In situ NMR spectroscopy: Inulin biomass conversion in ZnCl2 molten salt hydrate medium - SnCl4 addition controls product distribution

Article abstract of DOI:10.1016/j.carbpol.2014.09.011

The dehydration of inulin biomass to the platform chemicals, 5-hydroxymethylfurfural (5-HMF) and levulinic acid (LA), in ZnCl₂ molten salt hydrate medium was investigated. The influence of the Lewis acid catalyst, SnCl₄, on the produ

Full text of DOI:10.1016/j.carbpol.2014.09.011

Carbohydrate Polymers 115 (2015) 439–443
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Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol
In situ NMR spectroscopy: Inulin biomass conversion in ZnCl2 molten
salt hydrate medium—SnCl4 addition controls product distribution
Yingxiong Wanga, Christian Marcus Pedersenb, Yan Qiaoa, Tiansheng Denga,
Jing Shia, Xianglin Houa,∗
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, 030001 Taiyuan,
People’s Republic of China
b Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 April 2014
Received in revised form 18 August 2014
Accepted 9 September 2014
Available online 18 September 2014
The dehydration of inulin biomass to the platform chemicals, 5-hydroxymethylfurfural (5-HMF) and
levulinic acid (LA), in ZnCl2 molten salt hydrate medium was investigated. The influence of the Lewis acid
catalyst, SnCl4, on the product distribution was examined. An in situ 1H NMR technique was employed
to follow the reaction at the molecular level. The experimental results revealed that only 5-HMF was
obtained from degradation of inulin biomass in ZnCl2 molten salt hydrate medium, while the LA was
gradually becoming the main product when the reaction temperature was increased in the presence of
the Lewis acid catalyst SnCl4. In situ NMR spectroscopy could monitor the reaction and give valuable
insight.
Keywords:
Inulin biomass
5-HMF
Levulinic acid
Molten salt hydrate medium
In situ NMR
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
2012). Due to its high d-fructose content, inulin is considered to
be an ideal source of preparing d-fructose derived chemicals (i.e.
The deterioration of environment and diminishing fossil
resources have given rise to a universal interest in sustainable
resources exploration and utilization (Bai, Xiao, & Sun, 2014; He
et al., 2014; Liu et al., 2014; Omari, Besaw, & Kerton, 2012; Pan
et al., 2013; Siankevich et al., 2014; Tao, Song, Yang, & Chou, 2011;
Yao et al., 2014; Zhang, Liu, & Zhao, 2012). Biomass has received
great interests recently because of their potential applications for
producing valuable liquid fuel and platform chemicals, such as
5-hydroxymethylfurfural (5-HMF) or levulinic acid (LA) (Climent,
Corma, & Iborra, 2014; Shi et al., 2014; Song, Fan, Ma, & Han, 2013;
van Putten et al., 2013).
5-HMF). Owing to its strong intra- and inter-molecular hydrogen
bonds between OH groups, it is difficult to dissolve and hydrolyze
inulin under mild conditions in aqueous solution. It is therefore
important to develop an environmentally benign medium for
dissolving inulin in order to effectuate the hydrolysis to d-fructose
and d-glucose, as well as the dehydration of the monosaccharides
into 5-HMF and/or LA in a “one pot” fashion.
Recently, molten salt hydrate medium, i.e. ZnCl2·4H2O (65 wt%
ZnCl2 solution), has been disclosed as an efficient reaction medium
for transforming lignocellulosic biomass (de Almeida et al., 2010;
Deng et al., 2012; Li, Spina, Moulijn, & Makkee, 2013) and chitin
biomass (Wang, Pedersen, Deng, Qiao, & Hou, 2013) into value-
added fine chemicals, such as 5-HMF and isosorbide. Compared
with dipolar aprotic organic solvents, ionic liquids or other reac-
tion media, the ZnCl2·4H2O system might be more promising due
to its low price, recyclable and the efficient downstream separation
of 5-HMF (Deng et al., 2012; Shi et al., 2014; Song et al., 2013; Wang
et al., 2013).
Inulin, which is also called ␣-d-glucopyranosyl-[␤-d-fructo
furanosyl](n−1)-d-fructofuranoside
or
d-fructofuranoside-[␤-
d-fructo furanosyl](n−1)-d-fructofuranoside, is a natural linear
polyfructan, and can be obtained in large quantities from plants
of the Compositae family such as Jerusalem artichoke, chicory and
dahlia (Barclay, Ginic-Markovic, Johnston, Cooper, & Petrovsky,
Precise product distribution control, for example of the well-
known platform chemicals, 5-HMF and LA, is a challenge for
biomass conversion. This is mainly due to the fact that the
∗
Corresponding author. Tel.: +86 351 4049501; fax: +86 351 4041153.
