Natural Abundance 17O MAS NMR and DFT simulations: New insights into the atomic structure of designed micas

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Natural Abundance 17O MAS NMR and DFT simulations: New insights into the atomic structure of

designed micas

Esperanza Pavón, Francisco Osuna, María Alba, Laurent Delevoye

To cite this version:

Esperanza Pavón, Francisco Osuna, María Alba, Laurent Delevoye. Natural Abundance 17O MAS

NMR and DFT simulations: New insights into the atomic structure of designed micas. Solid State

Nuclear Magnetic Resonance, Elsevier, 2019, 100, pp.45 - 51. �10.1016/j.ssnmr.2019.03.006�. �hal-

03096767�

1

Natural Abundance ¹⁷ O MAS NMR and DFT Simulations: New

2

Insights into the Atomic Structure of Design Micas

3 4

Esperanza Pavón

, Francisco J. Osuna

, María D. Alba

and Laurent Delevoye

5

6

1

Instituto Ciencia de los Materiales de Sevilla- Departamento de Química Inorgánica, 7

CSIC-Universidad de Sevilla. Avda. Américo Vespucio, 49. 41092, Sevilla. Spain 8

2

Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité 9

de Catalyse et de Chimie du Solide, F-59000 Lille, France 10

E-mail: esperanza.pavon@gmail.com. Tel.: +34 954 48 9546 11

12 13

ABSTRACT 14

Combining

O Magic-Angle Spinning (MAS) NMR at natural abundance with DFT 15

calculations is a promising methodology to shed light on the structure and disorder in 16

tetrahedral sheets of designed micas with enhanced properties. Among brittle micas, 17

synthetic mica is an important alternative to natural ones with a swelling sheet-like 18

structure that results in many applications, by exploiting unique characteristics.

19

Lowenstein’s rule is one of the main chemical factor that determines the atomic structure 20

of aluminosilicates and furthermore their properties. In the present article,

O MAS NMR 21

spectroscopy is used to validate (or not) the agreement of the Lowenstein’s rule with the 22

distribution of Si and Al sites in the tetrahedral sheets of synthetic micas.

O MAS 23

spectra of synthetic high-charged micas exhibit two regions of signals that revealed two 24

*

*Phone: +34 954 48 9546. E-mail: esperanza.pavon@gmail.com

distinguishable oxygen environments, namely Si-O-X (with X=Si, Al

, Mg) and Al

-O- 25

Y (Y=Mg or Al

). DFT calculations were also conducted to obtain the

O chemical shift 26

and other NMR features like the quadrupolar coupling constant, C

, for all of the oxygen 27

environments encountered in the two model structures, one respecting the Lowenstein’s 28

rule and the other involving Al

-O-Al

and Si-O-Si environments. Our DFT calculations 29

support the

O assignment, by confirming that Al

-O-3Mg and Al

-O-Al

oxygen 30

environments show chemical shifts under 30 ppm and more important, with quadrupolar 31

coupling constants of about 1 MHz, in line with the spectral observation. By quantifying 32

the

O MAS NMR spectra at natural abundance, we demonstrate that one of the synthetic 33

mica compositions does not meet the Lowenstein’s rule.

34 35

Keywords:

O NMR, DFT, natural abundance, Design mica, Lowenstein’s rule, atomic structure 36

37

38

39

1. Introduction 40

The preparation of synthetic clays, such as Na-n-Mica [1, 2], with 1) swelling 41

capacity, 2) tuned high charge density and 3) charged with specific heteroatoms, has 42

opened new possibilities in many areas of chemistry and material sciences due to their 43

potential use as a catalyst support and novel adsorbent of hazardous and radioactive 44

cations [3]. For example, the chemical properties of the clays will be enhanced by a 45

controlled incorporation of heteroatoms. In this context, it was demonstrated that 46

aluminum substitution improves the chemical retention of toxic cations such as heavy 47

metals and hydrocarbons [4]{Alba, 2001 #2}[5].It also changes the Lewis/Brönsted 48

acidity, modifying their catalytic properties [6]. However, still today many of the 49

structural aspects that can affect the properties of the silicates are unknown.

