【作者】 鄢红；

【导师】 段雪；

【作者基本信息】 北京化工大学 ， 应用化学， 2009， 博士

【摘要】 基于结构的可调变性,层状复合金属氢氧化物（LDHs）在催化和吸附、分子容器、药物缓释、光、电、磁功能材料等诸多领域引起了研究者的广泛关注,取得了长足进展。众所周知,材料的性能与其结构密切相关,故LDHs的结构设计与构筑是推动其进一步发展和应用的基础。然而,目前人们对LDHs结构构筑的认识仅仅建立在有限的经验判据之上,很多实验结果和现象得不到合理的解释。本论文围绕长期实践中遇到的问题展开,旨在通过理论计算,从原子及电子层次上揭示LDHs微观结构与宏观物性之间的相关机制,解释、澄清实验科学中若干争议性问题,提出设计、合成LDHs的更为合理的判据,从而为超分子插层结构LDHs材料的进一步发展奠定理论基础。论文立足于LDHs材料的主体结构,从主、客体两方面对LDHs的结构影响因素进行了系统的理论研究。内容涉及:层板金属元素的性质、组成、排列的有序无序性、层板堆积方式及层间客体对体系稳定性所起的作用;建立了分子簇及周期性两种模型。采用密度泛函方法（DFT）对分子簇模型的几何构型、结合能、形成能、稳定化能、自然键轨道（NBO）、振动性质进行计算;采用基于DFT的赝势平面波方法对周期性固体模型进行了结构、晶胞优化、晶格能及电子态密度（DOS）的计算。以LDHs层板的最小模板基元MO₆八面体[M（OH₂）₆]ⁿ⁺¹（M=金属离子,n=1,2,3,4）结构优化为基础,提出了构筑LDHs层板的八面体变形度判据:变形度小于1°的金属离子易引入层板,而大于10°的则很难引入层板形成稳定结构。这一判据反映了金属离子的配位环境,从科学本质上分析了金属离子引入LDHs层板的可能性,对“离子半径相近”经验规则无法解释的实验事实进行了合理的解释。计算所得的MO₆八面体相关结构参数已研制成数据库。对含闭壳层离子Mg²⁺,Ca²⁺,Zn²⁺,Cd²⁺和开壳层离子Mn²⁺,Fe²⁺,Co²⁺,Ni²⁺,Cu²⁺的[M₂Al（OH₂）₉（OH）₄]³⁺团簇进行了计算,阐明了二价离子的电子结构对相应的LDHs的结构及其稳定性起到决定性作用。结合能变化规律与实验所得的LDHs的相对稳定性高度一致:开壳层体系中稳定性最高的是2Ni-Al结构,最低的是2Cu-Al结构;闭壳层体系中2Mg-Al和2Zn-Al的结构最稳定。澄清了LDHs,特别是含Ni和Cu的LDHs结构稳定性的本质原因。对[Mg_n-1M（OH₂）_n+6（OH）_2n-2]³⁺（n=3～6）和[Mg_n-1M（OH₂）_2n-2（OH）_2n-2]³⁺（n=7）（M=Al,Ga）团簇进行了计算,提出了纯相LDHs稳定存在的M²⁺/M³⁺比（R）范围为2～5。在R＜5时三价离子对体系的结构及稳定性起主导作用,而且R值越小越稳定;而当R≥5时,LDHs相的稳定性下降,Mg²⁺占据了团簇中绝大多数八面体的中心位置,导致体系的结构与Mg（OH）₂类似。对“纯相LDHs在R=2～4之间存在”的经验规则进行了合理的解释。无序性Mg-Al-Cl-LDH晶胞的计算结果表明Mg-Al-LDH可能的存在范围是在Mg/Al比R=2～4。由于Al-O-Al键的存在,其稳定性随Mg/Al比的增大而升高。这是由于R值增大（Al含量减少）,导致金属离子3s、3p和Cl的2p电子密度减小而使HOMO-LUMO能隙变大,故体系趋于稳定。该计算结果不但证实了Lowenstein规则的正确性,即:在稳定的水滑石构型中应当避免出现Al-O-Al键,而且表明在R值较低时,Mg-Al-LDH中金属离子存在有序的排布方式。三种层板堆积方式（3R,2H,1H）下无序性Mg-Al-Cl-LDH晶胞的计算表明,相同的Mg/Al比,3R堆积方式最稳定。这是由于3R体系中堆积层数最多,原子数目增多,金属离子的3s、3p电子密度增大,导带价电子能级降低所致。计算并对比了3R Mg-Al-LDH层板与含阴离子（Cl^-）的无序性Mg-Al-Cl-LDH晶胞,表明阴离子的插层显著降低了晶胞参数、键长和键角,使层间距减小,层板厚度增加,体系稳定性增强。这是由于阴离子Cl^-进入层间区域造成Cl的2p电子和O的2p电子离域性增强,即主体与层间客体相互作用增强所致。本论文工作使得LDHs类材料的设计、合成与构筑从经验的、尝试性的工作提升到了一个理论高度,能够为实验工作者提供丰富的理论信息和有益的指导,对合成新型的LDHs类功能材料奠定一定的理论基础。更多还原

