Determination of active sites on Na₂SiO₃ and Li₂SiO₃ catalysts for methanol dissociation and methoxide stabilization concerning biodiesel production

Surfaces of sodium and lithium silicates are theoretically investigated to account for the role of dissimilar atoms, towards the methanol adsorption/dissociation within the transesterification reaction producing biodiesel in heterogeneous processes. The definition of active site is elucidated on these materials on the basis of the following calculations: electronic density, electrostatic potential, charges, geometrical parameters and density of states (total and projected) computed with periodic Density Functional Theory (DFT). The structural requirements of a catalytic surface for biodiesel production are analyzed relying on different atomic configurations of the highest intensity peaks, according to experimental X-ray analysis. This information reveals that the alkaline metals (Na, Li) on the surface participate in the methanol adsorption, subsequently; Si acts as Lewis acid site to stabilize the methoxide anion, while the oxygen atoms perform as Brönsted basic sites to abstract the proton from methanol. This constitutes the early stages of the transesterification mechanism, and possibly the rate-controlling step (RCS) of the biodiesel generation. In the sodium silicate, the alkaline metal possesses the ability to stabilize the methoxide anion through electrostatic interactions unlike the lithium silicate.