Unravelling the Mechanism of Sol–Gel Process: a Key Stage in the Production of Silica Aerogels from Alkoxysilanes

The mechanism of the sol–gel synthesis, a key step in the preparation of silica aerogels (SA) from alkoxysilanes, is studied on example of a BF 3 ‐catalyzed process. “Stop‐flow” NMR without MAS is chosen for in situ monitoring of the reaction. Using this procedure, the reaction mixture is maintained at r.t. for a specified time before being frozen to −80°C, at which point all chemical transformations cease, and subsequently studied via NMR. It allows continuous structure studying of both soluble low‐molecular‐weight intermediates and the successively formed nanoparticles (NPs) within the sol and solid materials (wet gel and SA) under unchanged conditions. 1D, 2D, and 3D 29 Si NMR are used as main method to determine the structure of the Si‐intermediates. An extended H‐bonded network between both low‐molecular‐ and high‐molecular‐weight Si‐intermediates allows detecting even short‐living and difficult‐to‐detect intermediates. The use of 1D and 2D 1 H, 11 B, 19 F, and 29 Si NMR experiments provides detailed information on the evolution of the BF 3 ‐precatalyst into B‐containing Lewis acids and F‐containing Brønsted bases as catalytic species. Their interactions with Si─OEt‐ and Si─OH‐containing intermediates are studied. Using rheology, dynamic light scattering (DLS) and DLS in an electric field, the observations obtained by NMR were confirmed independently.