A nuclear magnetic resonance investigation of high surface area silica-aluminas

Abstract

A combination of conventional and multiple pulse nuclear magnetic resonance (NMR) techniques has been used to examine the hydroxyl protons on a series of high surface area silica-aluminas whose composition varied from 0–100% SiO 2. Protons remaining on these materials after being calcined at 500 °C are found to exist as immobile and widely separated hydroxyl groups at room temperature. The concentrations of both SiOH and AlOH groups are reported as a function of adsorbent composition. With the high silica content samples, 100-75% SiO 2, no AlOH groups were detectable, while on the 50% and lower SiO 2 content samples most surface protons were in AlOH groups; thus, there is indication of a major change in local structure between 75 and 50% silica content. The isotropic part of the chemical shift tensor for hydroxyls on pure SiO 2 is −4.2 ppm relative to TMS and the anisotropy extracted from the powder pattern assuming an axially symmetric tensor is +6.9 ppm, substantially smaller than those found for hydroxyl groups in the solid state.