Synthesis of different structured FePO4 for the enhanced conversion of methyl cellulose to 5-hydroxymethylfurfural

Article abstract of DOI:10.1039/c7ra09186a

FePO₄ catalysts with branch-like, flower-like, and spherical morphologies were synthesized for the conversion of methyl cellulose to 5-hydroxymethylfurfural (5-HMF) via a hydrothermal route. The molar ratio of Fe³⁺ and H₂PO₄^- ions in the reaction system played a crucial role in the morphology of FePO₄. Compared with flower-like, spherical and amorphous FePO₄, branch-like FePO₄ presented a better catalytic performance in the cellulose conversion and 5-HMF yield. The branch-like FePO₄ retained a branch structure after recycling five times in the bi-phasic reaction process. The insolubility of low temperature and partial dissolution of elevated temperature were responsible for the excellent catalytic activity of the FePO₄ phase-change catalyst. The combined effect of H⁺ ions and iron species generated from the hydrolysis of FePO₄ can be favorable for the enhanced yield of 5-HMF.

Full text of DOI:10.1039/c7ra09186a

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Synthesis of different structured FePO4 for the
enhanced conversion of methyl cellulose to
5-hydroxymethylfurfural
Cite this: RSC Adv., 2017, 7, 51281
a
a
a
a
b
*
*
Yong Liu, Zili Li, Yaohui You, Xiaogang Zheng and Jing Wen
FePO4 catalysts with branch-like, flower-like, and spherical morphologies were synthesized for the
conversion of methyl cellulose to 5-hydroxymethylfurfural (5-HMF) via a hydrothermal route. The molar
ratio of Fe3+ and H2PO4 ions in the reaction system played a crucial role in the morphology of FePO4.
ꢀ
Compared with flower-like, spherical and amorphous FePO4, branch-like FePO4 presented a better
catalytic performance in the cellulose conversion and 5-HMF yield. The branch-like FePO4 retained
a branch structure after recycling five times in the bi-phasic reaction process. The insolubility of low
temperature and partial dissolution of elevated temperature were responsible for the excellent catalytic
activity of the FePO4 phase-change catalyst. The combined effect of H+ ions and iron species generated
from the hydrolysis of FePO4 can be favorable for the enhanced yield of 5-HMF.
Received 19th August 2017
Accepted 30th October 2017
DOI: 10.1039/c7ra09186a
rsc.li/rsc-advances
imidazolium chloride ([C4C1im]Cl, [C2C1im]Cl, and [C4C1im]
HSO4) present superior medium for the conversion of biomass
1. Introduction
feedstock due to their specic chemical and physical proper-
ties.18–25 Nevertheless, the hybrid reaction system of ionic
liquids and metal salts is not suitable for the large-scale
synthesis due to the high energy consumption, high cost, and
environmental contamination.
Due to the growing environmental awareness of greenhouse gas
and the diminishing supply of petroleum resources, the cata-
lytic conversion of renewable biomass feedstock into fuels,
chemicals, and solvents is an alternative route to relieve the
reliance on fossil resources.1,2 Many efforts focus on the devel-
opment of innovative strategies for the transformation of
biomass into chemicals to conquer the drawbacks including
extensive, environmentally harmful, and high-cost pretreat-
ment. Conversion of cellulosic biomass to 5-hydrox-
ymethylfurfural (5-HMF) has triggered increasing attention.3–5
5-HMF, as a platform chemical, can be converted into valuable
chemicals and fuels such as 2,5-furandicarboxylic acid, 2,5-
dimethylfuran, and levulinic acid.6–10
Compared with fructose and glucose, cellulose composed of
linear glucose polymer chains is an ideal feedstock for the
synthesis of 5-HMF due to its low cost and abundant
resources.11,12 However, the poor solubility of cellulose and the
formation of humins in aqueous media and organic solvents
leads to the inferior conversion of cellulose and the low yield of
5-HMF in the catalytic system assisted with mineral acid.13–17
Ionic liquids combined with metal salts have been conrmed to
break down the inferior conversion of cellulose into 5-HMF.
