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

Abstract

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) 25Mg 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 27Al NMR), the ordering increasing with an increase in Al content. 1H 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.

Investigating Local Structure in Layered Double Hydroxides with 17O NMR Spectroscopy

“…[ 26 ] Consistent with this assertion, the 17 O single pulse NMR data can also be well fi t with these Mg 3 OH/Mg 2 AlOH ratios and the NMR parameters in Table 1 ( Figure S10, Supporting Information). The mole percentages of Mg 3 OH and Mg 2 AlOH sites in the hydroxide layers of OH − anion-containing LDHs extracted from the 17 O NMR data are in good agreement with the predictions made assuming cation ordering with Al-O-Al linkage avoidance and thus the results from 25 Mg NMR and 1 H NMR [ 14,15 ] investigations of NO 3 − -containing LDHs (Figure 5 b). No MgAl 2 OH environment was observed and a different trend in the percentage of Mg 2 AlOH was found in our study compared to the results from Cadars and co-workers [ 13 ] at high Al mole percentages (>30%), again we believe their observation of the Al-O-Al defects originates from the different preparation methods.…”

“…[ 13 ] Cation ordering was further supported by the 25 Mg NMR data obtained by Sideris et al at an ultrahigh magnetic fi eld, where it was argued that the presence of defects in some samples was likely a consequence of the synthesis method. [ 14,15 ] Large 1 H-1 H homonuclear dipolar couplings are present in these systems, [ 17 ] and ultrafast spinning rates (i.e., >40 kHz) [ 18 ] were required for the NO 3 − -containing LDHs to allow resolution of the major resonances. [13][14][15] With different anions in the interlayer regions, for example, the carbonate ion, CO 3 2− , however, the resonance becomes much broader and the spectral resolution decreases signifi cantly due to the diffi culty in removing larger 1 H-1 H homonuclear dipolar coupling and/or chemical exchange between the different proton species.…”

“…[ 14,15 ] Large 1 H-1 H homonuclear dipolar couplings are present in these systems, [ 17 ] and ultrafast spinning rates (i.e., >40 kHz) [ 18 ] were required for the NO 3 − -containing LDHs to allow resolution of the major resonances. [13][14][15] With different anions in the interlayer regions, for example, the carbonate ion, CO 3 2− , however, the resonance becomes much broader and the spectral resolution decreases signifi cantly due to the diffi culty in removing larger 1 H-1 H homonuclear dipolar coupling and/or chemical exchange between the different proton species. [ 14 ] Therefore, new approaches are needed to study the local structure of LDHs and perform double resonance experiments to investigate the fundamental interactions between cations and anions that defi ne the supramolecular chemistry in LDHs.…”

Recent advances on the utilization of layered double hydroxides (LDHs) and related heterogeneous catalysts in a lignocellulosic-feedstock biorefinery scheme

“…20 Additionally, the uniform distribution of the M 2+ and M 3+ cations in the layered structure makes these materials excellent precursors for the preparation of highly dispersed metal-supported catalysts. [52][53][54][55][56][57] This preparation approach requires the introduction of the catalytically active species as substituted cations in the structure of the LDHs during the synthesis, followed by calcination and reduction steps. It allows the formation of well-dispersed metal NPs on a mixed oxide matrix (MMO).…”

Relative Humidity-Induced Reversible Hydration Of Sulfate-Intercalated Layered Double Hydroxides

“…Evidence in favor of cation ordering is provided by an array of techniques such as extended X-ray absorption fine structure (Vucelic et al, 1997;Bigey et al, 1997;Roussel et al, 2000Roussel et al, , 2001, magic angle spinningnuclear magnetic resonance (Sideris et al, 2008(Sideris et al, , 2012Cadars et al, 2011), and X-ray diffraction (XRD) (Krivovichev et al, 2010;Radha and Kamath, 2013). Cation ordering generates a large unit cell with a = Hn6a o (n = 3 for x = 0.33) (Hofmeister and Platen, 1992).…”