Organic crystal templating

Fig. 77. Schematic drawing of the liquid-crystal templating mechanism. Hexagonal arrays of cylindrical micelles form (possibly mediated by the presence of silicate ions), with the polar groups of the surfactants (light grey) to the outside. Silicate species (dark grey) then occupy the spaces between the cylinders. The final calcination step burns off the original organic material, leaving hollow cylinders of inorganic material [473]...

Fig. 3. Schematic representation of the formation of MCM-41 by the liquid-crystal templat-ing mechanism, (a) Hexagonal array of cylindrical micelles (b) the same, with silicate species between the cylinders and (c) hollow cylinders of MCM-41 after thermal elimination of organic material. (Rouquerol, Rouquerol and Sing, 1999).

Different mechanisms for the interactions between the silicate precursors and the organic template as well as the liquid crystal templating mechanisms have been discussed (Ying, Mehnert, and Wong, 1999 Tanev and Pinnavaia,... 

Several approaches towards the synthesis of hierarchical meso- and macro-porous materials have been described. For instance, a mixture that comprised a block co-polymer and polymer latex spheres was utilized to obtain large pore silicas with a bimodal pore size distribution [84]. Rather than pre-organizing latex spheres into an ordered structure they were instead mixed with block-copolymer precursor sols and the resulting structures were disordered. A similar approach that utilized a latex colloidal crystal template was used to assemble a macroporous crystal with amesoporous silica framework [67]. 

This technique involves the interactions between chemical reagents (liquid or gas) and the organic guest template in the pores and channels of a microporous crystal under mild conditions, and this detemplating technique may result in free pores with the whole structure of the compound remaining intact. The selection of the chemical reagents and the related chemical reactions should be based on the mildness of the reaction conditions and the ease of product separation. 

Although crystallization does occur at low temperatures, hydrothermal methods are often used to speed up the reaction by raising the temperature to 160-250 C. Numerous templating agents have been used in the formulation of zeolite materials. Microporous materials (highly crystalline, pores <12 A) often use organic bases such as TEA (tetraethyl-ammonium), while mesoporous materials (larger pores but lower crystallinity, pores >12 A) often use liquid crystal templates ... 

A combined experimental and theoretical study is carried out to show the effect of various organic amine templates on the synthesis of AlPO-5 fi-om the same gel composition. It is shown that under mild conditions, TEAOH is the best template. However, after addition of HF to the synthesis gels, TPA/HF and TEAOH/HF give better AIPO-5 crystals than MCHA/HF or TEA/HF. 

A comparative study on using various organic amine templates is presented towards a better understanding of the template s role in AlPO synthesis. Under mild synthesis conditions, TEAOH is the best candidate to prepare AlPO-5. This is supported by computational results. The addition of HF in low concentration (0.5 M) enhances the crystallization of the AlPO-5 phase. Moreover, HF affects the crystallization in different ways in the presence of organic templates. TEAOHTfF and TPA/HF are the best combinations to synthesize AlPO-5 crystals. 

Recently, inqjortant efforts have been made to fabricate photonic crystals by colloidal crystal templating [1-5], Colloidal crystals are self-organized arrays of silica or latex spheres, having the periodicity required for photonic band gaps. The basic idea is to use a colloidal crystal as a template, infiltrate the interstitial spaces between the spheres with another material and, then, remove the spheres by chemical etching or combustion. Since the resulting macroporous structures have a complete photonic bandgap [6,7], these materials have many applications in photonics. 

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