5-HMF is a selective transformation product of cellulose (polysaccharide) through consecutive acidic hydrolysis, isomerization, and dehydration, which, at the same time, has been awarded to be the promising candidate of the “top 10 + 4” bio-based key platform compounds by the U.S. Department of Energy because it is capable of being a bridge between biomass feedstock and value-added industrial chemicals (Hu et al., 2020; Thoma et al., 2020). 5-HMF is a five-membered heterocyclic compound comprising one furan ring as the parental skeleton and one hydroxymethyl and aldehyde group located at 2, 5 positions that is permitted to be converted efficiently into various high-quality fuels and value-added chemicals by the oxidation and/or reduction reactions for the hydroxymethyl and/or aldehyde group(s).

The oxidation/reduction of 5-HMF can yield furanic derivatives such as 2, 5-furandicarboxylic acid (FDCA), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 5-formyl-2-furancarboxylic acid (FFCA), 2, 5-diformylfuran (DFF), 2, 5-bis(aminomethyl)furan (BAMF), (tetrahydrofuran-2, 5-diyl)dimethanol (THFDM), 2, 5-dimethyl furan (DMF), and 5-methylfuran-2-carbaldehyde (MFC), which undergo oxidation/reduction of formyl and/or hydroxymethyl group(s) of 5-HMF to various extents (Figure 3).

In Brønsted acid catalysis, Brønsted acids are often considered to be potential catalysts in catalysis because of enabling the release of hydrogen ions (H⁺) in an aqueous solution via dissociation. H⁺ can protonate oxygen atom(s) in the 1, 4-glycosidic bond and/or pyranose ring. Subsequently, by undergoing the half-chair conformational isomerization of the oxygen atom neighboring C₁ position of the anhydroglucose unit, it can bring about the formation of carbenium ion, presenting the cleavage of the C-O bond. Finally, in the presence of a water molecule, the carbenium ion interacted with the water molecule, enabling to reconstruct of the anomeric center to form D-glucose. After the formation of D-glucose, the isomerization of D-glucose into fructose occurs successively in acidic surroundings (Scheme 2) (Rinaldi and Schüth, 2009; Van Putten et al., 2013; Yamabe et al., 2013; Kang et al., 2018; Zhao et al., 2021). In this process, the critical intermediate of 1, 2-enediol is formed via a 1, 2-hydride shift that occurred in glucose. And then, as following steps of two 1, 2-eliminations, one 1, 4-elimination, and a ring closure in 1, 2-enediol, the promising production of 5-HMF is converted successfully from cellulose. Totally, the aforementioned pathway of 5-HMF converted from cellulose can be summarized by the following steps: 1) the protonation of oxygen atoms in the 1, 4-glycosidic bond and/or pyranose ring; 2) the cleavage of carbon–oxygen bond; 3) nucleophilic attack of water; 4) isomerization of glucose into fructose; and 5) successive dehydration (Zhao et al., 2021).

In comparison with Lewis acid catalysis, Lewis acids inhering empty orbits can accept electron pairs in chemical concepts (Marqués et al., 2021). In other words, acidic sites of Lewis acids are capable of being transformed into Brønsted acidic sites by combination with pairs of electrons donated from protic solvent, which can be responsible for the isomerization of glucose into fructose (Delidovich and Palkovits., 2015; Xu et al., 2017). In previous research studies, Lewis acidic sites of catalysts utilized for the aldose–ketose isomerization were conducted well because they could effectively promote the formation of enol intermediates (Charmot and Katz, 2010; Choudhary et al., 2013). Therefore, the general mechanism for Lewis acid-catalyzed isomerization can be summarized in two steps: one is the enolization of aldose, and the other is the 1, 2-hydride shift, both of which favor the isomerization of aldose into ketose. Moreover, in terms of glucose–fructose isomerization with Lewis acid, the isomerization promoted by Lewis acid (M_xL_y) should first follow the interaction of L of M_xL_y with H of hydroxyl groups attached to the glucose skeleton. Afterward, under heating conditions, the formation of the five-membered complex occurs immediately, resulting from the coordination between M (Lewis acidic site) and two oxygen atoms (electron donors) in glucose. Then, the complex is authorized to be transformed into the corresponding enol intermediate through undergoing ring opening, which is subsequently isomerized into fructose. Eventually, as for the step of 5-HMF converted from fructose as an isomer of glucose, fructose is permitted to be directly dehydrated into 5-HMF with the assistance of the Brønsted acidic site (Scheme 3) (Hu et al., 2009; Zhao et al., 2021).

