Morphology and porosity characteristics control of SBA-16 mesoporous silica. Effect of the triblock surfactant Pluronic F127 degradation during the synthesis

The use of a reaction mixture, characterized by high dilution (H2O/TEOS = 495), low to medium acid concentration (0.4-1.6 M HCl) and the presence of a cosurfactant (CTMABr), allows the synthesis of micron-sized particles of SBA-16 type mesoporous silica with different morphologies and structural characteristics. With a F127/TEOS molar ratio around 0.005 and a temperature between 70 °C and 95 °C, perfect rhombododecahedra (∅ 2-8 μm) are obtained. High acid concentration (1.6 M HCl) and/or temperature higher than 100 °C produce a smoothening of the edges and lead to the formation of spherical particles. This morphology is also favoured by low F127/TEOS molar ratios and corresponds to a disordered mesoporous structure. At a temperature of 40 °C or lower, the gel-like particles with poorly organized mesostructure are difficult to transform into well-organized materials. High F127/TEOS molar ratios lead to the same result. These different behaviours can be rationalized taking into account the strength of the interactions between the surfactant and the silica species which depends on the conformation of the hydrophilic poly-ethylene-oxide chains. Temperature and acid concentration are the main parameters to modify the conformation. But these two parameters are also responsible of the degradation of the Pluronic F127.