1 Introduction
Aromatic hydrocarbon compounds such as toluene, ethylbenzene and p-xylene (collectively known as triphenyl) are important bulk chemicals in the chemical industry, which are widely used in the production of polymers, solvents, pharmaceuticals, pesticides and spices. With the rapid development of economic and industrial globalization, the demand for aromatic compounds is increasing dramatically every year. At present, aromatic hydrocarbons are mainly obtained by catalytic reforming or steam cracking of petroleum. However, the sustainable production of aromatic hydrocarbons is affected by the non-renewable nature of fossil resources and environmental requirements
[1]. Therefore, the development of new technologies using renewable biomass to replace fossil energy is becoming increasingly important
[2]. In the past decades, many methods have been developed to obtain aromatic compounds, such as methanol aromatization, CO
2 conversion, direct conversion of lignocellulose, synthesis of C2 compounds and so on
[3][4][5][6]. However, the yield of triphenyl obtained by these methods is low due to the fact that the products obtained usually contain other linear or cyclic compounds. Therefore, it is still a serious challenge to obtain aromatic hydrocarbon monomers with high selectivity from biomass
[7].
The Diels-Alder cycloaddition reaction can combine two molecules with carbon-carbon double bonds in the conjugated system into a cyclic molecule, achieving 100% carbon atom economy
[8]. In recent years, researchers have proposed to use furan biomass as conjugated dienes to synthesize aromatic compounds by Diels-Alder cycloaddition reaction with dienophiles such as ethylene and propylene
[9]. There are many kinds of derivatives of Furan biomass, such as 2,5-Dimethylfuran (DMF), 2-Methylfuran (MF), Furan (FU), Furfuryl Alcohol (FA), Furfural (FUR) and 5-hydroxymethylfurfural (HMF) as shown in Table 1, which have wide sources and low cost
[10]. Taking FUR as an example, its annual output in China can reach more than 700000 tons, but people have not carried out high-value conversion and utilization, and its innovative application is highly anticipated
[11]. In recent years, domestic and foreign scholars have reported the breakthrough research progress in the conversion of furan-based biomass into aromatic compounds under the action of various catalysts such as molecular sieves, metal oxides and ionic liquids. The types of reaction precursors and solvent systems have been continuously enriched, various catalytic models have been developed, and the types of products have been expanded from triphenyl to more functional aromatic compounds
[12]. However, most of these reactions rely on high energy consumption reaction conditions of high temperature and high pressure, and there is a problem of many by-products caused by "one-pot method".
The research on the conversion of furan-based biomass to aromatic compounds has important scientific value and application prospects, and will become an important research field in green chemistry in the future. In this paper, the common principles and influencing factors of various furan-based biomass to produce aromatic compounds were summarized, and the characteristics of different furan-based biomass conversion reactions were summarized, and the key steps and the latest research results in the catalytic conversion of furane-based biomass to aromatic compounds were analyzed.In this paper, the future route of molecular conversion of biomass to aromatic compounds is prospected, and it is expected that a mild catalytic system with low energy consumption and high yield can be constructed in the future to realize the partial substitution of renewable energy for fossil energy.
2 Reaction Mechanism
2.1 Reaction Mechanism
The catalytic conversion of furan-based biomass into aromatic compounds generally consists of two tandem reactions, and the reaction mechanism is shown in Figure 1:1) Diels-Alder cycloaddition reaction of furan-based biomass with dienophiles to form six-membered ring intermediates; 2) that cycloaddition intermediate is convert into an aromatic hydrocarbon compound through dehydration aromatization. The first step, Diels-Alder cycloaddition reaction, is the core step of this synthetic route, which is mainly carried out under thermal excitation. It is generally believed that when the five-membered heterocyclic ring of furan biomass participates in the Diels-Alder cycloaddition reaction, the steric hindrance of the substituent beside the heteroatom should not be too large, and the dienophile should contain active carbon-carbon double bonds or triple bonds
[8].
Brønsted acid catalysts have good catalytic dehydration ability for the six-membered ring intermediate produced by Diels-Alder cycloaddition reaction, but strong Brønsted acid catalysts usually promote the uncontrolled alkylation and oligomerization reactions, resulting in the decrease of the selectivity of the target product and the formation of carbonization coke deposited on the surface and in the pores of the catalyst
[13]. In comparison, Lewis acid has a weak dehydration effect on intermediates, but has much lower activity for the hydrolysis and condensation side reactions of furan biomass, which can better stabilize the reaction intermediates, reduce side reactions and improve product selectivity
[14].
