Identification and Quantification of Defects in the Cation Ordering in

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Identification and Quantification of Defects in the Cation Ordering in Mg/Al Layered Double Hydroxides Sylvian Cadars,*,† Geraldine Layrac,‡ Corine Gerardin,‡ Micha€el Deschamps,† Jonathan R. Yates,§ Didier Tichit,‡ and Dominique Massiot† †

CEMHTI CNRS UPR3079, Universite d’Orleans, 1D Avenue de la Recherche-Scientifique, 45071 Orleans Cedex 2, France UMR 5253 CNRS/ENSCM/UM2/UM1, Equipe Materiaux Avances pour la Catalyse et la Sante, Institut Charles Gerhardt Montpellier, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France § Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom ‡

bS Supporting Information ABSTRACT: Cation ordering is believed to have crucial effects on many of the physicochemical properties that make layered double hydroxides (LDHs) materials of considerable interest as host structures for drug delivery systems, nanocomposite materials, or for catalysis. Here we first unambiguously confirm that solid-state 1H NMR at fast (6065 kHz) magic-angle spinning (MAS) can be used to distinguish and quantify the different local MgnAl3nOH (n = 1, 2, and 3) environments of hydroxyl groups in LDH layers. By combining different solidstate 1H and 27Al one- and two-dimensional NMR measurements with first-principles calculations, we demonstrate that, although globally ordered, the cation distribution in Al-rich Mg/Al-2 LDHs contains detectable amounts of Al clustering. Though small, the fraction of Al atoms misplaced with respect to the perfectly ordered cation arrangement (where AlAl pairs are avoided) could be quantified. Their number is shown to counterbalance the number of misplaced Mg atoms for a Mg/Al ratio of 2 and to strongly decrease for reduced Al contents. This establishes that, although not favored, AlOAl linkages are not excluded in Al-richer LDH materials, a finding that will strongly impact our understanding of the local acidity of these materials and their widely exploited anion exchange and reconstruction properties. KEYWORDS: layered double hydroxides, NMR, cation distribution, density functional theory

1. INTRODUCTION Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds, by reference to the naturally occurring hydrotalcite mineral, are a family of anionic clays with a large variety of compositions and physicochemical properties.1 LDHs are described by the general formula [M2þ(1x)M3þx(OH)2]xþ (An)x/n 3 mH2O, where M2þ and M3þ are divalent and trivalent metal cations, respectively (e.g., M2þ = Zn2þ, Mg2þ, Co2þ, Ni2þ, and/or Cu2þ; M3þ = Cr3þ, Ga3þ, or Al3þ), and An are organic or inorganic compensation anions (e.g., Cl, NO3, OH, SO42, CO32). The molar ratio x of the trivalent cation [x = M3þ/ (M2þ þ M3þ)] determines the overall positive charge of the layers and hence the number of compensation anions located in the interlayer space. LDHs have found various applications as antiacids, flame retardants, and stabilizing agents of PVC, for example. The range of applications was later extended thanks to the development of nanomaterials in which LDHs act as host structures for different anionic guest entities. Such materials have been used for the controlled delivery of drugs2 and as pigments,3,4 optical devices,5,6 nano- and macro-fillers in polymer composites,7 and templates for the confinement of coordination polymers.8 Particular attention has been paid to the exploitation of LDHs for r 2011 American Chemical Society

heterogeneous catalysis, where they are used as precursors of acido-basic MII(MIII)O mixed oxides (obtained by thermal decomposition) or supported metal catalysts or to stabilize catalytically active intercalated complexes.912 At the molecular level, LDHs consist of single layers formed by a hexagonal “honeycomb” arrangement of metal cations with hydroxyl groups pointing alternatively up and down perpendicular to the plane of the sheet, as illustrated in the hypothetical LDH structure sketched in Figure 1,a,b. The layer structure of LDHs is therefore similar to that of Mg(OH)2 brucite, with the additional presence of counteranions in the interlayer space to compensate the charge introduced by the M3þ cations. Until recently, there were mostly contradictory speculations concerning order and/or disorder in the cation distribution in LDH layers.13 Long-range ordering was rarely observed, while strong evidence for local ordering was obtained by XRD and EXAFS spectroscopy in some cases such as Zn/Cr, Cu/Cr, Mg/Ga, and Mg/Fe LDHs.1417 The distribution of cations in the layers Received: January 5, 2011 Revised: March 7, 2011 Published: May 17, 2011 2821

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Figure 1. (a) Side and (b) top views of a hypothetical layered double hydroxide (LDH) structure with Mg/Al = 2 and ordered Mg/Al repartition. Blue spheres represent intercalation anions (NO3 in the samples studied here). Each hydroxyl group is typically located above a triangle formed by three cationic sites, typically two Mg and one Al for an ordered Mg/Al distribution, in which case it is referred to as an Mg2AlOH moiety, as indicated with a black triangle in panel b. Each Al atom is located in a hexagon formed by six Mg atoms to form an Al(Mg)6 moiety as shown by the dotted hexagon in panel b. (c) X-ray diffraction pattern of the LDH sample Mg/Al-2. The (00l) reflections at ca. 2θ = 10° and 20° indicate an interlayer spacing of 8.81 Å.

presumably affects the arrangement of compensating anions in the interlayer space and consequently the design of highly ordered hybrids and the homogeneity of distribution of catalytically active sites in the mixed oxides. Random distributions would preferentially induce segregation of single MIIO and M2IIIO3 oxides during calcination, and thus have a detrimental influence on the so-called memory effect, that is, the intrinsic ability of MII(MIII)O mixed oxides [particularly Mg(Al)O] to recover the layered hydroxide structure when immersed in an anion-containing aqueous solution. The memory effect is extensively used, for example, to obtain by reconstruction in pure water Mg/Al LDHs intercalated with OH. These correspond to the mineral meixnerite, which exhibits the unique properties of Brønsted-type catalysts.18 When single oxides are segregated, reconstruction leads, in addition to meixnerite, to the formation of Mg(OH)2 and Al(OH)3 phases and may induce noncontrolled and irreproducible catalytic behavior. In summary, the various roles played by the arrangement of M2þ and M3þ cations within LDH frameworks point to the paramount importance of being able to characterize their distribution. Mg/Al-y LDHs, where y stands for the Mg/Al ratio of the structure [y = (1 x)/x, where x is the molar ratio of M3þ as defined in the general formula above], are known to exist for a range of y values, with a minimum of 2 pointing to the maximum level of substitution of Mg cations by Al cations for which AlOAl linkages can be avoided in the hydroxide layers. That is possible only if the layer structure consists of an ordered arrangement of the cations, in which Mg ions form a hexagonal two-dimensional (2D) pattern. In this case, a single type of OH and Al moieties are present: Mg2AlOH and Al(Mg)6 environments, respectively, as illustrated by the black triangle and dashed

