Niobium phosphotungstates: Excellent solid acid catalysts for the dehydration of fructose to 5-hydroxymethylfurfural under mild conditions

Article abstract of DOI:10.1039/c8ra05940c

The efficient conversion of carbohydrates to 5-hydroxymethylfurfural (5-HMF) under mild conditions represents a very attractive and promising method of producing important building blocks. In this work, niobium phosphotungstates, with Nb/P molar ratios of 0.6, 1.0, 2.0 and 4.0 (NbPW-06, NbPW-1, NbPW-2, and NbPW-4, respectively) have been prepared by a facile, one-pot, alcohol-mediated thermal process and used for the direct conversion of fructose to 5-HMF. By adding a certain amount of Nb, the surface of the catalyst became enriched in P, and this enrichment was associated with the presence of surface P-OH groups that offered Br?nsted acid sites that can activate superficial hydrogen species to facilitate 5-HMF generation. Pyridine-FTIR confirmed the presence of Br?nsted and Lewis acid sites, which might play important roles in the dehydration of fructose to 5-HMF. Furthermore, polar aprotic solvents were well-suited for the conversion, and higher yields of 5-HMF were obtained in polar aprotic solvents than in nonpolar solvents. A 5-HMF yield of 96.7% with complete fructose consumption was obtained over NbPW-06 in DMSO at 80 °C after 90 min. In addition, NbPW-06 could be recycled several times without a significant decrease in the catalytic activity. A catalytic mechanism for this reaction was proposed. Moreover, this catalytic system can also be utilized for the dehydration of sucrose and inulin to 5-HMF in satisfactory yields. This study establishes an important platform for the further design of Nb-containing catalysts for the production of 5-HMF from carbohydrates under mild conditions.

Full text of DOI:10.1039/c8ra05940c

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PAPER
Niobium phosphotungstates: excellent solid acid
catalysts for the dehydration of fructose to 5-
hydroxymethylfurfural under mild conditions†
Cite this: RSC Adv., 2018, 8, 32423
a
b
a
a
a
Guo Qiu, Xincheng Wang, Chongpin Huang,* Yingxia Li and Biaohua Chen
The efficient conversion of carbohydrates to 5-hydroxymethylfurfural (5-HMF) under mild conditions
represents a very attractive and promising method of producing important building blocks. In this work,
niobium phosphotungstates, with Nb/P molar ratios of 0.6, 1.0, 2.0 and 4.0 (NbPW-06, NbPW-1, NbPW-2,
and NbPW-4, respectively) have been prepared by a facile, one-pot, alcohol-mediated thermal process and
used for the direct conversion of fructose to 5-HMF. By adding a certain amount of Nb, the surface of the
catalyst became enriched in P, and this enrichment was associated with the presence of surface P–OH
groups that offered Brønsted acid sites that can activate superficial hydrogen species to facilitate 5-HMF
generation. Pyridine-FTIR confirmed the presence of Brønsted and Lewis acid sites, which might play
important roles in the dehydration of fructose to 5-HMF. Furthermore, polar aprotic solvents were well-
suited for the conversion, and higher yields of 5-HMF were obtained in polar aprotic solvents than in
nonpolar solvents. A 5-HMF yield of 96.7% with complete fructose consumption was obtained over NbPW-
ꢀ
0
6 in DMSO at 80 C after 90 min. In addition, NbPW-06 could be recycled several times without
Received 12th July 2018
Accepted 7th September 2018
a significant decrease in the catalytic activity. A catalytic mechanism for this reaction was proposed.
Moreover, this catalytic system can also be utilized for the dehydration of sucrose and inulin to 5-HMF in
satisfactory yields. This study establishes an important platform for the further design of Nb-containing
catalysts for the production of 5-HMF from carbohydrates under mild conditions.
DOI: 10.1039/c8ra05940c
rsc.li/rsc-advances
A variety of catalysts, including both homogeneous and
heterogeneous catalysts, have been employed for the conversion
1
. Introduction
With the rapid growth of the world economy, the high depen- of fructose to obtain 5-HMF. Several homogeneous catalysts,
1
2–14
15
dence on fossil fuels has caused serious environmental problems such as mineral or organic acids,
Lewis acids, inorganic
16
17–19
and requires groundbreaking improvements in clean technology salts, and ionic liquids,
have been found to be effective for
1,2
that will take advantage of renewable energy sources. Biomass the synthesis of 5-HMF from fructose. However, the isolation of
represents an abundant and renewable carbon resource that can these catalysts from the reaction products and subsequent
be transformed into liquid fuels and value-added chemicals. The recovery are major impediments to their potential industrial
catalytic conversion of biomass has been the subject of intense application. Therefore, heterogeneous catalysts that can be
3–6
research efforts in recent years. As a promising raw material separated from the products by ltration have been developed
with numerous industrial applications, carbohydrates have been for the conversion of fructose to 5-HMF, for instance solid
2
0
21
employed in diverse areas of the food, chemistry, paper and heteropolyacid Cs2.5
H
0.5PW12
O
40
,
3 4
sulfated zirconia, Fe O –
7
22
23
24
pharmaceutical industries. One of the key building blocks SBA–SO
derived from carbohydrates is 5-hydroxymethylfurfural (5-HMF), acidic resin,
which has been identied as one of the most important biomass- works (MOFs), covalent organic frameworks (COFs), and
3
H, tungstated zirconia, zirconium oxophosphate,
25,26
27
and tin-beta zeolite, metal–organic frame-
28
29
3
0
derived platform compounds and is widely utilized in the nanostructured niobium oxide. These catalysts are efficient
8–11
synthesis of various chemicals.
and recyclable in the dehydration of fructose, and these nd-
ings have inspired further studies on heterogeneous catalysts.
