Research Article
pubs.acs.org/journal/ascecg
NMR Study of the Hydrolysis and Dehydration of Inulin in Water:
Comparison of the Catalytic Effect of Lewis Acid SnCl4 and Brønsted
Acid HCl
Yan Qiao,†,§ Christian Marcus Pedersen,‡ Dongmei Huang,† Wenzhi Ge,† Mengjie Wu,∥ Chunyan Chen,∥
Shiyu Jia,∥ Yingxiong Wang,*,∥ and Xianglin Hou*,∥
†
Analytical Instrumentation Center, §State Key Laboratory of Coal Conversion, and ∥Shanxi Engineering Research Center of
Biorefinery, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, People’s Republic
of China
‡
Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
S Supporting Information
*
ABSTRACT: Various NMR techniques were employed to study
the catalytic performance of the Lewis acid SnCl4 and the
Brønsted acid HCl in the conversion of inulin to value-added
compounds by hydrolysis and subsequent dehydration. The
hydrolysis of inulin was examined to reveal the catalytic abilities
of SnCl4 besides its intrinsic acidity by in situ 1H and 13C NMR at
25 °C. The dehydration reaction of inulin with SnCl4 as catalyst
was followed by high temperature in situ 1H NMR at 80 °C. The
fructose moieties were dehydrated to 5-(hydroxymethly)furfural
(5-HMF), but the glucose fragment of inulin was inactive for
dehydration reaction under this condition. The formation of 5HMF and its transformation into formic acid and levulinic acid
through a rehydration reaction could be monitored by in situ
NMR spectroscopy. Moreover, diffusion ordered spectroscopy NMR revealed that the Lewis acid ion, Sn4+ interacts with the
inulin model compounds, i.e., sucrose and fructose. The synergistic effects of complexation and acidity from the hydrolysis of
SnCl4 results in a higher catalytic ability of this Lewis acid catalyst compared with a Brønsted acid.
KEYWORDS: Inulin, In situ NMR, DOSY NMR, SnCl4, 5-HMF, Levulinic acid
■
INTRODUCTION
Biorefining of biomass is a sustainable process to supply both
liquid fuels and chemicals with great potential. With further
improvements this could meet the increasing energy demand
and address increased environmental concerns.1−5 Two steps,
hydrolysis and dehydration, are essential in the preparation of
platform chemicals through biorefinery lignocellulosic biomass,
inulin and chitin.6−11 In the initial hydrolysis step, the
glycosidic bonds of biopolymers are hydrolyzed, and
monosaccharides such as fructose, glucose, and glucosamine
are obtained. In the following dehydration step, these
hydrolytic products are dehydrated to value-added platform
chemicals.2 Mineral acids and Lewis acid metal ions are the
most commonly utilized homogeneous acid catalysts for these
two biorefinery steps.7,10,12−15
Our previous studies revealed that the ZnCl2 molten salt
hydrate (ZnCl2·4H2O, 65 wt % ZnCl2 solution) is an ideal
aqueous reaction medium for converting lignocellulosic
biomass,13 chitin,16 and inulin,17 into value added chemicals,
such as 5-(hydroxymethly)furfural (5-HMF)13,16 and levulinic
acid (LA).17 These studies also showed that SnCl4 could
promote the conversion of inulin, cellulose and hemicellulose,
© 2016 American Chemical Society
but inhibited the conversion of chitin due to the too strong
chelation between the amino groups of glucosamine and the
Lewis acid ion Sn4+.5,9,16,18,19 An even more exciting result was
the product distribution of inulin conversion, where 5-HMF
and LA could be manipulated by the use of SnCl4 as
cocatalyst.17 Interestingly, the ZnCl2 molten salt hydrate
could hydrolyze sucrose to glucose and fructose with the aid
of SnCl4 even at room temperature (Figure S1). However, the
exact same condition, do not hydrolyze the glycosidic bond in
other disaccharides, such as cellobiose and maltose (Figures S2
and S3). These results suggested that SnCl4 is a unique catalyst
for biorefinery in the aqueous phase, and its full potential has to
be studied in detail. For example, it is interesting to reveal why
and how SnCl4 could break the glycosidic bond of sucrose.
According to the research of Enslow et al. and Choudhary et al.,
it has been proven that the Lewis acidic ions, such as Sn4+ and
Cr3+, could hydrolyze and release the H+ ion (Brønsted acid) in
aqueous media.20,21 Therefore, it is reasonable to presume that
Received: February 23, 2016
Revised: April 22, 2016
Published: April 25, 2016
3327
DOI: 10.1021/acssuschemeng.6b00377
ACS Sustainable Chem. Eng. 2016, 4, 3327−3333
Research Article
ACS Sustainable Chemistry & Engineering
spectrometer, the 1H NMR (frequency at 400.13 MHz) and 13C NMR
(frequency at 100.61 MHz) at 25 °C were obtained at certain reaction
times to monitor the hydrolysis reaction progress. For the in situ
dehydration of inulin at 80 °C, 0.037 mmol inulin and 0.0185 mmol
SnCl4·5H2O were dissolved in 0.2 mL D2O and injected into a 5 mm
heavy wall NMR tube. After transferring the sample into the NMR
spectrometer, which was already preheated to 80 °C, the solution was
kept for 2 min to reach equilibrium. When the temperature inside the
NMR spectrometer was stabilized at 80 °C, 1H NMR (8 scans) and
13
C NMR (128 scans) spectra were recorded at certain reaction times.
