Organic carbonate synthesis from CO₂ and alcohol over CeO ₂ with 2-cyanopyridine: Scope and mechanistic studies

The combination system of CeO₂-catalyzed carboxylation and 2-cyanopyridine hydration (CeO₂ + 2-cyanopyridine system) is effective for the direct synthesis of organic carbonates from CO₂ and alcohols. This catalyst system can be applied to various alcohols to afford the corresponding carbonates in high alcohol-based yields. The hydration of 2-cyanopyridine over CeO₂ rapidly proceeds under the low concentration of water, which can remove the water from the reaction media. Since the reaction is limited by the chemical equilibrium, the removal of water remarkably shifts the chemical equilibrium to the carbonate side, leading to high carbonate yields. In addition, 2-picolinamide that is produced by hydration of 2-cyanopyridine forms an intramolecular hydrogen bonding between H atom of the amide group and N atom of the pyridine ring, which weakens the adsorption of 2-picolinamide on CeO₂ by reduction of the acidity. The reaction mechanism of DMC formation in CeO₂ + 2-cyanopyridine system is also proposed.