1 Introduction
The demand for fuels and chemicals in modern society is increasing, and carbon dioxide emissions continue to rise. All mankind is facing the severe challenge of environmental problems caused by global warming, so people begin to devote themselves to exploring and utilizing various forms of renewable energy. Biomass is a renewable non-fossil carbon energy source, which is considered to be an ideal substitute for traditional fossil resources because of its environmental protection and abundant reserves. Using lignocellulose to produce biofuels and biochemicals is a promising research because of its low cost, which can significantly reduce carbon dioxide emissions and achieve effective recycling of biomass waste
[1⇓~3]. Lignocellulose can be hydrolyzed into C5 and C6 monosaccharides, which can be used to synthesize various biomass platform compounds such as furfural, 5-hydroxymethylfurfural, lactic acid and levulinic acid. These functionalized platform compounds can be further catalyzed to fuel additives and value-added chemicals.
furfural, an important biomass platform molecule, was first isolated by D Döbereiner et al. In 1832 and named furfural by Fownes et al. In 1845
[4]. Furfural can be obtained by acid hydrolysis of hemicellulose from biomass resources such as corn stalks, wheat straw or bagasse. Xylan in hemicellulose is first depolymerized to pentose (xylose), which is dehydrated by acid catalysis to produce furfural
[5,6]. The world produces about 500,000 tons of furfural annually, of which the main production capacity is located in China. The current market for furfural is worth approximately $1.8 billion and is growing at 5% per year. Figure 1 shows the number of papers related to furfural and furoic acid published in recent years.
In order to improve the added value of products, more and more enterprises focus on the development of downstream products of furfural. About 70% of furfural is used to produce furfuryl alcohol (FA), which is an important raw material in the synthesis of furan resin. Furfuryl resin is widely used in thermosetting polymer-based composites, cement, adhesives, coatings, casting resins, epoxy resins, diluents, modifiers of phenolic and urea-formaldehyde resins, etc. Furfural can be converted into a variety of C4 and C5 molecules through selective hydrogenation and oxidation, such as 2-methylfuran, levulinic acid, furoic acid, γ-valerolactone, cyclopentanone and 1,4-pentanediol (Figure 2)
[7,8]. Among them, furoic acid, as an important downstream product of furfural, has important applications in food manufacturing, material preparation, optical technology, drug synthesis, and can be used to synthesize preservatives, plasticizers, thermosetting resins, spices, and a variety of drugs (Figure 3)
[9].
Hydrogenation of furoic acid can be used to synthesize tetrahydrofurancarboxylic acid, an important drug intermediate, and then to prepare a variety of drugs that have been used clinically. The furoate can be obtained by the esterification reaction of furoic acid, and can be used as a modifier and a flavoring agent in the food, beverage and cosmetic industries. The derivatives of furoic acid, furoyl chloride and furoamide, can be used to prepare a variety of fine chemicals and bio-based polymers. At the same time, furoic acid can also be used to synthesize a new bio-based material monomer 2,5-furandicarboxylic acid (FDCA) by immobilized carbon dioxide, and then to synthesize a bio-based furan material poly (ethylene 2,5-furandicarboxylate) (PEF)
[10]. PEF is considered to be an improved packaging material with excellent water, oxygen and carbon dioxide barrier properties, which can prolong food storage time and reduce processing temperature. The crude product of furoic acid can be sublimated to obtain high-purity furoic acid crystals, which can be directly used as food preservatives and bactericides. At the same time, due to the good structural characteristics of furoic acid crystal, it can be used as nonlinear optical materials (NLO) in optical equipment
[11].
In this paper, the research progress in the preparation of furoic acid by catalytic oxidation of furfural in recent years was reviewed, and the homogeneous/heterogeneous catalytic systems were classified and discussed.The application prospect of heterogeneous catalyst in the industrial preparation of furoic acid was highlighted, and the direction worthy of attention and research in the preparation of furoic acid was prospected.
