Structural Investigation of Zn(II) Insertion in Bayerite, an Aluminum

Article pubs.acs.org/IC

Structural Investigation of Zn(II) Insertion in Bayerite, an Aluminum Hydroxide Suraj Shiv Charan Pushparaj,† Nicholai Daugaard Jensen,† Claude Forano,‡,§ Gregory J. Rees,∥ Vanessa Prevot,‡,§ John V. Hanna,∥ Dorthe B. Ravnsbæk,† Morten Bjerring,⊥ and Ulla Gro Nielsen*,† †

Department of Physics, Chemistry, and Pharmacy, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark Institut de Chimie de Clermont-Ferrand, Université Clermont Auvergne, Université Blaise Pascal, BP 10448, F-63000 Clermont-Ferrand, France § CNRS, UMR 6296, F-63178 Aubiere, France ∥ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, U.K. ⊥ Department of Chemistry and Interdisciplinary Nanoscience Center, Aarhus University, Gustav Wiedsvej 14, 8000 Aarhus C, Denmark ‡

S Supporting Information *

ABSTRACT: Bayerite was treated under hydrothermal conditions (120, 130, 140, and 150 °C) to prepare a series of layered double hydroxides (LDHs) with an ideal composition of ZnAl4(OH)12(SO4)0.5·nH2O (ZnAl4-LDHs). These products were investigated by both bulk techniques (powder X-ray diffraction (PXRD), transmission electron microscopy, and elemental analysis) and atomic-level techniques (1H and 27Al solid-state NMR, IR, and Raman spectroscopy) to gain a detailed insight into the structure of ZnAl4-LDHs and sample composition. Four structural models (one stoichiometric and three different defect models) were investigated by Rietveld refinement of the PXRD data. These were assessed using the information obtained from other characterization techniques, which favored the ideal (nondefect) structural model for ZnAl4-LDH, as, for example, 27Al magic-angle spinning NMR showed that excess Al was present as amorphous bayerite (Al(OH)3) and pseudoboehmite (AlOOH). Moreover, no evidence of cation mixing, that is, partial substitution of Zn(II) onto any of four Al sites, was observed. Altogether this study highlights the challenges involved to synthesize pure ZnAl4-LDHs and the necessity to use complementary techniques such as PXRD, elemental analysis, and solidstate NMR for the characterization of the local and extended structure of ZnAl4-LDHs.

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INTRODUCTION Layered double hydroxides (LDHs), anionic clays, find widespread applications in many areas of materials science ranging from catalysis1 and energy storage to energy conversion.2 Moreover, they are used in environmental remediation3 and drug delivery4 due to their unique anionexchange properties.5 Two classes of LDHs are known. The first can be viewed as divalent metal hydroxides (M(II) (OH)2) with partial substitution of the divalent ions (M(II)) by trivalent cations (M(III)) accompanied by insertions of anions and water in the interlayer, that is, “hydrotalcite-type” named after the mineral hydrotalcite [Mg6Al2(OH)16(CO3)·4H2O]. A large family of hydrotalcite-type LDHs with the general chemical composition of [M(II)1−xM(III)x An−x/n mH2O] (where M(II) = Mg2+, Ca2+, Zn2+, Fe 2+, Co2+, ..., M(III) = Al3+, Fe3+, Co3+, Ga3+, and An− is an inorganic or organic anion) have been developed. The flexible chemistry of the cation layers in combination with a vast number of possible anions ranging from simple anions such © XXXX American Chemical Society

as halides or oxyanions to complex organic anions or biomolecules account for their popularity and widespread applications.6 The second class, Al(OH)3-LDHs, is based on insertion of mono- or divalent metal ions into the vacancies in aluminum hydroxides (Al(OH)3-LDHs, Figure 1a,d) with the excess positive charge of the layer balanced by anions in the interlayers (Figure 1b), which also contain a variable amount of water. Hydrotalcite-LDHs have been subject to numerous studies exploring their chemical composition, structure, and properties as well as further modifications creating nanocomposites7 and two-dimensional materials.8 In contrast only limited studies have been reported for the Al(OH)3-family of LDHs. The structures of gibbsite and bayerite, the two most commonly synthesized polymorphs of Al(OH)3, both consist of dioctahedral layers, where one-third of the cation sites are Received: June 14, 2016

