Valorization of an underused sugar derived from hemicellulose: Efficient synthesis of 5-hydroxymethylfurfural from mannose with aluminum salt catalyst in dimethyl sulfoxide/water mixed solvent

Article abstract of DOI:10.1039/c7ra07803j

Converting saccharides into 5-hydroxymethylfurfural (5-HMF) has attracted more and more research interest under the background of global energy shortage. A lot of effort has been devoted to the conversion of fructose and glucose. However, other underused sugars receive very limited attention, although some of them have considerable reserves in nature as well. In this work, mannose, a major component from hemicellulose, was effectively converted into 5-HMF under mild conditions. AlCl₃·6H₂O exhibited superior activity among the tested catalysts, achieving a maximum 5-HMF yield of 60% in a dimethyl sulfoxide (DMSO)/water mixed solvent at 130 °C within 45 min. Adding an appropriate amount of water in DMSO could suppress some side reactions while preserving the reactivity of mannose dehydration. Mannose showed a comparable reactivity to fructose in the employed catalytic system. The studied system was also effective in the conversion of di/trisaccharides such as cellobiose and melezitose into 5-HMF. A number of control experiments with different catalysts and additives were conducted to elucidate the preliminary mechanism.

Full text of DOI:10.1039/c7ra07803j

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Valorization of an underused sugar derived from
hemicellulose: efficient synthesis of 5-
hydroxymethylfurfural from mannose with
aluminum salt catalyst in dimethyl sulfoxide/water
mixed solvent†
Cite this: RSC Adv., 2017, 7, 39221
a
a
b
Songyan Jia, * Xinjun He and Zhanwei Xu
Converting saccharides into 5-hydroxymethylfurfural (5-HMF) has attracted more and more research
interest under the background of global energy shortage. A lot of effort has been devoted to the
conversion of fructose and glucose. However, other underused sugars receive very limited attention,
although some of them have considerable reserves in nature as well. In this work, mannose, a major
component from hemicellulose, was effectively converted into 5-HMF under mild conditions.
AlCl
3
$6H
2
O exhibited superior activity among the tested catalysts, achieving a maximum 5-HMF yield of
ꢀ
6
0% in a dimethyl sulfoxide (DMSO)/water mixed solvent at 130 C within 45 min. Adding an appropriate
amount of water in DMSO could suppress some side reactions while preserving the reactivity of
mannose dehydration. Mannose showed a comparable reactivity to fructose in the employed catalytic
system. The studied system was also effective in the conversion of di/trisaccharides such as cellobiose
and melezitose into 5-HMF. A number of control experiments with different catalysts and additives were
conducted to elucidate the preliminary mechanism.
Received 15th July 2017
Accepted 1st August 2017
DOI: 10.1039/c7ra07803j
rsc.li/rsc-advances
ꢀ
12
ꢁ
31.5% of fructose at equilibrium in water at 31 C), which are
1
. Introduction
13
proposed as the active species, so fructose can be readily
5
-Hydroxymethylfurfural (5-HMF) represents a versatile bio- converted into 5-HMF with various catalysts, such as mineral
1
based platform compound. 5-HMF can be rened into varieties acids, metal salts, zeolites, ionic liquids and other functional-
14–19
of high value added ne chemicals and transport fuels through ized materials.
By contrast, glucose remains recalcitrant to
oxidation, reduction, etherication, amination, alkylation and most catalysts used for fructose conversion. Glucose almost
2–8
12
condensation. The synthesis of 5-HMF from carbohydrates is exists in the form of a- and b-pyranose. As shown in Scheme 1,
consistent with the scope of sustainable development in the it is generally accepted that glucose undergoes a two-step
1
future, and thus, 5-HMF has attracted widespread attention. 5- tandem reaction to produce 5-HMF (isomerization of glucose
1
1,20,21
HMF was recognized as an important and potential platform into fructose and dehydration of fructose into 5-HMF).