2 Catalytic oxidation system
There are some problems in furoic acid industry in China, such as small production scale, small quantity and relatively backward production process, so it is necessary to develop cheap and environmentally friendly catalytic oxidation methods to meet the urgent production needs. The traditional preparation methods of furoic acid mainly include equivalent oxidant oxidation, Cannizarro disproportionation and catalytic oxidation. Among them, the use of equivalent oxidants such as potassium permanganate, potassium dichromate, sodium hypochlorite and sodium peroxide has some limitations, such as more three wastes, low selectivity, low utilization rate of oxidant atoms, and the safety of oxidant itself. Cannizarro disproportionation is a simple reaction, which can convert two molecules of furfural into one molecule of furoic acid and one molecule of furfuryl alcohol under alkaline conditions. However, the separation of equivalent furfuryl alcohol produced in the reaction is cumbersome, and its structure is unstable, so the separation cost is relatively high
[12]. At present, the research of furoic acid is mainly focused on catalytic oxidation. Catalytic system mainly includes homogeneous catalytic system, heterogeneous catalytic system and biological oxidation system. The reaction conditions of the three types of catalytic systems will be described below.
2.1 Homogeneous oxidation system
According to the different oxidants required in the reaction process, the reaction of furfural to furoic acid catalyzed by homogeneous catalytic system is described from oxygen, air, t-BuOOH and other oxidants as oxygen sources, and the relevant results are summarized in Table 1.
2.1.1 Oxygen as oxidant
In the oxidation of furfural, the choice of oxidant is crucial. Air or oxygen is used as a green oxidant, which is cheap and environmentally friendly, and water is the only by-product in the reaction.
Li et al. Tried the combination of CuCl
2, CuCl, CuBr
2 and various ligands (bipyridine, phosphine ligand, IMes, IPr, SIMes, SIPr, etc.) to test the oxidation of aldehydes, and finally found that the catalyst prepared by the more electron-rich SIMes ligand had the best oxidation activity for the reaction (Fig. 4)
[13]. The separation yield of furoic acid was 99% by using [Cu(acac)
2]/SIMes catalyst in sodium hydroxide aqueous solution at 50 ℃ for 12 H. Although the preparation process of the [Cu(acac)
2]/SIMes catalyst is relatively simple, the price of 1,3-bis (2,4,6-trimethylphenyl) -4,5-dihydroimidazolium chloride (SIMes · HCl), the raw material for preparing the catalyst, is relatively expensive, resulting in a relatively high cost of the catalyst (Table 1, No.1).
Metal-free organocatalysts are seen as an effective alternative to metal complex catalysts represented by [Cu(acac)
2]/SIMes catalysts
[14]. Among all organic molecule-based catalysts, the N-heterocyclic carbene (NHC) family has been developed as one of the most promising catalyst classes for the oxidation of aldehydes to carboxylic acids. Tiwari et al. developed an efficient triazolium-NHC catalyst by screening a variety of N-heterocyclic carbene catalysts and base additives. In the presence of base DABCO, furfural can be oxidized to furoic acid in tetrahydrofuran solvent at room temperature for 16 H with a yield of 96% (Fig. 5)
[15]. The triazolium-NHC catalytic system has high catalytic activity and carboxylic acid product selectivity for several types of aldehydes with oxidation challenges, including ortho-substituted aromatic aldehydes, highly electron-rich aromatic aldehydes, and indole-3-carbaldehyde. The reaction conditions of this scheme are mild, and it can also be used for gram-scale synthesis (Table 1, No.2).
2.1.2 Air as oxidizer
It is the most ideal and economical way to develop an efficient catalytic system to achieve efficient oxidation in aqueous system under air conditions. In the study of using air as oxidant, Tang et al. Reported that in the presence of sodium hydroxide, aldehyde was directly oxidized to acid by using air as oxidant, water as solvent, and Ir complex as catalyst, in which the yield of furoic acid reached 90%, and the product could achieve high purity without purification by column chromatography (Figure 6)
[16]. The study of the reaction mechanism shows that the aerobic oxidation involves highly reduced iridium hydride, which can capture oxygen to give iridium hydroxide. Interestingly, this catalyst was also used for the reduction of aldehydes under acidic conditions (Table 1, number 3).
Studer et al. Developed an efficient NHC and Ru (II) -based redox catalyst, which can realize the oxidative esterification of aromatic aldehydes and heterocyclic aldehydes to prepare the corresponding esterification products in alcohol solvents (Table 1, No.4)
[17]. Using NHC alone as a catalyst, furfural can be selectively oxidized to furoic acid by air in acetonitrile solvent at room temperature with a yield of 86% (Fig. 7).
