A magnetically separable SO₄/Fe-Al-TiO₂ solid acid catalyst for biodiesel production from waste cooking oil

A novel magnetic SO₄/Fe-Al-TiO₂ 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 TiO₂ 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 SO₄/Fe-Al-TiO₂ 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 SO₄/Fe-Al-TiO₂ catalyst. FTIR confirmed the presence of surface sulphate groups SO₄²⁻ and S₂O₇²⁻/S₃O₁₀²⁻, 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. SO₄/Fe-Al-TiO₂ 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.