Recyclable Rh-PVP nanoparticles catalyzed hydrogenation of benzoic acid derivatives and quinolines under solvent-free conditions

Article abstract of DOI:10.1016/j.catcom.2019.02.019

Various transition metal nanoparticles, prepared by microwave-assisted alcohol reduction method were examined for hydrogenation of benzoic acid to cyclohexanecarboxylic acid under solvent-free conditions. Rh metal was the most effective catalyst over other metal catalyst. The catalyst showed moderate to high yield for the hydrogenation of substituted benzoic acid and substituted quinolines. Rh-PVP was recycled four times with a minor loss in catalytic activity.

Full text of DOI:10.1016/j.catcom.2019.02.019

Accepted Manuscript
Recyclable Rh-PVP nanoparticles catalyzed hydrogenation of
benzoic acid derivatives and quinolines under solvent-free
conditions
Chandan Chaudhari, Hirotaka Imatome, Yoshihide Nishida,
Katsutoshi Sato, Katsutoshi Nagaoka
PII:
S1566-7367(19)30053-6
DOI:
https://doi.org/10.1016/j.catcom.2019.02.019
CATCOM 5630
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
6 December 2018
14 February 2019
19 February 2019
Please cite this article as: C. Chaudhari, H. Imatome, Y. Nishida, et al., Recyclable Rh-
PVP nanoparticles catalyzed hydrogenation of benzoic acid derivatives and quinolines
under solvent-free conditions, Catalysis Communications, https://doi.org/10.1016/
j.catcom.2019.02.019
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ACCEPTED MANUSCRIPT
Recyclable Rh-PVP nanoparticles catalyzed hydrogenation of benzoic acid
derivatives and quinolines under solvent-free conditions
a,*
a
Chandan Chaudhari chandanchaudhari@oita-u.ac.jp, Hirotaka Imatome , Yoshihide
a
a,b
a,*
Nishida , Katsutoshi Sato , Katsutoshi Nagaoka nagaoka@oita-u.ac.jp
a
Department of Integrated Science and Technology, Faculty of Science and Engineering,
Oita University, 700 Dannoharu, Oita 870-1192, Japan
b
Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura,
Kyoto 615-8520, Japan
*
Corresponding authors at: Department of Integrated Science and Technology, Faculty of
Science and Engineering, Oita University, 700 Dannoharu, Oita 870-1192, Japan.

ACCEPTED MANUSCRIPT
Abstract
Various transition metal nanoparticles, prepared by microwave-assisted alcohol reduction
method were examined for hydrogenation of benzoic acid to cyclohexanecarboxylic acid
under solvent-free conditions. Rh metal was the most effective catalyst over other metal
catalyst. The catalyst showed moderate to high yield for the hydrogenation of substituted
benzoic acid and substituted quinolines. Rh-PVP was recycled four times with minor loss
in catalytic activity.
Keywords: Heterogeneous catalyst, Rhodium, Nanoparticles, Hydrogenation, Benzoic
acid, Quinolines

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1
. Introduction
The chemoselective hydrogenation of benzoic acid to cyclohexanecarboxylic acid
CCA) has great significance in chemical and pharmaceutical industries. The
(
pharmaceutical drugs such as ansatrienin can be synthesized by using cyclohexyl
carboxylic acid [1]. Moreover, the formation of cyclohexanecarboxylic acid by
hydrogenation of benzoic acid is a crucial step for the production of caprolactam from
toluene in SNIA process (developed by SNIA S.p.A, Italy) [2]. Traditionally, the
hydrogenation of benzoic acid and its derivatives was carried out with stoichiometric
amount of Na-K alloys [3]. Several Supported monometallic (Pd, Ir, Rh, Ru and Ni)
catalysts have been reported for hydrogenation of benzoic acid [4-18]. Bimetallic alloy
based catalysts (Pd-Au, Ru-Pt, Ru-Pd, Ni-Pd-B and Ni-Zr-B) were also examined [19-24].
However, some of them suffer from use of additives [24, 25], high temperature (200 °C)
[
[
18], high pressure (>20 bar)[5-7, 12-15, 18, 19, 22-25], high catalyst loading (> 1 mol%)
4, 9, 10, 17, 20] and non-recyclable catalyst (Table S1) [7-14, 23, 24]. In addition, the
solvent was necessary for selective hydrogenation of benzoic acid to cyclohexane
carboxylic acid (Scheme 1) [4-25].
Scheme 1. Hydrogenation of benzoic acid
Metal nanoparticles exhibit a wide range of applications, particularly in the field
of catalysis [26]. Among them, polymer or ionic liquids stabilized nanoparticles are
located at the border of homogeneous and heterogeneous catalysis. The intrinsic catalytic
activity of these systems is easy to understand compared to metal nanoparticles supported
on metal oxides because substrate and support direct interactions can be eliminated. Rh
metal demonstrated excellent activity for the hydrogenation reactions.
Polyvinylpyrrolidone (PVP) stabilized Rh nanoparticles were reported for the biphasic
hydrogenation of benzene but deactivation of catalyst was observed due to aggregation of

