Study On The Synthesis Of 2,4-toluene Dicarbamate From Methoxycarbonylation In The Free Phosgenation Method

2,4-toluene dicarbamate is the precursor of 2,4-toluene diisocyanate (TDI), which is the key intermediate for the manufacture of polyurethanes and useful in various fields of applications such as foam, fibre, plastic, dope, leather, pesticide and latex. Because of the extreme toxicity phosgene method used traditionally, several synthetic non-pjosgene methods of 2,4-toluene diisocyanate have been described. Among them, methoxycarbonylation of 2,4-toluene diamine with dimethylcarbonate (DMC) followed by decomposition of 2,4-toluene dicarbamate (TDC) to TDI is a promising process. In fact, DMC is currently produced on a large scale by oxidative carbonylation of methanol. The above two processes combined with each other to make a green and clean synthesis route accord with the requirement for modern chemical engineering industry. In this paper, TDC was synthesized by methoxycarbonylation of 2,4-toluene diamine and DMC in the presence of several catalysts. GC-MS and ion cataclasm mechanism showed that the side products of the methoxycarbonylated reaction included TDC N-methylated products, N-methylated toluene mono-carbamate, TDA N-methylated products and urethanes. The high performance liquid chromatograh analytic method was set up through the investigation of different mobile phases and chromatograph conditions. Various catalysts were investigated and the evaluation results showed that Yb(OTF)₃ catalyst had a better activity and the test proved that the catalyst could be reused. The Lewis acidity of Ln(OTF)₃ (Ln=La, Nd, Yb) and their catalytic activities showed the Lewis acidity of Ln(OTF)₃ catalysts drive dynamically the methoxycarbonylation. The stronger Lewis acidity of catalyst, the higher TDC yield and selectivity were gained. When Pb(OAC)₂ was used as catalyst for the reaction, the 100% conversion of 2,4-toluene diamine and 97.7% yield of dimethyltoluene-2,4-dicarbamate were attained at 443K in 4h. XRD showed that most of Pb(OAC)₂ converted into Pb₃(CO₃)₂(OH)₂ after reaction. Pb₃(CO₃)₂(OH)₂ could be regenerated and showed the same activity as the original Pb(OAC)₂ catalyst.