无机/有机包覆磁性纳米载体表面离子液体桥连负载TEMPO的结构调控与性能研究

Chemoselective oxidation of alcohols to carbonyl compounds is an important reaction in organic synthesis.In view of economic and environmental points, the use of metal-free catalysts for selective oxidation of organic compounds is attracting more and more attention. Stable free nitroxyl radicals,typically 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO),play an increasingly important role in this area. Normally, the efficient transformation of alcohols to carbonyl compounds or carboxyl acids under mild condition include the use of 1mol% of TEMPO as a catalyst and stoichiometric amount of a terminal oxidant.Various oxidants are successful for the alcohol oxidation, however, the isolation of TEMPO from the reaction mixture or products needs tedious procedure.To simplify the products purification and catalyst recovery, the immobilized/supported TEMPO catalyst seems alternative. Unlike conventional micro-size particles, the activity of superparamagnetic nanoparticles (Fe3O4 SPNs) immobilized TEMPO can be benifited from keeping catalyst particle size as small as possible regardless of catalyst recovery.However, naked Fe3O4 SPNs is susceptible to aggregate in clusters and be corroded under the TEMPO mediated oxidation, which results in a significant loss of their superparamagnetic strength and recovery times. Moreover, the limitation of supported amount of active points decreases the activity of this kind of heterogeneous catalysts despite of the nano-size effect. In our previous study, a protocol for coating Fe3O4 SPNs with a poly(styrene-co-chloromethyl polystyrene) layer was applied to successfully prepare (Fe3O4) core/(Polymer) shell supported TEMPO, which exhibits much more reused times with promise performance.In this project,a silica/polymer double coating strategy will be used to enhance nano-magenetic supporter's stability in a liquid reaction solution. The TEMPO will be immobilized on the supporter through a unique surface linkage procedure-ionic liquid (IL) bridge.The super-hydrophobic polymer shell/ionic liquid adjustable surface enables the catalyst to have a feature of high enrichment and good miscibility with reactants, and it is believed that this enrichment effect and phase transfer behavior of ionic liquid enhance the catalyst activity; the resulting catalyst could show identical or even higher activity than their corresponding homogeneous analogues. Experiments of nano-structure regulation, surface modification, IL brush thickness and TEMPO grafting density adjustment will be carried out to investigate the relationship between the structure and the performance (affinity adsorption of reagents, catalyst activity and recyclability, ect.) of catalyst. The outcomes of this original research could offer a novel procedure for catalyst immobilization and recycling, promoting the development of the green oxidation technology and polymer interface science.

2,2,6,6-四甲基哌啶氮氧自由基 (TEMPO) 是一种重要的有机小分子催化剂，可催化次氯酸盐、双氧水、氧气等绿色氧化剂将有机醇高效地氧化为相应的羰基化合物。纳米磁核固载化可实现外磁场下催化剂的快速分离,是实现TEMPO催化醇氧化这一绿色反应经济性和环保性的有效途径。本研究针对现有纳米磁核固载TEMPO方法存在的稳定性较差、活性降低的不足，采用二氧化硅/聚苯乙烯双层包覆提高纳米磁核在反应液相环境中的稳定性；对催化剂载体表面性质、TEMPO负载量和负载方式进行调控，利用底物在载体表面选择性富集、离子液体聚合物刷相转移促进和负载量优化提高纳米磁核负载型催化剂活性；通过催化醇氧化反应研究其结构与催化性能和磁分离效果之间的关系，制备出可快速磁分离、高活性和高稳定性的TEMPO催化剂。研究结果将为纳米磁核载体负载催化剂性能的提高提供新的研究方法和实验思路。

醇选择性氧化为相应的酮、醛等羰基化合物是工业合成和基础研究中最重要的化学反应之一。传统的醇氧化方法主要采用化学计量的高价态的过渡金属盐（如Collin试剂Cr(IV)等），这类氧化剂成本高、选择性低且反应过程会产生重金属污染物，研究和探索绿色、高效的非重金属氧化剂及有机催化剂已成为醇氧化反应的热点。项目针对具有工业化应用价值的代表性有机小分子催化剂TEMPO的分离回收问题，利用纳米顺磁性材料高比表面积可快速磁响应的原理，构建了一类具有核/壳结构、有机/无机复合、表面性质可调的高效磁响应TEMPO负载型催化剂。系统研究了可聚合纳米SiO2包覆磁核的结构调控以及结构层次与磁核在酸碱环境中稳定性的规律；制备了活性氯甲基含量可控交联核/壳型纳米磁性微球，通过离子液体桥连修饰的策略制备咪唑型离子液体直接（单点）桥连的TEMPO催化剂以及可控自由基聚合方法制备了海胆型TEMPO催化剂；系统研究了离子液体桥连修饰的纳米磁性TEMPO催化剂的磁响应、液/液相界面的吸附特性；采用Anelli-Montanari氧化体系评价催化剂的活性与底物的普适性；探究催化剂纳米结构、表面微环境对催化剂活性、稳定性和回收效率的影响规律；揭示了磁性纳米TEMPO相界面催化剂的强化协同催化醇氧化的反应机理。项目的研究结果和意义主要体现在：无机/有机双层包覆的策略可显著提高纳米磁核在反应液环境中的稳定性，可有效解决TEMPO磁力分离回收问题并提高磁核负载催化剂套用稳定性，为有机小分子催化剂的固载化提供新的思路和方法；离子液体桥连长链烷基接枝TEMPO是调控负载型纳米催化剂表面性质的有效方法，纳米载体可有效稳定油相/水相界面，非均相液/液中负载型TEMPO纳米催化剂总反应活性是小分子TEMPO的数倍；纳米磁性相界面催化剂的反应强化和催化机理为新型催化剂的结构设计、液/液非均相的过程强化提供了理论上的依据。项目资助下共发表SCI论文13篇（11篇IF > 3.0），EI论文2篇，3篇稿件在审，培养在站博士后1名，指导毕业博士研究生2名，在读博士生2名，毕业硕士生4名，在读硕士生1名。