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Solvent System and Conversion Mechanism of 5-Hydroxymethylfurfural Preparation from Glucose

Yu Yin , Chunhui Ma , Wei Li , Shouxin Liu

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (10) : 1856 -1873.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (10) : 1856-1873 DOI: 10.7536/PC200852
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Solvent System and Conversion Mechanism of 5-Hydroxymethylfurfural Preparation from Glucose
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Abstract

5-Hydroxymethylfurfural(5-HMF) is regarded as the most developing and potential platform in recent years. The hydrolysis of cellulose to glucose and then to5-HMFis the key steps in the use of biomass resources to produce green platform compounds. Understanding the catalytic conversion process from glucose to5-HMFis of great significance for improving the potential utilization of biomass resource. Solvent systems and outfield coordination used for the preparation of5-HMFfrom glucose was introduced in this work, focusing on the mechanism of the conversion from glucose to5-HMFincluding the isomerization of glucose to fructose and the dehydration of fructose to 5-HMF. Single-phase solvents, ionic liquids, biphasic solvents and deep eutectic solvent was used in5-HMFproduction. The biphasic solvents system composed of ionic liquid and organic solvent is the most effective reaction solvent system, which can quickly transfer the5-HMFto the organic phase, reduce side reactions and increase the yield of 5-HMF. Outfield coordination of ultrasound vibration, microwave radiation and pressure field accelerate mass transfer and heat transfer, greatly shorten the reaction time and improve the reaction efficiency through the synergistic reaction with the reaction solvent. The improvement of5-HMFyield and the stability of intermediates require further research.

Contents

1 Introduction

2 Solvent systems

2.1 Single-phase solvent

2.2 Ionic liquids

2.3 Biphasic systems

2.4 Deep eutectic solvents

3 Outfield assistance

3.1 Ultrasound vibration

3.2 Microwave radiation

3.3 Pressure field

4 Mechanism of 5-HMF formation

5 Conclusions

Graphical abstract

Keywords

glucose / 5-hydroxymethylfurfural / solvent systems / microwave field

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Yu Yin, Chunhui Ma, Wei Li, Shouxin Liu. Solvent System and Conversion Mechanism of 5-Hydroxymethylfurfural Preparation from Glucose. Progress in Chemistry, 2021, 33(10): 1856-1873 DOI:10.7536/PC200852

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1 Introduction

Finding renewable biomass resources that can replace fossil resources is an important way to solve the problems of global energy shortage and environmental pollution[1⇓⇓⇓~5]. 5-HMF can be produced by the dehydration of biomass hexose under acidic conditions, and is listed as one of the 12 high-value-added biomass-derived chemicals by the U.S. Department of Energy.
5-HMF has aldehyde group, hydroxyl group and furan ring in its molecular structure, so it has high chemical activity. It can be used to synthesize a variety of chemical products, such as 2,5-furandicarboxylic acid (FDCA) by selective oxidation, 2,5-Dimethylfuran (DMF) and 5-Ethoxymethylfurfural by hydrogenation or etherification[6⇓⇓⇓~10].
Cellulose can produce a series of small organic molecules represented by glucose through a certain depolymerization pathway, and then the conversion of glucose to the next step is the main reaction process for the preparation of energy and platform compounds. Therefore, it is of great significance to further understand the conversion pathway from glucose to target products for the chemical conversion of cellulose. Many scholars have focused on the preparation of 5-HMF from glucose and achieved some results[11⇓⇓⇓~15]. According to previous studies, the yield and selectivity of 5-HMF are closely related to the choice of solvent system in the reaction, and the synergistic effect of external field system can also improve the reaction rate to a certain extent. In this paper, the research results and their effects on the catalytic conversion of glucose to 5-HMF in different solvent systems and external field systems in recent years are reviewed, and the conversion mechanisms of glucose to 5-HMF in different systems are systematically summarized.

2 Olvent system

In the preparation of 5-HMF, the solvent system affects the yield and selectivity of 5-HMF[17,18]. The suitable reaction solvent should have the advantages of environmental protection, low price, strong solubility of raw materials, and can improve the yield and selectivity of the target product 5-HMF to a certain extent and reduce the formation of side reactions and by-products[19⇓⇓⇓⇓⇓⇓~26]. At present, the solvent systems for the preparation of 5-HMF can be summarized as single-phase solvents (water, organic solvents), ionic liquids, biphasic solvents and eutectic solvents.

2.1 Ingle-phase solvent

2.1.1 Aqueous solvent

The conventional preparation system of 5-HMF uses water as the reaction solvent (Table 1). Glucose dehydration is a typical acid-catalyzed reaction, so in the 1970s, the preparation of 5-HMF mostly used homogeneous catalysts, such as inorganic acids, Lewis acids and salts. Heeres et al. Used water as solvent and H2SO4 as catalyst. With the increase of reaction time at 140 ℃, the concentration of 5-HMF increased first, and then further converted to LA[27]. In 2013, Hanefeld et al achieved 19% glucose conversion and 38% 5-HMF selectivity by using water as solvent and glucose as raw material at 195 ℃ for 80 min without any catalyst[28]. Ma et al. Found that the conversion of glucose to 5-HMF was a zero-order reaction in aqueous solvent system with CrCl3·6H2O, AlCl3 and FeCl3·6H2O as catalysts, and the reaction time and temperature would affect the yield of the final product[29]. The highest yield of 5-HMF was obtained when CrCl3·6H2O was used as catalyst. Faungnawakij et al systematically studied the effects of catalyst H3PO4 and sodium hydroxide on the conversion of glucose to 5-HMF under hydrothermal conditions, and found that sodium hydroxide could promote the catalytic conversion of glucose to fructose and tautomeric intermediates[30].
Heterogeneous catalysts are insoluble in the reaction solvent, and the selectivity of the reaction and the yield of the target product can be improved by adjusting the pore structure of the catalyst and the number, proportion and strength of Lewis acid sites and Brønsted acid sites on the surface of the catalyst. Therefore, the preparation of 5-HMF using heterogeneous catalysts in aqueous systems has attracted increasing attention. Faungnawakij et al prepared 5-HMF using phosphate CaP2O6 and α-Sr(PO3)2 as catalysts under hydrothermal conditions. It was found that the yield of 5-HMF increased with the increase of reaction temperature and reached the highest at 220 ℃[30]. However, the phosphorus in the solid phosphate catalyst is dissolved in water in the form of salt, which leads to the easy deactivation of the catalyst in the reaction process, thus reducing the yield of the target product. Jiang et al designed and synthesized a functionalized heterogeneous catalyst C16H3PW11Cr with both Lewis acidity and Brønsted acidity, and catalyzed the conversion of glucose to 5-HMF in aqueous solvent. It was found that the micellar structure composed of a large number of hydrophobic groups on the surface of the catalyst was the main reason for the improvement of 5-HMF yield[31]. Hara et al. Used Nb2O5·nH2O as catalyst in water solvent and reacted at 120 ℃ for 3 H to obtain 5-HMF with a yield of 12.1%[32]. When the Nb2O5·nH2O treated with H3PO4 was used as the catalyst, the side products were reduced and the yield of 5-HMF was greatly increased (52.1%). Amin et al prepared Fe/HY molecular sieve by impregnation method as catalyst, and reacted in water solvent at 120 ℃ for 240 min to obtain 5-HMF with a yield of 11.4%[33].
Although water is an ideal solvent which is environmentally friendly, cheap and easy to obtain, a large number of studies have shown that the yield of 5-HMF has not been significantly improved even if the temperature and reaction time are continuously adjusted. In addition, due to the hydrolysis of 5-HMF in aqueous solution at high temperature, which leads to the formation of by-products LA and formic acid (FA), the single-phase aqueous solvent system still has some limitations in the preparation of 5-HMF.

