Citation: Cerón Ferrusca, M.; Romero, R.; Martínez, S.L.; Ramírez-Serrano, A.; Natividad, R. Biodiesel Production from Waste Cooking Oil: A Perspective on Catalytic Processes. Processes 2023, 11, 1952. https://doi.org/10.3390/ pr11071952 Academic Editors: Dicho Stratiev, Dobromir Yordanov and Aijun Guo Received: 15 May 2023 Revised: 17 June 2023 Accepted: 21 June 2023 Published: 28 June 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). processes Review Biodiesel Production from Waste Cooking Oil: A Perspective on Catalytic Processes Montserrat Cerón Ferrusca 1 , Rubi Romero 1, * , Sandra Luz Martínez 2 , Armando Ramírez-Serrano 2 and Reyna Natividad 1, * 1 Chemical Engineering Laboratory, Centro Conjunto de Investigación en Química Sustentable, UAEM-UNAM, Universidad Autónoma del Estado de México, Km 14.5 Toluca-Atlacomulco Road, Toluca 50200, Mexico; mcerfer96@gmail.com 2 Faculty of Chemistry, Universidad Autónoma del Estado de México, Toluca 50120, Mexico; slmartinezv@uaemex.mx (S.L.M.); aramirezs75@hotmail.com (A.R.-S.) * Correspondence: rromeror@uaemex.mx (R.R.); rnatividadr@uaemex.mx (R.N.) Abstract: Presently, the use of fossil fuels is not ecologically sustainable, which results in the need for new alternative energies such as biodiesel. This work presents a review of the classification of the lipidic feedstocks and the catalysts for biodiesel production. It also presents the pros and cons of the different processes and feedstocks through which biodiesel is obtained. In this context, cooking oil (WCO) has emerged as an alternative with a high potential for making the process sustainable. A detected limitation to achieving this is the high content of free fatty acids (FFA) and existing prob- lems related to homogeneous and heterogeneous catalysts. To overcome this, the use of bifunctional catalysts is being evaluated by the scientific community. Thus, this work also explores the advances in the study of bifunctional catalysts, which are capable of simultaneously carrying out the esterification of free fatty acids (FFA) and the triglycerides present in the WCO. For the sake of an improved understanding of biodiesel production, flow diagrams and the mechanisms implied by each type of process (enzymatic, homogenous, and heterogeneous) are provided. This article also highlights some of the challenges in catalyst development for sustainable biodiesel production from low-grade raw materials. Keywords: biodiesel; waste cooking oil; bifunctional catalysts; transesterification 1. Introduction 1.1. Biodiesel Global Scenario The energy model that currently prevails is highly dependent on the usage of fossil fuels and supports various sectors such as transportation, industry, and agriculture, among others [1]. However, this model has become less and less viable due to the reduction in the non-renewable energy source, its increasing price, and the fact that this type of fuel favors an elevation in greenhouse gas emissions, some of which have been shown to have an impact on people’s health as they are related to various types of cancer [2–4]. As stated by the United Nations’ Sustainable Development Goals (SDG), regarding sustainable energy access (SDG7) and climate change (SDG13), several countries have started to take actions to reduce carbon emissions by at least 43% by 2030 and to not reach an increase in global temperature of 1.5 ◦ C between 2030 and 2050 (IPCC, 2018) [5]. The aforementioned problems have motivated the development of other cleaner energy options. An example is biodiesel, because of its capability to be produced from vegetable and/or animal fats while also being able to be used in diesel engines without major changes because it poses physicochemical characteristics similar to those of high- performance diesel [6–8]. This biofuel improves engine performance in addition to having cleaner combustion because it has an oxygenated molecule that allows it to reduce CO 2 emissions by up to 80% [6,9]. Processes 2023, 11, 1952. https://doi.org/10.3390/pr11071952 https://www.mdpi.com/journal/processes