Identification of Cation Clustering in Mg-Al Layered Double Hydroxides Using Multinuclear Solid State Nuclear Magnetic Resonance Spectroscopy

A combined X-ray diffraction and magic angle spinning nuclear magnetic resonance (MAS NMR) study of a series of layered double hydroxides (LDHs) has been utilized to identify cation clustering in the metal hydroxide layers. High resolution (multiple quantum, MQ) Mg-25 NMR spectroscopy was successfully used to resolve different Mg local environments in nitrate and carbonate-containing layered double hydroxides with various Al for Mg substitution levels, and it provides strong evidence for cation ordering schemes based around Al-Al avoidance (in agreement with Al-27 NMR), the ordering increasing with an increase in Al content. H-1 MAS double quantum NMR spectroscopy verified the existence of small Mg3OH and Mg2AlOH clusters within the same metal hydroxide sheet and confirmed that the cations gradually order as the Al concentration is increased to form a honeycomb-like Al distribution throughout the metal hydroxide layer. The combined use of these multinuclear NMR techniques provides a structural foundation with which to rationalize the effects of different cation distributions on properties such as anion binding and retention in this class of materials.