Hydrogenation of dicarboxylic acids to diols over Re-Pd catalysts

Article abstract of DOI:10.1039/c6cy00335d

A Re-Pd/SiO₂ (Re/Pd = 8) catalyst was applied to hydrogenation of dicarboxylic acids (succinic acid, glutaric acid and adipic acid) to diols. In the hydrogenation of dicarboxylic acids, ex situ liquid-phase (in only 1,4-dioxane solvent) reduced Re-Pd/SiO₂ showed much higher activity than in situ liquid-phase (in the mixture of dicarboxylic acid and 1,4-dioxane) and gas-phase reduced ones, in which the in situ liquid-phase reduced catalyst has been reported to show good activity in the hydrogenation of monocarboxylic acids. High diol yields (71-89%) were achieved in the hydrogenation of dicarboxylic acids on the ex situ liquid-phase reduced catalyst at 413 K. Lactones and hydroxycarboxylic acids were first formed as intermediates in the reaction of C4-C5 and ≥C6 dicarboxylic acids, respectively. Characterization using XRD, XPS and XAS indicates that ex situ liquid-phase reduced catalysts with high activity contains comparable amounts of Re⁰ and Reⁿ⁺ species, both of which have been reported to be necessary for good performance. The amount of Reⁿ⁺ species on the in situ liquid-phase reduced catalysts is much larger than that of surface Re⁰ species. This result suggests that the presence of dicarboxylic acids suppresses the reduction of Re species to Re⁰ on the calcined catalysts while that of monocarboxylic acids does not, which leads to the low activity in the hydrogenation of dicarboxylic acids on in situ liquid-phase reduced catalysts.

Full text of DOI:10.1039/c6cy00335d

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Hydrogenation of dicarboxylic acids to diols over
Re–Pd catalysts†
Cite this: DOI: 10.1039/c6cy00335d
a
a
a
b
Yasuyuki Takeda, Masazumi Tamura, Yoshinao Nakagawa, Kazu Okumura
a
and Keiichi Tomishige*
A Re–Pd/SiO
acid and adipic acid) to diols. In the hydrogenation of dicarboxylic acids, ex situ liquid-phase (in only 1,4-di-
oxane solvent) reduced Re–Pd/SiO showed much higher activity than in situ liquid-phase (in the mixture
2
(Re/Pd = 8) catalyst was applied to hydrogenation of dicarboxylic acids (succinic acid, glutaric
2
of dicarboxylic acid and 1,4-dioxane) and gas-phase reduced ones, in which the in situ liquid-phase
reduced catalyst has been reported to show good activity in the hydrogenation of monocarboxylic acids.
High diol yields (71–89%) were achieved in the hydrogenation of dicarboxylic acids on the ex situ liquid-
phase reduced catalyst at 413 K. Lactones and hydroxycarboxylic acids were first formed as intermediates
in the reaction of C4–C5 and ≥C6 dicarboxylic acids, respectively. Characterization using XRD, XPS and
XAS indicates that ex situ liquid-phase reduced catalysts with high activity contains comparable amounts of
0
n+
Received 12th February 2016,
Accepted 6th April 2016
Re and Re species, both of which have been reported to be necessary for good performance. The
n+
amount of Re species on the in situ liquid-phase reduced catalysts is much larger than that of surface
0
Re species. This result suggests that the presence of dicarboxylic acids suppresses the reduction of Re
DOI: 10.1039/c6cy00335d
0
species to Re on the calcined catalysts while that of monocarboxylic acids does not, which leads to the
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low activity in the hydrogenation of dicarboxylic acids on in situ liquid-phase reduced catalysts.
1c,5,6
reactions
because 1,4-BuD is an important raw material for
Introduction
thermoplastic polymers such as polybutylene succinate (PBS)
1h
Biomass is an abundant and environmentally friendly re-
source in place of petroleum because biomass is the only re-
newable resource with organic carbons, and the conversion
of biomass to valuable chemicals is an important and ur-
and polybutylene terephthalate (PBT). However, 1,4-BuD is
conventionally produced by the transformation of petroleum
7
such as the catalytic hydrogenation of maleic anhydride, and
biomass-derived 1,4-BuD has been required for the production
1
8
gent research topic. Among various biomass-derived
of biopolymers. In addition, other dicarboxylic acids such as
chemicals, succinic acid (SUC) has recently attracted much
attention as a promising platform chemical because SUC
adipic acid (ADI) and glutaric acid (GLU) can be produced in
the oxidation of a mixture of cyclohexanol and cyclohexanone
derived from petroleum, and hydrogenation of these dicarbox-
2
9
can be easily produced by the fermentation of sugar (poten-
3
tial production of SUC: 64 000 tons per year (in 2015) ) and
ylic acids can also give diols which are important materials for
8b
the production cost of the fermentation method is esti-
mated to be much lower than that of the conventional
the production of polymers. Therefore, effective catalysts for
the hydrogenation of biomass- and petroleum-derived dicarbox-
ylic acids to diols are desired to be developed.
4
method of SUC production from petroleum. Various trans-
formation methods of SUC have been developed, which can
produce valuable chemicals such as 1,4-butanediol (1,4-
BuD), N-methyl-2-pyrrolidone, 2-pyrrolidone, succinimide,
Effective
homogeneous
catalysts
such
as
4
H -
Ru (CO) (PBu ) (ref. 6m) and heterogeneous catalysts such
4
8
3 4
as Re O , Ru–Sn-based catalysts, M/C (M = Pd, Pt, Rh) and Re
2
7
5
6a–l
succinic esters and maleic acid. Among these methods, hy-
species + noble metal catalysts
for the hydrogenation of
drogenation of SUC to 1,4-BuD is one of the important
dicarboxylic acids to diols have been reported. From the view-
point of the recovery and reusability of the catalysts, hetero-
geneous catalysts are more preferable than homogeneous
a
Department of Applied Chemistry, School of Engineering, Tohoku University,
6
l
6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan.
ones. In the hydrogenation of SUC, the Re catalyst obtained
E-mail: tomi@erec.che.tohoku.ac.jp
by in situ reduction of Re O has been known to be effective
b
2 7
Department of Applied Chemistry, Faculty of Engineering, Kogakuin University,
for the reaction for a long time, providing 94% yield of
1-24-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo 163-8677, Japan
6
a
1
,4-BuD (483 K, 25 MPa H , no solvent). In the case of
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
2
c6cy00335d
supported catalysts, Re species + noble metal catalysts
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2
−1
provided high yields of 1,4-BuD (Re(3.6 wt%)–Pd(2 wt%)/
pregnating silica (G-6, BET surface area 535 m g , Fuji
Silysia Chemical, Ltd.) with an aqueous solution of RuCl3
6
h
TiO : 83% (solvent: H O), Pd(1 wt%)–Re(4 wt%)/TiO : 86%
2
2
2
6
j
(
solvent: 1,4-dioxane and
H
2
O),
Re(0.3 mol%)–Ru(0.3
3
(Kanto Chemical Co., Ltd.), RhCl (Soekawa Chemical Co.,
Ltd.), PdCl (Wako Pure Chemical Industries, Ltd.), H PtCl
6
(Kanto Chemical Co., Ltd.), H IrCl (Furuya Metals Co., Ltd.),
2 6
mol%)/mesoporous carbon (MC): 71% (solvent: 1,4-dioxa-
2
2
6
k
ne) ) under the conditions of T = 433–473 K and P = 6.9–
H
2
2
5 MPa. In the hydrogenation of mixtures of SUC, GLU and
ADI, the Ru(5 wt%)–Sn(3 wt%)–Re(5 wt%)/C catalyst (453 K,
5 MPa H , solvent: H O) provided high yields of 1,4-BuD
(NH
4 6 7
) Mo O24 (Wako Pure Chemical Industries, Ltd.),
NH ReO (Soekawa Chemical Co., Ltd.), (NH ) W O (Wako
4
4
4 10 12 42
1
Pure Chemical Industries, Ltd.) and TiCl3 aq. (20%, Wako
Pure Chemical Industries, Ltd.). After impregnation, the
obtained wet catalysts were dried at 383 K for 12 h, and then
they were calcined in air at 673 K for 3 h. M″–M′/SiO cata-
2
lysts were prepared by impregnating M′/SiO catalysts after
2
2
(75%), 1,5-pentanediol (98%) and 1,6-hexanediol (96%),
6
l
respectively. In addition, Re species + noble metal catalysts
have been reported to be also active in the hydrogenation of
maleic acid and monocarboxylic acids
2
1
0
11–13
to alcohols.
Therefore, catalysts including Re species are efficient for the
hydrogenation of dicarboxylic acids to diols.
drying at 383 K for 12 h. In the case of M″–Pd/SiO catalysts,
the loading amount of M″ is represented as the molar ratio
of M″ to Pd (for example, M″/Pd = 8). On the other hand, in
2
In our previous work, we also reported that the Re–Pd/SiO
2
catalyst with a high Re loading amount (Re = 14 wt%, Pd = 1
wt%, Re/Pd = 8) showed high selectivity and yield of
the case of Re–M′/SiO catalysts (M′ = Ru, Rh, Ir and Pt), the
loading amount of M′ is represented by the molar ratio of Re
to M′ (for example, Re/M′ = 8). The Re loading amount of the
2
1
-octadecanol (94%) in the hydrogenation of stearic acid (STA)
under comparatively mild conditions (T = 413 K, P = 8 MPa,
Re–Pd/SiO (Re/Pd = 8) catalyst is the same as that of the Re/
H
2
2
11,12
solvent: 1,4-dioxane).
Characterization of the Re–Pd/SiO
2
SiO
2
(Re = 14 wt%) catalyst. The preparation method for Re–
0
11
catalyst suggests the effective structure of the Re surface mod-
Pd/SiO is the same as that in our previous work. All cata-
2
n+
ified with Re (n = 3 or 4) species, which was responsible for
lysts were used in powdery form.
the high activity. In addition, the catalyst activity was greatly
influenced by the reduction method of the catalyst; liquid-
phase reduction in the reaction mixture (in situ reduction
method) showed much higher activity for the reaction than ex
situ gas-phase reduction. A similar tendency was also observed
Activity tests
Activity tests were performed using a stainless steel autoclave
(190 ml) with an inserted glass vessel. The catalyst was set in
the glass vessel with a spinner, succinic acid (SUC, Wako
Pure Chemical Industries, Ltd., >99.5%) and an appropriate
amount of 1,4-dioxane (Wako Pure Chemical Industries, Ltd.,
>99.5%) as a solvent. The pretreatment of the catalyst was
described below. The solvent was selected because of the sol-
ubility of the carboxylic acids used in this study. The sub-
strates are listed in Table S1 in the ESI.† The glass vessel with
the reagents was inserted into the autoclave, and after sealing
the reactor, the air content in the autoclave was purged with
1 MPa H2 (99.99%, Showa Denko K.K.) three times. The
autoclave was heated to the desired temperature (typically,
6a
in the case of Re O , where in situ reduction of Re O pro-
2
7
2 7
vided a higher 1,4-BuD yield (94%) than ex situ reduction
35%) in the hydrogenation of SUC. These results indicate that
(
the structure of Re-based catalysts is largely influenced by the
reduction method of the catalysts, and the selection of an ap-
propriate reduction method is essential to obtain high perfor-
mance of the catalysts in the objective reaction.
