Simple and green route for preparation of tin phosphate catalysts by solid-state grinding for dehydration of glucose to 5-hydroxymethylfurfural (HMF)

Article abstract of DOI:10.1039/c7ra10083c

This work demonstrated a simple and green method for the synthesis of tin phosphate (SnP) catalysts for glucose to HMF transformation. In this process, SnP catalysts with various P/Sn mole ratios were prepared by solid-state grinding of a mixture of SnCl₄·5H₂O and NaH₂PO₄·2H₂O at room temperature, followed by drying with or without calcination. Physicochemical properties of the obtained SnP catalysts were investigated using N₂-physisorption, XRD, FT-IR, Py-IR, NH₃-TPD, and XPS techniques. The characterization studies showed that varying P/Sn ratio affected the textural and surface acid properties of the SnP catalysts. The activity of these catalysts was evaluated for dehydration of glucose for HMF production in water using recyclable low boiling point organic solvent THF as HMF extracting solvent. The experimental results revealed that the catalytic activity of SnP catalysts depended on the P/Sn ratio. Under the optimized reaction conditions, 98% glucose conversion, and 61% HMF yield were achieved over as-synthesized SnP-1 catalyst with P/Sn mole ratio of 1.0. The high catalytic activity of the SnP-1 catalyst can be attributed to its high acidity and the presence of water tolerant Br?nsted and Lewis acid sites. Furthermore, the catalytic activity of SnP-1 catalyst is retained even after four cycles indicating its good catalytic stability.

Full text of DOI:10.1039/c7ra10083c

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PAPER
Simple and green route for preparation of tin
phosphate catalysts by solid-state grinding for
dehydration of glucose to 5-hydroxymethylfurfural
(HMF)†
Cite this: RSC Adv., 2017, 7, 48501
Kasanneni Tirumala Venkateswara Rao, Sadra Souzanchi, Zhongshun Yuan,
*
*
Madhumita B. Ray and Chunbao (Charles) Xu
This work demonstrated a simple and green method for the synthesis of tin phosphate (SnP) catalysts for
glucose to HMF transformation. In this process, SnP catalysts with various P/Sn mole ratios were
prepared by solid-state grinding of a mixture of SnCl4$5H2O and NaH2PO4$2H2O at room temperature,
followed by drying with or without calcination. Physicochemical properties of the obtained SnP catalysts
were investigated using N2-physisorption, XRD, FT-IR, Py-IR, NH3-TPD, and XPS techniques. The
characterization studies showed that varying P/Sn ratio affected the textural and surface acid properties
of the SnP catalysts. The activity of these catalysts was evaluated for dehydration of glucose for HMF
production in water using recyclable low boiling point organic solvent THF as HMF extracting solvent.
The experimental results revealed that the catalytic activity of SnP catalysts depended on the P/Sn ratio.
Under the optimized reaction conditions, 98% glucose conversion, and 61% HMF yield were achieved
over as-synthesized SnP-1 catalyst with P/Sn mole ratio of 1.0. The high catalytic activity of the SnP-1
catalyst can be attributed to its high acidity and the presence of water tolerant Brønsted and Lewis acid
sites. Furthermore, the catalytic activity of SnP-1 catalyst is retained even after four cycles indicating its
good catalytic stability.
Received 11th September 2017
Accepted 11th October 2017
DOI: 10.1039/c7ra10083c
rsc.li/rsc-advances
production of polymers, fuel and value added chemicals
through different chemical reactions.6 For instance, oxidation
1. Introduction
of 5-HMF produces 2,5-furandicarboxylic acid (FDCA), a mono-
mer for the production of bio-based polyethylene fur-
andicarboxylate (PEF) polymer, a promising bio-replacement
for polyethylene terephthalate (PET) PET plastic.6 Further-
more, HMF can be upgraded to bio-fuels such as 2,5-dime-
thylfuran (DMF) and 5-ethoxymethylfurfural (EMF), by
hydrogenation and etherication, respectively.3,7 HMF also can
be used for the production of C7–C15 liquid alkanes via a series
of aldol condensation, dehydration and hydrogenation reac-
tions.8 Recently, our group reported a one-pot synthesis of
phenol-hydroxymethyl furfural (PHMF) resin by reacting phenol
with HMF derived from glucose in situ in the presence of CrCl2/
CrCl3 and tetraethylammonium chloride catalysts.9
According to literature review,6 catalytic conversion of
glucose to HMF involves tandem process. In the rst step,
glucose isomerizes to fructose in the presence of base, enzymes
or Lewis acids and while Brønsted acid facilitate the dehydra-
tion of fructose to HMF. Up to now, innumerable homogeneous
and heterogeneous catalysts have been developed for glucose to
HMF conversion. Homogeneous catalysts including ionic
liquids,10 lanthanide salts in ionic liquids,11 metal salts with
mineral acids.12 Despite the advantages, there are certain
With fast depletion of fossil deposits, increased demand for
energy and chemicals, and growing environmental concerns,
nding sustainable and economical methods for the synthesis
of chemicals and fuels is one of the key challenges across the
globe.1 Biomass, a carbon neutral renewable energy resource, is
widely available on earth with an annual production of
170 billion metric tons.2 Lignocellulose biomass consists of
cellulose, hemicellulose and lignin as its main constituents.