A

DOI: 10.1021/acs.inorgchem.6b01436 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Crystal structure of bayerite (a) before and (b) after Zn(II) incorporation. (c) ZnAl4-LDHs morphology in TEM. (d) A single cation layer in the reactant bayerite, which by incorporation of Zn(II) into half the vacancies, is converted to a ZnAl4-LDH. (e) A stoichiometric ZnAl4-LDH has four crystallographic inequivalent Al (Al1, Al2, Al3, and Al4) and is isostructural to the mineral nickelalumite.28 (f) Two possible defects in the ZnAl4-LDH structure: (i) substitution of Zn(II) on the Al1 site, as proposed by ref 21 (Zn2), and (ii) a “Zn1” vacancy (point defects), which creates a bayerite-like environment.

vacant (Figure 1d), that is, a composition of Al2□(OH)6, where □ denotes a vacancy. They differ only in the stacking sequence of adjacent layers, which are ABBAAB9 and ABAB10 for gibbsite and bayerite, respectively. Monovalent ions such as Li+ may fill 100% of the vacancies resulting in the ideal chemical composition of [LiAl2(OH)6]+[An−]1/n·nH2O. Both inorganic and organic Li+ salts have been used as starting materials for insertion of Li+ into gibbsite11−17 and bayerite.11,12,18,19 In 2004 a new subclass of Al(OH)3-LDHs was obtained by incorporation of divalent cations (M(II) = Ni(II), Co(II), Zn(II), and Cu(II)) filling up to half the vacancies in the parent gibbsite structure.20 This was achieved by hydrothermal treatment of a physical mixture of activated gibbsite with an excess of M(II) salts.20 Following this, a few more Al(OH)3-type LDHs with divalent metal and anions including Zn(II) with sulfate (SO42−) and nitrate (NO3−) as anions in bayerite21,22 and Mg(II), Co(II), and Ni(II) in gibbsite with chloride in the interlayer23,24 were reported. The majority of these studies reported the presences of impurities and that preparation of pure Al(OH)3-LDHs phases is very challenging. On the basis of this limited number of studies, it is clear that optimization of the synthesis conditions is crucial to minimize the presence of impurities.23,24 Therefore, the majority of studies have focused on the synthesis of the material20,23−25 and investigations of the anion-exchanges properties,23,26,27 whereas only two studies have reported detailed crystallographic data.21,25 Currently, the most detailed structural study of Zn(II) insertion into bayerite with sulfate in the interlayer has been reported by Britto and Kamath.21 Powder X-ray

diffraction (PXRD) showed the material to be isostructural with the mineral nickelalumite (ideally [Ni□]Al4(OH)12(SO4)· 3H2O, space group P21/n),28 hence the name “ZnAl4-LDH”. Thus, the ideal crystal structure of ZnAl4-LDH contains a single Zn site and four crystallographic inequivalent Al sites (Al1, Al2, Al3, and Al4), as illustrated in Figure 1e. The presence of four Al sites in ZnAl4-LDH is due to a doubling of the a-axis as compared to bayerite (two Al sites). However, Britto and Kamath reported excess Zn (20% excess from stoichiometric) and sulfate deficiency, which was modeled as Zn substitution on only one of the Al sites (Al1, Figure 1f).21 Thus, a chemical composition of (Zn□)(Al 3.6 Zn 0.2 □ 0.2 )(OH) 12 (SO 4 ) 0.6 · 3.6(H2O) was proposed based on PXRD refinement in combination with elemental analysis (metal ions and sulfate) and Fourier transform infrared (FT-IR) spectroscopy.21 In contrast the hypothesis of Zn(II) substitution in the aluminum sites in gibbsite was ruled out based on 27Al triple-quantum magic-angle spinning (3QMAS) NMR even though the PXRD refined structure suggested cation mixing.25 Thus, ambiguities remain with respect to the structure at the atomic level of LDHs obtained by insertion of divalent cations into Al(OH)3’s. The properties of Al(OH)3-LDHs are controlled by the atomiclevel structure, where the ordering of cations in the metal oxide layer and anions govern the reactivity and anion-exchange properties, respectively.29 To gain further insight into the local and extended (longrange) structure of ZnAl4-LDHs and determine the Zn(II) occupancy on the possible crystallographic positions in the bayerite layer, we performed a detailed study of Zn(II) B

