High density aviation fuel is a synthetic liquid hydrocarbon with a density greater than 0.80 G/mL, which is widely used in aerospace fields, such as aircraft, rockets, missiles, spacecraft and satellites^[1]. The flight performance of an aircraft mainly depends on the density and volumetric calorific value of the fuel used. There is a positive linear correlation between fuel density and volumetric heating value, and the volumetric heating value (VNHOC) increases with the increase of fuel density^[2]. When the volume of the fuel tank of the aircraft is constant, the mass calorific value of the fuel is basically unchanged. At this time, the higher the density of the fuel, the greater its volume calorific value, and the greater the driving force provided; In the case of a certain driving force, the higher the density of the fuel, the smaller the volume of the fuel tank it occupies, which can effectively improve the space utilization of the aircraft^[3]. At the same time, because most spacecraft need to be used in low temperature and pressure environment, the fuel is required to have low freezing point and appropriate viscosity, so the synthesis of high-density fuel with excellent comprehensive performance is the key to improve the flight performance of spacecraft^[4⇓~6].

The petroleum-based aviation fuels developed in the early stage are mainly paraffins and monocycloalkanes, which have the problem of low density. The density of aviation kerosene, which is mainly composed of paraffins, is about 0.78 G/mL, while the density of rocket kerosene, which contains a large amount of naphthenes, is only 0.83 G/mL, which can not meet the requirements of high-performance aircraft. The polycyclic hydrocarbon fuel synthesized later has higher density and volumetric calorific value. For example, RJ-4 is a mixture of two isomers (endo-bridge and exo-exo) prepared by hydrogenation of dimethyl dicyclopentadiene. The density of RJ-4 is 0.927 G/mL, the volumetric calorific value is 39.0 MJ/L, and it also has a suitable flash point and low temperature performance^[7]. JP-10, exo-tetrahydrodicyclopentadiene (exo-THDCPD), has a density of 0.94 G/mL, a freezing point of -79 ℃, and a volumetric calorific value of 39.6 MJ/L, which is 15% higher than that of traditional aviation kerosene. RJ-5, also known as perhydrogenated norbornadiene dimer, is a mixture of several condensed cycloalkanes with low carbon ratio, which is obtained by polymerization, isomerization and hydrogenation of norbornadiene (NBD) under the catalysis of 5% Rh/C with a mass fraction of 15%. The density of RJ-5 reaches 1. 08 G/mL, the volumetric calorific value reaches 44. 9 MJ/L, the freezing point is too high (> 0 ℃), and the viscosity is as high as 2000 mm²/s（-40℃）. When it is too cold, it will immediately freeze into a solid, causing the fuel system to shut down^[8]. High density fuel with density of 0. 968 ~ 1.033 G/mL and freezing point below -40 ℃ can be obtained by blending RJ-5 with other low freezing point fuels^[9]. Since then, researchers have developed a series of petroleum-based high-density aviation fuels with density > 1 G/mL and excellent low-temperature performance (freezing point < -40 ℃).

As the raw material of petroleum-based aviation fuel directly comes from fossil resources, the CO₂ produced by its combustion process is one of the main reasons for the current greenhouse effect. With the gradual depletion of fossil resources and the increasing greenhouse effect, in order to protect the environment and achieve the sustainable development of human society, the preparation of high-density aviation fuel from renewable resources, especially biomass resources, has attracted increasing attention. Biomass resource is renewable, carbon neutral and environmentally friendly, which indicates that it is an ideal alternative to fossil energy^[10]. At present, terpenoids and lignocellulose are the main biomass raw materials for the synthesis of biomass high-density aviation fuel. Turpentine is an important source of terpenoids, mainly including pinene, limonene, carene and so on. The density of the fuel obtained from pinene by C-C bond coupling reaction is more than 0. 90 G/mL, but the global annual output of turpentine is only 300,000 tons, which limits its application in the synthesis of high-density aviation fuel. Lignocellulose, represented by crop straw and forestry waste, is a cheap and abundant biomass resource in nature. A large number of biomass platform molecules and their derivatives can be obtained from lignocellulose by fermentation, hydrolysis, pyrolysis, gasification and other methods, such as cyclopentanone, cyclohexanone, furfural, methylfurfural, hydroxymethylfuran, phenol derivatives, vanillin, 2.5-hexanedione, acetone, ethanol, cyclopentanol, cyclohexanol, p-benzoquinone, benzyl alcohol, isophorone, alkyl aromatic aldehyde, isoprene, etc. (Fig. 1)^{[11⇓⇓~14]}.

