One-pot synthesis of 2,5-diformylfuran from fructose using a magnetic bi-functional catalyst

Article abstract of DOI:10.1039/c6ra01549b

A magnetic bi-functional WO₃HO-VO(salten)-SiO₂@Fe₃O₄ nanocatalyst was prepared to directly synthesize 2,5-diformylfuran (2,5-DFF) from fructose. The chlorosilylated SiO₂@Fe₃O₄ (Cl-SiO₂@Fe₃O₄) nanoparticles served as the platform for the two functionalities. Tungstic acid was generated via protonation of sodium tungstate, which was directly attached on the platform via nucleophilic -Cl displacement. Meanwhile, oxovanadium was complexed with a salten ligand which was functionalized on the Cl-SiO₂@Fe₃O₄. Characterization results confirmed the successful preparation of the WO₃HO-VO(salten)-SiO₂@Fe₃O₄ nanocatalyst. Under the optimal one-pot system, tungstic acid-mediated fructose dehydration afforded 82% 5-hydroxymethylfurfural (5-HMF) in 1 h. Upon co-oxidant H₂O₂ addition, in situ 5-HMF oxidation by the activated oxoperoxovanadium species produced 71% of 2,5-DFF after 15 h under ambient air. The stability of 5-HMF formation was found critical to 2,5-DFF production. Aside from the catalytic efficiency and process simplicity, the WO₃HO-VO(salten)-SiO₂@Fe₃O₄ nanocatalyst was readily retrieved magnetically and re-used multiple times with marginal losses in its activity.

Full text of DOI:10.1039/c6ra01549b

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RSC Advances
One-pot synthesis of 2,5-diformylfuran from fructose using a magnetic bi-
functional catalyst
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Neha Mittal, Grace M. Nisola, Lenny B. Malihan, Jeong Gil Seo, Hern Kim, SeongꢀPoong Lee,
WookꢀJin Chung*
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Energy and Environment Fusion Technology Center, Department of Energy Science and
Technology, Myongji University, Myongji-ro 116, Namdong, Cheoingu, Yongin-city, Gyeonggi-
Do, 449-728, Republic of Korea
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__________________________________________
*Corresponding Author: Tel: +82ꢀ31ꢀ336ꢀ8471, Fax: +82ꢀ31ꢀ337ꢀ2902
EꢀMail: wjc0828@gmail.com
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Abstract
A magnetic biꢀfunctional WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 nanocatalyst was prepared to
directly synthesize 2,5ꢀdiformylfuran (2,5ꢀDFF) from fructose. The chlorosilylated SiO2@Fe3O4
(ClꢀSiO2@Fe3O4) nanoparticles served as the platform for the two functionalities. Tungstic acid
was generated via protonation of sodium tungstate, which was directly attached on the platform
via nucleophilic ꢀCl displacement. Meanwhile, oxovanadium was complexed with a salten ligand
which was functionalized on the ClꢀSiO2@Fe3O4. Characterization results confirmed the
successful preparation of WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 nanocatalyst. Under optimal oneꢀpot
system, tungstic acidꢀmediated fructose dehydration afforded 82% 5ꢀhydroxymethylfurfural (5ꢀ
HMF) in 1 h. Upon coꢀoxidant H2O2 addition, in-situ 5ꢀHMF oxidation by the activated
oxoperoxovanadium species produced 71% of 2,5ꢀDFF after 15 h under ambient air. The
stability of 5ꢀHMF formation was found critical to 2,5ꢀDFF production. Aside from the catalytic
efficiency and process simplicity, the WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 nanocatalyst was
readily retrieved magnetically and reꢀused multiple times with marginal losses in its activity.
Keywords: 2,5ꢀDiformylfuran, Fructose, 5ꢀHydroxymethylfurfural, Oxidation, Dehydration, Biꢀ
functional magnetic catalyst
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1. Introduction
Production of biomassꢀbased platform chemicals typically involves a multiꢀstep conversion of
the substrate through sequential transformations of bridging molecules. A common practice is to
carry out individual reactions as each step requires a suitable downꢀstream process and a unique
catalyst, which functions in a narrow set of reaction conditions and solvent system. This rigorous
procedure ensures that the desired product quality and yield are achieved.1,2
Particular interest is currently focused on 2,5ꢀdiformylfuran (2,5ꢀDFF), a specialty chemical with
a wide array of utility as a precursor of fungicides, pharmaceuticals, ligands of various polyꢀ
Schiff bases, crossꢀlinking agents, and polymers.3 The 2,5ꢀDFF can be produced via a twoꢀstep
reaction using biomassꢀderived carbohydrates as substrates (Scheme 1). Initial step would
require dehydration of C6ꢀcarbohydrates like fructose into 5ꢀhydroxymethylfurfural (5ꢀHMF), a
valuable key intermediate molecule.1,4,5 In the second step, the isolated 5ꢀHMF can be selectively
oxidized to afford 2,5ꢀDFF.6,7
<<Insert Scheme 1>>
Fructose dehydration into 5ꢀHMF has been commonly performed with several types of acid
catalysts1,4 either as homogeneous inorganic mineral acids, metal chlorides or heterogeneous5
ionꢀexchange resins, supported heteropolyacids, zeolites, sulphonic acid functionalized polymer
and carbon nanotubes, graphite oxide, mesoporous silica and magnetic silica supported acid
catalysts. Likewise, various reaction systems which employed homogenous catalysts6 like
Pb(OAc)4ꢀpyridine, pyridinium chlorochromate, K2Cr2O7ꢀDMSO, Co/Mn acetates, 2,2,6,6ꢀ
tetramethylpiperidineꢀ1ꢀoxide, V2O5 and Cu(NO3)2/VOSO4 and heterogeneous catalysts7 like
vanadylphosphate, porous manganese oxide, V2O5ꢀzeolite, silverꢀmanganese oxide,
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Mn3O4/Fe3O4, TEMPOꢀSBAꢀ15, RuꢀPVPꢀCNTs and Ruꢀcovalent triazine frameworks were
reported for the selective 5ꢀHMF oxidation to 2,5ꢀDFF.