2.1.3 T-BuOOH as oxidant
The oxidation of furfural to furoic acid was carried out by homogeneous catalyst, and the most commonly used oxidant was tert-butyl peroxide. Sekar et al. Reported the oxidation of aldehydes to the corresponding carboxylic acids using CuCl catalyst in acetonitrile solvent at room temperature with aqueous peroxy-tert-butyl alcohol as the equivalent oxidant (fig. 8
[18]; Table 1, serial number 5). The DABCO-CuCl complex /O
2/TEMPO(2,2,6,6- tetramethylpiperidine oxide) catalytic system was first used, but the reaction did not afford even traces of the corresponding carboxylic acid. In the presence of TEMPO catalyst, the yield of carboxylic acid was improved by using 30% H
2O
2 instead of oxygen. When 30% of H
2O
2 was replaced by anhydrous t-BuOOH (5 mol/L in decane), all raw materials were consumed at room temperature for only 6 H, and the isolated yield of furoic acid was 91%, and other benzene ring and aliphatic substrates could also give good yields of carboxylic acids. They unexpectedly found that the real catalyst in this reaction was ligand-free CuCl. CuCl alone as a catalyst gave the same effect. The catalytic system is simple, the catalyst is cheap and easy to obtain, and the scheme has a good industrial application prospect if the anhydrous t-BuOOH is optimized to avoid being used as an equivalent oxidant or the amount of the anhydrous t-BuOoh is reduced to the greatest extent.
Inspired by the Cu
II-salen complex /30%H
2O
2 catalytic system, the bivalent copper Cu (Ⅱ) catalytic oxidation system has attracted the interest of researchers. Chakraborty et al. Converted various aromatic, aliphatic, conjugated aldehydes and alcohols into corresponding carboxylic acids and ketones at room temperature using 70% t-BuOOH aqueous solution as solvent under the catalysis of 5%CuBr
2, in which the yield of furoic acid was 92% after 1.5 H of reaction under the catalytic system (Fig. 9
[19]; Table 1, serial number 6).
Chakraborty et al. Have done a lot of research work on the oxidation of aldehydes, using a catalytic amount of Bi
2O
3 to achieve the oxidation of furfural to furoic acid in 70% t-BuOOH aqueous solution. This method does not use ligands and other additives, has high product selectivity, and does not involve cumbersome post-treatment and purification operations (Table 1, No.7)
[20].
On the basis of the above work, Chakraborty et al. Continued to study periron-based catalysts with catalytic activity and environmental friendliness (Table 1, No.8)
[21]. They used a commonly used redox indicator [Mohr's salt oxidized to Mohr's salt as an oxidation catalyst (Fig. 10). In the presence of a catalytic amount (10 mol%) of Mohr's salt, Mohr's salt, 70% t-BuOOH and other oxidants were screened in DMSO and water solvents, and it was found that DMSO as a solvent gave the best results. The conversion of furfural was 100% and the yield of furoic acid was 92% when 5 equivalents of 70% t-BuOOH aqueous solution was used as oxidant in DMSO solvent at 80 ℃ for 3. 5 H. The reaction system is also suitable for other aldehyde substrates and has good functional group tolerance.
Hypervalent iodine reagents, such as Dess Martin periodinane (DMP), 2-iodoxybenzoic acid (IBX) and iodobenzene, are widely used in organic synthesis as highly selective equivalent oxidation reagents of alcohols to aldehydes or ketones. Although DMP and IBX are practical and popular, they are relatively expensive and explosive, and these unavoidable drawbacks limit their widespread use. Elias et al. Developed a stable metal-free I
2/NaOH catalytic system using water as solvent for the catalytic oxidation of alcohols or aldehydes to carboxylic acids (Table 1, number 9)
[22]. Through control experiments, they found that the iodine dioxide (IO
2) species generated from the oxidation of iodine oxide (IO) by tert-butyl hydroperoxide (TBHP) could be the active catalytic species of the reaction. The method does not require chromatographic purification, is suitable for large-scale synthesis, and has good substrate compatibility. Using this catalytic system, 90% yield of furoic acid was obtained from furfural, and 77% yield of furoic acid was obtained from furfuryl alcohol (Fig. 11).
2.1.4 Other oxidizer
In addition to the common oxidants mentioned above, researchers have also studied other oxidants. Under solvent-free conditions, Gholizadeh et al. Reported that furfural could be effectively oxidized to furoic acid using pyridinium chlorochromate (PCC)
[23]. As a mild oxidant, PCC has high oxidation efficiency, but its disadvantage is that the reaction time is too long and PCC itself is toxic (Fig. 12; Table 1, serial number 10).
