Catalytic Transfer Hydrogenation of Biomass-Derived Ethyl Levulinate into Gamma-Valerolactone Over Graphene Oxide-Supported Zirconia Catalysts

Article abstract of DOI:10.1007/s10562-019-02835-2

Abstract: The transformation of biomass-derived intermediates into value-added chemicals and liquid fuels is of great importance in sustainable chemistry. In this study, graphene oxide supported ZrO₂ (ZrO₂/GO) was found to be an active heterogeneous catalyst for the transfer hydrogenation of ethyl levulinate to γ-valerolactone (GVL) with iso-propanol as the hydrogen donor. Several important reaction parameters such as the hydrogen donor, the reaction temperature and the catalyst loading were studied in detail with the aim to get a high yield of GVL. It was found that the structure of alcohols had a great effect towards the activity of the ZrO₂/GO catalyst and the selectivity of GVL. Iso-propanol was the best hydrogen donor for the transfer hydrogenation of ethyl levulinate to GVL. The highest GVL yield reached 91.7% with an ethyl levulinate conversion of 96.2% under optimal reaction conditions. More importantly, the ZrO₂/GO catalyst demonstrated a high stability without the loss of its catalytic activity during the recycling experiments, which should be due to the strong interaction between GO and ZrO₂. Graphical Abstract: The graphene oxide supported ZrO₂ (ZrO₂/GO) catalyst showed high activity for the transfer hydrogenation of ethyl levulinate to GVL with a high yield up to 91.7%.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02835-2

Catalysis Letters
https://doi.org/10.1007/s10562-019-02835-2
Catalytic Transfer Hydrogenation of Biomass‑Derived Ethyl Levulinate
into Gamma‑Valerolactone Over Graphene Oxide‑Supported Zirconia
Catalysts
1
1
1
2
2
1
1
Jinhua Lai · Shuolin Zhou · Xianxiang Liu · Yongjun Yang · Jing Lei · Qiong Xu · Dulin Yin
Received: 17 March 2019 / Accepted: 13 May 2019
©
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
The transformation of biomass-derived intermediates into value-added chemicals and liquid fuels is of great importance in
sustainable chemistry. In this study, graphene oxide supported ZrO (ZrO /GO) was found to be an active heterogeneous
2
2
catalyst for the transfer hydrogenation of ethyl levulinate to γ-valerolactone (GVL) with iso-propanol as the hydrogen donor.
Several important reaction parameters such as the hydrogen donor, the reaction temperature and the catalyst loading were
studied in detail with the aim to get a high yield of GVL. It was found that the structure of alcohols had a great eꢀect towards
the activity of the ZrO /GO catalyst and the selectivity of GVL. Iso-propanol was the best hydrogen donor for the transfer
2
hydrogenation of ethyl levulinate to GVL. The highest GVL yield reached 91.7% with an ethyl levulinate conversion of 96.2%
under optimal reaction conditions. More importantly, the ZrO /GO catalyst demonstrated a high stability without the loss of
2
its catalytic activity during the recycling experiments, which should be due to the strong interaction between GO and ZrO .
2
Graphical Abstract
The graphene oxide supported ZrO (ZrO /GO) catalyst showed high activity for the transfer hydrogenation of ethyl levulinate
2
2
to GVL with a high yield up to 91.7%.
O
O
O
O
O
Yield = 91.7 %
i-PrOH
COOH
HOOC
O
ZrO /GO
COOH
2
ZrO2
Keywords Non-noble metal catalyst · Transfer hydrogenation reaction · Ethyl levulinate · γ-Valerolactone · Sustainable
chemistry
1
Introduction
The gradual depletion of fossil fuel resources has forced the
world to seek new routes for the production of chemicals
and fuels from the renewable resources [1]. In contrast to
other renewable resources, the non-food biomass is the only
carbon-containing renewable resources, which can provide
Jinhua Lai and Shuolin Zhou contributed equally to this work.
*
Xianxiang Liu
lxx@hunnu.edu.cn
Extended author information available on the last page of the article
Vol.:(0
1
123456789)
3

J. Lai et al.
both chemicals and fuels to the world [2, 3]. Therefore, great
attention has been paid to the development of the eꢀective
methods for the transformation of biomass into biofuels and
value-added chemicals, which has emerged as one of the
promising ways to alleviate the current reliance on fossil
fuel sources [4, 5].
