TiO2-Supported Re as a General and Chemoselective Heterogeneous Catalyst for Hydrogenation of Carboxylic Acids to Alcohols

Article abstract of DOI:10.1002/chem.201604762

TiO₂-supported Re, Re/TiO₂, was found to promote selective hydrogenation of carboxylic acids having aromatic and aliphatic moieties to the corresponding alcohols. Re/TiO₂showed superior results compared to other transition-metal-loaded TiO₂and supported Re catalysts for selective hydrogenation of 3-phenylpropionic acid. 3-phenylpropanol was produced in 97 % yield under mild conditions (5 MPa H₂at 140 °C). Contrary to typical heterogeneous catalysts, Re/TiO₂does not lead to the formation of dearomatized byproducts. The catalyst is recyclable and shows a wide substrate scope in the synthesis of alcohols (22 examples; up to 97 % isolated yield).

Full text of DOI:10.1002/chem.201604762

A Journal of
Accepted Article
Title: TiO2-Supported Re as a General and Chemoselective
Heterogeneous Catalyst for Hydrogenation of Carboxylic Acids to
Alcohols
Authors: Takashi Toyao, S. M. A. H. Siddiki, Abeda S. Touchy, Wataru
Onodera, Kenichi Kon, Yoshitsugu Morita, Takashi Kamachi,
Kazunari Yoshizawa, and Ken-ichi Shimizu
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To be cited as: Chem. Eur. J. 10.1002/chem.201604762
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10.1002/chem.201604762
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COMMUNICATION
TiO2-Supported Re as a General and Chemoselective Heterogeneous
Catalyst for Hydrogenation of Carboxylic Acids to Alcohols
Takashi Toyao,*[a,b] S. M. A. H. Siddiki,[a] Abeda S. Touchy,[a] Wataru Onodera,[a] Kenichi Kon,[a]
Yoshitsugu Morita,[c] Takashi Kamachi,[c] Kazunari Yoshizawa,[b, c] Ken-ichi Shimizu,*[a,b]
Abstract: TiO2-supported Re, Re/TiO2, has been found to
promote selective hydrogenation of aromatic and aliphatic
carboxylic acids to alcohols. Re/TiO2 is superior to other
transition metal-loaded TiO2 and supported Re catalysts for
selective hydrogenation of 3-phenylpropionic acid. This process
produces 3-phenylpropanol in a 97% yield under mild condition
(5 MPa H2 at 140 ºC). Contrary to typical heterogeneous
catalysts, Re/TiO2 does not give dearomatized byproducts. The
catalyst is recyclable and shows wide substrate scope toward
the alcohol forming process (22 examples; up to 97% isolated
yield).
limited to one example, and thus, the system lacks the generality.
Recently, a biocatalytic system has been found to promote the
selective hydrogenation of carboxylic acids.[8] Although this
would be an interesting strategy, development of more general
systems are desired. In this regard, more recently, several
research groups described new homogeneous catalysts for
selective hydrogenation reactions of carboxylic acids that
contain aromatic rings.[9] For example, Leitner et al. reported that
Ru(triphos)(TMM) (triphos
= tridentate phosphine; TMM =
trimethylene methane) can promote selective hydrogenation
reaction of benzoic acid to benzyl alcohol.[9a] Also, Beller et al.
