Transesterification of sunflower oil on single step sol–gel made Al2O3 supported CaO catalysts: Effect of basic strength and basicity on turnover frequency

The activities of single step sol–gel made calcium oxide on alumina catalysts were studied as a function of CaO loading, methanol/oil molar ratio and the amount of the catalyst in the transesterification of sunflower oil at 50 °C. Also, the turnover frequency (TOF) of the catalysts was calculated to better understand the relationship between the basicity/basic strength and the catalytic activity. From volcano curve (TOF vs. basic strength), it was found that under 50 °C and methanol/oil molar ratio of 9, 60% CaO/Al2O3 had the highest turnover frequency, 0.028 s−1, whereas 85% CaO/Al2O3 showed the highest biodiesel yield, ∼96.6%, but TOF obtained on it was 0.012 s−1. It seemed that 60% CaO catalyst had the proper basic strength to obtain the highest TOF.

► CaO/Al2O3 single step sol–gel made catalysts for sunflower transesterification.
► Sunflower transesterification at 50 °C.
► Basicity and basic strength using CO2 temperature programmed desorption.
► Volcano plot using turnover frequency and basic strength.
► High turnover frequency does not mean high biodiesel yield at 50 °C.
► 60% CaO/Al2O3 has high TOF, 0.028 s−1, but low biodiesel yield, ∼80%.