Hydrogenation of succinic acid over supported rhenium catalysts prepared by the microwave-assisted thermolytic method

Article abstract of DOI:10.1039/c5cy00004a

A series of metallic Re/C catalysts were prepared with the microwave-assisted thermolytic method by using decacarbonyldirhenium [Re₂(CO)₁₀] as a precursor for the hydrogenation of succinic acid. The results of FT-IR, UV-Vis, XRD, HRTEM, ICP, TPR and CO chemical adsorption showed that the as-prepared catalysts had well-dispersed rhenium nanoparticles on activated carbon. Changing irradiation time or rhenium loadings could effectively adjust the properties of Re/C catalysts which exhibited good catalytic performance for the hydrogenation of succinic acid. There were plenty of active sites on Re/C catalysts for the high concentration hydrogenation of succinic acid, and increasing temperature or pressure improved catalytic activity at a defined scope. From the kinetic study of succinic acid catalytic hydrogenation, there was a certain converting relationship between the different intermediates and the product distribution which could be controlled by variation of reaction time. This journal is

Full text of DOI:10.1039/c5cy00004a

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Hydrogenation of succinic acid over supported
rhenium catalysts prepared by the microwave-
assisted thermolytic method†
Cite this: DOI: 10.1039/c5cy00004a
a
a
a
b
a
Xin Di, Zhengfeng Shao, Chuang Li, Wenzhen Li and Changhai Liang*
A series of metallic Re/C catalysts were prepared with the microwave-assisted thermolytic method by
using decacarbonyldirhenium ijRe IJCO)10] as a precursor for the hydrogenation of succinic acid. The results
2
of FT-IR, UV-Vis, XRD, HRTEM, ICP, TPR and CO chemical adsorption showed that the as-prepared cata-
lysts had well-dispersed rhenium nanoparticles on activated carbon. Changing irradiation time or rhenium
loadings could effectively adjust the properties of Re/C catalysts which exhibited good catalytic perfor-
mance for the hydrogenation of succinic acid. There were plenty of active sites on Re/C catalysts for the
high concentration hydrogenation of succinic acid, and increasing temperature or pressure improved cata-
lytic activity at a defined scope. From the kinetic study of succinic acid catalytic hydrogenation, there was a
certain converting relationship between the different intermediates and the product distribution which
could be controlled by variation of reaction time.
Received 2nd January 2015,
Accepted 3rd February 2015
DOI: 10.1039/c5cy00004a
www.rsc.org/catalysis
the synthesis of films and engineering plastics. The main
application of BDO is in the production of THF, which
Introduction
Considering the rapid consumption and limited reserves of
fossil fuels such as oil and gas, there is an urgent need to find
alternatives to mitigate such a situation. Of all the renewable
energies available today, biomass-derived energies and
chemicals seem to be a good choice. According to the data
released by the U.S. Department of Energy, succinic acid
approximately contributes to 40% of the total BDO produc-
tion volume.
7
,8
A series of catalysts have been developed for the hydroge-
nation of succinic acid. The activity of the catalysts is highly
2
,5,9
10–13
affected by metal species.
ruthenium
In fact, palladium
and
1
4–16
based catalytic systems are the most studied
belongs to
a
new class of bio-derived building-block
among various hydrogenation catalysts because of their high
activity and selectivity. The bimetallic catalysts exhibit differ-
ent physical and chemical properties when adopting the sec-
chemicals, which could replace the current maleic anhydride
C4 platform. The production of succinic acid could achieve
1
17
18
19
2
0 000–30 000 tonnes per year. Therefore, it is economically
ond metal as an additive, such as Ru–Co, Ru–Re, Pt–Re
2
0
and technically feasible to produce more valuable chemicals
using succinic acid as raw material. The succinic acid can be
converted into many valuable products by hydrogenation,
and Pd–Re based catalysts. It is well known that supported
rhenium catalysts can effectively reduce both carboxyl and
2
1
carbonyl compounds.
It is reasonable to propose that
such as γ-butyrolactone (GBL), tetrahydrofuran (THF),
rhenium-based catalysts should also have a positive effect on
hydrogenation of succinic acid. However, little systematic
research was performed on the hydrogenation of succinic
acid over supported Re catalysts.
