LiCl-promoted-dehydration of fructose-based carbohydrates into 5-hydroxymethylfurfural in isopropanol

Article abstract of DOI:10.1039/d0ra08737h

The carbohydrate-derived 5-hydroxymethylfurfural (HMF) is one of the most versatile intermediate chemicals, and is promising to bridge the growing gap between the supply and demand of energy and chemicals. Developing a low-cost catalytic system will be helpful to the production of HMF in industry. Herein, the commercially available lithium chloride (LiCl) and isopropanol (i-PrOH) are used to construct a cost-effective and low-toxic system, viz., LiCl/i-PrOH, for the preparation of HMF from fructose-based carbohydrates, achieving ～80% of HMF yield under the optimum conditions. The excellent promotion effect of LiCl on fructose conversion in i-PrOH could be attributed to the synergistic effect of LiCl with i-PrOH through the LiCl-promoted and i-PrOH-aided dehydration process, and the co-operation of LiCl and i-PrOH for stabilizing the as-formed HMF by hydrogen/coordination bonds, giving a low activation energy of 68.68 kJ mol-1 with a pre-exponential factor value of 1.2 × 104 min-1. The LiCl/i-PrOH system is a substrate-tolerant and scalable catalytic system, fructose (scaled up 10 times), sucrose, and inulin also give 73.6%, 30.3%, and 70.3% HMF yield, respectively. Moreover, this system could be reused 8 times without significant loss of activity. The readily available and low-toxic LiCl, the sustainable solvent (i-PrOH), the renewable starting materials, and the mild reaction conditions make this system promising and sustainable for the industrial production of HMF in future.

Full text of DOI:10.1039/d0ra08737h

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LiCl-promoted-dehydration of fructose-based
carbohydrates into 5-hydroxymethylfurfural in
isopropanol†
Cite this: RSC Adv., 2021, 11, 1404
a
Zhenzhen Li,b Lili Chena and Junjiang Tenga
*
Hao Ma,
The carbohydrate-derived 5-hydroxymethylfurfural (HMF) is one of the most versatile intermediate
chemicals, and is promising to bridge the growing gap between the supply and demand of energy and
chemicals. Developing a low-cost catalytic system will be helpful to the production of HMF in industry.
Herein, the commercially available lithium chloride (LiCl) and isopropanol (i-PrOH) are used to construct
a cost-effective and low-toxic system, viz., LiCl/i-PrOH, for the preparation of HMF from fructose-based
carbohydrates, achieving ꢀ80% of HMF yield under the optimum conditions. The excellent promotion
effect of LiCl on fructose conversion in i-PrOH could be attributed to the synergistic effect of LiCl with i-
PrOH through the LiCl-promoted and i-PrOH-aided dehydration process, and the co-operation of LiCl
and i-PrOH for stabilizing the as-formed HMF by hydrogen/coordination bonds, giving a low activation
energy of 68.68 kJ molꢁ1 with a pre-exponential factor value of 1.2 ꢂ 104 minꢁ1. The LiCl/i-PrOH system
is a substrate-tolerant and scalable catalytic system, fructose (scaled up 10 times), sucrose, and inulin
also give 73.6%, 30.3%, and 70.3% HMF yield, respectively. Moreover, this system could be reused 8 times
without significant loss of activity. The readily available and low-toxic LiCl, the sustainable solvent (i-
PrOH), the renewable starting materials, and the mild reaction conditions make this system promising
and sustainable for the industrial production of HMF in future.
Received 14th October 2020
Accepted 22nd December 2020
DOI: 10.1039/d0ra08737h
rsc.li/rsc-advances
HMF production. It is found that fructose-based carbohydrates
(fructose, sucrose, inulin, etc.) are the prevalent feedstocks due
Introduction
to their much easier dehydration process in the presence of acid
catalysts.6,7 So far, diverse catalysts involving strong mineral acids
(HCl, H2SO4, H3PO4, etc.), strong metal-based Lewis acids (Cr3+,
Sn4+, Fe3+, etc.), and solid acid catalysts (acidic zeolites, cation-
exchanged resins, and others) have been developed for the
conversion of fructose-based carbohydrates into HMF.6,8 Although
these acid catalysts have excellent catalytic performance for
carbohydrates conversion, they still suffer from one or more per-
sisting drawbacks such as equipment corrosion (mineral acids),
environmental toxicity (Lewis acids), difficulty in product separa-
tion and catalyst recycling (mineral acids and Lewis acids), tedious
workup procedures (solid acids), and so on.6,8–10 Therefore, it is
pressing to explore new safer, milder, and less environmentally
impactful catalysts that are in line with the concept of sustainable
chemistry to catalyze carbohydrates to produce HMF.