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hexagon in the top view of the hypothetical structure in Figure 1b. Any defect in these hexagonally ordered Mg/Al-2 layers must lead to the appearance of additional OH and Al environments and especially of AlOAl pairs. In materials with Mg/Al ratios higher than 2, defects in the cation arrangement are not necessarily associated with AlOAl pairs. For a long time, there has been no direct way to access the M2þ and M3þ cation distribution, and this has remained an important open question. Powder X-ray diffraction techniques are not able to investigate local ordering of the cations within the layers because the expected superreflections are hidden due to the presence of stacking and/or turbostratic disorder between the layers and also because of the similarities in the scattering length densities of most M2þ and M3þ cations.13 Solid-state nuclear magnetic resonance (NMR) experiments have been used to study the local environments of octahedral Al3þ moieties1923 in Al-containing LDH materials or the intercalated species by use of 13C NMR.21,24 They provided insights into the extent of local ordering inside or between the hydroxide layers. However, the nature of the disorder, and whether it could be attributed to variations of the atomic composition (chemical disorder) or of the local geometry (topological disorder), has remained largely unknown. In particular, no direct information on the cation distributions has been obtained until very recently, when Sideris et al.25 demonstrated that 1H NMR can provide key elements to solve this issue in Mg/Al LDHs. The breakthrough was the collection of 1H NMR spectra of Mg/Al LDH materials under the highest magic-angle spinning (MAS) frequencies available nowadays, that is, 6070 kHz, which increases 1H spectral resolution through significant reduction of strong dipolar interactions between abundant protons. In the case of Mg/Al LDHs, this led to the separation of 1H NMR signatures of different framework hydroxyl groups. Besides the dominant OH environments surrounded by two Mg2þ and one Al3þ cations (the socalled Mg2AlOH moieties), the presence in the Mg/Al-2 sample of Mg3OH moieties (OH groups linked to three Mg cations) was also established. The quantification revealed that Mg3OH moieties represent less than 3% of the hydroxyl moieties in Mg/Al-2 LDHs. This led to the conclusion that Mg2þ and Al3þ cations are not randomly distributed but adopt instead an ordered honeycomb arrangement, in which pairs of adjacent Al atoms are excluded.25 The absence of AlOAl linkages in the hydroxide layers was deduced from the absence of a 1H NMR peak that could be assigned to hydroxyl groups surrounded by two Al3þ and one Mg2þ cations (MgAl2OH moieties). These interesting observations remain puzzling in the sense that, in a pure Mg/Al-2 LDH containing a major contribution of Mg2AlOH sites, the presence of small amounts of Mg3OH environments must be accompanied by the presence of Al-rich counterparts (i.e., MgAl2OH sites). The existence of MgAl2OH sites, which were not identified by Sideris et al.,25 thus remains an open question. The first objective of our work was thus to further and unambiguously confirm the assignment of the 1H NMR signatures of Mg2AlOH and Mg3OH moieties proposed by Sideris et al.25 by combining chemical analyses and 1H and 27Al solidstate NMR measurements with first-principles calculations of NMR parameters. While we support their main conclusion that the cation distributions in Mg/Al LDHs are indeed globally ordered, we then demonstrate that in addition to the Mg-rich Mg3OH defects in the OH environments, Al-rich counterpart environments are present, as is expected for a Mg/Al ratio of 2. These Al2MgOH species are then quantified and shown to match 2822

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Chemistry of Materials the amount of Mg3OH moieties, but their contribution to 1H NMR spectra rapidly disappears for increasing Mg/Al ratios. This indicates that although M2þ and M3þ cations indeed arrange in a globally ordered pattern, there are nevertheless quantifiable amounts of defects in their distribution.

2. EXPERIMENTAL SECTION 2.1. Sample Preparations and Compositions. Mg/Al LDHs with Mg/Al = 2, 2.5, 3, and 3.5 (x = 0.33, 0.28, 0.25, and 0.22, respectively) were synthesized by a conventional coprecipitation method at constant pH under ambient atmosphere. In each case, 200 mL of aqueous solution of Mg(NO3)2 3 6H2O (0.42, 0.54, 0.63, and 0.74 M, respectively) (Panreac, 98%) and Al(NO3)3 3 9H2O (0.21 M) (Fluka, g98%) was added dropwise at a rate of 2 mL 3 min1 at room temperature into a beaker. Simultaneously, an appropriate volume of NaOH (2 M) (Panreac, 98%) was added at a controlled rate to maintain the pH close to 10 by use of a pHSTAT Titrino (Metrohm, France) apparatus. After complete precipitation, the gel obtained was refluxed at 353 K for 24 h. It was then repeatedly washed with distilled water at 298 K (∼6 L) and finally dried overnight at 353 K. Thereafter, these solids are referred to as Mg/Al-y with y = 2, 2.5, 3, and 3.5. 2.2. Characterization Methods. Elemental analyses were performed by the Service Central d’Analyze (CNRS, Vernaison, France). X-ray powder diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer using Cu KR1 radiation (λR = 1.541 84 Å, 40 kV, and 50 mA). Data were collected between 2° and 70° (2θ), with a step size of 0.02° and a counting time of 0.2 s/step. NMR 1H, 1H{27Al}, and 1H1H experiments were conducted on a Bruker Avance III 750 wide-bore spectrometer operating at 1H and 27Al frequencies of 750.13 and 193.46 MHz, respectively, with a 1.3 mm double-resonance probe at a MAS frequency of 64 kHz. Quantitative 1H MAS experiments were collected by use of a spinecho with a short echo duration (8 rotor periods, or 125 μs) to remove probe background signal with negligible effects on the relative intensities of the sample 1H signals. A recycling delay of 2.5 s was used. The same pulse sequence was used to collect the T2-filtered spectra, with echo durations of 200 rotor periods or 3.125 ms. For 1H{27Al} cross-polarization (CP) MAS experiments, polarization transfer from 27Al to 1H was achieved by use of amplitude ramps (50100% of the maximum amplitude) on the proton channel (see pulse sequence in Supporting Information, Figure S1a). For experiments collected with CP contact times of 200 μs and 10 ms, signal was accumulated over 256 and 128 transients, respectively, with a recycling delay of 1 s. Total elimination of residual 1H magnetization from direct 1H excitation was achieved by a saturation of the 1H signal before the initial 27 Al excitation pulse, using a series of 10 90° pulses spaced by a delay decreasing from 20 to 2 ms by steps of 2 ms. Complete elimination was carefully verified for each CP contact time by running a control experiment with the radiofrequency (RF) power of the initial 27Al excitation pulse set to zero. Two-dimensional 1H{1H} dipolar recoupling experiments were collected via a novel approach referred to as dipolar homonuclear homogeneous Hamiltonian (DH3) double-quantumsingle-quantum (DQ-SQ) experiment to establish through-space double-quantum correlations between nearby protons. The pulse sequence used is shown in Supporting Information, Figure S1b, and the general principle of this experiment is described in detail elsewhere.26 The ability of the DH3 method to reliably establish spatial proximities between 1H or 19F nuclei at spinning frequencies higher than ca. 50 kHz was demonstrated on organic (L-alanine) and inorganic (β-BaAlF5) crystalline systems of known structures. DQ crosspeak intensities for each pair of sites i and j were found to correlate well with the sum of rij3 over all rij internuclear distances shorter than 10 Å (although some effects of geometry are observed for the shortest distances). In the particular case of the LDH samples, the DH3 experiment was found to