Synthesis of GVL from levulinic acid and its ester via the
use of formic acid as the hydrogen donor was generally per-
formed over noble metal catalysts such as Au and Ru cata-
lysts [19]. Obviously, the use of noble metals and the cor-
rodibility of formic acid under harsh conditions have limited
the application of formic acid for the transfer hydrogenation
of levulinic acid and its ester. Compared with formic acid,
the use of alcohols as the hydrogen source for the transfer
hydrogenation of levulinic acid and its ester into GVL has
received much more attention in recent years. Up to now,
several catalytic systems have been developed for the trans-
fer hydrogenation reactions of levulinic acid and its esters to
GVL using various alcohols as the hydrogen source, includ-
Through careful design of catalytic systems, many kinds
of important chemicals as well as the liquid fuels have been
successfully derived from biomass resources [6]. Among
them, there has been a growing attention on the synthesis
of γ-valerolactone (GVL), owing to its versatile application
in many ꢁelds [7]. GVL has been identiꢁed as a green and
renewable solvent for chemicals reaction, and it was reported
that GVL sometimes could improve the performance of bio-
mass conversion and organic transformations in terms of
the catalytic eꢂciency as well as the product puriꢁcation
®
ing homogeneous Ru complexes, RANEY Ni, and Lewis
acids especially Zr based catalysts [20, 21]. However, some
drawbacks still need to overcome, such as harsh reaction
temperatures, long reaction time, low catalyst reactivity and
stability. Therefore, the development of new eꢂcient and
cost-eꢀective methods for the production of GVL is a highly
attractive task.
[
8, 9]. GVL can be used as an additive suitable for liquid
fuels, perfumes, and food [7]. More importantly, GVL can
serve as the intermediate for the production of gasoline and
diesel fuels (e.g., C8–C18 alkanes and 2-methyltetrahydro-
furan) and valuable chemicals, such as 1,4-pentanediol,
methyl pentenoate [10], and ionic liquids [11]. Owing to
the wide application of GVL, the synthesis of GVL from
renewable carbohydrates or the biomass-derived chemicals
have been extensively studied in recent years [12–14]. The
direct and simple way to produce GVL is the hydrogenation
of levulinic acid (LA) and its esters, which can be readily
synthesized from lignocellulosic biomass through multiple
catalytic steps in the presence of acid catalysts. However, the
hydrogenation step posed a challenge to researchers with the
combined eꢀect of the carbonyl and ester groups on selective
hydrogenation.
Herein we reported the synthesis and characterization of
a graphene oxide supported ZrO catalyst (ZrO /GO), con-
2
2
taining Lewis acidic zirconium sites and Bronsted acidic
carboxyl and hydroxyl groups, under hydrothermal method.
The as-synthesized ZrO /GO catalyst showed high catalytic
2
activity in the transfer hydrogenation of ethyl levulinate into
GVL. Obviously, our developed methods demonstrated some
advantages such as the use of easily preparative non-noble
metal catalyst with low cost, and did not use the explosive
hydrogen.
The catalytic reduction of levulinic acid and its esters
have been generally been performed in the presence of
metal catalysts, especially noble metal catalysts by the use
of hydrogen [15, 16]. Although hydrogen is a clean hydrogen
source, these catalytic systems demonstrated some draw-
backs such as the low solubility of molecular hydrogen in
most solvents, the need of high pressure, the use of expen-
sive noble metal catalyst, and the low stability of the metal
catalyst. For example, Yang et al. prepared porous carbon
nanoꢁbers encapsulated Ru nanoparticles for the transfer
hydrogenation of LA into GVL, which was performed at
2 Experimental Section
2.1 Materials and Methods
All other chemicals were purchased from Aladdin Chemicals
Co. Ltd. (Beijing, China). All solvents were purchased from
Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
All of the chemicals and solvents were used directly without
puriꢁcation.
2.2 Catalyst Preparation
1
50 °C and 45 bar H pressure [17].
2
In recent years, catalytic reactions with other hydrogen
Graphene oxide (GO) was prepared by a modiꢁed Hummer’s
method [22]. Brieꢃy, graphite, NaNO and 98.0 wt% H SO
donors such as formic acid and alcohols instead of hydrogen
have received much interest [18]. This process can avoid
the use of explosive hydrogen; thus, it seems to be more
economical and much safer. In the process of the transfer
hydrogenation of levulinic acid and its esters into GVL, the
hydrogen donors were mainly used formic acid and alcohols.
It is highly attractive to perform the transfer hydrogenation
with formic acid, as it is also renewable from carbohydrates.
3
2
4
were ꢁrstly stirred at 0 °C for 30 min. Then KMnO was
4
added into the mixture slowly and the mixture was continu-
ously stirred at 0 °C for 2 h. After the addition of KMnO ,
4
the mixture was stirred for 2 h at 0 °C, subsequently at 25 °C
for 5 days. After that, 98 wt% H SO was added dropwise
2
4
and then stirred at 98 °C for 2 h. The temperature was then
reduced to 25 °C and the mixture was stirred for another
1
3

Catalytic Transfer Hydrogenation of Biomass-Derived Ethyl Levulinate into…
2
h. Then, the solid product was collected by centrifugation,
2.4 General Procedure of the Transfer
and washed with 3 wt% H SO and 3 wt% HCl. Finally, the
Hydrogenation of Ethyl Levulinate into GVL
2
4
solid product was dried in a vacuum oven at 40 °C to obtain
graphite oxide.
