Selective conversion of biomass-derived levulinic acid to ethyl levulinate catalyzed by metal organic framework (MOF)-supported polyoxometalates

Article abstract of DOI:10.1016/j.apcata.2019.01.004

The esterification of levulinic acid (LA) and ethanol into ethyl levulinate is an attractive biomass conversion process since the product EL has wide applications as food additive, fragrance and fuel. Herein, a metal-organic framework (MOF)-supported phosphomolybdic acid [Cu-BTC][HPM] was synthesized with 1,3,5-Benzenetricarboxylic acid, copper nitrate and phosphomolybdic acid in a one-step process at ambient temperature. The synthesized [Cu-BTC][HPM] was used for the catalytic esterification of levulinic acid to EL in ethanol, and showed excellent activity with a high EL yield close to 100% at 120 °C for 6 h, which should be ascribed to the uniform dispersion of HPM embedded in the MOF. The [Cu-BTC][HPM] catalyst could keep stable crystal structure and active component contents, and thus exhibited good stability in recycling process.

Full text of DOI:10.1016/j.apcata.2019.01.004

AppliedCatalysisA,General572(2019)168–175
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Selective conversion of biomass-derived levulinic acid to ethyl levulinate
catalyzed by metal organic framework (MOF)-supported polyoxometalates
Tianmeng Guo, Mo Qiu, Xinhua Qi⁎
Agro-Environmental Protection Institute, Chinese Academy of Agricultural Sciences, No. 31, Fukang Road, Nankai District, Tianjin, 300191, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Levulinic acid
Biofuel
Esterification
Metal-organic frameworks
Polyoxometalates
Biomass
The esterification of levulinic acid (LA) and ethanol into ethyl levulinate is an attractive biomass conversion
process since the product EL has wide applications as food additive, fragrance and fuel. Herein, a metal-organic
framework (MOF)-supported phosphomolybdic acid [Cu-BTC][HPM] was synthesized with 1,3,5-
Benzenetricarboxylic acid, copper nitrate and phosphomolybdic acid in a one-step process at ambient tem-
perature. The synthesized [Cu-BTC][HPM] was used for the catalytic esterification of levulinic acid to EL in
ethanol, and showed excellent activity with a high EL yield close to 100% at 120 °C for 6 h, which should be
ascribed to the uniform dispersion of HPM embedded in the MOF. The [Cu-BTC][HPM] catalyst could keep
stable crystal structure and active component contents, and thus exhibited good stability in recycling process.
1. Introduction
generally environmentally friendly and are easily separated from re-
action system, which can be a substitute for homogeneous catalysts.
With the gradual depletion of fossil fuels and the increasing demand
for energy, it is urgent to develop renewable and sustainable energy
resources to meet current need for fine chemicals, chemical materials
and fuels [1–5], among which, biomass, as one of the renewable re-
sources with abundant reserves and widespread distribution, has at-
tracted significant attention. Levulinic acid (LA), a high value-added
chemical derived from biomass, can be obtained from cellulose by acid-
catalyzed hydrolysis process [6]. Since it’s an organic acid with both
ketone carbonyl and carboxyl groups, it has good reaction activity and
can be used to produce levulinate esters, γ-valerolactone, 2-methylte-
trahydrofuran and other fuel additives through various reactions such
as esterification, redox, substitution and polymerization [7–9]. Among
various products, ethyl levulinate (EL) is particularly worthy of atten-
tion, since its physicochemical properties are similar to fatty acid me-
thyl esters in biodiesel, and it can be used as a new liquid fuel additive
to partially replace petroleum. In addition, it’s also an important che-
mical feedstock for the production of food flavor [8,10,11].
EL can be efficiently produced by esterification of LA with ethanol
[12] (Scheme 1). In previous studies, inorganic acids such as sulfuric
acid [13], hydrochloric acid, and p-toluenesulfonic acid have been
widely used in esterification reactions to achieve high yields. However,
these conventional homogeneous catalysts inevitably have dis-
advantages such as difficulty in recycling, equipment corrosion, and
environmental pollution [10]. In contrast, heterogeneous catalysts are
Common solid acids used for esterification include sulfonated zir-
conium dioxide [14], zeolites [15], heteropoly acids (HPAs) [16], etc.