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compare favorably (in terms of signal-to-noise and/or spectrum quality with respect to t1 noise in particular) to the few recoupling techniques applicable at such spinning frequencies, such as the so-called “back-to-back”27,28 or SPIP29 experiments. The name of this DQ recoupling method, DH3, was introduced to avoid confusion with the refocused incredible natural abundance double quantum transfer experiment (INADEQUATE),30 which employs an identical sequence of RF pulses but is based on a through-bond transfer mechanism and probes the existence of chemical bonds. The DH3 experiment, in contrast, is used in radically different situations (i.e., strongly coupled multispin systems) to probe spatial proximities based on a through-space transfer mechanism (due to threespin terms in the homonuclear dipolar Hamiltonian; see ref 26). Here, the best spectrum quality was obtained for a half-spinecho delay τ = 1.0 ms. A total of 128 points were acquired in the indirect dimension, within 192 transients each. The recycling delay was 1 s, leading to a total experimental time of 7 h. NMR 27Al experiments were collected on an Avance 300 wide-bore spectrometer operating at 1H and 27Al frequencies of 300.17 and 78.21 MHz respectively, with a 4 mm double resonance probe at a MAS frequency of 14 kHz. Quantitative single-pulse 27Al MAS NMR experiments were conducted with an excitation pulse of 1.11 μs corresponding to a π/18 flip for Al(NO3)3 in solution and a recycling delay of 1 s. For Hahn echo experiments, central-transition-selective 90° and 180° pulses of 18 and 36 μs were employed, respectively, with a total echo duration of two rotor periods (143 μs) and a recycling delay of 1 s. For 27Al27Al double-quantum (DQ) filtered experiments, dipolar recoupling between nearby 27Al nuclei was achieved via a variant proposed by Wang et al.31 of the symmetry-based32,33 R221 recoupling sequence first applied by Eden et al.34 for the excitation of double quantum coherences between the central transitions of a homonuclear pair of half-integer quadrupolar nuclei (see pulse sequence in Supporting Information, Figure S1c,d). Best efficiency of the experiment is obtained for a radio frequency irradiation power providing a nutation frequency of the 27Al central transition of 7 kHz (i.e., νR/2), and a total duration of the DQ excitation and reconversion blocks of 8 rotor periods (i.e., 0.57 ms). The selective 180° pulse for the elimination of single-spin DQ coherences34,35 was set to 30 μs. The spectrum was collected with 8192 transients and a recycling delay of 1.2 s, leading to a total experimental time of 3 h. Heteronuclear 1 H decoupling was applied during acquisition with SPINAL64 decoupling scheme,36 at a 1H nutation frequency of 80 kHz with a pulse duration of 5.8 μs, and with continuous wave (CW) decoupling during recoupling blocks, at the same nutation frequency of 80 kHz. 2.3. Computations. First-principles calculations with periodic boundary conditions were achieved by use of the CASTEP code,37,38 which relies on a plane-wave-based density functional theory (DFT) approach. The electron correlation effects are modeled by the PBE generalized gradient approximation.39 For geometry optimizations we employed “ultrasoft” pseudopotentials40 and a plane-wave cutoff energy of 600 eV. Convergence thresholds were set to 105 eV/atom for the total energy, 3 102 eV/Å for the maximum ionic force, and 103 Å for the maximum ionic displacement. The default “on-the-fly” pseudopotentials of CASTEP 5.0 were used, and they are described in Supporting Information, Table S1. The NMR calculations were performed by the gauge including projector-augmented wave approach (GIPAW),41,42 at the same cutoff energy of 600 eV, which led to calculated 1H and 27Al NMR shieldings converged within less than 0.1 ppm. Supercell dimensions for the LDH model were a = b = 9.162 Å and (unless mentioned otherwise) c = 26.43 Å, and R = β = 90° and γ = 120°. They were kept fixed during geometry optimizations. To accommodate partial Mg/Al occupancies, the a and b parameters were taken as 3 times those of a published structure of the naturally occurring LDH hydrotalcite (intercalated with carbonate) with an Mg/Al ratio of 2.43 The c parameter was taken from the (00l) reflections measured by XRD on the sample prepared by coprecipitation with nitrate anions studied 2823



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here. A 3 3 1 MonkhorstPack44 (MP) grid was used to sample the Brillouin zone. Calculated 1H chemical shifts relative to tetramethylsilane (TMS) were obtained by use of brucite Mg(OH)2 as a secondary reference (experimental shift with respect to TMS δref = 0.0 ppm).25 Specifically, the powder neutron diffraction structure of brucite45 was optimized with fixed cell parameters (a = b = 3.149 79 Å, and c = 4.770 20 Å, R = β = 90° and γ = 120°) by use of CASTEP in similar conditions as above and a 9 9 6 MP grid. The NMR 1H shielding σref was then calculated under the same conditions, and all calculated 1H chemical shifts reported here are then given as δcalc = σref σcalc þ δref.

3. RESULTS AND DISCUSSION The compositions and structures of the LDH samples were examined by elemental and XRD analyses. The elemental analyses reported in Table 1 show that the molar ratios x = Al3þ/ (Mg2þ þ Al3þ) are very close to the nominal values. In all samples, the positive charge of the layers is equilibrated by NO3 anions provided by the metal salts and by small amounts of carbonates dissolved in the aqueous synthesis mixture. The XRD patterns present the expected profiles for well-crystallized LDH compounds generally indexed in a hexagonal lattice with R3m rhombohedral symmetry (see Figure 1c for sample Mg/Al-2 and Supporting Information Figure S2b for samples Mg/Al-y with y = 2.5, 3, and 3.5). Two intense and symmetric (00l) harmonics are observed below 2θ = 30°, while broad and asymmetric (0kl) peaks are observed in the 2θ range from 30° to 70°. The position of the 003 peak in Mg/Al-2 sample leads to a basal spacing of 8.81 Å. A linear relationship is observed between the lattice a parameter obtained from the position of the (110) peak at about 2θ = 60° (a = 2d110) and the molar fraction of trivalent cation x (Supporting Information, Figure S2b). This is in agreement with decreasing a values normally observed in LDHs when x decreases in the range from 0.35 to 0.2 due to the smaller ionic size of Al3þ as compared to Mg2þ.46 Importantly, this proves that the Al3þ contents within the layers are in agreement with the nominal ones and that no extra segregated phase exists in the material, whatever the Mg/Al ratio. Solid-state 1H NMR measurements conducted at very fast (i.e., > 60 kHz) rotation of the sample around the so-called magic angle (i.e., 54.74° with respect to the static magnetic field) make it possible in most H-containing solids to distinguish and assign a large range of chemical environments. This is a direct consequence of the increase in resolution resulting from the reduction of the strong dipoledipole interactions between naturally abundant 1H nuclei in such conditions. Figure 2a shows the 1H MAS spectrum of the Mg/Al-2 LDH material, where different peaks correspond to different proton environments, including framework hydroxyls and interlayer water molecules, while relative intensities are expected to account directly for the relative amounts of these species.25 Sideris et al.25 assigned the dominant peak at 3.8 ppm to Mg2AlOH species and the shoulder at ca. 4.6 ppm to intercalated and/or adsorbed water. The small peak at

