Enhancing the Catalytic Properties of Ruthenium Nanoparticle-SILP Catalysts by Dilution with Iron

Article abstract of DOI:10.1021/acscatal.6b00796

The partial replacement of ruthenium by iron ("dilution") provided enhanced catalytic activities and selectivities for bimetallic iron-ruthenium nanoparticles immobilized on a supported ionic liquid phase (FeRuNPs@SILP). An organometallic synthetic approach to the preparation of FeRuNPs@SILP allowed for a controlled and flexible incorporation of Fe into bimetallic FeRu NPs. The hydrogenation of substituted aromatic substrates using bimetallic FeRuNPs@SILP showed high catalytic activities and selectivities for the reduction of a variety of unsaturated moieties without saturation of the aromatic ring. The formation of a bimetallic phase not only leads to an enhanced differentiation of the hydrogenation selectivity, but even reversed the order of functional group hydrogenation in certain cases. In particular, bimetallic FeRuNPs@SILP (Fe:Ru = 25:75) were found to exhibit accelerated reaction rates for C=O hydrogenation within furan-based substrates which were >4 times faster than monometallic RuNPs@SILP. Thus, the controlled incorporation of the non-noble metal into the bimetallic phase provided novel catalytic properties that could not be obtained using either of the monometallic catalysts.

Full text of DOI:10.1021/acscatal.6b00796

Research Article
pubs.acs.org/acscatalysis
Enhancing the Catalytic Properties of Ruthenium Nanoparticle-SILP
Catalysts by Dilution with Iron
Kylie L. Luska,† Alexis Bordet,§ Simon Tricard,§ Ilya Sinev,∥ Wolfgang Grunert,∥ Bruno Chaudret,§
̈
and Walter Leitner*,†,‡
†Institut fur Technische und Makromolekulare Chemie, RWTH Aachen University, Worringerweg 2, 52074 Aachen, Germany
̈
‡Max-Planck-Institut fur Kohlenforschung, 45470 Mulheim an der Ruhr, Germany
̈
̈
§Laboratoire de Physique et Chimie des Nano-Objets, Universite
de Rangueil, 31077 Toulouse, France
́
de Toulouse, LPCNO, INSA, UPS, CNRS-UMR5215, 135 Avenue
∥Lehrstuhl fur Technische Chemie, Ruhr-Universitat Bochum, Universitatsstraße 150, 44801 Bochum, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: The partial replacement of ruthenium by iron
(“dilution”) provided enhanced catalytic activities and selectiv-
ities for bimetallic iron−ruthenium nanoparticles immobilized
on a supported ionic liquid phase (FeRuNPs@SILP). An
organometallic synthetic approach to the preparation of
FeRuNPs@SILP allowed for a controlled and flexible
incorporation of Fe into bimetallic FeRu NPs. The hydro-
genation of substituted aromatic substrates using bimetallic
FeRuNPs@SILP showed high catalytic activities and selectivities
for the reduction of a variety of unsaturated moieties without
saturation of the aromatic ring. The formation of a bimetallic
phase not only leads to an enhanced differentiation of the
hydrogenation selectivity, but even reversed the order of functional group hydrogenation in certain cases. In particular, bimetallic
FeRuNPs@SILP (Fe:Ru = 25:75) were found to exhibit accelerated reaction rates for CO hydrogenation within furan-based
substrates which were >4 times faster than monometallic RuNPs@SILP. Thus, the controlled incorporation of the non-noble
metal into the bimetallic phase provided novel catalytic properties that could not be obtained using either of the monometallic
catalysts.
KEYWORDS: bimetallic nanoparticles, iron, ruthenium, supported ionic liquid phases, selective hydrogenation
reported Fe NPs as effective catalysts for the reduction of C−C
multiple bonds in alkenes and alkynes under mild con-
ditions;19−25 however, the hydrogenation of other unsaturated
functional groups has been challenging to achieve.26−29 On the
contrary, ruthenium is an effective catalyst for the hydro-
genation of various polar unsaturated functionalities (e.g.,
aldehydes, ketones, esters, imines, nitriles, etc.). Thus, the
partial replacement (“dilution”) of ruthenium with iron, in the
formation of bimetallic catalysts, could provide an appropriate
balance between the cost and performance of Ru-based
catalysts. Bimetallic FeRu NPs have been reported as catalysts
for Fischer−Tropsch synthesis30−33 and the water−gas shift
reaction;34 however, only a few reports have investigated their
use in hydrogenation reactions.35−39 Chaudret and co-workers
have recently reported the preparation of colloidal FeRu NPs
and found that the Fe:Ru ratio influenced the selectivity in
hydrogenation catalysis.35
INTRODUCTION
■
Ruthenium is widely used as a metal component in catalysts for
hydrogenation and hydrogenolysis processes. Molecular and
material systems provide a suitable balance between catalytic
activity and stability1 that has been attributed to the ability to
undergo relatively thermoneutral exchange reactions with
various donor atoms (e.g., oxygen, nitrogen, carbon).2 Its
tolerance to oxygen donor functionalities has stimulated
intensive research toward the application of ruthenium-based
catalysts in the conversion of biomass, as these feedstocks
possess high carbon-to-oxygen ratios and are transformed in the
presence of water.3−17 Although, ruthenium is considerably
cheaper than other noble metals such as platinum, palladium,
and rhodium, there has been significant interest in its
replacement with the third-row homologue iron as an
abundant, inexpensive, and environmentally benign alternative
to the noble metal catalysts.
