Synthesis of Novel Core-Shell Ti/Zn-FeSi Sulfated Nanocatalysts for Biodiesel Production from Waste Cooking Oils

There is urgent need to transition away from fossil fuel-based modes of transportation. Although biodiesel has been argued as a cleaner burning and lower carbon impact fuel compared to conventional diesel, common animal/vegetable fatty feedstocks present undesirable food versus fuel competition. Upgrading waste cooking oil (WCO) could be a solution providing resilient catalysts are developed that tolerate high free fatty acid (FFA) and moisture content typically present. Acidic nanocatalysts can offer both high FFA tolerances and easier recoverability of heterogeneous catalysis alongside improved reactivities typically only achievable via conventional homogeneous base catalysis.

This work explores six novel mixed metal oxide nanocatalysts. Core 100nm SiO₂ nanospheres were coated with intermediate α-Fe₂O₃ shell around which wurtzite ZnO or anatase TiO₂ was coprecipitated. Surface functionalisation with 0.1M, 0.55M or 1.0M chlorosulfonic acid then increased Brønsted-Lowry acidity. While many sulfated metal oxide nanocatalysts for biodiesel production have been explored, to date this specific core-shell configuration remains unreported in literature. Various analytical methods were utilised to determine nanocatalyst properties: BET, CHONS, DLS-Zetapotential, FTIR, STEM-EDX, TGA-DSC, XPS and pXRD. Characterisation confirmed production of highly crystalline nanoscale catalysts loaded with surface sulfonic acid.

Oleic acid FFA was explored via Design of Experiments approach, with 0.1M-TiFeSi and 0.55M-TiFeSi showing highest reactivity. Approximately ~100% methyl oleate yield was obtained with 0.55M-TiFeSi after 6.75 hrs, 60 °C, 3.19 wt% catalyst and 16:1 methanol to oleic acid. In two-step reaction with degraded WCO, 0.55M-TiFeSi pretreatment before KOH transesterification gave +30.6% yield enhancement. While 4 cycles of 0.55M-TiFeSi esterification alone could achieve similar yields to one step KOH route, the two step enhancement effect ended after first run. FTIR confirmed loss of S-O, S=O and H-O-H peaks indicating acid leaching explained weak reusability. Future work will optimise biodiesel production via two step pathway and explore methods to reduce leaching or regenerate acidity.