Hydrogenation of succinic acid to 1,4-butanediol over Re-Ru bimetallic catalysts supported on mesoporous carbon

Article abstract of DOI:10.1016/j.apcata.2014.11.029

A series of Re-Ru bimetallic catalysts supported on mesoporous carbon (denoted as (0.6 - x)Re-xRu/MC) were prepared by a single-step surfactant-templating method and a subsequent incipient wetness impregnation method with a variation of ruthenium loading (x, mol%), and they were applied to the liquidphase hydrogenation of succinic acid to 1,4-butanediol (BDO). The effect of metal content on the catalytic activities and physicochemical properties of (0.6 - x)Re-xRu/MC catalysts was investigated. It was found that a Re-Ru miscible phase was formed in the catalysts during the reduction process, and it was responsible for strong interaction between rhenium and ruthenium. It was also revealed that reducibility, metal dispersion, and oxidation state of (0.6 - x)Re-xRu/MC catalysts were affected by Re:Ru molar ratio. In particular, the oxidation state was closely related to the hydrogen adsorption behavior of the catalysts. The amount of weak hydrogen-binding sites increased with increasing the ratios of metallic rhenium (Re⁰) and ruthenium (Ru⁰) with respect to total metallic species in the reduced (0.6 -x)Re-xRu/MC catalysts. Catalytic performance in the hydrogenation of succinic acid to BDO(0.6-x)Re-xRu/MC showed a volcano-shaped trend with respect to Re:Ru molar ratio. This result was well correlated with the amount of weak hydrogen-binding sites of the catalysts. Among the catalysts tested, 0.3Re-0.3Ru/MC with the largest amount of weak hydrogen-binding sites showed the best catalytic performance in the BDO production by hydrogenation of succinic acid.

Full text of DOI:10.1016/j.apcata.2014.11.029

Applied Catalysis A: General 490 (2015) 153–162
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Hydrogenation of succinic acid to 1,4-butanediol over Re–Ru
bimetallic catalysts supported on mesoporous carbon
Ki Hyuk Kang, Ung Gi Hong, Yongju Bang, Jung Ho Choi, Jeong Kwon Kim, Jong Kwon Lee,
∗
Seung Ju Han, In Kyu Song
School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744,
South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 September 2014
Received in revised form
A series of Re–Ru bimetallic catalysts supported on mesoporous carbon (denoted as (0.6 − x)Re–xRu/MC)
were prepared by a single-step surfactant-templating method and a subsequent incipient wetness
impregnation method with a variation of ruthenium loading (x, mol%), and they were applied to the liquid-
phase hydrogenation of succinic acid to 1,4-butanediol (BDO). The effect of metal content on the catalytic
activities and physicochemical properties of (0.6 − x)Re–xRu/MC catalysts was investigated. It was found
that a Re–Ru miscible phase was formed in the catalysts during the reduction process, and it was respon-
sible for strong interaction between rhenium and ruthenium. It was also revealed that reducibility, metal
dispersion, and oxidation state of (0.6 − x)Re–xRu/MC catalysts were affected by Re:Ru molar ratio. In par-
ticular, the oxidation state was closely related to the hydrogen adsorption behavior of the catalysts. The
1
7 November 2014
Accepted 20 November 2014
Available online 26 November 2014
Keywords:
Hydrogenation
Succinic acid
0
1
,4-Butanediol
amount of weak hydrogen-binding sites increased with increasing the ratios of metallic rhenium (Re )
0
Bimetallic catalyst
Re–Ru catalyst
and ruthenium (Ru ) with respect to total metallic species in the reduced (0.6 − x)Re–xRu/MC catalysts.
Catalytic performance in the hydrogenation of succinic acid to BDO over (0.6 − x)Re–xRu/MC showed
a volcano-shaped trend with respect to Re:Ru molar ratio. This result was well correlated with the
amount of weak hydrogen-binding sites of the catalysts. Among the catalysts tested, 0.3Re–0.3Ru/MC
with the largest amount of weak hydrogen-binding sites showed the best catalytic performance in the
BDO production by hydrogenation of succinic acid.
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
,4-Butanediol (BDO) is a versatile chemical that can be used
BDO production moves toward the utilization of renewable energy
sources such as biomass. In this respect, conversion of succinic acid
to BDO by catalytic hydrogenation has attracted recent attraction
as a promising process, because succinic acid can be obtained from
bio-refinery process [5].
Hydrogenation of succinic acid to BDO occurs via two-step
hydrogenation reactions as shown in Fig. 1. Succinic acid is first
transformed into ␥-butyrolactone (GBL) by hydrogenation, and
then BDO or tetrahydrofuran (THF) is formed through consec-
utive hydrogenation of GBL [4]. For the catalytic conversion of
succinic acid, various noble metal catalysts such as Pd [7–9], Pt
[10], Rh [10], Ru [11,12], and Re [13] have been investigated. Among
these catalysts, rhenium has been considered as the most efficient
monometallic catalyst for the selective formation of BDO. However,
several studies have shown that rhenium alone was not sufficient
to obtain high yield for BDO [5,14–17].
1
in a wide range of industrial applications. BDO has been used as
an organic solvent and a fine chemical for production of adhesives,
fibers, and polyurethanes [1–4]. Recently, BDO has received much
attention as an important raw material for thermoplastic polymers
such as polybutylene succinate (PBS) and polybutylene terephtha-
late (PBT) [5]. As the consumption of these polymers is growing
faster in electronics and automobile industries, in particular, the
global demand for BDO is expected to increase rapidly.
