Raman and GC-MS study of the initial stage of the hydrolysis of tetramethoxysilane in acid and base catalyzed sol-gel processes

29Si NMR and UV−Raman Investigation of Initial Oligomerization Reaction Pathways in Acid-Catalyzed Silica Sol−Gel Chemistry

“…The reduction in molecular mass by hydrolysis of ethoxide substituents causing vibrations to shift to higher wavenumbers turned out as a convenient guide for the assignment of Raman signals. 13,14 Here, we report an investigation of silica polycondensation under strongly acidic conditions at low r-values by in situ 29 Si NMR and UV-Raman. Low hydrolysis ratios offer the advantage of limited hydrolysis and slow reaction kinetics allowing investigation with 29 Si NMR.…”

“…[5][6][7] Crucial parameters of the silica sol-gel processes are the water:alkoxide molar hydrolysis ratio or r-value, the pH, and the nature of the alkoxide. 5,[8][9][10][11][12][13][14][15][16][17] In general terms, homogeneous acid catalysts direct silica polymerization toward linear polymers and materials having low fractal dimension, while basic catalysts generally favor formation of tridimensional highly branched silicate networks. 8,15 Raman spectroscopy and 29 Si NMR are frequently used for investigation of silica polymerization.…”

“…8,15 Raman spectroscopy and 29 Si NMR are frequently used for investigation of silica polymerization. 13,14,[18][19][20][21][22][23][24][25][26][27] In alkaline media, silicon displays a complex polyanion chemistry. 21,[28][29][30] Identification of a large number of polyanion species in alkaline media has been achieved through a combination of NMR spectroscopy and molecular modeling.…”

Vibrational spectra and structure of methoxysilanes and products of their hydrolysis

“…Si(OMe) 4Àn (OH) n [2,5]. Attempts to characterize these compounds by assigning IR bands in the n as SiO 4 frequency range (1000-800 cm À1 ) to the hydrolysis products were made [7][8][9], but certain difficulties exist in this way, since, in contrast to the n s SiO 4 mode, asymmetric SiO stretching vibrations strongly interact with other modes, primarily with SiOH deformations.…”

“…The first step of these processes is the hydrolysis of alkoxysilanes followed by the condensation of hydrolysis products (silanols) with formation of siloxane networks [1]. Changes in the vibrational spectra of alkoxysilanes during these processes are frequently used for the monitoring of these processes [1][2][3][4][5][6][7][8][9]. For this purpose, the n s SiO 4 mode of tetraalkoxysilanes (the most common precursors), which appears in the Raman spectra as a strong band, is regularly used for the characterization of the stepwise hydrolysis as a first phase of sol-gel processes [2][3][4][5].…”