Review on synthesis and properties of high-energy-density liquid fuels: Hydrocarbons, nanofluids and energetic ionic liquids

A neural network potential energy surface assisted molecular dynamics study on the pyrolysis behavior of two spiro-hydrocarbons

Spiro-hydrocarbons are potentially a type of novel alternative jet fuel due to their high density and net heat of combustion. In this work, the pyrolysis study of two spiro-hydrocarbons (spiro[cyclopropane-1,6'-tricyclo[3.2.1.02,4]octane] (C10H14) as Fuel 1 and spiro[bicyclo[2.2.1]heptane-2,1'-cyclopropane] (C9H14) as Fuel 2) is performed via molecular dynamics (MD) simulations, with a neural network potential energy surface (NNPES), deep potential (DP) model, adopted. The data set for the DP model of each fuel is constructed after 31 and 27 iterations, respectively. The high precision of the DP model is demonstrated, and the temperature transferability of each model is observed. The overall pyrolysis performance is evaluated with the fuel decomposition rate, showing that both fuels have comparable gas-reactivity to commercial aviation fuels, such as JP-10. The reaction networks of initial pyrolysis for Fuels 1 and 2 are constructed, and the contribution of each pathway is discussed. Fuel 1 tends to form an unsaturated six-membered ring structure, while Fuel 2 generates unsaturated open-chain hydrocarbons. Further analyses of the MD results provide time-evolution information on each component in the pyrolysis species pool. Compared to Fuel 1, the initial pyrolysis of Fuel 2 leads to more hydrogen, alkenes, and alkanes, as well as fewer monocyclic aromatic hydrocarbons (MAHs), demonstrating a reduced tendency for afterward coking. This work might contribute to the development of the mechanism of the two spiro-hydrocarbons and guide the research of other similar structural fuels.

Continuous isomerisation of 2,5-dimethylfuran to 2,4-dimethylfuran over Ga-silicate

2,4-dimethylfuran has a rare disubstitution pattern in the five-membered heterocyclic furan ring that is highly interesting chemically but challenging to access synthetically. We present a heterogeneously catalysed route to synthesise 2,4-dimethylfuran from commonly available 2,5-dimethylfuran using a zeolite packed-bed flow reactor. As supported by DFT calculations, the reaction occurs inside the zeolite channels, where the acid sites catalyse proton transfer followed by migration of a methyl group. The zeotype Ga-silicate (MFI type) appears superior to an aluminium-containing ZSM-5 by demonstrating higher selectivities and slower catalyst deactivation. This work provides new opportunities for the continuous valorisation of bio-feedstock molecules in the perspective of the emerging biorefinery era.

Biosynthesis Progress of High-Energy-Density Liquid Fuels Derived from Terpenes

High-energy-density liquid fuels (HED fuels) are essential for volume-limited aerospace vehicles and could serve as energetic additives for conventional fuels. Terpene-derived HED biofuel is an important research field for green fuel synthesis. The direct extraction of terpenes from natural plants is environmentally unfriendly and costly. Designing efficient synthetic pathways in microorganisms to achieve high yields of terpenes shows great potential for the application of terpene-derived fuels. This review provides an overview of the current research progress of terpene-derived HED fuels, surveying terpene fuel properties and the current status of biosynthesis. Additionally, we systematically summarize the engineering strategies for biosynthesizing terpenes, including mining and engineering terpene synthases, optimizing metabolic pathways and cell-level optimization, such as the subcellular localization of terpene synthesis and adaptive evolution. This article will be helpful in providing insight into better developing terpene-derived HED fuels.

Energy Applications of Ionic Liquids: Recent Developments and Future Prospects

Ionic liquids (ILs) consisting entirely of ions exhibit many fascinating and tunable properties, making them promising functional materials for a large number of energy-related applications. For example, ILs have been employed as electrolytes for electrochemical energy storage and conversion, as heat transfer fluids and phase-change materials for thermal energy transfer and storage, as solvents and/or catalysts for CO2 capture, CO2 conversion, biomass treatment and biofuel extraction, and as high-energy propellants for aerospace applications. This paper provides an extensive overview on the various energy applications of ILs and offers some thinking and viewpoints on the current challenges and emerging opportunities in each area. The basic fundamentals (structures and properties) of ILs are first introduced. Then, motivations and successful applications of ILs in the energy field are concisely outlined. Later, a detailed review of recent representative works in each area is provided. For each application, the role of ILs and their associated benefits are elaborated. Research trends and insights into the selection of ILs to achieve improved performance are analyzed as well. Challenges and future opportunities are pointed out before the paper is concluded.

