Hydrogenation of succinic acid to tetrahydrofuran (THF) over ruthenium-carbon composite (Ru-C) catalyst

Article abstract of DOI:10.1016/j.apcata.2013.10.029

Ruthenium-carbon composite (Ru-XC) catalysts prepared by a single-step surfactant-templating method were pre-graphitized at different temperature (X = 200, 250, 300, 350, and 400 C), and they were applied to the liquid-phase hydrogenation of succinic acid to tetrahydrofuran (THF). The effect of pre-graphitization temperature on the catalytic performance of Ru-XC catalysts (X = 200, 250, 300, 350, and 400 C) was investigated. It was observed that Ru-XC composite catalysts showed different textural properties depending on pre-graphitization temperature. In the liquid-phase hydrogenation of succinic acid to tetrahydrofuran (THF), conversion of succinic acid and yield for THF showed volcano-shaped trends with respect to pre-graphitization temperature. In other words, an optimal pre-graphitization temperature was required to achieve maximum catalytic performance of Ru-XC catalysts. Yield for THF in the hydrogenation of succinic acid increased with decreasing ruthenium particle size of Ru-XC catalysts. Among the catalysts tested, Ru-300C, which had the smallest ruthenium particle size, showed the highest yield for THF.

Full text of DOI:10.1016/j.apcata.2013.10.029

Applied Catalysis A: General 469 (2014) 466–471
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Hydrogenation of succinic acid to tetrahydrofuran (THF) over
ruthenium–carbon composite (Ru–C) catalyst
Ung Gi Hong, Jeong Kwon Kim, Joongwon Lee, Jong Kwon Lee, Ji Hwan Song, Jongheop Yi,
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 11 July 2013
Received in revised form 12 October 2013
Accepted 15 October 2013
Available online 27 October 2013
Ruthenium–carbon composite (Ru–XC) catalysts prepared by a single-step surfactant-templating method
were pre-graphitized at different temperature (X = 200, 250, 300, 350, and 400 ◦C), and they were
applied to the liquid-phase hydrogenation of succinic acid to tetrahydrofuran (THF). The effect of pre-
graphitization temperature on the catalytic performance of Ru–XC catalysts (X = 200, 250, 300, 350, and
400 ◦C) was investigated. It was observed that Ru–XC composite catalysts showed different textural prop-
erties depending on pre-graphitization temperature. In the liquid-phase hydrogenation of succinic acid to
tetrahydrofuran (THF), conversion of succinic acid and yield for THF showed volcano-shaped trends with
respect to pre-graphitization temperature. In other words, an optimal pre-graphitization temperature
was required to achieve maximum catalytic performance of Ru–XC catalysts. Yield for THF in the hydro-
genation of succinic acid increased with decreasing ruthenium particle size of Ru–XC catalysts. Among
the catalysts tested, Ru-300C, which had the smallest ruthenium particle size, showed the highest yield
for THF.
Keywords:
Hydrogenation
Succinic acid
Tetrahydrofuran
Ruthenium
Mesoporous carbon
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
materials. Among various supporting materials, carbon has been
widely employed due to its well-developed porosity, non-toxicity,
Tetrahydrofuran (THF) is a very important chemical in various
polymer industries. THF can be converted into polytetramethylene
ether glycol (PTMEG) and tetrahydrothiophene [1–3]. THF is also
widely used as a solvent in the production of polyvinyl chloride
(PVC) and paints. THF is currently produced via several petrochemi-
cal processes such as hydrogenation of maleic anhydride (MAN) [4],
dehydration of 1,4-butanediol (BDO) [5], and oxidation of butadi-
ene (BD) [6]. However, all these feedstocks (MAN, BDO, and BD)
are obtained from fossil fuels, leading to several problems such
as unstable price and environmental contamination. Therefore,
demand for finding a new and clean platform that can replace these
feedstocks has continuously increased [7–9].
Recently, conversion of succinic acid to THF has attracted much
attention, because of the increase of succinic acid production
in the biorefinery process [10,11]. Succinic acid is a cheap and
bio-based chemical that can be converted into THF by hydrogena-
tion reaction. It is known that ruthenium-based catalyst is one
of the efficient catalysts for selective formation of THF through
hydrogenation of succinic acid [12–14]. Many researches on ruthe-
nium catalysts have been focused on finding suitable supporting
and hydrophobic property [15–17].
