Identification of Cation Clustering in MgâAl Layered Double

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Identification of Cation Clustering in Mg−Al Layered Double Hydroxides Using Multinuclear Solid State Nuclear Magnetic Resonance Spectroscopy Paul J. Sideris,† Frédéric Blanc,†,‡ Zhehong Gan,§ and Clare P. Grey*,†,‡ †

Department of Chemistry, State University of New York, Stony Brook, New York 11790, United States Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom § National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States ‡

S Supporting Information *

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 carbonatecontaining 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. KEYWORDS: hydrotalcite, brucite, layered double hydroxides, LDHs, 25Mg MQMAS, 1H DQ NMR, cation clustering

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anions in the interlayer region, the M3+/(M2++M3+) ratio typically varying between 19 and 33%. Physisorbed and interlayer water is also present, which can hydrogen-bond to the metal hydroxide layers or the charge-balancing anions. Naturally occurring hydrotalcite has a chemical formula Mg6Al2(OH)16(CO3)·4H2O. Synthetic LDHs can be made using a variety of metals in place of Mg and Al, the most common preparation method being via a coprecipitation route. Additionally, various anions can be introduced into the structures either during the synthesis or more commonly via a subsequent anion-exchange. The relative ease of synthesis, tunable chemical composition, and unique structure type make LDHs attractive materials for several applications, such as anion-exchangers, polymer and catalyst supports, and drug delivery systems.1 Previous studies have focused on determining anion affinities and adsorption capacities of materials and drawing correlations from the average, long-range structure of the LDHs obtained through diffraction techniques. Determination of the local structure of the metal hydroxide layers and interlayers has been particularly challenging. Many structural

INTRODUCTION Hydrotalcite-like layered double hydroxides (LDHs) are a class of compounds composed of sheets of close-packed, edgesharing divalent (M2+) and trivalent (M3+) metal hydroxide octahedra stacked along the crystallographic c axis (Figure 1). The octahedral units are corner shared such that the hydroxyl groups are oriented perpendicular to the metal layer, forming triangular lattices above and below the metal ions. Excluding cation vacancies and crystal edges, each hydroxyl group is bonded to three metal atoms in the layer (Figure 1C). Due to the similarities in the local structure, hydrotalcite and related LDHs are often compared with the mineral brucite (Mg(OH)2), which is also a layered double hydroxide containing neutral layers solely composed of close-packed Mg2+ hydroxide octahedra. In brucite, the MgO6 octahedra are slightly compressed along the crystallographic c axis, giving rise to D3d symmetry as opposed to the ideal Oh local geometry of an octahedral structural unit. This distortion increases the O−O and Mg−Mg distances in the metal hydroxide layer, resulting in nonideal Mg−O−Mg angles of 96.7° and 83.3°. These distortions are retained when Mg2+ is substituted by a trivalent cation. The presence of a metal ion with an oxidation state greater than two induces an overall positive charge on the brucite-like sheets; this is compensated by the incorporation of © 2012 American Chemical Society

Received: February 3, 2012 Revised: May 21, 2012 Published: May 30, 2012 2449

dx.doi.org/10.1021/cm300386d | Chem. Mater. 2012, 24, 2449−2461

Chemistry of Materials

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metal hydroxide sheets chosen by these authors did not contain cation ordering schemes consistent with a hexagonal or rhombohedral supercell (Figure 2).13 An earlier study

Figure 1. (A) Ideal cation arrangement of the metal hydroxide sheet for the highest Al content LDH Mg0.67Al0.33(OH)2(NO3)·yH2O, cation ordering being a direct consequence of Al−O−Al avoidance. The aluminum hydroxide octahedra (shown in light gray) adopt a honeycomb arrangement throughout the metal hydroxide sheet. (B) Possible local Mg environments in Mg/Al LDHs derived from doping the metal sites in a brucite-like configuration (center) and assuming no Al−O−Al linkages. The dark and light colored polyhedra are Mg(OH)6 and Al(OH)6 units respectively. (C) Environments created by the substitution of a Mg defect into the ordered honeycomb array.

Figure 2. Examples of possible superstructures derived from a triangular lattice defining the metal hydroxide layers of LDHs and assuming no Al−O−Al bonds; Mg and Al atoms are shown as dark and white circles, respectively. Brucite-like environments are shown in (A). The layer can be described by a rhombohedral cell whose sides correspond to the metal−metal distance a. The ordered, honeycomb arrangement present in the 33% Al-doped sample is shown in (B), corresponding to a rhombohedral superlattice with sides of length a√3. For ordered LDHs with Al content less than 33%, various superlattices can be envisioned including, for 25% Al, a rhombohedral cell with sides 2a (C) and an orthorhombic cell with sides 2a (D).

aspects of LDHs, such as the cation and anion arrangements, the effect of hydration, and the extent of polytypism, are a matter of debate in the literature.2−10 Furthermore, the extent and role of hydrogen bonding and the interaction of anions with the layers is poorly understood. In order to tailor these materials for specific applications, a detailed understanding of the local environment, in particular the cation distribution, is critical. Hofmeister and von Platen argued that all LDHs were cation ordered, but this could not be easily verified through experiments because of high pseudosymmetry, microcrystallinity, and stacking faults.11 Simulations of cation ordering sequences with slight defects or variations in the cation sequence show a dramatic reduction in the supercell reflection intensities. Rebours et al. have investigated a series of carbonate-containing Mg−Al and Mg−Ga LDHs using diffraction methods, radial distribution functional analyses, and extended X-ray absorption fine structure (EXAFS) analyses.12 In their study, the Mg−Al system did not show any evidence of cation ordering, even when the Al content approached 33%. These results were in contrast to the Mg−Ga LDHs, which showed a supercell reflection peak at approximately 4.63 Å for Mg/Ga = 2. The authors provided two possible explanations for the discrepancy: (1) since the difference in ionic radii between Mg2+ and Ga3+ is smaller than between Mg2+ and Al3+, distortions in the corresponding octahedra are less pronounced in the former system, which may facilitate an ordered arrangement of cations and (2) both types of LDHs contain ordered cations, but the similar scattering power of isoelectronic Mg2+ and Al3+ prevent the observation of the reflections of a superlattice. A computational study involving Mg3Al1(OH)8-Cl LDHs showed that configurations with adjacent Al cations were energetically less favorable, but the cation arrangements on the