Accelerating the Design of High-Energy-Density Hydrocarbon Fuels by Learning from the Data

In the ZINC20 database, with the aid of maximum substructure searches, common substructures were obtained from molecules with high-strain-energy and combustion heat values, and further provided domain knowledge on how to design high-energy-density hydrocarbon (HEDH) fuels. Notably, quadricyclane and syntin could be topologically assembled through these substructures, and the corresponding assembled schemes guided the design of 20 fuel molecules (ZD-1 to ZD-20). The fuel properties of the molecules were evaluated by using group-contribution methods and density functional theory (DFT) calculations, where ZD-6 stood out due to the high volumetric net heat of combustion, high specific impulse, low melting point, and acceptable flash point. Based on the neural network model for evaluating the synthetic complexity (SCScore), the estimated value of ZD-6 was close to that of syntin, indicating that the synthetic complexity of ZD-6 was comparable to that of syntin. This work not only provides ZD-6 as a potential HEDH fuel, but also illustrates the superiority of learning design strategies from the data in increasing the understanding of structure and performance relationships and accelerating the development of novel HEDH fuels.

Design Strategy and Application of Deep Eutectic Solvents for Green Synthesis of Nanomaterials

The first report of deep eutectic solvents (DESs) was released in 2003 and was identified as a new member of ionic liquid (IL), involving innovative chemical and physical characteristics. Using green solvent technology concerning economical, practical, and environmental aspects, DESs open the window for sustainable development of nanomaterial fabrication. The DESs assist in different fabrication processes and design nanostructures with specific morphology and properties by tunable reaction conditions. Using DESs in synthesis reactions can reduce the required high temperature and pressure conditions for decreasing energy consumption and the risk of environmental contamination. This review paper provides the recent applications and advances in the design strategy of DESs for the green synthesis of nanomaterials. The strategy and application of DESs in wet-chemical processes, nanosize reticular material fabrication, electrodeposition/electrochemical synthesis of nanostructures, electroless deposition, DESs based nano-catalytic and nanofluidic systems are discussed and highlighted in this review.

Synthesis of Cyclopentadiene and Methylcyclopentadiene with Xylose or Extracted Hemicellulose

Cyclopentadiene (CPD) and methylcyclopentadiene (MCPD) are important intermediates that have been widely used in the production of high-energy-density rocket fuels, polymers and valuable chemicals. Currently, CPD and MCPD are produced from fossil energies at very low yields, which greatly limits their application. As a solution to this problem, we disclose an alternative two-step bio-route to access CPD and MCPD using xylose or extracted hemicellulose as the feedstock. In the first step, cyclopentanone (CPO) was directly produced by the selective hydrogenolysis of xylose or extracted hemicellulose over a commercial Ru/C catalyst in an acid-free toluene/NaCl aqueous solution biphasic system. In the second step, CPO was selectively converted to CPD by a cascade hydrodeoxygenation/dehydrogenation reaction over zinc molybdate catalysts. When methanol was introduced with CPO and hydrogen, MCPD was selectively obtained by a cascade dehydrogenation/aldol condensation/selective hydrodeoxygenation reaction over zinc molybdate catalysts.

Influence of Adding Carbonaceous Fuels to Ionic Liquids on Propellant Properties

To make ionic liquids (ILs) accessible and economical, ethylene glycol was mixed in 1-ethyl-3-methylimidazolium-dicyanamide ([EMIm]DCA) to obtain droplets that could experimentally collide white fuming nitric acid. To investigate the ignition delay (ID) time theoretically in terms of hydrodynamics, alcohol fuels and kerosene were used as combustibles, while the intermiscibility between them and nitric acid (HNO₃) was calculated using the ternary phase-field method alongside finite element analysis. The specific impulses of blend fuels were calculated by a thermodynamic method and compared to ILs. When the droplet was ethylene glycol/[EMIm]DCA with a 2.1 mm diameter and a 1.69 m/s colliding velocity, the ID time was the shortest. Kerosene was not an applicable additive for [EMIm]DCA owing to its lower intermiscibility with ILs and HNO₃ than alcohol fuels; alcohol fuels, however, were appropriate. The concentration of ethylene glycol in the oxidizer pool increased faster than the concentration of propylene glycol, triggering more rapid hypergolic ignition in the first 50 ms. The protocols regarding the hypergolic ignition conditions were verified, i.e., the size of the droplet had to be minute when the colliding velocity was as fast as possible; this was carefully calculated using ethylene glycol. According to thermodynamic calculations, the addition of alcohol fuels can improve the specific impulse of fuels, with ethylene glycol performing the best. The feasibility of adding alcohol fuels to ILs was confirmed via experiments and thermodynamic computations, with the simulation results providing some guidance on selecting the experimental or engineering conditions or both.

