Selective Hydrogenation of Benzofurans Using Ruthenium Nanoparticles in Lewis Acid-Modified Ruthenium-Supported Ionic Liquid Phases

Article abstract of DOI:10.1021/acscatal.9b05124

Ruthenium nanoparticles immobilized on a Lewis-acid-functionalized supported ionic liquid phase (Ru?SILP-LA) act as effective catalysts for the selective hydrogenation of benzofuran derivatives to dihydrobenzofurans. The individual components (nanoparticles, chlorozincate-based Lewis-acid, ionic liquid, support) of the catalytic system are assembled using a molecular approach to bring metal and acid sites in close contact on the support material, allowing the hydrogenation of O-containing heteroaromatic rings while keeping the aromaticity of C6-rings intact. The chlorozincate species were identified to be predominantly [ZnCl₄]^2- anions using X-ray photoelectron spectroscopy and are in close interaction with the metal nanoparticles. The Ru?SILP-[ZnCl₄]^2- catalyst exhibited high activity, selectivity, and stability for the catalytic hydrogenation of a variety of substituted benzofurans, providing easy access to biologically relevant dihydrobenzofuran motifs under continuous flow conditions.

Full text of DOI:10.1021/acscatal.9b05124

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Research Article
Selective Hydrogenation of Benzofurans Using Ruthenium
Nanoparticles in Lewis Acid-Modified Ruthenium-Supported Ionic
Liquid Phases
Sami El Sayed, Alexis Bordet, Claudia Weidenthaler, Walid Hetaba, Kylie L. Luska, and Walter Leitner*
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ABSTRACT: Ruthenium nanoparticles immobilized on a Lewis-
acid-functionalized supported ionic liquid phase (Ru@SILP-LA)
act as effective catalysts for the selective hydrogenation of
benzofuran derivatives to dihydrobenzofurans. The individual
components (nanoparticles, chlorozincate-based Lewis-acid, ionic
liquid, support) of the catalytic system are assembled using a
molecular approach to bring metal and acid sites in close contact
on the support material, allowing the hydrogenation of O-
containing heteroaromatic rings while keeping the aromaticity of C6-rings intact. The chlorozincate species were identified to be
predominantly [ZnCl4]2− anions using X-ray photoelectron spectroscopy and are in close interaction with the metal nanoparticles.
The Ru@SILP-[ZnCl4]2− catalyst exhibited high activity, selectivity, and stability for the catalytic hydrogenation of a variety of
substituted benzofurans, providing easy access to biologically relevant dihydrobenzofuran motifs under continuous flow conditions.
KEYWORDS: supported ionic liquid phases, ruthenium nanoparticles, Lewis acid, selective benzofurans hydrogenation, continuous flow
benzofurans.5 In general, however, state-of-the-art catalysts
INTRODUCTION
■
for this reaction still show severe limitations such as low
The catalytic reduction of functional groups (alkene, carbonyl,
imine, nitro, etc.) in the side chains of aromatic substrates has
attracted considerable attention in the past decade for the
production of aromatic structures in the chemical industry, and
hydrogenation selectivity,2 narrow substrate scope,2−5,13,21,22
low stability,3,5,13 or are not suitable for continuous flow
applications.2−5,13,21,22
To address this synthetic challenge, we describe in this paper
especially in commodities, fine chemicals, agrochemicals, and
pharmaceuticals.1−7 The selective hydrogenation of bicyclic
the preparation, characterization, and application of a new
multifunctional catalyst system for the selective hydrogenation
heteroaromatic compounds (quinolines, indoles, benzopyrans,
of benzofuran derivatives. Ruthenium NPs were combined
benzofurans, etc.) is especially attractive because of the
with a Lewis acid-functionalized ionic liquid on a solid support
presence of these motifs in various natural products, bioactive
