Conversion of xylose and xylan into furfural in biorenewable choline chloride-oxalic acid deep eutectic solvent with the addition of metal chloride

Article abstract of DOI:10.1016/j.cclet.2014.03.029

An environmentally benign processing approach for furfural production from xylose and xylan under very mild conditions (353-373 K) was developed with the addition of metal chlorides in ChCl-oxalic acid (a deep eutectic solvent (DES)) synthesized from cheap and renewable starting materials). ChCl-oxalic acid acted as both a Br?nsted acid catalyst and a reaction medium in this catalytic route. In addition, a biphasic system with methyl isobutyl ketone as an extracting reagent (DES/MIBK) to further increase furfural yield was also proposed. This processing approach for producing furfural eliminated the large energy consumption for high pressure saturated steam and the generation of acidic effluent, which was very difficult to handle. The whole catalytic system was more environmentally friendly compared with the commercial process for furfural production.

Full text of DOI:10.1016/j.cclet.2014.03.029

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CCLET-2915; No. of Pages 5
Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Conversion of xylose and xylan into furfural in biorenewable choline
chloride–oxalic acid deep eutectic solvent with the addition of metal
chloride
a
b,
Lu-Xin Zhang a, Han Yu a, Hong-Bing Yu a, , Ze Chen , Lei Yang
*
*
a College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China
b College of Chemistry, Nankai University, Tianjin 300071, China
A R T I C L E I N F O
A B S T R A C T
Article history:
An environmentally benign processing approach for furfural production from xylose and xylan under
very mild conditions (353–373 K) was developed with the addition of metal chlorides in ChCl–oxalic acid
(a deep eutectic solvent (DES)) synthesized from cheap and renewable starting materials). ChCl–oxalic
acid acted as both a Brønsted acid catalyst and a reaction medium in this catalytic route. In addition, a
biphasic system with methyl isobutyl ketone as an extracting reagent (DES/MIBK) to further increase
furfural yield was also proposed. This processing approach for producing furfural eliminated the large
energy consumption for high pressure saturated steam and the generation of acidic effluent, which was
very difficult to handle. The whole catalytic system was more environmentally friendly compared with
the commercial process for furfural production.
Received 22 November 2013
Received in revised form 4 February 2014
Accepted 26 February 2014
Available online xxx
Keywords:
Furfural
Xylose
Xylan
ß 2014 Hong-Bing Yu and Lei Yang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All
rights reserved.
DES
1. Introduction
on the development of a variety of different catalysts [4,5] and
reaction medium [6,7]. Besides, systems using simultaneous
As petrochemical resources deplete in an alarming rate,
producing fuel and other high-value chemicals from renewable
biomass has generated sustained interest worldwide. It is an
important and challenging path for biomass utilization using
selective catalytic routes to process the carbohydrate fractions of
lignocellulose to produce valuable platform chemicals [1]. Furfural,
derived from lignocellulosic biomass, is a very versatile and key
bio-based platform chemical used for the production of important
non-petroleum-derived chemicals and the development of new
generation of bioplastics [2]. Besides, it is a biofuel precursor and
also in high demand in industries including plastics, petroleum
refining, agrochemical and pharmaceutical industries [3]. Indus-
trial process for furfural production via mineral acid treatment of
hemicellulose and high pressure saturated steam stripping of
furfural suffered from disadvantages such as low efficiency, high
energy consumption, equipment corrosion, severe yield loss due to
side-reactions and the generation of acidic effluent, etc. Alternative
approaches for furfural production investigated recently focused
stripping (e.g., N2 stripping [8]) or extraction (e.g., water/organic
solvent biphasic systems [9–11]) for furfural have also been
proposed. All the above-mentioned approaches have improved
furfural yield and selectivity. However, the production of furfural
with minimal carbon footprint route is still
a challenging
undertaking. Water is definitely a cheap and green solvent for
furfural formation. However, the use of water as reaction medium
suffers from disadvantages such as low furfural yields and the
generation of a large amount of acidic effluent. Recently, due to the
negligible vapour pressure, ionic liquids have been widely
considered as ‘‘green’’ solvents instead of water or other organic
solvents for many reactions including furfural production
[7,12,13]. Besides, other unique properties of ionic liquid such
as high thermal stability, non-flammability, and good solvent
power also make ionic-liquid-phase reactions useful for producing
biofuels and biochemicals. However, the inherent toxicity of
commonly used ILs has not been investigated thoroughly. More
importantly, the application of commonly used ionic liquids on
industrial scale is limited, owing to their high costs. More recently,
deep eutectic solvents (DES) have drawn increasing attention and
are revealed to be a promising alternative to ILs as green media
[14,15]. DESs are mixtures of two or three compounds that are
capable of forming eutectic liquids, in which choline chloride
*
Corresponding authors.
