MPV reduction of ethyl levulinate to γ-valerolactone by the biomass-derived chitosan-supported Zr catalyst

Article abstract of DOI:10.1039/d0nj02667k

Herein, we used the biopolymer chitosan as a support to synthesize a biomass-derived catalyst (chitosan-Zr) to achieve GVL in 97% yield from MPV reduction of EL, by using isopropanol as a hydrogen source. The catalyst system is also applicable to the reduction of various organic compounds with carbonyl groups. Additionally, we have proposed a possible mechanism for this reaction based on the systematic investigation towards the reaction. Moreover, the recycle and reuse experiment showed that this chitosan-Zr exhibited long-life catalytic performance and can maintain its high catalytic performance even after five runs of recycle and reuse experiments.

Full text of DOI:10.1039/d0nj02667k

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Full Paper
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MPV Reduction of Ethyl Levulinate to γ-Valerolactone by
Biomass-derived Chitosan-Supported Zr Catalyst
Tianlong Wang, Hai Xu,* Jianghua He* and Yuetao Zhang*
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State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University,
Changchun, Jilin, 130012, China.
*Corresponding author E-mail: xuhai@jlu.edu.cn; hjh2015@jlu.edu.cn; ytzhang2009@jlu.edu.cn
Abstract
Herein, we employed the biopolymers chitosan as support to synthesize a biomass-
derived catalyst (chitosan-Zr) to achieve GVL in 97% yield rom MPV reduction of EL,
by using isopropanol as hydrogen source. The catalyst system is also applicable to the
reduction of various organic compounds with carbonyl group. Additionally, we have
proposed a possible mechanism for this reaction based on the systematic investigation
towards the reaction. Moreover, the recycle and reuse experiment showed that this
chitosan-Zr exhibited long-life catalytic performance and can maintain its high catalytic
performance even after five runs of recycle and reuse experiments.
Keywords
Biomass conversion
•
Chitosan-Zr
•
Ethyl levulinate
•
MPV reduction γ-Valerolactone
•
1

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1. Introduction
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Due to the depletion of fossil resources along with the intensifying worldwide
problems, such as air pollution, global warming and land desertification, resulting from
the excessive consumption of fossil resources,1 exploring the renewable resources
from biomass has become a sustainable strategy to tackle the crisis facing our society.2
As the most abundant renewable resource in nature, the valorization of lignocellulose
(composed of cellulose, hemicellulose and lignin) can provide plenty of bio-fuels and
fine chemicals.3-8 Ethyl levulinate (EL) originated from the acid-catalyzed alcoholysis
of cellulose or hemicellulose, has become an important biomass-based platform
chemical for the synthesis of various value-added compounds,9,10 including succinic
acid,11 valeric acid,12 5-nonanone,13 1,4-pentanediol,14 N-containing lactams15 and
γ-valerolactone (GVL).16 Among them, GVL has shown its wide application in the
production of both liquid fuels17 and renewable monomers, such as α-methylene-γ-
valerolactone18-20 and caprolactam.21 In addition, GVL can serve as green solvents for
organic reactions owing to its high boiling point and low toxicity.22,23 Therefore, the
transformation of EL into GVL is recognized as one of the most significant steps in
biomass valorization.24 Previously, production of GVL from EL mainly used H2 as
hydrogen source and was typically catalyzed by metal-based catalysts, such as Au,25
Ir 26 , Ru, 27 Pt, 28 Pd. 29 , 30 Although these catalyst systems have shown good
performance, their practical applications were greatly restricted by the requirement of
high H2 pressure and high catalyst loading of precious metal.
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By using alcohol as both the green solvent and hydrogen source, Meerwein-
Ponndorf-Verley (MPV) reduction has been recognized as a safe and promising strategy
in biomass conversion31. Catalysts based on Zr,32-42 Hf,43-45 Ru,46 and Sn47 were
recently reported to be effective for MPV reduction. In 2011, Dumesic et al. employed
commercial ZrO2 to achieve a 92% yield of GVLfrom the hydrogen transfer of levulinic
32
acid under 300 psi N2 whereas Lin group utilized Zr(OH)4 to catalyze the MPV
reduction of EL to furnish GVL in 88% yield at 200 ºC.34 Several efficient biomass-
2

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derived catalyst systems have been developed to overcome the harsh reaction condition.
