Synthesis of biodiesel from residual oil extracted from spent bleaching earth using spent bleaching earth-supported catalyst

Abstract

Proper disposal of spent bleaching earth (SBE) is much anticipated to create a circular economy. In this study, a two-pronged approach was employed to utilise SBE from palm oil refinery for its suitability to act as second-generation biofuel feedstock and catalyst. The SBE was first wet-impregnated and doped with potassium hydroxide (KOH) to further enhance the catalytic activity. The hexane-extracted bleached oil of SBE at conditions of 40 g SBE, 1:4 ratio of oil to n-hexane, and 6 hr extraction time was subjected to different transesterification parameters i.e., catalyst loading, oil: methanol ratio, reaction temperature, and reaction time. The obtained catalysts were characterized using Fourier transform infrared spectroscopy, thermogravimetric analysis, field emission scanning electron microscopy with energy dispersive X-ray, temperature programmed desorption of carbon dioxide (TPD-CO2) and Brunauer-Emmett-Teller (BET) techniques. The N2 adsorption-desorption and TPD-CO2 analyses indicated that SBE/KOH (600 °C) had the highest attainable BET surface area (22.62 m2/g) and basicity (25.82 cm3/g). SBE/KOH (600 °C) exhibited the highest biodiesel yield (80.3%) at the optimised reaction conditions: (80 °C, 2 wt% catalyst loading, 1:20 oil: methanol ratio and 2 h reaction time) compared to 67.3% via the unmodified control. The kinetics study revealed that the experimental results followed the first order reaction kinetics. The activation energy, enthalpy (ΔH) and entropy (ΔS) were determined to be 90.06 kJ/mol, 6.92 × 10−4 kJ/mol and −0.0414 kJ/mol K, respectively while the Gibbs free energy (ΔG) was 15.443 kJ/mol. Overall, SBE has enormous potential as one of the alternative sources for second-generation biofuel production.