Resin-based sulfonated solid acids as potent heterogeneous catalysts have achieved good results in biomass conversions, resulting from the presence of sulfonic groups (-SO₃H) on the frameworks of catalysts. The sulfonic groups are essentially Brønsted acidic sites, with strong acidity. In recent years, some novel resin-based sulfonated acid catalysts have been reported in the application of biomass conversions to high-value chemicals, with good results. The novel Cl-containing resin-based solid acid catalyst (Cl_0.3-S-R) bearing with -Cl and -SO₃H was synthesized by a simple hydrothermal method through the polymerization of o-chlorophenol and p-hydroxybenzenesulfonic with formaldehyde. Sulfonated solid acid with 1.47 mmol/g SO₃H density on the external surface was thought to be an effective catalyst for corn stover conversion to 5-HMF and furfural production. Notably, cellulose is a major component of corn stover. In experimental results, the Cl-containing sulfonated acid catalyst exhibited a superior catalytic activity in the catalytic transformation of corn stover to 5-HMF in a 1,4-dioxane/H₂O biphasic system, yielding a product of 5-HMF in a yield of 43.8% and the by-product of furfural in a yield of 38.1% (Figure 4) (Yang et al., 2021).

Afterward, with the assistance of microwave irradiation, employment of the catalyst SC-FAR-800 to the selective conversions of fructose, glucose, cellobiose, and cellulose into 5-HMF was exhibited to be in effect to varying extent, with the yields of 5-HMF being on 89.35, 38.17, 42.6, and 14.73%, respectively, indicating that fructose is a more appropriate feedstock for the synthesis of 5-HMF (Figure 5) (Huang T. et al., 2022). Moreover, the SC-FAR-800 is a furfuryl alcohol resin-based sulfonated acid catalyst, containing multiple sulfonic acid moieties with an acid density of 3.43 mmol/g. Moreover, by the techniques of SEM and BET, the irregular mesoporous structure of the sulfonated solid acid SC-FAR-800 was observed to be a specific surface area of 32.56 m²/g, an average pore size of 0.018 cm³/g, and a pore volume of 3.25 nm, which could be accountable for the efficient conversions of biomass.

Carbon is the most basic element in bio-organism. Carbon-based materials such as activated carbon (AC), graphite, hydrothermal carbon, graphene, and carbon nanotube (CNT) have been in popularity in the design of novel functional carbon-based materials because carbon-based materials as potential supporters are capable to be in charge of the construction of novel frameworks. The novel Nb–carbon composite is a class of bifunctional carbon-based catalysts in possession of varying amounts of Brønsted and Lewis acid sites, which are able to be prepared by means of the hydrothermal carbonization method. Relying on the determination of catalytic activity for the hydrolysis and dehydration of cellulose, the agglomerated particle of Nb/C-50 can effectively enhance the selectivity of 5-HMF, which was achieved with a 53.3% yield of 5-HMF in a THF/H₂O biphasic system at 170°C for 8 h (Figure 6) (Li Z. et al., 2018).

With the development of effective approaches to conversions of biomass, the eco-friendly and energy-efficient methods for cellulose conversion to 5-HMF were developed using modified activated carbon (AC) immobilized with metal ions. Activated carbon (AC) is a common carbon-based material. Via the treatment of diluted acid H₂SO₄, H₃PO₄, and HCl, it can, respectively, gain modified activated carbon supporters (ACS, ACP, and ACH). These supporters undergoing the immobilization of metal ions (M⁺ⁿ = Cr⁺³, Fe⁺³, Cu⁺², Zn⁺², K⁺, and Al⁺³) in aqueous media were successfully transformed into carbon-based catalysts M-ACS/ACP/ACH. In the identification of catalytic activity, a potent metal carbon-based Cr-ACS catalyst was identified to be effective for 5-HMF conversion from cellulose, assisting ionic liquid [Bmim]Cl as a medium. Of course, it is emphasized here that immobilized metal ions can effectively improve Lewis acidic sites on the surface of catalysts (Tyagi et al., 2018).