2.1.2 Organic solvent

The preparation of 5-HMF in water is often accompanied by the formation of by-products such as FA, LA and black rot, which reduces the yield of 5-HMF[34⇓~36]. In addition, the separation and purification of the reaction product from the reaction system is still a problem to be solved. In order to improve the yield of 5-HMF, many scholars have studied the organic solvents and mixed solvents instead of water solvents, or added the corresponding catalysts to inhibit the degradation of 5-HMF and improve the yield of 5- HMF[37⇓⇓⇓~41].
(1) aprotic organic solvent
Dimethyl sulfoxide (DMSO), N, N-Dimethylacetamide (DMAC), Tetrahydrofuran (THF) are the common aprotic organic solvents used in the preparation of 5-HMF[42⇓⇓⇓⇓⇓~48]. DMSO is the most frequently reported aprotic solvent for the dehydration of hexoses. In recent years, nuclear magnetic resonance spectroscopy and theoretical calculations have shown that DMSO affects the conversion of glucose to 5-HMF in the following ways: (1) at 150 ℃, it ensures that the main tautomers of fructose are β-D-fructofuranose and α-fructofuranose[49,50]; (2) providing a sulfoxide group (— S = O) as a catalytically active site in a catalyst-free reaction system[51⇓⇓~54]; (3) promoting proton transfer in a Brønsted acid catalyzed system[55]; (4) assist in that dehydration of fructose by preferential hydrogen bonding to the hydroxyl group[54]; (5) Rehydration of the reaction product is prevented by interaction with the hydroxyl and carbonyl groups of 5-HMF[56]. Ma et al prepared 5-HMF from glucose catalyzed by CrCl3·6H2O in DMSO at 130 ℃ for 480 min, and the yield of 5-HMF was 54. 43%[29]. When the concentration of glucose was increased from 5% to 10%, the yield of 5-HMF was increased from 43.9% to 48.29%. At high glucose concentrations, the rate of conversion of 5-HMF to insoluble putrescine is higher than that of the conversion itself, so the yield of 5-HMF begins to decrease with subsequent increases in glucose concentration. Xue et al. Used SnClx(x=2,4) as a catalyst and added a series of quaternary ammonium salts to promote the conversion of glucose[57]. Tetrabutylammonium iodide (TBAB) showed the best catalytic performance and the highest yield of 5-HMF was 69. 1% when DMSO was used as solvent and the reaction temperature was 100 ℃. The chelate formed by glucose and SnCl4 is more stable in the reaction than the SnCl2 (fig. 2). The effects of different reaction solvents on the yield of 5-HMF were investigated. When the catalyst was SnCl4-TBAB, high yields of 5-HMF were obtained in DMAC and N, N-Dimethylformamide (DMF), but the yield of 5-HMF from glucose was only 23. 8% in N-Methylpyrrolidone (NMP).
Wang et al. Found that Brønsted acidity is an important factor affecting the selectivity of 5-HMF in DMSO solvent system[58]. Yang et al. Suggested that Lewis acidity could promote the formation of 5-HMF in organic solvent systems[59]. Yang et al. Synthesized a series of nano-mesoporous Al2O3-B2O3 catalysts by evaporation-induced self-assembly of aluminum isopropoxide and boric acid. The number of Brønsted and Lewis acid sites of the catalysts was changed by changing the molar ratio of Al2O3 to B2O3, and DMSO was used as the reaction solvent. The yield of 5-HMF was 41. 4% when the reaction temperature was 140 ℃ and the reaction time was 120 min at Al/B=5∶5(Al5B5). When the amount of DMSO increased to a certain extent, the yield of 5-HMF did not increase. The yield of 5-HMF remained at 32. 2% after the catalyst was recycled five times Al5B5. Carbon material-based catalysts can be modified by functional modification to adjust the acid sites and number of catalysts, so they have become commonly used catalysts or catalyst supports in organic solvent reaction systems[60⇓⇓~63]. Zhou and Ma et al. Prepared biomass carbon-based solid acid catalysts C-SO3H and B-SO3H from microcrystalline cellulose and bagasse by incomplete carbonization and sulfonation, respectively[64]. Using DMSO as the reaction solvent, the C-SO3H showed better catalytic performance than the B-SO3H. When C-SO3H was used as catalyst, the conversion of glucose was 73. 29%, the selectivity and yield of 5-HMF were 13. 78% and 10. 10% respectively at 130 ℃ for 480 min. Jadhav et al. synthesized β-cyclodextrin -SO3H carbon solid acid catalyst (β-cyclodextrin-SO3H) with cyclodextrin as carbon source and p-toluenesulfonic acid as sulfonation reagent, and used it to catalyze the conversion of glucose to 5-HMF in DMSO[65]. The yield of 5-HMF was 47% at 180 ℃ for 300 min. The catalyst recovered from the reaction system was reused at least three times without any significant effect on the yield of the product.
Except DMSO, other polar aprotic organic solvents have poor solubility for glucose, so a certain amount of water should be added to form a single-phase solvent system. Zhang et al. Synthesized a new solid p-hydroxybenzenesulfonic acid formaldehyde resin solid acid (SPFR) by direct hydrothermal method, and converted glucose to 5-HMF in water/γ-valerolactone (γ-GVL) system[66]. The yield of 5-HMF was 33% at 190 ℃ for 30 min. The catalyst SPFR could be reused for 5 times, and the yield of fructose to 5-HMF still reached 64. 2% in the fifth cycle. The -SO3H on the surface of SPFR was leached by H+ ions during the reaction, which resulted in the decrease of 5-HMF yield. Li et Al. Synthesized a Al3+ modified formaldehyde-p-hydroxybenzenesulfonic acid resin catalyst (Al-SPFR) by hydrothermal method and used it for the conversion of glucose[67]. The yield of 5-HMF was 47.4% with Al-SPFR as catalyst in the H2O/γ-GVL biphasic system. Huang et al used aminopropyltrimethoxysilane (ATS) as a binder to prepare a silica heterogeneous catalyst (SiO2-ATS-PTA) loaded with phosphotungstic acid (PTA), and converted glucose to 5-HMF in acetone/water system[68]. It was found that the yield of 5-HMF was affected by the content of organic phase in the solvent. When the ratio of acetone/water (V/V) in the reaction solvent increased gradually, the yield of 5-HMF increased gradually, and when the ratio of acetone/water was 1. 0/5. 0 (V/V), the highest yield of 5-HMF was 78%. When water was used as the solvent, the yield of 5-HMF was only 52. 21%. Using room temperature ionic liquid as the reaction solvent and [C2OHMIM]BF4 as the catalyst, the kinetic calculation results show that the activation energy of the conversion of glucose to 5-HMF is 55.77 kJ·mol-1, and the pre-exponential factor is 1.6×104min-1. In Huang's work, the kinetic calculation shows that the activation energy of the reaction is reduced to 30.25 kJ·mol-1, and the pre-exponential factor is only 1.08×107min-1. This is due to the fact that the diffusion rate in the water/acetone solvent is faster than in the room temperature ionic liquid solvent.
(2) Protic organic solvent
Common protic organic solvents used for the conversion of glucose to 5-HMF include ethanol, isopropanol (iPrOH), and the like. These protic organic solvents can associate with hydrogen bonds or form coordinated cations, so they are often used with other solvents to promote the catalytic conversion of glucose. Yang et al. Used glucose as raw material, AlCl3 as catalyst, and ethanol-water mixed solution (water content was 10 wt%) as reaction solvent[69]. The yield of 5-HMF was 70% when the reaction was carried out at 160 ℃ for 15 min. Zhang et al. studied the effect of reaction conditions on the yield of 5-HMF in the iPrOH/DMSO system. When SBA-15 molecular sieve (CeO2-2SZ@SBA-15) loaded with sulfated zirconia (SZ) was used as the catalyst, due to the high Brønsted/Lewis acid strength and relatively low base strength of SZ, the yield of 5-HMF could reach 66.5% and the selectivity of 5HMF was 70.8% at 120 ℃ for 360 min[70]. In the reaction system, iProOH can effectively replace the DMSO solvent to reduce its harm to the environment.
Although the problem of further degradation of 5-HMF in aqueous solvent can be solved by using organic solvents, which can effectively inhibit the occurrence of side reactions and reduce the content of by-products, the widely used organic solvents have poor solubility for carbohydrates, which limits the amount of reactants and hinders the large-scale production of organic solvents. In addition, organic solvents generally have high boiling points, which increases the difficulty of separation and purification of 5-HMF, so there are still outstanding problems in the preparation of 5-HMF in single-phase organic solvent system.

