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 3d5/2 XPS 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/MoO2 is multiply recyclable and with 69 ± 2% of the Mo sites catalytically significant by a titration poisoning assay. Overall, rGO/MoO2 shows promise as a well-defined, carbon-supported, efficient, and scalable single-site hexavalent metal dioxo heterogeneous catalytic system.

Synthesis and structure-activity characterization of a single-site MoO2 catalytic center anchored on reduced graphene oxide

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