We have previously investigated the liquid-phase hydrogena-
tion of succinic acid over novel metal catalyst impregnated on
mesoporous carbon prepared by a surfactant-templating method
[14,18]. We found that carbon support prepared by a surfactant-
templating method retained high surface area and large pore
volume, leading to excellent physical properties. For this reason,
novel metal catalyst impregnated on mesoporous carbon showed
a considerable catalytic performance in the hydrogenation of suc-
cinic acid to THF [18]. However, novel metal catalyst impregnated
on mesoporous carbon required a number of filtration and drying
steps. Furthermore, novel metal particles did not well interact with
carbon framework.
It has been reported that structure of carbon framework
prepared by a self-assembly method can be controlled by pre-
graphitization treatment [19,20]. If metal–carbon interaction is
properly formed, therefore, metal dispersion of metal–carbon com-
posite prepared by a single-step surfactant-templating method
may be controlled. In other words, metal particle size of
metal–carbon composite can be controlled by changing pre-
graphitization temperature. Furthermore, metal–carbon compos-
ite prepared by a single-step surfactant-templating method with
pre-graphitization can not only achieve a fine metal dispersion
but also provide a simple preparation route. Therefore, it is
∗
Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415.
E-mail address: inksong@snu.ac.kr (I.K. Song).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.10.029

U.G. Hong et al. / Applied Catalysis A: General 469 (2014) 466–471
467
expected that ruthenium–carbon composite prepared by a single-
step surfactant-templating method can serve as an excellent
catalyst in the hydrogenation of succinic acid to THF, if it is properly
pre-graphitized.
In this work, ruthenium–carbon composite (Ru–XC) catalysts
prepared by a single-step surfactant-templating method were
pre-graphitized at various temperatures (X = 200, 250, 300, 350,
and 400 ◦C) to control carbon structure and ruthenium disper-
sion. The Ru–XC composite catalysts were characterized by N2
adsorption–desorption isotherm, XRD, HR-TEM, and EDX, CO
chemisorption analyses. Liquid-phase hydrogenation of succinic
acid to THF was carried out over Ru–XC catalysts in a batch reactor.
The effect of pre-graphitization temperature on the textural prop-
erties and catalytic activities of Ru–XC catalysts was investigated.
Ru-400C
Ru-300C
Ru-200C
0.2
2. Experimental
0
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
2.1. Preparation of ruthenium–carbon composite catalysts
Fig. 2. Nitrogen adsorption–desorption isotherms of Ru–XC catalysts reduced at
450 ◦C.
Fig. 1 shows the schematic procedures for the preparation of
ruthenium–carbon composite (Ru–XC, X = 200, 250, 300, 350, and
400 ◦C) catalysts. A carbon precursor (sucrose, 8 g, Aldrich), P123
(tri-block copolymer surfactant, 16 g, Aldrich), and ruthenium pre-
cursor (RuCl3, 1.3 g, Kojima chemical) were dissolved in 4 M HCl
solution (300 ml) at 40 ◦C for 4 h under stirring. After a silica pre-
cursor (TEOS, 36 ml, Aldrich) was slowly added into the solution,
the mixture was constantly stirred for 3 h. After the mixture was
dried at 100 ◦C for 48 h, the resulting brown powder was calcined
at various temperatures (200, 250, 300, 350, and 400 ◦C) for 4 h
at a heating rate of 5 ◦C/min in a nitrogen stream (100 ml/min)
for pre-graphitization. The calcined powder was then washed and
filtered with de-ionized water, and subsequently, it was dried at
80 ◦C to obtain a ruthenium/carbon slurry/silica composite. For the
complete dehydration of surfactant, 2 g of composite was treated
with 40 ml of 0.4 M H2SO4 solution for 12 h. The resulting brown
solid was then carbonized at 800 ◦C for 4 h at a heating rate of
5 ◦C/min in a nitrogen stream (100 ml/min). The resultant was fur-
ther treated with 5 wt% HF solution to remove silica template. After
it was dried overnight at 80 ◦C, the dried ruthenium–carbon com-
posite was reduced with a mixed stream of hydrogen (2.5 ml/min)
and nitrogen (47.5 ml/min) at 450 ◦C prior to the catalytic reac-
tion. The ruthenium–carbon composite catalysts pre-graphitized
at different temperature (200, 250, 300, 350, and 400 ◦C) were
denoted as Ru–XC (Ru-200C, Ru-250C, Ru-300C, Ru-350C, Ru-
400C), where X represented the pre-graphitization temperature in
Celsius.
