Study on the ammonia-catalyzed hydrolysis kinetics of single phenyltriethoxysilane and mixed phenyltriethoxysilane/tetraethoxysilane systems by liquid-state 29Si NMR

In situ 29Si liquid-state nuclear magnetic resonance was used to investigate the ammonia-catalyzed hydrolysis and condensation of the single phenyltriethoxysilane (PTES) systems and the mixed tetraethoxysilane (TEOS)/PTES systems dissolved in methanol. By varying the molar ratio of the PTES, water and ammonia in the initial solutions, the hydrolysis rate constants for PTES in single precursor systems were disclosed as well as the corresponding reaction orders by fitting the concentration curves of the intermediate species as functions of time. Due to the cooperation of inductive and steric effect, PTES shows a low reaction activity. Under ammonia catalysis, the hydrolysis reaction orders of TEOS and PTES in the mixed precursor systems all retained the first-order, which is similar to single precursor systems. The hydrolysis rate constants of TEOS and PTES in the mixed systems were larger than the values of TEOS and PTES in their single precursor systems, respectively. Another important result was: the reaction orders of both ammonia and water increased to different extent for TEOS and PTES in mixed systems. Hydrolysis and condensation kinetics showed more compatible hydrolysis–condensation relative rates between TEOS and PTES, which affected remarkably the final microstructure of silica particles.