Experimental and theoretical analysis revealing the underlying chemistry accounting for the heterogeneous transesterification reaction in Na₂SiO₃ and Li₂SiO₃ catalysts

The influence of sodium and lithium content in the metasilicate crystal structure (SiO₃²⁻) is herein analyzed concerning the heterogeneous transesterification reaction for biodiesel production. Na₂SiO₃ and Li₂SiO₃ were characterized structurally and microstructurally. The catalyst content was evaluated between 1 and 5 wt %, where the maximum conversions to FAME (∼99%) were obtained using 3 wt % of Na₂SiO₃ during 1 h at 65 °C. After some cyclic experiments, it was determined that Na₂SiO₃ possesses a better stability and consequently reutilization capacity than Li₂SiO₃ in terms of its triglycerides conversion to FAME. DFT calculations were then used to analyze these experimental differences, revealing significant differences between these two catalysts in terms of energy, geometrical configuration, and electronic structure. It was found that three active sites are required on both catalytic surfaces to overcome the methanol deprotonation, which is herein suggested as the rate-controlling step of the entire transesterification mechanism. In one site, the oxygen atom of methanol approaches, while Lewis acid and Brønsted base sites are needed for the methoxide anion stabilization and proton stabilization, respectively.