Catalytic conversion of inulin and fructose into 5-hydroxymethylfurfural by lignosulfonic acid in ionic liquids

Article abstract of DOI:10.1002/cssc.201100588

In this work, we found that lignosulfonic acid (LS), which is a waste byproduct from the paper industry, in ionic liquids (ILs) can catalyze the dehydration of fructose and inulin into 5-hydroxymethylfurfural (HMF) efficiently, which is a promising potential substitute for petroleum-based building blocks. The effects of reaction time, temperature, catalyst loading, and reusability of the catalytic system were studied. It was found that a 94.3% yield of HMF could be achieved in only 10 min at 100°C under mild conditions. The reusability study of the LS-IL catalytic system after removal of HMF by ethyl acetate extraction demonstrated that the catalytic activity decreased from 77.4 to 62.9% after five cycles and the catalytic activity could be recovered after simply removing the accumulated humins by filtration after adding ethanol to the LS-ILs. The integrated utilization of a biorenewable feedstock, catalyst, and ILs is an example of an ideal green chemical process.

Full text of DOI:10.1002/cssc.201100588

DOI: 10.1002/cssc.201100588
Catalytic Conversion of Inulin and Fructose into
5-Hydroxymethylfurfural by Lignosulfonic Acid in Ionic
Liquids
Haibo Xie,*[a, b] Zongbao K. Zhao,[a, b] and Qian Wang[a]
In this work, we found that lignosulfonic acid (LS), which is
a waste byproduct from the paper industry, in ionic liquids
(ILs) can catalyze the dehydration of fructose and inulin into 5-
hydroxymethylfurfural (HMF) efficiently, which is a promising
potential substitute for petroleum-based building blocks. The
effects of reaction time, temperature, catalyst loading, and re-
usability of the catalytic system were studied. It was found
that a 94.3% yield of HMF could be achieved in only 10 min at
1008C under mild conditions. The reusability study of the LS–IL
catalytic system after removal of HMF by ethyl acetate extrac-
tion demonstrated that the catalytic activity decreased from
77.4 to 62.9% after five cycles and the catalytic activity could
be recovered after simply removing the accumulated humins
by filtration after adding ethanol to the LS–ILs. The integrated
utilization of a biorenewable feedstock, catalyst, and ILs is an
example of an ideal green chemical process.
Introduction
The development of the sustainable chemical industry requires
the use of sustainable chemical feedstocks, green solvents,
green catalysts, and sustainable energy sources for the produc-
tion of valuable products through chemical reactions; these
are key points of the recognized 12 principles of green chemis-
try.[1]With increasing concerns about global warming and
energy security due to the overdependence on nonrenewable
fossil resources, biomass, which is the largest renewable
carbon source on the planet, has been regarded as an impor-
tant alternative source for the production of fuels, chemicals,
and materials.[2] Taking lessons from chemical production from
fossil resources, currently, greenness is considered as one of
the important aspects of new strategies for biomass conver-
sion. For example, 5-hydroxymethylfurfural (HMF) has been
recognized as a versatile and key precursor for the production
of fine chemicals, polymeric materials, and biofuels.[3] In recent
years, the preparation of HMF through the dehydration of bio-
mass-based sugars has received much attention, especially in
ionic liquids (ILs), due to the good solubility of carbohydrates
in ILs.[3] Since Zhao et al.[4] reported that chromium(II) chloride
could catalyze the dehydration of glucose into HMF in 1-ethyl-
3-methylimidazolium chloride ([Emim]Cl) with a good yield,
much effort has been devoted to developing more efficient
and environmentally friendly processes for the conversion of
cellulose and glucose into HMF.[5] Compared to glucose and
other carbohydrates, the production of HMF from fructose has
high efficiency and selectivity. Fructose can be obtained by in-
dustrial isomerization of glucose or hydrolysis of inulin, which
exists in many plants, such as in the roots of chicory.[6] There-
fore, research into the production of HMF from fructose has
been studied extensively for a long time. Many catalysts, such
as acidic resin,[7] Lewis acids,[8] mineral acids,[9] organic carbox-
ylic acids,[5a] and acidic ILs,[10] have been applied to this conver-
sion successfully. Regardless of these developments, it remains
a challenge to obtain HMF from carbohydrates in terms of effi-
ciency and greenness. With the aim of developing a green
pathway to use biomass in our lab, we are devoted to using
green solvents and developing new green catalysts for this
purpose.[11]
Lignosulfonic acid (LS) is also called sulfonated lignin, which
is a highly cross-linked polyphe-
nolic polymer containing sulfonic
acid groups. It is an abundant re-
newable resource and a waste by-
product from the paper industry
(Figure 1).[12] The development of
high-value applications of LS or
lignin is one of the most impor-
tant directions of modern biorefi-
nery.[13] Recent research demon-
strated that they can be used as
Figure 1. Representative
proton-transfer materials for fuel
cells[14] or electrochemistry materi-
als due to their sulfonic acid func-
structure of LS.
tionality.[15] However, as an acidic renewable polymer, the appli-
cation of LS as an acidic catalyst is seldom reported.