At present, a number of preparation methods are reported
for supported rhenium catalysts. However, most of the
methods require the modification of the supports, and the
inevitable use of solvents in the preparation process. More-
over, it is difficult to control the preparation steps and obtain
2
22
1
,4-butanediol (BDO) and others. All these chemicals find
3
expansive use in many fields. To name a few, THF can be
used as a high-performance solvent and GBL is an impor-
tant chemical intermediate in the manufacture of pyrrolidone
4
,5
6
derivatives. BDO can be a raw material for many organic syn-
theses, including polybutylene succinate (PBS) and poly-
butylene terephthalate (PBT) resins which may be used for
a
Laboratory of Advanced Materials and Catalytic Engineering, Dalian University
highly
dispersed
catalysts.
The
microwave-assisted
of Technology, Dalian 116024, China. E-mail: changhai@dlut.edu.cn;
Fax: +86 411 84986353
thermolytic method, which is generally more environmentally
friendly than conventional wet preparation methods, has
attracted the increasing attention of various groups for the
b
DCBE, Biorenewables Research Laboratory, Iowa State University,
Iowa 50011, USA
2
3–25
syntheses of supported catalysts.
The microwave-assisted
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy00004a
thermolytic method avoids the use of solvents and has a
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better heat transfer effect than conventional methods. Cata-
lysts prepared by the microwave thermolytic method could
exhibit good textural properties and show some distinct cata-
lytic properties. For example, Ni et al. successfully prepared
supported ruthenium catalysts by the microwave-assisted
thermolytic method and the as-prepared catalysts showed
superior catalytic properties in the hydrogenation of cinnam-
microwave irradiation. The catalysts were soaked in n-octane
for 24 h and then the solution was monitored by FT-IR and
UV-Vis.
Powder X-ray diffraction (XRD) patterns of Re/C catalysts
were obtained using a Rigaku XRD diffractometer (Rigaku
D/Max-RB, Rigaku, Japan) using CuKα (λ = 1.54178 Å) as the
radiation source, operated at 40 kV and 100 mA with a scan
2
6
−1
aldehyde.
Hence, it is expected that highly dispersed
speed of 10° min .
supported rhenium catalysts could also be prepared with the
microwave-assisted thermolytic method.
The contents of rhenium in Re/C catalysts were analyzed
by inductively coupled plasma atomic emission spectroscopy
(ICP, Optima 2000DV, PerkinElmer, USA). CO chemical
adsorption was measured with an Autosorb-iQ (Quantachrome,
USA). The catalysts were reduced for 1 h under hydrogen
at 300 °C, then cooled to 30 °C and CO adsorption was
measured.
Transmission electron microscopy (TEM) and high-
resolution transmission electron microscopy (HRTEM) were
performed with a Philips CM200 FEG TEM (Philips, Japan).
The composition distribution analysis (HAADF-STEM and ele-
mental mapping) was performed in STEM mode in combina-
tion with energy dispersive X-ray spectroscopy (EDX) using a
DX4 analyzer system (EDAX) in the same microscope.
Temperature-programmed reduction (TPR) was carried out
In this work, supported rhenium catalysts were prepared
with the microwave-assisted thermolytic method by using
decacarbonyldirhenium ijRe IJCO) ] as a precursor. The influ-
2
10
ence of microwave irradiation time and rhenium loadings on
catalytic activity was investigated in detail. The structure and
textural properties of the catalysts were characterized and
discussed with their catalytic performance in the aqueous
phase hydrogenation of bio-derived succinic acid. Meanwhile,
the kinetics of succinic acid catalytic hydrogenation was also
investigated.
