Chengxiang Shi,Jisheng Xu,Lun Pan,Xiangwen Zhang,Ji-Jun Zou,*

1 Key Laboratory for Green Chemical Technology of the Ministry of Education,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,China

2 Collaborative Innovative Center of Chemical Science and Engineering (Tianjin),Tianjin 300072,China

Keywords:High-energy-density Fuel Alkane Petroleum Cyclopentadiene Coal

ABSTRACT High-energy–density (HED) fuel is specifically pivotal to improve the performance of volume-limited aircrafts.The widely used HED fuels composed of polycyclic hydrocarbons are mainly synthesized from petroleum feedstocks.In order to ensure abundant supply,alternative resources such as coal should be considered.Herein,we summarize the synthesis methods and properties of typical HED fuels by using petroleum-derived cyclopentadiene (CPD) as key feedstock through dimerization,cycloaddition,hydrogenation and isomerization/photoisomerization reactions,and then propose a blueprint for synthesizing HED fuels from coal.The method to produce CPD from coal is analyzed and feasibility is demonstrated according to theoretical calculations and reported results.This review provides a novel route for synthesis of HED fuels from coal.

1.Introduction

High-energy–density(HED)fuels with higher density and volumetric net heat of combustion (NHOC) can provide more propulsion energy without increasing volume of fuel tank [1,2].This is particularly attractive for air vehicles like high-thrust rockets,long-range missiles and supersonic aircrafts that have higher and higher requirements for range and load,while they suffer from restrictions on the volume of fuel tanks.HED fuel is not only necessary to meet the propulsion requirements of advanced aircraft,but also effective to improve the performance of current aircrafts at low cost.

Unlike conventional refined fuels that consist of linear and monocyclic hydrocarbons,synthetic HED fuels are composed of polycyclic hydrocarbons,which afford high density and volumetric energy [2].In addition,the synthetic polycyclic fuel molecule can be controlled to provide better physical and chemical properties[3].So far,typical HED fuels widely used in aerospace such as JP-10,RJ-5 and RJ-7 are mainly synthesized from petroleum-based feedstocks.However,the reserves of petroleum resources are relatively small and decreasing day by day,especially in China.In order to ensure abundant supply,more resources should be well investigated and developed.Due to the relative abundance of coal reserves in China,synthesizing HED fuels from coal is an alternative strategy.

This review firstly summarizes the synthetic methods of typical HED fuels mainly derived from petroleum feedstocks.Polycyclic hydrocarbons in typical HED fuels,such as JP-10,RJ-7 and RJ-5,are mainly synthesized through dimerization,cycloaddition,hydrogenation and isomerization/photoisomerization by using dicyclopentadiene (DCPD) and norbornadiene (NBD) as reactants.DCPD is the natural dimer of cyclopentadiene (CPD),NBD can be synthesized by Diels-Alder [4+2] cycloaddition between CPD and acetylene.Hence,CPD is the initial feedstock for synthesis of HED fuels.According to theoretical calculations,dehydrogenation of coal-based C5fraction to produce CPD is feasible.Then,a novel synthetic approach to realize the production of coal-based HED fuels is proposed,and the critical procedure,i.e.dehydrogenation of C5molecules like pentane,pentadiene and cyclopentane/cyclopentene to CPD,is discussed particularly.

2.Synthesis of Typical High-Energy-Density Fuels

The most important and main compositions of current typical and widely used HED fuels,such as JP-10,RJ-5 and RJ-7,are tetrahydrodicyclopentadiene (THDCPD),tetrahydrotricyclopentadiene (THTCPD),and hydrogenated norbornadiene dimer (HNBDD),respectively.THDCPD can be obtained by hydrogenation of DCPD,THTCPD can be achieved by hydrogenation of tricyclopentadiene (TCPD),which is synthesized by cycloaddition of DCPD and CPD.H-NBDD is from dimerization and hydrogenation of NBD.In addition,quadricyclane (QC),a hypergolic HED fuel with high energy and density,can also be obtained from NBD through photoisomerization.In general,synthesis of HED fuels involves two steps:cycloaddition of cyclic olefins to form polycyclic olefins,and then hydrogenation of polycyclic olefins to form polycyoalkanes,which have higher storage stability and hydrogen content.When necessary,the spatial configuration of polycyoalkanes can be further controlled through isomerization to get better lowtemperature properties,such as low freezing point and low viscosity.

