Recent catalytic innovations in furfural transformation

Kangyu Zhao, Bin Wen, Qing Tang, Feng Wang, Xianxiang Liu*, Qiong Xu and Dulin Yin
National & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of Resources, Key Laboratory of the Assembly and Application of Organic Functional Molecules of Hunan Province, Hunan Normal University, Changsha 410081, China. E-mail: lxx@hunnu.edu.cn

First published on 16th May 2024

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1. Introduction

Fossil fuels have traditionally served as the primary raw materials for chemical industry production. However, it's essential to recognize that fossil fuels are finite and non-renewable resources. In recent years, as the pace of modernization and industrialization has quickened, the global demand for energy has surged, leading to the gradual exhaustion of fossil fuel reserves.¹ Moreover, the environmental challenges stemming from the utilization of high-carbon energy sources like oil and coal have grown progressively dire. In the face of escalating energy demands in our modern society, coupled with ever-stricter carbon emissions standards and the persistent pursuit of economic prosperity, the shift towards non-fossil, renewable energy sources has emerged as a prominent and vital trend.² Within the framework of carbon peak and carbon neutrality objectives, energy development has ushered in a new stage, signifying the gradual shift from the fossil fuel-dominated development era to a non-fossil energy era. The global consensus is now firmly set on seeking green energy solutions for sustainable development.^3–5

Biomass is a unique form of energy wherein solar energy is captured and stored within green plants through photosynthesis, either directly or indirectly. It is Earth's exclusive and renewable carbon source, originating from a wide spectrum of materials and possessing substantial reserves.⁶ With its environmentally sustainable nature and rapid regeneration cycle, biomass can undergo chemical processes to produce a variety of valuable products.⁷ Consequently, biomass stands out as a pivotal choice in addressing the energy crisis.^4,8–10 Leveraging renewable lignocellulosic biomass resources to generate high-value chemicals, like the conversion of various biomaterials such as straw and fruit shells into premium fuels and fine chemicals, emerges as a vital strategy in addressing future energy and environmental challenges. This approach has garnered growing attention from both academic and industrial sectors.^11–14

In this context, furfural has garnered recent attention as a leading value-added chemical sourced from biomass. It is notably considered a key product in lignocellulosic biorefineries. Lignocellulosic biomass mainly refers to a class of natural products composed of cellulose, hemicellulose, and lignin, which can be hydrolyzed into C6 sugars (such as glucose and fructose) and C5 sugars (like xylose and arabinose) under the influence of acid catalysis.¹⁵ Furfural, recognized as one of the leading 30 platform compounds derived from biomass by the U.S. Department of Energy, is primarily obtained through the hydrolysis and dehydration of xylan, a prominent component of hemicellulose. The process of converting xylose/xylan into furfural is a thoroughly established method, with its initial industrial application dating back to 1921 when the Quaker Oats Company pioneered its commercialization. In general, raw materials containing a certain amount of pentose can be used to produce furfural. Due to the technical influence, high-valued utilization of furfural is still very limited. As a vital chemical raw material, furfural could serve various purposes, acting as a selective extractant, solvent, vulcanizing agent, anesthetic, intermediate for pesticides, a component in resin products, and a key ingredient for drug, cosmetic, and spice manufacturing. What's more, the inherent chemical reactivity of furfural, owing to its furan rings and aldehyde functional groups, positions it as a versatile biomass-derived platform compound. Realizing the high-value utilization of furfural and further expanding its applications in downstream, value-added products carry substantial scientific and industrial significance.^16,17

Several excellent reviews on furfural conversion have been published, most of them focusing on biofuel applications.^4,10,18,19 This article provides a systematic and comprehensive review of the progress of research on the catalytic transformation of furfural, a process leading to the production of valuable downstream products, such as furfuryl alcohol (FOL), tetrahydrofurfuryl alcohol (THFA), levulinic acid (LA), γ-valerolactone (GVL), 2-methylfuran (2-MF), 2-methyltetrahydrofuran (2-THMF), pentanediols (PeDs), cyclopentanone (CPO), and nitrogen-containing compounds with a special emphasis on recent advancements in heterogeneous catalytic material, green synthesis system and advanced technology in the last five years. Furthermore, possible reaction mechanisms for the conversion of furfural to a variety of high-value chemicals are discussed. Meanwhile, the challenges and opportunities for the upgrading of furfural to high-value chemicals are revealed, providing theoretical guidance for expanding the application value of furfural and pointing out the direction for further research in the future.

2. Furfural production

Furfural is a product of the natural dehydration of xylose, a monosaccharide found in large quantities in the hemicellulosic fraction of lignocellulosic biomass, so biomass containing a certain amount of xylose can be used to produce furfural.²⁰ The most important raw materials for the industrial production of furfural are corn cobs, rye straw, cottonseed hulls, barley straw, sugarcane bagasse, rice hulls, peanut hulls, wood, etc. These plant-based biomass materials have a high content of pentosan, which can satisfy the high furfural yield.²¹ Among these, bagasse and corn cobs account for more than 98% of the raw materials used in the production of furfural because they are inexpensive and readily available from sugarcane and maize processing plants. The finished product has a wide range of usage and is an effective solution to the problem of wastage and disposal of agricultural resources. Lignocellulosic biomass, including agricultural and forestry residues, is particularly suitable for large-scale production of renewable fuels and chemicals.²² The main furfural production processes at home and abroad include Quaker Oats process, Agrifuran process, Petrole-Chimie process, Escher Wyss process and Rosenlew process.^23,24 Furfural is produced industrially using batch or continuous reactors in which the hemicellulose is treated with acid, liquefied in the presence of solvents and catalysts to obtain hemicellulose derivatives (mainly xylose), which are subsequently further dehydrated at temperatures in the range of 150 °C to 240 °C to obtain furfural (Fig. 1).²⁵

In industrial scale, furfural production usually consists of four main steps: hydrolysis of hemicellulose, dehydration of xylose, recovery of furfural, and finally sequential purification. At present, furfural industrial production usually uses batch reactors or continuous reactors. The whole furfural production process technology flow is about (Fig. 2):²⁴ (1) first, corn cobs with no spoilage or mold were selected as raw materials for furfural production and crushed, as the natural moldy and deteriorated corn cobs would result in decreased polysaccharide content. After crushing, the particle size of the raw material is reduced, which increases the surface area of the raw material and thus the reaction speed. (2) Corn cobs with dilute acid are added and stirred well, then passed through a hydrolysis reaction in a hydrolysis pot. This process is difficult to operate at room temperature, so the actual production process is carried out at high temperature and pressure. The whole process temperature is controlled between 145 °C and 230 °C. Typically, high pressure steam is used to provide the heat required for the reaction, while another important role of steam is to remove furfural from the reaction system in time, thus avoiding the occurrence of side reactions. (3) Subsequently, distillation is performed to concentrate the dilute furfural solution, thereby enhancing the furfural concentration. After the dilute furfural solution is evaporated several times, the concentrated furfural vapors are led through the top of the tower, while the residual liquid is discharged from the bottom of the tower. The export steam enters the primary furfural collector through a condenser and divides into two layers, the upper layer is the saturated solution of furfural dissolved in water, containing 7–10% furfural content, and the lower layer of furfural is the concentration of raw furfural at about 90%. This process requires the temperature at the top of the tower to be maintained at 94–97 °C, the temperature at the bottom of the tower to be controlled at 98–103 °C, and the temperature of the distillate should be below 55 °C. (4) Due to the low purity of crude furfural, which contains high boiling point and low boiling point substances as well as water, its application is restricted and requires further refining. Usually, the refining is performed by decompression distillation or water vapor distillation, and the purity of furfural can be as high as 98% after the purification process.

On the other hand, from the microscopic point of view of the chemical reaction, liquefaction of hemicellulose to xylose takes place in the following steps: (1) glycosidic bond protonation of xylan in acidic media. (2) The carbon–oxygen bond is cleaved to form carbon positive ions and monosaccharides, respectively. (3) The carbon positive ion binds to water and the positive charge migrates to eventually produce monosaccharides and protons. (4) The proton continues to participate in xylan liquefaction until the glycosidic bond reaction is complete.²⁶

The reaction mechanism of xylose dehydration is very complex, and the following three different pathways are mainly considered according to different catalytic environments (Fig. 3): (i) the xylose is first ring-opened to form an enol under Lewis acid catalysis, then undergoes dehydration isomerization to form a diolefinic intermediate, and finally dehydration cyclization to form furfural. (ii) Catalyzed by Brønsted acid, the oxygen atom at the C¹ or C² position is protonated and undergoes dehydration and intracyclic rearrangement to form furfural directly. Or (iii) isomerization of xylose to form xylulose under the combined action of Lewis acid and Brønsted acid, which in turn undergoes dehydration cyclization to form furfural.²⁷

Catalysts for furfural preparation can be divided into homogeneous catalysts and heterogeneous catalysts. The main types of homogeneous catalysts include common organic and inorganic acids, inorganic salts and metal ions, etc. The heterogeneous catalysts mainly refer to the solid acid catalysts, and the common solid acid catalysts mainly include molecular sieves, metal oxides, ion-exchange resins, phosphates and acidic heteropolyacids. The most widely used and technologically mature catalysts in the industry nowadays are H₂SO₄²⁸ and HCl,²⁹ which catalyzed the production of furfural and can obtain high yield, but there are problems such as the difficulty of separating and recovering the products, the product contamination, and the loss of furfural yield due to the longer residence time. So, some people tried to use metal salt catalysts instead of acid catalysts. Leitner and María et al. have shown that a biphasic medium consisting of an aqueous solution of FeCl₃ and NaCl with biomass-derived 2-THMF is an effective catalytic system for xylose dehydration.³⁰ This method was able to achieve a maximum furfural yield of 71% in the presence of 20 wt% NaCl additive. Similar to the Fe system, AlCl₃ catalyzed the conversion of xylose to >99% at 140 °C in the biphasic medium water/tetrahydrofuran (THF), with furfural yields up to 30% within 5 min.

Compared with homogeneous catalysts, heterogeneous catalysts can improve the conversion and selectivity of reactions by adjusting their acidity, acid amount and structure, thus becoming a hot research topic.^10,31 Solid acid catalysts have many advantages in terms of activity, selectivity, catalyst life, and ease of recycling and reuse, and have been widely studied owing to they can directly replace liquid acids. O'Neill et al. used HZSM-5 catalyst to catalyze xylose in the aqueous phase at 200 °C to obtain 46% furfural yield.³² Yang et al. prepared a charcoal-based solid acid catalyst, SC-GCA-800, for the preparation of furfural by catalyzing the conversion of xylose in 1,4-dioxane using 4-anilinosulfonic acid as a sulfonating agent in the sulfonation of calcium gluconate, and achieved a furfural yield of 76.9% in 40 min at 140 °C.³³ Qi et al. used cellulose as the char source, impregnated with FeCl₃, and then sulfonated with sulfuric acid to produce a magnetic char-based solid acid catalyst, which effectively achieved the conversion of xylose to furfural in a water–methyl isobutyl ketone biphasic solvent system, reaching the highest yield of furfural of 79.0% at 190 °C.³⁴ The development of heterogeneous solid acid catalysts has helped to increase furfural production, thus providing an opportunity for the furfural industry to explore more valuable downstream products.^18,35

With the application of corn cobs as the raw material for production in China 30 years ago, production costs have been greatly reduced and China has gradually become the largest exporter of furfural and its processed products. In recent years, the furfural production process has become more and more perfect with the progress of social development and the growth of science and technology. Furfural has been vigorously promoted by many fields and industries, and its development prospects are very broad. However, there are still some problems in the actual production process, such as the environmental impact of the waste residue, hydrolysis pot wastewater and distillation tower top exhaust gas in the production process, the low efficiency of furfural production and so on. So the entire production process needs to be continuously developed and improved. It has improved the productivity while promoting growing sophistication of production process, manpower, material resources and other production costs to do an effective reduction in the overall development of agriculture to play a positive role in driving. At the same time, furfural and its derivatives require significant improvements in yield and production strategies to compete with petroleum-based products as renewable alternatives. The furfural industry is a basic industry for chemical products, and its production and demand are generally showing an upward trend from year to year. In China, for example, according to reported statistics, China's furfural production increased to 512200 tons in 2019 from 282000 tons in 2015, and demand increased to 493500 tons from 256500 tons in 2015, and it is expected that China's furfural production is expected to reach 600000 tons in 2023, and demand is expected to reach 590000 tons.³⁶ Furfural production may be greater than demand, so it is necessary and important to study the conversion of furfural to other downstream products.