Because platform molecules and their derivatives generally have less carbon atoms, biomass fuels directly prepared from these compounds mainly include paraffins and monocycloalkanes, which have low density and volume calorific value, and can not meet the requirements of high-performance spacecraft. According to the molecular structure-activity relationship, the properties of hydrocarbons are determined by their molecular structure. The density of paraffins is lower and that of cycloalkanes is higher.The synthesis of hydrocarbons with polycyclic structures (linked ring, spiro ring, fused ring and bridged ring) is one of the important means to improve the density of fuels, and the construction of polycyclic structures can be achieved by the commonly used C-C bond coupling method^{[15⇓⇓~18]}. At the same time, aiming at the problem that the comprehensive performance of the biomass multi-ring high-density aviation fuel is poor due to the fact that the increase of the freezing point is greater than the increase of the density, the problem is preliminarily improved by introducing alkyl into the molecule and synthesizing a petroleum-based high-density fuel or a multi-component mixed fuel. In this paper, the latest progress in the synthesis of polycyclic hydrocarbon high density aviation fuels from platform molecules and their derivatives is reviewed. Firstly, the commonly used C-C bond coupling methods for the construction of polycyclic structures are introduced.Including aldol condensation, alkylation, aldol condensation-hydrodeoxy-intramolecular alkylation, Diels-Alder reaction, photocatalytic 2 + 2 cycloaddition, rearrangement reaction, etc^[19⇓~21]. The influence factors of catalyst on C-C bond coupling reaction were discussed. Finally, a large number of performance data of high density fuel were summarized, and the influence of molecular structure and fuel composition on the comprehensive performance of fuel was discussed. Through the summary of the above work, it is expected to provide reference for the future research on the synthesis of biomass high-density aviation fuel.

Aldol reaction is an important method to construct C-C bond between aldehydes or ketones. Cyclopentanone, as a platform molecule, can be prepared by selective hydrogenation of furfural, while cyclohexanone can be prepared by hydrogenation of phenol, both of which have the characteristics of wide sources. Cyclopentanone and cyclohexanone have five- and six-membered ring structures, respectively, and the number of carbon atoms and carbon rings can be effectively increased by aldol condensation. At present, there are many reports on the synthesis of polycyclic hydrocarbon high density aviation fuel from cyclic ketones^[22]. Zhang et al. Used magnesium-aluminum hydrotalcite (MgAl-HT) to catalyze the self-aldol reaction of cyclopentanone to obtain 2-cyclopentylidene cyclopentanone intermediate, and then used Ni-SiO₂ to catalyze hydrodeoxygenation (HDO) to obtain dicyclopentane (Figure 2A)^[23]. The density of dicyclopentane is 0.866 G/mL, the volumetric calorific value is 36.7 MJ/L, and the freezing point is -38 ℃. Zhang et al. Synthesized tricyclopentane with 80% carbon yield by aldol condensation of cyclopentanone and HDO catalyzed by Pd/MgAl-HT and Ni/Hβ in a dual-bed continuous flow reactor. They found that the preparation method of Ni/Hβ had a great influence on the dispersion and HDO activity of Ni. Tricyclopentane has a density of 0.91 G/mL, which is higher than that of dicyclopentane, but it can only be used as an additive due to its high viscosity (Figure 2b)^[24]. Zhang et al. Synthesized tetracyclopentane in 70% yield from cyclopentanone via three steps of aldol condensation-selective hydrogenation, aldol condensation, and hydrodeoxygenation^[25]. Tetracyclopentane has a density of 0.943 G/mL and a freezing point of -39.5 ℃, and can be mixed with JP-10 (Fig. 2C).

In addition to the self-aldol reaction of cyclic ketones to achieve the construction of polycyclic structures, the cross-aldol reaction between different cyclic ketones or between cyclic ketones and biomass aromatic aldehydes can be used to synthesize polycyclic high-density aviation fuels with different ring structures. Zou et al. Obtained a multi-component intermediate compound by preparing a highly stable, efficient and waterproof phosphotungstic acid HPW/SBA-16 to catalyze the cross aldol condensation reaction of cyclopentanone and cyclohexanone, and then obtained a multi-component polycyclopentane, polycyclohexane, poly (cyclopentane-cyclohexane) -type high-density fuel mixture by HDO (Fig. 3)^[26]. They made HPW uniformly dispersed on SBA-16 through alkylimidazole anchoring, and at the same time, the catalyst was hydrophobic due to the protection of hydrophobic alkyl chain to prevent the shedding of polyoxometalate. The density of the fuel is 0. 905 G/mL, the freezing point is below-50 ℃, and the volumetric calorific value is 38. 67 MJ/L, which is higher than that of the fuel prepared by the aldol condensation of cyclic ketones, and has certain application potential.

Li et al. Used protonic acid to catalyze the cross-aldol reaction of cyclopentanone and vanillin to obtain the intermediate compound, and then used Pd/HY to catalyze HDO to obtain the final product (Fig. 4)^[27]. They found that H₂SO₄ had the highest catalytic efficiency. The aldol reaction between cyclopentanone and vanillin does not occur under alkaline conditions, which is mainly due to the fact that the H atom on the hydroxyl group of vanillin is easier to leave than the H atom on the α position of cyclopentanone. The high density fuel obtained in this work has a density of 0. 943 G/mL and a freezing point of-35 ℃, and its low temperature performance needs to be further improved.

