29Si CPMAS NMR and near-IR study of sol–gel microporous silica with tunable surface area

High-resolution solid-state NMR and IR techniques were used to investigate the surface structure of microporous silica with tunable surface areas, obtained by a sol gel method acting on a variation of gelation time. ²⁹Si MAS NMR and Med-IR evidenced a similar number of silanol groups in all the silicas. Near-IR results revealed the existence of a different ratio of hydrogen bonded silanols and free silanols as a function of surface area. ²⁹Si CP-MAS experiments, including CP spin dynamics studies, were employed to probe hydrogen bonding and local structural environments of various hydroxyl groups of silica surface. These studies showed that for high surface area silicas both free and hydrogen bonded silanols are present at the surface of the samples and the amount of free silanol groups increases with the increasing of the surface area of the samples.