E-mail addresses: hongbingyu1130@sina.com (H.-B. Yu),
yangleishanxi@2008.sina.com (L. Yang).
http://dx.doi.org/10.1016/j.cclet.2014.03.029
1001-8417/ß 2014 Hong-Bing Yu and Lei Yang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: L.-X. Zhang, et al., Conversion of xylose and xylan into furfural in biorenewable choline chloride–oxalic
acid deep eutectic solvent with the addition of metal chloride, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.03.029

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(ChCl) is a commonly used component [16]. Deep eutectic solvents
share many characteristics with ILs and have added advantages of
low price, low toxicity, bio-degradability, environmental friendli-
ness, ease to prepare on large scale and the elimination of the
preliminary purification step. ChCl-based DESs could be obtained
by mixing choline chloride with substances having hydrogen bond
donors (urea, carboxylic acids, alcohols, etc.). In this manuscript,
ChCl–oxalic acid, characterized by low toxicological risks, low
pollution, reduced mobility, bulk renewable and acidic property,
was selected as an alternative media for ‘‘greener’’ furfural
production processes.
analyzed using HPLC. Reactions heated by microwave irradiation
were conducted according to the procedures reported in our
previous work [12].
2.3. Procedure for the reaction in the DES/MIBK biphasic system
Experiments were carried out in 10 mL sealed thick-walled
glass vessels in a preheated oil bath with magnetic stirring. In a
typical experiment, the reactor was charged with required
amounts of feedstock, metal chloride, ChCl–oxalic acid and MIBK,
these materials were then mixed using a magnetic stirrer and
heated to the desired temperature and time. After reaction, the
reactor was cooled to room temperature by cooling water
immediately. The DES phase and organic phase were separated
and collected for analysis.
As for the catalysts employed in the production of furfural, both
Brønsted acid and Lewis acid catalysts catalyze the reactions in
furfural formation [17]. Recently, metal chloride as catalyst
[12,18,19] or additive [20,21] for the conversion of lignocelluloses
into furfural have been reported. Metal chloride clearly accelerated
the reaction rate of the furfural formation from xylose and it was
found that both metal cation and Clꢀ ion are responsible for this
reaction [20,22]. Additionally, although it had been established
that metal chlorides had pronounced influence on xylose
isomerization, the presence of Brønsted acid was recently found
to facilitate both the dehydration reactions and the furfural
selectivity [20]. Thus the combination of a metal chloride and an
acid would probably achieve better furfural yields. The ChCl–oxalic
acid mixture could play the role of both reaction medium and
Brønsted acid catalyst, and it is also a Clꢀ ion provider. In addition,
industrial processes for furfural production are commonly carried
out at temperatures >423 K. If the reactions could be conducted at
lower temperatures, energy saving could be substantial. Based on
all the reasons above, and with the aim to produce furfural in a
milder and more environmentally friendly manner, metal chlor-
ides enhanced production of furfural in ChCl–oxalic acid under
mild conditions were investigated in this work. Moreover, one
approach to inhibit the formation of humins and thus promote
furfural yield and selectivity is to extract furfural simultaneously
using an organic solvent as extractant (low boiling solvent is
energetically more advantageous, such as MIBK). Therefore
reactions in a DES/MIBK biphasic system was also studied in this
work to selectively extract furfural from the DES phase to organic
phase in order to further enhance the furfural yield.