investigated the lowest energy configurations of two-dimensional Coulombic alloys of the form A1−xBx, where A and B represent ions with two different oxidation states that were allowed to occupy different sites on a triangular lattice. The authors of this earlier study identified certain supercells that were more stable for a given A/B ratio and suggested that these calculations would be relevant to the cation distributions in the LDHs, since each metal hydroxide layer in an LDH is isolated from the subsequent layers by the anions in the interlayer gallery, the dominant intralayer interaction therefore arising from Coulombic repulsions of the trivalent metals. A large number of superstructures of a triangular lattice were considered, particularly those of a class of supercells defined by a√3, where a√3 represents the nearest neighbor distance between the trivalent cations B leading to a composition 1/r (Figure 2B). For r = 4, corresponding to an Al-content of x = 0.25 in our notation, superstructures corresponding to nontriangular superlattices (e.g., the rectangular superlattice shown in Figure 2D) were only slightly less favorable energetically. For a cation ratio equal to 1/3, the honeycomb superstructure of trivalent cations was determined to be the most energetically favorable, which is consistent with the experimental observations for Mg/Ga LDHs.12 Other simulations have shown that although the honeycomb lattice is energetically favorable up to x = 0.33, domains with both triangular and rectangular superlattices can, in principle, be present on the metal hydroxide sheets at lower concentration of Al.14 Magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy is a well established technique to tackle structural challenges of a wide range of solid state materials.15,16 It is particularly well-suited to elucidating local environments in Mg1−xAlx LDHs, since all atoms have at least one spin-active nucleus (17O, 25Mg, and 27Al are spin I = 5/2 and 1H spin I = 1/2). The NMR spectra I > 1/2 nuclei are dominated by the 2450



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sites, corresponding to Mg-rich and Al-ordered honeycomb clusters in the LDHs, is observed. The ultrafast 1H MAS NMR data are consistent with our previous report and the Mg MQMAS data, the latter showing a gradual loss of the brucitelike Mg environment in favor of an Al honeycomb-ordered local environment as the Al-content of the metal hydroxide layers increases to 33%. 1H double quantum (DQ) MAS NMR spectroscopy, a 1H−1H correlation experiment that determines the through-space proximities of the various rigidly bound protons, confirms that the brucite-like Mg environments do not arise from a separate phase but exist within the metal hydroxide sheets.

quadrupolar interaction (between the intrinsic nuclear electric quadrupole moment and the surrounding electric field gradient (EFG)). This interaction limits the spectral resolution even under MAS, where second-order perturbation terms are still present.16,17 However, the multiple-quantum (MQ) MAS experiment of Frydman et al.18,19 removes the second-order quadrupolar broadening term, allowing high resolution spectra of half-integer quadrupolar nuclei to be obtained and providing a two-dimensional experiment from which the number of unequivalent sites can, in principle, be directly obtained. Although 25Mg NMR studies are particularly challenging (as 25 Mg has a low natural abundance (10.13%), a low gyromagnetic ratio, and a relatively large quadrupole moment), several studies have now been reported for a large range of materials,20−31 including natural and synthetic carbonatecontaining hydrotalcite, brucite,21 and our preliminary investigation of LDHs.32 25Mg MQMAS studies are even more scarce but have been successfully used to characterize Mgadensosine-5′-triphosphate complex,33 Mg-doped perovskite,34 various glasses,35 and a few other inorganic materials.36,37 In our previous publication using 1H NMR spectroscopy under ultrafast MAS conditions,32 we determined that the Al cations in nitrate-containing LDHs were arranged in a nonrandom fashion on the metal hydroxide sheets, such that Al−O−Al linkages were avoided. The 1H spectra were dominated by three resonances assigned to two hydroxyl resonances, corresponding to Mg3−OH and Mg2Al−OH environments, and to water. The presence of only two hydroxyl environments (and importantly the absence of MgAl2−OH and Al3−OH environments) in the lower Al-doped materials, as opposed to a binomial distribution, was consistent with a nonrandom cation distribution in the layers. The Mg2Al−OH hydroxyl group dominated the spectrum of the 33% Al-doped LDH sample, and only a very weak signal from the Mg3−OH hydroxyl groups (∼ 3% of the total signal from the 2 hydroxyl groups) was seen, consistent with a honeycomb arrangement of Mg and Al in the brucite-like layers, as shown in Figure 1A. As further support to the nonrandom arrangement of cations, high field 25Mg MAS NMR spectra were acquired on a series of LDHs corresponding to the low, mid, and high Al dopant levels. The 33% Al-doped sample displayed an axially symmetric second-order line shape with a quadrupole coupling constant of approximately 4.6 MHz. 25Mg MQMAS of the compound confirmed the presence of a single Mg site. Our results are in qualitative agreement with an 1H ultrafast MAS NMR report by Cadars et al.,38 although these authors also observe additional resonances, which they assigned to MgAl2− OH hydroxyl groups due to Al clustering and resulting from local defect on an otherwise perfectly cation-ordered structure. As discussed more below, we ascribe the presence of the additional local environments to the different preparation methods used in the two studies. Cadars et al.38 used samples extracted directly after the coprecipation step, while we used samples that were subjected to an additional hydrothermal treatment, which may anneal out some defects and result in the most thermodynamically favored structure. In this manuscript, we extend our 25Mg MQMAS study of the local and long-range structure of Mg/Al LDHs and include compounds with Al contents of less than 33%. MQMAS NMR experiments39 and powder X-ray diffraction are used to understand the cation distributions in Mg/Al LDHs with low (19%), mid (25%), and high (33%) Al concentrations with both carbonate and nitrate anions. Resolution of multiple Mg

