Influence of a Lewis acid and a Br?nsted acid on the conversion of microcrystalline cellulose into 5-hydroxymethylfurfural in a single-phase reaction system of water and 1,2-dimethoxyethane

Article abstract of DOI:10.1039/c7ra13387a

5-Hydroxymethylfurfural (HMF) is a typical dehydration product of C6 carbohydrates, and it can be converted into a series of chemicals and liquid fuels. In this study, an advanced low-boiling single-phase reaction system consisting of water and 1,2-dimethoxyethane (DMOE) was proposed for the production of HMF from microcrystalline cellulose (MCC). AlCl₃ and H₃PO₄ were selected as the Lewis acidic catalyst and Br?nsted acidic catalyst, respectively, and the influence of these two catalysts on the conversion behavior of MCC was studied. The results showed that MCC could be selectively converted into HMF or levulinic acid (LA) by altering the solvent composition. As for the composition of the catalyst, high AlCl₃ content favored the generation of HMF, whereas high H₃PO₄ content could decrease the HMF yield and promote the formation of glucose and fructose. The highest HMF yield of 49.42% was obtained at an AlCl₃-H₃PO₄ ratio of 1:0.8. GC-MS analysis suggested that much MCC was transformed into furans and cyclopentenones in the presence of AlCl₃, while anhydrosugars tended to be generated with a high H₃PO₄ proportion in the catalyst. Besides, FTIR analysis of the insoluble humin formed during MCC conversion indicated that AlCl₃ could also facilitate the depolymerization of MCC.

Full text of DOI:10.1039/c7ra13387a

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Influence of a Lewis acid and a Brønsted acid on the
conversion of microcrystalline cellulose into 5-
hydroxymethylfurfural in a single-phase reaction
system of water and 1,2-dimethoxyethane
Cite this: RSC Adv., 2018, 8, 7235
*
Haizhou Lin, Jingping Chen and Hao Xu
Yuan Zhao,
Shurong Wang,
5-Hydroxymethylfurfural (HMF) is a typical dehydration product of C6 carbohydrates, and it can be
converted into a series of chemicals and liquid fuels. In this study, an advanced low-boiling single-phase
reaction system consisting of water and 1,2-dimethoxyethane (DMOE) was proposed for the production
of HMF from microcrystalline cellulose (MCC). AlCl3 and H3PO4 were selected as the Lewis acidic
catalyst and Brønsted acidic catalyst, respectively, and the influence of these two catalysts on the
conversion behavior of MCC was studied. The results showed that MCC could be selectively converted
into HMF or levulinic acid (LA) by altering the solvent composition. As for the composition of the catalyst,
high AlCl3 content favored the generation of HMF, whereas high H3PO4 content could decrease the
HMF yield and promote the formation of glucose and fructose. The highest HMF yield of 49.42% was
obtained at an AlCl3–H3PO4 ratio of 1 : 0.8. GC-MS analysis suggested that much MCC was transformed
into furans and cyclopentenones in the presence of AlCl3, while anhydrosugars tended to be generated
with a high H3PO4 proportion in the catalyst. Besides, FTIR analysis of the insoluble humin formed during
MCC conversion indicated that AlCl3 could also facilitate the depolymerization of MCC.