In the same year, the supporter of graphite-like mesoporous carbon material Sibunit had been documented by Gromov et al. (2018). Virtually, the supporter is also a carbon-based material. As a result of its inherent mesoporous properties, the carbon Sibunit treated by the following procedures of sulfonation, oxidation, and additional reaction for all the oxidized carbons sulfonated at 200°C was developed for the successive solubilization, hydrolysis, and dehydration of cellulose into 5-HMF in a one-pot reaction. The depolymerization of microcrystalline cellulose began at 180°C in water, in the presence of sulfonated carbon Sibunit solid acid. The main product 5-HMF could be achieved with the maximum yield of 22%, which was possibly ascribed to the formation of acidic groups (sulfuric, phenolic, and carboxylic groups) on the external surface of the carbon Sibunit (Figure 7) (Gromov et al., 2018).

Similarly, novel supporters of hydrothermal carbons (HTCs) as sugar-derived carbon materials prepared from monosaccharides are allowed to be obtained by suitable hydrothermal carbonizations (including reaction temperature and time). Subsequently, the resulting HTCs following the sulfonation can be transformed into potential novel carbon-based solid acids. The carbon-based acid (HTC220-6-SO₃H) acting as a representative example was afforded through the hydrothermal carbonization of glucose for 6 h at 220°C, and subsequently the sulfonation for 15 h at 150°C. The HTC220-6-SO₃H solid acid acting as a novel catalyst exhibited relatively high catalytic activity for the selective hydrolysis of cellulose and dehydration of fructose, indicating that sulfuric acidity (Brønsted acidity) can be responsible for the conversion to glucose and 5-HMF, with 43.63 and 20.29% yields of glucose and 5-HMF, respectively (Wataniyakula et al., 2018).

Bio-carbon is one of the most promising carbon-based materials. Cellulose, acting as a representative bio-carbon, is usually considered to be a biomass raw feedstock for biomass conversions and an effective supporter for carbon-based catalysts. In recent studies, the supporter of cellulose via moderate formylation was facilely developed for cellulose formate (CF) production. The CF production with a special net-like structure and a high degree of formyl substitution was successfully applied to the selective conversions of cellulose biomass, assisting co-catalyst of HCl-AlCl₃ and co-solvent of DMSO-H₂O (Figure 8) (Jin et al., 2021). The exhibitions of conversion and selectivity to 5-HMF appeared to be excellent, as compared to monomeric glucose as another bio-carbon supporter (Table 1).

To the best of our knowledge, the framework of zeolite presents a promising constitution for a novel solid catalyst. In general, once non-/noble metal ions are inhered on the external surface of a zeolite structure, the catalytic activity of metal–zeolite material will be effectively improved, due to the enhancement of pore volumes and Lewis acidic sites on the external surface of the metal–zeolite occurred. In 2020, the access gateway of non-/noble metal ions (Cu⁺² and Cr ⁺³) loaded on the external surface of ZSM-5 zeolite particles to the establishment of Cu-Cr-based zeolite was achieved by means of the ion exchange method. The highly crystalline Cu–Cr/ZSM-5 zeolite applied to the catalytic conversion of glucose into 5-HMF was smoothly initiated under suitable conditions, with good performance in conversion and selectivity (Chung et al., 2020).

Catalytic activity is always the key index for evaluating the performance of catalysts. Some classic zeolites of HY, Hβ, H-mordenite, and HZSM-5 as conventional solid catalysts are further discussed in catalytic activities for biomass conversions. In 2021, Zheng et al. (2021) reported on selective conversions of cellulose and starch over classic zeolites. The experimental results indicated that Hβ zeolite having appropriate Brønsted and Lewis acid sites becomes an effective promoter for furfural conversion from cellulose and starch. Simultaneously, HY zeolite with weak acidity is a flexible access to the transformation of 5-HMF from starch. H-mordenite and HZSM-5 zeolites bearing fewer Lewis acid sites on their external surfaces enable to inhibit the isomerization from glucose to fructose. On the whole, the generation of 5-HMF is closely correlated to the acid properties of zeolites. Simultaneously, it is confirmed that the acidity of zeolites can determine the target product formation.