However, the search for efficient catalysts frequently leads to
new homogeneous and heterogeneous catalysts of increasing
complexity, sometimes overlooking common, commercially
available materials that can be used in catalysis directly will be
more practical. For example, lithium chloride (LiCl) is weak
Lewis acid with lower environmental risk, and commonly used
as an additive to privileged cellulose solvents, i.e., LiCl/N,N-
dimethylacetamide (LiCl/DMAc).11 The Li+ and Clꢁ played the
key role for cellulose dissolution in LiCl containing solvent
5-Hydroxymethylfurfural (HMF), a well-known six-carbon mole-
cule, is identied as one of the renewable and versatile interme-
diates for the production of a variety of chemicals and alternative
fuels, such as 5-ethoxymethylfurfural, 2,5-furandicarboxylic acid,
2,5-dimethylfuran, and 2,5-bis(alkoxymethyl)furans, most of which
are derived from fossil feedstocks traditionally, due to its three
functional moieties of hydroxymethyl (–CH2OH), formyl (–CHO)
and a furan ring.1 Undoubtedly, the carbohydrate-derived HMF
has been regarded as one of the bridges between conventional
fossil resources and renewable carbons.2,3 In recent years, catalytic
conversion of renewable biomass into HMF has attracted much
attention, and developing sustainable and efficient methods and
chemical processing is still demanded with the rapidly diminish-
ing supply of fossil feedstock.4,5
In general, the six-carbon sugars, such as glucose, fructose,
sucrose, and starch, are commonly used as the feedstocks for
aCollege of Chemistry, Guangdong University of Petrochemical Technology, Maoming
525000, P. R. China. E-mail: thma@gdupt.edu.cn
bSchool of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, P. R. China
† Electronic supplementary information (ESI) available: Recycling of LiCl/i-PrOH
system, NMR spectrum and HPLC prole of the isolated HMF, and the data for
kinetics analysis. See DOI: 10.1039/d0ra08737h
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systems, and could keep strong interaction with the hydroxy has been constructed consequently. The promotion effect of
groups (–OH) attached in cellulose chains.12–14 Recently, Binder LiCl, the effect of reaction conditions, the extent of the substrate
et al.15,16 found that the LiCl was an additive for promoting types are evaluated systematically.
carbohydrates dehydration reactions in the presence of toxic
CrCl2 or CrCl3 catalyst; and with the aid of LiCl, the yield of HMF
elevated signicantly from fructose, glucose, and cellulose
substrates in DMAc solvents; Chen et al.17 studied the promotion of
Experimental
Materials
LiCl on CrCl2, SnCl4 or SnCl2 catalyzed dehydration of glucose into
D-Fructose (99%), sucrose (99%), inulin (97%), glucose (99%),
HMF in the caprolactam solvent, achieving acceptable HMF yield
of 55–67% with the suitable amount of LiCl additive. Moreover,
maltose (99%), starch (99%), cellulose (99%), HMF (99%), and
CH3OH (high-performance liquid chromatography, HPLC) were
LiCl could also act as a catalyst in catalytic ring-opening poly-
supplied by the J&K Chemical Company (Beijing, China); the
merization of lactide in the presence of hydroxyl-containing
analytical graded LiF (99%), LiCl (99%), LiBr (99%), LiI (99%),
NaCl (99%), KCl (99%), MgCl2 (99%), CaCl2 (99%), NH4Cl
(99%), and i-PrOH (99%) were purchased from Sinopharm
compounds,18 catalytic Friedel–Cras reaction of electron-rich
aromatic compounds with ethyl glyoxylate,19 and catalytic
coupling reactions of propylene oxide and CO2 to produce
Chemical Reagent Co., Ltd. (Shanghai, China). All reagents are
propylene carbonate (PC) with the help of imidazolium halide.20
used as received without further pretreatment.
