【作者】 常铮；

【导师】 段雪；

【作者基本信息】 北京化工大学 ， 应用化学， 2006， 博士

【摘要】 水滑石类化合物又称层状双金属氢氧化物（Layered Double Hydroxides，简称LDHs），是一类具有特殊结构的层状材料。它独特的结构有利于多种功能性离子的引入，扩大了此类材料的应用范围，尤其是其作为催化剂和催化剂前体的应用更引起人们的普遍关注。而稀土因为特殊的电子结构排布使其在催化、发光、磁性材料等领域都具有广泛的应用。因此本论文提出利用LDHs的结构特征，引入稀土元素铈（Ce）对其进行修饰，通过插层组装制备出新型含稀土LDHs材料。由于受晶格定位效应的制约，金属离子（稀土）在LDHs层板和层间相互高度分散，经过焙烧即可得到高分散多元金属（稀土）氧化物。在苯酚湿式催化氧化反应中，多元金属氧化物表现出较好的催化性能，组分间的协同效应得到发挥。而苯酚是工业含酚废水的主要成份之一，它的有效氧化和去除使得含稀土LDHs材料在工业废水处理方面具有潜在的应用价值。本文首先采用自行设计的恒定条件下连续共沉淀装置制备出粒径分布窄、形貌均匀的纳米尺寸LDHs材料。详细研究了多种制备条件对ZnAl-CO₃-LDHs结构和形貌的影响，其中金属离子浓度和混合溶剂对溶液的过饱和度影响较为显著。这种方法在控制LDHs晶粒尺寸、形状、团聚程度和比表面积等性能的同时，极大地缩短了制备周期，具有较好的工业应用前景。本论文重点进行了稀土元素配合物阴离子在LDHs结构中的插层组装研究。将配合物[Ce（dipic）₃]^3-引入到Zn₂Al-NO₃-LDHs以及其它具有不同层板组成元素及比例的多种LDHs主体结构中，然后进行不同稀土元素配合物客体[Ce（DTPA）]^2-和[Eu（dipic）₃]^3-在Zn₂Al-NO₃-LDHs中的插层组装。通过主、客体的变化来研究插层过程中两者相互作用及其对插层过程、插层结构的影响。通过配合物[Ce（dipic）₃]^3-与LDHs层间NO₃^-的阴离子交换反应，将稀土元素Ce引入到LDHs结构中，制备出[Ce（dipic）₃]^3-插层LDHs。通过多种表征方法的研究，发现配合物阴离子在层间以与主体层板倾斜的方位排布。在一些阴离子交换过程中，配合物[Ce（dipic）₃]^3-容易发生部分分解生成dipic^2-和Ce³⁺(Ce⁴⁺)，导致dipic^2-插层LDHs副产物的出现。通过配合物[Ce（DTPA）]^2-与Zn₂Al-NO₃-LDHs前体的离子交换反应制备出另外一种含稀土元素Ce的LDHs材料，结构研究表明其层间配合物阴离子以与主体层板几乎垂直的方位排布。采用近似离子交换方法，将稀土元素Eu通过配合物[Eu（dipic）₃]^3-引入到Zn₂Al-NO₃-LDHs结构中，这种超分子插层结构明显提高产物的热稳定性，并具有较好的发光性能。对含稀土LDHs材料的热分解过程进行研究，探讨制备条件（如主体层板金属元素种类和比例、离子交换条件、焙烧条件）与焙烧产物的层状结构、晶粒尺寸及其分布、孔结构、比表面积、组分与组成等结构参数间的关系。Ce的存在明显提高插层LDHs材料的热稳定性：热分解过程中高分散的CeO₂能够提高尖晶石的形成温度，以及活性组分金属氧化物的热稳定性。含稀土LDHs材料经过不同热处理过程制备出系列新型高分散多元金属氧化物，尤其是500℃焙烧6h的[Ce（dipic）₃]^3-插层CuZnAl-LDHs的层状特征结构完全消失，形成二价金属氧化物、二氧化铈和Cu-Ce-O固溶体的混合物。利用共沉淀方法制备出含铈铜基LDHs杂化材料，它是由CuZnAl-LDHs和CeO₂两相组成的混合物，Ce未被引入到LDHs正八面体结构中，但在LDHs颗粒表面形成了均匀分散。利用离子交换方法将配合物[Ce（dipic）₃]^3-引入到CuZnAl-NO₃-LDHs层间得到具有插层结构的含铈铜基LDHs。在其焙烧产物中，Ce的出现明显提高催化剂的苯酚湿式催化氧化活性，影响氧化产物的分布。而在共沉淀方法制备的含铈铜基LDHs杂化材料的焙烧产物中，Ce的出现加强苯酚的深度氧化，减少催化剂中各金属元素的流失。两种催化剂在苯酚氧化反应中表现出的催化性能差异与材料的结构特征以及金属元素间相互作用有关，比表面积的大小和Cu-Ce-O固溶体的生成是对催化活性产生影响的两个竞争因素。更多还原

【Abstract】 Layered double hydroxides （LDHs） and rare earth elements （REEs） have both attracted extensive attention because of their structures and properties. The layered structure of LDHs makes it possible for many functional ions to be incorporated and the uniform dispersion of metal-organic anions in interlayer galleries or metal cations in layers extends the applications of layered materials to a wide variety of fields, whilst the electronic structure of REEs offers many advantages in the areas of optical, magnetic and catalytic materials.Catalytic wet oxidation （CWO） is a promising technique for destruction of organic pollutants, of which phenol is a simple representative, in water under mild conditions. Common heterogeneous catalysts for CWO can be divided into three series: noble metals, transition metals （especially copper） and rare earths, each of which has their own advantages and shortcomings. Co-existence of two types of catalyst components may show a higher catalytic efficiency. It is, therefore, of interest to prepare hybrid materials based on ZnAl-or CuZnAl-LDHs incorporating rare earth elements such as Ce or Eu in order to investigate the potential synergistic effect between the two components and the resultant influence on catalytic properties.A continuous co-precipitation method under steady-state conditions was first developed for the preparation of nanometer-size LDH particles using Zn₂Al（OH）₆（CO₃） _0．