Ionic liquids such as [Bmim]Br, [Bmim]Br, [EMIM]Cl, and
Biphasic reaction systems26,27 and heterogeneous catal-
ysis28–30 have been developed to address above challenges. These
strategies are more suitable and cost effective for the large-scale
transformation of cellulose into 5-HMF. However, these
synthesis approaches usually suffer from the serious pollution
and low 5-HMF yield if the cellulose and catalysts are not pre-
treated in biphasic system.31,32 This's ascribed to the insufficient
contact and interaction between active sites of solid acid and
insoluble cellulose in water and most organic solvents.33,34
Therefore, the phase transfer catalyst has been developed to
achieve high conversion of cellulose into 5-HMF.35–38 The phase
catalyst is characterized by the excellent activity of homoge-
neous acid catalyst and the effective separation and recovery of
heterogeneous solid acid catalyst.39,40 Cellulose hydrogenation
reaction assisted with H2WO4 and Ru/C presents the maximized
yield of ethylene glycerol due to the effective contact between
insoluble cellulose and soluble H2WO4 catalyst.29
Iron phosphate (FePO4) is a promising and inexpensive
phase transfer catalyst for the efficient catalytic conversion of
carbohydrates (such as glucose, fructose, and cellulose) into 5-
HMF in biphasic system.36,37 FePO4 bulks are dissolved and
transferred from solid state into the aqueous phase at high
aCollege of Chemistry and Chemical Engineering, Neijiang Normal University,
Neijiang, Sichuan 641100, China. E-mail: zhengxg123456@163.com; Fax: +86 0832
2341577; +86 0971 7762180; Tel: +86 0832 2341577; +86 0971 7762180
bKey Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake
Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining
810008, China. E-mail: wj580420@163.com
ꢁ
temperature (>140 C), and re-precipitated to form solid bulks
aer cooling down to room temperature. Hence, this insolu-
bility at room temperature and the partial dissolution at
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elevated temperatures of FePO4 can surmount the drawback of and then stirred at room temperature for 2.0 h. The above
recycling and separation of homogeneous catalysts. The archi- mixture was placed into a 200 mL autoclave under the given test
tecture of FePO4 may be crucial to the physical and chemical conditions. Aer cooled down to room temperature, the prod-
properties for the catalyzed synthesis of 5-HMF from cellulose ucts were centrifuged, washed, and separated into solid residue,
during biphasic reaction process compared with CrPO4 and aqueous phase, and organic phase. The liquid fraction was
CrCl3 catalysts.20,28,38 This is attributed to the accelerated diffu- analyzed by the high-performance liquid chromatography. The
sion rate of molecules and the effective exposure of active sites catalytic stability of FePO4 for the methyl cellulose converted
ꢁ
to reactants. For example, the ordered mesoporous structure of into 5-HMF at 160 C for 80 min was performed for ve cycles
Nb4W4 exhibits high amounts of Brønsted and Lewis acid sites, under similar conditions. Before adding to the next reaction
leading to the excellent catalytic performance in glucose process, the used FePO4 bulks were centrifuged, washed with
conversion into 5-HMF.39 Due to the mesoporous structure and water three times, dried at 60 ꢁC for 12 h, and calcined at 500 ꢁC
strong Brønsted acid sites, the mesoporous Nb2O5–WO3 and for 3 h.
Ta2O5–WO3 catalysts also presented remarkable catalysis
performance.40,41 However, the new insights into the effect of
2.4. Characterization and analysis
FePO4 microstructure on the chemical transformations of
XRD patterns of as-obtained FePO4 were measured by a Bruker
biomass feedstock remains elusive. The fundamental studies in
D8 Advance X-ray Powder Diffractometer. FI-IR spectra were
the effect of FePO4 structure on the conversion of cellulose to 5-
obtained by a Shimadzu® FTIR spectrometer, IRPrestige-21
HMF in biphasic catalytic system is also scarce.
model. SEM images were observed using a Hitachi S-3400
This work focused on the controllable synthesis of FePO4 for
eld emission electron microscope. XPS spectra were recorded
the catalytic conversion of methyl cellulose into 5-HMF. The
on a Thermo Fisher Scientic Escalab 250 spectrometer. Fe3+
branch-like, ower-like, and sphere FePO4 were prepared in
species in aqueous solution was analyzed via the Inductively
a hydrothermal system with different molar ratio of FeCl3 and
Coupled Plasma Optical Emission Spectrometer (ICP-OES) on
KH2PO4. The catalytic mechanism of FePO4 was also proposed
Varian 710-ES equipped for axial viewing with a 1.12 megapixel
for the conversion of methyl cellulose to 5-HMF.