Yin et al. Showed that Brønsted-Lewis acid bifunctional catalysts could efficiently catalyze the aromatization reaction of DMF and ethylene to produce p-xylene at high temperature and high pressure
[15]. Both Lewis acid sites and Brønsted acid sites in the NbO
x-based catalyst can accelerate the cycloaddition reaction and dehydration reaction, respectively, and they synergistically promote the aromatization reaction. The appropriate ratio of Brønsted to Lewis acid can increase the conversion of DMF to 99%. However, excessive Brønsted acid sites and Lewis acid sites will lead to the increase of by-products and the decrease of product selectivity
[16]. For the reaction catalyzed by Brønsted-Lewis acid, although the reaction mechanism is controversial, it is generally agreed in the existing research work that the two acids can achieve better catalytic effect
[17].
2.2 Possible side reaction
Due to the existence of competitive reactions and the easy decomposition of substrates, some organic and inorganic by-products are usually produced in the reaction process of furan-based biomass conversion to aromatic compounds, resulting in unsatisfactory selectivity of target aromatic compounds
[18]. It is worth noting that side reactions vary with reaction models and experimental conditions, which greatly increases the difficulty of studying the influence of product selectivity and brings challenges to the actual optimization of reactions.
At present, the "one-pot" route is mostly used in this kind of conversion reaction, and the intermediate does not need to be separated and purified, which is very beneficial from the economic and environmental point of view. However, because reactants, intermediate products, target products and by-products all coexist in the same reactor, side reactions are more easily induced, and even polymers which are difficult to accurately detect are produced
[19]. These by-products transform and compete with each other, which aggravates the complexity of the product. Generally speaking, there are three main types of side reactions that may occur: 1) hydrolysis, isomerization or hydrogenation of furan biomass or intermediates; 2) certain species polymerize with one another to form oligomers during the reaction; 3) that product undergo an isomerization reaction, or the product reacts with other molecule to cause side chain alkylation of the product. Fig. 2 take that reaction of DMF and ethylene as an example.
The results showed that the coke formed by the deposition of oligomerization products would block the pores and active sites on the surface and in the inner pores of the catalyst, resulting in the decrease of catalytic activity or complete deactivation
[20]. In the conversion process, it is particularly important to note that the water produced by the dehydration reaction of the cycloaddition intermediate will lead to the hydrolysis of the furan ring
[21]. The formation of hydrolysis products and their adsorption on the catalyst surface will reduce the surface activity of the catalyst, so water should be removed or isolated during the reaction to avoid causing other adverse side reactions. To sum up, the selectivity of the target product depends largely on the reaction conditions and the nature of the side reactions. Inhibition of side reactions is essential to ensure the economic and technical feasibility of the whole process, which is the primary problem to be solved in the conversion application.
3 Influential factors
Efficient and selective conversion of furan-based biomass into aromatic compounds has important industrial application value.Clear understanding of various influencing factors in this conversion process is a key link to effectively improve the yield of target products. Previous studies have shown that the catalyst composition, solid acid content, solvent effect, electrophilic (nucleophilic) selection of dienes and other factors will have a significant impact on the preparation of aromatic compounds.
3.1 Olid acid catalyst component
Catalytic reaction can be divided into heterogeneous catalysis and homogeneous catalysis. Compared with heterogeneous catalysis, homogeneous catalysis has better reaction efficiency, but its application is limited because it is difficult to separate the catalyst from the product. In this paper, the effects of several solid acid catalysts on the catalytic reaction system will be discussed.
3.1.1 Molecular sieve catalyst
To date, the most frequently reported catalysts in this study are molecular sieves with weak and medium strength acids, such as H-BEA
[22]. Dispersed acidic sites and complex and diverse pore structures on the surface of molecular sieves provide abundant reaction sites for the adsorption and transformation of furan molecules, and the strength, density and types of these surface acidic sites are closely related to the constituent units of molecular sieves. The acidic sites of different molecular sieves have different types and strengths, which also correspond to different catalytic abilities. In addition, the same molecular sieves with different Si/Al ratios have different surface acid strength and micropore distribution. Among them, the higher the ratio of silicon to aluminum, the higher the activity of surface hydroxyl. However, the effect of Si/Al ratio on the catalytic activity is still controversial. Kim et al. studied the Diels-Alder cycloaddition reaction of MFI-type molecular sieves with different silica-alumina ratios, and found that the Brønsted acid site density of MFI-type molecular sieves with high silica-alumina ratio was higher than that of the materials with low silica-alumina ratio, and the internal pores were more conducive to species diffusion, and the yield of p-xylene increased approximately linearly with the increase of silica-aluminum ratio
[23]. However, Williams et al believed that the catalytic performance of molecular sieves was not related to the ratio of silicon to aluminum
[24]. They found that HY zeolites with different silica-alumina ratios had significantly different Brønsted acid site densities, but showed the same catalytic reaction rate.
