Highly active niobium-loaded montmorillonite catalysts for the production of 5-hydroxymethylfurfural from glucose

Article abstract of DOI:10.1039/c9gc01225g

Bio-based platform chemical 5-hydroxymethylfurfural (5-HMF) can be converted into large-scale, high value-added compounds generally obtained from petroleum. Hence, developing an efficient catalytic system to transform glucose (a cheap and abundant branch of biomass) into 5-HMF has drawn significant attention. Earlier reports have concentrated on a two-step conversion of glucose to 5-HMF involving glucose isomerization into fructose and subsequent dehydration. In this work, efficient heterogeneous niobium-loaded montmorillonite (Nb-MMT) catalysts containing Lewis and Br?nsted acid sites have been synthesized by a readily cation-exchanged synthetic method and were used to convert glucose and showed high selectivity and conversion. An optimum 5-HMF yield of 70.52% with a 99% conversion of glucose was achieved in a biphasic solvent of methyl isobutyl ketone (MIBK)/water at 170 °C for 3 h. The catalytic system also exhibited an excellent activity in the conversion of disaccharides and polysaccharides to 5-HMF with satisfactory yields. In addition, Nb-MMT was effectively reused four times without significant loss of activity. Nb-MMT also featured good catalytic activity and selectivity towards carbohydrate conversion in water. Furthermore, the reaction mechanism for the transformation of glucose to 5-HMF over Nb-MMT was investigated by density functional theory (DFT) calculations, and the results showed that niobium oxyhydroxide species of Nb-MMT played an essential role in glucose conversion, and the catalytic mechanism was a synergetic proton transfer process via hydrogen bonding. This study paves the way for improvements in Nb-containing solid acid catalysts for conversion of carbohydrates.

Full text of DOI:10.1039/c9gc01225g

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Green Chemistry
PAPER
Highly Active Niobium-Loaded Montmorillonite Catalysts for the
Production of 5-Hydroxymethylfurfural from Glucose
Guo Qiu, a Chongpin Huang,*a Xiuliang Sun*a and Biaohua Chen b
Received 00th January 20xx,
Accepted 00th January 20xx
Bio-based platform chemical 5-hydroxymethylfurfural (5-HMF) can be converted into a large scale of high value-added
compounds generally obtained from petroleum. Hence, developing an efficient catalytic system to transform glucose (a
cheap and abundant branch of biomass) into 5-HMF has drawn significant attention. Earlier reports have concentrated on a
two-step conversion of glucose to 5-HMF involving glucose isomerization into fructose and subsequent dehydration. In this
work, efficient heterogeneous niobium-loaded montmorillonite (Nb-MMT) catalysts containing Lewis and Brønsted acid
sites have been synthesized by a readily cation-exchanged synthetic method, and were used to convert glucose and showed
high selectivity and conversion. The optimum 5-HMF yield of 70.52% with 99% conversion of glucose was achieved in a
biphasic solvent of methyl isobutyl ketone(MIBK)/water at 170 oC for 3 h. The catalytic system also exhibited excellent
activity in the conversion of disaccharides and polysaccharides to 5-HMF with satisfactory yields. In addition, Nb-MMT was
effectively reused four times without significant loss of activity. Nb-MMT also featured the good catalytic activity and
selectivity towards the carbohydrate conversion in water. Furthermore, the reaction mechanism for the transformation of
glucose to 5-HMF over Nb-MMT was investigated by density functional theory (DFT) calculations, and the results showed
that niobium oxyhydroxide species of Nb-MMT played an essential role in glucose conversion, and the catalytic mechanism
was a synergetic proton transfer process via hydrogen bonding. This study paves the way for improvements in Nb-containing
solid acid catalysts for conversion of carbohydrates.
DOI: 10.1039/x0xx00000x
www.rsc.org/
to its low cost and abundance.3, 10 The conversion of glucose to 5-
HMF is a two-step process, involving the isomerization of glucose
into fructose and subsequent dehydration.12 Glucose-fructose
isomerization is a significant step in forming 5-HMF and can be
catalyzed by Lewis acids in low yields. The isomerization process is
challenging due to the stability of the glucose ring. On the other
hand, the subsequent dehydration of fructose occurs by the catalysis
of Brønsted acid.13 Thus, it is necessary to design a catalyst that can
efficiently catalyze the isomerization of glucose to fructose and
subsequent dehydration of fructose to 5-HMF.