2.2 Ionic liquid

Ionic liquids, also known as room temperature ionic liquids, are salts composed of organic cations and inorganic anions, which are liquid at low temperature or room temperature, also known as low temperature molten salts. The ionic liquid has good solubility for various carbohydrates, extremely low saturated vapor pressure, good chemical stability and wide electrochemical stable potential window[71⇓⇓~74]. Ionic liquids can be used not only as reaction solvents, but also as catalysts in the catalytic degradation of biomass. In recent years, ionic liquids have been increasingly used as green solvents and efficient catalysts for the conversion of glucose to 5-HMF (Table 3).

2.2.1 Conventional ionic liquid

The research on the conversion of glucose to 5-HMF using ionic liquids as reaction solvents can be traced back to 2007. Zhang et al. First achieved efficient conversion of glucose in ionic liquid 1-ethyl-3-methylimidazolium chloride ( [EMIM] Cl) using metal chlorides as catalysts[75]. In the absence of any catalyst, the yield of 5-HMF was only about 3% when glucose was reacted in [EMIM] Cl at 180 ℃ for 180 min. Among all the metal chlorides tested, CrCl2 showed the best catalytic performance, and the yield of 5-HMF was as high as 70% when glucose was reacted at 100 ℃ for 180 min. This discovery reveals the important role of ionic liquids as reaction solvents in the preparation of 5-HMF. Zhao et al. explored the effect of different ionic liquids, such as 1-butyl-3-methylimidazolium chloride ( [BMIM] Cl), 1-hexyl-3-methylimidazolium chloride ( [HMIM] Cl), 1-octyl-3-methy-limidazoliumchloride ( [OMIM] Br), and 1-decyl-3-methy-limidazoliumchlorine ( [DMIM] Br) on the 5-HMF yield: the 5-HMF yield decreased from 38.4% to 23.3% as the alkyl chain in the[76]. The yield of 5-HMF was the highest when [BMIM] Cl was used as the reaction solvent. Chen et al., inspired by the related research of Zhang et al., used the zero-valent chromium carbonyl complex (Cr0-NP)Cr(CO)6 as a catalyst to catalyze the conversion of glucose in [EMIM] Cl (Fig. 3), and the yield of 5-HMF could reach 50%[77][76]. Kim et al. Used 1,1-decane-1,10-bis (3-ethylpyridine) dibromide ([C10(EPy)2]2Br)) as the reaction solvent and CrCl2 as the catalyst to catalyze the conversion of glucose to 5-HMF at 110 ℃[78]. When the reaction time was 120 min, the yield of 5-HMF was 47%.
Liu et al. examined the effects of modified metal ions and different cation exchange resins on the direct conversion of glucose to 5-HMF in [BMIM] Cl[79]. The results showed that the Cr3+-D001-CC catalyst obtained by modifying D001-CC cation exchange resin with CrCl3 by ion exchange method had the best catalytic performance. Akmaz et al. prepared Cr3+ exchanged Cr-bentonite and Cr-montmorillonite catalysts[80]. Using [BMIM] Cl as reaction solvent, the conversion of glucose catalyzed by Cr-montmorillonite was carried out at 140 ℃ for 20 min, and the yield of 5-HMF was 45%. The yield of 5-HMF catalyzed by Cr-bentonite was up to 62. 6% at 150 ℃ for 10 min. Koc et al. Used Cr-USY and Cr-β-zeolite as catalysts in [BMIM] Cl, and the yields of 5-HMF were 54. 5% and 58. 8%, respectively, at 130 ℃ for 60 min[81].
Li et al. Synthesized a series of metal phosphates using P123 as a template through the soft template method[82]. Using 1-ethyl-3-methylimidazolium bromide ( [EMIM] Br) as the solvent, the reaction was carried out at 120 ℃ for 180 min. Compared with CrPO, FePO, ZrPO and NbPO, SnPO had the best catalytic effect, and the yield of 5-HMF reached 58. 3%. Ju et al designed and synthesized a catalyst with high Lewis acidity and low Brønsted acidity by treating Al2O3 with different concentrations of sodium hydroxide[83]. The ratio of Lewis acidity to Brønsted acidity varies with the concentration of alkali, and the Al2O3-b-0.05 catalyst obtained by 0.05 mol·L-1NaOH treatment has the best catalytic performance. When [EMIM] Br was used as the reaction solvent and the reaction was carried out at 120 ℃ for 180 min, the yield of 5-HMF was 34. 5% and the conversion was 56. 7%.

