SBA-15 mesoporous silica highly functionalized with propylsulfonic pendants: A thorough physico-chemical characterization

Organosulfonic modified SBA-15 silicas are promising catalysts for a wide spectrum of organic reactions. In particular, their catalytic activity seems to be effective only when large amounts of functional groups are anchored to the silica skeleton. However, there is evidence that the ordered mesostructure is retained only for contents of sulfonic units not exceeding 20 mol%. In this paper, for the first time to our knowledge, we fully address the gradual structural disordering mechanisms of the SBA-15 silica functionalised with increasing amounts of propylsulfonic groups in the compositional range 0-70 mol%. This investigation was carried out by means of a multi-technique wide approach including thermal analysis, TEM, XRD, N2 adsorption, FTIR/Raman and solid-state multinuclear NMR. We obtained detailed information on the structural modifications of these hybrid systems as a consequence of the sulfonation process. In particular, we showed that high functionalization degrees lead to inhomogeneous materials made of inorganic or low-functionalized clusters where the SBA-15 structure is likely retained, and hybrid clusters where the hexagonal mesoporous structure is destroyed. Moreover, full conversion of -SH into -SO3H groups is efficient until 20 mol% functionalization. In fact for higher sulfonation degrees, the formation of S-S bridges becomes a competitive mechanism, which is predominant in the sample SO3H70%, where only about 20% of the introduced organic moieties are actually oxidized. Therefore, to push sulfonic functionalization above 20 mol% is not necessarily useful, at least for applications where ordered mesoporous structure is mandatory, such as in catalysis or in membranes for polymer fuel cells.