介孔材料的介观结构与形貌控制

【作者】 胡钧锴；

【导师】 唐颐；

【作者基本信息】 复旦大学 ， 物理化学， 2009， 硕士

【摘要】 尽管介孔材料的研究历史只有十几年,其独特的结构与性质吸引了来自很多领域的科学家。由于介孔材料具有较好的水热稳定性,高比表面,高孔隙率,孔径尺寸相对较大并且在一定范围内可调,使得其在大分子催化、生物过程、选择吸附、功能材料等多方面有广泛的应用前景。研究表明,除了孔道形貌与尺寸外,介孔材料的宏观结构也对其应用有着重要影响。不同形貌的介孔材料有着不同的外表面积,孔道长短也不尽相同,因而其传质阻力有很大差别;同时,材料的大小和形貌也决定了其操作和分离的难易成度。因此,对介孔材料的形貌控制长期以来一直是介孔材料研究领域的热点。本论文将针对目前制备介孔材料的两大方法分别对制备的介孔材料进行形貌和孔道控制。对于经典以表面活性剂胶束为模板制备介孔材料的溶胶凝胶法,我们选取相对研究较少的HMS(hexagonal mesoporous silica)为研究对象。HMS因其三维联通的孔道而相对其他直孔道介孔材料更利于客体分子的孔道内扩散,使其在吸附及催化领域有着广泛应用。然而,同样由于这种空间无序的蠕虫状孔道,使得HMS材料采取各向同性生长,因而其形貌变得难以调控。在第二章中,我们发现了一种同时控制HMS形貌和介观结构的简便方法:通过调变体系醇水比及表面活性剂和共溶剂的种类,可以得到荷叶状、交错薄片、土豆状囊泡、空心囊泡、裂口球等一系列呈现规律形貌变化的HMS材料。通过对其深入表征我们发现以上各形貌都含有层状和蠕虫状两种不同的介孔,且不同宏观形貌的介孔空间分布也不相同。在对体系进行深入研究后我们提出了一套机理来解释这一特殊的形貌变化规律和双孔结构。反应开始时的层状/囊泡状胶束决定了最终产物的宏观形貌。双孔结构的形成是由反应过程中层状结构向蠕虫状结构的转化导致的,其根本原因是由于表面活性剂浓度的降低、醇浓度的上升与硅酸根聚合度的提高造成的。在层状相水平方向的介观结构演化会在初始骨架的基础上影响产品的宏观形貌。除此之外,我们还用粒子成核和生长速度的差异揭示了不同醇浓度下氧化硅微球/纳米粒子的尺寸分布变化。另一种制造介孔的新型方法是基于微孔分子筛来制备微孔-介孔多级孔道沸石材料。这类材料含有的介孔可以使客体分子更容易进入材料内部进行吸附和催化,同时其孔壁由晶化的沸石组成,含有有丰富的微孔和大量的酸性位。使得其具有更为优异的催化性能。在诸多制备微孔-介孔多级孔道沸石材料的方法中,聚合物诱导胶体凝聚法(polymerization-induced colloid aggregation,PICA)由于易于控制,合成效果好而受到越来越多的关注。第三章通过使用未经洗涤的纳米沸石为原料使用PICA法制备了微孔-介孔多级孔道沸石微球,大大简化了操作步骤并使PICA法得以应用于低硅铝比纳米沸石的组装。我们还研究了不同反应位脲醛聚合的速度差异对产品形貌的影响,并通过控制体系酸度和尿素、甲醛含量成功制备了微孔-介孔多级孔道沸石空心球。最后,我们还以丁烯烷基化反应为探针反应,对比了多级有序孔道β沸石球及普通β沸石的催化活性差别,验证了微孔-介孔多级有序结构的优势。更多还原

【Abstract】 Although the discovery of mesoporous materials no more than 20 years, their novel structures and properties have attracted a great many of scientist from many research areas. Because of its high thermal stability, large surface area and porosity, tunable and relatively lager pore diameter, the mesoporous materials have been widely used in many areas, such as macromolecular catalysis, biological process, selectively adsorption, functional materials, etc. Many researches show that beside of their mesoporsity, the morphology is also extremely important for their practical applications. This is because that the morphological difference could lead to the variation of outer surface area and pore length, which would influence the mass diffusion. Beside of this, the size and morphology also determine whether the material is easy to handle. Therefore, the morphological control has long been a highlight in the research region of mesoporous materials. In this thesis, we will focus on two main preparation methods to control both morphology and porosity of mesoporous materials.As for the classical sol-gel method which uses surfactant micelle as template, we select HMS (hexagonal mesoporous silica) for our study. The 3-dimensional (3D) wormholelike or spongelike framework in HMS greatly favors adsorption and catalysis for its easy accessibility of the active sites on its pore walls. However, this 3D disordered wormholelike porosity also leads to the isotropy growth of HMS, and so the less tenability of its morphology. In Chapter 2, a series of HMS with uniform sizes and versatile morphologies, such as lotus-leaf-like flake, aggregated flake, potato-like vesicle, hollow vesicle and cracked sphere, are successfully obtained by simply change the alcohol/water ratio and alcohol or surfactant type. We also found that these materials have the regional distributed lamellar-wormholelike dual-mesoporosity. After systematically investigation the influence of each component, we have proposed a mechanism to interpret these phenomena. The morphology of the product is mainly determined by the initial surfactant micelle. Moreover, with the progress of reaction, the increase of alcohol content, decrease of surfactant concentration and the increase of silica polymerization degree would induce the mesophase evolution of product from 2D lamella-like to 3D wormholelike meso-structures, leading to the formation of dual meso-structured HMS products. This meso-structure evolution at horizontal direction of lamella structure could influence the initial formed scaffold and thus the morphology of final product. Beside of this, the size changes of HMS sphere and the formation of mesoporous nanoparticle at high alcohol/water ratio can be explained by the competition between nucleation and particle growth.Another new method to generate mesopore is the preparation of micro/meso-porous hierarchical structured zeolite materials based on the microporous zeolite. The mesopores in this kind of materials could favour the guest molecule to access the active site in the material, while the large number of acidic site generated by the framework of zeolite would offer it an outstanding catalytic activity. Among varies of methods, polymerization-induced colloid aggregation (PICA) have attract more and more attention. In Chapter 3, by using unwashed nanozeolite as building blocks, we have successfully obtained micro/meso-porous hierarchical structured zeolite spheres by PICA. Compared with the classical method, the procedural of our reformative method is much easier, and this method could also used for the assembly of zeolite with low Si/Al ratio, which would dealuminization if use classical method. We also studied the influence of polymerization speed on the products’ morphology and successfully generate the micro/meso-porous hierarchical structured zeolite hollow spheres by tuning the acidity and concentration of urea and formaldehyde. Finally, we have compared the catalytical activity of hierarchicalβzeolite hollow sphere and commercialβzeolite on the Isobutane/2-Butene Alkylation. And the results show that the hierarchical structure have the advantages on the mass diffusion and coke avoidance.更多还原