Biodiesel Synthesis from Used Vegetable Oils using Sulfonated Nitrogen Doped Bifunctional Catalyst

The scalability of biofuels is limited by the high cost and ethical concerns associated with edible oils, whereas inedible oils with elevated free fatty acids can undergo saponification using conventional basic catalysts. Multistep processes (using acid and base catalyst stepwise) to overcome this remain commercially unattractive. Acid–base bifunctional catalyst serves as a viable alternative owing to the synergistic role of its active sites for one-step transesterification-esterification under mild reaction conditions. Herein, N-doped sulfonated biochar catalyst was designed through a glucose–urea-mediated hydrothermal carbonization, followed by activation and sulfonation using ZnCl2 and 4-benzenediazonium sulfonate (4-BDS), respectively, thereby enabling precise modulation of both acidic and basic active sites. The optimized SNBC catalyst exhibited high acidity (5.16 mmol g–1) and basicity (4.57 mmol g–1), enabling one-pot transesterification–esterification of used vegetable oil. Consequently, a maximal 98% biodiesel yield was attained under mild reaction environments (82.3 °C, 19.2:1 methanol to oil molar ratio, 5 wt % catalyst loading, 25.1 min), meeting the key specifications of ASTM D6751 and EN 14214. Moreover, the catalyst exhibited improved catalytic stability with comparatively better reusability (sixth cycle, >90%) than reported bifunctional catalysts. Green metrics assessment indicated environmentally benign nature of the process. Life Cycle Cost Analysis evaluated the biodiesel production cost of 0.59 USD L–1 underscoring the potential of SNBC as a sustainable and cost-effective catalyst for scalable biodiesel production.