2.2.2 Functionalized ionic liquid

The greatest advantage of ionic liquids lies in their designability. Through the design of anions and cations, their physical and chemical properties can be changed to meet the different requirements of separation or catalysis, such as melting point, viscosity, density, hydrophilicity, thermal stability, acidity and alkalinity. These ionic liquids are also called functionalized ionic liquids[84,85]. Functionalized ionic liquids have the general advantages of ionic liquids and the specific functions of functional groups. de Souza et al. Designed and synthesized imidazolium ionic liquids ([C16MIM]Cl)) containing long alkyl chains as solvents for the conversion of glucose to 5-HMF[86]. When the reaction was carried out at 120 ℃ for 12 min, the conversion of glucose was 81%, the yield of 5-HMF was 31%, and the selectivity of 5-HMF was 38%. Han et al designed and synthesized three kinds of 1,8-Diazabicyclo [5.4.0] undec-7-ene (DBU) -based ionic liquids. When CrCl3·6H2O was used as the catalyst, the CrCl3·6H2O/Bu-DBUCl system showed excellent catalytic performance for the conversion of glucose to 5-HMF[87]. The yield of 5-HMF was 64% at 100 ℃ for 180 min. The presence of tertiary nitrogen in the cation provides Lewis basic properties to the ionic liquid, which can promote the dehydration of glucose and produce 5-HMF through the hydrogen bond formed directly between the hydroxyl group of glucose and the tertiary nitrogen atom of the ionic liquid. Inspired by this, Han et al designed and synthesized DBU-based ionic liquids containing Benzene sulfonate (BS) anions again[88]. The yield and selectivity of 5-HMF could be increased to 78. 6% and 84. 2%, respectively, when the reaction was carried out at 100 ℃ for 180 min in the presence of Et-DBUBS and CrCl3 as the catalyst. The presence of BS anion can increase the catalytic activity of the catalyst and the selectivity of the product 5-HMF by forming a hydrogen bond between BS and glucose.
In addition, typical functionalized ionic liquids commonly used in the conversion of glucose to 5-HMF also include acidic ionic liquids, such as Brønsted acidic ionic liquids and Lewis acidic ionic liquids. Functionalization is usually carried out by anion exchange with inorganic or organic acids of different strengths, or by introducing acidic groups to cations to make them acidic and thus have stronger catalytic activity[89⇓~91]. Ma et al. Realized the direct conversion of glucose to 5-HMF in [BMIM] Cl using 1-butylsulfonate-3-methylimidazolium chloride ionic liquid ([C4SO3HMIM]Cl)) as a catalyst, and the total reducing sugar (TRS) yield was as high as 90% (Figure 4)[92].
Ramli et al. Synthesized acidic functionalized ionic liquid 1-sulfo-3-methylimidazolium tetrachloroferrate ( [SMIM][FeCl4]) by 1-sulfo-3-methylimidazolium chloride ( [SMIM] [Cl]) and FeCl3·6H2O[93]. The yield of 5-HMF was 24% at 110 ℃ for 300 min. Kim et al. Synthesized three kinds of ionic liquids [GLY(MIM)3][PF6]3, [GLY(MIM)3][Cl]3 and [GLY(MIM)3][Br]3 containing three cations by anion exchange method using Tris (3-methylimidazole) trimethyl sulfonate [GLY(MIM)3][OMs]3 as precursor[21]. When [GLY(MIM)3][OMs]3 was used as catalyst and solvent, the yield of 5-HMF was 16% at 110 ℃ for 180 min in nitrogen atmosphere. However, the yield of 5-HMF was 58% at 140 ℃ for 300 min when [GLY(MIM)3][Cl]3 was used as catalyst and CC-SO3H was used as co-catalyst. Kim et al believed that the high conversion of glucose and high yield of 5-HMF were mainly due to the ion exchange between CC-SO3H and [GLY(MIM)3][Cl]3 and the release of a large number of H+ ions.
Although ionic liquids have many advantages over other types of reaction solvents, and ionic liquids can be used as reaction solvents to obtain high yields of 5-HMF under mild conditions, the cost of traditional ionic liquids is relatively high, and the cost of ionic liquid reaction system is much higher than that of other reaction systems. The synthesis of functionalized ionic liquids is complex and energy-intensive, and 5-HMF obtained in the ionic liquid system is difficult to be separated from the system. In addition, the reusability of the solvent is low, and the recovery is quite difficult, so it is not conducive to its large-scale use in industry.

2.3 Biphasic solvent

In the single-phase reaction system, the yield of 5-HMF decreased because 5-HMF was very unstable in the reaction process and was easy to hydrolyze into by-products such as LA and produce a large number of black rot products. At the same time, it is difficult to separate and extract 5-HMF in the single-phase reaction system. Therefore, if 5-HMF is separated from the reaction system in time, the yield and selectivity of 5-HMF can be improved[94⇓⇓⇓~98]. Therefore, in recent years, more and more researchers have focused on the production of 5-HMF in a two-phase system. The biphasic system takes advantage of the high hydrophilicity of sugars and the relative hydrophobicity of 5-HMF. When glucose is converted to 5-HMF in the reaction phase, 5-HMF is immediately transferred to the organic phase[99]. The concentration of 5-HMF in the reaction phase can always be maintained at a low level, thereby reducing the generation of side reactions and avoiding further decomposition into by-products. In addition, the separation of the product from the reaction system was achieved, greatly increasing the yield and selectivity of 5-HMF (fig. 5). Table 4 summarizes several reports on the preparation of 5-HMF from glucose in a biphasic solvent system.

