Synthesis of calcium oxide@magnesium oxide nanocatalyst for the transesterification of waste cooking oil

Environmental concerns of fossil-based fuel sources has driven progress towards several sustainable alternatives, such as methyl esters, also known as ‘biodiesel’. The synthesis of biodiesel is done by transesterification, assisted by either homogenous or heterogeneous catalysis. Biodiesel obtained from homogenous catalysis requires purification procedures which yields a significant amount of waste water and the catalyst cannot be reused, thereby driving research towards a reusable alternative in heterogeneous catalysis. This study aims to synthesize calcium oxide and magnesium oxide nanocomposite (CaO@MgO) using microwave-assisted solution combustion technique for heterogeneous catalysis. Sodium hydroxide and potassium hydroxide were used for homogeneous catalysis, to aid the study. The synthesis of biodiesel from both types of catalysis was carried out via transesterification of waste cooking oil. The biodiesel yield was 96.5% for heterogeneous catalysis with a reaction time of 4 h and 3 wt. % catalyst, while the yield was 96.0% for homogenous catalysis with a reaction time of 1 h and 1 wt. % catalyst, and it was found to be easier to separate the former catalyst from the products, utilizing less water in the process. The density, viscosity, and calorific value were similar for biodiesel synthesized via both types of catalysis; however, the cloud point and turbidity were higher for biodiesel synthesized via CaO@MgO nano catalyst. This was however taken care of by precipitation using citric acid and centrifugation. Subjecting this catalyst to ethanol washing and ultra-sonication rendered it reusable up to three times, thereby justifying its use as a sustainable alternative.