Mg/Al Ordering in Layered Double Hydroxides Revealed by Multinuclear NMR Spectroscopy

Abstract

The anion-exchange ability of layered double hydroxides (LDHs) has been exploited to create materials for use in catalysis, drug delivery, and environmental remediation. The specific cation arrangements in the hydroxide layers of hydrotalcite-like LDHs, of general formula Mg 2+ 1– x Al 3+ x OH 2 (Anion n– x / n )· y H 2 O, have, however, remained elusive, and their elucidation could enhance the functional optimization of these materials. We applied rapid (60 kilohertz) magic angle spinning (MAS) to obtain high-resolution hydrogen-1 nuclear magnetic resonance ( 1 H NMR) spectra and characterize the magnesium and aluminum distribution. These data, in combination with 1 H- 27 Al double-resonance and 25 Mg triple-quantum MAS NMR data, show that the cations are fully ordered for magnesium:aluminum ratios of 2:1 and that at lower aluminum content, a nonrandom distribution of cations persists, with no Al 3+ -Al 3+ close contacts. The application of rapid MAS NMR methods to investigate proton distributions in a wide range of materials is readily envisaged.