Esterification as well as transesterification of waste oil using potassium imbued tungstophosphoric acid supported graphene oxide as heterogeneous catalyst: Optimization and kinetic modeling

Abstract

For one pot esterification as well as transesterification of waste oil (WO), K+ impregnated tungstophosphoric acid (TPA) was supported over graphene oxide (GO) and used as a heterogeneous catalyst. The structure of the catalyst was evaluated by a powder X-ray diffraction (XRD) study. The elemental oxidation state and catalyst composition was confirmed by X-ray Photoelectron Spectroscopy (XPS) analysis. The temperature-programmed desorption (TPD) technique supported the existence of acidic and basic sites on the surface of the catalyst. Being acidic, TPA probably catalyzes the esterification while K+ transesterification activity to the catalyst. Beneath the optimized reaction conditions, i.e. loading of catalyst 10 wt% (concerning oil), methanol: oil molar ratio of 9:1 with reaction temperature of 65 °C, >98.5% (±0.34) biodiesel was produced within 1.5 h of reaction period. By using centrifugation, the catalyst was extracted from the reaction mixture and recycled six times. The metal level in the reaction mixture was determined to be less than 2 ppm, supporting the stability of the catalyst. Additionally, the catalyst was found to be active even in the existence of free fatty acid (up to 8.76 wt%) and moisture (up to 4.0 wt%) contents. The positive values of enthalpy of activation (ΔH‡) and free energy of reaction (ΔG‡) highlighted the endothermic and non-spontaneous nature of the reaction. The kinetic modeling study, using MATLAB software (version R2021b), suggests that the reaction of WO with the synthesized catalyst was found to follow a (pseudo) first-order kinetic equation. A negative value of entropy of activation (ΔS‡) indicates that the reaction of triglyceride (T) to diglyceride (D) follows the associative pathway. In contrast, the diglyceride to monoglyceride (M) and monoglyceride to glycerol (G) conversion progressed via dissociative route.