Although adequate yields have been achieved from each independent reaction, the use of 5ꢀHMF
as an isolated precursor is impractical due to its difficult separation from highꢀboiling point
reaction solvents and unꢀstable nature.4c Thus, an alternative approach is to directly produce 2,5ꢀ
DFF from fructose. Tandem reactions, which include dehydration and oxidation in a oneꢀpot
system could significantly simplify the whole process, reduce the production cost, and render the
production route environmentally benign.
Few studies have reported oneꢀpot 2,5ꢀDFF synthesis from fructose using a duo of catalysts such
as (1) Amberlystꢀ15 combined with Ru/HT, (2) AlCl3.6H2O/NaBr with supported vanadium
oxide, (3) protonated graphitic carbon nitride (gꢀC3N4) with vanadium doped gꢀC3N4, (4)
magnetic sulphonic acid with magnetic hydroxyapatite, and (5) Amberlystꢀ15 with polyanilineꢀ
VO(acac)2.7c,8ꢀ11 Recently, more advanced catalysts with biꢀfunctional properties such as (1) Pdꢀ
V@MILꢀ101 (MOFs),12 (2) protonated and vanadium doped gꢀC3N4,9 and (3) heteropolyacids
containing Molybdenum (H3PMo12O40),13a combined with Cesium (Cs3HPMo11VO40)13b have
also been reported; the combination of two catalysts on a single material platform further
simplified the oneꢀpot 2,5ꢀDFF system.
But so far, acceptable 2,5ꢀDFF yields and facile oneꢀpot reaction systems are yet to be achieved.
The lack of reaction “switch” that activates the required catalyst for the subsequent reaction
suggests unregulated production of intermediates like 5ꢀHMF, which could limit 2,5ꢀDFF
production.9b Also, the use of high boiling point solvents like DMSO and DMF requires energyꢀ
intensive and complex product isolation techniques.8,12,13b While some biꢀfunctional
nanocatalysts were recycled via filtration or centrifugation, these postꢀreaction steps add cost to
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the entire production.9,13b With these current limitations, a robust recyclable catalyst with dual
functionality, that can be easily retrieved via a simple technique, must be developed.
Furthermore, the use of nanoparticles (NPs) as support could provide a pseudoꢀhomogeneous
character wherein significant activity can be achieved despite the heterogeneity of the catalyst
system. In this regard, magnetic iron oxide NPs (Fe3O4) are attractive catalyst supports due to
their low cost, easy synthesis and functionalization, high surface area, and facile separation by an
external magnetic force.7g,14ꢀ19
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Thus herein, a biꢀfunctional catalyst with magnetic property was developed for direct synthesis
of 2,5ꢀDFF from fructose. The biꢀfunctional catalyst was anchored with (1) tungstic acid for
fructose dehydration and (2) oxovanadium complex for 5ꢀHMF oxidation. Both active sites were
easily attached covalently on the magnetic NPs (Scheme 2). Tungstic acid was selected due to its
reportedly high catalytic activity.20,21 Meanwhile, the oxovanadium complex is a dormant moiety
which can be activated upon addition of H2O2.22 This reaction ‘switch’ mechanism could mimic
the sequential dehydration and oxidation of conventional twoꢀstep 2,5ꢀDFF synthesis in a oneꢀ
pot system.
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<<Insert Scheme 2>>
2. Experimental
2.1. Materials
Iron (II) chloride tetrahydrate (>99% FeCl2•4H2O), iron (III) chloride hexahydrate (>99%
FeCl3•6H2O), tetraethylorthosilicate (98% TEOS), (3ꢀchloropropyl)ꢀtriethoxysilane (95%
CPTES), salicylaldehyde (99%), diethylenetriamine (>98%), sodium tungstate dihydrate (>99%
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Na2WO4•2H2O), and 5ꢀhydroxymethylfufural (98% 5ꢀHMF) were purchased from Acros
Organics. Vanadium (IV) oxide sulfate hydrate (97% VOSO4•xH2O), p-toluenesulfonic acid
monohydrate (98.5% pꢀTSA), Dꢀ(ꢀ) fructose (99%), and dry toluene (99.8%) were obtained from
SigmaꢀAldrich (USA). 2,5ꢀdiformylfuran (>98% 2,5ꢀDFF) was supplied by Tokyo Chemicals
(Japan). Hydrochloric acid (35ꢀ37% HCl) was bought from Showa Chemicals whereas other
reagents and solvents including ammonia hydroxide (25ꢀ28% NH3•H2O), hydrogen peroxide
(30% H2O2), sodium sulfate (99%), anhydrous ethanol (99.9% EtOH), nꢀhexane (95%), ethyl
acetate (99.8% EtOAc), and isopropanol (99.5%) were purchased from Dae Jung Chemicals
(South Korea). All chemicals were of reagent grade or higher, and were directly used without
further purification.
2.2. Bi-functional catalyst preparation
2.2.1. Silica coated magnetic nanoparticle synthesis
Magnetite (Fe3O4) NPs were prepared via coꢀprecipitation (Scheme 2) as described elsewhere.23
Aqueous solution of FeCl3·6H2O and FeCl2·4H2O (2:1 mole ratio) was continuously stirred (400
rpm) under N2 blanket at 85 oC. Ammonia water (28% NH3·H2O) was then added dropwise until
pH = 9. After 4 h, the black precipitates were collected and washed with deionized (DI) water
(pH = 7) and EtOH. The NPs were vacuumꢀdried for 12 h to obtain Fe3O4 (1) NPs.