Iron nanoparticles (Fe NPs) have long been known to
possess activity in Fischer−Tropsch synthesis,18 while recent
publications also have demonstrated their activity for the
hydrogenation of organic substrates.19−27 Several groups have
Received: March 18, 2016
Revised: May 2, 2016
© XXXX American Chemical Society
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In general, the preparation of bimetallic NPs has been an
effective strategy to tune the catalytic properties of heteroge-
neous catalysts,40−42 in which the major challenge in this field
concerns the development of synthetic methodologies that
permit control over the structure of the bimetallic NPs (e.g.,
metal oxidation state, NP size and distribution, atomic
distribution) and thus an ability to tailor the catalytic properties
of bimetallic NPs.42 In our present work, we have adapted an
organometallic approach for the synthesis of bimetallic NPs,
previously reported by some of us,35 toward the preparation of
iron−ruthenium NPs (FeRu NPs) embedded in a supported
ionic liquid phase (SILP) (Figure 1).
achieved according to a previous reported procedure and
involved the condensation of [1-butyl-3-(3-triethoxysilylprop-
yl)-imidazolium]NTf2 with dehydroxylated SiO2.49 The syn-
thesis of bimetallic FeRu NPs immobilized on a SILP
(FeRuNPs@SILP) involved the in situ reduction of a
mesitylene solution (5 mL) of {Fe[N(Si(CH3)3)2]2}2 and
[Ru(cod) (cot)] (total metal loading = 0.20 mmol) in the
presence of the SILP (0.50 g) under an atmosphere of H2(g) (3
bar) at 150 °C (Scheme 1). Upon stirring the suspension under
Scheme 1. Synthesis of Iron−Ruthenium Nanoparticles
Immobilized on a Supported Ionic Liquid Phase
(FeRuNPs@SILP)
an atmosphere of H2(g) for 18 h, the mixture transformed from
a light green to a dark black color indicating the coreduction of
the organometallic precursors and the formation of FeRu NPs.
This synthetic procedure allowed for a facile variation of the
Fe:Ru ratio by adjusting the quantities of {Fe[N(Si-
(CH3)3)2]2}2 and [Ru(cod)(cot)] used for the synthesis, in
which a series of bimetallic FeRuNPs@SILP was prepared:
Ru100, Fe20Ru80, Fe25Ru75, Fe33Ru67, Fe60Ru40, and Fe100 (see
the Supporting Information (SI) for complete SILP and
FeRuNP@SILP synthetic and characterization data).
Scanning electron microscopy with energy dispersive spec-
troscopy (SEM/EDS) was used to confirm the Fe:Ru ratio and
the total metal loading (1 wt % total metal loading) (Table S1),
while TEM confirmed the presence of small and well-dispersed
NPs in all cases (2.9−3.5 nm) (Figure S1). Further, both
metals appear well-dispersed throughout the particles as
evidenced by elemental mapping using scanning transmission
electron microscopy with energy dispersive spectroscopy
(STEM/EDS) (Figure 2).
Figure 1. Schematic representation of bimetallic iron−ruthenium
nanoparticles immobilized on a supported ionic liquid phase
(FeRuNPs@SILP) as catalysts for the selective hydrogenation of
substituted aromatic substrates.
The design of our bimetallic catalysts involved the covalent
attachment of an imidazolium ionic liquid (IL) onto silica to
decorate the inorganic support material with a stabilization
matrix for FeRu NPs generated from organometallic complexes
as precursors. Although these surface layers are of course no
longer “liquid phases”, immobilized IL-type structures are
commonly known as SILPs43−47 and have been investigated as
supports of homogeneous44−48 and heterogeneous cata-
lysts.6,49−54 For NP-based systems, the SILP allows for (a)
facilitated access of the substrates and removal of products from
the active metal sites; (b) enhanced NP stability through the
combination of electrosteric stabilization of the IL and steric
protection of the silica support; and (c) molecular tuneability of
the NP reactivity by modifying the IL structure.17 The
organometallic approach toward the synthesis of FeRu NPs
stabilized on a SILP (FeRuNPs@SILP) provides a facile
method to tailor the quantity of Fe introduced into the
bimetallic FeRu NPs by controlling the ratio between the metal
constituents,35 while the SILP permitted a retention of the
materials characteristics of the NPs (i.e., NP size, shape,
distribution, atomic distribution). This enabled us to system-
atically vary the Fe:Ru ratio in this matrix, providing intriguing
evidence that the resulting catalysts possessed exclusive and
partly enhanced catalytic properties that differ from those of the
Fe and Ru monometallic counterparts. Extensive character-
ization of the materials, including X-ray absorption spectros-
copy (XAS), allowed for a rationalization of the influence of the
Fe:Ru ratio on the catalytic properties of FeRuNPs@SILP.