The transfer hydrogenation of ethyl levulinate into GVL
was performed in a stainless steel 40 mL Parr batch reac-
tor. A representative procedure was as follows: ethyl levuli-
The ZrO /GO catalyst was prepared via hydrothermal
2
methods. 150 mg of ZrOCl ·8H O was dissolved in 50 mL
2
2
of water and 1 g of GO support was slowly added into the
nate (1 mmol), ZrO /GO (40 mg) catalyst and iso-propanol
2
ZrOCl solution with continuous stirring for 30 min. The
(10 mL) were charged in the reactor. The air in the reactor
2
mixture was then transferred into a 100 mL Teꢃon-lined
stainless steel reactor which was heated at 160 °C for 12 h
in a muꢄe furnace. Upon heating for the set time, the reac-
tor was cooled down to room temperature and the material
was ꢁltered to obtain solid powder. The ꢁltered material
was subsequently washed with water and alcohol, and dried
at 80 °C in a vacuum oven overnight. The as-synthesized
was exchanged with nitrogen for ꢁve times and sealed under
N pressure (1.0 MPa). Then the autoclave was heated from
2
room temperature to 180 °C within 10 min and then the reac-
tion was performed at 180 °C for 3 h. After cooling the reac-
tor to room temperature, the reaction mixture was ꢁltrated,
and the clear solution was analyzed by gas chromatography.
material was denoted as ZrO /GO.
2.5 Recycling of the Catalyst
2
After reaction, the ZrO /GO catalyst was collected by an
2
2
.3 Catalyst Characterization
external magnet and it was washed with water and ethanol
for three times, respectively. Then it was dried at 100 °C over
night. The spent catalyst was reused for the next time under
the same identical condition as the ꢁrst run. These processes
were repeated for ꢁve times.
Transmission electron microscope (TEM) images were
2
obtained using an FEI Tecnai G -20 instrument. The sam-
ple powder was ꢁrstly dispersed in ethanol and dropped onto
copper grids for observation. X-ray powder diꢀraction pat-
terns of samples were determined with a Bruker advanced
D8 powder diꢀractometer (Cu Kα). The scan ranges were
3 Results and Discussion
1
0°–80° with 0.016° steps, respectively. X-ray photoelec-
tron spectroscopy (XPS) was conducted on a Thermo VG
scientiꢁc ESCA MultiLab-2000 spectrometer with a mono-
chromatized Al Kα source (1486.6 eV) at constant analyzer
pass energy of 25 eV. The binding energy was estimated
to be accurate within 0.2 eV. All binding energies (BEs)
were corrected referencing to the C1s (284.6 eV) peak of
the contamination carbon as an internal standard. UV–Vis-
ible absorption spectra and the molecular absorption spectra
were recorded on a Shimadzu ultraviolet and visible spec-
trophotometer (Kyoto, Japan).
The ZrO /GO catalyst was prepared via hydrothermal
2
methods in the presence of GO. During the hydrothermal
process, ZrOCl ·8H O was transformed into ZrO , which
2
2
2
was then deposited on the surface of the GO support. The
heterostructure of the ZrO /GO catalyst can be veriꢁed by
2
the morphological analyses. Figure 1 shows the typical
TEM images of graphite oxide and the as-prepared ZrO /
2
GO catalyst. ZrO particles was clearly observed on the sur-
2
face of the GO support, and no ZrO particles are scattered
2
out of the GO support, which indicates that there exists a
Fig. 1 TEM image of the as-
prepared ZrO /GO catalyst
2
1
3

J. Lai et al.
strong interaction between ZrO particles and the support.
2
300000
O 1s
(
a)
The strong interaction between the ZrO particles and the
2
C 1s
2
2
50000
00000
GO support should be possibly derived by the interaction
between the oxygen-containing functional groups such as
hydroxyl group and carboxylic acid groups on the surface of
the GO and the ZrO particles [23]. The TEM results clearly
150000
Zr 3p1/2
Zr 3s
Zr 3p3/2
2
indicates that this method is eꢀective for the preparation
1
00000
50000
0
of GO supported ZrO particles. However, ZrO particles
2
2
Zr 3d
were observed to have a wide size distribution. The enlarged
Zr 4p
TEM image of the ZrO /GO catalyst in Fig. 1 shows ZrO
2
2
particles are almost in an oval morphology, and the large
8
00
600
400
200
0
ZrO particle was composed of the small-sized nanoparti-
2
Binding energy (eV)
cles with dark dots. On the other hand, graphene oxide has
a layer structure, which is believed to be composed of many
individual sheets [24].