Among heterogeneous catalysts, niobium compounds and
a
State Key Laboratory of Chemical Resource Engineering, Beijing University of related materials have recently attracted much attention as
Chemical Technology, Beijing 100029, China. E-mail: huangcp@mail.buct.edu.cn;
industrial catalysis due to their intrinsic acidity and redox
For instance, high yields (86.2%) of 5-HMF were
achieved with a Nb O -catalyzed conversion of fructose in
DMSO aer 2 h at 120 C. Sulfated mesoporous niobium oxide
provided a higher yield of 5-HMF (up to 88%) and showed better
Fax: +86 10 6441 9619; Tel: +86 10 6441 2054
31,32
properties.
b
Beijing Key Laboratory of Fuels Cleaning and Advanced Catalytic Emission Reduction
2
ꢀ
5
Technology, Beijing Institute of Petrochemical Technology, Beijing 102617, China
33
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra05940c
1
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34
recyclability than what was observed with commercial Nb
2
O
5
.
purchased from J&K Scientic Ltd. (Beijing, China). HPW,
Xue et al. developed a novel Nb-containing catalyst by the phosphomolybdic acid (HPMo), HSiW, niobium(V) chloride,
reaction of niobium chloride and nitrilotris (methyl- niobium(V) oxide, cesium nitrate, chromium(iii) chloride, and
enephosphonic acid), and that catalyst provided a 5-HMF yield stannic chloride were purchased from the Alfa Aesar Company
of 85.6% from the dehydration of fructose in a DMA–NaBr (Shanghai, China). All reagents were used as received without
11
mixture aer 1.5 h. However, niobium compounds catalyst further purication.
existed problems such as complex preparation, higher reaction
To elucidate the structures of the niobium phosphotungstate
temperature and the existence of by-products. Therefore, these materials, powder X-ray diffraction (PXRD) (D8FOCUS, Bruker),

ndings inspired us to develop a promising and readily X-ray photoelectron spectroscopy (XPS) (ESCALAB-250, Ther-
prepared Nb-containing catalytic system for the conversion of moFisher Scientic), Fourier transform infrared spectroscopy
ꢀ
fructose to 5-HMF under mild conditions (less than 100 C) with (FTIR) (TENSOR 27, Bruker), transmission electron microscopy
3
1
high throughput.
(TEM) (JEM-3010, JEOL), and solid-state P magic angle spin-
Keggin-type heteropolyacids (HPAs) have well-dened ning (MAS) nuclear magnetic resonance (NMR) spectroscopy
structures with a central cation (heteroatoms: P, As, Si, Ge, B, (AV300, Bruker) were employed. The coordination of niobium
etc.) surrounded by polyanions. They are protonic acids with and phosphorus and the structure of the tungsten species were
35,36
strong Brønsted acidity,
in many reactions.
and they have been widely utilized examined by UV-vis diffuse reectance spectroscopy (DRS) with
In the conversion of biomass, their a Shimadzu UV-3600 spectrophotometer, and BaSO was
37,38
4
reactivity was attributed to their acidity, and their other prop- utilized as a reference. The type of acid and its strength can be
erties vary signicantly based on their structural characteristics clearly differentiated by pyridine-FTIR spectroscopy. The total
and heteroatoms. Recently, Xiao et al. reported that H PW O
number of acid sites on the niobium phosphotungstate mate-
3
12 40
(
HPW) and H
4
SiW12O40 (HSiW) could convert fructose to 5-HMF rials was determined by temperature-programmed desorption
with high selectivity (99%) in an ionic liquid solvent, 1-butyl-3- of ammonia (NH
3
-TPD) (OmniStar, MS200). Thermogravimetric
ꢀ
39
ꢀ
methyl imidazolium chloride, within 5 min at 80 C. However, analysis (TGA) was carried out in the range of 50–500 C using
the homogeneous HPA catalysts suffer from complicated sepa- a Perkin-Elmer TGA 7 thermal analyzer.
ration and recycling procedures. Hence, there is great interest in
the design of heterogeneous catalyst containing hetero-
polyanions for the production of 5-HMF from fructose. For
example, Cs2.5H0.5PW O with a Keggin cluster synthesized
2
.2 Catalysts preparation
NbPW was prepared by an alcohol-mediated heating method. In
a typical synthesis, a certain amount of H 40 was dis-
12
40
+
3
PW12O
via Cs -mediated proton exchange can convert fructose to 5-
solved in ethanol (90 mL), and the mixture was stirred for 2 h at
4
mL) and subjected to ultrasonic treatment for 10 min. The
NbCl5 solution was then added dropwise to the stirred
H PW O solution, and the resultant mixture was then stirred
for another 12 h. The catalyst was obtained aer rotary evapo-
ration at 50 C and drying at 120 C overnight. Finally, the
catalyst was calcined at 300 C in static air for 3 h. These as-
prepared materials were designated NbPW-06, NbPW-1,
NbPW-2, and NbPW-4 based on the mole ratio of NbCl
H PW O Different types of HPW-based metal catalysts and
Nb-based heteropolyacid salt catalysts were synthesized using
hydrothermal methods and an alcohol-mediated heating
method, respectively.
ꢀ
20
HMF with a nal selectivity of 94.7% in 60 min at 115 C. Fan
et al. reported that a high yield of 5-HMF (77.7%) with a selec-
tivity of 93.8% could be obtained from fructose aer 60 min
ꢀ
0 C. A certain amount of NbCl
5
was dissolved in ethanol (50
ꢀ
40
using Ag PW12O40 at 120 C. These achievements indicate that
3
3
12 40
heteropolyanions are potential candidates for the catalytic
conversion of fructose to 5-HMF.
