Assessing the Nature of Active Sites on Nanodiamonds as Metal-Free Catalysts for the EB-to-ST Direct Dehydrogenation Using a Catalytic Approach

The oxygen-free alkane direct dehydrogenation (DDH) is a greener, safer, and more selective alternative to oxygen co-fed conditions to produce olefins. Carbon-based catalysts have significantly boosted this branch of catalysis by providing cheaper, robust, durable, and more environmentally friendly single-phase materials as valuable substitutes of a variety of alkaline- or alkaline earth-promoted transition metal oxides. In particular, nanodiamonds (NDs) rank among the most effective and selective metal-free systems investigated so far for the alkane-to-alkene conversion under either oxidative or direct dehydrogenation conditions. Although important structural/compositional/activity relationships for this class of sp2/sp3 C-hybrids have already been unveiled for the exothermic oxidative dehydrogenation (ODH) process, many issues still remain to be addressed for the more challenging oxygen-free direct dehydrogenation process. In particular, the mechanism and nature of active sites in carbon catalysts employed for the alkane steam-free DDH still remain a controversial matter of debate because of the lack of studies under harsher operative conditions typically required by this endothermic process. Here, we report on the chemico-physical and morphological properties of nanodiamond samples before and after their use as catalysts in the model ethylbenzene-to-styrene dehydrogenation using oxygen and oxygen-free conditions. The combination of the catalytic outcomes with the extensive characterization of these metal-free systems led us to speculate on the nature of oxidized carbons as catalytically active sites in the DDH process. Density functional theory (DFT) calculations have finally been used to corroborate our hypotheses, providing support to the role of ortho-quinone (oQ) groups at the edge of cubic-sp3 NDs as the oxidized carbon source (active sites) for DDH.