STRUCTURAL STUDIES OF HIGH SURFACE-AREA SILICAS

Neutron scattering data for various high surface area silicas are presented. Significant changes are observed in the position and intensity of the first sharp diffraction peak in the interference function Qi(Q), and this has been interpreted in terms of differing concentrations of three-membered rings in these silicas. Comparison of the Qi(Q) and G(r) distributions with the same functions calculated from computer-generated continuous random network models suggest significantly different ring statistics characterising each silica depending on its method of synthesis. These results are supported by Si-29 MAS-NMR experiments in which a peak at approximately - 105 ppm relative to tetramethylsilane is required to adequately fit the spectra of samples in which the neutron data suggest a significant content of three-membered ring structures. Changes in the concentration of three-membered rings in the silica samples did not correlate with surface area or pore size. The work presented suggests that the synthesis route and thermal history of a porous solid will have a significant effect on the structure of the silica network, and hence on the adsorption and mobility of species interacting with the silica surface.