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MATERIALS AND METHODS

Sample Preparation. The nitrate-containing LDHs were prepared by the conventional coprecipitation method by dissolution of the desired amounts of Mg(NO3)2·6H2O (Aldrich, 95%) and Al(NO3)3·9H2O (Aldrich, 95%) in distilled water (approximately 40 mL) to obtain the desired Mg/Al molar ratios. A 1 M solution of NaOH or NH4OH was used for the co-precipitation reaction. The LDHs were transferred to a 125 mL Teflon-lined autoclave and heated at 125 °C for 3 days unless otherwise noted. The samples were then vacuum-filtered and dried at 80 °C in an oven. The carbonatecontaining LDH, having an ordered anion stacking arrangement, was prepared using the urea hydrolysis method as described in more detail elsewhere.40−44 No measures were taken to control the relative humidity or carbonate uptake. The LDH samples were labeled as follows: MgAl-x-An−, where x denotes the desired molar Al-dopant level and An− is the interlayer anion. Inductively coupled plasma (ICP) analysis (Galbraith Inc. Laboratories, Knoxville, Tennessee) was performed on MgAl-x-NO3n− to determine the Mg and Al contents. All samples have the desired nominal composition except the 33% sample, for which the Al content was found to be 35%. This is ascribed to a minor Al oxy/hydroxide phase impurity and possibly Mg vacancies, since the Al content is higher than the theoretical maximum, assuming Al−Al avoidance. X-ray Diffraction. Powder X-ray diffraction (PXRD) data were collected at the National Synchrotron Light Source at Brookhaven National Laboratory on the X16C beamline (0.6984 Å) and on a Scintag Instruments X-ray diffractometer (1.54 Ǻ of Cu Kα). To compare data sets, all diffraction patterns were converted to Cu Kα radiation. A unit cell refinement was conducted on the reflections with the least amount of asymmetrical broadeningnamely the 003, 006, 110, 113, and 116 reflectionsusing the Jade software package. A Rietveld refinement45 was performed on brucite using the GSAS/ EXPGUI software package.46,47 NMR Spectroscopy. 25Mg triple quantum MAS (3QMAS) NMR spectra were obtained at 19.6 T on a Bruker DRX 830 MHz spectrometer at the National High Magnetic Field Laboratory (NHMFL) equipped with a home-built single-channel 4 mm MAS probe-head tuned to 51.00 MHz.39 A shifted-echo48 pulse sequence was used with a soft-pulse-added-mixing (SPAM) enhancement pulse49 where indicated. A total of 18−24 rotor-synchronized t1 increments were used with the number of transients typically varying between 4800−24 576 and recycle delays either 0.3 or 0.4 s, yielding acquisition times of 5−43 h/spectrum (Table S1, Supporting Information). All pulse durations were optimized on the samples. The rf-field amplitude ν1 was 50 kHz during the shifted-echo sequence, and 95 kHz during SPAM-MQMAS.39 The MAS frequency νr was set to 10 kHz. The 25Mg shifts were externally referenced to MgSO4 in water at 0 ppm. Sine-bell functions were used to apodize the data before the shearing transformation. The spectra along F1 were referenced according to a “unified representation” described by Amoureux et al.50 The anisotropic slices of the MQMAS data were simulated using the STARS program.52 Single pulse 27Al MAS NMR was performed using a double resonance Varian T3 4 mm probe on an InfinityPlus 360 MHz spectrometer at 8.45 T (93.81 MHz). An rf-field of approximately 90 2451



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kHz and sample spinning of 15 kHz (±2 Hz) were used for each sample. Short pulses of 0.4 μs (∼ π/4 flip angle) were used to irradiate the entire signal manifold more efficiently. Typically, 4096 transients were collected with a recycle delay of 1 s. An exponential function of 100 Hz was used to apodize the data. Spectral simulations were performed with the program STARS.52 1 H “ultrafast” MAS NMR spectra were collected at 11.7 T on an InfinityPlus Varian 500 MHz spectrometer equipped with a 1.3 mm HX MAS probe-head (built by Ago Samoson and co-workers) spinning the sample at νr ∼ 55 kHz. Spectra were recorded with four transients using a simple one pulse sequence with pulses at an rf-field amplitude of 100−125 kHz. The plastic components in our probe produce a background signal centered at approximately 7.0 ppm. 1H double quantum (DQ) spectra were acquired at 14.1 T on an Avance II 600 MHz spectrometer equipped with a Bruker 2.5 mm HX probehead spinning the sample at νr = 35 kHz. Multiple quantum (MQ) excitation and reconversion were performed by using the Back to Back (BaBa) sequence51 (ν1 = 62.5 or 125 kHz) with a MQ excitation block of four rotor periods. A total of 128 t1 increments were collected with 8 or 16 transients each. Quantitative recycle delays of 10 s were used. The 1H chemical shifts were externally referenced to TMS at 0 ppm. NMR data were processed with MatLab and/or the WinNuts software package. Simulations of the 25Mg MQMAS, 27Al MAS NMR data, and 1H spectral deconvolution were performed using STARS52 or MatNMR.53

from this data are given in Table 1. A change in the stacking sequence of the metal hydroxide layers upon Al doping into the Table 1. Lattice Parameters Extracted from Cell Refinements of the LDH Phasesa sample

a, b (Å)

c (Å)

Mg(OH)2 MgAl-19-NO3− MgAl-25-NO3− MgAl-33-NO3− MgAl-33-CO32−

3.144(1) 3.07(1) 3.05(1) 3.04(1) 3.03(1)

4.769(2) 23.95(9) 25.05(5) 26.94(3) 22.56(1)

a Using the most intense reflections, 003, 006, 110, 113, and 116 for the NO3− compounds and 003, 006, 012, 015, 118, 110, 113, and 116 2θ reflections for the CO32−-containing compound.

brucite-like sheets and incorporation of the anions in the interlayer is observed and is associated with a change in space group from P3̅m1 (164) (Mg(OH)2) to R3̅m (166). As the amount of Al is increased, the lattice parameter a, defining the average metal−metal distance, decreases due to the smaller ionic radius of Al3+ in comparison to Mg2+. The increase in the lattice parameter c is correlated to the Al content of the nitratecontaining LDHs (Table 1) and the resulting increase in the number of interlayer anions, which are accommodated in the structure to charge-balance the Al3+. The reflections of nitratecontaining LDHs fall into three categories: (i) a pair of intense reflections at low angles, (ii) three asymmetrically broad peaks at mid angles, and (iii) three relatively sharp reflections at high angles. In contrast, the carbonate-containing LDHs all have very sharp reflections, even in the LDH mid angle region. The reflections at low angles are indexed to the basal 00l reflections and are readily observed for all samples. The unit cell a parameter was estimated from the 110 reflection (Table 1). Crystallographic information is usually difficult to obtain from the mid angle reflections, primarily because of their asymmetry and breadth. These reflections are usually indexed as 01l or 10l, depending on the polytype. Ideally, the intensities and systematic absences of certain classes of reflections can be used to determine the stacking sequence of the LDH. Oftentimes, the different stacking sequences or polytypes are intergrown in a random manner corresponding to turbostratic disorder, implying that the cations are completely uncorrelated in the c direction. A more detailed analysis of the diffraction pattern of each LDH is described below. Rietveld refinements of brucite gave cell parameters (Table 1) in good agreement with previously reported values.54 Upon introduction of Al in the metal hydroxide layers to form MgAl19-NO3−, the average metal−metal distance in the metal layers is reduced by approximately 0.07 Å relative to brucite. The low angle and high angle reflections are slightly sharper and more intense than for the higher Al-doped nitrate-containing LDHs, which may be indicative of larger particle sizes. A weak, partially resolved, 101 reflection at approximately 2θ = 35° is observed, implying some ordering of the stacking arrangements of the metal hydroxide layers. The reflections between 32° ≤ 2θ ≤ 50° are slightly higher in intensity relative to the other nitratecontaining LDHs, which is consistent with a slightly higher degree of correlation along the crystallographic c axis. MgAl-25-NO3− displays 00l reflections that are shifted to lower 2θ and a 110 reflection shifted to higher 2θ relative to the MgAl-19-NO3−, consistent with a higher Al-content in the layers. The reflections are noticeably broader, which may

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RESULTS AND DISCUSSION X-ray Diffraction. The powder X-ray diffraction data of LDHs are shown in Figure 3 and the cell parameters refined

Figure 3. Powder X-ray diffraction patterns of the layered double hydroxides. The reflections marked with an asterisk (*) correspond to NaNO3 used in the synthesis reaction. The observation of any weak reflections from aluminum (oxy-)hydroxide impurities in the 25° ≤ 2θ ≤ 35° region is hampered by the background from the capillary used in this measurement. A diffraction pattern of a 33% Al sample (collected without using a glass capillary) showing very weak reflections from these minor impurities is given in Figure S1, Supporting Information. 2452



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Figure 4. Natural abundance 25Mg 3QMAS spectrum of brucite. The dashed line shows the anisotropic slice of the single Mg environment simulated with parameters given in Table 2.

