# Biosynthesis Progress of High-Energy-Density Liquid Fuels Derived from Terpenes
## 2 Properties of HED Fuels Derived from Terpenes
### Monoterpene-Derived Fuels  
Monoterpenes have two isoprene units and can be divided into linear or cyclic types, of which the cyclic type is more widely used. The main components of turpentine oil extracted from pine resin include \(\alpha\)-pinene, \(\beta\)-pinene, and small amounts of camphene and limonene (with a density of 0.841-0.859 g/mL) [16]. Due to the compact structure of \(\alpha\)-pinene and \(\beta\)-pinene, the hydrogenated products catalyzed with Ni-SiO\({}_{2}\) and Pd-Al\({}_{2}\)O\({}_{3}\) (the yield of products was over 95%) could be used as HED fuel with a density of 0.86 g/mL, a kinematic viscosity of 13 mm\({}^{2}\)/s at \(-\)40 \({}^{\circ}\)C and a freezing point below \(-\)75 \({}^{\circ}\)C [17]. [PERSON] et al. used Nafion acid to catalyze the dimerization of \(\beta\)-pinene with a yield of 90%. The resulting pinene dimers were directly hydrogenated to produce a better HED fuel with a density of 0.938 g/mL and a volumetric NHOC of 39.5 MJ/L, which is similar to JP-10 [16]. The \(\alpha\)-pinene dimer and camphene dimer showed similar performance, including their density, volumetric NHOC, viscosity and freezing point, since during the polycondensation of \(\alpha\)-pinene it preferentially forms camphene rather than dimers [16]. Limonene also has good properties, especially low-temperature performance. It was reported that mixing diesel fuel with 10% hydrogenated limonene can effectively reduce the cloud point and viscosity of the blended fuel, indicating that limonene can be used as a substitute for traditional fuels without changing the engine [18]. The volumetric NHOC of limonene was close to JP-10, reaching 39.15 MJ/L, while the density of limonene was lower than that of JP-10 [19]. The dimerization of limonene could improve the density to 0.914 g/mL; however, due to the lack of a compact structure, the density of the limonene dimer was inferior to that of the ppinene or camphene dimers [20]. In addition to pure pinene, limonene and camphene, crude turpentine was also dimerized with catalysts, including Nafion SAC-13, MMT-K10 and HPW/MCM-41, to generate a high-density turpentine dimer fuel (TDF), of which the product's composition was similar to that of a pure pinene dimer: the density was up to 0.93 g/mL and the volumetric NHOC value was close to JP-10 [16; 20; 21]. The above research clearly shows that the direct dimerization of monoterenes or crude turpentine to generate HED fuels can take full advantage of the monomers. Although the combustion value and density of dimer fuels are very high, the viscosity is too high to be applied at a low temperature [21]. To solve this problem, the dimer HED fuel was mixed with other low-viscosity components, including JP-8 and JP-10, resulting in mixed fuels with a superior combustion value and low-temperature performance [17; 21]. Cyclopropanation has been widely studied due to its significant impact on fuel density and heating value. As early as the Soviet era, a cyclopropanation fuel named Syntin had been used in rocket engines, which could effectively increase the payload [22]. In 2022, [PERSON] et al. synthesized a class of polycyclopropanated fatty acid methyl ester fuels (POP-FAMEs), the densities and volumetric NHOC values of which exceeded those of JP-10 [23]. The volumetric energy of terpen could also be improved further through cyclopropanation. [PERSON] et al. discovered that the density and volumetric NHOC of the cyclopropanated products of limonene and turpentine were improved by 3-4% [24]. In addition, [PERSON] et al. effectively cyclopropanated the monoterpene myrcene with a conversion rate of 69.8%; the resulting product had a density of 0.85 g/cm\({}^{3}\), a volumetric NHOC of 36.89 MJ/L, a freezing point below \(-\)70 \({}^{\circ}\)C and a specific impulse higher than that of RP-1 and JP-10 [25].