Balasubramaniam et al. Used equivalent sodium chlorite as an oxidant in acetonitrile solvent to achieve high yield preparation of furfural to furoic acid under mild conditions, and the yield of furoic acid reached 90% (Table 1, No.11)
[24]. Homogeneous Ni-based catalysts developed by Ghashang et al., using 30%H
2O
2 as an oxidant, can convert various aromatic, aliphatic, and conjugated aldehydes into the corresponding carboxylic acid or carboxylate derivatives (Fig. 13; Fig
[25]. Table 1, serial number 12). When hydrogen peroxide was used as oxidant, the yield of furoic acid was 76% in acetonitrile solvent, and the yield of methyl furoate was 72% in methanol solvent.
Menon et al. Developed a carbon dioxide-mediated NHC catalyzed aldehyde to acid method without the introduction of additional oxygen
[26]. The introduction of carbon dioxide did not lead to the conversion of aromatic aldehydes to aromatic ketoacids, but to aromatic carboxylic acids. The mechanism study showed that carbon dioxide participated in the change of reaction intermediates during the reaction. The catalytic system significantly promoted the conversion of furfural to furoic acid, resulting in a 65% yield of furoic acid (fig. 14; Table 1, serial number 13).
Goldberg et al. Developed a more electron-donating (hexamethylbenzene) Ru
Ⅱ complex catalyst to achieve efficient oxidation of aldehydes to carboxylic acids by aldehyde water shift reaction (AWS) using H
2O as an oxidant at 95 ° C in the presence of 0.4 mol% Ru catalyst (Fig. 15
[27]; Table 1, serial number 14). It was found that the use of electron-rich arene ligands improved the selectivity of AWS reaction, while the strong binding bidentate auxiliary ligands greatly reduced the reactivity, and the catalyst activity decreased with the increase of acid concentration in the reaction system.
As can be seen from the results summarized in Table 1, the researchers developed a variety of homogeneous systems that exhibited different oxidation activities in the presence of different oxidants. As far as the reported systems are concerned, although the use of equivalent oxidant can achieve higher product yield, the atom economy is poor and the emission of three wastes is more. There are many studies on the method of using tert-butyl peroxide as oxidant and the corresponding catalytic system, but considering the atom economy and industrial application prospects, the development of efficient homogeneous system under air or oxygen conditions is more attractive, and the cheap, efficient and easily prepared catalyst is the focus of research. Aldehyde-water exchange reaction has great advantages in atom economy, safety and cost, but the expensive use of catalyst, catalyst recycling and low product yield are still the main problems faced by this method.
2.2 Heterogeneous oxidation system
Heterogeneous catalyst is the first choice for most industrial production. Heterogeneous catalyst can be easily separated from the product and can be reused. Heterogeneous catalysts used in recent years for the oxidation of furfural to furoic acid are summarized in Table 2 below.
2.2.1 Au-based catalyst
Giorgianni et al. Obtained a high-performance Au/ZTC catalyst for the selective oxidation of furfural to furoic acid by depositing Au nanoparticles on β-ZTC carbon prepared from BETA zeolite,The only weak acid site on the support hinders the acetalization and esterification pathway, which is beneficial to the selective oxidation of the aldehyde function on the furan ring to carboxylic acid, and 90% furfural conversion and quantitative furoic acid selectivity can be achieved (Table 2, No.1)
[28]. Han et al. Found that the conversion of furfural to furoic acid could be achieved using AuNPs/TiO
2 catalyst in Na
2CO
3 aqueous solution under ultraviolet (UV) and visible light
[29]. The yields of furoic acid were 92% and 96%, respectively, when the reaction was carried out under 350 ~ 400 nm ultraviolet light or 420 ~ 780 nm visible light for 8 H. At the same time, furfuryl alcohol was used as the raw material, and the reaction time was extended to 48 H, and the yield of furoic acid was 90% under two light sources (Table 2, No.2). The catalyst has good reusability and shows high oxidation selectivity to furoic acid. Au/TiO
2 catalysts are easy to prepare, but relatively expensive. It is a challenge to develop cheaper non-noble metal or organic photocatalysts and design more universal photocatalytic reactors. Wojcieszak et al. Prepared a core-shell SiO
2@Au@TiO
2 catalyst and realized the air oxidation of furfural to furoic acid under alkali-free conditions
[30]. Compared with conventional supported SiO
2@TiO
2@Au catalysts, the catalytic activity of these materials is improved by a factor of one hundred. It was found that the core-shell structure played a key role in the catalytic activity of the catalyst, which could prevent the irreversible adsorption of furoic acid on the active metal Au and the deactivation of the catalyst, improve the metal-oxide interaction, and form new catalytic active sites around the Au-TiO
2 interface. Under the optimal reaction conditions, the conversion of furfural reached 100% and the selectivity of furoic acid reached more than 99% by using 0.25%SiO
2@Au@TiO
2 catalyst (Table 2, No.3).
