Two-Stage Optimization of Oleic Acid Esterification via Response Surface Methodology and Green Chemistry Metrics Selection for Biodiesel from WCO

Abstract Biodiesel production from used cooking oil (WCO) is a sustainable alternative to fossil fuels, but its high free fatty acid (FFA) content (>5 wt %) makes direct basic transesterification difficult. This study presents an integrated two-stage strategy that optimizes the acid esterification of oleic acid (the dominant FFA in WCO) using response surface methodology (RSM, Box–Behnken) and then evaluates downstream transesterification, while reporting green chemistry metrics alongside conversion. The RSM model identified an optimum at a methanol-to-oleic acid ratio of 8.44:1, 2 wt % H2SO4, and 363 min. Kinetic validation showed that conversion reaches ∼94% within 90 min and then plateaus, indicating a practical endpoint that reduces energy and time with minimal yield loss. Applied to a WCO feed (8.12 wt % FFA), the pretreatment reduced FFA to ∼2.4 wt %, enabling transesterification of the base to ∼98 wt % FAME with <0.5 wt % residual FFA. The final biodiesel met ASTM D6751 and EN 14214 specifications and exhibited a favorable FAME profile (∼75.6% methyl oleate). Green chemistry analysis clarified the trade-offs of these results, with the esterification showing high theoretical atom economy (∼94%), while the transesterification provided a low E-factor (∼0.26) and high atom efficiency (∼87.7%) in a wash-free gravity settling process with methanol recovery/reuse. Treatment of crude glycerol as a byproduct further improves the material efficiency. The results demonstrate a robust and scalable pathway for WCO biodiesel that combines statistical optimization with sustainability assessment, delivering high conversion with reduced material intensity and waste, and offering an adaptable framework for other high FFA feedstocks.