Lignocellulose biomass has long been a main energy source for
mankind, but it can also be a renewable chemical source. For
example, polysaccharides in cellulose and hemicellulose can
undergo hydrolysis to yield sugar monomers that can be con-
verted into platform chemicals such as 5-hydroxymethylfurfural
(HMF) and furfural by acid catalyzed dehydration reactions.3,4
5-HMF has been identied as one of the top 14 most promising
high-value bio-based chemicals by the US Department of
Energy,5 which can be used as a versatile platform molecule for
Department of Chemical and Biochemical Engineering, Western University, London,
Ontario, Canada N6A 5B9. E-mail: cxu6@uwo.ca; mbhowmic@uwo.ca
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra10083c
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drawbacks associated with these processes, such as difficult catalyst loading on glucose conversion and HMF selectivity were
catalyst recovery, high cost of ionic liquids, and corrosion when investigated.
mineral acids are used. To overcome these challenges inex-
pensive heterogeneous bi-functional solid acid catalysts are
more desirable for large scale industrial applications from the
2. Materials and methods
2.1 Materials
perspectives of simple recovery and reusability of the catalysts,
and low cost compared to homogeneous catalysts. In addition,
processes using heterogeneous catalysts and water as a reaction
medium for glucose to HMF conversion, are more cost effective
and environmentally benign. Watanabe et al. reported titanium
and zirconium oxide catalysts for the dehydration of glucose at
200 ꢀC, where HMF yields were below 20%.13 HMF yield of 53%
was obtained with glucose using Sn-beta zeolite and HCl cata-
lysts in a THF/H2O–NaCl bi-phasic system.14 As high as 49% and
58% HMF yield was obtained using modied niobium and
tantalum oxide catalyst, respectively, with glucose at different
reaction temperatures.15,16 Nijhuis et al. reported HMF selec-
Reagent grade SnCl4$5H2O (98%), NaH2PO4$2H2O (99%),
D-glucose (99.9%), 5-hydroxymethyl furfural (99%), trimethyl-
amine (>99%) and micro crystalline cellulose were purchased
from Sigma-Aldrich. Tetrahydrofuran (THF, HPLC grade), water
(HPLC grade), acetonitrile (HPLC grade) were obtained from
Caledon Laboratories. All reagents were used as received
without any further purication.
2.2 Methods
2.2.1. Catalyst preparation. A series of tin phosphate cata-
tivity in the range of 30–60% obtained with glucose using lysts, abbreviated as SnP catalysts, with different P/Sn mole
aluminium, zirconium, titanium and niobium based phosphate ratios, was prepared by solid-state grinding. In a typical
catalysts again in a water/MIBK (or 2-methyltetrahydrofuran) synthesis, a mixture of 3.506 g SnCl4$5H2O and 1.56 g of
biphasic system.17 Titanium hydrogenphosphate was reported NaH2PO4$2H2O was ground manually in a mortar for 15 min at
to catalyze glucose to HMF in water/THF bi-phasic system, room temperature. The resulting mixture was then oven dried
producing HMF at a yield 35% at 60% glucose conversion.18 in air at 100 ꢀC for 12 h. The obtained white solid was then
Atanda et al. reported HMF yields of 81% at glucose conversion washed with deionized water followed by oven drying again at
of 97% using phosphate TiO2 catalyst at 175 ꢀC in water-butanol 100 C for 12 h. This as-synthesized catalysts were denoted as
ꢀ
bi-phasic system.19 HMF yield of 63% from glucose in DMSO SnP-1. Similarly, other SnP catalysts by varying the P/Sn mole
was obtained at 140 C using chromium phosphate catalyst.20 ratio (0.5, 1.5 and 2.0) were synthesized in the same procedure
ꢀ
Very recently various other heterogeneous bi-functional solid as described above, and denoted as SnP-0.5, SnP-1.5, and SnP-2.
2ꢁ
acid catalysts including SO4 /Al2O3–SnO2,21 Nb-SBA-15,22 All as-synthesized catalysts were directly used without any
ordered mesoporous Nb–W oxide,23 SAPO-34,24 and calcium further treatment. It was proved that extraction of as-
phosphate,25 have been exploited for HMF production. synthesized SnP catalysts with dilute HCl did not remove any
Although, all these catalysts were efficient for HMF production tin from the samples, conrming the absence of Sn2+ ions and
but the preparation of these catalysts oen have disadvantages the presence of Sn4+ ions in as-synthesized catalysts samples.
such as multistep processing, need for the delicate pH control Moreover, the SnP-1 catalyst was also calcined at various calci-
and generation of vast amount of environmental waste. In view nation temperatures (300–500ꢀ), denoted as SnP-1-300, SnP-1-
of this, simple and environmental benign process for synthesis 400 and SnP-1-500. For comparison, SnP-1 catalyst was also
of highly efficient bi-functional solid acid catalyst for produc- prepared using the wet-chemical method reported elsewhere.30
tion of 5-HMF from glucose is highly attractive.
This catalyst was denoted as SnP-1-L.
Heterogeneous tin phosphate (SnP) catalyst containing both
2.2.2. Catalyst characterization. Powder X-ray diffraction
Brønsted and Lewis acid sites are efficient for dehydration of (XRD) patterns of the catalysts were measured using an X-ray
glucose to HMF.26,27 Generally, tin phosphates were prepared diffractometer (Rigaku RINT 2500, Tokyo, Japan) with
ꢀ
using a conventional wet-chemistry route. Compared with the Ni-ltered Cu Ka radiation (l ¼ 1.5406 A) with a scan speed of
wet-chemistry route, simple solid-state grinding at a low 2ꢀ minꢁ1 and a scan range of 2–80ꢀ at 30 kV and 15 mA. N2
temperature is more attractive and greener catalyst preparation physisorption was performed using Quadrasorb SI automated
route for synthesis of tin phosphate catalysts. In some recent surface area and pore size analyzer. Prior to N2 adsorption
studies, solid heterogeneous catalysts prepared via the solid- measurement, the sample was degassed at 200 ꢀC until a stable
state grinding method, demonstrated better activities than static vacuum of less than 5 ꢂ 10ꢁ3 Torr was achieved. Subse-
those prepared by wet-chemistry route.28,29 To the best of our quently, N2 adsorption was performed at ꢁ196 ꢀC. The surface
knowledge, there is no report on preparation of tin phosphate area of the catalysts was measured using multi point BET in the
catalysts with the solid-state grinding route for conversion of relative pressure range of 0.05–0.3. The total pore volume was
glucose to HMF.
estimated at a relative pressure of 0.99.
