1 Introduction
As we all know, biomass is abundant on the earth, and it is a kind of renewable energy. It can be used to prepare bulk chemicals, fine chemical products and polymer materials, and related research is of great significance. The U.S. Department of Energy has discovered that 5-hydroxymethylfurfural (HMF) is a cornerstone of biochemistry
[1]. As a biomass-derived platform compound, HMF can be prepared by the dehydration of biomass-derived carbohydrates, and can be used in the synthesis of polymers, the preparation of liquid fuel precursors, and the preparation of drugs. Because HMF has multiple functional groups, it can be used as an intermediate to convert into a variety of high-quality fuels and high value-added chemicals through selective oxidation or hydrogenation processes, which is considered to be a bridge between biomass resources and petroleum industry.
HMF can be selectively oxidized to form DFF (2,5-furandicarboxaldehyde), FFCA (5-formyl-2-furancarboxylic acid), FDCA (2,5-furandicarboxylic acid), HMFCA (5-hydroxymethyl-2-furancarboxylic acid) and other important fine platform compounds, which play an important role in the chemical industry. DFF can be used to synthesize some drugs, antifungal agents, chemical products and polymers containing furan rings
[2]. FDCA is one of the 12 important fine platform compounds of biomass, which can be used in the preparation of synthetic drugs, metal-organic framework materials and organic synthesis
[3]. FFCA is an important intermediate for the oxidation of DFF to FDCA, which has great application potential in the fields of medicine and furan-based polyester.
In recent years, more and more attention has been paid to the selective oxidation of HMF due to the wide application of its oxidation products, and many efficient catalytic systems have been developed. In this paper, the research results in the past five years are summarized, and the selective oxidation of HMF and the direct conversion of biomass to HMF derivatives via HMF as an intermediate are introduced. In addition, the research prospect of selective oxidation of HMF was prospected to provide a reference for further research.
2 HMF Overview
2.1 Properties of HMF
HMF is a yellowish solid at room temperature, a needle-like crystalline state at low temperature, and a reddish brown liquid at high temperature. It has very strong ultraviolet absorption because of its low melting point and large conjugated system in the molecule. It is less soluble in nonpolar solvents, but more soluble in water and polar solvents. In addition, the chemical properties of HMF are more active, for example, it is sensitive to strong acids, strong bases and strong oxidants, and it is also a heat-sensitive substance; The reaction such as polymerization or degradation is easy to occur; Formic acid and levulinic acid (LA), which are by-products of hydration reaction, can be effectively controlled by carrying out the reaction in an anhydrous system. The intermediate of HMF oxidation reaction will polymerize to produce insoluble Humin and soluble polymer. However, the formation of such by-products can not be effectively controlled in both aqueous and organic reaction systems.
2.2 Reaction Pathway of Catalytic Oxidation of HMF
HMF is derived from the degradation of biomass carbohydrates (cellulose, glucose, fructose, etc.). The degradation mechanism of how carbohydrates are converted into HMF has been discussed in detail by Li et al
[4]. Generally, cellulose is hydrolyzed to obtain glucose, which is isomerized to fructose, and then three molecules of water are removed from fructose to obtain HMF
[5,6].
Because HMF contains abundant functional groups, it can be selectively oxidized to produce high value-added chemicals such as DFF, HFCA, FFCA and FDCA, as shown in Figure 1
[7,8]. However, a small amount of by-products such as LA, formic acid and FA are inevitably produced in these reaction pathways. Therefore, the study of HMF directional oxidation is of great significance for the development and utilization of biomass resources.
3 Study on Selective Oxidation of HMF
In recent years, researchers have focused on the selective oxidation of HMF by thermocatalysis and photocatalysis. At present, more reports focus on the selective oxidation of HMF to DFF and FDCA via a thermocatalytic pathway, and FFCA is an intermediate for the oxidation of DFF to FDCA, which has considerable potential in medicine, but has not attracted wide attention.
3.1 Thermal-catalytic selective oxidation of HMF
3.1.1 Noble metal catalyst
The noble metal catalyst has good stability and high selectivity, and shows high catalytic effect on the selective oxidation of alcohol hydroxyl groups. According to the existing reports, researchers use molecular oxygen or green air as oxidant, and noble metal catalysts such as Ru, Pt, Pd, Au and Rh have been applied to catalyze HMF and show good catalytic activity.
(1) Ru-containing catalyst
Ru is an active metal with Lewis acid sites, which has been widely used in selective oxidation. Ru is typically disper on supports such as metal oxide and carbon-based materials. Pichler et al. prepared 5%Ru/ZrO
2 catalysts for the selective oxidation of HMF to FDCA
[9]. It was found that the conversion of HMF was almost complete and the yield of FDCA was up to 97% when the reaction was carried out at 120 ℃ for 16 H under 10 bar O
2 with H
2O as solvent. In addition, they believed that the ZrO
2(239 m
2·g
-1) with high surface area could be prepared by surface casting method, which was conducive to the formation of small Ru particles of 0. 8 ~ 1 nm, and the high dispersion of Ru particles on the support with large specific surface area was the key factor for the high reaction activity of the catalyst. It can be seen that the particle size of Ru plays an important role in the catalytic performance. In addition, the MgAlO support also has a high specific surface area, and the yield of FDCA can reach 99% at 160 ℃ for 4 H under 90 psi O
2 with (200 m
2·g
-1),2%Ru/MgAlO catalyst
[10]. However, due to the high acidity of the formed FDCA, the carrier is severely dissolved during the reaction and cannot be reused. In addition to the high catalytic performance of metal oxide supports, the noble metal catalysts supported on carbon-based materials also have satisfactory results. Liu et al. studied the efficient conversion of high concentrations of HMF to FDCA in a DMSO/H
2O mixed solvent with a yield of 93% using 5 wt% Ru/C as the catalyst and a weak base NaHCO
3,FDCA
[11]. Similarly, Zheng et al. studied a 5 wt% Ru/C catalyst with a yield of 97.3% Mg(OH)
2,FDCA with the addition of a strong base
[12]. They believe that the addition of alkaline substances is not only a neutralizer of FDCA, but also a promoter of the oxidation of HMF to FDCA. Therefore, alkaline substances play an important role in the complete conversion of HMF to FDCA.