The strength and density of the acid on the surface of the molecular sieve can also be adjusted by introducing heteroatoms. After the introduction of heteroatoms, the structure of the molecular sieve will not change greatly. However, the difference of ionic radius and electronegativity of heteroatoms will affect the pore size distribution and acid-base properties of zeolites, and further affect their adsorption behavior and catalytic performance
[25]. Among them, the promotion effect of non-metallic element phosphorus on the reaction is obvious to all
[26]. The superior performance of phosphorus-containing molecular sieves may be the lower rate of formation of byproducts such as alkylated aromatics and oligomers. Cho et al. Showed that phosphorus-impregnated molecular sieves produced a highly branched pore structure, which provided active sites for effective dispersion and facilitated transport channels
[27]. As shown in fig. 3, the two molecular sieves P-BEA and P-SPP showed excellent performance compared to other atom doping, with nearly 97% yield of p-xylene at 99% conversion. In addition to the incorporation of heteroatoms, Kasipandi et al. Also reported that zirconium phosphate grafted SBA-15 catalyst, highly active and stably ordered zirconium phosphate was uniformly distributed on the surface of the molecular sieve
[28]. Therefore, the grafting of zirconium phosphate can not only produce a larger specific surface area, but also promote the synergistic effect between Lewis acid and Brønsted acid sites, which is beneficial to the aromatization reaction.
In addition, the introduction of metal atoms on the surface of molecular sieves can not only change the type of molecular sieve solid acid, but also affect the reaction path. Rohling et al. Found that under the catalysis of Brønsted HY molecular sieve with high acidity, the amount of by-product 2,5-hexanedione produced was almost the same as that of p-xylene, and the selectivity of p-xylene was only 15%
[29]. When the bases (Li, Na, K, Cs, Rb) were introduced, the Brønsted acidity in the molecular sieve was weakened, so the hydrolysis of furan ring initiated by Brønsted acid was inhibited, in which KY was reduced to the greatest extent, and the selectivity to p-xylene was increased to 41%. Chang et Al. Synthesized p-xylene by aromatization of DMF with ethylene over Sn-BEA, Zr-BEA, Ti-BEA and Al-BEA catalysts, respectively
[14]. Compared with the other three catalysts, Al-BEA had the lowest conversion and selectivity. The reason may be that Sn-BEA, Zr-BEA and Ti-BEA have Lewis acid sites, while Al-BEA has strong Brønsted acid sites. The stronger Brønsted acid effectively catalyzes the hydrolysis reaction to produce more 2,5-hexanedione and alkylation by-products. These results indicate that the metal atoms modified on the surface of molecular sieves can change the type of acid sites to a certain extent and control the product distribution.
Another notable feature of that molecular sieve catalyst is the adjustable particle size and porosity. The number of channels, the structure of channels and the active sites of zeolites are closely related to their catalytic activity. In the selective conversion of products, the restriction effect of pore channels in the framework of molecular sieves plays an important role
[30]. Some of the molecules in the product are too bulky to diffuse out of the molecular sieve pore windows. For example, ZSM-5 molecular sieve catalyst, its window only allows substances with smaller molecular weight to diffuse out of the reaction zone, so it has higher selectivity for monocyclic aromatic hydrocarbons. Byproducts with too large volume will be deposited in the pores and outlets of the catalyst, and some molecular sieves with special structures will also contribute to the catalytic reaction. For example, H-SAPO-34 with smaller pore size exhibited lower DMF conversion, but good selectivity and high stability due to its smaller Brønsted/Lewis acid ratio and pore hierarchical structure
[31].
In a word, researchers have a broad and mature understanding of molecular sieve catalysts, and these classical solid acid catalysts show high conversion and complex and diverse products in the catalytic conversion of furan-based biomass to aromatic compounds. Recently, with the continuous modification of solid acid molecular sieves, other types of catalysts have gradually attracted the attention of researchers.
3.1.2 Other types of catalyst
Compared with the molecular sieve system, the research results of catalytic conversion of biomass molecules to aromatic hydrocarbons by composite oxides, phosphates and heteropolyacids as catalysts are limited at this stage.However, because the composition and modification method of this kind of catalyst are more adjustable and efficient, the development of this kind of catalyst as a new type of efficient catalyst has attracted much attention.
