DFT investigations for “Fischer” esterification mechanism over silica-propyl-SO3H catalyst: Is the reaction reversible?

For the first time, the mechanism of Fischer esterification between acetic acid and ethanol over silica-functionalized propylsulfonic acid (silica-propyl-SO 3 H) catalyst was explored by means of computational modeling techniques. For this purpose, 6-edge-atom cage-like cluster comprising Si–O–Si sequences has been selected to represent the surface of the catalyst. The results indicate that the reaction goes through concerted transition states. In all optimized structures no proton (H + ) transfer occurs from catalyst to the substrates and the role of the catalyst is via the activation of the substrates through the formation of strong hydrogen bonds (H-bonds). Furthermore, the energetic diagram demonstrates that the activation energies of forward (esterification) and reverse (ester hydrolysis) reactions only differ by 0.3 kcal/mol. The quantum theory of atoms in molecules (QTAIM) analysis was performed to investigate the nature of H-bonds. Another mechanism in which the catalyst has been acted as the Bronsted acid/base was also provided for this reaction.