Figure 5. Natural abundance 25Mg SPAM-3QMAS spectrum of MgAl-19-NO3−. The dashed lines show the anisotropic slices taken at three F1 shifts and simulated with parameters given in Table 2. The slice corresponding to Mg(OMg)4(OAl)2 (δ1 = 47.5 ppm) is representative of the second-order line shape obtained over the 46−50 ppm range.

higher Al content. The ordering along c is increased relative to the 25% sample, but the sample remains poorly ordered in the a,b plane. MgAl-33-CO32− has very sharp and well-defined reflections (Figure 3), even in the midangle region, indicating larger particle sizes and a highly ordered stacking arrangement. The sample appears to have no stacking faults as there is no asymmetrical broadening of the 01l reflections and similar amplitudes of the 110 and 113 reflections. The most striking feature that distinguishes the phase-pure carbonate-containing LDH from its nitrate counterpart is the relatively small distance

indicate a smaller particle size and disorder in the interlayer region. The latter suggestion is consistent with the presence of a larger number of anions in the interlayer, which would alter the nitrate packing arrangement relative to the MgAl-19-NO3− sample, a change in nitrate packing occurring to minimize the anion−anion repulsion. The high angle peaks are relatively weak and broad due to a distribution in the unit cell a and b parameters. Disorder in the layer stacking is evident from a relatively weak and broad 113 reflection, relative to the 110 reflection. A further increase in the c parameter and decrease in the a parameter is seen in MgAl-33-NO3−, consistent with the 2453



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Table 2. 25Mg NMR Parameters Extracted from the 3QMAS Spectra of LDHa sample brucite MgAl-19-NO3−

MgAl-25-NO3−

MgAl-33-NO3−32 MgAl-33-CO32−

site

relative intensityb

δ1 (ppm)

δ2 (ppm)

δiso,cs (ppm)

CQ (MHz)

η

Mg(OMg)6 brucite-like Mg(OMg)6 Mg(OAl)3(OMg)3 Mg(OAl)2(OMg)4 brucite-like Mg(OMg)6 Mg(OAl)3(OMg)3 Mg(OAl)2(OMg)4 Mg(OAl)3(OMg)3 Mg(OAl)3(OMg)3

100 19 61 20 10 44 46 100 100

23.5(2) 26.6(4) 40.6(4) 47.5(4) 28.4(2) 42.5(4) 46.0(4) 38.9(4) 40.6(4)

−12.5 (1) −8(1) −36(1) −43(2) −5(1) −34(1) −40(2) −35(1) −32(1)

10.2(2) 13.8(5) 12.2(6) 14(1) 16.0(5) 14.2(6) 14.1(9) 11.5(6) 13.7(6)

3.1(1) 3.1(1) 4.6(1) 4.9(3) 3.0(1) 4.5(2) 4.8(3) 4.6(1) 4.5(1)

0.0 0.0(1) 0.0(1) 0.2(1) 0.0 0.0(1) 0.2(1) 0.0(1) 0.0(1)

δ1 and δ2 are the center of gravity of the peak in the isotropic (F1) and anisotropic (F2) dimensions, respectively. δiso,cs is the isotropic chemical shift and is determined by δiso,cs = (17δ1 + 10δ2)/27. CQ and η are the quadrupolar coupling constant and asymmetry parameter respectively. They are obtained by fitting of the anisotropic cross sections. bRelative intensity of the 25Mg peaks obtained from the deconvolution of the F1 dimension of the 25Mg MQMAS NMR spectra. See text for discussion of errors. a

Figure 6. Natural abundance 25Mg SPAM-3QMAS spectrum of MgAl-25-NO3−. The dashed lines show the anisotropic slices of the three Mg environments simulated with parameters given in Table 2. The slice corresponding to Mg(OMg)4(OAl)2 (δ1 = 46 ppm) is representative of the second-order line shape obtained over the 45−50 ppm range.

charge density and can lie parallel to the metal hydroxide sheets, resulting in smaller interlayer spacing. NMR Spectroscopy: 25Mg MQMAS NMR. A single Mg environment is seen in the 25Mg 3QMAS spectrum of brucite (Figure 4), in agreement with the crystal structure. The site could be fitted with NMR parameters that are consistent with Mg in 6-fold symmetry, as published previously.21 There are no anions in the interlayer gallery, and thus, the quadrupole parameters result primarily from the intrinsic distortions from octahedral symmetry of the Mg hydroxide octahedra that define

between the metal hydroxide layers, despite the high Al content. The difference in the c-parameter between LDHs with interlayer nitrates and carbonates is primarily attributed to the different anion packing arrangement.55 Since more nitrate anions need to be packed into the interlayer space for charge compensation at a given layer charge, the higher Al content materials contain a staggered arrangement of the anions and, consequently, a larger interlayer space for high Al-content materials. In comparison, the carbonate anions have a higher 2454



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Figure 7. Natural abundance 25Mg SPAM-3QMAS spectrum of MgAl-33-CO32−. The dashed line shows the anisotropic slice of the single Mg environment simulated with parameters given in Table 2.

