催化剂的结构(上)催化剂: 结构与活性关系 催化剂的选择性分四个层次: 化学选择性(chemo-selectivity), 立体选择性(stereo-selectivity), 对映体选择性(enantio-selectivity), 和局域选择性(regio-selectivity)。催化剂选择功能的实现依赖于催化剂的结构电子的或几何的结构。 当代催化化学的中心论题是提高催化剂的专一性(specificity), 因而催化剂的结构设计至关重要。 一. 分子筛催化剂 (chemo & stereo) 固体酸催化剂, 唯一被证明晶体结构与选择性有良好关系(shape selective, 择形催化)的催化剂 二. 担载催化剂 (chemo) 载体多是氧化物, 被担载组分多是过渡金属和/或氧化物, 是典型的非晶体系. 三. 非担载固体催化剂 (chemo) 原理上同担载催化剂. 四. 络合催化剂 (chemo, stereo, enantio & regio) 均相催化, 典型的立体选择性体系, 可实现不对称选择催化(asymmetric catalysis)和官能团识别(discrimination, 生物酶).催化剂: 结构与活性关系 沸石是具有骨架结构的硅铝酸盐，沸石骨架形成的空腔被较大的离子或水分子占据。这些离子或水是近乎自由的，可以被离子交换或可逆地脱去。n开放的沸石骨架是通过共享顶部的四面体位构建的，Si4+/Al3+交替出现在四面体位，但两个铝不能相邻，即没有AlOAl链接。显然, AlO4四面体需要阳离子的参与以保持电中性，这是形成酸性位的化学本质。所以，沸石/分子筛是典型的固体酸催化剂。1. 分子筛催化剂分子筛酸性位的形成 H+ H+ H+ O O O O O / / / / Si Al Si Si 分子筛的结构特点:1. 晶体结构; 2. 骨架由多元环组成; 3. 有笼(cage)和孔道(channel); 4. 酸性随硅铝比变化而变化.粘土膨润土,高岭土,水滑石复合氧化物 沸石，分子筛盐硫酸盐镁锌铜及过渡金属盐磷酸盐铝等及过渡金属盐固体酸SO42-/ZrO2,SO42-/SiO2阳离子交换树脂交换型分子筛,磺化酚醛树脂固体酸/碱催化剂的种类 分子筛的人工合成始于40年代，主要是合成低/中硅铝比的沸石。60年代Mobil合成了高硅沸石ZSM-5并迅速在炼油工业中获得大规模应用，由此开始了分子筛合成与应用的新时代。ZSM-5有分子级的孔道，亲油疏水，水热稳定性高。分子筛的人工合成 沸石(zeolite)在真空下脱水得到分子筛(molecular sieves)，因为其规整的孔道结构可以“筛分”分子。现在倾向于把所有这一类有规整孔道的骨架化合物统称为分子筛。典型人工合成分子筛示意 Supercage (1.3 nm)左：八面沸石右：丝光沸石(mordenite)Si/Al3Y型分子筛分子筛的人工合成分子筛的人工合成择形选择性择形选择性催化剂孔径（）2,6-/2,7-异构比2,6-%SiO2/Al2O360132Zeolite L7.10.822Zeolite Y7.3137C Mordenite7.02.770ZSM-55.5lowlow 分子筛的结构-活性关系: 1. 分子筛的酸性不是晶体性质而是表面的从而是非晶特性. 2. 硅铝混合显然是改变了O-H键的键强, 原理同担载催化剂. 3. 分子筛的立体选择性(择形)是晶体的结构所致. Metal oxides, particularly those of the transition metals, are widely used as catalysts. For this purpose, they are often supported on other oxides such as aluminas (m.p. 2000 oC) and silicas (m.p. 1700 oC) of different types. The oxide supports have high melting points and surface areas and are somewhat amorphous in nature. 2.氧化物的表面化学小结：吸附与络合之化学原理比较 2.1 简评表面化学In comparison with studies on othermaterials,studiesofcatalystspaymuchmoreattentiontothebehavioronthesurfaceorattheatomiclevel.Duetothispoint,althoughcatalysis studies and others investigate thesamematerialsinthesameframework(frommacrotomesotomicro),the strategies and the methods are greatly different from each other. Surface chemistry/physics has opened up a new world in the last 40 years. Initially, scientists believed that studies on ideal surfaces would finally give us a key to open the black box of catalysis. Great progress has also been made in the studies on ideal oxide surfaces in the