Rhenium-Loaded TiO2: A Highly Versatile and Chemoselective Catalyst for the Hydrogenation of Carboxylic Acid Derivatives and the N-Methylation of Amines Using H2 and CO2

Article abstract of DOI:10.1002/chem.201702801

Herein, we report a heterogeneous TiO₂-supported Re catalyst (Re/TiO₂) that promotes various selective hydrogenation reactions, which includes the hydrogenation of esters to alcohols, the hydrogenation of amides to amines, and the N-methylation of amines, by using H₂ and CO₂. Initially, Re/TiO₂ was evaluated in the context of the selective hydrogenation of 3-phenylpropionic acid methyl ester to afford 3-phenylpropanol (p^H2 =5 MPa, =5 MPa, T=180 °C), which revealed a superior performance over other catalysts that we tested in this study. In contrast to other typical heterogeneous catalysts, hydrogenation reactions with Re/TiO₂ did not produce dearomatized byproducts. DFT studies suggested that the high selectivity for the formation of alcohols in favor of the hydrogenation of aromatic rings is ascribed to the higher affinity of Re towards the COOCH₃ group than to the benzene ring. Moreover, Re/TiO₂ showed a wide substrate scope for the hydrogenation reaction (19 examples). Subsequently, this Re/TiO₂ catalyst was applied to the hydrogenation of amides, the N-methylation of amines, and the N-alkylation of amines with carboxylic acids or esters.

Full text of DOI:10.1002/chem.201702801

A Journal of
Accepted Article
Title: Rhenium-Loaded TiO2: A Highly Versatile and Chemoselective
Catalyst for the Hydrogenation of Carboxylic Acid Derivatives and
the N-Methylation of Amines Using H2 and CO2
Authors: Takashi Toyao, Hakim S. M. A. Siddiki, Yoshitsugu Morita,
Takashi Kamachi, Abeda S. Touchy, Wataru Onodera,
Kenichi Kon, Shinya Furukawa, Hiroko Ariga, Kiyotaka
Asakura, Kazunari Yoshizawa, and Ken-ichi Shimizu
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To be cited as: Chem. Eur. J. 10.1002/chem.201702801
Link to VoR: http://dx.doi.org/10.1002/chem.201702801
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Chemistry - A European Journal
10.1002/chem.201702801
Rhenium-Loaded TiO : A Highly Versatile and Chemoselective
2
Catalyst for the Hydrogenation of Carboxylic Acid Derivatives and the
N-Methylation of Amines Using H
2
and CO
2
Takashi Toyao,*[a,b] S. M. A. H. Siddiki, Yoshitsugu Morita, Takashi Kamachi,[b,c,e] Abeda S.
[a]
[c,d]
[
a]
[a]
[a]
[a,b]
[a]
[a]
Touchy, Wataru Onodera, Kenichi Kon, Shinya Furukawa,
Kazunari Yoshizawa,[b, c] Ken-ichi Shimizu,*[a,b]
Hiroko Ariga, Kiyotaka Asakura,
[a]
Dr. T. Toyao, Dr. S. M. A. H. Siddiki, Dr. A. S. Touchy, W. Onodera, Dr. K. Kon, Dr. S. Furukawa, Dr.
H. Ariga, Prof. Dr. K. Asakura, 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
[b]
[c]
[d]
[e]
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)
Present address Dr. Y. Morita
Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27
Kasuga, Bunkyo-ku (Japan)
Present address Dr. T. Kamachi
Department of Life, Environment and Materials Science, Fukuoka Institute of Technology (FIT), 3-
3
0-1 Wajiro-Higashi, Higashi, Fukuoka 811-0295 (Japan)
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Chemistry - A European Journal
10.1002/chem.201702801
Abstract
Herein, we report a heterogeneous TiO
2
-supported Re catalyst (Re/TiO ) that promotes
2
various selective hydrogenation reactions, e.g. hydrogenation of esters to alcohols,
hydrogenation of amides to amines, and N-methylation of amines using H and CO . Initially,
Re/TiO was evaluated in the context of the selective hydrogenation of 3-phenylpropionic acid
2
2
2
methyl ester to afford 3-phenylpropanol (pH2 = 5 MPa, T = 180 ºC), where revealed a superior
performance relative to other catalysts explored in this study. In contrast to other typical
heterogeneous catalysts, Re/TiO
suggested that the high selectivity for the formation of alcohols in favor of the hydrogenation of
aromatic rings, should be ascribed to the higher affinity of Re toward the –COOCH group
relative to the benzene ring. Moreover, Re/TiO shows a wide substrate scope for the reaction
19 examples). Subsequently, Re/TiO was applied to the hydrogenation of amides and the N-
methylation of amines with H and CO . Furthermore, Re/TiO promotes the N-alkylation of
amines with carboxylic acids or esters in the presence of H
2
did not produce dearomatized byproducts. DFT studies
3
2
(
2
2
2
2
2
.
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Introduction
The discovery and development of efficient methods for the selective reduction of
challenging functional groups such as carboxylic acids, amides, and esters have for many
years presented formidable challenges, despite their importance and the high demand for a
variety of applications in the pharmaceutical and fine-chemical industry, as well as in biomass
[
1]
conversion. The challenges are associated predominantly with the fact that these carboxylic
acid derivatives are usually thermodynamically stable and kinetically inert due to the low
electrophilicity of the carbonyl carbon atom.[2] Although stoichiometric quantities of strong
reducing agents have conventionally been used for this process,[3] the inevitable workup
procedures, which include a hazardous quenching step, filtration, and repeated extractions, as
well as the concomitant formation of undesirable waste are problematic, especially on an
industrial scale. These issues could potentially be overcome by catalytic hydrogenations that
use molecular hydrogen, as this would be an environmentally benign alternative.[4] It is
therefore hardly surprising that substantial effort has been devoted to the development of such
hydrogenation processes in order to realize environmentally more friendly systems, and some
of these processes have already become industrial viable. However, since those carboxylic
acid derivatives are among the most difficult carbonyl substrates to be hydrogenated, harsh
reaction conditions are usually required to transform these substrates.[5] For instance, a
conventional copper-chromite-based catalytic system requires very severe conditions (pH2 > 20
MPa, T = 250 ºC) and high catalyst loadings (~20 wt%).[6] The harsh conditions typically
required for heterogeneous catalytic systems often go hand in hand with low selectivity and
efficiency,[7] and the yields of these processes are frequently limited by over-reductions.