(
b)
8000
6000
4000
2000
0
184.9 eV
ZrO /GO
2
To obtain insights into the local structure of Zr species
supported on the surface of GO, X-ray diꢀraction (XRD) of
182.7 eV
the ZrO /GO catalyst were performed. In the XRD patterns
2
of the ZrO /GO catalyst (Fig. 2), a strong peak at 2θ=24.8°
2
was present in the XRD spectrum of the ZrO /GO catalyst,
2
and this peak was assigned to the (002) plane of graphite
carbon, which was the characteristic peak of the graphite
structure [22]. In addition, a weak peak at 2θ = 43.7 sug-
gests the formation of intralayer condensation for the carbon
materials in the graphite layers [22]. As far as the crystal
1
92 190 188 186 184 182 180 178 176
Binding energy (eV)
structure of ZrO , it has three phases including tetragonal
2
phase, cubic phase, and monoclinic phase. Besides the char-
acteristic peaks of the GO, three peaks with a weak intensity
at 2θ=30.3°, 31.5°, 34.8° and 50.3° were attributed to the
XRD peaks of the monoclinic phase (JCPDS NO. 37-1484)
Fig. 3 XPS survey scan spectrum of the ZrO /GO catalyst (a), and
2
the XPS spectrum of the Zr 3d in the ZrO /GO catalyst (b)
2
[
24, 25].
three elements of Zr, C and O without no impurity such
as Cl. In addition, the Zr 3d peaks in Fig. 3b can be ꢁtted
into two peaks with the binding energies at 182.7 eV and
184.9 eV, which were the characteristic binding energy of
The valence state of the ZrO /GO catalyst was further
2
characterized by XPS. As shown in the survey scan spec-
trum of the ZrO /GO catalyst Fig. 3a, these peaks are cor-
2
4
+
responded to the binding energy of Zr, C and O, which
Zr [24]. Thus, the XPS results also revealed that ZrO
2
indicates that the ZrO /GO catalyst is composed of the
was successfully formed by the hydrothermal treatment
of ZrOCl ·8H O, and loaded on the surface of the GO
2
2
2
support.
Diꢀuse reꢃectance UV–Vis spectra of the ZrO /GO
2
•
8
6
4
2
00
00
00
00
0
catalyst and the GO support were shown in Fig. 4. The GO
♦
•
ZrO2
GO
support showed no distinct absorption peak in the UV–Vis
region. However, after loading the ZrO particles on the
2
surface of the GO, there exists two distinct peaks in the
UV region for the ZrO /GO catalyst. The broad absorp-
♦
2
tion band around 205–250 nm due to the ligand-to-metal
2
−
4+
♦
charge transfer from an O to Zr ion of highly dispersed
4
+
Zr in an isolated tetrahedral conꢁguration [24]. Further a
distinct absorption band at 280–410 nm can be attributed
♦
•
♦
to the well dispersed ZrO nanoparticles [24, 25]. These
2
1
0
20
30
40
50
60
70
80
results conꢁrmed the presence of highly dispersed ZrO
2
Degree (2θ)
species in ZrO /GO catalysts.
2
Fig. 2 XRD patterns of the GO/ZrO catalyst
2
1
3

Catalytic Transfer Hydrogenation of Biomass-Derived Ethyl Levulinate into…
that GVL was stable at the reaction temperature of 180 °C.
When the reaction was performed in n-propanol, the con-
version of ethyl levulinate increased to 28.3% in compari-
son with ethanol (Table 1, Entry 3). However, the selec-
tivity of GVL in n-propanol decreased to 59.4% (Table 1,
Entry 3). The main byproduct was n-propyl levulinate,
which was also generated by the transesteriꢁcation of
ethyl levulinate with n-propanol. These results in Entries
1
1
0
0
0
.6
.2
.8
.4
.0
GO
O /GO
Zr
ZrO
2
2
1
–3 revealed that the transesteriꢁcation of ethyl levuli-
nate became much more diꢂcult for the alcohols with an
increase of the carbon atoms due to the increase of the
steric hindrance. Meanwhile, the transfer hydrogenation
ability increased in an order of methanol, ethanol, and
n-propanol.