Procedure for DOSY NMR Experiments. The samples for
DOSY experiments were prepared in a 5 mm NMR tube by dissolving
sucrose (0.04625 mmol) or fructose (0.0925 mmol) in 0.4 mL D2O
containing SnCl4·5H2O (0.04625 mmol) or HCl solution (adjusted to
pH 0.6). DOSY measurements were performed with the Bruker
standard bipolar pulse longitudinal eddy current delay (BPPLED)
pulse sequence on a Bruker AV-III 400 MHz NMR spectrometer (9.39
T), using a 5 mm PABBO BB/19F-1H/D probe with z gradient coil
producing a maximum gradient strength of 0.504 T m−1. All the
measurements were performed at 25 °C and at a gas flow rate of 400 L
h−1 without sample spinning. The gradient strength was incremented
in 32 steps from 2% to 95% of its maximum value in a linear ramp.
The diffusion time (Δ) was 100 ms. The duration of the pulse field
gradient (δ/2) was adjusted in a range of 600−2000 μs in order to
obtain 2−5% residual signal with the maximum gradient strength. The
experimental data was processed using Bruker TopSpin 3.1 software.
General Procedure for Dehydration in Aqueous and
Biphasic Conditions. For the inulin dehydration reaction, using
SnCl4·5H2O and HCl in an aqueous solution at 80 °C, the reaction
mixture was prepared by 5.55 mmol monosaccharide unit (∼1 g) in 30
g reaction medium, which was heated in an oil bath under continuous
stirring. At certain times during the reaction period, an aliquot of the
reaction mixture was taken out and the reaction stopped by cooling in
an ice bath. The 5-HMF and LA yields during the reaction were
qualitatively analyzed by 1H NMR spectroscopy using maleic acid as
an internal standard, following the procedure described by Rundlöf et
al.40 The NMR parameters are as follows: pulse program for
acquisition, zg; D1, 60.0 s; DS, 2; NS, 16; P1, 9.7 μs; PLW1, 18 W;
AQ, 4.09 s. The baseline was corrected by “Automatic using
polynomial of degree ABSG” or “Automatic alternate algorithm”
methods.
the inherent acid from the hydrolysis of Sn4+ ion may provide
catalytic ability for the selective glycosidic bond cleavage in
inulin conversion.17 However, the reason for the catalytic ability
beyond the inherent acidity of Sn4+ is unclear and, hence, needs
to be elucidated. In other words, what is the difference between
mineral acids and Lewis acid metal ions during the biorefining
of inulin?
Nuclear magnetic resonance (NMR) has been developed as a
versatile technique for the investigation of biomass conversion
due to the following features: first, commonly used reaction
solvents (water, DMSO, chloroform, acetone, and methanol)
are cheap and readily available in deuterated form;22 second,
the nuclei usually contained in the biomass samples or catalysts,
such as proton, carbon, boron, silicon, and phosphorus, are
NMR active.8,23 The development of the hardware and software
of NMR has made 2D NMR, such as 1H−1H correlation
spectroscopy (COSY), 1H−13C heteronuclear single quantum
coherence (HSQC), 1H−13C heteronuclear multiple bond
correlation (HMBC), and 1H−15N HSQC, routine measurements.5 Advanced in situ NMR has recently been utilized to
track the reaction intermediates in biomass conversion without
the need for quenching the reaction.18,24−29 Recently, another
2D NMR technique, diffusion ordered spectroscopy (DOSY)
NMR, has been used as a versatile tool in the field of chemistry,
biology, and the environment.3,30−35 This NMR method
measures diffusion rates in an NMR sample and thereby
provides a diffusion coefficient (D). Thus, it provides a pseudo2D NMR spectrum with the chemical shift as the first
dimension (F2) and D for the second dimension (F1).36 The
DOSY method characterizes the interaction between molecules,
analyzes compound mixtures, and monitors complex formation.
With the aid of a calibration curve this method can be used to
calculate the formula weight of the complex in the solution
state.37−39
In this report, two aqueous catalytic reaction systems, i.e.
SnCl4 solution and HCl solution, with the exactly same pH
values, were utilized. Consequently, the role of the Lewis acid
SnCl4 could be compared with the Brønsted acid HCl. Various
in situ NMR techniques were employed to follow the SnCl4
catalyzed hydrolysis and dehydration reactions of inulin, and
the DOSY NMR technique was used to show a strong and
specific interaction between Lewis acid ion Sn4+ and the
reactants (sucrose and fructose, which are model compounds
for inulin).