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Rh nanoparticles [27]. In 2016, Philippot and co-workers studied Rh-PVP catalyzed
hydrogenation of benzoic acid using water as solvent [28]. However, benzoic acid
derivatives and recyclability of catalyst were not investigated. Very recently, our group
synthesized PVP stabilized Rh nanoparticles by microwave-assisted alcohol as reduction
method [29]. This catalyst showed good recyclability for the synthesis of secondary
imines from nitriles. As part of continuing interest in the applications of the Rh
nanoparticles, we report herein recyclable Rh nanoparticles catalyzed selective
hydrogenation of benzoic acid with different functional groups under solvent-free
conditions. In addition, the catalyst was applied to selective hydrogenation of quinolines
to tetrahydroquinolines which are important bioactive compounds. Good to excellent
yields of hydrogenated carboxylic acid derivatives and quinolines were obtained.
2
. Experimental
2
.1. Catalyst preparation
All metal precursors were purchased from Wako Pure Chemical Industries, Ltd. Japan.
Ru, Rh, Pd, Ir and Pt nanoparticles were synthesized by microwave-assisted alcohol
reduction method [29]. In typical procedure, 0.25 mmol of metal chloride and 1.25 mmol
of polyvinylpyrrolidone (molecular weight M ≈ 40000) were dissolved in 20 ml of
ethylene glycol. After purging by Argon, the vessel was sealed with cap and heated to
(180 °C) with microwave irradiation (400 W) for 15 min. The obtained colloidal
suspension was precipitated using diethyl ether and acetone followed by washing with
water and drying under vacuum at 40 °C.
2
.2. Catalyst characterization
The particle size of metal nanoparticles was determined by transmission electron
microscopy (TEM) using a JEM-2010 transmission electron microscope (JEOL, Japan)
operated at an accelerating voltage of 200 kV. The amount of PVP on metal nanoparticles
was calculated by using thermogravimetric analysis (ThermoPlus; Rigaku, Japan) from
room temperature to 1000 °C under air (flow rate: 200 mL/min). X-ray photoelectron
spectroscopy (XPS) analysis was carried out using an ESCA-3400 (Shimadzu, Kyoto,
Japan). The binding energy was calibrated using a C 1s binding energy of 284.6 eV. High-