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Inorganic Chemistry Table 1. Synthesis Conditions, Al Content, and the Phase(s) Identified for the Four ZnAl4-LDHs Samples

a

sample

time, h

T, °C

pHend

% Ala

phases by PXRD (wt %)

ZnAl-120

24

120

4.5(1)

75.4(8)

ZnAl-130

24

130

4.6(1)

77.3(7)

ZnAl-140 ZnAl-150

24 24

140 150

4.6(1) 4.6(1)

82.9(6) 87.4(6)

71(3) ZnAl4-LDH 14(2) bayerite 15(2) gunningite 91(3) ZnAl4-LDH 5(2) bayerite 4(2) gunningite 100 ZnAl4-LDH 50(3) ZnAl4-LDH 50(3) pseudoboehmite

particle shape (TEM) platelike

platelike + acicular

platelike + acicular platelike + acicular

ICP-OES. with scanning rate of 1°/min. Rietveld refinements were performed using the Fullprof software.31 The backgrounds were described by linear interpolation between selected points (21 points), while pseudoVoigt profile functions were used to fit the diffraction peaks. The structure of nickelalumite28 was used as starting model for the refinement with Ni(II) replaced by Zn(II) to give a composition of ZnAl4(OH)12(SO4)·3H2O. For all refinements profile parameters, unit cell parameters and the overall atomic displacement parameter Bov were refined along the atomic positions of Zn, Al, and O (O5−O16, Table 2) in the cation layers, while the positions of the sulfate anions were refined as rigid bodies (Figure S1 and O1−O4 in Table 2). The positions of O in the water molecules (O17−O19, see Table 2),

insertion in bayerite. Our investigations involved both optimization of the synthesis conditions (reaction temperature) and detailed characterization of the products. PXRD, elemental analysis (metal ion content), and transmission electron microscopy (TEM) were used to assess the bulk (long-range) properties, whereas 1H and 27Al solid-state NMR (SSNMR) spectroscopy in combination with FT-IR and Raman spectroscopy provided detailed insight into the atomic-level structure. Recently, we successfully used this approach to understand how the synthesis condition affects the composition of hydrotalcitetype ZnAl LDHs on both the atomic and bulk scales.30 A ZnAl4-LDH sample without crystalline impurities, according to PXRD, was obtained and used for Rietveld refinement of the crystal structure. Four different structural models were assessed using atomic-level techniques (SSNMR, Raman, and FT-IR) combined with first-principles calculations of the SSNMR parameters of the structural model supported by the experimental data.

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Table 2. Structural Data for ZnAl4(OH)12(SO4)·3(H2O) Obtained from Rietveld Refinement of the PXRD Diffractogram using Structural Model 1 (see Table S2)a crystal system

monoclinic

space group unit cell parameters (Å)

