Direct Synthesis Of Methylene Diphenyl Dicarbamate From Aniline,Dimethyl Carbonate And Formaldehyde

Methylene diphenyl dicarbamate?MDC?is an intermediate for the preparation of methylene diphenyl diisocyanate?MDI?.At present the synthesis of MDI using dimethyl carbonate?DMC?and anline as the raw material is a promising synthetic method.However,the drawback of this method is the multistep process to obtain MDI.To simplify the process,the integration between methyl N-phenyl carbamate?MPC?synthesis and its condensation is studied in this paper.The interactions between MPC synthesis and MPC condensation were investigated.It was found that when Zn?OAc?₂ and H?existed at the same time,MPC synthesis and its condensation were both inhibited,because aniline was prone to adsorb on the strong acid sites of the H?catalyst,which was beneficial for the N-methylation of aniline.Zn?OAc?₂was also found to adsorb on the H?surface to form Si-O-Zn bonds during the condensation,decreasing the quantity of acid sites with medium acid strength on the H?and reducing the catalytic performance consequently.When formaldehyde and water existed at the same time,aniline conversion and MPC selectivity both decreased obviously,because of the DMC hydrolysis to methanol with the water in formaldehyde.Also,methanol could react with Zn?OAc?₂ to form ZnO,which had no catalytic performance for MPC synthesis.Additionly,the coordination of the O atom in carbonyl group of formaldehyde also with the Zn²⁺inhibited the MPC synthesis.One-pot synthesis of MDC by dynamic operation was studied.Suitable reaction conditions were as follows:n?DMC?:n?aniline?=46,n?Zn?OAc?₂?:n?aniline?=0.05,at a temperature of 170?,a reaction time of 8h.And then HCHO and H?was added,the reaction condition was:n?HCHO?:n?aniline?=0.22,m?H??:m?aniline?=0.8,at a temperature of 160?,a reaction time of 3h.Under these conditions,aniline conversion was100%and the MDC yield was 73.9%.The deactivated H?and Zn?OAc?₂ could be regenerated by an aqueous acetic acid solution,and the catalytic performance of the regenerated catalyst was almost the same as that of the fresh catalyst.Zn?OAc?₂+H?was designed as the catalyst for the one-step synthesis of MDC.Optimal reaction conditions were as follows:n?DMC?:n?AN?:n?HCHO?=46:1:0.15,n?Zn?OAc?2?:n?aniline?=0.15,m?H??:m?aniline?=0.8,and reaction temperature was 120?for 6h.Under these conditions,aniline conversion was 86.9%and the MDC yield was 19.1%.In order to reduce the contact probility of aniline and H?,H?@SiO₂ was prepared.Compared with H?catalyst,MPC selectivity increased to 80.3%over H?@SiO2.However,the decrease of acid amount and acid strength lead to its poor catalytic activity for MPC condensation.