RESULTS AND DISCUSSION
Synthesis and Characterization of FeRuNPs@SILP. The
preparation of a supported ionic liquid phase (SILP) was
■
Figure 2. (a) High-resolution TEM of Fe25Ru75NPs@SILP and
STEM/EDS elemental mappings of (b) Fe−K (yellow), (c) Ru−L
(purple), and (d) the overlay of Fe−K and Ru−L.
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Further characterization of the mono- and bimetallic NPs@
SILP was achieved using X-ray absorption fine structure
(XAFS) measurements which showed spectral features that
were dependent on the Fe content of the bimetallic NPs (see
the SI for complete XAFS data analysis). The Ru K-edge X-ray
absorption near-edge structure (XANES) spectra of mono-
metallic Ru100NPs@SILP and bimetallic Fe25Ru75NPs@SILP
were similar to a Ru foil reference and thus indicated a
predominately zerovalent state for both catalysts (Figure 3a).55
Figure 4. Fe K-edge spectra of the (a) XANES and (b) EXAFS regions
for Fe100NPs@SILP (blue), Fe60Ru40NPs (purple), and Fe25Ru75NPs
(red).
and EXAFS data for higher Fe loadings provided evidence for
the presence of both Fe oxide and metallic Fe within
monometallic Fe100NPs@SILP and bimetallic Fe60Ru40NPs@
SILP. In contrast, the EXAFS spectrum of Fe25Ru75NPs@SILP
contained a much weaker Fe−O backscattering event at 1.5 Å,
as well as two additional signals at 1.95 and 2.45 Å. Modeling of
the Fe25Ru75NPs@SILP Fe−K and Ru−K EXAFS spectra
(involving Fe−O, Fe−Fe, Fe−Ru, Ru−Ru, and Ru−Fe
backscattering) was successful and therefore provided evidence
for the formation of Fe0 included in a bimetallic FeRu phase
(Figure S8). Lastly, the EXAFS data was employed to
determine the atomic distribution (J), also known as alloy
extent, within the bimetallic FeRu NPs using an approach
developed by Hwang and co-workers (see the SI for details on
atomic distribution calculations).58 From these calculations it
was determined that the atomic distributions within
Fe60Ru40NPs@SILP and Fe25Ru75NPs@SILP were similar and
provided evidence for a homophilic bimetallic structure; a
bimetallic phase possessing a higher extent of Ru and Fe
homoatomic interactions than for a perfect bimetallic structure
(Table S3).
Figure 3. Ru K-edge spectra of the (a) XANES and (b) EXAFS
regions for a Ru foil reference (black), Ru100NPs@SILP (blue),
Fe25Ru75NPs (red), Fe60Ru40NPs (purple), and a RuO2 reference
(light blue).
In contrast, Fe60Ru40NPs@SILP showed spectral features
similar to RuO2, which suggests that the Ru may have been
partly oxidized in NPs with high Fe content. The Fourier-
transformed Ru K-edge extended X-ray absorption fine
structure (EXAFS) spectra for mono- and bimetallic NPs
displayed dominate signals originating from Ru−Ru back-
scattering (2.30 Å, Figure 3b). The bimetallic NPs also
exhibited a shoulder signal at 1.80 Å resulting from Ru−Fe
backscattering, where the magnitude of the Ru−Fe scattering
signal was larger for Fe60Ru40NPs@SILP in comparison to
Fe25Ru75NPs@SILP.
In conclusion, these results demonstrate that the organo-
metallic approach provides an effective method for the
controlled preparation of supported bimetallic FeRu NPs in
various compositions. This synthetic procedure allowed for a
facile variation of the Fe:Ru ratio by adjusting the quantities of
the organometallic precursors, {Fe[N(Si(CH3)3)2]2}2 and
[Ru(cod)(cot)]. XAFS measurements showed that the
corresponding FeRuNPs@SILP possessed homophilic bimet-
allic structures independent of the Fe:Ru ratio, whereas the Fe
oxidation state varied with the NP composition. While the
formation of an FeRu bimetallic phase was dominant at low Fe
contents, high Fe contents provided a mixture of Fe oxide and
FeRu alloy. This dependence of the Fe oxidation state on the
Fe:Ru ratio was not observed for the analogous colloidal FeRu
NPs35 and may at least partly result from the presence of the
SILP. Thus, this method allowed for supported bimetallic FeRu
NPs to be prepared under mild conditions (150 °C, 3 bar H2)
with the ability to tune the properties of the NPs, while
traditional syntheses of supported FeRu bimetallic catalysts
require high temperature reduction using H2 (>450 °C) and
high Ru contents to favor the formation of bimetallic FeRu
phases.30−33
The Fe K-edge XANES spectra of bimetallic Fe25Ru75NPs@
SILP (Figure 4a) possessed a broad pre-edge signal (7115 eV)
similar to that of Fe foil (Figure S4); however, the spectral
features above the absorption edge differed from that of the Fe
reference. Similar changes within Fe K-edge XANES spectra
were reported for Fe in bimetallic NPs with other noble metals
such as Pt56 and Rh57 and were ascribed to formation of alloy-
type structures. In the present series, the XANES of samples
with higher Fe loadings, Fe60Ru40NPs@SILP and Fe100NPs@
SILP, contained clear indications for oxidation of Fe as the
spectra contained sharp pre-edge signals and an absorption
edge with a stronger intensity drop above a more intense white
line. While the pre-edge region was similar to that of a Fe2O3
reference, the characteristics of the absorption edge differed,
and thus support for FeRu bimetallic formation was sought in
the EXAFS region.
