Bulk and surface properties of commercial kaolins

Four different commercial kaolins have been characterized by specific surface measurement, X-ray diffraction, transmission electron microscopy, SEM–EDX analysis, TGA measurements, DR–UV–Vis–NIR spectroscopy, transmission IR spectroscopy of the samples as such and after acetonitrile adsorption. A correlation between surface acidity and structural and chemical properties was attempted. Kaolinite surface was almost neutral, with a very small amount of medium acidic centers. These centers gave rise to a weak protonic acidity arising from bridging hydroxy-groups on exposed basal planes (exposed gibbsite-like layers), and silanol groups likely formed on non-basal planes, as well as medium strength Lewis acid sites likely exposed on non-basal planes. Lewis and Brønsted sites may also arise from mineral impurities (e.g. anatase, amorphous silica–alumina).