Conversion of carbohydrates into 5-hydroxymethylfurfural catalyzed by ZnCl2 in water

Article abstract of DOI:10.1039/c2cc00122e

The incompletely coordinated zinc ions in the concentrated aqueous ZnCl₂ solution catalyze the direct conversion of carbohydrates into 5-hydroxymethylfurfural, and a moderate HMF yield up to 50% can be achieved.

Full text of DOI:10.1039/c2cc00122e

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COMMUNICATION
Conversion of carbohydrates into 5-hydroxymethylfurfural catalyzed by
ZnCl2 in waterw
Tiansheng Deng,ab Xiaojing Cui,ac Yongqin Qi,a Yinxiong Wang,a Xianglin Hou*a and
Yulei Zhu*ac
Received 7th January 2012, Accepted 11th April 2012
DOI: 10.1039/c2cc00122e
The incompletely coordinated zinc ions in the concentrated aqueous
ZnCl2 solution catalyze the direct conversion of carbohydrates into
5-hydroxymethylfurfural, and a moderate HMF yield up to 50%
can be achieved.
Biomass shows potential as an alternative feedstock to fossil
resources for a variety of chemicals and fuels.1 Carbohydrates
including cellulose and semicellulose are the main components
of biomass. However, they cannot be applied directly as fuels
Scheme 1 Transformation of cellulose into 5-hydroxymethylfurfural
due to their high oxygen contents. Many research efforts are
aimed at exploring routes to simply and efficiently lower their
(HMF).
oxygen contents.2 One of the attractive routes is dehydration of
carbohydrates to produce platform chemical 5-hydroxymethyl-
It is reported that the catalytic effects of metal ions stem from the
fact that they could coordinate with the hydroxyl groups of glucose
furfural (HMF).3
Presently, production of HMF is primarily started from
to form an enediol structure, which is considered as the key
intermediate in the isomerization of glucose to fructose.8–12,14 This
carbohydrates such as fructose, glucose and cellulose. HMF
suggested to us that Zn2+ ions could play the same role in
production from fructose is effortless and high HMF yields
catalyzing the conversion of glucose to HMF, since the Zn2+ ions
in the concentrated aqueous ZnCl2 solutions could coordinate with
two neighboring hydroxyl groups of carbohydrates.15,16 We
also noticed that the aqueous ZnCl2 solution with a concentration
Z60 wt% can dissolve cellulose well by forming a Zn–cellulose
complex.17 Besides, it exhibits strong acidity which would enable
the hydrolysis of cellulose and the dehydration of fructose to
HMF. In addition, it is much less expensive than ionic liquids
and organic reaction media, and it is far less toxic. Hence, it shows
potential as a green and economical reaction medium in the
industrial production of HMF from bio-based carbohydrates
(Scheme 1). Herein, we report using concentrated aqueous ZnCl2
solution as the reaction medium to produce HMF in a single step
from carbohydrates including cellulose, glucose, sucrose, maltose,
starch and fructose.
can be obtained,4 whereas effective production of HMF from
glucose is challenging because the isomerization of glucose
into fructose is usually achieved in the reaction systems with
powerful basicity5 while the dehydration of fructose into HMF
demands an acidic environment.6 Two strategies for this problem
include: (1) the application of acid–base solid catalysts7 and (2) the
introduction of metal salt catalysts including lanthanide ions,8
chromium ions,9 germanium ions,10 stannum ions11 and aluminium
ions.12 It is more challenging to convert cellulose into HMF since
dissolution is required beforehand. Ionic liquids and N,N-dimethyl-
acetamide containing lithium chloride (DMA–LiCl) are capable of
dissolving cellulose well9 and hence applied as the reaction media to
convert cellulose into HMF. Despite their effectiveness toward
HMF production, they are expensive.13 Therefore, it is still
indispensable to develop a more economical reaction system
for the large-scale production of HMF.