■
■
RESULTS AND DISCUSSION
Catalytic Effect of SnCl4 and HCl in the Hydrolysis of
Inulin. In our previous study of the catalytic conversion of
inulin into 5-HMF and LA, it was found that the sucrose
hydrolysis in ZnCl2 molten salt hydrate with a catalytic amount
of SnCl4 (0.5 mol equivalent to the substrate unit), gave a
NMR signal from the anomeric position (102.9 ppm, glucosylattached fructosyl C2 of sucrose, designated as F−C2) which
disappeared gradually within 96 h as shown in the 13C NMR
spectra (Figure S1). Correspondingly, the signals indicating the
hydrolysis products, i.e. glucose (95.0 ppm, C1 of β-D-glucose,
designated as β-pyr-C1) and fructose (97.5 ppm, C2 of βpyranose tautomer of D-fructose, designated as β-pyr-C2),
appeared and increased gradually. Under the exactly same
reaction conditions but without SnCl4, the glycosidic linkage of
sucrose was found to be stable in the ZnCl2 molten salt hydrate.
According to results by Choudhary et al. and our previous
reports,17,20,21 the hydrolysis of Lewis acid metal ion, SnCl4,
liberated HCl and thereby lowered the pH. The pH of the two
aqueous solvent systems, i.e. ZnCl2 molten salt hydrate with
and without SnCl4, were measured as pH 0.6 (with SnCl4) and
pH 1.9 (without SnCl4), respectively. On this basis it is easy to
take it for granted that the enhanced acidity of the reaction
medium containing SnCl4 contributed to the sucrose hydrolysis
reaction. However, according to previous reports about the use
EXPERIMENTAL SECTION
Materials. Inulin (α-D-glucopyranosyl-[β-D-fructofuranosyl](n−1)-Dfructofuranoside, practical grade, degree of polymerization 9.4, capped
with glucose at the reducing end of polyfructosyl chain), sucrose
(99.8%), fructose (99.5%), methyl isobutyl ketone (MIBK), HCl
solution (37%), and SnCl4·5H2O were obtained from Shanghai crystal
pure Co., Ltd. Deuterium oxide (D2O, 99.9 atom % D) was supplied
by Cambridge Isotope Laboratory. All reagents utilized in this work
were used as received without further purification.
Procedure for in situ NMR Experiments. The in situ hydrolysis
and dehydration reaction of inulin were monitored by a Bruker AV-III
400 equipped with an autosampler and TopSpin 3.1 and IconNMR
software. The chemical shift for 1H NMR was referenced to 4.77 ppm
(solvent residual signal from D2O). In a typical hydrolysis experiment,
sucrose or inulin containing 0.185 mmol mL−1 monosaccharide unit,
was dissolved in the reaction medium. A SnCl4·5H2O solution (0.0925
mmol mL−1, pH 0.6) or HCl solution (pH 0.6), was added to prepare
the reaction mixture. The aqueous reaction mixture was injected into a
5 mm NMR tube, and a D2O loaded 5 mm coaxial insert was
employed to lock the NMR field. After transferring the tube into the
3328
DOI: 10.1021/acssuschemeng.6b00377
ACS Sustainable Chem. Eng. 2016, 4, 3327−3333
Research Article
ACS Sustainable Chemistry & Engineering
of SnCl4 as the catalyst, we have good reason to believe that
there is an additional contribution to the catalysis, besides the
release of Brønsted acid.15,21,41
To verify this and to optimize the reaction system, sucrose,
the smallest inulin model, was examined to check the catalytic
performance of SnCl4 (Lewis acid catalyst, pH 0.6, without
ZnCl2·4H2O) for the hydrolysis reaction. Sucrose hydrolysis
catalyzed by diluted HCl (Brønsted acid catalyst, pH 0.6,
without ZnCl2·4H2O) was also performed for comparison. The
obtained in situ 1H and 13C NMR spectra for the sucrose
hydrolysis to glucose and fructose in these two reaction media,
are displayed in Figures 1 and 2, respectively.
Figure 2. In situ 13C NMR spectra for the hydrolysis reaction of
sucrose in Sn4+ and HCl aqueous solutions 25 °C. (a) 13C NMR in
Sn4+ solution; (b) 13C NMR in HCl solution. Signals for sucrose,
glucose, and fructose are indicated by S (103.6 ppm), G (95.9 ppm),
and F (98.1 ppm), respectively.
small amount of sucrose left in the aqueous solution according
to the weak intensity of signal at 5.23 ppm. Meanwhile, the
corresponding signals from glucose (4.48 ppm, β-H1) and
fructose (3.85 ppm, β-fur-H3) appeared and increased
correspondingly. Finally, the hydrolysis reaction of sucrose in
the SnCl4 solution was complete at 24 h, and no further
reactions, such as dehydration for 5-HMF and then rehydration
for levulinic acid were observed. The Lewis acid SnCl4 show a
high selectivity for hydrolysis of the glycosidic bond of sucrose.
As comparison, this hydrolysis reaction was performed under
the same condition except that the reaction medium was
changed to HCl aqueous solution (pH 0.6). The stacked in situ
1
H NMR spectra (Figure 1b) showed that the intensity of
signal for sucrose (5.23 ppm) decreased gradually, in the
meantime the intensity of signals of those obtained
monosaccharide intermediates, including glucose and fructose,
increased accordingly. Surprisingly the signal for sucrose was
still observable at 24 h and disappeared completely at 48 h.
