Calcium oxide functionalized with strontium as heterogeneous transesterification catalyst for biodiesel production

• Four different preparation methods were compared to functionalize CaO with Sr.
• Preparation methods significantly affected the catalyst stability.
• The basicity was found to parallel catalytic activity in transesterification.
• High catalytic activity of 0.5Sr/Ca-ICP900 was attributed to the SrxCa1−xO and SrO2.

The application of calcium oxide (CaO) as transesterification catalyst for biodiesel production is restricted due to the requirement of long reaction time and high molar ratio of methanol to oil. To solve this problem, CaO was functionalized with strontium (Sr/Ca catalyst) through the solid mixing method, wet impregnation method, co-precipitation method and improved co-precipitation method, respectively. Stability of the prepared catalyst, including the homogeneous contribution of leached active sites and the catalyst reusability, was investigated to screen the optimal preparation method, and the improved co-precipitation method was proved to be the most ideal method for the strongest interaction between Sr and Ca. The Sr/Ca catalysts were then characterized by X-ray diffraction (XRD), thermogravimetric analysis–derivative thermogravimetric (TGA–DTG), Hammett indicator (basic strength and basicity) and Fourier transform infrared spectroscopy (FTIR). Moreover, effects of the activation temperature and Sr/Ca molar ratio on catalytic activity were concerned. At activation temperature of 900 °C and Sr/Ca molar ratio of 0.5, the maximum conversion 98.31% of palm oil to fatty acid methyl esters could be achieved under the mild transesterification condition of 5 wt.% catalyst amount (based on oil weight) and 9 molar ratio of methanol to oil at 65 °C for 30 min.