In 55–65 wt% aqueous ZnCl2 solutions, the conversion of
glucose was high whereas the HMF yield was relatively low
(Table 1). Humins that are produced from glucose and their
dehydration products were also observed (Table S1, ESIw),4
whereas levulinic acid was not detected. The reactivity of glucose
towards HMF was also explored in a 1.6 wt% HCl aqueous
solution whose acidity is identical to that of 63 wt% ZnCl2.
However, a negligible HMF yield was obtained. This indicates that
the conversion of glucose to HMF is catalyzed by the Zn2+ ions.
Previous studies indicated that the Zn2+ ions in aqueous ZnCl2
solutions could coordinate with oxygen atoms of carbohydrates to
a Institute of Coal Chemistry, Chinese Academy of Sciences,
Taiyuan, People’s Republic of China. E-mail: houxl@sxicc.ac.cn,
zhuyulei@sxicc.ac.cn; Fax: +86 351 4041153;
Tel: +86 351 4049501
b Graduate University of Chinese Academy of Sciences,
Beijing, People’s Republic of China
c State Key Laboratory of Coal Conversion,
Institute of Coal Chemistry, Chinese Academy of Sciences,
Taiyuan, People’s Republic of China
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc00122e
c
5494 Chem. Commun., 2012, 48, 5494–5496
This journal is The Royal Society of Chemistry 2012

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Table 1 Conversion of glucose and cellulose to HMFa
The formation of the above-mentioned complex in high
concentrated ZnCl2 solutions should have resulted from
the coordination state of Zn2+ ions. The Zn2+ ions could
coordinate with groups/ligands containing electron pairs. In a
low concentrated ZnCl2 solution, H2O molecules are abundant,
like in the case of the 32 wt% one which has a H2O : ZnCl2 molar
ratio of 16 : 1. A Zn2+ ion can readily combine with 6 H2O
Solvent,
Substrate wt%
Cocatalystb,
mol/mol
Conv. HMF yield/
mol%
T/1C (%)
Glucose
ZnCl2, 32
ZnCl2, 63
ZnCl2, 55
ZnCl2, 60
ZnCl2, 63
ZnCl2, 65
HCl, 1.6
ZnBr2, 76
ZnBr2, 44
ZnCl2, 63 FeCl2, 1 : 1
98
98
Trace Trace
81.7
94.3
95.7
99.7
98.3
Trace Trace
82.3 29.1
Trace Trace
96.4
95.6
96.5
93.3
94.3
80.5
60.5
88.5
97.6
96.7
96.1
13.0
13.8
14.9
16.1
15.7
110
110
120
120
98
120
98
120
molecules to reach a complete coordination sphere, [Zn(H2O)6]2+
.
These coordinated H2O are unlikely to be replaced by carbo-
hydrates, since the coordination between the hydroxyl group(s)
of carbohydrates and zinc is weaker than that between H2O and
zinc. This implies that [Zn(H2O)6]2+ is ‘‘inert’’ toward carbo-
hydrates. While in concentrated ZnCl2 solutions, there are not
enough H2O molecules for all of the Zn2+ ions to reach the
complete coordination. For example, the H2O : Zn molar ratio
of the 63 wt% ZnCl2 solution is 4.5, indicating the incomplete
coordination of the Zn2+ ions in this solution. The incompletely
coordinated Zn2+ ions are able to coordinate with one or more
hydroxyl groups of carbohydrate. The interaction of incompletely
coordinated Zn2+ ions with H2O and glucose molecules was
supported by IR characterization (Fig. S3–S5, ESIw). It is also
reasonable to deduce that the possibilities for a Zn2+ ion to
coordinate with two hydroxyl groups of a carbohydrate would
be largely enhanced with the increase of the ZnCl2 concentration.
This is likely to be the reason for the observation of such
coordination between Zn2+ and carbohydrate in a 67 wt% ZnCl2
solution whose H2O : ZnCl2 molar ratio is approximately 3.7.16
Furthermore, the effects of HCl and several metal ions on
HMF yield were investigated (Table 1). The addition of HCl
promoted the HMF yield. The added chromium also improved
the HMF yield, but its promotional effect is less pronounced. The
introduction of Fe2+, Mn2+ and Li+ decreased the HMF yield.
