3.2.1. Effect of calcination temperature
To determine the influence of the calcination temperature on the activity of the eggshell-derived catalyst, eggshell was calcined at different temperatures between 200 °C and 1000 °C and then tested for the transesterification of soybean oil to produce biodiesel. These results showed that eggshell sample calcined above 800 °C was the most active catalyst. A yield of 97–99% was obtained in the presence of eggshell sample calcined above 800 °C. When the calcination temperature was 700 °C, a yield of 90% was achieved, whereas, a low level of activity was observed when the calcination temperature was