# Utilization of Polymeric Materials toward Sustainable Biodiesel Industry: A Recent Review
## 3 Polymeric Catalysts  
Limitless catalysts were reported in the literature to catalyze the conversion process of triglycerides into alkyl esters. The conversion is done through esterification and/or transesterification chemical reactions illustrated in Figure 4 [58]. The mechanism of both reactions has similarities and differences. In acidic esterification, carbonyl group of triglycerides reacts with the catalyst to convert triglycerides into diglycerides in the first reaction. Diglycerides are converted into monoglycerides in the second reaction, and esters in the third reaction. In contrast, the catalyst reacts with alcohol in basic transesterification to form alkoxy group. Then, the three reactions take place between alkoxy group and triglycerides to be converted into diglycerides, monoglycerides, and finally the alkyl esters [9; 59]. Catalyst functions to speed these processes 4000 times [60]. In general, catalysts were classified into homogenous and heterogenous catalysts. Homogenous catalysts are chemical compounds that have the solubility characteristic in either lipids or alcohol during biodiesel production. They offered high efficiency in mass transfer of the catalytic group. That is why the homogenous-catalyzed transesterification does not consume much time. However, contamination of the final products constitutes a drawback for homogeneous catalyst. Consequently, a purification process is highly needed to separate catalyst from resultants. However, the reusability of homogeneous catalysts is not promising. In contrast, heterogeneous catalysts are insoluble compounds in lipids nor alcohol. Therefore, sepa \begin{table}
\begin{tabular}{l c c} \hline \hline \multicolumn{1}{c}{**Physicochemical Properties**} & **Unit** & **Value** \\ \hline Density at 20 \({}^{\circ}\)C & Kg/m\({}^{3}\) & 900 \\ Viscosity 20 \({}^{\circ}\)C & mm\({}^{2}\)/s & 4 \\ Pour point & \({}^{\circ}\)C & 1 \\ Flash point & \({}^{\circ}\)C & 68.5 \\ Heating value & MJ/kg & 25.1 \\ Cetane number & - & \textless{}1 \\ Octane number (RON) & - & 62.2 \\ Oxygen content & \%wt & 41 \\ Oxidative stability at 100 \({}^{\circ}\)C & h & 8.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The physicochemical properties of polymeric biodiesel (HBME) (adapted from [20; 22; 57]). rating heterogeneous catalyst from resultants is not challengeable which saves resources and time in the purification process. In addition, the reusability property is potentially attainable. Compared with homogenous catalysts, heterogenous catalysts consume much time to catalyze the conversion process of triglycerides into biodiesel since they offered poor mass transfer characteristics of the catalytic group [61; 62]. Polymers materials offered efficient catalysts for biodiesel production. Indeed, the polymeric catalysis is rapidly expanding in accordance with the revolution of green chemistry. The performance efficiency of polymeric based catalysts comes from the interior porous structure of polymers that provides a wide surface area and diffusible capability, selectivity functions of polymers materials, high stability of polymers compounds, and the unique chemical composition of polymers which allows for some adjustments [24]. Polymeric catalysts are produced through chemical, natural, and waste derived composites. All these composited approaches proved high efficiency in catalyzing triglycerides and transesterification and FFA esterification, particularly for the second-generation feedstock oils. They provide high mass transfer of the catalytic group during reaction which allows for high conversion yield of fatty acids methyl esters (FAME) within less reaction time. Moreover, polymers, both homogeneous and heterogeneous, migrate completely to glycerol byproduct. In this case, purification process of produced biodiesel is unrequired. In addition, the recovered catalyst is reusable for several cycles with high conversion percentages as reported in the literature [10; 23].