2.2. Characterization
Nitrogen
adsorption–desorption
isotherms
of
ruthe-
nium–carbon composite (Ru-200C, Ru-300C, and Ru-400C)
catalysts were obtained with an ASAP-2010 instrument
(Micromeritics). Surface areas and pore volumes of the com-
posite catalysts were calculated using the BET equation and the
BJH model, respectively. XRD patterns of Ru–XC catalysts were
obtained with a Rigaku D-Max2500-PC instrument using Cu-K␣
˚
radiation (ꢀ = 1.541 A) operated at 50 kV and 100 mA. Ruthenium
dispersion was examined by HR-TEM (Jeol, JEM-3000F), EDX
(Tecnai F20, FEI), and CO chemisorption (BELCAT-B, BEL Japan)
analyses.
2.3. Hydrogenation of succinic acid to THF
Liquid-phase hydrogenation of succinic acid (SA) to THF was
carried out over Ru–XC catalysts (X = 200, 250, 300, 350, and 400 ◦C)
in a batch reactor at 240 ◦C and 80 bar (H2). Succinic acid (0.5 g) and
a reduced Ru–XC catalyst (0.2 g) were charged into an autoclave
(150 ml). 1,4-Dioxane (50 ml) was used as a solvent for the reaction.
The reactor was purged with nitrogen to remove air, and then it was
pressurized up to 40 bar using hydrogen. After heating the reactor
to reaction temperature (240 ◦C), hydrogen pressure was raised up
to 80 bar. The catalytic reaction was carried out for 4 h. During the
reaction, reaction mixture was stirred at 500 rpm in order to avoid
mass transfer limitation. Reaction products were analyzed using a
gas chromatograph (Younglin, ACME-6100) equipped with a FID
(flame ionization detector). In order to investigate the stability of
the catalyst after the reaction, the amount of ruthenium leaching
was measured by ICP-AES (PerkinElmer, Optima-4300DV) analyses.
3. Results and discussion
3.1. Textural properties of Ru–XC catalysts
Fig. 2 shows the nitrogen adsorption–desorption isotherms of
Ru–XC (X = 200, 300, and 400 ◦C) catalysts. All the Ru–XC cata-
lysts showed type-IV isotherm and type-H4 hysteresis loop. With
increasing pre-graphitization temperature, more apparent type-H4
hysteresis loop indicating the existence of micropore-like meso-
pores [21] was observed.
Textural properties of Ru–XC (X = 200, 250, 300, 350, and 400 ◦C)
catalysts are summarized in Table 1. Average pore size of Ru–XC
Fig. 1. Schematic procedures for the preparation of Ru–XC composite catalysts.

468
U.G. Hong et al. / Applied Catalysis A: General 469 (2014) 466–471
Table 1
Textural properties of Ru–XC catalysts.
Surface area
(m2/g-cat.)
Micropore surface area
Smicropore/Smesopore
Pore volume
(cm3/g-cat.)
Average pore
size (nm)
(Smicropore) (m2/g-cat.)
Ru-200C
Ru-250C
Ru-300C
Ru-350C
Ru-400C
412
430
472
533
574
193
221
258
311
362
0.47
0.51
0.55
0.58
0.63
0.69
0.72
0.74
0.78
0.87
6.8
6.7
6.3
6.0
5.7
Table 2
Ruthenium particle sizes of Ru–XC catalysts.
catalysts was in the mesopore range (5.7–6.8 nm), and average pore
size of Ru–XC catalysts gradually decreased with increasing pre-
graphitization temperature. Surface area and pore volume of Ru–XC
catalysts increased with increasing pre-graphitization tempera-
ture. It has been reported that surface area of carbon increases with
increasing pre-graphitization temperature due to the increase of
micropore formed by thermal activation [19,22]. Therefore, surface
area and pore volume of Ru–XC catalysts increased with increasing
pre-graphitization temperature, while average pore size of Ru–XC
catalysts decreased with increasing pre-graphitization tempera-
ture. The ratio of micropore surface area with respect to mesopore
surface area of Ru–XC catalysts also increased with increasing pre-
graphitization temperature. This indicates that pre-graphitization
at high temperature may cause destruction of mesopore, leading to
the increase of microporosity.
