【作者】 朱文明；

【导师】 王野；

【作者基本信息】 厦门大学 ， 物理化学， 2008， 硕士

【摘要】 本学位论文针对以氧气为氧化剂的丙烯环氧化多相催化反应,主要从催化性能及结构特征两个方面,详细研究了碱金属离子对CuO_x-SiO₂催化剂的修饰作用,并对反应中铜的状态和反应机理进行了探讨。研究发现,溶胶—凝胶法制备的K⁺修饰的CuO_x-SiO₂催化剂可以有效地催化丙烯环氧化反应。K⁺修饰后,CuO_x-SiO₂催化剂上主要的部分氧化产物由丙烯醛转变为环氧丙烷。改变多种影响因素（铜含量、铜源、碱金属盐修饰剂及K/Cu摩尔比等）对催化剂进行优化,发现K⁺-5 wt%CuO_x-SiO₂（K/Cu=0.2）催化剂表现出最佳的丙烯环氧化催化性能。该催化剂在反应前并不需要预还原,且在高氧分压的反应条件下,当丙烯的转化率为0.2%时,PO选择性高达78%;当丙烯转化率提高至4.0%时,PO的选择性仍能维持在27%,该催化剂显著优于迄今国内外所报道的其它含铜催化剂。虽然该催化剂也存在失活现象,但主要是由于积炭引起的,通过在550℃下氧气气氛活化处理可以使得催化剂再生。XRD、HRTEM和UV-Vis等表征结果表明,CuO_x-SiO₂催化剂中铜物种主要以CuO微晶和高分散的CuO_x纳米小簇或孤立的Cu（Ⅱ）离子形式存在。经过K⁺修饰后,催化剂中铜物种的配位状态基本没有发生改变,仍然为八面体配位状态,但CuO微晶的粒径明显减小,铜物种的分散度显著提高,这暗示着K⁺和CuO微晶之间存在着较强的相互作用。我们认为这种强相互作用可能对PO的生成有利。H₂-TPR表征表明,K⁺的修饰使得铜的可还原性能明显降低,晶格氧活性受到抑制,这可能进而抑制了丙烯分子中甲基氢的氧化。NH₃-TPD和CO₂-TPD研究发现,CuO_x-SiO₂催化剂经过K⁺修饰后,催化剂表面的Lewis酸性位显著减少,并产生了一定的碱性位。催化剂中酸性位的减少大大抑制了生成的环氧丙烷发生裂解、异构、聚合及进一步的深度氧化反应。这也通过以环氧丙烷作为反应物,考察其在反应条件下的转化得到证实。我们认为催化剂表面的酸碱性质对环氧丙烷的形成起到至关重要的作用。XRD、EPR和FT-IR研究表明反应条件下催化剂上生成了Cu（Ⅰ）位,且高浓度的Cu（Ⅰ）位对环氧化有利。我们首次提出Cu（Ⅰ）位是丙烯环氧化的活性位,我们推测Cu（Ⅰ）位可能起到活化氧气、生成对丙烯环氧化有利的亲电性氧物种的作用。更多还原

【Abstract】 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（Ⅱ）O clusters or isolated Cu（Ⅱ） 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（Ⅱ） and Cu（Ⅰ）,and a higher concentration of Cu（Ⅰ） favored C₃H₆ epoxidation.For the first time,we have clarified that Cu（Ⅰ） sites can function as the active sites for the epoxidation of propylene by oxygen.Cu（Ⅰ） sites are proposed to account for the activation of oxygen,generating an electrophilic oxygen species for C₃H₆ epoxidation.更多还原