Molecular and Surface W-Sites through 31P-Detected and Dynamic Nuclear Polarization Surface-Enhanced 183W NMR...

Supported tungsten catalysts are widely employed in industrial olefin metathesis reactions. However, further optimization of their performance requires detailed insight into the local structure of the active sites. Accessing this information remains challenging because of the limited site resolution and surface sensitivity of conventional spectroscopic techniques. While 183W solid-state NMR spectroscopy could in principle offer direct information on the tungsten coordination environment, its practical application is severely limited because of intrinsically low sensitivity. Herein, we demonstrate that the 183W NMR signatures of molecular and supported alkylidene complexes are readily accessible through indirect 31P detection. Through this strategy, we are able to identify two distinct Lewis acidic tungsten surface sites on WOx/SiO2 catalysts and resolve their structural characteristics through their NMR fingerprints. A fraction of the total number of sites (ca. 30%), which correspond to isolated dioxo sites, are able to coordinate PMe3, a requirement for NMR detection; these sites can likely also coordinate other small molecules, to possibly generate active sites. These findings provide a structural basis for understanding the catalytic activity and the rational design of supported metathesis catalysts.