Introduction
Heterogeneous catalysis has developed alongside homogeneous catalysis and brings the advantage of separation and re-use of the catalyst. In particular, the synthesis of porous silicates has been explored extensively with some successful commercialization, such as zeolites (Editorial in Nature Materials, 2020; Busca and Gervasini, Reference Busca and Gervasini2020). These materials provide strong acid sites for heterogeneous catalysis processes as well as good thermal stability. For example, faujasite is dehydrated below 400°C and no further transformation occurs until 770°C (Földvári, Reference Földvári2011). Clay minerals are also attractive porous materials. Their catalytic activity is sometimes referred to as ‘enzyme-like’ due to the presence of weaker acid sites compared with zeolites. Thus, clay minerals can be used to catalyze ‘mild’ reactions, where strong acid sites might not be necessary (Theng, Reference Theng2018). The most frequently used commercial clay-based acid catalyst is ‘K10’, which is obtained from a montmorillonite by calcination and washing with mineral acid. The resulting catalyst presents a strong Brønsted acidity (Cornélis and Laszlo, Reference Cornélis and Laszlo2004). Although natural clay mineral deposits are widespread, they are often heterogeneous in terms of their mineralogical composition, implying the need for post-treatments for purification and acid functionalization, as with the K10 commercial clay. On the contrary, synthesis of clay minerals allows us to obtain a relatively pure phase with a controlled chemical composition.
Among the clay minerals, saponite has the greatest thermal stability (Földvári, Reference Földvári2011) and is straightforward to synthesize (Kloprogge, Reference Kloprogge1999; Jaber et al., Reference Jaber, Komarneni, Zhou, Bergaya and Lagaly2013). Its general chemical composition can be written as Na x-yMg3-y Al y(Si4-x Al x)O10(OH)2·nH2O, where Mg2+ and Al3+ ions surrounded by O2– and OH – groups are at the center of a sheet of octahedra. The octahedral sheet is sandwiched between two tetrahedral sheets composed of Si4+ and Al3+ atoms as shown in Fig. 1. Isomorphic substitution of Mg2+ for Al3+ in the octahedral sheet or Si4+ for Al3+ in the tetrahedral sheet results in the overall negative layer charge, which is compensated by positive cations in the interlayer space. Previous studies have reported and discussed the acidic surface properties of saponite (Okada et al., Reference Okada, Arimitsu, Kameshima, Nakajima and MacKenzie2006; Bisio et al., Reference Bisio, Gatti, Boccaleri, Marchese, Bertinetti and Coluccia2008; Gómez-Sanz et al., Reference Gómez-Sanz, Morales-Vargas, González-Rodríguez, Rojas-Cervantes and Pérez-Mayoral2017; Tkachenko et al., Reference Tkachenko, Kustov, Kapustin, Mishin and Kuperman2017). In particular, Bisio et al. (Reference Bisio, Gatti, Boccaleri, Marchese, Bertinetti and Coluccia2008) reported evidence that a mild acid treatment resulted in the formation of sites, the acidity of which was similar to that observed in strong acid zeolites.
Schematic representation of the saponite structure.
Synthetic saponites are relatively easy to produce. Several extensive reviews have been published, summarizing various synthesis conditions (Kloprogge, Reference Kloprogge1999; Zhang et al., Reference Zhang, Zhou, Lin, Tong and Yu2010; Carniato et al., Reference Carniato, Gatti and Bisio2020; Ponce and Kloprogge, Reference Ponce and Kloprogge2020). However, the formation mechanisms of saponite are still not well understood. Recent studies by Besselink et al. (Reference Besselink, Stawski, Freeman, Hövelmann, Tobler and Benning2020) and Blukis et al. (Reference Blukis, Schindler, Couasnon and Benning2022) reported the formation of saponite nuclei from a magnesium hydroxide and aluminosilicate gel. Baldermann et al. (Reference Baldermann, Dohrmann, Kaufhold, Nickel, Letofsky-Papst and Dietzel2014) suggested that brucite-type (magnesium hydroxide) octahedral template sheets were formed first. They suggested a subsequent condensation of dissolved Si-O tetrahedra upon the brucite sheet. They noted that multiple dissolution-precipitation processes followed, leading to the reorganization of the structure and resulting in the formation of saponite structures. Another study by Zhang et al. (Reference Zhang, Petit, He, Villiéras, Razafitianamaharavo, Baron, Tao and Zhu2020), which investigated the saponite crystal growth process, focused on the evolution of the saponite crystallite size with