【作者】 陈晓东；

【导师】 李映伟；

【作者基本信息】 华南理工大学 ， 物理化学， 2019， 博士

【摘要】 金属有机骨架（Metal-organic Frameworks,MOFs）是由氧、氮等多齿有机配体与无机的二级结构单元构筑而成的一类新型多孔材料。由于具有大的比表面积和高的孔隙率等优异的物理化学特性,在过去二十年里,MOFs材料在催化应用方面展现了巨大的潜力和特殊的优势。MOFs自身具有丰富的配位不饱和的金属位点和功能化的有机配体可以作为酸/碱催化剂,但受催化活性及稳定性限制,催化应用范围不广,设计和制备高性能的MOFs基催化剂对推进MOFs材料在催化领域的实际应用具有重要意义。本论文以MOFs作为载体修饰催化活性位或作为前体制备衍生多孔碳复合材料,设计和制备了一系列具有高催化活性、高稳定性的新型MOFs基催化剂,应用于重要的工业加氢、环境污染物催化转化及能源转换的反应中,并研究了催化剂结构与催化性能之间的关系。论文的主要研究内容和结果如下:选择同时具有高孔隙率、高结构稳定性以及Lewis酸性位的MIL-101为载体,通过胶体沉淀法负载高活性的Pd纳米颗粒。制得的Pd/MIL-101具有双功能催化剂的特性,在Lewis酸性及Pd催化活性的协同作用下,可以同时催化硝基的还原、苯环的加氢以及氨基的交叉偶联反应进行,首次实现了从硝基苯到二环己胺的高效转化。此外,研究还发现溶剂的极性对硝基苯加氢反应具有重要作用,可以通过调节溶剂的极性得到苯胺类衍生物或二环己胺类衍生物。以聚苯乙烯小球（PS）为模板,合成具有核壳结构的、含有不同Zn²⁺/Co²⁺比例的PS@Zn_1-xCo_x-ZIF材料。通过在高温下煅烧,制备了一系列具有空心结构的Co/N共掺杂的多孔碳材料Co-HNCS-x。前体材料中合适的Zn/Co摩尔比可以使其衍生多孔材料同时具备大的比表面积和孔体积、高的N含量、超细的Co纳米颗粒及较高的石墨化程度等优势。而空心结构则可以降低扩散阻力、增加催化活性位点的暴露。在ORR反应中,Co-HNCS-0.2表现了显著提升的催化活性,其半波电位达到了0.82 V,远优于单金属ZIFs衍生的Co-HNCS和HNCS的空心材料,也优于双金属ZIFs衍生的Co-NC-0.2实心材料。此外,该催化剂还显示了4电子的反应机理,而且具有良好的稳定性。发展了简单、高效的三聚氰胺与MOF共煅烧的方法,制备了同时具有3D互联的介孔结构以及Fe原子高度分散的Fe@NMC-x催化剂。前体中加入适量的三聚氰胺可以使Fe-ZIF-8衍生材料产生丰富的介孔结构,并提升其Fe-N_x活性组分的密度。而过量的三聚氰胺则会使Fe-ZIF-8衍生材料转化为碳纳米管,比表面积降低,活性位点减少。其中,当三聚氰胺与Fe-ZIF-8质量比为1:1时,所得到的Fe@NMC-1具备了结构和组成上的优势,在酸碱介质中都能以4电子过程高效地催化ORR反应的进行,而且还表现了比商业Pt/C催化剂更优的稳定性和甲醇耐受性。该研究为制备高效的非Pt基ORR催化剂提供了一条可行的途径。基于杂原子掺杂能显著改变多孔碳材料的表面化学性质以及特殊的空心介孔结构能促进反应物的快速扩散、提升材料内部活性位点利用率的考虑,通过直接煅烧ZIF-8与三聚氰胺和苯硼酸的混合物,制备了具有空心介孔结构的B,N共掺杂的多孔碳材料。将材料应用于对硝基苯酚的还原反应中,结果表明,B原子和N原子掺杂都能促进催化活性的提升,而B,N共掺杂则具有更显著的提升效果。其中BN/HMC-0.05表现了最优的催化活性,能在3.5 min内将对硝基苯酚完全反应,超过其他多孔碳复合材料;而且材料还显示了较好的稳定性。更多还原