Ruthenium particle
size (nm)
Ruthenium particle
size (nm)
Ruthenium particle
size (nm) determined
by HR-TEM
determined by
determined by CO
chemisorption
XRDa
Ru-200C
Ru-250C
Ru-300C
Ru-350C
Ru-400C
10.4
4.9
3.6
5.5
7.7
15.1
6.0
4.8
6.3
13.7
14.3
6.0
3.3
6.5
10.6
a
Calculated using the Debye–Scherrer equation.
explained by metal sintering resistance [31–33]. With a stronger
ruthenium–carbon interaction, Ru–XC catalysts (X = 200, 250, and
300 ◦C) showed a stronger resistance toward ruthenium sinter-
ing, resulting in smaller particle size of ruthenium. In case of
Ru–XC catalysts (X = 300, 350, and 400 ◦C), however, ruthenium
particle size became larger with increasing pre-graphitization tem-
perature, due to collapse of mesopore. Pre-graphitization at high
temperature provides not only strong ruthenium–carbon interac-
tion but also increase of micropore. As presented in Table 1, the
ratio of micropore surface area with respect to mesopore surface
area of Ru–XC (X = 300, 350 and 400 ◦C) catalysts increased with
increasing pre-graphitization temperature. In other words, meso-
porosity of Ru–XC decreased with increasing pre-graphitization
temperature. It is known that mesoporosity is also responsible
for enhancing dispersion of metal particle in the metal-support
system [18]. Although Ru-350C and Ru-400C retained stronger
interaction between ruthenium and carbon framework than Ru-
300C, therefore, they did not show smaller ruthenium particle size
than Ru-300C. These results indicate that both ruthenium–carbon
interaction and mesoporosity of Ru–XC catalysts are responsible
for achieving fine dispersion of ruthenium incorporated with car-
bon framework. Thus, an optimal pre-graphitization temperature
for the treatment of Ru–XC was required to yield small ruthenium
particles.
Fig. 4 shows the HR-TEM images of Ru–XC catalysts (X = 200,
250, 300, 350, and 400 ◦C) reduced at 450 ◦C. In the Ru–XC catalysts
(X = 200, 250, and 300 ◦C), ruthenium particle size became smaller
with increasing pre-graphitization temperature. In the Ru–XC cat-
alysts (X = 300, 350, and 400 ◦C), however, ruthenium particle size
became larger with increasing pre-graphitization temperature. In
order to ensure the trend of ruthenium particle size of Ru–XC cat-
alysts, sizes of 200 metallic ruthenium particles in the reduced
catalysts were directly measured from HR-TEM images, as sum-
marized in Table 2. It was observed that the trend of average
ruthenium particle size determined by HR-TEM was well consistent
with the trend of ruthenium particle size determined by XRD and
CO chemisorption. Among the Ru–XC catalysts, Ru-300C catalyst
showed the smallest ruthenium particles (less than 5 nm), while
Ru-200C catalyst showed the largest ruthenium particle size (above
10 nm).
3.2. Crystalline structure and ruthenium dispersion of Ru–XC
catalysts
Fig. 3 shows the XRD patterns of unreduced and reduced Ru–XC
(X = 200, 250, 300, 350, and 400 ◦C) catalysts. It was observed that all
the Ru–XC catalysts exhibited a weak shoulder at 2Â = 25◦, which
was attributed to graphitic carbon [23]. Unreduced Ru–XC cata-
lysts showed the characteristic XRD peaks of metallic ruthenium
(2Â = 38◦, 42◦, 44◦, and 58◦), bulk ruthenium oxide (2Â = 28◦, 34◦,
and 57◦), and ruthenium chloride (2Â = 48◦) [24]. This means that
washing step and carbonization step were not sufficient to form
complete metallic ruthenium. On the other hand, the reduced
Ru–XC catalysts did not show XRD peaks of ruthenium oxide
or ruthenium chloride but showed metallic ruthenium peaks at
2Â = 38◦, 42◦, 44◦, and 58◦. These results indicate that ruthenium
species in the Ru–XC catalysts were completely reduced into metal-
lic ruthenium during the reduction process.