2.2.2 Co-based catalyst
Li et al. Developed the Co
4HP
2Mo
15V
3O
62/H
2O
2 catalytic system used in ionic liquid [TEBSA][BF
4] for the conversion of aldehydes to carboxylic acids and esters (Table 2, No.4)
[31]. Furoic acid can be separated by simple organic solvent extraction, and the catalytic system can be recycled. At the same time, the catalytic system is compatible with a variety of aromatic and aliphatic aldehyde substrates, and has a high selectivity for carboxylic acid products (Fig. 16).
Xu et al. Developed a Co-N-C catalyst to realize the highly selective conversion of furfuryl alcohol to furoic acid, and obtained a furoic acid yield of 96.7% at 120 ℃, oxygen pressure of 1 MPa, and water for 1 H (Table 2, No.5)
[32]. In recent years, supported metal complex catalysts have been widely used in organic synthesis. Over the past decade, acetylacetonate metal complexes have become versatile catalysts for a variety of reactions. Clark et al. Prepared a series of heterogeneous catalysts by combining metal acetylacetonates [Co(acac)
2, Cu(acac)
2, Pd(acac)
2, Ru(acac)
3, Mn(acac)
3] with organically modified 3-aminopropyl silica to study the oxidation activity in aqueous solvent
[33]. They found that the oxidation activity of SiO
2-Co(acac)
2 was the highest. Under solvent-free and air conditions, 85% furoic acid yield can be obtained by using SiO
2-Co(acac)
2 catalyst at 50 ℃ for 5 H, and 80% furoic acid yield can be obtained by using aqueous solution at 70 ℃ for 7.5 H (Table 2, No.6).
2.2.3 Cu-based catalyst
Wei et al. Reported a method for the catalytic oxidation of aldehydes with inorganic ligand-supported Cu catalyst (NH
4)
4[CuMo
6O
18(OH)
6] in water under atmospheric oxygen conditions
[34]. The catalyst was synthesized from copper sulfate with ammonium molybdate in water at 100 ° C by a simple one-step method, avoiding the use of expensive, toxic, air/moisture-sensitive, and commercially unavailable organic ligands. The catalyst has one Cu (Ⅱ) core and six Mo (Ⅵ) centers arranged in the plane. Furfural was completely converted to furoic acid in 99% yield under oxygen in water solvent. The activity of the copper-based catalyst did not decrease significantly after six consecutive uses (Table 2, No.7). CuO and Ag
2O/CuO catalysts can effectively oxidize furfural to furoic acid. Under the optimal reaction conditions, Ag
2O/CuO and CuO catalysts with a size of 150 nm can obtain furoic acid in a yield of nearly 100%. Ag
2O/CuO catalyst is easy to prepare and has a low cost, but the hydrothermal stability and repeatability of the catalyst are poor (Table 2, No.8)
[35]. Xu Xianlun et al. Synthesized nano-CuO catalysts with different morphologies in water by different methods, among which the prepared rod-like CuO catalyst showed good oxidation activity, and the yield of furoic acid reached 91% when furfural was added at one time (Table 2, No.9)
[36].