Product 1 was acid treated in 0.1 M HCl (2 g 100 mLꢀ1) for 10 min and then washed with DI
water before coating with SiO2. The NPs were dispersed in (5+1 v/v) EtOH/H2O solution,
followed by pH adjustment to 10 using NH3·H2O (28%). The SiO2 precursor (2 mL TEOS) was
fractionally added and reacted for 6 h under N2 blanket at room temperature (RT). The collected
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dark brown NPs were washed thrice with EtOH and finally vacuumꢀdried to obtain SiO2@Fe3O4
(2).
2.2.2. Chloroꢀfunctionalization of SiO2@Fe3O4 (3)
In 200 mL of N2ꢀblanketed dry toluene, 1.0 g of 2 was dispersed under constant agitation.
Silylating agent CPTES (1.5 mmol in 50 mL dry toluene) was added dropwise and refluxed for
24 h. The silylated NPs were collected, washed with toluene and EtOH, and vacuumꢀdried to
obtain ClꢀSiO2@Fe3O4 (3).
2.2.3. 3ꢀ[N,Nꢀbisꢀ3ꢀ(salicylidenamino)ethyltriamine] (Salten) (A) synthesis
The Salten ligand was prepared by first mixing salicylaldehyde (2.0 mmol) with pꢀTSA (0.05
mmol) followed by adding diethylenetriamine (1.0 mmol). The mixture was reacted via openꢀ
beaker microwave irradiation (300 W) for 3 min. The crude product was diluted with EtOAc and
washed with DI water. The organic layer was dried over sodium sulfate, filtered, and
concentrated via rotary evaporation. The yellow oily product was washed with hexane and
vacuumꢀdried to afford product (A).24
2.2.4. Synthesis of SaltenꢀClꢀSiO2@Fe3O4 (4)
A dispersion of 3 (1.0 g) in dry toluene (100 mL) was constantly stirred with Salten ligand
solution (0.4 mmol in 20 mL dry toluene) and refluxed for 3 h in N2 blanket. The collected NPs
were washed with toluene and EtOH, and vacuumꢀdried to obtain SaltenꢀClꢀSiO2@Fe3O4 (4).
Reactant composition during Salten ligand attachment was controlled to ensure partial
functionalization; a portion of Clꢀterminated sites were left unreacted and reserved for tungstic
acid attachments as detailed in section 2.2.6.
2.2.5. Vanadium oxide complexation with SaltenꢀClꢀSiO2@Fe3O4
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A dispersion of 4 (1.0 g) in dry EtOH (100 mL) was constantly stirred with VOSO4·xH2O
solution (0.5 mmol in 25 mL EtOH) and refluxed for 3 h to ensure VO2+ complexation with the
Salten ligand groups. The VO2+ modified NPs were magnetically retrieved, washed with EtOH,
and vacuumꢀdried to obtain the nanocatalyst VO(Salten)ꢀClꢀSiO2@Fe3O4 (5).
2.2.6. Tungstic acid functionalization on nanocatalyst (5)
Sodium tungstate dihydrate was functionalized on the remaining Clꢀterminated sites of 5 via
nucleophilic displacement of the chlorine moiety. Dispersion of 5 (1.0 g) was constantly stirred
in nꢀhexane (100 mL), Na2WO4•2H2O (0.3 mmol) was then added and refluxed for 12 h in N2
blanket. After 12 h, the reaction mixture was cooled to RT and the magnetically retrieved NPs
were washed with nꢀhexane (20 mL) and DI water (15 mL). The sample was acidified in 0.1 M
HCl (10 mL) to protonate the sodium tungstate and obtain tungstic acid. This was indicated by
an increase in pH of the dispersion from 1 to 5. The acidꢀtreated sample was magnetically
retrieved, washed with DI water, and vacuumꢀdried to afford the biꢀfunctional magnetic
nanocatalyst WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 (6).
2.3. Preparation of mono-functional catalysts
2.3.1. WO3HOꢀSiO2@Fe3O4 acid catalyst (7)
The acid nanocatalyst was prepared by anchoring Na2WO4•2H2O on ClꢀSiO2@Fe3O4 (3) instead
of VO(Salten)ꢀClꢀSiO2@Fe3O4 (5) (Scheme S1). Subsequent reactions detailed in Section 2.2.6
were followed to obtain WO3HOꢀSiO2@Fe3O4 (7).
2.3.2. VO(Salten)ꢀSiO2@Fe3O4 (8) oxidation catalyst
The oxidation catalyst was prepared similarly with 4 as detailed in Section 2.2.4 except that all
chlorine moieties were reacted with A to obtain Saltenꢀsaturated NPs (SaltenꢀSiO2@Fe3O4) (4a
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Scheme S2). Subsequent VO complexation procedure was performed according to Section 2.2.5
to obtain VO(Salten)ꢀSiO2@Fe3O4 (8).
2.4. Catalyst characterization
Wideꢀangle Powder Xꢀray diffraction (XRD) patterns were recorded on PANalytical X’pertꢀPro
diffractometer, operated at a 40 kV and 40 mA. The FTꢀIR spectra of the samples were scanned
in Varian spectrophotometer using standard KBr disc method between wavenumbers 400 and
4000 cmꢀ1 at RT. Thermogravimetric analysis (TGA) was carried out in SCINCO TGA (Nꢀ1000)
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from 35 C to 1000 C at 10 C/min ramping rate, in N2 atmosphere (50 mL/min). Nitrogen
adsorptionꢀdesorption isotherms were obtained using BELSORP MiniꢀII (Japan) surface area
analyzer at 77K. The samples were outgassed in vacuo at 100 oC for 24 h prior to the adsorption
experiments. The samples were inspected under Field Emission Transmission Electron
Microscope (FEꢀTEM TECNAI G2 F30). The magnetic strength of NPs was measured on a
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vibrating sample magnetometer (VSM) at 25 C from ꢀ15,000 to 15,000 Oe. Vanadium content
was quantified by inductively coupled plasma mass spectrometer (ICPꢀMS 7500, Agilent, USA).
