphine FIL stabilized Pd NPs on styrene hydrogenation catal-
ysis [32]. In this system, both FIL 1 and 2 feature a very similar
activity, with a slightly better reproducibility for 1 after run 4. This
any stabilizer, the IL solvent alone [BDMI]NTf , afforded clusters of
2
small particles (Fig. 2A and Table 1, entry 1). These collections of
particles did not allow the precise measurement of the size of indi-
vidual particles. 1:Rh (Fig. 2B and Table 1, entry 2) and 2:Rh (Fig. S1C
37] and Table 1, entry 3) consisted of monodisperse small NPs of
.4 ± 0.6 nm and 2.0 ± 0.5 nm respectively. When PPh3 was used as
a stabilizer (Table 1, entry 4) even smaller particles of 1.5 ± 0.4 nm
were obtained (Fig. 2C). The 1:Rh* system also afforded much fewer
particles than 1:Rh preventing us from measuring their size with
precision (Fig. S1E [37] and Table 1, entry 5). We noticed during this
2
4
[
2
synthesis that RhCl was not as soluble as Rh(allyl) in the IL, which
3
3
may explain this observation. During TEM imaging, EDX measure-
ments were conducted to confirm that the visible particles were
composed of Rh metal (Fig S2 [37]).
Table 2
Toluene hydrogenation catalyzed by Rh NPs.a
Entry
Catalyst
% Toluene conversionb
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
Cycle 6
Cycle 7
Cycle 8
Cycle 9
Cycle 10
1
2
3
4
5
None:Rh
1:Rh
2:Rh
PPh3:Rh
1:Rh*
91
72
32
9
95
93
98
68
0
63
96
100
94
90
100
100
97
86
100
89
100
42
40
100
95
100
64
59
100
94
100
65
50
100
93
89
49
53
95
100
99
74
100
100
93
0
19
56
70
52
a
◦
Reaction conditions: Rh precursor (0.05 mmol), ligand (0.05 mmol), IL (1 mL), toluene (5.0 mmol, 100 equiv.), p[H2(g)] = 40 bar, T = 75 C, t = 3 h.
Determined by GC analysis.
b

S.A. Stratton et al. / Catalysis Today 183 (2012) 96–100
99
Fig. 4. TEM images of Rh NPs after 1 catalytic cycle: (A) none:Rh, (B) 1:Rh, and (C) PPh3:Rh.
effect is smaller but consistent with what we observed with Pd [32].
As can be seen, we do observe some induction period lasting for
avoid aggregation of the NPs while allowing for a high catalytic
activity (Fig. 4B).
2
–4 cycles depending on the sample. This conditioning phase could
We then underwent a study of the effect of the Rh precur-
sor on NP synthesis and performance, specifically to compare
result from: (1) an incomplete reduction of the Rh precursor during
NP synthesis, which allowed the continued formation of NPs dur-
ing the initial hydrogenation cycles; or (2) a restructuring of the NP
surface under catalytic conditions, which produces more active cat-
alytic sites. Janiak and co-workers have reported on using various
transition metal NPs stabilized by ILs as hydrogenation catalysts
in which a conditioning phase was also observed [36]. In order to
test these hypotheses, we performed the synthesis of 1:Rh under a
Rh(allyl)3 with a more traditionally used precursor, RhCl . Based
3
on the performance of 1:Rh, we selected 1 to form NPs employing
RhCl ·3H O, 1:Rh*. The activity for this sample was considerably
3
2
lower than that of 1:Rh (Table 2, entry 5). TEM analysis revealed
the presence of very few particles after synthesis, but only aggre-
gates were visible after cycle 1 and cycle 10 (Fig. S1F [37], Table 1,
entry 5). These results are explained by the much lower solubility
of RhCl3 in the IL solvent compared to the allyl precursor, result-
ing in much fewer active small NPs. The catalytic activity of 1:Rh*
increased in later hydrogenation cycles, which could result from
the dissolution of a small quantity of precursor by toluene into the
IL. Like for none:Rh, aggregates are associated with lower activ-
ity and irregular results. Rh(allyl)3 was introduced by Chaudret’s
group as a clean NP precursor, that decomposes easily under H2
pressure [17], and in our hands it proves a more efficient precursor
than RhCl3.
◦
higher temperature (75 C) to favor the reduction of the precursor.
