Single cobalt sites in mesoporous N-doped carbon matrix for selective catalytic hydrogenation of nitroarenes

A supported cobalt catalyst with atomically dispersed Co-N_x sites (3.5 wt% Co) in a mesoporous N-doped carbon matrix (named Co@mesoNC) is synthesized by hydrolysis of tetramethyl orthosilicate (TMOS) in a Zn/Co bimetallic zeolitic imidazolate framework (BIMZIF(Co,Zn)), followed by high-temperature pyrolysis and SiO₂ leaching. A combination of TEM, XRD XPS and X-ray absorption spectroscopy studies confirm the absence of cobalt nanoparticles and indicate that these highly dispersed cobalt species are present in the form of Co-N_x. The exclusive formation of Co-N_x sites in the carbon matrix is attributed to the presence of a large amount of Zn and N in the BIMZIF precursor together with the presence of SiO₂ in the pore space of this framework, extending the initial spatial distance between cobalt atoms and thereby impeding their agglomeration. The presence of SiO₂ during high-temperature pyrolysis is proven crucial to create mesoporosity and a high BET area and pore volume in the N-doped carbon support (1780 m² g⁻¹, 1.54 cm³ g⁻¹). This heterogeneous Co@mesoNC catalyst displays high activity and selectivity (>99%) for the selective hydrogenation of nitrobenzene to aniline at mild conditions (0.5–3 MPa, 343–383 K). When more challenging substrates (functionalized nitroarenes) are hydrogenated, the catalyst Co@mesoNC displays an excellent chemoselectivity to the corresponding substituted anilines. The presence of mesoporosity improves mass transport of reactants and/or products and the accessibility of the active Co-N_x sites, and greatly reduces deactivation due to fouling.