Solid-state nanocasting synthesis of ordered mesoporous CoNx–carbon catalysts for highly efficient hydrogenation of nitro compounds

Abstract

Selective catalytic hydrogenation of nitro compounds (NCs) is an attractive challenge with significant research being focused on the development of cobalt (Co)-based nanocatalysts. Herein, in order to achieve high activity and selectivity for the catalytic hydrogenation of NCs and identify the essential active Co-containing sites, a facile solid-state nanocasting approach is developed for the controllable synthesis of CoN_x-doped ordered mesoporous carbon materials (denoted as CoN_x-OMCs). Compared with the previous nanocasting synthesis of mesoporous catalysts, the current method requires no solvent and relies on melting and interfacial chemical interactions between silica and the precursors for loading and casting, and chemical coordination among the precursors for the formation and dispersion of the active sites. The resulting CoN_x-OMCs possess high surface areas (∼941 m² g⁻¹), ordered mesopores (∼4.0 nm), high N content (∼6.8 wt%) and abundant CoN_x sites and fine metallic Co nanoparticles. With molecular H₂ as the reducing agent, the optimized catalyst delivers very attractive catalytic activities (100% conversions), selectivities (close to 100% selectivities) and stability (no obvious performance decay after cycling) in the hydrogenation of a series of NCs carrying diverse groups in aqueous solutions under mild conditions. A comparative study clearly reveals that the CoN_x sites, not the metallic Co nanoparticles, are the key active sites for the hydrogenation of NCs. The CoN_x sites are found to preferentially adsorb nitro groups, thus activating them and promoting their reduction. A detailed study reveals that the high catalytic performance relies on the synergistic cooperation of the catalyst composition and structure, which are tuneable by adjusting the synthetic conditions.