Furthermore, the multifunctional zeolite catalyst (Ru/HY-SO₃H) was developed by the introduction of Lewis and Brønsted acidic sites on the surface of zeolite for the selective cellulose conversion to 5-HMF, which was successfully prepared under metal immobilization and sulfonation (Wang et al., 2021b). Under light illumination and low temperature (120°C), the Ru/HY-SO₃H, with the help of an ionic liquid/methyl isobutyl ketone biphasic medium, was capable of being a potent solid acid catalyst for the production of 5-HMF converted from cellulose. More significantly, the external surface of Ru/HY-SO₃H irradiated by high-intensity light could be initiated on the plasmon resonance effect for the direct cellulose conversion, with a 48.4% yield of 5-HMF as selective production. Nevertheless, as compared to precursors of Ru/HY-SO₃H, the promotions for selective conversion to 5-HMF by HY and Ru/HY were being on less in yields under the same conditions (Figure 9). It is implied that the formation of the plasmon resonance effect on the external surface of Ru/HY-SO₃H can enhance more acidic sites for the selective conversion of microcrystalline cellulose.

Pham et al. (2020) disclosed a series of bifunctional sulfonated mesoporous silica materials used for the conversion of cellulose into 5-HMF in the Teflon-lined stainless steel reactor. These mesoporous silica catalysts (Zr-MCM-41) synthesized following the in situ method and sulfonation were given varied Brønsted acidic sites, which were identified by NH₃-TPD analysis. Under heating for 2 h at 170°C, the sulfonated acid of S-15Zr-MCM-41 (73.44 wt% in silicon, 14.78 wt% in zirconium, and 11.78 wt% in sulfur) was allowed to be 70.2 and 16.4% in the conversion of cellulose and selectivity of 5-HMF, which were more than MCM-41 without sulfonation being on the conversion and selectivity of 15.2 and 1.3%.

Heteropoly acids (HPAs) are a type of unique combination of hydrogen cations and polyoxometalate anions, which are composed of transition metal–oxygen anion clusters. Stable heteropoly acid (HPA) with strong Brønsted acidity and mild Lewis acidity is reckoned as an effective solid acid that is significant for the efficient conversion of renewable biomass to valuable chemicals. In 2015, a novel ionic crystal of metal-based HPA was emerged in the conversion of monosaccharides into 5-HMF. Cs₂[Cr₃O(OCC₂H₅)₆(H₂O)₃]₂[α-SiW₁₂O₄₀] acting as an HPA ionic crystal was witnessed to be a novel heterogeneous acid catalyst, with identifying components (W, 49.37; Cr, 6.83; Cs, 5.85; Si, 0.61 wt.%), in the dehydration of fructose or glucose into 5-HMF. The effects for fructose or glucose selectively dehydrated into 5-HMF were being on desirable results that were, respectively, 86 and 56% in yields of 5-HMF in DMSO media. In further investigation, the experimental results were demonstrated that lower Brønsted acidity of the HPA ionic crystal profits for the stabilization of 5-HMF with weak polarity, which may be a rational explanation for better catalytic performance of the HPA ionic crystal, as compared to conventional HPA (H₄SiW₁₂O₄₀) (Yi et al., 2015). In 2020, a novel strategy for synthesizing a series of temperature-responsive HPA catalysts (Ch_nH_5-nCeW₁₂O₄₀, n = 1–5) was proposed by Lai et al. The Ce-based HPA of ChH₄CeW₁₂O₄₀ was responsible for one-pot production of 5-HMF from cellulose, with the achievement of 67.5% in yield in a biphasic system (Figure 10) (Lai et al., 2020).