Kuwahara et al. [14] designed a series of sulfated zirconia catalysts
with different amounts of silicon. When a 10 wt.%-Si catalyst was used,
the conversion of LA reached 77.5% at 70 °C in10 h reaction time. The
study also showed that the catalysts with extremely high acid amounts/
densities wasn’t the most effective catalyst for the reaction, leading to a
conclusion that in addition to the amount of acidic sites, the availability
of acid sites determined by the structural properties of the catalytic
materials, and the accessibility between the reaction substrate and the
acid site were also key factors affecting the catalytic effect. Among
them, heteropolyacids are highly valued due to their high proton mo-
bility, stability, fast electron transfer capability and excellent physico-
chemical structure. Especially for Keggin type HPAs, usually show
stronger Bronsted acidity compared with traditional mineral acids
[17–19]. Nevertheless, relatively small surface areas and the high so-
lubility of HPAs in polar solvents limit their applications in liquid-phase
reactions to a certain extent. Considering the above drawbacks, it is of
great significance to search structurally stable carriers to stabilize the
HPAs. In this way, the catalyst can be conveniently separated from the
reactants and the recyclability is enhanced. Nandiwale et al. synthe-
sized a catalyst loading 15% (w/w) DTPA (dodecatungstophosphoric
acid, one kind of HPAs) on H-ZSM-5 (zeolite), which showed good
performance in the esterification process of LA with 94% LA conversion
⁎ Corresponding author.
E-mail address: qixinhua@nankai.edu.cn (X. Qi).
https://doi.org/10.1016/j.apcata.2019.01.004
Received 23 September 2018; Received in revised form 7 January 2019; Accepted 9 January 2019
Availableonline09January2019
0926-860X/©2019PublishedbyElsevierB.V.

T. Guo et al.
AppliedCatalysisA,General572(2019)168–175
instrument. The Fourier-transformed infrared spectroscopy (FTIR)
spectrum of the catalyst was observed using a Nicolet iN10 micro in-
frared spectrometer. The scanning electron microscope (SEM) images
were obtained on a Gemini Sigma 300 scanning electron microscope.
The transmission electron microscope (TEM) images and the distribu-
tion of elements were characterized using a JEOL JEM-2010 transmis-
sion electron microscope with energy-dispersive X-ray spectroscopy
(EDX). N2 adsorption-desorption isotherms were measured at 77 K
using a Micromeritics ASAP 2020, and the specific surface area and
pore size distribution of Cu-BTC and [Cu-BTC][HPM] were analyzed by
the Autosorb IQ analyzer from Quantachrome Instruments, using the
Brunauer-Emmett-Teller (BET) method, the Barrett-Joiner-Halenda
(BJH) formula (for [Cu-BTC][HPM]) and the Horvath-Kawazoe (HK)
formula (for Cu-BTC).
Scheme 1. Esterification of LA with ethanol into EL.
and 100% EL selectivity [20]. A series of zirconia-supported catalysts
with different mass ratios of phosphotungstic acid were tested for EL
production. The results showed that under optimal conditions, 97.3% of
EL yield can be observed [21].
In recent years, as a new type of multifunctional porous material,
metal organic framework (MOF) has attracted much attention due to its
novel structure, good stability, and uniformity of pore structure. It has a
wide range of applications in gas adsorption, storage and separation,
especially in the field of catalysis [22–24]. Ligand-functionalized MOFs
have been used to esterification [25]. Given the good pore structure
characteristics of MOF, it should be an excellent support for the active
ingredient for the esterification reaction. Therefore, in this work, Cu-
BTC (Cu2+ as the coordination metal, benzene-1,3,5-tricarboxylate as
the ligand) loaded with phosphomolybdic acid (HPM) was used for the
esterification of LA. Cu-BTC (HKUST-1) was first synthesized with
1,3,5-Benzenetricarboxylic acid and copper nitrate in a mixture of
ethylene glycol and water by Chui et al. [26]. Since then many scientists
have conducted in-depth research on it. Compared to the other MOFs
materials, Cu-BTC has suitable pore size for HPM. Thus, the active in-
gredient HPM can be immobilized within the pores of the MOF, which
should contribute greatly to the stability and recyclability of the cata-
lyst.