In 2015, Han et al. synthesized a porous Zr-phytic catalyst to achieve 95.4% yield of
GVL from MPV reduction of EL at 150 ºC.35 In 2018, by using 2,5-furandicarboxylic
acid as ligand, Fang group synthesized Hf-FDCA catalyst to obtain GVL in 98% yield
from EL.44 More recently, Leng group employed the tannin-derived zirconium-
containing porous hybrid to produce GVL in 89.2% yield from the transformation of
EL.42 These literatures indicated that the great potential of biomass-based catalysts for
MPV reduction. Currently, the development of stable, low-cost and easy-synthesized
biomass-derived catalysts are still highly demanded to achieve the efficient
transformation of EL into GVL.
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As the renewable resources originating from the diacylation of chitin, chitosan is
the second abundant biopolymer in nature next to cellulose.48 There is around 1600
million tons of chitin produced in marine ecosystems every year in the world,49 but they
are still highly underutilized. Currently, chitosan was mainly employed in the field of
biomaterials and as adsorption for heavy metal ions.50 There exist many -NH2 and -OH
groups in the structure of chitosan, which endows it with good stability and strong
chelating ability for metal ions. In addition, the combination of these -NH2 and -OH
groups with central metal may exhibit both acidic and basic sites, which have been
reported to play an important role for MPV reduction reaction.44 Based on these results,
we anticipate the chitosan-based catalysts might be suitable for MPV reduction of EL
to GVL. Here we report the synthesis and characterization of biomass-derived chitosan-
metal catalysts. These catalysts were examined for the MPV reduction of EL to GVL
and chitosan-Zr showed the best catalytic performance among the investigated catalysts.
Additionally, this catalyst system could be applied to the reduction of many aromatic
and aliphatic carbonyl compounds. The investigation towards the reaction mechanism
for transformation of EL to GVL was also included into this study.
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Scheme 1 The transformation of EL to GVL and application of GVL.
2. Experimental Section
2.1 Materials and measurements
Levulinic acid (LA) (98%), ethyl levulinate (EL) (98%), methyl levulinate (ML)
(98%), butyl levulinate (BL) (98%), isopropyl levulinate (IPL) (98%), ZrCl4 (98%),
RuCl3 (98%), CoCl2 (98%), NiCl2 (98%), chitosan (medium viscosity, 98%), chitin
(98%) were purchased from Energy. Ammonia (28 wt% in water), ZrO2 (98%), furfural
(98%), 5-hydromethylfurfural (98%), 4-methoxybenzaldehyde (98%), cinnamaldehyde
(98%), cyclohexanone (98%), octanal (98%), acetophenone (98%) were purchased
from J&K Scientific Ltd. Solvents were provided by Adamas and directly used without
purification.
1
All products were characterized by H NMR spectroscopy and GC analysis
compared with standard compounds. The conversion and yield of all compounds were
measured by GC analysis using naphthalene as internal standard, which was performed
on an Agilent 6890 system equipped with a flame ionization detector (FID) and a DB-
WAX capillary column (15 m × 0.32 mm × 0.25 µm). The temperature of the injector
and detector was set at 250 °C, and the oven was started at 60 ºC, and was held for 5
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min, raised to 70 ºC at a rate of 5 ºC/min and held for 10 min, then raised to 80 ºC at a
rate of 2 ºC/min and stayed for 4 min. At last, the temperature was continuously raised
to 160 ºC at a rate of 5 ºC/min. NMR spectra were recorded on a Bruker Avance II 500
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(500 MHz, H) instrument at room temperature. Chemical shifts for H spectra were
referenced to internal solvent resonances and reported as parts per million relative to
SiMe4.
