过氧铌/钽酸盐离子液体的构建及其催化氧化活性中心的演变规律

Catalytic oxidation is a critical approach to obtaining epoxides. The epoxidation process with hydrogen peroxide as an oxidant and polyoxometalate (POM) as the catalyst has an advantage of mild reaction condition and high yield of the epoxides. Although tremendous progress has been achieved in the field, there still remain many challenges. The first is that the evolution of catalytically active peroxide species is not understood clearly during the reaction; The second is that POM could degrade into smaller peroxide species and thus result in leaching in the presence of hydrogen peroxide; The third is that there might be a difficulty in separation of product from catalyst. On the basis of the other groups’ and our previous work, in this project, we attempt to construct the novel functionlized ionic liquids, which consists of organic cations and monomeric peroxoniobate/peroxotantalate anions coordinated/stabilized by various ligands. These new functionalized ionic liquids will be employed for the epoxidation of allylic alcohols and olefins. The properties of the functionalized ionic liquids can be regulated according to the hydrophilicity/hydrophobicity, aggregation structure and redox properties by coupling the organic cations and monomeric peroxoniobate/peroxotantalate anions appropriately. In this project, we try to reveal the structure evolution of catalytic active peroxoniobate/peroxotantalate anions in the course of the reaction in order to understand the relationship of structure and catalytic activity by the combination of experiments and chemical calculation (Density Function Theory, DFT), and thus establish the common rules of the monomeric peroxoniobate/peroxotantalate anion-functionalized ionic liquids in catalyzing the epoxidation of olefin/allylic acohol. This project not only offer an deep insight into the nature of catalytically active sites for the organic ligand-stabilized peroxoniobate/peroxotantalate anion-functionalized ionic liquid, and also opens new approach towards a facile, rational strategy to modify the properties of ionic liquids with optimized catalytic activity and stability, leaching resistance and separation performance.

催化选择氧化是生产环氧化合物的关键技术。以双氧水为氧化剂，多金属氧酸盐为催化剂的环氧化过程具有条件温和、产物收率较高的优点。目前存在的问题是对活性过氧中心在反应中演化规律认识不清晰，多金属氧酸盐降解导致组分流失、产物分离困难等。基于以上缺陷和申请人前期对多金属氧酸盐催化剂的研究基础，本项目拟以催化烯烃/烯醇（不对称）环氧化反应为研究对象，构筑一系列单核过氧铌/钽酸根阴离子功能化的离子液体，得到氧化-还原性、亲水-疏水性、聚集结构等性质可以精细调控的催化新体系。本项目试图揭示功能化离子液体阴、阳离子相互作用本质，结合原位表征和理论计算的方法，系统研究在反应过程中过氧铌/钽酸根阴离子结构的动态演变规律，从而认识过氧金属物种和催化性能之间的构效关系。本项目不仅阐明催化环氧化的反应机制，利用新型离子液体解决催化体系的稳定性问题，同时提升催化剂分离性能，为拓展离子液体催化体系的应用提供可借鉴的思路。

烯醇及烯烃的环氧化反应在工业上具有重要的意义，反应生成的环氧化物是化工生产的重要中间体。由于硅钛分子筛存在大量微孔，其对小分子烯烃具有较好的活性，对于长碳链烯烃存在导致扩散受阻而活性较低，容易阻塞孔道, 从而导致TS-1失活。另外，尤其在实际反应条件下，分子筛上Ti活性中心流失，也导致催化剂失活。本项目针对以上实际问题，结合课题组先前的研究基础，设计了一些列过氧金属多酸（Nb/Ta/V）功能化的离子液体催化剂，高效催化烯烃及烯醇的环氧化反应。实验研究与理论计算结合表明：1）金属多酸阴离子中的金属过氧物种（M-h2-O2）可与烯醇中的羟基的氢键作用，促使环氧化反应发生；2）a-羟基羧酸或者CO2均可与过氧金属阴离子配位，可以明显降低金属-过氧物种的电荷密度，利用过氧物种对烯烃的亲电进攻反应，导致更高的环氧化反应活性，研究发现这些催化剂也对苯胺氧化偶联制取氧化偶氮苯也呈现高活性以及选择性；3）含氟有机铵盐可以稳定、修饰铌氧簇，研究发现氟离子对铌中心起到强烈的配位稳定作用，有机阳离子则起到平衡电荷和调节疏水性的作用，得到的铌氧簇催化剂对于硫醚的氧化。烯醇环氧化反应具有极高的催化效率；4）吡啶甲酸功能化离子液体可作为配体与钒配位而形成了稳定钒氧簇，通过离子液体和钒摩尔比的调节可以实现钒状态的调控，钒氧簇催化剂中主要存在为低聚态（三聚体），其为催化活性钒物种，并且在H2O2存在时也没有分解为单核钒络合物；5）利用超分子相互作用，冠醚可以与简单过氧铌酸铵反应构超分子离子液体，其对烯烃、烯醇环氧化呈现高活性，且非常稳定。通过本项目的研究，充分利用了离子液体可设计的优点，实现了高效催化环氧化、清洁分离等一系列反应过程，为将来工业化应用奠定了良好的基础。