Conversion of glucose into 5-hydroxymethylfurfural catalyzed by acid-base bifunctional heteropolyacid-based ionic hybrids

Article abstract of DOI:10.1039/c7gc03821f

The design of stable acid-base bifunctional heterogeneous catalysts has become crucial for efficient catalytic conversion of renewable biomass to high value-added chemicals. Heteropolyacids (HPAs) are of great interest due to their unique tunable structures and suitable acidity. In this study, a series of acid-base bifunctional ionic hybrid catalysts [MimAM]_nH_3-nPW₁₂O₄₀ (n = 1-3) were synthesized by the ion exchange method using an amino-functionalized imidazolium ionic liquid and H₃PW₁₂O₄₀ as precursors. The introduction of the ionic liquid resulted in a varied Lewis-Br?nsted acidity for [MimAM]_nH_3-nPW₁₂O₄₀. The synergistic effect of dual acidic properties endowed [MimAM]H₂PW₁₂O₄₀ with more efficiency for glucose dehydration in THF/H₂O-NaCl with 53.9% HMF yield. A unique sequential hydrogen transfer mechanism is proposed to understand the efficient heterogeneous catalysis. Among them, the absence of fructose and levoglucosenone in glucose dehydration may come from the formation of stabilized intermediates 1,2-enediol and anhydrosugars due to the existence of an effective hydrogen bond between O-H and OW on the heteropoly anion; the reversible process between glucose dehydration and anhydrosugar hydration is responsible for the absence of HMF during the early stage of reaction. This work provides a deep understanding of the HPA-catalyzed conversion of glucose to HMF.

Full text of DOI:10.1039/c7gc03821f

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PAPER
Conversion of glucose into 5-hydroxymethylfurfural catalyzed by
acid-base bifunctional heteropolyacid-based ionic hybrids
Received 00th January 20xx,
Accepted 00th January 20xx
Pingping Zhao*, Yunyun Zhang, Yong Wang, Hongyou Cui*, Feng Song, Xiuyu Sun and Lipeng
Zhang
DOI: 10.1039/x0xx00000x
Design of stable acid-base bifunctional heterogeneous catalysts has become crucial for efficient catalytic conversion of
renewable biomass to high value-added chemicals. Heteropolyacids (HPAs) are of great interest due to their unique
www.rsc.org/
tunable structures and suitable acidity. In this study, a series of acid-base bifunctional ionic hybrid catalysts [MimAM]
40 (n = 1-3) were synthesized by ion exchange method using amino-functionalized imidazolium ionic liquid and
40 as precursors. The introduction of ionic liquid resulted in a varied Lewis-Brønsted acidity for [MimAM]
40. The synergistic effect of dual acidic properties endowed [MimAM]H 40 more efficiency for glucose
dehydration in THF/H O-NaCl with 53.9% HMF yield. A unique sequential hydrogen transfer mechanism is proposed to
n 3-
H
n
PW12
PW12
PW12
O
H
3
O
n 3-
H
n
O
2
PW12O
2
understand the efficient heterogeneous catalysis. Among them, the absence of fructose and levoglucosenone in glucose
dehydration may come from the formation of stabilized intermediate 1,2-enediol and anhydrosugars due to the existence
of an effective hydrogen bond between O-H and O=W on heteropoly anion; the reversible process between glucose
dehydration and anhydrosugars hydration is responsible for the absence of HMF during the reaction early stage. The work
highlights
a
deep
understanding
of
the
HPAs-catalyzed
conversion
of
glucose
to
HMF.
recovery and adjustable acidity. In this context, H-zeolite,
6-23
ammonium polymer, metal oxide and metal HPAs salts
1
1
. Introduction
have been employed as catalysts for the dehydration of
The resource shortage and global warming have attracted
much attention in the transformation of biomass renewable
1
glucose. Among them, Ag-containing H PW O salts showed
9
3
12 40
1
-4
high catalytic activity to HMF, while their singleness in catalyst
structure and leaching of catalyst active species in polar
reaction medium as well as their uncomprehended reaction
mechanism still need to be further improved. Recently HPA-
based ionic hybrids were studied extensively as the catalysts
for various acid-catalyzed reactions in view of their strong
energy into useful chemicals and fuels. For all available
biomass resources, the carbohydrates are regarded as one of
the most important raw materials for the production of 5-
hydroxymethylfurfural (HMF),
a very versatile platform
5
-7
towards fuels and chemicals. Efficient transformation of
fructose to HMF has been achieved, in which high yield (over
2
4-27
acidity and tailorable chemical structure.
Moreover, it was
9
0%) has been obtained with homogeneous or heterogeneous
8
,9
found that [MIMPS] PW O and poly(VMPS)-PW HPA-based
3 12 40
acid catalyst. Regretfully, fructose is not an ideal feed stock
because of its high price and less abundance in the nature,
while glucose is the most abundant monosaccharide and the
cheapest hexose making it a more promising candidate for the
ionic hybrids could efficiently catalyze fructose dehydration to
8,29
2
HMF in good yields.
However, when glucose was used as
the starting feedstock in the catalytic systems, only very poor
HMF yields were obtained. Therefore, a clear understanding of
the reaction pathways occurring in the HPAs-catalyzed
conversion of glucose to HMF, as well as the new exploration
on highly efficient HPA-based ionic hybrid catalysts for
2-15 heterogeneous glucose dehydration, will be of great
1
0,11
production of HMF.
By now, a great variety of acid catalysts have been tested
in HMF production from glucose. Although mineral acids,
metal chlorides, ionic liquids (ILs) and HPAs give high HMF
1
yields but lead to defects in recycling and eco-friendliness.
significance.
Solid catalysts are the best candidates due to their facile
It is well known that the synergistic effect between
Brønsted (B) acid and Lewis (L) acid in catalyst system can
3
0
effectively promote glucose or cellulose dehydration to HMF.
