Synthesis and thermal stability of dimethyl adamantanes as high-density and high-thermal-stability fuels

Fuel 260 (2020) 116424

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Synthesis and thermal stability of dimethyl adamantanes as high-density and high-thermal-stability fuels

T

Jiawei Xiea,b, Tinghao Jiaa,b, Si Gonga,b, Ning Liua,b, Genkuo Niea,b, Lun Pana,b, ⁎ ⁎ Xiangwen Zhanga,b, , Ji-Jun Zoua,b, a b

Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Dimethyl-adamantane High density fuel High thermal stability Acid catalytic rearrangement

Alkyl-adamantane fuels are appealing for aerospace vehicles due to their high density, good low-temperature performance and thermal stability. Here we reported a facile route to synthesize dimethyl-adamantane fuels via solvent-free rearrangement. The thermodynamic preference of six products (dimethyl-adamantane isomers) was revealed by Density Functional Theory (DFT) calculation and experiment, and the reaction pathway was further discussed. The reaction conditions including catalyst, catalyst dosage, temperature, and solvent were optimized, with the dimethyl-adamantane yield of 88.7%. We synthesized two typical dimethyl-adamantane fuels (with different compositions of isomers) by adjusting the reaction conditions and then evaluated their properties. It is found the two fuels show different fuel properties (e.g., density, heating value, thermal stability) due to the difference in methyl substitution position, although both with high density and good low-temperature fluidity. Specifically, the dimethyl-adamantane with methyl on the tertiary carbon shows better thermal stability than JP10 and decalin. This work presents dimethyl-adamantanes as good high-density and high-thermal-stability fuels, and suggests the position of methyl substitution on adamantane has considerable effect on their properties.

⁎ Corresponding authors at: Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China. E-mail addresses: [email protected] (X. Zhang), [email protected] (J.-J. Zou).

https://doi.org/10.1016/j.fuel.2019.116424 Received 23 March 2019; Received in revised form 9 October 2019; Accepted 12 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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[5.2.1.02,6]decane, and evaluated the thermal stability using pressure differential scanning calorimetry (PDSC) and jet fuel thermal oxidation tester (JFTOT) tests. The result shows that dimethyl-adamantane has high density (> 0.9 g/mL), good thermal stability (better than JP-10) and low-temperature fluidity (freezing point < −60 °C, viscosity < 23.4 mm2/s at −20 °C), which thus is promising as high-density and high-thermal-stability fuel.

1. Introduction The propulsion performance of engines, including turbine, turbofan, ramjet, rocket and combined engine, is deeply dependent on the fuel characteristics (e.g., density, heating value, low-temperature properties and thermal stability) [1–3]. Volume-limited aerospace vehicles (like missiles, rockets, etc.) impose specific constraints on density of fuels because their fuel tank must be designed as small as possible to restore enough space for other components. Therefore, the most desired features such as high density and high heating value are determining factor of flight range and payload. The first high-density fuel is RJ-4, and the most successful high-density fuel is JP-10 [4]. They show largely increased density compared with kerosene like JP-8, but the thermal stability is not much improved. There are also some additional requirements for practical operation. For example, low-temperature property (freezing point and viscosity) specifications ensure easy flow under frigid high-altitude conditions [4–7]. Meanwhile, the development of high Mach number aerospace vehicles leads to the demand for fuel with high thermal stability, because the jet fuels should play dual roles of energy source and coolant over wide ranges of operation conditions in terms of temperature and pressure [8]. Therefore, efforts are underway to improve the thermal stability of jet fuels. For example, an additive package has been developed for JP-8 to produce JP-8 + 100 fuel, which offers a 55 °C increase in the thermal stable temperature (from 163 °C to 218 °C) [9,10]. Further, a highly processed hydrocarbon-based kerosene fraction, i.e., JP-7, is stable up to 287 °C [11]. Considering that thermal stability strongly correlates with the component of fuels, regulating the composition is more effective to improve the thermal stability. Generally naphthenic fuels demonstrate good thermal stability, such as coal-based JP-900, mainly composed of decalin, has a maximum thermal stable temperature of 480 °C, but the density is relatively low (0.870 g/mL) compared with high-density fuels (Table 1) [12]. Therefore, a fuel with both high density and high thermal stability is more desired. Notably, alkyl-adamantane with compact adamantane core and alkyl substituted groups forming a beautiful three-dimensional structure meets the demands for both high density and high stability [13]. Particularly, since diamondoids are the most stable structures in thermodynamics with the lowest free energy, alkyl-adamantanes are expected to afford a high thermal stability. Different from adamantane and diamantane (including other diamondoid homologs) with high melting point (> 200 °C), the alkyl chains on the adamantane core can transfer it into liquid with satisfactory low-temperature characteristics such as low freezing point and low viscosity, thus the alkyl-adamantane is showing great potential as component of jet fuels [14,15]. Analogous to the classical acid-catalyzed synthesis of adamantane [16,17], the rearrangement of polycyclic hydrocarbons with > 10 carbon is a facile route to synthesize alkyl-adamantanes [18–20]. Notably, dimethyl-tricyclo[5.2.1.02,6]decane (DMTCD) with 12 carbon (main component of RJ-4), is ideal precursor to produce alkyl-adamantanes (i.e., dimethyl-adamantane, DMAM). Different from previous ionic liquid catalysis with tremendous dosage [21], acid catalysis (simply use AlCl3) makes the synthesis of dimethyl-adamantane fuels more feasible. Further, there is no report on the thermal stability of dimethyl-adamantane fuels. Therefore, in this work, we synthesized dimethyl-adamantanes by catalytic rearrangement of dimethyl-tricyclo