2
Herein, we applied the Re–Pd/SiO (Re = 14 wt%, Pd = 1
wt%, Re/Pd = 8) catalyst to the hydrogenation of dicarboxylic
acids to diols, and we found that the catalyst was also effec-
tive for the reaction. The catalyst system was applicable to
the hydrogenation of various dicarboxylic acids to afford the
corresponding diols in high yields (71–89%). In this catalyst
system, the selection of the reduction method was very im-
portant to obtain high activity of the catalyst, and the ex situ
liquid-phase reduction method, where the catalyst was re-
duced in only 1,4-dioxane solvent without any substrates,
showed higher activity in the hydrogenation of SUC than the
other reduction methods. The catalyst was also characterized
by means of X-ray diffraction (XRD), X-ray absorption spectro-
scopy (XAS) and X-ray photoelectron spectroscopy (XPS). The
dependence of the catalytic activity on the reduction method
was discussed on the basis of the catalyst structure.
2
413 K), and the H pressure was increased to the appropriate
pressure. The stirring rate was 500 rpm (magnetic stirring).
After the reaction, the reactor was cooled down in a water
bath, and the gas contents were collected in a gas bag. The
liquid contents were transferred to a vial, and the catalyst
was separated by centrifugation. The standard reaction condi-
tions for the hydrogenation of SUC to 1,4-BuD are as follows:
1.0 g of SUC (8.5 mmol), 19 g of 1,4-dioxane, 100 mg of cata-
lyst, 8.0 MPa hydrogen pressure (at 413 K), 413 K, 4 h. Reduc-
tion of the catalysts was conducted by three different
methods: (1) ex situ liquid-phase reduction without SUC
(exL), (2) in situ reduction with SUC (inL) and (3) gas-phase
reduction (G). In method (1), the calcined catalysts were re-
duced in an autoclave with a spinner and 1,4-dioxane (19 g)
without the substrate. After sealing the reactor, the reactor
Experimental
Catalyst preparation
2
was purged with 1 MPa H and then heated to 413 K. The au-
M′/SiO (M′ = Pd, Ru, Rh, Ir and Pt, 1 wt%) and M″/SiO (M″
toclave was pressurized to 8 MPa H , and the content was
2
2
2
=
Re, Mo, W and Ti, 14 wt%) catalysts were prepared by im-
stirred for 1 h. The catalysts are called “ex situ liquid-phase
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reduced catalysts” and denoted, for example, as Re–Pd(exL,
4
13), where exL means “ex situ liquid-phase” and 413 means
the reduction temperature. After cooling down to room tem-
perature, the autoclave was opened and the substrate was
added into the autoclave reactor in N atmosphere, where all
2
the procedures were carried out without exposure of the cata-
lyst to air. The catalyst samples after the activity test are de-
noted as Re–Pd(exL, 413, Reaction) catalyst. In method (2),
the calcined catalyst was in situ reduced during the heating
step of the activity test by hydrogen in the presence of the
substrate and the solvent. The catalysts are called “in situ
liquid-phase reduced catalysts” and denoted, for example, as
Re–Pd(inL, 413), where inL means “in situ liquid-phase”. In
addition, the samples after the activity test are denoted as
Re–Pd(inL, 413, Reaction) catalyst. In method (3), the cal-
cined catalysts were reduced in the gas phase under hydro-
(4)
where nsubstrate and nproduct are the carbon numbers in a sub-
strate and a product, respectively. Products were also identi-
fied using GC-MS (QP-2010, Shimadzu). The mass balance
was also confirmed for each result and the difference in mass
balance was in the range of the experimental error (±5%), un-
less otherwise noted.
Catalyst characterization
Characterization of the catalysts was carried out in a manner
similar to that of our previous report. The detailed methods
are described in the ESI.†
X-ray diffraction (XRD) patterns were recorded using a dif-
fractometer (MiniFlex600, Rigaku). Cu Kα (λ = 0.154 nm,
45 kV, 40 mA) radiation was used as the X-ray source. The
catalyst samples after the reaction or reduction were trans-
ferred to an atmosphere separator (air-sensitive sample
1
1
gen flow. The calcined catalysts were reduced in H flow
2
−
1
(
100% H
system and a glass tube. The obtained catalysts are called
gas-phase reduced catalysts” and denoted, for example, as
2
, 30 mL min ) at 473 K for 1 h by using a gas flow
“
Re–Pd(G, 473), where G means gas-phase. The reduced cata-
lyst and the substrate were introduced into an autoclave un-
der N atmosphere in order to avoid exposing the catalyst to
2
air. The samples after the activity test are denoted as Re–
Pd(G, 473, Reaction) catalyst.
2
holder, Rigaku Corporation) using a glove bag under N at-
mosphere to avoid exposing the samples to air. The particle
The products were analyzed using gas chromatographs
Shimadzu GC-2025 and GC-2015) equipped with an FID. An
size (d) on the catalysts was calculated by using Scherrer's
1
4
(
equation. The XRD patterns were fitted by some Gaussian
1
1,15
HP-FFAP capillary column (diameter 0.25 mm, 30 m) was
used for the separation of products in the liquid-phase such
as γ-butyrolactone (GBL), 1,4-butanediol (1,4-BuD), tetrahydro-
furan (THF), butyric acid (BA) and 1-butanol (BuOH). SUC
was analyzed using a high performance liquid chromato-
graph (Shimadzu Prominence) equipped with a UV-vis detec-
tor and a refractive index detector. An Aminex HPX-87H col-
umn (Bio-Rad) was used for the separation. A Porapak N
functions
expressed.
XAS was carried out at the BL01B1 station of SPring-8 with
the approval of the Japan Synchrotron Radiation Research
Institute (JASRI; proposal no. 2014A1119, 2014B1248,
2015B1068). The storage ring was operated at 8 GeV, and a Si
(111) single crystal was used to obtain a monochromatic
X-ray beam. Ion chambers for I0 were filled with 100% N2
and 100% Ar for Re L -edge and Pd K-edge measurements, re-
3
(diameter 3.0 mm, 3.0 m) packed column was used for the
2
spectively. Ion chambers for I were filled with 50% N + 50%
analysis of gaseous products such as CO , CH and C H . The
Ar and 100% Kr for Re L -edge and Pd K-edge measurements,
2
4
2
6
3
conversion, selectivity, averaged conversion rate (v) [mmol
respectively. The samples after the reaction and reduction
were prepared under the same conditions. After cooling, the
prepared catalyst powder was transferred to a commercially
−1
−1
−1
gcat h ] and quasi-formation rate of 1,4-BuD (r1,4-BuD) [h ]
are calculated on a carbon basis and defined in eqn (1)–(4):
(
1)
available polymer UV/Vis cell (UV-Cuvette micro 8.5 mm,
BRAND GMBH + CO KG) in a glove bag filled with nitrogen
to avoid exposure to air. The thickness of the cell filled with
the powder was 2 mm to give an edge jump of 0.4–1.1 and
(
2)
3)
3
0.4–1.0 for Re L -edge and Pd K-edge measurement, respec-
tively. The extended X-ray absorption fine structure (EXAFS)
data for Re L -edge were collected in a transmission mode
3
(
measurement, and those for Pd K-edge were collected in a
fluorescence mode measurement. Analyses of EXAFS and
X-ray absorption near edge spectral (XANES) data were
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performed using a computer program (REX2000 ver. 2.6.0,
Rigaku Corp.).
duction is the most effective in the hydrogenation of SUC. In
the case of hydrogenation of STA under similar reaction con-
1
1
X-ray photoelectron spectroscopy (XPS) experiments were
conducted with an AXIS-ULTRA DLD (Shimadzu Co., Ltd.)
using monochromatic Al Kα X-ray radiation (hν = 1486.6 eV)
operated at 20 mA and 15 kV at room temperature under
ditions (413 K, 8 MPa H
2
), the in situ liquid-phase reduced
catalyst, Re–Pd(inL, 413), also showed an averaged conversion
−
1
−1
rate of 4.5 mmol gcat
h
as high as that of the Re–Pd(exL,
cat−
−1 −1
1
−1
413) catalyst (3.9 mmol g
lar to that (5.4 mmol gcat h ) over the Re–Pd(exL, 413) cata-
h ), and the activity was simi-
−
8
1
0
Pa. The binding energy was calibrated with C 1s (284.6
eV). The catalysts after the reaction or reduction were trans-
ported to the analysis chamber in nitrogen atmosphere to
avoid any exposure to air. Analysis of XPS data was performed
by using the computer program CasaXPS ver. 2.3.15 (Casa
Software, Ltd.). XPS patterns were fitted by the Voigt func-
lyst in the hydrogenation of SUC. The difference in the suit-
able reduction method for the Re–Pd/SiO catalyst between
2
hydrogenation of STA and that of SUC can be derived from
the difference in the substrate (SUC or STA), which implies
that the presence of SUC suppresses the activation of the Re–
Pd catalyst with H2.
1
6
tion. Re 4f7/2 and Re 4f5/2 are separated by 2.43 eV and have
a relative peak area and a full width at half maximum
To confirm that Re–Pd/SiO catalysts are truly effective for
2
1
7
(FWHM) with a fixed ratio of 4 : 3 and 1 : 1, respectively.
the hydrogenation of SUC, the reaction was performed using
the ex situ liquid-phase reduced Re–M′/SiO (M′: Ru, Rh, Pd,
2
Ir and Pt; Re = 14 wt%, Re/M′ = 8) and M″–Pd/SiO (M″: Re,
2
Results and discussion
Mo, W and Ti; Pd = 1 wt%, M″/Pd = 8) catalysts (Table 2).
