Bifunctional heterogeneous catalysts for enhanced single-step biodiesel production from high free fatty acid-containing oils: A review

The biodiesel industry primarily relies on homogeneous strong base catalysts. However, these catalysts are sensitive to free fatty acid (FFA) levels above 2 wt% in oils. These oils require acid-catalyzed esterification of FFA prior to transesterification. Consequently, the entire biodiesel production process becomes a two-step. The first step is FFA esterification with acid catalysts, followed by triglyceride transesterification with base catalysts. Therefore, this two-step process increases biodiesel production costs and complexity. Bifunctional catalysts, such as acid–base bifunctional and Lewis-Brønsted acid bifunctional, can eliminate the two-step esterification and transesterification processes by carrying them out simultaneously. Several reviews have addressed bifunctional catalysts for biodiesel production; this review provides an integrated perspective on the role of FFAs in catalyst selection, the limitations of base-catalysis for high-FFA oils, and the potential of acid and bifunctional catalysts for single-step biodiesel production. Therefore, this review highlights trends in catalysts for biodiesel production, their overall advantages and limitations for biodiesel production from high FFA-containing oils, and focuses mainly on the current state and use of bifunctional heterogeneous catalysts for single-step biodiesel production from high FFA-containing oils. In addition, biomass-based heterogeneous catalysts and common heterogeneous catalyst synthesis methods are also discussed. Furthermore, this review paper outlines future research directions to address gaps in the field of bifunctional catalysts for sustainable biodiesel production from non-edible feedstocks.