Experimental
Catalyst preparation
using
a chemical adsorption instrument (Micromeritics
Autochem 2910, Quantachrome, USA). The samples were
Re nanoparticles on activated carbon [C] (surface area: 1502
firstly treated with inert gas for 1 h at 200 °C, and then the
2
−1
3
−1
m g , pore volume: 0.7 cm g , pore diameter: 1.9 nm) cat-
alysts were prepared with the microwave-assisted thermolytic
−1
samples were heated to 800 °C at a speed of 10 °C min . The
gas mixture is composed of 10% hydrogen and 90% argon at
method using decacarbonyldirhenium ijRe
2
IJCO)10] (>99.9%,
−1
a flow rate of 100 mL min . The peak area of TPR profiles
Acros Organics Co. Ltd., USA) as a precursor. Before the prep-
aration of catalysts, the active carbon was pre-treated with
HNO (10 M) for 4 h at 90 °C. Re IJCO)10 and active carbon
3 2
were simultaneously added into an agate mortar and then
mechanically mixed for 20 min. The homogeneous mixture
was transferred into a fluidized quartz tube reactor which
was then put in a microwave reactor. The homogeneous mix-
ture was fluidized with argon for 2 h at room temperature to
make sure that the reaction mixture was kept under an inert
atmosphere. Afterwards, the microwave reactor was set at
was used for quantitative analysis and the peak area of the
Re/C-5 catalyst was used as the standard. The relative hydro-
gen consumption was calculated according to the following
equation:
peak area of Re/C-X
Relative hydrogen consumption of Re/C-X 
peak area of Re/C-5
2
.45 GHz with a power of 800 W and then kept for certain
minutes. Finally, the resultants were cooled to room tempera-
ture under argon. The catalysts prepared with different
microwave irradiation times and the same Re loading (4.0
wt.%) were identified as Re/C-X (X = 1, 3, 4, 5, 7, 10 and 15),
where X represents microwave irradiation time. Adjusting the
ratio of Re IJCO)10 to C, a series of catalysts with different Re
2
loadings and the same irradiation time (5 min) could be pre-
pared. These catalysts were identified as Re/C-Y% (Y = 2, 3, 4,
Catalyst evaluation
Hydrogenation of succinic acid was performed in a Hastelloy
alloy autoclave (50 mL) equipped with a magnetic stirrer, a
heater and a controller. Before reaction, the catalysts were
pretreated in a flow of hydrogen gas at 300 °C for 90 min.
The reaction solution consisted of 18.0 g of deionized water,
2
.0 g of succinic acid and 0.2 g of reduced catalyst. Then the
autoclave was purged with H to the desired pressure (typi-
cally 8.0 MPa), heated to the desired temperature (typically
40 °C) and maintained at that temperature for 10 h with
2
5
, 6 and 8), in which Y denotes the rhenium loadings.
2
−
1
continuous stirring (800 rpm min ). After reaction, the reac-
tor was cooled to room temperature. The catalysts were fil-
tered and dried at 120 °C for 4 h. The selectivity and conver-
sion of the reaction product was analyzed by gas
chromatography (Tianmei GC7890, China) and liquid chro-
matography (Waters 1525, USA), respectively.
Catalyst characterization
Fourier transform infrared spectroscopy (FT-IR, Nexus Euro,
Thermo Fisher Nicolet, USA) and ultra-violet-visible spectro-
photometry (UV-Vis, U4100, Hitachi, Japan) were used to
monitor the decomposition profiles of Re IJCO) during the
2
10
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Results and discussion
Characterization of Re/C catalysts
Fig. 1 shows the FT-IR and UV-Vis spectra of octane,
2
Re IJCO)10 and supported rhenium catalysts with different
microwave irradiation times. As shown in Fig. 1(a), the bands
−
1
−1
−1
−1
at 2071 cm , 1975 cm , 2017 cm and 590 cm are charac-
teristic absorption peaks of Re IJCO)10. The bands at 2071
are assigned to CO horizontal
2
−
1
−1
cm
stretching vibration in Re
is assigned to CO vertical stretching vibration in Re
and 1975 cm
−
1
2
IJCO)10 and the band at 2017 cm
IJCO)10
The band at 590 cm is caused by Re–C stretching vibration
2
.
−
1
2
7
in Re
2
IJCO)10
.
