Nanostructured CeO2 with different morphologies for methoxycarbonylation of 2,4-toluene diamine with dimethyl carbonate

Abstract

The methoxycarbonylation of 2,4-toluene diamine (TDA) to dimethyl toluene-2,4-dicarbamate (TDC) is an important reaction step in the phosgene-free synthesis of toluene diisocyanate (TDI). To further clarify the catalytic mechanism and enhance the catalytic performance of CeO₂, a series of CeO₂ catalysts with different morphologies were prepared and their catalytic performance in TDA methoxycarbonylation was evaluated. The results show that spherical cerium dioxide (CeO₂-S) has better catalytic performance. Combining characterization methods such as TEM, XRD, BET, TPD, XPS, Raman spectroscopy and H₂-TPR, the reasons for the differences in the catalytic performance of the different CeO₂ catalysts were analyzed, and it is found that the total acid amount, medium-strong acid sites, lattice oxygen content and specific surface area of CeO₂ are positively correlated with their catalytic performance. Under optimized conditions, TDA was completely converted, the yield of TDC reached 86.8%, and the total yield of methyl carbamates (including monomethyl carbamate and TDC) reached as high as 96.5%. CeO₂-S could be reused four times without significant change in activity. Through designed experiments, it is determined that both pathways with methyl 3-amino-4-methyl-N-phenyl carbamate (4-TMC) and methyl 2-methyl-5-amino-N-phenyl-carbamate (2-TMC) as intermediates could eventually generate TDC. Finally, a synergistic catalytic mechanism of acid sites and lattice oxygen in CeO₂ for the methoxycarbonylation of TDA was established.