P21/n a = 10.2862(4) b = 8.9066(3) c = 17.1158(4) β = 95.529(3)° 1560.78(8) z

EXPERIMENTAL SECTION

Materials Synthesis. Aluminum chloride hexahydrate (AlCl3· 6H2O, Sigma-Aldrich), zinc sulfate heptahydrate (ZnSO4·7H2O, Sigma-Aldrich), and 25% aqueous ammonia (Prolabo; reagent grade) were used without any purification. The water content of the metal salts was determined by thermal−gravimetric analysis prior to synthesis. Decarbonized water was obtained by bubbling nitrogen gas (dry) into boiling Milli-Q water. A Metrohm-905 titrator was used for controlled addition of the metal salt and base solutions as well as to control the pH during the reaction. The solid product was collected by centrifugation and subsequently sonicated in decarbonized water to remove dissolvable impurities. This was repeated three times before the solid product was collected and dried in an oven at 65 °C for 36 h. The two-step synthesis begins with synthesis of bayerite followed by insertion of Zn(II) using hydrothermal treatment was used to prepare ZnAl4-LDHs, as reported by Britto and Kamath.21 Bayerite was prepared by a slight modification of earlier reported procedures19 (25 wt % ammonia instead of 30 wt %). Aqueous ammonia (10.7 mL of 25 wt %) was added at the rate of 0.5 mL/min to 150 mL of an aqueous solution of 0.28 M AlCl3 under stirring at ambient temperature. When the pH reached 7, an additional 95 mL of aqueous ammonia (3 mL/ min) along with 83 mL of water (2.7 mL/min) was added to reach a pH of 11, which ensured complete precipitation of Al(OH)3.19 The white slurry obtained was aged for 12 h, at ambient conditions upon which the product was isolated and subsequently dried. The purity of bayerite was confirmed by PXRD and 27Al SSNMR (Figures S1 and S2, Table S1). In the second step, 40 mL of a 2.8 M solution of ZnSO4 and 0.50 g of bayerite were mixed in an 80 mL Teflon-lined autoclave and treated hydrothermally at the conditions reported in Table 1. At the end of synthesis the solid product was isolated and dried. Powder X-ray Diffraction. PXRD diffractograms were collected on a Rigaku-Miniflex 600 powder X-ray diffractometer with Cu Kα radiation (λ = 1.5408 Å) using a step size of 0.01° from 2θ = 5° to 90°

unit cell volume (Å3) atom x Zn1 Al1 Al2 Al3 Al4 S1 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12 O13 O14 O15 O16

0.7553(5) 0.0138(11) 0.5027(13) 0.2571(11) 0.2306(10) 0.4911(6) 0.5305(6) 0.9221(6) 0.6158(6) 0.4138(6) 0.5869(15) 0.9199(17) 0.6468(18) 0.835(2) 0.1173(16) 0.1892(15) 0.652(2) 0.8780(19) −0.1290(19) 0.586(2) 0.391(2) 0.3373(15)

y 0.5045(5) 0.3281(12) 0.6731(11) 0.1677(11) −0.1541(12) 0.1137(5) 0.0325(5) 0.4965(5) 0.1710(5) 0.2465(5) 0.493(3) 0.488(2) 0.348(2) 0.324(2) 0.150(2) −0.014(2) 0.701(2) 0.6970(19) 0.2006(19) 0.798(2) −0.204(2) 0.015(2)

0.4912(3) 0.5048(7) 0.4908(7) 0.5023(7) 0.4987(8) 0.7423(3) 0.6730(3) 0.2903(3) 0.7868(3) 0.7155(3) 0.5406(7) 0.4338(7) 0.4342(9) 0.5664(9) 0.5629(9) 0.4353(9) 0.4451(9) 0.5327(9) 0.4505(9) 0.5580(10) 0.4552(9) 0.5548(9)

a

The occupancies were not refined. Moreover, the atomic positions for O17−O19 (water molecules) and all H atoms were not refined and are omitted here. C

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Inorganic Chemistry positions of all H atoms, and the individual atomic displacement factors from the single-crystal starting model, were not refined. This structural model is referred to as Model 1 (see Table S2). To investigate the possibility of vacancies and/or mixed cation occupancies three other structural models were tested (see Table S2), namely, Model 2, in which the occupancy of Zn was refined freely, Model 3, in which the occupancies of Zn and the SO4-group were refined freely, and Model 4, in which the occupancies of Zn, all Al, and the SO4 group were refined freely. The obtained compositions and agreement factors for all structural models are listed in Table S2.

computational details can be found in the Supporting Information.