ꢀ
ꢀ
In the present study, solid niobium phosphotungstate cata-
lysts were prepared by a simple alcohol-mediated thermal
approach, and the effects of the amount of Nb on the acidity
and activity were investigated. The main reaction parameters
when using different heterogeneous catalysts have been
studied. The as-prepared NbPW materials were excellent and
efficient heterogeneous catalyst for the dehydration of fructose
to 5-HMF using DMSO as the solvent under mild conditions.
This method also provides an alternative for the production of
ꢀ
5
to
3
12 40.
2
.3 General procedure for the conversion of carbohydrates
5-HMF in high yields under mild conditions, which makes it an
to 5-HMF
important platform for the development of Nb-containing
catalyst for the conversion of carbohydrates to 5-HMF.
In the dehydration reaction, a sealed tube with a capacity of
15 mL was charged with certain amounts of carbohydrates,
catalyst, and solvent, placed in a temperature-controlled oil
bath, and stirred at a constant rate of 300 rpm using an internal
stirrer. The temperature was monitored by a thermocouple in
2
. Experimental section
2.1 Materials and instrumentation
5-HMF (98%), D-fructose (99%), sucrose (99%) and inulin (99%) contact with the solution. Aer the reaction, the mixture was
were purchased from the Aladdin Chemistry Reagent Company immediately quenched by submersion in an ice water bath,
Shanghai, China). Methanol, ethanol, n-butanol (butan-1-ol), ltered through a 0.2 mm syringe lter, and then decanted into
sec-butanol (butan-2-ol), dimethyl sulfoxide (DMSO), dimethy- a volumetric ask using water as a diluent prior to analysis.
lacetamide (DMA), dimethylformamide (DMF), methyl isobutyl The product of the reaction was analyzed by high-
ketone (MIBK), acetone and N-methyl pyrrolidone (NMP) were performance liquid chromatography (HPLC) using a system
(
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equipped with a UV detector and a C18 column. A mixture of
deionized water/methanol (60/40 v/v) was used as the mobile
ꢁ
1
phase with a ow rate of 1 mL min . The residual carbohy-
drates were analyzed by HPLC with a Shodex IR detector and
a Hypersil APS-2 column. The column temperature was set at
ꢀ
3
5 C, and water was used as the mobile phase at a ow rate of
ꢁ
1
0.2 mL min
.
3. Results and discussion
3
.1 Characterization of the niobium phosphotungstate
The XRD patterns of the niobium phosphotungstate samples
NbPW-06, NbPW-1, NbPW-2, and NbPW-4) are displayed in
(
Fig. 1 and S7,† and NbPW-06 and NbPW-1 showed X-ray dif-
fractograms typical of the body-centered cubic secondary
structure of Keggin anions, compared to HPW. The patterns
ꢀ
ꢀ
Fig. 2 FTIR spectra of NbPW.
showed characteristic diffraction peaks at 10.38 , 25.38 , and
ꢀ
3
4.68 , which were very similar to those of HPW. Notably,
a signicant reduction in the intensity of the characteristic bridge oxygen, between edge-sharing WO
diffraction peaks and a slight upward shi were observed with tively. The Keggin structure of HPW was maintained in the
octahedra), respec-
6
36
increasing Nb content. The decrease in the value of X-ray NbPW materials.
3
1
diffraction peaks is likely due to the niobium insertion into
The solid-state P MAS NMR spectra of NbPW-06 were
35
the secondary Keggin structure. With increasing Nb content, shown in Fig. 3(a). The sharp resonance at ꢁ16.00 ppm was
the characteristic peaks of the HPW in the XRD patterns of assigned to the chemical shi (dp) of phosphorus, and the peak
NbPW-2- and -4 become less obvious, which could be attributed showed an obvious shi to more negative values compared to
42
to the existence of some changes in the structure of the NbPW-2- the corresponding signal from HPW (dp ¼ ꢁ15.0 ppm); this
and -4. Only one broad diffraction peak was observed for NbPW- shi indicated the progressive deprotonation of phosphorus
11
43
2
and NbPW-4, indicating they were amorphous. The inten- atoms is common during the synthesis. A resonance peak at
sities of the characteristic diffraction peaks of the calcined ꢁ12.23 ppm was observed in the spectrum of the calcined
catalysts were slightly lower than those of the as-synthesized NbPW-06, and it may be a result of the interaction between the
5
+
catalysts.
Keggin tungstophosphate anion and the Nb ion aer calci-
The FTIR spectra of both the as-synthesized and the calcined nation. Fig. 3(b) presents the UV-vis DRS spectra of NbPW and
NbPW are shown in Fig. 2. The Keggin structure is general HPW. The spectra of both the NbPW materials and HPW show
believed to be an edge-sharing octahedral that contains a PO
4
two absorption bands, and the peak at 251 nm may be attrib-
41
tetrahedron surrounded by four W
spectra show sharp bands at 1080 cm that correspond to tungstophosphate anion [PW12
asymmetric P–O stretching vibrations, while the band at corresponds to the formation of small clusters of bulk WO
O
3 9
groups. Obviously, the utable to the oxygen–metal charge transfer that occurs at the
ꢁ
1
3ꢁ
40
O ] , and the peak at 320 nm
3
ꢁ1
5
96 cm can be attributed to P–O bending vibrations. The particles, indicating the presence of octahedral coordination in
ꢁ
1
ꢁ1
ꢁ1
bands at 984 cm , 885 cm , and 810 cm were assigned to
W–O (terminal oxygen), W–O –W (inter-bridge oxygen,
between corner-sharing WO octahedra), and W–O –W (intra-
d
c
6
e
31
Fig. 3 (a) P NMR spectra of NbPW-06. (b) UV-vis DRS spectra of
Fig. 1 The X-ray diffraction patterns of HPW, NbPW-06, and NbPW-1. NbPW.
