Morphology-dependent catalytic performance of Co3O4 nanomaterials in the oxidative dehydrogenation of tetrahydroquinolines

Abstract

Nanostructured metal oxides with controlled particle morphology are considered to be highly effective for heterogeneous catalysis owing to their unique surface, acid–base, and redox properties. Here, we report the synthesis of shape-controlled Co₃O₄ nanocatalysts for the oxidative dehydrogenation of N-heterocycles to their corresponding aromatic derivatives. Various Co₃O₄ nanostructured catalysts, namely spherical-like (Co₃O₄-SP) and cubic (Co₃O₄-C), along with randomly shaped nanoparticles (Co₃O₄-NP), were prepared via template-free hydrothermal methods and thoroughly characterized using several analytical techniques. Among them, the Co₃O₄-SP material exhibited superior catalytic performance in the oxidative dehydrogenation of 1,2,3,4-tetrahydroquinoline (THQ), achieving 95% conversion of THQ with 100% selectivity to quinoline, which is due to the optimum amount of Co³⁺ species (Co³⁺/Co²⁺ = 0.515) and acid sites (0.192 mmol g⁻¹), along with the oxygen vacancy sites. In contrast, the Co₃O₄-NP and Co₃O₄-C catalysts gave 72% and 51% conversions of THQ, respectively, although 100% quinoline selectivity was achieved in both cases. The substrate scope was further extended to diverse N-heteroaromatic compounds (10 examples), delivering good to excellent yields under mild reaction conditions. Notably, the Co₃O₄-SP catalyst exhibited excellent reusability, with negligible loss in its catalytic activity over five cycles. The evaluated green chemistry metrics further demonstrated the sustainability of the developed Co₃O₄-SP-catalyzed oxidative dehydrogenation of N-heterocycles.