Surface postsynthesis grafting

Azobenzene has also been added to the internal surface of mesoporous films and powders through postsynthesis grafting.58,60-62 The effect of oligomer size on azobenzene isomerization kinetics was studied for branched oligomer-modified azobenzene derivatives in solution and covalently grafted to nanoporous silica 61 the size of the dendrimer had little effect on the thermal cis to trans isomerization rate in solution... 

Table 1 summarizes the surface properties of the various mesoporous titanosilicate samples. The BET surface areas of these titanosilicates (850-1250 mg ) was typical for that shown by mesoporous materials. The BET surface area and BJH average pore diameter is found to decrease after Ti grafting onto Ti-MCM-41 sample. The postsynthesis grafted Ti is expected to react with the surface silanol groups of the walls in a random fashion in the most accessible sites near the pore mouth and wider pores. As a consequence there is clear decrease in pore size after titanium grafting. The pore size distribution is also found to become narrower around the average pore diameter. The BET surface areas of disordered mesoporous materials is found to be lower than that of the ordered MCM-41 type materials. BJH average pore diameter of mesoporous titanosilicates TiO-SiO(l) and TiO-SiO(2) prepared by modified sol-gel method is much lower than that of disorderd mesoporous Ti-Meso and ordered Ti-MCM-41 samples crystallized hydrothermally. But all these samples show a narrow pore size distribution. All the samples exhibit isotherms of type IV, typical of mesoporous materials, with a H2 hysteresis loop (Fig. not shown). 

Surface-functionalized ionosilicas are generally synthesized either via postsynthesis grafting or hydrolysis-cocondensation reactions involving trialkoxysily-lated ionic precursors. Scheme 16.5 illustrates the two different synthetic methods involving a monosilylated imidazolium precursor 3-methyl-l-(3-(trie-thoxysilyl)propyl)-imidazolium chloride. 

Scheme 16.5 Synthesis of surface-functionalized silicas via postsynthesis grafting or hydrolysis-cocondensation reactions.

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