Herein, we report a novel application of LS as an acidic cata-
lyst for the dehydration of fructose and inulin into HMF in 1-
butyl-3-methylimidazolium chloride ([Bmim]Cl) (Scheme 1). The
combination of using biorenewable resources as feedstocks
[a] Prof. H. Xie, Prof. Z. K. Zhao, Q. Wang
Dalian Institute of Chemical Physics
CAS, 457 Zhongshan Road Dalian (P.R. China)
Fax: (+86)411-84379211
E-mail: hbxie@dicp.ac.cn
[b] Prof. H. Xie, Prof. Z. K. Zhao
Dalian National Laboratory for Clean Energy
Dalian 116023 (P.R. China)
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H. Xie et al.
zation of HMF,[18] as evidenced
by the formation of visually ob-
served black humins. The yields
were sustained at around 85%
after 15 min.
It was found that the applica-
tion of [Bmim]Cl as a solvent
was significant and promoted
the dehydration of fructose into
HMF. Other conventional sol-
vents, such as DMA, DMSO, and
NMP, were also relatively good
because the dehydration reac-
tion was slower (Figure 2). HMF
yields of 50.9, 30.9, and 16.6%
are achieved in 10 min under
identical conditions in DMSO,
NMP, and DMA, respectively.
Scheme 1. Conversion of fructose and inulin into HMF catalyzed by LS in ILs.
A maximum HMF yield of 79.6%
was obtained in DMSO after
40 min. Clearly, NMP and DMA
and catalysts, using green solvents, and the production of
valuable chemicals presents an ideal green chemical process.
are less effective in this study with HMF yields of 52.7 and
39.6%, respectively, after 60 min. As shown in Figure 3, ion
chromatography analysis demonstrated that the conversion of
fructose and selectivity to HMF were 98.3 and 95.0%, respec-
tively, after 10 min at 1008C in ILs, and the dehydration of fruc-
tose into HMF was almost quantitative. As the yields decreased
Results and Discussion
Lignin is an amorphous, polyphenolic polymer arising from the
polymerization of phenylpropanoid monomers, including coni-
feryl, sinapyl, and p-coumaryl alcohol. It was found that ILs of
the [Bmim]+ cation with a range of anions, such as [MeSO4]À,
ClÀ, BrÀ, and PF6À, had good solubility to kraft pulp lignin. The
solubility was up to 312 gLÀ1 at 508C in [Bmim][MeSO4],
whereas the solubility decreased to 13.9 gLÀ1 in [Bmim]Cl even
at 758C.[16] Lignosulfonic acid is sulfonated lignin; in this study,
it was found that it was easy to prepare a 2.5 wt% solution of
LS in [Bmim]Cl at 1008C in 5 min and a brown–red homogene-
ous solution was obtained. However, an increase in LS concen-
tration from 2.5 to 4.0 wt% required more time. Elemental
analysis of the LS showed that there was one sulfonic acid
group in every two C9 lignin representative units; therefore,
the LS can be regarded as an acidic polyphenolic polymer with
a degree of substitution of 0.5. The full solubilization of LS in
[Bmim]Cl results in an acidic solution, which is anticipated to
be potential reaction media for acid-catalyzed conversions. The
dehydration of carbohydrate to produce valuable chemicals,
such as HMF, is a typical acid-catalyzed process;[3] furthermore,
the catalytic conversion of carbohydrate is attracting extensive
attention because of the good solubility of carbohydrates in
ILs and the unique properties of ILs for catalysis.[17] The first
thought that comes to mind is whether LS is an effective cata-
lyst for the dehydration of carbohydrates. As shown in
Figure 2, an HMF yield of 88.5% was achieved in only 5 min
when 10 wt% of fructose was added into the 2.5 wt% LS–
[Bmim]Cl solution at 1008C. Prolonging the reaction time to
10 min resulted in an increase in HMF yield of 93.4%. However,
further prolongation of the reaction time led to decreased
yields; this was likely to result from degradation and polymeri-
Figure 2. The effect of solvents on the catalytic conversion of fructose into
HMF by LS. Experimental conditions: solvent (2.0 g), fructose (0.2 g), LS
(0.05 g), 1008C. DMA=dimethylacetamide, DMSO=dimethylsulfoxide, and
NMP=N-methyl-2-pyrrolidone.
