Among mesoporous silica; SBA-15 and MCM-41, which is having more isolated silanols (Q3 sites) suitable for grafting trialkoxy organic functional group?
It depends on synthesis condition in which you follow.
For example, the SBA-15 properties can tune their properties by varying the synthesis temperature.
Obviously SBA-15 have better stability than MCM-41 so most of the author prefer SBA-15
The silanol content also varies with calcining temperature, with higher temperatures leading to a lower silanol denisty. In the lab, I've noticed that slightly more organic groups can be introduced into SBA-15 than commercial MCM-41. Molar ratios of Si : trialkoxysilate of up to ~5 :1 could be achieved for calcined SBA-15 at 550 C. The organic group content could be increased by using solvent-extracted materials instead of calcined silicas.
Tuning the synthesis conditions is one factor to improve OH density. MCM synthesis is tedious than SBA to tune. However, Removal of template is as important as tuning the reaction conditions to improve Si-OH.
The nature of as-synthesized material is vital for higher silanol concentrations. organo-inorganic interactions in MCM-41 and SBA-15 are different. MCM-41 has strong direct electrovalent(S+I-) interactions between surf. and silica, whereas SBA-15 has H+ mediated interaction. The condensation is also very slow in acid mediated synthesis. Template removal is comparitively easy in case of later and hence a better silanol content.
Ultrasonication, Solvent extraction and microwave digestion of template show higher silanol concentrations compared to thermal treatments in Silicas (SBA-n, KIT-n, etc.). These kind of materials are of great value in nanocasting too.
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