Received 16th December 2017
Accepted 7th February 2018
DOI: 10.1039/c7ra13387a
rsc.li/rsc-advances
abundant on the earth, and it account for almost 50 wt% in
biomass.9 Cellulose consists of a linear chain of several hundred
1 Introduction
to many thousands of b-1,4-glycosidic bond linked glucose
units. Recent efforts have been primarily focused on the
conversion of glucose and fructose to HMF in order to reveal the
mechanisms involved in the production of HMF.10,11 However,
in a practical point of view, cellulose is a much more reasonable
feedstock for large-scale HMF production, even if it still faces
several challenges such as its rm crystal structure and multiple
side reactions.12
In a common conversion process from cellulose to HMF,
cellulose is rstly hydrolyzed to form oligosaccharides and
glucose, which was usually catalyzed by a Brønsted acid catalyst;
then, with the aid of a Lewis acid catalyst, the isomerization of
glucose occurs to form fructose, and fructose will undergo
consecutive dehydration reactions to form HMF.13 Actually,
glucose can also be directly dehydrated to generate HMF, but it
is usually more difficult for this route to occur than the isom-
erization–dehydration route owing to the different conformer
distribution of glucose and fructose solution.14,15 Consequently,
it is a commonly recognized strategy to combine Lewis acid and
Brønsted acid in the catalytic conversion of C6 carbohydrates to
HMF. In addition to the catalyst, a good reaction medium is also
an important aspect for efficient HMF production, since it could
affect the distribution of intermediates and the selectivity of
different products.16,17
In recent years, along with the rapid economic expansion all
over the world, there is an increasing demand for energy and
chemicals in human society.1,2 Furan compounds, which can be
directly derived from biomass carbohydrates through dehydra-
tion, are very important precursors for many value-added
chemicals and fuels.3,4 For example, a single dehydration
process of C6 carbohydrates can form 5-hydroxymethylfurfural
(HMF), which is an important biorenery building block since it
is crucial to the synthesis of a series of valuable chemical
intermediates and liquid fuels like 2,5-furan dicarboxylic acid
(FDCA), 2,5-dimethyl furan (DMF) and levulinic acid (LA).5,6
FDCA-derived polymer polyethylene furanoate (PEF) is used in
the production of bottles, bers and lms; DMF can be an
additive in current petroleum fuels; GVL is a hydrogenation
product of LA and can be upgraded to fuel additives and jet
fuels.7,8 Therefore, it is of great importance to develop efficient
and economical reaction systems for the production of HMF.
Lignocellulosic biomass is one of the most promising feed-
stocks for HMF production as it can be easily obtained from
nature. Especially, cellulose is a kind of macromolecular poly-
saccharide that is the most widely distributed and the most
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Zheda Road 38,
Hangzhou 310027, China. E-mail: srwang@zju.edu.cn; Tel: +86 571 87952066
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Table 1 presents an overview of recent progress in C6 because of fewer side reactions that occurred in non-aqueous
carbohydrates dehydration for HMF production in the presence solvent.34 AlCl3 and H3PO4 were chosen as the Lewis acidic
of different solvents and catalysts. The synergistic effect of catalyst and the Brønsted acidic catalyst, respectively. The effect
Lewis acidic catalyst and Brønsted acidic catalyst was reported of water–DMOE ratio on product selectivity was investigated,
to be pivotal for efficient production of HMF and its important followed by the inuence of reaction temperature and time.
derivative LA from C6 carbohydrates.18–22 Zhang et al. found that Then the catalytic effect of Lewis acid and Brønsted acid was
Lewis acidic AlCl3 and Brønsted acidic maleic acid could couple studied in depth by adjusting their mole ratio. The liquid-phase
with each other to form catalytic complexes, which were able to products and the solid insoluble humins formed at different
favor the hydride shi in glucose for isomerization leading to AlCl3–H3PO4 mole ratios were analyzed by gas chromatography-
enhanced reaction rates and HMF selectivity.22 In general, Lewis mass spectrometry (GC-MS) and Fourier Transform Infrared
acid could promote the isomerization reaction from glucose, Spectroscopy (FTIR), respectively.
while Brønsted acid is signicant for the hydrolyzation of
polysaccharides and the dehydration of monosaccharides.19
AlCl3 was reported to be an attractive Lewis acidic catalyst
among the metal chlorides for the conversion of glucose to
HMF,21,22 and it also has great potential for commercial HMF
2 Experimental section
2.1 Materials
production due to its low toxicity and low cost.23 Water is the
Glucose, fructose, microcrystalline cellulose (MCC), HMF, LA
most recommended solvent for its availability, but actually it
and DMOE were purchased from Aladdin Industrial Corpora-
was usually combined with polar aprotic solvent so as to get rid
tion (Shanghai, China). AlCl3$6H2O and H3PO4 were purchased
of side reactions (i.e., HMF rehydration).11 In recent years ionic
from Sinopharm Chemical Reagent (Shanghai, China). All
liquids have been a focus of attention because it could over-
chemicals were of analytical grade and were directly used
come the physical and biochemical barriers of the cellulose
without purication.
