【作者】 华春燕；

【导师】 单永奎；

【作者基本信息】 华东师范大学 ， 物理化学， 2007， 博士

【摘要】 多元多金属含氧簇合物(POM)化学是一门古老而又充满活力的分支学科。在近两个世纪里,不仅对它的基础理化科学问题进行了深入的研究,而且在许多领域中也得到了广泛的应用。基于多元多金属含氧簇合物合成方法已知,组成、结构和理化性质表征手段成熟,功能性质研究透彻的事实,人们把它视为实现催化剂人工设计的最合适材料之一,试图以多元多金属含氧簇合物为平台,按照人们的意愿,在分子和原子的水平上,剪裁组装,设计制备催化剂,得到具有满足人们要求特性的材料。本论文就是基于这一点,选择了合成方法已知、结构一定的多金属含氧簇合物Keggin型的[SiMoll-nWnM(H2O)O39]m-为材料的分子设计模型,通过系统地改变杂多阴离子的组成,进而改变其氧化还原行为,得到多金属含氧簇合物的组成和氧化还原性质之间的一系列关系。另一方面,多金属含氧簇合物的氧化还原行为也受外部条件影响。在实验过程中,保证阴离子结构相同、组成一定的前提下,我们选择在三种不同介质——水、有机、离子液体体系——中探讨其氧化还原行为。具体内容如下:(1)利用已知文献方法,合成了具有相同结构的系列多金属含氧簇合物[SiMoll-nWnM(H2O)O39]m-。通过元素分析、ICP对其组成进行了表征;利用IR表征其结构,单晶X-射线衍射确定其晶体结构;XPS确定所含元素的价态;利用Py-IR、NH3-TPD研究化合物的酸性和酸量。确定合成的多金属含氧簇合物符合实验要求。(2)在配位原子种类确定的前提下,我们利用电化学法首先考察了配位原子数目的改变对Keggin型二元混配(Mo/W)杂多阴离子的氧化还原性质的影响。实验发现,在杂多阴离子结构中引入钼原子,硅钼钨杂多阴离子的第一还原电位随着铝原子数的增加呈现先增加至极值而后缓慢下降的趋势。而且,与SiW12和SiMO12相比,硅钼钨杂多阴离子的第一还原电位都大幅度地向正电位方向移动。这可利用Mo的强氧化性和硅铝钨杂多阴离子中钼钨之间的协同效应来解释。并且,随着硅钼钨杂多阴离子中的钨原子数目的增加,可逆性呈现下降趋势。(3)进而在杂多阴离子骨架中分别掺入七种不同电负性的过渡金属离子,进一步探讨不同过渡金属离子自身及对杂多配体的氧化还原性质的影响。取代后的杂多阴离子氧化能力降低,可逆性下降,这主要是由于取代后阴离子结构松散、负电荷增加所致。不同过渡金属离子取代的多元多金属含氧簇合物的第一还原电位之间也存在差别。这主要与过渡金属离子的电负性有关。电负性是表征原子在分子中吸引电子能力的一个物理量。当进入阴离子骨架的金属离子具有较强的电负性时,金属离子吸引电子的能力较强,使得杂多阴离子上的负电荷密度相应较小,氧化能力增强。(4)除了通过改变杂多阴离子的内部因素来实现氧化还原性质的调控以外,外部因素对其氧化还原性质也有重要的作用。通过研究在三种不同介质——酸性水溶液;有机体系;室温离子液体——中杂多阴离子的电化学行为,我们可以明显看出溶剂性质也能调变其氧化还原性。在离子液体中,杂多阴离子的氧化还原电位比DMF体系正,比水体系负。这主要由溶剂的极性决定。水的极性最大,而离子液体则介于水和DMF之间。另一方面,离子液体由于高粘度而导致杂多阴离子的氧化还原电流下降,升高温度可以克服这一缺点。因此,离子液体是代替有机溶剂的理想绿色溶剂。(5)尽管组成各异,但是通过比较可以发现,它们在不同介质中的第一还原电位的变化值是相似的。这进一步证明了杂多阴离子的骨架结构在其与溶剂分子的相互作用过程中扮演着主要角色。因此,杂多阴离子结构的变化,能使性质发生质的改变;而在相同结构中组成的调变仅仅使其性质有量的改变。(6)通过对以多元多金属含氧簇合物为催化剂的异丙醇气相反应的研究发现,这些四元取代多金属含氧簇合物,无论是对反应的催化活性还是选择性都随着配合物的组成变化而变化,并显出很大的可调性和多变性。反应氧化产物选择性与催化剂组成之间的分布较有规律,但这个规律与我们在电化学实验中得到的氧化还原能力次序并不一致。分析原因,我们认为电化学实验仅仅反映多金属含氧簇合物在一特定体系中的氧化还原性质,对于电极反应过程中的电子传递与通过气固异相反应体现的氧化还原催化活性之间的异同,我们还未十分清楚,有待于进一步研究。不过,在判断某反应能否被上述多金属含氧簇合物之一催化或者在选择合适的介质时,由电化学得到的氧化还原电位值能起到借鉴作用。这些实验结果为多元多金属含氧簇合物催化剂的人工设计的意义提供了有益的实验数据和理论基础。更多还原

