Co₃O₄/Nitrogen-Doped Graphitic Carbon/Fe₃O₄Nanocomposites as Reusable Catalysts for Hydrogenation of Quinoline, Cinnamaldehyde, and Nitroarenes

Developing efficient, robust, and highly recyclable catalysts with the ability to separate products conveniently for industrially important hydrogenation reactions is a major challenge. Edges of nanoparticles possessing selective catalytic properties while the completely exposed metal particles are devoid of this attribute is a known fact. Herein, the preparation and evaluation of a Co3O4/N-Gr/Fe3O4 magnetic heterostructure composed of Co3O4 nanoparticles surrounded by nitrogen-doped graphitic carbon derived from ZIF-67 on an Fe3O4 support is described. Wrapping Co3O4 nanoparticles with porous nitrogen-rich graphitic carbon increases their catalytic selectivity and durability. Co3O4/N-Gr/Fe3O4 is obtained by pyrolysis of metal-organic frameworks, ZIF-67(Co) with magnetic Fe3O4 nanoparticles under nitrogen. Scanning electron microscopy reveals Fe3O4 as uniform octagonal microcrystals (∼450 nm) and transmission electron microscopy (TEM) shows graphitic carbon layers around the core Co3O4 nanoparticles on Fe3O4 microcrystals. TEM using a high-angle annular dark-field with spherical aberration (Cs) correction shows the core-shell structure of Co3O4/N-Gr nanocrystals (∼20 nm) with the graphitic carbon layers surrounding the core Co3O4 nanoparticles on Fe3O4 microcrystals. The resulting Co3O4/N-Gr/Fe3O4 construct produces a stable and reusable catalyst for the selective hydrogenation of structurally diverse N-heteroarenes. Particularly, quinoline was quantitatively hydrogenated to 1,2,3,4-tetrahydroquinoline (py-THQ) at 120 °C under 40 bar of H2. The wide applicability of Co3O4/N-Gr/Fe3O4 was tested for selective hydrogenation of cinnamaldehyde to hydrocinnamaldehyde (HCAL) with >99% selectivity. Also, the tolerance of functional groups in the reduction of nitroarene was evaluated. The benefit of the ability to produce py-THQ was demonstrated by extending the protocol for the synthesis of bioactive molecules, that is, a tubulin polymerization inhibitor with a 94% yield. The robust nature of the Co3O4/N-Gr/Fe3O4 construct was demonstrated through multiple cycles of simple separation and reuse.