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2.2 Synthesis and characterization of catalysts
Chitosan-Zr is prepared according to the similar synthetic strategy as reported for
chitosan-Ru.51 400 mg chitosan was suspended in 10 mL ethanol and treated under
ultrasound for five minutes to make it dispersed uniformly. Then, a solution of 100 mg
ZrCl4 dissolved in ethanol was slowly added to the suspension. Afterwards, 28%
ammonia was added dropwise to the solution to maintain its pH between 8 and 9. Finally,
the solution was refluxed overnight at 80 ºC. Other metal-based catalysts were prepared
in a similar manner as described for the synthesis of chitosan-Ru,51 by reacting chitosan
with the corresponding metal chlorides (CoCl2, NiCl2 and RuCl3) in the aqueous
solution at room temperature and adjusting the pH by ammonia. The synthesized
catalyst was directly collected by filtration and dried for 5 h at 50 ºC in vacuum.
Scheme 2 The synthesis and structure of Zr-chitosan.
The content of metal in supported catalyst was measured by inductively coupled
plasma atomic emission spectroscopy (ICP-AES) on a Perkin-Elmer Optima 3300 DV
ICP instrument. Scaning electron microscope (SEM) and Energy Dispersive X-Ray
Spectroscopy (EDS) was performed on SU 8020 instrument. Powder X-ray diffraction
(PXRD) spectra of chitosan-M were examined on a Rigaku D/Max 2550 diffractometer
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using Cu Kα radiation (λ = 1.5418 Å). Fourier transform infrared spectrometer (FT-IR)
spectra were recorded on Bruker VERTEX 80 V in wavenumber range of 400-4000 cm-
1 with KBr as background. Thermal properties were measured by thermogravimetry
analysis (TGA) on a TA Q50 instrument under N2 atmosphere at a heating rate of 10
ºC/min from room temperature to 800 ºC. X-Ray Photoelectron Spectroscopy (XPS)
characterization was performed on an ESCALAB 250 spectrometer, and the binding
energies of the XPS spectra was referred to the C 1s (284.5 eV) as internal reference.
Temperature programmed desorption of CO2 and NH3 (CO2-TPD and NH3-TPD) were
conducted on autochem II 2920 and the results were obtained from 50 ºC to 200 ºC. N2
adsorption-desorption isotherms of chitosan-Zr was measured by using a TriStar 3000
(Micromeritics) equipment.
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2.3 Catalytic activity
MPV reduction of EL to GVL was carried out in a 10 mL thick wall tube with a
Teflon screw cap under air atmosphere. In a typical procedure, 0.25 mmol EL in 1 mL
isopropanol and 60 mg chitosan-Zr (15 mol% Zr content relative to substrate) were
mixed in the reactor. Then, the reactor was transferred into an oil bath (Heidolph,
Germany) and stirred at 250 rpm. After the reaction was completed, the reactor was
cooled to room temperature and the solution was analyzed by 1H NMR and GC spectra.
2.4 The leaching and recycle and reusable experiments
In leaching experiments, chitosan-Zr catalyst was separated from the reaction
solution by filtration after reacting for 2 h at 160 ºC. The remaining reaction solution
was reacted for another 10 h. In recycle and reusable experiments, chitosan-Zr catalyst
was collected by centrifugation and washed by ethanol for three times. After dried at
50 ºC in vacuum for 5 h, the recovered catalyst was used for the next run.
3. Results and Discussion
3.1 Characterization of the catalyst
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Fig. 1 Characterization of catalyst: (a) SEM image of chitosan-Zr. The overlay of (b) Powder XRD
spectra, (c) FT-IR spectra and (d) TG curves of both chitosan and chitosan-Zr. (e) NH3-TPD and (f)
CO2-TPD spectra of chitosan-Zr.
Chitosan-Zr is synthesized according to the similar synthetic strategy as reported
for chitosan-Ru.51 Analysis of the filtrate showed the existence of NH4Cl, where the
+
NH4 cations was from the ammonia used to adjust the pH value of the solution and Cl-
anions was from ZrCl4. The structure of chitosan-Zr catalyst was proposed as shown in
scheme 2. ICP detection showed that the mass fraction of Zr element in chitosan-Zr was
6.0 wt% (Table S1), indicating that Zr has been loaded onto chitosan. SEM images
revealed that chitosan-Zr is amorphous structure composed of particles with varied
sizes (Fig. 1a). Besides, the N2-adsorption–desorption experiments of chitosan-Zr (Fig.