3. Reaction pathway of furfural conversion

Furfural is a furan compound similar to 5-hydroxymethylfurfural, also known as 2-furfuraldehyde. Since furfural contains many functional groups such as aldehyde group outside the ring, double bond inside the furan ring and ether bond, thus it has high reactivity. Furfural can be reduced to alcohol, reduced amination to amine, decarbonylation oxidation to carboxylic acids.^37,38 In addition, the furan rings can also participate in hydrogenation, oxidation, halogenation, ring-opening rearrangement, and other reactions to produce various important furan derivatives.^39–41 Based on the above chemical properties, furfural can be used to prepare many chemical products, so it has a wide range of application value in industrial production. Furfural is also an important raw material for the production of transportation fuels, lubricants, gasoline additives and aviation fuel blends. Therefore, among many chemicals derived from biomass, furfural is recognized as one of the most valuable chemicals.⁴²

Furfural can be converted into a series of C4 and C5 molecules by a variety of catalytic processes, which are indispensable components in the production of liquid fuels and fuel additives (Fig. 4). Besides fuels and fuel additives, furfural can be converted into other valuable chemicals such as PeDs, GVL, butanediol and butyrolactone. Among them, FOL and THFA are simple hydrogenation products of furfural. Hydrogenolysis of the carbonyl group on the side chain of furfural after hydrogenation can produce 2-MF or 2-THMF.⁴³ The furan ring in furfural can also be ring-opened and hydrogenated to produce PeDs. Reductive amination of furfural side-chain carbonyls to furfurylamine (FAM) and other C5 chemicals.⁴

In industrial production, hydrogenation is one of the important intermediate steps in the synthesis of high-demand products (flavors, pharmaceuticals, food preservatives, etc.). There are three ways to hydrogenate furfural: (1) Gas-phase hydrogenation: this is a traditional technology and the main reactor is a trickle bed reactor, which is a type of fixed-bed reactor. Hydrogen and feedstock are parallel flowed through a bed of solid particulate catalysts for gas–liquid–solid phase reaction. Trickle bed reactors commonly operate under pressure to promote mutual dissolution of the gas–liquid phases. Hydrogen and feedstock undergo full contact and hydrogenation reaction within the catalyst micropores. Furfural gas-phase hydrogenation is generally carried out at a temperature greater than 200 °C and under 3–6 MPa H₂. In the gas-phase hydrogenation process, reaction activity and selectivity are not only affected by the catalyst, but also by the reaction temperature. Different temperature ranges produce different products, and higher temperatures make it easier to break the C–C bond.⁴⁴ (2) Liquid-phase hydrogenation: this process involves dissolving sufficient hydrogen in the liquid-phase recycle material to provide for the hydrogenation reaction, so there is no need for a hydrogen recirculation system for the process. When hydrogen is dissolved in the feedstock oil and enters the reactor, mass transfer between the gas and liquid phases is avoided, the influence of the wetting factor is eliminated, the reactor temperature gradient is reduced, and the catalyst bed is close to isothermal operation. Moreover, the material flows unidirectionally through the catalyst bed, with a small temperature difference between the catalyst beds, fewer hydrocracking reactions, and higher liquid yields. At the same time, it makes the process flow simpler. The reaction conditions for liquid-phase hydrogenation are generally milder than those for gas-phase, with temperatures generally around 100–210 °C and H₂ pressures generally 2–8 MPa.⁴⁵ (3) Liquid-phase transfer hydrogenation: it uses a source of hydrogen obtained from a solvent or other additives. A more commonly used hydrogen source is isopropanol (2-PrOH), which is also as a solvent for the reaction. In the entire process, as the hydrogenation reduction reaction conflicts with the oxidation reaction, some of the reaction substrates may also be susceptible to oxidation. Therefore, liquid phase transfer hydrogenation usually requires nitrogen protection to exclude the influence of oxygen on the reduction reaction.⁴⁶

In addition to the hydrogenation of the aldehyde group outside the furfural furan ring with the furan ring, there is also the possibility of hydrogenolysis reactions. Hydrogenolysis is usually a reaction in which a carbon–heteroatom bond (or carbon–carbon bond) is broken in a reduction reaction and hydrogen replaces the departing heteroatom (carbon atom) or group. Hydrogenolysis reactions are often accompanied by high temperatures and high pressures, under which the active metal is susceptible to sintering and coking, resulting in a decrease in catalytic activity.⁴⁷ Metal oxide (e.g. ZrO₂, TiO₂, CeO₂) loaded metal-based catalysts are a common form of catalyst combination. They can enhance the dispersion of metal particles while ensuring the catalyst's structural stability and resistance to loss of activity, and provide Brønsted acid or Lewis acid sites. This promotes the activation and breaking of carbon–heteroatom bonds for a more efficient hydrogenolysis process. In the hydrogenolysis reaction of furfural, in the presence of Brønsted acid or Lewis acid, breakage between C–O bonds occurs mainly. Breakage of the C–O bond outside the furan ring may yield 2-MF and 2-THMF. The ether bond on the furan ring may also undergo hydrolytic breakage to yield pentanediol or rearrangement after breakage in the presence of Brønsted acid to produce LA.

The aldehyde group in furfural can also be used in reductive amination reaction to produce amines, amines are used in a wide range of applications such as pharmaceuticals, rubber and pesticides.⁴⁸ Among them, primary amine compounds are an important synthetic intermediates, which can be widely used in the production of plastics, resins and drugs, and so on.^49,50 The typical methods for synthesis of primary amines include ammonolysis of haloalkane compounds, hydrogenation reduction of nitrile compounds, addition of olefins to ammonia, nitro hydrogenation reduction on benzene rings, and reductive amination of carbonyl compounds.⁵¹ Reductive amination of aldehydes and ketones for the synthesis of primary amines is one of the most widely used methods for the preparation of primary amines due to the advantages of easy operation, wide range of applicable reactions, and wide source of substrate aldehydes/ketones.⁵² Depending on whether or not intermediates such as imines need to be isolated during synthesis, carbonyl compound reductive amination reactions are classified into two types: indirect reductive amination and direct reductive amination. As the indirect reductive amination is usually free of competitive reduction reactions, the selectivity of the reductant is less required. But most of the imine intermediates are very unstable and difficult to isolate, direct reductive amination has been preferred.⁵³ The synthesis of primary amines by direct reductive amination of aldehydes/ketones carbonyl compounds usually involves two steps: (1) imidization, the dehydration and condensation of aldehydes/ketones with ammonia donors (ammonia gas, liquid ammonia, and aqueous ammonia, etc.) to produce imine intermediates. (2) Sialylation hydrogenation, the hydrotreating of imines in the presence of a reducing agent to produce primary amines. The direct reductive amination of carbonyl compounds to synthesize high yield primary amines is challenging because many side reactions can be carried out simultaneously and by-products such as Schiff bases, secondary amines and alcohols are often present in the system.⁵⁴

4. Hydrogenation of furfural to FOL and THFA

4.1 Conversion of furfural to FOL

FOL is the most important industrial chemical derived from furfural,^55,56 accounting for nearly 65% of the annual global production of furfural.⁵⁷ FOL is an important chemical intermediate, widely used in the preparation of pesticides, synthetic plastics and phenolic resins, etc.⁵⁸ It can also be further converted into biofuel molecules, resins, synthetic fibers, and a number of fine chemical products, such as vitamin C, lysine and so on.^59,60 Currently, there is an increasing demand for FOL but a large capacity gap, so efficient catalytic conversion of biomass-based furfural to FOL is of great research significance. However, furfural has a variety of unsaturated functional groups, which poses a considerable challenge for the preparation of FOL by highly selective hydrogenation.

In recent years, researchers have focused on the study of new green and stable catalysts, including noble metal-based catalysts like Pt, Pd and Ru (Table 1),^56,61,62 non-noble metals are mainly used Cu, Co and Ni (Table 2).^63–68 The hydrogen sources used for hydrogenation are usually derived from molecular hydrogen (H₂), methanol, 2-PrOH. The following is an overview of the catalytic reaction systems for the selective hydrogenation of furfural for the preparation of FOL and their latest research progress in recent years from the perspective of both noble and non-noble metal-based catalysts, respectively.

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4.2 Conversion of furfural to THFA

THFA, another product of the selective hydrogenation of furfural, is a chemical produced by the deep hydrogenation of furfural.⁸⁹ It is a colorless and transparent liquid that is widely used as a green industrial solvent in agriculture, printing inks, and industrial and electronics cleaning. It is also used as a substitute for biofuels or fuel additives because of its physical and chemical similarities with kerosene in some parts.⁹⁰ THFA can also be used as a biofuel or fuel additive substitute. THFA can be produced from furfural or FOL by intramolecular CO and CC hydrogenation. Typically, it is produced from furfural by a two-step catalytic process over a supported metal catalyst, starting with the intramolecular CO hydrogenation of furfural to form FOL, followed by the intramolecular CC hydrogenation of FOL to THFA (Table 3).⁹¹

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Noble metal catalysts tend to be highly active, not easily lost, and require mild reaction conditions. For the simultaneous hydrogenation of CC and CO double bonds, the use of multi-metal co-catalysis is also an important idea. Synthesis of bimetallic or polymetallic central catalysts by doping non-noble metal catalysts into noble metal catalysts is the current research hotspot for furfural total hydrogenation catalyst design. The Pd-based bimetallic alloy catalyst (Pd₁Cu₉), prepared by the previously mentioned Chen team by reducing a metal precursor dissolved in an aqueous solution of the sugar-based surfactant GluM, efficiently converts furfural to FOL.⁷⁷ Replacing Cu with Ni in the bimetallic, the Pd₁Ni₃ alloy prepared can catalyze the full hydrogenation of furfural to generate THFA with high selectivity, and the reaction was carried out for 4 h at 50 °C and 1 MPa H₂ with GluM aqueous solution as solvent, and the selectivity of THFA was 96.2%. Wu et al. prepared Pt(3)Ni(3)/C catalysts by distributing small particles of PtNi alloy on activated carbon. Electron-rich metal singlet Pt promotes hydrogen dissociation and facilitates the hydrogenation of furfural, and the furan ring adsorbs on the Ni surface, which together synergistically enhance the catalytic effect.⁹² The furfural conversion was 99% and the THFA yield was 93% after 12 h of reaction at 35 °C and 2 MPa H₂ with H₂O as solvent. Ruan et al. prepared PdNiCo/N-CNTs catalysts by loading trimetallic PdNiCo on nitrogen-doped carbon nanotubes, which showed good catalytic performance due to its own PdNiCo alloy nanostructures as well as electronic synergistic effects between PdNiCo metal nanoparticles and N-CNTs. THFA could be obtained in 97% yield by reacting in ethanol at 120 °C and 3 MPa H₂ for 3 h.⁹¹