2.2.4 Fe-based catalyst
Wei et al. Prepared an iron-based heteropolyacid oxometalate [ [N(C
4H
9)
4]
3-[FeMo
6O
18(OH)
3{(OCH
2)
3CNH
2}](Fe
IIIMo
6) for the oxidation of aldehydes in aqueous solvent
[37]. The catalyst is a polymolybdate supported by a single Fe metal, which is composed of six-edge-shared octahedra. In the study of catalyst activity, they found that the presence of acidic additive NH
4Cl had a strong inhibitory effect on the oxidation reaction, and changing the catalyst loading had little effect on the product yield. The highest yield of furoic acid was 97% at 50 ℃ for 8 H under atmospheric oxygen. The experimental results showed that the activity of the catalyst did not decrease significantly after recycling, showing a high catalyst recycling performance (Table 2, No.10). Massi et al. Developed a method for the oxidation of aldehydes with an oxidation system composed of a heterogeneous triazolium catalyst and iron (II) phthalocyanine (Fig. 17)
[38]. The catalytic system can selectively convert 5-hydroxymethylfurfural (HMF) to 5-hydroxymethyl-2-furoic acid (HMFCA), and efficiently oxidize furfural to furoic acid with a yield of 90% (Table 2, No.11). HMF and furfural can also be directly converted into their corresponding ester, amide and thioester derivatives, which further illustrates the application potential of heterogeneous N-heterocyclic carbene catalysts in the field of biomass.
2.2.5 Other catalyst
With MnO
2 as the catalyst, the system could convert furfural to furoic acid with high selectivity in the presence of NaHCO
3 aqueous solution and oxygen (Fig. 18). The MnO
2 catalyst can be repeatedly used for several times without loss of activity by simple centrifugal separation. At the same time, the catalytic system can also oxidize 5-hydroxymethylfurfural (HMF) to 2,5-difurandicarboxylic acid (FDCA) in water (Table 2, No.12)
[39]. Rhee et al. Air oxidized furfural to furoic acid in a methanol-water mixed solvent using a Pd/C catalyst in the presence of sodium borohydride and potassium hydroxide with a furoic acid yield of 84% (Table 2, No.13)
[40]. The sodium borohydride in the catalytic system plays a role in reducing Pd/C,
The presence of a strong base favors the availability of carboxylic acid products. Chakraborty et al. Used 1 mol% ceric ammonium nitrate [Ce(NH
4)
2(NO
3)
6](CAN) as a catalyst in acetonitrile solvent at room temperature and 70% t-BuOOH aqueous solution as an oxidant to obtain 93% furoic acid yield in 15 min (Table 2, No.14)
[41].
Shen Jingru et al. used a new β-cyclodextrin derivative as a catalyst to oxidize furfural to furoic acid at 50 ℃ for 1 H under acidic conditions, with a conversion rate of 97%. The catalyst could be precipitated in organic solvents such as acetone and recycled. The catalyst could be reused for 4 times without significant decrease in activity (Table 2, No.15)
[42]. Thakur et al. Supported VO(acac)
2 on a TiO
2 to prepare a heterogeneous TSV catalyst. In the presence of hydrogen peroxide, the catalyst showed good catalytic activity for the oxidation of furfural to furoic acid or furoate. After 4 H of reaction at room temperature, the yield of furoic acid was 86% in acetonitrile, and the yield of methyl furoate was 75% in methanol (Table 2, No.16)
[43]. Kim et al. Developed a TEMPO-functionalized metal-organic framework (MOF) catalyst to achieve one-pot oxidation of alcohols to carboxylic acids through the addition and removal of heterogeneous MOF-TEMPO catalyst (Fig. 19)
[44]. Through the introduction of this method, the problems of aerobic oxidation of alcohols catalyzed by TEMPO and difficult self-oxidation of aldehydes under free radical inhibition can be effectively solved, and the two-step oxidation process can be effectively connected in series to realize the preparation of various carboxylic acid products (Table 2, No.17).
It can be seen from Table 2 that, in general, many supported noble metal catalysts, such as SiO2@Au@TiO2, AuNPs/TiO2, Pd/C and Ag2O/CuO catalysts, have been reported in the study of heterogeneous catalysts, which have high production costs. Nanometer copper oxide is easy to prepare and has low cost, but the reusability of the catalyst is poor, the reaction needs a certain amount of alkali (such as sodium hydroxide) to maintain the stability of the catalyst, and the use of alkali will cause a large amount of salt in the post-treatment process, which will pollute the environment, so it is very important to avoid the use of alkali. In recent studies, ionic liquids have been used as solvents in Co4HP2Mo15V3O62 systems, but the cost of the solvents is relatively high. In addition, t-BuOOH as an oxidant, the product yield is high, the reaction time is short, and no additional alkali is needed, but the separation and purification of t-BuOOH is difficult, the cost of large-scale use is high, and there are some potential safety hazards. Supported catalysts with core-shell structure have great advantages in catalyst stability and activity, and the development of cheap core-shell active metal catalysts is an important direction of future research. It is still a challenge to realize the conversion of furfural to furoic acid through green oxidation technology, and it is still of great research value to develop new heterogeneous catalytic systems to realize the aerobic oxidation of important bio-based platform molecules to produce high value-added chemicals.