2. Experimental section 2.1. Chemicals Dimethyl-dicyclopentadiene (> 95 wt%) was purchased from Creasyn Finechem Co., Ltd. Dimethyl-tricyclo[5.2.1.02,6]decane (DMTCD) was obtained by hydrogenating dimethyl-dicyclopentadiene. Anhydrous AlCl3 (99%) was obtained from Aladdin Co., China. Anhydrous AlBr3 (98%) and anhydrous FeCl3 (analytically pure) were supplied by Meryer (Shanghai) Chemical Technology Co., Ltd. Sulfuric acid (95–98%), phosphoric acid (85%), sodium hydroxide (96%), anhydrous magnesium sulfate (99%) and dichloromethane (99%) were obtained from Tianjin Yuanli Chemical Technology Company. All the chemicals were used without further purification. 2.2. Rearrangement reaction and analysis The reaction was carried out in a 25 mL three-neck flask equipped with a mechanical stirrer and reflux condenser under nitrogen atmosphere at ambient pressure. 15 g DMTCD was added into the flask placed in the oil bath, and then a defined amount of catalyst (AlCl3, AlBr3, FeCl3, H2SO4, H3PO4) was added when the temperature reached the set value. That time was regarded as the beginning of the reaction. After the reaction, the resultant mixture was neutralized with aqueous sodium hydroxide, deionized H2O, and then brine. The organic layer was dried over anhydrous magnesium sulfate. Under optimized condition, the reaction is scaled up to 250 mL to synthesize fuel for measurement of fuel properties. Quantitative analysis was conducted using Shimadzu GC-2010 Plus gas chromatograph equipped with a SH-Rtx-5MS capillary column (30 m × 0.25 mm × 0.25 µm) and a flame-ionization detector. Nitrogen was used as the carrier gas. The oven temperature was held at 60 °C for 2 min, ramped to 90 °C at 5 °C/min, held for 30 min, and then ramped at 30 °C/min to a final oven temperature of 280 °C, which was held for 10 min. Qualitative analysis of the product was conducted using Shimadzu GCMS-QP2020 gas chromatography-mass spectrometry (GC–MS) equipped with a Rtx-5MS capillary column (50 m × 0.25 mm × 0.25 µm). The oven temperature was held at 60 °C for 5 min, ramped to 100 °C at 5 °C/min, held for 40 min, and then ramped at 30 °C/min to a final oven temperature of 280 °C, which was held for 5 min. 2.3. Measurements of fuel properties The density was measured by Mettler Toledo DE40 density meter according to ASTM D4052. Freezing point was measured according to ASTM D2386, and kinematic viscosity was determined using capillary viscometer according to ASTM D445. The net heat of combustion was

Table 1 Basic properties of various jet fuels. Property

JP-7

JP-8/JP-8 + 100

JP-900

JP-10

Density at 15 °C (g/mL) Freezing point (°C) Viscosity at −20 °C (mm2/s) Thermal stability (JFTOT) test conditions Ref.