Influence of Reduction Temperature on the Structure and Naphthalene Hydrogenation Saturation Performance of Ni2P/Al2O3 Catalysts

Jet fuel rich in hydroaromatics and cycloalkanes could be derived from direct coal liquefaction oil via the hydrogenation saturation process. Developing an efficient catalyst to transform naphthalene hydrocarbons to hydroaromatics and cycloalkanes with high selectivity plays a significant role in realizing the above hydrogenation saturation process. In this work, Ni2P/Al2O3 catalysts were prepared at different reduction temperatures via the thermal decomposition of hypophosphite. We investigated the influence of reduction temperature and the results showed that reduction temperature had an important impact on the properties of Ni2P/Al2O3 catalysts. When the reduction temperature was 400 °C, the Ni2P particle size of the Ni2P/Al2O3 catalyst was 3.8 nm and its specific surface area was 170 m2/g. Furthermore, the Ni2P/Al2O3 catalyst reduced at 400 °C obtained 98% naphthalene conversion and 98% decalin selectivity. The superior catalytic activity was attributed to the smaller Ni2P particle size, higher specific surface area and suitable acidity, which enhanced the adsorption of naphthalene on Ni2P/Al2O3 catalyst.

Assembling Silver Cluster-Based Organic Frameworks for Higher-Performance Hypergolic Properties

Increasing research efforts have been focused on developing next-generation propellants. In this work, we demonstrated that assembling zero-dimensional (0D) silver clusters with energetic ligands into 3D metal organic frameworks (MOFs) not only inherited the short ignition delay (ID) time of the alkynyl-silver cluster but also significantly increased the output energy. Among them, the open cationic framework of ZZU-363 incorporating counter NO₃ ^- ions achieved a considerably reduced energy barrier and eventually the shortest ID time (26 ms), together with the highest volumetric energy density (40.4 kJ cm^-3) and specific impulse (263.1 s), which is far superior to traditional hydrazine-based propellants. The underlying mechanisms are clearly revealed by theoretical calculations. This work opens a venue to significantly enhancing the hypergolic activity of metal clusters and MOFs.

Synthesis of Ni/NiAlOx Catalysts for Hydrogenation Saturation of Phenanthrene

The saturation of octahydrophenanthrene was the rate-determining step in the hydrogenation process from phenanthrene to perhydrophenanthrene, which was due to the steric hindrance and competitive adsorption of octahydrophenanthrene. In this work, a series of Ni/NiAlO_x catalysts with a uniform electron-deficient state of Ni derived from the nickel aluminate structure was synthesized to overcome the disadvantage of noble catalyst and the traditional sulfided catalysts in the saturation hydrogenation process of phenanthrene. Results showed that the catalyst calcinated at 650°C possessed more Ni²⁺ (∼98%) occupying octahedral sites and exhibited the highest r_obs (1.53 × 10^-3 mol kg^-1 s^-1) and TOF (14.64 × 10^-3 s^-1) for phenanthrene hydrogenation. Furthermore, its ability to overcome steric hindrance and promote the rate-determining step was proven by octahydrophenanthrene hydrogenation. Comparing the evolution of hydrogenation activity with the change in the electronic structure of surface Ni sites, it was shown that the increase of metallic electron deficiency hindered the π-back bonding between surface Ni and aromatic rings, which was unfavorable for aromatic adsorption. As a result, the phenanthrene hydrogenation saturation performance can be enhanced by stabilizing the electron-deficient state of surface Ni on an optimal degree.

Single-Pulse Shock Tube Experimental and Kinetic Modeling Study on Pyrolysis of a Direct Coal Liquefaction-Derived Jet Fuel and Its Blends with the Traditional RP-3 Jet Fuel

