Mechanistic study of the selective hydrogenation of carboxylic acid derivatives over supported rhenium catalysts

Article abstract of DOI:10.1039/c9cy01404g

The structure and performance of TiO₂-supported Re (Re/TiO₂) catalysts for selective hydrogenation of carboxylic acid derivatives have been investigated. Re/TiO₂ promotes selective hydrogenation reactions of carboxylic acids and esters that form the corresponding alcohols, and of amides that generate the corresponding amines. These processes are not accompanied by reduction of aromatic moieties. A Re loading amount of 5 wt% and a catalyst pretreatment with H₂ at 500 °C were identified as being optimal to obtain the highest catalytic activity for the hydrogenation processes. The results of studies using various characterization methods, including X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM), indicate that the Re species responsible for the catalytic hydrogenation processes have sub-nanometer to a few nanometer sizes and average oxidation states higher than 0 and below +4. The presence of either a carboxylic acid and/or its corresponding alcohol is critical for preventing the Re/TiO₂ catalyst from promoting production of dearomatized byproducts. Although Re/TiO₂ is intrinsically capable of hydrogenating aromatic rings, carboxylic acids, alcohols, amides, and amines strongly adsorb on the Re species, which leads to suppression of this process. Moreover, the developed catalytic system was applied to selective hydrogenation of triglycerides that form the corresponding alcohols.

Full text of DOI:10.1039/c9cy01404g

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Mechanistic study of the selective hydrogenation of carboxylic acid
derivatives over supported rhenium catalysts
Takashi Toyao,*a,b Kah Wei Ting,a S. M. A. Hakim Siddiki,a Abeda. S. Touchy,a Wataru
Onodera,a Zen Maeno,a Hiroko Ariga-Miwa,a Yasuharu Kanda,c Kiyotaka Asakura,a Ken-ichi
Shimizu*a,b
a Institute for Catalysis, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
b Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura, Kyoto 615-
8520, Japan
c Applied Chemistry Research Unit, College of Environmental Technology, Graduate School of
Engineering, Muroran Institute of Technology, 27-1 Mizumoto, Muroran 050-8585, Japan
*Corresponding authors
Takashi Toyao, Ken-ichi Shimizu
E-mail: toyao@cat.hokudai.ac.jp, kshimizu@cat.hokudai.ac.jp, TEL: +81-11-706-9164
†Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x.
1

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Abstract
The structure and performance of TiO2-supported Re (Re/TiO2) catalysts for selective
hydrogenation of carboxylic acid derivatives have been investigated. Re/TiO2 promotes selective
hydrogenation reactions of carboxylic acids and esters that form the corresponding alcohols,
and of amides that generate the corresponding amines. These processes are not accompanied
by reduction of aromatic moieties. A Re loading amount of 5 wt% and a catalyst pretreatment
with H2 at 500 °C were identified as being optimal to obtain the highest catalytic activity for the
hydrogenation processes. The results of studies using various characterization methods,
including X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), X-ray photoelectron
spectroscopy (XPS), and scanning transmission electron microscopy (STEM), indicate that the
Re species responsible for the catalytic hydrogenation processes have sub-nanometer to a few
nanometer sizes and average oxidation states higher than 0 and below +4. The presence of
either a carboxylic acid and/or its corresponding alcohol is critical for preventing the Re/TiO2
catalyst from promoting production of dearomatized byproducts. Namely, although Re/TiO2 is
intrinsically capable of hydrogenating aromatic rings, carboxylic acids, alcohols, amides, and
amines strongly adsorb on the Re species, which leads to suppression of this process. Moreover,
the developed catalytic system was applied to selective hydrogenation of triglycerides that form
the corresponding alcohols.
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1. Introduction
Hydrogenation reactions of carboxylic acids, esters, and amides are receiving increased
attention because of their use in a variety of applications in the pharmaceutical and fine-chemical
industry, as well as for upgrading bio-based chemical feedstocks.1–5 Although significant
progress has been made in recent years, the development of catalysts that promote these
processes remains a formidable task. The difficulties are predominantly a consequence of the
fact that carboxylic acid derivatives are kinetically unreactive and thermodynamically stable as
a result of the low electrophilicity of the carbonyl carbons.6 Although stoichiometric amounts of
strong reducing agents are typically and effectively used to accomplish these reactions, use of
these agents has serious drawbacks in terms of reagent/functional group compatibility, and poor
atom economy and safety especially when the processes are performed on an industrial scale.7
Alternative approaches that utilize catalytic reduction with H2 are undoubtedly more attractive
because they are considerably more atom economic, environmentally benign and cost-effective.