Most recently, the LiCl has been used as a phase-transfer catalysts
to synthesize the thin Co2P nanosheets for oxygen evolution
reaction,21 and has also accelerated the cross-coupling of aryl
Typical procedure for the dehydration of fructose
In a typical experiment, 0.9 g of fructose (10 mmol), 0.42 g of
LiCl (10 mmol) and 10 mL of i-PrOH were charged into a 25 mL
polytetrauoroethylene (PTFE) lined hydrothermal autoclave
reactor, and then the reactor was ushed with nitrogen (N2)
stream for 1 min to displace the air. Finally, the reactor was
capped hermetically and immersed into an oil bath preheated
to the designed temperature (90–130 ꢃC) for reaction with
magnetic stirring (500 rpm). When the designed reaction time
(0.5–3.0 h) was over, the reactor was cooled down to room
temperature in cool water bath.
chlorides with aryl triates in the presence of Ni/Pd-based multi-
metallic system.22 The outstanding unexpected performance of
LiCl in catalysis makes it an alternative additive or promoter for
the production of HMF from fructose-based carbohydrates.
As known that HMF is formed from the acid-catalyzed
dehydration of hexoses by elimination of three water mole-
cules. Hence, the existence of water not only depressed the
dehydration of hexoses into HMF, but also accelerated the
rehydration of HMF into levulinic acid (LA) and formic acid
(FA), resulting in the lower HMF yield and selectivity.1,23 In order
to shield the reactive HMF from the aqueous acidic environ-
ment, thereby preventing side reactions, the dehydration reac-
tions are usually carried out in non-aqueous solvents, such as
dimethyl sulfoxide (DMSO), dimethylformamide (DMF), ionic
liquids, or mixtures. Since then, high yields of HMF have been
reported in ionic liquids and high-boiling-point organic
solvents.6,23 However, the high cost, toxicity of the solvents, and
the difficulty in recycling the above reaction media due to the
problematic isolation of HMF present major challenges for
large-scale biorenery applications.6,24 In this context, the low
boiling point non-aqueous solvents for efficient conversion of
sugars into HMF should be developed. Fortunately, the renew-
able isopropanol (i-PrOH) was found to be effective to mediate
the dehydration reaction efficiently to produce HMF.25,26 It has
low boiling point and low environmental toxicity, and thus
enables a simple production and isolation of HMF, and offers
Typical analysis procedure
The amount of unreacted fructose and the yielded HMF were
determined by using HPLC. Aer reaction, the resulting mixture
was diluted to 100 mL for qualitative and quantitative analysis
on an Agilent 1200 HPLC system equipped with a HPX-87H
column (300 ꢂ 7.8 mm, 5 mm, 5 mM H2SO4 solution as the
mobile phase at 0.6 mL minꢁ1) and RID detector for unreacted
substrates, and a C18 reversed phase column [250 ꢂ 4.6 mm, 5
mm, 1 : 4 (v/v) CH3OH/H2O as the mobile phase at 0.6
mL minꢁ1] and a UV detector (284 nm) for HMF. The unreacted
fructose and HMF amounts were determined through the
external standard method using commercially available stan-
dard substrates. Conversion of fructose (Conv., mol%), yield of
HMF (YHMF, mol%), and selectivity of HMF (SHMF, mol%) were
calculated according to eqn (1)–(3). The same process was
repeated 3 times to minimize error.
a
new opportunity for
a
large-scale economically viable
promising
process.24,27 Hence, the i-PrOH will be
a
ꢀ
ꢁ
environment-friendly solvent for HMF production with the LiCl
additive. In addition, it is found that the performance of LiCl for
catalysis will be improved by the combination of alcohols.18,19 It
is hypothesized reasonably that the LiCl-promoted and i-PrOH-
aided system will be more efficient for conversion of fructose-
based carbohydrates into HMF.
MRF
MF
Conv: ðmol%Þ ¼ 1 ꢁ
ꢂ 100%
(1)
(2)
(3)
MHMF
YHMF ðmol%Þ ¼
ꢂ 100%
MF
YHMF
Herein, in consideration of all above issues and in connec-
tion with our research program centering on the conversion of
SHMF ðmol%Þ ¼
ꢂ 100%
Conv:
sugars into HMF,28–31 a LiCl-promoted and i-PrOH-aided system where MF, MRF, and MHMF are the amounts (mol) of the original
for the dehydration of fructose-based carbohydrates into HMF fructose, residual fructose, and HMF, respectively.