hydrolysis reaction.24 Although the above solvent systems
exhibited satisfying performance for HMF formation, product
isolation could be a challenge for the downstream chemical
processing. For instance, biphasic solvents required a large sum
of organic extractant, and ionic liquids were expensive and hard
to be evaporated out of the solution.6 Hence a green solvent with
low boiling point was demanded to establish a single-phase
reaction system in large-scale HMF production.25
Herein, a single-phase reaction system composed of renew-
able 1,2-dimethoxyethane (DMOE), water, AlCl3 and H3PO4 was
proposed for HMF production from microcrystalline cellulose
(MCC). DMOE is a renewable organic solvent that can be
synthesized from bio-based ethylene glycol,33 and it is a polar
aprotic solvent miscible with water. DMOE has low boiling
point (84.6 ꢀC) that is lower than commonly used polar aprotic
solvents like dimethylformamide (DMF, 153 ꢀC), dimethyl
sulfoxide (DMSO, 189 ꢀC) and g-valerolactone (GVL, 207 ꢀC),
and therefore it requires a small amount of energy for separa-
tion through distillation. Moreover, studies have shown that
polar aprotic solvent could improve HMF yields signicantly,
2.2 Procedure for conversion of cellulose to HMF
In a typical reaction process, a mixture of cellulose (10 mmol
(1.8 g) based on monosaccharide units), solvent (water–DMOE
at a specied volume ratio; 50 mL) and catalysts (AlCl3 and
H3PO4) were placed in the Parr reactor (Parr Instruments,
Moline, IL, USA; 100 mL). For different catalyst compositions,
the mass of AlCl3 ranged from 0 to 0.133 g (0 to 1 mmol), while
the mass of H3PO4 ranged from 0 to 0.098 g (0 to 1 mmol). The
reactions were carried out under mechanical agitation in a N2
atmosphere, and the stirring rate was maintained at 300 rpm.
Aer the preset reaction temperature and time were reached,
heating was stopped and the reactor was cooled by airow.
Samples were ltered through a 0.22 mm syringe lter prior to
high performance liquid chromatography (HPLC) and GC-MS
analyses. Each experiment was repeated three times, and the
resulting mean value and standard deviation are shown in the
gures.
Table 1 C6 carbohydrates dehydration to HMF in acidic reaction systems
Reactant
Solvent (additive)
Catalyst
Temp. (ꢀC)
Time (h)
HMF yield (%)
Ref.
Fructose
Glucose
Glucose
Water/isopropanol
Water/DMOE
Water/GVL
Water (NaCl)
Water/MIBK
Water/THF (NaCl)
Water (NaCl)
Water/THF
CO2
AlCl3
PTSA-POM/Sn-b
AlCl3/HCl
LPSnP-1
Sn-Mont
AlCl3/CPME
H2SO4
NaHSO4/ZnSO4
Cr([PSMIM]HSO4)3
180
150
140
170
150
160
190
170
160
130
2
67.1
58.5
60.1
62.0
32.0
39.1
42.0
44.0
53.0
53.0
16
26
19
27
28
29
21
30
31
32
0.75
0.5
3
0.33
3
60
2
1
Glucose
Cellulose
Cellulose
Cellulose
Cellulose
Cellulose
Cellulose
Water/THF
[BMIM]Cl
5
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by GC-MS. The results showed no decomposition behavior of
DMOE, and therefore DMOE was stable in the reaction system.
