Article pubs.acs.org/JAFC
Conversion of Xylose into Furfural Using Lignosulfonic Acid as Catalyst in Ionic Liquid Changyan Wu,† Wei Chen,† Linxin Zhong,† Xinwen Peng,*,† Runcang Sun,*,†,‡ Junjie Fang,† and Shaobo Zheng† †
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, China Institute of Biomass Chemistry and Utilization, Beijing Forestry University, Beijing, China
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ABSTRACT: Preparation of biopolymer-based catalysts for the conversion of carbohydrate polymers to new energies and chemicals is a hot topic nowadays. With the aim to develop an ecological method to convert xylose into furfural without the use of inorganic acids, a biopolymer-derived catalyst (lignosulfonic acid) was successfully used to catalyze xylose into furfural in ionic acid ([BMIM]Cl). The characteristics of lignosulfonic acid (LS) and effects of solvents, temperature, reaction time, and catalyst loading on the conversion of xylose were investigated in detail, and the reusability of the catalytic system was also studied. Results showed that 21.0% conversion could be achieved at 100 °C for 1.5 h. The method not only avoids pollution from conventional mineral acid catalysts and organic liquids but also maked full use of a byproduct (lignin) from the pulp and paper industry, thus demonstrating an environmentally benign process for the conversion of carbohydrates into furfural. KEYWORDS: xylose, lignosulfonic acid, furfural, ionic liquid
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of xylose into furfural.18 However, it remains a challenge to obtain furfural from carbohydrates in terms of greenness and efficiency. Lignosulfonic acid (LS), an abundant waste byproduct from sulfite papermaking, is a polyphenolic polymer with sulfonic acid groups (Scheme 1). Due to its dispersion, adsorption, and
INTRODUCTION With the awareness of climate change and the depletion of fossil fuel reserves, the replacement of current hazardous processes and nonrenewable resources with environmentally friendly benign technologies and sustainable green bioresources is necessary.1 Lignocellulosic biomass is a starting material for transportation fuels and chemical intermediates because of its renewability, biodegradability, good availability, and low-cost nature. Moreover, production of fuels and chemicals from renewable biomass has the potential to generate lower greenhouse gas emissions compared to the combustion of fossil fuels. The challenge for the effective utilization of these sustainable resources is to develop cost-effective processing methods or novel catalysts to transform highly functionalized carbohydrate moieties into value-added chemicals.2,3 Furfural derivatives, such as furfural and 5-hydroxymethylfurfural (HMF), are highly valuable platform chemicals used in the manufacture of a wide range of important chemicals, oil refining, plastics, pharmaceuticals.4,5 Furfural can be produced from renewable biomass resources by acid-catalyzed conversion of pentoses and especially xylose (the primary five-carbon sugar).6−8 Presently, mineral acids, such as sulfuric acid, are used as catalysts in several industrial processes for furfural production under homogeneous conditions. Due to the high cost and inefficiency of reusing these catalysts and the hazardous properties of liquid acids, as well as other drawbacks including corrosion and safety problems, there has been a great effort to replace toxic catalysts and media by “green” alternatives. Alternative investigations focus on the conversion of xylose by novel catalysts, for example, oxides,9 chlorides,10 and heterogeneous solid acids11−13 in monophase (supercritical fluids,9 organic solvents14) or biphasic systems.15−17 Microwave irradiation3,7 and other systems with simultaneous stripping with nitrogen have been successfully applied in the conversion © 2014 American Chemical Society
Scheme 1. Representative Structure of LS
surface activity properties, LS has been utilized as a protontransfer material,19 electrochemistry material,20,21 concrete water reducing agent,22 and other petroleum chemicals. Recently, the use of lignosulfonic acid as a catalyst has received much interest including the conversion of inulin and fructose into 5-hydromethylfurfural.23 However, to the best of our knowledge, LS has not been investigated in the dehydration of xylose to furfural. In the present work, we used LS as acid catalyst to convert xylose into furfural (Scheme 2). Because of the good catalytic activity and selectivity, ionic liquids (ILs) as novel solvents to substitute conventional organic solvents have been utilized in various fields.24−26 Recently, the preparation of furfural through the dehydration of Received: Revised: Accepted: Published: 7430
May 22, 2014 July 8, 2014 July 9, 2014 July 9, 2014 dx.doi.org/10.1021/jf502404g | J. Agric. Food Chem. 2014, 62, 7430−7435
Journal of Agricultural and Food Chemistry
Article
Catalyst Reactions. In a typical procedure, LS (50 mg) was added into [BMIM]Cl (2 g) to obtain a well-proportioned and stable LS−IL solution in 15 min at 80 °C. Then xylose (200 mg) was added into the mixture and oil-bathed at 120 °C for 1 h. The reaction mixture was diluted with deionized water/methanol (1:4 by volume) to a certain concentration for HPLC detection range. Identification and Quantitation with HPLC. Furfural was quantified by external standard calibration curve method, which was performed on an Agilent 100 HPLC system. The sample was separated by using a reversed-phase C18 column (200 × 4.6) at 40 °C with a detection wavelength of 283.40 nm. The optimized mobile phase consisted of methanol and deionized water with a volume ratio of 30:70. The flow rate was fixed at 1.0 mL/min. The seven concentrations of furfural (0.005, 0.0025, 0.001, 0.0005, 0.00025, 0.0001, and 0.00005 mol/L) were selected for HPLC analysis to determine the standard calibration curve. The content of furfural was obtained directly by calibration curves.