used Ru(acac)3(triphos) (acac
=
acetylacetonate) as
a
Reduction of carboxylic acids to form alcohols is a key synthetic
transformation for pharmaceutical and fine-chemical industries
as well as for biomass conversion.[1] Conventionally, this
reaction has been performed by stoichiometric amounts of
strong reducing agents such as lithium aluminum hydride or
lithium triethylborohydride.[2] The use of these strong reductants
has serious drawbacks in terms of reagent compatibility, poor
atom economy and safety. Selective catalytic hydrogenation of
carboxylic acids is a straight-forward and effective synthetic
method to obtain alcohols because the conditions are mild and
water is the sole byproduct.[3] However, carboxylic acid groups
are thermodynamically and kinetically stable owing to the low
electrophilicity of the carbonyl carbon. Because of this feature,
these substances are among the most difficult carbonyl
substrates to hydrogenate.[4] So much effort has been devoted
to hydrogenation of carboxylic acids employing heterogeneous
and homogeneous catalysts in order to tackle this difficult
process, and consequently, some processes have become
industrial viable.[5] Although these processes have already been
in practical use and there has been substantial progress in
recent years,[6] the required harsh conditions often lead to low
selectivity to desired products. In particular, hydrogenation of
aromatic rings occurs rather than hydrogenation of carboxylic
acid groups when both moieties are present in substrates
(Scheme 1).[4] There are a few reports describing selective
hydrogenation of carboxylic acids containing aromatic rings
using heterogeneous catalysts (Ru and Sn co-loaded Al2O3
catalysts).[7] In these reports, however, the substrate scope is
homogeneous catalyst in the presence of a Lewis acid to
promote the selective hydrogenation reaction for various
carboxylic acids.[9b] Saito et al. utilized a Ru[9c] and Re[9d]
complexes as homogeneous catalysts for this process. A
homogeneous Co-complex, comprised of Co(BF4)2·6H2O and a
triphos ligand, was developed by Bruin et al for promoting
selective catalytic hydrogenation of carboxylic acids to the
corresponding alcohols.[9e] These processes represent important
contributions to the synthesis of alcohols. In all cases, however,
homogeneous catalysts were utilized that generally suffer from
product separation and catalyst recycling. From the viewpoint of
sustainable chemistry and large-scale applications, catalyst
employed for this selective hydrogenation reaction should be
heterogeneous and readily recyclable.
In the investigation described below, we uncovered a
general and versatile heterogeneous catalytic system,
comprised of TiO2-supported Re (Re/TiO2), which promotes
selective alcohol forming hydrogenation reactions of carboxylic
acids that contain aromatic and aliphatic moieties (Scheme 1).
Unlike previously-developed catalysts, TiO2-supported Re has
the advantage of being heterogeneous and, therefore, it can be
readily recovered and recycled. Moreover, the catalyst is easily
prepared by using a facile impregnation method. The new
[a]
Dr. T. Toyao, Dr. S. M. A. H. Siddiki, Dr. A. S. Touchy, W. Onodera,
Dr. K. Kon, Prof. Dr. K. Shimizu,
Institute for Catalysis, Hokkaido University, N-21, W-10, Sapporo
001-0021 (Japan)
E-mail: toyao@cat.hokudai.ac.jp, kshimizu@cat.hokudai.ac.jp
Dr. T. Toyao, Dr. T. Kamachi, Prof. Dr. K. Yoshizawa, Prof. Dr. K.
Shimizu
Elements Strategy Initiative for Catalysis and Batteries, Kyoto
University, Katsura, Kyoto 615-8520 (Japan)
Dr. Y. Morita, Dr. T. Kamachi, Prof. Dr. K. Yoshizawa
Institute for Materials Chemistry and Engineering and International
Research Center for Molecular Systems, Kyushu University,
Fukuoka 819-0395 (Japan)
[b]
[c]
Scheme 1. Methods for hydrogenation of carboxylic acids
containing aromatic ring(s).
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201604762
Chemistry - A European Journal
COMMUNICATION
S3 and S4). The results
of EDX analysis show
that Re is present and
Table 1. Hydrogenation of 3-phenylpropionic acid catalyzed by various catalyst.a
that
dispersed
it
is
over
highly
the
Yield [%]
entirety of the TiO2
particles. In addition,
Entry Catalyst
Conv. [%]
2a
60
0
3a
40
0
4a
0
5a
0
6a
0
7a
0
1
7
0
4
0
0
0
0
0
0
TEM
measurements
1
Re/TiO2
Pt/TiO2
Ir/TiO2
100
100
96
59
100
91
4
show that Re clusters
on TiO2 have 1-2 nm
sizes, suggesting that
2
0
64
1
34
16
6
3
18
0
34
6
14
45
65
90
0
they
dispersed on TiO2.