1.6 ppm was attributed to Mg3OH environments in reference to the 1H chemical shift of 0 ppm measured in the same conditions for brucite Mg(OH)2 (the shift to lower ppm values being interpreted as a direct effect of the substitution of the next-nearest Al3þ by Mg2þ). With increasing Mg/Al ratios, the peak assigned to Mg3OH at 1.6 ppm was found to shift to lower frequencies (0.9 and 0.8 ppm for Mg/Al = 3 and 4, respectively)25 and to grow in relative intensity, consistent with the increasing fraction of Mg cations. At the same time, the peak associated with the dominant Mg2AlOH moieties at 3.8 ppm was also found to shift (2.9 and 2.4 ppm for Mg/Al = 3 and 4, respectively)25 and to decrease in relative intensity with increasing Mg/Al ratio. Although this assignment was fully consistent with the trends observed as a function of the Mg/Al ratio, additional evidence is needed to ensure that the proposed interpretation of ordered cation distributions in the hydroxide framework is fully supported. One way to confirm the assignment of different framework hydroxyl protons is to probe the existence of spatial proximities to Al atoms. Such proximities were established by Sideris et al.25

Figure 2. Solid-state 1H NMR spectra of LDH sample Mg/Al-2. (a) MAS Hahn-echo spectrum collected with spinecho duration 2τ = 8τR. (b, c) 1H{27Al} CP-MAS spectra showing only 1H nuclei in close proximity to Al atoms. Cross-polarization contact times of (b) 200 μs and (c) 10 ms are used to probe short (across AlOH moieties only) and longer (typically 0.51 nm) distances, respectively. (d) T2-filtered echo MAS spectrum, where the long spin echo 2τ = 3.125 ms selectively reduces relative contributions from the rapidly relaxing and/or dephasing 1H signal of water at 4.6 ppm, hence revealing the small signal contributions underneath. All spectra were collected at 17.6 T with fast spinning (64 kHz) of the sample at the magic angle.

Table 1. Elemental Analyses and Suggested Structural Formulas of Samples sample

Mg (%)

Al (%)

C (%)

N (%)

formula

Mg/Al-2

18.43

9.89

0.30

3.87

Mg/Al-2.5 Mg/Al-3

21.85 23.50

9.56 8.80

0.683 0.439

3.36 3.55

[Mg0.68Al0.32(OH)2][(NO3)0.25(CO3)0.022] 3 0.74 H2O [Mg0.72Al0.28(OH)2][(NO3)0.19(CO3)0.045] 3 0.32 H2O [Mg0.75Al0.25(OH)2][(NO3)0.194(CO3)0.028] 3 0.23 H2O

Mg/Al-3.5

24.37

7.93

0.640

2.50

[Mg0.78Al0.22(OH)2][(NO3)0.140(CO3)0.041] 3 0.39 H2O

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Chemistry of Materials for Mg2AlOH moieties using 1H{27Al} transfer of populations in double resonance (TRAPDOR)47 experiments. Due to experimental limitations, however, these were collected at a low MAS frequency (5 kHz) and magnetic field (8.4 T), where dramatically broadened 1H lines lead to severe spectral overlap and large uncertainties on peak positions (probably on the order of (0.5 ppm). Here, spatial proximities between 1H and 27Al nuclei are probed instead by use of simple 1H{27Al} cross-polarization (CP) MAS experiments under the same conditions of very fast (64 kHz) MAS and high magnetic field (17.6 T) as used for the echo experiment in Figure 2a. Hence, these experiments are expected to provide similar information as the TRAPDOR experiments,25 but with a considerably increased spectral resolution. Figure 2 panels b and c show two such 1H{27Al} CP-MAS spectra collected on the Mg/Al-2 LDH. As shown in Figure 2b, at short (200 μs) CP contact times, magnetization transfers between 27Al and 1H moieties occur only across short distances, such as between Al atoms and adjacent OH moieties in Mg2AlOH environments (typically ca. 2.5 Å), as confirmed by the presence of the intense peak at 3.8 ppm. The absence of a peak at 4.6 ppm is also consistent with its previous attribution to intercalated and/or adsorbed water molecules, which are located too far from the framework 27Al nuclei to permit magnetization transfer. The same applies to Mg3OH framework hydroxyl groups, which are associated with AlH distances of at least 4 Å. At longer (10 ms) contact times (Figure 2c), the 27Al magnetization is transferred across longer distances, and an additional peak at 1.6 ppm clearly appears as a result, which again is fully consistent with its assignment to Mg3OH moieties. This assignment of 1H MAS NMR peaks to Mg2AlOH and Mg3OH moieties can be further confirmed by DFT calculations of 1H NMR chemical shifts. Calculation can also be used to predict the expected shifts of other local structural units potentially present in Mg/Al LDHs, such as framework MgAl2OH hydroxyl groups expected in the presence of cation disorder. LDH materials are not straightforward to model due to the occupation of cationic sites by the two cations (67% Mg and 33% Al for Mg/Al = 2), the presence of intercalated charge-compensation anions, and the unknown location of water molecules. In this work calculations were performed with the following approximations: (i) Water molecules were ignored. (ii) Unless mentioned otherwise, the basal spacing was fixed at 8.81 Å (i.e., c = 26.43 Å), which is the value obtained from XRD data of sample Mg/Al-2. (iii) Values of the unit cell a and b parameters were taken from the reported hydrotalcite structure43 and also kept fixed. (iv) The unit cell consists of an alternation of three layers each shifted by (a/3, b/3, c/3), where a and b correspond to the vectors between two adjacent cationic sites, while the basal spacings were scaled such that c/3 = 8.81 Å. (v) Occupancies of cationic sites in the structure with ordered Mg/Al distribution were treated as follows: the Al atoms were placed directly above those of the layer underneath (albeit with the shift of a/3 and b/3 between cationic sites of adjacent layers; see point iv). (vi) The NO3 anions were initially placed with the N atom vertical to the Al atom, with an initial orientation parallel to the layer and O atoms pointing toward the AlOH groups underneath. (Other configurations with the NO3 molecules located systematically above the Al

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Figure 3. Optimized supercell for the disordered model of LDH intercalated with nitrate, with Mg/Al = 2. (a) Side view: only one layer out of three contains defects in the Mg/Al distribution. (b) Top view of the disordered layer and the NO3 ions of the interlayer space underneath. In this model, cationic defects correspond to a single permutation of a pair of adjacent Al and Mg sites per supercell, as indicated by the black dashed circles. The resulting Mg3OH and MgAl2OH defects are indicated by green and blue triangles, respectively.

atom of the layer underneath were explored and found to yield similar results, since the nitrate would tend to move away from the misplaced Al atoms to form an ordered arrangement of intercalated molecules.) The structure obtained after geometry optimization in the case of an ordered cation distribution and Mg/Al = 2 is shown in Supporting Information, Figure S3, with a brief description of the corresponding calculated 1H NMR chemical shifts. With the same basic hypotheses, defects were introduced in the structure by simply permuting a pair of adjacent Al and Mg atoms in one of the three layers. The resulting structure, after geometry optimization, is shown in Figure 3, with substituted Al and Mg sites indicated by black dashed circles in Figure 3b. One such permutation generates four Mg3OH and four MgAl2OH hydroxyl sites per supercell, as illustrated by the green and blue triangles, respectively. The 1H chemical shifts calculated for this LDH model are reported in Figure 4a as a function of the associated (shortest) H-bond length to nearby O atoms. A strong relationship clearly appears, which suggests that H-bond lengths are the primary contribution to the 1H chemical shifts. Similar trends have been reported previously for phosphinic acid derivatives, at the organicinorganic interfaces in silica-based hybrid materials, or in other organic systems, for example, based on DFT calculations4850 and/or experimental data.5154 Interestingly, however, we see that the nature of the nearby cations also seems to substantially contribute to the 1H chemical shifts. The values calculated for Mg3OH moieties (green triangles) appear substantially below the trend described by Mg2AlOH moieties (red squares), which again is consistent with the assignment of 2825