molecules, and pharmaceuticals.3−5,8,9 Whereas the catalytic
using a molecular approach to assemble the individual key
components of the active material in a flexible and controlled
hydrogenation of N-containing bicyclic heteroaromatics has
been widely studied,3,4,9−12 the development of catalysts able
manner. Our approach is based on the deposition of well-
defined NPs from organometallic precursors directly on ionic
to efficiently and selectively hydrogenate O-containing
heteroaromatics remains a challenge.2−4,8,11,13 Methods
liquid-type layers that are covalently grafted on silica supports
(supported ionic liquid phases, SILPs). This method allows to
providing access to the dihydrobenzofuran motifs are however
generate multifunctional catalytic systems with tailor-made
highly desirable, as these structures are key building blocks for
reactivity.5,7,23−29 In this study, we present a version of such
materials where ruthenium NPs are stabilized on a SILP
consisting of an imidazolium-based Lewis acidic ionic liquid
covalently grafted on silica. The resulting material is denoted
Ru@SILP-LA, in which Ru defines the metal, SILP the IL-
grafted silica, and LA the Lewis acidic species (Figure 1). The
the production of molecules possessing antioxidant,14−16
antibacterial,15,17 anti-inflammatory,15,18 and cytotoxic15,19,20
activities (Scheme 1).
The selective hydrogenation of benzofurans to dihydroben-
zofuran derivatives requires the use of specific catalysts
possessing the ability to hydrogenate the furan ring while
leaving the six-membered aromatic ring untouched. In this
context, recent efforts led to the development of a few catalytic
systems with promising activity.1−5,8,10,13 Notably, Dyson et al.
reported the synthesis of Rh nanoparticles (NPs) in a Lewis
acidic ionic liquid and their use for the hydrogenation of
various heteroaromatics including a limited selection of
Received: November 26, 2019
Revised: January 10, 2020
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Scheme 1. Selective Partial Hydrogenation of Bicyclic Heteroaromatics: (a) N-Containing Substrates, (b) Benzofuran
Derivatives: Goal of This Study, (c) Examples of Bioactive Molecules Containing the Dihydrobenzofuran Motif
signals correspond to the Si atoms of IL bound to the SiO2
surface and thus provide evidence for the covalent attachment
of the IL on the silica support. Transmission IR spectroscopy
showed bands for the C−H stretch of the imidazolium and
aliphatic moieties (3145 and 2955 cm−1) and the symmetric
ring stretch of the imidazolium ring (1560 and 1450 cm−1)
(Figure S2). The presence of Lewis acid sites on SILP-LA was
probed using pyridine-DRIFT-IR, which evidenced the
characteristic band at 1630−1600 cm−1 (Figure S3).32
The Ru loading on Ru@SILP-LA was determined to be 0.02
mmol/g (0.20 wt %) by inductively coupled plasma atomic
absorption spectroscopy (ICP−AAS), well in agreement with
the theoretical value. The Brunauer−Emmett−Teller (BET)
surface of the support does not change upon Ru-loading
(Table S1). Analysis of the Ru 3d region in the X-ray
photoelectron spectroscopy (XPS) spectra indicate that the Ru
NPs are in the metallic state (BE Ru 3d5/2 = 280.5 eV; Figure S
4). Analysis of Ru@SILP-LA by scanning transmission electron
microscopy with high-angle annular dark-field (STEM-
HAADF) shows that the NPs are small (1.9 nm) and well-
dispersed within the SILP (Figure 2a). Low-magnification
STEM-HAADF−energy-dispersive X-ray (EDX) elemental
mapping confirms that the ionic liquid is homogeneously
Figure 1. Schematic representation of the multifunctional catalysts
based on ruthenium NPs on a Lewis acidic-supported ionic liquid
phase (Ru@SILP-LA)
materials proved highly active and selective for the hydro-
genation of a wide range of substrates bearing a variety of
functionalities and showed excellent stability under continuous
flow conditions.
