Epoxidation Of Propylene By Molecular Oxygen Over Potassium Ion-Modified CuO_x-SiO₂ Catalysts

This dissertation contributes to the studies on the epoxidation of propylene by molecular oxygen over alkali metal ion-modified CuO_x-SiO₂ catalysts.The structures and catalytic performances of the catalysts with and without alkali metal ions have been investigated in detail.The working state of copper and the reaction mechanism have also been discussed.A simple K⁺-modified CuO_x-SiO₂ catalyst prepared by a sol-gel method was found to be efficient for the epoxidation of propylene by oxygen.The main partial oxidation product was switched from acrolein to propylene oxide(PO) after the K⁺ modification.The K⁺-5 wt%CuO_x-SiO₂(K/Cu=0.2) catalyst,which was optimized by investigating the influences of various factors such as copper content,copper sources,nature of modifiers and K/Cu molar ratios,showed the best catalytic performance for propylene epoxidation.The catalyst did not require prereduction,and PO selectivity reached 78%at a C₃H₆ conversion of 0.2%,and it still could be sustained at 27%at a C₃H₆ conversion of 4.0%under O₂-rich condition,significantly higher than other Cu-based catalysts reported so far for the epoxidition of propylene by molecular oxygen.The main cause for catalyst deactivation was carbon deposition, and the catalyst can be regenerated in the presence of an oxygen-containing gas flow at 550℃to remove the coke.Characterizations by XRD,HRTEM and UV-Vis suggested that the copper species existed as crystalline CuO and highly dispersed Cu(â…¡)O clusters or isolated Cu(â…¡) ions in the CuO_x-SiO₂ catalysts.Our EPR characterizations suggested that copper was in octahedral coordination in the CuO_x-SiO₂ catalysts both with and without K⁺ modification.However,the mean size of CuO particles in the CuO_x-SiO₂ catalysts was significantly decreased and the dispersion of copper species was highly enhanced after K⁺ modification,indicating the existence of strong interaction between K⁺ and copper oxide species.It was speculated that the strong interaction may be beneficial to PO formation.The H₂-TPR characterizations suggested that the reducibility of copper species became difficult after the modification by K⁺,and this probably caused the inhibition of the reactivity of lattice oxygen,leading to the suppressing of the allylic oxidation.NH₃-TPD and CO₂-TPD measurements revealed that the Lewis acidity was remarkably decreased and the basicity appeared on the CuO_x-SiO₂ catalysts after K⁺ modification.We believed that the decrease in the acidity on catalyst surface inhibitted the cracking,isomerization,oligomerization and combustion of PO.This was confirmed by further experiments using PO as a reactant under O₂-rich conditions.The surface acid-base properties of catalysts may play a crucial role in PO formation.XRD,EPR measurements combined with FT-IR studies of adsorbed CO revealed that the catalyst after reactions comprised both Cu(â…¡) and Cu(â… ),and a higher concentration of Cu(â… ) favored C₃H₆ epoxidation.For the first time,we have clarified that Cu(â… ) sites can function as the active sites for the epoxidation of propylene by oxygen.Cu(â… ) sites are proposed to account for the activation of oxygen,generating an electrophilic oxygen species for C₃H₆ epoxidation.