Fe K-edge EXAFS showed a significant Fe−O scattering
signal at 1.5 Å similar to that for the Fe2O3 reference (Figure
S5) and a weak signal for metallic Fe−Fe backscattering at 1.95
(shoulder signal) and 2.30 Å for Fe60Ru40NPs@SILP and
Fe100NPs@SILP, respectively (Figure 4b). Thus, the XANES
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Hydrogenation of Substituted Aromatic Substrates.
The hydrogenation of CC and CO functional groups in
substituted aromatic substrates was used to probe the catalytic
properties of monometallic Fe-, Ru-, and bimetallic FeRu-based
SILP materials (Table 1). Monometallic Fe100NPs@SILP
The selectivity of the CC, CO, and aromatic hydro-
genation in the α,β-unsaturated ketone benzylideneacetone (5)
to the products 5a−d was used to gain further insight into the
catalytic properties of the bimetallic FeRuNPs@SILP (Table
2). The monometallic Ru100NPs@SILP proved a highly active
Table 1. Hydrogenation of Substituted Aromatic Substrates
Using Monometallic and Bimetallic NPs@SILP
Table 2. Hydrogenation of Benzylideneacetone (5) Using
FeRuNPs@SILP
a
a
b
product yield (%)
entry
catalyst
Ru100
conversion (%)
5a
5b
5c
5d
1
2
3
4
5
>99
>99
>99
96
0
0
0
0
0
0
0
0
0
>99
0
Fe20Ru80
>99
99
12
0
Fe25Ru75
0
0
Fe33Ru67
84
5
0
Fe60Ru40
5
0
c
6
Fe100 + Ru100
>99
0
0
>99
a
Reaction conditions: FeRuNPs@SILP (40 mg, 0.016 mmol total
metal loading), mesitylene (0.5 mL), benzylideneacetone (0.4 mmol),
b
H2 (20 bar), 120 °C, 18 h. Determined by GC using tetradecane as
an internal standard. Physical mixture of Fe100NPs@SILP (10 mg)
c
and Ru100NPs@SILP (30 mg) (0.016 mmol total metal loading, Fe:Ru
ratio = 25:75).
catalyst for the deep hydrogenation of 5 toward the completely
saturated product 5d (entry 1). The introduction of small
quantities of Fe into the Ru NPs, Fe20Ru80NPs@SILP and
Fe25Ru75NPs@SILP, controlled the reaction toward the
formation of the aromatic alcohol 5c in quantitative yields
under identical conditions (entries 2 and 3). Further increasing
the quantity of Fe in Fe33Ru67NPs@SILP provided high
selectivity toward the production of aromatic ketone 5a at
almost quantitative conversion (entry 4). However, the catalytic
activity collapsed almost completely at Fe:Ru ratios above
50:50, as evidenced with Fe60Ru40NPs@SILP (entry 5).
The hydrogenation of 5 was also performed using a physical
mixture of the monometallic Fe100NPs@SILP and Ru100NPs@
SILP catalysts (Table 2, entry 6). Using a Fe:Ru ratio of 25:75
provided a quantitative yield of the completely saturated
alcohol 5d, similar to those results obtained using only
Ru100NPs@SILP (Table 2, entry 1). Furthermore, a hot
filtration test using Fe25Ru75NPs@SILP demonstrated that
hydrogenation ceased immediately once the reaction mixture
was separated from the solid material, excluding the leaching of
soluble molecular clusters as the active species. Therefore, the
unique catalytic properties of the FeRuNP@SILP catalysts are
directly correlated with the presence of the bimetallic FeRu
phase in the SILP matrix.
a
Reaction conditions: NPs@SILP (40 mg, 0.016 mmol total metal
loading), mesitylene (0.5 mL), substrate (0.4 mmol), H2 (20 bar), 120
°C (except for substrate 4 which used 100 °C), 18 h. Determined by
GC using tetradecane as an internal standard. Decomposition to
mixture of unidentified products. Remainder of reaction mixture was
b
c
d
composed of N,N-dibenzylmethylamine.
