August 13, 2012
These catalysts allow the direct synthesis of diphenyl carbonate. An economical direct synthesis of diphenyl carbonate (DPC) remains elusive. DPC is a key intermediate used to make polycarbonates without the need for toxic phosgene. The current circuitous commercial process for making DPC usually requires the production of dimethyl carbonate, which then undergoes a reaction with phenol to make DPC. The reason for this indirect route is that direct oxidative carbonylation of phenol to produce DPC has relatively poor yield and selectivity.
Z. W. Huang, C. W. Chang, and C. J. Tsai disclose a catalyst system and operating conditions for the direct oxidative carbonylation of phenol that make DPC with good productivity and selectivity. The catalyst system consists of a palladium halide with one or more nitrogen-containing heterocyclic compounds as cocatalysts.
The reaction is carried out in a 1-L high-pressure vessel. The temperature is preferably 70–100 °C, and the pressure 6–12 kg/cm2 (0.59–1.18 MPa). In an example, 2.4 mol phenol, 1.39 mmol Mn(acac)2, 12 mmol n-Bu4NI, and 8.78 mmol of NaOH are added to the reactor. PdCl2 catalyst (265 ppm palladium based on total reactants) is also added. The air in the reactor is displaced with a CO–O2 mixed gas at a pressure of 10 kg/cm2 (0.98 MPa), and the temperature is raised to 80 °C.
Under these conditions, phenol conversion is 5.9% and selectivity to DPC is 98.9%. When various nitrogen-containing heterocyclic compounds are added as cocatalysts, the conversions increase substantially, and good selectivity is maintained. For example, when carbazole is added in the same quantity as the catalyst, phenol conversion increases to 7.2%, and selectivity is 100%. When 3,6-diaminocarbazole is added, phenol conversion grows to 10.4% with 100% DPC selectivity. (China Petrochemical Development Corp. [Taipei, Taiwan]. US Patent 8,212,066, July 3, 2012; Jeffrey S. Plotkin)