# Bifunctional Heterogeneous Catalyst for Biodiesel Production from Waste Vegetable Oil
## 1 Introduction
The inorganic solid catalysts that can be utilized for the transesterification reaction are the basic character, acidic character, and bifunctional (acidic-basic character); the latter are efficiently able to catalyze esterification and transesterification simultaneously. Heterogeneous bifunctional catalysts (solid base-solid acid) are very advantageous as they can be recycled, reused, and regenerated, with less energy consumption, and do not form soaps. They also simplify the glycerol purification and can be easily separated from the reaction products. Since they are used in powder form or in pellets, they also give better FAME yields in shorter reaction time and temperatures, in addition to being tolerant to FFAs and water in the feedstock [3; 8; 10]. Transesterification reactions with basic heterogeneous catalysts are mostly used for methyl ester or biodiesel production, and it is essential to use feedstock such as animal fats or vegetable oils, which contains low amounts of FFA [7; 10]. Free fatty acids and water have less of an effect on solid-acid catalysts; hence, they do not dissolve in the feedstock, as well as in the alcohol, and they can be easily separated through filtration and reuse. Such catalysts are efficient for the esterification of FFA and triglycerides [3; 11]. When the feedstock has a high content of FFAs or water, the alkali catalyst may react with the FFAs to generate soaps, while the water hydrolyzes the triglycerides to produce diglycerides and more FFAs [3; 10]. The formation of soap during reactions causes catalyst consumption, a reduction in ester yield, and prevents the separation of glycerol from methyl ester [12]. Studies have reported the use of solid heterogeneous catalysts such as metal oxides of alkaline earth metals (Ba, Be, Ca, Mg, Sr), \(\gamma\)-alumina, hydrotalcites, mixed oxides, and zeolites [13]. CaO, a solid basic catalyst, is economical and mostly utilized because of its high activity and strength with a long lifespan [14]. A strong acid catalyst such as hydrochloric, sulfuric, or phosphoric acid, heteropoly acid impregnated on various supports (zirconia, silica, activated carbon, and alumina), Nafion-NR50,WO\({}_{3}\)-ZrO\({}_{2}\), and SO\({}_{4}\)-ZrO\({}_{2}\) are used for transesterification because of their possession of sufficient acid sites on the surfaces with diverse strengths of Lewis acidity [15; 16]. As an alternative to the use of solid-acid catalysts for FFA esterification and solid-basic catalysts for the transesterification of triglycerides, the use of a solid catalyst with an acidic-basic character has been suggested. Bifunctional catalysts act as a base and an acid at once and can mediate simultaneous (esterification and transesterification) reactions for methyl ester production [17]. Other currently developed catalysts for the production of methyl esters are biomass pyrolysis by-products, which include biochar, fly ash, sugars, etc. [18]. In this study, a two-step catalytic FAME production from waste palm oil (WPO) and waste sunflower oil (WSO) was performed using calcium oxide (solid base) impregnated on alumina (solid acid) catalyst. Alumina (Al\({}_{2}\)O\({}_{3}\)) was selected as a support because of its large pore size and pore volume, high specific surface area, and thermal and mechanical stability. The main focus of the study was to determine the optimum reaction conditions and the effect of different catalyst loading of CaO/Al\({}_{2}\)O\({}_{3}\) after calcination on the FAME yield during the transesterification of WSO and WPO. Therefore, it is plausible that the use of a bifunctional heterogeneous catalyst such as CaO/Al\({}_{2}\)O\({}_{3}\), which is reusable in the transesterification of readily available WSO and WPO, can result in the enhancement and optimization of economically viable biodiesel (FAMEs).