The
9
compound as early as 1990. Entering the 21st century, the representative catalysts for glucose conversion typically include
pioneering work reported by Dumesic et al. and Zhang et al., an active metal center (e.g. Cr, Sn, Al, Ge), which is mainly used
1
1,13,22–24
respectively, has set off new studies toward the conversion of in the form of metal salt.
biomass into 5-HMF worldwide.
As a result of the above work,
10,11
some related polymer feedstocks such as inulin and cellulose
Fructose and glucose are the most common carbohydrates have been exploited as well.
25–28
1
for 5-HMF production. Fructose exists in a considerable
proportion of furanoses (e.g. a- and b-furanose account for
a
College of Chemical Engineering, Shenyang University of Chemical Technology,
Shenyang, Liaoning, 110142, China. E-mail: jiasongyan@126.com; Tel:
+
86-24-89386342
b
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian,
Liaoning, 116023, China
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra07803j
Scheme 1 Schematic path on the conversion of glucose and mannose
into 5-HMF.
1
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In addition to fructose and glucose, other underused sugars were purchased from Energy Chemical (China). SnCl
4 2
$5H O
receive far less attention in the aspect of 5-HMF synthesis, (99%), H BO (99%), AlCl3 (99%), Al(NO ) $9H O (98%),
3
3
3 3
2
although some of them have considerable reserves in nature. As Al (SO ) $18H O (99%), dimethyl sulfoxide (DMSO, 99%),
2
4 3
2
shown in Scheme 1, mannose is the C-2 epimer of glucose. Al(OH) (99%), glycerol (99%) and 1,2-propanediol (99%) were
3
29
Mannose is one of the major hexoses in hemicellulose. For purchased from Sinopharm (China). Hydrochloric acid (HCl,
instance, mannose can account for ꢁ10% of the dry weight of 37 wt%) and sulfuric acid (H SO , 98 wt%) were provided by
pine wood. Efficient conversion of mannose into 5-HMF would a local supplier. All of the commercial chemicals were used as
provide reference for the utilization of hemicellulose. However, received. Puried water (H O) with a resistivity of 18.2 MU cm
2
4
30
2
information on such conversion still remains limited at present. was produced by an ultra-pure water system (Taoshi Brand,
Binder et al. previously demonstrated that mannose showed China).
a similar reactivity to that of glucose in N,N-dimethylacetamide
(
DMA)–lithium chloride/bromide (LiCl/LiBr) with CrCl or CrCl
2 3
2.2 Reaction procedure
as the catalyst. A 5-HMF yield of 69% was achieved in DMA–LiBr
Typically, mannose (60 mg), catalyst (10 mol% to mannose) and
solvent (1 mL) were added into a reaction vial with a magnetic
stir bar. The vial was sealed and inserted into a heating block.
The mixture was stirred at a speed of ꢁ400 rpm at the reaction
temperature. Aer a specied time, the vial was taken out and
immersed in an ice-water bath to quench the reaction. Then,
ꢀ
31
at 100 C aer 2 h. De et al. obtained a 5-HMF yield of 45%
from mannose in DMA–LiCl with a H-form TiO catalyst at
40 C under microwave. Most recently, Wrigstedt et al. gained
2
ꢀ
32
1
a 5-HMF yield of 69% in a potassium bromide (aq.)—g-valer-
olactone biphasic system with CrCl $6H O and Amberlyst-38 at
3
2
ꢀ
33
1
60 C under microwave. Bhanja et al. synthesized a bifunc-
1
mL of H
dard) were added into the vial. A small amount of reaction
mixture was taken out and further diluted with H O. All the
2
O and a certain amount of glycerol (internal stan-
tionalized mesoporous SBA-15 catalyst which converted
mannose into 5-HMF in dimethyl sulfoxide (DMSO) with a yield
34
2
ꢀ
of 61% at 135 C under microwave. Flannelly et al. investigated
the conversion of mannose and glucose in sulphuric acid
solution. Both hexoses underwent comparable conversions with
levulinic acid yields of ꢁ45%, a followed product through the
samples were analyzed by high performance liquid chroma-
tography (HPLC) aer ltration.