Recently, researchers have also focused on the design of catalysts involving only non-noble metals in an attempt to improve the yield by changing the catalyst structure and adjusting the adsorption configuration of furfural on the catalyst. In general, nickel species are active on both the furan ring (CC) and the carbonyl group (CO) of the furfural molecule, whereas copper species are highly active only on the carbonyl group of the furfural molecule,⁹³ and both non-noble metals are also commonly used in furfural hydrogenation reactions. These two types of non-noble metals are also commonly used in furfural hydrogenation reactions. At present, non-noble metal catalysts have not been applied to the direct transfer hydrogenation of furfural to THFA, but there have been many studies on the use of non-noble metal catalysts for gas-phase hydrogenation. Meng et al. utilized metal nitrates or carbonates to prepare two catalysts with highly exposed Ni(111) facets loaded on polymetallic oxides with hydrotalcite as the precursor. The Ni/MMO–CO₃ showed the only selectivity for THFA, with 99% yield in 2-PrOH at a reaction temperature of 120 °C and 3 MPa H₂ for 3 h. The reaction temperature of Ni/MMO–CO₃ was also shown to be the only selective agent for THFA. The characterization revealed that terrace sites are predominant on Ni/MMO–CO₃ surface, which gives furfural a parallel adsorption conformation on the catalyst and facilitates the activated adsorption of CO groups and CC bonds in the furan ring.⁹⁴ Ranaware et al. used cobalt and cobalt oxides loaded on N-doped carbon nanotubes over catalysts (Co/CoO_x@N-CNTs), the absence of Brønsted acid sites in ethanol and the involvement of three hydrogen atoms in the ethanol molecule led to the predominance of THFA as a major product during furfural hydrogenation, and under optimal reaction conditions, the yield of THFA could achieve 99.5%.⁹⁵ Fu et al. employed 3D floral MOF Ni-BDC as a sacrificial template to prepare a carbon-embedded nickel-based catalyst Ni/C-400. With a highly exposed Ni(200) facet and highly dispersed surface NiO species, the catalyst exhibited excellent catalytic performance for the total hydrogenation of furfural, achieving an excellent THFA yield of 98.5% at 80 °C and 1 MPa H₂. The NiO species on the surface has a greater ability to adsorb furfural than Ni particles, while the Ni(200) surface is able to adsorb more dissociated H atoms, which allows the catalyst to achieve both the key to high conversion and high selectivity.⁹⁶ Sheng et al. synthesized precursors of nickel citrate complexes using citric acid as a carbon source and complexing agent, and subsequently prepared Ni-based catalysts (Ni-CA-400) with carbon-covered structure by pyrolysis, which possessed a large specific surface area, abundant defective sites in the carbon layer, and uniform Ni nanoparticles. In furfural hydrogenation reaction, Ni-CA-400 gave a high yield of 99.8% THFA at 80 °C under mild conditions. Such excellent catalytic performance is attributed to the unique carbon-coated structure of the catalyst, which not only effectively avoids the migration and aggregation of the active nickel particles during the preparation process, but also protects the metallic nickel nanoparticles from oxidative exposure to the air, which can lead to catalyst deactivation.⁹⁷

From the above studies, it can be seen that the structure of the catalyst and the size of the catalyst active center have an important influence on the type of adsorption of the reaction substrate, which leads to different selectivity of the hydrogenation reaction. Therefore, identifying the catalytic properties of each active site is crucial for designing hydrogenation catalysts. Meanwhile, alloy structures with high catalytic surface area and tunable surface energy can lead to high catalytic selectivity and activity. How to control the composition of bimetallic alloys to regulate furfural hydrogenation to generate high-value-added derivatives is an important strategy for catalyst design, and a great challenge in catalyst design at present.

4.3 Reaction mechanism

In the existing studies, there are two reaction routes for metal-mediated catalytic hydrogenation and Lewis acid-mediated intermolecular hydride transfer, corresponding to the active components of the metal site and Lewis acid site, respectively.⁹⁸

Xu et al. investigated the mechanism of furfural hydrogenation on Pt_n/CoAlO_x (Fig. 5a). Molecular hydrogen first adsorbed on the metal sites after activation to activated hydrogen and then added to the subsequent reaction. Due to the differences in the gathering of Pt species, furfural has different adsorption modes on Pt single atoms or Pt clusters. The formation of the final product of the furfural hydrogenation reaction is largely influenced by the adsorption selectivity of the CC and CO bonds. The CO of furfural can be adsorbed on both single-atom and clusters, but the CC bond in the furan ring can only be adsorbed on Pt clusters. The selective adsorption of the CO bond on single-atom leads to the unsaturated FOL product, whereas, because of the simultaneous adsorption of CC and CO, the saturated hydrogenation product, THFA, is generated on Pt nanoclusters.⁷⁰ Ranaware et al. investigated the mechanism of THFA generation from furfural hydrogenation on Co/CoO_x@N-CNTs (Fig. 5b and c). The O of the aldehyde group in the furfural molecule is adsorbed on the Co^δ+ center. Furfural was selectively hydrogenated to FOL by H ions dissociated from H₂ at the metal Co⁰ site and H⁺ ions dissociated from ethanol on the O_V–Co₃O₄ surface. Subsequently, FOL was adsorbed on the CoO_x surface and H ions dissociated from H₂ and ethanol saturated the furfuryl ring to produce THFA.⁹⁵

In summary, the mechanism of furfural hydrogenation for the preparation of FOL can be summarized as follows:⁶² (1) in metal-mediated hydrogenation, activated H atoms are first adsorbed onto the metal sites, and then the adsorbed H atoms are added to the CO bond to form an O–H bond; (2) whereas in the Lewis-acid-mediated transfer hydrogenation with alcohols as the hydrogen donor as well as the solvent is transferred according to the MPV mechanism. The MPV mechanism starts with the Lewis acid sites. The adsorbed 2-PrOH and furfural on the L acid site form a propanol–furfural six-membered ring configuration, through which the β-H of the 2-PrOH is transferred to the carbonyl C atom of the furfural (Fig. 5d).

In addition to the commonly assumed pathway of THFA via the carbonyl group of furfural firstly hydrogenated to produce FOL followed by further hydrogenation of the furan ring, Huang et al. found that there is another pathway from furfural hydrogenation to THFA via the THFAL intermediate followed by carbonyl group hydrogenation to produce THFA (Fig. 6).⁹⁸ Huang estimated the activation energy for each step experimentally and computationally. Since Ru/ZrO₂ is well known as an active catalyst for CO reduction, the apparent E_a of furfural over this catalyst after the first step of pathway 1 was calculated to be about 56 kJ mol⁻¹. The second step of pathway 1 was then tested on a Pd/Al₂O₃ catalyst and the apparent E_a was found to be about 44 kJ mol⁻¹. This suggests that the decisive speed step in pathway 1 is the reduction of the CO bond. Using the same method, the apparent E_a of the first step of pathway 2 was first tested on Pd/Al₂O₃, and the apparent E_a of furfural to THFAL was about 21 kJ mol⁻¹. Then after a kinetic study, the apparent E_a for the hydrogenation of THFAL to THFA was tested and obtained to be 30 kJ mol⁻¹. From this, Huang deduced that the linkage structure of the furan ring and the carbonyl group may reduce the hydrogenation activity of the CO bond, and that the hydrogenation of the CO bond of THFAL overcomes a much lower energy barrier than that of pathway 1. Of course, for rapid reactions with such low activation energies, the observed conversion may be affected by mass transfer limitations rather than catalyst activity, which would mask the actual differences between different catalysts, leading to false conclusions. This is also something that researchers need to pay more attention to in the studies.

The proposal of two pathways and the kinetic studies provided a new understanding of the multimetallic synergistic catalytic hydrogenation.

5. Catalytic conversion of furfural to LA and GVL

5.1 Conversion of furfural to LA

LA is one of the 12 most valuable platform compounds identified by the U.S. Department of Energy,⁹⁹ has a wide range of potential for the synthetic production of a variety of high-energy chemicals, and can be produced by dehydration or/and hydrolysis reactions from C5 or C6 compounds.¹⁰⁰ In the molecule of LA, a carbonyl group, a carboxyl group, and α-hydrogen are contained, which can be reacted as carboxylic acid and ketone, and through chemical reactions such as esterification, halogenation, hydrogenation, oxidative dehydrogenation, condensation, etc., a variety of fine chemicals are produced, which are widely used in the industries of fuel additives, agriculture, pharmaceuticals and cosmetics, and also used as oxidizing additives of clean fuels, components of fragrances, and green solvents.¹⁰¹ The conversion of furfural to LA consists mainly of two reactions: hydrogenation and acid-catalyzed ring opening, in which metal active centers or Lewis acid centers are often needed to catalyze the reductive hydrogenation reaction, and Brønsted acid centers are needed to catalyze the hydrolysis and ring-opening reaction and thus the conversion to LA. Therefore, the catalytic system for one-step preparation of LA from furfural needs to have two or more of the above mentioned catalytic active centers. The catalytic reaction system for the one-step preparation of LA from furfural requires two or more of the above catalytic centers (Table 4).

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5.2 Conversion of furfural to GVL

GVL is a five-membered cyclic ester, the five-membered ring consists of four carbon atoms and one oxygen atom, it is very stable at room temperature and normal pressure, with low volatility, low toxicity, good stability, etc. It is suitable for use as a liquid fuel, biodiesel additive, and also has a wide range of applications as green solvents and food additives, and at the same time, it is an important precursor for the production of transportation fuels, fine chemicals and polymers.^106,107 GVL is considered to be one of the most promising biomass-based platform compounds, which is expected to receive increasing attention in academia and industry as a renewable biomass energy source material to replace fossil energy sources.^108,109

The Meerwein–Ponndorf–Verley (MPV) reduction reaction pathway allows the use of inexpensive metal oxides with Lewis acid/base properties instead of precious metal catalysts, and the use of secondary alcohols as hydrogen donors instead of high-pressure H₂ enables selective hydrogenation of LA or furfural to the corresponding products via the transfer hydrogenation pathway.^110–112 Solid acids, such as H-type zeolites (HZSM-5), heteropoly acid and even metal oxides (e.g., γ-Al₂O₃) are very active in the ring-opening reaction of furan rings. Hydrothermally stable molecular sieves such as beta zeolite and ZSM-5 zeolite with good mass transfer effect are commonly used as carriers for this reaction, and the introduction of active centers such as Zr, Hf compounds with Lewis acid sites and phosphotungstic acid (HPW) with Brønsted acid sites are commonly used for designing highly efficient bifunctional catalysts (Table 5).^113–120

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6. Catalytic conversion of furfural to 2-MF

2-MF is an important organic chemical intermediate due to its special physicochemical properties. It can be used as a biofuel and also has high application value in the energy industry. It can be blended with gasoline, thereby reducing gasoline consumption, and 2-MF has a higher combustion efficiency than ethanol. Besides, it is widely used in the production of anti-malarial drugs, insecticides, polyurethane, pharmaceuticals and so on. Especially in the pharmaceutical industry, 2-MF can be used to make vitamin B1, chloroquine phosphate, primaquine phosphate and many other drugs. At the same time, it is also a great intermediate of spice used in food, cigarette flavors. By using 2-MF as the basis, a series of polymer compounds can be synthesized, and it is also a very good organic solvent, with good production prospects. The production of prepared 2-MF requires the hydrodeoxygenation of the CC bond of the furfural side chain, and it has been explored mainly in the gas phase at elevated temperatures based on Pd,^135–137 Ru,¹³⁸ Ni,^139–141 Co^140,142,143 and Cu^144–147 based catalysts (Tables 6 and 7). Presently, many researchers have reported on the preparation of 2-MF from furfural.

