Cobalt catalysts: Very efficient for hydrogenation of biomass-derived ethyl levulinate to gamma-valerolactone under mild conditions

Article abstract of DOI:10.1039/c4gc00482e

Hydrogenation of ester levulinate to gamma-valerolactone (GVL) is an interesting reaction in biomass conversion to produce value-added chemicals. Exploration of efficient and robust catalysts is crucial for large-scale application. In this work, we conducted the reaction catalyzed by a Co catalyst, and it was found that commercially available Co₃O₄ was very efficient for this reaction under mild conditions after reduction by H ₂. The effects of temperature, hydrogen pressure, amount of the catalyst used, and reaction time on the yield of GVL were studied. Under optimized conditions, the yield of GVL could reach 98%. The catalyst could be reused at least 10 times without notable loss of the activity and selectivity. The catalyst was characterized by scanning electron spectroscopy (SEM), transmission electron spectroscopy (TEM), X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (XRD). It was demonstrated that the metallic Co⁰ was the active species for the hydrogenation reaction. As far as we know, this is the first work conducting the reaction using Co as the catalyst. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00482e

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Cobalt catalysts: very efficient for hydrogenation
of biomass-derived ethyl levulinate to gamma-
valerolactone under mild conditions
Cite this: DOI: 10.1039/c4gc00482e
Huacong Zhou, Jinliang Song, Honglei Fan, Binbin Zhang, Yingying Yang, Jiayin Hu,
Qinggong Zhu and Buxing Han*
Hydrogenation of ester levulinate to gamma-valerolactone (GVL) is an interesting reaction in biomass
conversion to produce value-added chemicals. Exploration of efficient and robust catalysts is crucial for
large-scale application. In this work, we conducted the reaction catalyzed by a Co catalyst, and it was
found that commercially available Co3O4 was very efficient for this reaction under mild conditions after
reduction by H2. The effects of temperature, hydrogen pressure, amount of the catalyst used, and reaction
time on the yield of GVL were studied. Under optimized conditions, the yield of GVL could reach 98%.
The catalyst could be reused at least 10 times without notable loss of the activity and selectivity. The cata-
lyst was characterized by scanning electron spectroscopy (SEM), transmission electron spectroscopy
(TEM), X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (XRD). It was demonstrated
that the metallic Co0 was the active species for the hydrogenation reaction. As far as we know, this is the
first work conducting the reaction using Co as the catalyst.
Received 19th March 2014,
Accepted 4th June 2014
DOI: 10.1039/c4gc00482e
www.rsc.org/greenchem
duction of LA.28–30 In the process of hydrogenation, the
H-donors can be H2,19 formic acid,31,32 or isopropanol.18
Introduction
Gradual depletion of fossil resources has stimulated the efforts Different metal catalysts have been applied in the hydrogen-
for the sustainable production of fuels and chemicals. Ligno- ation of LA and its esters, such as Ru, Rh, Pd, Pt, Ir, Re, Au,
cellulose, the main component of biomass, is an abundant Cu, and Ni.20,33–36 Although many noble metal catalysts show
and renewable carbon source for sustainable production of excellent catalytic performances, the high cost and relatively
bulk chemicals and fuel additives.1–3 Through biological or low abundance limit their large-scale application to a certain
chemical approaches, lignocellulose and its derivatives can be degree.19,32 Furthermore, most of the reported catalysts often
converted into value-added chemicals, such as various require a multi-step preparation or pretreatment process.20,37
alcohols,4–6 short chain hydrocarbons,7,8 gluconic acid,9 Obviously, it is still highly desirable to develop non-precious,
5-hydroxymethylfurfural (HMF),10–12 lactic acid,13 pentanoic economically feasible, and easy-preparing catalysts using
acid esters,14 2,5-dimethylfuran,15 levulinic acid,16 gamma- abundant metals for GVL production from biomass-derived
valerolactone (GVL),17,18 etc. Among these platform molecules, platforms.19,38,39
GVL is considered as one of the most appealing molecules as a
fuel additive, solvent, liquid fuel, and ideal precursor for the ped for the conversion of LA and its esters to GVL. RANEY®
production of more valuable chemicals.19,20
Ni, the representative of the Ni-based catalysts, has been
Several transition metal-based catalysts have been develo-
Various biomass molecules have been reported for the pro- applied in the hydrogenation of LA into GVL since 1947.40 The
duction of GVL, such as cellulose,21 C6-sugars,22,23 furfural application of Ni-based catalysts was also followed by other
alcohol,18,24 etc. The hydrogenation of levulinic acid (LA) and researchers.41,42 An efficient catalytic transfer hydrogenation
its esters can generate GVL.18,25–27 Comparatively, the hydro- (CTH) process with isopropanol as the hydrogen source was
