volume 19 number 3 7 february 2017 pages 539–868...

GreenChemistryCutting-edge research for a greener sustainable futurersc.li/greenchem

ISSN 1463-9262

PAPER Rafael Luque, Yingwei Li et al. Effi cient one-pot fructose to DFF conversion using sulfonated magnetically separable MOF-derived Fe

3 O

4 (111) catalysts

Volume 19 Number 3 7 February 2017 Pages 539–868

Green Chemistry

PAPER

Cite this: Green Chem., 2017, 19, 647

Received 22nd July 2016,Accepted 14th September 2016

DOI: 10.1039/c6gc02018f

rsc.li/greenchem

Efficient one-pot fructose to DFF conversion usingsulfonated magnetically separable MOF-derivedFe3O4 (111) catalysts

Ruiqi Fang,a Rafael Luque*b and Yingwei Li*a

Aerobic oxidation of carbohydrate-derived 5-hydroxymethylfurfural (HMF) into high added-value

2,5-diformylfuran (DFF) has attracted much attention in recent years. However, the direct synthesis of DFF

from cheap and abundant carbohydrates via HMF as an intermediate through a one-pot process is highly

desirable but challenging. In this work, we have developed a highly efficient and recyclable non-noble

heterogeneous catalyst for one-pot conversion of fructose into DFF with extremely high yields (>99%).

The catalyst was prepared by a simple pyrolysis method using Fe-based metal–organic frameworks (MOF)

as a template and sulfur powder as a dopant. The pyrolysis of the MOF template under interactions of

Ostwald ripening and Kirkendall effects led to the formation of uniform octahedral Fe3O4 nanoparticles

with exposed (111) crystal facets highly dispersed on sulfur doped carbon. The superior selectivity to DFF

over the designed Fe-based catalyst is related to the low adsorption energy of DFF on the support as well

as the existence of non-oxidized sulfur that makes the catalytic system less oxidative.

1. Introduction

Faced with the increasing energy consumption and fossilenergy exhaustion, scientists have been focusing on alternativeenergies in recent decades.1–3 Compared to crude oil, biomasshas emerged as a unique carbon reservoir for the productionof biofuels, chemicals and sustainable energy.4–7 The complex-ity of biomass is exemplified in the presence of certainfractions (cellulose, lignin) from which a number of functionalcompounds denoted as platform chemicals can be derived.2,4

With the investigation of novel catalysts and reaction routes,key platform molecules including 5-hydroxymethylfurfural(HMF) and furfural (FFA) can be of high interest in theproduction of high added-value chemicals.4,8–10

HMF is a representative biomass platform molecule derivedfrom carbohydrates, which could also be converted into impor-tant compounds such as 2,5-diformylfuran (DFF). DFF has ahigh significance in pharmaceuticals, fungicides and func-tional polymer production. However, the reported routes forDFF production are mostly based on aerobic oxidation of pureHMF.11–15 From the viewpoint of atom economy and scale-upproduction, the bio-industry still calls for robust routes trans-

ferring original biomass directly into final products simul-taneously satisfying energy, economic and environmentalrequirements.16–18 The direct synthesis of DFF from cheap andabundant carbohydrates via HMF as an intermediate througha one-pot process is highly desirable.19–21

Acidic sites as well as an inert atmosphere have beenreported to be favorable for sugar dehydration, while mole-cular oxygen is necessary for HMF oxidation.22–27 In thisregard, Takagaki et al. developed a novel route using acidicABS-15 and Ru/HT as catalysts for one-pot fructose andglucose conversion, obtaining 49% and 25% DFF yields fromfructose and glucose, respectively.28 Fu and co-workers tookadvantage of Fe3O4-SBA-SO3H and KMn8O16·nH2O in catalyz-ing fructose conversion into DFF and achieved a yield as highas 80%.29 However, those reaction systems suffered from unde-sired side reactions and a large formation of byproducts (e.g.,FFCA and FDCA). On the other hand, the stepwise addition ofcatalysts complicates a potential implementation for industrialproduction. To date, the development of facile and highlyselective one-pot strategies for fructose conversion into DFFhas been still challenging.