1. Introduction
Growing concerns about the severity of global energy crisis and
environmental pollution have caused the search for alternatives to
finite fossil resources to efficiently supply energy and chemicals
based on renewable feedstock.1 Biomass resources are regarded as
clean, abundant and renewable energy sources, which have been the
focus as substitutes for fossil fuels to effectively alleviate the
environmental pollution and excessive consumption of fossil energy
confronting humanity critically.2-4 One of the most important
branches of non-edible biomass is carbohydrates which can be
converted to 5-hydroxymethylfurfural (5-HMF) through specific
catalytic process.5 Considered as one of the top 10 value-added,
potentially versatile platform chemicals obtained from biomass, 5-
HMF can be widely utilized in the production of a variety of non-
petroleum high-value chemicals.6-8
One conventional synthetic route to prepare 5-HMF is through
the acid-catalyzed conversion of hexoses. Among all hexoses,
fructose is the most efficient raw material producing higher yields of
5-HMF.9, 10 However, industrial applications of fructose for 5-HMF
synthesis are limited due to cost considerations.11 Instead, glucose is
regarded as a potential feedstock for the production of 5-HMF due
Through the continuous efforts of researchers, a variety of
catalysts have been used for high-yield conversion of glucose to 5-
HMF. Some homogeneous catalysts such as mineral acids,14 Lewis
acids,15 could be used to obtain higher 5-HMF yield, but there were
serious drawbacks in terms of the separation and recovery of the
catalyst. To overcome these difficulties, some groups have tried to
develop heterogeneous catalysts for converting glucose to 5-HMF,16
such as cation exchange resins,17 metal oxides,18-20 phosphates,21, 22
and zeolites.23 Among these,
a kind of niobium-containing
heterogeneous catalyst with high stability and strong acidic property
has shown excellent performance in the transformation of glucose-
based substrates to 5-HMF; in this system, Nb5+ ions serve as the
preferred Lewis acid sites for isomerization.24, 25 Zhang et al. found
that porous niobium phosphate (NbOPO4) with tunable acidity and
L/B acid ratios could be a kind of effective solid acid catalyst (39.3%
yield of 5-HMF) in a one-pot conversion of glucose to 5-HMF in
aqueous solution.26 Li et al. reported that niobia/carbon (Nb/C)
composites could actively catalyze glucose transformation to 5-
a. State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, Beijing 100029, China. E-mail: huangcp@mail.buct.edu.cn;
sunxl@mail.buct.edu.cn. Fax: +86 10 6441 9619; Tel: +86 10 6441 2054
b. College of Environmental and Energy Engineering, Beijing University of Technology,
Beijing 100124, China.
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Green Chemistry
HMF.27 However, preparation of these catalysts was complex, and
Pyridine-adsorbed FTIR spectroscopy and VtieewmAprteicrleaOtunrlinee-
hydrofluoric acid (HF) must be used during preparation. HF is a programmed desorption of NH3 (NH3-TPD) (DOOmI:n1i0S.t1a0r3,9M/CS92G0C00)1w22e5rGe
colorless, poisonous, corrosive, and volatile liquid; it is harmful to employed to determine the acid type and total acid sites of Nb-MMT.
humans and the environment, so developing synthetic methods that Structural information of Nb-MMT was obtained from X-ray
do not involve potentially toxic reagents is one step closer to diffraction (XRD) (D8FOCUS, Bruker), Fourier transform infrared
achieving green syntheses. To meet environment protection and safe spectroscopy (FTIR) (TENSOR 27, Bruker), X-ray photoelectron
production of 5-HMF from glucose, a green economical and effective spectroscopy (XPS) (ESCALAB-250, ThermoFisher Scientific), scanning
catalyst system is strongly required.
electron microscopy (SEM, Hitachi S-3400N), and Brunauer-Emmett-
On the other hand, the mechanism of 5-HMF synthesis from Teller surface area measurements (BET) (SORPTOMATIC 1990,
glucose has already been the subject of debate for some time. Thermo Electron Corporation). Elemental analysis of Nb-MMT was
Recently, computational chemistry has provided a new train of qualitatively determined by the X-ray fluorescence analysis (XRF)
thought for this field.28-30 The results of density functional theory (XRF-1800, Shimadzu). The compositions of niobium species was
(DFT) calculation show glucose isomerization occurs by proton analyzed by UV-vis diffuse reflectance spectroscopy (DRS) fitted with
transfer or intramolecular hydride; aldose is deprotonated and
a spectrophotometer (UV-3600, Shimadzu). In situ diffuse
isomerized through a series of oleate intermediates in the presence reflectance infrared Fourier transform spectroscopy (in situ DRIFTS)
of a base.1 Earlier reports have been generally concentrated upon a tests of NH3 adsorption were conducted to identify the presence of
two-step pathway for 5-HMF synthesis from glucose. Lewis acids Nb-MMT hydroxyl groups using an FTIR (Nicolet NEXUS 6700)
promote an intramolecular hydride shift and the open form of equipped with an MCT/A detector; the Nb-MMT was pretreated at
o
glucose is converted to the open form of fructose. Brønsted acids 350 C for 30 min under a nitrogen flow to remove any adsorbed
then facilitate dehydration of fructose by removing three water impurities.
32
molecules.31,
However, there are comparatively few reports 2.2 Catalysts preparation
combining DFT calculations with experimental results on niobium- Initially, calcium montmorillonite (MMT) was modified by niobium
containing catalysts. Therefore, an accessible and direct pentachloride with different niobium amounts (0.25 wt%, 0.5 wt%, 1
transformation mechanism of glucose to 5-HMF on niobium- wt%, 2 wt%, and 3 wt%). The ethanol solutions of NbCl5 with
containing catalysts was needed.
different concentrations were added to the MMT suspensions with
Herein, we develop a new and efficient niobium-exchanged constant stirring at 40 °C for 24 h. Then, the mixture was filtered,
montmorillonite (Nb-MMT) catalyst, in two-phase solvent system to washed with ethanol to remove excessive chlorides. Next, the
catalyze the conversion of glucose to 5-HMF with a high yield. sample was dried at 100 °C overnight. Finally, the collected was
Montmorillonite K-10 clay is a versatile low-cost stratified silicate ground into powder with a mortar and pestle, designated as Nb-
mineral with a three-layer structure, and can exchange with metal MMT. Moreover, a part of sample was calcined in static air for 3 h.