2.3 Biological oxidation process
Biotransformations are usually carried out under mild reaction conditions, and biocatalysts have excellent selectivity, biodegradability, and environmental friendliness, and biocatalytic oxidation can be used as a promising option to complement or even replace chemical methods.
Li et al. Reported the biocatalytic oxidation of furyl platform molecules to the corresponding carboxylic acids using the newly isolated strain Comamonas testosteroni SC1588
[45]. Dormant cells were found to be better biooxidation catalysts compared to growing cells. The strain showed high tolerance to furfural compounds (up to 180 mmol/L). Under the conditions of 50 mmol/L furfural, 30 mg/mL microbial cells, 4 mL phosphate buffer (200 mmol/L, pH 7. 0), 20 mmol/L histidine, and 30 ℃ for 96 H, the highest yield of furoic acid was 93%. Due to its high detoxification efficiency and furfuryl tolerance, the strain may have broad application potential in the biodegradation of lignocellulose hydrolysates, while it cannot use both glucose and xylose as carbon sources, thus avoiding sugar loss in the process of biological detoxification. P P Érez et al. Used Nocardia corallina B-276 whole cells to study the oxidation of furfuryl alcohol and furfural to furoic acid in batch culture and quiescent cells in potassium phosphate buffer (0.1 mol/L, pH 7.0)
[46]. 2-Furfuryl alcohol (1. 35 G/L) was converted to furoic acid with a yield of 81% in a 3 L bioreactor after 24 H, and the yield of 2-furoic acid reached 98% in 21 H by using quiescent cells. When the ratio of substrate to cells was 1 ∶ 3.5 (w/w), the yield of furoic acid was 88% after 8 H of reaction. de Mar María et al. Used the commercially available immobilized lipase Candida antarctica (Immo-CAL-B) as a biocatalyst to form peracid in situ by adding a total of 9.6 equivalents of aqueous hydrogen peroxide (30% V/V) in a mixed solvent of 50 mmol/L furfural, 10 mg/mL CAL-B catalyst, and ethyl acetate: tert-butanol = 1:1 (V/V), followed by oxidation of furfural to obtain a 91% yield of furoic acid
[47]. In which that alkyl ester act as the acyl donor.
Mutti et al. Oxidized aldehydes using three recombinant aldehyde dehydrogenases (ALDH-Bov, ALDH-Ec, and ALDH-PP) derived from bacterial and bovine lenses (fig. 20)
[48]. These enzymes are used in highly purified form in combination with nicotinamide oxidase (NOX), which consumes molecular oxygen from air to give catalytic NAD
+ species. They studied the reaction using lyophilized whole cell and quiescent cell biocatalysts. The study showed that the carboxylic acid product was uniquely detected and isolated with a chemoselectivity of over 99%. The method has good substrate compatibility, avoids the use of toxic or unsafe oxidants, strong acids and strong bases, and does not have other classical side reactions (such as halogenation of unsaturated functional groups, Dakin-type oxidation, etc.). Aldehyde dehydrogenases are important catalysts for the chemoselective oxidation of aldehydes to carboxyl groups, and future studies will focus on improving the tolerance and long-term stability of the enzymes to substrate concentrations, in order to make these enzymes more widely used.