ꢀ
35
2.3 Analysis method
hydration of 5-HMF, at 150 C aer 6 h. Guo et al. studied the
conversion of mannose in aqueous medium with Nb W cata- HPLC was performed on a Shimadzu LC-16 system equipped
4
4
ꢀ
36
lyst at 120 C, while the 5-HMF yield was lower than 10%. The with a Shimadzu RID-20 refractive index detector and an Agilent
above studies used either complicated system or microwave, Hi-Plex ligand exchange column (H-form, 300 ꢂ 7.7 mm). A
and it will still be necessary to develop facile and effective 0.005 M aqueous solution of H
approaches to produce 5-HMF by mannose and other under- phase with a ow rate of 0.65 mL min . The column and
2
SO
4
was used as the mobile
ꢃ1
ꢀ ꢀ
used sugars.
detector temperatures were 65 C and 50 C, respectively.
In this work, we were committed to explore effective Glycerol was added as the internal standard for quantitative
approaches to convert mannose into 5-HMF. We employed calculations. It should be specied that the retention times of
a series of metal salts and mineral acids as the catalysts. Some 1,3-dihydroxyacetone and glycerol are overlapped under the
aluminum salts showed good catalytic activities in a DMSO/H O analysis conditions, and 1,3-dihydroxyacetone could be an
2
mixed solvent. Some parameters such as water content, catalyst isomerization product from glyceraldehyde. Thus, 1,2-pro-
type and loading, reaction temperature, reaction time, feed- panediol was used as an alternative internal standard for the
stock type were investigated in detail. A number of control control experiments with glyceraldehyde, 1,3-dihydroxyacetone
experiments with different catalysts or simple model and glycerol. The conversion and product yield data were
compounds were also conducted to facilitate understanding the calculated by mole.
reaction mechanism.
3
. Results and discussion
2. Experimental
3.1 Catalyst screening test
2.1 Materials
29
Mannose is the C-2 epimer of glucose, implying that a catalyst
D-Mannose (99%), D-glucose (99%), D-fructose (99%), D-melezi- enabling the conversion of glucose into 5-HMF could be the
tose monohydrate (99%), 5-hydroxymethylfurfural (5-HMF, potential candidate in this work. According to previous report
1
1,22,23,37–43
9
(
(
9%), MnCl
99%), FeCl
99%), ZnCl
2
$4H
$6H
(99%), AlCl
2
O (99%), NiCl
O (99%), LaCl
$6H O (98%), InCl
2
$6H
2
O (99%), FeCl
O (99%), CoCl
$4H
2
$4H
$6H
2
O
O
on glucose conversion,
a wide range of metal salts and
3
2
3
$7H
2
2
2
mineral acids were employed as the catalysts for the preliminary
2
3
2
3
2
O (99%), 1,3- test.
dihydroxyacetone (97%) and levulinic acid (99%) were
purchased from Aladdin (China). D-Cellobiose (98%) and 130 C for 30 min. As shown, the conversion of mannose did not
Fig. 1 illustrates the results of catalyst screening in DMSO at
ꢀ
CuCl $2H O (99%) were purchased from Alfa Aesar (China). essentially occur in the absence of a catalyst, with very low
2
2
CrCl $6H O (98%) and DL-glyceraldehyde (90%) were purchased conversion (ꢁ5%), and no 5-HMF was detected. MnCl $4H O,
3
2
2
2
from Sigma-Aldrich (China). YbCl
triuoromethanesulfonate (Al(OTf)
3
$6H
2
O (99%), aluminum FeCl
2
$4H
2
O, FeCl
3
$6H
2
O, CuCl
2
$2H
2
O, CoCl
2
$6H
2
O and H
3
BO
3
3
, 99%) and AlBr
3
(99%) did not catalyze the formation of 5-HMF, possibly because these
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solvent. As shown in Fig. S1,† adding little water obviously
improved the dissolution of AlCl $6H O. Therefore, the effect of
3
2
water content in DMSO was next examined.
Five DMSO/H O solutions with volume fractions of 1000/0,
2
ꢃ1
9
50/50, 900/100, 800/200 and 600/400 (mL mL
)
were
employed for this investigation. The molar fractions of water
) in the solutions were calculated to be 0, 0.17, 0.3, 0.5 and
(c
w
0
.72. Fig. 2 presents AlCl
3
$6H
2
O catalyzed conversion of
ꢀ
mannose in DMSO with different amounts of water at 130 C.
