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Carbonate-Hydrogenocarbonate Coexistence and Dynamics in Layered Double Hydroxides Arnaud Di Bitetto, Gwendal Kervern, Erwan Andre, Pierrick Durand, and Cédric Carteret J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12192 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 23, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Carbonate-Hydrogenocarbonate Coexistence and Dynamics in Layered Double Hydroxides Arnaud Di Bitettoa,b, Gwendal Kervernb, Erwan Andréa, Pierrick Durandb, and Cédric Cartereta,∗ a

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

Villers-lès-Nancy, France. b

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

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

ACS Paragon Plus Environment


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ABSTRACT

Carbonated layered double hydroxides were fully characterized by vibrational spectroscopies, powder X-ray diffraction and solid-state NMR tuning the cations, the layers charge density and the preparation method to get original structural and dynamical features within the materials. It clearly appears that carbonate and hydrogenocarbonate coexist in the same interlayer after contact with air, and also that the hydrogenocarbonate quantity is correlated to the MII/MIII molar ratio constituting a strong pH probe of the interlayer space. Likewise, these two species are involved in an exchange process with atmospheric carbon dioxide, and hydrogenocarbonate proves to be the key parameter of exchange kinetics. These crucial results, extended to various cationic couples, could lead to new alternatives for carbon dioxide storage.


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INTRODUCTION Layered double hydroxides (LDH) are a class of synthetic or natural anionic clays with brucitic sheets holding divalent and trivalent metallic cations situated in hydroxides octahedra. Because of the positive charge of the layers, hydrated anions must be intercalated between them. Their general chemical formula is MII1-xMIIIx(OH)2(An-)x/n · yH2O, where MII is a divalent metallic cation (Mg2+, Zn2+, Ca2+, Co2+...), MIII a trivalent metallic cation (Al3+, Ga3+, Fe3+...),1-3 An- an interlayer anion (CO32-, Cl-, NO3-, ClO4-...)4-6 and x, the layers charge density ranges between 0.20 and 0.33 for most of cationic couples.7 Because of their high composition flexibility, LDH and their derivatives have been used in several fields including catalysis,8-12 depollution13-17 and vectorization.18-21 Moreover, these materials are of utmost relevance because of their anion exchange capacity which enables to tune the interlayer space without any modification of the brucitic sheets. Nowadays, the role of natural LDH in the sequestration of CO2 is subject to environmental interest.22-24 A recent work realized by Ishihara et al. highlighted a dynamic exchange between atmospheric carbon dioxide and intercalated carbonate anions,25 and its dependence with the layers charge density.26 However, more information is necessary on the exchange mechanism in order to develop a CO2 trap with that kind of material. Likewise, many recent Solid-State NMR studies have presented various results on structural and dynamical aspects in the chemistry of LDH. Some of the most recent investigations based on modern innovations revealed deep insights in the structure of the cationic sheets of MgII/AlIII- ZnII/AlIII- and MgII/GaIII-based LDH materials including cationic order and layer/interlayer interface.27-31 Moreover, structure and dynamics of adsorbed and intercalated anions as well as water molecules were widely studied by Solid-State 1

H, 13C, 15N and 35Cl NMR spectroscopy.32-36


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In this paper, we investigate carbonated MgII/AlIII-based LDH with MgII/AlIII molar ratios of 2, 3 and 4, with a full characterization leading to new and relevant structural features about these materials. Dynamic aspect, in particular exchange between intercalated anions and atmospheric carbon dioxide is studied and rationalized thanks to various techniques including Powder X-Ray Diffraction, Infrared and Raman vibrational spectroscopies, and Solid-State

13

C NMR. These

results are compared with those obtained in the case of other cationic couples (MgII/GaIII- and MgII/FeIII-based LDH). The main goal of this project is to establish relationships between structural and dynamical features of carbonated LDH. In a broader perspective, understanding the exchange mechanisms between organic and inorganic carbon in layered materials could change our knowledge about carbon cycle, and lead to the design of new materials dedicated to carbon dioxide storage.