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S6) only showed extremely low adsorption for N2, indicating that it is a nonporous
structure. These results suggested that the reaction was catalyzed by the active sites on
the surface of chitosan-Zr. EDS analysis revealed that the uniform distribution of Zr, C
and O element (Fig. S1). As shown in Fig. 1b, powder XRD spectra indicated that
chitosan-Zr is similar with that of chitosan. FT-IR spectra indicated that both the
chitosan-Zr and chitosan had the similar structure (Fig. 1c). The peaks at 1599 cm-1 and
1420 cm-1 were assigned to the vibration of -NH2 and -OH groups respectively in
chitosan. Compared with chitosan, the weakened signal in chitosan-Zr might be
attributed the losing of H atoms during the synthesis (Scheme 2). As shown in XPS
spectrum (Fig. S2), the energy peaks at 182.4 eV and 184.8 eV were ascribed to the Zr
3d5/2 and Zr 3d3/2, indicating of Zr loaded on chitosan is tetravalent.39 There was a little
shift between chitosan-Zr and chitosan in their O 1s and N 1s energy peaks (Fig. S2c
and Fig. S2d), which further confirmed that that Zr4+ was interacted with -NH2 and -
OH groups. TG analysis (TGA) of chitosan and chitosan-Zr both exhibited two obvious
weight loss (Fig. 1d) and indicated that chitosan-Zr catalyst can maintain the framework
up to 230 ºC. The CO2 and NH3-temperatured program desorption (TPD) spectra
showed that chitosan-Zr had both acidic (0.31 mmol/g, Fig. 1e) and basic (0.25 mmol/g,
Fig. 1f) catalyst sites. Compared to chitosan (Fig. S3a, 0.12 mmol/g amount for acidic
sites; Fig. S3b, 0.27 mmol/g amount for basic sites), the higher content of acidic sites
in chitosan-Zr might be resulted from the incorporation of Zr4+ species. In comparison,
chitin-Zr showed a lower Lewis acidic and basic sites than chitosan-Zr. (Fig. S4a, 0.24
mmol/g amount for acidic sites; Fig. S4b, 0.19 mmol/g amount for basic sites).
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3.2 Screening the catalysts for MPV reduction of EL to GVL
Next, we examined the effectiveness of various catalysts for MPV reduction of EL
to GVL at 160 ºC for 8 h (Table 1). Both the blank experiment and chitosan itself were
ineffective for this transformation (Table 1, entries 1-2). The employment of ZrO2 led
to the conversion of 10% of ELto GVLin 7% yield (Table 1, entry 3) whereas switching
to ZrCl4 produced GVL in 47% yield along with the generation of byproduct isopropyl
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levulinate (IPL), probably due to the absence of basic sites (Table 1, entry 4). To our
delight, both chitosan-Zr and chitosan-Hf can quantitatively convert EL into GVL in
around 97% yield (Table 1, entries 5 and 6) and exhibited a TOF value of 0.81 h-1.
Switching to Chitin-Zr prepared from the same approaches resulted in the GVL yield
decreased to 66% with a TOF value of 0.55 h-1(Table 1, entry 7). As revealed by the
temperature programmed desorption of CO2 (CO2-TPD) experiment, the basic sites of
chitin-Zr (0.19 mmol/g, Fig. S4a,) is less than that of chitosan-Zr (0.25 mmol/g, Fig.
1f). It is reported that These basic sites are significantly important for the hydrogen
transfer process in MPV reduction.39, 44 Therefore, chitin-Zr with the less basic sites
showed a decreased catalytic efficiency in comparison with chitosan-Zr. Utilizing other
chitosan-based catalysts (chitosan-M, M = Co, Ni, Ru) exhibited a much lower TOF
value (<0.1 h-1, Table 1, entries 8-10) compared with chitosan-Zr. From an economic
perspective, chitosan-Zr was selected as catalyst to examine the following MPV
reduction of EL with different hydrogen donors. It turned out that using primary
alcohols, i.e. methanol or ethanol, and tertiary alcohols, i.e. tert-butanol, as solvent only
furnished GVL in lower yield (Table 1, entries 11-13). However, the employment of 2-
butanol produced GVL in comparable yield as isopropanol did (Table 1, entry 14),
suggesting that secondary alcohol is a highly effective hydrogen donors and its β-H is
essentially important for MPV reduction.39
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Table 1 Various catalysts for the MPV reduction of EL to GVL.[a]
GVL
Conv.