The tungstic acid sites in nanocatalyst (6) was measured using acidꢀbase titration. The catalyst
was treated with standardized aqueous NaOH solution. The remaining excess (unꢀneutralized)
NaOH was used to titrate an oxalic acid standard using phenolphthalein as the indicator to
determine the concentration of remaining NaOH.21
2.5. 2,5-DFF production from fructose
Known amounts of fructose substrate, magnetic nanocatalyst, H2O2 (if added), and isopropanol
solvent were added in a 100 mL round bottom flask fitted with a mechanical stirrer and water
cooled condenser. The temperature was kept constant in an oil bath and the reaction mixture was
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mechanically stirred to facilitate the oneꢀpot reaction. At certain time intervals, aliquots of the
reaction mixture were collected and the nanocatalyst was magnetically separated. The liquid
sample was dried under reduced pressure, dissolved in DI water and then analyzed in high
pressure liquid chromatography (HPLC).
2.6. Product analysis and quantification
Samples were preꢀfiltered through a syringe filter (0.2 ꢀm Nylon). The Waters HPLC was
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equipped with Waters 2414 refractive index (RI) detector maintained at 40 C and Biorad
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Aminex HPXꢀ87H ionꢀexclusion column (300×7.8 mm) kept at 60 C. A 5 mM H2SO4 mobile
phase was delivered at a flow rate of 0.6 mL/min. The concentrations of 2,5ꢀDFF (i.e. [DFF])
were quantified against calibration curves of standard samples. The amount of produced 2,5ꢀDFF
(WDFF) and corresponding reaction yields were calculated according to Eqs. (1) and (2) where V
is the sample volume, MW is the molecular weight of fructose (180.16 g/mole) or 2,5ꢀDFF
(124.09 g/mole) while WRM, WAL, and WFructose are the masses of the reaction mixture, aliquot
for analysis, and fructose, respectively.
[DFF] × V(mL) × WRM (mg)
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(1)
(2)
WDFF (mg) =
WAL (mg)
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WDFF
MW
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Fructose 
2,5ꢀ DFF yield (%) =
×100
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WFructose
MWDFF
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2.7. Isolation and purification of 2,5-DFF
After catalyst removal, the vacuumꢀdried reaction sample (under reduced pressure) was washed
with hexane and reꢀcrystallized in ethyl acetate to obtain the pale white crystalline 2,5ꢀDFF
product (0.1464 g, isolated yield = 60%). Identification of the product 2,5ꢀDFF was confirmed
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by H NMR on a Varian 400MHz NMR spectrometer in deuterated chloroform (CDCl3) as
shown in Fig. S2. 1H NMR (CDCl3): δ 7.30 (s, 2H), 9.81 (s, 2H) ppm.
2.8. Recycling of catalyst (6)
After reaction, the biꢀfunctional magnetic nanocatalyst was magnetically retrieved from the
mixture. The used catalyst was washed with isopropanol to remove traces of reaction debris and
then dried at RT before reꢀuse as detailed in Section 2.5.
3. Results and discussion
3.1. Synthesis and characterization of bi-functional catalyst
The XRD patterns (Fig. 1) verified the crystallinity of bare Fe3O4 which exhibited six wellꢀ
resolved diffraction peaks at 2θ = 30.6°, 36.0°, 43.5°, 54.1°, 57.7° and 63.2°. These peaks
correspond to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) reflection planes, which is
consistent with a cubic inverse spinel structure as reported in literature (JCPDS 19ꢀ0629).25
Meanwhile, the identical XRD pattern of SiO2@Fe3O4 with bare Fe3O4 and absence of
characteristic peaks for SiO2 indicate the amorphous nature of the SiO2 coating layer.26 Likewise,
attachments of Salten ligand and WO3HO moieties on SiO2@Fe3O4 neither altered the textural
property of Fe3O4 (Fig. 1 WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4), which confirms the structural
preservation of the bulk NP support after multiꢀfunctionalization steps.
<<Insert Figure 1>>
While the SiO2 shell was not indicated in the XRD results, its presence was confirmed via TEM
(Fig. 2). Bare Fe3O4 NPs (Fig. 2a) were spherical with average nanosize diameter (Dave) of 9 ±
0.9 nm (Fig. 2b). Its SAED pattern (inset: Fig. 2a), which corroborated the XRD result (Fig. 1),
further ascertained the monoꢀcrystalline nature of Fe3O4. The SiO2@Fe3O4 apparently had
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similar morphology (Fig. 2c) with the bare Fe3O4 but close inspection (inset: Fig. 2c) revealed
the SiO2 layer with an average thickness of 1.36 ± 0.6 nm, ranging from 0.82 – 1.92 nm; this is
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consistent with the increased Dave of SiO2@Fe3O4 to 11.8 ± 0.8 nm (Fig. 2b). Statistical
comparison of the average diameters of the two samples via unpaired tꢀtest reveal their
significant difference (p<0.0001), which further affirms the presence of SiO2 layer in
SiO2@Fe3O4. Additional confirmatory test via EDS confirmed the presence of SiO2 layer on
SiO2@Fe3O4 (Fig. 2d).
Though the nanocatalyst WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 appeared similar with SiO2@Fe3O4
(Fig. S1a), it had the largest Dave (12.4 nm) among the samples (Fig. S1b). Based on the
measured Dave from TEM, the surface area (SA = πr2) ratio of spherical bare Fe3O4 and
nanocatalyst was estimated to be around 1.90. On the other hand, the BET SA of WO3HOꢀ
VO(Salten)ꢀSiO2@Fe3O4 decreased to 50.9 m2/g (vs. 100.2 m2/g of bare Fe3O4) from which the
SA ratio was 1.96. The very close SA ratios from TEM and BET SA results strongly suggest
growth in size of the nanocatalyst due to functionalization.
