Micro/Macroporous System: MFI-Type Zeolite Crystals with Embedded Macropores
Micro/Macroporous System: MFI-Type Zeolite Crystals with Embedded Macropores
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DOI：
10.1002/adma.201404493
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发表时间：
2015-02-11
影响因子：
29.4
通讯作者：
Schwieger, Wilhelm
中科院分区：
文献类型：
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作者：
Machoke, Albert G.;Beltran, Ana M.;Schwieger, Wilhelm
尽管如此，由于缺乏直接在沸石晶体中引入大孔的简单方法，因此嵌入大孔的微孔沸石晶体的开发和利用仍然是一个挑战。目前，在沸石中引入大孔的方法主要有球体模板化、[4]合成后改性、[4]和大孔支架模板化在这些技术中，使用硬模板如乳胶球或二氧化硅球已被广泛采用，以形成不同的大孔沸石结构，如三维有序的沸石大孔结构[6]或空心沸石胶囊[7]然而，硬模板技术仍然受到沸石前驱体的硬模板润湿性低，沸石合成条件下模板热稳定性差，沸石壁厚度难以控制，[6]和水热合成后模板去除等因素的限制为了克服这些限制所采取的策略导致了多步骤的过程，这些过程需要使用表面改性技术和伪固态转化过程来提高这些模板与前驱体溶液的润湿性，并防止模板在热液转化过程中熔化，[6]。此外，分子筛网络形成所需的大量模板（模板/ sio2摩尔比大于0.35）[6,7,9]被用于控制沸石壁的厚度，并在合成后采用苛刻的条件去除硬模板这些缺点仍然阻碍着大孔沸石的发展。制备具有额外大孔的沸石组件而不使用任何外部模板的另一种策略是使用介孔二氧化硅颗粒（MSPs）作为大孔形成的二氧化硅源和牺牲模板然而，目前可用的方法[10 - 12]是通过一层一层的沉积过程[12]将沸石种子包覆在这些msp上，形成中空的沸石胶囊[10]或具有孤立大孔的3D结构，或者它们还需要额外的二氧化硅源来形成多晶沸石相，该沸石相围绕着相互连接的大孔[11]沸石种子的制备和在MSPs上的涂层使这一过程变得费力。因此，没有方法可以制备具有接近经典形态的沸石晶体和嵌入在沸石基质中的晶内大孔系统。为了克服现有大孔沸石组合的局限性，在我们的自然环境中经常遇到具有等级组织系统的mfi型沸石晶体这种分级系统主要与结构特性（如树干、骨骼）或流体动力学特性（如肺或血液循环）有关这些系统能够最大限度地提高运输过程的效率，这一直是它们在从浇水系统到催化反应器的不同人工系统中实施的灵感。沸石晶体可以看作是一个小型催化反应器的集合，它们的微孔提供了大的比表面积，活性位点的确定环境以及每个单孔入口的形状选择性因此，沸石属于当今使用的最重要的催化材料。然而，由于反应物质在微孔内传输缓慢，它们在催化中的应用受到限制。为了尽量减少这些传输限制，减少扩散路径长度是非常可取的。纳米沸石或沸石体系的制备
Despite of this, the development and utilization of microporous zeolite crystals with embedded macropores is still a challenge due to the lack of simple methods to introduce macropores into zeolite crystals directly. Currently, macropores are being introduced in zeolites through sphere templating,[4] postsynthetic modifications,[4] and templating with macroporous supports.[5] Among these techniques, the use of hard templates like latex spheres or silica spheres has been widely adopted to form different macroporous zeolitic structures like 3D-ordered zeolitic macroporous structures [6] or hollow zeolite capsules.[7] However, the hard templating technique is still limited by the low wettability of the hard templates with zeolite precursors,[7] poor thermal stability of the template under zeolite synthesis conditions,[6] difficulties in controlling the thickness of the zeolitic walls,[6] and the template removal after hydrothermal synthesis.[8] The strategies adopted to overcome such limitations have resulted into multistep procedures that require the use of surface modification techniques and pseudosolid-state transformation procedures to improve the wettability of these templates with the precursor solution and to prevent melting of the template during the hydrothermal transformation,[6] respectively. In addition, huge amounts of template for the formation of the zeolite network (template/SiO 2 molar ratios above 0.35)[6, 7, 9] have been used to control the thickness of zeolitic walls, and harsh conditions have been employed to remove the hard template after the synthesis.[8] These drawbacks are still retarding the development of macroporous zeolites. Another strategy to prepare zeolite assemblies with additional macropores without utilizing any external template is the use of mesoporous silica particles (MSPs) both as a silica source as well as a sacrificial template for macropore formation.[2] However, currently available methods [10–12] involve the coating of these MSPs with zeolite seeds via a layer-by-layer deposition procedure [12] and result into hollow zeolite capsules [10] or 3D structures with isolated macropores or they additionally need an extra silica source to form a polycrystalline zeolitic phase, which surrounds the interconnected macropores.[11] The preparation and coating of zeolite seeds on MSPs make this procedure laborious. Thus, no procedure is available to prepare zeolite crystals with a nearly classical morphology and an intracrystalline macropore system that is embedded in the zeolitic matrix. To overcome the limitations of existing macroporous zeolite assemblies, crystals of MFI-type zeolites with aHierarchically organized systems are commonly encountered in our natural environment.[1] Such hierarchical systems are related mostly to structural properties (eg, stem of trees, bones) or fluid dynamic properties (eg, in the lung or the blood circle).[1] The ability of these systems to maximize the efficiency of transport processes has always been an inspiration for their implementation in different artificial systems reaching from watering systems up to catalytic reactors. Zeolite crystals can be regarded as an assembly of miniaturized catalytic reactors with their micropores providing large specific surface area, a defined environment of active sites as well as shape selectivity at each single pore entrance.[2] Thus, zeolites belong to the most important catalytic materials used today. However, their utilization in catalysis is limited due to the slow transport of the reacting species within the micropores. In order to minimize these transport limitations, it is highly desirable to reduce the diffusion path lengths. The preparation of either nanozeolites or zeolitic systems with …