(%)
Alkyl Levulinate
yield (%)[b]
TOF
Entry
Catalyst
Solvent
(h-1)[c]
Yield
(%)
selectivity
(%)
1
2
3
4
Blank
Chitosan
ZrO2
isopropanol
isopropanol
isopropanol
isopropanol
0
0
0
0
0
0
0
-
0
-
10
75
0
7
70
63
0.06
0.45
ZrCl4
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Chitosan-Zr
Chitosan-Hf
Chitin-Zr
isopropanol
isopropanol
isopropanol
isopropanol
isopropanol
isopropanol
methanol
100
100
73
7
3
2
4
2
0
0
0
0
2
2
97
97
66
0
97
97
90
0
0.81
0.81
0.55
-
7
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8
Chitosan-Co
Chitosan-Ni
Chitosan-Ru
Chitosan-Zr
Chitosan-Zr
Chitosan-Zr
Chitosan-Zr
9
0
0
0
-
10
11
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12
92
66
30
100
5
41
12
99
81
97
0.04
0.09
0.54
0.20
0.81
11
65
24
97
ethanol
tert-butanol
2-butanol
[a]
Reaction conditions: 0.25 mol EL, 15 mol% chitosan-Zr (metal Zr relative to substrate), 1 mL
isopropanol, 160 ºC, 8 h. [b] by-product alkyl Levulinate produced from the transesterification ofEL
with the solvent. [c] TOF is calculated as mol (produced GVL)/[mol (total metal added) × time (h)].
Conversion and yield were determined by GC spectra with naphthalene as internal standard.
3.3 Effect of the catalyst loading
The effect of the catalyst loading of chitosan-Zr was examined for transformation
of EL to GVL performed in isopropanol at 160 ºC for 8 h (Fig. 2). A 5 mol% catalyst
loading of chitosan-Zr (relative to substrate) only converted 42% of EL to GVL in 40%
yield. Increasing the catalyst loading to 10 mol% or 15 mol% led to the significantly
enhanced GVL yield of 82% and 97%, respectively. The GVL yield produced by 20
mol% catalyst loading is comparable with that obtained by 15 mol% of chitosan-Zr.
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Fig. 2 Effect of the catalyst loading of chitosan-Zr catalyst. Reaction conditions: 0.25 mol EL, the
catalyst loading was given in mol% of Zr content relative to the substrate, 1 mL isopropanol, 160
ºC and 8 h.
3.4 Effects of temperature and reaction time
The influence of temperature (from 140 ºC to 180 ºC) and reaction time (up to 12
h) were also examined on MPV reduction of EL to GVL. As shown in Fig. 3a, after
heating reaction at 140 ºC for 12 h, a 75% yield of GVL was obtained. Increasing
temperature to 160 ºC (Fig. 3b), GVL yield was drastically enhanced to 97% within 8
h. No further improvement was observed for heating reaction with prolonged reaction
time of 12 h. The amounts of byproduct IPL initially increased, and then gradually
decreased to 2% yield along with reaction time. Further increasing to 180 ºC (Fig. 3c),
although the reaction rate is faster, GVL yield is 90%, probably due to more side
reactions at high temperature. Therefore, the optimal result was achieved for our
chitosan-Zr system by heating reaction at 160 ºC for 8 h.
It was reported that MPV reduction of alkyl levulinate to GVL is a first-order
reaction.52 Kinetic studies of the conversion of EL to GVL were performed ranging in
temperature from 140 ºC to 180 ºC. Fig. 3d revealed that the plot for ln(Ct/C0) vs
reaction time obtained for each temperature is linear. According to the Arrhenius plot
(Fig. 3e), we can obtain an apparent activation energy (Ea) of 63.2 KJ/mol for this
reaction by plotting the logarithm of the reaction rate constant (lnK) against the
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reciprocal of the temperature (1/T), which is similar with those reported for MPV
reduction catalyzed by Ru-TPPTS (61.0 KJ/mol), 53 Ti-Beta (69.0 KJ/mol)42 and
hafnium–lignosulfonate (64.6 KJ/mol)35 catalyst system, indicating that chitosan-Zr is
also the active catalyst for the reduction of EL to GVL.