50

The Si-Al distribution in the tetrahedral sheet has been studied for decades and 51

according to Lowensteins' rule, Al

-O-Al

linkages in clay mineral frameworks are 52

forbidden. Indeed, quantum chemical studies have shown that a combination of Al

-O- 53

Al

corner sharing tetrahedral linkage and Si-O-Si linkage require higher energy than 54

two Si-O-Al

linkages [7, 8], indicating the instability of Al

-O-Al

linkages [9].

55

However, it has been observed that this Al

-O-Al

avoidance principle is not followed 56

in several aluminosilicates, both in layered and framework silicates [10-12]. In mica-type 57

materials, Alba et al. reported five different silicon environments in Na-4-Mica using

Si 58

MAS NMR, which suggested either a violation of Lowenstein´s rule or a Al/Si ratio lower 59

than 1 [5].

60

In order to remove this uncertainty,

O NMR could be an appropriate technique to 61

provide a more direct evidence of a Lowenstein rule violation in Na-4-Mica. The growing 62

interest of

O MAS NMR is due to the large chemical shift range for this nucleus (>1000

63

ppm) associated with the sensitivity of the quadrupolar coupling to the local geometry 64

[13, 14]. Nevertheless, as a quadrupolar nucleus (I > 1/2),

O is subjected to large 65

anisotropic broadening, even under magic-angle spinning (MAS) conditions. Moreover, 66

due to its low natural abundance (0.037%), the acquisition of

O NMR spectra may 67

become challenging and it usually requires

O enriched samples, high magnetic field and 68

often long acquisition times. As a consequence, only few

O NMR data on layered 69

silicates have been reported so far, even though the deduced structural information would 70

be of high interest.

71

The first direct evidence of Al

-O-Al

sites were reported by Stebbins et al. [10] in 72

crystalline aluminates, showing a violation of the Lowenstein rule using

O MAS NMR.

73

This work used

O isotopic enrichment of the samples, prior to the NMR investigations.

74

However, in addition to being expensive,

O isotopic enrichment might be challenging 75

from the synthesis point of view as it would require, in our study,

O enriched SiO

and 76

Al(OH)

as precursors, and further heating at high temperature (900 ºC). In a recent 77

publication, Pavón et al. [15] showed that it was possible to obtain

O MAS NMR spectra 78

of Na-4-Mica, at natural abundance, using high-field (18.8 T). They provided the first 79

evidence of Lowenstein’s rule violation in clay minerals by assigning the NMR signal 80

observed between 20 and 30 ppm to apical oxygen sites Si/Al

–O–3Mg and to Al

–O–

81

Al

. 82

Today, experimental NMR is often combined with density functional theory (DFT) 83

calculations of parameters to obtain a deeper understanding of the relation between NMR 84

data and the local structure and disorder. This is of great interest in the present context of 85

natural abundance

O NMR spectra of low sensitivity. CASTEP, a planewave, 86

pseudopotential DFT code, has been successfully applied to the calculation of the

O 87

NMR parameters in crystalline and amorphous inorganic systems [16-19]. In the case of

88

disorder or site substitutions, it is possible to propose models using larger unit cells in 89

order to highlight the diversity of oxygen sites present in the structure [20].

90

In this paper, we investigated the Si-Al distribution within the tetrahedral sheet of a 91

new family of swelling high-charged micas by combining experimental

O NMR data 92

and first-principle calculations of NMR parameters. The main objective was, for the first 93

time, to identify and quantify the different oxygen sites, using NMR spectra collected in 94

the challenging context of

O MAS NMR at natural abundance. The experimental 95

strategy consisted in the use of three trioctahedral 2:1 phillosilicates with different 96

tetrahedral substitution of Si by Al. One of those samples was a natural trioctahedral 2:1 97

phyllosilicate, saponite, in which the Lowenstein´s rule is obeyed and, it is therefore 98

considered here as the reference. The two other samples are the swelling high-charged 99

micas, Na-n-Mica (n=2 and 4), where n is the Al content on tetrahedral sheet based on 100

the O

F

unit cell, which are the target samples of the present study.