2.4. Quantification procedure for furfural and xylose
Furfural was determined using HPLC (Agilent 1200) with an
Ultraviolet Detector and a XDB-C18 column at 280 nm. The column
oven temperature was maintained at 303 K. The mobile phase was
acetonitrile/water (15/85, v/v) at
a flow rate of 1 mL/min.
Quantitative analysis of xylose was performed using HPLC (Waters
1525) equipped with a refractive index detector (Waters 2412) and
an aminex HPX-87H column. The column oven temperature was
maintained at 338 K. H2SO4 (5 mmol/L) was used as the mobile
phase at a flow rate of 0.6 mL/min. For HPLC analysis, the samples
were filtered with a syringe filter (0.2
Conversion of xylose and yields of products were defined as
follows:
mm) prior to analysis.
moles of xylose reacted
mole of starting xylose
xylose conversion ¼
ꢃ 100%
(1)
moles of furfural produced
moles of starting xylan
furfural yield ðfrom xylanÞ ¼
ꢃ 100%
(2)
(3)
moles of furfural produced
moles of starting xylose
furfural yield ðfrom xyloseÞ ¼
ꢃ 100%
2. Experimental
2.1. Materials
3. Results and discussion
Xylan (from birch wood, ꢁ90%) was purchased from Sigma–
We started our studies by investigating the effect of trivalent
metal chlorides in ChCl–oxalic acid in converting xylose into
furfural at 373 K. CrCl3ꢂ6H2O, FeCl3ꢂ6H2O, AlCl3ꢂ6H2O, CeCl3ꢂ7H2O
and LaCl3ꢂ7H2O were selected as a co-catalyst. As shown in Fig. 1,
furfural was formed in ChCl–oxalic acid without an additional
metal chloride affording a furfural yield of 14.6%, indicating that
DES acted as both a Brønsted acid catalyst and a reaction media.
The furfural yields could be improved by combining a metal
chloride as the co-catalyst. Amongst all the trivalent metal
chlorides tested, AlCl3ꢂ6H2O proved most efficient in producing
furfural from xylose, with a 32.4% furfural yield achieved at 74.3%
xylose conversion at 373 K in 30 min. For other trivalent metal
chlorides, the furfural yields obtained under the typical reaction
conditions followed the order of FeCl3ꢂ6H2O > CrCl3ꢂ6H2O >
CeCl3ꢂ6H2O > LaCl3ꢂ7H2O, leading to furfural yields and xylose
conversions in the range of 16%–23% and 61–69%, respectively.
The mechanism of furfural formation from xylose is still
debated and has not been unequivocally established. Specifically,
furfural production mechanism may differ when the reaction was
catalyzed by different catalysts under different reaction condi-
tions. In this manuscript, a double catalytic effect was achieved by
Aldrich Ltd.
D
-Xylose (ꢁ98%) and choline chloride (ꢁ99%) were
purchased from Acros. CrCl3ꢂ6H2O, FeCl3ꢂ6H2O, AlCl3ꢂ6H2O,
CeCl3ꢂ7H2O, LaCl3ꢂ7H2O and oxalic acid were purchased from
Tianjin Jiangtian Chemical Co., Ltd. (Tianjin, China). All other
chemicals were purchased from Sigma–Aldrich and used without
further purification. ChCl–oxalic acid was synthesized according to
procedures reported in the literature [23,24]. The molar ratio of
choline chloride to oxalic acid was following the report by Hu et al.
[24].