In the initial phase
this effort,
hydrogenation reactions
of 3-phenylpropionic
are
highly
4
Ru/TiO2
Rh/TiO2
Pd/TiO2
Ag/TiO2
Cu/TiO2
Ni/TiO2
Co/TiO2
TiO2
0
5
0
0
0
20
0
6
0
0
0
of
7
0
0
0
0
8
1
0
0
0
0
0
acid (1a) were used to
9
0
0
0
0
0
0
screen the properties of
10
11
0
0
0
0
0
0
various
catalysts.
0
0
0
0
0
0
Reactions were carried
aReaction conditions: 2 mol% catalyst, 1 mmol phenylpropionic acid, no solvent, 5 MPa H2, 140 °C, 6
h. Conversion and yields were determined by using GC with n-dodecane as an internal standard.
bThe reaction was performed after calcination at 500 °C in the air. Full data for the catalyst screening
are given in the Supporting Information.
out using 1 mmol of 1a
and
2 mol% of the
catalysts in a stainless
autoclave (10 cm3)
under 5 MPa H2 at 140
catalyst displays high activity under relatively mild conditions
and it does not require the use of additives. Density functional
theory (DFT) calculations suggest that high affinity of Re toward
–COOH group is an origin of the high selectivity for alcohol
formation without promoting hydrogenation of aromatic rings.
Finally, the broad carboxylic acid substrate scope of the process
promoted by Re/TiO2 makes it ideally suited for use in synthetic
organic chemistry.
ºC for 6 h, following pretreatment of the catalyst with H2 at
500 °C. The results of this study, summarized in Tables 1 and
S1 (Supporting Information), show that the Re/TiO2 produced 3-
phenylpropanol (2a) and the corresponding ester (3a). 3a was
formed via an esterification reaction of the starting carboxylic
acid by the formed alcohol. On the contrary, low yields of the
alcohol and ester were observed when other precious metal (Pt,
Ir, Ru, Rh and Pd) containing catalysts were employed (entries
2-6). This was caused by competitive formation of products
produced via dearomatization of benzene ring such as 3-
cyclohexyl-propionic acid (4a), 3-cyclohexyl-propan-1-ol (5a)
and 3-cyclohexyl-propionic acid 3-cyclohexyl-propyl ester (6a)
and/or over-reduction to the alkane such as propyl-cyclohexane
(7a). Ag-, Cu-, Ni-, Co-loaded TiO2 catalysts did not promote the
reduction reaction (entries 7-10). In addition, no reaction took
place when TiO2 was used as the catalyst (entry 11). It was also
found that Re/TiO2 gave 2a and 3a much more efficiently than
other Re-based catalysts on other supports (entries 13-22 in
Table S1 in the Supporting Information). Also, NH4ReO4 as well
as non-supported Re metal and its oxides (ReO2 and Re2O7)
were not effective catalysts for this hydrogenation
reaction(entries 23-26 in Table S1). The results of the
screening study clearly demonstrate that the combination of Re
as the catalytically active species and TiO2 as the support leads
to a catalyst that effectively promotes selective alcohol forming
hydrogenation reactions of carboxylic acids without formation of
the undesired products such as alkanes and dearomatized
compounds.
The Re/TiO2 catalyst was synthesized by using a facile
impregnation method employing NH4ReO4 (Aldrich) and TiO2
(JRC-TIO-8 supplied from Catalysis Society of Japan). Typically,
0.72 g of NH4ReO4 was added to a glass vessel (500 ml) with
100 ml of deionized water (concentration of Re = 0.027 mol/l).
After sonication for 1 minute to completely dissolve NH4ReO4,
TiO2 (9.5 g) was added to the solution and kept stirring for 15
min with 200 rpm at room temperature. This was followed by
evaporation to dryness at 50 °C, and by drying at 90 °C under
ambient pressure for 12 h. The prepared material was calcined
at 500 °C in air for 3 h. For each experiment, the active catalysts
were prepared by reduction in a Pyrex tube under a flow of H2
(20 cm3 min-1) at 500 °C for 0.5 h. Various characterization
techniques were utilized to gain insight into structural features of
Re/TiO2. Figure S1 shows the temperature programmed H2-
reduction (H2-TPR) profile of Re-loaded TiO2 after calcination.