genation of ester levulinate to GVL is more attractive because developed using RANEY® Ni as the catalyst.18 ZrO2 was also
the production of ester levulinate from lignocellulose has a reported to be an efficient catalyst in the CTH process for GVL
higher yield and a facile separation process than the pro- production, but the reaction temperature was higher.38 The
catalysts of Cu/SiO2 with tuneable Cu loadings were also
applied to convert LA to GVL.37 Up to now, most of the
reported transition metal-based catalysts needed relatively
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry,
high reaction temperatures, such as 150–220 °C for ZrO238 and
Chinese Academy of Sciences, Beijing 100190, China. E-mail: Hanbx@iccas.ac.cn;
265 °C for Cu/SiO2 catalysts.37
Tel: +86-10-62562821
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Herein, we carried out the first work for hydrogenation of charged into the reactor to the desired pressure, and then the
ethyl levulinate (EL) into GVL using the Co catalyst. It was stirrer was started. After reaction, the reactor was cooled in
found that commercial Co3O4 could catalyze the reaction water to quench the reaction and the organic phase was
efficiently under mild reaction conditions after being reduced extracted from the mixture by ethyl acetate. The sample was
under a H2 atmosphere. There are some obvious advantages of analyzed by a gas chromatograph (GC, HP 4890) equipped
this catalyst for GVL production, such as the relatively simple with a flame ionization detector (FID), and dodecane was used
preparation process, high stability and efficiency, and the use as the internal standard. Identification of the products and the
of an abundant metal as the catalyst.
reactant was done using a GC-MS (SHIMADZU-QP2010) as well
as by comparing the retention times with respective standards
in GC traces. In the reusability experiments, the catalyst was
separated from the liquid by an external magnet, washed with
ethanol and dried at room temperature under vacuum, and
then reused for the next run without further reduction.
Experimental
Materials
Ethyl levulinate (98%), gamma-valerolactone (98%), and
dodecane (99%) were purchased from J&K Chemical. All the
transition metal oxides were A. R. grade and provided by Sino-
pharm Chemical Reagent Co. Ltd. H2 (99.99%) was provided
by the Beijing Analytical Instrument Company. All the chemi-
cals were used as received.
Results and discussion
Screening and characterization of the catalysts
In our studies, Ni, Fe, and Co obtained from the metal oxides
by hydrogen reduction were used as the catalysts for the hydro-
genation of EL to GVL. The results are shown in Table 1. It can
be seen that the Ni showed a moderate hydrogenation activity
with a GVL yield of 35% (entry 1). In contrast with the Ni cata-
lyst, the Fe catalyst showed no activity for the hydrogenation
reaction (entry 2). Interestingly, the Co showed high activity
with a GVL yield up to 94% (entry 3). Different cobalt oxides,
Co3O4 and CoO, were also attempted as catalysts in the hydro-
genation of EL. It was found that both Co3O4 and CoO showed
very low activity (entries 4, 5). It can be deduced that the
metallic Co0 was active for the reaction, which was verified in
the following characterizations.
Catalyst preparation
Before being used, the Co3O4, NiO, and Fe3O4 were reduced to
the metallic state by H2. For the reduction of Co3O4 and NiO, a
metal oxide was reduced in pure H2 flow (15 mL min−1) at
atmospheric pressure by increasing the temperature from
25 °C to 400 °C at the rate of 5 °C min−1, and maintained at
this temperature for 1 h before cooling to room temperature.
For the reduction of Fe3O4, the metal oxide was reduced in
pure H2 flow (15 mL min−1) at atmospheric pressure by
increasing the temperature from 25 °C to 500 °C at the rate of
5 °C min−1, and maintained at this temperature for 1 h before
cooling to room temperature.
In order to understand well the structure of the catalyst,
some characterizations were conducted. Firstly, the mor-
phology of the catalyst before and after reduction was charac-
terized by SEM and TEM techniques (Fig. 1). From SEM
images (Fig. 1a, 1b), the surface of the catalyst was rather
smooth and pyknotic before H2 reduction, while it turned
rough and porous after reduction. The TEM images further
Catalyst characterization
The structural properties of the catalysts were characterized by
transmission electron microscopy (TEM, JEOL JEM-2100F) and
scanning electron microscopy (SEM, HITACHI S-4800). Powder
X-ray diffraction (XRD) patterns were recorded on a Rigaku D/
max-2500 X-ray diffractometer using Cu Kα radiation (y =
0.15406 nm). The tube voltage was 40 kV and the current was
200 mA. X-ray photoelectron spectroscopy (XPS) was performed
on the Thermo Scientific ESCALab 250Xi using a 200 W mono-
chromated Al Kα radiation. The 500 μm X-ray spot was used for
XPS analysis. The base pressure in the analysis chamber was
about 3 × 10−10 mbar. Typically, the hydrocarbon C 1s line at
284.8 eV from adventitious carbon was used for energy
referencing. The analysis of Co concentration in the reaction
solution was conducted on ICP-AES (VISTA-MPX).
