Study On Reaction Mechanism Of Synthesis Of Toluene Dicarbamate Catalyzed By Zinc Acetate And Its Deactivation As Well As Reuse

The synthesis of toluene diisocyanate(TDI) using dimethyl carbonate(DMC) instead of phosgene is a non-phosgene route which is clean, mild, and has the most industrialized potential. The first step of this technology is preparation of toluene dicarbamate(TDC), hence the experimental study on the reaction mechanism of synthesis of TDC catalyzed by zinc acetate, as well as the deactivation and reuse of zinc acetate has a great theoretical and applied significance. Firstly, the mechanism of synthesis toluene-2,4-dicarbamate from toluene-2,4-diamine and dimethyl carbonate catalyzed by zinc acetate was investigated by Fourier transform infrared spectroscopy. The result showed that the new coordination complex at first was formed by dimethyl carbonate and anhydrous zinc acetate, so dimethyl carbonate's carbonyl group was activated. The amidos of toluene-2,4-diamine which was a nucleophilic reagent attacked the carbon of above carbonyl group, and the methoxycarbonylation reaction occuried to produce toluene-2,4-dicarbamate and methanol.Secondly, the equilibrium constant of deactivation reaction from zinc acetate and methanol at 433.15K was 3.155×104 by thermodynamic calculation. It was found that the synthesis reaction of TDC was obviously faster than the deactivation reaction of zinc acetate by experimental study. The result showed that zinc acetate was not totally deactivated in the process of synthesis reaction of TDC, so zinc acetate always acted as a catalyst in the reaction. A extraction method was applied to remove Zn2+ of the residual zinc acetate catalyst from TDC, and the extraction conditions were optimized. The better extraction conditions were: water bath temperature 80℃, mixing time 1h, amount of water 50ml, stirring speed 250r/min. Under the extraction conditions above, the content of Zn2+ in TDC was decreased to less than 1%.Finally, through optimizing reaction conditions for synthesis of TDC catalyzed by zinc acetate, and integrating the characterizing results of XRD of zinc oxide prepared by the zinc acetate deactivation reaction above, the better integrated reaction conditions were as follows : reaction temperature 160℃, reaction time 5h, m(TDA):m(Zn(OAc)2)=1:0.16, n(TDA):n(DMC)= 1:30. Under these conditions, TDC yield was 97.6%, and nano-zinc oxide particle was homoplastical sphere with a diameter about 8nm, and specific area 59.91m2?g-1. The synthesis reaction of methyl N-cyclohexyl carbamate(MCHC) from cyclohexylamine and dimethyl carbonate was studied in the presence of zinc oxide catalysts which were prepared by four different methods. The results showed that the maximal MCHC yield of the reaction catalyzed by the nano-zinc oxide prepared by zinc acetate deactivation reaction was 44.2%. Big specific surface area and the surface modified of organic groups of the nano-zinc oxide were important factors for it's high catalytic activity.