Continuous-flow Hydrogenation Of Aromatic Methyl Carbamate And Cracking Process

Alicyclic isocyanate is the core raw material to synthesize high-grade polyurethane with excellent characteristics such as light stability,non-yellowing,weather resistance and high mechanical properties,and is one of the core raw materials for the production of high-grade polyurethane.The current industrial production adopts the phosgene method,and there are potential safety hazards and environmental pollution problems such as highly toxic phosgene,by-product hydrochloric acid,chlorine-containing products and harsh reaction conditions.To this end,the research on the synthesis process of alicyclic isocyanates with intrinsic safety and environmental friendliness is carried out in this paper,which plays an important role in the green development of the polyurethane industry.Firstly,in view of the lack of thermodynamic data of the studied system,various estimation methods such as Benson method and Joback method were used to analyze the thermodynamics of the aromatic methyl hydrogenation reaction of alicyclic methyl carbamate and the cracking reaction of alicyclic methyl carbamate.The basic thermodynamic data of aromatic methyl hydrogenation(MPC,TDC,MDC),and its aromatic hydrogenated alicyclic methyl carbamate and cracked alicyclic isocyanate were estimated.The relationship between reaction heat,free enthalpy and equilibrium constant and temperature was calculated.Then,on the basis of thermodynamic analysis,the catalytic hydrogenation of aromatic methyl carbamate was studied by using a continuous-flow fixed bed reactor.The supported Rh/γ-Al2O3 metal catalyst was prepared,and the effect of operating conditions on the hydrogenation of methyl phenylcarbamate(MPC)was investigated by single factor experiments.Then,the operating conditions were optimized by response surface method.The results were: 5%Rh/Al2O3,76 ℃,2.0 MPa,20 min-1 as gas phase space velocity.Continuous operation under this condition for 10 h,the selectivity of methyl cyclohexylcarbamate(MCHC)was 100%,and the conversion of MPC is from 88.0% to 100%.The optimization results of the operating conditions for the hydrogenation of Methyl Toluene Dicarbamate(TDC)and Methyl Diphenylmethane Dicarbamate(MDC)are as follows: 5%Rh/Al2O3,60 ℃,2.0 MPa,and 20 min-1 as gas phase space velocity.Continuous operation under this condition for10 h,the results showed the selectivity of Methylcyclohexanedicarbamate(HTDC)is100%,the conversion of TDC is from 60.2% to 100%,and the selectivity of methyl dicyclohexylmethane dicarbamate(HMDC)is from 91.4% to 94.9%,the conversion of MDC is from 70.9% to 99.9%.The fresh and used catalysts in the reaction of MPC hydrogenation were characterized by various means such as ICP,BET and XRD.It was found that the activity of the catalyst decreased due to the adhesion of the product and the increase of the particle size of the active metal.Finally,the catalytic cracking process of HTDC was studied.The thermal cracking reaction of HTDC was analyzed by TGA/DSC technology,and the initial and rapid cracking temperatures of HTDC were determined,and the thermal cracking temperature of HTDC was lower than that of TDC.Qualitative and quantitative analysis of methylcyclohexane diisocyanate(HTDI)by ethanol derivatization method.The optimized reaction operating conditions were as follows: 240 °C,dioctyl phthalate as solvent,liquid paraffin as heat carrier,Zn as catalyst,and the catalyst concentration 0.008 g/m L.Under these conditions,the reaction was carried out for 1.5h,and the yield of HTDI was 50.8%.