Table 1 Screening of catalysts for GVL productiona
Entry
Catalysts
Conversion/%
Selectivity/%
Yield/%
1
2
Ni
Fe
36
0
97
0
35
0
Reaction
3
Co
CoO
Co3O4
99
2
2
95
50
100
94
1
2
The hydrogenation reaction was performed in a 10 mL Teflon-
lined stainless steel autoclave equipped with a magnetic
stirrer. In a typical experiment, 1 g ethyl levulinate (EL) and a
suitable amount of the catalyst were introduced into the
reactor. After removing the air via a vacuum pump, the reactor
4b
5b
a Typical reaction conditions were as follows until otherwise stated: EL,
1 g; catalyst, 0.1 g; reaction temperature, 130 °C; H2 pressure, 3.3 MPa;
reaction time, 3 h. b Commercial oxides were used directly without
was placed in a constant temperature air bath. Hydrogen was reduction.
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Fig. 1 SEM images of the Co3O4 before (a) and after (b) reduction, and
TEM images of the catalyst after reduction (c). The insets show larger
magnification.
showed that the surface of the reduced catalyst had a loose
and fibrous texture (Fig. 1c), which is favourable for enhancing
the activity of the catalyst and the mass transfer of the reaction
species.
Fig. 3 XRD patterns of the catalysts reduced at different temperatures:
(a) the original Co3O4; (b) 300 °C; and (c) 400 °C.
XPS measurement was conducted to detect the chemical
valence change of cobalt on the surface of the catalyst after
reduction. The Co 2p spectra of the Co3O4 without reduction
and reduced under typical conditions are given in Fig. 2. The
decomposed spectrum of Co 2p could provide detailed infor-
mation on the surface composition of Co3O4. On the basis of
the published Co 2p spectra of Co3O4,43,44 the peak at the
binding energy (BE) of 779.5 eV could be assigned to a CoII/III
2p3/2 configuration, while the peak at 794.6 eV could be attri-
buted to the CoII/III 2p1/2 configuration (Fig. 2a). The weak
peaks at 788.1 eV and 804.7 eV represented the shake-up peaks
for CoII/III 2p3/2 and CoII/III 2p1/2, respectively. No peak for the
metallic Co was observed in the Co 2p spectrum. These results
indicated that it was the oxidation state of cobalt on the
surface of Co3O4 without reduction. After Co3O4 was reduced
by H2, new peaks at 778.4 eV and 793.5 eV were observed
(Fig. 2b). These two peaks could be assigned to Co0 2p3/2 and
Co0 2p1/2, respectively.45 Thus, the metallic Co appeared on the
surface of the catalyst after reduction, which verified our above
speculation.
(PDF# 48-1719), while the peaks at 41.4°, 44.5° and 47.4° could
be assigned to (010), (002) and (011) planes of the hcp-Co
phase (PDF# 05-0727). This result was in agreement with the
previous report that Co3O4 was reduced first to CoO and then
to the metallic Co.45 With partial reduction of Co3O4, the
activity of the catalyst increased to a moderate level with a GVL
yield of 52%. For the case of 400 °C, Co3O4 was totally reduced
to the metallic Co because the peaks for Co3O4 disappeared.
Besides the peaks for the hcp-Co phase, a new weak peak at
51.4° appeared, which could be assigned to the fcc-Co phase
(PDF# 15-0806).46 The catalyst obtained at this reduction temp-
erature exhibited the highest activity with a GVL yield up to
98%. These results verified further that the metallic Co0 was
the active species for the hydrogenation process.
The effect of catalyst dosage
The effect of catalyst dosage is given in Fig. 4. When the ratio
of the catalyst to the substrate (EL) was 5 wt%, the GVL yield
increased with the increasing of reaction time and reached
up to 98% within 13 h. The reaction time could be shortened
To gain an insight into the bulk phase change of the cata-
lyst after reduction, the XRD patterns of the catalysts were
adopted (Fig. 3). It was found that the catalysts reduced under
low temperatures (<200 °C) retained the structures of the origi-
nal Co3O4 (JCPDS database PDF# 42-1617). When increasing
the reduction temperature to 300 °C, new peaks appeared in
the XRD patterns. The peaks at 36.4°, 42.5° and 61.5° corres-
ponded to (111), (002) and (022) reflections of the CoO phase
Fig. 2 Co 2p spectra of Co3O4 without reduction (a), and reduced by
Fig. 4 Influence of the catalyst dosage on the yield of GVL. Reaction
H2 at 400 °C (b).
conditions: EL, 1 g; 130 °C; H2 pressure, 3 MPa.
