Effect of the Si/Zr molar ratio on the synthesis of Zr-based mesoporous molecular sieves

Highly ordered Zr-based mesoporous molecular sieves were synthesized via a surfactant-templated method and the effect of the Si/Zr molar ratio on the crystalline structure, textural properties and surface acidity were studied by XRD, FTIR, TEM and ²⁹Si MAS-NMR techniques. FTIR spectra show that the intensity of the band around 890 cm^-1 which corresponds to the vibration of Si-O-Zr bond was increased with increasing of the zirconium content, therefore, this band may be used as an indicator of the degree of the zirconium incorporation into the Si-framework. When the zirconium content increased in the materials, the Q³/Q⁴ value obtained from ²⁹Si MAS-NMR was linearly increased, whereas, the intensity the XRD peaks was gradually reduced; as a result, the pore wall thickness of the resultant materials was gradually increased, the surface area and the structural regularity were diminished. In order to obtain Zr-MCM-41 with highly ordered mesostructure and large surface area, proper Si/Zr molar ratio is a key factor, e.g., Si/Zr should be no less than 10. It was also found that the Brønsted acid sites which resulted from charge unbalance or local structure deformation due to the Zr⁴⁺ incorporation into the vicinity of the hydroxyls carrying silicon were created on the surface of the Zr-MCM-41 solids; strong Brønsted acidity could be formed on the solid with high zirconium content.