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angle annular dark field scanning transmission electron microscopy images (HAADF-
STEM) were obtained using JEM-ARM200F (JEOL, Japan) operated at 200 kV.
2
.3. Typical procedures for catalytic hydrogenation
All organic compounds (Tokyo Chemical Industry, Japan) were used without further
purification. Hydrogenation reactions were carried out in 10 ml autoclave with a glass
tube inside equipped with magnetic stirrer. In a typical experiment, 1 mmol of substrate
and catalyst (1 mol %) were placed in the reactor and sealed. After being sealed,
hydrogen was introduced to reactor and inside of the reactor was purged for multiple
times. The reactor was pressurized at 5-10 bar by hydrogen and heated to 100°C -180°C.
After completion of reaction, the reactor was cooled to room temperature and
depressurized. The obtained reaction crude washed with acetone several times and
resultant solution was filtered by using micro filters (Sartorius RC 0.20 μm). Then, the
filtrate solution was concentrated by using rotary evaporation. Conversion and yield of
1
product were obtained by H NMR analysis using 1, 3, 5-trimethoxybenzene as an
1
internal standard. H-NMR spectra were recorded using Brucker UltraShield 400 plus
operating at 400 MHz.
2
.4. Catalyst recycling
After the reaction, the reaction crude was washed by acetone several times. The catalyst
was recovered by centrifugation and dried at 50°C for 12 h under vacuum. The dried
catalyst was reused for the next run.
3
. Results and Discussion
We chose hydrogenation of benzoic acid as model reaction for the optimization of
reaction conditions (Table 1). First, various noble metals nanoparticles (Ru, Rh, Pd, Ir and
Pt) were used for hydrogenation of benzoic acid to cyclohexanecarboxylic acid. Among
the tested catalysts, Rh-PVP catalyst showed the highest yield (96 %) (entries 1-6). In
previous reports, Wang [23] and Bai [25] demonstrated that highly polar solvent such as
water was played a crucial role in selective hydrogenation of aromatic ring in benzoic
acid. Water facilitated to aromatic ring for the adsorption on catalyst surface and avoided

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unwanted adsorption of carboxyl group on catalyst. Contrary to this reports, Rh-PVP
catalyst showed high yield (96%) of cyclohexane carboxylic acid in the absence of
solvent. Polar and non polar solvents were found to be less effective for the
hydrogenation of benzoic acid.
a
Table 1 Hydrogenation of benzoic acid by metal catalysts
PVP
wt %)
82
Particle size
Solvent
(1 ml)
Neat
Neat
Neat
Neat
Neat
Neat
Conv.
(%)
99
80
70
53
63
32
48
20
1
Yield
(%)
Entry
1
Catalyst
c
d
b
(
(nm)
Rh-PVP
Rh-PVP
Pt-PVP
5.4±1.0
5.4±1.0
3.9±0.4
1.9±0.5
3.8±1.1
4.8±0.7
5.4±1.0
5.4±1.0
5.4±1.0
5.4±1.0
5.4±1.0
--
96
80
70
53
34
21
46
13
1
e
2
82
72
78
83
84
82
82
82
3
4
5
6
7
8
9
Ir-PVP
Ru-PVP
Pd-PVP
Rh-PVP
Rh-PVP
Rh-PVP
Rh-PVP
Rh-PVP
no catalyst
Water
Methanol
CH CN
3
1
0
1
82
82
--
THF
Cyclohexane
Neat
66
47
0
57
35
0
1
12
a
b
Reaction conditions: substrate (1 mmol), Metal (1 mol%), 150 °C, 1 h, H (10 bar). NMR
2
c
d
Yield. Calculated by thermogravimetric analysis. Determined by transmission electron
e
microscopy. Rh catalyst (0.5 mol%)
To demonstrate the general applicability of Rh nanoparticles, we carried out
hydrogenation reaction of various substituted benzoic acid and derivatives under
aforementioned reaction conditions (Table 2). The substituted benzoic acid bearing
electron donating group such as methyl gave 99% yield, and benzoic acid bearing
electron withdrawing trifluoromethyl group delivered in 60% yield (entries 2 and 3). Due
to reductive dechlorination reaction, the hydrogenation of para-chlorobenzoic acid
proceeded to cyclohexanecarboxylic acid as final product instead of para-
chlorocyclohexanecarboxylic acid (entry 6). The catalyst was useful for hydrogenation of