E-mail address: houxl@sxicc.ac.cn (X. Hou).
http://dx.doi.org/10.1016/j.carbpol.2014.09.011
0144-8617/© 2014 Elsevier Ltd. All rights reserved.

440
Y. Wang et al. / Carbohydrate Polymers 115 (2015) 439–443
reaction pathway and 5-HMF rehydration into LA is effected
profoundly by the following factors, including the properties of
extensively utilized Lewis acid catalysts (Pagán-Torres, Wang,
Gallo, Shanks, & Dumesic, 2012), the concentration of Lewis acid
or Brønsted acid concentration (Omari et al., 2012), the syner-
getic effect of these two type of acid catalyst (Yang, Fu, Mo, & Lu,
2013), and even the intrinsic Brønsted acidity originated from the
hydrolysis of Lewis acid cation (Choudhary et al., 2013). So, it is an
interesting job to disclose the influence of Lewis acid catalyst addi-
tion on the product distribution of biomass degradation in ZnCl2
molten salt hydrate medium. It is will be benefited to the higher
product yields and efficient separation.
According to previous experimental results, SnCl4 played a
completely different role in the degradation of chitin biomass
and cellulose biomass (Hu, Zhang, Song, Zhou, & Han, 2009;
Omari et al., 2012; Wang et al., 2013). For chitin biomass-ZnCl2
molten salt hydrate reaction system, the reaction was seriously
inhibited by Sn4+ due to the strong chelation between NH2 groups
of d-glucosamine and Sn4+ (Wang et al., 2013). However, the
degradation of glucose was promoted by this Lewis acid catalyst
(unpublished results). Hence, in order to investigate whether a con-
trol of the distribution of products including 5-HMF and LA could
be achieved, the conversion of inulin biomass in ZnCl2 molten salt
hydrate with and without SnCl4 as a co-catalyst was investigated
systematically.
2. Experimental
2.1. Materials
Inulin (Practical grade, degree of polymerzation is ∼9, capped
with glucose at reducing end of polyfructosyl chain, the 1H and
13C NMR were measured and listed in Figs. S1 and S2), sucrose
(analytical grade, 99.8%), glucose (analytical grade, 99.5%) and fruc-
tose (analytical grade, 99.5%) were obtained from Shanghai crystal
pure Co., Ltd. ZnCl2, SnCl4·5H2O and maleic acid were purchased
from Beijing Chemical Reagent Company. Deuterium oxide (D2O,
99.9 atom% D) was supplied by Cambridge Isotope Laboratory. Dou-
ble distilled water was used in all experiments. All reagents utilized
in this work were used as received without further purification.
2.2. General reaction procedure
In a typical dehydration experiment, the reaction mixture was
prepared by 5.6 mmol monosaccharide unit and 30 g ZnCl2 molten
salt hydrate medium with or without 2.8 mmol SnCl4. This reac-
tion mixture was heated in an oil bath under continuous stirring.
At certain times during the reaction period, aliquot of the reaction
mixture was taken out from the reaction flask and immediately
emerged in an ice bath to quench the reaction. A gradually dark-
ening of the initial clear solution to black slurry was usually
observed under the applied conditions. The yields of 5-HMF and
LA, as well as the rate of substrate consumption during the reac-
tion, were quantitatively analyzed by 1H NMR spectroscopy using
maleic acid as internal standard substance, following the procedure
described by Rundlöf et al. (2010) Maleic acid with concentration
of 1.0574 mg/mL in D2O was prepared as a standard solution, then
0.1 mL of reaction mixture was mixed with 0.4 mL this standard
solution to give the 1H NMR sample. 5-HMF and LA yields were
determined by the following formula: yield = (moles of 5-HMF or
LA)/(moles of monosaccharide unit) × 100%.
Fig. 1. Product and substrate concentrations as a function of the reaction time at
different temperatures for sucrose degradation. Concentration: 1 g sucrose in 30 g
ZnCl2 molten salt hydrate medium (ZnCl2·4H2O). (a) 80 ◦C; (b) 100 ◦C; (c) 120 ◦C.
5-HMF yield ꢀ; LA yield ꢁ; fructose concentration ᭹; glucose concentration ꢀ.