RESULTS AND DISCUSSION
■
Synthesis of the Catalyst. The SILP-LA support material
was synthesized via a two-step procedure involving first the
condensation of a silane-functionalized imidazolium IL, [1-
butyl-3-(3-triethoxysilylpropyl)imidazolium]Cl, on dehydroxy-
lated silica to form the corresponding SILP-Cl following an
established procedure (see Supporting Information, Scheme
S1).6 In a second step, SILP-LA was obtained through
impregnation with ZnCl2 (0.74 equivalents relative to the IL
loading). The synthesis of Ru NPs on SILP-LA was performed
following previously reported procedures.26,30,31 This involved
the wet impregnation of SILP-LA with a solution of [Ru(2-
methylallyl)2(cod)] in dichloromethane. After removal of the
solvent in vacuo, the impregnated SILP-LA was heated at 100
°C under an atmosphere of H2 (100 bar) for 18 h. Upon
subjection to the reductive environment, the color of the
material turned from white to gray (see Supporting
Information, Scheme S2).
Characterization. Solid-state 29Si NMR of SILP-LA
(Figure S1) shows the presence of two types of Si species:
(1) tetrafunctionalized (Q) signals at −109 ppm (Q4
=
Si(OSi)4) and −102 ppm (Q3 = Si(OSi)3OH); and (2)
trifunctionalized signals at −58 ppm (T2 = R−Si(OSi)2OR′)
and −53 ppm (T1 = R−Si(OSi) (OR′)2). The T2 and T1
Figure 2. (a) STEM-HAADF and (b) EDX elemental mapping
(STEM-HAADF−EDX) of Ru@SILP-LA, (c) Zn and (d) Cl
elemental mapping.
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Figure 3. XPS-spectra of the Cl 2p region for (a) SILP-Cl, (b) SiO2−ZnCl2, (c) SILP-LA, and (d) Ru@SILP-LA.
present on the support (Figure S5). Interestingly, STEM-
HAADF−EDX (Figures 2b−d and S6) indicates that the
concentration of Zn and Cl is about twofold higher around the
Ru NPs than in the NP-free regions, suggesting a strong
interaction between the chlorozincate anions and the metal
particles.
However, the presence of small quantities of free Cl− and
Lewis acidic [Zn2Cl6]2− species on the catalyst is possible, as
these anions were reported to exist in equilibrium with
[ZnCl4]2−.34−36 These species are not visible by XPS, but this
can be due to their low concentration and/or to the time scale
of the equilibria between anions, which is faster than the XPS
measurement.34 We thus do not exclude that small amounts of
[Zn2Cl6]2− can contribute significantly to the Lewis acidity of
the catalyst.
Although the use of Lewis acidic ionic liquids5,33 and the
dynamic equilibria controlling the anionic speciation of
2−
chlorozincate anions [ZnxCl2x+2
]
in pure ionic liquid has
been well studied,34 there are currently no reference data for
SILP materials. Therefore, SILP-LA and Ru@SILP-LA were
characterized by XPS, with a focus on the Cl 2p region to
investigate the nature of the chlorozincate anion(s) (Figure 3).
XPS spectra for the starting SILP-Cl and for SiO2
impregnated with ZnCl2 (denoted SiO2−ZnCl2) are provided
as references to support the discussion. The Cl 2p spectrum of
the starting SILP-Cl (Figure 3a) evidences the presence of a
major Cl species (197.9 and 199.5 eV; Figure 3a), together
with traces of a second species (200.5 and 202.1 eV) probably
corresponding to nongrafted ionic liquid remaining after the
washing step. In contrast, only one Cl species is visible after
impregnation of the SILP-Cl with ZnCl2 to produce SILP-LA
(Figure 3c). The binding energies of Cl 2p3/2 and 2p1/2 in
SILP-LA (198.5 and 200.1 eV respectively) are significantly
different from those observed for SILP-Cl (197.9 and 199.5
eV; Figure 3a) and SiO2−ZnCl2 (199.4 and 201.0 eV; Figure
3b), suggesting the formation of a Cl-containing species
different from Cl− and ZnCl2. The same species is observed
after deposition of the Ru NPs (Ru@SILP-LA; Figure 3d).