The intensity of these characteristic peaks
gradually disappeared with increasing irradiation time, indic-
ative of the Re IJCO) decomposition into metallic Re. Most
2
10
of the Re
2
IJCO)10 were completely decomposed when irradia-
tion time is above 3 minutes. From Fig. 1(b), the bands at
2
Re
76 nm and 309 nm are ascribed to the existence of CO in
2
8
2
IJCO)10
.
The results of UV-Vis in Fig. 1(b) are consistent
with the results of FT-IR.
Fig. 2 shows the XRD patterns of supported rhenium cata-
lysts at different microwave irradiation times and with differ-
ent Re loadings. According to standard X-ray diffraction pat-
terns (JCPDS no. 05-0702), diffraction peaks at 2θ = 42.9, 40.5
and 37.6° should be observed in these catalysts, which were
Fig.
2
XRD patterns of supported Re catalysts with different
irradiation times and different Re loadings.
indexed as the (101), (002) and (100) planes of metallic Re.
However, Fig. 2(a) reveals that no discernible characteristic
diffraction peak due to metallic Re was observed but the dif-
fraction peaks of active carbon. This indicates that the rhe-
nium particles are too small to be resolved (less than 4 nm)
and/or highly dispersed on the surface of active carbon. In
addition, the characteristic diffraction peaks of Re almost
coincided with that of active carbon, which would also cover
the peaks of Re. When increasing the Re loading of catalysts,
no significant reflection was observed from XRD patterns in
Fig. 2(b), indicating that the rhenium particles still exhibited
well dispersion and small particle size at high Re loading.
To further study the structure and composition distribu-
tion, Fig. 3 and Fig. S1† show the TEM, HRTEM images and
STEM of supported Re catalysts. The TEM image in Fig. 3(a)
confirmed that the metal particles were well dispersed on Re/C
catalysts, and exhibited non-symmetrical shapes with particle
size ranging from 1 to 4 nm. The catalysts prepared with
5
minutes microwave irradiation time (Re/C-5) showed better
dispersion and smaller particle size than other catalysts
Re/C-7 and Re/C-10) in Fig. S1.† When increasing irradiation
(
time, the agglomeration of particles took place as a result of
the extra energy provided by the microwave irradiation. It can
be seen from the particle size distribution in Fig. S1† that the
particle size moderately increases with extended irradiation
Fig. 1 FT-IR (a) and UV-Vis (b) spectra of liquid from Re/C-X (X = 1–10)
dissolved in octane, Re IJCO)10 and octane.
2
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could be adjusted by varying the microwave time in the
microwave-assisted thermolytic method.
Homogeneous Re, C and O distributions in the sample are
confirmed by the STEM dark field image and the correspond-
ing elemental maps are shown in Fig. 3(e). The existence of
oxygen may be due to the oxygen-containing groups on active
carbon obtained from its pre-treating process by HNO solu-
3
tion. Meanwhile the elemental maps also demonstrate the
well dispersion of rhenium particles.
The results of CO chemical adsorption and ICP are shown
in Table 1. The CO chemical adsorption indicates that there
are more active sites on the surface of Re/C catalysts prepared
1
3
by the microwave-assisted method than other Re/C catalysts
and rhenium particles exhibited good dispersion. The
amounts of active sites firstly increased because more
Re IJCO) decomposed into Re metal when increasing irradi-
2
10
ation time and then decreased as a result of agglomeration of
particles when prolonging irradiation time. But the phenome-
non of agglomeration was only moderate after 7 minutes of
irradiation since no obvious change was observed for CO
adsorption capacity and dispersion. These results could also
be verified by TEM images.
2
H -TPR tests were performed to investigate the change in
reduction state of catalysts as a function of temperature. One
peak was detected at around 370 °C in all catalysts prepared
with different irradiation times from Fig. 4. This hydrogen
consumption is assigned to the reduction of oxidized Re
which is caused by re-oxidation of metallic rhenium after
2
9
exposure to oxygen in the air.
