It causes less greenhouse gases emissions due to its renewable nature; hence, it is non-toxic and environmentally friendly. In addition to environmental benefits, the production of biodiesel also offers socioeconomic advantages as it provides job opportunities in various economic sectors.  
Biodiesel is defined as alkyl esters synthesized from triglycerides present in vegetable oil or animal's fat in the presence of a suitable catalyst under optimum reaction conditions [1; 2]. In a chemical reaction, triglycerides are first converted into diglyceride and then diglycerides are converted into monoglyceride; the end products are glycerol and biodiesel. This reaction is governed by a specific catalyst, used to enhance the rate of reaction and minimize the reaction time. For biodiesel synthesis, catalysts of chemical or biological nature are used and their selection is essentially determined by Free Fatty Acid (FFA) content of feedstock oil.  
1. Chemical-based catalysts can be either homogeneous or heterogeneous depending on their solubility in reaction media;
2. Recently, Nanotechnology has attracted researchers to synthesize nano-sized catalysts with large catalytic surface and meet the demands of industrial applications;
3. Biological catalysts comprised of enzymatic systems such as free lipases [3].  
With the increasing demand for biodiesel, research is being focused on proposing new routes for biodiesel production. These routes will lead to high conversion yields without posing damage to climate and at the same time viable economically. The production capacity of biodiesel from animal fats and vegetable oil has 8.3 billion gallon per annum and is increasing day by day. Figure 1 depicts production share (%) of countries in the base period. It is expected that biofuel will account up to 27% of liquid fuel supply across the globe. Agricultural countries that cultivate different oil yielding crops have favorable climatic conditions and enormous fertile lands, have made biofuel synthesis largely conducive compared to other feedstocks.  
Figure 1: World biodiesel production, 2021 (Percentage refers to production share of countries in the base period) Source: OECD/FAO. (2021). OECD-FAO agricultural outlook 2021–2030.  
Biodiesel is utilized in the form of blended fuel with petroleum diesel fuel known as BXX, where XX denotes the percentage of biodiesel in the blended fuel. Many countries have already applied different portions of biodiesel in the blended fuel [4]. Worldwide, biodiesel is used either in pure form (100%) represented by B100 or mixed with petro-diesel (biodiesel-petrodiesel blending) such as 5% (B5), 20% (B20) and 80% (B80) [5].  
Our current study is focused on a comprehensive review of various techniques to synthesize catalysts used for sustainability of biodiesel synthesis processes with special emphasis on transesterification. The novelty angle of this article is to survey the works of researchers on the advancement of distinctive types of catalysts, for example, homogeneous, heterogeneous, nano sized catalysts (especially clay derived nanocatalysts and Plant and animal based nanocatalysts) and biocatalysts utilized for biodiesel generation to date. This article compares the reasonableness and associated challenges within the transesterification with an accentuation on the catalysts and their reusability. Moreover, this review also compares different methodologies to synthesize catalysts that are in turn used for sustainable production eco-green biodiesel at large scale. This analysis will offer an insight into catalyst design and the most optimal combination of three main factors in biodiesel synthesis process: catalyst type, oil type and reaction conditions in order to enhance the biodiesel yield, eventually.