BDO has been produced through several conventional routes:
hydrogenation of maleic anhydride [6], isomerization of propylene
oxide [6], and acetoxylation of butadiene [4]. These processes rely
on petrochemical feedstocks derived from fossil fuel. Due to the
limited amount of fossil fuel, however, current research trend for
In an attempt to improve BDO production by hydrogenation of
succinic acid, Re-based bimetallic catalysts, including Re–Pt/C [14],
Re–Pd/C [14,15], Re–Pd/TiO2 [16], and Re–Ru/C [15,17], have been
investigated. Nonetheless, the researches have rarely elucidated
∗ Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415.
E-mail address: inksong@snu.ac.kr (I.K. Song).
http://dx.doi.org/10.1016/j.apcata.2014.11.029
0
926-860X/© 2014 Elsevier B.V. All rights reserved.

1
54
K.H. Kang et al. / Applied Catalysis A: General 490 (2015) 153–162
Fig. 1. Reaction pathways for hydrogenation of succinic acid.
the effect of interaction between rhenium and other metal on the
selective formation of BDO from succinic acid. This is because com-
bination of rhenium and noble metal causes difficulty in structural
and chemical analyses. For example, rhenium can be miscible with
noble metals such as Pt, Pd, and Ru to form a solid-solution due to
their similar atomic sizes and surface energies [18–20], which com-
plicates characterization. Moreover, since rhenium does not cause
dissociative hydrogen chemisorption at low temperature [21,22],
either modified hydrogen chemisorption or CO chemisorption
method is essential for determining metal dispersion of Re-based
catalyst. In this respect, a systematic investigation on the effect
of interaction between rhenium and other metal on the catalytic
activities and physicochemical properties of Re-based bimetallic
catalyst would be worthwhile.
In this work, a series of Re–Ru bimetallic catalysts with different
metal content were supported on mesoporous carbon, and they
were applied to the liquid-phase hydrogenation of succinic acid
to BDO. The effect of metal content on the catalytic activities and
physicochemical properties of the catalysts was investigated. The
catalysts were characterized by nitrogen adsorption–desorption,
TPR, XRD, CO chemisorption, TEM, STEM-EDX mapping, XPS, and
two metals was fixed at 0.6 mol% in all the samples to maintain
the same number of active sites. The acetone solution containing
metal precursors was then introduced to MC by an incipient wet-
ness impregnation method. After drying the impregnated sample
◦
◦
at 60 C, it was calcined at 500 C for 4 h with a heating rate of
◦
5 C/min under N flow (50 ml/min) to remove chlorine and organic
2
impurities. The prepared Re–Ru bimetallic catalysts were denoted
as (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6), where x rep-
resented the ruthenium content (mol%).
2.2. Characterization
Textural properties of calcined (0.6 − x)Re–xRu/MC (x = 0, 0.15,
0.3, 0.45, and 0.6) catalysts were investigated by nitrogen
adsorption–desorption measurements using a BELSORP-mini II
instrument (BEL Japan). Surface areas of the catalysts were cal-
culated by the Brunauer–Emmett–Teller (BET) method. The Re:Ru
molar ratios of the catalysts were determined by ICP-MS analy-
ses (ELAN 6100, Perkin-Elmer SCIEX). Temperature-programmed
reduction (TPR) analyses of the calcined catalysts were conducted
in a flow reactor system equipped with a quartz reactor. 10 mg of
◦
the catalysts was pretreated with N flow (50 ml/min) at 200 C for
2
H -TPD analyses.
2
1
h, and subsequently, TPR profiles were obtained using a thermal
2
. Experimental
conductivity detector (TCD) at temperatures ranging from room
◦
◦
temperature to 600 C with a heating rate of 5 C/min under 5%
H /N flow (50 ml/min). X-ray diffraction (XRD) patterns of the
2.1. Preparation of (0.6 − x)Re–xRu/MC catalysts
2
2
reduced catalysts were collected by a D-Max2500-PC diffractome-
ter (Rigaku) using Cu-K␣ radiation (ꢀ = 1.541 A˚ ) operated at 50 kV
A series of Re–Ru bimetallic catalysts supported on mesoporous
carbon were prepared by a single-step surfactant-templating
method and a subsequent incipient wetness impregnation method.
For the preparation of mesoporous carbon (MC), 5 g of P123 copoly-
mer (Sigma–Aldrich) was dissolved in deionized water (130 ml)
at room temperature under vigorous stirring. 2.1 g of sucrose
and 100 mA. In order to investigate the metal surface area, metal
dispersion, and average metal particle size of the reduced catalysts,
CO chemisorption experiments were performed using a BELCAT-B
instrument (BEL Japan). 10 mg of calcined catalyst was reduced at
◦
◦
500 C for 4 h with a heating rate of 5 C/min, and then 5% CO/He
◦
(
(
Sigma–Aldrich) as a carbon precursor and 20 ml of HCl solution
35%) were then added into the solution. Subsequently, 1.9 ml
mixed gas was periodically injected at 100 C. Metal surface area,
metal dispersion, and average metal particle size were calculated
from the amount of carbon monoxide adsorbed on the reduced cat-
alyst by assuming that one carbon monoxide molecule occupies
one surface metal atom. Morphology and particle size distribution
of the reduced catalysts were examined by transmission electron
microscopy (TEM) analyses (JEM-3010, JEOL). The particle size was
calculated on the basis of projected area of particle in the TEM
image by assuming that the shape of metal particle is sphere. The
projected area, A, was converted to particle diameter, D, using
of H SO solution (95%) was added into the solution to pro-
2
4
mote later cross-linkage of P123 with tetraethoxysilane (TEOS).
After maintaining the solution with stirring for 1 h, 9.3 ml of TEOS
(
Sigma–Aldrich) as a structure-directing agent was slowly added
◦
into the mixed solution. The resulting solution was stirred at 37 C
for 24 h, and it was then kept at 100 C for 24 h without stirring
◦
for self-assembly of micelle structure. The resultant was dried at
◦
◦
1
00 C for 48 h, and then carbonized at 800 C for 4 h at a heat-
◦
1/2
ing rate of 5 C/min in a nitrogen stream (50 ml/min). The obtained
carbon–silica composite was then treated with 300 ml of HF solu-
tion (5%) for 24 h to remove silica template, and it was finally
filtered and dried. The resulting mesoporous carbon was denoted
as MC.
