# Diversity of Potential (Bio)Technological Applications of Amino Acid-Based Ionic Liquids
## 2 Bioapplications of AA ILs: An Overview
### AA-ILs with Application in the Processing of Biomaterials
#### 2.6.2 Biodiesel Production  
Biodiesel consists of long-chain fatty alkyl esters and is produced from renewable bio-resources such as vegetable oils, animal fats, and waste cooking oils [119]. Production typically involves the process of transesterification and the subsequent separation of the products into biodiesel and glycerol (a by-product). The transesterification reaction requires a catalyst, which can be a classical catalyst such as an alkali catalyst (e.g., KOH or CaO), an acid (e.g., H2 SO4, HCl, or HNO3), or a lipolytic enzyme [120]. However, each of these catalysts has drawbacks related to stability, efficiency, environmental impact, or cost. In the last two decades, ILs, especially those based on AAs, have shown excellent potential in biodiesel production. Table 1 summarizes the main results obtained in biodiesel production using AA-based ILs as catalysts. The type of AA anion seems to be crucial for the catalytic activity of ILs in the transesterification reaction. In three series of ILs containing [Ch], [TBA], or [TMA] cations with the same AA anion, the tested ILs show comparable activity under identical reaction conditions [121]. In contrast, within a series of ILs with the same cation but different AA anions, the activity varies considerably. For example, while transesterification of sunflower oil in the presence of 6% [Ch][Arg] yields 99% biodiesel, replacement of the anion with Ala, Ser, Leu, Phe, His, Met, or Gly significantly reduces the yield to 0.9-6% [121]. The same trend was observed in the transesterification of soybean oil with [Ch][AA] [123]. Similarly, [PERSON] et al. observed the highest catalytic activity for [N222][Arg] within a series of [N222][AA] tested in the methanolysis reaction of soybean oil [122]. The transesterification of triglycerides with methanol is a typical nucleophilic substitution reaction [121]. A correlation between the basicity of the ILs and their catalytic activity was observed, with more basic \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Catalyst** & **Reaction Conditions** & **Biodiesel Yield** & **Reference** \\ \hline
[TBA][Arg] & Sunflower oil: methanol (1:9 mole ratio); 6\% & Fatty acid esters & [121] \\
[Ch][Arg] & (w/w on oil base) catalyst; 60 min; 80 \({}^{\circ}\)C & 98.8–99.8\% & [121] \\ \hline
[N2222][Arg] & Soybean oil: methanol (1:10 molar ratio); 20\% & Fatty acid esters 98.4\% & [122] \\ \hline
[Ch][Arg] & Soybean oil: methanol (1:9 molar ratio); 15 \% & Fatty acid esters 98.6\% & [123] \\ \hline
[Ch][Arg] & Soybean oil: glycerol (1:2 molar ratio), 10\% & Monoacylglycerolglycerols & [124] \\ catalyst (based on soybean oil weight); 12 h; & (biosulffactant) & [124] \\  & 100 \({}^{\circ}\)C & 60\% & \\ \hline
[TMA][Arg] & Soybean oil: glycerol (1:2 molar ratio), 10\% & Monoacylglycerolglycerols & [124] \\  & catalyst (based on soybean oil weight); 30 min; & (biosulffactant) & [124] \\  & 105 \({}^{\circ}\)C & 65\% & \\ \hline
[Ch][Arg] & Castor oil: glycerol (1:6 molar ratio); 12 \% & Monoacylglycerolglycerols & \\
[TBA][Arg] & catalyst (based on castor oil); 120 min; 100 \({}^{\circ}\)C & (biosulffactant) & [125] \\
[TMA][Arg] & & 65–83\% & \\ \hline
1-n-heptyl-3-methylimidazolium alanine [C\({}_{7}\)mim][Ala] & Soybean oil: methanol (1:23 molar ratio); 8\% & Fatty acid esters 40 \% & [126] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Examples of AA-ILs with excellent transesterification activity. ILs acting as more effective catalysts and stabilizing the nucleophile (in this case, methoxyl anion) [121]. In addition, similar to what was observed for the transesterification reaction, the glycerolysis reaction of soybean oil is at least 10 times more effective when using a catalyst based on the Arg anion compared to ILs containing other amino acid anions [125]. The rate of transesterification and the resulting biodiesel yield also depend on the composition of the fat.