DSM as a probe for the characterization of modified mesoporous silicas

Two methods were applied for the functionalization of mesoporous silica materials (MCM-41 and SBA-15). For the acidic functionalization with sulfonicpropyl groups a direct synthesis method was used. For the basic functionalization with aminopropyl groups a post-synthesis methodology was used. Sample characterization was performed by small angle X-ray powder diffraction, nitrogen adsorption and desorption and transmission electron microscopy. Direct functionalization has a high impact on the structural characteristics, decreasing BET surface area and changing its pore size distribution. Trans-4-[(4-dimethyl-amino)styryl]-1-methylpyridinium iodide (DSM+) was used for the acid base characterization of the synthesized materials. Ground state absorption spectra and laser induced time resolved emission spectra have shown the existence of two forms of the probe: DSM+ and the protonated, DSMH2+. Lifetime distribution analysis confirmed the presence of these two species in all materials, but with different relative amounts depending on the surface characteristics. All these findings clearly show that DSM+ is an excellent probe capable of providing important information regarding the internal surfaces of these materials, with channels of nanometer dimensions, namely the different acid properties of the active sites of these silica based hosts.