EXPERIMENTAL SECTION Samples preparation Deionized water was degassed by boiling for at least 1h with nitrogen bubbling. As previously described,37,38 a coprecipitation method at constant pH was used for the synthesis of carbonated layered double hydroxides with MgII/MIII molar ratios of 2, 3 and 4 (MIII = AlIII, GaIII or FeIII). A solution of 0.4 mol/L of MgIICl2 and MIIICl3 in the desired ratio was added dropwise to a solution containing 0.25 mol/L of [12C] or [13C] Na2CO3 in deionized water at room temperature under nitrogen flow, and pH was maintained at 11 by addition of 1 mol/L NaOH during the synthesis. The materials obtained underwent a hydrothermal treatment at 100°C for 20h, and were then centrifuged and washed 3 times with deionized water. Solids were finally dried under nitrogen flow at room temperature for 24h. Samples are called CO3@MgAl and 13CO3@MgAl in the case


4

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of 13C-labelled materials; CO3@MgGa, 13CO3@MgGa, CO3@MgFe and 13CO3@MgFe for other cationic couples. Carbonate to chloride exchange was carried out following the method proposed by Iyi et al. for MgII/AlIII-based LDH:39 0.662 mmol of CO3@MgAl 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 1h at 50°C. The materials obtained were centrifuged and washed with ethanol once, and dried in air at 50°C for 15h. Samples are called ex-Cl@MgAl. Chloride to carbonate exchange was carried out dispersing 0.662 mmol of ex-Cl@MgAl and 3.31 mmol of Na2CO3 in 40 mL of deionized water and pH was fixed at 9, 10 or 11. The mixture was stirred for 1h at 25°C under nitrogen flow, and samples were centrifuged and washed 3 times with deionized water, and dried under nitrogen flow for 24h. Materials obtained are called ex-CO3@MgAl. Hydrated LDH were previously stored under nitrogen flow and were in contact with air at t = 0.

Characterization techniques Powder X-Ray Diffraction patterns were recorded with a Panalytical X’Pert Pro MPD diffractometer in reflection geometry using a tube with Cu radiation (Kα1 = 1.5406 Å), a Ge(111) incident-beam monochromator, 0.02 rad Soller slits, programmable divergence and anti-scatter slits (the irradiated area was fixed to 10mm x 10mm) and an X’Celerator detector. Data were collected from finely ground samples with a sample holder spinner and continuous rotation of sample to improve statistical representation of the sample. All the powder diffraction data were collected using the same strategy, i.e. data collection was carried out in the same scattering angle


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range between 5 and 80° with a 0.0167°step and a speed of 1°/min. The measurement time being the same for all the diffractogramms, the intensity is not normalized with the counting time. Lattice Parameters were extracted from Cell Refinements of the LDH Phases. MII/MIII molar ratios were measured by Induced Couple Plasma Atomic Emission Spectroscopy (ULTIMA-horiba Jobin-Yvon). 10 mg of LDH were dissolved into 1% nitric acid. Two rays were used for each element (279.553 nm and 285.213 nm for magnesium, 237.312 nm and 396.152 nm for aluminum, 294.364 nm and 417.206 nm for gallium, 238.204 nm and 256.940 nm for iron). Carbon quantities were determined by a Carbon Sulphur Analyser (Leco SC 144 DRPC) and metals amounts were measured by Induced Couple Plasma Atomic Emission Spectroscopy (Thermo Fischer ICap 6500) for CO3@MgAl. For each MgII/AlIII molar ratio, 4 samples were analyzed. C/Al molar ratios determined were used for the quantification of carbonate and hydrogenocarbonate in the interlayer space. Detailed calculi are presented in supporting information S-1. Raman spectra were recorded on a Jobin Yvon T64000 spectrometer equipped with a nitrogen cooled Charged Coupled Device detector and a confocal microscope. Samples are placed on a glass and mounted in the focal plane of an Olympus X50 objective (N.A = 0.55). The spot area is around 2 µm². A 532 nm exciting radiation was used with a laser power at 50 mW for the Al/Ga LDHs and less than 1 mW for the Fe LDHs to prevent their degradation. The spectral resolution is about 4 cm-1 and the precision on the wavenumber is lower than 1 cm-1. Infrared studies were performed on a FT-IR spectrometer Thermo Nicolet 8700 at 25°C with a resolution of 2 cm-1. Mid-infrared spectra were recorded using an ATR accessory (GladiATRTM with diamond crystal, Pike Technologies) with MCT or DTGS detector. Exchange kinetics


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C

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C was monitored following the shift of the antisymmetric stretching modes of carbonate

between 1340 and 1380 cm-1 and hydrogenocarbonate between 1580 and 1620 cm-1. Details are given in supporting information S-1. Solid-state 13C CP-MAS NMR experiments and 13C relaxation times measurements were done on a Bruker Avance III 600 MHz spectrometer equipped with a 4 mm CP-MAS probe. CP-MAS spectra were recorded at 25°C at a spinning speed of 2 kHz and 12.50 kHz, with a recycle delay of 5 s. CP was optimized with 2 ms of contact time, spinal 64 proton decoupling (rf pulses of 65 kHz) was used, and 256 scans were recorded. T1 relaxation times were measured with a SATRECT sequence at 25°C.