Therefore, substantial effort has been made for the development of these hydrogenation
reactions, and some are industrially viable.7 However, because carboxylic acid derivatives are
the most difficult substrates to hydrogenate, harsh reaction conditions are usually required.8 For
instance, successful operation of a conventional Cu-Cr-based catalytic system for these
hydrogenations requires the use of highly severe conditions (T = 250 °C, pH2 > 20 MPa) and
high active metal loadings (ca. 20 wt%).9 Moreover, these processes often are unselective and
inefficient as a consequence of over-reduction. In particular, most catalytic systems are limited
because they preferentially promote hydrogenation of aromatic systems over carboxylic acid
groups.10–14 Because aromatic systems are present in many organic substances, this problem
seriously limits the use of those hydrogenation processes in synthetic organic chemistry.
Catalysts comprised of platinum-group-metal complexes have played a leading role in
carrying out selective hydrogenations.15–28 However, the chemical and thermal stabilities of
these catalysts are typically poor, and separation of catalysts from products is relatively difficult.
Another drawback of these homogeneous catalysts is that they often require the use of
stoichiometric quantities of additives. As a result, the design and development of highly versatile
heterogeneous catalysts for selective hydrogenation of carboxylic acids and their derivatives are
highly desirable goals in organic chemistry.
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In this context, we have recently reported that TiO2-supported Re (Re/TiO2) is a
heterogeneous catalyst that, in the absence of additives, promotes selective hydrogenation
reactions of various carboxylic acid derivatives that contain aromatic moieties.29,30 In the
processes, carboxylic acids and esters are converted to corresponding alcohols, while amides
are transformed to the corresponding amines. Importantly, this heterogeneous catalytic system
does not promote hydrogenation of aromatic groups but preferentially hydrogenate carboxylic
acid derivative groups. In addition to these simple hydrogenation processes, this catalyst can be
employed in N-alkylation reactions of amines, which proceed via the conversion of carboxylic
acid (or esters) to the corresponding alcohols followed by reaction of the amine with the
generated alcohol. Furthermore, low temperature (T = 150 °C) hydrogenation of CO2 to
methanol31 and the N-methylation reactions of amines, using CO2 and H2 as a methylation
reagent, can be promoted using Re/TiO2.
Although the earlier studies led to the development of a highly active and selective
catalyst for hydrogenation of carboxylic acid derivatives, they did not reveal the precise nature
of the catalytically active Re species. Moreover, little is known about the nature of the active
species in a number of valuable Re promoted transformation32–42 despite having been the
subject of scientific investigations for years.43–55 The main reasons for this are that Re often is
highly dispersed on supports and is difficult to be reduced, and it can exist in a variety of oxidation
states.56–65 This deficiency makes it highly desirable to carry out a comprehensive study of
supported Re catalysts that focuses on gaining insight into a fundamental understanding of the
catalytically active Re species.
In this study, we describe the results of a detailed mechanistic study of selective
hydrogenation reactions of carboxylic acids and their derivatives catalyzed by Re/TiO2.
Important information was gained in this investigation using scanning transmission electron
microscopy (STEM) and in situ methods such as X-ray diffraction (XRD), X-ray absorption fine
structure (XAFS), and X-ray photoelectron spectroscopy (XPS). The results show that for
effective hydrogenation processes the Re species in the catalyst should have sub-nanometer to
a few nanometer sizes and an average oxidation state that is higher than 0 and below +4.
Furthermore, the co-presence of either the carboxylic acid derivatives and/or the corresponding
products is important for preservation of the high selectivity of these catalytic reactions so that
byproducts arising from reduction of aromatic moieties are not formed. The results of this
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investigation have not only provided mechanistic insight into Re/TiO2-promoted hydrogenations,
they also led to a better understanding of the behavior of Re on the solid surfaces, which are
known to be interesting catalytically active systems.
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2. Experimental
Materials and preparation of the catalysts
Chemicals were obtained from commercial suppliers and used as received. TiO2 (ST-01, 188
m2 g-1) was purchased from Ishihara Sangyo Co., Ltd. Al2O3 (124 m2 g-1) was prepared by
calcination of γ-AlOOH (Catapal B) obtained from Sasol for 3 h at T = 900 °C. SiO2 (Q-10, 300
m2 g-1) was purchased from Fuji Silysia Chemical Ltd. The carbon support (296 m2 g-1) was
supplied by Kishida Chemical.
Precursors of Re(5)/TiO2 (Re = 5 wt%) and Re(5)/Support (Support = Al2O3, SiO2, and
Carbon) were prepared by mixing the corresponding support materials with appropriate metal
sources, i.e., aqueous solution of NH4ReO4. In a typical preparation of Re(5)/TiO2, NH4ReO4
(0.72 g) was added to a round-shaped glass vessel (500 mL) that contain 100 mL of deionized
water. After sonication for 1 min, TiO2 (9.5 g) was added and the resulting solution was stirred
at 200 rpm for 15 min at r.t. The reaction mixture was then evaporated at 50 °C, and the residue
was dried at 90 °C in air for 12 h. The obtained material was calcined at 500 °C in air for 3 h.
Prior to each experiment, the catalyst was prepared by reducing those precursors in a glass tube
(T = 500 °C, t = 0.5 h) under a flow of H2. Other catalysts were also prepared in the same manner.
