Effect of boron content on 1,4-butanediol production by hydrogenation of succinic acid over Re-Ru/BMC (boron-modified mesoporous carbon) catalysts

Article abstract of DOI:10.1016/j.apcata.2016.06.037

A series of Re-Ru bimetallic catalysts supported on mesoporous boron-modified carbon (denoted as Re-Ru/xBMC, x?=?B/C molar ratio) were prepared by a single-step surfactant-templating method and a subsequent incipient wetness impregnation method, and they were used for liquid-phase hydrogenation of succinic acid to 1,4-butandiol (BDO). The effect of boron addition on the catalytic activities and physicochemical properties of Re-Ru/xBMC catalysts was investigated. It was found that the addition of boron into carbon support affected surface area, metal dispersion, and reducibility of rhenium and ruthenium species in the Re-Ru/xBMC catalysts. It was also observed that boron species in carbon framework existed in several different phases such as substituted boron, partial oxidized boron, and boron oxide. In particular, the amount of substituted boron species was closely related to the hydrogen adsorption behavior of Re-Ru/xBMC catalysts. The amount of weak hydrogen-binding sites increased with increasing the amount of substituted boron species of the catalysts. Yield for BDO in the hydrogenation of succinic acid showed a volcano-shaped trend with respect to B/C molar ratio. This result was in good agreement with the amount of weak hydrogen-binding sites of the catalysts. It was revealed that TOF_BDO increased with increasing the amount of weak hydrogen-binding sites of Re-Ru/xBMC catalysts. Among the catalysts, Re-Ru/0.04BMC with the largest amount of weak hydrogen-binding sites served as an efficient catalyst in the selective formation of BDO by hydrogenation of succinic acid.

Full text of DOI:10.1016/j.apcata.2016.06.037

Accepted Manuscript
Title: Effect of boron content on 1,4-butanediol production by
hydrogenation of succinic acid over Re-Ru/BMC
(boron-modified mesoporous carbon) catalysts
Author: Ki Hyuk Kang Seung Ju Han Jong Won Lee Tae
Hyeop Kim In Kyu Song
PII:
S0926-860X(16)30351-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.06.037
APCATA 15933
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
25-5-2016
22-6-2016
30-6-2016
Please cite this article as: Ki Hyuk Kang, Seung Ju Han, Jong Won Lee, Tae Hyeop Kim,
In Kyu Song, Effect of boron content on 1,4-butanediol production by hydrogenation of
succinic acid over Re-Ru/BMC (boron-modified mesoporous carbon) catalysts, Applied
Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.06.037
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Applied Catalysis A: General (Revised manuscript-clean)
Effect of boron content on 1,4-butanediol production by hydrogenation of
succinic acid over Re-Ru/BMC (boron-modified mesoporous carbon) catalysts
Ki Hyuk Kang, Seung Ju Han, Jong Won Lee, Tae Hyeop Kim, 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
*Corresponding author
(Tel: +82-2-880-9227, Fax: +82-2-889-7415, E-mail: inksong@snu.ac.kr)
1

Graphical Abstract
100
14
12
10
8
Re-Ru/0.04BMC
Re-Ru/0.02BMC
80
60
40
20
0
Re-Ru/0.08BMC
Re-Ru/0BMC
6
Re-Ru/0.12BMC
4
2
0
0
0.02
0.04
0.06
0.08
0.10
0.12
B/C molar ratio
2

Research Highlights
- Re-Ru bimetallic catalysts supported on boron-modified carbon were prepared.
- Liquid-phase hydrogenation of succinic acid to 1,4-butanediol (BDO) was conducted.
- Yield for BDO showed a volcano-shaped trend with respect to B/C molar ratio.
- TOFBDO increased with increasing the amount of weak hydrogen-binding sites.
ABSTRACT
A series of Re-Ru bimetallic catalysts supported on mesoporous boron-modified carbon
(denoted as Re-Ru/xBMC, x = B/C molar ratio) were prepared by a single-step surfactant-
templating method and a subsequent incipient wetness impregnation method, and they were
used for liquid-phase hydrogenation of succinic acid to 1,4-butandiol (BDO). The effect of
boron addition on the catalytic activities and physicochemical properties of Re-Ru/xBMC
catalysts was investigated. It was found that the addition of boron into carbon support
affected surface area, metal dispersion, and reducibility of rhenium and ruthenium species in
the Re-Ru/xBMC catalysts. It was also observed that boron species in carbon framework
existed in several different phases such as substituted boron, partial oxidized boron, and
boron oxide. In particular, the amount of substituted boron species was closely related to the
hydrogen adsorption behavior of Re-Ru/xBMC catalysts. The amount of weak hydrogen-
binding sites increased with increasing the amount of substituted boron species of the
catalysts. Yield for BDO in the hydrogenation of succinic acid showed a volcano-shaped
3

