Sustainable Processes Reusing Potassium-Rich Biomass Ash as a Green Catalyst for Biodiesel Production: A Mini-Review

To mitigate the emissions of greenhouse gases (GHGs) from fossil fuels, the use of biodiesel and its sustainable production have been receiving more attention over the past decade, especially for the reuse of waste cooking oils and non-edible oils as starting feedstocks. For the biodiesel production process, the suitability of a green catalyst is a core function in the transesterification reaction. Heterogeneous (solid-state) catalysts are generally superior to homogeneous (liquid-state) catalysts due to several significant advantages such as no saponification products formed, recyclability, and less equipment corrosion. Recent studies also revealed that heterogeneous solid base catalysts were widely used for the production of biodiesel. Furthermore, the use of biomass-based ash derived from herbaceous and agricultural biomass is increasing rapidly because of its environmental sustainability, high biodiesel yield, and low catalyst cost. To highlight alternative catalysts from biomass residues, this mini-review paper thus focused on a summary of various heterogeneous potassium-rich ash materials, which were used as green catalysts for the sustainable production of biodiesel. Due to the abundant quantity and chemical compositions, it was found that ash derived from cocoa pod husk may be the most commonly used solid base catalyst for producing biodiesel in the literature. Finally, future perspectives on biodiesel production by adopting emerging technologies and using high-potassium (K) biomass ash as a green catalyst were also addressed.