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nitrogen-containing heterocyclic carboxylic acid (entry 7). The hydrogenation of
benzamide and para-methylbenzamide leads to corresponding products with good yields
(70-85 %) (entries 8 and 9). Moderate to high yields (53-98 %) were obtained when
hydrogenation reactions of ethyl benzoate and para-substituted methyl benzoate were
carried out (entries 10-12). The catalyst was also applicable for the synthesis of
industrially important cyclohexyldicarboxylic ester from hydrogenation of dimethyl
phthalate (entry 13).
Encouraged by benzoic acid hydrogenation results, we studied the applicability of Rh-
PVP catalysts for the selective hydrogenation of quinolines to 1,2,3,4-tetrahydroquinoline.
Hydrogenation of quinolines has gained much attention for the synthesis of
tetrahydroquinoline-derived natural products and bioactive compounds [30]. The first row
transition metal (Mn, Co and Ni) nanoparticles were investigated for the hydrogenation of
quinoline using lithium metal and water as hydrogen source. In compared to lithium and
water system, molecular hydrogen is an atom-economical source of hydrogen.
Unsupported Rh catalyst has been reported for direct hydrogenation of quinolines using
molecular hydrogen [32-35]. However, these catalytic methods require high pressure (30
bar H ) and organic solvents (Table S2).
2
Using our Rh nanopaticles, quinoline was easily hydrogenated to 1,2,3,4-
tetrahydroquinoline under mild pressure (5 bar H ). Electron donating (-OCH ) and
2
3
withdrawing (-Cl) bearing quinolines were hydrogenated to corresponding
tetrahydroquinoline and decahydroquinoline (entry 2 and 3).
a
Table 2 Hydrogenation of benzoic acid derivatives using Rh-PVP
b
Entry
Substrate
Products
Temp. Time Conv.
Yield(%)
(cis/trans)
(
ºC)
(h)
(%)
1
150
150
1
99
96
2
5
99
99(62:38)

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3
120
160
150
180
2
1
96
85
90
99
96(62:38)
50(59:41)
60(70:30)
99
c
4
5
6
1
24
7
8
9
150
150
180
150
24
24
24
3
78
85
88
98
95
75
85
70(65:35)
98
10
1
1
2
3
180
6
95(71:29)
1
1
180
150
24
6
61
89
53(83:17)
81(78:22)
a
b
c
Reaction conditions: substrate (1 mmol), Rh (1 mol%), H (10 bar). NMR Yield. 0.5
2
mol % catalyst.

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a
Table 3 Hydrogenation of quinolines by Rh-PVP.
b
Entry
Substrate
Products
Conv.
%)
Yield
(%)
(
1
99
94
c
2
3
95
83
c
92
78
a
b
Reaction conditions: substrate (1 mmol), Rh (1 mol%), 100 °C, 2 h, H (5 bar). NMR
2
c
Yield. Decahydroquinoline was obtained as byproduct.
We investigated reusability of Rh nanoparticles for hydrogenation of benzoic acid at
optimized reaction condition. The recovered catalyst was recycled three times with
st
nd
slightly loss in catalytic activity from 1 to 2 cycle (Fig.2). ICP-AES analysis showed
that the Rh amount in reaction mixture was below the detection limit (10 ppb).
Fig. 2. Recycle study for hydrogenation of benzoic acid by Rh catalyst.
In previous reports, deactivation of unsupported Rh nanoparticles which was synthesized
by conventional alcohol reduction method was observed for hydrogenation of arene
reactions [35]. In that case, morphological changes such as aggregation headed to catalyst
deactivation. To study morphological changes, HAADF-STEM images of fresh and

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recovered Rh catalyst after first reaction were taken (Fig. 3). Although Rh nanoparticles
were partially aggregated, the catalytic activity was not changed significantly. Probably,
reaction solvent has some role in catalyst deactivation. During reaction, PVP could be
dissolve in water and resultant Rh nanoparticles with less amount of PVP are easily
aggregated. By contrast, the removal of PVP was suppressed in our solvent-free
conditions, which provides the recyclability of our Rh catalyst. More detailed exploration
about the rate of aggregation is necessary and it will be objective of future works.
Fig.3.HAADF-STEM images of fresh (A,B) and used (C,D) Rh catalyst.
Moreover, electronic states of fresh and aggregated Rh nanoparticles were examined
by XPS analysis (Fig. 4).The binding energy of fresh Rh catalyst was slightly shifted to
lower binding energy than that of Rh bulk, indicating charge transfer from PVP to Rh
occurs. The binding energy of recovered Rh catalyst was identical to that of fresh Rh
nanoparticles, indicating that during the reaction electronic state of Rh nanoparticles
remained unchanged. Therefore, we concluded slight loss of activity after 1st cycle was
due to aggregation of Rh particles.