trend with respect to B/C molar ratio. This result was in good agreement with the amount of
weak hydrogen-binding sites of the catalysts. It was revealed that TOFBDO increased with
increasing the amount of weak hydrogen-binding sites of Re-Ru/xBMC catalysts. Among the
catalysts, Re-Ru/0.04BMC with the largest amount of weak hydrogen-binding sites served as
an efficient catalyst in the selective formation of BDO by hydrogenation of succinic acid.
Keywords: Hydrogenation; Succinic acid; 1,4-Butandiol; Re-Ru catalyst; Boron-modified
carbon
1. Introduction
1,4-Butanediol (BDO) is an important raw material for producing polybutylene
terephthalate (PBT) which is mostly used in automobile and electronic industries [1-3].
Traditionally, BDO has been produced via petrochemical-based routes such as Reppe [4],
Mitsubishi-Kasei [5], and Arco processes [6]. Recently, hydrogenation of succinic acid to
BDO has attracted much attention as an economical process, because development of bio-
refinery process has increased the production of succinic acid [7,8].
Succinic acid is hydrogenated to -butyrolactone (GBL), which is further hydrogenated
to BDO or tetrahydrofuran (THF), as shown in Fig. 1. In order for selective production of
BDO through hydrogenation of succinic acid, therefore, development of efficient catalysts for
minimizing the formation of THF is required. In this respect, several catalysts based on noble
metals such as Re [9,10], Ru [11,12], Rh [13], Pd [14-16], and Pt [17] have been investigated.
Among these catalysts, combination of rhenium and ruthenium species showed an excellent
activity because synergistic interaction between two metals significantly enhanced hydrogen
adsorption capacity [18,19].
4

To improve physicochemical properties of metal catalysts, various support materials
have been used in the dehydrogenation of succinic acid. For example, TiO2 was suggested as
a support for Re-based catalysts to increase stability under hydrothermal conditions, which
prevented metal leaching from the catalysts [15,20,21]. In case of Pd catalysts supported on
silica materials such as SBA-15 and MCM-41, product selectivity was highly dependent on
their pore geometry [14]. In particular, carbon is known as a suitable support for noble metal
catalysts due to its high surface area and acid resistant nature [18,19,22,23].
However, only a few studies have focused on the modification of carbon support for
hydrogenation of succinic acid to BDO. It has been reported that addition of non-metal
elements such as sulfur, boron, and phosphorous into carbon affects physicochemical
properties of carbon support [24-30]. In particular, boron can be easily substituted for carbon
atom during carbon growth process, which significantly changes structural and electronic
properties of carbon lattice [24,25]. Thus, it is expected that boron-containing carbon
effectively interacts with active metal species to enhance reducibility and hydrogen
adsorption behavior of supported metal catalyst [24-26]. Therefore, a systematic investigation
on the effect of boron addition into carbon on the catalytic activities and physicochemical
properties of Re-Ru bimetallic catalyst supported on boron-modified carbon in the
hydrogenation of succinic acid would be worthwhile.
In this work, a series of Re-Ru bimetallic catalysts supported on mesoporous boron-
modified carbon were prepared by a single-step surfactant-templating method and a
subsequent incipient wetness impregnation method. The prepared catalysts were applied to
the liquid-phase hydrogenation of succinic acid to BDO. The effect of boron addition on the
physicochemical properties and catalytic activities of the catalysts in the hydrogenation of
succinic acid was investigated.
5

