Chemoselective hydrogenation of arenes by PVP supported Rh nanoparticles

Article abstract of DOI:10.1039/C6DT03668F

Polyvinylpyrrolidone-stabilized Rh nanoparticles (RhNPs/PVP) of ca. 2.2 nm in size were prepared by the hydrogenation of the organometallic complex [Rh(η³-C₃H₅)₃] in the presence of PVP and evaluated as a catalyst in the hydrogenation of a series of arene substrates as well as levulinic acid and methyl levulinate. The catalyst showed excellent activity and selectivity towards aromatic ring hydrogenation compared to other reported transition metal-based catalysts under mild reaction conditions (room temperature and 1 bar H₂). Furthermore, it was shown to be a highly promising catalyst for the hydrogenation of levulinic acid and methyl levulinate in water leading to quantitative formation of the fuel additive γ-valerolactone under moderate reaction conditions compared to previously reported catalytic systems.

Full text of DOI:10.1039/C6DT03668F

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Chemoselective hydrogenation of arenes by PVP supported Rh nanoparticles
Mahmoud Ibrahima,b, Raju Poreddyc, Karine Philippot*,a,b, Anders Riisager*,c, Eduardo J. Garcia-
Suarez*,c
a CNRS, LCC (Laboratoire de Chimie de Coordination), 205 Route de Narbonne, BP44099, 31077
Toulouse Cedex 4, France
b Université de Toulouse, UPS, INPT, 31077 Toulouse Cedex 4, France
c Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of
Denmark, DK-2800 Kgs. Lyngby, Denmark
* Corresponding authors (edgar@kemi.dtu.dk; karine.philippot@lcc-toulouse.fr)
Abstract
Polyvinyl pyrrolidone-stabilized Rh nanoparticles (RhNPs/PVP) of ca. 2.2 nm in size were prepared by
hydrogenation of the organometallic complex [Rh(η3-C3H5)3]) in the presence of PVP and evaluated as
catalyst in the hydrogenation of a series of arene substrates as well as levulinic acid and methyl
levulinate. The catalyst showed excellent activity and selectivity towards aromatic ring hydrogenation
compared to other reported transition metal-based catalysts under mild reaction conditions (room
temperature and 1 bar H2). Furthermore, it showed to be a highly promising catalyst for the
hydrogenation of levulinic acid and methyl levulinate in water leading to quantitative formation of the
fuel additive γ-valerolactone under moderate reaction conditions compared to previously reported
catalytic systems.
Keywords
rhodium, nanoparticle, catalysis, hydrogenation, biomass, levulinic acid, methyl levulinate

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Introduction
The hydrogenation of aromatic C-C double bonds to form cyclohexane derivatives is one of the most
important reactions in both petrochemical and pharmaceutical industries. For instance, the
1
chemoselective hydrogenation of benzoic acid to cyclohexane carboxylic acid is a crucial step in the
fiber industry for the synthesis of ɛ-caprolactam from toluene in the Snia Viscosa process (developed
by SNIA S.p.A., Italy).2 Furthermore, cyclohexane carboxylic acid is a key intermediate in the
synthesis of high demanding pharmaceuticals such as praziquantel and ansatrienin (Figure 1).3,4,5
OH
O
O
N
H
O
O
O
O
N
O
N
NH
O
O
Figure 1. Praziquantel (left) and Ansatrienin (right)
The high relevance of benzoic acid hydrogenation in both bulk and fine chemistry industries has
attracted a lot of attention from the scientific community to develop catalysts for the chemoselective
hydrogenation of benzoic acid to cyclohexane carboxylic acid. However, hydrogenation of aromatic C-
C double bonds, compared to olefinic C-C double bonds, is a very challenging transformation since it
generally requires severe reaction conditions. Moreover, a selective catalyst is needed which only
hydrogenates the aromatic ring while keeping other functional groups present in the molecule intact.
Noble transition metal catalysts, either homogeneous or heterogeneous systems, including Ru-Pt, Rh/C,
Pd/C, Pd-Ru have been extensively studied for arene hydrogenation and generally found to exhibit high
activities.2,6,7,8,9 Among them, Rh/AC (AC = activated carbon) and Pd/AC catalysts show superior
catalytic activity compared to other supported transition metal catalysts.10 Pd/AC are traditionally used
for ɛ-caprolactam production in the chemical fiber industry11 and for terephthalic acid refining in the
polyester industry.12
It is worth to point out that the process for benzoic acid to cyclohexane carboxylic acid hydrogenation
with Pd/AC catalysts suffers from a number of limitations, i.e. high temperature (150 °C) and high H2