A basic understanding of the high-temperature pyrolysis process of jet fuels is not only valuable for the development of combustion kinetic models but also critical to the design of advanced aeroengines. The development and utilization of alternative jet fuels are of crucial importance in both military and civil aviation. A direct coal liquefaction (DCL) derived liquid fuel is an important alternative jet fuel, yet fundamental pyrolysis studies on this category of jet fuels are lacking. In the present work, high-temperature pyrolysis studies on a DCL-derived jet fuel and its blend with the traditional RP-3 jet fuel are carried out by using a single-pulse shock tube (SPST) facility. The SPST experiments are performed at averaged pressures of 5.0 and 10.0 bar in the temperature range around 900-1800 K for 0.05% fuel diluted by argon. Major intermediates are obtained and quantified using gas chromatography analysis. A flame-ionization detector and a thermal conductivity detector are used for species identification and quantification. Ethylene is the most abundant product for the two fuels in the pyrolysis process. Other important intermediates such as methane, ethane, propyne, acetylene, and 1,3-butadiene are also identified and quantified. The pyrolysis product distributions of the pure RP-3 jet fuel are also performed. Kinetic modeling is performed by using a modern detailed mechanism for the DCL-derived jet fuel and its blends with the RP-3 jet fuel. Rate-of-production analysis and sensitivity analysis are conducted to compare the differences of the chemical kinetics of the pyrolysis process of the two jet fuels. The present work is not only valuable for the validation and development of detailed combustion mechanisms for alternative jet fuels but also improves our understanding of the pyrolysis characteristics of alternative jet fuels.

Photocatalytic Upgrading of Lignin Oil to Diesel Precursors and Hydrogen

Producing renewable biofuels from biomass is a promising way to meet future energy demand. Here, we demonstrated a lignin to diesel route via dimerization of the lignin oil followed by hydrodeoxygenation. The lignin oil undergoes C-C bond dehydrogenative coupling over Au/CdS photocatalyst under visible light irradiation, co-generating diesel precursors and hydrogen. The Au nanoparticles loaded on CdS can effectively restrain the recombination of photogenerated electrons and holes, thus improving the efficiency of the dimerization reaction. About 2.4 mmol g_catal ^-1  h^-1 dimers and 1.6 mmol g_catal ^-1  h^-1 H₂ were generated over Au/CdS, which is about 12 and 6.5 times over CdS, respectively. The diesel precursors are finally converted into C16-C18 cycloalkanes or aromatics via hydrodeoxygenation reaction using Pd/C or porous CoMoS catalyst, respectively. The conversion of pine sawdust to diesel was performed to demonstrate the feasibility of the lignin-to-diesel route.

Highly Efficient Hydroisomerization of Endo-Tetrahydrodicyclopentadiene to Exo-Tetrahydrodicyclopentadiene over Pt/HY

The fast deactivation caused by serious formation of coke is a major challenge in catalytic isomerization of endo-tetrahydrodicyclopentadiene (endo-THDCPD) into exo-tetrahydrodicyclopentadiene (exo-THDCPD) over the HY zeolite. In order to suppress the coke formation for the isomerization process, the conventional HY zeolite was modified with Pt at 0.3 wt %. Then, the hydroisomerization of endo-THDCPD into exo-THDCPD was evaluated over a fixed-bed reactor. The catalytic stability of Pt/HY was greatly enhanced in comparison to that of the HY zeolite. The Pt/HY catalyst provided 97% endo-THDCPD conversion and 96% selectivity for exo-THDCPD without deactivation after 100 h. Moreover, the formation mechanism of coke on the HY zeolite during the isomerization process was proposed based on the results of the coke analysis. It was indicated that the coke was generated from the oligomerization and condensation of olefin species, which originated from the β-scission reaction or hydride transfer reaction of intermediates. The lower coke formation over Pt/HY was attributed to the lower amount of coke precursors, which could be hydrogenated by activated H₂ over Pt sites. Therefore, Pt on Pt/HY and H₂ were two crucial factors in efficiently enhancing the catalytic stability of the HY zeolite for this isomerization reaction.

Pomegranate-like Core-Shell Ni-NSs@MSNSs as a High Activity, Good Stability, Rapid Magnetic Separation, and Multiple Recyclability Nanocatalyst for DCPD Hydrogenation

A novel pomegranate-like Ni-NSs@MSNSs nanocatalyst was successfully synthesized via a modified Stöber method, and its application in the hydrogenation of dicyclopentadiene (DCPD) was firstly reported. The Ni-NSs@MSNSs possessed a high specific area (658 m²/g) and mesoporous structure (1.7-3.3 nm). The reaction of hydrogenation of DCPD to endo-tetrahydrodicyclopentadiene (endo-THDCPD) was used to evaluate the catalytic performance of the prepared materials. The distinctive pomegranate-like Ni-NSs@MSNSs core-shell nanocomposite exhibited superior catalytic activity (TOF = 106.0 h^-1 and STY = 112.7 g·L^-1·h^-1) and selectivity (98.9%) than conventional Ni-based catalysts (experimental conditions: Ni/DCPD/cyclohexane = 1/100/1000 (w/w), 150 °C, and 2.5 MPa). Moreover, the Ni-NSs@MSNSs nanocatalyst could be rapidly and conveniently recycled by magnetic separation without appreciable loss. The Ni-NSs@MSNSs also exhibited excellent thermal stability (≥750 °C) and good recycling performance (without an activity and selectivity decrease in four runs). The superior application performance of the Ni-NSs@MSNSs nanocatalyst was mainly owing to its unique pomegranate-like structure and core-shell synergistic confinement effect.