2. Experimental
2.1. Preparation of Re-Ru/xBMC catalysts
The synthesis of mesoporous boron-modified carbon was done according to our
previously reported methods [19] with slight modification. Copolymer P123 (Sigma-Aldrich)
and aqueous HCl were initially dissolved in distilled water with vigorous stirring. After P123
was completely dissolved, sucrose (Sigma-Aldrich) and aqueous H2SO4 were added into the
homogeneous solution. Subsequently, a known amount of boric acid as a structural modifier
was introduced into the solution. During this process, B/C molar ratio was adjusted to be 0,
0.02, 0.04, 0.08, and 0.12. TEOS (Sigma-Aldrich) was then added into the mixed solution,
and the solution was kept at 37 ˚C under vigorous stirring for 1 day for self-assembly reaction.
The resulting white solution was heated to 100 ˚C and it was maintained at the same
temperature for 1 day. Afterward, the dark brown suspension was transferred to unsealed
bottle and it was dried at 100 ˚C for 2 days. Carbonization was done at 800 ˚C for 4 h in a
flow of N2. The residual silica in the solid resultant was removed by HF solution. Template-
free carbon product was filtered, and finally, it was washed with distilled water and ethanol.
The resulting mesoporous boron-modified carbon was designated as xBMC (x = 0, 0.02, 0.04,
0.08, and 0.12), where x represented B/C molar ratio.
A set of Re-Ru catalysts supported on mesoporous boron-modified carbon (BMC) were
prepared by an incipient wetness impregnation method. The acetone solution containing
rhenium chloride (Sigma-Aldrich) and ruthenium chloride (Sigma-Aldrich) was introduced to
xBMC supports for impregnation. According to the previous study [19], an optimal molar
6

ratio of Re:Ru was modulated to be 1:1. The loading of each metal species was fixed at 0.3
mol%. After the impregnation, the resultant samples were dried in air overnight, followed by
calcination at 500 ˚C at 4 h using a N2 stream. The prepared catalysts were denoted as Re-
Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12).
2.2. Characterization
N2 adsorption-desorption experiments (BELSORP-mini II, BEL Japan) were carried out
to examine textural properties of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts.
Before the analyses, the catalysts were pretreated at 150 ˚C for 3 h using a vacuum pump.
Brunauer-Emmett-Teller (BET) equation was used for calculation of surface area. Inductively
coupled plasma mass spectroscopy (ICP-MS) analyses (ICPS-1000IV, Shimadzu) were
conducted to determine metal compositions of the catalysts. Crystalline structures of the
reduced catalysts were studied by X-ray diffraction (XRD) analyses. All the Re-Ru/xBMC
catalysts were reduced at 500 ˚C for 4 h under a hydrogen stream, and then they were brought
to sample vials in the glove box to avoid air exposure. Finally, the reduced catalysts were
quickly moved to sample holder of the XRD chamber. The phases of metal species were
identified by D-Max2500-PC (Rigaku) diffractometer. CO chemisorption experiments of the
reduced Re-Ru/xBMC catalysts were conducted. For the analyses, all the catalysts were
reduced and pulse technique using carbon monoxide was performed by BELCAT-B (BEL
Japan) apparatus. The obtained data were used to calculate surface area, particle size, and
dispersion of metal species. Transmission electron microscopy (TEM) and energy dispersive
X-ray spectroscopy (EDX) mapping analyses were conducted using JEM-2100F (JEOL). The
apparatus was operated at 200 kV with a nanoprobe of field emission gun. The reducibility of
7

calcined catalysts was investigated by temperature-programmed reduction (TPR) analyses. A
gas stream containing 5% H2/N2 was applied within the temperature range of room
temperature-600 ˚C with a ramping rate of 5 ˚C/min. H2 consumption was recorded by a
thermal conductivity detector (TCD). Raman spectroscopy analyses (T64000, Horiba) were
carried out within the range of 1000-2000 cm-1 using Ar laser to examine graphite carbon
structure. X-ray photoelectron spectroscopy (XPS) measurements were carried out using
AXIS-HIS (KRATOS) apparatus containing high vacuum chamber. The base chamber
pressure was operated at 10-10 Torr. All the measurements were performed with Mg/Al X-rays
under characteristic energy of 1500 eV. Hydrogen temperature-programmed desorption (H2-
TPD) experiments were conducted to measure the amount of hydrogen uptake and hydrogen-
binding strength of reduced Re-Ru/xBMC catalysts using a BELCAT-B (BEL japan). For this,
10 mg of each catalyst was charged into a quartz cell, and it was reduced at 500 ˚C for 4 h
with a hydrogen stream. After the saturation of catalyst surface by hydrogen, H2-TPD
analyses were conducted by increasing cell temperature from 200 ˚C to 700 ˚C at a ramping
rate of 5 ˚C/min.
2.3. Hydrogenation of succinic acid to BDO
Hydrogenation of succinic acid was conducted in a high pressure autoclave reactor of
200 ml equipped with an electrically heated jacket and an agitator with a magnetic driver.
Before the experiment, the reactor was purged with N2 gas to remove air. The reactor was
loaded with 50 ml of 1,4-dioxane (an inert aprotic solvent) and 0.25 g of succinic acid. Then,
0.1 g of catalyst reduced at 500 ˚C for 4 h was charged in the reaction vessel. After additional
purging with N2 gas, the reactor was heated to 200 ˚C and H2 gas was injected to 80 bar. The
8

