Sol-gel process: influence of the temperature on the textural properties of organosilsesquioxane materials

The hydrolytic sol-gel polymerization of a variety of molecular precursors, (RO)(3)Si-X-Si(OR)(3) with different structural features, has been investigated varying the temperature from -20 to +110 degrees C. "Rigid" precursors (R = Me, X = 1,4-C6H4, ferrocene-1,1'-diyl), "semi-rigid" precursors (R = Me, X = CH2CH2-1,4-C6H4CH2CH2; R = Et, X = CH2CH2CH=CHCH=CHCH2CH2) and alkylene-bridged precursors [R = Me, X = (CH2)(6), (CH2)(10); R = Et, X = (CH2)(2), (CH2)(14)] were studied. A molecular precursor containing three Si(OMe)(3) groups, 1,3,5-(MeO)(3)SiC6H3, was also considered. The reactions were performed using ionic and non-ionic catalysts. The ionic ones were a nucleophile [tetrabutylammonium fluoride (TBAF)], an acid (HCl) and a base (NaOH), and the non-ionic ones were nucleophiles [dimethylaminopyridine (DMAP), N-methylimidazole (NMI)]. The solvent employed was THF. Reactions were studied in MeOH only with TBAF as catalyst. In all cases the temperature appeared to be a fundamental parameter and had a drastic influence on the specific surface area and the porosity of the resulting solids. Particularly at low temperatures (-20 to 0 degrees C), the solids were mainly microporous with a weak mesoporous contribution without narrow pore size distribution. At higher temperature (20 to +110 degrees C), the solids exhibited very high specific surface areas and were mesoporous with a narrow pore size distribution. The specific surface area, the porosity and the pore size were highly dependent on the temperature. This work came in our study on the kinetic control of the texture.