CCD detector. The residue on the used FePO4 was conducted in
a TA Q5000 instrument with a He ow rate of 50 mL minꢀ1
.
2. Experimental
2.1. Reagents
Products were analyzed using an Agilent 1200 high-
performance liquid chromatography equipped with an UV
detector (280 nm) and a column (Zorbax SB-C18). 5-HMF and
furfural were the main product and byproduct of methyl cellu-
lose conversion, respectively. 5-HMF yield (Y) was dened as: Y
(5-HMF) % ¼ (moles of 5-HMF generated)/(glucose unit moles
of methyl cellulose), where the mass of 5-HMF was the total
content of 5-HMF in the water and organic phases.
Ferric chloride (FeCl3), potassium dihydrogen phosphate
(KH2PO4), polyvinyl pyrrolidone (PVP, Mw ¼ 58 000), absolute
ethanol (C2H5OH), methyl cellulose (1.0 ꢂ 105 mPa s, CAS:
9004-67-5), tetrahydrofuran (THF, C4H8O), and sodium chloride
(NaCl) were purchased from Aladdin Industrial Corporation.
These chemicals were of analytical grade without any further
purication.
3. Results and discussion
3.1. Characterization of FePO4
2.2. FePO4 preparation
FePO4 with branch-like, ower-like, and sphere structure were
prepared by a hydrothermal route with different molar ratios of
FeCl3 and KH2PO4 (1 : 3, 2 : 3, and 3 : 3). Typically, 2.0 mmol
FeCl3, 6.0 mmol KH2PO4, and 1.5 g PVP were dissolved in 60 mL
ethanol solution (50 wt%) and stirred at room temperature for
3 h. Then, the above solution was transferred into a 100 mL
The XRD patterns of FePO4$2H2O generated via a hydrothermal
route and FePO4 with different morphologies (including
branch-like, ower-like, and sphere structure) were shown in
Fig. 1. The typical peaks of FePO4$2H2O with different
morphologies appeared at 2q ¼ 26.7ꢁ, 27.6ꢁ, 28.9ꢁ, 34.7ꢁ, 39.6ꢁ,
44.5ꢁ, 48.6ꢁ, 52.1ꢁ, 55.2ꢁ, 57.6ꢁ, 64.0ꢁ, and 73.3ꢁ, which were
matched well with crystalline FePO4$2H2O phases (JCPDS no.
002-0250).42,43 The peaks of FePO4 samples at 2q ¼ 20.2ꢁ, 25.8ꢁ,
26.7ꢁ, 28.9ꢁ, 35.5ꢁ, 38.6ꢁ, 41.4ꢁ, 48.5ꢁ, 58.2ꢁ, and 65.7ꢁ were
ascribed to the (100), (011), (012), (111), (110), (112), (200), (114),
(212), and (124) planes of hexagonal FePO4 samples (JCPDS no.
29-0715), respectively.42–44 The (011) plane of FePO4 at 25.8ꢁ
showed stronger peak intensity compared with other peaks in
ꢁ
Teon-lined stainless autoclave and treated at 170 C for 12 h.
Aer cooling to room temperature, the obtained sample was
centrifuged, washed with deionized water, dried at 60 ꢁC for
10 h, and calcined at 500 ꢁC for 3 h. Amorphous FePO4 was
prepared by the precipitation route.
2.3. Methyl cellulose conversion
The conversion of methyl cellulose into 5-HMF over FePO4 XRD pattern (Fig. 1), indicating that the FePO4 samples
catalysts were performed in a 200 mL stainless steel autoclave prepared via the hydrothermal route favored the growth along
under the N2 atmosphere of 5 bar and the given reaction the (011) plane. The minor peak around 30.4ꢁ of fresh FePO4
temperature. Typically, 3.0 g NaCl and the given content of was ascribed to the typical peak of Fe4(P2O7)3 phase (JCPDS no.