As described in Fig. 1, the solvent composition has a great
inuence on the typical product distribution from cellulose. In
pure water solvent, the main product formed from cellulose
were LA with a high yield of 39.01% and FA with a yield of
14.94%, while the yield of glucose, fructose and HMF remained
at pretty low levels (<0.5%). Theoretically, LA would be
produced in a 1 : 1 ratio with formic acid from HMF rehydra-
tion. However, this ratio could be greater than, equal to or less
than 1 : 1 in experiments under different conditions.35 For
instance, Asghari et al. found that when catalyzed by hydro-
chloric acid, FA could decompose faster than LA aer they were
generated from HMF rehydration, leading to a much higher
yield of LA than that of FA.36 However, when the organic solvent
DMOE was gradually added into the solution until the DMOE–
water ratio reached 7 : 1, the yield of LA and FA showed a steep
drop while the yield of HMF rose rapidly to 48.27%. Besides,
both the yield of glucose and fructose increased at rst and
decreased thereaer, and their maximum yields of 8.20% and
5.35% were arrived at a DMOE–water ratio of 1 : 1, respectively.
The results suggested signicant effects of solvent on the
degradation route of cellulose when catalyzed by H3PO4 and
AlCl3.
In pure water solvent, cellulose could be hydrolyzed
adequately with the help of Brønsted acid to generate glucose,
and subsequently the glucose was catalyzed by Lewis acid to
undergo isomerization reaction. Aerwards the formed fructose
was dehydrated to form HMF in the presence of Brønsted acid,
and HMF was easily rehydrated in the water-rich environment,
resulting in the formation of LA and FA. Thus, the synergistic
catalytic effect of Lewis acid and Brønsted acid in aqueous
solvent could be utilized to transform cellulose or glucose into
LA. For example, Yang et al. achieved a LA yield of 49.8% in
a reaction system containing H3PO4 and CrCl3.37 Nemoto et al.
used H3PO4 and AlCl3 as catalysts to convert cellulose into LA
and a maximum LA yield of 50% was obtained.38 Along with the
2.3 Analytical methods
The ltered solution was analyzed on a Dionex HPLC system
(Dionex, Sunnyvale, CA, USA) equipped with a Bio-Rad Aminex
HPX-87H column (Bio-Red, Hercules, CA, USA) and an RI 2000
refractive index detector (Schambeck SFD GmbH, Bad Honnef,
Germany). H2SO4 solution (pH 2.5) was used as the mobile
phase, and its ow rate and the corresponding column
temperature were kept at 0.6 mL minꢁ1 and 60 ꢀC, respectively.
The concentrations of glucose, fructose, LA, formic acid (FA)
and HMF were determined by reference to standard calibration
curves. The yields of hexoses (glucose and fructose), LA, FA and
HMF were calculated as follows:
moles of hexose formed
Hexose yield ¼
ꢂ 100%
moles of hexose units in cellulose
moles of LA formed
LA yield ¼
ꢂ 100%
moles of hexose units in cellulose
moles of FA formed
FA yield ¼
ꢂ 100%
moles of hexose units in cellulose
moles of HMF formed
HMF yield ¼
ꢂ 100%
moles of hexose units in cellulose
In order to understand the process of glucose decomposi-
tion, the ltered samples were also analyzed by GC-MS (Thermo
Scientic, Trace DSQII) equipped with a DB-wax capillary
column (30 m ꢂ 0.25 mm ꢂ 0.25 mm). Helium (99.999%) at
a ow rate of 1 mL minꢁ1 was used as carrier gas. The GC oven
temperature was programmed to increase from 40 ꢀC (1 min) to
240 ꢀC (20 min) at 8 ꢀC minꢁ1 heating rate. The MS detector was
operated in electron ionization (EI) mode (70 eV) with a scan
range of m/z 35–450. All detected chemicals were identied by
comparison with the NIST (National Institute of Standards) MS
library.
2.4 Humins isolation and analyses
Insoluble polymeric humins from cellulose conversion process
were obtained as follows. Aer reaction, the samples were
ltered through a 0.22 mm organic lter membrane to obtain
the insoluble residue. The remaining solid residues aer
ꢀ
washing with water were dried at 105 C for 2 h, and the dry
solid was considered as insoluble humins. FTIR spectra of the
humins were recorded from 400 to 4000 cmꢁ1 at resolution of
4 cmꢁ1 on a Nicolet 5700 FTIR spectrometer by averaging 36
scans. The yield of humin was calculated as the mass of cellu-
lose divided by the mass of humins.