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Fig. 4.XPS spectra of fresh and used Rh catalyst.
Conclusions
In summary, we have demonstrated a successful example of Rh catalyzed
hydrogenation of benzoic acid and quinolines under solvent-free conditions. Compared to
previous reports, this catalyst does not require solvent to control selectivity for
cyclohexanecarboxylic acid from benzoic acid. The catalyst was tolerant to different
functional group of benzoic acid. With a minor loss in catalytic activity, Rh-PVP catalyst
was recycled successfully.
Acknowledgment
This work was financially supported from "Accelerated Innovation Research Initiative
Turning Top Science and Ideas into High-Impact Values (ACCEL)"of Japan Science and
Technology Agency (JST), Japan. The authors thank Assistant Professor Kohei Kusada
for XPS measurements and T. Yamamoto, T. Toriyama, Prof. S. Matsutmura for HAADF-
STEM analysis.

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Recyclable Rh-PVP nanoparticles catalyzed hydrogenation of benzoic acid
derivatives and quinolines under solvent-free conditions
a
a
a
ab
Chandan Chaudhari,* Hirotaka Imatome, Yoshihide Nishida, Katsutoshi Sato,
Katsutoshi Nagaoka*
a
a
Department of Integrated Science and Technology, Faculty of Science and Engineering,
Oita University, 700 Dannoharu, Oita 870-1192, Japan.
Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura,
b
Kyoto 615-8520, Japan.
*
Corresponding authors
Katsutoshi Nagaoka, Chandan Chaudhari
a
Department of Integrated Science and Technology, Faculty of Science and Engineering,
Oita University, 700 Dannoharu, Oita 870-1192, Japan
E-mail: nagaoka@oita-u.ac.jp, chandanchaudhari@oita-u.ac.jp
Tel.: +81-97-554-7896
Fax : +81-97-554-7979
Abstract
Various transition metal nanoparticles, prepared by microwave-assisted alcohol reduction
method were examined for hydrogenation of benzoic acid to cyclohexanecarboxylic acid
under solvent-free conditions. Rh metal was the most effective catalyst over other metal
catalyst. The catalyst showed moderate to high yield for the hydrogenation of substituted
benzoic acid and substituted quinolines. Rh-PVP was recycled four times with minor loss
in catalytic activity.
Keywords: Heterogeneous catalyst, Rhodium, Nanoparticles, Hydrogenation, Benzoic
acid, Quinolines
1
. Introduction
The chemoselective hydrogenation of benzoic acid to cyclohexanecarboxylic acid
(CCA) has great significance in chemical and pharmaceutical industries. The
pharmaceutical drugs such as ansatrienin can be synthesized by using cyclohexyl
carboxylic acid [1]. Moreover, the formation of cyclohexanecarboxylic acid by
hydrogenation of benzoic acid is a crucial step for the production of caprolactam from

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toluene in SNIA process (developed by SNIA S.p.A, Italy) [2]. Traditionally, the
hydrogenation of benzoic acid and its derivatives was carried out with stoichiometric
amount of Na-K alloys [3]. Several Supported monometallic (Pd, Ir, Rh, Ru and Ni)
catalysts have been reported for hydrogenation of benzoic acid [4-18]. Bimetallic alloy
based catalysts (Pd-Au, Ru-Pt, Ru-Pd, Ni-Pd-B and Ni-Zr-B) were also examined [19-24].
However, some of them suffer from use of additives [24, 25], high temperature (200 °C)
[
[
18], high pressure (>20 bar)[5-7, 12-15, 18, 19, 22-25], high catalyst loading (> 1 mol%)
4, 9, 10, 17, 20] and non-recyclable catalyst (Table S1) [7-14, 23, 24]. In addition, the
solvent was necessary for selective hydrogenation of benzoic acid to cyclohexane
carboxylic acid (Scheme 1) [4-25].
Scheme 1. Hydrogenation of benzoic acid
Metal nanoparticles exhibit a wide range of applications, particularly in the field
of catalysis [26]. Among them, polymer or ionic liquids stabilized nanoparticles are
located at the border of homogeneous and heterogeneous catalysis. The intrinsic catalytic
activity of these systems is easy to understand compared to metal nanoparticles supported
on metal oxides because substrate and support direct interactions can be eliminated. Rh
metal demonstrated excellent activity for the hydrogenation reactions.
Polyvinylpyrrolidone (PVP) stabilized Rh nanoparticles were reported for the biphasic
hydrogenation of benzene but deactivation of catalyst was observed due to aggregation of
Rh nanoparticles [27]. In 2016, Philippot and co-workers studied Rh-PVP catalyzed
hydrogenation of benzoic acid using water as solvent [28]. However, benzoic acid
derivatives and recyclability of catalyst were not investigated. Very recently, our group
synthesized PVP stabilized Rh nanoparticles by microwave-assisted alcohol as reduction
method [29]. This catalyst showed good recyclability for the synthesis of secondary
imines from nitriles. As part of continuing interest in the applications of the Rh
nanoparticles, we report herein recyclable Rh nanoparticles catalyzed selective