More interestingly, Ru-containing catalysts have also found good application in the selective oxidation of HMF to DFF. Wang et al. Studied Fe
3O
4@SiO
2-NH
2-Ru(Ⅲ) magnetic catalyst, the loading of Ru was 1.4 wt%, the reaction time was 4 H at 120 ℃, the conversion of HMF was 99.3%, and the yield of DFF was 86.4%
[13]. When the reaction time was extended to 16 H, the conversion of HMF was 99. 7% and the yield of DFF was 86. 8%. The magnetic catalyst can be recycled for 6 times and still has good catalytic activity. In addition, zirconium phosphide (ZrP) is a very practical inorganic material, which has the advantages of high thermal mechanical stability and water resistance, and has been applied to the oxidation of alcohols to ketones and aldehydes
[14][15]. Yang et al combined ZrP with Ru to obtain ZrP-Ru catalyst, and the yield of DFF was 62% in p-chlorotoluene solvent at 130 ℃ for 12 H
[16]. In particular, Mishra et al. studied a pure MnCo
2O
4 catalyst with a yield of only 70.1% of DFF
[17]. By a simple coprecipitation method, the loading of 1. 8% Ru is beneficial to improve the yield of DFF, and the highest yield can reach 98. 3%. On this basis, Mishra et al. Also synthesized 4%Ru/MnCo
2O
4 catalyst, which was beneficial to the deep oxidation of HMF by increasing the loading properly, and the FDCA yield of 99.1% could be obtained
[18]. It can be seen that the Ru loading can control the reaction degree of HMF and the target product.
(2) Other catalysts containing Pt, Pd, Au, Rh, etc.
In addition to Ru-containing catalysts, Pt-based catalysts supported on different metal oxides also showed high catalytic activity for HMF conversion in moderately alkaline aqueous solution
[19]. In the absence of alkali, only 10% of FDCA was produced by Pt/ZrO
2 catalyst, because the pH value was too low and the carboxylic acid product precipitated on the catalyst, which hindered the reaction. The addition of weak base in the reaction system can effectively prevent the catalyst from being blocked. It is clear that alkali plays a key role in the oxidation of HMF. The yield of FDCA was 84% when HMF was oxidized by Pt/TiO
2 catalyst in the presence of weak base Na
2CO
3 at 40 bar air. If NaHCO
3,FDCA is added, the yield can reach 95%. If the preparation method is changed and the Pt/ZrO
2 catalyst is synthesized by the atomic layer deposition method, the uniform and highly dispersed Pt particles can be produced, and the FDCA yield of 97.3% can be obtained without adding additional alkali, which may be due to the fact that the uniform and highly dispersed Pt active sites are the main factors ensuring high selectivity
[20]. In addition, Pt supported on other supports also showed good catalytic activity. For example, Zhou et al studied the Pt/Fe
3O
4/rGO catalyst, and the yield of FDCA could reach 98% after 8 H of reaction under mild conditions
[21].
In addition, noble metals such as Pd and Au also showed good catalytic activity in the selective oxidation of HMF. For example, Rathod et al prepared Pd/CC catalyst by glucose carbonization method, introduced Pd metal on the surface of the catalyst, reacted at 140 ℃ for 30 H, showed high catalytic activity, and the yield of FDCA could reach 85%
[22]. Zhang et al. Used magnetic Pd nanocatalyst to selectively oxidize HMF, and the yield of FDCA reached 92. 9% at 100 ℃ for 6 H
[23]. Antonyraj et al. obtained a yield of 93.2% of FDCA in Na
2CO
3 aqueous solution by simply loading Au-Pd nanoparticles on a basic anion exchange resin (AER), with the nanoparticles dispersed on the surface of the resin or within the spheres
[24]. Kim et al. Prepared an Au-supported CeO
2 catalyst, which can oxidize HMF to 90% -95% FDCA. Its high catalytic performance is attributed to the existence of OH
- and Lewis acid sites on the CeO
2[25].
It can be seen that the above noble metal catalysts show good catalytic oxidation performance of HMF. From the economic point of view, Ru is cheap compared with noble metals Pt, Pd, Rh and Au, and it also shows excellent performance in the reaction. Different target products can be obtained by adding additional weak base (NaHCO3, Na2CO3) or strong base (NaOH). The product can also be adjusted by controlling the loading of Ru. Less loading is beneficial to the formation of DFF, while increasing the loading is beneficial to the deep oxidation of HMF to FDCA. Therefore, Ru catalyst is a very attractive choice and one of the most studied noble metals.
3.1.2 Non-noble metal catalyst
Compared with noble metal catalysts, non-noble metal catalysts are relatively cheap and have abundant valence States, so they show better catalytic activity in catalytic oxidation reactions. In addition, the d-band electronic structure of non-noble metals enriches the performance of catalysts. By doping trace elements, the d-band structure of metals can be changed, so that their catalytic activity can reach the catalytic level of noble metals. Therefore, in recent years, there are more and more studies on the catalytic oxidation of HMF by non-noble metal catalysts, among which Mn, V, Ti, Fe, Ce and other catalysts are more studied.
(1) Mn-containing catalyst
Mn-containing catalyst doped with trace element
MnO
x have become a promising class of oxidation catalysts due to their multiple oxidation States and complex electronic and spin structures. Attractive DFF selectivities can be achieved on MnO
x catalysts by protonation of oxygen defects or lattice oxygen. Therefore, the catalytic activity can be improved by doping trace elements to promote the formation of more defect sites. Yuan et al prepared the Ce-doped MnO
x catalyst by the soft template method. The conversion of HMF and the yield of DFF were 98.4% and 94.7%, respectively, at 100 ℃ for 10 H in N, N-dimethylamide solvent
[26]. Ce doping effectively improves the catalytic activity, which can be attributed to the following reasons: ① the electron cloud density around Mn in the catalyst decreases and the valence state increases; (2) that alkalinity of the catalyst is strong by introducing Ce; ③ The catalyst has more defects and more pores (Fig. 2). Similarly, Tong et al. Doped Cu in the MnO
2, the conversion of HMF was 86.0% and the selectivity of DFF was 96.1%
[27]. XPS characterization results show that Cu ions can be doped into the lattice of MnO
2, which is conducive to the exchange of Mn
4+ and Mn
3+ and the migration of lattice oxygen, thus showing better catalytic activity. In addition, N element is also an effective anionic dopant in metal oxides because of its low ionization energy and similar ion diameter to oxygen. Ke et al. studied N-doped manganese oxide (N-MnO
2) catalysts with nearly 100% selectivity to DFF
[28].