3 Results and discussion
3.1 Effect of solvent composition
Fig. 1 Effect of solvent composition on the product distribution from
cellulose conversion (180 ꢀC, 120 min, 0.02 M AlCl3, 0.02 M H3PO4,
At rst, the mixtures of water–DMOE (7 : 1) and AlCl3/H3PO4
catalysts before and aer heating at 180 ꢀC for 2 h were analyzed 36 mg mLꢁ1 cellulose).
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ꢀ
increasing proportion of DMOE and the decreasing water further increased to 190 C, the HMF yield slightly dropped to
content, the generated HMF could be protected by the 46.35%. The yields of glucose and fructose were both lower than
increasing non-aqueous solvent (DMOE) concentration and the 1.5%, indicating a nearly complete conversion of the mono-
hydrogen-bond interaction between DMOE and HMF.26 In this saccharides coming from cellulose hydrolysis. Little LA and FA
way the formation of LA and FA was inhibited and the reactive were formed (<2%) due to the impeded secondary reactions
HMF could be preserved.
involving HMF in the presence of DMOE. The generation trend
In pure organic solvent, all of these products were hardly of LA with rising temperature coincided with that of HMF, and
formed, and the yield of HMF was as low as 5.96% (not shown in its yield reached a maximum value at 180 ꢀC. At higher reaction
Fig. 1). It was extremely difficult for cellulose to be hydrolyzed temperatures LA and HMF could become much more reactive to
towards monosaccharides without water, and therefore further undergo side reactions (i.e., condensation), which might be the
reactions would hardly occur. In spite of this, there was still cause of the uctuant tendency of HMF and LA in Fig. 2a. The
a little HMF that could be detected, which might come from yield of FA at different temperatures followed the same
other reaction paths. Weingarten et al. reported the production tendency as that of LA. As for the monosaccharides, the glucose
of HMF from cellulose in polar aprotic solvents without the yield showed a decline with increasing temperature, while the
presence of water, and they deemed that the cellulose rstly yield of fructose remained almost unchanged. The product
degraded to form levoglucosan, which was then dehydrated to distribution from cellulose at different reaction time was
generate HMF.30
depicted in Fig. 2b. It was obvious that the generation of HMF,
LA and FA followed an increasing tendency with the reaction
time going by, and both of them reached the peak values at
120 min. Aer 120 min the yield of HMF began to fall because
HMF was easily rehydrated to form byproducts like formic acid
and humins.39 Similarly, LA and FA could also undergo some
side reactions (fragmentation and polymerization) in acidic
reaction conditions.40 The yields of glucose and fructose were
below 0.5% and showed a slow uctuation with the highest
yields achieved at 90 min.
3.2 Effect of reaction temperature and time
As shown in Fig. 2a, when the reaction went up from 170 ꢀC to
ꢀ
180 C, the yield of HMF grew from 33.62% to 48.27%, corre-
spondingly. Nevertheless, when the reaction temperature
3.3 Effect of Lewis acid and Brønsted acid on liquid-phase
product
Aer cellulose was hydrolyzed to form glucose, the synergistic
catalytic effect between Lewis acid and Brønsted acid is required
for an efficient production of HMF from glucose, as shown in
Fig. 3.
In the conversion process of cellulose, the synergistic cata-
lytic effect between Lewis acid and Brønsted acid is required for
an efficient production of HMF. The product distribution from
cellulose conversion with different Lewis acid (AlCl3)–Brønsted
acid (H3PO4) ratios is presented in Fig. 4. When only AlCl3 was
added, the HMF yield reached up to 40.24%, indicating that
AlCl3 had good catalytic performance on the HMF production
from cellulose in the proposed water–DMOE solvent system.