Herein, we report a novel non-noble heterogeneous catalystfor one-pot conversion of fructose into DFF. The catalyst wassimply synthesized through pyrolysis by using a Fe-containingmetal–organic framework (MIL-88B) as a sacrificial templateand S powder as a dopant. The obtained Fe-based catalyst pos-sessed small quantities of acidic sulfur-derived functionalgroups with highly uniform octahedral nanoparticles featuring

aState Key Laboratory of Pulp and Paper Engineering, School of Chemistry and

Chemical Engineering, South China University of Technology, Guangzhou 510640,

China. E-mail: [email protected] de Química Orgánica, Universidad de Córdoba, Edif. Marie Curie,

Ctra Nnal IVa, Km 396, E14014 Córdoba, Spain. E-mail: [email protected]

This journal is © The Royal Society of Chemistry 2017 Green Chem., 2017, 19, 647–655 | 647

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Fe3O4 (111) exposed crystal facets. The proposed Fe systemexhibited quantitative yields for DFF from fructose. Thesuperior selectivity to DFF resulted from the interactions ofthe low adsorption energy of DFF on the Fe3O4 (111) crystalfacets and the low oxidation ability of the non-oxidizedS element doped in the amorphous carbon matrix. In addition,the magnetic catalyst could be easily separated after the reac-tion and reused over 6 runs without any significant reactivityloss.

2. Experimental

All reagents were of analytical grade and were used withoutfurther purification.

2.1. Synthesis of MIL-88B

In a typical synthesis, FeCl3·6H2O (1.62 g) and H2BDC (0.996 g)were dissolved in DMF (30 mL) at room temperature to form aclear solution. The mixture was sealed in a 100 mL autoclaveto crystalline at 373 K for 12 h. The resulting brown powderwas washed with DMF several times and then dried undervacuum at 323 K overnight.

2.2. Synthesis of Fe/C–S materials

Fe/C–S materials were prepared as follows: a mixture ofMIL-88B (Fe) (0.5 g) and S powder (1.0 g) was heated at acertain temperature for 5 h with a heating rate of 1 °C min−1

from room temperature under an argon atmosphere. After theheating treatment, the resultant sample was treated at 200 °Cfor 12 h under vacuum. The as-prepared material was denotedas Fe/C–S (ca. 0.3 g).

The Fe/C–S catalyst was dispersed in aqua regia for 24 h tocompletely remove Fe. The obtained solid was washed withethanol and collected by centrifugation, and then dried at200 °C for 12 h under vacuum. The as-synthesized materialwas denoted as Fe/C–S(H+).

2.3. Catalyst characterization

Powder X-ray diffraction (PXRD) patterns of the samples wereobtained on a Rigaku diffractometer (D/MAX-IIIA, 3 kW) usingCu Kα radiation (40 kV, 30 mA, λ = 0.1543 nm). BET surfacearea and pore size measurements were performed with N2

adsorption/desorption isotherms at 77 K on a MicromeriticsASAP 2020 M instrument. Before measurements, samples weredegassed at 100 °C for 12 h. The iron contents in the sampleswere measured quantitatively by atomic absorption spec-troscopy (AAS) on a HITACHI Z-2300 instrument. Elementalanalysis was performed on Elementar Vario EL III equipmentby weighing samples of 0.2–0.3 mg and packing with an alu-minium foil for the measurement. Raman spectra wererecorded on a LabRAM Aramis Raman Spectrometer (HORIBAJobin Yvon). The surface acidity was measured in a dynamicmode by means of a pulse chromatographic technique of thegas phase (200 °C) adsorption of 2,6-dimethylpyridine (DMPY,Brønsted sites) as a probe molecule. The magnetic properties

of the catalyst were measured on a Quantum Design physicalproperty measurement system (PPMS-9).