1H NMR was referenced to 4.77 ppm of deuterated solvent residual
proton, HDO. A reaction mixture of sucrose in ZnCl2·4D2O (65 wt%
ZnCl2 in deuterium oxide solution) was prepared in a 5 mm heavy
wall tube, with the same concentration of substrate and catalyst
(SnCl4) as in the general reaction procedure. Before it was trans-
ferred into the NMR spectrometer, which was already preheated
to 100 ◦C, the solution was kept at room temperature for 10 min
to reach equilibrium. When the temperature inside the NMR spec-
trometer probe was stabilized at 100 ◦C again after sample loading,
1H NMR (ns = 8) spectra were recorded at certain times during the
measurement.
2.3. Procedure for in situ NMR study
The reaction was followed by in situ NMR using a Bruker AV-
III 400 with a 1H frequency of 400.13 MHz. The chemical shift for

Y. Wang et al. / Carbohydrate Polymers 115 (2015) 439–443
441
Fig. 2. Product and substrate concentrations as a function of the reaction time at different temperatures for inulin biomass degradation. Concentration: 1 g substrate in 30 g
ZnCl2 molten salt hydrate medium (ZnCl2·4H2O), SnCl4/substrate unit = 0.5 (mol/mol). (a) sucrose, 80 ◦C; (b) sucrose, 100 ◦C; (c) sucrose, 120 ◦C; (d) fructose, 100 ◦C; (e) inulin,
100 ◦C. 5-HMF yield ꢀ; LA yield ꢁ; fructose concentration ᭹; glucose concentration ꢀ.
3. Results and discussion
If the reaction of sucrose was performed at 80 ◦C (Fig. 1(a)) in
ZnCl2·4H2O without SnCl4 added, the hydrolysis of sucrose was fin-
ished within 5 min. and the highest concentrations of d-glucose and
d-fructose during the reaction process were reached (the chemi-
cal structure identification for sucrose, glucose and fructose during
reaction are presented in Section 3.3). When reaction time was
prolonged from 10 min to 300 min, the concentration of d-glucose
and d-fructose decreased from 93.5% and 69.3% to 3.9% and 6.5%,
respectively. Moreover, the 5-HMF yield increased gradually dur-
ing the first 90 min of reaction time, the highest 5-HMF yield of
3.5% was achieved at 90 min; 5-HMF yield decreased gradually
when reaction time was prolonged from 90 min to 600 min. The
trend of 5-HMF yield over reaction time is quite similar to our
3.1. Sucrose degradation in ZnCl2 molten salt hydrate medium
Initially, the ZnCl2 molten salt hydrate was preferred for dissolv-
ing inulin according to our previous work, (Deng et al., 2012; Wang
et al., 2013) and it showed that ZnCl2·4H2O acts as good solvent for
inulin as well as being an acidic reaction medium (Fig. S3).
Sucrose can be considered as the smallest inulin (Barclay et al.,
2012), so the hydrolysis and dehydration of sucrose were inves-
tigated as a model system under various reaction conditions. The
quantitative 1H NMR technique was employed to follow the sub-
strate consumption rate and to quantify product yields.

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Y. Wang et al. / Carbohydrate Polymers 115 (2015) 439–443
previous report with d-glucose and d-glucosamine as substrates.
(Deng et al., 2012; Wang et al., 2013) When the temperature was
increased to 100 ◦C and 120 ◦C (Fig. 1(b) and (c)), the hydrolysis of
sucrose finished immediately (in less than 5 min), and the yield of
5-HMF increased from 3.5% at 80 ◦C to 5.3% at 100 ◦C, and 8.1% at
120 ◦C.
One important thing should be addressed, i.e. only 5-HMF was
observed within the investigated reaction time and temperature
range in the ZnCl2·4H2O sucrose system, and no LA was observed
(Deng et al., 2012; Wang et al., 2013). LA is the rehydration product
of 5-HMF under acidic condition (van Putten et al., 2013).
3.2. Inulin biomass degradation in ZnCl2 molten salt hydrate
medium with SnCl4
If SnCl4 (0.5 equiv.), as a Lewis acid catalyst, was added into the
reaction mixture, the yields of 5-HMF at each reaction temperature
were consistently higher than the corresponding reactions without
Sn4+. For example, at 100 ◦C, the yield of 5-HMF was 14.6% with
Sn4+ added (Fig. 2(b)), while it was 5.3% for the reaction without
Sn4+ added when reaction temperature is 100 ◦C.
