Synthesis And Properties Of Alkyl Diamondoid Fuels With High-Oxidation-Stability And High-Energy-Density

The interest on high-oxidation-stability and high-energy-density alkyl diamondoid fuels has been inspired in recent years.Since aerospace vehicles(like missiles,rockets,aircrafts)are always volume-limited,high energy density of fuels becomes the principal determining factor of flight range and payload.Meanwhile,future supersonic and hypersonic aircraft engines will encounter more aerodynamic heating than current engines.Thus,jet fuel will serve the dual role as a propellant to provide propulsion energy and as a coolant to circulate through the airframe and engine components to remove extra heat.Under these conditions,the fuel will experience more thermal stress.Therefore,a high degree of oxidation stability is very important for supersonic and hypersonic flight.The compact and symmetric diamondoid molecular structure possesses high density,consisting of two kinds of active carbon atoms,which is easy to achieve oriented synthesis of alkyl diamondoid derivatives,and tune the fuel properties.Based on the molecular structure-activity relationship of hydrocarbon fuels,several alkyl diamondoid fuels including dimethyl adamantane fuels,polymethyl adamantane fuels,mono-substituted ethyl/propyl adamantane fuels,and C15 alkyl diamondoid fuel were synthesized via skeletal rearrangement,alkylation,and modified Grignard reaction.Dimethyl adamantane fuels were synthesized via AlC13-catalyzed rearrangement of dimethyl tetrahydrodicyclopentadiene with a yield of 88.7%.Density Functional Theory(DFT)calculations were conducted to evaluate the relative electronic energies and Gibbs free energy change values of the six dimethyl adamantane isomers.Dimethyl adamantane isomers were transferred to 1,3-dimethyl-adamantane through methyl migration from secondary carbon to tertiary carbon.Two typical dimethyl adamantane fuels(with different composition of isomers)were synthesized by adjusting the reaction conditions,with the density of 0.922 g/mL and 0.903 g/mL,and good low-temperature fluidity(freezing point are-75℃ and-60℃,viscosity at-20℃ are 23.4 mm2/s and 13.4 mm2/s,respectively).The position of methyl substitution on adamantane has considerable effect on their properties.Specifically,1,3-dimethyl-adamantane(oxidation onset temperature is 212.4℃)with methyl on the tertiary carbon shows better oxidation stability over JP-10(oxidation onset temperature is 199.2℃).Polymethyl adamantane fuels,including 1,3,5-trimethyl-adamantane and 1,3,5,7tetramethyl-adamantane,were synthesized via AlBr3-catalyzed alkylation of 1,3dimethyl-adamantane with the yield of 60%and 80%,respectively.The alkylation pathway was discussed.The oxidation stability of methyl-substituted adamantane series were evaluated via pressure differential scanning calorimetry and rapid small-scale oxidation test.The results indicate that the oxidation stability is correlated to the carbon type available on the molecule in the order of quaternary carbon>primary carbon>secondary carbon>tertiary carbon.Notably,1,3,5,7-tetramethyl-adamantane has an oxidation induction time of 1232.3 min and an oxidation onset temperature of 234.0℃,which exhibits very high oxidation stability.More importantly,blending 1,3,5,7tetramethyl-adamantane in JP-10 fuel can significantly retard the oxidation(blend of 30 wt%1,3,5,7-tetramethylamantane can improve the oxidation induction time by 37.2%).Mono-substituted ethyl/propyl adamantane fuels including 1-ethyl-adamantane(0.940 g/mL),2-ethyl-adamantane(0.957 g/mL),and 1-propyl-adamantane(0.932 g/mL)were synthesized via modified Grignard coupling with high yields(>75%).The process was modified via concentrating the Grignard reagents and altering the solvent.The combustion characteristics(minimum ignition temperature and ignition delay time)of alkyl diamondoids were investigated by the hot-plate ignition test.Due to the contribution of alkyl substituents and diamondoid core,mono-substituted ethyl/propyl adamantane fuels possess better combustion characteristics(minimum ignition temperature<300℃)over JP-10(minimum ignition temperature is 370℃).Both 1ethyl-adamantane and 1-propyl-adamantane present better oxidation stability over JP10.Also,the specific impulse of alkyl diamondoids were revealed by theoretical calculation,the results suggest that specific impulse of 1-ethyl-adamantane,2-ethyladamantane are higher than JP-10 fuel.C15 high-density alkyl diamondoid fuel mainly composed of methyl-1,2tetramethyleneadamantane was synthesized via AlCl3-catalyzed rearrangement of tetrahydrotricyclopentadiene,and its comprehensive properties were revealed.The high-temperature and solvent-free conditions are favorable to the reaction,and the content of alkyl diamondoids in the liquid phase can reach 85.6%catalyzed by low dosage of catalyst.The AlCl3 catalytic skeletal rearrangement affords hydrocarbons with a density of 0.976 g/mL,a heating value of 40.6 MJ/L,and a freezing point less than-70℃,showing higher oxidation stability over JP-10(oxidation onset temperature is 201.7℃ and oxidation induction time is 246.9 min).The minimum ignition temperature of C15 alkyl diamondoid fuel is 275℃ possessing better combustion characteristics over other alkyl diamondoid fuels.Finally,the relationships between the alkyl diamondoid structure and fuel properties,e.g.density,freezing point,viscosity,oxidation stability,combustion characteristics,and specific impulse were discovered and summarized.