Although protonic acid has high catalytic efficiency, its application is limited due to its strong corrosiveness, so it is necessary to develop green catalysts for aldol reaction. Ma et al. Used ethanolamine lactate ionic liquid (LAIL) to catalyze the cross-aldol reaction of cyclopentanone and vanillin to obtain a mixture of monocondensation and dicondensation in 95.2% yield (Fig. 5)^[28]. They found that the synergistic effect of —NH₂ in ethanolamine and — OH in lactic acid promoted the aldol reaction. Finally, HDO was catalyzed by Pd/Nb₂O₅ to give the final product in 83.6% yield with 8.4% tricyclic component and 75.2% bicyclic component. The density of the high-density fuel mixture obtained in this work is 0.89 G/mL, the freezing point is below -60 ℃, and the low temperature performance is good.

In addition to cyclic ketone compounds and biomass aromatic aldehydes, chain diketones are used as raw materials to obtain cyclic ketenes through intramolecular aldol condensation, and then polycyclic hydrocarbon high-density aviation fuels can also be obtained through multi-step conversion. Zhang et al. First prepared 2,5-hexanedione by cellulose hydrolysis, and then catalyzed 2,5-hexanedione to undergo aldol condensation-hydrogenation reaction with Cu₂Ni/MgO-p under hydrogen atmosphere to obtain 3-methylcyclopentanone, which further underwent aldol condensation and HDO to obtain a mixture of methyldicyclopentane and methyltricyclopentane (Fig. 6)^[29]. The obtained fuel has a density of 0. 88 G/mL, a freezing point of-48 ℃, and a viscosity of 2.28 mm²/s(25℃), and can be used as aviation fuel or additive.

Isophorone is a ketene compound with a six-membered ring structure, which can be obtained by the trimerization of the platform molecule acetone, and can be used as a raw material for the synthesis of cyclohexane-type high-density fuel^[30]. Zhang et al. Obtained multi-methyl substituted bicyclohexane through Pd/C-catalyzed selective hydrogenation, NaOH-catalyzed aldol condensation, and Ni/SiO₂ catalyzed hydrodeoxygenation in three steps. The fuel has a density of 0.858 G/mL and a freezing point of − 51 ° C, which is significantly lower than that of bicyclohexane (1.2 ° C) (Fig. 7)^[31].

Alkylation reaction is an important method of carburization, which has been widely used in biomass high density fuel synthesis due to its mild reaction conditions and high yield. Turpentine is the main source of terpenes, and its main component is pinene. Because of the bicyclic structure and unsaturated C = C bond in pinene, high-density fuel with a density of more than 0.90 G/mL can be obtained by dimerization under acidic conditions. For example, Harvey et al. Used the superstrong solid acid Nafion to catalyze the isomerization and dimerization of β-pinene with a yield of more than 90% (Fig. 8)^[32]. The fuel obtained by PtO₂ hydrogenation of the dimerization product has a density of 0. 94 G/mL, a volumetric calorific value of 39. 5 MJ/L, and a pour point of-30 ℃. Recently, Yuan et al. Reported the efficient dimerization of α-pinene catalyzed by Hβ molecular sieve^[33]. Under the optimal reaction conditions, the conversion of α-pinene was 99. 9%, and the yield of dimerization product was 84. 5%. The yield of the dimerization product was still more than 60% after 8 cycles of the recovered Hβ. The fuel obtained by pinene dimerization has high viscosity and poor low temperature performance. At the same time, the source of pinene is very limited and the yield is low, which limits its wide application.

Lignin, as a major biomass component, is abundant in reserves. Phenolic derivatives are the main products of lignin degradation. Due to the existence of hydroxyl and methoxyl groups, phenolic derivatives have high alkylation activity and are easy to achieve C-C bond coupling. Zou et al. Synthesized ethyl-substituted dicyclohexylmethane high-density fuel from benzyl alcohol and 4-ethylphenol via phosphotungstic acid (HPW) -catalyzed alkylation, Pd/C and HZSM-5 catalyzed HDO (Fig. 9)^[34]. Phosphotungstic acid has a large number of acidic sites and a Keggin structure, so that polar molecules can be adsorbed in the solvate formed by phosphotungstic acid, which is beneficial to the efficient conversion of benzyl alcohol, so it can efficiently catalyze the alkylation reaction. The fuel mixture obtained by distillation comprises 91% of 1- (cyclohexylmethyl) -3-ethylcyclohexane, 1- (cyclohexylmethyl) -2-ethylcyclohexane and isomers; 7% dicyclohexylmethane and 2% alkyl migration products, the mixture has a density of 0.873 G/mL, a freezing point of -42 ℃, and a volumetric calorific value of 37.27 MJ/L.

The freezing point of fuel determines the use temperature of fuel. On the basis of improving the density of multi-ring hydrocarbon fuel, the freezing point of fuel can be reduced to a certain extent by synthesizing multi-component fuel mixture, thus improving the comprehensive performance of fuel^[35]. Zou et al. Synthesized a multi-component polycyclic hydrocarbon high-density fuel by one-pot method using lignin oil model compounds phenol and cyclopentanol as raw materials, Hβ molecular sieve and Pd/C as catalysts (Fig. 10)^[36]. The density of the blended fuel synthesized in this work is 0.88 G/mL, and the freezing point is lower than-75 ℃, which is significantly lower than that of the single component fuel. The lowering of the freezing point of the fuel is mainly caused by cyclopentylcyclohexane.