0.779–0.806 −43.3 8.0 355 °C/300 min MIL-DTL-38219D and [11]

0.775–0.840 −47 8.0 260 °C/150 min MIL-DTL-83133G and [9,10]

0.870 −65 7.5 – [8]

0.934–0.943 −79 8.8 300 °C/150 min MIL-DTL-87107E

2

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measured by IKA-C6000 isoperibol Package 2/10 Calorimeter according to ASTM D240-02. The oxidation onset temperature was measured by pressure differential scanning calorimetry (PDSC, NETZSCH DSC 204 HP Instruments, ASTM E2009), coupled to a pressure cell. 10 mg sample was weighed in an aluminum pan and placed in the sample chamber, and an identical empty pan was placed on the cell platform as a reference. The experiments were performed at 3500 kPa (500 psi) under 50 mL/min air flow. The cell was heated from 50 °C to 300 °C at a heating rate of 10 °C/min. The oxidation onset temperature was determined by intersection of the extrapolated baseline and tangent to the maximum exothermic peak. The thermal stability was assessed by jet fuel thermal oxidation tester (JFTOT) in accordance with ASTM D3241, carried on at 355 °C for 5 h. The filterable liquid particulates are monitored by pressure drop through a filter (dP filter), and surface deposits on the test tubes were assessed by visual rating after the test.

Table 2 The relative electronic energy values of six dimethyl-adamantane isomers in Fig. 1 and the corresponding ΔG values, calculated at different levels. Dimethyladamantane isomers

I II III IV V VI

B3LYP/6-31g(d)

M052X/6-31g(d)

M052X/6-311g(d,p)

Erel (kJ/ mol)

ΔG (kJ/ mol)

Erel (kJ/ mol)

ΔG (kJ/ mol)

Erel (kJ/ mol)

ΔG (kJ/ mol)

0 11.1 17.5 23.0 23.8 33.9

−53.6 −45.2 −38.0 −27.2 −19.8 −11.9

0 12.8 16.7 26.5 27.2 31.3

−67.6 −63.2 −55.2 −43.9 −42.5 −30.9

0 13.4 17.3 27.8 28.6 32.1

−68.4 −63.6 −57.2 −43.7 −42.1 −32.5

The total electronic energy of I calculated using B3LYP/6-31 g(d), M052X/631g(d), and M052X/6-311g(d,p) is −469.35774221, −469.30979367, and −469.42861864 a.u., respectively.

2.4. Numerical experiments

dimethyl-adamantane (III, 1,2-DMAM), 2,6-dimethyl-adamantane (IV, 2,6-DMAM), 2,4-dimethyl-adamantane (V, 2,4-DMAM) and 2,2-dimethyl-adamantane (VI, 2,2-DMAM). We also observed 1-ethyl-adamantane (VII, 1-EAM), 2-ethyl-adamantane (VIII, 2-EAM, mass spectra see Fig. S2), and some branched decalins formed by disproportionation and demethylation reactions (Fig. 1), however their amount is very low in all cases. DFT calculations were conducted to evaluate the thermodynamic priority of six dimethyl-adamantane isomers and predict the reaction pathway, as seen in Table 2. The relative electronic energies (Erel) and Gibbs free energy change values of the six isomers show the similar trend. Gibbs free energy change for the formation of I is more negative than others, and the total electronic energy of I is lower than other isomers, therefore I is expected to be the most preferred in thermodynamics. Similarly, the thermal preference follows the order of I > II > III > IV > V > VI. The six isomers may transfer to each other because the methyl group may migrate from one carbon to another. Therefore experiment was conducted to explore the reaction pathway (Fig. 2). During the reaction, DMTCD mainly rearranges to dimethyladamantanes (I–VI). With the prolonged reaction time, DMTCD is converted completely with negligible byproducts. Interestingly, among six dimethyl-adamantane isomers, I is the final product as reaction proceeds. We further monitored the change of the isomers with reaction time (Fig. 2(a)). The transformation of II + III (both with similar thermodynamics preference), IV, and V + VI (both with similar thermodynamics preference) show volcano-shaped tendency, indicating that II–VI are the intermediates in the methyl migration (from secondary carbon to tertiary carbon) of dimethyl-adamantanes. Notably, I increases in the whole process, indicating that it is the most stable isomer. The V + VI disappears first while II + III disappears in the end, demonstrating that the preference is in the order of I > II and III > IV > V and VI, which is in line with the DFT calculation. Therefore, we proposed a reaction pathway as shown in Fig. 2(b). As the reaction proceeds, the reactant DMTCD rearranges to II–VI first, and the methyl on the secondary carbons of isomers IV, V and VI will migrate to tertiary carbon and secondary carbon, generating II and III. Finally, all the isomers II–VI are converted to the most stable isomer I, of which two methyl substituted on tertiary carbons.

