MAS NMR and XRD study on the vanadium site of vanadium silicate mesoporous molecular sieve MCM-41

A wide range (10<Si/V) of mesoporous vanadium silicate molecular sieves with the MCM-41 structure have been synthesized using vanadyl sulfate as the source of vanadium and characterized by XRD, V-51 MAS NMR and Si-29 MAS NMR. The increase of the unit cell parameter and the decrease of Q(3)/Q(4) ratio of Si-29 spectra with the vanadium content suggest the incorporation of vanadium in the framework of MCM-41 structure. V-51 MAS NMR demonstrates that vanadiums in as-synthesized V-MCM-41 are present in the chemical environment of octahedra and octahedral vanadium is decreased and tetrahedral vanadium is increased inversely with raising the calcination temperature. Though the thermal treatment in rotor of hydrated sample resulted in the change from tetrahedral environment to octahedral one and the steaming and the acid treatment affect to the chemical environment of vanadium, the spectrum similar to originally calcined sample is regenerated after recalcination. This indicates that the vanadium is belong to the framework in a relatively exposed site. The best quality XRD pattern of the product of Si/V=27 may be attributable to heterogeneous nucleation mechanism. V-MCM-41,s having the Si/V ratio lower than 20 are completely collapsed after calcination.