Structure-directing agents zeolite materials

Microporous materials are typified by natural and synthetic zeolites that are crystalline 3D aluminosilicates with open channels or cages. Synthetic and structural concepts of zeolites have to a large extent shaped the development of microporous materials during the past 50 years. For example, the use of organic structure-directing agents in the synthesis of high-silica zeolites and their all-silica polymorphs contributed to... 

The mesoporous materials discussed here comprise silicates and aluminosilicates that are formed from synthesis conditions comparable to zeolites. The main difference with the latter is the use of supra-molecular assemblies of surfactant molecules, e.g. alkyltrimethylammonium ions, as the structure directing agents. Due to the large dimensions of these spherical or cylindrical micelles, the framework of the silicates is not so well crystallized as in the case of zeolites. This causes lower framework stability as well as weaker Bronsted acidity. Upon calcination, large pores of uniform diameter (say 4-10 nm, depending on the surfactants used) become accessible. 

The vapour-phase transport method was also successfully used for the preparation of membranes and bulk material of the following zeolites MCM-22 membrane (used for the alkylation of toluene with MeOH to p-xylene),[68] FER-membrane (for the separation of benzene/p-xylene),[69] EMT crystals using a crown ether as structure-directing agent,[70] and zeolite beta (BEA).[71] In the latter two examples, a conversion of agglomerated nanoparticles to highly crystalline zeolite was observed. 

Fig. 4 top) Compilation of structure directing agents (SDA) which all lead to the formation of MTN-type materials, bottom) Zeolite framework types obtained with quinuclidine as SDA under different synthesis conditions. 

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