Autotransformation and ageing of acid-treated montmorillonite catalysts: a solid-state 27Al NMR study

Abstract

Solid-state ²⁷Al magic-angle spinning (MAS) NMR has been used to study ageing processes in acid-treated clay catalysts. Acid-treated montmorillonite has been aged under conditions of high humidity. Elemental analysis has shown that Al migrates from the clay lattice to ion-exchange sites on the clay surface (autotransformation). Simultaneously, the cation-exchange capacity of the clay falls. NMR has shown that migration of Al from octahedral and tetrahedral lattice sites occurs at comparable rates. Despite exhibiting the same chemical shifts, hydrated Al³⁺(narrow-line spectrum) can be distinguished from octahedral lattice Al (broad line) under appropriate conditions. Hydrated Al³⁺ ions arising from autotransformation appear to occupy two distinct sites, and possibly other asymmetric sites in which the Al centres are NMR invisible.