Single crystals mesoporous silicates

The continuous effort of numerous academic and industrial laboratories around the world has resulted in recent years in successful synthesis of a number of new porous materials including new structural types of zeolites and zeolypes, siliceous and non-siliceous mesoporous molecular sieves, mesoporous zeolite single crystals, and micro/mesoporous or micro/macroporous composite materials of different chemical compositions. As a consequence of the success of basic research in this area, zeolites have found new industrial applications. 

In 2002, Chiang and coworkers[124] developed a new scheme for the confined synthesis of TPA-silicalite nanocrystals. The surfactant cetyltinmethylammonium bromide (CTAMeBr) (in ethanol solution) was added to the single- and double-heated TPA-silicalite precursor sols (SHPS and DHPS), and the mixture was flocculated at a certain pH value to collect the nano-size silicate species in the precursors, and then dried. The dried precursor/surfactant hybrid was pressed into pellets and then steamed in a stainless steel autoclave at 110 150°C for 7 36 h. Finally, the product was calcined to remove the surfactants and TPA. The particle sizes of silicalite-1 produced in this method are about 30 nm. The study indicates that the nanoparticles collected by surfactants already exhibit the structural features of MFI. They crystallize entirely to form silicalite-1 nanocrystals after steam treatment at 110 150 °C. This new solid-phase approach provides a way to synthesize MFI nanocrystals without the problem of separation and collecting nanocrystals from suspension, and it also avoids the large consumption and cost of special mesoporous templates used in the confined-synthesis methods. 

With respect to applications, there will certainly be more and more investigations where ordered mesoporous materials are used as catalysts or catalyst supports. However, the more skeptical note of the section on catalysis shall be repeated here In many cases, much cheaper and simpler alternatives exist, and the properties of ordered mesoporous materials are not so much superior to justify the higher effort of their synthesis. On a longer time scale, non-siliceous compositions will probably be used more frequently in catalysis. If one analyzes the catalytic processes implemented today, the majority is not based on silica as catalyst or support, and the single most important area of aluminumsilicates, acid catalysis by zeolites, seems to be less attractive for ordered mesoporous aluminumsilicates, unless a crystallization of the walls to zeolitic structures or the assembly of such materials from colloidal zeolites to enhance the acid strenght becomes possible. 

Organic cations are able to complex silicate anions to form silicate complexes. These complexes can condense to form ultimately microporous zeolite structures. Different templat-ing molecules may form different microcavities. Hence zeolite synthesis is an example of template-directed synthesis. Mesoporous materials can be prepared by using nanometersized micelles preorganized as hquid crystals as a template rather than a single molecule template. 

---