101 102

2. Experimental details 103

2.1. Sample description.

104

The selected aluminosilicates that are used in the present study, namely Saponite, Na- 105

2-Mica and Na-4-Mica, belong to the trioctahedral 2:1 phyllosilicate family (Table 1).

106

They are all made of one sheet of octahedral coordinated cation (Mg

) sandwiched 107

between two sheets of tetrahedral cations (Si

/Al

). Al is substituted to Si in the 108

tetrahedral sheet, leading to a layer charge deficit. Among the three samples studied here, 109

Saponite is the least charged silicate, followed by Na-2-Mica, and finally Na-4-Mica. This 110

latter compound exhibits the highest layer charge with a nominal Si/Al ratio of 1.

111

Natural trioctahedral layered aluminosilicates, saponite, contain basal oxygens (Si-O- 112

Si and Si-O-Al

), apical (Si-O-3Mg and Al

-O-3Mg) and oxygens from the hydroxyl

113

groups. Synthetic micas, Na-n-Mica (n=2 and 4), belong to the fluorophlogopite family, 114

which means that instead of hydroxyl groups, the octahedral coordination of the Mg 115

cation is balanced with fluorine ions. In Table 1 the relative ratios of the expected oxygen 116

environments in the different samples are, as well, presented. For the saponite, it is 117

deduced from its theoretical structure and for mica samples from the analysis of

Si MAS 118

NMR spectra [5].

119

A procedure similar to that described by Alba et al. [5] was employed to synthesize 120

Na-2-Mica and Na-4-Mica. Starting materials were SiO

from Sigma (CAS no. 112945- 121

52-5, 99.8% purity), Al(OH)

from Riedel-de Haën (CAS no. 21645-51-2, 99% purity), 122

MgF

from Aldrich (CAS no. 20831-0, 98% purity), and NaCl from Panreac (CAS no.

123

131659, 99.5% purity). Stoichiometric amounts of the reagents were mixed and grounded 124

vigorously before heating up to 900 °C in a Pt crucible for 15 h. After cooling, the solids 125

were washed with deionized water and dried at room temperature. Saponite sample, 126

supplied by the Source Clays Repository of the Clay Mineral Society, with particle 127

diameters of less than 2 μm, was used after removal of carbonates and organic matter.

128

Their structural characteristics are displayed in Table 1.

129

In order to remove the contribution of the water signal from the

O NMR spectra, 130

all the samples were dried at 300 ºC overnight and then, transferred to a sealed container 131

in order to avoid rehydration. NMR rotors were carefully filled in an inflatable glove box 132

to avoid contact with air humidity.

H MAS-NMR experiments and XRD patterns were 133

systematically carried out to check the absence of water signal.

134 135

2.2. NMR experiments.

136

NMR spectra were acquired on a Bruker Avance III 18.8T spectrometer (

O, 137

108.48 MHz) using a 4 mm HX Thalahassee probehead.

O excitation was achieved by

138

applying a central-transition selective π/2 pulse of 9 μs. The spinning frequency was 10 139

kHz, and the recycle delay was set to 1s after optimization. A total of 149504, 89216, 140

56240 transients were added for Saponite, Na-2-Mica and Na-4-Mica, respectively. Pulse 141

sequences, such as double-frequency sweep (DFS) designed for signal enhancement of 142

quadrupolar nuclei, showing large C

-dependency [21], were avoided to preserve 143

quantitative information.

O chemical shift was given in ppm with respect to

O signal 144

of water at 0 ppm.

145 146

2.3. Calculations methods.

147

First principles calculations with periodic boundary conditions were performed 148

using the CASTEP Code [22, 23], which relies on a plane-wave based density functional 149

theory approach (DFT). Electron correlation effects were modeled using the PBE 150

generalized gradient approximation (GGA) [24]. Geometry optimizations were 151

performed in several steps gradually increasing the precision to achieve an optimal cutoff 152

energy of 700 eV. The default pseudopotentials [25] in CASTEP were used (Version 6.1 153

of Material Studio). Convergence thresholds were set to 5x10

eV/Å for the total energy.