D = 2(A/ꢁ) . To confirm the detailed distribution of rhenium and
ruthenium of the reduced catalysts, energy dispersive X-ray spec-
troscopy (EDX) mapping analyses were conducted using a scanning
transmission electron microscopy (STEM) apparatus (JEM-2100F,
JEOL). Binding energies and surface atomic compositions of rhe-
nium and ruthenium in the reduced catalysts were examined by
X-ray photoelectron spectroscopy (XPS) analyses using a AXIS-HSI
instrument (KRATOS) equipped with a Mg/Al anode source. For
XPS analyses, the calcined catalysts were reduced using an ex situ
For co-impregnation of rhenium and ruthenium onto MC sup-
port, known amounts of ReCl5 (Sigma–Aldrich) and RuCl ·xH O
3
2
(
Sigma–Aldrich) as metal precursors were dissolved in 5 ml of ace-
tone. During this process, Re:Ru molar ratio was adjusted to be 1:0,
.75:0.25, 0.50:0.50, 0.25:0.75, and 0:1, while the total loading of
◦
0
reduction system at 500 C for 4 h under 5% H /N flow (50 ml/min),
2
2

K.H. Kang et al. / Applied Catalysis A: General 490 (2015) 153–162
155
and the catalysts were then transported to glass jar with sample
holder in argon atmosphere glove box to minimize air exposure.
After outgassing the glass jar in a vacuum oven, the sample holder
was transferred to the XPS chamber as quickly as possible. All
the XPS spectra were calibrated using C 1s peak (284.5 eV) as a
reference. H temperature-programmed desorption (H -TPD) anal-
0
.6Re/MC
0.45Re-0.15Ru/MC
0.3Re-0.3Ru/MC
0
0
.15Re-0.45Ru/MC
.6Ru/MC
2
2
yses of the reduced catalysts were conducted using a BELCAT-B
instrument (BEL Japan). 10 mg of calcined catalyst was reduced at
◦ ◦
00 C for 4 h with a heating rate of 5 C/min under 5% H /Ar flow
2
5
(
50 ml/min), and then purged with Ar flow (50 ml/min) for 10 min
◦
◦
at 500 C. After cooling the reduced catalyst to 200 C under Ar
flow (50 ml/min), 5% H /Ar mixed gas (50 ml/min) was injected for
2
◦
◦
3
0 min at 200 C. To remove physisorbed hydrogen, the sample was
purged at 200 C with Ar flow (50 ml/min), and subsequently, H -
2
TPD measurements were conducted within temperature range of
◦
◦
2
00–700 C at a heating rate of 5 C/min under Ar flow (50 ml/min).
2.3. Hydrogenation of succinic acid to BDO
Liquid-phase hydrogenation of succinic acid to BDO was con-
ducted in a stainless steel autoclave reactor with a volume of
0
0.2
0.4
0.6
0.8
1.0
2
00 ml. Prior to the reaction, the catalysts were reduced using an
Relative pressure (P/P0)
◦
ex situ reduction system at 500 C for 4 h with a heating rate of
5
◦
C/min under 5% H /N2 flow (50 ml/min). In order to avoid air
2
Fig. 2. Nitrogen adsorption–desorption isotherms of (0.6 − x)Re–xRu/MC (x = 0, 0.15,
.3, 0.45, and 0.6) catalysts.
exposure, reduced catalyst (0.1 g), succinic acid (0.25 g), and 1,4-
dioxane (50 ml, an inert aprotic solvent) were charged into the
reactor in an argon atmosphere glove box. The closed reactor filled
with argon was then mounted to the autoclave chamber as quickly
as possible. After purging the reactor with nitrogen, it was pressur-
ized up to 50 bar using hydrogen. The sealed autoclave was heated
0
values. Total metal loadings were also well fixed at 0.6 mol%. All
the catalysts retained high surface area (>812 m /g), large pore
volume (>1.06 cm /g), and large average pore diameter (>5.0 nm),
which means that mesoporous carbon structure were successfully
formed by a single-step surfactant-templating method. Interest-
ingly, surface area and pore volume of the catalysts decreased with
increasing rhenium loading. This might be because pore blockage
by rhenium was more severe than that by ruthenium due to the
difference in atomic radius of rhenium (137 pm) and ruthenium
2
3
◦
to the reaction temperature (200 C), and then pressurized up to
8
0 bar using hydrogen. Catalytic reaction was conducted with con-
stant stirring (700 rpm) for 7 h. After the reaction, the reactor was
cooled to room temperature and depressurized. Reaction products
were analyzed with a gas chromatograph (Younglin, ACME-6100)
equipped with a flame ionization detector (FID). Conversion of
succinic acid, selectivity for product, and yield for product were
calculated according to the following equations.
(134 pm).
mole of succinic acid reacted
mole of succinic acid supplied
3.2. Reduction behaviors of (0.6 − x)Re–xRu/MC catalysts
Conversion of succinic acid (%) =
Selectivity for product (%) =
× 100
(1)
mole of product formed
mole of succinic acid reacted
In order to investigate the reduction behaviors of
(0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts,
TPR measurements were conducted as shown in Fig. 3. It was
found that 0.6Re/MC and 0.6Ru/MC catalysts exhibited asym-
× 100
(2)
(3)
conversion of succinic acid × selectivity for product
Yield for product (%) =
100
◦
◦
Turnover frequency (TOF) and TOF for BDO (TOFBDO) were cal-
culated on the basis of moles of succinic acid converted at ca. 10%
conversion and moles of BDO formed at ca. 70% yield for GBL,
respectively. The moles of surface metal atoms used for TOF and
TOFBDO calculation were obtained by CO chemisorption experi-
ments.