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C-13C 2D CP-MAS EXSY experiments were acquired on a

Bruker Avance III 300MHz spectrometer equipped with a 4 mm CP-MAS probe with temperature-control from -10°C to 30°C, at a spinning speed of 10 kHz. CP was optimized with 2 ms of contact time, and no recoupling was applied during the mixing time of 200 ms. Spinal 64 proton decoupling (rf pulses of 80 kHz) was used and 16 scans were recorded. 13C-NMR spectra were externally referenced to the -CH2- signal of adamantane (38.5 ppm relative to TMS). Spectra were treated on GSIM with a line-broadening of 25 Hz.

RESULTS AND DISCUSSION Structural features Carbonated layered double hydroxides were synthesized by coprecipitation at pH = 11 under nitrogen flow, and metallic salts solutions were prepared with MgII/AlIII molar ratios of 2, 3 and 4. MgII/AlIII molar ratios of materials, measured by ICP-AES, are very close to expected values (2.02, 3.03 and 3.98). Samples are called CO3@MgAl-2, CO3@MgAl-3 and CO3@MgAl-4 for 12

C materials with MgII/AlIII molar ratios of 2, 3 and 4 respectively. PXRD patterns of


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CO3@MgAl (figure 1) exhibit the characteristic reflexions of these materials and highlight the presence of only one phase, with a 3R1 polytype and a very good crystallinity. Lattice parameters a (3.05 Å, 3.07 Å and 3.08 Å for CO3@MgAl-2, CO3@MgAl-3 and CO3@MgAl-4 respectively) and c (7.63 Å, 7.77 Å and 7.96 Å for CO3@MgAl-2, CO3@MgAl-3 and CO3@MgAl-4

(113)

(110)

(018)

(015)

(012)

(003)

(006)

respectively) were determined for each sample.

CO3@MgAl-2

Intensity (a.u.)

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CO3@MgAl-3

CO3@MgAl-4 10

20

30

40

50

60

70

80

2θ (°)

Figure 1. PXRD patterns of CO3@MgAl for different MgII/AlIII molar ratios.

The slight increase of a with the MgII/AlIII molar ratio can be ascribed to the higher ionic radius of Mg2+ (r(Mg2+) = 72 pm) compared to the one of Al3+ (r(Al3+) = 54 pm), and the expansion of interlayer space is related to the decrease of the layers charge density as previously explained.37,40 Infrared and Raman spectra of CO3@MgAl are shown in figures 2 and 3. The vibrational modes of the brucitic sheet are assigned to the intense absorptions between 400 and 1000 cm-1 in infrared spectra and to the intense bands between 400 and 600 cm-1 in the Raman spectra.41,42 The symmetry point group of an unperturbed carbonate anion is D3h and the expected vibrations modes in Raman and infrared spectra are: the symmetric stretch CO3, noted ν1, Raman active at ~1060 cm-1; the out-of-plane deformation, noted ν2, IR active at ~880 cm-1; the antisymmetric


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stretch CO3, noted ν3, IR and Raman active at ~1400 cm-1; the in-plane deformation mode, noted ν4, IR and Raman active at ~ 685 cm-1. The vibrational spectrum of interlayer carbonate reveals an apparent breakdown in these selection rules. The infrared forbidden ν1 appears in the infrared spectrum (figure 2, table S2). In addition the ν3 vibration appears to be split into two bands in both the infrared and Raman spectra at all MgII/AlIII ratios (figures 2 and 3, table S2).43 These observations for interlayer carbonate are similar to those made for alkali carbonate aqueous solutions.44-47 The distortion of the D3h symmetry of the aqueous carbonate has been ascribed to asymmetric hydration of the carbonate so that the antisymmetric stretching mode is broadened in the Raman and infrared spectra and split into two band components.46 brucitic sheet

2-

-

νa, CO2HCO3

ν3CO3

-

δCOHHCO3

-

νC-OHHCO3

Absorbance

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ν1CO23 (a)

(b)

(c) 400

600

800

1000

1200

1400

1600

Wavenumber (cm-1)

Figure 2. Infrared spectra in the spectral range 400-1800 cm-1 of CO3@MgAl for different MgII/AlIII molar ratios. a) CO3@MgAl-2, b) CO3@MgAl-3 and c) CO3@MgAl-4.


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ν1CO23

ν4CO23

νa, CO2HCO-3

δCOHHCO-3

νC-OHHCO-3

δCO2HCO-3

γC-OHHCO-3

-

νs, CO2HCO3

brucitic sheet

Raman intensity (a. u.)