2
00
300
400
500
600
700
800
Wavelength (nm)
Fig. 4 Diꢀuse reꢃectance UV–Vis spectra of the samples
As shown in Table 1, a high ethyl levulinate conver-
sion of 63.2% and a high GVL selectivity of 92.7% were
obtained when the reaction was performed in iso-propanol
3
.1 Catalytic Transfer Hydrogenation of Ethyl
over the ZrO /GO catalyst (Table 1, Entry 4). Compared
2
Levulinate with Diꢀerent Kinds of Alcohols
the results between the use of n-propanol and iso-pro-
panol, the side reaction of the transesteriꢁcation of ethyl
levulinate was greatly inhibited, which was due to the large
steric hindrance of iso-propanol. Meanwhile, the high con-
version of ethyl levulinate by the use of iso-propanol as the
hydrogen donor indicated that the secondary alcohol (iso-
propanol) was much more active than the primary alcohols
as a hydrogen donor for the transfer hydrogenation, pos-
sibly due to it giving a high stability intermediate after
release of the hydrogen atoms. Acetone as the dehydroge-
nation product was also determined, and its content was
equal to the amount of GVL. However, iso-butanol with
a similar structure of iso-propanol produced a lower con-
version of ethyl levulinate, but with a slight higher selec-
tivity of GVL due to its larger steric hindrance in com-
parison with iso-butanol (Table 1, Entry 5). As expected,
no conversion of ethyl levulinate was observed when the
transfer hydrogenation of ethyl levulinate was conducted
with tert-butanol as the hydrogen donor (Table 1, Entry
6). The reason should be that there was no hydrogen
close to the hydroxyl group in tert-butanol to produce the
Firstly, the transfer hydrogenation of ethyl levulinate into
GVL was studied over the ZrO /GO catalyst with diꢀerent
2
hydrogen donors, and the results are listed in Table 1. The
reactions were performed at 180 °C with the hydrogen
donors as the reaction solvents. It was observed that the
structure of the hydrogen donors showed a great eꢀect on
the conversion of ethyl levulinate as well as the selectiv-
ity of GVL. When the reaction was performed in metha-
nol, there was no aim product of GVL (Table 1, Entry
1
). However, a relative high ethyl levulinate conversion
of 65.7% was observed in methanol (Table 1, Entry 1).
Methyl levulinate was observed to be the byproduct, which
was formed via the transesteriꢁcation of ethyl levulinate
with methanol. A low conversion of ethyl levulinate was
observed in ethanol, but GVL was obtained with the high-
est selectivity of 98.6% (Table 1, Entry 2). The highest
selectivity of GVL in ethanol was due to the fact that the
there was no transesteriꢁcation of ethyl levulinate in etha-
nol (Table 1, Entry 2). In addition, it also demonstrated
Table 1 The catalytic transfer
Entry
Hydrogen donors
Catalyst
Conversion (%)
Yield (%)
Selectivity (%)
hydrogenation of ethyl
levulinate over diꢀerent
hydrogen donors
1
2
3
4
5
6
7
8
9
Methanol
Ethanol
ZrO /GO
5.7
7.4
28.3
63.2
42.9
5.8
10.6
3.5
43.2
0
0
2
ZrO /GO
7.3
16.8
58.6
24.8
0
–
–
98.6
59.4
92.7
97.8
0
–
–
2
n-Propanol
Iso-propanol
Iso-butanol
tert-Butanol
Iso-propanol
Iso-propanol
Iso-propanol
ZrO /GO
2
ZrO /GO
2
ZrO /GO
2
ZrO /GO
2
GO
–
ZrO2
41.0
94.9
a
Reaction conditions: ethyl levulinate (1 mmol), ZrO /GO (20 mg), solvent (10 mL), 180 °C, 3 h
2
1
3

J. Lai et al.
corresponding the carbonyl compounds. Therefore, iso-
propanol was the best solvent and hydrogen donor for the
transfer hydrogenation of ethyl levulinate into GVL over
Conversion
Selectivity
1
00
8
0
the ZrO /GO catalyst.
2
Furthermore, the transfer hydrogenation of ethyl levuli-
nate into GVL was also performed in the presence of GO.
However, no GVL was produced in the presence of GO after
6
0
4
0
3
h at 180 °C (Table 1, Entry 7). These results clearly indi-
cated that the Lewis acid site of Zr was the active site for
the transfer hydrogenation of ethyl levulinate to GVL with
alcohols as the hydrogen donor. However, a low conversion
of ethyl levulinate in 10.6% was also observed after 3 h at
2
0
0
120
140
160
180
200
1
80 °C in the presence of GO (Table 1, Entry 7). A control
Temperture (đ)
experiment was also performed in the absence of GO or
ZrO /GO, which still gave a very low ethyl levulinate con-
2
Fig. 5 The eꢀect of the reaction temperature on the transfer hydro-
version of 3.5% (Table 1, Entry 8). The product was iden-
tiꢁed to be iso-propyl levulinate either in the presence of
GO or without any catalyst. The higher conversion of ethyl
levulinate in the presence of GO suggested that GO demon-
strated the catalytic activity towards the transesteriꢁcation,
because of the acidic groups such as hydroxyl-, carboxylic
acid groups to activate the ester group in ethyl levulinate
genation of ethyl levulinate. Reaction conditions: ethyl levulinate
(1 mmol), ZrO /GO (20 mg), iso-propanol (10 mL), 3 h
2
3 h (Fig. 5). These results suggested that an increase of the
reaction temperature accelerated the dehydrogenation of iso-
propanol and the followed hydrogenation of ethyl levulinate
into GVL. Nevertheless, the selectivity of GVL remained
at a high level in the range from 92.7 to 96.4%. The high
selectivity of GVL was possibly due to the ꢃowing reasons
as described above. On the one hand, the transesteriꢁca-
tion of ethyl levulinate with iso-propanol was diꢂcult due
to the steric hindrance at diꢀerent reaction temperatures. On
the other hand, GVL was stable in our reaction system even
at the high temperature of 200 °C.