The size and morphology of the materials were studied byhigh-resolution scanning electron microscopy (HR-SEM).Transmission electron micrographs (TEM) andhigh-angle annular dark-field scanning transmissionelectron microscopy (HAADF-STEM) were recorded on a JEOLJEM-2010F instrument equipped with EDX analysis (BrukerXFlash 5030 T) operated at 200 kV. Samples were suspended inethanol and deposited straightaway on a copper grid prior toanalysis.

ATR-IR measurements were carried out on a Thermo FisheriS10 equipped with a liquid nitrogen cooled MCT detector.The spectra were obtained by averaging 32 scans at the resolu-tion of 1 cm−1. The thin film of the catalyst powder depositedon the ZnSe element for the ATR-IR spectroscopic study wasprepared as follows. A suspension of ca. 50 mg of the catalystpowder and 0.1 mmol of HMF or DFF) in 2 mL ethanol wasstirred overnight to eliminate any agglomeration, after which,the slurry was dropped onto the ZnSe internal reflectionelement (IRE) for recording the spectra. The IR spectrum ofthe pure catalyst in the solvent was acquired as thebackground.

XPS measurements were performed in a ultra-high vacuum(UHV) multipurpose surface analysis system (Specs™ model,Germany) operating at pressures <10−10 mbar using a conven-tional X-ray source (XR-50, Specs, Mg Kalpha, 1253.6 eV) in a“stop-and-go” mode to reduce potential damage due to sampleirradiation. Detailed Fe high-resolution spectra (pass energy25 and 10 eV, step size 1 and 0.1 eV, respectively) were recordedwith a Phoibos 150-MCD energy analyzer. Binding energieswere referenced to the C 1s line at 284.6 eV from adventitiouscarbon. Deconvolution curves for the XPS spectra wereobtained using software supplied by the spectrometermanufacturer.

2.4. General procedures for one-pot fructose conversion

One-pot fructose conversion was carried out in a stainlesssteel reactor. Fructose (1.2 mmol), catalyst (metal, 20 mol%)and ethanol (2 mL) were added into the reactor. The sealedreactor was purged several times with nitrogen to removeair and then maintained at 3 MPa at room temperature.After that, the reactor was heated up to 140 °C with astirring speed of 900 rpm. During the reaction, samples weretaken and analyzed by HPLC (Agilent 1260), and the firstsample was taken when the reaction temperature reached140 °C. For the recyclability test, the catalyst was separatedby using a magnet, washed with ethanol and then reuseddirectly.

The conversion of fructose (mol%), HMF selectivity (mol%)and yield of the products were calculated as follows:

Fructose conversion

¼ 1� Moles of fructoseMoles of fructose loaded

� �� 100%

Paper Green Chemistry

648 | Green Chem., 2017, 19, 647–655 This journal is © The Royal Society of Chemistry 2017

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. View Article Online


Product selectivity ¼Moles of product

Moles of fructose converted� 100%

HMF yield ¼ Moles of HMFMoles of fructose loaded

� 100%

3. Results and discussion

Scheme 1 shows the typical synthesis route of Fe/C–Smaterials. Firstly, MIL-88B (MIL: Matériaux de I’InstitutLavoisier) was prepared according to a literature reportedhydrothermal method.30 After the addition of a certainamount of the dopant S powder and thorough grinding, themixture was transferred into a quartz boat and thermallytreated at 500 °C for 5 h under an Ar atmosphere. The pyrolysistemperature was selected based on the thermogravimetricanalysis (TGA) of MIL-88B which showed that the structure col-lapses at ca. 400 °C.30 The obtained black powder was denotedas Fe/C–S. For comparison, Fe/C without S doping wassynthesized by using the same strategy. The Fe, C and S con-tents were determined by AAS and elemental analysis and the

results are summarized in Table 1. Compared with Fe/C, theFe content almost remained unchanged after doping withsulfur (ca. 8 wt%), while the C content slightly decreased.

