Catalysis and Inhibition of Transesterification of Rapeseed Oil over MgO-CaO

The catalytic transesterification of rapeseed oil toward biodiesel production was examined over four mixtures of MgO-CaO oxides as heterogeneous catalysts. The mixed oxidic system (100-x)MgO-(x)CaO, with x = 0, 1, 2, and 5 was synthesized via co-precipitation with ammonia in the presence of poly-vinyl-alcohol (PVA). The solid catalysts were characterized by X-ray powder diffraction (XRD), X-ray fluorescence spectroscopy (XRF), diffraction laser scattering (DLS), N-2 adsorption-desorption porosimetry, and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) techniques. The reaction took place in a stirred batch reactor where rapeseed oil was mixed with methanol, n-hexane, and the solid catalyst. Pure MgO oxide showed high catalytic activity, and the methyl ester yield was 97.7% while the gradual addition of CaO resulted in low methyl ester yield in the range 5-63%. This strong inhibition of the catalytic reaction was described phenomenologically by a standard deactivation formula. The rapid drop of activity was attributed to the extensive flocculation of primary MgO crystallites triggered by the addition of CaO (similar to 270 times larger). In this case, the reactants and/or products of the reaction suffer considerable diffusion limitations, probably of anomalous nature, in their way toward the active surface of MgO nanoparticle catalysts buried deep into the labyrinth of flocs made up of several hundreds of such active - but scarcely available - crystallites.