An Analysis of a New Approach to Sol-gel Synthesis: The Reaction of Formic Acid with Teos

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FTIR spectroscopy was investigated as a means of monitoring the reactions of formic acid and tetraethoxy silane, TEOS, at different temperatures and molar ratios of formic acid to TEOS, r. FTIR spectra of the reactions at r values of 1, 2, and 6 showed that increasing the molar ratio and temperature significantly increased the rates of hydrolysis and condensation. An activation energy of 10.5 0.6 kcal/mole was determined for the r= 6 system.
29Si NMR was used to monitor the reaction of formic acid with TEOS at molar ratios of 1, 2, and 6. The increase in reaction rate with increasing molar ratio was clearly evident in the silica NMR spectra. The low concentration of monomeric species containing hydroxy groups was deduced from NMR spectra. Proton NMR spectroscopy was utilized in identifying the byproducts of the reaction of formic acid with TEOS. Ethanol, ethyl formate, and SiOOCH groups were easily identified by NMR. Ethyl formate was initially the major byproduct; however, ethanol became the major low molecular weight species as the reaction proceeded. Information on the oligomeric structures was gathered using USAXS and mass spectrometry. USAXS was used to obtain radii of gyration for the r = 6 system which increased from 5.4 nm to 9.6 nm as the reaction progressed. An analysis was performed on the mass spectrum of the reaction of formic acid with TEOS at r =6, 35 minutes. Most oligomeric species contained at least one ring, and the maximum number of silicons was 11. Furthermore, the mass spectrum indicated that OR groups were the predominant groups attached to the silica oligomers. The analyses performed on the reaction of TEOS with formic acid provided possible explanations for the increased rate of gelation. The presence of fewer cyclic oligomers in the early stages of reactions was observed in 29Si NMR spectroscopy. 29Si NMR spectroscopy also indicated an increase in the rates of condensation for the reaction of formic acid with TEOS. The results of the analyses on the reaction of formic acid with TEOS were used to propose a kinetic model.