Probing the Dynamics of Layered Double Hydroxides by Solid-State

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Probing the Dynamics of Layered Double Hydroxides by Solid-State Al NMR Spectroscopy 27

Arnaud Di Bitetto, Erwan Andre, Cédric Carteret, Pierrick Durand, and Gwendal Kervern J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b13106 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 14, 2017

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The Journal of Physical Chemistry

Probing the Dynamics of Layered Double Hydroxides by Solid-State 27Al NMR Spectroscopy Arnaud Di Bitetto†,‡, Erwan André†, Cédric Carteret∗,†, Pierrick Durand‡ and Gwendal Kervern∗,‡ †

Université de Lorraine, UMR 7564 (UL-CNRS) LCPME, 405 rue de Vandœuvre, F 54600

Villers-lès-Nancy, France. ‡

Université de Lorraine, UMR 7036 (UL-CNRS) CRM2, BP 70239 Boulevard des Aiguillettes,

F 54506 Vandœuvre-lès-Nancy, France. * [email protected] ; [email protected]

ACS Paragon Plus Environment

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ABSTRACT In order to shed light on molecular dynamics and structure in layered materials,

27

Al NMR

spectra of layered double hydroxides (LDH) were investigated varying the layer charge density, the cations of the sheets, the interlayer anions, the hydration state and the temperature. This study reveals that most of the broadening of 27Al satellite transitions in LDH is due to dynamics within the interlayer space rather than the chemical environment of

27

Al in the sheets, i.e. cation

disorder. This finding provides a new solid-state NMR tool to probe dynamics in aluminumbearing layered materials which does not require tensor calculations, which is based on direct acquisition spectra and which provides long range information as the 27Al spectra are sensitive to dynamics that occur 3-5 Å away from the observed nuclei.


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INTRODUCTION Layered double hydroxides (LDH) are a class of synthetic or natural layered materials whose structure derives from brucite (Figure 1). In this structure, Mg2+ ions are six-fold coordinated to OH-. These octahedra share edges with their neighbors to form an electrically neutral two dimensional sheet. The substitution of divalent by trivalent cations with similar radii creates positive charges within the layer. The interlayer region accommodates charge-balancing anions and water molecules.1,2 LDH are capable of incorporating a wide range of cations in the hydroxide layer, most frequently in divalent and trivalent states.1,2 There is even less restriction concerning the anions that come in the interlayer space.3 These materials can be described by their general formula MII1-xMIIIx(OH)2x+ (An-x/n, zH2O)x- where MII and MIII represent the divalent and trivalent cations, An− the interlayer anion. The x value is defined as the molar fraction of MIII cations and therefore corresponds to the layer charge of LDH. This value is a key parameter of LDH chemistry, and many properties such as the anionic exchange capacity depend on it.1-3

Figure 1. Perspective view of LDH structure: example of hydrated MgAl-0.33-Cl-. Mg and Al octahedra are green and gray, respectively; interlayer Cl- ions are in violet.


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The opportunity to tune the composition, the structure and the texture leads to the design of refined materials with desired properties for various applications such as catalysis, drug delivery, and environmental remediation.4,5 Furthermore, these materials are of considerable geological relevance because of their anion exchange capacity, which can affect the mobility of chemical species in the environment. In recent papers by Ishihara et al., it has been shown that LDH materials not only have the capacity to store CO2 under the form of carbonate anions captured in cationic mineral layers, but also that this type of material literally breathes and exchanges the interlayer anionic carbonate with the atmosphere.6,7 This type of behavior bears some potential for the development of carbon dioxide storage materials but, in order to make a proper CO2 trap, more information is needed on the exchange mechanism that lies behind this breathing phenomenon. In several papers, Kirkpatrick et al. have investigated the anions in LDH especially focused on dynamics by solid-state NMR. Studies include nitrate, carbonate ion and chloride in related Li2Al and Mg3Al-LDH.8-13 The presence of

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Al nuclei in LDH materials makes them an easy

target for NMR studies thanks to 27Al’s high sensitivity in NMR.14-18 NMR of quadrupolar nuclei has many features that make it suitable for dynamical and structural studies. For instance, measuring first or second order quadrupolar interaction can give crucial information on the nuclei’s chemical environment and dynamical behavior.19-24 NMR studies on LDH have shown that, as the electric charge density of the mineral layer decreases, there is a broader distribution of

