# Bifunctional Heterogeneous Catalyst for Biodiesel Production from Waste Vegetable Oil
## 1 Introduction  
The ever-growing rate of energy production and consumption with its attendant release of greenhouse gases cause ever-increasing adverse environmental effects, which in turn affect the well-being of humans [1]. The majority of the energy consumed globally originates from petroleum, charcoal, and natural gas. Nevertheless, these sources are limited and could possibly be depleted in the next centuries [2, 3]. Therefore, the decline in global fossil energy resources will result in an increase in the crude oil prices and make renewable and sustainable energy sources more attractive. There is a necessity to find clean and renewable energy supplies, which is environmentally benign, to substitute current fossil energy sources. Biodiesel produced from the transesterification of triglycerides molecules in vegetable oils, animal fats, and waste oils such as yellow grease and soybean soapstock) is a promising alternative energy source because of its lower emissions of particulate matter and greenhouse gases [4]. The first generation of biodiesel was produced using oils obtained from oil-bearing crops, leading to the food-for-fuel debate. Plant feedstock majorly made up of lignocellulosic materials (second-generation), though abundant and cheaper than food crops, are still considered to be costly for large-scale production of biodiesel. The growing interest in waste vegetable oils and waste animal fats is being fueled by the fact that it offers a cost-effective way to deal with the growing global generation of waste vegetable oils and its considerably lower cost compared to virgin oils [4]. The production of fatty acid methyl esters (FAME) can be mediated by homogeneous and heterogeneous catalysts. Homogeneous catalysts exist in the same liquid phase as the reaction mixture, whereas heterogeneous catalysts are in a different phase, generally as a solid [1]. Homogeneous base and acid catalysts are widely used owing to their simplicity in application, good performance, and cost effectiveness. Alkaline catalysts such as KOH and NaOH are mostly utilized apart from sodium or potassium carbonates and alkoxides because of their high solubility in methanol (forming potassium and sodium methoxide [KCH\({}_{3}\)O and CH\({}_{3}\)NaO]) and the high yield of methyl ester that can be obtained under mild conditions when quality feedstock [\(<\) 1 FFA (wt %) and \(<\) 0.5 moisture (wt %)] are utilized [5; 6], although this may demand a number of hours for the reaction to complete. However, homogeneous acid catalysts have a longer reaction time, higher temperature (80-100 \({}^{\circ}\)C), and unwanted technical equipment failure, which are disadvantageous to its application. Furthermore, to increase triglyceride conversion, an excessive quantity of methanol, for example, a molar ratio of methanol/oil higher than 12:1, is required [7], whereas the reaction is conventionally performed at a temperature of 60-65 \({}^{\circ}\)C under atmospheric pressure with a molar ratio of alcohol/oil at 6:1. An acid catalyst is mostly taken as a pretreatment stage in order to reduce the time of reaction when converting free fatty acids (FFA) to esters, which is followed by the addition of a base catalyst for the transesterification step to convert triglyceride into esters. The homogeneous-catalyzed transesterification method is costly, especially in the separation of the catalyst from the reaction mixture, the purification of methyl esters, high energy consumption, and corrosiveness. Heterogeneously-catalyzed transesterification methods have been applied for methyl ester production [8; 9]. This process requires simple purification steps and no neutralization. Moreover, heterogeneous catalysts are more environmentally friendly, sustainable, and neither involve catalyst recovery nor aqueous treatment stages.