showed almost negligible activity for the hydrogenation of β-
methylstyrene (1) or acetophenone (2) as only a 4%
conversion of 1 to propylbenzene (1a) and no conversion of
2 were observed. As expected, monometallic Ru100NPs@SILP
were highly active catalysts, leading to the hydrogenation of not
only the olefin and carbonyl groups, but also of the aromatic
moieties. Thus, 1 and 2 were hydrogenated to the completely
saturated products propylcyclohexane (1b) and 1-cyclo-
hexylethanol (2b) in quantitative yields, respectively. In sharp
contrast, bimetallic Fe25Ru75NPs@SILP possessed high activ-
ities for CC and CO hydrogenation, but not for the
aromatic ring as 1 and 2 were converted almost completely to
propylbenzene (1a) and 1-phenylethanol (2a) over the
bimetallic FeRu catalyst.
Monometallic and bimetallic catalysts were also tested for the
hydrogenation of N-benzylidenemethylamine (3) and furfural
(4) (Table 1). Fe100NPs@SILP were inactive for the hydro-
genation of both 3 and 4. Treatment of 3 under the standard
reaction conditions in the presence of Ru100NPs@SILP led only
to complex mixtures of unidentified decomposition products,
while Fe25Ru75NPs@SILP produced high yields of N-
benzylmethylamine (3a) (83%). Hydrogenation of 4 using
Ru100NPs@SILP provided a mixture of furfuryl alcohol (4a)
(53%) and tetrahydrofurfuryl alcohol (4b) (47%), whereas a
quantitative yield of 4a was obtained using the bimetallic
Fe25Ru75NP@SILP catalyst.
Next, a competition experiment was performed in which the
CC and CO moieties of benzylideneacetone (5) were
separated into individual substrates (Scheme 2). The hydro-
genation of β-methylstyrene (1) and 4-phenyl-2-butanone (5a)
over Fe25Ru75NPs@SILP produced propylbenzene (1a) and 4-
phenyl-2-butanol (5c) in >99% and 86% yields, respectively.
Thus, the bimetallic catalyst can simultaneously hydrogenate
CC and CO moieties, wherein the reduction of CC
bonds occurs preferentially.
Overall, these results indicate that the dilution of Ru by Fe
greatly enhances the differentiation of the hydrogenation
activity for the functional moieties vs the aryl rings. Fe contents
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under shaken and not magnetically stirred conditions in order
to reduce mechanical degradation of the catalyst. Under shaken
conditions, the catalytic activity and selectivity for
Fe25Ru75NPs@SILP toward aromatic products 6a and 6c
were maintained for at least two cycles of catalysis (Table 3,
entries 6 and 7). Characterization of Fe25Ru75NPs@SILP after
catalysis showed no significant NP growth or aggregation by
TEM (Table S2) and a stable bimetallic structure as evidenced
by EXAFS (Figures S6−S7 and Table S3).
The reaction profiles for the hydrogenation of 6 using
Fe25Ru75NPs@SILP and Ru100NPs@SILP reveal that the
different selectivities are not caused by a simple decrease of the
general hydrogenation activity upon dilution with iron, but
result from a concurrent increase and decrease of the activity for
certain functional groups (Figure 5). Both catalysts possess a
Scheme 2. Competition Experiment between β-
Methylstyrene (1) and 4-Phenyl-2-Butanone (5a)
a
a
Reaction conditions: Fe25Ru75NPs@SILP (40 mg, 0.016 mmol total
metal loading), mesitylene (0.5 mL), 4-phenyl-2-butanone (0.4
mmol), β-methylstyrene (0.4 mmol), H2 (20 bar), 120 °C, 9 h.
in the range 25−33% essentially shut down the hydrogenation
of aromatic rings while maintaining practically useful activities
for the CC and CO groups. This allows excellent
selectivities in the order CC > CO ≫ arene. Next, we
moved to multifunctional heteroaromatic substrates, where the
intrinsic reactivity of the functional groups over monometallic
Ru NP catalysts follow the order CC > heteroarene > C
O.59
Furfuralacetone (6) was selected to examine the selective
hydrogenation of furan-based substrates as encouraging results
were obtained in the reduction of furfural (4) using the
bimetallic FeRuNPs@SILP. Furfuralacetone also represents an
important lignocellulose-derived platform chemical obtained as
the primary aldol condensation product of furfural and acetone;
a strategy that allows for pentose-derived intermediates to be
upgraded to higher carbon products and fuel compo-
nents.17,59−62 Under the standard conditions of the present
study, hydrogenation of 6 catalyzed by Ru100NPs@SILP
provided a mixture of the tetrahydrofuran products 6b and
6d in 35% and 65% yields, respectively (Table 3, entry 1). In
Table 3. Hydrogenation of Furfuralacetone (6) using
a
FeRuNPs@SILP
b
product yield (%)
entry
catalyst
conversion (%)
6a
6b
6c
6d
1
2
3
4
5
Ru100
>99
>99
>99
>99
4
0
0
35
0
0
94
94
16
0
65
6
Fe20Ru80
Fe25Ru75
0
0
6
Fe33Ru67
84
4
0
0
Fe60Ru40
0
0
c
6
Fe25Ru75(first cycle)
Fe25Ru75(second cycle)
>99
>99
13
13
0
81
83
6
d
7
0
4
a
Reaction conditions: FeRuNPs@SILP (40 mg, 0.016 mmol total
metal loading), mesitylene (0.5 mL), furfuralacetone (0.4 mmol), H2
b
Figure 5. Reaction profile for the hydrogenation of furfuralacetone (6)
using (a) Ru100NPs@SILP and (b) Fe25Ru75NPs@SILP. Reaction
conditions: Conversion >99%, FeRuNPs@SILP (40 mg, 0.016 mmol
total metal loading), mesitylene (0.5 mL), furfuralacetone (0.4 mmol),
H2 (20 bar), 100 °C.