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Al environment14,16,17 and that this distribution also has an effect on the inhomogeneous

linewidth of the 27Al solid-state NMR resonances15 but so far, nothing has been investigated on the effects of dynamics on 27Al spectra. It is possible to study the homogeneous line-broadening


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induced by first-order quadrupolar interaction fluctuation on satellite transitions (ST) to investigate local dynamics on quadrupolar nuclei.19,20,25-27 In this study, we investigate the influence of variable charge density (0.33 ≤ x ≤ 0.20), cations in the sheets (MgAl1-yGay with 0 ≤ y ≤ 1), interlayer anions (carbonate and chloride), hydration state, and temperature on the 27Al NMR spectra of Mg/Al-based LDH materials. We propose to show the first proof that chemical environment distribution is not the only phenomenon that causes the broadening of satellite transitions of 27Al resonances in LDH. We show that most of this broadening is due to dynamics within the interlayer space, that it is easy to measure with a simple direct acquisition experiment, and that interlayer anions and water play a key-role in this mechanism.

EXPERIMENTAL METHODS As previously described28,29 co-precipitation at low supersaturation was performed by slow addition (0.3 mL·min-1) of 200 mL of solution containing 0.4 mol·L-1 of MgIICl2 and MIIICl3 (MIII = AlIII or GaIII) in the desired ratio into 200 mL of basic solution containing 0.25 mol·L-1 of Na2CO3. The pH was kept constant at 11 by simultaneous addition of NaOH (1 mol·L-1) solution using an automatic titrator device (736 GP Titrino, Metrohm). The mixture obtained was divided by 4, and each fraction underwent a hydrothermal treatment at 100°C for 20 hours (in a recipient of 100 mL), and was then centrifuged and washed 3 times by deionized water. Solids were finally dried in air at room temperature for 24 hours. Carbonate to chloride exchange was carried out following the method proposed by Iyi et al. for MgAl-based LDH:30 0.662 mmol of carbonate-containing LDH were dispersed in 50 mL of ethanol. Then 81 µL of 37% HCl were added and exchange was performed under nitrogen flow, with vigorous stirring for 1 hour at


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50°C. The materials obtained were centrifuged and washed by ethanol once, and dried in air at 50°C for 15 hours. LDH samples were labeled as follows: MgAl1-yGay-x-An- , with y the ratio rate of Al substitution by Ga, x the layer charge, An- the interlayer anion either carbonate or chloride. Samples were dehydrated under vacuum (10-4 mbar) at 100°C for 1 hour and 15 hours in the case of chloride and carbonate anions respectively. 27

Al high-speed MAS solid-state NMR was performed on a Bruker Avance III 600 MHz

spectrometer equipped with a 2.5 mm CPMAS probe tuned to the

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Al resonance of 156 MHz.

Spectra were collected at a spinning speed of 25 kHz, with a recycle delay of 1 s and a single 10° rf-pulse of 0.25 µs of duration (γB1/2π = 100 kHz). Optimized spinal 64 proton decoupling was used with rf-pulses of 4.6 µs of duration. Probe temperature was set at 298 K for classical spectra acquisition, and between 210 K and 350 K for variable temperature spectra acquisition. 512 scans (1024 scans when necessary) were recorded for MgAl-x-An- samples, 2048 for MgAl0.5Ga0.5-x-An- and 16384 for MgAl0.1Ga0.9-x-An-. Spectra were referenced to Al(NO3)3 0.1 mol·L-1 at 0 ppm and FID were treated on GSIM31 without any line-broadening. The satellites Full Width Half Height (FWHH) was extracted using the Peak Fitting module of the Origin SoftwareTM. We chose the Pseudo-Voigt peak function type 1 since the line shape of satellites can change from Gaussian to Lorentzian (because the homogeneous broadening dominates the linewidth in the materials where mobility induces strong homogeneous relaxation in 27Al satellite transitions). The quadrupolar parameters were extracted from simulations using D. Massiot’s software DMFit.32 All the CQ and ηQ measurements and simulated spectra are available in supporting information. The quadrupolar coupling constant, CQ showed slight variation upon layer charge, intercalated anion, temperature or gallium content i.e between 1.30 and 1.50 MHz,