Figure 4. (a) Relationship between 1H chemical shifts, calculated via plane-wave-based DFT for the LDH model shown in Figure 3 and corresponding H-bond lengths to the next-nearest oxygen atoms. Mg2AlOH, Mg3OH, and MgAl2OH moieties are indicated as red squares, green triangles, and blue circles, respectively. (b) Correlation plot between experimental and calculated 1H shifts for previously assigned25 Mg2AlOH and Mg3OH moieties in Mg/Al LDHs and for the Al-rich MgAl2OH defects whose assignment is demonstrated here. Calculated shifts correspond to the center of mass of the points calculated for each type of moiety from the LDH model described in the text, while bars indicate the standard deviations of calculated 1H chemical shifts, which are most likely averaged by dynamics of the nitrate in the interlayer space at room temperature.

Mg3OH proposed by Sideris et al.25 MgAl2OH moieties (blue circles), on the other hand, are located slightly above the trend observed for Mg2AlOH moieties (red squares) and seem to be associated with generally stronger hydrogen bonds. Both hydrogen-bond strength and the nature of adjacent cations thus contribute to shifting the 1H chemical shifts calculated for MgAl2OH moieties to higher frequencies than Mg2AlOH moieties. An important observation is that the calculated 1H shifts are very much scattered (Figure 4a). For example, we found a standard deviation of 1.6 ppm for the 1H shifts calculated for Mg2AlOH sites, as compared to the experimental full width at half-maximum (fwhm) of 0.6 ppm (Figure 2b). A similar scatter was already observed in the case of the corresponding ordered model (see details in Supporting Information, Figure S4a). Most likely, this is partly a consequence of hypotheses i and ii above. At ambient temperature, a substantial amount of water is located in the interlayer space of LDHs. Increasing the temperature to between 150 and 200 °C (reversibly) reduces the basal spacing of LDHs by more than 10%, concomitant with the removal of water molecules.5557 Since our model does not include intercalated

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water molecules, one needs to consider the effect of reducing the c parameter down to the values observed for the LDH dehydrated at 200 °C. Systematic studies of these effects are described in Supporting Information (Figures S3 and S4). They indicate that our use of the basal spacing of the hydrated materials accounts for only a part of the large range of hydrogen-bond lengths and, as a result, of calculated 1H chemical shifts in our LDH model. Despite these limitations, important information can be obtained by focusing on the average 1H chemical shift values calculated for Mg3OH, Mg2AlOH, and MgAl2OH sites. This assumes in fact that fast (on the NMR time scale, i.e., 106 s or faster) rotational and/or translational dynamics of the nitrate anions in the interlayer space lead, at room temperature, to averaging of most of the 1H chemical shift distributions calculated (at 0 K) for our LDH model. This is not unreasonable, especially when water is present in the interlayer space, leaving more space for the reorientations of the nitrate anions. The average chemical shifts calculated for Mg3OH and Mg2AlOH moieties, 1.9 and 4.0 ppm, respectively, are in good agreement with the experimental values of 1.6 and 3.8 ppm and consistent with the assignment proposed by Sideris et al.25 The corresponding experimental and calculated shifts are reported in Figure 4b (green triangles and red squares, respectively). The predicted average value calculated for the Al-rich MgAl2OH moieties is between 5.9 and 6.0 ppm for LDH models with c higher than 8.4 Å and 4.8 ppm for the model with c = 7.9 Å (Supporting Information, Figure S4b). Although no 1H NMR signal was observed by Sideris et al.25 in this chemical shift range, which largely overlaps with the water signal, we demonstrate below that it can be revealed in Mg/Al-2 sample and unambiguously assigned to MgAl2OH defects. A close examination of the 1H{27Al} CP-MAS spectra in Figure 2b,c shows small but nevertheless undoubtedly present shoulders slightly above 5 ppm. Similarly, the MAS echo spectrum of Figure 2a shows an extended foot in this region, although it is harder to see in this case because of the overlapping peak due to water molecules (at ca. 4.6 ppm). It turns out that, due to the mobility of water molecules and to the strong intramolecular dipoledipole coupling between its protons, the water signal at 4.6 ppm relaxes and/or dephases very rapidly. As a result, its intensity can be selectively reduced relative to the signals of framework hydroxyl groups by running a T2-filtered experiment, where the spectrum is collected after a long enough spin echo, during which the signal of each site will dephase with its own characteristic time, depending (among other things) on its mobility and residual couplings to nearby nuclei. The spectrum collected with a spin echo of total duration 3.125 ms (Figure 2d) shows that the peak due to water has almost completely vanished in this spectrum, revealing a peak centered at 5.3 ppm. A strong indication that the small 1H peak at 5.3 ppm corresponds to MgAl2OH entities in Mg/Al-2 LDH is provided by double-quantum 1H1H recoupling experiments. These experiments are used to establish spatial proximities between protons through the observation of cross peaks that appear at the frequency of each site in the direct (horizontal) dimension and at the sum of their frequencies in the indirect (vertical) dimension. Figure 5 shows the result of such an experiment, designated as the dipolar homonuclear homogeneous Hamiltonian (DH3) DQ-SQ experiment in reference to the transfer mechanism on which it is based.26 In the particular case of strongly coupled protons (or 19F nuclei) at very high MAS 2826



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Figure 5. NMR 2D 1H1H DH3 spectrum of the LDH sample Mg/ Al-2, collected at 17.6 T at a MAS frequency of 64 kHz. Correlation peaks indicate spatial proximities between protons and establish, for example, that the peak at 5.3 ppm belongs to the LDH structure, consistent with its assignment to MgAl2OH moieties (see text).

frequencies, magnetization exchange occurs through space via residual dipoledipole couplings (rather than through chemical bonds via scalar couplings, as is the case with the refocused INADEQUATE,30 an experiment that uses an identical sequence of RF pulses). This experiment is based on spin echoes, during which similar transverse relaxation and/or dephasing mechanisms to those described for the T2-filtered experiment (Figure 2d) occur concomitantly with the magnetization transfer between nearby 1H nuclei through dipolar interactions involving multiple spins. Since these echoes are of comparable overall length to that used in the T2-filtered experiment (Figure 2d), the cross peak(s) involving water molecules do not appear in this spectrum. As a result, correlation peaks between the dominant Mg2AlOH moieties at 3.8 ppm and both the Mg3OH peak at 1.6 ppm and the peak at 5.3 ppm are clearly resolved. This establishes that the peak at 5.3 ppm is associated with species that belong to the LDH layers, which again is consistent with its assignment to MgAl2OH moieties, in agreement with the DFT calculation results. We note in addition the presence of an autocorrelation cross peak indicating proximities between Mg3OH moieties, as expected since the presence of a misplaced Mg generates several adjacent Mg3OH groups (see Figure 3b). The position of this peak (1.3 ppm), however, is slightly shifted with respect to the main Mg3OH position (1.6 ppm) for a reason that we do not clearly understand. Such MgAl2OH moieties could in principle be probed by 27 Al27Al correlation NMR spectroscopy. The 27Al MAS spectrum of the Mg/Al-2 LDH is shown in Figure 6a, with spinning side bands describing a rather well-defined first-order quadrupolar interaction. The central transition region, observed via a Hahn echo experiment in Figure 6b, shows the signature of octahedral 27Al environments. The sharp peak at ca. 10 ppm accounts for a large fraction of sites with a quite regular octahedral symmetry resulting in a small quadrupolar interaction. The best fit to the experimental central transition gives δiso = 11.7 ppm and |CQ| = 1.56 MHz, with ηQ fixed to 0. In addition, more distorted environments are also revealed by the broad shoulder centered at ca. 0 ppm corresponding to distributed quadrupolar interactions, yielding the typical asymmetric line shape expected in such cases.58 We note that both peaks are also clearly visible in the 27Al MAS experiment of Figure 6a upon magnification of the central transition region (not shown). This is consistent with previous 27Al NMR studies of Mg/Al LDHs.2123

