Direct conversion of cellulose to 5-hydroxymethylfurfural (HMF) using an efficient and inexpensive boehmite catalyst

Article abstract of DOI:10.1016/j.carres.2019.06.010

High efficiency conversion of cellulose to 5-hydroxymethylfurfural (HMF) remains a challenge today. A simple solid acid catalyst Boehmite (γ-AlOOH) with high hydrothermal stability was prepared and used as sole catalyst for the direct conversion of cellulose into HMF in mixed reaction solvents of ionic liquid 1-buthyl-3-methylimidazolium chloride (BmimCl) and dimethyl sulfoxide (DMSO). This was aimed at developing an efficient and inexpensive catalyst for the production of HMF. The effects of factors such as water, solvent, catalyst load, temperature and reaction duration were investigated. An impressive HMF yield of 58.4% with 97.2% cellulose conversion was obtained at 160 °C after 2 h. More importantly, the catalyst γ-AlOOH was reused several times without loss of its catalytic properties. After five reaction runs, an HMF yield of 47.8% with 91.0% conversion was also obtained. In addition, the catalyst γ-AlOOH displayed excellent catalytic effects on the degradation of other carbohydrates. High yields of HMF from other carbohydrates such as glucose (61.2%), starch (62.7%) and inulin (70.5%) were achieved using γ-AlOOH as the catalyst. The proposed catalytic method shows a promising potential for HMF preparation, especially for industrial-scale HMF production from renewable bioresources.

Full text of DOI:10.1016/j.carres.2019.06.010

CarbohydrateResearch481(2019)52–59
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Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Direct conversion of cellulose to 5-hydroxymethylfurfural (HMF) using an
efficient and inexpensive boehmite catalyst
Zhe Tang, Jianhui Su*
School of Chemistry & Chemical Engineering, Yancheng Institute of Technology, Yancheng, Jiangsu Province, 224051, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
High efficiency conversion of cellulose to 5-hydroxymethylfurfural (HMF) remains a challenge today. A simple
solid acid catalyst Boehmite (γ-AlOOH) with high hydrothermal stability was prepared and used as sole catalyst
for the direct conversion of cellulose into HMF in mixed reaction solvents of ionic liquid 1-buthyl-3-methyli-
midazolium chloride (BmimCl) and dimethyl sulfoxide (DMSO). This was aimed at developing an efficient and
inexpensive catalyst for the production of HMF. The effects of factors such as water, solvent, catalyst load,
temperature and reaction duration were investigated. An impressive HMF yield of 58.4% with 97.2% cellulose
conversion was obtained at 160 °C after 2 h. More importantly, the catalyst γ-AlOOH was reused several times
without loss of its catalytic properties. After five reaction runs, an HMF yield of 47.8% with 91.0% conversion
was also obtained. In addition, the catalyst γ-AlOOH displayed excellent catalytic effects on the degradation of
other carbohydrates. High yields of HMF from other carbohydrates such as glucose (61.2%), starch (62.7%) and
inulin (70.5%) were achieved using γ-AlOOH as the catalyst. The proposed catalytic method shows a promising
potential for HMF preparation, especially for industrial-scale HMF production from renewable bioresources.
Cellulose
HMF
Boehmite
Heterogeneous catalysis
Carbohydrates
1. Introduction
catalysts. Studies have shown that homogeneous acid catalysts are ef-
ficient in the degradation of cellulose to HMF. These catalysts are mi-
Factors such as diminishing petroleum sources and growth in de-
mand for energy and chemicals have aroused extensive interest in the
attainment of sustainability in conversion of abundant and renewable
biomass into platform chemicals [1,2]. Studies have identified 5-hy-
droxymethylfurfural (HMF) as an important biomass platform chemical
since it is widely used as a versatile building block for synthesis of fine
chemicals, medicines, bio-fuels and monomers of polymers [3,4].
