Study On The Dehydration Of Zn?OAc?₂·2H₂O And Catalytic Synthesis Of Toluene Dicarbamate

Toluene dicarbamate?TDC?is the intermediate for non-phosgene synthesis of toluene diisocyanate?TDI?.Zn?OAc?₂ is an efficient catalyst for the synthesis of TDC from dimethyl carbonate?DMC?and 2,4-diaminotoluene?TDA?.During the reaction process,Zn?OAc?₂easily deactivated into ZnO,and DMC played the role of reactant and solvent at the same time,which caused a problem with relatively large amount DMC.In this paper,the related problems about the synthesis of toluene dicarbamate catalyzed by organozinc salts,dehydrated by Zn?OAc?₂·2H₂O,were studied,which has a great theoretical and applied significane.The effects of reaction conditions on the catalytic activity of Zn?OAc?₂ for catalytic synthesis TDC and the composition of deactivated catalysts were investigated.The results showed that the reaction was basically completed in 5 h,and the prolonged time may cause side reactions.The yield of TDC increases with the increase of reaction temperature,but when the temperature reached 190°C,a obvious side-reaction occurred and the product properties even changed.The amount of catalyst had a great influence on the yield of TDC.The yield of TDC reached 88.2%after reacted for 5 h at 180°C when the amount of catalyst was m?TDA?:m?Zn?OAc?₂?=1:0.4.The deactivated catalyst was mainly ZnO,a layered hydroxyl double salt Zn₅?OH?₈?CH₃COO?₂·2H₂O was also produced at a lower temperature?120?or 140??.The amount of catalyst and reaction temperature affected the crystalline of ZnO.The main factor influenced the formation of crystalline ZnO was the amount of catalyst,and crystalline ZnO was formed when the amount of catalyst was m?TDA?:m?Zn?OAc?₂?=1:0.32 at 160?.The stirring method,crystal structure of zinc acetate,and the presence of Zn₄O?OAc?₆ in the catalyst may be responsible for the yield of TDC.The change of composition during the heat treatment of Zn?OAc?₂·2H₂O were investigated.The results showed that the dehydration process only occurred at 100°C.But when the temperature increased to 120°C,Zn₄O?OAc?₆ initially formed;Zn₄O?OAc?₆ and ZnO were generated after heat treatment at 160°C for 6 h,and the main product was ZnO after 12 h.With the increasing of heat treatment temperature,the formation rate of Zn₄O?OAc?₆ also increased.The catalytic activity of different amounts of Zn?OAc?₂ and Zn₄O?OAc?₆ on TDC were studied.When the relative amounts?area ratio?was 1:2.07,the yield of TDC was 94.4%after reacted at 180?for 7 h,Zn₄O?OAc?₆could play better catalytic activity at relatively higher temperatures.The influences of various solvents on the synthesis reaction of TDC were investigated.The results showed that the weak polar solvents were more conducive to the synthesis of TDC,among which aromatic hydrocarbons?toluene and paraxylene?and aromatic esters?dimethyl phthalate and dioctyl phthalate?showed a better solvent effect.The dioctyl phthalate could be used as TDC decomposition reaction solvent,and when it was used as a solvent,under the reaction conditions as:n?TDA?:n?DMC?=1:10,m?TDA?:m?Zn?OAc?₂?=1:0.27,reaction temperature 170°C,reaction time 4 h,the yield of TDC reached 87.0%.It was noteworthy that when the amount of DMC was too low,N-methylation reaction was more likely to occur,leading to the yield of TDC reducing significantly.