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Cyclopropane derivates are appealing to synthesis of high‐energy‐density fuels because of the high strained energy of the three‐numbered ring. The catalytic cyclopropanation of olefin with diazomethane is very effective to construct the carbocycle. The majority of the catalysts employed are nonrecyclable homogeneous compounds. Herein, we report cyclopropanation of polycyclic olefins catalyzed by heterogeneous Pd/C. The optimal cyclopropanation conditions were explored utilizing dicyclopentadiene as model substrate and a series of polycyclic olefins were cyclopropanated with high yield. Additionally, the catalyst has good recyclability and stability, as shown by characterizations as well as the fact that no inactivation happens after cyclopropanation. The synthesized cyclopropane derivates exhibit good fuel characteristics like high density (1.006–1.087 g/cm3), high volumetric net heat of combustion (42.58–46.45 kJ/cm3), good low‐temperature and combustion performance. This work shows industrial potential for cyclopropanation of polycyclic olefins with diazomethane. [ABSTRACT FROM AUTHOR]

[Display omitted] • Relationship between fuel molecular structure and pyrolysis performance was investigated. • Pyrolysis conversion of the fuels is highly associated with their C + H atom numbers. • The heat sink of the fuels is closely correlated with n H/C 6.9 × n H+C 3.8. High-energy-density endothermic hydrocarbon fuels (HDEHFs) are the ideal on-board coolants for the thermal management of advanced aircrafts. However, the pyrolysis performace of high-energy-density (HED) fuels with different molecular structures remains ambiguous. Herein, we investigated the pyrolysis performance of five HED fuels with different cyclic structures, i.e., bicyclohexyl (BCH), perhydrofluorene (PHF), decahydronaphthalene (DHN), exo -tetrahydrodicyclopentadiene (exo -THD) and cyclopropyl-tetrahydrodicyclopentadiene (CTHD), to reveal the molecular structure-reactivity relationship. The pyrolysis conversion (650 °C, 4 MPa) of the tested fuels is in the order of CTHD > PHF > BCH > DHN > exo -THD, which is highly associated with their C + H atom numbers, n H+C (except for CTHD with high-tension ring). With the analysis of product distribution as well as DFT calculation, the decomposition reaction pathways of the fuels were proposed. The heat sink of the fuels follows the order of BCH > PHF > DHN > exo -THD > CTHD, which is closely correlated with n H/C 6.9 × n H+C 3.8. Meanwhile, the fitting results between molecular structures and physicochemical properties of the fuels were also demonstrated. This study could provide a guidance on the design and development of HDEHFs fuels for the high-performance aerospace vehicles with long flight distance and hypersonic speed. [ABSTRACT FROM AUTHOR]

• HPW/MCM-41 catalyst can catalyze the Simmons-Smith cyclopropanation. • Cyclopropanation activity and the acid site amounts have a positive correlation. • HPW/MCM-41 is an efficient catalyst for the synthesis of high-energy–density fuel. Simmons-Smith cyclopropanation commonly promoted by CF 3 COOH is one important method for transforming olefin into high-energy–density cyclopropyl fuel, but it brings problems such as high cost and harm to environment. In this work, a series of highly active solid acid catalysts HPW/MCM-41 is prepared by impregnation method for the cyclopropanation of dicyclopentadiene. The composition, structural and acidity of catalyst were characterized by TEM, XRD, NH 3 -TPD, etc. The 5 % HPW/MCM-41 contains more Dawson-type P 2 W 18 (H 6 P 2 W 18 O 62) and defective-type P 2 W 21 (H 6 P 2 W 21 O 71) that can provide the most abundant acid sites. This catalyst shows the best catalytic activity, the conversion and bicyclic product selectivity for cyclopropanation of dicyclopentadiene are 93.4 % and 55.7 %, respectively, both higher than that of CF 3 COOH. Moreover, such catalyst shows good catalytic activity for several olefin substrates, and can be recycled in several runs. This work provides a green and efficient catalyst for cyclopropanation reactions for synthesis of high-energy–density fuel, pesticide and medicine. [ABSTRACT FROM AUTHOR]

