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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].