It was found that the reduction temperature didn't change
with increasing microwave irradiation time, which demon-
strated that the interaction between Re and supports was
almost the same in these catalysts. As shown in Table 2, the
catalysts with different irradiation times exhibited the same
hydrogen consumption. It indicated that Re/C-X catalysts
showed a similar phase state which was not influenced by
the preparation conditions such as irradiation time. Besides,
a broader peak was observed at high temperature, between
500 and 700 °C, which can be attributed to carbon methana-
tion. The active carbon supports were pretreated with nitric
acid solution to produce more functional groups on its sur-
face, because of that, partial carbon on the surface of sup-
ports can easily react with hydrogen. Meanwhile, the exis-
tence of the Re phase might also enhance the reduction of
Fig. 3 TEM image (a), HRTEM image (b), HAADF-STEM (c), (d) and ele-
mental mapping (e) of Re/C-5 catalyst which is pre-reduced by H .
2
time. The lattice spacing (2.11 Å and 2.39 Å) observed in
HRTEM images clearly points to hexagonal rhenium ((101)
with 2.11 Å and (100) with 2.39 Å based on JCPDS 05-0702;
Fig. 3(b)).The phenomenon of agglomeration usually couldn't
be avoided in the preparation process of catalysts, especially
for the microwave-assisted thermolytic method which pos-
sesses a high-energy feature. However, most of rhenium par-
ticles are well dispersed on the Re/C catalyst and exhibit
small non-symmetrical shapes except for a small number of
larger nanoparticles caused by agglomeration of rhenium
metallics from the images of HAADF-STEM (Fig. 3(c) and (d)).
Combined with the results in Fig. S1 ,† the particle morphology
3
0
functionalized carbon on the surface of catalysts.
Table 1 CO-absorb and ICP results of supported Re catalysts
−1
a
Catalysts
Re loading in theory (wt.%)
Actual Re loading (wt.%)
CO adsorption capacity (μmol g
)
Dispersion (%)
Active carbon
Re/C-3
Re/C-5
Re/C-7
Re/C-10
—
—
1.0
45.0
52.7
45.1
49.2
—
4.0
4.0
4.0
4.0
4.0
3.8
3.9
3.9
20.9
25.8
21.5
23.5
a
Irreversible adsorption of CO calculated per each metal atom in Re/C catalyst.
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sites in return. From the results of CO chemical adsorption and
TEM images, the influence of this phenomenon was not very
obvious when further increasing irradiation time. Coincided
with that, the conversion of succinic acid changed only little
when irradiation time was above 7 minutes.
With increasing Re loading (from 2 wt.% to 8 wt.%), the
conversion of succinic acid increased from 34.1% to 94.5%
with the selectivity to GBL decreasing from 93.2% to 19.8%
and the selectivity to THF increasing from 5.7% to 66.7%.
The conversion of succinic acid had a great leap at 4 wt.% Re
resulting from the lack of active sites when rhenium loading
was very low. Furthermore, the change in conversion was
small, compared to a significant change in selectivity when
Re loading was above 4%. When the conversion of succinic
acid was above 90%, the concentration of succinic acid was
much lower than that of GBL and most of the active sites
were occupied by it. The transformation of succinic acid then
slowed down, resulting in a small change in conversion of
succinic acid and increased selectivity to THF when Re load-
ing was above 4 wt.%.
Fig. 4
2
H -TPR profiles of supported Re catalysts.
Table 2 Relative hydrogenation consumption of supported Re catalysts
Re/C-5
1.00
Re/C-10
1.01
Re/C-15
0.99
Effect of succinic acid concentration. The effects of
succinic acid concentration on conversion and products are
demonstrated in Table 4. High conversion (>90%) could be
obtained even though the concentration was increased up to
Relative hydrogen consumption
Hydrogenation of succinic acid
1
0 wt.% which was higher than the concentration in indus-
6
Effects of microwave irradiation time and rhenium load-
ing on catalytic performance. The activity of different cata-
lysts in hydrogenation of succinic acid is summarized in
Table 3. In the aqueous phase hydrogenation of succinic
acid, the catalytic activity firstly increased and then decreased
with increasing microwave irradiation time. The highest con-
version of succinic acid (91.0%), with 59.2% selectivity to
GBL and 33.0% selectivity to THF, was detected on Re/C-5.