Yin et al. Synthesized p-xylene in a green and efficient way by using a composite catalyst whose active components are mainly Nb
2O
5 and NbOPO
4[15]. A large number of sheet-like nanostructures in the composite catalyst make a large number of pores around the nanosheets, which provide active sites for the adsorption of reaction substrates. Metal phosphates (metal phosphorus oxides) having amorphous or crystalline properties may also participate in the reaction as solid acid catalysts, and they generally have high stability and regenerability. In recent years, stannic phosphate and zirconium phosphate have been used to catalyze the conversion of furan molecules to aromatic compounds. Feng et al. Used highly stable tin phosphate (SnPO) as a catalyst, and the tetrahedral coordination of Sn (IV) in SnPO made the catalytic activity of benzene series more effective
[32]. Ni et al. Have extended the catalyst range by using the Lewis acid catalyst trifluoromethanesulfonic acid metal salt
[33].
Heteropolyacids have strong acidity and can be used in acid catalytic reactions at moderate temperatures. The catalytic research of heteropolyacids has also attracted wide attention
[34]. Fikri et al. supported phosphotungstic acid (HPW), silicotungstic acid (HSiW) and phosphomolybdic acid (HPMo) on various oxide supports (SiO
2, TiO
2, ZrO
2), and the study showed that the Brønsted acid strength of HPW and HSiW supported on SiO
2 was the highest compared with the combination of other supports and heteropolyacid
[35]. The strong acidity can accelerate the dehydration reaction and shift to the cyclization step to a greater extent, which makes HPW and HSiW supported on the SiO
2 have higher selectivity for terephthalic acid esters in the product than other combinations. It is worth noting that the strong interactions between different heteropolyacids and supports are different, for example, the Keggin structure of HPW on ZrO
2 and TiO
2 is decomposed, while the Keggin structure of HPW on SiO
2 surface can be well maintained.
With the development of biomass conversion research, various catalysts have been extended into this system. Considering the diversity of elements or structures in solid acid catalysts, the range of reaction substrates can be further expanded. At the same time, the synergistic effect between the acid sites and the pore structure can promote the catalytic reaction. In short, solid acid catalysts are inherently stable, efficient, and designable. Therefore, solid acid catalysts will continue to play an important role in this field.
3.2 Solvent effect
In the heterogeneous reaction system composed of solid acid catalyst, reactant and solvent, the interaction among solute, solvent and catalyst plays an important role
[36]. These interactions affect the reaction equilibrium and reaction rate, especially the formation of interaction regions between hydrophobic (hydrophilic) solvents and solutes and catalysts, which have a great impact on the reaction rate and selectivity
[37]. Previous studies have shown that there is an obvious solvent effect in the Diels-Alder reaction process. Theoretical calculations show that the appropriate solvent can reduce the energy of the transition state and intermediate, but there is no solvent that can reduce the activation energy barrier enough to achieve the reaction without catalyst
[38].
The solvents used in this reaction can be roughly divided into polar solvents, non-polar solvents and ionic liquids. The hydrogen bond formed between the oxygen atom on the furan ring or the hydroxyl group on the side chain and the polar solvent molecules affects the progress of the catalytic reaction. Wijaya et al. investigated the effects of different solvents such as non-polar solvent (n-heptane), polar aprotic solvent (dioxane, tetrahydrofuran) and polar protic solvent (isopropanol) on the yield of the target product in the study of selective conversion of DMF to p-xylene using silica-alumina aerogel catalyst
[39]. The rate of dehydration reaction in n-heptane was much higher than that in polar aprotic solvent, and the yield of the product was increased. Notably, when alcohols (ethanol, propylene glycol, etc.) Are used as polar protic reaction solvents, the intermolecular hydrogen bonding strength is increased. If isopropanol is used as a solvent, the charged intermediate produced in the dehydration process can be more stable and the reaction can be more easily promoted.
Dehydration reaction is inevitable in this conversion process, and the generated water will further cause side reactions such as hydrolysis. The adverse effects of water can be reduced by using some non-polar solvents. In the current research, aliphatic solvent n-heptane is mostly used as solvent
[40]. Xiong et al. Found that n-heptane increased the selectivity of benzene series to 75%, and slightly increased the content of alkylation by-products while reducing the hydrolysis and oligomerization by-products of furan biomass
[41]. Williams et al. Used n-heptane as a solvent to adjust the dielectric properties of the solution to obtain the best product selectivity
[24]. The amount of water adsorbed by the catalyst in n-heptane decreased to about
compared with that in the absence of solvent, which indicated that the local hydrophobicity of the catalyst surface increased in n-heptane solvent, resulting in the reduction of water-related side reactions on the catalyst surface.