Therefore, the synthesis of 5-HMF from carbohydrates has attracted a
lot of attention. Generally, excellent yields of HMF can be obtained
using simple monosaccharides like fructose and glucose as reactants
under various catalytic systems. However, apart from serving as re-
sources in food industries, monosaccharides are expensive and not
readily available [5]. In contrast, cellulose is an abundant, less ex-
pensive and non-edible carbohydrate, and a more promising material
for use in the production of HMF [6]. However, the robust structure of
cellulose and instability of HMF under harsh reaction conditions create
some problems in the degradation of cellulose to HMF, often resulting
in poor yield of HMF [7–9].
neral acids [13], heteropolyacids [14], ionic liquids [15], and metal
chlorides [16–18]. However, the use of homogenous acid catalysts gives
rise to serious problems related to corrosion of equipment, separation of
catalysts from products, and waste treatment [19]. On the other hand,
heterogeneous acid catalysts have numerous advantages such as easy
separation and recycling, low cost, and low environmental hazards
[20,21]. Therefore, various heterogeneous catalysts such as metal
oxides [22], zeolites [23], sulfonated carbon-based materials [24],
polymers [25], fused salts [26], metal-organic frameworks [27] and
resins [28] have been developed and used to degrade cellulose or sac-
charides into HMF. Among these heterogeneous catalysts, metal oxides
are the most commonly used due to their property-tunable solid acids
[29]. Aluminum oxide (Al2O3) has been reported to possess high ac-
tivity in the production of HMF [30–34]. For instance, Sampath [30]
investigated the effect of Al2O3 on the conversion of glucose to HMF. A
High HMF yield of 56% was obtained in DMSO system at 130 °C after
3 h in the presence of some amount of copper chloride (CuCl2,
0.6 mmol). It has been discovered that the reaction system could also
convert disaccharides and starch into HMF. Garcia-Sancho [32] found
that Al2O3, when used as support for Nb2O5, enhanced the dehydration
of xylose. However, the hydrothermal stability of Al2O3 is low. Thus, it
Decades of research have demonstrated that acid catalysts accel-
erate the degradation of cellulose [10–12]. This process consists of two
categories: homogeneous acid catalysis and heterogeneous acid
* Corresponding author.
E-mail address: sujianhui999_888@163.com (J. Su).
https://doi.org/10.1016/j.carres.2019.06.010
Received 18 March 2019; Received in revised form 9 June 2019; Accepted 17 June 2019
Availableonline21June2019
0008-6215/©2019PublishedbyElsevierLtd.

Z. Tang and J. Su
CarbohydrateResearch481(2019)52–59
is easily converted to its precursor boehmite (γ-AlOOH) at temperatures
above 150 °C in hot water systems [35]. This is not beneficial for the
degradation of cellulose since the conversion of cellulose often requires
addition of some water to accelerate the hydrolytic reaction at tem-
peratures usually higher than 150 °C.
2.3. Catalyst characterization
Powder XRD patterns were determined using Panalytical X-ray
Powder diffractometer (XRD) with Cu-Kα radiation (λ = 1.5406 Å).
The data were recorded at a 2θ range of 5° to 80°. Fourier transform
infrared (FT-IR) analysis was done with Nexus Nicolet Fourier
Transform spectrometer (Madison, Wisconsin, USA) in the range of
wavelengths of 4000–500 cm−1. Specific surface area and pore size
were determined using Quantachrome Micromeritics Instrument with
ASAP 2020 system (Norcross, GA, USA), Brunauer-Emmett-Teller (BET)
equation and Barrett-Joyner-Halenda (BJH) method, as appropriate.
Morphology and microstructure were assessed through transmission
electron microscopy (TEM, JEM-2100UHR; JEOL Ltd., Tokyo, Japan).
Thermal gravimetric analyses (TGA) of the samples were performed on
TA thermal gravimetric analyzer at a heating rate of 10 °C/min under
air-atmosphere (SDT Q600, Newcastle, Delaware, USA). Lewis and
Brønsted acidic sites were investigated via FT-IR pyridine adsorption
technique (Py-FTIR; Nicolet, Madison, Wisconsin, USA). The IR spectra
were recorded at a resolution of 4 cm−1 in the range of 1700 cm−1 to
Boehmite (γ-AlOOH) is one of the Al2O3 precursors with high hy-
drothermal stability; it is easy to prepare, and it is cheap [36,37].
Studies by Takagaki et al. [38] revealed that γ-AlOOH can catalyze the
conversion of glucose to HMF in water systems. This finding inspired
the idea that γ-AlOOH may be an efficient catalyst in the conversion of
cellulose or other carbohydrates to HMF. To the best of our knowledge,
there are few reports on conversion of cellulose or other carbohydrates
to HMF using γ-AlOOH as catalyst.