<<Insert Figure 2>>
The thermal stability and presence of organic moieties on WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4
catalyst were examined by comparing the TGA curves of the NPs (Fig. 3). Initial weight loss
(1.4 – 2.0%) in all samples below 200 oC is attributable to the evaporation of physically adsorbed
(or crystalline) water.14 The bare Fe3O4 experienced only 7.2% of total weight loss while that of
SiO2@Fe3O4 had similar value (data not shown). On the other hand, SiO2@Fe3O4 functionalized
with CPTES organic linker (3) had a total weight loss of 9.8% while that of the nanocatalyst
WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 was 12.3%. The higher weight loss in both samples indicates
the presence of organic functional groups (i.e. CPTES linker and Salten ligand), which were
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thermally decomposed from 200 to 800 oC. Thus, TGA results suggest the successful preparation
of WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4. Moreover, the minimal weight loss in the nanocatalyst
o
below 200 C indicates its thermal stability and feasible application at its typical operating
condition of 100 oC.14
<<Insert Figure 3>>
The FTIR results provided a more definite indication of the functional groups present on the
modified Fe3O4 NPs (Fig. 4). The characteristic peaks of bare Fe3O4 (Fig. 4a) at 578 cmꢀ1 was
due to the FeꢀO stretching.27 The silica coating layer in SiO2@Fe3O4 sample was confirmed by
the SiꢀOH stretching at 3425 cmꢀ1 and SiꢀOꢀSi bonds (antisymmetric) at 1080 cmꢀ1 (Fig. 4b).7h
Peak appearances at 2978 and 2916 cmꢀ1 in ClꢀSiO2@Fe3O4, due to asymmetric and symmetric
CꢀH stretching of propyl chain of the linker, indicate successful silylation of SiO2@Fe3O4 with
CPTES (Fig. 4c). Peaks at 1489 cmꢀ1 and 1458 cmꢀ1 were due to the aromatic C=C stretching of
the ligand in SaltenꢀClꢀSiO2@Fe3O4 (Fig. 4d). Likewise, the vibrational signal at 1627 cmꢀ1 was
due to the C=N stretching of the Salten ligand.28 However, the complexation of vanadyl sulfate
with SaltenꢀClꢀSiO2@Fe3O4 (5) showed weak intensities at 980 cmꢀ1 (for V=O) and 555 cmꢀ1 (for
VꢀN) which might have been masked by strong SiꢀOꢀSi and FeꢀO bands (Fig. 4e).29 The final
nanocatalyst product exhibited sharp signal for W=O bond at 956 cmꢀ1 and WꢀO bond at 902 cmꢀ
1 21,30
,
suggesting the successful synthesis of the biꢀfunctional WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4
nanocatalyst.
<<Insert Figure 4>>
Overall characterization results confirm the successful preparation of WO3HOꢀVO(Salten)ꢀ
SiO2@Fe3O4. The spherical biꢀfunctional nanocatalyst was found to contain 0.4 mmol/g of
VO(Salten) complex, from the V content of ICPꢀMS results, and 0.25 mmol/g tungstic acidic
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sites (WO3OHꢀ) based on acidꢀbase titration. No material deterioration is anticipated if it is used
at elevated temperature (~100oC).
While the preparation of WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 was found successful, its utility as a
heterogeneous biꢀfunctional nanocatalyst was further assessed in terms of its recovery from the
reaction mixture. Primarily, its magnetic character must remain sufficiently strong to facilitate its
convenient magnetic separation from the reaction mixture. Magnetization curves (Fig. 5) of bare
Fe3O4 and WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 exhibited no hysteresis loop, suggesting the
superparamagnetic nature of both NPs.14 Compared with the bare Fe3O4 (67.2 emu/g), the
reduced magnetization saturation (Ms) value of the nanocatalyst (41.6 emu/g) was due to its
increased mass and size after SiO2 coating and functionalization. Nonetheless, the Ms value of
the nanocatalyt was still reasonably strong for its easy and efficient magnetic separation from the
reaction mixture (inset: Fig. 5).
<<Insert Figure 5>>
3.2. Catalytic performance of mono-functional catalysts (7) and (8)
To elucidate the performance of the biꢀfunctional catalyst, the role of each catalytic component
was decoupled by synthesizing the monoꢀfunctionalized nanocatalysts which were solely
anchored with either tungstic acid (WO3HOꢀSiO2@Fe3O4) or VO(Salten) complex [VO(Salten)ꢀ
SiO2@Fe3O4] (Schemes S1 and S2). Acidꢀbase titration result revealed 0.64 mmol/g of tungstic
acid in WO3HOꢀSiO2@Fe3O4 (7) while ICPꢀMS analysis of V estimated 0.70 mmol/g of
VO(Salten) complex in VO(Salten)ꢀSiO2@Fe3O4 (8).