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phase decreased substantially. Therefore, the current synthetic
method is suitable for enriching P species on the surface.
3
.2 Acidity measurement
The number of total acid sites and acid strength of the catalysts
were investigated by NH -TPD. The weak-, medium-, and strong-
acid sites were measured according to their NH desorption
3
3
ꢀ
ꢀ
ꢀ
temperatures (150–300 C, 300–500 C, and 500–650 C,
respectively) (Fig. S4†). The concentrations of total acid sites on
the calcined NbPW materials decreased with increasing Nb
3
content (Table 1). As shown in Fig. S4 and S5,† the NH -TPD
proles of NbPW catalysts showed desorption peaks of weak-
acid sites, which indicated the NbPW catalysts were weak-acid
salts. In addition, there is a higher concentration of total acid
sites on the as-synthesized NbPW-06 than on the calcined
Fig. 4 Nb 3d (a), P 2p (b) and W 4f (c) core level spectra of NbPW
material. The binding energy scale was calibrated with respect to the C
1
s peak.
ꢁ1
ꢁ1
NbPW-06 (1.32 mmol g and 0.89 mmol g , respectively). The
properties of the acid sites were monitored by using FTIR
spectroscopy aer pyridine chemisorption. As shown in Fig. 5,
44
the extraframework. Notably, the peak at 320 nm blue-shied
to 302 nm for the NbPW-06 sample; this was likely not due to
the quantum connement effect because the sizes of the NbPW-
ꢁ
1
characteristic absorption bands at 1540 and 1635 cm , corre-
sponding to the Brønsted acid sites, were observed in the
45
06 samples were not comparable with their Bohr atomic radii.
49
spectra of the NbPW samples. The band at approximately
The TG analysis of the NbPW material is shown in Fig. S1.†
ꢁ
1
1
445 cm , consistent with pyridine molecules the adsorbed at
The calcined catalysts showed no weight loss except the physi-
cally adsorbed water. Moreover, the TG analysis revealed that
the niobium phosphotungstate exhibited high thermal stability.
Furthermore, the high resolution XPS spectra of the niobium,
phosphorus and tungsten in the NbPW material are shown in
Fig. 4. The Nb 3d spectrum involves two characteristic peaks
attributable to Nb 3d5/2 and Nb 3d3/2 at approximately 207.8 eV
and 210.5 eV, respectively; this is characteristic of pentavalent
the Lewis acid sites, was clear, whereas the corresponding
bands in the spectra of NbPW-06 and NbPW-1 were very
50,51
ꢁ1
weak.
Additionally, a band at 1490 cm , which could be
attributed to the overlap of the Brønsted and Lewis acid sites,
52
was also observed in the spectra of the catalysts. The intensity
ꢁ
1
of the band at 1540 cm
remained almost constant with
ꢁ
1
increasing temperature, while the bands at 1445 cm and
1490 cm gradually decreased in intensity, indicating that the
Brønsted acid sites are stronger than the Lewis acid sites
ꢁ
1
46
niobium (Fig. 4(a)). The peak located at a binding energy of
34.4 eV was associated with the phosphorous in P–OH species
Fig. 4(b)), and this peak conrmed the presence of P–OH
1
(
(Fig. 5(a) and (b)). As seen in Table 1, all NbPW samples
exhibited high Brønsted acidity, and a progressive decrease in
the density of total Brønsted acid sites and a progressive
increase in the density of total Lewis acid sites were observed
increasing niobium loading, corresponding to the fact that
Lewis acid sites were generated by the incorporated niobium
species in the NbPW samples. The relative ratio of Brønsted and
Lewis acid sites could be calculated by integrating the areas
groups on the surface of the prepared catalyst; the supercial
enrichment of these groups appeared as an increased number
47
of Brønsted acid sites. The peaks at 35.7 eV and 37.8 eV were
assigned to W 4f7/2 and W 4f5/2, at 35.7 eV and 37.8 eV were
assigned to W 4f7/2 and W 4f5/2, respectively (Fig. 4(c)), indi-
cating a blueshi in the binding energies compared with those
48
of pure HPW; the shi may be due to the interaction between
ꢁ
1
ꢁ1
3
ꢁ
under the bands at 1540 cm and 1455 cm . The B/L ratio is
mainly affected by the surface elements, and the more P species
on the surface, the greater the Brønsted acid site ratio (Table 1).
However, the content of P species in the bulk phase is much
lower than that of the surface, suggesting that Brønsted acid
sites originate from surface P species; more specically, the
sites likely originate from P–OH moieties, because P–OH groups
the Keggin unit of the tungstophosphate anion [PW12
the metal cation Nb .
O
40
]
and
5
+
The SEM images conrmed that the NbPW was amorphous
Fig. S2†), whereas the TEM images of NbPW-06 revealed the
(
presence of spherical particles (Fig. S3†). The compositional
analysis of NbPW by X-ray uorescence (XRF) and XPS is shown
in Table 1. The phosphorus to niobium(V) molar ratios obtained
from XRF analysis decreased gradually from 0.53 to 0.07 with
increasing Nb content, and a similar trend was observed from
the XPS analysis. However, there was signicant difference
between the total atomic composition and the surface atomic
47
can serve as sources of H, encouraging a spillover effect.