The geometry optimization and total electronic energy (Etotal) calculations on possible dimethyl-adamantane isomers and Gibbs free energy change (ΔG) for the formation of these isomers were performed using density functional theory with the M052X and B3LYP methods at the level of 6-31 g(d) and 6-311 g(d,p). The total electronic energies were corrected for zero point vibrational energies (ZPE). Gibbs free energy changes were calculated as the difference between the Gibbs free energy of the products and reactants. All the theoretical methods were applied using restricted level of theory. All the computations were performed by using Gaussian 03 software package program. 3. Results and discussion 3.1. Rearrangement of DMTCD As shown in Fig. 1, the reactant, i.e. dimethyl-tricyclo[5.2.1.02,6] decane (DMTCD) is a mixture of various configurations because the methyl group can be positioned on different position of tricyclodecane core, including 1,2-dimethyl-, 1,3-dimethyl, 1,4-dimethyl-tricyclo [5.2.1.02,6]decane, etc. And the rearranged products contain six major isomers (mass spectra see Fig. S1), including 1,3-dimethyl-adamantane (I, 1,3-DMAM), 1,4-dimethyl-adamantane (II, 1,4-DMAM), 1,2-

3.2. Optimization of reaction conditions The effects of reaction conditions including catalyst, catalyst dosage, reaction temperature, and solvent were studied to establish an optimal operation. From the discussion above, as long as extend the reaction time, II–VI can completely be converted to I. Hence, regulating the reaction time can adjust the composition of the dimethyl-adamantanes (I or II–VI). We tested several acids (AlCl3, AlBr3, FeCl3, H2SO4, H3PO4) and found only AlCl3 and AlBr3 show considerable

Fig. 1. Chromatogram of product produced from rearrangement of DMTCD (I–VI, dimethyl-adamantane isomers; VII and VIII, ethyl-adamantane isomers; ●, DMTCD isomers; ♦, branched decalin). 3

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(b)

H3C

CH3 DMTCD Feedstock

+ 2,2-DMAM VI

2,4-DMAM V

+ 2,6-DMAM IV

1,2-DMAM III

1,4-DMAM II

1,3-DMAM I

Fig. 2. (a) Composition-time profiles of dimethyl-adamantanes in the rearrangement of DMTCD. Reaction condition: 15 g DMTCD, 50 wt% AlCl3, 60 °C. (b) Proposed rearrangement pathway.

Fig. 3. Effect of (a) catalyst and (b) AlCl3 dosage on the rearrangement of DMTCD. Reaction condition: (a) 15 g DMTCD, 20 wt% cat., 60 °C, 10 h; (b) 15 g DMTCD, 60 °C.

shown in Fig. 4(a). As the reaction temperature increases from 40 °C to 60 °C, the yield of dimethyl-adamantanes increases obviously from 56.1% to 88.7%. However, further increasing reaction temperature results in a decline in yield from 88.7% of 60 °C to 73.4% of 100 °C. The reason may be that higher temperature causes intensive reaction and produces viscous tar covering the catalyst surface, leading to the deactivation of catalyst. Fig. 4(b) shows the effect of solvents. Since the rearrangement reaction is initiated by the generation of carbonium ion, we checked the polar halide solvents such as 1,2-dichloroethane and

catalytic activity. Overall, AlCl3 is a better choice with high yield of 88.7% (Fig. 3(a)). The reaction rate accelerates as the AlCl3 dosage increases, and the time to reach equilibrium is shortened (Fig. 3(b)). With enough reaction time (> 10 h), 20 wt% catalyst dosage can also obtain the same yield of ~90%. The yield does not change when further extending the reaction time, but the dimethyl-adamantanes finally isomerize to I via methyl migration (from secondary carbon to tertiary carbon as shown in Fig. 2(b)). We further studied the effect of temperature on this reaction, as

