MAS NMR study of surface-modified calcined kaolin

Calcined kaolins, modified by a silane coupling agent, were analyzed by magic angle spinning nuclear magnetic resonance (MAS NMR) and compared to unmodified samples. The results show no chemical shift for 29Si, whereas there is a marked change for 27Al. The chemical shifts of 5.44 and 65.69 ppm of 27Al in unmodified samples are respectively moved to 3.8–4.4 and 54.6–59.9 ppm after modification. This is attributed to changes in the chemical environment around the surface Al ions on the calcined kaolin resulting from chemical bonding of the silane coupling agent molecules to the Al. The results demonstrate that MAS NMR is a useful technique for characterizing the surface modification mechanisms of minerals.