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Figure 2.3 Hydrolysis of triglyceride leading to FFA formation.

Alcohols used can be polar (such as ethanol and methanol) or nonpolar (such as 2‐propanol or butanol). Although polar alcohols are capable of providing good fuel yields during transesterification, they are not miscible with the oils, and it has been reported by Karmakar et al. in their works with castor–karanja oil blend that 2‐propanol provides better conversion compared to methanol when used simultaneously, due to being miscible [1]. This can lead to the speculation that while activation energy for the nonpolar system would be lower (from reduced diffusion resistance), the reaction rate would be lower since nonpolar compounds are less reactive. Conversions of this nature can be facilitated with the use of alkali or enzyme catalysts, and while alkalis are used more frequently, the system is left vulnerable to saponification due to the presence of FFAs in most feedstock as depicted in Figure 2.2. Triglycerides are also degraded into FFAs at elevated temperatures in the presence of water, through a process termed as “hydrolysis,” depicted in Figure 2.3.

      Other modes of transesterification include the use of supercritical fluids heated to 300–400 °C under 10–30 MPa; superheated alcohols, which are heated to above 150 °C and injected to hot oil (above 250 °C); and enzyme catalysts under slightly elevated temperatures [19]. However, these processes are not without drawbacks, discussed in later sections in detail. The formed fatty acid alkyl esters (FAAE) have a much lower viscosity compared to the parent oil due to their smaller molecular sizes, which gives the fuel its ability to be compatible in diesel engines. This process is applicable to all feedstock in which glycerides exist and thus can be used for a wide variety of sources [2].

2.5 Conversion Techniques

      When it comes to conversion of reactive components of an oil into fuel‐grade products, there are a surprisingly large number of approaches available. Here, we discuss only the most commonly used techniques that are still in use for research as well as in commercial biodiesel synthesis.

      2.5.1 Traditional Conversion Approaches

      These approaches include acids, bases, enzymes, or other novel catalysts, which have been successfully used in batch or semicontinuous studies by researchers to obtain biodiesel or for the pretreatment of feedstock prior to actual conversion from a wide variety of edible and nonedible oils including WCO [3]. Usually, the use of homogeneous catalysts can provide greater conversion in a single round, but for process economics it is always better to dope the necessary functional groups derived from acid, base, enzyme, or transition metals (among other novel catalysts) and then use them as they can typically be reused for a few times before being discarded or recharged. An in‐depth discussion about this is presented in the following sections.

      2.5.1.1 Acid Catalysis

      The acid‐catalyzed conversion is termed as “esterification” and involves the conversion of FFAs present in the oil to esters and water using polar alcohols such as methanol or ethanol [6]. Along with esterification, transesterification of glycerides also takes place albeit to a smaller degree. Also the effect of nonpolar alcohols such as 2‐propanol had been studied, which revealed that while 2‐propanol is able to enhance the acid‐catalyzed transesterification, it has absolutely no impact in FFA conversion, which resulted in the formed esters becoming rancid with 72 h upon storage [6]. Usually, mineral acids (such as HNO₃, H₂SO₄, or HCl) are used as they provide greater reactivity compared with other weaker acids.

The esterification process involves adding H⁺ to the O atom in a carboxyl group, which is followed by the alcohol performing a nucleophilic attack on the fatty acid aided by the protonated acid catalyst, which results in breakage of a H₂O molecule and a proton (adds back to the acid catalyst), resulting in ester formation [2]. Heterogeneous doped catalysts are preferred for reusability (verified up to four uses in our studies) before needing to be recharged with fresh acid. It was also shown in our other reported works with both MFL and Delonix regia char that the inert carbon supports are able to efficiently go through multiple rounds of redoping since the supports can withstand mechanical stress from agitation during reactions [4, 5]. Various other researchers have performed extensive studies on a wide variety of oils including WCO through the use of both homogeneous acids as well as acid‐doped inert supports; a select few of which are summarized in Table 2.2. An interesting point to note is that this process is not sensitive to the presence of small amounts of water, since hydrolysis produces FFAs, which are readily esterified [2].

      2.5.1.2 Alkali Catalysis

The use of bases in transesterification is probably the most common practice of conversion of compatible feedstock (low in FFA and moisture) into esters. The conversion involves formation of an alkoxide ion from the base and the alcohol, which then targets the triglyceride (for example), attacking the carbonyl carbon, and forming a tetrahedral shaped intermediate. These compounds then undergo reaction with an alcohol molecule, undergoing a structural rearrangement during the process to give off an ester molecule and leading to the formation of a diglyceride [2]. This process will now repeat itself twice to finally yield glycerol as a by‐product along with 2 mol of alkyl ester [28], as shown in Figure 2.1. The process cannot tolerate even traces of FFA and moisture except for KOH, which is why it is favored over other bases. The process can also include homogeneous and heterogeneous catalysts, with doped basic groups being more stable compared to acidic groups [1, 6]. Many edible and nonedible oils (including

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