Characterization of porosity of ammonia catalysed alkoxysilane silica

Monodispersed silica spheres were obtained by hydrolysis and condensation 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 analysis and Si-29 nuclear magnetic resonance reveal a high silanol concentration, close to 5.4 mmol OH g(-1), despite a very small specific surface area of most samples, determined by Kr adsorption. This contrast is explained by an ultramicroporous structure of the beads. Pores smaller than 1 nm have been directly observed on transmission electron micrographs of ultramicrotomed slices of these beads. This porosity also explains both the low density measured by decaline picnometry and the vibration mode observed in Raman spectra below 10 cm(-1) for particles 10 nm diameter. Some samples of monodispersed silica beads display peculiarities. 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 picnometry is still smaller than for usual samples. Thermal treatments and reaction with NaOH illustrate various properties of these microporous silica spheres. (C) 1997 Elsevier Science B.V.