Characterization of the catalysts
STEM images were recorded on a JEM-ARM200F microscope (JEOL). The Cs-corrector
CESCOR (CEOS GmbH, Germany) was used.
In situ XRD (Rigaku Ultima IV) measurements were conducted using a CuKα radiation
source. Samples were heated under a flow of 5% H2/N2 (30 mL min-1) at temperatures up to 500
°C with a heating rate of 10 ˚C/min. XRD patterns were recorded under a flow of 5% H2/N2 (30
mL min-1) at temperatures between r.t.-500 ˚C. Ex situ XRD was performed by using a CuKα
radiation source with a Rigaku Miniflex. Temperature programmed reduction by H2 (H2-TPR)
was performed by using BELCAT II (MicrotracBEL).
Ti K-edge and Re L3-edge XAFS measurements were performed at the BL01B1 line (SPring-
8) using a Si(111) double crystal monochromator (proposal No. 2018B1126). For in situ analysis,
samples in pellet form (φ10 mm) were introduced into a quartz cell equipped with Kapton film
windows and gas lines. Samples were heated under a flow of 5% H2/He (100 mL min-1) at
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temperatures up to 700 °C with a heating rate of 10 ˚C/min. Spectra were recorded under a flow
of 5% H2/He (100 mL min-1) at temperatures between 100-700 ˚C. A measurements on Re
metallic powder as a reference was made using a sample that is sealed in a glove bag filled with
N2 after H2 reduction at 900 ˚C in order to ensure that surface ReOx is not formed. The extended
XAFS (EXAFS) analysis was carried out using the REX ver. 2.5 program (RIGAKU). The Fourier
transformation of the k3-weighted EXAFS from k space to R space was performed over a k range
of 3–12 Å−1. Curve-fitting analysis was carried out using the parameters for the Re–O and Re–
Re shells provided by FEFF6.
XPS was recorded on an EA 125 X-ray photoelectron spectrometer (Omicron) having a
modified UHV chamber employing Mg Kα radiation (1253.6 eV). Binding energies calibration
were accomplished by using the O 1s peak energy (532.0 eV) of TiO2. Samples were subjected
to measurements after reduction under a flow of H2 in a glove box filled with N2. The glove box
was connected directly to the XPS chamber to avoid exposing the reduced samples to air.
Catalytic hydrogenation reactions
The following procedure is representative for reductions of carboxylic acids and amides. After
being reduced with H2 at T = 500 °C (cf. Preparation of the Catalysts), the catalyst (2 mol% with
respect to the loaded metal), and a mixture of 3-phenylpropionic acid (1.0 mmol) and n-dodecane
(0.29 mmol) were added to an autoclave with 10 mL volume. The mixture was stirred
magnetically (T = 140 °C, pH2 = 5 MPa). The resultant mixture was diluted with 2-propanol and
then analyzed by using GC (Shimadzu GC2014 having a capillary column UA+-1 from Frontier
Laboratories Ltd.) and GC-MS (Shimadzu GCMS-QP2010 with the same capillary column). GC
analyses with n-dodecane as the internal standard were employed for yield determination. Yields
are given relative to the starting substrate. Conversion was calculated using the following
equation (1).
퐶표푛푣푒푟푠푖표푛 (%) = (1 ― 퐴푚표푢푛푡 표푓 푑푒푡푒푐푡푒푑 푠푢푏푠푡푟푎푡푒 (푚푚표푙)) × 100
(1)
퐴푚표푢푛푡 표푓 푙표푎푑푒푑 푠푢푏푠푡푟푎푡푒 (푚푚표푙)
The following procedure is representative for reduction of triglycerides. After being reduced
with H2 at T = 500 °C (cf. Preparation of the Catalysts), the catalyst (2 mol% with respect to the
loaded Re for the number of carboxylic acid ester moieties), and a mixture of tristearin (0.33
mmol) and n-dodecane (0.29 mmol) were added to the autoclave. The reaction mixture was
stirred magnetically (T = 220 °C, pH2 = 5 MPa). The resultant mixture was analyzed by using the
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GC and GC-MS. GC analyses with n-dodecane as the internal standard were employed for yield
determination. Yields are given relative to the number of carboxylic acid ester moieties. Detailed
procedures for product separations and NMR analysis is given in the Supporting Information.
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3. Results and discussion
Characterization of the supported Re catalysts
Re(5)/TiO2, Re(5)/Al2O3, Re(5)/SiO2, and Re(5)/Carbon catalysts were characterized by using
in situ XRD, STEM, and XAFS analysis. Figure 1 shows the XRD patterns for the catalysts
arising from measurements carried out under a flow of 5% H2/N2 (30 mL min-1). The XRD pattern
of the Re(5)/TiO2 catalyst is essentially identical to that of pristine TiO2 even after H2 reduction
at 500 °C. Peaks from Re species were also not observed for Re(5)/Al2O3 and Re(5)/Carbon.
