Lead Oxide-Catalyzed Methoxycarbonylation Of 2,4-Toluene Diamine

The synthesis of toluene dicarbamate (TDC) through methoxycarbonylation of toluene diamine (TDA) is an important step to toluene diisocyanate (TDI) by non-phosgene route. The study on the catalysts used in this reaction has a great theoretical and applied significance. In this paper, the catalytic performance of lead oxide was investigated in the synthesis of TDC from the methoxycarbonylation of TDA and dimethyl carbonate (DMC).First of all, suitable preparation conditions for lead oxide were determined: Pb(NO3)2 as precursor, calcination temperature 600℃and calcination time 4h. The results of activity evaluation and XRD analysis showed that orthorhombic PbO had higher catalytic performance. Under the suitable reaction conditions of molar ratio of TDA to DMC 1:30, weight percentage of catalyst 6%, reaction temperature 180℃and reaction time 3h, the yield of TDC was 82.5%. In addition, the recovered lead oxide catalyst remained higher activity after five runs, which suggested that lead oxide had stable catalytic performance.A possible reaction mechanism in the presence of lead oxide was discussed. Lead oxide interacted with water contained in DMC to form–PbOH, and then the–PbOH interacted with DMC to form–PbOCOOMe, which reacted with TDA to form TDC and–PbOH regenerated. In order to reduce the amount of lead oxide, supported lead oxide was prepared. The suitable preparation conditions were as follows: SiO2 as support, loading of PbO 30%(wt), calcination temperature 500℃and calcination time 4h. Under the suitable reaction conditions of molar ratio of TDA to DMC 1:35, weight percentage of catalyst 5.4%, reaction temperature 180℃and reaction time 3h, the yield of TDC was 83.9%. The catalytic performance of catalyst dropped after first run but remained stable in the following four runs.Pb-PbO/SiO2 was prepared from PbO/SiO2 by reduction under the the conditions of pure hydrogen atmosphere, reduction temperature 450℃and reduction time 4h. Pb-PbO/SiO2 showed high activity to the synthesis of TDC from TDA and DMC, suggesting that it may be a promising catalyst for the synthesis of TDC from one-step reaction of TDA, methanol and urea.