For example, an surfactant type B-L acid bifunctional ionic
hybrid Cr[(DS)H was revealed to be very effective in
cellulose dehydration, affording 52.7% of HMF yield. Porous
2
40 3
PW12O ]
3
1
coordination polymer PCP(Cr)-SO
3
H associated with L acid
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Cr(III) was capable of converting glucose to HMF with a yield of accompanied by energydispersive X-ray spectrometry (EDS;
3
2
8
0.7%. Although high yields could be achieved in the system, accelerating voltage: 20 kV) was utilized to study the elemental
the toxic heavy metal catalysts raise environmental concerns. distribution. Ultraviolet and visible light diffuse reflection
Recently, the ammonium polymer P-BnNH Cl (P-BnNH spectra (UV−vis−DRS) were recorded in the range of 190−800
HCl) and an integrated supported enzyme/solid acid catalyst nm on a double beam Jasco spectrophotometer. Barium
+
2
3
1
7
3
3
system were reported to be excellent acid-base bifunctional sulfate was used a reflectance standard. A Thermo Elemental
catalysts for glucose dehydration to HMF. Wherein, it was X7 ICP-MS equipped with a laser unit Thermo New Wave
demonstrated that amino group could effectively promote UP/213 was used for the W element analyses in the filtrate. TG
sequential hydrogen transfer via an 1,2-enediol intermediate. analysis was carried out with a STA409 instrument in dry air at
o
-1
Inspired by these facts, it is rational to prepare B-L acid a heating rate of 10 C min .
bifunctional ionic hybrid catalysts by pairing amino-
functionalized IL cations with HPA-anions such that the
2
.2. Catalyst preparation
catalyst simultaneously contains B acid site from HPA and L
acid site from amino in a composite structure. The resulting
2.2.1. Preparation of Ionic Liquid Precursors
composites might be highly efficient heterogeneous catalysts The amino-functionalized IL [MimAM]Br was prepared
3
according to our previous work. Methylimidazole (8.21 g,
4
for glucose dehydration to HMF.
Herein, we synthesized a series of amino-functionalized 0.10 mol) and 2-bromoethylamine hydrobromide (20.49 g,
o
HPA-based ionic hybrid catalysts, [MimAM]
-3), containing strong B acidity and some L acidic properties
Scheme S1). By use of different numbers of the IL cation 1-
n
H3-nPW12O
40 (n = 0.10 mol) were dissolved in acetonitrile (50 mL) at 80 C for 12
h under nitrogen atmosphere with stirring. On completion, the
solvent was removed by distillation, and the residue was
recrystallized in ethanol to afford the [MimAM]Br·HBr as a
white solid. NaOH was added into the aqueous solution of the
above solid for neutralization, followed by the evaporation
1
(
+
aminoethyl-3-Methylimidazolium [MimAM] , the molar ratio
of B and L acidity could be tailored, and the relationship
between the acidity of the catalysts and their activity in
conversion of glucose to HMF can be investigated. A
3
under vacuum. Methanol (20 mL) and CHCl (2 mL) were
added into the resulting mixture to precipitate sodium salt.
After filtration, the filtrate was evaporated to give the
2
tetrahydrofuran (THF)/H O-NaCl biphasic solvent system was
used to achieve a high yield of HMF from glucose. A possible
sequential hydrogen transfer mechanism for the HPAs-
catalyzed conversion of glucose to HMF was proposed. The
roles of intermediate fructose, 1,2-enediol and anhydrosugars
were studied by characterization of catalysts and systematic
experiments. Furthermore, the reusability of catalyst was also
surveyed.
[
MimAM]Br product as yellow oil (Yield: 60%). The analogue
ILs, dimethylimidazole-ethylamine bromine [MiDimAM]Br and
pyridine-ethylamine bromine [PyAM]Br were prepared
accordingly.
The alkyl-functionalized IL [EMim]Br was synthesized
3
5
according to the previous literature. Methylimidazole (8.21 g,
.10 mol) and bromoethane (10.9 g, 0.10 mol) were dissolved
0
o
in ethanol (50 mL) at 80 C for 3 days with stirring. The
resulting solution was concentrated under vacuum and then
2
. Experiment
washed with CH
93%).
2 2
Cl to give the [EMim]Br as a white solid (Yield:
2.1 Materials and methods
All chemicals including glucose (99.5%), THF (99.9%), NaCl 2.2.2. Preparation of HPA-based ionic hybrids
99.8%), methylimidazole (99.8%), phosphotungstic acid and 2-
(
HPA-based ionic hybrids were prepared from the reaction
bromoethylamine hydrobromide (99.9%) were purchased from between ionic liquid precursors and HPA. In a typical
Sinopharm Chemical Reagent Co. Ltd and used without further preparation, [MimAM]Br (5.0 mmol) and H PW O (5.0 mmol)
3
12 40
purification.
Fourier transform infrared (FT-IR) spectra were recorded aqueous solution of H
on a Nicolet 360 FT-IR instrument (KBr discs) in the 4,000-400 solution, followed by stirring at room temperature for 6 h. The
were dissolved in deionized water (20 ml), respectively. The
3
PW12O40 was added into [MimAM]Br
-
1
formed precipitate was then filtered, washed with water, and
dried in vacuum, giving the product [MimAM]H PW. With the
above procedure, [MimAM] HPW and [MimAM] PW were
prepared by using the corresponding molar ratio of reactants,
respectively. [MimAM] PW (Na PW) was prepared by
employing the same IL [MimAM]Br but using Na
absence of B acid site) in place of H 40. The analogue
hybrids [EMim]H PW, [DiMimAM]H PW, [PyAM]H PW,
MimAM]H PMo, [MimAM]H SiW and [MimAM]H PMoV
cm region. Pyridine FT-IR spectra were obtained on an Avatar
30 Fourier Spectrometer with the KBr pellet technique, the
samples pre-absorbed with pyridine adequately under vacuum
3
2
2
3
o
2
at 50 C, and excess pyridine was blown off with N flow.