The degradation of cellulose to HMF is a tandem reaction involving
hydrolysis of cellulose into glucose, isomerization of glucose to fruc-
tose, and dehydration of fructose to HMF [39]. Hence, the first reaction
step (the hydrolysis) is very important. However, due to the robust
structure of cellulose, cellulose is sparingly soluble in water and most
organic solvents [7–9]. Hence, the choice of a suitable solvent is also
crucial in the production of HMF from cellulose. Research has de-
monstrated that ionic liquid (IL) is a promising solvent for dissolution of
cellulose due to the strong interaction between its anion and hydroxyl
of cellulose [40–42]. Although IL is more expensive than common
solvents, it can be recycled many times or mixed with other solvents
e.g. DMSO so as to reduce its cost, since it has high thermal and che-
mical stability and low vapor pressure [43].
1400 cm−1
.
Prior to measurements, the samples were degassed in air at 150 °C
for 5 h, cooled down, and then adsorbed in saturated pyridine atmo-
sphere at room temperature for 5 h. After adsorption, the infrared
spectrum was recorded with sample temperature fixed at 150 °C while
outgassing. The number of Lewis acid sites was determined on the basis
of the integral absorbance of the characteristic band at 1450 cm−1
,
using integrated molar extinction coefficient 2.22 cm mmol−1 [44,45].
In the present study, γ-AlOOH was first employed to degrade cel-
lulose into HMF using IL (1-butyl-3-methylimidazolium chloride,
BmimCl) and DMSO as the reaction solvents. Reaction conditions such
as reaction duration, reaction temperature, catalytic loading, water
content and reaction solvents were carefully investigated. This was with
the aim of establishing optimum reaction parameters for high catalytic
efficiency in the conversion of cellulose to HMF, and understanding the
possible reaction mechanism(s). Furthermore, the catalyst was char-
acterized using many technologies such as XRD, FT-IR, Py-IR, TEM, BET
and TG.
2.4. Typical procedure in the production of HMF from cellulose
The experiments were carried out in an 18 mm × 180 mm glass tube
with a lid. In a typical reaction process, 100 mg microcrystalline cel-
lulose (MCC) was added to a mixed solution consisting of 4.0 g IL
(BmimCl) and 2.0 g DMSO under magnetic stirring. When MCC was
dissolved in the IL-containing solvents, 100 mg of the catalyst (γ-
AlOOH) and 1.0 mL deionized water were added to the reaction. The
resultant solution was heated to 160 °C and reaction was allowed for
2.0 h, with magnetic stirring at 800 rpm. The reaction mixture was
quickly cooled to room temperature through addition of 20 mL cold
deionized water. After centrifugation at 10000 rpm for 5 min, the clear
solution and solid were separated, and the solid was weighed. The clear
solution was used to analyze the distribution and contents of the re-
action products. The solid was washed several times in deionized water
and ethanol, dried and weighed. This first weight of solid was desig-
nated W1. Then, the solid was further washed thrice using γ-valer-
olactone, dried and weighed. This second weight of the solid was
tagged W2. The dried solid was used as spent catalyst for the next re-
action on recyclable experiments of the catalysts. The difference in the
weight between W1 and W2 was the mass of residual cellulose, based on
the fact that γ-valerolactone can dissolve cellulose, while water and
ethanol cannot dissolve cellulose [46]. Hence the conversion of cellu-
lose was calculated according to the following equation:
2. Experimental
2.1. Materials
The ionic liquid (IL) 1-buthyl-3-methylimidazolium chloride
(BMIMCl, AR) was purchased from Shanghai Cheng Jie Chemical Co.
Ltd. (Shanghai, China), while HMF standard sample (AR) was acquired
from Aladdin Reagent Co. Ltd. (Shanghai, China). Microcrystalline
cellulose (MCC) (average particle size 50 μm) was supplied by
Sinopharm Chemical Reagent Co. Ltd. (Beijing, China). All other re-
agents were analytically pure pharmaceuticals from Sinopharm
Chemical Reagent Co. Ltd, and were used without purification.
2.2. Preparation of catalyst
Mass of residual cellulose
Mass of starting cellulose
Cellulose conversion(%) = 100 %
2.5. HPLC analysis
× 100%
The catalyst (γ-AlOOH) was prepared via hydrothermal synthesis.