Aer H3PO4 was gradually added to the solution, the HMF yield
showed an ascent at rst, and it arrived at the highest value of
49.42% when the AlCl3–H3PO4 mole ratio was 1 : 0.8. The main
reason was that H3PO4 could promote the hydrolysis process of
cellulose to generate monosaccharide (glucose). In this way, the
following isomerization reaction of glucose catalyzed by AlCl3
would be facilitated. When the AlCl3–H3PO4 mole ratio
decreased to 1 : 1, the HMF yield dropped slightly to 48.27%.
Noticeably, as the proportion of AlCl3 further declined, the HMF
yield fell considerably to 36.73%, 27.33% and 13.46% at the
AlCl3–H3PO4 ratio of 0.8 : 1, 0.5 : 1 and 0.1 : 1, respectively. It
could be concluded that H3PO4 had few catalytic effects on the
formation of HMF, because H3PO4 mainly behaved as
a Brønsted acid to provide hydrogen ions for the hydrolysis and
Fig. 2 Effect of reaction temperature (a) and time (b) on the product
distribution from cellulose conversion (DMOE–water (7 : 1), 120 min
(a), 180 ꢀC (b), 0.02 M AlCl3, 0.02 M H3PO4, 36 mg mLꢁ1 cellulose).
3ꢁ
dehydration reactions. Although PO4 could be recognized as
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Fig. 3 Transformation of cellulose to HMF catalyzed by Lewis acid and Brønsted acid.
6.17% and 1.02%, respectively, resulting in much more
unreacted glucose caused by insufficient AlCl3. When no AlCl3
existed, the yields of glucose and fructose reached 5.70% and
0.99%, respectively, and the corresponding HMF yield was as
low as 13.46%. The decreased yield of glucose from 6.17% to
5.70% could be explained by the enhanced dehydration reaction
towards anhydrosugars when catalyzed solely by H3PO4, which
was evidenced by the following GC-MS analysis. Besides,
glucose might also undergo a variety of acid-catalyzed side
reactions to generate undesired products such as humins,
which led to its consumption.26
In order to provide a deeper insight into the catalytic
mechanism of Lewis acid and Brønsted in the water–DMOE
solvent system, typical product distribution in the liquid phase
at different AlCl3–H3PO4 mole ratio was analyzed by GC-MS. The
products formed from the catalytic conversion of cellulose were
mainly divided into four categories, namely oxygenated
aliphatics, furans, cyclopenten-1-ones and anhydrosugars. The
distribution variation of these chemical families at different
AlCl3–H3PO4 mole ratios is shown in Fig. 5.
Fig. 4 Effect of AlCl3–H3PO4 mole ratio on the product distribution
from cellulose conversion (DMOE–water (7 : 1), 180 ꢀC, 120 min,
36 mg mLꢁ1 cellulose).
a conjugate base and facilitate the isomerization reaction of
glucose to generate fructose, its Lewis acidity was actually much
lower than AlCl3, leading to decreased HMF yield without the
presence of AlCl3. Yin et al. also reported unsatisfactory HMF
yield of 20.72% from cellulose with only a Brønsted acidic
catalyst (hydrochloric acid).41 Consequently, for the purpose of
high-efficiency production of HMF from cellulose, the Lewis
acid–Brønsted acid ratio needed to be optimized to take full
advantage of their synergistic effect.
As for the oxygenated aliphatics, besides LA and formic
acid, they mainly consisted of hydroxyacetones and
As for the formation of LA and FA, it can be seen from Fig. 4
that the yields of LA and FA were both around 1% when the
AlCl3 concentration was not less than H3PO4. As the share of
AlCl3 decreased, the yields of LA and FA also dropped slowly,
which could be attributed to the lower HMF yield in the pres-
ence of excess H3PO4.
Obviously, the yields of glucose and fructose were pretty low
(<0.5%) when the AlCl3–H3PO4 mole ratio was higher than 1 : 1,
and at these ratios fructose was always more than glucose. This
suggested that most of the glucose formed from cellulose
hydrolyzation was converted into fructose with Lewis acidic
AlCl3 as the catalyst, and then the fructose was dehydrated to
generate HMF. As the AlCl3–H3PO4 mole ratio decrease to
0.5 : 1, the yields of glucose and fructose rose signicantly to
Fig. 5 Effect of AlCl3–H3PO4 mole ratio on the product distribution of
chemical families from cellulose conversion (HMF, LA, glucose and
fructose are not depicted).