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pressure (150 bar) conditions as well as excessive catalyst consumption due to its fast deactivation.13
Tsung et al. have recently reported that Pt nanocrystals are catalytically active in the hydrogenation of
pyrrole to n-butylamine and that both the activity and the selectivity strongly depend on the shape and
size of Pt nanocrystals.14 A similar behavior was observed by Hu et al. using Pd nanocrystals in citral
and cyclohexene hydrogenation.15 Yoon et al. investigated a bimetallic Rh-Pd/CNT (CNT = carbon
nanotube) catalyst for benzene hydrogenation which displayed a much higher activity compared to
Rh/CNT or Pd/CNT monometallic catalysts alone. The superior activity was attributed to a synergistic
effect between Rh and Pd, but the reaction system required elevated pressure (10 bar) in order to
observe sufficient activity.16
Metal nanoparticle catalysis has progressed significantly during the last 25 years.17 However, a major
drawback often reported when using nanoparticles in solution catalysis is drop in catalytic activity and
selectivity associated to particle agglomeration due to a lack of stability with time and/or reaction
conditions. Accordingly, intensive research has been focused on developing approaches to increase
particle stability and improve catalytic performance. In particular, numerous supports and/or stabilizers
(polymers or ligands) that could limit nanoparticle evolution during their application as catalysts in
solution are studied. Among polymers employed as stabilizers of metal nanoparticles, polyvinyl
pyrrolidone (PVP) efficiently controls the particle growth and contributes to obtain an uniform metal
nanoparticle dispersion in the polymer matrix. For these reasons, PVP-stabilized Rh nanoparticles
(RhNPs) have attracted attention for arene hydrogenation in both ionic liquids and water as reaction
media at relatively low temperatures.18,19
In this work, we report results on the mild hydrogenation of benzoic acid and its derivative benzamide
to cyclohexane carboxylic acid and cyclohexane carboxamide at room temperature and atmospheric H2
pressure with a RhNPs/PVP nanocatalyst using water as a green and eco-friendly solvent. Furthermore,
the RhNPs/PVP nanocatalyst has been evaluated in aqueous phase hydrogenation of levulinic acid and
its methyl ester to γ-valerolactone.
Experimental part
Materials
All manipulations were carried out under inert atmosphere using standard Schlenk techniques or in
glove-box if not otherwise stated. Tetrahydrofuran (THF) and n-pentane were purchased from Sigma-

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Aldrich purified by column-filtration and degassed by means of three freeze-pump-thaw cycles before
use. Other solvents and/or reagents were purchased from Sigma-Aldrich and used without further
purification except for PVP that was dried under vacuum over P2O5 at 80 °C for 3 days prior to use.
The rhodium precursor [Rh(η3-C3H5)3] was synthesized from RhCl3.3H2O (Johnson Matthey) and
CH2=CHCH2MgCl (Sigma-Aldrich; solution 2.0 M in THF) following a published preparation
method.20 Hydrogen gas (Alphagaz) was purchased from Air liquid.
Synthesis of RhNPs/PVP
[Rh(η3-C3H5)3] metal precursor (150 mg, 0.663 mmol) was placed in a Fisher-Porter bottle and left
under vacuum for 30 min. The Fisher-Porter bottle was then cooled to -80 °C and a THF solution (150
mL) containing 680 mg of PVP (MW = 40000 g/mol) was added to the rhodium complex leading to a
yellow solution. The reaction mixture was then let to warm up to room temperature before
pressurization with H2 (3 bar). After 10 min, the yellow solution turned into a black colloidal solution
indicating the formation of NPs. The solution was left for stirring under H2 atmosphere for additional
16 h to ensure complete decomposition of the metal precursor. Then excess of H2 was removed under
vacuum and the colloidal solution concentrated to 10 mL before addition of n-pentane in order to
precipitate the RhNPs. The resulting RhNPs/PVP solid was washed twice with n-pentane, filtered and
dried under vacuum overnight allowing obtaining a crystalline dark grey powder (~600 mg) that was
kept under inert atmosphere until use.
Characterization of RhNPs/PVP
TEM and HR-TEM analyses
Samples for transmission electron microscopy (TEM) and high resolution HR-TEM analyses were
prepared by slow evaporation of a drop of the crude colloidal solution deposited onto a holey carbon-
covered copper grid under argon (in glove-box). TEM and HR-TEM analyses were performed on a
MET JEOL JEM 1400 microscope operating at 120 kV and a JEOL JEM 2010 microscope working at
200 kV with a resolution point of 2.5 Å and equipped for Energy-Dispersive X-ray spectroscopy (EDX;
Noran), respectively. Enlarged micrographs were used for treatment with ImageJ software to obtain a