Based on previous literature reports by Licence et al., these
data strongly suggest the presence of [ZnIICl4]2− anions on
SILP-LA and Ru@SILP-LA.34 This hypothesis is further
supported by the elemental analysis (ICP−AAS) which gives
Zn/Cl, N/Cl, and Zn/N molar ratios of ca. 1/4, 1/1, and 1/4,
respectively (see Table S1 for details). Based on these results,
the M@SILP-LA synthesized here can be formulated as a Ru@
SILP-[ZnCl4]2− material which exhibits Lewis acidity.
Interestingly, changing the amount of ZnCl2 introduced in
the impregnation step (0.50/0.67/0.74 equiv relative to the
ionic liquid) influenced neither the nature of the chlorozincate
species formed (XPS) nor the Zn and Cl content (ICP−AAS)
on the material (Table S2), suggesting that after the formation
of [ZnIICl4]2− on the SILP, the remaining ZnCl2 does not react
further and is removed during the washing step. This is in
contrast with the case of pure ionic liquids, where the chemical
2−
structure of the anion [ZnII Cl2n+2
]
is tunable and directed
n
by the stoichiometric ratio of the ionic liquid and ZnCl2.34
Catalytic Study. The catalytic properties of Ru@SILP-
[ZnCl4]2− were first investigated in batch conditions using
benzofuran (1) as a model substrate (Figure 4).
The substrate (benzofuran 1) was completely converted
after 8 h, producing dihydrobenzofuran (1a) in high yield
(88%), which was essentially retained over the course of the
reaction. Compounds 1b and 1c were identified as minor
byproducts. The amount of 1bwhich resulted from the full
hydrogenation of the substrateremained below 5% even at
prolonged reaction time, demonstrating a very effective
suppression of the aromatic hydrogenation by the Lewis acid
modification. 1c was formed from 1 and/or 1a through
hydrogenolysis, a transformation frequently observed in
hydrogenation reactions catalyzed by transition-metal
NPs.37,38 Based on these promising preliminary results, the
catalyst’s activity and stability were studied under continuous
flow conditions (Figure 5). Continuous flow experiments
involved passing a solution of benzofuran (0.05 M in decalin)
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SILP-[ZnCl4]2− is active, selective, and highly stable for the
hydrogenation of benzofuran in continuous flow conditions.
The catalytic study with Ru@SILP-[ZnCl4]2− was further
pursued by exploring the substrate scope of this reaction
considering various functionalized benzofurans (Table 1). A
nonfunctionalized Ru@SILP catalyst (Ru NPs immobilized on
an imidazolium-based SILP with NTf2 as anion, see Figure S8
for details) was used for comparison.
Using Ru@SILP-[ZnCl4]2−, benzofuran derivatives with
electron-donating (substrates 2 and 3) and electron-with-
drawing (4 and 5) substituents on the six-membered ring were
hydrogenated as selectively as benzofuran (1), giving the
corresponding dihydrobenzofurans products in more than 90%
yield (85−90% isolated yields). Introducing a methyl group in
positions 2 and 3 on the furan ring (6 and 7) made the
hydrogenation slightly more challenging, but good yields (80
and 75%, respectively) could still be obtained after adapting
the reaction conditions. Interestingly, Ru@SILP-[ZnCl4]2−
was also able to hydrogenate very selectively bergapten (8),
a natural psoralen derivative employed in several biomedical
applications,39,40 giving product 8a with excellent selectivity
(96%) in high yield (95% NMR yield, 84% isolated yield). In
contrast, hydrogenation of substrates 1−8 with Ru@SILP
resulted in all cases in lower conversions and/or selectivities
toward the formation of the desired products, outlining the
importance of the combination of Ru NPs and Lewis acid sites.