2.1.High-energy-density fuels from dicyclopentadiene

2.1.1.Exo-tetrahydrodicyclopentadiene

DCPD composed of two isomers:endo-andexo-DCPD mainly comes from by-products of petroleum pyrolysis and catalytic cracking to ethene [4].Full hydrogenation ofendo-DCPD,whose mass fraction is as about 99% DCPD,can obtainendotetrahydrodicyclopentadiene (endo-THDCPD).However,the freezing point ofendo-THDCPD (80 °C) is too high to be used as liquid fuel,while the freezing point of its isomer (exo-THDCPD) is as low as–79 °C [5].Hence,endo-THDCPD needs to be converted toexo-THDCPD through isomerization to meet the low-temperature property requirements of aviation fuel.The synthesis process ofexo-THDCPD,major composition of JP-10,is illustrated in Fig.1.The first step,i.e.hydrogenation of DCPD toendo-THDCPD,is generally conducted with metals as catalysts.Noble metals or their alloys,such as Pt,Pd,Ru,etc.,are very effective catalysts for hydrogenation reactions at modest temperature and pressure,but their high cost and rare reserves restrict their wide application.Accordingly,transition metals like Ni with much lower cost are widely investigated and applied in the hydrogenation reaction of DCPD.But high temperature is quite necessary to achieve sufficient conversion of DCPD due to the low activity of Ni.While,at high temperature,DCPD will decompose to CPD that will be hydrogenated to cyclopentane at H2atmosphere,which will significantly reduce selectivity of THDCPD.Therefore,further research on improving the activity of transition metal at moderate temperature is more desirable.

Zouet al.investigated the hydrogenation performance of amorphous Ni-alloy(SRNA-4)and compared it with conventional Raney Ni catalyst in hydrogenation of DCPD [6].As shown in Fig.2,the conversion of DCPD can reach 97.9%in 4 h at 110°C by using Raney Ni as catalyst,but the yield of THDCPD is very low (2.9%) and hardly increases with reaction time.While for SRNA-4,the conversion of DCPD quickly reaches 96.5%in 0.5 h at 110°C,and the yield of THDCPD increases continuously with reaction time,indicating good low-temperature performance of SRNA-4.Moreover,the authors concluded that the hydrogenation reaction of DCPD is a consecutive process as shown in Fig.3.In addition,oxygendeficient tungsten oxide(WO3-x)has been proved to be a more efficient transition metal catalyst than SRNA-4 for hydrogenation.By using WO3-xas hydrogenation catalyst,both conversion of DCPD and selectivity of THDCPD beyond 99% were achieved [7].

Isomerization ofendo-THDCPD was initially performed by using Brönsted acid and Lewis acid such as sulfuric acid,AlCl3[8,9].But utilization of these homogeneous acid catalysts would cause a lot of environmental problems,so heterogeneous catalysts such as zeolites have been explored.Among the zeolites that have been studied in isomerization ofendo-THDCPD,zeolite HY has the best performance due to its larger pores [5,10].The selectivity ofexo-THDCPD can reach 98.4%by using fluorine modified HY as catalyst,meanwhile,conversion ofendo-THDCPD is up to 94.0%[10].Acidic ionic liquids(ILs)were also used to catalyze isomerization ofendo-THDCPD toexo-THDCPD[11,12].The strong acidity and polarity of ILs make the isomerization reaction take place easily under mild conditions.Both conversionendo-THDCPD of and selectivityexo-THDCPD can exceed 98% [11].

Besides the above process,it is recently found thatendo-DCPD can be isomerized toexo-DCPD through thermal treatment or catalytic reaction.And then,the resultedexo-DCPD can be hydrogenated toexo-THDCPD,which provides a new route to synthesize JP-10 through isomerization and hydrogenation(Fig.4).

Zhanget al.reported the thermal isomerization ofendo-DCPD toexo-DCPD at high temperature and pressure for the first time[13].Hanet al.reported that several commercial acidic zeolites effectively promote isomerization ofendo-DCPD [14],and Hβ and HY exhibit higher performance due to their large three-dimensional porous structures.Subsequently,Zouet al.prepared Al-grafted ordered mesoporous silica sample (Al-MCM-41),which shows higher performance (Fig.5) and better coke tolerance capability in isomerization ofendo-DCPD [15].The better activity of Al-MCM-41 could be ascribed to its ordered and large pores that allow the reactants and products to diffuse freely and provide better tolerance for coke deposition.

