Non-phosgene synthesis of isocyanates based on CO2: Synthesis of methyl N-phenyl carbamate through coupling route with lead compound catalysts

The synthesis of methyl N-phenyl carbamate (MPC), the key intermediate for the non-phosgene synthesis of isocyanates based on CO2, was studied from the reaction of dimethyl carbonate and N,N′-diphenyl urea under pressure. A variety of solid catalysts were screened, which showed lead compounds such as PbO, PbO2, PbCO3, and 2PbCO3·Pb(OH)2 had excellent catalytic activity. The characterization of XRD showed that the screened lead compound catalysts after reaction were changed into the same lead compound with the group –PbOH. With the increase of recycling times of the used lead compound catalysts, the selectivity of MPC and conversion of DPU were still about 99%. And the characterization of XRD and FTIR showed that new lead compound, which was produced with the increase of recycling times of the used lead compound catalysts, was also a lead compound with –PbOH. So the above results indicated that the real lead catalyst was the compound containing –PbOH. A possible reaction mechanism was discussed with the lead compound catalysts.