Designed synthesis, zeolite structures

The combination of synthesis and modification techniques gives us a chance to rationally design or tailor zeolite structures. For example, we can increase shape selectivity by modifying or eliminating active sites on the external surface of zeolite crystals. Although this outside surface may represent only 2-5 % ot the total surface area, acid sites located there are more accessible to reacting molecules than acid sites in the pores. As these catalytic sites are not shape selective, they catalyze a disproportionate amount of non-shape selective reactions. 

At which extent the traditional definition of zeolites is still valid Are zeolite scientists still dealing with "crystalline aluminosilicates containing pores and cavities of molecular dimensions" [ 1 J, or did they create new materials original enough to render this time-honoured definition obsolete Indeed, the zeolite community has pushed afar the borders of his field of interest, as any healthy body of scientists has to do. Zeolite researchers presently deal with nanopores instead of micropores, self-assembly instead of synthesis, and they prepare periodical structures from any comer of the periodical table, well beyond the limits of the class of ordered silicates. The evolution of the subject (and of the vocabulary used to describe it) has been astounding and somewhat refreshing but the core of the activity of the zeolite scientist is still the same as it was when Barrer described the first documented synthetic zeolite in 1948 [2] to apply up-to-date characterisation techniques to the design, synthesis and application of periodical self-assembled objects. 

Computerizing the information on zeolite catalysts have been attempted successfully and different databases concentrate on specific properties[25-28]. The information in several databases are shown in Table 3. Our approach[29] involves retrieval of information from the database and additionally an expert system approach is followed to derive a set of conditions to achieve one s goal in the synthesis of zeolites. The structure of the system is designed to perform three salient fiinctions as shown in Fig. 7. The first function is to provide access to a large database of physico-chemical properties and crystallographic information of all reported zeolite[30]. The second function provides for the synthesis of zeolites - the most logical route for the synthesis of a desired zeolite structure is provided. The third function is a graphic tool application to simulate X-ray powder diffraction patterns for zeolite phases with different amount and nature of purity. 

SDAs are featured for their structure diversity and controllability to meet the objective of designed synthesis of zeolites with specific structures. Studies show that SDAs with proper shape, polarity, charges, and flexibility are the keys to achieve the design of novel zeolite structures [63]. Nowadays, many zeolite structures have been synthesized by using novel SDAs. Besides, hierarchical zeolites can be synthesized with bifunctional templates. Some representative works will be discussed later. 

The information presented above suggests that the design of zeolite pore structures is feasible if pure-silica syntheses are used with water-soluble organics that do not decompose at syndiesis conditions diat may last for several months. For most practical cases, faster synthesis times and the addition of heteroatoms, e.g. Al, are desired. Small amounts of alkali-metal cations and heteroatoms will most likely be acceptable. However, large amounts of alkali-metal ions and heteroatoms will alter the reaction chemistry sufficiently to override or modify the structure-directing effects of the organic species. 

At this point in time a completely designed synthesis of a zeolite has not occurred. However, the evidence presented here paints a bright picture for the future. The design of organic structure-directing agents and molecular building blocks is now a... 

Structure-based Design of Templates for Zeolite Synthesis... 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

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