Catalytic activity of highly ordered mesoporous VMCM-48 Original Research Article

Mesoporous VMCM-48 molecular sieves with Si/V (molar) ratios of 25–200 were synthesized hydrothermally and systematically characterized by various analytical and spectroscopic techniques, viz., XRD, N2 sorption, TG-DTA, TEM, EPR, 51V MAS-NMR, FT-IR, and DRUV-VIS. XRD studies suggest that the substitution of vanadium occurs in the silicate framework structure of MCM-48. TEM and ED investigations confirm the highly ordered cubic structure of VMCM-48. EPR and 51V NMR studies indicate the presence of pentavalent vanadium ions in tetrahedral framework positions, while DRUV-VIS spectra show their existence in two different environments. The catalytic activity of these well-characterized (both in the calcined and the washed forms) materials was evaluated for cyclohexane oxidation under mild reaction conditions. All these catalysts gave high substrate conversion and excellent product (cyclohexanol) selectivity. Furthermore, unlike many other vanadium-based heterogeneous catalysts reported, the mesoporous VMCM-48 catalysts show minimal leaching of the active vanadium species. This was confirmed by washing, recycling and quenching experiments where only a small amount of vanadium ions leaches out in the case of calcined samples, while vanadium was not detected for the washed ones. Finally, the catalytic activity of VMCM-48 was also compared with mesoporous VMCM-41 as well as microporous VS-1 catalyst. The results indicate that the former showed superior activity to those of the latter two, such superiority could, however, be directly related to the amount of vanadium incorporated in MCM-48 being larger than the amounts in MCM-41 and MFI structures.