time. Their study elucidated the suggestion that crystals grew rapidly from 15 to 45 nm during the first 5–15 days; after that time the size did not evolve significantly. These studies have thus provided important insights into the crystal-growth process of saponite, but they did not focus on the link between element speciation in the solution and in the solid; nor did they investigate precursor compounds leading to saponite formation. The geometry of the Si and Mg ions is identical in aqueous or in solid phases, i.e. tetrahedral or octahedral, respectively. This is completely different for the aluminum, the coordination of which is pH-dependent. Below pH6, Al chemical entities in the aqueous solution are octahedrally coordinated, but above pH6 they become tetrahedrally coordinated. Indeed, previous studies proved experimentally that the type of clay mineral obtained depends on the solution pH (Criouet et al., Reference Criouet, Viennet, Baron, Balan, Buch, Delbes, Guillaumet, Remusat and Bernard2023). This suggested that the coordination of Al in the solution would have an influence on its coordination in the resulting clay mineral phase (de Kimpe et al., Reference de Kimpe, Gastuche and Brindley1961; Frank-Kamenetzkij et al., Reference Frank-Kamenetzkij, Kotov and Tomashenko1973). Regarding the coordination of Al in saponite, the study by Gebretsadik et al. (Reference Gebretsadik, Mance, Baldus, Salagre and Cesteros2015) reported that a high pH (>13) resulted in a saponite clay mineral with a higher IVAl:VIAl ratio compared with the synthesis at pH8. However, the reported results were not discussed in terms of the Al speciation in the solution. The authors focused instead on the influence of different additives for the saponite synthesis. Also, different chemicals were used as a source of Si, Al, and Mg, depending on the target pH, which might lead to different mechanisms.
In this context, the coordination of Al in the saponite could be suggested to depend on the Al coordination in the solution during the synthesis. This point was investigated in the current study by preparing a series of synthetic saponite-like materials with various OH:Si initial molar ratios. The differences in the precursor solution pH was expected to have an influence on the nature of the product obtained. A detailed characterization of the resulting solids was carried out by studying the coordination and the local environment of Si, Mg, and Al atoms using XANES spectroscopy. In particular, focus was placed on the Al by coupling nuclear magnetic resonance (NMR) and XANES spectroscopies to investigate a possible link between the coordination of Al in the solution and in the solid.
Materials and methods
Chemicals
Magnesium nitrate 99% (Mg(NO3)2·6H2O), sodium metasilicate (Na2SiO3), and sodium hydroxide 97% (NaOH) were purchased from Sigma Aldrich. Aluminum nitrate 98–102% (Al(NO3)3∙9H2O) was purchased from Carlo Erba. Deionized water (18.2 MΩ·cm) was used for all syntheses.
Synthesis of the saponite clay-like mineral materials
Seven samples of saponite-like mineral were synthesized using a hydrothermal treatment by varying the molar ratio of OH:Si from 0.18 to 2.14 (Table 1). The precursor was prepared by mixing 0.2 M Mg(NO3)2, 0.2 M Al(NO3)3, and 0.2 M Na2SiO3/NaOH solutions with an Al:Si molar ratio of 0.33 and a Mg:Si molar ratio of 0.75. The NaOH was added to the 0.2 M Na2SiO3 solution prior to mixing in such a way that the NaOH/Na2SiO3 initial molar ratio varied between 0.1 and 2.0. After precipitation, the suspension was homogenized over a period of 3 h by stirring under ambient conditions and then calcined at 450°C prior to hydrothermal treatment. At the maximum temperature, the holding time was 1 h after a heating rate of 5°C min–1 and a cooling rate of <10°C min–1 afterwards. The calcined precursor was transferred to a mineralization bomb with a Teflon liner; 70 mL of 0.01 M NaOH solution was added, and the mineralization bomb was sealed tightly and set at 200°C for 5 days. The solid/solution ratio was ~0.09. The amount of OH– added by means of 0.01 M NaOH solution was included in the overall OH:Si molar ratio reported in Table 1. After the hydrothermal treatment, samples were recovered, washed three times with the deionized water by centrifugation at 8000 rpm (9946×g) for 10 min, and then dried in the oven at 60°C. The pH of the supernatant recovered after synthesis was measured at 19°C using an Orion™ ThermoFisher electrode (pH ±0.01) calibrated with two buffer solutions at pH7.00 and pH10.00.