last ten years. How much the research of surface science based on ideal surfaces influence the science of catalysis is still an open question, however.2.1 简评表面化学Phenomenon:IdealsurfaceisnotperfectPhenomenon:ReconstructionFigure 8. The hydrogen induced paired new reconstruction of Ni110.78 Part a shows the direction of the pairing displacements of the top layer atoms; b shows a plan view of the reconstructed surface, while c shows a cross sectional view. Hydrogen atoms have been omitted for clarity.a)b)c)Phenomenon:ReconstructionFigure 9. The Cu110-(23)-N structure.105 The small dark circles represent the N atoms, the white circles represent the pseudo-100layer,and the shaded circles represent the underlying 110 substrate.Part a shows a plan view of the surface, while b displays a cross sectional view through the surface.The outermost, 100-like, layer is buckled by 0.52 by the registry mismatch with the second layer.a)b)Phenomenon:ReconstructionandRelaxationConcept:SiteConcept:Ensemble(domain)岩盐（100）：MgO/CaO/MnO/NiO/CoO(Henrich/Cox, Cambridge, 1994)2.2 氧化物的表面化学钙钛矿(100)：SrTiO3/BaTiO3/WO3/NaXWO3(Henrich/Cox, Cambridge, 1994)2.2 氧化物的表面化学钙钛矿(111)：SrTiO3/BaTiO3/WO3/NaXWO3(Henrich/Cox, Cambridge, 1994)2.2 氧化物的表面化学金红石(110)：TiO2/SnO2/RuO2(Henrich/Cox, Cambridge, 1994)2.2 氧化物的表面化学金红石(100)：TiO2/SnO2/RuO2(Henrich/Cox, Cambridge, 1994)2.2 氧化物的表面化学刚玉：Al2O3/Ti2O3/V2O3/-Fe2O3(Henrich/Cox, Cambridge, 1994)2.2 氧化物的表面化学纤维锌矿：ZnO(Henrich/Cox, Cambridge, 1994)2.2 氧化物的表面化学尖晶石：MoO3/Cu2O/V2O5/PbO/-Al2O3/(M3O4)(Henrich/Cox, Cambridge, 1994)2.2 氧化物的表面化学Steps on oxide surfaces (Henrich/Cox, Cambridge, 1994)2.2 氧化物的表面化学2.3Concept:Two-dimensionalorderAn example of two -dimension long distance ordered (Gallowy,Benitez and Salmeron)2.3Concept:Two-dimensionalorder2.3Concept:Two-dimensionalorder2.4Concept:Geometry2.5 Limitations of Characterization The comparison of CO hydrogenation catalyst ZrO2/Al2O3 catalyst between XRD and radial structure functionOXIDESZrO2CeO2TiO2-Al2O3MgOSiO2Bulkbondingcovalentcovalentioniccovalentioniccovalentredoxpartly yesCationicSiteionicradius() 0.79 0.92 0.680.51 0.660.42site-symmetryZrO7ZrO8TiO6AlO4AlO6MgO6SiO4O2defectsnoneyes yes noneyesnoneoxidationstate4+4+4+3+2+4+2.6 小结OXIDESZrO2CeO2TiO2-Al2O3MgOSiO2SurfaceProperty bondingcovalentexposedfaceClose-packed(110)60%(100)(111)(110)(100)Mg2+-O2-alteredfunctionalgroupO2-O2-/OH-O2-/OH-OH-OH-OH-arealowlowhigherhighlowhigh2.6 小结