Therefore, NbPW-06 is characterized by a larger number of total
Brønsted acid sites than the other synthesized compounds.
composition, indicating that the P species were supercially 3.3 Effect of varying the composition of the catalyst on the
enriched when niobium atoms are incorporated into the selectivity of 5-HMF from fructose
framework of the catalysts. The P/W ratios determined from
XRF did not vary consistently with increasing Nb content, and
these values were much lower than those obtained from XPS
analysis, indicating that the content of P species in the bulk
The efficacies of catalysts with different compositions, metals,
and heteropolyacids on the dehydration of fructose in DMSO
ꢀ
were investigated at 80 C using a reaction time of 3 h (Table 2).
A small amount of 5-HMF was obtained using the blank
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reference catalyst (Table 2, entry 1). When a heteropolyacid
(
Table 2, entry 2–4) was used, the yield of 5-HMF was relatively
+
high, suggesting that H ions have a positive effect on the
dehydration of fructose, which is consistent with the results
39
reported by Xiao et al. According to our results, a very similar
yield of 5-HMF was obtained using either homogeneous NbCl
Table 2, entry 5) or heterogeneous Nb (Table 2, entry 6) as
5
(
2 5
O
5
+
the catalyst, indicating that Nb could promote the conversion
of fructose. Among the prepared NbPW catalysts, NbPW-06
(Table 2, entry 7) exhibited excellent catalytic activity in the
dehydration of fructose and provided complete conversion of
fructose and 96.7% yield of 5-HMF. As expected, as the Nb
content increased, the yield of 5-HMF did not increase (Table 2,
entry 8–10). The catalytic activity of NbPW-06 was higher than
those of the other prepared catalysts, which was attributed to its
higher acidity and appropriate content of Lewis and Brønsted
acid sites required for the dehydration of fructose. In addition,
the enhanced catalytic activity of NbPW-06 can be attributed to
the presence of more Brønsted acid sites, and the yield of 5-
HMF gradually increased with increasing total number of
Brønsted acid sites. This conrmed the benecial effect of
Brønsted acidity for catalytic systems for the dehydration of
fructose.
When using phosphomolybdic acid or HSiW as the raw
material, the prepared NbSiW-08 (entry 12, Table 2) performed
better than NbPMo-06 (entry 11, Table 2), but its catalytic
activity was slightly lower than that of NbPW-06. Furthermore,
all the phosphotungstate and silicotungstate species used in
such reactions have exhibited better catalytic conversion results
than that of phosphomolybdate.
Further investigations have been carried out by introducing
4
+
3+
3+
various metal ions such as Sn , Cr , and Ce (entry 13–15,
ꢀ
Table 2). The use of SnPW-075 and CrPW-1 at 80 C lead to 5-
HMF yields of 76.5% and 81.5%, respectively, whereas a yield of
35.9% can be obtained using CePW-1 as a catalyst. These results
suggest that the metal ions have a profound inuence in the
success of fructose dehydration, which can be attributed to the
various metal ions playing different roles in the conversion of
Fig. 5 IR spectra of pyridine adsorbed on (a) NbPW-06 and (b) NbPW-
ꢀ ꢀ ꢀ
1
after evacuation at (i) 100 C, (ii) 150 C and (iii) 200 C. Absorbance
peaks characteristic of Brønsted (B), Lewis (L), strong Lewis (SL) and
weak Lewis (WL) sites are indicated.
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a
Table 2 The results of fructose conversion after 3 h
As-synthesized
Calcined
Entry
Catalyst
Blank
Fructose conv. (%)
5-HMF (%)
Selectivity (%)
Fructose conv. (%)
5-HMF (%)
Selectivity (%)
1
2
3
4
5
6
7
8
9
10.3
99.7
70.4
99.3
99.1
99.5
99.4
99
98.4
98.1
75.8
99.5
99.4
99.6
64.5
<0.5
83.6
46.1
86
<0.5
83.9
65.5
86.6
76
—
—
—
—
—
—
—
—
—
—
—
—
96.7
89
77.6
73.4
32.7
79.4
76.5
81.5
35.9
—
—
—
—
—
—
96.7
90.1
79
75.5
46.3
80.7
76.8
81.7
60.5
H
H
H
3
3
4
PW12
PMo12
SiW12
O
40
40
40
O
O
NbCl
Nb
5
75.3
73.7
86.4
78.8
72.5
67.3
41.3
81.6
69.4
76.8
32.1
2
O
5
74
87
NbPW-06
NbPW-1
NbPW-2
NbPW-4
NbPMo-06
NbSiW-08
SnPW-075
CrPW-1
100
79.6
73.8
68.6
54.5
82.1
69.8
77.2
49.8
98.7
98.2
97.2
70.6
98.3
99.6
99.7
59.3
1
1
1
1
1
1
0
1
2
3
4
5
CePW-1
a
ꢀ
Conditions: 56 mM fructose, 50 mg catalyst, 80 C, 5 mL DMSO, 3 h.
4
+
fructose. According to previous reports, Sn , as a Lewis acidic 3.4 Effect of varying the solvent on the dehydration activity
ion, can interact with the fructose molecule, and this interac-
First, the inuence of different solvents on the fructose
conversion was evaluated (Table 3). The yields and selectivities
tion and complex formation can promote the dehydration
53
reaction. Chromium atoms are the main contributors to the
Lewis acidity of the catalyst, and the incorporation of Cr into the
easily separable solid catalyst lessens the hazardous effects of
for 5-HMF varied remarkably in different solvents, suggesting
the solvent effect was substantial. The high 5-HMF selectivity
and yield in the fructose dehydration conducted DMSO have
been explained (see Table 3, entry 1); DMSO can prevent the
rehydration of 5-HMF to levulinic acid and formic acid and the
formation of humins through the preferential coordination of
DMSO to the 5-HMF, and DMSO can prevent fructose from
undergoing reactions other than dehydration to 5-HMF through
3
+
54,55
Cr and promote 5-HMF formation.