The surface areas as well as porosities of the Fe/C–Smaterials were characterized by performing N2 adsorption/desorption measurements (Fig. 1a). Compared with Fe/C, theBET surface areas of Fe/C–S remained unchanged. In addition,the MOF-derived materials clearly showed both microporousand mesoporous structures. No obvious differences could beobserved after S doping, indicating that S doping did not sig-nificantly influence the surface areas and porosity.

The PXRD patterns of the materials are shown in Fig. 1b.Characteristic peaks at 30.2° and 35.6° indicated the formationof a Fe3O4 phase (JCPDS no. 65-3107) through pyrolysis at ahigh temperature. For Fe/C–S, there was a significant widepeak from 10° to 20°, indicating the existence of amorphouscarbon derived from the organic ligands of MIL-88B. No othersignificant difference could be observed between Fe/C andFe/C–S patterns, confirming that the structure was mostlypreserved after sulfonation.

Raman spectroscopy was also carried out for the materialsand the results have been summarized in Fig. 1c. Two peaks at1280 and 1568 cm−1 were assigned to the D band and G band,respectively. The defects and distortion of carbon layers wererelated to the D band, and contradictorily, the crystalline andgraphitic structure of the carbon layers could be analyzedthrough the G band.

As a result, the intensity ratio ID/IG was utilized as a para-meter of configurationally randomness.31 In general, introdu-cing heteroatoms (e.g. N or S) into carbon-based materialsleads to an increment of randomness and thus a higher ID/IGratio.32,33 The ID/IG value of Fe/C–S (1.246) was higher thanthat of Fe/C (1.122), confirming that S atoms were successfullyintroduced into carbon layers of the Fe-based catalyst.34

The morphology analysis was carried out using ScanningElectron Microscopy (SEM) and Transmission ElectronMicroscopy (TEM). The parent MIL-88B crystals exhibited anoctahedral morphology with some rough surface (Fig. 1d).After carbonization at 500 °C for 5 h, the surfaces of Fe/C andFe/C–S became rougher and increasingly disordered (Fig. 1eand f). As shown in Fig. 2a, the MIL-88B template collapsed toform shape-irregular aggregated metal oxide clusters. However,after sulfur doping, ordered octahedral particles couldbe obtained with an average diameter of ca. 200 nm (Fig. 2band c). No significant aggregation could be observed, with(111) Fe3O4 as the only interplanar fringe exposed (4.8 Åcharacteristic for the Fe3O4 (111) facet), confirming theScheme 1 Synthetic strategy for the design of Fe/C–S catalysts.

Table 1 Characterization results of catalysts

Material

Element content (wt%)

SBET (m2 g−1) SLangmuir (m

2 g−1) Vpore (cm3 g−1) DMPY (μmol g−1)Fe C S

Fe/C 54 17 0 291 395 0.53 0Fe/C–S 53 13 8 248 351 0.51 18

Green Chemistry Paper


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existence of S heteroatoms only in the carbon layers (Fig. 2d).Elemental mappings and EDX line scanning revealed the highdispersion of Fe, O, C and S elements on Fe/C–S (Fig. 3a–e).The low-level S element was homogeneously dispersed in thecarbon layers. The slight aggregation of sulfur in Fig. 3e wascaused by density changes and could be revealed in the linescan results (Fig. 3f).

We assume that the formation of octahedral Fe3O4 nano-particles was attributed to the Kirkendall effect and Ostwaldripening during thermolysis.35–37 At high temperatures andunder an inert atmosphere, the MOF template underwent car-bonization resulting in its depolymerization. According to the

Kirkendall effect, metal ions as well as carbon atoms derivedfrom the organic ligands of MIL-88B migrated out at differentspeeds. Carbon atoms migrated out quickly for the formationof amorphous carbon (Scheme 1).38 Reactions between carbonatoms and doped S atoms took place and oxidized sulfur func-tional groups were generated, leading to an acidic environmentwhich prevented the aggregation of iron ions compared toneutral or even alkaline ones. As the temperature of thermo-lysis was not high enough (500 °C), carbothermal reduction ofthe oxidized sulfur functional groups would take place to gene-rate non-oxidized thioether and thiophene functionalgroups.39 Compared to carbon atoms, however, the migrating

Fig. 1 (a) Nitrogen adsorption/desorption isotherms and pore size distribution, (b) powder XRD patterns, and (c) Raman spectra of Fe/C (blue) andFe/C–S (red). SEM images of MIL-88B (d), the Fe/C catalyst (e), and the Fe/C–S catalyst (f ).



