Materials Transactions, Vol 56, No (2015) pp 1434 to 1440 Special Issue on Nanostructured Functional Materials and Their Applications © 2015 The Japan Institute of Metals and Materials Magnetic Poly(Vinylsulfonic-co-Divinylbenzene) Catalysts for Direct Conversion of Cellulose into 5-Hydroxymethylfurfural Using Ionic Liquids Trung-Dzung Nguyen1, Huy-Du Nguyen2, Phuong-Tung Nguyen1 and Hoang-Duy Nguyen1,3,+ Institute of Applied Materials Science, Vietnam Academy of Science and Technology, Hochiminh City, Vietnam Department of Chemistry, HCM University of Science, Vietnam National University, Hochiminh City, Vietnam Department of Chemistry, National Taiwan University, Taipei 106, Taiwan Mesoporous poly(vinylsulfonic-co-divinylbenzene) (VS-DVB) and magnetic polymer (VS-DVB/CoFe2O4) are prepared and used as solid acidic catalysts to directly transform cellulose into 5-hydroxymethylfurfural (5-HMF) The characteristic and morphology of the polymers were examined by Fourier transformed infrared spectroscopy, X-ray diffraction, vibrating sample magnetometer, field-emission scanning microscope, and transmission electron microscopy The yield of 5-HMF can reach as high as 98% from the dehydration of glucose using CrCl3·6H2O catalyst in tetrabutylammonium chloride at 120°C for 90 Cellulose conversion using the prepared VS-DVB in 1-butyl-3-methyl imidazolium chloride at 120°C for 180 showed high yields of 50% glucose and 10% 5-HMF An enhancement in 5-HMF yield was observed as reaction time increased A combination of VS-DVB/CoFe2O4 and CrCl3·6H2O in ionic liquids was employed at optimal conditions for cellulose conversion Magnetic catalysts were readily separated from resulting products in the magnetic field, as well as recycled and reused with negligible loss in activity Glucose and 5-HMF yields were determined through high-performance liquid chromatography analysis [doi:10.2320/matertrans.MA201539] (Received January 29, 2015; Accepted May 7, 2015; Published June 19, 2015) Keywords: magnetic acidic-catalysts, cellulose conversion, 5-hydroxymethylfurfural, poly(vinylsulfonic-co-divinylbenzene) Introduction The catalytic conversion of biomass into 5-hydroxymethylfurfural, a key renewable chemical for biochemical and biofuel production, has attracted increased attention owing because of the rising global demand for energy and environmental benefits.1,2) Ionic liquids (ILs) have been employed in transformation processes to form homogeneous carbohydrate solutions that result in enhancement of breaking hydrogen and ¢-1,4-glycosidic bonds.3,4) High 5-HMF yield was prepared from the dehydration of fructose (100% 5-HMF yield), glucose (60%­80% 5-HMF yield), or sucrose (76% 5HMF yield) using metal halide catalysts in ILs under mild conditions of 100°C­130°C.5­8) As cellulose is a major source of glucose and the most abundant photosynthetically fixed carbon resource in nature, numerous studies have focused on the direct conversion of cellulose into 5-HMF Impressive HMF yields of 48%­54% were obtained from untreated lignocellulosic biomass and purified cellulose using N,N-dimethylacetamide-LiCl solvent as a solvent in the presence of chromium chloride, 1-ethyl-3-methylimidazolium chloride (EMIMC), and HCl acid as co-catalyst at 140°C.9) A single-step process of cellulose conversion into 5HMF catalyzed by a pair of metal chlorides CuCl2­CrCl2 dissolved in EMIMC, showed a 5-HMF yield of 55% at 120°C in h reaction time.10) Another combination of CrCl2­ RuCl3 in EMIMC provided nearly 60% HMF yield at 120°C in h.11) Zhang and colleagues12) presented the direct conversion of cellulose into 5-HMF (47% yield) at 120°C using a catalytic system of CrCl2/zeolite/[BMIM]Cl in which solid acid zeolite with moderate acidity was employed to