Fig. 4. Effect of (a) temperature and (b) solvent on the rearrangement of DMTCD. Reaction condition: (a) 15 g DMTCD, 20 wt% AlCl3, 10 h; (b) 15 g DMTCD, 20 wt% AlCl3, 50 wt% solvent, 60 °C. 4

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Table 3 Typical fuel properties of alkyl-adamantane fuels and some liquid hydrocarbon fuels. Fuel

Dimethyl-adamantane A Dimethyl-adamantane B 2-Ethyladamantane 2-Propyladamantane 2-Butyladamantane 1-Pentyladamantane Diamondoids derived from adamantane and heptane Diamondoids derived from adamantane and nonane Jet A-1 JP-10 RJ-4 RJ-5

Density at 15 °C (g/mL)

0.903 0.922 0.954 0.933 0.928 0.915 0.919 0.912 0.775–0.840 0.94 0.94 1.08

Freezing point (°C)

−60 < −75 −24 −32 – −22 – – −47 < −110 < −40 >0

Kinematic viscosity (mm2/s) 40 °C

−20 °C

2.8 3.6 5.8 6.1 7.2 6.7 3.2 4.9 – 2.3 – –

13.4 23.4 57.7 72.9 111.9 109.3 20.5 60.6 8.0 8.8 60 (−40 °C) –

Heating value (MJ/L)

Ref.

37.7 38.7 40.2 39.2 39.6 39.0 38.6 38.5 33.2–36.0 39.6 39.0 44.9

This work This work [14]

[15] ASTM D1655 [4]

1,2-dichlorobenzenen that may be helpful to form the carbonium ion transition state and dissolve AlCl3 well [22,23]. In presence of polar halide solvents, the reaction is accelerated with the yield of dimethyladamantanes increasing from 61% to 87% at 7 h, indicating solvent benefits the reaction. However, with prolonged time (10 h), the same yield is also obtained under the solvent-free condition, so a solvent-free reaction is used in this work to lower the cost. 3.3. Basic properties of dimethyl-adamantane fuels Under the optimal conditions, a scaled synthesis was conducted with 120 g DMTCD, 20 wt% AlCl3 at 60 °C. By controlling the reaction time to 23 h and 10 h, Dimethyl-adamantane A fuel consisting of 85.6% I, 5.5% II–VI, along with remaining DMTCD, and Dimethyl-adamantane B containing 3.3% I and 73.2% II–VI, along with remaining DMTCD were obtained, respectively. Table 3 shows that different methyl substitution position results in different fuel properties. The density of Dimethyl-adamantane B is 0.922 g/mL, higher than that of Dimethyladamantane A (0.903 g/mL). Also, the heating value of Dimethyl-adamantane B (42.0 MJ/kg) is higher than that of Dimethyl-adamantane A (41.8 MJ/kg). Dimethyl-adamantane A and Dimethyl-adamantane B are colorless liquid with freezing point of −60 °C and ≤−75 °C, respectively. In general, the center of symmetry in the molecule has an effect on freezing point. For example, para-xylene has a much higher freezing point than ortho- or meta-xylene, while cyclohexane, benzene, and paraxylene have comparable freezing point. Therefore, the absence of symmetry center in Dimethyl-adamantane B can decrease the freezing point. Notably, both Dimethyl-adamantane A and B show better lowtemperature performance than reported diamondoid fuels (Fig. 5). It can be seen that the substituent group position affects the property of dimethyl-adamantane fuels, including density, heating value, and lowtemperature performance, as well as thermal stability discussed next.