On the other hand, peaks from Re species are present in the XRD pattern of Re(5)/SiO2 after
pretreatment with H2 at 500 °C. The results indicate that Re forms crystalline and sufficiently
large species that can be observed by using XRD only on the SiO2 support. The ex situ XRD
patterns for these catalysts (Figure S1 in the Supporting Information) are consistent with those
arising from the in situ measurements.
Figure 2 shows bright field STEM (BF-STEM) images and high-angle annular dark-field
STEM (HAADF-STEM) images of Re(5)/TiO2, following exposure to air as a consequence of the
experimental setup. Inspection of the images shows that Re on TiO2 is highly dispersed and has
particle sizes below 2 nm. Also, some Re species exist as clusters with a few Re atoms and
single atoms in the TiO2 matrix. Viewing the HAADF-STEM images of Re(5)/Al2O3, Re(5)/SiO2,
and Re(5)/Carbon, displayed in Fig. 3, shows that Re is relatively dispersed on the Al2O3 surface
and Re clusters with sizes below 1 nm exist. In addition to these highly dispersed species,
aggregated Re species with 2-5 nm sizes are present. In Re(5)/SiO2 and Re(5)/Carbon, Re
exists mainly in the form of aggregated particles with sizes larger than 5 nm although there still
are some atomically dispersed Re.
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Figure 1. In situ XRD patterns of Re(5)/TiO2, Re(5)/SiO2, Re(5)/Al2O3, and Re(5)/Carbon measured under
a flow of 5% H2/N2 (30 mL min-1) up to 500 °C with a heating rate of 10 ˚C/min.
Figure 2. BF(left)- and HAADF(right)-STEM images for Re(5)/TiO2. The Re(5)/TiO2 sample was reduced
at 500 °C under a flow of H2. Due to the experimental setup, the sample was exposed to air before
measurements.
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Figure 3. HAADF-STEM images for Re(5)/SiO2, Re(5)/Al2O3, and Re(5)/Carbon. All the catalysts were
reduced at 500 °C under a flow of H2. Due to the experimental setup, the samples were exposed to air
before the measurements.
Figure 4 shows Re L3-edge XANES and EXAFS spectra. The in situ XANES spectra of
the Re samples (Fig. 4A-E) were recorded under a flow of 5% H2/He (100 mL min-1) at various
temperatures between 100-700 ˚C together with the spectra of reference compounds measured
at room temperature. The XANES spectra of all the catalysts treated with H2 at temperatures up
to 200 ˚C corresponded to that of Re2O7, indicating that the Re species in these samples were
in the highest oxidation state (VII). The white line intensity for Re(5)/TiO2 decreased by the H2
reduction at 300 ˚C while those of the other Re species decreased at 400 ˚C. These results
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indicate that the valence states of the Re species decreased at 200-400 ˚C. It is worth mentioning
here that Re on TiO2 starts to be reduced at the lowest temperature (below 300 ˚C) among the
samples measured in this study. A comparison of XANES spectra of Re on different supports is
given in Fig. 4E. The spectra were recorded using samples treated with H2 at 500 ˚C because
catalysts treated in this manner have the highest activities in hydrogenation reactions of
carboxylic acid derivatives.29,30 The results show the edge position lie between those for metallic
Re and ReO2 as references while the white line intensities are all higher than that of ReO2. These
results indicate that all the catalysts contain oxidized Re species. In addition, as shown in Fig.
S2, a prominent peak was observed at ca. 10549 eV and appeared with the order of Re(5)/TiO2
< Re(5)/Al2O3 < Re(5)/Carbon < Re(5)/SiO2. It was reported that the peak intensity increases
with increase in the Re particle sizes.50
Fourier transforms (FT) of k3-weighted EXAFS spectra of the supported Re catalysts are
displayed in Fig. 4F. The spectra were recorded at room temperature on samples not exposed
to air after H2 pretreatment at 500 ˚C. The distances and coordination numbers (CNs) for the
Re–O and Re–Re shells were determined by curve-fitting analysis of the EXAFS data (Table 1).
The EXAFS spectra of Re(5)/TiO2 and Re(5)/Al2O3 contain features associated with both Re–O
and Re–Re. Re(5)/TiO2 has large Re–O contributions, suggesting that the Re species on TiO2
is highly dispersed. On the other hand, the spectra of Re(5)/SiO2 and Re(5)/Carbon are
comprised of a single Re–Re contribution at 2.75 Å with a CN of 10.4 and 10.7, respectively.
The high CNs of Re–Re observed for Re(5)/SiO2 and Re(5)/C indicate that most of the Re
species form large Re particles. These observations are consistent with results from XRD and
STEM measurements.