promote cellulose hydrolysis and slow down HMF product decomposition The remarkable improvement in 5-HMF + Corresponding author, E-mail: nhduy@iams.vast.vn yield of up to 89% by using high loading CrCl2 catalyst in EMIMC under anhydrous conditions was studied at 120°C in h.13) Recently, CrCl3, a compound with higher environmental stability than that of the strongly reductive CrCl2, was studied for the 5-HMF production from cellulose in EMIMC solvent under microwave irradiation, through which a 5HMF yield of 60% was obtained.14,15) The CrCl3 and LiCl with : molar ratio in BMIMC demonstrated a high 5HMF yield of 62% at 140°C under microwave irradiation for 40 min.16) The direct conversion of cellulose into 5-HMF (54% yield) using CrCl3 catalyst in BMIMC heated at 150°C through a conventional water-bath was developed by Qi et al.7) In recent years, a high 5-HMF yield at approximately 70% was produced from cellulose through an efficient twostep process in which cellulose hydrolysis into glucose was catalyzed by a strong acidic cation exchange resin through the gradual addition of water into EMIMC, and then, CrCl3 was used to catalyze hydrolysis products into 5-HMF at 110°C.17) However, solid catalysts should be readily isolated from solid residues after reaction for the catalysts to be regenerated and reused in further conversion process Magnetic porous silica particles were studied for the efficient hydrolysis of starch and cellulose into glucose.18) Results showed that the catalyst can be easily separated from the reaction system by magnetic force and undergo 3-times repeated use without significant loss in activity In this study, poly(vinylsulfonic-co-divinylbenzene) and magnetic polymers (VS-DVB/CoFe2O4) with meporous structure were prepared using a reverse micelles method Effects of the prepared catalysts and CrCl3·6H2O on the conversion of cellulose into glucose and 5-HMF in ionic liquids under mild conditions were studied and discussed The catalysts demonstrated high stability and recyclability in the cellulose conversion process, without reducing activity after several cycles Magnetic Poly(Vinylsulfonic-co-Divinylbenzene) Catalysts for Direct Conversion of Cellulose into 5-Hydroxymethylfurfural Experimental Procedure 2.1 Chemicals D-Glucose (96%, Aldrich), cellulose (microcrystalline powder, Aldrich), 5-hydroxymethylfurfural (5-HMF 99%, Aldrich), vinylsulfonic acid sodium salt solution (VS 25%, Aldrich), divinylbenzene (DVB 86%, Aldrich), sodium dodecyl sulfate (SDS 99%, Aldrich), hexadecane (HD 99%, Merck), benzoyl peroxide (BPO 75%, Acros Organics), polyvinyl alcohol (PVA, MW ³140.000, 87%­89% hydrolyzed, HIMEDIA Co.), sorbitan monooleate (Span 80, Shanghai Chemical Reagent Co., China), FeCl3 (98%, Merck), CoCl2·6H2O (97%, Acros Organics), CrCl3·6H2O (96%, Aldrich), tetrabutylammonium chloride (TBAC 99%, Aldrich), 1-butyl-3-methyl-imidazolium chloride (BMIC 95%, Aldrich), acetone nitrile (HPLC grade, Scharlau), NaOH (98.9%, Acros Organics), and H2SO4 (98%), C2H5OH (99%), CH2Cl2 (99.7%), n-hexane (95%), ethylacetate (99.5%) purchased from Chemsol Co Vietnam, were used as received Preparation of magnetic polymers (VS-DVB/ CoFe2O4) CoFe2O4 nanoparticles coated with oleic acid were synthesized according to the co-precipitation method from aqueous salt solutions Fe3+ and Co2+ in alkaline medium.19) DVB-VS polymers were prepared following the reverse micelles method20) with various mole ratios of VS to DVB ³0.5 : 1, : 1, and : Magnetic DVB-VS polymers were prepared in the presence of CoFe2O4 nanoparticles A 1.2 gram mixture of PVA and SDS with weight ratio of : 0.2 was added to 100 mL distilled water containing 5.24 g VS solution (VS:DVB ³1 : 1) and 0.5 g CoFe2O4 (aqueous phase) Another solution that