The variation in selectivity could be tuned by varying the
amounts of active sites on catalysts. Increasing irradiation
trial production to our knowledge. The selectivity to GBL
was improved with increasing succinic acid content in aque-
ous solution and the selectivity to THF was just the opposite.
This phenomenon could be explained by investigating the
reaction mechanism as shown in Scheme 1. Hydrogenation
of succinic acid involves consecutive reaction steps and GBL
was proposed to be the intermediate product. The catalysts
have sufficient active sites for hydrogenation of succinic acid
and for further hydrogenation of GBL at low acid concentra-
tion. However, at high acid concentration, the reaction needs
more active sites and most of active sites are occupied by
succinic acid which suppressed further hydrogenation of GBL.
Effect of reaction temperature and pressure. From the
results in Table 5, increasing temperature (from 200 °C to
1
3
time until 5 min, there are more Re IJCO)10 that decomposed
2
to Re metal, as already proven by CO chemical adsorption
studies. However, the particles that agglomerate by overly pro-
longing irradiation time would decrease the amounts of active
Table 3 Hydrogenation of succinic acid on different catalysts
a
Selectivity (%)
Conversion
(%)
Catalyst
Reactant
GBL
THF
BDO
NBA
NPA
Re/C-3
Re/C-4
Re/C-5
Re/C-7
Re/C-10
Re/C-15
Re/C-2%
Re/C-3%
Re/C-4%
Re/C-5%
Re/C-6%
Re/C-8%
SUC
SUC
SUC
SUC
SUC
SUC
SUC
SUC
SUC
SUC
SUC
SUC
70.3
72.3
91.0
78.5
84.1
82.6
34.1
48.4
91.0
92.3
92.9
94.5
83.3
82.6
59.2
78.1
70.3
75.2
93.2
90.0
59.2
41.8
43.6
19.8
14.5
15.1
33.0
19.3
25.1
21.6
5.7
0.4
0.6
4.0
0.5
1.6
1.2
0.2
0.3
4.0
8.4
5.9
7.1
1.3
1.3
2.9
1.4
2.2
1.6
0.5
0.7
2.9
3.7
4.0
5.0
0.5
0.4
0.9
0.7
0.8
0.4
0.4
0.5
0.9
1.6
1.1
1.4
8.5
33.0
44.5
45.4
66.7
a
GBL = γ-butyrolactone; THF = tetrahydrofuran; BDO = 1,4-butanediol; NBA = n-butanol; NPA = n-propanol. Reaction conditions: 20 g solution
concentration = 10 wt.%), 0.2 g catalyst, 240 °C, 8.0 MPa, 800 rpm, 10 h.
(
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Table 4 Effect of succinic acid concentration on conversion and product
compared with increasing temperature, but the extent of the
impact was much smaller. Besides that, not only was the
succinic acid hydrogenolysis accelerated, but also could the
by-products such as NBA and NPA be slightly increased with
the increase in temperature and pressure. These results indi-
cate that the catalyst can generate fewer by-products in the
hydrogenation of succinic acid.
Kinetics of succinic acid hydrogenation. Our group has
also studied the kinetic relationship between different reac-
tants over supported rhenium catalysts. It was found that the
conversion was 81.3% and the main products were THF
a
distribution over Re/C-5 catalysts
Selectivity (%)
Succinic acid
Conversion
concentration (wt.%) (%)
GBL THF BDO NBA NPA
5
7
1
1
1
95.5
91.4
91.0
77.1
69.6
17.7 65.5 9.9
52.5 38.1 5.8
59.2 33.0 4.0
79.2 18.2 0.5
82.4 15.6 0.7
5.3
2.8
2.9
1.7
1.0
1.6
0.8
0.9
0.4
0.3
0
2
5
a
Reaction conditions: 20 g solution (concentration = weight of
succinic acid × 100/weight of solution), 0.2 g catalyst, 240 °C, 8.0 MPa,
00 rpm, 10 h.