2.3.1 Aqueous solution/organic solvent

At present, most biphasic reaction solvents for preparing 5-HMF from glucose use aqueous solution as reaction phase, organic solvent as extraction phase, solid acid as catalyst, and a certain amount of inorganic salt solution (usually NaCl solution) is added to the reaction system to increase the distribution coefficient of the reaction phase and the extraction phase. Davis et al. Prepared 5-HMF by one-pot conversion of glucose in water/1-butanol and water/THF systems using tin-Beta zeolite (Sn-Beta) as catalyst[100]. Meanwhile, the pH of the reaction was adjusted to 1 by HCl, and NaCl was added to the reaction system to increase the distribution coefficient of 5-HMF in butanol solution (fig. 6). When the ratio of water to butanol was 1 ∶ 3 (V/V), the selectivity of 5-HMF was only 26. When a certain amount of NaCl was added to the reaction system, the selectivity of 5-HMF was increased to 55%. NaCl as an electrolyte changes the intermolecular binding interactions between the liquid components, thereby increasing the immiscibility between the aqueous and organic phases and thus increasing the selectivity of 5-HMF. In the water/THF/NaCl reaction system, the selectivity of 5-HMF was increased to 72% and the conversion of glucose was 79% at 180 ℃ for 80 min.
Rohani et al. Used sulfonated activated carbon (SO3H-OAC) as a catalyst to prepare 5-HMF in a THF/H2O-NaCl biphasic system[101]. The results showed that the concentration of 5-HMF in the organic phase increased gradually with the increase of reaction time, while the concentration of 5-HMF in the aqueous phase only remained in a very low range. The extraction coefficients (kEX1 and kEX2) of 5-HMF in both organic and aqueous phases decreased with the increase of reaction temperature, but the equilibrium extraction coefficient (kEX1/kEX2) was always greater than 1, which proved that the biphasic system could effectively transfer 5-HMF from aqueous phase to organic phase. Zhao et al. Synthesized a series of acid-base bifunctional ionic hybrid catalysts by ion exchange method using amino-functionalized imidazole ionic liquid and heteropolyacid as precursors [MimAM]HnPW12O40(n=1~3][102]. The conversion of glucose and the yield of 5-HMF in the biphasic system were significantly higher than those in the aqueous and organic solvent systems. The conversion of glucose was up to 99.8% and the yield of 5-HMF was 53.9% when the reaction was carried out at 160 ℃ for 7.5 H. Du et al. Synthesized a modified porous PCP(Cr)-SO3H·Cr(Ⅲ) solid catalyst by one-pot self-assembly of sulfonic acid/phosphotungstic acid, and converted glucose to 5-HMF in a H2O/THF/NaCl system[103]. In the reaction system of NaCl solution/THF = 1 ∶ 2 (V/V), the conversion of glucose was nearly 100% and the yield of 5-HMF was as high as 80. 7% when the reaction was carried out at 180 ℃ for 240 min. When DMSO was used as reaction solvent, the yield of 5-HMF was only 9.3%. The addition of the organic phase can effectively extract 5-HMF from the reaction system and reduce the generation of LA and FA by the continuous reaction of 5-HMF in the reaction system, thereby reducing the yield of 5-HMF. 5-HMF was not detected in the aqueous phase and the yields of LA (1.8%) and FA (4.4%) were very low. In recent years, other research progress on the conversion of glucose to 5-HMF catalyzed by THF/H2O-NaCl biphasic system is shown in Table 4[104,105].
In the H2O/MIBK system, Zeng et al. Catalyzed the conversion of glucose with betaine base catalyst (B-Cl) derived from betaine sugar industry and AlCl3·6H2O[106]. At 170 ℃ for 30 min, the yield of 5-HMF was 64%, and the yield of LA was less than 4%. In addition, the catalytic effect of B-Cl/SnCl4·5H2O was also studied. The yield of 5-HMF decreased to 25% and the yield of LA increased to 12%. Zeng et al. supported S O 4 2 - /ZrO2(SZ) on a H+ zeolite (SZ-HM) and used to compare the catalytic conversion of glucose in a single-phase system and a two-phase system (fig. 7)[107]. In the NaCl/H2O/DMSO(DMSO∶NaCl=1.5∶3.5) single-phase reaction system, due to the mutual solubility of DMSO and NaCl solution, the existence of NaCl in the reaction system would affect the separation of 5-HMF, and the yield of 5-HMF was only 61. 14%. The yield of 5-HMF was increased to 90% in the NaCl/H2O/GVL(GVL∶NaCl=5.0∶1.5) biphasic reaction system.
Sun et al. Used highly efficient heterogeneous niobium supported montmorillonite (Nb-MMT) containing both Lewis and Brønsted acid sites as a catalyst in a MIBK/H2O biphasic system to obtain a glucose conversion rate of 99% and a 5-HMF yield of 70.52%[108]. Dumesic's group used NaCl aqueous solution of pH 2. 5 as the aqueous phase and 2-sec-butylphenol as the organic phase to form a two-phase system. When AlCl3 and HCl were used as catalysts, the yield of 5-HMF was 62%, and the extraction rate of the organic phase could reach 97%[109]. At the end of the reaction, the Lewis and Brønsted acid catalysts (AlCl3 and HCl, respectively) were still in the aqueous phase, and when THF was used as the organic phase, the presence of 30% HCl in the organic phase was detected. They then explored the catalytic conversion of glucose to 5-HMF in a biphasic system consisting of an aqueous NaCl solution and a series of gamma-lactones[110]. In which monosaccharide is dehydrate in an aqueous phase and then 5-HMF is extracted into an organic phase, a saturated NaCl solution as an aqueous phase can reduce that solubility of the organic solvent and 5-HMF and improve the efficiency of the extraction of the organic phase. In the water/γ-lactone single phase system, the selectivity of 5-HMF decreased significantly.