(20 bar), 100 °C, 18 h. Determined by GC using tetradecane as an
internal standard. 1st cycle, 100 °C, 18 h, shaken at 240 rpm. 2nd
cycle, 100 °C, 18 h, shaken at 240 rpm.
c
d
sharp contrast, the bimetallic catalysts Fe20Ru80NPs@SILP and
Fe25Ru75NPs@SILP remained highly active catalysts, but now
produced the aromatic alcohol 6c in excellent yields of 94%
(entries 2 and 3). Again, increasing the Fe content further
allowed a shift in the product selectivity toward the saturated
ketone 6a with Fe33Ru67NPs@SILP (entry 4), but an almost
complete lose in activity for Fe contents above 50%
(Fe60Ru40NPs@SILP, entry 5).
very high activity toward the hydrogenation of the CC bond
as >99% conversion of 6 was achieved after 1 h. The
monometallic Ru100NPs@SILP catalyst exhibited a substantial
arene hydrogenation activity from the beginning of the reaction
as a mixture of the furanic (6a, 28%) and tetrahydrofuranic
ketone (6b, 62%) was already formed after 1 h. The yield of the
fully saturated ketone 6b reached a maximum after 4 h and a
slow consumption of 6b to aliphatic alcohol 6d occurred over
the course of the reaction. In stark contrast, the Fe25Ru75NP@
The recyclability of the bimetallic Fe25Ru75NP@SILP catalyst
was evaluated briefly for the hydrogenation of 6 conducted
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SILP catalyst showed a very selective formation of aromatic
ketone 6a after 1 h, which was then smoothly converted to
aromatic alcohol 6c. Only small quantities of the tetrahy-
drofuran-based ketone 6b and alcohol 6d were observed
throughout the course of the reaction and >90% yield of
aromatic alcohol 6c was obtained after 12 h.
Reaction rates for the CO hydrogenation of intermediates
6a and 6c by Fe25Ru75NPs@SILP and Ru100NPs@SILP were
determined to be 0.107 and 0.025 M/h, respectively (Figure
S9). These data demonstrate that that the hydrogenation of the
heteroarene is suppressed and the ketone hydrogenation is
enhanced at the same time, when ca. one-fourth of ruthenium
is replaced with iron. As summarized in Scheme 3, the
group was significantly enhanced with the bimetallic catalyst in
case of furan-based substrate 6.
This indicates that dilution of an active noble metal catalyst
by a non-noble metal, which cannot form an active catalyst by
itself, can result not only in improved relative selectivities, but
also in enhanced absolute activities at least for hydrogenation
catalysts. Taking this into account may open intriguing
opportunities in the current quest for the replacement of
noble metals in catalysis (e.g., in the area of biomass
conversion). Naturally, direct substitution of noble metals will
often not be possible due to lack of catalytic activity of the
corresponding third-row metals. The results presented here
show that controlled dilution through formation of bimetallic
catalysts can not only help to reduce the required amount of
noble metals, but also may be used as a strategy to access
catalytic materials with improved catalytic properties that
cannot be achieved by either of the individual metals alone.
Scheme 3. Major Reaction Pathways for the Hydrogenation
of Furfuralacetone (6) Using Bimetallic Fe25Ru75NPs@SILP
a
(Blue) and Monometallic Ru100NPs@SILP (Red) Catalysts
EXPERIMENTAL SECTION
■
Safety Warning. High-pressure experiments with com-
pressed H2(g) must be carried out only with appropriate
equipment and under rigorous safety precautions.
General. The supported ionic liquid phase (SILP) was
synthesized as previously reported.49 If not otherwise stated,
the synthesis of the SILP and FeRu NPs immobilized on SILPs
(FeRuNPs@SILP) were carried out under an inert atmosphere
using standard Schlenk techniques or within a glovebox. After
synthesis, SILPs and FeRuNPs@SILPs were stored under an
inert atmosphere. All catalyst solutions were prepared under an
inert atmosphere. Furfuralacetone was synthesized according to
a known literature method.63 {Fe[N(Si(CH3)3)2]2}2 and
[Ru(cod)(cot)] were obtained from NanoMePS. Furfuralace-
tone and benzylideneacetone were purified by sublimation, and
furfural was distilled, prior to use. Mesitlyene (from VWR
Prolabo, 99%) was dried over alumina desiccant and degassed
via freeze−pump−thaw cycling. All other chemicals and
solvents were purchased from commercial sources and used
without purification.