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6.1 Noble metal catalysts

Noble metal-based catalysts such as Ru, Re, Pd etc. for the hydrodeoxygenation of furfural to 2-MF have been studied. Yu et al. reported a metal pre-chelate-assisted assembly catalyst strategy, embedding extremely small nanosized Ru clusters in synthesized hollow carbon spheres (Ru@Shell-HCSs). Ru@Shell-HCSs formed a special channel after chelation, which modified the electronic effect of Ru clusters through the interfacial charge transfer mechanism. The electron-deficient property of Ru clusters promoted the intermediate FOL hydrogenolysis, which improved the selectivity of 2-MF. On the other hand, the channel of the catalyst extended the residence time of furfural in the channel by lengthening the path of furfural's diffusion, which accelerated the overall hydrogenation reaction speed. 2-MF yield reached a maximum of 79.1%.¹⁴⁸ Kumaravel et al. prepared a bifunctional catalyst on SBA-16, loaded with Ru and Ni bimetals (Ru_0.8Ni_0.2SB-16) using in situ hydrothermal method and showed 100% conversion of furfural and 88% yield of 2-MF after 4 h at 180 °C, 0.1 MPa H₂. The synergistic effect of metal and acidic sites on the surface of the catalyst is responsible for the high selectivity towards 2-MF. In addition, the catalysts prepared using in situ hydrothermal method were highly stable compared to those prepared by other methods. Ru_0.8Ni_0.2SB-16 exhibited no leaching of Ni and Ru in 5 cycles of use.¹⁴⁹ Nagpure et al. designed Ru-based catalysts loaded with nitrogen-doped mesoporous carbon (Ru/NMC), which were able to completely convert furfural and yield 87% of 2-MF in a 10 h reaction at 140 °C using isopropanol as a hydrogen source. During the study it was found that the incorporation of nitrogen in the mesoporous carbon framework helps to stabilize the Ru metal nanoparticles on the support and hence, improves the metal–support interaction and with increasing N-content in the catalyst leads to formation of smaller size of Ru nanoparticles. When the mass fraction of nitrogen in the catalyst was 11.4%, the catalytic performance was the best.¹⁵⁰

Zhou et al. prepared a 5Cu3Re/Al₂O₃ bifunctional catalyst using a conventional impregnation method. Using isopropanol as the hydrogen donor, 94.0% yield of 2-MF was achieved at 220 °C for 4 h. The excellent performance of the bimetallic CuRe/Al₂O₃ catalysts can be attributed to the synergistic effect of the Al₂O₃ carrier, metal Cu and Re species. In particular, Al₂O₃ has abundant acid–base sites and large specific surface area, and the dehydrogenation of isopropanol and the hydrogenation of furfural to FOL can be enhanced with the introduction of Cu on Al₂O₃ surface. The modification of Re species, on the other hand, promotes the further hydrogenolysis of FOL to 2-MF. In addition, the strong interaction of ReO_x with Cu and Al₂O₃ results in the high stability of the bimetallic catalysts. The selectivity of 2-MF and the catalyst activity did not change obviously during five cycles.¹⁵¹

The above studies have brought some inspirations for future development and research on noble metal catalysts. (1) Developing more catalysts for green solvents such as H₂O, isopropanol, etc. (2) Doping non-precious metals to construct bifunctional catalysts to reduce the cost and at the same time to realize the improvement of catalytic performance. (3) Designing the catalyst structure on the nanoscale and constructing the active sites on the atomic level.

6.2 Non-noble metal catalysts

Single-metal catalysts are limited in the preparation of 2-MF from furfural for a variety of reasons. For example, in most cases, it is difficult to produce 2-MF by furfural preparation over Ni-based catalysts due to the excessive hydrogenation of the furan ring and the decarbonation reactions. The hydrogenolysis activity of Cu on C–O bonds is relatively low compared to the selective hydrogenation of aldehyde functional groups. Therefore, it is often necessary to add a second metal to the catalyst, commonly such as Mo and Fe, to compensate for the limitations exhibited by single-metal based catalysts in the reaction.^153,154 On the basis of this, a large number of bimetallic and even trimetallic-based catalysts have been reported in the gas-phase reaction system.

Liu et al. prepared a NiMo  intermetallic compound catalyst (NiMo IMC) based on a layered bimetallic hydroxide precursor, which exhibited outstanding catalytic performance for the hydrodeoxygenation of furfural to 2-MF. 99% yield of 2-MF was achieved by reacting at 200 °C for 4 h at a rather low hydrogen pressure (0.1 MPa).¹⁵⁵ Characterization showed that the atomically-ordered Ni and Mo sites in the catalyst determined the furfural molecule exhibits a unique adsorption pattern, with the CO group binding to adjacent Ni–Mo sites (via Mo–O and Ni–C bonds). Furan rings exhibit weak, tilted physisorption on the NiMo IMC(331) surface through the Ni–C bond. This unique configuration promotes the activated adsorption of CO groups, whereas the adsorption of the furan ring is greatly inhibited, which leads to the inhibition of saturated hydrogenation of the furan ring and the breakage of the C–O bond outside the ring. Wang et al. also synthesized the catalyst Ni/Fe₃O₄ from layered bimetallic hydroxides. When the Ni/Fe ratio was 1, the yield of 2-MF reached 78.4% under mild conditions (100 °C, 1 MPa H₂, 4 h).¹⁵⁶ Characterization revealed that the Ni²⁺/Fe³⁺ ratio was closely related to the dispersion and surface acidity of the catalyst. The surface-exposed active metal Ni and the strong acidity of the carrier surface favored the activation of the CO group of furfural. Meanwhile, the strong interaction between Ni particles and Fe₃O₄ carriers enhanced the stability of the catalyst. Wang et al. prepared catalysts of bimetallic Ni–Cu alloy loaded on ZSM-5 zeolite by a simple impregnation method. Through previous studies, it was demonstrated that the Cu/ZSM-5 catalyst was able to modify the adsorption mode of the CO bond in furfural, avoid saturation of the furan ring by presenting a tilted adsorption, but the conversion was low. While adding Ni to Cu/ZSM-5 in appropriate amount can improve the conversion and enhance the adsorption of furfural and hydrogen molecules at the same time. In addition, the acidity of the zeolite surface favors the cleavage of the C–O bond in the intermediate FOL. After 30 min of reaction at 220 °C, the yield of 2-MF was 78.8%.¹⁵⁷ Gong et al. synthesized a nickel–cobalt bimetallic catalyst (NiCo–MgAlO) catalyst using a co-precipitation method, achieving 100% furfural conversion and 92.3% 2-MF selectivity.¹⁴⁰ Bimetallic Ni–Co–MgAlO catalysts with uniform Ni–Co alloy nanoparticle compositions and more medium acid sites than single-metal catalysts are able to promote C–O bond breaking to improve 2-MF selectivity due to their oxytropism. Dou et al. prepared a CuCo alloy bimetallic catalyst (CuCo/NC) packed in a hollow nitrogen-doped carbon cage using ZIF-67 as a sacrificial template. After characterization and DFT calculations, it was shown that introduced Cu species, on the one hand, modified the electronic structure of Co, leading to a change in the adsorption configuration of furfural from parallel to vertical on the surface of the catalyst, which successfully impeded the hydrogenation of the furan ring. On the other hand, the formation of CuCo(111) crystalline face promotes hydrogen dissociation, as well as the cleavage of the C–O single bond, which lowers the diffusion barrier of hydrogen. The two aspects together promote the formation of 2-MF. Under relatively mild reaction conditions (150 °C, 1.5 MPa H₂ and 4 h), the yield of 2-MF was as high as 95.7%.¹⁵⁸ An et al. investigated transition metal oxide-loaded Cu catalysts prepared by sol–gel method with Mo as an additive (Cu–Mo/CoO_x) to catalyze the hydrodeoxygenation of furfural to produce 2-MF. At Cu/Mo molar ratio of 3, furfural conversion exceeded 99% and 2-MF yield was as high as 92% at 180 °C and 2 MPa H₂.¹⁴⁴ XRD, XPS and CO-DRIFTS showed that the addition of Mo not only accelerated the formation of CuCo alloys in the catalysts, but also due to the electron transfer from Co to Mo. Enhanced the positive electronegativity of Co^δ+, which in turn affected the adsorption of aldehyde groups on the carriers. The abundant Lewis acid sites on CoO_x promoted C–O cleavage, leading to format more 2-MF. Koley et al. prepared Cu-based/mixed metal oxide catalysts (Cu/CuFe₂O₄@C-A) loaded on carbon using metal–organic skeletons as sacrificial templates. In this case, Cu can help furfural hydrogenation to alcohols, while the metal oxides can contribute to the hydroxyl removal step in the hydrodeoxygenation process.¹⁵⁹ In Cu/CuFe₂O₄@C-A catalyst, the charge transfer between the carbon substrate and the metal oxides promotes the stabilization of the active metal sites in the ordered carbon framework, which contributes to the catalytic efficiency. About 90% yield of 2-MF was obtained by reacting at 165 °C, 1.5 MPa H₂ for 5 h.

For preparation of 2-MF by catalytic hydrogenation and dehydration of furfural, liquid-phase hydrogenation is usually prone to side reactions and has lower selectivity. Niu et al. using isopropanol as a hydrogen donor, produced 2-MF by catalytic transfer hydrogenation of furfural in the liquid phase via CuZnAl catalyst. The results show that the intermediate FOL can be generated immediately in the reaction system. Subsequent C–O cleavage is the decisive step in the overall reaction. This step depends on the amount of Cu in the catalyst. The synergistic effect of Cu⁰ and Cu⁺ species on the catalyst surface, as well as the CuAl₂O₄ phase, gives the catalyst a good catalytic performance. For 4 h of reaction at 180 °C, the 2-MF yield was 72%.¹⁶⁰ More et al. investigated MFe₂O₄ (M = Cu²⁺, Ni²⁺, Fe²⁺) antispinel catalysts and prepared several kinds of catalysts with magnetic properties and easy separation. Among them, CuFe₂O₄ was able to achieve 99.4% furfural conversion and 97.6% 2-MF selectivity at 200 °C over 1.5 h. In contrast, NiFe₂O₄ and Fe₃O₄ were mainly producing the hydrogenation product FOL. Upon characterization and analysis, it was found that the acidity of the catalyst is derived from the Lewis acid site, and its strength is directly related to the binding energy of Feⁿ⁺ ions in the MFe₂O₄. The Cu²⁺ in CuFe₂O₄ has a stronger interaction with isopropanol, while the Fe³⁺ acts as active Lewis acid site centers, both of which work together to improve the catalytic activity of the catalyst.

6.3 Reaction mechanism

Preparation of 2-MF from furfural typically through FOL intermediate, so the reaction basically consists of two steps, hydrogenation of the aldehyde group of furfural and breaking of the extra-ring C–O bond. So far, many studies have been done on the mechanism of the catalytic conversion of furfural to 2-MF. Liu et al. propose reaction mechanism for the hydrodeoxygenation of furfural to 2-MF in the presence over NiMo intermetallic compound by in situ FT-IR and DFT calculations (Fig. 9a):¹⁵⁵ (1) the CO bond of the furfural molecule is activated by adsorption through Mo–O and Ni–C bonds, and the furan ring occurs weakly tilted adsorption. At the same time, hydrogen molecules dissociate on neighboring Ni atoms to form active hydrogen atoms. (2) The CO bond is hydrogenated to produce the intermediate product FOL. (3) C–O bond in FOL cleavage to form methylidene furan and hydroxyl group. (4) Methylene furan and hydroxyl group combine with active hydrogen atoms to form 2-MF and H₂O. (5) 2-MF molecule desorbs from the catalyst surface.

