One Pot Esterification as Well as Transesterification of Waste Cooking Oil using Potassium and Phosphotungstic Acid Supported Silica: Optimization and Kinetic Modeling

In order to prepare the heterogeneous catalyst, phosphotungstic acid (PTA, H 3 PW 12 O 40 ) and K + were supported over silica matrix for simultaneous transesterification as well as esterification of waste cooking oil (WCO). The catalyst structure was determined by powder X-ray diffraction (XRD) analysis. The composition and the elemental oxidation state were analyzed by X-ray photoelectron spectroscopy (XPS) study, while the surface morphology was recognized by the high resolution transmission electron microscopy (HR-TEM). The presence of acidic as well as basic sites over the catalyst surface was studied by the temperature programmed desorption (TPD) technique. PTA, being acidic, is expected to provide the esterification while potassium oxide is responsible for the transesterification activity of the catalyst. In order to optimize the biodiesel yield, the effect of various reaction parameters was studied. At a 10 wt % catalyst content relative to oil, 12 : 1 methanol to oil molar ratio and 65°C reaction temperature, 98% fatty acid methyl ester (FAME) yield was obtained within 3 h of reaction duration. The catalyst was recovered via centrifugation from the reaction mixture and recycled during 6 consecutive runs. The metal concentration in the reaction mixture was found to be less than 2 ppm to verify the catalyst stability. Moreover, the catalyst was found to be effective even in the presence of high free fatty acid (up to 8.8 wt %) and moisture (up to 4 wt %) contents to prove its usefulness for biodiesel production even from low-quality vegetable oils. The kinetic modeling analysis showed that the reaction of waste oil with the prepared catalyst followed a first-order kinetics and was temperature dependent. Using the Arrhenius equation, activation energy and frequency factor were found to be 33.7 kJ/mol and 26.0 s –1 respectively. The prepared biodiesel fuel fulfill the parameters stipulated as per EN 14214 standard.