13
C NMR spectroscopy was also performed in order to
follow the sucrose hydrolysis in both reaction media. The
signals located at 103.6, 95.9, and 98.1 ppm represent the
sucrose (glucosyl-attached fructosyl C2 of sucrose, designated
as F−C2), glucose (β-C1 of β-D-glucose, designated as β-pyrC1), and fructose (β-C2 of β-pyranose tautomer of D-fructose,
Figure 1. In situ 1H NMR spectra for the hydrolysis reaction of
sucrose in Sn4+ and HCl aqueous solution at 25 °C. (a) 1H NMR in
Sn4+ containing solution; (b) 1H NMR in HCl solution. Signals for
sucrose, glucose, and fructose are indicated by S (5.23 ppm), G (4.48
ppm), and F (3.85 ppm), respectively.
In the case of sucrose hydrolysis catalyzed by SnCl4 in water,
(Figure 1a) the following proton signals were chosen: 5.23 ppm
(fructosyl-attached glucosyl H1 of sucrose, designated as G-H1),
4.48 ppm (β-H1 of β-D-glucose, designated as β-H1), and 3.85
ppm (β-H3 of β-furanose tautomer of D-fructose designated as
β-fur-H3). These signals represent sucrose, glucose, and
fructose, respectively. According to the stacked 1H NMR
spectra listed in Figure 1a, the intensity of the signal for sucrose
(5.23 ppm) decreased gradually, which means the hydrolysis
reaction of sucrose is promoted under the present reaction
conditions. As the reaction time increased to 12 h, there is only
3329
DOI: 10.1021/acssuschemeng.6b00377
ACS Sustainable Chem. Eng. 2016, 4, 3327−3333
Research Article
ACS Sustainable Chemistry & Engineering
designated as β-pyr-C2) respectively. Due to the large chemical
shift width, there is no problem with signal overlapping in 13C
NMR spectra, and all these signals could therefore be assigned
and monitored during the reaction. These results of the in situ
13
C NMR are agreed with 1H NMR and could support the
conclusions based on the 1H NMR study. The two catalytic
reaction system show obviously different capabilities for the
sucrose hydrolysis reaction, as it is completed within 24 h (for
Lewis acid catalyst, SnCl4), whereas it takes 48 h (for Brønsted
acid catalyst, HCl), when keeping the same pH value of 0.6.
Moreover, 13C NMR show clearly, besides the β-pyranose (98.1
ppm) the signals corresponding to β-furanose (101.5 ppm) and
α-furanose (104.5 ppm) tautomers of fructose, which are
important intermediates for the formation of 5-HMF.5 No
detectable side reactions or products were observed by 1H or
13
C NMR in both SnCl4 and HCl media.
Next, inulin (caped by glucose side, with average polymerization degree of 9.4) was explored in order to check the
difference between the two reaction systems described above
(Figures S4 and S5). Although the biopolymer inulin has a
complex molecular structure, the in situ 1H (Figure S4) and 13C
NMR (Figure S5) results disclosed that inulin follows the same
hydrolysis pattern as sucrose. The inulin was completely
hydrolyzed within 24 h with Lewis acid SnCl4 as catalyst, while
48 h was necessary to reach the same level in the HCl aqueous
solution.
These experimental indicate that there is an additional effect
beyond the Brønsted acidity generated from the hydrolysis of
SnCl4the Lewis acidity of Sn4+.
Catalytic Effect of SnCl4 and HCl for the Dehydration
of Inulin. To study additional effects of using SnCl4 as the
catalyst, high temperature in situ 1H NMR, was carried out to
follow the dehydration reaction of inulin at 80 °C. The
obtained time-progression in situ 1H NMR spectra were
stacked and are presented in Figure 3. From the NMR study it
According to our previous spectra assigned for the
dehydration products, the characteristic peaks found (at 9.80,
2.90, and 8.57 ppm) are signals from 5-HMF, LA, and formic
acid (FA), respectively. These signals were therefore used to
follow the formation of products. As shown in Figure 3, the
signal corresponding to 5-HMF, 9.80 ppm, appeared after 20
min and increased progressively with a maximum after 315 min,
where after it then decreased gradually. The signals
representing LA (2.90 ppm) and FA (8.57 ppm), appeared
after 65 min and increased continuously with time.
Interestingly, no signals of byproducts, such as soluble
oligomers could be detected. This was further supported by
the color of the NMR-samples before and after the in situ NMR
experiment (Figure S6). It is yellow after reaction but did not
turn into brown slurry as with the reactions in ZnCl2 molten
salt hydrate.13,16 The acidity provided by SnCl4 is sufficient for
the catalytic hydrolysis and dehydration of inulin at 80 °C,
however this acidity might be low enough to limit side
reactions, such as further condensation reactions of the
products into humins. Besides this, the intensity of the signal
corresponding to glucose was unchanged during the whole in
situ reaction processes, which indicate the glucose formed, from
the hydrolysis, but is inactive for dehydration under the
conditions. This result is different from previous reports,9,21
where glucose and glucosamine could be turn into 5-HMF
efficiently at a higher reaction temperature (140 or even 170
°C) using SnCl4 in an aqueous solution. At the reaction
temperature of 80 °C, the aldose to ketose isomerization
(glucose to fructose) is too slow and therefore the glucose
cannot be converted.
Although the above-mentioned in situ NMR experiment
could describe the inulin dehydration at the molecular level, the
5-HMF and LA yields could not be monitored accurately.