reaction was done at 700 rpm for 5 h. After the reaction, the aqueous samples taken from the
reactor were analyzed with a gas chromatograph equipped with a capillary column (DB-624)
and a flame ionization detector (FID). Catalytic performance was evaluated by the following
equations.
mole of succinic acid reacted
Conversion of succinic acid (%) =
× 100
(1)
mole of succinic acid supplied
mole of product formed
× 100
Selectivity for product (%) =
(2)
mole of succinic acid reacted
Yield for product (%) = (Conversion of succinic acid) × (Selectivity for product) / 100 (3)
3. Results and discussion
3.1. Textural properties of Re-Ru/xBMC catalysts
To investigate textural properties of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12)
catalysts, nitrogen adsorption-desorption analyses were conducted. Fig. 2 shows the nitrogen
adsorption-desorption isotherms of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts.
All the catalysts showed IV-type isotherm with H3-type hysteresis loop, indicating that well-
developed mesopores and slit-shaped carbon structure were formed in the catalysts. Detailed
textural properties of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts is
summarized in Table 1. Actual Re:Ru and B/C molar ratios in the Re-Ru/xBMC catalysts
were in good agreement with the designed values. In addition, all the catalysts retained
9

mesopores larger than 6.3 nm, representing successful preparation of mesoporous boron-
modified carbon supports. It was also found that surface area and pore volume of Re-
Ru/xBMC catalysts decreased with increasing B/C molar ratio. This is because boron was
introduced into carbon framework during the preparation step by retarding the formation of
pore structure.
3.2. XRD, CO-chemisorption, and STEM-EDX analyses of Re-Ru/xBMC catalysts
Fig. 3 shows the XRD patterns of reduced Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and
0.12) catalysts. All the catalysts exhibited broad diffraction peaks at 23.5˚ and 43.8˚,
corresponding to graphitic carbon phase. Interestingly, neither rhenium phase nor ruthenium
phase was observed in the XRD patterns of the catalysts. This means that rhenium and
ruthenium species were finely dispersed on the surface of the catalysts. Moreover, no distinct
boron-related peaks were observed in the XRD patterns, indicating that boron species were
homogeneously distributed in the carbon framework.
The above result was further confirmed by CO chemisorption analyses of reduced Re-
Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts, as listed in Table 2. It was found that
all the catalysts retained metal particles less than 6 nm. It is interestingly to note that metal
surface area and metal dispersion decreased while average metal particle size increased with
increasing B/C molar ratio of Re-Ru/xBMC catalysts. This might be due to the decrement of
surface area with increasing B/C molar ratio, as discussed in Section 3.1.
STEM images of Re-Ru/0.04BMC catalyst and EDX mapping images with
distributions of rhenium, ruthenium, and boron species are shown in Fig. 4. It was found that
each element was clearly distinguishable in the EDX mapping images. In particular, it was
10

