Synthesis of superacid sulfated TiO₂ nanowires for esterification of waste cooking oil

Sulfated TiO₂ nanowires (STNW) were prepared by molten salt method from anatase TiO₂ followed by sulfatation using ammonium sulfate precursor. The phases, morphologies, surface properties and acidity of the STNW were characterized using X-ray powder diffraction (XRD), scanning electron microscope-energy dispersive X-ray (SEM–EDX), nitrogen physisorption, and temperature-programmed desorption of ammonia (NH₃-TPD), respectively. STNW has a rutile structure and shows a needle-like one-dimensional (1D) nanowires morphology with mean crystal size calculated from Scherrer equation was 53.35 nm. The catalytic activity of the as-prepared STNW for esterification of waste cooking oil (WCO) was performed in comparison to sulfated anatase TiO₂ (STA). The STNW catalyst exhibited higher conversion and yield compared to STA. Moreover, the reaction rate of STNW catalyst was 1.3 times higher than STA although the total acidity of STNW catalyst was slightly lower than STA. The superior catalytic activity of STNW was corresponds to its higher temperature acid sites of STNW compared to STA, indicating a super acidity of the catalyst. The optimum reaction conditions of 3 h reaction time, 1:26 molar ratio of WCO to methanol, 3 wt% catalyst loading to WCO and 65 °C reaction temperature give the catalytic conversion of 99.5%. The result implies that STNW act as promising acidic heterogenous catalyst for the esterification of free fatty acids with good reusability. Graphical abstract: [Figure not available: see fulltext.].