The mannose conversion remained at 81–86% in the systems of
c # 0.3, and it slightly decreased to 71% when c reached 0.5.
w
w
However, the mannose conversion sharply dropped to 17% with
further increasing c to 0.72. Because the formation of 5-HMF
from mannose is a dehydration reaction, water of course will
inhibit such process. However, DMSO/H O binary mixtures
w
2
49
w
retain DMSO-like properties at c < 0.5, which could be the
Fig. 1 Conversion of mannose in the presence of different catalysts in
DMSO at 130 C for 30 min.
reason that relatively high conversions (71–86%) were obtained
ꢀ
ꢃ1
in DMSO/H O of 950/50, 900/100 and 800/200 (mL mL ). The 5-
2
HMF yield showed a similar trend as mannose conversion, and
it kept at 44–56% in the systems of c # 0.5. A maximum 5-HMF
w
catalysts were unable to isomerize mannose into a proper
intermediate, such as fructose. Although 5-HMF could be
ꢃ1
yield of 56% was obtained in DMSO/H
2
O of 950/50 (mL mL ) (c
w
¼
0.17). The conversion was also monitored in anhydrous
produced by employing NiCl $6H O, LaCl $7H O, ZnCl2,
2
2
3
2
ꢃ1
DMSO and DMSO/H
2
O of 950/50 (mL mL ) with reaction time at
$6H O and AlCl showed
YbCl
large difference between mannose conversion and 5-HMF yield
over FeCl $6H O, CuCl $2H O and H SO indicated that
3 2 2 4
$6H O and H SO , the 5-HMF yields were still low. The
ꢀ
130 C. As shown in Fig. S2,† AlCl
3
2
3
close activities in DMSO, of which the conversion rate was
3
2
2
2
2
4
ꢃ1
a little faster than that in DMSO/H O of 950/50 (mL mL ) within
2
a signicant level of side reactions occurred. The side products
20 min. Adding water may improve the hydrolysis of AlCl3-
were proposed to be undesired oligomers, polymers and
$
6H O, but water itself is deleterious to the dehydration of
2
44
humins.
By contrast, CrCl $6H O and InCl $4H O led to moderate
mannose into 5-HMF, accounting for the slower conversion rate
3
2
3
2
ꢃ1
in DMSO/H
2
O of 950/50 (mL mL ) within short time. As re-
yields of 5-HMF (ꢁ22%). The reaction with SnCl $5H O ach-
4
2
ported, an appropriate amount of water could suppress some
side reactions such as inter- and intra-molecular dehydration of
ieved an improved 5-HMF yield (ꢁ44%). Among the tested
catalysts, AlCl $6H O exhibited the superior catalytic perfor-
mance with ꢁ84% conversion and a 5-HMF yield of ꢁ52%. The
anhydrous AlCl was also tested, giving very close results to
AlCl $6H
3
2
48
sugar to a certain degree, leading to an improved 5-HMF yield
of 60% as shown in Fig. S2.† Fig. S3† showed the chromato-
3
grams on the conversion of mannose in DMSO and DMSO/H O
2
3
2
O (ꢁ87% conversion and ꢁ51% 5-HMF yield). In
ꢃ1
ꢀ
of 950/50 (mL mL ) without catalyst at 130 C for 1 h. The results
consideration of the hydroscopicity of AlCl , leading to difficulty
3
for weighing, AlCl $6H O was employed for subsequent studies.
3
2
As reported, some metal salts (e.g. CrCl , AlCl ) may hydrolyze
3
2
3
+
+
into metal ion species (e.g. M(OH)
2
or M(OH) ) and
companion acids during the dehydration of glucose into 5-
45–47
HMF.
It was proposed that the metal ion species mainly
played a role in isomerizing glucose into fructose, while the
companion acid catalyzed the subsequent dehydration of fruc-
45,48
tose into 5-HMF.