FePO4 and methyl cellulose were added to the mixed solution of 24-0526).42,45 Additional diffraction peaks of impurity were not
45 mL deionized water and 135 mL THF (TMF solution of 75%), detected in XRD patterns of FePO4, meaning that the as-
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like FePO4 were also assigned to the symmetric stretching mode
of PO4 group. Due to the difference in morphology, the peaks
below 600 cmꢀ1 of all FePO4 samples were related with the
different Fe–O and P–O bending and stretching modes, leading
to the different catalytic activity.
Fig. 3 presents the SEM images of FePO4 with different
morphologies. The branch-like, ower-like, and sphere FePO4
were successfully achieved with the FeCl3/KH2PO4 molar ratio of
1 : 3, 2 : 3, and 3 : 3, respectively. The branch-like FePO4
(Fig. 3A–D) was consisted of four main branch, of which the
length and the width were 10 mm and 2 mm, respectively. The
ower-like FePO4 with the particle diameter of around 10 mm
(Fig. 3E–H) was regularly formed from small branch structure of
200 nm. Sphere FePO4 (Fig. 3I–L) with a diameter of 15 mm was
consisted of small nanoparticles (<50 nm). The branch-like and
ower-like FePO4 exhibited smooth surface structure (Fig. 3D
and H), while the sphere FePO4 was rough surface structure
(Fig. 3L).
Fig. 1 XRD patterns of FePO4$2H2O and FePO4.
The formation of different structured FePO4 was related with
the molar ratio of Fe3+ and H2PO4ꢀ ions in hydrothermal system.
Surfactant PVP serving as the structure-directing molecule also
played a crucial role in the synthesis of FePO4 with desired
morphologies.46–48 FePO4 nanocrystallites generated from Fe3+
prepared sample is pure FePO4. The typical peaks of FePO4
phases and FePO4$2H2O phases were detected in used branch-
like FePO4 bulks for the conversion of methyl cellulose into 5-
HMF aer ve cycled times. It's suggested that the new crys-
talline bulks were generated via the dissolution and recrystal-
lization of branch-like FePO4 in the hydrothermal system.
As shown in Fig. 2, the bands around 1630 cmꢀ1 as well as
the bands approximately 3400 cmꢀ1 of all FePO4 samples were
ascribed to the O–H vibration of adsorbed water molecules,
while these typical bands of branch-like FePO4$2H2O exhibited
at 1635 and 3328 cmꢀ1. Compared with amorphous FePO4
bulks, the branch-like, ower-like, and sphere FePO4 samples
presented slight difference in FT-IR spectrum. The asymmetric
stretching vibration of PO4 group was detected at around
1090 cmꢀ1. The bands at 1020 and 940 cmꢀ1 arisen from the
3ꢀ
and PO4 ions were adsorbed by PVP and then aggregated in
a certain direction. Due to the reduced surface free energies of
the special crystal facets, the well-matched FePO4 particles with
similar surface energy were assembled into oriented structure
with the increase teꢀmperature.46,49 The increasing H+ ions
released from H2PO4 could destroy the intrinsic crystal struc-
ture such as branch-like structure and change the surface energy
of FePO4 nanocrystallites. These escaping nanocrystallites went
into solution of excess H+ ions, nucleated and grew in a prefer-
ential direction with the assistance of surfactant, leading to the
formation of ower-like and sphere FePO4.49 The molar ratio of
Fe3+ and H2PO4ꢀ ions thus affected the morphology of FePO4 in
PVP-assisted hydrothermal system.
symmetric PO4-stretching mode associated with the Q0 PO4
3ꢀ
tetrahedral.38 The peaks at 898 cmꢀ1 of amorphous and ower-
As shown in Fig. 4A–C, the used branch-like FePO4 aer rst
cycle time retained four main branch structure with less
nanosheets due to the partial dissolution in reaction system at
high temperature. The branch structure of FePO4 was destroyed
aer the h cycle time under same conditions, and small
particles randomly and gradually precipitated on the branch
surface from the aqueous phase (Fig. 4D–F). It's indicated that
FePO4 was partially dissolved at elevated temperature and
randomly precipitated again in cooling process.