• Two spirocyclopropane high-energy–density fuels have been synthesized from cyclopentadiene. • Optimal cyclopropanation conditions were explored to obtain high yield. • The reaction process for cyclopropanation of CPD has been proposed. • Cyclopentadiene-based spirocyclopropane high-energy–density fuels exhibit high energy density, specific impulse and excellent low-temperature properties. High-energy–density fuels play a vital role in the development of the aviation industry because they can provide more propulsion energy than conventional liquid fuels. Constructing more five- or six-membered rings in fuel molecules is an efficient way to increase fuel density. However, as the number of rings increases, the hydrogen content of fuel molecules gradually decreases, resulting in a decrease in the mass calorific value, which limits the further improvement of the volumetric calorific value, meanwhile, the low-temperature characteristics of the fuel become worsen. Considering the high strain energy stored in cyclopropane moiety and the good cryogenic properties of spiro fuel, in this work, two spirocyclopropane high-energy–density fuels have been synthesized from cyclopentadiene by phase transfer catalytic cyclopropanation and palladium catalytic cyclopropanation, and the reaction conditions have been optimized. The synthesized fuels possess heat value and specific impulse higher than the widely used high-energy–density fuel JP-10. Additionally, their kinematic viscosity at low temperatures is only about one-third that of JP-10. The outstanding properties of the synthesized strained fuels suggest their good potential as high-energy–density fuels. [ABSTRACT FROM AUTHOR]

The exploration to increase the fuel energy density is stimulated by the payload and cruising range concerns in aerospace industry. The high-density fuels are always rationally designed for the advanced volume-limited aircrafts to fulfill more complicated missions. Alkyl adamantanes gradually become a prospective class of high-energy-density fuels. Herein, in this work, we reported an approach to synthesize asymmetric cyclopentyl adamantane via AlCl 3 -catalyzed alkylation of adamantane and cyclopentene with an attractive combination of a high density of 0.990 g/mL as well as a low freezing point of −30 °C. The obtained 1-cyclopentyl-adamantane was well designed with the cyclic substituent (enhance the density) and asymmetric structure (lower the freezing temperature). Subsequently, the comprehensive fuel performance of cyclopentyl adamantane, i.e. , density, freezing point, low-temperature viscosity, heating value, and combustion properties were evaluated. It was shown that both density and combustion properties of cycloalkyl adamantane were superior to the typical high-density JP-10 fuel. In addition, the properties of binary fuel mixture (1-cyclopentyl-adamantane and JP-10) demonstrated that cyclopentyl adamantane could be used as a promising blendstock with conventional jet fuel to enhance the energy density and promote the combustion performance of the final fuel blends. [Display omitted] • Asymmetric cycloalkyl adamantane was synthesized via alkylation. • Cyclopentyl adamantane owned higher density than reported adamantanes and JP-10. • Relationships between molecular structure and properties were discovered. • Cyclopentyl adamantane was a novel hydrocarbon fuel for aerospace propulsion. [ABSTRACT FROM AUTHOR]

High-energy-density (HED) liquid fuels are critical for volume-limited aerospace vehicles to extend the flight range and increase the payload, and can be used as an energetic additive for conventional fuels. This review will summarize the synthesis and properties of HED fuels along with challenges appearing especially in the past decade. The currently used multi-cyclic HED fuels are introduced first with a focus on more efficient and greener synthesis routes. Then the synthesis of new multi-cyclic, strained and caged fuels with higher energy is summarized, mainly including the reaction mechanism and catalysts involved. After that, HED biofuels synthesized from biomass feedstock including terpene, hemicellulose, cellulose and lignin are comprehensively reviewed. Nanofluid HED fuels are further introduced as an alternative way to increase the energy density of liquid fuels by adding energetic nanoparticles. Hypergolic energetic ionic liquids are then summarized as a special alternative for toxic and hazardous hydrazine. Finally we will emphasize the problems need to be resolved in the synthesis and application of HED fuels. This review article will be helpful to explore and develop better approach and process for HED-fuel synthesis and upgrade for advanced propulsion applications. This work has been selected by the Editors as a Featured Cover Article for this issue. [ABSTRACT FROM AUTHOR]