3
3
Elemental analyses were performed on a CHN elemental
analyzer (Vario EL cube) and inductively coupling plasma
spectrometer (TCP, Jarrell-Ash 1100). XRD patterns were
collected on a Bruker D8 Advance powder diffractometer using
Ni-filtered Cu Ka radiation source at 40 kV and 200 mA, with a
3
PW12O
40
(
3
PW12O
2
2
2
[
2
2
4
2
were prepared based on similar procedures using the
corresponding precursors.
o
-1
o
scan rate of 2 min from 5 to 50 . Scanning electron
microscopy (SEM; Hitachi S-4800, accelerating voltage: 5 kV)
2
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Moreover, the synthesis scheme of amino-functionalized
3
. Results and Discussion
HPA-based ionic hybrid catalysts and various counterparts was
illustrated in Scheme S1.
3
.1. Characterization of catalysts
The elemental analyses of [MimAM]
n
H3-nPW are given in Table
S1. The C, H, N contents correspond well with the calculated
results. The SEM-EDX analysis of C, N, O, P and W performed
2.3. Catalyst evaluation
For the glucose dehydration to HMF in dual solvent of H O- on an extended area of a solid sample of [MimAM]H
2
PW (Fig.
NaCl/THF system, a mixture of glucose (0.1 g), NaCl (0.37 g) S1). The ratio between N and P is approximately 2.9, pointing
and catalyst (30 μmol) was added into water (4 mL) and THF (8 to a formula of [MimAM]H PW composed of one amino-
mL). Then it was subjected to reaction in a steel autoclave functionalized IL cation MimAM and a HPA anion H
2
2
+
-
2
PW . The
2
PW possesses a
o
lined with Teflon for 7.5 h at 160 C with magnetic stirring. TG curve (Fig. 1A) indicates that [MimAM]H
o
After the reaction, the reaction mixture presented three layers: thermally stable structure up to 420 C. A slight weight loss can
o
the upper was the organic phase containing the desired be observed at temperature less than 180 C due to the
product HMF, the medium was an aqueous phase where NaCl, release of moisture and constitutional water, and the drastic
o
traces HMF, unreacted glucose and byproduct anhydrosugars weight loss ranging from 420 to 600 C is attributed to the
were dissolved in, and the bottom was composed of the decomposition of organic IL-cation moiety and the subsequent
byproduct humins and the catalyst. Anhydrosugars were collapse of the inorganic Keggin POM structure. The total
o
analyzed with a gas chromatography-mass spectrometry weight loss of nearly 6.4% in the range of 420-600 C is
instrument (GC-MS; PerkinElmer, Clarus SQ 8) equipped with a approximately consistent with the theoretical data (7%) on the
capillary column, TC-1701 (GL Sciences, length 60 m, i.d. 0.25 basis of the formula [MimAM]H
μm, and film thickness 0.25 μm). Quantitative analysis of HMF the chemical composition for the hybrid.
and glucose were conducted on a HPLC (Shimadzu LC-10A),
Fig. 1B illustrates the FT-IR spectra of pure H
which was equipped with a HPX-87H column and a refractive [MimAM]H PW. For H 40, four characteristic vibration
index detector. Each test was done in triplicate and the bands for Keggin structure of PW12
2
PW, verifying the rationality of
3
PW12O40 and
2
3
PW12O
3
−
O
40 anion are observed at
1080, 974, 896, 811 cm (curve a), assignable to the stretching
), yield of HMF (y) and selectivity of vibration of the central oxygen ν(P-O ), terminal oxygen
HMF (s) were calculated based on the following equations:
ν(W=O ), inter-octahedral oxygen ν(W-O -W), and intra-
octahedral oxygen ν(W-O -W), respectively.
-1
reported data was their average value.
Glucose conversion (
x
a
t
b
3
7
0
Glu
c
As for
C
-C
Glu
(1)
x =
×100
0
[MimAM]H PW (curve b), the four Keggin featured bands are
2
C
Glu
also observed with only slight shifts, and the bands at 3114,
-1
3175 and 1448-1704 cm are assigned to stretching vibrations
CHMF
(2)
y =
×100
theory
C
HMF
of amino groups and imidazolium ring, indicating the
formation of the HPA-based ionic hybrid by strong ionic band
y
s = ×100
-1
x
interactions. In addition, the weak band at 1041 cm is a
branched peak from the stretching P-O vibration band at 1080
-1
cm , which implies the distortion of the symmetric Keggin
(
3)
0
Where
C
Glu
and CGlu are the initial concentration of glucose
and the concentration at reaction time t separately, in unit of
1
00
(A)
b
1080
974
(B)
-
mol L ;
1
theory
is the theoretical concentration of HMF
HMF
C
896
811
1
704 1448
9
9
8
6
3
114
assuming that the consumed glucose is completely converted
a
3175
1041
to HMF, while CHMF denotes the actual concentration of HMF
in the mixture, in unit of mol L .
-
1
94
2
To test the catalytic recycling of [MimAM]H PW, the
9
2
1
00 200 300 400 500 600 700
3500 30002000
1500
cm
1000
)
500
catalyst after reaction was separated by filtration, washed with
o
hot water of 80 C three times and then dried under vacuum at
o
00 C for 6 h. The recovered catalyst was reused in the next
o
-1
Temperature/ C
Wavenumbers
(
(C)
(D)
110
B
L
L+B
1
a
run.
a
b
1
10
c
2.4. The total acid content evaluation
b
d
Titration was used to evaluate the total acid content of the
1
0
20 30 40 50 60 70 80
1540 1520 1500 1480 1460 1440
3
6
-1
solids. In brief, 0.05 g of the solid was suspended in 45 mL of
acetonitrile and stirred for 3 h. The density of acid sites in the
catalysts was then measured by titration with a solution of n-
2 Theta/degrees
wavenumbers (cm )
Fig. 1 (A) TG curves of [MimAM]H
MimAM]H
2
PW; (B) FT-IR spectra (a) H
PW; (C) XRD patterns of (a) H 40 and (b) [MimAM]H
D) FT-IR spectra for pyridine adsorbed by [MimAM] 3-nPW (a) n = 0 (b) n =
3
PW12O40 (b)
[
(
2
3
PW12O
2
PW;
-1
butylamine in acetonitrile (0.05 mol.L ) using anthraquinone
pKa = −8.2) as the indicator.
n
H
(
1 (c) n = 2 (d) n = 3.