Notably, Hayashi et al. Suggested that the selective oxidation of HMF to FDCA is largely dependent on the crystal structure of the MnO
2[29]. Density functional theory (DFT) reveals that the hole formation energy of planar oxidation sites is higher than that of curved oxidation sites in α-MnO
2 and γ-MnO
2. The β-MnO
2 and the λ-MnO
2 are formed by planar and curved sites, respectively, and have lower hole formation energy, so they are a good oxidant. In addition, the β-MnO
2 has a high specific surface area, which can significantly improve the catalytic activity. If Pd atoms are introduced into the MnO
2, on the one hand, the introduction of Pd creates larger interface sites and induces good electron-metal carrier interaction, thus activating HMF
[30]; On the other hand, the doping of Pd into the MnO
2 produces more abundant oxygen holes and effectively activates oxygen. The synergistic effect of atomic Pd and the surrounding MnO
2 sites effectively promoted the oxidation of HMF, and the yield of FDCA reached 88% when O
2 was used as oxidant.
Multi-metal composite Mn-containing catalyst
In addition to improving the reaction performance of Mn catalysts by doping, other metal oxides can also be used to form composite catalysts with MnO
x in a certain proportion, such as Mn-Fe, Mn-Co and so on. Liu et al prepared Mn
6Fe
1O
x catalyst by changing the molar ratio of Mn/Fe, and the conversion of HMF and the selectivity of DFF could reach 97% and 98% respectively at 110 ℃ for 5 H
[31]. The possible reaction route for the catalyst to convert HMF to DFF in the presence of oxygen is shown in fig. 3. First, HMF is adsorbed on the Mn
4+ sites (Lewis acid) on the catalyst surface and shares a pair of lone electrons with the oxygen atom in its hydroxyl group. At the same time, the hydrogen atom in the hydroxyl group is fixed by the O
2- site near the Mn
4+, which leads to the cleavage of the O — H bond and the formation of the alcoholoxy intermediate. Then, in molecular oxygen, the Mn
3+ can be oxidized to Mn
4+, forming the Mn
4+-O
2- acid-base pair again. The introduction of α-Fe
2O
3 can significantly increase the number of active sites (Mn
4+-O
2-) and the activity of lattice oxygen, thus improving the oxidation performance of HMF. The study of Mn-Co bimetallic oxidation is more common. Through a simple thermal decomposition method, Wang et al prepared a Mn
0.5Co
0.5O binary complex oxide with CoMnO
3 new phase. Using Mn and Co nitrates as precursors, the Mn-Co catalyst was obtained by treating at 400 ℃ for 4 H. When the molar ratio of Mn/Co was 1 ∶ 1, the catalyst showed better catalytic performance. After 2 H reaction, the conversion of HMF was 42.6% and the selectivity of DFF was 98%
[32]. Through a series of characterization, it was proved that Mn-Co binary oxides could increase the proportion of Mn
4+/Mn
3+ and expose more lattice oxygen, thus improving the catalytic activity. Ding et al. Prepared spinel CoMn
2O
4 hollow spheres by solvothermal method. When the molar ratio of Co/Mn was 2:3, the conversion of HMF was 41.6%, and only DFF was observed in the product
[33]. The high selectivity of DFF may be due to the fact that the catalyst has enough exposed active sites to oxidize HMF to DFF, and the produced DFF can quickly leave the surface of the catalyst to avoid further oxidation. On the other hand, the appropriate Co/Mn molar ratio promotes the process of electron transfer, thus significantly improving the performance of the catalyst.
The Mn-Co bimetallic oxide can catalyze the oxidation of HMF to obtain the product DFF, and another product FDCA can be obtained by adjusting the molar ratio of the metals. Rao et al. Prepared a highly active and inexpensive Co-Mn mixed oxide by a simple solid-phase grinding method
[34]. It was found that the order of adding precursors in the preparation process played a key role in the yield of reaction. When the molar ratio of Co/Mn was 0.25,NaHCO
3/HMF=2,120℃, the reaction time was 5 H, and Co was added to the MnO
x as a promoter, the mixed oxide formed was Co-Mn-0.25, and the yield of FDCA could reach 95%. If Mn was added to the CoO
x as a promoter, the mixed oxide formed was Mn-Co-0.25, and the yield of FDCA was only 34%. HMF could not be oxidized by CoO
x alone. The yield of FDCA was 46.1% when the doped MnO
x was used as catalyst. It is clear that in the selective oxidation of HMF, MnO
x acts as the main catalyst and Co acts as the promoter. In addition, Gao et al. Prepared a Co
3O
4NPs modified Mn
xCo catalyst by Co-precipitation method. The molar ratio of Mn/Co was 0.2, and the selectivity of FDCA reached 88%, showing good catalytic activity
[35]. This is mainly attributed to the fact that Mn
4+, as Lewis acid sites, adsorb hydroxyl groups in HMF to produce oxygen vacancies, thus promoting the adsorption of O
2-, which dissociates at the vacancies to form highly active O
latt species. Secondly, the adsorbed hydrogen produced by the dehydrogenation of the hydroxyl group can react with the species to form H
2O, thereby accelerating the oxidation reaction. Meanwhile, the Co
3O
4NPs with small and uniform particle size can be used as Bronsted acid sites to promote the formation of COOH group intermediates and FDCA. It can be concluded that the Lewis and Bronsted acid sites on the catalyst surface and the redox between the Mn
4+ and the Mn
3+ enhance the oxidation mobility, which is the key to achieve high FDCA yield. Similarly, Gawade et al. Used spinel magnetic nanoparticles (MNPs) and tert-butyl hydroperoxide as oxidant to selectively oxidize HMF to FDCA, with a yield of 85%, showing good photocatalytic activity, mainly due to the change of manganese oxidation state
[36].
