Characterization of reactive sites in supported catalysts by 51V/15N rotational echo double resonance NMR spectroscopy: formation of phenylimido groups at surface-bound oxovanadium sites.

Silica-supported oxovanadium groups were reacted in a gas-solid reaction with aniline at 175 degrees C. The reaction was clean as monitored in situ by UV-vis spectroscopy and resulted in the elimination of water as the principal product of the reaction and the disappearance of the terminal V=O stretch in the Raman spectrum. 15N MAS solid-state NMR spectroscopy showed only a single nitrogen-containing species on the surface. Proton-dephased 15N NMR showed only weak attenuation of its intensity, indicating that there are no protons directly bonded to the nitrogen. The formation of a vanadium-imido covalent bond was characterized by 51V/15N rotational echo double resonance NMR spectroscopy where the quadrupolar 51V nucleus was monitored and the spin-1/2 15N nucleus was dephased.