It was also observed that ruthenium peak intensity of reduced
Ru–XC catalysts decreased in the order of Ru-200C > Ru-400C > Ru-
350C > Ru-250C > Ru-300C. This result indicates that ruthenium
particles of Ru-300C catalyst were most finely dispersed on the
carbon framework. In general, dispersion of ruthenium catalyst
supported on metal oxide is determined by hydrogen chemisorp-
tion measurement [25]. In case of ruthenium catalyst with carbon
material, however, it was difficult to determine the accurate dis-
persion of ruthenium due to considerable hydrogen spillover to
carbon material [26–28]. In order to determine the accurate disper-
sion of ruthenium in the Ru–XC catalysts, therefore, particle size of
ruthenium was calculated by Debye–Scherrer equation (using XRD
peaks) [29,30] and CO chemisorptions.
Table 2 shows the average ruthenium particle sizes of Ru–XC cat-
alysts determined by XRD and CO chemisorption measurements.
Ruthenium particle sizes of Ru–XC catalysts determined by XRD
and CO chemisorption measurements showed the same trend with
respect to pre-graphitization temperature. In the Ru–XC catalysts
(X = 200, 250, and 300 ◦C), ruthenium particle size became smaller
with increasing pre-graphitization temperature. In other words,
ruthenium particle size became smaller with increasing interac-
tion between ruthenium and carbon framework. This might be
Fig. 5 shows the enlarged HR-TEM image of Ru-300C catalyst
and its EDX image obtained by mapping on ruthenium. TEM result

U.G. Hong et al. / Applied Catalysis A: General 469 (2014) 466–471
469
Fig. 3. XRD patterns of (a) unreduced and (b) reduced Ru–XC catalysts.
shows that small ruthenium particles (less than 4 nm) were well
incorporated on the carbon framework in the Ru-300C compos-
ite catalyst. EDX image of Ru-300C catalyst obtained by mapping
on ruthenium also clearly supports that ruthenium was finely dis-
persed on the carbon support.
hydrogenolysis and oxidative-hydrogenation of succinic acid, and
small amounts of acetic acid and butyric acid were also produced as
by-products. It is known that ruthenium is one of the most efficient
catalysts for hydrogenolysis and oxidative-hydrogenation [34,35].
Therefore, the side reaction (formation of ethanol) competitively
occurred together with hydrogenation of succinic acid (formation
of THF) over the ruthenium–carbon composite catalyst. As shown
in Fig. 6, it should be noted that both conversion of succinic acid
and yield for THF showed volcano-shaped trends with respect to
pre-graphitization temperature. This means that an optimal pre-
graphitization temperature was required for maximum production
of THF. Among the catalyst tested, Ru-300C catalyst showed the
highest conversion of succinic acid (91.2%) and the highest yield for
THF (46.4%), while Ru-200C catalyst showed the lowest conversion
3.3. Hydrogenation of succinic acid to THF over Ru–XC catalysts
Catalytic performance of Ru–XC catalysts (X = 200, 250, 300,
350, and 400 ◦C) in the liquid-phase hydrogenation of succinic acid
to THF is shown in Fig. 6. THF (41–51% selectivity) together with
␥-butyrolactone (24–36% selectivity) and 1,4-butanediol (6–8%
selectivity) were produced as main products via hydrogenation
of succinic acid. Ethanol was produced as a major by-product via
Fig. 4. HR-TEM images of Ru–XC catalysts reduced at 450 ◦C.

470
U.G. Hong et al. / Applied Catalysis A: General 469 (2014) 466–471
Fig. 5. (a) HR-TEM image of reduced Ru-300C catalyst and (b) its EDX image obtained by mapping on ruthenium.
