must be easily available, cheap, and sustainable. Feedstock is selected on the basis of biodiesel production that must be compatible to chemical composition and properties of feedstock to be used, percentage per dry biomass, agricultural potential, yield per hectare, and geographical region of that feedstock [27]. For example, soybean oil, palm oil, coconut oil, and rapeseed oil are mainly used as feedstock in US, tropical countries like Indonesia, coastal areas, and European countries, respectively. Cultivation and climate conditions of the feedstock production area are also considered for its selection [28]. Depending on the nature, there are two types of feedstock for biodiesel production. First is the lipid raw material and second includes alcohol feedstock. Lipid sources can be divided into three categories, i.e., oils derived from plant sources (edible and non-edible oils), waste oils (waste cooking oils, industrial wastewater, lard, yellow grease, and animal fats), and oils from oleaginous microorganisms such as bacteria, fungi, and microalgae [29]. Properties of biodiesel like cold filter plugging point and oxidation stability are determined from the feedstock used for production. Feedstock properties like moisture content, impurities, content, and composition of free fatty acids (FFAs) affect the performance of engine [27, 28]. Composition of fats and oils including monoglycerides, diglycerides, and triglycerides are used for biodiesel production. Utilization of edible plant oils as feedstock is an expensive way for biodiesel production that leads to imbalance in food market and industry. It is also associated with some environmental problems like disruption of vital soil resources and deforestation due to mass propagation [29]. In order to solve problems linked with edible plant oils, the best alternate is the production of second-generation biodiesel which is produced by using non-edible (inedible) feedstock which are more favorable than edible oils due to reduction in cost and waste pollution, lower aromatic, sulfur contents, and high calorific value [8]. Inedible oils involve inedible plant oils, industrial waste, cooking oils, animal fats, and microalgal oils. Inedible oil producing plants have certain remarkable features that make them favorable to use, for example, they can be managed to grow in arid and semi-arid conditions and they do not require fertilizers and moisture for growth [24].

Repeated use of fried vegetable oils at high temperature leads to the production of waste cooking oils. Moreover, chemical composition of waste cooking oil is totally dependent on the oil from which it is derived. Hydrogenation, oxidation, and polymerization are the main chemical reactions that lead to production of very toxic and detrimental compounds for consumption. Fatty acid content of these oils lies in the range of 0.5% to 15% which is very much higher than refined oil having fatty acid content less than 0.5%. The waste cooking oil is known as yellow grease if the fatty acid content is less than 15% and it is called low value brown grease if the fatty acid content is higher than 15% [28]. Animal waste products like lard, tallow, animal fat, poultry fat, fish oil, and pork fat are also very effective feedstock for biodiesel production [30, 31]. Animal-based bio-diesel is a good lubricating agent and has high percentage of saturated fats which decreases sedimentation risk and low temperature fluidity. Moreover, it increases oxidative stability and cold filter plugging point of biodiesel which are the characteristics of good quality biodiesel. Utilizing these waste materials is an effective solution to encounter waste disposal. Apart from all these mentioned advantages of non-edible or waste oils, there are also some shortcomings or disadvantages, for example, low oil yield, higher carbon residue, unsaturated fatty acid content, and low volatility [29]. In some cases, large plantation land for inedible oils is required compared to edible ones, e.g., Pongamia pinnata and Jatropha has 2–50 folds less oil yield per hectare than palm oil so that is why they require much area to meet the demand [31]. Because of the drawbacks associated with second-generation biodiesel, scientists are looking for more efficient methods for biodiesel production. Biodiesel production using oleaginous microorganisms like bacteria, algae, microalgae, and fungi are considered as the future of biodiesel production that can meet global biodiesel demand for transportation fuels and other energy consuming applications [32]. Microbial oils are better than other plant oils because of their short life cycle and rapid growth, less requirement of space, labor, and easier scaling [33–35]. Microalgae as a feedstock is very effective because of its enormous advantages like they have high oil yield, can grow in salty and waste waters, use of non-arable land, and growth in 24 hours so multiple harvesting in a year is possible. If we give land area for microalgal growth then according to an estimate, only 2% of the US cropping land is enough for meeting 1/3 demand of US transportation fuels and less than 5% land is required to completely replace all transportation fuels [36–38]. Moreover, dry algal biomass can accumulate more than 80% oil without water and they have a capacity to produce oil yield 250 times greater than soybean water free oil [35]. Some examples of microalgae used for biodiesel production are Botryococcus sp., Cylindrotheca sp., Schizochytrium sp., Chlorella sp., and Nitzschia sp.