XANES spectra showed that all the Re species have valence higher than 0 whereas
EXAFS spectra showed the Re–Re contribution which suggests formation of metallic Re
particles. The Re species in those catalysts would be mixture of Re particles and highly
dispersed Re species which should have Re–O bond. Although the fraction of oxidized Re
species estimated from the XANES spectra was not so different among the studied catalysts,
results of EXAFS showed strong support dependence on the dispersion of Re. True reasons for
this inconsistency are not clear. However, origins of the observed XANES spectra would be
related to not only the valence states but also the complex structures of Re.50
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Ti K-edge XANES spectra of Re(5)/TiO2 were also recorded on samples treated with H2
at temperatures up to 700 ˚C (Figure 5). Inspection of the spectra shows that the edge position
shifts slightly to lower energy with increasing reduction temperature, which indicates that Ti
species in the support are also reduced by the H2 pretreatment. Figure S3 shows H2-TPR
profiles of Re(5)/TiO2, Re(5)/SiO2, Re(5)/Al2O3, and Re/Carbon before the H2 reduction
treatment. The peaks corresponding to the H2 consumption appeared around temperatures of
200-400 °C for all the samples. It was also found that the H2 consumption peak for Re(5)/TiO2
appeared at the lowest temperature and that for Re(5)/Al2O3 appeared at the highest
temperature among the samples tested. These features are consistent with the results of in situ
Re L3-edge XANES spectra measured during the H2 reduction.
Figure 4. In situ Re L3-edge XANES spectra of (A) Re(5)/TiO2, (B) Re(5)/Al2O3, (C) Re(5)/Carbon, and
(D) Re(5)/SiO2. Samples were heated under a flow of 5% H2/He (100 mL min-1) up to 700 °C with a
heating rate of 10 ˚C/min. XANES spectra for reference samples were recorded at room temperature. (E)
Comparison of XANES spectra of Re on different supports which were measured at 500 ˚C under the
flow of 5% H2/He (100 mL min-1). (F) Fourier transforms of k3-weighted EXAFS oscillations recorded at
room temperature. The samples were not exposed to air after the H2 reduction at 500 ˚C by sealing the
samples in a glove bag filled with N2.
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Table 1. Curve-fitting analysis of the Re L3-edge EXAFS of the Re(5)/TiO2, Re(5)/Al2O3, Re(5)/Carbon,
and Re(5)/SiO2 catalysts.
Sample
Re(5)/TiO2
Shell
Re
O
Re
O
CN a
3.4
2.7
8.6
2.0
R (Å) b
2.62
2.02
2.74
1.98
2.75
2.75
σ (Å) c
0.09
0.09
0.08
0.09
0.07
0.07
Rf (%) d
4.1
Re(5)/Al2O3
3.3
Re(5)/Carbon
Re(5)/SiO2
Re
Re
10.4
10.7
2.3
1.7
a Coordination number. b Bond distance. c Debye-Waller factor. d Residual factor.
Figure 5. In situ Ti K-edge XANES spectra of Re(5)/TiO2. A sample was heated under a flow of 5% H2/He
(100 mL min-1) up to 700 °C with a heating rate of 10 ˚C/min. The spectrum of Ti foil was recorded at
room temperature.
XPS analysis was performed in order to gain further information about the oxidation states
of the Re species on TiO2. Figure 6 shows spectra of the Re 4f region of Re(5)/TiO2 before and
after reduction with H2 at 500 ˚C, along with a spectrum after H2 reduction at 700 ˚C. It should
be noted that spectra were recorded on reduced samples directly after H2 reduction without
being exposed to air by using a glove box that is connected to the XPS chamber. The spectra
are comprised of signals associated with both 4f7/2 and 4f5/2 core levels. The spectrum of
Re(5)/TiO2 prior to H2 reduction contains two Re components in 4f7/2 peaks at ca 46.1 eV and
ca 44.0 eV, which are attributed to Re7+ and Re6+ species, respectively.30 In contrast, a peak at
ca 40.7 eV is present in the spectrum of Re(5)/TiO2 after H2 reduction at 500 ˚C, which is
assigned to a 4f7/2 peak of a Re0 species.61 In addition, contributions from oxidized Re species
such as Re4+, Re2+, and Re훿+ (훿 ∼ 1) can be present in the spectra of the samples even after H2
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reduction.47 Moreover, analysis of the spectra shows that Re species in Re(5)/TiO2 exist in
further reduced states after H2 reduction at 700 ˚C. These results indicate that, in a manner that
is consistent with the XAFS observations, H2 reduction induces formation of low-valent Re
species including those in the metallic state. Importantly, however, not all the Re species are
reduced to the metallic state even when H2 reduction is carried out at 700 ˚C.30
Figure 6. XP spectra of the Re 4f region of Re(5)/TiO2 before and after the reduction with H2 at 500 and
700 ˚C. The spectra before and after the reduction 700 ˚C are from our previous report.30
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Selective hydrogenation of carboxylic acids and their derivatives
Hydrogenations reactions of 3-phenylpropionic acid (1a) were explored in order to assess the
activities of the various catalysts investigated by using various characterization methods.