includes 1.3 g DVB, 0.2 g BPO initiator, and 1.0 g span80 was prepared as the oil phase Thereafter, the oil phase was dispersed into the aqueous phase under vigorous stirring and was heated at 75°C for h Thereafter, dark brown polymers were separated from the reaction solution by using an external magnet bar The obtained beads was extracted with boiling acetone for 24 h Then, 1.0 g of magnetic polymer was ion exchanged by using a solution of 20 mL dichloromethane and mL H2SO4 (98%) in h Finally, the powder was washed with water and ethanol, and then dried at 60°C in air 1435 (a) (b) 2.2 2.3 Characterization Resulting polymers were characterized via X-ray diffraction (XRD, D2PHASER:Cu-K¡ radiation, Bruker AXS, Germany) Fourier transform infrared (FTIR) spectra were recorded using a Bruker Equinox 55 FTIR spectrometer Thermogravimetric analysis (Perkin-Elmer TGA7, Model2960, USA) was performed to examine the thermal durability of polymers A vibrating sample magnetometer (VSM, EZ11, Microsene, USA) was used to measure the hysteresis loops of magnetic catalysts at room temperature Transmission electron microscope (TEM, JEOL JEM 1400, Japan) and field-emission scanning microscope­energy dispersive X-ray analysis (FESEM-EDX JSM-6700F, JEOL) were employed to evaluate the size and element component of catalysts The N2 adsorption/desorption isotherms of catalysts (degassed at Fig (a) HPLC chromatograph plot of 5-HMF solution with various concentrations; (b) Standard curve of authentic 5-HMF in deionized water 170°C for h) were recorded using Quantachrome NOVA 1000e Surface area was determined using the Barrett­ Emmet­Taller (BET) method within the P/P0 range of 0.05­ 0.30 Pore diameter and volume were calculated by the Barrett­Joyner­Halenda method applied to the adsorption branch of the isotherm The acidic site amount (mmol H+ g¹1) of the catalysts was determined using the acid-base titration method 2.4 Catalytic tests 2.4.1 Glucose conversion 50 mg glucose and 7.5 mg CrCl3·6H2O (10 mol% with respect to glucose) were added into a 50 mL glass tube (ACE Glass Inc., USA) containing 500 mg ionic liquid (BMIC or TBAC) The mixture was sonicated for and heated at the desired temperatures from 30 to 180 Then, the mixture was immediately cooled down to room temperature 5-HMF was extracted from the reaction mixture by using ethyl acetate (5 mL © 2) Standards and samples of 5-HMF were analyzed at room temperature by using HPLC Agilent 1100 with a UV detector (­ = 285 nm) and advanced chromatography technologies ACE-C18 (150 mm â 4.6 mm, 3.5 àm) A mixture of acetone nitrile (ACN) and water with a ratio of : 95 in volume was used as mobile phase at 0.6 mL minạ1 rate The 5.0 àL injection volume was employed As shown in Fig 1, the 1436 T.-D Nguyen, H.-D Nguyen, P.-T Nguyen and H.-D Nguyen (a) (a) (b) (b) Fig (a) HPLC chromatograph plots of 5000 mg L¹1 glucose solution (dash-dot line), and 5000 mg L¹1 glucose solution in presence of 500 mg BminCl (solid line), (b) Standard curve of authentic glucose in deionized water calibration curve was obtained from various concentrations of 5-HMF standard solutions (5.0, 10.0, 25.0, 50.0, 100.0, and 200.0 mg L¹1) The 5-HMF yield was calculated as 5-HMF yield (%) = (moles of 5-HMF/initial moles of glucose) © 100 2.4.2 Cellulose conversion A 50 mL ACE glass tube containing 500 mg ionic liquid (BMIC or TBAC) and 50 mg cellulose was sonicated for 15 Catalyst was added into the tube Thereafter, the mixture was sonicated for and heated at 110°C­120°C from 30 to 180 After 180 of cellulose hydrolysis in accordance with the aforementioned procedure, 7.5 mg CrCl3·6H2O was added into the reaction to improve the 5HMF yield The mixture was heated at 120°C in 60 After the reaction, each sample was immediately