To understand the promotional effect of HCl on the HMF yield,
the kinetics of glucose conversion in the 63 wt% aqueous ZnCl2
solution was studied, both alone and with HCl (for details see
ESIw). The added HCl increased drastically the value of k/kg,
indicating that HCl promotes glucose selectively converting
into HMF (Table 2). However, an expected large improvement
in the HMF yield was not observed. This is because the degradation
of HMF was not slowed down by HCl, as indicated by the slight
change in the k0. From this point of view, a satisfactory HMF yield
could be obtained by suppressing the degradation of HMF.
To achieve this goal, a two-phase method was carried out in
this work aiming at transferring the formed HMF away from
the reaction medium as soon as possible (details in ESIw). The
ZnCl2 reaction system that showed the maximal HMF yield
was first chosen as the aqueous phase. It is surprising to find
that the introduction of organic phase drastically decreased the
HMF yield from 32.3% to 5.2% (Fig. 1a). After the reaction,
Zn2+ ions were detected in the organic phase, indicating the
15.8
14.1
14.2
15.3
25.5
32.3
12.3c
25.6
17.7
25.4
24.9
11.5
15.0
22.1
30.4d
20.8
15.3
53.3
38.8
32.8
29.3
ZnCl2, 63 MnCl2, 1 : 1 120
ZnCl2, 63 LiCl, 1 : 1
ZnCl2, 63 LiBr, 1 : 1
ZnCl2, 63 HCl, 0.8 : 1
ZnCl2, 63 HCl, 1 : 1
ZnCl2, 63 HCl, 1 : 1
ZnCl2, 63 HCl, 1.1 : 1
ZnCl2, 63 CuCl2, 1 : 1
120
120
120
120
120
120
120
ZnCl2, 63 CrCl3, 0.5 : 1 120
ZnCl2, 63 CrCl3, 0.3 : 1 120
Cellulose ZnCl2, 63
120
ZnCl2, 63 CrCl3, 0.6 : 1 120
ZnCl2, 63 HCl, 1 : 1
ZnCl2, 63 HCl, 1 : 1
ZnCl2, 63 HCl, 0.8 : 1
120
120
120
120
120
120
120
120
ZnCl2, 63 HCl, 1.2 : 1
Fructose ZnCl2, 63 HCl, 1 : 1
97.3
96.5
95.2
Sucrose
Maltose
Starch
ZnCl2, 63 HCl, 1 : 1
ZnCl2, 63 HCl, 1 : 1
ZnCl2, 63 HCl, 1 : 1
a
Reaction system: 100 g ZnCl2 aqueous solution, 2 g carbohydrate.
Cocatalyst loading is relative to moles of monosaccharide in carbo-
b
c
hydrates. The glucose concentration is 15 wt%. The reaction system
d
was treated at 70 1C for 12 h before reaction.
form different complexes, and their coordination state was depen-
dent on the concentration of ZnCl2. At high concentrations, a
complex with a Zn2+ ion coordinating with two neighboring
hydroxyl groups of a carbohydrate was reported.15,16 Noticeably,
the concentration of ZnCl2 solution in the previous work is
67 wt%,16 which is at the same level as our work. Therefore, it is
reasonable to deduce that such a complex could also form in our
concentrated aqueous ZnCl2 solutions, which facilitates the iso-
merization of glucose into fructose and consequently the synthesis
of HMF. This assumption was proved by the increase and decrease
of fructose as a function of reaction time in the 63 wt% ZnCl2
solution during the conversion of glucose (Fig. S2, ESIw). To
further verify this hypothesis, the conversion of glucose to HMF
was tested in a 32 wt% aqueous ZnCl2 solution. This was based on
acknowledgement that the above-mentioned complex is unable to
form in low-concentrated ZnCl2 solutions. As expected, the con-
version of glucose and the HMF yield in the 32 wt% ZnCl2
solution are negligible (Table 1), and no fructose was detected
during the reaction. This assumption was further proved by a
negligible HMF yield in the 44 wt% ZnBr2 solution whose
H2O : Zn molar ratio is identical to that of the 32 wt% ZnCl2
solution (16 : 1). Noticeably, a higher HMF yield was obtained in
the 76 wt% ZnBr2 solution whose H2O : Zn molar ratio (4 : 1) is
similar to that of the 65 wt% ZnCl2 one. This result indicates that
although the halogen ions alone show no catalytic activity, they
could affect the catalytic performances of the active Zn2+ ions.