The procedure for preparing the γ-AlOOH was as follows: 40 mL of
0.75 M NH4HCO3 was slowly added to 10 mL of 1.5 M Al(NO3)3·9H2O,
with magnetic stirring at 800 rpm. Then, 25% ammonia water was
added to the clear solution until the pH reached 9. Thereafter, the so-
lution was transferred into a 100-mL Teflon-lined stainless autoclave,
sealed, and then heated at 150 °C for 12 h. After the autoclave was
naturally cooled to room temperature, the resultant solid was collected
through centrifugation and washed three times with deionized water.
Finally, the catalyst (γ-AlOOH) was obtained through vacuum-drying at
80 °C for 24 h.
Following filtration through a 0.22 μm filter, the collected clear li-
quids were analyzed using High Performance Liquid Chromatography
(HPLC, Agilent 1200, USA). The concentrations of HMF were quantified
with HPLC equipped with an ultraviolet detector (UV-9600, Beijing,
China) at ultraviolet wavelength of 284 nm. The eluent was a mixture of
methanol and water at a volumetric ratio of 2:8, and the flow rate was
0.6 mL min−1. The concentrations of glucose, fructose, levulinic acid
53

Z. Tang and J. Su
CarbohydrateResearch481(2019)52–59
(LA) and formic acid (FA) were also determined using the same HPLC
outfitted with a refractive index (Agilent G1362A) and an Bio-Rad ion-
exclusion column (300 mm × 7.8 mm, Aminex HPX-87H, Beijing,
China) at 60 °C. The mobile phase was 0.004 M H2SO4 at a flow rate of
0.45 mL min−1. The yields of HMF and glucose were calculated ac-
cording to the following equations:
Thermogravimetric analysis of γ-AlOOH was also carried out. As
shown in Fig. S4, three stages of weight loss were observed. Stage 1 was
at temperature range of 30–190 °C, which was attributable to the vo-
latilization of the adsorbed waters on the surfaces of γ-AlOOH [47].
However, the weight loss was slight (less than 2.0%). Stage 2 wt loss
occurred at temperature range of 190–710 °C, and accounted for most
of the weight loss (up to 41.0%). This weight loss was due to trans-
formation as a result of dehydration of γ-AlOOH to γ-Al2O3 [48]. Stage
3 wt loss occurred above 710 °C, and was due to crystal transformation
from γ-Al2O3 to α-Al2O3 [49]. The stage-3 weight loss was also very
slight and negligible (less than 0.5%). These results indicate that at
reaction temperatures was below 190 °C, γ-AlOOH was stabilized and
did not change to other phases or decompose.
Glucose yield (%)
Moles of glucose in product
Moles of cellulose load in the reaction (base on glucose unit)
=
× 100%
HMF yield (%)
Fig. 2 shows results of Py-FTIR for γ-AlOOH. Only the absorption
Moles of HMF in product
Moles of cellulose load in the reaction (base on glucose unit)
peaks of Lewis acid sites appeared, which were at 1592 cm−1
,
=
1486 cm−1, and 1450 cm−1. There were no pyridine absorption peaks
of Brønsted acid sites (at 1540 cm−1) [35]. The results demonstrate that
γ-AlOOH acted as a Lewis acid catalyst without the Brønsted acid sites.
The amount of acid and acid site density per site of γ-AlOOH were about
83 μmol g−1 and 0.2 per nm2, respectively, using integrated molar
extinction [44,45].
× 100%
3. Results and discussion
3.1. Characterization of the catalyst
3.2. Results of degradation of cellulose into HMF
The XRD pattern of the catalyst (γ-AlOOH) is depicted in Fig. S1. All
the diffraction peaks were due to the characteristic peaks of an or-
thorhombic γ-AlOOH (JCPDS Card 021–1307). No other peaks were
observed, which indicated that γ-AlOOH was successfully synthesized.
The FT-IR spectroscopy was carried out in order to confirm the for-
mation of γ-AlOOH (shown in Fig. S2). All bands of characteristic
groups in γ-AlOOH were in Fig. S2. The bands at 3310, 3090, 1072 and
1162 cm−1 were assigned to the characteristic vibrations of νas(Al)O–H,
νs(Al)O–H, δs Al–O–H and δas Al–O–H, respectively, while the bands at
751, 630, and 484 cm−1 belonged to the vibration mode of AlO6.
