Synthesis of fatty acid methyl ester via transesterification of waste frying oil by a zinc-modified pumice catalyst: Taguchi approach to parametric optimization

Abstract

Methanolysis of waste frying oil (WFO) was conducted to synthesize fatty acid methyl esters (FAME) using zinc-modified pumice (ZMP) as a heterogeneous catalyst. The ZMP catalyst was prepared via incipient wetness impregnation method and characterized by various spectroscopic techniques such as Brunauer–Emmett–Teller (BET) surface area, scanning electron microscopy (SEM), energy dispersive X-ray (EDX), X-ray fluorescence (XRF), Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). Taguchi design approach was employed to optimize the transesterification process variables, which include reaction temperature (50–70 °C), reaction time (1–3 h), methanol/WFO molar ratio (6:1–12:1) and catalyst loading (1–3 wt%). The transesterification reaction kinetic and impact of reaction temperature on the reaction rate were investigated. The kinetic data obtained at different temperatures were evaluated using zero-order and pseudo-first-order models. Among the process variables studied, FAME yield was mostly affected by the reaction temperature, methanol/WFO molar ratio, and catalyst loading. The maximum FAME yield of 91.05 wt% was attained at the optimum reaction temperature of 60 °C, 3 wt% catalyst loading, and 12:1 methanol/WFO molar ratio. The better activity exhibited by the ZMP catalyst in the transesterification reaction was attributed to the interaction among several metal oxides present in the catalyst as confirmed by the EDX, XRF and XRD analyses. The catalyst stability study showed that the ZMP could be reused up to four times. The pseudo-first-order model gave the best fit with the catalytic reaction data, and the activation energy was 50.61 kJ mol−1.