Kinetics of interaction of 3-amimopropyltriethoxysilane on a silica gel surface using elemental analysis and diffuse reflectance infrared Fourier transform spectra

In order to study the kinetics of interaction of 3-aminopropyltriethoxysilane (APTS) on silica gel, silica gel samples reacted with APTS were prepared by sampling the reaction mixture from the reactor at various time intervals, and the concentrations of 3-aminopropylsilyl (APS) groups modified on the silica gel surface were determined by elemental analysis. From the reaction time dependence of the APS concentration, it has been found that, after an initial fast reaction of APTS and silica gel, a slower second reaction and a much slower third reaction subsequently occur. For the three processes, the reaction rate constants (k(obs)/s(-1)) have been estimated and found to be k(obs)(i) = 1.00 x 10(-1) s(-1) for the initial reaction, k(obs)(s) = 6.91 x 10(-3) s(-1) for the second reaction and k(obs)(1) = 4.20 x 10(-4) s(-1) for the third reaction. The diffuse reflectance infrared Fourier transform (DRIFT) spectra were measured for the same samples. Bands at 3734-3739 cm(-1) were observed in the OH stretch mode region, and were found to decrease in intensity with reaction time, reflecting the rate of reaction of APTS on the silica gel. Analysis of an OH stretching band has also furnished reaction rate constants that are very similar to those obtained from the time dependence of the APS concentration. A mechanism for the three reaction processes is postulated.