2.4. The procedure for the catalytic esterification of LA to EL
In a typical experiment, a certain amount of LA (0.25 mmol,
0.5 mmol, 1 mmol and 2 mmol) was dissolved into 10 ml ethanol, then
the mixture was transferred to a 50 ml Teflon lined stainless steel au-
toclave followed by the addition of a certain amount of the prepared
catalyst (20 mg, 40 mg and 80 mg). The reaction was carried out with
magnetic stirring (1000 r/min) at a given temperature (80 °C, 100 °C
and 120 °C) for a period of time (2–14 h). Once the reaction was com-
pleted, the autoclave was quickly cooled with ice water to end the re-
action. After the solid was separated by centrifugation and diluted with
anhydrous ethanol, the supernatant was quantified with
a GC
(Shimadzu) equipped with an Rtx-1 capillary column (30
m × 0.25 mm) and a flame ionization detector (FID) with N2 as carrier
gas. The column temperature was 100 °C, and the vaporizer tempera-
ture was set to 260 °C.
The yield of ethyl levulinate was calculated by the following for-
mula [27]:
2. Experimental
2.1. Chemicals and reagents
molar amount of EL generated
initial molar amount of LA added
Yield of ethyl levulinate =
× 100%
Copper acetate monohydrate (99.95%), L-glutamic acid (99%), le-
vulinic acid (99%) were obtained from Shanghai Macklin Biochemical
Co., Ltd. (Shanghai, China). 1,3,5-Benzenetricarboxylic acid (99%) was
bought from Beijing Innochem Technology Co., Ltd. (Beijing, China).
Phosphomolybdic acid hydrate (AR) was received from Shanghai
Yuanye Biological Technology Co., Ltd. (Shanghai, China). Ethyl al-
cohol (99.8%) was provided by Concord Technology Co., Ltd. (Tianjin,
China). All chemicals were directly used without further purification.
The recovered catalyst was washed with ethanol several times for
reuse. Each reaction was repeated three times, and the reproducibility
of EL yields was within 3% in standard deviation.
3. Results and discussion
3.1. Characterization of the prepared MOF-supported HPM
2.2. Preparation of Cu-BTC and [Cu-BTC][HPM]
Normally, the preparation of most of the MOFs is carried out at a
high temperature by hydrothermal method. Herein we prepared cata-
lysts Cu-BTC and [Cu-BTC][HPM] which were composed of nano-
particle units by stirring at room temperature according to the litera-
ture [28]. When an ethanol solution containing an organic ligand and
an aqueous solution containing a metal salt were mixed together, with
the rapid deprotonation of ligands, MOF could be instantly generated
by self-assembly.
The crystal phases of samples are confirmed by X-ray diffraction
(XRD) and showed in Fig. 1. There is nearly no difference of the dif-
fraction patterns between the as-prepared Cu-BTC in this work and that
reported in the literature [29], demonstrating that in this way, Cu-BTC
could be successfully synthesized under mild room temperature. Com-
paring the prepared [Cu-BTC][HPM] with the original Cu-BTC, the
obvious differences of diffraction peaks appeared in the range of 5-10°.
The peaks at around 7° and 9° (2 Theta) had significant decreases in the
intensity, corresponding to (200) and (220) crystallographic planes of
Cu-BTC, respectively. It was proved that HPM was not simply presented
on the surface of the MOF, but also was embedded in the pore cage of
the Cu-BTC and affected the crystallization of Cu-BTC, thus led to the
changes in the diffraction peak of [Cu-BTC][HPM]. No obvious HPM
diffraction peaks indicated that HPM was highly uniformly dispersed
In a typical preparation process, 1 mmol copper acetate mono-
hydrate, 0.5 mmol L-glutamic acid and 300 mg of phosphomolybdic
acid hydrate were added to 40 ml of distilled water, and the mixture
was stirred vigorously to get a clear solution. Then, 140 mg of 1,3,5-
benzenetricarboxylic acid dissolved in 40 ml of ethanol was added to
the previous prepared solution while stirring continuously. After that,
the mixed solution was stirred for 12 h with magnetic stirring to obtain
a blue-green precipitate, named as [Cu-BTC][HPM]. For comparison,
Cu-BTC was also obtained by the same method, but no phosphomo-
lybdic acid was added during the preparation. After these two pre-
cipitates were separated from the solution by centrifugation, the sam-
ples were washed with ethanol for three times and dried in a vacuum
oven at 70 °C overnight.
2.3. Characterization of the catalysts
The crystallographic information was obtained by the powder X-ray
diffraction (XRD) with
a Japan Rigaku D/MAX-2500 X-ray dif-
fractometer (Cu Kα radiation, 40 kV, 100 mA). Elemental composition
of the catalysts was determined by ICP-OES on an Agilent 730 ICP-OES
169

T. Guo et al.
AppliedCatalysisA,General572(2019)168–175
system without HPM, the crystal nucleation rate was faster, and thus
tended to form crystals of smaller particles.