This was further confirmed with the use of a commercial Ru/C
catalyst for the hydrogenation of substrate 3, which also gave a
significantly lower yield for the product 3a (62 vs 85% isolated
yield, Table S5).
Figure 4. Reaction/time-profile for the hydrogenation of benzofuran
(1) using Ru@SILP-[ZnCl4]2-. Reaction conditions: Ru@SILP-
[ZnCl4]2− (75 mg, 0.00224 mmol), substrate (1.68 mmol, 750
equiv), decalin (0.5 mL), H2 (10 bar), 150 °C. Composition of the
reaction mixture determined by GC using tetradecane as an internal
standard.
Using Ru@SILP-[ZnCl4]2−, all the substrates considered
were selectively hydrogenated, whereas the C6-membered
aromatic ring was conserved, showing that the catalyst can
accommodate a wide range of functionalities. In addition, time
profiles performed with Ru@SILP and Ru@SILP-[ZnCl4]2−
using benzofuran as the substrate show that the presence of the
Lewis acid does not only hinder the hydrogenation of arenes,
but also accelerates the hydrogenation of the furan moiety
(Figure 6).
Figure 5. Hydrogenation of benzofuran (0.05 M in decalin, 0.5 mL·
min−1, residence time = 1 min) using Ru@SILP-[ZnCl4]2− (537 mg,
0.016 mmol Ru) under continuous flow conditions at 175 °C and 10
bar H2 (gas flow rate = 35 N mL·min−1). Product composition and
yields were determined by GC using tetradecane as an internal
standard.
The versatility of the Ru@SILP-[ZnCl4]2− catalyst was
further studied in continuous flow operation. Solutions
containing substrate 1 or 3 were alternatively passed in a
hydrogen stream over the catalyst bed using a H-Cube Pro
continuous flow system from ThalesNano. Substrate 1 was
used for 2 h before switching to substrate 3 that was kept as
well for 2 h. This substrate “switch” was performed three times
in a row to study the impact of real-time repeated substrate
changes on the catalyst’s properties (Figure 7). Under
optimized reaction conditions (see Table S6 for optimization
steps), the substrate feed could be switched back and forth
between substrates 1 and 3 to alternatively produce 1a and 3a
in more than 70% yield. The consecutive 2 h cycles
corresponded to a total of 12 h time-on-stream without
significant changes in activity, selectivity, or stability of the
catalyst, outlining once again its robustness and versatility in
the hydrogenation of benzofuran derivatives.
through a cartridge packed with Ru@SILP-[ZnCl4]2− (537
mg) using a H-Cube Pro from ThalesNano. The influence of
the reaction temperature, substrate flow and H2 pressure on
the catalytic activity and selectivity was investigated, whereas
the hydrogen flow rate was maintained at 35 N mL·min−1.
After a screening of the parameters (see Table S3), the
reaction conditions were fixed to 175 °C, 10 bar H2, and 0.5
mL·min−1 (residence time = 1 min).
High conversion (75−85%) and selectivity (85−95%)
corresponding to a nearly constant yield of ca. 70% toward
the formation of 1a were maintained over a period of 6 h
without any noticeable decay (Figure 5). TEM characterization
of the catalyst after 6 h on stream did not show a significant
change in the size and dispersion of the Ru NPs (1.9 0.4 nm;
Figure S7). In addition, ICP measurements did not evidence
any leaching of the metals (Ru and Zn) or of the IL, and the
textural properties of the catalyst were conserved according to
BET analysis (Table S4). These results indicate that Ru@
CONCLUSIONS
■
A Lewis acidic-supported ionic liquid phase material (SILP-
LA) was synthesized and used as support for ruthenium NPs
(Ru NPs) deposited from organometallic precursors. The
resulting material was fully characterized, revealing the
formation of small and well-dispersed Ru NPs on the support,
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a b
,
Table 1. Selective Hydrogenation of Benzofuran Derivatives Catalyzed by Ru@SILP-[ZnCl4]2−
a
b
Results obtained using Ru@SILP are given for comparison. Reaction conditions: Ru@SILP-[ZnCl4]2− (75 mg, 0.00224 mmol Ru), decalin (0.5
c
d
mL), 16 h. 24 h. 1,4-dioxane (1.0 mL), 32 h. Composition of the reaction mixture determined by GC using tetradecane as an internal standard.