In addition, due to the oxidizability of CeO
2 oxides, the mixed oxides of Ce and other abundant metals in the crust are also well applied in the field of selective oxidation of HMF. Ventura et al. Studied that MgO-CeO
2 mixed oxides prepared by high-energy ball physical mixing method could selectively oxidize HMF to 98% DFF by adding 7 mL water with O
2 as oxidant
[37]. However, MgO will react with water to form Mg(OH)
2 in the reaction, which will change the basic properties of the catalyst even if it is not leached out, and reduce the activity of the catalyst after a period of reaction. However, the Mg(OH)
2 is dehydrated and converted into MgO by the recovery calcination, so that the initial activity is substantially recovered. In addition, they also studied the effects of metal oxides such as CuO and CeO
2 on the oxidation of HMF to FFCA. When 7 mL H
2O was added, the reaction temperature was 383 K, and CuO was used alone, the yield of FFCA was 40%
[38]. The yield of FFCA was 76.6% over pure CeO
2 catalyst. The yield of FFCA can reach 90% when the CuO-CeO
2 synthesized by high-energy ball physical mixing method is used as the catalyst. It should be noted that this catalyst also suffers from deactivation due to the deposition of organic matter on its surface rather than leaching, but the surface can be cleaned by calcining the organic matter, so that the activity can be restored to some extent. The above catalysts are obtained by physical mixing, and all of them have deactivation. They can only oxidize HMF to DFF or FFCA, but can not further oxidize to FDCA, which may be attributed to the fact that the alkalinity in the reaction system is not enough to oxidize it deeply. Han et al. Prepared MnO
x-CeO
2 mixed oxides by co-precipitation method. When the molar ratio of Mn/Ce was 6:1, the yield of FDCA was 91%, and the catalyst had little loss of activity after being reused for five times
[39]. Increasing the proportion of Mn can effectively promote the further oxidation, and more europium ions on the surface of the catalyst as active sites play a key role in the oxidation of HMF. The yield of the MnO
x-CeO
2 catalyst prepared by Hayashi et al. Can reach 91% without adding basic NaHCO
3,FDCA
[40]. From the above analysis, it is found that the reaction degree can be effectively controlled by adding an appropriate amount of alkaline substance. More interestingly, when the three oxides are in the right proportion, they also show good activity for the oxidation of HMF without the addition of basic substances. Nocito et al. Studied MgO-MnO
2-CeO
2 mixed oxides, using O
2 as oxidant and H
2O as solvent, the yield of DFF could reach 94.9% after 6 H reaction
[41]. The yield of DFF reached 95. 2% after 10 H reaction. It is worth noting that no deep oxidation of HMF was observed even after a longer reaction time, and the increased time only increased the selectivity of DFF. In addition, they also studied the catalytic activity of single oxide, and obtained 88.7% FFCA by using CeO
2 alone. Using MgO alone is not conducive to the selective oxidation of HMF, but will lead to the decomposition of HMF into formic acid and humic acid. MgO is mainly used to mix with other metal oxides to adjust the basicity of the catalyst and effectively control the reaction route and selectivity. The MnO
2 has strong oxidizability, and 75.5% FDCA, 23.5% FFCA and a small amount of LA can be obtained. Similarly, Ventura et al. studied a CuO·MnO
2·CeO
2 catalyst with a yield of 71% of FDCA
[42]. The repeatability of the catalyst is not very good, which is attributed to the deactivation of the catalyst due to the reduction of Cu
2+ to Cu during the reaction, and the catalyst needs to be calcined at 823 K to recover its activity. Therefore, compared with MgO, CuO is a better oxidant, which can effectively promote the further oxidation of HMF.
(2) Other catalysts containing V, Ti, Cu, etc.
In addition to the application of Mn-containing catalysts in the oxidation of HMF, V-containing catalysts are also effective in the oxidation of alcohols, which has attracted the attention of some researchers. Zhao et al. Synthesized vanadium dioxide embedded mesoporous carbon spheres (V-CS) by hydrothermal method. After optimizing the conditions, HMF was almost completely converted, and the yield of DFF could reach 99%
[43]. As the catalyst has a micro/mesoporous structure, the specific surface area of the catalyst is large, and the VO
2 serving as an active catalytic center is highly dispersed in a mesoporous carbon matrix, so that reactants and products are allowed to freely move in the mesopores, and the contact between the reactants and the active sites is enhanced, thereby being beneficial to improving the catalytic activity of the catalyst. Lai et al. Prepared vanadium phosphate (VPO) heterogeneous catalysts with different V/P molar ratios to selectively oxidize HMF to DFF in liquid phase
[44]. When V/P molar ratio is 0.25, the yield of DFF can reach 83.6%. Yan et al. Synthesized nano-array vanadium oxide, which catalyzed the selective oxidation of HMF to DFF, with the conversion and selectivity of 93.7% and 95.4%, respectively
[45]. The V-based catalysts showed good catalytic performance for the oxidation of HMF, but the structure and reaction of the catalysts became more complex because of the rich coordination mode of V
[46]. There are two kinds of vanadate structures: monovanadate and polyvanadate, and their crystal structures are related to their valence States. However, the crystal structure and the type of the active site have not been well studied.
As we all know, nano-TiO
2 has the advantages of low cost, easy preparation, convenient separation and good catalytic performance, and is widely used in photocatalytic degradation of organic pollutants, alcohol oxidation and other fields. Gupta et al. studied the sulfonated carbon catalyst doped with Ti (Ⅳ), which showed good catalytic performance, and the DFF yield of 88% could be obtained at 70 ℃, which was mainly attributed to the oxidation state of + 4 titanium and the strong Bronsted acid site of -SO
3H
[47].
