1. Diskussionsforum: Ausbreitungsphänomene in Natur, Technik und Gesellschaft 1 H Solid-state NMR Investigations of Calcination and Rehydration of Montmorillonite and Kaolinite Powder 1Dieter Freude by Nina Dvoyashkina, Seungtaik Hwang, Christian Chmelik, Jörg Kärger, Dieter Freude Oven for calcination, 1 d at 700 °C Calcination and rehydration in the open MAS rotor. At right rotors without and with cap, outer diameter is 4 mm. Rehydration in a desiccator over a solution of ammonium chloride. The room temperature was 20.0 ± 0.5 °C. This saturated aqueous solution creates 80% RH at 20 °C. 100% RH correspond to 23.1 mbar at 20 °C. Thus the water pressure in the desiccator was 18.5 mbar.

2. Dehydroxylation and rehydroxylation D. Muñoz-Santiburcio, M. Kosa, A. Hernández-Laguna, C. I. Sainz-Díaz, and M. Parrinello: Ab Initio Molecular Dynamics Study of the Dehydroxylation Reaction in a Smectite Model, J. Phys. Chem. C 2012, 116, 12203−12211. 2Dieter Freude Muñoz-Santiburcio et al. conclude that the mechanism of proton migration across the octahedral vacancy is more favorable than the migration between the octahedral cation pair. The most remarkable feature about this mechanism is that the migration is assisted by the dislocation of one tetrahedral Si, which stabilizes the charge in the proton releasing oxygen. Figure 1. Dehydroxylation reaction in a dioctahedral phyllosilicate. H atoms in black, O atoms in red, and octahedral cations in pink. We would be able to support the rehydroxylation (RHX) dating of archaeological ceramics, if we could observe the proton migration by NMR techniques. However,...

3. What are we looking at? Montmorillonite 3Dieter Freude Kaolinite pubs.usgs.gov/of/2001/of01-041/monstru.jpg Philippe F. Weck, Eunja Kimb and Carlos F. Jové-Colóna: Relationship between crystal structure and thermo-mechanical properties of kaolinite clay: beyond standard density functional theory, Dalton Trans., 2015, 44,12550 We are interested in hydroxyl groups, but we look at the superposition of the hydroxyl proton signal and the water proton signal. The latter is strongly broadened by homonuclear dipolar interaction for hydrated clay materials.

4. 1 H NMR literature in this field 4Dieter Freude A magic-angle spinning frequency of 64 kHz was necessary, in order to obtain well-resolved signals of inter-layer water and hydroxyl groups. In addition, the samples were synthesized without impurities and calcined at 100 °C which reduces the hydration. The "dehydrated sample" was dehydrated at 100 °C under vacuum. In conclusion, 1 H solid-state NMR studies of hydrated clays are quite laborious - in other words, quite expensive. Sylvian Cadars, Régis Guégan, Mounesha N. Garaga, Xavier Bourrat, Lydie Le Forestier, Franck Fayon, Tan Vu Huynh, Teddy Allier, Zalfa Nour, Dominique Massiot: New Insights into the Molecular Structures, Compositions, and Cation Distributions in Synthetic and Natural Montmorillonite Clays, Chem. Mater. 2012, 24, 4376−4389 Figure 4. Solid-state NMR 1 H spectra of synthetic Na-mont- morillonite, collected at 17.6 T, at a MAS frequency of 64 kHz.