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speed of Fe ions was slow enough to be ignored because of thelarger atom size and heavier weight. Fe ions still remained andFe3O4 nanoparticles were formed.40 According to Ostwaldripening, octahedral nanocrystals with the (111) crystal planeswere preferred to be cubic due to the lower surface energy ofsuch nanoentities ({111} < {110} < {100} planes).

DFT calculations to support surface binding energies wereperformed to further verify the rationality of the optimalgrowth orientation of Fe3O4, as shown in Fig. 4. The initialstage for the crystal growth requires atomic nucleation on the

Fig. 2 TEM images of Fe/C (a), Fe/C–S (b), and HRTEM images (c, d) ofthe Fe/C–S catalyst.

Fig. 4 The optimized surface slab models of the (a) (100), (b) (110) and(c) (111) facets for Fe3O4. Red and purple balls represent oxygen and ironatoms, respectively.

Fig. 3 HAADF-STEM image (a), corresponding elemental mappings of (b) Fe, (c) O, (d) S, (e) Fe + O + C + S, and (f ) line scan patterns of the Fe/C–Scatalyst.

Table 2 Reaction results of one-pot fructose conversion over differentcatalystsa

Entry Catalyst Fe : fructose (mol%) Conversion (%)

Yield (%)

HMF DFF

1 Fe/C 20 70 10 —2 Fe/C–S 10 75 2 723 Fe/C–S 20 >99 — >994 Fe/C–S 30 >99 — >995b Fe/C–S 20 >99 >99 —6b Fe/C 20 73 20 107c Fe/C–S 5 99 — 99

a Fructose (1.2 mmol), catalyst (metal, 20 mol%), ethanol (2 mL),100 °C, N2 (1 bar) for 2 h and then changed to O2 (1 bar) for 3 h.bWithout changing the atmosphere. c 120 °C, N2 (3 bar) for 6 h andthen changed to O2 (3 bar) for 8 h.



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substrate, followed by the orientated growth along the moststable facet. The energy values shown for different facets rep-resent the energy to be released when the next periodic cellbinds to the current given facet per unit area. The extensionalong the (111) facet of Fe3O4 after nucleation was found asthe optimum thermodynamically directed orientation becauseof the exergonic process with −0.197 eV Å−2.

Following characterization, Fe-based materials wereemployed as catalysts in the one-pot fructose conversion toDFF. Table 2 shows the reaction results using different cata-lysts. Fe-based catalysts exhibited a high reactivity in fructosedehydration under an N2 atmosphere while Fe/C–S gave abetter selectivity to DFF without any byproduct in a sub-sequent HMF oxidation (entries 1 and 3). With the incrementof catalyst loading, fructose conversion was significantlyenhanced at extremely high DFF selectivities (entries 2–4). Theresults reported in this work are remarkably improved com-pared to those previously reported (Table 3), with the activityof the Fe/C–S catalyst being comparable to or even higher thanthat of noble-metal catalysts under environmentally friendlyreaction conditions.

Further studies confirmed the high reactivity and selectivityof Fe/C–S in dehydration. Under optimized reaction con-ditions, fructose was quickly dehydrated into HMF within2 hours (>99% HMF yield, without changing the atmosphere,

Fig. 5 Yield as a function of reaction time. Reaction conditions: fruc-tose (1.2 mmol), catalyst (metal, 20 mol%), ethanol (2 mL), 100 °C,N2 (1 bar) for 2 h and then changed to O2 (1 bar) for 3 h.