5·2H₂O as prototype. The effects of varying the operating conditions on the structural and textural properties of LDHs were studied, including total cation concentration, solvent, residence time, pH and intercalation anion. Increasing either the cation concentration or the fraction of ethylene glycol （EG） in EG/H₂O mixtures affects salt solubility and supersaturation, which results in smaller crystallites, larger surface areas and more amorphous compounds. The new method employs a short residence time of less than 15 min, allows large-scale production and maintains a constant supersaturation level in the reactor, and was shown to be a promising alternative to the conventional batch method.The second part of this research was to prepare a series of novel REE-containing LDH materials by anion-exchange or co-precipitation methods, and to investigate the effects of varying experimental parameters in order to optimize the product properties. Using the anion-exchange method, pyridine-2,6-dicarboxylic acid （H₂dipic） and diethylenetriaminepentaacetic acid （DTPA） were employed as the ligands in REE-containing complex anions. [Ce（dipic）₃] ^3- complexes were firstly introduced into ZnAl-NO₃-LDH and other LDH precursors with varying layer metal ions like CuZnAl-NO₃-LDH, and then different REE-containing complexes such as [Ce（DTPA）]^2- and [Eu（dipic）₃] ^3- were incorporated into LDHs. The products were characterized by many physicochemical techniques, including XRD, IR, UV, ICP, BET, TEM, TG/Mass, in situ HT-XRD and XPS.Ce-containing ZnAl-LDHs prepared by an anion-exchange method with [Ce（dipic）₃] ^3- were found to be always mixed with a CO₃^2--containing LDH. The intercalation of [Ce（dipic）₃] ^3- in the layered host was confirmed by an increase in interlayer spacing to 1．24 nm. Geometrical considerations suggest the complex has a tilted orientation between the layers. However, a fraction of the [Ce（dipic）₃] ^3- anions decomposed into dipic^2- and Ce³⁺ (or Ce⁴⁺) ions during the exchange process, resulting in formation of a dipic^2--containing LDH. Different experimental conditions were optimized, including the host layer composition, charge density and other synthesis parameters such as temperature, pressure, reagent concentration, and reaction time. The most promising product was obtained with a ratio of layer cations M²⁺/M³⁺ = 2, with a [Ce（dipic）₃] ^3- concentration around 5 mmol/1 and an exchange period around 10 h at room temperature. For some other host matrix compositions, the decomposition of [Ce（dipic）₃] ^3- and co-intercalation of CO₃^2- could both be reduced. The interlayer spacing values varied for different layer metals. Another Ce-containing ZnAl-LDHproduct was synthesized by a ion-exchange process with [Ce（DTPA）]^2-. The product showed characteristics of a well-crystallized LDH and the interlayer spacing was enlarged to 1．