3.2.1. Fructose dehydration activity of WO3HOꢀSiO2@Fe3O4
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The initial step of 2,5ꢀDFF formation involves fructose dehydration to 5ꢀHMF.The catalytic
activity of WO3HOꢀSiO2@Fe3O4 (0.032 mmol tungstic acid) in 1.0 mmol fructose was observed
for 2 h to yield 5ꢀHMF. Isopropanol was used as reaction solvent; it readily dissolves fructose,
minimizes 5ꢀHMF esterification,2 and eliminates liquidꢀliquid 5ꢀHMF extraction, as it can be
easily evaporated in vacuo. The reaction was maintained at 80 oC due to the 5ꢀHMF degradation
at higher temperatures.4a
As a Brønsted acid catalyst, WO3HOꢀSiO2@Fe3O4 supplies the proton that activates the
anomeric –OH site in fructose which initiates the triple dehydration reaction.20,31ꢀ33 It is believed
that the released protons are captured back by the tungstic acid sites, which closes the catalytic
cycle for fructose dehydration.20,32 Time profile (Fig. 6a) reveals a gradual decline in fructose
concentration concomitant with the steady increase in 5ꢀHMF until a maximum yield of 86.3%
and 90% fructose conversion was achieved after 60 min. No rehydration products4c were
detected even after 120 min when the 5ꢀHMF yield eventually declined to 27.6%. However, the
decrease in 5ꢀHMF yield after 60 min could be due to the onset of polymeric humins formation,
a degradation product of 5ꢀHMF.4a,34 Polymeric humins are typically formed in a highly acidic
environment (i.e. surplus acid supply from WO3HOꢀSiO2@Fe3O4) and prolonged reaction time
at an elevated temperature. This finding suggests that regulation of acid catalyst loading is
critical to minimize the extent of 5ꢀHMF degradation.
<<Insert Figure 6>>
3.2.2. 2,5ꢀDFF production from 5ꢀHMF by VO(Salten)ꢀSiO2@Fe3O4
The second step for 2,5ꢀDFF production from fructose involves 5ꢀHMF oxidation. The catalytic
activity of VO(Salten)ꢀSiO2@Fe3O4 (0.035 mmol VO(Salten) complex) in 1.0 mmol 5ꢀHMF
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with the aid of H2O2 (1.0 mmol) as coꢀoxidant in isopropanol was examined for 24 h. To
alleviate overꢀoxidation of 5ꢀHMF, initial reaction was performed at RT.35
Oxidation of 5ꢀHMF is facilitated by the intermediate oxoperoxovanadium species which are
conveniently generated via activation of the VO(Salten) complex in the presence of H2O2.36
Time profile (Fig. 6b) shows the gradual formation of 2,5ꢀDFF while the consumption of 5ꢀHMF
was rapid and complete albeit overꢀoxidation products remained undetected. Production of 2,5ꢀ
DFF eventually stalled after 24 h at which a maximum yield of 68.9% was achieved. The nonꢀ
concurrent rates of product generation and substrate consumption could be due to the limiting
effects of intermediate formation, which consequently hampered the final step of 2,5ꢀDFF
formation. Since the reaction was performed at RT, oxidation temperature increase is probably
needed to accelerate either the generation of oxoperoxovanadium species or the oxidation of 5ꢀ
HMF by oxoperoxovanadium complex. Nonetheless, the absence of overꢀoxidation products
along with the decent 2,5 DFF yield makes the VO(Salten)ꢀSiO2@Fe3O4 a suitable and selective
5ꢀHMF oxidation catalyst.
Overall results from monocatalyst activities demonstrate the individual function and
effectiveness of each component of the biꢀfunctional catalyst. To figure out the synergy between
the tungstic acid and VO(Salten) complex, oneꢀpot reactions using the biꢀfunctional nanocatalyst
WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 was carried out and tuned to maximize the 2,5ꢀDFF yield
from fructose at the shortest period possible.
3.3. One-pot synthesis of 2,5-DFF from fructose
Initial oneꢀpot 2,5ꢀDFF synthesis from 1.0 mmol fructose (Fig. 7a) was performed using 100 mg
of the biꢀfunctional nanocatalyst (0.025 mmol tungstic acid and 0.04 mmol of VO(Salten)
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complex). Fructose dehydration was initiated at 80 C wherein a maximum 5ꢀHMF yield of 82.2%
and 94.2% fructose conversion were achieved after 1 h. This reaction behavior coincided with
that of the acidic monocatalyst (WO3HOꢀSiO2@Fe3O4). The slightly lower 5ꢀHMF yield can be
ascribed to the lower level of tungstic acid supply from the biꢀfunctional nanocatalyst (0.025
mmol vs. 0.032 mmol in Section 3.2.1).
o
After 1 h of fructose dehydration, the reaction mixture was cooled to RT (25 C) and then
injected with H2O2 to initiate 5ꢀHMF oxidation into 2,5ꢀDFF. The 5ꢀHMF gradually declined
and reached a minimum value of 4.8% after 24 h. Coupled with 5ꢀHMF consumption is the
gradual production of 2,5ꢀDFF which reached 58.9% yield after 24 h oxidation (i.e. 25 h overall
reaction). Albeit the supply of VO(Salten) complex was higher in the current experiment (0.04
mmol vs. 0.035 mmol in Section 3.2.2), the 2,5ꢀDFF yield was lower than that from the
monocatalyst VO(Salten)ꢀSiO2@Fe3O4. Apart from humins formation due to 5ꢀHMF
degradation, the accessibility of VO(Salten) complex in WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 to the
5ꢀHMF or H2O2 might have been reduced due to the simultaneous presence of tungstic acid sites.
Moreover, the lower yield of 5ꢀHMF which was the substrate of 2,5ꢀDFF might have also
contributed to its lower production. Nonetheless, the findings from this reaction exhibited the
potential production of 2,5ꢀDFF from fructose viaꢀone pot reaction using the developed biꢀ
functional WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4.