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Table 1 Catalytic performance of different catalysts in glucose dehydration
a
to HMF .
framework due to the formation of hydrogen bonds between
38
the terminal oxygen of PW and amino groups. Both strong
ionic bond and hydrogen bond interactions may account for
Solubility
Yield of
Entry Catalyst
in reaction HMF (%)
2
the solid-state of [MimAM]H PW, and it is sparingly soluble in
1
2
3
4
5
6
7
8
9
1
1
1
1
None
--
3.9
dimethyl sulfoxide (DMSO) but is insoluble in most common
solvents, such as water, alcohols, methyl isobutyl ketone
[MimAM]Br
Soluble
4.6
H
3
PW12
O
40
Soluble
49.4
53.9
37.4
20.2
8.5
(MIBK), THF, and acetonitrile.
[MimAM]H
2
PW
Insoluble
Insoluble
Insoluble
PW) Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
2
Fig. 1C shows the XRD patterns of [MimAM]H PW and
[MimAM]
[MimAM]
[MimAM]
2
3
3
HPW
PW
3 3
H PW12O40. H PW12O40 displays a set of well-resolved sharp
diffraction peaks for a HPA crystal, an index of the typical
secondary cubic structure of Keggin anions (curve a). The
PW(Na
3
[EMim]H
2
PW
7.8
[
2
MimAM]H PW (curve b) preserves the similar strong Bragg
o
[PyAM]H PW
2
24.3
8.1
peaks (2θ = 8.90, 17.8, 20.8, 25.7, 27.8, 33.1 and 34.4 ) with
slight shifts of peak locations. The major peak at 8.90 is
tentatively assigned to the [110] phase, located at 10.4 for
0
1
2
3
2
[DiMimAM]H PW
[MimAM]H
[MimAM]H
2
PMo
SiW
37.9
30.2
25.8
2
3
parent H PW12O40. This shift of the [110] phase reflects an
[MimAM]
3
H
2
PMoV
2
Insoluble
expanded unit cell due to the replacement of protons with the
a
3
9
Reaction conditions: 0.1 g glucose, 0.37 g NaCl and 30 μmol catalyst in 12
o
bulky organic cation. The other variations of the hybrid
MimAM]H PW may arise from the slight change of the long-
range order of the original secondary cubic structure of Keggin B acidity of catalyst is essential for glucose dehydration to HMF.
anions. These characterizations indicate that [MimAM]H PW is Nonetheless, H PW O was homogeneous catalyst due to the
mL solvent (H
2
O : THF=1 : 2) at 160 C for 7.5 h. The conversion of glucose
[
2
was close to 100% under the reaction conditions.
2
3
12 40
+
composed of IL cations [MimAM] and heteropoly anions. The good solubility in the reaction media, hindering the recycling
IL cations are ionically bonded to the heteropoly anions, and reusing of it. In contrast, the newly synthesized hybrid
2
7
reaching a certain polycrystalline structure. This is also catalyst [MimAM]H PW resulted in
a
liquid-solid
heterogeneous reaction system and demonstrated a 53.9%
The total acid content of the hybrid catalysts yield of HMF (entry 4). Such activity is much better than the
MimAM] 3-nPW are also measured by titration with n- homogeneous H PW O , which may be contributed by the
butylamine (Table S2). The [MimAM]
different amounts of IL cations [MimAM] have different acid hybrid catalyst. This is also
contents in the order of H 40 > [MimAM]H PW > heterogeneous heteropolyacid catalyst in dual solvent because
MimAM] HPW > [MimAM] PW. The different surface acidic the production of HMF with significant yields in the catalytic
sites of [MimAM] 3-nPW are determined by pyridine conversion of glucose is difficult in water, although it can be
adsorption infrared spectroscopy (Fig. 1D). The band for B acid achieved in ionic liquid.
2
consistent with the SEM image in Fig. S2.
[
n
H
3
12 40
n
H3-nPW samples with synergistic relation between B acid and L acid in the solid
+
a
comparable result for
3
PW12
O
2
[
2
3
4
1
n
H
1
5
-1
-
sites can be observed at 1538 cm , while the band at 1447 cm
1
To clearly illustrate the relationship between acidity and
corresponds to the L acid sites. In addition, a band at 1486 catalytic activity in HMF production, three control samples,
cm correlates to both B and L acid sites. These confirm the [MimAM] HPW (entry 5), [MimAM] PW (entry 6) and
-1
40
2
3
n
3 3
coexistence of B acid and L acid sites in [MimAM] H3-nPW. [MimAM] PW (Na PW) (entry 7), were prepared using
+
Based on the peak intensities, it can be seen that the B acid different numbers of the IL cations [MimAM] or different
sites increase and L acid sites decrease in the following order: heteropoly anion. Interestingly, their catalytic activities are
H
3
PW12
O
40 > [MimAM]H
2
PW > [MimAM]
2
HPW > [MimAM]
3
PW, much distinct. The yield of HMF followed as a sequence of
which shows the acid intensity is related to the number of [MimAM]H PW (53.9%)
>
[MimAM] HPW (37.4%)
2
>
2
+
[
MimAM] ions. Thus, it can be concluded that changing the [MimAM] PW (20.2%)
> [MimAM] PW (Na PW) (8.5%),
3
3
3
number of organic groups can control the molar ratio of the B suggesting that acid catalyst with higher B acidity may be
and L acids contents for [MimAM]
beneficial to enhance the HMF yield. However, the control
sample [EMim]H PW with the similar acidity to
MimAM]H PW without L acid site afforded only 7.8% HMF
n
H3-nPW.