Figure 6. (ac) 27Al NMR spectra of the LDH material Mg/Al-2, conducted at 7.0 T and a MAS frequency of 14 kHz. (a) Direct excitation 27 Al MAS spectrum and (on top) magnification of the satellite transition region on both sides of the central transition. (b) Hahn echo 27Al NMR spectrum (and magnification on top), focusing on the central-transition region. (c) 27Al27Al DQ-filtered spectrum revealing only pairs (or clusters) of Al atoms in close spatial proximity. (d) 27Al NMR central transition spectrum calculated for the disordered LDH model of Figure 3. (eg) Details of the individual 27Al signals calculated for (e) Al(Mg)6 sites corresponding to ordered Al environments and for (f) Al(Mg)4(Al)2 and (g) Al(Mg)5(Al) environments generated by the permutation of one Al and one Mg site in the disordered LDH model of Figure 3. Line broadenings of 100 Hz were applied to all calculated 27Al contributions. Intensities are normalized to identical areas. The calculated spectrum in panel d results from the summation of all contributions in panels eg.

By comparison, a DQ-filtered 27Al experiment conducted with a symmetry-based dipolar recoupling technique (Figure 6c) is expected to reveal only pairs or clusters of 27Al nuclei giving rise to 27Al27Al dipoledipole couplings. (A 2D version of this experiment, shown in Supporting Information, Figure S5, confirms that the double-quantum filter works as expected since only diagonal DQ signals are observed.) The most remarkable feature of the DQ-filtered spectrum (Figure 6c) as compared to the Hahn echo spectrum (Figure 6b) is the emphasis of the broad peak due to the fraction of distorted Al environments at ca. 0 ppm. In an ideal ordered LDH framework with Mg/Al = 2, each 2827


Chemistry of Materials Al atom would be surrounded by six Mg atoms [i.e., Al(Mg)6 moieties, see Figure 1b] and, in the next coordination sphere, by six equidistant Al neighbors located at 5.3 Å. As illustrated in Figure 3b, the replacement of a Mg by an Al atom would give rise via AlOAl linkages to Al atoms connected to one, two, or three Al atoms with AlAl distances of 3.1 Å [i.e., Al(Mg)5(Al), Al(Mg)4(Al)2, and Al(Mg)3(Al)3 moieties, respectively, the latter being expected only when misplaced Al and Mg atoms are not adjacent]. The increase in relative intensity of the broad peak at 0 ppm thus suggests that it could correspond to such Al(Mg)5(Al), Al(Mg)4(Al)2, and/or Al(Mg)3(Al)3 moieties. The less-symmetric environment of these moieties as compared to Al(Mg)6 moieties is consistent with their larger and more distributed quadrupolar interactions. These assignments of the sharp 27Al peak at 10 ppm to Al(Mg)6 moieties and of the broad peak at 0 ppm to Al(Mg)5(Al) and Al(Mg)4(Al)2 moieties resulting from defects in the cation ordering are supported by DFT calculations of 27Al NMR parameters. Figure 6d shows the 27Al NMR spectrum calculated from the same model LDH system as used above (Figure 3), and Figure 6eg shows the decomposition of this calculated spectrum into individual contributions calculated for Al(Mg)6, Al(Mg)4(Al)2, and Al(Mg)5(Al) moieties, respectively. Despite large distributions of calculated isotropic shifts and even more so of calculated quadrupolar interactions (Figure 6eg), it is clear that Al(Mg)6 moieties contribute only to the sharp peak of the central transition region, since these symmetric environments yield small quadrupolar couplings |CQ| between 1.4 and 2.0 MHz. Al(Mg)4(Al)2 and Al(Mg)5(Al) moieties yield significantly larger quadrupolar interactions (|CQ| of 3.4 MHz and between 4.5 and 4.7 MHz, respectively) and clearly contribute instead to the broader and more distributed peak at 0 ppm. This is consistent with the previous assignment based on changes in relative intensities of these two peaks between the 27Al Hahn echo and DQ-filtered spectra (Figure 6b,c). As for 1H NMR data, the larger distributions of 27Al NMR parameters in the calculated spectra as compared to experimental spectra can be attributed to the reorientation and/or translation dynamics of the nitrate anions in the interlayer space and the resulting averaging of chemical shifts and quadrupolar interactions at room temperature. In summary, 1H MAS, 1H{27Al} CP-MAS, 2D 1H DH3 DQ-SQ, and 27Al DQ-filtered NMR experiments and DFT calculations all indicate that Al-rich defects also exist in nonnegligible amounts in Mg/Al-2 LDH. The MgAl2OH hydroxyl moieties associated with these defects can be assigned to a peak at 5.3 ppm in the fast MAS 1H spectrum, while the resulting Al(Mg)4(Al)2 and Al(Mg)5(Al) moieties account for the distorted (and disordered) Al(VI) environments at ca. 0 ppm observed in previous studies.2123 On the basis of this knowledge, the relative amounts of Mg2AlOH, Mg3OH, and MgAl2OH moieties can be determined from deconvolution of the quantitative 1H MAS spectrum. Figure 7a shows the details of the best fit to the experimental spectrum for LDH sample Mg/Al-2. The position of the peak associated with MgAl2OH moieties could be fixed at 5.3 ppm on the basis of the 2D 1H DQ-SQ recoupling experiment (Figure 5) despite the largely overlapping water peak at 4.6 ppm. For reasons that remain unclear, it was necessary to properly account for the peak corresponding to Mg2AlOH moieties in all 1H spectra at our disposal to use two components. It is clear (from two-dimensional spin-diffusion recorded at short mixing times, not shown) that the signals of both Mg2AlOH and Mg3OH moieties are largely dominated by dispersions of

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Figure 7. Best fits (in red) to the quantitative 1H echo-MAS NMR spectra (in black) of LDH samples (a) Mg/Al-2, (b) Mg/Al-2.5, (c) Mg/ Al-3, and (d) Mg/Al-3.5. Below are shown the different Gaussian/ Lorentzian contributions to each fit, with intercalated and/or surfaceadsorbed water in gray and MgAl2OH, Mg2AlOH, and Mg3OH framework hydroxyl groups in blue, orange-red, and green, respectively. Mg2AlOH and Mg3OH sites were decomposed into two peaks (whose significance is discussed in the text) when found necessary in order to improve the quality of the fits and provide more accurate quantifications. In each case, the fits were based on series of 1H{27Al} CP-MAS, T2-filtered, and 2D 1H DH3 DQ-SQ experiments to ensure accurate positions of otherwise overlapping peaks.