The N2 absorption and desorption isotherm of γ-AlOOH is depicted
in Fig. 1. It can be seen that the isotherm was a type IV isotherm with a
type H2 hysteresis loop above 0.4 P/P0, indicating the existence of
meso-pores and relatively uniform distribution of pore size. The pore
size distribution is also shown in Fig. 1 (the inset image). Most pores
were of size 2.9 nm. Only a few pores were up to 10.2 nm. The other
textural properties of γ-AlOOH can be seen Table 1. In order to in-
vestigate the morphology and surface structure of the γ-AlOOH, TEM
was carried out, and its image is displayed in Fig. S3. It was discovered
that γ-AlOOH consisted of a great number of irregular nanosheets with
mean length of about 200 nm and the surfaces of the nanosheets were
not smooth. There were a vast number of small pores, which might
explain the observation that most of pores were 2.9 nm in size.
Table 2 shows the results of comparison of catalytic effectiveness
between γ-AlOOH and current best-acting catalysts as regards conver-
sion of cellulose to HMF. The results indicated that γ-AlOOH (entry
14–16) exhibited excellent catalytic effects in the degradation of cel-
lulose to HMF. In the absence of the catalyst γ-AlOOH, HMF yield (entry
16) was only 11.3%, with low cellulose conversion of 55.7%. However,
after adding the catalyst, a higher HMF yield of 58.4% was achieved
with high cellulose conversion (97.2%). The performance of γ-AlOOH
was similar to that of the homogenous catalytic systems of IL and metal
chlorides (Entry 3,4) and bi-phase catalytic systems with metal chlor-
ides (entry 2), but higher than those of the catalytic systems using IL as
the only catalysts (entry 5,6), and normal heterogeneous catalysts
(entry 9, 10, 13). Furthermore, it was lower than that of novel catalytic
systems, including mixture of novel strong acid IL and metal chlorides
(entry 7), novel hybrid solid polymer (entry 8), novel metal-doped
carbon materials (entry 11) and a new material (entry 12). The results
suggest that the catalytic effect of γ-AlOOH was similar to those of
metal chlorides. In previous studies [17,44,45], the catalytic me-
chanism involved in the metal chloride-catalyzed conversion of cellu-
lose to HMF was shown to entail acceleration of the hydrolysis of cel-
lulose to glucose with acid generated from their hydration, promotion
of isomerization of glucose to fructose and dehydration of fructose to
HMF, due to their Lewis acidity. The catalyst γ-AlOOH is a typical Lewis
acid catalyst. Therefore, the possible catalytic mechanism of γ-AlOOH
involves Lewis acid-induced enhancement of glucose isomerization to
fructose, and fructose dehydration into HMF. Furthermore, the surface
hydroxyl groups of γ-AlOOH might produce weak acidity and accelerate
the hydrolysis of cellulose into glucose with assistance of IL. However,
the use of homogenous metal salts (metal chlorides) lead to serious
problems such as equipment corrosion and waste treatment. Studies
have shown that γ-AlOOH is a readily available, low-cost heterogeneous
catalyst which is easily separated from products, and is suitable for
large-scale production of HMF, unlike the novel catalysts ((entries 7, 8,
11 and 12) which are either complex to prepare or expensive.
3.3. Optimization of reaction parameters
3.3.1. Reaction temperature and time
In order to obtain optimized HMF yield, the degradation of cellulose
to HMF was monitored as a function of time at different temperatures.
Since the hydrolysis, isomerization and dehydration reactions in the
three-step tandem for conversion of cellulose to HMF are endothermic
Fig. 1. N2 adsorption and desorption isotherms and Pore size distribution
(inset) of the γ-AlOOH.
54

Z. Tang and J. Su
CarbohydrateResearch481(2019)52–59
Table 1
Textural and acid properties of γ-AlOOH.
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore size (nm)
3.1
Amount of acida (umol g−1
83
)
Density of acid sites (sites per nm2)
0.2
316
0.2
a
Pyridine-IR
and time were 160 °C and 2 h, respectively. These were used in sub-
sequent discussion of other reaction parameters.
3.3.2. Catalyst loading
With regard to the loading of the catalyst γ-AlOOH as shown in
Fig. 4, cellulose conversion and HMF yield were increased at increased
doses of catalyst. When the amount of γ-AlOOH was 0.1 g, the yield of
HMF was 58.4%, and cellulose conversion was 97.2%. Further increases
in dose of γ-AlOOH led to decrease in HMF yield and selectivity, as well
as increases in side reaction products (LA and FA). These suggest that
excess catalyst would lead to decomposition of HMF into side products.