Therefore, sucrose dehydration in a glass vessel was studied
with two homogeneous catalysts, SnCl4 and HCl aqueous
solution (pH 0.6) at 80 °C. The obtained results from the
dehydration are listed in Figure 4. The yields of 5-HMF and LA
using an aqueous solution containing SnCl4 were 10.6% and
16.4% after 12 h (720 min), respectively. However, the yields in
HCl aqueous solution were only 6.2% for 5-HMF and 2.2% for
LA. It was also observed that glucopyranose was not converted
to 5-HMF and/or LA in any of these aqueous media, which is
Figure 3. Stacked in situ 1H NMR spectra for the inulin dehydrated
into the platform molecules, 5-HMF, FA, and LA: solvent SnCl4·
containing solution pH 0.6; reaction temperature 80 °C.
is clear that no signal corresponding to the anomeric protons in
inulin could be detected after 10 min reaction, i.e. the
hydrolysis step of inulin was complete within 10 min at 80
°C. The signals from the intermediate monosaccharides, i.e.
glucose (5.56 ppm) and fructose (4.43 ppm), were however
detected.
Figure 4. Time evolution of sucrose degradation in SnCl4· solution,
HCl solutions, and the biphasic reaction system composed by SnCl4·
containing solution and MIBK (30 mL): reaction temperature 80 °C;
reactant concentration 1 g substrate in 30 g solvent; MIBK volume 30
mL.
3330
DOI: 10.1021/acssuschemeng.6b00377
ACS Sustainable Chem. Eng. 2016, 4, 3327−3333
Research Article
ACS Sustainable Chemistry & Engineering
agreed with the in situ NMR results. In order to check and
increase the yield of products, the inulin hydrolysis and
dehydration reactions in a biphasic system with methyl isobutyl
ketone (MIBK) as the organic phase were performed. Under
these conditions the 5-HMF yield increased significantly to
30.5%, due the rehydration reaction of 5-HMF to LA and FA
being prevented by extraction of products into the organic
phase. Moreover the yield of 5-HMF in the aqueous phase was
7.1% whereas no LA could be detected. The biphasic reaction
system lowered the side reactions and promoted the 5-HMF
yield significantly.14
DOSY-NMR to Study the Interaction between Sn4+
Species and the Substrate. As shown in the above sections
SnCl4 could match the catalytic ability of the Brønsted acid
HCl. Furthermore, there is an additional catalytic ability of
SnCl4 during the hydrolysis and dehydration of the inulin. This
could be caused by a strong interaction between the Lewis acid
ion Sn4+ and the inulin substrate, catalyzing the inulin
conversion.15,21,42 However, no direct and strong evidence
has supported this suggestion so far.
According to the Stokes−Einstein equation, the diffusion
coefficient of a species in solution is dependent on its weight
and size. If a strong interaction between the Sn4+ and the
reactants (sucrose and fructose) occur and a complex is formed,
this larger and heavier complex species will diffuse more slowly
than the substrate. Correspondingly, a smaller diffusion
coefficient will be obtained in the DOSY NMR spectra’s F1
dimension.30,43 Initially, diffusion coefficients for the model
compound for hydrolysis reaction, sucrose, in SnCl4 aqueous
solution (pH 0.6) and HCl solution (pH 0.6) were measured
(Figure 5a and b). Normally, in 2D DOSY NMR spectra, large
molecules with a smaller D appear in the upper portion of the
spectrum, while small molecules with larger D appear in the
bottom of the spectrum. By comparing the spectra, it is clear
that the diffusion coefficient of sucrose in SnCl4 containing
solution (3.9 × 10−10 m2 s−1) is smaller than that in HCl
solution (4.8 × 10−10 m2 s−1), which means sucrose in SnCl4
solution diffuses more slowly than in HCl solution.
This suggests a complex formation between the sucrose
(molecular weight of 342) and Sn4+ (ionic weight of 119), and
this strong interaction promotes the breaking of the glycosidic
bond. However, on the basis of the present experimental
results, the formula weight of complex, or the ratio between
sucrose and Sn4+ in the complex, cannot be extracted.
Subsequently, we employed the monomeric model compound
of inulin, fructose, to check the diffusion coefficient in SnCl4
aqueous solution (pH 0.6) and HCl solution (pH 0.6). As
expected, the DOSY NMR spectra (Figure 5c and d) showed
that the diffusion coefficient of fructose in the SnCl4 containing
solution (4.8 × 10−10 and 5.3 × 10−10 m2 s−1) is smaller than
that in the HCl solution (5.8 × 10−10 m2 s−1). Notably, there
are two sets of DOSY NMR signals for fructose in the SnCl4
containing solution. The signals in the top of the spectrum
(Figure 5c) belong to the β-furanose tautomer, while the
signals in the bottom of the spectra come from the most βpyranose tautomer.44 Thus, DOSY NMR results suggest that
there is a stronger interaction between the Lewis acidic ion,
Sn4+ and the β-furanose tautomer. The β-furanose has been
shown to be important for 5-HMF formation through
dehydration. In addition to this effect, the binding of the
Lewis acid metal ion Sn4+ to the substrate and the resulting
complex formation weaken the glycosidic bonds, leading to a
Figure 5. 2D DOSY NMR spectra obtained at 25 °C. (a) Sucrose with
SnCl4 (D = 3.9 × 10−10 m2 s−1, SD = 2.424 × 10−3); (b) sucrose with
HCl (D = 4.8 × 10−10 m2 s−1, SD = 1.103 × 10−2); (c) fructose with
SnCl4 (the upper line D = 4.8 × 10−10 m2 s−1, SD = 3.780 × 10−4, the
bottom line D = 5.3 × 10−10 m2 s−1, SD = 4.231 × 10−4); (d) fructose
with HCl (D = 5.8 × 10−10 m2 s−1, SD = 1.179 × 10−2).
faster hydrolysis and dehydration of inulin to produce
monosaccharides and thereafter the desired platform molecules.
