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 BF₃-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 ²⁹Si 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 ¹H, ¹¹B, ¹⁹F, and ²⁹Si NMR experiments provides detailed information on the evolution of the BF₃-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.