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statistical size distribution and the nanoparticle mean diameter. FFT treatments were carried out with
Digital Micrograph Version 1.80.70 to determine NPs crystalline structure.
ICP-AES analyses
The Rh content of the RhNPs/PVP catalyst was quantified by inductively coupled plasma atomic
emission spectroscopy (ICP-AES) using a Thermo Scientific iCAP 6300 DUO spectrometer with a 3-
channel, 12-roller pump and a 27.12 MHz solid state RF plasma generator. Mineralization of the
RhNPs/PVP sample was performed in aqua regia solution.
XPS analyses
X-ray photoelectron spectroscopy (XPS) data were acquired with a ThermoScientific system at room
temperature using Al-Kα radiation (1484.6 eV) as the excitation X-ray source. During analysis the
pressure in the chamber was maintained at 2⋅10-10 mbar. The XPS measurements were then performed
in the electron binding energy ranges corresponding to Rh 3d excitations and C, N and O 1s
excitations. All samples were measured twice and the reported values are the average of the two
analyses.
General hydrogenation procedure
All reactions were performed in a 300 mL stainless steel high pressure autoclave equiped with an
electrically heated jacket, a turbine stirrer with a variable speed magnetic driver and a liquid sampling
line. In a typical reaction, the reactor was loaded with the substrate (0.5 mmol), water (50 mL) and
catalyst (10 mg). Afterwards, the reactor was pressurized with H2 (4 bar) three times followed by
evacuation to displace residual air in the reactor. The autoclave was then filled with H2 (1 or 10 bar),
heated to the desired temperature and the reaction initiated by stirring the mixture at 320 rpm. After the
selected reaction time the autoclave was cooled down to room temperature. An aliquot of the reaction
mixture was subsequently analyzed by gas chromatography (GC-FID) using a chromatograph (Agilent
6890N) equiped with an HP-5 capillary column (Agilent, J&W, 30 m x 320 μm). Gas chromatography
coupled to mass spectrometry (GC-MS) was also performed on Agilent 6850N equipment fitted with
the same capillary column.

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Results and Discussions
Characterization of RhNPs/PVP
Rhodium nanoparticles stabilized by PVP polymer (RhNPs/PVP) were synthesized following the so-
called “organometallic approach” previously developed by some of the authors.21,22 This method allows
to prepare well-controlled NPs that display interesting catalytic properties.23 The organometallic
complex [Rh(η3-C3H5)3] was used as the rhodium source, and decomposed in THF solution at room
temperature under H2 atmosphere at mild conditions (3 bar H2).24 The main advantage of this method is
the formation of propane as the only by-product, which is inert towards the metallic surface and easily
removed, thus allowing to get uncoordinated metal NPs in the PVP stabilizing matrix. The PVP matrix
permits to have NPs that are both soluble in organic solvents and water.
The RhNPs/PVP were characterized by electron microscopy at low (TEM) and high resolution (HR-
TEM) coupled with energy-dispersive X-ray spectroscopy (EDX) techniques to determine the
dispersion of the particles inside the PVP polymer and their mean diameter as well as crystalline
structure. TEM analysis performed on the crude colloidal solution confirmed the presence of small,
spherical, and very well-dispersed Rh NPs with a mean size of ca. 2.2 (± 0.9) nm. The particles were
found to be homogeneous in shape with a quite narrow size distribution (Figures 2a and 2c). HR-TEM
analysis (Figure 2b; [011] atomic planes) revealed well-crystallized NPs with a face centered cubic
(fcc) packed structure as expected for metallic Rh(0). The EDX analysis showed a clear signal for
elemental Rh as well as well-identified signals for C and N atoms from the PVP polymer that embeds
the particles (Figure 2d).

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Figure 2. (a) TEM and (b) HR-TEM images of the as-prepared RhNPs/PVP; (c) NPs size distribution histogram; (d) EDX
analysis spectrum.
The oxidation state and the surrounding chemical environment of RhNPs/PVP were examined by X-ray
photoelectron spectroscopy (XPS). Figure 3a show XPS spectra with the signals corresponding to C, N
and O from the PVP used as stabilizer and Rh (elemental survey). A signal found at about 200.5 eV
(inset Figure 3b) is attributed to contamination of Cl from the organometallic precursor. However, it is
quite insignificant since it is only about 0.13% of the corresponding intensity compared to the C1s level
signal. Figure 3c shows the 3d core level XPS spectra for RhNPs/PVP. The lower binding energy
signals at 307.8 eV (3d5/2) and at about 311.9 eV (3d3/2) confirm the metallic state of the Rh NPs. The
higher binding energy peak at 313-315 eV (3d3/2) suggests the presence of rhodium oxide species, and
comparison with earlier investigations are in agreement with Rh3+ species in Rh2O3.25 The presence of
such species can be attributed to partial oxidation during sample preparation and/or sample introduction
to the spectrometer. However, even if such rhodium oxide species are present, they are likely to be
reduced under the hydrogenation reaction conditions to Rh(0). A small signal is further detected at 304