Moreover, Ce-based
heteropolyacid salt catalysts have exhibited high catalytic
activities in the conversion of fructose at higher temperatures.
20,35,40
According to previous reports,
the NbPW catalyst showed
more excellent catalytic activity under mild condition, indi-
cating NbPW was a preferred catalyst with high efficiency of
energy utilization, less pollution, lower energy consumption.
Notably, the calcined material of each phosphotungstate cata-
lyst tended to provide much higher yields of 5-HMF than the
corresponding as-synthesized catalysts, which is in contrast
with the results reported for phosphomolybdate and silico-
tungstate compounds.
57
the specic coordination of DMSO to fructose. In DMA, DMF,
NMP, and acetone (see Table 3, entry 2–5) the conversions of
fructose and the yields of 5-HMF were relatively high, which
could be attributed to these polar aprotic solvents being well
suited mediums for the reaction, effectively inhibiting side
reactions, and easily solubilizing the reactants. With MIBK as
the solvent (see Table 3, entry 6), NbPW-06 showed poor
The catalytic conversion of fructose to 5-HMF by NbPW within
6
min was studied (Table S1†). Conversions of fructose of
approximately 50% were obtained, and the yields of 5-HMF grad- Table 3 Conversion of fructose to 5-HMF with different solventsa
ually decreased with increasing Nb content in the catalyst. On one
hand, the effect of the Nb loading was investigated by calculating
Experimental parameter
the turnover frequencies (TOF based on total acid sites). On the
Entry Solvent
Conversion/% Yield/% Selectivity/%
other hand, the TOFs for each of the catalysts were different,
suggesting that the observed changes in reactivity with various
amounts of Nb could be attributed to a fundamental change in the
reactivity of the available acid sites. NbPW-06 gave the highest
TOF, and it was approximately 4.5 times higher than the TOF
obtained in a previous study. In addition, further decreases in the
amount of incorporated Nb did not enhance the catalytic efficiency
because the syntheses of catalysts with low Nb contents are lower
yielding, making it difficult to produce heterogeneous catalyst with
low Nb concentrations. Therefore, NbPW-06 can be considered the
optimum catalyst as it provides the most suitable content of acid
sites for the production of 5-HMF.
1
2
3
4
5
6
7
8
9
1
1
DMSO
DMA
DMF
NMP
Acetone
MIBK
sec-Butanol
n-Butanol
[EMIM]Br
100
87.1
91.8
96.7
89.5
91.5
92.8
22.4
50.6
59.9
96.7
31.3
33.5
70.3
33.9
1.3
26.3
0.9
5.3
96.7
35.9
36.5
72.8
37.8
1.5
28.3
4.1
10.5
13.0
7.2
56
0
1
H
2
O
7.8
5.9
DMSO/H
2
O (4 : 1) 82.9
a
ꢀ
Conditions: 56 mM fructose, 50 mg NbPW-06, 80 C, 3 h.
32428 | RSC Adv., 2018, 8, 32423–32433
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catalytic activity, and a large amount of humin was formed.
Fig. 6(a) shows the inuence of reaction temperature on the
Protic solvents, such as sec-butanol, facilitated the conversion of conversion of fructose to 5-HMF with NbPW-06 as the catalyst.
fructose; however, the yields and selectivities of 5-HMF were not Indeed, the fructose conversion and the 5-HMF yield were
high in these solvent (see Table 3, entry 7). These poor results strongly inuenced by the reaction temperature. As tempera-
may have been the result of side reactions in the sec-butanol. ture increased, the rate of fructose conversion increased, and
Moreover, the fructose dehydration was not efficient and the a high conversion of fructose (99%) was achieved within 20 min
ꢀ
yield of 5-HMF was less than 10% in n-butanol, [EMIM]Br and at 80 C. At lower temperatures, the rate was substantially lower;
ꢀ
water (see Table 3, entry 8–10) due to the formation of humins for example, at 60 C, a reaction time of 2 h was required to
3
and rehydration of 5-HMF to levulinic acid and formic acid.
reach 98% fructose conversion. Obviously, lower yields of 5-
According to a previous report, the presence of water plays a key HMF and slower reaction rates were obtained when the reaction
ꢀ
role in the dehydration of fructose when DMSO was as the temperature was below 80 C. The yield of 5-HMF increased
58
solvent. Nevertheless, the yield of 5-HMF was not as good as signicantly when the reaction temperature was increased from
ꢀ
ꢀ
we expected because NbPW-06 showed poor catalytic activity in 60 C to 80 C, and the highest 5-HMF yield (96.7%) was ach-
ꢀ
DMSO/H
2
O (4 : 1 v/v). Briey, DMSO was carefully chosen as the ieved at 80 C. The higher the temperature reaction required the
optimum solvent to obtain high yields of 5-HMF.
shortest reaction time to obtain the optimum yield. However,
ꢀ
when the reaction temperature was above 80 C, the yields of 5-
ꢀ
3.5 Effect of the processing conditions for the NbPW-06-
HMF were lower than what was achieved at 80 C, which could
catalyzed conversion of fructose to 5-HMF
be attributed to the formation of a trace amounts of black
humin oligomer. The color of the reaction solution also deep-
ened at elevated temperature, although to a lesser extent than
was observed at 60 C. Therefore, 80 C was the optimal
temperature under these conditions.