【Abstract】 Layered double hydroxides（LDHs） have received considerable attention as a good template for the design and assembly of functional host-guest materials.However,some generally accepted empirical rules for the LDHs synthesis can not give rational explanation for many experimental findings.Since the close relationship between the material properties and structures has been confirmed thus far,the theoretical studies in this dissertation were carried out in order to understand the microstructure of LDHs at the scale of atomic and electronic,explain the unclear experimental facts,and find out more rational rules for the design and preparation of LDHs materials.The molecular cluster and periodic solid models have been used for the calculation of the electronic property of metal cations in LDHs layer, the composition,ordering and stacking pattern of layers as well as the property of interlayer anions.The geometry,binding,formation and stabilization energy,natural bond orbital and vibration property of the cluster model were computed by density functional theory（DFT） method. In the case of the periodic model,the calculations were performed using the plane-wave pseudopotential implementation of DFT.The initial template unit of LDHs layers,MO₆([M（OH₂）₆]ⁿ⁺（M= metal cation,n=1,2,3,4）,has been investigated.A distortion-angle rule was proposed that the metal cations with the distortion angle（θ） smaller than 1°are easily incorporated into LDHs layers to form stable structures, while those withθlarger than 10°are difficult to be introduced into LDHs layers.Some of the experimental findings which can not be explained by the empirical ion-size rule can be well understood by the calculation-based rule.A database of the calculated geometric parameters has been thus established.The[M₂Al（OH₂）₉（OH）₄]³⁺ clusters(M=divalent cation Mg²⁺,Ca²⁺, Mn²⁺,Fe²⁺,Co²⁺,Ni²⁺,Cu²⁺,Zn²⁺ or Cd²⁺) have been studied.The calculated binding energies are in good agreement with the relative stability of the experimental results for the corresponding LDHs.The 2Ni-Al cluster shows the highest stability among the open-shelled-cation-containing clusters,while the stability of the 2Cu-Al cluster is the weakest.The 2Mg-Al and 2Zn-Al cluster are the most stable ones among the closed-shelled-cation-containing clusters.The electronic structure of divalent cations plays a more significant role in the structure and stability of the corresponding LDHs layers rather than ionic size.The[Mg_n-1M（OH₂）_n+6（OH）_2n-2]³⁺（n=3～6） and[Mg_n-1M（OH₂）_2n-2 （OH）_2n-2]³⁺（n=7）（M=Al,Ga） clusters have been investigated.The stability of both MgAl and MgGa cluster increases with the decrease of M²⁺/M³⁺ ratio（R）.The trivalent cation(Al³⁺ or Ga³⁺) plays a more significant role than Mg²⁺ in the microstructure and bonding properties of the corresponding Mg_n-M（Ⅲ）（M=Al,Ga） clusters,especially when R<5; while Mg²⁺ becomes a dominant factor when R≥5.These findings imply the R range（R=2～5） for the stable formation of pure LDH phase.The calculation of disordered unit cell of Mg-Al-Cl-LDH shows that the probable R（Mg/Al ratio） range for the stable formation of pure LDH phase is R=2～4.The stability of the Mg-Al-Cl-LDH unit cell increases upon increasing R due to the Al-O-Al bond therein.The Al content decreases with the increase of R,which leads to the decrease of electron density of 3s and 3p in metal cation and that of 2p in Cl.Consequently, the HOMO-LUMO energy gap as well as the systematical stability increase.From this point of view,the Lowenstein rule was substantiated that the Al-O-Al bond is absent in LDHs layers.The disordered unit cells of Mg-Al-Cl-LDH with three different stacking sequence（3R,2H,1H） were calculated respectively.The results reveal that system with 3R polytype is the most stable one with the same Mg/Al ratio.This finding is related to the reason that the conductive band energy levels decline with the increase of electron density of 3s、3p of metal cation in 3R system. The 3R Mg-Al-Cl-LDH is more stable than that of Mg-Al-LDH-layer due to the intercalation of Cl anion.This stability also leads to the significant decrease of unit cell parameters,bond angles, bond angles and basal spaces in Mg-Al-Cl-LDH,along with the increase of the layer thickness.These changes result from the superposition of the electron density of 2p in Cl and that in O,which increases the dislocation of 2p electrons as well as the interaction between host layers and interlayer anions.The theoretical calculations in this dissertation not only provided a number of theoretic information of LDHs structure,but also clarified some unclear experimental facts.This work therefore will play an important role to instruct the design and preparation of LDHs or related materials with prospective applications.更多还原