• HED fuel is synthesized by photoinduced [2 + 2] cycloaddition of norbornene. • The fuel shows high density of 1.024 g/mL and high volumetric NHOC of 42.98 MJ/L. • The photosensitized processes contain triplet energy transfer, cyclization and relaxation. • The pseudo reaction kinetics parameters and reaction rate equations are obtained. High-energy–density (HED) fuels have attracted extensive attention for volume-fixed aerospace vehicles because they can provide more propulsion energy to increase the payload and range of aerospace vehicles. Herein, we report a route for synthesis of HED fuel, pentacyclo[8.2.1.14,7.02,9.03,8]tetradecane (PCTD) with strained structure, by photoinduced intermolecular [2 + 2] cycloaddition of norbornene (NBE). The triplet quenching and phosphorescence quenching results indicate that NBE cycloaddition is conducted through the photosensitized processes containing triplet–triplet energy transfer, cyclization and relaxation. The reaction conditions are optimized to obtain the high photocatalytic activity, with the NBE conversion and PCTD selectivity reaching 78.7% and 94.4%, respectively. Moreover, a kinetic model is proposed with the reaction rate equation being correlated with the incident light flux, with the pseudo reaction kinetics parameters and reaction rate equations obtained. Importantly, PCTD shows high density of 1.024 g/mL and high volumetric net heat of combustion of 42.98 MJ/L, much higher than those of JP-10. In addition, blending with JP-10 or QC can significantly improve cryogenic properties of blended fuels, which is very promising for the practical application. [ABSTRACT FROM AUTHOR]

High-energy-density (HED) fuels have drawn substantial attention for volume-limited aerospace vehicles because they can provide high propulsion energy. Herein, we report a route for synthesis of HED fuel pentacyclo[5.3.0.02,5.03,9.04,8]decane (PCD) with strained structure by photoinduced intramolecular [2 + 2] cycloaddition of dicyclopentadiene (DCPD). The reaction conditions, like photosensitizer, solvent, concentration, temperature and light intensity, were optimized for achieving the best photochemical performance, with the high DCPD conversion of 95.89% and high PCD selectivity of 62.08%. The triplet quenching and phosphorescence quenching results show that DCPD cycloaddition proceeds under the photosensitizer photoexcitation, triplet-triplet energy transfer, cyclization and relaxation. Moreover, a reaction kinetic model is established with the reaction rate constant being correlated with light intensity, with the pseudo reaction kinetics parameters and reaction rate equations obtained. As a caged fuel, PCD possesses both high density (1.066 g/cm3) and high volumetric neat heat of combustion (44.43 MJ/L), which is very promising to blend with current high density liquid fuels to obtain high-performance fuels for practical application. • PCD is synthesized by a one-step photoinduced intramolecular [2 + 2] cycloaddition. • The triplet sensitization mechanism was proposed. • A reaction kinetic model correlated with light intensity is established. • PCD exhibits both high density and high volumetric neat heat of combustion. • PCD is a promising high-energy additive for practical application. [ABSTRACT FROM AUTHOR]