The monometallic Pd catalyst, Pd(exL, 413), showed no activ-
ity for the reaction (Table 2, entry 1). On the other hand, the
monometallic Re catalyst, Re(exL, 413), showed moderate ac-
tivity with high selectivity to GBL at 4 h (Table 2, entry 2).
The addition of noble metals (Ru, Rh, Pd, Ir and Pt) to the Re
catalyst enhanced the catalytic activity (Table 2, entries 3–7),
and these catalysts also showed high selectivities to GBL
(≥88%) at 4 h. On the other hand, the Re–Pd(exL, 413) cata-
lyst showed lower selectivities to hydrogenolysis products (BA
and BuOH) than the other catalysts (Table 2, entries 3–7). In
terms of the averaged conversion rate, Ir is a more effective
noble metal than Pd (Table 2, entries 5 and 6). To assess the
catalytic performance of the Re–Pd(exL, 413) and Re–Ir(exL,
413) catalysts, the activity test was conducted at longer reac-
tion time (24–96 h) (Table 2, entries 8–11). The Re–Pd(exL,
413) catalyst provided 1,4-BuD in 89% yield at 96 h (Table 2,
entry 8), but the Re–Ir(exL, 413) catalyst showed higher selec-
tivities to by-products such as n-butane and BuOH at 24–96 h
than the Re–Pd(exL, 413) catalyst (Table 2, entries 9–11) and
n-butane and BuOH became the major products at 96 h
(Table 2, entry 11). According to previous reports, rhenium
Catalytic performance of Re–Pd and related catalysts with
different reduction methods in hydrogenation of succinic
acid (SUC)
First, the effect of the reduction methods was investigated on
the catalysts of Re–Pd/SiO (Re = 14 wt%, Pd = 1 wt%, Re/Pd
2
=
2
8) and Re/SiO (Re = 14 wt%) in the hydrogenation of SUC
(Table 1) because SUC is different from STA in terms of acid-
ity and hydrophobicity. In situ liquid-phase reduction ((inL, x)
series, the catalysts were reduced in SUC + 1,4-dioxane), gas-
phase reduction ((G, x) series, the catalysts were reduced un-
der hydrogen flow) and ex situ liquid-phase reduction ((exL,
x) series, the catalysts were reduced in only 1,4-dioxane) were
applied in this reaction. The detailed reduction methods are
described in the experimental section. The Re–Pd(exL, 413)
catalyst showed a higher averaged conversion rate (v) (5.4
−
1
−1
mmol g
h ) than the Re–Pd(inL, 413) catalyst (0.8 mmol
cat
−1
−1
−1 −1
gcat h ) and Re–Pd(G, 473) catalyst (2.2 mmol gcat h )
(Table 1, entries 1–3) with high selectivity to GBL, which is
an intermediate of the formation of 1,4-BuD as discussed
1
c,5,6h
below.
On the other hand, THF, which can be produced
6
k
from GBL, was not observed in the case of all the catalysts,
although GBL was observed. In addition, a similar tendency
was also observed over the Re catalysts (Re-supported SiO2)
1
8b–e,g
oxide-modified noble metal catalysts, such as Rh–ReO ,
x
1
8k
18i,j
18a,f,h,l,m
Ru–ReO
x
,
Pt–ReO
x
and Ir–ReO
x
,
were effective
1
8a,b,f,h–l
(Table 1, entries 4–6). As a result, the ex situ liquid-phase re-
for the hydrogenolysis of C–O bonds in glycerol,
Table 1 Effect of the reduction methods on SUC hydrogenation over Re–Pd/SiO
2
and Re/SiO
2
catalysts
Selectivity/%
a
v /mmol
−
1
−1
Entry
Catalyst
Conv./%
GBL
1,4-BuD
THF
BA
BuOH
g
cat
h
1
2
3
4
5
6
Re–Pd(inL, 413)
Re–Pd(G, 473)
Re–Pd(exL, 413)
Re(inL, 413)
Re(G, 473)
Re(exL, 413)
3.9
10
26
<1
<1
7.0
94
94
96
—
—
94
1.8
5.1
3.0
—
—
3.9
0.0
0.0
0.0
—
—
0.0
3.8
0.8
0.9
—
—
1.6
0.0
0.0
0.3
—
—
0.9
0.8
2.2
5.4
<0.2
<0.2
1.5
SUC, succinic acid; GBL, γ-butyrolactone; 1,4-BuD, 1,4-butanediol; THF, tetrahydrofuran; BA, butyric acid; BuOH, 1-butanol. Reaction conditions:
5
8
wt% SUC solution 20 g (SUC 1 g, 1,4-dioxane 19 g), catalyst (Re = 14 wt%, Re/Pd = 8) amount 0.1 g, reaction temperature 413 K, H
.0 MPa, reaction time 4 h. Averaged conversion rate.
2
pressure
a
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Table 2 Results of SUC hydrogenation over various M″–M′/SiO
Paper
2
catalysts
Selectivity/%
a
v /mmol
−1
−1
Entry
M″–M′ catalyst
Time/h
Conv./%
GBL
1,4-BuD
THF
BA
BuOH
n-Butane
Others
g
cat
h
b
1
2
3
4
5
6
7
8
9
Pd(exL, 413)
Re(exL, 413)
4
4
4
4
4
4
4
96
24
48
96
4
4
4
4
<1
7.0
12
22
26
—
—
—
—
—
—
—
<0.2
1.5
2.5
4.6
5.4
7.0
4.4
—
—
—
—
<0.2
<0.2
0
94
95
94
96
88
94
3.1
56
4.0
0.1
—
3.9
2.0
3.0
3.0
6.4
2.9
89
27
32
0.0
—
0.0
0.0
0.0
0.0
0.3
0.0
0.2
0.7
1.2
0.7
—
1.6
2.1
2.4
0.9
4.2
2.7
0.0
2.0
0.0
0.0
—
0.9
0.7
0.8
0.3
1.5
0.7
7.6
12
54
22
—
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.6
6.8
60
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.0
1.7
17
Re–Ru(exL, 413)
Re–Rh(exL, 413)
Re–Pd(exL, 413)
Re–Ir(exL, 413)
Re–Pt(exL, 413)
Re–Pd(exL, 413)
Re–Ir(exL, 413)
Re–Ir(exL, 413)
Re–Ir(exL, 413) c
Ti–Pd(exL, 413)
33
21
100
>99
100
100
<1
<1
0
10
11
12
13
14
15
—
—
—
—
—
—
—
—
c
W–Pd(exL, 413)
Mo–Pd(exL, 413)
Re(exL, 413) + Pd(exL, 413)
—
—
>99
—
—
—
—
—
—
—
—
—
—
—
—
c
d
3.0
0.6
SUC, succinic acid; GBL, γ-butyrolactone; 1,4-BuD, 1,4-butanediol; THF, tetrahydrofuran; BA, butyric acid; BuOH, 1-butanol; Others, methane
and ethane. Reaction conditions: 5 wt% SUC solution 20 g (SUC 1 g, 1,4-dioxane 19 g), catalyst amount 0.1 g (M″ = 14 wt%, M″/M′ = 8, M′ = Ru,
a
b
Rh, Pd, Ir and Pt, M″ = Re, Mo, W and Ti; support = SiO
2
), reaction temperature 413 K, H
2
pressure 8.0 MPa. Averaged conversion rate. Pd =
c
d
1
wt%. Pd = 1 wt%, M″/Pd = 8. Physical mixture of ReO /SiO (0.1 g) and Pd/SiO (0.1 g) catalysts.
x
2
2
1
8d,e,g
tetrahydrofurfuryl alcohol
and so on in the aqueous
with high
molar ratios of Re to Ir such as Re/Ir = 1–3 showed high activ-
the Re–Pd(exL, 413) catalyst was determined to be the suit-
able catalyst for the hydrogenation of SUC to 1,4-BuD under
the reaction conditions (T = 413 K, PH2 = 8 MPa). In addition,
the Re and Pd amounts on the Re–Pd(exL, 413) catalyst were
optimized by changing the molar ratio of Re/Pd, which
resulted in the Re–Pd(exL, 413) catalyst with Re = 14 wt%
and Pd = 1 wt% (Re/Pd molar ratio = 8) being the most pref-
erable for the reaction (Fig. S1 and S2 in the ESI†). The opti-
phase. In addition, it is characteristic that Ir–ReO
x
1
8a
ity in glycerol hydrogenolysis
compared to the case of Rh–
1
8b–d
x
ReO .
This tendency suggests that the combination of Re
with Ir at high molar ratios of Re/Ir can result in high activity
for hydrogenolysis of C–O bonds compared to the combina-
tion with other noble metals. It should be noted that Ir–
ReO -based catalysts were effective in the formation of mono-
mized Re–Pd/SiO catalyst in the hydrogenation of SUC is the
same as that in the hydrogenation of STA.
x
2
1
9b,c,f
19d,f
11,12
alcohols and alkanes from cellulose,
hemi-cellulose,
1
9a,f
19e,f
sugar alcohols,
and sugars
by the hydrogenolysis of
multiple C–O bonds. In fact, the main products obtained for
the Re–Ir(exL, 413) catalyst in SUC hydrogenation were
strongly influenced by the reaction time, and the main prod-
uct changed from shorter to longer reaction time as follows:
GBL → 1,4-BuD → BuOH → n-butane. The reaction route of
Hydrogenation of various carboxylic acids over Re–Pd
catalysts with different reduction methods
The hydrogenation of several dicarboxylic acids and related
compounds was studied under the conditions of 413 K,
1
,4-BuD → BuOH → n-butane is due to C–O hydrogenolysis.