Furthermore, Zou et al. used lignin oil and cyclopentanol as raw materials to synthesize a mixed fuel of polycyclopentane and polycyclohexane by HY molecular sieve and Pd/C catalytic alkylation and HDO (Fig. 11)^[37]. The fuel has a density of 0.91 G/mL, a freezing point below -60 ℃, and a volumetric calorific value of 39.0 MJ/L. It was found that the catalyst with 5. 3 molar ratio of SiO₂/Al₂O₃ in HY had the best performance, which could effectively catalyze the alkylation of model substrate and lignin oil, and the conversion of phenol monomer in lignin oil could reach 80%. This work provides a convenient way to co-convert lignocellulose depolymerization products into high-density fuels, which is more in line with the application requirements.

Compared with the intermolecular alkylation reaction, which can only couple the existing ring structure in the platform molecule, the intramolecular alkylation reaction can form a new ring structure to achieve the purpose of ring addition, such as the synthesis of condensed cycloalkanes, thereby significantly improving the density of fuel. Zou et al. Obtained perhydrofluorene with high selectivity by intramolecular alkylation and hydrogenation of 2-benzylphenol catalyzed by Pd/C^[38]. Molecular sieve catalyst HZSM-5 can accelerate the deoxygenation reaction, while Pd/C is beneficial to the hydrogenation reaction. In the presence of HZSM-5, the -OH and -H of 2-benzylcyclohexanol undergo dehydration to form a carbon-carbon double bond; In the presence of Pd/C alone, the — OH of 2-benzylcyclohexanol and the — H on the benzene ring undergo a dehydration reaction to form a ring closure product (Fig. 12). Perhydrofluorene has a density of 0. 959 G/mL and a calorific value of 40. 1 MJ/L, but it can only be used as an additive because of its high viscosity (（1752 mm²/s, 20 ℃) and high freezing point (-15 ℃).

Since 2-benzylphenol cannot be obtained directly from biomass, Zhang et al. Used biomass-derived methylbenzaldehyde and cyclohexanone as raw materials to obtain intermediate compounds through intermolecular aldol reaction catalyzed by ethanolamine acetate (EAOAc) ionic liquid.Then an intramolecular alkylation ring-closure reaction is carried out under the catalysis of Pd/C, and finally 1-methylperhydrofluorene and 3-methylperhydrofluorene are obtained through hydrogenation (Figure 13)^[39]. The hydroxyl and amino groups in ethanolamine acetate ionic liquid are beneficial to the aldol reaction of methylbenzaldehyde and cyclohexanone, and the space distance between hydroxyl and amino groups and the number of hydroxyl groups also have great influence on the aldol reaction. The densities of 1-methylperhydrofluorene and 3-methylperhydrofluorene are 0. 99 and 0. 96 G/mL, respectively, which are the highest densities of biomass high-density fuels reported so far. However, because their freezing points are -22 ℃ and -3 ℃, respectively, and their viscosities are also high, they can only be used as additives.

Li et al. Prepared a protonated titanium dioxide nanotube (PTNT) solid acid to catalyze the aldol condensation of p-tolualdehyde, o-tolualdehyde and acetone, and obtained the intermediate compounds of α, β-unsaturated ketones in 76% yield^[40]. Compared with protonated layered titanium dioxide and protonated titanium dioxide nanowires, they found that the high activity of PTNT was mainly attributed to the special nanotube morphology, large surface area, high acid site number and acid strength. Finally, Pt/C was used to catalyze HDO and intramolecular alkylation to obtain dimethyl substituted octahydroindene (fig. 14). The densities of the fuels obtained in this work are 0. 94 G/mL and 0. 91 G/mL, respectively, and the freezing points are below-40 ℃, which have good application prospects.

The solid base can efficiently catalyze the aldol reaction of aldehydes and ketones. Li et al. Catalyzed the solvent-free aldol reaction of methylbenzaldehyde and a series of biomass ketones by solid base K₂CO₃/Al₂O₃ to obtain the intermediates of α, β-unsaturated ketones, and then catalyzed the hydrogenation deoxygenation and intramolecular alkylation cyclization by Pt/C in aqueous phase to obtain alkyl-substituted octahydroindenes^[41]. They found that the high activity of K₂CO₃/Al₂O₃ is mainly due to its high surface area and concentration of basic sites. At the same time, they used Amberlyst-15 resin to catalyze the aldol reaction of alkylbenzaldehyde and methyl isobutyl ketone to obtain intermediate compounds, and then used commercial Ru/C to catalyze HDO and intramolecular alkylation to obtain alkyl-substituted octahydroindene (Fig. 15)^[42]. The good performance of Amberlyst-15 resin is mainly attributed to its large surface area, high amount of acid sites and acid strength. The alkyl substituted octahydroindene fuel obtained in this work has a density of 0.895 ~ 0.902 G/mL and a freezing point below -40 ℃.