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0.5
0.0
(e)
y= -7600x + 16.96
-0.5
-1.0
-1.5
Ea= 63.2 KJ/mol
0.0022
0.0023
0.0024
1/T (K-1
)
Fig. 3 Effects of reaction temperature and time at (a) 140 ºC, (b) 160 ºC and (c) 180 ºC. (d) Kinetics
profiles and (e) Arrhenius plot for the reduction of EL to GVL. Conditions: 0.25 mmol EL, 15 mol%
chitosan-Zr, 1 mL isopropanol.
3.5 The scope of substrates
After the identification of highly efficient chitosan-Zr catalyst system, we started
to investigate whether this catalyst system could be applied to the reduction of different
organic aldehyde compounds (Table 2). We are very glad to see that chitosan-Zr can
achieve GVL in excellent yield by using substrates having similar structure with EL,
such as levulinic acid, methyl levulinate and butyl levulinate (Table 2, entries 1-3). For
the biomass-derived carbohydrates, such as furfural and 5-hydroxymethylfurfural, also
could be reduced to furfuryl alcohol and 2,5-dihydromethylfuran, respectively after
heating at 120 ºC for 4 h (Table 2, entries 4 and 5). This chitosan-Zr system is also
highly effective for the MPV reduction of various carbonyl compounds. Such as the
employment of 4-methoxyl benzaldehyde, octyl aldehyde and cyclohexanone as
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starting materials led to the production of their corresponding reduced products in up
to 99% excellent yield at 120 ºC (Table 2, entries 6-8), showing that our system is
applicable to the reduction of both aromatic and aliphatic carbonyl compounds.
Switching to cinnamaldehyde led to cinnamyl alcohol in 97% yield (Table 2, entry 9),
indicating that chitosan-Zr exhibited high selectivity towards reducing C=O bond rather
than C=C bonds. For ketone compound such as acetophenone, 160 ºC is needed for
high product yield of 1-phenylethanol (Table 1, entry 10), probably due to the large
hindrance of ketone structure.35
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Table 2 Reduction of various carbonyl compounds catalyzed by chitosan-Zr.[a]
Temp.
(ºC)
Time
(h)
Conv.
(%)
Yield
(%)
Entry
Substrate
Product
1
2
160
160
8
8
>99
>99
96
97
3
160
8
>99
89
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5
120
120
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>99
>99
96
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120
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>99
99
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8
120
120
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4
>99
>99
99
97
9
120
160
4
4
>99
>99
97
98
10
[a] Conditions: 0.25 mmol substrate, 15 mol% chitosan-Zr, 1 mL isopropanol. Yields were determined by GC spectra
with naphthalene as internal standard.
3.6 Leaching experiment and Recycle and reuse experiments
A leaching experiment was conducted to confirm the heterogeneity of chitosan-Zr
catalyst (Fig. 4a). After heating at 160 ºC for 2 h, the solid catalyst was removed by
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filtration and the filtrate reacted for another 10 h. Fig. 4a clearly revealed that the GVL
yield remained unchanged as shown in. ICP analysis indicated that the filtered chitosan-
Zr only showed about 0.1 wt% loss of Zr species compared to the fresh prepared catalyst,
suggesting that Zr was almost not leached into the solution. Furthermore, the reusability
of chitosan-Zr was verified by the recycle and reuse experiment. Upon the completion
of the reaction, the catalysts solids were collected by centrifugation, then washed by
ethanol for three times and dried at 50 ºC for 5 h in vacuum. After that, it was used for
the next run. It turned out that chitosan-Zr can maintained its catalytic performance
even after five runs of recycle and reuse experiments, without obvious decrease in the
selectivity and yield of GVL (Fig. 4b). Moreover, ICP results showed that the mass
fraction of Zr is 5.7 wt% for the recovered chitosan-Zr after five runs of recycle and
reuse experiments (Table S1). It is similar with that for the fresh prepared catalyst (6.0
wt %), indicating no obvious loss of Zr in chitosan-Zr. The recovered chitosan-Zr was
also characterized by FT-IR and TG analysis Fig. S5, showing that the thermal stability
and structure of the recovered chitosan-Zr are similar with the fresh-prepared one.