Especially when aromatic rings and carboxylic acid groups are present in the substrates, the
preferential hydrogenation of the former relative to the latter represents a severe problem for
most catalytic systems.[8] Since aromatic rings are frequently encountered in many organic
molecules including natural compounds, this restriction seriously limits the utility of such
hydrogenation processes in synthetic chemistry, both on an academic and industrial scale.
In recent years, substantial progress has been made to promote the selective
hydrogenation of carboxylic acid derivatives using homogeneous catalytic systems. Efforts
have especially been devoted to the selective hydrogenation of carboxylic acids,[9] of esters to
alcohols,[10] and of amides to amines.[11] It is worth pointing out that the homogeneous
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Chemistry - A European Journal
10.1002/chem.201702801
catalysts used in these reductions selectively hydrogenate the targeted functional groups,
while they do not compromise any aromatic rings in the substrates, and that these processes
thus represent highly important contributions to the toolkit of selective hydrogenations of
carboxylic acid derivatives. However, the homogeneous catalysts used in these cases
generally suffer from product separation issues and limited potential for catalyst recycling. This
is especially problematic with respect to sustainability considerations and large-scale
applications, and ideally, catalysts employed for selective hydrogenations should be
heterogeneous and readily recyclable. Another shortcoming of the aforementioned
homogeneous catalysts is that they are often highly substrate specific and frequently require
stoichiometric amounts of additives. Consequently, a highly versatile heterogeneous catalyst
for the reduction of a broad variety of carboxylic and carbonic acid derivatives represents a
highly desirable research target in order to develop a flexible platform for the transformation of
diverse functionalities. It is therefore not surprising that substantial efforts have been made to
realize selective hydrogenations promoted by heterogeneous catalysts.[12] Even though various
types of catalysts have been proposed,[8,12] heterogeneous catalysts normally suffer from
competitive product formation, i.e., dearomatization of aromatic rings and/or over-reductions to
alkanes diminish the chemoselectivity compared to homogeneous catalysts. The selective
hydrogenation of carboxylic acid derivatives with heterogeneous catalysts in the absence of
dearomatization remains challenging and represents a long-standing research objective with
benefits in academic organic synthesis and the industrial production of bulk chemicals. A few
heterogeneous catalytic systems can promote such selective hydrogenations of carboxylic acid
derivatives in the absence of dearomatizations. For instance, a German patent claims the
selective hydrogenation of amides using activated Co and Ni catalysts.[13] Broadbent et al.
have reported the selective hydrogenation of carboxylic acid derivatives including carboxylic
acids, esters, and amides catalyzed by rhenium black, which was prepared in situ from Re
oxides.[14] Although these seminal contributions are undoubtedly remarkable, these catalytic
systems typically require high pressures and temperatures (pH2 > 20 MPa; T > 200 ºC),
especially for the transformation of esters and amides. In addition, their substrate scopes
remained limited for each reaction. More recently, heterobimetallic Ru/Sn catalysts loaded on
Al
2
O
3
have been reported to promote the hydrogenation of carboxylic acids that contain
-based catalysts that
aromatic rings.[15] Furthermore, Lu et al. have reported Cu/ZnO-Al
2
O
3
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Chemistry - A European Journal
10.1002/chem.201702801
convert amino acid esters without dearomatization into the corresponding amino alcohols.[16]
However, in these reports, the substrate scope and the applicability toward various carboxylic
acid derivatives of these hydrogenations is limited, and thus, the development of more general
and efficient systems is strongly desirable. In this context, we have very recently reported that
a heterogeneous TiO
2
-supported Re catalyst (Re/TiO ) promotes the selective hydrogenation
2
of various carboxylic acids that contain aromatic moieties to the corresponding alcohols under
relatively mild conditions (pH2 = 5 MPa, T = 140–180 ºC; 22 examples; yield ≤ 97%), whereby
additives are not required.[17] It should be noted here that this heterogeneous catalytic system
does not produce any dearomatized byproducts, despite the difficulty of hydrogenating the –
COOH group.[18] The high chemoselectivity of the Re/TiO
catalytic system toward the
2
selective hydrogenation of carboxylic acids to alcohols under concomitant compatibility of
aromatic rings are features rarely seen for both homogeneous and heterogeneous
hydrogenation catalysts. Although selective hydrogenations of carboxylic acids that contain
aromatic groups to alcohols over heterogeneous catalysts have been developed, a highly
versatile heterogeneous catalyst for the reduction of a broad variety of carboxylic acid
derivatives would be desirable in order to meet the demand for the development of a general
catalytic system for these challenging transformations.[9a]
This study is concerned with the selective hydrogenation of carboxylic acid esters and
amides that contain aromatic moieties using the aforementioned Re/TiO
2
catalyst. In contrast
to previously developed catalysts for these reactions, Re/TiO is heterogeneous and can
2
accordingly be recovered and recycled with relative ease, while the synthesis of the catalyst is
straightforward using a facile impregnation method. Even under relatively mild conditions,
Re/TiO displays high activity and does not require additives. Density functional theory (DFT)
2
calculations suggested that the high selectivity for the generation of alcohols, which leaves
aromatic rings unaffected, should be attributed to the higher affinity of Re toward the –
COOCH group than to aromatic rings. In addition to the simple hydrogenation of esters and
3
amides, the N-alkylation of amines via hydrogenation was also accomplished. The latter
reaction involves the hydrogenation of carboxylic acid esters to the corresponding alcohols in
the presence of H
Furthermore, the N-methylation of amines using H
The broad range of organic transformations, based on the selective hydrogenation of
2
, followed by N-alkylation of the amines with the generated alcohol.
2
and CO has been achieved using Re/TiO .
2
2
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Chemistry - A European Journal
10.1002/chem.201702801
carboxylic acid derivatives promoted by Re/TiO
desirable tool for synthetic organic chemistry.