The conversion of mannose into 5-HMF is
a dehydration process, and the produced water will promote the
hydrolysis of metal salts. Accordingly, the latter four metal salts
may possess the aforementioned properties during the reaction,
45–47
leading to the production of 5-HMF.
3
.2 Effect of water content in DMSO
During the catalyst screening tests, AlCl
solubility in DMSO and some AlCl
3
$6H
2
O exhibited a poor
precipitation
3
$6H O
2
appeared aer reaction. The other tested metal salts could
dissolve well in DMSO. It is speculated that the undissolved
3 2
AlCl $6H O may limit 5-HMF yield. To overcome this problem,
we attempted to add an appropriate amount of water as co- solvents at 130 C for 30 min.
Fig. 2 Catalytic conversion of mannose in varied DMSO/H
2
O mixed
ꢀ
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demonstrated that anhydrous DMSO could promote the the tested aluminum salts generally worked well for the
formation of mannose dimer, while adding a little water sup- production of 5-HMF except Al (SO ) $18H O. Average conver-
2
4 3
2
pressed such side reaction. Compared with anhydrous DMSO sion of ꢁ80% and 5-HMF yield of ꢁ45% could be obtained in
system, the completely dissolved AlCl $6H O did not signi- the presence of Al(NO ) $9H O, Al(OTf) and AlBr , respectively.
3
2
3 3
2
3
3
cantly increase the yield of 5-HMF. DMSO/H
2
O binary mixtures As compared, no improved 5-HMF yield was achieved with other
50
display H O-like properties at c > 0.5, especially at c
2
w
w
$ 0.7,
3 2
aluminum salts than AlCl $6H O. These results indicate that
which may lead to the inferior mannose conversion and 5-HMF the actual active catalyst enabling the conversion of mannose
ꢃ1
yield in DMSO/H
the conversion of mannose with SnCl
InCl $4H O in varied DMSO/H O solutions, respectively. Al (SO ) $18H O could be that the active aluminum species as
2
O of 600/400 (mL mL ). Fig. S4–S6† present into 5-HMF should be attributed to an aluminum species.
4
2 3 2
$6H O, CrCl $6H O and One reason accounting for the inferior performance of
3
2
2
2
4 3
2
However, the addition of water signicantly suppressed the discussed was not effectively produced. Moreover, the steric
formation of 5-HMF with increasing cw in the systems. hindrance of anion should be taken into account as well, while
As compared above, AlCl $6H O was chosen as the catalyst this point needs more in-depth studies.
3 2
for the subsequent investigations and the DMSO/H
2
O of
Fig. 3b illustrates the conversion of mannose with different
AlCl $6H O loadings. As shown in Fig. 3b, no 5-HMF was
detected in the absence of a catalyst. The mannose conversion
ꢁ3%) was lower than that of blank test (ꢁ5%) in Fig. 1 due to
the suppression of water. Only 1 mol% AlCl $6H O loading
ꢃ1
950/50 (mL mL ) solution was further employed.
3
2
(
3.3 Effect of catalyst type and loading
3
2
A number of aluminum salts were then employed to examine
their effects on the conversion of mannose into 5-HMF. In order
to maintain a 10 mol% ratio of aluminum atom to mannose,
could lead to 63% conversion with a 5-HMF yield of 35%. With
increasing catalyst usage, the mannose conversion grew in an
upward trend, while the 5-HMF yield was found stagnant at
AlCl $6H O loading $10 mol%. Higher usage of catalyst may
5
mol% Al (SO ) $18H O was added herein. As seen in Fig. 3a,
2 4 3 2
3
2
Fig. 3 Effect of aluminum salts (a) and catalyst loadings (b) on the Fig. 4 Effect of reaction temperature and time on the conversion of
ꢀ
ꢃ1
conversion of mannose into 5-HMF at 130 C for 30 min.
2
mannose into 5-HMF in DMSO/H O of 950/50 (mL mL ).