In the present work, tin phosphate catalysts were prepared
Fourier transform infrared (FT-IR) spectra of the catalysts
using solid state grinding as a simple and green catalyst prep- were recorded on a PerkinElmer IR spectrometer using the KBr
aration method. The catalytic performance of the catalysts was disc method. The nature of acid sites (Brønsted and Lewis) of
evaluated for the production of HMF from glucose in a water/ the catalysts was determined by pyridine adsorbed FT-IR spec-
THF biphasic system. Effects of the P/Sn ratio and other reac- troscopy. Prior to the pyridine adsorption, the catalysts samples
tion parameters such as reaction time, temperature, and were dried at 150 ꢀC for 1 h. To the dried samples (ꢃ50 mg each)
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in a glass vial, 0.1 mL of pyridine was added directly. Then, the
excess pyridine was removed by heating the sample at 120 ꢀC for
1 h. Aer cooling the sample to room temperature, FT-IR
spectra of pyridine adsorbed catalysts samples were recorded.
X-ray photo electron spectroscopy (XPS) measurements were
conducted on an Axis Nova spectrometer using a mono-
chromatic Al K (alpha) source (15 mA, 14 kV). The XPS instru-
ment was calibrated using Au as standard. For energy
calibration, the carbon 1s photoelectron lines were used. The
carbon 1s binding energy was taken as 285 eV. The Kratos
charge neutralizer system was used on all specimens.
3. Results and discussion
3.1 N2 physisorption
Specic surface area of a heterogeneous catalyst plays a pivotal
role in its activity since catalytic reactions normally take place
on the surface of the catalyst. Besides, the total pore volume of
the catalyst is also an important factor affecting the reaction
rate. Table 1 represents the BET surface area and total pore
volume of various SnP catalysts prepared by solid-state grinding
method. It can be seen that the as-synthesized SnP catalysts
with a P/Sn mole ratio ranging from 0.5 to 2.0 exhibit relatively
low specic surface areas (<40 m2 gꢁ1). Among them, the SnP-1
catalyst with P/Sn mole ratio of 1.0 exhibits the highest specic
surface area of 38 m2 gꢁ1. The SnP-0.5 catalyst has similar BET
specic surface area (34 m2 gꢁ1) as the SnP-1 catalyst, and
further increasing the P/Sn mole ratio from 1.0 to 1.5 and 2.0
decreases the surface area of the catalyst. The total pore volume
of the catalysts also follows the same trend. The decrease in
surface area and total pore volume of the SnP catalysts at higher
P/Sn mole ratio (>1) is likely due to an excess phosphate
content, which might block the pores of the catalyst, as also
observed by Khder et al.32 Liu et al. reported effects of P/Al mole
ratio on thermal stability, surface area and pore volume of the
aluminum phosphate (AlPO) catalysts,33 where a higher P/Al
ratio (>1.1) led to signicant decreases in surface area, pore
volume and thermal stability of AlPO catalysts.
Total acidity of the catalysts was measured by NH3-TPD. NH3-
TPD measurement was carried out on a ChemBET Pulsar TPR/
TPD automated chemisorption analyzer. In a typical experi-
ment, about 0.1 g of the sample was pre-treated at 300 ꢀC for 1 h
under a ow of helium (99.9%, 120 mL minꢁ1). Aer pretreat-
ment, the sample was saturated with anhydrous ammonia at
100 ꢀC for 10 min and subsequently ushed with He at the same
temperature to remove any physisorbed ammonia. Then, TPD
ꢀ
analysis was carried out from ambient temperature to 500 C
with a heating rate of 10 ꢀC minꢁ1 and the desorbed ammonia
was measured by thermal conductivity detector.
2.2.3. Catalytic activity tests. The dehydration experiments
of glucose was carried out in a two-phase reaction system con-
sisting of water/THF (1 : 3 v/v). NaCl was added to the reaction
medium to sustain a biphasic reaction medium as well as
improve the efficiency of HMF extraction by the organic layer.31
In a typical experiment, a custom-made micro scale reactor was
charged with 0.06 g of catalyst, 0.15 g of glucose substrate in
NaCl saturated water (3 mL) and THF (9 mL). The temperature
of the reaction was maintained at 155–185 ꢀC. Upon completion
of the reaction, the reaction mixture was rapidly quenched to
room temperature, the catalyst was separated by centrifugation
and the supernatant liquid of both phases was collected for
analysis. Both the aqueous and the organic phases were
The calcined SnP-1 catalysts obtained at various calcination
temperatures (300–500ꢀ), have a similar BET surface area
(32–34 m2 gꢁ1), although slightly lower than that of the as-
synthesized SnP-1 catalyst (38 m2 gꢁ1). The slight decrease in
surface area at an increased calcination temperature might be
due to an increase in the catalyst's crystallinity.
3.2 FT-IR
analyzed using high performance liquid chromatography The structural evaluation of SnP catalysts was characterized by
(HPLC) (Waters 2690 HPLC) equipped with refractive index FT-IR spectroscopy. FT-IR results of the as-synthesized SnP
detector (Waters 410) and UV detectors (Waters 484) using catalysts with different P/Sn mole ratios are presented in Fig. 1.
a XBridge Amide, 3.5 mm, 4.6 ꢂ 250 mm analytical column. The sharp FT-IR band observed at 1060 cmꢁ1 is corresponding
80/20 acetonitrile/water with 0.2% triethylamine was used as to the asymmetric stretching of Sn–O–P bond vibration.34
A
mobile phase with a ow rate of 0.6 mL minꢁ1. The column broad IR band appeared between 3300–3700 cmꢁ1 is attributed
temperature was maintained at 35 ꢀC. Glucose conversion, HMF to the asymmetric stretching vibrations of O–H groups, arising
selectivity and yield were calculated according to the following from the interlayer water and P–OH groups.35,36 As such, it is
equations.