Figure 3. HMF yields, selectivity, and conversion of fructose catalyzed by LS
in ILs. Experimental conditions: [Bmim]Cl (2.0 g), fructose (0.2 g), LS (0.05 g),
1008C.
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Catalytic Conversion of Inulin and Fructose
tion. Further increasing the catalyst loading to 80 mg resulted
in a decrease in HMF yield to 86.7%; this demonstrated that
the higher catalyst concentration could facilitate both the de-
hydration of fructose and the degradation of HMF. Although
the conversion of fructose increased to 98.8%, the selectivity
decreased to 87.8%.
The effect of temperature was also studied (Figure 6). Lower
reaction rates were observed when the reaction was per-
formed at 808C, for example, an HMF yield of only 53.7% was
Figure 4. Conversion of fructose, selectivity, and yields of HMF in different
solvents. Experimental conditions: solvent (2.0 g), fructose (0.2 g), LS (0.05 g),
1008C, 40 min.
after 10 min, the selectivity also decreased to 85.9% after
40 min.
The conversion of fructose in DMSO, NMP, and DMA was
consistent with the trends in HMF yields in these solvents and
a series of comparative results are shown in Figure 4. It was
found that although the reaction rate was slower in DMSO
than that in [Bmim]Cl, a satisfactory conversion of 97.4% and
a moderate selectivity of 81.7% were achieved in 40 min,
which led to a yield of 79.6%. Although the conversion of fruc-
tose was up to 91% in NMP, an HMF yield of only 52.4% was
obtained due to a low selectivity of 57.6%. In DMA under iden-
tical conditions, only 72.4% conversion and 48.3% selectivity
were obtained; these values were consisted with the low HMF
yield of 35.0%. The results demonstrate that [Bmim]Cl and
DMSO can suppress the formation of humins during the dehy-
dration of fructose into HMF catalyzed by LS; furthermore, ILs
can facilitate the dehydration process, which presents a faster
dehydration reaction than that in traditional organic solvents
and is consistent with previous publications.[19]
Figure 6. The effect of reaction temperature on the yield of HMF from fruc-
tose catalyzed by LS. Experimental conditions: [Bmim]Cl (2.0 g), fructose
(0.2 g), LS (0.05 g), 1008C, 10 min.
obtained in 10 min; however, HMF yields were 93.4 and 88.0%
in 10 min at 100 and 1208C, respectively. High temperature
not only facilitated the dehydration of fructose to HMF, but
also facilitated the formation of humins, as proved by the
lower yields obtained when the reaction was performed at
1208C. Although a low reaction rate was observed at 808C, the
yield reached 89.7% after 40 min, which was higher than those
at 100 and 1208C after 40 min. This further demonstrates the
effect of reaction temperature on the formation of humins.
The tolerance for the concentration of feedstock is an impor-
tant aspect to evaluate a catalytic conversion process from an
economical point of view. The effect of fructose concentration
was also investigated, and the results are shown in Figure 7. It
was found that the HMF yield decreased with increasing fruc-
tose concentration. For example, the yields were 92.7, 88.0,
84.1, and 73% in 10 min, respectively, when 5, 10, 25, and
50 wt%, respectively, of fructose were used. After 40 min, the
remaining fructose was analyzed by ion chromatography, and
the results showed that over 98% of fructose was consumed;
With the aim of optimizing the catalytic system, the effect of
catalyst loading was studied (Figure 5). An HMF yield of only
Figure 5. The effect of catalyst loading on the dehydration of fructose into
HMF in ILs. Experimental conditions: [Bmim]Cl (2.0 g), fructose (0.2 g), LS
(0.05 g), 1008C, 10 min.
75.2%, a selectivity of 77.0%, and a conversion of 97.7% were
obtained when 10 mg of LS (0.5 wt% LS–IL solution) was used
for 10 min. It seems that 20–50 mg of LS (1.0–2.4 wt% LS–IL
solution) is a favorable catalyst loading for satisfactory fructose
conversion, HMF yield, and selectivity, for example, a yield of
90.3%, fructose conversion of 98.7%, and a selectivity of
91.5% was obtained for the dehydration in 1.0 wt% LS–IL solu-
Figure 7. The effect of fructose concentration on the dehydration of fructose
into HMF in ILs catalyzed by LS. Experimental conditions: [Bmim]Cl (2.0 g),
LS (0.05 g), 1208C.
