## 2 p-C Terenenoid Biosynthetic Pathway  
The fermentative production pathway of p-C can be approached from a non-traditional perspective. Marine bacteria that live in extreme environments have unique biosynthetic pathways, unlike typical fermentative organisms. The production of biosurfactants increases the bioavailability of substrates, which requires lipophilic substances such as p-C [33]. The p-C contained in citrus waste also provides an environmentally friendly alternative to prevent environmental pollution problems. Although citrus waste can cause environmental pollution, it can be utilized as a building block for new chemicals by producing p-C compounds. It can be utilized in food processing and pharmaceutical industries and is considered safer than conventional synthetic compounds [34].  
Traditionally, however, the biosynthesis of p-C begins with limonene. The limonene used in the derivatization step of the dehydrogenated product, p-C, is produced by microbial systems, which motivates the development of renewable biomass through the biosynthetic pathway of terpenoids. Limonene, a natural product, is synthesized by limonene synthase as a cyclization of geranyl pyrophosphate (GPP) in either the methyleryth-thiritol 4-phosphate (MEP) pathway or the mevalonate (MVA) pathway [35; 36; 37; 38]. In the terpenoid pathway of limonene, the biosynthetic pathway to 1,8-cineole has been shown in Escherichia coli in particular, using 1,8-cineole as a synthase in place of limonene synthase. Palladium (Pd) has been utilized for the conversion of 1,8-cineole to p-C [39; 40]. A two-step mechanism using atmospheric oxygen as a green oxidant is also an efficient method for obtaining p-C, with the catalytic conversion of natural mordenite from limonene to p-C initiated by a non-catalytic process starting from limonene isomerization over natural mordenite [41; 42]. The acidity is further enhanced by the use of acetic acid (CH3 COOH), hydrochloric acid (HCI), sulfuric acid (HESO\({}_{4}\)), and nitric acid (HNO\({}_{3}\)) [43].  
The biosynthesis of p-C from plants implies the integration of biological and chemical catalysis for the efficient production of p-C and produces the precursors limonene and 1,8-cineole via the mevalonate pathway. Using hydrogenating/dehydrogenating metals to convert p-C showed that 1,8-cineole the optimal biological precursor with low toxicity [44]. A favorable alternative route of p-C via biosynthesis, from biological intermediates to chemical synthesis, is the application of limonene dehydrogenation. This is where Pd-based catalysts are used to convert limonene to p-C, and palladium/silicon dioxide (Pd/SiO\({}_{2}\)) catalysts (where palladium is immersed in silicon dioxide) can be used to perform limonene dehydrogenation to improve the yield of the product. It has also been added to palladium catalysts immersed in ZSM-5 (a specific form of zeolite) to show the biosynthesis of p-C with the catalytic activation of p-C [45; 46; 47; 48]. A favorable alternative route for p-C via biosynthesis, from biological intermediates to chemical synthesis, is the application of limonene dehydrogenation. This is the most commonly used palladium-bearing metal and metal oxide catalyst and performs limonene dehydrogenation over Pd/SiO\({}_{2}\) catalysts [49]. The transition between these cycles is shown in Figure 1.  
Figure 1: Limonene to p-Cymene cycle conversion.