Multinuclear high-resolution solid-state nuclear magnetic resonance studies of amorphous silica-aluminas

Commercial silica-aluminas contain aluminium atoms tetra- and hexacoordinated to oxygen. After dehydration at 1 Pa at temperatures above 370 K, their¹H MAS NMR spectra show silanol groups and OH groups bonded to aluminium atoms but no acidic bridging OH groups. Spectra of samples dehydrated at lower temperature give clear evidence for such acidic OH groups.²⁹Si MAS and CP-MAS NMR spectra of silica-aluminas with low silica concentration show mainly SiO₄ tetrahedra bonded to four others and silicon atoms, bonded to OH groups, whose²⁹Si chemical shift is upfield of that generally attributed to terminal SiOH or geminal Si(OH)₂. The environment of SiO₄ tetrahedra in the sample with SiO₂:Al₂O₃ = 75:25, prepared by coprecipitation, is random.