Modified Amberlyst 15 for continuous glycerol-free biodiesel production from rice bran oil fatty acid distillate

This study developed a modified Amberlyst 15 (A15) catalyst for the production of glycerol-free biodiesel from rice bran oil fatty acid distillate (RBOFAD), a low-quality feedstock with high free fatty acid content. A15 was selected as the catalyst platform because of its strong Brønsted acidity and good stability; however, its direct application is limited by textural constraints and restricted access to acid sites. To overcome these limitations, A15 was modified through thermal treatment followed by acid-site regeneration to enhance both pore accessibility and catalytic functionality. The optimized modified catalyst exhibited a 140.5% increase in surface area and an 11.6% increase in acid density compared with the as-received A15. The catalyst was then applied to the continuous production of biodiesel, using dimethyl carbonate as a glycerol-free acyl acceptor. The effects of key operating variables were systematically investigated and optimized using a design-of-experiments approach. Under optimal conditions of 120 °C, a residence time of 60 min, and a DMC-to-RBOFAD weight ratio of 3 : 1, the modified catalyst achieved a % FAME of 87.5%. In addition, the catalyst showed good preliminary operational stability over 30 h of time-on-stream. The influence of Zn loading on the modified A15 catalyst was also evaluated, but it did not further improve its catalytic performance. Benchmarking against related catalyst-based biodiesel production systems showed that the modified A15 catalyst provided competitive performance under substantially milder operating conditions. These findings demonstrate that catalyst modification combined with acid-site regeneration is a practical and effective strategy for improving continuous glycerol-free biodiesel production from low-quality, high-FFA feedstocks.