The induction period was shortened to 1 catalytic cycle but was
not completely eliminated, suggesting that the first hypothesis is
more likely occurring in this system. In order to understand if the
particles were undergoing major size or shape alteration under cat-
alytic conditions, we analyzed them again by TEM after cycle 1 and
1
0 (Table 1, entries 2 and 3). We observed a subtle decrease in size
for 1:Rh and no measurable change for 2:Rh. The unstabilized NPs,
none:Rh, behaved very differently. Before catalysis all visible par-
ticles were small and collected into large clusters (Fig. 2A). The high
activity in the initial runs was attributed to these small particles,
which may be better dispersed in solution than what is visible on
the TEM grid. The image after 1 cycle (Fig. 4A) clearly evidenced that
some of these clusters tend to fuse into larger, ill-defined particles.
After extended recycling runs, complete aggregation was observed
by TEM and also plating of metal on the reaction vessel demon-
strating the instability of the system over time. This correlates with
irregularities observed for the long-term activity of none:Rh. Many
reports evidenced that only small sized Rh NPs feature catalytic
activity towards hydrogenation, which supports our observations
One further study was conducted in the optimization of the
catalytic conditions and involved a low temperature hydrogena-
tion of toluene. A catalytic run of the 1:Rh system was carried out
◦
at 35 C, 40 bar H (g), for 24 h and resulted in 27% conversion of
2
toluene to methylcyclohexane and thus the NPs are even active at
low temperatures.
A CS2 poisoning test was also carried out on the 1:Rh sample
to determine whether a homogeneous mechanism was involved
in the hydrogenation of toluene [36]. The addition of 0.5 equiv. of
CS2 to the IL phase after NP synthesis rendered the NP catalyst
completely inactive in the hydrogenation of toluene. This result
[
39]. The better and more consistent results of the FIL systems evi-
dence the important role that stabilization plays for the long-term
catalytic efficiency of Rh NPs in ILs [22].
In order to better understand the role of the phosphine ligand
in NP stabilization and activity, we studied PPh3 stabilized NPs,
PPh :Rh (Table 2, entry 4). Their catalytic activity was also excellent
3
after a steep induction period. The activity of this system, which is
comparable to the FIL systems, can be attributed to the presence of
small particles collected into clusters (Fig. 2C). This confirms that
phosphine ligands are providing excellent stabilization to the Rh
NPs; however, the 3 P NMR analysis of the product supernatant
after cycle 1 revealed that PPh3 had leached into the organic layer
during the extraction of the product. This observation is consistent
1
with the poor solubility of PPh3 in [BDMI]NTf . While we did not
2
observe any statically relevant decrease of catalytic activity during
1
0 cycles (Table 2, entry 4), this leaching may cause destabiliza-
tion of the system in the long-term and may affect the activity or
recyclability of the system. On the contrary, FILs 1 and 2 are immo-
bilized within [BDMI]NTf2 and provide sufficient stabilization to
Fig. 5. Hydrogenation of other arene substrates. Reaction conditions: Rh pre-
cursor (0.05 mmol), ligand (0.05 mmol), IL (1 mL), toluene (5.0 mmol, 100 equiv.),
p[H (g)] = 40 bar, T = 75 ◦C, t = 3 h. Determined by GC analysis.
2

1
00
S.A. Stratton et al. / Catalysis Today 183 (2012) 96–100
supports a heterogeneous mechanism for the hydrogenation of
arenes employing these Rh NP catalysts and removes the possibility
of an active homogeneous species in the system.
In order to understand the scope of the reaction, our best cata-
lyst, 1:Rh, was employed in the reduction of more demanding arene
substrates (Fig. 5). The first substrate investigated was styrene,
which was reduced to ethylcyclohexane in high yields, reaching
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4
. Conclusion
Phosphine FILs are effective stabilizers in the formation of bipha-
[
sic Rh NP catalysts. This new class of ligands increased both the
catalytic activity and lifetime of the Rh NP catalysts beyond those
stabilized by an unfunctionalized IL solvent or a traditional organic
stabilizer for the hydrogenation of toluene to methylcyclohexane.
This observation was rationalized by the size and shape analysis
of the NPs before and after catalysis. The heterogeneous nature of
catalysis was also demonstrated. Furthermore, the FIL stabilized Rh
NPs were also active catalysts in the reduction of more challeng-
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catalytic activity of Rh NPs stabilized by phosphine FILs and also
introduces Rh(allyl)3 as an efficient Rh NP precursor in ILs.
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Acknowledgements
[
[
We thank the Natural Science and Engineering Research Council
of Canada (NSERC), the Canada Foundation for Innovation (CFI), the
Canada Research Chairs (CRC), the Fonds de Recherche sur la Nature
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Center for Green Chemistry and Catalysis (CGCC) and McGill Uni-
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