CHARACTERIZATION OF POROSITY OF AMMONIA CATALYZED ALKOXYSILANE SILICA

Monodispersed silica spheres were obtained by hydrolysis and condensat ion of a tetraalkoxysilane in concentrated ammonia solutions and their structure was studied. The particle size, in the range 10-730 nm, was observed by transmission electron microscopy. Thermogravimetric analy sis and Si-29 nuclear magnetic resonance reveal a high silanol concent ration, close to 5.4 mmol OH g(-1), despite a very small specific surf ace area of most samples, determined by Kr adsorption. This contrast i s explained by an ultramicroporous structure of the beads. Pores small er than 1 nm have been directly observed on transmission electron micr ographs of ultramicrotomed slices of these beads. This porosity also e xplains both the low density measured by decaline picnometry and the v ibration mode observed in Raman spectra below 10 cm(-1) for particles 10 nm diameter. Some samples of monodispersed silica beads display pec uliarities. Type I Kr isotherms, associated with very long equilibrium times and high surface areas up to 210 m(2) g(-1) with Kr and 385 m(2 ) g(-1) with N-2, prove their microporosity. The density obtained by p icnometry is still smaller than for usual samples. Thermal treatments and reaction with NaOH illustrate various properties of these micropor ous silica spheres. (C) 1997 Elsevier Science B.V.