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RESULTS AND DISCUSSION Synthesis Optimization. Hydrothermal conditions (pressure and elevated temperatures) are necessary to force the insertion of M(II) in bayerite. Thus, hydrothermal reactions at autogenous pressure was conducted at four different temperatures (120, 130, 140, and 150 °C), as summarized in Table 1. These samples will be referred to as ZnAl-T, where T indicates the reaction temperature (T = 120, 130, 140, and 150 °C) at which the sample was hydrothermally treated for 24 h. The samples were first characterized by PXRD to identify and quantify the crystalline phases present. Initially, ZnAl-150, which was prepared by earlier reported conditions,21 contained approximately equal amounts of pseudoboehmite (aluminum oxy hydroxide, AlOOH), and ZnAl4-LDH, the target product, according to PXRD and 27Al SSNMR. Thus, the reaction time was increased to 48 h, but similar quantities of pseudoboehmite impurities were determined by PXRD (data not shown) implying that the reaction temperature was too high for LDHs synthesis, as evidenced by the formation of boehmite.24 At the lowest temperatures (120 and 130 °C), reflections from bayerite, the starting material, and gunningite (ZnSO4·H2O) are seen along with ZnAl4-LDH, whereas the reflections from pseudoboehmite are not observed, cf., Figure S3. However, Rietveld refinements of the PXRD diffractograms of ZnAl-140 showed that all reflection can be assigned to ZnAl4-LDH (Figure 2). ICP-OES provided insight into the relative

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PHASE COMPOSITION AND SAMPLE MORPHOLOGY The Zn and Al content was obtained by inductively coupled plasma-optical emission spectroscopy (ICP-OES) after dissolving the sample (0.1 g) in concentrated nitric acid. The FT-IR spectra were collected on a PerkinElmer 1720 FT-IR spectrometer and recorded in the range from 400 to 1800 cm−1 with a resolution of 1 cm−1 with the sample dispersed in KBr. Raman spectra were recorded from 150 to 1500 cm−1 at room temperature using a confocal micro-Raman spectrometer (T64000 Jobin Yvon) with an excitation wavelength of 514.5 nm (argon ion laser line, Spectra Physics 2017), and a spectral resolution of 1 cm−1, fitted with a charge coupled device (CCD) multichannel detector cooled by liquid nitrogen to 140 K and coupled with a Olympus confocal microscope. TEM images were taken using a Hitachi 7650 microscope at an acceleration voltage of 80 kV. The samples were dispersed in ethanol and stirred for 1 h, and then a single droplet of the suspension was applied to a 400 mesh holey carbon-coated copper grid and left to dry in air. Solid-State NMR Spectroscopy. Single-pulse 1H and 27Al MAS, the latter quantitative, as well as 27Al 3QMAS NMR spectra, were recorded on a Bruker 950 MHz (22.3 T) spectrometer using a 2.5 mm MAS probe yielding 35 kHz spinning frequency. Background subtractions were performed for the 1H MAS NMR spectra prior to analysis to compensate for a large probe background. The 27Al 3QMAS spectra were recorded using the three-pulse z-filter sequence.32 Quantitative single-pulse 27Al spectra were recorded on an Agilent INOVA spectrometer operating at 14.1 T (600 MHz for 1H, 1.6 mm triple resonance probe, and 35 kHz spinning) and on a Varian Infinity+ spectrometer operating at 7.1 T (300 MHz for 1H, 2.5 mm probe, and 30 kHz spinning). The quantitative 27Al MAS NMR spectra were recorded with short (10−15°) pulses using a 70−120 kHz radiofrequency field (rf). One molar AlCl3 (δiso(27Al) = 0.0 ppm) for 27Al and water (δiso(1H) = 4.6 ppm) was used as references except at 7.1 T, where the octahedral 27 Al site in yttrium aluminum garnet (δiso(27Al) = 0.7 ppm)33 was used as a secondary reference. SSNMR spectra were analyzed using QuadFit,34 VnmrJ 4.2A, and Topspin 2.1.