To observe the distribution of HPM on Cu-BTC more intuitively, the
distribution of Cu, P, and Mo in the catalyst was characterized with EDX
mapping (Fig. 4), which further demonstrates that HPM is highly dis-
persed in the metal-organic framework due to the uniform pore struc-
ture of Cu-BTC.
Fig.5 shows the adsorption-desorption isotherms of Cu-BTC and
[Cu-BTC][HPM] samples, which shows that Cu-BTC had the type I
isotherm, with specific surface area of 1577 m2/g. The pore size dis-
tribution diagram shows that most of the pore diameters are centred at
0.7 nm within 0–2 nm, which is the ordered microporous structure in
Cu-BTC. The hysteresis loop appeared around p/p0 = 0.9 is probably
caused by the amorphous pores generated by the interconnection of Cu-
BTC particles [36,37], as shown in the meso/macropore size distribu-
tion of Cu-BTC in Fig. S1. The nitrogen adsorption-desorption isotherm
of the [Cu-BTC][HPM] is a typical type IV isotherm, and there is a
significant hysteresis loop at P/P0 ˜ 0.6–1.0, indicating that [Cu-BTC]
[HPM] had many mesopores. According to the crystal growth theory
proposed in References [36,37], the [Cu-BTC][HPM] particles are
composed of sub-micron sized particles, and amorphous stacked pores
are formed among these particles, corresponding to the existence of
amorphous mesopores in [Cu-BTC][HPM]. The addition of HPM influ-
ences the synthesis environment, therefore the generated size of or-
dered pores and amorphous stacked mesopores are also different be-
tween [Cu-BTC] and [Cu-BTC][HPM]. Only the ordered pores in Cu-
BTC framework can interact with HPM molecules to load them well.
Besides, the specific surface of [Cu-BTC][HPM] sharply decreased to
57 m2/g, which should be ascribed to the introduction of HPM as guests
occupied the pores of Cu-BTC, this phenomenon also demonstrates that
HPM is embedded in the pores rather than attached to the surface [38].
Table 1 showed the textural properties of Cu-BTC and [Cu-BTC][HPM].
Fig. 1. XRD patterns of Cu-BTC and [Cu-BTC][HPM].
within the MOF structure.
The FT-IR spectra of the Cu-BTC and [Cu-BTC][HPM] samples are
illustrated in Fig.2. The absorption band at 1585 cm−1 can be attrib-
uted to the stretching vibration of the C]C bond in the benzene ring.
The in-plane bending vibration of the C–H in the benzene ring and the
C–OeCu stretching vibration cause absorption bands at 1112 cm-1 and
1045 cm-1, respectively [30], while the band around 750 cm-1 re-
presents the out-plane bending vibration of the C–H group in the ben-
zene ring. The band at 1644 cm-1 can be assigned to the stretching vi-
bration of the C]O in the carboxyl group. The origin of the band at
1448 cm−1 is the in-plane bending vibration of the OeH, and the band
at 1374 cm-1 corresponds to the stretching vibration of the C–O [31,32],
which further proves the existence of the carboxyl group. Moreover, the
bands at 730 cm-1 and 491 cm-1 are related to the stretching vibration of
Cu-O [33,34], and the absorption bands in the range of 800-1100 cm−1
are the characteristic bonds of HPM [35].
3.2. Effect of reaction time and temperature on the catalytic conversion of
LA to EL by [Cu-BTC][HPM] catalyst
Then, the prepared catalysts were applied for the catalytic conver-
sion of LA to EL. When the [Cu-BTC][HPM] catalyst (40 mg) was used
for the esterification of LA (0.5 mmol) in ethanol (10 ml), a high EL
yield of 92.4% could be obtained at 120 °C in 4 h reaction time. On the
other hand, when the reactions were carried out in the absence of ad-
ditive or in the presence of Cu-BTC as catalyst, no EL product could be
detected under the same reaction conditions (120 °C, 4 h) (Table 2).
Besides, a control experiment using pure HPM as catalyst was carried
out, and the used amount of HPM was based on the content of HPM in
the used [Cu-BTC][HPM]. It was found that the yield of EL was similar
to that of using [Cu-BTC][HPM] as catalyst (Table 2). However, com-
pared with HPM, [Cu-BTC][HPM] as catalyst can be effectively recycled
in this reaction system. Thus, the as-prepared [Cu-BTC][HPM] catalyst
exhibited good catalytic activity for the esterification of levulinic acid
to ethyl levulinate.
