Sol-gel synthesis of microporous amorphous silica from purely inorganic precursors

Aqueous sodium silicate solutions and hydrochloric acid were used to prepare silica gels without any additives. In order to establish the optimum conditions for the preparation of microporous and monodisperse gels, the sol–gel reaction has been studied as a function of pH, concentration of silicate, time, and temperature. From 29Si NMR spectroscopy it was found that polycondensation sets on immediately after mixing the reagents, with a continuous increase of the fraction of 4-connected silicate tetrahedra (Q4) at the expense ofQ1andQ2. Polycondensation continues beyond the gelation point for 1 ≤ pH ≤ 7, but not for 7 ≤ pH ≤ 10. Two mechanisms were also observed for the gelation kinetics which is about second order with respect to [Si] for pH ≤ 7, whereas for pH > 7, the order increases rapidly with pH. Gels formed at pH > 7 had an electrostatic nature and redissolved in water, whereas for pH ≤ 7, gelation was irreversible as expected for a gel where individual colloid particles are linked by chemical (≡Si—O—Si≡) bonds. From nitrogen adsorption, it is found that the pore size depends mainly upon pH, the finest pores being obtained for pH 2–3 (BET surface area 650–700 m2/g, porosity 35%, pore radius 5–11 Å).