Analytics. Solution-state nuclear magnetic resonance
(NMR) spectra were recorded on a Bruker AV300, 400, or
600 MHz spectrometer. 1H and 13C NMR spectra were
calibrated to TMS using the residual solvent signal. High-
pressure experiments were performed using in-house engi-
neered 10 and 20 mL stainless steel finger autoclaves. Catalytic
reactions were performed in glass inlets using a magnetic stirbar
(600 rpm) and an aluminum heating block. Recyclability
experiments were performed using a GFL Orbital Shaker
(Model 3017) operating at 240 rpm. Gas chromatography
(GC) was performed on a Thermo Scientific Chromatograph
Tace GC Ultra equipped with a CP-Wax 52 CB column from
Agilent. Brunauer−Emmett−Teller (BET) measurements were
performed on a Quadrasord SI automated Surface Area and
Pore Size Analyzer from Quantachrome Instruments and data
analysis using QuadraWin 5-04. Transmission electron micro-
scopic (TEM) images were collected using a JEOL JEM 1400
operated at 120 kV and using a tungsten filament. Scanning
electron microscopy with energy dispersive spectroscopy
(SEM/EDS) was performed on a JEOL JSM 7800F operated
at 10 kV with a thermally assisted Schottky electron gun and
equipped with a Bruker XFlash 6|60 detector (silicon drift
detector technology). Scanning transmission electron micros-
copy with energy dispersive spectroscopy (STEM/EDS) was
obtained on a Hitachi S-5500 ultrahigh-resolution cold field
a
Note: thickness of the arrows qualitatively reflects the relative rates of
the individual steps.
bimetallic catalyst thus opens an alternative parallel reaction
pathway in the consecutive hydrogenation sequence. The
“diluted” catalyst enables the highly selective and efficient
synthesis of compound 6c, a species that could not be targeted
with monometallic Ru-based catalysts due to the intrinsic
functional group selectivity of the Ru NPs.59
SUMMARY AND CONCLUSIONS
■
The present study demonstrates that partial substitution of iron
for ruthenium can result in enhanced catalytic activities and
selectivities for the hydrogenation of multifunctional aromatic
and heteroaromatic substrates. An organometallic synthetic
approach to prepare bimetallic iron−ruthenium nanoparticles
immobilized on a supported ionic liquid phase (FeRuNPs@
SILP) provided a molecular tool to control the ratio of Fe:Ru
with all other material characteristics remaining largely
identical. At high Fe contents (above 50%), a high degree of
oxidized Fe- and Ru-species was found within these materials,
while at Fe contents in the range of ca. 20−30% the metallic
state was retained for both metals in what appears to be a well-
dispersed FeRu bimetallic phase.
The structural characteristics are also reflected in the catalytic
properties of these materials. The monometallic Fe and the
bimetallic FeRuNPs@SILP with iron content >50% were
practically inactive for all hydrogenation reactions under
investigation. In contrast, the materials with ca. 20−30% Fe
content showed excellent hydrogenation activity, but with a
very distinct reactivity profile as compared to the monometallic
Ru particles. The typically high catalytic activity for arene and
heteroarene groups of the ruthenium metal was suppressed
almost completely upon dilution with iron, whereas very
significant activities for CC, CO, and CN groups were
still observed with the bimetallic particles. Most significantly,
the activity for the hydrogenation of the ketone to the alcohol
3724
DOI: 10.1021/acscatal.6b00796
ACS Catal. 2016, 6, 3719−3726

ACS Catalysis
Research Article
emission scanning microscope with a Thermo Scientific
NORAN System 7 X-ray Microanalysis unit at an acceleration
voltage of 30 kV. X-ray absorption fine structure (XAFS)
measurements were carried out on the CLÆSS beamline of the
ALBA synchrotron facility (Barcelona, Spain).
reactor was cooled in an ice bath, carefully vented and the
organic phase was removed by decantation. The reaction
mixture was analyzed via GC using tetradecane as an internal
standard. The determination of the reaction profile for the
hydrogenation of furfuralacetone was performed as individual
experiments as outlined above and were stopped after the
desired reaction time. Reaction rates for the CO hydro-
genation of intermediates 6a and 6c were determined from the
GC data of the reaction profiles. For recycling experiments, the
reaction was performed using a shaker plate operating at 240
rpm. After catalysis, the reaction mixture was decanted from the
catalyst under an inert atmosphere and additional furfuralace-
tone and mesitylene were added directly to the catalyst. Note:
care must be taken to prepare all catalyst solutions under an
inert atmosphere as exposure of the bimetallic catalysts to
atmospheric conditions prior to catalysis can alter their catalytic
properties (Table S4).