Following pretreatment of the catalysts with H2 at 500 ˚C, reactions were conducted using 1
mmol of 1a and 2 mol% (based on metal content) of the catalysts in a stainless autoclave (10
mL) under 5 MPa H2 at 140 ºC for 3 h. The results, summarized in Table 2, show that the reaction
promoted by Re(5)/TiO2 produces 3-phenylpropanol (2a) and the corresponding ester (3a),
which is formed via an esterification reaction of the starting carboxylic acid with the formed
alcohol (Entry 1). Hydrogenation reaction of 1a catalyzed by Pt(5)/TiO2 also generates 2a and
3a in relatively high yields. In contrast, use of Pt(5)/TiO2 as a catalyst leads to competitive
formation of undesired products such as 3-cyclohexyl-propionic acid (4a), 3-cyclohexyl-propan-
1-ol (5a) and 3-cyclohexyl-propyl 3-cyclohexyl-propioniate (6a), which are produced by benzene
ring reductive dearomatization reactions (Entry 2). These products are also formed in reactions
promoted by other precious metal (Ir, Ru, Rh, and Pd) containing catalysts and
propylcyclohexane is generated when the reaction time is extended(Entries 3-6).29 In processes
in which Re catalysts such as Re(5)/SiO2, Re(5)/Al2O3, and Re(5)/Carbon are employed, only
2a and 3a are produced (Entries 7-9), suggesting that Re does not catalyze reactions that form
dearomatized products. Although reactions catalyzed by these Re-based catalysts display good
selectivity for reduction of the carboxylic acid group rather than the aromatic moiety, the yields
are highest when Re(5)/TiO2 is used. We also found that the desired alcohol 2a is efficiently
generated via a pathway involving initial formation of 3-phenylpropyl 3-phenyl-propionate (3a) in
yields that reach 97% after 24 h.29 The results clearly demonstrate that the combination of Re
as the catalytically active species and TiO2 as the support leads to a catalytic system that
promotes selective and efficient hydrogenations of carboxylic acids to form alcohols, and that
this system does not promote generation of undesired side-products such as alkanes and
dearomatized compounds. It should also be noted that we have confirmed that the
hydrogenation reactions proceed with heterogeneous nature in our previous studies.29,30
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Table 2. Heterogeneous catalytic hydrogenation of 3-phenylpropionic acid.a
O
O
O
O
Catalyst
O
OH
OH
OH
OH
O
5 MPa H2, 140 oC,
neat, 3 h
1a
4a
5a
6a
2a
3a
Yield (%)
Entry Catalysts
Conv. (%)
2a
3a
4a
5a
6a
1
2
3
4
5
6
7
8
9
Re(5)/TiO2
Pt(5)/TiO2
Ir(5)/TiO2
Ru(5)/TiO2
Rh(5)/TiO2
Pd(5)/TiO2
Re(5)/Al2O3
Re(5)/Carbon
Re(5)/SiO2
36
66
6
18
13
0
0
0
0
7
5
1
18
24
6
1
0
0
6
8
2
0
20
0
0
65
52
0
0
0
0
3
0
0
0
0
0
0
0
0
6
0
0
1
0
0
0
0
1
66
52
13
13
3
aReaction conditions: 2 mol% metal, 1 mmol phenylpropionic acid, no solvent, pH2 = 5 MPa, T = 140 °C,
t = 3 h. Yields were determined by GC using n-dodecane as the internal standard.
Control experiments were conducted to identify the origin of the high selectivity of
Re(5)/TiO2 in catalyzing hydrogenation reactions that form alcohols rather than those lead to
reduction of benzene rings. In our previous studies, we employed density functional theory (DFT)
calculations to show that the selectivity of hydrogenations of carboxylic acid derivatives strongly
depends on the relative adsorption affinities of substances containing benzene rings, and COOH
or COOCH3 groups.29,30 In the present effort, we performed individual catalytic hydrogenation
reactions of (i) propylbenzene, (ii) propylbenzene + lauric acid (dodecanoic acid), and (iii)
propylbenzene + lauryl alcohol (dodecanol) using Re(5)/TiO2 as the catalyst. The results
displayed in Fig. 7 show that Re(5)/TiO2 promotes hydrogenation (dearomatization) of
propylbenzene to form propylcyclohexane. However, when a mixture of propylbenzene and
lauric acid is used, dearomatization reaction of the former substance does not take place while
lauric acid is preferentially reduced to generate the corresponding alcohol and lauryl laurylate.
Moreover, no reaction occurs using a mixture of propylbenzene and lauryl alcohol. These results
show that Re(5)/TiO2 is capable of promoting hydrogenation of benzene rings but this ability is
suppressed in the presence of carboxylic acids and alcohols. This finding suggests that, owing
to its oxophilic nature,37 Re species in the catalytic system strongly adsorb carboxylic acids and
alcohols, which suppresses its participation in benzene ring reductive dearomatization reactions.