Hence, the optimal dose of γ-AlOOH was 0.1 g, which was used for
subsequent experiments.
3.3.3. Reaction solvents
From Table 2, it can be seen that in the absence of DMSO (entry 14),
an HMF yield of 51.3% was achieved with IL (BMIMCl) in the presence
of γ-AlOOH, which meant that the reaction solvent had a great effect on
the degradation of cellulose. As shown in Fig. 5, among the single
solvents, IL produced the best catalytic performance in which the HMF
yield and selectivity were 51.3% and 54.1%, respectively. With other
solvents (DMSO, acetone, THF and DMF), the HMF yields were very
little (not more than 3.0%), which might be attributed to the robust
structure of cellulose, which is sparingly soluble in water and in most
common organic solvents [7–9]. However, cellulose was easily dis-
solved in IL due to the strong interaction between the IL anion and
hydroxyl groups of cellulose [40–42]. However, in view of high cost
and high viscosity of IL, some common solvents were introduced in the
system. Studies have shown that an aprotic solvent could maintain the
Fig. 2. Py-FTIR spectrum of the γ-AlOOH.
[58,59], elevated temperature was beneficial for the generation of
HMF, as shown in Fig. 3. A maximum HMF yield and selectivity (58.4%,
60.1%) were obtained at 160 °C for 2 h. Further elevation of reaction
temperature to 170 °C and 200 °C led to decomposition and condensa-
tion of HMF into other compounds after 2 h reaction time. The HMF
yield and selectivity were decreased correspondingly. Furthermore, as
depicted in Fig. 3, prolongation of the reaction over optimized time was
not beneficial for the production of HMF, indicating that longer reac-
tion time would lead to side-reactions. Hence, the optimal temperature
Table 2
Overview of HMF Yields from the degradation of Cellulose in the Best-Behaving Catalytic System.
Entry Substrate
Solvent
Catalyst
Ta (°C)
Time
Cb (%) Yc (%)
Ref.
16
1
2
3
Cellulose (40 mg)
THF(3 mL)-H2O (1 mL)- NaCl
THF(6 mL)-H2O (1 ml)
BmimCl (1 g)
AlCl3·6H2O (0.1 mmol)
NaCl (30 wt%)
180 (microwave) 0.5 h
200
150 (microwave) 10 min
99.0
37.0
Cellulose (50 mg)
Cellulose
6 h, 8 h > 99
56.1,58.9 17
CrCl3·6H2O (15mg0
> 99
54.0
49
（50 mg）
4
5
6
7
Cellulose (50 mg)
Cellulose (0.2 g)
Cellulose (0.1 g)
Cellulose (0.35 g)
EmimCl (0.5 g)
CuCl2/CrCl2 (37μmol/g, XCuCl2 = 0.17)
120
150
120
160
8 h
> 99
77.1
57.5
52.7
53
50
14
51
15
H2O (4 mL)
Cr((DS)H2PW12O40
)
(0.06 mmol)
2 h
3
BmimCl (2.0 g)
Cr((PSMIM)HSO4)3 (0.05 g)
((C4SO3Hmim)(CH3SO3) (1.5 mL) +CuCl2
(0.1 mol/L)
5 h
95.0
EmimAc(10 mL) +H2O (0.2 mL)
3.5 h
> 99
69.7
8
Cellulose (0.1 g)
Cellulose (0.25)
Cellulose(0.1 g)
Cellulose (1 g)
MIBK(5 mL)-H2O (0.5 mL)
H2O (10 mL)
ChH2PW12O40
140
190
170
200
8 h
4 h
8 h
1 h
87.0
67.0
> 99
> 99
75.0
46.0
53.3
85.0
52
53
54
55
9
Bimodal-HZ-5 (0.50 g)
Nb–C-50 (0.1 g)
10
11
THF (6 mL) –H2O (2 mL)-NaCl
H2O (45 mL)
Nin/Cs (200 mg)
+H2(6 MPa)
12
Cellulose (0.1 g)
THF (4 mL) –H2O (1 mL)-NaCl
HfO(PO4)2.0
190
4 h
69.8
89.7
56
(0.03 g)
13
14
Cellulose (0.1 g)
Cellulose (0.1 g)
BmimCl (2.0 g) +H2O (1.2 mL)
BmimCl (4.0 g) + H2O (1.0 mL)
Cr-ACS (0.05 g, XCr3+ = 5%)
γ-AlOOH (0.1 g)
120
160
1 h
2 h
> 99
94.8
49.0
51.3
57
This
work
This
work
This
work
15
16
Cellulose (0.1 g)
Cellulose (0.1 g)
BmimCl (4.0 g) +DMSO(2.0 g) +H2O γ-AlOOH (0.1 g)
160
160
2 h
2 h
97.2
55.7
58.4
11.3
(1.0 mL)
BmimCl (4.0 g) +DMSO(2.0 g) +H2O Absence
(1.0 mL)
a
b
c
reaction temperature.
conversion of cellulose.
yield of HMF.