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which would lead to a second order quadrupolar broadening of ca. 300 Hz, which is around the minimum linewidth (CT) observed in our spectra. Sideris et al. have reported quadrupolar coupling parameters, CQ = 1.55 MHz and ηQ = 0, from a single simulation based on an axial site for Al, without mentioning neither the charge nor the anion.17 This reported CQ constant is close from our CQ values. Our asymmetry parameter values are non-zero, which is compatible with a symmetry site of Al not perfectly D3d. Indeed, taking into account the cation ordering, the best structure refinement is obtained with the space group C2/m (two different Al-O distances and various O-Al-O angles all different of 90°).33 We observe an increase in the asymmetry parameter as the charge decreases for MgAl-x-CO32-, which we attribute to the decrease in symmetry of the

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Al environment in lower-charge materials. Upon temperature variation, the

measured CQ and ηQ vary without discontinuity. We therefore assumed that no major structural change was occurring in the temperature range used for our experiments,34 and that the effect of the CQ and ηQ variation on the linewidth was negligible. Pure recrystallized KBr was used for magic-angle calibration (79Br sideband pattern) between each LDH sample change (no significant deviation of the magic angle was detected with temperature) and for effective samples temperature (Tsample) measurements using 79Br’s chemical shift as explained by Thurber et al.35

RESULTS AND DISCUSSION The focus on this study now goes on the structure of LDH layers and its impact on the

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Al

NMR signal. The layers are made of metallic hydroxide octahedra that can be assembled in more or less ordered fashion. Physical law that drives this assembling is that electrostatic repulsion prevents trivalent octahedra to share a common hydroxide. As a consequence of this


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phenomenon, the structure of LDH materials does not allow for a ratio of more than 1 trivalent metal for 2 divalent metals. In this case, the structure has to be fully ordered, but as the charge decreases, there is more and more space around the MIII centers, and it is possible to generate 2D structures that respect the MII/MIII ratio as well as the electrostatic repulsion rule, but with a distribution of environments for the MIII centers (as can be seen in fig S1). From the 27Al NMR point of view, this increasing number of possible environments with the Mg/Al ratio should lead to a distribution of 27Al frequencies, thus increasing the linewidth of 27Al resonances.

Figure 2. Evolution of the 27Al MAS-NMR spectra (Tsample = 311 K) of hydrated MgAl-x-CO32LDH as a result of layer charge modification (x = 0.33, 0.25 and 0.20) with a zoom on a satellite side band (at -98 kHz).

This observational prediction almost fits with the one-dimensional observation of the

27

Al

MAS-NMR spectra of the base-material with carbonate counter-ions for various layer charges: as can be seen on figure 2, as the charge decreases, the

27

Al linewidth of satellite transitions

increases (the composition, the label, and the structural analysis (powder XRD) of the materials


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are reported in supporting information). The highest charge material (x = 0.33) has the sharpest satellites, as expected from the cation ordering in the metal hydroxide sheets. More disorder is expected for the lower Al content samples, along with a noticeable increase in the linewidth of satellite transitions, particularly for the sample x = 0.20. This behavior would seem to be compatible with an increase of the disorder-based distribution of chemical shifts and quadrupolar interactions but the width of the central transition (CT) which should be influenced by such a distribution as well remains unaffected.

Figure 3. Evolution of the 27Al MAS-NMR spectra (Tsample = 311 K) of hydrated MgAl1−yGay0.33-CO32- LDH as a result of Al substitution by Ga (y = 0.0, 0.5 and 0.9) with a zoom on a satellite side band (at -98 kHz).

To probe the effects of disorder on the

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Al spectra, we generated environmental disorder in

the sheets of layered materials thanks to partial to total Al3+ → Ga3+ substitution. The gallium content in the synthesized materials varies in the full range 0-100% for the three layer charges (table S2). The PXRD data confirmed well crystallized materials with the same polytype 3R1


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(figure S3, table S3). The evolution of the cell parameter a revealed perfect solid solutions Ga/Al for the three layer charges (figure S4). We observed very few modifications of satellite transitions in

27

Al NMR spectra (figures 3 and S9), and the precise simulation of

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Al spectra

using DMFit32 shows no significant modification of the quadrupolar coupling (table S5) upon variation of the gallium content. We can conclude that disorder in the sheets is not the only cause of 27Al satellite transitions broadening.