metrical reduction peaks at 340 C and 264 C, respectively; the
former was attributed to the reduction of rhenium species [24],
while the latter was attributed to the reduction of ruthenium
species [25]. The reduction peak of 0.45Re–0.15Ru/MC catalyst
moved toward lower temperature and showed more narrow shape
than that of 0.6Re/MC. In case of bimetallic catalysts containing
noble metals such as Pd, Pt, and Ru, the noble metals can affect
the reduction of the other metal due to hydrogen transfer from
their reduced species [26–29]. Therefore, it can be inferred that
hydrogen adsorbed on reduced ruthenium species was transferred
to unreduced neighboring rhenium species during the reduction
process, which promoted the reduction of rhenium species. It
is interesting to note that (0.6 − x)Re–xRu/MC (x = 0.15, 0.3, and
0.45) catalysts retained only one reduction peak corresponding to
co-reduction of rhenium and ruthenium without any reduction
peaks for bulk rhenium and ruthenium species, indicating that
most of ruthenium atoms were distributed in the periphery of
unreduced rhenium atoms throughout the catalyst. Furthermore,
the reduction bands of (0.6 − x)Re–xRu/MC (x = 0.15, 0.3, and 0.45)
catalysts were changed to the symmetrical shape compared to
those of 0.6Re/MC and 0.6Ru/MC. It is known that the symmetrical
peak shape can be induced from uniform reduction process caused
3
. Results and discussion
3
.1. Textural properties of (0.6 − x)Re–xRu/MC catalysts
Textural properties of (0.6 − x)Re–xRu/MC (x = 0, 0.15,
0
.3, 0.45, and 0.6) catalysts were examined by nitrogen
adsorption–desorption measurements as shown in Fig. 2. All
the catalysts exhibited IV-type isotherms indicative of meso-
porous structure. H3-type hysteresis loops were also observed in
the isotherms, which were attributed to the capillary condensation
of nitrogen molecules in the well-developed mesopores of carbon
support [23]. Detailed textural properties of (0.6 − x)Re–xRu/MC
(
x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts are listed in Table 1. It
was found that the actual Re:Ru loadings and molar ratios of the
prepared catalysts were in good agreement with the designed

1
56
K.H. Kang et al. / Applied Catalysis A: General 490 (2015) 153–162
Table 1
Textural properties of (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts.
Re:Ru loading (mol%)a
Re:Ru molar ratio
Surface area (m /g)
2
b
Pore volume (cm /g)c
3
Pore diameter (nm)d
Sample
0
0
0
0
0
.6Re/MC
0.59:0
1:0
812
838
861
899
923
1.06
1.07
1.14
1.18
1.19
5.2
5.1
5.3
5.3
5.0
.45Re–0.15Ru/MC
.3Re–0.3Ru/MC
.15Re–0.45Ru/MC
.6Ru/MC
0.44:0.15
0.29:0.31
0.15:0.45
0:0.6
0.74:0.26
0.49:0.51
0.25:0.75
0:1
a
b
c
Determined by ICP-MS measurement.
Calculated by the BET equation.
Total pore volume at P/P0 = 0.99.
Average pore diameter.
d
Table 2
TPR and CO chemisorption results of (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts.
Catalyst
Amount of hydrogen uptake
mol-H2/mol-metal)a
Metal surface area
(m /g-catalyst)
Metal dispersion (%)b
Average metal particle
size (nm)
2
b
b
(
0
0
0
0
0
.6Re/MC
2.73
3.29
4.10
3.50
3.03
3.15
3.62
4.74
4.09
3.36
17.6
20.3
26.9
23.2
19.2
7.9
6.8
5.1
5.9
6.9
.45Re–0.15Ru/MC
.3Re–0.3Ru/MC
.15Re–0.45Ru/MC
.6Ru/MC
a
Calculated from peak area of TPR profiles in Fig. 3.
Calculated from CO chemisorption measurement by assuming a stoichiometry factor of CO/metalatom = 1.
b
by homogeneity of metal particle size [30]. Thus, it is inferred
that the particles of both rhenium and ruthenium species were
homogeneously formed in the Re–Ru bimetallic catalysts. The
amounts of hydrogen uptake calculated from the TPR profiles of
[32]. Therefore, it is difficult to determine the reduction degree of
(0.6 − x)Re–xRu/MC (x = 0.15, 0.3, 0.45, and 0.6) catalysts by only
TPR results, and this issue will be further discussed in Section 3.5.
(
0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts are
summarized in Table 2. It was found that (0.6 − x)Re–xRu/MC
x = 0.15, 0.3, and 0.45) catalysts showed the larger amount of
3
.3. Crystalline structures of reduced (0.6 − x)Re–xRu/MC
catalysts
(
hydrogen uptake than 0.6Re/MC and 0.6Ru/MC, indicating that the
interaction between rhenium and ruthenium species might mod-
ify the reducibility of both metal species. However, the amount
of hydrogen uptake of (0.6 − x)Re–xRu/MC (x = 0.15, 0.3, 0.45,
and 0.6) catalysts determined by TPR measurements exceeded
the theoretical ratio of hydrogen with respect to rhenium (3.5)
and ruthenium (2) for complete reduction [31]. This was due to
hydrogen spillover induced by metal surface on carbon material
Crystalline phases of the reduced (0.6 − x)Re–xRu/MC (x = 0,
.15, 0.3, 0.45, and 0.6) catalysts were examined by XRD measure-
ments as presented in Fig. 4. All the catalysts showed diffraction
peaks for graphitic carbon structure at 2ꢂ = 23.5 and 43.8 [33]. The
diffraction peaks corresponding to (1 0 1) planes of metallic rhe-
nium (dashed line in Fig. 4) and ruthenium (solid line in Fig. 4) were
0
◦
◦
Re (101)
Ru (101)
0
.6Re/MC
0
0
.45Re-0.15Ru/MC
0
.3Re-0.3Ru/MC
.15Re-0.45Ru/MC
0
.6Ru/MC
20
30
40
50
60
70 80
2
Theta (degree)
Fig. 4. XRD patterns of reduced (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6)
Fig. 3. TPR profiles of (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts.
catalysts.