Inspection of the profile shows that large H2 consumption
occurred at around 300 °C corresponding to the reduction of the
Re species. X-ray diffraction (XRD) patterns of calcined and
reduced catalyst were found to be essentially identical, with
peaks arising from TiO2 being the only ones observed (Fig. S2).
Energy dispersive x-ray (EDX) as well as transmission electron
microscope analyses were performed to evaluate the element
dispersion and particle size of Re in the Re/TiO2 catalyst (Fig.
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and H2 pressure, which level at around 500 ºC and 5 MPa,
respectively. Note that 2a yield was only 3% when the reaction
was performed after calcination at 500 °C in the air without H2
reduction as a pretreatment (entry 12 in Table S1). From the
results of the effect of the H2 reduction temperature, Re species
having low valence states including metallic Re would be
catalytically active sites for the hydrogenation reaction although
it is difficult to identify the exact catalytically active sites for the
hydrogenation reaction at the current time. Taking these
observations into consideration, a Re loading of 5 wt.%,
pretreatment temperature of 500 ºC and H2 pressure of 5 MPa
were employed as optimized conditions.
The time course of the hydrogenation reaction of 3-
phenylpropionic acid, carried out under the optimized conditions,
is given in Fig. 2. Inspection of the profile shows that 3-
phenylpropanol (2a) was efficiently generated from 3-
Figure 1. Adsorption energies of acetic acid (left) and benzene
(right) on Re (001), ReOx, Pd (111) and Rh (111). Most stable
adsorption structures of acetic acid and benzene adsorbed on
Re (001) are shown (Side view).
phenylpropionic acid (1a) via
a pathway involving initial
formation of 3-phenylpropyl 3-phenyl-propionate (3a). Note that
3a was formed as an intermediate product by esterification of 1a
and 2a. The yield of 2a reached 97% after 24 h. 3-
Phenylpropionaldehyde, an anticipated intermediate in the
process, along with over-reduced products were not observed
even after 24 h. The reaction was also performed on a 10 times
larger scale (i. e., 10 mmol 1a) under the optimized conditions,
as shown in Scheme 2. Column chromatographic separation of
the crude product mixture gave 2a in an isolated yield of 87%,
indicating high scale-up potential of this catalytic process.
Moreover, recycling test of Re/TiO2 was performed to show its
potential use as a recyclable heterogeneous catalyst. Following
the reaction carried out, the catalyst was separated, washed
with isopropanol, dried in air and then reused for an ensuing
reaction. The desired alcohol was obtained in excellent yield for
the 2nd run (97%). This is the same yield obtained for the 1st
In order to identify the origin of the high selectivity of the
Re/TiO2 catalyst toward alcohol formation over dearomatization
of benzene ring, DFT calculations were conducted (See
Supporting Information for detailed computational methods).
Inspired by a report describing that the product selectivity in
nitrostyrene hydrogenation depends strongly on relative
adsorption affinity of its functional groups,[10] adsorption affinity
of –COOH and benzene moieties with catalyst surfaces were
investigated by employing acetic acid and benzene as probe
molecules. In addition to Re, metals promoting hydrogenation of
aromatic rings rather than carboxylic acid groups such as Pd
and Rh were examined for comparison. Moreover, partly
oxidized Re (ReOx) was also modelled by adsorbing an oxygen
atom on the metal surface and investigated in the same manner
since oxidized Re species could be catalytically active sites.[11]
Note that most stable and common planes for each metal were
utilized for this investigation. Most stable adsorption structures of
benzene and acetic acid adsorbed on Re (001), ReOx, Pd (111)
and Rh (111) surfaces are displayed in Figure S5 and S6,
respectively. As demonstrated in Fig. 1, adsorption energies of
benzene on Re (001) and ReOx are lower than those on Pd
(111) and Rh (111). In contrast, adsorption energies of acetic
acid on Re (001) and ReOx are higher than those on Pd (111)
and Rh (111). Namely, Re and ReOx have relatively high affinity
with –COOH groups compared to Pd and Rh, whereas the Re-
based surfaces have lower affinity with benzene moiety in
comparison with the other metal surfaces, indicating that Re has
oxophilic nature.[12] This relatively high affinity of Re toward –
COOH could be one reason to explain the high selectivity of
Re/TiO2 for hydrogenation of 3-phenylpropionic acid.
