Environmental issues and population growth have increased the consumption of fossil fuels and expanded efforts to find alternative fuels. Biodiesel is a degradable fuel made from edible and non-edible vegetable oils, animal fats, and algae [1_3]. According to the ASTM standards, biodiesel is a combination of long chain mono alkyl esters of fatty acids from the reaction of an alcohol with renewable lipids (the most important sources are animal fats and vegetable oils) [4_6]. Suspended particles and unburned hydrocarbons emitted by diesel vehicles are toxic and carcinogenic and pose a threat to human health. The presence of 11–15% oxygen in the molecular structure of biodiesel in the combustion process in engines accelerates and reduces pollutants such as carbon monoxide, as well as other pollutants and particulate matter. This finding, i.e., the effect of using biodiesel on reducing toxicity and air pollution, has been approved by reputable international organizations [7_10].
The higher flash point of biodiesel makes it a safer fuel than diesel [11]. Large-scale biodiesel production is facing two problems including the cost of feed and the production process. In order to reduce the cost of biodiesel production, alternative feedstocks such as frying oil, animal fats, low-cost nonedible vegetable oils, and industrial waste oil can be used [12_16]. Vegetable oils must be modified for use in diesel engines because their high viscosity has a detrimental effect on the engine. There are four main ways to modify vegetable oils and improve their properties for use in diesel engines. These methods include microemulsion, transesterification, dilution and pyrolysis (thermal cracking), which reduce viscosity [17, 18]. Among these methods, transesterification has received the most attention among researchers. The complete triglyceride transesterification reaction consists of three steps. These steps include formation of diglycerides, monoglycerides, and finally glycerol. In each step, an ester is released as a product [19, 20]. Homogeneous or heterogeneous acidic, basic, or enzymatic catalysts can be used to perform this reaction. However, in the presence of alkali catalysts, if the oil contains large amounts of free fatty acids (FFAs) or even more than 0.5% wt, an unfavorable saponification reaction occurs. Separating the formed soap is difficult, and the reaction yield is greatly reduced.
Therefore, it is better to use acidic and enzymatic catalysts for the transesterification reaction for feedstocks that contain a high fatty acids content and water, because these catalysts are not sensitive to water and fatty acids [19,21_24].