Figure 2. PXRD data of ZnAl-140 (Yobs) and the final result from Rietveld refinement (Ycalc) using the stoichiometric model of ZnAl4(OH)12(SO4)·3H2O (Model 1). ZnAl-140 contains no crystalline impurities according to PXRD unlike all other samples (ZnAl-120, ZnAl-130, and ZnAl-150) studied; cf. Figure S3.

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concentrations of Zn and Al (Table 1). On the basis of the theoretical formula of a ZnAl4-LDHs, ZnAl4(OH)12(SO4)· 3H2O, 80% of the metal ions (molar concentration) are Al corresponding to a Zn/Al molar ratio of 1:4. However, the Al content (molar percent) will be reported instead of the commonly used Zn/Al ratio, as even minor experimental uncertainties in the ICP-OES data drastically change the Zn/Al ratio from its mean value. For example, one percent uncertainty in relative concentration (80(1)% Al and 19(1)% Zn) results in a Zn/Al ratio of 1:4.00(25). Thus, any deviation from 80 mol % Al indicates the presence of other phases (impurities) and/or defects (vacancies and/or cation substitution) in the ZnAl4LDH. The ICP-OES results shows excess Al (more than 80 mol

FIRST-PRINCIPLES DENSITY FUNCTIONAL THEORY GAUGE-INCLUDING PROJECTOR AUGMENTED WAVE CALCULATIONS A starting model for the crystal structure was constructed using the unit cell parameters reported earlier for a stoichiometric ZnAl4-LDHs21 and the atomic coordinates for nickelalumite,28 i.e., the same starting model as used in the PXRD refinement. All density functional theory (DFT) calculations were performed using the CASTEP 8 code, which employs Kohn− Sham DFT methodology using periodic plane-waves under the ultrasoft pseudopotential approximation.35 The remaining D

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the sulfate content in ZnAl4-LDH. Thus, we only report the metal ion concentration in the LDHs.

% Al) for ZnAl-140 (82.9%) and ZnAl-150 (87.4%), whereas ZnAl-120 and ZnAl-130 are Zn-rich, in agreement with quantification of the crystalline phases by analysis of PXRD data (Table 1). The bulk Al composition determined from ICPOES for ZnAl-120 and ZnAl-130 are 75.4(8) mol % and 77.3(7) mol %, respectively, which fits with previously reported values (∼73.7 mol %) for ZnAl4-LDHs.21 Since no indications of crystalline impurities such as bayerite, pseudoboehmite, or gunningite were observed in the previously reported PXRD study, the excess Zn was attributed to partial occupation of Zn (∼20%) into Al1 site (Zn2 site in Figure 1f).21 The results above clearly demonstrate that the purity of the ZnAl4-LDH is very sensitive to even small variations in the reaction temperature leading to unwanted byproducts (Figure 3). The