2
should render this catalyst system a highly
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Chemistry - A European Journal
10.1002/chem.201702801
Results and Discussion
The Re/TiO
and TiO . The catalyst was characterized by X-ray diffraction (XRD) analysis and transmission
electron microscopy (TEM) (Figures S1 and S2). The XRD patterns of Re/TiO exhibited
peaks that arise exclusively from TiO , while peaks associated with Re species were not
observed. TEM measurements showed that the Re clusters on TiO exhibit particle sizes
below 2 nm. These results suggest that the Re species are highly dispersed on TiO , similar to
our previous study.[19] The X-ray photoelectron spectroscopy (XPS) analysis revealed the
oxidation states of the Re species on TiO . Figure 1 shows the XPS spectra of the Re 4f
region of Re/TiO before and after reduction with H at T = 700 ˚C. It should be noted that the
reduced samples were measured directly after the H reduction without any exposure to air by
using a glove box that was connected to the XPS chamber. The obtained spectra include
signals due to both 4f7/2 and 4f5/2 core levels. The spectrum of Re/TiO prior to the H reduction
2
catalyst was prepared using a facile impregnation method based on NH
4
ReO
4
2
2
2
2
2
2
2
2
2
2
2
showed two Re components that contain 4f7/2 peaks at ~46.1 eV, which is assigned to Re+VII
species with an atomic concentration of 56%, and at ~44.0 eV, which is attributed to Re+VI
species with an atomic concentration of 44% (Fig. 1(a)). On the other hand, a strong peak at
0
~
40.7 eV assignable to a 4f7/2 peak of Re species was observed for Re/TiO
2
after the
reduction, as shown in Fig. 1(b). The result suggests that the major part of the Re species
were reduced to a metallic state after the reduction. Minor contributions from oxidized Re
species such as Re+III and Re+IV could also be observed even after the H
reduction.[20] These
2
results indicate that the H
2
reduction induces the formation of low-valent Re species that
include the metallic state.
The ring-stretching region of the FT-IR spectra for pyridine adsorbed both on Re/TiO
and pristine TiO
is shown in Figure 2. Consistent with previous FT-IR studies,[21] the spectra
for both Re/TiO
2
2
-1
2
and TiO exhibit a strong band at 1445 cm , which is due to pyridine that
2
-1
coordinates to the Lewis-acidic sites, whereas a band at 1540 cm , which should be due to the
+
pyridinium ion (PyH ), was not observed in significant intensity. This result indicates that both
samples contain Lewis but not Brønsted acid sites. The spectra furthermore revealed that the
-1
intensity of the band at 1445 cm is higher for Re/TiO
introduced Re species contain Lewis acid sites.
2
than for TiO , which suggests that the
2
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Chemistry - A European Journal
10.1002/chem.201702801
Figure 1. XPS spectra of the Re 4f region of Re/TiO
2
(a) before and (b) after the reduction with
2
H at T = 700 ˚C. Red line: raw spectra; dark gray line (dashed): total; pale gray line: Ti 3p;
6+
7+
green line: Re ; navy blue line: Re .
Figure 2. Ring-stretching region of the FT-IR spectra of pyridine adsorbed on Re/TiO and
2
TiO
2
after the H
2
reduction. Measurements were carried out at T = 100 °C under a He flow.
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Hydrogenations of 3-phenylpropionic acid methyl ester (1a) were carried out in order to
screen the properties of various catalysts. Following pretreatment of the catalysts with H at
00 °C, reactions were carried out using 1 mmol of 1a and 2 mol% of catalyst in a stainless
2
7
3
autoclave (V = 10 cm ; pH2 = 5 MPa; T = 180 ºC; t = 24 h). The obtained results are
summarized in Table 1. Re/TiO produced 3-phenylpropanol (2a) and 3-phenylpropyl 3-
2
phenyl-propionate (3a) in yields of 96% and 3%, respectively. It was expected that 3a is
formed via a transesterification between the starting ester and the alcohol generated during the
reaction, but it should also be noted that methanol was also detected as a product. In contrast,
merely low yields of the alcohol were observed when other precious metal (Ru, Pt, Ir, Rh, or
Pd) catalysts were used (entries 2-6). This should mainly be attributed to the formation of
byproducts such as propyl-benzene (4a), 3-cyclohexyl-propionic acid (5a), 3-cyclohexyl-
propan-1-ol (6a), 3-cyclohexyl-propionic acid 3-cyclohexyl-propyl ester (7a), propyl-
cyclohexane (8a), and ethyl-cyclohexane (9a) via the competitive hydrogenation of benzene
rings and over-reduction to alkanes. Such side reactions should be expected, as the
hydrogenation of the aromatic ring should proceed easier than the far more demanding
reduction of the ester group. The reaction was not promoted by Ag-, Cu-, Ni-, or Co-loaded
TiO
used as the catalyst (entry 11). Moreover, we observed that Re/TiO
more efficiently than other Re-based catalysts on other supports (entries 13-24). In a control
experiment, we confirmed that NH ReO , as well as non-supported Re metal and its oxides
e.g. ReO and Re ) did not catalyze this hydrogenation reaction (entries 25-28). These
results clearly demonstrate that the combination of Re as the catalytically active species and
TiO as the support leads to an efficient catalyst system that selectively promotes the
2
catalyst systems (entries 7-10). In addition, no reaction occurred when pristine TiO
2
was
2
afforded 2a and 3a much
4
4
(
2
2
O
7
2
hydrogenation of esters to form alcohols, whereby the generation of undesired products such
as alkanes and dearomatized compounds is minimized.