Figure 4. Evolution of the

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Al MAS-NMR spectra (Tsample = 311 K) of hydrated MgAl-x-Cl-

LDH as a result of layer charge modification (x = 0.33, 0.25 and 0.20) with a zoom on a satellite side band (at -98 kHz).

In addition to that, the same 27Al NMR experiments done after replacing the carbonate anions with chloride ions (table S1) give the results shown on figure 4: the trend that was observed in accordance with the expectation of an increased disorder in lower-charge material seems to fail as, in this case, the linewidth of satellite transitions decreases with the charge of the hydroxide layers. On figure S2, we compare the results of a series of powder X-Ray diffraction experiments


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made on carbonate and chloride LDH with various layer charges. These experiments show that the structure of the anion-exchanged LDH presents little difference with the starting material in the interlayer distance, and no difference in the hydroxide layer structure itself. Given these experimental data, we can safely assume that the overall structure is preserved. We can therefore deduce that the nature of the anions in the interlayer space has a strong influence on satellite transitions’ linewidth (while the CT is not or hardly affected).

Figure 5. Evolution of the 27Al MAS-NMR spectra (Tsample = 311 K) of dehydrated (a) MgAl-xCO32- and (b) MgAl-x-Cl- LDH as a result of layer charge modification (x = 0.33, 0.25 and 0.20) with a zoom on a satellite side band (at -98 kHz).

The variations in Full Width at Half Height (FWHH) can be also partially ascribed to the water content within the layers. Figure 5 contains the 27Al NMR spectra of dehydrated MgAl-x-CO32and MgAl-x-Cl- . In particular, the behavior of the FWHH of

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Al ST spinning sidebands upon

charge variation for Cl- anions is reversed with respect to the behavior in the hydrated compound. This shows that the interlayer domain (anion and water) has more influence on the 27Al NMR ST


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linewidth than the cationic Al3+ order in the sheets. Hou et al. previously reported that chloride environment varies significantly with hydration state.12 So we expect that less water in the interlayer space implies less mobility for chloride ions and therefore trend reversal in the

27

Al

satellite for dehydrated materials.

Figure 6. Effect of sample temperature on

27

Al satellite transitions’ linewidth of hydrated (a)

MgAl-x-CO32- and (b) MgAl-x-Cl- (satellite at -98 kHz).

Figure 6 reports the FWHH for the 27Al NMR satellite signals of MgAl-x-CO32- and MgAl-xCl- LDH under increase of temperature (effective sample temperature was estimated with KBr as explained by Thurber et al.35). Again, two opposite trends can be observed from ambient temperature: the carbonated LDH linewidth of 27Al satellite's spinning sidebands increases with temperature while that of the chloride-exchanged material decreases as it was already seen by Sideris et al. in the case of nitrate anions (MgAl-0.19-NO3-).17 This difference in trends can also be observed while decreasing the temperature, but in the case of MgAl-0.33-Cl- LDH, a maximum is reached in satellite transitions' linewidth around 250 K.


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These behaviors with respect to temperature variations support the notion that the linewidth of 27

Al’s satellite transitions is in great part governed by fluctuation of first-order quadrupolar

interaction induced by molecular motion within the interlayer space while the central transition, unaffected by first order quadrupolar coupling, remains almost constant under temperature variations. The relatively narrow range of CQ values measured for all our samples at various temperature indicates two things: first, the second order quadrupolar remains weak at temperatures explored in this study, and cannot be the cause of the broadening and second, since the phenomenon observed here only affects the satellites and is homogeneous, it can only be caused by molecular dynamic-based fluctuations of the first-order quadrupolar interaction in 27Al resonances. Similar behavior was predicted and observed by several other groups.20,26 Some of the proposed models predict narrow linewidth in high- and low-mobility regimes, and a significant increase of the linewidth in the intermediate situation, as can be seen on figure 6b for MgAl0.33-Cl- LDH materials. Thus the fact that the linewidth of satellites goes through a maximum in the case of chloride-exchanged LDH is a signature of a regime switching from high-mobility (the linewidth of satellites decreases as the mobility increases) to low-mobility regime (the tendency is reversed) at low temperatures which is consistent with a thorough slowing of the molecular dynamics within the interlayer space at low temperatures. This transition could not be observed with carbonate-based LDH materials since the observed behavior at ambient temperature seems to be that of a low-mobility regime, and required to increase temperature above 360 K. However, we feared that such a high temperature would dehydrate the material, making it impossible to get to the high-mobility regime.