As an important green solvent and functional fluid, ionic liquids (ILs) have attracted much attention because of their high polarizability, good hydrogen bond donor ability and a large number of acid active sites, which are considered to be the solvents that may greatly affect the process of Diels-Alder cycloaddition
[42][43]. The common cations are [EMIm]
+, [N
1444]
+, [N
122,102]
+, [P
14]
+ and so on.The anions include [BF
4]
- and [PF
6]
-. The composition of the ion is different, the performance is also different. Ni et al. Used acidic ionic liquid 1-butyl-3-methylimidazolium hydrogen sulfate as solvent in the conversion reaction of DMF and acrylic acid, and found that the conversion of DMF reached 87% and the total yield of the product reached 78% only at room temperature for 1 H
[33]. Other ionic liquids do not achieve good aromatics yields because they are not in the optimal acidity range or are trapped in steric hindrance. Compared with other high temperature and high pressure conditions, it is a breakthrough to prepare aromatic compounds in ionic liquids at room temperature and normal pressure. However, the application of ionic liquids is limited due to the problems of high cost, high viscosity and low safety.
To sum up, the choice of solvent greatly affects the experimental results, and the appropriate solvent can not only accelerate the reaction rate, but also improve the product selectivity. At present, a large number of organic solvents used in the reaction system not only increase the reaction volume, but also bring negative effects on the separation of subsequent products and the environment. It is expected to develop greener solvents to realize the conversion and utilization of biomass molecules.
3.3 Selection of dienophile
Ethylene and propylene are often used as dienophiles for cycloaddition reactions, but these gas molecules usually come from fossil fuels, which makes the synthesis route of aromatic hydrocarbons still rely on non-renewable energy sources. In addition, it is found that ethylene is not the most ideal dienophile because of its high pressure, harsh reaction conditions and low reactivity
[44]. Compared with ethylene, the reaction of propylene molecules with furan-based biomass can reduce coke and have a higher yield of aromatics
[45].
In order to reduce the cost, alkane can also be used as a raw material, and the alkane can be dehydrogenated under a dehydrogenation catalyst and then connected in series with a Diels-Alder cycloaddition reaction. Qi et al. Observed a two-step cascade reaction path of dehydrogenation and addition of high pressure propane at 823 K over bifunctional mixed catalysts CrO
x/Al
2O
3 and ZSM-5
[46]. As shown in fig. 4, propane is dehydrogenated on a CrO
x/Al
2O
3 to generate propylene, and the generated propylene is subjected to addition reaction with furan in situ in the micropores of the ZSM-5 molecular sieve to selectively generate aromatic hydrocarbons.
In recent years, the work of using renewable ethanol and methanol as raw materials to replace olefins has been developed rapidly. The short-chain alcohol molecules can be subjected to dehydration reaction on the acidic or basic sites of the catalyst to generate the olefin compounds
[47]. Teixeira et al. Used a batch reactor to dehydrate ethanol over HUSY molecular sieve at 573 K to produce ethylene in situ, and then synthesized aromatic compounds with DMF in a one-pot method
[48]. This green and sustainable synthetic route greatly reduces the deposition of the by-product benzofuran on the catalyst. Recent studies have shown that methanol can be rapidly converted to olefins over the catalyst, improving the selectivity of benzene series. Zhu et al. Proposed the use of methanol coupling to produce olefins, which can further form toluene and xylene with MF at 723 K
[49]. Furthermore, Wang et al also demonstrated that dimethyl ether was an important and unavoidable intermediate in this reaction, and dimethyl ether could be completely converted to olefins by increasing the temperature
[50].
The electron donating (withdrawing) effect of substituents on dienophiles plays an important role in the Diels-Alder cycloaddition reaction, which is not only related to the reaction efficiency, but also affects the product distribution. Electron-withdrawing substituents lower the frontier orbital energy, while electron-donating substituents do the opposite
[51]. It is generally believed that dienophiles with electron-withdrawing substituents and dienes with electron-donating substituents can promote the reaction. For example, the carboxyl group of acrylic acid and the aldehyde group of acrolein, as electron-withdrawing substituents, can lower the energy of the frontier orbital to accelerate the reaction
[52]. Salavati-fard et al. Used furan and acrylic acid (or methyl acrylate) to synthesize benzoic acid, and realized the combination of Diels-Alder cycloaddition reaction and dehydration reaction
[53]. Shiramizu et al. Used DMF and acrolein as reactants to react in two steps at low temperature, and the yield of p-xylene could reach 34%
[54]. Therefore, it is an important way to find and synthesize dienes and dienophiles that are conducive to cycloaddition reactions to enhance the conversion of furan biomass.