cations to form many effective heterogeneous catalysts for a variety These samples were labelled as xNb-MMT-y, where x and y represent
of reactions on account of large specific surface area and excellent the Nb loading and the calcination temperature, respectively.
cation-exchange capacity.33-35 These prepared Nb-MMT catalysts are 2.3 Dehydration of carbohydrates to 5-HMF
economically viable for large-scale 5-HMF production due to low In a typical dehydration reaction, carbohydrate, catalyst and 5 mL
cost, facile synthesis, high catalytic activity, and good chemical and solvent (monophasic or biphasic) were added to a sealed 15 mL tube,
environmental stability. A new catalytic mechanism for the direct which was located in the temperature-controlled oil bath and stirred
conversion of glucose to 5-HMF was verified by DFT calculations, at approximately 300 rpm. After the reaction, the mixture was
demonstrating that the active site was niobium oxyhydroxide species immediately put into the ice water and quenched to room
of Nb-MMT that played an essential role in glucose conversion; and temperature. Then removed the catalyst by filtering with a 0.2-μm
the catalytic mechanism is a synergetic proton transfer process by syringe filter. The solvents in different phases were diluted with
hydrogen bonding.
different diluents prior to analysis. The filtered catalyst was washed
at least three times with ethanol, and was dried overnight at 100 °C
for recycling.
2. Experimental Section
The analysis of the product was performed by HPLC (Agilent
1200, USA) attached with a C18 column and a UV detector. The
mobile phase was a mixture of methanol/deionized water (40/60
v/v; flow rate 1 mL/min). HPLC using a Hypersil APS-2 column and a
Shodex IR detector was employed to analyze residual carbohydrates,
and water was the eluent with a flow velocity of 0.2 mL/min at 35 °C.
2.4 DFT calculation
Quantum chemistry based on density functional theory (DFT) was
utilized to verify the catalytic mechanism for the transformation of
glucose to 5-HMF over Nb-MMT. Wyoming montmorillonite, which
has been extensively studied at home and abroad, was constructed
in the Materials Studio 7.0.36 Si-O double ring close to active sites,
was adopted in this work (Fig 1). The active sites Nb oxyhydroxide
formed were simplified as [NbO(OH)] model. Through the use of the
2.1 Materials and instrumentation
5-HMF (98%), glucose (99%), fructose (99%), cellobiose (99%),
sucrose (99%), starch (99%), cellulose (99%), inulin (99%) and
montmorillonite K-10 were analytical grade and obtained from the
Aladdin Reagent Limited Company (Shanghai, China).
Methylisobutylketone
(MIBK),
dimethylsulfoxide
(DMSO),
dimethylacetamide (DMA), dimethylformamide (DMF), N-
methylpyrrolidone (NMP), acetone, methanol, and ethanol were
purchased from J&K Scientific Limited Company. (Beijing, China).
Niobium (V) chloride and NaCl were obtained from the Alfa Aesar
Chemical Ltd. (Shanghai, China). All reagents were used as supplied
without further purification.
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which expanded the layer spacing, and coordination of the interlayer
View Article Online
DOI: 10.1039/C9GC01225G
ions to arise polyhydroxyniobium. In addition, the diffraction peak at
6.17° of Nb-loaded MMT slightly shifts to a lower angle (6.05°),
suggesting the layer spacing increase of MMT.39 According to
previous reports, some metal oxide pillars would be generated from
calcination of metal ion-exchanged layered clays, resulting in the
increase of lamellar distance. The calcined Nb-MMT emerged
inorganic pillarization process, and the corresponding oxides would
be formed with the transformation of polyhydroxyniobium.39 No
peaks of niobium compounds appeared in XRD patterns, because the
loaded niobium compounds were usually amorphous and did not
produce diffraction peaks.
Fig. 1 Structure diagram of the computational model. (a) niobium-
loaded montmorillonite. (b) optimized Si-O double ring model.
The FTIR spectra of Nb-MMT materials are shown in Fig. 3. There
were several characteristic bands in the FTIR spectra, consistent with
the original structure of MMT. The band at approximately 1635 cm-1
corresponded to the adsorption of water molecules on MMT, the
adsorption bands at approximately 1096 cm-1, 1040 cm-1 and 520 cm-
1
were associated with the vibrations of Si-O and the adsorption
bands at 915 cm-1 was assigned to the Al-O vibrations.40,
No
41
significant differences were observed in FTIR spectra of Nb-MMT
with different Nb loadings. The structure of MMT was stable in Nb-
MMT catalysts. After calcination, some absorption peaks at 1096 and
1040 cm-1 shifted slightly due to the interaction between the
framework of montmorillonite layers and niobium.
Fig. 2 X-ray diffraction patterns of (a) MMT, (b) 0.25Nb-MMT, (c)
0.5Nb-MMT, (d) 1Nb-MMT, (e) 2Nb-MMT, (f) 3Nb-MMT, (g) 1Nb-
MMT-500, and (h) 1Nb-MMT-900.