The XPS spectra of different structured FePO4 were exhibited
in Fig. 5. The binding energies (BE) of Fe 2p and P 2p of these
FePO4 samples were not shied, while the BE of O 1s were
slightly shied, as shown in Fig. 5A–C. For branch-like FePO4
(Fig. 5D), the peaks at 726.1, 712.4, and 717.9 eV were assigned
to the Fe 2p1/2, Fe 2p3/2, and satellite signal peaks, respec-
tively.43,46 It's suggested that the Fe(III) element presented in
fresh FePO4. The P 2p peak (Fig. 5E) was located at 133.7 eV,
which was in agreement with previous work.43 Two oxygen
signals of branch-like FePO4 generated from lattice oxygen and
hydroxyl oxygen were observed at 531.5 and 533.0 eV in O 1s
spectrum (Fig. 5F). As shown in Fig. 5C, the O 1s peaks (lattice
Fig.
2 FT-IR spectra of branch-like FePO4$2H2O and different
structured FePO4.
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Fig. 3 SEM images of branch-like (A–D), flower-like (E–H), and sphere (I–L) FePO4.
oxygen) of sphere, ower-like, and amorphous FePO4 were
located at 531.9, 531.7, and 531.4 eV, respectively. This slight BE
variation of O 1s peaks was attributed to the difference in
atomic ratio of lattice oxygen and hydroxyl oxygen of FePO4 with
different morphologies.
3.2. Conversion of methyl cellulose into 5-HMF
The phase-transfer catalysts such as H2WO4 and FePO4 are
effective for the cellulose converted into 5-HMF in biphasic
system composed of organic solvents and water.35,37 In the NaCl-
assisted biphasic system, the cellulose contacted with soluble
Fig. 4 SEM images of used branch-like FePO4 after first (A–C) and fifth (D–F) cycled times.
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Fig. 5 XPS spectra of different structured FePO4 (A–C) and branch-like FePO4 (D–F).
catalyst is converted into 5-HMF in the water phase, and then structured FePO4, the branch-like FePO4 exhibited the best
the obtained 5-HMF is rapidly extracted into organic phase to catalytic activity for the conversion of methyl cellulose. This
avoid side reaction.28,47,48 The optimal conversion conditions of could be related with the contact efficiency between dissoluble
methyl cellulose into 5-HMF were investigated in a biphasic Fe3+ ions and insoluble methyl cellulose molecules. The release
reaction system combined with different structured FePO4.
rate of Fe3+ ions in water phase was greatly affected by the
The 5-HMF yield increased with the increase of FePO4 morphology of FePO4 at high temperature. FePO4 bulks were
content ranged from 0.03 to 0.18 g, and slightly decreased when gradually dissolved in water phase with increasing reaction time
the FePO4 dosage increased from 0.18 to 0.24 g, as shown in at specied temperature (such as 160 ꢁC), leading to an
Fig. 6. In contrast with amorphous, sphere, and ower-like
FePO4, the branch-like FePO4 was much more suitable for the
methyl cellulose converted into 5-HMF. The highest 5-HMF
yield was observed in the presence of 0.18 g branch-like FePO4.
It could be ascribed to the effective contact between insoluble
methyl cellulose and soluble Fe3+ ions in the biphasic
system.36,37 The inferior 5-HMF yield for the conversion of
methyl cellulose was obtained by the solid catalyst at low reac-
tion temperature. With the increasing FePO4 amount, the
excess Lewis acid sites formed upon elevating temperature (160
ꢁC) was likely to catalyze 5-HMF to undesired products such as
furfural, formic acid and levulinic acid, leading to a lower 5-
HMF yield.6,20 Due to the sealed chamber and specic reaction
conditions in hydrothermal system, it's difficult to detect the
pH values of catalytic reaction process in this work. Aer cool-
ing to room temperature, the pH values of these systems
(Table 1) were slightly changed from 5.13 to 6.03 due to the poor
insolubility of FePO4.
The 5-HMF yield increased with the reaction time varied
from 30 min to 80 min, while decreased in a reaction time range
of 80–150 min, as shown in Fig. 7. Among these different
Fig. 6 Effect of FePO4 content on the 5-HMF yield and pH value
(methyl cellulose concentration of 2.5 g Lꢀ1, reaction temperature of
160 ꢁC, reaction time of 80 min, THF solution volume of 180 mL, and
NaCl dosage of 3.0 g).