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REFINEMENT OF THE STRUCTURE OF ZnAl4-LDH The PXRD data of ZnAl-140, the product obtained from the hydrothermal synthesis at 140 °C (Figure 2), confirms the formation of ZnAl4-LDH, which is isostructural to nickelalumite, and all Bragg peaks can be indexed in a monoclinic unit cell (space group P21/n), as reported earlier.21 To gain further insight into the atomic-level structure of ZnAl4-LDH, four structural models were tested by Rietveld refinement (see Table S2) including the fully stoichiometric composition ZnAl4(OH)12(SO4)·3(H2O), i.e., Model 1. From the point of view of PXRD and based on the Rietveld refinements, no definite conclusion on the possible inclusion of Zn(II) on the Al sites in the Al(OH)3 bayerite layer can be drawn, as the agreement factors are independent of the Zn, Al, and sulfate occupancies, as PXRD mainly provides insight into the longrange structure. The unit cell parameters here (Table 2) are slightly larger than those earlier reported,21 which may reflect the formation of fewer vacancies on the Zn1 site in our ZnAl4LDHs sample compared to the previous study.21 Models 2 and 4 are based on vacancies on both the Zn and Al positions, but they do not include SO4-vacancies; hence, in the case of cationsite vacancies (models 2 and 4) other charge-compensation mechanisms are needed. Also, note that model 4 does not yield Al-occupancies below 1, which implies very limited (or no) Zn substitution on the Al sites in contrast with previous reported data. Thus, the atomic coordinates and unit cell parameters for the stoichiometric ZnAl4-LDH structure (model 1) are reported (Tables 2 and S2), as this model is in the best agreement with the results achieved from other characterization techniques (vide infra).

Figure 3. Relative concentration of the target material ZnAl4-LDHs and crystalline impurities (bayerite, pseudoboehmite, and gunningite) as a function of reaction temperature determined from PXRD (Table 1).

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SAMPLE MORPHOLOGY BY TRANSMISSION ELECTRON MICROSCOPY TEM images clearly illustrate changes in the sample morphology, as the reaction temperature is varied from 130 to 150 °C (Figure 4). Ill-defined aggregated platelets characteristic of LDHs12 with a size ranging from 100 to 500 nm are observed for all samples, whereas the thickness of the individual particles cannot be determined as the platelets preferentially orient parallel to the grid. In addition, needleshaped microfibrils are also present. Higher temperatures clearly promote the formation of these acicular-shaped particles characteristic of pseudoboehmite.38 The presence of pseudoboehmite implies a partial transformation of the reactant

PXRD data for ZnAl-140 contains only reflections from ZnAl4LDH, cf. Figure 2, although elemental analysis showed an excess of Al (83 mol % Al). We will in the following mainly focus on this sample. The experimental results obtained by combining PXRD, SSNMR, ICP, and TEM as well as Raman and FT-IR spectroscopies showed that all samples contained multiples phases (vide infra). Thus, we could not unambiguously obtain the complete chemical composition of the LDHs. For example, sulfate-rich impurities and pseudoboehmite’s strong affinity for sulfate36,37 hampered the determination of

Figure 4. Representative TEM images of three ZnAl4-LDHs samples obtained at (a) 130 °C (ZnAl-130), (b) 140 °C (ZnAl-140), and (c) 150 °C (ZnAl-150). (insets) Images at a higher magnification illustrating the needle-shaped boehmite impurities present in all samples. E

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obtained: (11.4(6) ppm, 3.1(4) MHz), (11.1(9) ppm, 3.3(5) MHz), (10.1(12) ppm, 5.6(6) MHz), and (9.0(7) ppm, 2.4(8) MHz). The site at δiso(27Al) = 11.4 ppm is assigned to pseudoboehmite, as it fits previous reported values for the single 27Al resonance,42 and TEM shows crystallites with a morphology characteristic for pseudoboehmite (Figure 4). Closer inspection of the site at δiso(27Al) = 9.0 ppm reveals that this site is likely composed of two sites, as a partial splitting of the resonances is seen at 22.3 T (Figure 6b), which cannot be ascribed to second-order quadrupolar line broadening due to the estimated PQ. This is further supported by analysis of 14.1 and 7.1 T data (not shown). These parameters were used as starting values for deconvolution of the single-pulse spectra recorded at 22.3, 14.1, and 7.1 T. However, two additional sites were needed to obtain a reasonable fit, and their NMR parameters sites are in agreement with previously reported values of bayerite,42 the starting material. Moreover, addition of a site with parameters for boehmite did not improve the simulations suggesting that the concentration is below the detection limit (