Dou et al. similarly deduced the reaction pathway for this reaction by theoretical calculations (Fig. 9b):¹⁵⁸ H₂ initially adsorbs on the active sites of Co and CuCo alloy and is dissociated into activated hydrogen. The carbonyl group of furfural molecule is firstly adsorbed vertically on the CuCo(111) interface and reacts with activated H atoms to form the intermediate FOL. Subsequently, the hydrodeoxygenation of FOL was realized due to the strong destructive effect of CuCo alloy on the C–O bond of FOL and the strong spreading effect of H species to obtain the final product 2-MF. Eventually, 2-MF was desorbed from the catalyst and the whole hydrodeoxygenation reaction process was successfully completed.

Li et al. then proposed a new concept – the important role of oxygen vacancies during the reaction (Fig. 9c).¹⁶¹ This is a very interesting and novel statement. They pointed out that the mechanism of the reaction could be: first, the oxygen vacancies enhance the adsorption of furfural by CoAl₂O₄, resulting in vertical adsorption of furfural on carrier. In this case, the O atom of the aldehyde group inserts into the oxygen vacancy, while the O atom of the furfuryl ring adsorbs on the Co active site. Subsequently, the activated hydrogen dissociates from Co⁰ and combines with the aldehyde group to form FOL, and then FOL desorbs from the catalyst. For FOL, the presence of oxygen vacancies does not change its adsorption configuration on the surface, but greatly increases its adsorption capacity. FOL sequentially undergoes C–O cleavage, hydroxyl hydrogenation, water desorption, and methylene furan hydrogenation to obtain 2-MF. By theoretical calculations, they suggest that the oxygen vacancies appropriately weaken the adsorption of activated hydrogen by Co in the neighboring active site, thus promoting the spillover of Co-desorbed activated hydrogen to FOL, and thus lowering the energy barrier from FOL to 2-MF.

Zhang et al. focused on transfer hydrogenation catalytic mechanism with methanol as the hydrogen donor (Fig. 9d).¹⁶³ First, methanol adsorption reforming occurs on CuO(200)/Cu₂O(111), splits methanol into CO and H₂. Due to the presence of H₂, Cu²⁺/Cu⁺ is partially reduced to Cu⁰ at 200 °C. In addition, a large amount of CuO_x remains in the catalyst, providing an indispensable acid site for furfural conversion. Subsequently, H atoms adsorbed at Cu⁰ attack the CO bond to obtain the intermediate product FOL. Then, the O atoms on the FOL hydroxyl group are activated by Brønsted acid site to form transition state E. And then the reactive H atoms attack the α-positions of O and C atoms to form transition state F with the help of the Brønsted acid site, and be cleaved into a methyl group and a H₂O molecule. The hydrogenolysis product 2-MF was finally obtained.

Overall, the adjustment of the adsorption configuration of furfural molecules on the catalyst surface and the coordination of Lewis acid and Brønsted acid sites on the catalyst carrier surface is very important for both gas-phase hydrogenation and transfer hydrogenation. In general, the furfural molecules should be controlled to maintain a tilted or even vertical adsorption on the surface, and at the same time, the adsorption on the CO bond should be strengthened as much as possible. This will benefit the generation of the intermediate FOL and effectively inhibit the saturated hydrogenation of the furan ring. The investigators’ study of the reaction mechanism for the preparation of 2-MF from furfural plays an important role in the subsequent directed design of catalysts and in the effective modulation of product selectivity.

7. Catalytic conversion of furfural to PeDs

PeDs, especially 1,2-PeD and 1,5-PeD, are valuable and widely used fine chemicals. 1,2-PeD is the most critical intermediate in the process of synthesizing propiconazole, which can prepare fungicides, and can also be used for synthetic skin care products, baby products, preservatives, polyester fibers, plasticizers, emulsifiers, etc. In industry, it can also be used as raw materials for the production of ink, surfactants and cosmetics.²⁰ 1,5-PeD and its downstream products are also widely used in the processing and preparation of saturated/unsaturated polyester resins, esters for synthetic lubricants, polyester polyols, elastomer plasticizers, advanced lubricant additives and other fine chemicals. In addition, it can also be used as an antifreeze and an environmentally friendly solvent in chemical synthesis.⁴³

With the continuous development of the economy, people's demand for PeDs is growing rapidly, but in recent decades, the traditional PeDs preparation is still mostly produced from petrochemical sources, which does not meet the green development concept, and there are widespread shortcomings such as serious equipment rot, high production costs and complex processes, which greatly restricts the large-scale production of PeDs.¹⁶⁴ Due to the limited production capacity, usually in short supply, so the efficient synthesis of PeDs has excellent development prospects. Compared with the route using fossil energy as raw material, the preparation of 1,5-PeD using biomass-based furfural as raw material requires fewer reaction steps, can avoid the use of toxic reagents, and has high atomic economy. However, because the synthesis technology is not mature and the product yield is low, how to develop a synthesis process route with low cost, mild reaction conditions and environmental friendliness is the key to breaking through the current PeDs production bottleneck and improving the PeDs yield, which has very important research value.¹⁶⁵

7.1 Noble metal catalysts

The catalysts used for selective catalytic hydrogenation of furfural for PeDs synthesis are mainly group VIII noble metal supported catalysts, such as Pt-, Pd-, Rh-, and Ru-based catalysts (Table 8). In order to realize the preparation of PeDs under low cost and mild reaction conditions, Xu et al. first reported Pt/Co₂AlO₄, a bifunctional catalyst with noble metal platinum loaded on a novel nanosized mesoporous cobalt-aluminum spinel with high specific surface area and appropriate alkalinity. The catalyst can directly catalyze furfural to 1,5-PeD via hydrogenolysis of the furan ring at a milder condition. The furfural conversion was more than 99%, the yield of 1,5-PeD was 30.6%, and the yield of 1,2-PeD was 10.3% at 110 °C and 1.5 MPa H₂ for 24 h. Xu used Li-modified Pt/Co₂AlO₄ to react at 140 °C and 1.5 MPa H₂ for 24 h, the 1,5-PeD yield increased to 34.9% and the 1,2-PeD yield increased to 16.2%. It is because Li⁺ can increase the low-coordinated oxygen ion density on the surface of the oxide support, so that the support has a higher basicity, and thus improves the selectivity of the target product.¹⁶⁶ Mizugaki et al. prepared the hydrotalcite-supported Pt nanoparticles (Pt/HT) by impregnation, reacted using 2-PrOH as the solvent at 150 °C and 3 MPa H₂ for 8 h. The yield of 1,2-PeD was 73%, and the furfural conversion was complete.¹⁶⁷ Barranca et al. used a layered double hydrotalcite (LDH)-derived catalyst (Pt@Co²⁺) in the form of mixed metal oxide (MMO) in ethanol at 150 °C and 3 MPa H₂ for 4 h, and the yield of 1,5-PeD was 47%.¹⁶⁸ Yeh et al. prepared a novel alumina supported Pt catalyst with high loading and uniform distribution of Pt nanoparticles by in situ synthesis of a PT-embedded metal–organic skeleton (Pt@Al₂O₃), at a low temperature (45 °C), the yield of 1,5-PeD was 75.2% using H₂O as solvent and NaBH₄ as reducing agent for 8 h. The good catalytic performance of the catalyst is attributed to the strong metal–support interaction between Pt and aluminum.¹⁶⁹

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Date et al. prepared Pd/MMT-K10 metallic acid two-center catalyst with montmorillonite as the carrier and used it for furfural hydrogenation to prepare 1,2-PeD. The yield of 1,2-PeD was 66% in 2-PrOH at 220 °C and 3.5 MPa H₂ for 5 h. Py-IR confirmed the presence of Brønsted acid in the catalyst, which leads to the cleavage of furan rings after the formation of the first intermediate of furfural hydrogenation.¹⁷⁰ Liu et al. studied the one-pot selective conversion of furfural to 1,5-PeD by two-step reaction on Pd-added Ir–ReO_x/SiO₂ catalyst. With water as solvent, the reaction was first performed in 6 MPa H₂ at 40 °C for 8 h, and then at 100 °C for 72 h under the same hydrogen pressure, the highest yield of 1,5-PeD was 71.4%.¹⁷¹

Noble metal Rh also has good hydrogenation activity, so Liu et al. introduced Rh on Ir–ReO_x/SiO₂ catalyst. Similar to the previous study, the highest yield of 1,5-PeD was 78.2%, under the hydrogen pressure of 6 MPa, first at 40 °C for 8 h, and then at 100 °C for 32 h, with water as solvent. The catalyst characterization results showed that the Rh–Ir–ReO_x/SiO₂ catalyst exhibits an Ir–Rh alloy particle structure partially covered by ReO_x substance, and the formation of Ir–Rh alloy is a necessary condition for high hydrogenation activity.¹⁷² Pisal et al. synthesized a bifunctional catalyst (Rh/OMS-2) with base and metal sites by hydrothermal method. The yield of 1,2-PeD was 87.0% at 160 °C and 3 MPa H₂ for 8 h. Based on the obtained results and the characterization of the catalyst, it is found that there is a metal–base synergistic effect between the catalyst base carrier and the active metal, and the highly selective production of 1,2-PeD can be achieved through the adsorption and dissociation of H₂ at the metal site and the adsorption type of the substrate at the alkaline site. Ru-based catalysts are also commonly used in the study of PeDs synthesis of furfural.¹⁷³ Upare et al. used Ru₃Sn₇ alloy-supported ZnO catalyst (Ru–Sn/ZnO) for catalytic conversion of furfural to 1,2-PeD, which can effectively limit the formation of 1,5-PeD and other hydrolytic byproducts. Furfural could be completely transformed and the yield of 1,2-PeD reached 84.2% in 2-PrOH at 150 °C and 3 MPa H₂ for 6 h. The highly selective formation of 1,2-PeD in furfural is attributed to the synergistic effect of Ru₃Sn₇ alloy phase and SnO_x on basic ZnO support.¹⁷⁴

For research use, in addition to the commonly used stainless steel batch reactors, fixed-bed reactors have also been used for catalyst performance testing. Fixed-bed reactors refer to filling particle solid catalysts or reactants in the reactor, forming a certain height of the stacked bed. And then the gas or liquid materials pass the gap between particles through the fixed bed layer at the same time, to realize the heterogeneous reaction process. Fixed-bed reactors have a wide range of applications in industry due to the simple structure, high reaction efficiency, full utilization of catalysts and controllability. Brentzel et al. prepared Ru–TiO₂ catalyst and developed a process to synthesize 1,5-PeD from furfural using dehydration/hydration, ring-opening tautomerization and hydrogenation reaction. With water as solvent, 1,5-PeD with 84% yield can be obtained at 110 °C, 2 MPa H₂ and H₂ flow rate of 40 mL min⁻¹.¹⁷⁵

7.2 Non-noble metal catalysts

Although noble metal-based catalyst has high catalytic activity, due to the limited resources and high price, it increases the preparation cost of catalysts and greatly limits the production scale of PeDs. In recent years, some non-noble metal-based catalysts, such as Ni, Co, and Cu, have been investigated for the preparation of PeDs by furfural hydrogenolysis (Table 9).