2.1.2.exo-Tetrahydrotricyclopentadiene

exo-Tetrahydrotricyclopentadiene (exo-THTCPD) with higher density (1.03 g·cm-3) and volumetric energy (43.2 MJ·L–1) is the main composition of RJ-7.THTCPD can be obtained by hydrogenation of TCPD[16],which can be achieved through cycloaddition of DCPD and CPD [17,18].Owing to that DCPD has bothendo-andexo-isomers,cycloaddition of DCPD and CPD may occur on NBbond or CP-bond,and CPD may be fused inendo-orexo-position,TCPD may have many different isomers(Fig.6)through many possible synthetic pathways,which makes the reaction mechanism of cycloaddition of DCPD and CPD very complicated.Liet al.predicted the product distribution of TCPD and speculated possible cycloaddition reaction mechanism through DFT computation[4].The predicted product preference is NB-endo>CP-endo>CP-exo>NB-exo,which is consistent with experimental result.However,when a catalyst is used in this cycloaddition reaction,the product distribution of TCPD is totally different.Three new adducts,namely,II(NBexo),IV(CP-exo),and VI(NB-exo)are produced,suggesting that the kinetically un-preferredexo-addition is facilitated by catalyst like zeolite.While,the relatively small pores of zeolite make cycloaddition reaction of DCPD/CPD only occur on the external surface of catalyst,resulting in low yield and selectivity of product.Accordingly,Denget al.prepared unique hierarchically porous HZSM-5 through an alkali-treatment method and applied it in cycloaddition of DCPD and CPD.The yield and selectivity of TCPD reach 36.3%and 84%,respectively [19].

TCPD also has two unsaturated bonds on norbornene ring and cyclopentene ring,but steric hindrance effect makes them more difficult to be hydrogenated than DCPD,resulting in more complex hydrogenation process.Zouet al.investigated the hydrogenation reaction of TCPD by using Pd-B/γ-Al2O3as catalyst.Although this Pd-B/γ-Al2O3catalyst is amorphous,it is thermally stable and shows stronger H2-adsorption ability and hydrogenation activity than crystal metals with comparative particle size [20].They also analyzed the hydrogenation products and reaction pathways.The products include dihydrotricyclopentadiene (DHTCPD) with the bond in norbornene ring saturated and completely saturated THTCPD.As shown in Fig.7,the concentrations of reactants and products vary with the reaction time.Within 10 min,the concentration of TCPD decreases quickly and conversion of more than 95% is obtained.Simultaneously,the concentration of DHTCPD reaches the highest value and declines after that.The concentration of THTCPD increases with the increase of reaction time.This tendency is a characteristic of consecutive reaction.Thus,the hydrogenation of TCPD takes place according to Fig.8.Zouet al.also studied the kinetics of TCPD hydrogenation to THTCPD over Pd-B/γ-Al2O3amorphous catalyst [21].The activation energies for the first and second step are 11.11 and 34.71 kJ∙mol-1,respectively,confirming that the first step is easy to occur,but the second step is difficult to take place.

Fig.1.Hydrogenation of endo-DCPD and isomerization of endo-THDCPD.

Fig.2.Conversion of DCPD and yield of THDCPD by using Raney Ni or SRNA-4 as hydrogenation catalyst at 110 °C and 1.5 MPa (hydrogen pressure).Source:Zou et al.,2008.Reproduced with permission of Elsevier [6].

2.2.High-energy-density fuels from norbornadiene

2.2.1.Quadricyclane

Quadricyclane is a strained and caged hydrocarbon,which is mainly synthesizedviaphotoisomerization of NBD[22,23].QC contains multi-cyclic structures,i.e.,two three-membered rings,one four-membered ring and two five-membered rings (Fig.9(a)).The C–C bond angles in the three-membered ring and fourmembered ring(in the range of 59.9°–90.0°)are much smaller than the normal C–C bond angle (109.5°) in hydrocarbon.With such unique structure,QC possesses very high mass and volumetric NHOC (44.35 MJ·kg-1and 43.55 MJ·L–1),which are much higher than those of the widely used HED fuel JP-10 (42.1 MJ·kg-1and 39.6 MJ·L–1) (Fig.9(b)).Importantly,QC with high density of 0.982 g·cm-3maintains very good low-temperature viscosity(0.03 Pa·s at-40°C)[24].In addition,QC is also an excellent hypergolic fuel due to its high strained energy and further high chemical reactivity[25].As shown in Fig.9(c),when the fuel and oxidant are ignited,extremely powerful and intense single flames come out.Particulaly,the ignition delay (ID) time of QC–N2O4binary system is only 29 ms,which is even shorter than the target ID time for practical applications.