2
B
[
2
3.2. Catalytic activity
yield (entry 8). This result signifies that the L acid site endowed
by amino group plays an important role in the glucose
dehydration to HMF.
Table 1 compares the performance of the various catalysts for
glucose dehydration to HMF. It can be seen that without any
catalyst or in the presence of [MimAM]Br, which can present
predominantly Lewis acidic sites, glucose could hardly be
converted to HMF under the employed reaction conditions
The catalytic kinetics of [EMim]H
PW) for the glucose dehydration to HMF were also
surveyed separately (Fig. S3). When [EMim]H PW was used as
2 3
PW and [MimAM] PW
(Na
3
2
(
entries 1,2). H PW O , which provides dominantly Brønsted
3 12 40
the catalyst, the HMF yield and glucose conversion increased
slowly, but the reaction rate was very low. It signifies that the
relatively weak B acids hardly catalyzed glucose dehydration to
HMF in the absence of L acids synergism, in good agreement
acidic sites, afforded 49.4% yield of HMF (entry 3), in good
agreement with the previously reported good activity of
1
H PW O in biphasic system. These indicate that strong
9
3
12 40
2
8
with
the
previously
reported.
While
for
4
| J. Name., 2017, 00, 1-9
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Journal Name
3
MimAM] PW(Na PW), the HMF yield increased at the first
ARTICLE
Table 2 The effect of organic solvent on the conversion of glucose and yield
a
of HMF .
[
3
reaction stage, and then decreased. In this respect, we note
that although L acid was claimed to be more selective than B
acid for glucose dehydration, it also promoted undesired
polymerization, degradation via retro aldol reaction to
Entry Solvent
mL)
Con. Sel. Yield of
(%) (%) HMF (%)
(
1
2
3
4
5
6
7
DMSO(12)
99.8 83.4 83.2
69.3 10.0 6.9
98.9 21.4 21.2
99.3 16.3 16.2
99.6 32.6 32.5
99.2 39.5 39.2
4
2
H2O(12)
byproducts in the absence of B acids synergism. Hence, the
synergistic effect of adjustable L and B acidity enabled
MIBK(12)
THF(12)
[
MimAM]H
2
PW12O40 to be more efficient in glucose
MIBK(8)/H O(4)
2
dehydration.
THF(8)/H O(4)
2
In order to more extensively investigate the effects of the
organic cations on glucose dehydration to HMF, other two
THF(8)/H O(4)-NaCl 99.8 54.0 53.9
2
a
Reaction conditions: 0.1 g glucose, 0.37 g NaCl and 30 μmol catalyst at
o
60 C for 7.5 h.
control catalysts, [PyAM]H
prepared by replacing the methylimidazolium ring of
MimAM]H PW with pyridinium and dimethylimidazolium,
respectively. HMF yield of 24.3% was obtained under the
catalysis of [PyAM]H PW (entry 9), inferior to [MimAM]H PW
as catalyst for the conversion of glucose to HMF. When
DiMimAM]H PW was used as the catalyst, the HMF yield
decreased further to 8.1% (entry 10). The cause for the lower
yields observed over [PyAM]H PW or [DiMimAM]H PW
2
PW and [DiMimAM]H
2
PW, were
1
solvents (entries 2-6), suggesting that water is essential for the
glucose dehydration probably due to the very poor solubility of
glucose in most of the organic solvents. Upon analysis of the
[
2
2
2
result in Table 2, it can be seen that THF/H
efficient reaction medium under our reaction conditions with
9.2% yield of HMF and 99.2% conversion of glucose (entry 6).
2
O was the most
[
2
3
Dumesic had reported that adding some sodium chloride was
46
able to improve the HMF yield in biphasic solvents. Therefore,
2
2
remains unclear. However, it is obvious that the observed
changes are attributable to a reduction in the acidity of the
catalysts, which is related to the loss of the acidic proton on C-
2
NaCl was added in THF/H O biphasic solvent so as to increase
immiscibility between the aqueous and THF phases. What is
more important is that by doing so HMF can be extracted from
the aqueous phase into the organic phase as soon as it formed,
and thus prevented from its secondary reactions. It is well
known that low concentration of HMF in the aqueous phase
could suppress the side reactions of HMF and thereby improve
2
position of the imidazolium cation. This is consistent with the
role played by C-2 hydrogen of the imidazolium cation in the
1
5
glucose dehydration reported in the previous literature.
For comparing the various heteropoly anions,
MimAM]H PMo, [MimAM]H SiW, and [MimAM] PMoV
[
2
2
3
H
2
+
2
16
the HMF yield. When reacting in 12 mL water-NaCl/THF
were synthesized with the same IL cation [MimAM] . All of
them were heterogeneous catalysts because of their insoluble
properties throughout the reaction, and showed lower
conversions of 37.9%, 30.2%, and 25.8%, respectively (entries
system with the volume ratio of aqueous NaCl solution to THF
2
equal to 1:2 and 30 μmol [MimAM]H PW as the catalyst at 160
o
C for 7.5 h, the HMF yield was promoted to 53.9% (entry 7)
with 99.8% conversion of glucose, much higher than the
reported yield of HMF (31.8%) when using a combination of
11-13). The effect of the heteropoly anions on the acidity of
these catalysts seems to be apparent as the catalytic activity
declined with the decrease in their acidity in the order of
TiO
2 2
-ZrO and Cs2.5H0.5PW12O40 as the catalyst and reacting in
47
the identical solvent system.