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In order to gain further information about the substrates governed selectivity
phenomenon, control hydrogenation reactions of amides were explored (Figure 8). Individual
catalytic hydrogenation reactions of (i) propylbenzene, (ii) propylbenzene + N,N-dimethyl-2-
phenylacetamide, and (iii) propylbenzene + N,N-Dimethylphenethylamine were performed using
Re(5)/TiO2 under more vigorous conditions (pH2 = 5 MPa and T = 200 °C). The results show that
catalytic hydrogenation of propylbenzene occurs under these conditions to produce
propylcyclohexane. On the other hand, hydrogenations of a mixture of propylbenzene and N,N-
dimethyl-2-phenylacetamide, and a mixture of propylbenzene + N,N-Dimethylphenethylamine
do not lead to formation of products arising by reductive dearomatization. Rather, selective
hydrogenation of the amide moiety occurs to generate the corresponding amine. The results,
which are in accord with those coming from studies of hydrogenation reactions of
propylbenzene, lauric acid, and lauryl alcohol described above, show that the presence of amide
or amine groups suppresses undesired dearomatization reactions.
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Figure 7. Catalytic hydrogenation of (A) propylbenzene, (B) propylbenzene + lauric acid (dodecanoic
acid), and (C) propylbenzene + lauryl alcohol (dodecanol) over Re(5)/TiO2. Reaction conditions: 2 mol%
Re, 1 mmol of each substrate, no solvent, pH2 = 5 MPa, T = 140 °C. Yields were determined by GC using
n-octane as the internal standard.
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Figure 8. Catalytic hydrogenation of (A) propylbenzene, (B) propylbenzene + N,N-dimethyl-2-
phenylacetamide, and (C) propylbenzene + N,N-Dimethylphenethylamine over Re(5)/TiO2. Reaction
conditions: 2 mol% Re, 1 mmol of each substrate, 1 mL of octane, pH2 = 5 MPa, T = 200 °C. Yields were
determined by GC using n-octane as the internal standard.
Next, the effect of H2 pressure and concentration of the carboxylic acid on the rate of the
Re(5)/TiO2 promoted hydrogenation of 3-phenylpropionic acid in n-octane was investigated (Fig.
9). The hydrogenation rate was defined as the sum of the alcohol formation rate and one half of
the ester formation rate. The results show that the rate of hydrogenation increases with
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increasing H2 pressure. The reaction order with respect to H2 pressure was calculated to be
+0.88, which suggests that H2 is involved in a kinetically important step. On the other hand, the
reaction order with respect to 3-phenylpropionic acid concentration was calculated to be +0.03,
indicating that the carboxylic acid is strongly adsorbed on the surface of the Re(5)/TiO2 catalyst.
This observation is in agreement with those made in the selectivity experiments described above.
Figure 9. Effect of (A) the H2 pressure and (B) 3-phenylpropionic acid concentration on the rate of
hydrogenation of 3-phenylpropionic acid catalyzed by Re(5)/TiO2. Hydrogenation rate = alcohol formation
rate plus one half the ester formation rate. Pre-treatment: H2 (30 mL min-1), 500 ˚C, 0.5 h; reaction
conditions: 2 mol% Re, 1 mmol 3-phenylpropionic acid, 1 mL n-octane as a solvent, 140 °C, 3 h.
Hydrogenation reactions of 3-phenylpropionic acid using Re(x)/TiO2 (x = 0.2, 1, 5, 10, and
20 wt%) were conducted in order to determine the optimal catalyst loading and to investigate the
effect of the size of the Re species on the process (Figure 10A). The highest rate of
hydrogenation was achieved using Re(5)/TiO2. The results indicate that the hydrogenation
process occurs more efficiently when the catalytic system contains Re species of sub-nanometer
to a few nanometer sizes (see HAADF-STEM images of Re(x)/TiO2 in Fig. 11). We also explored
hydrogenation reactions of 3-phenylpropionic acid promoted by Re(5)/TiO2 following
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pretreatment with H2 at different temperatures (200, 300, 500, 700, and 900 ˚C). The results
displayed in Fig. 10B show that Re(5)/TiO2, pre-treated at 500 ˚C with H2, is optimal for this
process. These observations along with those made in the catalyst characterization studies
using techniques such as XANES and XPS demonstrate that the average oxidation state of the
Re species that promote hydrogenation should be higher than 0 and below +4. At this time, we
do not know the precise nature of the catalytically active Re species, and the catalyst may
contain a variety of Re species in different oxidation states. We also cannot exclude the
possibility that these mixtures of Re species in different oxidation states and the interfaces
between them are required for the efficient progression of the reaction. It should also be noted
that reductive pretreatment at temperatures beyond 500 ˚C leads to sintering of the Re species
(see HAADF-STEM images in Fig. 12) and a lowering of the activity of the catalyst.
Figure 10. Effect of (A) the Re loading and (B) pretreatment reduction temperature on the hydrogenation
of 3-phenylpropionic acid catalyzed by Re(x)/TiO2 (Re(5)/TiO2 was used for the reduction temperature
dependence). Hydrogenation rate = the sum of the alcohol formation rate and one half the ester formation
rate. Pre-treatment: H2 (30 mL min-1), 500 ˚C, 0.5 h; reaction conditions: 2 mol% Re, 1 mmol 3-
phenylpropionic acid, no solvent, 5 MPa H2, 140 °C, 3 h.