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Chemistry - A European Journal
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Table 1. Heterogeneous catalytic hydrogenation of 3-phenylpropionic acid methyl ester.a
Yield [%]
Entry Catalysts
Conv. [%]
2a
3a
4a
0
4
0
0
0
4
0
0
0
0
0
0
16
1
1
0
4
0
1
4
0
3
0
0
0
0
0
0
5a
0
0
0
0
22
6
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
6a
0
44
2
0
0
5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
7a
0
18
27
0
5
19
0
0
0
0
0
0
0
0
0
0
0
0
0
8a
0
0
2
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
9a
0
16
0
0
9
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
Re/TiO
Ru/TiO
2
2
99
83
82
39
36
34
3
0
0
0
0
18
90
60
57
16
96
0
50
10
2
0
1
0
0
3
0
0
28
0
0
2
0
0
0
0
8
0
20
16
1
10
4
23
18
9
Pt/TiO
Ir/TiO
Rh/TiO
2
2
2
2
2
Pd/TiO
Ag/TiO
Cu/TiO
2
Ni/TiO
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
Co/TiO
2
0
0
TiO
Re/TiO
2
b
2
10
74
39
39
15
12
10
8
7
6
3
0
Re/Carbon
Re/-Al
Re/-Al
Re/ZrO
Re/HZSM-5(22) 29
Re/SiO
Re/SiO
2
O
3
2
O
3
2
2
16
31
35
15
6
1
0
7
1
2
-Al
2
O
3
Re/HY(5.5)
Re/Nb
Re/CeO
Re/MgO
Re/SnO
NH ReO
Metallic Re
ReO
Re
0
0
0
0
0
0
0
0
2
O
5
2
0
0
0
0
0
0
0
2
0
6
1
2
c
4
4
2
2
1
2
2
O
7
1
0
a
Reaction conditions: 2 mol% catalyst, 1 mmol phenylpropionic acid methyl ester, 3 mL octane,
p
H2 = 5 MPa, T = 180 °C, t = 24 h. Yields were determined by GC using n-dodecane as the
b
c
internal standard. The reaction was performed after calcination at T = 500 °C in air. The
reaction was performed without pretreatment.
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Chemistry - A European Journal
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DFT calculations were conducted in order to identify the origin of the high selectivity of
Re/TiO toward the formation of alcohols relative to the dearomatization of benzene rings. We
2
have previously reported that the selectivity of the hydrogenation of carboxylic acid derivatives
strongly depends on the relative adsorption affinities of their functional groups, specifically on
the benzene rings and the –COOH groups.[17] Accordingly, we examined the relative
adsorption affinities of –COOCH
methyl acetate and benzene as probe molecules. In addition to Re, metals that promote the
hydrogenation of aromatic rings rather than that of –COOCH groups such as Pd and Rh were
3
and benzene moieties on the catalyst surfaces, employing
3
examined for comparison. It should be noted that the most stable and common planes for each
metal were used for this investigation. The most stable adsorption structures of methyl acetate
adsorbed on the Re(0001), Pd(111), and Rh(111) surfaces are displayed in Figures S3-5. As
shown in Figure 3, the adsorption energy of benzene on Re(0001) is lower than that on
Pd(111) or Rh(111). In contrast, the adsorption energy of methyl acetate on Re(0001) is higher
than that on Pd(111) or Rh(111). To rationalize this trend, we calculated the projected density
of states (PDOS), which is shown in Figure 4. The HOMO-like (highest occupied Kohn-Sham
orbital) states of both methyl acetate and benzene are broadened (cf. blue line in Figure 4),
indicating that all systems exhibit a forward-donation from the HOMO of methyl acetate and
benzene to the metal surfaces. As Re(0001) has more unoccupied d band than Pd(111) and
Rh(111),[22] the adsorption energy on the Re(0001) surface should be higher than that on the
Pd(111) or Rh(111) surfaces. This more open d band feature, which is located just above the
Fermi level (0 eV in the figures), should thus be responsible for the strong adsorption of methyl
acetate on Re(0001) relative to the other surfaces. In contrast to methyl acetate, benzene
adsorbs weaker on the Re(0001) surface than on the Pd(111) or Rh(111) surfaces. This
should be due to the weaker back-donation from the occupied d-band of the metal surface to
the empty * orbitals (LUMOs) of benzene.[23] This feature can be observed in the PDOS as
follows. Upon the adsorption on the surface, the peaks for the LUMO-like (lowest unoccupied
Kohn-Sham orbital) state of benzene are significantly broadened relative to the position
observed for benzene in the gas phase, indicating a strong back-donation from the metal
surfaces to the LUMO of benzene (cf. blue line in Figure 3). In addition, adsorbed benzene
molecules are more negatively charged than adsorbed methyl acetate molecules on each
metal surface (Table S1). Furthermore, benzene adsorbed on the Re(001) surface is less
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Chemistry - A European Journal
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negatively charged in comparison to benzene adsorbed on the Pd(111) or Rh(111) surfaces,
indicating a weaker back-donation from the Re(001) surface to benzene compared to Pd(111)
and Rh(111). These results reasonably explain why the Re surface exhibits a higher affinity
toward the –COOCH
3
group and a lower affinity toward the benzene moiety relative to the
for the hydrogenation of
-phenylpropionic acid methyl ester. This relatively high affinity of Re toward –COOCH should
for the hydrogenation of 3-
other metal surfaces, and consequently the high selectivity of Re/TiO
2
3
3
be one reason to explain the high selectivity of Re/TiO
2
phenylpropionic acid methyl ester. We also considered geometric factors of the adsorbates as
well as the aforementioned electronic structures. These features can be evaluated by the
adsorption modes and the C–C bond distances of the optimized structures of benzene on the
surfaces and in the gas phase (Figure 5). The calculated C–C bond distances for benzene in
the gas phase (1.40-1.41 Å) are consistent with those obtained from previous DFT studies.[24]
On the Re(0001) surface, the adsorption mode where the center of the aromatic ring is located
above the hollow position has been identified as the most favorable. Although the calculated
C–C bond distances of benzene on the Re(0001) surface (1.44-1.45 Å) increase upon
adsorption, the six C–C bonds remain almost equivalent, which is similar to the gas phase.
Conversely, the bridge site was found to be most favorable adsorption site for benzene on the
Pd(111) and Rh(111) surfaces.[25] Moreover, the six C–C bonds are no longer equivalent and
consist of two short (cf. C–C bonds 2 and 5 in Figure 4) and four long bonds (C–C bonds 1, 3,
4
, and 6 in Figure 4). This result indicates that the adsorption of benzene on Pd(111) and
2
3
Rh(111) leads to a sp to sp rehybridization, which transforms the adsorbate from an aromatic
planar mode to a 1,4-cyclohexadiene-like conformation.[26] Specifically, benzene distorts to a
higher degree on the Pd(111) and Rh(111) surfaces compared to the Re(0001) surface, which
represents another important factor explaining the higher affinity of the former to aromatic rings,
and why Pd and Rh metals hydrogenate benzene rings, while Re does not.