43
3 3 3 5 2
H PW > H PMo >H SiW > H PMoV .
The other parameters affecting the glucose dehydration
to HMF, including the ratio of THF to water, catalyst amount,
temperature and reaction time, were also investigated (Fig. S4).
The optimal reaction conditions were determined to be for 0.1
3.3. Optimization of reaction conditions
In order to obtain high HMF yield, different organic solvents
had been evaluated for their effect on the glucose dehydration,
including DMSO, MIBK, water, and THF as well as some
combination with water (Table 2). In pure DMSO solvent, as
high as 83.2% of HMF yield was achieved with 99.8% glucose
conversion (entry 1), which might be related to the co-catalysis
g glucose, 30 μmol [MimAM]H
2
PW catalyst, 0.37 g NaCl in 12
o
mL solvent (H O : THF=1 : 2) at 160 C reacting for 7.5 h.
2
The scope of this study was further extended to examine
the conversion of different substrates (Table S3) over
[
MimAM]H
substrates could be converted with different conversions and
.2%, 23.5%, 12.4% of HMF yields were obtained from
2
PW in water-NaCl/THF. It can be seen that all
4
4
and excellent solubility for glucose.
Unfortunately,
8
[MimAM]H PW is soluble in DMSO, which makes it become a
2
cellulose, sucrose and starch, respectively (entries 2-4). It is
worthwhile to note that only 40.1% HMF yield was obtained in
the case of fructose dehydration (entry 5), much lower than
that in the case of glucose in our reaction conditions. This is
very interesting because a common academic recognition is
that fructose is easier to form HMF than glucose via
homogeneous catalyst. Besides, the high boiling point of
DMSO makes the separation and purification of HMF difficult.
Therefore, DMSO is not an ideal candidate solvent albeit the
higher HMF yield.
In comparison to reaction in aqueous solution, both the
glucose conversion and HMF yield were significantly enhanced
when reacting in the biphasic solvents, agreeing well with the
29
dehydration reaction under acidic catalysts.
To confirm our result, the kinetic behavior of the
dehydration reaction to HMF of fructose and glucose in water-
NaCl/THF system were surveyed separately (Fig. 2). The
4
5
results in the previous report. Meanwhile, pure organic
solvent gave lower yields of HMF in comparison to biphasic
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ARTICLE
Journal Name
on catalyst [MimAM]H
2
PW was first investigated. 30μmol
6
0
0
(A)
100
(B)
100
60
45
30
15
0
[MimAM]H PW was added to a 12 mL water-NaCl/THF solution,
2
5
9
9
9
8
6
4
8
0
which dissolved 0.1g glucose in advance, and then stirred
o
vigorously at 25 C for 24 h. The concentration of glucose
40
3
0
0
60
2
reduced 4.2%, which suggested the [MimAM]H PW hardly
2
4
0
92
90
Yield of HMF
Conversion of fructose
10
0
Yield of HMF
Conversion of glucose
adsorbed glucose. Therefore, we deduced that the
0
50
100
150
200
250
3
4
5
6
7
8
9
dehydration of glucose to HMF over [MimAM]H
PW. (A) undergo a mechanism without fructose as a metastable
2
2
PW might
Reaction time (min)
Reaction time ( h )
Fig. 2 Kinetic behavior of different reaction over [MimAM]H
fructose dehydration and (B) glucose dehydration. Reaction conditions: 0.1
g substance, 0.37 g NaCl and 30 μmol catalyst in 12 mL solvent (H O :
intermediate. Nevertheless, there should be another new
intermediate, which postpone the formation of HMF. That is to
say, the glucose dehydration to HMF in our catalytic system
might differ from the traditional reaction mechanism, in which
glucose needs to be isomerized to fructose, followed by
2
o
THF=1 : 2) at 160 C.
dehydration reaction of fructose to HMF occurred surely much
faster than the reaction of glucose to HMF. What this means is
that if fructose was an intermediate in the dehydration of
glucose, it should be detected in the reaction products. In fact,
3
0
dehydration to HMF.
2 4
we really detected the existence of fructose when H SO was
used as the catalyst of glucose dehydration. Even though the
capability to observe fructose was sufficient, fructose was
never observed during the whole dehydration process of
glucose.
3
.4. Catalytic mechanism
The mechanism for the amine-catalyzed glucose isomerization
has been demonstrated previously, involving the formation of
1
7
a key intermediate 1,2-enediol and anhydrosugars. In
addition, Bell et al. did not also observe any fructose during
glucose dehydration to HMF catalyzed by heteropoly acid in
ionic liquid system. It was attributable to the formation of 1,2-
Furthermore, another interesting finding (Fig. 2B) was
that only a tiny amount of HMF was observed even when more
than 90% of glucose was converted after reacting for 3 h. After
that, the yield of HMF increased rapidly with further
prolonging the reaction time. While the conversion of glucose
approached 100% after 6 h, the HMF yield still further
increased until a maximum at 7.5 h, and then decreased.
Inspired by these findings, the adsorption behavior of glucose
1
5
enediol intermediate stabilized by the heteropoly anion.
With the evidence of the aforesaid results for [MimAM]H PW,
2
we suggest a unique heterogeneous sequential hydrogen
Anhydrosugars
OH
OH
O
OH
OH
+
H
2
O
HO
L acid HO
B acid
OH
-
H
2
O
HO
HO
OH
OH
Intramolecular
hydrogen transfer
Glucose
1,2-enediol
N
OH
O
H
OH
OH
N
HO
L acid
B acid
HO
OH
HO
OH
2
-H O
H3N
HO
O
OH
H
O
W
O
W
O
O
Intramolecular
hydrogen transfer
OH
OH
OH
O
O
O
O
B acid
-H O
2
L acid HO
Intramolecular
hydrogen transfer
HO
HO
OH
O
O
HO
NH2
O
W
W
O
O
O
Intramolecular
rearrangement
N
N
O
Intramolecular
hydrogen transfer
O
O
O
L acid
HO
OH
HO
B acid
+
HO
O
O
2
-H O
HMF
NH
2
N
N
O
O
O
intermolecular
hydrogen transfer
intramolecular
hydrogen transfer
n-
W
W
=[MimAM]
n
H
3-nPW
=H3-nPW12
O
40
=Ionic effect
=Hydrogen bond
=
=
O
H
O
2
Scheme 2 Proposed sequential hydrogen transfer mechanism for [MimAM]H PW catalyzed glucose dehydration to HMF in water-NaCl/THF.