The initial molar ratio of OH:Si and solution pH after the synthesis
Characterization of the solid phases
The solids prepared were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), 27Al solid NMR spectroscopy, and XANES spectroscopy at Si, Al, and Mg K-edges.
XRD analysis was carried out using a Panalytical X’Pert Pro MPD diffractometer with CuKα radiation (λ=1.5418 Å) from 2 to 70°2θ with a step size of 0.017°2θ and a time per step of 218 s. The total time for the acquisition was 75 min. Samples were prepared using a back-loading technique.
The TEM was performed using a JEOL ARM200F model operating at 200 kV. Samples were deposited on copper grids after ultrasonic dispersion in chloroform. The samples were very likely to degrade upon beam exposure so, to avoid this, the images were taken very quickly, adjusting the focus elsewhere on the sample, away from the recorded image. No degradation of the sample during the image acquisition was assumed, therefore.
The 27Al MAS NMR spectra were recorded at room temperature using a Bruker Avance NEO 400 MHz spectrometer operating at B0=9.4 T giving a 27Al Larmor frequency of 104.23 MHz. The instrument was equipped with a Bruker 4 mm double-channel probe and ZrO2 rotors and operated with a π/12 radio frequency pulse length of 0.5 μs, a recycle delay of 1 s, and a spinning rate of 14 kHz. The chemical shifts were referred to an external [Al(H2O)6]3+ standard in an AlCl3 aqueous solution. The decompositions of the spectra were performed using the DMFit software (Massiot et al., Reference Massiot, Fayon, Capron, King, Le Calvé, Alonso, Durand, Bujoli, Gan and Hoatson2002).
XANES experiments at the Mg, Al, and Si K-edges were carried out on the LUCIA beamline of the SOLEIL synchrotron (Vantelon et al., Reference Vantelon, Trcera, Roy, Moreno, Mailly, Guilet, Metchalkov, Delmotte, Lassalle, Lagarde and Flank2016). The X-ray beam was monochromatized using a pair of potassium titanyl phosphate (KTP) (011) crystals. Spectra were collected with steps of 2 eV (Mg and Al) or 4 eV (Si) below the edge, of 0.2 eV in the edge region, and of 1 eV (Mg) or 0.5 eV (Al and Si) beyond the edge, at a counting time of 6 s (Mg) or 4 s (Al and Si) per step. Powder samples were prepared as 13 mm diameter pellets that were transferred into the experimental chamber under a primary vacuum. Spectra were acquired in fluorescence mode using a 60 mm2 mono-element silicon drift diode detector (Bruker) and corrected for detector dead time. The data processing, the normalization, and the analysis were performed with the Athena software (Ravel and Newville, Reference Ravel and Newville2005).
Calculation of the Al3+ solubility and speciation in solution
The speciation and the solubility of the aluminum in aqueous solution was calculated using the same principle as described previously for silica by Dzene et al. (Reference Dzene, Doggaz, Dutournié, Inoué, Abdelmoula, Jourdain, Le Meins, Brendlé, Martin and Michau2024). The overall concentration, C 0, of Al was considered to be 0.028 mol L–1, corresponding to the amount of Al introduced to prepare the precursor. The following constants were used: pKa (AlOH2+/Al3+)=5.0, pKa (Al(OH)2+/Al3+)=10.1, pKa (Al(OH)3/Al3+)=16.9, and pKa (Al(OH)4–/Al3+)=22.7 from the PHREEQC database (Ball and Nordstrom, Reference Ball and Nordstrom2001). Amorphous Al(OH)3 was considered as a precipitate with pKs=10.8. Other possible phases, gibbsite and boehmite, had lower pK values of 7.6 and 8.1, respectively.
