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

In recent years, there has been increasing interest in developing cost-efficient, fast, and user-friendly 17O enrichment protocols to help understand the structure and reactivity of materials using 17O NMR. Here, we show for the first time how ball milling (BM) can be used to selectively and efficiently enrich the surface of fumed silica, which is widely used at the industrial scale. Short milling times (up to 15 min) allowed modulation of the enrichment level (up to ca. 5%) without significantly changing the nature of the material. High-precision 17O-compositions were measured at different milling times using LG-SIMS. High-resolution 17O NMR analyses (including at 35.2 T) allowed clear identification of the signals from siloxane (Si-O-Si) and silanols (Si-OH), while DNP analyses, performed using direct 17O polarization and indirect 17O{1H} CP excitation, agreed with selective labeling of the surface. Information on the distribution of Si-OH environments at the surface was obtained from 2D 1H-17O D-HMQC correlations. Finally, the surface-labeled silica was reacted with titania and using 17O DNP, their common interface was probed and Si-O-Ti bonds identified.