Introducing a Novel Method for Probing Accessibility, Local Environment, and Spatial Distribution of Oxidative Sites on Solid Catalysts Using Trimethylphosphine

Trimethylphosphine (TMP) is demonstrated as a suitable P-31 MAS NMR probe molecule for determining accessibility, environment, and spatial distribution of oxidation-active oxidic metal species on solid catalysts quantitatively. It oxidizes to trimethylphos-phine oxide (TMPO) at oxygen donor sites, which is demonstrated for oxides of copper, manganese, cobalt, and molybdenum. At loadings < 2 wt % of Mo a direct quantitative correlation between TMPO quantity and accessible metal oxide content is observed. Exceeding 2 wt % results in a gradual agglomeration and thus decreases the amount of available oxidative sites, probed as a decay of the amount of TMPO formed. Additionally, the spatial distribution of oxides neighboring species could be inferred. The solid TMPO deposited near MoOx species was very sensitive to extra framework aluminum (EFAL) as well as Bronsted acid sites in close proximity, depending on Mo loading. Thus, the TMP method provides unprecedented insights into the surface chemistry of oxidative metal oxide catalysts.