Amorphous SiO2–Al2O3 supported Co3O4 and its catalytic properties in cyclohexane nitrosation to ɛ-caprolactam: Influences of preparation conditions

The amorphous SiO2–Al2O3 supported Co3O4 catalysts were prepared by impregnation method and characterized by SEM, BET, XRD, XPS, NH3-TPD and FT-IR. The influences of the preparation conditions including the loading content of cobalt and the calcination temperature were studied in detail. The results show that the supported Co3O4/SiO2–Al2O3 catalysts possess both Lewis and Brønsted acid sites and the amount of the Brønsted acid sites increases with the increment of the loading content of cobalt. The 20% Co3O4/SiO2–Al2O3 catalyst calcinated at 500 °C shows relative larger surface area and pore diameter, smallest and uniform particle size, well dispersed of Co3O4. And it has the largest amount of Brønsted acid sites because it occupies the highest content of cobalt and Co2+. The relationship between the catalytic performances and the characterization results suggests that the Brønsted acid sites are the selective sites for the cyclohexane nitrosation to ɛ-caprolactam. The 20% Co3O4/SiO2–Al2O3 calcinated at 500 °C gives the best result of conversion of 11.2% and ɛ-caprolactam selectivity of 83.3%, which is the best result since one step synthesis of ɛ-caprolactam from cyclohexane nitrosation was developed.

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► The best results were obtained since cyclohexane nitrosation to CPL was developed.
► Brønsted acid sites are supposed to be the selective sites for the formation of CPL.
► Selectivity to CPL is improved when oleum is replaced by concentrated sulfuric acid.
► The 20% Co3O4/SiO2–Al2O3 calcinated at 500 °C shows the best catalytic performance.