154

The maximum ionic force and displacement were 1x10

eV/ Å and 5x10

Å, 155

respectively. The Brillouin zone was sampled using a Monkhorst-Pack grid with a 156

separation of 0.05 Å

giving 16 irreducible points. Dispersion corrections, important in 157

systems with weak intermolecular interactions, were not considered because 1) no 158

hydrogen bonds are found in Mica samples and 2) the high amount of Si/Al substitutions 159

generates a strong electrostatic interaction, that become more important for the crystal 160

cohesion than the dispersive forces.

161

The NMR calculations were performed after the geometry optimization using the 162

Gauge Including Projector Augmented Wave approach (GIPAW) [26] and the same

163

energy cutoff of 700 eV. Simulations of calculated spectra were achieved by SIMPSON 164

program [27].

165

In order to reference the calculated chemical shifts and to compensate for the 166

systematic errors predicted by DFT calculations, a family of silicates of known structure 167

and known NMR parameters were used [19, 28] (Table S1). The corresponding plot is 168

displayed in Fig.1S, and the correlation function derived is obtained with the following 169

equation: δ

(

O) =-0.89·δ

(

O) + 228 (in ppm).

170 171

3. Results and discussion 172

Figure 1 presents the

O MAS NMR spectra, recorded at natural abundance for the 173

three samples. All possible oxygen environments overlap, making the assignment 174

uncertain. However, the spectra for synthetic Mica samples can be separated into two 175

regions of signals: a broad region, labelled A, between 50 and 30 ppm and a narrower 176

contribution, region B, between 30 and 15 ppm.

177

On the basis of previous studies [29-31] and on our recent communication [15], 178

regions A and B can be assigned to specific

O environments. The group of overlapping 179

resonances between 30 and 50 ppm are assigned to basal oxygen sites, Si-O-Si and Si-O- 180

Al

[30, 32]. Apical oxygens Si-O-3Mg, which link tetrahedral and octahedral sheets, 181

are found at higher chemical shift values as stated in the literature [18] and coincide with 182

the signal at around 50 ppm.

183

The last oxygen environment, Al

-O-3Mg, that is present in the structure, was 184

previously reported considering the electronegativity only [15]. The Al

-O-3Mg sites 185

should feature smaller C

and smaller isotropic chemical shift values than that of Si-O-

186

3Mg environments. Therefore, the signal in region B in both the Na-4-Mica and the Na- 187

2-Mica spectra can be empirically assigned to Al

-O-3Mg sites (Fig.1b and c).

188

27

Al MAS NMR in Na-4-Mica revealed the presence of aluminum in octahedral 189

coordination (Fig. 3S). This environment (15% of total aluminum) might come from 1) 190

aluminum hosted in the octahedral sheet and substituting Mg, or 2) from an impurity. If 191

the first option is assumed, then Al

-O-Al

and Si-O-Al

should be considered. Table 192

1 shows the percentage of those environments in this case (1.63 and 2.1 % of total, 193

respectively) that are well below the detection limits and, hence, make those contributions 194

impossible to be observed in the present challenging context.

195

Note that the two regions observed in mica are shifted to higher frequency in saponite 196

17

O MAS NMR spectrum, probably due to the presence of OH

groups in the framework 197

instead of F

ions in the mica structure.

198

Hydroxyl groups are usually associated with large quadrupole coupling constants of 199

about 7 MHz [29] that give rise to resonances, which are dominated by the second-order 200

quadrupolar broadening [17]. Therefore, their detection is a challenge here in the context 201

of

O NMR experiments at natural abundance. Thus, we do not expect to be able to detect 202

the hydroxyl sites, which account for only 16.7 % of the

O NMR signal in saponite.

203

Stebbins et al. [10] detected Al

-O-Al

environments at an isotropic chemical shift 204

of approximately 21 ppm, with a relatively large C

value of 3.8 MHz, in a stilbite zeolite 205

with a ratio Si/Al=3. It was at similar chemical shifts than Al-O-Al site connecting pairs 206

of AlO

in crystalline NaAlO

and in an Al-rich aluminosilicate glass [10]. Occurring for 207

about 11.25 % in Na-4-Mica, the

O NMR signal at 23 ppm (region B) could also be 208

assigned to Al

-O-Al

sites.