(
Table 1, Entries 7 vs. 8). Furthermore, catalytic transfer
hydrogenation of ethyl levulinate to GVL was also carried
out in the presence of ZrO at 180 °C. Although GVL was
2
obtained in a high selectivity, the conversion of ethyl lev-
ulinate in the presence of ZrO was much lower than that
2
in the presence of ZrO /GO (Table 1, Entry 9). The higher
2
conversion of ethyl levulinate over the ZrO /GO catalyst in
2
comparison with bulk ZrO should be that ZrO particles
2
2
were homogeneously dispersed on the surface of GO, and
Furthermore, the kinetics of the transfer hydrogenation
thus much more active sites of ZrO were exposed, resulting
of ethyl levulinate over the ZrO /GO catalyst was studied.
2
2
a higher reaction rate. In addition, as discussed above, the
acidic groups in GO should also play a role in accelerating
the transfer hydrogenation of ethyl levulinate into GVL, as
the acidic sites in GO can activate the carbonyl group in
ethyl levulinate.
Kinetic experiments on the transfer hydrogenation of ethyl
levulinate were performed at three diꢀerent temperatures of
140, 150 and 160 °C. In our reaction system, iso-propanol
is used as the reaction solvent, so it was in large excess.
Therefore, the reaction kinetics of the transfer hydrogenation
of ethyl levulinate should not be aꢀected by the concentra-
tion of iso-propanol. Hence, the transfer hydrogenation of
ethyl levulinate can be considered as a pseudo ꢁrst order
reaction to the substrate of ethyl levulinate. Figure 6 depicts
the plot of ln(C /C ) versus time for the transfer hydrogena-
3
.2 Catalytic Transfer Hydrogenation of Ethyl
Levulinate at Diꢀerent Temperatures
With iso-propanol as the best hydrogen donor for the transfer
t
0
hydrogenation of ethyl levulinate into GVL over the ZrO /
tion of ethyl levulinate over the ZrO /GO catalyst, in which
2
2
GO catalyst, the reaction conditions were further optimized
in order to get the high yield of GVL. First, the eꢀect of the
reaction temperature on the transfer hydrogenation of ethyl
C is the initial concentration of ethyl levulinate, and C
0
t
represents the concentration of ethyl levulinate at certain
reaction time point. As shown in Fig. 6, the pseudo-ꢁrst
order kinetics were observed for the transfer hydrogenation
of ethyl levulinate into GVL at three diꢀerent reaction tem-
peratures. The reaction rate constants for the transfer hydro-
genation of ethyl levulinate were determined to be 0.02722,
levulinate with iso-propanol was studied over the ZrO /GO
2
catalyst, and the results are shown in Fig. 5. It was found
that the reaction temperature showed a great eꢀect on the
conversion of ethyl levulinate, but the selectivity of GVL
was not greatly inꢃuenced by the reaction temperature.
Ethyl levulinate conversion greatly increased from 5.8%
at a low temperature of 120 °C to 80.6% at 200 °C after
−
1
0.12425 and 0.17532 h for the temperatures of 140, 150
and 160 °C, respectively. The variation of the rate constant
(k) with temperature was inserted in Fig. 6. According to
1
3

Catalytic Transfer Hydrogenation of Biomass-Derived Ethyl Levulinate into…
Fig. 7. A general trend was observed as follows: the higher
the loading of the ZrO /GO catalyst was, the higher the con-
0
.0
2
version of ethyl levulinate was. The higher ethyl levulinate
conversion with the higher loading of the ZrO /GO catalyst
2
was due to the presence of much more catalytic sites, which
accelerated the transfer hydrogenation of ethyl levulinate to
GVL. Interestingly, the eꢀect of the catalyst loading on ethyl
levulinate conversion at a low content showed a much more
distinct eꢀect than that at high loading range. For example,
the conversion of ethyl levulinate greatly increased from the
-0.2
-0.4
-0.6
k = 0.02772 T=413K
-
-
-
-
1.6
2.0
2.4
2.8
k = 0.12425 T=433K
k = 0.17532 T=453K
Ea=47.69kJ/mol
33.9% with 10 mg of the ZrO /GO catalyst to 63.2% with
2
20 mg of the ZrO /GO catalyst. Then it gradually increased
0
.0022 0.0023 0.0024
2
1
/T (1/K)
from 63.2% with 20 mg of the ZrO /GO catalyst to 93.4%
2
with 50 mg of the ZrO /GO catalyst. This is due to the fact
0
1
2
3
4
2
that the concentration of the substrates on the active sites of
Time (h)
the ZrO /GO catalyst was much higher with the lower load-
2
ing than that with a higher loading of the catalysts. There-
fore, the increasing trend of the ethyl levulinate conversion
became weaker at a high catalyst loading range. However,
the selectivity of GVL remained stable around 90%. The
stable selectively of ethyl levulinate revealed the catalyst
loading have no great eꢀect on the catalyst selectivity, and
the selectivity was inꢃuenced by other important parameters
as discussed above such as the type of alcohols.