SEM images in Fig. 3 shows the synthesized [Cu-BTC][HPM] con-
sisted of particles roughly 700 nm in average size. As shown in the
images, the morphology of these sub-micron-sized particles is typical
octahedral structure and truncated octahedral structure. The size and
morphology of the synthesized crystals were affected by the acidity and
alkalinity of the solution system. Therefore, in the Cu-BTC synthesis
Reaction temperature and reaction time are normally important
factors influencing the reaction process. The effect of reaction tem-
perature and reaction time on the catalytic conversion of LA to EL by
the prepared [Cu-BTC][HPM] catalyst was examined, and the results
are illustrated in Fig.6. Increasing the reaction temperature or
prolonging the reaction time allowed the esterification reaction to
proceed more thoroughly.
In specific experiments, when the reaction was conducted at 80 °C,
the conversion of LA to EL was so slow that only a maximum EL yield of
53.1% was obtained in 14 h reaction time. When the reaction tem-
perature was increased to 100 °C, it took only four hours to reach the EL
yield of 60.2%, and LA could almost be completely converted to EL in
12 h. When the reaction temperature was further increased to 120 °C,
the yield of EL was close to 100% in 6 h reaction time, affording nearly
100% EL selectivity. According to the above experimental results, it can
Fig. 2. FTIR spectra of Cu-BTC and [Cu-BTC][HPM].
170

T. Guo et al.
AppliedCatalysisA,General572(2019)168–175
Fig. 3. SEM images of (a) Cu-BTC and (b) [Cu-BTC][HPM].
3.3. Effect of catalyst dosage on the catalytic conversion of LA to EL by [Cu-
BTC][HPM] catalyst
The effect of the used amount of catalyst on the catalytic conversion
of LA into EL was also investigated, and the results are shown in Fig. 7.
The results suggested the reaction rate and product yield were posi-
tively correlated with the amount of catalyst used. When 20 mg [Cu-
BTC][HPM] catalyst was used in the reaction, an EL yield of 79.5%
could be achieved at 120 °C in 4 h reaction time. When the dosage of the
catalyst was increased to 40 mg, the EL yield increased to 92.4% under
the same reaction conditions. With the further increase of the catalyst
amount to 80 mg, the LA was almost completely converted to EL (100%
yield) at 120 °C for 4 h.
Fig. 4. EDX mapping of the [Cu-BTC][HPM], elemental mapping of (a) Cu, (b)
P and (c) Mo.
The reaction mechanism for the selective conversion of LA to EL in
ethanol can be elaborated from the following aspects. The active
component of the used catalyst is HPM, a strong Bronsted acid with
hydrogen ion as active center. At the beginning of the catalytic reaction,
protons attack levulinic acid to form carbonium ions. The nucleophilic
oxygen atom of ethanol then attacks the levulinic acid intermediate
[42], resulting in the formation of oxonium ions. The new oxonium ions
produced by the proton transfer in the oxonium ions are further de-
hydrated, and the product ethyl levulinate is produced after deproto-
nation [42]. During the reaction, the metal-centered Cu, which has an
unsaturated coordination number, can provide a certain amount of
Lewis acid sites to adsorb levulinic acid, meanwhile increases the
be summarized that increasing the reaction temperature is more con-
ducive to the improvement of the EL yield than prolonging the reaction
time. Under the applied reaction conditions, EL is the only product, and
common by-products such as angelica lactones, ϒ-valerolactone [39]
and ethanol inter-molecular etherification products were not detected,
which is consistent with the results reported in the references
[27,40,41]. The esterification process of levulinic acid is an equilibrium
reaction, but the addition of a sufficient amount of ethanol allows the
reaction to proceed to the direction of EL formation, and the proper
acidity of the catalyst also avoids the occurrence of other side reactions.
171

T. Guo et al.
AppliedCatalysisA,General572(2019)168–175
Fig. 5. N2 adsorption-desorption isotherms and pore size distribution curves (inset) of (a) [Cu-BTC] and (b) [Cu-BTC][HPM].
Table 1
The textural properties of Cu-BTC and [Cu-BTC][HPM].
Sample
BET surface area (m2/g)
Micropore area (m2/g)
Total pore volume (cm3/g)*
Mesopore pore diameter (nm)**
Cu-BTC
[Cu-BTC][HPM]
1577
57
1493
26
1.13
0.23
25.8
11.4
* BJH adsorption cumulative volume of pores between 1.7 nm and 300 nm diameter.