55

Z. Tang and J. Su
CarbohydrateResearch481(2019)52–59
Fig. 3. Effects of reaction time under different temperatures on the HMF yield (a), cellulose conversion (b) and HMF selectivity. Reaction condition: 0.1 g cellulose;
0.1 g γ-AlOOH; 2.0 g DMSO, 4.0 g BMIMCl and 1.0 mL H2O.
Fig. 5. Effect of solvent on degradation of cellulose. Reaction condition: 0.1 g
cellulose; 0.1 g γ-AlOOH; 6.0 g solvent and 1.0 mL H2O; 160 °C; 2 h.
Fig. 4. Effect of catalyst loading on degradation of cellulose. Reaction condi-
tion: 0.1 g cellulose; 2.0 g DMSO, 4.0 g BMIMCl and 1.0 mL H2O; 160 °C; 2 h.
yield and cellulose conversion. Excessive decrease in IL decreased the
solubility of cellulose remarkably and affected the conversion of cel-
lulose to HMF.
ability of IL to solubilize cellulose [43]. Moreover, DMSO has the ca-
pacity to stabilize HMF and inhibit its decomposition [30,60]. Thus,
DMSO was chosen as the added solvent. As showed in Fig. 5, the ad-
dition of DMSO had great effect on the degradation of cellulose. When
the mass ratio of DMSO was 37.0%, the performance of the reaction
solvents (DMSO and IL) was optimum at 58.4%, with cellulose con-
version of 97.2%. Further increases in DMSO caused decreases in HMF
3.3.4. Effect of amount of H2O added
The amount of H2O added has a great influence on the degradation
of cellulose to HMF. Water not only acts as a cross-catalyst or reactant
in the hydrolysis of cellulose, it also acts as an inhibitor to the product
HMF (by increasing decomposition rate HMF into side-products like LA
56

Z. Tang and J. Su
CarbohydrateResearch481(2019)52–59
(12.4%) was obtained. Correspondingly, further increases in the
amount of H2O led to decreases in the amount of glucose produced, but
the amounts of other products were increased. When the H2O:cellulose
ratio was to 10:1, highest yield and selectivity of HMF (58.4% and
60.1%, respectively) were obtained. However, further increases in the
amount of H2O caused increased decomposition of HMF and decreased
solubility of cellulose in IL, with corresponding decreases HMF yield
and selectivity. Hence, the optimum H2O:cellulose ratio was 10, and the
optimum amount of H2O was 1.0 g (1.0 mL).
From the above results, it can be concluded that the optimum re-
action conditions were 0.1 g cellulose, 0.1 g γ-AlOOH, 2.0 g DMSO,
4.0 g BMIMCl,1.0 mL H2O, 160 °C, 2 h.
3.4. Distribution of the products
To understand the possible reaction route and distribution of other
products, the distribution of the products was investigated at optimum
reaction conditions. As depicted in Fig. 7, HMF yield and cellulose
conversion increased along with increase in reaction time; highest yield
of HMF was 58.4% at 120 min, with cellulose conversion of 97.2%.
Further extension of reaction time to 300 min had very little effect on
the conversion of glucose. However, HMF yield decreased remarkably
to 15.3%, which might be attributed to its decomposition, consistent
with variations in the two major side products LA and FA. Glucose and
fructose, the predominant intermediates, were detected during the re-
action, and achieved high yields of 13.5% and 6.9% at reaction times of
80 min and 100 min, respectively. This indicates that the degradation of
cellulose to HMF was a tandem reaction involving hydrolysis of cellu-
lose to glucose, isomerization of glucose to fructose, and dehydration of
fructose to HMF.
Fig. 6. Effect of H2O on degradation of cellulose. Reaction condition: 0.1 g
cellulose; 0.1 g γ-AlOOH; 2.0 g DMSO and 4.0 g BMIMCl; 160 °C; 2 h.