In addition, in terms of catalytic dehydration of fructose to 5-HMF by HPA, the newly plausible pathway for PW₁₂-ILs-C₄-HNS-catalyzed dehydration process of fructose to 5-HMF in DMSO was proposed by An Z. et al. (2019) (Scheme 4) (An Z. et al., 2019).

As shown in Scheme 3, the media molecule of DMSO is first activated by an active proton originating from PW₁₂-ILs-C₄-HNS. Then, the interactions between the activated DMSO and fructose happened through the formation of hydrogen bonds. With the release of H₂O, the intermediate of enol A is formed. Enol A is transformed to corresponding aldehyde B (2, 5-anhydro-D-mannose) via isomerization. Afterward, the dehydrated product C is authorized to transform into 5-HMF via successive dehydration in the presence of PW₁₂-ILs-C₄-HNS and activated DMSO. PW₁₂-ILs-C₄-HNS as a novel PHA is a multicomponent solid acid, composed of phosphotungstic acid (H₃PW₁₂O₄₀) and organosilica ((EtO)₃ Si-ILs-C₄) via immobilization to form corresponding organosilica hollow nanospheres. In terms of the organosilica hollow nanospheres, it was clearly figured out that H₃PW₁₂O₄₀ of PW₁₂-ILs-C₄-HNS HPA is a Brønsted acid site, enabling strong electrostatic interactions with ILs. In practical biomass conversion application, the PW₁₂-ILs-C₄-HNS PHA exhibited excellent catalytic capacity and selectivity for acid-catalyzed dehydration of fructose to 5-HMF, under the conditions of DMSO as good media and heating for 2 h at 100°C. Moreover, the data of 93.7 % yield of 5-HMF and over six catalytic cycles were adequately witnessed that PW₁₂-ILs-C₄-HNS PHA is a cost-effective and environmentally benign catalyst (An S. et al., 2019). Similarly, glucose as a common monosaccharide, used as biomass feedstock to be upgraded into high value-added chemicals, has attracted extensive attention (Han et al., 2018; Li X. et al., 2018; Wei and Wu., 2018). The establishment of HPW-Nb₂O₅ calcined at 300°C to the selective conversion of glucose into 5-HMF was promptly documented in 2020 (Siqueira Mancilha Nogueira et al., 2020). The HPW-Nb₂O₅ was prepared through the combination of Nb₂O₅ (HY-340) as a support with H₃PW₁₂O₄₀ as an active phase and subsequently calcination at 300°C. In optimizing reaction conditions, reaction temperature, reaction media, substrate concentration, and catalyst amount as essential factors were evaluated via Taguchi’s L₁₆ experimental design. As a result, glucose at a concentration of 50g/L was allowed to be efficiently converted to 5-HMF (40.8% in yield), loading the HPW-Nb₂O₅ PHA (5%, w/v), with acetone–water media (1:1, v/v in ratio) at 160°C.

In addition to aforementioned heterogeneous solid acids, other solid acids are still able to exhibit excellent catalytic activity for biomass conversions. The 2Al/SBA-15 catalyst containing 9.70 wt% in Al with a high amount of medium acid sites was permitted to be synthesized by means of the atomic implantation method. Because of the incorporation of appropriate Al amount into the framework of SBA-15, solid acid could present medium and strong acidic sites. Moreover, the catalytic activity and selectivity for the 2Al/SBA-15 were testified in the degradation of cotton cellulose to 5-HMF under mild hydrothermal conditions, with the performance of 2Al/SBA-15 in cellulose transformation with 5-HMF yield and selectivity of 68.5 and 62.1%, which was better than that of 3Al/SBA-15 (12.56 wt% in Al) with 46.03 and 64.49% in yield and selectivity of 5-HMF. The cause might be attributed to the formation of Al₂O₃ particles covering the acid sites created after the second Al layer deposition (Pham et al., 2019).