1.4 Methods in Biodiesel Production

There can be many ways for biodiesel production but esterification and transesterification are the two most widely used methods. Esterification is the reaction of FFAs and alcohol to make FAAEs and water is released, while transesterification is the reaction of triglycerides or triacylglycerols (TAGs) with alcohol to make FAAE and glycerol is produced as by-product [9]. Transesterification is slower than esterification process because of its multiple steps or reactions. It is a three-step process to convert TAGs into FAAE. In the first step, TAG reacts with one molecule of alcohol to produce one molecule of FAAE and diacylglycerol (DAG). In second step, DAG further reacts again with one molecule of alcohol to produce one molecule of FAAE and monoacylglycerol and in the last step monoacylglycerol is converted into one molecule of glycerol and FAAE after reacting with an alcohol molecule. In each of these three steps, FAAEs are produced and in total one molecule of TAG and three molecules of alcohol are consumed to produce three molecules of FAAE and one molecule of glycerol [6–10]. Transesterification is a reversible reaction, and in order to make the reaction go forward to produce more biodiesel, we have to supply alcohol in large excess so that the reaction equilibrium shifts toward the product [36, 37].

1.5 Types of Catalysts Involved in Biodiesel Production

      Biodiesel production process is carried by either catalytic or non-catalytic methods. Non-catalytic methods include use of alcohols or supercritical fluids or ionic liquids in the reaction system to produce biodiesel but mostly catalytic methods have been used for last 2 or 3 decades because of their advantages over non-catalytic methods [38]. Catalytic methods can be categorized into chemical homogenous catalysts, solid heterogenous catalysts, and biocatalysts.

      1.5.1 Chemical Homogenous Catalysts

Chemical homogenous catalysts include combination of base and acid catalysts. NaOH, KOH, and methoxides are the base catalysts while HCl and H₂SO₄ are the acid catalysts [39]. Acid catalysts are mostly used to overcome the problem of FFAs in the reaction system but the rate of trans esterification by acid catalyst is slower than alkaline or base catalysts [8]. Chemical catalytic processes either alkaline or acid catalysis both have several disadvantages. Alkaline catalysis provides high conversion of triacyl glycerol into the alkyl esters in a very short time but it has many drawbacks. Alkaline catalysis is very prone to FFA concentration (>2.5%) in the reaction system because high FFA concentration results in saponification reaction producing soaps and leads to loss in enzymatic activity and makes difficult to separate transesterification by-product, i.e., glycerol from bio-diesel. Hence, biodiesel yield decreases. Moreover, it needs high energy requirement [40]. To counter FFA problem, acid catalysts are used, e.g., sulfuric acid but it also causes some technical problems regarding separation and purification of glycerol. Moreover, acid catalysis is a slow process compared to alkaline process. Reactors, pipelines, and other equipment are badly affected by acid catalysts because of their corrosive nature that can increase the cost of biodiesel production [41].

      1.5.2 Solid Heterogeneous Catalysts

      Solid heterogeneous catalysts include acid heterogenous catalysts and base heterogenous catalysts. Solid acid heterogenous catalysts include heteropolyacid catalysts (HPAs), mineral salts, acids, and cationic exchange resins. Among these, titanium oxide, sulfonic ion exchange resin, tin oxide, sulfonated