In this study, the yields of 5-HMF in the NbPW material-
catalyzed reactions were superior to those catalyzed by other
heteropoly salts under mild conditions. Because NbPW-06
possesses a lower niobium content and is less expensive than
other NbPW material, NbPW-06 was chosen for further focused
studies on the conversion of fructose to 5-HMF.
ꢀ
ꢀ
Fig. 6(b) presents the effect of the initial fructose concen-
tration on the fructose conversion and 5-HMF yield. Notably,
Fig. 6 Effect of (a) reaction temperature, (b) substrate concentration, and (c) catalyst loading on the conversion of fructose to 5-HMF
ꢀ
(
conditions: [fructose]
o
¼ 56 mM, NbPW-06 ¼ 50 mg, 5 mL DMSO, 80 C).
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the fructose conversion was independent of the initial fructose
concentration, suggesting that the dehydration of fructose was
a rst-order reaction with respect to fructose. However, the yield
of 5-HMF was a function of the initial fructose concentration,
and a lower initial concentration of fructose between 56 mM to
167 mM lead to a higher yield of 5-HMF. Low substrate
concentrations tend to obtain high 5-HMF yields, which could
be attributed that it was easier for the substrate molecules to
diffuse in the solvent, thus accelerating the shi of reaction
equilibrium to 5-HMF. When higher initial concentrations of
fructose were employed, the reaction solution became deep
yellow probably due to a side reaction.
The inuence of the amount of NbPW-06 catalyst loading
ꢀ
was investigated in the dehydration of fructose at 80 C, and the
Fig. 7 (a) Effect of calcination temperature on the catalytic activity of
results are shown in Fig. 6(c). Obviously, higher catalyst loading
NbPW-06; (b) reusability of NbPW-06 catalyst (conditions: [fructose]
o
ꢀ
resulted in higher reaction rates, higher fructose conversion ¼ 56 mM, NbPW-06 ¼ 50 mg, 5 mL DMSO, 80 C).
and higher 5-HMF yield. Fructose conversion and 5-HMF yield
were maintained at 99% and 73%, respectively, when the cata-
lyst loading was decreased to 12.5 mg. In addition, higher nal
yields of 5-HMF were detected with higher NbPW-06 concen-
trations; hence, 50 mg of catalyst was selected for obtaining
high yields of 5-HMF.
ꢁ1
5
4.09 kJ mol , which is much lower than that reported by
ꢁ1
Moreau et al. (141 kJ mol ) when using zeolites as the cata-
lyst. The low activation energy of the NbPW-06-catalyzed
system can minimize energy consumption and undesirable
sugar degradation pathways, allowing the dehydration of
59
60
fructose to be conducted at moderate temperatures.
3.6 Effect of calcination temperature on the catalytic activity
of NbPW-06
3.8 Catalyst recyclability
To investigate the effect of calcination temperature on the cata-
lytic activity of NbPW-06, NbPW-06 was calcined at different To study the long-term stability of the catalyst, four consecutive
ꢀ
temperatures (100–600 C) aer treatment. The catalytic activities reactions were carried out with the NbPW materials. Aer the
of the NbPW-06 samples calcined at varying temperatures are reaction, the NbPW catalyst was separated through centrifuga-
ꢀ
ꢀ
shown in Fig. 7(a). The NbPW-06 calcined at 100–500 C exhibi- tion and recovered by washing and vacuum drying (120 C), and
ted an excellent performance in the conversion of fructose to 5- the recovered catalyst was weighed to ensure a constant
HMF with yields greater than 70% and selectivities greater than substrate-to-catalyst ratio (Fig. 7(b)). NbPW-06 was recycled four
ꢀ
9
0%. A calcination temperature of 300 C is suitable for NbPW- times without a signicant decrease in the activity since a 94.0%
0
6, which possessed high catalytic activity and achieved the yield of 5-HMF and a 98% fructose conversion were observed
highest yield and selectivity for 5-HMF. Desirable activity can be aer four cycles. The slight decrease in the yield of 5-HMF can
ꢀ
achieved with NbPW-06 material aer calcination at 300
C
be attributed to the minor decrease in the activity of the NbPW
because of its appropriate acid strength and acid contents. In materials or partial humins formation during the later reaction
contrast, less than 5% yield of 5-HMF was obtained from fructose cycles. The somewhat higher weight-loss ratio of the recycled
ꢀ
when NbPW-06 samples calcined at temperatures up to 600 C NbPW-06 (Table S2†) suggests a small amount of carbon
were used, and this change might be caused by some deactiva- deposition is occurring on the NbPW-06. The number of total
tion on the surface of the catalyst due to the higher temperature; acid sites decreased slightly during each cycle (Table S2†),
3
ꢁ
some distinct structural changes in these materials were sug- demonstrating that the tungstophosphate anion [PW12
40
O ]
gested by the observed FTIR patterns (Fig. S6†).
possessed excellent stability in DMSO. Thus, the NbPW-06
catalyst can be effectively separated and reused for the conver-
sion of carbohydrates.
3.7 Kinetic model
A kinetic analysis of the fructose conversion catalyzed by NbPW-
Table 4 Reaction rate constants (k) of fructose conversion at different
reaction temperatures using NbPW-06
0
6 in DMSO under mild conditions was performed, and the
a
12
fructose conversion was identied as a rst-order reaction.