The SEM images (Fig. S1) confirmed that there were some lamellar
structure; the original layered structure of MMT was remained intact
in Nb-MMT, indicating that the intercalation of niobium and the
calcination process did not destroy the layer structure. XPS spectra
of niobium in Nb-MMT samples are illustrated in Fig. 4a. The Nb 3d
spectrum comprises two peaks centered at around 207.5 eV and
210.2 eV that correspond to Nb 3d5/2 and Nb 3d3/2, respectively,
Fig. 3 FTIR spectra of (a) MMT, (b) 0.25Nb-MMT, (c) 0.5Nb-MMT, (d)
1Nb-MMT, (e) 2Nb-MMT, (f) 3Nb-MMT, (g) 1Nb-MMT-500, and (h)
1Nb-MMT-900.
LYP correlation function (B3LYP) in Gaussian 09, the DFT algorithm
was associated with hybrid density functional methods.37 For Si, O, H
and C atoms, the 6-31G(d) basis set was utilized to calculate the
geometry optimization (GO), transition state (TS), and intrinsic
reaction coordinate (IRC), whereas the high-level 6-311++G(d, p)
basis set was needed for energy calculation.38 The higher-level
LANL2DZ basis set was used for the calculations of Nb atom. In
addition, the GO models were identified as stationary points via the
frequency calculations (zero imaginary frequencies). The TS clusters
were tested by both frequency calculations (one imaginary
frequency) and IRC path analysis.
3. Results and Discussion
3.1 Characterization of Nb-MMT materials
The XRD patterns of Nb-MMT materials are displayed in Fig. 2. The
characteristic diffraction peak at approximately 26.9° was consistent
with the framework of montmorillonite, which indicated that the
cation-exchanged synthetic process had not destroyed the structure
of the montmorillonite layer. The relative intensity of the diffraction
peak at 29.7° decreased with increased Nb loading; this decrease was
attributed to the intercalation of niobium into montmorillonite layer,
Fig. 4 (a) Nb 3d core level spectra of Nb-MMT materials. (b) XPS
spectra of O 1s of 1Nb-MMT-900 sample. (c) UV-vis DRS spectra of
Nb-MMT material (inset: enlarged graph corresponding to two
peaks).
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Table 1 Characterization of Nb-MMT materials
View Article Online
DOI: 10.1039/C9GC01225G
Elemental content (wt%)b
Total acid sites Weak acid sites Strong acid sites
Catalyst
SBET (m2 g-1)
(mmol g-1)a
(mmol g-1)a
(mmol g-1)a
Si
Al
Ca
Na
Nb
MMT
77.46
2.06
1.36
0.86
0.64
0
2.06
0.96
0.49
0.38
34.53 7.38
34.73 7.56
35.01 7.23
34.56 7.49
4.16
2.54
2.27
2.13
0.18
0.17
0.20
0.19
0.00
1.09
1.52
1.85
1Nb-MMT
125.33
0.4
1Nb-MMT-500 168.49
0.37
1Nb-MMT-900 215.24
0.26
a Determined from ammonia NH3-TPD. b Determined by XRF.
which are characteristic of pentavalent niobium.42 The O 1s XPS decreased gradually along with the temperature increased from 50-
spectrum (Fig. 4b) of the 1Nb-MMT-900 sample is composed of two 300 °C.47 This suggested that Brønsted and Lewis acid sites coexisted
peaks, centered at approximately 530.9 and 534.1ꢀeV. The major simultaneously in Nb-MMT catalysts and Brønsted acid sites were
peak at 530.9ꢀeV was consistent with O 1s Si-O, Al-O or Nb-O bonds strongly bound up with the existence of Nb-OH groups. The type of
in sample, while the minor peak of higher binding energy (534.1ꢀeV) acid site on Nb-MMT materials was also monitored by pyridine
was responsible for the hydroxyl OH- group of Nb-OH in the adsorption IR spectra. Fig. 5(b) shows pyridine adsorption IR spectra
interlayer.43 The UV-vis DRS spectra of Nb-MMT are presented in Fig. of 1Nb-MMT. The characteristic absorption band at 1543 cm-1,
4c. The spectra of four MMT materials show a band at 246 nm; this corresponding to the Brønsted acid sites, originated from adsorption
band was assigned to the Si-O framework of montmorillonite, while of pyridine molecules at Nb-OH group.48 As the temperature
the niobium modified MMT catalysts show a weak electronic increased to 200 °C, the corresponding band intensity exhibited a
absorption peak at 236 nm arising from ligand-to-metal charge slight decrease; this could be attributed to the stability of Nb-OH
transfer transitions of oxygen-tetrahedral Nb(V).44 The coordinated groups in the sample. Furthermore, the band at approximately 1446
Nb(V) species are often considered to be the catalytically active sites. cm-1 related to Lewis acid sites was strong, which were in accordance
As seen in Table 1, niobium ions were successfully substituted with pyridine molecules interact with niobium ions.49 An adsorption
for calcium and sodium ions, and inserted into the montmorillonite band at 1490 cm-1 was ascribed to pyridine connected with both
layers. Besides, the Nb amounts of different Nb-MMT samples based Brønsted and Lewis acid sites.40 These results showed that Nb-MMT
on XRF were in good agreement with theoretical values (Table S1 and possessed
S2). The Nb content (wt%) of the calcined catalyst was higher due to
the disappearance of some impurities after calcination. The
incorporation of Nb species into the montmorillonite framework
increased the specific surface area, which may be due to facilitation
of stripping the montmorillonite layers upon incorporation of
niobium into the montmorillonite. Additionally, the density of total
acid sites and acid strength of Nb-MMT materials were determined
by NH3-TPD, and the amount of total acid sites decreased after
chemical modifications (Table 1). The niobium modified MMT had
fewer acid sites than MMT, and the NH3-TPD of Nb-MMT catalysts
showed desorption peaks of weak-, strong-acid sites; these were
examined based on NH3 desorption temperatures (150-300 °C, 300-
600 °C, respectively). The intensity of both weak acid sites and strong
acid sites declined with increasing calcination temperature, because
the calcination process promoted the transformation of
polyhydroxyniobium to niobium oxide, leading to the decrease of the
OH- groups from Nb-OH in the interlayer.