** BJH adsorption average pore diameter (4 V/A).
172

T. Guo et al.
AppliedCatalysisA,General572(2019)168–175
Table 2
Comparison of catalytic performance of various catalysts in this work.
Catalyst
Catalyst amount (mg)
LA conversion (%)
EL yield (mol %)
No catalyst
Cu-BTC
[Cu-BTC][HPM]
HPM
0
0
0
100
100
0
0
92.4
93.5
40
40
9
Reaction conditions: 0.5 mmol LA, 10 ml ethanol, 120°C, 4h.
Fig. 8. Influence of the initial LA loading amounts on the EL yield. Reaction
conditions: 40 mg [Cu-BTC][HPM], 10 ml ethanol, 120 °C, 4 h.
Fig. 6. Influence of reaction temperature and reaction time on the catalytic
conversion of LA to EL by the [Cu-BTC][HPM] catalyst. Reaction conditions:
0.5 mmol LA, 40 mg catalyst, 10 ml ethanol.
Fig. 9. Time-dependent changes in EL yields under different initial LA con-
centrations. Reaction conditions: 40 mg [Cu-BTC][HPM], 10 ml ethanol, 120 °C.
and almost 100% EL yield could be obtained at 120 °C in 4 h reaction
time. When the LA amount was increased to 0.5 mmol, the EL yield
could be remained at 92.4%, indicating that there were adequate cat-
alytic active sites for the conversion of LA to EL. When the LA loading
amount was increased to a higher level to 1 mmol and 2 mmol, the EL
yields decreased to 77% and 74%, respectively, in 4 h reaction time,
ascribing to the deficiency of the active component HPM for more
substrate. After the reaction time was prolonged to 8 h and12 h, the
highest EL yield of 89% and 81% could be achieved with LA dosage of
1 mmol and 2 mmol, respectively (Fig. 9). Thus, the as-prepared [Cu-
BTC][HPM] catalyst was applicable to the high initial LA loading for
Fig. 7. Influence of the [Cu-BTC][HPM] amounts on the EL yield. Reaction
conditions: 0.5 mmol LA, 10 ml ethanol, 120 °C, 4 h.
electrophilic properties of the carboxylic acid carbon atoms, to promote
the esterification reaction.
Table 3
Performance of different catalysts in the esterification process of LA to EL.
Catalyst
Temp. (oC)
Time (h)
EL yield (mol %)
Ref.
3.4. Effect of initial substrate loading amount
20-HPW/Zr
UiO-66
H4SiW12O40/SiO2
HPA/Silica
150
60
75
78
120
3
2
6
10
6
97
> 99
67
78
> 99
[21]
[25]
[43]
[42]
The effect of initial LA loading amount (0.25–2 mmol) on the cat-
alytic conversion of LA to EL by the [Cu-BTC][HPM] catalyst was in-
vestigated (Fig. 8). It can be seen that when a substrate loading amount
as low as 0.25 mmol was applied, LA was selectively converted to EL,
[Cu-BTC][HPM]
This work
173

T. Guo et al.
AppliedCatalysisA,General572(2019)168–175
available active sites and EL yields.
4. Conclusions
In this work, a MOF-supported phosphomolybdic acid catalyst [Cu-
BTC][HPM] was successfully synthesized using a one-step method at
ambient temperature, with a metal-organic framework (Cu-BTC) as the
carrier and a polyoxometalate HPM as the active component. The as-
prepared [Cu-BTC][HPM] was used as catalyst for the esterification of
biomass-derived LA to EL in ethanol, and achieved excellent results in
that the yield of EL was close to 100% at 120 °C in 6 h reaction time.
Owing to the highly ordered and suitable pore structure of Cu-BTC and
the intense interaction between HPM and Cu-BTC, HPM is stably pre-
sent in the MOF structure and has exhibited good performance in re-
cycling test compared to homogeneous catalysts.
Acknowledgements
The authors appreciate the financial support from the National
Natural Science Foundation of China (NSFC, No. 21876091, 51508284,
21577073), the Natural Science Fund for Distinguished Young Scholars
of Tianjin (No. 17JCJQJC45500), the Natural Science Foundation of
Tianjin (No. 16JCZDJC33700), and Central Public Research Institutes
Basic Funds for Research and Development (Agro-Environmental
Protection Institute, Ministry of Agriculture, 22060302008023).
