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A novel magnetic SO 4 /Fe-Al-TiO 2 solid acid catalyst was synthesized for biodiesel production via the (trans)esterification of waste cooking oil (WCO). The nanocomposite catalyst was prepared by the sequential functionalisation of commercial rutile/anatase mixed phase TiO 2 nanoparticles (NPs) with alumina as a buffer layer, and subsequently hematite to impart magnetic character, prior to sulfation with chlorosulfonic acid to introduce Brønsted acidity. XRD showed that the SO 4 /Fe-Al-TiO 2 catalyst comprised titania (rutile and anatase phases), aluminium sulphate, and hematite nanoparticles, while electron microscopy revealed the layer-by-layer assembly of these components within the SO 4 /Fe-Al-TiO 2 catalyst. FTIR confirmed the presence of surface sulphate groups SO 4 2− and S 2 O 7 2− /S 3 O 10 2− , creating a predominantly Brønsted acid catalyst with high acid loading. The catalyst achieved 96% fatty acid methyl ester (FAME) yield from WCO after 2.5 h of reaction at 90 °C, using 3 wt% of the magnetic catalyst, and a methanol:oil molar ratio of 10:1. SO 4 /Fe-Al-TiO 2 was also effective for feedstocks containing up to 20 wt% of free fatty acid (FFA), and showed excellent stability for WCO (trans)esterification over 10 recycles. [ABSTRACT FROM AUTHOR]