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hydrogenation of benzoic acid with different functional groups under solvent-free
conditions. In addition, the catalyst was applied to selective hydrogenation of quinolines
to tetrahydroquinolines which are important bioactive compounds. Good to excellent
yields of hydrogenated carboxylic acid derivatives and quinolines were obtained.
2
. Experimental
2
.1. Catalyst preparation
All metal precursors were purchased from Wako Pure Chemical Industries, Ltd. Japan.
Ru, Rh, Pd, Ir and Pt nanoparticles were synthesized by microwave-assisted alcohol
reduction method [29]. In typical procedure, 0.25 mmol of metal chloride and 1.25 mmol
of polyvinylpyrrolidone (molecular weight M ≈ 40000) were dissolved in 20 ml of
ethylene glycol. After purging by Argon, the vessel was sealed with cap and heated to
(180 °C) with microwave irradiation (400 W) for 15 min. The obtained colloidal
suspension was precipitated using diethyl ether and acetone followed by washing with
water and drying under vacuum at 40 °C.
2
.2. Catalyst characterization
The particle size of metal nanoparticles was determined by transmission electron
microscopy (TEM) using a JEM-2010 transmission electron microscope (JEOL, Japan)
operated at an accelerating voltage of 200 kV. The amount of PVP on metal nanoparticles
was calculated by using thermogravimetric analysis (ThermoPlus; Rigaku, Japan) from
room temperature to 1000 °C under air (flow rate: 200 mL/min). X-ray photoelectron
spectroscopy (XPS) analysis was carried out using an ESCA-3400 (Shimadzu, Kyoto,
Japan). The binding energy was calibrated using a C 1s binding energy of 284.6 eV. High-
angle annular dark field scanning transmission electron microscopy images (HAADF-
STEM) were obtained using JEM-ARM200F (JEOL, Japan) operated at 200 kV.
2
.3. Typical procedures for catalytic hydrogenation
All organic compounds (Tokyo Chemical Industry, Japan) were used without further
purification. Hydrogenation reactions were carried out in 10 ml autoclave with a glass
tube inside equipped with magnetic stirrer. In a typical experiment, 1 mmol of substrate

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and catalyst (1 mol %) were placed in the reactor and sealed. After being sealed,
hydrogen was introduced to reactor and inside of the reactor was purged for multiple
times. The reactor was pressurized at 5-10 bar by hydrogen and heated to 100°C -180°C.
After completion of reaction, the reactor was cooled to room temperature and
depressurized. The obtained reaction crude washed with acetone several times and
resultant solution was filtered by using micro filters (Sartorius RC 0.20 μm). Then, the
filtrate solution was concentrated by using rotary evaporation. Conversion and yield of
1
product were obtained by H NMR analysis using 1, 3, 5-trimethoxybenzene as an
1
internal standard. H-NMR spectra were recorded using Brucker UltraShield 400 plus
operating at 400 MHz.
2
.4. Catalyst recycling
After the reaction, the reaction crude was washed by acetone several times. The catalyst
was recovered by centrifugation and dried at 50°C for 12 h under vacuum. The dried
catalyst was reused for the next run.
4
. Results and Discussion
We chose hydrogenation of benzoic acid as model reaction for the optimization of
reaction conditions (Table 1). First, various noble metals nanoparticles (Ru, Rh, Pd, Ir and
Pt) were used for hydrogenation of benzoic acid to cyclohexanecarboxylic acid. Among
the tested catalysts, Rh-PVP catalyst showed the highest yield (96 %) (entries 1-6). In
previous reports, Wang [23] and Bai [25] demonstrated that highly polar solvent such as
water was played a crucial role in selective hydrogenation of aromatic ring in benzoic
acid. Water facilitated to aromatic ring for the adsorption on catalyst surface and avoided
unwanted adsorption of carboxyl group on catalyst. Contrary to this reports, Rh-PVP
catalyst showed high yield (96%) of cyclohexane carboxylic acid in the absence of
solvent. Polar and non polar solvents were found to be less effective for the
hydrogenation of benzoic acid.