magnesium hydroxide octahedron and a loss of the C3 rotational axis through the Mg atom. Tentatively, this site is assigned to Mg environments with two aluminum atoms in the first cationic coordination shell (i.e., Mg(OAl)2(OMg)4). This could be observed either in regions of the sample with local ordering that are similar to those shown in Figure 2C and D or, more likely, as defects in predominantly honeycomb ordered structures (Figure 1C). Intensity is also observed with δ1 > 40 ppm (approximately 38 ppm) and with an associated secondorder quadrupolar line shape that is smaller than the line shape ascribed to Mg(OAl)3(OMg)3. This intensity is tentatively assigned to Mg(OAl)(OMg)5 or possibly a second Mg(OAl)2(OMg)4 site. Given that the overlap with the Mg(OAl)3(OMg)3 was so severe, we chose not to analyze this site further. At least three sites could again be resolved in the 25Mg SPAM MQMAS spectrum of MgAl-25-NO3− (Figure 6), at similar F1 positions to those observed in the MgAl-19-NO3− sample (δ1 = 28.4 ppm, 42.5 ppm, and 46.0 ppm). In contrast to the MgAl-19-NO3− sample, the dominant Mg site (δ1 = 42.5 ppm) could now be accurately simulated and has a much smaller distribution in parameters. The site is consistent with a Mg atom surrounded by three Al atoms, found in the Alordered LDH (Mg/Al = 2/1) and strongly suggests the presence of at least short-range honeycomb cation ordering in the 25% Al-doped sample. The third site is more clearly resolved for the 25% Al sample, but it is similar in intensity to the same site in the 19% sample. Again, it is the most asymmetric of the three sites and is characterized by a larger CQ, suggesting that it likely arises from sites associated with cation arrangements that give rise to nonaxial symmetry (Table 2). Since the quadrupole moment of Mg is relatively large and is clearly sensitive to ordering in the first cation coordination shell, another possibility is that the third site is due to a Mg hydroxide octahedron interacting with an anion of higher charge density, such as the CO32− anion, since we took no precautions to exclude this anion from the synthesis mixture. However, it is not clear why this site would not be present in similar quantities in the 33% Al nitrate sample (see the

the structure. The presence of an axially symmetric (η = 0) second-order quadrupolar line shape is consistent with the lowering of symmetry of the Mg(O)6 octahedra from Oh to D3d. A distortion of the quadrupolar line shape was observed, which we ascribe largely to the difficulty in exciting the powder line shape uniformly in a MQMAS experiment (as discussed in the Supporting Information). At least three different magnesium environments can be resolved in the 25Mg SPAM-3QMAS spectrum of MgAl-19NO3−, as shown in Figure 5 and Table 2. The site at δ1 = 26.6 ppm (Table 2) has parameters very close to the ones obtained for pure brucite (Table 2) and is thus assigned to a brucite-like environment in the metal hydroxide sheets of the LDH, that is, an environment that is devoid of Al atoms in at least the first cationic coordination shell with local configuration Mg(OMg)6. Since the asymmetry parameter extracted for this site is zero, it seems unlikely that this site arises from an environment such as Mg(OMg)5(OAl). The most intense site, observed at δ1 = 40.6 ppm (Table 2), is associated with a much larger quadrupole coupling constant than the Mg(OMg)6 environment and has NMR parameters that are similar to those obtained previously for MgAl-33-NO3− (Figure S2, Supporting Information and Table 2).32 The model we proposed for this latter compound contains Mg atoms surrounded by three Al atoms and three other Mg atoms throughout the entire hydroxide sheet resulting in the configuration Mg(OAl)3(OMg)3 (Figure 1). Although the presence of this local environment in MgAl-19NO3− is entirely plausible, it is important to note that the fit to the experimental spectrum is not good, and in particular, the absence of the lower frequency discontinuity reflects a distribution of quadrupole coupling constants.56 This is ascribed to the variety of local ordering schemes that may be present in an LDH sheet at this composition and possibly some different local environments whose signals are buried or overlap with this resonance. A third, weaker site at higher frequency (δ1 ∼ 47.5 ppm; Table 2) could also be observed, confirming the presence of sites corresponding to different Mg−Al arrangements. The asymmetry parameter estimated from the line shape of this third resonance implies an increased distortion of the 2455



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sensitive to the quadrupolar interaction, hindering an accurate quantitative determination. Hence, the relative intensities can only be used for a qualitative comparison. Note, we did use these relative intensities to simulate the 1D 25Mg MAS NMR data presented in our earlier publication, and a reasonable agreement was obtained (Figure S3, Supporting Information). Unfortunately, the 1D line shapes are distorted by distributions in the quadrupolar interactions, the distortions being particularly severe for the MgAl-19-NO3− sample, and an accurate fit of the line shapes means that we have to introduce even more variables to describe the line shapes of the three overlapping sites, bringing into question any site populations extracted from these simulations. The fits of the isotropic dimension of the MQMAS data (and the 1D 25Mg MAS NMR)32 shows a very straightforward trend for the brucite-like peak at δ1 ∼ 24−28 ppm, the resonance decreasing monotonically with increasing Al-content. If we assume that the 25% sample is fully ordered but with cation schemes given in Figure 2C and D, only Mg environments nearby 2 Al atoms are predicted. Thus, the significant presence of both Mg(OMg)3(OAl)3 and Mg(OMg)6 sites indicates that these ordering schemes do not predominate in this sample. Second, we consider a honeycomb ordered lattice but with the Al atoms (on the Al sublattice) randomly replaced by Mg so as to create the final stoichiometry of the sample (as illustrated for high Al contents in Figure 1C), that is, 43, 25, and 1% of all Al atoms are replaced in a honeycomb lattice by Mg for the 19, 25 and 33% Al samples, respectively. In this case, it is relatively straightforward (as described in some detail in the Supporting Information) to predict probabilities for the occurrence of the Mg(OMg)6 and Mg(OAl)3(OMg)3 resonances of 24 and 12 and essentially 0% (Mg(OMg)6), and 15, 38, and 97% (Mg(OAl)3(OMg)3), respectively. These values are in good agreement with the experimentally observed intensities for the two higher Al content samples, given the errors associated with these measurements. In contrast, a completely random distribution of Mg and Al cations on the brucite triangular lattice results in calculated probabilities of 28, 18, and 9%, (Mg(OMg)6) and 7, 13, and 22% Mg(OMg)3(OAl)3 for the 19, 25, and 33% Al samples. This model overestimates the intensity of the Mg(OMg)6 resonance and underestimates the intensity of the Mg(OAl)3(OMg)3 resonances for the higher content Al samples. The relative intensity of the Mg(OMg)6 resonance (experimental intensity, 19%) is underestimated for the 19% Al sample, while the Mg(OMg)3(OAl)3 resonance (61%) is overestimated, with both models. The former error may occur because the MQMAS experiment was optimized on the 33% samples and thus for a site with a larger CQ value. The latter error suggests that the resonances from environments such as the Mg(OMg)5(OAl) may be buried under the Mg(OMg)3(OAl)3 resonance. Despite the errors associated with these fits, the spectra are consistent with partial to complete honeycomb ordering, at least for the higher Al content samples. 27 Al MAS NMR Spectroscopy. Figure 9 shows the 27Al MAS NMR spectra of the nitrate- and carbonate-containing LDHs. A single simulation is shown with NMR parameters δiso = 11.8 ppm, CQ = 1.55 MHz, η = 0 in agreement with an axial site for Al. The simulation only takes into account the quadrupole interaction and was performed using ideal pulses. A key observation is the lack of a large variation in the magnitude of the quadrupole coupling throughout the nitrate series, as is evident from the similarity in position of the discontinuities in

