Direct Synthesis of Diphenyl Carbonate from Phenol and Carbon Dioxide Over Ti-Salen-Based Catalysts.

Various metal-salen catalysts were prepared for use in the direct synthesis of diphenyl carbonate (DPC) from phenol and carbon dioxide. We found that metal-salen complexes containing titanium as central metal species retained suitable Lewis acid property for the reaction. It was revealed that the catalytic activity of Ti-salen complexes could be controlled by introducing appropriate substituents into salen ligand. Insertion of phosphonium salts into para-position of aromatic aldehyde of salen ligand enhanced solubility of the catalyst in the methanol-phenol solution, and tert-butyl substituent in the salen ligand induced selective formation of DPC due to steric effect. In addition, introduction of various bridging groups into salen ligand caused change in electronic property of central metal atom. Among the catalysts tested, Ti-(t-butyl)salphen(PPh3)Cl showed the best catalytic performance at 100 °C and 60 bar. The catalytic system utilizing Ti-(t-butyl)salphen(PPh3)Cl catalyst was then optimized by conducting the reaction at various reaction temperatures and pressures.