Because NBD can only absorb the light in ultraviolet region(～230 nm),the reaction efficiency of direct photoisomerization of NBD to QC is very low.In order to use high-wavelength light and promote isomerization efficiency,utilization of photosensitizers is necessary.Hammondet al.first reported the photosensitized isomerization of NBD to QC through triplet sensitizers like acetophenone and benzophenone [26].Whereafter,many photosensitizers have been investigated for isomerization of NBD,including homogeneous photosensitizers(such as carbonyl compounds,transition metal compounds and some metal complexes)and heterogeneous photosensitizers (for instance zinc and cadmium oxides and sulfides,modified zeolites,metal-doped TiO2and Ti-containing MCM-41) [27–29].

2.2.2.Hydrogenated norbornadiene dimer

Norbornadiene is first polymerized to form a dimer,and then hydrogenated to obtain hydrogenated norbornadiene dimer with very high density of 1.08 g·cm-3and volumetric NHOC of 44.9 MJ·L–1[1] (Fig.10).H-NBDD is the major constituent of RJ-5,which is the first liquid hydrocarbon fuel with density higher than 1.0 g·cm-3.Dimerization of NBD can be performed in the presence of homogeneous catalysts like complexes of rhodium,cobalt,iron,nickel,ruthenium,iridium and other transition metals.However,the high price and difficulties in recovery and reuse of homogeneous catalysts restrain their development and practical applications.Accordingly,more economic and efficient heterogeneous catalysts,such as Rh/C,Rh/zeolite and zeolite have been explored for dimerization of NBD.

Fig.3.Consecutive hydrogenation of DCPD.

Fig.4.Isomerization of endo-DCPD and hydrogenation of exo-DCPD.

Fig.5.Comparison of endo-DCPD conversion and exo-DCPD selectivity by using different catalysts (Al-MCM-41 (30) or Hβ).Source:Zou et al., 2012.Reproduced with permission of Elsevier [15].

Gol’dshlegeret al.investigated dimerization of NBD by using various heterogeneous Rh/zeolite catalysts and achieved up to 99% selectivity of hexacyclic dimers,though the conversion of NBD is less than 46.7%[30].Moreover,they concluded that the acid sites of zeolites facilitate hydration of NBD with water,which only produces alcohols and ethers.While,introduction of Rh into Hzeolite can suppress acid-catalyzed reaction of NBD and facilitate dimerization of NBD.In other words,Rh localized on the outside surface of zeolite is the active center for dimerization of NBD.However,Jeonget al.conducted dimerization of NBD only in the presence of zeolite or mesoporous aluminosilicate without metals[31].As shown in Fig.11,the conversion of NBD over four catalysts is almost the same,while HY zeolite shows higher yield and selectivity of NBD dimer.The authors suggested that the higher selectivity of NBD dimer is due to the appropriate pore structures and Brönsted acid sites of HY catalyst.Nevertheless,the real active center is controversial for dimerization of NBD.

Fig.6.Cycloaddition of CPD and DCPD and the possibly generated TCPD isomers.

Fig.7.Variation of concentration of reactant,hydrogenated intermediate and product with reaction time in hydrogenation of TCPD.Source:Zou et al., 2007.Reproduced with permission of Elsevier [20].

3.Blueprint for Synthesizing High-Density Fuels from Coal

In some countries or regions where petroleum reserve is far less abundant than coal,synthesizing fuels from coal is a very attractive strategy,which can meet the growing demand for fuel.Burgesset al.proposed the coal-based jet fuel through a two-stage liquefaction process [32].On this basis,Balsteret al.prepared JP-900 fuel with a high density of 0.87 g∙cm-3that can remain stable for a long time without decomposition or carbon deposition at 482 °C by using major components of refined coal tar and light circulating oil[33].Coal tar derived from dry distillation of coal contains plentiful polycyclic aromatic hydrocarbons that can be hydrogenated to polycyclic alkanes with higher density and heat value.In addition,through indirect liquefaction technology,Sasol in South Africa has produced a coal-based total Fischer-Tropsch synthetic jet fuel (S-5),which has been commercially available [34].Preparation method and properties of typical jet fuels are summarized in Table 1,from which one can see that the density of fuels from liquification of coal is lower than that of synthetic HED fuels.

Table 1Preparation method and properties of typical aviation hydrocarbon fuels

Fig.8.Consecutive hydrogenation of TCPD.