• Jet-fuel-range fuel is synthesized by dimerization reaction using acidic zeolites. • Hβ-25 shows the best performance with high NBE conversion and dimers selectivity. • The obtained fuel has high density and high volumetric neat heat of combustion. • The dimers fuel shows lower ignition temperature and shorter delay time than JP-10. Given a volume-fixed oil-tank, the advanced fuels with high energy density will increase the payload and range of the aircraft. Herein, we synthesized the high-energy–density liquid fuel via catalytic dimerization of norbornene by the acidic zeolites. Importantly, we reveal the relationship between the structural properties (i.e. , B/L acid site ratio and porous structure) of acidic zeolites and the conversion and selectivity of dimerization, and demonstrate the mechanism and kinetics of isomerization-dimerization two-step process. Among the investigated catalysts, Hβ-25 exhibits the best activity with high norbornene conversion and acceptable dimers selectivity, which is attributed to the synergy effect of appropriate B/L acid site ratio and suitable pore size. After hydrogenation, the obtained fuel has high density (0.978 g/cm3) and high volumetric neat heat of combustion (41.49 MJ/L), which are much higher than those of widely used JP-10 fuel. This work provides a promising route to produce high-energy–density fuel for practical application. [ABSTRACT FROM AUTHOR]

• The effects of phenolic antioxidants on high-energy–density fuel thermal oxidation and deposition were investigated. • The addition of antioxidants can significantly inhibit fuel oxidation. • TBDP and TBMP show better performance in antioxidation than BHA, DTBP and BHT. • The addition of phenolic antioxidants cause more severe deposition. The addition of antioxidants is normally adopted to improve the thermal oxidation stability of jet fuel, but the detailed antioxidation mechanism is still ambiguous. Herein, the inhibition of exo -tetrahydrodicyclopentadiene (the main component of JP-10) toward thermal oxidation and deposition by phenolic antioxidants (2,6-di- tert -butyl-4-methyphenol (BHT), 6- tert -butyl-2,4-dimethyphenol (TBDP), 2,6-di- tert -butylphenol (DTBP), 2- tert -butyl-4-methoxyphenol (BHA) and 2- tert -butyl-4-methylphenol (TBMP)) was investigated to reveal antioxidation mechanism and screen the best antioxidant for high-energy–density fuel. The theoretical and experimental results show that the addition of all phenolic antioxidants can significantly inhibit fuel oxidation and prolong the shelf life. Importantly, TBDP and TBMP show better performance than other antioxidants since they can scavenge more radicals per antioxidant molecule. However, antioxidants form dimer products via the addition between phenoxyl radicals and alkyl radicals during short-term thermal stress, which has a higher tendency to produce deposits. Consequently, the addition of phenolic antioxidants causes more severe deposition when jet fuel serves as the coolant in aircrafts. This work would guide the rational design of antioxidant for practical application in high-energy–density fuel. [ABSTRACT FROM AUTHOR]

• Cyclopropanated α- and β-pinane with high yields were synthesized through cyclopropanation. • Cyclopropanated α- and β-pinane show higher specific impulses and densities than reported rocket fuels. • Cyclopropanated α- and β-pinane with high-energy-density can be used as advanced rocket fuels. With the rapid development of aerospace industry, it is significantly necessary to develop advanced rocket fuels with high density, high specific impulse and good combustion performance. In this work, we report the synthesis of cyclopropanated pinanes through cyclopropanation of pinene. The reaction conditions including reaction time, solvent type, halohydrocarbon and molar ratio of zinc carbenoid to pinene were optimized. The basic physical properties of fuel, including density, kinematic viscosity, freezing point, calorific value, and combustion performance were tested, and the specific impulses of cyclopropanated pinanes were also calculated. Importantly, the cyclopropanated α- and β-pinane possess high density of 0.90 and 0.91 g/mL, specific impulse of 330.58 and 330.39 s, which results in higher volumetric specific impulse than reported rocket fuels. They also show good low-temperature performance and better ignition and combustion performance than JP-10. These excellent performances indicate that the cyclopropanated α- and β-pinane have great application prospect as high-energy rocket fuel. [ABSTRACT FROM AUTHOR]

