The formation of calcium glycerolate as an active species in the synthesis of biodiesel. A DFT study

Numerous studies have reported that the addition of glycerol at the onset of an oil transesterification reaction with methanol, catalyzed by CaO, significantly accelerates the reaction. The role of glycerol, which is also a reaction product, is not yet fully understood. The formation of glycerolate structures, in both solid and polymeric forms, has been proposed to actively contribute to the reaction rate. In previous works, we have simulated the formation of incipient OH and ethoxide species on the CaO during ethanol adsorption, and bidentate species between formic acid and the surface. In the present work we have investigated, using DFT-based simulations, the possible initial stages in the formation of glycerolate on CaO (001) surfaces. Our results indicate the bonding of two OHs from glycerol with the surface. We also detected the possible rupture of these OHs by a strong decrease in the bonding order (BO), up to 75%. At the same time, the formation of O_glycerol―Ca bonds and the decrease in the BO of the surface Ca―O bonds of up to 61% are detected. A charge transfer from the surface to the molecule is also observed. Additionally, a detailed analysis of the changes in the geometry and electronic structure of the glycerol and the surface before and after adsorption is conducted.