4 Conversion of biomass furan ring molecules to aromatic hydrocarbons
Biomass molecules containing furan groups, such as FUR, MF, DMF and FU, can be produced in large quantities by dehydration of many inexpensive renewable natural compounds (such as agricultural wastes such as corncobs)
[55]. Base on that electronegativity and polarity of the functional groups on the side chain of the furan heterocycles, their activities for the conversion reaction are also different, as shown in Table 2. In the following, the research work of several typical furan-based biomass molecules with different side chain functional groups as precursors to participate in the conversion reaction will be analyzed and compared.
4.1 Preparation of Benzene Series by DMF
DMF is a kind of renewable biomass molecule with high application potential, which can be obtained from HMF by hydrodeoxygenation. It has the advantages of high energy density, high octane number and low oxygen content, and is the most widely used furan biomass for the preparation of benzene series
[63].
There are two symmetrical methyl functional groups in the side chain of DMF, and the methyl substituents have electron-donating conjugation effect and induction effect. At the same time, the methyl substituent can stabilize the charged intermediate in the dehydration process of the cycloadduct and promote the dehydration of the cycloadduct. This is also the reason why DMF is the first choice for the catalytic conversion of furan biomass to aromatic compounds. Song et al. studied the conversion reaction of several furan derivatives with ethylene catalyzed by haloacetic acid, and found that compared with other furan derivatives (MF, FU, etc.), DMF as a reactant in 1,4-dioxane solvent at 473 K achieved the highest conversion rate and benzene series yield of 72. 1% and 49. 9%, respectively
[64].
Improving the yield of benzene homologues is the key to ensure the economic and technical feasibility of the whole system, so most studies focus on improving the yield of target products. Zhao et al. Optimized the catalyst structure and catalytic conditions, so that the conversion of DMF reached 100% at 573 K and ethylene pressure of 4.0 MPa, and even the selectivity of p-xylene reached 94%
[65]. The mechanism of DMF conversion was studied in detail by DFT theoretical calculations. On this basis, the reaction system is constantly enriched, the reaction conditions are gradually changed from harsh to mild, various high-performance, stable and recyclable catalytic materials are constantly expanded, and the reaction forms tend to be diversified. Salavati-fard et al. Synthesized renewable phthalic anhydride by cycloaddition of DMF and maleic anhydride
[66]. Heterogeneous Bi-BTC, designed and prepared by Yeh et al., successfully catalyzed the synthesis of p-xylene from DMF and acrylic acid at a mild temperature of 433 K (92% yield)
[67]. Bi-BTC has both microporous and mesoporous structures, and the Bi
3+ in Bi-BTC acts as a Lewis acid to promote the decarboxylation and dehydration reactions.
The reaction mechanism of DMF to BTEX has been studied by experiments and theoretical calculations, but how to realize this reaction under mild and green conditions still needs further study. The development of efficient catalytic systems with simple steps and mild reaction conditions is of great significance for the industrialization of renewable aromatic hydrocarbon production based on furan biomass.
4.2 Preparation of Benzene Series by MF
MF is a kind of furan biomass with stable structure, which is usually prepared by hydrodeoxygenation of FUR or FA molecules. As an important intermediate in organic synthesis, it is mainly used in the manufacture of drugs such as primaquine phosphate and liquid fuels
[68]. Compared with DMF, MF is less used in the synthesis of aromatic hydrocarbons.
Compared with DMF molecule, MF molecule lacks a methyl group with electron-donating effect on the side chain, so its reactivity decreases. It was found that under the same reaction conditions, the conversion and yield of MF were much lower than those of DMF
[69]. Green et al. Found that when MF and ethylene were used as raw materials at a reaction temperature of 523 K, the selectivity of toluene in the product on H-BEA molecular sieve was not more than 46%
[58]. In addition, there will be some new by-products in the reaction system. Further theoretical calculation shows that the activation energy for the cleavage of the C — O bond of the cycloaddition intermediate with only one methyl group on the side chain is 14.7 kcal/mol higher than that with methyl groups on both sides, and the dehydration reaction of the intermediate is difficult to carry out, thus affecting the whole reaction process.