Wang et al. Prepared sulfated titanium dioxide nanofibers (STNFs) and applied them to the aldol condensation of vanillin and cyclohexanone to obtain the condensation product in 81% yield^[43]. The good catalytic performance of STNFs is mainly attributed to its high acid strength and high Bronsted/Lewis ratio. Finally, the condensation product of vanillin and cyclohexanone was converted into a mixture of dicyclohexylmethane and perhydrofluorene under the synergistic catalysis of Pd/C and HY. The formation of perhydrofluorene is due to hydrodeoxygenation and intramolecular alkylation. The mixture has a density of 0.95 G/mL, a volumetric calorific value of 39.3 MJ/L, and a freezing point of -17 ℃, and its low temperature performance needs to be further improved (Fig. 16).

Diels-Alder reaction is an important method for the construction of six-membered rings in organic synthesis. Diels-Alder reaction is used as a key step in the synthesis of petroleum-based high-density fuels (JP-10, RJ-4) to construct polycyclic structures, and Diels-Alder reaction can also be used in the synthesis of biomass high-density fuels. Spiral compounds have high density and volumetric calorific value, and also have low freezing point, so they can be used as high-density fuels. Zou et al. Synthesized a spirocyclic compound intermediate by Mannich-Diels-Alder reaction catalyzed by molecular sieve HZSM-5 using petrochemicals cyclopentadiene and platform molecules cyclopentanone and formaldehyde as raw materials, and finally obtained a polycyclic hydrocarbon fuel with both spirocyclic and bridged ring structures through HDO (Fig. 17)^[44]. The Si/Al ratio in HZSM-5 zeolite has a great influence on the catalytic effect of Mannich-Diels-Alder, which is mainly determined by the ratio of Lewis acid to Blauster acid sites (L/B) in the zeolite. The spiro compound fuel has a density of 0. 952 G/mL, a volumetric calorific value of 40. 18 MJ/L, a freezing point of -53 ℃, and a moderate viscosity of （61.9 mm²/s, -40 ℃), which has great application potential.

At the same time, Zou et al. Synthesized a kind of polycyclic hydrocarbon high-density fuel mixture by using petrochemical dicyclopentadiene and platform molecule 2-methylfuran as raw materials, using HY molecular sieve to catalyze Diels-Alder reaction and Pd/C to catalyze HDO (Fig. 18)^[45]. Among the catalysts screened (HY, HZSM-5, Hβ and LaY), they found that HY had the highest conversion (88.5%) and the best selectivity (DCMF, 30%; CPMF,31%; TCPD, 23%) due to the large surface area, micropore volume and suitable pore size of HY. The density of the blended fuel is as high as 0. 984 G/mL, the freezing point is -58 ℃, the viscosity is 166.5 mm²/s（-20℃）, and the volumetric calorific value is 41. 96 MJ/L, so the comprehensive performance of the blended fuel is good. This work provides a method for the preparation of multi-ring high density fuel from the combination of bio-based feedstock and petroleum-based feedstock.

Due to the good comprehensive performance of the early developed petroleum-based high-density fuels (RJ-4, JP-10, etc.), in order to improve the performance of biomass-based high-density fuels, the synthesis of petroleum-based high-density fuels from platform molecules has become one of the recent research hotspots. As early as 2011, Harvey et al. used linalool as the raw material and RCM reaction as the key step to synthesize methylcyclopentadiene, the monomer raw material of RJ-4, and further obtained RJ-4 through Diels-Alder reaction, hydrogenation and isomerization (Figure 19)^[46]. This is the first report on the synthesis of RJ-4 from biomass feedstock. This method has the advantages of solvent-free and low energy consumption, but it is difficult to achieve large-scale industrial production due to the limitation of linalool source and the use of expensive Grubbs-Hoveyda catalyst in the reaction.

In 2019, Zhang et al. Synthesized JP-10 with a total carbon yield of 63% from easily prepared furfuryl alcohol through a six-step reaction^[47]. Furfuryl alcohol was first catalyzed by CaO to obtain 4-hydroxycyclopentenone through Piancatelli rearrangement, then catalyzed by Raney Ni to obtain 1,3-cyclopentanediol, followed by dehydration catalyzed by solid acid H-USY to obtain cyclopentadiene, then cyclopentadiene was autopolymerized to dicyclopentadiene at 120 ° C, followed by hydrogenation to obtain tetrahydrodicyclopentadien, and finally isomerized by molecular sieve La-Y to obtain JP-10 (Fig. 20). At the same time, they analyzed the cost of the petroleum-based synthetic route and biomass-based synthetic route of JP-10. By comparison, it is found that the biomass-based synthesis route has great economic advantages, and with the further reduction of the price of furfuryl alcohol, the price of JP-10 synthesis through the biomass-based route will be further reduced.

