Enriched montmorillonite K10 with Ca-Mg mixed oxides as a reusable catalyst for waste cooking oil biodiesel synthesis

The utilization of low-quality feedstock (like waste cooking oil, WCO) and an efficient heterogeneous catalyst is the key to ensuring sustainable production of biodiesel. This current study considered Ca-Mg mixed oxides modified montmorillonite K10 (CaO-MgO@MK10) as solid base catalyst for fatty acid methyl ester (FAME) synthesis via methanolysis of WCO. The CaO-MgO@MK10 catalysts, which were formulated by coprecipitation-impregnation route and followed by calcination at different temperatures (600, 700, 800 and 900 °C), were thoroughly characterized using various analytical techniques. The strong interaction between aluminosilicate content of MK10 and Ca/Mg metals enhanced the basicity of the CaO-MgO@MK10 sample calcined at 700 °C (CaO-MgO@MK10–700) and facilitated a better exposure of oxygen atoms on the catalyst surface which contributed to the formation of methoxide anions as a nucleophilic attack on the triglyceride molecules. This unique attribute contributed towards accelerating the methanolysis of WCO to achieve 97.8% FAME yield at 80 °C for 3 h with WCO/CH₃OH molar ratio of 1:15 and CaO-MgO@MK10–700 loading of 2.5 wt%. The transesterification kinetic data fitted well to the pseudo-first-order model with rate constant of 0.926 h⁻¹ at 80 °C and 87.56 ± 5.83 kJ∙mol⁻¹ activation energy. Additionally, the CaO-MgO@MK10–700 was reused six times, with almost 80% conversion to FAME at the 6th cycle, indicating that the catalyst was relatively stable and had reusability potential.