Fig. 3. Stacked 1H NMR spectra of in situ NMR study. Solvent: ZnCl2·4D2O with
SnCl4; substrate: sucrose; reaction temperature: 100 ◦C.
Unexpectedly, the 5-HMF rehydration product LA was also
detected in this reaction system with Sn4+. Correspondingly, formic
acid (FA), which is the widely utilized hydrogen resource for trans-
fer hydrogenation reaction, was also observed. Surprisingly, the
main product was switched from 5-HMF to LA gradually when the
reaction temperature was increased from 80 ◦C to 120 ◦C. At 80 ◦C,
the maximum yield for 5-HMF and LA were 12.4% and 0.28%, respec-
tively, with 5-HMF being the main product (Fig. 2(a)). When the
reaction temperature was increased to 120 ◦C, the highest yields
were 11.6% and 25.7% for 5-HMF and LA, respectively, with LA
being the main product (Fig. 2(c)). An apparent reaction time and
temperature dependence were observed for the sucrose hydroly-
sis, monosaccharide consumption, 5-HMF formation and LA yields.
Thus, the above experiments demonstrate that in the ZnCl2 molten
salt hydrate medium, the Lewis acid catalyst Sn4+ is not only effec-
tive for the conversion of substrate to 5-HMF, but it also promote
the 5-HMF rehydration to LA and FA (Omari et al., 2012).
Other model compounds, i.e. d-fructose and the biopolymer
inulin, were also used as feedstock for producing 5-HMF and LA in
ZnCl2 molten salt hydrate medium with the Lewis acid co-catalyst
SnCl4. The highest 5-HMF yields obtained from d-fructose and
inulin at 100 ◦C were 15.5% (Fig. 2(d)) and 15.9% (Fig. 2(e)), respec-
tively. The highest LA yields from fructose and inulin were 5.4% and
4.6%, respectively. The results above clearly demonstrate that sub-
strate consumption, 5-HMF and LA yields, approximately follow
the same trend in the reactions for bothd-fructose and inulin. This
suggests that the conversion of biopolymer into monosaccharides
through breaking the glycosidic bond was faster than the dehydra-
tion of monosaccharide. The rate determining step for the inulin
degradation is therefore the dehydration step, and not the glyco-
sidic bonds hydrolysis.
of sucrose (Figs. S4–S7), fructose (Figs. S8–S11) and glucose (Figs.
S12–S15) could be assigned (there are chemical shift differences
for these experimental samples and authentic samples due to the
solvent effect). A peak with chemical shift of 5.51 ppm is assigned
to ␣-H1 of d-glucose. The signal at 3.70 ppm belongs to the ␤-
H1 of d-fructose. The peak at 4.27 ppm is the typical signal of the
glucosyl-attached fructosyl H3 of sucrose, which can be utilized
for showing if the glycosidic bond of sucrose still remains or not
during the reaction. All NMR signals (ı = 5.51, 3.70 and 4.27 ppm)
mentioned above were selected to indicate the formation of inter-
mediate monosaccharides and the consumption of all substrates
including sucrose, d-glucose and d-fructose. According to the 1H
NMR, 2D 1H-1H COSY, 1H-13C HSQC spectra of reaction mixture
contains of 5-HMF and mixture rich of FA and LA (Figs. S16–S23), the
characteristic peaks found at ı = 9.50, 8.45 and 2.40 ppm are signals
owing to 5-HMF, FA and LA, respectively, and they are designated
to follow the product formation of these compounds.
As shown in Fig. 3, the intensity for signal of sucrose at 4.27 ppm
decreased quickly and became almost negligible after 20 min, and
the peaks of d-glucose (ı = 5.51 ppm), d-fructose (ı = 3.70 ppm),
5-HMF (ı = 9.50 ppm), FA (ı = 8.45 ppm) and LA (ı = 2.40 ppm)
appeared simultaneously. Moreover, the signals of by-products,
for instance, the soluble oligomers, were not strong enough to be
detected during the reaction. It was revealed that the hydrolysis
of the substrate in the present reaction using the reaction medium
containing the co-catalyst SnCl4 is highly efficient. With the reac-
tion proceeding, the H1 signals of intermediate monosaccharides
including d-glucose (ı = 5.51 ppm) and d-fructose (ı = 3.70 ppm)
became weaker and finally negligible. Meanwhile, the intensity
of 5-HMF signals (ı = 9.50 ppm) increased constantly until 80 min
where after it then decreased gradually, but the intensity of signals
attribute to FA (ı = 8.45 ppm) and LA (ı = 2.40 ppm) were increas-
ing constantly all the time. These data suggested that the rate of
formation of 5-HMF is slower than 5-HMF rehydration into LA and
FA.