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Table 1 Effect of FePO4 amounts on the conversion of methyl
cellulosea
FePO4
amount (g)
Fe3+
Furfural
yield/%
pH
Samples
amountb (mg)
valuec
Amorphous
Flower-like
Sphere
0.18
0.18
0.18
0.24
0.21
0.18
0.15
0.12
1.56
0.95
1.38
1.14
0.97
0.83
0.72
0.61
3.89
3.64
3.52
4.35
4.27
3.34
3.56
3.73
5.92
5.37
5.67
5.04
5.13
5.26
5.85
6.03
Branch-like
a
Conditions: methyl cellulose concentration of 2.5 g Lꢀ1, reaction
temperature of 160 ꢁC, reaction time of 80 min, THF solution volume
b
of 180 mL, and NaCl dosage of 3.0 g. Fe3+ amount in water phase
c
aer cooling to room temperature was detected by ICP-OES. pH
value of solution system aer cooling to room temperature.
Fig. 8 Effect of reaction temperature on the 5-HMF yield (methyl
cellulose concentration of 2.5 g Lꢀ1, reaction time of 80 min, FePO4
content of 0.18 g, THF solution volume of 180 mL, and NaCl dosage of
3.0 g).
leading to the increase of Fe3+ irons contacting with methyl
cellulose. High level of reaction temperature favored the
hydrolysis of FePO4 to form Fe3+ irons, which were likely to
catalyze 5-HMF into formic acid and levulinic acid in the water
phase.
The methyl cellulose concentration as well as FePO4
amounts was important for the 5-HMF yield in aqueous solvent.
As shown in Fig. 9, 5-HMF yield decreased with the increasing
concentration of methyl cellulose under same conditions,
which was agreed with previous works.36,37 Compared with the
amorphous, ower-like, and sphere FePO4, the branch-like
FePO4 exhibited better catalytic activity for methyl cellulose
ꢁ
due to its higher soluble ability at 160 C in water phase. The
Fig. 7 Effect of reaction time on the 5-HMF yield (methyl cellulose
concentration of 2.5 g Lꢀ1, reaction temperature of 160 ꢁC, FePO4
content of 0.18 g, THF solution volume of 180 mL, and NaCl dosage of
3.0 g).
controllable branch-like structure could effectively restrain the
increasing Lewis acid sites (Fe3+ ions). It's reported that cellu-
lose could be converted into furfural by Lewis acid catalysts
such as Fe3+, Zn2+, Ca2+, and Cr3+ ions.26,32,51,52 These strong
Lewis acid sites facilitated the formation of xylose from the
retro-aldol reaction of fructose, and further converted xylose
into furfural, leading to a lower 5-HMF yield.53,54
The effect of temperature on the 5-HMF yield over different
morphological FePO4 was shown in Fig. 8. 5-HMF yield
increased with the reaction temperature ranged from 130 to
ꢁ
160 C, while decreased with the increasing reaction tempera-
ture from 160 to 200 ꢁC. Compared with amorphous FePO4,
regular structured FePO4 presented excellent catalytic activity,
especially the branch-like morphology. It's suggested that
FePO4 microstructure was responsible for the soluble rate of
FePO4 to release soluble Fe ions and H+ ions. The soluble rate of
branch-like FePO4 consisted of nanosheets (Fig. 3A–D) was
higher than that of other structures at elevated temperature,
Fig. 9 Effect of methyl cellulose concentration on the 5-HMF yield
(reaction temperature of 160 ꢁC, reaction time of 80 min, FePO4
content of 0.18 g, THF solution volume of 180 mL, and NaCl dosage of
3.0 g).
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Table 2 Effect of methyl cellulose concentration on the residue and
glucose/fructose molecules to form humins during the dehy-
dration process of methyl cellulose.37,53,54 As listed in Table 2,
higher concentration of methyl cellulose induced to the
degradation of 5-HMF to humins, thereby lowering the 5-HMF
yield.
furfural yielda
Residue
Methyl cellulose
amountbꢀ1
(mg gcat )
Furfural
yield/%
Samples
concentration (g Lꢀ1
)
The catalyst stability of FePO4 for the conversion of methyl
cellulose was performed ve cycles at 160 ꢁC for 80 min. The 5-
HMF yield (Fig. 10A) decreased and the solid residue (Fig. 10B)
increased aer ve cycle times. It's ascribed to the dissolved
FePO4 could not completely transfer into the solid phase,
leading to the FePO4 mass lose aer cooling to room tempera-
ture.36,37 The residue deposited on the branch-like FePO4
surface was less than that of amorphous, ower-like, and sphere
FePO4, indicating that the branch-like FePO4 retained superior
catalytic activity compared with other structured FePO4. As
shown in Fig. 4, the solid residue was from the unreacted
methyl cellulose and the humins generated from 5-HMF.36,37 It's
clearly illustrated that the FePO4 was partially dissolved at the
evaluated temperature, and then randomly redeposited on the
FePO4 surface in the temperature-fall period. The new diffrac-
tion peaks of FePO4$2H2O (Fig. 1) was detected in XRD pattern
of used branch-like FePO4, which arisen from the recrystalli-
zation of dissolved FePO4.