Figure 2. Time course of the hydrogenation reaction of
phenylpropionic acid catalyzed by Re/TiO2. Substances in the
plot are 3-phenylpropionic acid (1a), 3-phenylpropanol (2a) and
3-phenylpropyl 3-phenyl-propionate (3a).
The Re/TiO2 catalyst system and reaction conditions for
hydrogenation of 3-phenylpropionic acid were further optimized.
Particularly, the effects of Re loading, pretreatment temperature
and H2 pressure were explored. The results given in Table S3-5
show that catalytic activity reached a maximum when a 5 wt.%
Re loading was utilized. The yields of the alcohol 2a and ester
3a increased with an increase in the pretreatment temperature
Scheme 2. Gram scale synthesis of 3-phenylpropanol through
hydrogenation of phenylpropionic acid over Re/TiO2.
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indicates that 0.1% of the loaded Re (0.002 mol% of
Re with respect to the reaction substrate) was leached
after the 24 h reaction. In addition, a leaching test was
performed, as shown in Figure S7. Specifically, we
observed that removal of the catalyst following a 3 h
period caused the hydrogenation reaction to cease,
Table 2. Hydrogenation of various carboxylic acids catalyzed by
Re/TiO2.a
Yield [%]
Alcoholb Ester
indicating only supported-Re species acts as
catalytically active site.
a
Substrate
Product alcohol
Entry
1
T [°C]
The substrate scope of the reaction was explored in
order to show the potential versatility of the Re/TiO2
catalyzed hydrogenation process. The results given in
Table 2 show that the catalytic system was capable of
promoting alcohol forming hydrogenation reactions of
a wide range of aromatic and aliphatic carboxylic
acids. Although some substrates such as sterically
hindered substrates displayed lower reactivity (entries
6 and 9 in Table 2), the use of increased reaction
temperature in these cases leads to high alcohol
yields. Notably, the substrate containing a chlorine-
substituted benzene ring did undergo the selective
hydrogenation reaction without change on the chlorine
group (entry 10 in Table 2). Moreover, the Re/TiO2-
catalyzed system was applicable to the hydrogenation
of substrates containing heterocyclic compounds to
yield the corresponding alcohols (entries 11 and 13
in Table 2) At present, some limitations of the process
do exist. For instance, olefin moieties were reduced
under the reaction conditions (entry 8 in Table 1).
However, the high chemoselectivity of the Re/TiO2-
catalyzed hydrogenation reaction and its concurrent
compatibility with aromatic rings are features rarely
seen with both homogeneous and heterogeneous
hydrogenation catalysts although there are more
number of papers for the selective hydrogenarion of
carboxylic acid derivatives such as esters[13] and
amides.[14]
140
97 (94)
94 (91)
93 (90)
93 (86)
90 (84)
93 (86)
92 (83)
94 (88)
2
6
0
0
8
4
2
2
2
3
4
5
6
7
8
140
140
140
160
160
160
160
9
160
160
160
91 (87)
97 (93)
87 (80)
4
0
2
10
11
Driven by the desire to replace fossil fuel
sources, much attention has been given to the
conversion of biomass materials into fuels and
chemicals.[15] Selective hydrogenation reactions of
carboxylic acids are important methods used for this
purpose because they can be employed to generate
fatty alcohols, which are widely used in the production
of surfactants, polymers and solvents.[16] To evaluate
the applicability of the Re/TiO2 catalyst to the goal of
producing fatty alcohols, hydrogenation reactions of
various fatty acids were explored. First, hydrogenation
of lauric acid was conducted as a model reaction
(Table S6 and S7, Supporting Information).