【Abstract】 Metal-organic frameworks（MOFs）are a new class of porous materials that are constructed by organic bridging ligands and inorganic secondary building units.Due to their excellent physical and chemical properties,such as large specific surface areas and high porosities,MOFs have shown great potential and special advantages in catalytic applications over the past two decades.Involving abundant coordination unsaturated metal sites and active sites on the functional organic ligands,pristine MOFs can serve as acid or base catalysts.However,due to the low catalytic activity and stability,their applications in catalysis are limited.Design and preparation of high-performance MOFs-based catalysts are of great significance for advancing the practical application of MOFs in the field of catalysis.In this thesis,MOFs are employed as support to incorporate catalytic active sites or as precursors to derive porous carbon composites.A series of new MOFs-based catalysts with high catalytic activity and high stability have been designed and developed for important catalytic applications,such as industrial hydrogenation,pollutant catalytic conversion and energy conversion.The relationships between the structural properties of the catalyst and the catalytic performance are also investigated in detail.The main contents and findings of the thesis are as follows:Due to its high porosity,high structural stability,and abundant Lewis acid sites,MIL-101 is selected as support to load highly active Pd nanoparticles through a colloid precipitation method.The prepared Pd/MIL-101 presents the characteristics of a bifunctional catalyst.Due to the synergistic effect of Lewis acidity and Pd,Pd/MIL-101 can simultaneously catalyze the reduction of nitro group,hydrogenation of benzene ring and cross-coupling reaction of amino groups.In consequence,the highly efficient conversion from nitrobenzene to dicyclohexylamine can be achieved.In addition,we find that the polarity of the solvents play an important role in the hydrogenation of nitrobenzene,and the aniline derivatives or the dicyclohexylamine derivatives can be obtained at will by adjusting the polarity of the solvent.A series of PS@Zn_1-xCo_x-ZIF with various molar ratio of Zn²⁺/Co²⁺and a core-shell structure are synthesized using polystyrene beads（PS）as template.By carbonizing the above materials at high temperature,a series of Co/N co-doped porous carbon materials（Co-HNCS-x）with hollow structure are prepared.Suitable Zn/Co molar ratio in the precursor material can endow the corresponding derived porous materials with large specific surface area and pore volume,high N content,ultrafine Co nanoparticles and high degree of graphitization.Moreover,the unique hollow structure can not only reduce the diffusion resistance but also increase the exposure of the catalytic active sites.When employed as ORR catalyst,Co-HNCS-0.2 exhibits significantly improved catalytic activity,whose half-wave potential reaches 0.82 V,which is far superior to that of single-metal ZIFs-derived Co-HNCS and HNCS hollow materials,and also superior to bimetallic ZIFs-derived Co-NC-0.2 solid material.Moreover,the catalyst shows 4 electron mechanism and good stability.A simple and efficient melamine and MOF co-calcination strategy is developed to prepare Fe@NMC-x catalysts with both highly dispersed Fe atoms and 3D interconnected mesoporous structure.An appropriate amount of melamine in the precursor allows the generation of rich mesopores and increased density of Fe-N_x moieties in the Fe-ZIF-8 derived materials.Excess amount of melamine in the precursor could convert the Fe-ZIF-8 derivative into carbon nanotubes,however,its specific surface area and the amount of active sites decrease.Among the Fe@NMC-x catalysts,Fe@NMC-1 possessing the advantages both in structure and composition,can efficiently catalyze the ORR reaction in both acid and alkaline media through the 4 electron pathway.In addition,it also exhibits much better stability and methanol tolerance than commercial Pt/C catalyst.These results would provide a viable approach for the preparation of highly efficient non-Pt-based ORR catalysts.It has been reported that doping heteroatoms in porous carbons can significantly change their surface chemistry and catalytic properties.On the other hand,constructing unique hollow mesoporous structure in catalysts can promote the rapid diffusion of the reactants and imrove the utilization of the internal active sites.To combine the above two merits in one catalyst,we have developed a facile strategy to prepared B,N co-doped hollow mesoporous carbons by directly calcining the composites of ZIF-8,melamine and phenylboronic acid.The materials are subsequently applied to the reaction of 4-nitrophenol degradation.The results show that both B and N atoms doping in porous carbon can promote the catalytic reaction,and B and N co-doping leads to much better catalytic performance than single atom（B or N）doping.Among them,BN/HMC-0.05 exhibits the best catalytic performance,which can convert 4-nitrophenol to 4-aminophenol completely within 3.5 min,surpassing other porous carbon composites.In addition,the catalyst also shows good stability.更多还原