2.3.2 Ionic liquid/organic solvent

Because ionic liquids are expensive and the initial concentration of reactants is low when ionic liquids are used as reaction solvents alone, ionic liquids in the current reaction system are often used as reaction solvents and catalysts, and cosolvents are used to increase the initial concentration of reactants. It is difficult to achieve industrial production, so researchers use low-cost organic solvents to partially or completely replace ionic liquids to obtain mixed solvent systems. The mixed solvent reduces the catalytic conversion cost on the basis of keeping the solubility of the ionic liquid. Chen et Al. Synthesized a series of polymer ionic liquids loaded with Al and used them as catalysts and solvents for glucose conversion, while organic solvents DMSO, DMF and DMAC were used as cosolvents[111]. When CrCl2 was used as catalyst, the yield of 5-HMF in [EMIM] Cl/organic solvent system was about 50%. Poly (3-butyl-1-vinylimidazolium) chloride (P [BVIM] Cl) showed the best catalytic activity among all the synthesized polymeric ionic liquids, and the yield of 5-HMF was increased to 65. 8% in the P[BVIM]Cl/DMF/CrCl2 system. The yield of 5-HMF was only 49% when the metal-loaded functional ionic liquid PIL-Al(P[BVIM]Cl-Et2AlCl) was used and DMF was used as the cosolvent. However, the recoverability test showed that the recoverability of the P[BVIM]Cl-Et2AlCl/DMF system was stronger, and the 5-HMF yield remained constant after 6 cycles. Saha et al. Used Zr(O)Cl2 as catalyst in [BMIM] Cl/MIBK biphasic system, and the yield of 5-HMF was 66%, while the yield of 5-HMF was only 42% in H2O/MIBK reaction system under the same reaction conditions[112]. It was confirmed that the addition of the ionic liquid in the [BMIM] Cl/MIBK biphasic solvent increased the dehydration rate of the reactant glucose and maximally reduced the formation of by-products. Ju et al. Used different metal chlorides as catalysts to catalyze the conversion of glucose in ionic liquid [BMIM] Br, organic solvent dimethyl ether (GDE), dimethyl carbonate (DMC), and [BMIM] Br/GDE and [BMIM] Br/DMC biphasic systems, respectively. It was found that the yield of 5-HMF was 64.5% and the conversion of glucose was close to 100% when SnCl4 was used as catalyst in [BMIM] Br[113]. Under the same reaction conditions, the yield of 5-HMF was only 18. 6% with GDE as the reaction solvent. In [BMIM] Br/GDE system, the yield of 5-HMF was 58.7%, while in [BMIM] Br/DMC biphasic system, the yield of 5-HMF could be increased to 64.2%. It is worth noting that [BMIM] Br/GDE system can contain up to 80 wt% glucose, so the biphasic system is more conducive to the production of 5-HMF in the actual biorefinery process than the single-phase ionic liquid. Smith et al. Catalyzed the conversion of glucose to 5-HMF in ethanol (EtOH) using AlCl3 as a catalyst at low temperature for a short time, at which time glucose was hardly converted to 5-HMF (yield < 2%)[114]. In [BMIM] Cl/EtOH biphasic system, the yield of 5-HMF can reach 11.9%.
The yield of 5-HMF can be significantly improved by the extraction of the product in the biphasic system, and the organic phase can be recycled after the product is separated, so that the reaction cost is reduced. With the continuous development of 5-HMF catalytic system, the biphasic system has excellent industrial application prospects.

2.4 Eutectic solvent

Eutectic solvents (DES) are usually composed of hydrogen bond donors such as carboxylic acids and polyols and hydrogen bond acceptors such as metal salts and metal salt hydrates, and their physical and chemical properties are similar to those of ionic liquids, which are gradually applied to the conversion of carbohydrates as a new green solvent (Table 5)[115].
Matsumiya et al. combined Dihydrogen citrate (ChDC) with different carboxylic acids to form a low-cost eutectic reaction system which can replace the Cr/ionic liquid reaction system, and used water as a cosolvent and B(OH)3 as a catalyst[116]. At 140 ℃ for 120 min, the yields of 5-HMF in ChDC/salicylic acid (Salicylic acid,SA)/B(OH)3 and ChDC/mandelic acid (Mandelic acid)/B(OH)3 were 26% and 28%, respectively. The yields of 5-HMF in ChDC/malic acid (Malic acid)/B(OH)3 and ChDC/glycolic acid (Glycolic acid,GA)/B(OH)3 were increased to 35% and 42%, respectively. The addition of co-solvent water reduced the formation of humic acid, so the yield of 5-HMF could be increased to about 60% in the reaction system of 240 min,ChDC/GA/B(OH)3/ water at 140 ℃. Fructose solution and choline chloride (ChCl) can form a biodegradable quaternary ammonium salt, which is a good eutectic solvent. Lin et al. Reacted at 150 ℃ for 30 min in DES composed of fructose solution/ChCl with dilute hydrochloric acid as catalyst, and the yield of 5-HMF was only 3.1%[117]. When CrCl3 was used as catalyst, the yield of 5-HMF was as high as 60.3%. Inspired by this, Lin et al. Used a two-phase system composed of ChCl and Methyl cyanide (MeCN) as the reaction solvent, in which MeCN was used as the extraction phase of the product 5-HMF[118]. At the same time, the yield of 5-HMF was only 12. 1% with Amberlyst-15 as catalyst at 130 ℃ for 120 min. Subsequently, AlCl3 and CrCl3 were loaded on Amberlyst-15 to obtain Amberlyst-15-AlCl3 and Amberlyst-15-CrCl3 catalysts, respectively. When the catalyst dosage was 40 wt%, the yield of 5-HMF from glucose catalyzed by Amberlyst-15-AlCl3 was 17%, while the yield of 5-HMF catalyzed by Amberlyst-15-CrCl3 could be increased to 57%. Zeng et al. Catalyzed the conversion of glucose to 5-HMF using a series of metal-loaded MCM-41 catalysts, M-D41, in a water/MIBK/ChCl reaction system (fig. 8)[119]. When Cr-D41, Zr-D41 and Al-41 were used as catalysts, the yields of 5-HMF were 44%, 34% and 52%, respectively.
The research on the preparation of 5-HMF in DES system is still in its infancy. Based on the unique advantages and solvent properties of DES, the development of a highly active, easily recovered and reusable catalyst suitable for DES solvent system will greatly increase the application potential of DES in the field of biorefining.

3 Outfield synergy

In the process of preparing 5-HMF by catalytic conversion of glucose, the commonly used external field synergy includes ultrasonic oscillation synergy, microwave radiation synergy and external pressure field synergy.