additional discussion in the Supporting Information). The most probable assignment of this site is a Mg atom surrounded by two Al atoms (as suggested for the 19% Al sample). The slightly improved resolution of both the Mg(OAl)3(OMg)3 and Mg(OAl)2(OMg)4 sites in the 25% doped sample is likely due to more pronounced cation ordering and the smaller concentration of sites with only one adjacent Al atom, resulting in fewer Mg local environments. The 25Mg SPAM MQMAS spectrum of the phase-pure, stacking-fault-free carbonate-containing 33% Al-doped LDH is shown in Figure 7, and like its nitrate counterpart, the isotropic dimension of this sample can be fit with a single peak, a slice along δ1 = 40.6 ppm giving quadrupolar parameters of δiso,cs = 13.7 ppm, CQ = 4.5 MHz, η = 0. A very small distribution of quadrupolar coupling parameters could be observed. The similarity of the CQ values of the major environments in the 25% and 33% Al samples, (4.5−4.6 MHz; Table 2), suggests only a small sensitivity of the CQ value to the counter-anion, providing further support that differences in CQ largely arise from different Al arrangements surrounding Mg. Figure 8 shows a stack plot of the isotropic dimensions from all the 25Mg MQMAS spectra, along with a deconvolution of

Figure 8. Deconvolution of the isotropic dimension F1 of the 25Mg MQMAS spectra. The solid black line is the isotropic spectrum, the dotted black line is the individual peaks, and the red line is the sum of each component.

the line shapes with the least number of components. A reasonable fit to all the spectra could be obtained with at least three resonances at δ1 ∼ 24−28, ∼ 42, and ∼46 ppm, with relative intensities tabulated in Table 2, providing justification for the choice of the position of the anisotropic slices in the 2D MQMAS data. It is important to stress that the relative intensities of the higher frequency resonances are extremely sensitive to the line widths used in the simulations (errors of at least 5−7% for the relative site population being estimated for the samples containing more than one environment). Furthermore, the intensity of multiple-quantum coherences is 2456



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The 27Al MQMAS spectra (Figure S5) of all three nitrate LDHs only contain a single sharp resonance, consistent with a single Al site and Al−Al avoidance. The MQMAS spectrum of the carbonate, however, contains both a sharp resonance due to a symmetric Al site, and a site with a similar chemical shift but with a distribution of quadrupolar parameters. 27Al NMR spectroscopy is sufficiently sensitive to resolve different sites in Al-hydroxides, particularly in bayerite, which contain sites with similar quadrupole coupling constants.57 Furthermore, 27Al studies of pristine and chemically modified Mg−Al LDHs by Vyalikh et al. show that Al NMR is sensitive to coordination changes of the Al polyhedra.58 In this paper, the authors observe two different octahedral aluminum environments with similar isotropic chemical shifts but different quadrupolar coupling constants (ΔCQ ∼ 2 MHz), whose origin is unclear. Given that, (i) our carbonate-containing LDH is the only sample displaying such a pronounced feature, (ii) the diffraction pattern of the sample does not show evidence of a phase impurity, and (iii) the corresponding 25Mg MQMAS spectrum displays a single Mg site consistent with Al−Al avoidance in the sample, the presence of two features in our sample is ascribed to slight variations in quadrupole parameters due to variations in the nature of the hydrogen bonding of the interlayer carbonate to the aluminum hydroxide units in the layers. The carbonates are likely to be more strongly hydrogen bonded than the nitrates as a result of both the anion charge and stacking arrangements, and thus, a greater degree of sensitivity of the 27Al resonance to anion ordering may be expected. It is also possible that cation vacancies may be present in the Mg sublattice of a honeycomb ordered array, particularly at high Al-content, a proposal that is consistent with Al avoidance. This proposal is also consistent with the observation of Al contents above 33% in some LDH samples. Al atoms in layers containing multiple vacancies are likely to show distributions of quadrupolar coupling constants and may contribute to the distribution seen in these spectra. In principle, 1 H NMR at very fast MAS conditions (see below) should be sensitive enough to detect these cation vacancies indirectly since they result in protons bound to an oxygen site that is coordinated to only two metal centers. These M−O−M′ oxygen sites should be noticeably more basic, and thus, the coordinated proton will resonate at a smaller chemical shift. 1 H MAS NMR Spectroscopy. At least three resonances are resolved for MgAl-x-NO3− (x = 19 and 25) (Figure 10) corresponding to Mg3−OH and Mg2Al−OH hydroxyl environments and structural water, as assigned previously.32 A resonance is observed at approximately 7.0 ppm in all the samples (even brucite), which is assigned to probe background, and is not observed for spectra acquired in some other probes. The resonances shift to slightly higher frequency with increased Al content and on replacing nitrate by carbonate, which is consistent with both the increased acidity of the hydroxyl protons, the nature of the stacking of the anions, and the propensity to hydrogen-bond to interlayer water or anions. Overall, the relative intensities of the spectra follow the trend outlined in our previous publication,32 which supports our initial assignments and the assertion of Al−O−Al avoidance in the layers of the LDHs. The MgAl-33-NO3− sample clearly shows no evidence of a substantial amount of Mg3OH hydroxyl groups, demonstrating a well cation-ordered metal hydroxide sheets with a negligible concentration of Mg defects in the honeycomb ordered layers. The line shape can be fit by using two resonances ascribed to Mg2AlOH and structural water. At a

Figure 9. 27Al MAS NMR spectra of the LDHs investigated in this study showing (A) the complete signal manifold, and (B) the central transition. The parameters used in the simulations shown in the figure are given in the text.

the line shapes. This is consistent with an Al local environment surrounded solely by Mg atoms in the first cation coordination shell. Slight variations in quadrupole parameters are most likely due to the degree of distortion of the surrounding Mg atoms or changes in the nature and strength of hydrogen bonding between the Al(OH)6 octahedra and interlayer species. The 33% Al nitrate-containing sample has the sharpest discontinuities, as expected from the complete cation ordering in the metal hydroxide layers. More disorder is seen for the lower Al content samples, along with a noticeable reduction in the intensities of the satellite transitions, particularly for the 19% sample. The decrease in intensity is partially ascribed to mobility of the water within the layers. Figure S4, in the Supporting Information contains the variable temperature27Al spectra of MgAl-19-NO3− and partially dehydrated MgAl-19NO3−, showing an increase in the intensity of the signal manifold as the temperature increases and when the water content is decreased. The carbonate-containing counterpart is unique: closer inspection of the central transitions of the LDHs (Figure 9B) shows that the carbonate-containing 33% Al sample has a broad shoulder centered at approximately 3 ppm. 2457



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in this more densely coupled system (which is a consequence of the reduced spacing between the hydroxide layers). Table 3. 1H NMR Spectral Assignment and Ratio of Hydroxyl Groups of Brucite and LDHs Having Different Al Contenta

sample

site

brucite

Mg3OH impurity Mg3OH Mg2AlOH water Mg3OH Mg2AlOH water Mg2AlOH water non-H-bonded Mg2AlOH or MgAlOH (vacancy) defect Mg2AlOH−CO32− water, urea, or bicarbonate and probe background)

MgAl-19-NO3−

MgAl-25-NO3−

MgAl-33-NO3− MgAl-33-CO32−

δiso,cs (ppm)

relative hydroxyl intensity (%)b

0.3 1.5 1.2 2.9 4.8 1.3 3.3 4.9 4.0 4.7 2.9

98 N/A 28 72 N/A 18 82 N/A 100 N/A negligible

5.0 7.0

100 N/A

a Obtained from 1H ultrafast MAS spectra and spectral deconvolution. The error of the isotropic chemical shift is ±0.1 ppm. bEstimated error in the integration is ±3%.