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Fig. 6 Liquid-phase products formation pathways for cellulose conversion in water–DMOE solvent reaction system.
hydroxybutanones, which came from the retroaldolization of reaction of furanose or the rearrangement of acyclic fragments
the intermediates or the fragmentation of acyclic intermedi- coming from monosaccharide retroaldolization. In general, it
ates.42–44 As the AlCl3–H3PO4 mole ratio decreased, the gener- was easy for furans to be formed in the presence of Lewis acid,
ation of oxygenated aliphatics was inhibited, suggesting whereas they presented a lower yield with Brønsted acid as the
a more profound effect of Lewis acid on the breakage of the catalyst. It could be inferred that reaction conditions con-
carbon chain. Furans were the primary products from cellu- taining sufficient Lewis acid would promote the generation of
lose degradation, and they covered a wide range of species furanose, and therefore the formation of furan products was
including HMF, furfural, 5-methylfurfural, 2,5-diformylfuran, greatly facilitated.
furfuryl alcohol and 5-acetoxymethyl-2-furaldehyde. This
The dominant species of cyclopenten-1-ones were
chemical family was mainly generated by the dehydration 2-cyclopenten-1-one and methyl cyclopentenolone. This kind of
Fig. 7 FTIR spectra and mass yields of insoluble humins formed at different AlCl3–H3PO4 mole ratios (humin 1–7 correspond to AlCl3–H3PO4
mole ratios at 1 : 0, 1 : 0.5, 1 : 0.8, 1 : 1, 0.8 : 1, 0.5 : 1, 0 : 1, respectively).
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product with 5-carbon rings might have been produced by in H3PO4-rich conditions (i.e., humin-6 and humin-7), while the
cyclization of chain intermediates containing multifunctional spectra of samples from AlCl3-rich conditions (i.e., humin-1,
groups. Similar to the furans, the generation of cyclopenten-1- humin-2, humin-3) were different from the spectrum of orig-
ones would be favored at high AlCl3–H3PO4 mole ratios, while inal cellulose. Especially, the peak intensities of humins from
they barely formed with single H3PO4 catalyst. The anhy- AlCl3-rich conditions at low wavenumbers became much weaker
drosugars mainly include levoglucosan, levoglucosenone, than that of cellulose, which indicated that AlCl3 could destroy
1,4:3,6-dianhydro-d-glucopyranose and oligosaccharides. These the original structure of cellulose to a higher degree than
sugars came from the Brønsted acid-catalyzed dehydration of H3PO4. Similar results were also reported by Ma et al. They
monosaccharides to which cellulose was hydrolyzed. Hence studied the effect of AlCl3 and HCl on cellulose conversion in
a large amount of anhydrosugars were produced when the high temperature liquid water, and AlCl3 was considered to
reaction was solely catalyzed by H3PO4, while their yields have a more evident impact on the cellulose degradation.52 In
decreased sharply with the addition of AlCl3. Fig. 6 describes the our previous research, we used glucose as a model compound of
reaction pathways involving the formation of liquid-phase cellulose to produce HMF in a similar reaction system of water–
products from cellulose. However, the related mechanisms are DMOE–AlCl3, and the obtained humins were mainly furan
complex and further work is needed for clarication.
polymers from dehydration/condensation reactions of glucose,
HMF and intermediates.26 Given that the majority of the
humins from cellulose in the current work was polysaccharide,
they could be recycled and reused in case of unnecessary waste.