2.2. Procedure for the reaction in the monophase system
Substrate sample (0.25 mmol), ChCl–oxalic acid (6 mmol based
on the acid) and a known amount of metal chloride were loaded
into a sealed glass reactor (10 mL). The mixture was then heated in
a preheated oil bath and stirred at different temperatures for the
desired time. When using xylan as the starting material, 10 mg of
H2O was added into each reactor. After the desired residence time,
the reaction was quenched by putting the reactor in cooling water
immediately. Samples were then diluted, filtered and then
Please cite this article in press as: L.-X. Zhang, et al., Conversion of xylose and xylan into furfural in biorenewable choline chloride–oxalic
acid deep eutectic solvent with the addition of metal chloride, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.03.029

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3
Table 1
Furfural formation from xylose in ChCl–oxalic acid in the addition of AlCl3ꢂ6H2O.a
Entry
Temperature
(K)
Reaction
Xylose
Furfural
yield (%)
time (min)
conversion (%)
1
2b
3
353
353
353
353
363
363
363
373
373
373
373
90
90
120
60
40
60
70
35
40
50
40
59.9 ꢄ 1.3
<1
14.1 ꢄ 0.2
Not detected
19.1 ꢄ 0.1
30.3 ꢄ 1.3
19.5 ꢄ 0.5
27.1 ꢄ 0.1
25.4 ꢄ 0.4
37.2 ꢄ 0.3
44.8 ꢄ 0.2
41.3 ꢄ 0.4
39.1 ꢄ 1.2
74.1 ꢄ 0.5
68 ꢄ 1.6
4c
5
62.1 ꢄ 0.4
79.1 ꢄ 1.2
86.7 ꢄ 1.6
79.3 ꢄ 0.4
84.8 ꢄ 0.6
92.3 ꢄ 1.5
90.7 ꢄ 0.5
6
7
8
9
10
11d
a
Reaction conditions: 6 mmol ChCl–oxalic acid, 0.25 mmol xylose, 0.125 mmol
AlCl3ꢂ6H2O.
b
c
Water was used as reaction medium.
Heated by microwave irradiation.
0.25 mmol AlCl3ꢂ6H2O was employed.
d
Fig. 1. Conversion of xylose into furfural in ChCl–oxalic acid in the addition of metal
chlorides. Reaction conditions: 6 mmol ChCl–oxalic acid, 0.25 mmol xylose,
0.125 mmol metal chlorides, 373 K, 30 min.
(Table 1, entry 3). Additionally, microwave irradiation resulted
in improved results (68% xylose conversion, 30.3% furfural yield in
1 h at 353 K) due to this heating method offered a means of rapid,
uniform, and selective heating, which resulted in accelerated
reaction rates [30,31]. Compared with traditional heating methods,
microwave method minimizes temperature gradients and has the
advantage of higher yields in a short reaction time. However, the
application of microwave irradiation on industrial scale remained
difficult, thus the subsequent experiments were conducted by
conventional heating. The yields of furfural improved when the
temperature was increased from 353 K to 373 K (furfural yields
increased from 14% to 44%, with shorter residence time), indicating
that temperature influenced the reaction significantly, and higher
temperature favored furfural formation [32]. In addition, further
increasing AlCl3ꢂ6H2O loading to 0.25 mmol at 373 K, the furfural
yield decreased (Table 1, entry 11) with increased solid formation.
This trend could be due to the fact that more AlCl3ꢂ6H2O
accelerated the formation of furfural but also promoted other
side reactions (mainly furfural degradation reactions such as
fragmentation of furfural, condensation reactions between furfural
and xylose or other intermediates, and resinification of furfural)
resulted in undesired by-products.
adding metal chlorides to ChCl–oxalic acid because both ChCl–
oxalic acid and metal chloride acted as catalyst in the production of
furfural from xylose (Scheme 1). It was known that metal chlorides
promote the enolization of xylose [25], and the presence of metal
chlorides induced xylose dehydration to form furfural via the
xylose isomerization to xylulose [10,26,27], which is much
different from the cases using individual Brønsted acid catalyst.
