Influence of kinetic parameters on the textural and chemical properties of silsesquioxane materials obtained by sol-gel process

Abstract

The hydrolytic sol-gel polymerization of a ‘rigid’ molecular precursor 1,4-C ₆ H ₄ [Si(OMe) ₃ ] ₂ 1 and a more ‘flexible’ one 1,4-C ₆ H ₄ [CH ₂ CH ₂ Si(OMe) ₃ ] ₂ 2 was investigated varying the experimental conditions. Two solvents, MeOH and THF, were employed. The influence of the catalyst has been determined by using TBAF (tetrabutylammonium fluoride) or NH ₄ F as nucleophilic catalysts, NH ₄ OH and NaOH as basic catalysts and HCl as acid catalyst. The effect of concentration of the precursor was also studied. Molecular precursor 1 led always to hydrophilic solids with similar degrees of condensation (63-67%). In all cases high specific surface areas and poor chemical reactivity towards Cr(CO) ₆ were observed and the solvent had an influence on the porosity. By contrast, the precursor 2 led to hydrophobic solids and the texture, the degree of condensation and the reactivity with Cr(CO) ₆ were strongly dependent on the solvent, the catalyst and the concentration. In MeOH no significant specific surface areas were observed, whereas in THF a high specific surface area was observed with TBAF catalysis at both precursor concentrations studied. Degrees of condensation were higher in THF. All the kinetic parameters involved in the hydrolytic sol-gel polymerization of molecular organosilicon precursors were of importance on the properties of the resulting solids, the main factor being the geometry of the organic unit.