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Figure 3. Adsorption energies of benzene and methyl acetate on Re(0001), Pd(111), and
Rh(111). The most stable adsorption structures of benzene and methyl acetate adsorbed on
Re(0001) are shown (side view).
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Figure 4. PDOSs of (a) isolated methyl acetate and methyl acetate adsorbed on the Re(0001),
Pd(111), and Rh(111) surfaces; (b) isolated benzene and benzene adsorbed on the Re(0001),
Pd(111), and Rh(111) surfaces; black line: total PDOS; blue line: PDOS of the adsorbates; red
line: PDOS of the slabs; in all plots: E = 0. The PDOS calculations for isolated methyl acetate
F
and benzene were performed by placing the molecules at the center of the vacuum space on
top of the Re(0001) slab.
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Figure 5. C–C bond distances in the optimized structures of benzene on the Re(0001),
Pd(111), and Rh(111) surfaces, as well as for benzene in the gas phase. Color code: gray: C;
white: H; blue: metal atoms.
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The Re/TiO
phenylpropionic acid methyl ester were further investigated. In particular, the effects of the Re
loading, the pretreatment temperature, and the H pressure were explored. The results
Tables S2-4) show that the catalytic activity reached a maximum for a catalyst loading of 5
wt% of Re. The yield of alcohol 2a increase with increasing pretreatment temperature and H
2
catalyst system and the reaction conditions for the hydrogenation of 3-
2
(
2
pressure, which level at T = ~ 500 ºC and pH2 = 5 MPa, respectively. It should be noted that the
yield of 2a was only 10% when the reaction was performed after calcination at T = 500 °C in
air without an H reduction pretreatment (entry 12, Table 1). The effects of the reaction
2
solvent and temperature were also investigated (Table S5 and S6). The reaction proceeds
efficiently to afford alcohols (yield: > 90%) in a variety of solvents including hexane, octane, p-
xylene, o-xylene, 1,4-dioxane, THF, and dimethoxyethane (DME). We found that the reaction
proceeds at lower temperature, but that temperatures of at least 180 °C are required to obtain
an alcohol yield of > 90% under the applied conditions.
The time-course plot of the hydrogenation of 3-phenylpropionic acid methyl ester in
(Figure 6) shows that 3-phenylpropanol (2a) was efficiently generated from 3-phenylpropionic
acid methyl ester (1a). It is noteworthy here, that 3a was formed as an intermediate from the
transesterification of 1a and 2a. The yield of 2a reached 96% after t = 24 h. However, 3-
phenylpropionaldehyde, which would be an anticipated intermediate in this process together
with other over-reduced products, was not observed even after t = 24 h. In order to examine
the reaction kinetics, a hydrogenation reaction employing 3a as the starting material was
carried out, and the obtained time-course plot is shown in Figure S6. An inspection of the
time-course plot shows that the hydrogenation of 3a is a slightly slower than that of 1a. In
addition, a recycling test of Re/TiO
heterogeneous catalyst system. Following the reaction, Re/TiO
isopropanol, dried in air, reduced with H at T = 700 °C, and subsequently reused for an
ensuing reaction. In this second run, the desired alcohol was obtained in excellent yield (97%),
indicating that the Re/TiO catalyst is recyclable. However, we found that the product alcohol
yield gradually decreased after the 4th recycling experiment. It should also be noted that a
second run without the reduction step using H afforded 2a in 75% yield, which is lower than
the yield obtained with an H reduction, indicating that the H reduction is necessary for optimal
2
was carried out to evaluate the potential reusability of this
2
was separated, washed with
2
2
2
2
2
recycling.[27] In order to further confirm the heterogeneous nature of the Re/TiO
system, a
2
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leaching test was performed (Figure S8), which revealed that the removal of the catalyst after t
3 h caused the hydrogenation to cease, indicating that only supported-Re species act as
=
catalytically active sites.
Figure 6. Time-course plot of the hydrogenation of 3-phenylpropionic acid methyl ester
catalyzed by Re/TiO ; 1a: 3-phenylpropionic acid methyl ester; 2a: 3-phenylpropanol; 3a: 3-
2
phenylpropyl 3-phenyl-propionate. Reaction conditions: 2 mol% Re, 1 mmol phenylpropionic
acid methyl ester, 3 mL octane, pH2 = 5 MPa.
The substrate scope of the reaction was explored in order to assess the potential versatility
of the Re/TiO
2
-catalyzed hydrogenation process. The results (Table 2) show that Re/TiO is
2
capable of promoting the hydrogenation of a wide range of carboxylic acid esters to alcohols.
Notably, substrates containing chlorine- or bromo-substituted benzene rings underwent the
selective hydrogenation with these functional groups remaining intact (entries 3, 7). Moreover,
Re/TiO can be used for the hydrogenation of substrates containing heterocyclic compounds to
2
yield the corresponding alcohols (entry 10). At present, this process is still somewhat limited
by e.g. olefin moieties in the substrates, which are reduced under the applied reaction
conditions (entry 5). Moreover, methyl benzoate was hydrogenated to toluene (97% yield)
over Re/TiO
2
at T = 180 °C and pH2 = 5 MPa after 24 h through an undesired over reduction.
-catalyzed hydrogenation and its
However, the high chemoselectivity of the Re/TiO
2
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concomitant compatibility with aromatic rings are rarely encountered features, especially for
heterogeneous hydrogenation catalysts.
Driven by the desire to replace fossil fuel sources, much attention has been given to the
conversion of biomass materials into fuels and chemicals.[28] Selective hydrogenations of
carboxylic acid esters are important methods in this context, as these can be employed to
generate fatty alcohols, which are widely used in the production of surfactants, polymers, and
solvents.[29] To evaluate the applicability of the Re/TiO
catalyst for the generation of fatty
2
alcohols, hydrogenation reactions of various fatty acid esters were explored. Fatty acid esters
with long or short aliphatic chains underwent efficient hydrogenation to produce the
corresponding alcohols in 67-92% yield under the previously established optimal conditions
(Table 2). These observations suggest that Re/TiO
2
should also be applicable for the
hydrogenation of bio-relevant carboxylic acid esters.