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all lower than the LiCl-promoted system (entries 6–9). These
results showed that the co-operation of Li+ and Clꢁ was crucial for
the dehydration of fructose into HMF in i-PrOH. These attributes
Bode well for the use of LiCl as an alternative additive for HMF
production. Furthermore, when the reported ammonium salt
(NH4Cl) with weak Brønsted acidity was used as the additive,33 the
fructose was converted completely, affording low HMF production
efficiency (entry 10), which might be caused by the occurrence of
many side-reactions (Maillard reactions) in the presence of sugars,
HMF, and NH3 from the decomposition of NH4Cl under the high
reaction temperature.34,35 Hence, the LiCl could act as an effective
additive to promote the conversion of fructose into HMF in the
subsequent experiments. It should be noted that the HMF selec-
tivity was less than 100%. The result could be attributed to the
complexity of the reaction and high reactivity of HMF, several side
reactions may occur, the most notable of which are the acid-
catalyzed HMF rehydration to levulinic and formic acids, HMF
Typical procedures for product separation and recycling of
LiCl/i-PrOH
Aer reaction, the i-PrOH was recovered rstly through rotary
evaporation due to its low boiling point (82.45 ꢃC, 1 atm),
resulting in a mixture of LiCl and HMF as viscous brown liquid.
The resulting mixture was extracted with 3 ꢂ 10 mL of ethyl
acetate (EtOAc) for isolation of HMF. The EtOAc phase con-
taining HMF was collected and evaporated to obtain the crude
HMF product as brown oil. The residual LiCl and recovered i-
PrOH (dried by Mg2SO4) could be reused into the next run
without further purication (ESI, Scheme S1†).
The structure of HMF was conrmed by proton nuclear
magnetic resonance (1H NMR) on a Bruker DRX 400 spec-
trometer using CDCl3 as the solvent (ESI, Fig. S1†); and the
purity of HMF was around 95% according to the HPLC quan-
titative analysis (ESI, Fig. S2†). The detailed procedures for the
purity analysis of isolated HMF was given in ESI.†
self-condensation
reactions,
and
HMF–fructose
cross-
polymerization, forming soluble and insoluble polymers named
humins conrmed by the brown mixture aer reaction.29–31,36
As known that the solvent also affected the dehydration
process of fructose conspicuously through coordinating or acti-
vating intermediates.7,37–40 Hence, the effect of organic solvent with
different structure on fructose dehydration with an aid of LiCl was
investigated inevitably. Fig. 1 showed that the organic solvents did
facilitate the dehydration of fructose with high degree ($90%) in
all cases; however, only bulky alcohols, such as n-PrOH, i-PrOH, n-
BuOH, and i-BuOH, could facilitate the formation of HMF from
fructose dehydration process, these results are in line with the
reported result by Zhang et al.,25 demonstrating that the dehydra-
tion reaction of fructose in bulky alcohols has a better selectivity to
HMF.41 Hence, the i-PrOH with low boiling point (82.45 ꢃC, 1 atm)
and toxicity is the promising solvent for dehydration of fructose
into HMF with the aid of LiCl.
Results and discussion
Initial salt additive and solvent testing
It has been reported that the chloride salts (chloride-containing
catalysts or additives) have superior performance in synthesis of
HMF from carbohydrates. Hence, the promotion of various low
or no toxic chloride additives, including LiCl, NaCl, KCl, MgCl2,
and CaCl2, in dehydration of fructose into HMF was studied
initially (Table 1).