cooled down to room temperature 5-HMF was extracted from the reaction mixture using ethyl acetate (5 mL © 2) Then, 10 mL deionized water was poured into the remaining mixture of IL and byproducts Magnetic catalysts, which were feasibly isolated from the magnetic field products, were re-acidified and reused Glucose amount was analyzed by a HPLC system with an Agilent 1260 RID detector and a LiChrospher NH2 (250 mm â 4.0 mm, àm) column Acetonitrile/water solution (90/10 V/V) was used as the flowing phase at 1.0 mL min¹1 The glucose concentration range was 500 mg/L­ 5000 mg/L The standard curve of the authentic glucose in (c) Fig (a) FTIR spectra and (b) XRD patterns of (¡) VS-DVB, (¢) CoFe2O4/OA, and (£) VS-DVB/CoFe2O4; (c) TGA curve of VS-DVB water is shown in Fig Product (glucose or 5-HMF) yields were calculated as Product yield (%) = ([Product]/[Cellulose]) © 100, in which [Product] was the concentration in ppm of glucose or 5-HMF obtained from the conversion, and [Cellulose] was the initial concentration in ppm of cellulose Results and Discussions 3.1 Characterizations of magnetic polymers Figure 3(a) shows the IR spectra of VS-DVB and VS- Magnetic Poly(Vinylsulfonic-co-Divinylbenzene) Catalysts for Direct Conversion of Cellulose into 5-Hydroxymethylfurfural Fig 1437 TEM images of (a) VS-DVB, (b), (c) VS-DVB/CoFe2O4, and (d) CoFe2O4 DVB/CoFe2O4 The asymmetric (¯as) and symmetric (¯s) stretching vibrations of ­SO3H groups were observed at 1172 cm¹1 and 1030 cm¹1 reflection bands, respectively The peak at 700 cm¹1 was considered because of the C­S stretching vibration.21) The FTIR spectra of CoFe2O4 coating olecic acid (CoFe2O4/OA) is also presented in Fig 3(a) Two sharp bands at 2922 cm¹1 and 2852 cm¹1 resulted from the ¯as and ¯s stretching vibrations of ­CHCH2 groups, respectively Two bands at 1457 cm¹1 and 1511 cm¹1 corresponded to the ¯as and ¯s stretching vibration bands of ­COO­ groups, respectively.22) The band at 580 cm¹1 ascribed to the Co/Fe­O stretching vibrations23) is observed in the spectra of CoFe2O4/OA and VS-DVB/CoFe2O4 Figure 3(b) depicts the XRD pattern of DVB-VS with a major peak at 2ª ³ 19° All diffraction peaks of CoFe2O4 nanoparticles matched well with the database of cubic spinel magnetite CoFe2O4 JCPDS No 001-1121 The XRD pattern of magnetic polymers revealed the characteristic diffraction peaks of DVB-VS and CoFe2O4 The thermal stability of the prepared VS-DVB is shown in the TGA curve (Fig 3(c)) with the major weight loss at 400°C The TEM image of the obtained VS-DVB beads with approximate round shapes and sizes approximately 100­ 200 nm in diameter are shown in Fig 4(a) TEM images of the VS-DVB/CoFe2O4 samples exhibit magnetic CoFe2O4 nanoparticles as small dark pots into polymer matrix (Figs 4(b) and 4(c)) CoFe2O4 nanoparticles with an average size of 10 nm that match the dark spots of magnetic polymer samples are observed in Fig 4(d) Figure 5(a) shows the elemental components Co, Fe, C, S, and O as shown in the EDX spectral image of VS-DVB/ CoFe2O4 Figure 5(b) demonstrates the N2 isotherms of the prepared polymers as type IV with clear hysteresis loop Isotherms showed mesoporous materials,23) in addition to the surface area (SBET), average pore diameter (Dp), and average pore volume (Vp), which were 332.0 m2 g¹1, 4.2 nm, and 0.44 cm3 g¹1 for VS-DVB, respectively These SBET, Dp, and Vp values for VS-DVB/CoFe2O4 were 166.3 m2 g¹1, 3.4 nm, and 0.33 cm3 g¹1, respectively Acid site amounts were 0.75, 1.20, 1.28, and 0.95 mmol H+/g for VS-DVB (0.5 : 1), VSDVB (1 : 1), VS-DVB (2 : 1), and VS-DVB (1 : 1)/CoFe2O4, respectively Figure 5(c) depicts the magnetization versus the applied magnetic fields (hysteresis curves) of CoFe2O4, VS-DVB/CoFe2O4, and spent VS-DVB/CoFe2O4 Permanent magnetization was almost unobserved for these samples, which suggested that they exhibited superparamagnetic behavior.24) The saturation magnetization values obtained at room temperature were 56.14, 37.19, and 25.91 emu g¹1 for CoFe2O4, VS-DVB/CoFe2O4, and used VS-DVB/CoFe2O4, respectively 3.2 Evaluating the catalytic ability of prepared polymers First, polymers with different acidic strengths were tested 1438 T.