Table 2. Catalytic hydrogenation of carboxylic acid esters to alcohols using Re/TiO
.a
2
O
Re/TiO (2 mol%)
2
R
OH
R
O
5 MPa H2
Entry
1
Substrate
Product
Temperature [°C]
Time [h]
24
Yield [%]
180
96 (93)
2
3
4
5
180
200
180
180
24
24
24
24
97 (93)
71 (57)
97 (92)
95 (90)
6
7
200
200
24
24
94 (91)
82 (76)
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8
9
180
180
24
24
97 (94)
89 (85)
1
0
1
200
200
48
48
85 (81)
92 (86)
1
12
13
14
15
16
17
200
180
180
180
180
180
48
24
24
36
24
36
94
90 (87)
89 (85)
92 (90)
67 (62)
90 (86)
1
8
9
180
180
36
36
90 (88)
76 (73)
1
aReaction conditions: 2 mol% Re, 1 mmol ester, 3 mL octane, 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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In order to show the potentially extended utility of the Re/TiO
2
catalytic system, we
examined the hydrogenation of amides to amines. This reaction has recently attracted much
interest, as it is as important for academic and industrial applications as it is challenging. In
addition, the selective hydrogenation of amides that contain aromatic moieties is considered
particularly difficult over heterogeneous catalysts.[12d] Prior to the exploration of the substrate
scope, a catalyst screening was carried using some promising catalysts based on the results
from the ester and carboxylic hydrogenation reactions. The hydrogenation of N-
benzylacetamide (1b) was selected, and pH2 = 5 MPa and T = 180 °C were employed for this
purpose. The results, summarized in Table S7 in the supporting information, show that
Re/TiO
yield. The results of the hydrogenation of various amides to amines using Re/TiO
MPa, T = 180-200 °C) is summarized in Table 3. Re/TiO enabled the selective hydrogenation
of various substrates that contain aromatic and aliphatic moieties. Importantly, Re/TiO does
2
exhibited the best activity and selectivity, and yielded N-ethylbenzylamine (2b) in 86%
2
(pH2 = 5
2
2
not lead to the formation of dearomatized byproducts when aromatic rings are present in the
substrate. It is also noteworthy that the required conditions to promote this transformation are
similar to the previously reported systems using precious metals such as Pt, Ru, or Rh.[8,12d,30]
Although some instances were observed, where olefin moieties were reduced (entry 11),
similar to the case of the ester hydrogenation, the wide substrate scope that includes in
particular amides that contain aromatic rings, can hardly be accessed with conventional
heterogeneous catalysts. The selective hydrogenation of the primary amide 2-
phenylacetamide was tested using the Re/TiO
After 24 h, diphenylamine and N-phenethyl-2-phenylacetoamide were obtained in 38% and
5% yield, respectively. The formation of these products could be feasible attributed to
2
catalyst under pH2 = 5 MPa and T = 180 °C.
4
coupling reactions of the starting 2-phenylacetamide and/or 2-phenyethylamine, which is a
well-known difficulty in the hydrogenation of primary amides.12d For the suppression of such
coupling reactions, further studies are required.
Depending on the type of catalyst, the hydrogenation of amides to amines can proceed
via two principal pathways: i) an initial reduction of the amide carbonyl group to a hemiaminal,
which leads to the formation of an alcohol and amine via cleavage of the C–N bond, followed
by an amination of the alcohol with the amine,[31] or ii) a simple deoxygenation of the amide. In
order to determine the reaction pathway operative in the present system, the time-course plots
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of the hydrogenation of N-benzylacetamide (1b) were recorded (Fig. S9). The plots show that
1
b is consumed under concomitant formation of N-ethylbenzylamine (2b), which is obtained in
6% yield after t = 24 h. It should be noted that the corresponding alcohol and amine were not
8
observed during the hydrogenation. This result indicates that the reaction should proceed via
the deoxygenation of the amide over the Re/TiO catalyst.
2
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Table 3. Catalytic hydrogenation of various amides to amines using Re/TiO
.a
2
O
R1
Re/TiO (2 mol%)
2
R1
N
R3
N
R3
5
MPa H2
R2
R2
Entry Substrate
Product
Temperature [°C]
Time [h]
Yield [%]
1
180
24
86 (78)
2
3
4
200
200
180
24
20
24
83
91
54
5
6
180
180
24
24
85
99
7
8
9
180
180
180
24
24
24
99
94
94
1
0
1
200
200
36
24
56
87
1
12
180
36
70
aReaction conditions: 2 mol% Re, 1 mmol amide, 3 mL octane, 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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Schemes 1 and 2 demonstrate the potential applicability of Re/TiO
2
toward the N-
alkylation of amines with carboxylic acids or esters in the presence of H . Traditionally, such
2
transformations have been accomplished by using stoichiometric amounts of metal
borohydrides.[32] Although recent efforts have enabled this challenging reaction to proceed
catalytically, these processes still depend on homogeneous catalysts and co-catalysts such as
trifluoromethanesulfonimide (HNTf ), or on silanes as reducing agents, which generally suffer
2
from a low atom efficiency and laborious workup procedures.[33] Herein, we report a
heterogeneous catalytic system that enables the one-pot N-alkylation of amines with carboxylic
acids or esters in the presence of H
2
. The reaction involves the hydrogenation of carboxylic
acids (or esters) to give the corresponding alcohols in the presence of molecular H
2
, followed
by an N-alkylation of the amines with the generated alcohols. The reaction with dimethylamine
and 3-phenylpropionic acid under pH2 = 5 MPa and T = 200 ºC furnished 3-phenylpropyl-
dimethylamine in 83% yield after t = 48 h. We moreover demonstrated that the N-Alkylation of
dimethylamine with 3-phenylpropionic acid methyl ester proceeded efficiently under similar
conditions, generating the desired product in 83% yield.
Scheme 1. N-Alkylation of dimethylamine with 3-phenylpropionic acid (pH2 = 5 MPa and T =
2
00 ºC) catalyzed by Re/TiO .
2
Scheme 2. N-Alkylation of dimethylamine with 3-phenylpropionic acid methyl ester (pH2 = 5
MPa and T = 200 ºC) catalyzed by Re/TiO
2
.