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eV and attributed to the 3d level with different chemical environment on the surface. The Rh metal
content in the RhNPs/PVP was determined by ICP-AES to be equal to 7.37 wt%.
Figure 3. (a) XPS spectra of the RhNPs/PVP; (b) Enlarged area of the Cl (2p) level; (c) 3d core level XPS of the
RhNPs/PVP.
Selective hydrogenation of arenes with RhNPs/PVP
O
O
O
OH
OH
H
NH2
OH
1
2
3
4
5
O
O
N
6
7
8
9
Figure 4. Arene substrates examined for the selective hydrogenation with RhNPs/PVP.
Benzamide (3 in Figure 4) was chosen as a first substrate for the optimization of the selective
hydrogenation reaction conditions, because of its low price compared to benzoic acid. As the nature of
the solvent (protic, aprotic, polar or apolar) is a well-known parameter influencing catalyst activity and
selectivity in hydrogenation reactions,26 three different solvents were initially tested; n-hexane,
methanol and water, for the benzamide hydrogenation to cyclohexane carboxamide under mild reaction

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conditions (80 °C, 1 bar H2, 2 h reaction time). Both conversion and selectivity were determined by GC
and GC-MS analysis and are reported in Table 1. Water, which has the least solubility of H2, gave a full
conversion of benzamide and 100% selectivity towards the formation of cyclohexane carboxamide
(Table 1, entry 3), whereas n-hexane, and methanol with relative high solubility of H2, were found to
be much less effective; no more than 39 and 71% conversion were achieved for n-hexane and
methanol, respectively (Table 1, entries 1-2). The study was then pursued with water as the reaction
solvent to analyze the influence of temperature. At 40 and 25 °C (Table 1 entries 4 and 5) the Rh
nanocatalyst displays the same high activity as at 80°C, yielding full conversion of benzamide with
100% selectivity towards the desired cyclohexane carboxamide product. Furthermore, a control
experiment with no catalyst was performed in order to ensure the indispensability of the catalyst for
benzamide hydrogenation to cyclohexane carboxamide. As expected, no reaction product was achieved
under the selected reaction conditions (Table 1, entry 6).
Table 1. Hydrogenation of benzamidea
Selectivityb
(%)
Entry
Solvent
Temperature Conversion
TOFc
(h-1)
13.9
25.4
35.7
35.7
35.7
0
(°C)
80
(%)
39
n-hexane
methanol
water
>99
1
2
80
71
>99
80
100
100
100
0
>99
3
water
40
>99
4
water
25
>99
5
6d
water
80
0
a Reaction conditions: 0.5 mmol substrate, 10 mg RhNPs/PVP catalyst (7.36 wt% Rh),
1.4 mol% substrate, 50 mL solvent, 2 h, 320 rpm, 1 bar H2. b Selectivity to cyclohexane
c
carboxamide. Calculated as mol cyclohexane carboxamide per mol Rh per hour. d No
catalyst.
After choosing the preferred reaction conditions regarding solvent and temperature (water and 25 °C)
the catalytic performance of the RhNPs/PVP catalyst was evaluated for hydrogenation of alternative
substrates. A series of eight other arene substrates containing diverse electron donating (EDG) or
electron withdrawing (EWG) groups were selected (Figure 4), and the obtained results are collected in
Table 2.