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H. Xie et al.
however, the corresponding HMF yields were 91.7, 85.9, 80.2,
and 74.0%. High concentrations of HMF in ILs can promote
the degradation and/or polymerization of HMF; this is support-
ed by the increasing amount of humins (0.007, 0.019, and
0.046 g) precipitated by the addition of water (20 mL) to the
reaction mixture.
Inulin is an indigestible oligosaccharide consisting of fruc-
tose and is available in large quantities. The hydrolysis of inulin
can produce fructose that can be further converted into HMF.
Both processes can be catalyzed by acids; therefore, the inte-
gration of the hydrolysis of inulin and the dehydration of fruc-
tose in one pot is logically possible and avoids the production
of fructose and the corresponding operation costs. Therefore,
the one-pot conversion of inulin into HMF has been investigat-
ed widely.[5a] In this study, we found that inulin had a good sol-
ubility in [Bmim]Cl at 100 or 1208C, and a 10 wt% viscous solu-
tion of inulin in ILs can be easily prepared in 5 min. Figure 8
demonstrates the relationship between HMF yields and reac-
Figure 9. Reusability of the catalytic system after HMF separation by ethyl
acetate extraction. Experimental conditions: [Bmim]Cl (2.0 g), LS (0.05 g),
fructose (0.5 g), 1208C, 10 min.
the humin formed is insoluble in ethanol.[20] Therefore, ethanol
(10 mL) was added into the LS–IL system after five cycles. The
precipitated humins were filtered and the catalytic system was
submitted to the next reaction after removal of ethanol and
a 76.7% yield of HMF was obtained (Figure 9, run 5), which
demonstrated that the LS–IL system could be reused after
simply removing the accumulated humins after five cycles.
Although many publications demonstrated that the separa-
tion of HMF from ILs could be fulfilled by extraction with ethyl
acetate and THF, more details of the real extraction process are
limited.[5a] Our results presented in Figure 10 show that the ex-
traction efficiency of ethyl acetate depends on the concentra-
tion of HMF in the IL. It was found that for the first run, there
Figure 8. Catalytic conversion of inulin into HMF by LS in ILs. Experimental
conditions: [Bmim]Cl (2.0 g), inulin (0.2 g), water (40 mL), LS (0.05 g), 1208C,
10 min.
tion times under identical conditions for the integrated catalyt-
ic conversion of inulin in a 2.5 wt% IL solution. It was found
that consecutive hydrolysis was very fast, with a maximum
yield of 47.0% achieved in only 5 min, and the yield could be
sustained at this level, even with prolonging the reaction time
to 90 min. It is worthy mentioning that, as the hydrolysis of
inulin needs the participation of water, the comparative results
in Figure 8 demonstrate that the addition of H2O (40 mL) does
not affect the reaction, with comparative yields obtained. It is
believed that the dehydration of one molecule of fructose pro-
duces three molecules of H2O, which is sufficient for the previ-
ous hydrolysis of inulin into fructose; therefore, external water
is not necessary.
Figure 10. The amount of HMF separated by extraction with ethyl acetate
(3ꢁ10 mL). Experimental conditions: [Bmim]Cl (2.0 g), LS (0.05 g), fructose
(0.5 g), 1208C, 10 min.
was 0.27 g HMF after the reaction, but only 0.08 g HMF could
be extracted with ethyl acetate (3ꢁ10 mL). After removing the
remaining ethyl acetate in the IL in vacuo, fresh fructose was
added into the LS–IL solution, and the mixture was treated
under identical conditions. With the accumulation of HMF in
the IL, the extraction efficiency increases. The amounts of HMF
extracted with ethyl acetate for runs 1–4 were 0.18, 0.25, 0.30,
and 0.28 g, respectively.