2
8 MPa H and 4 h using Re–Pd catalysts (Re = 14 wt%, Pd =
In contrast, it has been reported that Pd–ReO showed no ac-
1 wt%) (Table 3). In the case of the reactions of SUC (C4) and
glutaric acid (GLU, C5) over the Re–Pd(exL, 413) catalyst, the
corresponding lactone was produced with high selectivity
(Table 3, entries 1 and 3). On the other hand, in the case of
adipic acid (ADI, C6) and suberic acid (SUB, C8) over the Re–
Pd(exL, 413) catalyst, the corresponding terminal-hydroxyl
carboxylic acid (HCA) was produced with high selectivity
(Table 3, entries 4 and 6). However, the reactivity of the dicar-
boxylic acids decreased with increasing carbon number of di-
carboxylic acids (Table 3, entries 1, 3, 4 and 6). On the other
hand, in the case of monocarboxylic acids (hexanoic acid
(HXA, C6), octanoic acid (OCA, C8), STA (C18)), such a drastic
decrease in reactivity with respect to the carbon number was
not observed (Table 3, entries 8, 11 and 12). In addition, it
was also confirmed that the concentration of the monocar-
boxylic acid HXA did not influence its averaged conversion
rate strongly (Table 3, entry 10). These results indicate that
x
1
8a
tivity in glycerol hydrogenolysis at 393 K,
and this property
can be connected to the high selectivity to 1,4-BuD in the hy-
drogenation of SUC. These results indicate that the Pd metal
is a suitable additive for the synthesis of 1,4-BuD by hydroge-
nation of SUC. In addition, Ti–Pd, W–Pd and Mo–Pd catalysts
showed no activity for the hydrogenation of SUC (Table 2, en-
tries 12–14), which indicates that Re species are essential for
the reaction. Taking into consideration that the Pd(exL, 413)
catalyst showed no activity, it is concluded that Re species
are the main active species on the Re–Pd catalysts for the re-
action. Moreover, the physical mixture of Re(exL, 413) and
Pd(exL, 413) catalysts (weight ratio of Re(exL, 413)/Pd(exL,
4
4
13) = 1) showed much lower activity than the Re–Pd(exL,
13) catalyst (Table 2, entries 5 and 15), suggesting that the
high activity of the Re–Pd(exL, 413) catalyst can be caused by
Re species and Pd species on the same support. Therefore,
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Table 3 Hydrogenation of several carboxylic acids and related compounds over Re–Pd(exL, 413) and Re–Pd(inL, 413) catalysts
Selectivity/%
Carbon
a
v /mmol
−
1
−1
Entry
Substrate
SUC
number
Catalyst
Conv./%
Lactone
HCA
Diol
Monoalcohol
Others
g
cat
h
1
2
3
4
5
4
Re–Pd(exL, 413)
Re–Pd(inL, 413)
Re–Pd(exL, 413)
Re–Pd(exL, 413)
Re–Pd(inL, 413)
Re–Pd(exL, 413)
Re–Pd(inL, 413)
Re–Pd(exL, 413)
Re–Pd(inL, 413)
Re–Pd(exL, 413)
Re–Pd(exL, 413)
Re–Pd(exL, 413)
Re–Pd(inL, 413)
Re–Pd(exL, 413)
Re–Pd(exL, 413)
Re–Pd(exL, 413)
Re–Pd(exL, 413)
Re–Pd(exL, 413)
26
3.9
22
96
94
93
3.6
9.5
—
—
—
—
—
—
—
—
—
—
—
61
95
—
—
—
3.2
82
82
95
95
—
—
—
—
—
—
—
2.9
—
0.0
0.0
0
3.0
1.8
3.1
11
8.7
5.4
5.5
—
—
—
—
—
—
94
95
92
33
3.7
100
0.3
0.0
0.0
0.5
0.0
0.0
0.0
99
99
99
96
97
0.9
3.8
0.6
2.8
0.0
0.0
0.0
0.5
1.0
1.4
3.7
3.0
3.6
0.0
0.0
7.2
0.0
1.0
0.0
5.4
0.8
4.1
2.6
0.2
0.7
0.2
6.6
6.2
8.0
7.7
3.9
4.5
7.7
13
GLU
ADI
5
6
15
1.4
5.0
1.7
31
28
19
44
46
53
26
b
6
SUB
HXA
8
6
b
7
8
9
c
10
11
12
13
14
15
16
17
18
HXA
6
8
18
OCA
STA
d
d
96
GBL
DVL
2-HBA
DMS
DVL + SUC
4
5
4
6
5.8
1.7
1.2
5.3
0.2
0.0
51
8.4
10
1.9
1.6
4.7
0.6
5/4
22 (SUC)
2.4 (DVL)
HCA, hydroxycarboxylic acid; Others, monocarboxylic acid, gas products and heavy products; SUC, succinic acid; GLU, glutaric acid; ADI, adipic
acid; SUB, suberic acid; HXA, hexanoic acid; OCA, octanoic acid; STA, stearic acid; GBL, γ-butyrolactone; DVL, δ-valerolactone; 2-HBA,
2
-hydroxybutyric acid; DMS, dimethyl succinate. Reaction conditions: substrate 1 g, 1,4-dioxane 19 g, Re–Pd(exL, 413) (Re = 14 wt%, Re/Pd = 8)
a
b
c
2
0.1 g, temperature 413 K, initial H pressure 8 MPa, reaction time 4 h. Averaged conversion rate. Carbon balance = 80–90%. HXA 2 g.
d
The data are referred from ref. 11.
the carbon number of carboxylic acids affected the reactivity
only in the case of dicarboxylic acids. The decrease of the av-
eraged conversion rate in the hydrogenation of dicarboxylic
acids can be connected to the structure or adsorption state of
dicarboxylic acids.
stearate was less reactive than the corresponding stearic acid
over the Re–Pd(inL, 413) catalyst. To clarify the reason why
the lactone, a cyclic ester, is reactive over the Re–Pd(exL, 413)
catalyst, dimethyl succinate (DMS) was applied to the reac-
tion over the Re–Pd(exL, 413) catalyst, giving lower reactivity
than SUC (Table 3, entry 17). Therefore, the high reactivity of
lactones can be attributed not to the difference in the reduc-
tion method but to the strain and/or accessibility of the cyclic
structure. On the other hand, 2-hydroxybutyric acid (2-HBA)
as a model substrate of hydroxycarboxylic acid (HCA) showed
lower reactivity than lactones and simple carboxylic acids
(Table 3, entry 16).
The activity of the Re–Pd(exL, 413) catalyst (Table 3, en-
tries 1, 4 and 6) was compared with that of the Re–Pd(inL,
4
13) catalyst (Table 3, entries 2, 5 and 7) in the hydrogena-
tion of dicarboxylic acids such as SUC (C4), ADI (C6) and
SUB (C8). The Re–Pd(exL, 413) catalyst showed higher activity
than the Re–Pd(inL, 413) catalyst regardless of the carbon
number of dicarboxylic acids, which supports that ex situ
liquid-phase reduction (exL) is effective for the Re–Pd/SiO
2
The time dependence in the hydrogenation of SUC and
ADI over the Re–Pd(exL, 413) catalyst was investigated (Fig. 1,
Table S2 in the ESI†). In the case of SUC hydrogenation, GBL
was produced as an intermediate first and the highest GBL
yield (85%) was obtained at a reaction time of 24 h. GBL was
subsequently hydrogenated to 1,4-BuD and the highest
1,4-BuD yield (89%) was obtained at a reaction time of 96 h.
The selectivity to BuOH increased after GBL was consumed
(selectivity to GBL <1.0% at 96 h). Production of
4-hydroxybutyric acid was not observed at all. Therefore, the
hydrogenation of SUC to 1,4-BuD proceeds by two consecu-
tive reactions (Scheme 1(A)): (i) hydrogenation of SUC to GBL
and (ii) hydrogenation of GBL to 1,4-BuD. Another important
point is that the rate of the subsequent hydrogenation to
1,4-BuD is not as high as that of SUC hydrogenation, al-
though the reactivity of GBL is higher than that of SUC as
shown in Table 3. To clarify the cause of the low reactivity of
GBL, the hydrogenation of the mixture of SUC and DVL was
catalyst in the hydrogenation of dicarboxylic acids. On the
other hand, in the case of hydrogenation of C6 and C18
monocarboxylic acids (Table 3, entries 8, 9, 12 and 13 (11)),
the activity of the Re–Pd(exL, 413) catalyst was comparable to
that of the Re–Pd(inL, 413) catalyst, indicating that the effect
of the carbon number is small in the hydrogenation of mono-
carboxylic acids. Therefore, the reduction state of the catalyst
will be largely dependent on the existence of dicarboxylic
acids, and the two carboxylic acid groups in the dicarboxylic
acids may suppress the reduction of the catalyst.
Hydrogenation of related compounds such as lactones
and α-hydroxycarboxylic acid was also studied using the Re–
Pd(exL, 413) catalyst (Table 3, entries 14–16). The lactones
(GBL (C4) and δ-valerolactone (DVL, C5)) showed higher reac-
tivity (Table 3, entries 14 and 15) than the corresponding di-
carboxylic acids (Table 3, entries 1 and 3) to provide diols
1
2
with high selectivity (≥94%). In our previous work, methyl
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Fig. 1 Reaction time dependence in the hydrogenation of (A) SUC and (B) ADI over the Re–Pd(exL, 413) (Re/Pd = 8) catalyst. Reaction conditions:
SUC or ADI 1 g, 1,4-dioxane 19 g, catalyst amount 0.1 g, reaction temperature 413 K, H pressure 8.0 MPa. (A) ■: conversion; ○: γ-butyrolactone; △:
,4-butanediol; ×: butyric acid; +: 1-butanol. (B) ■: conversion; □: 6-hydroxyhexanoic acid; ○: ε-caprolactone; △: 1,6-hexanediol; ×: hexanoic acid;
2
1
+: 1-hexanol; ◊: others. SUC, succinic acid; ADI, adipic acid. The data were obtained through independent experiments at different reaction times.
Scheme 1 The reaction routes of (A) SUC hydrogenation and (B) ADI hydrogenation.
investigated (DVL + SUC, Table 3, entry 18). It is demon-
strated that the reactivity of DVL drastically decreases in the
presence of SUC. These results suggest that hydrogenation of
GBL on the Re–Pd(exL, 413) catalyst was suppressed by the
presence of SUC, which can explain the behavior of the reac-
tion time dependence (Fig. 1A). This result may be explained
by the stronger adsorption of the carboxylic acid than the lac-
tone, and a similar tendency was observed in our previous
work on hydrogenation of carboxylic acids, where hydro-
genolysis of alcohols was drastically suppressed in the pres-
ous studies, in the hydrogenation of GLU and ADI, Ru-based
catalysts such as Ru(2 wt%)–Sn(2.35 wt%)/Al O (ref. 6c) and
2
3
Ru(5 wt%)–Sn(3 wt%)–Re(5 wt%)/C (ref. 6l) also gave high
yields of 1,5-pentanediol (Ru–Sn/Al O : 75.6% and Ru–Sn–Re/
2
3
C: 98%) and 1,6-HxD (Ru–Sn/Al O : 89.4% and Ru–Sn–Re/C:
2
3
96%) under the conditions of T = 453–523 K and PH2 = 6.5–15
MPa. Therefore, these results indicate that the Re–Pd(exL,
413) catalyst can selectively hydrogenate dicarboxylic acids to
diols in high yields (71–89%) under relatively mild conditions
(T = 413 K, PH2 = 8 MPa).