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dramatically by increasing the catalyst dosage. Under a 15 wt%
catalyst dosage, the reaction proceeded fast in the first two
hours with a GVL yield of 89%, and the GVL yield increased to
97% within 8 h. Thus, a catalyst dosage of 15 wt% was
adopted to obtain acceptable reaction rate and GVL yields.
We also studied whether the reaction could proceed under
homogeneous catalysis. In the experiment, the solid Co cata-
lyst was removed using an external magnet after a reaction
time of 2 h under a 5 wt% dosage and the solution was
allowed to react under the same reaction conditions (Fig. 4). It
was found that no more GVL was produced after the removal
of the catalyst, proving that the solid Co particles played the
catalytic role.
The effect of the reaction temperature
The effect of reaction temperature on GVL yield is shown in
Fig. 5. It can be seen that reaction temperature affected the
GVL yields dramatically. In the low temperature range
(<100 °C), the catalyst exhibited low activity with a GVL yield
less than 21% within 2 h. Increasing the temperature could
enhance the yield significantly. The GVL yield could reach
89% within 2 h at 130 °C. Thus, a temperature of 130 °C is
high enough to obtain satisfactory reaction rate and GVL
yields. This temperature is lower than that required for the
reaction catalyzed by Ir/C (150 °C),41 ZrO2 (150–220 °C),38 and
Cu/SiO2 (265 °C).37
Fig. 6 Catalytic performances at different H2 pressures. Reaction con-
ditions: EL, 1 g; catalyst, 0.15 g; reaction temperature, 130 °C; reaction
time, 2 h.
Reusability of the catalyst
The reusability of the catalyst was investigated under the
experimental conditions that the yield of GVL was controlled
around 50%. After reaction, the catalyst could be separated by
centrifugation or simply applying a magnet because the cata-
lyst could respond to an external magnetic field (Fig. 7). The
results are given in Fig. 8, for which the catalyst was recycled
by an external magnetic field. The catalyst could be reused at
least ten times without considerable change in GVL yields. The
Co leaching during the reactions was investigated by determin-
ing the Co concentration in the reaction solutions via ICP, and
the results were also given in Fig. 8. It can be seen that the Co
concentrations in the reaction solutions were extremely low
(<0.42 ppm) during the ten recycling times. This may be attri-
buted to solvent-free and weak polarity of the reactant and the
product. To investigate the morphology and structural change
of the catalyst after reuse for 10 times, the recycled catalyst was
characterized by SEM, XPS, and XRD (Fig. 9). From the SEM
images (Fig. 9a and 9b), it can be seen that the porous struc-
The effect of H2 pressure
The effect of H2 pressure on the yield of GVL is shown in
Fig. 6. A moderate GVL yield (59%) was observed even under
low H2 pressure (1 MPa). Increasing the H2 pressure could
improve the reaction rate. The GVL yield could be as high as
96% within 2 h at a H2 pressure of 4 MPa. The results showed
that under similar conditions, the performance of the Co cata-
lyst in this study is comparable with or even better than those
of the reported catalysts, such as ZrO2, RANEY® Ni, Pd/C, Rh/
C, Pt/C, Re/C, etc.38,41,42
Fig. 5 Effect of reaction temperature on the yield of GVL. Reaction
conditions: EL, 1 g; catalyst, 0.15 g; H2 pressure, 3 MPa; reaction time,
2 h.
Fig. 7 Recycling of the catalysts using an external magnet.
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Conclusion
The hydrogen-reduced commercial Co3O4 has been found to
be very active and selective in the hydrogenation of EL to GVL
without using any solvent. Under the optimized conditions, EL
could nearly be quantitatively converted into GVL with a high
yield up to 98% under mild temperature and H2 pressure.
Further study has indicated that metallic Co0 is the active
species in the hydrogenation reaction. The catalyst can be
reused at least 10 times without decreasing the yield of GVL
considerably. We believe that the efficient and stable catalyst
has great potential of application with obvious advantages.
Acknowledgements
Fig. 8 The performance of the catalyst in the reuse experiments. Reac-
tion conditions: EL, 1 g; catalyst, 0.05 g; 130 °C; H2 pressure, 3 MPa;
reaction time, 3 h.
The authors thank the National Natural Science Foundation of
China. This work was supported by the National Natural
Science Foundation of China (21173234, 21133009, 21021003),
and Chinese Academy of Sciences (KJCX2.YW.H30). The
authors also thank Prof. Fen Liu, Dr Zhijuan Zhao, and Dr
Xiaoyu Zhang, Analysis and Measurement Laboratory of XPS,
Institute of Chemistry, CAS, for their help in XPS measurement
and data analysis.
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