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Table 2. Hydrogenation of alternative arenesa
Entry
Substrate
Conversion
(%)
74
Yield Selectivityb
TOFc
(h-1)
26.4
35.7
35.7
35.7
33.2
30.6
35.7
35.7
35.7
(%)
73
99
97
99
92
85
99
99
20
(%)
>98
>99
>97
>99
>98
>98
>99
>99
20
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
100
100
100
93
86
100
100
100
a Reaction conditions: 0.5 mmol substrate, 10 mg RhNPs/PVP (7.36 wt% Rh), 1.4
b
mol%, 50 mL solvent, 25 °C, 2 h, 320 rpm, 1 bar H2. Selectivity to ring
c
hydrogenation product. Calculated as mol cyclohexane carboxamide per mol
Rh per hour.
The RhNPs/PVP catalyst showed excellent activity with all of the tested arene substrates resulting in
conversions in the range 74-100 % and selectivity >97 % to the desired ring hydrogenated products
except for acetophenone 9 (Table 2, entries 1-9). Instead, the product selectivity of cyclohexyl methyl
ketone from 9 was only 20 % and 1-phenylethanol (keto group hydrogenation) was observed as the
major product, thus indicating preferential reduction of the carbonyl group, instead of ring
hydrogenation, for this substrate.27 Notably, the oxo group contained in benzamide 3 and benzoic acid 4
were not reduced under the applied reaction conditions. These observations corroborate with previously
reported results with Rh NPs stabilized on carbon nanofibers (CNF) under similar reaction conditions.28
As shown in Table 2, no clear differences were found at the applied reaction conditions in the catalytic
hydrogenation of the arene substrates despite their different EDG or EWG. It has been proposed that
EDG can facilitate faster hydrogenation as the result of a much stronger adsorption of the substrates on
the metal surface. However, stronger adsorption can also be detrimental because of an increase of the
hydrogenation energy barrier due to π-complexation of the aromatic ring.28,29 Thus, these opposing
effects probably make the effect of EDG or EWG unclear.30 In terms of TOF the catalytic performance

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of the prepared RhNPs/PVP catalyst is similar to the results obtained by Barbaro and co-workers with
resin-supported RhNP catalysts under mild reaction conditions (1 bar and RT).31 Other RhNPs-based
catalytic systems have been reported to be more active in the hydrogenation of arenes than the one
presented in this work, but usually the reaction conditions are more harsh in terms of temperature (> 60
ºC) and H2 pressure (> 10 bar). Moreover, the present work use water as the solvent making the overall
process greener than other reported catalytic systems.31-33
Encouraged by the good performance of the RhNPs/PVP catalyst in the selective hydrogenation of
arenes, the hydrogenation of levulinic acid and methyl levulinate was further explored as a plausible
route to obtain γ-valerolactone from biomass (Scheme 1). The reaction conditions were selected on the
basis of our recently developed catalytic system with Au core and Pt shell NPs stabilized on graphitized
carbon black (Au@Pt/G-CB).28,34 Thus, the reactions were performed at 120 °C under 20 bar of H2 for
20 h in water as solvent.
O
H2
O
+
O
ROH
R
O
NPs
Rh/PVP
O
=
or
CH3
R
H
Scheme 1. Hydrogenation of levulinic acid and methyl levulinate.
In both of the examined reactions 100 % conversion and selectivity >99 % to γ-valerolactone were
achieved with the RhNPs/PVP catalyst. These initial results are highly promising since significantly
more harsh reaction conditions (200 °C, 30-40 bar H2) are generally reported in literature for these
reactions to achieve similar yields of γ-valerolactone with catalysts containing transition metals like Pt,
Ru and Rh.29,30,35,36 More detailed exploration of the γ-valerolactone formation from methyl levulinate
and derivatives will be the subject of future work.
Conclusions
Well-defined PVP-stabilized Rh nanoparticles of ca. 2.2 nm in size were obtained via the
organometallic method, namely by hydrogenation of the organometallic complex [Rh(η3-C3H5)3]) in
the presence of PVP, and then evaluated as catalyst in the hydrogenation of a series of arene substrates.

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The RhNPs/PVP catalyst showed excellent activity and selectivity towards aromatic ring
hydrogenation compared to other reported transition metal-based catalysts at room temperature and 1
bar H2, except for acetophenone where 1-phenylethanol was formed as the major product. In addition,
the prepared RhNPs/PVP nanomaterial showed to be a highly promising catalyst for the hydrogenation
of levulinic acid and methyl levulinate in water, leading to quantitative formation of the fuel additive γ-
valerolactone under moderate reaction conditions in opposite to previously reported catalytic systems.
Recycling tests of the RhNPs/PVP catalyst are currently under investigation to validate its long-term
stability.
Acknowledgements
The authors thank V. Collière at the “Centre de microcaractérisation Raymond Castaing - UMS 3623 in
Toulouse” for HR-TEM analysis. CNRS and UPS-Université de Toulouse are acknowledged for
financial support and a MESR PhD fellowship (M. Ibrahim). The UNIK research initiative Catalysis
for Sustainable Energy is also acknowledged for financial support of a PhD fellowship (R. Poreddy).
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20 bar H2, 120 ºC
20h, H2O
Rh@PVP NPs
1 bar H2, RT
2h, H2O
R1 = H, C(O)H, OH, C(O)NH2, C(O)OH, CH2OH, CH3, OCH3; R2 = H, CH3; Z = N, C