1
1
ence of carboxylic acids. On the other hand, in the case of
ADI hydrogenation, 6-hydroxyhexanoic acid (6-HHA) was
mainly produced as an intermediate with 82% selectivity at
the initial reaction time of 4 h. The selectivity to 6-HHA de-
creased with increasing reaction time after the reaction time
of 4 h, and the selectivity to 1,6-HxD increased. The highest
To ascertain whether the reaction proceeds on the surface
of the Re–Pd catalyst, a leaching test was conducted. The hy-
drogenation of SUC was carried out under the same reaction
conditions for 1 h to achieve 6.4% conversion, and then the
catalyst was removed from the reaction mixture by filtration
(Fig. S3 in the ESI†). The filtrate was heated again at 8 MPa
1
,6-HxD yield of 74% was achieved at a reaction time of 96 h.
H for 2 h. The SUC hydrogenation reaction with the filtrate
2
Therefore, the hydrogenation of ADI to 1,6-HxD proceeds by
two consecutive reactions (Scheme 1(B)): (i) hydrogenation of
ADI to 6-HHA and (ii) hydrogenation of 6-HHA to 1,6-HxD.
Judging from the selectivity to 1,6-HxD at low ADI conversion,
the hydrogenation of 6-HHA to 1,6-HxD was not suppressed
by the presence of ADI in comparison with the hydrogenation
of SUC. In addition, hydrogenation of GLU provided DVL as
an intermediate first and then a high yield of 1,5-pentanediol
did not proceed further. These results indicate that SUC hy-
drogenation is catalyzed by the heterogeneous Re–Pd catalyst.
On the other hand, the leaching amount of Re and Pd after
the reaction (4 h) was found to be 3.5% and 0.4%, respec-
tively, by using inductively coupled plasma atomic emission
spectrometry (ICP-AES). The reusability of the catalyst was
also tested (Table S3 in the ESI†). The method was similar to
1
1
that for STA hydrogenation, and the detailed method is
(71%) was obtained in 120 h (Table S2 in the ESI†). In previ-
described in the ESI.† The averaged conversion rate (ν)
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decreased with increasing number of uses, while high selec-
tivity was maintained. The degree of activity decrease in SUC
hydrogenation is larger than that in STA hydrogenation.
same Re loading amount, and the kinetics of hydrogenation
of ADI and GLU over the Re–Pd(exL, 413) catalyst were inves-
tigated. The results were obtained under the conditions
where the conversion of the substrate was below 30%. Fig. 2
shows the effect of dicarboxylic acid (SUC, GLU and ADI) con-
centration on the activity of Re–Pd(exL, 413) and Re(exL, 413)
catalysts. The effect was investigated by changing the amount
of 1,4-dioxane and dicarboxylic acids (concentration: 3 to 30
wt%), and the detailed results are listed in Table S4 in the
ESI.† The reaction orders with respect to the SUC concentra-
tion over the Re–Pd(exL, 413) and Re(exL, 413) catalysts were
estimated to be +0.1 and +0.4, respectively, suggesting that
SUC is more strongly adsorbed on the Re–Pd(exL, 413) cata-
lyst than on the Re(exL, 413) catalyst. The presence of Pd can
1
1
The leaching amount of Re and Pd species in SUC hydroge-
nation was larger than that in STA hydrogenation. At present,
the decrease in averaged conversion rate is due to not only
leaching of the Re species but also structural changes in the
active sites because the structures of the active sites are sensi-
tive to reduction conditions as discussed in the following sec-
tion. Further investigation into the catalyst reuse method is
necessary for this catalyst system.
Kinetic studies of hydrogenation of various dicarboxylic acids
The effect of reaction temperature on the hydrogenation of
SUC over the Re–Pd(exL, 413) (Re/Pd = 8) catalyst was investi-
gated. The results are listed in Table 4. The averaged conver-
sion rate of the Re–Pd(exL, 413) catalyst increased with in-
creasing reaction temperature, while the high selectivity to
GBL (≥96%) was maintained at any reaction temperature.
The maximum 1,4-BuD yields over the Re–Pd(exL, 413) cata-
lyst at 413, 433 and 453 K were obtained by reaction time op-
timization and were determined to be 89, 84 and 74%, re-
spectively (entries 6–8). These results indicate that the
maximum 1,4-BuD yield decreased with increasing reaction
temperature. In addition, the quasi-formation rates of
1
,4-BuD (r1,4-BuD) from SUC at 413, 433 and 453 K, which were
calculated on the basis of the metal loading amount, were
−
1
0
.9, 2.3 and 5.3 h , respectively. The quasi-formation rates
6
a
over Re O (483 K, 25 MPa H ), Re(3.6 wt%)–Pd(2 wt%)/
TiO (433 K, 15 MPa) and Pd(1 wt%)–Re(4 wt%)/TiO (473
2 2
K, 6.9 MPa, liquid-flow reaction) are also listed in Table 4.
The Re–Pd(exL, 413) catalyst showed a similar or higher
quasi-formation rate under milder conditions, compared with
the reported catalysts.
2
7
2
6
h
Fig. 2 Effect of substrate concentration on the hydrogenation of
succinic acid (SUC), glutaric acid (GLU) and adipic acid (ADI) over the Re–
Pd(exL, 413) (Re/Pd = 8) and Re(exL, 413) catalysts. Reaction conditions:
6
j
3
–30 wt% dicarboxylic acid solution, 1,4-dioxane solvent 5.7–20 g, dicar-
boxylic acid 0.60–1.0 g, catalyst 0.060–0.10 g, reaction temperature
13 K, H pressure 8.0 MPa, reaction time 4 h. The detailed data are
4
2
The reaction kinetics of hydrogenation of SUC over the
Re–Pd(exL, 413) and Re(exL, 413) catalysts, which have the
shown in Table S4 .† ■: SUC (Re–Pd(exL, 413)); ♦: GLU (Re–Pd(exL, 413));
●: ADI (Re–Pd(exL, 413)); ×: SUC (Re(exL, 413)). ν: averaged conversion rate.
Table 4 Effect of reaction temperature on SUC hydrogenation over the Re–Pd(exL, 413) catalyst
Selectivity/%
/MPa Time/h Conv./% GBL 1,4-BuD THF BA
a
b
v /mmol
r
/h
1,4-BuD
−
1
−1
−1
Entry Catalyst
Temp./K
H
2
BuOH
0.0 0.0
g
cat
h
1
2
3
4
5
6
7
8
9
1
1
Re–Pd(exL, 413)
373
393
413
433
453
413
433
453
483
433
473
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
25
4
4
4
2.5
2
96
36
14
4
2.0
6.6
26
31
34
100
100
100
100
100
99
97
97
96
97
97
3.1
7.3
7.0
—
3.2
2.4
3.0
1.4
1.2
89
84
74
94
0.0
0.0
0.0
0.0
0.4
0.2
0.3
1.3
—
0.4
1.4
5.4
11
30
—
—
—
—
—
—
—
—
—
Re–Pd(exL, 413)
Re–Pd(exL, 413)
Re–Pd(exL, 413)
Re–Pd(exL, 413)
Re–Pd(exL, 413)
Re–Pd(exL, 413)
Re–Pd(exL, 413)
0.9 0.0
0.9 0.3
1.1 0.4
1.1 0.4
0.0 7.6
0.0 8.6
0.0 18
—
—
—
c
—
0.9
2.3
5.3
0.4
1.9
1.6
d
Re
3.6 wt% Re–2 wt% Pd/TiO
1 wt% Pd–4 wt% Re/TiO
O
2 7
6.0
—
—
e
f
0
1
2
15
48
—
—
—
83
86
—
—
2
6.9
—
SUC, succinic acid; GBL, γ-butyrolactone; 1,4-BuD, 1,4-butanediol; THF, tetrahydrofuran; BA, butyric acid; BuOH, 1-butanol. Reaction condi-
a
tions: SUC 1 g, 1,4-dioxane 19 g, catalyst 0.1 g (molar ratio of SUC (mol) to loading amount of total metal (mol) = 100) (entries 1–8). Averaged
b
c
d
conversion rate. Quasi-formation rate of 1,4-BuD; calculated on the basis of the total metal loading amount. Catalyst 0.05 g. The data are
e
listed from ref. 6a. Molar ratio of SUC (mol) to loading amount of total metal (mol) = 1.6. The data are listed from ref. 6h. Molar ratio of SUC
mol) to loading amount of total metal (mol) = 110. The data are listed from ref. 6j. This reaction was conducted under liquid flow (flow 5.6
mL h , catalyst 4.12 g).
f
(
−1
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be connected to the enhancement of SUC adsorption on the
catalyst surface, which was also supported by our previous
work on the hydrogenation of STA using the Re–Pd(inL, 413)
Pd(exL, 413) catalyst and that the produced hydride is the ac-
tive species for the hydrogenation of SUC.
1
1
1
1
Characterization of Re–Pd and related catalysts with different
catalyst. In addition, the reaction orders with respect to the
initial GLU and ADI concentration over the Re–Pd(exL, 413)
catalyst were estimated to be −0.1 and −0.5, respectively.
These results suggest that the dicarboxylic acids with longer
alkyl chains on the catalyst surface can decrease the activity
of the Re–Pd(exL, 413) catalyst, which can be connected to
the lower reactivity of dicarboxylic acids with longer alkyl
chains (Table 3, entries 6 and 8). One possible interpretation
is that the carboxylic groups of the long dicarboxylic acids
are easier to be adsorbed on the catalyst surface than those
reduction methods
Various characterization techniques (XRD, XPS and XAFS)
were applied to the catalysts in order to compare the struc-
tures of the Re–Pd/SiO
2
catalysts with different reduction
methods (exL, inL, G).
Fig. 4 shows the XRD patterns of Re–Pd and Re catalysts
with different reduction methods. The sharp peaks at 20.2°,
26.3° and 31.6° observed in Fig. 4d can be assigned to SUC
(ICDD 00-049-2106). The signal due to amorphous SiO
was
2
2
0
of the short dicarboxylic acids, which suppresses the ad-
observed around 20° in all XRD patterns. In our previous
1
1
sorption of H
acids.