6
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DOI: 10.1039/C7GC03821F
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PAPER
Book2_C[156]: slope = -2.89517
1
080
1041
974 896
(
A)
(B)
Fresh [MimAM]H
PW
2
2
transfer mechanism for [MimAM]H PW catalyzed glucose
c
Recycled [MimAM]H PW
2
dehydration to HMF in water-NaCl/THF system, as shown in
Scheme 2.
b
a
811
1052
The whole process is summarized as follows: under the
action of B acid site endowed by heteropoly anion, glucose
ring was opened via the protonation of the bridged O1 atom.
At the same time, some glucose itself dehydrated to form the
anhydrosugars; then the deprotonation proceeded in the
effect of L acid site from amino group to form the key
intermediate 1,2-enediol, which was stabilized by formation of
958
1
400 1200 1000
800
600
200
300
400
500
-
1
Wavenumbers (cm )
Wavelength (nm)
Fig.
MimAM]H
vis-DRS of [MimAM]H
3
(A) FT-IR spectra (a) [MimAM]H
2
PW (b) the first recycled
[
2
PW after 3 h and (c) the 5th recycled [MimAM]H
2
PW. (B) UV-
2
PW and the first recycled [MimAM]H PW after 3 h.
2
-1
hydrogen bonds between heteropoly anion and hydroxyl 1052 cm in recycled [MimAM]H
group, and it was accompanied by the solid catalyst Whereas the IR spectrum of the 5th recycled [MimAM]H
2
PW after 3 h (curve b).
PW
2
intramolecular hydrogen transfer between protonated amino are very similar to that of the fresh one. These phenomena
cation and heteropoly anion; afterwards, 1,2-enediol suggest that the existence of effective hydrogen bonds
intermediate was dehydrated with the synergistic effects of between W=O and O-H of 1,2-enediol intermediate as well as
dual acidic properties to form HMF; on the other hand, the solid catalyst intramolecular hydrogen transfer between
formed anhydrosugars, which were stabilized by the protonated amino cation and heteropoly anion during the
heteropoly anion, were continually rehydrated to glucose and glucose dehydration. That is, the absence of fructose in this
then further transformation into HMF due to the reversible mechanism is due to the formation of intermediate 1,2-
process between glucose dehydration and anhydrosugars enediol stabilized by the heteropoly anion through effective
hydration.
In the proposed mechanism, the B acid site from
hydrogen bonds and intramolecular hydrogen transfer.
UV-vis diffuse reflectance spectroscopy (UV-vis-DRS) was
heteropoly anion plays a crucial catalytic role that sets in used to determine the coordination geometry and chemical
motion the chain reaction of glucose dehydration, while L acid arrangement of the metal species on the surface of catalyst
site from the amine group plays an auxiliary role which because the band gap energies, which can be derived from the
facilitates the subsequent hydrogen transfer through the spectra, are known to depend on the coordination symmetry
5
0,51
hydrogen bonds as well as the interactions between IL cation of the metal involved with the hydrogen transfer.
and heteropoly anion. Thus, distinct from [MimAM]H PW, the shows the UV−vis−DRS spectra of [MimAM]H PW and the first
PW after 3 h. The absorption edge
g) can be estimated from the x-axis intercept of the tangent
catalysts [MimAM] H3-nPW, the yield of HMF declined with the at the steep rising part of the spectra (as shown in Fig. 3(B),
Fig. 3(B)
2
2
analogues [MimAM]Br and [MimAM] PW (Na PW), which are cycle recovered [MimAM]H
2
3
3
B acid-free, hardly afforded any HMF (Table 1, entries 2,7). For
(λ
n
5
2
decrease of B acid site (Table 1, entries 3-6).
Moreover, the key reaction pathways in this proposed edge of about 384 nm, corresponding to a band gap of 3.23 eV.
mechanism is the absence of fructose and formation of 1,2- While the first recycled [MimAM]H PW after 3 h ( g = 2.72 eV,
2
the dotted lines). [MimAM]H PW displays an absorption
2
E
enediol, which can be stabilized by the heteropoly anion 456 nm) gives a narrowing of the band gap. The difference of
through effective hydrogen bonds between O-H on 1,2-enediol above optical band gaps is related to solid catalyst
and O=W on heteropoly anion. In fact, amine-catalyzed intramolecular hydrogen transfer between protonated amino
glucose dehydration to HMF via an 1,2-enediol pathway is cation and heteropoly anion during glucose dehydration,
4
8
feasible as evidenced by the corresponding DFT calculations.
What's more, it has been reported that heteropoly acid or band to the empty 4d W conduction band.
WO was able to stabilize the reaction intermediate of glucose
It was interesting to see that the yield of HMF increased
rapidly only when glucose had almost completely been
arising from excitation of an electron of the 2p oxygen valence
3
1
7,49
dehydration.