• Three families of polycyclic hydrocarbons were proposed for high-energy-density fuels candidate. • The physicochemical and propulsion properties of polycyclic hydrocarbons were estimated. • The relationship between the propulsion performance and hydrocarbon structure characteristics was established. • The strained rings in molecules improves the density and specific impulse of polycyclic hydrocarbons. Polycyclic hydrocarbons have great potential as advanced high-energy-density (HED) fuels, and their physicochemical and propulsion properties are highly important for propulsion purposes. In this work, the fuel properties including density, melting point, boiling point, critical temperature/pressure, enthalpy of vaporization, enthalpy of fusion, enthalpy of combustion and specific impulse of three families of polycyclic hydrocarbons (a, cyclopropyl and methyl-substituted derivatives of tri-, tetra- and penta-cyclic hydrocarbons; b, [n]prismanes; and c, homocubanes) were evaluated by combining the group contribution and quantum chemical methods. The applied method properly predicts the physicochemical properties of polycyclic hydrocarbons, which are highly dependent on their molecule structures. For category (a), the hydrocarbons with low H/C ratio generally have higher density, volumetric specific impulse and volumetric combustion heat. Importantly, for all three families of polycyclic hydrocarbons, introducing additional strained rings (cyclopropyl ring or cyclobutyl ring) in molecules remarkably improves their density and specific impulse, which are highly appropriate for volume-limited aerospace vehicles to extend the flight range and increase the payload. The study on the relationship between the propulsion performance and structure characteristics of these hydrocarbons will give the general rule for the rational design of HED fuels. [ABSTRACT FROM AUTHOR]

Design and choice of fuels continue to be improved for aviation propulsion, and alkyl adamantanes have attracted great attention due to their high density, good low-temperature properties and potential characteristics. Herein, we reported a facile route to synthesize mono-substituted alkyl adamantanes via modified Grignard coupling (followed with one-step hydrodeoxygenation), including 1-ethyl-adamantane, 2-ethyl-adamantane, and 1- n -propyl-adamantane with high yields (> 75%). The process was modified via concentrating the Grignard reagents and altering the solvent, and one-step hydrodeoxygenation catalyzed by commercial Pd/C and zeolites was developed. Subsequently, comprehensive characteristics of alkyl diamondoid series, such as oxidation stability and combustion properties, were evaluated using pressure differential scanning calorimetry, rapid small-scale oxidation test, and hot-plate ignition test. Also, the specific impulse of alkyl diamondoids were revealed by theoretical calculation. The high-density alkyl diamondoids present adequate oxidation stability and possess better combustion characteristics over JP-10, thus can be considered as potential high-energy-density fuel for aerospace propulsion. [Display omitted] • Mono-substituted alkyl adamantanes are synthesized via modified Grignard coupling. • Alkyl diamondoids exhibit better comprehensive fuel properties than JP-10. • Relationships between molecular structure and properties are discovered. • Alkyl diamondoids are promising high-energy-density fuel for aerospace propulsion. [ABSTRACT FROM AUTHOR]

• Catalytic steam reforming of high-energy–density (HED) fuels was investigated. • Steam reforming reaction can significantly improve the heat sink of HED fuels. • The effects of H/C and (H/C)*M a on steam reforming reactivity were studied. This work investigated the steam reforming (SR) performance of high-energy–density (HED) fuels including exo- tetrahydrodicyclopentadiene (exo -THD), exo -tetrahydrotricyclopentadiene (exo -THT), 1,3-dimethyladamantane (DMA) and quadricyclane (QC), aiming to provide sufficient heat sink for hypersonic flight. The fuel conversion decreases in the order of QC > DMA ≈ exo -THD > exo -THT, while the molecular-structure-dependent catalyst stability trend is DMA > exo -THD > QC ≈ exo -THT, which are related to the H/C value, carbon deposition and exposure of active Ni species after carbon deposition. Importantly, the catalytic SR reaction can significantly improve the total heat sink of HED fuels. Especially, DMA and exo -THD with higher H/C and (H/C)*M a exhibit excellent performance on activity and stability, on which high total heat sink, total heating value and coke-resistance are achieved. This work provides the molecule-SR heat sink relationship for the design of the effective cooling system for the hypersonic flight. [ABSTRACT FROM AUTHOR]