Fig.9.(a) Molecular structures of QC,(b) photoisomerization reaction of NBD to QC and property comparison of QC and JP-10.Source:Zou et al., 2020.Reproduced with permission of Wiley [23].(c) Ignition process of QC with different oxidants.Source:Pan et al., 2014.Reproduced with permission of Royal Society of Chemistry [25].

Fig.10.Dimerization of NBD and hydrogenation of NBDD.

Fig.11.Comparison of conversion of NBD,yield and selectivity of NBDD in dimerization of NBD over various catalysts.Source:Jeong et al., 2017.Reproduced with permission of Springer [31].

As described in Section 2,the main compositions of typical HED fuels are polycyclic alkanes derived from DCPD and NBD(Figs.1,4,6,10).DCPD is the natural dimer of CPD,and NBD can be obtained through cycloaddition of CPD and acetylene.Hence,CPD is the initial and critical feedstock for synthesis of HED fuels.If CPD can be obtained from coal,it will greatly improve the utilization rate of coal,increase economic benefits and ensure the abundant supply of HED fuels.

In indirect coal liquefaction process,the product distribution of chain alkanes through Fischer-Tropsch synthetic method is adjustable[37,38],and C5fraction can be achieved with highest selectivity (Fig.12) [39].Accordingly,we conduct theoretical thermodynamic equilibrium calculation of dehydrogenation ofnpentane.As shown in Fig.13,the main dehydrogenation product is CPD at high temperature and atmosphere pressure without regard to pyrolysis and polymerization side reactions.When reaction temperature is low,the formation of pentenes,cyclopentane,cyclopentene and pentadiene is observed.Increasing reaction temperature brings in the increase of CPD yield and decrease of other C5compounds.So C5molecules besidesn-pentane can also be transformed to CPD and higher reaction temperature is preferred for synthesis of CPD.

Fig.12.Products distribution of hydrocarbons through Fischer-Tropsch synthetic method with H-β/Co/Al2O3-MX catalyst.Shadow column:branched alkane;blank column:alkene;black column:normal alkane.Source:Tsubaki et al., 2009.Reproduced with permission of Elsevier [39].

Herein we propose a blueprint for synthesizing HED fuels from coal,the overall process is illustrated in Fig.14.Firstly,coal is transformed to liquid oil through indirect liquefaction technology.Secondly,C5fraction is collected by distillation.Thirdly,C5compound is dehydrogenated to produce CPD,which is applied to synthesis DCPD and NBD through carbon–carbon coupling reactions.Finally,these polycyclic hydrocarbons are converted to HED fuels through hydrogenation and/or isomerization.As mentioned above,coal-to-liquid technologies and preparation of HED fuels have been extensively investigated and applied in industry.Therefore,the critical procedure is dehydrogenation of C5compound in the whole process.Considering that the composition of C5fraction is complex which contains pentane,pentene,pentadiene,and cyclo-C5,the introduction of C5compound dehydrogenation is divided into the following three parts according the type of reactant.

3.1.CPD from dehydrogenation of pentane

Generally,alkenes and aromatics are common dehydrogenation products of alkanes.Pineset al.studied the dehydrogenation ofnpentane catalyzed by chromia-alumina at 520 °C and atmospheric pressure [40].The liquid products are mostlyn-pentene,methylbutene and isopentane,no significant content of CPD is mentioned.Inuiet al.[41]and Kanazirevet al.[42]tried the catalytic dehydrocyclization ofn-pentane,but the products are aromatics and light hydrocarbon.In 1994,a patent described the conversion ofnpentane to cyclopentene [43],in which a dual temperature stage process is required.n-Pentane is reacted in the first reactor at 575 °C catalyzed by Pt/Sn-ZSM-5.The product effluent is then transferred to the second reactor at 45 °C equipped with Pd/Sn-ZSM-5,which is used to selectively saturate dienes to monoalkenes.The final product contains 52.0% cyclopentene and 11.5%cyclopentane.It can be inferred that plenty of CPD may exist in effluent of the first reactor.Recently,ExxonMobil applied a series of patents [44–59] for the conversion of pentane to cyclic C5with catalysts such as SiO2/K2SiO3/Pt,ZSM-5(Na)/Pt,ZSM-5(Na)/Pt/Ag,[Fe]ZSM-5(Na)/Pt and ZSM-5(K)/Pt.For example,when ZSM-5(Na)/SiO2/Pt was applied as the catalyst and mixed gas of C5H12/H2/He (34.5/34.5/137.9 kPa) as the reactant,39% selectivity and 30% yield of CPD could be achieved at 600 °C.