Fig. 10. Recycle performance of [Cu-BTC][HPM]. Reaction conditions:
0.5 mmol LA, 40 mg catalyst, 10 ml ethanol, 120 °C for 4 h.
Table 4
Element contents of [Cu-BTC][HPM] before and after the reaction by ICP*.
Elements
Contents (mg/g)
Appendix A. Supplementary data
Before the reaction
After the reaction
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcata.2019.01.004.
Cu
Mo
151.9
151.6
158.6
150.8
* Uncertainty = 2%.
References
the production of EL in ethanol, and should be beneficial to the prac-
tical application.
[1] D.M. Alonso, J.Q. Bond, J.A. Dumesic, Green Chem. 12 (2010).
[2] P. Gallezot, Chem. Soc. Rev. 41 (2012) 1538–1558.
[3] C. Xu, R.A. Arancon, J. Labidi, R. Luque, Chem. Soc. Rev. 43 (2014) 7485–7500.
[4] J.J. Bozell, G.R. Petersen, Green Chem. 12 (2010).
The catalytic performance of [Cu-BTC][HPM] was also compared
with the other heteropolyacid heterogeneous catalysts and MOF ma-
terials reported in the other published literatures as shown in Table 3,
and it shows that the catalyst [Cu-BTC][HPM] exhibited excellent
performance for the production of EL from LA in ethanol.
[5] B. Liu, Z. Zhang, ACS Catal. 6 (2015) 326–338.
[6] S. Alipour, H. Omidvarborna, RSC Adv. 6 (2016) 111616–111621.
[7] F.H. Isikgor, C.R. Becer, Polym. Chem-Uk 6 (2015) 4497–4559.
[8] F.D. Pileidis, M.M. Titirici, Chemsuschem 9 (2016) 562–582.
[9] L. Yan, Q. Yao, Y. Fu, Green Chem. 19 (2017) 5527–5547.
[10] J. Zhang, S. Wu, B. Li, H. Zhang, Chemcatchem 4 (2012) 1230–1237.
[11] A. Démolis, N. Essayem, F. Rataboul, ACS Sustain. Chem. Eng. 2 (2014) 1338–1352.
[12] C. Liu, Q. Feng, J. Yang, X. Qi, Bioresour. Technol. 255 (2018) 50–57.
[13] F.R. Ma, M.A. Hanna, Bioresour. Technol. 70 (1999) 1–15.
[14] Y. Kuwahara, W. Kaburagi, K. Nemoto, T. Fujitani, Appl. Catal. A Gen. 476 (2014)
186–196.
3.5. Catalyst recycling performance for conversion of LA to EL
The recycling performance of the catalyst was tested at 120 °C for
4 h. The spent catalyst was separated by centrifugation, and washed
with ethanol, and then dried before reuse. The fresh catalyst could af-
ford an EL yield of 92.4%. In the subsequent two cycles of use, EL yield
decreased slightly but still maintained a relatively high yield of 89.5%
when the reaction proceeded to the third time, as shown in Fig. 10. The
catalyst after reuse was characterized by XRD, FTIR and ICP, and there
was no distinct difference could be observed in their XRD patterns and
FITR spectra (Figures S2 and S3), confirming that the catalyst still
maintains its crystal structure and properties after use. The element
contents of the catalyst were analyzed by ICP to further examine
whether the catalyst had changed after the reaction. The results are
shown in Table 4. It reveals that after three times of the reaction, the
change of Cu and Mo contents were almost negligible, which indicated
that the Cu in the MOF did not precipitate and the MOF structure re-
mained stable. Meanwhile the content of Mo, representing HPM, did
not decrease, proving that HPM had no loss within the Cu-BTC struc-
ture. These analysis results demonstrate that the [Cu-BTC][HPM] cat-
alyst was stable in the reaction system and gave fairly constant EL
product yields. The slight decrease in EL yield in the recycled runs
should probably be ascribed to the occupation of active site on the
catalyst by LA or EL that was unable to be completely removed during
the washing process, and thus resulted in the slight decrease of the
[15] C.R. Patil, P.S. Niphadkar, V.V. Bokade, P.N. Joshi, Catal. Commun. 43 (2014)
188–191.
[16] K. Manikandan, K.K. Cheralathan, Appl. Catal. A Gen. 547 (2017) 237–247.
[17] M.N. Timofeeva, Appl. Catal. A Gen. 256 (2003) 19–35.