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Wijaya et al. prepared Ni–Y₂O₃ catalyzed hydrogenolysis of furfural by coprecipitation method. 2-PrOH was used as solvent and at 150 °C, 2 MPa H₂ for 72 h, the yield of 1,5-PeD was 46.0% and that of 1,2-PeD was 1.9%, showing good selectivity of 1,5-PeD. When changing the support, Ni–La(OH)₃ catalyzed furfural hydrogenolysis under the same conditions, 1,5-PeD yield increased to 55.8%, 1,2-PeD yield was 2.8%, also maintained a good 1,5-PeD selectivity. It is proved by characterization that Y³⁺ or La³⁺ has strong interaction with furfural carbonyl, but weak interaction with oxygen in furan ring, which is conducive to the selective breaking of C–O bond.¹⁷⁶ Bretzler et al. used Ni/W_xC/SiO₂ in 2-PrOH, at 200 °C, 6.5 MPa H₂ for 6 h, the combination of Ni and W_xC in the catalyst to open the furan ring to produce 1,2-PeD, the final yield of 21%.¹⁷⁷ Shao et al. synthesized bimetallic Ni–Fe catalysts (Ni_1.6Fe_0.4MgAl). The results show that the lower Fe content of catalyst contributes to the formation of mesoporous structure, but the higher Fe content destroys the LDH structure and leads to the decrease of surface area. Fe can also react with the metal Ni to form Ni–Fe alloys, which reduces the catalytic activity of further hydrogenation of furan rings in FA, so furfural will have the opportunity to produce PeDs through hydrogenolysis reaction. In ethanol, at 170 °C, 4 MPa H₂ for 3 h, the yield of 1,5-PeD was 31%, and that of 1,2-PeD was 25.9%.¹⁷⁸ Kurniawan et al. prepared Ni–CoO_x–Al₂O₃ with the same hydrotalcite structure, and in ethanol at 160 °C and 3 MPa H₂ for 6 h, the yield of 1,5-PeD reached 47.5%. Ni⁰ promotes the dissociation and adsorption of H₂ molecules, and the liberated H atoms are then transferred to the oxygen vacancy CoO_x (O_V–CoO_x) site through the hydrogen overflow mechanism. Then the H atom attacks the CO group, and finally the furan ring is cleaved to form 1,5-PeD.¹⁷⁹ Shao et al. synthesized a layered bimetallic oxide Cu_1.8Mg_1.2Al catalyst with highly dispersed Cu particles and adjustable base site, and at 140 °C and 6 MPa H₂ for 10 h to obtain a 1,2-PeD yield of 46%. The different content of Mg in the catalyst promoted the dispersion of copper oxide and the exposure of metal copper, weakened the interaction between copper oxide and the support, inhibited the agglomeration of metal copper substances, and promoted the catalytic activity.¹⁸⁰ Fu et al. prepared CuMgAlO catalyst by urea decomposition method and at 150 °C, 6 MPa H₂ for 6 h, the furfural conversion was 84.1%, and the selectivity of 1,2-PeD and 1,5-PeD was 54.9% and 27.8%, respectively. Through the characterization results, it was found that the ratio of Cu⁺/Cu²⁺ and the amount of acid and base played a key role in the catalytic performance of the catalyst during the calcination process, indicating that the selectivity of PeDs depends on the synergistic effect of Cu⁺ and acid and base properties.¹⁸¹ Hai et al. synthesized a series of Cu_xCo_3−xAl hydrotalcite based catalysts with different Cu/Co molar ratios. When the Cu/Co molar ratio was 1:29 (Cu_0.1Co_2.9Al), the furfural conversion was 100% and the total PeDs yield was 51.1% after reaction in ethanol at 140 °C and 4 MPa H₂ for 6 h. The yield of 1,5-PeD was 41.1%.¹⁸²

The preparation of PeDs by furfural catalyzed conversion has also been studied using non-noble metal-based catalysts in fixed-bed reactors. Gavilà et al. attempted to convert furfural directly to 1,5-PeD in a spinel-structured Co–Al catalyst.¹⁸³ After the reduction of spinel at 500 °C, the selectivity of FOL was high. However, at a higher reduction temperature (700 °C), furfural can be catalyzed to directly generate PeDs. The yield of 1,5-PeD was 30% after 8 h with 2-PrOH as the solvent at 150 °C and 3 MPa H₂. The difference in catalyst activity at different reduction temperatures was attributed to the formation of cobalt nanoparticles in different states on the surface after high temperature reduction.

7.3 Reaction mechanism

At present, there are two main ways for furfural to prepare PeDs (Fig. 10): (1) the CO bond in furfural was hydrogenated to produce THFA, which was further opened to get PeDs. (2) The CO group in furfural is reduced to form the FOL intermediate, and the FOL unsaturated furan ring undergoes ring-opening reaction, followed by further hydrogenation reaction to obtain PeDs selectively. The C⁵–O¹ bond and C²–O¹ bond on furan ring can generate 1,2-PeD and 1,5-PeD, respectively.

Date et al. investigated the reaction mechanism of furfural on the catalyst Pd/MMT-K10 to generate 1,2-PeD (Fig. 11a). It was deduced experimentally that the reaction started with the hydrogenation of the carbonyl group of furfural at the Pd site to produce FOL, followed by the extraction of H⁺ from MMT-K10 via the O of the furan ring (I) and simultaneous semi-hydrogenation of the ring to give the intermediate shown in (II), which is adsorbed on the surface of the catalyst via the C⁵–Pd bond. Such an adsorbed conformation allowed the subsequent hydrolytic breaking of the C⁵–O¹ bond, followed by the formation of 1,2-PeD.¹⁷⁰ Kurniawan et al. explored the mechanism of furfural conversion to 1,5-PeD on N_4.5C_3.5A1(1:2)-R (Fig. 11b). First, the H₂ molecule adsorbed on the Ni⁰ site dissociates into H atoms overflowing into the CoO_x site. Subsequently, the O¹ atom in the furfural carbonyl group adsorbs on the Co^δ+ site. The two H atoms then attack the C–O group of the adsorbed furfural to produce FOL, while the O_V–CoO_x on the catalyst surface will preferentially induce the breaking of the C²–O bond. After ring opening, H attacks the double bond and O² atom in the furan ring to produce 1,5-PeD.¹⁷⁹ The mechanisms of furfural ring-opening to produce PeDs are various so far, but it has been proved that the hydrogen overflow effect and the presence of oxygen vacancies are favorable for the reaction, so the design and preparation of catalysts on this basis will be beneficial to further improve the selectivity of PeDs.

In general, the direct conversion of furfural to PeDs is difficult and the yield is relatively low. This is because the adsorption type of furfural on the catalyst surface is related to many factors, and the reaction course may be conducive to the occurrence of other hydrogenation or hydrogenolysis reactions, which is not conducive to further ring opening. There are still some shortcomings in the current research, such as high cost of noble metal catalysts, harsh reaction conditions of non-noble metals, low product yield, poor stability, and unclear catalytic reaction mechanism. Although some research progress has been made compared to the past, there are still several problems to be solved in converting furfural directly to PeDs: (1) the specific action mechanism of mixed-valent oxide species in the reaction state, as well as related experiments and theoretical verification. (2) The non-noble metal-based catalyst used to avoid high-cost waste of noble metals has a relatively low yield of direct conversion of furfural to PeDs with high selectivity. (3) The selective ring-opening of furan rings in furfural is highly dependent on the adsorption types of furan rings and their intermediates as well as the order of hydrogenation and ring-opening.

8. Catalytic conversion of furfural to CPO

CPO is an important fine chemical with a wide range of applications. It is a raw material for the pharmaceutical and spice industries, and can be used to prepare a new type of spice, methyl dihydrojasmonate, and is also used in rubber synthesis, biochemical research, and as an insecticide.¹⁸⁴ Many methods are available for the synthesis of CPO, and currently the main methods are adipic acid pyrolysis and cyclopentene oxidation.^185–187 However, the raw materials for both methods are from fossil raw materials. In recent years, with the increasing depletion of petroleum resources, the synthesis of CPO through biomass-based chemicals has gradually become the focus of attention. Therefore, the synthesis of CPO by furfural not only solves the problem of shortage of raw materials in the traditional preparation method, but also improves the utilization of biological resources, which is in line with the requirements of sustainable development of green chemistry.

Some studies showed that, in organic solvents, furfural hydrogenation is easy to form FOL, THFA, 2-MF, 2-THMF and other by-products, and furfural preparation of CPO reaction, water is essential, and weakly acidic or neutral aqueous solution is conducive to the generation of CPO.^188–190 The following is a review and analysis of the reactions of furfural hydrogenation to CPO catalyzed by metal-based catalysts in aqueous media in recent years (Table 10).

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8.1 Noble metal catalysts

The noble metal catalysts that have been reported for furfural preparation of CPO are Pd-based, Pt-based, and Au-based. Pd-based catalysts are the most commonly used catalysts for furfural to prepare CPO. Wang et al. successfully prepared Pd/UiO-66-NO₂ catalysts, with 98.9% furfural conversion and 96.9% CPO selectivity under the conditions of 150 °C, 1.0 MPa H₂ pressure and 5 h reaction time.¹⁹¹ Gao et al. designed a bifunctional catalyst (Pd/7.74% Y₂(Sn_0.65Al_0.35)₂O_7−δ/Al₂O₃) with highly selective CO hydrogenation and Lewis acid-catalyzed ring rearrangement, which showed a furfural conversion of 99.9% at 150 °C, H₂ pressure of 4 MPa for 6 h. The CPO yield was 98.1%.¹⁹² Deng et al. synthesized three Lewis acidic zirconium-type carriers A₂B₂O₇ with the same crystal structure but different metals to modulate the Lewis acidity and surface properties of zirconium–titanium ores by inserting different kinds of A- and B-site metals. The selectivity of furfural for CPO was 84% through the synergistic action of Pd, acid site and oxygen vacancy.¹⁹³ Similarly, Tong et al. prepared Pd/La₂Ti₂O₇ catalyst, the support well-regulated Lewis acidity and metal–support interaction, and obtained more than 82% CPO yield through hydrogenation ring rearrangement reaction.¹⁹⁴ Yuan et al. prepared Pd/CeO₂/SiO₂ catalysts by a multistep deposition method, where Pd was confined to defective CeO₂ nanoclusters that were dispersed on high surface area SiO₂ with strong acidity. Furfural is vertically adsorbed on the oxygen-rich vacancies of Pd/CeO₂, which contributes to the selective hydrogenation of the carbonyl group, and the formed FOL gives CPO by acid-catalyzed furan ring rearrangement on SiO₂. Under the conditions of 150 °C, 2 MPa and 3 h, the conversion of furfural was 93% and the selectivity of CPO was 84%.¹⁹⁵

Liu et al. used porous carbon doped with heteroatoms as the support to prepare Pt/C catalyst. Due to its graded porous structure, high content of nitrogen and oxygen functional groups, highly dispersed Pt nanoparticles, good water dispersibility and reaction stability, it showed good activity for the hydrogenation of furfural. When Pt/NC-BS-800 is used as catalyst, the CPO yield was >76% at 150 °C and 3 MPa H₂.¹⁹⁶

Au-based catalyst has a weak catalytic hydrogenation ability, it is suitable for the conversion of furfural to CPO without producing other side reactions such as furan ring hydrogenation. Zhang et al. used anatase TiO₂ nanorods with only mild Lewis acidic sites as carriers to load Au. At 160 °C, 4 MPa H₂ pressure for 15 h, the conversion of furfural was 99%, and the CPO yield was >99%. The combination of Ru and acid center is also the preferred catalytic system for the preparation of CPO from furfural.¹⁹⁷ Fang et al. designed a supported Ru nanoparticle catalyst (Ru/MIL-101) that achieved complete conversion of furfural within 2.5 h at 160 °C and 4.0 MPa H₂, with a selectivity of >96%.¹⁹⁰