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accelerate more side reactions or 5-HMF degradation, which thermodynamic properties as glucose based on the fact that it is
may explain why 5-HMF yield stopped increasing. Herein, the the C-2 epimer of glucose, which accounts for the signicantly
AlCl $6H O loading of 10 mol% was selected for the next accelerated conversion at higher temperatures. The slight
3
2
ꢀ
investigation.
decrease of 5-HMF yield at 140 C indicated that 5-HMF could
degrade aer long time. Moreover, since it is difficult to sepa-
rate either catalyst or 5-HMF from the homogeneous system,
the catalyst circulation test is outside the scope of this work at
present.
3
.4 Effect of reaction temperature and time
In order to get an optimized 5-HMF yield, the conversion of
mannose was monitored as a function of time at different
temperatures. As illustrated in Fig. 4, the conversion of
ꢀ
mannose could proceed steadily at 110 C, reaching 68% with 3.5 Comparison with other saccharides
a 5-HMF yield of 44% aer 75 min. When the reaction was
conducted at 130 C, the mannose conversion and 5-HMF yield
Fructose and glucose are the isomers of mannose, while both of
the two saccharides are the most common carbohydrates for 5-
HMF production. We then compared the reactivity of mannose,
ꢀ
almost reached the maximum of 96% and 60%, respectively,
ꢀ
aer 45 min. Further elevating temperature to 140 C did not
fructose and glucose. Fig. 5 presents the reactivity comparison
improve the 5-HMF yield but obviously shortened reaction time
to ꢁ30 min to achieve the optimum. As mentioned above,
dehydration of glucose into 5-HMF is considered to undergo
a two-step tandem reaction, including the isomerization of
glucose into fructose and the dehydration of fructose into 5-
ꢃ1
in DMSO/H
1
2
O of 950/50 (mL mL ) with AlCl
3
$6H
2
O catalyst at
ꢀ
30 C. As shown, fructose exhibited the fastest reaction rate
with 92% conversion and 56% 5-HMF yield aer only 20 min,
36
which was consistent with that reported by Guo et al. However,
mannose showed almost comparable conversion and 5-HMF
yield to fructose with extending a bit of time.
1
1,20,21
HMF.
Both the isomerization of glucose and dehydration
51,52
of fructose are endothermic.
Mannose should have similar
Disaccharide and trisaccharide were also tested in the
system. Fig. 6 illustrates the production of 5-HMF from cello-
ꢀ
biose and melezitose over time at 130 C. Both cellobiose and
melezitose produced 5-HMF steadily, achieving a yield of 46%
and 54%, respectively, aer 120 min. The 5-HMF formation
from di/trisaccharide was slower than that from mono-
saccharide, because the former feedstocks needed to be
hydrolyzed rst. In addition, melezitose can be partially
hydrolyzed to glucose and turanose, the latter of which is an
53
isomer of sucrose. As reported, sucrose had a superior reac-
34,54
tivity to cellobiose,
which may be one reason that melezitose
showed a better performance than cellobiose.
3.6 Mechanistic insights
A number of control experiments were conducted to get some
mechanistic insights on the catalytic conversion of mannose.
Fig. 5 Comparison on the conversion of mannose, fructose and Fig. 6 The production of 5-HMF from cellobiose and melezitose in
ꢃ1
ꢀ
ꢃ1
ꢀ
glucose in DMSO/H
2
O of 950/50 (mL mL ) at 130 C.
DMSO/H O of 950/50 (mL mL ) at 130 C.
2
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a
Table 1 Results on the control experiments
(Table 1), glycerol did not substantially affect the conversion of
mannose as compared with entry 1. However, glyceraldehyde
did suppress the formation of 5-HMF, indicating that the
catalyst coordinated with the glyceraldehyde part of mannose.
1,3-Dihydroxyacetone could be produced as an isomerization
product from glyceraldehyde. Next, 1,3-dihydroxyacetone was
also tested as an additive. As shown in entry 10 (Table 1), 1,3-
dihydroxyacetone suppressed the production of 5-HMF as well.
As mentioned above, fructose can also be considered as a 1,3-
dihydroxyacetone attached to a glycerol. This result may imply
that catalyst promote the conversion of fructose, derived from
mannose isomerization, by interacting with 1,3-dihydroxyace-
tone subunit of fructose.