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a
Table 1 Hydrogenation of benzoic acid by metal catalysts
PVP Particle size
Solvent
(1 ml)
Neat
Neat
Neat
Neat
Neat
Neat
Conv. Yield
Entry Catalyst
c
d
b
(
wt %)
82
82
72
78
83
84
82
82
82
82
82
--
(nm)
(%)
99
80
70
53
63
32
48
20
1
(%)
96
80
70
53
34
21
46
13
1
1
Rh-PVP
Rh-PVP
Pt-PVP
5.4±1.0
5.4±1.0
3.9±0.4
1.9±0.5
3.8±1.1
4.8±0.7
5.4±1.0
5.4±1.0
5.4±1.0
5.4±1.0
5.4±1.0
--
e
2
3
4
5
6
7
8
9
Ir-PVP
Ru-PVP
Pd-PVP
Rh-PVP
Rh-PVP
Rh-PVP
Rh-PVP
Rh-PVP
no catalyst
Water
Methanol
CH CN
3
10
THF
Cyclohexane
Neat
66
47
0
57
35
0
11
12
a
b
Reaction conditions: substrate (1 mmol), Metal (1 mol%), 150 °C, 1 h, H (10 bar). NMR
2
c
d
Yield. Calculated by thermogravimetric analysis. Determined by transmission electron
e
microscopy. Rh catalyst (0.5 mol%)
To demonstrate the general applicability of Rh nanoparticles, we carried out
hydrogenation reaction of various substituted benzoic acid and derivatives under
aforementioned reaction conditions (Table 2). The substituted benzoic acid bearing
electron donating group such as methyl gave 99% yield, and benzoic acid bearing
electron withdrawing trifluoromethyl group delivered in 60% yield (entries 2 and 3). Due
to reductive dechlorination reaction, the hydrogenation of para-chlorobenzoic acid
proceeded to cyclohexanecarboxylic acid as final product instead of para-
chlorocyclohexanecarboxylic acid (entry 6). The catalyst was useful for hydrogenation of
nitrogen-containing heterocyclic carboxylic acid (entry 7). The hydrogenation of
benzamide and para-methylbenzamide leads to corresponding products with good yields
(70-85 %) (entries 8 and 9). Moderate to high yields (53-98 %) were obtained when
hydrogenation reactions of ethyl benzoate and para-substituted methyl benzoate were
carried out (entries 10-12). The catalyst was also applicable for the synthesis of
industrially important cyclohexyldicarboxylic ester from hydrogenation of dimethyl
phthalate (entry 13).

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Encouraged by benzoic acid hydrogenation results, we studied the applicability of Rh-
PVP catalysts for the selective hydrogenation of quinolines to 1,2,3,4-tetrahydroquinoline.
Hydrogenation of quinolines has gained much attention for the synthesis of
tetrahydroquinoline-derived natural products and bioactive compounds [30]. The first row
transition metal (Mn, Co and Ni) nanoparticles were investigated for the hydrogenation of
quinoline using lithium metal and water as hydrogen source. In compared to lithium and
water system, molecular hydrogen is an atom-economical source of hydrogen.
Unsupported Rh catalyst has been reported for direct hydrogenation of quinolines using
molecular hydrogen [32-35]. However, these catalytic methods require high pressure (30
bar H ) and organic solvents (Table S2).
2
Using our Rh nanopaticles, quinoline was easily hydrogenated to 1,2,3,4-
tetrahydroquinoline under mild pressure (5 bar H ). Electron donating (-OCH ) and
2
3
withdrawing (-Cl) bearing quinolines were hydrogenated to corresponding
tetrahydroquinoline and decahydroquinoline (entry 2 and 3).
a
Table 2 Hydrogenation of benzoic acid derivatives using Rh-PVP
b
Entry
1
Substrate
Products
Temp. Time Conv.
Yield(%)
(cis/trans)
(
ºC)
(h)
(%)
150
150
1
99
96
2
3
5
99
99(62:38)
120
160
150
2
1
1
96
85
90
96(62:38)
50(59:41)
60(70:30)
c
4
5

