Review of advances in bifunctional solid acid/base catalysts for sustainable biodiesel production

Biodiesel has attracted significant attention as a potential renewable and environmentally friendly substitute for petroleum-derived diesel fuel. Conventionally, commercial biodiesel is produced via liquid base-catalyzed transesterification of triglycerides components of oil/fat with short-chain alcohols. However, homogeneous base catalysts are very sensitive to free fatty acids present in low-quality oil feedstocks, requiring additional acid pretreatment and neutralization steps which not only cause environmental pollution but also increase the overall cost of biodiesel production. Encouragingly, solid catalysts can provide a green, efficient, and economical pathway for biodiesel production using low-cost oil feedstocks such as waste cooking oil. The present review discusses recent advances in the synthesis of bifunctional solid acid/base catalysts for sustainable biodiesel production with a focus on solid catalysts bearing acid-base bifunctionality or Bronsted-Lewis dual acidity and the synergistic effect of the active sites for simultaneous esterification of free fatty acids and transesterification of triglycerides. Furthermore, this review article also provides future research directions that can guide scientists to address research gaps hampering the synthesis of effective bifunctional solid acid/base catalysts for sustainable biodiesel production.