209

In order to clarify this latter point, DFT calculations were carried out. Two different 210

models for Na-4-Mica were constructed, based on a natural talc structure (ICSD 100682) 211

and modified as follows. First, the unit cell parameters a and b were changed to the values 212

obtained experimentally by XRD [5]. The basal distance was then adjusted to 9.84 Å, i.e., 213

the value found by XRD study in a dehydrated sample [33]. In the talc structure, the 214

tetrahedral layer consisted of Si atoms only, whereas in the Na-4-Mica the ratio Si/Al was 215

equal to 1. For that reason, half of the Si atoms were replaced by Al atoms. To compensate 216

for the layer charge, Na atoms were introduced in the hexagonal holes formed by the 217

tetrahedral arrangement. Finally, in order to produce a fluorinate silicate, OH sites were 218

exchanged by F ions. The resulting structure is a model for Na-4-Mica for which Si and 219

Al are fully ordered within the tetrahedral sheet. Oxygen sites are found solely in Si-O- 220

Al

environments in the tetrahedral sheet, and both in Si-O-3Mg and Al

-O-3Mg 221

environments in the octahedral sheet. In this case, basal oxygens account for 75% of all 222

the oxygen sites, whereas Si-O-3Mg and Al

-O-3Mg account 12.5% respectively as the 223

ratio Si/Al is equal to 1. This model will be referred to as Structure I in the following 224

sections (Figure 2).

225

The second model is constructed to include Al

-O-Al

sites in the tetrahedral sheet, 226

by replacing one Si cation by an Al cation within the tetrahedral sheet. This substitution 227

leads to the presence of three different basal oxygen sites as depicted in Fig. 2a, b and c.

228

This model will be called Structure II in the following sections.

229

Chemical shifts and quadrupolar parameters obtained by DFT GIPAW calculations 230

for structures I and II are displayed in Table 2 and Table 3, respectively. They were used 231

as starting values to simulate the experimental spectra (vide infra). For both structures, 232

the calculated values can be separated into two groups: the first group corresponds to 233

tet tet tet

low chemical shifts (<30 ppm); the second group includes the sites Si-O-3Mg, Si-O-Al

235

and Si-O-Si, which experience C

above 3 MHz and chemical shifts above 30 ppm.

236

For structure I, Si-O-Al

and Si-O-3Mg calculated chemical shifts and quadrupolar 237

parameters are in good agreement with those found in the literature [29]. Chemical shifts 238

are in the 30 to 50 ppm range and C

values are found between 3 to 3.5 MHz (Table 2).

239

As far as we know, there are no studies reporting NMR parameters for Al

-O-3Mg apical 240

oxygens. The calculated chemical shift for this oxygen environment at around 23 ppm, 241

and the low C

value (0.88 MHz) are in line with the presence of a narrow resonance in 242

region B for Na-4-Mica as well as for Na-2-Mica though at a lesser extent [15]. However, 243

this environment cannot explain by itself the intensity of the resonance in region B, as the 244

expected relative ratio in Na-4-Mica (12.5% or 9.5%, Table 1) is smaller than that 245

observed experimentally (27%).

246

Structure II is characterized by a perturbation in the periodicity of the silicate network 247

and consequently, an increase of the possible

O sites (Table 3). Each apical oxygen 248

(Al

-O-3Mg and Si-O-3Mg) is now spread into four different sites, slightly differing by 249

their geometrical environment. Their chemical shifts and C

values are in the same range 250

than that observed in Structure I.

251

Figure 3 shows the correlation between C

and δ

parameters for all oxygen sites 252

present in Structure II. Our calculated values are compared to the experimental data found 253

in the literature for different types of layered silicates (Fig. 2S). First, calculated values 254

are in accordance with previous experimental data for Si-O-Si and Si-O-Al

sites. Si-O- 255

3Mg sites exhibit slightly higher chemical shift than Si-O-Al

sites but similar 256

quadrupolar coupling constants and are in accordance with the only Si-O-3Mg site found 257

in talc [29]. If Al

-O-Al

sites are found in the same chemical shift region than in the

258

previous studies of framework aluminosilicates, their quadrupolar coupling constant is 259

significantly smaller. However, the obtained values are in good agreement with the Al

- 260

O-Al

bridging oxygen site first evidenced by Stebbins et al. ( δ

= 19 ppm and P

=1.85 261

MHz) [10]. Finally, Al

-O-3Mg site are in the same chemical shift than that reported for 262

Al

-O-Al

however their quadruplar coupling constant is smaller. Up to our knowledge, 263

no Al

-O-3Mg site has been reported before.