both
Brønsted
and
Lewis
acidity.
In situ DRIFTS spectra of NH3 adsorption over 1Nb-MMT are
shown in Fig 5(a). Three positive bands at approximately 3377 cm-1,
3259 cm-1 and 3156 cm-1 were in line with N-H stretching vibration
modes.45 The bands at 1441 and 1683 cm-1 were in accordance with
+
the asymmetric and symmetric bending vibrations of ionic NH4
species interacting with Brønsted acid sites, although the bands at
1597 cm-1 and 1217 cm-1 originated from the bending vibrations of
NH3 species connected to Lewis acid sites.46 Meanwhile, a negative
Fig. 5 In situ DRIFTS spectra of NH3 adsorption (a) and IR spectra of
pyridine adsorption (b) over 1Nb-MMT catalyst.
band at about 3688 cm-1 were consistent with the interaction of Nb-
+
OH groups with NH3 to generate NH4 , and the relative intensity
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(Table 2, entry 1), indicating it was tough for glucoseVcieownAvreticrlseioOnnlinine
the absence of catalyst. The yield of 5-HMF was zero when
Table 2 Summary of glucose conversion and 5-HMF yield
DOI: 10.1039/C9GC01225G
catalyzed by various catalystsa
unmodified Ca-MMT was employed (Table 2, entry 2), suggesting
that Ca-MMT has no effect on the conversion of glucose, which are
in agreement with the results reported by Wang et al.50 The yield of
5-HMF and the conversion of glucose increased synchronously with
the increment of Nb content in the range of 0.25-1 (Table 2, entries
3-5), showing the introduction of niobium promoted the
transformation of glucose. However, with further increasing the Nb
loading, the yield of 5-HMF was not increased as expected (Table 2,
entries 5-7). This is because the excess Nb possibly led to
agglomeration at the active sites during the reaction. The 1 wt%
loading of Nb in the catalyst was appropriate for obtaining higher
yield of 5-HMF, owing to the proper distribution of the active
components in the interlayer of montmorillonite. Previous reports
suggested an appropriate amount of Nb-OH and an optimal ratio of
L/B acid sites were also responsible for the excellent catalytic activity
of 1-Nb-MMT.12 In addition, a similar tendency was also observed in
the calcined Nb-MMT catalysts (Fig S3), hence the 1 wt% amount of
Nb was chosen as the optimal loading under the reaction conditions.
Almost complete conversion of glucose and a 70.52% yield of 5-HMF
was obtained when 1Nb-MMT-900 was used as the catalyst (Table 2,
entry 9), which exhibited excellent catalytic activity in 5-HMF
synthesis. However, 55.6% and 58.4% yields of 5-HMF were provided
(Table 2, entries 5 and 8) when it was catalyzed by 1Nb-MMT and
1Nb-MMT-500, respectively, demonstrating that calcination
promoted the increase of the specific surface area and exposure of a
large number of catalytic active sites, and the yield of 5-HMF
increased as the calcination temperature increased. The catalytic
activity of 1Nb-MMT-900 was superior to other catalysts, due to its
more catalytic active sites required for glucose isomerization and
subsequent dehydration. It is worth noting that lower conversion of
glucose and 0% yield of 5-HMF was gained with homogeneous NbCl5
(Table 2, entry 10) as the catalyst, showing that Nb5+ could make no
effort in 5-HMF synthesis. Other niobium-containing heterogeneous
catalysts used in converting glucose to 5-HMF are summarized in
Table S3. Comparatively speaking, the catalytic activity of Nb-MMT
material was clearly superior to other niobium-containing catalysts
under optimal reaction conditions due to its high stability and strong
acidic properties. Moreover, one clear advantage of Nb-MMT
catalysts is their ease of synthesis relative to other catalysts and that
HF was unnecessary for preparation.
