Synthesis Of Methy N-phenyl Carbamate Over Zn?OAc?₂/SiO₂ Catalyst Prepared By Solvothermal Impregnation

Methyl N-phenyl carbamate?MPC?is an important organic intermediate for synthesis of diphenylmethane diisocyanate?MDI?which is the major raw material for polyurethane synthesis.Although Zn?OAc?₂ exhibits better catalytic performance for MPC synthesis,it is prone to deactivate.In order to increase the stability of Zn?OAc?₂,Zn?OAc?₂/SiO₂catalyst was prepared by solvothermal impregnation and its catalytic performance in MPC synthesis was investigated in this paper.When Zn?OAc?₂/SiO₂ was prepared at 160?for 6h,the reaction conditions for MPC was optimized.And the optimal reaction conditions consist of a reaction temperature of190?,a reaction time of 5 h,n?DMC?:n?AN?of 20:1 and m_??:m_{Zn?OAc?2/SiO2} of 0.2.Under these conditions,the aniline conversion was 97.2%and the MPC selectivity was 89.4%.React-IR was used to monitor the preparation of Zn?OAc?₂/SiO₂ catalyst and it was characterized by XRD?FTIR?TG and so on.The results showed that Zn?OAc?₂?ZnO and Zn₄O?O₂CCH₃?₆ are present on the surface of Zn?OAc?₂/SiO₂ catalyst.Zn?OAc?₂ and Zn₄O?O₂CCH₃?₆ are the active sites.The stability of Zn?OAc?₂/SiO₂ was investigated and the results showed that when Zn?OAc?₂/SiO₂ was used for the seventh time,aniline conversion and MPC yield were found to be 79.1%and 79.2%,respectively.The reason for the improvement of the stability is the existence of Zn4O?O2CCH3?6 and the monolayer dispersed Zn?OAc?₂.The reaction between the active component and the methanol is inhibited due to the steric hindrance effect after the active component supported.The reason for the deactivation of Zn?OAc?₂/SiO₂ is the formation of ZnO during the reaction.The deactivated catalyst has the equal activity as the fresh catalyst after being regenerated by acetic acid aqueous solution.The adsorption kinetics and thermodynamic of Zn²⁺adsorption on carrier SiO₂ was studied during the catalyst preparation.The effect of temperature on the adsorption was significant,and the increase of temperature was beneficial to the adsorption process.The thermodynamic parameters of?G,?S and?H in the adsorption process were calculated,repectively.The negative change in free energy??G?and positive change in enthalpy??H?indicate that the adsorption is spontaneous and endothermic.The adsorption behavior of SiO₂ on Zn²⁺can be described by Langmuir and Freundlich isotherm.Langmuir isotherm can predict the maximum adsorption capacity of the monomolecular layer.The maximum adsorption capacities,calculated by Langmuir isotherm,were found to be 6.046×10²mg/g,6.345×10²mg/g,1.049×10³mg/g,1.810×10³mg/g at the temperature of140?,150?,160?,170?,respectively.The adsorption of Zn²⁺on the carrier SiO₂ is in accordance with the pseudo-second-order kinetics model.The particle diffusion during adsorption is important controlling factor for the adsorption rate of Zn²⁺on SiO₂.