8
(
89.5%) and BDO (6.2%) when using GBL as the reactant;
however, when the reactant was THF or BDO, less or no GBL
was produced, as shown in Table 6. This indicates that SUC
is firstly hydrogenated to GBL and then proceeds further to
hydrogenation of GBL. From the results, the conversion was
9
3.6% and the main product was THF (93.1%) when using
BDO as the reactant, compared to a very low conversion
1.7%) with THF as the reactant. The yield of NBA (0.6 and
(
NPA (0%) was much lower when using THF as the reactant
than the yield of NBA (4%) and NPA (0.6%) when BDO was
used as the reactant. This shows that the main products of
GBL hydrogenolysis are THF and BDO, at the same time BDO
could be further hydrogenated to THF, NBA and NPA, on the
contrary, THF could not be further converted to other prod-
ucts. The cyclodehydration of BDO to THF could be
influenced by acidic properties of catalysts and by reaction
3
1,32
temperature.
vated carbon, the activated carbon was pre-treated with
HNO solution which increases the functional groups such as
To remove impurities and modify the acti-
3
3
3
hydroxyl groups on the surface of Re/C catalysts. The graft
of functional groups onto the surface of active carbon could
increase the acidity of support and promoted the cyclo-
dehydration of BDO to THF. In addition to the influence of
acidity, the high temperature (above 200 °C) could also accel-
erate cyclodehydration of BDO in our experiment. The effort-
less cyclodehydration of BDO was proved by the results
above. A reasonable reaction route is shown in Scheme 1.
The time course of succinic acid hydrogenolysis is shown
in Fig. 5. The relative concentration of SUC decreased with
increasing reaction time. The reaction rate of succinic acid
decreased after 10 h as a result of its lowered concentration.
The relative concentration of GBL increased firstly, then
passed a maximum and gradually dropped. When reaction
time was below 3 h, it was found that the consumption rate
Scheme
1 Main reaction route of succinic acid catalytic
hydrogenation with supported rhenium catalysts.
2
40 °C) would observably promote the conversion of SUC
from 46.2% to 91.0%) which was accompanied by the
increase and decrease in the selectivity to THF (from 7.8% to
3.0%) and GBL (from 90.2% to 59.2%), respectively. For this
(
3
consecutive reaction (Scheme 1), the transformation of inter-
mediate product GBL was also simultaneously enhanced
when the catalytic activity was improved by increasing tem-
perature. Increasing pressure could exhibit a similar rule
Table 5 Hydrogenation of succinic acid over Re/C-5 in different reaction conditionsa
Reaction conditions
Selectivity (%)
Conversion
(%)
Temperature (°C)
Pressure (MPa)
GBL
THF
BDO
NBA
NPA
2
2
2
2
2
00
20
40
40
40
8.0
8.0
8.0
7.0
6.0
46.2
70.1
91.0
85.6
80.3
90.2
82.6
59.2
69.4
79.2
7.8
14.9
33.0
26.6
17.6
0.8
1.1
4.0
1.1
1.0
0.8
1.1
2.9
2.1
1.7
0.4
0.3
0.9
0.8
0.5
a
Reaction conditions: 20 g solution (concentration of SUC = 10 wt.%), 0.2 g catalyst, 800 rpm, 10 h.
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Table 6 Hydrogenation of succinic acid and its hydrogenated products over supported Re catalystsa
Selectivity (%)
Conversion
Catalyst
Reactant
(%)
SUC
GBL
THF
BDO
NBA
NPA
Re/C-5
Re/C-5
Re/C-5
Re/C-5
SUC
GBL
BDO
THF
91.0
81.3
93.6
1.7
—
0
0
59.2
—
2.2
0
33.0
89.5
93.1
—
4.0
6.2
—
2.9
3.4
4.1
0.9
0.9
0.6
0
0
62.6
37.4
a
Reaction conditions: 20 g solution (concentration = 10 wt.%), 0.2 g catalyst, 240 °C, 8.0 MPa, 800 rpm, 10 h.
Fig. 6 Aqueous hydrogenation of succinic acid over Re/C-5 catalysts
with respect to recycle run.
Fig. 5 Time course of succinic acid hydrogenolysis over Re/C-5
catalysts. (Reaction conditions: 20 g solution (concentration = 10 wt.%),
0
.2 g catalyst, 240 °C, 8.0 MPa, 800 rpm).
small amount of fresh catalysts which were used to make up
for the loss of catalysts were added into the autoclave reactor.
