Development of non-precious metal hydrogenation catalysts under mild conditions is of great significance to modern chemical industry. In this work, Co@ZIF-8 based on metal-organic framework material was synthesized using 2-methylimidazole, zinc nitrate hexahydrate and cobalt acetylacetonate as precursors under solvothermal reaction condition at 120 ℃ for 6 h. Nitrogen-doped carbon with spatial confinement effect of Co catalyst (Co@ZIF-8(C)) with Co-N_x as active site was successfully prepared by high temperature pyrolytic reduction at 1000 ℃ under nitrogen atmosphere. The catalyst was systematically characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, raman spectroscopy, inductively coupled plasma emission spectrometry, fourier transform infrared spectroscopy, and nitrogen adsorption and desorption test. The analysis showed that Co in the synthesized Co@ZIF-8(C) catalyst was uniformly dispersed as atomic clusters confined in nitrogen-doped carbon carrier with content 1.8% (w). The doping nitrogen provides sites for anchoring the metal Co to form Co-N_x sites. Large specific surface area and mesoporous structure of the ZIF-8 carrier increased contact probability between substrate and active site. Strong interaction between the carrier and metal and spatial confinement effect improved the performance of Co clusters for catalytic transfer hydrogenation of nitro compounds. When 10 mg catalyst, 0.2 mmol nitrobenzene, 0.8 mmol AB (NH₃BH₃), 3 mL methanol and 2 mL deionized water as solvents were added to reaction system at 30 ℃, the yield of aniline could reach more than 99% after 10 min. Catalytic performance of this catalyst was better than that of ZIF-8(C), ZIF-67(C) and other Co catalysts recently reported. In addition, nitro compounds with different functional groups were also obtained in high yields under this system except for those with larger functional groups. Cycling and hot filtration experiments demonstrated that the Co catalyst has excellent stability and recyclability. This research provides a simple and efficient method of Co catalyst for reduction of nitro compounds.

Key words: Co atom cluster, metal-organic framework, transfer hydrogenation, ammonia-borane, nitro compound