Results and Discussion
Structure of the synthesized material
For all samples, X-ray diffraction patterns showed peaks at 19.5, 34–38, and 60.4°2θ (4.56, 2.64–2.36, and 1.53 Å, respectively) (Fig. 2). These peaks were characteristic of a trioctahedral clay mineral as expected for the saponite structure (Brindley and Brown, Reference Brindley and Brown1982). The stacking of layers was characterized by diffraction peaks at 7.2 and 29.4°2θ (12.2 and 3.04 Å, respectively) corresponding to (001) and (004) planes. These peaks were well defined for the samples synthesized at pH12.7 and pH12.2, suggesting a relatively large coherent scattering domain in the c direction. For samples synthesized at a lower pH, the 00ℓ reflections were absent. This was also accompanied by a broadening of the (02,11) reflection, which indicated a decrease in the coherence of the lattice structure in the a–b plane, probably due to a decrease of the average size of the particles at low pH. This was confirmed by TEM observations, which showed a layered structure for all samples (Fig. 3; see also Fig. S1 in the Supplementary material). The XRD analysis and TEM observations confirmed the synthesis of a saponite-like material for all pH values. However, the particle length and the number of stacked layers decreased with decrease in synthesis pH. This indicated that the pH played a major role in the growth of these particles. Therefore, the objective was to explore the structure of these different compounds on a local scale by probing the atomic environment of Mg, Si, and Al atoms. The Mg K-edge XANES spectra (Fig. S2A in the Supplementary material) presented three intense peaks at the near-edge energy region from 1307 and 1323 eV (denoted as A, B, and C) followed by two broad contributions in the post-edge region (labeled as D and E). The triplet was characteristic of Mg arranged in octahedral sheet-like structure such as in brucite and phyllosilicate minerals (Dien et al., Reference Dien, Mingsheng and Murata1999), corresponding to a combination of an electronic transition from 1s to an empty bound 3p state with multiple scattering resonances (Ildefonse et al., Reference Ildefonse, Calas, Flank and Lagarde1995). The shape, the absorption intensity ratio, and the energy position of the peaks were in good agreement with what was expected for Mg sites in trioctahedral smectites as shown recently by Inoue et al. (Reference Inoue, Nishiki, Fukushi, Suma, Sato, Sakuma, Tamura, Yokoyama, Shimbashi, Mizukami, Unami, Noji, Kitajima, Fukaya, Takeichi, Yamashita, Suga and Takahashi2023).
Powder XRD patterns of synthetic saponite-like materials as a function of synthesis pH.
TEM images of synthetic saponite-like materials as a function of synthesis pH. The image of the sample with pHf =9.7 is given in the Supplementary material (Fig. S1).
The Si K-edge spectra of the resulting saponite-like materials were all identical (Fig. S2B in the Supplementary material) with a main absorption peak A at ~1846 eV related to the electronic transition from 1s to an empty bound 3p state (Li et al., Reference Li, Bancroft, Fleet and Feng1995). By comparison with reference spectra, broad modulations were noticed beyond 1850 eV (labeled as B to E) related to the multiple scattering effect (Andrini et al., Reference Andrini, Moreira Toja, Gauna, Conconi, Requejo and Rendtorff2017) and were close to the expected structure of a trioctahedral smectite by comparison with the reference spectrum of hectorite (pure SiMg smectite). However, the small component located at 1852 eV was also present in a dioctahedral smectite (montmorillonite SWy-2) and could be a spectral signature of 3d-like transitions due to the substitution of Al atoms within the tetrahedral sheet. Overall, the similarity of the spectra indicated that both the structural environment of Si (tetrahedral) and Mg (octahedral) were not affected during the synthesis regardless of the pH of the reaction. This led only to the possibility of a change in the aluminum environment to explain the structural differences observed by the TEM and the XRD.
Coordination of aluminum in the saponite-like structure
The coordination of Al in saponite-like materials was determined by two independent techniques: X-ray absorption spectroscopy at the Al K-edge and 27Al solid-state NMR spectroscopy. The Al K-edge XANES spectra with pHf are presented in Fig. 4A. A clear evolution was observed due to a variation of the coordination number of Al atoms with varying pH, the electric dipolar transitions of the Al core electrons (1s) to the first empty electronic bound states (2p) being different in tetrahedral IVAl sites compared with octahedral VIAl sites (Ildefonse et al., Reference Ildefonse, Cabaret, Sainctavit, Calas, Flank and Lagarde1998; N’Guessan et al., Reference N’Guessan, Joussein, Courtin-Nomade, Paineau, Soubrand, Grauby, Robin, Cristina, Vantelon, Launois, Fondanèche, Rossignol, Texier-Mandoki and Bourbon2021). Typically, for the sample synthesized at pH=12.7, the XANES spectrum consisted of an intense edge feature at 1566 eV (A) and four resonances at 1571.6, 1575, 1581, and 1587 eV (B, C, D, and E, respectively). The comparison with mineral references (Fig. S3 in the Supplementary material) revealed that the main absorption peak at 1566 eV (A) was close to the position expected for IVAl sites, suggesting a predominance of these sites at pH=12.7.
XANES spectra at the Al K-edge of synthetic saponite-like materials (A) and the result of the linear combination fitting with the proportion of the different Al coordination as a function of pHf (B).