Table 2 Effect of HCl on the reaction rate constantsa
d
HCl/glucose mol/mol
kgb/minꢀ1
kc/minꢀ1
k0 /minꢀ1
k/kg
0
1 : 1
0.0904
0.0146
0.0397
0.0134
0.0051
0.0044
0.44
0.92
a
b
Reaction temperature: 120 1C. kg: rate constant of glucose conver-
c
d
sion. k: rate constant of HMF formation. k0: rate constant of HMF
degradation.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5494–5496 5495

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In summary, the Zn2+ ions with incomplete coordination
can be created by simply modifying the concentration of
ZnCl2 in water. These Zn2+ ions could coordinate with the
carbohydrates and catalyze their transformation into fructose
and further into HMF. The HMF yield for bio-based carbo-
hydrates ranges from 30% to 50% under a mild reaction
condition. Especially, using the two-phase method a HMF yield
of 40% is obtained when the carbohydrate content in the
aqueous phase is high (15 wt%). Furthermore, active carbon is
applied to efficiently remove the byproducts and recycle the
concentrated aqueous ZnCl2 solution. All these results indicate
the potential of the concentrated aqueous ZnCl2 solution as a
green and economical reaction solution to produce HMF from
bio-based carbohydrates on a large scale.
Fig. 1 Effect of glucose concentration in the aqueous solution on
the HMF yield in two-phase reaction system: (a) 2 wt%; (b) 10 wt%;
(c) 12.5 wt%; (d) 15 wt%. Reaction conditions: 150 ml MIBK, 20 g 63 wt%
ZnCl2 aqueous solution, HCl/glucose = 1 : 1 (mol : mol), T = 120 1C.
This work was financially supported by the Major State
Basic Research Development Program of China (973 Program)
(No. 2012CB215305).
transfer of Zn2+ ions from aqueous phase to organic phase.
However, this transfer of Zn2+ ions was not observed in the
32 wt% ZnCl2 aqueous phase–organic phase system. This
phenomenon implies that only the Zn2+ ions with a certain
coordination state, namely, the incompletely coordinated
Zn2+ ions, are able to coordinate with the oxygen atoms in
methylisobutylketone (MIBK) and hence ‘‘dissolve’’ in the
organic phase. They could catalyze the degradation of HMF
in MIBK, and consequently lead to a lower HMF yield.
Hence, it is of vital importance to ‘‘lock’’ the Zn2+ ions in
the aqueous phase to improve the HMF yield. To solve this
problem, we increase the glucose content in the aqueous phase,
and reason that the Zn2+ ions with incomplete coordination
could be saturated by glucose molecules and thus be locked in
the aqueous phase, as the coordination between glucose and
Zn2+ ions is stronger than that between MIBK and Zn2+ ions.
This assumption was confirmed by the substantial improvement
in HMF yield with the gradual increase in the glucose content in
the aqueous phase (see Fig. 1b–1d). A HMF yield of 40% can be
reached with a glucose concentration of 15 wt%, and maintained
at this level (Fig. 1d).
Notes and references
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HMF yield to 30.4%. The incompletely coordinated Zn ions in
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c
5496 Chem. Commun., 2012, 48, 5494–5496
This journal is The Royal Society of Chemistry 2012