Preparation of Tripodal Vanadium Oxo-Organometallic Species Supported on Silica, [( SiO)(3)V(=O)], for Selective Nonoxidative Dehydrogenation of Propane

In this work, a strategy based on the grafting of a vanadium molecular complex, [V(=O)(OtBu)(3)], (1), bearing tert-butoxide ligands, on a partially dehydroxylated silica, followed by a thermal treatment has been used for the preparation of supported tripodal V oxo complex, [( SiO)(3)V(=O)], active species, for important chemical processes. The molecular complex reacts with a silica partially dehydroxylated at 200 degrees C (SiO2- 200) to lead to bipodal [( SiO)(2)V(=O)(OtBu)] (2) as major surface species, while monopodal surface species [( SiO)V(=O)(OtBu)(2)] (3) are obtained on SiO2-700. These materials were thermally treated to form active supported tripodal vanadium oxo species by releasing isobutene via a beta-H transfer, leading to V-OH intermediate. The latter species lead to the targeted tripodal oxo species [( SiO)(3)V(=O)] after restructuration of the support and will be represented as materials (2a) and (3a). All of the supported vanadium complexes were characterized by diffuse reflectance infrared Fourier transform (DRIFT), UV-vis, solid-state NMR, and X-ray absorption near edge structure (XAN ES) spectroscopies. In particular, the structure of isolated vanadium oxo tripodal species, [( SiO)(3)V(=O)], was confirmed by V-51 NMR with a major peak at -670 ppm, as reported in the literature. The catalytic property of (2a) and (3a) was evaluated for the direct propane dehydrogenation and showed promising performances that can be solely assigned to the isolated vanadium tripodal species.