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ν3CO23 (a) (b) (c)

400

600

800

1000

1200

1400

1600

Wavenumber (cm-1)

Figure 3. Raman spectra in the spectral range 400-1800 cm-1 of CO3@MgAl for different MgII/AlIII molar ratios. a) CO3@MgAl-2, b) CO3@MgAl-3 and c) CO3@MgAl-4.

In the vibrational spectra of CO3@MgAl-3 and especially CO3@MgAl-4 (figures 2c and 3c) we can observe additional vibrational features at 628, 673, 1018, 1298, 1346, 1615 cm-1 in Raman and 1005, 1295, 1620 cm-1 in infrared assigned to the hydrogenocarbonate anion.44-47 These spectra highlight that both anions, carbonate and hydrogenocarbonate, coexist in the interlayer and that the amount of hydrogenocarbonate increases with the MgII/AlIII molar ratio. The Raman and Infrared data of aqueous and interlayer carbonate and hydrogenocarbonate are reported in table S2 with the assignments of the modes. We can notice that the wavenumbers for interlayer species are close from the ones of aqueous species, revealing a quite similar perturbation of the anions. In order to investigate the influence of the samples storage on the hydrogenocarbonate amount in the interlayer space, we synthesized samples avoiding contact with air. Evolution after contact with air was followed by Raman spectroscopy for CO3@MgAl-2, CO3@MgAl-3 and CO3@MgAl-4. The CO2 content in air was measured at ~500 ppm, and the relative humidity at ~30 % with small fluctuations in time. No significant change was observed on the layers part of the materials since Mg-OH and Al-OH vibrations at about 470 and 545 cm-1 respectively are


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unaffected (see complete Raman spectra in figure S2), but the anionic part is modified after contact with air in the case of CO3@MgAl-3 and CO3@MgAl-4 (figure 4). In addition of a first peak at 1065 cm-1, in the spectral range of the symmetric stretching vibrational modes of carbonate anion, a new band assigned to hydrogenocarbonate appears at ~1020 cm-1 after few minutes in the case of CO3@MgAl-3, and equilibrium is almost reached after 30 minutes with no further modifications. In the case of CO3@MgAl-4, that second band is already present at t = 0, and equilibrium is reached after only few minutes. Similar observations were obtained by Infrared spectroscopy (figure S3).

Raman intensity (a.u.)

(a)

t = 8 weeks

t=0 960

1000

1040

1080

1120

Wavenumber (cm-1)

(b)


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t = 1 week

t = 30 min

t=0 960

1000

1040

1080

1120

Wavenumber (cm-1)


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(c)


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t = 1 week

t = 30 min

t=0 960

1000

1040

1080

1120

Wavenumber (cm-1)

Figure 4. Contact with air (atmospheric CO2 at ~500 ppm and Relative Humidity at ~30 %) followed by Raman spectroscopy (HCO3-/CO32- region) for a) CO3@MgAl-2, b) CO3@MgAl-3 and c) CO3@MgAl-4.

Quantification of carbonate and hydrogenocarbonate was done after one week of contact with air thanks to both Raman spectroscopy and chemical analysis. Results are presented in table 1 and decomposition of Raman spectra are given in figure S4. It clearly appears that after contact with air, CO3@MgAl-2 contains almost only carbonate, but that the amount of hydrogenocarbonate increases for CO3@MgAl-3 (~25 % of HCO3-) and becomes predominant in the case of CO3@MgAl-4 (~65 % of HCO3-). Carbon/Metal (C/M) molar ratio for each sample was also determined (table 1). We notice that a higher MgII/AlIII molar ratio does not affect the amount of carbon stored as one could have thought because of the lower charge of hydrogenocarbonate, making the higher MgII/AlIII molar ratio the better for CO2 storage. Table 1. Quantification of carbonate and hydrogenocarbonate of CO3@MgAl after one week of contact with air by Elemental Chemical analysis and Raman spectroscopy. Samples

Elemental Chemical analysis C/M theo.†

C/M exp.

C/Al theo.†

C/Al exp.

Raman spectroscopy 2-

CO3 (%)

HCO3 (%)

0.17 0.17 0.50 0.50 100 0 CO3@MgAl-2 0.13 0.16 0.50 0.60 80 20 CO3@MgAl-3 0.10 0.16 0.50 0.85 30 70 CO3@MgAl-4 † Determined supposing the presence of only carbonate in the interlayer space.


-

CO32(%)

HCO3(%)

>95 75 35

CarbonateâHydrogenocarbonate Coexistence and ... - ACS Publications

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