[18] W. Deng, Q. Zhang, Y. Wang, Dalton Trans. 41 (2012) 9817–9831.
[19] X. Zhang, Y. Li, L. Xue, S. Wang, X. Wang, Z. Jiang, ACS Sustain. Chem. Eng. 6
(2017) 165–176.
[20] K.Y. Nandiwale, S.K. Sonar, P.S. Niphadkar, P.N. Joshi, S.S. Deshpande, V.S. Patil,
V.V. Bokade, Appl. Catal. A Gen. 460–461 (2013) 90–98.
[21] N.A.S. Ramli, D. Sivasubramaniam, N.A.S. Amin, Bioenerg Res. 10 (2017)
1105–1116.
[22] H. Furukawa, K.E. Cordova, M. O’Keeffe, O.M. Yaghi, Science 341 (2013) 1230444.
[23] A.J. Howarth, Y. Liu, P. Li, Z. Li, T.C. Wang, J.T. Hupp, O.K. Farha, Nat. Rev. Mater.
1 (2016).
[24] H.C. Zhou, J.R. Long, O.M. Yaghi, Chem. Rev. 112 (2012) 673–674.
[25] F.G. Cirujano, A. Corma, F.X. Llabrés i Xamena, Catal. Today 257 (2015) 213–220.
[26] S.S.Y. Chui, S.M.F. Lo, J.P.H. Charmant, A.G. Orpen, I.D. Williams, Science 283
(1999) 1148–1150.
[27] D.R. Fernandes, A.S. Rocha, E.F. Mai, C.J.A. Mota, V. Teixeira da Silva, Appl. Catal.
A Gen. 425 (2012) 199–204.
[28] Z. Wang, Q. Chen, Green Chem. 18 (2016) 5884–5889.
[29] H.B. Wu, B.Y. Xia, L. Yu, X.Y. Yu, X.W. Lou, Nat. Commun. 6 (2015) 6512.
[30] J. Hu, H. Cai, H. Ren, Y. Wei, Z. Xu, H. Liu, Y. Hu, Ind. Eng. Chem. Res. 49 (2010)
12605–12612.
[31] W. Huang, X. Zhou, Q. Xia, J. Peng, H. Wang, Z. Li, Ind. Eng. Chem. Res. 53 (2014)
11176–11184.
[32] S. Lin, Z. Song, G. Che, A. Ren, P. Li, C. Liu, J. Zhang, Microporous Mesoporous
174

T. Guo et al.
AppliedCatalysisA,General572(2019)168–175
[39] E.S. Sankar, V. Mohan, M. Suresh, G. Saidulu, B.D. Raju, K.S.R. Rao, Catal.
Commun. 75 (2016) 1–5.
[40] J.A. Melero, G. Morales, J. Iglesias, M. Paniagua, B. Hernandez, S. Penedo, Appl.
Catal. A Gen. 466 (2013) 116–122.
Mater. 193 (2014) 27–34.
[33] C.H. Specht, F.H. Frimmel, Phys. Chem. Chem. Phys. 3 (2001) 5444–5449.
[34] L.T.L. Nguyen, T.T. Nguyen, K.D. Nguyen, N.T.S. Phan, Appl. Catal. A Gen. 425
(2012) 44–52.
[35] G.R. Rao, T. Rajkumar, B. Varghese, Solid. State. Sci. 11 (2009) 36–42.
[36] H. Du, J. Bai, C. Zuo, Z. Xin, J. Hu, CrystEngComm. 13 (2011) 3314–3316.
[37] T. Tsuruoka, S. Furukawa, Y. Takashima, K. Yoshida, S. Isoda, S. Kitagawa, Angew.
Chem. Int. Ed. Engl. 48 (2009) 4739–4743.
[41] K.Y. Nandiwale, S.K. Sonar, P.S. Niphadkar, P.N. Joshi, S.S. Deshpande, V.S. Patil,
V.V. Bokade, Appl. Catal. A Gen. 460 (2013) 90–98.
[42] G. Pasquale, P. Vázquez, G. Romanelli, G. Baronetti, Catal. Commun. 18 (2012)
115–120.
[38] C.-Y. Sun, S.-X. Liu, D.-D. Liang, K.-Z. Shao, Y.-H. Ren, Z.-M. Su, J. Am. Chem. Soc.
[43] K. Yan, G. Wu, J. Wen, A. Chen, Catal. Commun. 34 (2013) 58–63.
131 (2009) 1883–1888.
175