e
Composition of the reaction mixture determined by NMR using mesitylene as a standard. (X = conversion/S = selectivity/Y = yield). Isolated
yields are given in parentheses.
Figure 6. Comparative rate studies for the hydrogenation of benzofuran (1) using Ru@SILP and Ru@SILP-ZnCl42−. Catalyst (75 mg, 0.00224
mmol Ru), substrate (1.68 mmol, 750 equiv), decalin (0.5 mL), H2 (10 bar), 150 °C. Composition of the reaction mixture determined by GC
using tetradecane as an internal standard.
in close interaction with the chlorozincate species. XPS analysis
shows that the NPs are in the metallic state, and suggests
based on previous literature reportsthat the chlorozincate
species consist predominantly of [ZnCl4]2− anions. The Ru@
SILP-[ZnCl4]2− catalyst was applied for the hydrogenation of
benzofuran derivatives, and was found to be highly active,
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Figure 7. Ru@SILP-[ZnCl4]2− (520 mg, 0.016 mmol Ru) under continuous flow conditions for the hydrogenation of benzofuran (175 °C, 10 bar
H2, 0.05 M in decalin, 0.5 mL·min−1, residence time = 1 min) and 5-methoxybenzofuran (175 °C, 20 bar H2, 0.0125 M in decalin, 0.5 mL·min−1,
residence time = 1 min), gas flow rate = 35 N mL·min−1.
selective, and stable for the selective hydrogenation of a broad
range of substrates and functional groups. Under continuous
flow conditions, benzofuran and 5-methoxybenzofuran could
be hydrogenated alternatingly over the same catalysts to
produce the corresponding partially hydrogenated products in
good yields. This demonstrates the versatility of the Ru@SILP-
[ZnCl4]2− catalytic system and its potential for the production
of dihydrobenzofuran derivatives under synthetically relevant
conditions. The molecular approach to the catalyst synthesis is
modular and flexible, providing general access to M@SILP-LA
systems. The results further substantiate the possibility to
utilize this methodology for the preparation of finely tuned
multifunctional catalysts to achieve challenging selectivities in
hydrogenation reactions.
Kylie L. Luska − Institut fur Technische und Makromolekulare
̈
Chemie, RWTH Aachen University, Aachen 52074, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.9b05124
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge financial support by the Max Planck
Society and by the Deutsche Forschungsgemeinschaft (DFG,
German Research Foundation) under Germany’s Excellence
StrategyExzellenzcluster 2186 “The Fuel Science Center”
ID: 390919832. Furthermore, the authors would like to thank
Josef Vaeßen (ITMC, RWTH Aachen) for BET absorption
measurements, Hannelore Eschmann and Elke Biener (ITMC,
RWTH Aachen) for GC measurements, Meike Emondts
(ITMC, RWTH Aachen) for solid-state NMR measurements,
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.9b05124.
̈
Norbert Pfander (MPI for Chemical Energy Conversion,
Mulheim an der Ruhr), Adrian Schluter (MPI for Kohlenfor-
schung, Mulheim an der Ruhr) for TEM analysis, and Rosalie
̈
̈
Complete synthesis, characterization, and catalysis data
for Ru@SILP-LA (PDF)
̈
̈
Dalhoff, Konstantin Krockert and David Kuß (ITMC, RWTH
Aachen) for their help for the experimental study.
AUTHOR INFORMATION
■
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