Hierarchical Multizeolitic Purification System for Upgrading Waste Frying Oil into High-Quality Biodiesel Feedstock

The development of sustainable biodiesel production depends on cost-effective and renewable feedstocks that minimize environmental impact without compromising fuel quality. Waste frying oil (WFO) is a promising alternative due to its abundance, yet its high content of water, free fatty acids (FFAs), and polymerized oxidation products severely hinders transesterification efficiency. This study introduces a hierarchical multizeolitic purification system engineered to sequentially eliminate these contaminants through targeted adsorption. Four zeolites with complementary pore architectures and acidities—4A (LTA), Beta (BEA), Y (FAU), and ZSM-5 (MFI)—were hydrothermally synthesized from rice husk ash and metakaolin and assembled in a serial adsorption column. Each zeolite was assigned a specific role: dehydration (4A), removal of polymerized and oxidized species (Beta), adsorption of long-chain FFAs (Y), and final polishing of aromatic and oxygenated residues (ZSM-5). The integrated process markedly enhanced WFO quality, reducing moisture from 1166 mg·kg⁻¹ to 200 mg·kg⁻¹, acid value from 2.65 to 0.59 mg KOH·g⁻¹, peroxide value from 14.16 to 4.9 meq O₂·kg⁻¹, and viscosity from 43.1 to 37.1 mm²·s⁻¹, while improving oxidative stability from 4.8 h to 6.7 h. Fatty acid profiling confirmed a reduction in polyunsaturated species and enrichment in thermally stable monounsaturated and saturated fractions. The proposed system demonstrates an efficient, scalable, and environmentally benign approach for converting waste oils into high-quality biodiesel feedstock, aligning with green chemistry and circular economy principles.