Eco-Friendly Biodiesel Synthesis via Microbubble-Aided Transesterification and Leaf-Derived Green Carbon Catalyst

This study presents an innovative and sustainable approach to biodiesel production by introducing Bismarckia nobilis leaf—an underutilized biomass—as a novel precursor for synthesizing a green carbon heterogeneous catalyst. The catalyst was developed through an optimized process involving controlled carbonization and acid treatment, and its application in a custom-designed bubble reactor significantly enhanced mass transfer and mixing efficiency compared to conventional batch systems. Green carbon was synthesized from waste Bismarckia nobilis leaf pretreated with hydrochloric acid and carbonized at several temperatures (250 °C, 350 °C, and 450 °C) and periods (60–120 min), followed by washing with different acids (hydrochloric acid, citric acid, and acetic acid). Characterization employing Fourier transform infrared spectroscopy for functional group analysis, Brunauer–Emmett–Teller for surface area measurement, and scanning electron microscopy for morphological evaluation revealed that the sample treated at 350 °C for 90 min and washed with hydrochloric acid exhibited the maximum surface area and well-developed mesoporosity, making it optimal for catalytic applications. This optimized waste Bismarckia nobilis leaf-derived green carbon served as a heterogeneous catalyst in the transesterification step of a dual-stage biodiesel production process, following acid-catalyzed esterification for free fatty acids reduction. The process achieved 92.86% biodiesel yield from waste cooking oil within 35 min using only 0.5 wt.% catalyst, outperforming most previously reported green catalysts under longer reaction times and higher catalyst loadings. This work presents a sustainable and economically viable approach for synthesizing a value-added functional catalyst from waste biomass through acid pretreatment and low-temperature carbonization, supporting waste valorization and biodiesel production.