Figure 10. Single pulse 1H NMR spectra at 11.7 T with a MAS frequency of 55 kHz. Peak positions and relative areas from the deconvolution are provided in Table 3. Asterisks (*) denote the probe background (see Figure S7, Supporting Information).

It is possible that the brucite-like environments in LDH, observed by 25Mg MQMAS experiments could arise from separate impurity components that are too small or too poorly crystallized to be detected in the PXRD measurements. In order to rule this hypothesis out and demonstrate that all of the 1 H environments are within the same LDH layers, 1H−1H double quantum (DQ) recoupling MAS NMR experiments were performed. This two-dimensional experiment correlates the 1H sites that are close in space to each other by recoupling the dipolar coupling, which is otherwise averaged to zero under MAS conditions. 1H sites close in space appear at the sum of their individual frequencies in the indirect F1 dimension.59 The 1H DQ spectra of MgAl-19-NO3− and MgAl-25-NO3− (Figure 11 A, B) are similar and contain two major peaks along the diagonal, one at approximately 1.1−1.4 ppm corresponding to the Mg3−OH environment and a second at 2.9−3.4 ppm corresponding to the Mg2Al−OH environment. The diagonal peak at 2.9−3.4 ppm is very intense, which implies a significant clustering of Mg2Al−OH hydroxyl groups. This clustering arises naturally from the presence of Al−Al avoidance as illustrated in Figure 1C, which shows an Al site surrounded by 6 Mg2Al−OH groups, 3 above and 3 below the metal oxide sheet. A strong diagonal peak at 1.1−1.4 ppm is also present from a small cluster of Mg3−OH hydroxyl groups. (The presence of a Mg(OMg)6 environment is similarly associated with at least 6 nearby Mg3−OH hydroxyl groups.) Strong offdiagonal peaks are present linking the two different hydroxyl clusters, providing compelling evidence that the two environments observed in the proton spectra are part of same metal hydroxide sheet of the LDH. No peaks corresponding to structural water in the LDHs are observed in the 25% and 33% Al LDHs, presumably as a result of molecular motion reducing or removing the dipolar coupling between these protons. A

spinning speed of 55 kHz and a magnetic field strength of 11.7 T, there is no clear indication of the resonance at 5.3 ppm that was assigned to MgAl2OH by Cadars et al.38 It is worth noting that MgAl-33-NO3− does contain trace amounts of aluminum hydroxide or oxyhydroxide impurities, which have hydroxyl groups with chemical shifts at approximately 5.3 ppm (see Figure S6, Supporting Information); however, these were not clearly resolved. The 1H MAS NMR spectrum (Figure 10) of the ordered carbonate-containing LDH has a very broad resonance at approximately 5.0 ppm assigned to the Mg2Al−OH hydroxyl groups, which are hydrogen bonded to interlayer carbonates and possibly water. There is a shoulder at approximately 7.0 ppm, which can be assigned to either the background signal, the hydrogen in bicarbonate, water, or strongly hydrogen-bonded interlayer water/carbonate (Figure S7, Supporting Information). A very weak resonance at lower frequencies (at approximately 2.9 ppm) needed to be included in the fits of the spectrum, which we assign to either Mg2AlOH groups that are not hydrogen-bonded to interlayer carbonate or possibly more basic protons coordinated to sites nearby Mg vacancies (i.e., to an Mg−O−Al linkage). The assumption of stronger hydrogen-bonding in this sample is consistent with the smaller metal hydroxide layer spacing over those for the nitrates and the higher charge on the carbonate ion. The pronounced peak breadth in this sample may be indicative of chemical exchange between the Mg2Al−OH hydroxyl group and interlayer species, but more likely, it is simply due to the overlap with the water peak and the difficultly in removing the homonuclear coupling 2458



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Figure 11. 1H DQ MAS NMR spectra of MgAl-19-NO3− (A), MgAl-25-NO3− (B), MgAl-33-NO3− (C), and MgAl-33-CO32− (D) obtained at 14.1 T with a MAS frequency of 35 kHz. The dashed line shows the diagonal, corresponding to the autocorrelated, peaks. Projections of the single (F2) and double quantum (F1) dimensions are shown above and next to the spectra, respectively. The asterisk (*) in (D) shows an artifact arising from the center spike of the transmitter frequency.

weak peak at δ1 = 4.9 ppm is observed in the 19% Al LDH sample, which is assigned to structural water. It is unlikely that this resonance is due to an MgAl2−OH hydroxyl group, as suggested by Cadars et al.38 for the higher Al content materials, since the Al-content is low. The main difference between the 19 and 25% Al materials is the decrease in intensity of the brucitelike component at the expense of the Mg2Al−OH region in MgAl-25-NO3−, consistent with the 1H and 25Mg NMR data. In contrast, the 1H DQ MAS NMR spectrum of MgAl-33NO3− (Figure 11C) displays one intense resonance along the diagonal, consistent with its assignment to the Mg2Al−OH environment, along with a second, extremely weak resonance at approximately 1.5 ppm due to Mg3OH. The intensity of the Mg2Al−OH resonance along the diagonal is consistent with the highly ordered Mg and Al cations in the 33% Al-doped sample. The Mg2Al−OH resonance is somewhat broadened in the double quantum dimension due to a chemical shift distribution. The distribution presumably arises from differences in the hydrogen-bonding to the interlayer water or anions. Very weak off-diagonal peaks linking the predominant resonance with the much weaker Mg3OH (1.5 ppm) resonance are present, indicating that the latter environment does not arise from an impurity. The 1H DQ MAS NMR spectrum of the carbonatecontaining LDH contains one predominant broad resonance at ∼4.7 ppm, as in the nitrate counterpart. A second peak at approximately 5.9 ppm can be clearly resolved in the 2D spectrum. At the given spinning speed and magnetic field strength, off-diagonal peaks linking the two resonances cannot