expected that the intensity of the band at 3500 cmꢁ1 increases
with increasing the P/Sn mole ratio from 0.5 to 1. While further
increasing P/Sn mole ratio from 1 to 1.5 and 2, however, there is
a slight decrease in intensity of the peak of asymmetric
stretching vibrations of O–H (the interlayer water and P–OH
groups), which might be caused by the increase in crystallinity
of the catalysts forming Sn–O–P structure with the increase of P/
Sn mole ratio, as evidenced by the XRD results (Fig. 2). The
sharp band at 1630 cmꢁ1 is ascribed to water H–O–H bending
vibration of water molecules coordinated with the phosphate
groups.36 Similarly, and the intensity of this band decreased
when the P/Sn ratio > 1, likely due to the increase in crystallinity
of the catalysts. The peak at 2400 cmꢁ1 is attributed to P–O–H
stretching vibration responsible for Brønsted acidity of the SnP
moles of glucose reacted
initial moles of glucose
Glucose conversion ð%Þ ¼
ꢂ 100
(1)
number of moles of HMF produced
number of moles of glucose reacted
HMF selectivity ð%Þ ¼
ꢂ 100
(2)
number of moles of HMF produced
initial number of moles of glucose
HMF yield ð%Þ ¼
ꢂ 100
(3)
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Table 1 Physicochemical properties of as-synthesized and calcined SnP catalysts
P/Sn mole
ratio
BET surface area
Total pore volume
Total acidity
Catalyst
(m2 gꢁ1
)
(cm3 gꢁ1
)
(mmol gꢁ1
)
B/L ratio (I1535/I1455)
SnP-0.5
SnP-1
SnP-1.5
SnP-2
SnP-1-300
SnP-1-400
SnP-1-500
0.5
1
1.5
2
1
1
34
38
15
9
34
34
32
0.07
0.13
0.09
0.07
0.13
0.11
0.16
0.98
1.23
0.67
0.52
1.13
0.58
0.29
0.448
0.542
0.295
0.152
0.402
0.323
0.113
1
catalysts.37 The peak intensies while increasing P/Sn mole ratio characteristic crystalline peaks, indicating amorphous nature of
from 0.5 to 1.0, but levels off at P/Sn mole ratio > 1.0. The FT-IR these catalysts, accounting for their higher BET surface areas
bands between 700–450 cmꢁ1 can be ascribed to the Sn–O bond (Table 1). The diffraction lines ascribed to the Sn–O–P
vibrations in Sn–O–P matrix.38 Besides, there are no IR bands compounds such as a-Sn(HPO4)2$H2O in SnP-1.5 and SnP-2
between 900–700 cmꢁ1, suggesting the absence of P–O–P bond catalysts are stronger those of the SnP-0.5 and SnP-1 catalysts,
in the catalysts. Thus, the FT-IR measurements of the as- suggesting increased crystallinity of these as-synthesized cata-
synthesized SnP catalysts with various P/Sn ratios from lysts at higher P/Sn mole ratio.35,39 As reported by Chen et al., the
0.5 to 2, prove the successful formation of the metal phosphate formation of a-Sn(HPO4)2$H2O was conrmed by solid state
Sn–O–P framework by solid-state grinding method. Further- reaction of SnCl4$5H2O and P2O5 at lower temperatures.40 In the
more, it was found that the calcined SnP-1 catalyst exhibits present study, the characteristic crystalline XRD peaks were
a similar FT-IR spectrum as that of the as-synthesized SnP-1 observed at 2q equal to 10, 20 and 27ꢀ in SnP-1.5 and SnP-2
catalyst (Fig. S1†), while the intensity of the bands decreases catalysts corresponding to layered structure of tin phosphate
with the increase of calcination temperature of the catalyst was reported in the literature.36 The increase in crystallinity of
particularly at 3500 and 1630 cmꢁ1 (related to water and O–H the catalysts at higher P/Sn ratio (>1) may cause a decrease in
groups), which can be due to the loss of surface adsorbed and surface area of the catalysts, as evidenced by the BET results
crystallized water molecules in calcination.
(Table 1). The surface hydroxyl groups and amorphous nature of
the catalyst would affect the acidity of the catalysts, hence play
an important role in the catalytic dehydration of glucose. As
clearly shown in Fig. 2, increasing P/Sn ratio (>1) or calcination
3.3 XRD
Powder X-ray diffraction (XRD) was employed to elucidate the temperature results in an increase in crystallinity of the catalyst
crystalline structure analysis of the catalyst samples. Fig. 2 relative to the as-synthesized SnP-1 catalyst, likely due to loss of
shows the XRD patterns of as-synthesized SnP catalysts with
adsorbed and crystallized water molecules, as conrmed by the
FT-IR results (Fig. 1 and S1†) where the IR bands at 3500 and
different P/Sn ratios, as compared with the calcined SnP-1
catalysts at different temperatures. It can be seen that as- 1630 cmꢁ1 corresponding to the stretching and bending vibra-
synthesized SnP-0.5 and SnP-1 catalysts do not exhibit strong tional modes of water molecules both decrease in intensity with
Fig. 1 FT-IR spectra of as-synthesized SnP catalysts.
48504 | RSC Adv., 2017, 7, 48501–48511
Fig. 2 XRD pattern of SnP catalysts. # a-Sn(HPO4)2$H2O.
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increasing the P/Sn ratio or calcination temperature. Thus, that the P–OH groups on the surface of SnP catalysts are the
a higher P/Sn ratio (>1) or higher calcination temperature would main source of acidic sites, which was actually supported by the
negatively inuence the activities of the SnP catalysts. On the highest activity for the as-synthesized SnP-1 catalyst among all
other hand, there is no indication of formation of pyrophos- catalysts testes (to be discussed later). The acidity values for all
ꢀ
phate phase although the catalyst was calcined at 500 C indi- catalysts are presented in Table 1 and the NH3-TPD desorption
cating the thermal stability of the as-synthesized SnP catalysts. proles for the calcined SnP-1 catalyst are shown in Fig. S2.†
Furthermore, there are no diffraction lines related to either The total acidity of the catalyst decreased with increasing the
SnCl4$5H2O or NaH2PO4 observed in the as-synthesized SnP calcination temperature, as similarly reported by another study
catalysts, indicating that the tin phosphate species (Sn–O–P) using tin phosphate catalyst.41 Similar observation (a higher
were successfully synthesized through the solid-state grinding calcination temperature led to a decrease in the intensity of the
method.
