Preparation of the Silica Gel Monolith by the Sol-Gel Method Using N, N-Dimethylformamide and the Vitrification of the Gel

Abstract

Monolithic dried silica gels were synthesized from tetramethoxysilane (TMOS) solutions using N, N-dimethylformamide (DMF) as a drying-control chemical additive. Solutions of the system TMOS-DMF-CH3OH-H2O-NH4OH (or HNO3) with varying amounts of catalyst, concentrations of DMF and [H2O]/[TMOS] molar ratios were reacted to obtain dried gel monolith, in order to find an optimum composition. A crack-free cylindrical rod of 32mm in diameter and 120mm in length could be obtained from the solutions of the composition 1 TMOS⋅1 DMF⋅2 CH3OH⋅10 H2O⋅5×10-4NH4OH in mole and some other compositions. The pore size distribution varied with the kind and amount of catalyst, the concentration of DMF, and the [H2O]/[TMOS] molar ratio. The beneficial role of DMF in producing crack-free dried gel monolith was attributed to the effect of keeping the average pore size of the gel large and of lowering the surface tension of liquid filling pores in the drying gel. When the composition of the starting solution was appropriate, the dried gel monolith could be converted into transparent silica glass without cracking and bloating by heating to 1050°C and holding there for more than two hours. The change of gel structure of the dried gel was investigated by infrared and Raman spectroscopy.