2
by the effect of the bulkiness of dicarboxylic
work, the Re–Pd(inL, 413, Reaction) catalyst showed broad
signals around 2θ = 40° in the hydrogenation of STA (Fig. S4d
0
0
Fig. 3 shows the effect of H pressure over the Re–Pd(exL,
in the ESI†), which were assigned to Re IJHCP), Re IJFCC) and
2
0
0
4
13) and Re(exL, 413) catalysts using SUC, GLU and ADI as
Pd IJFCC). We proposed that Re is one of the main compo-
nents of the active species in the hydrogenation of STA. In
the hydrogenation of SUC, the Re–Pd(exL, 413, Reaction) cat-
alyst showed broad signals around 2θ = 40° and sharp signals
substrates, and the details are listed in Table S5 in the ESI.†
The reactions were performed in the range from 2 MPa to
8
2 2
MPa H . The reaction orders with respect to H pressure
0
were estimated to be about +1 (+1.0 to +1.2) in all cases. The
positive reaction orders indicate that the reaction rate was
strongly influenced by H pressure, meaning that the dissoci-
2
assignable to Re IJHCP) at 2θ = 37.8, 40.6, 42.9, 56.9 and 67.6°
(Fig. 4a). On the other hand, the Re–Pd(inL, 413, Reaction)
catalyst showed very small signals around 40° (Fig. 4b). The
Re–Pd(G, 413, Reaction) catalyst showed broad signals
around 40° (Fig. 4c). In the hydrogenation of ADI, the Re–
Pd(exL, 413, Reaction) catalyst also showed much stronger
broad signals around 40° and sharp signals for Re IJHCP)
than the Re–Pd(inL, 413, Reaction) catalyst (Fig. S4c and S5d
in the ESI†), and this tendency is similar to that in the hydro-
ation of H or the reaction of the dissociated hydrogen spe-
2
cies is involved in the rate-determining step. Considering
that the dissociation of H is very fast on the Pd or Re metal
2
0
surface, the reaction of the dissociated hydrogen species will
be the rate-determining step. According to our previous
1
1,18g,21
works,
the reaction order of almost +1 with respect to
H2 pressure can be interpreted that hydrogen is hetero-
lytically dissociated to proton (H ) + hydride (H ) on the Re–
genation of SUC. As for the Re/SiO catalysts, the Re(exL, 413,
2
+
−
Reaction) catalyst also showed broad signals around 2θ = 40°
0
and sharp signals for Re IJHCP) (Fig. 4d), but the Re(inL, 413,
Reaction) catalyst hardly showed broad and sharp signals
(
Fig. 4e). The Re(G, 473, Reaction) catalyst also showed broad
signals around 40° (Fig. 4f).
Fig. 3 Effect of H
2
pressure on the hydrogenation of succinic acid
(
SUC), glutaric acid (GLU) and adipic acid (ADI) over the Re–Pd(exL,
4
13) (Re/Pd = 8) and Re(exL, 413) catalysts. Reaction conditions: 5 wt%
Fig. 4 XRD patterns of the Re–Pd (Re/Pd = 8) and Re catalysts after
the reduction and the SUC reaction. (a) Re–Pd(exL, 413, Reaction), (b)
Re–Pd(inL, 413, Reaction), (c) Re–Pd(G, 473, Reaction), (d) Re(exL, 413,
Reaction), (e) Re (inL, 413, Reaction), (f) Re (G, 473, Reaction), (g) Re–
Pd(exL, 413) and (h) Re(exL, 413). Reaction conditions are described in
dicarboxylic acid solution 20 g (dicarboxylic acid 1 g, 1,4-dioxane 19 g),
catalyst amount 0.1 g, reaction temperature 413 K, reaction time 4 h
(
Re(exL, 413) 16 h), H
in Table S5.† ■: SUC (Re–Pd(exL, 413)); ♦: GLU (Re–Pd(exL, 413)); ●: ADI
Re–Pd(exL, 413)); ×: SUC (Re(exL, 413)). ν: averaged conversion rate.
2
pressure 2–8 MPa. The detailed data are shown
(
2
Table 1. Peak assignment: ○: SiO ; △: Re(HCP); ▽: Pd.
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On the other hand, a broad signal was observed around 40°
over both Re–Pd(inL, 413, Reaction) and Re–Pd(exL, 413, Reac-
tion) catalysts in the case of the hydrogenation of HXA and STA
Re–Pd(exL, 413) and Re–Pd(inL, 413, Reaction) catalysts. The
only signals due to Pd 3d5/2 and 3d3/2 core levels were also ob-
served at 335.1–335.3 and 340.5–340.7 eV, respectively (Fig. S7 in
the ESI ,† Table 6), for all the three reduced Re–Pd catalysts, and
(
Fig. S4e–h in the ESI†). Therefore, ReO species over the Re–
x
0
6g,i,11,22
Pd/SiO catalyst were reduced by the ex situ liquid-phase reduc-
these signals can be assigned to Pd species.
Therefore,
2
tion method, but not by the in situ liquid-phase reduction
method in the hydrogenation of dicarboxylic acids, although the
the Pd species on the surface of the catalysts was in the metallic
state. Fig. 5 and S8† show the XPS profiles of the Re 4f region,
and asymmetric signals were observed. The lines at 40.2–40.6,
reduction of ReO species took place by the in situ liquid-phase
x
0
reduction method in the hydrogenation of monocarboxylic
41.2–41.6, 42.3–42.7 and 45.4–45.8 eV were assigned to Re ,
3+
4+
6+
6g,i,11,17,22–24
acids. This result means that the reduction of ReO species was
Re , Re and Re , respectively.
These XPS data
x
suppressed by the presence of dicarboxylic acids. In addition,
were deconvoluted into these Re species in a manner similar
11
no ReO
x
species (ReO
2
, ReO
3
2 7
and Re O ) were observed in the
to that in our previous study. Table 6 lists the results of the
deconvolution of the XPS data. In the case of the Re–Pd(inL,
413, Reaction) and Re(inL, 413, Reaction) catalysts, the signals
range of 2θ = 20–30° for all the patterns, which indicates that
ReO species were amorphous or highly dispersed on SiO . In
x
2
4+
6+
order to clarify the state of the Re metal and Pd metal, the
broad signals were fitted using the signals of Pd metal,
can be almost deconvoluted into Re and Re species (area ra-
4+
6+
tio of Re + Re to total Re >94%) (Fig. 5D and S8D,† Table 6,
entries 1 and 4). On the other hand, in the case of the Re–
Pd(exL, 413, Reaction) and Re(exL, 413, Reaction) catalysts, the
0
0
Re (HCP) and Re (FCC) (Fig. S5 in the ESI†). The fitting analy-
sis was conducted in a manner similar to that in our previous
11
0
3+
study. The results of XRD fitting analysis are listed in Table 5.
signals can be deconvoluted into three components (Re , Re
0
4+
The particle sizes of Re (HCP) on the ex situ liquid-phase re-
and Re ) (Fig. 5F and S8F ,† Table 6, entries 3 and 6), and the
0
0
duced catalysts (8–9 nm) after the reaction are larger than those
Re is the major species on the catalyst surface (area ratio of Re
to total Re ≥58%). These results are supported by XRD analysis.
In addition, in the case of the Re–Pd(exL, 413) and Re(exL, 413)
catalysts, the signals can be deconvoluted into four components
on the catalysts reduced with other methods (2–3 nm) (entries
0
1
–6), and the larger Re (HCP) particles were also observed on
the catalysts after ex situ liquid-phase reduction (Fig. 4g and h
and Table 5, entries 7 and 8). These results indicate that the ex
situ liquid-phase reduction method can increase the particle size
0
3+
4+
6+
(Re , Re , Re and Re ) (Fig. 5E and S8E ,† Table 6, entries 2
0
3+
and 5) and the contribution of Re and Re was lower than that
after the reaction. These results indicate that further reduction
of the Re species proceeds slightly during the reaction.
0
of Re (HCP). Field emission scanning transmission electron
microscopy (FE-STEM) and energy-dispersive X-ray (EDX) analy-
sis of Re–Pd(exL, 413, Reaction) were also conducted (Fig. S6 in
the ESI†). Similar to the TEM image of Re–Pd(inL, 413, Reac-
XAS analyses were performed with the Re and Re–Pd cata-
lysts after the reduction and SUC reaction. The Pd K-edge
XANES patterns of all the catalysts were similar to that of the
Pd foil (Fig. S9 in the ESI†), which indicates that the Pd spe-
cies was reduced to the metallic state. This result was also
verified by Pd K-edge EXAFS analysis (Fig. S10 and Table S6
in the ESI†). The formation of the Pd metal was also
11
tion) reported in the previous work, small particles were ob-
served in Fig. S6(A) ,† which seems to agree with the results of
XRD analyses (Fig. 4 and Table 5), while large particles were not
almost observed. In addition, from the TEM-EDX analysis, Re
species were highly dispersed over the catalyst surface and were
located near the Pd metal particles, suggesting some interactions
3
supported by the XPS results. Re L -edge XANES analyses of
between the ReO and Pd metal species.
the Re–Pd and Re catalysts are shown in Fig. S11 in the ESI.†
x
In order to analyze the state of Re species on the catalyst sur-
face, XPS analysis was conducted. Fig. 5, S7 and S8† and Table 6
show the results of XPS analyses for the Re(exL, 413, Reaction),
Re(exL, 413), Re(inL, 413, Reaction), Re–Pd(exL, 413, Reaction),
The average valence of the Re species can be estimated from
the white line area in the Re L -edge XANES, and the
3
obtained average Re valence is listed in Table 7. The average
valences of the Re–Pd(inL, 413, Reaction), Re–Pd(exL, 413,
2
5
Table 5 Results of the fitting analyses of the XRD patterns of Re–Pd and Re catalysts after reduction and SUC hydrogenation
4
XRD area/10 degree × counts
Size of Re
metal (dRe)/nm
0
0
Total
Re IJHCP)
(42.7°)
Re IJFCC)
(40.0°)
Size of Pd
metal/nm
Entry
Pattern (Fig.)
Catalyst
HCP
FCC
a
1
4(a) and S5(a)
4(b) and S5(a)
Re–Pd(exL, 413, Reaction)
Re–Pd(inL, 413, Reaction)
Re–Pd(G, 473, Reaction)
Re(exL, 413, Reaction)
Re(inL, 413, Reaction)
Re(G, 473, Reaction)
Re–Pd(exL, 413)
7.2
11
6.5
—
—
—
8.9
1.7
2.1
8.1
3.2
2.6
7.1
8.5
1.8
1.4
2.9
1.7
—
3.3
1.9
1.8
17.3
3.2
18.8
11.7
1.0
16.0
15.9
9.1
5.0
1.6
14.3
6.8
1.0
12.5
5.7
12.3
1.6
4.5
5.0
—
3.5
10.2
3.2
a
2
a
3
4(c)
4(d)
4(e)
4(f)
4(g)
4(h)
a
4
a
5
a
6
7
8
7.2
—
Re(exL, 413)
5.8
a
Reaction conditions are described in Table 1.