Brønsted and Lewis acid sites) was obtained from the pyridine
FT-IR measurements to be presented in the subsequent section.
3.4 NH3-TPD
3.5 Pyridine-adsorbed FT-IR
As widely reported, the total acidity of a catalyst plays a prom-
inent role in determining its catalytic performance in glucose to
HMF conversion.14–21 The NH3-TPD patterns of as-synthesized
SnP catalysts with different P/Sn mole ratios are shown in
Fig. 3, where the NH3-TPD measurements were carried out in
the temperature range of 100–600 ꢀC. The total acidity of these
catalysts was calculated by integration of the NH3 desorption
peaks, and the values are presented in Table 1. In the case of
SnP-0.5 catalyst, only one broad NH3 desorption peak was
detected in the range of 120–450 ꢀC, peaked at about 225 ꢀC
(Tmax), representing the presence of weak-to-medium acidity.
For SnP-1 catalyst, the Tmax is slightly shied to a higher
temperature region with marked increase in the peak intensity,
indicating a much greater total acidity (1.23 mmol gꢁ1 for SnP-1
vs. 0.98 mmol gꢁ1 for SnP-0.5, as shown in Table 1). With SnP-
1.5 and SnP-2 catalysts, a further increase in the P/Sn ratio from
1 to 2, two ammonia desorption peaks with lower intensity
peaks were observed between the temperature range of 150–
550 ꢀC. Whereas, compared with that of SnP-1, the total acidity
of the SnP-1.5 and SnP-2 catalysts decrease signicantly, which
could be due to their increased crystallinity (Fig. 1 and 2).
Combining the results of NH3-TPD and FT-IR, it can be
concluded that there is a strong relationship between the total
acidity and the P–OH groups on catalyst surface. It is apparent
FT-IR spectra of pyridine adsorption were recorded to probe
accessible surface acid sites. Fig. 4 shows the pyridine-adsorbed
FT-IR spectra of the as-synthesized SnP catalysts with different
P/Sn mole ratios. The presence of lone pair of electrons on
nitrogen in a pyridine molecule can coordinate with both metal
atom and acidic proton on the catalyst surface to show different
characteristic FT-IR stretching vibrations in the nger print
region of 1400–1600 cmꢁ1
.
The FT-IR band at 1536 cmꢁ1 is
42
assigned to characteristic protonation of pyridine molecule
onto Brønsted acidic sites, while the IR band at 1458 cmꢁ1 is
assigned to pyridine molecule coordinated to Lewis acidic metal
atom.41 The presence of strong band at 1488 cmꢁ1 is due to
pyridine adsorbed onto both Brønsted and Lewis acidic sites.43
The Brønsted acidity arises from the presence of geminal P–OH
groups, whereas Lewis acidity originates from the electron
decient Sn4+ sites. As shown in Fig. 4, the as-synthesized SnP-
0.5 and SnP-1 catalysts exhibit strong Brønsted and Lewis
acidity compared to SnP-1.5 and SnP-2 catalysts with a higher P/
Sn mole ratio, which is in a good agreement with the total
acidity of the catalysts quantied by NH3-TPD measurements
(Fig. 3 and Table 1). The decrease in acidity of the SnP catalysts
with a too high P/Sn mole ratio is possibly due to loss of surface
Fig.
4 Pyridine adsorbed FT-IR spectra of as-synthesized SnP
Fig. 3 NH3-TPD pattern of as-synthesized SnP catalysts.
catalysts.
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Table 2 Surface atomic composition of the SnP-1 catalyst determined
by XPS
hydroxyl groups as evidenced from FT-IR results (Fig. 1). The B/
L ratios are calculated and presented in Table 1 based on the
intensity of Brønsted (B) and Lewis acid (L) sites obtained from
the absorbance at 1536 and 1458 cmꢁ1, respectively. The B/L
ratio is maximum for the as-synthesized SnP-1 catalyst and
decrease thereaer with a further increase in P/Sn mole ratio.
Fig. S3† illustrates the pyridine-adsorbed FT-IR spectra of the
SnP-1 catalyst calcined at different temperatures, revealing
decreases in both Brønsted and Lewis acidic sites with
increasing the calcination temperature.
Surface atomic composition (atom%)
P/Sn mole
Catalyst
Sn
P
O
Na
Cl
ratio
SnP-1 (fresh)
SnP-1 (spent)
12.5
10.7
9.0
9.3
51.5
58.3
3.5
4.6
8.4
13.6
0.72
0.86
and phosphorous.30 The broad peak of O 1s spectra might also
be attributed to the surface hydroxyl groups, contributing to the
total acidity of the catalyst.45 Similarly, the photoelectron
spectra of P 2p also shows a broad peak (126–134 eV), resulted
from the P–O bonds on the surface that can be enhanced by the
phosphorus atoms bonded to the hydroxyl groups, contributing
to the acid sites on the catalyst surface.44 However, the binding
energy of Sn 3d, O 1s and P 2p shied slightly towards lower
values compared to that reported in the literature.44 This might
be due to the possible inuence of residual sodium and chlo-
ride ions on the catalyst surface. The surface atomic composi-
tion of as-synthesized SnP-1 catalyst is presented in Table 2,
determined based on XPS peak areas of individual elements on
the catalyst surface. The surface P/Sn atomic ratio was calcu-
lated accordingly for both fresh and spent catalysts, and the
ratio is 0.72 (fresh SnP-1) and 0.86 (spent SnP-1), both slightly
lower than 1.0 (the bulk P/Sn mole ratio) though.
3.6 XPS
XPS measurements were performed to examine the valence
states of individual elements presented in the catalyst samples
and the surface atomic composition. The XPS spectra of tin,
phosphorus and oxygen for the as-synthesized SnP-1 catalyst are
shown in Fig. 5. The core level spectra of Sn 3d consist of two
broad but distinct binding energy peaks, at 480–488 eV and
488.5–497 eV, corresponding to 3d5/2 and 3d3/2, respectively.
