AUTOTRANSFORMATION AND AGING OF ACID-TREATED MONTMORILLONITE CATALYSTS - A SOLID-STATE AL-27 NMR-STUDY

Solid-state Al-27 magic-angle spinning (MAS) NMR has been used to stud y ageing processes in acid-treated clay catalysts. Acid-treated montmo rillonite has been aged under conditions of high humidity. Elemental a nalysis has shown that Al migrates from the clay lattice to ion-exchan ge sites on the clay surface (autotransformation). Simultaneously, the cation-exchange capacity of the clay falls. NMR has shown that migrat ion of Al from octahedral and tetrahedral lattice sites occurs at comp arable rates. Despite exhibiting the same chemical shifts, hydrated Al 3+ (narrow-line spectrum) can be distinguished from octahedral lattice Al (broad line) under appropriate conditions. Hydrated Al3+ ions aris ing from autotransformation appear to occupy two distinct sites, and p ossibly other asymmetric sites in which the Al centres are NMR invisib le.