264

Figure 4 represents the simulated

O MAS NMR spectra for Na-2-Mica and Saponite, 265

together with the experimental one. The best-fit simulation was obtained using the DFT- 266

calculated NMR parameters of Structure II (Table 3), which includes Si-O-Si sites as 267

expected in the structure of these two aluminosilicates. No Al

-O-Al

sites were 268

considered for the modelling and relative ratios of the different oxygen sites were set at 269

values close to those given in Table 1. The δ

, C

and η

NMR parameters after fitting 270

are summarized in Table 4. In Na-2-Mica the DFT calculated C

and δ

values were 271

only slightly modified within a reasonable range except for Si-O-Al

and Si-O-Si 272

chemical shift that was increased by about 6 ppm in the fitting. Fitting saponite spectrum 273

generates the displacement of all the chemical shifts between 4 and 11ppm from the 274

calculated DFT values and 6 ppm from the Na-2-Mica fitting values, probably due to the 275

presence of OH

groups in the structure.

276

As expected if the Lowenstein’s rule is respected, the experimental

O MAS NMR 277

spectra of Na-2-Mica and saponite can be fully explained considering 4 different oxygen 278

sites: Si-O-Si, Si-O-Al

, Si-O-3Mg and Al

-O-3Mg.

279

Figure 5a shows the comparison of the experimental

O MAS spectrum (solid black 280

line) and the calculated spectrum (solid blue line) for Na-4-Mica using DFT calculated 281

values from structure I. The calculated spectrum is obtained by summation of all

282

individual contributions (dotted lines), taking into account their expected relative 283

proportion (75% for Si-O-Alt

and 12.5% for Si-O-3Mg and Al

-O-3Mg, respectively).

284

A reasonable agreement is found between the experimental and calculated spectra using 285

Structure I for region A for which two contributions, Si-O-Al

and Si-O-3Mg are 286

sufficient. One should note that Si-O-3Mg site largely overlap with the Si-O-Al

sites, 287

due to similar quadrupolar coupling constants that provoke a line broadening.

288

Consequently, low spectral resolution is observed. Below 30 ppm in region B, NMR 289

parameters for Al

-O-3Mg sites fit perfectly the experimental data. Remarkably, the 290

narrow resonance is in line with a small C

value (< 1 MHz), as deduced from the DFT 291

calculations. However, as previously mentioned, the theoretical proportion of Al

-O- 292

3Mg sites is not sufficient by itself to explain the intensity of the resonance observed in 293

the experimental spectrum.

294

Structure II was then proposed in order to add explanatory contributions from Si-O- 295

Si and Al

-O-Al

sites, which would then suggest a violation of the Lowenstein’s rule 296

for the Na-4-Mica sample. Figure 5b shows the best-fit simulation of

O MAS NMR 297

spectrum for Na-4-Mica using all possible oxygen environments. In region A, the NMR 298

signal is due to the combination of the basal oxygens Si-O-Al

and Si-O-Si, and the 299

apical arising from Si-O-3Mg. In region B, the signal is now the result of two 300

contributions, i.e., the apical Al

-O-3Mg site and the basal oxygen Al

-O-Al

site, with 301

relative proportions close to those expected.

302

4. Conclusion 303

In the present work, we demonstrated that in a synthetic swelling high charged mica, 304

Na-4-Mica, the Lowenstein’s rule is violated. Additionally, our findings reveal the 305

viability of using

O MAS NMR at natural abundance on synthetic high-charged

306

silicates. By combining the experimental work and DFT calculations, it is now possible 307

to offer a complete description of the heteroatoms distribution within the mica sheets, 308

distribution that depends on the total layer charge. In the Na-2-Mica (Si/Al < 1), the Si 309

and Al atoms are randomly distributed obeying the Lowenstein’s rule. However, in the 310

Na-4-Mica (Si/Al =1), the most stable structure is produced with a completely freed Si/Al 311

distribution, even with the formation of Al

-O-Al

bonds of unfavorable enthalpy cost.