3.3. In situ NMR characterization of the sucrose dehydration
using SnCl4
To gain insights into the reaction pathways at the molecular
level, and elucidate the role of Lewis acid catalyst, Sn4+, during the
inulin biomass degradation in ZnCl2 molten salt hydrate, molecu-
lar level monitoring was performed by in situ 1H NMR technology.
The sucrose dehydration reaction was carried out in ZnCl2·4D2O
with a catalytic amount of SnCl4 at 100 ◦C, and the obtained time-
progression 1H NMR spectra were stacked and presented in Fig. 3.
According to the 1H NMR spectrum at the initial stage of the
The signals earlier reported for reaction intermediates in the
conversion of d-glucose and d-fructose into 5-HMF, such as
3-deoxy-glucosone and (4S,5R)-4-hydroxy-5-hydroxymethyl-4,5-
dihydrofuran-2-carbaldehyde, are not observed from the in situ
spectra (Jadhav, Pedersen, Soiling, & Bols, 2011). Presumably, these
possible intermediate compounds are too unstable or with too
short life time in this ZnCl2 molten salt hydrate medium to be
observed. As a result, the observation and identification of the
reaction, and 1D 1H NMR, 13C NMR, and 2D 1H-1H COSY, 1H-13
HSQC spectra of authentic samples, the following proton signals
C

Y. Wang et al. / Carbohydrate Polymers 115 (2015) 439–443
443
reaction intermediate compounds are difficult within the NMR
time resolution (Akien, Qi, & Horvath, 2012; Kimura, Nakahara, &
Matubayasi, 2011). These results are in agreement with reports
of hexose monosaccharide dehydration in the aqueous medium
(Kimura et al., 2011).
He, Y., Hoff, T. C., Emdadi, L., Wu, Y., Bouraima, J., & Liu, D. (2014). Catalytic conse-
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into 5-hydroxymethylfurfural catalyzed by a common Lewis acid SnCl4 in an
ionic liquid. Green Chemistry, 11, 1746–1749.
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4. Conclusions
Kimura, H., Nakahara, M.,
& Matubayasi, N. (2011). In situ kinetic study
Our results show that the degradation of inulin biomass in
ZnCl2 molten salt hydrate medium can selectively afford the 5-
HMF; whereas the ratio between LA and 5-HMF can be controlled
precisely by taking advantage of the Lewis acid catalyst (SnCl4)
in combination with the reaction temperature. In situ NMR spec-
troscopy was demonstrated to be a convenient method to follow
the reaction pathways of inulin biomass degradation. The results
of this study will pave the way for improved control of the biomass
degradation products and support the possible industrial applica-
tion for platform molecular production.
on hydrothermal transformation of d-glucose into 5-hydroxymethylfurfural
through d-fructose with 13C NMR. Journal of Physical Chemistry A, 115,
14013–14021.
Li, J., Spina, A., Moulijn, J. A., & Makkee, M. (2013). Sorbitol dehydration into isosor-
bide in a molten salt hydrate medium. Catalysis Science and Technology, 3,
1540–1546.
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and Brønsted acid catalysts in water in a biphasic reactor with an alkylphenol
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Acknowledgements
Pan, T., Deng, J., Xu, Q., Xu, Y., Guo, Q.-X., & Fu, Y. (2013). Catalytic conversion
of biomass-derived levulinic acid to valerate esters as oxygenated fuels using
supported ruthenium catalysts. Green Chemistry, 15, 2967–2974.
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The authors would like to acknowledge the financial support
from the Major State Basic Research Development Program of China
(973 Program) (no. 2012CB215305), the National Natural Science
Foundation of China (no. 21106172), and the Natural Science Foun-
dation for Youths of Shanxi (2012021009-2, 2013021008-7).
Appendix A. Supplementary data
Siankevich, S., Savoglidis, G., Fei, Z., Laurenczy, G., Alexander, D. T. L., Yan,
N., et al. (2014).
A novel platinum nanocatalyst for the oxidation of 5-
Supplementary material related to this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.carbpol.
2014.09.011.
Hydroxymethylfurfural into 2,5-Furandicarboxylic acid under mild conditions.
Journal of Catalysis, 315, 67–74.
Song, J., Fan, H., Ma, J., & Han, B. (2013). Conversion of glucose and cellulose
into value-added products in water and ionic liquids. Green Chemistry, 15,
2619–2635.
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