■
CONCLUSIONS
The catalytic abilities of Lewis acid SnCl4 and Brønsted acid
HCl for the biorefining of inulin were evaluated by NMR.
SnCl4 exhibits a stronger catalytic effect than the HCl for both
hydrolysis and dehydration reaction of inulin. The small
diffusion coefficients of sucrose and fructose in SnCl4 solution
were determined by 2D DOSY NMR. This suggests that a
strong interaction between the substrate and the Lewis acidic
ion Sn4+, and this interaction and complex formation promote
both the hydrolysis and dehydration reaction of inulin. This is
an additional function of Lewis acid catalyst Sn4+ together with
the intrinsic Brønsted acidity derived from its hydrolysis. Our
research results have shed light on the role of a Lewis acid in
biomass conversion.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acssuschemeng.6b00377.
In situ NMR data of sucrose hydrolysis in ZnCl2·4D2O
solution with a catalytic amount of SnCl4 and 1H and 13C
NMR spectra for inulin hydrolysis, and pictures before
and after inulin hydrolysis reaction mixture (PDF)
3331
DOI: 10.1021/acssuschemeng.6b00377
ACS Sustainable Chem. Eng. 2016, 4, 3327−3333
Research Article
ACS Sustainable Chemistry & Engineering
■
in a Biphasic Reactor with an Alkylphenol Solvent. ACS Catal. 2012, 2,
930−934.
(15) Tian, G.; Tong, X.; Cheng, Y.; Xue, S. Tin-catalyzed efficient
conversion of carbohydrates for the production of 5-hydroxymethylfurfural in the presence of quaternary ammonium salts. Carbohydr. Res.
2013, 370, 33−37.
(16) Wang, Y.; Pedersen, C. M.; Deng, T.; Qiao, Y.; Hou, X. Direct
conversion of chitin biomass to 5-hydroxymethylfurfural in concentrated ZnCl2 aqueous solution. Bioresour. Technol. 2013, 143, 384−
390.
(17) Wang, Y.; Pedersen, C. M.; Qiao, Y.; Deng, T.; Shi, J.; Hou, X.
In situ NMR spectroscopy: inulin biomass conversion in ZnCl2 molten
salt hydrate medium-SnCl4 addition controls product distribution.
Carbohydr. Polym. 2015, 115, 439−443.
(18) Hu, S.; Zhang, Z.; Song, J.; Zhou, Y.; Han, B. Efficient
conversion of glucose into 5-hydroxymethylfurfural catalyzed by a
common Lewis acid SnCl4 in an ionic liquid. Green Chem. 2009, 11,
1746−1749.
(19) Ren, J.; Wang, W.; Yan, Y.; Deng, A.; Chen, Q.; Zhao, L.
Microwave-assisted hydrothermal treatment of corncob using tin(IV)
chloride as catalyst for furfural production. Cellulose 2016,
DOI: 10.1007/s10570-016-0898-x.
(20) Choudhary, V.; Mushrif, S. H.; Ho, C.; Anderko, A.; Nikolakis,
V.; Marinkovic, N. S.; Frenkel, A. I.; Sandler, S. I.; Vlachos, D. G.
Insights into the interplay of Lewis and Bronsted acid catalysts in
glucose and fructose conversion to 5-(hydroxymethyl)furfural and
levulinic acid in aqueous media. J. Am. Chem. Soc. 2013, 135, 3997−
4006.
(21) Enslow, K. R.; Bell, A. T. SnCl4-catalyzed isomerization/
dehydration of xylose and glucose to furanics in water. Catal. Sci.
Technol. 2015, 5, 2839−2847.
(22) Kimura, H.; Nakahara, M.; Matubayasi, N. Solvent effect on
pathways and mechanisms for D-fructose conversion to 5-hydroxymethyl-2-furaldehyde: in situ 13C NMR study. J. Phys. Chem. A 2013,
117, 2102−2113.
(23) Chen, X.; Chew, S. L.; Kerton, F. M.; Yan, N. Direct conversion
of chitin into a N-containing furan derivative. Green Chem. 2014, 16,
2204−2212.
(24) Amarasekara, A. S.; Owereh, O. S.; Ezeh, B. Interactions of Dcellobiose with p-toluenesulfonic acid in aqueous solution: a (13)C
NMR study. Carbohydr. Res. 2011, 346, 2820−2822.
(25) Akien, G. R.; Qi, L.; Horvath, I. T. Molecular mapping of the
acid catalysed dehydration of fructose. Chem. Commun. 2012, 48,
5850−5852.