These results obtained from both the leaching and recycle and reuse experiments
indicated that chitosan-Zr can maintain the heterogeneous and stability during reaction.
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Fig. 4 (a) Leaching and (b) recycle and reuse experiments of chitosan-Zr for the reduction of EL to
GVL. Reaction conditions: 0.25 mmol EL, 15 mol% chitosan-Zr, 1 mL isopropanol, 160 ºC.
3.7 Mechanistic study
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As discussed above, chitosan-Zr has both acidic (0.31 mmol/g) and basic sites
(0.25 mmol/g), which mainly originated from the metal Zr4+ species and N or O atoms
in chitosan-Zr catalyst, respectively. It is reported that pyridine and benzoic acid could
interact with acidic and basic sites to block the catalytic sites of catalyst, respectively.
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We utilized pyridine and benzoic acid to poison the acidic and basic sites of chitosan-
Zr to examine their influence on this reaction. The addition of benzoic acid to reaction
resulted in the drastically decreased GVL yield of 16% (Table 3, entry 3), whereas the
employment of pyridine led to the GVL yield slightly decreased to 76% (Table 3, entry
2), probably due to the weaker poisoning effect of pyridine. 55 The poisoning
experiments demonstrated that both the acidic and basic sites of chitosan-Zr are
significantly important for the reactivity of MPV reduction. In combination with the
previous literature,44 a plausible mechanism of the MPV reduction of EL to GVL is
proposed as shown in scheme 3. In which, EL molecules are initially absorbed to
chitosan-Zr surface and formed a six-member transitional intermedia through the
coordination with the activated isopropanol. After that, EL is reduced to ethyl-4-
hydroxypentanoate through hydrogen transfer while the same equivalent of isopropanol
molecules was transformed into acetone. Ethyl-4-hydroxypentanoate is converted to
GVL along with release of one ethanol molecule promoted by the acid site of chitosan-
Zr. Moreover, 1H NMR spectra (Fig. S8) revealed the existence of leaving ethanol and
acetone molecules in the reaction mixture, which further confirmed the reaction
mechanism.
Table 3 Effect of additives on the conversion of EL to GVL. [a]
Entry
Additive
-
EL conversion (%)
GVL yield (%)
1
2
3
100
81
97
76
16
Pyridine[b]
benzoic acid[c]
19
[a] Reaction conditions: 0.25 mmol EL, 15 mol% chitosan-Zr, 1 mL isopropanol. 160 ºC, 8 h. [b] 50
μL. [c] 50 mg.
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Scheme 3 Reaction mechanism proposed for the chitosan-Zr-catalyzed MPV reduction of EL to
GVL.
4. Conclusions
In summary, a biomass-derived chitosan-Zr catalyst was synthesized and highly
effective for MPV reduction of EL by using isopropanol as hydrogen source, furnishing
GVL in up to 97% yield. The poisoning experiments demonstrated that both the acidic
and basic sites in chitosan-Zr play significantly important roles in MPV reduction.
Detailed experimental data coupled with kinetic studies led to the plausible mechanism
proposed for the reduction of EL to GVL. Both the leaching and recycle and reuse
experiments indicated that chitosan-Zr can maintain the heterogeneous and stability
during the reaction. Furthermore, this method is also applicable to the reduction of
various aromatic and aliphatic carbonyl compounds. All these results indicated the great
potential of this simply synthesized and low-cost chitosan-Zr catalyst in the reduction
of carbonyl compounds in biomass valorization and organic synthesis.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant
no. 21975102, 21871107, 21774042).
Statement of Data availability
All data generated or analyzed during this study are included in this published article
[and its supplementary information files].
Conflicts of interest
The authors declare no competing financial interest.
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Graphical Abstract
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MPV Reduction of Ethyl Levulinate to γ-Valerolactone by
Biomass-derived Chitosan-Supported Zr Catalyst
Tianlong Wang, Hai Xu,* Jianghua He* and Yuetao Zhang*
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State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University,
Biomass-derived chitosan-supported Zr catalyst with dual acid-base properties exhibits
highly efficient performance towards MPV reduction of ethyl levulinate to γ-
valerolactone.
20