K.H. Kang et al. / Applied Catalysis A: General 490 (2015) 153–162
157
Fig. 5. TEM images and particle size distributions of reduced (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts.
clearly observed in the XRD patterns of 0.6Re/MC and 0.6Ru/MC
catalysts, respectively, although the diffraction peak of 0.6Ru/MC
was overlapped with that (43.8 ) of the carbon support. On the
and ruthenium particles were finely dispersed in the mesoporous
carbon framework. It is noteworthy that average metal particle
sizes of (0.6 − x)Re–xRu/MC (x = 0.15, 0.3, and 0.45) catalysts were
smaller than those of 0.6Re/MC and 0.6Ru/MC. This might be
because aggregation of each rhenium and ruthenium particle was
effectively suppressed by forming a Re–Ru metallic bond during
the reduction process, as discussed in the XRD results.
◦
other hand, any diffraction peaks for rhenium and ruthenium
species were not detected in the (0.6 − x)Re–xRu/MC (x = 0.15, 0.3,
and 0.45) catalysts. This result might be because particle sizes of
metallic rhenium and ruthenium in the catalysts were too small
to be detected by XRD. According to the literatures [18–20], rhe-
nium and ruthenium species can interact in all range of chemical
composition due to their similar crystal structure, atomic radius,
and electronegativity. Thus, it is inferred that the interaction
between rhenium and ruthenium species affected the develop-
ment of particles during the reduction process, which might induce
the decrement of metal particle size of Re–Ru bimetallic catalysts
compared to rhenium or ruthenium monometallic catalyst.
The above result was further confirmed by TEM analyses. Fig. 5
shows the TEM images and particle size distributions of reduced
(0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts. All
the catalysts retained an ordered mesoporous carbon structure
and well-dispersed metal particles as presented in the TEM
images. However, (0.6 − x)Re–xRu/MC (x = 0.15, 0.3, and 0.45)
catalysts showed more narrow particle size distributions than
0.6Re/MC and 0.6Ru/MC, indicating that metal particles of Re–Ru
bimetallic catalysts were uniformly distributed, as discussed in
the TPR results. It is noteworthy that all the reduced catalysts
retained a few metal particles which were larger than pore sizes
of the calcined catalysts (Table 1). This means that some metal
particles grown during the reduction process caused the partial
pore blockage of carbon supports. It was revealed that average
metal particle size measured by TEM decreased in the order of
0.6Re/MC > 0.6Ru/MC > 0.45Re–0.15Ru/MC > 0.15Re–0.45Ru/MC >
0.3Re–0.3Ru/MC, which was well consistent with the trend of
average metal particle size determined by CO chemisorption.
However, average metal particle sizes of all catalysts determined
by CO chemisorption were larger than those measured by TEM.
This overestimation might be because the catalysts were partially
3.4. Metal dispersion of reduced (0.6 − x)Re–xRu/MC catalysts
To investigate the effect of Re–Ru interaction on the metal
dispersion, CO chemisorption measurements for the reduced
(
0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts were
conducted. As listed in Table 2, metal surface area and metal dis-
persion increased in the order of 0.6Re/MC < 0.6Ru/MC < 0.45Re–
0.15Ru/MC < 0.15Re–0.45Ru/MC < 0.3Re–0.3Ru/MC, while aver-
age metal particle size decreased in the order of 0.6Re/MC > 0.6Ru/
MC > 0.45Re–0.15Ru/MC > 0.15Re–0.45Ru/MC > 0.3Re–0.3Ru/MC.
It was found that all the catalysts retained small metal particle
size in the range of 5.1–7.9 nm, indicating that metallic rhenium

1
58
K.H. Kang et al. / Applied Catalysis A: General 490 (2015) 153–162
Fig. 6. STEM-EDX images of reduced 0.3Re–0.3Ru/MC catalyst obtained by mapping on rhenium and ruthenium. (For interpretation of the references to color in text near
the figure citation, the reader is referred to the web version of this article.)
◦
were divided into Re (40.6 eV), Re3+ (41.4 eV), Re4+ (42.3 eV), and
0
reduced under the reduction condition at 500 C for 4 h. Among the
6+
catalysts, 0.3Re–0.3Ru/MC showed the largest metal dispersion
and smallest average metal particle size.
Re (45.3 eV) species [36,37]. On the other hand, the Ru 3p3
spectra were assigned to metallic Ru (462.1 eV) and Ru
/2
4+
0
In order to confirm the distribution of rhenium and ruthenium
species, STEM-EDX analyses were carried out. Fig. 6 showed the
STEM and EDX mapping images for one metal particle (white arrow
in Fig. 6) of reduced 0.3Re–0.3Ru/MC catalyst. Rhenium and ruthe-
nium species were distinguishable in the EDX mapping images. It
is noticeable that rhenium atom (blue dot in Fig. 6) and ruthenium
atom (red dot in Fig. 6) were co-presented in the metal particle
domain of 0.3Re–0.3Ru/MC catalyst. On the basis of TPR, XRD, and
EDX mapping results, it is believed that a Re–Ru miscible phase
was formed in the reduced 0.3Re–0.3Ru/MC catalyst and this was
responsible for strong interaction between metallic rhenium and
ruthenium species.
