Template reactions/synthesis zeolites

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

Zeolite materials with tunable size and volume of mesopores can be prepared by using dispersed carbon black particles with narrow distribution of their sizes as inert mesoporous matrix or as secondary template. In such confined space for synthesis the crystallization of zeolite gel occurs inside the interparticle voids of carbon matrix [10,11,12]. In the case of generation of mesopores by secondary templating by means of addition of carbon black into the reaction mixture, zeolite crystals are formed around carbon particles [13]. After burning off a carbon matrix or carbon particles, zeolite crystals with a controlled pore size distribution and a crystalline micro-mesoporous hierarchical structure are prepared. 

The conversion of porous glass into the zeolites with MFI- and FER-structure has been performed following various synthesis routes, with or without a template addition. Tetrapropylammoniumbromide (TPABr) or propylamine (PA) have been used as a so-called structure directing template. Starting reaction mixtures expressed in mole ratios of the oxides are given below ... 

The role of the template in the synthesis is not merely as a porogen on the contrary, it is also responsible for many key functions [5, 9, 10]. The template (typically cationic) balances the negative charge that characterizes zeolitic framework, due to the isomorphic substitution of Si(IV) by Al(III), prearranges the secondary building units (SBUs) toward the zeolitic framework, improves the gel synthesis conditions, especially the solubility of the silica precursors, and favors the thermodynamics of the reaction by stabilizing the porous zeolite framework. 

In this study, we have shown that both alcohol and D20 have an Important effect on the nucleation and crystal growth of zeolites with Si/Al ratios between 1-2. In the case of alcohol, the formation of large pore zeolites such as zeolites X or Y is markedly accelerated at low alcohol levels. We attribute this to a stabilization of the cation-water complex and structured H20 which act as templates. However, at high alcohol levels, the structure of water disintegrates and leads to the formation of more condensed zeolites such as sodalite or cancrinite. Synthesis of zeolite A in D20 is slower than that in water, which primarily arises from the primary and secondary isotope effect during the condensation polymerization reactions necessary for zeolite growth. 

Synthesis of silica-based materials with controlled skeleton structures, such as zeolites, requires controlling the structure of oligomeric silicate species at the first reaction step. Organic quaternary ammonium ions, which are known as organic templates in zeolite synthesis (1 ), have a role in making up the specific structures of silicate anions, whereas silicate anions randomly polymerize in aqueous solutions containing alkali metal ions, resulting in the presence of silicate anions with different structures. 

Published(8) and our own findings that sodalite cages in ZK-4 all contain one TMA suggest that the role is as a template for sodalite cage formation. Templating of sodalite cages apparently is not required for synthesis of zeolite A (Si/Al ratio of one) because the reaction is facile in the absence of TMA. 

The favorable influence of organic molecules in the synthesis of MFI-type zeolites is widely recognized. For example. aluminium-containing ZSM-5 can be obtained also in the absence of an organic template ( LJJK but the synthesis of silicate I seems to require the presence of an organic molecule in the reaction mixture ( 3K... 

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