Interestingly, Cu-containing catalysts have also been well used in the conversion of HMF. Wei et al. Studied the copper-acetonitrile complex and obtained 97% yield of DFF at 80 ℃ for 18 H
[48]. Among them, the formation of Cu(CH
3CN)
4+ complexes in aqueous solution plays an important role in promoting the oxidation of HMF through the redox cycle of Cu
2+ and Cu
+.
In general, non-noble metals have been well applied to selective oxidation of HMF due to their unique electronic structures. At present, non-noble metal catalysts containing V and Ti have also been studied, which have appropriate specific surface area and are conducive to the formation of DFF. In particular, Mn-based catalysts have great potential for the selective oxidation of HMF. By doping Cs, N, Cu and other elements, the surface area and defect sites can be properly increased, thus improving the catalytic activity of manganese oxide. If a proper preparation method is selected, or a composite catalyst is formed with other metals in a certain proportion, different metal elements have a synergistic effect and can effectively promote the electron transfer, and simultaneously, more defect sites appear on the surface of the catalyst and more lattice oxygen is exposed, so that the catalytic activity is effectively improved. So far, Mn-Fe and Mo-Co have been studied more. In addition, it is worth noting that CeO2 is a very attractive metal oxide, which is beneficial to the oxidation of HMF to DFF and FDCA when it is reasonably combined with MnO2, CuO and MgO and added with appropriate alkaline substances. Therefore, CeO2 is one of the most studied metal oxides at present.
3.2 Photocatalytic selective oxidation of HMF
At present, the selective oxidation of HMF mostly uses the thermal catalytic process. However, this process requires high reaction temperature, high energy consumption, high O2 pressure and high alkalinity. In recent years, more and more researchers have paid attention to the photocatalytic process, which is driven by light, mild reaction conditions, energy saving and green environmental protection, and has great potential to solve the process problems of thermal catalysis. However, there are few studies on the photocatalytic oxidation of alcohols, especially the conversion of HMF, which mainly has the problems of low efficiency and low selectivity.
Among the few catalysts for photocatalytic oxidation of HMF, the most studied one is the g-C
3N
4 catalyst. The g-C
3N
4 has a moderate band gap (E
g=2.7 eV) and a good response to visible light. Moreover, it has the advantages of rich surface properties, excellent stability and adjustable electronic structure, and is a potential photocatalytic candidate material, which is widely used in photocatalytic degradation, hydrogen production, oxidation and other fields. Compared with the most widely used TiO
2, the conduction band potential of g-C
3N
4 is more negative (-1.3 eV), which has a higher ability to activate O
2 and can more easily reduce O
2 to superoxide radical ·O
2-. At the same time, the higher valence band position (1. 4 eV) of g-C
3N
4 makes it difficult to generate non-selective hydroxyl radical · OH in liquid phase, so it has potential application in the photocatalytic oxidation of HMF. Wu et al. prepared a g-C
3N
4 by simply calcining melamine, which can photocatalytically convert HMF to DFF
[49]. The addition of water in the preparation process is beneficial to the increase of the specific surface area and the formation of a pore structure, and inhibits the recombination of photogenerated electrons and holes, thereby improving the photocatalytic performance. Under visible light irradiation, the yield of DFF was 26. 7% and the selectivity was 85. 6% after 6 H reaction with O
2. They also explained the mechanism of photocatalytic oxidation of HMF to DFF by g-C
3N
4.The photogenerated electrons produced by the g-C
3N
4 in the conduction band can reduce the O
2 to ·O
2-, and the formed ·O
2- remain on the surface of the g-C
3N
4 and combine with it. At the same time, in the oxidation process, the basic characteristics of g-C
3N
4 materials also help HMF release h
+ to form alkoxide anions, which then react with holes through electron transfer to form free radicals, which recombine with superoxide groups to obtain corresponding aldehydes and release H
2O
2, which can further oxidize HMF to FDCA.
However, the photogenerated electrons and holes of the g-C
3N
4 are easy to recombine, and the photocatalytic performance can be effectively improved by modifying the photogenerated electrons and holes, such as combining with metal oxides to form a composite catalyst. The WO
3 has low cost, is a transition metal oxide which is stable and low in price, and has high solar energy absorption efficiency as an n-type semiconductor photocatalyst. Zhang et al. Prepared the WO
3/g-C
3N
4 by a simple calcination method, and the formed composite structure is conducive to the rapid separation and transfer of photogenerated electrons and holes between the catalyst interfaces. Compared with pure g-C
3N
4, it has better photocatalytic performance
[50]. The conversion of HMF and the selectivity of DFF were 27. 4% and 87. 2%, respectively, when the catalyst was 10.5%WO
3/g-C
3N
4, the reaction time was 6 H under visible light irradiation with oxygen, and the solvent was a mixture of toluene trifluoride and acetonitrile. In addition, the free radical trapping experiment showed that the main species of selective oxidation of HMF to DFF were not hydroxyl radicals, but superoxide radicals and holes. In the reaction process, the reaction conditions (such as solvent, pH value, etc.) Have a great influence on the main products. Zhu et al. Synthesized g-C
3N
4/NaNbO
3 photocatalyst. In aqueous solution, the main oxidation product of weak base NaHCO
3,HMF was FFCA, the conversion rate of HMF was 35.8%, and the selectivity of FFCA could reach 87.4%
[51]. Xu et al. Used CoPz/g-C
3N
4 catalyst to selectively oxidize HMF to FDCA with O
2 as oxidant under simulated sunlight irradiation, which had high photocatalytic activity
[52]. The yield of FDCA was 96.1% at pH = 9.18 and normal temperature and pressure. But only DFF was produce under that acidic condition of pH 4.01. They believe that g-C
3N
4 itself is not suitable as a catalyst for selective oxidation, because under experimental conditions, g-C
3N
4 produces hydroxyl radical active species, which can trigger the direct oxidation of HMF to CO
2 and H
2O. On the other hand, CoPz activates the O
2 to produce singlet oxygen
1O
2, which makes the oxidation of HMF to FDCA more controllable, but the yield is lower. If CoPz is dispersed on the g-C
3N
4 carrier, the interaction between CoPz and the g-C
3N
4 carrier can not only improve the active sites of CoPz, but also inhibit the generation of hydroxyl radicals in the g-C
3N
4, thereby significantly improving the photocatalytic activity, and simultaneously reducing the aggregation of hydrophobic CoPzmolecules in an aqueous solution, which is beneficial to the recycling of the catalyst.
