# Biosynthesis Progress of High-Energy-Density Liquid Fuels Derived from Terpenes
## 1 Introduction  
High-energy-density (HED) liquid fuel is a type of liquid propellant with higher density and volumetric net heat of combustion (NHOC) than conventional distilled fuel. It can be widely used in aerospace and national defense fields, such as spacecraft, rockets and missiles. HED fuel can provide more propulsion energy to improve aircraft range and speed and/or increase the payload. It can also effectively reduce the volume of fuel tanks, thereby making the aircraft smaller and more maneuverable [1; 2; 3]. Thus far, various HED fuels have been successfully synthesized and applied, obtained through polymerization, hydrogenation, isomerization and purification using petroleum-based raw materials. For example, RJ-4 (RJ for ramet) is the earliest synthesized polycyclic HED fuel with a density of 0.927 g/mL and a volumetric NHOC of 39.0 MJ/L, and is obtained from methylcyclopentadiene [1; 4]. RJ-5 and RJ-7 are two currently reported HED fuels with densities exceeding 1 g/mL. RJ-5, with a density of 1.08 g/mL and a volumetric NHOC of 44.9 MJ/L, is synthesized from dicyclopentadiene [1]. RJ-7 is a mixed fuel with a density of 1.01 g/mL and a combustion value of 42.1 MJ/L [5]. However, their high freezing points limit further applications. JP-10, a high-density cruise missile fuel used extensively, is synthesized from dicyclopentadiene, with a density of 0.94 g/mL and a volumetric NHOC of 39.6 MJ/L. It can provide 13% more energy than kerosene (such as Jet A) [6]. The addition of HED fuels can increase missile range by 50% and improve carrying capacity by 17% without pollution [7; 8]. In addition, high-tension cage hydrocarbons and nanoparticles can also be added to HED fuels to further improve fuel performance [9; 10]. Although petroleum-based HED fuels have been widely used, the development of green synthesis of HED fuels has received widespread attention due to the shortage of petroleum resources and the environmental pollution crisis. The replacement of petroleummaterials with biomass materials to achieve renewable and green fuels is an important research direction. Currently, the main sources of HED biofuels include terpenes and lignocellulose [3]. Among them, terpenes are favored due to their compact polycyclic structure and specific spatial configuration. terpenes have unsaturated bonds, which can be modulated by direct hydrogenating or increasing the number of rings, providing more possibilities for the design of HED fuels with great application prospects [3]. Although terpenes are widely present in plants, their content is low, and they cannot be obtained in sufficient quantities for fuel applications through traditional extraction methods [11]. With the development of genetic engineering, systems biology and synthetic biology, researchers have discovered a production mode of synthesizing terpene-based fuels by constructing microbial cell factories to achieve green, low-cost and sustainable future manufacturing. Some microorganisms have been metabolically engineered from the aspect of isopropanol pathways for producing diverse terpenes, especially the two representative microbial hosts _Escherichia coli_ and yeast. _E. coli_ is the most widely studied prokaryotic microorganism model. It has a fast growth rate and can use a variety of substrates for aerobic or anaerobic growth. Moreover, diverse genetic manipulation tools and its clear genetic background have also contributed to _E. coli_ acting as an important model microbial chassis for producing terpenes [12]. Yeasts, such as _Saccharomyces cerevisiae_ and other non-conventional yeasts, including _Yarrowia lipolytica_ and _Kluyveromyces lactis_ et al., are single-celled eukaryotic microorganisms famous for their capabilities to ferment different sugars into targets. In terms of the ease and productivity of fermentation, yeasts exhibit an edge over bacterial strains since they have the ability to grow in more inexpensive media and have higher resistance to inhibitors [13].