All catalysts were prepared by a dry impregnation method using an aqueous solution of KNO3 and CaO powder. Firstly, 10.0 g of the commercial CaO (powder form) was dried at 100 °C for 3 h to remove the physisorbed water. Then, it was mixed with an aqueous solution of KNO3 (25 mL and the amount of KNO3, for example, was 3.22 g) in a glass flask. After stirring for 15 min, the mixture was set for 24 h at room temperature, and then dried at 100 °C for 4 h to remove water through evaporation. Subsequently, the mixture was grinded and sieved using the standard sieve with 100 mesh. The obtained solid powder was calcined at specified temperature (for example, 700 °C) for 5 h under static air. A heating ramp of 5 °C/min was employed in this step. Finally, the obtained catalyst was used in the transesterification of GL with DMC. All the prepared catalysts are denoted as KNO3/CaO (n%, m), where n% is the weight percentage of KNO3 loaded on CaO and m is the calcination temperature. It should be noted that the KNO3 loading was calculated using the corresponding amount of K2O with KNO3, which was obtained by the initial amount of KNO3 based on that there has equal K element amount between K2O and KNO3.
The K2O/CaO(15%, 700) catalyst also was prepared by mechanical mixing method. Firstly, the commercial KNO3 was calcined at 900 °C for 8 hours with a heating ramp of 5 °C/min to obtain the K2O [34]. Then, 10.0 g of the commercial CaO (power form) and 1.5 g of the prepared K2O were mixed and grinded adequately for 30 min. The mixture was dried at 100 °C for 4 h, and then was grinded and sieved using the standard sieve with 100 mesh. The obtained solid powder was calcined at 700 °C for 5 h under static air with a heating ramp of 5 °C/min. Finally, the obtained catalyst was also used in the transesterification of GL with DMC.