(464.3 eV) species [38]. It was found that the areas of Re 4f and
Ru 3p3/2 spectra decreased with decreasing the rhenium and
ruthenium loading, respectively. On the basis of deconvoluted
peak areas of Re 3f7/2 and Ru 3p3/2 spectra, the oxidation state
ratios and surface atomic ratios of rhenium and ruthenium
were quantified as summarized in Table 3. It was found that
0
Re /Re
increased in the order of 0.6Re/MC < 0.15Re–0.45Ru/MC
total
0
< 0.45Re–0.15Ru/MC < 0.3Re–0.3Ru/MC,
while
Ru /Ru
total
increased in the order of 0.6Ru/MC < 0.15Re–0.45Ru/MC < 0.45Re–
0
0
0.15Ru/MC < 0.3Re–0.3Ru/MC. It was also found that (Re + Ru )/C
increasedintheorderof0.6Re/MC < 0.6Ru/MC < 0.45Re–0.15Ru/MC
< 0.15Re–0.45Ru/MC < 0.3Re–0.3Ru/MC, which was in good agree-
ment with the trend of metal surface area determined by CO
chemisorption (Table 2). This demonstrates that all the catalysts
retained different total reduction degree of rhenium and ruthe-
3.5. XPS study of reduced (0.6 − x)Re–xRu/MC catalysts
0
0
nium species. In addition, (Re + Ru )/C increased with decreasing
average metal particle size determined by TEM (Fig. 5), indicating
that total reduction degree of both two metals was closely related
to the average particle size of the catalysts. It is important to note
It is known that the oxidation state of metal species in the
catalyst served as an important factor determining the catalytic
activity [34,35]. Accordingly, the oxidation states of reduced
(
0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts were
examined by XPS analyses. Fig. 7 shows the XPS spectra for Re 4f
Fig. 7(a)) and Ru 3p3/2 (Fig. 7(b)) levels of the catalysts. The Re 4f
0
0
that (Re + Ru )/C ratios of (0.6 − x)Re–xRu/MC (x = 0.15, 0.3, and
0
.45) catalysts were higher than those of 0.6Re/MC and 0.6Ru/MC.
(
According to the shrinking core model [39–41], the reduction of a
metal particle is limited by the intraparticle hydrogen diffusivity,
spectra were deconvoluted into Re 4f7/2 (solid line in Fig. 7(a)) and
Re 4f5/2 (dashed line in Fig. 7(a)) peaks, and then Re 4f7/2 spectra

K.H. Kang et al. / Applied Catalysis A: General 490 (2015) 153–162
159
Fig. 7. XPS spectra of (a) Re 4f and (b) Ru 3p3/2 levels in the reduced (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts.
which means that the reduction degree of metal species can be
enhanced by decreasing the particle size. Therefore, it is believed
that the Re–Ru bimetallic catalysts with smaller particle size were
moreeffectivelyreducedthanrheniumor rutheniummonometallic
catalyst.
6
07
2
83
MC
6
05
5
23
5
4
60
0
.6Re/MC
6
07
3.6. Hydrogen adsorption study on reduced (0.6 − x)Re–xRu/MC
4
62
48
catalysts
0
.45Re-0.15Ru/MC
It has been reported that the ability of hydrogen adsorption
on the active sites plays a crucial role in the catalytic hydrogena-
470
613
5
59
tion reaction [42,43]. Thus, H -TPD measurements were carried
2
0
.3Re-0.3Ru/MC
out to explain the affinity of metal species toward hydrogen in
the reduced (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6)
catalysts. In this study, hydrogen was injected to the reduced
6
13
5
48
4
56
◦
0.15Re-0.45Ru/MC
0.6Ru/MC
catalysts at 200 C, which was identical to the reaction tempera-
ture, to thermally activate the dissociative adsorption of hydrogen
molecules on metallic rhenium and ruthenium [21,22,44]. Fig. 8
612
5
07
4
01
shows the H -TPD profiles of reduced (0.6 − x)Re–xRu/MC (x = 0,
2
0
.15, 0.3, 0.45, and 0.6) catalysts. For comparison, H -TPD profile
2
2
00
300
400
500
600
700
of MC support was also presented. The H -TPD profile of MC
2
◦
support exhibited a broad miner peak at 283 C and a main peak
Temperature (˚C)
◦
at 607 C; the former corresponded to the adsorption of hydrogen
Fig. 8. H2-TPD profiles of reduced (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6)
molecules, while the latter was caused by gaseous products such
as H , CH , C H , and C H formed by reaction of carbon with
catalysts and mesoporous carbon (MC) support.
2
4
2
2
2
4
Table 3
XPS analyses results of reduced (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts.
Ratio of rhenium speciesa
Ratio of ruthenium speciesa
Surface atomic ratio (×10 )
3
Catalyst
0
Re␣+/Retotalb
Re6+/Retotal
0
Ru4+/Rutotal
(Re0 + Ru )/C
0
Re /Retotal
Ru /Rutotal
Retotal/C
Rutotal/C
0
0
0
0
0
.6Re/MC
0.46
0.61
0.67
0.53
–
0.26
0.24
0.25
0.16
–
0.28
0.15
0.08
0.31
–
–
–
4.12
5.03
4.75
1.51
–
–
1.90
4.66
8.21
6.19
3.72
.45Re–0.15Ru/MC
.3Re–0.3Ru/MC
.15Re–0.45Ru/MC
.6Ru/MC
0.88
0.92
0.81
0.65
0.12
0.08
0.19
0.35
1.81
5.46
6.65
5.73
a
Calculated from deconvoluted peak area of XPS spectra in Fig. 7.
b
3
≤ ˛ ≤ 4.