These two peaks are characteristic of tetravalent Sn4+ 44 The O 1s
.
spectra of the SnP-1 catalyst show a broad peak between 524–
533 eV which could be attributed to oxygen bonded to both tin
4. Catalytic activity
The catalytic performance of the as-synthesized SnP catalysts
prepared by solid-state grinding method at low temperatures
was evaluated for dehydration of glucose to HMF in a water/THF
biphasic system. A biphasic solvent system has several advan-
tages over most of the single-phase organic solvent systems for
HMF production.46 Since the boiling point of THF is low
(normal boiling point 66 ꢀC), it can easily be recovered by simple
evaporation aer reaction. Moreover, the bi-phasic solvent
system can avoid the HMF degradation and prevent formation
of soluble and insoluble humins through reactions of HMF with
the reaction intermediates from glucose. These secondary
reactions would decrease the HMF yield and selectivity.
4.1 Effects of reaction temperature
With the as-synthesized SnP-1 catalyst, the effects of reaction
temperature on glucose conversion and HMF yield were evalu-
ated at various temperatures from 155–185 ꢀC at a specic
reaction time (1 h), and the results are shown in Fig. 6. The
reaction temperature had played a very signicant role on both
glucose conversion and HMF selectivity and yield. At 155 ꢀC,
only 66.5% of glucose conversion and 22% HMF yield was ob-
tained. As presented in Fig. 7, besides HMF, fructose was also
considerably detected in the reaction products at temperatures
below 175 ꢀC, suggesting the reaction proceeds through rst
isomerization of glucose to fructose over Lewis acidic Sn4+ sites
followed by fructose dehydration to HMF on Brønsted acidic
sites.47–49 Fructose was not identied at 185 ꢀC probably due to
Fig. 5 (a) Sn 3d, (b) O 1s and (c) P 2p XPS spectra of the as-synthesized
SnP-1 catalyst.
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ꢀ
temperature increased further to 185 C. The maximum HMF
yield (61%) was obtained with the for the SnP-1 catalyst at
ꢀ
ꢀ
175 C. Therefore, 175 C was selected as the reaction temper-
ature to study the effects of other process parameters.
4.2 Effects of P/Sn mole ratio and calcination temperature of
the catalyst
Effects of the P/Sn mole ratio for the as-synthesized SnP cata-
lysts was tested for glucose dehydration to HMF at 175 ꢀC at 1 h
reaction time, and the results are presented in Fig. 8. From the
gure, clearly the catalytic activity of these catalysts, in partic-
ular their HMF selectivity, is considerably inuenced by the P/
Sn mole ratio. As shown in Fig. 7, high glucose conversion
(95–100%) and HMF selectivity (50–65%) were achieved over the
SnP-0.5 and SnP-1 catalysts with a P/Sn mole ratio of 0.5 and 1,
respectively. The conversion of glucose as well as HMF selec-
tivity decreased with further increase of P/Sn ratio to 1.5 and 2.0.
The maximum glucose conversion (98%) and HMF selectivity
(>60%) were obtained with the as-synthesized SnP-1 catalyst
(P/Sn ¼ 1), leading to the maximum HMF yield 61% and highest
ever reported on SnP catalysts in spite of high catalyst loading in
the present protocol.26,27 The variation in catalytic activity with
the P/Sn mole ratio for the SnP catalysts could be explained
based on the characteristics of the catalysts as discussed in the
previous sections, in particular with respect to the acidity of the
catalysts. As presented previously (Table 1, Fig. 3 and 4), the as-
synthesized SnP-1 catalyst has high concentration of total acid
sites and the presence of strong Brønsted and Lewis acidic sites
on the catalyst surface, which may account for its high catalytic
activity (in terms of glucose conversion and HMF selectivity).
The reduced HMF yield at a higher P/Sn ratio (>1) might be due
to the decrease in surface area, low IB/IL ratio, and low acidity on
the catalyst surface (Table 1). Moreover, in situ generated
SnP catalyst was also tested under present experimental
conditions by adding tin salt (SnCl4$5H2O) phosphorus salt
(NaH2PO4$2H2O) with mole ratio of P/Sn equal to 1, afforded
27% HMF yield from the total conversion of glucose, illustrating
Fig. 6 Effects of reaction temperature on glucose dehydration over
SnP-1 catalyst. Other reaction conditions: glucose/catalyst 2.5 : 1 w/w,
NaCl 0.525 g, water 3 mL, THF 9 mL, 1 h.
its complete transformation to HMF. It has also been reported
that levulinic acid and formic acid formed by rehydration of
HMF could be produced during the dehydration of glucose to
HMF.49 However, in the present work these by-products were
not detected, possibly due to the fact that the acidity of the SnP
acid catalyst is not strong enough to catalyze rehydration of
HMF. This was veried by conducting a control experiment with
HMF and the same quantity of catalyst at 175 ꢀC for 1 h, in the
same biphasic reaction medium, and levulinic acid was not
detected either, suggesting that rehydration of HMF is negli-
gible with the as-synthesized SnP-1 catalyst. This result is sup-
ported by a study by Jimenez-Morales et al. where HMF was not
rehydrated over a mesoporous tantalum phosphate catalyst.50
The carbon balance is below 100% over SnP-1 catalyst, beside
HMF and fructose, the other by-products are organic phase
soluble and insoluble humin substances which are not detected
by HPLC analysis.
As clearly shown in the Fig. 6, when increasing the temper-
ꢀ
ature from 155 to 175 C the conversion of glucose and HMF
selectivity and yield all increased simultaneously, while the
performance of the catalyst declined as the reaction
Fig. 7 Effects of P/Sn mole ratio for the SnP catalysts on dehydration Fig. 8 Effects of calcination temperature for the SnP-1 catalyst on
reaction of glucose. Other reaction conditions: glucose/catalyst dehydration of glucose. Other reaction conditions: glucose/catalyst
2.5 : 1 w/w, NaCl 0.525 g, water 3 mL, THF 9 mL, 175 ꢀC, 1 h.
2.5 : 1 w/w, NaCl 0.525 g, water 3 mL, THF 9 mL, 175 ꢀC, 1 h.