ꢁ1
The values of the pseudo-rst-order rate constant (k) at different Entry
Temp. (K)
k (min
)
Correlation coefficient
temperatures (T) were obtained by plotting ln(1 ꢁ x) vs. time (t)
1
333
343
353
363
373
0.0709
0.1287
0.2690
0.3625
0.5753
0.9909
0.9935
0.9833
0.9855
0.9941
(
where x is the conversion of fructose). The value of k increases
2
3
4
5
gradually with increasing temperature, indicating that the
fructose conversion is promoted by elevated reaction tempera-
ture (Table 4). Table 5 shows the kinetic parameters of the
NbPW-06-catalyzed fructose conversion, and the apparent acti-
vation energy of the fructose conversion in this system was
a
Conditions: 56 mM fructose, 50 mg NbPW-06, 5 mL DMSO, 5 min.
32430 | RSC Adv., 2018, 8, 32423–32433
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Table 5 Kinetic parameters of the fructose conversion using NbPW-
0
6
Parameter
Value
Reaction order, n
Activation energy, E
Pre-exponential factor, A (min
Correlation coefficient
1
ꢁ
1
a
(kJ mol
)
54.09
2.3 ꢂ 10
0.9864
ꢁ
1
6
)
Scheme 1 Proposed reaction mechanism for the conversion of
fructose to 5-HMF.
3.9 Effect of various substrates on the dehydration activity
NbPW-06 is a solid catalyst with Brønsted acid sites that can
catalyze this hydrolysis reaction, and it allows a tandem
hydrolysis-dehydration pathway for converting disaccharides
and polysaccharides into 5-HMF in a one-pot method. In
addition, NbPW-06 is very active in the dehydration of sugars
containing fructose units. Furthermore, when this catalytic
system was applied to the dehydration of other substrates to 5-
HMF, satisfactory yields were achieved from sucrose and inulin
(Table 6). The conversion could reach over 95%, and the yield
and selectivity were more than 50%, which indicated that Scheme 2 Possible pathway for the conversion of fructose to 5-HMF
in DMSO over NbPW-06 under mild condition.
NbPW-06 could play an important role in the hydrolysis and
dehydration of disaccharides and polysaccharides.
HMF through the gain and loss circulation of the hydrogen
proton on the surface of the NbPW-06. There was no levulinic
3.10 Mechanism of 5-HMF formation in DMSO catalyzed by
acid (LA) or formic acid (FA) detected under the optimal
conditions; therefore, the proposed reaction pathway does not
include the rehydration of 5-HMF to LA and FA. The 96.7% yield
of 5-HMF from fructose conrmed that the Brønsted acid sites
on NbPW-06 were sufficiently active to catalyze the dehydration
under mild conditions. Scheme 2 presents the proposed reac-
tion pathways for the conversion of fructose to 5-HMF over the
NbPW-06 catalyst in DMSO including the dehydration of fruc-
tose to 5-HMF on the Brønsted acid sites. Additionally, no
insoluble humins were observed, and the color of the reaction
solution remained light yellow under mild conditions because
the lower temperature minimized the side reactions and could
not support the formation of the by-products. The rehydration
of 5-HMF to LA and FA is prevented in this reaction system;
hence, it allows 5-HMF to be produced directly from fructose
with high selectivity.
NbPW-06 under mild conditions
NbPW-06 showed excellent catalytic activity in our reaction
system under mild conditions. To a certain degree, the role of
niobium is to change the distribution of P species in the bulk
phase and on the surface of the catalyst. The incorporation of
Nb species during catalyst formulation enriches the P species
on the surface of the catalyst. Brønsted acid sites originate from
P species, and more specically from P–OH groups, enriched on
the surface of the catalyst. The proposed reaction mechanism
for the conversion of fructose to 5-HMF is presented in Scheme
1. The rst dehydration step from a cyclic D-fructose would yield
the enol form of intermediate 2, whose formation was the rate-
limiting step. The hydrogen proton from P–OH on the surface of
NbPW-06 could accelerate the rst dehydration of D-fructose,
and the protonation and deprotonation occur at different
functional groups on D-fructose, so that the hydrogen proton on
NbPW-06 could be at an equilibrium. This step is followed by
two consecutive b-dehydrations in the ring to form the 5-HMF, 4. Conclusions
which was observed to proceed slower than when starting from
fructose. Generally, D-fructose lost three water molecules to 5-
This work offered a preliminary study on the feasibility of using
NbPW materials as highly active catalysts for the conversion of
fructose to 5-HMF under mild conditions. A high yield (96.7%)
Table 6 Catalytic conversion of various substrates to 5-HMF by
a
ꢀ
NbPW-06
could be achieved from fructose in DMSO at 80 C with a reac-
tion time of 1.5 h. The loading of Nb impacted the activity of the
NbPW catalyst, and NbPW-06 showed the best performance
because it provided the optimum number of Brønsted acid sites
for this dehydration reaction. The incorporation of Nb species
changes the distribution of P species in the bulk phase and at
the surface of the catalyst, and additional Nb promotes the
enrichment of P–OH groups on the surface of the catalyst,
Experimental parameter
Entry
Substrate
Conversion/%
Yield/%
Selectivity/%
1
2
3
Fructose
Sucrose
Inulin
99.6
96.7
98.2
96.7
52.2
53.3
97.1
54.0
54.3
a
ꢀ
Conditions: 56 mM substrate, 50 mg NbPW-06, 5 mL DMSO, 80 C, which generates Brønsted acid sites that facilitate the produc-
3
h.
tion of 5-HMF. A possible reaction pathway involving the Nb-
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Conflicts of interest
There are no conicts to declare.
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