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6
180
24
99
99
7
8
150
150
24
24
78
85
75
85
9
180
150
24
88
70(65:35)
10
3
6
98
95
98
1
1
2
3
180
95(71:29)
1
1
180
24
6
61
53(83:17)
150
89
81(78:22)
a
b
c
Reaction conditions: substrate (1 mmol), Rh (1 mol%), H (10 bar). NMR Yield. 0.5
2
mol % catalyst.
a
Table 3 Hydrogenation of quinolines by Rh-PVP.
b
Entry
Substrate
Products
Conv.
%)
Yield
(%)
(
1
99
94

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c
2
3
95
83
c
92
78
a
b
Reaction conditions: substrate (1 mmol), Rh (1 mol%), 100 °C, 2 h, H (5 bar). NMR
2
c
Yield. Decahydroquinoline was obtained as byproduct.
We investigated reusability of Rh nanoparticles for hydrogenation of benzoic acid at
optimized reaction condition. The recovered catalyst was recycled three times with
st
nd
slightly loss in catalytic activity from 1 to 2 cycle (Fig.2). ICP-AES analysis showed
that the Rh amount in reaction mixture was below the detection limit (10 ppb).
Fig. 2. Recycle study for hydrogenation of benzoic acid by Rh catalyst.
In previous reports, deactivation of unsupported Rh nanoparticles which was synthesized
by conventional alcohol reduction method was observed for hydrogenation of arene
reactions [35]. In that case, morphological changes such as aggregation headed to catalyst
deactivation. To study morphological changes, HAADF-STEM images of fresh and
recovered Rh catalyst after first reaction were taken (Fig. 3). Although Rh nanoparticles
were partially aggregated, the catalytic activity was not changed significantly. Probably,
reaction solvent has some role in catalyst deactivation. During reaction, PVP could be
dissolve in water and resultant Rh nanoparticles with less amount of PVP are easily

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aggregated. By contrast, the removal of PVP was suppressed in our solvent-free
conditions, which provides the recyclability of our Rh catalyst. More detailed exploration
about the rate of aggregation is necessary and it will be objective of future works.
Fig.3.HAADF-STEM images of fresh (A,B) and used (C,D) Rh catalyst.
Moreover, electronic states of fresh and aggregated Rh nanoparticles were examined
by XPS analysis (Fig. 4).The binding energy of fresh Rh catalyst was slightly shifted to
lower binding energy than that of Rh bulk, indicating charge transfer from PVP to Rh
occurs. The binding energy of recovered Rh catalyst was identical to that of fresh Rh
nanoparticles, indicating that during the reaction electronic state of Rh nanoparticles
remained unchanged. Therefore, we concluded slight loss of activity after 1st cycle was
due to aggregation of Rh particles.

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Fig. 4.XPS spectra of fresh and used Rh catalyst.
Conclusions
In summary, we have demonstrated a successful example of Rh catalyzed
hydrogenation of benzoic acid and quinolines under solvent-free conditions. Compared to
previous reports, this catalyst does not require solvent to control selectivity for
cyclohexanecarboxylic acid from benzoic acid. The catalyst was tolerant to different
functional group of benzoic acid. With a minor loss in catalytic activity, Rh-PVP catalyst
was recycled successfully.
Acknowledgment
This work was financially supported from "Accelerated Innovation Research Initiative
Turning Top Science and Ideas into High-Impact Values (ACCEL)"of Japan Science and
Technology Agency (JST), Japan. The authors thank Assistant Professor Kohei Kusada
for XPS measurements and T. Yamamoto, T. Toriyama, Prof. S. Matsutmura for HAADF-
STEM analysis.

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Graphical abstract