Conversion 5-HMF yield
(%)
d
Entry Catalyst
Additive
(%)
1
2
3
4
5
6
7
8
AlCl
HCl
Al(OH)
HCl + Al(OH)
3
$6H
2
O
None
None
None
None
HCl (10 mol%)
HCl (20 mol%)
HCl (30 mol%)
Glycerol
80
48
5
51
6
0
b
3
c
3
47
5
e
e
AlCl
AlCl
AlCl
AlCl
3
3
3
3
$6H
$6H
$6H
$6H
2
2
2
2
O
O
O
O
46/68e
25/38e
50/71
15/26
e
e
50/66
11/17
80
56
52
36
32
(
100 mol%)
Glyceraldehyde
100 mol%)
Dihydroxyacetone 51
100 mol%)
9
1
AlCl
3
3
$6H
$6H
2
2
O
O
(
0
AlCl
(
4. Conclusions
a
Conditions: 60 mg of mannose with catalyst (10 mol% to mannose)
ꢃ1
was added into total 1 mL of DMSO/H
2
O (950/50, (mL mL )) mixed In this work, we demonstrated that mannose could be effec-
ꢀ
b
solvent and heated at 130 C for 20 min. 30 mol% (to mannose) HCl
was added. 30 mol% (to mannose) HCl and 10 mol% (to mannose)
were added. The number in the brackets represents the
dosage of additive (with respect to mannose by mol). The results
tively converted into 5-HMF in a DMSO/H
2
O mixed solvent
c
d
under mild conditions. AlCl $6H O exhibited the superior
3
2
Al(OH)
3
e
activity among the tested catalysts. Adding an appropriate
amount of water in DMSO can suppress some side reactions
while preserving the reactivity of mannose dehydration.
Mannose showed a comparable reactivity to that of fructose in
this catalytic system. A maximum 5-HMF yield of 60% could be
were obtained aer 30 min.
Due to the existence of water, AlCl
3
$6H
2
O may undergo hydro-
. To clarify
ꢀ
obtained at 130 C within 45 min. The studied system was also
lysis into HCl, aluminum complexes and Al(OH)
3
effective for the conversion of other di/trisaccharides into 5-
HMF. An active aluminum species from the hydrolysis of
AlCl $6H O was turned out to play a key role in converting
which component catalyzed the conversion, both HCl and
Al(OH) were tested as the catalyst, respectively. It was assumed
that AlCl $6H O (10 mol% to mannose) completely hydrolyzed,
then the amount of formed HCl should be 30 mol% to
mannose. As listed in Table 1 (entry 2), HCl promoted the
3
3
2
3
2
mannose into 5-HMF. The active aluminum species primarily
interacted with the glyceraldehyde subunit of mannose and 1,3-
dihydroxyacetone subunit of fructose to achieve the nal
formation of 5-HMF. Mannose is a major sugar component of
hemicellulose. The results provided herein will offer useful
information on the valorization of hemicellulose and even
lignocellulose in the future.
conversion of mannose. However, both HCl and Al(OH)
inactive to the formation of 5-HMF (entries 2 and 3). Vlachos
et al. and Hu et al. demonstrated that AlCl could hydrolyze into
3
were
3
2
+
+
an active aluminum complex, Al(OH) or Al(OH)
2
, which
4
5–47
played a role in isomerizing glucose.
As a result, the active
aluminium complex was also proposed to be the key species for
converting mannose into 5-HMF. As given in entry 4, the
performance of mannose conversion appeared to be similar to
the one with HCl alone, possibly because the active aluminium
complex was not formed to an appreciable extent under this
Acknowledgements
The work was supported by the Natural Science Foundation of
Liaoning Province (China) (No. 2015020633), Project of Educa-
tion Department of Liaoning Province (China) (No. L2015421),
the National Natural Science Foundation of China (No.
condition. Partial hydrolysis of AlCl
and aluminum complexes as the products. As seen in entries 5–
(Table 1), the 5-HMF yield signicantly decreased with
increasing the usage of HCl, even though the reaction time was
3 2
$6H O could produce HCl
21306186).
7
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