Experimental parameter
Entry
Catalyst
Conversion/%
Yield/%
1
2
No catalyst
Ca-MMT
7.8 ± 0.8
52.0 ± 2.5
60.7 ± 1.2
76.1 ± 0.9
80.5 ± 0.2
78.2 ± 2.3
77.6 ± 0.4
95.2 ± 2.6
99.0 ± 0.8
64.2 ± 0.5
0 ± 0
0 ± 0
3
0.25Nb-MMT
0.5Nb-MMT
1Nb-MMT
36.8 ± 0.4
45.3 ± 0.8
55.6 ± 0.7
48.3 ± 0.5
45.6 ± 1.8
58.4 ± 0.3
70.5 ± 2.0
0 ± 0
4
5
6
2Nb-MMT
7
3Nb-MMT
8
1Nb-MMT-500
1Nb-MMT-900
NbCl5
9
10
aReaction conditions: 66.6 mM glucose, 40 mg catalyst, 170 oC, 3 h, 5 mL of
reaction solution, methyl isobutyl ketone/saturated sodium chloride solution
(MIBK-NaCl) =70 : 30 (v/v).
3.3 Determination of suitable processing conditions for 1Nb-
MMT-900-catalyzed glucose conversion to 5-HMF
Obviously, the conversion of glucose to 5-HMF was strongly
influenced by the reaction temperature and the investigated results
are shown in Fig. 6(a). With increasing temperature, the glucose
conversion rate was increased remarkably, and a great glucose
conversion (above 60%) was gained within 60 min at 170 °C. Under
lower temperatures, the conversion rate was reduced considerably;
for instance, a reaction time of 2 h resulted in only a 20% glucose
conversion at 140 °C. Fig. 6(b) displays the influence of reaction
temperature on 5-HMF yield, increasing significantly as the
temperature went from 140 °C to 170 °C. In addition, the highest
value of 5-HMF yield was rising with the increased temperature, and
a 29.26% yield of 5-HMF with greater than 60% glucose conversion
was acquired at 170 °C for 1 h. At a higher temperature (180 °C), the
Fig. 6 Effect of reaction temperature on conversion of glucose (a) and
yield of 5-HMF (b) (conditions: 66.6 mM glucose, 40 mg 1Nb-MMT-
900, 170 oC, 5 mL of reaction solution, MIBK-NaCl =70 : 30 (v/v)).
3.2 Comparison of catalyst activity and effect of Nb-MMT catalyst
on conversion of glucose to 5-HMF
The efficacies of Nb-containing catalysts on glucose conversion to 5-
HMF in biphasic solvent were studied at 170 °C for 3 h (Table 2). No
5-HMF formation was observed using the blank reference catalyst
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reaction time for maximum yield was shorter, but the yield of 5-HMF were employed in the reaction. There was an apparent change of 5-
View Article Online
was lower than what was obtained at 170 °C, owing to the formation HMF yields in diverse monophasic sDolOvIe:n10ts.1,03e9l/uCc9iGdaCt0in12g25G
of insoluble humins. Hence, the reaction temperature of 170 °C was considerable solvent effect (Fig. 7a). After reacting at 170 °C for 3 h,
chosen as the optimum reaction temperature. the yield of 5-HMF (below 40%) in monophasic solvents is drastically
a
The change of glucose conversion and 5-HMF yield under lower than that in two-phase solvents. Concretely, the glucose
different initial glucose concentration was studied (Table S4, entries conversion and 5-HMF yield in water reached to 63.0% and 18.4%,
1, 2 and 3). Notably, glucose transformation to 5-HMF was partially respectively, suggesting 1Nb-MMT-900 possessed stable catalytic
affected by the initial concentration of glucose, and glucose activity in water and trace amount of levulinic acid (LA) and formic
conversions at different initial concentrations were above 98%. acid (FA) were detected arising from 5-HMF rehydration in the
Moreover, a yield of 50.6% for 5-HMF was obtained at 133 mM of presence of H2O. With MIBK as the solvent, 1Nb-MMT-900 showed
initial glucose concentration. The lower the initial glucose poor catalytic activity, which may be attributed to the lower solubility
concentration between 66.6 mM to 133 mM, the higher the yield of of substrate. In NMP, DMA, DMF, and DMSO, glucose conversions
5-HMF, which is likely due to substrate molecules with lower were relatively high, because these polar aprotic solvents were well
concentration diffuse easily in the reaction medium, speeding up the suited for dehydration, and readily dissolved reactants.
shift of chemical equilibrium to 5-HMF. In contrast, a partial loss of Nevertheless, the occurrence of side reactions and the formation of
5-HMF at higher initial concentration was observed owing to the humins were responsible for low 5-HMF yield, and it required a large
appearance of side-reactions and undesirable by-products.
The effect of the dose of 1Nb-MMT-900 on 5-HMF synthetic
quantity of energy when 5-HMF separated from these high boiling-
point solvents. In addition, insoluble black humus in DMSO may
originate from condensation reaction between 5-HMF and glucose.51
A biphasic solvent containing MIBK and saturated sodium chloride
water was utilized to replace the single-phase system with
satisfactory results and a 70.52% yield of 5-HMF was obtained;
simultaneously, the color of the reaction solution also was clear
yellow with the organic phase being deeper in color relative to the
inorganic phase (Fig. S5). In biphasic reaction system, 5-HMF was
gradually produced in the inorganic phase with continuous
extraction into the organic phase to achieve the separation of
substrates and products; as a result, it helped facilitate 5-HMF yield
prominently.
reaction is presented in Table S4, entries 1, 4, 5 and 6. Obviously, the
increase of 1Nb-MMT-900 dosage could greatly accelerate the
reaction rate. Under the same reaction conditions, the addition of
more 1Nb-MMT-900 led higher 5-HMF yield and glucose conversion.
