Vol.11 no.3 2019

Research paper : Challenge towards synthesis of non-silica-based hybrid mesoporous materials (T. KIMURA)−114−Synthesiology - English edition Vol.11 No.3 (2018) 2.1 From silica to its hybrid (organosilica)For cases in which synthesis is started from silica starting materials such as tetra-alkoxysilane, there are sufficient findings in silicate chemistry.[6] This means that the synthesis is done in an environment in which hydrolysis and polycondensation reactions can be controlled easily in solution. Moreover, because basic scientific understanding that conditions and reaction processes of dissolved silicate species can be traced by 29Si NMR has grown, it is actually possible to synthesize diverse types of mesoporous silica. Inorganic-organic compounding of the framework in silica materials can be relatively easily achieved, even by using silane compounds bridged with organic groups as starter materials.[7] Although reactivity of silane compounds may change, findings of silicate chemistry can be used fully, such as synthesizing by selecting either acidic or alkaline conditions. As shown in Fig. 4, there are reports of mesoporous organosilica synthesis from silane compounds bridged by various organic groups. Also, inorganic and organic groups are arranged alternately at molecular scale, and intermolecular interaction between organic groups within silane compounds may give periodicity to the arrangement of the organic and inorganic species.[8][9] Simple organic groups are only expected to have the role of merely making pore surfaces hydrophobic. On the other hand, there is work done on utilizing the functionality or designability of organic groups themselves. For example, technology to capture light energy is being studied by forming a metal complex using bipyridine arranged as footholds on framework surfaces.[10]2.2 From silica to non-silica: Difculty to synthesize non-silica-based mesoporous materialsThe knowledge of silicate chemistry does not apply at all to the synthesis of non-silica-based mesoporous materials. Here, I shall explain using oxides such as alumina and titania as representatives of non-silica-based materials. Non-silica-based inorganic starters have severe reactions in solution. Therefore, chemical modifiers are used or non-aqueous systems are applied for reactions, and various other measures are reported as control methods to delay the general sol-gel reaction. However, combining such reaction control technology to the concerted organization route (Fig. 3) to obtain precursors for mesoporous materials has not been done fully. Since efforts are only mainly applied to the initial stage of the reaction, subsequent bond formation among inorganic species cannot be controlled. Therefore, self-assembly and framework formation are not adjusted to an appropriate speed, and inorganic materials are precipitated without being fully incorporated into amphiphilic organic molecules. Such behavior can be understood as follows: strength of bond energy (covalent bond) of inorganic framework >> energy of interaction (electrostatic interaction, hydrogen bond, etc.) of inorganic species and amphiphilic organic molecule > power of energy of self-assembly. To obtain a mesoporous structure with high structural order, how to control bond formation of inorganic species in solution is the most important component.For non-silica-based materials for which reactions are difficult to control, precision is required to control the reaction in solution. In other words, the fact that most reports for alumina and titania are limited to films shows Fig. 3 Formation mechanism of mesoporous silica: Liquid-crystal templating and cooperative organization routes①Reactivity controlof inorganic species②Design of interaction with amphiphilic organic molecule③Understanding ofself-assembling behavior④Adjustment of condensation reaction of inorganic species during the formation of liquid-crystal-like structure⑤Removal of amphiphilicorganic molecules(calcination, extraction, etc.)⑥Process design assumingdeveloping application(powder, lm, etc.)Hydrophilic partHydrophobic part (lipophilic region)Formation of liquid-crystal structureFormation of liquid-crystal-like structureInsertion of silicate speciesOligomerMonomerAmphiphilic organic moleculeSoluble silicate species※Liquid-crystal template route※Cooperative organization route

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