Furthermore, Zou et al. Developed a new five-step method to prepare RJ-4 and its mixed fuel from 5-methylfurfural^[48]. Firstly, 5-methylfurfural is subjected to reductive ring opening to obtain 2,5-hexanedione under the catalysis of Ni₂P and HZSM-5, then the 2,5-hexanedione is subjected to intramolecular aldol condensation to obtain 3-methylcyclopent-2-enone under the catalysis of sodium hydroxide, and then 3-methylcyclopent-2-enol is obtained through Luche reduction;Then 3-methylcyclopent-2-enol was dehydrated and polymerized to produce a mixture of dimethyldicyclopentadiene and trimethyltricyclopentadiene under the catalysis of HZSM-5, and finally RJ-4 and its mixed fuel were obtained by catalytic hydrogenation of Ni₂P (Fig. 21). Because 5-methylfurfural can be prepared from cellulose in large quantities and is abundant, this work opens up a new way to synthesize petroleum-based high-density fuel RJ-4. However, Luche reduction requires excessive NaBH₄, which increases the production cost, and there are potential safety hazards in the reaction process, so it is not suitable for large-scale production.

Because RJ-4 and JP-10 are synthesized from methylcyclopentadiene and cyclopentadiene, the core work of synthesizing RJ-4 and JP-10 from platform molecules is to study how the platform molecules are efficiently converted into methylcyclopenta-diene and cyclopenta-diene. Harvey et al. Used the platform molecule 2,5-hexanedione to obtain methylcyclopentadiene in three steps^[49]. First, 2,5-hexanedione underwent KOH-catalyzed aldol condensation to give 3-methylcyclopent-2-enone, then RuCl₂(PPh₃)₃ catalyzed selective hydrogenation of 3-methylcyclopent-2-enone under NH₂(CH₂)₂NH₂ and basic conditions to give 3-methylcyclopent-2-enol, and finally AlPO₄/MgSO₄ catalyzed dehydration to give methylcyclopentadiene (Figure 22 A). Due to the use of noble metal homogeneous catalysts in the selective hydrogenation, the synthesis cost of this route is high. Zhang et al. Catalyzed the selective hydrodeoxygenation of 3-methylcyclopent-2-enone to prepare methylcyclopentadiene in one step at 400 ° C under atmospheric hydrogen atmosphere with metal oxide MoO₃/ZnO as catalyst, and the yield was as high as 70%, which provided a great possibility for the efficient synthesis of RJ-4 with platform molecules (Fig. 22 B)^[50]. The results show that the ZnMoO₃ sites formed in the reduction of MoO₃/ZnO preferentially adsorb C = O, which leads to the improvement of hydrogenation selectivity. Zhang et al. Catalyzed the intramolecular aldol condensation and selective hydrodeoxygenation of 2,5-hexanedione to prepare methylcyclopentadiene in one step with an overall yield of up to 65% using Zn₃Mo₂O₉ as a catalyst at 400 ° C under atmospheric hydrogen atmosphere (Fig. 22c)^[51]. It has been shown that Zn₃Mo₂O₉ has a higher specific surface area than MoO₃/ZnO, which is easier to produce oxygen vacancies to adsorb C = O, so it has a higher activity for intramolecular aldol condensation/selective hydrodeoxygenation, and the stability of this catalyst is also higher.

In 2023, Li et al. Synthesized methylcyclopentadiene and cyclopentadiene in two steps from xylose or extracted hemicellulose^[52]. First xylose or extracted hemicellulose was directly converted to cyclopentanone by Ru/C-catalyzed hydrogenolysis in a biphasic system of toluene/20 wt% NaCl aqueous solution. The presence of NaCl in the solvent not only inhibits the hydrogenation of xylose to xylitol, but also the hydrogenation of furfural (from the dehydration of xylose) to tetrahydrofurfuryl alcohol, both of which promote the hydrogenolysis of xylose or extracted hemicellulose to cyclopentanone in high yield (or high selectivity). Cyclopentadiene was obtained by hydrodeoxygenation and dehydrogenation of cyclopentanone catalyzed by zinc molybdate (ZMO). When methanol and hydrogen are added to the reaction system, dehydrogenation, aldol condensation, and selective hydrogenation deoxygenation occur to obtain methylcyclopentadiene (fig. 23). This work uses cheap and abundant hemicellulose raw materials to synthesize cyclopentadiene and methylcyclopentadiene, which opens up a new way for the synthesis of petroleum-based high-density fuels from biomass.

Lignin, as a major biomass component, can be directly depolymerized into quinones, which have unsaturated C = C double bonds and ring structures, and can be used to synthesize polycyclic high-density fuels through Diels-Alder reaction. Li et al. Synthesized dimethyldecalin fuel from 2-methyl-2,4-pentanediol and p-benzoquinone through a three-step reaction (Fig. 24)^[53]. Firstly, the deep eutectic solvent (DES) prepared by choline chloride (ChCl) and methanesulfonic acid was used to catalyze the dehydration of 2-methyl-2,4-pentanediol to diene compounds with high selectivity. It was found that the acidity of methanesulfonic acid determined the high catalytic efficiency of this eutectic solvent. Then, the Diels-Alder reaction of the diene compound and p-benzoquinone in the absence of catalyst gave the addition product of C12. Finally, the final product was obtained by HDO catalyzed by physically mixed Pd/C and HY. The dimethyl decalin high density fuel has a density of 0.91 G/mL and a freezing point of -48 ~ -37 ℃.