Fig. 6 shows the result for aqueous hydrogenation of
succinic acid over Re/C-5 catalysts with respect to recycle run.
A slight drop of catalytic activity was detected in our experi-
ment after five times recycle. No significant leaching of rhe-
nium was detected by ICP-AES analysis from the result in
Table S1.† It indicates that the Re/C-5 catalysts prepared with
the microwave-assisted method which relies on the
thermolytic effect of microwave irradiation could serve as a
stable catalyst in the aqueous hydrogenation of succinic acid.
of SUC was equal to the production rate of GBL, which was
almost the only product. Combined with Scheme 1, step 1
was firstly accelerated. When the reaction proceeded to above
2
0 h (SUC was almost completely consumed), the consump-
tion rate of GBL was equal with the production rate of BDO
and THF. By increasing concentration of GBL, step 2 and step
4
are activated. THF is the main final product because of the
fact that BDO could be easily hydrogenated to THF (step 6).
That is also the reason why the selectivity or relative concen-
tration of THF continues to increase with increasing pres-
sure, temperature, time and active sites. All these results
show that GBL is an intermediate product which could
achieve a maximum yield when reaction time is appropriate,
and THF is the final product whose yield could exceed 40%
accompanied by less by-products when extending the reac-
tion time. In conclusion, the product distribution could also
be adjusted by varying the reaction time at a certain scope
from the results of kinetic studies.
Stability of Re/C catalyst. To investigate the stability and
reusability of the catalyst, recycle tests for hydrogenation of
succinic acid over Re/C-5 catalysts were performed for five
times in our experiment. After reaction, most of the reaction
liquid was separated from the catalyst by centrifugation and
only a little part was left to protect catalysts from oxidation.
Then, the fresh reaction liquid, separated catalysts and a
Conclusions
The supported rhenium catalysts can be prepared by the
microwave-assisted thermolytic method. The use of organic
solvents can be avoided and thus this method provides a
route to a more environmentally friendly process. The cata-
lysts exhibit good dispersion of rhenium nanoparticles and
efficient performance for the catalytic hydrogenation of
succinic acid in aqueous solution. The catalytic properties of
Re/C can be modified by changing Re loading, leading to
improved conversion and selectivity. The agglomeration of Re
particles caused by the thermal effect due to extra energy
results in a decreased activity of Re/C catalysts. In addition,
the catalytic hydrogenation of succinic acid on supported
rhenium catalysts (Re/C-5) is a consecutive reaction and
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exhibits certain conversion relationship between different
substances. Further studies demonstrated that different
product distributions could be obtained by the variation of
12 B. Tapin, F. Epron, C. Especel, B. K. Ly, C. Pinel and M.
Besson, ACS Catal., 2013, 3, 2327–2335.
13 Z. Shao, C. Li, X. Di, Z. Xiao and C. Liang, Ind. Eng. Chem.
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14 U. G. Hong, H. W. Park, J. Lee, S. Hwang and I. K. Song,
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1
3
reaction time. According to our research in the past and
1
2,15,16,19
the literature,
Re/C catalysts prepared with the
microwave-assisted thermolytic method exhibit more superior
transformation performance of GBL which is the intermediate
product to Pd based catalysts, and generate fewer by-products
than Ru based catalysts when used for hydrogenation of
succinic acid.
16 U. G. Hong, J. K. Kim, J. Lee, J. K. Lee, J. H. Song, J. Yi and
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1
1
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7 R. M. Deshpande, V. V. Buwa, C. V. Rode, R. V. Chaudhari
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Acknowledgements
We gratefully acknowledge the financial support provided by
the National Natural Science Foundation of China (no.
2
1428301 and 21373038), the Fundamental Research Funds
for the Central Universities IJDUT13RCIJ3)41) and the Postdoc-
toral Science Foundation of China (2013M541220). We thank
Dr. Bingsen Zhang, from the Institute of Metal Research, Chi-
nese Academy of Sciences, for technical support in HRTEM
and STEM test.
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