Fig. 5. Viscosity of dimethyl-adamantane fuels from −40 °C to 40 °C.

for the test (Fig. 6(b)). The antioxidant obviously increases the OOT of all fuels, but keeps the order unchanged. In view of non-isothermal PDSC measurements, Dimethyl-adamantane A is the most stable fuel among the tested fuels. Generally, quaternary carbon is more stable than tertiary carbon [30–32]. Dimethyl-adamantane A is more thermally stable than Dimethyl-adamantane B because it mainly consists of I with 2 quaternary carbons and 2 tertiary carbons, while II-VI in Dimethyl-adamantane B contain only 0–1 quaternary carbon and 4–6 tertiary carbons. And for DMTCD with two methyl groups on secondary carbons of tetrahydrodicyclopentadiene core (JP-10), shows the lowest thermal oxidative stability due to the presence of four unstable tertiary carbons and absence of stable quaternary carbon. Therefore, methyl substitution on the tertiary carbon of adamantane can significantly improve the thermal stability. Finally, we compared the thermal oxidation stability of Dimethyladamantane A and JP-10 (both with100 ppm BHT) by JFTOT tests at 355 °C for 5 h. For Dimethyl-adamantane A, the deposit rating of tube indicates level 3–4, which is lower than the tube rating of JP-10 (> 4) under the test condition. And the variation of filter pressure gradient (dP filter) is 0 kPa. Although the tube rating slightly exceeds the specification of JP-7 (3, MIL-DTL-38219D), the dP filter is far below the specification of JP-7 (25 mm Hg). This result shows that the dimethyladamantane has good potential as high-density and high-thermal-stability fuel.

3.4. Thermal stability of dimethyl-adamantane fuels The thermal stability of dimethyl-adamantane fuels were measured with PDSC and JFTOT tests. The non-isothermal PDSC method can be used to determine the onset oxidation temperature (OOT, ASTM E2009), which gives a fresh look at the oxidation stability [24–29]. Fig. 6(a) depicts non-isothermal PDSC for five typical fuels without any antioxidant, including DMTCD (main component of RJ-4), Dimethyladamantane A, Dimethyl-adamantane B, JP-10, and decalin (main component of JP-900 [8]). The heat flow remains relatively constant until oxidation begins, after which exothermic peak can be detected [25]. The OOT follows the order of Dimethyl-adamantane A > decalin ≥ JP-10 > Dimethyl-adamantane B > DMTCD. The higher the OOT, the better the thermal oxidative stability. We further added 100 ppm antioxidant (butylated hydroxytoluene, BHT) into the samples 5

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Fig. 6. Non-isothermal PDSC curves for dimethyl-adamantane fuels (a) without antioxidants and (b) with 100 ppm BHT.

4. Conclusions

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We reported the synthesis of dimethyl-adamantanes by facile acid catalytic rearrangement of dimethyl-tricyclo[5.2.1.02,6]decane as potential high-density and high-thermal-stability fuels. 1,3-dimethyladamantane is the most thermodynamic preferred among the six dimethyl-adamantanes, and other isomers will transfer to it through methyl migration from secondary carbon to tertiary carbon. Under optimal reaction conditions, the yield of dimethyl-adamantanes is as high as 88.7%. Two typical dimethyl-adamantane fuels, i.e., Dimethyladamantane A (mainly 1,3-dimethyl-adamantane) and Dimethyl-adamantane B (mainly 1,4-, 1,2-, 2,6-, 2,4-, and 2,2-dimethyl-adamantane) were obtained by controlling the reaction time, which show density of 0.903 g/mL and 0.922 g/mL, respectively, both with good low-temperature fluidity. Moreover, Dimethyl-adamantane A shows better thermal stability than JP-10 and decalin. Therefore, the dimethyl-adamantanes are good high-density fuels, and especially the Dimethyladamantane A has advantage of both high-density and high-thermalstability. Also, this work suggests the position of methyl substitution position on adamantanes has considerable effect on the properties of fuels, thus providing a way to tune the properties of alkyl-adamantanes. 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 The authors appreciate the supports from the National Natural Science Foundation of China (21808162), the 63rd batch of China Postdoctoral Science Foundation (2018M631743) and China Postdoctoral Innovation Talent Support Program (BX20180212). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116424. References [1] Edwards T. Liquid fuels and propellants for aerospace propulsion: 1903–2003. J Propul Power 2003;19(6):1089–107. [2] Pan L, Deng Q, Xiutianfeng E, Nie G, Zhang X, Zou J-J. Synthesis chemistry of highdensity fuels for aviation and aerospace propulsion. Prog Chem 2015;27(11):1531–41. [3] Rawson PM, Stansfield C-A, Webster RL, Evans D, Yildirim U. The oxidative stability of synthetic fuels and fuel blends with monoaromatic blending components. Fuel

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