Metal cation could promote the conversion of xylose to furfural by
bonding with an oxygen atom. In this work, the furfural yields
obtained by different metal chlorides varied when these metal
chlorides were added in equal molar dosage, mainly because metal
cations catalyze the reaction of carbohydrates into furfural
proportionally to their ionization potential [22].
AlCl3ꢂ6H2O produced superior yields in the preliminary tests
and hence was selected for further assessments. The effects of
temperature, residence time on the reaction were studied, and the
results are shown in Table 1. At 353 K, a furfural yield of 14.1% was
obtained in 90 min. However, when water was used as the reaction
medium as a control, furfural was not produced (Table 1, entry 2).
The results indicated that the use of ChCl–oxalic acid as reaction
media improved the dehydration of xylose to furfural at 353 K. One
reason for this observation could be the acidity of the ChCl–oxalic
acid system while another possibility could be that nonaqueous
media was beneficial furfural formation [28,29]. Further increase
of the reaction time at 353 K gave higher furfural yield
Consequently, to further expand the range of feedstock for
furfural production in ChCl–oxalic acid with the addition of metal
chlorides, the conversion of xylan into furfural was investigated.
Specifically, the effect of FeCl3ꢂ6H2O was also compared to that of
AlCl3ꢂ6H2O. The transformation of xylan to furfural was found
unappreciable at <373 K in aqueous media before. Binder et al. [26]
Scheme 1. Putative mechanism for the conversion of xylose into furfural catalyzed by both metal chloride and ChCl–oxalic acid. Me3+ represent trivalent metal cations.
Please cite this article in press as: L.-X. Zhang, et al., Conversion of xylose and xylan into furfural in biorenewable choline chloride–oxalic
acid deep eutectic solvent with the addition of metal chloride, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.03.029

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Table 2
Conversion of xylan into furfural in the addition of metal chlorides in ChCl–oxalic
acid.a
Entry
Reaction time (min)
Co-catalyst
Furfural yield (%)
1
2
50
50
50
70
70
50
None
13.3 ꢄ 0.1
39.8 ꢄ 0.5
22.8 ꢄ 0.3
38.4 ꢄ 0.2
30.1 ꢄ 0.2
34.9 ꢄ 0.2
AlCl3ꢂ6H2O
FeCl3ꢂ6H2O
FeCl3ꢂ6H2O
AlCl3ꢂ6H2O
AlCl3ꢂ6H2O
3
4
5
6b
a
Reaction conditions: 6 mmol ChCl–oxalic acid, 0.25 mmol xylan, 10 mg water,
0.125 mmol metal chloride, 373 K.
b
0.25 mmol AlCl3ꢂ6H2O was used.
reported a moderate furfural yield from xylose with acid and
chromium catalysts and halide additives at 373 K in DMA.
However, when they used similar conditions to convert xylan
into furfural, the yields were very low (1%). In this manuscript, it is
worth noting that moderate furfural yields were achieved from
xylan at 373 K (Table 2). This result indicated the advantage of
ChCl–oxalic acid in terms of xylan (or drawing an analogy to the
xylose polymers) conversion into furfural at relatively low
temperature (e.g., 373 K). As shown in Table 2, without metal
chlorides, a furfural yield of 13.3% from xylan was obtained due to
the acidic characteristics of ChCl–oxalic acid. FeCl3ꢂ6H2O achieved
a comparable furfural yield (38.4%) to that of AlCl3ꢂ6H2O (39.8%) at
equal molar dosage, but with longer residence time (Table 2,
entries 2 and 4). The catalytic effect of metal cation is of great
interest and worth studying in the future. Furfural yield decreased
from 39.8% to 30.1% when the reaction time further prolonged
from 50 min to 70 min in the presence of AlCl3ꢂ6H2O (Table 2,
entries 2 and 5), probably due to the formation of undesired
compounds at longer residence time. Increasing AlCl3ꢂ6H2O dosage
to 0.25 mmol did not result in an increased furfural yield under the
typical reaction conditions (Table 2, entries 2 and 6), mainly due to
side reactions were accelerated by the excess AlCl3ꢂ6H2O.
