Study On The Properties Of Cu₂O Catalysts Supported Over MgO And MgO-based Composite Oxides For Dehydrogenation Of Cyclohexanol To Cyclohexanone

Catalytic dehydrogenation of cyclohexanol to cyclohexanone is an industrially important reaction in the manufacture of caprolactam, which is used as a material for the synthesis of nylon. But the dehydrogenation efficiency depends on the properties of the catalysts. So it has great significance to develop highly active and thermostable dehydrogenation catalysts.In this dissertation, MgO and CeO₂ were prepared by amorphous citrate method, the MgO-based composite supports MgO-SiO₂0, MgO-ZrO₂20 and MgO-CeO₂20 were prepared by amorphous citrate method and co-precipitation. The supported Gu₂O catalysts were prepared by chemical reduction combined with impregnation method. Cyclohexanol dehydrogenation to produce cyclohexanone was selected as probe reaction to study their catalytic properties. The catalytic properties of Cu₂O catalyst supported on MgO prepared by amorphous citrate method were compared with that supported on the commercial MgO for cyclohexanol dehydrogenation. And the effects of transition metals Fe, Co, Ni, Mo, Mn on the properties of Cu₂O/MgO(B) were studied. The effects of different MgO-based composite supports, different preparation methods of composite support and different proportion of MgO/CeO₂ in composite support on the properties of supported Cu₂O catalyst for cyclohexanol dehydrogenation were also studied. The catalysts were characterized by means of N₂ adsorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), CO₂ temperature programmed desorption (CO₂-TPD), cyclohexanol temperature programmed desorption (cyclohexanol-TPD) and cyclohexanone temperature programmed desorption (cyclohexanone-TPD). The results showed that (1) The activity of Cu₂O/MgO(B) was higher than that of Cu₂O/MgO(C). The main reason for it might be that the specific surface area of MgO prepared by amorphous citrate method was larger than that of the commercial MgO which was beneficial to the dispersion of the active components. The catalysts Mn-Cu₂O/MgO(B) and Ni-Cu₂O/MgO(B) exhibited higher catalytic activity than Cu₂O/MgO(B). The reason for it might be that Mn and Ni interacted with Cu₂O, resulting in the reduction ability of Mn and Ni declined, while that of Cu₂O increased. Therefore, Ou₂O in Mn-Cu₂O/MgO and Ni-Cu₂O/MgO was more stable than that in Cu₂O/MgO. And the addition of Mn and Ni into Cu₂O/MgO catalyst weakened the strength of the adsorption sites over the surface of the catalyst facilitating the dehydrogenation process. (2) The specific surface areas of the three MgO-based composite supports prepared by amorphous citrate method were large which was beneficial to the dispersion of the active components. And the order of them was MgO-SiO₂20> MgO-ZrO₂20> MgO-CeO₂20. But the activity of Cu₂O catalyst supported on MgO-CeO₂20 was higher than that of Cu₂O supported on MgO-ZrO₂20 and MgO-SiO₂20, and the selectivity to cyclohexaone on it was also the highest among them. This was perhaps that the Cu₂O catalysts supported on the MgO-based composite support had medium strong adsorptinn sites and strong adsorption sites on the catalyst surface, but Cu₂O catalyst supported on MgO-SiO₂20 with the largest specific surface area among the three composite supports had only the strongest adsorption strength of cyclohexanol which resulted in its lowest activity. (3) Compared with the composite support MgO-CeO₂20 (cp) prepared by co-precipitation, Cu₂O catalyst supported on MgO-CeO₂20 (B) prepared by amorphous citrate method showed higher dehydrogenation activity. This was mainly associated with the bigger surface areas of MgO-CeO₂20 (B) than MgO-CeO₂20 (cp) and more sites of medium strong adsorption existed on the surface of Cu₂O/MgO-CeO₂20(B) catalyst, which was favorable to the dehydrogenation process. (4) The activity of Cu₂O/MgO-CeO₂ was higher than that of Cu₂O/MgO(B) and Cu₂O/CeO₂ when the mass ratio of CeO₂ in the composite support is up to 10%. Although CeO₂ had the biggest specific surface areas, Cu₂O supported on CeO₂ showed the lowest activity. The reason was that the strong adsorption sites on Cu₂O/ CeO₂ played a dominant role in the adsorption of reactant and products. Cu₂O/ CeO₂ had the fewest number of weak basic sits over the surface of catalyst and Cu₂O could be reduced easily. Compared with catalyst Cu₂O/MgO(B), Cu₂O/MgO-CeO₂10 exhibited higher activity . The reason for it might be that the addition of CeO₂ into MgO enhanced the strength and number of weak basic sites, diminished the adsorption strength of reactant and product on the medium strong adsorption sites of catalyst Cu₂O/MgO. All the catalysts showed high selectivity for cyclohexanone because of the basic sites over them.