Table 3 Comparison of the reaction results of one-pot fructose conversion catalyzed by different systems

Ref. Catalyst T (°C), atmosphere Fructose conversion (%) HMF yield (%) DFF yield (%)

28 ABS-15 + Ru/HT 100 99 71 —4.4 wt% N2, 1 barRu/HT 120 99 34 494.4 wt% O2, 1 bar

29 Fe3O4-SBA-SO3H 110 99 80 —0.75 equiv. Air, 1 barFe3O4-SBA-SO3H + H0.2K0.8Mn8O16·nH2O 110 >99 0 800.75 equiv. O2, 1 bar

This work Fe–S/C (S 8 wt%) 100 >99 — >9920 mol% N2 (1 bar) for 2 h and then

changed to O2 (1 bar) for 3 h

Fig. 6 XPS spectra of the C 1s (a), Fe 2p (b) and S 2p (c) regions ofFe/C–S.



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entry 5), significantly improved compared to dehydrationactivity obtained for Fe/C (entry 6).

It should be noted that the reaction could also proceed wellat a low catalyst loading (e.g., 5 mol%) by slightly altering thereaction conditions, giving 99% fructose conversion with 99%DFF yield within 14 h (entry 7). Furthermore, to examine therole of Fe3O4 in the oxidation reaction, we tested the activity ofFe/C–S(H+) that was prepared by immersing the Fe/C–S catalystin aqua regia to completely remove the Fe species. Fe/C–S(H+)was not active for HMF oxidation, indicating that Fe oxideswere essential for the oxidation process.

The details of fructose conversion using Fe/C–S under opti-mized reaction conditions were further investigated. The yielddistribution of the one-pot reaction at different times isdepicted in Fig. 5. Fructose was initially quantitatively de-hydrated to HMF, with yields reaching >99% within 2 hours.A switch to molecular oxygen resulted in a significant decreasein the HMF yield with an obvious DFF production. HMF wasrapidly oxidized by adjusting the time of reaction to achievequantitative yields for DFF in less than 3 h.

To further interpret the high activity of the Fe/C–S catalyst,X-ray photoelectron spectroscopy measurements were carriedout to identify the different species in the catalyst (Fig. 6). Inthe C 1s spectrum, the peaks at 284.5, 284.8 and 285.2 eVcould be assigned to the CvC, C–H and C–C bonds derivedfrom the organic ligands of MIL-88B, respectively.41 Twocharacteristic peaks at 707.8 and 710.6 eV in the Fe 2p3/2region indicated the existence of both Fe2+ and Fe3+ species inthe catalyst.42 In the S 2p region, the S 2p1/2 peak at 161.5 eVcould be attributed to S–S bonds, while the strong S 2p3/2signal at 163.5 eV could be assigned to active S species with S–C bonds in the material. Weak signals at 167.9 and 168.8 eVcould be attributed to the trace amounts of oxidized sulfurspecies, in good agreement with previous literature reports.39

In situ ATR-IR experiments for HMF and DFF adsorptionwere carried out to simulate the reaction conditions and figureout the adsorption of the furanic derivatives. In Fig. 7a, the

strong HMF characteristic bands at 1180 cm−1, 1518 cm−1 and1677 cm−1 (corresponding to C–O–C bonds in furan rings,CvC bonds in furan rings and CvO stretching bonds) couldbe visualized. The intensity of these bands was significantlyhigher on Fe/C–S, indicating a stronger HMF adsorption onFe/C–S catalysts.43,44 However, there was no significant inten-sity change as shown in Fig. 7b, resulting from a weak adsorp-tion of DFF on both Fe-based catalysts, being potentially thereason for the high DFF selectivity obtained in the systemsand the non-existent generation of by-products.

Furthermore, DFT calculations conducted to furthersupport these findings indicated that the oxygen atomslocated in the (111) facet are negatively charged with −0.43e byMulliken population (or −0.16e by Hirshfeld population).As shown in Fig. 8, the negatively charged oxygen species werefavorable for HMF oxidation. Nevertheless, the two formylgroups in the DFF molecule strengthen the electrostatic repul-sive interaction between the catalyst surface and the DFF mole-cule, indicating that DFF can be easily desorbed from the cata-lyst (not strongly adsorbed on the surface), suppressing over-oxidation to side products.

