Study On One-Step Catalytic Synthesis Of Methylene Diphenyl Dicarbamate

Methylene diphenyl dicarbamate (MDC) is an intermediate in the synthesis of methylene diphenyl diisocyanate (MDI), and MDI can be used as the raw material for the production of polyurethane. The synthesis of MDI using dimethyl carbonate (DMC) is environmentally friendly. Nevertheless, the process is complicated, for it comprises three steps: synthesis of methyl N-phenyl carbamate (MPC), reaction of MPC with methylenating agents to form MDC, and decomposition of MDC to prepare MDI. If MPC synthesis can be coupled and performed together with its condensation, the process can be greatly simplified. This work reports the one-step synthesis of MDC over bifunctional catalysts.The one-step synthesis of MDC using aniline and DMC as raw materials was studied. ZrO2/SiO2 and H4SiW12O40/SiO2 were used as the catalyst in MPC synthesis and MPC condensation respectively. Formaldehyde was chose as a methylenating agent and DMC could be used as a solvent for the system.A bifunctional catalyst H4SiW12O40-ZrO2/SiO2 was developed for the direct synthesis of MDC from aniline, DMC, and formaldehyde solution. The effects of reaction time, reaction temperature, H4SiW12O40 loading and DMC/aniline/formaldehyde molar ratio were studied. Suitable reaction conditions were as follows: DMC/aniline/formaldehyde molar ratio was 20/1/0.05, ZrO2 loading was 1%, H4SiW12O40 loading was 10%, and reaction temperature was controlled by stages (i.e., reaction at 170℃for 7h and then at 100℃for 4.5h). Under these conditions, the yield of MDC was 24.9%. FT-IR of the catalyst adsorbed with pyridine was carried out to characterize the H4SiW12O40-ZrO2/SiO2. From the result, it could be concluded that Lewis acid sites and Br?nsted acid sites existed on the surface of catalyst at the same time, which were the active sites for MPC synthesis and MPC condensation respectively. It could be conjectured that the acid sites of H4SiW12O40 were occupied by aniline which resulted in the difficulty for MPC condensation with formaldehyde.With a view to reducing the attachment of aniline to the acid site of H4SiW12O40 and H4SiW12O40 leaching, a capsule catalyst ZrO2/SiO2@H4SiW12O40 was prepared through sol-gel process. When gel time was 50h, the catalyst showed high activity. The better reaction conditions were as follows: DMC/aniline/formaldehyde molar ratio was 20/1/0.05, reaction at 170℃for 7h and then at 100℃for 4.5h. Under these conditions, the yield of MDC was 9.2%. The spacial structure and geometrical parameters of reagents and produces were simulated and computed by Material Studio solfware, it showed that reagents and produces could diffuse freely in the pore of catalyst. Compared with H4SiW12O40-ZrO2/SiO2, ZrO2/SiO2@H4SiW12O40 catalyst showed better stability on MDC synthesis.