3.5. Recycling potential of γ-AlOOH
The re-usability of γ-AlOOH is not only very important for evalu-
ating the stability of γ-AlOOH, but also necessary for large scale pro-
duction of HMF from cellulose. Leaching of acid sites and deactivation
are the two major problems of the catalyst associated with HMF pro-
duction. Acid site leaching experiments were carried out via the fil-
tration of γ-AlOOH after running the cellulose degradation for about
60 min at 160 °C, and then the reaction was prolonged for 180 min in
the absence of the catalyst γ-AlOOH. As shown in Fig. 8a, HMF yield
was not increased, and no further reaction was detected, suggesting that
the no acid sites in γ-AlOOH were leached. The stability of γ-AlOOH
was also investigated under the reaction conditions indicated in section
2.4. After the reaction, the catalyst γ-AlOOH was recovered via filtra-
tion, washed with water, ethanol and γ-valerolactone, and dried
Fig. 7. Distribution of the products. Reaction condition: 0.1 g cellulose; 0.1 g γ-
AlOOH; 2.0 g DMSO, 4.0 g BMIMCl and 1.0 mL H2O; 160 °C.
and FA). Hence, after the addition of H2O, the rate of hydrolysis of
cellulose was increased, and the total reaction rate was promoted cor-
respondingly, causing the amount of all products to increase, as shown
in Fig. 6. When the H2O:cellulose ratio was to 4:1, rate of hydrolysis of
cellulose to glucose was maximum, and the highest yield of glucose
Fig. 8. Time-yield plots of cellulose to HMF conversion, existence γ-AlOOH (solid circle) and removing γ-AlOOH (hollow circle) after 60 min (a) and recycling
experiments of the catalyst (b). Reaction condition: 0.1 g cellulose; 0.1 g catalyst; 2.0 g DMSO, 4.0 g BMIMCl and 1.0 mL H2O; 160 °C; 2 h.
57

Z. Tang and J. Su
CarbohydrateResearch481(2019)52–59
Table 3
Catalytic conversion of various sugars into HMF over the γ-AlOOH.
Entry
Sugars
Solvents
Temperature (°C)
Time (h)
Conversion (%)
HMF Yield (%)
1
2
3
4
5
6
Cellulose
glucose
glucose
glucose
Starch
IL (4.0 g) +DMSO(2.0 g) +H2O (1.0 mL)
DMSO (2.5 g)
160
130
130
130
160
160
2
3
3
3
2
2
97.2
98.6
96.9
95.3
> 99
> 99
58.4
61.2
41.1
45.2
62.7
70.5
IL (2.5 g)
IL + DMSO (2.5 g, IL/DMSO, 2:1 W:W)
IL (4.0 g) +DMSO(2.0 g) +H2O (1.0 mL)
IL (4.0 g) +DMSO(2.0 g) +H2O (1.0 mL)
Inulin
Raw materials: 0.1 g cellulose, 0.1 g γ-AlOOH.
overnight under vacuum at 100 °C. Then, the spent catalyst was used in
the next reaction as a test of its recyclability. As shown in Fig.8b, γ-
AlOOH performed well without loss of its catalytic properties. After five
reaction runs, an HMF yield of about 47.8% was obtained, with cellu-
lose conversion of 91.0%, which might be attributed to its high hy-
drothermal stability. The fresh and spent catalysts after five times of
reuse were further characterized using XRD and FT-IR. From the pat-
terns of XRD (Fig. S6a) and FT-IR (Fig. S6b), it can be seen that the
peaks of the spent γ-AlOOH were almost the same with those of fresh γ-
AlOOH. Furthermore, the XRD patterns of the spent γ-AlOOH at dif-
ferent reaction times at 160 °C were also investigated. As shown in Fig.
S7, their diffraction peaks were almost the same with those of the fresh
γ-AlOOH in Fig. S6a. These results indicate that γ-AlOOH possesses very
good hydrothermal stability.
Acknowledgments
This work was supported financially by the Talent Instruction
Projects of the Yancheng Institute of Technology (KJC2014015 and
XJ201724), the Provincial Education Program of Jiangsu
(17KJD530002), and Jiangsu Overseas Visiting Scholar Program for
University Prominent Young & middle-aged Teachers and Presidents.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.carres.2019.06.010.
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A simple solid acid catalyst (γ-AlOOH) was prepared and employed
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