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Re/TiO
. The use of CO
methanol, formic acid, and other value-added chemicals has recently received substantial
attention.[34] Among the various methods reported, especially the use of a CO
/H mixture as a
methylation reagent for the synthesis of fine chemicals seems highly promising.[35] This
reaction involves the initial formation of formate from CO and H , a subsequent condensation
with amines (or NH ), and a hydrogenation of the generated amides to N-methylated
amines.[36] Taking this reaction mechanism into consideration, Re/TiO
, which is an effective
2
also promotes the N-methylation of various amines in the presence of CO
2
and
H
2
2
as a renewable carbon resource for the production of chemicals such as
2
2
2
2
3
2
catalyst for the hydrogenation of amides, may potentially also be applicable to this
transformation. The feasibility of the catalytic system toward such N-methylation reactions was
examined by performing the reaction with N-methylaniline (1c) under pCO2 = 1 MPa, pH2 = 5
MPa, and T = 200 °C. After t = 24 h, the desired product, N,N-dimethylaniline (2c), was
generated in 98% yield (Figure S10). The effect of the reaction temperature was investigated
(Table S8), and the results show that temperatures of at least 200 °C are necessary for the
progression of the reaction under the applied conditions. A comparison of the catalytic
performance was carried out under the same conditions (pCO2 = 1 MPa, pH2 = 5 MPa, T =
2
00 °C, and t = 24 h) and the results are summarized in Table S9. Among the catalysts
explored in this study, the Re/TiO catalyst afforded the highest yield for 2c. The Re/TiO
catalyst was also compared to the supported Au catalyst (Au/Al ), which is the state-of-the-
art heterogeneous catalyst.[34d] Using Re/TiO
under the same condition (0.5 mol% Re, pCO2
MPa, pH2 = 6 MPa, T = 140 °C, and t = 7 h) afforded 2c in 25% yield, suggesting that further
2
2
2
O
3
2
=
2
improvements regarding the activity are required for the future. However, compared to Au-
based catalysts, Re-based catalysts would be more attractive from an economic perspective.
The substrate scope for this system was also explored and is summarized in Table 4. Re/TiO
effectively catalyzes the N-methylation of various amines under the aforementioned conditions.
Re/TiO catalyzed not only the transformation of substituted anilines, but also that of
2
2
tetrahydroquinolines (entries 6, 7). Furthermore, aliphatic secondary amines were also
converted to the corresponding tertiary amines in 70-99% yield.
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Table 4. Catalytic N-methylation of amines with CO
2
and H
2
over Re/TiO
N
2
.a
H
Re/TiO (2 mol%)
2
N
R1
R2
R1
R2
Time [h]
24
5 MPa H + 1 MPa CO2
2
Entry Substrate
Products
Temperature [°C]
Yield [%]
1
200
98 (91)
2
3
200
200
24
24
78
92
4
5
6
200
200
180
36
24
24
84
85
66
7
200
36
85
8
9
200
200
24
24
99
99
10
200
24
70
aReaction conditions: 2 mol% Re, 1 mmol amine, 3 mL octane, pCO2 = 1 MPa and 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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Conclusions
We developed a heterogeneous catalytic system based on Re and TiO
2
that promotes the
selective hydrogenation of esters to alcohols and of amides to amines. Most importantly, this
Re/TiO catalyst did not promote dearomatization reactions during the hydrogenations. The
results of DFT calculations suggested that this high chemoselectivity of Re/TiO should be
2
2
ascribed to the high affinity of Re toward carboxylic acid derivatives relative to benzene rings.
This result should provide an important design guideline for the development of advanced
catalysts for this process. Re/TiO
2
also effectively catalyzed the N-alkylation of amines with
via an initial hydrogenation of carboxylic acids
or esters) to afford the corresponding alcohols, followed by N-alkylation of the amines with the
carboxylic acids or esters in the presence of H
2
(
generated alcohols. In addition, Re/TiO efficiently catalyzed the N-methylation of amines
2
using a H
2
/CO mixture. Given the importance of these processes for both academic organic
2
synthesis and the industrial production of bulk chemicals, the presented system is a versatile
and selective hydrogenation catalyst that should find widespread applications.
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Experimental section
Materials and Catalyst Preparation
Organic and inorganic compounds were purchased from common commercial suppliers
Tokyo Chemical Industry, Kanto Chemical, Wako Pure Chemical Industries, Nacalai Tesque,
and SigmaAldrich) and used without further purification. TiO (ST-01) was obtained from
Ishihara Sangyo Co., Ltd., while MgO (JRC-MGO-3) and SiO
3.75 wt%) were supplied by the Catalysis Society of Japan. Conversely, γ-Al
(
2
2 2
-Al O
3
(JRC-SAL-2, Al
2
O
3
=
1
2
O
3
2
and -Al O
3
were prepared by calcination of γ-AlOOH (Catapal B Alumina, Sasol) for 3 h at T = 900 and
100 °C, respectively. CeO was prepared by calcination (T = 600 °C, t = 3 h, in air) of CeO
supplied from Daiichi Kigenso Kagaku Kogyo Co., Ltd (Type A). SiO (Q-10) was supplied by
Fuji Silysia Chemical Ltd., while Nb was prepared by calcination (T = 500 °C, t = 3 h) of
niobic acid (CBMM). The standard carbon support, Vulcan XC72 (210 m g ), was
commercially supplied by Cabot Carbon Black. ZrO was prepared by calcination (T = 500 °C, t
3 h) of a hydroxide of Zr. SnO was prepared by calcination (T = 500 °C, t = 3 h) of H SnO
Kojundo Chemical Laboratory Co., Ltd.). H-ZSM-5 (SiO /AI = 22) and HY (SiO /AI
.5) were obtained from TOSO Co., Ltd. NH ReO and metallic Re were purchased from
SigmaAldrich. ReO and Re were supplied from Hydrus Chemical Inc. and Strem
Chemicals Inc., respectively.
