Direct ¹⁷O Isotopic Labeling of Oxides Using Mechanochemistry

Abstract

While 17 O NMR is increasingly being used for elucidating the structure and reactivity of complex molecular and materials systems, much effort is still required for it to become a routine analytical technique. One of the main difficulties for its development comes from the very low natural abundance of 17 O (0.04%), which implies that isotopic labeling is generally needed prior to NMR analyses. However, 17 O-enrichment protocols are often unattractive in terms of cost, safety, and/or practicality, even for compounds as simple as metal oxides. Here, we demonstrate how mechanochemistry can be used in a highly efficient way for the direct 17 O isotopic labeling of a variety of s-, p-, and d-block oxides, which are of major interest for the preparation of functional ceramics and glasses: Li 2 O, CaO, Al 2 O 3 , SiO 2 , TiO 2 , and ZrO 2 . For each oxide, the enrichment step was performed under ambient conditions in less than 1 h and at low cost, which makes these synthetic approaches highly appealing in comparison to the existing literature. Using high-resolution solid-state 17 O NMR and dynamic nuclear polarization, atomic-level insight into the enrichment process is achieved, especially for titania and alumina. Indeed, it was possible to demonstrate that enriched oxygen sites are present not only at the surface but also within the oxide particles. Moreover, information on the actual reactions occurring during the milling step could be obtained by 17 O NMR, in terms of both their kinetics and the nature of the reactive species. Finally, it was demonstrated how high-resolution 17 O NMR can be used for studying the reactivity at the interfaces between different oxide particles during ball-milling, especially in cases when X-ray diffraction techniques are uninformative. More generally, such investigations will be useful not only for producing 17 O-enriched precursors efficiently but also for understanding better mechanisms of mechanochemical processes themselves.

Sustainability Assessment of Mechanochemistry by Using the Twelve Principles of Green Chemistry

“…[101] Notably, LAG has also enabled the establishment of practical and economic isotope labelling protocols. [102][103][104][105] But not only liquids have been used as auxiliaries in ball milling reactions, the addition of catalytic amounts of solid ionic salts has also been a common practice. [106] Additionally, silica, alumina, sodium chloride or sodium sulfate rank among the most often used solid additives in ball milling reactions.…”

“…Moreover, in situ released liquid products in mechanochemical transformations have been observed to accelerate chemical reactions, [100] thus indicating the possibility for LAG catalysis (Figure 4c) [101] . Notably, LAG has also enabled the establishment of practical and economic isotope labelling protocols [102–105] …”

Field‐stepped ultra‐wideline NMR at up to 36 T: On the inequivalence between field and frequency stepping

“…The paramagnetic doping is necessary because a duration of T 1 is necessary for the cw NMR magnetization to reach a steady state. CS referencing of most nuclei is carried out by measuring the 17 O resonance frequency of a D 2 O sample and calculating the IUPAC recommended reference frequencies [39]. When performing field-stepped measurements, the magnetic field can be regulated by adjusting the 7 Li lock frequency accordingly, but only within the frequency bandwidth of the 7 Li resonance circuit.…”

“…The National High Magnetic Field Laboratory of the United States has built a seriesconnected hybrid (SCH) magnet that can reach fields up to 36 T ( 1 H resonance frequency higher than 1.5 GHz) [4]. Openly accessible to researchers worldwide, this magnet has demonstrated its resolution and sensitivity benefits for quadrupolar nuclei in a variety of samples since being commissioned [5,[8][9][10][11][12][13][14][15][16][17][18].…”

Labeling and Probing the Silica Surface Using Mechanochemistry and 17O NMR Spectroscopy**

“…In a recent study, we showed that various oxides including silica can be labeled in 17 O using a user-friendly and costefficient 17 O enrichment protocol involving mechanochemistry. [33] More specifically, it had been demonstrated that using the liquid-assisted grinding (LAG) approach, fumed silica (Aerosil) can be efficiently enriched in 60 minutes while working under ambient temperature and pressure, which is a clear advantage compared to previous protocols. [33] However, these milling conditions were found to induce some changes in composition and texture of the silica phase, such as a decrease in the specific surface area (by a factor of � 3), and the presence of impurities coming from the milling jar (e. g. Fe and Cr).…”