Acidic Ionic Liquids for Catalytic Transesterification and Esterification in Biodiesel Production

ABSTRACT Acidic ionic liquids (AILs) are especially suitable for processing acidic oil feedstocks, as they exhibit tolerance toward free fatty acids (FFAs) and water and are capable of catalyzing both transesterification of triglycerides and esterification of FFAs in biodiesel production. This review systematically examines both homogeneous and heterogeneous AIL catalytic systems. For homogeneous systems, we analyze Lewis AILs, Brønsted AILs, and Brønsted–Lewis AILs, discussing their synthesis routes, acidity regulation strategies, and catalytic mechanisms. In heterogeneous systems, we comprehensively review AILs functionalized on various supports, including silica materials, magnetic nanomaterials, polymeric frameworks, and metal–organic frameworks (MOFs), with particular focus on their immobilization strategies, structural stability, and enhanced recyclability. Furthermore, we extend to incorporate emerging approaches beyond conventional catalyst analysis such as hierarchical pore design, energy‐assisted intensification, and machine learning (ML) optimization. Finally, the main challenges of AILs currently faced in the field of biodiesel production are also put forward.