Mesoporous post-synthetic modification

Post-synthetic modification methods (route A in Figure 36.2) rely on the presence of silanol groups on the inside of the pore walls. The simplest type of post-synthetic modification is the hydrophobization of the calcined mesoporous materials by reaction with alkylchloro- or alkylalkoxy-silanes, - " but functional groups and metal complexes can also be attached. - " ... 

Many studies have aimed to prepare mesoporous solids with active titanium, either by direct synthesis or by post-synthetic modifications. Although some of the advantages of crystalline framework solids are lost, the larger pore size increases markedly the size limit of molecules that can be converted by this catalytic chemistry, and active and selective catalysts have been prepared. In addition, such solids can be used with organic peroxides (such as tert-butylhy-droperoxide) that do not produce water as an inhibiting by-product. 

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. 

Metal-Organic Framework Materials covers topics describing recent advances made by top researchers in MOFs including nanoparticles and nanoscale frameworks, mesoporous frameworks, photoreactive frameworks, polyrotaxane frameworks, and even edible frameworks, as well as functionalized frameworks based on porphyrins, fluorine, and alumimun. In addition, the volume features aspects on mechanochemical synthesis and post-synthetic modification, which provide discussions on new vistas... 

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. 

---