Study On The Non-phosgene Synthesis Of Hexamethylene-1,6-diisocyanate Over Heterogeneous Catalyst

CO₂ utilization to produce useful chemicals is an important research topic of huge industrial interest in green synthesis.Hexamethylene-1,6-diisocyanate?HDI?is one of the most widely used aliphatic diisocyanate because of its unique properties and high demand in industries for various high-end applications.Non-phosgene synthesis of HDI has attained considerable interest in the field of green chemistry because it can not only substitute the highly toxic phosgene method,but also provide an approach for CO₂ utilization by using dimethyl carbonate?DMC?as carbonylation reagent.In this dissertation,aiming at resolving the key problems that exist in the two-step non-phosgene synthesis of HDI:?1?synthesis of hexamethylene-1,6-dicarbamate?HDC?by methoxycarbonylation of HD A with DMC;?2?thermal decomposition of HDC to HDI,various heterogeneous catalysts are designed and studied.Furthermore,characterization of catalysts,the relationships between physicochemical properties and catalytic performances,reusability of catalysts,effects of reaction parameters,catalytic mechanisms,reaction kinetics as well as product separation and purification are discussed in detail.Thus,both process and theoretical bases are supplied for the non-phosgene synthesis of HDI in this dissertation.The achievements and progress in this dissertation are as follows:?1?Synthesis of HDC by methoxycarbonylation of HDA with DMC was conducted over the bulk and hybrid heteropoly acid catalyst for the first time.The reaction network was established.The performances of bulk and hybrid heteropoly acid catalysts were evaluated.Results showed that the H₄[SiW₁₂O₄₀]catalyst revealed the best performance compared with other catalysts.The catalysts were systematically characterized by XRD,FTIR,SEM and NH3-TPD techniques.It was found that the acidic properties were responsible for the high performance of the H₄[SiW₁₂O₄₀]catalyst.Under the optimized reaction conditions,HDA conversion and HDC selectivity could be reached 93.2%and 64.8%,respectively,with only 3.8%by-products selectivity.A possible reaction mechanism was also proposed.Moreover,HDC was obtained with a purity of 99.9%by using the recrystallization separation method.?2?Synthesis of HDI by thermal decomposition of HDC was carried out over metal oxide supported catalyst.Different metal oxide supported catalysts were prepared by incipient wetness impregnation?IWI?,PEG-additive?PEG?and deposition precipitation with ammonia evaporation?DP?methods and screened.The catalyst screening results showed that Co₃O₄/ZSM-525 catalysts prepared by different methods showed different performances in the order of Co₃O₄/ZSM-525?PEG?>Co₃O₄/ZSM-525?IWI?>Co₃O₄/ZSM-525?DP?.The physicochemical properties of Co₃O₄/ZSM-525 catalyst were characterized by XRD,FTIR,N₂ adsorption-desorption measurements,NH3-TPD and XPS.Results showed that the superior catalytic performance of Co₃O₄/ZSM-525?PEG?catalyst was attributed to its relative surface content of Co³⁺,surface lattice oxygen content and total acidity.Under the optimized reaction conditions,the HDC conversion and HDI yield could reach 100%and 92.8%,respectively,without polymerization by-products.The Co₃O₄/ZSM-525?PEG?catalyst could be facilely separated from the reaction mixture,and reused for 5 successive runs without degradation in catalytic performance.A possible reaction mechanism was purposed based on the physicochemical properties of the Co₃O₄/ZSM-525 catalysts.Moreover,HDI was obtained with a purity of>99%by the thermal decomposition of HDC obtained from the methoxycarbonylation of HDA with DMC,using molecular distillation method.?3?The kinetics of the thermal decomposition of HDC to HDI in the presence of CO3O4/ZSM-525?PEG?catalyst were then studied.Kinetic parameters were calculated by using concentration profiles at different reaction temperatures.The activation energy and pre-exponential factor for the thermal decomposition of HDC to HMI and HMI to HDI were 39.67 kJ.mol^-1,3.23 × 102 min^-1 and 114.54 kJ.mol^-1,1.35 × 1011 mol^-0.49.L^0.49.min^-1 respectively.The kinetic model was found in good agreement with the experimental data.The results showed that the thermal decomposition of HDC to HMI required less activation energy and easy to carry out compared with the thermal decomposition of HMI to HDI.Thus,the higher reaction temperature will be beneficial for the decomposition of HMI to HDI.?4?A set of bimetallic?Zn-Co?oxide supported ZSM-5 catalysts was prepared by PEG-additive method,which was further studied in the thermal decomposition of HDC to HDI in this dissertation.The physicochemical properties of the catalysts were investigated by FTIR,XPS,XRD,N₂ adsorption-desorption measurements,SEM,EDS and NH3-TPD techniques.Results showed that the ZnCo₂O₄ spinel oxide was formed on the ZSM-5 support and provided synergetic effect between Zn and Co species for the bimetallic oxide supported catalyst.Their catalytic performances were then studied.Results showed that the Zn-2Co/ZSM-5 catalyst showed excellent catalytic performance due to the good synergetic effect between Co and Zn species,which provided a suitable contribution of acidic sites.HDC conversion of 100%with HDI selectivity of 91.2%and by-products selectivity of 1.3%could be achieved within short reaction time of 2.5 h over Zn-2Co/ZSM-5 catalyst.The reusability of the Zn-2Co/ZSM-5 catalyst showed that loss in the catalytic performance was due to the ZnO leaching into reaction mixture which led structural defects in the ZnCo₂O₄ cubic spinel structure.