found that boron (purple dots in Fig. 4) species were successfully introduced into carbon
support during the single-step surfactant-templating preparation step. It was also observed
that rhenium (yellow dots in Fig. 4) and ruthenium (blue dots in Fig. 4) atoms were
homogeneously distributed throughout the catalyst.
.
3.3. TPR analyses of Re-Ru/xBMC catalysts
TPR measurements were conducted to examine reduction behavior of Re-Ru/xBMC (x
= 0, 0.02, 0.04, 0.08, and 0.12) catalysts, as shown in Fig. 5. It was observed that all the
catalysts showed one reduction peak at around 282-341 ˚C despite the presence of two noble
metals. According to the previous work [19], rhenium and ruthenium species are
simultaneously reduced because pre-reduced ruthenium species can transfer hydrogen to
unreduced metal species. Therefore, it is believed that rhenium and ruthenium species in the
Re-Ru/xBMC catalysts were co-reduced during the reduction process. It is interesting to note
that the peak shape became broader with increasing B/C molar ratio. Moreover, the peak
moved toward higher temperature with increasing B/C molar ratio. In particular, peak
temperature of Re-Ru/xBMC (x = 0.08 and 0.12) was significantly higher than that of Re-
Ru/xBMC (x = 0, 0.02, and 0.04). The amount of hydrogen uptake calculated from reduction
peak area of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts is listed in Table 2. It
was found that the amount of hydrogen uptake exceeded the theoretical values for rhenium
(3.5) and for ruthenium (2) due to hydrogen spillover by metal species [30]. It is noteworthy
that Re-Ru/xBMC (x = 0.08 and 0.12) showed rapid decrement of the amount of hydrogen
uptake compared to Re-Ru/xBMC (x = 0, 0.02, and 0.04). From the result, it can be inferred
that an excessive boron addition into carbon support aggravated particle distribution and
11

reducibility of rhenium and ruthenium species in the Re-Ru/xBMC catalysts.
3.4. Raman and XPS analyses of Re-Ru/xBMC catalysts
In order to examine boron-modified carbon structure of Re-Ru/xBMC (x = 0, 0.02, 0.04,
0.08, and 0.12) catalysts, Raman spectroscopy analyses were carried out as shown in Fig. 6. It
was found that all the catalysts showed two characteristic peaks at 1350 cm-1 and 1600 cm-1;
the former corresponds to D band while the latter corresponds to G band of sp2-carbon
material [31,32]. It is known that the ratio of D band/G band intensity (ID/IG) represents the
degree of sp2-carbon structure [31-34]. In other words, the decrement of ID/IG ratio indicates
the increment of substituted boron species in the sp2-carbon lattice. The ID/IG ratio calculated
from Raman spectra of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts is
summarized in Table 2. It was revealed that ID/IG ratio decreased in the order of Re-
Ru/0.12BMC > Re-Ru/0.08BMC > Re-Ru/0BMC > Re-Ru/0.02BMC > Re-Ru/0.04BMC.
Therefore, it can be said that Re-Ru/0.04BMC catalyst retaining the smallest ID/IG ratio
would have more substituted boron species in the carbon lattice.
XPS analyses were carried out to confirm chemical state of boron species of Re-
Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts. Fig. 7 shows the XPS spectra for B 1s
level of Re-Ru/xBMC catalysts. The B 1s spectra was deconvoluted by Gaussian curve-
fitting method, and the peaks were assigned to substituted boron (BC3) at 191.2 eV, partially
oxidized boron (BN3, BCO2, and BC2O) at 192.6 eV, and boron oxide (BOx) at 193.6 eV,
respectively [35,36]. It was found that the total area of B 1s spectra increased with increasing
B/C molar ratio of Re-Ru/xBMC catalysts. The deconvoluted areas for B 1s spectra of Re-
Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts were re-calculated into ratio of boron
12

species and surface atom ratio, as summarized Table 3. It was found that Btotal/C ratio
increased with increasing boron content in the catalysts. It was also revealed that Bsubstituted/C
ratio increased in the order of Re-Ru/0.12BMC < Re-Ru/0.08BMC < Re-Ru/0.02BMC < Re-
Ru/0.04BMC, in good agreement with the result of Raman spectroscopy analyses. Among the
catalysts, Re-Ru/0.04BMC catalyst showed the highest Bsubstituted/C ratio.
3.5. Hydrogen adsorption study on Re-Ru/xBMC catalysts
For quantitative comparison of hydrogen adsorption capacity of Re-Ru/xBMC (x = 0,
0.02, 0.04, 0.08, and 0.12) catalysts, H2-TPD measurements were conducted. Fig. 8 shows the
H2-TPD profiles of reduced Re-Ru/xBMC catalysts. According to the previous study [19], the
peak above 600 ˚C in the H2-TPD profiles was due to hydrogen spillover or gaseous products
such as methane, ethylene, and acetylene formed by thermal reaction of carbon material. On
the other hand, hydrogen desorption peaks below 600 ˚C were attributed to dissociatively
adsorbed hydrogen on the reduced rhenium and ruthenium species. Thus, H2-TPD profiles of
the catalysts were deconvoluted into weak hydrogen-binding site (400-500 ˚C), strong
hydrogen-binding site (500-600 ˚C), and the others (>600 ˚C). The two peaks below 600 ˚C
were only considered for the calculation of hydrogen uptake capacity by metal species. The
amount of desorbed hydrogen calculated from H2-TPD profiles of Re-Ru/xBMC (x = 0, 0.02,
0.04, 0.08, and 0.12) catalysts is summarized in Table 4. It was found that total amount of
desorbed hydrogen decreased with increasing B/C molar ratio of the catalysts. This trend was
well consistent with the trend of metal surface area and metal dispersion in Table 2. It was
also found that the amount of weak hydrogen-binding sites increased in the order of Re-
Ru/0.12BMC < Re-Ru/0.08BMC < Re-Ru/0BMC < Re-Ru/0.02BMC < Re-Ru/0.04BMC, in
13

