Microwave-assisted synthesis of a sulfonated carbon-based catalyst for efficient esterification of oleic acid into biodiesel

Abstract

Carbon-based solid acid catalysts have emerged as an efficient alternative for the esterification of non-edible oils into biodiesel. However, their preparation often relies on post-synthetic sulfonation with harsh acids or high-energy-demanding methods, such as hydrothermal carbonization. In this work, a sulfonated carbon catalyst derived from sucrose and p-toluenesulfonic acid was developed via a one-pot hydrothermal microwave-assisted method, in search of a more sustainable and energy-efficient catalyst route. A synthesis study was performed by varying the sucrose : p-toluenesulfonic mass ratio (1 : 0.5–1 : 2), temperature (140–220 °C), and reaction time (10–30 min). The synthesis conditions 1 : 1.5 mass ratio, 180 °C, and 20 minutes were selected as optimal as they provided a favourable balance between catalytic activity and energy consumption, achieving 90.2% conversion under preliminary esterification conditions (1 : 9 oleic acid-to-methanol, 90 °C, 3 h, 5 wt% catalyst). Characterization confirmed the successful incorporation of sulfonic groups (–SO₃H density of 0.20 mmol g⁻¹). Esterification parameters were further optimized using a one-variable-at-a-time approach, evaluating the effects of oil-to-methanol molar ratio (1 : 6–1 : 30), catalyst loading (1–7 wt%), temperature (70–90 °C), and time (0.5–5 h). Under optimal conditions (1 : 18 molar ratio, 5 wt%, 90 °C, 30 min), a conversion of 91% was achieved, demonstrating strong performance at short reaction times. Reusability tests showed a significant drop in performance after the second cycle, likely due to the leaching of active sites. However, the catalyst's low cost, fast synthesis, and strong performance under mild conditions render it a valuable option for more sustainable biodiesel production.