From Table 1, it could be seen that the above chlorides could
promote the conversion of fructose with high conversion degree
(>80%); however, only LiCl could afford the good HMF yield of
up to 79.1% (entries 1–5). These results showed that the Clꢁ
anions could promote the fructose conversion, and only the Li+
could induce the dehydration of fructose into HMF through
stabilizing the HMF with association between the Li+ cations
and the carbonyl groups (–CHO) of HMF.32 The contribution of
Clꢁ has also been identied, when the anion of LiCl was
replaced by Fꢁ, Brꢁ, Iꢁ and SO42ꢁ, the conversion efficiency was
It is reported that the impurities, such as metal species (Fe3+
,
Cu2+, etc.), in the catalytic system will improve the dehydration
Table 1 Conversion of fructose into HMF with various additivesa
HMF
Selectivity
Entry
Additive
Conversion (%)
Yield (%)
(%)
1
2
3
4
5
6
7
8
LiCl
NaCl
KCl
MgCl2
CaCl2
LiF
LiBr
LiI
100
79.1 ꢄ 2.3
22.4 ꢄ 1.9
41.2 ꢄ 1.1
0.9 ꢄ 0.1
18.2 ꢄ 3.4
Trace
53.1 ꢄ 1.6
26.7 ꢄ 1.8
ND
79.1 ꢄ 2.3
27.2 ꢄ 1.5
41.6 ꢄ 0.9
0.9 ꢄ 0.1
18.6 ꢄ 3.0
—
55.1 ꢄ 0.7
29.6 ꢄ 1.2
—
82.4 ꢄ 2.2
99.0 ꢄ 0.6
100
97.6 ꢄ 2.2
29.7 ꢄ 2.9
96.3 ꢄ 1.5
90.2 ꢄ 2.5
40.5 ꢄ 4.3
100
9
10
11
Li2SO4
NH4Cl
—
32.1 ꢄ 2.3
32.1 ꢄ 2.3
96.3 ꢄ 3.9
6.2 ꢄ 0.9
6.4 ꢄ 1.1
Fig. 1 LiCl-promoted dehydration of fructose into HMF in various
solvents (reaction condition: 1.8 g of fructose (10 mmol), 0.42 g of LiCl
(10 mmol), 10 mL of solvent, 130 ꢃC for 2 h).
a
Conditions: 1.8 g of fructose (10 mmol), 10 mmol of additive, 10 mL of
i-PrOH, 130 ꢃC for 2 h.
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of fructose,42 hence to conrm the potential inuence of LiCl did promote the formation of HMF from dehydration of
impurities in the LiCl/i-PrOH system, the inductively coupled fructose. Further increasing the LiCl dosage to 10 mmol, the
plasma optical emission spectrometer (ICP-OES, Optima 8300, HMF yield could reach the acceptable value of 68.1%. Moreover,
PerkinElmer) was used to detect the existence of other metal increasing the dosage of LiCl above 10 mmol provided no
species, and no metal impurities could be detected in the improvement of HMF yield with complete conversion of fruc-
reaction system. Hence, we can believe that LiCl and i-PrOH are tose. These results above showed that the 10 mmol of LiCl was
active components for fructose dehydration to produce HMF.
the sufficient amount for promoting HMF formation through
fructose dehydration, and 10 mmol of LiCl was set to the suit-
able dosage for the subsequent experiments.
Effect of process parameters
Next, we sought to optimize the reaction temperature and
duration. Fig. 2B showed that the high reaction temperature
facilitated not only the fructose dehydration but also the formation
of HMF, when the reaction temperature was elevated to 130 ꢃC, the
peak value of HMF yield of 79.1% was achieved. However, further
elevating the reaction temperature would cause the lower yield of
HMF because of HMF decomposition at elevated temperatures
(>130 ꢃC).29 Fig. 2C showed that at the reaction temperature of
130 ꢃC, the reaction time also played the key role for HMF
production. As the reaction time prolonged to 1.5 h, the fructose
would be converted completely. Meanwhile, the HMF yield pre-
sented an increasing trend as the reaction time prolonged, and
reached the peak value of 79.1% at 2.0 h. Further prolonging the
reaction time, the HMF yield declined, reecting the weak stability
of HMF at higher reaction temperature (>130 ꢃC) due to some side-
reactions occurred as reaction time extended.30,43 Hence, to obtain
the higher HMF yield, the optimized reaction temperature and
time would be set to 130 ꢃC for 2 h.
With appropriate additive (LiCl) and solvent (i-PrOH) in hand,
and to nd the best experimental conditions to carry out this
dehydration reactions, the process parameters such as LiCl
loading, temperature, reaction time, and fructose intake were
further considered, and the results are present as Fig. 2.