46 nm.Eu-containing LDHs with [Eu（dipic）₃] ^3- anions in the interlayer galleries were prepared under the same conditions as for [Ce（dipic）₃] ^3--intercalated LDHs and the structure and properties of the products were similar. The immobilized luminescent materials had excellent stability and luminescence properties. Eu³⁺ was used as a structural probe to study the interaction between the layered host and the complex guest by monitoring its luminescence properties. The structural information obtained can reasonably be transposed to [Ce（dipic）₃] ^3--intercalated LDHs.On the basis of studies with REE-containing ZnAl-LDH materials, CuZnAl-Ce（dipic）-LDHs with cerium ions located in the interlayer galleries were synthesized by the ion-exchange method. The decomposition of [Ce（dipic）₃] ^3- and co-intercalation of CO₃^2- were relatively insignificant with this host matrix. Another type of CeX-LDHs （where X represents Ce content） was prepared by a co-precipitation method. It was demonstrated that the products were a mixture of CuZnAl-LDH and CeO₂, with cerium uniformly dispersed on the surface of small LDH particles.Thermal behavior of Ce-containing CuZnAl-LDHs was found to be influenced by the presence of cerium oxides and the temperature of formation of spinel phases was significantly increased. The effects of varying some experimental conditions such as preparation method, layer composition, calcination temperature/time and Ce/Al molar ratio on the specific surface area were investigated. When the product CuZnAl-Ce（dipic）-CLDH with a stoichiometric Ce/Al ratio was calcined at 500℃for 6 h, a composite Cu-Ce-O solid solution consisting of mixed metal oxides and having a large specific surface area was obtained.The final part of research was to prepare a series of new catalysts by calcination of Ce-containing CuZnAl-LDHs, to study the catalytic abilities of these materials in the phenol oxidation reaction and investigate the interaction between Cu and Ce centers in the catalysts. It was shown that the difference in catalytic performances of the two types of Ce-containing CuZnAl-CLDHs is related to the structure and composition of the catalysts. For CuZnAl-Ce（dipic）-CLDHs obtained by calcination of [Ce（dipic）₃] ^3--intercalated CuZnAl-LDHs, the presence of cerium significantly improved the catalytic activity and control over the product distribution in phenol oxidation. The uniform dispersion of Ce-complexes in the interlayer galleries of the LDH precursors results in the presence of a Cu-Ce synergistic effect in the Cu-Ce-O solid solution. The strong interaction between Cu and Ce enhances the catalyst performance. For CeX-CLDHs obtained by calcination of the mixture of CuZnAl-LDH and CeO₂, the presence of cerium enhanced deep oxidation of phenol and reduced the extent of leaching of metal elements, resulting in improved catalyst selectivity and stability. The interaction between Cu and Ce in the two different phases was weak, not enough to give significant increase in catalytic activity, but sufficient to stabilize the active component, Cu, against leaching.更多还原