chemical shifts, which could be due for example to variations in the (dynamically averaged) distance to the nearest intercalated anions or water molecules. Quantification results are summarized in Table 2. The Mg/Al-2 sample was found to contain about 6% ( 2% Mg3OH moieties, which is larger than observed by Sideris et al.25 (3%) for a similar sample. This may indicate a stronger disorder in the cation arrangement in our sample rather than a deviation from the expected Mg/Al stoichiometry, since the combination of elemental analyses and the relationship between a parameter and composition proved that the layers have the nominal composition. The newly identified MgAl2OH moieties characteristic of adjacent Al sites represent 10% ( 4% of the total amount of framework hydroxyl sites. Although estimated uncertainties are larger for these moieties because of the overlap with the water peak at 4.6 ppm, their amount is 2828



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Table 2. Percentage Amounts of LDH Framework Hydroxyl Moieties As Determined by 1H NMR Together with Values Predicted for a Hypothetical Ordered Structure experimenta (%)

ordered model (%) Mg2AlOH

sample Mg/Al-2

Mg3OH

MgAl2OH b

10(4)

Mg2AlOH b

Mg3OH 6(2)b

100

0

Mg/Al-2.5

86

14

0

84(5)

16(5)

Mg/Al-3

75

25

0

73(5)c

27(5)c

Mg/Al-3.5

67

33

0

64(3)

36(3)

85(6)

Retained values are those obtained for fits with as many free parameters as could be reasonably used while achieving convergence. Errors were then estimated by conducting series of fits with different Gaussian to Lorentzian (G/L) ratios of various peaks and considering the variations observed among all fits of acceptable quality. The respective ratios indeed have a strong influence on the relative areas of the peaks and thus represent the main potential source of error on the quantifications. b Deviations from 100% of the total result from the averaging rule. c For sample Mg/Al-3, series of fits of equally good quality were obtained depending on the (fixed) G/L ratio used for the dominant peak assigned to Mg3OH moieties at 1.2 ppm (between 0.55 and 1). The proportion of Mg3OH species varied as a result between 22% and 31%, from which the value of 27(5) % was retained. a

comparable to that of Mg3OH entities. This suggests that the stoichiometry of framework cation sites is indeed that of the nominal composition. Thus, although the Mg/Al distribution is globally ordered, the small amounts of Mg3OH sites reflect the presence of some defects in the cation distributions rather than excess framework Mg2þ. These numbers give an estimation of the defects from the point of view of the hydroxyl groups. More relevant to the materials structure and properties are the proportion of Al3þ and Mg2þ cations that are misplaced with respect to a perfectly ordered distribution with Mg/Al = 2, which can be calculated on the basis of the quantification of hydroxyl groups. Two limiting cases must be considered: (i) all substitutions correspond to permutations of Al and Mg atoms in adjacent sites, and (ii) substitutions never occur on sites that are adjacent. Case i is illustrated in Figure 3b by the model considered for DFT calculations, which shows that the permutation of a single pair of adjacent Al and Mg sites generates four Mg3OH (green triangles in Figure 3b) and four MgAl2OH sites (blue triangles in Figure 3b). On the other hand, case ii generates six Mg3OH and six MgAl2OH sites. This can be expressed as ndefects ndefects 1 1 M2þ M3þ ¼ ¼ ðcase iÞ or ðcase iiÞ defects defects 8 12 nOH nOH On the basis of the general composition of nitrate-intercalated LDH materials [M2þ(1x)M3þx(OH)2]xþ(A)x 3 mH2O, with x = 0.33 for Mg/Al = 2, the proportion of Al defects may be written as ! ! ndefects ndefects xdefects nOH defects OH M3þ M3þ ¼ xM3þ ¼ nM3þ x nðM2þ þ M3þ Þ ndefects OH where xdefects is the fraction of hydroxyl defects (Mg3OH and OH MgAl2OH) relative to the total amount of framework hydroxyl groups nOH (i.e., 16% ( 7%), and nOH/n(M2þ þ M3þ), the ratio between framework hydroxyl groups and cationic sites, is equal to 2. This means that the proportion of Al atoms that are

misplaced with respect to their expected ordered positions may range between 8% (case i) and 12% (case ii). Similarly, the proportion of Mg sites that do not occupy their expected ordered position is given by !" # ndefects ndefects xdefects nOH defects OH M2þ M2þ ¼ xM2þ ¼ nM2þ ð1 xÞ nðM2þ þ M3þ Þ ndefects OH with the fraction of Mg relative to the number of cationic sites (1 x) = 67%, which yields 4% and 6% in limiting cases i and ii, respectively for sample Mg/Al-2. The difference between cases i and ii is within the quantification uncertainties, and we can conclude that the proportions of Al and Mg sites that are not in their ordered positions are 10% ( 4% and 5% ( 2%, respectively. Although these defect concentrations do not contradict the previous claim of ordered cation distributions in Mg/Al-2 LDHs,25 they are definitely not negligible and probably affect the macroscopic properties of these materials. One should expect a decrease in the number of adjacent Al, and thus of MgAl2OH moieties with increasing Mg/Al ratios. Fast MAS 1H experiments were conducted for samples Mg/Al-y with y = 2.5, 3.0, and 3.5 to specifically monitor the evolution of the signatures of MgAl2OH species around 5.3 ppm. The spectra are shown in Figure 7bd along with the corresponding deconvolutions. As observed previously,25 the relative intensities of the peaks associated with Mg2AlOH and Mg3OH moieties change accordingly with the Mg/Al ratio. In addition, the peaks progressively shift to lower chemical shift values due to modifications of the chemical environment (in-plane second coordination sphere, reduction of the number of compensating anions in the interlayer space). For all samples with Mg/Al larger than 2, no signal contribution is observed that can potentially be attributed to the MgAl2OH moieties, either in the 1D quantitative echo-MAS experiments (Figure 7bd) or in 2D 1H1H DH3 experiments (Supporting Information, Figure S6ac). This is in marked contrast with the observation made for sample Mg/ Al-2, where the peak attributed to MgAl2OH moieties was clearly visible in the 2D DH3 correlation experiment (Figure 5). As shown in Supporting Information, Figure S6b,c, no signals other than those of water, Mg2AlOH, and Mg3OH moieties were observed for other samples with Mg/Al > 2.5 in either 1H dipolar DQ-SQ, 1H T2-filtered (dotted lines), or 1H{27Al} CP-MAS (dashed lines) experiments. This is consistent with the expected absence of the AlOAl linkage when the Al content is reduced. Table 2 shows the comparisons of relative quantifications of all framework hydroxyl moieties for the samples studied here and, for comparison, the relative amounts of these species expected for an ordered cation distribution. We observe that, contrary to the case of sample Mg/Al-2, where significant deviations to the ordered model are observed due to the presence of MgAl2OH and Mg3OH defects, for Mg/Al ratios larger than 2 the quantifications are in good agreement with the prediction of the ordered model, consistent with the observations of Sideris et al.25 and with the absence of peaks that could be assigned to MgAl2OH species, as established here. In theory, 27Al NMR experiments could be used to support the conclusions drawn on the basis of quantitative 1H NMR, since we established that the two distinct sites observed for the Mg/Al-2 sample pointed respectively to isolated Al(Mg)6 moieties and to Al atoms next to others [i.e., Al(Mg)5(Al), Al(Mg)4(Al)2, and potentially Al(Mg)3(Al)3 moieties]. In practice, however, quantitative 27Al MAS NMR spectra and 27Al Hahn echo spectra 2829