The bimetallic supported catalyst also showed high activity in the catalytic hydrogenation of furfural to prepare CPO. The strong bimetallic synergistic effect between the two metal components was used to improve the catalytic performance of the catalyst. Deng et al. synthesized a Pd/Cu-BTC catalyst with Lewis acidity. Due to the high dispersion of Pd nanoparticles, furfural was almost completely transformed and CPO was more than 90% selective at 150 °C and 4.0 MPa H₂ for 24 h.¹⁹⁸ Li et al. found that with Pd/Fe-MIL-100 as catalyst, furfural conversion was 99%, and CPO selectivity was 93.1% at 150 °C, 4.0 MPa H₂ for 6 h. When catalyst was reused for 5 times, its catalytic activity did not decrease.¹⁹⁹ They also found that, using Pd/FeZn-DMC as catalyst, the furfural conversion was 99.9% and the CPO selectivity was 96.6% under the condition of 150 °C, 4.0 MPa H₂ for 6 h. The catalyst was reused for 6 times, and the catalytic performance was almost unchanged.²⁰⁰ Wang et al. synthesized a core–shell Pd–Co@UiO-66 catalyst with furfural conversion of 99% and CPO selectivity of 96% at 120 °C and 3 MPa H₂ for 12 h.²⁰¹ Zhang et al. used PdZn/ZnO as catalyst at 120 °C and 4.0 MPa H₂ for 12 h, and found that the catalyst showed catalytic generality for the synthesis of CPO from furfural compounds (i.e., furfural, 5-hydroxymethylfurfural and 5-ethyl furfural) and furanone (i.e., 2-furanmethyl ketone and 2-furanethyl ketone), the yield was over 90%.²⁰²

8.2 Non-noble metal catalysts

Non-noble metal catalysts commonly used in the preparation of CPO for furfural include Cu-based, Ni-based, and Co-based catalysts, among which Cu-based and Ni-based catalysts are the most commonly used non-noble metal catalysts (Table 11). Zhang et al. used Cu/ZnO catalyst to react at 150 °C and 1.5 MPa H₂ for 4 h, and the CPO yield was 91.3%.²⁰³ Tian et al. studied furfural hydrogenation on Ni/SiO₂ catalyst with a yield of 83.5% at 160 °C and 3 MPa H₂ for 3 h.²⁰⁴ Gao et al. introduced P into Ni/Al₂O₃ to modify the distribution of acid sites and regulate the activity of metal sites. The balanced distribution of hydrogenation sites and acid sites in the catalyst, and their regulatory activities for hydrogenation and acid catalytic reactions are essential for the selective conversion of furfural to CPO. The CPO yield was 85.5% at 3 MPa H₂ and 150 °C for 2 h.²⁰⁵ Gong et al. used metal nanoparticles encapsulated in nitrogen-doped carbon nanotubes (Co@NCNTs) to react at 140 °C and 4 MPa H₂ for 5 h, and achieved high yield synthesis of furfural to CPO, with CPO yield of 75.3%.²⁰⁶ Wang et al. prepared N-doped carbon nanotubes (CoNP@N-CNTs) of Co nanoparticles. The furfural conversion was 100% and the CPO selectivity was 95% at 0.5 MPa H₂ and 160 °C for 8 h.²⁰⁷ Li et al. synthesized a novel nitrogen-doped capped catalyst (Co@NC) for catalyzing the selective hydrogenation of furfural in the aqueous phase to produce CPO. 86.5% yield of CPO was obtained at 200 °C and 3 MPa H₂. In this process, FOL was considered as the main intermediate of the reaction. The N atom as an electronegative heteroatom in the catalyst not only enhances the adsorption of furfural but also provides a weak Lewis acid site for the generation of CPO.²⁰⁸

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The single metal-based catalyst showed good catalytic effect on the conversion of furfural to CPO. However, some inherent defects of single metals, such as the activity of Cu is usually low, requiring harsh reaction conditions, on the contrary, the highly active Ni catalyst is easy to lead to excessive hydrogenation of FOL to form THFA, limiting the further improvement of comprehensive catalytic performance. Therefore, combining different catalytic active centers to regulate the electronic structure through metal–metal interactions is a good way to achieve high-performance catalysts.

Zhou et al. prepared Cu_0.4Mg_5.6Al₂ hydrotalcite catalyst by co-precipitation and calcination. The yield of CPO was 98.1%, the reaction temperature was 180 °C, the initial hydrogen pressure was 0.2 MPa, and the reaction time was 4 h.²⁰⁹ Pan et al. prepared magnetic Cu/Fe₃O₄ nanoparticles by coprecipitation. The maximum yield of CPO reached 91% at 3 MPa H₂ and 170 °C for 4 h. The obvious interaction between the two active metals, Cu and Fe, is the main reason for the high selectivity of the catalyst for CPO.²¹⁰ Zhang et al. synthesized the CuNi/Al-MCM-41 catalyst and reacted it at 2.0 MPa H₂ and 160 °C for 5 h, obtaining 99.0% furfural conversion and 97.7% CPO selectivity. The small amount of aluminum highly dispersed in MCM-41 acts as an anchor and ensures the formation of highly dispersed CuNi bimetallic nanoparticles, which is conducive to the improvement of catalytic properties.²¹¹ Liu et al. synthesized a bimetallic NiCu nanoparticle catalyst with surface oxygen modification and reacted at 140 °C and 1 MPa H₂ for 1 h, CPO yield of about 89.5%.²¹² Jia et al. used the bimetallic synergism between Ni and Fe in NiFe/SBA-15 catalyst to obtain a CPO yield of 90% at 160 °C, 3.4 MPa H₂ for 6 h.¹⁸⁹ Huang et al. prepared a multifunctional catalyst (xCo–yNi@NC) with highly dispersed Co–Ni alloy nanoparticles embedded in a porous carbon nitrided matrix by pyrolysis of a MOF template. Co–Ni alloy showed a strong bimetallic synergistic effect, and the introduced N species affected the physicochemical properties of the catalyst, thus promoting the catalytic performance. At 150 °C and 1.5 MPa H₂ for 6 h, the CPO yield was 92.5%.²¹³ Lin et al. synthesized the catalyst Ni₃Sn₂–ReO_x/TiO₂ by continuous impregnation method with 92.5% CPO selectivity at 140 °C and 3.0 MPa H₂. Characterization demonstrated that the Ni₃Sn₂ phase significantly inhibited the overhydrogenation of the furan ring, whereas the Lewis acid site on ReO_x activated the hydroxyl group of FOL and induced it to undergo rearrangement.²¹⁴

8.3 Reaction mechanism

The reaction mechanism of furfural's synthesis of CPO is divided into two steps: Step 1: furfural selectively hydrogenates to FOL. Step 2: under the action of water, the FOL is opened and rearranged to produce CPO. The reaction rate of FOL open loop rearrangement hydrogenation to CPO is faster, and step 1 is the rate control step (Fig. 12).

Mironenko et al. used D₂O as a solvent for furfural hydrogenation over a 1% Pd/CNTs catalyst. It was found that furfural was hydrogenated to produce FOL, and then FOL was hydrogenated to produce the product CPO after Piancatelli rearrangement. The intermediate products of the reaction were detected as FOL, 3-hydroxycyclopentenone (HCPO), 4-hydroxy-2-cyclopentenone (HCPEO), etc. It was inferred that the reaction process involved hydrogenation, hydrolysis, intramolecular aldol condensation and hydroxylation. From this, the reactions were deduced that hydrogenation, hydrolysis, intramolecular aldehyde condensation, dehydration and hydrogenation reactions occurred during the reaction (Fig. 13).²¹⁶ The reaction mechanism was then deduced as follows: selective hydrogenation of furfural to produce FOL, hydrolysis and dehydration of FOL to produce 2-pentene-1,4-dione, intramolecular aldol condensation to produce HCPEO, final hydrogenation to produce HCPO, and final hydrogenation and dehydration to produce CPO.

In the catalytic hydrogenation of furfural to prepare CPO, the metal active site and acid center on the catalyst are the key points of the catalytic reaction, and the temperature, pressure and reaction time of the reaction system all affect the selectivity of CPO. Currently, the gap between production and demand of CPO is large, and the preparation of CPO from biomass-based furfural is a green and environmentally friendly new technology in line with the development trend of atomic economy. This technology has very significant economic and social value. Finding and developing efficient catalytic systems for selective conversion of furfural to CPO remains a focus of future research. Non-noble metals are cheaper and easier to obtain than precious metals, and the catalytic effect of non-noble metal catalysts is better. Cu has the characteristics of high activity and selectivity in the catalytic reaction, and Ni also has better catalytic hydrogenation performance. However, due to the poor stability of catalyst and poor reuse, the research on non-noble metal-based catalyst is mainly to improve its stability and reusability. Bimetal catalyst has better catalytic performance than single catalyst, and the interaction between bimetals can further promote furfural hydrosynthesis of CPO, which has a wider application prospect.

9. Catalytic conversion of furfural to nitrogen-containing compounds

Nitrogen-containing compounds, as one of the important types of compounds, are widely used as precursors for agrochemicals and drugs.^217,218 Preparation of renewable, high-value nitrogen-containing chemicals from biomass will not only increase the product diversity of biorefineries, but also improve their economic viability. Considering the wide range of applications of organic nitrogen-containing chemicals in industry and daily life, the development of open regenerative synthetic chains of nitrogen-containing chemicals still requires a great deal of research.²¹⁹ With furfural as the substrate, the nitrogen-containing compounds generated are dominated by FAM, and further ring-opening rearrangement of FAM to form piperidine, and furfural to ring-open to form pyrrole after removing the carbonyl group (Fig. 14).

9.1 Catalytic conversion of furfural to FAM

As a biomass platform molecule available for large-scale industrial production, the realization of highly selective amination of furfural to FAM can fill the market gap for future pharmaceutical synthesis intermediates.^220,221 Among various synthetic methods, reductive amination has become one of the most applied methods for the preparation of nitrogen-containing compounds due to the wide availability and low price of aldehydes and ketones. Currently, a variety of homogeneous and heterogeneous catalysts have been reported for the catalytic synthesis strategy of carbonyl compounds by reductive amination. Among them, homogeneous catalysts are difficult to separate and reuse, so reductive amination using heterogeneous catalysts has been a hot research topic in recent years (Table 12).

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9.2 Catalytic conversion of furfural to other nitrogen-containing compounds

Pyrrole is an important nitrogen-containing chemical widely used as a precursor for fine chemicals, drugs, catalysts and materials.²⁴⁰ Since furan amination requires 400–450 °C to occur, but the excessive temperature (>350 °C) is not suitable for furfural decarbonylation to furan, which is one of the difficulties for this route. Therefore, at present, the one-step decarbonylation–amidation of furfural to pyrrole with NH₃ and H₂ has not been achieved.²⁴¹ It was reported that Pd-based catalysts are effective for furfural decarbonylation for the preparation of furan, while the conversion of furan to pyrrole often occurs in the presence of acid catalysts.²⁴² Song et al. used ultra-small metal nanoparticles encapsulated within zeolite channels to form a core–shell structure to prepare Pd-coated and confined in zeolite pores for the one-step conversion of furfural to pyrrole via a tandem decarbonylation–amidation reaction on a stationary bed in the Pd@S-1 and H-beta zeolite catalytic system. The Pd@S-1 catalyst was used at 460 °C with furfural weight hourly space velocity of 0.2 h⁻¹, NH₃ flow rate of 12 mL min⁻¹, H₂ flow rate of 20 mL min⁻¹, and time-on-stream for 12 h at 0.1 MPa, followed by H-beta zeolite catalyst, the final yield of pyrrole could be obtained in 75%.²⁴³ This study achieved one-step decarbonylation–amidation of furfural to pyrrole in the presence of NH₃ and H₂ for the first time (Fig. 16a).