1
2
2
2
2
O
5
2
-1
2
=
2
2
3
(
2
2
O
3
2
2
O =
3
5
4
4
2
2
O
7
Precursors of M/TiO
wt% Re; MOx = metal oxides, zeolites, or carbon) were prepared by mixing the support
material with the metal sources, i.e., an aqueous solution of NH ReO , nitrates of Ag, Ni, Cu,
Co, RuCl or IrCl ·nH O, or aqueous HNO solutions of Pt(NH (NO , Rh(NO , or
Pd(NH (NO . Typically, for the preparation of Re/TiO , 0.72 g of NH ReO were added to a
glass vessel (500 mL) containing 100 mL of deionized water ([Re] = 0.027 M). After sonication
for 1 minute to completely dissolve the NH ReO , TiO (9.5 g) was added to the solution, which
2
(M = 5 wt% Re, Pt, Ir, Rh, Ru, Pd, Ag, Cu, Ni, or Co) and Re/MOx (5
4
4
3
3
2
3
)
3 2
3
)
2
)
3 3
)
3 2
)
3 2
2
4
4
4
4
2
was then stirred (200 rpm) for 15 min at room temperature. Subsequently, the reaction mixture
was evaporated to dryness at T = 50 °C, followed by drying at T = 90 °C under ambient
pressure for t = 12 h. The thus obtained material was calcinated (T = 500 °C, t = 3 h, in air).
For each experiment, the active catalyst was prepared by reduction in a quartz tube (T =
3
-1
7
00 °C, t = 0.5 h) under a flow of H
2
(20 cm min ). The other M/TiO catalysts were prepared
2
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Chemistry - A European Journal
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in the same manner as described above, except that the H
2
reduction of M/TiO was carried
2
out at T = 500 °C.
Characterization of the Catalysts
X-ray photoelectron spectroscopy (XPS) was carried out on an Omicron EA 125 X-ray
photoelectron spectrometer, equipped with a modified UHV chamber using Mg Kα (1253.6 eV)
radiation. Binding energies were calibrated based on the O1s peak energy of TiO
2
(532.0 eV).
The reduced samples were measured after the reduction using H in an N -filled glove box that
2
2
was directly connected to the XPS chamber in order to avoid exposure of the samples to air.
FT-IR spectra were recorded on a JASCO FT/IR-4200 with an MCT (Mercury-
Cadmium-Telluride) detector. Samples (40 mg) were pressed to obtain self-supporting pellets
(
 = 2 cm), which were placed in the quartz FT-IR cell with CaF
2
windows connected to a
conventional gas flow system. Prior to the measurements, the sample pellets were subjected
3
-1
to a pretreatment (T = 700 °C, t = 0.5 h) under a flow of H (20 cm min ), followed by cooling
2
to T = 100 °C under He. Subsequently, 0.3 mmol liquid pyridine per g of catalyst were injected
into the He flow, which was preheated to T = 150 °C, before the vaporized pyridine was fed
into the FT-IR cell. Then, the FT-IR disk was purged with He for t = 600 s, and the IR
measurement was carried out. Spectra were measured by accumulating 15 scans (resolution:
-1
4
cm ). A reference spectrum was taken at T = 100 °C under a He flow, which was subtracted
from each spectrum.
Catalytic Reactions
A following procedure for the hydrogenation reactions can be considered representative: after
the reduction with H at T = 700 °C (cf. Catalyst Preparation), the catalyst (2 mol% with respect
2
to the Re loading), and a mixture of 3-phenylpropionic acid methyl ester (1.0 mmol), octane (3
3
mL), and n-dodecane (0.29 mmol) were added to a stainless steel autoclave (10 cm ). The
resulting mixture was magnetically stirred (T = 180 °C, pH2 = 5 MPa). The products were
+
analyzed by GCMS (SHIMADZU GCMS-QP2010 with an Ultra ALLOY capillary column UA -1;
Frontier Laboratories Ltd.) and GC (Shimadzu GC-14B with an Ultra ALLOY capillary column
+
UA -1; Frontier Laboratories Ltd.). Yields were determined by GC analysis using n-dodecane
as the internal standard relative to the starting esters. All products were subjected to column
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Chemistry - A European Journal
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chromatography on silica gel 60 (spherical, 63-210 m, Kanto Chemical Co. Ltd.;
1
eluent:hexane/ethyl acetate = 90/10, v/v), and subsequently analyzed by GC and GCMS. H
and 13C NMR analysis was carried out for those products that were isolated successfully. H
and 13C NMR spectra were recorded at ambient temperature on a JEOL-ECX 600
spectrometer ( H: 600.17 MHz, 13C: 150.92 MHz), using tetramethylsilane as the internal
standard. Isolated yields were determined relative to the starting esters.
1
1
Reactions for the recycling experiments were performed in a manner similar to that
described above. Following the reaction (pH2 = 5 MPa, T = 180 ºC, t = 24 h), the catalyst was
separated, washed with isopropanol, dried in air, reduced with H
reused for an ensuing reaction. It should be noted that the recycling reaction was carried out
after H reduction as in the case of the initial run. A leaching test was performed in a manner
2
at T = 700 °C and then
2
comparable to the recycling test. After t = 3 h, the catalyst was separated by centrifugation.
Subsequently, the separated solution was transferred to the reactor, which was recharged with
2
H (pH2 = 5 MPa), followed by heating (T = 180 °C) and magnetic stirring for t = 21 h.
Computational Methods
All calculations were performed with the CASTEP code[37] in the Materials Studio of Accelrys
Inc. The Perdew−Burke−Ernzerhof (PBE) generalized gradient functional[38] was employed for
the exchange-correlation energy. The plane-wave basis set with a cutoff energy of 250 eV was
used for the system with periodic boundary conditions. Ultrasoft pseudopotentials were used to
describe the electron–ion interactions. The Brillouin zone was sampled with (3 × 3 × 1)
Monkhorst–Pack[38] k-points. The Re(0001), Pd(111), and Rh(111) surfaces were modeled by a
supercell slab that consists of a (3 × 3) surface unit cell with four atomic layers in accordance
with previously reported procedures.[17] It should be noted that the most stable and common
planes were used for each metal. The (0001) surface, which was used for Re, has a
hexagonal-close-packed (hcp) structure, while the (111) surface, which was used for Pd and
Rh, exhibits a face-centered-cubic (fcc) structure. The slab was separated in vertical direction
by a vacuum space (height: 15 Å). The top two layers of the surface were fully relaxed,
whereas the bottom two layers were fixed at the corresponding bulk positions. The adsorption
energy (Eads) was calculated as the difference in energy between the molecule absorbed on
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