-D Nguyen, H.-D Nguyen, P.-T Nguyen and H.-D Nguyen (a) (a) (b) (b) (c) (c) Fig (a) EDX patterns of VS-DVB/CoFe2O4; (b) N2 adsorption­ desorption isotherm of ( ) VS-DVB and ( ) VS-DVB/CoFe2O4; and (c) Magnetization curves of ( ) CoFe2O4, ( ) VS-DVB/CoFe2O4 and ( ) used VS-DVB/CoFe2O4 Fig (a) Cellulose conversion into glucose using 50 mg VS-DVB in different ionic liquids at 110°C; Cellulose conversion into (b) glucose and (c) 5-HMF using various VS-DVB contents in BIMC ionic liquid at 120°C for cellulose conversion in a BMIC solvent at 110°C for 30 Glucose yields were 1.2% and 5.0% by using VSDVB (0.5 : 1) and VS-DVB (1 : 1), respectively These results indicated that the cellulose hydrolysis depended heavily on the acidic strength of the catalyst Therefore, VS-DVB (1 : 1) was used for further conversions Effects of TBAC and BMIC solvents on the cellulose conversion using VS-DVB (1 : 1) at 110°C with different reaction times are shown in Fig 6(a) Glucose yields gradually increased with reaction time (30 min­120 min) BMIC was a more effective solvent for the production of glucose from cellulose compared with TBAC Maximum glucose yields were 8.0% and 23.0% at 110°C in 120 by using TBAC and BMIC, respectively Reaction temperature similarly pre- sented a significant influence on glucose production Results are shown in Fig 6(b) The enhancement in glucose yield of 55.0% was obtained at 120°C in 30 reaction using BMIC solvent Lower glucose yields were reached at increasing reaction time at high temperature, which can be attributed to the dehydration of glucose into 5-HMF As depicted in Fig 6(c), the 5-HMF yield increased with the hydrolysis time of cellulose from 30 to 180 Figure 6(c) similarly presents the unremarkable effect of VS-DVB content on the 5-HMF yield In the presence of 50 mg and 75 mg catalysts, the yield of 5-HMF can reach 8.8% and 8.0% at 120 min, and 10.5% and 8.3% at 180 with 120°C, respectively Any further reaction time increase resulted in lower 5-HMF yields, which can be due to the condensation of 5-HMF.25) Magnetic Poly(Vinylsulfonic-co-Divinylbenzene) Catalysts for Direct Conversion of Cellulose into 5-Hydroxymethylfurfural 1439 low-cost ionic liquid at mild reaction conditions qualify TBAC as a potential solvent for 5-HMF production from glucose Effects of VS-DVB/CoFe2O4 catalysts on conversion cellulose into glucose and 5-HMF using TBAC and BMIC solvents at 120°C for 180 are presented in Fig 7(b) Yields of 28.0% glucose and 1.5% 5-HMF were obtained using 50 mg VS-DVB/CoFe2O4 in BMIC However, lower yields of glucose (7.8%) and 5-HMF (0.1%) were derived using magnetic catalyst in TBAC Results highlighted TBAC as a powerful solvent for glucose conversion contrary to cellulose conversion This finding may be due to the weak solubility of cellulose in TBAC Figure 7(b) shows the improved 5-HMF yield when 7.5 mg CrCl3·6H2O was added into the reaction at 120°C for 60 as well Yields of 23.0% glucose and 5.5% 5-HMF or 14.3% glucose and 1.3% 5HMF were obtained by using BMIC or TBAC, respectively Given the efficient catalysis of TBAC and CrCl3·6H2O systems for the glucose conversion into 5-HMF promoted the cellulose conversion into glucose, the increase in both glucose and 5-HMF yields were observed in using TBAC and VS-DVB/CoFe2O4-Cr (Fig 7(b)) After the reaction, VS-DVB/CoFe2O4 was