Piperidine is used as a condensing agent and solvent in organic synthesis, and can also be used in the manufacture of local anesthetics, painkillers, fungicides, wetting agents, epoxy resin curing agents, rubber vulcanization accelerators and so on.²⁴⁴ There are few studies on the direct one-step preparation of piperidines from furfural, a reaction cascade that starts with reductive amination of furfural, followed by hydrocyclic cleavage, dehydrogenation, intramolecular reductive amination and final hydrogenation to produce piperidines, especially the selective C–O cleavage of the furan ring under reductive amination conditions, which is the most key reaction step. Therefore, the key to realizing this new strategy lies in the fine design of multifunctional catalysts.²⁴⁵ In order to solve this challenge, Qi et al. targeted the design and synthesis of a Ru₁Co_NP/HAP catalyst (Ru species are sufficiently isolated by Co atoms and form surface single-atom Ru₁Co_NP alloy structure) which showed the highest yield of piperidine of 93% in the presence of NH₃ and H₂. Kinetic investigation verified the reaction pathway, initially, furfural was reductively aminated to produce FAM, then hydrogenated to tetrahydrofurfurylamine (THFAM), and after 6 h of reaction at 100 °C, and then warmed up to 180 °C for 14 h of reaction, the cyclic ether bond of THFAM was broken, and rearrangement to produce piperidine was initiated (Fig. 16b). DFT calculations demonstrated that the surface structure of Ru₁Co_NP facilitated the direct ring opening of THFAM, as well as the rapid conversion of the intermediate 5-amino-1-pentanol to the product piperidine. This study achieved a one-step conversion of furfural to piperidine for the first time.²⁴⁶

Indole is widely contained in lemon oil, citrus oil, grapefruit peel oil, jasmine oil and other essential oils, and can be widely used in jasmine, lilac, gardenia, lotus, narcissus and other floral flavors. Derivatives of indole are widely distributed in nature, and many natural compounds contain indole ring in their structure, and some indole derivatives are closely related to life activities, so indole is also a very important heterocyclic compound. The new efficient and sustainable synthetic method using the one-step conversion of furfural to indole offers the possibility of producing indole from renewable and environmentally friendly biomass resources. Yao et al. used HZSM-5 (Si/Al = 25) as catalyst and the maximum carbon yield of indole was 20.79% at 650 °C, an airspeed of 1.0 h⁻¹, and the NH₃ to furfural molar ratio of 2. Based on the experiments, Yao also verified a possible reaction pathway for the conversion of furfural to indole: furfural first reacts with ammonia to form imine, which subsequently undergoes a cleavage reaction to produce furan after decarbonization. After that, the furan continues to react with ammonia to form pyrrole, and is finally converted to indole by the Diels–Alder reaction, a dehydration reaction between furan and pyrrole (Fig. 16c).²⁴⁷

So far, noble metal catalysts can obtain high yields of primary amines under milder reaction conditions. The non-noble metal catalysts are inexpensive but have the problems of complicated preparation process, high loading and harsh reaction conditions. The particle size, nature, structure and specific surface area of the carriers have a significant influence on the catalytic performance of the catalysts. The heterogeneous catalysts for the catalytic reductive amination of carbonyl compounds to synthesize primary amines still have difficulties of low reusability of the catalysts due to the susceptibility of the metals to agglomeration, leaching of the metals, and poisoning of the metal active centers. There are also few reports on the generation of other N-heterocyclic compounds from biomass, but in fact nitrogen-containing compounds are usually of higher value compared to oxygenated chemicals and are widely used in the production of pharmaceuticals and agrochemicals.²⁴⁸ Therefore, the development of simple, efficient, highly selective and stable non-noble metal catalysts to catalyze the preparation of nitrogen-containing compounds from furfural is a major trend.

10. Others

Besides the above products, furfural can be synthesized into high-quality fuel precursors by other reactions. Among them, aldol condensation reactions have received extensive attention in the synthesis of high-quality fuels. Since it occurs under mild conditions, and can be promoted by acidic or basic catalysts, reduce the O/C ratio. Compared to hydrocarbon fuels, oxygenated fuels are more suitable for combustion in the inner cylinder to promote the depth of combustion and to reduce the amount of oxygen supplied and the amount of slag, exhaust, etc., produced due to incomplete combustion. At the same time, condensation reactions are capable of lengthening carbon chains and are one of the most mature reactions used to construct CC bonds, which are important types of reactions in organic chemical synthesis and biochemistry.^249,250

Acetone is often used as a substrate for the reaction of aldol condensation with furfural. A high concentration of moderately strong basic sites enables the successful condensation of furfural with acetone.²⁵¹ Most of the industry uses aqueous sodium hydroxide and ammonia for the furfural condensation reaction with acetone. However, in order to have further development in terms of recoverability, separation and purification, heterogeneous solid base catalysts have been developed and investigated. Solid base catalysts in aldol condensation include basic or alkaline earth metal hydroxides, magnesium organosilicates, zeolites, alumina, and magnesium oxide (Table 13).²⁵² Al–Zr mixed oxides have excellent activity for the acetalization reaction of furfural with acetone. But generally, mixed oxides may exhibit severe deactivation, which is associated with furfural deposition of compounds on the catalyst surface and/or leaching of active substances.²⁵³ Much work has been done to improve the stability of Al–Zr mixed oxides. Yuan et al. developed a simple low-temperature solution phase induction strategy for manufacturing Al–ZrO₂ composites with tunable surface acid–base properties and high specific surface area. In the Al–ZrO₂ composite, the catalyst with an Al/(Al + Zr) molar ratio of 0.5 had the highest activity, the furfural conversion was as high as 92.0% and the 4-(2-furyl)-3-buten-2-one (FAc) selectivity was 98.7%.²⁵⁴ The synergistic interaction of basic and acidic sites on the catalyst surface is crucial for the superiority of the catalyst in aldol condensation. Similar to Al–Zr oxides, mixed oxides such as Mg–Al have been developed as heterogeneous catalysts for the condensation of furfural and pyruvic aldehydes.^255–257 Arhzaf et al. prepared a series of MgAl–CO₃ type hydrotalcite materials, and the mixed oxide with a Mg/Al ratio of 3.5 (Mg_3.5Al–O), prepared from hydrotalcite calcined at 450 °C, was the most alkaline and catalytically active of the series of catalysts.²⁵⁸ A molar ratio of furfural/acetone of 1/10 was added to the reactor and the reaction was carried out at 90 °C for 2 h. The conversion of furfural and the selectivity of FAc were optimized. The conversion was 98% and the selectivity to FAc was 78%. It realized a highly selective condensation reaction in the solvent-free condition.

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Desai et al. made some studies on the reaction mechanism of the condensation of furfural with acetone (Fig. 17). The catalyst alkalinity they prepared varied with different molar ratios of Mg and La. When the Mg, La molar ratio was 3:1, the activity of the catalyst increased significantly due to the presence of medium to strong basic sites in the catalyst. At the beginning of the reaction, an acetone and a furfural adsorbed on the basic and acidic sites of the mixed oxide catalyst, respectively. Then, the enol ester formed on the surface of the catalyst attacks the carbonyl group of the furfural, which subsequently dehydrates to form FAc.²⁵⁹

Overall, increasing the distribution of basic sites can effectively improve the catalyst activity in the furfural condensation reaction with acetone.²⁵¹ But most of the catalysts will be susceptible to deactivation due to the deposition of compounds on their surfaces during the reaction process. So, improving the stability of the catalysts, as well as optimizing the structure of the catalysts or/and adjusting the surface properties of the catalysts to address issues such as deactivation are of greater interest. As of now, the aldol condensation reactions of biomass-derived oxygenated furan compounds, such as furfural, have been less studied by heterogeneous acid catalysts compared to heterogeneous base catalysts. However, furan-containing compounds are usually produced from cellulose or hemicellulose under acidic conditions. To develop effective acidic catalysts for aldol condensation one-pot conversion from biomass furan oxide compounds, and hence eliminating neutralization and purification operations in the production process, is worth further exploration and research.

11. Conclusions and perspectives

Fossil fuel depletion and the environmental problems caused by their misuse have led to extensive research on biofuels in recent years. Furfural, in the last decades, has gained attention as a promising contender in the field of sustainable chemical production, mainly due to its ability to be produced from cheap lignocellulosic biomass materials. Its high reactivity has led it to be recognized as one of the most valuable platform chemicals obtained from biomass. Research on the conversion of furfural into a range of fine chemicals, especially within the framework of green chemistry and the pursuit of sustainable development, has attracted great interest and holds great promise.

The high-value compounds into which furfural is converted can generally be divided into two categories: biomass-based chemicals and biofuels. When converting furfural into biofuel compounds, it is generally necessary to selectively remove oxygen atoms on carbonyl carbon to remove oxygen-containing functional groups and improve its octane number to meet the needs of fuel. The conversion of furfural into high-value biological chemicals is generally to retain oxygen atoms on carbonyl carbon, and the conversion of furfural into oxygen-containing compounds can make better use of this biomass-derived molecule with highly functional groups. In addition, by using furfural as a renewable platform compound, high-value compounds can be produced in fewer steps and are more environmentally friendly than by using fossil resources as raw materials, in line with the current green chemistry concept of sustainable production. This review systematically summarized the latest research progress in the efficient conversion of furfural to other high-value compounds, introduced a variety of efficient and green noble and non-noble metal catalytic systems, especially for catalytic reactions in green solvents, such as H₂O. In multiphase catalytic systems, the conversion of furfural to value-added chemicals such as LA, GVL, PeDs and CPO is a very valuable study because these production pathways are highly atomistic-economic, sustainable, and very much in line with the requirements of green development.

However, the following key issues still need to be addressed in future research on the conversion of furfural to other value-added chemicals: (1) Catalyst performance: due to the complexity of the conversion pathway, and the competing side reactions, little research has been done on some high-value chemicals such as PeDs, pyrroles, and piperidines. Researchers are working to develop more efficient and selective catalysts for the targeted conversion of furfural to a variety of high-value products. These catalysts can control reaction conditions, improve product selectivity, and minimize by-product generation. (2) Catalyst design: most studies lack deeply rooted mechanisms and catalytic design strategies specifically explored and validated for the conversion of furfural to chemicals. Using theoretical modeling, kinetic studies and characterization tools, the role played by each active component and active site of the catalyst in the catalytic process can be verified, which facilitates the proposal of catalyst design strategies to further improve the reaction efficiency and target product selectivity. (3) Green catalytic technologies: green catalytic technologies aim to reduce energy consumption, minimize waste generation, and enhance the environmental friendliness of reactions. Therefore, the development of green catalytic technologies is crucial for furfural conversion processes, such as research into catalyst design based on renewable resources and the use of environmentally friendly solvents. (4) Exploration of novel catalytic mechanisms: furfural conversion to high-value compounds usually involves multiple reaction steps, but the synergistic effect between different metals of multimetallic composite catalysts and the adsorption configuration of the reaction substrate on the catalyst surface are not well understood and lack of corresponding theoretical validation. Confronted with problems such as the controversial reaction mechanism of some reactions, and because many target products become intermediates of other reactions under different conditions, how to control the selectivity of the target products becomes a major challenge. Therefore, when designing highly selective and active bifunctional catalysts, each step of the reaction should be matched with the corresponding active site and reaction conditions. (5) Expanding new downstream products: the multiple functional groups of furfural and its high reactivity make it a strong potential for development as a functionalized renewable platform compound. Besides the developed downstream products such as FOL, THFA, LA, GVL, PeDs, CPO, FAM, 2-MF, 2-THMF, etc., more value-added chemicals used in various fields can continue to be explored in the future to maximize the value of furfural.

In summary, prospects for catalytic technologies for downstream products of furfural include advancements in selective catalytic conversion, optimization of multiphase catalytic systems, catalyst regeneration and recycling, application of green catalytic technologies, exploration of novel catalytic mechanisms, and exploration of other downstream products. The continuous development of these technologies will drive the effective utilization of furfural and promote the development of sustainable chemical industries.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (22278121), Hunan Provincial Natural Science Foundation of China (2023JJ30390) and the Science and Technology Planning Project of Hunan Province (2018TP1017).

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