good agreement with the trends of Raman and XPS results. Among the catalysts, Re-
Ru/0.04BMC showed the largest amount of weak hydrogen-binding sites. This indicates that
hydrogen can be weakly bound and easily diffused to adsorbed reactant on the metal surface
of Re-Ru/0.04BMC catalyst.
On the basis of XPS and H2-TPD results of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and
0.12) catalysts, it is suggested that hydrogen adsorption behavior of active metal species was
closely related to the amount of substituted boron species in the catalysts. Fig. 9 shows the
relationship between the amount of weak hydrogen-binding sites and Rsubstituted/C ratio of
reduced Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts. It was revealed that the
amount of weak hydrogen-binding sites increased with increasing Bsubstituted/C ratio of the
catalysts. It is known that the substitution of boron into carbon lattice induces negatively
charged carbon atom due to lower electronegativity of boron (2.04) than carbon (2.55), which
increases electron density of active metal species [37]. Therefore, it is believed that electron-
rich rhenium and ruthenium induced by boron-containing carbon support became more
favorable for the formation of weak hydrogen-binding sites.
3.6. Hydrogenation of succinic acid to BDO over Re-Ru/xBMC catalysts
Catalytic performance of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) in the liquid-
phase hydrogenation of succinic acid to BDO at 200 ˚C and 80 bar after a 5 h-reaction is
summarized in Table 5. It was found that all the catalysts selectively produced BDO rather
than THF due to the synergistic effect between rhenium and ruthenium species [19]. However,
only Re-Ru/0.04BMC catalyst yielded BDO as a major product while the other catalysts
produced GBL as a major product in the reaction. TOF and TOFBDO were calculated to
14

evaluate the active sites of the catalysts, as listed in Table 5. Fig. 10 shows the yield for BDO
and TOFBDO over Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts plotted as a
function of B/C molar ratio. It was revealed that yield for BDO and TOFBDO showed volcano-
shaped curves with respect to B/C molar ratio. Yield for BDO and TOFBDO increased in the
order of Re-Ru/0.12BMC < Re-Ru/0.08BMC < Re-Ru/0BMC < Re-Ru/0.02BMC < Re-
Ru/0.04BMC. This indicates that an optimal B/C molar ratio was required for maximum
BDO production by hydrogenation of succinic acid over Re-Ru/xBMC catalysts. Moreover,
TOFBDO of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts was well correlated
with the amount of weak hydrogen-binding sites, as shown in Fig. 11. It was found that
TOFBDO increased with increasing weak hydrogen-binding sites of Re-Ru/xBMC catalysts.
This indicates that hydrogen adsorption capacity of the catalysts plays an important role in
determining the catalytic performance. In other words, weakly adsorbed hydrogen can be
easily diffused on the metal surface, and consecutively, can be easily supplied to the adsorbed
reactant, which promotes the selective production of BDO via GBL in the hydrogenation of
succinic acid. Among the catalysts, Re-Ru/0.04BMC with the largest amount of weak
hydrogen-binding sites showed the highest TOFBDO. Thus, Re-Ru/0.04BMC catalyst served
as an effective catalyst in the hydrogenation of succinic acid to BDO.
4. Conclusions
A series of Re-Ru bimetallic catalysts supported on mesoporous boron-modified carbon
(Re-Ru/xBMC, x = B/C molar ratio) were prepared for use in the liquid-phase hydrogenation
of succinic acid to BDO. The effect of boron addition on the catalytic activities and
physicochemical properties of Re-Ru/xBMC was investigated. All the catalysts retained well-
15