However, the cost of MF as the raw material is lower than that of DMF, and it has a stronger application prospect from the economic point of view. Therefore, people hope to improve the current situation of low MF conversion efficiency by regulating the dienophiles involved in the conversion reaction and the corresponding catalysts. Zheng et al. Showed that the yield of aromatics produced by coupling conversion of MF and methanol over ZSM-5 was higher than that of ethylene as a dienophile
[70]. As shown in fig. 5, the synergistic effect among the methanol-to-olefin reaction, the Diels-Alder cycloaddition reaction of furan and olefin, and the alkylation reaction of benzene series and methanol is the reason for the improvement of MF and methanol coupling conversion. In addition, the introduction of stronger electron-withdrawing groups on the dienophile is also beneficial to the reaction. Thiyagarajan et al. First demonstrated the route of cycloaddition of MF with maleic anhydride at 573 K using a physically mixed solid catalyst of H-Y (or H-VUSY) molecular sieve and Pd/C to prepare reproducible phthalic anhydride with a yield of nearly 90%
[71].
To sum up, the preparation of BTEX by MF provides a new chemical route for the conversion of renewable biomass resources into aromatic compounds with high added value. In addition to the target products such as toluene and benzene, aromatic di (tri) carboxylic acids such as phthalic acid, isophthalic acid and trimellitic acid can also be synthesized directly. With the development of this research, it will become another important branch of biomass conversion.
4.3 Benzene homologues prepared by HMF
HMF is an important platform chemical for obtaining biofuels and chemicals
[72]. HMF has a wide range of chemical activities and can be further converted into value-added chemicals, including 2,5-furandicarboxylic acid and DMF
[73][74]. In general, HMF can be hydrodeoxygenated to produce DMF for subsequent conversion to aromatic compounds
[75].
If HMF can be directly used to prepare BTEX without intermediate conversion reaction steps, it will be of great economic significance and broad application prospects. At present, in the production route using HMF as substrate, it is an important challenge to realize the hydrodeoxygenation reaction of HMF to produce DMF intermediate without introducing additional hydrogen. In order to solve this problem, Tao et al. Used formic acid as a reversible hydrogen storage material, and the protons and electrons generated in situ directly reduced HMF to DMF under the action of noble metals, and then converted to p-xylene under the action of supported catalysts
[76]. Although the above reaction can achieve extremely high yields, the reduction step to convert HMF to DMF requires a high-cost noble metal catalyst, which makes the commercial implementation of this route challenging. Jia et al. Used HMF to prepare several intermediates by direct decarbonylation, and the intermediates were integrated into phthalic anhydride by MoO
3/Cu(NO
3)
2 catalyst under mild conditions
[77]. Ogunjobi et al. Reported a new catalytic dehydration route over molecular sieves. At 573 K and 6 MPa, the partially oxidized intermediates of HMF were directly converted to aromatic products with ethylene via a "one-pot" route
[78].
A variety of intermediate products may be formed in the route of direct conversion of HMF to aromatics, which has the problems of poor catalyst recovery and disproportion between yield and cost
[79]. Due to the possibility of polymerization of HMF with high concentration, the low concentration of substrate used in most conversion processes leads to the difficulty of product separation, which increases the cost of separation and concentration. How to overcome these problems will determine whether HMF can be applied to the synthesis of aromatic hydrocarbons on a large scale.
4.4 Preparation of benzene series by FUR
As a cheap and renewable furan derivative which can be obtained from lignocellulose in large quantities, FUR has a wide range of applications in oil refining, plastics, pharmaceuticals and agrochemical industries
[80,81]. The catalytic conversion of FUR as a biomass platform molecule to high-value chemicals has attracted much attention in recent years. There are three ways to produce aromatics using FUR as a feedstock. First, FUR is most commonly converted to MF or furan by hydrodeoxygenation followed by aromatization
[82,83]. Secondly, the electron-withdrawing effect of the aldehyde group reduces the nucleophilicity of the furan ring of FUR, so that the furan ring is not easy to undergo hydrolysis and Diels-Alder cycloaddition reaction, so the aldehyde group on the side chain of FUR can be functionalized first and then converted and utilized. As shown in fig. 6, Scodeller et al. selectively synthesized aromatic compounds using FUR and acrylonitrile
[60]. The advantage of this route is that the reaction temperature is only 303 ~ 333 K. Secondly, the stable acetal group of the furfural glycol acetal intermediate can protect the highly reactive formyl group from side reactions, and the acetal can be stabilized in a higher concentration reaction solution
[84]. The enhanced alkalinity of the solution is beneficial to the stable existence of the acetal substance, and the intermediate product can be continuously degraded under the condition of low alkalinity
[85]. Above 353 K, the retro-Diels-Alder cycloaddition becomes the dominant reaction
[86].
