Efficient dehydration of fructose to 5-hydroxymethylfurfural over sulfonated carbon sphere solid acid catalysts

Highlights

• Sulfonated carbon as synthesized by a modified two-step method under mild condition.

• Spherical shape was preserved which is independent of sulfonation solution concentration.

• This carbon-based solid acid showed excellent activity in fructose dehydration to HMF.

• 90% HMF yield was obtained at 160 °C after 1.5 h reaction time duration.

Abstract

A carbon-based solid acid catalyst was prepared via hydrothermal method using glucose as carbon precursors and aqueous solution of H₂SO₄ as sulfonation agent. The as-synthesized solid acid catalyst was attempted in the catalytic dehydration of fructose to 5-hydroxymethylfurfural (HMF). The effects of acid site density, reaction time, solvents, catalyst amount, temperature and mole ratio of catalyst to substrate were investigated. Under the optimum reaction conditions, the HMF yield of 90% was achieved in dimethylsulfoxide (DMSO) solvent at 160 °C after 1.5 h reaction time duration. The solid acid catalyst can be separated from the reaction mixture after reaction and reused without substantial loss in catalytic activity.

Introduction

Biomass is one of promising renewable and sustainable alternatives for energy and chemical production. As an energy source, biomass can either be utilized directly via combustion to produce heat or indirectly after converting to various biofuels. In addition, researchers have recently made considerable progress in transforming biomass to chemicals such as carbohydrates [1], [2], [3], and subsequently converting these carbohydrates to valuable intermediates and polymeric materials. Among all the carbohydrates derived from biomass (primarily cellulosic components of biomass), glucose and fructose are economically suitable to be employed as feedstock for the production of downstream value-added chemicals [4], [5], [6]. Several chemical processes have been developed to convert glucose and fructose to chemicals, for instance, increasing interest has been devoted to the dehydration of fructose to produce chemical building blocks such as 5-hydroxymethylfurfural (HMF). HMF has been identified as a versatile platform molecule which can be transformed to valuable chemicals such as 2,5-furandicarboxylic acid (FDCA) 2,5-diformyfuran (DFF) and 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) that are suitable starting materials for the preparation of polymers [7].

HMF was first separated from the reaction mixture of fructose, sucrose and oxalic acid in 19th century. Nowadays, HMF is mainly produced by the dehydration of monosaccharides. The dehydration process to produce HMF is remarkably complex due to the possible side reactions. For instance, the cross-polymerization of HMF leads to the formation of colored soluble polymers and insoluble brown humins. The dehydration of fructose to HMF can be catalyzed by protonic acid as well as by Lewis acid [8], [9]. HCl, H₂SO₄ and H₃PO₄ are the most common acids for fructose dehydration to produce HMF. Sulfuric acid afforded a HMF yield as high as 53% in sub-critical water at 250 °C [10], [11]. HCl and H₃PO₄ can also catalyze fructose dehydration in subcritical water, resulting in a HMF yield of 40–50% [12]. Organic acids such as oxalic acid, levulinic acid and p-toluenesulfonic acid were attempted as well [13], [14], [15], [16]. In addition, Ionic liquids were also designed and applied in the conversion of carbohydrates to HMF [17]. In order to improve the HMF yield, a two-phase reactor was developed in the presence of HCl as the catalyst. Adding dimethylfulfoxide (DMSO) and ploy (1-vinyl-2-pyrrolidinone) (PVP) to the reaction mixture significantly suppressed the undesired side reactions [18], and 80% HMF selectivity at 90% conversion was reported for a 10 wt.% fructose solution. Although the fructose conversion and HMF selectivity can be enhanced by optimizing the reaction parameters, there are still concerns using the above-mentioned liquid acids as catalysts: these conventional homogeneous acid catalysts are difficult to be separated from the reaction mixture, resulting in the product purification and catalyst recycling issues.

Solid acid catalysts can overcome the drawbacks of homogeneous catalysts and have been attempted in fructose dehydration. Furthermore, solid acid catalysts are capable of tuning the surface acidity and working at harsh reaction conditions [19]. The fructose conversion of 76% and HMF selectivity of 92% were achieved over de-aluminated H-form mordenite at 165 °C in a solvent consisting of water and MIBK [20], [21]. In the presence of vanadylphosphate (VOP), a 40% yield of HMF was obtained in a 6 wt.% aqueous fructose solution [22]. Introducing different trivalent metal cations enhanced the VOP catalytic activity; the yield and selectivity increased to 50% and 87%, respectively over a Fe-containing VOP catalyst. Nb-based catalysts such as Nb₂O₅, niobium phosphate (NbPO₄) and sulfated mesoporous niobium oxide also exhibited high catalytic activity [23], [24], [25], [26]. Zr- and Ti-based catalysts with different structures were tried in the dehydration of fructose, HMF yield of 47% was obtained within 4 h at 130 °C over SO₄²⁻/ZrO₂Al₂O₃ with ZrAl molar ratio of 1:1 [27]. Ion-exchange resin Amberlyst-15 has also been reported as the catalyst for fructose dehydration in aqueous solutions [28].

The carbon-based solid acids, possessing high stability, low cost and abundant strong protonic acid sites on surfaces, have been widely used in hydrolysis, esterification and condensation reactions [29]. In particular, carbon sphere (CS) solid acid catalysts can be prepared by direct sulfonation of CS generated from various carbon precursors such as sugars, polycyclic aromatic compounds, resins, activated carbon, bio-char and lignin [30], [31], [32]. In the typical synthesis of CS, glucose, sucrose, fructose or cellulose was heated to 400–600 °C under N₂ flow to produce black powder. The obtained black powder was then heated in concentrated sulfuric acid or fuming sulfuric acid at 150–200 °C [33], [34], [35]. In addition to SO₃H groups on CS surfaces, there were also PhOH and COOH functionalities, resulting in superior performances in liquid-phase acid-catalyzed reactions. Sulfonated CS afforded excellent catalytic performances compared to the sulfonated amorphous glassy carbon, activated carbon and natural graphite [36], [37], due to the compact carbon structure of these precursors and the lack of functional groups, particularly acid sites on the surfaces [36], [38]. In this work, a modified preparation of carbon-based solid acid under mild conditions was developed. The CS was prepared by hydrothermal carbonization of glucose at 180 °C which was remarkably lower than the temperature in other CS synthetic routes. The resulted CS was sulfonated by sulfuric acid aqueous solutions instead of concentrated H₂SO₄ or fuming sulfuric acid. Catalytic results showed that the catalysts afforded high activity for the dehydration of fructose to HMF. Under an optimized condition, fructose was converted into HMF with 90% yield at 160 °C after reaction duration of 1.5 h.