Amorphous
Flower-like
Sphere
2.5
2.5
2.5
3.5
3.0
2.5
2.0
1.5
96.5
75.2
83.5
98.4
65.7
43.8
36.9
32.6
3.89
3.64
3.52
7.35
5.76
3.34
4.58
4.20
Branch-like
a
Conditions: FePO4 amount of 0.18 g, reaction temperature of 160 ꢁC,
reaction time of 80 min, THF solution volume of 180 mL, and NaCl
b
dosage of 3.0 g. Residue amount deposited on catalyst surface was
detected by TGA/DTA.
dissolution rate of FePO4, and further affect the H+ ions
released from the hydrolysis of Fe3+ ions. In addition, 5-HMF
molecules were likely to react with and cross-polymerize
There were three steps for the conversion of methyl cellulose
into 5-HMF over FePO4 catalyst in the biphasic system, as
shown in Fig. 11. The methyl cellulose was depolymerized into
glucose monomers by acid catalysts, then the glucose was iso-
merized into fructose by Lewis acid sites such as Fe3+ ions, and
nally the fructose was dehydrated into 5-HMF over H+ ions.37,55
The Lewis acid sites (Fe3+ ions) and Brønsted acid sites (H+ ions)
generated from the hydrolysis of FePO4 are thus suitable for the
methyl cellulose converted into 5-HMF.36,37 It's reported that the
ion strength and temperature were crucial for the H+ ions and
+
soluble ions (Fe(OH)2 and Fe(OH)2+) released from the hydro-
lysis of Fe3+ ions.55,56 With the elevating temperature, FePO4 can
partially dissolve into Fe3+ ions, and then hydrolyzed to soluble
iron species and H+ ions. These soluble iron species can
isomerize glucose into fructose, and H+ ions can facilitate
methyl cellulose to generate glucose and catalyze fructose to 5-
HMF. This indicated that the dissolved FePO4 catalyst served as
a “homogeneous” catalyst for the conversion of methyl cellu-
lose. In addition, the low contact efficiency between insoluble
methyl cellulose and solid FePO4 active sites was also respon-
sible for the heterogeneous catalysis reaction of methyl
Fig. 10 Catalytic stability of FePO4 for the conversion of methyl
cellulose into 5-HMF (methyl cellulose concentration of 2.5 g Lꢀ1
,
reaction temperature of 160 ꢁC, reaction time of 80 min, FePO4
content of 0.18 g, THF solution volume of 180 mL, and NaCl dosage of Fig. 11 Catalytic mechanism of FePO4 for the conversion of methyl
3.0 g).
cellulose into 5-HMF.
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cellulose into 5-HMF.36,37,50 It's noticed that the FePO4 mass loss
leaded to the decreasing 5-HMF yield in the recycling process
(Fig. 10). It's reasonable believed that the homogeneous catal-
ysis of FePO4 played a crucial role in the conversion of methyl
cellulose.
7 L. K. Ren, L. F. Zhu, T. Qi, J. Q. Tang, H. Q. Yang and
C. W. Hu, ACS Catal., 2017, 7, 2199.
8 S. Jia, X. He and Z. Xu, RSC Adv., 2017, 7, 39221.
9 J. Jae, W. Zheng, R. F. Lobo and D. G. Vlachos,
ChemSusChem, 2013, 6, 1158.
10 H. Xin, T. Zhang, W. Li, M. Su, S. Li, Q. Shao and L. Ma, RSC
Adv., 2017, 7, 41546.
11 L. Hu, G. Zhao, X. Tang, Z. Wu, J. Xu, L. Lin and S. Liu,
Bioresour. Technol., 2013, 148, 501.