Modification of the solution pH induced a progressive shift of the A component to a higher energy with an increase in the B component contribution at 1571.5 eV. This peak was characteristic of VIAl sites in phyllosilicates (Ildefonse et al., Reference Ildefonse, Cabaret, Sainctavit, Calas, Flank and Lagarde1998), suggesting an upward trend in the proportion of VIAl as the pH of the solution was reduced. The other contributions related to the multiple scattering from atomic shells were attenuated (C and D) or slightly affected (feature E). The XANES spectra of the two outermost samples (pHf=6.7 and pHf=12.7) corresponded to compounds with a mixed coordination number (IVAl+VIAl). The possibility of fitting the intermediate curves by linear combination using the spectral signatures of the two extreme compounds, therefore, was tested. The fits are given in Fig. S4 in the Supplementary material. An excellent agreement between the experimental and fitted curves was obtained, revealing an evolution of the proportion of IVAl sites in the resulting saponite-like materials with increasing synthesis pH (Fig. 4B).
In order to obtain a quantitative analysis, 27Al solid-state NMR spectroscopy was performed. The results showed that for all samples, coordinations of aluminum, tetrahedral and octahedral, were identified by chemical shifts of 65 and 7 ppm, respectively (Fig. 5). The proportion of tetrahedral aluminum increased with increasing synthesis pH (Fig. 5), from 57% for the sample at pHf=6.7 up to 81% for the sample at pHf=12.7. This observation was in agreement with a previous study (Gebretsadik et al., Reference Gebretsadik, Mance, Baldus, Salagre and Cesteros2015), where 84% of the tetrahedral Al was obtained for the sample at pH=13. In the same study, for the sample prepared at pH=8, 72% of the tetrahedral Al was obtained. This value was greater than observed in the present study somewhere between 64 and 62%. However, the overall trend that the amount of tetrahedral aluminum increased with pH was the same for both studies. Note that in Gebretsadik et al. (Reference Gebretsadik, Mance, Baldus, Salagre and Cesteros2015) the sample synthesized at pH=8 used different precursors compared with the current study, which could possibly explain the difference in the behavior. Interestingly, despite the fact that the initial Al:Si molar ratio was different for both studies (0.18 in Gebretsadik et al. (Reference Gebretsadik, Mance, Baldus, Salagre and Cesteros2015) and 0.33 in the present study), the proportion of IVAl:VIAl was of the same order of magnitude.
27Al solid state NMR spectra of synthetic saponite-like materials as a function of synthesis pH.
The amount of tetrahedral aluminum determined by the NMR spectroscopy agreed very well with the amount determined by XANES spectroscopy (Fig. 6). The data points represented the same trend, with the exception of the sample at pH=11.7. For this sample, the amount of tetrahedral Al determined by NMR spectroscopy was smaller compared with that determined by XANES spectroscopy. Overall, both techniques allowed us to determine the amount of IVAl, and showed a continuous evolution from octahedral to tetrahedral aluminum without intermediate species such as the penta-coordinated aluminum (Gilson et al., Reference Gilson, Edwards, Peters, Rajagopalan, Wormsbecher, Roberie and Shatlock1987; Wang et al., Reference Wang, Jiang, Lafon, Trébosc, Duk Kim, Stampfl, Baiker, Amoureux and Huang2016).
Comparison of IVAl:TOTAl determined from the NMR (red) and the XANES (blue) data.