(26) Amarasekara, A. S.; Wiredu, B. A comparison of dilute aqueous
p-toluenesulfonic and sulfuric acid pretreatments and saccharification
of corn stover at moderate temperatures and pressures. Bioresour.
Technol. 2012, 125, 114−118.
(27) Zhang, J.; Weitz, E. Anin SituNMR Study of the Mechanism for
the Catalytic Conversion of Fructose to 5-Hydroxymethylfurfural and
then to Levulinic Acid Using13C Labeledd-Fructose. ACS Catal. 2012,
2, 1211−1218.
(28) Frihed, T. G.; Bols, M.; Pedersen, C. M. Mechanisms of
glycosylation reactions studied by low-temperature nuclear magnetic
resonance. Chem. Rev. 2015, 115, 4963−5013.
(29) Zhang, J.; Yu, X.; Zou, F.; Zhong, Y.; Du, N.; Huang, X. RoomTemperature Ionic Liquid System Converting Fructose into 5Hydroxymethylfurfural in High Efficiency. ACS Sustainable Chem.
Eng. 2015, 3, 3338−3345.
(30) Subramanian, H.; Jasperse, C. P.; Sibi, M. P. Characterization of
Bronsted acid-base complexes by (1)(9)F DOSY. Org. Lett. 2015, 17,
1429−1432.
(31) Neufeld, R.; Stalke, D. Accurate molecular weight determination
of small molecules via DOSY-NMR by using external calibration
curves with normalized diffusion coefficients. Chem. Sci. 2015, 6,
3354−3364.
(32) Bjorneras, J.; Nilsson, M.; Maler, L. Analysing DHPC/DMPC
bicelles by diffusion NMR and multivariate decomposition. Biochim.
Biophys. Acta, Biomembr. 2015, 1848, 2910−2917.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected] (Y.W.).
*E-mail: [email protected] (X.H.).
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was the Research Project Supported by Shanxi
Scholarship Council of China (2015-123) and the Natural
Science Foundation of China (No. 21472247). Y.Q. thanks the
Chinese Academy of Sciences (2013YC002) and the Youth
Innovation Promotion Association of Chinese Academy of
Sciences (2011137) for financial support. C.M.P. acknowledges
the CAS President’s International Fellowship Initiative
(2015VMB052).
■
REFERENCES
(1) Farran, A.; Cai, C.; Sandoval, M.; Xu, Y.; Liu, J.; Hernaiz, M. J.;
Linhardt, R. J. Green solvents in carbohydrate chemistry: from raw
materials to fine chemicals. Chem. Rev. 2015, 115, 6811−6853.
(2) van Putten, R. J.; van der Waal, J. C.; de Jong, E.; Rasrendra, C.
B.; Heeres, H. J.; de Vries, J. G. Hydroxymethylfurfural, a versatile
platform chemical made from renewable resources. Chem. Rev. 2013,
113, 1499−1597.
(3) Ge, W.; Zhang, J. H.; Pedersen, C. M.; Zhao, T.; Yue, F.; Chen,
C.; Wang, P.; Wang, Y.; Qiao, Y. DOSY NMR: A Versatile Analytical
Chromatographic Tool for Lignocellulosic Biomass Conversion. ACS
Sustainable Chem. Eng. 2016, 4, 1193−1200.
(4) Caes, B. R.; Teixeira, R. E.; Knapp, K. G.; Raines, R. T. Biomass
to Furanics: Renewable Routes to Chemicals and Fuels. ACS
Sustainable Chem. Eng. 2015, 3, 2591−2605.
(5) Qiao, Y.; Pedersen, C. M.; Wang, Y.; Hou, X. NMR Insights on
the Properties of ZnCl2 Molten Salt Hydrate Medium through Its
Interaction with SnCl4 and Fructose. ACS Sustainable Chem. Eng.
2014, 2, 2576−2581.
(6) Barclay, T.; Ginic-Markovic, M.; Johnston, M. R.; Cooper, P. D.;
Petrovsky, N. Analysis of the hydrolysis of inulin using real time 1H
NMR spectroscopy. Carbohydr. Res. 2012, 352, 117−125.
(7) Foo, G. S.; Van Pelt, A. H.; Krötschel, D.; Sauk, B. F.; Rogers, A.
K.; Jolly, C. R.; Yung, M. M.; Sievers, C. Hydrolysis of Cellobiose over
Selective and Stable Sulfonated Activated Carbon Catalysts. ACS
Sustainable Chem. Eng. 2015, 3, 1934−1942.
(8) Jiang, F.; Zhu, Q.; Ma, D.; Liu, X.; Han, X. Direct conversion and
NMR observation of cellulose to glucose and 5-hydroxymethylfurfural
(HMF) catalyzed by the acidic ionic liquids. J. Mol. Catal. A: Chem.
2011, 334, 8−12.
(9) Omari, K. W.; Besaw, J. E.; Kerton, F. M. Hydrolysis of chitosan
to yield levulinic acid and 5-hydroxymethylfurfural in water under
microwave irradiation. Green Chem. 2012, 14, 1480−1487.