100
100
80
60
40
20
0
Conversion of succinic acid
Yield for THF
Conversion of succinic acid
Yield for THF
80
60
40
20
0
1
2
3
Recycle run
Ru-200C Ru-250C Ru-300C Ru-350C Ru-400C
Fig. 8. Result for liquid-phase hydrogenation of succinic acid over Ru-300C
catalyst with respect to recycle run (reaction temperature = 240 ◦C, reaction pres-
sure = 80 bar, reaction time = 4 h).
Fig. 6. Catalytic performance of Ru–XC catalysts in the liquid-phase hydrogenation
of succinic acid to THF (reaction temperature = 240 ◦C, reaction pressure = 80 bar,
reaction time = 4 h).
size (with increasing ruthenium dispersion) of Ru–XC catalysts.
This result strongly supports that ruthenium particle size of Ru–XC
catalysts served as an important factor determining the catalytic
performance in the hydrogenation of succinic acid to THF.
of succinic acid (61.0%) and the lowest yield for THF (25.2%). It
is believed that the excellent catalytic performance of Ru-300C
catalyst was due to the smallest ruthenium particle size.
In order to investigate the effect of ruthenium particle size of
Ru–XC (X = 200, 250, 300, 350, and 400 ◦C) catalysts on the catalytic
activity in the hydrogenation of succinic acid, a correlation between
ruthenium particle size of Ru–XC catalysts and yield for THF was
established. Fig. 7 shows the correlation between ruthenium parti-
cle size of Ru–XC catalysts (determined by XRD) and yield for THF in
the hydrogenation of succinic acid. The correlation clearly shows
that yield for THF increased with decreasing ruthenium particle
3.4. Stability and reusability of Ru–XC catalyst
To investigate the stability and reusability of the catalyst, recy-
cle tests for hydrogenation of succinic acid over Ru-300C catalyst
were performed three times. Fig. 8 shows the result for liquid-phase
hydrogenation of succinic acid to THF over Ru-300C catalyst with
respect to recycle run. It was found that both fresh and spent cat-
alysts showed similar catalytic activity (more than 95% of recycle
efficiency). Furthermore, no significant ruthenium leaching (less
than 1.0 ppm) was detected by ICP-AES analysis after each reaction
test. Thus, Ru-300C catalyst served as a stable and reusable catalyst
in the liquid-phase hydrogenation of succinic acid to THF.
Ru-300C
45
Ru-250C
40
Ru-400C
Ru-350C
4. Conclusions
35
Ruthenium–carbon composite (Ru–XC) catalysts prepared by
a surfactant-templating method were pre-graphitized at differ-
ent temperature (X = 200, 250, 300, 350, and 400 ◦C), and they
were applied to the liquid-phase hydrogenation of succinic acid
to THF. Among the Ru–XC (X = 200, 250, 300, 350, and 400 ◦C) cat-
alysts, Ru-300C catalyst showed the smallest ruthenium particle
size due to both moderate ruthenium–carbon interaction and well
developed mesoporous structure. In the hydrogenation of succinic
acid, both conversion of succinic acid and yield for THF showed
30
25
Ru-200C
11
10
9
8
7
6
5
4
3
Ruthenium particle size (nm)
Fig. 7. A correlation between ruthenium particle size of Ru–XC catalysts and cat-
alytic activity in the hydrogenation of succinic acid to THF.

U.G. Hong et al. / Applied Catalysis A: General 469 (2014) 466–471
471
volcano-shaped trends with respect to pre-graphitization temper-
ature; an optimal pre-graphitization temperature for the treatment
of Ru–XC composites was required to achieve maximum yield for
THF. Yield for THF in the hydrogenation of succinic acid increased
with decreasing ruthenium particle size of Ru–XC catalysts. Thus,
ruthenium particle size played a key role in determining the cat-
alytic performance in the hydrogenation of succinic acid to THF
over Ru–XC catalysts. In the recycle test, Ru-300C catalyst served
as a stable and reusable catalyst for hydrogenation of succinic acid
to THF.
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Acknowledgements
This subject is supported by Korea Ministry of Environment as
“Converging Technology Project (202-091-001)”
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