Room-Temperature Suzuki–Miyaura Reaction Catalyzed by Pd Supported on Rare Earth Oxides: Influence of the Point of Zero Charge on the Catalytic Activity

Abstract

Five Pd/REO (rare earth oxide) compounds (Pd/La2O3, Pd/CeO2, Pd/Pr6O11, Pd/Sm2O3, and Pd/Gd2O3) have been successfully used as precatalysts in the Suzuki–Miyaura C–C cross-coupling reaction of aryl bromides with arylboronic acids in ethanol/water at r.t. All Pd/REO showed high activity and selectivity. The effective catalyst arises from palladium leaching from the REO support. We have demonstrated that the activity of each Pd/REO (the decreasing order is Pd/La2O3 > Pd/Pr6O11 > Pd/Gd2O3 > Pd/Sm2O3 ≫ Pd/CeO2) is strictly related to the corresponding PZC (point of zero charge) value. Accordingly, it can be reasonably argued that the metal is released in solution in the form of Pd2+, and higher is the amount of positive charge on the surface in catalytic conditions, higher is the concentration of Pd2+ ions in solution and faster is the formation of the coupling product. Also the recycling of each Pd/REO is strictly connected with the PZC value of the REO support. As a matter of fact, Pd/CeO2, which has the lowest PZC value (6.7), shows the best reusability, according to its lower tendency to release Pd2+ ions in solution.