Synthesis of MDI from Dimethyl Carbonate over Solid Catalysts

The catalytic synthesis of methylene diphenyl-4,4‘-diisocyanate (MDI) consists of three steps. Starting from the catalytic reaction of aniline and dimethyl carbonate (DMC), methyl phenyl carbamate (MPC) is formed. Then MPC condenses with formaldehyde to produce dimethyl methylene diphenyl-4,4‘-dicarbamate (MDC). Last, MDC is decomposed to MDI. For the first step, the properties of supported zinc acetate catalysts on different supports have been examined. Supported zinc acetate catalyst on activated carbon (AC) or α-Al 2 O 3 shows good catalytic properties. Over Zn(OAc) 2 /AC catalyst, MPC yield reaches 78% and the selectivity is 98%. For the second step, when zinc chloride is used as a catalyst and nitrobenzene as a solvent, MDC yield can reach 87.4%. The catalytic activity of AC-supported ZnCl 2 catalyst, which is calculated based on 1 mol of ZnCl 2, is much higher than that of homogeneous ZnCl 2 . For the third step, zinc powder and its organic salts show higher catalytic activity; MDI yield is 87.3% over zinc powder catalyst.