Silica post-synthetic modification

Zeolites are formed by crystallization at temperatures between 80 and 200 °C from aqueous alkaline solutions of silica and alumina gels in a process referred to as hydrothermal synthesis.15,19 A considerable amount is known about the mechanism of the crystallization process, however, no rational procedure, similar to organic synthetic procedures, to make a specifically designed zeolite topology is available. The products obtained are sensitive functions of the reaction conditions (composition of gel, reaction time, order of mixing, gel aging, etc.) and are kinetically controlled. Nevertheless, reproducible procedures have been devised to make bulk quantities of zeolites. Procedures for post-synthetic modifications have also been described.20 22... 

In contrast to these post-synthetic modifications, it is also possible to functionalize the pore walls directly during the synthesis, as was first shown by Mann and co-workers [7,8] and Stucky and coworkers [9], who used trialkoxysilanes R-Si(OR )3. In our approach, such R Si(OR )3 molecules substitute for part of the TEOS. After hydrolysis, they serve as additional framework components during the hydrothermally induced condensation. An essential condition for this approach is that the trialkoxysilane does not destroy the micellar arrangement of the surfactant, which gives rise to the mesostructure. In mesostructures produced in this way, the R residues should be covalently linked to the silica walls. After the synthesis, the organic surfactant molecules can be removed by extraction so that a modified mesoporous material should remain. For example, when using phenyltrimethoxysilane (PTMOS), phenyl groups may become attached to the walls of the mesopores these can be utilized for further modifications, e.g. the immobilization of metal complexes. 

The change of composition of mesoporous materials can be done by direct synthesis and post-synthesis modification. Now, the composition of mesoporous materials can be extended to nonsilica oxides, phosphates, sulfides, even metals. The study of nonsilica mesoporous materials started much later than that for silica-based materials. The main reasons include the hydrolysis and condensation reactions of transition metal precursors is difficult to control the inorganic wall easily crystallizes and results in the loss of mesostructures the synthetic procedure is difficult to repeat. 

---