5. 1 H NMR comparison of fresh and at 700 °C calcined samples 5Dieter Freude Samples were provided by Vincent John Hare. The spectra of the fresh samples are blue. All spectra show spinning sidebands. The MAS rotation frequency of 10 kHz corresponds at a Larmor frequency of 750 MHz to a sideband distance of 13.3 ppm. The center of gravity of the centerband of the kaolinite sample has the chemical shift of 1.7 ppm corresponding to the chemical shift of hydroxyl groups. For montmorillonite the value is 2.4 ppm which indicates an increased water contribution. Water has a stronger homonuclear interaction compared to adjacent hydroxyl groups. This explains the stronger line broadening for montmorillonite. Montmorillonite Kaolinite The red spectra were measured after calcination. The area under the line is proportional to the concentration of hydrogen atoms for all 1 H NMR spectra. For kaolinite this means that less then 0.1% of the atoms remained; the very weak single line could be even caused by the Kel-F®-caps of the rotor. For montmorillonite we have a residual concentration of 0.85 ± 0.15% with respect to the fresh sample due to AlOH groups (narrow sideband pattern with the centerband at 1.4 ppm).

6. Sample rehydration 6Dieter Freude On the right hand side we see the blue spectrum of the fresh sample and the green spectrum taken after 3 months after calcination. The total concentration of hydrogen seems to be similar. But the blue spectrum points to hydroxyl groups and the green one to water molecules. Spectra on the left hand side (from the bottom to the top) were measured straightaway after calcination, 1 d after calcination, 1 month after calcination and 3 months after calcination. We see that the residual hydroxyl signal at 1.6 ppm disappears after one day, and a water signal at 4.8 ppm, which reaches the half of its maximum value just after one day, increases within one month and remains constant for the next month. Montmorillonite In conclusion, rehydration increases the water content. But the sample is not rehydroxylated.

7. A 1 H solid-state NMR problem for broad lines: The probe head signal 7Dieter Freude Green spectrum is the difference between blue and red and corresponds to rehydrated montmorillonite. The green spectrum has a full width at half maximum of fwhm ≈ 28 kHz. The broadening results from homonuclear dipolar spin-spin interaction. The signal shape can be approximated by a Lorentzian line. In this case we have a transverse relaxation time T 2 Hahn echo = T 2 = 1 / fwhm ≈ 11 µs. Blue spectrum is the measured one. Montmorillonite without sample spinning Red spectrum is taken with an empty rotor. The signal comes from the probe head. The base line roll of all spectra is a minor methodical artifact.

8. Can we use fast magic-angle spinning and echoes for signal separation? 8Dieter Freude All spectra on this page were measured with MAS frequency of 25 kHz (instead of 10 kHz used in the experiments shown on the previous slides) with a 2.5-mm-rotor and a MAS stator which costs twice the price of a 4-mm-stator. On the left hand side we see spectra of the rehydrated kaolinite which were obtained with a Hahn echo with pulse distances between 40 µs (top) and 7 ms (bottom). For longer pulse distances the 1.13-ppm- signal becomes dominant. Unfortunately, the 2.5-mm-rotor has Vespel®-caps with a proton signal. On the right side we have three spectra with a pulse distance of 1 ms. The upper spectrum is montmorillonite, the middle spectrum is kaolinite and the spectrum on the bottom is the empty rotor. We see that the 1.13-ppm- signal rather corresponds to the caps than to hydroxyl groups. In conclusion, 25 kHz spinning and the use of Hahn echoes cannot separate water and hydroxyl signals for hydrated clay materials.

9. PFG NMR on hydrated clay materials? 9Dieter Freude MAS PFG NMR uses gradient strengths up to 2 T/m. Traditional PFG NMR can be performed in Leipzig with gradients up to 37 T/m. A minimum r. f. pulse distance of 1 ms is necessary in the two-pulse sequence Hahn echo for the application of a pulsed field gradient. The spectra above were measured without MAS rotation and with an echo pulse distance of 15 µs (blue) and 1 ms (red). The intensity of the red signal is very low compared to the blue one. But the essential point is that it seems identical with the cap signal. We expected this result from the estimated value T 2 Hahn echo ≈ 11 µs. In conclusion, after a /2-1ms- -1ms pulse sequence is no signal left, and thus PFG NMR experiments are not feasible for rehydrated clay materials. Montmorillonite Kaolinite

10. Acknowledgement 10Dieter Freude Thanks go to Vincent John Hare, since he provided the samples, and to Jürgen Haase for providing the NMR equipment.