Catalytic Synthesis Of Aromatic (Aliphatic) Dicarbamate

Isocyanates are very important organic intermediates. At present, the main industrial preparation involves the use of phosgene that is extremely poisonous to human and environment, and produces a large amount of hydrochloric acid as the by-product. Thus, developing a clean and safe non-phosgene route with the use of dimethyl carbonate (DMC) has become much attention. The thermal decomposition of carbamates for the preparation of isocyanates is a prospective method for industrial applications. This technology consists of the synthesis and decomposition of aromatic (aliphatic) dicarbamates. If it is combined with the synthesis of DMC by gas-phase oxidative carbonylation of methanol, the high economic achievement can be acquired. In this paper, the synthesis of aromatic (aliphatic) dicarbamates is studied in detail.Firstly, the synthesis of diaminodiphenylmethane (MDA) is studied, with aniline (AN) and formaldehyde as raw materials, hydrochloric acid as the catalyst. The synthesized MDA was characterized by means of melting-point test, FTIR and1HNMR spectra. The optimal conditions are2.55of n(aniline)/n(formaldehyde) molar ratio,28.55%of HC1concentration,100℃of reaction temperature,35min of reaction time, by which the best yield of MDA can reach85.6%(wt%). Above-described process has advantages of mild reaction conditions and easy product separation.Secondly, the preparation of aromatic dicarbamates are investigated, in which the catalysts include CH30Na, NaOAc, Zn(OAc)?, Pb(OAc)2, and so on. Zn(OAc)2was found to be highly active. The synthesized aromatic carbamates were structurally characterized by means of melting-point test, FTIR and1HNMR spectra. Effects of reaction temperature, DMC dosage, reaction pressure, reaction time and dosage of various catalysts on the yield of aromatic carbamates were also discussed. When using Zn(OAc)2as the catalyst,96.40%yield of MDC was achieved at180℃in2h. with n(DMC)/n(MDA)=20:l,n(catalyst)/n(MDA)=0.05;85.37%yield of3,3’-Dimethyldiphenyl carbamate was produced at170℃in7h, with n(DMC)/n(3,3’-Dimethylbenzidine)=20:1, n(catalyst)/n(3,3’-Dimethylbenzidine)=10:1.Finally, aliphatic dicarbamates using DMC and alkylamines as raw materials was optimally synthesized and characterized, in which CH3ONa and Pb(OAc)2were found to be optimal catalysts. Using CH3ONa as the catalyst,94.03%yield of HDC was produced at80℃in8h, with n(DMC)/n(HDA)=6:1, n(catalyst)/n(HDA)=0.19. Using Pb(OAC)2as the catalyst,92.13%yield of trimethylhexamethylenedicarbamate was achieved at80℃in15h,with (DMC)/n(trimethylhexamethylenediamine)=10:1,n(catalyst)/(trimethylhexamethylenediamine) =10:1;96.0%yield of m-Phenyl dimethyl dicarbamate was attained at180℃in2h, with n(DMC)/n(MXDA)=20:1, n(catalyst)/n(MXDA)=10:1.