3.1 Ultrasonic Oscillation

Ultrasound is essentially a mechanical longitudinal wave with a frequency equal to or higher than 20 kHz. The propagation of this mechanical longitudinal wave causes a sinusoidal change in the static pressure in the liquid medium, causing cavitation in the reaction system[120]. Cavitation has both physical and chemical effects on the reaction system, which enhances the mass transfer in the reaction system and produces highly reactive free radicals in the cavitation bubbles, thus accelerating the reaction rate[23,121⇓~123].
Muhammad et al. studied the one-pot preparation of 5-HMF from glucose in ionic liquid [BMIM] Cl by using probe ultrasound method, and found that the yield of 5-HMF was 42% in 10 min (Fig. 9a), and 41% could be obtained by Conventional heating, but the reaction time needed to be increased to 180 min[123]. However, the yield of 5-HMF decreased as the reaction time increased, indicating that 5-HMF was degraded under prolonged sonication to form a putrescence (fig. 9B).
So far, the use of ultrasound-assisted is not widespread, but it can not be denied that it has great potential in the preparation of platform compounds such as 5-HMF. Although this method has proved to be very effective on a laboratory scale, scaling up to factory production is still a huge challenge in terms of power consumption, heat consumption and energy loss.

3.2 Microwave radiation

In general, microwave digesters produce efficient internal heating by generating ultrashort electromagnetic waves at a frequency of 2450 MHz to directly couple microwave energy to molecules of solvents, reactants, or catalysts in the reaction mixture[124⇓~126]. Compared with traditional heating methods, microwave-assisted heating has shown remarkable advantages, such as improving the yield of reaction products, accelerating the reaction rate and greatly shortening the reaction time, and reducing the energy consumption in the reaction process[127⇓⇓~130].
Appels et al. studied the effect of microwave on the preparation of 5-HMF in a biphasic system HCl solution/MIBK by kinetic analysis, and evaluated the effect of microwave on the yield of 5-HMF using two reactors of the same shape[36]. A borosilicate glass reactor was used to heat the mixture directly by microwaves, and the mixture in the other reactor was heated by convective heat transfer due to the absorption of microwaves by the silicon carbide (SiC) wall (fig. 10). The results showed that the temperature profiles of the two reactions were almost the same. Kinetic calculations showed that the rate of isomerization of glucose to fructose increased by a factor of 2.5 under microwave irradiation. The rate constant obtained in the glass container was 2.9866 h-1,SiC, and the rate constant in the container was only 1.1950 h-1. Comparing the energy consumption in glass container and SiC container at 177 ℃ for 240 min in microwave heating, microwave heating consumes more energy than traditional heating, but the yield of 5-HMF increases from 35% to 51%. Tsang et al. compared the effects of different biphasic systems on the microwave-assisted conversion of glucose to 5-HMF[38]. The propylene carbonate (Propylene Carbonate,PC)/H2O, the GVL/H2O, the acetone /H2O and the water are used as reaction solvents, and the SnCl4 is used as a catalyst. The yields of 5-HMF were 26%, 23%, 10% and 3% at 120 ℃ in 10 min.

3.3 Applied pressure field

The addition of the pressure field is mainly realized by a high-pressure reaction kettle, in a closed reaction vessel, water is used as a solvent, and the degradation-grade synthesis of many biomass resources can be realized by the pressure of the water at a certain temperature[131,132]. In general, the preparation of 5-HMF by catalytic conversion of glucose needs to be carried out under the condition of concentrated acid, but under the condition of high temperature and high pressure, the target product can be prepared in a dilute acid environment, thereby reducing the pollution of waste acid to the environment and the pressure of recovering the waste acid, and being beneficial to improving the conversion rate of the reactant glucose.
Ordonez et al. combined a homogeneous catalyst (HCl) with a heterogeneous catalyst (beta-zeolite) to convert glucose to 5-HMF in an autoclave with significant improvement[133]. At 140 ℃ for 240 min, the yield of 5-HMF was 0.06 mol·L-1,24 h, and the conversion of glucose was almost complete (94%). The amount of HCl used in the reaction was only 200 ppm. Compared with the control reaction, the activation energy of hydrothermal conversion of glucose was significantly reduced, and the yields of putrescine and anhydroglucose (AHG) were significantly reduced[134]. Torres-Torres et al. Synthesized different TiO2-ZrO2 mixed metal oxide catalysts by sol-gel method, and studied the effect on the yield of 5-HMF at 175 ℃ and 30 bar (Ar atmosphere)[20]. The TZ5050-C2H4O2 catalyst was prepared by adding acetic acid (C2H4O2) during the synthesis of TiO2-ZrO2, and a certain amount of NaCl was added to the THF/H2O biphase system to increase the distribution coefficient in the biphase system. The yield of 5-HMF from glucose was 76%. The TZ5050-C2H4O2 could be reused 6 times under the same reaction conditions and the yield of 5-HMF remained stable. Li et al. Prepared phosphotungstic acid (PHA) catalyst Fe3O4@SiO2/PHA supported on magnetic nanoparticles Fe3O4 coated with SiO2, and used it to synthesize 5-HMF from glucose[19]. The yield of 5-HMF was 30. 4% when the reaction mixture was reacted at 130 ℃ for 180 min in nitrogen atmosphere with DMF as the reaction solvent. The catalyst was recovered by an external magnet after the reaction and directly reused in the next reaction without further treatment, and no significant change in activity was observed after the catalyst was recycled for 4 times.

