Kinetics of the acid-catalyzed hydrolysis of tetraethoxysilane (TEOS) by 29Si NMR spectroscopy and mathematical modeling

Abstract

Tetraethoxysilane (TEOS) is widely used to synthesize siliceous material by the sol–gel process. However, there is still some disagreement about the nature of the limiting step in the hydrolysis and condensation reactions. The goal of this research was to measure the variation in the concentration of intermediates formed in the acid-catalyzed hydrolysis by 29Si NMR spectroscopy, to model the reactions, and to obtain the rate constants and the activation energy for the hydrolysis and early condensation steps. We studied the kinetics of TEOS between pH 3.8 and 4.4, and four temperature values in the range of 277.2–313.2 K, with a TEOS:ethanol:water molar ratio of 1:30:20. Both hydrolysis and the condensation rate speeded up with the temperature and the concentration of oxonium ions. The kinetic constants for hydrolysis reactions increased in each step kh1 < kh2 < kh3 < kh4, but the condensation rate was lower for dimer formation than for the formation of the fully hydrolyzed Si(OH)4. The system was described according to 13 parameters: six of them for the kinetic constants estimated at 298.2 K, six to the activation energies, and one to the equilibrium constant for the fourth hydrolysis. The mathematical model shows a steady increase in the activation energy from 34.5 kJ mol−1 for the first hydrolysis to 39.2 kJ mol−1 in the last step. The activation energy for the condensation reaction from Si(OH)4 was ca. 10 kJ mol−1 higher than the largest activation energy in the hydrolytic reactions. The decrease in the net positive charge on the Si atom contributes to the protonation of the ethoxy group and makes it a better leaving group.