Chemistry of Materials collected for samples Mg/Al-y with y > 2 (Supporting Information, Figure S7) show considerably increased broadening of the dominant 27Al peak as compared to the Mg/Al-2 sample, with the asymmetry characteristic of distributed quadrupolar interactions.58 This points to increased topological or chemical [e.g., composition of the second coordination sphere of Al(Mg)6 moieties] disorder, which makes it impossible to tell whether additional small peak(s) may be present underneath. Interestingly, Benito et al.21 recently found that microwave treatments of terephthalate-intercalated Mg/Al and Zn/Al LDHs caused the ratio between symmetric and distorted Al(VI) environments—that is, the sharp 27Al peak at ca. 10 ppm and the broad 27Al shoulder at ca. 0 ppm, respectively assigned here to Al(Mg)6 and to Al(Mg)5Al and/or Al(Mg)4(Al)2 environments —to increase progressively with treatment time. Concomitantly, they observed a gradual increase in sample crystallinity and a slight increase of the M2þ/M3þ ratio, indicating the release of some Al3þ cations. For the longest irradiation times, the 27Al NMR signal from distorted Al(VI) environments in some samples was found to completely vanish, consistent with the elimination of less-stable Al clusters in the layers. Similarly, samples intercalated with a different carboxylate anion (oxalate) exhibited M2þ/M3þ ratios of 2.7 or above in the layers, whatever the microwave treatment duration, and were found to display only a single sharp Al(VI) environment (at ca. 9 ppm). This is again fully consistent with the assignment of 27Al NMR signals proposed here and with the existence of Al clustering at low (smaller than 2.5) Mg2þ/Al3þ (and possibly Zn2þ/Al3þ) ratios. Furthermore, it indicates that postsynthesis treatments such as microwave aging can lead to the preferential removal of Al atoms misplaced with respect to the ordered cation arrangement. In addition, the relative amounts of ordered and distorted Al(VI) environments and their evolution during the treatments were found to vary with the nature of the intercalated (during synthesis) carboxylate anions, suggesting that the intercalated anions can affect cation ordering in the layers. These observations open important perspectives for the precise control of the extent of cation ordering in LDH materials during or after synthesis.

4. CONCLUSIONS The cation distributions in LDH materials can be characterized by 1H solid-state NMR under very fast MAS at high magnetic fields. This recent finding25 is strengthened here by the confirmation that framework Mg3OH species (hydroxyl groups surrounded by three Mg atoms) indicating Mg-rich regions can be observed and quantified. Definitive identification of the 1H NMR signature of these moieties was obtained in the present work by a combination of chemical analyses, various 1H and double resonance 1H{27Al} solid-state NMR experiments, and DFT calculations of 1H chemical shifts. Furthermore, we demonstrated for the first time the presence in MgAl LDH materials of additional 1H NMR signal contributions that are assigned to MgAl2OH moieties (framework hydroxyl groups surrounded by two Al3þ and one Mg2þ). These correspond to the Al-rich counterparts expected in the presence of Mg3OH moieties to maintain the stoichiometric ratio Mg/Al = 2. This new type of MgAl2OH defects was quantified and their amount was found to match the amount of Mg3OH species, indicating that the small amount of Mg2þ clustering observed in our work and by Sideris et al.25 in fact truly corresponds to defects in the

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otherwise globally ordered cation distribution, rather than to excess Mg2þ in the hydroxide framework. These defects may correspond to the substitution of up to ca. 10% ( 4% and ca. 5% ( 2% of the framework Al3þ and Mg2þ sites, respectively. Such amounts of defects are believed to have an impact in several important applications of LDHs, through their influence on the ability of the materials to reconstruct by “memory effect” or on the extent of ordering of the intercalated anionic species in nanocomposites formed by anionic exchange. Our ability to reliably quantify the extent of cation ordering in LDH materials opens considerable perspectives for its precise control and as a result for optimized tuning, at the molecular level, of the structures and properties of LDH-based nanocomposites.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional text, one table, and seven figures showing NMR pulse sequences; details of pseudopotentials used for DFT calculations; TEM of sample Mg/Al-2 and XRD patterns of samples Mg/Al-y with y = 2.5, 3.0, and 3.5; structure of LDH model with ordered cation distribution; effects of interlayer spacing on calculated 1H NMR shifts; 2D NMR 27Al DQ-SQ spectrum of sample Mg/Al-2; 2D NMR 1H DQ-SQ spectra of samples Mg/Al-y with y = 2.5, 3.0, and 3.5; and 1D 27Al echo-MAS NMR spectra of samples Mg/Al-y with y = 2.0, 2.5, 3.0, and 3.5. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from the French TGE RMN THC Fr3050 for conducting the research is gratefully acknowledged. We thank Dorothee Berthomieu (Montpellier, France) for helpful discussions. DFT calculations were performed with the cluster of the “Centre de Calcul Scientifique en Region Centre” (Orleans, France). ’ REFERENCES (1) Layered Double Hydroxides: Present and Future; Rives, V., Ed.; Nova Science Publishers, Inc.: New York, 2001. (2) Khan, A. I.; Lei, L. X.; Norquist, A. J.; O’Hare, D. Chem. Commun. 2001, 2342–2343. (3) Bauer, J.; Behrens, P.; Speckbacher, M.; Langhals, H. Adv. Funct. Mater. 2003, 13, 241–248. (4) Laguna, H.; Loera, S.; Ibarra, I. A.; Lima, E.; Vera, M. A.; Lara, V. Microporous Mesoporous Mater. 2007, 98, 234–241. (5) Zlatanova, K.; Markovsky, P.; Spassova, I.; Danev, G. Opt. Mater. 1996, 5, 279–283. (6) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399–438. (7) Leroux, F.; Taviot-Gueho, C. J. Mater. Chem. 2005, 15, 3628–3642. (8) Coronado, E.; Marti-Gastaldo, C.; Navarro-Moratalla, E.; Ribera, A. Inorg. Chem. 2010, 49, 1313–1315. (9) Tichit, D.; Coq, B. CATTECH 2003, 7, 206–217. (10) Sels, B. F.; De Vos, D. E.; Jacobs, P. A. Catal. Rev.—Sci. Eng. 2001, 43, 443–488. (11) Tichit, D.; Gerardin, C.; Durand, R.; Coq, B. Top. Catal. 2006, 39, 89–96. (12) Cervilla, A.; Corma, A.; Fornes, V.; Llopis, E.; Palanca, P.; Rey, F.; Ribera, A. J. Am. Chem. Soc. 1994, 116, 1595–1596. 2830


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Identification and Quantification of Defects in the Cation Ordering in

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