<<Insert Figure 7>>
3.3.1. Temperature control for 5ꢀHMF oxidation
At the same reactant composition and fructose dehydration condition, the effect of reaction
temperature during 5ꢀHMF oxidation was investigated to improve the 2,5ꢀDFF yield as inferred
in Section 3.2.2. Increase in temperature is expected to expedite 5ꢀHMF oxidation due to
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improved kinetic mobility and contact frequency of the substrate with the catalyst at elevated
temperature. Furthermore, the system would require shorter time to cool from 80 oC down to the
desired temperature for 5ꢀHMF oxidation. Increasing the oxidation temperature from RT to 50
o
oC (Fig. 7b) significantly improved the maximum 2,5ꢀDFF yield to 67.3%, even higher at 60 C
which afforded 71.1% 2,5ꢀDFF (Fig. 7c) and 95.5% conversion of 5ꢀHMF. Moreover, both
reactions achieved the maximum yields at shorter reaction period (16ꢀ19 h) which suggests
o
accelerated 5ꢀHMF oxidation at elevated temperature. But further increase to 70 C (Fig. 7d)
reduced the 2,5ꢀDFF yield to 51.1% albeit the almost complete 5ꢀHMF conversion. This suggests
that the reaction selectivity was reduced as 5ꢀHMF degradation to humins was promoted at
higher reaction temperature. Thus in the oneꢀpot reaction using the biꢀfunctional catalyst,
temperature control of individual steps was found necessary to maximize 2,5ꢀDFF yield and
substrate conversions. In the current system, fructose dehydration at 80 oC and 5ꢀHMF oxidation
o
at 60 C provided the highest 2,5ꢀDFF yield from fructose after 16 h of reaction, hence was
adopted in subsequent experiments.
3.3.2. Effect of WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 dosage
Under optimal reaction conditions, the dosage of the biꢀfunctional nanocatalyst was varied to
elucidate its effect on the 2,5ꢀDFF yield from fructose. Dosage was reported in terms of solid
nanocatalyst/liquid volume (S/L) ratio. Thus in a 20 mL reaction volume, experiments were
performed at S/L ratios = 4 to 6 (mg/mL) wherein higher S/L means higher nanocatalyst dosage.
The lowest S/L ratio afforded only 70.1% 5ꢀHMF after 1 h and 57.1% 2,5ꢀDFF after another 15
h in oneꢀpot system (Fig. 8a). Increase in S/L ratio to 5 enhanced both yields to 82.7% 5ꢀHMF
and 71.1% 2,5ꢀDFF without changing any other reaction parameter. However, increase of S/L
ratio = 6 reduced the 5ꢀHMF (77.8%) and 2,5ꢀDFF (65.7%) yields.
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To elucidate the effect of nanocatalyst dosage, 5ꢀHMF yield was plotted against 2,5ꢀDFF yield
(Fig. 8b). The strong correlation between the values (r2 = 0.99) indicates that 2,5ꢀDFF production
was mainly dependent on the availability of 5ꢀHMF. This supports the earlier finding (Section
3.3) that 2,5 DFF yield at fixed 5ꢀHMF was not dependent on the amount of available VO(Salten)
complex. Thus, this suggests that it is the availability of tungstic acid sites (i.e. controlled via biꢀ
functional catalyst dosage) that influenced 5ꢀHMF production, which in turn affected the 2,5ꢀ
DFF yield. At low S/L ratio = 4, insufficient tungstic acid sites led to low 5ꢀHMF production
whereas a harsher acidic environment at S/L ratio = 6 hampered the 5ꢀHMF yield perhaps due to
the observed humins formation. The lower 5ꢀHMF yields in both S/L ratios consequently
resulted in subꢀoptimal 2,5ꢀDFF yields. The highest 5ꢀHMF yield and 2,5ꢀDFF production at S/L
= 5 suggests that this catalyst dosage provided the most suitable tungstic acid level for the
reaction (Fig. 8b).
<<Insert Figure 8>>
3.3.3. The significance of H2O2 addition
The importance of H2O2 was elucidated by varying its amount in the reaction system. In its
absence, no 2,5ꢀDFF was produced (Fig. 9) while the slight consumption of 5ꢀHMF was due to
humins production. This finding further asserted the role of H2O2 as an oxidation “switch” or as
a VO(Salten) complex activator (Section 3.2.2).22,36 Addition of 0.5 mmol H2O2 resulted in
partial oxidation of 5ꢀHMF which afforded 45.2% of 2,5ꢀDFF yield. This suggests that
insufficient amount of H2O2 added generated inadequate oxoperoxovanadium species which
were directly involved in 5ꢀHMF oxidation. Adjustment in H2O2 dosage to 1.0 mmol improved
the 2,5ꢀDFF yield to 71.1% while further increase to 2.0 mmol reduced 2,5ꢀDFF yield, which can
be ascribed to 5ꢀHMF decomposition. Coincidentally, the most suitable H2O2 dosage of 1.0
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mmol in 20 mL reaction volume was the same as those employed in earlier experiments hence no
further increase in 2,5ꢀDFF yield was observed.
Results from oneꢀpot synthesis of 2,5ꢀDFF from fructose using WO3HOꢀVO(Salten)ꢀ
SiO2@Fe3O4 as a biꢀfunctional nanocatalyst reveal that temperature control in each reaction step,
nanocatalyst dosage via S/L ratio, and H2O2 addition were critical in achieving the maximum 5ꢀ
HMF yield (82.2%) and 2,5ꢀDFF production (71.1%).
<<Insert Figure 9>>
3.4. Recycling of WO3HO-VO(Salten)-SiO2@Fe3O4 catalyst
Recycling and isolation of any catalyst are essential characteristics for its probable industrial
application. The nanocatalyst WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 was readily retrieved due to its
magnetic core support. After each reaction, it was magnetically separated from the mixture,
washed with isopropanol, and then dried for further use. The performance of the biꢀfunctional
nanocatalyst was reliable even after repeated use (Fig. 10). After five cycles of reaction, the
decline in 2,5ꢀDFF yield was very minor (≅ 5%) relative to the first cycle. Furthermore, the FTIR
of used nanocatalyst (Fig. S3) showed consistent signature peaks with that of fresh WO3HOꢀ
VO(Salten)ꢀSiO2@Fe3O4 (Fig. 4f). This suggests resilience of the nanocatalyst to deterioration
despite its exposure to different chemical conditions during the reaction. The consistent
performance of WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 after several uses demonstrates its potential as
an economically viable route for oneꢀpot synthesis of 2,5ꢀDFF from biomassꢀbased
carbohydrates like fructose.