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This dynamic signature in the 27Al NMR signal can be interpreted as a strong difference in the dynamics within the anionic layers of chloride and carbonate-containing LDH materials, in agreement with previous results.8,13 Since the high-mobility regime appears at much lower temperature in chloride-exchanged materials, we know that interlayer molecular mobility is much higher in these materials, and, according to figure 6b, that it decreases as the charge increases. Since carbonate-based materials are in the low-mobility regime for

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Al satellite

transitions, we can infer from figure 6a that again, mobility in these materials decreases as the charge increases. This seems quite natural as the electrostatic interactions increase with the charge density, making motion more difficult for the species in the interlayer space, whatever the nature of the anion. The huge difference between mobility at a given temperature in carbonate and chloride materials can be explained by the fact that carbonate species ions can be involved in hydrogen bonding, generating a more solid anion-water network within the interlayer space. In addition, the fact that chloride-exchanged materials exhibit the same trend as nitrate anions17 clearly shows that interlayer mobility seems to be close for both anions, and that it is much higher in nitrate containing materials than in carbonated ones, which is again in agreement with Kirkpatrick et al. results.8,13 A more quantitative work involving molecular dynamics simulations is underway to give a clear and quantitative interpretation of these phenomena. The trend in 27Al satellite transitions’ linewidth not matching the expectations does not put into question the increased disorder as the charge decreases. The key point of this study is the following: there is a strong difference in the behavior between ST and the CT of

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Al NMR

signals. This difference cannot be explained by static quadrupolar interaction: if there was a second order effect, the central transition would be affected as well, and in the first order approximation, the quadrupolar interaction affects the shape of the sideband pattern, but not the


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linewidth of the satellite’s spinning sidebands. This is reinforced by the fact that, while keeping the layers’ structure identical, we can change this behavior by changing the counter ion. Since there is no chemical shift distribution and no visible second order quadrupolar interaction, the only explanation that remains is that the difference in coherence lifetime for 27Al signals in LDH materials is due to a fluctuation of an interaction that does not affect the central transition but affects the satellites. The fluctuation of first order quadrupolar interaction would be the only possible phenomenon to explain this.

CONCLUSIONS In summary, the NMR study of quadrupolar nuclei provides powerful tools for characterization of dynamics within solid materials, but it is more common to see studies that take advantage of the direct measurement of residual quadrupolar19-24 than resonance’s linewidth.26,27 In this case, we showed that there is a dynamical effect that appears in the 27Al resonances of LDH materials, and that this effect is counter-anion dependent. Since understanding dynamics within LDH materials seems to rise as a crucial problem, and given the fact that 27Al nuclei are located at the heart of the mineral layers and yet are sensitive to dynamics within the interlayer space, we think that this phenomenon needs to be advertised, as its understanding will eventually provide the scientific community with a universal tool for future studies in this domain.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge on the ACS Publications website at:


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Composition of materials (table S1, table S2), structure of materials (figure S1), powder X-Ray Diffraction experiments and results (figures S2, S3 and S4, table S3), NMR spectra simulations (figures S5, S6 and S7, tables S4 and S5) and NMR complementary results (figures S8, S9, S10 and S11).

AUTHOR INFORMATION Corresponding Author * [email protected] ; [email protected] Author Contributions The manuscript was written through contributions of all authors. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We would like to acknowledge Prof. Sabine Bouguet-Bonnet, Dr. Axel Gansmüller, Dr. Laurent Le Pollès, Dr. Pierre Florian and Dr. Luminita Duma for fruitful discussions about dynamics measurements with relaxometry and quadrupolar couplings. We would like to acknowledge Ms. Claire Génois for ICP-AES analysis on our samples. Financial support was received from the French Ministry of Higher Education (MRES) and the French National Scientific Centre (CNRS). ADB acknowledges the French Ministry of Higher Education for PhD grant.


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Probing the Dynamics of Layered Double Hydroxides by Solid-State

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