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 Mg2+(1-x)Al3+(x)OH2(Anion(n-)(x/n)).yH2O, 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 (1H NMR) spectra and characterize the magnesium and aluminum distribution. These data, in combination with 1H-27Al double-resonance and 25Mg 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 Al3+-Al3+ close contacts. The application of rapid MAS NMR methods to investigate proton distributions in a wide range of materials is readily envisaged.

Effects of Composition and Structure of Mg/Al Oxides on Their Activity and Selectivity for the Condensation of Methyl Ketones

“…Previous studies suggest that as-synthesized hydrotalcite possesses a single axially symmetric Mg(OMg) 6 environment similar to that observed for pure brucite, Mg(OH) 2 . 19 The one-dimensional 25 Mg MAS NMR spectrum of hydrotalcite pretreated at 623 and 823 K shows the presence of a single, relatively broad peak at 23 ppm, suggesting the formation of a poorly crystalline MgO phase ( Figure S4). 28,35,36 However, due to the broadness of the spectrum it is difficult to conclude whether there is only one Mg environment or whether other resonances are hidden under the broad one pulse spectrum.…”

“…17 In as-synthesized hydrotalcite, Al has six Mg atoms at nearest neighboring positions, connected through OH groups, whereas Mg has both Al and Mg at these positions. 18,19 Once calcined at elevated temperatures, hydrotalcites become effective catalysts for various carbon−carbon bond forming reactions, such as aldol, Knoevenagel, Michael, and Claisen-Schmidt condensation. 16,17,20−24 The catalytic competence of hydrotalcites after calcination is a result of the high surface area and moderate acidity/basicity of the resultant Mg− Al mixed oxides.…”

Exchange-biased NiFe2O4/NiO nanocomposites derived from NiFe-layered double hydroxides as a single precursor

“…level [19], and the mechanism of the transformation from the LDH precursor to the final composites has been investigated in detail [20][21][22]. The structure of the LDH precursor belongs to the hexagonal group, and the cations on the (00l) facets are arranged in an ordered "hexagon" formation.…”

Uncovering the Local Magnesium Environment in the Metal–Organic Framework Mg2(dobpdc) Using 25Mg NMR Spectroscopy

“…This situation is exacerbated by 25 Mg concentrations of only 0.21 25 Mg per nm 3 in Mg2(dobpdc) as compared for example to 5.3 for MgO. We addressed these problems by performing SSNMR experiments at high magnetic field [19][20][21][22][23][24][25][26][27][28] and using the double frequency sweep (DFS) technique to enhance the sensitivity, 34,45 though the quality of the echo spectra are still very poor. The Quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) method has been extensively used to acquire the spectrum of insensitive quadrupolar nucleus.…”

“…25 Mg SSNMR spectroscopy is capable of identifying these Mg coordination environments, because the interaction between the nuclear quadrupole moment of 25 Mg (spin I = 5/2) and the local electric field gradient (EFG) is highly sensitive to the spatial arrangement of neighboring atoms around the 25 Mg nucleus. [16][17][18][19][20][21][22][23][24][25][26][27] Moreover, 25 Mg SSNMR spectroscopy can probe the local disorder around Mg centers which are unavailable in diffraction- 25 Mg. The refocusing pulse was still 180° for maximum signal.…”