Molecularly-derived single-site heterogeneous catalysts can bridge the understanding and performance gaps between conventional homogeneous and heterogeneous catalysis, guiding the rational design of next-generation catalysts. While impressive advances have been made with well-defined oxide supports, the structural complexity of other supports and the nature of the grafted surface species present an intriguing challenge. Here, we report the synthesis and detailed characterization of a highly active single-site molybdenum dioxo catalyst anchored on a well-ordered reduced graphene oxide support (rGO/MoO₂). The chemical and physicochemical properties of the precursor, the rGO support, a crystallographically characterized molecular model compound, and how the former two evolve on catalyst chemisorption, are extensively characterized by NMR spectroscopy, ICP-OES, XRD, XPS, DRIFTS, Raman spectroscopy, N₂ physisorption, NH₃-TPD, aqueous contact angle, Mo EXAFS, and DFT computation. The NH₃-TPD experiments reveal that the anchored MoO₂ moiety does not have strong acidity, while the Mo 3d_5/2XPS assigns the oxidation state as Mo(VI), and the PXRD shows little change in rGO periodicity on MoO₂ grafting. Contact angle analysis shows that MoO₂ grafting consumes rGO surface polar groups, producing a more hydrophobic surface. The rGO/MoO₂ DRIFTS assigns features at 959 cm^-1 and 927 cm^-1 to the symmetric and anti-symmetric Mo=O stretching modes of an isolated cis-(O=Mo=O) moiety. Moreover, the Mo EXAFS rGO/MoO₂ structural data are consistent with isolated (C-O)₂-Mo(=O)₂ species having two Mo=O multiple bonds and two Mo-O single bonds at distances of 1.69 Å and 1.90 Å, respectively. These metrical data are in excellent agreement with the model compound and the DFT computational results. In reductive carbonyl coupling reactions at TOFs approaching 1.81x10³ h^-1, rGO/MoO₂is multiply recyclable and with 69 ± 2% of the Mo sites catalytically significant by a titration poisoning assay. Overall, rGO/MoO₂shows promise as a well-defined, carbon-supported, efficient, and scalable single-site hexavalent metal dioxo heterogeneous catalytic system.

Leighton Jones

Appearances