312

313

Acknowledgements 314

We would like to thank the CNRS, Université de Lille, Chevreul Institute (FR2638), 315

Région Hauts de France, Ministère de l'Education Nationale de l'Enseignement 316

Supérieure et de la Recherche and FEDER for financial support. This work was made 317

possible through the use of the computational resources provided by the Centre de 318

Ressources Informatiques (CRI) of the Université de Lille ST. Financial support from 319

the TGIR-RMN-THC Fr3050 CNRS for conducting the research is also gratefully 320

acknowledged. The authors thank the Junta de Andalucía (Spain) and FEDER (Proyecto 321

de Excelencia de la Junta de Andalucía, project P12-FQM-567), the Spanish State 322

Program R+D +I oriented societal challenges and FEDER (Project MAT2015-63929-R) 323

for financial support. Dr. Pavón received support from Andalucía Talent Hub Program, 324

co-funded by the EU in 7FP, Marie Skłodowska-Curie actions and the Junta de Andalucía.

325

F.J. Osuna received support from the training researcher program associated with the 326

excellence project of Junta de Andalucía (P12-FQM-567).

327 328

Appendix A. Supplementary material 329

Supplementary data associated with this article can be found, in the online version 330

331

References 332

333

334

Table 1

335 336 337 338

Table 1: Samples studied in this work, their structural theoretical formulae and the kind of 339

substitution they exhibit. Relative proportion of oxygen sites for all the samples, deduced either 340

from their theoretical structure (Saponite samples) or from the analysis of the

Si MAS NMR 341

spectra (Mica samples). For the Mica systems, the amount of Al

-O-Al

linkages was determined 342

using the

Si NMR spectra of Na-n-Mica [5] following the method described elsewhere [34]. In 343

Na-4-Mica two different calculations for the proportion of apical oxygens is represented: a) no 344

aluminum in the octahedral sheet is considered as ideally expected and b) the aluminum in 345

octahedral coordination observed in the

Al MAS-NMR spectrum (15% of the total aluminum) 346

[5] is considered to be in the octahedral sheet.

347 348 349 350

351 352 353

†

Supplied by the Source Clays Repository of the Clay Minerals Society.

‡

Synthesized by Pavon et al, following Alba et al. Chemistry of Materials 2006, 18 (12), 2867- 2872

Silicate Chemical formulae

Oxygen sites

Basal Oxygens Apical Oxygens

Si -O-

Si

Si -O- Al

Al

-O- Al

Si -O- 3Mg

Si -O- 2Mg1Al

Al

-O- 3Mg

Al

-O- 2Mg1Al

OH

Saponite

(Na

K

Ca

)[Si

Al

8

] [Mg

Fe

] O

(OH)

55.5 6.94 - 18.75 - 2.08 - 16.7 Na-2-

Mica

Na

[Si

Al

]Mg

O

F

48.3 26 - 18.75 - 6.25 - -

Na-4-

Mica

Na

[Si

Al

]Mg

O

F

13.94 47.7 5

11.2 5

12.5 - 12.5 - -

12.01 2.1 9.25 1.63 -

a

b

Table 2

354 355 356 357

Table 2.

O NMR parameters in Na-4-Mica as deduced from DFT calculations for the 358

different oxygen sites in Structure I with the NMR parameters obtained from modelling 359

the spectrum.

360 361

17

O sites

DFT Modelling

σ

, ppm

σ

-ref, ppm

C

,

MHz η δ, ppm C

,

MHz η % Al

-O-3Mg 230.92 23.5 0.88 0.50 27.3±0.2 1.80±0.03 0.5±0.2 13.2 Si-O-3Mg 207.50 43.4 3.13 0.11 43±2 3.6±0.4 0.4±0.2 13.3

Si-O-Al

216.50 35.3 3.38 0.19

217.86 34.1 3.43 0.49 42.0±0.3 3.0±0.1 0.6±0.05 73.5 214.25 37.3 3.47 0.41

362

363

Table 3

364 365

Table 3.

