One-pot transesterification and esterification of waste cooking oil via ethanolysis using Sr:Zr mixed oxide as solid catalyst

Abstract

Mixed oxides of Sr and Zr were prepared by the co-precipitation method and examined as heterogeneous catalysts for the one-pot esterification and transesterification of waste cooking oil with ethanol for the production of fatty acid ethyl esters (FAEE). The mixed oxide of Sr:Zr with a 2 : 1 atomic ratio calcined at 650 °C showed optimum activity among the prepared catalysts. The catalyst possesses both acidic and basic sites, and hence was able to perform simultaneous esterification and transesterification of free fatty acid containing vegetable oils. The transesterification activity was found to be a function of basic sites of the catalyst which in turn depends on calcination temperature and Sr:Zr atomic ratio. A pseudo-first-order kinetic equation was applied to evaluate the kinetics of the 2Sr:Zr-650 catalyzed ethanolysis of waste cottonseed oil and the activation energy (E_a) for the reaction was observed as 48.17 kJ mol⁻¹. The thermodynamic activation parameters of the reaction were evaluated from the Eyring-Polanyi equation and the values of ΔG^‡, ΔH^‡ and ΔS^‡ were found to be 88.23 kJ mol⁻¹, 45.97 kJ mol⁻¹ and −121.37 J mol⁻¹ K⁻¹, respectively. These values suggest that the 2Sr:Zr-650 catalyzed reaction is endothermic, unspontaneous and follows the associative mechanism. The catalyst was recovered by simple filtration from the reaction mixture and reused in four cycles without any significant loss in activity as well as metal leaching from the catalyst in reaction mixture.