CoAPO-5和MnAPO-5分子筛的合成、表征及在环己烷选择氧化反应中的应用

A series of cobalt or manganese incorporated aluminophosphates (CoAPO-5 and MnAPO-5) with different metal contents were hydrothermally synthesized. Various methods such as XRD, SEM, nitrogen adsorption, thermogravimetric analysis and UVVisible diffusion reflectance spectra were used to characterize the structure and crystallinity of the aluminophosphates as well as the chemical state of Co or Mn in the framework of molecular sieves. Moreover, the selective oxidation of cyclohexane was investigated with CoAPO-5 and MnAPO-5 as catalyst. The metal content incorporated in the molecular sieves has a vital influence on the structure, crystallinity and templates removal of CoAPO-5 and MnAPO-5. High crystallinity was obtained with a Me/P (Me = Co or Mn) mole ratio lower than 0.10, while higher metal content may result in a great deal of extra framework species. The tetrahedral Co2+ or Mn2+ in the lattice framework of molecular sieves was evidenced, which could be partially oxidized to Co3+ or Mn3+ to form redox centers upon calcination. The catalytic tests proved that CoAPO-5 and MnAPO-5 are effective catalysts for the selective oxidation of cyclohexane to cyclohexanol and cyclohexanone at low temperature (130℃) using molecular oxygen as oxidant. MnAPO-5 exhibits higher activity for alkane oxidation than CoAPO-5, but gives more products of deep oxidation. The products distribution was also affected by the reaction duration and the contents of the metal ions. CoAPO-5 with a Co/P mole ratio of 0.05 exhibited the best activity and selectivity; after the reaction at 130℃ lasted for 24h, the selectivity to cyclohexanol and cyclohexanone reached 88.5% with the cyclohexane conversion of 14.7%.