In addition to the hafnium-based catalysts, some other catalytic systems have also shown good photocatalytic activity for the oxidation of HMF. Bismuth-based catalysts have also been applied in the field of selective oxidation due to their stable crystal structure, excellent optical and electrical properties. Among them, Bi
2MoO
6 is an active member of bismuth-based semiconductors with appropriate band gap and layered structure. However, the photocatalytic performance of pure Bi
2MoO
6 is limited, which may be due to the serious recombination of photoinduced charges and the insufficient utilization of visible light. Xue et al. Studied Fe
3+/Bi
2MoO
6 nanosheets, the highest conversion of HMF under visible light was 32.6%, and the selectivity of DFF could reach 95.3%
[53]. The ·O
2- of the superoxide radical formed on the conduction band and the h
+ of the valence band can both participate in the reaction to enable the HMF to generate the contact of a close interface between the DFF,Fe
3+ cluster and the Bi
2MoO
6 nanosheet, which is beneficial to expanding the absorption range of visible light and separating photogenerated carriers, and Fe ions undergo a process of oxidation and reduction through a Fe
3+/Fe
2+ in the reaction process. Similarly, in the Nb
2O
5-800 catalyst for photocatalytic oxidation of HMF reported by Zhang et al., Nb also underwent a process of valence change during the reaction, but the reaction mechanism on the Nb
2O
5 catalyst was different from it. HMF was adsorbed on the surface of the Nb
2O
5 to form an alcohol oxide, which reduced the band gap of the Nb
2O
5 and enhanced its absorption of visible light. DFF was produced with high selectivity through the oxidation and reduction of Nb
5+/Nb
4+[54].
Noble metals can not only convert HMF by thermal catalysis, but also effectively convert HMF to DFF in photocatalytic reaction. Ma et al. Reported that Au-Ru nanoparticles supported on reduced graphene oxide showed good photocatalytic performance
[55]. In the absence of any base, HMF can be effectively oxidized to DFF by using Au
0.5Ru
2.5/rGO catalyst in toluene solvent under the irradiation of 0.5 W/cm
2 when the solution is heated to 80 deg C, and the yield can be as high as 91%. Compared with monometallic Au/rGO or Ru/rGO, the Au-Ru/rGO catalyst showed better ability to adsorb O
2- and react with them to generate superoxide radicals. The interaction between the reactant and the catalyst can be enhanced by the transfer of graphene photoexcited electrons from Ru to Au nanoparticles, thereby reducing the activation energy of the reaction. Interestingly, heating is necessary in the reaction, and if only light is used, the reaction performance is low, which shows that this is a light-heat synergistic catalytic process, but they have not done in-depth research.
Manganese oxide, a metal oxide, has also been used in the photocatalytic conversion of HMF. Giannakoudakis et al. synthesized new manganese oxide nanorods on the basis of ultra-low controlled precipitation
[56]. The key to the preparation of the catalyst lies in the control of the morphology, and the Mn
2+ is induced to oxidize to form a Mn
4+ in the dark by dropping the KMnO
4 at an ultra-slow speed, thereby facilitating the nucleation of MnO
2 nanoparticles, and finally aggregating to form nanorods. Under the optimized conditions, the conversion of HMF was 89% and the selectivity of DFF was close to 94% under UV irradiation at 29 ℃, and the selectivity of DFF was close to 100% under UV irradiation at 39 ℃. The conversion of HMF was 83% and the selectivity of DFF was 93% at 29 ℃ with visible light as the light source. Therefore, compared with visible light, ultraviolet light has a more obvious positive effect. The slight increase of temperature caused by light irradiation can further improve the catalytic efficiency, but the effect of light irradiation itself is greater than that of temperature increase, and the latter has a synergistic effect on irradiation by increasing the reaction rate.
Some researchers also found that in the process of photocatalytic water decomposition, HMF can replace the commonly used electron capture agents such as methanol or triethanolamine, and its oxidation product DFF can be obtained at the same time of hydrogen production. Meng et al. Prepared the NiS/Zn
3In
2S
6 catalyst by hydrothermal method. This bifunctional catalyst can promote the separation of photogenerated electrons and holes, and more importantly, both photogenerated electrons and holes can participate in the reaction to generate H
2 and DFF respectively. Under visible light, the selectivity of DFF can reach 94.1%, but its yield is low, only 129 umol/G · H (< 1%)
[57]. The Zn
0.5Cd
0.5S-P photocatalyst reported by Ye et al can be used for the oxidation of HMF and the decomposition of water at the same time. The conversion of HMF can reach 40% and the selectivity of DFF is 65%
[58]. Battula et al. Found that Pt deposited on porous g-C
3N
4 could also be used for both hydrogen production and HMF oxidation, with a DFF yield of 13.8%, but there was no reaction in the pure g-C
3N
4 catalyst, and Pt was necessary in this process
[59]. Therefore, from the point of view of sustainable development and green environmental protection, the reaction of coupling photocatalytic hydrogen production and biomass conversion is of great significance both in theoretical research and practical production.
In general, there are few reports on the photocatalytic oxidation of HMF to DFF, FFCA and FDCA using O2 as oxidant under mild conditions. It is worth noting that the g-C3N4 shows good application potential due to its suitable energy band structure.The selective oxidation products of HMF can be effectively adjusted by changing the morphology or combining with metal oxides to form composite catalysts, as well as adjusting the reaction conditions.