The recycling of the LS–IL system was investigated after ex-
tracting HMF with ethyl acetate (3ꢁ10 mL) and adding fresh
fructose to the mixture. The HMF yields for each reaction run
in ILs and the amount of HMF extracted by ethyl acetate are
shown in Figures 9 and 10, respectively. The results in Figure 9
show that, after reusing the LS–IL catalytic system four times,
the HMF yield decreases from 77.4 to 62.9%, which demon-
strates that the humins formed and accumulated during the
reaction affect the catalytic activity of LS in ILs. Hu et al. report-
ed that the accumulated residues can be precipitated from the
IL phase by adding ethanol because ILs are soluble, whereas
Conclusions
The dissolution of LS in ILs is efficient for the acid-catalyzed de-
hydration of fructose into HMF and even inulin into HMF in
a one-pot reaction. The yield of HMF from fructose was up to
93.4% in only 10 min under mild conditions. The reusability
study of the LS–IL catalytic system after removing HMF with
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Catalytic Conversion of Inulin and Fructose
aqueous solution of acetic acid with a volume ratio of 15:85. The
ethyl acetate extraction demonstrated that the catalytic activity
decreased from 77.4 to 62.9% after five cycles, and the catalyt-
ic activity can be recovered after simply removing the accumu-
lated humins by filtration after adding ethanol to the LS–ILs.
The combination of using renewable feedstocks, renewable or-
ganic catalysts, green solvents, and the production of valuable
platform chemicals in this study presents an ideal chemical
conversion process following the principles of green chemistry.
We believe that this highly efficient and green route for the
production of valuable chemicals from biomass has great po-
tential for applications.
flow rate was set at 1.0 mLminÀ1
.
Determination of fructose
The ion chromatography system was from Dionex. The hardware
consisted of an ICS-2500 system equipped with a GP50 gradient
pump, an ED50A integrated amperometry detector, a CarbonPac
PA10 guard column (4 mmꢁ50 mm), a high capacity CarbonPac
PA20 analytical column (3 mmꢁ150 mm), and a 25 mL sample
loop. Samples were eluted with 30 mm NaOH at a rate of
0.5 mLminÀ1
.
Keywords: biomass · green chemistry · ionic liquids · 5-
hydroxymethylfurfural · lignosulfonic acid
Experimental Section
Materials
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The ionic liquid (IL) 1-butyl-3-methylimidazolium chloride
([Bmim]Cl) was supplied by Lanzhou Greenchem ILS, LIPC, CAS
(Lanzhou, China). Drying was performed under vacuum at 808C for
10 h to remove absorbed water in the IL. Fructose was purchased
from Sanland-Chem International (Xiamen, China). Inulin was pur-
chased from Acrose. Acetonitrile (HPLC grade) was purchased from
Merck (Darmstadt, Germany). All other chemicals were supplied by
local suppliers and used without further purification. The sodium
salt of LS was purchased from Sigma Aldrich. The acidic cation ex-
change resin (001ꢁ7) was purchased from the Chemical Plant of
Nankai University (Tianjin, China).
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Preparation of lignosulfonic acid
The acidic resin was activated in a saturated aqueous solution of
NaCl for 1 day, followed by treatment with a 2.5 wt% aqueous so-
lution of NaOH for 80 min, then it was washed with distilled water
until the pH became 7, and finally it was treated with a 5 wt%
aqueous solution of HCl for 12 h. Then the resin was transferred to
a column and washed with deionized water until the eluent had
a pH value of 7. The sodium salt of lignosulfonic acid (LS, 10.0 g)
was dissolved in water (200 mL), and the solution was allowed to
flow through the acidic resin column at a speed of 60 drops per
minute. The acidic eluent was collected and yellow LS (7.8 g) was
obtained after freeze-drying the solution. Yield: 78.3%; elemental
analysis of [C18H22.4O11SN0.14]n: found: C 47.77, H 4.94, O 39.14,
S 7.69, N 0.46.
5-Hydroxymethylfurfural production
In a typical run, LS (50 mg) was added to [Bmim]Cl (2.0 g) to pro-
duce a homogenous LS–IL solution in 5 min. Fructose (0.2 g) was
added to the mixture, which was heated at a specified tempera-
ture for the desired time under atmospheric pressure with a mag-
netic stirrer. Samples were withdrawn, diluted with water, centri-
fuged at 10000 rpm for 5 min, and analyzed by means of HPLC.
Determination of 5-Hydroxymethylfurfural
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873–877.
HPLC analysis of 5-Hydroxymethylfurfural (HMF) was performed on
a Dionex system (Dionex, CA, USA) equipped with Dionex P680
four-unit pump and UVD 170U detector. The samples were separat-
ed by using a reversed-phase C18 column (200ꢁ4.6 mm) at l=
280 nm. The column temperature was maintained at 308C. The op-
timized mobile phase consisted of acetonitrile and a 0.1 wt%
Received: September 22, 2011
Revised: December 1, 2011
Published online on April 19, 2012
ChemSusChem 2012, 5, 901 – 905
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