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that ex situ prepared SnP catalyst was more active than in situ 175 ꢀC and 1 h. As shown in Fig. S4,† varying the catalyst
generated SnP catalyst. loading, by changing the glucose/catalyst weight ratio (w/w)
As discussed previously, the surface hydroxyl groups on SnP remarkably inuenced both glucose conversion and HMF
catalysts are the main source of Brønsted acid sites. The surface selectivity and yield. As shown in the gure, with increasing
hydroxyl groups on SnP catalysts are temperature sensitive, and glucose/catalyst weight ratio from 2.0 to 3.0, or in other words
they could diminish by calcination, inuencing the catalysts' decreasing catalyst loading, glucose conversion decreases
activity. The catalytic performance of the SnP-1 catalyst calcined continuously as expected. Whereas, there is an optimal catalyst
at different temperatures between 300 to 500 ꢀC is compared loading for HMF selectivity and yield, corresponding to glucose/
with that of the as-synthesized catalyst, as shown in Fig. 8. It can catalyst weight ratio of 2.5. Above this optimum value, a higher
be seen that the HMF selectivity decreased with increasing catalyst loading or more catalytic active sites led to a decreased
calcination temperature of the catalyst, and the glucose HMF selectivity and yield, which might be caused by the
conversion dropped from 98 to 80% and HMF yield decreased promoted side-reactions between the HMF and glucose or other
from 61 to 35% aer calcination of the as-synthesized SnP reaction intermediates, resulting in the formation of humin
catalyst at 500 C. This proves that Brønsted acidic sites (asso- substances.51 More investigation on the possible cause of the
ꢀ
ciated with the hydroxyl groups) on the catalyst surface played optimal catalyst loading and effective measures to restrict the
a signicant role in dehydration of glucose to HMF.
side-reactions will be of one of our future research interests.
Next, effects of initial glucose concentration (or substrate
concentration) were examined with the as-synthesized SnP-1
catalyst under the best conditions obtained by far (glucose/
catalyst 2.5 : 1 w/w, NaCl 0.525 g, H2O 3 mL, THF 9 mL, 175 ꢀC,
and 1 h). From an economical and industrial point of view, it is of
interest to employ a higher substrate concentration for higher
HMF productivity and process throughput. As such, two levels of
substrate concentration (wt% glucose in water) were tested,
5 wt% and 10 wt%, and the results are presented in Fig. S5.† As
clearly shown in the gure, when the reaction was performed
with 10 wt% glucose concentration, the HMF yield was decreased
to 42% compared with >60% HMF yield with 5 wt% glucose
concentration although approx. 100% glucose conversion was
obtained under both substrate concentrations (Fig. 12). Such
sharp decrease in the catalyst's HMF selectivity again may be due
to the formation of humins via condensation polymerization of
HMF with glucose and other reactive intermediates.52
4.3 Effects of reaction time, catalyst loading and initial
glucose concentration
Inuence of reaction time on dehydration of glucose over the as-
synthesized SnP-1 catalyst was investigated under the following
reaction conditions: glucose/cat. 2.5 : 1 w/w, NaCl (0.525 g), H2O
ꢀ
(3 mL), THF (9 mL) and 175 C. The results are illustrated in
Fig. 9. Clearly, the reaction time has a marked effect on both
glucose conversion and HMF yield. The results demonstrated
that although the glucose conversion increased with increasing
reaction time, both the HMF selectivity and yield maximized at
approx. 60 min. With a longer reaction time, secondary reac-
tions were likely promoted, to form by-products such as soluble/
non-soluble humins, although not detectable by HPLC, leading
to substantial decreases in both HMF selectivity and yield. As
shown in the gure, the HMF yield decreased from 61 to 45% as
the reaction time increased from 60 to 90 min. According to
these results, the best reaction time for HMF production with
4.4 Effect of catalyst preparation method
ꢀ
the as-synthesized SnP-1catalyst at 175 C was approx. 60 min.
Furthermore, we examined the effects of catalyst loading on
glucose conversion and HMF selectivity and yield during
glucose dehydration over the as-synthesized SnP-1 catalyst at
Encouraged by the efficacy of solid-state synthesis of SnP-1
catalyst for converting glucose to 5-HMF, we compared the cata-
lytic activity of SnP-1 catalyst prepared by the conventional liquid-
state as a reference catalyst with solid-state SnP-1 catalyst. As
shown in Fig. 10, there is a marginal difference in catalytic activity
between these catalysts, indicating the superiority of SnP-1 cata-
lyst prepared via solid-state method to the SnP-1 catalyst prepared
by liquid-state method. The difference in catalytic activity was
mainly attributed to the difference in acid properties of the
catalysts. The total acidity of the catalysts was measured by NH3-
TPD as shown in Fig. S6.† It reveals that SnP-1-L catalyst showed
a lower concentration of total acid sites (0.64 mmol gꢁ1),
compared to SnP-1 catalyst prepared by solid-state method
(Table 1). The structural properties of SnP-1-L catalyst was well
reported in the literature and the total acidity of SnP-1-L catalyst
is comparable to that reported in the literature.30
4.5 One-pot conversion of micro crystalline cellulose to HMF
Compared with glucose, cellulose or raw lignocellulosic
biomass is certainly more economical substrate for HMF
production as it is the most abundant naturally occurring
Fig. 9 Influence of reaction time on dehydration of glucose over the
as-synthesized SnP-1 catalyst. Other reaction conditions: glucose/
catalyst 2.5 : 1 w/w, NaCl 0.525 g, water 3 mL, THF 9 mL, 175 ꢀC.