To verify the proposed reaction pathways, we present the transformed in Fig. 2B. Notably, anhydrosugars were also
IR spectra of fresh [MimAM]H PW and recycled observed in the system (Fig. S5) and the content in the
2
-1
[
MimAM]H PW in Fig. 3(A). The new band at 958 cm is reaction mixture increased firstly and then decreased as
2
assigned to the split peak from the vibration of W=O at 974 glucose content decreased. Nevertheless, levoglucosenone,
-1
cm in the first recycled [MimAM]H PW after 3 h (curve b). which was generally produced via anhydrosugars further
2
-1
The weak band at 1041 cm , assignable to vibrations of P-O in dehydration, was not detected during the whole reaction. Thus,
we conjectured that anhydrosugars could also be stabilized by
fresh [MimAM]H PW, shifts to a much higher wavenumber
2
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ARTICLE
Journal Name
the heteropoly anion through effective hydrogen bonds
between O-H on anhydrosugars and O=W on heteropoly anion
to prevent the further dehydration. That is to say, glucose was
converted into intermediate 1,2-enediol and anhydrosugars in
70
Without fresh catalyst
Adding fresh catalyst
6
5
4
0
0
0
2
the effect of acid-base bifunctional catalyst [MimAM]H PW,
which were envisioned to be stabilized by the heteropoly
anion. Thus, there was no HMF formed at the early period of
glucose dehydration. After that, 1,2-enediol dehydration
formed HMF with further prolonging the reaction time. In the
meanwhile, glucose gradually decreased to facilitate
anhydrosugars reversible hydration, it led to the yield of HMF
increased rapidly until a maximum.
30
2
1
0
0
0
1
2
3
4
5
Reaction run
2
Fig. 4 Catalytic recycling of [MimAM]H PW for glucose dehydration to HMF.
Reaction conditions: 0.1 g glucose, 0.37 g NaCl and 30 μmol catalyst in 12
o
2
mL solvent (H O : THF=1 : 2) at 160 C for 7.5 h.
In addition, the hydrogen bonds between the acidic
proton of imidazolium ring and hydroxyl group exist in all
intermediates and transition states, it is able to stabilize all
recovery of the catalyst in each run, the decline in the HMF
yield was unnoticeable. This phenomenon suggests that
4
8
these compounds and facilitates this reaction. That’s why the
yield of HMF decreased when the imidazolium ring was
replaced by pyridine or dimethyl imidazolium. (Table 1, entries
[
2
MimAM]H PW is a reusable heterogeneous catalyst in water-
NaCl/THF for glucose dehydration to HMF.
8,9).
2
Based on the above mentioned analyses, [MimAM]H PW
catalyzed glucose dehydration is via 1,2-enediol and
anhydrosugars intermediates reaction mechanism. The
heteropoly anion of catalyst plays a dominant role for the
stabilization of intermediates. The rationality of hydrogen
transfer mechanism for HPAs-catalyzed conversion of glucose
to HMF was verified by the comparison of experimental results
and characterization.
Conclusions
The acid-base bifunctional ionic hybrid catalysts [MimAM] H
n
3-
PW O (n = 1-3) were synthesized, characterized, and tested
12 40
n
for glucose dehydration to HMF. The synergistic effect of
adjustable Lewis and Brønsted acidity enabled
MimAM]H PW O to be more efficient in catalyzing glucose
[
2
12 40
dehydration to HMF in THF/H O-NaCl with 53.9% yield at
2
9
9.8% conversion. A unique sequential hydrogen transfer
3
.5. Recycling test
mechanism is proposed to understand the heterogeneous
catalysis of glucose dehydration. The rationality of reaction
pathways, involving the absence of fructose and
levoglucosenone as well as the formation of 1,2-enediol and
anhydrosugars, were verified by the comparison of
experimental results and characterization. This work is helpful
for understanding the HPAs-catalyzed conversion of glucose to
HMF and the future development of effective functional HPA-
based catalysts for converting renewable biomass into
valuable chemicals.
To examine the durability of this catalyst, the recycling tests of
[MimAM]H PW in water-NaCl/THF system for glucose
2
dehydration to HMF were preformed, as shown in Fig. 4. After
each cycle, the used catalysts were recovered by filtration,
o
washed with distilled water, and dried at 100 C. Afterwards
the spent catalyst, without adding fresh catalyst to
compensate the catalyst loss in recovering, was used directly
for next run. The yield of HMF decreased slowly from 53.9% to
42% after five successive runs. To clarify the reason for the
decrease of yield, the IR spectrum of the 5th cycle recovered
catalyst was recorded and found to be very similar to that of
the fresh one (Fig. 3). The elemental analysis over the 5th cycle
Acknowledgements
recovered catalyst testified to be C: 2.11, N: 1.05, H: 0.32 The authors thank the National Natural Science Foundation of
Table S1), very close to that for the fresh catalyst, i.e., no China (No. 21506118, 21476132, and 51574160) and
(
significantly enhanced C content was observed for the 5th Shandong Province Foundation for Outstanding Young
cycle recycled catalyst. Further, 0.56 ug/L=0.003 umol/L W was Scientist (No. BS2014CL030).
observed in the reclaimed filtrate of first run by ICP-MS
analysis, suggesting that there is the leaching of active sites
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reaction and recovery rather than the formation of coking on
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| J. Name., 2017, 00, 1-9
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DOI: 10.1039/C7GC03821F
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Green Chemistry
Page 10 of 10
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DOI: 10.1039/C7GC03821F
Graphical Abstracts
Conversion of glucose into 5-hydroxymethylfurfural catalyzed by
acid-base bifunctional heteropolyacid-based ionic hybrids
Pingping Zhao*, Yunyun Zhang, Yong Wang, Hongyou Cui*, Feng Song, Xiuyu Sun and Lipeng
Zhang
Glucose
1,2-enediol
1,2-enediol
Intramolecular
hydrogen transfer
O
H
W
O
O
W
O
O
2
-3H O
=
[MimAM]
n
H
3-nPW12
O
40
=H
3-nPW12
O
n-
40
O
HO
O
Intermolecular
hydrogen transfer
Intramolecular
hydrogen transfer
HMF
Acid-base bifunctional ionic hybrids were employed as catalysts for glucose
dehydration to HMF via a unique sequential hydrogen transfer route.