Synthesis of bio-based methylcyclopentadiene via direct hydrodeoxygenation of 3-methylcyclopent-2-enone derived from cellulose

The exploration of highly efficient processes to convert renewable biomass to fuels and value-added chemicals is stimulated by the energy and environment problems. Herein, we describe an innovative route for the production of methylcyclopentadiene (MCPD) with cellulose, involving the transformation of cellulose into 3-methylcyclopent-2-enone (MCP) and subsequent selective hydrodeoxygenation to MCPD over a zinc-molybdenum oxide catalyst. The excellent performance of the zinc-molybdenum oxide catalyst is attributed to the formation of ZnMoO₃ species during the reduction of ZnMoO₄. Experiments reveal that preferential interaction of ZnMoO₃ sites with the C=O bond instead of C=C bond in vapor-phase hydrodeoxygenation of MCP leads to highly selective formations of MCPD (with a carbon yield of 70%).

Synthesis of methylcyclopentadiene (MCPD) with abundant biomass is appealing. Here the authors show that cellulose-derived 3-methylcyclopent-2-enone can be directly hydrodeoxygenated to MCPD over a partially reduced Zn-Mo oxide catalyst.

Thermal Conductivity and Stability of Hydrocarbon-Based Nanofluids with Palladium Nanoparticles Dispersed by Modified Hyperbranched Polyglycerol

Palladium nanoparticles, which were prepared by modified hyperbranched polyglycerol (mHPG) as stabilizers, can be dispersed well in nonpolar organic solvents and form highly stable nanofluids. The influences of three mHPG products modified with cyclohexanethiol (CSHPG), dodecanethiol (DSHPG), and octadecanethiol (OSHPG) on the preparation and stability of the palladium nanoparticles were investigated. The stability and thermal conductivity enhancement of the hydrocarbon-based nanofluids with Pd@mHPG (Pd@CSHPG, Pd@DSHPG, and Pd@OSHPG) compared to the corresponding base fluid were investigated at different temperatures. The average diameters of nanoparticles stabilized by CSHPG, DSHPG, and OSHPG are within 2.7-3.6 nm. The palladium nanoparticles could be dispersed well in the nonpolar base fluid such as decalin. The nanofluids with Pd@DSHPG and Pd@OSHPG could remain stable for up to 330 days at room temperature. The nanofluid with Pd@DSHPG or Pd@OSHPG could be stable for more than 24 h at 110 °C. The thermal conductivity of the nanofluids improved with increasing temperature and the mass fraction of nanoparticles compared to the corresponding base fluid. The long alkyl chain-modified HPG can give better protection for nanoparticles from agglomeration and assist metal nanoparticles in enhancing the thermal conductivity of nanofluids.

Bio-Based Cycloalkanes: The Missing Link to High-Performance Sustainable Jet Fuels

The development of sustainable energy solutions that reduce global carbon emissions, while maintaining high living standards, is one of the grand challenges of the current century. Transportation fuels are critical to economic development, globalization, and the advancement of society. Although ground vehicles and small aircraft are beginning a slow transition toward electric propulsion with energy sourced from solar radiation or wind, the extreme power requirements of jet aircraft require a more concentrated source of energy that is conveniently provided by liquid hydrocarbon fuels. This Review describes recent efforts to develop efficient routes for the conversion of crude biomass sources (e. g., lignocellulose) to cycloalkanes. These cycloalkanes impart advantageous properties to jet fuels, including increased density, higher volumetric heat of combustion, and enhanced operability. The combination of bio-based cycloalkanes and synthetic paraffinic kerosenes allows for the preparation of 100 % bio-based fuels that can outperform conventional petroleum-based fuels. In this Review methods are described that convert biomass-derived small molecules, including furfural, furfuryl alcohol, 5-hydroxymethylfurfural, cyclic ketones, phenolics, acyclic ketones, cyclic alcohols, furans, esters, and alkenes to high-density cycloalkanes. In addition to describing the chemical transformations and catalysts that have been developed to efficiently produce various cycloalkanes, this Review includes summaries of key fuel properties, which highlight the ability to generate fuels with customized performance metrics. This work is intended to inspire other researchers to study the conversion of sustainable feedstocks to full-performance aviation fuels. An acceleration of this research is critical to reducing the carbon footprint of commercial and military aviation on a timescale that will help blunt the impacts of global warming.