Fig. 6 The reaction kinetics for the hydrogenation of ethyl levulinate
to GVL at diꢀerent temperatures. Reaction condition: the ZrO /GO
2
catalyst (20 g), temperature (413–453 K), ethyl levulinate (1 mmol),
speed of agitation (1000 rpm), iso-propanol (10 mL)
the Arrhenius plot, the activation energy (Ea) was calcu-
−
1
lated to be 47.69 kJ mol over the ZrO /GO catalyst for
2
the transfer hydrogenation of ethyl levulinate into GVL by
iso-propanol. There were few studies on the calculation of
activation energy for transfer hydrogenation of ethyl levuli-
nate into GVL by iso-propanol.
Furthermore, the time course of the transfer hydrogena-
tion of ethyl levulinate into GVL was studied at the reaction
temperature of 180 °C with the ZrO /GO catalyst loading
2
of 20 mg. As depicted in Fig. 8, the molar percentage ethyl
levulinate gradually decreased during the reaction process,
while the molar percentage of GVL gradually increased. As
shown in Fig. 8, the conversion rate of ethyl levulinate was
much sharper at an early reaction stage than the latter reac-
tion stage. The reason should be that a higher concentra-
tion of ethyl levulinate at an early reaction stage resulted in
3
.3 Eꢀect of Catalyst Loading on the Catalytic
Transfer Hydrogenation
Then the eꢀect of the catalyst loading on the transfer hydro-
genation of ethyl levulinate into GVL was studied at the
reaction temperature of 180 °C, and the results are shown in
100
Ethyl levulinate
GVL
100
Conversion
Selectivity
80
80
60
60
40
40
20
20
0
0
10
20
30
40
50
0
2
4
6
8
10
Catalyst amount (mg)
Time (h)
Fig. 7 Eꢀect of the catalyst amount on the transfer hydrogenation of
ethyl levulinate. Reaction condition: ethyl levulinate (1 mmol), speed
of agitation (1000 rpm), iso-propanol (10 mL), 180 °C and 3 h
Fig. 8 Time course of the transfer hydrogenation of ethyl levulinate
into γ-GVL. Reaction conditions: ethyl levulinate (1 mmol), ZrO /GO
2
(20 mg), iso-propanol (10 mL), and 180 °C
1
3

J. Lai et al.
Scheme 1 Transformation of
H O
2
ethyl levulinate (EL) into GVL
O
OH
O
O
O
OH
O
O
EL
4-HPA
GVL
1
00
Conversion
Selectivity
1
00
8
0
0
0
0
8
0
0
0
0
6
4
2
6
4
2
0
0
1
2
3
4
5
6
0
2
4
6
8
10
Cycle number
Time (h)
Fig. 9 The stability of the ZrO /GO on the transfer hydrogenation
Fig. 10 The stability of the ZrO /GO on the transfer hydrogenation
2
2
of ethyl levulinate into γ-GVL. Reaction conditions: ethyl levulinate
of ethyl levulinate into γ-GVL. Reaction conditions: ethyl levulinate
(
1 mmol), ZrO /GO (20 mg), iso-propanol (10 mL), 180 °C and 6 h
(1 mmol), ZrO /GO (20 mg), iso-propanol (10 mL), 180 °C and 3 h
2
2
a higher reaction rate. During the whole reaction process,
the selectivity of GVL remained around 92%. Besides the
byproduct generated from the transesteriꢁcation of ethyl lev-
ulinate with iso-propanol. No other products were observed
during the reaction process as determined by gas chroma-
tography. It has been generally accepted that a product from
the hydrogenation of carbonyl group in ethyl levulinate is the
intermediate during the transformation of ethyl levulinate
into GVL (Scheme 1). However, this intermediate was not
detected during the reaction process, suggesting it was not
stable, which fast underwent the cyclisation/lactonization
to generate the aim product of a stable ꢁve-member ring
product of GVL.
and the selectivity of GVL were almost the same for the six
runs. These results indicated that the as-prepared ZrO /GO
2
catalyst was stable during the reaction process without the
loss of its catalytic activity.
Furthermore, a hot ꢁltration experiment was also conduct
to further conꢁrm the high stability of the ZrO /GO catalyst.
2
After the reaction for 3 h at 180 °C, the ZrO /GO catalyst
2
was removed. Then the liquid solution was continuously
to be heated at 180 °C for 10 h. As shown in Fig. 10, the
conversion of GVL remained stable without any increase,
which further conꢁrmed that the ZrO /GO catalyst was
2
highly stable during the reaction process without the leach
of the active sites into the reaction solution. All of the results
conꢁrmed that our transfer hydrogenation of ethyl levulinate
into GVL was a real heterogeneous reaction.
