# Biosynthesis Progress of High-Energy-Density Liquid Fuels Derived from Terpenes
## 2 Properties of HED Fuels Derived from Terpenes
### Sesquiterpene-Derived Fuels  
Sesquiterpenes are more complex in structure than monoterenes and have also received widespread attention in fuel applications. The linear sesquiterpene farnesene has a lower density and NHOC than its hydrogenated product farnesane, which is not suitable for use as a HED fuel alone but can be used as a jet fuel [32]. [PERSON] et al. found that the volumetric NHOC and density of the monocyclic sesquiterpene bisabolene were very similar to D2 diesel and could be used as a substitute [33]. [PERSON] et al. tested the properties of the bicyclic sesquiterpenes, including valencene, caryophyllene and premnaspirodiene,and the results showed that the hydrogenated fuels of the three compounds had a high volumetric NHOC (37.01-37.78 MJ/L) but lower density [34]. Subsequently, cryophyllene was catalyzed into mixtures of isomers with Nafion SAC-13, and the mixture fuel had a density of 0.85-0.9 g/mL and a volumetric NHOC of 37.07-39.30 MJ/L [34]. They also mixed the cryophyllene isomers with a hexene dimer at a 40:60 ratio to obtain blended fuel with a density of 0.806 g/mL, a volumetric NHOC of 34.73 MJ/L and a kinematic viscosity of 8.3 mm\({}^{2}\)/s at \(-20\)\({}^{\circ}\)C, which are close to JP-5 [35].  
Cedarwood oil (CWO) is a mixture of sesquiterpenes, mainly composed of tricyclic sesquiterpenes, including thuiopsene, \(\alpha\)-cedrene, \(\beta\)-cedrene and cedrol [36]. [PERSON] et al. hydrogenated CWO containing 32% \(\alpha\)-cedrene, 7% \(\beta\)-cedrene, 51% thuiopsene and some other minor sesquiterpenes and obtained HCWO fuels with a density of 0.917 g/mL and a volumetric NHOC of 39.13 MJ/L, which are similar to JP-10. However, the obtained fuels had a high viscosity, with a kinematic viscosity of 54 mm\({}^{2}\)/s at \(-20\)\({}^{\circ}\)C, which is higher than that of JP-8 (\(<\)8.0 mm\({}^{2}\)/s) or JP-10, and a kinematic viscosity of 6.2 mm\({}^{2}\)/s at 40 \({}^{\circ}\)C, which is higher than that of traditional diesel (2.1-4.1 mm\({}^{2}\)/s) [37]. Furthermore, by catalyzing cedrol to generate cedrane, which possesses better properties, including higher density, a higher volumetric NHOC value and lower viscosity (with kinematic viscosity 33 mm\({}^{2}\)/s at \(-20\)\({}^{\circ}\)C and 4.8 mm\({}^{2}\)/s at 40 \({}^{\circ}\)C) than that of HCWO, it exhibited great potential for practical applications [37].  
Moreover, some studies have investigated the properties of sesquiterpene-derived fuels using theoretical calculations. It was reported that the theoretical predicted gravity NHOC values of three tricyclic sesquiterpene hydrogenation products, epi-isozizane, pentalenane and \(\alpha\)-isocomane, were 42.584 MJ/kg, 42.609 MJ/kg and 42.783 MJ/kg, respectively, which are close to the commercial jet fuel Jet-A1 [38]. [PERSON] et al. also found that the volumetric NHOC of saturated prespatane was 41.46 MJ/L, which is better than that of JP-10 [39]. [PERSON] et al. analyzed 33 compounds from fungi and discovered that five sesquiterpenes, longifolene, \(\beta\)-cubebene, \(\alpha\)-ylangene, \(\beta\)-patchoul and \(\alpha\)-santalene, have great potential to be applied for HED fuels [40]. Recently, our research team systematically calculated the combustion properties of 122 sesquiterpene skeletal compounds already reported and found that many hydrogenated products of these molecules have superior properties to JP-10 in terms of their volumetric NHOC value, density and other characteristics. Subsequently, the actual combustion performance of pentalenene and presilphiperfol-1-ene was tested, and the results showed that the hydrogenated pentalenane and presilphiperfol-1-ane have volumetric NHOC values of 40.55 MJ/L and 39.24 MJ/L, respectively, which were close to the theoretical predicted values [32].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Fuel** & **Structure** & **NHOC/(MJ/L)** & **Density/(g/mL)** & **Viscosity/mm\({}^{2}\)/s** & **Freezing Point\({}^{c}\)C** & **Ref.** \\ \hline \(\beta\)-Pinene & \(\cancel{\text{\text{\text{\text{\text{\text