【Abstract】 Polyoxometalates （abbreviated to POM） are "metal-oxygen molecules", and have been proven to be useful catalytic materials in fundamental studies as well as in practical synthetic processes. POM were suitable materials for both the catalyst design of practical processes and for fundamental researches regarding heterogeneous catalysis, owing to that the relationships among the following four levels of information can be established on the molecular basis, namely, （i） catalytic performance; （ii） chemical and physical properties; （iii） molecular and bulk composition and structure; and （iv） method of synthesis of catalysts. These relationships are most useful for the molecular design of the solid materials and as model system for fundamental research, providing opportunities for mechanistic studies on the molecular level. At the same time, polyoxometalates compounds become increasingly important for applied catalysis. However, the practical catalyst prepared by molecular design had hitherto not been realized. This may be because the understanding and study of their relationship between the composition, structure, chemical properties, physical function and the manipulation in synthesis were not quite enough for the practical and academic requirement.Therefore, in this work, a series of the multicomponent heteroply silicates with Keggin structure [SiMo_11-nW_nM（H₂O）O₃₉]^m- has been synthesized by a controllable method. The correlation between the redox properties and composition of the POM were investigated by cyclic voltammetric method in a systematic way. On the other hand, we also perform a comparative study on the electrochemical behaviors of [SiMo_11-nW_nM（H₂O）O₃₉]^m- in different media （aqueous solution, organic solvent and ionic liquids） by using cyclic voltammetry.The main contents are as follows:（1） According to the literature, a series of [SiMo_11-nW_nM（H₂O）O₃₉]^m- with the same structure have been synthesized. The POM were characterized by elemental analysis, ICP, XPS, IR, NH₃-TPD, Py-IR etc.. The results show that the experimental data are consistent with the theoretical values.（2） When increasing the number of the molybdenum atoms containing in anion, the first reduction wave potential of the tungstomolybdosilic heteropoly anion moves toward more positive potential, which exhibits the first sharply increasing and then slowly decreasing tendency. This phenomenon may be ascribed to the more reducible character of molybdenum and the synergistic effect of the molybdenum and the tungsten in the tungstomolybdosilic anions.（3） Incorporation of the transition metal ion depressed the oxidizing ability of the tungstomolybdosilic anion, owing to increasing the negative charge of the whole heteropolyanions. For a given oxidation state of transition metals, the oxidizing ability of the transition metal-substituted tungstomoJybdosilic anion increases with increasing electro negativity of the transition metal element.（4） In ionic liquids, the potentials for the reduction processes are more negative than that found in aqueous solution, but much more positive than those in DMF solvent. It was proposed that the potentials for the reduction process may be correlated with the solvent polarity. The tungstomolybdosilicates are most difficult to reduce in the least polar solvent medium DMF and are easiest to reduce in the most polar medium H₂O. Therefore, room-temperature ionic liquids, which could dissolve inorganic and organic compounds, are the best candidate for ideal replacement for volatile organic solvent in electrochemistry.（5） Though cyclic voltammograms of POM did exhibit remarkable difference in the medium change from one to another, it can be found that the change value of the first reduced peak potential for these heteropoly compounds almost were similar. This fact testifies that the electric charge and the solid geometry of heteropoly anions play main role in the interaction of the heteropoly anions and solvent molecules, except for the action of the solvent molecules.（6） Decomposition of isopropanol was performed to clarity the properties of POM as bifunctinoal catalysts, and the only products detected were propene and acetone. The experimental results show that the relationship between the composition of the POM and selectivity of oxidation product is not consistent with that obtained from electrochemical investigation. However, the process is still under investigation.更多还原