Lu et al. Developed a vanadium-supported titania （V-TiO₂） catalyst to catalyze the tandem Diels-Alder reaction of isoprene and p-benzoquinone to construct a tricyclic structure^[54]. They found that the higher proportion of V⁴⁺/V⁵⁺ on the surface of titanium dioxide promotes the formation of tricyclic compounds. Under the optimal conditions, the yield of tricyclic intermediate reached 77. 2%. Finally, dimethyltetradecahydroanthracene was obtained by HDO catalyzed by Ru/C and HZSM-5. The combustion heat value of dimethyltetradecahydroanthracene is 45. 7 MJ/kg, which has certain application potential. This work expands the application of lignin compounds in high-density fuel synthesis (Fig. 25).

Solar energy is an inexhaustible source of energy, and it is of great significance to synthesize biomass high-density fuel by photocatalytic reaction. Compared with traditional thermochemistry, the conditions of photoreaction are milder and the selectivity is higher. Photocatalytic 2 + 2 reaction can be used to construct four-membered ring structure with higher energy and improve the performance of fuel^[55,56]. Zou et al. used isophorone and cyclohexene as raw materials to synthesize condensed ring high-density fuel through photocatalytic 2 + 2 cycloaddition reaction and HDO (Fig. 26)^[57]. They used isophorone as photosensitizer and reaction reagent, and the conversion rate of isophorone reached 95% and the selectivity reached 93% under the irradiation of 300 – 375 nm high-pressure mercury lamp with the light intensity of 231 mW/cm² for 9 H. At 365 nm, the quantum yield of the co-addition of isophorone and cyclohexene is 88%, while the quantum yield of the self-addition of isophorone is only 16%, indicating that the activation energy of the co-addition of isopherone and cyclohexene is lower than that of the self-addition of isopherone. The density of the coaddition product is 0.903 G/mL, the freezing point is -55 ℃, and the volumetric calorific value is 38.77 MJ/L; The self-addition product has a density of 0.892 G/mL, a freezing point of -40 ℃, and a volumetric calorific value of 38.58 MJ/L. The presence of methyl can significantly reduce the freezing point of the compound, but too much methyl can increase the freezing point of the compound, reduce the density and deteriorate the performance.

Furthermore, Zou et al. Synthesized spiro high-density fuel by photocatalytic 2 + 2 cycloaddition reaction using isophorone and β-pinene as raw materials (Fig. 27)^[58]. They used isophorone as photosensitizer and reaction reagent, and the conversion of isophorone reached 92% and the selectivity reached 96% under the irradiation of 300 – 375 nm high-pressure mercury lamp with the light intensity of 210 mW/cm² for 9 H. The addition product has a density of 0. 911 G/mL, a freezing point of-51 ℃, and a volumetric calorific value of 38. 67 MJ/L.

The carbon skeleton rearrangement reaction can construct complex spiro, fused ring and other structures, thereby improving the comprehensive performance of the fuel. Zou et al. Synthesized spiro high density fuel with pinacol rearrangement as the key step^[59]. Pinacol was prepared from cyclohexanone and cyclopentanone by reductive coupling reaction catalyzed by TiCl₄ and Zn, followed by pinacol rearrangement catalyzed by SnCl₄ to give spiro [5,6] dodecan-7-one and spiro [4,5] decan-6-one in 79% and 94% yields, respectively, and finally spiro [5,6] dodecane and spiro [4,5] decane were obtained by Wolff-Kishner reduction in 83% and 90% yields, respectively. The fuel test results show that the freezing points of spiro [4,5] -decane and spiro [5,6] -dodecane are -76 ℃ and -51 ℃, respectively, which are much lower than those of dicyclopentane and dicyclohexane, and the improvement of low temperature performance is very significant.

Li et al. Used the carbocation formed by cyclopentanol to polymerize and rearrange to obtain the precursor compound, and finally synthesized decalin by hydrogenation^[60]. Cyclopentene was first formed from cyclopentanol catalyzed by Amberlyst-36, and then cyclopentane carbocation was formed, followed by successive polymerization and rearrangement reactions to obtain a mixture of C₁₀ and C₁₅ with a carbon yield of 74%, and finally a mixture of C₁₀ and C₁₅ was obtained by Pd/C catalytic hydrogenation reaction, in which the selectivity of decalin reached 77% (Fig. 29). The fuel obtained in this work has a density of 0.896 G/mL and a freezing point of-37 ℃, and can be used as a high-density fuel.

Zou et al. Used cyclopentanol, cyclohexanol and methylcyclohexane, methylcyclopentane to synthesize methyl-substituted decalin and ethyl-substituted decalin by dehydration, alkylation, carbocation rearrangement and hydrogen transfer reaction under the catalysis of H₂SO₄ at room temperature (Fig. 30)^[61]. In the reaction process, the conversion rate of the cyclic alcohol reaches 100%, the conversion rate of the methyl cyclopentane is 65%, and the selectivity of the obtained methyl substituted decalin is 87%. The methyl or ethyl substituted decalin obtained in this work has a freezing point as low as -110 ℃, a density of 0. 87 ~ 0.88 G/mL, and a volumetric calorific value of more than 37 MJ/L, and can be directly used as a high-density fuel.