(10) Zhao, H.; Holladay, J. E.; Brown, H.; Zhang, Z. C. Metal
chlorides in ionic liquid solvents convert sugars to 5-hydroxymethylfurfural. Science 2007, 316, 1597−600.
(11) Zhuo, K.; Du, Q.; Bai, G.; Wang, C.; Chen, Y.; Wang, J.
Hydrolysis of cellulose catalyzed by novel acidic ionic liquids.
Carbohydr. Polym. 2015, 115, 49−53.
(12) Caratzoulas, S.; Davis, M. E.; Gorte, R. J.; Gounder, R.; Lobo, R.
F.; Nikolakis, V.; Sandler, S. I.; Snyder, M. A.; Tsapatsis, M.; Vlachos,
D. G. Challenges of and Insights into Acid-Catalyzed Transformations
of Sugars. J. Phys. Chem. C 2014, 118, 22815−22833.
(13) Deng, T.; Cui, X.; Qi, Y.; Wang, Y.; Hou, X.; Zhu, Y.
Conversion of carbohydrates into 5-hydroxymethylfurfural catalyzed
by ZnCl2 in water. Chem. Commun. 2012, 48, 5494−5496.
(14) Pagán-Torres, Y. J.; Wang, T.; Gallo, J. M. R.; Shanks, B. H.;
Dumesic, J. A. Production of 5-Hydroxymethylfurfural from Glucose
Using a Combination of Lewis and Brønsted Acid Catalysts in Water
3332
DOI: 10.1021/acssuschemeng.6b00377
ACS Sustainable Chem. Eng. 2016, 4, 3327−3333
Research Article
ACS Sustainable Chemistry & Engineering
(33) Simpson, A. J.; McNally, D. J.; Simpson, M. J. NMR
spectroscopy in environmental research: from molecular interactions
to global processes. Prog. Nucl. Magn. Reson. Spectrosc. 2011, 58, 97−
175.
(34) Rodrigues, E. D.; da Silva, D. B.; de Oliveira, D. C.; da Silva, G.
V. DOSY NMR applied to analysis of flavonoid glycosides from Bidens
sulphurea. Magn. Reson. Chem. 2009, 47, 1095−1100.
(35) Cohen, Y.; Avram, L.; Frish, L. Diffusion NMR spectroscopy in
supramolecular and combinatorial chemistry: an old parameter–new
insights. Angew. Chem., Int. Ed. 2005, 44, 520−554.
(36) Bakkour, Y.; Darcos, V.; Li, S.; Coudane, J. Diffusion ordered
spectroscopy (DOSY) as a powerful tool for amphiphilic block
copolymer characterization and for critical micelle concentration
(CMC) determination. Polym. Chem. 2012, 3, 2006−2010.
(37) Guang, J.; Hopson, R.; Williard, P. G. Diffusion CoefficientFormula Weight (D-FW) Analysis of (2)H Diffusion-Ordered NMR
Spectroscopy (DOSY). J. Org. Chem. 2015, 80, 9102−9107.
(38) Su, C.; Hopson, R.; Williard, P. G. Characterization of
cyclopentyllithium and cyclopentyllithium tetrahydrofuran complex.
J. Am. Chem. Soc. 2013, 135, 12400−12406.
(39) Li, W.; Chung, H.; Daeffler, C.; Johnson, J. A.; Grubbs, R. H.
Application of (1)H DOSY for Facile Measurement of Polymer
Molecular Weights. Macromolecules 2012, 45, 9595−9603.
(40) Rundlöf, T.; Mathiasson, M.; Bekiroglu, S.; Hakkarainen, B.;
Bowden, T.; Arvidsson, T. Survey and qualification of internal
standards for quantification by 1H NMR spectroscopy. J. Pharm.
Biomed. Anal. 2010, 52, 645−651.
(41) Oliveira, R. C.; Hammer, P.; Guibal, E.; Taulemesse, J.-M.;
Garcia, O. Characterization of metal−biomass interactions in the
lanthanum(III) biosorption on Sargassum sp. using SEM/EDX, FTIR,
and XPS: Preliminary studies. Chem. Eng. J. 2014, 239, 381−391.
(42) Lew, C. M.; Rajabbeigi, N.; Tsapatsis, M. One-Pot Synthesis of
5-(Ethoxymethyl)furfural from Glucose Using Sn-BEA and Amberlyst
Catalysts. Ind. Eng. Chem. Res. 2012, 51, 5364−5366.
(43) Weisheim, E.; Reuter, C. G.; Heinrichs, P.; Vishnevskiy, Y. V.;
Mix, A.; Neumann, B.; Stammler, H. G.; Mitzel, N. W. Tridentate
Lewis Acids Based on 1,3,5-Trisilacyclohexane Backbones and an
Example of Their Host-Guest Chemistry. Chem. - Eur. J. 2015, 21,
12436−12448.
(44) Barclay, T.; Ginic-Markovic, M.; Johnston, M. R.; Cooper, P.;
Petrovsky, N. Observation of the keto tautomer of D-fructose in D(2)
O using (1)H NMR spectroscopy. Carbohydr. Res. 2012, 347, 136−
141.
3333
DOI: 10.1021/acssuschemeng.6b00377
ACS Sustainable Chem. Eng. 2016, 4, 3327−3333