In addition, some scholars have successfully constructed the route from FUR alone to benzene homologues by using FUR as a single reactant. Tachibana et al. Used a variety of catalysts to dehydrate FUR on a P
2O
5 to produce maleic anhydride, and then the maleic anhydride was aromatized with FU obtained by catalytic cracking of FUR to produce terephthalic acid
[87].
This series of reaction routes are still being developed and improved, but the complicated routes and low selectivity caused by the complex molecular structure can not be ignored. From the perspective of economic benefits and industrial prospects, the direct preparation of BTEX from FUR will also be a hot and difficult research topic in the future.
4.5 Preparation of Benzene Series from Other Furan Biomass
In addition to the common furanoid biomass mentioned above, other furanoid biomass have also been used in this reaction model. Although there are few reports, these reactions also provide a useful reference for fully understanding the reaction rules of furan biomass
[88].
FU is the simplest oxygen-containing five-membered heterocycle, which can be obtained by decarbonization of furfural. The conversion of FU and propylene over the treated GaZSM-5 catalyst was only 24%, but the product selectivity was 96%
[89]. Uslamin et al. Realized the co-aromatization of FU and methanol over HZSM-5 zeolite
[61]. Due to the good coupling reaction performance between FU molecules and benzene rings, FU is easy to further polymerize with the product to form benzofuran when used as a raw material. Compared with other furans, the yield and selectivity of the catalytic conversion of FU to BTEX are lower, and the experimental conditions are more stringent
[90].
As the hydrogenation product of FUR, the conversion of FA to benzene homologues is also a feasible and competitive way. FA can be converted to MF by hydrodeoxygenation, followed by cycloaddition and aromatization. In addition, FA can directly undergo Diels-Alder cycloaddition reaction with dienophiles to form aromatic compounds
[62]. In the experiment of Zheng et al., the aromatics selectivity of FA and methanol conversion reached 24.2%, of which toluene accounted for nearly half
[70].
To sum up, the route of preparing benzene homologues from furans is closely related to the molecular structure of furans, resulting in different benzene homologues. It will be an important development direction in this research field to break through the reaction barriers as far as possible and develop a route to synthesize certain aromatic compounds from cheap raw materials.
5 Conclusions and outlook
The production of high-value aromatic compounds from renewable furan-based biomass, which can reduce the dependence on fossil energy, has become a research hotspot in the field of green chemistry in recent years. The existing research results show that a variety of furan molecules can obtain aromatic compounds through tandem Diels-Alder cycloaddition reaction and aromatization dehydration reaction. However, due to the strong reactivity of furan molecules and the complexity of tandem catalytic reactions, there are still some problems to be solved urgently. The following aspects need to be further studied:
(1) reducing the generation of byproducts. The occurrence of side reactions is an important reason for the low selectivity of products, and the selection of appropriate experimental conditions can effectively reduce the occurrence of side reactions. In recent years, ionic liquids have attracted much attention as solvent systems, which have shown high selectivity in biomass alkylation. At the same time, the efficient solvent-free reaction route will be an important direction for future development.
(2) applying an economical and friendly reaction substrate. The starting material of catalytic conversion is changed into a substrate with lower price, and dehydration, hydrodeoxygenation and other reactions are connected in series with the aromatization reaction through a one-pot method, so that the method has higher economic value and industrial application prospect. In such a reaction system, converting HMF or FUR into an acetal form to prevent its self-polymerization reaction can increase the concentration of the reactants and obtain a higher yield.
And (3) designing a catalytic reaction system and a catalytic model with high efficiency and low energy consumption. Intermolecular hydrogen transfer, dehydration and acid-catalyzed Diels-Alder cycloaddition reaction can be completed orderly and efficiently under mild conditions, which is the key point to realize the industrial application of biomass conversion. Among them, the "one-pot" synthesis route coupling a variety of reactions involves a variety of catalytic reaction paths. Although there are many side reactions, the reaction process is economical and convenient, and there is still a broad research space.
National Natural Science Foundation of China(51972327)
National Natural Science Foundation of China(51772312)
National Natural Science Foundation of China(51961165107)
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