Based on the characterization and reaction results above,the unprecedented reactivity of the Fe/C–S catalyst could be

Fig. 7 ATR-IR spectra of different catalysts after the adsorption of (a) HMF and (b) DFF.

Fig. 8 HMF and DFF molecules adsorbed over the (111) facet of Fe3O4.



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related to different elemental states of sulfur functional groupsproviding the optimum dehydration activity in the systems andwell distributed iron oxides derived from MOFs with exposed(111) crystal facets. In the catalytic system, fructose was firstdehydrated into HMF catalyzed by oxidized sulfur functionalgroups under a nitrogen atmosphere. The generated HMF wasstrongly adsorbed on the catalyst, oxidized by iron oxides fol-lowing the Mars–van Krevelen mechanism: one HMF moleculewas adsorbed and oxidized by the lattice oxygen of iron oxides.Subsequently, the absent lattice oxygen was immediately filledby molecular oxygen.12 Compared to the undoped Fe/C cata-lyst, Fe/C–S exhibited an extremely high DFF selectivity withthe existence of large amounts of non-oxidized S in the carbonlayers. Non-oxidized S are believed to make the catalytic systemless oxidative, suppressing side reactions and the generationof over-oxidation products (e.g., FFCA and FDCA). Additionally,the observed weak adsorption of DFF on the catalysts isbelieved to contribute to no by-product formation.

Fe/C–S exhibited good magnetic properties (Fig. 10), withthe possibility to be magnetically separated from the reactionmixture after the reaction. After magnetic separation from thereaction system, the Fe/C–S catalyst was washed several timeswith ethanol and dried under vacuum. The recyclability experi-ments of the Fe/C–S catalyst were conducted under optimizedconditions. After six runs, no significant loss in activity andselectivity could be observed (Fig. 9). In addition, leachingexperiments were carried out after 4 h of the reaction (Fig. 9).The liquid phase was filtered off to quantify any potentialleached Fe species using AAS. To our delight, no metal leach-ing could be quantified in the filtrate (<0.5 ppm), which alsoexhibited no reactivity in the selected process under optimizedreaction conditions, confirming the truly heterogeneousnature of the reaction as well as the high stability of theFe–C–S system in the selective conversion of fructose to DFF.

4. Conclusions

Uniform S doping Fe-based catalysts were successfully syn-thesized through a facile pyrolysis method using MIL-88B as asacrificial template and S powder as a dopant. The obtainedoctahedral Fe nanoparticles on sulfur-doped carbon exhibitedan excelling reactivity in the one-pot fructose conversion withcomplete fructose conversion and >99% DFF selectivity undermild conditions. We found that the fully exposed Fe3O4 (111)crystal facets were in favor of HMF oxidation and non-oxidizedelemental sulfur present in the carbonaceous matrix made thesystem less oxidative, resulting in a superior selectivity to DFF.In addition, a strong HMF adsorption and weak DFF adsorp-tion on Fe/C–S also enhanced HMF conversion and suppressedthe generation of by-products. This work represents a novelsynthetic route to prepare uniform and shaped metal nano-particles from MOFs and provides new insights into the selec-tive transformation of biomass using recyclable heterogeneouscatalysts.

Fig. 9 Reusability test (left) and metal leaching test (right) of the Fe/C–S catalyst. Reaction conditions: fructose (1.2 mmol), catalyst (metal, 20 mol%),ethanol (2 mL), 100 °C, N2 (1 bar) for 2 h and then changed to O2 (1 bar) for 2 h.

Fig. 10 Hysteresis loop of the Fe/C–S catalyst.



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Acknowledgements

This work was supported by the National NSF of China(21322606, 21436005 and 21576095), the State Key Laboratoryof Pulp and Paper Engineering (2015TS03), FRFCU (2015ZP002and 2015PT004), and Guangdong NSF (2013B090500027 and2016A050502004).

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volume 19 number 3 7 february 2017 pages 539–868...

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