Biodiesel production using heterogeneous catalyst derived from natural calcite stone: Study of the effect of Mg–Zr doping and reaction conditions

The aim of this study is to examine the impact of doping Mg–Zr on CaO derived from natural calcite, which are utilized as heterogeneous catalysts to produce biodiesel through the transesterification reaction of canola oil and methanol. The catalysts were synthesized via the wet impregnation method with varying concentrations of Mg–Zr (2.5, 5, 7.5, and 10 wt%) over the calcite, (Mg/Zr mass ratio of 2:1), followed by calcination at 600 °C. Physicochemical analyses, encompassing XRD, BET, SEM, EDX, and TGA, were employed for catalyst characterization. The samples' basicity was determined through titration. Operational parameters, including catalyst loading, methanol-to-oil ratio, reaction time, and temperature, were systematically explored. The stability and reusability of the optimal catalyst were also assessed. With the increase in the mass ratio of Mg–Zr to CaO, the total number of base sites on the catalyst increases, and maximum is achieved for the 7.5%(Mg–Zr)/CaO catalyst compared to other catalysts. Also observed a similar pattern in the variation of biodiesel yield, indicating a strong correlation between catalytic activity and the overall number of basic sites present on the catalyst surface. Results indicated that, under specific conditions (7 wt% catalyst loading, a methanol-to-oil molar ratio of 12:1, and a reaction temperature of 60 °C for a 3-h duration), the highest content of fatty acid methyl esters (FAME) reached 96.7% over on 7.5%Mg–Zr/CaO. The optimum catalyst exhibited robust stability and reusability across four consecutive reaction cycles, with the produced biodiesel meeting the standards of EN 14214 and ASTM D6751.