4 Mechanism of 5-HMF formation from glucose

In general, the mechanism of preparing 5-HMF from glucose can be mainly divided into two types: (1) glucose is isomerized to fructose under the action of Lewis acid, enzyme or alkali catalyst, and then three molecules of water are removed from fructose under the action of Brønsted acid to form 5-HMF; (2) glucose is first removed one molecule of water to form anhydrolevoglucose, and then two molecules of water are removed to form 5-HMF (Figure 11)[135⇓⇓⇓⇓⇓⇓~142]. However, black insoluble Humin and soluble polymers are easily produced during the reaction, and 5-HMF is further hydrolyzed to produce by-products such as LA and FA[24⇓~26,135,143].
The isomerization of glucose under acid catalysis is the key step to determine the yield and selectivity of 5-HMF[144,145]. At present, the mechanism of glucose isomerization to fructose is that glucose first reaches the equilibrium of α-glucose and β-glucose through mutarotation, and then forms enediol intermediates through H+ transfer (H+shift) and Protonation (Protonation) and isomerizes to fructose[75]. In the second step, there are two routes for the dehydration of fructose to 5-HMF. The first route is Acyclic intermediates, in which fructose first removes one molecule of water to form a cyclic intermediate of fructofuranose, and then fructofuranose continues to remove two molecules of water to form 5-HMF. The second mechanism is the formation of 5-HMF from enol intermediates via the Cyclic intermediates route. Similarly, under acidic conditions, fructose does not form Cyclic intermediates, but directly removes three molecules of water in the form of a straight chain to form 5-HMF (Fig. 12)[146⇓~148].
Zhang et al. Suggested that the CrCl 3 - in the [EMIM]CrCl3 composed of [EMIM] Cl and CrCl2 caused the optical isomerization of glucose through proton transfer, thus achieving an equilibrium between α-glucopyranose and β-glucopyranose in the reaction system (fig. 13)[75]. Glucose is then isomerized to fructose by the transfer of hydrogen from the formyl group to form an enol intermediate.
de Souza et al. Proposed a complete reaction pathway for the dehydration of fructose to 5-HMF via a cyclic route (fig. 14)[86]. During this reaction, the dehydration of fructose is first protonated by the hydroxyl group at position 2 of the furan ring in the molecule, thus removing a molecule of water. The spontaneously dehydrated protonated fructofuranose is then rearranged to form the intermediate enol. And finally deprotonate and removing two molecule of water to obtain 5-HMF. Li et al. studied the catalytic performance of [BMIM] Br/2,5-furoic acid (FDCA) in the process of fructose conversion to 5-HMF, and proposed the catalytic effect of ionic liquid on fructose dehydration (Fig. 14)[148]. [BMIM] Cl can exist in two forms (fig. 15A). Compared with compound I, compound II contains N cations, which have missing electron orbitals and minimal steric hindrance, but the missing electron orbitals of N cations can be further stabilized by Br atoms through strong nucleophilic interactions. The missing N electron orbital in the low sterically hindered ionic liquid II plays a key role in the conversion of fructose to 5-HMF (fig. 15 B). The coordination of the oxygen atom in fructose with the ionic N atom promotes the removal of water molecules, thus forming 5-HMF. Since there is almost no unstable basic compound I in [BMIM] Br, [BMIM] Br shows excellent catalytic performance in the conversion of fructose to 5-HMF without using any other catalyst.
The key step in the synthesis of 5-HMF from fructose via a straight-chain intermediate route is the formation of a 1,2-enol intermediate, followed by the removal of three molecules of water to form 5-HMF[149]. The reaction involves the formation of 3-deoxyglucose-2-ene, the tautomer of which forms 3-deoxy-D-glucuronone (3-DG) (fig. 16). Bols et al. Investigated and compared the production of 5-HMF from D-fructose, D-glucose and 3-DG, respectively (fig. 17)[150]. When 3-DG was used as the raw material, the reaction rate was obviously increased. Therefore, 3-DG is likely to occur as an intermediate in the formation of 5-HMF from fructose via a ring-opening route. While the formation of 5-HMF can be considered as a 1st and 2nd order reaction compared to the formation of the corresponding substituted halogenated furans. With fructose as the starting material, the 1,2-enediol intermediate can also be isomerized to 2,3-enediol, followed by triple dehydration to form 2-hydroxyacetylfuran and decarbonylation of 3,4-deoxyglucose, leading to the formation of furfural.
Based on the above studies, Mu et al. Calculated the conversion of glucose to 5-HMF in [BMIM] Cl ionic liquid using density functional theory (DFT)[151]. The reaction mechanism is divided into two parts: glucose isomerization and fructose dehydration. In the first part, a five-membered ring chelate of a metal atom and glucose is used as a key intermediate. The second part is the sequential release of three water molecules from fructose. The first water removal is the rate controlling step in this section, while the second and third water removal are exothermic steps. The metal center has a significant effect on the stability of the intermediate and the change in Gibbs free energy associated with the reaction step. The reactivity of metal chlorides decreases in the order of WCl3>MoCl3>CrCl3>FeCl3. Zhao et al. calculated the reaction process of the catalytic conversion of glucose to 5-HMF by DFT using aluminum alkoxide Al(OMe)3 as the catalyst (fig. 18)[152]. The isomerization of glucose to fructose involves three steps: (1) ring opening of cyclic pyranose 1 to its open-chain isomer IM4; (2) isomerization from the open-chain form of glucose to the open-chain isomer of fructose IM7 via intermolecular hydrogen bonding interactions; and (3) formation of the closed cyclic fructose IM11. The highest free energy in this process is the 23.9 kcal·mol-1, which is low enough to achieve this reaction under mild conditions. Fructose dehydration also involves three steps, including three consecutive dehydration processes, from A to the formation of 1,2-enol compound B, to the formation of acrolein compound D, and finally to 5-HMF 3. The free energy of this reaction is 31.2 kcal/mol, so this step needs to be carried out at high temperature. In addition, the metal-ligand bifunctional interaction in the Al(OMe)3 catalyst plays an important role in stabilizing the intermediates and transition States in the reaction process.
Chermahini et Al. Synthesized Al-containing silicon material Al-KCC-1 as a catalyst by co-condensation method, and studied the effects of Brønsted acid sites and Lewis acid sites on the catalytic conversion of glucose to 5-HMF (Fig. 19)[153]. They believe that the dehydration reaction of glucose involves two steps: first, Lewis acid sites on the catalyst isomerize glucose to fructose, and then Brønsted acid sites on the catalyst dehydrate fructose to 5-HMF.

5 Conclusion

Understanding the process of catalytic conversion of glucose to 5-HMF is the basis for understanding the directional depolymerization and reorganization of biomass resources such as cellulose to produce energy platform compounds. In recent years, a variety of reaction solvent systems and external field synergy have been gradually applied to the preparation of 5-HMF from glucose. In the two-phase reaction system, the problem of separation and purification of 5-HMF is greatly solved through the transfer of products between the reaction phase and the extraction phase. At present, there are more and more studies on the reaction mechanism of glucose to 5-HMF, but the instability of intermediates and the complexity of the reaction process limit the further study of this mechanism.
In order to realize the efficient and economical conversion of biomass to 5-HMF, the following breakthroughs are needed: developing green and friendly reaction solvent systems, exploring new catalysts for the preparation of 5-HMF with high yield and selectivity; To clarify the mechanism of directional depolymerization and efficient recombination of biomass resources, to understand the mechanism of tautomerism of intermediate products, and to provide strong support for the industrial production of 5-HMF.

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Funding

Fundamental Research Funds for the Central Universities(2572017ET02)

National Natural Science Foundation of China(31890773)

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