O NMR parameters in Na-4-Mica as deduced from DFT calculations for the 366

different oxygen sites present in Structure II with the NMR parameters obtained from 367

modelling the spectrum.

368

369 370 371

17

O sites

DFT Modelling

σ

, ppm σ

-ref,

ppm C

, MHz η δ, ppm C

, MHz η %

Al

-O-3Mg 226.23 26.4 0.97 0.25 27.45±0.01 1.61±0.02 0.6±0.2 10.1

230.72 22.4 0.75 0.52

228.06 24.8 0.88 0.46

230.97 22.2 0.85 0.49

Al

-O-Al

230.22 22.8 1.18 0.37 25.7±0.6 1.77±0.01 0.4±0.5 5.4

231.39 21.8 1.21 0.82

Si-O-3Mg 206.63 43.9 3.16 0.1 42.4±0.2 2.988±0.009 0.58±0.03 15.7

221.50 39.5 2.99 0.1

207.11 43.5 3.04 0.09

211.38 39.7 2.99 0.16

Si-O-Al

216.84 34.8 3.31 0.22 42.1±0.3 3.28±0.01 0.51±0.02 55.2

215.56 35.9 3.47 0.14

217.84 33.9 3.39 0.53

217.83 33.9 3.40 0.38

215.55 35.9 3.43 0.44

213.67 37.6 3.44 0.33

213.10 38.1 3.46 0.34

210.79 40.2 3.27 0.37

Si-O-Si 201.96 48.1 5.12 0.21 45.7±0.5 4.90±0.01 0.23±0.03 13.6

202.79 47.3 5.21 0.32

Table 4

372 373

Table 4.

O NMR parameters in Na-2-Mica and saponite after fitting the spectrum 374

using as starting values those obtained from DFT calculations in Structure II.

375 376 377

Na-2-Mica Saponite

17

O site δ, ppm C

MHz η % δ, ppm C

MHz η %

Al

-O-3Mg 27.3±0.3 1.9±0.1 0.6±0.2 5.54 34.0±0.5 1.8±0.2 0.6±0.6 2.24

Al

-O-Al

- - - - - - - -

Si-O-Mg 44. ±2 3.9±0.5 0.5±0.2 19.52 48.0±0.3 3.00±0.09 0.2±0.1 21.74 Si-O-Al

42±1 3.3±0.3 0.54±0.08 29.0 48±1 3.3±0.3 0.32±0.07 7.76 Si-O-Si 54±1 6.1±0.2 0.3±0.1 45.94 52.1±0.3 5.09±0.06 0.27±0.07 68.27

378

379

380

Figure 1

381

382 383

Fig. 1. Natural abundance

O MAS NMR spectra recorded at 18.8 T for (a) Saponite, (b) Na-2- 384

Mica and (c) Na-4-Mica. Regions A and B are related to NMR signals found above and below 385

30 ppm, respectively.

386

387

388

389

390

391

392

393

Figure 2

394 395 396 397 398 399 400 401 402 403 404 405 406

Fig.2. Schematic representation of the structural models for Na-4-Mica. Structure I is 407

constructed under the assumption of a perfect Si/Al ordering within the tetrahedral layer.

408

Structure II is built forcing the presence of Al

-O-Al

and Si-O-Si environments within 409

the tetrahedral layer. a-c: possible environments for basal oxygens.

410

411

Figure 3

412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428

Fig. 3. DFT-calculated

O C

and δ

of oxygen sites using Structure II. Top : 429

17

O MAS NMR spectrum of Na-4-Mica giving a guideline for the chemical shift regions.

430

431

Figure 4

432 433 434 435 436 437

438 439 440

441 442 443 444 445 446

Fig. 4. Experimental (solid line) and DFT simulated

O MAS spectra (solid blue 447

line) using Structure II for a) Na-2-Mica and b) Saponite. The individual contributions of 448

the oxygen sites are displayed with dotted lines.

449 450

a) b)

Figure 5

451 452 453 454 455 456

457

Fig. 5 Na-4-Mica experimental (solid line) and DFT simulated

O MAS spectra (solid 458

blue line) obtained using (a) Structure I and (b) Structure II. Each individual site 459

is represented by dotted lines.

460 461 462

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