Relationship between aluminum coordination in aqueous and solid phases
Considering different equilibrium constants of aluminum species in an aqueous solution given in the ‘Materials and methods’ section, the continuous black line in Fig. 7 represents the percentage of Al(OH)4– in solution with respect to other aqueous species as a function of pH. However, a fraction of all available aluminum is in solid form between pH4 and pH10. The dashed black line represents the percentage of Al(OH)4– considering all the available aluminum which is distributed between the solid and the aqueous species. As the IVAl:TOTAl trend was similar between NMR and XAS results (Fig. 6), only NMR results were plotted in Fig. 7. Above pH10, when all available aluminum was in soluble form as Al(OH)4–, the amount of tetrahedral aluminum in saponite-like materials increased with increase in the final synthesis pH. The increase in tetrahedral Al correlated with the appearance of and increase in H2SiO42– species as well (Dzene et al., Reference Dzene, Doggaz, Dutournié, Inoué, Abdelmoula, Jourdain, Le Meins, Brendlé, Martin and Michau2024). Between pH7 and pH10, Al(OH)4– was the major aqueous species, but most of the aluminum was in solid form as Al(OH)3. In this pH range, the percentage of IVAl:TOTAl in the saponite-like material was very similar, between 60 and 65%. Below pH7, the percentage of Al(OH)4– in the aqueous solution decreased, as did the amount of IVAl:TOTAl in saponite-like materials. The equilibrium constants corresponded to those at room temperature. Indeed, precipitation of the precursor took place at room temperature. The molar balance of OH– confirmed this hypothesis. The amount of OH– introduced was insufficient for hydroxylation of Al, Mg, and Si species separately (i.e. Al(OH)3, Mg(OH)2, and H4SiO4). This suggested that the first hydroxylated compounds formed precursor species such as HO-Mg-O-Si-(OH)3 or (HO)2-Al-O-Si-(OH)3. The formation of precursor species released OH–, which then allowed condensation reactions to proceed. The calcination of the precursor then served to ‘fix’ it in such conditions. During hydrothermal treatment, dissolution-precipitation reactions occurred. One could assume that the precursor prepared at room temperature served as a template for the precipitation and subsequent crystal growth of clay-like particles during hydrothermal treatment. The hydrothermal conditions (temperature of 200°C and autogenic pressure of <15 bar) favored the crystal-growth process. Such an assumption, however, would need to be verified by in situ measurements. Nevertheless, if one assumes that the precipitation of precursors at room temperature serve as a template for the growth of clay-like particles, Fig. 7 revealed good correlation between the Al speciation in an aqueous solution and the Al coordination in the precipitated solid phase. Previous studies (Baldermann et al., Reference Baldermann, Dohrmann, Kaufhold, Nickel, Letofsky-Papst and Dietzel2014; Besselink et al., Reference Besselink, Stawski, Freeman, Hövelmann, Tobler and Benning2020; Zhang et al., Reference Zhang, Petit, He, Villiéras, Razafitianamaharavo, Baron, Tao and Zhu2020; Blukis et al., Reference Blukis, Schindler, Couasnon and Benning2022) focused on the saponite crystal-growth process after precursor formation. It would be interesting to investigate the complete process, bridging the gap between the formation stage of the precursor compounds and the subsequent formation of saponite crystals.
The percentage of tetrahedral aluminum as a function of pH: versus the total amount of the aqueous aluminum (continuous line); versus the total amount of aluminum considering both aqueous and solid forms (dashed line); and the percentage of tetrahedral aluminum in saponite-like materials determined by NMR spectroscopy (filled triangles).
Conclusions
The aim of this work was to investigate whether there could be a correlation between the coordination of Al in the solid phase and Al coordination in solution. Characterization of the solid phase (saponite-like material) was done by two independent spectroscopic techniques: NMR and XANES at the Al K-edge. The proportion of tetrahedral Al obtained from both techniques was in excellent agreement. Comparison of these results with Al speciation in the solution as a function of pH revealed that above pH10, the amount of IVAl in the material increased. This pH domain corresponded to the presence of Al in soluble form as Al(OH)4–. Between pH7 and pH10, the proportion of the tetrahedral Al in the material changed only slightly, suggesting equilibrium between Al(OH)3(s) and Al(OH)4–(aq).
Supplementary material
The supplementary material for this article can be found at http://doi.org/10.1017/cmn.2024.41.
Author Contributions
Liva Dzene: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - Original Draft, Writing - Review & Editing, Visualization, Project administration, Funding acquisition
Patrick Dutournié: Methodology, Formal analysis, Writing - Review & Editing
Séverinne Rigolet: Methodology, Formal analysis, Investigation, Resources, Writing - Review & Editing
Loïc Vidal: Methodology, Formal analysis, Investigation, Resources, Writing - Review & Editing
Benedikt Lassalle-Kaiser: Methodology, Resources, Writing - Review & Editing
Delphine Vantelon: Methodology, Resources, Writing - Review & Editing
Erwan Paineau: Conceptualization, Methodology, Formal analysis, Investigation, Writing - Original Draft, Writing - Review & Editing, Visualization, Funding acquisition.
Acknowledgements
The XRD, TEM, and NMR spectroscopy experiments were performed on the technical platforms at IS2M. The authors are very grateful to L. Michelin for her contribution.
Financial support
The authors acknowledge Synchrotron SOLEIL for providing beamtime under proposal 20210095.
Competing interest
The authors declare none.
Data availability statement
Data will be made available on request.