First, we investigated the effect of LiCl loading on HMF
yields (Fig. 2A). Without adding LiCl, more than 85% of fructose
could be converted into unknown species accompanying with
about 2.7% of HMF yielded. This result showed that i-PrOH
facilitated the fructose conversion, but not to form HMF. The
unknown species have been found to be the intermediates,
which could be further converted into HMF with the addition of
catalyst according to the controlled experiments. When the LiCl
was added into the reaction system, the HMF yield was
improved signicantly and the conversion degree of fructose
could reach 100% at the appropriate LiCl loading ($5 mmol).
For examples, when 2.5 mmol of LiCl was added, the conversion
degree of fructose was elevated to 94.0%, however, the HMF
yield was improved to remarkable yield of 45.0%, implying the
Last, from a practical and economical point of view, the
initial fructose intake is also one of the crucial factors for HMF
Fig. 2 The influence of reaction conditions ((A) 10 mmol of fructose, 10 mL of i-PrOH, 120 ꢃC for 2 h; (B) 10 mmol of fructose, 10 mmol of LiCl,
10 mL of i-PrOH, 2 h; (C) 10 mmol of fructose, 10 mmol of LiCl, 10 mL of IPA, 130 ꢃC; (D) 10 mmol of LiCl, 10 mL of i-PrOH, 130 ꢃC for 2 h).
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factor for LiCl-promoted fructose dehydration to HMF could be
obtained through Arrhenius plot generated from rst-order kinetic
constants (Fig. 3, S3 and S4†), and be summarized in Table S1.†
It could be found that the activation energy for fructose
dehydration in LiCl-promoted and i-PrOH aided system was
only 68.68 kJ molꢁ1 with a pre-exponential factor value of 1.2 ꢂ
104 minꢁ1, this result revealed that the dehydration reaction of
fructose occurred easily in the presence of LiCl in i-PrOH (kinetic
control), and also implying that the LiCl-promoted and i-PrOH
meditated system is suitable for production of HMF from fruc-
tose. Furthermore, based on the results above (Table 1, Fig. 1–3,
Table S1 and Fig. S3, S4†), the mechanism of this dehydration was
also proposed, and illustrated as Scheme 1.
In a word, the efficient production of HMF from fructose in
LiCl/i-PrOH system is achieved through the LiCl-promoted and
i-PrOH-aided dehydration of fructose into HMF, which is
further stabilized by LiCl and i-PrOH through the formation of
hydrogen bonds and coordination bonds,13,32,45–49 affording
satisfactory yield of HMF.
Fig. 3 Arrhenius plots for LiCl-promoted fructose dehydration in i-
PrOH.
production in industrial process. Looking at the plot pictured in
Fig. 2D revealed that fructose afforded good conversion degree in
all cases (ꢀ100%). However, the HMF yield declined with
increasing of initial fructose concentration, and at the 30 mmol of
fructose intake, the lowest value of HMF yield of 69.9% reached
frustratedly. These results could be attributed to the fact that
increasing the fructose intake led to higher collision probability of
fructose and generation of HMF, leading to self- or cross-
polymerization and formation of humins,31,44 and thus resulting
in the low HMF yield at high fructose concentration. Nevertheless,
the HMF yield is still acceptable with the moderate fructose intake,
achieving 79.7% of HMF yield at a fructose intake of 10 mmol.
Hence, the appropriate fructose intake in this work is set as
10 mmol, which is more meaningful in the process economy.
Therefore, on the basis of the analysis of the process
parameters, the optimal conditions were found to be 10 mmol
of LiCl dosage at 130 ꢃC for 2.0 h with 10 mmol of fructose
intake, affording acceptable HMF yield of 79.1%.
Substrate scope
To study the generality of LiCl-promoted and i-PrOH aided
system for production of HMF, a series of substrates, such as
fructose in large scale, glucose, sucrose, maltose, inulin, starch
and cellulose, were loaded into this reaction system. It was
found that this reaction system was suitable for HMF produc-
tion in large scale, because when the reaction system was scaled
up by 10 time, the acceptable HMF yield of 73.6% could still be
achieved (Table 2, entry 2), reecting the potential practicability
of this system for the production of HMF in industrial process.