readily isolated from the products through a magnet The spent catalyst was regenerated and reused for a new reaction cycle Glucose and 5-HMF yields reached 25% and 1.4% in the second and third cycles, respectively (Fig 7(c)) However, the cellulose conversion in the fourth and fifth cycles was only 50% compared to the cellulose conversion in the first cycle The decrease in production may be due to the trapping of byproducts inside the porous structure of the catalyst, leading to reduced activity sites The saturation magnetization ³25.9 emu g¹1 (as shown in Fig 5(a)) and porosity at SBET ³ 97.7 m2 g¹1, VP ³ 0.15 cm3 g¹1, and DP ³ 5.1 nm were retained in the magnetic catalysts used (a) (b) (c) 3+ Fig Glucose conversion into 5-HMF using Cr catalyst in different ionic liquids at 120°C; (b) Cellulose conversion into glucose and 5-HMF using VS-DVB/CoFe2O4 and VS-DVB/CoFe2O4-Cr in different ionic liquids; (c) Reused VS-DVB/CoFe2O4 catalytic performance on the cellulose conversion in BMIC ionic liquid at 120°C in 180 Therefore, optimal reaction conditions at 120°C in 180 were found for the conversion of cellulose into glucose (50%) and 5-HMF (10.5%) using VS-DVB in the BMIC solvent The dehydration of glucose into 5-HMF in TBAC and BMIC solvents using CrCl3·6H2O (15 mass% with respect to glucose) at 120°C are displayed in Fig 7(a) In this case, BMIC exhibited a less effective performance than TBAC The 5-HMF yields of 26.0% and 6.6% were obtained by using TBAC and BMIC at 120°C in 10 min, respectively (Fig 7(a)) Increases in 5-HMF yields with time reaction were observed in Fig 7(a) as well The 5-HMF yields can reach up to 12.5% in BMIC in 30 and 98% in TBAC during a 90 reaction Efficient glucose conversion in a Conclusion Mesoporous VS-DVB and magnetic polymer VS-DVB/ CoFe2O4 were prepared by using the reverse micelles method Efficient cellulose hydrolysis resulting in 50% glucose and 10% 5-HMF yields were obtained through VSDVB polymer in BMIC ionic liquid under mild conditions The catalytic system of low-toxicity CrCl3·6H2O and inexpensive ionic liquid TBAC were revealed as excellent glucose conversion approaches with an impressive HMF yield of 98% at 120°C in 90 reaction In the presence of VS-DVB/CoFe2O4, cellulose conversion amounted to 28% glucose and 1.5% 5-HMF yields by using BMIC solvent Magnetic catalysts can be reused several times without obvious deactivation Furthermore, a significant improvement in 5-HMF yields was observed when CrCl3·6H2O was added into the cellulose conversion process using magnetic catalyst and ionic liquid Results showed VS-DVB/CoFe2O4 as a potential catalyst in directly converting cellulose into valuable chemicals as glucose and 5-HMF in ionic liquids Acknowledgment This study was funded by the Vietnam National 1440 T.-D Nguyen, H.-D Nguyen, P.-T Nguyen and H.-D Nguyen Foundation for Science and Technology Development (NAFOSTED) under grant number 104.01-2012.50, as well as the Young Scientists Program of Vietnam Academy of Science and Technology under grant VAST.ĐLT.07/12-13 REFERENCES 1) C H Zhou, X Xia, C X Lin, D S Tong and J 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Characterizations of magnetic polymers Figure 3(a) shows the IR spectra of VS-DVB and VS- Magnetic Poly(Vinylsulfonic-co-Divinylbenzene) Catalysts for Direct Conversion of Cellulose into 5-Hydroxymethylfurfural. .. to the condensation of 5-HMF.25) Magnetic Poly(Vinylsulfonic-co-Divinylbenzene) Catalysts for Direct Conversion of Cellulose into 5-Hydroxymethylfurfural 1439 low-cost ionic liquid at mild reaction... magnetic catalysts used (a) (b) (c) 3+ Fig Glucose conversion into 5-HMF using Cr catalyst in different ionic liquids at 120°C; (b) Cellulose conversion into glucose and 5-HMF using VS-DVB/CoFe2O4