be resolved, reinforcing our tentative assignment that the resonance arises from strongly hydrogen-bonded structural water, or possibly bicarbonate ions or a trace amount of urea. Our results should be contrasted to those of Cadars et al.,38 who used a higher magnetic field and a faster MAS frequency to perform 1H DQ MAS NMR experiments (using a different scheme for the recoupling of the 1H dipolar coupling). They resolved a peak at approximately 5.3 ppm in the 33% Alsubstituted material, which was correlated with the Mg2AlOH resonance. This resonance was assigned to a MgAl2−OH hydroxyl group. The percent contribution of this environment to the total line shape was estimated to be approximately 10%. We observed no clear evidence of this environment in our samples, although we do note that we see a shoulder at a similar shift position (5.0 ppm) in the one pulse spectra of the carbonate-containing LDH, which we assigned as either structural water, a Mg2Al−OH hydroxyl group interacting with a carbonate anion, or trace amounts of urea from the synthesis. However, unlike the study of Cadars et al.,38 no clearcut correlation is seen in the 2D correlation spectrum. It is possible that we have not observed this resonance due to our lower magnetic field strengths (although we note we did not see clear evidence of this site in our previous study performed with faster MAS rates and at higher field strengths).32 Most likely, the discrepancy originates from the differences in sample preparation. Cadars et al.38 used samples following synthesis via the coprecipitation method, while our nitrate-containing LDHs were hydrothermally treated in an autoclave following 2459



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nuclear and multidimensional NMR experiments can be used to rationalize local structures in layered double hydroxides with different cation concentrations. This work can be extended to study, for example, different anion-exchanged compounds to elucidate structural changes and anion layer interactions that may affect anion uptake and retention.

coprecipitation, which may help to anneal out the Al−Al defects.

■

CONCLUSIONS A combination of synchrotron X-ray diffraction and NMR spectroscopy have been used to characterize nitrate and carbonate-containing LDHs. Multiple Mg sites have been resolved using state of the art 25Mg MQMAS NMR spectroscopy at high fields for a series of layered double hydroxides materials (LDHs) with an Al cation ratio less than 33%. Three sites could be observed and assigned as follows: one site has a quadrupole coupling constant comparable to the site in brucite (∼3 MHz) and is assigned to a Mg(OMg)6 brucite-like environment, a second has a quadrupole coupling constant comparable to that in the 33% Al-doped sample (∼4.5 MHz) and is assigned to Mg symmetrically surrounded by 3 Mg and 3 Al ions, Mg(OAl) 3(OMg) 3, and the third environment is characterized by a larger quadrupole coupling constant (∼5 MHz) and, more importantly, nonaxial symmetry. As the Al content increases, the component corresponding to the brucite-like environment decreases as expected, while the environment corresponding to the nonaxially symmetric site is maximum in the sample with intermediate Al contents. The larger quadrupole coupling constant and nonzero asymmetry parameter signifies a larger distortion of the metal hydroxide octahedra, beyond the already-existing due to compression in the layer. This distortion most likely arises from the more distorted environment Mg(OAl)2(OMg)4, a cation arrangement that removes the (local) axial symmetry of the Mg(OAl)3(OMg)3 environment. Although detailed electronic calculations of various super cells are required to extract the electric field gradient and assign the various Mg sites resolved using MQMAS definitively, a model based on Mg substitution onto a honeycomb ordering can be used to rationalize the MQMAS spectra at the higher Al contents. 1 H MAS data obtained at high spinning rates is a successful method with which to resolve the different hydroxyl resonances. The shifts of the individual Mg3−xAlxOH (x = 0,1) move to higher frequencies as the Al content increases, which we ascribed to increased hydrogen bonding with the interlayer anions. Additional broadening of the peaks or slight shifts could also be attributed to changes in the hydrogen bonding networks of the materials, which depend on the concentration, type, orientation, and charges of the chargebalancing anions and their proximity to the metal hydroxide layers as well as the hydration state, that is, interlayer water content of the material. 1H DQ MAS NMR has been used to examine the hydroxide ordering. Peaks along the diagonal demonstrate the presence of small Mg3−OH and Mg2Al−OH clusters in MgAl-19-NO3− and MgAl-25-NO3−. Furthermore, off-diagonal cross peaks are also observed indicating that the different hydroxyl environments are close in space and thus located in the same hydroxide layer, rather than originating from separate phases. The single, predominant diagonal peak in the 1H DQ MAS NMR spectrum of MgAl-33-NO3− confirms cation ordering. Consistent with both the 25Mg and 1H spectra, the 27Al NMR spectra of the nitrate LDHs are consistent with a single Al environment and Al avoidance. In the carbonate sample, however, the stronger hydrogen bonding between the anions and the hydroxyl groups results in a greater distribution of 27Al quadrupolar parameters along with a shift of the Mg2AlOH resonance from 4.0 ppm in the nitrate LDH to 5.0 ppm in the carbonate. This work demonstrates how multi-

■

ASSOCIATED CONTENT

S Supporting Information *

Additional discussion of the 25Mg MQMAS data, methods to calculate the probabilities of different Mg configurations, X-ray diffraction pattern of a representative MgAl-33-NO3− sample showing weak reflections of impurity phases (Figure S1), 25Mg 3QMAS spectra of 25Mg-enriched MgAl-33-NO3− (Figure S2), simulations of the 25Mg MAS NMR spectra of MgAl-19-NO3− and MgAl-25-NO3− (Figure S3), variable temperature 27Al NMR of MgAl-19-NO3− and partially dehydrated MgAl-19NO3− (Figure S4), 27Al 3Q MAS spectra of MgAl-x-NO3− (x = 19, 25, and 33%) and MgAl-33-CO32− (Figure S5), α-Gibbsite (a polymorph of Al(OH)3) (Figure S6), and 1H MAS NMR spectrum of the background signal of the 1.3 mm HX MAS probe (Figure S7). Table S1 gives the experimental details used to acquire the 25Mg 3QMAS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank P. Vishnu Kamath for insightful discussions, particularly involving cation vacancies, and the Kamath Laboratory for kindly providing the carbonate LDH sample. P.J.S. thanks Peter W. Stephens and Kevin Stone for assistance with data collection at the NSLS and Martine Ziliox for assistance with the NMR measurements. Support was provided by the Center for Environmental Molecular Sciences, funded by NSF Grant No. CHE-0021934 and the CRC (Grant No. CHE-0714183). The NMR facility at the National High Magnetic Field Laboratory is supported by NSF and the state of Florida (DMR-0084173). P.J.S. acknowledges the Graduate Assistance in Areas of National Need (GAANN) Fellowship for financial assistance. F.B. thanks the European Commission framework FP7 for a Marie Curie International Incoming fellowship (Grant number 275212) and Clare Hall, University of Cambridge, UK for a Research Fellowship.

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