4 Biomass Oxidation with HMF as Intermediate
A sustainable future for the chemical industry requires feedstocks based on renewable energy, rather than the continuous consumption of non-renewable feedstocks, but the conversion of nature's five and six carbon compounds is still a challenge
[60]. It is worth noting that the dehydration of biomass to form HMF mostly requires DMSO solvent, and at least 100 ℃ is needed to produce HMF. In DMSO solvent, the yield of HMF is higher. However, DMSO is a solvent with a high boiling point, which means that high energy is required to separate the catalyst, and it is not advantageous for industrial production. If water is used as the solvent, the HMF yield is obviously not high. In addition, the heterogeneous catalyst has adjustable specific surface area and acidic sites, and the acidity can be adjusted by the ratio of Lewis acid and Bronsted acid, thereby being beneficial to the conversion of biomass to HMF. At present, zirconium phosphate, metal oxides (TiO
2, SnO
2, WO
3, ZrO
2), OMC, cerium phosphate, S-AlMo and other catalysts have been applied to the research of fructose dehydration
[61][62⇓~64][65][66][67]. Catalysts such as metal oxides, cation exchange resins, zeolites, metal-organic framework compounds, HNbWO
6, PW, SCHOP, SnPCP@MnO
2-PDA, tungstate catalysts and the like are applied to catalyze glucose dehydration
[68][69][70⇓~72][73][74][75][76][77][78]; Catalysts such as niobium catalysts and Bronsted-Lewis acidic ionic liquids are used to dehydrate cellulose
[79,80][81]. However, these catalytic processes still have many problems, such as (1) the reaction temperature is still relatively high, at least more than 100 ℃; (2) that reaction time is long; (3) The yield of HMF is not high.
In view of the above problems, because the separation of HMF requires high energy and its price is much higher than that of biomass, which is not in line with the current requirements of sustainable development, it is undoubtedly a more green and economically feasible method to convert biomass directly into HMF derivatives efficiently and economically. Among them, glucose and fructose, two abundant hexose molecules, can be used as potential raw materials to convert them into HMF derivatives (DFF, FFCA, FDCA) by selecting appropriate catalysts, which have great application prospects and reduce the burden of energy shortage in the future.
The conversion of fructose to DFF is usually carried out by dehydration to form HMF, which is an acid catalyzed reaction process, while the conversion of HMF to DFF is an oxidation reaction process. Therefore, in order to use fructose to produce HMF derivatives in one step, a bifunctional catalyst with the combination of acid catalysis and oxidation is needed. At present, molybdenum-based, iron-based, graphene oxide, vanadium-based, Amberlyst-15, Pt/C, Co-Al hydrotalcite and other catalysts show good catalytic activity for the conversion of fructose to DFF and FDCA
[82⇓⇓⇓⇓~87][88,89][90,91][92,93][94,95][96][97]. For example, the presence of Bronsted acid sites in the catalyst facilitates the dehydration of fructose to form HMF, and the presence of Mo oxidation species facilitates the selective oxidation of HMF to form DFF. Zhou et al used glucose and phosphomolybdic acid as precursors of carbon and molybdenum oxide to prepare MoO
x/CS catalysts by hydrothermal carbonization
[82]. The yield of DFF was 78% at 160 ℃ for 2 H in DMSO. In the reaction, the effect of MoO
x on the dehydration of fructose can be ignored, and its main role is the selective oxidation of HMF. They also compared it with Al
2O
3/MoO
x catalyst prepared by traditional reduction method, and found that HMF yield was higher on this catalyst, but no DFF was formed, which confirmed that the synergistic effect between the acid sites on the carbon surface and the oxidation sites of molybdenum oxide was needed to convert fructose into DFF. Similarly, Zhao et al. First prepared ZS by sulfonating ZrO
2 with sulfuric acid, and then loaded MoO
3 on ZS to synthesize MZS bifunctional catalyst. The yield of DFF reached 74% after 10 H reaction at 150 ℃, and ZS had certain acidity, which was beneficial to the dehydration of fructose
[83].
Fe
3O
4 nanoparticles are attractive catalyst supports for selective oxidation of biomass due to their low cost, ease of synthesis and functionalization, high surface area, and ease of separation by magnetic materials. Mittal et al. Prepared a magnetic bifunctional WO
3HO-VO-SiO
2@Fe
3O
4 nanocatalyst to directly oxidize fructose to DFF
[88]. Under the optimized conditions, fructose was directly dehydrated at 80 ℃ for 1 H to form 82% HMF. The activated vanadium oxide species oxidized the in-situ HMF to form 71% DFF after the addition of co-oxidant H
2O
2,60℃ for 15 H. In addition, the catalyst has good stability and basically has no loss of activity after being recycled for five times, which is mainly due to the good magnetism of the Fe
3O
4 and is beneficial to recycling. Fang et al. Used a simple pyrolysis method to prepare the catalyst, using an iron-based metal-organic framework (MOF) as a template and sulfur powder as a dopant
[89]. The results showed that the yield of DFF was more than 99% under the conditions of 2 mL ethanol, 100 ℃, 1 bar N
2 for 2 H, and then 1 bar O
2 for 2 H. It was found that the exposed crystal faces of the Fe
3O
4(111) were favorable for the oxidation of HMF, and the unoxidized elemental sulfur in the carbonaceous matrix reduced the oxidation degree of the system, resulting in better selectivity for DFF.
Interestingly, in the selective oxidation of HMF to FDCA, the addition of certain alkaline substances is beneficial to improve the selectivity, which is also well applied in the conversion of fructose to FDCA. Chen et al. Studied the conversion of fructose to FDCA in a one-pot two-step method. In DMSO solvent, Amberlyst-15 was used to catalyze the dehydration of fructose, and the reaction was carried out at 120 ℃ for 1 H, the yield of HMF was 97.1%
[98]. HMF was then directly oxidized in a solvent with a (K
2CO
3)H
2O/DMSO of 3:1 to form 91% FDCA. In this catalytic system, the ratio of H
2O/DMSO, the type of base, the ratio of base to HMF and the reaction temperature are all important factors affecting the reaction rate and selectivity. Therefore, it can be inferred that the synergistic effect of alkaline aqueous solution and DMSO plays a key role in improving the yield of FDCA. It is well known that high temperatures are required for the dehydration of fructose in DMSO solvents. It is worth noting that De et al. Speculate that DMSO forms a strong acid during thermal cracking, which may act as a catalyst for the reaction, and that the addition of bromide can increase the selectivity of the catalyst
[99]. Therefore, if DMSO is used as the solvent, this catalytic process may be involved, especially when halides are present, which should be taken into account.
