molecules Review Advances in Enzyme and Ionic Liquid Immobilization for Enhanced in MOFs for Biodiesel Production Reem Shomal 1 , Babatunde Ogubadejo 1 , Toyin Shittu 1 , Eyas Mahmoud 1 , Wei Du 2 and Sulaiman Al-Zuhair 1,3, *   Citation: Shomal, R.; Ogubadejo, B.; Shittu, T.; Mahmoud, E.; Du, W.; Al-Zuhair, S. Advances in Enzyme and Ionic Liquid Immobilization for Enhanced in MOFs for Biodiesel Production. Molecules 2021, 26, 3512. https://doi.org/10.3390/ molecules26123512 Academic Editors: Rajkumar Kore, Arvind H. Jadhav and Hui Wang Received: 24 April 2021 Accepted: 6 June 2021 Published: 9 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). 1 Chemical and Petroleum Engineering Department, UAE University, Al Ain 15551, United Arab Emirates; 201350343@uaeu.ac.ae (R.S.); 201990035@uaeu.ac.ae (B.O.); 201990179@uaeu.ac.ae (T.S.); emahmoud@uaeu.ac.ae (E.M.) 2 Department of Chemical Engineering, Tsinghua University, Beijing 100084, China; duwei@tsinghua.edu.cn 3 National Water and Energy Center, UAE University, Al Ain 15551, United Arab Emirates * Correspondence: s.alzuhair@uaeu.ac.ae; Tel.: +971-371-353-19 Abstract: Biodiesel is a promising candidate for sustainable and renewable energy and extensive research is being conducted worldwide to optimize its production process. The employed catalyst is an important parameter in biodiesel production. Metal–organic frameworks (MOFs), which are a set of highly porous materials comprising coordinated bonds between metals and organic ligands, have recently been proposed as catalysts. MOFs exhibit high tunability, possess high crystallinity and surface area, and their order can vary from the atomic to the microscale level. However, their catalytic sites are confined inside their porous structure, limiting their accessibility for biodiesel production. Modification of MOF structure by immobilizing enzymes or ionic liquids (ILs) could be a solution to this challenge and can lead to better performance and provide catalytic systems with higher activities. This review compiles the recent advances in catalytic transesterification for biodiesel production using enzymes or ILs. The available literature clearly indicates that MOFs are the most suitable immobilization supports, leading to higher biodiesel production without affecting the catalytic activity while increasing the catalyst stability and reusability in several cycles. Keywords: biodiesel; metal–organic frameworks; immobilization; lipases; ionic liquids 1. Introduction The need to protect the environment from fossil fuel emissions together with the con- tinuously growing energy needs has led to focus on renewable energy sources [1–9]. Apart from their expected depletion in the future, fossil fuels have unstable prices, intensifying the search for more sustainable and reliable energy sources [10]. Ideal fuel substitutes should possess better properties than conventional fuels, such as renewability, nontoxicity, biodegradability, and less-than-zero release of harmful gases into the environment [11–13]. Possible alternative energy sources include sunlight, wind, and biofuels [14]. Biodiesel is gaining increasing recognition worldwide due to the abundance of vari- ous possible feedstocks [15–20] and its superior properties compared to petroleum diesel, including better cetane number, higher flash point, and zero sulfur content [21]. These benefits along with its almost direct use in the diesel engine have encouraged the replace- ment of petroleum diesel with biodiesel [7,10,21–26]. Biodiesel is mainly produced by the transesterification of triglycerides and the esterification of free fatty acids (FFA) found in vegetable oils and animal fats [27]. However, in biodiesel production, the feedstocks should be carefully selected and the production process should be optimized for economic competitiveness with petroleum diesel production. For instance, use of waste oil instead of pure vegetable oil can effectively reduce production costs. However, such feedstocks suffer from inconsistent availability and collection complexity [14,28–33]. Microalgae appear to be the most promising feedstock for biodiesel production, as they can generate high lipid Molecules 2021, 26, 3512. https://doi.org/10.3390/molecules26123512 https://www.mdpi.com/journal/molecules