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Figure 11. HAADF-STEM images for Re(x)/TiO2 (x = 0.2, 1, 10, 20 wt%) reduced at T = 500 °C under a
flow of H2. Due to the experimental setup, the samples were exposed to air before images were recorded.
Figure 12. HAADF-STEM images for Re(5)/TiO2 without reduction and reduced at T = 300, 700, and 900
°C under a flow of H2. Due to the experimental setup, the samples were exposed to air before images
were recorded.
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Selective hydrogenation of triglycerides
Because it is highly desired to replace fossil fuels as chemical and energy sources, much
attention has been given to the conversion of biomass materials into fuels and chemicals.66–72
Among biomass materials, triglycerides serve as the most important feedstocks for the synthesis
of fatty amines, amides, nitriles and alcohols used in the production of surfactants, polymers and
other useful commodity chemicals.73 However, benign manufacturing processes for the direct
transformation of triglycerides into these chemicals are not well developed. In this study, we
demonstrated the potential applicability of Re(5)/TiO2 catalyzed hydrogenations to direct
transformation of triglycerides into the corresponding alcohols. As shown in Table 3,
hydrogenation reactions of a variety of triglycerides with different chain lengths (C4-C18), using
Re(5)/TiO2 pretreated with 5 MPa H2 at 500 °C, leads to formation of the corresponding alcohols
without any solvents.
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Table 3. Catalytic hydrogenation of triglycerides to form alcohols using Re(5)/TiO2.a
O
O
Re(5)/TiO2
R
O
O
O
R
3
R
OH
5 MPa H2, neat
O
R
Entry Substrate
Product
Temperature (°C) Time (h) Alcohol yield (%)
O
O
C17H35
O
O
O
C17H35
C17H35
OH
1
2
3
230
230
230
30
30
24
89 (82)
89 (84)
90 (86)
O
C17H35
O
O
O
C15H31
O
O
O
O
C15H31
C15H31
OH
O
O
C15H31
O
C13H27
O
O
C13H27
C13H27
OH
C13H27
O
O
C11H23
O
O
O
C11H23
C11H23
OH
4
230
30
84 (81)
O
C11H23
O
O
O
C9H19
O
O
O
O
C9H19
C9H19
OH
5
6
230
230
24
24
86 (80)
66 (62)
O
O
C9H19
O
C7H15
O
O
C7H15
C7H15
OH
C7H15
O
O
C5H11
O
O
O
C5H11
C5H11
OH
OH
7
8
220
220
36
24
82 (78)
87 (83)
O
C5H11
O
O
C3H7
O
O
C3H7
C3H7
O
O
C3H7
aReactions were performed under pH2 = 5 MPa. Yields were determined by GC using n-dodecane as the
internal standard. Yields of isolated products are given in parenthesis.
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4. Conclusions
In the effort described above, we carried out a detailed mechanistic study of selective
hydrogenation reactions of carboxylic acids and their derivatives promoted by TiO2-supported
Re (Re/TiO2) catalysts. The results show that the optimal Re loading and H2 pretreatment
temperature are 5 wt% and 500 °C, respectively. Detailed investigations using XRD, XAFS, and
XPS as well as the STEM method were carried out to identify the catalytically active Re species.
The results indicate that the active Re species should have sub-nanometer to a few nanometer
sizes and average oxidation states higher than 0 and below +4. Also, we found that Re on a
TiO2 support is more highly dispersed than it is on the other supports such as SiO2, Al2O3, and
Carbon. Furthermore, we observed that the Re/TiO2 promotes hydrogenation of propylbenzene
to form propylcyclohexane, indicating this catalytic system has the intrinsic ability to promote
hydrogenation of aromatic rings. However, when either lauric acid or lauryl alcohol (or an amide
or amine) is present in the reaction mixture, reductive dearomatization reactions do not take
place. These observations suggest that strong adsorption of carboxylic acids and alcohols (or
amides and amines) to the catalyst, caused by the oxophilic nature of Re, suppresses the
dearomatization process. This finding is in accord with results of kinetic investigations, which
showed that the order of the reaction with respect to the carboxylic acid is +0.03. Finally, in a
study aimed at demonstrating that the catalytic system can be used for conversion of biomass
materials into value-added chemicals, we observed that Re/TiO2 can be employed to convert
triglycerides into the corresponding alcohols.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the JST-CREST project JPMJCR17J3, KAKENHI grants
17H01341, 18K14057, and 18K14051 from JSPS and by MEXT projects "Elements Strategy
Initiative to Form Core Research Center" and IRCCS. The authors deeply thank the technical
division of the Institute for Catalysis (Hokkaido University) for manufacturing experimental
equipment and the technical staffs of Research Institute for Electronic Science, Hokkaido
University for help with STEM observations. XAFS measurements were performed at the BL-
01B1 facility of SPring-8 at the JASRI (proposal No. 2018B1126).
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