Kinetic and thermodynamic issues in the early stages of sol-gel processes using silicon alkoxides

An understanding of the chemical processes that take place in the earliest stages of a sol-gel preparation can provide the potential to better control microstructural evolution of a catalyst. While the desired catalyst properties depend on specific details of a catalytic application, in general one wants at least to control textural and chemical homogeneity. Silica provides an excellent test system for the study of sol-gel processes starting from alkoxide precursors as it can exhibit a wide variety of structure and has been extensively studied. In this review the features of tetraethoxysilane (TEOS) polymerization as observed by Si-29-NMR spectroscopy are summarized. Trends in hydrolysis and condensation with increasing oligomer size are identified. The kinetics and equilibrium of these reactions, metastability and phase separation are reviewed. Finally we suggest a comprehensive reaction engineering picture of TEOS polymerization with special focus on the crossover between gelation and precipitation. Selected comments on other alkoxides, non-alkoxides, and on multicomponent formulations are also offered.