Li et al. Reported the selective synthesis of furancarboxylic acids from the corresponding aldehydes by recombinant E. Coli cells expressing 3-succinylsemialdehyde-pyridine dehydrogenase (SAPDH) from Pseudomonas testosteroni SC1588
[49]. The effects of induction and reaction conditions on the whole-cell catalytic oxidation of furfural (FF) were investigated. They found that the yield and selectivity of furoic acid were significantly improved by using high temperature induction, but the reaction time was slightly longer, and the recombinant strain showed good catalytic activity in a narrow pH range (7 ~ 8). The recombinant strain could tolerate about 100 mmol/L furfural, and the yield of furoic acid was 97%. Li et al. Constructed a two-enzyme cascade system consisting of galactose oxidase (GOase) and alcohol dehydrogenases (ADHs), and screened ADHs including carbonyl reductase from Streptomyces coelicolor (ScCR) and Synechocystis sp
[50]. And horse liver ADH enzymes (SADH and HLADH). This two-enzyme cascade oxidation system has low O
2 dependence, higher atom efficiency and product controllability, excellent universality for a variety of furyl substrates, and no addition of catalase (fig. 21). Li et al. Synthesized carboxylic acid products from the biological oxidation of aldehyde substrates by cofactor-engineered Escherichia coli cells
[51]. They introduced NADH oxidase (NOX) into E. Coli containing aldehyde dehydrogenase (ALDH) to promote the regeneration of intracellular NAD
+, thereby significantly enhancing the catalytic oxidation ability of ALDH. These engineered biocatalysts are able to efficiently oxidize various aromatic aldehydes. More importantly, they exhibit high substrate tolerance to toxic furan substrates. Escherichia coli, vanillin dehydrogenase and NOX were co-expressed (E. coli _ CtVDH1 _ NOX), which could efficiently oxidize > 240 mmol/L furfural and 5-hydroxymethylfurfural (HMF) to 2-furoic acid and 5-hydroxymethyl-2-furoic acid with production efficiencies of 2.0 and 3.7 G/Lh.
Although the biocatalytic oxidation method has achieved many remarkable and gratifying results, it still needs further improvement to achieve economical large-scale production of furoic acid. Since the toxicity of furaldehyde can strongly inhibit the activity of biocatalysts, it is of great significance to develop biocatalysts with higher tolerance and stability and apply them to the production process of furoic acid and related products. Secondly, in order to achieve efficient production of furoic acid, the cascade enzyme reaction process should be further optimized to connect the upstream and downstream processes, avoid the accumulation of intermediate products, and simplify the downstream extraction process. At the same time, efficient mutants with high activity and high yield should be systematically designed and constructed through random or site-directed mutagenesis of enzyme engineering and genetic engineering and optimization of expression system. In order to reduce the production cost, enzyme and whole cell immobilization methods need to be investigated to obtain economically viable, stable, highly active, reusable immobilized biocatalysts.
3 Conclusion and prospect
At present, the preparation of furoic acid by furfural oxidation should focus on the development of new heterogeneous catalytic systems which are mild, inexpensive and can be used in alkali-free environment. We believe that more research on the following aspects may have some inspiration for the green synthesis of furoic acid:
(1) Develop environmentally friendly catalytic systems. It is necessary to develop efficient, low-cost and stable catalysts to achieve aerobic oxidation. Compared with noble metal catalysts, non-noble metal catalysts have been less studied for aerobic oxidation. Although some results have been achieved, further research is needed. In addition, the catalytic system with air as the oxidant should be mainly developed. Air has many advantages over other oxidants, such as high atom economy and only water as a by-product. However, a cheap and efficient catalytic method with molecular oxygen as the terminal oxidant is still lacking. Therefore, non-noble metal catalysts utilizing molecular oxygen as an oxidant in aqueous systems are worth exploring.
(2) The stability of catalyst is very important. Among the reported examples of organometallic catalysts, various metal ligands, phosphine ligands or nitrogen heterocyclic carbenes are widely used, which are susceptible to oxidative self-degradation, which is a common problem in transition metal catalysis. In addition, the stability of the catalyst depends on the interaction between the active metal species and the support. The support should be stable under the reaction conditions. It is clear from the available studies that the role of the support plays a decisive role in achieving high conversion and selectivity. Improving the hydrothermal stability of heterogeneous catalysts, especially under alkali-free conditions, through the design and development of catalyst structures, is an urgent problem to be solved.
(3) The alkali-free oxidation of furfural by catalytic methods such as photocatalysis, electrocatalysis, ultrasonic treatment, microwave heating and biological method is an alternative scheme worthy of further exploration. The basic principle of green chemistry is to promote the use of renewable raw materials and safer solvents and auxiliaries to develop less dangerous chemical synthesis. Under mild conditions, it is imperative to use a variety of new catalytic methods to improve process efficiency and prevent waste generation.
Major Science and Technology Projects of Anhui Province(18030701157)
Anhui Natural Science Foundation Project(2008085QB63)
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