Synthesis of Iron:Ruthenium Nanoparticles.
Fe60Ru40NPs@SILP. A solution of {Fe[N(Si(CH3)3)2]2}2 (45.2
mg, 0.060 mmol) in mesitlyene (2 mL) was combined with a
solution of [Ru(cod)(cot)] (25.2 mg, 0.080 mmol) in
mesitlyene (2 mL) in a Fischer−Porter bottle (70 mL). The
SILP (500 mg) was added to the solution of metal precursors
along with mesitylene (1 mL), and the suspension was stirred
under argon at rt for 30 min. The Fischer−Porter bottle was
evacuated and backfilled with H2(g) and the suspension was
stirred at 150 °C under H2(g) (3 bar) for 18 h. Under this
reducing environment, a dark black powder and clear
supernatant were obtained indicating the immobilization of
FeRu NPs onto the SILP. FeRuNPs@SILP were isolated by
decantation of mesitlyene. The black powder was then washed
with fresh mesitylene (5 mL) and dried in vacuo at 45 °C for 1
h. Samples were prepared for TEM by ultrasound dispersion of
FeRuNPs@SILP in mesitylene under ambient atmosphere. A
drop of the suspension was deposited onto a copper TEM grid
with an amorphous carbon support film and dried in vacuo
prior to analysis. The NP size and distribution was determined
from the measurement of >150 spherical particles chosen in
arbitrary areas of enlarged micrographs. Reaction profile data
was obtained from individual experiments stopped after the
desired time.
Fe33Ru67NPs@SILP. A solution of {Fe[N(Si(CH3)3)2]2}2
(24.8 mg, 0.033 mmol) in mesitlyene (2 mL) was combined
with a solution of [Ru(cod)(cot)] (42.2 mg, 0.134 mmol) in
mesitlyene (2 mL) in a Fischer−Porter bottle (70 mL). The
remainder of the reaction was performed as outlined for
Fe60Ru40@SILP.
Fe25Ru75NPs@SILP. A solution of {Fe[N(Si(CH3)3)2]2}2
(18.8 mg, 0.025 mmol) in mesitlyene (2 mL) was combined
with a solution of [Ru(cod)(cot)] (47.3 mg, 0.150 mmol) in
mesitlyene (2 mL) in a Fischer−Porter bottle (70 mL). The
remainder of the reaction was performed as outlined for
Fe60Ru40@SILP.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.6b00796.
Complete synthesis, characterization, and catalysis data
for FeRuNPs@SILP (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: leitner@itmc.rwth-aachen.de. Fax: (+49) 241-80-
22177. Tel.: (+49) 241-80-26481.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was performed as a part of the Cluster of Excellence
“Tailor-Made Fuels from Biomass”, which is funded by the
Excellence Initiative of the German federal and state govern-
ment to promote science and research at German universities.
The authors would like to thank Simon Cayez (INSA de
Toulouse) for his assistance with SEM/EDS measurements,
Karl-Josef Vaeßen (ITMC, RWTH Aachen University) for
N2(g) adsorption measurements, Hans-Josef Bongard (Max-
Fe20Ru80NPs@SILP. A solution of {Fe[N(Si(CH3)3)2]2}2
(15.1 mg, 0.020 mmol) in mesitlyene (2 mL) was combined
with a solution of [Ru(cod)(cot)] (50.5 mg, 0.160 mmol) in
mesitlyene (2 mL) in a Fischer−Porter bottle (70 mL). The
remainder of the reaction was performed as outlined for
Fe60Ru40@SILP.
Planck-Institut fur Kohlenforschung) for STEM/EDS analysis,
̈
and Dr. Nils Theyssen (Max-Planck-Institut fur Kohlenfor-
̈
schung) for his generous support. K.L.L. would like to thank
Deutscher Akademischer Austaush Dienst (DAAD) for
financial support.
Ru100NPs@SILP. A solution of [Ru(cod)(cot)] (63.1 mg,
0.200 mmol) in mesitlyene (4 mL) was combined with SILP
(500 mg) and mesitlyene (1 mL). The remainder of the
reaction was performed as outlined for Fe60Ru40@SILP.
Fe100NPs@SILP. A solution of {Fe[N(Si(CH3)3)2]2}2 (75.3
mg, 0.100 mmol) in mesitlyene (4 mL) was combined with
SILP (500 mg) and mesitlyene (1 mL). The remainder of the
reaction was performed as outlined for Fe60Ru40@SILP.
Hydrogenation of Substituted Aromatic Substrates.
As an example, FeRuNPs@SILP (40 mg, 0.016 mmol total
metal loading), furfuralacetone (55 mg, 0.4 mmol), and
mesitylene (0.5 mL) were combined in a glass insert within a
glovebox and placed in a high-pressure autoclave. Upon
pressurization with H2(g), the reaction mixture was magnetically
stirred (600 rpm) at 100 °C in an aluminum heating block
under 20 bar H2(g) for 18 h. Once the reaction was finished, the
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