When the 1Nb-MMT-900 dose was reduced to 10 mg, glucose
conversion and 5-HMF yield were guaranteed at 91.5% and 44.8%,
respectively, indicating the excellent catalytic activity of 1Nb-MMT-
900.
3.4 Catalyst recyclability
In order to evaluate the recycling performance of 1Nb-MMT-900
catalyst, the recovery and reuse experiments were carried out. After
the reaction, 1Nb-MMT-900 was collected by centrifugation and
filtration, and the precipitate was washed and then dried (100 °C).
The catalyst was weighed after recovery to certify that the ratio of
reactants and catalysts remained the same as that in the original
reaction. The results showed 5-HMF yield and glucose conversion
after four cycles reached to 66.2% and 94.4%, respectively,
demonstrating 1Nb-MMT-900 was reused four times without a
remarkable loss of the catalytic activity and possessed excellent
stability in biphasic solvent (Fig. S3). Furthermore, a slight reduction
in 5-HMF yield and glucose conversion appeared which might be
attributed to the formation of trace amount of by-product and a
slight decline of activity after four cycles. XRD patterns (Fig. S4(a))
and FTIR spectra (Fig. S4(b)) verified the structural stability of 1Nb-
MMT-900 after three consecutive runs. And the decrease of the
relative intensity of the diffraction peak and a slight shift of the peak
at 6.05° in the XRD patterns were observed after four cycles, possibly
due to a tiny quantity of carbon deposition in the layer of the catalyst.
Meanwhile, XPS spectra (Fig. S4(c)) showed no valence change of
niobium and a slight reduction of Nb content after the 4th cycle;
these results are in accord with the reduction trend of Nb content
determined by XRF (Table S5) due to leaching of a small amount of
Nb during the reaction process. Therefore, effective recovery and
recyclability of 1Nb-MMT-900 were feasible for the conversion of
glucose.
Fig. 7 (a) Glucose conversion and 5-HMF yield in various solvents by
1Nb-MMT-900. (b) The influence of aqueous sodium chloride on the
reaction (conditions: 66.6 mM glucose, 40 mg 1Nb-MMT-900, 170 oC,
3 h).
3.5 Catalyst activity in different solvent for glucose conversion
To investigate the role of solvents in glucose conversion, a series of
single-phase solvents and two-phase solvents with different ratios
6 | J. Name., 2012, 00, 1-3
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The effect of the ratio of organic phase to inorganic phase on the oxyhydroxide species were formed in water.53 The hVyidewroAgrteicnle bOonlninde
5-HMF yield is shown in Table S6. In MIBK or saturated sodium connection between hydroxyl groups of gluDcOosI:e10a.n10d3N9/bC-9OGHC0g1ro22u5pGs
chloride solution single-phase solvents, glucose conversions and 5- strengthens the affinity of substrate for Nb-MMT catalysts, leading
HMF yields were comparatively low; this is because the simultaneous to active sites Nb species approachable to glucose molecules.54 On
existence of reactants and products in the same phase arises readily the theoretical level, DFT calculation was used to validate our
the condensation reactions, indicating the necessity of two-phase hypotheses and look more deeply into the mechanism of glucose
solvent. Obviously, the ratio of organic phase to inorganic phase was converting to 5-HMF in Nb-MMT-catalyzed systems. Based on
not proportional to the yields of 5-HMF. When the ratio was 1, the 5- previous reports of experiment and theory about the 5-HMF
HMF yield in MIBK was higher than that in water (NaCl), illustrating synthesis from glucose, it is widely viewed that the favored pathway
that organic solvents have a better capacity of 5-HMF molecules than initially goes through an enediol intermediate from glucose; the
inorganic solvents. To sum up, the optimum volume ratio (3.5:1.5) of enediol intermediate converts to fructose and subsequently 5-
56
MIBK and water (NaCl) was selected through comparative analyses.
HMF.55,
On the basis of DFT calculation results, a plausible
To further explore the influence of aqueous sodium chloride on mechanism for glucose transformation to 5-HMF with Nb-MMT as
the reaction, a series of solvents with different concentrations of the catalyst and water as the solvent is shown in Scheme 1 and Fig.
sodium chloride were used in dehydration reactions. (Fig. 7b) 8. The formation of fructose, captured during experimental runs (Fig
Compared with pure water system, glucose conversion and 5-HMF S6), could validate the feasibility and validity of DFT calculation
yield were both increased in the water (NaCl), which may be due to results.57 According to previous work,1 the pathway for glucose
the fact that Cl- has salting-out effect in the solution and can greatly isomerization was related to three steps: 1) the ring opening of
promote the reaction rate, thus improving the separation rate of 5- glucopyranose to the acyclic form of glucose; 2) enol-ketone
HMF. Notably, the 5-HMF yields in organic phase were elevated with isomerization; 3) the ring closing of acyclic fructose to
the increase of NaCl concentration in aqueous phase. The saturated fructofuranose. Firstly, H atom of Nb-OH attacks the ring O5 atom of
sodium chloride solution instead of pure aqueous solution in two- the glucose by hydrogen-bonding interaction, leading to the rupture
phase system effectively altered the interaction of molecular bonds the of C1-O6 bond and the protonation of the ring O5, accompanied
in liquid components, and improved the immiscibility of aqueous and by the adjacent terminal O atom of Nb-O as a proton acceptor and
organic phases, thereby enhancing the distribution coefficient of 5- the formation of aldehyde groups on C1 atom (M1),58 which is a
HMF in organic phases. In a word, saturated sodium chloride water cooperative process with an activation barrier of 14.69 kcal/mol (Fig
as inorganic phase was optimal in the glucose dehydration.