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recyclability of the as-synthesized SnP-1 catalyst was evaluated
according to reported procedure in the literature.19,54 Aer
initial glucose dehydration reaction (glucose/catalyst 2.5 : 1 w/
ꢀ
w, NaCl 0.525 g, H2O 3 mL, THF 9 mL, 175 C, 1 h), the cata-
lyst was recovered by centrifugation, washed rst with methanol
and then with water for three to four times and then oven dried
at 100 ꢀC for more than 12 hours. The retrieved catalyst was
reused without any post treatment. Repeating the same catalyst
recycling-reuse cycles for 5 times and the results are shown in
Fig. 12. Both aqueous and organic phases of the reaction solu-
tion were analyzed to quantify the total amount of HMF
formation in each run. During all these experiments, no fresh
catalyst was supplied to compensate the loss of recovery. It can
be observed from the gure that the catalytic activity of the as-
synthesized SnP-1 catalyst in terms of HMF yield slightly
decreases from ꢃ60% to ꢃ52% aer ve cycles. Although
leaching of the Sn4+ ions in the catalyst is unavoidable in the
process, the above results on recyclability of the catalyst
demonstrate that the metal ions leached into the reaction
medium was insignicant.
Fig. 10 Effect of catalyst preparation method on glucose to HMF
conversion. Other reaction conditions: glucose/catalyst 2.5 : 1 w/w,
NaCl 0.525 g, water 3 mL, THF 9 mL, 175 ꢀC, 1 h.
feedstock.53 In the present study, we tested the catalytic effi-
ciency of the as-synthesized SnP-1 catalyst toward hydrolysis-
dehydration of microcrystalline cellulose for HMF production.
Under the same glucose dehydration reaction conditions as
described above (i.e., substrate/catalyst 2.5 : 1 w/w, NaCl
4.7 Performance of the as-synthesized SnP-1 catalyst
compared with some solid acid catalysts reported in the
literature
ꢀ
0.525 g, water 3 mL, THF 9 mL, 175 C and 1 h), conversion of
microcrystalline cellulose with the as-synthesized SnP-1 catalyst
was very poor, producing only 10% HMF yield (Fig. 11). The
poor reactivity of cellulose may be due to the ineffective depo-
The catalytic performance of the as-synthesized SnP-1 catalyst
for dehydration of glucose to HMF in a biphasic reaction
medium is compared with that of a few solid acid catalysts re-
ported in the literature. The use of Sn-beta zeolite catalyst in
combination with HCl achieved good yield for HMF (56.8%) but
comparatively low glucose conversion (79%).14 Moreover, the
use of homogeneous HCl catalyst led to product contamination
and hence tedious clean up procedure was required to purify
the product. The activities of mesoporous tantalum oxide and
phosphate catalysts are much poorer, with HMF yield of only
ꢀ
lymerization (hydrolysis) of cellulose at 175 C with the SnP-1
catalyst. Indeed, when the reaction temperature was increased
to 190 ꢀC, HMF yield increased signicantly to 24%, suggesting
that the SnP-1 catalyst could be promising catalyst for one-pot
conversion of cellulose or other complex carbohydrates into
HMF, while the reaction conditions need to be optimized.
4.6 Catalyst recyclability
The advantage of using a heterogeneous catalysis for chemical
process is the recyclability of the catalyst. In this work, the
Fig. 11 Comparison of catalytic performance of the as-synthesized
SnP-1 catalyst in the conversion of glucose or cellulose to HMF. Other Fig. 12 Recyclability of the SnP-1 catalyst for conversion of glucose to
reaction conditions: substrate/catalyst 2.5 : 1 w/w, NaCl 0.525 g, water HMF. Other reaction conditions: glucose/catalyst 2.5 : 1 w/w, NaCl
3 mL, THF 9 mL, 175 ꢀC or 190 ꢀC, 1 h.
0.525 g, water 3 mL, THF 9 mL, 175 ꢀC, 1 h.
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Table 3 Comparison of catalytic activity of the prepared SnP-1 catalyst with some solid acid catalysts reported in the literature
Paper
Catalysts
Solvents
Temp. (ꢀC)
Time (min)
Glucose conversion (%)
HMF yield (%)
Ref.
SnP-1
H2O/THF
H2O/THF
H2O/MIBK
H2O/MIBK
H2O/MIBK
H2O/butanol
H2O/THF
H2O/THF
175
180
175
170
195
175
160
165
60
70
90
120
150
180
90
98
79
69
56.3
87
97
91.7
93.5
61
56.8
23
32.8
36
81
This work
Sn-beta, HCl
meso-Tantalum oxide
meso-Tantalum phosphate
meso-Al-MCM-41
P–TiO2
14
52
50
51
19
53
22
meso-AlSiO
Nb-SBA-15
63.1
61.5
180
23% and 32.8% and glucose conversion of 69% and 56.3%, and green method via solid-state grinding without need of
respectively.50,52 Conversely, mesoporous aluminium doped energy-intensive calcination treatment. The new preparation
MCM-41 catalyst realized good glucose conversion (87%), but method for SnP catalysts is easy to scale up and these catalysts
exhibited low HMF yield (36%).51 As shown in Table 3, could be useful for large scale continuous production of 5-HMF
phosphated TiO2 (P–TiO2) catalyst exhibited very high glucose from glucose, which is currently under study in our laboratory.
conversion (97%) and HMF yield (81%) but at a much longer
reaction time (180 min),19 as compared with the glucose
conversion (98%) and HMF yield (61%) at a much shorter
Conflicts of interest
reaction time (60 min) with the as-synthesized SnP-1 catalyst in
this work. Mesoporous AlSiO catalyst55 and Nb-SBA-15 catalyst22
obtained similarly high HMF yield (62–63%) and glucose
conversion (92–94%) at 160–165 ꢀC for a longer time (90 and
190 min, respectively). Although, these catalysts showed good
catalytic performance, preparation of these catalysts involved
wet impregnation and high-temperature calcination for activa-
tion of the catalysts. In contrast, the as synthesized SnP-1
catalyst in this work is simple to prepare without the need of
high-temperature activation process.
There are no conicts to declare.
Acknowledgements
The authors acknowledge the nancial support from NSERC
Discovery Grants and the funding from BioFuelNet Canada,
a Network of Centres of Excellence. One of the authors (C. X.)
also gratefully acknowledges the funding from NSERC/
FPInnovations Industrial Research Chair program and the
ORF-RE Program in Forest Biorenery.
5. Conclusions
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