They used reduced graphene oxide-assisted H₂SO₄ to catalyze the one-pot synthesis of methyl- and cyclopentyl-substituted decalin mixtures from methylcyclopentane and cyclopentanol. The reduced graphite oxide is used as an additive to promote the emulsification of the reactant in the hexasulfuric acid, thereby improving the carbon yield of the reaction. In the reaction, the conversion of methylcyclopentane is 54.9%, the carbon yield of the product is 83.2%, and the selectivity is 97.3%. The fuel mixture obtained in this work has a density of 0.90 G/mL and a freezing point as low as below − 72 ° C^[62]. This is a simple, effective, and low-cost method to synthesize biomass high-density fuel (Fig. 31).

This paper summarizes the performance of a range of biomass polycyclic hydrocarbon high density aviation fuels, including linked, fused, spiro, and bridged naphthenic fuels, as well as multi-component blends (see Table 1). It can be seen from the table that among the cyclic fuels synthesized from cyclic ketones by aldol condensation, the density of cyclohexane fuels is higher than that of cyclopentane fuels, while the freezing point of cyclopentane fuels is generally lower; When the number of rings reaches or exceeds 3, the density of the fuel exceeds 0.90 G/mL, but the freezing point and viscosity will increase significantly, which is not conducive to the use at low temperatures; The introduction of alkyl substituents (especially methyl groups) on the ring can significantly reduce the freezing point of the fuel, but the density of the fuel is slightly reduced. The density of condensed ring fuels is generally high, especially the density of fuels with 6-5-6 condensed ring structure is higher than that of commonly used petroleum-based high-density fuels, up to 0.99 G/mL, but it has the problems of high viscosity and high freezing point, so it is not suitable for direct use as fuel; The freezing point of the fuel can be reduced by introducing methyl into the fused ring, so that the low temperature performance is gradually improved, but further improvement is needed. Decalin fuels generally have low freezing points and simple synthesis methods, but the density of fuels needs to be further improved to improve their application value. Spiral fuels have lower freezing point and higher density, but the synthesis is more complex, so the process needs to be simplified to improve efficiency. Bridged ring fuels have the best overall performance, but most of them need to be synthesized from cyclopentadiene or methylcyclopentadiene. At present, important progress has been made in the synthesis of cyclopentadiene or methylcyclopentadiene from platform molecules, which has promoted the synthesis of petroleum-based high-density fuels with excellent performance from platform molecules. From the aspect of fuel composition, the density and freezing point of the fuel can be properly adjusted by adjusting the proportion of fuels with different properties in the multi-component mixed fuel, so as to gradually improve the comprehensive performance of the fuel. For example, the density of multi-component and multi-ring fuels prepared from cyclic ketones, cyclic alcohols and phenol derivatives exceeds 0. 90 G/mL, and the freezing point is as low as -70 ℃, which is significantly improved compared with the comprehensive performance of single-component fuels.

As an important liquid jet propellant, high-density aviation fuel plays an important role in improving the flight performance of aerospace vehicles. With the gradual depletion of fossil resources and the increasing greenhouse effect, the use of biomass resources to synthesize high-density aviation fuel is also in line with the current national strategic requirements for sustainable economic and social development. Because there is a linear positive correlation between the volume calorific value and density of high-density fuel, and the molecular structure of fuel determines the density of fuel, the density of biomass fuel can be effectively improved by building a multi-ring structure through C-C bond coupling. The commonly used C-C bond coupling methods in organic synthesis, including aldol reaction, intermolecular and intramolecular alkylation reaction, Diels-Alder reaction, photopromoted 2 + 2 cycloaddition reaction and rearrangement reaction, have been widely used in the synthesis of biomass high-density fuels, and a series of high-density fuel products have been obtained. However, the current synthetic biomass high-density fuel generally has the problem of low comprehensive performance, that is, it can not take into account both high density and low freezing point. In order to improve the performance of biomass high density fuels, petroleum-based high density fuels (JP-10, RJ-4), polycycloalkane hybrid fuels containing multi-components, and alkyl-substituted bi- or tricyclic fused ring fuels have been synthesized by using platform molecules recently, so that their densities exceed 0. 90 G/mL and freezing points are below-50 ℃, which improves the comprehensive performance of biomass high density fuels to a certain extent.

In general, biomass high-density fuel synthesis is in the development stage, and the comprehensive performance of the fuel needs to be further improved. In the future, it is necessary to further study the relationship between fuel performance and fuel molecular structure, so as to rationally design fuel molecules; By further studying the relationship between fuel performance and fuel composition, multi-component blended fuel is designed and synthesized to balance the contradiction between fuel density and freezing point and improve the comprehensive performance of fuel; Further improve the efficiency of fuel synthesis, develop multi-functional catalysts and one-pot synthesis process more suitable for industrial production, and improve the market competitiveness of biomass fuels. It is hoped that through the above work, the green and sustainable development of biomass energy will be gradually realized.