Compared with fructose, the conversion of glucose is relatively complex, which requires several steps, such as isomerization of Lewis acid sites, dehydration of Bronsted acid sites and selective oxidation of oxygen sites. The first step, the isomerization of glucose to fructose, usually requires a metal catalyst to convert it to fructose, such as aluminum, tin, vanadium or chromium-based compounds. In DMSO/DG/water, the conversion of glucose was 68% and the selectivity of fructose was 72% when NaAlO
2 was used as catalyst
[100]. The selectivity of fructose was 70% with Snβ catalyst in aqueous solution
[101]. Zhang et al. Studied the one-pot two-step method to convert glucose into DFF,AlCl
3-6H
2O/NaBr with vanadium compound as catalyst and O
2 as oxidant. The yield of DFF was 35% ~ 48%
[102]. Xiang et al. studied the one-pot two-step conversion of glucose to DFF with CrCl
3-6H
2O/NaBr/NaVO
3-2H
2O catalyst. The yield of DFF was 55% in DMF solvent
[103]. Takagaki et al. studied the conversion of glucose to DFF in a one-pot three-step process, first isomerizing glucose to fructose with hydrotalcite, then dehydrating fructose to HMF with Amberlyst-15, and finally selectively oxidizing HMF to DFF with Ru/HT catalyst
[104]. The yield of DFF was 25% in DMF solvent.
Generally speaking, DMSO and DMF are used as solvents and O2 as oxidant to convert biomass (fructose and glucose) into DFF and FDCA by one-pot multi-step method. There are relatively more reports on the directional oxidation of fructose to DFF with bifunctional catalysts, but few reports on the oxidation to FDCA. Because the conversion mechanism of glucose is more complex and difficult than that of fructose, it has not received more attention from researchers. Fortunately, Mo oxide species have good oxidizing properties and are usually combined with Bronsted acid catalysts to form bifunctional catalysts, which are well used in the conversion of fructose to DFF.
5 Conclusion and prospect
HMF is one of the raw materials for the preparation of fine platform compounds from biomass.It is a feasible strategy to develop an economical and efficient catalyst for the oxidation of HMF to achieve high selectivity and high yield of various derivatives. The catalyst systems for HMF oxidation mainly include noble metal catalysts and non-noble metal catalysts. Hydroxyl group in HMF can be selectively oxidized to aldehyde group to obtain the product DFF, in which non-noble metal-based catalysts such as Ru, Mn, V, Ce and mixed oxide catalysts have been widely studied. The catalysts for the deep oxidation of HMF to FDCA are mainly based on noble metals such as Pt, Pd, Au, Ru and non-noble metals such as Mn. In addition, the controllable synthesis of multi-metal oxides with certain acidity and alkalinity, such as MgO, CeO2, CuO, MnO2 and other metal composite oxides, is conducive to the selective oxidation of HMF. The CeO2 is mainly used for the oxidation of HMF to form FFCA,MnO2, and the main product formed on the CuO catalyst is FDCA. The combination of MgO and the above metal oxides is beneficial to the synthesis of DFF by adjusting the pH value. On the other hand, photocatalysis, which is driven by light, has great potential to solve the problems of thermal catalysis process, and is a green and feasible catalytic technology. However, there are few studies on selective oxidation of HMF through photocatalysis. Among them, the selective oxidation of HMF to DFF, FFCA and FDCA by g-C3N4 based photocatalysts has shown good research potential. Although the yield of the target product is not high enough to reach the level of industrial application, it is necessary to modify it, explore other kinds of oxidation photocatalysts, and combine the characteristics of solid acid catalysts.It has a good response to visible light or natural light, and the efficient and directional oxidation of HMF through photocatalytic or photothermal synergistic catalysis is a direction worthy of further study by researchers in the future. Finally, bifunctional catalysts with suitable acid sites and oxygen sites are mostly used for biomass conversion, such as molybdenum, iron, graphene oxide, Amberlyst-15 and transition metals for direct conversion of fructose to DFF.
In recent years, the research on HMF oxidation has attracted much attention, but there are still many problems. The author believes that it is worth discussing the challenges of selective oxidation of HMF, which mainly include the following points: ① Weak and strong bases need to be introduced into the reaction system. Although the introduction of alkaline substances is conducive to the reaction, it is not conducive to the separation of the target product. At the same time, such more severe reaction conditions increase the equipment requirements in industrial production, that is, increase the equipment cost. Thus, obtaining the desired product in high yield under mild conditions remains a challenge. (2) Although noble metal catalysts such as Pt, Pd, Au and Ru show good catalytic activity in the reaction, they are expensive and inevitably agglomerate during the reaction, resulting in inactivation. Therefore, the non-noble metal catalysts with lower price are more worthy of further study. However, although the cost of non-noble metals is cheaper than that of noble metals, their catalytic performance is not as good as that of noble metals, and there are problems of leaching of active components and growth of metal nanoparticles. (3) It is very difficult to get a higher yield only by illumination, and a process of light-heat synergistic catalysis is needed, but there is no in-depth study in this area. (4) For the direct conversion of biomass to HMF, most of them need to be in organic solvents, and the boiling point of organic solvents is relatively high, which is not conducive to the separation of catalysts in the later stage. Therefore, there are few studies on the direct conversion of biomass to HMF derivatives.
Based on the current problems, the following aspects should be paid attention to in the future research: ① controllable design and synthesis of catalysts, which can efficiently convert HMF into different oxidation products under mild reaction conditions without adding additional base; (2) continuing to study non-noble metals, avoiding the leaching of active components, and synthesizing a stable catalyst; (3) selective oxidation of HMF by green photocatalysis, development of efficient visible-light catalysts, and in-depth study of the reaction mechanism to further guide the design of catalysts; (4) Design bifunctional (multifunctional) catalysts to catalyze the one-step conversion of biomass to the downstream oxidation products of HMF, and explore efficient and convenient technologies for separating organic solvents and products.
Natural Science Foundation of Zhejiang Province(LY16B030002)
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