Study On Synthesis Of Toluene-2,4-Dicarbamates Via Urea Route

The synthesis of toluene diisocyanate (TDI) starting from urea is a clean and economicroute. The preparation of toluene dicarbamates using urea as a starting material is an importantstep in the manufacture of TDI via urea route. Hence the study on this reaction has a greattheoretical significance and application value.Firstly, thermodynamic analysis of the reaction system for synthesizing dimethyl toluene-2,4-dicarbamate (TDC(Me)) via urea route was studied. The result showed that the mainreaction was endothermic and its _rG^o was -61.7 kJ.mol^-1at 298K, which suggested that highequilibrium conversion could be expected. Secondly, LC-MS was used in qualitative analysis ofthe constitutes in the reaction system for synthesing TDC(Me). The main side-reactions weremethylation reactions of 2,4-toluene diamine (TDA) and 3-amino-4-methylphenyl urea (TU).The possible reaction pathway was presumed based on LC-MS result and the thermodynamicanalysis conclusion. Additionally LC-MS was also used in qualitative analysis of the reactionsystem for the synthesis of dipropyl toluene-2,4-dicarbamate (TDC(Pr)). The main side-reactions were condensation reactions between 3-amino-4-methyl N-propylphenylcarbamate(TMC) with TU or toluene 2,4-bisurea (TBU). Lastly, the effects of reaction conditions onTDC(Me), TDC(Et) and TDC(Pr) synthesis reaction were investigated. The optimal conditionsfor the synthesis of TDC(Me) were as follows: reaction temperature 180℃, reaction time 16h,n(TDA):n(Me)=1:88, n(TDA):n(Urea)=1:2.5. Under these conditions, TDA conversion was93.6%, and TDC(Me) yield and selectivity were 13.4% and 14.3%, respectively. The optimalconditions for the synthesis of TDC(Et) were: reaction temperature 180℃, reaction time 4h,n(TDA):n(Et)=1:100, n(TDA):n(Urea)=1:2.5. Under these conditions, TDA conversion was78.7%, and TDC(Et) yield and selectivity were separately 43.2% and 55.0%. The optimalconditions for TDC(Pr) synthesis were similar as those for TDC(Et) synthesis: reactiontemperature 170℃, reaction time 4h, n(TDA):n(Pr)=1:84, n(TDA):n(Urea)=1:3.0. Underthese conditions TDA conversion was 95.3%, and TDC(Pr) yield and selectivity were 66.1%and 69.4%, respectively.