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Fig. 5 The results of XPS for Re 4f over the Re–Pd catalysts ((A, D) Re–Pd(inL, 413, Reaction), (B, E) Re–Pd(exL, 413), (C, F) Re–Pd(exL, 413, Reaction)).
(
A–C): Raw data, (D–F): analysis results. Reaction conditions: 5 wt% SUC solution 20 g (SUC 1 g, 1,4-dioxane 19 g), catalyst amount 0.1 g, reaction tem-
0
3+
4+
6+
2
perature 413 K, H pressure 8.0 MPa, reaction time 4 h. Black line: raw spectra; black dotted line: 4f7/2 for Re , Re , Re and Re ; gray dotted line:
0
3+
4+
6+
4f5/2 for Re , Re , Re and Re ; gray broken line: the result of fitting; gray line: background. The fitted values are shown in Table 6.
Table 6 The results of XPS analyses of Re–Pd (Re/Pd = 8) and Re catalysts after reduction and SUC hydrogenation
0
S
0
S
3+
S
4+
S
6+
Pd
Re
Re
Re
Re
S
a
b
b
b
b
Spectra
Entry (Fig.)
B.E.
Re/Pd /eV
B.E. Area
B.E. Area
B.E. Area
B.E. Area
ReS,total
0
Catalyst
/eV
ratio/% /eV
ratio/% /eV
ratio/% /eV
ratio/% /Pd
S
c
1
2
3
4
5
6
S7A, 5A and 5D Re–Pd(inL, 413, Reaction)
S7B, 5B and 5E Re–Pd(exL, 413)
8
8
8
—
—
—
335.1
335.3 40.6 44
335.2 40.2 58
—
—
—
—
—
—
—
42.3 95
42.7 21
42.6 14
42.6 86
42.5 27
42.6 25
45.8 4.9
45.8 20
52
62
34
—
—
—
41.6 15
41.2 29
—
41.4 16
41.4 15
c
S7C, 5C and 5F Re–Pd(exL, 413, Reaction)
—
—
c
S8A and S8D
S8B and S8E
S8C and S8F
Re(inL, 413, Reaction)
Re(exL, 413)
40.3 6.2
40.4 41
40.3 60
—
45.8 8.0
45.4 16
c
Re(exL, 413, Reaction)
—
—
a
b
c
B.E.: binding energy for Pd 3d5/2
.
B.E.: binding energy for Re 4f7/2
.
Reaction conditions: 5 wt% SUC solution 20 g (SUC 1 g, 1,4-dioxane
2
19 g), catalyst amount 0.1 g, reaction temperature 413 K, H pressure 8.0 MPa, reaction time 4 h.
Reaction) and Re–Pd(G, 473, Reaction) catalysts were deter-
mined to be +3.7, +1.2 and +1.3, respectively (Table 7, entries
SUC reaction. On these samples, ReO, Re–O and Re–Re bonds
were detected. In particular, the valence of Re can also be esti-
mated approximately from the CN of ReO and Re–O (valence
5
, 7 and 8), which indicates that the degree of reduction of
11,18e,26
Re species on the Re–Pd(exL, 413, Reaction) and Re–Pd(G,
of Re = 2 × CNReO + 1 × CNRe–O).
of the Re valence from EXAFS analyses agreed well with the Re
valence from Re L -edge XANES as listed in Table 7. It is verified
The calculated values
4
4
73, Reaction) catalysts is higher than that on the Re(inL,
13, Reaction) catalyst.
3
Fig. 6 and Table 7 show the results of Re L -edge EXAFS
analysis of the Re and Re–Pd catalysts after the reduction and
that the local structure of the Re species on the in situ liquid-
phase reduced catalysts was more oxidized than that on the ex
3
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Table 7 Valence of Re determined from XANES analysis and curve fitting results for the Re L
3
-edge EXAFS analysis of Re–Pd (Re = 14 wt%, Pd = 1 wt%)
and Re (Re = 14 wt%) catalysts after reduction and SUC hydrogenationa
b
c
d
−1
e
−1
f
g
Entry
Catalyst
Valence of Re
3.8
Shells
CN
R /10 nm
σ /10 nm
ΔE
0
/eV
f
R /%
h
1
Re(inL, 413, Reaction)
ReO
Re–O
Re–Re
ReO
Re–O
Re–Re
Re–O
Re–Re
Re–O
Re–Re
ReO
Re–O
Re–Re
ReO
Re–O
Re–Re
Re–O
Re–Re
Re–O
0.3
3.3
1.3
0.3
1.1
7.4
1.1
8.0
1.6
7.3
0.3
3.3
1.4
0.2
1.5
7.2
1.1
6.2
1.7
7.4
4
1.77
2.06
2.60
1.76
2.07
2.73
2.08
2.75
2.06
2.75
1.76
2.08
2.60
1.74
2.04
2.70
2.13
2.67
2.09
2.73
1.73
2.74
0.094
0.096
0.076
0.095
0.093
0.060
0.093
0.060
0.092
0.061
0.095
0.098
0.060
0.095
0.095
0.095
0.099
0.099
0.096
0.074
0.060
0.060
−15.0
3.4
−11.1
−12.0
6.3
6.3
1.9
i
2
Re(exL, 413)
1.6
−0.3
h
3
Re(exL, 413, Reaction)
1.1
1.4
3.7
9.5
2.6
1.9
5.1
0.7
5.2
1.0
h
4
Re(G, 473, Reaction)
h
5
Re–Pd(inL, 413, Reaction)
Re/Pd = 8
−15.0
5.5
−11.8
13.2
4.9
−2.9
15.4
−7.2
8.0
0.4
0.0
0.0
i
6
Re–Pd(exL, 413)
Re/Pd = 8
1.6
1.5
h
7
Re–Pd(exL, 413, Reaction)
Re/Pd = 8
Re–Pd(G, 473, Reaction)
Re/Pd = 8
1.2
1.3
4.7
1.4
h
8
Re–Re
ReO
Re–Re
4
NH ReO
4
Re powder
12
a
b
c
d
Fourier filtering range: 0.092–0.350 nm. Valence of Re was determined from XANES analysis (Fig. S11). Coordination number. Bond
e
f
g
distance. Debye–Waller factor. Difference in the origin of photoelectron energy between the reference and the sample. Residual factor.
h
i
Reaction conditions: 5 wt% SUC solution 20 g (SUC 1 g, 1,4-dioxane 19 g), cat. 0.2 g, 413 K, 8 MPa, 2 h. The catalysts were only reduced by
the ex situ liquid-phase reduction method.
0
situ liquid-phase reduced catalysts, which is also supported by
the results of XRD and XPS. The CNRe–Re is determined by two
factors: the molar ratio of Re metal to total Re species and the
particle size of Re metal (Table S7 in the ESI†). The particle size
of Re metal can be obtained from the XRD results (Table 4).
Using these values, we determined the molar ratio of Re metal
to total Re. The calculation of bulk and surface molar ratio of
Re species was conducted using the results of the valence of
phase reduced catalysts. In addition, the amount of ReS species
on the in situ liquid-phase reduced catalysts was much smaller
than those on the other catalysts, which may lead to the low
intensity or absence of the signal of Re species in the results of
XPS analysis (Table 6). In our previous work, it was reported that
the amount of CO adsorbed on the Re–Pd(G, 473) catalyst is
lower than that of the calculated surface metals (Re + Pd ),
suggesting that the surface of Re and Pd particles on the Re–
Pd catalyst in STA hydrogenation was partially covered with Re
species. Therefore, it is suggested that, in the case of SUC hy-
drogenation, the surface of Re and Pd particles on the Re–Pd
catalysts can be also partially covered with Re species. In our
previous work, it was proposed that the catalytically active spe-
0
0
S
0
S
0
0
n+
n+
bulk Re (XANES), the molar ratio of Re species (XPS) and the
0
11
dispersion of Re particles (XRD). The calculation methods were
11
0
0
also the same as those used in our previous report, and the
n+
details are shown in the ESI (Table S7†). The order of the molar
0
ratio of Re to total Re (Table S7†) is as follows: Re–Pd(G, 473,
Reaction) (0.71) ≈ Re(exL, 413, Reaction) (0.69) ≈ Re–Pd(G, 473,
Reaction) (0.64) ≈ Re–Pd(exL, 413, Reaction) (0.63) ≫ Re–
Pd(inL, 413, Reaction) (0.11) ≈ Re(inL, 413, Reaction) (0.08), and
this trend agreed with the XRD peak area due to the Re metal as
listed in Table 5, assuming that the diffraction intensity of
cies on the Re–Pd(inL, 413) catalyst for hydrogenation of STA
0
n+ 11
can be formed by the combination of Re with Re . There-
0
n+
fore, the species composed of Re with Re will be also a cata-
lytically active one in the case of the hydrogenation of dicarbox-
3
+
4+
0
3+
ylic acids. Suitable molar ratios of (Re + Re )/Re
S
or (Re
+
0
0
4+
0
0
Re IJHCP) and Re IJFCC) is the same. Table 8 lists the estimation
Re )/(PdS + Re ) in terms of the generation of the active sites
S
of the surface metal amount determined by the above catalyst
for hydrogenation of dicarboxylic acids can be obtained on the
ex situ liquid-phase reduced catalysts. This interpretation is also
supported by the previous results in the hydrogenation of STA
(2.2) (Table 8, entry 9). In the case of the in situ reduced cata-
3+
4+
characterization. The ratio of the Re + Re amount to the total
3
+
4+
0
0
surface metal amount ((Re + Re )/(PdS + Re ); assuming
S
n+
n+
that Re
S
= Re because of the high dispersion) on the ex situ
3+
4+
0
0
liquid-phase reduced catalysts (≈1) is much lower than that on
the in situ liquid-phase reduced catalysts (>7) and is higher
than that on the Re–Pd(G, 473, Reaction) catalyst (0.38). These
results indicate that the degree of reduction of Re species on
the ex situ liquid-phase reduced catalysts is located between
those on the in situ liquid-phase reduced catalysts and gas-
S S
lysts, the surface molar ratio of (Re + Re )/(Re + Pd ) is
very high (>7) (Table 8, entries 1 and 5), which leads to low ac-
0
tivity because of the lack of Re species. In contrast, in the case
n+
of the Re–Pd(G, 473) catalyst, the ratio of Re to surface metals
(0.38) was lower than those for the other catalysts (Table 8, entry
n+
4), which also leads to low activity because of the lack of Re
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3
Fig. 6 Results of Re L
EXAFS oscillations. (II) Fourier transform of k -weighted Re L
calculated data (dotted line), Fourier filtering range: 0.092–0.350 nm. (a) Re powder, (b) Re(inL, 413, Reaction), (c) Re(exL, 413), (d) Re(exL, 413, Reaction),
e) Re(G, 473, Reaction), (f) Re–Pd(inL, 413, Reaction), (g) Re–Pd (exL, 413), (h) Re–Pd(exL, 413, Reaction), (i) Re–Pd(G, 473, Reaction) and (j) Re
3
-edge EXAFS analysis of Re–Pd (Re/Pd = 8) and Re (Re = 14 wt%) catalysts after the reduction and SUC reaction. (I) k -Weighted
3
−1
3
-edge EXAFS, FT range: 30–120 nm . (III) Fourier filtered EXAFS data (solid line) and
(
2 7
O .