<<Insert Figure 10>>
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4. Conclusions
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The biꢀfunctional magnetic nanocatalyst WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 which contained two
types of active sites was successfully prepared through a simple functionalization procedure. The
nanocatalyst had a stable structure which can withstand the needed reaction temperature. Its dual
catalytic performance is attributable to a reaction “switch” mechanism, which afforded
sequential reactions. The catalyst effectively produced 2,5ꢀDFF from fructose in a oneꢀpot
system with reasonable yields under mild reaction conditions. The nanocatalyst was readily
separated from the reaction mixture by an external magnet and it performed consistently for
several cycles of reaction without any form of deterioration. These findings strongly indicate the
industrial potential of WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 nanocatalyst for direct 2,5ꢀDFF
production from fructose. Overall findings can provide insights for the development of oneꢀpot
systems for the syntheses of biomassꢀderived specialty chemicals.
5. Acknowledgements
This research was supported by Basic Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of Education (2009ꢀ0093816) and by the
Ministry of Science, ICT & Future Planning (2015R1C1A2A01054605). The authors would like
to thank Korea Basic Science Institute (KBSI), Seoul for the use of TEM and Korea Institute of
Science and Technology (KIST), Seoul for the use of VSM.
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Scheme Captions:
Scheme 1. Oneꢀpot, twoꢀstep conversion of fructose to 2,5ꢀdiformylfuran.
Scheme 2. Synthesis of biꢀfunctional WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 (6) nanocatalyst.
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Figure Captions:
Fig. 1. Powder XRD patterns of Fe3O4, SiO2@Fe3O4 NPs and WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4
catalyst.
Fig. 2. TEM images of (a) Fe3O4, (b) SiO2@Fe3O4 NPs, and their (c) size distribution histograms
and (d) EDAX analysis. The insets show (a) SAED pattern of Fe3O4 NPs and (b) HRTEM image
of silica coating over the Fe3O4 core.
Fig. 3. TGA curves of Fe3O4, ClꢀSiO2@Fe3O4 NPs and WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4
catalyst.
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Fig. 4. FTꢀIR spectra of (a) Fe3O4, (b) SiO2@Fe3O4, (c) ClꢀSiO2@Fe3O4, (d) SaltenꢀClꢀ
SiO2@Fe3O4, (e) VO(Salten)ꢀClꢀSiO2@Fe3O4 and (f) WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4
catalyst.
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Fig. 5. Magnetization curves of Fe3O4 NPs and WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 catalyst. The
inset shows the magnetic behavior of the catalyst after reaction.
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Fig. 6. (a) Dehydration of fructose using WO3HOꢀSiO2@Fe3O4 catalyst. Reaction conditions:
o
fructose (1.0 mmol), catalyst (50 mg, 0.032 mmol tungstic acid), isopropanol (solvent), 80 C
and, (b) Oxidation of 5ꢀHMF using VO(Salten)ꢀSiO2@Fe3O4 catalyst. Reaction conditions: 5ꢀ
HMF (1.0 mmol), catalyst [50 mg, 0.035 mmol VO(Salten)], H2O2 (1.0 mmol), isopropanol
(solvent), RT. *C/Co = Final concentration/Initial concentration.
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Fig. 7. Reaction profiles of oneꢀpot conversion of fructose to 2,5ꢀDFF using WO3HOꢀ
VO(Salten)ꢀSiO2@Fe3O4 biꢀfunctional catalyst at different temperatures (oxidation step).
Reaction conditions: fructose (1.0 mmol), catalyst [100 mg, 0.04 mmol VO(Salten) and 0.025
mmol tungstic acid], H2O2 (1.0 mmol, when added), isopropanol (solvent), 80 oC (for initial 1 h).
*C/Co = Final concentration/Initial concentration.
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Fig. 8. (a) Effect of S/L ratio (catalyst dosage/reaction volume) on oneꢀpot conversion of
fructose to 2,5ꢀDFF using WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 catalyst and (b) Plot of dependence
of 2,5ꢀDFF yield on 5ꢀHMF produced in the reaction with respect to S/L ratio. Reaction
conditions: fructose (1.0 mmol), catalyst [0.4 mmol/g VO(Salten) and 0.25 mmol/g tungstic
o
o
acid], H2O2 (1.0 mmol, added after 1 h), isopropanol (solvent), 80 C (for initial 1 h) and 60 C
(for subsequent 15 h).
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Fig. 9. Effect of H2O2 dosage on oneꢀpot conversion of fructose to 2,5ꢀDFF using WO3HOꢀ
VO(Salten)ꢀSiO2@Fe3O4 catalyst. Reaction conditions: fructose (1.0 mmol), catalyst [100 mg,
0.04 mmol VO(Salten) and 0.025 mmol tungstic acid], H2O2 (added after 1 h), isopropanol
(solvent), 80 oC (for initial 1 h) and 60 oC (for subsequent 15 h).
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Fig. 10. Recyclability of WO3HOꢀVO(Salten)ꢀSiO2@Fe3O4 catalyst. Reaction conditions:
fructose (1.0 mmol), catalyst [100 mg, 0.04 mmol VO(Salten) and 0.025 mmol tungstic acid],
o
o
H2O2 (1.0 mmol, added after 1 h), isopropanol (solvent), 80 C (for initial 1 h) and 60 C (for
subsequent 15 h).
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One-pot, two-step conversion of fructose to 2,5-diformylfuran.
77x46mm (600 x 600 DPI)

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Synthesis of bi-functional WO3HO-VO(Salten)-SiO2@Fe3O4 (6) nanocatalyst.
362x226mm (150 x 150 DPI)