Oxygen-functionalized melem as a stable heterogeneous catalyst for free fatty acid esterification in low-grade biodiesel feedstocks

The feasibility of biodiesel production relies on efficiently utilizing low-grade feedstocks rich in free fatty acids (FFAs). However, high FFA content deactivates base catalysts via saponification, requiring pre-treatment with acid catalysts. This study synthesized oxygen-functionalized melem via a single-step acid treatment, employing it as a heterogeneous acid catalyst for FFA esterification. Characterization (SEM, XRD, FTIR, XPS, and solid-state ¹³C NMR) and density functional theory (DFT) calculations identified NO₂, NO, and CO functional groups as key acidity-enhancing sites, achieving 94.3 % oleic acid conversion. The catalyst demonstrated excellent stability over five cycles with minimal active site leaching. Sequential esterification-transesterification with acid-treated melem and Na-GCN resulted in over 90 % biodiesel conversion. DFT calculations confirmed NO₂ functionalization as the most active site (1.47 eV). Implementation in a continuous packed-bed reactor maintained stable FFA conversion for over 100 h. These results highlight oxygen-functionalized melem as a costeffective, durable catalyst for biodiesel production from low-quality feedstocks, advancing sustainable fuel processing.