Scheme 2. Illustration for the Production of Furfural from Xylose in [BMIM]Cl
xylose in IL has received much attention.6,27,28 The aim of this work was to perform the conversion of xylose using IL ([BMIM]Cl) as a pertinent solvent and LS as catalyst. The combination of using biomass-based feedstock and catalyst as well as green solvent presents a pathway to realize the green conversion of xylose into high value-added chemicals.
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MATERIALS AND METHODS
RESULTS AND DISCUSSION Characterization of LS Catalyst. The FT-IR spectrum of LS is shown in Figure 1. Three new bands appeared at 1200−
Chemicals and Reagents. Dendrocalamus membranaceus (DmM) was obtained from Kunming, China. The composition (%, w/w) of DmM was 45.68% cellulose, 25.60% hemicelluloses, 26.33% lignin, 1.53% ash, and 1.40% wax on a dry weight basis. Xylose was supplied by J&K Scientific Ltd. Furfural standard, N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and N-methylpyrrolidone (NMP) were purchased from Aladdin Reagent Co., Ltd. [BMIM]Cl (99%) was purchased from Zhongke Kate Industry & Trade Co., Ltd. Acidic ionexchange resin, acetonitrile, and 1,4-dioxane were purchased from Shanghai Chemical Reagent co., Ltd. Toluene and methanol were supplied by National Medicine Group Chemical Reagent Co., Ltd. Lignosulfonate was prepared by sulfonating the lignin according to the former literature;29 lignin was obtained by bamboo kraft pulping process and purified from black liquor as reported.29,30 The Mw of the obtained lignosulfonate was about 534 g/mol determined by gel permeation chromatography (GPC). THF was used as eluent with the flow rate of 1.0 mL/min. The lignosulfonate concentration was 1.0 g/ L. Distilled and deionized water was obtained in-house from a Heal Force Smart ultrapure water system. Catalyst Preparation. The synthesis of LS was prepared according to the literature23 with slight modifications. The acidic resin was activated in a saturated aqueous solution of NaCl for 1 day, followed by the treatment of 2.5 wt % NaOH aqueous solution for 80 min, and then washed with distilled water until pH 7.0, and finally it was treated with 5.0 wt % HCl aqueous solution for 12 h. Afterward, the resin was transferred to a column and washed with deionized water until the eluent reached pH 7.0. The sodium salt of LS (3.0 g) was dissolved in water (50 mL), and the solution was allowed to flow through the acidic resin column at a speed of 20 drops per minute. The acidic eluent was collected and then freeze-dried for 12 h to obtain LS product. Catalyst Characterizations. Fourier transform infrared (FT-IR) spectra was conducted on a Vector-33 FT-IR spectrometer in the wavenumber range of 400−4000 cm−1. The thermal stability of lignosulfonate and LS was determined by using an SDT-Q500 thermogravimetric analyzer (TGA) with temperature varying from room temperature to 700 °C in a nitrogen atmosphere and at a heating rate of 20 °C/min. A Zeiss EVO 18 (Jena, Germany) scanning electron microscope (SEM) was also used to investigate the morphology of lignosulfonate and LS. An Agilent 1100 (Santa Clara, CA, USA) high-performance liquid chromatograph (HPLC) was used to determine the yields of furfural. Titration of −SO3H in LS. LS (100 mg) was added into a NaCl (2.00 mol/L, 20 mL) aqueous solution, sonicated for 1 h, and then centrifuged. The solution was titrated by standard NaOH solution (0.05 mol/L) with phenophthalein as indicator. The average value of −SO3H content was determined to be 1.87 mmol/g. The sulfur content in LS was determined to be 1.93 mmol/g by elemental analysis, which was very similar to the result of titration. These results showed that sulfur species on the sample were mainly in the form of sulfonic acids.31
Figure 1. FT-IR spectrum for the lignosulfonic acid and lignosulfonate.
1250, 1010−1100, and 650 cm−1, which were attributed to the OSO asymmetric and symmetric stretching vibration and S−O stretching vibration of sulfonic groups (−SO3H), respectively.32 The peak at 1712 cm−1 is attributed to −COOH. The absorbance at 3200−3500 cm−1 can be assigned to the O−H stretching vibration of LS, which is consistent with the former literature.31 The peaks at 2837 and 2935 cm−1 belonged to the extensional vibration of −CH−, indicating that the phenol structure of LS is connected by methylene of the aldehyde structure.33 The absorptions at 1460, 1511, and 1615 cm−1 are characteristic peaks of benzene rings. The results of TGA are illustrated in Figures 2 and 3. Because of the evaporation of water, when the temperature was >100 °C, the TG curves of LS and lignosulfate fell slowly. LS began to pyrolyze when the temperature was >140 °C and decomposed quickly at higher temperature. The thermal stability of lignosulfonate was much better when the temperature was
