Esterification of Benzaldehyde Catalyzed by Alkaline Earth Oxides

When benzaldehyde was treated at 120-200 C over BeO, MgO, CaO, SrO or BaO without any solvent, a large amount of benzyl benzoate and a very small amount of benzyl alcohol were found as the raction products. The time variation of the ester formation gave S-shaped curves including induction periods (Fig.1). After the induction period, the reaction obeyed the first order rate equation with respect to the concentration of benzaldehyde (Fig.3). The induction periods were shortened remarkably by the addition of benzyl alchohol (Fig.6). The catalytic activity was found to increase in order of BeO MgO CaO SrO BaO (Table 1)and the activation energy and entropy were 9.4 kcal/mol and -49 e. u. for CaO and 54 kcal/mol and 54 e. u. for SrO, respectively. A very good correlation was found between the catalytic activity and surface basicity measured by an indicator method (Fig.4). ln the case of CaO, the highest activity was observed for CaO obtained by calcining Ca(OH)2 at 400 C or by thermal decomposition of CaCO3 at 900 C (Table 2), theugh CaO obtained by calcining Ca(OH)2at 900 C did not show any activity. On the basis of the observed results, it has been concluded that the active species of MgO and CaO are the metal alcoholates formed by the reaction of benzyl alcohol with the catalyst surface and the formation of the alcoholate is facilitated by the basic sites(O2- and OH-)on the surface. The reaction mechanism was also discussed.