## 3 Types of Catalysis in Biodiesel Production
### Esterification  
Esters are substances which form as a result of elimination of water molecule during a slow reaction at 25 \({}^{\circ}\)C between carboxylic acid and an alcohol. Generally, esters are slow melting solids or liquids (all methyl and alkyl esters) and are lighter than water. These have lower boiling points compared to the alcohols or acids (both having H-bond) of relative molecular weight because hydrogen bonding is not possible in esters. hese are usually liquids even though parent acid is a solid and all these above-mentioned physical properties are attributed to loss of hydrogen bonding in esters. All esters are in soluble in water and soluble in organic solvents such as ethyl acetate. Esters have a wide range of applications such as in medicines, flavoring agents, paint removers and part of our daily diet as found naturally in oils and fats [22]. Esters are prepared via direct \"esterification\" of carboxylic acid or their derivatives with alcohol or phenol [23]. It involves the refluxing of mixture of above-mentioned compounds in the presence of catalyst that is a mineral acid such as dry HCl and H\({}_{2}\)SO\({}_{4}\), as shown in Equation (1):  
\[\text{RCOOH}\,+\,\text{R}^{\prime}\text{OH}\,\xrightarrow{\text{H}_{2}\text{ SO}_{4}}\,\text{RCOOR}^{\prime}+\,\text{H}_{2}\text{O} \tag{1}\]  
The treatment of feedstock is achieved via this reaction for the reduction in concentration of free fatty acids in oils and fats that results in the production of ester: biodiesel and H\({}_{2}\)O.  
This is the first order reaction and factors affecting the rate of reaction include reaction temperature, oil to alcohol molar ratio, catalyst concentration and reaction time. The mechanism of acid esterification involves s typical acid- catalyzed nucleophilic substitution at acyl carbon atom as shown in Figure 3a.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{2}{c}{**Type of Catalyst**} & \multicolumn{2}{c}{**Examples**} & \multicolumn{2}{c}{**Features**} \\ \hline \multirow{3}{*}{Acids} & Homogeneous & \
\begin{tabular}{c} Sulphuric acid, Sulphonic acid, \\ Hydrochloric acid \\ \end{tabular} & Better catalysis but corrosive and slow reaction rate \\ \cline{2-4}  & Heterogeneous & Bronsted and Lewis acids, keggin & Reusable and recoverable but severe reaction \\  & & heteropolyacids & and costly \\ \hline \multirow{3}{*}{Bases} & Homogeneous & Potassium hydroxide, Sodium hydroxide & High reaction rate, non-corrosive, cheap and readily available but high FFA cause soap \\  & and Sodium methylate & formation and cannot be reused \\ \cline{2-4}  & Heterogeneous &
\begin{tabular}{c} Carbonates and oxides of alkali metals \\ and alkaline earth metals \\ \end{tabular} & Reusable, easy purification, minimum waste \\  & & and alkaline earth metals & production but costly and leaching on recovery \\ \hline \multirow{2}{*}{Biocatalysts} & Enzymes & Lipases & Highly purified product, easily separable, mild reaction but highly expensive, deactivation of enzyme due to methanol \\ \hline \hline \end{tabular}
\end{table}
Table 3: Types of catalysts used for biodiesel synthesis [20].  
The protonation of fatty acid followed by nucleophilic attack results in the formation of tetrahedral intermediate. It undergoes molecular rearrangement and ester (biodiesel) is formed after elimination of water molecule. The alkaline esterification mechanism has not been presented in this review because the alkali-based catalysts form soap when they react with fatty acid and alcohol.