developed mesopores and slit-shaped carbon structure. Metal surface area and metal
dispersion decreased with decreasing B/C molar ratio due to the decrement of surface area of
Re-Ru/xBMC catalysts. It was found that an excessive addition of boron into carbon structure
decreased reducibility of rhenium and ruthenium species in the Re-Ru/xBMC catalysts.
Raman and XPS analyses showed that the amount of substituted boron species was changed
with a variation of B/C molar ratio. It was revealed that the trend of Bsubstituted/C ratio was in
good agreement with the trend of weak hydrogen-binding sites of Re-Ru/xBMC catalysts.
The amount of weak hydrogen-binding sites increased with increasing Bsubstituted/C ratio of the
catalysts. Yield for BDO and TOFBDO in the hydrogenation of succinic acid to BDO over Re-
Ru/xBMC showed volcano-shaped curves with respect to B/C molar ratio. It was also found
that TOFBDO increased with increasing the amount of weak hydrogen-binding sites of Re-
Ru/xBMC catalysts. This indicates that catalytic performance of Re-Ru/xBMC was closely
related to the amount of weak hydrogen-binding sites. Among the catalysts, Re-Ru/0.04BMC
with the largest amount of weak hydrogen-binding sites showed the highest TOFBDO
.
Acknowledgements
This work was supported by the Global Frontier R&D Program on Center for
Multiscale Energy System funded by the National Research Foundation under the Ministry of
Education, Science and Technology, Korea (20110031575).
16

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Figures
Fig. 1. Reaction pathways for hydrogenation of succinic acid.
Fig. 2. Nitrogen adsorption-desorption isotherms of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08,
and 0.12) catalysts.
Fig. 3. XRD patterns of reduced Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts.
Fig. 4. STEM-EDX images of reduced Re-Ru/0.04BMC catalyst obtained by mapping on
rhenium, ruthenium, and boron.
Fig. 5. TPR profiles of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts.
Fig. 6. Raman spectra of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts.
Fig. 7. XPS spectra of B 1s level in the Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12)
catalysts.
Fig. 8. H2-TPD profiles of reduced Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts.
Fig. 9. A correlation between Bsubstituted/C ratio and the amount of weak hydrogen-binding
sites of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts.
20

Fig. 10. Correlations between B/C molar ratio and yield for BDO and between B/C molar
ratio and TOFBDO of Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts.
Fig. 11. A correlation between TOFBDO and the amount of weak hydrogen-binding sites in the
Re-Ru/xBMC (x = 0, 0.02, 0.04, 0.08, and 0.12) catalysts.
21

Figure
HO
OH
1,4-Butanediol
(BDO)
O
O
HO
O
OH
O
O
-Butyrolactone
Succinic acid
(GBL)
Tetrahydrofuran
(THF)
Fig. 1

Re-Ru/0BMC
Re-Ru/0.02BMC
Re-Ru/0.04BMC
Re-Ru/0.08BMC
Re-Ru/0.12BMC
0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Fig. 2

Re (101)
Ru (101)
Re-Ru/0BMC
Re-Ru/0.02BMC
Re-Ru/0.04BMC
Re-Ru/0.08BMC
Re-Ru/0.12BMC
70 80
20
30
40
50
60
2 Theta (degree)
Fig. 3

250 nm
250 nm
B
250 nm
250 nm
Re
Ru
Fig. 4

282
283
Re-Ru/0BMC
Re-Ru/0.02BMC
Re-Ru/0.04BMC
Re-Ru/0.08BMC
Re-Ru/0.12BMC
286
318
341
0
100
200
300
400
500
600
Temperature (°C)
Fig. 5

D band
G band
Re-Ru/0BMC
Re-Ru/0.02BMC
Re-Ru/0.04BMC
Re-Ru/0.08BMC
Re-Ru/0.12BMC
1000
1200
1400
1600
1800
2000
Raman shift (cm-1)
Fig. 6

Partially oxidized B
Boron oxide
Substituted B
Re-Ru/0.02BMC
Re-Ru/0.04BMC
Re-Ru/0.08BMC
Re-Ru/0.12BMC
188
190
192
194
196
Binding energy (eV) of B 1s
Fig. 7

Re-Ru/0BMC
Re-Ru/0.02BMC
Re-Ru/0.04BMC
Re-Ru/0.08BMC
Re-Ru/0.12BMC
200
300
400
500
600
700
Temperature (°C)
Fig. 8