8a). M1 to M2 is the stretching process of acyclic glucose chain.
3.6 Conversion of various substrates on Nb-MMT-900 catalyst
The 1Nb-MMT-900 catalyst was robust in the synthesis of 5-HMF
from glucose, which establishes the feasibility of employing other
carbohydrates as raw material (Table 3). Remarkably, 5-HMF yields
from the transformation of fructose, sucrose or inulin in DMSO were
higher than the biphasic MIBK-NaCl system, because organic solvents
could be used to protect these fructose-containing substrates from
undergoing side reactions through the specific coordination of
solvent molecule to ketose molecule (Table 3, entries 1-3). On the
contrary, when maltose, cellobiose, starch and cellulose (composed
of glucose units) were employed as substrate, the 5-HMF synthesis
in the biphasic solvent was more preferable than that in DMSO (Table
3, entries 5-8). With regard to aldose, humins were more prone to
form in single-phase solvents, as a result of the condensation of
substrates and products. Biphasic solvents allowed substrates to
react in water phase with 5-HMF generated gradually entered the
organic phase, thus achieving the separation effect. Moreover, the
yield of 5-HMF from mono/di-saccharides was greater than 45%,
indicating Brønsted acid sites (Nb-OH) of 1Nb-MMT-900 played a
very important role in hydrolysis and dehydration. Surprisingly, a 5-
HMF yield could reach to 33.76% with the reaction time of 3 h in the
transformation of starch, one of the cheapest and most abundant
renewable carbohydrates. These results were satisfactory owing to
the difficulty in the hydrolysis of polysaccharides to glucose in
previous reports.52
Table 3 Catalytic conversion of various substrates to 5-HMF by
1Nb-MMT-900.
DMSO
Conv.
(%)
MIBK-NaCl
Entry
Substrate
Yield
(%)
Conv.
Yield
(%)
(%)
99.1
78.2
80.42
99.0
96.8
97.6
54.68
80.42
1
2
3
4
5
6
7
8
Fructose
Sucrose
Inulin
Glucose
Maltose
Cellobiose
Cellulose
Starch
99.6
95.1
99.6
81.8
74.2
94.9
10.3
30.6
73.49
57.83
65.16
27.96
32.5
15
37.4
39.98
35.39
70.52
46.27
47.51
10.51
33.76
1.3
6.98
(Reaction conditions: 66.6 mM carbohydrate, 40 mg 1Nb-MMT-900, 170 oC,
3 h).
3.7 DFT calculation on glucose conversion to 5-HMF
The existence of Brønsted acid sites Nb-OH groups as catalytically
active sites was verified by in situ DRIFTS spectra of NH3 adsorption
and IR spectra of pyridine adsorption over Nb-MMT. When Nb-MMT
enters into biphasic solvent, isolated mononuclear niobium
Scheme 1 The mechanism of glucose conversion in the presence
of Nb-MMT.
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DOI: 10.1039/C9GC01225G
Fig 8. Reaction free energy diagram for glucose conversion to 5-HMF on the [NbO(OH)] group on the surface of Nb-MMT.
Similarly, driven by hydrogen bonding,59 the H atom of Nb-OH conversion of the unstable M3 to ketose (M5, the acyclic form of
remains close to O1 atom of the aldehyde group to generate 1,2-enol fructose) takes place through the second enol-ketone isomerization
(M3), which is the rate-limiting step. The activation barrier for the step, which is endothermic by 27.52 kcal/mol (Fig 8b). The H atom of
ketone-enol isomerization step is 29.38 kcal/mol. Subsequently, Nb-OH attacks the carbonyl O atom of M5, resulting in ring closure
8 | J. Name., 2012, 00, 1-3
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to produce the five-member ring fructofuranose (M7) with an 2. W. J. Liu, W. W. Li, H. Jiang and H. Q. Yu, Chem. Rev., 2017,
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DOI: 10.1039/C9GC01225G
activation barrier of 17.05 kcal/mol. The process of glucose
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isomerization to fructose is completed, following by a three-step 3. C. García-Sancho, I. Fúnez-Núñez, R. Moreno-Tost, J.
dehydration reaction.60 (Fig 8c) The C2-OH bond on the five-member
ring was aggressed by H atom of Nb-OH to form the first water
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time, the cleavage of C3-OH bond on the ring facilitates the formation
of the second water molecule and C=C bond on the ring with an
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2960.
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4. Conclusions
This study offers a preliminary finding on the possibility of
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biphasic system, which will act as an efficient and green route
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This research was sponsored by the National Natural Science
Foundation of China under grant number 21476021.
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Graphic abstract
Niobium-loaded montmorillonite was employed as a highly efficient catalyst for the
catalytic conversion of glucose to 5-HMF in biphasic solvent.