4. Conclusion
The ower-like, branch-like, and sphere FePO4 catalysts were
successfully prepared via a hydrothermal route. Compared with 12 H. Tang, N. Li, F. Chen, G. Li, A. Wang, Y. Cong, X. Wang and
amorphous, sphere, and ower-like FePO4, the branch-like T. Zhang, Green Chem., 2017, 19, 1855.
FePO4 exhibited excellent catalytic activity and stability for the 13 R. Weingarten, A. Rodriguez-Beuerman, F. Cao,
conversion of methyl cellulose into 5-HMF in the biphasic
system. It's attributed to the Lewis acid sites (soluble iron
J. S. Luterbacher, D. M. Alonso, J. A. Dumesic and
G. W. Huber, ChemCatChem, 2014, 6, 2229.
species) and Brønsted acid sites (H+ ions) generated from the 14 R. Weingarten, J. Cho, R. Xing, W. C. Conner Jr and
dissolved FePO4 at elevated temperature. The synergistic effect
G. W. Huber, ChemSusChem, 2012, 5, 1280.
between iron species and H+ ions was favorable for the excellent 15 Z. Hu, Y. Peng, Y. Gao, Y. Qian, S. Ying, D. Yuan, S. Horike,
catalytic activity for the methyl cellulose converted into 5-HMF.
This dissolved FePO4 induced to the formation of FePO4$2H2O
N. Ogiwara, R. Babarao, Y. Wang, N. Yan and D. Zhao, Chem.
Mater., 2016, 28, 2659.
phase and the deterioration of branch-like structure during the 16 M. I. Alam, S. De, B. Singh, B. Saha and M. M. Abu-Omar,
phase-transfer catalytic process. The insolubility at room Appl. Catal., A, 2014, 486, 42.
temperature and the dissolubility at high temperature were 17 S. K. R. Patil, J. Heltzel and C. R. F. Lund, Energy Fuels, 2012,
suitable for the potential application of FePO4 in large-scale
conversion of methyl cellulose into 5-HMF.
26, 5281.
18 E. A. Pidko, V. Degirmenci and E. J. M. Hensen,
ChemCatChem, 2012, 4, 1263.
19 H. Liu, H. Wang, Y. Li, W. Yang, C. Song, H. Li, W. Zhu and
W. Jiang, RSC Adv., 2015, 5, 9290.
Conflicts of interest
There are no conicts to declare.
20 S. Siankevich, Z. Fei, R. Scopelliti, P. G. Jessop, J. Zhang,
N. Yan and P. J. Dyson, ChemCatChem, 2016, 9, 2089.
21 H. Guo, A. Duereh, Y. Hiraga, T. M. Aida, X. Qi and
R. L. Smith Jr, Chem. Eng. J., 2017, 323, 287.
Acknowledgements
The authors gratefully acknowledge the nancial support of this 22 S. Siankevich, Z. Fei, R. Scopelliti, G. Laurenczy, S. Katsyuba,
work by the National Natural Science Foundation of China N. Yan and P. J. Dyson, ChemCatChem, 2014, 7, 1647.
(Grant No.: 21506103 and 51608512), the Science and Tech- 23 M. E. Zakrzewska, E. Bogel-Lukasik and R. Bogel-Lukasik,
nology Support Program of Sichuan Province (Grant No.:
Chem. Rev., 2011, 111, 397.
2015GZ0170), the Major Training Program of the Education 24 Q. Wang, K. Su and Z. Li, Mol. Catal., 2017, 438, 197.
Department of Sichuan Province (Grant No.: 15CZ0026 and 25 Y. Shen, J. Sun, Y. Yi, M. Li, B. Wang, F. Xu and R. Sun,
17CZ0019), and Key Laboratory of Fruit Waste Treatment and
Bioresour. Technol., 2014, 172, 457.
Resource Recycling of the Sichuan Province College (Grant No.: 26 B. Kassanov, J. Wang, Y. Fu and J. Chang, RSC Adv., 2017, 7,
KF17003).
30755.
27 X. Guo, J. Tang, B. Xiang, L. Zhu, H. Yang and C. Hu,
ChemCatChem, 2017, 9, 3218.
28 S. Q. Xu, X. P. Yan, Q. Bu and H. A. Xia, RSC Adv., 2016, 6,
8048.
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