Fig.13.Possible reaction processes of n-pentane dehydrogenation and products distribution based on theoretical thermodynamic equilibrium calculations.

Fig.14.Block flow diagram for synthesis of HED fuels from coal.

3.2.CPD from dehydrogenation of pentadiene

C5dienes are major products of pentane dehydrogenation and can also be applied to prepare CPD.Kennedyet al.proposed the synthesis route of piperylene to CPD [60].They found that more than 20% recycle yield of CPD is achieved at 600–620 °C,1.33–4.00 kPa without catalyst.Higher temperature and pressure result in unfavorable cracking and polymerization.Packings or catalysts like activated silica,silicon carbide,fused alumina,jack chain and chromia-alumina improve the conversion [61–65].However,the per pass yields of CPD are less than 9%.Pt-carbon or Al2O3-Cr2O3-K2O was found to accelerate dehydrocyclization of dienes [66].The yield of CPD is 18% converted by piperylene at 600 °C and 2.67–3.33 kPa.Marcinkowski researched dehydrogenation of various C5compounds at 650 °C and atmospheric pressure [67].The isomers of piperylene,i.e.,trans-1,3-pentadiene andcis-1,3-pentadiene,show different reaction characteristics.The conversion oftrans-1,3-pentadiene is much higher thancis-1,3-pentadiene,but the selectivity of CPD exhibits opposite trend.Considering thatcis-configuration is the intermediate structure of cyclization reaction,high temperature is necessary.Increasing temperature from 550–650 °C to 700 °C can adjust the main product from cyclopentene to CPD too,as indicated by Fel’dblyumet al.[68].Introduction of steam can inhibit the cracking and carbon deposition,and improve the selectivity of CPD.Hydrogen sulfide can be applied as a promoter too,which may take part in the cyclization or removal of hydrogen atoms on C5,and further research is necessary to explain the effect of it.

3.3.CPD from dehydrogenation of cyclopentane/ cyclopentene

Cracking of cyclopentane has a long history and can be traced back to 1934 when F.E.Frey attempted the pyrolysis of cyclopentane at 574°C and 10.1 kPa,but the products are C1–C3mostly[69].Grosseet al.used chromium oxide on alumina to transform cyclopentane at 500°C and 25.3 kPa,8.9%yield of CPD is achieved[70].Several reported catalysts like Al2O3-Cr2O3-K2O and Pt-carbon can improve the reaction,nevertheless,yield of CPD is unsatisfactory (lower than 20%) [66].

Pyrolysis of cyclopentene was firstly reported by Riceet al.[71]and it is found that 20%of cyclopentene is consumed at 850°C and 1.33 kPa,and CPD is obtained with selectivity as high as 93%.While when the reactant is replaced by cyclopentane,cracking instead of dehydrogenation becomes the main reaction.High temperature,negative pressure and short contact time are preferred for the improvement of the selectivity of CPD [72].Catalysts like SiO2-ZrO2-Al2O3[73]and Al2O3-Cr2O3-K2O[66]were applied to catalyze the conversion of cyclopentene too.Catalytic cracking is the main reaction for the former,polymer formation is marked,and carbon deposit is large.The latter promotes the production of CPD with yield as high as 58% at 600 °C and diminished pressure (2.67–3.33 kPa).

4.Conclusions and Perspective

The properties and preparation methods of HED fuel molecules,including THDCPD,THTCPD,QC and H-NBDD that are widely applied as aviation fuel components,are summarized in this review.Cycloaddition,hydrogenation and isomerization are key reactions for the synthesis of THDCPD and THTCPD.Photoisomerization of NBD results in QC,dimerization and hydrogenation of NBD can obtain H-NBDD.The main feedstocks for these syntheses are DCPD and NBD,which are derived from petroleum-based CPD.In order to ensure abundant supply,achieving CPD from other sources with large reserves such as coal should be developed.According to theoretical calculations and experimental results,dehydrogenation of coal-based C5fraction to CPD is completely feasible.Moreover,the present research results show that the selectivity and yield of CPD can be improved by adjusting the catalysts,promoters and reaction conditions.Nevertheless,the research of dehydrogenation of C5to CPD is insufficient and needs more work.Also,the integration of dehydrogenation,separation and subsequent reactions should be considered to develop largescale and efficient production of HED fuels from coal.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China(21978200)and Scientific Research of the Ministry of Education of China (6141A02033522).