Fig. 2. Furfural yield in MIBK in the recycle of ChCl–oxalic acid and AlCl3ꢂ6H2O. The
condition was the same as that of entry 1 in Table 3, under the typical reaction
condition, xylose conversion >99%.
was still in the EDS phase. Before reuse, the DES–AlCl3ꢂ6H2O was
dried at 353 K in a vacuum drier to remove water. Subsequently,
the DES–AlCl3ꢂ6H2O phase was reused by adding new substrate
(0.5 mmol xylose) into the reactor directly. The results of recycling
the DES and AlCl3ꢂ6H2O (Fig. 2) indicated that the ChCl–oxalic acid
and AlCl3ꢂ6H2O remained active when they were reused over five
consecutive cycles.
4. Conclusion
In summary, both monophase (DES) and biphase (DES/MIBK)
routes for processing xylose/xylan to produce furfural were
developed in the presence of trivalent metal chlorides in ChCl–
oxalic acid. In the monophase reactions, the addition of metal
chlorides (especially AlCl3ꢂ6H2O) in ChCl–oxalic acid resulted in
improved furfural yields and moderate furfural yields (14%–44%)
Subsequently, we concentrated on how to promote furfural
yield. In order to design a process that provides furfural in higher
yields, side reactions would have to be suppressed. It has been well
studied that biphasic systems facilitated the continuous product
removal by extracting furfural to the organic phase once it is
produced [33] and thus reduced the side reactions and increased
the furfural yield and selectivity. Given all this, biphasic systems
also offered the possibility of a higher initial feedstock loading. So
the biphasic system (DES/MIBK) was investigated in this work for
furfural formation from xylose and xylan by increasing the initial
substrate loading to 0.5 mmol. As expected (Table 3), simultaneous
furfural extraction by MIBK significantly increased the furfural
yields in all cases and the highest furfural yield of 60.4% from
xylose was achieved at 373 K for 60 min. Moreover, in the biphasic
system, ChCl–oxalic acid is not extractable by MIBK and ChCl–
oxalic acid and AlCl3ꢂ6H2O could be reused after separating the
MIBK phase from the reactor and further extracting the product in
the DES using fresh MIBK (3 mL ꢃ 3). After extraction, AlCl3ꢂ6H2O
were obtained under very mild conditions (353–373 K).
A
processing strategy of biphasic system (DES/MIBK) was also
studied, and furfural yields obtained from xylose and xylan in the
presence of AlCl3ꢂ6H2O were 60.4% and 55.5%, respectively. The
DES/AlCl3ꢂ6H2O phase could be reused to maintain a stable
catalytic effect for five successive runs in the biphasic system.
However, ChCl–oxalic acid was less stable above 379 K in this
catalytic process, which appeared to be the key limiting issue of
this approach. The development of acidic green DES with better
thermostability should be emphasized in the future work.
Acknowledgment
The authors gratefully acknowledge the Major National Science
& Technology Projects of China on Water Pollution Control and
Treatment (No. 2012ZX07501002-001) for the financial support.
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Please cite this article in press as: L.-X. Zhang, et al., Conversion of xylose and xylan into furfural in biorenewable choline chloride–oxalic
acid deep eutectic solvent with the addition of metal chloride, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.03.029

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Please cite this article in press as: L.-X. Zhang, et al., Conversion of xylose and xylan into furfural in biorenewable choline chloride–oxalic
acid deep eutectic solvent with the addition of metal chloride, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.03.029