Table 8 Estimation of the surface metal amount
Amount/mmol g
cat−
1
d
v /mmol
0a
S
b
S
b
S
0c
S
0
S
0
S
3+
4+
−1 −1
Entry Catalyst
Pdtotal Pd
Retotal ReIJHCP)
ReIJFCC)
Re
Pd
+ Re
Re + Re
g
cat
h
1
2
3
4
5
6
7
8
9
Re–Pd(inL, 413, Reaction) 0.094 0.0096 0.75
0.032
0.033
0.021
0.26
0.027
0.048
0.051
0.20
0.039
0.22
0.26
0.060
0.0
0.13
0.18
0.043
0.064
0.071 0.081
0.64
0.17
0.27
0.13
0.63
0.20
0.23
0.22
0.42
7.9
0.8
—
5.4
2.2
<0.2
—
1.5
<0.2
4.5
Re–Pd(exL, 413)
0.094 0.015
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.75
0.25
0.28
0.32
0.27
0.29
0.34
0.64
0.94
0.38
24
1.2
1.0
Re–Pd(exL, 413, Reaction) 0.094 0.016
Re–Pd(G, 473, Reaction)
Re(inL, 413, Reaction)
Re(exL, 413)
Re(exL, 413, Reaction)
Re(G, 473, Reaction)
0.094 0.015
—
—
—
—
—
—
—
—
0.027 0.027
0.17
0.23
0.24
0.19
0.17
0.23
0.24
0.20
0.9
2.2
e
Re–Pd(inL, 413, Reaction) 0.094 0.017
0.12
a
0
0
The amount of surface Pd metal atoms [Pd
S
] is calculated from both the dispersion (D) and total Pd metal atoms [Pdtotal ]. D was calculated
b
from the XRD particle size (dXRD, Table 5) by using the equation: dXRD [nm] = 1.12 (Pd)/D. The amount is determined from the molar ratio
described in Table S7. Re
after STA hydrogenation.
c
0
S
0
0
d
e
= Re (HCP)
S
+ Re (FCC)
S
.
Averaged conversion rate at <30% conversion (Table 1). The results for the catalyst
1
1
species. On the other hand, the Re(G, 473, Reaction) catalyst
is easily covered by ReO species. Therefore, the amount of the
x
0
n+
gave Re species with a comparable amount to Re species
exposed Re metal on the catalyst will be decreased in this case,
3+
4+
0
S
0
S
3+
4+
0
0
S
(
(Re + Re )/(Re + Pd ) = 0.9) (Table 8, entry 8). In our previ-
although the molar ratio of (Re + Re )/(PdS + Re ) was esti-
ous work, it was reported that the amount of Re metal on the
surface of the Re(G, 473) catalyst is much lower than that on the
Re–Pd(G, 473) catalyst from the results of CO adsorption amount
mated to be about +1.
From the above catalyst characterization results and the
discussion on the hydrogenation of dicarboxylic acids, the
structures of the Re–Pd(exL, 413, Reaction), Re–Pd(inL, 413,
11
of these catalysts, suggesting that the surface Re metal species
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4
+
0
0
Reaction) and Re–Pd(G, 473, Reaction) catalysts (Fig. 7) and
the reaction mechanism of hydrogenation of SUC over Re–
Pd(exL, 413) are proposed (Scheme S1 in the ESI†). In the
case of the Re–Pd(exL, 413, Reaction) catalyst, the particle
size of Pd is 7.2 nm (Table 5) and the surface of Pd particles
Re )/(Pd + Re ) = 7.9) (Fig. 7(C)). Dicarboxylic acids sup-
S S
press the reduction of Re species, although monocarboxylic
acids do not. At present, the mechanism of the suppression
of catalyst reduction by dicarboxylic acids is not elucidated,
and further investigation into the interaction between the di-
carboxylic acids and the Re (and Pd) species on the calcined
catalyst is necessary. In the case of the Re–Pd(G, 473, Reac-
tion) catalyst in the hydrogenation of dicarboxylic acids, the
n+
3+
4+
is partially covered with Re (Re + Re ). The amount ratio
0
0
3+
4+
6+
of Re IJHCP) : Re IJFCC) : Re : Re : Re was estimated to be
0
1
8 : 45 : 25 : 12 : 0.0, and the main species of Re is fcc. The
Re species form 1.8 nm Re IJFCC) particles and 8.9 nm
Re IJHCP) particles. The model structure is depicted in
Fig. 7(A) where the Re IJHCP) particles with a size of 8.9 nm
0
0
0
0
3+
4+
6+
ratio of Re (HCP) : Re (FCC) : Re : Re : Re was estimated to
0
0
be 54 : 17 : 7.2 : 10 : 12, and the Re species form 2.1 nm
0
0
0
Re (HCP) particles and 1.9 nm Re (FCC) particles. It is char-
3
+
4+
0
0
were not included because of their small contribution. The
S S
acteristic that the molar ratio of (Re + Re )/(Pd + Re ) on
the Re–Pd(G, 473, Reaction) catalyst is clearly lower than that
on the Re–Pd(exL, 413, Reaction) catalyst (Fig. 7(D)). The ra-
0
0
surface of both Re IJHCP) and Re IJFCC) particles can be also
n+
3+
4+
3+
partially covered with Re (Re + Re ) species ((Re
+
4+
0
0
3+
4+
0
0
Re )/(Pd
S
+ Re
S
) = 0.94). This structure is similar to the Re–
S S
tio of (Re + Re )/(Pd + Re ) can be dependent on the de-
Pd(inL, 413, Reaction) catalyst in the hydrogenation of STA
gree of reduction of Re species, and the medium level of ratio
can be connected to the good performance of the Re–Pd(exL,
413, Reaction) catalyst considering that the catalytically active
1
1
(
Fig. 7(B)). On the other hand, in the case of the Re–Pd(inL,
13, Reaction) catalyst in the hydrogenation of dicarboxylic
acids, the particle size of Pd is 11 nm (Table 5) and the sur-
4
0
n+
species consist of Re and Re . From the kinetic studies, the
n+
3+
n+
face of Pd particles is partially covered with Re (Re
+
main role of Re species is to promote the heterolytic disso-
4
+
0
0
3+
4+
Re ). The amount ratio of Re (HCP) : Re (FCC) : Re : Re
Re was estimated to be 5.3 : 5.3 : 0.0 : 85 : 4.0, and the Re
species was rather minor, although the Re species form 1.7
nm Re (HCP) particles and 1.4 nm Re (FCC) particles ((Re
:
2
ciation of H to produce reactive hydride species and protons
6+
0
0
n+
at the interface between Re and Re , and this role is also
0
suggested by the results of STA hydrogenation on Re–Pd(inL,
0
0
3+
11
+
413). In addition, the role of Pd is to promote the reduction
2
Fig. 7 Model structures of the Re–Pd/SiO catalysts. (A) SUC hydrogenation, Re–Pd(inL, 413, Reaction); (B) STA hydrogenation, Re–Pd(inL, 413,
1
1
Reaction); (C) SUC hydrogenation, Re–Pd(exL, 413, Reaction); (D) SUC hydrogenation, Re–Pd(G, 473, Reaction). Particle sizes are listed in Table 5,
and the ratios of the species are listed in Tables 8 and S7.†
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n+
0
of Re to Re and strengthen the interaction between the
carboxyl group and the catalyst surface as suggested in our
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1
previous report on STA hydrogenation. Scheme S1† illus-
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SUC to 1,4-BuD on the basis of the similarity of the reaction
behavior to that of STA hydrogenation and focuses on the
role of Re –Re active sites, with the role of Pd not included.
SUC or GBL are adsorbed on Re near the Re –Re interface
step I, V), and then they are hydrogenated by the hydride
species (step III, VII) which is produced from the heterolytic
1
1
0
n+
0
0
n+
(
dissociation of H at the interface (step II, VI).
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2
014, 98, 987–1000.
Conclusions
3 I. Fukawa, ARC Report, RS-978, 2014, https://www.asahi-
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The catalytic activity of Re–Pd/SiO and Re/SiO catalysts in
2
2
4
Myriant Corporation, Succinic Acid & Derivatives SBU, http://
www.myriant.com/pdf/myriant-succinic-acid-customer-presentation-
english.pdf, (accessed February 2016).
the hydrogenation of dicarboxylic acids is influenced by the
reduction methods. The ex situ liquid-phase reduction is
more effective than the in situ liquid-phase and gas-phase re-
ductions for the hydrogenation of dicarboxylic acids, while,
for the hydrogenation of monocarboxylic acids, in situ liquid-
phase reduction is also effective. High diol yields in the hy-
drogenation of dicarboxylic acids on the ex situ liquid-phase
reduced catalysts are obtained and the diols are formed via
lactones or hydroxycarboxylic acids as reaction intermediates.
Catalyst characterization indicates that the ex situ liquid-
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phase reduced Re–Pd/SiO has Pd , Re IJHCP), Re IJFCC), Re
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Considering that the combination of Re species with Re
species has been proposed as the active site, it is suggested
0
n+
n+
0
0
S
that the medium amount ratio of Re /(PdS + Re ) (= 0.94)
such as that obtained in the ex situ reduced catalysts is suit-
able for hydrogenation of carboxylic acids. In addition, the
n+
amount of Re species on the in situ liquid-phase reduced
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Acknowledgements
A part of this work is supported by the JSPS KAKENHI “Grant-
in-Aid for Scientific Research (A)” (26249121) and “Grant-in-
Aid for JSPS Fellows” (263554). We also appreciate the Techni-
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XPS measurements.
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