1
60
K.H. Kang et al. / Applied Catalysis A: General 490 (2015) 153–162
1.0
Conversion of succinic acid
Yield for GBL
Yield for BDO
100
1.0
0.8
0.6
0.4
Yield for THF
8
0
0
.3Re-0.3Ru/MC 0.8
0
.45Re-0.15Ru/MC
0
.15Re-0.45Ru/MC
60
0
.45Re-0.15Ru/MC
0.3Re-0.3Ru/MC
40
0
.6
.4
0
.6Ru/MC
0
.15Re-0.45Ru/MC
20
0
0
.6Re/MC
0
0
1
2
3
4
5
6
7
0
20
40
60
Reaction time (h)
Amount of weak hydrogen-binding sites (μmol-H /g)
2
Fig. 10. Conversion of succinic acid and yields for GBL, BDO, and THF with time on
Fig. 9. Correlations between the amount of weak hydrogen-binding sites and
stream in liquid-phase hydrogenation of succinic acid over 0.3Re–0.3Ru/MC catalyst
0
at 200 ◦C and 80 bar.
Re /Retotal ratio and between the amount of weak hydrogen-binding sites and
0
Ru /Rutotal ratio of reduced (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) cata-
lysts.
–
0.15Ru/MC < 0.3Re–0.3Ru/MC, which was different from the
trend of the total amount of hydrogen-binding sites.
spillover hydrogen on carbon surface, as evidenced by the TPR
results [32,33,45]. Interestingly, when rhenium or ruthenium
was added into the MC support, new peaks corresponding to
From XPS and H -TPD results of the reduced (0.6 − x)Re–xRu/MC
2
catalysts, it was found that the trend of weak hydrogen-binding
0
0
sites was well matched with the trend of Re /Re
and Ru /Ru
total
total
◦
the dissociatively adsorbed hydrogen (400–600 C) appeared
ratios (Table 3). Fig. 9 shows the relationships between the amount
◦
instead of the peak for molecularly adsorbed hydrogen (<300 C).
0
of weak hydrogen-binding sites and Re /Re
ratio and between
total
In order to distinguish dissociatively adsorbed hydrogen in the
0
the amount of weak hydrogen-binding sites and Ru /Ru
ratio
total
H -TPD profiles of the reduced (0.6 − x)Re–xRu/MC catalysts,
2
of reduced (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) cat-
therefore, H -TPD profiles were deconvoluted into three peaks
2
alysts. It was revealed that the amount of weak hydrogen-binding
in terms of desorption temperature; weak hydrogen-binding site
0
0
sites increased with increasing Re /Re
and Ru /Ru
ratios,
total
total
◦ ◦
400–500 C), strong hydrogen-binding site (500–600 C), and the
(
suggesting that the metallic ratio was a crucial factor determining
the hydrogen adsorption behavior of (0.6 − x)Re–xRu/MC catalysts.
In other words, the synergistic interaction formed by Re–Ru metal-
lic bond increased the amount of weak hydrogen-binding sites with
respect to total active sites in the (0.6 − x)Re–xRu/MC catalysts.
It is well known that the weakly bound hydrogen atoms on the
metallic surface are highly mobile due to their low stability, and
therefore, they can be easily and continuatively delivered to the
adsorbed reactant molecules [46,47]. Thus, it is expected that a
catalyst retaining larger amount of weak hydrogen-binding sites
would be more favorable for continuous supply of hydrogen to
adsorbed succinic acid.
◦
◦
others (>600 C). The other peaks appearing above 600 C might
be due to mesoporous carbon. Thus, weak hydrogen-binding
sites and strong hydrogen-binding sites were only considered for
quantification. The amounts of these two hydrogen-binding sites
calculated from the deconvoluted results of H -TPD profiles of the
2
catalysts are summarized in Table 4. Total amount of hydrogen
desorbed from the reduced (0.6 − x)Re–xRu/MC catalysts increased
in the order of 0.6Re/MC < 0.6Ru/MC < 0.45Re–0.15Ru/MC <
0
.15Re–0.45Ru/MC < 0.3Re–0.3Ru/MC. This trend was well consis-
0
0
tent with the trend of (Re + Ru )/C determined by XPS analyses
Table 3), indicating that larger total amount of desorbed hydrogen
(
resulted from more reduced state of the catalysts. On the other
hand, the amount of weak hydrogen-binding sites increased in
the order of 0.6Ru/MC < 0.6Re/MC < 0.15Re–0.45Ru/MC < 0.45Re
3
.7. Hydrogenation of succinic acid to BDO over
(
0.6 − x)Re–xRu/MC catalysts
Table 4
In order to ensure the reaction pathways presented in Fig. 1,
H2-TPD results of reduced (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6)
catalytic hydrogenation of succinic acid to BDO was carried out at
catalysts.
◦
2
00 C and 80 bar with time on stream. 0.3Re–0.3Ru/MC catalyst
with the largest amount of weak hydrogen-binding sites was used
as a model catalyst for the reaction. Fig. 10 shows the conversion of
succinic acid and yields for GBL, BDO, and THF as a function of time.
It was found that succinic acid was completely converted after 5 h,
and yield for GBL exhibited a volcano-shaped curve with respect to
the reaction time because GBL was transformed to BDO or THF by
consecutive hydrogenation. It is noteworthy that BDO was domi-
nantly produced from GBL, indicating that BDO can be selectively
formed from succinic acid over 0.3Re–0.3Ru/MC catalyst. The yield
for BDO showed the maximum value (71.2%) after 7 h-reaction.
Catalyst
Amount of desorbed hydrogen
␮mol-H2/g-catalyst)a
Total
(
Weak site
Strong site
(500–600 C)
◦
◦
(400–500 C)
0
0
0
0
0
.6Re/MC
7.3
15.5
18.9
23.2
28.4
35.6
22.8
47.3
80.1
52.1
41.0
.45Re–0.15Ru/MC
.3Re–0.3Ru/MC
.15Re–0.45Ru/MC
.6Ru/MC
28.4
56.9
26.7
5.4
a
Calculated from deconvoluted peak area of H2-TPD profiles in Fig. 8.