However, this system was only suitable for fructose-based
carbohydrates, and when the glucose, maltose, starch, and
cellulose were used as the feedstocks, the yield of HMF could be
neglected, these results can be attributed to the weak ability of
LiCl for catalytic isomerization of glucose into fructose (Table 2,
entries 3, 5, 7 and 8), which is the key step for formation of HMF
from glucose.50 This specialty was conrmed by the sucrose as
the raw material due to 30.3% of HMF derived from fructose
moiety of sucrose (Table 2, entry 4). Hence, this system showed
the positive effect for synthesis of HMF from fructose-based
Kinetics analysis and mechanism hypothesis
To understand the LiCl-promoted and i-PrOH meditated dehy-
dration of fructose into HMF deeply, the kinetic analysis of this
reaction was performed. The activation energy and pre-exponential
Table 2 Substrate range of this LiCl-promoted systema
HMF
Entry
Substrate
Conversion (%)
Yield (%)
Selectivity (%)
1
2b
3
4
5
6
7
8
Fructose
Fructose
Glucose
Sucrose
Maltose
Inulin
100
100
13.7 ꢄ 1.9
60.5 ꢄ 2.3
20.5 ꢄ 2.6
—
79.1 ꢄ 2.3
73.6 ꢄ 3.1
4.2 ꢄ 1.0
30.3 ꢄ 2.2
2.8 ꢄ 0.6
70.3 ꢄ 1.2
<0.1
79.1 ꢄ 2.3
73.6 ꢄ 3.1
30.9 ꢄ 3.2
31.5 ꢄ 1.8
13.8 ꢄ 1.1
—
Starch
Cellulose
0.5 ꢄ 0.1
—
—
0
ND
a
Conditions: sugar units 10 mmol, LiCl 10 mmol, i-PrOH 10 mL, 130 ꢃC
for 2 h. b Sugar units 100 mmol, LiCl 100 mmol, i-PrOH 100 mL, 130 ꢃC
for 2 h.
Scheme
1 The proposed dehydration mechanism of fructose
promoted by LiCl in i-PrOH medium.
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feedstocks, respectively. Moreover, this catalytic system could
be reused 8 times without signicant loss of activity. From the
perspective of green and sustainable chemistry, this work
demonstrates green benets not only constructing sustainable
catalytic system, but also demonstrating green production of
HMF from biomass. The readily available and low-toxic LiCl, the
sustainable i-PrOH solvent, the renewable starting materials,
and the mild reaction condition make the LiCl/i-PrOH system
promising and sustainable for industrial production of HMF in
future.
Conflicts of interest
The authors declare no competing nancial interest.
Fig. 4 Recycle experiments (reaction condition: 1.8 g of fructose (10
mmol), 0.42 g of LiCl (10 mmol), 10 mL of solvent, 130 ꢃC for 2 h).
Acknowledgements
The authors gratefully acknowledge the nancial support of the
Natural Science Foundation of China (21808043), the Natural
Science Foundation of Guangdong Province, China
(2018A030307055, 2020A151501145), the Ordinary University
Innovation Project of Guangdong Province, China
(2018KTSCX151, 2017KQNCX131), and the College Students
Innovation and Entrepreneurship Training Program of China
(733571).
carbohydrates, even though the famous biopolymer of fructose,
inulin, used as the feedstock, the excellent yield of HMF (70.3%)
could still be achieved (Table 2, entry 6). Therefore, the LiCl-
promoted and i-PrOH aided system is the potential candidate
choice for production of HMF from fructose-based
carbohydrates.
Recyclability of LiCl and solvent
Notes and references
Reusability is a critical parameter to evaluate the catalyst
property, especially in viewpoint of practical application. With
the above information in mind, we investigated the possibility
of reusing the LiCl/i-PrOH system for conversion of fructose
into HMF (Fig. 4).
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As shown in Fig. 4, using recycled solvent and LiCl directly
for the dehydration reaction, the yield of HMF is still retained at
68.3% in the 8th cycle, indicating a quite low activity lost in each
cycle. It is interesting to see that the yield of HMF in some
recycles was even higher than the rst one, which can be
attributed to the retention of HMF and residual unreacted
fructose from the previous cycle.47,51,52 This simple product
isolation and solvent recycling process makes the reaction
system particularly suitable for industrial operation.
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© 2021 The Author(s). Published by the Royal Society of Chemistry
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