Pretreatment temperature of 500 ºC and H2 pressure
of 5 MPa were employed as optimized conditions as is
12
13
14c
160
95 (83)
0
180
180
87 (84)
96 (82)
2
2
aReaction conditions: 2 mol% Re, 1 mmol substrate, no solvent, 5 MPa H2,
24 h. Conversion and yields were determined by using GC with n-dodecane
as an internal standard. bIsolated yields are shown in parentheses. c The
reaction was performed for 36 h.
run, indicating that the Re/TiO2 catalyst is recyclable. Note that
the case with the above-described reactions. Further studies
show that lauryl alcohol was produced in a 90% yield when the
hydrogenation reaction was conducted using 5 MPa H2 at 160
ºC for 24 h (Figure S8). In addition, fatty acids bearing both long
and short aliphatic chains did undergo efficient hydrogenation to
produce the corresponding alcohols under these conditions (79-
the recycling reaction was carried out after H2 reduction as is the
case with the 1st run. If the H2 reduction was not performed, the
alcohol yield was 60%, indicating that the catalyst needs to be
reduced for recycling. In order to further confirm heterogeneous
nature of Re/TiO2, inductively coupled plasma atomic emission
spectroscopy (ICP-AES) analysis was carried out. The result
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10.1002/chem.201604762
Chemistry - A European Journal
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99% alcohol yield), as shown in Table S8. These observations
suggest that the catalytic system developed in this effort is
applicable to bio-relevant carboxylic acid hydrogenation
reactions.
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In summary, we developed a heterogeneous catalytic system
that promotes selective alcohol forming hydrogenation reactions
of carboxylic acids under mild conditions without the need for
additives. The Re/TiO2 catalyst, which is readily prepared by
using a facile impregnation method, could be utilized for
selective hydrogenation of carboxylic acids that contain aromatic
groups. DFT calculations suggest that the high selectivity for
alcohol formation of the Re-based catalyst is originated from
high affinity of Re with –COOH group over benzene ring. This
finding would be a design guide to develop better catalysts for
this process. Importantly, the catalyst was easily recyclable as a
heterogeneous catalyst. In view of the importance of the process
in both organic synthesis and industrial bulk chemical production,
the new method for carboxylic acid hydrogenation should find
wide utility and the findings should stimulate further studies
targeted at the design of catalysts for selective hydrogenation
reactions.
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Acknowledgements
This work was supported by KAKENHI Grant numbers
JP16H06595, JP26289299, JP24109014, JP15K13710 and
JP15K05431 from Japan Society for the Promotion of Science
(JSPS), the Ministry of Education, Culture, Sports, Science and
Technology of Japan (MEXT) Projects of “Integrated Research
on Chemical Synthesis” and “Elements Strategy Initiative to
Form Core Research Center” and JST-CREST “Innovative
Catalysts”. TEM, STEM-EDX and ICP-AES analyses were
carried out at the OPEN FACILITY, Hokkaido University.
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Keywords: Hydrogenation of carboxylic acid • Alcohol
synthesis • Heterogeneous catalysis • Rhenium
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10.1002/chem.201604762
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
TiO2-supported Re (Re/TiO2) has
Takashi Toyao,* S. M. A. H. Siddiki,
Abeda S. Touchy, Wataru Onodera,
Kenichi Kon, Yoshitsugu Morita,
Takashi Kamachi, Kazunari
enabled
selective
catalytic
hydrogenation of carboxylic acids
having aromatic and aliphatic
moieties to the corresponding
alcohols under 5 MPa H2 pressure at
140-180 °C. The Re/TiO2 catalyst
Yoshizawa, Ken-ichi Shimizu,*
Page No. – Page No.
TiO2-Supported Re as a General and
can
be
recyclable
catalyst
as
a
and
Chemoselective
Heterogeneous
heterogeneous
Catalyst for Hydrogenation of
Carboxylic Acids to Alcohols
demonstrate wide substrate scope
with preparative useful yields.
This article is protected by copyright. All rights reserved.