K.H. Kang et al. / Applied Catalysis A: General 490 (2015) 153–162
161
Table 5
Performance of (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts in liquid-phase hydrogenation of succinic acid at 200 C and 80 bar for 7 h.
◦
TOF (h 1)a
−
TOFBDO (h )b
−1
Catalyst
Conversion of succinic acid (%)
Selectivity (%)
Yield for BDO (%)
GBL
BDO
THF
0
0
0
0
0
.6Re/MC
73.1
100
100
100
45.2
88.7
39.8
18.1
44.3
97.5
7.7
52.2
71.2
48.9
1.8
3.6
7.9
10.7
6.8
5.6
52.2
71.2
48.9
0.8
25.4
46.1
61.2
38.6
13.4
2.05
6.04
8.51
5.23
0.29
.45Re–0.15Ru/MC
.3Re–0.3Ru/MC
.15Re–0.45Ru/MC
.6Ru/MC
0.7
a
Calculated as moles of succinic acid converted per moles of surface metal atom per hour (at ca. 10% conversion of succinic acid).
Calculated as moles of BDO produced per moles of surface metal atom per hour (at ca. 70% yield for GBL).
b
The catalytic performance of (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3,
that the trend of TOFBDO was closely related to the hydro-
gen adsorption behavior of the catalysts. Accordingly, TOFBDO
of (0.6 − x)Re–xRu/MC catalysts was correlated with the amount
of weak hydrogen-binding sites as shown in Fig. 11. It was
revealed that TOFBDO increased with increasing the amount of
weak hydrogen-binding sites. This indicates that weak hydrogen-
binding sites of the catalysts served as the crucial active sites for
the selective formation of BDO from hydrogenation of succinic acid.
This is because weak hydrogen-binding site can provide excellent
hydrogen supply to the reactants adsorbed on the catalysts, as dis-
0
.45, and 0.6) in the liquid-phase hydrogenation of succinic acid to
◦
BDO at 200 C and 80 bar for 7 h is summarized in Table 5. It was
found that 0.6Re/MC and 0.6Ru/MC catalysts produced GBL as a
major product, while (0.6 − x)Re–xRu/MC (x = 0.15, 0.3, and 0.45)
catalysts produced BDO as a major product in the reaction. This
result indicates that Re–Ru bimetallic catalysts were more effec-
tive for consecutive hydrogenation of succinic acid to BDO via GBL
than rhenium or ruthenium monometallic catalyst. It should be
noted that TOF and TOFBDO of (0.6 − x)Re–xRu/MC (x = 0.15, 0.3,
and 0.45) catalysts were also higher than those of 0.6Re/MC and
cussed in the H -TPD results. Among the catalysts, 0.3Re–0.3Ru/MC
2
0
.6Ru/MC, which means that the Re–Ru bimetallic catalysts were
catalyst with the largest amount of weak hydrogen-binding sites
showed the highest TOFBDO in the hydrogenation of succinic acid
to BDO.
structure sensitive in the hydrogenation of succinic acid to BDO. In
other words, the catalytic activity of (0.6 − x)Re–xRu/MC was sig-
nificantly affected by the ability of active sites rather than by the
amount of total active sites. TOF and TOFBDO increased in the order
of 0.6Ru/MC < 0.6Re/MC < 0.15Re–0.45Ru/MC < 0.45Re–0.15Ru/MC
3.8. Stability and reusability of 0.3Re–0.3Ru/MC catalysts
<
0.3Re–0.3Ru/MC. Among the catalysts tested, 0.3Re–0.3Ru/MC
To investigate the stability of 0.3Re–0.3Ru/MC catalyst, reusabil-
ity of the catalyst was tested. According to the previous works
12,13], it was revealed that the decrement of catalytic activity in
−1
−1
showed the highest TOF (61.2 h ) and TOFBDO (8.51 h ). In partic-
ular, the catalytic performance of 0.3Re–0.3Ru/MC was comparable
to that of reported Re-based bimetallic catalysts [14–17]. This
implies that 0.3Re–0.3Ru/MC served as a promising catalyst for the
selective formation of BDO from succinic acid.
[
the hydrogenation of succinic acid was mainly attributed to the
leaching of active metal species under the harsh reaction condi-
tions. For the recycle test, the spent catalyst was separated from
liquid product by filtration, washed with deionized water, and dried
From the trend of TOFBDO (Table 5), it was revealed that an
optimal Re:Ru molar ratio was required for the maximum BDO pro-
duction by hydrogenation of succinic acid over (0.6 − x)Re–xRu/MC
◦
in a vacuum oven at 60 C after each reaction test. The remaining
liquid containing leached metal species was collected for ICP-MS
analysis. Fig. 12 shows the recycle results of 0.3Re–0.3Ru/MC cat-
alyst in the liquid-phase hydrogenation of succinic acid to BDO at
(
x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts. It was also found
1
0
8
6
4
2
0
Conversion of succinic acid
Selectivity for BDO
Yield for BDO
0
.3Re-0.3Ru/MC
1
00
0
.45Re-0.15Ru/MC
7
5
2
5
0
5
0
0
.15Re-0.45Ru/MC
0
.6Re/MC
0
.6Ru/MC
0
15
30
45
60
Fresh
Recycle 1
Recycle 2
Amount of weak hydrogen-binding sites (μmol-H /g)
Recycle run
2
Fig. 11. A correlation between TOFBDO and the amount of weak hydrogen-binding
sites in the reduced (0.6 − x)Re–xRu/MC (x = 0, 0.15, 0.3, 0.45, and 0.6) catalysts.
Fig. 12. Results for liquid-phase hydrogenation of succinic acid to BDO over
0.3Re–0.3Ru/MC catalyst with respect to recycle run at 200 C and 80 bar for 7 h.
◦

1
62
K.H. Kang et al. / Applied Catalysis A: General 490 (2015) 153–162
Table 6
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