3.4 Effect of Lewis acid and Brønsted acid on insoluble
humins
The mass of insoluble humins were measured. As shown in
Fig. 7, it is obvious that the smallest amount of insoluble
4 Conclusion
humins was obtained when the AlCl3–H3PO4 mole ratio was This research focused on the catalytic effects of Lewis acid AlCl3
1 : 1. Hence a combination of both Lewis acid and Brønsted and Brønsted acid H3PO4 on the conversion process from
acid is required in order to suppressing side reactions that led cellulose to HMF. Firstly, the inuence of solvent composition
to humin formation. For the sake of further understanding the (water–DMOE ratio) on cellulose conversion catalyzed by both
effects of catalysts on cellulose conversion when catalyzed by AlCl3 and H3PO4 was investigated. The results showed that
Lewis acid and Brønsted acid in water–DMOE solvent, the cellulose tended to form LA in pure water solvent, while the
insoluble polymer humins were analyzed by FTIR. The FTIR yield of HMF signicantly increased as the proportion of DMOE
spectra of the humins formed at different AlCl3–H3PO4 mole went up. In pure DMOE solvent, the reaction pathway of cellu-
ratios are shown in Fig. 7. Humin 1–7 correspond to AlCl3– lose towards HMF was severely hindered. Consequently, cellu-
H3PO4 mole ratios at 1 : 0, 1 : 0.5, 1 : 0.8, 1 : 1, 0.8 : 1, 0.5 : 1, lose could be selectively converted into HMF or LA by altering
0 : 1, respectively. It could be found that the humin structures at the solvent composition.
different catalyst ratios were similar to each other. The
The conversion results of cellulose at different AlCl3–H3PO4
absorption peaks at 3345 cmꢁ1 and 1318 cmꢁ1 correspond to mole ratios were further analyzed. It was found that AlCl3-rich
the stretching and bending of hydroxyl groups, respectively, environment favored the formation of HMF, and the highest
which widely exist in cellulose structure; the peak at 2905 cmꢁ1 HMF yield 49.42% was achieved at an AlCl3–H3PO4 mole ratio of
is ascribed to C–H stretching, and this structure indicates the 1 : 0.8. In H3PO4-rich conditions, the generation of HMF was
existence of methyl and methylene; the absorption at 1640 cmꢁ1 greatly inhibited and more glucose and fructose would be
can be attributed to the bond water in samples.45,46
produced. Moreover, the liquid-phase products were analyzed
The peaks at 1431 cmꢁ1 and 1220 cmꢁ1 come from the by GC-MS, and they could be mainly separated into oxygenated
bending and stretching of –CH2– structure in sugar rings, aliphatics, furans, cyclopenten-1-ones and anhydrosugars. AlCl3
respectively; intensive and complex absorptions appear at 1200– was able to facilitate the formation of furans and cyclopenten-1-
1000 cmꢁ1, and this range mainly correspond to the stretching ones, whereas more anhydrosugars would be produced in the
of the glycosidic linkage (C–O–C) and C–OH in the sugar presence of high H3PO4 content. FTIR analyses showed that
rings;45,47,48 the signal at 1162 cmꢁ1 is from the C–O–C asym- some of the original polymer structures in cellulose were
metric bridge vibration in the b-glycosidic linkage of cellu- preserved in the insoluble humins, and high proportion of AlCl3
lose;49,50 the absorption at 899 cmꢁ1 corresponds to the C1 group in catalysts could noticeably promote the cellulose degradation.
frequency or ring frequency, which is also characteristic of b-
glycosidic linkages between the sugar units;51 the absorption
Conflicts of interest
band at 710 cmꢁ1 is assigned to Ib crystalline phase in cellu-
lose.46 The above absorption peaks have similar characteristics, The authors declare that they have no competing interests.
and they indicate the fundamental framework of poly-
saccharides remained in the humins. This might be explained
by the lacking of water in the reaction system, leading to
Acknowledgements
insufficient hydrolyzation of cellulose.
This work was supported by the National Science Fund for
In addition, further analysis of the results revealed similarity Distinguished Young Scholars (51725603), the National Natural
between the FTIR spectra of cellulose and the humins obtained Science Foundation of China (51476142), the National Science
This journal is © The Royal Society of Chemistry 2018
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