The long-term stability and reusability of a heterogeneous
catalyst are of great importance for the practical application
to reduce the manufacturing cost. Hence, the stability of the
ZrO /GO catalyst was studied. The catalytic transfer hydro-
2
genation of ethyl levulinate into GVL was used as the model
4 Conclusion
reaction to study the recycling experiments of the ZrO /GO,
2
which was performed at 180 °C. After reaction, the ZrO /
In conclusion, we have developed a new method for the
transfer hydrogenation of biomass-derived ethyl levulinate
into GVL using iso-propanol as the hydrogen donor via a
tandem MPV reduction and cyclisation/lactonization over
2
GO catalyst was separated from the reaction mixture by cen-
trifugation, so as to avoid the loss of the weight of the ZrO /
2
GO catalyst. The spent catalyst was subsequently washed
with water and ethanol, respectively. Then the spent cata-
a non-noble catalyst. The ZrO /GO catalyst can be simply
2
lyst was dried in a vacuum oven at 80 °C. The spent ZrO /
prepared via the hydrothermal treatment of ZrOCl ·8H O in
2
2
2
GO catalyst was carried out under the identical conditions
with the ꢁrst run. These steps were repeated for ꢁve runs.
As shown in Fig. 9, the conversion of the ethyl levulinate
the presence of GO, and the as-prepared ZrO /GO demon-
2
strated high activity towards the transfer hydrogenation of
ethyl levulinate for the GVL production. Various parameters
1
3

Catalytic Transfer Hydrogenation of Biomass-Derived Ethyl Levulinate into…
were optimized for the transfer hydrogenation of ethyl levuli-
nate into GVL. It was found the structure of hydrogen donors
showed a great eꢀect on the ethyl levulinate conversion as
well as the GVL selectivity. Iso-propanol was observed to
be the best hydrogen donor, which produced the highest
selectivity of GVL. A full conversion of ethyl levulinate
and a high selectivity of GVL of 94.8% were obtained at the
reaction temperature of 180 °C for 10 h by the use of 20 mg
7. Zhang ZH (2016) Chemsuschem 9:156–171
8
.
Yu IKM, Tsang DCW, Yip ACK, Hunt AJ, Sherwood J, Shang J,
Song H, Ok YS, Poon CS (2018) Green Chem 20:2064–2074
Mellmer MA, Sanpitakseree C, Demir B, Bai P, Ma KW, Neurock
M, Dumesic JA (2018) Nat Catal 1:199–207
9
.
10. Zeng FX, Liu HF, Deng L, Liao B, Pang H, Guo QX (2013)
Chemsuschem 6:600–603
1
1. Orha L, Tukacs JM, Gyarmati B, Szilagyi A, Kollar L, Mika LT
(
2018) ACS Sustain Chem Eng 6:5097–5104
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of the ZrO /GO catalyst. Kinetic studies indicated that the
2
transfer hydrogenation of ethyl levulinate into GVL was
1
1
4. Osawa T, Tanabe Y (2018) Catal Lett 148:824–830
5. Yang WJ, Cheng HY, Zhang B, Li Y, Liu T, Lan ML, Yu YC,
Zhang C, Lin WW, Fujita S, Arai M, Zhao FY (2016) Green Chem
18:3370–3377
sensitive to the reaction temperature with activation energy
−
1
of 47.69 kJ mol .The ZrO /GO catalyst was highly stable
2
and could be reused for several times without the loss of its
1
1
6. Kubo Y, Kakizaki D, Kogo M, Magatani Y (2016) Supramol
Chem 28:91–97
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Acknowledgements The authors gratefully acknowledge the ꢁnan-
cial support of the National Natural Science Foundation of China
1
1
(
21606082), the Scientiꢁc Research Fund of Hunan Provincial Educa-
tion Department (17C0951), Hunan Provincial Natural Science Foun-
dation of China (2018JJ3334), Hunan Provincial Innovation Foundation
for Postgraduate (CX2018B295), and the Opening Fund of National &
Local Joint Engineering Laboratory for New Petro-chemical Materials
and Fine Utilization of Resources (KF201803).
2
2
0. Chia M, Dumesic JA (2011) Chem Commun 47:12233–12235
1. Tang X, Hu L, Sun Y, Zhao G, Hao W, Lin L (2013) RSC Adv
3
:10277–10284
2
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tain Chem Eng 6:3773–3782
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Aꢀliations
1
1
1
2
2
1
1
Jinhua Lai · Shuolin Zhou · Xianxiang Liu · Yongjun Yang · Jing Lei · Qiong Xu · Dulin Yin
1
2
National & Local Joint Engineering Laboratory for New
Petro-Chemical Materials and Fine Utilization of Resources,
Key Laboratory of the Assembly and Application of Organic
Functional Molecules of Hunan Province, Hunan Normal
University, Changsha 410081, People’s Republic of China
Chenzhou Gao Xin Material Co., Ltd., Chenzhou 423000,
Hunan, People’s Republic of China
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