D-glucose, a complete hydrolysate of cellulose, is usually used as biomass raw material to synthesize refining industrial substances in biomass conversions. Actually, the conversion of glucose into 5-HMF is a crucial intermediate step for cellulose being converted to 5-HMF. Considering the convenient separation of a catalyst from the mixture, a magnetic solid acid Fe₃O₄@SiO₂/PHA was prepared by phosphotungstic acid (Brønsted acid) supported on magnetic nanoparticles (Fe₃O₄) coated by SiO₂ (Figure 11) (Wang Y. et al., 2019). The magnetic solid acid in possession of good recovery by an external magnet could be on the run for four cycles in the glucose conversion to 5-HMF, without catalytic activity declining significantly.

In addition, heterogeneous catalysis asks for the catalyst and catalytic substrate to be in a varying phases and interacted on the external surface of the catalyst. Briefly, the catalyst is disabled to be dissolved in the solvent in the catalytic system. Therefore, inorganic metal salts are considered to be potential heterogeneous catalysts in the organic solvent. In 2019, an efficient strategy for one-pot conversion of microcrystalline cellulose into 5-HMF was introduced by the employment of non-/noble metal salts (e.g., FeCl₃, RuCl₃, VCl₃, TiCl₃, MoCl₃, and CrCl₃) in a biphasic system. The catalytic results demonstrated that RuCl₃ as a potent catalyst exhibits excellent performance in the selectivity and conversion, with 83.3 and 87.5% in yield and selectivity of 5-HMF, in the NaCl aqueous/butanol biphasic system. Notably, the decrystallization and cleavage of 1, 4-glycosidic bonds in cellulose were able to be promoted through the interaction between transition metal chloride and cellulose, and subsequently, the consecutive isomerization, dehydration, and elimination of glucose being transformed into 5-HMF occurred (Yan et al., 2019).

In the same year, a hydrothermal solid acid (Yb₆(BDC)₇(OH)₄(H₂O)₄) containing both bridging hydroxyls and metal-coordinated waters was introduced by Burnett et al. in glucose biomass conversions. The stable ytterbium metal–organic framework in possession of Brønsted and Lewis acid sites was considered to be a bifunctional catalyst, by which the conversion of glucose to 5-HMF was achieved successfully with 70% in selectivity in aqueous media (Burnett et al., 2019). With a similarity to the promotion of glucose into 5-HMF by Yb₆(BDC)₇(OH)₄(H₂O)₄ in water, transition metal–oxide nanosheet aggregates, such as HNbWO₆, HNb₃O₈, and HTiNbO₅, were prepared by means of successive exfoliation and aggregation of layered metal oxides, which were good for yielding 5-HMF under hydrothermal conditions. In the conversion performance for 5-HMF, the catalytic capacities of these aggregated nanosheets were much better than that of ion-exchange resins and H-form zeolites under the same conditions. Meanwhile, it found that HNbWO₆ nanosheets with an acid amount of 0.34 mmol/g exhibited higher selectivity for glucose conversion in the H₂O–toluene biphasic system than fructose conversion (Takagaki, 2019). As a developing catalytic pathway of biomass conversion, a catalytic fast pyrolysis of cellulose biomass yielding value-added platform chemical 5-HMF was initiated over zirconium–tin mixed metal oxides (ZrO₂–SnO₂). The ZrO₂–SnO₂ metal oxide with 15 wt% Zr loading was mainly in charge of the increase in 5-HMF yield and selectivity, in a catalyst-to-cellulose ratio of 2/1, at 350°C as a pyrolysis temperature. In testifying the catalytic scope of ZrO₂-SnO₂-15 as a catalyst, various saccharides, such as cellobiose, maltose, glucose, and mannose, used as biomass stocks for selective conversion to 5-HMF were, respectively, evaluated under the same conditions. The conversion results indicated that the effects for the 5-HMF yield of glucose and mannose are slightly different, with 5-HMF yields of 12.16 wt% and 11.07 wt%. Maltose and cellobiose are deemed to be a dimer of amylose and cellulose, including, respectively, α- and β-O-4 glycosidic bonds. In their selectively catalytic pyrolysis, the yield of selective product 5-HMF from maltose and cellobiose were, respectively, 14.63 wt% and 14.21 wt% over ZrO₂-SnO₂-15 catalysts, which may be more conductive for α- and β-O-4 glycosidic bonds (Li Y. et al., 2021).